UNIVERSITY OF CALIFORNIA MEDICAL CENTER LIBRARY SAN FRANCISCO GIFT OF SAN FRANCISCO COUNTY MEDICAL SOCIETY HUMAN ANATOMY The skeleton in relation to the contour of the body. 4279 HUMAN ANATOMY INCLUDING STRUCTURE AND DEVELOPMENT AND PRACTICAL CONSIDERATIONS BY THOMAS DWIGHT, M.D., LL.D. J. PLAYFAIR McMURRICH, PH.D. KMAN PROFESSOR OF ANATOMY IN HARVARD PROFESSOR OF ANATOMY IN THE UNIVERSITY O UNIVERSITY MICHIGAN CARL A. HAMANN, M.D. GEORGE A. PIERSOL, M.D., SC.D. J. WILLIAM WHITE, M.D., PH.D., LL.D. JOHN RHEA BARTON PROFESSOR OF SURGERY IN THE UNIVERSITY OF PENNSYLVANIA WITH SEVENTEEN HUNDRED AND THIRTY-FOUR ILLUSTRATIONS, OF WHICH FIFTEEN HUNDRED AND TWENTY-TWO ARE ORIGINAL AND LARGELY FROM DISSECTIONS BY JOHN C. HEISLER, M.D. PROFESSOR OF ANATOMY IN THE MEDICO-CHIRURGICAL COLLEGE EDITED BY GEORGE A., PIERSOL VOL. I. FOURTH EDITION. PHILADELPHIA & LONDON J. B. LIPPINCOTT COMPANY Copyright, 1906, by J. B. Lippincott Company. Copyright, 1907, by J. B. Lippincott Company. Copyright, 1908, by J. B. Lippincott Company. Copyright, 1911, by J. B. Lippincott Company. Copyright, 1913, by J. B. Lippincott Company. Entered at Stationers' Hall, London, England. All Rights Reserved. ELECTROTYPEO AND PRINTED BY J. B. LIPPINCOTT COMPANY, PHILADELPHIA, U.S.A. PREFACE. THE preparation of this work was undertaken with three chief considerations in mind. i. The presentation of the essential facts of human anatomy, regarded in its broadest sense, by a descriptive text which, while concise, should be sufficiently com- prehensive to include all that is necessary for a thorough understanding not only of the gross appearances and relations of the various parts of the human body, but also of their structure and development. 2. Adequate emphasis and explanation of the many and varied relations of anatomical details to the conditions claiming the atten- tion of the physician and surgeon. 3. The elucidation of such text by illustrations that should portray actual dissections and preparations with fidelity and realism. To the first of these ends the co-operation of several American teachers of anatomy was enlisted, whose contributions have been welded into a homogeneous whole. Dr. Thomas Dwight has written the description of the skeleton, including the joints, and that of the gastro-pulmonary system and of the accessory organs of nutrition. Dr. Carl A. Hamann has contributed the account of the cerebro-spinal and sympathetic nerves. Dr. J. Playfair McMurrich has supplied the systematic description of the mus- cular and of the blood- and lymph-vascular system. Dr. George A. Piersol has written the introductory, histological and embryolog- ical paragraphs throughout the work and contributed the description of the central nervous system, including the deep relations of the cranial nerves, of the organs of special sense, of the carotid, coccygeal and aortic bodies, and of the uro-genital system. The second desideratum adequate consideration of the practical applications of anatomy has been ensured by the co-operation of Dr. J. William White, whose ripe experience, both as a surgeon and as a teacher of surgery, has enabled him to point out with unusual force the relations of anatomy to the requirements of the practitioner, and to associate for the benefit of the student anatomical facts with those conditions, resulting from injury or disease, that these facts elucidate. While no attempt has been made to cover the field of operative surgery, brief descriptions of operative methods have been given when they have seemed necessary to complete the study of an anatomical region or of an important organ. Occasion- ally a relatively rare operation has been included because of the exceptional practical importance of the subject from an anatomical standpoint. The writer of the Practical Considerations has aimed at presenting, in connection with each organ or system, enough facts illustrative of the dependence of the diag- nostician and practitioner upon anatomical knowledge to awaken interest and to com- bat the tendency to regard anatomy as something to be memorized during student days and forgotten when examinations are over. Even when such facts do not seem at a first glance to come within the scope of a text-book of anatomy, it will be found that a careful comparison of this text with the descriptive portion of the book will show a real and practical relation between them a relation which, once established 72827 vi PREFACE. in the minds of the student and the physician, will make it easier for the former to learn his anatomy and for the latter to remember and apply it. Dr. White desires to acknowledge fully his obligations to the existing treatises on applied anatomy and to the various text-books and encyclopedias on surgery and medicine from which many valuable suggestions were gathered. To Drs. Gwilym G. Davis and T. Turner Thomas, his thanks are due for a careful search for possible errors, for friendly criticism, and for help in the selection of illustrations. The illustrations for the anatomy a matter of fundamental importance in a work of this character have received most conscientious attention. The determination to produce a series of original drawings that should faithfully record the dissections and preparations as they actually appear, and not as diagrammatic figures, has involved an expenditure of time and painstaking effort that only those having experience with similar tasks can appreciate. When it is stated that considerably more than two thousand original drawings have been made in the preparation of the figures illus- trating the work, some conception will be had of the magnitude of this feature. In the completion of this labor the editor has been most fortunate in having the assistance of Dr. John C. Heisler, to whose skill and tireless enthusiasm he is indebted for the admirable dissections from which most of the illustrations of the muscles, blood-vessels, nerves, perineum and inguinal region were drawn, as well as for many suggestions for and revision of the drawings themselves. Professor Gwilym G. Davis has also rendered valuable assistance in supplying the dissections for the drawings relating to the Practical Considerations, as well as in supervising that portion of the artist's work. In addition to the numerous dissections and preparations made especially for the illustrations, advantage has been taken of the rich collections in the museums of the Medical Department of the University of Pennsylvania, of the Harvard Medical School and of the Wistar Institute of Anatomy, which were kindly placed at the editor's service. Records of the dissections, in many cases life size, were made in water colors chiefly by Mr. Hermann Faber, whose renditions combine faithful drawing with artistic feeling to a degree unusual in such subjects. The records not made by the last-named artist are from the brush of Mr. Ludwig E. Faber. The translations of the colored records into black and white, from which the final blocks have been made, as well as the original drawings of the bones and of the organs, have been made by Mr. Erwin F. Faber. To the conscientious and tireless 'efforts of this artist are due the technical beauty that distinguish these illustrations. Mr. J. H. Emerton drew the joints, as well as some figures relating to the gastro-pulmonary system, from dissections and sections supplied by Professor Dwight. The numerous illustrations representing the histological and embryological de- tails throughout the work, and in addition the sections of the brain-stem under low magnification, are by Mr. Louis Schmidt. In all cases sketches with the camera lucida or projection lantern or photographs have been the basis of these drawings, the details being faithfully reproduced by close attention to the original specimens under the microscope. Notwithstanding the unusually generous allotment of drawings from original dissections and preparations, advantage has also been taken of a number of illus- trations which have appeared in special monographs or in foreign journals or works. With very few exceptions such borrowed illustrations have been redrawn and modi- fied to meet the present requirements, due acknowledgment in all cases being given. PREFACE. vn The editor gratefully acknowledges the many kindnesses shown by a number of his associates. Dr. William G. Spiller generously placed at his disposal a large collection of microscopical preparations of the central nervous system, from which drawings of selected sections were made. To Dr. George Fetterolf the editor is indebted for valuable assistance in preparing for and seeing through the press the section on the peripheral nervous system. The collaboration of Dr. Edward A. Shumway very materially facilitated the preparation of the description of the eye, which received only the editor's revision. Likewise, Dr. Ralph Butler, by placing in the editor's hands a painstaking review of the more recent literature on the ear and preliminary account of that organ, greatly lightened the labor of writing the text. Further, Dr. Butler supplied the microscopical preparations from which several of the drawings were made. In addition to assuming the preparation of the index a no insignificant undertaking in a work of this character Dr. Ewing Taylor gave valu- able assistance in the final revision of the first hundred pages of the book. The editor is indebted to Dr. W. H. F. Addison for repeated favors in preparing special microscopical specimens. Dr. T. Turner Thomas kindly assisted in locating cross- references. This opportunity is taken to express full appreciation and thanks to the various authors and publishers, who so kindly have given permission to use illus- trations which have appeared elsewhere. Very earnest consideration of the question of nomenclature led to the conclusion, that the retention, for the most part, of the terms in use by English-speaking anatomists and surgeons would best contribute to the usefulness of the book. While these names, therefore, have been retained as the primary terminology, those adopted by the Basle Congress have been included, the BNA synonyms appear- ing in the special type reserved for that purpose. The constant aim of the editor has been to use the simplest anatomical terminology and preference has always been given to the anglicized names, rather than to the more formal designations. Although in many cases the modifications suggested by the new terminology have been followed with advantage, consistent use of the Basle nomenclature seems less in accord with the conceded directness of English scientific literature than the enthusi- astic advocates of such adoption have demonstrated. The editor desires to express his appreciation of the generous support given him by the publishers and of the unstinted facilities placed by them at his, disposal throughout the preparation of the work. UNIVERSITY OF PENNSYLVANIA, SEPTEMBER, 1907. CONTENTS. VOL. I. INTRODUCTION. Relation of Anatomy to Biology . . Subdivisions of Anatomical Study PAGE I General Plan of Vertebrate Construction Descriptive Terms THE ELEMENTS OF STRUCTURE. The Elementary Tissues The Cells and Intercellular stances The Embryonal Cell Vital Manifestations Metabolism Growth Reproduction Sub- Irritability The Animal Cell Structure of the Cytoplasm Structure of the Nucleus . . The Centrosome Division of Cells Mitotic Division . . PAGE i 6 6 7 8 9 10 ii Amitotic Division 14 EARLY DEVELOPMENT. The Ovum 15 The Spermatozoon 16 Maturation of the Ovum 16 Fertilization of the Ovum 18 Segmentation of the Ovum 21 The Blastoderm and the Blastodermic Layers 22 Derivatives of the Blastodermic Layers. . 24 The Primitive Streak and the Gastrula. . 24 The Significance of the Primitive Streak . . 25 The Fundamental Embryological Pro- cesses 26 The Neural Canal 26 The Notochord 27 The Ccelom 28 The Somites 29 The Foetal Membranes 30 The Amnion 30 The Serosa 31 The Vitelline Sac 32 The Allantois and the Chorion 32 The Human Foetal Membranes 35 The Amnion and Allantois 35 The Chorion 41 The Amniotic Fluid 41 The Umbilical Vesicle 42 The Deciduae 44 The Trophoblast 46 The Decidua Vera 46 The Decidua Placentalis 48 The Placenta 49 The Umbilical Cord 53 The After-Birth 55 Development of Body-Form 56 The Stage of the Blastodermic Ves- icle 56 The Stage of the Embryo 56 The Visceral Arches and Fur- rows 59 The Development of the Face . . 62 The Stage of the Fcetus 63 THE ELEMENTARY TISSUES. The Epithelial Tissues 67 Squamous Epithelium 68 Columnar Epithelium 69 Modified Epithelium 70 Specialized Epithelium 70 Endothelium 71 The Connective Tissues 73 The Cells of Connective Tissue 73 The Intercellular Constituents 74 Fibrous Tissue 74 Reticular Tissue 75 Elastic Tissue 76 Development of Connective Tissue . 77 Tendon 77 Adipose Tissue 79 Cartilage 80 The Connective Tissues Continued Hyaline Cartilage 80 Elastic Cartilage 81 Fibrous Cartilage 82 Development of Cartilage 82 Bone 84 Chemical Composition 84 Physical Properties 85 Structure of Bone 85 Bone Marrow 90 Red Marrow 91 Yellow Marrow 93 Development of Bone 94 Endochondral Bone 94 Membranous Bone 98 Subperiosteal Bone 98 THE SKELETON, INCLUDING THE JOINTS. The Axial Skeleton 103 The Appendicular Skeleton 104 General Considerations of the Bones. .. . 104 General Considerations of the Joints .... 107 The Spinal Column 1 14 The Thoracic Vertebrae 115 CONTENTS. The Thoracic Vertebrae Continued The Cervical Vertebrae 116 The Lumbar Vertebrae 117 Peculiar Vertebrae 119 Dimensions of Vertebrae 122 Gradual Regional Changes 122 The Sacrum 1 24 The Coccyx 127 Development of the Vertebrae 128 Variations of the Vertebrae 131 Articulations of the Vertebral Column . . 132 Ligaments Connecting the Bodies . . 132 Ligaments Connecting the Laminae and the Processes 133 Articulations of the Occipital Bone, the Atlas and the Axis 135 The Spine as a Whole 138 Dimensions and Proportions 141 Movements of the Head 142 Movements of the Spine 142 Practical Considerations : The Spine. .. . 143 Curvature of the Spine 144 Sprains, Dislocations and Fractures . 144 Landmarks 146 The Thorax 149 The Ribs 149 The Costal Cartilages 153 The Sternum 155 Articulations of the Thorax 157 The Anterior Thoracic Articulations. 158 The Intersternal Joints 159 The Costo-Sternal Joints 160 The Interchondral Joints 160 The Costo-Vertebral Articulations. . 160 The Thorax as a Whole 162 The Movements of the Thorax 165 Practical Considerations : The Thorax . . 167 Deformities 167 Fractures and Disease of the Ribs . . 169 Landmarks 170 The Skull 172 The Cranium 172 The Occipital Bone 172 The Temporal Bone 176 The Tympanic Cavity 183 The Sphenoid Bone 186 The Ethmoid Bone 191 The Frontal Bone 194 The Parietal Bone 197 The Bones of the Face 199 The Superior Maxilla 199 The Palate Bone 204 The Vomer 205 The Lachrymal Bone 207 The Inferior Turbinate Bone 208 The Nasal Bone 209 The Malar Bone 209 The Inferior Maxilla 211 The Temporo-Maxillary Articulation. ... 214 The Hyoid Bone 216 The Skull as a Whole. 216 The Exterior of the Cranium 218 The Interior of the Cranium 220 The Architecture of the Cranium . . . 220 The Face 222 The Orbit 222 The Nasal Cavity 223 The Accessory Pneumatic Cav- ities 226 The Architecture of the Face . . . 228 The Anthropology of the Skull 228 Practical Considerations : The Skull .... 235 The Cranium 235 Pract. Consid. : The Skull Continued Malformations 235 The Wormian Bones 236 Diseases of the Cranial Bones . . 237 Fractures 238 Landmarks 240 The Face 242 Deformities and Fractures 243 Dislocation of the Jaw 246 Landmarks 246 The Bones of the Upper Extremity 248 The Shoulder-Girdle 248 The Scapula 248 Practical Considerations 253 Malformations 253 Fractures and Disease 254 Landmarks 255 Ligaments of the Scapula 256 The Clavicle 257 Practical Considerations 258 Malformations 259 Fractures and Disease 259 Landmarks 260 The Sterno-Clavicular Articulation . 261 The Coraco-Clavicular Ligament . . . 262 Movements of the Clavicle and Scap- ula 262 Surface Anatomy of the Shoulder- Girdle 263 Practical Considerations 263 The Sterno-Clavicular Articula- tion 263 The Acromio-Clavicular Articu- lation 264 The Humerus 265 Practical Considerations 270 Malformations 270 Separation of the Epiphyses .... 271 Fractures and Disease 273 The Shoulder-Joint 274 Practical Considerations 278 Dislocations and Diseases 278 Landmarks 280 The Ulna 281 Practical Considerations 285 Malformations 285 Fractures 286 Landmarks 287 The Radius 287 Practical Considerations 293 Malformations 293 Fractures and Disease 294 Landmarks 296 The Radio-Ulnar Articulations 297 The Forearm as a Whole 299 The Elbow-Joint 301 Practical Considerations 305 Dislocations and Disease 305 Landmarks 308 The Bones of the Hand 309 The Carpal Bones 309 The Metacarpal Bones 314 The Phalanges 317 Practical Considerations 319 The Carpus 319 The Metacarpus 319 The Phalanges 320 Landmarks 320 Ligaments of Wrist and Metacarpus .... 320 Movements and Mechanics of Wrist and Carpo-Metacarpal Articu* lations 326 Surface Anatomy of Wrist and Hand 328 CONTENTS. Practical Considerations: The Wrist- Joint Landmarks The Joints of the Carpus, Metacarpus and Phalanges The Bones of the Lower Extremity The Pelvic Girdle The Innominate Bone Joints and Ligaments of the Pelvis. . The Sacro-Iliac Articulation. . . . The Symphysis Pubis The Sacro-Sciatic Ligaments . . The Pelvis as a Whole Mechanics of the Pelvis Surface Anatomy Practical Considerations: The Pelvis . . Malformations Fractures and Disease Landmarks Joints of the Pelvis The Femur Surface Anatomy Practical Considerations: The Epiphyses Fractures and Disease Landmarks The Hip-Joint Practical Considerations Outward or Posterior Luxations. .. . Inward or Anterior Luxations Congenital Luxation Disease of the Hip- Joint The Framework of the Leg The Tibia Practical Considerations Separation of the Epiphyses Fractures and Disease Landmarks . . 329 330 630 332 332 332 337 338 339 339 34i 342 345 345 345 346 349 350 352 360 363 366 367 374 375 377 380 380 382 382 387 387 389 39 The Fibula Practical Considerations Separation of Upper Epiphysis. . . . Fractures and Disease Landmarks Connections of the Tibia and Fibula. . . . The Bones of the Leg as one Apparatus The Patella The Ligamentum Patellae The Knee-joint Practical Considerations : The Knee- joint Dislocations Subluxation of Semiiunar Cartilages Disease of Knee-joint The Patella The Bones of the Foot The Tarsal Bones The Metatarsal Bones The Phalanges Practical Considerations : The Foot- Bones Fracture, Dislocation and Disease. . Landmarks The Ankle-joint The Articulations of the Foot Intertarsal Joints Tarso-Metatarsal Joints Metatarso-Phalangeal Joints Synovial Cavities The Foot as a Whole Surface Anatomy Practical Considerations: The Ankle- joint Dislocations and Disease Tarsal, Metatarsal and Phalangeal Joints Landmarks . . 393 393 394 39 6 396 397 398 400 400 409 409 411 412 416 419 419 428 432 436 437 437 438 440 445 446 447 447 447 449 450 45i 453 THE MUSCULAR SYSTEM. Muscular Tissue in general 454 Nonstriated or Involuntary Muscle. . 454 Structure 455 Development 457 Striated or Voluntary Muscle 457 General Structure 458 Structure of the Muscle-Fibre. . 459 Cardiac Muscle 462 Development of Striated Muscle. 465 Myomeres and their Modifica- tions 467 General Consideration of the Muscles. . . . 468 Attachments 468 Form 469 Fasciae 470 Tendon-Sheaths 470 Bursae 471 Classification 471 Nerve-Supply 473 The Branchiomeric Muscles 474 The Trigeminal Muscles 474 Muscles of Mastication 474 Submental Muscles 477 Trigeminal Palatal Muscle 479 Trigeminal Tympanic Muscle 479 The Facial Muscles 479 Hyoidean Muscles 480 Platysma Muscles 480 Superficial Layer 481 Deep Layer 486 Practical Considerations : Muscles and Fasciae of Cranium 489 The Scalp 489 The Face 492 Landmarks 494 The Vago- Accessory Muscles 495 Muscles of Palate and Pharynx 495 Muscles of Larynx . . Vol. II 1824 Trapezius Muscles . 499 The Metameric Muscles 502 The Axial Muscles 502 Orbital Muscles 502 Fasciae of Orbit 504 Movements of Eyeball 505 Hypoglossal Muscles 506 The Trunk Muscles 507 The Dorsal Muscles 507 Transverso-Costal Tract 508 Transverso-Spinal Tract 511 The Ventral Muscles 515 Abdominal Muscles 515 Rectus Muscles 516 Obliquus Muscles 517 Ventral Aponeurosis 521 Inguinal Canal 523 Anterior Abdominal Wall. . 525 Hyposkeletal Muscles 526 Practical Considerations : The Abdo- men 526 The Loin 530 Xll CONTENTS. Practical Considerations Continued Landmarks a nd Topography o f Abdomen 531 Anatomy of Abdominal Incisions. . . 535 Examination of Abdomen 537 The Thoracic Muscles 538 Rectus Muscles 538 Obliquus Muscles 538 Hyposkeletal Muscles 542 The Cervical Muscles 542 The Deep Cervical Fascia 542 Rectus Muscles 543 Obliquus Muscles 546 Triangles of the Neck 547 Hyposkeletal Muscles 548 Practical Considerations : The Neck. .. . 550 Cervical Fascia and its Spaces 551 Landmarks 554 The Diaphragm 556 The Pelvic and Perineal Muscles 558 Pelvic Fascia 558 Obturator Fascia 559 Pelvic Muscles 559 Perineal Muscles 562 The Appendicular Muscles 566 The Muscles of the Upper Limb 568 Muscles extending between Axial Skele- ton and Pectoral Girdle 568 Pectoral Fascia 568 Preaxial Muscles 568 Postaxial Muscles 571 The Axilla 574 Muscles passing from Pelvic Girdle to Brachium 575 Preaxial Muscles 575 Postaxial Muscles 575 Practical Considerations : Muscles and Fascia of Axilla and Shoulder. . 579 Fracture of Clavicle 579 Dislocation of Shoulder-Joint 582 The Brachial Muscles 585 Preaxial Muscles 585 Postaxial Muscles 588 Practical Considerations : Muscles and Fascia of the Arm 589 Fractures of Humerus 590 The Antibrachial Muscles 591 Preaxial Muscles 592 Postaxial Muscles 598 Practical Considerations : The Forearm. 603 The Muscles of the Hand 606 Deep Fascia 606 The Muscles of the Hand Continued Preaxial Muscles 607 Muscles of First Layer 607 Muscles of Second Layer 610 Muscle of Third Layer 610 Muscles of IV and V Layers. ... 6n Postaxial Muscle 613 Pract. Consid. : The Wrist and Hand. . . 613 Palmar Abscesses 616 Dislocation of Thumb 617 Surface Landmarks of Upper Extremity . 618 The Muscles of the Lower Limb 623 Muscles extending from Pelvic Girdle to Femur 623 Preaxial Muscles 623 Postaxial Muscles 630 The Femoral Muscles 633 Fascia Lata 633 Preaxial Muscles 636 Postaxial Muscles 639 Practical Considerations : Muscles and Fasciae 641 The Buttocks 641 The Hip and Thigh 642 Fractures of the Femur 644 The Knee 645 Hursae of Popliteal Region 646 The Crural Muscles 647 The Crural Fascia 647 Preaxial Muscles 648 Superficial Layer 649 Middle Layer 651 Deep Layer 654 Postaxial Muscles 655 The Muscles of the Foot 659 The Plantar Fascia 659 Preaxial Muscles 659 First Layer 660 Second Layer 662 Third Layer 662 Fourth and Fifth Layers 663 Postaxial Muscles 665 Practical Considerations : Muscles and Fasciae 665 The Leg 665 The Ankle and Foot 666 Club-Foot 667 Surface Landmarks of Lower Extremity 669 The Buttocks and Hip 669 The Thigh 670 The Knee 671 The Leg 671 The Ankle and Foot 672 THE VASCULAR SYSTEM. THE BLOOD- VASCULAR SYSTEM. The Structure of Blood-Vessels 673 The Arteries 675 The Veins 677 The Capillaries 678 The Blood 680 General Characteristics 680 Blood-Crystals 680 The Colored Blood-Cells 68 1 The Colorless Blood-Cells 684 The Blood-Plaques 685 Development oi Blood- Vessels and Cells. . 686 The Heart 689 General Description 689 Position and Relations 692 Chambers of the Heart 693 Architecture of the Heart-Muscle. . . 700 Structure 702 Blood- Vessels and Lymphatics 703 Nerves 704 Development 705 Practical Considerations: The Heart. .. . 710 Valvular Disease 711 Rupture and Wounds 713 The Pericardium 714 CONTENTS. Xlll Practical Considerations: The Pericar- dium 717 The General Plan of the Circulation .... 719 THE ARTERIES 719 General Plan of Arterial System .... 720 The Pulmonary Aorta 722 The Systemic Aorta 723 The Aortic Arch 723 Practical Considerations: Aortic Arch and Thoracic Aorta 726 Surface Relations 726 Aneurisms 727 The Coronary Arteries 728 The Innominate Artery 729 Practical Considerations 729 The Common Carotid Arteries 730 Practical Considerations 731 The External Carotid Artery 733 Practical Considerations 733 Branches of External Carotid Ar- tery 734 The Internal Carotid Artery 746 Practical Considerations 747 Branches of Internal Carotid Ar- tery 748 Anastomoses of Carotid System. . . . 753 The Subclavian Artery 753 Practical Considerations 756 Branches of Subclavian Artery 758 The Axillary Artery 767 Practical Considerations 769 Branches of Axillary Artery 771 The Brachial Artery 773 Practical Considerations 775 Branches of Brachial Artery 777 The Ulnar Artery 778 Practical Considerations 780 Branches of Ulnar Artery 781 The Radial Artery 785 Practical Considerations 786 Branches of Radial Artery 787 The Thoracic Aorta 791 Branches of Thoracic Aorta 792 The Abdominal Aorta 794 Practical Considerations 796 The Visceral Branches 797 The Parietal Branches 805 The Common Iliac Arteries 807 Practical Considerations 807 The Internal Iliac Artery 808 Practical Considerations 810 Branches of Internal Iliac Artery. . . . 810 The External Iliac Artery 818 Practical Considerations 819 Branches of External Iliac Artery. . . 820 The Femoral Artery 821 Practical Considerations 824 Branches of Femoral Artery 826 Anastomoses of Femoral Artery ... 831 The Popliteal Artery 831 Practical Considerations 832 Branches of Popliteal Artery 833 The Posterior Tibial Artery 835 Practical Considerations 836 Branches of Posterior Tibial Artery . 838 The Anterior Tibial Artery 842 Practical Considerations 842 Branches of Anterior Tibial Artery, 844 The Dorsal Artery of the Foot 845 Development of the Arteries 846 THE VEINS 850 General Characteristics 850 Classification 852 The Pulmonary System 852 The Pulmonary Veins 852 The Cardinal System 854 The Cardiac Veins 854 The Superior Caval System 857 Vena Cava Superior 857 Practical Considerations 858 The Innominate Veins 858 Practical Considerations 859 Tributaries of Innominate Veins .... 859 The Internal Jugular Vein 861 Practical Considerations 863 Tributaries of Internal Jugular Vein . 863 The Sinuses of the Dura Mater 867 Practical Considerations 869 The Diploic Veins 874 Practical Considerations 875 The Emissary Veins 875 Practical Considerations 876 The Cerebral Veins 877 Practical Considerations 878 The Ophthalmic Veins 879 Practical Considerations 880 The External Jugular Vein 880 Practical Considerations 881 Tributaries of External Jugular Vein 882 The Subclavian Vein 884 Practical Considerations 885 Veins of the Upper Limb 886 The Deep Veins 886 The Superficial Veins 889 Practical Considerations 891 The Azygos System 893 The Azygos Vein 893 Tributaries 893 Practical Considerations 895 The Hemiazygos Vein 895 The Accessory Hemiazygos Vein. . 895 The Intercostal Veins 896 The Spinal Veins 897 Practical Considerations 898 The Veins of the Spinal Cord 898 The Inferior Caval System 898 Vena Cava Inferior 899 Practical Considerations 900 Tributaries of Inferior Cava. .. . 901 Practical Considerations. . . 904 The Common Iliac Veins 905 The Internal Iliac Veins 905 Tributaries of Internal Iliac .... 905 The External Iliac Vein 909 Tributaries of External Iliac. . . . 909 The Veins of the Lower Limb 910 The Deep Veins 910 The Superficial Veins 914 Practical Considerations of Iliac Veins and Veins of Lower Limb 917 The Portal System 919 The Portal Vein 919 Tributaries of Portal Vein 920 Practical Considerations 925 Development of the Veins 926 The Fcetal Circulation , 929 THE LYMPHATIC SYSTEM. General Consideration 931 Lymph-Spaces 931 Lymph-Capillaries 933 Lymph- Vessels 934 Lymph-Nodes 935 Structure of Lymphoid Tissue 936 Development of Lymphatic Vessels and Tissues 939 XIV CONTENTS. The Thoracic Duct 941 Practical Considerations 944 The Right Lymphatic Duct 945 The Lymphatics of the Head 945 The Lymph-Nodes 945 The Lymph- Vessels 949 Practical Considerations 955 The Lymphatics of the Neck 957 The Lymph-Nodes 957 The Lymph-Vessels 958 Practical Considerations 959 The Lymphatics of the Upper Limb .... 961 The Lymph-Nodes 961 The Lymph-Vessels 963 Practical Considerations 965 The Lymphatics of the Thorax 966 The Lymph-Nodes 966 The Lymph- Vessels 968 Practical Considerations 971 The Lymphatics of the Abdomen 972 The Lymph-Nodes 972 The Lymph-Vessels 976 The Lymphatics of the Pelvis 983 The Lymph-Nodes 983 The Lymph-Vessels 984 Practical Considerations 990 The Lymphatics of the Lower Limb .... 991 The Lymph-Nodes 991 The Lymph-Vessels 993 Practical Considerations 994 VOLUME I. GENERAL CONSIDERATIONS THE CELL EARLY DEVELOPMENT THE ELEMENTARY TISSUES THE SKELETON THE BONES THE ARTICULATIONS THE MUSCULAR SYSTEM THE VASCULAR SYSTEM THE HEART THE ARTERIES THE VEINS THE LYMPHATICS HUMAN ANATOMY. ANATOMY is that subdivision of morphology the science of form as contrasted with that of function or physiology which pertains to the form and the structure of organized beings, vegetal or animal. Phytotomy and Zootomy, the technical names for vegetal and animal anatomy respectively, both imply etymologically the dissocia- tion, or the cutting apart, necessary for the investigation of plants and animals. The study of organized bodies may be approached, evidently, from several stand- points. When the details of the structure of their various tissues and organs par- ticularly is investigated, such study constitutes General Anatomy or Histology, fre- quently also called Microscopical Anatomy, from the fact that the magnifying lens is used to assist in these examinations. The advantages of comparing the organization of various animals, representing widely different types as well as those closely related, are so manifest in arriving at a true estimate of the importance and significance of structural details, that Comparative Anatomy constitutes a department of biological science of far-reaching interest, not merely for the morphologist, but likewise for the student of human anatomy, since we are indebted to comparative anatomy for an intelligent conception of many details encountered in the human body. Devel- opmental Anatomy, or Embryology, also has been of great service in advancing our understanding of numerous problems connected with the adult organism by tracing the connection between the complex relations of the completed structures and their primitive condition, as shown by the sequence of the phases of development. These three departments of anatomical study general, comparative, and developmental anatomy represent the broader aspects of anatomical study in which the features of the human body are only incidents in the more extended contemplation of organized beings. The exceptional importance of an accurate knowledge of the body of man has directed to human anatomy, or anthropotomy , so much attention from various points of view that certain subdivisions of the subject are conveniently recognized ; thus, the systematic account of the human body is termed Descriptive Anatomy, while when the mutual relations and peculiarities of situation of the organs located in par- ticular parts of the body especially claim attention, such study is spoken of as Topo- graphical or Regional Anatomy. Consideration of the important group of anatomi- cal facts directly applicable to the diagnosis and the treatment of disease constitutes Applied Anatomy. General Plan of Construction. Vertebrate animals, of which man rep- resents the most conspicuous development of the highest class, fishes, amphibians, reptiles, birds, and mammals being the recognized subdivisions of the vertebrata, possess certain characteristics in common which suffice to distinguish the numerous and varied members of the extended group. The fundamental anatomical feature of these animals is the possession of an axial column, or spine, which extends from the anterior or cephalic to the poste- rior or caudal pole and establishes an axis around which the various parts of the elongated body are grouped with more or less symmetry. While this body-axis is usually marked by a series of well-defined osseous segments constituting the ver- tebral column of the higher animals, among certain of the lower fishes, as the sharks or sturgeons, the axial rod is represented by cartilaginous pieces alone ; in fact, the tendency towards the production of a body-axis is so pronounced that the formation HUMAN ANATOMY. of a primitive axis, the notochord, takes place among the early formative processes of the embryo. In addition to the fundamental longitudinal axis, vertebrate animals exhibit a transverse cleavage into somatic or body-segments. While such segmentation is rep- resented in the maturer conditions by the series of vertebrae and the associated ribs, the tendency to this division of the body is most marked in the early embryo, in which the formation of body-segments, the somites, takes place as one of the primary developmental processes. Although these primary segments do not directly corre- spond to the permanent vertebrae, they are actively concerned in the formation of the latter as well as the segmental masses of the earliest muscular tissue. In man not only the skeleton, but likewise the muscular, vascular, and nervous systems are affected by this segmentation, the effects of which, however are most evident in the structure of the walls of the thoracic portion of the body-cavity. Disregarding the many variations in the details of arrangement brought about by specialization and adaptation, the body of every vertebrate animal exhibits a fundamental plan of construction in which bilateral symmetry is a conspicuous fea- ture. Viewed in a transverse section passing through the trunk, the animal body FIG. i. Neural arch Neural tube Spinal cord Vertebral axis Epidermi Corium Parietal mesoblas Costal segment Parietal mesoblast Aorta Parietal mesothelium Visceral mesothelium Entoblastic epithelium Subepithelial mesoblast Visceral mesoblast Diagrammatic plan of vertebrate body in transverse section. {Modified from IViedersheim.) may be regarded as composed primarily of the axis, formed by the bodies of the vertebrae, and two tubular cavities of very unequal size enclosed by the tissues con- stituting the body-walls and invested externally by the integument (Fig. i). The smaller of these, the neural tube, is situated dorsally, and is formed by the series of the vertebral arches and associated ligaments ; it surrounds and protects the great cerebro-spinal axis composed of the spinal cord and the specialized cephalic extremity, the brain. The larger space, the visceral tube corresponding to the body- cavity, or ccelom, lies on the ventral side of the axis and contains the thoracic and abdominal viscera, including the more or less convoluted digestive-tube with its accessory glandular organs, the liver and the pancreas, and the appended respiratory tract, together with the genito-urinary organs and the vascular and lymphatic appa- ratus. The digestive-tube, which begins anteriorly at the oral orifice and opens posteriorly by the anus, is extended by two ventral evaginations giving rise to the respiratory tract and the liver, a dorsal glandular outgrowth representing the pan- creas. The sexual and urinary glands and their ducts primarily occupy the dorsal wall of the body cavity. The vascular system consists essentially of the ventrally placed contracting dilatation, the heart, divided into a venous and an arterial com- DESCRIPTIVE TERMS. partment, and the great arterial trunk, the aorta, the major part of which occupies the dorsal wall of the space. The elongated typical vertebrate body terminates anteriorly in the cephalic segment, posteriorly in the caudal appendage ; between these two poles extends the trunk, from which project the ventrally directed limbs, when these appendages exist. Just as the axial segments, represented by the bodies of the vertebrae, take part, in conjunction with the neural arches, in the formation of the neural canal, so do these segments also aid in forming the supporting framework of the ventral body-cavity in connection with the series of ribs and the sternum. Descriptive Terms. The three chief planes of the vertebrate body are the Sagittal, the transverse, and the frontal. The sagittal plane, when central, passes through the long axis of the body vertically and bisects the ventral or anterior and the dorsal or posterior surfaces. The transverse plane passes through the body at right angles to its long axis and to the sagittal plane. The frontal plane passes vertically but parallel to the anterior or ventral surface, being at right angles to both the sagittal and transverse planes (Fig. 2.) The vertical position of the long axis in the human body is unique, since man, FIG. 2. FIG. 3. Human embryo showing primary relations of limbs, a, a, preaxial surfaces ; 6, 