V* I I E> R.A RY OF THE U N 1 V E. R_ S 1 TY Of ILLINOIS 636.1 Ml 6 a AGRICULTURE EH This volume cannbt be bound or repaired ecause of brittle paper . Reshelving in the olloving manner is considered a temporary easure. 1. _Tvin£ a nd/or wrapping and tying. 2. Storage in acid free envelope, document box. ect. ONSIDER THE FOLLOWING ALTERNATIVES: 1. 2 . 3 . k. \ 5 . Replacement Reprint Withdrawal Photocopying, Microfilm, Microfiche, ect. Storage for future preservation if bibliographic research determines it is valuable to our collection. (If transferred to Storage in Stacks , it must be last copy in system) 1, tf ~ : — -»w w — m — --VF.WL i c AFft - 197b - M 1 1 m ?. o 19 nst L161 — 0-1096 r!trf — f •>» ■ \J */■ n\r ' usan l~ i n i -i- • Digitized by the Internet Archive in 2016 with funding from University of Illinois Urbana-Champaign Alternates https://archive.org/details/anatomyofhorsedi00mcfa_0 I i { i { *. i DISSECTION GUIDE J. MTADYEAN, M.B., C.M., B.Sc., MEMBER OF THE ROYAL COLLEGE OF VETERINARY SURGEONS, LECTURER ON ANATOMY AT THE ROYAL (DICK’S) VETERINARY COLLEGE, EDINBURGH. W. & A. K. JOHNSTON, EDINBURGH AND LONDON. All rights reserved.'] W. AND A. K. JOHNSTON, PRINTERS, EDINBURGH AND LONDON. i(o.\ > V Co 8 . UNIVEfKiTV Of ILLINOIS Li.h^nf TO 1LLIAM TURNER, M.B., LL.D., F.R.S., PROFESSOR OF ANATOMY IN THE UNIVERSITY OF EDINBURGH, AS A TRIBUTE TO HIS EMINENCE AS AN ANATOMICAL TEACHER. PLATES. .ATE. 1. Pectoral Region ...» , 2. Pectoral Region . 3. Brachial Plexus .... 4. Shoulder — Outer Aspect . 5. Shoulder and Arm— Inner Aspect G. Shoulder, Arm, and Fore-arm— Inner Aspect 7. Fore limb — Outer Aspect . [ 8. Shoulder, Arm, and Fore-arm— Outer Aspect I 9. Metacarpus and Digit — Inner Aspect 10. The Foot . hi. Joints and Ligaments of Fore limb |l2. Thigh — Inner Aspect Jl3. Thigh— Inner Aspect 14. Thigh — Inner Aspect [15. Hip and Thigh .... 16. Hip and Thigh .... 17. Leg — Inner Aspect [118. Leg — Outer Aspect 19. Metatarsus and Digit— Outer Aspect 20. Chest-wall and Back 21. Chest-wall and Back 22. Thoracic Cavity — Left Side i 23. Heart and Great Vessels — Left Side 24. Heart and Great Vessels— Right Side 25. Thoracic Cavity— Right Side 26. Thoracic Cavity — Right Side 27. Neck and Intermaxillary Space . 28. Neck 29. Side of Face .... 30. Cavity of the Mouth 31. Cavity of the Mouth, Pharynx, etc. 32. Tongue, Pharynx, etc. 33. Brain — Inferior Aspect 34. Brain — Superior Aspect . 35. Brain— Lateral and 4th Ventricles, etc. . 36. Brain — Ganglia of the Base 37- Male Perinseum .... 38. Abdominal Wall 39. Abdominal Wall 40. Abdominal Wall 41. Intestines and Anterior Mesenteric Artery 42. Intestines and Mesenteric Arteries 43. Coeliao Axis, etc. .... 44. Abdominal Viscera, etc. 45. Sublumbar Region and Diaphragm 46. Male Pelvis .... 47. Genito-urinary Organs of Male 48. Lumbo-sacral Plexus i Page. Facing 2 . 4 . 6 . 9 . 12 . 14 . 18 . 25 . 28 . 39 . 47 . 57 . 59 . 61 . 64 . 66 . 71 . 74 . 78 . 95 . 96 . 105 . 107 . 112 . 116 . 119 . 145 . 148 . 181 . 185 . 190 . 199 . 243 . 245 . 249 . 252 . 276 . 287 . 289 . 291. . 304 . 306 . 316 . 319 . 322 . 342 . 345 . 349 7^oo- ~ 2 > PEEFACE I want of an illustrated topographical treatise on equine anatomy has the experience of the author, been a great barrier to the effieie iching of that all-important branch of veterinary education. In this rk the object of the author has been to place m the hands of veterma y ideuts a dissection guide comparable, in some degree, to the text-boo the service of the practical student of human anatomy The order of section laid down is that which the author has found to he most advan- ,eous, and he has attempted to describe with accuracy and moderate ness the different organs as they present themselves m that order us description is largely supplemented by the ilhistrations, which are complete that almost every organ in the body is delineated. Th ajority of these dictations are original, being faithful portraits of he thor’s own dissections. It is hoped that they will prove useful to the ndent, in the first place, as a plan and a guide m his work, an condly, as a means by which he may afterwards summon up a mental cture of his own dissections. While the book is specially designed for use in the dissecting-room, le author ventures to hope that it may also be serviceable to the jterinary practitioner. Special care has been taken in portraying those ■gions that possess a surgical interest, and the illustrations furnish a >ady means by which the surgeon may refresh his memory regarding tie obiects to be met in the course of an operation. The greater number of the original drawings were made m thedissectmg- oom of the Royal (Dick’s) Veterinary College, by Mr J. Bayne, artist, few were executed by Mr R. S. Reid, artist; and the remainder by lr R. H. Potts, veterinary student. A few of the illustrations were lirectly drawn on stone by Messrs W. & A. K. Johnston To all of hese gentlemen the author is much indebted for the clearness and idelity with which they have delineated the various objects. The source of each of the borrowed illustrations is duly acknowledge :lse where, but the author is constrained to make special mention of •hose from the systematic text-hook of Professor Chauveau, who ;enerously consented to the copying of as many of his figures as nug »e thought useful for this work. VI PREFACE. To insure accuracy, the author has been careful to compare the rd it of his own dissections with the descriptions of other writers, and ) r especially with the works of Percivall, Leyh, and Chauveau, to whit ‘h begs to express his indebtedness. To Professor Turner the author is under deep obligation for’ tl revision of the chapters on the brain, the eye, and the ear, and for rmic] esteemed suggestions regarding other points. Finally, the author’s best thanks are due to his brothers Gavin an< Andrew, who have carefully revised the entire proof-sheets, and t< Mr. T. Barker, veterinary student, for assistance in making the index. Royal (Dick’s) Veterinary College, Edinburgh, October, 1884 . TABLE OF SYNONYMS OF MUSCLES. IX i «.2 .a pp 5 2 s .2 ft Jh £ Sh » Ph .PTo cS o o ft O O M o o £ pp 4 o o eg Sts «- a 3 s-c ^ ft. c ’s* 4> « : r - . Ow£o2SQh3S l§ 2 ft 4) u 5 re § m s m S S 53 f4 "5b bO e6 . -g ©'a 2 eg eg w ft PP A OOP d co ft fn d ft 3 S 2*3 ft . o pS^-S-S s P S g g rn <-h Hift" M 02 . SPh 2 © 1 > ^ , bo"£ Pi o O 41 I P02- £•8 © ft ft ^o j ft 2§ g 5 np S Id'S 1 2 F=5 S 1 43 O eg -a PP _ _ £ O re *s © §,!z SSPn v^PCi ■d © o 9 © Pd 3 * © 2 •b fewHis 2 Pi 0) 4 g 4 S' ■5 a § o gs § > u ^ ft g ft> ft >• a s o ^ d 2 d £ bO > (T) o .^5 m 2 o o bD .S Pftdl d i ft © © PS 43 d « TJ 8 ft> d U u fH gH ^ ^ rd © .2 O ft ft S" 1 ft 1 '© g ft O eg 2 WMWW $i o eg .• r.id 9 -ft 2 Sg’ggi 45 2 « a© £2 pp o g d^ bo S> .« . © © § d o boTP E; a a g e.| g cgc3c30)il)OOOO^5 ooooooooooo eg ft 5 P 2 2 Table of Synonyms of Muscles — continued. x w U .2 eg X P -40 2 "- 1 a g 3.3 ^2 o ^ fa's So 1=1 Cgr^ o c- fl o X eg a o & >-4 c3 ^ O CQ O li I” II -p> -u> e8 a © o cs |3 g . ®a s . §>a>3 sf§ & f3 cs £ bo.” £ § as 'eg § 'eg O - © ”-5 Gg ^7 s a § eg eg ©*72 g •£ © §| § g 6 PhI'E § a-§ a* glr«r^ s 2 S g g I|-I| ■S^g? i ell § Z2 g02 a .s s o. eg g eg 3 03 m m &§,s ns ? n (3 r- 1 bo bo © eg g g &p o eg eg f3 © p£3 rG eg a ^-a "g o««- s* U ° g o O n 2 72 “ o -+=> ^ -t- 3 ■+= © x -g X X -2 © g © © X o © o O,— i ■fl £/a 'a g • -2:3-2 -2 © § aG2 a a o .a 1 w |i § S 2J 2 © £ a X 9 K X 2 2 a 2 II Hi omSo5 a a S © © 3S333 .©.GO ^ 43 (9-P S ^ O eg JV -- r-H £ § § § s s hS Kj K! 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Ph— 1 p ^ 9 p.a-s © .> Pi 03 ,— i P eg .a a b-2 1.S GC !>> & ■+3 P 02 02 .5 e3 2 1 O .S Or^ © © io to d P . eg eg 03 r-H r-H © eg eg bO-P X fl ftft <2 tj— I «4H «oo P h CO o c* * o © © © O O .rH -rH cc © © © p p Q 02 02 S3-P ^ N 02 N O © 03 03 .P © *“* CP r* P p rO P p ©“skrxxp (jrO D Sj id !U « B 4 P S3 S3 S3 S3 S3 O © S .a ® 1 g p a • : s a^* 6 _© © S “Ta A 02 ^ 03 -+0 O &-2’C 2 eg > eg s P tg P © © 6 | Sh f* p Sh sh ,p , © Q3 (D © © rH S 552 S OOOOO W ©S3 M W J *g £ . O Hi © & P M -_J2 :p p o © p a ©.Segegegrjcgeg eg eg © © © © © NIN) > > o © « s § « oa .2 3^ be bn a p p a OOP Table of Synonyms of Muscles — continued. xii .2 £ 03 ■+? 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P3 W r n > r& © .2 | « si " eg » » o rippooSgo 5 S 52 ffl »' St £ ft § pi S3 TS hL 3 M M g^oa) . 2.2 0 43 5 - in U « t-i <-. ™ w cScg©©OOO03 'bog S > jt 4 H O o © *4 2 £ £ .PS «S cS "g M 03 o ’-g ’-§ ^'pp’Bh M 2 ^.ce xiii o P • go go go f* h O !j J S | .a O Ph P - 1 M ^ ^ ^'S ‘3 ^ «2 ■+? 3 3 8 A "°3 £ 53 53 +3 .2 £j P o> • -fd CG P 3 ?H rrj p P OQ .Xj I jfJHi a, , 3 2 ts >)0 ci * Hs^^s o © 6 § P © bo §£> <3 8 ©_2 8 8 g 2 ^ 0 ^ &W Ah 02 M O ^ M cc * © .2 g © § ■+3 g § ’g4 3 o M 11 s >> • rt §.2 J3 6 O « 6 S'S S N © © HP4 <3 • © bop* ’8 § * 80 U, U, [S [S 2 ^ N S s <5 <1 P r d c 3 bO g S g ®g w S . 2 .2 •-! p. a a ® « © 53 a 1 1 § CO 3 2 I s §' s ^ 4 > © < 5 J g P©©©cScg©p©«'U© i _ 50 © o "S "d *bs j-< *+ "53 5 bs ® bs bi bs -p CZ 2 i~a i ^i k ~i K HHH K N'^ k ^AH , ^ K i K H 2 Z 2 o iiii 2532 ^ p ’a, g I |& ^02 g.a c« cS £ £ =3 >> t>>^ A2 -O 2 2 .5 o "S’.d 2 £ m.g 2 -i Sis iS © > o a a a 2 1 0 C 3 e 6 C 3 w o3 ®«6 £» ” ^2 o .§3'b0 ? ® § .2 o'o o op g bo * * pi P rH rt O O) I 2 2 2 © © © xjimm r- ro S' 5 ? © aj 1—, -3 _2 2 ^ q3 q=l bO -2 S 9 t? £ 2 2 - * .a O 2 ( 3 d ^ 2 s pa . _ >?2A p/8‘8 8 'eL g nj Q bOrd B ft(jcdcficf3 »1 2 -.r^SSOOOO c 3 c 3 © ' , ASSS 5 a 533 S^^^^“ ll,a>a> >-' GJ -|£ h ? hCOC ©< X>?H r-H r — I r-H r-H ^ rb r » . ^ ©©©©BBo-p>-p>-*p-(j-*odddSd mmmuimmmx/ixnmmmmmxfiuim P, O 2 P © © H H- Table of Synonyms of Muscles — continued . xiv M £L| . P3 P H s M M P P § 2 «* § g^, 2 ■— 1 -2 2 1 M^l .» P'P o ^ S 2 a> a' +» o Ph Pr (H <-j -rH 'g .« „• jg 2 II 8 O o 2 + .2 eg a 2 §§> ►» N 2 eg . m-i s ° ^ S H fi fi g g S S 4 O' ») W W o 4) (D '■p vs vs >z "e -S SSSC-S® ft a ‘SJ M OD q5 .2 _g eg O a> -4^ p 2 s £ P ;2.2 £ eg eg VMM o Sh 5h i-H Q <3 a> .P 2 6 .2 8 *-3 2 > > _jO cq co r 7t fH r-< > n too 2 2 S P P *p o 1 g U H H § ^.s§ M M eg eg rp bo §> S k*^ N A few words are here necessary in explanation of the system of nomen- clature used throughout this work. Although reluctant to add to the confusion already prevailing in the nomenclature of veterinary anatomy, the author has not conformed to any of the systems in general use. The system here employed is based on the principle of naming each object after the homologous object in human anatomy. So far, indeed, as any of the systems in use can be said to follow a principle, it is that just stated ; but the violations of the principle are numerous, and, in most cases, appear to have been dictated by the merest caprice. The most vicious form of departure from the principle is that in which terms adopted from human anatomy are employed to designate not the actual homologues, but other parts having, it may be, some faint resemblance in shape or otherwise to the objects bearing these names in the human subject. This method is indefensible, siuce it tends to produce the greatest confusion, and, if generally adopted, would render a comparison of the anatomy of any two animals an impossibility. Many such terms have long been in use, but it is hoped that they are not ineradicable. In cases where objects appear to be without homologues in human anatomy, new names must, of course, be found. In only a few of these instances, however, has the author employed terms of his own invention, preferring, in general, to adopt some of those already in use. The greatest diversity of names, it will be found, exists in the case of muscles, and the following table of synonyms has been compiled for the convenience of those already familiar with the terms employed in some other works. i CONTENTS. :o :• CHAPTER I. DISSECTION OF THE ANTERIOR LIMB. Page. p Pectoral Region and the Axilla 1 |e Outer Scapular Region . . 8 er Aspect of the Shoulder and 10 r Aspect of the Shoulder and rm ...... 17 Fore-arm .... 19 Metacarpus and Digit . . 27 The Foot The Shoulder- joint The Elbow-joint The Knee The Fetlock-joint The Pastern- joint The Coffin- joint CHAPTER II. Page. . 35 . 43 . 44 . 46 . 50 . 52 . 52 DISSECTION OF THE POSTERIOR LIMB. lb Inner Aspect of the Thigh . 56 e Hip and Outer Aspect of the ’high 63 3 Leg 70 The Metatarsus and Digit . . 77 The Stifle-joint .... 81 The Tarsus ...... 86 The Tarso-metatarsal articulation . 90 1 20 122 125 CHAPTER III. DISSECTION OF THE 5 Chest-wall and Back 94 i Cavity of the Thorax 100 3 Lungs 103 3 Pericardium .... 105 3 Heart 106 3 Nerves and Vessels of the left ide of the Thorax 108 e Nerves and Vessels of the right ide of the Thorax 116 BACK AND THORAX. Examination of the Lung . 122 Dissection of the Heart . 123 Structure of the Heart . . 129 Articulations of the Ribs . 132 Intervertebral Joints and Liga- ments . 134 The Spinal Cord . . 136 xviii CONTENTS. CHAPTER IV. DISSECTION OF THE ! HEAD AND NECK. Page. The Under part of the Neck . 143 The Tongue . . . .1 The Upper part of the Neck . 151 The Hard Palate . . .9 Ligaments and Articulations of the The Soft Palate . . . 1 Neck posterior to the Dentata . 158 The Pharynx, the Hyoid Bone, ar The External Ear .... 159 the Base of the Skull . The Parotideal Region . 164 The Orbit .... The Region of the Poll . 168 The Occipito - atlantal Articuls The Intermaxillary Space 170 tion The Appendages of the Eye . 172 The Atlanto-axial Articulation The Face 176 The Temporo-maxillary Articulat The Pterygo-maxillary Region and The Cavity of the Nose . the Region of the Guttural Pouch 184 CHAPTER V. DISSECTION OF THE LARYNX. Cartilages of the Larynx 224 . Muscles of the Larynx . Articulations, Ligaments, and Mem- Nerves of the Larynx branes of the Larynx . 226 1 Interior of the Larynx . CHAPTER VI. DISSECTION OF THE BRAIN. Membranes of the Brain 233 The Cerebellum .... Arteries of the Brain 236 The Cerebrum The Medulla Oblongata . 238 The Cranial Nerves The Pons Varolii . 240 CHAPTER VII. DISSECTION OF THE EYEBALL. / Dissection of the Eyeball . CHAPTER VIII. DISSECTION OF THE EAR. The Middle Ear . 267 | The Internal Ear . CHAPTER IX. DISSECTION OF THE PERIN2EUM IN THE MALE. The Perinseum . . . . 274 The Prepuce .... The Scrotum , 277 The Penis .... The Testicle and Epididymis . . 279 CONTENTS. XIX CHAPTER X. DISSECTION OF THE ABDOMEN. Page. Page. The Abdominal Wall 285 Structure of the Liver , . 330 The Cavity of the Abdomen . 294 Structure of the Spleen . . 332 The Sublumbar Region . 320 Structure of the Pancreas . 333 Structure of the Stomach 328 Structure of the Kidney . 333 CHAPTER XI. DISSECTION OF THE PELVIS. The Hip-joint and the Ligaments The Tail . 360 of the Pelvis .... 338 Joints and Ligaments of the The Cavity of the Pelvis 340 Sacrum and Coccyx . . 363 Reproductive Organs in the Female. 351 Fig. 1 . ) 18. 19. 20 . 21 . 22 . 23. 24. 25. 26. 27. 28. 29. 30. Page. Dissection of the Metacarpus and Digit, showing the Tendons and their Synovial Sheaths ........ Muscles of the Tail, deep Muscles of the Hip, and Pelvic Ligaments . Flexor Metatarsi Muscle ........ Femoro-tibial Ligaments ........ Ligaments of the Tarsus ........ Diagrams showing Disposition of the Pleurae ..... Floor of the Thorax ........ Termination of the Air Passages in the Lung ..... Diagram of the two Cavities of the Eight Side of the Heart Diagram of the two Cavities of the Left Side of the Heart Root of the Common Aorta laid open ...... View of a partial Dissection of the Fibres of the Left Wall of the Ventricles in a Sheep’s Heart ......... Two cos to- vertebral, and two intervertebral Joints, viewed from below Two costo-vertebral, and two intervertebral Joints, viewed from above View of the Membranes of the Spinal Cord ..... Portion of Spinal Cord with the Roots of the Nerves .... Transverse Section of Spinal Cord of Calf ..... Ligamentum Nuchse and Deep Muscles of the Neck .... Auricular Muscles and Nerves of a Mule ...... Cartilages of the Nose ........ Hard Palate ......... Muscles of the Eyeball ........ Muscles of the Eyeball ........ Longitudinal Section of the Head, showing the Cavities of the Mouth, Nose and Pharynx ......... Transverse Section through the Nasal Chambers .... Larynx, side view ......... Larynx, side view ......... Larynx, back view ......... Interior of the Larynx, seen from behind ..... Arteries of the Brain . . . 33 68 76 83 87 101 120 122 125 128 129 131 132 133 138 139 141 156 161 176 201 209 210 217 218 228 229 230 231 236 XX CONTENTS. Page. 31. Corpus Callosum and inner face of cerebral Hemisphere. . . . 248 32. "View of the Lower Half of Right Adult Human Eye, divided horizontally through the middle ........ 258 33. Choroid Membrane and Iris exposed by the removal of the Sclerotic and Cornea .......... 261 34. Diagram of the membranous Labyrinth ...... 269 35. Transverse Section through the Tube of the Cochlea .... 271 36. Vertical Section through the Wall of the Duodenum, showing the Glands of Brunner .......... 309 37. Diagrammatic View of a small portion of the Mucous Membrane of the Colon 310 38. Vertical Section of the Coats of a Pig’s Stomach .... 329 39. Stomach, Everted and Inflated ....... 330 40. Longitudinal Section of a Portal Canal, from the Pig .... 331 41. Transverse Section through the Hepatic Lobules .... 332 42. Cut Surface of Horse’s Spleen, Trabecular Framework .... 333 43. Vessels of the Kidneys and Uriniferous Tubules .... 335 44. Bladder and Intrapelvic portion of the Urethra opened from below . . 348 45. Section of the Cat’s Ovary ....... 353 46. Right Ovary and Fallopian Tube ....... 354 47. Generative Organs of the Mare, viewed from above .... 356 48. Muscles of the Tail, Deep Muscles of the Hip, and Pelvic Ligaments . . 361 DISSECTION OF THE CHAPT THE PECTORAL REGION AND THE AXILLA. As the first step in the examination of the fore limb, the student should dissect the structures which pass between the trunk and the ventral aspect of the limb. Position . — The subject should be placed on the middle line of its back, and its limbs should be forcibly drawn upwards and outwards by ropes running over pulleys fixed to the ceiling. If only one side is being dissected, the subject may be inclined as in Plate 1. This will put the muscles and other structures on the stretch, and thus facilitate their dissection. Surface-marking . — In the fore part of the pectoral region the student will notice the well-marked prominence formed by the anterior super- ficial pectoral muscle. Between this muscle and the lower edge of the mastoido-humeralis there is a groove in which will afterwards be dissected the cephalic vein and a branch of the inferior cervical artery. Extending inwards from the point of the elbow is a prominent fold of skin over the hinder edge of the posterior superficial pectoral muscle. Directions . — An incision through the skin, but not deeper, is to be made along the middle line of the sternum, from the ensiform cartilage as far forwards as the cariniform cartilage. From the middle of this incision another is to be carried transversely outwards, and terminated a little beyond the elbow-joint. Where this second incision stops, another is to be made across the inner face of the fore-arm. Beginning at the point where these incisions meet, the student should raise and turn outwards the two flaps of skin, so as to denude the superficial pectoral muscle. In doing this, it may be noticed that here, as in other unexposed situations, the skin is comparatively thin. Beneath the skin is the subcutaneous fascia, and search is to be made in it for the cutaneous nerves of this region. Cutaneous Nerves. A nerve of considerable size, derived from the 6th cervical nerve (Plate 1), crosses the groove between the mastoido- humeralis and the anterior superficial pectoral, and distributes branches to the skin over the latter muscle and part of the posterior superficial pectoral. Other small cutaneous twigs, which are branches of the inter- B 2 THE ANATOMY OF THE HORSE. costal nerves, appear near the middle line, and are directed transversely outwards. Directions . — The surface of the superficial pectoral muscles should now be carefully cleaned by the removal of the subcutaneous fascia ; and this operation should be conducted by beginning at the anterior or posterior border of the muscle and working parallel to the direction of the muscular fibres. When this has been effected, a line will be seen on the surface of the muscle ; and by dissecting carefully down on this line, the student will be able to separate the anterior from the posterior part of the muscle. Search is to be made, in the groove already men- tioned, for the cephalic vein, and the fat is to be carefully removed from the vein and its accompanying arterial branch. Superficial Pectoral Muscle ( Pectoralis transversus of Percivall). — This muscle is divided, though not very distinctly, into two portions, which may be distinguished as the anterior superficial pectoral and the pos- terior superficial pectoral. The Anterior Superficial Pectoral (Plate 1) arises from the first two or three inches of the inferior border of the sternum, its posterior fibres overlapping the anterior part of the next muscle. It is inserted into the external lip of the musculo-spiral groove. The Posterior Superficial Pectoral (Plate 1) arises from the inferior border of the sternum from within an inch of its anterior end as far back as a point behind the 6th costal cartilage, and from a fibrous cord which joins the muscle along the middle line to its fellow of the opposite side. It is inserted into the superficial fascia which descends on the inner face of the fore-arm ; and a few of its anterior fibres, forming a band about one inch in breadth, are inserted along with the preceding muscle into the external lip ^f the musculo-spiral groove. At the elbow-joint the muscle covers the posterior radial vessels and the median nerve, but these are not to be exposed at present. Action. — The superficial pectoral muscle is an adductor of the limb at the shoulder, and the posterior division of the muscle is also a tensor of the fascia of the fore-arm. Directions. — Both divisions of the muscle are now to be cut across near their origin, and dissected carefully from the subjacent deep pectoral ; and while this is being done, search is to be made for their nerves, which Come from the brachial plexus by passing between the two divisions of the deep pectoral muscle. In reflecting the muscle, the dissector will cut many small branches of the external or internal thoracic vessels. The reflected muscles are now to be fastened outwards with chain and hooks, and the dissection of the deep pectoral is to be undertaken after the cephalic vein has been examined. The Cephalic Vein (Plate 1). This is the upward continuation of one of the divisions of the internal subcutaneous vein of the fore-arm. DISSECTION OF THE .ANTERIOR LIMB. 3 It ascends in the groove between the anterior superficial pectoral and the mastoido-humeralis. In the inner third of this groove it lies on the anterior deep pectoral, in company with a branch of the inferior cervical artery. It empties itself into the jugular about two inches from the lower end of that vessel. Deep Pectoral Muscle. — This consists of two distinct divisions, which may be distinguished as the anterior deep pectoral and the posterior deep pectoral. The Posterior Deep Pectoral (. Pectoralis magnus of Percivall) (Plate 2) is a muscle of large size ; and its posterior part, being subcutaneous, was visible before reflection of the superficial pectoral. It arises from the abdominal tunic covering the external oblique and the straight muscles of the abdomen ; from the tips of the cartilages of the 5th, 6th, 7th, and 8th ribs, and from the immediately subjacent lateral surface of the sternum. It is inserted into the inner tuberosity of the humerus, into the tendon of origin of the biceps, and into the fascia which retains that muscle in the bicipital groove. By its deep face the muscle serves to bound the axillary space ; while its upper border is closely united to the panniculus, and bordered by the subcutaneous thoracic nerve and vessels. The Anterior Deep Pectoral ( Pectoralis parvus of Percivall) (Plate 2) arises from the cartilages of the first four ribs, and from the immediately subjacent lateral surface of the sternum; and, being carried upwards in front of the supraspinatus nearly as far as the cervical angle of the scapula, it is somewhat loosely inserted into the fascia covering the last- named muscle. This insertion is concealed by the mastoido-humeralis, and will be better seen in the dissection of the muscles on the outer surface of the scapula (Plate 4). The deep face of the muscle forms part of the inferior boundary of the axilla. Action. — The two divisions of the deep pectoral have the same action, which is to pull the shoulder-joint, and thus the whole limb, backwards. When the limbs are fixed, the muscle may to some extent act as a muscle of inspiration. Directions. — The deep pectoral muscles are now to be severed care- fully about midway between their origin and insertion, and the cut portions are to be turned outwards and inwards. Their nerves, which come from the brachial plexus, will be found entering their deep face; and care is to be taken of the external thoracic and inferior cervical arteries. By the reflection of these muscles, the axilla is exposed. Owing to the limited power of abduction at the shoulder- joint of the horse, the dissection of the space is attended with much greater difficulty than in man. The best method of procedure is as follows: — All the pectoral muscles having been cut across, the limb is to be forcibly separated from the chest-wall; and, to permit this to a sufficient extent, it may be necessary to cut the mastoido-humeralis in 4 THE ANATOMY OF THE HORSE. front of the shoulder. On looking into the space, the dissector will now see it occupied by a large amount of loose, areolar connective-tissue, which envelops its contents, and facilitates the play of the shoulder on the wall of the thorax. This areolar tissue must be cleaned away from the axillary vessels and the brachial plexus of nerves, but most of the branches of these will be more conveniently followed after separation of the limb from the trunk. The Axilla corresponds to the arm-pit of the human subject, and is the important space across which the large vessels and nerves for the supply of the fore limb are transmitted. j Boundaries of the space . — In the natural movements of the limb, and before dissection, the space can hardly be said to have any existence except at its lower part ; but in the dissected condition it may be observed to have the following boundaries. On its outer side are the subscapularis, teres major, and (in part) latissimus dorsi muscles. The inner side of the space is formed by the anterior part of the chest-wall covered by the serratus magnus, lateralis sterni, and intercostal muscles. Inferiorly the space is enclosed by the deep pectoral muscles, and there the space is most extensive. Superiorly the outer and inner boundaries meet at the insertion of the serratus magnus into the scapula. The anterior limit of the space may be taken as formed by the mastoido- humeralis and the reflected portion of the anterior deep pectoral ; while posteriorly the space is closed by the panniculus carnosus and skin where these are carried from the wall of the thorax over the outer aspect of the shoulder. The Axillary Artery (Plates 3 and 5) begins within the thorax. On the left side it arises as one of the terminal branches of the anterior aorta; while on the right it is a branch of the arteria innominata. It leaves the chest and reaches the axilla by turning round the anterior border of the first rib, below the inferior insertion of the scalenus. It crosses the axillary space, inclining downwards and backwards ; and at the anterior border of the teres major tendon it is directly continued as the brachial artery. In this course it gives off four vessels, viz., inferior cervical, external thoracic, suprascapular, and subscapular ; but only the first two are to be followed at present. The Inferior Cervical Artery (Plates 1 and 2) arises from the front of the axillary where that vessel turns round the first rib. After a course of about two inches it bifurcates, its superior branch passing between the mastoido-humeralis and the subscapulo-hyoideus, while the inferior division passes into the groove between the mastoido-humeralis and the anterior superficial pectoral, where it has already been seen in company with the cephalic vein. The External Thoracic Artery (Plate 3) arises about the same point as the preceding, but from the opposite side of the parent vessel; and Post, superficial pectoral PECTORAL REGION DISSECTION OF THE ANTERIOR LIMB. 5 passing backwards in relation to the axillary surface of the deep pectorals, it distributes branches to these, and also to the superficial pectorals. A slender branch from it accompanies the subcutaneous thoracic vein to the panniculus carnosus. The Axillary Vein is the upward continuation of the brachial vein, and is, at its lower part, posterior to the artery ; but at the anterior border of the first rib it is below the artery, and it here joins the jugulars and the axillary vein of the opposite side, thus forming the anterior vena cava. Directions. — The axillary vessels may now be cut as they turn round the first rib, and the limb may be further abducted to facilitate the dissection of the brachial plexus, which, in its first step, should be undertaken by the dissectors of the limb and of the neck conjointly. The Brachial Plexus (Plate 3) is composed of the nerves for the supply of the fore limb. It is formed by the inferior primary branches of the last three cervical (6th, 7th, and 8th) and first two dorsal nerves. These, however, do not enter into it in equal proportions. The 6th cervical sends only a very slender branch to it, while the 7th and 8th, after detaching a communicating filament to the sympathetic, are wholly expended in it. The 1st dorsal is, with the exception of a similar communicating filament and a slender intercostal branch, also entirely expended in it, but the 2nd dorsal gives off, besides the usual communicating branch, a considerable intercostal nerve before joining the plexus. These roots of the plexus converge towards each other, and come out as a flat fasciculus between the upper and lower portions of the scalenus. In descending to this point, the dorsal roots of the plexus turn round the anterior border of the first rib, leaving on it a smooth impression near its upper end. The several roots anastomose in an intricate fashion, contributing to the formation of the various branches of the plexus, in proportions that the student will not be able to trace accurately in the course of an ordinary dissection. The manner in which the several roots of the plexus comport themselves is liable to slight variation, but the following is probably as common as any other : — I. The root from the 6th cervical nerve is a slender branch detached from the division which that nerve sends to aid in the formation of the phrenic. Passing obliquely back- wards on the scalenus muscle, it resolves itself into three divisions — or rather its fibres are traceable in three groups, viz., — 1. To the suprascapular nerve ; 2. to the anterior root of the median ; 3. to join branches from all the other roots of the plexus in forming a broad, flat fasciculus from which arise the subscapular, circumflex, and musculo-spiral nerves. II. The root from the 7th cervical nerve gives a branch to the nerve for the serratus magnus, and then divides its fibres in three directions, viz., — 1. To the above-mentioned fasciculus giving off the subscapular, circumflex, and musculo-spiral nerves ; 2. to the suprascapular nerve ; 3. to the phrenic, anterior root of the median, and nerve for the anterior deep pectoral muscle. III. The root from the 8th cervical nerve gives a branch to the nerve for the serratus magnus, and then sends its fibres in three directions, viz., — 1. To the before-mentioned 6 THE ANATOMY OF THE HORSE. flat fasciculus giving off the subscapular, etc. ; 2. to join the cord from which arise the posterior root of the median, the ulnar, and the subcutaneous thoracic nerve ; 3. to the anterior root of the median and the nerve for the anterior deep pectoral muscle. IY. The roots from the 1st and 2nd dorsal nerves unite to form a common cord which divides its fibres in two directions, viz., — 1. To join the above-mentioned cord giving off the posterior root of the median, etc. ; 2. to join the broad fasciculus from which arise the subscapular, etc. The following is a list of the branches of the plexus : — 1. The phrenic or diaphragmatic nerve (in part). 2. The suprascapular nerve. 3. Nerves to the pectoral muscles. 4. The nerve to the subscapularis. 5. Nerves to the serratus magnus and levator scapulae (cervical portion of the serratus), the latter only in part. 6. The circumflex nerve. 7. Nerves to the teres major and latissimus dorsi. 8. The musculo-spiral nerve. 9. The median nerve (two roots). 10. The ulnar nerve. 11. The subcutaneous thoracic nerve. The Phrenic Nerve. This nerve is formed by the union of two, or sometimes three, branches. The inconstant branch comes from the 5th cervical; the other two come from the 6th and 7th respectively. The root from the 6th nerve gives off a branch to the brachial plexus, and then unites on the scalenus with the root from the 5th — when that is present. The single cord resulting passes obliquely backwards and downwards, and at the lower edge of the scalenus it joins with the root from the 7th nerve. This last comes from the fore part of the brachial plexus. The trunk of the nerve, as thus formed, passes backwards between the axillary artery and its inferior cervical branch, and enters the thorax. It is the motor nerve to the diaphragm. The Nerves to the Levator Anguli Scapulae and Rhomboideus. In Plate 3 two nerves are seen at the upper edge of the scalenus. They are not, strictly speaking, branches of the brachial plexus ; but come from the inferior primary branch of the 6th nerve, and pierce the muscle either together or separately. They are distributed to the levator anguli scapula?, and the posterior of the two is continued in that muscle to reach the rhomboideus. The Nerve to the Serratus Magnus is formed by the union of two branches, which pierce the upper division of the scalenus before uniting. These are branches of the 7th and 8th nerves respectively. By their fusion there is formed a broad nerve, which passes backwards on the surface of the serratus, distributing its filaments upwards and downwards. Before fusion, the branch from the 7th gives off a nerve which is distributed to both the levator and the serratus. Dra-wu fcPrintrd "by Vf. 8t A K. Johnston. Edinburgh & London DISSECTION OF THE ANTERIOR LIMB. 7 The Subcutaneous Thoracic Nerve (Plates 1 and 3) derives its fibres from the dorsal roots of the plexus and from the 8th cervical, but princi- pally from the former. It accompanies the spur vein to near the flank, being distributed with perforating intercostal branches on the deep face of the panniculus carnosus. A branch from it unites with perforating branches from the 2nd and 3rd intercostal nerves, and turns round behind the limb, to be distributed to the panniculus over the shoulder and arm. The Nerves to the Pectoral Muscles have already been referred to. The nerve to the anterior deep pectoral leaves the fore part of the plexus, deriving its fibres from the 7th and 8th cervical nerves. The nerve to the superficial pectoral muscle (both divisions) derives its fibres from both roots of the median. In general, there are two nerves to the posterior deep pectoral. The first — to the anterior part of the muscle, comes off with the posterior root of the median, the other — to the posterior part of the muscle, comes off in common with the subcutaneous thoracic. Directions.— The remaining nerves of the brachial plexus can be more satisfactorily followed after separation of the limb from the trunk, and the dissector should therefore now proceed as follows : — Pass a cord round the nerves of the plexus as they emerge from between the two divisions of the scalenus, and then cut the roots of the plexus as near their points of origin as possible. Cut also the axillary artery and vein at the first rib. This will allow the limb to be carried well out from the trunk, so as to expose the serratus magnus and levator anguli scapulse, which are now to be cleaned. Serratus Magnus and Levator Anguli Scapulce. — These muscles are, in the horse, not very distinctly marked off from each other, and have therefore been frequently described as one muscle under the first name. The Serratus Magnus (Plate 4) arises from the outer surfaces of the eight (or nine) anterior ribs, its eight slips of origin forming a curved, serrated line which gives to the muscle its name. The posterior four of these slips inter-digitate with slips of origin of the external oblique muscle of the abdomen (Plate 39), and are overspread by the abdominal tunic. It is inserted into a triangular area on the ventral surface of the scapula near its dorsal angle, and, in common wuth the next muscle, into another triangular area at the cervical angle. Action. — It pulls the dorsal angle of the scapula downwards and backwards on the chest- wall, causing the shoulder-joint at the same time to move upwards and forwards ; but when the limbs are fixed, it can become a muscle of inspiration, pulling the ribs upwards and for- wards. In the standing posture of the animal at rest, the chest is, in a manner, slung on the fore limbs by means of the right and left ser- ratus muscles. 8 THE ANATOMY OF THE HORSE. The Levator Anguli Scapulae (Plate 4) arises from the trans- verse processes of the last four cervical vertebra ; and its fibres converge to be inserted into the triangular area on the ventral surface of the scapula near its cervical angle, in common with the anterior fibres of the preceding muscle, from which it is not distinct. The two muscles, taken together, have a well-marked fan-like arrangement, having an extensive convex border where they take origin, while they converge to a comparatively narrow point at their insertion. Action . — The levator anguli scapulae carries the articular angle of the scapula backwards by pulling the cervical angle forwards ; but when the scapula is fixed, the right and left muscles, acting together, can raise the cervical portion of the spinal column, or the single muscle can incline it to one side. The Subscapulo-hy oid. This muscle, which arises from the sub- scapular fascia, is described with the dissection 'of the neck. THE OUTER SCAPULAR REGION. Position . — The muscles which pass between the shoulder and the trunk, on the outer aspect of the former, must next be dissected ; and, to permit this, the subject must be placed in an entirely new position. The standing posture of the animal is the best for this purpose ; and it may be imitated by suspending the subject to a stout iron rod provided with chains and hooks, and capable of being raised or lowered by means of a system of pulleys or a small windlass. Surface-marking . — About the centre of the region to be dissected the student will feel the spine of the scapula, the most prominent part of which is its tubercle. In a well-nourished, sound horse the spine should not be very distinctly, visible, but in an emaciated animal, or in one whose scapular muscles are atrophied as an accompaniment of joint- disease, it forms a very prominent ridge. Directions . — An incision through the skin is to be made along the spine of the scapula from the withers to the middle of the arm, where a transverse incision is to be made from the anterior to the posterior border of the limb. Another incision is to be carried along the middle line of the back, and prolonged forwards along the neck by the dissector of that region, and backwards to the lumbar region by the dissector of the back. The dissectors of the three regions should here work together, the skin being turned down as a single flap from the neck and anterior half of the shoulder, and as another flap from the back and posterior half of the shoulder. The skin, it will be observed, is thicker than in the pectoral region, and it has the panniculus carnosus attached to its inner surface. Care must be taken not to remove this panniculus with the skin. The Panniculus Carnosus is the muscle which enables the horse to PLATE IV Splenius Levator ang. scapulae Ant. deep pectoral iihomboideus (dorsal part) anticus Ant. deep Mastoido-1 Deltoid Splenius Levator ang. scapulae Serratus magnus magnum medium Drawn & Printed "by W. ScA.JC Johneton. Edinburgh &. London SHOULDER — Outer Aspect DISSECTION OF THE ANTERIOR LIMB. 9 twitch its skin, and thus remove offending insects. It is most extensive over the thorax and abdomen, but it is here carried over the muscles covering the scapula and humerus. Before the muscle passes on to the limb, it sends an aponeurotic layer inwards between the limb and the chest-wall. At its upper border this layer is provided with a small tendon, which becomes inserted into the inner tuberosity of the humerus, and which will be seen when the limb is dissected from the trunk. A nerve will be seen ramifying in the scapulo-humeral part of the panni- culus. This turns round the posterior border of the limb ; and, as already seen, it is formed by the union of the subcutaneous thoracic with some perforating intercostal nerves. Directions. — The panniculus is now to be dissected away from the limb; and in doing this in front, care is to be taken of the thin cervical trapezius muscle, which might be mistaken for a portion of the panniculus. The Trapezius in the horse has its muscular substance interrupted by a tendinous portion, and is therefore sometimes described as two separate muscles, distinguished as the cervical and the dorsal trapezius. The Cervical Trapezius (Plate 4) arises from the funicular por- tion of the ligamentum nuchae ; and it is inserted into the tubercle on the spine of the scapula, while its most anterior fibres are continuous with an aponeurosis covering the scapular muscles. Both the deep and the superficial face of the muscle have a thin, adherent, fibrous covering, the direction of whose fibres is at right angles to that of the muscular fibres. Action. — It draws the scapula forwards and upwards. The Dorsal Trapezius (Plate 4) is continuous with the preced- ing by the aponeurotic centre already mentioned. It arises from the summits of a few of the anterior dorsal spines, and is inserted into the tubercle on the scapular spine. Action. — It pulls the scapula backwards and upwards. Directions. — Both divisions of the trapezius are now to be severed close to their origin, and reflected downwards ; and while this is being- done, search is to be made for the branches of the 11th, or spinal accessory, nerve, which enter their deep face. The muscles which were covered, wholly or in part, by the trapezius, will now be exposed. These are : the splenius, the levator anguli scapulae, the supraspinatus, the infraspinatus, the anterior deep pectoral, the latissimus dorsi, and the rhomboideus. It will be remembered that in the dissection of the pectoral region the anterior deep pectoral could not be followed to its termination. The reflected portion of the muscle is here seen (Plate 4), but is partly covered by the insertion of the mastoido-humeralis. The Latissimus Dorsi (Plate 4). Though neither the origin nor the insertion of the muscle is found here, attention should be given to 10 THE ANATOMY OF THE HORSE. it as it is being exposed by the dissector of the back. It arises by an aponeurotic tendon from the series of vertebral spines, beginning about the 4th dorsal, and extending backwards to the last lumbar. This tendon is succeeded by a thick muscular portion, which contracts and passes in between the limb and the trunk, where it will afterwards be followed to its insertion into the internal tubercle of the humerus. Its anterior fibres will be noticed to play over the dorsal angle and cartilage of prolongation of the scapula. Action. — It is a flexor and an inward-rotator of the shoulder-joint. The Rhomboideus (Plate 4), like the trapezius, comprises a cervical and a dorsal portion. The cervical part is an elongated, narrow muscle, which extends as far forward as the axis, and arises from the funicu- lar part of the ligamentum nuchae. Its fibres take a very oblique direction downwards and backwards, and are inserted into the anterior part of the cartilage of prolongation on its inner surface, being there confounded with the insertion of the levator anguli scapulae. The dorsal portion consists of fibres which arise from the anterior dorsal spines, and pass in a nearly vertical direction to be inserted into the inner surface of the cartilage of prolongation, behind the fibres of the cervical division. It will be recollected that the nerve to these muscles passes from the 6th cervical nerve, and reaches its destination by traversing the levator anguli scapulae. Action . — To pull the scapula upwards and forwards on the chest-wall. The Mastoid o-humeralis, or Levator Humeri (Plate 4). This muscle, in the greatest part of its extent, is found in the head and neck, where it takes its origin from the mastoid crest and the trans- verse processes of the first four cervical vertebrae ; but attention must here be given to its insertion , which is into the external lip of the musculo- spiral groove, after covering the shoulder-joint. It receives here some branches from the circumflex nerve. Action. — It is an extensor and inward-rotator of the shoulder-joint. When the limb is fixed, it bends the neck laterally. Directions. — The limb may now be detached from the trunk by severing the attachment of the rhomboideus, serratus magnus, levator anguli scapulae, mastoido-humeralis, and latissimus dorsi, the last being cut where it plays over the angle of the scapula. Pieces of clean cloth saturated with some preservative solution should be placed on the outer aspect of the shoulder where the skin has been removed, while the dissector proceeds to examine the structures over the inner surface of the scapula and humerus. INNER ASPECT OF THE SHOULDER AND ARM. Directions. — The dissector should now identify the terminal portions of the muscles already dissected, and cut them off within an inch or DISSECTION OF THE ANTERIOR LIMB. 11 two of their insertion, except in the case of the latissimus dorsi, which is to be left at its present length until its nerve and artery have been followed. The posterior superficial pectoral should be cut away on a level with the olecranon, but care is to be taken not to disturb the vessels and nerves which it covers. The aponeurosis which the panniculus sends within the shoulder will now be observed, and, at its upper border, a small glistening band passing to be inserted into the internal tuberosity of the humerus. The next step is to dissect out the axillary and brachial vessels, and the remaining branches of the brachial plexus ; and this is an operation demanding time and care. While an assistant holds the nerves on the stretch, the fat and areolar connective-tissue which surround them and the vessels, are to be cleaned away piecemeal, always proceeding from the main trunks to the branches. In doing this, the dissector will meet two groups of lymphatic glands. Brachial Lymphatic Glands. The upper group consists of a cluster placed behind the brachial vessels, on a level with the middle of the humerus. The lower group consists of one or two glands in relation to the vessels, just above the elbow-joint. The Axillary Artery (Plates 5 and 6). This vessel has already been seen passing in a curved direction from the anterior border of the 1st rib, across the inner aspect of the shoulder-joint, where it rests above the terminal insertion of the posterior deep pectoral, and on the tendon of the subscapularis. It passes on to the teres major, and is continued as the brachial artery. In this course it gives off four vessels, viz., the inferior cervical, external thoracic, suprascapular, and subscapular. The first two have already been dissected in the axilla. The Suprascapular Artery (Plate 5) is a small, tortuous vessel springing from the upper surface of the axillary artery about the middle of its extra-thoracic course. It passes upwards for a short distance, and then divides into branches, the longest of which passes over the subscapularis to reach the anterior deep pectoral. A branch passes in between the subscapularis and the supraspinatus, while smaller branches are expended in the tendons about the shoulder. The Subscapular Artery (Plates 5 and 6) is a comparatively large vessel, and beyond its origin the parent trunk is much reduced in calibre. It arises at the interstice between the subscapularis and teres major muscles ; and, disappearing between these muscles, it ascends behind the glenoid border of the scapula, as far as its dorsal angle. It gives off a considerable number of vessels that cannot at this stage be completely followed, but near its origin it will be seen to throw off a branch which runs upwards and backwards on the latissimus dorsi (Plate 5). The Brachial Artery (Plates 5 and 6) is the direct continuation of the 12 THE ANATOMY OF THE HORSE. axillary, which changes its name when it passes on to the teres major. It descends in a nearly vertical direction to the lower extremity of the humerus, where, above the inner condyle, it divides to form the anterior and posterior radial arteries.* In its course it crosses the direction of the humerus obliquely, and rests successively on the tendons of the teres major and latissimus dorsi, the small head of the triceps, and the bone. In front of it is first the coraco-humeralis, and then the biceps ; but these are separated from it by the median nerve, which is in close contact with the vessel. Behind the artery is the satellite vein, posterior to which is the ulnar nerve. Its collateral branches are : the pre-humeral, the deep humeral, the ulnar, the nutrient artery of the humerus (sometimes), and innominate muscular branches. The Pre-humeral or Anterior Circumflex Artery (Plate 6) arises at the tendon of the teres major, and passes in front of the humerus, between the upper and lower insertions of the coraco-humeralis, to terminate in the biceps or the mastoido-humeralis. Some of its fine twigs may anastomose with divisions of the posterior circumflex. The Deep Humeral Artery (Plates 5 and 6) arises at the lower border of the latissimus dorsi tendon, and soon splits into three or four branches, the larger of which perforate the large head of the triceps extensor cubiti, while the smaller supply the small and medium heads of the same muscle. A branch is continued round behind the humerus, in company with the musculo-spiral nerve, to the front of the elbow-joint, where it anastomoses with branches of the anterior radial. This branch will not be followed at present. Muscular Branches of the Brachial. The largest and most constant of these is a vessel of considerable size which penetrates the lower part of the biceps (Plate 6). The Ulnar artery and the two terminal branches of the brachial will be followed in the dissection of the fore-arm. The Brachial Vein is a large vessel which ascends behind the artery, and receives branches that for the most part correspond to those of the artery. It receives also the subcutaneous thoracic or spur vein. Directions . — As the brachial vein generally contains a large quantity of blood which exudes from the smaller cut branches, it will contribute to the neatness and cleanness of the dissection if the dissector will carefully remove the vein and all its branches before he proceeds to follow the nerves. The Brachial Plexus. The mode of formation of the plexus has already been explained, and the student will recollect that he has already followed branches from it to the levator anguli scapulse, serratus magnus, and pectoral muscles, as well as the subcutaneous * In Plate 6 the termination of the brachial artery has been pulled slightly forwards in order to show the origin of the anterior radial artery. PLATE V Mastoido-humeralis Post, radial artery Superficial pectoral Subscapularis Supraspinatus Suprascapular nerve Nerve to subscapularis Nerve to latissimus dors! Circumflex nerve Suprascapular artery- Axillary artery- Post. deep pectoral Ant. deep pectoral Median nerve Brachial artery- Biceps latissimus dorsi •Teres major Nerve to teres major f Artery to \latissimus dorsi Subcut. thoracic nerve ubscapular artery Musculo-spiral nerve Deep humeral artery Caput parvum —Ulnar nerve Ulnar artery Scapulo-ulnaris Cutaneous branch of ulnar nerv Drawn 8c. Printed 'by'W ScAK. Johnston. Edinburgh 8c London SHOULDEE AND ARM— Inner Aspect DISSECTION OF THE ANTERIOR LIMB. 13 thoracic nerve, and the filament furnished by the plexus to the phrenic nerve. He can now easily identify and trace the following branches : — The Nerve to the Latissimus Dorsi (Plate 5) derives its fibres from the 8th cervical and the dorsal roots of the plexus. The Nerve to the Teres Major (Plate 5) — one or more filaments, generally deriving fibres, in common with the circumflex nerve, from the 7th and 8th cervical roots (with possibly some fibres from the 6th). The Nerve to the Subscapularis (Plate 5) derives its fibres from all the cervical roots of the plexus. The Circumflex Nerve (Plates 5 and 6). Its fibres come from the 7th and 8th cervical roots, and possibly also from the 6th. It turns round behind the shoulder-joint in company with the posterior circumflex artery ; and on the outside of the joint it supplies branches to the teres minor, deltoid, mastoido-humeralis, and skin (Plate 7). It gives a twig to the small scapulo-humeral muscle. The Suprascapular Nerve (Plate 5), deriving' its fibres from the 6th, 7th, and 8th cervical roots, passes into the interstice between the subscapularis and the supraspinatus. It then turns round the anterior border of the scapula ; and gaining its dorsal surface, is expended in the supraspinatus and subspinatus muscles (Plate 8). The Musculo-spiral Nerve (or radial nerve) (Plates 5 and 6) is, at its origin, the thickest of the nerves of the brachial plexus. Deriving its fibres from the 7th and 8th cervical, and from the dorsal roots of the plexus, it passes downwards and backwards on the subscapularis and teres major muscles, and some little distance behind the axillary vessels, from which it is separated by the ulnar nerve. On reaching the deep humeral artery, it disappears in front of the large head of the triceps, and is continued round the humerus in the musculo-spiral groove, where it rests on the brachialis anticus (humeralis externus), and, after- wards, at the posterior or outer border of that muscle. It reaches the front of the elbow-joint, being here deeply placed between the brachialis anticus inwardly, and the origin of the great extensor of the metacarpus outwardly. Before the nerve disappears behind the humerus, it gives branches to the great and small heads of the triceps, and a long branch which passes backward to divide under the scapulo-ulnaris for the supply of that muscle. Behind the limb it supplies the medium head of the triceps and the anconeus, and furnishes a few cutaneous branches which perforate the caput medium, or emerge at its lower part, to be distributed to the skin of the outer side of the fore-arm, below the elbow. The termination of the nerve will afterwards be followed in the fore-arm, where it supplies the extensor muscles and the flexor metacarpi externus. The Ulnar Nerve (Plates 5 and 6) derives its fibres from the dorsal roots of the brachial plexus. At first it lies close behind the main 14 THE ANATOMY OF THE HORSE. vessels ; but as it passes downwards, it recedes from them, and passing under cover of the scapulo-ulnaris, it reaches the space between the olecranon and the inner condyle. Thence it descends to the back of the fore-arm, where it will subsequently be dissected. At present it is seen to give off only one branch, which disappears within the superficial pectoral muscle, and afterwards becomes distributed to the skin of the fore-arm (Plate 5). The Median Nerve (Plates 5 and 6) is formed by the union of two roots. The anterior of these comes from the 6th, 7th, and 8th cervical, while the posterior is derived from the 8th cervical and the 1st dorsal. These roots gives off some pectoral twigs, and then unite by forming a loop in which the axillary artery rests. The nerve then descends in front of the axillary artery and its brachial continuation, and will afterwards be seen to accompany the posterior radial artery. The following branches of the nerve may be found at present The Nerve to the Biceps and C oraco-humeralis comes off close below the union of the two roots of the median, or from the anterior root above the point of union. It passes between the upper and lower insertions of the coraco-humeralis, supplying that muscle and terminating in the biceps. Musculo-cutaneous branch. — This is given off from the median about the middle of the humerus ; and passing underneath the biceps, it divides into a muscular branch for the brachialis anticus, and a cutane- ous branch for the front of the fore-arm. Directions. — The muscles of this region should now be examined in the order of their description. The Latissimus Dorsi (Plates 5 and 6). The insertion of this muscle into the inner tubercle of the humerus is here seen. About an inch or two from its termination the tendon gets a twist which alters the direc- tion of its surfaces, and brings it to be inserted in front of the termina- tion of the teres major on the same tubercle. Action. — The muscle is a flexor and an inward-rotator of the shoulder- joint. The Teres Major (Plate 5). It arises from the dorsal angle of the scapula, and from an aponeurosis between it and the subscapularis. It is inserted into the internal tubercle of the humerus, its terminal tendon resting in the twist formed by the tendon of the latissimus dorsi muscle. Action. — It is a flexor and an inward-rotator of the shoulder. The Scapulo-Ulnaris (Plate 5). This is a thin, flat muscle which rests on the inner surface of the triceps, and is provided, in front and above, with a thin, transparent tendon. It arises from the posterior border of the scapula, and is inserted into the posterior border of the olecranon, and into the fascia of the fore-arm. At its lower extremity the muscle covers the ulnar vessels and nerves. PLATE VI Suprascapular Musculo-spiral nerve Circumflex nerve Ulnar nerve Roots of median nerve Axillary Nerve to and Coraco-humeralis Coraco-humera Brachial art. Median nerve Musculo-cutaneoue Ulnar art Art. to Flexor meta carpi Post, radial art. Int. plantar nerve Subscapularis major art. Art. to Latissimus dorsi Latissimus dorsi magnum Beep humeral art. Prehumeral art. Caput parvum Ant. radial art. Ulnaris accessorius head head of ) Flexor metacarpi . of j medius Flexor perforans Ulnar artery nerve Flexor perforatus Small metacarpal Large metacarpal art. Branch of median to ext. plantar nerve Branch of ulnar to ext. plantar nerve Ext. plantar nerve !Draym & Printed 'by'W. &.A.K Johnston, Edmbiirgh fe London SHOULDER, ARM, AND FORE- ARM— Inner Aspect DISSECTION OF THE ANTERIOR LIMB. 15 Action. — To extend the elbow-joint, and tense the fascia of the fore- arm. The Triceps Extensor Cubiti. This is an immense muscular mass which, with the preceding, fills up the angle formed behind the shoulder-joint. It has three divisions or heads, which may be distin- guished as the caput magnum, the caput medium, and the caput parvum. The Caput Magnum , or large head (Plate 5), forms a great mass which is seen on both the outside and the inside of the limb. It arises from the dorsal angle and glenoid (posterior) border of the scapula ; and it is inserted into the olecranon, there being a synovial bursa between the summit of that eminence and the tendon. The Caput Parvum , or small head (Plate 5), is, when compared with the preceding, a very small muscle. It arises from the shaft of the humerus below and behind the internal tubercle, and it is inserted into the olecranon. The Caput Medium , which is not now visible, will be dissected with the outside of the shoulder. Action of the triceps. It is an extensor of the elbow-joint, and acts as a lever of the first order, the joint, which represents the fulcrum, being between the power and the weight. The large head is also a flexor of the shoulder. The Subscapularis (Plates 5 and 6). This muscle is lodged in the fossa of the same name on the ventral surface of the scapula, and it arises from the whole extent of that fossa. It is inserted into the inner tuberosity of the humerus, a small synovial bursa being interposed between the tendon and the bone. The tendon is crossed by the origin of the coraco-humeralis, and another small bursa is here interposed between the tendons. Above its insertion it is closely related to the capsular ligament of the joint. The muscle is partly united in front with the supraspinatus, and behind with the teres major. Action. — It is an adductor of the shoulder. The Coraco-humeralis (or coraco-brachialis) (Plates 5 and 6). This, which is rather a small muscle, arises from a small tubercle on the inner side of the coracoid process of the scapula. It has two insertions , the first into the inner surface of the shaft of the humerus above the internal tubercle, the second into a line which begins on a level with the internal tubercle, and runs down the anterior surface of the shaft near its inner border. Between these two insertions, the pre-humeral artery and the nerve to the biceps pass. The tendon of origin of the muscle comes out between the supraspinatus and subscapularis muscles, and the posterior border of the muscle is related to the brachial artery. Action . — To adduct and flex the shoulder. The Biceps (Plates 5 and 6). This muscle receives its name in the 16 THE ANATOMY OF THE HORSE. human subject from its having two heads of origin. It is also known as the flexor brachii or coraco-radialis. It arises from the whole of the coracoid process of the scapula with the exception of the tubercle on its inner side, which is for the coraco-humeralis. Its strong tendon of origin emerges from between the outer and inner tendons of the supraspinatus, and passes over the shoulder-joint, a pad of fat separating its deep face from the capsular ligament of the joint. The tendon, which is of fibro- cartilaginous consistency, then plays over the bicipital groove of the humerus, on which its deep face is moulded, and a synovial bursa facilitates the movements of the tendon in the groove. The central portion of the muscle, which is thick and fusiform, has numerous tendinous intersections, and is traversed throughout by a fibrous cord. It rests on the anterior face of the humerus, and at its lower end terminates by a tendon which, passing over the anterior ligament of the elbow-joint (to which it is adherent), is inserted into the bicipital tuberosity of the radius. The tendon is partly covered by the internal lateral ligament of the elbow. The muscle has a second insertion, in the shape of a strong fibrous band, detached from the main tendon to blend with the sheath of the extensor metacarpi magnus, and deep fascia on the front of the fore-arm. Action. — To flex the elbow-joint, and make tense the fascia of the fore-arm. In the first of these actions it is a good example of a lever of the third order, where the power is applied between the fulcrum — represented by the elbow-joint, and the weight — represented by the distal portion of the limb. The fibrous cord which traverses the muscle is a mechanical extensor of the shoulder-joint, as long as the elbow is kept extended by the triceps extensor cubiti. Directions . — The teres major from the shoulder upwards should now be removed, in order to follow more thoroughly the course of the sub- scapular artery with its branches, and to expose the small scapulo- humeral muscle, which lies on the capsular ligament behind the joint ; but care should be taken, in dissecting the tendons in the neighbour- hood of the joint, to preserve the capsular ligament intact. The Subscapular Artery (Plate 6) springs from the axillary trunk at the interstice between the subscapularis and teres major muscles, and disappearing from view, runs upwards at the posterior, border of the scapula. It gives off as its most important branches : — 1. A Muscular branch of considerable volume which passes backwards and upwards on the deep face of the latissimus dorsi. 2. The Posterior circumflex artery, which turns round behind the shoulder, passing through a triangular space bounded by the teres major, caput magnum, and scapulo-humeralis gracilis. At the outer side of the joint (Plate 7) it appears between the caput magnum, caput medium, and teres minor, and is covered by the deltoid. It splits into branches DISSECTION OF THE ANTERIOR LIMB. 17 which are distributed to these muscles and the supraspinatus (Plate 7). It is accompanied by the circumflex nerve. 3. Other branches of the subscapular are as follows : — A few inches above the origin of the posterior circumflex, a vessel is detached which passes backwards, and divides to supply the caput magnum. A number of smaller branches come off from the anterior aspect of the vessel, and are distributed on both surfaces of the scapula. One of these supplies the nutrient artery of the scapula. The Scapulo-humeralis Gracilis is a very slender muscle. It arises from the scapula above the rim of its glenoid cavity ; and passing over the capsular ligament of the shoulder, on which some of its fibres seem to terminate, it insinuates itself between the fibres of the brachialis anticus (humeralis externus), and is inserted into the posterior surface of the shaft of the humerus. It is supplied by a small nerve from the circumflex. Action . — The muscle is too inconsiderable in size to exercise any appreciable action on the joint over which it passes, and, probably, its function is to raise the capsular ligament and prevent its injury during flexion of the joint. OUTER ASPECT OF THE SHOULDER AND ARM. Directions . — The limb is now to be turned over, and the muscles and other structures on the outer side of the scapula and humerus are to be dissected. Scapular Fascia. — This is a strong, glistening, fibrous covering which is spread over the muscles on the dorsum of the scapula, affording by its inner surface an origin to many of their fibres. When traced upwards, it is seen to be inserted into the scapula or its cartilage of pro- longation; while before, behind, and inferiorly, it becomes less fibrous, and is continuous with the fascia covering the muscles on the inner surface of the scapula and the outer aspect of the arm. It furnishes septa to pass between the subjacent muscles, and it is adherent to the tubercle on the scapular spine. If an attempt be made to dissect it off these muscles, they will be exposed with a rough surface, showing that they there take origin from the inner aspect of the fascia. The Deltoid Muscle (scapular portion) (Plates 4 and 7). This muscle was by Percivall erroneously termed the teres minor. It is not the homologue of either of the teres muscles of human anatomy, but is, most clearly, the representative of that part of the deltoid muscle which in man takes origin from the scapula. A linear depression which traverses the muscle corresponds to an imperfect division of it into an anterior and a posterior portion. It arises by its anterior portion from the 18 THE ANATOMY OF THE HORSE. scapular fascia, and by its posterior portion from the dorsal angle of the scapula. It is inserted into the deltoid (external) tubercle of the humerus. Action . — To abduct the humerus, and rotate it outwards. Acting- with the teres major, it is also a flexor of the shoulder. Directions . — The last-mentioned muscle should be carefully cut at the level of the shoulder, and reflected upwards and downwards. This will expose the divisions of the circumflex vessels and nerve, branches of which will be seen entering the muscle, and it will at the same time bring into view the next muscle. The Teres Minor (Plates 7 and 8). ( This small muscle arises from the posterior border of the scapula, from the rough lines at the lower part of the infraspinous fossa, and from the small tubercle on the outer rim of the glenoid cavity. Its tendon, which is crossed by a glistening- band of fascia, is inserted into the lower half of the ridge running upwards from the deltoid tubercle to the external tuberosity. Action. — The same as the preceding muscle. The Infraspinatus (subspinatus, or postea-spinatus) (Plates 7 and 8) occupies the greater part of the fossa of the same name. It arises from the whole extent of the fossa, and from the inner surface of the scapular fascia. It possesses two tendons of insertion, the outer of which passes over the convexity of the external tuberosity, a synovial bursa being interposed, and is inserted into the upper half of the ridge connecting that tuberosity to the deltoid tubercle. If this tendon be cut where it plays over the convexity, the synovial bursa will be opened, and, at the, same time, the inner insertion of the muscle into the inside of the convexity will be exposed. This inner tendon is more fleshy than the outer, and is in contact with the capsular ligament of the shoulder. Action. — It abducts the humerus, and rotates it outwards. The Supraspinatus (antea-spinatus) (Plates 7 and 8) fills the whole of the fossa of the same name, and takes origin from it as well as from the scapular fascia. It is bifid interiorly, having an inner tendon inserted into the internal tuberosity at its highest point, and an outer tendon inserted into the corresponding point of the external tuberosity. These two tendons are in contact with the capsular ligament of the joint, and the tendon of origin of the biceps emerges from between them. Action. — It is an extensor of the shoulder-joint. Directions. — The outer aspect of the triceps extensor cubiti is here seen; and when its surface has been cleaned, a line will be observed running from the shoulder to the point of the elbow. Careful dissection downwards into the mass, along this line, will separate the caput mag- num (already described) from the caput medium, which lies below it. While the surface of the muscle is being cleaned, some small cutaneous PLATE VII -Infraspinatus Deltoid -Biceps Deep flexors Caput magnum Caput medium Extensor metacarpi mag. Ulnaris accessorius Ulnar nerve and artery- Flexor metacarpi ext.< Band from carpus to extensor ^ suffr. tendon Subcarpal ligament Flexor perforatus tendon Flexor perforans tendon Suspensory ligament Supraspinatus -Teres minor —Circumflex vessels and nerves Extensor suffraginis Interosseous artery Extensor metacarpi obliq. Extensor pedis tendon Extensor suffr. tendon Slip from suspensory ligament to extensor pedis Drawn Sc. Printed byW. ScA.K Johnston, Edinburgh Sc. London FORE-LIMB— Outer Aspect DISSECTION OF THE ANTERIOR LIMB. 19 nerves from the musculo -spiral will be found to pierce the muscle, or emerge at its lower edge, and become distributed to the outer side of the fore-arm. These should, as far as possible, be preserved. The Caput Medium (Plates 7 and 8) arises , by a short aponeurotic ten- don, from a curved line beginning on the deltoid tubercle and continued upwards to the external tuberosity. It is inserted into the olecranon. Action. — Like the other divisions of the triceps, this muscle is an extensor of the elbow-joint. Directions. — By raising the lower edge of the last muscle and dissect- ing upwards, the anconeus will be partly exposed ; but to effect a com- plete and natural separation of the two muscles, is a matter of some difficulty. The Anconeus (Plates 7 and 8) is a small muscle which lies above the olecranon fossa, and there covers the synovial membrane of the joint, a pad of fat being interposed. It arises from the margin of the fossa, and is inserted into the olecranon on its outer and anterior aspect. Action. — To assist in extending the elbow, and at the same time to raise the synovial membrane and prevent its injury between the bones. Directions. — If the caput medium be now severed at its origin, and turned backwards, the musculo-spiral nerve and some branches of the deep humeral artery will, as already described, be found turning round the humerus in the musculo-spiral groove, which is mainly filled by the brachialis anticus muscle. The Brachialis Anticus muscle (Plate 8), also known as the humeralis obliquus or externus, is lodged in the furrow of torsion on the shaft of the humerus. The muscle has its origin on the posterior aspect of the shaft of the humerus below its articular head. Its tendon, which cannot be followed at present, passes in front of the elbow-joint, and is afterwards reflected under the internal lateral ligament of the joint, to be inserted into the radius and ulna. Action . — To flex the elbow-joint. the fore-arm. Surface-marking. — At the elbow-joint the olecranon process of the ulna is distinctly seen ; but the shafts of the bones of the fore-arm are clothed with muscles, except at the lower third of the inner border of the radius, where the bone is subcutaneous. On the outer side of the front of the elbow-joint a large muscular mass is formed by the extensor metacarpi magnus and the anterior extensor of the digit (extensor pedis). In the living animal (in which it is preferable to study these surface-markings) this is more distinctly visible, and the tendons of these muscles and that of the lateral extensor (extensor suffraginis) may be distinctly traced. On the inner side of the elbow-joint one may feel the tendon of insertion of the biceps; and just behind the tendon the posterior 20 THE ANATOMY OF THE HORSE. radial vessels and the median nerve may be felt as they lie on the bone under cover of the posterior superficial pectoral, and they may be made to roll under the finger. This should be practised, as the posterior radial artery is a convenient vessel at which to feel the pulse. The internal subcutaneous vein crosses the inner face of the fore -arm obliquely upwards and forwards ; and in the living animal, pressure at the upper part will distend the vessel and bring it into view. At the outer side of the carpus the prominence formed by the pisiform bone may be seen and felt. On the inner surface of the fore-arm, at its lower third, the skin presents an oval-shaped, horny callosity, vulgarly termed the chestnut. This is largest in coarse-bred animals. Directions. — The skin is now to be carefully removed from the fore -arm and carpus, and the cutaneous nerves and vessels are to be sought. Cutaneous Nerves. (1) At the front of the elbow-joint (Plate 8) the cutaneous division of the musculo-cutaneous branch of the median appears from beneath the biceps, and splits into two branches, one accompanying the anterior, the other the internal, subcutaneous vein ; (2) a little way below the elbow, on its inner aspect, the cutaneous branch of the ulnar (Plate 5) appears from beneath the insertion of the posterior superficial pectoral, and divides for the supply of the skin of the back of the fore-arm on both its outer and its inner side ; (3) perforating the caput medium, or emerging at its lower edge, are some twigs from the musculo-spiral nerve, which are distributed to the skin of the outer side of the fore-arm beneath the elbow ; (4) on the outer side of the carpus (Plate 8) are the ramifications of a cutaneous branch of the ulnar, which comes out be- tween the tendons of the external and oblique flexors of the metacarpus. Subcutaneous Veins. — 1. The Median or Internal subcutaneous vein begins at the inner side of the carpus, where it continues upwards the internal metacarpal vein. It crosses the fore -arm obliquely upwards and forwards, in company with a cutaneous nerve already described, and divides into the cephalic and basilic veins. The Cephalic vein has already been seen ascending in the groove between the mastoido- humeralis and the anterior superficial pectoral to terminate in the jugular. The Basilic vein pierces the posterior superficial pectoral to concur in forming the brachial vein. 2. The Anterior subcutaneous or radial vein is much smaller than the preceding vessel. It begins at the front of the carpus, and, ascending on the middle line of the fore-arm, it empties itself into the cephalic or the median vein. Directions. — The thin superficial fascia in which these nerves and vessels are distributed should be removed to show the deep fascia. Deep Fascia of the fore-arm. — This is spread in the form of a close- fitting fibrous envelope around the fore -arm. Above it receives an DISSECTION OF THE ANTERIOR LIMB. 21 insertion from the biceps, and another from the scapulo-ulnaris ; below it is continued over the carpus to form sheaths for the tendons ; while by its deep face it furnishes septa to pass between the muscles of the fore-arm. Directions . — The dissection of the back of the fore-arm is now to be undertaken. The before -mentioned fascia is to be incised along the lines of separation of the muscles, and these are to be cleaned and isolated. The remaining portion of the posterior superficial pectoral muscle, which covers the posterior radial vessels and the median nerve at the inner side of the elbow, is to be removed ; and care is to be taken of the ulnar vessels and nerve, which are placed beneath the deep fascia, on the middle line at the back of the limb. The Ulnar Artery (Plates 6 and 7) is a collateral branch of the brachial, from which it comes off at the lower border of the caput parvum. It descends parallel to the lower border of that muscle, to the space between the olecranon and the inner condyle, where it is covered by the scapulo- ulnaris. It here places itself in company with the ulnar nerve ; and, crossing beneath the ulnar origin of the middle flexor of the metacarpus, it descends to the carpus by following the tendon of the ulnar portion of the deep flexor (ulnaris accessorius), being placed between the external and oblique flexors of the metacarpus. At the upper limit of the carpus it concurs in the formation of the supracarpal arch, by joining a branch detached from the large metacarpal artery. In this course it gives off — (1) the nutrient artery to the humerus (sometimes); (2) articular branches to the elbow-joint ; (3) muscular branches in the neighbour- hood of the joint, to the scapulo-ulnaris, caput parvum, and posterior superficial pectoral; (4) cutaneous branches to the skin on the inner side of the fore-arm. The Ulnar Vein accompanies the artery and nerve, and at the elbow concurs in the formation of the brachial vein. The Ulnar Nerve (Plates 6 and 8) has already been partly described in the dissection of the arm. At the lower part of that region it crosses the ulnar artery, with which it places itself in company between the ole- cranon and the inner condyle. It here gives off branches to the following muscles : — (1) the anterior head of the middle flexor of the metacarpus; (2) the ulnar head of the same muscle ; (3) the superficial flexor of the digit (perforatus) ; (4) the ulnar origin of the deep flexor (ulnaris accessorius). In the fore-arm it descends in close company with the vessels of the same name, and at the carpus it gives off the cutaneous branch already described (page 20). At the upper border of the pisiform bone, and beneath the tendon of the middle flexor,* it joins a branch from the median to form the external plantar nerve. * In Plates 6 and 9 the termination of the nerve has been pulled slightly forwards to show its junction with the branch from the median. 00 THE ANATOMY OF THE HORSE. The Posterior Radial Artery (Plate 6) is one of the terminal branches of the brachial. It is so much larger than the other terminal branch (the anterior radial), that it might be described as the direct continuation of the brachial, whose direction it prolongs. Beginning above the inner condyle, it descends on the bone, and then lies over the internal lateral ligament of the elbow-joint, and posterior to the tendon of insertion of the biceps. It is here covered by the posterior superficial pectoral, and is related to the median nerve, which lies close behind it, and to its satellite veins. At this point it is favourably placed for taking the pulse, and its situation and relations should be carefully noted. After crossing the elbow, it inclines forwards and disappears with the median nerve between the radius and the internal flexor of the meta- carpus. In this position it descends to within a short distance of the carpus, where it divides into two terminal branches of unequal size — the large and small metacarpal arteries. It gives off* the following collateral branches : — 1. Articular Branches to the elbow-joint. 2. The Interosseous Artery of the fore-arm, which reaches the outside of the limb by passing through the radio-ulnar arch. It then descends along the outer side of the line of junction of the radius and ulna (Plate 7), where it will be followed in the dissection of the front of the fore-arm. 3. Muscular Branches to the flexors of the metacarpus and digit. 4. Cutaneous Branches . The Posterior Radial Veins. The artery is accompanied by three or four satellite veins, which surround it and the nerve, and anastomose freely with each other. They begin at the carpus, where they anasto- mose with the metacarpal veins, and at the elbow-joint they unite with the basilic and ulnar veins to form the brachial vein. They receive branches corresponding more or less exactly to those of the artery. The Median Nerve in the fore-arm (Plate 6). This nerve has already been followed in the dissection of the arm, where it was seen descending in front of the brachial artery. It preserves the same relationship to the first few inches of the posterior radial artery, but at the elbow it crosses the artery superficially to take up a posterior position. Below the joint it again changes its position by mounting on the surface of the artery, or it may even again place itself in front. At a variable point in the fore-arm it terminates by dividing into two branches, one of which is continued as the internal plantar nerve, while the other joins the ulnar to form the external plantar. In the subject from which Plate 6 was taken, the division took place considerably above the middle of the fore- arm, but more frequently it oocurs in the lower third. Immediately below the elbow the nerve furnishes a branch to the internal flexor of the metacarpus, and branches to the deep flexor of the digit (humeral and radial heads). DISSECTION OF THE ANTERIOR LIMB. 23 Directions. — The muscles on the back of the fore -arm must now be learnt. These consist of the three flexors of the metacarpus, and the two flexors of the digit. The Flexor Metacarpi Internus (Plate 6). This muscle lies along the inner edge of the posterior surface of the radius, where it conceals the posterior radial vessels and the median nerve. It arises from the inner condyle of the humerus, just behind the point of origin of the internal lateral ligament, where it is confounded with the origin of the middle flexor. It terminates inferiorly in a long, slender tendon, which, after passing through a synovial sheath at the inner side of the carpus, is inserted into the head of the inner small metacarpal bone. Action. — It is a flexor at the carpal articulations — i.e ., it flexes the manus on the fore-arm. The Flexor Metacarpi Medius (Plate 6). This muscle descends in contact with the posterior edge of the internal flexor. It has two heads of origin — an anterior and a posterior. It arises by its anterior head just behind the origin of the preceding muscle, and by its posterior head from the upper part of the posterior edge of the olecranon. After a course of three or four inches these two heads unite, and the single inferior tendon is inserted into the upper border of the pisiform bone. The ulnar nerve and vessels pass beneath the posterior or ulnar head of the muscle. Action. — The same as the preceding muscle. The Flexor Metacarpi Externus (Plates 7 and 8) is situated at the outer side of the back of the fore-arm, having the lateral extensor of the digit (extensor suffraginis) in front of it, while behind it is separated from the last-described muscle by the ulnar division of the deep flexor of the digit (ulnaris accessorius). It arises from the lowest point of the outer ridge bounding the olecranon fossa. At its lower end it has two inser- tions , viz., (1) into the upper border of the pisiform bone, where it is confounded with the insertion of the middle flexor ; (2) by a cord-like tendon which, after descending in a synovial sheath formed inwardly by the oblique groove on the outer surface of the pisiform bone, is inserted into the head of the external small metacarpal bone. Action . — Like the preceding two muscles. Directions . — The three flexors of the metacarpus surround the flexors of the digit, and they should be cut about their middle and reflected to bring these latter into view. The Superficial Flexor of the Digit (flexor pedis perforatus) (Plate 6) arises , by a tendon common to it and the deep flexor, from the lower extremity of the ridge bounding the olecranon fossa on the inside. Its muscular belly contains much tendinous tissue, and cannot without difficulty be separated from the deep flexor, on which it rests. At the lower part of the radius its muscular portion is succeeded by a tendon, 24 THE ANATOMY OF THE HORSE. which, after being reinforced by a fibrous band from the back of the radius, passes through the carpal sheath behind the carpus, and is ultimately inserted by a bifid tendon into the second phalanx. The examination of this and the succeeding muscle, from the carpus down wards, must be postponed till the dissection of the metacarpus and digit is undertaken. Action. — The muscle flexes successively the pastern, fetlock, and carpal joints. The Deep Flexor of the Digit (flexor pedis perforans) (Plate 6). This muscle is situated in contact with the posterior surface of the radius, and consists of three divisions, which may be distinguished as the humeral, the radial, and the ulnar portions. The humeral or main division arises , in common with the preceding muscle, from the lower extremity of the ridge bounding the olecranon fossa on the inside. The radial portion, or radialis accessorius , is deeply placed, and arises from the back of the radius. The ulnar division, or ulnaris accessorius , is placed beneath the deep fascia of the fore-arm, where it lies between the external and oblique flexors of the metacarpus, and is accompanied by the ulnar nerve and vessels. It arises from the summit and posterior border of the olecranon. These three divisions unite above the carpus, and have a common tendon which passes through the carpal sheath, and is ultimately inserted into the os pedis. Action. — It flexes successively from below upwards the inter-phalan- geal joints, the fetlock, and the carpus. Directions. — The front of the fore-arm must now be dissected ; and here it will be convenient to turn attention in the first place to muscles ; but while these are being isolated, care is to be taken of the interosseous vessels, which descend along the lateral extensor at the outer side of the region, and of the tendon of the oblique extensor where it crosses over the tendon of the extensor metacarpi magnus above the carpus. The Extensor Metacarpi Magnus (Plates 7 and 8) corresponds to the long and short radial extensors of the wrist in the human subject. It is a powerful muscle, having at its upper end a massive muscular belly, which tapers downwards, and terminates a few inches above the carpus in a tendon. It arises from the anterior and upper part of the outer ridge of the olecranon fossa (the outer condyloid ridge), where this ridge bounds the musculo-spiral groove ; and by a second tendon, in common with the extensor pedis, from a depression which is placed external to the coronoid fossa. Its inferior tendon lies in the largest and most internal of the vertical grooves at the lower end of the radius ; and after gliding over the front of the carpus in a synovial sheath, it is inserted into a special tubercle on the upper end of the large metacarpal bone at its inner side. Action. — It extends the manus on the fore-arm. PLATE VIII Infraspinatus, Supraspinatus Deltoid Bicepi Musculo-cutaneous nerve ) (cutaneous division) ) Extensor metacarpi > magnus ) Ant. radial artery Extensor metacarpi ) obliquus f •; |ij|| Extensor pedis Suprascapular nerve Branches from subscapular artery Teres minor Circumflex nerve Caput magnum aput parvum Caput medium Brachialis anticus Musculo-spiral nerve Anconeus Ulnaris accessorius Flexor metacarpi ext. .Extensor suffraginis Ulnar nerve . Cutaneous branch ' Branch to ext. plantar nerve Printed by W &.A.K. Johnston. EAm'bnrfh fc .London SHOULDER, ARM, AND FORE-ARM— Outer Aspect ( Chauveau ) DISSECTION OF THE ANTERIOR LIMB. 25 The Extensor Metacarpi Obliquus (Plates 8 and 9). This is the re- presentative of the extensor muscles of the thumb in man. It arises from the outer side of the radius ; and its tendon, after passing obliquely downwards and inwards over that of the great extensor, is inserted into the head of the inner small metacarpal bone. It lies in an oblique groove at the lower end of the radius, where the play of its tendon is facilitated by a small synovial bursa. Action. — Like the preceding muscle. The Extensor Pedis, or anterior extensor of the digit (Plate 7), represents the extensor communis digitorum of man. At its origin it lies immediately to the outer side of the extensor metacarpi magnus, but at the lower part of the fore-arm the extensor metacarpi obliquus emerges from between the two muscles. It arises , by a tendon common to it and the extensor metacarpi magnus, from a depression external to the coronoid fossa ; also from the external lateral ligament of the elbow, and the external tuberosity at the upper end of the radius. It consists of two parallel portions of unequal size, and these are succeeded by two tendons which lie close together, but are distinct from each other. These tendons pass in common through a vertical groove at the lower end of the radius, and over the front of the carpus, where they are pro- vided with a synovial sheath. In the dissection of the metacarpus and digit, the tendons will be pursued to their insertion, the outer and smaller * joining the tendon of the extensor suffraginis, while the inner and main tendon becomes inserted into the pyramidal process of the os pedis. Action. — This muscle extends in succession the interphalangeal joints, the fetlock, and the carpus. The Extensor Suffraginis, or lateral extensor of the digit (Plates 7 and 8), is a smaller muscle than the extensor pedis, to the outer side of which it lies. It is the homologue of the extensor of the little finger in man. It arises from the external lateral ligament of the elbow, from the external tuberosity at the upper end of the radius, from the line of junction of the radius and ulna, and from the outer border of the radius. Its tendon passes first through a vertical groove on the external tuber- osity at the lower end of the radius, then through a synovial sheath at the outer side of the carpus, and it will subsequently be followed to its insertion into the first phalanx. Action. — It is an extensor of the fetlock and of the carpus. Directions. — The nerves and bloodvessels on the front of the fore-arm must next be sought, and in order to fully expose them, some of the foregoing muscles must be cut. The biceps is to be cut about its This is sometimes termed the muscle of Phillips. Occasionally there occurs, to the inner side of the preceding, another and smaller fasciculus, with a slender tendon which joins the main tendon before reaching the carpus. This is the muscle of Thiernesse. 26 THE ANATOMY OF THE HORSE. middle in order to follow the anterior radial artery ; and by dissecting deeply down in front of the elbow, between the brachialis anticus and the extensor metacarpi magnus, the artery will be found to meet the musculo -spiral nerve. The extensor metacarpi magnus is to be cut about its middle and carefully reflected in order to follow the artery, which lies in relation to the deep face of the muscle ; and the extensor pedis is to be similarly reflected to trace the termination of the mus- culo-spiral nerve. The Anterior Radial Artery (Plate 8) is the smaller terminal branch of the brachial. It separates at an acute angle from the pos- terior radial, and passes forwards beneath the biceps and then beneath the brachialis anticus. It meets the musculo-spiral nerve in the inter- space between the brachialis anticus and the extensor metacarpi magnus, and afterwards descends on the anterior surface of the radius, where it is covered by the last-mentioned muscle. It terminates at the carpus by anastomosing inwardly with branches from the posterior radial, and outwardly with the interosseous artery of the fore- arm. It supplies articular branches to the elbow, and muscular branches to the muscles on the front of the fore-arm. The Interosseous Artery of the fore -arm (Plate 7) is a branch given off by the median at the back of the fore -arm. It comes out- wards through the radio -ulnar arch, and descends along the extensor suffraginis, terminating in slender branches in front of the carpus. It supplies articular branches to the elbow ; the nutrient artery of the radius ; and muscular twigs to the extensor suffraginis, extensor pedis, and extensor metacarpi obliquus. The anterior radial and interosseous arteries are, generally, compara- tively slender vessels, but they are liable to some variation in size and distribution, and the one may partly supplant the other. Veins. Satellite veins of the same names run in company with the foregoing arteries. The Musculo-spiral Nerve in the fore-arm (Plate 8). In the dissec- tion of the axilla and arm, this nerve has already been seen as a large trunk descending from the brachial plexus, and taking a spiral course be- hind the humerus. It reaches the front of the elbow, where it meets the radial artery in the interspace between the brachialis anticus inwardly, and the origin of the extensor metacarpi magnus outwardly. It here gives off branches to the extensor metacarpi magnus, extensor pedis, extensor suffraginis, and flexor metacarpi externus ; and, much reduced in size, it descends between the shaft of the radius and the extensor pedis, and terminates in the extensor metacarpi obliquus. The nerve to the flexor metacarpi externus is furnished after the branches to the extensor pedis, and passing outwards between the latter muscle and the bone, it penetrates its muscle at the radio-ulnar arch. DISSECTION OF THE ANTERIOR LIMB. 27 Directions . — In this stage of the dissection the student will be better able to trace the musculo-cutaneous branch of the median nerve, and the insertions of the biceps and brachialis anticus muscles (see pages 16 and 19). When these have been examined, he may, as the next step, either dissect the articulations of the shoulder and elbow (pages 41 and 43), or he may saturate the parts already dissected with some preservative solu- tion, and postpone the examination of these- joints till after the dissec- tion of the metacarpus and digit. THE METACARPUS AND DIGIT. The distal portion of the horse’s fore limb, beyond the lower extremity of the radius, is technically termed the manus, as it corresponds to the hand of man. The carpus, or, as it is commonly but erroneously termed, the knee, of the horse corresponds to the wrist of the human subject. The portion of the limb between the carpus and the fetlock, representing the palmar portion of man’s hand, is called the metacarpus ; while the rest of the limb, beyond the fetlock, is the digit, and is the homologue of man’s middle-finger. Surface-marking . — By flexing the carpal and fetlock joints, the splint bones may be felt at the back of the metacarpus. Behind the bones in the same region lie the flexor tendons, the subcarpal ligament, and the suspensory ligament. These, whose edges may be more or less distinctly seen in a well-bred animal, have the relation to each other shown in Plate 7. Behind the fetlock-joint is a tuft of hair in which will be found a horny spur or ergot , which is largest in coarse-bred animals. By manipulation, the flexible lateral cartilages may be felt above the hoof, in the region of the heels. Directions . — The entire remaining portion of skin should now be care- fully removed from the limb. Should it be intended to study from the same preparation the parts contained within the hoof, this must, before the removal of the skin, be detached by force in the manner described on page 35. The various structures are now to be defined by dissection in the order of the following description ; and while the vessels and nerves are being cleaned, care must be taken of the small lumbricales muscles, which lie on the tendon of the deep flexor above the fetlock. The palmar arterial arches cannot be fully exposed at this stage of the dissection, but it is convenient to describe them here, from their relationship to the vessels of the region. The same applies to the large metacarpal artery and the plantar nerves behind the carpus, all of which can be fully traced in the examination of the carpal sheath (page 33). The Large Metacarpal Artery (Plate 9). This is the largest artery in the part of the limb now exposed, and is, by means of its ter- 28 THE ANATOMY OF THE HORSE. minal branches, the main vessel of supply to the digit. It has already been seen at its origin, as the larger of the two terminal branches of the posterior radial artery ; and, indeed, from its volume and direction, it might be described as the direct continuation of that vessel. From its point of origin at the lower end of the radius, it descends in company with the flexor tendons, by passing behind the carpus and beneath the carpal arch. Emerging from beneath the last-named structure, it con- tinues to descend on the inner side of the flexor tendons until a little above the fetlock, where it sinks slightly inwards to bifurcate into the digital arteries. From the carpus downwards the artery is related to the internal metacarpal vein, which ascends in front of it, and to the internal plantar nerve, which is in contact with it posteriorly. The relative position of the three structures should be carefully noted in reference to the higher operation of neurotomy. Only two of its collateral branches are of sufficient size to merit description, and both are somewhat irregular in their origin. The first of these comes off near the origin of the parent vessel, and may come from the posterior radial itself. It crosses behind the lower extremity of the radius, and anastomoses with the termination of the ulnar artery to form the supracarpal or superficial palmar arch. The second is an un-named vessel which springs from the large metacarpal at or near its point of bifurcation, and divides into branches that ascend to anastomose with the interosseous metacarpal arteries. The Supracarpal or Superficial Palmar Arch is formed behind the lower extremity of the radius, by the junction of the above-mentioned branch of the large metacarpal artery with the termination of the ulnar. The convexity of the arch is turned downwards, and from it there arise several branches. The largest and most regular of these descends within the carpal arch, and joins the small metacarpal artery to form the subcarpal or deep palmar arch, which will be dissected at a later stage. The Digital Arteries (Plates 9 and 10) are the terminal branches of the large metacarpal artery. They separate at an acute angle, the outer one passing above the fetlock, between the deep flexor and the suspensory ligament. Each passes over the side of the fetlock-joint, and descends at the edge of the flexor tendons as far as the inner face of the basilar process, where it bifurcates to form the plantar and pre- plantar arteries. Each artery is related in front to the vein of the same name, and behind to the posterior branch of the plantar nerve. The anterior branch of the same nerve crosses the vessel at the fetlock ; while other twigs cross over the artery and form the middle branch, which will be found between the artery and vein, or resting on the former. Crossing these vessels and nerves obliquely, is a small glistening ligamentous cord (Plate 9) which stretches PLATE IX Ant. interosseous artery- Suspensory ligament Interosseous muscle Lumbricalis Flexor metacarpi int. Post, radial art. Int. plantar nerve Small metacarpal art. Extensor metacarpi obliquus Flexor metacarpi med. Branch of median to ext. plantar nerve to supracarpal arch. Large metacarpal art. Branch from ulnar to ext. plantar nerve -Ext. plantar nerve Int. plantar nerve Large metacarpal art. metacarpal vein Flexor perforatus Flexor perforans Oblique branch from int. to ext. plantar nerve Extensor pedis Band from suspensory ligament to extensor pedis Middle digital nerve Ant. digital nerve Post, digital nerve Digital art. Digital vein Drawn & Printed toyW. ScAK. Johnston, Edinburgh lit London METACARPUS AND DIGIT— Inner Aspect DISSECTION OF THE ANTERIOR LIMB. 29 downwards and forwards from the horny spur behind the fetlock, becoming attached within the wing of the os pedis. A knowledge of these relationships is of importance for the performance of the lower operation of neurotomy. The collateral branches of the digital arteries are : — 1. At different levels numerous small branches for the skin, tendons, or articulations. Among these may be included the rameaux echelonnes of Bouley (Plate 10). These branches, some of them of considerable size, spring from the posterior aspect of the artery, and anastomose across the back of the digit with corresponding branches from the oppo- site side, forming arches arranged like the steps of a ladder. 2. The Perpendicular Artery , which comes off at a right angle about the middle of the first phalanx, and divides almost immediately into an ascending and a descending set of branches, both of which are distri- buted on the front of the first phalanx. Branches from each of these sets anastomose with corresponding vessels from the opposite side. 3. The Artery of the Plantar Cushion. 4. Vessels forming the Coronary Circle. The last two, as well as the terminal branches of the digital arteries, will be described in connection with the foot. The Small Metacarpal Artery (Plate 9). This, the smaller terminal branch of the posterior radial artery, descends behind the knee and towards its inner side. It is superficially placed to the fibrous band completing the carpal arch, while the large metacarpal lies beneath that structure. In company with it is the first part of the median vein. At the level of the head of the inner metacarpal bone it crosses to the outer side by passing between the suspensory ligament and the subcarpal ligament, or check -band furnished from the back of the carpus to the tendon of the deep flexor. It here anasto- moses with a branch already described as descending from the supra- carpal arch. In this way the subcarpal arch is formed. The Subcarpal or Deep Palmar Arch gives off the following two pairs of arteries : — 1. The Anterior or Dorsal Interosseous Metacarpal Arteries. — These are small vessels (Plate 9), one on each side of the limb, which turn forward round the heads of the small metacarpal bones, and descend in the grooves between these bones and the large metacarpal. They supply the skin and subjacent structures on the front of the metacarpus, and anastomose above the fetlock with divisions of the artery springing from the large metacarpal at its point of bifurcation. 2. The Posterior or Palmar Interosseous Metacarpal Arteries . — These descend on the edge of the suspensory ligament, each being internally placed to the small metacarpal bone of its own side. They anastomose like the preceding, and supply small branches to the suspensory ligament 30 THE ANATOMY OF THE HORSE. and flexor tendons. One of them gives off the nutrient artery of the large metacarpal bone. They are of unequal size, the outer being the larger. The Digital Veins (Plate 9). These are the satellites of the digital arteries, in front of which they ascend. They drain away the blood from the venous plexuses within the hoof, and, uniting with one another above the fetlock, they form an arch between the deep flexor and the suspensory ligament. From this arch spring the metacarpal veins. The Metacarpal Veins are three in number : — 1. The Internal Metacarpal Vein (Plate 9), which is the largest of the three, ascends in front of the large metacarpal artery, on the inner edge of the flexor tendons. At the inner side of the back of the carpus it is continued as the median vein. 2. The External Metacarpal Vein is similarly disposed on the outside of the flexor tendons, in company with the external plantar nerve. At the carpus it divides into several anastomosing branches, which are continued as the ulnar and posterior radial veins. 3. The Interosseous or Deep Metacarpal Vein is an irregular vessel ascending between the suspensory ligament and the inner splint bone. At the back of the carpus it breaks up into branches that anastomose with the external and internal metacarpal veins. The Plantar Nerves ( metacarpal nerves of Percivall). — These are the nerves which confer sensibility on the digit, and which, in their main trunks, or in one of their terminal branches, are cut in the operation of neurotomy. They must therefore be dissected with great care, and the stildent must make himself thoroughly acquainted with their situation and relations. The Internal Plantar Nerve (Plate 7). This is one of the ter- minal branches of the median nerve. Beginning at a variable point above the carpus, it passes within the carpal arch, in close company with the large metacarpal artery, both resting on the side of the deep flexor tendon. Here the nerve crosses beneath the artery, to place itself behind it. Throughout the metacarpal region the same relationship is preserved, the nerve lying immediately behind the artery, in front of which is the internal metacarpal vein. Just above the fetlock the artery sinks in somewhat more deeply than the vein and nerve, and thereby allows these to approach each other. In the higher operation of neurotomy the nerve is cut a little way above the fetlock, and before it divides. About the middle of the metacarpus it gives off a consider- able branch which winds obliquely downwards and outwards behind the flexor tendons, to join the external plantar nerve an inch or more above the button of the splint tone. At the level of the sesamoid bones the trunk of the nerve divides into three digital branches, which are distinguished as anterior, middle, and posterior. These are of very unequal size, the posterior being much the largest, and also the most DISSECTION OF THE ANTERIOR LIMB. 31 important, as it is the nerve which is cut in the lower operation of neurotomy when performed for navicular arthritis. The middle is the smallest and most irregular, and all three branches are in close relation- ship with the digital vessels. The Anterior branch descends in front of the vein, distributes cutane- ous branches to the front of the digit, and terminates in the coronary cushion. The Middle branch, which is small and irregular, descends between the artery and vein. It is generally, as in Plate 9, formed by the union of several smaller branches which cross forwards over the artery before uniting, and it terminates in the sensitive laminae and coronary cushion. The Posterior branch lies close behind the artery, except at the fetlock, where the nerve is almost superposed to the artery. It accompanies the digital artery into the hoof, and passes with the preplantar branch of that vessel to be distributed to the os pedis and the sensitive laminae. Within the hoof it gives off several branches, which for the most part accompany the arteries. The External Plantar Nerve (Plate 9). This is formed by the fusion of the termination of the ulnar nerve with one of the terminal branches of the median. These two branches unite at the upper border of the pisiform bone, beneath the middle flexor of the metacarpus. Behind the carpus the nerve inclines downwards and outwards, in the texture of the annular ligament that completes the carpal sheath. In the metacarpal region it occupies, bn the outside of the limb, a position on the flexor tendons analagous to that of the internal plantar nerve on the inside. Unlike the latter nerve, however, it is accompanied by only a single vessel — the external metacarpal vein, which lies in front of it. An inch or more above the button of the splint bone it is joined by the oblique branch from the internal nerve. In the higher operation of neurotomy it is cut at the same point as the inner nerve. At the level of the sesamoid bones it divides into three digital branches, exactly similar to those of the internal nerve already described. The plantar nerves give filaments to the lumbricales and interossei muscles, and to the suspensory ligament. Directions . — The student must now pursue the dissection of the following muscles which have already been dissected in the fore -arm, viz., the extensor pedis and extensor suflraginis on the front of the limb, and the superficial and deep flexors behind. In addition to these, there are the lumbricales and interossei muscles, which entirely belong to this region; and, as they are of small size, and might easily be overlooked, their dissection must be first undertaken. The Lumbricales Muscles (Plate 9) receive their name in the human hand from their resemblance to a common earthworm. In the 32 THE ANATOMY OF THE HORSE. horse they are of small but very variable size. Frequently they contain but little muscular tissue, but now and again a subject is met in which they are very distinct. They are two in number, one being placed on each side of the deep flexor tendon, above the fetlock. The fibres of the small muscular belly arise from the side of the deep flexor, and terminate in a small tendon which is lost in the tissue beneath the horny spur of the fetlock. The Interossei Muscles (Plate 9). These are -the representatives of the muscles which, in the human hand, fill up the interspaces of the metacarpal bones, and give lateral movement to the fingers. In the horse they are two in number, and are extremely rudimentary. Each is to be sought to the inner side of the small metacarpal bone of its own side, between that bone and the edge of the suspensory ligament. Each has at its upper end a small muscular belly taking origin from the neighbourhood of the head of the small metacarpal bone. It is succeeded by a long, slender, nerve-like tendon, which at the fetlock blends with the band sent from the suspensory ligament to the extensor pedis tendon, or with the connective-tissue on the side of the joint. The interossei and lumbricales muscles are of great interest to the comparative anatomist, but, from their small size, they can have no appreciable effect on the movements of the digit. The Tendon of the Extensor Suffraginis (Plate 7) is to be followed from the point below the carpus to which it has already been dissected. The flat tendon, after crossing the carpus, descends to the outer side of the anterior surface of the large metacarpal bone. As it passes over the fetlock-joint, it becomes somewhat broader, and its play over the anterior ligament of the joint is facilitated by means of a small synovial bursa. Immediately below the joint it is inserted into the fore part of the upper end of the first phalanx. In the region of the metacarpus the tendon receives on each side a reinforcing band. The outer band comes from the external side of the carpus ; the inner is detached from the extensor pedis tendon. Action . — The muscle is primarily an extensor of the digit on the meta- carpus. When contraction is carried beyond this, it extends the meta- carpus on the fore-arm. The Tendon of the Extensor Pedis (Plate 7). This tendon, after throwing off the slip to the extensor suffraginis, descends over the front of the metacarpus and digit, and lies on the middle line. Its play over the anterior ligament of the fetlock is facilitated by a small synovial bursa ; while, over the front of the interphalangeal joints, the synovial membrane is directly supported by the deep face of the tendon, there being no anterior ligament for these joints. At the middle of the first phalanx the tendon is joined on each side by a strong band which descends obliquely over the side of the fetlock from the suspensory ligament. DISSECTION OF THE ANTERIOR LIMB. 33 finally inserted into the pyramidal process of the os of the muscle is to extend the third The tendon is pedis. Action . — The first action phalanx on the second, and then the second on the first. When con- traction is continued, it produces successively extension of the fet- lock and of the carpus. Directions . — The tendons on the back of the metacarpus and digit must next be dissected ; and as a preliminary step, the carpal and metacarpo-phalangeal sheaths formed in connection with these tendons should be examined. The Carpal Sheath (Fig. 1) is the tubular passage through which the flexors of the digit are transmitted behind the carpus. It is formed in front by the back of the carpus covered by the posterior common ligament of that joint. Behind it is bounded in its outer third by the pisiform bone, and in its inner two-thirds by a strong fibrous band representing the anterior annular ligament of the human wrist. This band stretches like an arch from the pisiform bone to the inner side of the carpus. It is continuous above with the deep fascia on the back of the fore-arm, of which it may be considered a thickened portion ; and below it becomes thinner, and is continued as the fascia on the back of the meta- carpus ( palmar fascia of man). The carpal sheath is provided with Dissection of the Metacarpus and Digit-, showing the Tendons and their Synovial Sheaths ( Chauveau ). 1. Synovial bursa of the extensor metacarpi magnus; 2. Superior cul-de-sac, or pouch, of the synovial membrane of the carpal sheath ; 2 1 , 2 1 . Inferior part of the same ; 3. Pouch of the radio- carpal synovial membrane, appearing as a hernia between the posterior common ligament and the outermost radio-carpal ligament ; 4. Synovial bursa of the extensor pedis ; 5. Protrusion of the synovial membrane of the fetlock-joint ; 6, 7, 8. Superior, middle, and inferior pouches of the synovial membrane of the metacarpo-phalangeal sheath ; 9. Inferior extremity of the same, exposed by the removal of the reinforcing sheath of the perforans tendon ; E. S. Extensor suffraginis ; S. L. Subcarpal ligament ; E. P. Extensor pedis ; S. S. Superior sesamoidean (suspensory) ligament ; F. Pa. Flexor perforans ; F. Pt. Flexor perforatus. . D 34 THE ANATOMY OF THE HORSE. a synovial membrane, which lines it, and is reflected over the flexor tendons to facilitate their gliding. If the fibrous band just described be cut, and a probe be passed upwards and downwards within the sheath, an idea of the extent of the synovial sac will be gained. It will be found to extend upwards for two or three inches above the carpus, and downwards as far as the middle of the metacarpus. Directions. — The fibrous band should be entirely removed in order to permit the examination of the tendons, and of the nerves and bloodvessels which accompany these within the sheath. The Metacarpo-phalangeal or Great Sesamoid Sheath (Fig. 1). This is a second synovial apparatus developed in connection with the flexor tendons. If a vertical incision be made through the superficial flexor just above the fetlock, and a probe passed into the incision, it will enter the synovial cavity, and may be pushed upwards for two or three inches above the fetlock, and downwards as far as the middle of the second phalanx. The synovial membrane lubricates the pulley-like surface formed by the sesamoid bones and the inter-sesamoid ligament, and is reflected on to the tendons. It is supported laterally by a fibrous expansion which, adhering to the superficial flexor behind, is inserted in front by three slips on each side, the highest insertion being into the sesamoid, and the other two into the first phalanx. At its lower extremity this synovial membrane meets that of the navicular sheath, and in front of the same point it is separated from the synovial capsule of the coffin- joint by a kind of partition of yellow fibrous tissue connecting the front of the perforans tendon to the back of the os coronse (Plate 10, fig. 2). The Superficial Flexor tendon (Plates 5, 9, 10, and 11). The tendon succeeds the fleshy portion of the muscle at the lower part of the fore-arm, and it is there reinforced by a fibrous band which springs from the back of the radius and is sometimes termed the superior carpal ligament , in contradistinction to the band which reinforces the tendon of the deep flexor below the carpus. The tendon passes through the carpal sheath in company with and behind the deep flexor, and then descends behind the metacarpus. Having arrived at the fetlock, there is formed in it a remarkable ring, through which the tendon of the deep flexor plays. It is in consequence of this arrangement that the superficial muscle is termed perforatus , and the deep one perforans. As already seen, the tendons are here enveloped by the synovial membrane of the meta- carpo-phalangeal sheath. At its extremity the tendon is bifid, and each slip is inserted into the upper extremity of the second phalanx on its lateral aspect. Action . — The muscle flexes successively the pastern, fetlock, and carpal joints. The Deep Flexor tendon (Plates 5, 9, 10, and 11) is, through- out its course, closely related to the preceding, in front of which it lies. DISSECTION OF THE ANTERIOR LIMB. 35 After descending through the carpal sheath, it is joined by a very strong fibrous band — th e inferior carpal ligament, which is the downward continua- tion of the posterior common ligament of the carpus. This fuses with the tendon about the middle of the metacarpus, and it is of consider- able importance, being frequently involved in what is commonly termed “ sprain of the back tendons.” In that condition it may be very distinctly felt by manipulating in front of the flexor tendons, just below the carpus. The tendon, as thus reinforced, descends between the suspensory liga- ment in front, and the perforatus tendon behind ; and at the fetlock it glides over the sesamoid pulley, and passes through the ring of the superficial flexor. It then passes between the terminal branches of the last-mentioned muscle, glides over the smooth surface on the back of the second phalanx, plays over the navicular bone, and finally becomes inserted into the semilunar crest of the os pedis. The terminal portion of the muscle, as well as the navicular sheath developed in connection with it, will be examined with the parts contained within the hoof. Action . — The muscle flexes successively the interphalangeal joints, the fetlock, and the carpus. THE FOOT. Directions . — By the term foot, as here applied, is meant the hoof and the parts contained within it. If it is intended to study this in a limb the whole of which is to be successively dissected, the student must proceed in the following manner. When the dissection of the fore-arm has been completed, and before the removal of the skin from the meta- carpus and digit, the hoof must be forcibly removed by the aid of a shoeing-smith’s hammer, toe-knife, and pincers. To facilitate this, the hoof may be heated in a fire, the skin of the digit being swathed in a wet cloth to prevent charring. This is the speediest method of removing the hoof, but it has the double disadvantage of destroying in great measure the hoof itself, and also the injection of the vessels, provided that has been executed. The following is a preferable method of pro- cedure : — Procure a foot severed a few inches above the fetlock, and inject the arteries and veins from the metacarpal vessels. When the injection has solidified, roll the foot in a piece of wet cloth, and bury it in a fermenting heap of stable manure. Decomposition will speedily set in, and after a week the preparation should be examined at intervals of two or three days, the metacarpal bone being fixed in a vice while forcible attempts are made to pull off the hoof. When this has been effected, the foot and removed hoof should be immersed for a day in a saturated solution of carbolic acid in water, to which a little methylated spirit may be added. This will speedily remove all odour of decom- position, and dissection may then be proceeded with. The Hoof (Plate 10, figs. 4 and 6). This is made up of the wall , the bars , the sole , and the frog. 36 THE ANATOMY OF THE HORSE. The Wall is that part of the hoof which is exposed when the foot rests in its natural position on a flat surface. It is divided, though not by any well-defined boundaries, into toe , quarters , and heels. The toe includes an area on each side of the middle line of the wall in front ; and it passes on each side into the quarter , which comprises the lateral region of the wall. Posteriorly the wall changes its direction, and disappears from view, forming an angular part, which is termed the heel. In reality, the wall does not stop at the heel, and it is this concealed continuation that is termed the bar. In a well-formed hoof the wall in the region of the toe slopes at an angle of about 50°. The External Surface of the wall is, in a state of nature, covered by a kind of epithelial varnish termed the periople , which is thickest at the top of the wall, just under the hair. This, which is a natural varnish provided to check evaporation and consequent cracking of the subjacent horn, is generally rasped away by the shoeing-smith. The internal sur- face of the wall is traversed in a vertical direction by the series of horny lamince. These number about five or six hundred ; and before separa- tion of the hoof, they were interleaved with the sensitive laminae to be presently described. The superior border of the wall shows a kind of gutter, termed the cutigeral groove , which is the mould left by the coronary cushion. The floor of this groove has a closely punctated appearance, each minute perforation being the upper end of one of the horn tubes of the wall, and lodging, in the natural state, one of the papillae of the coronary cushion. The inferior border embraces the sole, and in the unshod animal comes into contact with the ground. The wall is thicker at the toe than at the quarters or heels ; and in each of these areas, it is thicker on the outside than in the correspond- ing area on the inside. The Bars. These are the reflected terminations of the wall behind the heels; and if the foot be turned up, the continuity will be distinctly seen. The Outer Surface of the bar, which is here seen, slopes towards the frog, and bounds outwardly the lateral lacuna of that body. It shows an inferior border , which runs towards the centre of the sole, but stops a little behind the point of the frog. The bars are also seen in the interior of the hoof, where they show an internal surface bearing horny laminae like those of the wall. The superior border of the bars is included between the frog and the sole, and blended with them. The Sole presents an inferior face , which is vaulted, and this inde- pendently of any paring to which the foot may have been subjected, as the horn of which it is composed exfoliates so as to give it this con- figuration naturally. The superior face is somewhat convex, and has a punctated appearance similar to that already seen in the cutigeral 1 groove. The minute holes lodge the papillae of the so-called sensitive sole, which is the horn secreting structure of this region. Anteriorly DISSECTION OF THE ANTERIOR LIMB. 37 the sole presents a convex border, which unites it intimately to the lower border of the wall, a line of whitish horn marking the junction of the two structures. Posteriorly it has a deep V shaped indentation, into the central point of which the frog penetrates, while behind that on each side it is related to the bar. The sole of the hind hoof is distinguished from that of the fore by being more vaulted, and by being more pointed (less circular) at the toe, this latter difference affecting also the form of the wall in the same region. The outer edge of the sole is more convex than the inner, which enables one to readily distinguish between a right and a left hoof. The Frog. This is a distinctly elastic mass of horn which, in a state of nature, projects sufficiently to come into contact with the ground, and thus give the animal a secure foothold. Its inferior surface shows posteriorly a shallow cleft, or depression, termed the median lacuna. The lateral lacunae lie at the sides of the frog, the outer boundary of each lacuna being formed by the bar. The superior surface shows, vertically over the median lacuna, a projection termed the frog -stay. On each side of the frog-stay this surface is depressed, and the whole is moulded on the plantar cushion. This surface is punctated, and the papillae of the plantar cushion are received into the minute apertures The posterior extremity , or base , of the frog consists of two rounded emi- nences — the bulbs , or glomes — separated from each other by the median lacuna. The anterior extremity , or point , is wedged into the centre of the sole. The lateral borders bring the frog into relation with the bars and the sole, and there is an intimate union with each of these at the point of contact. Minute Structure of the hoof. The entire hoof is an aggregation of modified epithelial cells, which here represent the horny layer of the epidermis. When a thin section across the wall, sole, or frog is examined, the horn substance is seen to be arranged in the form of tubes, cemented together by an intertubular substance, and containing within their lumen a quantity of intratubular material. All of these — tubular, intertubular, and intratubular — are composed of modified epithelial cells, differing in the three situations in the direction of the cells, their state of aggregation, or the presence or absence of contained pigment. The tubes of the wall are straight, and extend parallel to the surface, from the coronary to the inferior edge of the wall. The tubes of the sole have the same disposition, but those of the frog are slightly flexuous. The upper end of each tube is occupied by an elongated vascular papilla, which belongs, in the case of the wall, to the coronary cushion ; in the periople, to the perioplic ring ; and in the sole and frog, to the sensitive structures of the same names. In the growing hoof the bond of connection between these papillated surfaces (which represent the corium of the skin) and the corresponding part of 38 THE ANATOMY OF THE HORSE. the hoof, is a stratum of soft protoplasmic epithelial cells by whose growth and multiplication the hoof-horn is formed. This stratum of cells represents the deepest cells of the rete mucosum in the skin, and it is by its ready decomposition that the bond of connection between the sensitive and insensitive structures is destroyed, permitting the extremity of the digit to be extracted from its horny investment. Directions . — The student should next turn his attention to the ex- tremity of the digit as exposed by the removal of the hoof, and he will find it to present a configuration not unlike the exterior of the hoof itself (Plate 10, figs. 1 and 5). And in the first place, let him examine that part which he will easily recognise as having been separated from the inner surface of the wall. This is traversed by a series of leaves which, in contradistinction to those already seen on the inner surface of the wall, are termed the sensitive laminae, and sometimes the podophyllous tissue. The Sensitive Laminae. Each lamina is fixed by one of its borders to the periosteum of the os pedis, and extends in a vertical direction from near the coronary cushion to the sharp edge of the bone, where it terminates in five or six long papillae. In the natural state the sensitive and the horny laminae are interleaved, and the former here represent the corium, or true skin. The laminae, it will be noticed, become pro- gressively shorter as they are traced backwards ; and at the end of the series on each side, and adjacent to the plantar cushion, there is a number of small leaves that were interleaved with the horny laminae of the bars. The Coronary Cushion. This is a projecting, cornice-like structure, placed above the laminae and below the limits of the skin of the digit. It fits into the cutigeral groove at the upper border of the wall, and its surface is closely set with long papillae which were received into the apertures found in that groove. These papillae give the coronary cushion a velvety pile, which may be rendered very evident by immers- ing the foot in water. If the coronary cushion be traced backwards, it will be seen to pass into the plantar cushion. Above the cushion is a narrow groove separating it from the periopolic ring. Below the cushion there is a narrow smooth space which runs between the cushion and the sensitive laminae. The coronary cushion is a modified portion of the corium, and through the agency of the cells which cover the surface of its papillae, the wall of the hoof is formed. The Perioplic Ring. This ring is composed of papillae like those of the coronary cushion, but smaller in size ; and it is by its agency that the periople which covers the exterior of the wall is formed. The Plantar Cushion. This is a fibro-elastic pad interposed between the horny frog and the terminal part of the perforans tendon. It possesses two faces, two borders, a base, and an apex. The lower face looks backwards as well as downwards when the foot rests on a flat Drawn & Finned by W. kA.X. Johnston. Edinburgh fcLoiu PLATE X. Fig. I. — The Digit with the Hoof removed, flexed and viewed from behind. A. Sensitive sole ; B. Sensitive laminae that were interleaved with the horny laminae of the bar ; F. The pyramidal body, or sensitive frog ; L. Lateral lacuna of the same ; M. Median lacuna of the same ; Q. Q. Fibrous sheath uniting the two branches of the perforatus ; R. Branches of the per- foratus passing to be inserted into the os coronae ; T. Tendon of the perforatus ; T'. Tendon of the perforans in its passage between the branches of the perforatus ; V. Reinforcing sheath of the plantar aponeurosis ; X. Attachment of the same to the side of the os suffraginis. Fig. II.— Vertical mesial Section of the Digit. PlQ.-nl'a.v- A. Os pedis ; B. Ge ron ary cushion ; C. Coffin-joint ; D. Navicular bone ; E. Os coronae ; F. Pastern-joint ; H. Branch of the perforatus at its insertion into the lateral aspect of the os coronae ; I. Insertion of the plantar aponeurosis into the semilunar crest ; K. Os suffraginis ; L. The perforatus tendon ; M. Ligament of yellow fibrous tissue which unites the anterior face of the perforans to the posterior face of the os coronae, and separates the inferior cul-de-sac of the great sesamoid sheath from that of the synovial membrane of the coffin-joint ; N. Protrusion of the synovial membrane of the corono-pedal joint between the navicular bone and the os pedis ; O. Small sesamoid sheath ; P. Synovial membrane of the coffin-joint in contact superiorly with the great sesamoid sheath, from which it is separated by the yellow transverse ligament M. ; T. Tendon of the perforans ; Y. Fetlock-joint. Fig. III. — Arteries of the Digit. A. A. Digital artery ; C. Perpendicular artery at its origin ; H. One of the posterior branches ( rameaux echelonnes), for the perforans tendon ; J. Another of the same ; K. Origin of the artery of the plantar cushion ; M. Origin of anterior branch of coronary circle ; M.' Posterior branch of the same circle ; R. Origin of preplantar artery ; S. Plantar artery in the plantar groove and in the os pedis, forming with the opposite artery the semilunar anastomosis ; V. V. Descending branches from the semilunar anastomosis. Fig. IV. — The Hoof — plantar aspect. P. P. Region of the toe ; S. Sole ; L. Frog ; A. Line indicating the junction of wall and sole ; B. Angle of inflexion of the wall, showing the continuity of the wall and the bar ; E. Inferior edge of the bar ; F. Lateral lacuna of the frog ; G. Bulbs of the frog ; Q. Median lacuna of the frog ; U. Regions of the quarters ; O. Regions of the heels. Fig. V. — Extremity of the Digit with the Hoof removed — viewed from the side. A. B. cushion with its villosities ; D. Groove between the ^ planfe r cushion and the perioplic ring ; E. Perioplic ring ; F. Inferior border of the pJiblar cushion ; G. Sensitive laminae, or podophyllous tissue ; H. Villosities which terminate the laminae. Fig. VI. — Antero-posterior mesial Section of the Hoof— showing its interior. M. Series of horny laminae ; O. Section of the wall ; P. Section of the sole ; S. Upper edge of the periople above the cutigeral groove ; T. Section of the frog ; X. Cutigeral groove. 40 DISSECTION OF THE ANTERIOR LIMB. surface, and it is moulded on the upper face of the horny frog, to which it has a close Resemblance in form. The central portion of the cushion is therefore sometimes termed the sensitive frog, and it is also known as the pyramidal body. It shows in front a single ridge, which posteriorly becomes divided into two by a deep median cleft for the reception of the frog-stay. This surface has a villous aspect, the papillae being imbedded in the foramina seen on the upper surface of the horny frog. The horny frog is formed by the agency of the cells covering these papillae. The upper face looks forwards as well as upwards, and is applied to the reinforcing sheath of the deep flexor tendon. The borders , which are right and left, bring the plantar cushion into relation with the inner surface of the lateral cartilages. The apex lies in front of the semilunar crest of the os pedis, with whose periosteum the tissue of the cushion is intimately blended. The base of the cushion consists of two thick rounded masses termed the bulbs of the plantar cushion. These are continuous in front with the ridges of the pyramidal body, and they present the same velvety aspect; while, on each side, the villous tissue joins the extremities of the coronary cushion. The Sensitive Sole. The student should next examine that part of the foot which, before separation of the hoof, came into contact with the upper surface of the horny sole, and which for that reason is termed the sensitive sole. It is of a roughly crescentic form, being penetrated by the pyramidal body behind ; and it is co-extensive with the plantar surface of the os pedis. Its connective-tissue basis is firmly adherent to the periosteum of the bone, while its free surface bears long papillae which penetrate the horn tubes of the sole. The horny sole is formed by the agency of the cells which clothe the papillae of the sensitive sole. Directions . — On manipulating the bulbs of the plantar cushion, the student will feel the lateral cartilages of the foot ; and one of these is to be exposed and defined by removing one half of the plantar cushion. The Lateral Cartilages. These are in the main composed of hyaline cartilage, though often erroneously termed the fibro-cartilages of the foot. As is common with fibro-cartilage in many other regions, it shows a transitional structure at its periphery, where its matrix becomes more or less fibrous. Each plate of cartilage possesses two faces, and four borders separated by four angles. The external face is convex and covered by a plexus of veins, some of which penetrate the plate and connect the plexus with another lying beneath it. The internal face is concave. Behind it is united to the plantar cushion, while anteriorly it protects the corono-pedal articulation ; and a cul-de-sac of the synovial membrane of the joint lies in direct contact with the cartilage, a fact which it is important to remember in connection with operations for “quittor.” The superior border is thin and flexible, and may be felt in the living animal. The digital vessels cross this border in passing into THE ANATOMY OF THE HORSE. 41 the foot. The inferior border is supported by the wing of the os pedis in front, while posteriorly it blends with the plantar cushion. The anterior border slopes downwards and backwards, and is blended with the antero- lateral ligament of the corono-pedal joint. TYvq posterior border is parallel to the anterior, and is covered by the plantar cushion. The four borders meet at four angles, of which the postero-superior one and the one diago- nally opposite are obtuse, while the other two are acute. In the disease termed “Side-bones,” the lateral cartilages lose their mobility, in consequence of their conversion into bone. The Bloodvessels of the Foot (Plate 10, fig. 3). These should be studied in an injected limb from which the hoof has been removed by the method of decomposition described at page 35. The arteries of the foot are derived from the digital artery, which has already been dissected in its descent towards the foot, where, within the wing of the os pedis, it divides into the plantar and preplantar arteries. Some of the collateral branches of the digital artery have already been described at page 29; but there remain for examination the artery of the plantar cushion and the coronary circle, as well as the plantar and preplantar terminal branches. The Artery of the Plantar Cushion arises from the digital, just as that vessel passes within the upper border of the lateral cartilage, and it passes obliquely downwards and backwards to its destination. Besides supplying the plantar cushion, it gives off a branch which turns forwards to concur in the formation of the circumflex artery of the coronary cushion. The Coronary Circle. Where each digital artery lies under cover of the lateral cartilage, it gives off an anterior and a posterior branch which inosculate on the middle line before and behind with the corre- sponding branches of the opposite side, and thus form an arterial circle. This circle closely embraces the os coronee ; and among the largest branches furnished by it, are two which emanate from its anterior half, and descend, one at each border of the extensor tendon, to aid in form- ing the circumflex artery of the coronary cushion. The Circumflex Artery of the Coronary Cushion ( Chauveau ). This is a slender vascular arch placed immediately above the coronary cushion, to which its branches are distributed. It is fed in front by the two above- mentioned vessels from the coronary circle, and behind, on each side, by the before-mentioned branch from the artery of the plantar cushion. The Preplantar Artery is the smaller of the two terminal branches of the digital. It passes forwards through the notch in the wing of the os pedis, and then along the preplantar groove on the laminal surface of that bone, where its branches are expended in the sensitive laminae. The Plantar Artery passes along the plantar groove to enter the foramen of the same name. Within the os pedis it inosculates with the corresponding vessel of the opposite side, forming the plantar arch , or 42 DISSECTION OF THE ANTERIOR LIMB. semilunar anastomosis. From this intra-osseous arch a great number of branches proceed. An ascending ( anterior laminal) set of these leave the os pedis by the numerous small foramina which cribble its laminal surface. A descending ( inferior communicating ) set escape from the bone by the series of larger foramina which open on the sharp edge separating its laminal and plantar surfaces. These inferior communi- cating arteries anastomose right and left with each other, and thus form the circinnflex artery of the toe. From the concavity of this artery branches pass backwards, and supply the tissue of the sole. The Veins of the Foot. — Intra-osseous vessels. Within the os pedis the arterial branches are accompanied by satellite veins. There is thus a semilunar venous anastomosis, to which small veins converge from the laminal surface of the bone. The blood from this sinus is drained away by a larger vessel which passes out by the plantar fora- men in company with the plantar artery, and joins the posterior part of the coronary plexus. Extra-osseous vessels. The foot is richly provided with a superficial system of vessels, which are arranged in the form of a close-meshed network having little or no communication with the deep set. This venous envelope of the foot is divided into a solar , a laminal (podophyllous), and a coronary plexus. Where the solar and laminal plexuses meet, a composite venous vessel runs in company with the circumflex artery of the toe. These two plexuses communicate freely with each other, and with the coronary plexus. This last consists of a central part, which underlies the coronary cushion, and of two lateral parts, which on each side ramify on both surfaces of the lateral cartilage. By the convergence of branches belonging to this cartilaginous division of the coronary plexus, the digital veins are formed ; and these drain away the blood from both the intra-osseous and extra-osseous systems of vessels. Directions. — The terminal portion of the deep flexor tendon, and the synovial apparatus developed in connection with it, should now be examined. The Deep Flexor tendon (Plates 10 and 11), when it reaches the upper border of the navicular bone, widens out to form what is called the plantar aponeurosis. This plantar aponeurosis plays over the navicular bone by means of the navicular sheath, and is covered posteriorly by a fibrous layer which ultimately blends with it. It becomes inserted into the semilunar crest of the os pedis, and into the bone behind that crest. The above-mentioned fibrous layer was first described by Bouley, and designated by him the reinforcing sheath of the perforans. This expansion is attached on each side by a slip to the lower half of the first phalanx, and it serves to maintain the plantar aponeurosis against the navicular bone. The Navicular or Small Sesamoid Sheath (Plate 10, fig. 2). This is a THE ANATOMY OF THE HORSE. 43 synovial apparatus developed in connection with the perforans tendon where it plays over the navicular bone. It lines the deep face of the tendon, and is reflected on to the navicular bone and interosseous liga- ment. It also extends above the navicular bone, where it is in contact with the synovial membrane of the coffin-joint and that of the metacarpo- phalangeal sheath. THE SHOULDER-JOINT. This joint is formed between the glenoid fossa of the scapula and the head of the humerus. It is enclosed by a single capsular ligament lined internally by the synovial membrane. The absence of lateral or other retaining ligaments in connection with the joint, is compensated for by the numerous tendons which pass from one bone to the other in close relation to the capsular ligament. These muscles are as follows : — the supraspinatus, infraspinatus, teres minor, biceps, and small scapulo- humeralis. The last passes over the joint behind, where some of its fibres seem to be inserted into the ligament. In front of the joint the tendon of the biceps is separated from the ligament by a pad of fat. Movements. — The joint belongs to the class of enarthrodial or ball- and-socket joints, and the amount of its mobility should be proved by manipulation before the removal of the muscles. If the scapula be kept fixed, it will be found that the humerus can be carried backwards so as to diminish the angle formed by the meeting of the bones. This is a movement of flexion. Or the humerus can be carried forward in the same plane as in the preceding movement, but increasing the angle. This is extension. Or again, the humerus may be moved in a lateral direction either outwards or inwards. When, in the living animal, it is carried inwards, the limb is thrown towards the middle plane of the body, and is said to be adducted. The opposite movement, by which the limb is carried outwards from the middle plane, is termed abduction. Two other movements are permitted in the joint, viz., rotation and circumduction. In rotation the humerus, without change of place as a whole, turns round its own axis. In circumduction the shaft of the humerus moves so as to describe the surface of a cone. (These different terms having been here defined at length, their application in the case of the other joints of the body will be readily understood). The shoulder-joint of the horse is thus possessed of considerable freedom of movement ; but still, the range of its mobility, owing to the absence of a clavicle, and to the different disposition of the pectoral muscles, is much more restricted than in the human arm. Directions . — The muscles which surround the joint must now be removed, care being taken not to cut the capsular ligament. The Capsular Ligament loosely surrounds the articular ends of the 44 DISSECTION OF THE ANTERIOR LIMB. bones, and may be conceived as having the form of a double-mouthed sack, one mouth being attached around the rim of the glenoid cavity, and the other at the periphery of the head of the humerus. The wall of this sack is comparatively thin, but in front it is strengthened by accessory fibres that pass in a divergent manner from the coracoid pro- cess to the outer and inner tuberosities. These correspond to the coraco-humeral ligament of man. Directions. — If, in the removal of the muscles, the ligament has been preserved perfectly intact, it will be noticed that though a considerable force be exerted to pull the articular surfaces from each other, they still remain in contact. If, however, an incision be made in the ligament, the air will be heard to rush into the joint, while the bones separate to the extent of half an inch or more. In the shoulder then, as in other joints, atmospheric pressure is to be included among the agents keeping the articular surfaces in contact. The capsular ligament is to be slit up so as to expose the smooth and glistening aspect of the synovial membrane, and the articular surfaces of the bones covered by articular cartilage. The Synovial Membrane lines the inner surface of the capsular ligament. It secretes the synovia, or joint oil, some of which will be seen escaping from the joint. THE ELBOW- JOINT (PLATE 11, fig. 1). This joint is formed by the lower extremity of the humerus and the upper extremities of the bones of the fore-arm. It possesses two lateral ligaments, and an anterior ligament which supports the synovial membrane in front ; but behind, there being no ligament, the synovial sac is directly supported by muscles. Movements. — This is a ginglymoid joint, the only movements being flexion and extension. Inflexion , while the humerus remains fixed, the bones of the fore-arm are carried forwards until the movement is arrested by the coronoid process passing into the fossa of the same name. In this movement the bones of the fore-arm do not move in the plane in which the humerus lies, but deviate a little outwards. The opposite movement is extension , in which the radius and ulna are carried back- wards until they are arrested by the tension of the lateral ligaments, and by the passage of the beak of the olecranon into the fossa of the same name. Directions . — The anterior and lateral ligaments are to be exposed and defined by removing the muscles from the front of the joint, but on the posterior aspect of the joint the muscles should not be removed at present. The External Lateral Ligament is a cord-like band which is fixed superiorly to a depression on the outer side of the lower extremity of the humerus, and to the ridge which forms the lower boundary of the THE ANATOMY OF THE HORSE. 45 musculo-spiral groove ; while interiorly it passes to be inserted into the external tuberosity at the upper end of the radius. The Internal Lateral Ligament forms a longer but more slender cord than the preceding, and passes from a small eminence on the outer side of the lower extremity of the humerus to be inserted into the shaft of the radius below the bicipital tuberosity. Some of the anterior fibres join the tendon of the biceps or the anterior ligament, while some of the posterior join the arciform fibres connecting the radius and ulna. The Anterior Ligament is of a membranous form. Its upper border is fixed to the humerus, its lower border to the radius, while its lateral borders blend with the lateral ligaments. Directions .-— The anterior and lateral ligaments should now be cut transversely about their middle in order to expose the interior of the joint. The Synovial Membrane will be seen to line the inner face of the anterior and lateral ligaments, but at the back part of the joint there is no ligament and the membrane is supported by the muscles. If the finger be passed backwards and upwards, it will enter a process of the synovial capsule which extends upwards into the olecranon fossa, where a pad of fat intervenes between it and the anconeus muscle. Just behind the external lateral ligament the membrane lines the origin of the flexor metacarpi externus. On the inner side of the joint, behind the internal lateral ligament, the membrane lines the tendons of origin of the middle and internal flexors of the metacarpus, and of the super- ficial and deep flexors of the digit. This disposition of the synovial capsule will be rendered more evident by cutting the above-mentioned muscles a few inches below the joint, and then turning their tendons of origin upwards. Directions . — The humerus being now completely severed from the radius and ulna, the mode of union of these latter bones should be examined. The Radio-ulnar Articulation. — In the adult animal the bones of the fore-arm are fused together below the radio-ulnar arch, by ossification of the interosseous fibres which in the young animal are interposed between the two bones. Above the arch, however, the fibres interposed between the bones do not ossify except in a very old animal, but persist as an interosseous ligament. The union of the two bones is further maintained by arciform fibres passing on each side from the one bone to the other, and blending with the lateral ligaments of the elbow. At the upper part of their opposed surfaces, the tw r o bones respond to each other by two small synovial facets, which, however, have no special synovial membrane, but are lubricated by processes from the synovial capsule of the elbow-joint. Movements . — These are inappreciable, the limb of the horse being fixed in a condition of pronation. 46 DISSECTION OF THE ANTERIOR LIMB. THE KNEE, OR CARPUS (PLATE 11, figS. 2 and 3). This is not a simple, but a composite, joint, and entering into its formation there are the carpal bones, the lower extremity of the radius, and the upper extremities of the bones of the metacarpus. The carpal bones are arranged in two rows, or tiers, and the bones of each row are firmly bound together and converted into a single piece by ligaments passing between the adjacent bones. A transverse joint is then formed between the upper and the lower tier. This may be called the inter- carpal joint, and it is secured by special ligaments passing between the two rows. Another transverse joint is formed between the lower row and the heads of the metacarpal bones ; and this, which has also got special ligaments, is termed the carpo-metacarpal articulation. A third transverse joint is formed between the lower end of the radius and the upper row. This, which is the radio-carpal joint, is also provided with special ligaments. Lastly, there are four ligaments which do not belong specially to any of these articulations, but secure the stability of the entire composite joint, and are therefore termed common. Movements. — The movements which take place at the carpus are -flexion and extension , and each of the transverse joints above-mentioned is a ginglymus. When these movements are executed, however, the three j oints do not participate in them in an equal degree. The largest share of the movement occurs at the radio-carpal articulation, and the smallest between the carpus and the metacarpus ; while, as regards the amount of movement, the inter-carpal transverse joint occupies an inter- mediate position. When the limb is flexed at the carpus, it will be noticed that the metacarpus and digit deviate a little outwards from the plane of the fore-arm. When the limb is fully extended the lateral ligaments are tightly stretched, and resist any attempts to produce abduction or adduction ; but these movements can be produced when the limb is fully flexed, in which position the lateral ligaments are relaxed. Lateral movement, however, is not executed at this joint in any appreci- able degree in the living animal. The gliding movement permitted between adjacent bones in each row is of importance, as tending to distribute pressure, and obviate the bad effects which would have been likely to result from concussion had each row been a single rigid mass. Directions. — The tendons which pass in relation to the joint before and behind should be removed, and the ligaments should be studied in the order of the following description. There are four ligaments common to the whole joint, viz., two lateral, an anterior, and a posterior. The External Lateral Ligament is a cord-like band composed of a deep and a superficial set of fibres, which slightly cross each other. It is fixed superiorly to the external tuberosity at the lower end of the PLATE XI Fig. VI Drawn & Printed "b yV. &.A.K. Johnston Ediribiirgh &. London JOINTS AND LIGAMENTS OF FORE LIMB PLATE XI. Fig. I. — Ligaments of the Elbow, seen from behind (Leyh). A. Ext. lateral ligament ; B. Int. lateral ligament ; C. C. C. Arciform ligaments ; D. Radio- ulnar arch. Fig. II. —Ligaments of the Carpus, front view ( Chauveau ). 1. 1. Ant. ligaments of upper row ; 2. An ant. ligament of lower row ; 3. 3. Ant. carpo- metacarpal ligaments ; 4. Int. lateral ligament ; 5. Ext. lateral ligament. Fig. III. — Ligaments of the Carpus, viewed from the outer side ( Chauveau ). 1.1.1. Ant. ligaments of upper row; 2. An ant. ligament of the lower row; 3. 3. Ant. carpo- metacarpal ligaments ; 4. An intercarpal ligament ; 5. Ext. lateral ligament ; 6. A radio-carpal ligament. Fig. IV. — Ligaments of the Fetlock, Pastern, and Coffin-Joints ; side view {Chauveau). 1. Superficial fasciculus of the ext. lateral ligament of the fetlock ; 2. 3. Sesamoid and phalangeal slips of the deep fasciculus of the same ligament; 4. 5.6. Upper, middle, and lower fibrous slips attaching the glenoidal fibro-cartilage to the os suffraginis ; 7. Lateral ligament of the pastern-joint ; S. Antero-lateral ligament of the coffin-joint ; 9. Postero-lateral ligament of the same joint. Fig. V. — Back of the Digit dissected to show the Tendons and Ligaments ( Eouley ). A. Antero-lateral ligament of the coffin-joint ; B. Insertion of extensor pedis tendon ; D. Postero- lateral ligament of the coffin-joint ; E. Divergent fibres of the same ligament passing to be attached to the wing of the os pedis and inner surface of the lateral cartilage ; F. Slip sent from suspensory ligament to extensor tendon ; P. Branch of bifurcation of the suspensory ligament ; R. Branch of perforatus ; T. Perforans emerging from between the branches of the perforates ; Y. Attachment of the reinforcing sheath of the perforans tendon to the side of the os suffraginis. Fig. VI.— Back of the Digit dissected to show the Tendons and Ligaments ( Bouley ). A. Superficial inferior sesamoidean ligament ; B. Highest slip attaching the glenoidal fibro- cartilage of the pastern-joint to the first phalanax ; O. Branch of perforatus ; P. Middle inferior sesamoidean ligament ; S. Insertion of plantar aponeurosis into semilunar crest ; T. Reinforcing sheath of the plantar aponeurosis ; X. Perforans tendon. Fig. VII. — Back of the Fetlock-joint (Modified from Bouley). A. Intersesamoid ligament ; B. B. Lateral bands of the middle inferior sesamoidean ligament ; C. Middle band of the same ligament, its upper attachment cut away to show D. the deep inferior sesamoidean ligament. 48 THE ANATOMY OF THE HORSE. radius ; and passing over the outside of the carpus, it furnishes slips to the cuneiform and unciform bones, and terminates on the head of the external small metacarpal bone. The ligament is perforated by a thecal canal in which the tendon of the extensor suffraginis plays. The Internal Lateral Ligament is fixed superiorly to the internal tuberosity of the radius, and interiorly to the heads of the large and inner small metacarpal bones, furnishing slips, as it passes over the carpus, to the scaphoid, magnum, and trapezoid bones. The Anterior Common Ligament has a flattened, four-sided form. It is fixed superiorly to the radius, and interiorly to the large metacarpal bone, while its lateral borders are united to the lateral ligaments. Its deep face is partly adherent to the carpal bones or their anterior ligaments, and partly it is lined by synovial membrane. The tendons of the extensor pedis and the extensors of the metacarpus play over its superficial face, where they are provided with synovial bursae. The ligament is somewhat loose when the joint is extended, and is put on the stretch during flexion. The Posterior Common Ligament is a much stronger ligament than the preceding. It is fixed above to the radius, and below to the large metacarpal bone. Its internal border mixes its fibres with the internal lateral ligament, while its outer border is blended in the same way with the most external of the intercarpal ligaments. Its anterior or deep face is very intimately united to the carpal bones, and its posterior face is smooth and lined by the synovial membrane of the carpal sheath. The subcarpal ligament , or fibrous band which reinforces the perforans tendon below the carpus, takes origin from the posterior common liga- ment, or may be described as the downward continuation of that ligament. Directions . — The anterior and lateral ligaments just described are to be carefully dissected away, and in removing the first of these, care is ; to be taken of the anterior bands connecting the bones in each row. Radio -carpal Ligaments. — There are three of these. The strongest of them is a thick cord that stretches obliquely downwards and inwards behind the carpus, and connects the radius and scaphoid. It will be ' seen, without removing the posterior common ligament, which covers it, by strongly flexing the joint and looking into it from the front. The second is a very slender ligament which is fixed to the radius beneath the preceding, and passes downwards to be attached to the pisiform and the interosseous ligament uniting the cuneiform and semilunar • bones. The third is situated at the outside of the carpus, where it connects the radius and the upper border of the pisiform bone, and is partly covered by the lateral ligament. The Inter-carpal Ligaments are also three in number. Two of i them are situated behind the joint, under cover of the posterior common DISSECTION OF THE ANTERIOR LIMB. 49 ligament, and will be seen without further dissection on flexing the joint and looking into it from the front. One of these connects the scaphoid to the magnum and trapezoid, the other joins the cuneiform and magnum. The third is a strong ligament situated at the outer side of the joint, where it is blended with the lateral ligament in front, and with the posterior common ligament behind. Its fibres are fixed superiorly to the pisiform bone, and inferiorly to the unciform and head of the external small metacarpal bone. The Carpo-metacarpal Ligaments are four in number — two anterior and two interosseous. One of the anterior ligaments is composed of two separate slips which connect the os magnum and large metacarpal bone. The other passes from the unciform to the head of the external small metacarpal bone, under cover of the lateral ligament. The two interosseous pass, one on each side, from the point of articulation of the large and small metacarpal bones, to join the interosseous ligaments connecting the bones of the lower row. Directions . — Attention may at this stage be given to the disposition of the synovial membranes of the carpus, which are three in number. Synovial Membranes. — 1. The radio-carpal synovial membrane not only facilitates the movements between the radius and the bones of the upper row, but also descends between the latter bones as far as their interosseous ligaments. 2. The inter-carpal synovial membrane, in the same way, belongs to the intercarpal transverse joint ; but it is also insinuated above, between the bones of the upper row as far as their interosseous ligaments, and descends in the same way below, between the adjacent bones of the lower row. It communicates with the next. 3. The carpo-metacarpal synovial membrane facilitates the movements between the lower row and the heads of the metacarpal bones, ascends between the adjacent bones of the lower row as far as their interosseous ligaments, and dips down to supply the articulations between the large and small metacarpals. Directions . — The radio-carpal, inter-carpal, and posterior common ligaments should now be cut transversely. The upper row will thus be isolated as a single piece for the examination of its special ligaments. The Ligaments of the Upper Row are three anterior, and three interosseous ; and they are extremely simple. The anterior ligaments are flattened bands connecting the adjacent bones in front, while the interosseous bands are very short and connect the contiguous surfaces of the bones. The Ligaments of the Lower Row are two anterior, and two inter- osseous; and they are disposed like those of the upper row. In examining these, the lower tier of bones must not be separated from the metacarpus, as that would involve the destruction, in part, of the suspensory ligament of the fetlock. E 50 THE ANATOMY OF THE HORSE. The Inter-metacarpal Articulations. The head of the large meta- carpal bone responds to one of the small metacarpals on each side by a small synovial joint lubricated by a process from the carpo-metacarpal synovial membrane. Below that point the union of the bones is main- tained by short interosseous fibres , which, in adult animals, are very frequently ossified. The lower extremities of the splint bones, however, for a short distance above the little knob that terminates them, remain freely movable, as may be felt by manipulation in the living animal. In addition to the interosseous fibres, the ligaments of the carpus which get inserted in common into the heads of both large and small metacarpal bones, contribute to the union of these bones. THE FETLOCK-JOINT (PLATE 11 , FIGS. 4 - 7 ). This, which is technically termed the metacar po-phalangeal articula- tion , is a ginglymoid joint ; and its articular surfaces are furnished by the lower extremity of the large metacarpal bone, the upper extremity of the first phalanx, and the two sesamoid bones. It corresponds to the joint at the knuckles in the human hand. Movements. — Flexion and extension are, in the natural state, the only movements executed at the joint; but by manipulation, slight lateral movements may be produced when the joint is fully flexed. In com- plete extension the digit is carried beyond the point at which it lies in a straight line with the metacarpus ( over-extension ), until the movement is arrested by tension of the suspensory ligament. Directions. — The tendons which pass in relation to the joint before and behind having been carefully removed, the ligaments should be dissected and studied in the order of their description. The Superior Sesamoidean or Suspensory Ligament. — The main por- tion of this ligament is lodged in the channel formed by the three meta- carpal bones, where it is related by its posterior face to the perforans tendon and its reinforcing band (subcarpal ligament). It has a double origin behind the carpus, viz., (1) by a superficial layer from the lower row of carpal bones, and (2) by a deeper layer from the upper end of the large metacarpal bone. (In the hind limb it has a similar origin from the tarsus and metatarsus). These two portions blend, and descend be- hind the metacarpus as a flattened band which bifurcates a few inches above the sesamoid bones. Each branch passes to the sesamoid bone of its own side, where a considerable proportion of its fibres become inserted ; while the rest is continued in the form of a band which crosses obliquely downwards and forwards over the side of the fetlock to join the extensor tendon on the front of the digit, and be continued with it to the os pedis. The ligament is composed of white fibrous tissue with a constant admix- ture of striped muscular tissue. The presence of muscular tissue here, points to the conclusion (strengthened by other considerations) that the DISSECTION OF THE ANTERIOR LIMB. 51 suspensory ligament is a muscle which, in the evolution of the horse, has undergone retrogressive changes, and lost its original function.* The Inferior Sesamoidean Ligaments. These are three in number, and may be distinguished as superficial, middle, and deep. The super- ficial ligament is fixed below to the glenoidal fibro-cartilage developed behind the superior articular surface of the second phalanx. It ascends as a flattened band behind the os suffraginis, where it is placed between the middle ligament and the tendon of the deep flexor ; and, widening a little, it is inserted into the base of the sesamoids and the intersesamoid ligament. By cutting the ligament about its middle, and reflecting it upwards and downwards, the middle ligament will be brought into view. The middle ligament consists of a median and two lateral bands. Each is fixed to the back of the os suffraginis, and ascends to be inserted into the base of the sesamoids. This should be cut and reflected like the preceding ligament, in order to expose the next. The deep ligament consists of a few short fibres disposed like the letter X, and fixed, on the one hand, to the upper part of the posterior surface of the os suffraginis, and, on the other, into the base of the sesamoid bones. This ligament supports the synovial membrane of the joint. The Lateral Ligaments of the fetlock-joint. Each comprises (1) a superficial fasciculus connecting the lower extremity of the large metacarpal bone to the upper extremity of the first phalanx ; and (2) a deep fasciculus attached, on the one hand, to the large metacarpal beneath the preceding, and, on the other, to the sesamoid and upper extremity of the first phalanx. The Anterior Ligament has a membranous, four-sided form. It covers the joint in front, and supports the synovial membrane by its deep face ; while the extensor pedis tendon passes over its superficial aspect, a synovial bursa being interposed. It is fixed above to the large metacarpal, below to the first phalanx, and on each side to the lateral ligament. Directions . — On one side of the joint the lateral ligament and the slip sent from the suspensory ligament to the extensor tendon must be removed to expose the next ligament. The Lateral Sesamoidean Ligaments. These are not to be con- founded with the lateral ligaments of the joint, by which they are partly covered. Each fixes the sesamoid of its own side to the upper extremity of the first phalanx. The Intersesamoid Ligamemt is the name given to the fibro-cartila- ginous tissue which unites the two sesamoids, and with them forms a pulley-like surface for the passage of the deep flexor tendon. The Synovial Membrane is supported in front by the anterior * According to Professor D. J. Cunningham ( Reports of the Challenger Expedition, Vol. V.), the ligament is the altered flexor brevis of the middle digit, the corresponding muscle in the human subject being the 1st plantar interosseous muscle. 52 THE ANATOMY OF THE HORSE. ligament, and on each side by the lateral ligament. Behind the joint it is supported below the sesamoids by the deep inferior sesamoidean ligament, but above these bones it is unsupported ; and when the synovial sac is distended, it bulges upwards between the branches of the suspensory ligament (Fig. 1, page 33). THE PASTERN-JOINT (PLATE 11). This joint, which is technically termed the first inter phalangeal articulation, is formed between the distal end of the os suffraginis and the proximal end of the os coronse. It is a ginglymus, or hinge joint, and corresponds to the second joint of the human finger. Movements. — As with the joint last described, the only natural movements ar efiexion and extension. Directions. — The tendon of the extensor pedis, which passes over the front of the joint, should be cut and reflected downwards. This will show that the tendon completes the joint in front, where it plays the part of an anterior ligament, and supports the synovial membrane. The lateral ligaments are next to be defined, and after these, the supplementary cartilaginous apparatus placed behind the joint. The Lateral Ligaments. Each of these stretches from the lower extremity of the first phalanx on its lateral aspect, to be inserted into the side of the os coronse, and beyond that point some of its fibres are continued downwards and backwards as the postero-lateral ligament of the second interphalangeal joint. The Glenoidal Fibro-Cartilage. This is a piece of fibro-cartilage fixed at the posterior edge of the upper articular surface of the os coronse. It serves to increase that surface, and its anterior face is moulded on the lower articular surface of the first phalanx, while its posterior face is smooth for the passage of the perforans tendon. Three fibrous slips pass from it on each side, and are attached to the first phalanx. The superficial inferior sesamoidean ligament is inserted into it, and the terminal insertion of the perforatus tendon is blended with it on each side. Synovial Membrane. This is supported in front by the extensor tendon, and on each side by the lateral ligament. Posteriorly it lines the glenoidal fibro-cartilage, and is prolonged upwards as a pouch behind the lower extremity of the first phalanx (Plate 10, fig. 2). THE COFFIN-JOINT (PLATE 11). This, the second interphalangeal joint , has three bones entering into its formation, viz. , the os coronse, the os pedis, and the navicular bone. It is a ginglymus, and corresponds to the first joint of the human finger. Movements. — Flexion and extension. DISSECTION OF THE ANTERIOR LIMB. 53 It possesses an interosseous ligament, and two pairs of lateral ligaments. The Interosseous Ligament is composed of short fibres passing from the inferior border of the navicular bone to the os pedis behind its artic- ular surface. The Antero-Lateral Ligaments. Each of these passes from the side of the os coronse to be inserted into the excavation at the side of the pyramidal process of the os pedis. The Postero-Lateral Ligaments. These seem to be the downward con- tinuations of the lateral ligaments of the pastern-joint. Passing from the side of the os coronse, each is inserted into the upper border of the nav- icular bone, and sends slips to the wing of the os pedis and inner surface of the lateral cartilage. Synovial Membrane. This is supported in front by the extensor tendon, and laterally by the lateral ligaments. A protrusion of it passes on each side between the antero-lateral and postero-lateral ligaments, and lies in relation to the deep face of the lateral cartilage. A third protru- • sion passes upwards posteriorly, between the navicular bone and the back of the os coronae (Plate 10, fig. 2). TABULAR VIEW OF THE MUSCLES OF THE FORE LIMB IN THEIR ACTION ON THE DIFFERENT JOINTS. Shoulder. Flexors Extensors { Deltoid. Coraco-humeralis. Latissimus dorsi. Teres major. Teres minor. Scapulo-humeralis gracilis (?) Large head of triceps. f Supraspinatus. 1 Mastoido-humeralis. ( Superficial pectoral. Adductors < Subscapularis. ( Coraco-humeralis. Rotators outwards Deltoid Teres minor Infraspinatus ( Deltoid. Abductors < Teres minor. ( Infraspinatus. Rotators inwards Flexors Flexors Flexors Flexors ( Flexor brachi. ) Bracliialis anticus. Mastoido-humeralis. Latissimus dorsi. Teres major. Elbow. { Triceps extensor cubiti. Anconeus. Scapulo-ulnaris. Carpus. /Flexor metacarpi externus. I Flexor metacarpi medius. < Flexor metacarpi internus. I Flexor perforans. ' Flexor perforatus. Extensors ( Extensor metacarpi magnus. ] Extensor metacarpi obliquus. j Extensor pedis. V Extensor suffraginis. Flexor perforans. Flexor perforatus. Fetlock. Extensors f Extensor pedis. ( Extensor suffraginis. Pastern. | Extensor —Extensor pedis. Coffin-joint. Flexor — Flexor perforans. | Extensor — Extensor pedis. ( Flexor perforans. \ Flexor perforatus. 54 THE ANATOMY OF THE HORSE. P, ft ft ft ft ft ft ft ft g a o o 2 Sh o « g 2 ft g 3 Si £ 'o D £3 > e8 H ft g o Jh !h gS ft ft ® O *2 .2 ce q ^ ^ .a i .3 J > ja ft ft ^ ft o o 03 ° c8 2 O fl Si _ _ PQ ft > * o ft o ft 1 £3 . 2 • 3 a) £3 I? co T3 £3 03 d S o H D 2 <«-. fc, g _ ° D g ftjHg ^ * Dft ° bO 2 - S.«§ .s °-l I 2 e3 5 £3 _ . D 2 , . ► • a •a. § a .a C Q <3 a; ^ ago £3 2ft w w SM g-l- .