6, postaxial ; s, s, somitic segments of trunk. Three principal planes of human body. T, T, transverse ; S, S, sagittal ; F, F, frontal. of all animals, is capable of habitually maintaining the erect posture with full exten- sion of the supporting extremities. The lack of correspondence between the actual position of the chief axis of man and the horizontal fore-and-aft axis of vertebrates in general results in discrepancies when the three principal planes of the human body are compared with those of other animals. Thus, the sagittal plane alone retains the relation, as being at right angles to the plane of the support, in all verte- brates, although in man its greatest expansion is vertical. The transverse plane in man is parallel with the supporting surface, while it is, obviously, at right angles to the corresponding plane in the four-footed vertebrate ; likewise, the frontal plane in man is vertical, while it is horizontal in other animals. The various terms employed in describing the actual position of the numerous parts of the human body and their relations to surrounding structures have been adopted with regard to the erect attitude of man and the convenience of the student of human anatomy ; hence, in many cases, they must be recognized as having a limited specific and technical application and often not directly applicable to other 4 HUMAN ANATOMY. vertebrates. Superior and inferior, upper and lower, as indicating relations towards or away from the head-end of the body, are, probably, too convenient and useful as expressing the peculiar relations in man to be readily relinquished, although when directly applied to animals possessing a horizontal body-axis they refer entirely to relations with the plane of support, the additional terms cephalic and caudal being necessary to indicate relations with the head- and tail-pole. Likewise, "anterior" and ' ' posterior, ' ' as referring respectively to the front and back surfaces of the human body, are more logically described as ventral and dorsal, with the advantage that these terms are directly applicable to all vertebrates. ' ' Outer' ' and ' ' inner, ' ' as expressing relations with the sagittal plane, are now largely replaced by the more desirable terms lateral and mesial respectively, external and internal being reserved to indicate relations of depth. Cephalic and caudal, central and peripheral, prox- imal and distal, are all terms which have found extensive use in human anatomy. Preaxial and postaxial, in addition to their general and obvious significance with reference to axes in common, have acquired a specific meaning with regard to the limbs, the appreciation of which requires consideration of the primary relations observed in the embryo. In the earliest stage the limbs appear as flattened buds which project from the side of the trunk and present a dorsal and ventral surface ; subsequently the limbs become folded against the body, the free ends being directed ventrally, while one border looks headward, the other tailward. If an axis corre- sponding to the transverse plane of the body be drawn through the length of the extremities, each limb will be divided into two regions, one of which lies in front of the axis, and is, therefore, preaxial, the other behind, or postaxial. On reference to Fig. 3 it is obvious that the preaxial border or surface of each limb is primarily directed towards the cephalic or head-end of the animal, and, conversely, that the postaxial faces the caudal or tail-end. These fundamental relations are of great im- portance in comparing the skeleton of the upper and lower extremities with a view of determining the morphological correspondence of the several component bones, since the primary relations become masked in consequence of the secondary dis- placements which the limbs undergo during their development. The terms homologue and analogue call for a passing notice, since an exact understanding of their significance is important. Structures or parts are homologous when they possess identical morphological values founded on a common origin ; thus, the arm of a man, the front leg of a dog, and the wing of a bat are homologues, because each represents the fore-limb of a vertebrate, although they differ in individual func- tion. On the other hand, the wing of a bat and that of a butterfly are analogous, since they are structures of functional similarity, although of wide morphological diversity. Homologue, therefore, implies structural identity, analogue implies functional similarity. Parts are said to be homotypes when they are serial homo- logues ; thus, the humerus and the femur are homotypes, being corresponding structures repeated in the same animal. Where parts possess both morphological and functional identity, as the wing of a bird and of a bat, they are analogous as well as homologous. THE ELEMENTS OF STRUCTURE. WHEN critically examined, the various organs and parts going to make up the complex economy of the most highly specialized vertebrate and, indeed, the same is true of all animals whose organization does not approach the extremely sirmole uni- cellular type are found to be constituted by the various combinations of a very small number of elementary tissues ; these latter may be divided into four funda- mental groups : Epithelial tissues ; Connective tissues ; Muscular tissues ; Nervous tissues. Of these the nervous tissues are most specialized in their distribution, while the connective tissues are universally present, in one or another form contributing to the composition of every organ and part of the body. The tissues of the circulatory system, including the walls of the blood-vessels and lymph-channels and the corpus- cular elements of their contained fluids, the blood and the lymph, represent special- izations of the connective tissues of such importance that they are often conceded the dignity of being classed as independent tissues ; consideration of the develop- ment of the vascular tissues, however, shows their genetic relations to be so nearly identical with those of the great connective-tissue group that a separation from the latter seems undesirable. Each of the elementary tissues may be resolved into its component morphologi- cal constituents, the cells and the intercellular substances. The first of these are the A FIG. 4. Nucleus Vacuole Pseudopod Endoplasm Vegetal food- inclusions Exoplasm A, unicellular animal (amoeba); B. embryonal cell, leucocyte. descendants of the embryonal elements derived from the division or segmentation of the parent cell, the ovum, and are highly endowed with vital activity ; the intercellu- lar substances, on the other hand, represent secondary productions, comparatively inert, since they are formed through the more or less direct agency of the cells. The animal cell may exist in either the embryonal, matured, or metamorphosed condition. The embryonal cell, as represented by the early generations of the direct off- spring of the ovum, or by the lymphoid cells or colorless blood-corpuscles of the adult, consists of a small, irregularly round or oval mass of finely granular gelati- nous substance the protoplasm in which a smaller and often indistinct spherical body the nucleus lies embedded. In the embryonal condition, when the cell is without a limiting membrane, and composed almost entirely of active living matter, the outlines are frequently undergoing change, these variations in shape being known as amoeboid movements, from their similarity to the changes observed in the outline of an active amoeba, the representative of the simplest form of animal life, in which 6 HUMAN ANATOMY. the single cell constitutes the entire organism, and as such is capable of performing the functions essential for the life-cycle of the animal. As the embryonal cell advances in its life-history, the conditions to which it is subjected induce, with few exceptions, further specializations, since in all but the lowest forms division of labor is associated with a corresponding differentiation and adaptation to specific function. Vital manifestations of the cell include those complex physico-chemical phenomena which are exhibited during the life of the cellular constituents of the body in the performance of the functions necessary for fulfilment of their appointed life-work. These embrace metabolism, growth, reproduction, and irritability. Metabolism, the most distinctive characteristic of living matter, is that process by which protoplasm selects from the heterogeneous materials of food those partic- ular substances suitable for its nutrition and converts them into part of its own sub- stance. Metabolism is of two forms, constructive and destructive. Constmctive metabolism, or anabolism, is the process by which the cell converts the simpler com- pounds into organic substances of great chemical complexity ; destructive metabolism, or katabolism, on the contrary, is the process by which protoplasm breaks up the complex substances resulting from constructive metabolism into simpler compounds. Vegetal cells possess the power of constructive metabolism in a conspicuous degree, and from the simpler substances, such as water, carbon dioxide, and inorganic salts, prepare food-material for the nutritive and katabolic processes which especially dis- tinguish the animal cell, since the latter is dependent, directly or indirectly, upon the vegetal cell for the materials for its nutrition. Growth, the natural sequel of the nutritive changes effected by metabolism, may be unrestricted and equal in all directions, resulting in the uniform expansion of the cell, as illustrated in the growth of the ovum in attaining its mature condition. Such unrestricted increase, however, is exceptional, since cells are usually more or less intimately related to other structural elements by which their increase is modi- fied so as to be limited to certain directions ; such limitation and influence result in unequal growth, a force of great potency in bringing about the differentiation and specialization of cells, and, secondarily, of entire parts and organs of the body. Familiar examples of the result of unequal growth are observed in the columnar elements of epithelium, the fibres of muscular tissue, and the neurones of the ner- vous system. Reproduction may be regarded as the culminating vital manifestation in the vegetative life-cycle of the cell, since by this process the parent element surrenders its individuality and continues its life in the existence of its offspring. While the details of the process by which new cells arise from pre-existing cells are reserved for consideration in connection with the more extended discussion of the cell to follow (see page 9), it may here be stated that reproduction occurs by two methods, the indirect or mitotic and the direct or amitotic. The first of these, involving the complicated cycle of nuclear changes collectively known as mitosis or karyokinesis, is the usual method; the second and simpler process of direct division, or amitosis, is now recognized as exceptional and frequently associated with conditions of im- paired vital vigor. Irritability is that property of living matter by virtue of which the cell ex- hibits changes in its form and intimate constitution in response to external impres- sions. These latter may originate in mechanical, thermal, electrical, or chemical stimuli to which the protoplasm of even the lowest organisms responds, or they may be produced in consequence of the obscure and subtle changes occurring within the protoplasm of neighboring elements, as illustrated by the reaction of the neurones in response to the stimuli transmitted from other nervous elements. THE ANIMAL CELL. Ever since the establishment of the Cell Doctrine, in 1838, by the announcement of the results of the epoch-making investigations of Schleiden and Schwann on "The Accordance of Structure and Growth of Animals and Plants, ' ' the critical examination of the cell has been a subject of continuous study. Notwithstanding the tireless enthu- STRUCTURE OF THE CELL. siasm with which these researches have been pursued by the most competent investi- gators and the great advance in our accurate knowledge concerning the intricate problems relating to the morphology and the physiology of the cell, much pertaining to the details of the structure and the life of the cell still remains uncertain, and must be left to the future achievements in cytology. The account here given of the mor- phology of the cell presents only those fundamental facts which at the present time may be accepted as established upon the evidence adduced by the most trustworthy observers. The more speculative and still unsettled and disputed problems of cy- tology, interesting as such theoretical considerations may be, lie beyond the purpose of these pages ; for such discussions the student is referred to the special works and monographs devoted to these subjects. An appreciation, however, of the salient facts of cytology as established by the histologists of the present generation is essen- tial not only for an intelligent conception of the structure of the morphological ele- ments, but likewise for the comprehension of the highly suggestive modern theories concerning inheritance, since, as will appear in a later section, the present views regarding these highly interesting problems are based upon definite anatomical details. FIG. 5. Cytoplasm Spongioplasm Hyaloplasm Metaplastic inclusions Exoplasm Endoplasm Nuclear membrane Nucleolus Centrosome surrounded by centrosphere Cytoplasm Cell-wall Diagram of cell-structure. In the upper part of the figure the granular condition of the cytoplasm is represented ; in the lower and left, the reticular condition. Notwithstanding the great variations in the details of form and structure, cells possess a common type of organization in which the presence of the cell-body or cyto- plasm, and the nucleus is essential in fulfilling the modern conception of a cell. The latter may be defined, therefore, as a nucleated mass of protoplasm. The term protoplasm, as now generally employed by histologists, signifies the organized substance composing the entire cell, and with this application includes both the cytoplasm and the nucleus. Structure of the Cytoplasm. The cytoplasm, or the substance of the cell- body, by no means invariably presents the same appearance, since it may be regarded as established that the constituents of this portion of the cell are subject to changes in their condition and arrangement which produce corresponding morphological varia- tions ; thus, the cytoplasm may be devoid of definite structure and appear homoge- neous ; at other times it may be composed of aggregations of minute spherical masses and then be described as granular, or, where the minute spheres are larger and con- sist of fluid substances embedded within the surrounding denser material of the cell, as alveolar ; or, again, and most frequently, the cytoplasm contains a mesh- work of fibrils, more or less conspicuous, which arrangement gives rise to the reticular con- dition. The recognition of the fact established by recent advances in cytology, that the structure of cytoplasm is not to be regarded as immutable, but, on the contrary, as capable of undergoing changes which render it probable that a cell may appear HUMAN ANATOMY. during one stage of its existence as granular and at a later period as reticular, has done much to bring into accord the conflicting and seemingly irreconcilable views regarding the structure of the cell championed by competent authorities. Whatever be the particular phase of structural arrangement exhibited by the cell, histologists are agreed that the cytoplasm consists of two substances, an active and & passive ; while both must be regarded as living, the vital manifestations of con- tractility are produced by the former. Since a more or less pronounced reticular arrangement of the active and passive constituents of cytoplasm is of wide occurrence in mature cells, this condition may serve as the basis for the description of the morphology of the typical cell. Critical examination of many cells, especially the more highly differentiated forms of glandular epithelium, shows the cytoplasm to contain a mesh-work com- posed of delicate fibrils and septa of the more active substance, the spongioplasm ; although conspicuous after appropriate staining, the spongioplastic net-work may be seen in the unstained and living cell, thereby proving that such structural details are not artefacts due to the action of reagents upon the albuminous substances com- posing the protoplasm. The interstices of the mesh-work are filled with a clear homogeneous semifluid material to which the name of hyaloplasm has been applied. Embedded within the hyaloplasm, a variable amount of foreign substances is frequently present ; these FIG. 6. A B Spermatogenic cells, showing variations in the condition and the arrangement of the constituents of the cyto- plasm and the nucleus ; the centrosomes are seen within the cytoplasm close to the nucleus. A, from the guinea-pig X 1685 (Meves} ; B, from the salamander X 5 (Meves) \ C, from the cat X 750 (von Lenhossek). include particles of oil, pigment, secretory products, and other extraneous materials, which, while possibly of importance in fulfilling the purposes of the cell, are not among its essential morphological constituents. These substances, which are inert and take no part in the vital activity of the cell, are termed collectively metaplasm. Cytoplasm consists, therefore, morphologically, of the spongioplasm and the hyaloplasm ; chemically, cytoplasm consists of certain organic compounds, salts and water. The organic compounds are grouped under the term' proteins, which are complex combinations of carbon, hydrogen, nitrogen, and oxygen, with often a small percentage of sulphur. The proteins of the cytoplasm contain little or no phosphorus. Structure of the Nucleus. The nucleus, during the vegetative condition of the cell, or the "resting stage," as often less accurately called, appears as a more or less spherical body whose outline is sharply defined from the surrounding cyto- plasm by a definite envelope, the nuclear membrane. Since the nucleus is the nutritive, as well as reproductive, organ of the cell, the fact that this part of the cell is relatively large in young and actively growing elements is readily explained. The nucleus consists of two parts, an irregular reticulum of nuclear fibres and an intervening semifluid nuclear matrix, therein resembling the cytoplasm. Exam- ined under high magnification, after appropriate treatment with particular stains, such as haematoxylin, safranin, and other basic dyes, the nuclear fibres are shown to be composed of minute irregular masses of a deeply colored substance, appropriately STRUCTURE OF THE CELL. 9 called chromatin in recognition of its great affinity for certain stains ; the chromatin particles are supported upon or within delicate inconspicuous and almost colorless threads of linin. The latter, therefore, forms the supporting net-work of the nuclear fibrils in which the chromatin is so prominent by virtue of its capacity for staining. The forms of the individual masses of chromatin vary greatly, often being irregular, at other times thread-like or beaded in appearance. Not infrequently the chromatin presents spherical aggregations which appear as deeply stained nodules attached to the nuclear fibres ; these constitute the false nucleoli, or karyosomes, as distinguished from the true nucleolus which is frequently present within the karyoplasm. Chemi- cally, chromatin, the most essential part of the nucleus, contains nuclein, a com- pound rich in phosphorus. The matrix, or nuclear juice, which occupies the interstices of the net-work, possesses an exceedingly weak affinity for the staining reagents employed to color the chromatin, and usually appears clear and untinted. It is probably closely related to the achroumtin and contains a substance described as paralinin. The nucleolus, or plasmosome, ordinarily appears as a small spherical body sometimes multiple lying among, but unattached to, the nuclear fibres ; its color in stained tissues varies, sometimes resembling that of the chromatin, although less deeply stained, but usually presenting a distinct difference of tint, since it responds readily to dyes which, like eosin or acid fuchsin, particularly affect the linin and cytoplasm. Concerning the exact nature, purpose, and function of the nucleolus much uncertainty still exists ; according to certain authorities, these bodies are to be regarded as storehouses of substances which are used in the formation of the chro- matin segments during division, while other cytologists attribute to the nucleolus a passive role, even regarding it as by-product which, at least in some cases, is cast out from the nucleus into the cytoplasm, where it degenerates and disappears. Since trustworthy observations may be cited in support of both of these conflicting views, definite conclusions regarding the exact nature of this constituent of the nucleus must be deferred. The nucleolus is credited with containing a peculiar substance known as pyrenin. The term amphipyrenin, as applied to the substance of the nuclear membrane, is of doubtful value. The Centrosome. In addition to the parts already described, which are con- spicuous and readily seen, the more recent investigations into the structure of cells show the presence of a minute body, the cen- trosome, which plays an important role in FIG. 7. elements engaged in active change, as con- spicuously during division and, in a lesser degree, during other phases of cellular activity. Ordinarily the centrosome escapes attention because, on account of its minute size and varia- ble staining affinity, it is with difficulty distin- guished from the surrounding particles. Its i> -.. usual position is within the cytoplasm, but the C ^ {, exact location of the centrosome seems to de- ' -.<, pend upon the focus of greatest motor activity, since, as shown by Zimmermann, this little C-/ m body, or bodies, being often double, is always found in that part of the cell which is the seat |v of greatest change ; thus, in a dividing ele- ment, the centrosome lies immediately related centimes (,, C ] in human%itheiium ; to the actively changing nucleus, while Within A, B, cells from gastric glands; C, from duo- ... , 'if -5" if it, denal glands; >, from tongue ; /, leucocyte with Ciliated epithelium it IS removed from the nu- centrosome X 625. (jr. W. Zimmermann.) cleus and is found closely associated with the contractile filaments which probably produce the movements of the hair-like ap- pendages. In recognition of the intimate relations between this minute body and the active motor changes affecting the morphological constituents of the cell, the cen- trosome may be regarded physiologically as its dynamic centre ; the name kino- centrum has been suggested by Zimmerman as best expressing this probable function of the centrosome. This little body is frequently surrounded by a clear 10 HUMAN ANATOMY. area or halo, the centrosphere or the attraction sphere, within which it appears as a minute speck, frequently being double instead of single. In recapitulation, the chief constituents of the animal cell may be tabulated as follows : f Meshwork Spongioplasm. Cytoplasm \ Ground-substance Hyaloplasm, containing inclusions, Meta- [ plasm. (Linin fibrils, f Nuclear reticulum consisting of -I Chromatin (containing Nu- ( clein). Nuclear matrix (containing Para/inin}. Nucleolus ( containing Pyrenin. ) Nuclear membrane. PROTOPLASM < | Centrosome Nucleus I DIVISION OF CELLS. Disregarding for the present, at least, the occurrence of direct fission as a means of producing new elements observed among the simplest forms of animal life, Diagram of mitosis. A, resting stage, chromatin irregularly distributed in nuclear reticulum; a, centrosphere containing double centrosome ; n, nucleolus. , chromatin arranged as close spirem ; c, c, centrosomes surrounded by achromatic radial striations. C, stage of loose spirem, achromatic figure forming amphiaster (amp). Z>, chro- matin broken into chromosomes ; nucleolus has disappeared, nuclear membrane fading ; amphiaster consists of two asters (a, a) surrounding the separating centrosomes, connected by the spindle (s). , longitudinal cleavage of the chromosomes which are arranged around the polar field (p) occupied by the spindle, f, migration of chromatic segments towards new nuclei, as established by centrosomes (c, c) \ ep, equatorial plate formed by intermingling segments. G, separating groups of daughter chromosomes (d, d) united by connecting threads (c t). H, daughter chromosomes (d, d) becoming arranged around daughter centrosomes which have already divided ; C, C, beginning cleavage of cytoplasm across plane of equatorial spindle. 7, completed daughter nuclei (D, D) ; cytoplasm almost divided into two new cells. (Modified from Wilson). or as an exceptional method among effete and diseased cells of the higher types, the production of new generations of cells may be assumed as accomplished for all DIVISION OF CELLS. ii varieties of elements by a complicated series of changes, collectively known as kar- yokinesis, or mitosis, especially affecting the nucleus. As already pointed out, in addition to presiding over the nutritive and chemical changes, the nucleus is par- ticularly concerned in the process of reproduction ; further, of the several morpho- logical constituents of the nucleus, the chromatin displays the most active change, since this substance is deeply concerned in transmitting the characteristics of the parent cell to the new elements. So essential is this substance for the perpetuation of the characteristics of each specific kind of cell that the entire complex mitotic cycle has for its primary purpose the insurance of the equal division of the chroma- tin of the mother cell between the two new nuclei, such impartial distribution of the chromatin taking place irrespective of any, or even very great, dissimilarity in the size of the daughter cells, the smaller receiving exactly one-half of the maternal chromatin. Mitotic Division. The details of karyokinesis, or mitosis, sometimes also spoken of as indirect division, include a series of changes involving the centrosome, FIG. 9. ABC D H 1 Chromatic figures in dividing cells from epidermis of salamander embryo. X 96- A < resting stage; B, close spireme ; C, loose spireme ; D, chromosomes (" wreath "), seen from surface; E, similar stage, seen in profile; F, longitudinal cleavage of chromosomes ; G, beginning migration of segments towards centrosomes ; //.separating groups of daughter segments ; /, daughter groups attracted towards poles of new nuclei, cytoplasm exhibits begin- ning cleavage. the nucleus, and the cytoplasm, which are conveniently grouped into four stages ; (i) the Prophases, or preparatory changes; (2) the Metaphase, during which the chromatin is equally divided ; (3) the Anaphases, in which redistribution of the chromatin is accomplished ; (4) the Telophases, during which the cytoplasm under- goes division and the daughter cells are completed. 12 HUMAN ANATOMY. In anticipation of the consideration of the details of mitosis, it should be pointed out that the process includes two distinct, but intimately associated and coinci- dent series, of phenomena, the one involving the chromatin, the other the centro- somes and the linin. While as a matter of convenience these two sets of changes are described separately, it must be understood that they take place simultaneously and in coordination. The purpose of the changes affecting the chromatin is the accu- rate and equal division of this substance by the longitudinal cleavage of the chroma- tin segments ; the object of the activity of the centrosomes and the linin is to supply the requisite energy and to produce the guiding lines by which the chromatin segments are directed to the new nuclei, each daughter cell being insured in this manner one-half of the maternal chromatin. The Prophases, or preparatory stages, include a series of changes which involve the nuclear substances and the centrosomes and result in the formation of the kary o kinetic figitre ; the latter consists of two parts, (i) the deeply staining chro- matin filaments, and (2) the achromatic figure, which colors but slightly if at all. The chromatin loses its reticular arrangement and, increasing in its staining affinities, becomes transformed into a closely convoluted thread or threads, constituting the " close skein ;" the filaments composing the latter soon shorten and thicken to form the " loose skein." The skein, or spireme, may consist of a single continuous fila- ment, or it may be formed of a number of separate threads. Sooner or later the skein breaks up transversely into a number of segments or chromosomes, which ap- pear as deeply staining curved or straight rods. A very important, as well as remark- able, fact regarding the chromosomes is their numerical constancy, since it may be regarded as established that every species of animal and plant possesses a fixed and definite number of chromosomes which appear in its cells ; further, that in all the higher forms the number is even, in man being probably twenty-four. . During these changes affecting the chromatin the nucleolus, or plasmosome, disappears, and, prob- ably, takes no active part in the karyokinesis ; the nuclear membrane likewise fades away during the prophases, the nuclear segments now lying unenclosed within the cell, in which the cytoplasm and the nuclear matrix become continuous. Coincident with the foregoing changes, the centrosome, which by this time has already divided into two, is closely associated with phenomena which include the ap- pearance of a delicate radial striation within the cytoplasm around each centrosome, thereby producing an arrangement which results in the formation of two stars or asters. The centrosomes early show a disposition to separate towards opposite poles of the cell, this migration resulting in a corresponding migration of the asters. In consequence of these changes, the retreating centrosomes become the foci of two systems of radial striation which meet and together form an achromatic figure known as the amphiaster, which consists of the two asters and the intervening spindle. Notwithstanding the observations which tend to question the universal importance of the centrosome as the initiator of dynamic change within the cell, as held by Van Beneden and Boveri, there seems to be little doubt that the centrosome plays an important role in establishing foci towards which the chromosomes of the new nuclei become attracted. The nuclear spindle, which originates as part of, or secondarily from the amphiaster, often occupies the periphery of the nucleus, whose limiting membrane by this time has probably disappeared. The delicate threads of linin composing the nuclear spindle lie within an area, the polar field, around which the chromosomes become grouped. The chromosomes, which meanwhile have arisen by transverse division of the chromatin threads composing the loose skein, appear often as V-shaped segments, the closed ends of the loops being directed towards the polar field which they encircle. Owing to this disposition, when seen from the broader surface, the chromosomes constitute a ring-like group, sometimes described as the mother wreath ; the same segments, when viewed in profile, appear as a radiating group of fibrils known as the mother star ; the apparent differences, therefore, be- tween these figures depend upon the point of view and not upon variations in the arrangement of the fibres. The Metaphase includes the most important detail of karyokinesis, namely, the longitudinal cleavage of the chromosomes, whereby the number of the latter is MITOTIC DIVISION. 13 doubled and the chromatin is equally divided. This division is the first step towards the actual apportionment of the chromatin between the new nuclei, each of which receives exactly one-half of the chromatin, irrespective of even marked inequality in the size of the daughter cells. Meanwhile the centrosomes have continued to separate towards the opposite poles of the cell, where, surrounded by their attraction spheres, each forms the centre of the astral striation that marks either pole of the amphiaster, the nuclear spindle being formed by the junction of the prolonged and opposing striae. The purpose of the achromatic figure is to guide the longitudinally divided chromosomes towards the new nuclei during the succeeding changes. The Anaphases accomplish the migration of the chromosomes, each pair of sister segments contributing a unit to each of the two groups of chromosomes that are passing towards the poles of the achromatic spindle ; in this manner each new nucleus receives not only one-half of the chromatin of the mother nucleus, but also the same number of chromosomes that originally existed within the mother cell, the numerical constancy of the particular species being thus maintained. Anticipating their passage towards the poles of the achromatic figure, the mi- grating chromatic segments, attracted by the linin threads, for a time form a com- pact group about the equator of the spindle known as the equatorial plate. As the receding segments pass towards their respective poles, the opposed ends of the sep- arating chromosomes are united by intervening achromatic threads, the connecting fibres. Sometimes the latter exhibit a linear series of thickenings known as the cell-plate or mid-body. The migration of the chromosomes establishes the essential features of the division of the nucleus, since the subsequent changes are only repe- titions, in inverse order, of the changes already noted. The Telophases, in addition to the final stages in the rearrangement of the chromatic segments of the new nuclei, including the appearance of the daughter wreath, the daughter skeins, the new nuclear membrane, and the nucleolus, witness the participation of the cytoplasm in the formation of the new cells. In these final stages of mitosis the cell-body becomes constricted and then divides into two, the plane of division passing through the equator of the nuclear spindle. Each of the resulting masses of cytoplasm invests a new nucleus and receives one-half of the achromatic figure consisting of a half-spindle and one of the asters with a centro- some. The new cell, now possessing all the constituents of the parent element, usually acquires the morphological characteristics of its ancestor and passes into a condition of comparative rest until called upon, in its turn, to enter upon the com- plicated cycle of mitosis. MITOTIC DIVISION. I. Prophases. A. Changes within the nucleus : Chromatic figure. Chromatin loses reticular arrangement, Close skein, Loose skein, Disappearance of nucleolus, Division of skein into chromosomes, Arrangement around polar field mother wreath, Disappearance of nuclear membrane. B. Changes within the cytoplasm : Achromatic figure. Division of centrosome, Appearance of asters, Migration of centrosomes, Appearance of spindle, Formation of amphiaster, Appearance of nuclear spindle and polar field. II. Metaphase. Longitudinal cleavage of chromosomes, i 4 HUMAN ANATOMY. III. Anaphases. Rearrangement of chromosomes into two groups, Migration of groups towards poles of amphiaster. Appearance of connecting fibres between receding groups, Construction of daughter nuclei. IV. Telophases. Constriction of cell-body appears at right angles to spindle, Chromosomes rearranged in daughter nuclei to form skeins, Reappearance of nuclear membrane, Reappearance of nucleoli, Complete division of cell-body, Daughter nuclei assume vegetative condition, Achromatic striation usually disappears, Centrosomes, single or divided, lie beside new nuclei. AMITOTIC DIVISION. The occurrence of cell reproduction without the foregoing complex cycle of karyokinetic changes is known as amitotic or direct division. That this process does take place as an exceptional method in the reproduction of the simplest forms of ani- mal life, or in the multiplication of cells within pathological growths or tissues of a transient nature, as the fcetal envelopes, may FIG. 10. be regarded as established beyond dispute. The essential difference between amitotic and the usual method of division lies in the fact that, while in the latter the chromatin of the nucleus is equally divided and the number of <|hromosomes carefully maintained, in direct division the nucleus remains passive and suffers cleavage of its total mass, but not of its indi- vidual components. Since the nucleus re- mains in the vegetative condition, neither the chromatic nor achromatic figure is pro- duced, the activity of the centrosome, when exhibited, being possibly directly expended in effecting a division of the cytoplasm, and inci- dentally that of the nucleus. In many cases the amitotic division of the nucleus is not ac- companied by cleavage of the cytoplasm, such processes resulting in the production of multi- In general, it may be assumed that cells which undergo direct division are elements destined to suffer premature degeneration, since such cells subserve special purposes and are not capable of perpetuating their kind by normal reproduction. Flemming has pointed out the fact that those leuco- cytes which arise by amitotic division, and therefore deviate from the usual mode of origin of 'these elements, are cells which are doomed to early death; this form of cell-division among the higher forms must be regarded, probably, as a secondary process. Decidual cells showing amitotic division of nucleus (A-D) ; in E an attempt at mitosis has occurred. X 410. nuclear and aberrant nuclear forms. EARLY DEVELOPMENT. THE human body with all its complex organism is the product of the differentia- tion and specialization of the cells resulting from the union of the parental sexual elements, the ovum and the spermatozoon. The Ovum. The maternal germ-cell is formed within the female sexual gland, the ovary, in which organ it passes through all stages of its development, from the immature differentiation of its early condition to the partially completed matura- tion of the egg as it is liberated from the ovary. The human ovum, in common with the ova of other mammals, is of minute size, being, as it is discharged from the ovary, about . 