§,§1 a "® a a^ft- .3 !h P D .2 « ft 43 2 h v s co •’-i 2 -ft , 1 * w : 1*8| OJ^-P ^ 2 ft 2 D D D D 2 ft 2 2 .S g .9 .9 a a fH “ ft 2 oft ft ft U 2 O c3 D .S'S CO ft o § *£ ' 22 a o ft Q ft 'S >“* « • I --3SS 2 ft 2 oo 2 § ft' Til $ * o ^ Smj M 2 S be ® gft 5* > co 2 ft o 5 S g D s -+3 2 ◄ . 2 2 g “ g > a 2 _bD $ ^ | ft g ® a 2 a 2 ■_g 5 . LJ W p OQ <4H i .2 |ft D ^ -2 - -+=> ?H *9 £ ^ ft ft o3 ft, O ft 03 ft D .2 -S oc 03 ft ft 2 • — .rH DISSECTION OF THE ANTERIOR LIMB. 55 I JJ?g i 1 o H3 O o o (B o ^ b b o m 3 CD P rf . . P3 PS o 53 *3.£.£ w £ 52 rs r3 2 p5 45 cu cl> S P P S S CHAPTER II. DISSECTION OF THE POSTERIOR LIMB. In the male subject, the dissection of the perinseum must be completed before the dissector of the hind limb can begin his operations. THE INNER ASPECT OF THE THIGH. Position . — The animal should be placed on the middle line of its back, and its hind limbs should be drawn forcibly upwards and outwards by ropes running over pulleys fixed to the ceiling. This is the position most convenient for allowing the dissection of both hind limbs to be pursued at the same time. If only one limb is being dissected, the rope may be unfastened from that limb, and the body allowed to incline to the same side, as in Plate 12. Surface-marking . — The internal saphena vein ascends on the inner aspect of the thigh ; and a few inches below the upper limit of the region, it dips in between the sartorius and gracilis muscles. Pressure at this point in the living animal will produce distension of the vessel, and render its course much more evident. Venesection is sometimes per- formed on this vessel. Above the point where the before-mentioned vessel disappears from view, the deep inguinal lymphatic glands are situated in the interstice between the sartorius and gracilis muscles. They here cover the femoral artery, and may be very distinctly felt in a case of lymphangitis , or “weed.” Directions . — An incision through the skin is to be carried down the middle line of the thigh, and terminated a few inches below the level of the stifle-joint. Here another incision is to be made across the inner aspect of the limb, from its anterior to its posterior border. These incisions, together with those already made in the dissection of the perinseum, will enable the dissector to reflect the skin as an anterior and a posterior flap. The student should then dissect the internal saphena vein with its accompanying artery and nerve, and the cutaneous nerves at the forepart of the region, which are derived from the lumbar nerves. Thereafter the surface of the sartorius and gracilis is to be cleaned, and these muscles are to be examined. The Internal Saphena Vein (Plate 12). This is a large vessel PLATE XII DJrawn limited ‘by'W &.A.K. Johnston. Edinburgh Sc London DISSECTION OF THE POSTERIOR LIMB. 57 formed on the inner side of the leg by the junction of an anterior and a posterior root, these being the upward continuations of the inner and outer metatarsal veins. In the thigh it inclines upwards and forwards on the surface of the gracilis, until it disappears between that muscle and the sartorius, to empty itself into the femoral vein. The Saphena Artery" (Plate 12). This artery lies in front of the vein. It is a long and slender vessel given off by the femoral artery about the middle of the femur. It comes out between the sartorius and gracilis, or it may pierce the edge of one of these muscles. It then descends in front of the saphena vein, and finally divides into two branches, which accompany the roots of that vessel. The Internal Saphenous Nerve (Plate 12) is a branch of the anterior crural, from which it is given off a little above the brim of the pelvis. At the crural arch (Poupart’s ligament) it descends in front of the femoral artery, to which and the sartorius muscle it supplies branches. It then divides into two cutaneous branches, which emerge from between the sartorius and the gracilis, in company with the saphenous artery and vein. The anterior half of the nerve gives off branches for the supply of the thigh in front of the vein, and is continued downwards over the forepart of the inner side of the leg, as far as the hock. The posterior half sends branches backwards for the supply of the pos- terior part of the thigh, and it then descends behind the anterior half. Cutaneous Branches from the lumbar nerves. These will be found at the forepart of the thigh, the largest (from the 3rd lumbar) being accompanied by the posterior division of the circumflex iliac artery. The Precrural Lymphatic Glands. These are superficially placed at the inner side of the front of the thigh, on the track of the above- mentioned branch of the circumflex iliac artery. Fascia. At the forepart of the region now exposed, the muscles are overspread by a strong membranous fascia, which is attached superiorly to the tendon of the external oblique muscle of the abdomen, at the line where it is reflected to form Poupart’s ligament. Round the anterior border of the thigh this fascia is continuous with the strong fascia lata ; but when traced backwards, it becomes less fibrous, and over the posterior part of the region it is thin and areolar. When it has been examined, the fascia is to be cleaned away from the subjacent muscles. The Deep Inguinal Ly t mphatic Glands (Plate 45) are ten or twelve in number, and form a chain connected by areolar tissue, and situated in the upper part of the interstice between the gracilis and sartorius muscles, and over the femoral vessels. The Sartorius (Plate 12). This is a somewhat slender muscle which at present can be dissected only in a part of its course. It is seen descending beneath Poupart’s ligament, from its point of origin within the abdominal cavity. It there takes origin from the iliac fascia (Plate 45). 58 THE ANATOMY OF THE HORSE. In the thigh it lies in front of the gracilis. About the middle of their line of apposition the saphena vessels and nerves emerge, but below that point the muscles are adherent to each other. It is inserted into the internal straight ligament of the patella. Action . — To adduct and flex the hip-joint. To a slight extent it may also rotate the limb inwards at the stifle. The Gracilis (Plate 12). This muscle does not possess the slender character from which it is named in human anatomy. It is a large, somewhat four-sided mass, forming the greater part of what is termed the flat of the thigh. A linear depression seen on the surface of the muscle when it is cleaned, is often mistaken by students for the line of separation between it and the sartorius. It arises from the lower face of the pubis and ischium close to the symphysis, and it is here united to its fellow of the opposite side. Interiorly it has a broad flat tendon, united in front to that of the sartorius. It is inserted with the sartorius into the internal straight ligament of the patella, and into a line on the tibia between its anterior and internal tuberosities. The posterior edge of its tendon is continuous with the deep fascia of the leg. A large branch from the external pudic veins traverses the muscle near its origin, and opens into the femoral vein; Action . — To adduct the hip, and rotate the limb inwards. Directions . — The two preceding muscles are to be carefully cut across about their middle, and turned upwards and downwards. On reflecting the proximal half of the gracilis, branches of the obturator nerve and deep femoral artery will be seen penetrating its deep face ; and, in the same way, twigs from the saphena nerve will be found entering the sartorius. The deep inguinal glands are to be removed, and the femoral vessels and anterior crural nerve are to be dissected. The Femoral Artery (Plate 13) is the main arterial trunk for the supply of the hind limb. It is the direct continuation of the external iliac, the brim of the pelvis being selected as the arbitrary line of division between the two vessels ; and, in like manner, it is directly con- tinued by the . popliteal artery, the vessel changing its name when it passes between the two heads of the gastrocnemius muscle. The lower third of the vessel, however, will not be exposed till the next stage of the dissection. The part of the vessel now seen begins at the pelvic brim, where it is seen issuing from beneath Poupart’s ligament. It there rests on the common termination of the iliacus and psoas magnus, having the sartorius in front and the pectineus behind. In the thigh it descends obliquely downwards and backwards, resting first on the com- mon termination of the iliacus and psoas magnus, and then on the vastus internus. It has the sartorius in front ; while posteriorly it is related first to the pectineus, and then to the adductor parvus. In this course it corresponds to the interstice between the gracilis and sartorius PLATE XIII Pectineus bmitendinosus 'Gracilis Gracilis Branches of obturator nerve and deep femoral art. Femoral art. Deep femoral artery [Common tendon of mag. and iliacus Prepubic art. Art. to quadriceps Loral art. Jnt. saphenous nerve Ant. crural nerve Poupart’s ligament rtorius irvus Vastus intemus Sartonus tus femoris /Articular branoh l of femoral art. | Prawn Jc Trent ei ty V. &A.X. Johnaton, E&mbnrgh Sc London THIGH— Inner Aspect DISSECTION OF THE POSTERIOR LIMB. 59 muscles, and is covered by the deep inguinal lymphatic glands. It is closely related to the femoral vein, which lies beneath and slightly pos- terior to it, except at the brim of the pelvis, where the vein is imme- diately posterior to the artery. In the present stage of the dissection the vessel disappears between the upper and lower insertions of the adductor magnus, where it will subsequently be followed. The following collateral branches of the femoral are here seen : — 1. The Profunda or Deep Femoral Artery. This branch is given off under Poupart’s ligament at the pelvic brim. At its origin it usually forms a short common trunk with the prepubic artery. It passes downwards and backwards under the pectineus, and will be followed in the next stage of the dissection. 2. Muscular Branches. The largest of these is a vessel of consider- able size for the supply of the quadriceps extensor cruris muscle. It comes off at about the same level as the profunda, which it generally exceeds in volume; and passing over the psoas magnus and iliacus, and under the sartorius, it penetrates between the rectus femoris and vastus interims, in company with the anterior crural nerve. Other innominate arteries of smaller size enter the vastus internus, pectineus, gracilis, sartorius, and adductors. 3. The Saphena Artery already described (page 57). 4. The Nutrient Artery of the Femur is given off at the tendon of insertion of the pectineus. 5. An Articular branch, of slender volume, descends between the vastus internus and adductor magnus to the stifle-joint. The Femoral Vein ascends in close company with the artery, and receives branches which correspond more or less exactly to those just described. At the brim of the pelvis it lies posterior to the artery, and is continued upwards as the external iliac vein. The Anterior Crural Nerve (Plate 13) is derived from the lumbo- sacral plexus. It descends between the psoas magnus and parvus ; and passing over the common termination of the iliacus and psoas magnus, where it is covered by the sartorius, it splits into a bundle of branches that together penetrate between the vastus internus and rectus femoris to supply the mass of the quadriceps extensor cruris. While under cover of the sartorius it gives off the internal saphena nerve already described. Directions . — The pectineus, adductor parvus, adductor magnus, and semimembranosus muscles are now to be cleaned and isolated. These muscles succeed each other from before to behind in the order named. Some little difficulty may be experienced in finding the line of separa- tion between the two adductors, but a reference to Plate 13 will prove of some assistance. Moreover, the fibres of the small adductor are of a paler colour than those of the adductor magnus. 60 THE ANATOMY OF THE HORSE. The Pectineus (Plate 13). This muscle has a distinctly conical form. It lies posterior to the femoral vessels, and the profunda artery disap- pears beneath it. It arises from the brim and inferior surface of the pubis, and it is there penetrated by the pubio-femoral ligament, from which some of its fibres take origin. Its tapering point is inserted into the shaft of the femur in the neighbourhood of the nutrient foramen. Action . — It adducts the limb, and flexes the hip. The Adductor Parvus (. Adductor brevis of Percivall) (Plate 13) is situ- ated between the pectineus and the great adductor. It arises from the inferior surface of the pubis, and is inserted into the posterior surface of the femur about its middle. Action . — It is an adductor and outward-rotator at the hip-joint. The Adductor Magnus ( Adductor longus of Percivall) (Plate 13) arises from the inferior surface of the ischium, and from the tendon of origin of the gracilis. It has two insertions , between which the femoral artery passes. 1. Its deeper fibres are inserted into the posterior surface of the femur, on a quadrilateral area above the smooth groove in which the femoral artery rests. 2. Its more superficial and longer fibres are inserted into the forepart of the supracondyloid crest. Action . — It is an adductor at the hip. The Semimembranosus ( Adductor magnus of Percivall) (Plate 13). This is a muscle of large size. It arises from the lower surface of the ischium, including its tuberosity, and by a small slip from the fascia investing the muscles of the tail. It is inserted into the inner condyle of the femur, behind the tubercle for the attachment of the internal lateral ligament of the stifle. Action. — Commonly, it is an adductor and extensor of the hip ; but when the femur is fixed, it acts as a lever of the first order, and assists in rearing. Directions . — The foregoing muscles must now be cut and partially removed as follows : — The semimembranosus is to be cut transversely, an inch or two above its insertion. The muscle is then to be raised upwards from the semi- tendinosus, on which it rests ; and in doing this, branches of nerves from the great sciatic will be found entering it in front. The central portion of the muscle may then be removed, leaving a few inches at its origin. The other muscles must be served in the same way, leaving only short portions at the origin and insertion, except in the case of the adductor parvus and upper half of the adductor magnus, whose common insertion into the back of the femur is to be entirely removed. Care is to be taken of the femoral artery where it rests on the bone, and in performing the dissection it will be well to refer to Plate 14 as a guide. In reflecting the upper portion of the great adductor, a branch of the obturator nerve will be found entering its deep face, after having PLATE XIV Pectineus Adductors. Obturator ext. of obturator nerve iacus femoris sat sciatic nervji Semitendiq^fi fanosus Drawn ^Printed li/H. &A.K Johnston, Edinburgh fc London THIGH— Inner Aspect DISSECTION OF THE POSTERIOR LIMB. •61 passed through the obturator externus muscle. Other branches of the same nerve will be found supplying the small adductor and the pecti- neus. The Femoral Artery (Plate 14). The remaining portion of this vessel is now exposed as it winds round behind the shaft of the femur, leaving its impress on the bone. It is seen passing in between the heads of the gastrocnemius muscle, at which point it takes the name of popliteal. In this part of its course it gives off only one vessel of note — -the femoro-popliteal. The Femoro-popliteal Artery. The point of origin of this branch marks the lower limit of the femoral artery. It passes backwards in a horizontal direction, and penetrates the semitendinosus. Near its origin it gives off a considerable branch which ascends behind the femur, sup- plying the biceps, and anastomosing with the profunda. Other branches descend from it to the gastrocnemius. Popliteal Lymphatic Glands. A few glands will be found on the track of the femoro-popliteal artery between the semitendinosus and biceps femoris muscles. The Profunda or Deep Femoral Artery. In the preceding stage of the dissection this branch of the femoral was seen at its origin. It passes downwards and backwards, between the adjacent edges of the iliacus and obturator externus, and under cover of the pectineus and adductor parvus. Above the insertion of the quadratus femoris it crosses behind the femur, where its terminal branches descend to supply the biceps. It also furnishes collateral branches to the pectineus, gracilis, and adductors. Veins. The foregoing arteries are accompanied by satellite veins of the same names. The Quadratus Femoris (Plates 14 and 16). This is a somewhat slender '•riband-shaped muscle. It arises from the lower surface of the ischium in front of the tuberosity, and it becomes inserted into an oblique line on the back of the femur, at the level of the third trochanter. Action . — It is an extensor and outward-rotator at the hip. The Obturator Externus (Plate 14). This muscle, which is coarsely fasciculated, covers the obturator foramen, and conceals the obturator nerves and vessels as they emerge from the pelvis. It is traversed by two branches of the obturator nerve, the posterior of which is for the great adductor, while the anterior splits into branches for the supply of the small adductor, pectineus, and gracilis. It arises from the lower surface of the pubis and ischium, and is inserted into the trochanteric fossa. Action . — It is an extensor and outward-rotator at the hip. Directions . — The nerves which emerge from the obturator externus should be traced through the substance of that muscle to their origin 62 THE ANATOMY OF THE HORSE. from the obturator nerve. The muscle itself may then be removed to expose the obturator vessels and nerve. The Obturator Artery (Plates 14 and 46). This vessel begins at the pelvic inlet as one of the terminal branches of the internal iliac. It leaves the pelvis by the obturator foramen, in company with a vein and nerve of the same name. At its point of emergence it is covered by the obturator externus, and it passes backwards between that muscle and the bone, and then curves downwards to terminate in the biceps and semitendinosus. It gives off the artery of the corpus cavernosum. The Obturator Vein passes into the pelvis by the obturator foramen, and aids in forming the internal iliac vein. The Obturator Nerve is a branch of the lumbo-sacral plexus. Emerging by the obturator foramen, it divides for the supply of the obturator externus, adductor parvus, adductor magnus, pectineus, and gracilis muscles. Directions . — In this stage of the dissection the great sciatic nerve is seen in its course downwards through the thigh. Its examination is more conveniently undertaken in the dissection of the hip and outer aspect of the thigh, but attention may also be given to it here. The Great Sciatic Nerve, which is a branch of the lumbo-sacral plexus, after passing through the hip (see Plate 16), descends in the thigh, behind the femur, where it is deeply enclosed between the biceps and semitendinosus outwardly, and the semimembranosus and great adductor inwardly. Under the name of the internal popliteal, it passes in between the two heads of the gastrocnemius. The following branches whose points of origin are not now visible, being situated in the hip, may be identified by reference to Plate 14: — (1) Branches to the biceps, semitendinosus, and semimembranosus ; (2) the external pop- liteal ; (3) the external saphenous. The last two will be again seen in the dissections of the hip, thigh, and leg. Directions . — The vastus internus, situated at the front of the thigh, should now. be examined. It is a division of the great muscular mass termed in man the quadriceps extensor cruris, whose other divisions — the rectus femoris 'and vastus externus — will be dissected with the outer aspect of the thigh. The dissection in this position of the limb will be completed by an examination of the common insertion of the iliacus and psoas magnus. The Vastus Internus (and Crureus*) (Plates 13 and 14) is a thick fleshy muscle whose fibres take origin from the internal surface and inner half of the anterior surface of the femur, meeting along the front of the femur the vastus -m^ernus, and with it forming a groove in which the rectus femoris rests. Its fibres are inserted into the inner ligament of * This is the name given to the fourth division of the quadriceps in human anatomy. The fibres that represent it in the horse are in no way separable from the inner vastus. Under the same name Percivall describes (inaccurately) the rectus parvus. DISSECTION OF THE POSTERIOR LIMB. 63 the patella, or into that bone along with the other divisions of the quadriceps. Action. — It is an extensor of the stifle-joint. Psoas Magnus and Iliacus (Plate 14). Only the terminal portion of each of these muscles is here seen. They are more fully displayed in the dissection of the sublumbar region, where the psoas magnus arises from the last two ribs, and the vertebrae from the 16th dorsal to the 5th lumbar (Plates 44 and 45). The iliacus arises from the iliac surface and external angle of the ilium, and from the sacro-iliac ligament. It presents a deep groove for the terminal portion of the psoas magnus. The two muscles pass downwards beneath Poupart’s ligament, and have a com- mon insertion into the small ( internal ) trochanter of the femur. Action. — These muscles flex the hip-joint, and rotate it outwards. THE HIP AND OUTER ASPECT OF THE THIGH. Position. — The animal should be suspended in imitation of the natural standing posture, by the means mentioned at page 8. Surface-marking . — A prominent feature of the region is the bony pro- jection formed by the external angle of the ilium ( angle of the haunch). The tuber ischii may also be felt by pressing deeply at the point of the hip. At the highest part of the croup the internal angle of the ilium ( angle of the croup) may be felt, and in the middle line the tips of the sacral spines are subcutaneous. In a lean animal a number of grooves are seen marking the divisions of the biceps and the line of opposition of that muscle with the semitendinosus (Plate 15). Directions. — An incision through the skin is to be carried along the middle line from the root of the tail as far forwards as the lumbar region, where a transverse incision is to be carried outwards and down- wards as far as the level of the angle of the haunch. Beginning at the middle line above, the dissector is to reflect the skin from the limb, as far as the middle of the leg. The first few inches of the skin will require to be raised by the use of the scalpel, and then an attempt may be made to tear it downwards off the limb — a method which will show the cutaneous nerves distinctly without further dissection. Cutaneous Nerves. 1. Appearing a few inches from the middle line, are some slender branches derived from the sacTal nerves. 2. Two or three branches of considerable size, derived from the lumbar nerves, pass backwards and downwards over the forepart of the gluteal region. 3. A few inches below the point of the hip a cutaneous branch derived from one of the posterior gluteal nerves appears from between the biceps and semitendinosus, and separates into a number of radiating filaments. 4. On a level with the stifle-joint the 'peroneal-cutaneous branch of the external popliteal nerve comes out through the biceps, and is distributed on the outer side of the leg. 64 THE ANATOMY OF THE HORSE. Directions. — The dissector should, in the next place, direct his atten- tion to the strong fascia covering the muscles in this region, after which the fascia must be removed, and the muscles cleaned and separated. Gluteal Fascia and Fascia Lata. The gluteal fascia forms a bluish- white covering over the muscles of the hip, and by its deep face affords origin to many fibres of the superficial and middle gluteal muscles. It is fixed above to the summits of the sacral spines and to the external angle of the ilium, and between these points it is continuous forwards with the tendon of the latissimus dorsi. It is prolonged downwards over the muscles of the thigh, where it takes the name of the fascia lata. This fascia lata receives in front the insertion of the tensor vaginae femoris muscle, and it should not be removed until that muscle has been examined. It forms a sheath for the muscles of the thigh, and is pro- longed downwards over the leg. From its inner face a septum is sent in between the vastus internus and the biceps, to join the tendon of the superficial gluteal muscle, and be inserted into the femur. The Tensor Vaginae Femoris (Plate 15). This muscle is situated at the forepart of the thigh, in front of the superficial gluteal muscle, from which it is somewhat difficult to separate it. It arises from the external angle of the ilium, and it is inserted into the fascia lata. Action. — It flexes the hip-joint. It also keeps the fascia lata tense, and mechanically aids in keeping the stifle-joint extended. Directions. — The gluteal fascia and the fascia lata are now to be removed. It is a matter of some difficulty to remove the former, as its deep face has the muscular fibres taking origin from it, and these are therefore exposed with a rough surface when it is removed. The Superficial Gluteus ( Gluteus externus of Percivall, part of the glu- teus maximus of human anatomy) (Plate 15). The outline of this muscle is not distinctly recognisable until the gluteal fascia has been removed. It is then seen to have some resemblance to the letter V, having in its upper border an indentation that divides it into an anterior and a posterior branch. The anterior branch arises from the external angle of the ilium ; the posterior from the gluteal fascia. Both converge to a common tendon, which is inserted into the third trochanter of the femur ( trochanter minor externus). From the posterior branch of the muscle an aponeurotic layer passes backwards beneath the biceps, to be inserted into the sacro-sciatic ligament and the tuber ischii. Action. — It is an abductor at the hip-joint. The Biceps Femoris (Plate 15). This is one of the largest muscles in the body. It arises from the sacral spines, the fascia enveloping the muscles of the tail, the sacro-sciatic ligament, the tuber ischii, and the gluteal fascia. Inferiorly it has three divisions, one of which is inserted into the anterior surface of the patella, a small synovial bursa being- interposed between the tendon and the bone, another into the tibial &. Printed 'by'W. &.A.K. JohHeton. Edinburgh ScLondi DISSECTION OF THE POSTERIOR LIMB. 65 crest, and the third into the fascia of the leg. Besides these, the muscle has an insertion into the circular mark behind the third trochanter of the femur, by means of a fibrous band detached from the deep surface of the muscle. Action. — The anterior half of the muscle, in virtue of its attachment to the patella, is an extensor of the stifle, and an abductor at the hip. The posterior half of the muscle, with its insertions into the tibia and fascia of the leg, is a flexor and an outward-rotator at the stifle. When the stifle-joint is kept extended, the lower end of the muscle becomes its fixed point, and it then extends the pelvis on the femur, and aids in rearing. The Semitendinosus (Plate 15). This muscle is placed at the posterior border of the hip and thigh, where it occupies a position between the last-described muscle and the semimembranosus. The muscle is bifid superiorly, where it arises by one division from the sacral spines and sacro-sciatic ligament, and by another and shorter branch from the tuber ischii. Interiorly it has a flat tendon, which is inserted into the tibial crest, and whose posterior border blends with the fascia of the leg. Action. — To flex the stifle and rotate the leg inwards. When the stifle is fixed, it can aid in rearing. The biceps femoris and semitendinosus muscles represent, apparently, the muscles of the same name in man, plus portions of the gluteus maximus. Percivall describes the semitendinosus as representing also the semimembranosus of man. Directions. — The biceps must be carefully severed at its origin, and pulled downwards. This will expose the aponeurotic layer that passes beneath it from the superficial gluteus. A branch from the posterior gluteal nerves should be found entering the last-named muscle by turn- ing forwards round the middle gluteus. Both branches of the super- ficial gluteus should then be thrown downwards in order to fully expose the next muscle. The Middle Gluteus ( Gluteus maximus of Percivall) (Plate 15) is a muscle of great size and strength. It was partly exposed before the removal of the superficial muscle. The fibres of the muscle arise from the aponeurosis of the common mass of the loins (longissimus dorsi), from the gluteal surface of the ilium, from the two ilio-sacral and the sacro-sciatic ligaments, and from the gluteal fascia. It has three distinct and constant insertions : 1. By a tendon, into the summit of the great trochanter. 2. By another tendon, which plays over the convexity of the same trochanter by means of a synovial bursa, and becomes inserted into the crest. 3. By a triangular fleshy slip, into the back of the trochan- teric ridge. Action. — To extend and abduct the hip. In the former of these 66 THE ANATOMY OF THE HORSE. actions, when the limb is free to move, the femur, and with it the whole limb, is carried backwards ; but when the femur is fixed, it raises the trunk, as in rearing. Directions . — The last-described muscle must be removed in order to expose the deep gluteus and the other structures which it covers. A deep incision should be made through the muscle along the crest of the ilium, and the muscle is to be turned down by severing its fibres at their origin. Care must be taken, in doing this, to avoid cutting the subjacent deep gluteus, whose fibres may be recognised, as soon as they are reached, by their insertion within the great trochanter. A reference to Plate 16 may here be useful. The semitendinosus is to be turned down in the same manner, by severing its superior attachments ; and this muscle and the biceps may be removed to the extent shown in the Plate. In performing this dissection, the gluteal nerves and vessels and the ischiatic vessels are unavoidably severed, but a look-out should be kept for these, and they should be cut about the points shown in the figure. The Gluteal Artery (Plate 16) is a branch given off from the internal iliac within the pelvis. After a very short course it splits into several branches, which, emerging by the great sacro-sciatic opening, are distributed to the gluteal muscles. The Ischiatic Artery (Plate 16) is one of the terminal branches of the lateral sacral artery, which, again, is a collateral branch of the internal iliac. It perforates the sacro-sciatic ligament near the edge of the sacrum, and is distributed in the biceps and semitendinosus. Veins of the same names accompany the foregoing arteries. The Internal Pudic Artery. The dissection of this artery belongs to another region, but the vessel is generally visible here in a part of its course. A few inches of it are represented in Plate 16, as showing faintly through the texture of the sacro-sciatic ligament. It is described at page 342. The Gluteal Nerves (Plate 16). These nerves, which are derived from the lumbo-sacral plexus, issue from the great sacro-sciatic opening in company with the gluteal vessels and the great sciatic nerve. They consist of an anterior and a posterior set. The Anterior gluteal nerves are three or four in number. One of them passes downwards and forwards between the middle and internal gluteal muscles, to reach the tensor vaginae femoris and anterior division of the superficial gluteus. Another branch passes downwards over the deep gluteus, to which it is distributed. One or two other branches supply the middle gluteus. The Posterior gluteal nerves are two in number — an upper and a lower. The upper nerve passes backwards on the sacro-sciatic liga- ment; and after giving branches to the posterior division of the PLATE XVI Drawn &.Prmled "by "W. Sc A. K. Johnston, Edinburgh t London HIP AND THIGH DISSECTION OF THE POSTERIOR LIMB. 67 superficial gluteus, and to the posterior fleshy slip of the middle gluteus, it enters the biceps femoris. The lower nerve, passing down- wards and backwards, divides into an outer and an inner branch ; the former, turning over the tuber ischii on its outer side, becomes cutane- ous at the back of the thigh about four or five inches below the tuber ; the latter, after giving twigs to the semitendinosus, joins a branch from the internal pudic nerve to be distributed to the perineal structures. The Great Sciatic Nerve (Plate 16). This is, at its point of origin, the largest nerve in the body. It is furnished by the lumbo- sacral plexus, and appears at the great sacro-sciatic opening as a broad riband. In its downward course in the hip it is covered by the middle gluteus, and rests in succession on the sacro-sciatic ligament, the gluteus internus, the gemelli and common tendon of the obturator internus and pyriformis, and the quadratus femoris. In the thigh it is included between the biceps and semitendinosus outwardly, and the semimembranosus and great adductor inwardly. The trunk of the nerve is continued as the internal popliteal nerve between the two heads of the gastrocnemius, where it will be followed in the dissection of the leg. It gives off in succession the following branches : — 1. A nerve for the supply of the obturator internus, pyriformis, gemelli, and quadratus femoris. This slender branch is given off about midway between the great and small sciatic openings, and it descends at the posterior border of the parent trunk, or between that and the ligament. The nerves to the quadratus and gemelli may arise from the sciatic independently, and the branch to the first of these muscles passes under the gemelli and the common tendon of the pyriformis and obturator internus. 2. The external 'popliteal nerve is a large branch that separates from the great sciatic about the level of the small sacro-sciatic opening ; and, descending in front of the parent nerve, it passes between the biceps and the outer head of the gastrocnemius, where it will be followed at a later stage. The peroneal cutaneous branch of this nerve has already been seen perforating the lower part of the biceps, at the level of the stifle. 3. A branch that divides to supply the semimembranosus and lower portions of the biceps and semitendinosus (Plate 14). 4. The ex- ternal saphenous nerve , which will be followed in the dissection of the leg. Directions. — The great sciatic nerve should now be cut at the upper border of the gluteus internus, and turned downwards with its branches. The gluteus internus, and the common tendon of the pyriformis and obturator internus, together with the gemelli, should be carefully cleaned and defined ; and to facilitate this, the limb should be rotated inwards as far as possible, by pulling the point of the hock outwards. This will put these muscles on the stretch. The Deep Gluteus, or gluteus internus (Plate 16, and fig. 2), is placed above the hip-joint, in immediate contact with the capsular ligament. 68 THE ANATOMY OP THE HORSE. It is a comparatively small muscle, with coarse fasciculi having a slightly spiral direction. It arises from the rough lines on the gluteal surface of the shaft of the ilium just above the cotyloid cavity, and from the supra- cotyloid ridge (superior ischiatic spine). It is inserted to the inner side of the convexity of the great trochanter. Action . — It is an abductor and inward-rotator at the hip-joint. The Obturator Internus and the Pyriformis (Plate 16, and fig. 2) are two muscles arising within the pelvis, the former taking origin from the bone around the obturator foramen, and the latter from the pelvic surface of the ilium. They have a common tendon, which emerges from the pelvis by the lesser sacro-sciatic opening, where it plays over a smooth portion on the external border of the ischium. The tendon is inserted into the trochanteric fossa. Action . — To produce outward rotation at the hip. The Gemelli. In Plate 16 a bundle of muscular fibres is seen at each edge of the above-mentioned common tendon. If this common Muscles of the Tail, deep Muscles of the Hip, and Pelvic Ligaments ( Chauveau ). 1. Erector coccygis ; 2. Curvator coccygis ; 3. Depressor coccygis ; 4. Compressor coccygis ; 5. Deep gluteus ; 6. Rectus parvus ; 7. Common tendon of obturator internus and pyriformis ; 8. Gemelli ; 9. Accessory fasciculus of the same ; 10. Quadratus femoris ; 11. Sacro-sciatic ligament ; 12. Great sacro-sciatic foramen ; 13. Superior ilio-sacral ligament ; 14. Inferior ilio-sacral ligament. tendon be cut where it appears at the lesser sciatic opening, and raised outwards, what previously seemed two distinct muscular bundles will now be seen to be the edges of a single flattened muscle, which arises from the ischium below and at the edges of the smooth surface for the DISSECTION OF THE POSTERIOR LIMB. 69 passage of the common tendon, and becomes inserted in common with that tendon.* Action. — The same as the two preceding muscles. Directions. — The tensor vaginae femoris and the gluteus internus should now be cut away, care being taken not to injure the capsular ligament, on which the latter muscle rests. The rectus femoris, vastus externus, and rectus parvus are then to be dissected. The last- mentioned muscle will be found by dissecting deeply into the upper part of the interstice between the other two muscles, and at the same point the iliaco-femoral artery will be found. The Iliaco-femoral Artery is one of the terminal branches of the internal iliac (Plate 48). It comes out between the iliacus and the shaft of the ilium, and penetrates between the rectus femoris and the vastus externus. The Rectus Femoris (Plate 18) arises by two heads — one from each of the depressions on the shaft of the ilium, above and in front of the cotyloid cavity. The central portion of the muscle is thick and fleshy, and rests in a groove formed by the two yasti, with which it is confounded at its lower extremity. It is inserted into the anterior face of the patella. Action. — To flex the hip-joint and extend the stifle. The Vastus Externus (Plate 18) arises from the outer surface of the femur, and from the outer half of the anterior surface of the same bone. Its fibres become inserted along with the rectus femoris into the patella. Action . — To extend the stifle. The Rectus Parvus (fig. 2) is very slender when compared with the muscles between which it is placed, being about the thickness of a human finger. It arises from the ilium, external to the pit from which the outer head of the rectus femoris takes origin. Passing in front of the capsular ligament of the hip-joint, to which it adheres, it insinuates itself between the two vasti muscles, and is inserted into the anterior surface of the femur. Action. — The muscle is of too slender a size to exert any appreci- able flexor action on the hip-joint, and probably its function is to raise the capsular ligament during flexion of the joint. (See footnote, page 62.) Directions. — The dissector is now in a position to detach the limb from the trunk, and this should be done by cutting through the bone and soft structures, below the level of the internal trochanter. It is necessary to make the section at this point, in order to leave the hip- joint and the common insertion of the iliacus and psoas magnus intact for examination by the dissector of the abdomen and pelvis. The limb * In man this muscle consists of two separate slips, and from this disposition it is named. I do not hesitate to give it the same designation here, although I have never found it double as it is usually described. 70 THE ANATOMY OF THE HORSE. having been removed, it should be placed on a table, and the cut muscles connected with it may be completely cut away after they have been identified. In doing this, a better opportunity will be afforded to observe accurately the insertion of each muscle. In remov- ing the lower portion of the biceps, particular care must be taken not to cut the external popliteal and external saphenous nerves, which are included between it and the outer head of the gastrocnemius (Plate 18). THE LEG. aS 'ur face-marking . — The bones of the leg are clothed by muscles except at the inner side of the limb, where the tibia is subcutaneous. This unprotected area of bone corresponds to the shin in man. The superficial muscles of the region (see Plates 17 and 18) form promi- nences more or less distinct, especially in the neighbourhood of the hock, where the various tendons stand out distinctly during the movements of the living animal. Position . — In the further dissection of the limb, it may be placed on a clean table, and laid on either side as may be convenient ; or a cord may be passed round the femur, and the limb suspended at such a height as just to permit the hoof to come into contact with the table. This latter method has the advantage of keeping the part clean ; and while dissection is being carried on, the leg may be steadied in any position by an assistant. Directions . — An incision through the skin is to be carried down the middle line of the limb on its inner side, and terminated a few inches below the hock, where a circular incision may be carried round the limb. The whole of the skin above the circular incision is then to be removed, and the cutaneous nerves and vessels of the region are to be examined. The Internal Saphena Vein. On the inner side of the leg, above the hock, two venous branches will be seen to converge and unite to form the internal saphena vein, which is continued up the leg to the thigh, where it has already been dissected. The vessels by whose union the main vein is formed, are the upward continuations of the internal and external metatarsal veins. Slender branches of the saphena artery accompany these veins. The External Saphena Vein. This vessel begins at the hock, where it communicates with the internal saphena vein, and with the posterior tibial vein. It ascends at the outer side of the gastrocnemius tendon, and, passing between that muscle and the biceps, it empties itself into the femoro-popliteal vein. Cutaneous Nerves. 1. The ramifications of the internal saphenous nerve cover the inside of the leg, and descend over the inside of the PLATE XVII -Post, tibial nerve Flexor perforans Post, tibial art. of flexor perforatus Cutaneous nerve from post, tibial Flexor accessorius sciatic nerve popliteal nerve Int. popliteal nerve saphenous nerve Fein or o-popli teal art. Popliteal Middle straight ligament Inner straight patellar Int. lateral ligament of Flexor Popliteus Outer head of gastrocnemius Inner head of gastrocnemius Femoral artery Drawn & 'Printed by "W iA K Johnston Edinburgh fc London LEG— Inner Aspect DISSECTION OF THE POSTERIOR LIMB. 71 hock. 2. The external saphenous nerve (Plate 18) is a branch of the great sciatic. It descends over the outer head of the gastrocnemius, where it is covered by the biceps, and is reinforced by a' branch from the external popliteal (or from the peroneal cutaneo'^ side of the metatarsus. 3. The cutaneous termination of the' mu.s&hlo'^ cutaneous division of the external popliteal nerve (Pla^e 18) piercea^he m deep fascia on the outer side of the limb at the lower ^ird of theAJ^; and, passing over the hock, it is distributed to the skin Os^the front of the metatarsus. Deep Fascia of the Leg. This forms envelope to the muscles of the region. Its inner face furnishes septa that pass in between the muscles ; and over the inner surface and crest of the tibia, it is adherent to the bone. Above it is continuous with the fascia lata and tendons of the gracilis and semitendinosus on the inside, and with the tendon of the biceps on the outside. As it passes over the hock it becomes thinner, and is continuous with the fascia of the meta- close-fitting, tarsal region. Directions — The muscles on the back of the leg may now be dissected, the vessels and nerves shown in Plates 17 and 18 being at the same time carefully preserved. By a reference to Plate 18, the student should note the position of the small soleus muscle, so as to avoid its removal with the fascia. The Gastrocnemius (Plates 17 and 18). At its origin this muscle consists of two distinct fleshy heads, which terminate in a single inferior tendon. The outer head arises from the outer lip of the supracondyloid fossa of the femur, the inner head from the supracondyloid crest. The cord-like tendon is joined by that of the soleus, and is inserted into the back part of the summit of the os calcis. When the hock is strongly flexed, the tendon for an inch or two above its insertion rests on the forepart of the summit, and a small synovial bursa is here interposed between the tendon and the bone. The tendon of the perforatus is at first beneath that of the gastrocnemius ; but, passing to the inner side, it places itself superficial to the latter, which it completely covers at the summit of the os calcis. In thus changing positions, the two tendons form a half twist, and indent each other like the strands of a rope. This tendon of the gastrocnemius corresponds to the firm tendon extending upwards from the human heel, and known as the tendo Achillis. Action. — To extend the hock-joint. The Soleus (Plate 18). In British veterinary text-books this muscle is erroneously termed plantaris. It is a small muscle of delicate texture, and it is often partially or entirely removed in cleaning the 72 THE ANATOMY OF THE HORSE. gastrocnemius. It arises from the head of the fibula, and its tendon joins that of the preceding muscle, which it assists in extending the hock. Directions. — The inner head of the gastrocnemius is to be severed at its origin, and turned downwards in the manner shown in Plate 17. The Superficial Flexor of the digit (flexor perforatus) (Plates 17 and 18) is remarkable in that, throughout nearly the whole of its extent, it exists as a strong tendinous cord with a sparing admixture of muscular fibres at its upper fifth only. It arises from the bottom of the supra- condyloid fossa; and, winding round the gastrocnemius tendon in the manner already described, it gains the summit of the os calcis, over the extreme posterior portion of which it plays by means of a synovial bursa. At the os calcis it detaches on each side a slip to be inserted into the bone. It is continued downwards in the meta- tarsal and digital regions in the same manner as the perforatus of the fore limb, becoming finally inserted by a bifid termination into the second phalanx. In front of the tendons of the superficial flexor and gastrocnemius there will be noticed a strong fibrous band, which is united to these muscles above, and inserted into the os calcis below, while laterally it is continuous with the deep fascia of the leg. Action. — It flexes successively the pastern and fetlock joints ; and, by its insertion into the os calcis, it is also an extensor of the hock-joint. It also plays an important part in mechanically maintaining the hock in a state of extension so long as the hip and stifle joints are kept extended by muscular contraction. The flexor perforatus of the horse is represented in man by two distinct muscles — the plantaris and the flexor brevis digitorum. Directions. — The deep layer of muscles at the back of the leg consists of the popliteus, the flexor perforans, and the flexor accessorius ; and these should now be examined as far as possible without disturbance of the vessels and nerves. The superficial muscles must therefore, in the meantime, be allowed to remain in position. The Popliteus (Plate 17). This muscle is placed immediately behind the stifle-joint, whose posterior ligament it covers. It arises by a tendon from the lower and most anterior of the two pits situated on the outer side of the external condyle of the femur. (The other pit is for the attachment of the external lateral ligament of the stifle, the ligament concealing the origin of the tendon.) The tendon is partly invested by the synovial membrane of the joint, and plays round the external semi- lunar cartilage, and over the articular surface of the tibia. The fibres of the muscle have an oblique direction downwards and inwards, and are inserted into the comparatively smooth triangular area at the upper part of the posterior surface of the tibia, and into the inner edge of the bone at the same level. The terminal portion of the popliteal artery is concealed by the muscle. DISSECTION OF THE POSTERIOR LIMB. 73 Action. — It flexes the stifle, and to a slight extent rotates it inwards. The Deep Flexor of the digit (flexor perforans) (Plates 17 and 18). This muscle is indistinctly divided into an outer and an inner division, the former being the larger of the two. It arises from the ridged area on the posterior surface of the tibia, from the external tuberosity at the upper end of the same bone, from the fibula, and from the interosseous membrane uniting the two bones. At the lower third of the tibia the muscular divisions are succeeded by tendons, which soon unite ; and the single tendon thus formed glides through the tarsal sheath at the inner side of the os calcis, and then descends at the back of the metatarsus and digit, to be inserted into the solar surface of the os pedis, in a man- ner exactly similar to the flexor perforans of the fore limb. Like that muscle, it receives, at the upper part of the metatarsus, a reinforcing or check band — the subtar sal ligament , which is the downward continua- tion of the 'posterior tarso-metatarsal ligament of the hock. This band is not so strong as the subcarpal ligament of the fore extremity. Action. — It flexes successively from below upwards the interphalangeal joints and the fetlock, and finally extends the hock. The Tarsal Sheath, through which the tendon passes at the inner side of the back of the hock, is formed outwardly by the grooved surface of the os calcis, in front by the posterior ligament of the tibio-tarsal articulation and by the posterior tarso-metatarsal ligament, and it is completed inwardly by a fibrous arch that converts the groove into a complete canal. An extensive synovial membrane here invests the tendon and lines the passage, extending upwards for a few inches at the lower extremity of the tibia, and downwards below the middle of the metatarsus. A dropsical condition of this synovial sac gives rise to the condition termed “ thorough-pin. ” The Flexor Accessorius (Plate 17) is a somewhat slender muscle extending obliquely downwards at the back of the leg, between the popliteus and the perforans. It arises from the back of the external tuberosity at the head of the tibia. Its tendon, which begins at the lower third of the leg, descends first in a groove on the deep flexor, and then through a synovial passage at the inner side of the tarsus, and finally blends with the tendon of the deep flexor at the back of the metatarsus. Action . — To assist the deep flexor. Directions . — The vessels and nerves of the region must now be noticed, and it will be convenient to begin with the latter. The Internal Popliteal Nerve (Plates 17 and 18) is the continuation of the great sciatic. It passes in between the two heads of the gastro- cnemius muscle, follows for a short distance the posterior border of the perforatus, and at the level of the lower border of the popliteus it is continued under the name of the posterior tibial nerve. The nerve fur- 74 THE ANATOMY OF THE HORSE. nishes branches to all the muscles at the back of the leg, viz., both heads of the gastrocnemius, the sole us, the perforatus, the popliteus, the perforans, and the flexor accessorius. The branch to the soleus gains its muscle by passing between the popliteus and the outer head of the gastrocnemius. The Posterior Tibial Nerve (Plate 17) continues the internal popliteal. It is at first deeply placed beneath the inner head of the gastrocnemius, where it crosses the perforatus. Becoming more super- ficial by emerging from beneath the first-named muscle, it descends on the inner side of the leg, in front of the tendo Achillis , being covered by the deep fascia of the leg. At the tarsus it bifurcates to form the external and internal plantar nerves. These accompany the perforans tendon through the tarsal sheath, and are continued through the meta- tarsal and digital regions like the corresponding nerves of the fore limb. The only collateral branches of the posterior tibial nerve are slender cutaneous filaments, one of which is shown in Plate 17, descending over the inner side of the hock. The External Popliteal Nerve and the External Saphenous Nerve cross the external head of the gastrocnemius on its outer side (Plate 18). The latter nerve has already been described, and the former should be preserved to be followed in the dissection of the front of the leg. Directions . — The outer head of the gastrocnemius and the perforatus should now be detached close to their origin, and turned downwards. This will expose the whole of the popliteus, which must be dissected carefully from the posterior ligament of the stifle and from the tibia, in order to follow the popliteal artery. The Popliteal Artery (Plate 17) is the direct continuation of the femoral. In veterinary anatomy the arbitrary line of distinction is usually drawn at the point where the vessel passes in between the heads of the gastrocnemius. It passes over the posterior ligament of the stifle, where it is covered by the popliteus ; and at the tibio-fibular arch it bifurcates to form the anterior and posterior tibial arteries. It gives off — (1) articular branches to the stifle, and (2) muscular branches to the superficial muscles at the back of the leg. The Posterior Tibial Artery (Plate 17) is much the smaller of the two terminal branches of the popliteal. In the first part of its course it is deeply placed beneath the popliteus and the deep and accessory flexors. As it descends, it becomes more superficial, and appears at the posterior border of the flexor accessorius, whose tendon it follows in the same posi- tion. A little above the hock it forms an S-shaped curve that brings it into company with the terminal part of the posterior tibial nerve ; and passing with that nerve into the tarsal sheath, it divides at the back of the hock into the two plantar arteries. The collateral branches of the posterior tibial are — (1) muscular branches to the deep muscles at the PLATE XVIII Annular bands of hock Rectus Vastus Ext. straight patellar ligament Middle straight patellar ligament Ext. lateral ligament of stifle Flexor Extensor Peroneus sciatic nerve xt. popliteal nerve Flexor perforans of gastrocnemius saphenous nerve Tendon of flexor perforatus Extensor brevis popliteal of peroneal cdtaneous nerve head of gastrocnemius branch to ext. saphenous nerve lateral ligament of patella Anterior tibial nerve usculo-cutaneous nerve to soleus Drawn ^Printed by W U K Johnston Edinburgh St London LEG — Outer Aspect / 1 DISSECTION OF THE POSTERIOR LIMB. 75 back of the leg ; (2) the nutrient artery of the tibia ; (3) a retrograde branch which, emanating from the second curve of the sigmoid flexure, ascends in front of the tendo Achillis ; (4) articular branches to the tarsus. Veins. The foregoing arteries run in company with satellite veins bearing the same names. Directions . — The front of the leg must now be dissected ; and as the first step, the muscles of the region should be cleaned and isolated. These are — the extensor pedis, the flexor metatarsi, and the peroneus. The first of these is superposed to the second on the front of the leg, while to the outer side of both is the smaller peroneus. In dissecting the tendons of these muscles in the region of the hock, care should be taken of three transverse fibrous bands that retain the tendons in posi- tion (Plates 18 and 19). The first of these bands is fixed by its extremities to the lower end of the tibia, and beneath it pass the tendons of the ex- tensor pedis and flexor metatarsi. The second is fixed outwardly to os calcis ; and, passing over the extensor pedis tendon, it is attached to the flexor metatarsi. The third retains the tendons of the extensor pedis and peroneus in position at the upper end of the large metatarsal bone, to which its extremities are attached. The Extensor Pedis (Plate 18). This muscle arises , in common with the tendinous portion of the flexor metatarsi (fig. 3, page 76), from the pit between the trochlea and external condyle of the femur. It has a thick, fusiform muscular belly, which at the krwer third of the leg is succeeded by a strong tendon. This passes over the front of the hock, and under the three annular bands just described. It then descends over the front of the metatarsus, where it receives the insertion of the short extensor of the digit, and is joined by the tendon of the peroneus. In the dissec- tion of the digit it will be pursued to its insertion into the pyramidal eminence of the os pedis. Action . — Jt extends in succession from below upwards the interphalan- geal joints and the fetlock, and finally flexes the hock. The Peroneus (Plate 18). This is a much smaller muscle than the preceding, to whose outer side it lies. Its muscular fibres have a penni- form arrangement, and arise from the external lateral ligament of the stifle, from the fibula, and from the aponeurotic septum between it and the deep flexor of the phalanges. Its tendon passes through the groove on the external tuberosity (external malleolus) at the lower end of the tibia, and then over the outer side of the hock, where it plays in a synovial canal formed in the external lateral ligament. Below the hock it is directed obliquely forward, and joins the tendon of the extensor pedis about the middle of the metatarsus. Action. — The same as the preceding muscle. Directions . — Cut the extensor pedis about the middle of the leg, and reflect it upwards and downwards to expose the next muscle. 76 THE ANATOMY OF THE HORSE. The Flexor Metatarsi (fig. 3). This muscle consists of two parallel portions — a superficial and a deep. The superficial division exists in the form of a tendinous cord with little or no muscular tissue, and arises , in common with the extensor pedis, from the pit between the trochlea and external condyle of the femur. This tendon of origin passes through the notch between the anterior and external tuberosities at the upper end of the tibia, and is there enveloped by the -■» synovial membrane of the femoro-tibial joint. In the leg the tendinous cord rests on the deep division of the muscle, and passes under the annular band at the lower extremity of the tibia, in company with the tendon of the extensor pedis. At the front of the hock it is perforated by the tendon of the deep division, and then bifurcates, one branching continuing downwards to be inserted into the upper extremity of the large metatarsal bone, the other deviating outwards to be inserted into the cuboid. The deep division of the flexor metatarsi rests on the tibia, and its muscular fibres arise from the upper part of the outer surface of that bone. At the lower end of the tibia it is succeeded by a tendon which perforates that of the superficial F IG . 3 division of the muscle, and divides, one branch Flexor Metatarsi Muscle P a ®® in S to be inserted into the head of the large ( Chauveau ). metatarsal bone, along with the large division of the superficial cord, while the other branch is carried inwards to be inserted into the cuneiform parvum. Action . — To flex the hock. In this action the superficial tendinous cord plays merely a mechanical part, flexing the hock when the stifle is flexed. The ■ External Popliteal Nerve (Plate 18). This nerve has already been seen in the hip and thigh. It is a branch given off by the great sciatic ; and, descending in front of the parent nerve, it passes between the biceps and the outer head of the gastrocnemius, where, a little behind the external lateral ligament of the stifle, it divides into the musculo-cutaneous and anterior tibial nerves. The Musculo-Cutaneous Nerve descends along the line of contact of the extensor pedis and peroneus, supplying filaments to the latter muscle. At the lower third of the leg, as has already been seen, the 1 . Superficial division of the muscle; 2. Its origin from the femur ; 3. Its cuboid branch ; 4. Its metatarsal branch ; 5. Deep division of the muscle ; 6. Its tendon passing through that of the superficial division ; 7. Cuneiform branch of this tendon ; 8. Metatarsal branch of the same ; 9. Extensor pedis ; A. Pero- neus ; B. Insertion of middle straight patellar ligament ; C. Femoral trochlea. DISSECTION OF THE POSTERIOR LIMB. 77 cutaneous division of the nerve pierces the deep fascia, and passes over the hock to supply the skin on the outer side of the metatarsus. The Anterior Tibial Nerve separates from the preceding at an acute angle, and a few inches below the stifle it passes under cover of the extensor pedis. It supplies twigs to the last-named muscle, the flexor metatarsi, and the short extensor ; and descends at the outer side of the tibial vessels, afterwards accompanying the large metatarsal artery to terminate in the skin on the outer side of the digit. The Anterior Tibial Artery (Plate 19). This, it will be recol- lected, is one of the terminal branches of the popliteal artery. Origi- nating behind the upper extremity of the tibia, it is here seen coming forwards through the tibio-fibular arch. It descends on the tibia, under cover of the flexor metatarsi, and accompanied by the nerve and vein of the same name. Gaining the front of the hock, it rests on the anterior tibio-tarsal ligament, covered by the flexor metatarsi and extensor pedis at their line of contact. Here it deviates outwards under the tendon of the last-mentioned muscle, and divides into two vessels of unequal size. The larger of these, which continues the direction of the parent vessel, is the large metatarsal artery ; the other is the perforating metatarsal artery ; and both will be dissected with the metatarsus. The anterior tibial artery gives off numerous un-named muscular branches to the extensor pedis, flexor metatarsi, and peroneus ; and articular branches to the hock. The Anterior Tibial Vein, which may be double, keeps close com- pany with the artery. It is formed at the front of the hock by the fusion of several rootlets. The largest of these is the upward continuation of the deep metatarsal vein, which comes forwards through the vascular ca nal between the tarsal bones. After passing backwards through the tibio-fibular arch, the anterior joins the posterior tibial vein to form the popliteal. THE METATARSUS AND DIGIT. The distal portion of the horse’s hind limb, beyond the lower extremity of the tibia, is technically termed th e pes, as it corresponds to the foot of man. The tarsus, or hock, represents the hu man ankle ; the part between the tarsus and fetlock corresponds to the body of the human foot, and is termed the metatarsus ; while the rest of the limb, beyond the fetlock, is the digit, and is the homologue of man’s third toe. Surface-marking . — Extending down the middle line in front is the tendon of the extensor pedis, which, a little below the tarsus, is joined obliquely by the tendon of the peroneus. Behind the metatarsus, and resting on the bone, is the suspensory ligament ; and behind that again are the deep and superficial flexors of the foot. The edges of these structures can be distinctly seen or felt in the living animal, and jn the dead subject they may be identified by a reference to Plate 19. 78 THE ANATOMY OF THE HORSE. At the upper part of the inner face of the metatarsus is a flattened horny callosity, or chestnut; and another horny excrescence, in the form of a spur, or ergot , is concealed in the tuft of hair behind the fetlock. By manipulation in the neighbourhood of the heels, the lateral cartilages may be felt. Directions . — Remove the entire remaining portion of skin from the limb ; and if it is intended to study on the same preparation the parts contained within the hoof, this must, before the removal of the skin, be detached by force in the manner described at page 35. The various structures are now to be defined by dissection, in the order of the following description ; and while the vessels and nerves are being cleaned, care must be taken of the small lumbricales muscles, which lie on the tendon of the deep flexor, above the fetlock. Cutaneous Nerves. — Descending over the inner side of the hock and metatarsus are twigs of the internal saphenous and posterior tibial nerves, and on the outer side of the same regions are branches of the external saphenous and musculo-cutaneous nerves. The Large Metatarsal Artery (Dorsalis pedis of man) (Plate 19) is the larger branch resulting from the division of the anterior tibial artery at the front of the tarsus. It inclines outwards and down- wards under the extensor brevis and the peroneus, and places itself in the groove formed on the outer side of the metatarsus by the junction of the large and outer small metatarsal bones. Along this groove it descends in company with the slender continuation of the anterior tibial nerve, until, a little above the button of the smaller bone, it passes to the back of the metatarsus by penetrating between the two bones. Finally, it bifurcates above the fetlock, between the two divisions of the suspensory ligament, to form the digital arteries. It gives off numerous un-named twigs to the skin, tendons, etc. Descending in the metatarsal region, there are other four arteries besides the vessel just described. They will be found, one at each side of the flexor tendons, in company with the vein and nerve, and another at each edge of the suspensory ligament, within the splint bone of the same side. All of these are branches of an arterial arch formed across the origin of the suspensory ligament from the back of the tarsus. The arch corresponds to the subcarpal arch of the anterior limb, and is formed as follows : — The Perforating Metatarsal Artery, the smaller branch resulting from the division of the anterior tibial artery, passes from the front to the back of the tarsus by the canal between the cuboid, scaphoid, and cuneiform bones. Here it unites with the outer and inner plantar divisions of the posterior tibial, which descend in the tarsal sheath, one on each side of the perforans tendon. Of the four vessels that spring from the arch thus formed, the two that descend with the plantar PLATE XIX Lumbricalis Flexor perforatus Gastrocnemius Lowest annular band, cut and reflected Flexor perforatus Flexor perforans Satellite art. of plantar nerve Int, metacarpal vein Digital art. Digital Coronary plexus Ant. tibial art. Peroneus Extensor pedis Flexor metatarsi Extensor brevis — Peroneus Extensor pedis Large metatarsal art. Suspensory ligament — Button ' ot splint-bone Slip from suspensory lig. to extensor pedis | -Perpendicular art. Drawn fc Printed Vy-W. kA.K. Johnston, Edinburgh & London METATARSUS AND DIGIT— Outer Aspect DISSECTION OF THE POSTERIOR LIMB. 79 nerves at the side of the flexor tendon are un-named and slender (Plate 19). The other two are termed the plantar interosseous metatarsal arteries. This may be regarded as the most typical arrangement of the arteries here, but it is not constant. Sometimes the inner plantar artery is directly continued as the satellite vessel of the internal plantar nerve in the metatarsus, the outer plantar artery alone uniting with the perforating metatarsal artery. The External Plantar Interosseous Artery is very slender. It descends, as beforesaid, between the outer splint bone and the edge of the suspensory ligament ; and above the fetlock it anastomoses with a recurrent twig from the large metatarsal artery. The Internal Plantar Interosseous Artery, a vessel of consider- able size, descends between the inner splint bone and the edge of the suspensory ligament. Above the lower extremity of that bone it inclines towards the middle of the limb to join the large metatarsal artery. It supplies the nutrient artery of the large metatarsal bone. The Digital Arteries (Plate 19). These arteries separate at an acute angle, in passing backwards between the branches of bifurcation of the suspensory ligament. For the remainder of their course they are identical with the homonymous vessels of the fore limb. For their description, turn to page 28. The Digital Veins (Plate 19). These are the satellites of the digital arteries, in front of which they ascend. They drain away the blood from the venous plexuses within the hoof ; and, uniting with one another above the fetlock, they form an arch between the deep flexor and the suspensory ligament. From this arch spring the metatarsal veins. The Metatarsal Veins are three in number : — 1. The Internal Metatarsal Vein ascends in front of the inner ed^e of the deep flexor tendon, in company with the internal plantar nerve and a slender artery. The vein is the most anterior of the three structures, and the slender artery is between the vein and the nerve. At the upper third of the metatarsus the vein deviates forwards, crossing the inner splint bone and the large metatarsal obliquely, to gain the inner side of the hock, above which it is continued as the anterior root of the internal saphena vein. The course of the vein over the hock is generally apparent in the living animal, and when very prominent it constitutes the so-called “blood-spavin.” 2. The External Metatarsal Vein (Plate 19) ascends on the inner edge of the deep flexor, having the same relationship to nerve and artery as the internal vein. After communicating with the deep vein, it is con- tinued through the tarsal sheath to become the posterior root of the internal saphena. 3. The Deep Metatarsal Vein ascends between the suspensory liga- ment and the large metatarsal bone ; and passing from the back to the 80 THE ANATOMY OF THE HORSE. front of the hock, by the vascular canal for the perforating metatarsal artery, it is continued as the anterior tibial vein. The Plantar Nerves. These nerves result from the bifurcation of the posterior tibial nerve when it gains the back of the tarsus. They accompany the perforans tendon in the tarsal sheath ; and diverging from one another, they descend in the metatarsal region, one at each side of the deep flexor tendon. Each is accompanied in the metatarsus by the metatarsal vein of that side, and by a slender artery from the vascular arch at the back of the tarsus. A little below the middle of the metatarsus the inner nerve detaches a considerable branch that winds obliquely downwards and outwards behind the flexor tendons to join the outer nerve above the level of the button of the splint bone. At the fetlock each nerve, coming into relation with the digital vessels, resolves itself into three branches for the supply of the digit. These are identical in their arrangement with the like branches of the plantar nerves in the fore limb, for the description of which, turn to page 30. The student must now pursue the dissection of the following muscles which have already been dissected in the leg, viz., the extensor pedis and peroneus on the front of the limb, and the superficial and deep flexors behind. In addition to these, there are the short extensor of the foot, the lumbricales, and the interossei, which entirely belong to this region ; and since they are of small size, and might easily be over- looked or injured, their dissection must be first undertaken. The Lumbricales (Plate 19) and Interossei Muscles. These exactly resemble the muscles of the same name in the anterior member. Turn, therefore, to the description of the latter given at page 31, substituting the word foot for handy toes for fingers , and metatarsal for metacarpal. The Short Extensor of the foot ( extensor brevis digitorum of man) (Plate 19). Look for this small muscle at the front of the tarsus, in the angle of union of the extensor pedis and peroneus tendons. It arises from the os calcis and astragalus, and is inserted into the united tendon of the above-mentioned muscles, to whose action it is auxiliary. The Extensor Pedis tendon (Plate 19) descends along the middle line of the limb in front, to be inserted into the pyramidal eminence of the os pedis. Above the middle of the metatarsus it receives on its outer side the tendon of the peroneus, and at the same point it is joined by the short extensor. A small synovial bursa is interposed between the tendon and the anterior ligament of the fetlock, but at the front of the interphalangeal joints the ligament supports directly the articular synovial membranes. At the middle of the first phalanx the tendon is joined on each side by a strong band that descends from the suspensory ligament. Action. — It extends in succession from below upwards the interphal- angeal joints and the fetlock, and finally it flexes the hock. DISSECTION OF THE POSTERIOR LIMB. 81 The Peroneus (Plate 1 9). The tendon of this muscle emerges from the thecal canal in the external lateral ligament of the tarsus, and joins the tendon of the last-described muscle about the middle of the metatarsus. Action . — The same as the preceding muscle. The Superficial Flexor (flexor pedis perforatus) (Plate 19). The tendon of this muscle, after playing over the os calcis, descends on the middle line of the limb to the back of the fetlock, where it forms a remarkable ring for the passage of the tendon of the deep flexor. Be- yond this point the tendon bifurcates, and each half is inserted into the upper extremity of the second phalanx, on its lateral aspect. In con- nection with the tendon of this and the next muscle there is developed an extensive synovial apparatus, termed the metatarso-phalangeal sheath, which exactly resembles the metacarpo-phalangeal sheath of the fore limb, described at page 34. Action . — It flexes successively the pastern and fetlock joints ; and, by its insertion into the os calcis, it is also an extensor of the hock-joint. It also mechanically maintains the hock in a state of extension as long as the hip and stifle joints are kept extended by their proper muscles. The Deep Flexor (flexor perforans) (Plate 19). The tendon of this muscle, after its passage through the tarsal sheath, descends between the suspensory ligament and the superficial flexor. At the fetlock it passes through the ring of the last-named muscle, descends behind the digit, plays over the navicular bone, and finally becomes inserted into the solar surface of the os pedis (see page 42). At the upper part of the metatarsus it receives the check band, or subtarsal ligament, which is analagous to the subcarpal ligament of the fore limb, but not so strong. Like the analagous structure in the fore limb, it is involved in sprain of the back tendons. A little lower the deep flexor is joined on its outer side by the tendon of the flexor accessorius. Action . — It flexes successively from below upwards the interphalangeal joints and the fetlock. Directions . — For the description of the foot, which is identical in the fore and hind limbs, turn to page 35. If the student has already dis- sected the foot in a fore limb, he may proceed at once to the articulations. THE STIFLE-JOINT (PLATES 17 AND 18). This corresponds to the knee-joint of man. It comprises — (1) the articu- lation between the patella and the femoral trochlea; and (2) the articula- tion between the condyles of the femur and the proximal end of the tibia. Directions . — The various structures in connection with the joint are to be examined in the order of the following description ; and in order to expose them, the muscles, fat, etc., are to be removed from around the joint, care being taken, in the first stage of the dissection, to pre- serve the thin femoro-patellar capsule intact. 82 THE ANATOMY OF THE HORSE. FEMORO-PATELLAR ARTICULATION. Movements. — This joint is commonly classified as an arthrodia. The movements (see page 43) of the patella on the trochlea, however, are not those of simple gliding, but of gliding with coaptation. In the latter movement, while the patella moves as a whole upwards or downwards, successive areas of its articular surface come into contact with the trochlea. These movements take place at the same time as the move- ments in the femoro-tibial articulation. In complete extension of that joint the patella lies at the upper part of the trochlea, and the three straight patellar ligaments are tense. When flexion takes place, these ligaments become relaxed, and the patella descends over the trochlea till it rests at its lower part. The ligaments of the joint are — one capsular, two lateral, and three straight. The Capsular Ligament is loose and membranous, and it supports the synovial membrane. It is attached, on the one hand, to the margin of the patellar articular surface, and, on the other, at the periphery of the trochlea. The Lateral Ligaments are two thin, riband-shaped bands, stretching, one on each side of the joint, from the femur to the patella. They serve to strengthen the capsular ligament, from which they are not distinct. The Straight Patellar Ligaments. These correspond to the single lig amentum patellae of the human knee. They are three in number, and are distinguished as external , middle , and internal. All three ligaments are attached superiorly to the anterior surface of the patella, the inner one having a fibro-cartilaginous thickening which extends the articular surface of the patella, and glides on the inner ridge of the femoral trochlea. The middle ligament lies on a deeper plane than the other two, and rests interiorly in the vertical groove on the anterior tuberosity of the tibia. It is inserted into the lower part of this groove, and a small synovial bursa is developed between the ligament and the bone above the point of insertion. The external and internal ligaments are inserted into the same tuberosity, one on each side of the attachment of the middle ligament. These three ligaments may be regarded as the terminal tendon of the quadriceps extensor cruris, whose action they transmit tc the bones of the leg. Synovial Membrane. This will be exposed by incising the capsular ligament. It lines the inner surface of that ligament, and extends upwards beyond the trochlea, forming a protrusion under the quad- riceps extensor cruris. Interiorly it is in contact with the synovial membranes of the femoro-tibial joint, and sometimes it communicates with them. It is a point worthy of notice in connection with the interior of the joint, that the inner ridge of the femoral trochlea is much higher than DISSECTION OF THE POSTERIOR LIMB. 83 the outer ; and when the patella is dislocated, it is carried outwards over the external ridge. THE FEMORO-TIBIAL ARTICULATION (FIG. 4 ). Movements. — This is a ginglymus, or hinge joint, in which the move- ments are principally flexion and extension. In extension the bones of the leg are carried forwards, but cannot be brought into a straight line Fig. 4. A. Femoro-tibial Ligaments, back view. 1. External lateral ligament ; 2. Internal lateral ligament ; 3. Inner semilunar fibro-cartilage ; 4. Outer semilunar fibro-cartilage, with 5, and 6, the femoral and tibial attachments (coronary ligaments) of its posterior extremity ; 7. Posterior crucial ligament ; 8. Anterior crucial ligament ; 9. Head of fibula. B. Anteroposterior Vertical Section of the Femoro-tibial Articulation to show the Crucial Ligaments. 1. The posterior crucial ligament ; 2. Anterior crucial ligament ; 3. External lateral ligament ; 4, 5, and 6, as in A. with the femur, the movement being arrested by tension of the lateral ligaments and of the anterior crucial ligament. The contrary movement, flexion, is finally arrested by tension of the posterior crucial ligament. A slight degree of lateral movement and rotation can be produced when the joint is flexed. Lateral Ligaments. — These are two strong fibrous cords, placed one on each side of the joint. The external is fixed above to the higher of the 84 THE ANATOMY OF THE HORSE. two pits on the external condyle of the femur, where it covers the origin of the popliteus from the lower pit. It descends over the external tuberosity of the tibia, a synovial bursa being interposed, and is inserted into the head of the fibula. The internal is longer, but more slender, than the preceding. It is fixed above to a small tubercle on the inner condyle, plays over the inner edge of the tibial articular surface, and is inserted into the internal tuberosity of the tibia. The Posterior Ligament is of a flattened, membranous character, and consists of a superficial and a deep layer, which are separable from each other superiorly, but blended below. Superiorly the ligament is attached to the posterior surface of the femur above the condjdes ; below it is inserted into the corresponding surface of the tibia, just below the margin of the articular surface ; while laterally its margins blend with the lateral ligaments. The superficial surface of the ligament is related to the popliteal vessels, and to the gastrocnemius, flexor perforatus, and popliteus muscles. Its deep face serves to support the synovial mem- branes of the joint, and is partly adherent to the semilunar cartilages and posterior crucial ligament. The ligament presents apertures for the transmission of vessels to the interior of the joint. Synovial Membranes. These are two in number, one for each condyle of the femur and corresponding part of the articular surface of the tibia. They are separated from each other by the crucial ligaments in the interior of the joint; while behind, and at the sides, they line the posterior and lateral ligaments. In front they are in contact with the synovial capsule of the femoro-patellar articulation, and are supported by a pad of fat, which separates them from the straight ligaments of the patella. A communication frequently exists in front between these synovial capsules and that for the gliding of the patella. These synovial membranes invest the semilunar cartilages ; and the external one covers, in addition, the tendon of origin of the popliteus, and the common tendon of origin of the flexor metatarsi and extensor of the digit. Directions . — The posterior ligament should now be cut away; and the patella being thrown down, the synovial membrane and fat should be removed from the front of the joint. The joint should then be strongly flexed, in order to expose, as far as possible, the crucial liga- ments in the intercondyloid groove. The rims of the semilunar cartilages and their coronary ligaments will at the same time be exposed. The Crucial Ligaments are two strong fibrous cords stretching between the femur and the tibia, and lodged in the intercondyloid groove. They cross one another somewhat like the limbs of the letter X, and hence their name. They are distinguished as anterior and posterior. The anterior , the most external of the two, is attached superiorly to the DISSECTION OF THE POSTERIOR LIMB. 85 intercondyloid groove, and to the external condyle of the femur where it bounds that groove. Its fibres have a slightly spiral arrangement, and extend downwards and forwards to be inserted into the summit of the tibial spine. The 'posterior ligament is longer than the anterior, and is fixed superiorly to the intercondyloid groove and inner condyle. It extends downwards and backwards to be fixed to a special tubercle on the back of the tibia below the rim of its articular surface. These two ligaments bind the femoral and tibial articular surfaces closely together, and at the same time restrict the movements of the joint, the anterior ligament being put upon the stretch during extension, and fin- ally arresting that movement, while the posterior ligament plays the same part with regard to flexion. The Inter-articular or Semilunar Fibro-cartilages. These are two crescentic or sickle-shaped pieces of fibro-cartilage, interposed between the condyles of the femur and the articular surface of the tibia. The convex margin of each is turned outwards, and is much thicker than the concave edge, which embraces the tibial spine, and is so thin as to be translucent. The lower surface of each is flattened to rest on the tibia, but the upper surface is hollowed to embrace the femoral condyle. They are fixed in position as follows : — The anterior extremity of the inner fibro-cartilage is fixed into an excavation in front of the tibial spine, while its posterior end is similarly fixed behind the spine. The outer cartilage is fixed by its anterior extremity in front of the spine, while its posterior extremity is bifid, having an upper slip inserted into a depression at the posterior part of the intercondyloid groove, and a lower into the rim of the tibial articular surface, partly under cover of the posterior interosseous ligament. These slips of insertion at the extremities of the fibro-cartilages are sometimes termed the coronary ligaments , three of which belong to the outer, and two to the inner, fibro-cartilage. Although these insertions serve to prevent the total displacement of the fibro-cartilages, some degree of movement is, never- theless, permitted to the latter; for it will be noticed, that during flexion they are, as it were, squeezed towards the front of the joint, while in extension they are carried backwards. Directions . — If the internal lateral ligament be now cut, and the internal condyle removed with the saw, a better view will be obtained of the crucial ligaments; after which, complete separation of the femur and tibia should be effected by cutting the remaining lateral ligament, the crucial ligaments, and the slip of insertion of the external fibro-cartilage at the back of the intercondyloid groove. This will expose thoroughly the semilunar fibro-cartilages. Tibio-fibular Articulation. In the horse the amount of movement permitted between the bones of the leg is very restricted, and not appreciable on the general movements of the limb. Where the head of 86 THE ANATOMY OF THE HORSE. the fibula is opposed to the rough diarthrodial facet on the external tuberosity at the upper end of the tibia, short and strong peripheral fibres pass between the two bones, and bind them closely together. An interosseous membrane extends across the tibio-fibular arch, and is perfor- ated by the anterior tibial vessels. Just above the aperture for the transmission of these vessels the fibres of the ligament are disposed in opposite directions, like the limbs of the letter X. Where the osseous substance of the fibula ceases, a fibrous cord begins, and this is carried downwards to the region of the external tuberosity at the lower end of the tibia, where it mixes its fibres with the external lateral ligament of the tibio-tarsal joint. THE TARSUS (FIG. 5). Several articulations are formed in the tarsus, or hock ; and these are of very unequal importance as regards the amount of movement permitted. The most important of them is that corresponding to the ankle-joint of man, which is formed between the astragalus and the lower extremity of the tibia ; and attention should first be given to the movements that take place here. This is one of the most typical ginglymoid joints in the body, the movements being limited to flexion and extension. It will be observed that in flexion the distal part of the limb does not move in the plane of the leg, but deviates a little out- wards, and that in extension the movement is arrested by tension of the lateral ligaments before the distal portion of the limb is brought into the same straight line as the leg. In the other articulations found in connection with the tarsus the movements are of a very restricted character, and are not concerned in the general movements of the limb. They, however, serve a no less important purpose in the joint, distributing and equalising pressure, and obviating the bad effects which concussion would have been likely to produce in the tarsus, had it been one rigid structure. Directions. — The ligaments of the tarsus are both numerous and com- plicated, and the best order of their dissection is that in which they are hereafter described. Since one set of ligaments must be removed in order to expose the following set, the dissector should not proceed with undue rapidity. Tibio-tarsal Ligaments. — These are four in number, viz., two lateral, an anterior, and a posterior. The External Lateral Ligament consists of a superficial and a deep fasciculus, which cross one another like the legs of the letter X. The superficial division, which is the larger of the two, is fixed superiorly to the posterior part of the external tuberosity at the lower end of the tibia, while interiorly its fibres are inserted into the astrag- alus, os calcis, cuboid, large metatarsal bone, and external small DISSECTION OF THE POSTERIOR LIMB. 87 metatarsal. It is perforated by the thecal canal for the passage of the peroneus tendon. The deep division of the ligament extends down- wards and backwards from its point of attachment to the forepart of the external tuberosity of the tibia, and it becomes inserted by distinct slips into the astragalus and os calcis. In order to expose it thoroughly, the superficial division should be cut at its point of attachment to the tibial tuberosity, and dissected downwards, the difference of direction serving to distinguish the fibres of the two divisions. The Internal Lateral Ligament is, like the preceding, a composite liga- ment, and consists of three divisions, which may be distinguished as Fig 5. A. — Ligaments of the Tarsus, front view. 1. Superficial fasciculus of the internal lateral ligament (cut) ; 2. Middle fasciculus of the same (two slips) ; 3. Deep fasciculus of the same ; 4. Superficial fasciculus of the external lateral liga- ment ; 5. Deep fasciculus of the same ; 6. Astragalo-metatarsal ligament ; 7. Canal for the perforat- ing metatarsal artery ; 8. Anterior cuboido-cunean ligament ; 9. Anterior cuboido-scaphoid liga- ment ; 10. Cuboid insertion of the flexor metatarsi. B. — Ligaments of the Tarsus, back view. 1. External lateral ligament ; 2. Internal lateral ligament ; 3. Tarso-metatarsal ligament ; 4. Fibro-cartilaginous thickening of the posterior ligament. 5. Calcaneo-metatarsal£ligament ; Subtarsal ligament, or check-band to perforans tendon ; 7. Suspensory ligament. v . superficial, middle, and deep. The superficial division, the largest of the three, is fixed, on the one hand, to the internal tuberosity at the lower end of the tibia, and, on the other, to the astragalus, scaphoid, 88 THE ANATOMY OF THE HORSE. large and small cuneiforms, and large and internal small metatarsal bones. The middle division is of intermediate size ; and in order to expose it, the superficial division must be cut, and dissected downwards. Above it is attached to the internal tuberosity of the tibia ; and, passing downwards and backwards, it is inserted by distinct slips into the astragalus and os calcis. The deep division is very slender, and stretches between the tibia and the astragalus, under cover of the middle fasciculus, which must be removed in order to expose it. The Anterior Ligament is membranous and four-sided. It is fixed above to the tibia ; and below to the astragalus, scaphoid, cuneiform magnum, and astragalo-metatarsal ligament ; while on each side it blends with the lateral ligament. The posterior surface of the ligament is lined by the synovial membrane of the joint. The anterior surface is related to the anterior tibial vessels, and to the flexor metatarsi and extensor pedis tendons. Towards its inner side the ligament is unsupported ; and hence, when the synovial membrane becomes dropsical, the distension shows at that point, constituting a “ bog- spavin. ” The Posterior Ligament is of a similar form to the preceding. It is fixed above to the tibia, below to the astragalus and os calcis, and at the sides to the lateral ligaments. Its anterior surface supports the synovial membrane of the joint ; while the posterior is lined by the synovial membrane of the tarsal sheath, and presents a fibro-cartila- ginous thickening where the perforans tendon plays over it. This tendon affords support to the posterior ligament, which therefore does not bulge so readily as the anterior ligament ; but in a case of extreme distension of the synovial membrane, the swelling shows itself at the back of the joint. The Synovial Membrane is supported by the anterior, posterior, and lateral ligaments ; and it communicates with the synovial membrane that lubricates the articulations between the os calcis and the astragalus on the one hand, and the cuboid and scaphoid on the other. It also sometimes supplies the two upper facets between the os calcis and astragalus. Directions. — The anterior and posterior ligaments should be incised in order to expose the synovial membrane ; and, thereafter, these and the lateral ligaments should be cut away. This will effect the separa- tion of the tibia ; and the next group of ligaments may then be examined. The following ligaments can hardly be classified as belonging specially to any one articulation or set of articulations. For the most part they bind together the series of tarsal bones, and also serve to bind these to the metatarsal bones. The Astragalo-metatarsal Ligament . — This is a flat, radiating ligament, DISSECTION OF THE POSTERIOR LIMB. 89 situated on the inner side of the tarsus. Its fibres are attached above to the tubercle on the inner side of the astragalus ; and, widening as it descends, it becomes inserted into the scaphoid, cuneiform magnum, and large metatarsal bone. The Calcaneo-metatarsal or Calccmeo-cuboid Ligament. — This is a strong, cord-like ligament, situated at the outer side of the back of the hock, and attached to the posterior border of the os calcis, the cuboid, and the head of the external small metatarsal bone. The Tarso-metatarsal Ligament will be seen covering the tarsal bones at the back of the hock. It forms a thick mass of fibrous tissue intimately adherent to these bones and to the heads of the metatarsal bones. Its inner border is blended with the lateral ligament of the tibio-tarsal joint ; and its outer with the calcaneo-metatarsal ligament. Below it is continued as the subtar sal ligament , which joins the per- forans tendon. The anterior face of the ligament, where not adherent to the bones, is lined by synovial membrane ; and its posterior face is similarly lined by the synovial membrane of the tarsal sheath. Directions . — At the front of the hock the point of a scalpel should be introduced into the articulation between the astragalus and the scaphoid ; and by cutting round the hock through the three ligaments just described, an attempt should be made to separate the astragalus and os calcis, as a single piece, from the rest of the tarsal bones. Before this can be effected, however, there must be cut an interosseous ligament , which is composed of short and strong fibres passing between the os calcis and astragalus on the one hand, and the cuboid and scaphoid on the other. At the same time the synovial membrane belonging to the articulations between these two sets of bones will be opened into. This capsule communicates in front with that of the tibio-tarsal joint, and “is prolonged superiorly between the calcis and astragalus, to lubricate two of the facets by which these bones come into contact. In addition, it descends between the cuboid and scaphoid bones, to form a prolongation for the anterior cuboido-scaphoid arthrodia.” — Chauveau. Ligaments uniting the Os Calcis and Astragalus. — There are four of these — a superior, two lateral , and an interosseous. The first of these is composed of fibres passing between the two bones above their surfaces of contact ; the lateral ligaments pass between them on each side ; while the interosseous ligament cannot be seen in its entirety, as it passes between the rough impressions on the surfaces of apposition of the bones, and must be cut before these can be separated. Directions. — Attention should next be turned to the following liga- ments, which bind together the other four tarsal bones. The Anterior Cuboido-scaphoid Ligament is of small size, and passes between the two bones from which it is named, above the entrance to the canal by which the perforating metatarsal artery passes through the 90 THE ANATOMY OF THE HORSE. hock. The same bones are joined by an interosseous ligament , which forms the roof of that canal. The Anterior Cuboido-cunean Ligament connects the cuboid and cunei- form magnum bones below the entrance to the above-mentioned vas- cular canal ; and an interosseous cuboido-cunean ligament forms the floor of the canal. The Scaphoido-cunean Interosseous Ligament joins the scaphoid and two cuneiform bones. The Intercunean Ligament passes between the two cuneiforms. These and the other interosseous ligaments are concealed in the interstices between the different bones which they bind together, and cannot be fully seen until the bones are separated. Synovial Membranes. — “There is a proper synovial membrane for the facets by which the scaphoid and cuneiform magnum bones corre- spond ; this synovial membrane belongs also to the two cuboido- scaphoid, and posterior cuboido-cunean arthrodiae. The anterior cuboido-scaphoid diarthrosis receives a prolongation from the synovial membrane between the os calcis and astragalus on the one hand, and the cuboid and scaphoid on the other. The play of the anterior cuboido-cunean, and inter-cunean facets is facilitated by two prolongations of the tarso-metatarsal synovial membrane.” — Chauveau. THE TARSO-METATARSAL ARTICULATION. An Interosseous Ligament binds the heads of the metatarsal bones to the tarsal bones with which these articulate, and the union is further secured by many of the ligaments, already dissected, which, though they belong to the hock, have points of insertion into the heads of the metatarsal bones. Synovial Membrane. — This not only supplies the tarso-metatarsal joint, but also ascends between the two cuneiforms, and into the anterior facet between the cuboid and cuneiform magnum. It also descends into the articulations between the large and small metatarsal bones. Directions . — For a description of the remaining joints of the hind limb (except the hip), turn to the description of the corresponding articula- tions of the fore limb (page 50). The hip-joint is described with the pelvis, at page 338. DISSECTION OF THE POSTERIOR LIMB. 91 TABULAR VIEW OF THE MUSCLES IN THEIR ACTION ON THE JOINTS OF THE HIND LIMB. Hip. /Sartorius. | Pectineus. I Psoas magnus. < Iliacus. j Tensor vaginae femoris. I Rectus femoris. \Rectus parvus (?) / Semimembranosus. ) Quadratus femoris. ) Middle gluteus. V Obturator externus. / Superficial gluteus, j Biceps femoris (anterior half), j Middle gluteus. V Deep gluteus. ( Sartorius. Gracilis. Adductor magnus. Semimembranosus. Pectineus. Adductor parvus, f Sartorius. ( Gracilis. /Deep gluteus. I Adductor parvus. I Quadratus femoris. J Psoas magnus. Rotators outwards s Iliacus. ( Obturator externus. Obturator internus. Pyriformis. Gemelli. Stifle. / Biceps femoris (posterior half). ■< Semitendinosus. ( Popliteus. / Vastus internus. ) Vastus externus. ^ Rectus femoris. V Biceps femoris (anterior half). Rotator outwards — Biceps femoris (posterior half). ( Semitendinosus. ( Popliteus. Hock. Flexors . Extensors Abductors Adductors Rotators inwards Flexors Extensors Rotators inwards Flexors Flexors . Flexors . Flexors . { Extensor pedis. Peroneus. Flexor metatarsi. Extensors . Fetlock. ( Flexor perforatus. ■\ Flexor perforans. Extensors . ( Flexor accessorius. Pastern. { Flexor perforatus. Flexor perforans. Extensors . Flexor accessorius. Coffin-joint. Flexor perforans. Flexor accessorius. Extensors . / Gastrocnemius. 1 Soleus. ■I Flexor perforatus. j Flexor perforans. 'Flexor accessorius. ( Extensor pedis, s Peroneus. ( Extensor brevis. { Extensor pedis. Peroneus. Extensor brevis. { Extensor pedis. Peroneus. Extensor brevis. 92 THE ANATOMY OF THE HORSE. SI 2 J - - - © eg © a> Ph Ph Ph Ph Ph pt< fsq 5 ns CQ • Sh O s a.§ O 2 2 eg J r-5 rs | P g < eg > S S 5 o o • 0.5p=P2 2 „ ^ eg eg O ?H ?H ^ r ce 2 2 c cj M s s 1 1 © ® a a , — -Ph txi i-h i_ ( bC‘c w S_ ® £ =3 * ' O “ • bo bo's -2 nS .. 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At the lower extremity of the parotid it is joined by a large branch — the submaxillary vein ; and it then passes into the jugular furrow, in which it descends to the entrance to the chest. The upper boundary of the furrow, it will now be observed, is formed by the mastoido-humeralis, and the lower by the sterno-maxillaris. In the upper half of this groove the vein rests on the subscapulo-hyoideus muscle, which there separates the vessel from the carotid artery ; but in the lower half the vein rests on the side of the trachea, and is in direct contact with the carotid, which lies above and slightly internal to it. The jugular of the left side differs from the right in being related, in the lower part of the groove, to the oesophagus as well as the trachea. At the entrance to the chest the right and left jugulars unite with one another and with the axillary veins, thus forming the initial portion of the anterior vena cava. The jugular receives, in the part of its course now exposed, the following branches : — 1. The Submaxillary or Facial vein, which joins the jugular at an acute angle in which lies the inferior extremity of the parotid gland. 2. The Thyroid vein, bringing blood from the thyroid body and larynx. 3. Innominate cutaneous , muscular, oesophageal, and tracheal branches, whose disposition is not constant. 4. The Cephalic vein, which enters the jugular near its termination. [The single jugular of the horse is generally said to be the representative of the external jugular of man ; the internal jugular, under that view, being undeveloped in the soliped. This I believe to be a mistake, and for the following reasons. The external jugular of man runs on the surface of the platysma (panniculus), and never beneath it as does the vein of the horse ; moreover, it is a vessel of very variable volume, being frequently small, and sometimes absent. On the other hand, the cervical part of the internal jugular of man has a situation exactly corresponding to that of the horse’s vein, save that the latter vessel is generally superficially placed to the suhscapulo-hyoid (omo-hyoid) ; and in the horse I have seen the jugular, otherwise normal, pass under that muscle, keeping com- pany with the carotid artery for the whole of its course.] The Sterno-maxillaris (Plate 27). This muscle corresponds to the inner portion of the sterno-mastoid of man. It arises from the carini- form cartilage of the sternum, and is inserted by a flat tendon into the angle of the inferior maxilla. In the lower half of the neck the muscle lies below the trachea, and covers the sterno-thyro-hyoideus muscle. In this position the right and left muscles are in contact, but about the middle of the neck they diverge, and cross obliquely upwards and forwards over the trachea and the subscapulo-hyoideus muscle. Its terminal tendon is included between the parotid and submaxillary glands. The upper edge of the muscle forms the lower boundary of the jugular furrow. In its lower part the muscle is thick and rounded, but it becomes more slender and flattened as it is traced upwards. L. 146 THE ANATOMY OF THE HORSE. Action. — To depress (flex) the head or give it a lateral inclination, according as the right and left muscles act singly or in concert. The Sterno-thyro-hyoideus (Plates 27 and 28). This is a long and slender muscle, extending along the lower face of the trachea, and closely applied along the middle line to its fellow of the opposite side. It takes origin from the cariniform cartilage of the sternum. About the middle of the neck its muscular substance is interrupted by a short tendinous portion, rendering the muscle digastric. Above this central tendon the muscle divides into two portions. The outer or thyroid band passes obliquely outwards and forwards between the trachea and the sub- scapulo-hyoideus muscle, and becomes inserted into the edge of the thyroid cartilage of the larynx. The inner or hyoid band is continued directly forwards in company with the corresponding branch of the opposite muscle, and becomes inserted into the body of the hyoid bone. Action — To depress the hyoid bone and larynx. The Subscapulo-hyoideus ( Omo-hyoid of man) (Plates 27 and 28). This is a thin, ribbon-shaped muscle having a breadth of three or four inches. It takes origin at the inner side of the scapula, from the fascia covering the subscapularis muscle. It then passes downwards and for- wards between the scalenus and rectus capitus anticus major muscles inwardly; and the supraspinatus, anterior deep pectoral, and mastoido- humeralis muscles outwardly. Appearing at the lower edge of the last-named muscle, to which it adheres closely, it passes between the jugular vein and carotid artery ; and crossing over the upper part of the trachea in a direction obliquely forwards and downwards, it applies itself at the outer edge of the hyoid band of the sterno-thyro-hyoideus, and becomes inserted along with that muscle into the body of the hyoid bone. In the lower part of the neck the ascending branch of the inferior cervical artery and the prescapular group of lymphatic glands are included between this muscle and the mastoido-humeralis. Action. — To depress the hyoid bone. Nerves. At the upper part of the neck a branch from the spinal accessory nerve enters the sterno-maxillaris, and branches from the 1st spinal nerve enter the sterno-thyro-hyoid and subscapulo-hyoid muscles. These, however, will be better dissected at a later stage. Directions. — The jugular vein should now be ligatured at the upper and lower ends of the jugular furrow, and the intermediate portion of the vessel should be cut away. The excised portion of the vein should be laid open to expose its valves. The part of the subscapulo-hyoideus which passes over the trachea may be cut out after the manner of Plate 28, and the sterno-maxillaris may be similarly treated. This will expose for examination the trachea, the oesophagus, the carotid artery, the pneumogastric and sympathetic nerves, and the recurrent nerve. Valves of Veins. Three or four valves are placed in the jugular vein. DISSECTION OF THE HEAD AND NECK. 147 Each valve is composed of two or three semilunar folds of the inner coat of the vein, the folds having a close resemblance to the semilunar seg- ments of the aortic valve (Fig. 1 1, page 129). Each flap with the adjacent part of the wall of the vein forms a small pouch with its mouth directed towards the heart. When the blood tends to regurgitate, it distends these pouches until the segments meet across the vessel and thus arrest the backward current. In most veins throughout the body similar valves are found; but they are most numerous in the veins of the limbs. In the small veins each valve may be composed of only a single flap. The following veins have few or no valves : — the pulmonary system of veins, the veins of the portal system, the hepatic veins, the anterior and posterior venae cavae, and the veins of the brain. The Trachea, or wind-pipe (Plate 28), begins beneath the altanto-axial articulation, where it is continuous with the larynx. It descends in the middle plane of the neck, beneath the spinal column ; and passing between the first two ribs, it gains the thorax, where it bifurcates to form the bronchi. In the neck the muscles of the region envelop the trachea, and are related to it as follows : — The longus colli is related to its upper aspect, the sterno-thyro-hyoideus extends along its lower face, the sterno-maxillaris crosses its direction obliquely upwards and forwards, the subscapulo-hyoideus crosses it obliquely downwards and forwards, and at the lower part of the neck it contacts on each side with the scalenus. It is also related to the oesophagus, the carotid artery, the jugular vein, and the pneumogastric, sympathetic, and recurrent nerves. The Thyroid Body or Gland (Plate 29) is related to the upper part of the trachea on each side, resting on its first four rings. The gland has a rounded form, and a reddish-brown colour; and it is richly supplied with blood, which it receives from the thyroid and thyro- laryngeal branches of the carotid artery. Sometimes a narrow isthmus connects the right and left glands across the lower face of the trachea. The gland has an investing capsule of fibrous connective-tissue, continuous with an internal trabecular framework. Under the microscope the sub- stance of the organ is seen to contain numerous spherical spaces, each lined by a single layer of epithelium, and filled by a viscid colloid material. The (Esophagus, or gullet (Plate 28), is a segment of the alimentary canal. It begins above the larynx, where it is continuous with the pharynx. It descends on the upper face of the trachea, and in the first few inches of its course it lies in the middle plane of the neck, being related to the longus colli muscle above. It soon, however, begins to deviate to the left side, so that below the middle of the neck it lies rather on the upper part of the left side of the wind-pipe. Maintaining this relationship, the two tubes enter the thorax in company, the gullet being prolonged through that cavity to pass by the foramen sinis- trum of the diaphragm into the abdomen, where it terminates in the 148 THE ANATOMY OF THE HORSE. stomach. The gullet is related to the muscles of the left side already- enumerated as contacting with the trachea, the sterno-thyro-hyoideus excepted. It is also related to the carotid artery, the jugular vein, and the pneumogastric, sympathetic, and recurrent nerves of the left side. The cervical part of the oesophagus has the external appearance of a volun- tary muscle, for which it is often mistaken at first sight by the student. The examination of its structure, as well as that of the trachea, must be postponed until the accompanying vessels and nerves have been examined. The Common Carotid Artery (Plate 28). This is the vessel that conveys the blood to the head and upper part of the neck. It begins on the under aspect of the trachea, at the entrance to the thorax, where it results from the bifurcation of a short vessel termed the cephalic trunk — a branch of the arteria innominata. It ascends in the neck in company with the trachea, and terminates above the cricoid cartilage of the larynx by dividing into the external carotid, the internal carotid, and the occipital artery. It thus crosses the trachea very obliquely, being at first on its under surface, then on its lateral aspect, and finally above it. It is in contact with the scalenus, longus colli, rectus capitis anticus major, and subscapulo-hyoideus muscles, the last- mentioned intervening between the artery and the jugular vein in the upper half of the neck. In the lower half of the neck the artery and vein are in direct contact, the carotid being above and slightly internal to the jugular. The common cord of the pneumogastric and sympathetic nerves is on the upper side of the artery, and the inferior laryngeal (recurrent) nerve is below it. At the entrance to the thorax the pre- pectoral group of lymphatic glands is in contact with the artery. The 1 left carotid differs from the right in being related for a considerable j] part of its course to the oesophagus, which separates it from the trachea. L The collateral branches of the carotid are as follows : — 1. Innominate and slender muscular , oesophageal , and tracheal branches. 2. The Thyroid artery, which arises a few inches behind the thyroid body, and passes obliquely forwards to enter the gland on its posterior aspect. Sometimes this artery is distributed mainly or entirely to the neighbouring muscles. 3. The Thyro-laryngeal artery. — This is the largest of its collateral branches. It arises a little in front of the preceding vessel, and passing to the inner side of the thyroid body, it divides in front of it into thyroid and laryngeal branches. The former turn back to enter the gland in front, while the latter pass to the larynx and pharynx. The terminal branches of the carotid are not to be followed at present. The Pneumogastric and Sympathetic Nerves in the neck (Plate 28). The pneumogastric, vagus, or 10th cranial nerve has its origin from PLATE XXVIII & Printed "by IT. &.A.K. Johnston. Edinburgh & London DISSECTION OF THE HEAD AND NECK. 149 the medulla oblongata. It leaves the cranium by the po^tarior par$ of the foramen lacerum, and inclining downwards and backwards on the guttural pouch, it meets the cervical cord of the sym^thetic, with which it becomes in nearly all cases closely united. The ^ohunoji cOrd resulting from the fusion of the two nerves descends in company wit'll the carotid artery, lying on the upper side of that vessel. At thp lower part of the neck the two nerves, in passing into the thora^^aain become separate. 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DISSECTION OF THE LARYNX. The larynx is a short tube forming the upper part of the windpipe. It is, however, not merely a part of the respiratory apparatus, but is also the organ of voice. It possesses a framework of cartilages, which are movably articulated together, and connected by ligaments or membranes. These cartilages are moved by muscles, some of which pass between the different cartilages and constitute an intrinsic group, while others pass between the cartilages and extraneous parts, and con- stitute an extrinsic group. Directions . — Provided the dissection has to be carried out on one larynx, the study of the muscles must precede that of the cartilages. When another larynx can be procured, it is more advantageous to reverse this order, removing the muscles from the first larynx in order to study the cartilages and their mode of union, and then using the other for the examination of the muscles and remaining structures. Even when the first method has to be followed, it is advisable, before proceeding to dissect the muscles, to read the description of the cartilages, which is therefore here put first. CARTILAGES OF THE LARYNX. These are five in number, viz., the cricoid, thyroid, and epiglottis, which are single ; and the pair of arytenoid cartilages. In man there are two additional pairs — two cornicula laryngis and two cuneiform cartilages. In the horse the first of these are amalgamated with the tips of the arytenoids, while the cuneiform cartilages are small, shot-like bodies included in the aryteno-epiglottic fold of mucous membrane. In the natural position of the animal at rest, the long axis of the larynx is oblique upwards and forwards. For convenience of description, however, we may assume it to be vertically placed, as indeed it is when the head and neck are extended (elevated) to the fullest degree. In this position the cartilages are related to one another as follows : — The cricoid is the lowest, and is connected to the first ring of the trachea. The thyroid is placed above this, and bounds the tube of the larynx in front and at the sides. The arytenoids surmount the cricoid behind, and the epiglottis is superposed to the thyroid in front of the upper aperture of the tube. DISSECTION OF THE LARYNX. 225 The Cricoid Cartilage has the form of a finger ring, from which it receives its name. The depth of the ring is greatest behind, where it presents a portion comparable to the bezel, or part of a ring in which the stone is set. The inner surface of the ring is smooth, and lined by the laryngeal mucous membrane. The outer surface of the bezel is divided by a vertical median ridge which increases the surface of origin of the posterior crico-arytenoid muscle. Towards the outer limit of this surface there will be seen on each side a little cavity which is smooth for articulation with the thyroid cartilage. The inferior border is notched in the middle line of the bezel, and is connected by ligament to the first ring of the trachea. The upper border has a wide notch in front ; and posteriorly, over the bezel, it shows a pair of smooth convex facets for articulation with the arytenoid cartilages. In texture the cricoid is composed of hyaline cartilage. The Thyroid Cartilage receives its name from covering the front and sides of the larynx like a shield. It consists of a median thickened portion, or body ; and two lateral plates — the alee, or wings. The body is known in human anatomy as “Adam’s apple” — the pomum Adami. The epiglottis is superposed to it, the two cartilages being united by elastic fibres. On each side it is continuous with the wings. Each ala is a rhomboidal plate of cartilage. The outer surface is slightly convex, and the inner is correspondingly concave. The upper edge of the cartilage is attached to the thyroid cornu of the hyoid bone by the thyro-hyoid membrane. The lower edge slightly overhangs the cricoid, and receives the insertion of the crico-thyroid muscle. The other two edges are directed obliquely, one backwards and upwards, the other forwards and downwards. The first of these receives the insertion of the palato- pharyngeus muscle ; the other, in receding from the corresponding edge of the opposite wing, leaves beneath the body a triangular gap which is occupied by the crico-thyroid membrane. Of the four angles of each plate, three demand mention. The supero-anterior angle is acute, and joins the ala to the body of the thyroid. The supero-posterior angle is obtuse, and carries a small bar of cartilage — the superior cornu of man — which is articulated to the extremity of the thyroid cornu of the hyoid bone. Close to the base of this process the plate is perforated by a foramen for the passage of the superior laryngeal nerve. The postero- inferior angle is acute like the first, to which it is diagonally opposite. It is drawn out a little, forming a projection — the inferior cornu of man — which is terminated by a convex facet for articulation with the cricoid. The thyroid is composed of hyaline cartilage. The Arytenoid Cartilages. These stand at the upper aperture of the larynx like the mouth of a pitcher, and from this resemblance they are named. They are irregular in shape, but each bears some resemblance to a three-sided pyramid. The inner surface of the pyramid is covered .Q 226 THE ANATOMY OF THE HORSE. by laryngeal mucous membrane ; the outer surface receives the insertion of the thyro-arytenoid muscle ; the posterior surface is covered by the arytenoid muscle. The base of the cartilage possesses within its area a smooth, depressed facet for articulation with the cricoid. Two of the angles of the base require particular notice, viz., the anterior angle, which is pointed, and projects horizontally forwards to receive the insertion of the vocal cord ; and the postero-external angle, which is thick and rounded, and receives the insertion of the crico-arytenoid muscles. The apex is directed upwards, and is prolonged by a slender piece of yellow fibro- cartilage representing the cornicula laryngis of man. This curves back- wards and inwards ; and with the corresponding process of the opposite side forms, behind the upper aperture of the larynx, the pitcher- like lip. Except in the apical prolongation, which is composed of yellow or elastic fibro-cartilage, the texture of the arytenoid is hyaline cartilage. The Epiglottis is shaped like an ovate, pointed leaf. Its anterior surface is concave in the vertical direction, and convex from side to side. Near the base it receives on the middle line the insertion of the hyo- epiglottideus muscle. The posterior surface has the converse configura- tion, and presents numerous pits in which are lodged mucous glands. The borders of the cartilage are convex, and they are free above, but below they are enveloped by the aryteno-epiglottic folds of mucous mem- brane. The apex is pointed, and curved forwards in the upright position of the cartilage. The base of the cartilage is expanded, and rests on the body of the thyroid. From each side of it an irregular bar of cartilage projects horizontally backwards. The epiglottis is composed of yellow fibro-cartilage. ARTICULATIONS, LIGAMENTS, AND MEMBRANES OF THE LARYNX. Mode of Union with the Hyoid Bone. The larynx is suspended to the base of the skull through the intervention of the hyoid bone, the tip of the thyroid cornu (heel process) of that bone being connected by liga- mentous fibres (without a synovial membrane) to the so-called superior cornu at the supero-posterior angle of the thyroid ala. The connection between the hyoid bone and the larynx is further maintained by the thyro-hyoid membrane , which is attached, on the one hand, to the body and thyroid cornua of the hyoid, and, on the other, to the body and upper edge of each wing of the thyroid cartilage. Mode of Union with the Trachea. The lower edge of the cricoid cartilage is connected to the first ring of the trachea by a fibro-elastic membrane — the crico-tracheal ligament . Union of the Cricoid and Thyroid Cartilages. The postero-inferior angle, or inferior cornu, of each thyroid ala is articulated to the concave facet on the bezel of the cricoid in a diarthrodial joint, provided with a DISSECTION OF THE LARYNX. 227 small capsular ligament , and lined by a synovial sac. The two cartilages are further united by the crico-thyroid membrane. This is a fibro-elastic structure consisting of a central and two lateral portions. The central portion is triangular and fills up the space between the adjacent edges of the right and left thyroid alse. It is attached by its sides to these edges, while by its base it is inserted into the upper border of the cricoid. Each lateral portion lies under cover of the laryngeal mucous membrane, and is shaped somewhat like a quadrant, having an inferior convex edge fixed to the margin of the cricoid in company with the central portion, an anterior edge confounded with the central portion, and an upper straight edge which is thin and free on the side of the larynx. This upper edge is the true vocal cord ; and since its fibres are attached in front to the angle of union of the thyroid alse, and posteriorly to the projecting anterior angle of the base of the arytenoid, it is also termed the thyro-arytenoid ligament. Vocal sounds are produced by the vibra- tion of the vocal cords. Movements. — The movements between the cricoid and thyroid cartil- ages take place around an imaginary horizontal axis passing through the right and left crico-thyroid joints, and in these movements either cartil- age may be supposed to remain fixed while the other revolves around the axis. It should be observed that these movements vary the distance between the angle of junction of the thyroid alse and the base of the arytenoids, and thus vary the tension of the true vocal cords, which stretch between these points. Union of the Cricoid and Arytenoid Cartilages. Each arytenoid cartilage is articulated by the concave facet on its base to one of the convex facets on the upper edge of the cricoid bezel. It is a diarthrodial joint, possessing a capsular ligament and a synovial sac. Movements . — The arytenoid cartilage swings like a door, around a vertical axis passing through the crico-arytenoid joint. When the cartilage is swung outwards, the true vocal cord, which is attached to the anterior angle of its base, is separated from the cord of the opposite side, and the glottis is widened. The glottis is narrowed by the opposite movement. Union of the Thyroid and Epiglottis. These cartilages are united by elastic fibres passing between them, and forming a kind of amphi- arthrosis. Movements. — Except during the act of deglutition, the epiglottis stands erect in front of the upper aperture of the larynx. During that act the cartilage is bent downwards and backwards so as to cover the aperture like a lid. This movement, however, is executed not exclusively at the joint between the two cartilages, but partly by a bending of the whole cartilage. At the close of the act of deglutition the epiglottis assumes the erect position, owing to its own elastic texture and the 228 THE ANATOMY OF THE HORSE. elastic fibres connecting it to the thyroid ; but in the horse this action is assisted by the hyo-epiglottideus muscle. THE MUSCLES OF THE LARYNX. Extrinsic Group. — This includes the sterno-thyroid, the thyro-hyoid, and the hyo-epiglottideus. The last of these is a single muscle ; the other two are double. The Sterno-thyroid Muscle (Fig. 26). See page 146. The Thyro-hyoid Muscle (Figs. 26 and 27). This is a dark-coloured, fleshy muscle taking origin from the thyroid cornu (heel process) of the hyoid bone, and inserted into an oblique line on the outer surface of the thyroid wing. Action. — Acting alone, the thyro-hyoid muscles would elevate the larynx between the thyroid cornua of the hyoid bone ; but when they act in concert with the sterno-thyroid, the thyroid cartilage will be steadied, and will serve as the fixed point for the crico-thyroid and thyro- arytenoid muscles. The Hyo-epiglottideus Muscle (Fig. 27) takes origin from the upper face of the body of the hyoid bone ; and passing back- wards in the middle line, it is inserted into the anterior surface of the epiglottis at its lower part. Its fibres are mixed with a quan- tity of fatty-elastic tissue. Action. — To assist the natural elasticity of the epiglottis in restoring the cartilage to the erect position at the close of the act of deglutition. Intrinsic Group . — This includes four pairs of muscles, viz., the crico-thyroid, the thyro-arytenoid, the posterior crico-arytenoid, and the lateral crico-arytenoid; and a single muscle — the arytenoid- eus. The Crico-thyroid Muscle (Fig. 26) arises from the side of the cricoid cartilage ; and its fibres, passing obliquely upwards and back- wards, are inserted into the lower edge of the thyroid wing. Action. — This muscle acts on the crico-thyroid joint, increasing the tension of the vocal cord by increasing the distance between the fore- part of the thyroid and the base of the arytenoid cartilage. In this Fig. 26. Larynx, Side View. 1. Glossal Process of Hyoid ; 2. Small Cornu ; 3. Great Cornu ; 4. Arytenoid Cartilage^ Thyjro- Hyoideus ; 6. Insertion of Sterno -Thyroid ; 7. Crico- Thyroideus ; $. Crico-arytenoid eus Posticus ; 9. 1st Ring of Trachea ; 10. Thyroid Body. ' DISSECTION OF THE LARYNX. 229 action either the cricoid or thyroid attachment may be the fixed point of the fibres. Directions . — The thyro-arytenoid and lateral crico-arytenoid muscles lie under cover of the thyroid wing, which must therefore be removed on one side. This is to be done by removing the thyro-hyoid and crico- thyroid muscles, disarticulating the crico-thyroid joint, and incising the ala a little behind the body of the thyroid, after the manner of Fig. 27. The Thyro-arytenoid Muscle (Fig. 27) consists of two parallel bundles, between which the mucous membrane of the ventricle of the larynx protrudes as a pouch. Its fibres arise from the inner surface of the thyroid wing near its junction with the body, and from the crico- thyroid membrane. The lower fibres are inserted into the outer surface of the arytenoid cartilage, while its higher fibres join those of the arytenoideus muscle. Action . — The muscle is antagonistic to the crico-thyroid, dimin- ishing the tension of the vocal cord by acting on the crico-thyroid joint. The Posterior Crico-ary- tenoid Muscle (Figs. 26 and 28). This is the most powerful of the intrinsic muscles. Its muscular tissue is dark red, and mixed with tendinous tissue. Its fibres take origin from the outer surface of the cricoid bezel, and are inserted into the prominent tubercle on the external angle of the arytenoid cartilage. Action . — To swing out- wards the arytenoid carti- lage, and thus to separate the vocal cords and dilate the glottis. The Lateral Crico-ary- tenoid Muscle (Fig. 27). This muscle is placed below the thyro-arytenoid, under concealment of the thyroid wing. Its fibres arise from the upper border of the side of the cricoid cartilage; and passing backwards and upwards, they become inserted into the same tubercle on the base of the arytenoid as the posterior muscle, and into the outer surface of the arytenoid in front of that tubercle. Fig. 27 . Larynx, Side View (thyroid ala removed). 1. Glossal Process of Hyoid ; 2. Cut Base of Thyroid Cornu ; 3. Small Cornu ; 4. Great Cornu ; 5. Epi glottis : 6. Arytenoid Cartilage; T^Jiit Wing of .Thyroid Carti- lage ; 8. Facet on Cricoid for Articulation wrETT Thyroid Cartilage ; 9. Pouch of Mucous Membrane from Ven- tricle of Larynx ; 10. and 11. Upper and Lower Bundles of Thyro -Arytenoideus ; 12. Crico-Arytenoideus Lateralis; 13. Crico-Arytenoideus Posticus ; 14. Tliyro-Hyoideus ; 15. Hyo-Epiglottideus ; 16. Thyroid Body ; 17. 1st Ring of Trachea. 230 THE ANATOMY OF THE HORSE. Action . — The muscle acts on the crico-arytenoid joint in a manner antagonistic to the preceding muscle, approximating the vocal cords and narrowing the glottis by swinging the arytenoid cartilage inwards. The Arytenoideus Muscle (Fig. 28). This may be regarded either as a single muscle, or as a double muscle whose right and left fibres meet at a median raphe. Its fibres are inserted on each side into the pos- terior surface of the arytenoid cartilage, and superiorly it is joined by the higher fibres of the thyro-arytenoid muscle. Action . — To approximate the right and left arytenoid cartilages, and thus narrow the glottis. * NERVES OF THE LARYNX. nerves are distributed to the -the superior and inferior laryn- Two larynx- geal nerves. The latter is also known as the recurrent nerve, and both are branches of the vagus, or 10th cranial nerve. The Superior Laryngeal Nerve has its origin described at page 194. It gives motor filaments to the crico-thyroid and crico-pharyngeus muscles ; and then pene- trating the thyroid wing by the foramen near its supero-posterior angle, the nerve splits into sensory branches distributed to the mucous membrane of the larynx, giving also twigs to the lining of the pharynx and oesophagus. The Inferior Laryngeal (Recurrent) Nerve has its origin and course described at page 149. It is the motor nerve to all the intrinsic muscles except the crico-thyroid, and it also gives some sensory twigs to the laryngeal mucous membrane. Fig. 28. Larynx, Back View. 1. Epiglottis ; 2. Arytenoid Carti- lage ; 3. Thyroid Cartilage ; 4. Ary- tenoideus ; 5. Crico- Arytenoideus Pos- ticus ; J5. Cricoid Cart ilage ; 7. 1st Ring of Trachea ; 8. Thyroid'Body. INTERIOR OF THE LARYNX. Directions . — A vertical incision should be made along the middle line of the larynx behind, severing the arytenoideus muscle and the bezel of the cricoid cartilage. By separating the lips of this incision, a view of the interior of the larynx from behind will be obtained, and this is to be supplemented by looking into the tube from its upper and lower apertures. The Superior Aperture of the larynx is a large orifice placed at the floor of the pharynx. It is bounded in front by the epiglottis, behind DISSECTION OF THE LARYNX. 231 by the pitcher-like lip of the arytenoid cartilages and the fold of mucous membrane uniting them, and laterally by the aryteno-epiglottic fold of mucous membrane. During degluti- tion the epiglottis is folded over the aperture, which it closes like a lid. The Lower Aperture is circum- scribed by the inferior edge of the cricoid cartilage, and is directly con- tinued by the lumen of the trachea. The Glottis, or Rima Glottidis. This is a third aperture, placed about the middle of the tube of the larynx, which it divides into an upper and a lower compartment. In its anterior two-thirds this opening lies between the right and left vocal cords, and in its posterior third it lies between the bases of the arytenoid cartilages. The size of the aperture is varied by the movements executed in the crico- arytenoid joints, as already seen ; and its form varies with its size. It can be completely closed by the apposition of its margins in the mesial plane. When it is only slightly opened, it is Interior of the Larynx, seen from behind a slit-like ancero-posterior aperture, 1. Epiglott is : 2. Aryte noid cartilage; 2 1 . widest at the centre ; when moderately *■£“ open, as in easy respiration, it has the s \^ tricle ofthe lar * nx ’ 6 - Sub-epiglottic form of an elongated isosceles triangle with the base behind ; when dilated to the fullest extent, it is lozenge- shaped. The Ventricles, or Sinuses, of the larynx. Each of these is a recess, or cavity, placed on the side of the larynx. The entrance to it lies above the vocal cord, whose free straight edge, covered by mucous membrane, forms the lower margin. The upper margin is formed by a concave fold of mucous membrane, containing in man a few fibres designated the false vocal cord. The cavity of the ventricle descends to the outer side of the true vocal cord, and a pouch of the mucous lining of the cavity passes out between the upper and lower divisions of the thyro-arytenoid muscle. The Sub-epiglottic Sinus is a depression beneath the base of the epiglottis, and provided with a lunated fold of mucous membrane. The Sub-arytenoid Sinus is a depression beneath the cri co-arytenoid joints. Fig. 29 . 232 THE ANATOMY OF THE HORSE. Mucous Membrane of the Larynx. This, which is continuous with the lining of the pharynx and trachea, is of a pale colour. It forms the aryteno-epiglottic folds, and lines the ventricle of the larynx. It is provided with numerous mucous glands. Its free surface is covered by an epithelium, which is ciliated except over the vocal cords and around the superior aperture, in which positions it is stratified and squamous. CHAPTER VI. DISSECTION OF THE BRAIN, OR ENCEPHALON. Directions . — The removal of the brain of the horse from its contain- ing cavity is a somewhat difficult operation, in consequence of the thickness of the cranial bones. Supposing the head of an animal recently killed to have been procured for the special purpose, the first steps are the disarticulation of the jaw on both sides, and the removal of the inferior maxilla. Next denude the cranial bones of the muscles and other soft structures, and with the saw remove on each side the zygomatic arch, the supraorbital process of the frontal, and the styloid process of the occipital. Estimating the thickness of the last-named bone at the poll, as much as possible of it may be sawn off without actually encroaching on the cranial cavity. Armed with a chisel, mallet, and strong bone-forceps, the student must now remove as much of the cranial wall as will enable him to extract the brain ; and he may do this by removing either the roof or the floor of the cavity. The first method is the speedier, but the latter has the advantage of permitting the roots of the cranial nerves, the pituitary body, and the cranial vessels to be better preserved. The dura mater is to be left as far as possible intact, but its attachments along the inter- frontal and interparietal sutures, and to the oblique ridge between the cerebral and cerebellar divisions of the cranial cavity, must be cut with the scalpel. When the forepart of the cavity is reached, the handle of the scalpel is to be used to scoop the olfactory bulbs out of the fossse in which they lie. The brain having been removed in its membranes, it should be laid with its base upwards on a broad strip of calico, and lowered into a vessel of methylated spirit or a ten per cent, solution of nitric acid in water. After a week’s immersion, it will be ready for examination. MEMBRANES, OR MENINGES, OF THE BRAIN. The brain, like the spinal cord, is surrounded by three envelopes : the dura mater, the arachnoid, and the pia mater. The Dura Mater is the external of these envelopes. It is a strong fibrous membrane, similar in structure to the spinal dura mater, with which it is continuous at the foramen magnum. It differs, however, from the same envelope of the spinal cord, in that it is closely adherent 234 THE ANATOMY OF THE HORSE. to the inner surface of the cranial bones, and forms for them an internal periosteum. All over its outer surface it is connected by slender fibrous processes and vessels to the bones ; but it is particularly adherent to these along the lines of the sutures, and at the margins of foramina. The meningeal vessels ramify on the outer surface of the membrane, and leave their impressions on the inner surface of the cranial bones. Sometimes the outer surface of the dura mater, on each side of the middle line above, shows numbers of granular processes — the Pacchion- ian bodies , which are developed from the subjacent arachnoid. Occa- sionally they are large enough to cause the partial absorption of the bones over them. The inner surface of the dura mater is smooth, in virtue of an endothelial layer representing the parietal layer of the arachnoid. This inner surface is closely applied to the brain contained within the other two membranes ; and along certain lines it detaches processes which pass inwards, and form partial partitions between the different divisions of the encephalon. These processes are : the falx cerebri and the tentorium cerebelli. The Falx Cerebri is a vertical, mesial, sickle-shaped process which dips in between the two hemispheres of the cerebrum. The convex upper edge of the process is attached to the cristagalli process, and to the inter- frontal and interparietal sutures. The concave lower edge is thin and lace-like, and rests free on the corpus callosum. The short posterior edge, or base, is straight, and is attached to the intracranial projection of the interparietal bone. The Tentorium Cerebelli is a vaulted partition extending transversely between the cerebrum and the cerebellum. In outline it is crescentic, having a superior convex, and an inferior concave, border. The former is attached on the middle line to the intracranial projection of the inter- parietal bone, and on each side of that its attachment descends obliquely forwards and downwards along the crest formed by the parietal and petrous temporal bones. The concave edge is free, and arches over the crura cerebri. The anterior surface of the membrane is convex, and the posterior ends of the cerebral hemispheres rest on it. The posterior surface is concave, and is in contact with the cerebellum. The Sinuses of the Dura Mater. — These are venous passages formed by the splitting of the dura mater. They are as follows : — The Superior Longitudinal Sinus is of considerable size, and is found in the falx cerebri at its attached or convex edge. Beginning at the crista galli process, it becomes larger as it passes backwards, and it terminates at the intracranial projection of the interparietal bone. The Inferior Longitudinal Sinus is small and inconstant. It extends along the free or concave edge of the falx ; and after receiving the veins of Galen, it is continued backwards in the tentorium cerebelli, terminating at the same point as the preceding sinus. DISSECTION OF THE BRAIN, OR ENCEPHALON. 235 Where the two foregoing sinuses meet, they form the whirlpool of Herophilus ( torcular Herophili), from which the blood is drained away by the transverse sinuses. The Transverse Sinuses pass right and left at the periphery of the tentorium cerebelli, and enter the parieto-temporal conduit. In that canal each is continued as the parieto-temporal confluent, from which the blood is drained away by the roots of the temporal veins. The Cavernous Sinuses. Each of these is placed in the dura mater at the side of the sella turcica of the sphenoid bone. Anteriorly each receives the ophthalmic vein, and posteriorly the right and left sinuses become continuous behind the pituitary gland. The venous arch which they thus form discharges its blood through the foramen lacerum basis cranii into the sub-sphenoidal confluent. The internal carotid artery traverses the cavernous sinus, and forms while in it a sigmoid curve. The Petrosal Sinuses are small, and pass in the tentorium cerebelli on each side, between the transverse and cavernous sinuses. The Occipital Sinuses. These are placed in or external to the dura mater lining the cerebellar division of the cranial cavity. They are continuous through the foramen magnum with the spinal sinuses, and their contained blood is drained away by a large vein that passes through the condyloid foramen to join the occipital vein. The Meningeal Arteries. These are derived from the meningeal branch of the ophthalmic artery , which enters the forepart of the cavity at the internal orbital foramen ; and from the great meningeal or spheno-spinous branch of the internal maxillary. The spheno-spinous artery enters by the foramen lacerum basis cranii, and, after detaching meningeal branches, enters the parieto-temporal conduit to anastomose with the mastoid artery. Some meningeal twigs are also furnished by the prevertebral branch of the occipital artery (page 191). The Meningeal Nerves. Filaments from the 4th, 5th, 9th, and 10th cranial nerves, and from the sympathetic, are said to have been traced to the dura mater. The Arachnoid. This, like the same membrane of the spinal cord, is a delicate transparent membrane. In structure and disposition it is comparable to a serous membrane. Its parietal layer is represented by the endothelial lining of the dura mater its visceral layer invests the brain and pia mater; and the parietal and visceral portions together enclose a space, which is the arachnoid cavity , or subdural space. The free surface of the membrane bounding this space is smooth and moist like a serous membrane. Between the visceral arachnoid and the pia mater another space is left, which is termed the subarachnoid space. This space is most evident over the intervals between the cerebral convolu- tions, and over surface depressions at the base of the brain, for at these points the arachnoid does not dip down to line the hollows, but bridges 236 THE ANATOMY OF THE HORSE. them over. The space is continuous with the same space in the spinal meninges, and contains the limpid cerebrospinal fluid. The Pia Mater, This is the vascular membrane of the brain. It consists of delicate areolar tissue and bloodvessels. It invests the brain closely, following all its surface irregularities. Behind the cerebral hemispheres it sends towards the interior of the cerebrum a wide process — the velum inter positum ; and where the cerebellum is superposed to the medulla oblongata, it forms on each side a thickened granular cord — the choroid plexus of the Jflh ventricle. These will be exposed at a later stage. ARTERIES OF THE BRAIN (FIG. 30 ). Three vessels are concerned in supplying blood to the encephalon, viz., the basilar, internal carotid, and ophthalmic arteries. The Arteries of the Brain. 1. Anterior branch of cerebro-spinal artery ; 2. Basilar artery ; 3, 3. Irregular branches to medulla and cerebellum ; 4. Posterior cerebellar arteries ; 5. Bifurcation of the basilar ; 6. Anterior cere- bellar arteries ; 7. Posterior cerebral arteries (more numerous and smaller than usual) ; 8. Internal carotid ; 9. Posterior communicating branch ; 10. Anterior branch of internal carotid, which divides to form 11 and 12 — the middle and anterior cerebral arteries ; 13. Single vessel formed by the union of 11 and 12, disappearing into great longitudinal fissure ; A. Medulla oblongata ; B. Pons Varolii ; C. Cerebellum ; D. Crus cerebri ; E. Corpus albicans ; F. Optic commissure ; G. Olfactory bulb ; H. Cerebral hemisphere. The Basilar Artery is formed on the middle line of the lower face of the medulla oblongata, by the union of two vessels. These are the DISSECTION OF THE BRAIN, OR ENCEPHALON. 237 anterior divisions of the right and left cerebro-spinal arteries, whose posterior divisions unite in the same manner to form the middle spinal artery. The basilar artery passes forwards in the median groove of the medulla, and crosses the pons, in front of which it bifurcates to form the posterior cerebral arteries. In its course the basilar artery detaches on each side, besides numerous vessels to the medulla and pons, the posterior cerebellar arteries. The Posterior Cerebellar Arteries are two in number, a right and left. They are detached at different levels from the basilar, behind the pons ; and they turn round the medulla to reach the cerebellum. The Posterior Cerebral Arteries diverge from each other in the inter- peduncular space ; and after being connected together by a short transverse branch of considerable volume, and by numerous smaller reticulate twigs, they are joined by the posterior communicating branch of the internal carotid. Each then turns outwards over the crus cerebri to gain the choroid plexus and the posterior part of the cerebral hemisphere. Behind the point at which the vessels are connected by the short transverse branch, they give off the anterior cerebellar arteries. Sometimes, as in Fig. 30, the posterior cerebral artery, instead of turning outwards as a single vessel, detaches from its outer side two or three branches which wind round the crus. The Anterior Cerebellar Arteries are variable in number and disposi- tion, and may arise as branches of the basilar artery. Generally there are two or three on each side, and they turn backwards and outwards over the crus cerebri to gain the front of the cerebellum. The Internal Carotid Artery. This vessel begins above the cricoid cartilage of the larynx, as one of the terminal branches of the common carotid. It passes upwards and forwards to the foramen lacerum basis cranii, being sustained in a fold of the guttural pouch, and accompanied by some nervous branches from the superior cervical ganglion of the sympathetic. Piercing the sub-sphenoidal sinus, it passes through the foramen into the cavernous sinus, within which it forms a sigmoid curve. It then leaves the sinus, and gaining the deep face of the dura mater, it divides at the margin of the sella turcica of the sphenoid bone into an anterior and a posterior branch. The latter, termed the 'posterior communicating artery , is reflected backwards to join the posterior cerebral artery. The anterior branch passes forwards, and at the outer side of the optic commissure divides into the middle and anterior cerebral arteries. The Middle Cerebral Artery passes outwards across the hemisphere, in the fissure of Sylvius. The Anterior Cerebral Artery unites in the mesial plane, above the optic commissure, with the corresponding vessel of the opposite side. The single vessel thus formed receives the meningeal branch of the 238 THE ANATOMY OF THE HORSE. ophthalmic artery, and turns round the anterior end of the corpus callo- sum to gain the great longitudinal fissure. Here it separates into a right and a left branch, each of which passes backwards along the flat face of the hemisphere. By the anastomosis of the two anterior cerebral arteries in front, and the junction of the posterior communicating artery on each side with the posterior cerebral, which results from the bifurcation of the basilar artery, a vascular circle is established around the pituitary body. This is termed the Circle of Willis, and its object is to keep up a free blood supply to the cerebrum, even should there be an obstruction in one of the main vessels forming the circle. Moreover, the internal carotid arteries of opposite sides are, before they divide, connected by a large transverse branch which further contributes to the freedom of the circulation. The Ophthalmic Artery is a collateral branch of the internal maxil- lary. It enters the cranial cavity from the orbit by the internal orbital foramen, along with the nasal branch of the ophthalmic nerve, and divides into meningeal and nasal branches. The Meningeal Branches of opposite sides give off branches to the dura mater, and then unite to form a single trunk which joins the middle cerebral arteries. The Nasal Branch passes through the cribriform plate to gain the nasal chamber. The Sympathetic Nerve. Two branches from the superior cervical ganglion accompany the internal carotid artery, and anastomose around it to form the carotid plexus. Within the cavernous sinus they form another plexus — the cavernous plexus. From these plexuses filaments pass to join the 3rd, 4th, 6th, and ophthalmic cranial nerves. A twig also joins the large superficial petrosal nerve from the 7th, to form the vidian nerve ; another passes to the lenticular ganglion, either separately or with the ophthalmic nerve ; and some filaments pass to the Gasserian ganglion. * The Brain , or Encephalon , consists of four principal parts, viz., the medulla oblongata, the pons Varolii, the cerebellum, and the cerebrum. The medulla is the division which is in direct continuity behind with the spinal cord. The pons projects as a thick transverse bar, or ridge, in front of the medulla. The cerebellum is superposed to both medulla and pons. The cerebrum lies in front of the other three segments, and is larger than these taken together. The weight of the whole brain in an average-sized horse is about twenty-three ounces. THE MEDULLA OBLONGATA, OR BULB (PLATES 35 AND 36). The medulla oblongata is continuous at the foramen magnum with the spinal cord, of which it appears to be the expanded anterior termination. DISSECTION OF THE BRAIN, OR ENCEPHALON. 239 It rests by its inferior face on the basilar process of the occipital bone ; and its superior face, which is concealed by the cerebellum, is depressed and forms the floor of the 4th ventricle. Its anterior extremity is limited by the pons Yarolii, and is its widest part. The middle line of the medulla above and below is traversed by lines which continue forwards the superior and inferior median fissures of the cord. The medulla is composed of both white and grey nerve matter. The former occurs at the exposed surface of the medulla, and its nerve fibres are for the most part longitudinal in direction, and are collected into tracts, or bundles. Thus, lying at each side of the inferior median fissure of the organ, there is a tract termed the inferior pyramid. To the outer side of this again, and isolated from it by a faint longitudinal groove, is a tract occupying the position of the olivary fasciculus and olivary body of human anatomy. More externally placed than the last, and forming a thick cord at each side of the medulla, is the restiform body ; while above the restiform body, and nearer the superior median fissure, is a more slender column of fibres termed the superior pyramid. The line of separation between the two last-mentioned tracts is very faint, and in the horse there is seldom or never any surface line of demar- cation between the restiform body and the olivary fasciculus. Where the medulla joins the cord, the inferior pyramids become narrow, and the inferior median fissure shallow or nearly obliterated ; and at that point there is a visible crossing of fibres from one side to the other, constituting the decussation of the pyramids. Towards the posterior part of the medulla its lateral aspect is crossed by superficial curved fibres — the arciform fibres , and immediately behind the pons Yarolii there is a band of transverse fibres termed the trapezium. Within the medulla some fibres pass across the median plane and connect its right and left halves. The grey matter of the medulla oblongata occurs in considerable amount at the floor of the 4th ventricle, where it will subsequently be exposed. Course of the Longitudinal Fibres of the Medulla Oblongata. The inferior pyramid is in part composed of fibres from the inferior column of the same side of the cord, but principally of fibres crossing from the opposite side of the cord at the decussation. These decus- sating fibres are furnished mainly by the lateral column, but partly also from the superior column. The fibres of the inferior pyramid are continued through the pons to the cerebrum. The olivary fasciculus of fibres is derived from the inferior column of the cord on the same side, and it is continued through the pons to the cerebrum. The restiform body derives its fibres from all three columns of the cord on the same side, but in greatest proportions from the superior 240 THE ANATOMY OF THE HORSE. column. It enters the cerebellum, of which it forms the posterior peduncle. The superior pyramid derives its fibres from the innermost part of the superior column of the cord on the same side, and its fibres are continued through the pons to the cerebrum. The medulla oblongata shows the superficial origin of the last seven cranial nerves. THE PONS VAROLII (PLATE 33). The pons Varolii rests on the basilar process, in front of the medulla oblongata. In front of it the crura cerebri appear. Its inferior face is convex in both directions, and has a faint median furrow. The superior face forms the anterior part of the floor of the 4th ventricle. Its extremities are curved upwards to enter the cere- bellum, of which they form the middle peduncles. The pons consists of Avhite and grey nerve matter. The nerve fibres of the white matter are arranged in two sets — a transverse and a longitudinal. The transverse fibres consist of the surface fibres of the pons, and of deeper fibres separated from these by the longitudinal set. It is these transverse fibres that curve upwards at either extremity of the pons to enter the cerebellum as its middle peduncle, and they accordingly play the part of a commissure to the right and left halves of the cerebellum. The longitudinal fibres are the forward continuation of the longitudinal fibres of the medulla oblongata, minus the restiform bodies. In front of the pons these longitudinal fibres are continued as the crura cerebri. The grey matter of the pons occurs within its substance, and at the floor of the 4th ventricle. To a group of pigmented nerve cells in the latter position, the term locus cceruleus is applied. The pons shows the superficial origin of the 5th cranial nerve, by two distinct roots springing from its lateral aspect. THE CEREBELLUM (PLATES 34 AND 35). The cerebellum is superposed to the medulla and pons, and lies under the supra-occipital division of the occipital bone. The tentorium cerebelli arches downwards in front of it, and isolates it from the posterior extremities of the cerebral hemispheres. It is traversed in the antero-posterior direction by two shallow grooves, which divide it into a middle and two lateral lobes. The middle lobe is the smallest, and is known as the vermiform lobe. When followed forwards, the vermiform lobe is seen to be reflected round the anterior aspect of the cerebellum to gain its lower surface at the roof of the 4th ventricle ; and it terminates by a blunt end about the middle of this surface. When followed posteriorly, the vermiform lobe behaves in the same way, terminating at the roof of the 4th ventricle by a blunt end opposed to the first. These reflected DISSECTION OF THE BRAIN, OR ENCEPHALON. 241 portions have a distinct resemblance to two caterpillars, and they may be distinguished as the anterior and 'posterior vermiform processes. The anterior vermiform process is adherent to the valve of Vieussens. Each lateral lobe is joined on its inferior aspect by three bundles of nerve fibres, which are termed the peduncles. The posterior peduncle is the termination of the restiform body, the middle peduncle is the reflected extremity of the pons, and the anterior peduncle passes forwards beneath the corpora quadrigemina. Besides the grooves which divide the cerebellum into its lobes, numerous smaller fissures occur over its surface, and divide the lobes into folia , or leaflets. The arrangement of these leaflets will be made much more evident by making an antero-posterior vertical section, at or near the mesial plane of the organ. The peduncles are to be cut as they enter the lower face of the lateral lobe, and the anterior vermiform process is to be carefully separated from the valve of Vieussens with the scalpel. This will enable one half or a little more of the cerebellum to be removed after the manner of Plate 35. The cerebellum contains both grey and white matter. The white matter forms a large mass in the interior, and from this mass large plates are given off towards the surface. From these primary plates proceed more numerous smaller secondary plates, and these again detach small terminal plates which end in the surface folia. In consequence of this disposition of the white matter, it presents on vertical section a strikingly arborescent appearance, to which the term arbor vitce is applied. The nerve fibres of the white matter are for the most part directly continuous with the peduncles; but some are proper to the organ, and connect different points of the grey matter. The grey matter of the cerebellum is spread over its surface, and also forms two independent masses within the central mass of white matter. These latter have the form of a corrugated capsule, and each is placed a little to one side of the mesial plane, and is known as the corpus dentatum of the cerebellum. The surface layer of grey matter invests the core of white matter within each leaflet, and also extends across the bottom of the fissures between adjacent leaflets. It consists of two strata : an outer grey layer , and an inner rust-coloured layer. The Fourth Ventricle (Plates 35 and 36). This is a space between the cerebellum above, and the medulla and pons below. Its boundaries are as follows : — Its floor is formed by the medulla and pons ; its roof by the valve of Vieussens, the under suface of the vermiform lobe, and the reflection of pia mater from the medulla to the cerebellum ; laterally it is bounded in its anterior third by the anterior peduncle of the cere- bellum, and in its posterior two-thirds by the restiform body. The widest part of the space is at the point where the peduncles enter the cerebellum, and it contracts towards both extremities. At the posterior . R 242 THE ANATOMY OF THE HORSE. extremity there is a minute hole, which is the entrance to the short tube that continues the central canal of the cord into the posterior end of the medulla. The pointed posterior end of the space is the calamus scriptorius of human anatomy, so named from its resemblance to a writing pen. The anterior end of the space lies under the valve of Vieussens, and leads into the aqueduct of Sylvius, which is a canal tunnelled beneath the corpora quadrigemina, and opening anteriorly into the 3rd ventricle. The floor of the cavity is traversed by a longitudinal mesial furrow, and it shows the grey matter of the medulla and pons. The cavity is lined by a ciliated epithelium, and it communicates by one or more minute apertures in its floor with the sub-arachnoid space. On each side of the cavity, between the cerebellum and the restiform body, there is a thickened piece of pia mater — the choroid plexus of the Jfth ventricle. The Valve of Vieussens is a delicate, translucent fold, placed at the anterior part of the roof of the 4th ventricle. The lateral edges of the valve are fixed to the anterior cerebellar peduncles, its anterior edge is attached behind the testes, and its posterior edge stretches across the anterior vermiform process. The upper face of the valve is adherent to the anterior vermiform process, and its lower face is free and forms the anterior part of the roof of the 4th ventricle. The 4th nerve arises in the valve, close behind the testes, the right and left nerves appearing continuous with one another across the middle line. THE CEREBRUM. Under the term cerebrum are included all the parts of the encepha- lon except the medulla, pons, and cerebellum. It forms a mass larger than these taken together, although the amount by which it ex- ceeds them is much less in the horse than in man. The inferior aspect of the cerebral mass is termed its base, and the student should begin by examining the objects to be seen there (Plate 33). The Crura Cerebri are two thick, round, white cords, which appear in front of the pons. At this point they are close together ; but as they proceed forwards, they diverge and form the posterior boundaries of a lozenge-shaped area — the interpeduncular space , which is completed in front by the optic tracts and commissure. Anteriorly each crus disappears into the cerebral hemisphere, but its point of termination is concealed by the optic tract. The crus is composed of a superficial and a deep layer of nerve fibres with an intermediate thin stratum of grey matter. The superficial layer of fibres is known as the crusta, and the deep is termed the tegmentum. The fibres of both layers are continuous posteriorly with the longitudinal fibres of the pons ; and they are trans- mitted in front to the optic thalami, corpora striata, and grey matter of the hemisphere. The corpora quadrigemina, which are superposed PLATE XXXIII Great longitudinal fissure between hemispheres of cerebrum Olfactory bulb Olfactory Peduncle Infundibulum, >tic (2nd) nerve Tuber cinereui ,Optic chiasma Optic tract Pituitary Pissure of Sylvius J Corpus albicans .3rd nerve Pons Tarini ' th nerve Portio intermedia Crus cerebri 8th nerve Great oblique fissure Pons Varolii 10th nerve Trapezium Lateral lobe of cerebellum 12th nerve -Medulla oblongata Inf. pyramid Decussation of Drawn & Printed ojV. &.A.K. Johnnnn. EAmkmrgh fe London BRAIN— Inferior Aspect DISSECTION OF THE BRAIN, OR ENCEPHALON. 243 to the crura, also receive some fibres. The grey matter of the crus contains nerve cells with dark pigment, and is therefore termed the locus niger. The crura cerebri show the superficial origin of the 3rd pair of nerves. The Optic Tracts. These are two white cords of nerve fibres which turn round the crura cerebri, and pass forwards and inwards to meet in the middle line and form by their fusion the optic commissure or chiasma. This commissure rests on the sphenoid bone, in front of the pituitary fossa ; and in front it gives off the diverging optic or 2nd nerves. The optic tracts form the anterior boundary of the interpeduncular space. The Pons Tarini is the grey matter in the posterior angle of the interpeduncular space. It is also known as the locus perforatus posticus, from its being penetrated by numerous vessels. The Corpus Albicans is a pea-like, white nodule placed on the middle line, about the centre of the interpeduncular space. As will subse- quently be learned, the body is formed by the reflection of the anterior pillars of the fornix. The Tuber Cinereum is a layer of grey matter between the corpus albicans and the optic commissure. It is perforated in its centre, and connected to the upper surface of the pituitary gland by a hollow tube of grey matter — the infundibulum. The Pituitary Body is a reddish-yellow, disc-shaped body, having a diameter about equal to that of a sixpence. It is thickest in its centre and thinnest at its rim. Its lower face rests on the sella turcica of the sphenoid bone ; and its upper face receives the insertion of the infundi- bulum, and covers the tuber cinereum, and, in part, the corpus albicans and optic commissure. Within its structure it comprises cells resembling those of the blood-vascular or ductless glands, and others that resemble nerve cells. In the foetus it is proportionally larger, and contains a cavity which communicates with the 3rd ventricle through the infundi- bulum. The pons Tarini, corpus albicans, and tuber cinereum form the floor of the 3rd ventricle , a cavity which the dissector will hereafter expose by working from the upper aspect of the cerebrum. The Lamina Cinerea, or Lamina Terminalis, is a thin, delicate layer of grey matter which is placed above and in front of the optic commis- sure. It is the anterior boundary of the 3rd ventricle. The Locus Perforatus Anticus is a spot of grey matter at each side of the optic commissure, penetrated by numerous vessels for the corpus striatum, which lies above the spot. The Fissure of Sylvius, is a faint and ill-defined groove which begins at the locus perforatus anticus, and extends outwards across the hemi- sphere. The Great Longitudinal Fissure. In front of the optic chiasma the 244 THE ANATOMY OF THE HORSE. cerebral mass is seen to be mesially divided by the great longitudinal fissure. This fissure, as will be better seen when the brain is viewed from above, is a great vertical mesial cleft extending the whole length of the cerebrum, which it partially divides into right and left halves, or hemispheres. The Olfactory Bulbs. The olfactory bulb is the white body situated at the anterior end of the hemisphere. It occupies the olfactory fossa at the forepart of the cranial cavity; and unless special care is taken in the removal of the brain, the bulb is apt to be separated from the hemi- sphere and left in that fossa. From the free surface of the bulb the delicate filaments of the olfactory (1st cranial) nerve pass through the cribriform plate of the ethmoid bone, and enter the nasal chamber. The bulb is hollow, having a central cavity that is in communication with the anterior cornu of the lateral ventricle. The Olfactory Peduncle is a short, thick, white cord immediately behind the bulb, and in direct continuity with it. The hemisphere is slightly depressed over the peduncle, the depression being termed the olfactory fissure. The peduncle divides posteriorly into the olfactory tracts. The Olfactory Tracts. These are two white diverging bands — an inner and an outer — that continue the olfactory peduncle backwards. The internal tract (inner olfactory root) is short, and passes backwards and inwards to the edge of the great longitudinal fissure. The external tract (outer olfactory root) is a much longer band which curves out- wards and backwards across the fissure of Sylvius, and then encircles outwardly the uncinate and hippocampal convolutions, to reach the tentorial aspect of the hemisphere, on which it is lost. At the fissure of Sylvius the tract seems to lose some of its fibres in front of the unci- nate convolution, and behind that point it becomes grey on its surface. Behind the angle of divergence of the olfactory tracts is a smooth and slightly convex area — the quadrilateral space of Paul Broca. The surface layer of this space consists of grey matter constituting the middle or grey olfactory root; and, according to Broca, it covers white fibres that connect the olfactory bulb to the crus cerebri and to the anterior cere- bral commissure (anterior white commissure of 3rd ventricle). If now the olfactory peduncle be raised from the olfactory fissure, it will be seen to be connected to the frontal lobe of the hemisphere by a lamina termed by Broca the superior olfactory root. This lamina is grey on its surface and white beneath, and if it be ruptured the cavity of the bulb and the communication between that cavity and the anterior cornu of the lateral ventricle will be brought into view. External to the outer olfactory tract, each hemisphere shows numerous winding worm-like ridges, termed convolutions , and internal to the pos- terior half of the same tract there is seen a thick ridge — the hippo- PLATE XXXIV oblongata Lateral fissure sat oblique ;ure Lateral fissure oblique fissure Great longitudinal fissure between hemispheres of cerebrum Lateral lobe of cerebellum lobe of cerebellum Crucial Drawn & Printed 'by'W. &A.K Johnston. Edinburgh & London BRAIN— Superior Aspect DISSECTION OF THE BRAIN, OR ENCEPHALON. 245 campcil convolution — which terminates behind the fissure of Sylvius in a nipple-like eminence — the uncinate convolution (mastoid lobule, or mammillary eminence). These will presently be more particularly described. Directions. — The student must now reverse the position of the brain, laying it with its base downwards, while he proceeds to examine its upper aspect. The Great Longitudinal Fissure is now seen in its entirety. It extends from the anterior to the posterior end of the cerebrum, and appears to completely separate the right and left hemispheres. In the natural state the fissure is occupied by the falx cerebri. Gently separate the contiguous margins of the hemispheres, so as to widen out the fissure. Except towards the hinder end of the fissure, this proceed- ing requires no dissection, but at that point the hemispheres are united on the middle. It is, however, a mere adhesion through the medium of pia mater. Separate the hemispheres here by traction, or by cutting carefully in the mesial plane. There will now be exposed (Fig. 31) a white body — the corpus callosum — which connects the hemispheres at the bottom of the great longitudinal fissure. At the same time there will be brought into view the opposed inner surfaces of the hemispheres. The Cerebral Convolutions. In his examination of the base of the brain, and more clearly now, the student will have observed that the surface of the hemisphere is not smooth, but traversed by numerous winding worm-like elevations. These are termed the cerebral convolu- tions or gyri; and the intermediate grooves or fissures are technically termed sulci. At first sight it might be supposed, as indeed was believed until a comparatively recent date, that the disposition of these convolutions is quite irregular and hap-hazard. Observation has shown, however, that such is far from being the case, and that the convolutions have a nearly, if not altogether constant, arrangement. In the human subject, indeed, the surface of the hemisphere has been accurately mapped, and each convolution named. In the brain of the horse the plan of these convo- lutions appears to be as uniform as in man ; and although, perhaps, the convolutions are not absolutely identical in any two brains, or even in the two hemispheres of the same brain, still the irregularities are so slight as to permit one to describe with considerable minuteness what might be termed a common plan. The mapping of the surface of the hemispheres derives its chief interest and utility from the discovery that definite areas are associated with particular functions, in such a way that when these areas are destroyed or injured there follows total loss or dis- turbance of these functions, and that in some cases the exercise of par- ticular functions can be brought about by applying stimuli to particular spots of the cerebral cortex. 246 THE ANATOMY OF THE HORSE. In the brain of man the hemisphere is primarily subdivided into five lobes, viz., frontal, parietal, occipital, temporo-sphenoidal, and central, the last being also known as the Island of Reil, or the Insula. The lines of separation between these lobes are certain well-marked fissures, distinguished from the sulci in general by their greater depth and constancy. In each lobe, again, the secondary sulci form the lines of separation between a definite number of convolutions. In the third edition of Professor Chauveau’s admirable work ( Traite d’Anatomie com - parec des Animaux domestiqucs) an attempt is made to describe the cerebral convolu- tions of the horse after the plan followed in human anatomy, and to establish an almost complete correspondence of these parts in the two brains. It appears to me, after very careful consideration, that except in a few points, an identity between convolutions in the two brains is not clearly indicated on anatomical grounds alone. That most of the convolutions of the human brain have corresponding convolutions in the brain of the horse is more than probable ; and experimental, pathological, or developmental evidence may yet place this correspondence beyond doubt. In the meantime, however, and pro- visionally, I think it preferable to describe the cerebral convolutions of the horse accord- ing to what appears the most natural plan. The surface of each hemisphere (excluding from present consideration its inner aspect) is divided into three lobes or areas, viz., an anterior lobe, a postero-superior lobe, and a postero-inferior lobe. This subdivision is effected by certain fissures (Plate 34), as follows : — 1. The Crucial Fissure. This is a short fissure which begins near the middle of each hemisphere where it margins the great longitudinal fissure. Passing outwards, it joins the great oblique fissure. The crucial fissure separates the anterior from the postero- superior lobe. In the right hemisphere of Plate 34 these lobes are connected across the fissure by a small annectent or bridging convolution. 2. The Great Oblique Fissure. This is the most pronounced fissure of the hemi- sphere. Beginning near the middle of the upper surface of the hemisphere, where it is continuous with the crucial fissure, it is directed obliquely outwards, downwards, and backwards, to reach the tentorial aspect of the hemisphere (Plate 33). It separates the postero-superior from the postero-inferior lobe. 3. The Lateral Fissure. This fissure begins on the upper surface of the hemisphere, at the point of junction of the crucial and great oblique fissures. It curves round the side of the hemisphere, with a slightly forward inclination ; and it separates the anterior from the postero-inferior lobe. 4. The Fissure of Sylvius.