25 millimetre in diameter. Ex- amined microscopically and after sectioning, the human ovum is seen to be enclosed within a distinct envelope, the zona pellucida, .014 millimetre in thickness, which in favorable preparations exhibits a radial striation, and hence is also named the zona radiata. This envelope at first was confounded with the proper limiting mem- brane of the cell, and for a time was erroneously regarded as corresponding to the FIG. ii. -Corona radiata Zona pellucida Germinal vesicle (nu- cleus) containing germ- inal spot (nucleolus) If Zone rich in deutoplasm Zone poor in deutoplasm fading into homogene- ous peripheral zone Human ovum from ripe Graafian follicle. X 170. (Nagel.) cell-wall. The nature of the zona pellucida is now generally conceded to be that of a protecting membrane, produced through the agency of cells surrounding the ovum. The substance of the ovum, the yolk, or vitellus, consists of soft, semifluid pro- toplasm modified by the presence of innumerable yolk-granules, the representatives of the important stores of nutritive materials present in the bird's egg. Critically examined, the vitellus is resolvable into a reticulum of active protoplasm, or ooplasm, and the nutritive substance, or deutoplasm. At times the yolk is limited externally by a very delicate envelope, the vitelline membrane, which usually lies closely placed, or adherent, to the protecting zona radiata ; sometimes, however, it is separated from the latter by a perivitelline space. The vitelline membrane is probably absent in the unfertilized human ovum. A large spherical nucleus, \h& germinal vesicle, approximately .037 millimetre in diameter, usually lies eccentrically within the yolk, surrounded by the distinct nuclear membrane. Within the germinal vesicle the constituents common to nuclei in 15 i6 HUMAN ANATOMY. FIG. 12. D Head Neck j Connect- I ing piece Tail general are found, including the all-important chromatin fibrils, nuclear matrix, and nucleolus ; the latter, in the original terminology of the ovum, is designated as the germinal spot, and measures about .005 millimetre in diameter. In addition to these more easily distinguished components of the maternal cell, the centrosome must be accepted as a constant constituent of the fully formed, but unmatured, ovum, although its presence may escape detection. The Spermatozoon. The male germ-cell, the spermatic filament, is produced by the specialization of epithelial elements lining the seminiferous tubules within the testicle. The human spermatozoon consists of three parts the ovoid head, the cylindrical middle-piece, which includes the slightly-constricted neck and the connecting -piece, and the attenuated and greatly extended tail ; of these, the head and middle-piece are the most important, since these parts contain respectively the chromatin and the centrosome of the cells from which the spermatic filaments are derived. The centrosome is represented by two minute spherical bodies, the neck-granules, which lie in the neck immediately beneath the head, at the extremity of the axial fibre ; the latter extends throughout the spermatozoon from the head to the termi- nation of the tail, ending as an extremely attenuated thread, the terminal filament. The tail corresponds to a flagellum and serves the purposes of propulsion alone, taking no part in the important changes produced in the ovum by the entrance of the male element. Maturation of the Ovum. Maturation, or ripening of the ovum, is that process by which the female element is prepared for the reception of the spermatozoon. It takes place, however, entirely independently of the influence of the male or of the probability of fertilization, every healthy ovum undergoing these changes before it becomes sexually ripe. About the time that the ovum is liberated from the ovary by the bursting of the Graafian follicle, as the sac which encloses the egg within the ovarian stroma is called, its nucleus engages in the complicated cycle already described as mitotic division. The nucleus migrates to the periphery of the ovum, loses its limiting membrane, and un- dergoes division, one pole of the nuclear spindle being located within the protrusion of protoplasm which has coincidently taken place. With the division of the nuclear chromatin, the protruded protoplasm becomes constricted and finally separated from the ovum; the minute isolated mass thus formed, containing one-half of the maternal chromatin, is \hefirstpolar body. Almost imme- diately the mitotic cycle is repeated, and again results in the con- striction and final separation of a minute cell, the second polar body. These two isolated portions of the ovum remain visible for a long time as small, deeply stained cells lying within the perivitelline space beneath the zona pellucida. With each division of the egg-cell, one-half of the chromatin passes to the polar body, the matured ovum consequently retaining but one-fourth of the original chromatin. While the latter is thus diminished at each division, the masses of chro- matin are reduced to one-half the normal quota of chromosomes, this reduction being effected just before the first polar division. The chromatin remaining within the ovum after the repeated division becomes collected within a new nucleus, which now takes a non-central position within the egg, and is henceforth known as the female pronucleus or egg-nucleus. After mat- uration the ovum is prepared for union with the spermatozoon, although in many cases the male sexual element has actually entered the ovum before the completion of the maturation cycle : should, however, impregnation not occur, the ovum passes along the oviduct into the uterus and is finally lost. The passage of the human egg from the ovary to the uterus occupies, probably, about eight days, a period corresponding closely to the length of time that the ovum retains its capability of fertilization. The significance of the extrusion of the polar bodies a process which occurs Terminal filament Diagram of human spermatozoon; a, neck- granules, representing the centrosome: fe, axial fibre, X 1800. (Meves.) EARLY DEVELOPMENT. 17 with great constancy in almost all animals, and, indeed, is probably represented in the development of vegetal organisms as well has been the subject of much dis- cussion and speculation. The most satisfactory explanation of the significance of maturation has been proposed by Van Beneden, Boveri, and others, based upon the comparison of the changes which take place in the development of the germ-cells of the two sexes. In order to appreciate the necessity and the meaning of maturation of the ovum, it will be of advantage to take a brief survey of the phenomena attending the devel- opment of the male sexual elements. The seminiferous tubules of the testicle are lined with epithelial cells, certain of which, known as the primary spermatocytes, FIG. 13. D Semi-diagrammatic representation of the formation of the polar bodies based upon observations of invertebrate ova (Asca^T^Physa} T. , nucleus; c, c, centrosomes; 5, nuclear spindle; p', p", first and second polar bodies; e, egg-nucleus. (After Kostanecki and Wierzejski. ) increase in size and undergo division, the daughter cells constituting the secondary spermatocytes. Each of the latter, in turn, gives rise to a new generation, the sper- matids, from which the spermatozoa are directly formed, the chromatm of the sperma- tid being stored within the head, and the centrosome forming the neck-granules within the middle-piece. The spermatozoon, therefore, represents the third generation and corresponds to the mature ovum. Turning to the phenomena of maturation, a parallel process is presented, since the ovarian egg, or primary oocyte, divides into two cells, the secondary oocytes, represented by the ovum and the first polar body, each of which receives one-half of the chromatin, notwithstanding that one of the daughter cells, the first polar body is disproportionately small ; the repetition of division effects a second distribution of i8 HUMAN ANATOMY. the chromatin, so that the mature egg, after the completion of maturation, represents the third generation, and is, therefore, morphologically equivalent to a spermatozoon. Attention has already been directed to the important fact that the cells of a given species contain a fixed, definite, and even number of chromosomes (page 12); hence, in their primary condition, each germ-cell contains the full complement of chromatin segments. Since, however, the new being arises from the elements derived from the segmentation of a cell to the nucleus of which both parents con- tribute an equal number of chromosomes, it follows that, unless some provision be made whereby the number of chromosomes in each germ-cell be reduced to one- Primordial germ-cell MALE Spermatogonia Primary sperma- tocyte Secondary sper- matocytes Spermatids Spermatozoa FIG. 14. Division-period ' Growth-period Maturation-period Primordial germ-cell FEMALE Oogoriia Primary oocyte (ovarian egg) Secondary oocytes (egg and first polar body) Mature egg and polar bodies Diagram illustrating the genesis of the male and female germ-cells. (After Boveri.) half the full number, the elements of the new being would be provided with double the number required to satisfy the normal complement for the particular species. In fact, such reduction of the chromosomes of the germ-cells does take place during the development of these elements, in consequence of which the ovum and the spermatozoon each contribute only one-half the number of chromosomes, the nor- mal quota being restored to the segmentation nucleus, and subsequently to the cells of the new being, by the sum of the contributions of both parents. Interpreted in the light of these considerations, maturation may be regarded as the means by which correspondence between the sexual cells is secured, and, further, the polar bodies may be considered as abortive ova. Fertilization of the Ovum. Impregnation, or fertilization of the ovum, includes the meeting of the male and female elements, the penetration into the sub- stance of the latter by the former, and the changes immediately induced by the presence of the spermatozoon within the egg. Coincidently with the rupture of the distended Graafian follicle, the surface of the ovary is embraced by the expanded fimbriated extremity of the oviduct, along the plications of which the liberated matured ovum is guided into the tube. It is highly probable that not an inconsiderable number of the ova discharged from the ovary fail to reach the oviduct and are lost in the abdominal cavity. Recent investigations have shown that both germ-cells contain particular acces- sory chromosomes, which are probably important factors in the determination of the sex of the new being. The spermatozoa overcome the obstacles offered within the narrow channels by the mucus and the opposed ciliary currents of the uterine and tubal mucous membranes by virtue of their long actively vibrating tails, and advance at a rate estimated at from 1.5 to 2.5 millimetres per minute ; it is therefore probable that the seminal cells accomplish the journey from the mouth of the uterus to the ovum in from eight to ten hours. Spermatozoa retain their vitality and fertilizing pow- ers for many days within the normal female genital tract ; repeated observation on the human subject has shown that this period may extend throughout an entire FERTILIZATION OF THE OVUM. menstrual cycle of twenty-eight days, a possibility to be remembered when calcu- lating the probable termination of pregnancy. Of the many millions of spermatic elements deposited within the vagina, only FIG. 15. B ff K M Fertilization of the ovum as illustrated lay sections of the eggs of the mouse. (Sobotta.) All the figures are magnified 437 diameters except D-G, in which the amplification is 1310 diameters. A-C, prophases of formation of first polar body (p) ; z, zona pellucida ; , nuclear figure; m, head of spermatozoon. >-G, entrance of spermato- zoon (j) into ovum and subsequent changes. H-M, sequence of changes during the formation, approach, and blend- ing of the male (m) and female (/) pronuciei ; p,p, polar bodies. an insignificant number ever reach the vicinity of the ovum. Notwithstanding that probably a number of spermatozoa penetrate the zona pellucida, normal fertili- zation in man and the higher animals is effected by a single seminal element. After 20 HUMAN ANATOMY. the entrance of the favored spermatozoon into the substance of the ovum, an effec- tual barrier to the penetration of additional seminal cells is presented by the thick vitelline membrane which immediately forms. The point at which the spermatozoon is about to enter the egg is indicated by a conical elevation, the receptive eminence, into which the male germ-cell sinks, the tail only partly entering the protoplasm of the egg and very soon disappearing. The position of the remains of the spermatozoon within the substance of the ovum is indicated by an ovoid body, the male pronudeus, which contains the chro- matin and centrosome of the paternal germ-cell. The sperm-nucleus and the egg- nucleus > as the male and female pronuclei are now often designated, usually break up into their respective chromosomes without fusing into a single segmentation FIG. 1 6. B C cont Early stages of segmentation as seen in ova of mouse, surface view. X 450. (Sobotta.) The external double tour represents the zona pellucida; the cell marked with X, the polar body. A, fertilized ovum at stage of the pronuclei; , two segmentation spheres of equal size; C, segmentation spheres of unequal size; Z>, three-cell stage resulting from division of larger sphere ; , stage of four spheres ; F, six ; G, eight ; //, sixteen ; /, twenty-five. nucleus. In this case the two groups of chromosomes unite in the first mitotic figure, the segmentation spindle (Fig. 17). After the fusion of the pronuclei, and just as segmentation is beginning, the fertilized ovum presents a clear oval area which contains the two groups of chromo- somes contributed by the germ-cells of both parents ; on opposite sides of the chro- matin figure are the centrospheres, each containing a centrosome and surrounded by a marked polar striation within the substance of the egg. The centrosomes now present within the ovum are usually both derived from the substance of the cen- trosome of the spermatid, which entered the ovum as the neck-granules within the middle-piece of the fertilizing spermatozoon. The role of the latter, therefore, is two- fold, to contribute the chromatin necessary to restore to the parent cell the normal SEGMENTATION OF THE OVUM. 21 complement of chromosomes, and to furnish the stimulus required to inaugurate the karyokinetic cycle of segmentation. Segmentation of the Ovum. The union of the male and female pronuclei and the resulting formation of the segmentation nucleus is followed immediately by the division of the ovum into two new elements ; each of these gives rise to two additional cells, which, in turn, produce following generations of segmentation cells, or blastomeres. This process of repeated division of the fertilized ovum and its H Early stages of segmentation as seen in sections of ova of mouse. X 500. (Sobotta.) A-D show the rearrange- ment of the chromosomes contributed by the male (m) and female (/) pronuclei as preparatory to the first cleavage of the fertilized ovum; f>, f>, polar bodies; ep, stage of equatorial plate; a, 6, daughter groups of chromosomes. E, F, the daughter cells arising from first cleavage. G, one cell (b) is larger and is preparing to divide. //, later stage of this division. /, stage of three segmentation spheres (a and c, c) resulting from this division. descendants constitutes segmentation, a process common to the development of all animals and plants above the very simplest. Study of the details of segmentation in the various classes of animals shows that a close relation exists between the character of the cleavage and that of the ovum with regard to the amount and distribution of the nutritive yolk, or deuto- plasm, present. In the human and mammalian egg the nutritive yolk particles are compara- tively meagre and are uniformly distributed throughout the vitellus ; in such eggs there is no aggregation of the food particles, hence such ova are termed homolecithal or with a homogeneous yolk. In the eggs of birds, reptiles, and fishes, on the con- 22 HUMAN ANATOMY. trary, the deutoplasm, or nutritive material, is collected towards one pole of the egg, while the protoplasm, or formative material, is limited to the other ; eggs in which these conditions obtain possess a distinctly polar yolk, and hence are known as telolecithal ova. These aggregations of the protoplasm and the deutoplasm con- stitute respectively the formative and the nutritive yolk, and correspond in position to the animal and the vegetative poles of the egg. In an additional class of eggs, the centrolecithal, the yolk occupies the centre of the ovum, being covered by a peripheral zone of formative material ; since such ova belong alone to certain in- sects and are not found among vertebrates, they possess limited interest to students of mammalian forms. Comparison of the behavior of these various groups of ova during segmen- tation shows that only eggs poor in deutoplasm, as the alecithal mammalian and amphibian ova, undergo complete cleavage during segmentation, those of the bird, reptile, and fish undergoing cleavage only within the formative yolk. Ova, there- fore, are classified according to the completeness of their division into those exhibit- ing complete segmentation and those undergoing partial segmentation ; the former are known as holoblastic, the latter as meroblastic. The embryologist further recog- nizes an equal and an unequal complete segmentation according to the equality or inequality of the cells, or blastomeres, resulting from the division of the ovum. Since the segmentation spheres derived from the mammalian egg may be regarded as practically of equal size, the egg of this class of animals, including the human ovum, is described as an homolecithal holoblastic ovum, undergoing equal segmenta- tion. It must be understood, however, that even in the segmentation of such ova FIG. 19. FIG. 18. Outer cells Zona pellucida Inner cells .Trophoblast Ectoblast Entoblast Diagram of early mammalian blastodermic vesicle, consisting of trophoblast and inner cell-mass. (After Van Beneden.) Trophoblast Zona pellucida Diagram of mammalian blastodermic vesicle ; inner cells differentiating into ectoblast and entoblast. (After Van Beneden.) the blastomeres very early exhibit inequality in size and in rapidity of division (Fig. 16), the effect of this differentiation being, that the more rapidly multiplying blas- tomeres are smaller than the more slowly dividing elements. It is of interest, in this connection, to note that the purest type of total equal segmentation is observed in the ovum of the lowest vertebrate, the amphioxus, an animal whose development has shed much light on many obscure problems in the embryology of the higher forms, including mammals and even man. The meroblastic bird's egg, on the contrary, undergoes cleavage only within a limited circular field at its animal pole ; it is said, therefore, to undergo partial dis- coidal segmentation. In contrast to this, the centrolecithal ova exhibit partial super- ficial segmentation, the peripheral zone of formative material alone undergoing cleavage. The Blastoderm and the Blastodermic Layers. The completion of segmentation in holoblastic ova results in the production of a mass of blastomeres, which is a solid sphere composed of mutually compressed segmentation cells ; to this sphere the older anatomists gave the name of the morula, or the mulberry mass. The solidity of the morula is temporary, since a cavity is soon developed within it. This cavity, often called the segmentation cavity, increases to such an extent that a THE BLASTODERMIC VESICLE. hollow sac is formed, walled by a single layer of cells, at one point on the inner sur- face of which is attached a small mass of cells. The outer, covering layer of cells is known as the trophoblast ; the small group of cells attached to the inner surface of the trophoblast is known as the inner cell-mass (Fig. 18). Examined from the sur- face, this aggregation of inner cells appears as an opaque circular field, the embryonic area, due to the increased thickness and consequently diminished transparency of the wall of the blastodermic vesicle at the place of attachment of the included cells. In the purest type of the blastodermic vesicle, that seen in the amphioxus (Fig. 26, A}, the sac consists of a single layer of blastomeres of almost uniform size ; the mammalian blastodermic vesicle, however, presents greater complexity, due to the unequal rate at which some of the segmentation cells divide and to the rapid increase in the size of the vesicle. The inner mass of germinal cells soon undergoes differentiation (Fig. 19) into two strata, an outer layer, closely applied to the trophoblast, and an inner layer. These layers are respectively the ectoblast and the entoblast, two of the three great primary blastodermic layers from which the embryo is differentiated. Coincidently with the formation of these germinal layers, the mammalian blas- todermic vesicle grows with great rapidity, increasing from a sphere of microscopic size to a vesicle of one or more millimetres in diameter. In consequence of this growth, the trophoblast undergoes great expansion, its cells becoming reduced to flattened elements, which, over the embryonic area, later disappear. In some ani- mals, as in the rabbit, the flattened trophoblast cells extend over the embryonic ectoblast and have been called the cells of Rauber. In such cases, therefore, the ectoblast is overlaid within the embryonic area by the cells of Rauber, but at the margin of the area, the embryonic ectoblast is continuous with the trophoblast form- ing the outer layer of the wall of the blastodermic vesicle. With the subsequent expansion of the blastodermic vesicle, the cells of Rauber disappear from the surface of the embryonic ectoblast, which then lies upon the surface of the vesicle (Figs. 20, 21). FIG. 21. FIG. 20. Entoblast Trophoblast Embryonic ectoblast Mesoblast Diagram of mammalian blastodermic vesicle ; the entoblast forms an almost complete inner layer. Entoblast Trophoblast Diagram of mammalian blastoderniic vesicle ; the mesoblast is just appearing as the third blastodermic layer. The early blastodermic vesicle at first consists of only two primary layers, the ectoblast and the entoblast ; this stage of development is appropriately termed that of the bilaminar blastoderm (Fig. 20); a little later, a third layer, the mesoblast, makes its appearance between the outer and inner blastodermic sheets ; this stage is designated as that of the trilaminar blastoderm ( Fig. 21). The early embryo, shortly after the formation of the blastodermic vesicle, con- sists of three layers of cells, the ectoblast, the mesoblast, and the entoblast. The histological characters of the outer and inner of these primary layers differ, almost 24 HUMAN ANATOMY. from the first, from those of the mesoblast, their component elements being more compact in arrangement and early manifesting a tendency to acquire the character- istics of covering cells or epithelium. The mesoblastic elements, on the contrary, soon assume irregular forms and are loosely held together by intercellular substance, thus early foreshadowing the special features which distinguish the subsequently differentiated connective tissues. This early distinction becomes more marked as differentiation proceeds, the epithelial tissues possessing elements of comparatively regular form, separated by minute amounts of intercellular substance ; the latter in the connective tissues, on the con- trary, becomes conspicuous on account of its excessive quantity and the resulting profound modifications in the physical character of the tissue; the cells of the con- nective tissues rapidly assume the irregularly stellate or triangular form so charac- teristic in young tissues of this class. Since the three primary layers give rise to all the tissues of the organism, a brief synopsis presenting these genetic relations here finds an appropriate place. DERIVATIVES OF THE BLASTODERMIC LAYERS. From the ectoderm are derived The epithelium of the outer surface of the body, including that of the conjunc- tiva and anterior surface of the cornea, the external auditory canal, to- gether with the epithelial appendages of the skin, as hair, nails, sebaceous and sweat-glands (including the involuntary muscle of the latter). The epithelium of the nasal tract, with its glands, as well as of the cavities communicating therewith. The epithelium of the mouth and of the salivary and other glands opening into the oral cavity. The enamel of the teeth. The tissues of the nervous system. The retina ; the crystalline lens, and perhaps part of the vitreous humor. The epithelium of the membranous labyrinth. The epithelium of the pituitary and pineal bodies. From the mesoderm are derived The connective tissues, including areolar tissue, tendon, cartilage, bone, den- tine of the teeth. The muscular tissues, except that of the sweat-glands and dilator pupillae. The tissues of the vascular and lymphatic systems, including their endothelium and circulating cells. The sexual glands and their excretory passages, as far as the termination of the ejaculatory ducts and vagina. The kidney and ureter. From the entoderm are derived The epithelium of the digestive tract, with that of all glandular appendages except those portions derived from ectodermic origin at the beginning (oral cavity) and termination of the tube. The epithelium of the respiratory tract. The epithelium of the urinary bladder and urethra. The epithelium of the thyroid and thymus bodies, the modified primary epithe- lium of the latter giving rise to Hassall's corpuscles. The Primitive Streak and the Gastrula. Examined from the surface during the formation of the primary layers, the mammalian blastodermic vesicle, as represented by that of the rabbit, presents a circular light-colored field, the embryonic area, which corresponds to the expansion of the original embryonic spot, the latter becoming larger with the extension of the ectoblast and the entoblast differentiated from the inner cell mass. At first circular, the embryonic area later becomes oval or pyriform in outline (Figs. 22, 23), the larger end corresponding with the cephalic THE EMBRYONIC AREA. 25 pole of the future embryo. In consequence of the proliferation of the ectoblastic cells, the embryonic area becomes differentiated into a central field, the embryonic shield, and a peripheral zone, the area pellucida, which by transmitted light appear respectively dark and light, owing to the varying transparency of the thicker cen- tral and thinner peripheral portions of the germinal field. FIG. 22. FIG. 23. -Area pellucida -Embryonic shield -T Embryonic shield / '' f 1 Area pellucida CTB Wall of blastodertnic vesicle. Wall of blastoder- mic vesicle Embryonic area of rabbit of about six and one- Embryonic area of rabbit of about seven days, seen from half days, seen from the surface by transmitted light. the surface. X 26. (Kollmann.) X 26. (Kollmann.) Coincidently with the assumption of the oval or pyriform outline, a linear thick- ening of ectoderm appears towards the smaller end of the embryonic area. This is the primitive streak, which grows backward from a terminal thickening, the node of Hensen, that marks its anterior extremity. The primitive streak indicates the direc- tion of the longitudinal axis of the future em- bryo and is modelled by a shallow furrow, FIG. 24. the primitive groove, due to the proliferation of the surrounding ectoderm. From the s/" sides of the primitive streak cells are budded ( off to form the mesoderm, which grows -Head process between the outer and inner germ-layers ; until it, finally, surrounds the blastodermic i Hensen'snode vesicle. At first, the mesoblast extends \ , . -4 Primitive streak laterally and posteriorly and, later, grows forward as two lateral wings, that embrace ^J Embryonic area the head-end of the embryonic area. While for a time attached only to the ectoderm, - Extra-embryonic the primitive streak subsequently fuses with Serrnk vesicle the entoderm, so that sections across the ; : Streak Show all the _ germ-layers blended Embryonic area of n^bit of about eight days, en The primitive Streak IS a transient organ and from the surface. X 20. (After Van Beneden.) later entirely disappears ; it contributes, however, the rapidly growing mesoblastic tissue, which later becomes related to the anal region and the tail-bud. The Significance of the Primitive Streak and the mode of formation of the mesoblast are vexed problems in embryology. A brief note on this topic will suffice here. In amphioxus, the lowest vertebrate, the immediate result of segmentation is a hollow sphere, ti\zblastula, filled with fluid, lined by a single layer of cells. Invagi- nation at one point of the wall of the blastula occurs, forming eventually a two-layered cup, the gastrula, the outer layer of which is the ectoblast, and the inner one the ento- blast. The cavity within the entoblast is the archenteron or primitive gut. The open- ing into the archenteron is the blastopore. Cells given off from the entoblast, near the blastopore, form a third layer, the mesoblast. Typical gastrulation does not occur in the higher animals, although in the early human embryo a canal appears, known as the neurenteric canal, the opening of which is often regarded as homologous with the blastopore. The primitive streak is regarded by some authorities, notably Hert- wig, as an elongated blastopore with lips fused. Since the primitive streak, the prod- uct of the outer germ-layer, is the principal primary source of the mesoblast, the latter may be regarded as indirectly derived from the ectoblast. A limited secondary and later production of mesoblast is attributed by some to the inner germ-layer. 26 HUMAN ANATOMY. THE FUNDAMENTAL EMBRYOLOGICAL PROCESSES. Shortly after the appearance of the primitive streak a structure, it will be remembered, which is transient and only indirectly takes part in the formation of the embryo proper a series of phenomena mark the earliest stages of the future new being. These changes are known as the fundamental embryological processes, and result in the formation of the neural canal, the notochord, and the somites. While described for convenience as separate processes, they progress to a great extent simultaneously. FIG. 25. Ectoblast soblast Transverse section through cephalic end of primitive streak of very young rabbit embryo. X too. The Neural Canal. The earliest indication of the embryo consists in the appearance of two slightly diverging folds (Fig. 27), enclosing the anterior end of the primitive streak, which are produced by a local proliferation and thickening of the ectoblast. These are the medullary folds and mark the beginning of the formation of the neural canal, from which the great cerebro-spinal nervous axis, together with its outgrowths, the peripheral nerves, is derived. The medullary folds at first border a shallow and widely open furrow (Fig. 28), the medullary groove ; A FIG. 26. B en entoblast ; m, mesoblast cell ; b, blastopore, leading into archenteron, the latter becomes rapidly deeper and narrower as the medullary folds increase in height and gradually approach each other. The approximation of the folds (Fig. 29) and subsequent fusion take place earliest at some distance behind the cephalic end of the groove, at a point which later corresponds to the upper cervical region of the spinal cord. After the closure of the groove and its conversion into the medullary canal (Fig. 32), the thickened and invaginated ectoblast forming the lining of the neural tube becomes separated from the outer layer of the embryo by the ingrowth of the THE NOTOCHORD. 27 FIG. 27. mesoblast. The subsequent differentiation of the walls of the neural tube will be more fully considered in connection with the nervous system ; suffice it here to state that the cephalic portion expands into the brain vesicles, and subsequently becomes the brain with the contained ventricles, while the remainder of the tube becomes the spinal cord, enclosing the minute central canal. The N o to chord. Coinci- dently with the formation of the med- ullary groove the entoblast opposite the bottom of that furrow exhibits proliferation and thickening ; the group of cells thus differentiated be- comes separated from the general mass of the inner layer and takes up a position immediately below the neural tube (Figs. 30, 31). This isolated column of entoblastic cells constitute the noiochord, or chorda dorsalis, the earliest Suggestion of Embryonic area ^T^bbit of about eight and one-half days, the Cardinal Vertebrate axis around seen from the surface. X24. (Kollmann.) which the parts of the early embryo are symmetrically arranged. While for a time constituting the sole longitudinal axis of the embryo, extending from a point near the cephalic pole, which corresponds later Medullary fold Medullary groove Wall of blastoder- mic vesicle Primitive streak Embryonic area Medullary fold FIG. 28. Groove Medullary fold Transverse section of rabbit embryo of about eight and one-half days. X 80. Future neural canal is represented by widely open groove. Amniotic sac FIG. 29. Closing neural canal Somatopleura Body-cavity Body-cavity Visceral mesoblast Entoblast Chorda Open gut-tube Splanchnopleura Transverse section of rabbit embryo of about nine and one-quarter days. X 80. Neural canal is just closing. to the base of the skull, to the caudal extremity, the notochord is but a temporary structure, and subsecmently is supplanted by the true vertebral column. It is 28 HUMAN ANATOMY. interesting to note, that in the connecting link between the vertebrates and invertebrates, the amphioxus, the notochord remains as the permanent and sole spinal axis. The history of the notochord in man and mammals presents three stages : (a) it exists as an unbroken cord which extends uninterruptedly through the series of cartilaginous vertebrae ; (b) the notochord suffers segmentation in such manner that the breaks in its continuity correspond to the vertebral bodies, conspicuous proliferation and local increase in its substance, on the contrary, marking the Transverse sections through axis of early human embryo of about fifteen days, showing formation of notochord from entoblast. High magnification. (After Kollmann.) n, neural canal; ch, cells forming notochord differenti- ating from entoblast (e) ; m, mesoblast ; s, early somite ; b, sections of primitive aortae. position of the intervertebral disks in which the chordal tissue during the first months after birth is represented by a considerable mass of central spongy substance ; (V) atrophy of the remains of the notochord, resulting in the entire disappearance of the chordal tissue within the vertebrse and the reduction of the proliferated intervertebral cell-mass to the pulpy substance existing within the intervertebral disks. The cephalic end of the notochord in man corresponds in position to the dorsum sellae, and marks the division of the skull into two parts, that lying in front of FIG. 32. Paraxial mesoblast Ectoblast Amniotic cavity Somatopleura Body-cavity Splanchno- Open Ento- Chorda Neural Visceral Body-cavity pleura gut blast tube mesoblast Transverse section of rabbit embryo of about nine and one-quarter days. X 80. Neural canal is now closed. the termination of the notochord, the prechordal portion, and that containing the notochord, the chordal portion ; the latter is sometimes described as the vertebral segment of the skull. The Coelom. The downward growth of the neural ectoblast and the upward extension of the chordal entoblast effect a division of the mesoblast along the embryonic axis into two sheets (Fig. 28). These latter undergo further division in consequence of the formation of a cleft within their substance, as the result of which the mesoblast becomes split into two layers enclosing a space, the ccelom, or primary body-cavity (Fig. 29). THE SOMITES. 