* This begins at the base of the brain (Plate 33), at the side of the optic commissure. It passes outwards as a faint and ill-defined depression in front of the uncinate convolution ; and crossing the outer olfactory tract, it divides into four branches, which, however, are mere sulci. One of these is directed backwards between the outer olfactory tract and the postero-inferior lobe ; another passes forwards between the inner olfactory tract and the anterior lobe ; a third ascends into the postero-inferior lobe ; and the fourth is directed forwards into the anterior lobe. In the first part of its course the fourth branch separates the adjacent convolutions of the anterior and postero- inferior lobes, being itself separated by a bridging convolution between these lobes from the lower extremity of the lateral fissure. To the outer side of the outer olfactory tract, at the point from which these branches of the Sylvian fissure radiate, there is a minute nodular convolution that is partially or entirely concealed from view until the adjacent convolutions are slightly separated. This seems to foreshadow the convolutions of the insula of man. The Anterior Lobe presents four convolutions 1. The First Anterior Convolution (Plate 33, 1. A) is seen on the under surface of the lobe. It lodges the olfactory peduncle in the olfactory fissure ; and when the peduncle is in position it shows an inner and an outer part, the former occupying the position of the gyrus rectus of human anatomy. 2. The Second Anterior Convolution begins on the under surfa ce of the lobe (Plate 33, * The first part of the fissure of Sylvius, as far as the outer olfactory tract, is sometimes and more correctly called the valley of Sylvius, and Broca restricts the term fissure of Sylvius to the third of the above-described branches. DISSECTION OF THE BRAIN, OR ENCEPHALON. 247 2. A), external to the preceding. It runs forwards and upwards round the extremity of the hemisphere, and abuts on the antero-marginal convolution (Plate 34). 3. The Third Anterior Convolution begins at the under surface of the lobe (Plate 33, 3. A) to the outer side of and behind the preceding. It curves upwards and forwards across the hemisphere, and reaches its upper aspect (Plate 34). Here it is reflected back- wards and inwards ; and turning upon itself, it descends to near the point from which it started, being connected at its termination by a bridging convolution to the first convolu- tion of the postero-inferior lobe. 4. The Antero-marginal Convolution (Plate 34, A. M.) lies at the forepart of the great longitudinal fissure, and is visible on both the upper and inner aspects of the lobe. Be- ginning at the crucial fissure (being sometimes connected to the postero-marginal convolu- tion of the postero-superior lobe), it passes forwards at the edge of the hemisphere, at the anterior end of which the first and second anterior convolutions abut upon it. The Postero-superior Lobe comprises three convolutions : — 1. The Postero-marginal Convolution (Plate 34, P. M.) extends along the margin of the lobe, appearing on both its upper and inner surfaces,- and lying in series with the antero- marginal convolution of the anterior lobe. Beginning at the crucial fissure, it passes backwards at the edge of the hemisphere, and reaches its tentorial aspect. 2. The First Oblique Convolution (Plate 34, 1. O) lies external to the preceding. Be- ginning in front, near the margin of the hemisphere, it passes obliquely backwards and outwards, and curves round the extremity of the lobe to reach its tentorial surface. 3. The Second Oblique Convolution (Plate 34, 2. O) passes with an oblique direction between the preceding convolution and the great oblique fissure, and reaches the tentorial surface of the lobe (Plate 33), from which it seems to be in part continued by the outer olfactory tract. The Postero-inferior Lobe. The sulci of this lobe are numerous and small, and it is difficult to divide it naturally into convolutions. For convenience of description, how- ever, two convolutions may be recognised in it : — 1. The First Postero-inferior Convolution (Plates 33 and 34, 1. P. I.) is four-sided, and contains within itself several short sulci. It lies behind the third convolution of the anterior lobe, to which it is connected by a bridging convolution. 2. The Second Postero-inferior Convolution (Plates 33 and 34, 2. P. I.) lies at the posterior part of the lobe, above the outer olfactory tract ; and its posterior extremity appears on the tentorial surface of the hemisphere. Like the preceding, it possesses numerous minor sulci within itself. Directions. — There still remains for examination the inner surface of each hemisphere. Separate the hemispheres as widely as possible along the great longitudinal fissure. At the upper edge of this fissure there will now be seen the inner aspect of the antero-marginal and postero-marginal convolutions already described, and between the lower edge of these and the corpus callosum there lies a thick convolution — the gyrus fornicatus. The Gyrus Fornicatus (Fig. 31). — This is comparable to a lobe,* rather than to a convolution. It is disposed in a great curve, or arch, from which it is named. It begins at the forepart of the under surface of the hemisphere, in front of the lamina cinerea, and here it is narrow and pointed. It bends round the anterior extremity {genu) of the corpus callosum, acquiring at its point of reflection a great increase in thickness. It passes backwards above the corpus callosum, and below the antero-marginal and postero- marginal convolutions. From the former body it is separated by the fissure of the corpus callosum , while the calloso-marginal fissure {great limbic fissure of Broca) separates it from the marginal convolutions above. In this part of its course the gyrus is distinctly divided into two tiers by a fissure that traverses it in its length. Posteriorly this fissure becomes very shallow, and the gyrus, losing its double character, turns round the posterior end {splenium) of the corpus callosum and reaches the tentorial surface of the hemisphere. At this point it becomes slightly constricted ; and after being connected with the convolu- * Paul Broca ( Anatomie comparee des circonvolutions cerebrates) considers that this part of the hemisphere represents not merely a lobe, but several lobes — that it is, in fact, the equivalent of all the rest of the cerebral cortex. He accordingly divides the surface of the hemisphere primarily into two great divisions — the great limbic lobe (gyrus fornicatus) and the convolutionary mass. 248 THE ANATOMY OF THE HORSE. tions of the postero-superior and postero-inferior lobes, it is directed forwards at the base of the brain (Plate 33, Hipp. con.), between the crns cerebri and the outer olfactory tract. Finally, it terminates in the nipple-like eminence already noticed (Unc. con.). The whole of this great convolution corresponds very closely to the gyrus fornicatus of human anatomy. Thus, the part which turns round the genu and rests on the upper surface of the corpus collosum is the callosal convolution ; the part from the splenium to the side of the cerebral crus is the hippocampal convolution; and the nipple-like eminence is the uncinate convolution. The hippocampal part of the gyrus fornicatus has a small process which projects forwards under the splenium ; and as the convolution curves forwards to emerge at the side of the crus, it rests on the optic thalamus. By carefully raising the convolution from the thalamus, there will be brought into view a fissure on the under aspect of the former. This is the hippocampal fissure , and it projects the convolution into the lateral ventricle as the hippocampus. Beyond this fissure the edge of the hippocampus is seen, margined by a thin-edged white band — the taenia hippocampi. The hippocampus and its taenia here form the upper boundary of the great transverse fissure of the brain, by which the pia mater of the hemisphere projects towards the interior of the cerebrum as the velum interpositum. Fig. 31. Corpus Callosum and Inner Face of the Cerebral Hemisphere. 1, 2. Right and left cerebral hemispheres ; 3. Cerebellum ; C. Corpus callosum ; G. F. Gyrus fornicatus (its callosal portion) ; C. M. Calloso-marginal fissure ; A. M. Antero-marginal convolu- tion ; P. M. Postero-marginal convolution. Directions . — With a large, thin-bladed, sharp knife a horizontal slice should be removed from the top of one or both cerebral hemispheres, down to the level of the corpus callosum. The hemisphere will now be seen to contain both grey and white matter. In the centre of the hemisphere the white matter forms a large mass connected with that of the opposite side by the corpus callosum. At the surface the mass sends a white core into each convolution. The PLATE XXXV Olfactory bulb Septum lucidum Corpus callosum ^Body of fomix jCorpus striatum Choroid plexus ^Tsenia hippocampi ^Hippocampus Nates Testes 4th nerve' Valve of Vieussens Ant. ped. of cerebellum Mid. ped. of cerebellum Post. ped. of cerebellum 8th nerve 1 4th ventricle Medulla oblongata Cerebellum Drawn & Printed "by "W. St A K Johnston, Edinburgh & London BRAIN— LATERAL AND 4TH VENTRICLES, Etc. DISSECTION OF THE BRAIN, OR ENCEPHALON. 249 great sheet of grey matter on the surface of the hemisphere invests the white core in each convolution, and also extends across the bottom of each fissure. The Corpus Callosum (Plate 35 and Fig. 31) is a great commissure of nerve fibres connecting the right and left hemispheres. It termin- ates behind in a thickened margin — the splenium ; and in front it is abruptly bent downwards and backwards, the bend being named the genu , and the reflected portion the rostrum. The rostrum becomes narrower as it descends, and is connected to the optic commissure by the lamina cinerea. Along the middle line of its lower face the corpus callosum is connected posteriorly with the fornix, and anteriorly with the septum lucidum ; and on each side it forms the roof of a cavity in the hemisphere — the lateral ventricle. Nearly all the fibres of the corpus callosum have a transverse direction ; but on each side of the longitudinal middle line of its upper face there are a few longitudinal fibres termed the striae longitudinales, or nerves of Lancisi. Directions. — If the corpus callosum be now cut through in the longi- tudinal direction, a little to one side of the middle line, and dissected outwards, the lateral ventricle will be exposed. The corpus callosum, it will now be seen, is thickest at its posterior extremity, and thinnest at its middle. The Lateral Ventricles (Plate 35) are two in number, one in each hemisphere. They are separated from one another along the middle line by the fornix and septum lucidum, but beneath the former body they communicate through the foramen of Monro. The central portion of each cavity is termed the body , and its prolongations before and behind are termed respectively the anterior and the descending cornu. On the floor of the body of the cavity the following objects will be noticed : — In front, a large pear-shaped grey eminence — the corpus striatum ; behind, another body of about the same size but white on its surface — the hippocampus ; between the corpus striatum and the hippocampus, a groove, in which there lies a red granular cord — the choroid plexus. Where the hippocampus bounds this groove, it is margined by a white band — the taenia hippocampi ; and if the choroid plexus be pulled gently backwards, another white band will be seen to margin the corpus striatum where it bounds the groove — this is the taenia semicircular is. * The anterior cornu is occupied by the base of the corpus striatum. It curves downwards and forwards into the anterior part of the hemisphere, where it communicates with the cavity of the olfactory bulb. The descending cornu contains the prolongations of the hippocampus and its taenia. It passes at first backwards and outwards, and then * The optic thalamus and taenia semicircularis are generally enumerated among the objects visible in the body of the lateral ventricle. In the brain of the horse, however, the choroid plexus completely conceals from view the optic thalamus, and in most cases also the taenia semicircularis. 250 THE ANATOMY OF THE HORSE. curves downwards, forwards, and inwards, terminating at the base of the hemisphere in the uncinate convolution. The ventricles are lined by a ciliated epithelium, which is continuous through the foramen of Monro with the lining of the 3rd ventricle. The Septum Lucid um is a thin, translucent partition between the two lateral ventricles. It is broadest in front, where it is attached to the rostrum, or reflected part of the corpus callosum. Its upper edge is attached to the corpus callosum, and its lower edge to the fornix ; and posteriorly these edges meet at an acute angle. The septum consists of white matter in its centre, with a layer of grey matter on each side. In man it contains a small isolated cavity — the 5th ventricle. The Fornix, or arch, is a mesially placed white band, consisting of a central part, or body, and two pairs of processes, or pillars. The body is flattened above and below, and broadest behind. Its upper face is adherent posteriorly to the corpus callosum, but in front it dips down and leaves beneath the forepart of the corpus callosum a space occupied by the septum lucidum. The under surface of the body rests on the velum interpositum, and at its anterior extremity arches over the foramen of Monro. The anterior 'pillars of the fornix are two white cords which descend in front of the foramen of Monro, being separated by a slight interval. Reaching the base of the brain, they turn on themselves, forming thus the corpus albicans, and they then enter the optic thalamus. The posterior pillars are broader and flatter, and not so well defined. Each in part bestows its substance on the surface of the hippocampus, and in part it descends along the anterior edge of that body as the tcenia hippocampi, or corpus fimbriatum. The Corpus Striatum (Plates 35 and 36). This is the large grey body already noticed in the body and anterior cornu of the lateral ventricle. In shape it is pyriform, having its broad end directed forwards and inwards, and its tapering end backwards and outwards to the roof of the descending cornu. The body comprises two masses of grey matter, separated from each other by intermediate white fibres which curve upwards and outwards from the cerebral crus. The upper mass of grey matter — termed the nucleus caudatus — is that which pro- jects into the lateral ventricle. The lower mass — the nucleus lenticu- laris — lies above the quadrilateral space already seen at the base of the hemisphere between the diverging olfactory tracts. The Taenia Semioircularis (Plate 36) is a narrow white band that extends between the corpus striatum and the optic thalamus. (Its relation to the thalamus will be better seen in the next stage of the dis- section.) The Hippocampus is the curved eminence already noticed in the body and descending horn of the lateral ventricle. It rests on the optic thalamus, from which it is separated by the velum interpositum. The DISSECTION OF THE BRAIN, OR ENCEPHALON. 251 ventricular aspect of the body is white, but the surface that rests on the optic thalamus is grey. The hippocampus is to be viewed as a convolu- tion of the cerebrum, being, in fact, an inward projection of the hippo- campal convolution already noticed (page 248). Directions . — The corpus callosum and septum lucidum should be cut away in order to see the upper surface of the fornix, which should then be divided transversely in its middle. The anterior part should be raised forwards and upwards to expose its anterior pillars and the foramen of Monro. The posterior part should be removed along with the hippocampus in order to bring into view the velum interpositum. The dissection will then assume the form of Plate 36 ; but the optic thalami and the pineal gland, there exposed, will be covered by the velum. The Velum Interpositum is a triangular fold of pia mater, continuous by its base with the pia mater on the hinder end of the cerebrum. Its apex lies at the foramen of Monro, and its lateral edges, fringed by the chor- oid plexuses, project towards the lateral ventricle through what is termed the great transverse fissure of the cerebrum. This is an arched cleft extend- ing over the optic thalami, from the extremity of the descending horn on one side to the same point on the other. Above it is bounded centrally by the fornix, and on each side by the hippocampus and its taenia. The velum interpositum is, like the pia mater in general, a vascular membrane ; and the choroid plexus of each side is a thickened and highly vascular portion of it. Along its centre the veins of Galen extend backwards, and unite to turn round the posterior extremity of the corpus callosum, and enter the inferior longitudinal sinus. The velum should now be raised from its apex backwards, when it will be seen to cover the optic thalami and the pineal gland, and care must be taken lest the latter be removed with it. The Optic Thalami. Each of these is a large grey-coloured body, superposed to the crus cerebri behind the corpus striatum, and in front of the corpora quadrigemina. Its upper surface is convex and covered by the velum interpositum. When followed outwards this surface changes its direction, looking backwards and downwards ; and it there forms part of the boundary of the descending horn of the lateral ventricle. Inwardly the right and left thalami are opposed to one another along the middle line, and they include between them the 3rd ventricle. In front each thalamus is separated from the corpus striatum by a groove, in which will now be seen more distinctly the taenia semicircularis. Behind, another groove isolates the thalamus from the nates. The Pineal Gland is a small, reddish, conical body, named from its resemblance to a pine cone. It stands by its base on the middle line between the optic thalami and the nates. From its base two white 252 THE ANATOMY OF THE HORSE. bands — the 'peduncles of the pineal gland — extend forwards along the groove between the two thalami ; and at the foramen of Monro each peduncle unites with the anterior pillar of the fornix to descend to the base of the brain, and concur in forming the corpus albicans. In structure the body presents some resemblance to lymphoid tissue, but it also contains some branched corpuscles which are possibly nerve cells. Imbedded in it is a quantity of gritty calcareous matter termed the acervulus cerebri , or brain-sand. The Third Ventricle is a narrow space whose sides are formed by the optic thalami. Its floor corresponds to the parts already examined in the interpeduncular space, viz., the pons Tarini, corpus albicans, and tuber cinereum. Its roof is formed by the velum interpositum covered by the fornix. In front it is bounded by the lamina cinerea, and it here communicates with the lateral ventricles by the foramen of Monro. Posteriorly the aqueduct of Sylvius enters it from the 4th ventricle. The cavity is crossed by three commissures : 1. The Anterior Commis- sure is a small white cord of nerve fibres stretching transversely between the corpora striata at the anterior end of the cavity, and immediately in front of the descending anterior pillars of the fornix. The fibres of the commissure are traceable through the corpora striata into the white matter of the hemispheres. 2. The Middle (soft) Commissure is com- posed of delicate grey matter cementing the inner surfaces of the thalami, and apt to be more or less ruptured in handling the brain. 3. The Posterior Commissure is white, like the anterior ; and its fibres connect the two thalami at the base of the pineal body, and immediately in front of the nates. The 3rd ventricle has a ciliated lining continuous with that of the 4th through the aqueduct of Sylvius, and with that of the lateral ventricles through the foramen of Monro. In the foetus the cavity com- municates through the tuber cinereum and infundibulum with the pituitary body. The Foramen of Monro, or Foramen Commune Anterius, is the common point of communication between the 3rd and lateral ventricles. It might be described as a short vertical shaft ascending from the fore- part of the 3rd ventricle, and opening under the fornix, which is thrown over it like an arch. Beneath this arch the lateral ventricles communi- cate with one another and with the 3rd ventricle. The Corpora Quadrigemina are two pairs of bodies superposed to the crura cerebri behind the optic thalami. The anterior pair of bodies, or nates , are larger than the posterior pair, or testes, from which they are separated by a groove. Between the right and left nates there is a well- defined groove, but the groove between the testes is faint or not observ- able. The nates are grey on their surface, but the testes are white. The bodies were named nates and testes from a fancied resemblance PLATE XXXVI Corpus striatum Peduncle of pineal body 4th nerve Valve of Vieussens- 8th nervt Body of fornix Ant. pillar of fornix Ant. ) Mid. v commissure Post. ) Taenia semicircularis' Optic thalamus P ineal body Nates Testis (peduncles of the Post. » cerebellum Drawr *. Printed Ijlf. kA K Johnston, Edinburgh fc London BRAIN— GANGLIA OF THE BASE DISSECTION OF THE BRAIN, OR ENCEPHALON. 253 to the hips and testicles of a man, hut these terms are far from express- ing the relative size of the two bodies. The Aqueduct of Sylvius, or iter, is a tunnel which, commencing posteriorly in the 4th ventricle, beneath the valve of Vieussens, extends forwards beneath the corpora quadrigemina, and opens into the hinder part of the 3rd ventricle. It possesses a ciliated lining con- tinuous with that of the ventricles which it connects. Optic Tracts and Corpora Geniculata. — The optic tracts have already been seen at the base of the brain, where they form the anterior boundaries of the interpeduncular space. When followed backwards, each tract will be found to turn round the crus cerebri, and join the optic thalamus. At the point of junction two eminences are placed, an outer, or anterior, and an inner, or posterior. These are named respectively the corpus geniculatum externum and internum. They are composed of grey matter from which some fibres of the optic tract pass. Other fibres of the tract come directly from the optic thalamus, and others from the corpora quadrigemina. THE CRANIAL OR ENCEPHALIC NERVES (PLATE 33). In the examination of the base of the brain, the roots of the cranial nerves have already been noticed, but it will be advantageous to describe them here as a series. The cranial nerves are distinguished by special names, and also by numerical designations. It must be observed, how- ever, at the outset that there are two different systems of enumeration in use among anatomists, the first of which recognises twelve, and the other nine, pairs of nerves. This diversity of nomenclature is apt to lead to confusion, but fortunately this confusion does not extend to veterinary anatomy, in which, both at home and abroad, the first and more natural of these methods is exclusively employed. This system is also that employed by human anatomists on the continent, but by British human anatomists the number of cranial nerves is stated as nine pairs. The following table exhibits in the central column the special names of the nerves, and in the side columns their numerical designations under the two systems : — 1st pair . Olfactory nerves .... . 1st pair. 2nd ,, . Optic nerves . 2nd ,, 3rd „ . Oculo-motor nerves . 3rd „ 4th „ . Pathetic or Trochlear nerves . 4th „ 5th ,, . Trifacial or Trigeminal nerves . 5th ,, 6th „ . Abducent nerves .... . 6th „ 7th ,, . Facial nerves (Portio dura) . ; pth „ 8th „ . Auditory nerves (Portio mollis) . 9th , , . Glosso-pharyngeal nerves • \ 10th „ . Pneumogastric or Yagus nerves . • >8th „ 11th „ . Spinal Accessory nerves J 12th ,, . Hypoglossal nerves . 9th , , The Olfactory or 1st nerve. The fibres of this nerve leave the surface 254 THE ANATOMY OF THE HORSE. of the olfactory bulb , and pass through the foramina of the cribriform plate to reach the summit of the nasal chamber. They are there distri- buted in the olfactory division of the lining membrane of that chamber. The Optic or 2nd nerve arises from the optic chiasma or commissure , and reaches the back of the orbit by passing through the optic foramen. Piercing the sclerotic and choroid tunics of the eyeball, its fibres radiate outwards and form one of the layers of the retina. As already seen, the optic chiasma is formed by the fusion of the optic tracts, each of which derives its fibres from the optic thalamus, corpora geniculata, corpora quadrigemina, and decussation of the pyramids. In the optic chiasma some of the fibres of each tract cross and are continued in the optic nerve of the opposite side. Some of the fibres of each optic tract, it is stated, cross in the chiasma and return to the brain by the opposite tract, while in the same way fibres pass from the one optic nerve to the other optic nerve. The Oculo-motor or 3rd nerve arises from the inner side of the crus cerebri by a number of bundles, the fibres of which are traceable to nerve cells in the corpora quadrigemina. The nerve leaves the cranium by the foramen lacerum orbitale, and reaches the orbit. The Pathetic, Trochlear, or 4th nerve appears to arise in the valve of Yieussens (Plate 35). Some of its fibres are decussate with those of the opposite nerve, and the others are traceable to nerve cells of the locus cseruleus, or of the corpora quadrigemina. Emerging from the valve, the nerve winds round the crus cerebri, and appears in front of the pons. It leaves the cranium by the minute pathetic foramen, and reaches the back of the orbit. It is the smallest of the cranial nerves. The Trifacial, Trigeminal, or 5th nerve springs out of the side of the pons by two roots. The outer and larger of these is termed the sensory root ; and its fibres are traceable to cells of the grey matter of the medula, pons, and locus cseruleus, and possibly also to the cerebellum. This root near its origin expands into a large ganglion — the Gasserian ganglion , beyond which it divides into three branches, viz., the ophthal- mic, superior maxillary, and inferior maxillary divisions. The inner or motor root of the 5th nerve is traceable to grey matter of the pons. It joins the inferior maxillary division of the sensory root. The superior maxillary division leaves the cranium by the foramen rotundum, the ophthalmic division by the foramen lacerum orbitale, and the inferior maxillary division by the forepart of the foramen lacerum basis cranii. The trifacial is the largest of the cranial nerves. The Abducent or 6th nerve. This nerve springs from the anterior part of the medulla, in line with the faint groove that limits outwardly the inferior pyramid. Some of its fibres issue from the groove between the pons and the medulla, while others penetrate the trapezium. The fibres of the nerve are traceable to a group of nerve cells in the medulla. DISSECTION OF THE BRAIN, OR ENCEPHALON. 255 The nerve reaches the orbit by passing through the foramen lacerum orbitale. The Facial or 7th nerve springs out of the medulla, close behind the pons, its fibres seeming to continue outwards the trapezium. Its rootlets are traceable to nuclei of grey matter in the medulla. The nerve is joined by a delicate filament — the portio intermedia — which appears between the roots of this and the next nerve. The 7th nerve enters the internal auditory meatus in company with the 8th nerve. Separating from that nerve, it passes along a canal in the petrous temporal bone — the aqueduct of Fallopius — from which it emerges by the stylo-mastoid foramen, under the parotid gland. Within the aque- duct of Fallopius the nerve forms a knee-shaped bend, and at that point it shows a minute ganglion — the geniculate ganglion — from which pro- ceed the great and small superficial petrosal nerves (pages 189 and 214). The Auditory or 8th nerve springs from the medulla, close behind the pons, and immediately external to the root of the 7th. It is here compounded of two roots — a superior and an inferior. The superior root (Plate 35) passes over the restiform body to the grey matter at the floor of the 4th ventricle. The inferior root springs out of the side of the restiform body, its fibres arising from nerve cells of that body or of the grey matter at the floor of the 4th ventricle, and possibly also from the cerebellum. The 8th nerve enters the internal auditory meatus, and penetrates to the internal ear. The Glosso-pharyngeal or 9th nerve springs out of the side of the medulla, a little behind the outer extremity of the trapezium. It is here compounded of two or three bundles, the outermost being in line with the roots of the next two nerves. The fibres emanate from nerve cells of the grey matter at the floor of the 4th ventricle. The nerve leaves the cranium by the posterior part of the foramen lacerum basis cranii, and at that point it shows a minute ganglion — the petrous ganglion , or the ganglion of Andersch, from which the nerve of Jacobson arises (page 269). The Pneumogastric, Vagus, or 10th nerve is formed by a number of rootlets which spring from the side of the medulla, behind and in line with the outermost fibres of the 9th nerve. Its fibres arise from nerve cells of the medulla. The nerve passes out of the cranium by the posterior part of the foramen lacerum basis cranii, and is joined by the inner division of the 11th nerve. As the nerve passes through the foramen it presents an enlargement — the upper ganglion , or ganglion of the root From this ganglion arises the auricular branch of the vagus, which penetrates to the aqueduct of Fallopius, where it anastomoses with the 7th nerve; afterwards emerging from the bone in company with that nerve, to be distributed to the mucous membrane of the external auditory process. 256 THE ANATOMY OF THE HORSE. The Spinal Accessory or 11th nerve comprises two sets of roots — a spinal and a medullary. The spinal roots appear along the lateral column of the cervical part of the spinal cord, in which they arise from a group of nerve cells towards the middle of the grey crescent. By the union of these roots there is formed a cord which travels upwards between the superior and the inferior roots of the cervical spinal nerves, becoming thicker as it ascends. This cord enters the cranial cavity by the foramen magnum, and is then joined by the medullary roots. The medullary roots spring out of the side of the medulla oblongata, behind and in line with the roots of the 10th nerve, the fibres arising from nerve cells at the floor of the 4th ventricle. These roots join the spinal part of the nerve, which then leaves the cranium by the foramen lacerum basis cranii, along with the 10th nerve. In the foramen of exit the trunk of the nerve resolves itself into two portions — an internal and an external. The internal portion joins the 10th nerve ; the external portion is that which has already been seen in the dissection of the neck (page 151). The Hypoglossal or 12th nerve is formed by the fusion of rootlets that spring from the lower face of the medulla, along the line that indi- cates the outer limit of the inferior pyramid. These roots are in series with the inferior roots of the spinal nerves ; and sometimes there is also present a superior root, in series with the superior roots of the same nerves, and provided with a minute ganglion. These roots arise from nerve cells of the medulla. The roots of the nerve perforate the dura mater, and unite in emerging from the cranium by the condyloid foramen. CHAPTER VII. DISSECTION OF THE EYEBALL. Directions . — Let the student procure three or four eyes of the. horse, or, failing these, of the ox. They should be excised from the orbit immediately after death, and as much as possible of the optic nerve should be preserved in connection with the eye. While an assistant holds the eye without squeezing it, the dissector should clean the optic nerve and the outer surface of the sclerotic with forceps and a sharp scalpel. One of the eyes so prepared should be completely frozen in a mixture of ice and salt, and it should then be bisected vertically with a large knife or fine saw. While still frozen, the section to which the optic nerve is attached should be fastened by a strong pin to a layer of solid paraffin at the bottom of a wide and shallow basin. It should be fastened with the cut surface upwards, the pin being passed vertically from the centre of that surface ; and the vessel should then be filled with water. The remaining segment should be laid on the freezing mixture, with its cut surface upwards. By an examination of both segments, the student should make out the following points : — The Globe or Ball of the eye approaches the spherical in form, as is expressed by these designations. On closer inspection, however, it will appear to be made up of two combined portions from spheres of different sizes. The posterior portion, forming about five-sixths of the ball, is a sphere of comparatively large size with a small segment cut off it in front ; and at this point there is applied to it the anterior portion, which, being a segment of a smaller sphere, projects at the front of the ball with a greater convexity than the posterior portion. The eyeball consists of concentrically arranged coats, and of refracting media enclosed within these coats. The coats are three in number, viz., (1) an external protective tunic made up of the sclerotic and cornea , (2) a middle vascular and pigmentary tunic — the choroid , (3) an internal nervous layer — the retina. The sclerotic is the white opaque part of the outer tunic, of which it forms about the posterior five-sixths, being co-extensive with the larger sphere already mentioned. The cornea forms the remaining one-sixth of the outer tunic, being co-extensive with the segment of the smaller sphere. It is distinguished from the sclerotic by being colourless and transparent. The choroid coat will be s 258 THE ANATOMY OF THE HORSE. recognised as the black layer lying subjacent to the sclerotic. It does not line the cornea, but terminates behind the line of junction of that coat with the sclerotic, by a thickened edge — the ciliary processes. At Fig. 32. View of the Lower Half of the Right Adult Human Eye, divided horizontally THROUGH THE MIDDLE. MAGNIFIED FOUR TIMES (A. ThomSOV). 1. The cornea ; 1'. Its conjunctival layer ; 2. The sclerotic; 2'. Sheath of the optic nerve passing into the sclerotic ; 3. 3'. The choroid ; 4. Ciliary muscle, its radiating portion ; 4'. Cut fibres of the circular portion ; 5. Ciliary fold or process ; 6. Placed in the posterior division of the aqueous chamber, in front of the suspensory ligament of the lens ; 7. The iris (outer or temporal side) ; 7'. The smaller, inner, or nasal side ; 8. Placed on the divided optic nerve, points to the arteria centralis retinae; 8'. Papilla optica at the passage of the optic nerve into the retina; 8". Fovea centralis retinae; r. The nervous layer of the retina; r\ The bacillary layer ; 9. Ora serrata, at the com- mencement of the ciliary part of the retina ; 10. Canal of Petit ; 11. Anterior division of the aqueous chamber, in front of the pupil ; 12. The crystalline lens, within its capsule ; 13. The vitreous humour ; a. a. a. Parts of a line in the axis of the eye ; b. b. b. b. A line in the transverse diameter. DISSECTION OF THE EYEBALL. 259 the line of junction of the sclerotic and cornea, the iris passes across the interior of the eye. This, which may be viewed as a dependency of the choroid, is a muscular curtain perforated by an aperture termed the pupil. The retina will be recognised as a delicate glassy layer, lining the greater part of the choroid. The refracting media of the eye are three in number, viz., (1) the aqueous humour — a watery fluid enclosed in a chamber behind the cornea ; (2) the crystalline lens (and its capsule) — a transparent soft solid of a biconvex form, and placed behind the iris ; (3) the vitreous humour — a transparent material with a consistence like thin jelly, and occupying as much of the interior of the eye as is subjacent to the choroid. Directions . — Another eye should be cleaned like the first, and used for the more particular examination of the sclerotic and cornea. The Sclerotic is a strong, opaque fibrous membrane which in great measure maintains the form of the eyeball, and protects the more delicate structures within it. Its anterior portion, which is covered by the ocular conjunctiva, is visible in the undissected eye, and is commonly known as the “white of the eye.” In form it is bell-shaped, and the optic nerve pierces it behind like a handle. The point of perforation, however, is not exactly at the centre of the summit of the bell, but a little to its inner side. When the nerve is cut oft' close to the sclerotic, the nerve-bundles appear as if passing through the apertures of a sieve, and to this appearance the term lamina cribrosa is applied. The sheath of the nerve passes on to the sclerotic around the point of perforation. In front the rim of the bell becomes continuous with the cornea. The outer surface of the membrane receives the insertion of the muscles of the eyeball. The inner surface (which will afterwards be exposed) is of a light brown colour, and is connected to the choroid by fine processes of connective-tissue — the lamina fusca. The coat is thickest over the posterior part of the eyeball, and is thinnest a little behind its junction with the cornea. Structure. — The sclerotic is composed of connective-tissue, there being a great preponderance of white fibres, but intermixed with these are some fine elastic fibres. The bundles of fibres, which are disposed both meridionally and equatorially, have a felted arrangement, but the surface fibres are mostly longitudinal. The texture of the sclerotic is only slightly vascular, the capillaries forming a wide-meshed network. It is most vascular just behind the cornea. The Cornea is the anterior transparent portion of the outer coat of the eyeball. It may be viewed as a part of the sclerotic specially modified to permit the passage of light into the interior of the eye. Its outline is elliptical approaching the circular, and its greatest diameter is transverse. At its periphery it joins the sclerotic by continuity of tissue ; and as the edge of the cornea is slightly bevelled, and has the 260 THE ANATOMY OF THE HORSE. fibrous sclerotic carried for a little distance forward on its outer surface, the cornea is generally said to be fitted into the sclerotic like a watch- glass into its rim. The venous canal of Schlemm runs circularly around the eyeball at the line of junction of the sclerotic and cornea. The anterior surface of the cornea is exquisitely smooth, and is kept moist by the lachrymal secretion. Its posterior surface forms the anterior boundary of the chamber in which the aqueous humour is contained. The cornea- is of uniform thickness ; and, as will afterwards be proved in removing it, it is very difficult to cut, being of a dense, almost horny consistence. When its normal convexity is disturbed, the cornea becomes opaque. Structure. — Save a few capillary loops at its margin, the cornea is without vessels. Its structure comprises the following layers, which are enumerated in order from the anterior to the posterior surface : — 1. The Anterior Epithelium is a stratified, pavement epithelium, continuous at the margin of the cornea with the conjunctival epithelium. 2. The Anterior Elastic Lamina (Bowman’s membrane). This is a structureless, elastic layer. It is extremely thin in the eye of the lower animals, but is better developed in the human eye. 3. The Substantia Propria. This, which forms the main thickness of the cornea, is composed of fibrous connective-tissue arranged in lamellae parallel to the surfaces of the cornea. Between adjacent lamellae there is left a network of spaces and branching canals, in which are found the branched corneal corpuscles. 4. The Posterior Elastic Lamina (Descemet’s membrane) is a thick, structureless, elastic layer. 5. The Posterior Epithelium is a single layer of polygonal cells. Directions. — A strong pin should now be passed through the optic nerve, and used to fasten the eye beneath the surface of water in a wide and shallow vessel, as already directed in the case of the frozen section. While one hand steadies the eye beneath the water, an incision is to be made with the other through the cornea, using for the purpose a very sharp scalpel. As soon as the incision is made, some of the aqueous humour will escape into the water, and may possibly be recog- nised by a slight inky discoloration, which is due to a post-mortem disintegration of the pigmented epithelium lining the cavity in which the humour is contained. Still keeping the eye under water, one blade of a pair of small scissors should be introduced within the incision, and the cornea should be excised immediately in front of its junction with the sclerotic. The iris will by this means be exposed, and the next step must be to remove a portion of the sclerotic so as to expose the sub- jacent choroid. Beginning at its anterior edge, it may be incised back- wards towards the optic nerve, snipping it bit by bit with the point of the scissors. Another incision may then be made parallel to the first, DISSECTION OF THE EYEBALL. 261 and about half an inch from it. The piece of sclerotic between the incisions may then be raised and turned backwards by destroying the slender processes, nerves, and vessels that connect it to the choroid. At the anterior edge of the piece of choroid thus exposed, and immediately behind the rim of the iris, there will be seen a whitish zone — the ciliary body , or annulus albidus. The Aqueous Humour occupies a chamber which is bounded in front by the posterior surface of the cornea ; and behind by the capsule and suspensory ligament of the lens, and by the ends of the ciliary processes. It is across this chamber that the iris extends, and the chamber is some- times described as being divided by the iris into two compartments, viz., an anterior, in front of the iris ; and a posterior, behind it. In the living eye, however, the posterior surface of the iris contacts with the lens-capsule, so as to leave only a narrow chink behind the attachment of the curtain to which the term posterior chamber may be applied. The aqueous humour is composed of water with a small proportion of common salt in solution. The Iris is a muscular pigmented curtain extended across the interior of the eye, and having about its centre an aperture termed the pupil. By variations in the size of this aperture, the amount of light trans- mitted to the retina is regulated. It varies somewhat in colour, but is most frequently of a yellowish-brown tint. Its anterior surface, which shows some lines con- verging to the pupil, is bathed by the aqueous humour, as is also its posterior surface immedi- ately internal to its attachment. The greater part of the posterior sur- face, however, is in con- tact with the capsule of the lens, and glides on it during the movements Fig. 33 . Choroid membrane and Iris exposed by the removal of the Sclerotic and Cornea ( Quain after Zinn). a. One of the segments of the sclerotic thrown back ; b. Ciliary- muscle ; c. Iris ; e. One of the ciliary nerves ; /. One of the vasa vorticosa or choroidal veins. of the curtain. The circumferential border is attached within the junction of the sclerotic and cornea. The inner border circumscribes the pupil, which varies in outline according to its size. When much contracted, the pupil is a very elongated ellipse, the long axis of which is in the line joining the nasal and temporal canthi of 262 THE ANATOMY OF THE HORSE. the eyelids; but when it is extremely dilated, the ellipse approaches the circular in form. Appearing at the upper margin of the pupil, there are generally two or three little sooty masses termed the corpora nigra . These are little dependent balls of the uvea , or pigmentary layer covering the back of the iris. Structure. — This comprises a connective-tissue stroma, muscular tissue, and an anterior and a posterior epithelium. The Stroma is a framework of connective-tissue, the fibres having a radial arrangement, and the corpuscles being branched and pigmented. The pigment varies in shade from yellow to dark brown or almost black. The Muscular Tissue is of the non-striated variety, and its fibres are arranged in two sets, viz., (1) the sphincter of the pupil , a narrow band around the pupil, and close to the posterior surface of the curtain ; (2) the dilator of the pupil , whose fibres begin at the attached edge of the curtain, and extend radially inwards to end in the sphincter. The size of the pupil is regulated by the state of contraction of these two muscles. When the action of the sphincter preponderates, the aperture is con- tracted ; when that of the dilator preponderates, the pupil is dilated. The Anterior* Epithelium is continuous at the attached edge of the iris with the posterior epithelium of the cornea. It is a single layer of pigmented cells. The Posterior Epithelium , or Uvea, comprises several layers of cells similarly pigmented ; and, as before stated, the corpora nigra are small dependent portions of it. In the eyes of albinos the iris is devoid of pigment ; and occasionally in the horse and dog the pigment is only present in spots, and the animal is then said to be “ wall-eyed.” Vessels. — The arteries of the iris are derived from the ciliary branches of the ophthalmic. They form at the circumference of the iris a larger circle, from which radial vessels pass inwards and form around the pupil a smaller circle. The veins have a similar disposition, and termin- ate in those of the choroid. In the foetus the pupil is closed by a vascular transparent membrane ■ — the membrana pupillaris, which disappears before birth. The Ciliary Muscle. This is a zone of non-striated muscular tissue which forms the outer layer of the ciliary body, and lies behind the circumferent edge of the iris. It consists (1) of an outer radiating set of fibres, which arise from the inner surface of the sclerotic close behind its line of junction with the cornea, and pass backwards to be inserted into the choroid and ciliary processes ; and (2) of an inner circular set, which surround the rim of the iris. When the radiating fibres contract, they pull forward the choroid coat and ciliary pro- cesses, and allow the lens to bulge forwards by slackening its tense suspensory ligament. This is the mechanism by which the eye is accommodated for near objects. DISSECTION OF THE EYEBALL. 263 The Choroid Coat. This is a bell-shaped, dark membrane which lines the sclerotic. Its outer surface, when exposed by the removal of the sclerotic, has a shaggy appearance due to the tunica fusca which unites the two coats. Between the two the ciliary vessels and nerves pass forwards. The inner surface of the choroid is lined by the layer of pigmented hexagonal cells belonging to the retina. Behind it is pierced by the optic nerve ; and in front it is continued as the ciliary processes, which form, as it were, the rim of the bell. Directions. — In the eye prepared to expose the iris and choroid, a segment of the former and of the ciliary muscle should be carefully and delicately removed with scissors, so as to lay bare a number of the ciliary processes. This is to be done while the eye remains immersed in water. The Ciliary Processes. These form a fringe around the slightly inverted rim of the choroid. They number upwards of a hundred, and each projects on the inner side of the rim, as a small swelling separated by depressions from the adjacent processes. The outer surface of each is covered by the ciliary muscle ; the inner surface rests in a depression on the suspensory ligament of the lens ; behind each is continuous with the texture of the choroid ; and in front it terminates in a rounded end which bounds in part the so-called posterior chamber of the aqueous humour, behind the peripheral part of the iris. Structure. — The choroid possesses a stroma of connective-tissue with ramifying corpuscles containing brown or black pigment — melanin. This stroma is lined internally by a structureless layer — the lamina vitrea, and it supports the vessels of the choroid. The arteries — which are derived from the ciliary branches of the ophthalmic — and the veins lie together in the outer part of the stroma, while the capillaries lie in its deeper part and form there the tunica Ruyschiana. The smaller veins converge in whorls — the vasa vorticosa — to join four or five principal trunks. The ciliary processes have the same structure as the choroid. Each contains a rich plexus of tortuous vessels. The branched cells at the anterior end of each process are without pigment. Over a considerable area on the inner surface of the choroid the pigment is absent ; and there the choroid shines with a peculiar iridescent, metallic appearance termed the tapetum lucidum. In the eyes of albinos the choroid is entirely free from pigment. The Ciliary Nerves are efferent branches of the lenticular ganglion. They perforate the sclerotic in company with the ciliary arteries, and run forwards between the sclerotic and cornea. They give branches to the cornea and ciliary muscle, and terminate in the iris. They contain sensory fibres, which are derived from the ophthalmic division of the 5th nerve ; motor branches to the ciliary muscle and sphincter muscle of the pupil, which come from the third nerve ; and motor fibres to the 264 THE ANATOMY OF THE HORSE. dilator muscle of the pupil, which are derived from the sympathetic system. Directions. — In the immersed eye from which the cornea and part of the sclerotic have been removed, the portion of choroid exposed is to be torn away with two pairs of forceps from the subjacent retina. The inner surface of the membrane will be seen, through the transparent vitreous humour, in the submerged half of the eye that was frozen. The Retina is the most delicate of the coats of the eyeball. It is formed by the radiation of the optic nerve on the inner surface of the choroid, and like that coat it is bell-shaped. Its external or choroidal surface is covered by a layer of hexagonal pigment cells, which were at one time referred to the texture of the choroid. Its inner surface is moulded on the vitreous humour. This surface shows a little to the inner side of the summit of the bell, or of the antero-posterior axis of the eyeball, a disc-like elevation — the papilla optica , which is the point at which the optic nerve begins to expand. In the centre of this spot the arteria centralis retinae appears, and divides into branches which radiate on the inner surface of the retina. The nervous structures of the retina terminate at a wavy line — the ora serrata — behind the ciliary processes ; but the retina is continued beneath these processes in the form of an epithelial layer — the pars ciliaris retinae , which forms the edge of the bell. In the human eye a yellow spot — the macula lutea — is placed a little external to the papilla optica, and almost exactly in the antero-posterior axis of the eyeball. This is not present in the eye of the horse or in any mammal lower than the quadrumana. The perfectly fresh retina is translucent and of a pale pink colour, but it speedily becomes opaque. In consistence it is delicate and jelly-like. Structure. — Ten distinct layers are described as composing the thick- ness of the retina. These enumerated from within to without are as follows : — The Membrana Limitans Interna. — This, although appearing as a distinct line in a transverse section, is not a distinct stratum, but merely the inner limiting line of a sustentacular framework — the radial fibres of Muller — which pervades and supports the nervous elements in the other layers of the retina. 2. The Layer of Nerve fibres. — This layer results from the radiation of the optic nerve, whose fibres at their point of entrance into the eye- ball lay aside their medullary sheath. 3. The Layer of Nerve Cells. — This is a single layer of multipolar nerve cells. 4. The Inner Molecular Layer is a thick stratum of fibres and inter- mediate granular matter. DISSECTION OF THE EYEBALL. 265 5. The Inner Nuclear Layer contains spindle-shaped or bipolar nerve cells with distinct oval nuclei and only a small amount of protoplasm. The inner and outer j>oles of the cells are continued through the 4th and 6th layers respectively. 6. The Outer Molecular Layer repeats the structure of the inner molecular layer. 7. The Outer Nuclear Layer contains spindle-shaped cells with con- spicuous nuclei and a small amount of protoplasm, the poles of the cells being prolonged as in the case of the similar elements in the inner nuclear layer. 8. The Membrana Limitans Externa. — This is the outer boundary of the sustentacular framework of fibres already mentioned. 9. The Layer of Rods and Cones , or the bacillary layer , is composed of two different kinds of elements. The longer elements, the rods, extend vertically between the 8th and 10th layers; the cones are much shorter than the rods, and do not reach so far as the next layer. 10. The Pigmented Epithelium. — This is a layer of polygonal pig- mented cells, generally six-sided. Directions. — The third eye should be transversely divided with a sharp scalpel, about half an inch behind the junction of the sclerotic and cornea. This should be done with the eye immersed in water. The posterior half, after removal of the vitreous humour, should be used for the better examination of the inner surface of the retina. The lens should be removed for examination from the anterior half. In the eye already used for the display of the retina, that coat should be in part removed, so as to display the vitreous humour with the lens imbedded in its anterior part. By a combined examination of all the preparations, the following points regarding the lens and vitreous body may be made out. The Lens is situated behind the pupil, and is contained within a capsule of its own. The Capsule is a close-fitting, firm, transparent membrane, which is four or five times thicker on the front than on the back of the lens. The anterior surface of the capsule forms the posterior boundary of the cavity in which the aqueous humour is contained, and the iris in its movements glides on it. At its periphery the suspensory ligament of the lens blends with it. The posterior surface is in contact with the vitreous humour. The lens is a transparent solid body of a biconvex shape, the convexity of its posterior surface being considerably greater than that of the anterior. It is maintained in a depression on the front of the vitreous humour by a suspensory ligament. This ligament, which is also known as the zonula of Zinn , arises behind and beneath the ciliary processes, where it is connected with the hyaloid membrane of the vitreous 266 THE ANATOMY OF THE HORSE. humour. It passes over the rim of the lens, and blends with the anterior part of the lens-capsule. Behind the rim of the lens the ciliary processes rest on the outer surface of the ligament ; and when these are removed, the ligament is there seen to have a fluted or plaited appearance, each plait fitting into the depression between two processes. At this same point the inner surface of the zonula forms the outer boundary of a triangular chink which runs round the lens behind its rim. This is the canal of Petit , which is bounded in front by the lens- capsule, behind by the hyaloid membrane of the vitreous humour, and outwardly by the zonula. Structure . — When removed from its capsule, the lens is found to be soft and pulpy in its outer portion, but its density increases in passing from the surface to the centre. Both its surfaces show some faint white lines radiating from the central point of the surface. The number of these lines varies in the adult, but in the foetus they are three in number, and each line on the posterior surface is in position midway between two of the anterior lines. A lens that has been hardened in spirit or by boiling may be broken down into concentric laminae like the coats of an onion. Each of these laminae is composed of long riband-shaped fibres. These lens-fibres when examined microscopically are seen to have finely serrated edges by which adjacent fibres are interlocked. The foetal lens is nearly spherical, it is of a reddish colour, and not quite transparent. In the young adult it is distinctly biconvex, firm, colourless, and transparent. With advancing age it tends to become flatter, denser, less transparent, and of a yellowish colour. The Vitreous Humour occupies four-fifths of the interior of the eye- ball. It is globular in form, with a depression in front for the lodgment of the lens. It is colourless, transparent, and of a consistency like thin jelly. It is enveloped by a delicate capsule — the hyaloid membrane , which is connected in front with the suspensory ligament of the lens, and ends by joining the capsule behind the lens. Structure . — The vitreous humour is composed of branched connective- tissue corpuscles in a jelly-like matrix. CHAPTER VIII. THE EAR. The organ of hearing consists of three divisions — the external, the middle, and the internal ear. The first of these comprises the osseous external auditory process, and the trumpet-like organ which collects the w r aves of sound and transmits them along that process to the middle ear. It is described at page 159. The middle and the internal ear are cavities excavated in the sub- stance of the petrous temporal bone. From their situation and the minuteness and intricacy of their parts, their dissection is extremely difficult. The student is therefore recommended to study the anatomy of these parts on the models and special dissections to which he is likely to have access, and by the aid of the fuller description given in systematic text-books. At the same time, an outline description will be here given, which the student may illustrate to himself by procuring two or three petrous temporal bones and dissecting them after they have been decalcified in a hydrochloric or chromic acid solution. THE MIDDLE EAR. The Middle Ear — called also the Tympanum, or drum of the ear — is a cavity of the petrous temporal bone. It contains air, and across it there stretches a chain of minute bones, which transmit the sound waves from the outer to the inner ear. The inner wall of the chamber is formed by that portion of bone in which the divisions of the internal ear are excavated, and it shows the following objects : — The promontory — a projection, or bulging, which corresponds to the first turn of the cochlea. Above the promontory, the fenestra ovalis — an opening which is closed by the base of the stapes (the innermost of the auditory ossicles). Below the promontory, another opening — the fenestra rotunda , which is closed by a thin membrane. A pin passed through the fenestra ovalis, would enter the vestibular division of the internal ear ; if passed through the fenestra rotunda, it would penetrate the scala tympani of the cochlea. The outer wall of the chamber is formed mainly by the membrana tympani. This is a thin, translucent membrane which forms the septum between the tympanum and the outer ear. 268 THE ANATOMY OF THE HORSE. The rim of the membrane is fixed in a groove of the bone. The mem- brane is slightly cupped towards the outer ear ; while its inner surface is convex, and has the handle of the malleus (the outermost ossicle) attached to it. The surfaces of the membrane are inclined so that the outer surface looks somewhat downwards, and the inner upwards. In structure the membrane comprises (1) a middle fibrous stratum, the fibres being arranged both radially and circularly, with (2) an outer and (3) an inner epithelial covering. The roof and the floor of the tympanum present nothing of interest. The former is the more exten- sive. The anterior extremity of the chamber shows a fissure by which air is admitted from the Eustachian tube. Through this opening also the mucous lining of the cavity is continuous with that of the Eustachian tube. The posterior extremity, and part of the floor and outer wall communicate with the cellular spaces of the mastoid protuberance. The Auditory Ossicles. — There are three of these, viz., the malleus, the incus, and the stapes. The Malleus, named from its resemblance to a hammer, is the largest bone. It possesses a head , a handle , and two processes. The head is articulated by a synovial joint to the stapes. The handle is fixed on the inner surface of the membrana tympani. The long process is slender, and projects forwards to be fixed in a slit of the petrous temporal. The short process is a mere projection of the root of the handle, and is fixed to the membrana tympani. The Incus is named from its supposed resemblance to an anvil, but it has more likeness to a human bicuspid tooth. It presents a body and two processes , or crura. The body has a saddle-shaped articular facet for the malleus. The short process is directed backwards to be fixed to the wall of the tympanum. The long process curves downwards and inwards to terminate in a rounded point — the orbicular process , which articulates with the head of the stapes. The Stapes is stirrup-shaped. It is the smallest bone, and possesses a head, a neck, a base, and two crura. The head is depressed for articula- tion with the orbicular process, and is succeeded by the slightly constricted neck. The base is a thin plate which closes the fenestra ovalis. The crura are slender rods of bone connecting the base and the neck. Muscles of the Ossicles . — These are two — the tensor tympani and the stapedius. (The so-called laxator tympani is now believed to be a ligament.) The Tensor Tympani arises from the petrous temporal bone near the Eustachian orifice, and it is inserted by a slender tendon into the handle of the malleus near its root. Action. — To tense the membrana tympani. The Stapedius arises within the pyramid — a small process of bone at THE EAR. 269 the back of the tympanum. Issuing from the pyramid, it is inserted into the neck of the stapes. Its tendon of insertion contains a small nucleus of bone. Action. — To regulate (diminish the excursions of) the movements of the stapes. Bloodvessels. The arteries of the tympanum are derived from the tympanic artery , a branch of the internal maxillary artery. Nerves. The chorda tympani branch of the 7 th nerve enters the cavity of the tympanum from the aqueduct of Fallopius; and passing across the membrana tympani it leaves the cavity by the styloid fora- men. The sensory nerves of the tympanum are derived from the tympanic branch (Jacobson’s nerve) of the glosso-pharyngeal. The Nerve to the Stapedius is a branch of the 7th. The Nerve to the Tensor Tympani comes from the 5th, through the otic ganglion. THE INTERNAL EAR. The Internal Ear, called also, from its complexity, the Labyrinth, consists of a series of chambers, or passages, in the petrous temporal bone, and of certain fluids and soft textures contained within these passages. The chambers, with the wall of condensed bone tissue which immediately surrounds them, con- stitute the osseous labyrinth ; the contained soft structures form the membranous labyrinth. The osseous labyrinth consists of three divisions; — the vestibule , the cochlea , and the semicircular canals , and each of these contains a division of the mem- branous labyrinth. The Vestibule. This is the central division of the labyrinth. It lies between the inner wall of the tympanum and the internal audi- tory meatus. In front it communicates with the scala vestibuli of the cochlea, and the semicircular canals open into it behind by five openings. On its outer wall, which separates it from the tym- panum, is the fenestra ovalis, closed by the base of the stapes. On its inner wall in front there is a depression — the fovea hemispherica — placed over the meatus auditorius internus, and pierced by minute foramina for the passage of the filaments of the auditory nerve. Behind the fovea hemispherica is a small slit which leads into the aqueductus vestibuli. The roof of the vestibule shows another depres- sion — the fovea hemi-elliptica. Fig. 34. Diagram of the Membranous Labyrinth. DC. Ductus cochlearis ; dr. Ductus reuniens ; S. Sacculus; U. Utriculus; dv. Ductus vesti- buli ; SC. Semicircular canals. ( Turner , after Waldeyer). 270 THE ANATOMY OF THE HORSE. Contained immediately within the osseous vestibule there is a quantity of limpid, serous fluid — the perilymph, which surrounds the parts of the membranous labyrinth here found. These are two delicate sacs — the saccule and the utricle. The Saccule is the anterior and smaller of the two sacs, and is lodged in the fovea hemispherica. It contains a fluid termed the endolymph. It communicates with the membranous canal of the cochlea by a minute tube — the canalis reuniens , and with the utricle by a Y shaped tube — the ductus vestibuli , the stem of which ends blindly in the aqueduct of the vestibule. The Utricle , placed above and behind the saccule, is lodged in the fovea hemi-elliptica. Like the saccule, it contains endolymph. It com- municates, as aforesaid, with the saccule ; and the five openings of the membranous semicircular canals open directly into it. The interior of both saccule and utricle is elevated into a ridge — the crista acoustica, in which are distributed the terminal filaments of the vestibular division of the auditory nerve. On this crest are certain peculiar cells, each having a peripheral hair-like process which projects into the endolymph, and a central process which is probably continuous with a filament of the auditory nerve. Here are also found the otoliths , which are minute calcareous particles imbedded in a jelly-like material. The Semicircular Canals are placed behind the vestibule. They are three in number and are distinguished as superior, posterior, and external. The two first have a vertical direction, while the latter is nearly hori- zontal. Each canal opens into the vestibule by a dilated extremity, termed the ampulla. The non-ampullated end of the external canal opens by an independent orifice into the vestibule, while the non-ampull- ated ends of the other two canals have a common opening into the same cavity. The three canals have thus five openings into the vestibule, and three of these openings are ampullated. The Membranous Semicircular Canals . — Contained immediately within the osseous canals is a quantity of perilymph, which surrounds the membranous canals. Each of these repeats the form of the osseous canal in which it is lodged ; and they communicate with the utricle by five openings, three of which are ampullated. The membranous canals contain endolymph, and the ampullated end of each is raised inwardly into a ridge, or acoustic crest , having hair cells, otoliths, and nerve terminations similar to those of the saccule and utricle. The Cochlea is named from its resemblance to a snail's shell. It has the form of a slightly tapering tube wound spirally two and a half times around a central axis — the modiolus. It is thus somewhat conical in form, the base lying inwards near the internal auditory meatus, from which point the axis of the cone is directed outwards, for- wards, and downwards to the apex. Projecting half way into the tube THE EAR. 271 of the cochlea is a lamina, or shelf, of bone — termed the osseous spiral lamina. The tube is thus imperfectly divided into two passages, termed respectively the scala tympani and the scala vestibuli. The separation between these two passages is rendered more complete, and a third passage is marked off, by certain membranous structures. These are the basilar membrane and Meissner's membrane. The basilar membrane stretches from the free edge of the osseous spiral lamina to the outer wall of the tube, where it joins a thickening of the lining of the tube, termed the spiral ligament. Meissner's Membrane is much more delicate, and stretches from the crista spiralis at the free edge of the osseous spiral lamina, obliquely upwards and outwards to the wall of the tube. Fig. 35. Transverse Section through the Tube of the Cochlea. m. Modiolus ; O. Outer wall of cochlea ; SV. Scala vestibuli ; ST. Scala tympani ; DC. Ductus cochlearis ; ?;iR. Membrane of Reissner ; bin. Basilar membrane ; sc. Crista spiralis ; si. Spiral liga- ment ; sg. Spiral ganglion of auditory nerve ; oc. Organ of Corti (Turner). The tube is thus divided into three passages, viz., the scala tympani , the scala vestibuli , and the scala intermedia. The Scala Tympani is the largest of the three passages, and is separated from the other two by the osseous spiral lamina and the basilar membrane. At the base of the cochlea it begins at the fenestra rotunda, by which, in the dried bone, it communicates with the 272 THE ANATOMY OF THE HORSE. tympanum. At the apex of the cochlea it communicates with the scala vestibuli by a small opening — the helicotrema. The Scala Vestibuli is separated from the preceding by the osseous spiral lamina, and from the scala intermedia by Reissner’s membrane. At the apex of the cochlea it communicates with the scala tympani by the helicotrema, and at the base it opens freely into the osseous vestibule. Like the vestibule, it therefore contains perilymph, and this passes also by the helicotrema into the scala tympani. The Scala Intermedia is the smallest but the most important of the three passages. It is the true membranous cochlea , and is called also the ductus cochlearis. It is separated from the scala vestibuli by the mem- brane of Reissner ; and from the scala tympani mainly by the basilar membrane, but partly by the osseous spiral lamina near its free edge. At the base of the cochlea it communicates by the slender canalis reuniens with the sacculus, and it thus contains endolymph. The terminal filaments of the cochlear division of the auditory nerve are distributed in the substance of the basilar membrane ; and on that surface of the membrane which is directed towards the scala intermedia, there occurs a peculiar arrangement of cells, termed the organ of Corti. The Organ of Corti. — When the basilar membrane is examined in transverse section, it is seen to support about the centre of the surface directed towards the scala intermedia a double row of elongated rod- like cells, termed Corti 1 s rods. The rods of the two rows, where they rest on the basilar membrane, are separated by a slight interval ; but they incline towards each other and meet at the opposite extremity, so as to enclose a minute canal — the canal of Corti. On the outer side of the external row of rods, the basilar membrane supports four or five rows of shorter cells, the free extremity of each of which bears a tuft of stiff, hair-like processes. In the same way the membrane supports a single row of hair-bearing cells on the inner side of the inner rods of Corti. On either side these hair -bearing cells are succeeded by cells which become progressively shorter and pass into the general columnar cell lining of the scala intermedia. A delicate cellular membrane — the membrana reticularis — is spread over the outer hair-bearing cells. Through apertures in this membrane, the tufts of hair-like processes project, in a manner comparable to tufts of grass springing through the interstices of a wire net. Still another membrane — the membrana tectoria — springs from the edge of the osseous spiral lamina between the lines of origin of the basilar and Reissner’s membranes, and passes outwards over the organ of Corti. The Auditory (8th) Cranial Nerve. This nerve enters the inter- nal auditory meatus in company with the 7th, which passes into the aqueduct of Fallopius. The 8th divides into two branches, one for the cochlea, the other for the vestibule and semicircular canals. The THE EAR. 273 filaments of the latter branch penetrate the minute foramina seen at the bottom of the internal auditory meatus, and are finally distributed in the saccule, utricle, and ampullated ends of the membranous semicircular canals. The cochlear branch penetrates the modiolus, and in its passage detaches twigs which pass outwards in the osseous spiral lamina to reach the basilar membrane. Within the spiral lamina there are numerous ganglion cells placed on the course of the nerve fibres. T CHAPTER IX. DISSECTION OF THE PERINEUM IN THE MALE* Under this section there will be described not only the perineum proper, but also the scrotum, testicle, prepuce, and penis. The dissec- tion of all these must precede that of the hind limb and abdomen, and it should therefore be begun without delay. THE PERINEUM. Position . — Place the animal on the middle line of its back, and draw its hind legs upwards and outwards by ropes running over pulleys fixed to the ceiling. The posterior extremity of the trunk should be level with, or project slightly over, the end of the table on which the subject rests. Empty the posterior part of the rectum, and stuff it with tow saturated in some preservative solution. A stitch should then be put through the edges of the anus. Surface-marking . — The deep boundaries of the perinseum are those of the outlet of the pelvis (page 341), but its superficial boundaries are as follows : — Above it is limited by the root of the tail, on each side it is bounded by the semimembranosus muscle, and inferiorly it is continued without any limit into the cleft between the thighs. On the middle line below the root of the tail is the anus. This forms an eminence more pronounced in the young, than in the old, animal. The integumental covering of the eminence is thin, puckered, and hair- less; and it is generally dark-pigmented. Passing between the rectum and the root of the tail on each side, and most distinct when the latter is forcibly elevated, there is a projection caused by the so-called suspen- sory ligament of the rectum. Beneath the anus there can be seen or felt a longitudinal prominence formed by the urethra ; and on the middle line of this, there is a median raphe which is prolonged between the thighs. Directions . — Make a mesial incision through the skin for a length of six inches below the anus. Carry this incision round the sides of the anus, and up to the root of the tail. Make another incision trans- versely from one tuber ischii to the other. These incisions will enable * The description of the perinseum in the female is incorporated with that of the pelvis. DISSECTION OF THE PERINEUM IN THE MALE. 275 sufficient skin to be raised as four triangular flaps. Around the anus there is a quantity of fat, whose amount varies with the condition of the subject, but is greater in the young, than in the old, animal. In this fat the perineal nerves are to be followed. Perineal Cutaneous Nerves. — 1. Hcemorrhoidal Branch of 5th Sacral Nerve. This nerve will be found emerging at the hinder edge of the coccygeal origin of the semimembranosus, and curving downwards and inwards at the root of the tail. It supplies the skin there, and gives some twigs downwards to the skin of the anus. 2. Hoemorrhoidal Nerve. The trunk of the hsemorrhoidal nerve, which cannot be reached at present (page 343), divides between the sacro-sciatic ligament and the retractor ani. Its branches are as follows : — 1. A branch appears at the inner side of the coccygeal origin of the semimembranosus, and is distributed at the side of the anus. 2. External to the preceding a branch perforates the semimembranosus; and descending over the tuber ischii, it is distributed at the side of the penis. 3. About an inch or two below the anus a branch appears near the middle line, and descends over the urethra. 3. Pudic Nerve. Ascending on the side of the anus, beneath the branches of the hsemorrhoidal nerve, are some twigs from the pudic nerve. They terminate in the skin and the sphincter ani. Perineal Fascia. The lower part of the perinseum is covered by two layers of fascia, viz., a superficial and a deep. The superficial layer is attached laterally to the fascia covering the muscles on the inside of the thigh, towards the anus it loses its aponeurotic character and becomes cellular, and interiorly it blends with the dartos. The deep layer is reflected upwards at each side of the penis, while above and below it loses its distinctness and becomes cellular. Directions . — These layers of fascia should be removed, and the parts should be cleaned after the manner of Plate 37. Beneath the deep layer a branch of the pudic nerve will be found descending on the acceler- ator urinae muscle. The transversus perinaei, if present (it was absent in the subject from which the Plate was taken), will be found concealing the internal pudic artery, and may be removed on one side. The Internal Pudic Artery. This vessel is a branch of the internal iliac artery (Plates 46 and 47). It descends obliquely along the side of the pelvis, on the inner side of the sacro-sciatic ligament or within its texture. At the small sacro-sciatic foramen it passes backwards and inwards to turn round the ischial arch. It penetrates the urethral bulb, immediately resolving itself into a number of branches that supply the erectile tissue of that body. Its position should be particularly noted with reference to the operation of lithotomy, in which, by making a mesial incision, the urethra may be opened without danger of wounding the artery. 276 THE ANATOMY OF THE HORSE. In this part of its course the vessel gives off small branches to the anus and to the erector penis muscle. The Internal Pudic Vein accompanies the artery. The Sphincter Ani Externus. The fibres of this muscle are of the striped variety, and they are circularly disposed around the anus. Above the anus the fibres are fixed at the root of the tail, and below it they unite to form a pointed slip inserted into the perineal fascia. The muscle should be removed in order to expose the next. The Sphincter Ani Internus. This is comprised between the outer muscle and the mucous membrane. Its fibres are circularly disposed like those of the external sphincter, from which they differ in being of the non-striped variety. They are, in fact, nothing more than the last of the circular muscular fibres of the rectum ; and in the horse they are not aggregated in the form of a ring, as they are in man. Action of the sphincters. — To maintain the anus closed except during the passage of excreta. The Retractor Ani ( Levator ani of human anatomy). This muscle is red like the external sphincter. It arises (but this cannot be seen at present) from the superior ischiatic spine, and from the inner surface of the sacro-sciatic ligament over the small sacro-sciatic foramen. Its fibres pass upwards and backwards, and terminate in tendinous slips that are insinuated beneath the anterior edge of the external sphincter. Action . — During the passage of faeces the anus is carried backwards and everted, and the action of this muscle is to carry the anus forwards and invert it after the act of defecation. The Retractor Penis. This muscle descends at the side of the rec- tum, immediately in front of the external sphincter, and under cover of the termination of the retractor ani, which must therefore be raised and turned forwards. The fibres of the muscle are non-striped, and they form a narrow riband which arises from the 1st and 2nd or 2nd and 3rd coccygeal bones. The right and left bands meet below the rectum, for which they thus form a kind of sling. They are then prolonged downwards on the middle line of the corpus spongiosum, on which they are lost near the extremity of the penis. Action . — To retract the penis within the prepuce when erection passes off. The Suspensory Ligament of the Rectum (Plate 46). This, although denominated a ligament, is composed of non-striped muscular tissue. It is derived from the longitudinal muscular fibres of the rectum, which it leaves in front of the external sphincter; and passing upwards, it becomes inserted into the 4th and 5th coccygeal vertebrae. It forms at the root of the tail a prominence which has already been referred to. The Transversus Perin^i. This muscle is not constantly present. It arises from the tuber ischii, behind the origin of the erector penis ; PLATE XXXVII & Printed. byW. ScA.X. Johnston, Edinburgh &. London DISSECTION OF THE PERINiEUM IN THE MALE. 277 and it passes transversely inwards to terminate on the middle line over the urethra, being confounded with its fellow of the opposite side, and with the first fibres of the accelerator urinse. Action . — To dilate the bulbous part of the urethra. The Accelerator Urin^:. These muscles (right and left) cover the sides and lower face of the urethra from the ischial arch to the free extremity of the penis. Along the inferior median line of that tube the right and left muscles are joined by an intermediate fibrous raphe. From this raphe the fibres pass round the urethra on each side, with a slightly forward inclination, and are lost on the upper aspect of the tube, but without reaching the middle line. Beneath the anus the first fibres of the muscle seem to arise from the retractor penis muscle, but elsewhere the retractor is superficial to the intermediate raphe of the right and left muscles. Action . — The muscles of opposite sides always act together ; and when they do so, they diminish the calibre of the urethra and expel its con- tents. In this way they are instrumental in the ejaculation of semen. In micturition the muscles ordinarily do not come into play until the close of the act, when they empty the urethra from behind to before. The necessity for this action exists because the expelling power of the bladder is lost as soon as its own cavity is emptied. The Erector Penis. This is a thick, dark-red muscle covering the crus penis. Its fibres arise from the inferior ischiatic spine (of the tuber), and they terminate on the crus. Action . — To aid in erecting the penis by compressing the crus and thus retarding the return of blood from the cavernous body of the penis. THE SCROTUM (PLATE 37). Position . — Let the subject remain in the dorsal position, but unfasten the rope from one of the hind limbs, and allow the trunk to incline to the same side. The loose limb should be fastened backwards out of the way. Directions . — Grasp the neck of the scrotum close to the wall of the abdomen, so as to tighten the skin over the testicle, and then tie a piece of soft cord round the constricted neck of the scrotum. This will facili- tate the dissection of the different layers of the pouch. The scrotum is the bag, or pouch, in which the testicles are sus- pended. It is laminated, and comprises the following layers : — 1. The Scrotal Integument. This is continuous with the surround- ing skin, of which it is a modified portion. It is thin, with short fine hairs, and numerous sebaceous glands, whose secretion renders it moist. It is traversed mesially by a raphe, continuous posteriorly wfith the median raphe of the perinseum. The scrotal integument forms a single bag for the two testicles. 278 THE ANATOMY OF THE HORSE. 2. The Dartos. If a portion of skin be removed from the scrotum, it will expose a reddish-yellow layer, composed of connective-tissue with many elastic fibres and a considerable quantity of involuntary muscular tissue. This is the dartos, and, like the remaining tunics of the testicle, it forms two distinct pouches, one for each testicle. In the mesial plane, over the median raphe, the right and left pouches are applied together and form the septum scroti; but superiorly they separate to allow the penis to pass between them. Traced upwards, the dartos is continuous around the external abdominal ring with the subcutaneous fascia. Under the contraction of the muscular tissue of the dartos, the scrotum becomes firm and wrinkled; during relaxation the scrotum is smooth and pendulous. 3. The Spermatic Fascia, continuous with the tendon of the external oblique tendon. 4. The Cremasteric Fascia, continuous with the internal oblique muscle. 5. The Infundibuliform Fascia, continuous with the transversalis fascia. 