29 The cleavage of the mesoblast, however, does not extend as far as the mid-line of the embryo, but ceases at some distance on either hand, thus leaving a tract of uncleft mesoblast on either side of the medullary groove and the chorda. The uncleft area constitutes the paraxial mesoblast (Fig. 32), which extends from the head towards the caudal pole and appears upon the dorsal surface of the embryo as two distinct ribbon-like tracts bordering the neural canal. Beyond the paraxial mesoblast, the cleft portions of the middle layer extend on either side as the lateral plates ; each lateral plate consists of two laminae, the one forming the dorsal and the FIG. 33. Neural canal Somite Chorda Intermediate cell- mass Primary gut-tube Parietal mesoblast Visceral mesoblast FlG. 34- Transverse section of human embryo of about fifteen days, showing early differentiation of somite. X 210. (Kollmann.) Transverse section of human embryo of about twenty-one days, showing differentiation of somite. X 90. (Kollmann.) FIG. 35. Muscle- plate Dorsal border of myotome Cutis-plate other the ventral boundary of the enclosed primary body-cavity ; in view of their subsequent relations to the formation of the body-walls and the digestive tube respectively, the dorsal mesoblastic lamina is appropriately named the parietal layer and the ventral lamina the visceral layer (Fig. 32). In the separation of these layers, which soon takes place in consequence of the dorsal and ventral folding occurring during the formation of the amnion and the gut-tube, the parietal mesoderm adheres to the ectoblast, in conjunction with which it constitutes the somatopleura (Fig. 29), the ecto-mesoblastic sheet of great importance in the production of the lateral and ventral body- walls. Similarly, the visceral mesoblast unites with the entoblast to form the splanchnopleura (Fig. 29), the ento-mesoblastic layer from which the walls of the primary digestive canal are formed. The Somites. The paraxial mesoblast at an early stage about the twentieth day in man exhibits indications of transverse division, in consequence of which this band-like area becomes differentiated into a series of small quadrilateral masses, the somites, or protovertebrcz. This segmentation of the embryonic mass appears earliest at some distance behind the cephalic end of the embryo, at a point which later corresponds to the beginning of the cervical region. The somites are seen to best advantage in the human embryo at about the twenty-eighth day (Fig. 71). The early somites, on transverse section, appear as irregular quadrilateral bodies, composed of mesoblast and covered externally by ectoblast, lying on either side of the neural canal and the notochord (Fig. 33). Each somite consists of a dorsomesial principal cell-mass, which is connected with the lateral plate by means of an intervening cell-aggregation, the intermediate cell-mass (Fig. 33). Subsequently, Differentiation of myotome of human embryo of about twenty-one days. X 525. (Kollmann.) 30 HUMAN ANATOMY. the latter becomes separated from the remaining portion of the somite and is probably identified with the formation of the segmented excretory apparatus of the embryo, the Wolffian body, and hence is known as the nephrotome. The principal mass, including the greater part of the somite proper, consists of an outer or peripheral zone of condensed mesoblast enclosing a core of looser struc- ture. The less dense mesoblastic tissue later breaks through the surrounding zone on the side directed towards the notochord and forms a fan-shaped mass of embryonic connective tissue which envelops the chorda and grows around the neural canal. The cell-mass derived from the core of the myotome constitutes the sclerotome, and directly contributes the tissue from which the permanent vertebrae and the associated ligamentous and cartilaginous structures arise. The remaining denser part, the myotome, which collectively forms a compressed C-like mass, becomes differentiated into a lateral and a mesial stratum (Fig. 35). The lateral stratum, sometimes called the cutis-plate, consists of several layers of closely packed elements. By some these cells are regarded as concerned in producing the connective tissue portion of the skin ; according to others they are in large part converted into myoblasts, which, with those of the mesial stratum, or muscle-plate, give rise to the voluntary muscles of the trunk. The genetic relations of the somite, therefore, may be expressed as follows : J Myotome muscle segment. Sclerotome axial segment. Nephrotome excretory gland segment. The number of somites of the human embryo is about thirty-seven, comprising eight cervical, twelve thoracic, five lumbar, five sacral, and from five to seven caudal segments. THE FCETAL MEMBRANES. The Amnion. With the exception of fishes and amphibians, animals whose development takes place in water, the young vertebrate embryo is early enveloped in a protecting membrane, the amnion. Animals possessing this structure, including reptiles, birds, and mammals, are classed, therefore, as amniota,'\n contrast to the anamnia, in which no such envelope is formed. An additional foetal appendage, the allantois, is always developed as a structure complemental Embryonic area /Embryonic ectoblast to the amnion ; hence the am- ,Mesoblast n j ota p OSSess b^h amn i on an( J ^Entobiast allantois. Since the development of the foetal membranes in man presents certain deviations from the process as seen in other mammals, due to pecu- liarities affecting the early human embryo, it is desirable -cavity of biastodermic f ? examine briefly the forma- intobiast vesicle tion of these structures as ob- -Trophobiast served in animals less highly specialized. Referring to the early an biastodermic vesicle. mammalian embryo, in which the biastodermic layers are arranged as somatopleura and splanchnopleura on either side of the embryonic axis and^the surrounding uncleft mesoderm, and extend as parallel sheets over the en- larging biastodermic vesicle, the first trace of the amnion appears as a duplicature of the somatopleura. The earliest indication of the process is seen slightly in front of the cephalic end of the embryo, the resulting head-fold being, however, soon fol- lowed by the appearance of the lateral and tail-folds. The rapid growth of these THE AMNION. FIG. 37- Serosa duplicatures of somatopleura from all sides results in the encircling of the embryo within a wall which increases in height until the prominent edges of the folds meet and coalesce over the dorsal aspect of the enclosed embryo. The folds of the amnion first meet over the head-end, from which point the union extends tailward, where, however, fusion may be delayed for some time. The line along which the junction of the folds takes place is known as the amniotic suture. The amnion thus forms a closed sac completely in- vesting the embryo and con- taining a fluid, the liquor amnii ; at first closely sur- rounding the embryo, the amniotic sac rapidly expands until its dimensions allow the enclosed foetus to turn freely, practically supported by the amniotic fluid, which pos- sesses a specific gravity of 1003. It has long been known that in certain forms, conspicuously in the chick, the amnion executes rhythmi- cal contractions, at the rate of ten per minute, whereby Ectoblast "I [Amnion Mesoblast) Exoccelom pen gut-tube Splanchnopleura Trophoblast 'Vitelline sac Entoblast Diagram showing fornnu75rroT"arnniotic folds and of gut-tube ; trans- verse section of axis of embryo. the embryo is swayed from end to end of the sac. From the manner of its formation, as folds of the somatopleura (Figs. 37 and 38), it is evident that the amnion consists of an inner ectoblastic and an outer mesoblastic layer. The Serosa, or False Amnion. Coincident with the fusion of the inner layers of the somatopleuric folds to form the closed sac of the amnion, the outer layers of the same folds unite to FIG. 38. Serosa -Amnion lotic sac Gut-tube Exocoelom produce a second external en- velope, the serosa, or false am- nion. The serosa soon becomes separated from the amnion by an intervening space to form the primitive chorion ; the latter, therefore, consists of ectoblastex- ternallyand mesoblast internally, the reverse of the disposition of these layers in the amnion. The outer surface of the mammalian primitive chorion the entire envelope formed of the serosa and the trophoblast is distinguished by prolifera- tion of the epithelial elements, which process results in the production of more or less con- spicuous projections or villi (Fig. 40), this villous condition being particularly well marked in man. The ectoblast of the primitive chorion takes no part in the formation of the body of the embryo, but, on the other hand, assumes an important role in establishing the earliest connection between the embryo and the maternal tissues and, later, participates in the formation of the placenta. The ectoblast of the Vitelline sac Trophoblast Entoblast Diagram showing formation of amniotic folds and vitelline sac ; longitudinal section of embryo. 32 HUMAN ANATOMY. primitive chorion is the direct derivative of the original ectodermal layer of the blastodermic vesicle beyond the embryonic region proper, a layer which, on account of this important nutritive function, has been called by Hubrecht the trophoblast. As already noted (Fig. 32), the cleft between the parietal and visceral layers of the mesoblast is the primary body -cavity or ccelom ; with the separation of these layers following the dorsal and the ventral folding associated respectively with the formation of the amniotic sac and the gut-tube, the intramesoblastic space becomes greatly expanded and extends between the amnion and primitive chorion. This large space is appropriated only to a limited extent by the future definite body- cavity, and hence is divisible into an embryonic and an extra-embryonic portion, or exoccelom (Fig. 38), which are temporarily continuous. The Vitelline Sac. While the somatopleura is engaged in producing the protecting amniotic sac, the splanchnopleura, composed of the entoblast and the adherent visceral layer of mesoblast, becomes approximated along the ventral sur- face of the embryo to define the primitive gut-tube by enclosing a part of the blastodermic vesicle ; the remaining, and far larger, portion of the latter cavity constitutes the vitelline sac, and corresponds to the yolk-sac of the lower forms. The constriction and separation of the gut-tube from the vitelline sac is accom- plished earliest at the Jj^JG^g^ cephalic and caudal ends -Primitive chorion o f t h e future alimentary canal, the intervening por- Ammon . . . , & *. tion remaining tor a time in widely open communi- -Ammotic sac . . . , cation with the yolk-sac. During the rapid diminu- -Gut-tube . ? . . , tion of the latter the com- munication becomes re- -Cceiom duced to a narrow channel, the vitelline diict, which persists as a slender stalk terminating at its distal end in the remains of the yolk- sac. In animals other than mammals in which a pla- Aiiantois centa is developed, the yolk-sac is the chief nutri- tive organ of the embryo ; Vitelline duct the mesoblastic tissue of the vesicle becomes vascu- Diagram showing completed amnion and serosa, beginning allantois and , . , . . . vitelline duct. larized by the development of the blood-vessels consti- tuting the vitelline circulation, of which the vitelline or omphalomes enteric arteries and veins form the main trunks. The contents of the yolk-sac as such do not directly minister to the nutrition of the embryo, but only as materials absorbed by the vitelline blood-vessels. In man and other high mammals the nutritive function of the yolk is at best insignificant, the vitelline sac of these animals representing the more important organ of their humbler ancestors. In the lowest members of the mammalian group, the monotremata, in which the large ova are comparatively rich in deutoplasm, the vitelline circulation is of great importance for nutrition, since it constitutes the sole means for this function until the immature animals are hatched and supplied with milk by the mother. In the kangaroo and opossum the yolk-sac at one point forms a disk-like organ, which, from the fact that it becomes provided with vascular villi that lie in contact with the uterine mucous membrane, is termed the vitelline placenta. The Allantois and the Chorion. Coincidently with the formation of the amnion, another fcetal appendage, the allantois, makes its appearance as an out- THE VITELLINE SAC. 33 growth from the caudal segment of the primary gut-tract. Although modified in man and certain mammals to such an extent that its typical form and relations are obscured, the allantois, when developed in a characteristic manner, as in the chick, assumes the appearance of a free vesicle connected with the embryo near its caudal pole by means of a narrow pedicle, the allantoic stalk. Since the allantois is an evagination from the primitive gut, its walls are formed by direct continuations of the primary layers enclosing the digestive canal, namely, a lining of entoblastic cells, reinforced externally by a layer of visceral mesoblast. Beginning as a wide bay on the ventral wall of the hind-gut, the allantois elon- gates and appears as a pyriform sac projecting from the embryo behind the attach- ment of the still large vitelline stalk (Fig. 39). It rapidly grows into the exoccelom, and in mammals expands in all directions until it comes into contact with the inner surface of the primitive chorion, with which it fuses to constitute the true chorion. The latter, sometimes spoken of as the allantoic chorion in contrast to the amniotic or primitive chorion, now becomes the most important envelope of the mammalian embryo on account of the role that it is destined to play in establishing the respira- FIG. 40. 'Primitive chorion -Amnion -Amniotic sac Allantois ^Vitelline sac Diagram showing villous condition of serosa, expanding allantois, and diminishing vitelline sac. tory and nutritive organ of the foetus, the placenta. After the fusion of the allantois with the primitive chorion to form the chorion, the villous projections upon the external surface of the latter become more highly developed, consisting of a core of mesoblastic tissue covered externally by the ectoblast. The primary purpose of the allantois, as a receptacle for the effete materials ex- creted by the Wolffian body of the early foetus, is soon overshadowed by its function as a respiratory organ ; this occurs with the appearance of the rich vascular supply within the chorion following the invasion of its mesoblastic tissue by the blood-vessels constituting the allantoic circulation. The latter includes the two allantoic arteries, which are extensions from the aortic stem of the embryo and convey venous blood, and the two allantoic veins, which return the oxygenated blood to the embryo and become tributary to the great venous segment of the primitive heart. The vascu- larization of the chorion extends to the highly developed villi occupying its outer surface in many mammalian forms, especially man. The vascular villi of the chorion, bearing the terminal loops of the blood-vessels conveying the foetal blood, are important structures on account of their intimate relations with the uterine mucous membrane (Fig. 41), in conjunction with which 3 34 HUMAN ANATOMY. they form a respirative and nutritive apparatus. The intimacy between the uterine mucous membrane and the chorionic tufts presents all degrees of association, from simple apposition, as seen in the sow, where the feebly developed and almost uni- formly distributed vascular projections are received within corresponding depressions in the richly vascular uterine tissue, to the firm and complex attachment found in the highly developed human placenta. FIG. 41. Villi of extraplacental chorion Gut-tube Ectoblast Amniotic meso blast Amniotic sac Space between am- nion and chorion Allantois Allantoic blood- vessels Allantoic sac Mesoblast Entoblast "Maternal blood-spaces "Decidua placentalis Diagram showing villous chorion, differentiation of placental area, and vascularization of chorion. In contrast with the chorion of those animals in which the nutritive relations between the maternal tissues and the embryo are uniformly distributed are the local specializations seen in the chorion of those types in which a placental area is de- veloped. The animals in which the latter condition obtains are known as placentalia, of which three subgroups are recognized depending upon the multiple (cotyledons), FIG. 42. A uU Diagrams illustrating the various types of development of the chorion. A, uniformly developed villi (hog, horse) ; B, multiple placentae or cotyledons (cow, sheep) ; C, zonular placenta (cat, dog) ; D, discoidal placenta (monkey, man). A-B comprise non-deciduate ; C-D. deciduate mammals. zonular, or discoidal form of the placenta, man and the apes representing the highest specialization of the last division. In its general plan of development, therefore, the placenta is formed of a foetal and a maternal portion, the former consisting of the vascular villi which are unusually well developed within a particular portion of the chorion, and the latter of the opposed uterine lining which becomes highly special- ized throughout a corresponding area and more or less intimately united with the THE HUMAN FCETAL MEMBRANES. 35 foetal structures. The mucous membrane of the entire uterine cavity, in many of the higher mammals, suffers profound change, and before the end of gestation becomes inseparably attached to the chorion even in its extent beyond the placental area ; in such animals the fused uterine and chorionic tissue constitute the decidiitz which, lined internally by the closely applied amnion, form the membranous envelope en- closing the foetus. After rupture consequent upon the expulsion of the foetus at the termination of pregnancy, the deciduae, including the specialized placental portion, are separated from the uterine wall and expelled as the membranes and the placenta which are known collectively as the after-birth. The foregoing sketch of the general development of the foetal membranes in the higher mammals must be now supplemented by consideration of the peculiarities encountered in the development of these structures in man. THE HUMAN FCETAL MEMBRANES. The young human embryo is distinguished by the very early formation of the amniotic cavity, by the precocious development of the mesoblast and extra- embryonic coelom, by the presence of the body-stalk and by the great thickening of the trophoblast. It must be remembered, in considering the formation of the human foetal membranes, that the earliest stages of development, to wit, fertilization, segmentation, the formation of the blastodermic vesicle, the earliest differentiation of the embryonic area and the formation of the amniotic cavity have not yet been observed on human specimens. Our knowledge of these processes is derived from a study of some of the lower types ; beyond these very early stages, however, the conditions in the human embryo have been subject to direct study. The Human Amnion, Amniotic Cavity and Allantois. The accompany- ing diagrams (Fig. 43) will serve to illustrate the process of formation of the foetal membranes in man. Of these five diagrams, A alone is purely hypothetical with reference to the human embryo. In diagram A the amniotic cavity is already indicated as a small cleft between the embryonic area below and a covering layer of cells above continuous with the trophoblast. This layer, the trophoblast, forms the outer covering of the entire vesicle. It is presumably already thickened at as early a stage as this diagram represents. Presumably also the surface of the trophoblast shows irregularities, for this tissue it is which comes into direct contact with the uterine mucous membrane and which, by its activities, forces its way into the maternal decidua. This latter process is known as implantation, a process which supposedly is taking place, if not completed, at about the stage of this diagram. Whether the trophoblastic layer in man is originally a thin single sheet of cells, as for instance is the case in the rabbit, or whether it is from the beginning thickened, we do not know. Certainly the thickened condition appears at a very early stage. The embryonic area shows the embryonic ectoblast proper, which is of small extent ; this ectoblast being so distinguished from the trophoblastic ectoblast. The ento- blast beneath is represented as already arranged in the form of a sac. Between the entoblast and ectoblast the mesoblast has made its appearance. It will be noted that in the diagram the entoblastic sac is much smaller than the outer trophoblastic vesicle. We do not know that this is really the condition when the entoblastic sac is first formed or only appears in conjunction with the great development of the extra embryonic coelom in the mesoblast. It is certainly not unreasonable to suppose that the former case is the true one. The early appearance of the amniotic cavity is to be explained in this way. After the blastodermic vesicle has reached the stage when the inner cell mass is attached to one point on the inner surface of the trophoblast, the formation of a cavity occurs in the region of the inner mass. This cavity, at first very small, has below it the cells of the inner mass, which soon become arranged into the two primary germ layers of the embryonic area, ectoblast and entoblast, while above the cavity is a layer of cells continuous with the trophoblast. Such a method of formation of the amniotic cavity has been observed in some of the lower forms, for instance, in a lemur by Hubrecht, and since the earliest human embryo accurately studied shows a completely closed amniotic cavity, while in 36 HUMAN ANATOMY. a very early stage of development it is a reasonable inference that in man such a process actually occurs. In diagram B the mesoblast has not only surrounded the entoblastic sac and the inner surface of the trophoblast, so enclosing the large extra-embryonic ccelom, but has invaded the layer of cells above the amniotic cavity, dividing this layer into two parts, the inner part going to form the ectoblast of the amnion, the outer part being a continuation of the- trophoblast of the chorion There is here evidently a very great development of the extra-embryonic ccelom. In explanation of this condition, it may be assumed that the entoblastic sac is at first much smaller than the trophoblastic covering of the vesicle ; that the mesoblast, shortly after its appearance, FIG. 43. VS Diagrams illustrating development of human foetal membranes. Stage A is hypothetical ; others are based on stages which have been actually observed. Red represents trophoblast ; purple, embryonic ectoblast ; gray, meso- blast ; blue, entoblast. ac, amniotic cavity; a/, allantois ; am, amnion; , body-stalk; ch, chorion; ee. embryonic ectoblast ; en, entoblast ; g, gut-tube ; m, mesoblast ; p, placental area ; t, trophoblast ; v, yolk-sac ; vs, yolk-stalk. develops a ccelom ; that the two layers of the mesoblast so formed grow separately around the vesicle ; the splanchnic layer around the entoblast, the somatic layer around the trophoblast, so enclosing between them as they grow, the considerable space which becomes, by this process, extra-embryonic body cavity. This diagram corresponds roughly to the condition of Peters' embryo (Fig. 44). The trophoblast is greatly thickened ; its outer surface very irregular, showing lacunae or spaces filled with maternal blood. This early intimate contact of the foetal tissue with the maternal blood permits nutrition of the young embryo from the maternal blood to be carried on through the trophoblast cells some time before the allantoic circulation and definite placenta are established. Hence the significance of this term trophoblast. THE HUMAN FCETAL MEMBRANES. 37 In the next diagram, (Fig. 43), C, the extra-embryonic coelom has invaded the sheet of mesoblast above the amniotic cavity to such an extent that the chorion is completely separated from the amnion and the body of the embryo except at one point, the posterior end of the body, where a solid stalk of mesoblast connects the chorion and embryo. This solid band of mesoblast is called the body-stalk. It represents, therefore, a primary and permanent connection between the chorion and the body of the embryo. A small diverticulum from the entoblastic sac growing into the mesoblast of the body-stalk marks the beginning of the allantois. As the diagram shows, the amnion is at first a comparatively small membrane overlying the embryonic area. The ectoblast of the amnion is on the inner side facing the embryo, the mesoblast on the outer side. In the chorion these layers are placed inversely, the mesoblast on the inner side, the ectoblast (trophoblast) outside. The space between amnion and chorion is seen to be a continuation of the extra-embryonic ccelom. In diagram Z7, the amnion has become considerably expanded in association with the growth of the body of the embryo and the accumulation of amniotic fluid. A constriction in the entoblastic sac has made its appearance, a constriction which separates the gut of the embryonic body from its appendage, the yolk-sac, the narrower connecting piece being known as the yolk-stalk, or sometimes as the vitello-intestinal duct. This constricted area is brought about by the rapid growth of the body of the embryo. In the early condition the entoblastic sac is attached to the embryonic body practically along its entire ventral surface. The body region grows very rapidly, particularly the head end, which comes to project from the entoblastic sac to a marked extent ; the tail end also projects somewhat. There is a corresponding growth of the gut within the body of the embryo. As a consequence of this process of expansion of the body, the area of attachment of the entoblast external to the body becomes relatively much reduced in size, occupying only a small portion of the ventral surface of the body, and a progressively smaller portion as the body increases in bulk. In other words, the narrow area of the yolk-stalk makes its appearance. In the diagram (D, a/) the allantois projects from the posterior end of the embryonic gut into the body-stalk. It will be noticed that the human allantois is never a free structure as it is in many of the lower types, where it grows from the body freely into the extra-embryonic coelom and only later becomes connected with the chorion to form the placenta, but that in man it grows directly into the body- stalk, where, outside of the body of the embryo, it is an insignificant structure. Inside the body, part of the allantois persists as the bladder. The urachus, a fibrous cord which in the adult passes from the top of the bladder to the umbilicus, is also a remnant of the allantois. The thick irregular projections of the trophoblast have received a core of mesoblast tissue, so forming the early chorionic villi. These villi, at the point of attachment of the body-stalk, the area where the placenta is developing, are increasing in size, while the villi over the remainder of the chorion are diminishing in size. In diagram E, the amnion has become greatly expanded. It lies closer to the inner surface of the chorion. In close association with this expansion of the amnion, and the accompanying growth of the body of the embryo, the structures which form the umbilical cord are so closely approximated that the area of the cord is clearly defined. These structures are the body-stalk containing the allantois and allantoic vessels, the yolk-stalk, and, bounding the other side of this area, the fold of the amnion from beneath the head. At first the body-stalk projects from beneath the extreme posterior end of the body of the embryo, but as growth in this part of the body advances and the tail projects more and more, the body-stalk is brought to the ventral surface of the abdominal region in close proximity to the yolk-stalk. The allantoic blood-vessels grow from the embryo through the body-stalk to the chorion, where they ramify in the chorionic villi. At first there is an extension of the ccelom about the yolk-stalk in the umbilical cord, but the mesoblast tissues of the structures of the cord soon fuse together, obliterating this cavity. The area of attach- ment to the abdomen of the umbilical cord becomes relatively very much reduced in size and is known in the adult, after the separation of the cord, as the umbilicus or navel. 38 HUMAN ANATOMY. The chorionic villi at the point of attachment to the chorion of the body-stalk are enlarged. These villi constitute the foetal portion of the placenta, the so-called chorion frondosum. They are imbedded in the maternal decidua, more specifically, the decidua basalts or placentalis. It must be remembered that the villi contain a core of mesoblast tissue in the stage represented by diagram , although this meso- blastic core is not shown in the figure, and that the allantoic blood-vessels run in M. Cap. U.E Sy ^- T.M. B.L. Sy. B.L. Sy. U.E. Cap. Tr. I.I. Ca. g. Section of mucous membrane, decidua, of a pregnant uterus containing imbedded in it an extremely young human embryonic vesicle, described by Peters, a, b, points of entrance of embrvonic vesicle ; B. L., blood lacunae ; B. Z., Bordering zone; Ca., capillary in uterine tissue; Cap., beginning of decidua capsularis ; Comp., compact tissue of uterine mucosa; ., embryo; g., gland of uterus : M , mesoblast; Sy., syncytium ; T. M., covering tissue over break in uterine surface; Tr., trophoblast ; U. ., epithelium of uterine mucosa. X 50 (Peters). this mesoblast: also that the villi are in reality considerably branched, not straight as in the diagram. The remainder of the chorion is acquiring a smooth surface and is commonly known as the chotion l&ve, as a means of distinguishing the extra- placental portion of this membrane. The yolk-sac, in man called the umbilical vesicle, at the extremity of the yolk-stalk, is retained usually in the placental area just beneath the amnion. It is possible to find the yolk-sac in nearly every placenta THE HUMAN FCETAL MEMBRANES. 39 Reflected amnion Medullary folds-^?- Medullary groove - by slightly stretching the umbilical cord at its insertion, when a fold appears containing no large vessels. This fold points to the position of the yolk-sac. To sum up, the chief peculi- arities of the human foetal mem- FIG. 45. branes are the following : 1 . The amniotic cavity is Viteiime sac- developed at a very early period apparently by a process of hollowing out in the region of the cells of the inner mass, and not by any folding process. The cells above this primi- tive amniotic cavity are later split into two portions by the entrance of the mesoblast and extra-embryonic ccelom ; the inner portion becomes the . ectoblast of the amnion, the outer portion is merely a part of the the trophoblast of the chorion. 2. The mesoblast and extra- embryonic ccelom are precociously developed at a very early period. 3. The body-stalk constitutes a primary and permanent connection between the embryo and the chorion. 4. The allantois, which, ex- ternal to the body of the embryo, is an insignificant structure, grows into the body-stalk and therefore is never a free vesicle. . . 5. The trophoblast is very early greatly proliferated and very early in intimate contact with the maternal blood. FIG. 46. Neurenteric canal- Primitive streak Belly-stalk Chorion __ Chorionic villi l ~~y~? r ~ r >- Dorsal surface of early human embryo, two millimetres in length. X 23. (After Spee.) The amnion has been divided and turned aside. Wall of vitelline sac Belly-stalk Neurenteric canal Primitive streak Chorion- Longitudinal section of human embryo represented in preceding figure. X 23. (After Spee.) Fur 44 paee *8, is a reproduction of the drawing of Peter's embryo and deserves special Attention. The figure shows a small portion of the mucous mem- brane of the uterus in which is imbedded the embryonic or choriomc vesicle. 