6. The Tunica Vaginalis Reflexa, a layer of serous membrane con- tinuous with the peritoneum of the abdominal cavity. In Plate 37 these layers are semi-diagrammatically represented as suc- ceeding each other like the coats of an onion. The dissector will probably be unable to discriminate layers 3, 4, and 5. The tunica vaginalis reflexa, he will recognise as a semitransparent layer which, when cut through, takes him into a smoothly lined pouch in which the testicle lies free. This is the sac of the tunica vaginalis , a diverticulum of the peritoneal cavity. In the foetus the testicles make their first appearance in the sub- lumbar region, close behind the kidneys. As development proceeds, they descend through the abdominal wall into the scrotum ; and hence the correspondence between the coverings of the testicle and the layers that compose the wall of the abdomen. If the dissector will now lay hold of the testicle, and endeavour to drag it out of the opening wdiich he has made in its coverings, he will bring into view the spermatic cord. The testicle, he will observe, is covered by a glistening serous membrane, the tunica vaginalis propria , which he can trace upwards on the cord. This spermatic cord contains the vessels, nerves, and excretory duct (vas deferens) of the testicle, which structures descend through the abdominal wall by an oblique passage termed the inguinal canal. In the upper part of this canal, which is not to be exposed at present, the tunica vaginalis propria of the cord is continuous with the tunica vaginalis reflexa. The Cremaster Muscle is continuous with the cremasteric fascia already described. It is a bright red muscle, placed in the cord beneath DISSECTION OF THE PERINEUM IN THE MALE. 279 its serous covering. It passes upwards through the inguinal canal, in which its connections will be observed at a later stage. The Spermatic Vessels. The spermatic artery is an important vessel from the haemorrhage to which it may give rise in castration. It is placed in the anterior part of the cord, and in a well-injected subject its remarkably convoluted disposition will be evident without dissection. The spermatic veins accompany the artery. They are large and tor- tuous. The Vas Deferens is the excretory duct of the testicle, and is placed at the posterior part of the spermatic cord, where it may be seen and felt as a thick, firm tube. Directions. — The student, having identified these different elements of the cord, may practise the operation of castration by any one of the common methods, taking care to sever the spermatic cord just above the epididymis, at the upper border of the testicle. The cord is to be left in the inguinal canal. THE TESTICLE AND EPIDIDYMIS (PLATES 46 AND 47). The Testicle is the gland that secretes the semen — the male fertil- izing fluid. In form it is ovoid. Its faces, right and left, are smooth and rounded; its inferior border is slightly convex and free; its upper edge is nearly straight, and is related to the epididymis. Its anterior extremity shows below the globus major of the epididymis a small cyst- like body — the pedunculated hydatid of Morgagni. The Epididimis is made up of the convolutions of the excretory tube of the testicle. It presents anteriorly an enlargement termed the globus major , and posteriorly a lesser enlargement termed the globus minor , the intermediate part being called the body. At the globus minor the tube loses its convoluted disposition, and is continued as the vas deferens, which, as already seen, becomes one of the constituents of the spermatic cord. Structure. The testicle has for its most external investment the tunica vaginalis propria. This, as already explained, is a serous mem- brane which passes on to the testicle from the cord, and is continuous with the peritoneum at the upper opening of the inguinal canal. It is, as it were, the visceral part of a serous membrane, the tunica vaginalis rejlexa — the inner lining of the bag in which the testicle lies free — being the parietal portion of the same membrane. This covering is thin and transparent, and closely adherent to the next covering — the tunica albuginea. The tunica albuginea is a complete envelope of dense, lamellated connective-tissue, containing some fibres of non-striped muscular tissue. Towards the upper and anterior part of the testicle, a strong process from the tunica albuginea passes into the interior of the gland. This is termed the corpus Highmori , or mediastinum testis; and 280 THE ANATOMY OF THE HORSE. between it and the inner surface of the tunic, numerous trabeculae pass, forming a framework for the gland, and dividing it into a number of conical compartments, or lobules , which lodge the seminal tubules. On the inner surface of the tunica albuginea, and on its trabeculae, the bloodvessels are distributed, forming the tunica vasculosa. Each seminal tubule begins either with a blind extremity, or by anas- tomosing with an adjacent tubule. The tubes are highly convoluted until they approach the mediastinum, where they unite to form a series of straight tubes — the tubuli recti , which enter the mediastinum and form in it a network — the rete testis. From this network arise a number of tubes termed the vasa efferentia , which perforate the tunica albuginea above the anterior end of the testicle. On leaving the gland, these become convoluted, forming little masses known as the coni vasculosi; and they then unite with one another until there results a single excre- tory tube, whose convolutions make up the globus major, body, and globus minor of the epididymis. The seminal tubules are composed of a membrana propria and an epithelial lining. The epithelium is arranged in several layers, and through the agency of the innermost cells — spermatoblast cells — the spermatozoa of the semen are produced. The tubuli recti and rete testis are lined by a single layer of columnar epithelium. The tubes of the vasa efferentia and epididymis have a wall that contains non-striped muscular fibres, and they possess a colum- nar ciliated lining. THE PREPUCE. The prepuce, vulgarly called the “ sheath,” is the involution of skin which lodges the free portion of the penis when that organ is non- erect. In this condition it consists of two layers — an external, similar to the surrounding integument, with which it is continuous; and an internal, which is intermediate in texture between skin and mucous membrane. The latter layer is smooth, destitute of hair, and provided with numerous preputial glands, which secrete a strong-smelling sebaceous material. This material facilitates the protrusion of the penis during erection; and, ordinarily, it accumulates in considerable amount within the prepuce. These two layers are continuous with one another at the orifice of the preputial cavity, and at the posterior end of the cavity the inner layer is continuous with the investment of the penis. Towards the orifice of the perputial cavity, two rudimentary tubercle-like teats are sometimes found. Lay hold of the extremity of the penis, and pull it forcibly forwards, at the same time pulling the prepuce backwards. This will obliterate the prepuce, as in erection, in which condition the inner layer of the prepuce becomes a part of the covering of the penis. Directions. — While the penis is pulled forwards out of the prepuce, carry a mesial incision through the skin from the perinseum to the DISSECTION OF THE PERINEUM IN THE MALE. 281 entrance of the prepuce, and reflect the skin on each side for three or four inches. Suspensory Ligaments of the prepuce. When the outer cutaneous layer of the prepuce is removed, there is exposed an elastic fibrous layer which descends into it on each side from the abdominal tunic. These are the suspensory ligaments of the prepuce. Vessels and Nerves. The cutaneous nerves of the prepuce and scro- tum are branches of the inguinal nerve or nerves. One or more of these, derived from the 2nd and 3rd lumbar nerves, descend through the inguinal canal. The arteries are branches of the subcutaneous abdominal artery. This vessel, which is a branch of the external pudic artery, passes forwards a few inches from the middle line. The trunk of the artery is to be left undisturbed at present. A rich plexus of veins exists in and around the scrotum. This plexus is drained by a comparatively small vein that accompanies the external pudic artery, and by a larger vessel which penetrates the gracilis to empty itself into the femoral vein. THE PENIS. Directions. — While the penis is pulled forwards, reflect the integu- mental covering from the upper face of its free portion, and follow back- wards its dorsal vessels and nerves. Dorsal Arteries of the penis (Plates 39 and 46). On each side there are two of these, distinguished as anterior and posterior. 1. The anterior dorsal artery of the penis is one of the terminal branches of the external pudic artery. It results from the bifurcation of that vessel imme- diately after its emergence from the inguinal canal, and after a course of a few inches it divides into an anterior branch which passes forwards on the free portion of the penis, and a posterior which passes backwards on the fixed portion, meeting and anastomising with the posterior dorsal artery. When the penis is non-erect, the anterior of these branches has a flexuous disposition, which permits it to be elongated without stretching when the organ becomes erect. 2. The posterior dorsal artery of the penis is a branch of the cavernous artery (from the obturator). It runs forwards on the dorsal aspect of the fixed portion of the penis, and anastomoses with the posterior division of the anterior dorsal artery. These arteries are mainly expended in branches to the cavernous and spongy portions of the penis, and they also give off some twigs to the prepuce. Dorsal Nerves of the penis. These nerves, right and left, accom- pany the dorsal vessels on the dorsum, or upper surface, of the penis. Each is the continuation of the pudic nerve, which reaches the penis by turning round the ischial arch. In proceeding forwards along the penis, the nerves are disposed in a flexuous manner to allow them to be 282 THE ANATOMY OF THE HORSE. adapted without stretching to the varying length of the organ. They emit numerous branches to the cavernous and spongy portions of the penis, and terminate in the glans. Suspensory Ligaments of the penis (Plate 46). These are two fibrous bands, right and left, which are attached superiorly to the tendon of origin of the gracilis, and below to the cavernous body of the penis. Directions . — The penis may now be freed as far as its posterior extremity, and its surface cleaned of vessels, nerves, and connective- tissue. On one side the erector penis muscle should be removed, to lay bare the crus and expose the artery of the corpus cavernosum. The Artery of the Corpus Cavernosum (Plate 46). This is a branch of the obturator artery, detached after the emergence of that artery from the obturator foramen. It passes backwards on the lower face of the ischium, and perforates the crus penis. It gives off as a collateral branch the posterior dorsal artery of the penis. The Penis (Plates 46 and 47) is the male organ of copulation. It begins at the ischial arch, where it is attached by its crura to the ischial tuberosities; and it terminates anteriorly in a free enlargement — the glans. It may be said to consist of a posterior fixed portion, and an anterior portion wdiich is free and protrusible. The former portion extends from the ischial arch to the scrotum; the latter, when the organ is non-erect, is lodged in the prepuce, but during erection the prepuce becomes obliterated, and this part of the penis then projects freely in front of the scrotum. The penis is compounded of three longitudinal and parallel columns, viz., two corpora cavernosa and a single corpus spongiosum. From the relationship of these to one another, the penis has been happily com- pared to a double-barrelled gun, the barrels being represented by the corpora cavernosa, and the ramrod by the corpus spongiosum. The Corpora Cavernosa. Each corpus cavernosum begins at the tuber ischii, to whose inferior ridge (inferior ischiatic spine) it is firmly attached under cover of the erector penis muscle. These constitute the roots, or crura , of the penis, and they converge towards each other and form a single mass which makes up the main thickness of the penis as far as the glans. The united corpora cavernosa have an upper flattened surface, or dorsum, along which the dorsal vessels and nerves pass. Their sides are smooth and slightly rounded, and interiorly they form a shallow median groove for the corpus spongiosum (Fig. 44). Anteriorly they terminate bluntly in the glans. The Corpus Spongiosum forms a much more slender column than the corpora cavernosa. It is traversed in the whole of its length by the extra-pelvic part of the urethra. This urethra, as will subsequently be seen, begins at the neck of the bladder, and its first few inches are intra-pelvic, being placed over the ischiatic symphysis. Turning round DISSECTION OF THE PERINEUM IN THE MALE. 283 the ischial arch, the intra-pelvic urethra becomes directly continuous with the extra-pelvic portion, and from the point of continuity onwards the urethra is enveloped in a sheath of erectile tissue, which is the corpus spongiosum. The corpus spongiosum forms at either of its extremities an enlargement. The posterior enlargement, which is situated at the ischial arch, is termed the bulb ; the anterior enlarge- ment is the glans 'penis. The glans forms the expanded free extremity of the penis, and it surrounds the blunt anterior end of the united corpora cavernosa. During erection the enlargement assumes a shape resem- bling, somewhat, the rose of a watering-can, having a prominent ridge — the corona glandis , behind which there is a slight constriction — the cervix. The front of this rose-like swelling presents a fossa from which the urethra projects for about half an inch as a free tube — the urethral tube. Above the base of the urethral tube there is the opening of a double cavity — the urethral sinus , which generally contains some of the partially inspissated secretion of sebaceous glands that open into the cavity. Interiorly the corona glandis is interrupted on the middle line by the suburethral notch. The corpus spongiosum as far as the glans is surrounded by the accelerator urinse muscle. Superiorly it fits into the groove on the lower aspect of the corpora cavernosa, and along its under aspect pass the retractor muscles of the penis. Directions. — Immediately in front of the junction of its crura, the penis should now be amputated, that the structure of its component parts may be examined. Structure of the corpora cavernosa. The corpora cavernosa possess a strong envelope of white fibrous tissue, termed the tunica albuginea. This tunica albuginea, besides forming a common envelope to the united bodies, sends inwards an incomplete mesial septum between the two — the septum pectiniforme. This septum when viewed laterally is seen to be perforated by numerous vertical slits, which give its processes a resemblance to the teeth of a comb ; hence the name. Besides the sep- tum pectiniforme, numerous small trabeculae pass into the interior of the corpora cavernosa, and by their anastomosis form a framework for these bodies. The trabeculae are composed of fibrous tissue with some bundles of non-striped muscular tissue. Between the trabeculae are innumer- able intercommunicating spaces, placed between the capillaries and the small veins. During erection the blood is poured into these spaces, and thus is brought about the increase in the size of the organ. At other times the blood passes in the ordinary manner from the capillaries to the venous radicles. In the crura and peripheral part of the cavernous bodies some of the small arteries terminate directly in these venous spaces. The small arteries are imbedded in the trabeculse, and when these are contracted, in the non-erect state, the 284 THE ANATOMY OF THE HORSE. arteries assume a coiled disposition, from which they receive the name arterice helicince. Structure of the corpus spongiosum. The structure of the spongy body resembles, somewhat, that just described. It possesses an envelope of fibrous tissue with trabeculae and a plexus of large veins. In its peripheral part, and in the bulb, it also contains true cavernous spaces, like those of the cavernous bodies but smaller. Structure of the spongy (or extra-pelvic) part of the urethra. This should be laid open on its under aspect with scissors. The lumen of the tube is not uniform. At the ischial arch (this will not be seen at present) it presents a dilatation ; and its calibre is again increased as it enters the glans, forming what is termed in man the fossa navicularis. The interior of the tube is lined by mucous membrane having simple columnar epithelium, except at its orifice, where it is stratified and squamous. The ducts of numerous small racemose glands open on the surface of the membrane. External to the mucous membrane the wall of the urethra is made up of non-striped muscular tissue, arranged as an inner circular and an outer longitudinal layer. CHAPTER X. DISSECTION OF THE ABDOMEN. Before this part can be begun in the male subject, the dissection of the perinseum (Chapter IX.) must be completed. THE ABDOMINAL WALL. Position . — The subject should be placed on the middle line of its back, or slightly inclined to one side, its limbs being drawn upwards and outwards by ropes and pulleys. The Mammary Glands, or the Udder. It is convenient to describe here these glands, since their dissection must precede that of the abdominal wall. They are organs peculiar to the female, occupying the position of the scrotum in the male. As regards their function, they may be viewed as an accessory part of the reproductive system, secreting the milk upon which the young animal subsists for some time after birth. It is only during the period of lactation that they become fully developed, and therefore a subject suited for the satisfactory display of their structure seldom presents itself in the dissecting-room. The glands are two in number, and are placed side by side on the middle line of the abdominal wall, in front of the pubes. They form here a single mass, with a wide and shallow mesial furrow between them. The term “ udder ” is used to include both glands. From the most prominent part of each, the mamilla, teat , or nipple , projects. This has the form of a short, flattened cone. Its free extremity is perforated by two or three orifices belonging to the large milk ducts by which the milk is extracted from the gland. The integumentary covering of both glands and teats is thinner than the surrounding skin, and it is generally black-pigmented. Moreover, the ordinary body hairs are absent over it, their place being taken by a fine down, except over the summit of the teat, where there are no hairs. It is richly provided with sebaceous and sudoriparous glands, whose secretion renders it moist. When the cutaneous covering of the gland is reflected, there is exposed a second envelope, composed of yellow elastic tissue. This covering detaches a number of processes into the interior of the gland 286 THE ANATOMY OF THE HORSE. between its main lobes, and on the mesial plane the elastic envelopes of the two glands are applied together, and form a kind of intermediate septum. A few strong slips of the same texture descend into the gland from the abdominal tunic, and play the part of suspensory ligaments. The secretory structure of the gland is arranged on the racemose type. If a bristle be passed into one of the orifices seen at. the extremity of the teat, it will pass upwards by the large milk duct, and enter a dilatation at the base of the teat, termed the galactopherous or lactiferous sinus. The secretion of milk during the period of lactation is constant, and the liquid accumulates in these reservoirs, to be drawn off by the young animal. The milk ducts and the sinuses are lined by a mucous membrane; and in the substance of the teat, between this mucous lining and the external skin, there are some fibres of non-striped muscular tissue, arranged both longitudinally and circularly. The circular fibres prevent the escape of the milk from the sinus. The milk enters each sinus from a number of tubes which, w r hen traced into the substance of the gland, divide and subdivide ; and the smallest ducts resulting from this subdivision lead up to the ultimate acini of the gland structure. These acini are lined by a secretory epithelium by whose agency the milk is formed. The arteries and veins of the glands are branches of the external pudic vessels. They undergo a great increase in size during lactation. The nerves of the gland are branches of the inguinal nerves. Directions . — Reflect the skin as shown in Plate 38. If the dissector of the fore limb be engaged with the pectoral region, the skin from the posterior part of that region will be turned back in a piece with that over the front of the abdomen. If not, the dissector of the abdomen must limit the skin which he is about to reflect, by an incision carried outwards from the ensiform cartilage to the point of the elbow. He will be guided in the same way towards the hind limb. Care must be taken not to reflect the panniculus with the skin. A slight degree of tympanitic distension of the intestines is favourable for the dissection of the abdominal wall. When excessive, however, as it often becomes, it interferes with the dissection, and is almost certain to rupture the diaphragm, or the abdominal wall before its dissection can be completed. This should be prevented by tapping the large intestine with a canula and trochar, making the puncture at the most prominent part. Cutaneous Nerves. In reflecting the skin, a multitude of small nerves will be seen on its inner surface. They are derived from the intercostal nerves. The Subcutaneous Abdominal Artery (Plate 38). Look for this vessel near the middle line, in the region of the prepuce or mammary gland. It is one of the terminal divisions of the external pudic artery, Subcutaneous thoracic (spur) vein ABDOMINAL WALL DISSECTION OF THE ABDOMEN. 287 and is distributed to the scrotum and prepuce (skin of mammary gland in mare), superficial inguinal glands, and skin, terminating a little in front of the umbilicus. The Subcutaneous Abdominal Vein runs in company with the artery. The Superficial Inguinal Lymphatic Glands (Plate 38). These form a small group close to the subcutaneous abdominal artery, at the side of the prepuce. The Subcutaneous Thoracic (Spur) Vein (Plate 38) will be found on the surface of the panniculus. The primary rootlets of the vein collect blood from the skin in front of the mamma or prepuce, and pass on to the surface of the panniculus, where they unite to form the trunk of the vein. This is at first lodged in a groove on the superficial aspect of the panniculus. It then perforates the muscle ; and gaining its deep face, it passes forwards towards the axilla (Plate 1), where it joins the brachial vein. The course of this vein is usually distinctly visible in the living animal. From its position it is liable to be injured in deep spurring, and hence one of its names. The Panniculus Carnosus (Plate 38). This is a thin extended sheet of muscular tissue, which is adherent to the deep surface of the skin over a large part of the abdomen and thorax, being continued also from the latter region over the outer aspect of the shoulder. The most posterior angle of the muscle is included in the fold of skin at the groin, but it does not reach the hind limb. From this angle the superior edge of the muscle (which will not at present be seen) slopes upwards with two or three wide sinuosities to near the spine in the dorsal region, while from the same point the posterior edge of the muscle slopes down- wards and forwards to a second angle which is rounded and placed from three to six inches external to the umbilicus. The inferior edge extends from this latter angle forwards towards the elbow. Anteriorly the muscle is continued over the scapular region, and sends also an apon- eurotic tendon between the fore limb and the chest-wall to be attached to the internal tuberosity of the humerus. The edges of the muscle are prolonged by a thin fascia which is attached superiorly to the vertebral spines, and below and behind is adherent to the abdominal tunic. The outer surface of the muscle is with difficulty separated from the skin, which indeed receives the insertion of its fibres. The muscular tissue of the panniculus is, as compared with striped muscles in general, of a pale colour. *■* Action . — It twitches the skin, and plays the part of a hand to the animal in removing offending insects. Directions . — Begin at the lower edge of the panniculus and raise it upwards from the subjacent structures. This is easy over the abdo- minal tunic, but anteriorly it is closely adherent to the edge of the deep pectoral muscle. The panniculus is not to be removed, but raised 288 THE ANATOMY OF THE HORSE. as far as is necessary to bring the origin of the external oblique muscle of the abdomen into view. Notice on the inner surface of the muscle ramifying nerves, and anteriorly the spur vein accompanied by a small branch of the external thoracic artery. Perforating Nerves. The nerves seen descending on the inner surface of the panniculus are perforating branches derived from the intercostal trunks, and from the last dorsal and first lumbar nerves. These perforating nerves appear along a curved line a few inches below the origin of the external oblique. They supply the panniculus, and give cutaneous twigs through it to the overlying skin. A perforating branch from the 2nd lumbar nerve appears close to the bony prominence of the haunch, and descends to the skin on the front of the thigh. A perforating branch from the 3rd lumbar nerve appears below the same bony prominence, and two inches below the point of exit of the preceding nerve. It is accompanied by a branch of the circum- flex iliac artery, with which it descends to the thigh, internal to the last described branch. The Subcutaneous Thoracic Nerve (Plate 1). This will be found running horizontally backwards on the inner surface of the panniculus, behind the shoulder, and in company with the vessels of the same name. It comes from the brachial plexus. Perforating Vessels. Small un-named branches, mostly branches of the intercostal vessels, appear at the same points as the nerves. The Abdominal Tunic (Plate 38). This is a great expansion of yellow elastic tissue which is spread over the inferior and lateral walls of the abdomen. It is nearly co-extensive with the external oblique muscle, to which it is adherent. It is thickest in its posterior part, near the linea alba ; and becomes gradually thinner as it is traced out- wards over the muscular part of the external oblique, and forwards beneath the posterior deep pectoral. Posteriorly it furnishes the sus- pensory ligaments of the prepuce, or analogous slips to the mammary gland. The tunic acts as an admirable elastic abdominal bandage, assisting the muscles to support the heavy abdominal viscera, and adapting the wall of the abdomen to the varying volume of its contents. Directions . — The abdominal tunic must be entirely removed. This is an operation requiring time and care, for the tunic is intimately adherent to the tendon of the external oblique muscle, especially in its anterior half. Transverse incisions should be made through it, taking care not to cut the fibres of the subjacent tendon, which will be recognised by its different colour and texture. Then seize the cut edges of the tunic with the forceps, and tear it off in strips forwards and backwards. Proceed in this way until the whole of it has been torn away. Muscles of the Abdominal Wall. On each side there are four of these, viz., the obliquus abdominis externus, the obliquus abdominis intemus, i & < A ◄ I & Printed byVT ScA.K. Join » ton, EfanVurgh 8c Londi DISSECTION OF THE ABDOMEN. 289 . the rectus abdominis, and the transversalis abdominis. They are stated in the order of their occurrence, the first being the most external. These muscles have not only to discharge the ordinary function of a muscle, but they have also to close in the abdominal cavity; and for this latter purpose, they are, with the exception of the rectus abdominis, peculiarly modified in form. Thus, the two oblique muscles and the transverse muscle have their tendons of insertion extended in the form of great fibrous or aponeurotic sheets, and the fibres in each of these tendons have a direction different from that of the others. The Linea Alba is the white mesial raphe, or band, which extends from the ensiform cartilage to the pubes. It is fibrous in structure, and is formed by the meeting of the aponeurotic tendons of the right and left muscles. A little behind its mid point is a puckered cicatrix — the umbilicus. The External Abdominal Ring (Plate 39). This is the lower orifice of the inguinal canal. It has the form of a slit in the tendon of the external oblique. The direction of the slit is oblique forwards and out- wards. The lips, or pillars, of the slit are simply fibres of the external oblique tendon. The inner angle or commissure is placed at the edge of the prepubic tendon. This prepubic tendon is a strong fibrous band by which the abdominal muscles get a common insertion into the anterior edge of the pubic bones, and from whose surface the pubio-femoral liga- ment of the hip-joint arises. The external abdominal ring gives passage in the male to the spermatic cord, the external pudic vessels, and the inguinal nerves. In the female it transmits merely the corresponding- vessels and nerves. The Obliquus Abdominis Externus (Plate 39). This consists of a muscular band at its antero-superior edge, and an aponeurotic tendon over the inferior and lateral parts of the abdomen. It arises by its muscular portion from the outer surface of the last fourteen ribs, and behind the last rib from the tendon of the latissimus dorsi. Its anterior slips of origin interdigitate with the serratus magnus. The muscular fibres are directed obliquely downwards and backwards, and are suc- ceeded by the aponeurotic tendon. The fibres of the tendon continue in the same direction, and become inserted into the linea alba, the prepubic tendon, and the external angle of the ilium ; while between the two last- mentioned points they are continued to form Poupart’s ligament. Along the line between these two points the fascia of the inside of the thigh is inserted to the surface of the tendon, and it must be cut in order to expose the ligament. It will then be observed that from the prepubic tendon to the bony prominence of the haunch, the fibres of the external oblique tendon, instead of becoming inserted into bone, curve upwards and forwards and are lost to view. It is these reflected fibres that constitute the ligament of Poupart (Plate 40), which may be described 290 THE ANATOMY OF THE HORSE. as having two extremities, two surfaces, and two edges. Its extremities are attached to the pubis and angle of the haunch respectively. Its anterior surface is concave, and directed towards the abdomen. This surface gives origin outwardly to fibres of the internal oblique muscle, and inwardly it forms the posterior wall of the inguinal canal. The posterior surface is convex, and forms an arch over the femoral vessels, the crural nerve, and the sartorius, iliacus, and psoas magnus muscles (Plate 13). Neither of the edges of the ligament has a distinct [exist- ence. The posterior or inferior edge is the line of continuity between the ligament and the tendon of the external oblique. At its anterior or superior edge the ligament becomes thin in texture, and disappears on the fascia covering the sublumbar muscles. All of these points cannot be made out at present, but they will become evident as the dissection proceeds. Action of the external oblique muscle. — When the right and left muscles act in concert, they bend the trunk, and arch the back. If the spine is fixed, they pull the ribs backwards and assist in expiration. If both the spine and ribs are fixed, they compress the abdominal viscera, and assist in urination, defecation, and parturition. If only one muscle acts, it bends the trunk or pelvis to the same side. The Inguinal Canal is the oblique passage in the abdominal wall through which the testicle descends in the young animal, and in which the spermatic cord is lodged in the adult. The external abdominal ring, which has already been examined, is the lower opening of the canal. Its upper orifice, which will be seen at a later stage, is termed the internal abdominal ring. The direction of the canal is oblique down- wards and inwards, and it is slightly curved with the concavity forwards. Introduce the finger into the canal and press on the posterior wall. This, it will be seen, is formed by the reflected portion of the external oblique tendon — in other words, by Poupart’s ligament. Rotate the hand, and press the finger on the anterior wall, at the same time separ- ating the edges of the external abdominal ring. The anterior wall will be seen and felt to be formed by muscular substance, viz., by the muscular part of the internal oblique. The canal gives passage in the male to the spermatic cord, the external pudic vessels, and the inguinal nerves. In the female it is much smaller, and transmits the corresponding vessels and nerves. The Spermatic Cord. See page 278. The External Pudic Artery (Plate 39) is one of the terminal divisions of the prepubic. In the inguinal canal it descends posterior and internal to the spermatic cord. After its emergence it divides into the subcutaneous abdominal artery, and the anterior dorsal artery of the penis. In the mare the latter branch is represented by the mammary artery. Transversalis abdominis ABDOMINAL WALL DISSECTION OF THE ABDOMEN. 291 The External Pudic Vein is proportionally smaller than the artery, which it accompanies. The Inguinal Nerves are derived from the 2nd and 3rd lumbar nerves, and are distributed to the prepuce, the scrotum, and the adjacent skin. Directions . — Incise the external oblique tendon, from the external angle of the ilium to the edge of the prepubic tendon. Reflect Poupart’s ligament towards the thigh, and hook it up after the manner of Plate 40. Then strip away the tendon of the external oblique from the subjacent internal oblique. This will be found easy in the region of the flank, where the tendon is related to the muscular part of the internal oblique ; but over the inferior part of the abdomen, and especially in front, where the tendons of the two muscles are applied to each other, the opera- tion is difficult, and in some parts impossible. In this proceeding the dissector has to guard against removing the thin tendon of the inner muscle along with the outer, and this he will best do by observing that the fibres of the inner tendon cross these of the outer at right angles, being directed downwards and forwards. Observe that anteriorly the two tendons are not simply in apposition, but actually interwoven — a disposi- tion of tendons which is unique, and one which greatly increases the strength of the abdominal floor. The muscular portion of the external oblique should be raised as far as the lower extremities of the ribs. A better view of the inguinal canal and its contents will now be obtained. The Obliquus Abdominis Internus (Plate 40) consists of a fan-shaped fleshy portion situated in the flank, and an aponeurotic tendon spread over the abdominal floor. It arises from the external angle of the ilium, and from the adjacent part of Poupart's ligament. It is inserted into the prepubic tendon and the linea alba by the inferior edge of its tendon, and by tendinous slips into the four or five last costal cartilages. In front of the lower end of the fourth last intercostal space, the aponeurotic tendon has a free edge which ordinarily lies under concealment of the line of overlapping costal cartilages. When the abdomen is tym- panitic, however, this edge is thrust outwards, and the transversalis muscle is exposed as in Plate 40. The posterior edge of the fan-like muscular portion lies in contact with Poupart’s ligament ; and the inguinal canal, as already seen, passes between the two structures. The highest fibres of its muscular part are parallel to the edge of a small muscle — the retractor costce — inserted into the last rib, under cover of the most posterior slip of the serratus posticus. This is described with the muscles of the back (page 96). Action . — Similar to that of the external oblique. Directions . — The internal oblique covers the transversalis and rectus abdominis muscles. The outer edge of the last may be seen through the thin tendon of the internal oblique, and through the same tendon the posterior abdominal artery may be seen if well injected 292 THE ANATOMY OF THE HORSE. (Plate 40). The circumflex iliac artery is on the deep surface of its muscular portion. In order to see these connections of the muscle to the most advantage, incise the muscle along the line of junction of the muscular fan and the tendon. Raise the muscular portion carefully, and hook it back. Strip away entirely the aponeurotic tendon, using the scalpel where the tendon is firmly adherent to the rectus abdominis. The Circumflex Iliac Artery. This is a branch of the external iliac artery, and will be better seen in the dissection of the sublumbar region (Plate 44). It has an anterior division whose branches are distributed to the internal oblique and transverse muscles in the flank, and a 'posterior division which, after giving some twigs to the oblique muscles, perforates them below the angle of the haunch, and descends to the thigh. The Posterior Abdominal Artery (Plate 40). This is a branch of the prepubic artery, beginning at the inner side of the internal abdominal ring. It places itself on the abdominal aspect of the internal oblique muscle, crosses behind and internal to the ring, and runs forwards to enter the rectus abdominis, in which, about midway between the sternum and the pubis, it anastomoses with the anterior abdominal artery. These arteries are accompanied by veins of the same names. The Rectus Abdominis (Plate 40). This muscle extends in the form of a broad band from the sternum to the pubis, at the side of the linea alba. To a large extent it separates the internal oblique and transverse muscles, but beyond its outer border these muscles are in contact in the flank and below the extremities of the ribs. The muscle is widest about its centre, and it is crossed from side to side by a number (about a dozen) of white lines — lineae transversce, which are caused by as many tendinous intersections of its muscular substance. It arises from the lower face of the sternum, and from the five costal cartilages behind the 4th. It is inserted into the anterior border of the pubis by the prepubic tendon. Action. — Similar to that of the oblique muscles. Nerves (Plate 40). At the lower ends of the last ten intercostal spaces, the intercostal nerves are prolonged beyond the rim of overlapping cartilages to pass between the straight and transverse muscles, giving fibres to both and also some perforating twigs to reach the skin. The last dorsal nerve (behind the last rib) has a similar distribution. The inferior primary branches of the 1st and 2nd lumbar nerves are similarly prolonged after furnishing twigs to the oblique muscles in the flank. Directions. — Cut the rectus abdominis transversely about the um- bilicus, and reflect it forwards and backwards from the subjacent trans- versalis. Look for the anterior abdominal artery on its deep face. The Anterior Abdominal Artery is one of the terminal branches of the internal thoracic artery. It appears at the side of the ensi- DISSECTION OF THE ABDOMEN. 293 form cartilage, where it turns round the 9th costal cartilage behind its tip. It runs backwards along the middle of the superior face of the rectus, giving off lateral branches, and terminating about midway between the sternum and pubis in branches which anastomose with those of the posterior abdominal artery. It is accompanied by a satellite vein. The Transversalis Abdominis (Plate 40). This muscle consists of a fleshy band at its origin, and of an aponeurotic tendon over the abdominal floor. In both of these the direction of the fibres is transversely downwards and inwards towards the linea alba. It arises by its fleshy portion from the lower extremities or cartilages of the asternal ribs (last ten), meeting here the origin of the diaphragm ; and from the transverse processes of the lumbar vertebrae. It is inserted by the inner edge of the aponeurotic tendon into the ensiform cartilage and the linea alba. The posterior edge of the tendon is thin and ill- defined. The inner surface of the entire muscle is related to the parietal peritoneum, there being interposed, however, a very thin layer of connective-tissue representing the fascia transversalis of man. Slender branches from the intercostal or asternal vessels run on the peritoneal surface of the muscle. Action . — Similar to that of the oblique muscles. Directions . — The abdominal cavity will be exposed by the removal of the transverse muscle and its peritoneal lining. If only one side of the abdominal wall has been dissected, the other side may now be used for the better display of things not satisfactorily made out in the first; and particularly, a portion of the abdominal wall in front of Poupart’s liga- ment should be turned back in its entire thickness, so as to expose its peritoneal aspect and the internal abdominal ring. The Internal Abdominal Ring (Plate 44) is the abdominal opening of the inguinal canal. As seen from the abdominal side, its posterior or outer edge is prominent, and corresponds to the edge of the muscular part of the internal oblique ; while the opposite boundary of the ring is flattened over the sublumbar muscles covered by the continuation of Poupart’s ligament. The student can now see the direct continuity between the peritoneum and the tunica vaginalis, the latter membrane passing directly into the inguinal canal, and forming a well-defined edge on the posterior and outer side of the entrance. It is by this opening that a portion of intestine or mesentery sometimes passes into the inguinal canal, or onwards into the scrotum, constituting an inguinal or scrotal hernia. The Prepubic Artery (Plate 44). This vessel arises from the femoral artery at the brim of the pubis, forming a short common trunk with the deep femoral branch. It crosses to the edge of the internal oblique, and divides into the external pudic and posterior abdominal arteries. The 294 THE ANATOMY OF THE HORSE. former enters the inguinal canal at a point internal to the internal abdominal ring. The latter passes behind the ring, and crosses it on the inner side. Both branches have already been followed, but the relation of the posterior abdominal artery to the ring should now be specially noted, as, in consequence of its position, an incision for the relief of a strangulated hernia must be made outwards to avoid wound- ing the vessel. The Spermatic Cord. The various structures which compose the spermatic cord meet at the internal abdominal ring. The vas deferens is seen turning inwards to enter the pelvis, and projecting the peri- toneum to form a small band, or fnenum, for itself. The vessels and nerves of the cord are to be left undisturbed, so that they may be followed to their source at a later stage. The Cremaster Muscle (Plate 44). The fibres of this muscle are now seen at their origin from the iliac fascia, where they are close to the muscular fibres of the internal oblique. They pass into the inguinal canal, where, separating but remaining connected by intermediate areolar tissue, they constitute the cremasteric covering of the cord and testicle. When the muscle contracts, it twitches the testicle upwards by shortening the spermatic cord. THE CAVITY OF THE ABDOMEN. Boundaries of the Cavity. — The abdomen is the largest of the visceral cavities of the body. It is placed behind the thorax, from which it is separated by the diaphragm; posteriorly it is directly continuous with the cavity of the pelvis; laterally and interiorly it is enclosed by muscu- lar, tendinous, and elastic textures making up what is generally termed the abdominal wall ; and superiorly it is bounded by the lumbar portion of the spine clothed by the sublumbar muscles. Contents of the Cavity. — The cavity is occupied mainly by the gastro- intestinal part of the alimentary tube, and its associated glands — the liver and the pancreas. Besides these, it lodges the spleen and the kidneys. In the female it contains the ovaries and the uterus (in part), and in the male the vas deferens passes through it. Divisions of the Cavity. — As a matter of convenience in describing the position of its contained organs, the cavity is arbitrarily divided into the following nine areas : — left hypochondriac epigastric right hypochondriac left lumbar umbilical right lumbar left iliac hypogastric right iliac This subdivision is quite arbitrary, the boundaries between these areas being certain imaginary planes. Thus, the three anterior regions are separated from the three middle regions by a transverse vertical plane passing through the lower end of the 15th rib, and the three middle DISSECTION OF THE ABDOMEN. 295 regions are separated from the three posterior regions by another trans- verse vertical plane passing through the external angle of the ilium (angle of the haunch). Again, each of these three regions — anterior, middle, and posterior — is further subdivided into a central and two lateral regions, this subdivision being effected by two vertical and parallel longitudinal planes, each passing through the centre of Poupart’s ligament. Directions. — The intestines of the horse, owing to their unwieldy size, and generally also to the weight of their contents, are extremely incon- venient to dissect. From the following description and the accompany- ing plates, the student should first learn how the intestinal tube is divided. He should then, with as little disturbance of the different intestines as possible, observe how they are disposed within the abdo- minal cavity. The Intestines (Plates 41 and 42). The intestinal tube begins at the pyloric orifice of the stomach, and it terminates on the surface of the body, at the anus. It is primarily divided into small and large intestines, and each of these is naturally or arbitrarily divided into segments. The Small Intestine comprises the first portion of the tube, and in a horse of medium size it measures about seventy -two feet in length. As is expressed by its name, it is of smaller calibre than the large intestine. Moreover, it is distinguished from nearly every part of the large intestine by having a smooth and regular contour when distended. The first two feet of the tube occupies a fixed position, and is termed the duodenum. It received this name because in man its length is about equal to the breadth of twelve fingers. The remainder of the small intestine has a comparatively loose mode of suspension ; and it is arbitrarily divided into jejunum and ileum, the former succeeding the duodenum, and measuring about thirty feet, the latter comprising the remainder of the tube — about forty feet. These terms are borrowed from human anatomy, where the term jejunum was applied in consequence of that portion of the intestine being generally found empty in the dead body, while the ileum was so designated on account of its convoluted disposition. The Large Intestine is, for the most part, of vastly greater calibre than the small ; and, unlike the latter, it has when distended, not a smooth, but a bosselated, surface. In a medium-sized animal it is about twenty- five feet in length. It is subdivided — and in a much more natural fashion than the small intestine — into caecum, colon, and rectum , the colon being further subdivided into double and single colon. When the muscles which enclose the abdomen below and on each side have been removed, it most commonly happens that only the large intestines are exposed, and consequently their examination must precede that of the small intestines. The CvECUM is the first of the large intestines. In an animal of 296 THE ANATOMY OF THE HORSE. medium size it measures about three feet in length, and when moderately distended it has a capacity of about four gallons. At one of its extremi- ties it is curved, forming what is termed the crook of the caecum , while the opposite extremity tapers to a blind point, from which the bowel is named. The bowel has a puckered appearance, which is most evident when it is distended. This is owing to the longitudinal muscular fibres of its wall being not uniformly distributed as they are in the small intes- tine, but collected into bands, which shorten the bowel by throwing it into folds. The terminal portion of the ileum (small intestine) joins the caecum on the concave side of the crook, and a few inches above the point of communication is the orifice by which alimentary matters are passed on to the colon. The crook of the caecum is fixed in the right sublumbar region by means of loose cellular tissue, and it is in contact with the right kidney and the pancreas. On its inner side it adheres by cellular tissue to the termination of the double colon, and the duodenum passes round it on the outer side. The remaining portion of the bowel extends downwards and forwards through the right hypochondriac region, terminating by its blind point in the epigastrium. The first portion of the large colon, which lies to its inner side, extends in the same direction, and the peritoneum in passing from the one bowel to the other forms a fold which has been termed the meso-ccecum. As the caecum is not adherent to the abdominal parietes except in the neighbourhood of its crook, it admits of some displacement ; and the student must therefore be prepared to find it deviating somewhat from the course just described. The Double or Large Colon. This bowel is termed double because when taken out of the abdomen it is arranged in the form of two paral- lel portions; but in order that it may be accommodated within the cavity, it has again to be doubled, so that in its natural disposition it presents four portions, which receive numerical designations. In an animal of medium size its length is about ten feet, and its capacity about sixteen gallons. It is puckered like the ceecum, and from the same cause. The 1st division of the bowel begins at the crook of the caecum, by an orifice of communication which is comparatively small. It extends downwards and forwards through the right hypochondriac region, bulg- ing laterally into the umbilical region ; and on reaching the epigastrium, the bow T el becomes bent on itself, forming what is termed, from its relation to the ensiform cartilage of the sternum, the suprasternal flexure. The angle of this flexure forms the point of separation between the 1st and 2nd portions of the double colon. The 2nd division, beginning at the suprasternal flexure, runs back- wards on the left side of the abdomen, occupying the hypochondriac, umbilical, and lumbar regions; and on approaching the entrance of the DISSECTION OF THE ABDOMEN. 297 pelvic cavity, the bowel forms in the iliac or hypogastric region a second flexure — the 'pelvic flexure , the angle of which marks the point of separa- tion between the 2nd and 3rd portions. The 1st and 2nd portions of the double colon have extensive contact with the abdominal wall, and they conceal the other two divisions of the bowel, which lie above them (in the natural standing posture). The 3rd division, beginning at the pelvic flexure, extends forwards along the left side of the abdomen, through the same areas as the 2nd portion, being closely bound to it, and lying immediately above it. On reaching the epigastric region, a third flexure is formed, in contact with the diaphragm, liver, and stomach, and from these relations named the diaphragmatic or g astro-hepatic flexure. This will be brought into view by grasping and pulling backwards the suprasternal flexure, above which it lies. The 4th portion begins at the angle of the diaphragmatic flexure, and passes backwards on the right side of the cavity, lying above the 1st division, and closely bound to it. On reaching the inner side of the crook of the caecum, to which it is adherent, it suddenly becomes much reduced in calibre, and is continued as the small or floating colon. The pelvic flexure of the colon should now be seized and carried for- wards, so as to place the bowel in the position shown in Plate 41. It will now be observed that the bowel is quite unattached except at its beginning and termination, where it adheres to the pancreas and the crook of the caecum. In this disposition the suprasternal and diaphragmatic flexures are obliterated, and the 1st and 4th portions are seen to be closely adherent to one another, and, in like manner, the 2nd and 3rd portions, except just at the pelvic flexure, where, in the angle of the flexure, a small space is bridged over by a racket-shaped piece of peritoneum. It will be noticed also that the intestine varies greatly in calibre at different points. Its greatest diameter is in its 4th portion, and its smallest about the centre of the 3rd. This narrow portion of the intestine is further distinguished from the rest by being not puckered, but plain, when distended. The Small or Floating Colon succeeds the double colon. It is much narrower than that bowel, indeed it does not greatly exceed in calibre the small intestine, from which, however, its coils are readily distin- guished by their puckered appearance. In a medium-sized animal it is about ten feet in length. It is disposed within the abdomen after the manner of the small intestine, being suspended at the free edge of a de- pendency of the peritoneum, termed the meso-colon or colic mesentery. It has a convoluted disposition, and occupies the left lumbar and iliac regions. Its last coil passes into the pelvic cavity, and is continued as the rectum. The Rectum is the terminal portion of the intestine, and is about two 298 THE ANATOMY OF THE HORSE. feet in length. It derives its name from its approximately straight course through the pelvic cavity, in connection with which it will be more fully described. Directions . — The coils of the jejunum and ileum should be arranged in the left flank after the manner of Plate 41. To get a view of the duodenum, the caecum should be thrown across the abdomen, with its point towards the left side. The duodenum will then be seen encircling the crook of the caecum on its outer side. Should the large intestine contain much ingesta, that should be evacuated through an incision across the pelvic flexure of the double colon and another at the point of the caecum. When the ingesta has been expelled, the bowels should be moderately inflated, and the cut ends ligatured. The Duodenum (Plate 44) is the first segment of the small intestine. Its length is about two feet, but it cannot be very well seen in its entirety at this stage of the dissection. It begins at the pyloric aper- ture of the stomach, where it is related to the posterior surface of the liver. It curves upwards and backwards across the lower face of the right kidney, and then sweeping round the crook of the caecum to its outer side, it crosses the spine behind the anterior mesenteric artery, and is continued as the jejunum. It is maintained in position by a narrow band of peritoneum, and in this fixity of position it is distinguished from the rest of the small intestine. The Jejunum and Ileum. These comprise the remaining portion of the small intestine, of which about thirty feet is arbitrarily appor- tioned to the former, and the remainder (about forty feet) to the latter. They are arranged in the form of numerous coils, which occupy the iliac, umbilical, and hypogastric regions. The coils are attached to the free edge of a fold of peritoneum called the great mesentery ; and inasmuch as this mesentery is of considerable breadth, they may move from place to place within the above-mentioned areas. When distended, they have not a puckered, but a smooth, surface. The terminal part of the ileum joins the crook of the csecum, into which it projects for a little distance, after the manner of a tap into a barrel; and at the point of entrance there is a valvular arrangement — the ileo-ccecal valve , to pre- vent regurgitation from the caecum into the ileum. The Peritoneum is the lining membrane of the abdominal and pelvic cavities. It belongs to the class of serous membranes, and, like all such membranes, it consists of a parietal and a visceral division, these being portions of one great sac. The parietal part is that which lines the abdominal walls, or parietes ; the visceral part invests the solid and hollow organs, or viscera, of the abdominal cavity. In virtue of this membrane, all the free surfaces that present themselves when the abdominal wall is removed, have a smooth and shining appearance. The surface of the membrane is covered by a layer of endothelial cells, DISSECTION OF THE ABDOMEN. 299 and these rest upon a layer of vascular connective-tissue. The object of the membrane is to facilitate the movements of the different ab- dominal organs on each other and on the walls of the cavity, and especially to facilitate the vermicular or peristaltic movements of the intestines. For this purpose the surface of the membrane is kept moist by a sparing amount of serous fluid, which gives to the membrane its glistening aspect. To trace the exact disposition of the peritoneum in the horse is very difficult, in consequence of the unwieldy character of the intestines. When the student has the opportunity he should examine the mem- brane in a foal, in which the different organs can be manipulated with ease. The parietal and visceral peritoneum, as has already been stated, form portions of one great sac, and the various abdominal viscera are external to this sac. The sac of the peritoneum, it must be observed, encloses not an actual, but merely a potential, cavity ; the inner surface of every portion of the sac being in contact with the same surface of another portion. To facilitate the understanding of this, let the student imagine the cavity of the abdomen (including the pelvis) as having its natural form, but deprived of all its contents, and completely lined by peritoneum, which, for simplicity’s sake, he may suppose to be elastic. The continuity of the membrane, and the fact that it formed a close sac would then be apparent. Now let him imagine a simple tube of intestine extending between this membrane and the spinal column, that is, outside the serous sac. Conceive next this tube of intestine let gradually down, until it extends through the cavity about its centre. In this descent the intestine would first surround itself with peritoneum ; and then, as it sank farther, it would stretch the membrane so as to form a kind of sling passing upwards to the point from which it started. The membrane would now have lost its simplicity, for it would have a parietal division continuing to line the abdominal walls, and a visceral portion surrounding the tube of intestine. Moreover, these two portions would be continuous with each other along the sling-like portion sus- pending the tube. Lastly, imagine the tube of intestine to grow and branch, so as to completely fill up the abdominal cavity, and obliterate the space between the parietal and visceral peritoneum. This, of course, would not destroy the continuity of the serous sac, although it would complicate it so that its continuity would be difficult to trace. All the organs, then, that actually project into the abdominal cavity get a more or less complete investment of visceral peritoneum ; and, in the case of each organ, this visceral covering is traceable on to a neigh- bouring organ, or on to the walls of the abdomen. Where organs are contiguous to each other or to the abdominal parietes, the peritoneum may pass directly from the one organ to another or to the abdominal 300 THE ANATOMY OF THE HORSE. parietes ; but, at other times, the connection between the parietal and visceral peritoneum is traceable along bands or folds analogous to the sling-like membrane that was formed in the imaginary case. These folds constitute the various mesenteries, omenta, and peritoneal liga- ments that will hereafter be described. Although there is but a single peritoneal sac, this sac is so disposed that it forms two compartments, termed respectively the greater and lesser cavities of the peritoneum, the latter being also known as the cavity of Winslow. The greater cavity is that which is exposed when the inferior wall of the abdomen is removed, the lesser cavity is situated behind the stomach, and is separated from the greater cavity mainly by the omentum. The Great or G astro-colic Omentum . — Passing backwards among the intestines, on the left side of the abdomen, there will have been noticed a large lace-like membrane, which is the great omentum , epiploon , or web. In order to examine its connection, the caecum and double colon should be thrown backwards over the right flank, and the coils of the single colon arranged over the left flank. The coils of small intestine should at the same time be gathered backwards and to the right. The omen- tum is composed of two layers of peritoneum, which include between them vessels, and a varying quantity of fat. This fat is deposited mainly along the course of the vessels, leaving, except in obese subjects, intervening transparent areas that are free from fat ; and it is from this arrangement that the membrane possesses a lace-like appearance. The two layers of the omentum may be distinguished as superficial and deep. When the superficial layer is traced backwards, it is seen to pass on to the terminal part of the double colon (4th part) and initial part of the single colon, covering the posterior aspect of these where they extend across the roof of the abdominal cavity. Behind these it passes back- wards along the roof of the abdominal cavity, from which it descends to envelop the small intestine, forming the great mesentery, and the float- ing colon, forming the colic mesentery. To the right, again, it passes directly on to the caecum and the double colon ; and after enveloping these intestines, it returns to the abdominal wall, to pursue its back- ward course to the pelvis. When followed forwards, the superficial layer reaches the convex curvature of the stomach, and the initial dila- tation of the duodenum ; and it passes over the anterior surfaces of these organs as visceral peritoneum. Passing off the duodenum and stomach, it next forms the anterior layer of the gastro-hepatic omen- tum, and thus reaches the posterior surface of the liver at the portal fissure. From that point it descends over the posterior surface of the liver as visceral peritoneum, and turns round the inferior edge of the gland to gain its diaphragmatic surface. It ascends on this surface ; DISSECTION OF THE ABDOMEN. 301 and where the liver and diaphragm are united, it passes from the former to the latter, on which it descends to the inferior wall of the abdomen. Along this it passes until it enters the pelvis, where it becomes con- tinuous with the same layer already followed backwards along the roof of the abdomen. In the male it is to be observed that the parietal peritoneum of the abdominal floor passes into the inguinal canal, and forms the tunica vaginalis of the testicle, the sac of which is a simple diverticulum of the great peritoneal sac. Returning again to the omentum, it will be noticed that its superficial layer, towards the left side, in passing forwards to gain the convex curvature of the stomach, encounters the spleen. Passing round that organ, it gives to it a visceral covering, and then continues its course to the stomach. The portion of omentum between the spleen and the left sac of the stomach is termed the g astro- splenic omentum. Now make a transverse opening about the centre of the great omen- tum, and introduce the hand through the opening. The hand is now in what is termed the cavity of Winslow, and the deep layer of the omen- tum is exposed. When this layer is traced forwards, it is seen to reach the convex curvature of the stomach, where, separating from the super- ficial layer, it passes over the posterior surface of the stomach, and initial dilatation of the duodenum. From these, again, it passes as the posterior layer of the g astro-hepatic omentum, and reaches the liver at the portal fissure. There it separates from the other layer of the gastro- hepatic omentum, and ascends on the liver. It turns round the superior edge of the gland, and passes from its anterior face to the dia- phragm, on which it ascends to the spine. The deep layer of the omen- tum is now to be followed in the backward direction. It is seen to reach the terminal part of the double colon, and the initial part of the single colon ; and, separating there from the superficial layer, it passes over the anterior aspect of these portions of intestine, and is reflected forwards on the under surface of the pancreas. It turns round the anterior edge of that gland, covers for a little distance its upper face, and then passes on to the spine, where it meets the same layer advanc- ing in the opposite direction. It is thus seen that the deep layer of the omentum, when traced in the antero-posterior direction, forms a con- tinuous layer ; and at first sight it does not appear to be continuous with the remainder of the peritoneum. As already stated, however, the peritoneum forms a single sac, and the before-mentioned layer is con- tinuous with the remainder of the serous membrane at a narrow opening termed the foramen of Winslow. To find this opening, pass the dorsal aspect of the left forefinger along the posterior surface of the lobulus caudatus of the liver, close to the spine ; and insinuate the point of the finger onwards towards the left (of the subject). At the same time pass the right hand up to the spine in the cavity of Winslow, and insinuate the 302 THE ANATOMY OF THE HORSE. forefinger towards the right, above and behind the pylorus. The tips of the forefingers of opposite hands can thus be made to meet, showing the continuity of the larger sac of the peritoneum, in which the left hand is, with the smaller sac, or cavity of Winslow, in which the right hand is. Perhaps the simplest way to get an understanding of the relationship of the two cavities, is to imagine the deep layer of the omentum to be suppressed. In that condition, the anterior aspect of the double and single colon at their point of junction, the pancreas, the posterior surface of the stomach and initial dilatation of the duodenum, the upper parts of the liver and diaphragm, and the roof of the abdomen for a short space behind the hiatus aorticus would be without a serous cover- ing. It may be supposed that to supply this deficiency, a pouch of the great sac of peritoneum has to be made. This pouch is made at the foramen of Winslow, the peritoneum being there thrust outwards towards the right, and expanded until it forms what has already been traced as the deep layer of the omentum. The foramen will be observed to have the following boundaries : — the base of the lobulus caudatus in front, the 4th part of the double colon behind, the free edge of the gastro-hepatic omentum below, and the posterior vena cava and right pillar of the diaphragm above. The Great Mesentery is the membrane that suspends the small intes- tine. Like the omentum, it is composed of two layers of peritoneum. These layers leave the spine at the root of the anterior mesenteric artery, being there continuous with the parietal peritoneum ; and they descend, one on each side of the branches of that artery, until they reach the intestine. At the concave edge of the bowel the two layers separate; and after encircling the tube as visceral peritoneum, they meet and become continuous at its convex or free border. Where the mesentery suspends the first part of the jejunum, it is continuous with the peritoneal freenum of the duodenum ; and at its opposite extremity, where it envelops the termination of the ileum, it passes on to the caecum At the latter point it will be observed that the two layers of mesentery do not become continuous around the convex border of the ileum, but are prolonged beyond that, so that the terminal portion of the small intestine is included in the mesentery some distance from its free edge. The Colic Mesentery . — This is the membrane that suspends the single or floating colon. It is composed of two layers of peritoneum, which leave the roof of the abdomen along a line extending from the root of the anterior mesenteric artery to the inlet of the pelvis. These two layers include between them the posterior mesenteric artery and its branches ; and after enveloping the single colon, they become continuous at its free edge. At its anterior extremity the colic mesentery is con- tinuous with the great omentum and with the great mesentery, and at the pelvic inlet it is continuous with the meso-rectum. DISSECTION OF THE ABDOMEN. 303 The Uterine Broad Ligaments . — These are the double peritoneal folds that suspend the uterus, ovaries, and Fallopian tubes. Each ligament leaves the roof of the abdomen in the lumbar region, and descends to the concave edge of the cornu, and to the side of the upper face of the body, of the uterus. At these points the layers of the ligament sepa- rate, and pass on to the uterus as its visceral covering. The ligaments are widely apart in front; but as they are traced backwards, they become narrower and nearer to each other. The Fallopian tube is sus- tained between the two layers of each ligament at its anterior edge, and here the fimbriated extremity of the tube opens into the sac of the peri- toneum. In the female, therefore, the peritoneum does not form a shut sac. Stretching between the ovary and the uterine cornu is a cord of non-striped muscular tissue — the ligament of the ovary — which forms the free edge of a small secondary fold of peritoneum. This forms with the adjacent part of the broad ligament a pocket-like cavity. On the outer side of the broad ligament another secondary fold extends as far as the internal abdominal ring, and contains a layer of non-striped muscular tissue corresponding to the round ligament of the human uterus. Besides some scattered fibres of non-striped muscle, the layers of the broad ligament include between them the uterine and ovarian vessels and nerves. The other peritoneal ligaments will be described in connection with the organs to which they belong. Peritoneal Pockets. The peritoneum, in passing from one organ to another, forms several remarkable pockets, one of which has been men- tioned above in connection with the ligament of the ovary. The exact position 6f the others will now be indicated. So far as I am aware, these have not hitherto been described. Nevertheless, they possess considerable interest, since, in the human subject, a coil of intestine has been known to become incarcerated in a similar pocket of peritoneum. 1. ‘The entrance to the first of these pockets will be found imme- diately in front of the base of the lobulus caudatus, which separates it from the foramen of Winslow. It is bounded by the anterior end of the right kidney, and by the lobulus caudatus and upper part of the right lotye of the liver. It extends inwards to near the spine between the diaphragm and the upper part of the right lobe of the liver. 2. Another pocket will be found a little to the left of the root of the interior mesenteric artery, the entrance to it being on the anterior surface of the mesentery suspending the first few inches of the jejunum. The poeket is bounded in part by this piece of mesentery, and in part by a peritoneal fold passing between the jejunum and the first part of the single colon. 3. Other two pockets will be found at the termination of the small intestine. Turn the point of the caecum backwards and to the right, 304 THE ANATOMY OF THE HORSE. and pull upon the terminal part of the jejunum. On each side of the point at which the latter perforates the csecal crook, there will be found a recess, the posterior (in this position) being the deeper. 4. Another considerable pocket will readily be found on the concave side of the caecal crook, being formed by the peritoneum in passing between the caecum and the beginning of the double colon. 5. Another but much smaller pocket will be found in the cavity of Winslow, above and in front of the first few inches of the single colon. Directions . — For the display of the mesenteric vessels and the sympa- thetic nerve, the intestines should first be disposed after the manner of Plate 41. When well injected, the arteries require but little dissection, and they are closely accompanied by the veins and nerves. The arteries of the caecum and colon should be taken where most conspicuous, and traced in both directions. Each of these vessels must be carefully dis- sected up to its point of origin, but only two or three of the arteries of the small intestine need be fully dissected. The whole intestinal tube with the exception of a short piece of the duodenum next the stomach, is supplied by the anterior and posterior mesenteric arteries, which are branches of the abdominal aorta. The first supplies the whole of the small intestine except the piece of duodenum specified; and it also sup- plies the caecum, the large colon, and a few inches of the beginning of the small colon. The remainder of the small colon, and the rectum are supplied by the posterior mesenteric artery. The Anterior Mesenteric Artery (Plate 41) comes off from the inferior aspect of the aorta at the 1st lumbar vertebra. It is only about an inch and a half in length, but it has a large calibre; and im old horses it often shews aneurismal dilatation. It divides into three terminal branches, which from their direction are distinguished as Deft, right , and anterior. The left distributes its branches to the whole of the small intestine except a few inches at the beginning of the duodenunji and about two feet at the end of the ileum ; the right supplies the terminal portion of the ileum, the entire caecum, and the double colon as \far as the pelvic flexure; and the anterior is distributed to the double c&lon beyond the pelvic flexure, and to the first few inches of the single colon. It is an assistance to the memory to study the different branches in t/he order of their distribution to the intestine, taking first those that supp‘ Ur the most anterior segment of the tube. 1. The Left Branch of the anterior mesenteric artery is no soone detached than it splits up into about fifteen or twenty arteries, which pass between the layers of the mesentery to supply the small intestine. Indeed, the left branch can scarcely be said to exist, for these arteries of the small intestine seem to spring from a common point of the anterior mesenteric trunk. As each artery approaches the intestine, it bifurcates, each branch inosculating with the corresponding branch of an adjacent PLATE XLI Retrograde colic art. -Diaphragmatic flexure Suprasternal flexure. Colon (4th part) •Ileum Pelvic flexure Colon (2nd part) Direct colic art. Colon (3rd part) Post, aorta st art. to single colon f Ant. div. of ant. mesenteric art. (Arteries to small (intestine Duodenum Small intestines Ilio-csecal art. .Great mesentery / ( ( / U Printed 'by'W. ScA.K. Johneton, Edinburgh fc London INTESTINES AND ANTERIOR MESENTERIC ARTERY ( Chauveau ) DISSECTION OF THE ABDOMEN. 305 artery to form an arch. From the convexity of these arches smaller vessels pass to each side of the intestine, and anastomose round it. At the anterior part of the tube two sets of superposed arches are formed before the ultimate vessels to the intestine are detached. The branch which is most anterior in point of distribution anastomoses with the duodenal branch of the coeliac axis, while the one which is most pos- terior anastomoses with the ileo-csecal artery from the right branch of the anterior mesenteric. 2. The Right Branch of the anterior mesenteric artery divides into four vessels, viz., the ileo-ceecal, the superior csecal, the inferior csecal, and the direct colic arteries. a. The Ileo-coecal Artery (Plate 41, for ilio-ccecal read ileo-ccecat) sup- plies the terminal portion of the ileum (about two feet in length), and inosculates ivith the last of the arteries from the left branch. b. The Superior C cecal Artery, in the present inverted position of the intestines, passes beneath the termination of the ileum to run along one of the longitudinal muscular bands of the csecum. It sometimes gives off the ileo-csecal artery as a collateral branch, and at the point of the csecum it anastomoses with the next vessel. It gives off branches right and left to the walls of the caecum. c. The Inferior Ccecal Artery, in the present position of parts, passes above the termination of the ileum to run along another of the muscular bands of the caecum. Besides collateral branches to the main portion of the bowel, it gives off the artery of the arch, which follows the concavity of the caecal crook and terminates on the beginning of the double colon. d. The Direct or Right Colic Artery . — This is a large vessel, receiving the first of these designations because the course of its blood stream is the same as that of the alimentary matters in the bowel. It supplies, by right and left collateral branches, the 1st and 2nd portions of the double colon, and anastomoses at the pelvic flexure with the retrograde colic artery. 3. The Anterior Branch of the anterior mesenteric artery divides after a very short course into two vessels of unequal size, viz., the retrograde colic artery and the first artery of the small colon. a. The Retrograde or Left Colic Artery, much the larger of the two, supplies successively the 4th and 3rd portions of the double colon, run- ning parallel to the direct colic artery, but carrying its blood in a direc- tion counter to the course of the alimentary matters in the intestine. b. The First Artery of the Small Colon supplies a short piece at the beginning of that bowel. It is included between the layers of the colic mesentery, and anostomoses with the first branch of the posterior mesenteric artery. Directions. — To display the posterior mesenteric artery, the small x 306 THE ANATOMY OF THE HORSE. colon must be spread out over the left flank after the manner of Plate 42. The Posterior Mesenteric Artery is a much smaller vessel than the anterior. It is a branch of the abdominal aorta, from which it is given off at the 4th lumbar vertebra. It passes in a curved direction between the layers of the colic mesentery and meso-rectum, and terminates near the anus in vessels which supply the end of the intestinal tube. From the convexity of its curve, which is directed downwards, about twelve or fourteen branches pass to supply the small colon (except a few inches at its beginning) and the rectum. The branches which supply the first half of the small colon divide and form arches by anastomosis in the mesentery, close to the bowel ; but the more posterior branches do not anastomose until they perforate the intestinal wall. The Intestinal Veins . — The blood which is brought to the intestines by the arteries just considered is carried away by vessels belonging to the 'portal system. These veins for the most part run in close company with the arteries, and receive the same names. The Anterior Mesenteric Vein is a very large vessel having tribut- aries which correspond almost exactly to the divisions of the artery of the same name. It joins the splenic and posterior mesenteric veins to constitute the vena portae. The Posterior Mesenteric Vein has its roots in the haemorrhoidal veins around the termination of the rectum, which veins, on the other hand, communicate with the internal pudic vein. After receiving blood from the walls of the rectum and small colon, the posterior mesenteric vein forms by union with the splenic a very short trunk which joins the anterior mesenteric to form the vena portae. Lymphatic Vessels of the Intestine. — In an ordinary dissecting-room subject the lymphatic vessels will not be visible unless the animal is emaciated and has been killed shortly after a meal, in which case the mesenteric vessels may be seen without dissection. They will be recog- nised as vessels with very thin walls and milky contents, coursing between the layers of the mesentery, from the intestine towards the anterior mesenteric artery. The lympathic vessels of the small intestine are called lacteals. The Lymphatic Glands of the Intestine are very numerous. Those of the small intestine are chiefly aggregated in the form of a cluster of about thirty included between the layers of the mesentery, near the anterior mesenteric artery ; but a number are placed lower down in the mesentery, along the course of the ileo-caecal artery. The glands of the caecum are distributed in the form of two chains along the track of the superior and inferior caecal arteries, and numerous glands are similarly placed on the colon along the course of the direct and retrograde colic arteries. Those of the small colon and rectum are, for the most part, PLATE XLII Printed ‘by'W. &A..K. Johneton, Edinburgh fc London Urinary bladder colic art. colic art. fasciculus of ant. mesenteric fasciculus of ant. mesenteric fasciculus of ant. mesenteric mesenteric art. art. Small intestine mesentery aorta mesente artery iliac art. Int. iliac art. INTESTINES AND MESENTERIC ARTERIES (Chauveau) DISSECTION OF THE ABDOMEN. 307 placed on the wall of the bowel, at the edge of the mesentery ; but a few are included between the layers of the colic mesentery. The lacteals from the small intestine and the lymphatic vessels from the large intes- tine traverse these various groups of glands on their course towards the receptaculum chyli. The Sympathetic Nerve. This nerve forms on the aorta, in front of the anterior mesenteric artery, a great network termed the Solar 'plexus. The solar plexus is at present concealed by the pancreas, but the student has to notice the anterior and posterior mesenteric plexuses, which are wholly or in part derived from it. The Anterior Mesenteric Plexus comprises numerous nerves already met in dissecting the branches of the anterior mesenteric artery. The nerves interlace around the arteries, and pass with them to gain the bowel, where they further interlace before penetrating its wall. The Posterior Mesenteric Plexus. — The branches of this plexus run in company with the divisions of the artery of the same name. Its nerves are derived in part from the aortic plexus, which is a backward continua- tion of the solar plexus, and in part from roots furnished by the lumbar cord of the sympathetic. The ultimate branches are distributed in the wall of the small colon and rectum. Directions. — The intestinal mass is now to be removed in the folio wing- manner. The ropes must be unfastened from the left limbs of the animal, while those on the right limbs are to be lengthened until the subject inclines considerably over to the left side. Two ligatures a few inches apart are to be passed round the duodenum where it encircles the crook of the caecum, and the bowel is then to be cut across between the liga- tures, the object of which is to keep the contents from escaping. Where the small colon joins the rectum, at the entrance to the pelvis, the bowel is to be served in the same way, and the colic mesentery is to be cut along its point of origin at the spine. Both large and small intestines are then to be thrown as far as possible outwards over the left flank. The next step must be to take the scalpel and carefully sever the con- nective-tissue adhesions between the csecal crook and colon on the one hand, and the sublumbar region and pancreas on the other. In doing this, the dissector must cut close to the wall of the bowel, and take especial care not to take away any portion of the pancreas, wdiich will be recognised by its dark colour. The operation will be favoured by the weight of the intestines, which tends to tear these connections. When the csecum and colon have been freed, it will be found that strong resist- ance to the removal of the intestines is still offered by the mesentery, or rather, by its included vessels. These must therefore be cut near the spine, and the entire mass will then slip over the left side, the omentum being cut or torn from its attachment to the colon. The intestines should now be spread out on a table ; and when the student has refreshed 308 THE ANATOMY OF THE HORSE. his memory regarding their form and connections with one another, he must proceed to examine their structure. This should be done by taking a short piece of the gut, slitting it up, and pinning it with its mucous surface downwards on a block of wood. Structure of the Small Intestine. The wall of the bowel is made up of four layers, viz., serous, muscular, submucous, and mucous. 1. The Serous Layer y the most external, is a part of the visceral peritoneum. It reaches the bowel by the mesentery, whose two layers separate at the concave border of the intestine, and pass round each side to meet and become continuous on its convex or free border. It is closely adherent to the subjacent muscular layer, wdiich it completely covers except at the line of separation of the two layers of the mesentery, where the vessels enter. It must be stripped off to expose the next coat. 2. The Muscular Coat is made up of two distinct sets of fibres: 1. Longitudinal fibres, which are most external, and form a thin layer uniformly spread along the wall. 2. Circular fibres, thicker than the preceding, and also spread over every part of the wall. These fibres are of the pale, non-striated variety. 3. The Submucous Coat is composed of loose areolar tissue uniting the muscular and mucous layers. In the duodenum it contains the glands of Brunner, which have the racemose type of structure, and are about the size of a hemp seed. Their ducts pass through the mucous membrane, and open on its free surface. Directions. — A few feet of the jejunum and about the same length of the ileum should be taken and slit up along the line of attachment of the mesentery. After the pieces have been gently washed, they should be spread on a flat surface with the peritoneal coat downwards. 4. The Mucous Membrane forms an inner lining to the intestine. It is a soft, velvety -looking membrane which, when healthy and fresh, has a pinkish-yellow colour. When a piece of intestine is floated in water, the mucous membrane is seen to be studded with short, thread-like projec- tions, to which the velvety appearance of the membrane is due. These are the intestinal villi. Each of them may be regarded as an upheaval of the mucous membrane, containing in its interior microscopic blood and lymph vessels, some non-striped muscular fibres, and a framework of lymphoid tissue. The villi are important agents in the absorption of nutrient particles from the contents of the bowel. They are found throughout the whole of the small intestine, but are more numerous in the jejunum than in the ileum. The free surface of the mucous mem- brane, including the villi, is formed by a single layer of columnar epi- thelium with goblet cells interspersed. Contained within the substance of the membrane are numerous microscopic tubular glands — the glands fo Lieberkdhn — whose mouths open on the free surface. The solitary glands are small spherical bodies about the size of a mustard seed. DISSECTION OF THE ABDOMEN. 309 M They are covered by the epithelium, and occur throughout the whole intestine, but are more numerous in the ileum than in the jejunum. They are composed of lymphoid tissue. The glands of Peyer , or, as they are commonly called, Peyer' s patches, are circular or oval patches formed by the aggregation of solitary glands. They are more numerous in the ileum than in the jejunum, their total number being about one hundred. They are distributed along the convex or free border of the intestine, and hence it was directed that the bowel should be opened along the attachment of the mesentery, so as to leave the patches intact. Directions . — The caecum, with the first few inches of the double colon and a like length at the end of the small intestine, should be separated from the rest of the intestinal mass. After the serous and muscular coats have been observed on the inflated ciecum, the bowel should be slit open on the convex side of its crook, the incision being extended to its point. The mucous surface is to be gently washed; and in connection with its study, the student is to examine the two orifices found on the concave side of the crook. Structure of the Large Intestine. Throughout nearly the whole of its length, the wall of the large bowel is made up of four coats, similar to those of the small infpsfinn V. Intestinal villi ; L. Layer of intestine. glands of Lieb erkuhn ; B. ABrun- 1 . The Serous Coat is derived from the “ e J’ s d. its excretory duct ; S.M. Submucous coat; M. Muscular peritoneum, but it forms here a less com- coat, plete investment than in the case of the small intestine, considerable areas of the wall being without this cover- ing. Thus, it is absent where the caecum and double colon adhere to the pancreas and abdominal parietes in the sublumbar region ; it is also absent where these two intestines adhere to each other, and where the parallel portions of the double colon come into contact; and lastly, as will be seen in the dissection of the pelvis, the terminal part of the rectum is without a peritoneal covering. 2. The Muscular Coat consists of two distinct layers — an external longi- tudinal and an internal circular. Throughout nearly the whole extent of the large intestine, the longitudinal fibres are not uniformly distri- Fig. 36. Vertical Section through the Wall of the Duodenum, show- ing the Glands of Brunner (Turner). 310 THE ANATOMY OF THE HORSE. buted over the wall, but are collected into distinct bands, the areas between the bands being provided only with circular fibres. When these bands contract, they shorten the intestine, and throw the wall of the bowel between them into alternate ridges and furrows. The number of these bands is different at different points. The caecum has four. The colon in its 1st part has also four. Three of these disappear on the 2nd part, so that at the pelvic flexure there is only a single band, on the concave side of the flexure. This single band is continued along the 3rd part, and near the diaphragmatic flexure other two bands originate. The 4th part has three bands. The single colon has two bands, one on each curvature, and these are continued on the first half of the rectum, but are lost on its terminal half, as will be seen in the dissection of the pelvis. The inner layer of circular fibres is uniformly distributed. 3. The Submucous Coat is a layer of loose areolar tissue uniting the muscular and mucous coats. 4. The Mucous Coat lines the cavity of the bowel. Its surface is covered by a single layer of columnar epithelium, and in its deeper part it contains solitary glands and glands of Lieberhuhn , similar to those of Diagrammatic View (magnified) of a small Portion of the Mucous Membrane of the Colon {Allen Thomson). A small portion of the mucous membrane cut perpendicularly at the edges is shown in perspective ; on the surface are seen the orifices of the crypts of Lieberkuhn or tubular glands, the most of them lined by their columnar epithelium, a few divested of it and thus appearing larger ; along the sides the tubular glands are seen more or less equally divided by the section ; these are resting on a wider portion of the submucous tissue, from which the blood-vessels are represented as passing into the spaces between the glands. the small intestine. No Peyer’s patches are found in it; and it is with- out villi. The foldings of the wall of the bowel produced by the longi- tudinal muscular bands involve all the coats, and the interior therefore shows the alternately ridged and furrowed appearance already seen on the exterior. DISSECTION OF THE ABDOMEN. 311 Orifices of the Crook . — These are the apertures of communication with the ileum, and with the large colon. At its termination the ileum pro- jects slightly into the interior of the caecum, and beneath the mucous membrane surrounding the orifice, there is developed a ring of mus- cular fibres. This fold of mucous membrane with its included muscular fibres constitutes the ileo-coecal valve. The opening of communication with the colon is considerably larger than the preceding, above which it is placed. Directions. — The student must now return to the parts left within the abdominal cavity, where, without further dissection, he will be able to examine the stomach, spleen, pancreas, and liver. Should the stomach be nearly empty, the ligature should be untied from the cut end of the duodenum, and by means of bellows that intestine and the stomach should be moderately inflated. To permit this, it will not be necessary to ligature the oesophagus. At the present stage the above-mentioned organs may be studied as regards their form, situation, and relations, their structure being postponed for future consideration. The Stomach (Plates 43 and 44) is the most dilated segment of the alimentary tube. When moderately distended, it will be seen to have the following configuration. It possesses an anterior and a posterior surface, both being smoothly rounded. It has a concave or lesser cur- vature, which is turned upwards and to the right; and a convex or greater curvature, which is directed downwards and to the left. The left extremity of the organ is much the larger, and is termed the cardiac extremity , or the fundus. The smaller right end is termed the pylorus. The stomach occupies the epigastric and left hypo- chondriac regions, and it will be observed to have the following connections. The anterior surface is related to both the liver and the diaphragm, and in the natural position looks upwards as well as forwards. The posterior face looks downwards as well as backwards, and before the removal of the intestines was related to these, and chiefly to the gastro-hepatic flexure of the double colon. The smaller curvature is fixed to the liver by means of the gastro-hepatic omentum. If, in the present inverted position of the animal, the greater curvature be pulled backwards, so as to separate the anterior surface from the liver and diaphragm, the oesophagus will be found entering the stomach at its lesser curvature, about midwaj^ between the central point of that curvature and the extremity of the fundus. The greater curvature is related in its left half to the spleen, and throughout the rest of its extent to the intestines, particularly to the suprasternal flexure of the colon, now removed. The right extremity, or pylorus, is directly continued into the duodenum, a slight constriction being the only outward mark of their separation. The left or cardiac extremity extends to the left beyond the insertion of the oesophagus, and is related 312 THE ANATOMY OF THE HORSE. to the pancreas and base of the spleen. The stomach is retained in position by continuity with the oesophagus and duodenum, and by certain folds of peritoneum, viz., the gastro-phrenic ligament, and the gastro-hepatic, gastro-splenic, and gastro-colic omenta. The gastro- phrenic ligament extends from the diaphragm to the stomach, around the oesophageal insertion. The gastro-hepatic amentum passes between the lesser curvature and the posterior fissure of the liver. The gastro- splenic omentum passes from the cardiac extremity to the spleen. The gastro-colic or great omentum is continuous with the preceding, and passes in the form of a loose fold from the greater curvature. It extends backwards and downwards, and then curves upwards to the roof of the abdominal cavity; and, as has already been explained (page 300), it separates the greater and lesser cavities of the perito- neum; and, inasmuch as in man it hangs downwards to float upon the intestines, it has been termed the Epiploon. Even in emaciated subjects, it contains between its layers a considerable quantity of fat. The Duodenum (Plates 43 and 44). A better opportunity to examine this part of the intestine is now afforded. Commencing in the epigas- trium, at the pyloric orifice of the stomach, it ascends across the posterior face of the right lobe of the liver, in passing into the right hypochon- drium. It then curves backwards in the right lumbar region, beneath the right kidney; and sweeping round the crook of the csecum, it crosses the spine and is continued as the jejunum. Its calibre is greatest just beyond the pylorus, and at this point it presents, when inflated, a small dilatation like a miniature stomach with its greater curvature superior. Throughout the whole of its course it is retained in position by a narrow band of peritoneum formed by the serous membrane as it passes to envelop the bowel. The right extremity of the pancreas rests against the duodenum, a few inches from the pylorus, and at that point the wall of the bowel is perforated by the bile and pancreatic ducts. The Spleen (Plates 43 and 44) is a bluish-purple solid organ placed in close proximity to the left sac of the stomach. In the horse it has a scythe-shaped outline. It presents an external face, which is slightly convex; an internal face, which is slightly concave and narrower than the outer; an anterior thick border; and a posterior border, which is sharp. Its surfaces are widest above, where they terminate in the base of the organ, and below they taper to the apex. The spleen is situated in the left hypochondriac region, and has the following relations : — Its outer surface is related to the diaphragm ; its inner surface contacts with the double colon ; its anterior border is penetrated by the vessels and nerves of the organ, and is related to the greater curvature of the stomach ; its posterior border is free, and is included between the intes- tines and the diaphragm ; its base is related to the pancreas and left DISSECTION OF THE ABDOMEN. 313 kidney. The spleen is retained in the left hypochondrium by the gastro- splenic omentum , and by a special splenic ligament. The g astro-splenic omentum forms a loose connection between the left half of the greater curvature of the stomach and the anterior border of the spleen. The splenic ligament is a fold of peritoneum developed at the base of the organ, and formed by the serous membrane in passing from around the anterior end of the left kidney to envelop the spleen. The Pancreas (Plate 44) is a body having a lobulated structure and a very irregular shape. It is placed across the roof of the abdominal cavity, its central portion underlying the last dorsal vertebrae. Its upper face is applied to the aorta, the coeliac axis, the vena cava, the pillars of the diaphragm, and the right kidney, and is partly covered by peritoneum. Its lower face towards the right is adherent to the crook of the caecum and the termination of the double colon, while to the left it is covered by peritoneum. Its anterior border is related to the stomach, the duodenum, and the liver. Its posterior border is related about its centre to the anterior mesenteric artery. Its right extremity, or head , is in contact with the duodenum; while the left extremity, or tail , is related to the base of the spleen. The entire thickness of the gland is perforated by the portal vein, which passes from its lower to its upper surface through what is named the pancreatic ring. The gland possesses two excretory ducts, both of which leave it at its right extremity. The main duct is named the duct of Wirsung, and it perforates the wall of the duodenum about six inches from the pylorus, and close by the point of entrance of the bile duct. The accessory duct is much smaller, and pene- trates the bowel at a point opposite the entrance of the duct of Wirsung. The healthy fresh pancreas has a greyish-yellow colour; but when decomposition sets in after death, this speedily changes to an almost black hue. The Liver (Plates 43 and 44) is the largest gland in the body. It forms the bile and discharges it into the duodenum. In health it has a reddish-brown colour and a moderately firm consistence. In form it is not comparable to any common object, and its irregularity of shape makes its description somewhat difficult. It should be observed, in the first place, that inferiorly the rim of the organ is deeply indented, or notched, and two of the largest of these notches serve to partially divide the gland into its three main lobes, viz., a right, a left, and a middle, or lobulus guadratus. Of these the middle lobe is always the smallest, and its inferior border shows two or three minor indenta- tions. The left lobe is generally the largest, but sometimes it is less than the right. The liver possesses a fourth lobe, in the form of a small projection of liver substance about the size of two or three of the human fingers, and situated at the upper part of the right lobe. This is the homologue of the lobulus caudatus of the human subject. 314 THE ANATOMY OF THE HORSE. The exact form of the liver will be more distinctly seen when it has been removed from the body ; but while it remains in situ , the student may endeavour to make out the following points: — Viewing the organ as a whole, it may be described as having an anterior and a posterior surface, and a circumference divisible into an upper and a lower border. The anterior surface is closely applied to the diaphragm, and is convex. The posterior vena cava, in descending from the spine to the foramen dextrum, passes between this surface and the diaphragm; and its course is marked on the liver by a vertical groove, which may be termed the anterior fissure. The posterior surface, when the organ is in situ , is concave ; but when the liver is removed from the body, this surface, like the anterior, is slightly convex. It presents the portal fissure (L. porta , a gate), by which the portal vein, hepatic artery, bile duct, and hepatic nerves and lymphatics enter the liver. The upper border shows about its centre a rounded notch for the reception of the short abdominal portion of the oesophagus. The lower border shows the sharper and deeper indentations dividing the liver into its three principal lobes, and the lesser indentations that partially subdivide the lobulus quadratus. The liver is situated in the epigastric and right and left hypo- chondriac regions. Its most important relations, besides those already mentioned, are as follows : — The anterior surface is applied to the diaphragm, the right lobe, which has the highest point of contact, being related to the most superior part of the muscular rim on the right side, the lobulus quadratus corresponding to the phrenic centre, while the left lobe touches the lowest point reached by the liver, and lies against the lower part of the muscular rim on the left side and the adjacent part of the tendinous centre. The posterior surface is related to the stomach, the duodenum, the gastro-hepatic flexure of the double colon, the pancreas, and the right kidney, the latter slightly indenting the upper part of the right lobe. If an attempt be made to pull the liver from its position, it will be found that this is opposed by certain folds of peritoneum which pass between it and the abdominal parietes. These are the ligaments of the liver, and they are named as follows : — The Right Lateral Ligament passes between the right lobe and the adjacent part of the phrenic rim. The Left Lateral Ligament attaches the left lobe to the phrenic centre. The Falciform or Suspensory Ligament attaches the lobulus quadratus to the diaphragm and to the abdominal floor a little to the right of the linea alba. Its posterior edge is concave and free, and contains the shrivelled remains of the umbilical vein — the so-called round ligament. DISSECTION OF THE ABDOMEN. 315 The Ligament of the Caudate Lobe is a small peritoneal fold passing between the anterior end of the right kidney and the lobulus caudatus. The Coronary Ligament . — If all the preceding ligaments be cut, and an attempt made to pull the liver out of position, it will be found that the gland is still firmly attached to the diaphragm by its anterior face. This adhesion takes place over an area that is traversed by the anterior fissure lodging the vena cava, and the peritoneum in passing between the gland and the phrenic centre on each side of this area constitutes the coronary ligament. Directions . — The cceliac trunk and its branches must now be prepared; and, coincidently with this, the bile duct, portal vein, and solar plexus must be dissected. The portal vein will be found passing through the pancreas to the transverse fissure; and emerging from the fissure, below the vein, is the bile duct, which passes to open into the duodenum close by the principal pancreatic duct. The cceliac axis is concealed by the pancreas, which must be carefully raised by dissection at its anterior border, and pulled backwards. The same dissection will expose the semilunar ganglia and the solar plexus, whose branches are to be traced in company with the arteries. In dissecting the vessels, the student will meet the lymphatic glands of the stomach, spleen, and liver. Lymphatics. The glands of the stomach form two groups, viz., (1) a few large glands situated at the lesser curvature, and (2) a number of smaller glands placed at the greater curvature. The glands of the liver also form two groups, viz., (1) a number situated in the posterior fissure, and (2) a group, between the portal vein and the pancreas. The glands of the spleen are placed on the course of the splenic vessels. The lymphatic vessels emanating from the stomach, liver, and spleen traverse these groups of glands; and after anastomosing with each other, they pass to the thoracic duct. The Bile Duct. This is the main duct for the conveyance of the bile from the liver to the intestine. It is formed at the portal fissure of the liver, by the union of secondary branches from the three principal lobes, and it passes between the layers of the gastro-hepatic omentum to penetrate the wall of the duodenum, about six inches from the pylorus. The excretory apparatus of the horse’s liver has the peculiarity — shared by a few other animals — of being without a gall-bladder. The Cceliac Axis (Plates 43 and 44) is a collateral branch of the abdominal aorta, arising from the inferior face of that vessel between the pillars of the diaphragm. It is less than an inch in length, and it divides into three branches : the gastric trunk, the hepatic artery, and the splenic artery. The Hepatic Artery is directed obliquely forwards, downwards, and to the right, to gain the posterior fissure of the liver, which it penetrates 316 THE ANATOMY OF THE HORSE. in company with the portal vein and the bile duct. At first imbedded in the pancreas, it then passes over the duodenum, and reaches its des- tination by passing between the layers of the gastro-hepatic omentum. It crosses the posterior vena cava, from which it is separated by the foramen of Winslow. It "gives off the following collateral branches : — 1. Pancreatic Branches. 2. The Right G astro-omental Artery , which is, at its origin, of larger volume than the continuation of the parent trunk, crosses behind the duodenum ; and placing itself in the texture of the great omentum, it is carried round the greater curvature of the stomach to inosculate with the left gastro-omental artery. It gives off the pyloric and duodenal arteries, besides numerous omental and gastric branches. The pyloric artery is detached from the right gastro-omontal artery near its origin, and sometimes it is a branch of the hepatic artery. It supplies the pylorus and the initial dilatation of the duodenum. The duodenal artery is detached from the right gastro-omental artery before that vessel crosses the duodenum ; and following the lesser curvature of the duodenum, in the narrow serous band that fixes the bowel, it meets, and inosculates with, the first artery from the left branch of the anterior mesenteric artery. The omental branches of the right gastro-omental are small and unimportant. The gastric branches pass from the concave side of the parent artery ; and bifurcating at the greater curvature, they are distributed to the right sac of the stomach on both its surfaces, where they anastomose with branches of the pyloric and gastric arteries. The Gastric Trunk is the central of the three terminal branches of the coeliac axis. After a course of a few inches downwards and forwards, it bifurcates to form the anterior and posterior gastric arteries. The Anterior Gastric Artery reaches the anterior surface of the stomach by crossing the lesser curvature immediately to the right of the oesophagus. The Posterior Gastric Artery descends to the lesser curvature of the stomach, where it divides into branches distributed on the posterior aspect of the organ. The Pleuro-oesophageal Artery is a vessel constantly present, but variable as regards its origin. It may arise from the gastric trunk or one of its branches, or from the splenic artery. Passing through the foramen sinistrum along with the oesophagus, it enters the thoracic cavity, and there anastomoses with the oesophageal arteries, supplying the pulmonary pleura at the base of the lung. The Splenic Artery is considerably larger than the gastric trunk or the hepatic artery. Under cover of the pancreas, it passes outwards between the left kidney and the cardiac extremity of the stomach. Beaching the spleen, it descends along the anterior border of that PLATE XLIII Int. iliac art. Pyloric art. Middle lobe of liver Right gastro- omental art. Right lobe of liver Duodenal art. ight sac of stomach Left lobe of liver Pleuro-oesophageal art. Ant. gastric art. ost. gastric art. eft sac of stomach Splenic artery Left gastro-omental artery Gastro-splenic omentui Spleen Duodenum Kidney Portal vein Suprarenal capsule Hepatic art. Renal artery Post, vena cava Ureter Gastric trunk Coeliac axis Suprarenal capsule Kidney Renal artery Ant. mesenteric art. Post, aorta Circumflex iliac art. Ext. iliac art. Printed. byW. ScA.K. Johnston. Edinburgh 8c London DISSECTION OF THE ABDOMEN. 317 organ, beyond which it is continued as the left gastro-omental artery, From its convex side it gives off many large splenic branches; and from its concave side it emits gastric branches , which pass in the gastro-splenic omentum to reach the great curvature of the stomach, where they bifurcate to be distributed to both surfaces of the left sac. The Left Gastro-omental Artery is the continuation of the splenic artery beyond the tip of the spleen. It passes in the texture of the great omentum to meet, and inosculate with, the right gastro-omental artery, advancing in the opposite direction. Besides omental branches , it emits gastric branches , which bifurcate at the great curvature of the stomach to be distributed to both its surfaces. The Portal Vein (Plates 43 and 44) is the trunk which collects the blood from the stomach, intestines, spleen, and pancreas, and conveys it to the liver, where, as will afterwards be described in connection with the liver structure, the vessel comports itself after the manner of an artery. The vessel is formed behind the pancreas, by the junction of the anterior mesenteric vein with a short trunk resulting from the union of the posterior mesenteric and splenic veins. It gains the upper face of the pancreas by passing through its substance, the perforation being termed the pancreatic ring ; and descending in the gastro-hepatic omentum to the posterior fissure of the liver, it penetrates the substance of the gland in company with the bile duct and hepatic artery. Anterior and Posterior Mesenteric Veins, satellites of the arteries of the same names, have already been described ; but there is no venous trunk corresponding to the coeliac axis, the companion veins of the divisions of that artery behaving as follows : — The Splenic Vein is the upward continuation of the left gastro-omental vein. After receiving the posterior gastric vein, it becomes one of the roots of the vena portie, previously forming a short trunk by union with the posterior mesenteric vein. The Anterior Gastric Vein joins the vena portae in the posterior fissure of the liver. The Right Gastro-omental Vein is continuous with the left vein of the same name, in the texture of the great omentum, opposite the middle of great curvature of the stomach. It receives gastric, omental, duodenal, pyloric, and pancreatic branches, all of which run in company with the arteries of the same names; and then, above the pancreas, it joins the portal vein. The (Esophageal Nerves. These nerves are the backward continu- ations of the vagus, pneumogastric, or 10th cranial nerves. They reach the abdominal cavity by passing through the foramen sinistrum of the diaphragm, in company with the oesophagus and the pleuro-oesophageal branch of the gastric artery. The inferior nerve forms at the lesser curvature of the stomach a plexus whose filaments pass mainly to the 318 THE ANATOMY OF THE HORSE. right sac; while the superior, after giving branches to the left sac, joins the solar plexus. The Splanchnic Nerves. On each side there are two splanchnic nerves — a great and a small. Both are formed by efferent branches of the dorsal portion of the sympathetic gangliated cord, and they reach the abdomen by passing between the diaphragm and the psoas parvus muscle. The great splanchnic nerves terminate in the semilunar ganglia ; the small nerves pass directly to the solar plexus, or they may be continued to the renal or the suprarenal plexus. The Semilunar Ganglia are the largest in the body. They are placed one at each side of the lower face of the aorta, between the coeliac and anterior mesenteric arteries. Each receives the great splanchnic nerve of its own side, and the two ganglia communicate by transverse branches across the lower face of the aorta. The efferent branches which proceed from them form the solar plexus. The Solar Plexus is an intricate network of nerves and ganglia. It is joined on each side by the lesser splanchnic nerve, and by the terminal fila- ments of the superior oesophageal nerve. From the plexus nerves pass to the abominal viscera, and in doing so they run in company with arteries. There is thus : a coeliac plexus , whose branches reach the liver, pancreas, spleen, and stomach, by accompanying the divisions of the hepatic, splenic, and gastric arteries ; a renal and a suprarenal plexus , which pass to the kidneys and suprarenal bodies ; an aortic plexus , continued back- wards on the aorta to join the posterior mesenteric plexus ; and an anterior mesenteric plexus , already described. Directions. — The form, situation, and relations of the kidneys, and the course of the ureters should now be examined. Without displacing the kidneys, the fat and peritoneum is to be stripped from their lower face, their vessels being carefully cleaned at the same time. The ureter will be found passing backwards from the notched inner border of each kidney, and it is to be followed backwards to the entrance to the pelvis. In close relation to each kidney is its suprarenal body, which, to prevent displacement, may be transfixed in position with a long pin. The Kidneys (Plates 44 and 47) are the two glandular bodies that secrete the urine. Each kidney occupies a position at the side of the vertebral column, on the inferior aspect of the loins, and at the roof of the abdominal cavity. In all except the most emaciated subjects, the kidneys are surrounded by a quantity of adipose tissue, which is so abundant in fat animals as to completely isolate them from surrounding objects. The most common shape of the mammalian kidney is so well known that it is popularly used as a descriptive term, objects having a similar form being described as “kidney-shaped.” Each kidney possesses two surfaces, two borders, and two extremities. The inferior surface is convex ; the superior, which is concealed at present, is almost flat. The outer border PLATE XLIV Ensiform cartilage Diaphragm. Right lobe) of liver L Falciform lig. of liver Middle lobe of liver Left lobe of liver Pancreas Right kidney Post, vena cavi From last dorsal. Transv. abdom Aortic plexus ?rom 1st lumbar nerve Psoas magnus ?rom 2nd lumbar nerve — J) ’rom 3rd lumbar nerve Int. oblique Inguinal nerves To pelvic plexus Lateral and inf. ligamen of bladder Stomach f-Ant. gastric art. Post, gastric art. Spleen Splenic Suprarenal body Ant. mesent. art. Renal vessels Left kidney Post, aorta Ureter Post, mesent. art. Circumf. iliac art. Int. iliac art. Ext. iliac art. Artery of cord Spermatic art. Cremaster m. Int. abdom. ring deferens repubic art. Rectum Bladder Drawn & Printed by V 8&A.K Johnston. Edinburgh fc London ABDOMINAL VISCERA, Etc. DISSECTION OF THE ABDOMEN. 319 is convex ; while the inner is concave, presenting a well-marked notch termed the hilus. From this hilus the ureter issues, and in its neigh- bourhood the renal vessels and nerves pass into or out of the kidney. The extremities are anterior and posterior, and both are rounded. It will at once be noticed, however (Plate 47), that although this description applies to both kidneys, they are far from being identical in shape. The right kidney has an outline somewhat like the “ heart ” of playing-cards, while the left has a decided resemblance to a haricot bean. The right has the longest transverse, but the shortest antero-posterior, diameter. The right is nearly symmetrical on each side of a line drawn from the hilus to the middle of the outer border; but if such a line be drawn on the left, the part in front of the line will be considerably smaller than the part behind it. Furthermore, it will be noticed that the two kidneys differ in situation, and in relations. The right kidney is the more anterior; and taking their relation to the skeleton, the difference may be expressed thus : the right extends from the middle of the third last intercostal space at its upper end to a point beneath the 2nd lumbar transverse process ; the left extends from the second last intercostal space to the 3rd lumbar transverse process. The right kidney is related by its upper face to the psoas muscles and to the rim of the diaphragm ; by its lower face it contacts with the pancreas and the crook of the caecum, and is partly covered by peri- toneum; its inner border is margined by the posterior vena cava, and is in contact with the right suprarenal capsule in front of the hilus (some- times behind); its outer border is in contact with the duodenum; its anterior extremity is in contact with the right lobe of the liver (which is slightly depressed for it) and with the lobulus caudatus, and to the latter a small fold of peritoneum passes from the lower face of the kidney. The left kidney has the same relations on its upper face as the right ; its lower face is covered by peritoneum, and is related to the small intestines ; its inner border is margined by the aorta, and is related to the left suprarenal body in front of the hilus; its anterior border is related to the left extremity of the pancreas ; and the anterior half of its outer or convex border is related to the base of the spleen. Renal Arteries. Each kidney receives blood from a large vessel — the renal artery — which is a branch of the aorta. Springing at a right angle from the parent trunk, the artery passes towards the hilus, where it divides into a number of branches that penetrate the kidney substance. The left artery is short, and passes directly to the hilus ; the right is longer, and passes between the psoas parvus muscle and the vena cava to reach its destination. It is also generally a little anterior to the left in its point of detachment, both being a little behind the trunk of the anterior mesenteric artery. 320 THE ANATOMY OF THE HORSE. The Renal Veins are as large in proportion as the arteries. They join the posterior vena cava, the right vein being the shorter and passing directly from the hilus, while the left crosses the lower face of the aorta behind the anterior mesenteric artery. The Renal Plexus of nerves interlace around the artery, and enter the kidney with its branches. The Suprarenal Capsules (Plates 44 and 47). These are two small solid bodies found in close relation to the kidneys, the right being between the vena cava and the inner border of the right kidney, and the left between the aorta and left kidney. They have an irregular elon- gated shape, and a slatey-brown colour. They are highly vascular, receiving branches from the mesenteric or renal arteries, and having veins that enter the posterior vena cava or the renal veins. They have also a rich nervous supply, receiving the suprarenal plexus — an offset from the solar plexus. Their substance consists of a cortical and a medullary portion. Each possesses a fibrous capsule continuous with an internal trabecular framework. The interspaces of these trabeculae contain nucleated polyhedral or branched cells, which in the medulla and innermost layer of the cortex frequently enclose yellowish-brown pigment. They are without ducts, and their function is not well known. The Ureters. The ureter is the tube which conveys the urine from the pelvis of the kidney to the urinary bladder. On the left side it has the following course. Beginning at the hilus, it is directed backwards and inwards across the lower face of the kidney to place itself at the side of the aorta, over (under, in the natural position) the psoas parvus muscle. Here it is crossed obliquely by the spermatic artery. It next curves a little outwards, crossing over the circumflex iliac artery and the artery of the cord, runs at the outer side of the external iliac artery, and then crosses it very obliquely to enter the pelvis. The right ureter has similar relations, except that it passes at the side of the vena cava instead of the aorta. Directions . — Pin each ureter in position immediately behind the kidney, and then cut it across. Remove carefully the liver, stomach, duodenum, spleen, pancreas, and kidneys, by cutting the oesophagus and the various ligaments, vessels, and cellular adhesions which retain these organs in position. Put them in carbolic solution, or procure fresh organs, to serve for the examination of their structure. In the meantime proceed to dissect the sublumbar region. THE SUBLUMBAR REGION. Directions . — Under this heading there will be described the abdomi- nal aorta and the vena cava, with their branches ; the inferior primary branch of the last dorsal nerve, and the corresponding branches of the first four lumbar nerves; the aortic plexus and gangliated lumbar cord DISSECTION OF THE ABDOMEN. 321 of the sympathetic nerve ; and, lastly, a group of muscles, comprising the iliacus, psoas magnus, psoas parvus, quadratus lumborum, and lumbar intertransverse muscles. The great arterial and venous trunks are mesially placed, and the aortic plexus is on the great artery. The other structures enumerated are the same on both sides of the body. One side may be used for the nerves and the arterial and venous branches, the other being reserved for the muscles. In the mare, after the ovarian and uterine vessels have been examined, the broad ligaments must be cut to allow of the ovaries and uterus being pushed into the pelvic cavity, where they are to remain until they can be dissected along with the other reproductive organs of the female. Lymphatic Glands. In cleaning the nerves and vessels, the follow- ing groups of lymphatic glands will be found : — 1. Sacral glands, between the right and left internal iliac arteries at the entrance to the pelvis. 2. Internal Iliac glands, between and around the roots of the external and internal iliac arteries on each side. 3. External Iliac glands towards the point of bifurcation of the circumflex iliac artery. 4. Lumbar glands, on the lower face of the aorta around the roots of the posterior mesenteric and spermatic arteries. These various groups of glands are placed on the course of the lymphatic vessels of the hind limb, pelvis, and spermatic cord ; and the efferent vessels from the most anterior group (lumbar) pass to enter the receptaculum chyli. The Posterior Aorta (Plates 44 and 45). The abdominal portion of this great artery appears close to the spine, between the two pillars of the diaphragm, the opening being termed the hiatus aorticus. It passes back- wards across the lumbar vertebral bodies, resting on the left pillar of the diaphragm and the inferior common ligament. At the 5th lumbar vertebra it terminates in four branches, two diverging to each side. These are the external and internal iliac arteries. On its right side the aorta is related to the vena cava. On the left it is related to the psoas parvus, the left lumbar sympathetic cord, and the left kidney and suprarenal capsule ; and the left ureter is beside or in actual contact with it. Besides the external and iliac arteries, which are described as its terminal branches, it gives off the following : — 1. Phrenic Branches (two or three) to the pillars of the diaphragm. They are given off at the hiatus aorticus. 2. Lumbar Arteries. There are six or seven of these on each side. The last comes from the lateral sacral artery, the second last from the internal iliac, and the others from the aorta. These last arise from the upper aspect of the vessel, and divide into two branches — a superior for the skin and muscles over the lumbar vertebrae, giving also a spinal twig through the intervertebral foramen; and an inferior which passes out- wards in the intertransverse spaces to the flank, where it anastomoses with the circumflex iliac artery in supplying the abdominal muscles. 322 THE ANATOMY OF THE HORSE. 3. The Middle Sacral Artery is an extremely slender vessel, and not always present. Search for it in the angle between the internal iliacs. Arising from the summit of that angle, it passes mesially backwards on the sacrum. It is of interest as representing the large vessel which in some animals continues the aorta to the coccygeal region. 4. The Coeliac Axis is detached as soon as the aorta passes through the hiatus aorticus. 5. The Anterior Mesenteric — the largest of the branches — is detached at the 1st lumbar vertebra. 6. The Renal Arteries , right and left, arise from the sides of the aorta at the articulation between the 1st and 2nd lumbar vertebrae. 7. The Spermatic Arteries , right and left, come off a few inches behind the renals, viz., between the 3rd and 4th lumbar vertebrae, and one generally a little in advance of the other. As seen in the dissection, each passes obliquely backwards and outwards over the ureter and circumflex iliac artery to gain the internal abdominal ring, where it joins the other constituents of the spermatic cord. In the cord it has a remarkably tortuous disposition; and, although a long vesssel, it detaches no branches of any size until it reaches the testicle. In the mare it is represented by the ovarian artery , which passes in a tortuous manner between the layers of the uterine broad ligament to reach the ovary. It gives off a uterine branch to the uterine horn. 8. The Posterior Mesenteric Artery. This vessel is usually detached at the 4th lumbar vertebra, a little behind the origin of the spermatics, but this relationship may be reversed. The External Iliac Artery is, speaking generally, the vessel of supply to the hind limb. It is regarded as a terminal branch of the aorta, and it has its root at the body of the 5th lumbar vertebra. It descends with a curved course at the pelvic inlet, and at the anterior border of the pubis it is directly continued as the femoral artery. It is placed immediately beneath the peritoneum, and each is related on its outer side to the psoas parvus, sartorius, and iliacus muscles, the tendon of the first of these separating it from the great crural nerve. On the inner side it is related successively to the common iliac and external iliac veins, the former separating it from the internal iliac artery. Its branches are: — 1. The Circumflex Iliac Artery. This is a large artery detached from the outer side of the external iliac close to its origin. It passes outwards across the psoas muscles ; and at the outer edge of the psoas magnus it divides into an anterior and a posterior branch. The former is distributed in the flank, beneath the internal oblique muscle; and the latter perforates the oblique muscles near the bony angle of the haunch, and descends to the thigh. 2. The Artery of the Cord (Plate 44). This is a slender vessel arising close to the preceding, or it may come from the aorta itself. PLATE XLV Ensiform cartilage Foramen dextrum t crus of diaphragm From last dorsal nerve— Retractor costae- Post, vena cava From 1st lumbar nerve From 2nd lumbar nerve Psoas parvus Psoas magnus I From 3rd lumbar nerve - Inguinal nerv I Urinary bladder Sartorius — lie lig. of Bladder L \ i inguinal glands _ Asternal artery - Phrenic sinus ((Esophagus in 1 foramen sinistrum — Post, aorta in hiatus Coeliac axis Left crus of diaphragm Ant. mesenteric art. — Renal art. - Psoas parvus .A lumbar art. Spermatic art. Quadratus lumborum -Post, mesenteric art. — An intertransverse muscle Circumflex iliac art. Ext. iliac art. Int. iliac art. — Last lumbar art. Ant. crural nerve -Iliacus Psoas magnus -Prepubic art. -Femoral art. Deep femoral art. Art. to quadriceps — i— Int. saphenous nerve Drawn & Printed bjr’W. ScA.K. Johnston . Edinburgh Sc London SUBLUMBAft REGION AND DIAPHRAGM DISSECTION OF THE ABDOMEN. 323 Parallel and internal to the spermatic artery, it passes to the internal abdominal ring, and is distributed to the spermatic cord. In the mare it is represented by the uterine artery . This, which is a much larger vessel, passes between the layers of the broad ligament to reach the uterus, being distributed to the body of that organ, and anastomosing anteriorly with the uterine branch of the ovarian artery, and posteriorly with the vaginal artery. 3. The Prepubic Artery (Plates 45 and 46). This vessel arises at the anterior border of the pubis, and marks the limit of the iliac and femoral arteries. It forms, at its origin, a short common trunk with the deep femoral artery. It is about two or three inches in length, and it passes on the anterior face of Poupart’s ligament to the posterior edge of the internal oblique, where, at the inner side of the internal abdominal ring, it divides into the posterior abdominal and external pudic arteries. The Internal Iliac Artery. This vessel may be described as the vessel for the supply of the pelvic walls and contents. It will be described with the pelvis. The Posterior Vena Cava (Plates 43, 44, and 45). This great venous trunk is formed to the right of the termination of the aorta, by the union of the two common iliac veins. It passes forwards along the right side of the lumbar vertebral bodies, until it reaches the upper border of the liver. Here it descends in the anterior fissure of the liver, being included between that organ and the diaphragm. Passing through the foramen dextrum, it enters the thorax. It is related on its left side to the aorta ; and on its right to the psoas parvus, ureter, kidney, suprarenal capsule, and lumbar sympathetic cord of the same side. It receives the following branches : — 1. Lumbar Veins, exactly corresponding to the arteries. 2. Phrenic Veins, or sinuses (2). These begin in the muscular rim of the diaphragm, and converge to the foramen dextrum, where they join the vena cava. They are distinctly visible without dissection in the tendinous centre of the diaphragm (Plate 45). 3. Spermatic Veins ( Ovarian in the mare). The right and left veins often unite before joining the vena cava. 4. Renal Veins, the left longer than the right. 5. Hepatic Veins. These join the vena cava while it lies in the anterior fissure of the liver. They discharge the blood of the portal system of veins, after it has circulated in the liver. Iliac Veins. There are external and internal iliac veins, with branches corresponding in all respects to the divisions of the homonymous arteries. The external and internal iliac veins of each side, however, unite and form a short trunk termed the common iliac vein, which is placed in the angle of separation between the external and internal iliac arteries. The right and left common iliac veins unite to form the posterior vena cava. 324 THE ANATOMY OF THE HORSE. The Receptaculum Chyli. Separate the aorta and vena cava at the origin of the anterior mesenteric artery, and look above them for this. It is the dilated commencement of the thoracic duct. It is formed by the union of a variable number of large lymphatic vessels, and it is continued forwards by the duct. This is a thin-walled vessel of small calibre which passes into the thorax between the pillars of the diaphragm, being generally to the right of the aorta. Last Dorsal and First Two Lumbar Nerves (Plates 44 and 45). The inferior primary branches of these nerves appear at the outer edge of the psoas magnus, the last dorsal being close behind the last rib, and the other two issuing in series behind it. These nerves have already been followed in the dissection of the abdominal wall, where they are distri- buted in the region of the flank to the abdominal muscies, panniculus, and skin. These lumbar nerves also furnish cutaneous branches to the inside and front of the thigh. 3rd Lumbar Nerve (Plates 44 and 45). The inferior primary branch of this nerve will be found in front of the circumflex iliac artery, emerging from between the psoas magnus and parvus muscles, after having penetrated the substance of the latter. It accompanies the posterior division of the circumflex iliac artery to the front of the thigh, where it is expended in cutaneous branches. Before it emerges, it gives branches to the psoas and quadratus muscles. Inguinal Nerves. There is considerable variation in the mode of formation of these, but that figured in Plates 44 and 45 is probably as common as any other. A nerve is there seen passing obliquely backwards over the circumflex iliac artery. It is formed by the union of two branches which emerge at the inner side of the psoas parvus, these being from the 2nd and 3rd lumbar nerves respectively. It divides into three sets of branches, viz. : — 1. Muscular , to the internal oblique. 2. Cremasteric , to the cremaster muscle. 3. Inguinal , which descend in the inguinal canal to supply the scrotum, prepuce (mammary gland in the female), and surrounding skin. The Lumbo-sacral Plexus (Plate 48). This is the plexus of nerves for the supply of the hind limb. Like the corresponding brachial plexus, the inferior primary branches of five nerves compose it, viz., the 4th, 5th, and 6th lumbar, and the 1st and 2nd sacral nerves. There is a loop of communication between the first of these and the 3rd lumbar, which to that extent also enters into the formation of the plexus. The majority of its branches fall to be dissected with the pelvis, and a complete account will then be given of it (page 349). In the meantime only the most anterior of its branches will be dissected. 1 . Branches to thepsoas magnus and iliacus. These are derived from the 4th lumbar root of the plexus, or from the loop between that and the 3rd. DISSECTION OF THE ABDOMEN. 325 2. The Anterior or great crural nerve (Plate 45). This is a large nerve which derives its fibres from the 4th and 5th lumbar roots, and from the loop between the 3rd and 4th. Emerging between the psoas magnus and parvus, it descends at the outer side of the external iliac artery, but separated from it by the tendon of the last-named muscle. It rests on the iliacus and psoas muscles, and crosses their common termination to end in a fasciculus of branches for the extensors of the leg. In this course it is covered by the sartorius muscle. It gives off as a branch the internal saphenous nerve , whose origin is about opposite the ilio-pectineal eminence. The Aortic Plexus of the Sympathetic Nerve (Plate 45). This is the backward continuation of the solar plexus. Its branches interlace around the aorta behind the kidneys, and unite with the posterior mesenteric plexus. It receives some of the efferent filaments of the lumbar ganglia. The Posterior Mesenteric Plexus is formed around the root of the artery of the same name. It is united in front with the aortic plexus, and receives efferent branches from the lumbar ganglia. Three sets of branches pass from it: — 1. Branches following the divisions of the posterior mesenteric artery. 2. Branches accompanying the spermatic artery, and forming the spermatic plexus. 3. Pelvic branches to join the pelvic plexus. The Sympathetic Gangliated Cord in the loins. This is the back- ward continuation of the dorsal cord. Beginning between the psoas parvus and the diaphragmatic crus, it extends backwards to the lumbo- sacral articulation, where it is directly continued by the sacral division of the cord. Each nerve will be found on the inner aspect of the psoas parvus muscle of the same side, the left nerve being related inwardly to the aorta; and the right for the greater part of its course to the vena cava, but for a short distance in front to the aorta. Six fusiform, greyish ganglia stud the cord, and from these proceed the various branches of the cord. These branches are : — 1. Communicating branches with the inferior primary divisions of the lumbar spinal nerves. 2. Branches to the aortic and posterior mesenteric plexuses. Iliac Fascia. This is the name given to the aponeurotic layer which covers the inferior face of the psoas magnus and iliacus muscles. It is densest and most adherent at the side of the pelvic inlet, and becomes more cellular as it is traced forwards and backwards. It is adherent inwardly to the psoas parvus tendon, and outwardly to the bony promi- nence of the haunch. Poupart’s ligament is adherent to its inferior fftfce, and it gives origin to the sartorius and cremaster muscles. The Psoas Magnus (Plates 44 and 45). This muscle is broad and flattened anteriorly, and thick and pointed behind, where it rests in 326 THE ANATOMY OF THE HORSE. a depression of the iliacus. It arises from the last two dorsal vertebrae and the under surfaces of the last two ribs at their upper part ; also from the lumbar vertebrae except the last, covering their transverse processes. It is inserted , in common with the iliacus, into the internal trochanter of the femur. Action. — It is a flexor and an outward-rotator at the hip -joint. When the hind limbs are fixed, the two muscles will arch the loins, or the single muscle will incline the trunk to the same side. The Psoas Parvus (Plate 45). This is a smaller and more tendinous muscle than the preceding, to whose inner side it is placed. It arises from the bodies of the last three or four dorsal and all the lumbar vertebrae. It is inserted into the ilio -pectineal eminence (of the ilium) at the side of the pelvic inlet. Action. — To flex the pelvis on the loins when both musles act ; or to incline it laterally when a single muscle acts. If the pelvis be fixed, it will execute the same movements on the loins. Directions. — Raise the outer edge of the psoas magnus, and remove it except its fibres of origin beneath the rim of the diaphragm and its conical tendon of insertion, as in Plate 45. The Iliacus (Plates 45 and 16). This is a powerful fleshy muscle which, when the psoas magnus muscle is in position, appears to consist of an outer and an inner portion. In reality, however, it is a single mass, with a deep groove in it for the terminal tendon of the psoas magnus. It arises from the entire iliac surface of the ilium, from its external angle, and from the sacro-iliac ligament. It is inserted into the inner trochanter of the femur, in common with the psoas magnus. Action. — The same as the psoas magnus. The Quadratus Lumborum (Plate 45). This muscle lies under cover of the great psoas muscle. Its most external and strongest fasciculus arises from the sacro-iliac ligament. It is inserted by this same fasciculus into the tips of the lumbar transverse processes, and into the hinder edge of the last rib. From the main fasciculus others pass inwards to the lumbar transverse processes, and to the under surfaces of the three last ribs, close to the spine. Action . — To assist in bending the loins to the side of the acting muscle. Both muscles, by fixing the last ribs, will enable the diaphragm to act to more advantage. Lumbar Intertransverse Muscles (Plate 45). These are thin muscular and tendinous strata connecting the edges of adjacent lumbar transverse processes. Action . — To assist in bending the loins to the side on which the muscles act. Directions . — An examination of the diaphragm will complete the dissection of the abdomen. Define its pillars attaching it to the lumbar DISSECTION OF THE ABDOMEN. 327 vertebrae, and clean the edges of its foramina. At its periphery, under the costal cartilages, follow the asternal vessels (Plate 45). The Asternal Artery. This is one of the divisions of the internal thoracic artery (Fig. 7, page 120). It passes from the thorax to the abdomen by perforating the rim of the diaphragm about the 9th chondro- costal joint. As here seen, it passes backwards at the rim of the diaphragm, in the interval between it and the origin of the transversalis abdominis, and terminates at the lower extremity of the 1 3th intercostal space. It has three sets of branches, viz., (1) ascending branches, which anastomose with the intercostal arteries of the spaces crossed ; (2) internal branches to the diaphragm; (3) descending branches, which run on the peritoneal surface of the transversalis abdominis muscle. The Asternal Vein accompanies the artery. The Diaphragm (Plate 45) is the muscle which serves as a partition between the thoracic and abdominal cavities. In outline it has some resemblance to the heart of playing-cards, the point being at the ensiform cartilage, and the base at the spine. Its general direction is oblique downwards and forwards. Its anterior or thoracic surface (Plate 22) is convex, covered by pleura, and related to the bases of the lungs. Its posterior surface is concave, covered for the greater part by peri- toneum, and related to the liver, stomach, spleen, and intestines. It consists of a muscular rim, two muscular pillars, or crura, and an aponeurotic centre. The Fleshy Rim is composed of soft muscular fibres, the lowest of which are attached outwardly to the upper face of the ensiform cartilage about one inch behind its junction with the sternum (Fig. 7, page 120). From this mid point the line of attachment of the rim rises on each side, the fibres taking origin from the cartilages of the last ten ribs, or from the ribs themselves above the chondro-costal joints. On each side these fibres meet, or are separated by only a narrow line from, the fibres of the transversalis abdominis at their origin ; and along the line of separa- tion the asternal artery runs. The muscular fibres are all directed from these points of origin inwards, where, along a denticulated line, they terminate in the tendinous centre. The Pillars , or Crura . — These are right and left. The right is the largest, and arises by a strong tendon from the lumbar vertebrae, through the medium of the inferior common ligament. Its muscular fibres terminate in. the tendinous centre, some of them diverging to the right, but without joining the muscular rim, while others descend to near the mid point. The left pillar has a similar origin from the left side of the lumbar vertebrae, and its fibres terminate in the tendinous centre, being sometimes continuous outwardly with the muscular rim. The Tendinous Centre is pearly white, and composed of glistening fibres interlacing in various directions. By the descent of the pillars 328 THE ANATOMY OF THE HORSE. into it, it is partially divided into right and left halves, or leaflets. Ligamenta Arcuata. — On each side of the pillars the rim of the diaphragm arches with a free edge over the apices of the psoas muscles, forming the so-called arcuate ligament. Foramina of the diaphragm. The Foramen Sinistrum. — This is a slit between the fibres of the right eras, formed slightly to the left of the mesial plane, and a little below the spine. It transmits the oesophagus, the oesophageal continuations of the vagus nerves, and the pleuro-oesophageal branch of the gastric artery. The Foramen Dextrum. — This is the aperture by which the posterior vena cava passes from the abdomen to the thorax. It is formed near the middle of the tendinous centre, but a little to the right of the mesial plane. The margins of the opening are closely adherent to the wall of the vein, and here the phrenic sinuses empty. The Hiatus Aorticus. — This is the opening between the right and left pillars, close to the spine. It gives passage to the posterior aorta, and to the initial portions of the thoracic duct and great azygos veins. Between the crus and the psoas parvus on each side the gangliated cord of the sympathetic passes, and a little outward the great splanch- nic nerve passes between the same muscle and the edge of the diaphragm. The asternal vessels penetrate the edge of the diaphragm at the 9th chondro-costal joint. Action of the Diaphragm . — The diaphragm is the principal muscle of inspiration. When it contracts, it moves backwards, and thus increases the antero-posterior diameter of the thorax. In this action it pushes back the abdominal viscera, and causes the abdominal wall to descend. The movements of the diaphragm affect principally its periphery, any great backward movement of the tendinous centre being prevented by the posterior vena cava, which passes like a ligament between the centre and the heart. STRUCTURE OF THE STOMACH. The wall of the stomach comprises four layers, viz., serous, muscular, submucous, and mucous. 1. The Serous Coat is a smooth, glistening covering derived from the eritoneum. It is united to the subjacent muscular coat by areolar tissue sometimes termed the subserous coat. 2. The Muscular Coat. — This can be best displayed on a stomach which has been boiled for a few minutes. If two such stomachs can be procured, one of them should be everted and moderately inflated, and then its mucous coat stripped off with fingers and forceps. From the other, similarly inflated, the peritoneum should be stripped off. The muscular fibres are disposed in three planes, viz., an outer longi- tudinal, a middle circular, and an inner oblique layer. Of these the DISSECTION OF THE ABDOMEN. 329 circular layer is found all over the organ, but the other layers are mainly confined to the left half. At the right extremity of the stomach the circular fibres are aggregated to form the sphincter-like 'pyloric ring . The fibres are of the non-striped variety. 3. The Submucous Coat is composed of areolar connective-tissue, in which the blood-vessels ramify before they pass into the next coat. 4. The Mucous Coat. — It is desirable to study this on the stomach of an animal recently killed. If possible, take such a stomach with about a foot of the duo- denum and a few inches of the oeso- phagus attached, and fasten the duodenum to a tap. Let water flow into the organ, and it will be noticed that, even when the stomach is much distended, none of the water escapes by the orifice of the gullet, although that is unligatured. This is an instructive experiment, as showing the difficulty or impossibility of vomition in the horse. Now allow the contents of the stomach to escape by the duo- denum ; and either evert the organ and inflate it, or incise it along its convex curvature. It will at once be noticed that the mucous lining is not the same throughout. The left or cardiac half of the cavity is lined by a mucous membrane termed cuticular ; the right or pyloric half has a totally different lining, termed villous. The cuticular portion is pale, harsh, without true gastric glands, but possessed of a few mucous follicles, and covered on its free surface by a thick stratified squamous epithelium. It is, in fact, an extension of the oesophageal mucous membrane, which it resembles in all respects. Towards the middle of the stomach it is separated from the villous half by an abrupt, raised, and slightly sinuous line of demarcation — the cuticular ridge. The villous half is rosy, soft, and velvety (but without villi), thickly beset with gastric glands, and possessed of a single layer of columnar epithelium. The gastric glands are of the tubular variety, and by the aid of a lens numbers of them may be seen opening together into pits, or alveoli , of the mucous mem- brane. The cuticular portion is but slightly vascular, but the villous portion is richly supplied with blood-vessels. In the collapsed organ the mucous membrane is thrown into folds, or rugae. The (Esophageal Orifice, it will now be seen, is very narrow, and obstructed by the mucous membrane gathered into folds. Vertical Transverse Section of the Coats of a Pig’s Stomach. 30 Diameters (from Kolliker ). a. Gastric glands ; b. Muscular layer of the mucous membrane ; c, Sub- mucous or areolar coat ; d. Circular muscular layer ; e. Longitudinal mus- cular layer ; f. Serous coat. 330 THE ANATOMY OF THE HORSE. The Pyloric Orifice is much larger, but capable of being completely closed by the pyloric ring of muscular fibres. Fig. 39. Stomach, everted and inflated. 1. Left (cardiac) sac with its cuticular mucous lining ; 2. Right (pyloric) sac with its villous mucous lining ; 3. Cuticular ridge ; 4. Termination of oesophagus ; 5. Initial part of duodenum ; 6. Pyloric ring. In the interior of the duodenum, about six inches from the pylorus, the openings of the bile and pancreatic ducts will be found. The orifices of the bile duct and duct of Wirsung are placed together on the con- cave side of the bowel, and are surrounded in common by a ring-like valvular fold of mucous membrane. The opening of the accessory pan- creatic duct is placed opposite to these. STRUCTURE OF THE LIVER. Lay the organ with its diaphragmatic surface downwards. Find the portal vein, hepatic artery, and bile duct, at the portal fissure, and trace them for a little distance into the liver. Invert the organ, and observe the course of the anterior vena cava in the anterior fissure, and the mouths of the hepatic veins which there discharge themselves into the cava. Tunics or Capsules of the liver. These are two in number : 1. A peritoneal coat , giving the free surface of the organ its smooth and glistening characters. 2. A tunica propria, or fibrous coat, placed beneath the preceding. All over the surface of the liver it sends inwards delicate processes that join the interlobular connective-tissue, and at the portal DISSECTION OF THE ABDOMEN. 331 fissure it furnishes a sheath that accompanies the portal vein, hepatic artery, and bile duct into the liver. This sheath is the capsule of Glisson. Lobules o f the Liver. — When a fresh-cut surface of the liver is examined, it shows a system of lines mapping it out into areas about the size of a pin’s head. These areas are sec- tions of the lobules of the liver, which are united together by interlobular connective - tissue. This interlobular connective-tissue is much more abundantly de- veloped in the pig, and, conse- quently, in that animal the lobu- lation of the liver substance is much more evident. A lobule may be viewed as having a frame- work of blood-vessels, in which are set the liver-cells. Between the adjacent cells the rootlets of the bile passages begin, and there are possibly also branches of nerves and lymphatic vessels. The liver is supplied with blood by two vessels. The first and much the larger of the two is the portal vein, the other is the hep- atic artery. The Portal Vein collects its blood from the stomach, intestines, spleen, and pancreas. Entering the liver at the portal fissure, this vein comports itself like an artery, in that it reduces itself by division and subdivision to branches that become progressively smaller until they terminate in a set of capillaries. In their course through the liver, the larger branches of the vein run in tunnels of the liver substance — the portal canals — which contain also branches of the hepatic artery and bile ducts, and are lined by Glisson’s capsule. The smaller branches of the portal vein are distri- buted in the interlobular connective-tissue, where, at the circumference of each lobule, they form an interlobular plexus. From this plexus capillary vessels penetrate the lobule, and form within it the intra- lobular plexus. The capillaries of this last plexus converge towards the axis of the lobule, and there empty themselves into what is termed the central vein of the lobule. This is the initial vessel of the hepatic system of veins, and at the base of the lobule it joins a larger vessel — Fig. 40. Longitudinal Section of a Portal Canal, CONTAINING A PORTAL VEIN, HEPATIC ARTERY, and Hepatic Duct, from the Pig (after Kiernan ). About 5 diameters. P. Branch of vena portae, situated in a portal canal, formed amongst the hepatic lobules of the liver ; p. p. Larger branches of portal vein, giving off smaller ones (i. i.), named interlobular veins ; there are also seen within the large portal vein numerous orifices of interlobular veins arising directly from it ; a. Hepatic artery ; cl. Biliary duct. 332 THE ANATOMY OF THE HORSE. the sublobular vein. By the union of these sublobular veins through- out the liver, the larger hepatic venous trunks are formed; and these, as already seen, enter the pos- terior vena cava in the anterior fissure of the liver. The Hepatic Artery is a branch of the cceliac axis. It enters the liver with the portal vein, and ramifies with it. It has three sets of branches : (1) capsular branches , to the tunica propria ; (2) vaginal branches , to Glisson’s capsule and the vessels within it ; and (3) inter- lobular branches , whose capil- pass into the lobule, Fig. 41 . Transverse Section through the Hepatic Lobules laries {Turner). i, i, i. Interlobular veins ending in the intralobular w h ere they help to form the capillaries ; c c. Central veins joined by the intra- intralobular pleXUS, and enter lobular capillaries. At a, a. the capillaries of one 1 ’ lobule communicate with those adjacent to it. the central vein. The capil- laries of the vaginal and capsular branches terminate in veins that join the portal vessels. The Liver Cells. — These are granular nucleated masses of protoplasm, often containing fat particles. They are arranged in columns between the strands of the intralobular plexus of capillaries. The Bile Passages begin within the lobule as a network of fine canals — the bile capillaries — tunnelled at the lines of apposition of the liver cells. At the periphery of the lobule these become continuous with interlobular bile ducts having a proper wall and a simple columnar epithelial lining. The interlobular bile ducts unite to form the larger ducts that accompany the blood-vessels in the portal canals, and these finally form the main bile duct, which passes in the gastro-hepatic omentum to perforate the wall of the duodenum. STRUCTURE OF THE SPLEEN. The spleen, like the liver, possesses two coats, viz., an outer serous or peritoneal coat , and a deeper fibrous tunic, or tunica propria. The latter is composed of white fibrous tissue with a considerable admixture of elastic and non-striped muscular fibres. It detaches from its inner sur- face a multitude of trabeculae , which by their anastomosis form a fibrous framework in the interior of the organ. The interspaces of this frame- work are occupied by a grumous material — the splenic pulp. If the cut surface of the spleen be washed beneath a tap, the pulp may be removed and the fibrous trabeculae rendered very evident. The Splenic Artery, a division of the coeliac axis, is a very large DISSECTION OF THE ABDOMEN. 333 vessel. Its branches enter at the concave border of the spleen, and cany with them sheaths derived from the fibrous tunic. These branches reduce themselves by division, and the smaller branches are remark- able in having the outer coat formed of lymphoid tissue Here and there this lymphoid tissue forms distinct swellings developed either uniformly around the arteries, or more or less to one side. These are the Malpighian bodies of the spleen. The arteries terminate in tufts of capillary vessels in the pulp. They are believed to have incomplete walls, allowing their contents to escape and form the pulp. The rootlets of the splenic vein begin in the same manner, having incomplete walls through which their lumen is continuous with the spaces lodging the pulp. Gradually their walls become thicker and complete, and adja- cent veins uniting on their course towards the anterior border form the large splenic vein , which is one of the main branches of the portal vein. The Splenic Pulp possesses a supporting network of retiform connective- tissue; and the meshes of this network are set with many lymphoid cells like the colourless corpuscles of the blood, and with red blood corpuscles, normal or in different stages of disintegration STRUCTURE OF THE PANCREAS. The pancreas is a compound tubular or racemose gland. It is com- posed of lobules held together by a connective-tissue framework. When the main ducts of the gland are traced backwards into the gland, they are found to be formed by the union of smaller ducts, and so on until the smallest ducts are reached. These begin in the alveoli , which are lined by secretory epithelium. STRUCTURE OF THE KIDNEY. The kidney is invested by a fibrous capsule . In health this can with- out difficulty be stripped off the kidney substance, to which it is con- nected only by delicate processes and vessels. If a horizontal section be made from the convex border to the hilus of the kidney, the organ will be seen to possess a cavity towards the hilus, termed the pelvis , and to Fig. 42. Cut Surface of Horse’s Spleen, TRABECULAR FRAMEWORK. 334 THE ANATOMY OF THE HORSE. consist of two different kinds of tissue — the cortical and the medullary substance of the kidney. The renal pelvis is a curved cavity, its extremities being termed the arms. On its outer side there is a horizontal ridge — the renal crest — on which the uriniferous tubules open, and on its inner side it is con- tinuous by a funnel-shaped opening with the lumen of the ureter. The cortical substance forms a layer beneath the capsule ; the medulla is disposed around the pelvis and is internal to the cortical substance. The cortex is about twice as thick as the medulla, but the two layers meet along a sinuous line, and slightly interpenetrate one another. It will be noticed that the two layers contrast with one another in the following respects : — The cortex is of a deep red colour, it is granular, friable, and studded with numerous small shining points — the Malpighian bodies . The medulla, on the other hand, is pale red, striated, and fibrous-looking, less friable than the cortex, and without any Malpighian bodies. Uriniferous tubules . — The largest tubes, or papillary ducts , open on the crest of the pelvis. If such a tube be traced, it will be found to pass outwards through the medulla, having a straight course, and branching dichotomously. The smaller tubes resulting from this division are called the collecting tubes ; and, still preserving their rectilinear course, they enter the cortex in bundles termed the pyramids of Ferrein. At the surface of these pyramidal bundles, the straight tubes curve out- wards in the cortex, and become dilated and tortuous, forming the intermediary or junctional tubules. Each of these is succeeded by a narrow straight tubule, which descends from the cortex to the medulla, where it forms a bend, or loop, and runs up again into the cortex. There is thus formed the looped tube of Henle , which is shaped like the letter U. Having re-entered the cortex, Henle’s tube becomes dilated and tortuous, constituting the convoluted tube , which becomes constricted and then expands into a bladder-like dilatation — Bowman's capsule. Bowmaris capsule surrounds a clue-like tuft of capillary vessels called the glomerulus , and the whole constitutes a Malpighian body. It is more natural, but less simple at first, to regard the tube as beginning not at the crest of the pelvis, but at Bowman’s capsule. The student should mentally work it out in that direction for himself. The urini- ferous tubules consist of a basement membrane with an epithelial lining. In the convoluted and intermediary tubes the cells are irregularly columnar, but their outlines are obscure ; in the descending limb of Henle’s tube (nearest the capsule of Bowman) the cells are flattened ; and elsewhere the cells lining the tubes are cubical or columnar. The Renal Vessels. The renal artery divides into a number of branches which penetrate the kidney near the hilus. Reaching the boundary line between the cortex and medulla, the arteries divide DISSECTION OF THE ABDOMEN. 335 and anastomose to form a series of arches from which both cortical and medullary vessels arise. The cortical or interlobular arteries are larger and more numerous than those for the medulla. They pass directly outwards towards the surface of the kidney, giving off lateral branches — -the vasa afferentia — to Bowman’s capsule, and terminal branches to the fibrous coat of the kidney. Each vas afferens pierces Bowman’s capsule, and resolves itself into the glomer- ulus , or capillary tuft. From this again the blood is led out of Bowman’s capsule by the vas ejferens. The vasa efferentia again resolve themselves into capillaries, and these form a network among the convoluted tubes. From this intertubular capillary network, small veins arise and pass to join the inter- lobular veins , running alongside Fig. 43. (modified from Turner). 1. Papillary duct ; 2. Collecting tube ; 3. Inter- mediary tube ; 4. Looped tube of Henle ; 5. Con- voluted tube ; 6. Bowman’s capsule ; A. Segment of artery forming renal arch ; B. Interlobular artery C. Afferent vessel of glomerulus ; D. Efferent vessel of the same ; E. Glomerulus ; F. Plexus formed by vasa efferentia ; G. Arteriolse rectse ; H. Interlobular vein. the arteries. These interlobular Vessels of the Kidneys, and uriniferous Tubules veins begin at the surface of the kidney by the convergence of a number of minute veins from the capsule — forming the stellate veins. The interlobular veins join venous arches disposed in the boundary layer between cortex and medulla, and from these arise the larger branches that finally unite to form the large renal vein at the hilus. The medulla is less vascular than the cortex. Springing from the arterial arches in the boundary layer are branches that break up into pencils of long straight arterioles — the arteriolce rectce. These pass with a rectilinear course between the straight tubules of the medulla, and break up into a wide-meshed capillary network around and between these tubules. Veins having a straight course like the arteries run in company with them, and join the venous arches in the boundary layer. Connective-tissue of the Kidney . — This exists very sparingly between the tubes in the cortex, but more abundantly in the medulla. 336 THE ANATOMY OF THE HORSE. £ > ^ S c 3 bn P GO iS nS S3 02 O) 'rf £ CD (D ^ 33 o « Q "S«a > §3 "SrH «8 •S 3 gs x* S3 o I ■+=» o 3 £ > .5 ’ 02 CD O o O) *4 o 33 *P« cc .3'3,-h S «S cS cS S3 jh £3 bO 02 S3 2 □Q § c3 CQ § 5> GO A c<3 Sh ’5b S3 o c3 "§ c3 > O) 32 S3 S° OQ 0 <3 Eh kJ « GO H O O « H DISSECTION OF THE ABDOMEN. 337 T5 O o ni J-H o -4^ £ Z CHAPTER XI. DISSECTION OF THE PELVIS. Under this heading there will be described not only the pelvic cavity and its contents, but also the tail and the hip-joint. Directions. — The dissection of the abdomen having been completed, the vertebral column should be sawn across or disarticulated about the middle of the lumbar region. If the directions given on page 69 have been attended to, the dissector of the pelvis should find the hip-joint intact, with the femur sawn across below the small trochanter, as in Fig. 48. The muscles or portions of muscles left around the hip-joint should be carefully removed, and the ligaments of the joint are to be dissected, noticing in the first place, however, its movements. THE HIP-JOINT AND THE LIGAMENTS OF THE PELVIS. The Hip-joint belongs to the class of enarthrodial or ball-and-socket joints. The bones that enter into its formation are the femur and the os innominatum, the former furnishing a rounded hemispherical head , and the latter a cup-like cavity — the acetabulum , or the cotyloid cavity. Movements. If the stump of the femur be grasped, it will be found to have a great freedom of movement. Thus, it can be flexed , extended , abducted , adducted , circumducted , and rotated. In flexion the femur is carried forwards so as to diminish the angle formed by that bone and the ilium. For the definition of the other terms see page 42. In the horse the hip-joint admits of a greater range of movement than any other joint of the limbs. The movement of abduction, however, is less free than it is in the other domestic animals, being, as will presently be seen, restricted by the pubio-femoral ligament. The joint possesses four ligaments, viz., capsular, cotyloid, pubio- femoral, and round ligaments. The Capsular Ligament has the form of a double-mouthed sac, attached, on the one hand, to the rim of the cotyloid cavity, and to the cotyloid ligament, and, on the other hand, to the periphery of the articular head of the femur. It is strengthened in front by an oblique band representing the ilio -femoral ligament of man. Its inner face is DISSECTION OF THE PELVIS. 339 lined by the synovial membrane of the joint, while its outer face is supported by the following muscles : — the deep gluteus above, the obturator externus below, the rectus femoris and the rectus parvus in front, and the gemelli behind. The ligament should be incised to show the synovial membrane, after which it may be removed entirely. The Synovial Membrane forms a complete internal lining to the capsular ligament, and also invests the pubio-femoral and round ligaments in the interior of the joint. The Cotyloid Ligament is a ring of fibro-cartilage fixed at the margin of the cotyloid cavity, which it serves to deepen for the reception of the femoral head. On the inner side of the joint, where the notch interrupts the rim of the cotyloid cavity, the ligament bridges over the gap, and to this portion of the ring the term transverse ligament is sometimes applied. This portion of the ligament, thus, converts the notch into a foramen, through which the pubio-femoral ligament enters the joint. The Pubio-Femoral Ligament. This ligament derives its fibres from the prepubic tendon of the abdominal muscles, the right and left ligaments intercrossing their fibres in front of the pubes. It is directed outwards and backwards, resting in a groove on the inferior surface of the pubis, and perforating the origin of the pectineus muscle. At the notch on the inner side of the cotyloid ligament, it enters the hip-joint by passing above (in the natural position) the so-called transverse ligament, and it terminates in the depression on the head of the femur. The ligament, being attached across the middle in front, is put upon the stretch when the limb is abducted, and therefore restricts that movement. The Round Ligament ( interarticular ligament , or ligamentum teres). This short and strong ligament is fixed above to the non-articular depression at the bottom of the cotyloid cavity, and below to the excava- tion on the head of the femur, being confounded at the latter point with the pubio-femoral ligament. It will be best displayed by cutting the transverse ligament and abducting the femur. Direction. — It is convenient to dissect at this stage the sacro-sciatic ligament, as it is necessary to remove it in order to display the pelvic contents. Along with it, there will be described two other ligaments — the superior and inferior ilio-sacral ligaments. The Sacro-sciatic Ligament (Plate 16, and Fig. 48). This is a large membranous ligament which forms the greater part of the lateral boundary of the pelvis. It is irregularly four-sided in form. Its upper edge, which is pierced by the ischiatic artery, is fixed to the lateral lip of the sacrum, and to the rudimentary transverse processes of the first one or two coccygeal bones; its lower edge is attached to the superior ischiatic spine and to the tuber ischii, and between these points it forms the upper boundary of the small sacro-sciatic foramen ; its anterior edge 340 THE ANATOMY OF THE HORSE. is short, and forms the posterior boundary of the great sacro-sciatic foramen; its posterior edge, much more extensive than the anterior, is thin, ill-defined, and united to the coccygeal origin of the semimem- branosus. Its outer surface is crossed by the great sacro-sciatic nerve, and is covered by the biceps femoris and semitendinosus muscles, which in part arise from it. Its inner surface is lined anteriorly by peritoneum, and is related posteriorly to the compressor coccygis and retractor ani muscles, some of whose fibres take origin from it. The internal pudic nerve and vessels cross this surface, or they may be partly embedded in the texture of the ligament. The Great Sacro-sciatic Foramen is an elliptical opening in the lateral wall of the pelvis, its anterior boundary being formed by the ischiatic edge of the ilium, and its posterior by the sacro-sciatic ligament. It transmits the gluteal nerves and vessels, and the great sciatic nerve. The Small Sacro-sciatic Foramen is an interval in the lower and posterior part of the lateral wall of the pelvis. Its upper edge is formed by the sacro-sciatic ligament ; its lower by the smooth and rounded external border of the ischium, between the tuber and the superior ischiatic spine. By this opening the common tendon of the obturator internus and pyriformis emerge from the pelvis, and the nerves to these muscles pass in. The Superior Ilio-sacral Ligament (Fig. 48) is cord-like, and passes between the internal angle of the ilium (the angle of the croup) and the summits of the sacral spines. The Inferior Ilio-sacral Ligament (Fig. 48) is membranous and triangular in form. Its anterior edge is fixed to the upper part of the ischiatic border of the ilium ; its lower edge is attached to the lateral lip of the sacrum ; its posterior or upper edge is ill-defined, being con- tinuous with the fascia investing the muscles of the tail. THE CAVITY OF THE PELVIS. Directions. — Fix the pelvis on a table, with the inlet looking upwards. Sponge out the cavity and distend the bladder with air or some preserv- ative fluid, tying the urethra to prevent its escape. The pelvis is not distinct from the abdominal cavity, but is merely a backward continuation of it. It is, in fact, that portion of the general cavity of the belly which is posterior to the bony circle formed by the sacrum, pubes, and ilio-pectineal lines. The plane of separation between the abdominal cavity proper and the pelvic cavity, is termed the inlet of the pelvis ; the posterior extremity of the pelvic cavity is termed its outlet. The inlet or brim of the pelvis is circumscribed by the promontory of the sacrum above, by the anterior margin of the pubic bones below, and by the ilio-pectineal line on each side. It looks downwards and forwards, DISSECTION OF THE PELVIS. 341 and it is considerably larger in the mare than the horse. In form it is nearly circular. The outlet of the pelvis is circumscribed by the first one or two coccygeal bones above, by the posterior edges of the ischial bones below, and by the posterior edge of the sacro-sciatic ligament on each side. In outline it is ovoid, with the broad end below ; and it looks backwards and upwards, being nearly parallel to the inlet. The Cavity of the Pelvis is the irregularly tubular passage between the inlet and the outlet. Its transverse section approaches the circular in front, but changes gradually to the oval as it is taken more posteriorly. For convenience of description, however, it may be said to have a roof, a floor, and two lateral walls. The roof is formed by the inferior surface of the sacrum and first one or two coccygeal bones. The floor is formed by the pubic and ischial bones. Each lateral wall is formed for a short space in front by the pelvic surface of the shaft of the ilium, and for the rest of its extent by the sacro-sciatic ligament. Contents of the Cavity . — These vary with the sex. In both sexes it contains the rectum, the urinary bladder, and the termination of the ureters, and numerous important vessels and nerves. In the male it lodges, besides these, the vasa deferentia (in part), the seminal vesicles, the prostate, Cowper’s glands, the ejaculatory ducts, and the prostatic and membranous portions of the urethra. In the female it lodges the posterior part of the uterus, the vagina, and the vulva. The Peritoneum. The serous lining of the abdominal cavity is con- tinued into the pelvis, whose walls and contents it in part covers. Thus, if it be followed backwards along the roof of the cavity, it will be seen to cover the lower face of the sacrum about as far as its 4th segment, but at that point it is reflected on to the rectum. Again, if the peri- toneum be traced over the pelvic brim at the pubes, it will be found to cover the floor of the pelvis for a short distance, and then to become re- flected on to the bladder. In the same way, along a curved line on the side of the pelvis between these two points, the peritoneum leaves the pelvic wall and passes on to the viscera. Since this reflection, however, takes place anterior to the posterior extremity, or outlet, of the cavity, it results that the pelvic viscera get at most only a partial covering of peritoneum. Thus, the rectum for a length of from four to six inches in front of the anus, the posterior extremity of the vesiculee seminales, and (in the collapsed state) nearly the half of the upper face of the bladder, and three-fourths of its lower face are without a serous covering. In the mare, in the same manner, the posterior part of the vagina and the whole of the vulva are without a serous covering. The peritoneum in passing on to the viscera forms certain folds, or ligaments. Thus, it forms below and on each side of the urinary bladder a double fold, the inferior and lateral ligaments of the organ (Plate 342 THE ANATOMY OF THE HORSE. 