4 o HUMAN ANATOMY. Between the points a, b in the figure lies the area through which the embryonic growth has made its way into the mucous membrane of the uterus, and, in consequence, the uterine epithelium in this area has disappeared. Above this small area there lies a covering mass of tissue ( T. M. ) mainly composed of blood, the result evidently of hemorrhage following the breaking of the mucosa of the uterus in this region. The chorionic vesicle as a whole is quite large, especially in proportion to the embryonic area E, the surface of which is covered with a distinct columnar epithelium. Surrounding the chorionic vesicle there are two kinds of tissue, which make a very striking feature of the picture. First, there is the thickened and very irregular trophoblast, the cells of which appear dark, and which forms the outer covering of the wall of the embryonic vesicle itself. Then there are numerous large blood-spaces or Amnion Umbilical or yolk-sac Chorion Human embryo of about twenty days, enclosed within the amnion. X 30. blood-lacunae lying among the irregular projections of the trophoblast. The maternal blood, therefore, in this very early condition bathes the trophoblast cells of the embryo, a relation very significant with reference to the nutrition of the embryo before the allantoic-placental circulation is established. The mesoderm {M) extends around the vesicle on the inner side of 'the trophoblast. In several places there are outgrowths of the mesoderm intQ the trophoblast, so indicating the beginnings of the villi of the chorion. It will be remembered that the cells of the trophoblast form the epithelial covering of the chorion. At several places in the figure the syncytial layer of the trophoblast Sy can be distinguished. The proportionally large cavity within the vesicle is extra-embryonic ccelom, a fact which can readily be verified by observing the relations of the mesoderm. The latter layer of tissue is seen to extend around the small yolk sac as the visceral layer of the mesoderm, while the layer of the mesoderm on the inner side of the trophoblast is of course the parietal layer, hence the cavity within these respective layers is the extra-embryonic ccelom, precociously developed for this early stage. There is a small amniotic cavity above the embryo. Between this cavity and the trophoblast the mesoderm extends as a solid sheet. There are one or two more points to be noted in the figure. In the areas THE HUMAN CHORION. FIG. Extraplacental area ( Chorion l&ve) marked B. Z., which are merely portions of the uterine mucosa lying against the trophoblast, the tissue is oedematous in character. This tissue is described by Peters as the bordering zone. In other portions of the mucous membrane there are seen parts of some of the uterine glands ("). In the region marked Cap., is seen the beginning of the decidua capsularis, growing in over the area through which the embryonic vesicle broke into the surface of the uterus. This layer, decidua capsu- laris, is at this stage scarcely developed, only the beginning of it is apparent. This embryo, described by Peters, is one of the youngest which has been accu- rately studied. The inner dimensions of the vesicle, as given by Peters, are as follows: 1.6 by o. 8 by 0.9 mm. The youngest human embryo is that described by Bryce and Teacher, and is probably several days earlier than the one recorded by Peters. In a gen- eral way, it presents the relations of the amniotic and vitelline sacs already described. The Human Chorion. The vascular chorionic villi, although becoming more complex by the addition of secondary branches, are for a time equally well developed over the external surface of the entire embryonic vesicle ; subsequently, from the end of the second month, a noticeable differentiation takes place, the villi included within the field that later corre- sponds to the placental area undergoing unusual growth and far outstripping those covering the remaining parts of the chorion. This inequality in the development of the villi led to the recognition of the chorion frondosum and the chorion l&ve, as the placental and non-placental portions of the chorion respectively are termed (Fig. 48). The vascular supply of the villi also shares in this differentiation, the vessels to those of the placental area becoming progres- sively more numerous, while, on the con- trary, those distributed to the remaining villi gradually atrophy as the chorion comes into intimate apposition with* the uterine tissue. When well developed, the chorionic villi possess a distinctive appear- ance, the terminal twigs of the richly branched projections being clubbed and slightly flattened in form. Their recogni- tion in discharges from the vagina often affords valuable information as to the ex- istence of pregnancy. The Amniotic Fluid. Theamnion at first lies closely applied to the embryo, but soon becomes separated by the space which rapidly widens to accommodate the increasing volume of the contained liquor amnii. The accumulation of fluid within the amniotic sac, which in man takes place with greater rapidity than in other mam- mals, results in the obliteration of the cleft between the chorion and amnion until the latter envelope lies tightly pressed against the inner surface of the chorion. The union between the two envelopes, however, is never very intimate, as even after the expulsion of the membranes at birth the attenuated amnion may be stripped off from the chorion, although the latter is then inseparably fused with the remaining portions of the deciduae. The amniotic fluid, slightly alkaline in reaction, is composed almost entirely of water ; of the one per cent, of solids found, albumin, urea, and grape-sugar are constituents. The quantity of liquor amnii is greatest during the sixth month of gestation, at which time it often reaches two litres. With the rapid increase in the general bulk of the foetus during the later months of pregnancy, the available space for the amniotic fluid lessens, resulting in a necessary and marked decrease in the quantity of the liquid ; at birth, less than one litre of amniotic fluid is usually present. Sometimes, however, the amount of the liquor amnii may reach ten Placental area ( Chorion frondosum) External surface of part of the human chorion of the third month ; the lower portion is covered with the highly developed villi of the placental area. HUMAN ANATOMY. litres, due to pathological conditions of the foetal envelopes ; such excessive secre- tion constitutes hydramnion. During the later months of pregnancy the foetus swal- lows the amniotic fluid, as shown by the presence of hairs, epithelial cells, etc. , within the stomach. In view of the composition of the fluid, consisting almost en- tirely of water, it seems certain that the introduction of the liquor amnii does not serve the purposes of nutrition ; on the other hand, it is probable, as held by Preyer, that the unusual demands of the fcetal tissues for water may be met largely in this manner. The source of the amniotic fluid in man has been the subject of much discus- sion. While it has been impossible to determine accurately the extent to which the mother participates in the formation of this fluid, it may be accepted as established that the maternal tissues are the principal contributors ; it is also probable that the foetus likewise aids in the production of the liquor amnii ; the latter, therefore, orig- inates from a double source, maternal and fcetal. The early amniotic fluid resembles FIG. 49. Umbilical vesicle Umbilical stalk Inner surface of chorion Umbilical cord Cut edge ot amnion- Masses of chorionic villi ' ~ v^e Human embryo of about thirty-three days. X 4. Amnion and chorion have been cut and turned aside. in appearance and chemical composition a serous exudate ; later, after the formation of the urogenital openings, the liquor amnii becomes contaminated, as well as aug- mented, by the addition of the fluid derived from the excretory organs of the foetus. During the later weeks of gestation the contents of the digestive tube are discharged into the amniotic sac as meconium. The Umbilical Vesicle. The umbilical vesicle, as the yolk-sac in man is termed, presents a reversed growth-ratio to the amnion and body-stalk since it pro- gressively decreases as these latter appendages become more voluminous. The early human embryo is very imperfectly differentiated from the large and conspicuous yolk-sac, with which its ventral surface widely communicates. With the advances made during the third week in the formation of the primitive gut, the connection between the latter and the vitelline sac becomes more definitely outlined in conse- quence of the beginning constriction which indicates the first suggestion of the later vitelline or umbilical duct (Fig. 47). By the end of the fourth week the connection THE UMBILICAL VESICLE. 43 between the umbilical sac and the embryo has become reduced to a contracted channel extending from the now rapidly closing ventral body-wall to the yolk-sac, which is still, however, of considerable size. The succeeding fifth (Fig. 50) and sixth weeks effect marked changes in the umbilical duct, now reduced to a narrow tube, which extends from the embryo to the chorion, where it ends in the greatly diminished vitelline sac. The lumen of the umbilical duct is conspicuous during the earliest months of gestation, but later disappears, the entoblastic epithelial lining remaining for a considerable time within the umbilical cord to mark the position of the former canal. The chief factor in producing the elongation of the umbilical duct is the rapid expansion of the amnion ; with the increase in the amniotic sac the distance between this envelope and the embryo increases, until the amnion fills the entire space within FIG. 50. Amnion Umbilical vesicle (yolk-sac) Inner surface of chorion Chorionic villi of outer surface Zhorionic sac of thirty-five day embryo laid open, showing embryo enclosed by amnion. the chorion, against which it finally lies. In consequence of this expansion, the attachment between the embryo and the amnion around the ventral opening, which later corresponds to the umbilicus, becomes greatly elongated and narrowed. At this point the tissues of the embryonic body-wall and the amniotic layers are directly continuous. The tubular sheath of amnion thus formed encloses the tissue and structures which extend between the embryo and the chorion, as the constituents of the belly-stalk, together with the umbilical duct and the remains of the vitelline blood-vessels ; the delicate mesoblastic layer of the amnion fuses with the similar tissue of the allantois, the whole elongated pedicle constituting the umbilical cord or funiculus. The latter originates, therefore, from the fusion of three chief com- ponents, the amniotic sheath, the belly-stalk, and the vitelline duct ; the belly-stalk. 44 HUMAN ANATOMY. as already noted, includes the allantois, with its blood-vessels, and diverticulum, while traces of the vitelline circulation are for a time visible within the atrophied walls of the umbilical duct. As gestation advances, the amnion and the chorion become closely related, but not inseparably united ; between these attenuated mem- branes lie the remains of the once voluminous yolk-sac, which at birth appears as an inconspicuous vesicle, from three to ten millimetres in diameter, situated usually several centimetres beyond the insertion of the umbilical cord. In cases in which the closure and the obliteration of the vitelline duct before birth are imperfectly effected, a portion, or even the whole, of the intra-embryonic segment of the canal may persist as a pervious tube. Although in extreme cases of faulty closure a passage may lead from the digestive tube to the umbilicus, and later open upon the exterior of the body as a congenital umbilical anus, the retention of the lumen of the vitelline duct is usually much less extensive, being limited to the prox- imal end of the canal, where it is known as Meckel 's diverticulum. The latter is con- nected with the ileum at a point most frequently about 82 centimetres (thirty-two inches) from the ileo-caecal valve. Such diverticula usually measure from five to 7.5 centimetres in length, and possess a lumen similar to that of the intestine with which they communicate. The foregoing envelopes, the amnion and the chorion, are the product of the embryo itself ; their especial purpose, in addition to affording protection for the deli- cate organism, is to aid in establishing close nutritive relations between the embryo and the maternal tissues, which, coincidently with the development of the foetal envelopes, undergo profound modifications ; these changes must next be considered. The Deciduae. The birth of the child is followed by the expulsion of the after-birth, consisting of the membranes and the placenta, which are separated from the uterine wall by the contractions of this powerful muscular organ. Close inspection of the inner surface of the uterus and of the opposed outer surface of the extruded after-birth shows that these surfaces are not smooth, but roughened, presenting evi- dences of forcible separation. The fact that the external layer of the expelled after- birth consists of the greater portion of the modified mucous membrane which is stripped off at the close of parturition suggested the name decidua for the mater- nal portion of the fcetal envelopes shed at birth. Since the deciduae are directly derived from the uterine mucous membrane, a brief sketch of the normal character of the last-named structure appropriately pre- cedes a description of the changes induced by pregnancy. The normal mucous membrane lining the body of the human uterus (Fig. 51) presents a smooth, soft, velvety surface, of a dull reddish color, and measures about one millimetre in thick- ness. The free inner surface is covered with columnar epithelium (said to be cili- ated) which is continued directly into the uterine glands. The latter, somewhat sparingly distributed, are cylindrical, slightly spiral depressions, the simple or bifur- cated blind extremities of which extend into the deeper parts of the mucosa in close relation to the inner bundles of involuntary muscle ; all parts of the tubular uterine glands are lined by the columnar epithelium. The muscular bundles representing the muscularis mucosas are enormously hypertrophied and constitute the greater part of the inner more or le^s regularly disposed circular layer of the uterine muscle. The unusual development of the muscular tissue of the mucous membrane reduces the submucous tissue to such an insignificant structure that the submucosa is gener- ally regarded as wanting, the extremities of the uterine glands being described as reaching the muscular tunic. The glands lie embedded in the connective-tissue complex, rich in connective-tissue elements and lymphatic spaces, that forms the tunica propria of the mucosa. With the beginning of pregnancy the uterine mucous membrane undergoes marked hypertrophy, becoming much thicker, more vascular, and beset with nu- merous irregularities of its free surface caused by the elevations of the soft spongy component tissue. These changes take place during the descent of the fertilized ovum along the oviduct and indicate the active preparation of the uterus for the reception of the ovum. According to the classical description of the encapsulation of the ovum (Fig. 52) by the uterine mucous membrane, the embryonic vesicle becomes arrested within THE DECIDU^E. 45 one of the depressions of the uterine lining, usually near the entrance of the ovi- duct, whereupon the adjacent mucosa undergoes rapid further hypertrophy, which results in the formation of an annular fold surrounding the product of concep- tion. This encircling wall of uterine tissue continues its rapid growth until the embryonic vesicle is entirely enclosed within a capsule of modified mucous mem- brane, known as the decidua reflexa, as distinguished from the decidua vera, the name applied to the general lining of the pregnant uterus. That portion of the uterine mucosa, however, which lies in close apposition to the embryonic vesicle, constituting the outer wall of the decidual sac, is termed the decidua serotina ; later it becomes the maternal part of the placenta. FIG. 51. Duct of gland Sllfli^ii m -'*-' >lJV-7- i .?ETS.-:.-- -.-->., ;...-: -..<. SEr;v \'?y,?'&.-. :.':>/.<> "'..:V^v:;S- . . . .. Spiral portion of gland Process of muscular tissue extending be- tween the glands Muscular tissue Uterine blood- vessel Uterine mucous membrane with part of muscular tissue. X 45- Our knowledge of the details regarding the encapsulation of the ovum has been materially advanced by the recent observations of Peters, who had the rare good fortune of carefully studying the details of the process at an earlier stage than any hitherto accurately investigated. The results of Peters' s observations lead to a somewhat modified conception of the early phases of the encapsulation of the ovum, as well as shed additional light on some of the vexed problems concerning the details of the formation of the placenta. According to these investigations, the embryonic vesicle, on reaching the uterine 4 6 HUMAN ANATOMY. FIG. 52. cavity and becoming arrested at some favorable point, usually in the vicinity of the oviduct, brings about a degeneration of the uterine epithelium over the area of contact. The disappearance of the epithelial lining is followed by sinking and em- bedding of the embryonic vesicle within the softened mucous membrane, the process being accompanied by erosion of some of the uterine capillaries and consequent hemorrhage into the opening representing the path of the ovum. The extravasated blood escapes at the point of entrance on the uterine surface and, later, forms a mushroom-shaped plug marking the position of the embedded ovum. The latter thus comes into closer relations with the maternal tissues at an earlier period than was formerly recognized. The Trophoblast. The ear- liest human embryonic vesicle that has been accurately studied, that of Bryce and Teacher, measuring only i millimetre in its greatest di- Diagrams representing relations of the uterine mucous mem- ameter, Was already enclosed CXter- brane to the embryonic vesicle, or ovum, during the embedding nallv bv a Conspicuous CCtoblastic of the latter, s, v, c, decidua serotina, vera, and reflexa, re- J . J . \ spectiveiy ; o, ovum. envelope, consisting of an outer and an inner cell-layer. This thick ectoblastic layer is evidently the proliferated trophoblast (page 31), a membrane so designated to indicate the important nutritive functions which it early assumes. Very early the trophoblast becomes honeycombed by the extension of the maternal vascular channels into the ectoblastic tissue (Fig. 53), which consequently is broken up into irregular epithelial trabeculae separating the maternal blood-spaces. The inner surface of the trophoblastic capsule presents numerous irregular depres- sions into which corresponding processes of the adjacent young mesoblast project ; this arrangement foreshadows the formation of the chorionic villi which soon become so conspicuous in the human embryonic vesicle. Coincidently with the invasion of the trophoblast by the vascular lacuna externally and the penetration of the FIG. 53. mesoblastic tissue internally, the pe- ripheral portions of the ectoblastic capsule undergo proliferation and extend more deeply into the sur- rounding maternal tissues. In con- sequence of the rapid growth of the embryonic vesicle, that part of the hypertrophied uterine mucosa which overlies the embedded embryonic vesicle soon becomes elevated and projects into the uterine cavity, thus giving rise to the structure described as the decidua reflexa, or, preferably, the decidua capsularis. The Decidua Vera. The changes which affect the uterine mu- cous membrane, the decidua vera, result in great thickening, so that the mucosa often measures nearly a centimetre ; this thickening, however, is most marked in the immediate vicinity of the embedded ovum, throughout the greater part of the uterus the decidua attaining a much less conspicuous hypertrophy. Towards the cervix the mucosa is least affected, and at the internal orifice of the cervical canal presents its normal appearance. Examina- tion of the decidua shows that the normal constituents of the uterine mucosa undergo hypertrophy which results in enlargement of the uterine glands (Fig. 54), as well as in increase of the intervening connective-tissue stroma. The enlargement of the glands is not uniform, but is limited to the middle and terminal or deeper parts of Mesoblast Trophoblast Diagram showing early stage of attachment between foetal and maternal tissues; invasion of trophoblast by maternal blood-vessels. (Peters.) THE DECIDUA VERA. 47 the tubular depressions ; the inner portions of the glands, directed towards the sur- face of the uterus, become elongated and lie embedded within a comparatively dense matrix. In consequence of these changes, the decidua in the vicinity of the ovum, where the hypertrophy is most marked, presents in section two strata, an inner compact and an outer spongy layer. The ciliated columnar epithelium that normally clothes the free surface of the uterus, and perhaps also the uterine glands, gradually disappears, the degeneration beginning before the end of the first month. The integrity of the cells lining the uterine glands is maintained for a longer period, but the glandular epithelium likewise, after a time, suffers, losing its columnar character and changing to small cubical or flattened elements, which, after appear- ing as shrunken columns during the fourth and fifth months, finally disappear during the latter half of gestation. An important exception, however, is to be noted in the behavior of the epithelium lining the deeper portion, or the fundus, of the glands next the muscular tissue ; the epithelium situated in this position does not participate in the atrophic changes above described, but retains more or less per- FIG. 54. Free surface Enlarged gland Blood-vessel Enlarged gland Uterine muscle Section of mucous membrane lining body of uterus (decidua vera) ; fourth month of pregnancy. (After Leopold.} fectly its normal condition to the close of pregnancy. After the expulsion of the decidual portion of the uterine mucous membrane, the epithelium remaining in the fundus of the glands becomes the centre of regeneration for the new lining of the uterus. The connective-tissue elements of the matrix surrounding the glands, especially in the compact layer in the vicinity of the ovum, undergo active proliferation, in consequence of which large spherical elements, the decidual cells, are produced. The latter, from .030 to .040 millimetre in diameter, in places are so densely packed that they assume the appearance of epithelium ; although most typical and nu- merous in the compact layer, they are, nevertheless, present in the spongy stratum, in this situation being more elongated and lanceolate in form. The decidua vera retains this general character during the first half of preg- nancy ; from this time on, however, the increasing volume of the uterine contents subjects the decidua to undue pressure, in consequence of which the hypertrophied mucosa undergoes the atrophic changes characteristic of the so-called second stage. These include a gradual reduction in the thickness of the decidua vera from nearly 4 8 HUMAN ANATOMY. one centimetre to about two millimetres, the disappearance of the ducts and open- ings of the uterine glands, and the conversion of the compact layer into a dense homogeneous stratum, in which the tightly compressed glands later entirely disap- pear. The spongy layer, on the contrary, retains the dilated gland-lumina, which, however, in consequence of pressure, are converted into irregular spaces arranged with their longest dimensions parallel to the uterine surface. The clefts next the FIG. 55. Amnion - Chorion Decidua reflexa Blood-space. Giant cell Enlarged lumen of glands Degenerating glandular epi- thelium ^ Blood-space of compact layer Spongy layer Muscle Section through foetal membranes and uterus at margin of the placenta ; sixth month of pregnancy. (After Leopold.) muscular tissue are clothed with well-preserved epithelium ; the lining cells of those towards the compact layer, on the contrary, early atrophy and disappear. The Decidua Placentalis. The decidua placentalis, or decidua serotina, being destined to contribute the maternal portion of the placenta, undergoes profound changes which particularly affect the blood-vessels of the mucosa. In addition to the initial general hypertrophy of the mucous membrane, which the placental decidua shares in common with other parts of the uterine lining, peculiar polynucleated ele- ments, the giant cells, make FIG. 56. their appearance during the fifth month ; by the end of preg- nancy they are found in large numbers within the basal plate and the septa of the placenta, although they are not wanting within the remains of the spongy layer. The giant cells are par- ticularly numerous in the im- mediate vicinity of the large blood-vessels. The relations between the ingrowing foetal trophoblastic tissue and the ma- ternal structures early become so intimate within the placental area that especial modifications are instituted destined for the production of the vascular arrangement by which the maternal and foetal blood-streams are brought into close relations. The proliferating trophoblastic tissue invades the stroma of the mucous mem- brane and encroaches upon the capillaries until the latter in places become ruptured, allowing the escape of the maternal blood, which thus is brought into direct contact with the trophoblast. The erosion effected by the blood, on the one hand, and the encroachment of the foetal mesoblast, on the other, gradually reduces the tropho- blastic stratum, which is broken up into narrow epithelial trabeculae separating the rapidly enlarging vascular lacunae, the primary representatives of the intervillous Sections of chorionic villi from placenta. X 170. a, b, small branches of umbilical artery and vein ; v, capillary vessels ; c, cell- aggregations of syncytium (d) ; m, mesoblastic stroma of villi. THE PLACENTA. 49 FIG. 57. Main stalk maternal blood-spaces of the placenta. The active outgrowth of the mesoblastic tissue of the chorion into the trophoblastic envelope results in the production of the characteristic villous condition distinguishing the early human embryonic vesicle. When sectioned, the well-developed chorionic villi are seen to be composed of two portions, (a) the central core of gelatinous connective tissue, containing nu- merous stellate cells and blood-vessels, repre- senting the fcetal mesoblast, and (6) the epi- thelial covering derived from the trophoblast. The investment of the villi consists of two layers, an inner stratum, next the connective- tissue core, composed of low, distinctly out- lined polyhedral cells, the chorionic epithe- lium, and an outer stratum, the syncytium, composed of an apparently continuous proto- plasmic layer, in which nuclei are visible, but definite cell boundaries are wanting. Irregu- larly distributed aggregations of nuclei, or cell-patches (Fig. 56), form slight elevations on the surface of the villi. The derivation of the outer layer, or syncytium, has been the subject of much discussion ; its close rela- tion to the maternal blood -spaces suggested a maternal origin to some investigators, while others regard it as a foetal production. The observations of Peters on the very early human ovum, already mentioned, conclusively show the correctness of the latter view, and lhat the syncytium is formed by the transformation of the trophoblast next the vascular lacunae (Fig. 58) ; the syncytium, as well as the remaining parts of the villi of the chorion, therefore, is of fcetal origin. The epithelium covering the villi of the pla- cental area early evinces a tendency towards regression, and by the fourth month exists only as isolated patches ; during the later stages, and particularly on the larger villi, the layer of chorionic epithelium disappears, the syncytium remaining as the sole attenuated covering of the connective-tissue core of the villi. In certain parts of its extent, especially where it covers the chorion and the decidua serotina, Isolated tuft of chorionic villi from placenta. X38. FIG. 58. Chorionic mesoblast Mesoblastic core of foetal villus Trophoblast Syncytium Maternal blood-space & Muscle ^^ Endothelium 'Maternal blood-vessel Diagram showing formation of placenta. (Peters.) as well as upon some of the villi, the syncytium undergoes degeneration and is replaced by a peculiar layer of hyaline refracting material known as canalized fibrin. The Placenta. The placenta constitutes, from the third month of intra- uterine life, the nutritive and respiratory organ of the fcetus. As seen at birth, it is of irregular discoidal form, concavo-convex in section, and measures from fourteen to eighteen centimetres in diameter and from three to four centimetres in thickness. 4 HUMAN ANATOMY. Its convex external or uterine surface is rough, owing to the separation from the deeper part of the lining of the uterus which has taken place at the termination of labor. This surface, moreover, presents a number of divisions, the cotyledons, de- nned by deep fissures. The inner or foetal surface is smooth, being covered by the amnion, and slightly concave. The weight of the fully developed placenta averages about 500 grammes. The position of the placenta is determined, evidently, by the point at which the ovum forms its attachment with the maternal tissues ; in the majority of cases this location is at the fundus of the uterus in the vicinity of the oviduct, right or left, the orifice of which becomes occluded by the expansion of the placental structures. Less frequently the placenta occupies the more dependent portions of the uterine wall and, in exceptional cases, its position is in the immediate vicinity of the internal mouth of the uterus ; in these latter cases the placenta may partially, or even com- pletely, grow over the latter opening, thus constituting the grave condition known as placenta prsevia. The general constitution of the placenta (Fig. 59), as consisting Uterine blood-vessels FIG. 59. Maternal blood-spaces Foetal villi Umbilical vesicl Decidua placentalis Allantois Decidua capsularis Amnion Chorion Decidua vera Interdecidual space Amniotic sac Diagram illustrating the relations of the fcetus, the membranes, and the uterus during the early months of pregnancy. of the fcetal and the maternal portions, has already been sketched ; it now remains to consider briefly the arrangement of these structures. The foetal portion of the placenta, the contribution of the chorion frondosum, soon becomes a mass of richly branching villi, the more robust main stalks of which are attached to the maternal tissue, while the smaller secondary ramifications are free, completely surrounded by the contents of the maternal blood-sinuses in which they float. In all cases the villous processes support the terminal loops of the foetal blood-vessels, the blood being conveyed to and from the placenta, along the umbil- ical cord, by the umbilical arteries and vein. Although coming into close relation, the syncytium and the meagre connective tissue surrounding the foetal capillaries alone intervening, the blood-streams of the mother and of the child never actually mingle ; the delicate septum, however, allows the free interchange of gases necessary for the respiratory function as well as the passage of nutritive substances into the fcetal circulation. THE PLACENTA. The maternal portion of the placenta is contributed by that portion of the uterine mucous membrane known as the decidua serotina ; its especial peculiarities consist in the intervillous blood-spaces, which may be regarded as derivations from the eroded maternal blood-vessels. As already described, the trophoblast and maternal tissues early come into close relation, and the capillary blood-vessels are opened by the invasion of the foetal tissue, which latter, in turn, is eroded and channelled out by the maternal blood which escapes upon the rupture of the blood-vessels of the mucosa. The extension of the blood-spaces thus originating constitutes the elaborate system of vascular lacunae, or intervillous spaces, forming so conspicuous a part of the fully developed placenta. In its earlier changes the decidua serotina closely resembles the decidua vera, presenting an inner compact and an outer spongy layer ; by the middle of preg- nancy, however, the previously enlarged glands have entirely disappeared in conse- quence of the atrophy induced by the increasing pressure caused by the augmenting volume of the uterine contents. When the placenta is detached from the uterus the FIG. 60. Stump of umbilical cord Chorion Villus Placenta xPlacental septum Decidua serotina '' Line of separation .Uterus Spiral branches of uterine artery Inner limit Arteries of muscle Section of placenta and uterus at the seventh month. (Ecker.) line of separation passes through the junction of the former spongy and compact layers ; according to Webster, however, the separation occurs in the compact layer. The condensed decidual tissue closing in the vascular lacuna, on the one hand, and covering the surface of separation, on the other, constitutes the basal plate. The latter is continued deeply within the placenta by connective-tissue portions, the septa placenta, which extend between the groups of chorionic villi, forming the cotyledons visible On the outer surface of the placenta as irregular lobules separated by deep furrows. These septa do not reach as far as the chorion except at the margin of the placenta, where they form a thin membranous sheet beneath the chorion, the subcho- rionic occluding plate of Waldeyer. Large, round, multinucleated elements, the giant cells, measuring from .04 to .08 millimetre in diameter, are present within the tissue of the maternal placenta, especially within the basal plate and the septa. At the margin the placental tissue becomes directly continuous with the foetal mem- branes, the chorion and the decidua being closely united. The numerous branches of the arteries supplying the uterus pierce the muscular tunic and gain the basal plate ; here the arterial vessels lose their muscular coat and 52 HUMAN ANATOMY. penetrate the placental septa as spirally directed channels of enlarged calibre bounded by endothelial walls. After a shorter or longer course within the septa, the arterial FIG. 61. Stalk of villus Intervillous blood-spaces Section of larger villus ntervillous blood-spaces Maternal blood-vesseT Maternal blox>d-space Maternal blood-vessel <;^-~ Basal plate -Torn surface at line of separation Section of human placenta at end of pregnancy. X 12. The foetal blood-vessels have been injected ; the maternal lood-spaces appear as clear areas surrounding the sections of the foetal villi. trunks open directly into the intervillous or intraplacental blood-spaces which are limited by the chonon and the villi on the one side and by the septa and basal plate THE UMBILICAL CORD. 53 on the other. Maternal capillaries are wanting within the placenta, since they have become early replaced by the intervillous lacunae. The maternal blood is carried away from these spaces by wide venous channels which pass directly from the lacunae through the placental septa into the basal plate, where they form net- works from which proceed the larger venous trunks. At the edge of the placenta the anastomosing cav- ernous spaces form an annular series of intercommunicating venous channels known collectively as the marginal sinus , into which empty numerous placental veins, on the one hand, and from which, on the other, pass tributaries to the larger veins of the uterus. FIG. 62. Umbilical vein Umbilical arteries Corrosion preparation of human placenta, showing general grouping of foetal vessels into lobules. The Umbilical Cord. The umbilical cord, or funiculus umbilicalis, which connects the body of the foetus with the placenta, thereby conveying the foetal blood to and from the respiratory and nutritive apparatus, is formed in consequence of the fusion of three originally distinct structures, the belly-stalk, the vitelline stalk, and the amnion. The first of these, in addition to forming the early attach- ment of the foetus to the chorion, supports the rudimentary allantoic canal and the allantoic, later umbilical, blood-vessels. The vitelline stalk encloses the diminish- ing vitelline duct and the remains of the vitelline blood-vessels, while surrounding these stalks the amniotic sheath gradually becomes more closely applied. These 54 HUMAN ANATOMY. three constituents of the cord lie embedded within the delicate stroma formed by the gelatinous connective tissue, the jetty of Wharton, surrounded externally by the common amniotic investment. The details of the cord must necessarily vary with the period of gestation, since the component structures undergo marked changes. On section of the funic- ulus at the end of pregnancy, the following features may usually be distinguished : (1) The amniotic sheath, which is closely united with the underlying connective tissue, except for a short distance beyond the umbilical opening, at which point the amnion may be separated as a distinct layer. (2) The jelly of Wharton forms the common ground-substance in which the remaining constituents of the cord lie embedded. This tissue corresponds to the mucoid type, and contains a generous distribution of stellate connective-tissue cells which form a reticulum by their anastomosing processes. (3) The umbilical blood-vessels two arteries and one vein are the most con- spicuous components of the cord, since their size increases with the demands made by the growing foetus. The markedly tortuous umbilical arteries usually entwine the single umbilical vein and slightly increase in lumen in their progress towards the placenta, in the immediate vicinity of which an anastomosis very constantly is to be found. Seldom in man, but always in certain mammals, as the mouse, the umbilical artery is single. According to His, even the youngest human cords possess only a single umbilical vein, except in the immediate vi- P , cinity of the placenta ; again, on entering umbilical vein t ^ ie body of the fcetus the single vessel is Remains of represented by two umbilical veins which, aiiantoic for a time, course within the abdominal wall. The right vein, however, soon un- dergoes atrophy, while the left takes part in the formation of the hepatic circulation. Valves have been described within the um- bilical vein. The latter shares with the pulmonary vein the distinction of conveying blood which has been oxygenated by respi- Remains of vitelline ratol 7 function. duct and vessels (4) The aiiantoic duct, as a distinct canal, is usually obliterated by the third Transverse section of umbilical cord of third month. mQnth Q{ ^^ ^ . ^ y^ however . ( atrophic remains, consisting of a narrow column of epithelial cells situated between the umbilical blood-vessels, are seen in sections of the cord taken from the vicinity of the navel. The stalk of the vitelline sac, or umbilical vesicle, enclosing the vitelline duct and supporting the vitelline, or omphalomesenteric, blood-vessels, is still present during the second month ; at this period it lies within the extension of the ccelom, which is continued into the young cord. With the early disappearance of this space the vitelline stalk and the associated structures disappear, and by the end of gesta- tion usually all traces of these structures have vanished from the cord. The most conspicuous details of the umbilical cord at birth, therefore, are the three umbilical vessels, embedded within the gelatinous connective tissue and invested by the sheath of amnion. The human umbilical cord is conspicuous on account of its exceptional length, which averages from fifty to sixty centimetres, while measuring only about twelve millimetres in thickness. The extremes of length include a wide range, varying from twelve to 160 centimetres (four and three-quarters to sixty-three inches). The cord almost constantly exhibits a torsion, the spirals passing from left to right when traced towards the placenta. In addition to the general twisting of the cord, which begins towards the close of the second month, the umbilical arteries display even more marked spiral windings, usually enclosing the somewhat less twisted umbilical vein. The cause of this conspicuous torsion is probably to be sought in the spiral growth of the umbilical blood-vessels, the twisting of the cord, as well as the revolutions of the fcetus, being secondary. THE AFTER-BIRTH. 55 While the attachment of the cord usually is situated near the middle of the placenta, it is seldom exactly central ; the insertion is subject to great variation, however, the eccentricity sometimes being so great that the cord is fixed to the periphery of the placenta, such disposition constituting insertio marginalis. Among the more exceptional variations in the arrangement of the cord are the cleft and the extraplacental attachment known respectively as insertio furcata and insertio vela- mentosa. In the former condition, where the cord divides before reaching the pla- centa, each limb conveys one of the umbilical arteries and a branch of the umbilical When the insertion of the FIG. 64. Artery Artery Vein Transverse section ot umbilical cord at end of pregnancy, taken from placental end ; the umbilical blood-vessels are em- bedded within the embryonal connective tissue. X 10. vein. cord is into the chorion entirely outside the placental area, in ex- ceptional cases being as far re- moved as the opposite pole of the membranous capsule, the umbilical vessels course within the non-vil- lous portions of the chorion until they reach the fcetal placenta. In addition to the true knots, which often occur and are due to the excursions of the fcetus, the um- bilical cord sometimes presents nodular thickenings and irregular constrictions, as well as projections formed by loops and varicosities of the blood-vessels. The After-Birth. The ex- pulsion of the child through the rupture in the enveloping mem- branes, which is produced by the powerful contractions of the uterine muscle at the close of pregnancy, is followed, after a short interval, by the separa- tion and expulsion of the "after-birth ;" under this term are included the placenta and the enveloping membranes. The latter, as will be understood from the fore- going consideration of the encapsulation of the fcetus, consist of three chief constit- uents, the remains of the decidua vera, the chorion, and the amnion ; the reflexa undergoes complete absorption. Since the decidua represents the shed portion of the modified uterine mucosa, the outer surface of the after-birth appears rough and studded with shreds of uterine tissue ; the inner surface of the decidua is so closely fused with the adjacent cho- FIG. 65. rion by means of delicate connective tissue that only a limited and uncertain separation is possible. The amnion, on the other hand, although attached to the chorion by bands of connec- tive tissue, may be peeled off the chorion with relative ease, since the union be- tween the two membranes is never firm. The inner ectoblastic surface of the amnion in contact with the fcetus is smooth and bathed in the liquor amnii. The external and unshed portion of the modified uterine mucosa contains the incon- spicuous remains of the epithelium lining the fundus of the glands : these elements are of the utmost importance for the regeneration of the glandular and epithelial tissues of the new uterine mucous membrane, since the reparation of these struc- tures, which is effected within a few weeks after labor, begins in the proliferation of the deeper glandular epithelium, which remains throughout pregnancy as the latent source of subsequent repair. Decidua Remains of uterine glands Uterine muscle Section through f ostal membranes and uterus at end of pregnancy. (After Leopold.) HUMAN ANATOMY. DEVELOPMENT OF THE GENERAL BODY-FORM. In considering the evolution of the external form of the human product of con- ception, it is convenient to recognize the three developmental epochs suggested by His, the stage of the ovum, the stage of the embryo, and the stage of the foetus. The Stage of the Blastodermic Vesicle. This stage, or the stage of the ovum, embraces the first two weeks of intra-uterine life, during which the initial phases of development, including fertilization, segmentation, and the formation of the blasto- FIG. 66. Villous chorion Embryo with amnion tii^i<&%g3 *liJrJ*t3A/2 ~3$f Umbilical vesi- cle ; blood- islands appear- ing Chorionic Early human embryonic vesicle of about thirteen days laid open, showing the young embryo (.37 millimetre long) attached to the wall of the serosa by means of the belly-stalk. X 25. (After Spec.) dermic vesicle, are completed, and the fundamental processes resulting in the differ- entiation of the medullary tube, the notochord, the somites, and the mesoblastic plates are begun. The early details of many of these processes have never been observed in man, but there is little reason to doubt that in its essential features the early human embryo closely follows the changes directly observed in other mammals. The Stage of the Embryo. The stage of the embryo, from the second to the fifth week, is distin- I?IG - 6 7- guished by the formation of organs essentially embry- onic and transient in char- acter, as the somites, the notochord, the Wolffian body, and the visceral arches. The earliest phase in the differentiation of the vertebrate body-form con- sists in the establishment of a dorsal tube by the appo- sition and fusion of the ectoblastic medullary folds, and a ventral tube by the approxi- mation and final union of the folds directly derived from the somatopleura. The dorsal, or animal, tube represents the early neural canal, and becomes the great cerebro-spinal nervous axis ; the ventral, or vegetative, tube, formed by the ventral extension and approximation of the somatopleura, constitutes the body-cavity, and encloses the primary gut and the associated thoracic and abdominal viscera, and the vascular system. The primitive gut-tube originates by the delimitation of a part of Mesoblast Belly-stalk Allantoic duct Umbilical sac- Section of preceding embryonic vesicle and embryo. X 25. (After Spee.) THE EARLY HUMAN EMBRYO. 57 the vitelline sac accomplished by the ventral approximation of the splanchnopleura, and for a time maintains a wide communication with the remains of the yolk-cavity. The early embryo, lying flatly expanded upon the blastodermic vesicle, becomes differentiated in form by the appearance of head- and tail-grooves, in consequence of 530 .0-0 I '5 which constriction the cephalic and the caudal poles of the body become defined and partially separated from the embryonal area ; the middle segment, however, em- bracing the widely open gut-tract, for a time remains closely blended with the vitel- line sac, of which, at first, the embryo appears as an appendage (Fig. 68, i and 2~). HUMAN ANATOMY. The more complete differentiation of the digestive tube and the ventral folding in of the body-walls change this relation, the rapidly decreasing umbilical vesicle soon becoming secondary to the embryo. At the close of the stage of the blastodermic vesicle about the fifteenth day the embryo possesses a general cylindrical body-form, the dilated cephalic pole being free, while the belly-stalk attaches the caudal segment to the chorion ; the amniotic sac invests the dorsal aspect, the large umbilical vesicle occupying the greater part of the ventral surface. Human FIG. 69. Otic vesicle Second visceral arch / Cephalic flexure -^- " Optic vesicle Maxillary process Mandibular proces: of first visceral arch Caudal end ol embryo Umbilical cord in section Lower limb-bud''" -Third visceral arch -Fourth visceral arch -Ai-Heart -Upper limb-bud Human embryo of about twenty-three days, drawn from the model of His. X 10. embryos of the fourteenth and fifteenth days (Fig. 68, 3 and 4) are distinguished by a conspicu- ous flexure opposite the attach- ment of the umbilical vesicle, the convexity being directed ven- trally, the deep corresponding concavity producing a marked change of profile in the dorsal outline. During these changes the expansion of the cerebral segments outlines the three pri- mary divisions of the cephalic portion of the neural tube, the anterior, the middle, and the posterior brain-vesicles. A little later a series of conspicuous bars, the visceral arches, appears as ob- liquely directed parallel ridges on either side of the head, immediately above the prominent heart-tube, which is now undergoing marked torsion. By the nineteenth day the dorsal concavity, which is peculiar to the human embryo, has entirely disap- peared, the profile of this part of the embryo presenting a gentle convexity ; the cephalic axis, however, exhibits a marked bend, the cephalic flexure, in the vicinity of the middle cerebral vesi- cle, in consequence of which the axis of the anterior cere- bral segment lies almost at right angles to that of the Cephalic flexure-/ Optic vesicle FIG. 70. Otic vesicle Mandibular process of first visceral arch Olfactory pit Umbilical cord Heart Upper limb-bud middle vesicle. The com- pletion of the third week finds the characteristic de- tails of the cephalic end of the embryo, the cerebral, the optic, and the otic vesi- cles, and the visceral arches and intervening furrows well advanced, with correspond- ing definition of the primitive heart and the umbilical stalk and vesicle. The limb-buds usually appear about this time, those of the upper ex- tremity slightly preceding those of the lower. The period between the twenty-first and the twenty-third days witnesses remarkable changes in the general appearance of the embryo ; in addition to greater prominence of the visceral arches, the cerebral segments, and the limb-buds, the embryonic axis, which, with the exceptions already noted, up to this time is only slightly curved, now undergoes flexion to such extent that by the twenty-third day the overlapping cephalic and caudal ends of the embryo are in close apposition, the body-axis describing rather more than a complete circle (Fig. 69). Cervical flexure Second visceral arch Third visceral arch Fourth visceral arch Lower limb-bud 1 ^ Human embryo of about twenty-five days, drawn from the model of His. X 10. THE VISCERAL ARCHES AND FURROWS. 59 From the twenty-third to the twenty-eighth day the excessive flexion gradually disappears, owing to the increased volume of the heart and the growth of the head, and by the end of the fourth week the embryo has acquired the most characteristic development of the embryonic stage (Fig. 71). The reduction in the curvature of the body-axis and the consequent separation of its poles and the raising of the head are accompanied by the appearance of four well-marked axial flexions, the cephalic, the cervical, the dorsal, and the sacral flexures (Fig. 71). The first of these, the cephalic, is an accentuation of the primary flexure, which is seen as early as the eighteenth day, and is indicated by the projection of the midbrain ; it corresponds in position to the future sella turcica. The second and very conspicuous bend, the cervical flexure, marks the caudal limit of the cephalic portion of the neural axis, and agrees in position with the subsequent upper cervical region. The dorsal and sacral flexures are less well defined, the former being situated opposite the upper limb-bud, where the cervical and dorsal series of somites join, the latter, near the lower limb-bud, corresponding with the junction of the lumbar and sacral somites. The cephalic segment at this stage presents numerous prominent details, the FIG. 71. Cervical flexure , Third visceral arch Otic vesicle-^^ >w /""^ ^First external visceral furrow Second visceral arch -Mandibular process of first Maxillary process of first ^.^p ^ |l \. "*S&> visceral arch visceral arch ^j^ .^A Dorsal flexure m Cephalic flexure- ^. . __ ^- TT Upper limb . -" m- v Eye- Olfactory pit Heart Umbilical cord- Lower limb "T Sacral flexure Human embryo of about twenty-eight days, drawn from the model of His. X 10. secondary cerebral vesicles, the forebrain, the interbrain, the midbrain, the hind- brain, and the after-brain, the visceral arches and furrows, the optic and otic vesicles, the olfactory pits, and the primitive oral cavity all being conspicuous. The heart ap- pears as a large protrusion, occupying the upper half of the ventral body-wall, on which the primary auricular and ventricular divisions are distinguishable. The somites form a conspicuous longitudinal series of paraxial quadrate areas, about thirty-seven in number ; they correspond to the intervertebral muscles, and may be grouped to accord with the primary spinal nerves, being, therefore, distinguished as eight cer- vical, twelve dorsal, five lumbar, five sacral, and five or more coccygeal somites. THE VISCERAL ARCHES AND FURROWS. Since the visceral arches are best developed in the human embryo during the last half of the third week, a brief consideration of these structures in this place is appropriate. The visceral arches in mammalian embryos constitute a series of five parallel bars separated by intervening furrows, obliquely placed on the ventro-lateral aspect of the cephalic' segment, occupying the region which later becomes the neck. They represent, in rudimentary development, the important branchial or gill- 60 HUMAN ANATOMY. apparatus of water-breathing vertebrates, in which the respiratory function is per- formed by means of the rich vascular fringes lining the clefts through which the water passes, thus permitting the exchange between the oxygen of the water and the carbon dioxide of the blood. Each arch is supplied by a blood-vessel, or aortic bow, which passes from the main ventral stem, the truncus arteriosus, through the substance of the visceral arch backward to unite with the similar bows to form the dorsal aorta. In aquatic vertebrates the aortic bows supply an elaborate system of secondary branchial twigs, which form rich capillary plexuses within the gills ; in air-breathing vertebrates, however, in which these structures are only rudimen- tary, the main stems, the aortic bows, are alone represented. With the loss of func- tion which follows the acquisition of aerial respiration in the higher vertebrates, the number of visceral arches is reduced from six, or even seven, as seen in fishes, to five, the fifth arch in man, however, being so blended with the surrounding struc- tures that it is not visible externally as a distinct bar. In their condition of great- est perfection, as in fishes, each visceral arch contributes an osseous bar, which forms part of the branchial skeleton ; these bony bars are represented in man and mammals by cartilaginous rods, which temporarily occupy the upper arches, for the most part entirely disappearing. When viewed in frontal section (Fig. 73), the mammalian visceral arches are seen as mesodermic cylinders imperfectly separated by external and internal grooves, the visceral furrows and the pharyngeal pouches respectively ; this arrangement emphasizes another modification following loss of function, namely, the conversion of the true visceral clefts of the lower forms into furrows, since in man and mammals the FIG. 72. fissures are closed by the occluding mem- brane formed by the apposition of the ectoblast and the entoblast at the bottom of the outer and inner furrows. Maxiiiarv 'i The First or Mandibular Arch process - --=4 early becomes differentiated into a short p . -.- /m. / . Second arch oral "alky / / x_Third arch u PP er or maxillary process and a longer Mandibular -xj, f j lower or mandibular process. The maxil- process -re Fourth arch , ... .. r ,, ,*. lary process, in conjunction with its fellow - 4; -Filth arch r f u -J i ^ s i of the opposite side and the fronto-nasal Head of human embryo of about twenty-one days, yW/vvcc whiVh H<=>c:<-p>nr1 Q a mprlian rrn seen from the side, showing visceral arches and external Process, WHlcn dCSCC visceral furrows, x 20. (After His.} jection from the head (Fig. 75), contrib- utes the tissue from which the superior and lateral boundaries of the oral cavity and the nasal region are derived. The mandibular process joins with its mate in the mid-line and gives rise to the lower jaw and other tissues forming the inferior boundary of the primary oral cavity. The latter in its original condition appears as a widely open space leading into the primi- tive pharyngeal cavity ; later the septum is formed which divides the oral from the nasal cavity. The mandibular process contains a cartilaginous rod, which for a time represents the corresponding bony arch of the visceral skeleton of lower types. The ventral and larger part of this rod, known as Meckel 's cartilage, entirely disap- pears, the lower jaw being developed independently around this bar of cartilage ; the upper end of the cartilaginous bar, however, persists and forms two of the ear- ossicles, the malleus and the incus. The Second or Hyoid Arch also contains a cartilaginous bar, from the ven- tral segment of which (known as the cartilage of Reichert*} is derived the smaller cornu of the hyoid bone ; the dorsal end of the bar, which is fused with the tem- poral bone, gives rise to the styloid process, the intervening portion of the cartilage persisting as the stylo-hyoid ligament. The cartilage of the second arch is also con- cerned in the formation of the stapes. The origin of this ear-ossicle is double, since the crura of the stapes are derived from the cartilage of the hyoid arch, while the base is contributed by the general cartilaginous capsule of the labyrinth. The char- acteristic form of the stapes is secondary and due to the perforation of the triangu- lar plate, the early representative of this bone, which thus acquires its characteristic stirrup-shape in consequence of the penetration of a minute blood-vessel, the perfo- rating stapedial artery, a branch of the internal carotid, which later disappears. THE VISCERAL ARCHES. 61 The Third or First Branchial Arch contains a rudimentary cartilaginous bar from which part of the body and the greater cornu of the hyoid bone are derived. I ne fourth and fifth arches, or second and third branchial, enclose rudimentary cartilaginous bars which early fuse into plates ; these unite along their ventral borders and give rise to the thyroid cartilage of the larynx. The External Visceral Furrows (Fig. 73), the representatives of the true clefts of the lower types, appear with decreasing distinctness from the first towards the fourth ; the third and fourth early suffer modification, so that by the twenty-eighth day the first and second furrows alone are clearly defined. The First Visceral Furrow, the hyomandibular cleft, undergoes obliteration except at its dorsal part, which becomes converted into the external auditory meatus the surrounding tissue giving rise to the walls of the canal and the external ear. The remaining clefts gradually disappear, becoming closed and covered in by the over- hanging corresponding arches ; this relation is particularly marked towards the FIG. 73. Optic vesicle Maxillary process Dorsal wall of primitive oropharynx Primitive oesophagus Upper end of body-cavity Right umbilical vein Tuberculum irnpar Primitive larynx IV Body-cavity Neural tube Upper half of human embryo of about eighteen days, drawn from His's models. X 45. A, dorsal wall of primitive oropharynx bounded by visceral arches, external and internal furrows. , anterior wall of primitive oropharynx, seen from behind. 1-5, sections of aortic arches; I-IV, external visceral furrows. caudal end of the series, where the sinking in of the arches and the included furrows produces a depression or fossa the sinus pracervicalis of His in the lower and lateral part of the future neck region. This recess subsequently entirely disappears on coalescence of the bordering parts ; sometimes, however, such union is defective, the imperfect closure resulting in a permanent fissure situated at the side of the neck, known as cervical fistula, by means of which communication is often established between the pharynx and the exterior of the body. Such communication must, however, be regarded as secondary, as originally the external furrows were sepa- rated from the primitive pharyngeal cavity by the delicate epithelial septum already mentioned as the occluding plate. Where entrance into the pharynx through the fistula is possible, it is probable that the septum has been destroyed as the result of absorption or of mechanical disturbance following the use of the probe. The Inner Visceral Furrows, or pharyngeal pouches, repeat the general arrangement of the external furrows. The first pharyngeal pouch becomes narrowed and elongated, and eventually forms the Eustachian tube ; a secondary 62 HUMAN ANATOMY. FIG. 74. Fronto-nasal process Olfactory pit Primitive oral cavity Mesial nasal process Maxillary process Mandibular process Head of human embryo of about twenty-seven days, showing boundaries of primitive oral cavity. X 7- (After His.) dorsal expansion gives rise to the middle ear, while the occluding plate separating the outer and inner furrows supplies the tissue from which the tympanic membrane is formed. The second furrow in great part disappears, but its lower portion con- tributes the epithelium of the faucial tonsil and the supratonsillar fossa. The fossa of Rosenmiiller is a secondary depression and probably does not represent the original furrow. The third and fourth pouches give rise to ventral entoblastic outgrowths from which the epithelial portions of the thymus and of the thyroid body are developed respectively. The last-named organ has an additional unpaired origin from the ento- blast forming the ventral wall of the pharynx in the vicinity of the second visceral arch. The Development of the Face and the Oral Cavity. The earliest suggestion of the primitive oral cavity is the depression, or stomod&um, which ap- Laterai nasal process pears about the thirteenth day on the ventral surface of the cephalic end of the embryo immediately beneath the ex- panded anterior cerebral vesicle. The oral pit at first is separated from the ad- jacent expanded upper end of the head- gut by the delicate septum, the pharyn- geal membrane, composed of the opposed ectoblast and the entoblast, which in this location are in contact without the inter- vention of mesoblastic tissue. With the rupture of the pharyngeal membrane, the deepened oral pit opens into the cephalic extremity of the head-gut, now known as the primitive pharynx. The formation of the face is closely associated with the growth and fusion of the upper visceral arches in conjunction with the surrounding parts of the ventral surface of the head. The first visceral arch, as already described, presents two divisions, the maxillary and the mandibular process. The latter grows ventrally and joins in the mid-line its fellow of the opposite side, to form, with the aid of the second visceral arches, the tissues from which the lower boundary and the floor of the mouth are derived. The upper and lateral boundaries of the primitive oral cavity and the differentiation of the nasal region proceed from the modification and fusion of three masses, the two lateral paired maxillary processes of the first visceral arches and the mesial unpaired fronto-nasal process, which descends as a conspicuous projection from the ventral surface of the anterior part of the head. The maxillary processes grow towards the mid-line and, in conjunction with the descending fronto-nasal projection, form the lateral and superior boundary of the primitive oral cavity (Fig. 74). Very soon the development of the future nares is suggested by the appearance of slight depressions, the olfactory pits, one on each side of the fronto-nasal process ; these areas constitute part of the wall of the forebrain, a relation which foreshadows the future close association between the olfactory mucous membrane and the cortex of the olfactory lobe. During the fifth week the thickened margins of the fronto-nasal process undergo differentiation into the mesial nasal processes, while coincidently the lateral portions of the fronto-nasal projection grow downward as the lateral nasal processes, these newly developed projections constituting the inner and outer boundaries of the rapidly deepening nasal pits. The line of contact between the lateral nasal process and the maxillary process is marked by a superficial furrow, the naso-optic groove, FIG. 75. Lateral nasal process Maxillary process First external vis- ceral furrow' Second visceral arch Third visceral arch Head of human embryo of about thirty-four days. (After His.) X5- THE STAGE OF THE FCETUS. which leads from the nasal pit to the angle of the eye ; this furrow, however, merely indicates the position of the naso-lachrymal duct which develops independently at the bottom of the primary groove. Reference to Figs. 74 and 75 emphasizes the fact that the nasal pits and the primitive oral cavity are for a time in widely open com- munication ; towards the close of the sixth week, however, the maxillary processes of the first arch have approached the mid-line to such an extent that they unite with the lateral margins of the fronto-nasal process as well as fuse with the lateral nasal processes above. Owing to this union of the three processes, the nasal pits become separated from the oral cavity, and with the appearance and FIG. 76. completion of the palatal sep- tum the isolation of the nasal fossae from the mouth is ac- complished. The lateral nasal processes contribute the nasal Naso-optic groove' Dorsum of nose Anlage producing nasal tip Nasal groove Oral surface of maxillary process Maxillary process of first arch Roof of oropharynx Portion of head of human embryo of about thirty-four days, showing roof of primitive oral cavity. X 10. (After His.) FIG. 77. alae, while from the conjoined mesial nasal process are devel- oped the .nasal septum and the bridge of the nose in addition to the middle portion of the upper lip and the intermaxil- lary segment of the upper jaw, the superior maxillary part of the latter being a derivative of the maxillary process of the first arch. Arrested development and imperfect union between the maxillary processes and the fronto-nasal process result in the congenital defects known as harelip and cleft palate, the degree of the malformation depending upon the extent of the faulty union. The Stage of the Foetus. The fifth week marks the completion of the period of development during which the product of conception has acquired the characteristic features of its embryonal stage ; beginning with the second month and continuing until the close of gestation, the succeeding stage of the fcetns is distin- guished by the gradual assumption of the external features which are peculiar to the young human form. In addition to the already mentioned changes affecting the visceral arches and frontal process in the development of the face, the fifth week witnesses the differentiation of the limbs into segments, the distal division of the upper extremity exhibiting indica- tions of the future fingers, which thus anticipate the appear- ance of the toes. The liver is already conspicuous as a marked protuberance occupying the ventral aspect of the trunk immediately below the heart. The head by this time has acquired a relatively large size, the prominent cephalic flexure which marks the position of the midbrain being par- ticularly conspicuous. At the end of the fifth week, or the thirty-fifth day, the foetus measures about fourteen millimetres in its longest dimension. The sixth week finds the foetus elongated with greater distinctness of the human form, the large size of the head, on which the cervical flexure is very evident, being highly characteristic when compared with corresponding stages of the lower mammals. The several constituents of the face become more perfectly formed, including the completion of the superior boundary of the oral cavity and its separation from the nasal pits by the septum resulting from the union of the fronto-nasal process with the maxillary processes ; the fusion of the latter with the lateral frontal processes now defines the external boundary of the nostrils of the still, however, broad and flattened nose, which lies immediately above the transverse cleft-like oral opening. The visceral arches are no longer visible as individual bars, having undergone com- plete fusion. The differentiation of the digits on both hands and feet has so far Head of human embryo of about seven weeks. X 5. (After Ecker.) 6 4 HUMAN ANATOMY. progressed that fingers and toes are distinctly indicated, although the fingers only are imperfectly separated. The first suggestion of the external genitals appears about the end of the sixth week. At this time the fcetus measures about nineteen millimetres. During the seventh and eighth weeks the fcetal form of the body and the limbs attain greater perfection, the large head becoming raised from the trunk and the toes, as well as fingers, being now well formed, although the rudiments of the nails do not appear until some time during the third month. At the close of the second month the extra-embryonic protrusion of the intestine through the umbilicus into the umbilical cord reaches its greatest extent. The genito-urinary system is repre- sented by the fully developed Wolffian body, the vesical dilatation of the allantoic duct, the separation of the cloaca into rectum and genito-urinary passage, the indif- Umbilical vesicle Umbilical stalk Inner surface of amnion Umbilical cord -_. Human embryo of about thirty-five days. X 4- Amnion and chorion cut and turned aside. ferent sexual gland, and the undifferentiated external genitals, consisting of the geni- tal eminence and the associated genital folds and genital ridges. The external ear has assumed its characteristic form, and the eyelids appear as low folds encircling the conspicuous eye, in which the pigmentation of the ciliary region is visible. Although the face is well formed, the nose is still flat, the lips but slightly prominent, and the palate not completely closed. The rapid growth of the brain results in the dispro- portionate size of the head, which at this stage almost equals the trunk in bulk. It is to be noted that by the close of the second month. the permanent organs are so far advanced that the subsequent growth of the fcetus is effected by the further de- velopment of parts already formed and not by the accession of new organs. The beginning of the second month marks the period of greatest relative growth; at the end of this month the fcetus measures about thirty millimetres in its longest dimension. STAGE OF THE FCETUS. 65 The third month is characterized by greater perfection of the external form, the rounded head is raised from the trunk so that a distinct neck appears, while the thorax and abdomen are less prominent ; the limbs, which are well developed with completed differentiation of the fingers and toes, provided with imperfect nails, now assume the characteristic fcetal attitude. The eyelids become united by the tenth week, remaining closed until the end of the seventh month. The cloacal opening becomes differentiated during the ninth and tenth weeks into the genito-urinary and FIG. 79. Umbilical cord Umbilical stalk Allantoic vessels Umbilical vesicle Human fcetus of about eight weeks. X 3%. Amnion has been cut and reflected, but still covers the umbilical vesicle and its stalk. the anal orifice, while during the eleventh and twelfth weeks the external genital organs acquire the distinguishing peculiarities of a definite sex. The greatest length of the fcetus, measured in its natural position and excluding the limbs, at the end of the third month, is about eighty millimetres ; its weight approximates twenty grammes. The fourth month witnesses augmented growth in the fcetus, which, how- ever, resembles in its general appearance the fcetus of the preceding month. The 5 66 HUMAN ANATOMY. extra-foetal portion of the intestinal canal, which at an earlier period passes into the umbilical cord, during the fourth month recedes within the abdomen. The differentiation of sex is still more sharply exhibited by the external organs : in the male the penis is acquiring a prepuce, and in the female the labia majora and the clitoris are becoming well developed. At the close of this period the foetus measures approximately 150 millimetres and weighs about 120 grammes. During the fifth month the first foetal movements are usually observed. The heart and the liver are relatively of large size. The decidua capsularis fuses with the decidua vera, thereby obliterating the remains of the uterine cavity. The meco- nium within the intestinal canal shows traces of bile. The advent of the fine hair, the lanugo, first upon the forehead and the eyebrows, and somewhat later upon the scalp and some other parts of the body, represents a conspicuous advance. Likewise adipose tissue appears in places within the subcutaneous layer. The approximate length, at the end of the fifth month, is twenty-three centimetres and the average weight about 320 grammes. The sixth month is characterized by complete investment of the body by lanugo and by the appearance of the vernix caseosa, the protecting sebaceous secre- tion which coats the body of the foetus to prevent as far as possible maceration of the epidermis in the amniotic fluid. The latter now reaches the maximum quantity, being contained within the large sac of the amnion. The sixth month is distin- guished by the conspicuous increase both in the size and weight of the fcetus, and is known, therefore, as the period of greatest absolute growth. At the close of the sixth month the fcetus measures approximately thirty-four centimetres in its longest dimension and weighs about 980 grammes. The seventh month is marked by progressive changes in the various parts of the fcetus, whereby the more advanced details become pronounced in the central nervous system and digestive tract. The length of the fcetus at the close of the seventh month approximates forty centimetres and its weight about 1700 grammes. The eighth month is occupied by the continued growth and general develop- ment, as part of which the fcetus acquires greater plumpness than before and a brighter hue of the integument, now entirely covered with vernix caseosa. The lanugo begins to disappear, while the scalp is plentifully supplied with hair ; the nails have reached, or project beyond, the tips of the fingers. By the close of the eighth month the foetus has attained a length of about forty-six centimetres and a weight of about 2400 grammes. The ninth month witnesses the gradual assumption of the characteristics of the child at birth, among which are the rounder contours, the extensive, although not complete, disappearance of the lanugo, except from the face, where it largely persists throughout life, the completed descent of the testicles within the scrotum, the approximation of the labia majora, the permanent separation of the eyelids, with well-developed lashes, and the presence of dark greenish meconium within the in- testinal canal. The umbilicus has reached a position almost exactly in the middle of the body. The average length of the fcetus at birth is about fifty centimetres, or twenty inches ; its average weight, while included between widely varying extremes, may be assumed as approximately 3100 grammes, or 6.8 pounds. The weight of the fcetus at term is materially influenced by the age of the mother, women of about thirty-five years giving birth to the heaviest children. The weight and stature of the mother probably also affect the weight of the child. Repeated pregnancies exert a pronounced effect upon the foetus, since the weight of the child reaches the maximum with the fifth gestation. The purpose of the preceding pages is to present an outline of the general developmental processes leading to the differentiation and establishment of the defi- nite body-form of the human embryo ; a more detailed account of the development of the various parts of the body is given in connection with the descriptions of the systems and the individual organs, to which the reader is referred. THE ELEMENTARY TISSUES. THE various parts and organs of the complex body may be resolved, in their morphological constitution, into a few component or elementary tissues, of which there are four principal groups, the epithelial, the connective, the muscular, and the nervous tissues. The first two of these may be discussed at this place ; the re- maining groups, the muscular and the nervous tissues, are considered most advan- tageously in connection with the muscular and nervous systems to which they are directly related and under which sections they will be found. THE EPITHELIAL TISSUES. The epithelial tissues include, primarily, the integumentary sheet of protecting cells covering the exterior of the body and the epithelium lining the digestive tube. Secondarily, they embrace the epithelial derivatives of the epidermis, such as the nails, hairs, and glands of the skin and its extensions, and the epithelial lining of the ducts and compartments of the glands formed as outgrowths from the primi- tive gut-tube, as well as the epithelium clothing the respiratory tract which originates as an evagination from the digestive canal. An apparent exception to the usual origin of the epithelial tissues from either the ectoblast or the entoblast is presented by the lining of the genito-urinary tract, since all the epithelium occurring in connection with these organs, as far as the bladder, is of mesoblastic origin, and hence genetically related closely with the extensive mesoblastic group of tissues. It is to be noted in this connection that the epithelium of the bladder and of a part of the urethra is derived from outgrowths of the primary gut, and therefore is entoblastic in origin. The primary purpose of epithelium being protection of the more delicate vascular and nervous structures lying within the subjacent connective tissue of the integument or of the mucous membrane, the protecting cells are arranged as a con- tinuous sheet, the individual elements being united by a small amount of inter- cellular substance. Epithelium contains no blood-vessels, the necessary nutrition of the tissue being maintained by the absorption of the nutritive juices which pass to the cells by way t of the minute clefts within the intercellular substance. Likewise, the supply of nerve-fibres within epithelium ordinarily is scanty, although in certain localities possessing a high degree of sensibility, as the cornea or tactile surfaces, the termi- nations of the nerves may lie between the epithelial elements. The epithelial tissues are frequently separated from the subjacent connective tissue by a delicate basement membrane, or membrana propria ; the latter, which may be regarded as a derivative or modification of the connective tissue, usually appears as a delicate subepithelial boundary, being particularly well marked beneath the epithelium of glands. According to the predominating form of the component cells, the epithelial tissues are best divided into two chief groups, squamous and columnar, with sub- divisions as shown in the following table : VARIETIES OF EPITHELIUM. I. SQUAMOUS : a. Simple, consisting of a single layer. b. Stratified, consisting of several layers. II. COLUMNAR : a. Simple, consisting of a single layer. b. Stratified, consisting of several layers. III. MODIFIED : a. Ciliated, b. Goblet, c. Pigmented. IV. SPECIALIZED : a. Glandular epithelium, b. Neuro-epithelium. 