44). The inferior ligament is a mesial fold attaching the bladder to the pubic symphysis, and to the middle line of the abdominal wall in front of the pubic brim. The lateral ligaments pass between the sides of the bladder and the lateral walls of the pelvis, and in the adult the free (anterior) edge of each contains the cord-like remains of the umbilical artery. Again, the peritoneum, in descending from the roof of the cavity to envelop the first part of the rectum, forms a suspensory fold — the meso- rectum, which is continuous in front with the colic mesentery. On each side the ureter and the vas deferens project narrow bands of peritoneum, and the right and left vasa deferentia where they lie above the bladder are connected by a triangular serous" fold which contains between its layers the prostatic vesicle. In the mare there are formed in an analogous manner the uterine ligaments described at page 303. Directions. — The pelvis should now be either laid on its side, or sus- pended in the natural position and at a convenient height. A side view of its contents is to be exposed by the following steps : — With the saw cut through the shaft of the ilium close above the cotyloid cavity. Make another section through the same bone immediately external to the sacro- iliac articulation. Remove the intermediate piece of bone, at the same time separating the peritoneum from its inner aspect. Find the internal pudic artery in the position shown in Plate 16. It will be either internal to the sacro-sciatic ligament or in its texture. Trace it forwards and backwards. It is accompanied by a satellite vein, and where the two vessels pass above the small sacro-sciatic foramen they are crossed out- wardly by the internal pudic nerve. This having been found should be followed upwards. Without injury to the nerve and vessels, the sacro-sciatic ligament may then be removed, taking care of the com- pressor coccygis and retractor ani muscles, which lie internal to the pos- terior part of the ligament. The Internal Pudic Artery (Plates 46 and 47) is a branch of the internal iliac, arising at the last lumbar vertebra. Entering the pelvis, it descends obliquely downwards and backwards across the side of the cavity, lying on the inner surface of the sacro-sciatic ligament or within its texture (Plate 1 6). At the small sacro-sciatic foramen it passes with an inward and backward direction, terminating in a manner that varies with the sex. In the male it turns round the ischial arch and reaches the perineeum, where it penetrates the urethral bulb. Besides slender hsemorrhoidal and perineal branches, it gives off the vesico-prostatic artery. The vesicogjrostatic artery arises about the neck of the bladder, and supplies the prostate, the vesicula seminalis, the posterior part of the bladder, and the terminal part of the vas deferens. Printed, by W. St. A K Johnston. Edinburgh Sc London DISSECTION OF THE PELVIS. 343 In the female the internal pudic terminates in haemorrhoidal, vulvar, and bulbous branches; and, instead of the vesico-prostatic, it gives off the vaginal artery , which is expended in the bladder, vagina, and cervix uteri, anastomosing with branches of the uterine artery. The Umbilical or Hypogastric Artery. In the adult (Plate 46) this is a comparatively small vessel arising from the internal pudic near its root. It is pervious only in the first few inches of its course, giving off a few twigs to the bladder, and being then continued as a solid cord at the free edge of the lateral ligament of the bladder. In the foetus, however, it is of great size, and carries the foetal blood to the placenta to be purified. The Internal Pudic Vein runs in company with the artery. It receives branches corresponding to those of the artery, and terminates in the internal iliac vein. The Pudic Nerve is derived from the 3rd sacral. Descending on the inner surface of the sacro-sciatic ligament, it crosses the internal pudic vessels superficially at the small sacro-sciatic foramen. Here it turns slightly inwards, and disappears beneath the ischio-urethral muscle. Having gained the lower face of the urethra, it turns round the ischial arch, and is continued as the dorsal nerve of the penis. Before leaving the pelvis, it detaches a perinceo-anal branch , which gives twigs to the muscles of the urethra and penis, and hemorrhoidal branches that pass upwards on the rectum to reach the anus, some of them appearing to terminate in the sphincter. These latter branches are crossed by de- scending branches from the hemorrhoidal nerve. The lower posterior gluteal nerve (Plate 16) gives fibres to both the trunk of the pudic nerve and its perineo-anal branch, and in some cases the latter derives the majority of its fibres from this source. In the female the pudic nerve terminates in branches to the labia, clitoris, and constrictor muscles of the vulva. The Haemorrhoid al Nerve is derived mainly from the 4th sacral. It descends on the inner face of the sacro-sciatic ligament, and (for a short distance) the compressor coccygis muscle. It supplies a twig to that muscle, and then penetrates it, or emerges between it and the retractor ani. It then divides into branches for the retractor and sphincter muscles of the anus, and for the skin of the perinaeum. The Retractor Ani. This muscle is described with the perinaeum (page 276), but it is here exposed in the whole of its extent. The Compressor Coccygis (Fig. 48) arises from the inner surface of the sacro-sciatic ligament, over the superior ischiatic spine. Passing back- wards and upwards, it is inserted into the last sacral and first two coccy- geal vertebrae. By its inner face it is related to the rectum, except close to its insertion, where the edge of the depressor of the tail intervenes. Action . — Acting with the opposite muscle, it forcibly depresses the 344 THE ANATOMY OF THE HORSE. tail, compressing it over the perinaeum. Acting singly, it inclines the tail to that side. Directions . — The preceding two muscles should be entirely removed. Above the rectum there will be found the terminal portion of the pos- terior mesenteric artery; and on its side, the pelvic plexus of nerves. The Posterior Mesenteric Artery (Plate 46) is a branch of the abdominal aorta. Its terminal portion enters the pelvic cavity between the layers of the meso-rectum; and passing backwards above the bowel, it terminates above the anus. In its backward course it detaches numerous branches to the wall of the rectum. The Posterior Mesenteric Vein runs in company with the artery. Its initial portion is formed at the posterior part of the rectum, by the union of haemorrhoidal veins, which communicate with like branches of the internal pudic vein. In the abdominal cavity it concurs in the formation of the portal vein. The Pelvic Plexus of the sympathetic nerve. This is an intricate network of nerves, placed on the side of the rectum, and distributing- branches to the pelvic viscera. It receives in front the offsets from the posterior mesenteric plexus, and above it is joined by branches from the inferior sacral nerves. In both sexes it distributes branches to the rectum and bladder; and, besides, it supplies branches to the prostate, vesicula seminalis, and vas deferens in the male, and to the vagina and uterus in the female. The Kectum (Plate 46) is the terminal segment of the large intes- tines. At the entrance to the pelvis it is directly continuous with the small colon, and it terminates at the anus. Its initial portion resembles the small colon in being puckered and of comparatively small calibre. Its terminal portion, on the other hand, is dilated and sac-like, forming a large pouch in which the faeces collect. In the male it is related inferiorly to the bladder, vesiculae seminales, vasa deferentia, prostate gland, and pelvic part of the urethra. In the female it is related on the same aspect to the vulva, vagina, and uterus. Structure . — The wall of the rectum resembles that of the large intes- tine in general (page 309), possessing serous, muscular, submucous, and mucous layers. As already seen, its peritoneal investment is incom- plete, its terminal portion being destitute of peritoneum, and connected by loose areolar tissue to contiguous organs. In front of the anus the longitudinal muscular fibres of the bowel form on each side a band that passes upwards to be inserted into the coccygeal vertebrae. This, which is termed the suspensory ligament of the rectum , forms a prominence at the root of the tail. At the anus the last of the circular muscular fibres form what is termed the internal sphincter. Developed in connec- tion with the termination of the rectum are two striped muscles — the sphincter ani extern us and the retractor ani. These are described at PLATE XLVII Post aorta Renal artery Supra-renal capsule Left kidney Umbilical art. Right kidney Testicle Vas deferens Cowper’s gland Crus penis -Retractor penis Printed by W. &.A.K. Johnston. EdinVurgh fc London GENITOURINARY ORGANS OF MALE (Chauveau) DISSECTION OF THE PELVIS. 345 page 276. In the male the retractor muscles of the penis (page 276) form a kind of sling for the rectum in front of the anus; and similar cords of involuntary muscular tissue unite below the rectum at the same point in the female, and terminate in the vulva. The Urinary Bladder (Plates 46 and 47) is the reservoir for the accumulation of the urine. The secretory action of the kidneys is constant ; and the urine, passing along the ureter, accumulates in the bladder, to be expelled at intervals. As now seen in its distended condition, the bladder is not wholly contained within the pelvic cavity, but projects a little • beyond the pubic brim. When empty, however, it lies entirely within the cavity, resting on the concave upper surface of the pubic bones. In form the distended viscus is ovoid. The broad end, which is free and directed forwards, is termed the fundus ; the narrow end has the opposite direction, and becomes continuous by a constricted neck with the urethra ; the sides of the bladder are related to the pelvic walls ; and the upper surface is related to the rectum, vasa deferentia, and vesiculse seminales in the male, and to the vagina and uterus in the female. It is maintained in position by the peritoneum, which gives it only a partial covering, and by its continuity with the urethra. As already noticed, the peritoneum in passing on to it forms the folds called the middle and lateral ligaments of the organ. The Ureters (Plates 46 and 47). Each tube having crossed the inlet of the pelvis, passes across its lateral wall, sustained by a narrow band of peritoneum. Finally, it is reflected inwards to perforate the upper wall of the bladder, a little in advance of its neck. Directions. — Should the subject be a mare, the dissector must now turn to page 351 et seq., where the urethra and reproductive organs of the female are described. The Urethra in the male (Plate 47). This is a long tube, extending from the neck of the bladder to the free extremity of the penis. The first few inches of the tube are contained within the pelvis, between the rectum and the ischiatic symphysis ; for the rest of its extent it is extra- pelvic, and amalgamated with the penis except at its termination, where it projects as a short tube from the glans penis. The intra-pel vie division of the tube is divided into the prostatic and membranous portions; the extra-pelvic division is called also the spongy portion. The prostatic portion includes the first inch or two of the tube behind the neck of the bladder, and it is embraced by the prostrate gland. The membranous portion comprises the next two or three inches, extend- ing as far as the ischial arch, where, at a very acute angle, it becomes continuous with the spongy portion. It is at this angle that the point of the catheter is likely to be arrested. Muscles. The membranous part of the urethra has connected with it two muscles. The first of these, termed Wilson’s muscle, or the 346 THE ANATOMY OF THE HORSE. constrictor urethrae , envelops the tube behind the prostrate gland, from which, indeed, it is not well defined. Its muscular fibres, of a pale red colour, comprise two sets, which extend across the urethra on its upper and lower faces respectively, and embrace the tube like an elliptical sphincter. The most posterior fibres of the muscle pass over Cowper’s glands. The other, termed the ischio-urethral muscle , consists on each side of a band whose fibres arise from the ischial arch, and pass to the urethra beneath Cowper’s gland, blending with Wilson’s muscle. Like the preceding, it is composed of pale red muscular fibres. Action. — These muscles are constrictors of the membranous urethra, and aid in the ejection of urine and semen. The spongy 'portion of the urethra, with its muscles — the transversus perinsei and accelerator urinse — has been already described as a con- stituent part of the penis (page 284). The Prostate Gland (Plates 46 and 47) embraces the neck of the bladder and the initial part of the urethra. It consists of a middle and two lateral lobes ; and in structure it is glandular, with a considerable admixture of striped muscular tissue. Its glandular texture consists of branching excretory tubes and acini, both having a columnar lining. Its ducts, as will be seen at a later stage, perforate the urethral wall, to which it is adherent. Cowper’s Glands (Plates 46 and 47). Each of these is placed at the side of the membranous urethra, just in front of the ischial arch. They are round, reddish-yellow, and (in the stallion) about the size of a hazel nut. They have the racemose type of structure, and their ducts perforate the adjacent wall of the urethra. The Vasa Deferentia (Plates 46 and 47). These are the excretory ducts of the testicles. As already seen, each is one of the constituents of the spermatic cord. Appearing at the internal abdominal ring, as a tube about the thickness of a goose-quill, it is reflected backwards to enter the pelvis. Crossing the direction of the ureter, it places itself on the upper surface of the bladder, and expands to four or five times its previous calibre, forming what is called the bulbous portion of the vas deferens. It then passes backwards beneath the vesicula seminalis ; and contracting again, it terminates under the prostate, by uniting out- wardly with the neck of the vesicula to form a short tube termed the ejaculatory duct. Where the vasa deferentia lie above the bladder, they are connected together by a peritoneal fold between whose layers there is contained the vesicula prostatica , or uterus masculinus. This is a short tube with a blind anterior end, and opening by its posterior extremity into the urethra. It is the homologue of the uterus and vagina of the female. The V EsicuLiE Semin ales (Plates 46 and 47). These bodies are placed between the rectum and the posterior part of the upper face of the DISSECTION OF THE PELVIS. 347 bladder. Each is a small ovoid sac, like a miniature bladder. The anterior end of the sac is rounded and free; the posterior end contracts, and unites with the vas deferens to form the ejaculatory duct. Only the anterior half of the vesicula is covered by peritoneum, which in passing between the two bodies forms a small triangular serous fold. The Common Ejaculatory Ducts. Each of these is a short tube formed under cover of the prostate, by the union, at a very acute angle, of the neck of the vesicula seminalis with the vas deferens. Its opening into the roof of the urethra will be presently exposed. Directions. — Carefully raise the fundus of the bladder, and cut its peritoneal and connective-tissue adhesions to the sides and floor of the pelvis. Free, in the same way, the membranous urethra at the ischial arch ; and cut the crus penis and its erector muscle from the tuber ischii. This will enable the dissector to remove from the pelvis the organs just described, while maintaining their mutual relations. Lay the bladder on a table with its upper or rectal aspect downwards, and open it by a mesial incision on its lower face. Carry the incision backwards into the urethra, so as to open the whole extent of its prostatic and membranous portions. Care must be taken that the incision in both bladder and urethra is on the inferior face. Structure of the Bladder. This comprises four coats : — 1. The Serous or Peritoneal Coat. This, as already seen, is an incom- plete investment. 2. The Muscular Coat is composed of bundles of non-striped fibres arranged in all directions. Compared with its condition in many other animals, this coat is very thin ; and its fasciculi in the distended bladder seem hardly to form a continuous layer. At the neck of the bladder some of the fibres have a circular disposition, forming the sphincter vesicce. 3. The Submucous Coat is composed of vascular areolar connective- tissue, and it loosely unites the muscular and mucous coats. 4. The Mucous Coat. This forms a complete internal lining for the bladder, and in the empty viscus it is thrown into folds, or rugae. Observe the slit-like orifices of the ureters, near one another and a little anterior to the urethral orifice (Fig. 44). Pass a probe or bristle into one of them, and notice that the ureter perforates the wall very obliquely — an arrangement which has a valvular action in preventing the regurgitation of urine from the distended bladder. Between the uretral and urethral orifices in the human subject is a triangular area — the trigone — over which the mucous membrane is smooth even in the contracted bladder. In the horse, however, this area is wrinkled like the rest of the surface. The epithelium of the mucous membrane is stratified and transitional. Structure of the Urethra. The spongy portion has been described 348 THE ANATOMY OF THE HORSE. with the penis. The prostatic and membranous portions have a mucous lining , external to which is a muscular coat of non-striped fibres. Observe the following points in connection with the interior of the intra-pelvic part of the urethra (Fig. 44). On the middle line of the roof of the tube, close behind the communication with the bladder, there is a mucous eminence — the colliculus seminalis , or verumontanum. In the gelding this is often small, and sometimes hardly recognisable, but in the stallion it is sometimes a considerable eminence, like the tip of the little finger. At each side of this projection is the orifice of the ejaculatory duct. These orifices in the stallion are sufficiently large to permit of the tip of the little finger being insinuated into them. This should be remembered, as the point of catheter, if not guided along the floor of the urethra, might easily pass into one of them. At the summit of the colliculus, and therefore on the middle line, is a very minute opening — the orifice of the uterus masculinus. Insert a fine bristle into it, and guide it on into the tube. On the wall of the urethra at each side of the colliculus, observe an irregular series of minute orifices which belong to the ducts of the prostate gland. Behind these on each side, notice another series of small open- ings with a linear arrangement. These are the Bladder and intrapelvic orifices of the ducts of Cowper’s glands. Insert portion of Urethra 1 ° opened from below bristles into a few of each set of openings, and 1 . Vas deferens; i'. Bulb- o u ^ e them on into the respective glands. Close ous part of the same ; 2. Peri- to the neck of the bladder the epithelium of the toneal fold joining the vasa r deferentia ; 3. Bladder ; 4. urethra is of the same character as in the bladder, Vesicula seminalis ; 5. Ori- fices of ureters ; 6. Prostate ; but behind that point it is simple and columnar. 7. Verumontanum with ori- _ T mi fices of ejaculatory ducts ; S. STRUCTURE OF THE VESICUHE SEMIN ALES. 1 he Orifice of prostatic vesicle; 0 -ci „ 9. Cowper’s gland ; io. Ori- walls of these are composed ol fibrous, fibro- Orifices^^ucte^o^c^wper^s muscular, and mucous layers ; and contain many no a sum ; u. Coitus spongi- tubular glands, which discharge their secretion osum with urethra in its i n to the cavity, where it mixes with the semen. The bulbous portion of each vas deferens has the same structure. Directions. — The student must now return to the pelvis, at the roof of which he is to dissect the lumbo-sacral plexus of nerves, and the branches of the internal iliac artery (Plate 48). Thereafter he is to examine the pyriformis and obturator interims muscles. PLATE XL VIII O DISSECTION OF THE PELVIS. 349 The Lumbo-sacral Plexus (Plate 48) is composed of the anastomos- ing nerve trunks for the supply of the hind limb. It is formed by the inferior primary branches of the last three lumbar (4th, 5th, and 6th) and first two sacral nerves, and it receives also a fasciculus from the corresponding branch of the 3rd lumbar nerve. Each of these roots emerges from the intervertebral foramen behind the vertebra after which it is named ; thus, the root from the 6th lumbar nerve emerges by the intervertebral foramen behind the 6th lumbar vertebra, the 1st sacral root by the first inferior sacral foramen, and so on. The branches of the plexus, taken in order from before to behind, are as follows : — 1. Iliaco-muscular Branches, for the psoas and iliacus muscles. Two of these are seen in Plate 48, one coming from the anterior root of the plexus, and the other from the anterior crural nerve. 2. The Anterior or Great Crural Nerve. In point of size, this is the second nerve of the plexus. It derives its fibres from the first two roots of the plexus (4th and 5th lumbar), and from the fasciculus furnished by the 3rd lumbar nerve. 3. The Obturator Nerve derives its fibres from the 4th and 5th lumbar roots of the plexus. It descends in company with the obturator vessels, resting on the pelvic surface of the ilium. Under cover of the obturator internus muscle, it passes through the obturator foramen and reaches the thigh. The 5th lumbar root, having given a branch to aid in the formation of the anterior crural, and another to the obturator nerve, is continued obliquely backwards between the internal iliac artery and the spine, to join a broad nervous fasciculus to which the remaining roots of the plexus (6th lumbar and first two sacral) contribute the whole of their fibres. The remaining branches of the plexus are divisions of this fasciculus. 4. The Anterior Gluteal Nerves. Three or four in number, these leave the pelvis and reach the hip by passing through the forepart of the great sciatic opening, with the gluteal vessels. 5. The Great Sciatic Nerve, the largest in the body, passes out into the hip through the great sciatic foramen, behind the preceding. 6. The Posterior Gluteal Nerves, distinguished as superior and inferior, pass out behind the great sciatic. The 3rd Sacral Nerve. The inferior primary branch of this nerve is continued as the internal pudic nerve, after giving a bundle of fibres to aid in the formation of the hgemorrhoidal nerve. The 4th Sacral Nerve receives the before-mentioned branch from the 3rd nerve, and is continued as the hgemorrhoidal nerve. The 5th Sacral Nerve gives a backward twig to the 1st coccygeal nerve, and is then expended in the skin of the anus and root of the tail. As in other regions of the spine, each of the inferior primary branches 350 THE ANATOMY OF THE HORSE. just considered communicates with the contiguous ganglion of the sym- pathetic cord, by one or more branches detached at the intervertebral foramen; and the sacral nerves send each a filament to the pelvic plexus. The Sympathetic Gangliated Cord in the sacral region. This is the direct backward continuation of the lumbar cord. It is placed on the inferior surface of the sacrum, internal to the inferior sacral foramina, the lateral sacral artery intervening between it and the inferior primary branches of the sacral nerves at their points of emergence. It possesses a ganglion opposite each of the first three sacral foramina; and, as before said, it communicates by filaments passing between these ganglia and the corresponding spinal nerves. The emergent branches of these ganglia are very slender, and pass to the cellular tissue beneath the sacrum, or to the contiguous blood-vessels. The cord terminates at the last ganglion, either abruptly, or by a filament passing on to the middle coccygeal artery. The Internal Iliac Artery (Plate 48). This is one of the terminal branches of the posterior aorta. Beginning at the intervertebral disc between the 5th and 6th lumbar vertebrae, it passes downwards and backwards across the articulation between the last lumbar transverse process and the sacrum, and then across the sacro-iliac articulation; and at the upper part of the ilio-pectineal line, a little above the eminence of the same name, it divides into the iliaco-muscular and obturator arteries. The vessel is covered by the peritoneum, and in the first inch or two of its course it is separated from the external iliac artery by the common iliac vein. The collateral branches of the internal iliac, taken in the order of their point of detachment, are as follows: — 1. The second last of the series of lumbar arteries arises from the in- ternal iliac at its root. It behaves like the lumbar branches of the aorta. Its upper division, much the larger of the two, passes upwards through the intervertebral foramen between the 5th and 6th lumbar vertebrae. 2. The Internal Pudic Artery. This is a considerable vessel having its origin at the last lumbar vertebra. Entering the pelvis, it descends at the ischiatic edge of the ilium, and then passes backwards in the texture of the sacro-sciatic ligament, or on its inner face. 3. The Lateral Sacral Artery leaves the parent trunk at the sacro- lumbar articulation, and passes backwards on the lower face of the sacrum, beneath or at the inner side of the inferior sacral foramina. A little behind the middle of the sacrum it divides into the ischiatic and lateral coccygeal arteries. The former, much the larger of the two, passes out through the edge of the sacro-sciatic ligament to reach the hip (Plate 16); the latter continues the direction of the lateral sacral artery to the tail. The inferior division of the 3rd sacral nerve appears in the angle of separation between these tw T o arteries. The collateral branches of the lateral sacral artery are : — (1) Branches entering the DISSECTION OF THE PELVIS. 351 intervertebral foramen between the last lumbar vertebra and the sacrum (last lumbar artery), and the first two or three inferior sacral foramina. Each of these enters the spinal canal, furnishes there a spinal branch, and then emerges by the corresponding superior foramen, and is dis- tributed to the overlying muscles and skin. (2) The middle coccygeal artery is an unpaired vessel, variable as to its origin, but generally, as in Plate 48, furnished by the right lateral sacral artery. It passes inwards to the middle line, and is continued backwards to the tail. 4. The Ilio-lumbar Artery. This artery is in series with the lumbar arteries, representing, as it were, the abdominal or inferior branch of the last lumbar artery. Arising from the outer side of the parent trunk, it passes outwards across the sacro-iliac joint, giving branches to the iliacus and psoas magnus muscles. Its terminal twigs may reach the gluteus maximus or the tensor vaginae femoris. 5. The Gluteal Artery, a large vessel, arises at the edge of the sacrum, and passes out into the hip by the great sacro-sciatic foramen, dividing into a number of branches as it escapes (Plate 16). The Iliaco-femoral Artery, one of the terminal branches of the internal iliac, passes downwards and outwards between the shaft of the ilium and the iliacus muscles, to reach the outer aspect of the thigh. It supplies the nutrient artery of the ilium. The Obturator Artery, the other terminal branch of the internal iliac, passes downwards and backwards on the pelvic surface of the ilium, at the anterior edge of the pyriformis muscle. Under cover of the obturator internus muscle, it passes through the obturator foramen and reaches the thigh. It is accompanied by a satellite vein, and by the obturator nerve, wdiich is placed anterior to the vessels. The tendon of the psoas parvus muscle is inserted in the angle of separation between this and the preceding artery. The Internal Iliac Vein collects the blood from the satellite veins of the foregoing arteries. It unites with the external iliac vein, forming the common iliac vein. The Obturator Internus and the Pyriformis. For a description of these muscles turn to page 68. REPRODUCTIVE ORGANS IN THE FEMALE. Comprised under this heading there are : the ovaries, the Fallopian tubes, the uterus, the vagina, and the vulva. The ovaries, the Fallopian tubes, and the uterus (in part) are abdominal organs, and their mode of suspension in that cavity has already been noticed. Their more com- plete examination can now be undertaken along with the dissection of the purely pelvic parts of the same apparatus, and at the same time it is convenient to examine the female urethra. The Ovaries, as already seen, are situated in the lumbar region of the 352 THE ANATOMY OF THE HORSE. abdominal cavity (see page 303). Each ovary is about half the size of the testicle — the corresponding organ of the male. In form it is ovoid, with a distinct depression on its upper surface — the hilus. At the hilus the nerves and vessels of the organ enter from the broad ligament of the uterus, and in its neighbourhood the expanded end of the Fallopian tube is attached by one of its fimbriae to the surface of the ovary. From the posterior extremity of the ovary a cord of non-striped muscular tissue — the ligament of the ovary — passes to the uterine cornu. The lateral surfaces, the inferior border, and the anterior end of the ovary are rounded and free. Structure of the Ovary. This comprises (1) an epithelial covering on the surface of the organ, (2) a fibrous framework, or stroma, and (3) Graafian follicles. 1. The Germinal Epithelium. — This is a single layer of short columnar cells with granular contents. In veterinary text-books the surface of the ovary is described as having a serous covering derived from the broad ligaments. The cells of this surface covering, however, are in marked contrast to the cells of the broad ligament, which have the ordinary flattened and transparent endothelial characters. The term germinal is applied to this layer because the ova, or germ-cells , are separated from it in the foetal ovary. 2. Th e Stroma is composed of fibrous connective-tissue with some bundles of non-striped muscular tissue. The blood vessels of the ovary ramify in it, and it surrounds the Graafian follicles. Around the hilus it is most vascular and open in texture, and this portion of the stroma is sometimes termed the zona vasculosa or the medullary substance , in contra- distinction to the peripheral cortical substance. A layer of condensed stroma without any Graafian follicles lies beneath the surface epithelium, and is sometimes termed the tunica albuginea of the ovary. 3. The Graafian Follicles , or Ovi-sacs. — These are vesicular bodies for the maturation and extrusion of the ova. A large-sized follicle possesses the following parts : — a. The wall of the follicle, composed of an inner delicate tunica propria , and an outer layer — the tunica fibrosa — derived from the surrounding stroma. b. The Membrana Granulosa. — This forms an epithelial lining to the wall of the follicle, and consists of several layers of cells. At one point these epithelial cells are heaped up to form the cumulus or discus proligerus, the cells of which surround the ovum. c. The Liquor Folliculi . — This is a fluid which fills up the remainder of the cavity of the follicle. The Ovum is a typical animal cell. It consists of an outer envelope — the zona pellucid a; protoplasmic cell-contents — the vitellus or yelk; a nucleus — the germinalvesicle; and, within the nucleus, a nucleolus — the germinal spot. DISSECTION OF THE PELVIS. 353 The Graafian follicles vary greatly in size. The smallest are imbedded in the cortical part of the ovary. These are of microscopic size, and differ from the larger follicles in having only a single layer of cells in the membrana granulosa, and in having no liquor folliculi. Follicles of intermediate size are placed more deeply in the ovary, and differ from the largest chiefly in the small amount of liquor that they contain. These differences of size represent different stages of development of the follicles, the largest being the most mature. When mature, a follicle occupies a considerable space in the substance of the ovary in the neigh- bourhood of the hilus. Finally it bursts through the surface of the Fig. 45 . Section of Cat’s Ovary, magnified (from Schrori ). 1. Outer covering of the ovary ; 2. Fibrous stroma ; 3. Superficial layer of fibro-nuclear substance; 3'. Deeper parts of the same ; 4. Blood-vessels ; 5. Ovi-sacs forming a layer near the surface ; 6. One or two of the ovi-sacs sinking deeper and beginning to enlarge ; 7. One of the ovi-sacs farther developed, now enclosed by a prolongation of the fibrous stroma, and consisting of a small Graafian follicle, within which is situated the ovum covered by the cells of the discus proligerus ; 8. A follicle farther advanced ; 8'. Another which is irregularly compressed ; 9. the greater part of the largest follicle, in which the following parts are seen ; a . Cells of the tunica granulosa lining the follicle ; b . The reflected portion named discus proligerus ; c . Vitellus or yelk part of the ovum, surrounded by the zona pellucida ; d . germinal vessicle ; e . Germinal spot. ovary, and the ovum, along with the liquor folliculi and part of the membrana granulosa, escapes and is caught by the expanded extremity of the Fallopian tube. The follicle then collapses, while it becomes in part filled with blood from the vessels opened by the rupture of its wall. The rupture then heals, and the follicle becomes converted into a 2 a 354 THE ANATOMY OF THE HORSE. yellowish body — the corpus luteum. In the early stage of a corpus luteum the cells of the membrana granulosa proliferate, while capil- laries extend into it from the wall of the follicle. Later on the blood- clot in the centre becomes decolorised, and the granulosa cells become fatty ; and finally the corpus luteum shrinks and disappears. The Parovarium, or the Organ of Rosenmuller. This is a minute body situated in the broad ligament, between the ovary and the Fallopian tube. It consists of a number of short convoluted tubules opening into a longitudinal tube, the latter representing the canal of Gcertner in the cow. The parovarium is the homologue of the epididymis of the male. The Fallopian Tubes, or Oviducts. The Fallopian tube is the duct for the conveyance of the ova from the ovary to the uterus. In its Fig. 40 . Right Ovary and Fallopian Tube. 1. Fallopian tube ; 2. Abdominal opening (fimbriated extremity) of the same ; 3. A probe intro- duced into the uterine opening of the tube ; 4. Ovary ; 5. Ligament of the ovary ; 6. Broad liga- ment of the uterus ; 7. Tip of uterine cornu laid open. course between these two organs the tube passes in a flexuous manner at the anterior border of the broad ligament. The ovarian extremity of the tube opens on the surface of an expansion whose rim is cut into a few short fringe -like processes — the fimbrice. Inwardly the rim of this expansion is fixed to the surface of the ovary near the hilus. The upper surface of the expansion is covered by a mucous membrane with delicate rugse that converge from its rim to its centre, where it shows the orifice of the tube — the ostium abclominale. The under surface DISSECTION OF THE PELVIS. 355 of the expansion is smooth and covered by peritoneum. The uterine extremity of the tube opens into the extremity of the uterine horn by a minute orifice — the ostium uterinum. Although the Fallopian tube bears to the ovary the relationship of an excretory duct, in that it conveys away the ova, it differs from all other excretory ducts in not having its lumen closely continuous with the interior of the gland whose secretion it conveys. Moreover, this discon- tinuity between the Fallopian tube and the ovary establishes an indirect communication between the sac of the peritoneum and the surface of the body, and brings about the single exception to the rule that serous membranes form perfectly close sacs. Structure of the Tube. The wall of the oviduct comprises the follow- ing layers, enumerated from without inwards, viz., (1) an outer serous coat , derived from the broad ligament ; (2) a coat of non-striped muscular tissue , arranged as an outer longitudinal and an inner circular set of fibres; (3) a submucous coat of vascular connective-tissue ; (4) a mucous coat , having a ciliated columnar epithelium. The lumen of the tube is narrowest at its uterine extremity and widest at the ovary. The Uterus, or womb, is the organ that receives the ovum, retains it during its development (provided it has been fertilised), and, finally, expels it at the expiration of the full term of pregnancy. In situation the organ is partly abdominal, and partly pelvic, and its mode of suspension by the broad ligaments has already been observed in connection with the peritoneum (page 303). The organ is single in its posterior portion, and bifid in front. The anterior bifurcations of the organ are termed its cornua or horns. At its anterior extremity each horn is pointed, and receives the uterine opening of the Fallopian tube. From this point the calibre of the horn gradually increases to its posterior end, where it opens into the body of the organ. Each horn shows a concave upper border at which the broad ligament reaches it, while its lower border is convex and free. The cornua are entirely abdominal in position, and are related to the intestines. The posterior single portion of the uterus comprises the body, and the neck , or cervix ; but this division is not apparent on the exterior. The body, placed in front, presents two faces, two borders, and two extremities. The upper face is slightly flattened and related to the rectum ; the lower face, also flattened, is related to the intestines in front, and to the bladder behind ; the borders, right and left, show the insertions of the broad ligaments ; the anterior extremity, or fundus, is the widest part of the body, and it is joined at each angle by the cornu ; the posterior extremity is continuous with the cervix. The body of the uterus is partly abdominal and partly pelvic in situation. 356 THE ANATOMY OF THE HORSE. The cervix is the extreme posterior part of uterus. It is directly continuous with the body in front ; and its posterior extremity, as will be seen when the organ is laid open, projects into the anterior extremity of the vagina. Fig. 47 . Generative Organs of the Mare, viewed from above. * 1, 1. Ovaries ; 2, 2. Fallopian tubes ; 3. Fimbriated extremity of the tube, outer face ; 4. The same, inner face, showing the abdominal orifice ; 5. Ligament of the ovary ; 6. Right cornu, intact ; 7. Left cornu, laid open ; S. Body of the uterus ; 9. Broad ligament ; 10. Os uteri (externum); 11. Interior of the vagina; 12. Meatus urinarius, with its valve 13 ; 14. Mucous fold, a vestige of the hymen ; 15. Interior of the vulva ; 16. Clitoris ; 17, 17. Labia of the vulva ; 18. Inferior commissure of the vulva. The • Vagina is a tubular organ which connects the uterus and the vulva. It is lodged entirely within the pelvis, being related to the rectum above, to the bladder and urethra below, and to the ureters and pelvic walls laterally. Its mode of connection with the two cavities that it connects will be ex- amined later on. Its average length is about nine or ten inches. The Vulva is the passage that continues the vagina back- wards, and opens on the surface of the body beneath the anus. The tube of the vulva is about five inches in length. It is united by cellular tissue to the rectum above, and to the pelvic floor below, while on each side it is related to the retractor ani muscle. Below and laterally it is covered by a layer of striped muscular tissue — the anterior constrictor of the vulva. The fibres of this muscle after embracing the tube of the vulva are lost on the sides of the rectum. The external opening of the vulva has the form of a vertical slit, and it is bounded at the sides by the labia , which meet above and below to form the commissures. The superior commissure is acute, and separated from the anus by a narrow interval. The inferior commissure is rounded, and immediately within it the clitoris is lodged. The labia are covered externally by skin, which is thin, almost destitute of hairs, and generally black-pigmented ; inwardly they are lined by mucous membrane ; and at their sharp edges these cutaneous and mucous coverings meet. If the cutaneous covering of the labia be removed, the posterior constrictor of the vulva will be exposed. This is a red muscle corresponding to the compressor bulbi of human anatomy. Its fibres are elliptically disposed DISSECTION OF THE PELVIS. 357 around the extremity of the vulva, being confounded with the sphincter ani above, while interiorly some of the fibres are attached to the base of the clitoris, and others are attached to the inner surface of the skin below the inferior commissure. When the muscle contracts, it constricts the orifice of the vulva. Its lower fibres may frequently be observed to contract after micturition, depressing the inferior commissure and exposing the clitoris, which is simultaneously erected. The Clitoris. This small erectile body is the homologue of the male penis minus the urethra. It is lodged within the inferior commissure of the vulva, and presents a base, or attached extremity, a body, and a free extremity. The base is bifid, and attached to the ischial arch by the branches, or crura , each crus being covered by a rudimentary erector clitoridis muscle — the homologue of the erector penis. The body of the clitoris, wdiich is from two to three inches in length, projects backwards and upwards, and is composed of right and left halves like the corpora cavernosa of the penis. The free extremity is formed by a rudimentary plans, which is provided with a mucous cap analogous to the prepuce. The clitoris is composed of erectile tissue resembling that of the penis. The Vestibular Bulb. This will be exposed by the removal of the posterior constrictor muscle. It is an erectile body composed of right and left halves, each of which is placed at the side of the vulvar cavity (the vestibule), between the posterior constrictor and the mucous membrane. Interiorly the two halves of the organ are in communica- tion wfith one another, and with the erectile tissue of the clitoris, and superiorly each terminates at the side of the vulva by a rounded end. The bulb is the homologue of the corpus spongiosum of the penis. Directions. — The pelvic viscera must now be removed to allow an examination of the structure and interior of the organs just considered. This is to be effected by cutting the meso-rectum and the peritoneal ligaments of the bladder, carrying the knife above the anus and below the inferior commissure of the vulva, and destroying the vascular and connective-tissue attachments of the various organs to the pelvic walls. The entire generative apparatus will thus be removed along with the urinary bladder and the rectum. The latter organ should be dissected from the vagina and vulva (for its structure see page 344), and the other viscera examined seriatim. The canal of the vulva and vagina is to be exposed by a mesial incision on the upper wall of these organs. The Canal of the Vulva. This, as already stated, is a tubular passage about five inches in length. When removed from the body and inflated, it assumes a large calibre, but ordinarily its walls are in contact. Tracing the canal in an order inverse to that followed 358 THE ANATOMY OF THE HORSE. in the previous description of parts, it may be said to begin on the surface of the body at the vertical slit already described, and to terminate in front by joining the tube of the vagina. In the adult animal there is little to mark the line of separation between the two passages, but in the young animal a membranous septum — the hymen — stretches between the two. This is occasionally seen also in the adult mare, and more frequently a few warty projections — the carunculoe myrtiformes — which are the shrunken remains of the hymen, stud the line of junction ; but very often the canal of the vulva passes without interruption into that of the vagina. The vulva is lined by a mucous membrane of a rosy, vascular tint. It possesses numerous mucous follicles ; and its free surface is formed by a stratified squamous epithelium, which, towards the external opening, is often pigmented in spots. The Meatus Urinarius. The urethra opens on the middle line of the floor of the vulva immediately behind its point of continuity with the vagina. The opening is surmounted by a large mucous fold — the valve of the meatus urinarius. This valve has its free edge directed backwards, and it serves to direct the flow of urine towards the exterior. Its presence must be remembered in passing the female catheter, the point of which should be made to press on the floor of the vulva as it is directed onwards. The meatus is of large size when compared with the same orifice in the male, since it readily admits two fingers. Directions . — Reverse the natural position of parts, laying the uterus, vagina, and vulva with their upper surfaces downwards, and open the bladder by a mesial incision on its lower (in the natural position) face. For an account of the structure of the bladder turn to page 347. The Urethra of the female is very much shorter, but considerably wider, than the corresponding tube of the male. Beginning as a funnel- like prolongation of the neck of the bladder, it passes backwards on the middle line of the lower face of the vagina, in whose wall it is partially imbedded ; and after a course of two or three inches it perforates the lower wall of the vulva, and opens by the meatus already described. The calibre of the tube is in correspondence with the large size of the meatus ; and v T ith slight stretching it will accommodate three fingers. The wall of the urethra is composed of connective-tissue, and non- striped muscular fibres circularly disposed ; and it is lined internally by a longitudinally folded mucous membrane with a stratified squamous epithelium. Structure and Interior of the Vagina. The tube of the vagina is about nine or ten inches in length. Posteriorly it joins the vulva, and anteriorly it embraces the cervix uteri. The connection between the cavities of the vagina and uterus is, thus,, not by simple continuity, DISSECTION OF THE PELVIS. 359 but the vaginal wall is carried forwards, so as to cause the os uteri to project freely into the forepart of the vaginal canal. The wall of the vagina comprises the following layers — 1. A Serous Coat . — This is only a partial covering, the posterior part of the organ being without a peritoneal investment. In the hinder part of the tube the place of the peritoneum is taken by connective-tissue uniting it to surrounding parts. This connective-tissue is loose and areolar towards the rectum ; but between the vagina and the bladder it is closer, and forms a more intimate bond between the two organs. 2. A Muscular Coat.—' This is composed of non-striped muscular tissue, continuous in front with the muscular coat of the uterus. Posteriorly the muscular tissue is reddish in tint, and passes into the anterior constrictor of the vulva. The fibres are arranged both longi- tudinally and circularly. 3. A Mucous Coat. This lines the tube inwardly, and it is longitudi- nally folded. It possesses numerous mucous glands, and its epithelium is stratified and squamous. It is of a pinkish, vascular tint, like the mucous lining of the vulva. Directions . — Lay open one of the horns of the uterus in its whole extent, and carry the incision along the body and cervix to the os. Structure and Interior of the Uterus. The interior of the uterus comprises the cavities of the body and cervix, and those of the horns. The Cavity of the Cervix begins posteriorly at the orifice of the tap- like projection already noticed at the forepart of the vaginal canal. This orifice is termed the os uteri externum , or, shortly, the os uteri. Ordinarily the orifice is closed, and forms a circular depression from which the folds of mucous membrane radiate outwards, and curve round the circular lip of the os, to be carried to the vaginal wall. In front the canal of the cervix passes gradually into the wider cavity of the body. (In the human subject the connection between the canal of the cervix and the cavity of the body is abrupt, constituting the os uteri internum .) The Cavity of the Body is triangular in form, with the base in front. At its posterior angle it passes into the canal of the cervix, and at each antero-lateral angle it is joined by the cavity of a horn. The Cavities of the Horns are conical and curved. Each is widest at its base, where it joins the cavity of the body ; and it tapers to its anterior extremity, in the centre of which it presents a small tubercle perforated by the uterine orifice of the Fallopian tube. The wall of the uterus comprises serous, muscular, and mucous layers : — 1. The Serous Coat is peritoneum, continuous with the layers of the broad ligaments. It completely envelops the organ. 2. The Muscular Coat is composed of non-striped fibres arranged as 360 THE ANATOMY OF THE HORSE. an external longitudinal, and an inner circular set. To compensate for the expansion of the uterine wall during pregnancy, and to provide a force to expel the foetus at parturition, there is, during pregnancy, both an increase in the size, and an addition to the number, of these muscular fibres. 3. The Mucous Coat forms a complete lining to the uterus. It is smooth, of a pale pink colour, and thrown into longitudinal wrinkles. The epithelium is simple, columnar, and ciliated, except in the posterior part of the cervix, where it is stratified and squamous, as in the vagina. In the cornua and body the mucous membrane is set with numerous utricular glands. The mouths of these glands open on the surface of the membrane, while their blind ends lie against the muscular coat. They lie obliquely in the membrane, and are branched at their deep ends. They are lined by a single layer of columnar ciliated cells. The mucous membrane of the cervix contains numerous mucous follicles, and the peculiar ovula Nabothi , which appear to be mucous glands distended into a vesicular form by their own clear secretion. In pregnancy these cervical glands secrete the mucous plug that closes the os uteri. Directions. — The student must now return to the dissection of parts remaining in the pelvis, beginning with the lumbo-sacral plexus (page 349). THE TAIL (FIG. 48). Directions. — Saw through the ilium that is still intact, making the section across the bone at the great sciatic foramen. By cutting the sacro-sciatic ligament on the same side, the sacro-coccygeal part of the spine, with the sacro-iliac joints, will be isolated. Dissect away the inferior ilio-sacral ligament, and remove the skin from the tail. The skin of the tail differs from that of the body in general in the greater length of its hairs. On its under surface, however, extending backwards from its root, there is a triangular area without hairs. Along the under surface of the tail, and especially in front, the skin is thin; but on its upper aspect and sides it is thick, and intimately adherent to the subjacent fascia. The muscles of the tail are enveloped by a strong coccygeal fascia which is continuous in front with the inferior ilio-sacral ligament. The isolation of the muscles can be readily effected near the root of the tail, but towards its tip they tend to blend with each other. In each half of the tail there are three muscles, viz., one above, one below, and one at the side. There are also three arteries — one on the middle line below, and one between the inferior and lateral muscles on each side. On each side there are two sets of nerves, one of which accompanies the lateral artery, while the other is on the upper aspect of the bones, DISSECTION OF THE PELVIS. 361 between the lateral and superior muscles. [Besides the three muscles now to be described, there is the compressor coccygis already dissected (page 343).] The Erector Coccygis (sacro-coccygeus superior). This muscle arises from the sides and summits of the sacral spines, and it is inserted by successive short tendons to the upper aspect of the coccygeal vertebrae. Action . — Acting with its fellow, to elevate the tail directly ; acting alone, to elevate the tail and incline it laterally. The Curvator Coccygis (sacro-coccygeus lateralis). This muscle seems to continue backwards the semispinalis of the loins. It arises from the last two lumbar spines and from the spines of the sacrum, and it is inserted into the lateral aspect of the coccygeal bones. Action . — To bend the tail to the side of the acting muscle. Muscles of the Tail, deep Muscles of the Hip, and Pelvic Ligaments ( Chauveau ). 1. Erector coccygis ; 2. Curvator coccygis ; 3. Depressor coccygis ; 4. Compressor coccygis ; 5. Deep gluteus ; 6. Rectus parvus ; 7. Common tendon of obturator internus and pyriformis ; 8. Gemelli ; 9. Accessory fasciculus of the same ; 10. Quadratus femoris ; 11. Sacro-sciatic ligament ; 12. Great sacro-sciatic foramen ; 13. Superior ilio-sacral ligament ; 14. Inferior ilio-sacral ligament. The Depressor Coccygis. Anteriorly this muscle consists of an outer and an inner portion, which Leyh describes as separate muscles. It arises from the lower face of the sacrum, beginning about the 3rd foramen. The slips of the inner portion are inserted into the first six coccygeal vertebree, while the outer portion extends to the extremity of the tail, and is provided with strong tendons of insertion. Action . — It inclines the tail laterally or depresses it, according as it acts alone or with the opposite muscle. 362 THE ANATOMY OF THE HORSE. Between the preceding two muscles a number of semi-independent fleshy fasciculi connect adjacent coccygeal bones. Leyh describes these separately as the intertransversales caudce. At the root of the tail, between the right and left depressors, the retractor muscles of the penis take origin from the 1st and 2nd or 2nd and 3rd coccygeal bones ; and behind these the so-called suspensory ligaments of the rectum are inserted (Plate 46). The Middle Coccygeal Artery (Plate 48). This is the largest artery of the tail.* It is an unpaired vessel, and in the great majority of cases it is a collateral branch detached from the inner side of the lateral sacral artery towards the middle of the sacrum. Sometimes it is detached in the same way from the left lateral sacral artery. Passing backwards and inwards on the lower surface of the sacrum, it places itself on the middle line, and extends in that position throughout the tail, lying under the coccygeal vertebrae, and between the right and left depressor muscles. In its backward course it gradually reduces itself by giving off lateral branches. The Lateral Coccygeal Artery (Plate 48). Each artery (right or left) is one of the terminal branches of the lateral sacral artery (the ischiatic artery being the other branch). Having its origin towards the middle of the sacrum, it passes backwards in the tail, crossing the sides of the coccygeal bones, between the depressor and curvator muscles, the former muscle separating it from the middle artery. It becomes smaller by the detachment of numerous collateral twigs, the largest of which pass upwards. Leyh designates this vessel the infero-lateral coccygeal artery, describing as the supero-lateral coccygeal artery what is, apparently, an unusually large branch of the first. Veins. The foregoing arteries are accompanied by veins of the same names. Coccygeal Nerves. There are five or six pairs of coccygeal nerves, and they are numbered according to the bones behind which they turn outwards, the first issuing behind the first coccygeal vertebra, and so on with the others. The first of them has a loop of communication with the last sacral. As they turn outwards, they divide into an upper and a lower branch corresponding to the superior and inferior primary branches of the spinal nerves in other regions. The branches of each of these sets are directed backwards, detaching slender filaments, and then applying themselves to the next nerve of the same set. In this way there are formed in each half of the tail two composite nerves, one of which accompanies the lateral artery, while the other runs on the upper aspect of the tail between the erector and curvator coccygis. These cords are expended in branches to the muscles and skin of the tail. * Leyh describes and figures this artery as being smaller than the lateral coccygeal, but that, certainly, is not usually the case. DISSECTION OF THE PELVIS. 363 Sacral Nerves. On the upper aspect of the sacrum the superior primary branches of the sacral nerves will be found at their points of emergence from the spinal canal, the first four issuing by the superior sacral foramina, and the last by the foramen between the sacrum and the first bone of the coccyx. These nerves are much smaller than the corresponding inferior primary branches ; and after giving twigs to the muscles on the side of the spine, they pass upwards to the skin of the croup. Slender branches of the lateral sacral artery issue from the spinal canal in company with them. JOINTS AND LIGAMENTS OF THE SACRUM AND COCCYX. The sacral portion of the spine in the adult animal does not present any joints between its constituent pieces, which are fused by anchylosis. The lumbar supraspinous ligament is prolonged on the summits of the sacral spines. This region, however, furnishes the important joint between the vertebral column and the skeleton of the hind limb — the sacro-iliac articulation. The Sacro-iliac Articulation. The bony surfaces that concur to form this are the auricular facet on the lateral aspect of the sacrum, and the corresponding facet on the pelvic surface of the ilium. The move- ments permitted in the joint are scarcely appreciable, as the student may prove by grasping the sacrum and the part of the ilium left in connection with it. Since this joint is the bond of connection between the skeleton of the trunk and that of the hind limb, in which, in loco- motion, the main propulsive efforts are originated, it is necessary that but slight movement should be permitted, as otherwise these efforts would not be transmitted with precision to the trunk. The stability of the joint is effected mainly by one ligament — the sacro-iliac, and to a less degree by the superior and inferior ilio-sacral ligaments and the sacro-sciatic ligament already described (page 339). The Sacro-iliac Ligament . — This ligament is composed of strong fibres passing between the sacrum and ilium, in close relation to the joint. It consists of an upper and a lower half, corresponding respectively to the anterior and posterior sacro-iliac ligaments of human anatomy. The former is much the stronger of the two ; and the necessity for its strength is apparent when one reflects that whatever weight is placed on the back and loins of the horse, tends to drive the sacrum downwards from its connection with the iliac bones, and that this tendency is rather favoured than otherwise by the form of the articular surfaces, which offer an arrangement comparable to an inverted arch. The bones should be disarticulated to show the articular surfaces. The joint is provided with a rudimentary synovial membrane. Sacrococcygeal and Inter-coccygeal Articulations. Ordinarily these are movable joints, the articular surfaces being the opposed 364 THE ANATOMY OF THE HORSE. extremities of the rudimentary vertebral bodies. These are connected by small intervertebral discs, which are shaped like a biconcave lens, since the bodies of the vertebrae are here convex on both extremities. The bones are also invested by a fibrous sheath, which may be supposed to represent the superior and inferior common ligaments of the back and loins. Movements . — The biconvex form of the vertebral centra, and the suppression of the different processes in this region give a great range and freedom of movement to the tail, which, provided with its appendage of hairs, is admirably fitted to protect the hind quarters of the animal from the attacks of insects. It is interesting to notice the absence of the panniculus carnosus over the area within which the tail is serviceable for this purpose. In animals above middle age it is not uncommon to find the sacro-coccygeal, and even the first intercoccygeal joint, anchylosed. INDEX. Abdomen, 285. Abdominal tunic, 288. Accessory duct of pancreas, 313, 330. Acervulus cerebri, 251. Annular cartilage, 164. Annulus ovalis, 126. Appendages of eye, 172. Aqueduct of Sylvius, 252. Aqueductus vestibuli, 269. Aqueous humour, 261. Arachnoid, cranial, 235. ,, spinal, 137. Arciform fibres of medulla, 239. Arteries — Anastomotic, 170, 191. Angular, 181. Anterior abdominal, 292. ,, aorta, 112. ,, auricular, 162, 167. ,, circumflex, 12. ,, deep temporal, 213. ,, dorsal of penis, 281. ,, gastric, 316. ,, laminal, 42. „ mesenteric, 304, 322. ,, radial, 26. ,, tibial, 77. Asternal, 327. Axillary, 4, 11, 113, 119. Basilar, 236. Brachial, 11. Bronchial, 105. Broncho-oesophageal, 118. Buccal, 183, 213. Csecal, 305. Centralis retinae, 211, 264. Cephalic, 119. Cerebellar, 237. Cerebral, 237. Cerebro-spinal, 140, 170, 236. Ciliary, 211. Circle of Willis, 238. Circumflex of coronary cushion, 41. „ iliac, 292, 322. ,, of toe, 42. Coeliac axis, 315, 322. Colic, 305. Arteries — continued . Common aorta, 112. „ carotid, 148, 191. Coronary, 123. „ circle, 29, 41. Deep humeral, 12. ,, femoral, 59, 61. Digital of fore limb, 28. ,, hind limb, 79. Dorsal, 113, 154. Dorso-cervicai, 118. Dorso-spinal, 96. Duodenal, 316. External carotid, 167, 192. ,, iliac, 322. ,, pudic, 290. ,, thoracic, 4, 113. Femoral, 58, 61. Femoro-popliteal, 61. Gastric, 316. Gluteal, 66, 351. Great meningeal, 188. Helicinse, 284. Hepatic, 315, 332. Hypogastric, 343. Ileo-caecal, 305. Iliaco-femoral, 69, 351. Ilio-lumbar, 351. Inferior cervical, 4, 113, 152. ,, communicating, 42. ,, dental, 185, 188. ,, labial, 181. Infraorbital, 213. Innominata, 118. Intercostal, 98, 112. Internal carotid, 191 , 237. ,, iliac, 323", 350. „ maxillary, 167, 187, 212. „ pudic, 66, 275, 342, 350. ,, thoracic, 113, 121. Interosseous of fore-arm, 22, 26. ,, metacarpal, 29. 366 THE ANATOMY OF THE HORSE. Arteries — continued. Ischiatic, 66, 350. Lachrymal, 175, 211. Large metacarpal, 28. ,, metatarsal, 78. Lateral coccygeal, 350, 362. „ sacral, 350. Left gastro-omental, 317. Lingual, 192, 200. Lumbar, 99, 321. Mammary, 290. Mastoid, 170, 191. Maxillo- muscular, 167, 182. Meningeal, 188, 191, 238. Mental, 185. Middle coccygeal, 351, 362. , , sacral, 322. , , spinal, 140. Nasal of ophthalmic, 220, 238. ,, submaxillary, 181. Nutrient of femur, 59. ,, humerus, 12, 21. ,, ilium, 351. ,, metacarpal bone, 30. ,, metatarsal bone, 79. ,, radius, 26. ,, scapula, 17. ,, tibia, 75. Obturator, 62, 351. Occipital, 169, 191. Occipito- muscular, 170. (Esophageal, 118. Of corpus cavernosum, 282. Of plantar cushion, 29, 41. Of quadriceps, 59. Of spermatic cord, 322. Ophthalmic, 211, 212, 238. Orbital, 176, 211. Ovarian, 322. Pancreatic, 316. Palato-labial, 180, 201, 213. Perforating metatarsal, 78. Perpendicular, 29. Pharyngeal, 192, 204, 205. Phrenic, 321. Plantar of digital, 41. ,, interosseous, 79. ,, of tibial, 74, 78. Pleuro-cesophagea, 316. Popliteal, 74. Posterior abdominal, 292. ,, aorta, 112, 118, 321. ,, auricular, 163. 167. ,, circumflex, 16. ,, communicating, 237. ,, deep temporal, 188. ,, dorsal of penis, 281. ,, gastric, 316. ,, mesenteric, 306, 322, 344. ,, radial, 22. ,, tibial, 74. Prehumeral, 12. Preplantar, 41. Prepubic, 293, 323. Prevertebral, 191. Arteries — continued. Profunda, 59, 61. Pterygoid, 186, 188. Pulmonary, 105, 111, 127. Pyloric, 316. Renal, 319, 322, 334. Retrograde of occipital, 170, 191. Right gastro-omental, 316. Saphenic, 57. Small metacarpal, 29. ,, metatarsal, 78. Spermatic, 279, 322. Spheno-palatine, 188, 213, 220. Spheno-spinous, 188. Splenic, 316, 332. Staphyline, 204, 213. Subcarpal arch, 29. Subcostal, 113. Subcutaneous abdominal, 281, 286. Submaxillary, 170, 181, 192. Submental, 171, 193, 198. Subscapular, 11, 16. Superior cervical, 113, 154. ,, dental, 213. ,, labial, 181. Superficial temporal, 167. Supracarpal arch, 28. Susrascapular, 11. Supraorbital, 175, 211. Thyroid, 148. Thyro-laryngeal, 148. Transverse facial, 167, 182. Tympanic, 188, 269. Ulnar, 21. Umbilical, 343. Uterine, 323. Vaginal, 343. Vertebral, 113, 157. Vesico-prostatic, 342. Articulations (see joints). Arytenoid cartilages, 225. Auricle, left, 128. ,, right, 125. Auriculo-ventricular groove, 106. ,, ,, opening, left, 129. „ „ „ right, 126. Axilla, 4. Barbs, 196. Bars of hoof, 36. Basilar membrane, 271. Bicuspid valve, 129. Bile duct, 315, 330, 332. Bladder, 345, 347. Bowman’s capsule, 334. ,, glands, 219. ,, membrane, 260. Brachial plexus, 5, 12. Brain, 233. Brunner’s glands, 308. Buccal gland, inferior, 184. ,, ,, superior, 183. Bulb, 238. INDEX. 367 Caecum, 29(5. Calamus scriptorius, 242. Canalis reuniens, 270. Canal of Corti, 272. „ Petit, 266. Carpal sheath, 33. Cartilages of ear, 164. ,, foot, 40. ,, larynx, 224. ,, nostril, 176. ,, trachea, 149. ,, tarsal, 175. Caruncula lachrymalis, 173. Carunculae myrtiformes, 358. Cavity of abdomen, 294. ,, nose, 216. „ pelvis, 340. ,, pharynx, 206. ,, thorax, 100. , , Winslow, 300. Central canal of spinal cord, 142. Cerebellum, 240. Cerebrum, 242. Chordae tendineae, 127, 129. Choroid coat, 263. „ plexus of 4th ventricle, 242. ,, ,, lateral ventricle, 249. Ciliary processes, 263. Circle of Willis, 238. Circumvallate papillae, 196. Clitoris, 357. Cochlea, 270. Colic mesentery, 302. Collecting tubules, 334. Colliculus seminalis, 348. Colon, large, 296. ,, small, 297. Columnae carneae, 126, 129. Columns of cord, 142. Commissures of cord, 142. ,, lateral ventricle, 252. Common ejaculatory ducts, 347. Conchal cartilage, 164. Coni vasculosi, 281. Conjunctiva, 174. Conus arteriosus, 127. ,, medullaris, 139. Convoluted tubules, 334. Convolutions of cerebrum, 245. Cornea, 259. Cornicula laryngis, 226. Corona glandis, 286. Coronary cushion, 38. Corpora cavernosa, 282. ,, geniculata, 252. ,, nigra, 262. ,, quadrigemina, 252. Corpus albicans, 243. ,, Arantii, 127. „ callosum, 248. ,, dentatum, 241. ,, Highmori, 279. „ luteum, 354. ,, spongiosuni, 282. Cowper’s glands, 346. Cricoid cartilage, 225. Crico-thyroid membrane, 226. Crista acoustica, 270. Crura of cerebrum, 242. Crura of diaphragm, 327. ,, penis, 282. Crusta, 242. Cuticular ridge, 329. Cutigeral groove, 36. Dartos, 278. Decussation of pyramids, 239. Descemet’s membrane, 260. Diaphragm, 327. Duct of Wirsung, 313, 330. Ducts of Rivinius, 197. Ductus ad nasum, 174, 177, 218. ,, cochlearis, 272. ,, vestibuli, 270. Duodenum, 298, 312. Dura mater, cranial, 233. ,, ,, spinal, 137. Ear, external, 159. ,, internal, 269. ,, middle, 267. Encephalon, 233. Endocardium, 125. Epicardium, 106. Epididymis, 279. Epiglottis, 226. Epigastric region, 294. Epiploon, 300, 312. Eustachian tubes, 187, 206. ,, valve, 126. External abdominal ring, 289. Eyeball, 257. Eyelids, 172. Fallopian tubes, 354. Falx cerebri, 234. Fascia— Coccygeal, 360. Cremasteric, 278. Gluteal, 64. Iliac, 325. Infundibuliform, 278. Lata, 64. Of fore-arm, 21. Of thigh, 57. Perineal, 275. Scapular, 17. Spermatic, 278. Fenestra ovalis, 267. ,, rotunda, 267. Filiform papillae, 196. Fimbriae of Fallopian tubes, 354. Fissures of spinal cord, 141. ,, cerebrum, 243, 245, 246. ,, liver, 314. Folia of cerebellum, 241. Foot, 35. Foramen dextrum, 328. ,, of Monro, 252. ,, of Winslow, 301. ,, ovale, 126. ,, sinistrum, 328. 368 THE ANATOMY OF THE HORSE. Foramina Thebesii, 125. Fornix, 250. Fossa ovalis, 126. Fovea hemielliptica, 269. ,, hemispherica, 269. Frsenum linguae, 196. Frog, 37. Fungiform papillae, 196. Galactopherous sinuses, 286. Ganglia — Andersch’s, 255. Ciliary, 212. Gasserian, 254. Geniculate, 255. Inferior cervical, 110, 117. Middle „ 109, 117. Superior ,, 195. Of roots of spinal nerves, 140. ,, root of vagus, 255. ,, trunk of ,, 194. Otic, 189. Petrous, 255. Semilunar, 318. Spheno-palatine, 214. t Gastric glands, 329. Gastro-colic omentum, 300, 312. Gastro-hepatic ,, 312. Gastro- splenic ,, 301, 312. Gland of Harder, 173. Glands of Bowman, 219. Gians penis, 283. Glisson’s capsule, 331. Globus major, 279. ,, minor, 279. Glomerulus, 334. Graafian follicles, 352. Great mesentery, 302. ,, omentum, 300, 312. ,, sesamoid sheath, 34. ,, transverse fissure of cerebrum, 257. Guttural pouches, 187. Gyrus fornicatus, 247. Hard palate, 200. Harderian gland, 173. Heart, 106, 123. Henle’s tubes, 334. Hiatus aorticus, 328. Hippocampus, 250. Hoof, 35. Horny laminae, 36. Hyaloid membrane, 266. Hymen, 358. Hypochondriac regions, 294. Hypogastric regions, 294. Ileo-csecal valve, 311. Ileum, 298. Iliac region, 294. Incus, 268. Inguinal canal, 290. Inferior pyramid, 239. Intestines, 295. Internal abdominal ring, 293. Interarticular cartilages of stifle-joint. 85. „ „ jaw, 216. Interpeduncular space, 242. Intervertebral substance, 136, 159. Iris, 261. Isthmus of fauces, 202. Jacobson’s organ, 219. Jejunum, 298. Joints— Atlanto -axial, 215. Basi-cornual, 207. Carpus, 46. Chondro -costal, 134. Chondro-sternal, 134. Costo-central, 132. Costo-transverse, 133. Crico-thyroid, 226. Crico -arytenoid, 226. Elbow, 44. Femoro-patellar, 82. Femoro-tibial, 83. Fetlock, 50. Hip, 338. Hock, 86. Intercoccygeal, 363. Intercornual, 207. Intermetacarpal, 50. Intervertebral, 134. Knee, 46. Occipito-atlantal, 214. Pastern, 52. Badio-ulnar, 45. Sacro-coccygeal, 363. Sacro-iliac, 363. Shoulder, 43. Stifle, 81. Tarsus, 86. Temporo-hyoideal, 207. Temporo-maxillary, 215. Thyro-epiglottic, 227. Tibio-fibular, 85. Kidneys, 318, 333. Labia vulvas, 356. Labial glands, 180. Labyrinth, 269. Lachrymal canals, 174. „ duct, 174, 177, 218 ,, gland, 173, 208. ,, puncta, 174. ,, sac, 174. Lacteals, 306. INDEX. Lamina cinerea, 243. ,, cribrosa, 259. Large intestine, 295, 309. Larynx, 224. Lateral cartilages of foot, 40. Lens, 265. Lieberkiihn’s glands, 308, 310. Ligaments — Anterior calcaneo-astragaloid, 89. ,, common of knee, 48. ,, cuboido-cunean, 90. ,, cuboido-scaphoid, 89. ,, of elbow, 45. ,, of fetlock, 51. ,, tibio-tarsal, 88. Antero -lateral of coffin-joint, 53. Arciform, 45. Arcuate, 328. Astragalo-metatarsal, 88. Atlanto-axial, 215. Broad of lung, 102. ,, uterus, 303. Calcaneo-metatarsal, 89. ,, -astragaloid, 89. Capsular, crico-arytenoid, 227. ,, crico -thyroid, 22 7. ,, chondro- sternal, 134. ,, hip, 339. ,, femoro-patellar, 82. ,, intertransverse lumbar, 135. ,, intervertebral, 135, 159. ,, temporo-maxillary, 215. Carpo-metacarpal, 49. Chondro-xiphoid, 134. Coronary of liver, 315. Costo-transverse, 133. Costo-vertebral, 132. Cotyloid, 339. Crucial of stifle, 84. Cruciform, 214. Denticulated, 138. External temporo-maxillary, 215. Falciform of liver, 315. Gastro-phrenic, 312. Hepatic, 314. Inferior atlanto-axial, 215. ,, of bladder, 342. ,, common, 135. ,, ilio-sacral, 340. ,, sesamoidean, 51. Interannular, 215. Interarticular, 133. Intercarpal, 48. Intercunean, 90. Interosseous calcaneo-astragaloid, 89. ,, costo-transverse, 133. ,, cuboido-cunean, 90. ,, cuboido-scaphoid, 90. ,, of coffin- joint, 53. ,, radio-ulnar, 45. ,, tarso-metatarsal, 90. Ligaments — continued. Interosseous tibio-fibular, 86. Intersesamoid, 51. Interspinous, 135, 159. Intervertebral substance, 136, 159. Lateral of bladder, 342. ,, elbow, 44, 45. ,, femoro-patellar, 82. ,, femoro-tibial, 83. ,, of fetlock, 51. ,, of knee, 46, 48. ,, of liver, 314. ,, of pastern, 52. ,, sesamoidean, 51. , , tibio-tarsal, 86. Nuclise, 158. Occipito-atlantal, 214. Patellar, 82. Posterior calcaneo-astragaloid, 89. ,, common of knee, 48. ,, femoro-tibial, 84. ,, temporo-maxillary, 215. ,, tibio-tarsal, 88. Postero-lateral of coffin- joint, 53. Pubio-femoral, 339. Badio-carpal, 48. Round, 339. Sacro-iliac, 363. Sacro-sciatic, 339. Scaphoido-cunean, 90. Splenic, 313. Stellate, 132. Straight patellar, 82. Styloid, 214. Subcarpal, 35, 48. Subflava, 135, 159. Subtarsal, 81, 89. Superior atlanto-axial, 215. ,, common, 135, 159. ,, ilio-sacral, 340. ,, sesamoidean, 50. Supraspinous, 135. Suspensory of liver, 314. ,, penis, 282. ,, prepuce, 281. ,, rectum, 276, 344. Tarso-metatarsal, 89. Thyro-arytenoid, 227. Tibio-tarsal, 86. Uterine broad, 303. Linea alba, 289. Lingual fibrous cord, 197. ,, glands, 197. Liver, 313, 330. Locus cseruleus, 240. ,, niger, 243. ,, perforatus anticus, 243. ,, ,, posticus, 243. Lumbar region, 294. Lungs, 103, 122. Lunula, 127. 2 B 370 THE ANATOMY OF THE HORSE. Lymphatics— Brachial, 11. Bronchial, 115. Cardiac, 115. Iliac, 321. Inguinal, deep, 57. ,, superficial, 287. Intestinal, 306. Lumbar, 321. (Esophageal, 115. Of liver, 315. Of spleen, 315. Of stomach, 315. Pharyngeal, 191. Popliteal, 61. Precrural, 57. Prepectoral, 115, 149. Prescapular, 152. Sacral, 321. Submaxillary, 170. Suprasternal, 122. Thoracic, 115. Tracheal, 115. Malleus, 268. Mammary glands, 285. Malpighian bodies of kidney, 334. ,, ,, spleen, 333. Meatuses of nose, 216. Meatus urinarius, 358. Mediastinum of thorax, 102. ,, testis, 279. Medulla oblongata, 238. Membrana nictitans, 173. ,, pupillaris, 262. ,, reticularis, 272. ,, tectoria, 272. ,, tympani, 267. Membranes of brain, 233. ,, spinal cord, 137. Mesentery, 302. Meso-rectum, 342. Mitral valve, 129. Moderator bands, 126. Modiolus, 270. Movements of joints, 43. Muscles — Adductor magnus, 60. ,, parvus, 60. Accelerator urinse, 277. Anconeus, 18. Anterior deep pectoral, 3. ,, superficial pectoral, 2. Arytenoideus, 230. Aryteno-pharyngeus, 205. Azygos uvulae, 204. Biceps, 15. Biceps femoris, 64. Brachialis anticus, 18. Buccinator, 179. Caput magnum, 15. Muscles — continued . Caput medium, 18. ,, parvum, 15. Cerato-hyoid, 205. Cervico-auriculares, 160. Ciliary, 262. Complexus, 154. Compressor coccygis, 343. Constrictors of vulva, 356. Coraco-humeralis, 15. Corrugator supercilii, 175. Cremaster, 278, 294. Crico-arytenoideus lateralis, 229. Crico-arytenoideus posticus, 229. Crico-pharyngeus, 205. Crico-thyroideus, 228. Crureus, 62. Curvator coccygis, 361. Deep flexor of digit (fore limb), 24, 34, 42. Deep flexor of digit (hind limb), 73, 81. Deep gluteus, 67. Deltoid, 17. Depressor labii inferior is, 179. ,, labii superioris, 180. ,, coccygis, 361. Diaphragm, 327. Digastricus, 166, 171, 190. Dilatator naris inferior, 179. ,, naris lateralis, 178. ,, naris superior, 178. ,, naris transversalis, 178. ,, pupillae, 262. Erector clitoridis, 357. ,, coccygis, 361. „ penis, 277. Extensor metacarpi magnus, 24. ,, ,, obliquus, 25. ,, pedis (fore limb), 25, 32. ,, ,, (hind limb), 75, 80. ,, suffraginis, 25, 32. External intercostal, 98. „ pterygoid, 185. Flexor accessorius, 73. ,, metacarpi externus, 23. ,, ,, internus, 23. ,, ,, medius, 23. ,, metatarsi, 76. ,, pedis perforans (fore limb), 24, 34. Flexor pedis perforans (hind limb), 73, 81. Flexor pedis perforatus (fore limb), 23, 34. Flexor pedis perforatus (hind limb), 72, 81. Gastrocnemius, 71. Gemelli, 68. Genio-glossus, 200. Genio-hyoideus, 200. Gracilis, 58. Great hyo-glossus, 199. Hyo-epiglottideus, 228. INDEX. 371 Muscles — continued. Hyoideus transversus, 199. Hyo-pharyngeus, 205. Iliacus, 63, 326. Inferior oblique of eye, 210. Infraspinatus, 17. Internal intercostal, 98. ,, pterygoid, 185. Interossei, 32, 80. Intertransversales of loins, 326. ,, ,, neck, 155. Ischio-urethral, 346. Lateralis sterni, 97. Latissimus dorsi, 9, 14, 94. Levator anguli scapulae, 8. Levatores costarum, 97. Levator labii superioris alseque nasi, 177. Levator labii superioris proprius, 178. Levator menti, 180. ,, palati, 204. ,, palpebrse superioris, 175. Longissimus dorsi, 96. Longus colli, 156. Lumbricales, 31, 80. Masseter, 180. Mastoido-auricularis, 162. Mastoido-humeralis, 10, 152, 166. Middle gluteus, 65. ,, hyo-glossus, 199. Mylo-hyoideus, 171, 198. Obliquus abdominis externus, 289. ,, ,, internus, 291. ,, capitis inferior, 169. ,, ,, superior, 169. Obturator externus, 61. ,, internus, 67. Occipito-styloid, 166. Orbicularis oris, 179. ,, palpebrarum, 175. Palato-glossus, 199. Palato-pharyngeus, 203. Panniculus, 8, 94, 144, 170, 177, 287. Parieto-auricularis externus, 160. ,, ,, internus, 162. Parotido-auricularis, 160. Pectineus, 60. Peroneus, 75, 81. Popliteus, 72. Posterior deep pectoral, 3. Posterior superficial pectoral, 2. Psoas magnus, 63, 325. ,, parvus, 326. Pterygo-pharyngeus, 205. Pyriformis, 67. Quadratus fern oris, 61. ,, lumborum, 326. Recti oculi, 209. Rectus abdominis, 292. ,, capitis anticus major, 155. Muscles — continued. Rectus capitis anticus minor, 208. ,, ,, lateralis, 208. ,, ,, posticus major, 169. ,, ,, posticus minor, 169. ,, femoris, 69. ,, parvus, 69. Retractor ani, 276. ,, costae, 96. ,, oculi, 209. ,, penis, 276. Rhomboideus, 10. Sartorius, 57. Scalenus, 156. Scapulo-humeralis gracilis, 16. Scapulo-ulnaris, 14. Scuto-auricularis externus, 162. ,, ,, internus, 162. Semimembranosus, 60. Semispinalis of back and loins, 97. „ colli, 155. Semitendinosus, 65. Serratus anticus, 95. ,, magnus, 7, 95. ,, posticus, 95. Short extensor of foot, 80. Small hyo-glossus, 199. ,, stylo -pharyngeus, 205. Soleus, 71. Sphincter ani, 276. ,, pupillae, 262. Splenius, 153. Stapedius, 268. Sterno-maxillaris, 145, 166. Sterno-thyro-hyoideus, 146. Stylo-glossus, 198. Stylo-hyoideus, 166, 190. Stylo-maxillaris, 166. Stylo-pharyngeus, 205. Subscapularis, 15. Subscapulo-hyoideus, 146. Superficial flexor of digit (fore limb), 23, 34. Superficial flexor of digit (hind limb), 72, 81. Superficial gluteus, 64. Superior oblique of eye, 210. Supraspinatus, 17. Temporalis, 185. Tensor palati, 203. ,, tympani, 268. ,, vaginae femoris, 64. Teres major, 14. ,, minor, 17. Thyro-arytenoid, 229. Thyro-hyoid, 228. Thyro-pharyngeus, 205. Trachelo-mastoid, 153. Transversalis abdominis, 293. ,, costarum, 96. Transversus perinaei, 276. Trapezius, 9. Triangularis sterni, 120. Triceps extensor cubiti, 15. Vastus externus, 69. ,, internus, 62. 372 THE ANATOMY OF THE HORSE. Muscles — continued. Wilson’s muscle, 346. Zygomatico-auricularis, 161. Zygomaticus, 179. Musculi papillares, 127, 129. ,, pectinati, 125, 128. Nares, inferior, 176, 218. ,, superior, 206, 218. Nates, 252. Navicular sheath, 42. Nerves — Abducent, 212, 254. Anterior auricular, 163. ,, crural, 59, 325, 349. ,, tibial, 77. Auditory, 255, 272. Auricular of 1st cervical, 164. „ 2nd „ 164. ,, 7th cranial, 163, 168. „ 10th „ 255. ,, lachrymal, 164. Brachial plexus, 5, 12, 109. Buccal, 184, 189. Cardiac, 110, 117, 124. Cervical of 7th cranial, 144, 163, 168. „ spinal, 152, 154, 169, 195. Chorda tympani, 189, 269. Ciliary, 212, 263. Circumflex, 13. Coccygeal, 362. Cranial, 253. Cremasteric, 324. Cutaneous of chest-wall and back, 94. ,, fore-arm, 20. ,, hip and thigh, 63. , , intermaxillary space, 17 0. ,, leg, 70. ,, metatarsus, 78. ,, neck, 143, 151. ,, pectoral region, 1. ,, perinseum, 275. , , prepuce, 281. ,, thigh, 57. Digital, 31. Dorsal of penis, 281. ,, spinal, 96, 99 Eighth cranial, 255, 272. Eleventh cranial, 140, 146, 151, 194, 255. External laryngeal, 194. ,, popliteal, 62, 67, 74, 76. ,, saphenous, 62, 67, 74. Facial, 168, 176, 182, 254. Fifth cranial, 254. First cranial, 219, 253. Fourth cranial, 212, 254. Glosso-pharyngeal, 193, 200, 206, 255. Gluteal, 66, 349. Great sciatic, 62, 67, 349. Gustatory, 186, 198. Nerves — continued. Haemorrhoidal, 275, 343. Hypoglossal, 193, 198, 256. Iliaco-muscular, 349. Inferior dental, 186, 189. ,, laryngeal, left, 109, 149, 230. „ „ right, 116, 149, 230. ,, maxillary, 188, 254. Infra-orbital, 183. Inguinal, 281, 291, 324. Intercostal, 99, 292. Internal popliteal, 73. ,, saphenous, 57, 325. Jacobson’s, 193, 255. Lachrymal, 211. Large superficial petrosal, 214. Lingual, 186, 189. Lumbar, 96, 99, 292, 324. Lumbo-sacral plexus, 324, 349. Median, 14, 22. Mental, 186. Middle auricular, 163. Musculo-cutaneous of median, 14. ,, ,, popliteal, 76. Musculo-spiral, 13, 26. Motor oculi, 176, 212, 254. Mylo-hyoid, 172, 186, 189. Nasal, 220. Ninth cranial, 193, 200, 206, 255. Obturator, 62, 349. Oesophageal, 317. Olfactory, 219, 253. Ophthalmic, 211, 254. Optic, 253. Orbital of superior maxillary, 176, 211. Palatine, 202, 213. Palpebro-nasal, 176, 211. Pathetic, 212, 254. Perforating, 288. Peroneal cutaneous, 67. Pharyngeal, of 9th, 193, 206. „ of 10th, 194. ,, of sympathetic, 195. Phrenic, 6, 108, 116. Plantar, of fore limb, 30. ,, of hind limb, 80. Pneumogastric, 108, 116, 194, 255. Posterior auricular, 163. ,, tibial, 74. Portio dura, 168, 176, 182, 254. ,, intermedia, 254. „ mollis, 255, 272. Pudic, 275, 343. Pulmonary, 109. Recurrent laryngeal, 109, 116, 149, 230. Sacral, 349. Sacral, 363. Second cranial, 253. Seventh cranial, 168, 176, 182, 254. Sixth cranial, 212, 254. INDEX. 373 N erves — cont inued. Small superficial petrosal, 189. Spheno -palatine, 214, 220. Spinal, 138. ,, accessory, 140, 146, 151, 194, 255. Splanchnic, 111, 318. Staphyline, 204, 214. Subcutaneous thoracic, 7, 288. Subzygomatic, 168, 183, 188. Superior dental, 213. ,, laryngeal, 194, 230. ,, maxillary, 213, 254. Supra-orbital, 176, 211. Suprascapular, 13. Sympathetic cord, cervical, 149, 195. „ „ dorsal, 111, 118. ,, ,, lumbar, 325. ,, ,, sacral, 350. Tenth cranial, 108, 116, 194, 255. Third cranial, 176, 212, 254. Trifacial or trigeminal, 254. Trochlear, 212, 254. Twelfth, 193, 198, 256 Ulnar, 13, 21. Vagus, 108, 116, 194, 255. Vertebral, 157. Vidian, 214. Nipple, 285. Nodulus Arantii, 127. Nostrils, 176. (Esophagus, 115, 150. Olfactory bulbs, 244. ,, cells, 219. Olivary body, 239. Omentum, gastro-hepatic, 312. ,, gastro-splenic, 301, 312. ,, great, 300, 312. Optic thalami, 251. ,, tracts, 243, 252. Orbicular process, 268. Organ of Corti, 272. ,, Jacobson, 219. ,, Rosenmiiller, 354. Osseous spiral lamina, 271. Os uteri, 359. Otic ganglion, 189. Otoliths, 270. Ovaries, 351. Oviducts, 354. Ovula Nabothi, 360. Ovum, 352. Pacchionian bodies, 234. Palate, hard, 200. ,, soft, 202. Palpebral tendon, 175. Papillae of tongue, 196. Papilla optica, 264. Pancreas, 313, 333. Parotid gland, 164. Parovarium 354. Pelvis, 338. Peduncles of cerebellum, 241. „ pineal gland, 251. Pedunculated hydatid of morgagni, 279. Penis, 281. Pericardium, 105. Perilymph, 270. Perinaeum, 274. Periople, 36. Perioplic ring, 38. Peritoneum, 298. ,, pockets of, 303. Pes anserinus, 182. Peyer’s patches, 309. Pharynx, 204. Pia mater, cranial, 236. ,, spinal, 138. Pillars of soft palate, 202. Pineal gland, 251. Pituitary gland, 243. ,, membrane, 219. Plantar cushion, 38. Pleurae, 101. Plexuses of Nerves — Anterior mesenteric, 307. Aortic, 325. Carotid, 238. Cavernous, 238. Cceliac, 318. Pelvic, 344. Posterior mesenteric, 307. Renal, 320. Solar, 318. Spermatic, 325. Suprarenal, 318. Pomum Adami, 225. Pons Tarini, 243. ,, Varolii, 240. Portal fissure, 314. Poupart’s ligament, 289. Prepuce, 280. Prostate, 346. Puncta lachrymalia, 174. Pupil, 261. Pylorus, 311. Pyramidal body, 40. Pyramids of Ferrein, 334. ,, medulla, 239. Quadrilateral space, 244. Receptaculum chyli, 324. Rectum, 297, 344. Reissner’s membrane, 271. Renal crest, 334. Rete testis, 280. Retina, 264. Right lymphatic duct, 119. Rivinius, ducts of, 197. Root of lung, 105. Roots of cranial nerves, 253. ,, spinal nerves, 140. Rostrum, 248. Saccule, 270. Scala intermedia, 272. ,, tympani, 271. ,, vestibuli, 272. 374 THE ANATOMY OF THE HORSE. Schneiderean membrane, 243. Sclerotic, 259. Scrotum, 277. Scutiform cartilage, 164. Semicircular canals, 270. Semilunar cartilages of stifle, 85. Seminal tubules, 280. Sensitive frog, 38. ,, laminae, 38. ,, sole, 40. Septum lucidum, 249. ,, nasi, 217. ,, pectiniforme, 283. ,, scroti, 278. Sheath, 280. Sinuses of dura mater, 234. ,, head, 219. ,, larynx, 231. ,, valsalva, 128, 129. Sinus, subsphenoidal, 207. ,, venosus, 107. Small intestines, 295, 308. ,, sesamoid sheath, 42. Soft palate, 202. Sole of hoof, 36. Solitary glands, 308. Spermatic cord, 278, 294. Spermatoblasts, 280. Spinal cord, 136. ,, nerves, 139. Spleen, 312, 332. Splenium, 248. Stapes, 268. Staphyline glands, 203. Stenson’s duct, 165, 171, 182, 184. ,, canal, 219. Stomach, 311, 328. Striae longitudinales, 249. Subarachnoid space, 138, 235. Subdural space, 137, 235. Sublingual gland, 197. , , ridge, 196. Submaxillary gland, 190. Subsphenoidal sinus, 207. Suburethral notch, 283. Suprarenal capsules, 320. Tail, 361. Taenia hippocampi, 250. Tapetum lucidum, 263. Teat, 285. Tegmentum, 242. Tentorium cerebelli, 234. Testes, 252. Testicle, 279. Thoracic cavity, 100. ,, duct, 114. Thymus gland, 115. Thyroid body, 147. ,, cartilage, 225. Tongue, 195. Tonsil, 202. Torcular Herophili, 235. Trabeculae carneae, 126. ,, of spleen, 332. Trachea, 115, 149. Trapezium, 239. Tricuspid valve, 127. Tuber cinereum, 243. Tubercle of Lower, 125. Tubuli recti, 280. Tunica albuginea, 279, 283. ,, Ruyschiana, 263. ,, vaginalis, 278, 279. ,, vasculosa, 280. Tympanum, 267. Udder, 285. Umbilical region, 294. Ureters, 320, 345. Urethra, female, 358. „ male, 282, 345, 347. Urethral sinus, 283. ,, tube, 283. Urinary bladder, 345, 347. Uriniferous tubules, 334. Uterus, 355, 359. ,, masculinus, 346. Utricle, 270. Utricular glands, 360. Uvea, 262. Vagina, ,356, 358. Valves— Auriculo-ventricular, left, 129. „ „ right, 127. Bicuspid, 129. Eustachian, 126. Mitral, 129. Of meatus urinarius, 358. Of Thebesius, 125. Of Vieussens, 242. Semilunar aortic, 129. ,, pulmonary, 127. Tricuspid, 127. Vasa deferentia, 279, 346. ,, efferentia, 280. ,, vorticosa, 263. Veins — Alveolar, 183, 213. Angular, 181. Anterior auricular, 163. ,, gastric, 317. ,, mesenteric, 306, 317. ,, subcutaneous of fore-arm, ,, tibial, 77. ,, vena cava, 114, 119. Asternal, 327. Axillary, 5. Azygos, great, 119. ,, small, 114. Brachial, 12. Bronchial, 105. Buccal, 183. Cephalic, 2, 145. Circumflex iliac, 292. Common iliac, 323. Cordis minimas, 125. Coronary of heart, 124. INDEX. 375 V eins — continued. Coronary plexus, 42. Digital, of fore limb, 30. ,, hind limb, 79. Dorsal, 114, 119, 155. External‘iliac, 323. ,, pudic, 291. Femoral, 59, 61. Gastric, 317. Gastro-omental, 317. Great vena azygos, 119. Hsemorrhoidal, 344. Hepatic, 323, 332. Inferior dental, 186. Intercostal, 99, 119. Internal iliac, 323, 351. ,, maxillary, 168, 186, 188. ,, pudic, 276, 343. ,, subcutaneous of fore-arm, 20. ,, saphena, 57, 70. ,, thoracic, 114, 119, 122. Jugular, 144, 167. Laminal plexus, 42. Lateral coccygeal, 362. Lingual, 186, 198. Lumbar, 99, 119, 323. Maxillo-muscular, 168, 182. Median, 20. Metacarpal, 30. Metatarsal, 79. Middle coccygeal, 362. Obturator, 62. Occipital, 170, 193. Of Galen, 251. Ophthalmic, 211, 213. Palatine, 202, 213. Phrenic, 323. Popliteal, 75. Portal, 317, 331. Y eins — continued. Posterior abdominal, 292. ,, auricular, 163, 168. ,, gastric, 317. ,, mesenteric, 306, 317, 344. ,, radial, 22. ,, tibial, 75. ,, vena cava, 119, 323. Pterygoid, 186. Pulmonary, 105. Renal, 320. Small vena azygos, 114. Solar plexus of foot, 42. Spermatic, 279, 323. Spinal, 141. Spheno-palatine, 213, 220. Splenic, 317. Spur, 287. Staphyline, 204. Subcutaneous abdominal, 287. ,, thoracic, 287. Vasa vorticosa, 263. Velum interpositum, 251. ,, pendulum palati, 202. Ventricle of brain, fourth, 241. ,, ,, lateral, 249. „ „ third, 252. Ventricle of heart, left, 128. ,, t „ right, 126. | Ventricles of larynx, 231. Ventricular grooves, 106. I Vermiform lobe, 240. Verumontanum, 348. Vesiculse seminales, 346, 348. Vesicula prostatica, 346. Vestibular bulb, 357. Vestibule of ear, 269. Villi of intestine, 308. Vitreous humour, 266. Vocal cord, 227. Vulva, 356, 357. Wall of hoof, 36. Wharton’s duct, 190, 197. Wir sung’s duct, 313. Zonula of Zinn, 265.