67 68 HUMAN ANATOMY. Squamous epithelium, when occurring as a single layer, is composed of flattened polyhedral nucleated plates which, when viewed from the surface, present a regular mosaic, sometimes described by the terms ' ' pavement' ' or ' ' tessellated. ' ' Such arrangement of the squamous type is unusual in the human body, the lining of the alveoli of the lungs, the posterior surface of the anterior capsule of the crystalline lens, the membranous labyrinth, and a few other localities being the chief places where a single layer of squamous cells occurs. The far more usual arrangement of such cells is several superimposed layers, this constituting the important group of stratified squamous epithelia. When FIG. 81. ,-">_ ^ "^ Vi - ^ f it --Til ^*i _^^* Wi^m Simple squamous epithelium from anterior capsule of crystalline lens. X4oo. Section of stratified squamous epithelium from anterior surface of cornea. X 500. seen in section, the deepest cells are not scaly, but irregularly columnar, resting upon the basement membrane by slightly expanded bases. The surface of the un- derlying connective tissue supporting this variety of epithelium is beset with minute elevations or papillae, which serve as advantageous positions for the terminations of the blood-vessels, as well as specialized nerve-endings. Owing to the more favored nutrition of the deepest stratum, the cells next the connective tissue exhibit the greatest vitality, and often are the exclusive source of the new elements necessary FIG. 82. FIG. 83. Isolated surface cells from epithe- lium lining the mouth. X 350. Epithelial cells from epider- mis, showing intercellular bridges. X6 75 - to replace the old and effete cells which are continually being removed at the free surface ; this loss is due not only to mechanical abrasion, but also to the displace- ment of the superficial elements by the new cells formed within the deeper layers. Passing from the basement membrane towards the free surface, the form of the cells undergoes a radical change. The columnar type belongs to the deepest layer alone ; the superimposed cells assume irregularly polyhedral forms and then gradu- ally expand in the direction parallel to the free surface to become, finally, converted into the large, thin scales so characteristic of the superficial layers of stratified epithelium. The position of the nucleus also varies with the situation of the cells, EPITHELIUM. 69 since within those next the basement membrane the relatively large nucleus the nutritive organ of the cell occupies the end nearest the subjacent connective tissue ; in the middle and superficial strata, the nucleus, comparatively small in size, is placed about the centre of the cell. The irregularly polyhedral cells of the deep or middle strata frequently are connected by delicate processes which bridge the intervening intercellular clefts ; when such elements are isolated, the delicate connecting threads are broken and the disassociated elements appear beset with minute spines, then constituting the prickle- cells. In certain localities, as in the urinary bladder, the columnar cells of the deepest layer rapidly assume the scaly character of the superficial strata ; such epithelium FIG. 85. FIG. Transitional epithelium from bladder of child. X 3o. Simple columnar epi- thelium from intestinal mucosa. X 750. Stratified columnar epithelium from vas deferens. X 500. possesses relatively few layers, and from the readiness with which the type of the cells changes, is often described as transitional epithelium; the latter cannot be regarded as a distinct variety, but only as a modification of the stratified scaly group. Columnar epithelium, when occurring as a single layer of cells, constitutes the simple columnar variety, which enjoys a much wider distribution than the cor- responding squamous group, the lining of the stomach and of the intestinal tube being important examples. When the single layer of such epithelial tissues is re- placed by several, as in the stratified columnar variety, the superficial cells alone FIG. 87. V..:-^-:: --*$ Ciliated border Stratified ciliated columnar epithelium from trachea of child. X 55- Ciliated epithelial cells. A, from intes- tine of a mollusk (cyclas) ; B, from nasal cavity of frog. X 75- (Engelmann.) are typically columnar. The free ends of the columnar elements not infrequently present specializations in the form of a cuticular border or of cilia, while their ends which rest upon the basement membrane are pointed, forked, or club-shaped. The intervals thus formed by irregularities of contour are occupied by the cells of the deeper stratum next the basement membrane. Each cell is provided with a nucleus, which is situated about midway between the ends of the superficial elements and nearer the base within the deeper ones. The surface cells often contain collections of mucous secretion which distend their bodies into conspicuous chalice forms known as goblet-cells, which occur in great profusion in the lining of the large intestine and the respiratory mucous membrane. HUMAN ANATOMY. FIG. 89. Goblet-cells from epithelium lining large intestine. X 500. Modified Epithelium. The free surface of the epithelium in many localities, as in the trachea, the inferior and middle nasal meatuses, and the uterus, is pro- vided with minute, hair-like vibratile processes, or cilia, which are produced by the specialization of the cytoplasm of the free end of the cell. The exact relations of the cilia to the cytoplasm are still matters of uncertainty, although the investigations of Engelmann and others on the ciliated epithelium of invertebrates render it prob- able that the hair-like processes attached to the cells of higher animals are also connected with intracellular fibrillae, which appear as delicate striations within the superficial and more highly specialized parts of the cells. In man and the higher mammals ciliated epithelium is limited to the columnar variety. The exact number of individual cilia attached to the free surface of each cell varies, but there are usually between one and two dozen such appendages. Their length, likewise, differs with locality, those lining the epididymis being about ten times longer than those attached to the tracheal mucous membrane. When favorable conditions obtain, including a suffi- cient supply of moisture, oxygen, and heat, ciliary motion may continue for many hours and even days. On surfaces clothed with columnar epithelium certain cells are distinguished by unusually clear cytoplasm and exceptional form and size ; these are the goblet-cells, the peculiar elliptical or chalice form of which results from the accumulation of the mucoid secretion elaborated within their protoplasm. When the distention becomes too great the cell ruptures in the direction of least resistance, and the secretion is poured out upon the surface of the mucous membrane as the lubricating mucus. The goblet-cells, therefore, may be regarded as unicellular glands, and represent the simplest phase in the specialization of glandular tissues. The protoplasm of epithelial cells often becomes invaded by particles of foreign substances ; thus, granules of fatty and proteid matters are very commonly encoun- tered, while the presence of granules of eleidin in certain cells of the epidermis char- acterizes the stratum granulosum. When the invading particles are colored, as when composed of melanin, the affected cells acquire a dark brown tint, and are then known as pigmented epithelium. Examples of such cells are seen in the retina and in the deeper cells of the epidermis in certain races. Specialized Epithelium. Reference has already been made to goblet-cells as representing unicellular glands ; these may be regarded, therefore, as instances of a temporary specialization of epithelium into glandular tissue. When the epithelial elements become permanently modified to engage in the elaboration of secretory substances, they are recognized as glandular epithelium. The cells lining the ducts and the ultimate compartments of glands are modified exten'sions of the epithelial investment of the adjacent mucous membrane. Their form and condition depend upon the degree of speciali- zation, varying from columnar to spherical and polyhedral, on the one hand, and upon the nature and number of the secre- tion particles on the other. The cells lining parts of certain glands, as those clothing the ducts of the salivary glands, or the irregular portion of the uriniferous tubules, exhibit a more or less pronounced striation ; cells presenting this peculiarity are termed rod-epithelium. The highest, and often exceedingly complex, specializations affecting epithelial tissues are encountered in connection with the neurones supplying the organs of special sense. The epithelium in these localities is differentiated into two groups of elements, the sustentacular and the perceptive ; to the latter the name of neuro- epithelium is applied. Conspicuous examples of such specialization are the rod- and cone-cells of the retina and the hair-cells of Corti's organ in the internal ear. A more detailed description of the glandular tissues is given with the digestive tract ; that of the neuro-epithelia with the organs of special sense. FIG. 90. Pigmented epithelium from human retina. X 435- ENDOTHELIAL TISSUES. Mesothelial cells from omentum of dog. X 300. Intercellular cement-substance stained by argentic nitrate. ENDOTHELIUM. The modified mesoblastic, later connective-tissue, cells that line serous surfaces, including those of the pericardial, the pleural, and the peritoneal divisions of the body- cavity, together with those of the blood- and lymph-vessels and the lymphatic spaces throughout the body, constitute endothelium. These spaces, in principle, are intramesoblastic clefts and the elements forming their lining are derivatives of the great connective-tissue layer. The endothelia, therefore, belong to the connective tissues and are properly regarded as modified elements of that class ; as FIG. 91. a matter of convenience, however, they may be considered at this place in connection with the epithelial tis- sues. The most striking difference in situation between the endothelia and the epithelia is found in the fact that the former cover surfaces not com- municating with the atmosphere, while the epithelial tissues clothe mucous membranes all of which are directly or indirectly continuous with the integumentary surface. A further contrast between these tis- sues is presented in their genetic re- lations with the primary blastodermic layers, since the epithelia, with the exception of those lining certain parts of the genito-urinary tracts which are derived from the mesoblast, are the trans- formations and outgrowths from the ectoblast and the entoblast, while the endo- thelia are direct modifications of the mesoblastic cells. The young mesoblastic cells bordering the early body-cavity become differenti- ated into a delicate lining, the mesothelium, and later give rise to the characteristic plate-like elements which constitute the FIG. 92. lining of the permanent serous sacs. The name mesothelium is sometimes retained to designate the permanent investment of the great serous cavities, as distinguished from the endothelium which clothes the vascular and other serous spaces. Seen in typical preparations, as ob- tained from the peritoneum after treatment with argentic nitrate and subsequent stain- ing with haematoxylin, the endothelial cells on surface view appear as irregularly polyg- onal areas mapped out by deeply tinted lines. The latter represent the silver- stained albuminous intercellular cement- substance which unites the flattened cells in a manner similar to that observed in simple squamous epithelium ; this superficial likeness is so marked that it has led to much confusion as to the proper classification of endothelium under the connective tissues. The lines of apposition are sinuous and less regular than between epithelial elements, in many cases appearing distinctly dentated. The exact form of the cells and the character of their contours, however, are not constant, since they probably depend largely upon the degree of tension to which the tissue has been subjected. Not infrequently the intercellular substance, at points where several endothelial cells are in apposition, shows irregular, deeply colored areas after silver staining ; Endothelial cells lining artery of dog, after silver staining. X 500. 72 HUMAN ANATOMY. these figures are described as stigmata o? pseudostomata, and by some are interpreted as indications of the existence of openings leading from the serous cavity into the subjacent lymphatics. Critical examination of these areas, however, leads to the conclusion that they are largely accidental, and due to dense local accumulations of the stained intercellular materials ; they are not, therefore, to be regarded as intercellu- lar passages. True orifices or stomata, however, undoubtedly exist in certain serous membranes, as in the septum between the peritoneal cavity and the abdominal lymph-sac of the frog, and, possibly, the peritoneal surface of the diaphragm of mammals. The positions of these stomata are marked by a conspicuous modification in the form and arrangement of the surrounding endothelial plates, which exhibit a radial disposition about the centres occupied by the stomata. The immediate walls of the orifices are formed by smaller and more granular elements, the guard or ger- minating cells, the contraction and expansion of which probably modify the size of the openings. Although the ectoblast and the entoblast are the germ layers which furnish great tracts of epithelium in the adult body, yet the mesoblast, the middle germ layer, also supplies distinct epithelial tissues. As it has been already pointed out, the epidermis, the epithelial portion of the skin, with its derivatives, is a product of the ectoblast. The epithelial lining of the mouth cavity as far back as the region of the palatine arches, and the epithelium of the anus are also of ectoblastic origin, since they are formed as in-pocketings of the outer germ layer during early embryonic life. With the exception of these areas, the epithelium lining the entire digestive tube, and that of its accessory glands, notably the liver and the pancreas, is of entoblastic origin. The same thing is true of the epithelium of the respiratory tract, since this entire tract is an outgrowth from the primitive intestine. But in the case of the uro-genital system, the epithelium there found, or most of it, is derived directly from the mesoblast. To be more specific, the Fallopian tubes (uterine tubes), uterus and vagina of the female, which have, of course, a distinct layer of epithelium on their inner surface, are formed from certain embryonic tubes known as the Miillerian ducts, which are derived from the mesoblast. The vas (ductus) deferens of the male is first represented in the embryo by a tube known as the Wolffian duct, which, with its epithelium, is also derived from the mesoblast. The sex-cells found in the sex -glands, which in the case of the male retain a distinct epithelial character, are apparently of mesoblastic origin. The ureter and part of the kidney are out- growths from the Wolffian duct and therefore mesoblastic, w.hile the rest of the kidney not formed in this way is also of mesoblastic origin. Hence, it is evident that distinct layers of epithelium are formed from all three germ layers, and that in this respect no peculiarity is attributable to any one of them. THE CONNECTIVE TISSUES. THE important group of connective substances, the most widely distributed of all tissues, is the direct product of the great mesoblastic tract ; the several members of this extended family are formed by the differentiation and specialization of the intercellular substance wrought through the more or less direct agency of the meso- blastic cells. The variation in the physical characteristics of the connective tissues is due to the condition of their intercellular constituents. During the period of em- bryonal growth these latter are represented by gelatinous, plastic substances ; a little later by the still soft, although more definitely formed, growing connective tissue, which, in turn, soon gives place to the yielding, although strong, adult areolar tissue. Grouped as masses in which white fibrous tissue predominates, the intercellular substance presents the marked toughness and inextensibility of tendon ; where, on the contrary, large quantities of yellow elastic tissue are present, extensibility is conspicuous. Further conden- sation of the intercellular sub- FIG. 93. stance produces the resistance encountered in hyaline carti- lage, intermediate degrees of condensation being presented by the fibrous and elastic varie- ties. In those cases in which the ground-substance becomes additionally impregnated with calcareous salts, the well-known hardness of bone or dentine is attained. Notwithstanding these variations in the density of the intercellular substance, the cel- lular elements have undergone but little change, the connective- tissue corpuscle, the tendon-cell, the cartilage- cell, and the bone- corpuscle being morphologically identical. The principal forms in which the connective substances occur may be grouped as follows : 1. Immature connective tissue, as the jelly of Wharton in the umbilical cord and the tissues of embryos and of young animals. 2. Areolar tissue, forming the subcutaneous layer and filling intermuscular spaces, and holding in place the various organs. 3. Dense fibre-elastic tissue, found in the fasciae, the sclera, the ligaments, etc. Where white fibrous tissue predominates and yellow elastic tissue is practically wanting, structures of the character of tendon or of the cornea are produced ; where, on the other hand, elastic tissue is in excess of fibrous tissue, highly extensible structures, as the ligamentum nuchae or the ligamenta subflava, result. 4. Cartilage, fibrous, elastic, and hyaline varieties. 5. Bone and dentine, in which impregnation of lime salts contributes character- istic hardness. 6. Reticulated connective tissue, occurring as the supporting framework in the lymphatic tissues, and as the interstitial reticulum of many organs. 7. Adipose tissue. The Cells of Connective Tissue. The cellular elements of the connective 73 Embryonal connective-tissue cells from the umbilical cord. X 5- 74 HUMAN ANATOMY. tissues are usually described as of two kinds, the fixed or connective-tissue cells proper, and the migratory or wandering cells. The latter, while frequently included among the elements of these tissues, are usually only migratory leucocytes which temporarily occupy the lymphatic clefts within the connective substance. FIG. 94. Young connective-tissue cells from subcutaneous tissue of cat embryo. X 590. FIG. 95. Granule-cells (mast-cells) from submucous tissue of mouth. X 1000. v, v, sections of blood-vessels. The typical connective-tissue cell, in its younger condition, possesses a flattened, plate-like body from which branched processes extend. With the completed growth of the tissue, the expanded, FIG. 6. often irregularly stellate, element contracts to the _. inconspicuous spindle cell commonly observed in adult areolar tissue. Granule-cells are addi- tional elements occasionally encountered in connective tissues. They are irregularly spherical in form and are dis- tinguished by conspicuous granules within their proto- plasm possessing a strong affinity for dahlia and other basic aniline stains. They include the plasma-cells of Waldeyer and the mast-cells of Ehrlich. Pigment-Cells. The fixed cells sometimes contain accumulations of dark parti- cles within their cytoplasm, the elements then appearing as large, irregularlybranched pigment-cells; these are con- spicuous in man within the choroid, the iris, and certain parts of the pia mater. The nucleus usually remains uninvaded, and hence appears as a lighter area within the dark brown, or almost black, cell-body. The Intercellular Constituents of the connective substances occur in three forms, -fibrous tissue, reticular tissue, and elastic tissue. Fibrous tissue consists morphologically of varying bundles of silky fibrils of Migratory leucocytes (Wandering cells) Fibrous tissue Section of subcutaneous tissue, showing the usual constituents of areolar tissue. X 300. FIBROUS TISSUE. 75 such fineness that they possess no appreciable width. The fibrils are united by and embedded within a semifluid ground-substance, which may be present in such meagre amount that it suffices only to hold together the fibrillae, or, on the other hand, it may constitute a large part of the entire intercellular tissue, as in the matrix of hya- FIG. 97. FIG. 98. Surface view of portion of omentum. X 130. Fi- brous and elastic tissue are arranged as a fenestrated membrane; the nuclei belong to the connective-tissue and the endothelial cells. FIG. 99. Pigrnented connective-tissue cells from choroid. x 400- line cartilage. Depending upon the dis- position of the bundles, fibrous tissue occurs in two principal varieties, areolar and dense connective tissue. The fibrous tissue of the areolar group is arranged in delicate wavy bun- dles which are loosely and irregularly in- terwoven, as seen in the subcutaneous layer, the intervening clefts being largely occupied by the ground-substance. In the denser connective tissues the fibrous tissue is disposed with greater regularity, either as closely packed, parallel bundles, as in tendon and aponeuroses, or as intimately felted, less regularly arranged, bands forming extended sheets, as in fasciae, the cornea, and the dura mater. The ground- substance uniting the fibrillae of dense connective tissues often contains a system of definite interfascicular lymph-spaces, which, in suitably stained prepara- tions, appear as irregularly stellate clefts that form, by union of their ramifications, a continuous net-work of channels for the conveyance of the tissue-juices throughout the dense connective substances ; in non-vascu- lar structures, as the cornea and the denser parts of bone, these systems of intercommunicating lymph-spaces serve to convey the nutritive sub- stances to the connective-tissue cells which lie within these clefts. Fibrous tissue yields gelatin on boiling in water, and is not digested by pan- creatin ; on the addition of acetic acid this tissue becomes swollen and trans- parent, the individual fibrillae being no longer visible. Reticular Tissue. The in- vestigations of Mall have emphasized the presence of a modified form of fibrous tissue in many localities, especially in organs rich in lymphoid cells. This variety of intercellular substance, known as reticular tissue or reticulum, consists of very fine fibrillae, either isolated or associated Cell-spaces of dense connective tissue from cornea of calf ; the surrounding ground-substance has been stained with argen- tic nitrate. X 525. 7 6 HUMAN ANATOMY. FIG. ioo. FIG. 101. as small bundles, which unite in all planes to form delicate net-works of great intri- cacy. In lymphatic tissues, where the reticulum reaches a typical development, the mesh-work contains the characteristic lymphoid elements and, in addition, supports the superimposed stellate connective-tissue cells which formerly were erroneously regarded as integral parts of the fibrillar net-work. Reticular tissue, associated with fibrous and elastic tissue, is also present in many other organs, as the liver, kidney, and lung. This modifica- tion of fibrous tissue differs from the more robustly developed form in the absence of the ground-substance and not yield- ing gelatin upon boiling in water (Mall); like fibrous tissue, the reticulum resists pancreatic digestion. The development of fibrous tissue has been a subject of much discussion re- garding which authorities are still far from accord. Two distinct views are held at the present time ; according to the one, Connective-tissue ceiis from cornea of calf which t h e fibres appear within' the originally occupy cell-spaces similar to those shown in preceding ~~. . / figure, x 525. homogeneous intercellular matrix of the early embryonal connective tissue without the direct participation of the cells, the fibres being formed as the result of a process somewhat resembling coagulation. This conception of the formation of the fibres of connective tissue, known as the indirect mode, is held to account for the earliest production of the fibrils in embry- onic tissue. The other view, held by Flem- ming, Reinke, and others, attributes an active participation of the young connective tissue cell, the peripheral zone of its protoplasm, known as ex- oplasm, being directly transformed into fibrillae. In consideration of ^^ the careful observations of Flem- ^S^WKfaVi^ ming, it is now widely believed that the method of formation of the fibres of connective tissue directly from the exoplasm of young con- nective tissue cells is the usual one. It is highly probable that the connective tissue cells are concerned in the production of the fibrous tissue, since these elements become much smaller as the formation of the fibrous tissue advances. Elastic tissue usually occurs as a net-work of highly refracting, homogeneous fibres lying among the bundles of fibrous tissue. The indi- vidual fibres are much thicker than the fibrillae of fibrous tissue and, although differing in width, maintain a- constant diameter until augmented by fusion with others. When disassociated, as in teased preparations, the elastic fibres assume a highly characteristic form, being wavy, bowed, or coiled. The proportion of elastic tissue in connective substances is, ordinarily, small ; in certain localities, however, as the ligamenta subflava of man, or especially the ligamentum nuchae of the lower mammals, almost the entire structure consists of bundles of robust fibres of elastic Fibrous and reticular connective tissue from human liver after pancreatic digestion. X 230. ELASTIC TISSUE. 77 tissue held together by a small amount of intervening fibrous tissue. In transverse section of such ligaments (Fig. 104), the individual elastic fibres appear as minute polygonal areas separated by the fibrous fibrillae and the associated connective-tissue cells. Within the walls of the large blood-vessels the elastic tissue is arranged as membranous expansions containing numerous FIG. 102. openings of varying size : these fenestrated mem- branes, as they are called, are probably formed by the junction and fusion of broad ribbon-like elastic fibres. Elastic tissue yields elastin upon FIG. 103. V Re-ticular connective tissue from lymph- node. X 350. The cells lie upon the fibrous tissue at the points of intersection. Portions of isolated elastic fibres from ligamen- tum nuchse of ox. X 375. FIG. 104. boiling in water, and disappears upon being subjected to pancreatic digestion, thus differing from fibrous and reticular tissue ;' by taking advantage of the especial affinity that elastic tissue possesses for certain stains, as orcei'n, a much wider and more generous distribution of elastic tissue has been established than was formerly appre- ciated. The development of elastic tissue has shared the uncertainty surrounding the mode of production of fibrous tissue, since here, as there, two opposed views have been held, one of a cellular and one of an independent origin. Accord- ing to the view of an independent origin, the older one, the elastic fibres first appear as rows of minute beads in the intercellular matrix. These linearly dis- posed beads gradually fuse, thus produc- ing the primary elastic fibres. According to the view of an intracelhdar origin, the one less generally accepted, the elastic fibres are derived directly from the exoplasm of the young connective tissue cells, as in the case of the white fibrils. The density of connective substances depends upon the amount and arrange- ment of the fibrous tissue ; the extensibility is determined by the proportion of elastic tissue present. When the former occurs in well-defined bundles, felted together into interlacing lamellae, dense and resistant structures result, as fasciae, the cornea, etc. ; in such structures the cement- or ground-substance within the interfascic- ular clefts usually contains the lymph-spaces occupied by the connective-tissue cells. Tendon. Tendon consists of dense connective tissue composed almost en- tirely of white fibrous tissue arranged in parallel bundles. The individual fibrillae Elastic fibres in section Interfibrillar connective tissue Nucleus of con nective-tissue cell Transverse section of ligamentum nuchae of ox. X 45- HUMAN ANATOMY. of the fibrous tissue, held together by cement-substance, are associated as compara- tively large primary bundles, which in turn are united by interfascicular fibrous FIG. 106. Longitudinal section of tendon from young subject; the tendon-cells are seen in profile between the bundles of fibrous tissue. X 300. Tendon-bundle Profile view Oblique view- Surface view, Tendon-bundles from tail of mouse, showing different views of the cells. X 300. FIG. 107. Blood-vessel within septa enclosing tertiary bundles substance and grouped into secondary bundles. The former, invested by a delicate areolar sheath and partially covered by plate-like cells, are held together by the septal extensions of the general connective- tissue envelope which surrounds the entire tendon ; the larger septa support the interfascicular blood-ves- sels and the lymphatics. The flattened connec- tive-tissue elements, here known as the tendon-cells, occur in rows within the clefts between the primary bundles, upon and between which the thin, plate-like bodies and wings of the tendon-cells expand. Seen from the surface, these cells appear as nucleated quadrate bodies (Fig. 106) ; viewed in longitudi- nal profile, the tendon-cells present narrow rectangular areas, while, when seen in transverse section, the same elements appear as stellate bodies, the extended limbs of which, often stretching in several planes, represent sections of the wing- plates. Examined in cross-section (Fig. 107), the cut ends of the primary tendon-bun- dles appear as light irregular polygonal areas, which, under high amplification, at Primary bundle Transverse section of a tendon, showing grouping of primary, secondary, and tertiary bundles of tendon-tissue. X 85. ADIPOSE TISSUE. 79 times exhibit a delicate stippling due to the transversely sectioned fibrillae. The interfascicular clefts frequently are represented, in such preparations, by stellate figures in which the sections of the tendon-cells, lying upon the primary bundles, can be distinguished ; the remaining portion of the stellate cleft is occupied by the Adipose tissue from omentum. X 160. The fat-cells are arranged as groups between the bundles of connective tissue. coagulated and stained interfascicular cement-substance. The larger divisions of the tendon, composed of the groups of secondary bundles, are separated by the septa prolonged inward from the general sheath investing the entire tendon. Ten- don is composed almost exclu- sively of fibrous tissue, elastic FIG. 109. fibres being practically absent. Adipose Tissue. The fatty material contained within the body is to a large extent en- closed within connective-tissue cells in various localities ; these modified elements are known as fat-cells, which, together with the areolar tissue connecting the cells and supporting the rich supply of blood-vessels, consti- tute the adipose tissue. The distribution of adipose tissue includes almost all parts of the body, although accumulations of fat are especially conspicuous in certain localities. Among the latter are the subcutaneous areolar tissue, the marrow of bones, the mesentery and the omentum, the areolar tissue surrounding the kidney, the vicinity of the joints, and the subpericardial tissue of the heart. On the other hand, in a few situations, in- cluding the subcutaneous areolar tissue of the eyelids, the penis and the labia minora, the lungs, except near their roots, and the interior of the cranium, adipose Peripheral zone of protoplasm enclosing oil- drop Young fat-cell Connective-tissue' cells Young fat-cells from subcutaneous tissue. X 550. 8o HUMAN ANATOMY. tissue does not occur even when developed to excess in other parts. As ordinarily seen, adipose tissue is of a light straw color and often presents a granular texture due to the groups of fat-cells within the supporting areolar tissue. Examined microscopically in localities where the fat-cells are not crowded, but occur in a single stratum and hence retain their individual form, adipose tissue is seen to be made up of relatively large, clear, spherical sacs held together by deli- cate areolar tissue. Unless treated with some stain, as osmic acid, Sudan III. or quinoline-blue, possessing an especial affinity for fat, the oily contents of the cells appear transparent and uncolored, and apparently occupy the entire cell-body. Critical study of the fat-cell, however, demonstrates the presence of an extremely thin enveloping layer of protoplasm, a local thickening on one side of the sac mark- ing the position of the displaced and compressed nucleus (Fig. 109). Fat-cells occur usually in groups, supported and held together by highly vas- cular connective tissue. In localities possessing considerable masses of fat, as be- neath the scalp and the skin, the cells are grouped into lobules which appear as yellow granules to the unaided eye ; in such localities the typical spherical shape of the individual fat-cells is modified to a polyhedral form as the result of the mutual pressure of the closely packed vesicles. In connective-tissue elements about to become fat-cells, isolated minute oil- drops first appear within the protoplasm ; these increase in size, coalesce, and grad- ually encroach upon the cytoplasm until the latter is reduced to a thin, almost inappreciable, envelope, which invests the huge distending oil-drop. The nucleus, likewise, is displaced towards the periphery, where it appears in profile as an incon- spicuous crescent embedded within the protoplasmic zone. After the disappearance of the fatty matters, as during starvation, the majority of fat-cells are capable of resuming the usual appearance and properties of connective-tissue corpuscles ; cer- tain groups of cells, the fat-organs of Toldt, however, exhibit an especial tendency to form adipose tissue, and hence only under exceptional conditions part with their oily contents. CARTILAGE. Cartilage includes a class of connective tissue in which the intercellular substance undergoes increasing condensation until, as in the hyaline variety, the intercellular matrix appears homogeneous, the constituent fibres being so closely blended that the fibrous structure is ordinarily no longer appreciable. Depending upon the differences presented by the intercellular matrix, three varieties of cartilage are recognized, hyaline, elastic, and fi bro u s. Considered in relation to the denser connective tissues, the description of fibrous cartilage, which differs but little from white fibrous tissue, should next follow ; since, however, the term ' ' cartilage' ' is usually applied to the hyaline variety, the latter will first claim attention. Hyaline cartilage, or gristle (Fig. no), enjoys a wide distribution, forming the articular surfaces of the bones, the costal cartilages, the larger cartilages of the larynx and the cartilaginous plates of the trachea and bronchi, the cartilages of the nose and part of the Eustachian tube. In the embryo the entire skeleton, with the exception of part of the skull, is mapped out by primary hyaline cartilage. The apparently homogeneous matrix of hyaline cartilage, after appropriate treatment, is resolvable into bundles of fibrous tissue ; ordinarily, however, these are so closely united and blended by the cementing ground-substance that the presence of the component fibrils is not evident. The cartilage-cells, as the connective-tissue elements which lie embedded within the hyaline matrix are called, are irregularly oval or spherical, nucleated bodies. They occupy more or less completely the interfascicular clefts, or lacuna, within which they are lodged. In adult tissue usually two or more cells share the same compartment, the group representing the descendants from the original occupant of the space. The matrix immediately surrounding the lacunae is specialized as a layer of different density, and is often described as a capsule ; a further differentiation of the ground-substance is presented by the more recently formed matrix, which CARTILAGE. 81 Perichondrium Young cartilage- cells Group of older cells. Cartilage-cells often stains with greater intensity, thereby producing the appearances known as the cell-areas. The lacunae of hyaline cartilage are homologous with the lymph- spaces of other dense forms of connective tissue ; although canals establishing com- munication between the adjacent lacunae are not demonstrable in the tissues of the higher vertebrates, it is not improbable that minute interfascicular passages exist which facilitate the access of nutritive fluids to the cells enclosed within the lacunae. The free surface of cartilage is covered by an envelope of dense connective tissue, the perichondrium ; the latter consists of an external fibrous layer of dense fibro-elastic tissue and an inner looser stratum or chondrogenetic layer, containing numerous connective-tissue cells. These are arranged in rows parallel to the sur- face of the cartilage and, during the growth of the tissue, gradually assume the characteristics of the cartilage-cells, being at first spindle-shaped and later ovoid and spherical. The young cartilage-cells thus formed become gradually separated by more extensive tracts of the newly deposited intercellular matrix ; as the groups of cells originating from the division of the original occupant of the lacuna recede from the perichondrial surface, they lose their primary parallel dis- FIG. no. position and become irregu- larly arranged and still further separated. Those portions of the ground-substance most re- mote from the perichondrium at times appear granular, this feature being intensified when, as in aged subjects, a deposition of calcareous matter takes place in these situations. In articular cartilage the su- perficial zone contains sparsely distributed groups of small cells arranged parallel to the free surface ; in the deeper strata these groups are replaced by elongated rows of larger ele- ments lying perpendicular to the articular surface. This columnar disposition of the car- tilage-cells is particularly evi- dent towards the underlying zone of calcified matrix. The blood-vessels of normal cartilage are usually limited to the periphery, within the perichondrium or the associated synovial membranes ; the nutrition of the cartilage is maintained by imbibition of the fluids through the matrix into lacunae, the existence of minute interfascicular canals being not impossible. In the thicker masses of the tissue, as in the cartilages of the ribs, nutrient canals exist in those portions most remote from the perichondrium ; these spaces contain a small amount of areolar tissue supporting the blood-vessels, which are, however, limited to the channels, the nutrition of the cartilage tissue being effected here, as at the periphery, by absorption through the matrix. Nerves have never been demonstrated within the cartilages, which fact explains the conspicuous insensibility of these tissues so well adapted to the friction, concus- sion, and compression incident to their function. Elastic cartilage, called also yellow elastic or reticular cartilage (Fig. in), has a limited distribution, occurring principally in the cartilages of the external ear, part of the Eustachian tube, the epiglottis, the cartilages of Wrisberg and of San- torini, and part of the arytenoid cartilages of the larynx. In its physical properties this variety differs markedly from hyaline cartilage, as it is dull yellowish in color 6 -Lacuna contain- ing nest of cells -Empty lacuna surrounded by hyaline matrix Transverse section of peripheral portion of costal cartilage, X 250. 82 HUMAN ANATOMY. and pliable and tough in consistence, in contrast to the bluish opalescent tint and comparative brittleness of the hyaline variety. The characteristic feature of the structure of the elastic cartilage is the presence of elastic fibres within the intercellular matrix. The cell-nests are immediately sur- rounded by limited areas of hyaline intercellular substance corresponding to the matrix of hyaline cartilage. The matrix intervening between these homogeneous fields, however, is penetrated by delicate, often intricate, net-works of elastic fibres extending in all directions. The connective-tissue cells lie within the lacunae, in the hyaline areas, and closely resemble the elements of hyaline cartilage. Elastic carti- lage possesses a perichondrium of the usual description. Fibrous cartilage, or fibro- cartilage (Fig. 112), as the fibrous variety is usu- ally designated, is found in comparatively few localities, the marginal plates and the interarticular disks of certain joints, the symphyses, the intervertebral disks, sesamoid cartilages, and the lining of bony grooves for tendons being its chief representatives. FIG. in. 'i* - // f^f {$&$ /* '/ A > . & .<-';''/ v\\ ^il^.lftl Cartilage-cells Hyaline areas Elastic net-work of intercellular tissue Lacuna contain- ing cell Section of elastic cartilage from the epiglottis. X 450. In its physical properties this tissue resembles both fibrous tissue and cartilage, pos- sessing the flexibility and toughness of the former combined with the firmness and elasticity of the latter. A proper perichondrium is wanting. In structure fibro- cartilage closely resembles dense fibrous tissue, since its prin- cipal constituent is the generally parallel wavy bundles of fibrous connective tissue ; among the latter lie small, irregularly disposed oval or circular areas of hyaline matrix which surround the cartilage-cells, singly or in groups. The number of cells and the proportion of fibrous matrix differ in various localities. The development of cartilage proceeds from the mesoblast, the cells of which undergo proliferation and, forming compact groups, become the embryonal cartilage- cells ; at first the latter lie in close apposition, since the matrix is wanting. During the later stages, when the masses of embryonal cartilage map out the subsequent skeletal segments, the cells are separated by a small amount of homogeneous matrix formed through the influence of these elements. DEVELOPMENT OF CARTILAGE. Hyaline area surrounding cartilage-cells Fibrous inter- cellular sub- stance -Cartilage-cells Section of fibrous cartilage from intervertebral disk. X 225. Cartilage grows in two ways : (a) by the expansion produced by the inter- stitial growth effected by the formation of new cells and the associated matrix, and () by the addition of the new tissue developed by perichon- p IG II2 . drial growth at the periphery of the cartilage from the chondro- genetic layer. The latter mode continues throughout the period of growth, and includes the di- rect conversion of the connec- tive-tissue cells of the perichon- drium intothecartilage elements, and the accompanying formation of new matrix. The development of the elastic fibres within the elastic cartilage is secondary, the matrix during the early stages of growth being hyaline. The elastic tis- sue first appears in the form of minute granules, which later fuse and become the elastic fibres ; this change first appears in the vicinity of the cartilage- cells, the elastic reticulum sub- sequently invading the more re- mote portions of the matrix. In the development of the fibro- cartilage, the fibres appear coincidently with the limited pericellular areas of hyaline substance. CHEMICAL COMPOSITION OF THE CONNECTIVE SUBSTANCES. Connective Tissue. The fibrils of white fibrous connective tissue consist of a substance known as collagen. The interfibrillar ground-substance contains mainly mucoid and the albuminous materials, serum globulin and serum albumin. Gelatin is the hydrate of collagen, and is obtained by boiling fibrous tissue with water, when the gelatin separates like a jelly on cooling. In the case of the yellow elastic fibres, elastin is found in place of collagen. In reticular tissue rcticulin is found. The latter substance contains phosphorus. These substances, namely, collagen with its hydrate gelatin, elastin and perhaps reticulin, are among those known as albuminoids, which are closely related to the true albumins, yet differ in some important respects. The albuminoids, for the most part, contain less carbon and more oxygen than the albumins proper. Cartilage. The fibres which are found in the matrix of fibre -cartilage and elastic cartilage are respectively composed of collagen and of elastin, just as they are in the corresponding connective tissues. According to His, the chemical composition of human cartilage is as follows : Costal cartilage. Articular cartilage. Water ' 67.67 73.59 Solids 32.33 26.41 Organic matter 30. 13 24.87 Mineral salts 2.20 1.54 In the mineral salts there is about 45 per cent, of sodium sulphate. A somewhat smaller percentage of potassium sulphate, and smaller amounts of the phosphates of sodium, calcium and magnesium, as well as of sodium chloride, are present. Adipose Tissue. The fats in the animal body are mainly the triglycerides of stearic, palmitic and oleic acid. There is found in man a comparatively large amount of olein. Small quantities of lecithin, cholesterin and free fatty acids are also found in fat tissue. 84 HUMAN ANATOMY. BONE OR OSSEOUS TISSUE. In the higher vertebrates, osseous tissue forms the bony framework, or skeleton, which gives attachment and support to the soft parts, affords protection to the more or less completely surrounded delicate organs, supplies the passive levers for the exercise of muscular action, insures stability, and maintains the definite form of the animal. In addition to contributing the individual bones composing the principal, and in man the only, framework, or entoskeleton, osseous tissue occurs in the lower ver- tebrates associated with the integument as an exoskeleton. Representatives of the latter are seen in the bony plates present in the skin of certain ganoid fishes, the dermal plates of crocodiles, the dorsal and ventral shields of turtles, or the dermal armor of the armadillo. Osseous tissue also exists within various organs in certain animals and then constitutes the splanchnoskeleton. Examples of the latter are furnished by the bony plates encountered in the sclerotic coat of the eyes of birds, in the diaphragmatic muscle of the camel, in the tongue of certain birds, in the heart of ruminants, in the nose, as the snout-bones of the hog, in the respiratory organs, as the laryngeal, tracheal, and bronchial bones of birds, and in the genital organs, as the penile bone of carnivorous and certain other mammals. True osseous tissue does not occur outside the vertebrates. Many invertebrate animals possess a skeletal framework, usually external but in some cases internal. Such a framework, however, consists of calcareous incrustations, hardened excre- tions or concretions composed principally of calcium carbonate and of silicious structures. These earthy or mineral hard parts of invertebrates are structureless deposits, so differing materially from the bone tissue of the higher vertebrates as well in structure as in chemical composition. Sometimes a deposit of calcareous material occurs in adult cartilage, a process entirely distinct from the formation of bone tissue. Familiar examples of such calcification are seen in the costal and some of the laryngeal cartilages. Chemical Composition. Bone is a dense form of connective tissue, the matrix of which is impregnated with lime salts ; it consists, therefore, of two parts, an animal and an earthy portion, the former giving toughness and the latter hardness to the osseous tissue. The animal or organic part of bone may be removed by calcination, leaving the inorganic constituents undisturbed. If a bone be heated in a flame with free access of air, the animal matter at first becomes charred and the bone black ; continued combustion entirely removes the organic materials, the earthy portion alone remain- ing. After such treatment, while retaining its general form, the bone is fragile and easily crushed, and has suffered a loss of one-third of its weight, due to the destruc- tion and elimination of the animal constituents. The latter, evidently, constitute one-third and the mineral matters two-thirds of the bone. The inorganic constitu- ents include a large amount of calcium phosphate, much less calcium carbonate, with small proportions of calcium fluoride and chloride, and of the salts of magnesium and sodium. The animal portion of the bone, on the other hand, may be separated from the inorganic salts by the action of dilute hydrochloric acid, which dissolves out the earthly constitutents ; after such treatment the bone, although retaining perfectly its form and details, is tough and flexible, a decalcified rib or fibula being readily tied into a knot. The animal constituents of bone yield gelatin upon prolonged boiling in water, therein resembling fibrous connective tissue. The composition of bone, according to Berzelius, is as follows : ORGANIC MATTER Gelatin and blood-vessels, 33-3 f Calcium phosphate, 51-04 | Calcium carbonate, ii-3Q INORGANIC MATTER -j Calcium fluoride, 2.00 Magnesium phosphate, ' 1.16 L Sodium oxide and sodium chloride, 1.20 100.00 PHYSICAL PROPERTIES OF BONE. FIG. 113. Physical Properties. Rauber has shown that a five-millimetre cube of com- pact bone of an ox when calcined will resist pressure up to 298 pounds ; when decal- cified up to 136 pounds ; under normal conditions up to 852 pounds, the pressure being applied in the line of the lamellae. It results from its composition that while bone is very hard and resistant to press- ure, it is also somewhat flexible, elastic, and capable of withstanding a tearing strain. It is remarkable that in many substances the power to resist a crushing strain is very different from that of resisting a tearing one. Thus, cast iron is more than five times as resistant to the former strain as to the latter, and wrought iron is nearly twice as resistant to the latter as to the former. Neither of these materials, therefore, is well fitted to resist both strains, since a much greater quantity must be used than would be needed were either material to be exposed only to the strain it is best able to with- stand. Bone, however, has the property of resisting both strains with approximately equal facility, its tearing limit being to its crushing limit about as 3 is to 4. This has the advantage that strength need not be obtained by great increase of weight, con- sequently the plan of bone structure com- bines lightness and strength. Structure of Bone. On sawing through a bone from which the marrow and other soft parts have been removed by ma- ceration and boiling, the osseous tissue is seen (Fig. 113) to be arranged as a pe- ripheral zone of compact bone enclosing a variable amount of spongy or cancellated bone. In the typical long bones, as the humerus or femur, the compact tissue al- most exclusively forms the tubular shaft enclosing the large marrow-cavity, the can- cellated tissue occupying the expanded extremities, where, with the exception of a narrow superficial stratum of compact bone, it constitutes the entire framework ; the clefts between the lamellae of the spongy bone are direct extensions of the general medullary cavity and are filled with mar- row-tissue. In the flat bones (Fig. 116), as those of the skull, the compact substance consists of an outer and inner plate, or tables, enclosing between them the cancel- lated tissue, or diplo'e, as this spongy bone is often termed. Short and irregular bones are made up of an inner mass of spongy bone covered by an external shell of compact substance which often presents local thickenings in order to insure additional strength where most needed. The cancellated bone consists of delicate bars and lamellae which unite to form an intricate reticulum of osseous tissue well calculated to insure considerable strength without undue weight ; in many positions, conspicuously in the neck of the femur (Fig. 374), the more robust lamellae are disposed in a definite manner with a view of meeting the greatest strains of pressure and of tension. Although composed of the same structural elements, compact and spongy bone differ in their histological details in consequence of the secondary modifications which take place during the conversion of the spongy bone, the original form, into the compact substance. To obtain the classic picture of osseous tissue, in order to study its general arrangement in the most typical form, it is desirable to examine thin ground sections of the compact substance cut at right angles to the axis of a long bone which has been macerated and dried, and in which the spaces contain air. Section of upper end of humerus, showing the external layer of compact bone surrounding the med- ullary cavity below and the spongy bone above. 86 HUMAN ANATOMY. The compact bone in such preparations, when examined under low ampli- fication (Fig. 114), is seen to be composed of osseous layers arranged as three chief groups : (#) the circumferential lamella, which extend parallel to the external and internal surfaces of the compact bone ; (b*) the Haversian lamella, which are disposed concentrically and form conspicuous annular groups, the Haversian systems, enclosing the Haversian canals ; and (V) the interstitial or ground lamella, which constitute the intervening more or less irregularly arranged bony layers filling up the spaces between the Haversian systems and the peripheral strata. FIG. 114. External circumferen- tial lamellce Haversian canal sur. rounded by Haver- sian lamella; xT*V.'*' r ''-' Interstitial lamellae vP^tt^'r *T^'i%** '^^^Q>'^^cS2^^ ~. *' ,iiV-fll^> ''^. 7^^;^'^" ^t^^^^Lw^ ^V j^ r _ -Internal circumferen- tial lamellae Transverse section of compact bone (metatarsal) ; the section has been ground and dried, hence the lacunae are filled with air. X 85. Each Haversian system consists of the concentrically disposed lamellae and the centrally situated channel, or Haversian canal, enclosing the ramifications of the medullary blood-vessels and associated marrow-tissue. Between the annularly arranged lamellae are seen small spindle-shaped or oval spaces, the lacuna, about .02 millimetre long, .01 millimetre wide, and .006 millimetre thick, from which ex- tend minute radiating channels, the canaliculi, establishing communication between the adjacent lacunae of the same Haversian system. The lacunae and the canaliculi constitute an intercommunicating net-work of lymph-spaces similar to those encoun- STRUCTURE OF BONE. tered in other forms of dense connective tissue. Since the lacunae are compressed oval cavities lying between the lamellae of the osseous matrix, when viewed in sec- tions which pass through the layers at right angles (Fig. 117), the lacunae present their narrower dimensions, appearing thus in profile as small lentiform spaces ; seen in sections, on the contrary, which pass parallel to the lamellae (Fig. 118), the lacunae are broader and more circular, the spaces with the canaliculi forming the spider-like figures so conspicuous in longitudinal sections of dried bone. The characteristic arrangement of the lamellae of the Haversian systems is due to the secondary formation of the osseous tissue during the conversion of the older spongy bone into compact tissue, the circumference of the system corresponding to the Haversian space in which the subsequent development of the concentric lamellae FIG. 115. Circumferential lamellae Interstitial lamellae Haversian canal Objiquely cut Haver- sian canal Longitudinal section of compact bone, ground and dried. X 85. took place. It follows, from this relation, that Haversian systems exist only in com- pact bone, since the necessary secondary deposit does not occur during the growth of the spongy or cancellous tissue. The lamellae of osseous tissue, when deprived of the mineral matters and exam- ined in thin fragments, often display the ultimate fibrous structure which they pos- sess, since they consist of delicate fibrils of fibrous tissue embedded within a ground- substance and associated into bundles which are arranged as crossing and interwoven layers. Within the Haversian lamellae the fibrous bundles cross generally at right angles, but in other locations they are less regularly and more acutely disposed. The perforating fibres of Sharpey (Fig. 119) consist of bundles of fibrous tissue which penetrate the lamellae in a direction perpendicular or oblique to their 88 HUMAN ANATOMY. surface, and thus pin or bolt the layers together. These fibres are especially numer- ous in the superficial lamellae beneath the periosteum, to which membrane they owe their formation, and with which many seem to be directly continuous. They are FIG. 116. 't ,*^^ ? r : ~ FIG. 117. Lacuna cut obliquely Canaliculi Section of frontal bone, showing the absence of Haversian systems. X 20. readily found on the surfaces of the lamellae of decalcified bone which have been forcibly separated. Although usually consisting of bundles of fibrous tissue, it is probable that in some cases the perforating fibres are elastic in nature. They are sometimes imperfectly calcified and leave, therefore, on drying, tubular canals, which pierce the lamella from the ex- terior of the bone. Since the perfo- rating fibres are associated genetically with the periosteum, they are never found in the secondary lamellae consti- tuting the Haversian systems. The Haversian canals are con- tinuations of the medullary cavity and serve the important purpose of con- veying the blood-vessels within the compact substance ; from these vessels the nutritive fluids pass into the peri- vascular lymph-spaces between the walls of the canal and the blood-ves- sels and thence, by way of the cana- Portion of adjacent Haversian systems cut transversely. liculi > W . hlch P 6n . intO theSG !y m P h - x 250. spaces, into the adjacent lacunae, and so on into the surrounding portions of the compact substance, the nutrition of which is thus maintained. Although the average size of the canals is about .05 millimetre, those next the medullary cavity are larger, some measuring . i millimetre or more in diameter, and contain, in addi- Lacuna in profile THE BONE-CELLS. 89 FIG. 118. Lacunae and canaliculi from dried bone cut parallel with the lamellae. X 300. tion to the blood-vessels, an extension of the marrow-tissue. The individual chan- nels are short, and communicate by oblique branches with adjacent canals (Fig. 115). The Haversian canals indirectly communicate with the external surface of the bone by means of the channels, or Volkmanri s canals, within the circumfer- ential lamellae, which open by minute orifices and receive vascular twigs from the periosteal blood-vessels (Fig. 122); the latter are thus brought into free anasto- mosis with the branches derived from the medullary vessels, the two constituting a freely communicating vascular net-work throughout the compact substance. The Bone-Cells. The details of osseous tissue thus far considered per- tain to the structure of the passive in- tercellular constituents of a dense con- nective tissue ; in addition to these, as in other forms of connective substances, the more active elements are the con- nective-tissue cells, here known as the bone-cells. As already pointed out, the lacunae and the canaliculi represent in- tercommunicating lymph-spaces, similar to those encountered in the cornea or other dense connective tissue ; as in the latter so also in the osseous tissue, the cellular elements occupy the lymph- spaces, the bone-cells lying within the lacunae. Since the classic pictures of bone are derived from ground sections of dried tissue, in such preparations the deli- cate bone-cells have shrunken and disappeared, and the lacunae contain, at best, only the indistinguishable remains of the cells mingled with debris produced during the preparation of the section ; the lacunae and the canaliculi in dried sections are filled with air, by reason of which condition they appear as the familiar dark, sharply defined, conspicuous spider- like figures. In order to study the bone- cells, the tissue after fixation is de- calcified and stained, and mounted in an approved preserving medium ; in consequence of such treatment the air is displaced from the spaces within the bone, which now appear faintly outlined, the delicate ramifi- cations of the canaliculi in places being almost invisible. The bone- cells, after being stained in such decalcified preparations, appear as small lenticular or stellate bodies within the lacunae (Fig. 121), which they almost entirely fill. Each cell- body consists of granular cytoplasm from which delicate processes ex- tend for a variable distance into the canaliculi, in favorable localities the protoplasmic processes sent out by adjacent bone-cells sometimes meeting. The deeply staining nucleus appears as a brilliant point within the stellate cell. The Periosteum. The external surface of bones is closely invested, except where covered with cartilage, with a fibrous membrane, the per^oste^lm, a structure of great importance during development and growth, and later for the nutrition and protection of the osseous tissue. During childhood an end of the immature bone may be broken off and yet held in place by the periosteum. The adult peri- osteum consists of two layers, an outer fibrous and an inner fibro-elastic ; when covering young bones, however, in which growth is actively progressing, the peri- FIG. 119. Semi-diagrammatic view of perforating fibres of Sharpey; the lamellae of decalcified bone have been partially separated. 90 HUMAN ANATOMY. osteum contains an additional stratum, the osteogenetic layer, which lies closely asso- ciated with the exterior of the bone. After growth has ceased, the osteogenetic layer becomes reduced to an inconspicuous stratum included as part of the fibro- elastic constituent of the periosteum. The fibrous layer is composed of closely placed bundles of fibrous connective tissue, and serves to support larger blood-vessels which break up within the deeper parts of the periosteum into the minute twigs entering the canals opening onto the surface of the bone. FIG. 120. i; ^-.-i.-,, Sharpey's fibres Lacuna Sharpey's fibres Oblique section of decalcified tibia, showing fibrous character of lamellae and groups of Sharpey's fibres. X 420. The fibro-elastic layer consists of a rich felt-work of elastic fibres, often arranged as several distinct strata ; the elastic tissue is separated from the surface of the bone by a layer of fibrous tissue comparatively rich in flat, plate-like connective-tissue cells, the remains of the elements of the osteogenetic layer. The inner surface of the periosteum is intimately attached to the osseous tissue by means of delicate processes of connective tissue which accompany the blood-vessels into the nutrient canals ; this relation persists from the continuity of the formative tissue of the young periosteum with the early marrow-tissue. FIG. 121. Between the fibrous bundles next the bone numerous cleft-like lymph-spaces exist ; these are imperfectly lined by the endothe- lioid connective-tissue cells and communi- cate with the lymph-channels within the bone. The osteogenetic layer, conspicuous during the development and growth of the osseous tissue, consists of delicate bundles of fibrous tissue and large numbers of connective-tissue cells of an embryonal type. Those next the growing bone as- sume a low, irregular columnar form, and are disposed in rows upon the surface of the developing osseous tissue ; since these cells are concerned in the production of the latter, they are appropriately termed osteoblasts. Later some of them become sur- rounded by the bony matrix, and are thus transformed into bone-cells. The osteo- genetic layer is rich in blood-vessels which, as the bone is formed, are continued into the primary marrow- cavities. The Marrow. The spaces in the interior of bones, whether the large medullary cavities surrounded by the compact substance forming the shaft of the long bones or the irregular interstices between the trabeculae composing the cancel- Bone-cells lying within the lacunae. X 700. THE RED BONE-MARROW. 91 lated tissue, are filled with bone- marrow. The latter also extends within the larger Haversian canals. Although originally only of one variety within the bones of the early skeleton, the marrow in the adult consists of two kinds, the yellow and the red. Thus, within the shaft of the long bones it consists of a light yellowish tissue, presenting the char- acteristics of ordinary adipose tissue, while within the spaces of the cancellated tissue at the ends of the same bones the marrow appears of a dull red color. In addition to the ends of the long bones, the localities in which red marrow especially occurs are the bodies of the vertebrae, the ribs, the sternum, the diploe of the cranium, and the short bones. Red Marrow. The ingrowth of the periosteal tissue and blood-vessels con- stitutes the primary marrow within the embryonal skeleton ; from this tissue the red marrow filling the young bones is directly derived. The red marrow is, therefore, Last formed lamella of bone m Periosteal blood- vessel passing into the bone Bone-cell within lacuna FIG. 122. ii ''V^M'i' 1 ;> its i **' IT 6 *1 ' .' !;. |.V. . m Dense fibrous layer '' m SM^v\:' : .'t'- i: i--r;^. w-mm : : Marrow-tissue continu- ous with periosteum Remains of osteogenetic layer V Section of young periosteum and subjacent bone. X 275. the typical and first formed variety within the foetus and the young animal ; subse- quently, that situated within the shaft of the long bones becomes converted into yellow marrow by the replacement of the majority of the marrow elements by fat- cells. The red marrow (Fig. 123), when examined in section after fixation and staining, presents a delicate reticulum of connective tissue which supports the numerous medullary blood-vessels and the cellular elements. Next the bone the fibrous tissue forms a thin membrane, the endosteum, lining the medullary cavity and the larger Haversian canals into which the marrow extends. This membrane is highly vascu- lar, its vessels joining those within the osseous canals on the one side and those of the marrow on the other. The delicate fibrous reticulum, in addition to the thin-walled blood-channels which it supports, contains within its meshes the several varieties of elements char- HUMAN ANATOMY. acteristic of the red marrow ; these are : (i) the marrorv-cells \ (2) the eosinophile cells, (3) the giant cells, and (4) the nucleated red blood-cells. The marrow-cells, or myelocytes, resemble the large lymphocytes of the blood, but may differ from the latter in their slightly larger size and in the possession of a relatively large round or oval nucleus which contains comparatively little chromatin; the presence of neutrophile granules within the cytoplasm of the marrow- cells affords an additional differential characteristic when compared with the large lymphocytes in which these granules are absent. The eosinophile cells occur in considerable numbers within the red marrow, and appear in varying stages of growth, as evidenced by their round mononudear, the indented transitional and segmented polymorphonuclear condition ; the cells con- taining the latter form of nucleus are most abundant and represent, probably, the mature elements. The giant cells, or myeloplaxes, are huge elements of irregular oval form, and contain simple or polymorphous nuclei. They represent specialized myelocytes, FIG. 123. Giant cell Marrow-cells Young red blood-cells Giant cell Blood-vessel Blood-vessel 2S^ cto^^E^f >> Connective-tissue reticulum Section of red marrow from epiphysis of young femur. X 300. and during the processes resulting in the removal of osseous tissue they are the osteoclasts which are actively engaged in effecting the absorption of the bony matrix. Ordinarily the giant cells occupy the central portions of the marrow ; when, however, they enter upon the role of bone-destroyers, they lie on the sur- face of the osseous trabeculae within the depressions known as How s hip' s lacuna (Fig. 128). The nucleated red blood-cells within the red marrow are concerned in the important function of renewing the colored cells of the blood, the red marrow being the chief seat in which this process takes place after birth ; hence the red marrow is classed as a blood-forming organ. The nucleated red blood-cells exist within the marrow in two forms, an older and a younger. The genetically older cells, the normoblasts, are the descendants of the embryonal nucleated blood- cells on the one hand and the indirect parents of the younger blood-elements on the other. The normoblasts possess relatively large nuclei, with chromatin reticulum and cytoplasm tinged with haemoglobin ; they are frequently observed during mitosis, since they gave rise to the second generation of nucleated red blood-cells. The latter, the erythro- THE YELLOW BONE-MARROW. 93 blasts, are directly converted into the mature, non-nucleated red blood-disks on the disappearance of their nucleus. In addition to a larger amount of haemoglobin in their cytoplasm, the erythroblasts differ from the normoblasts in the possession of a deeply staining nucleus, in which the chromatin no longer appears as a reticulum. It is usual to find isolated groups of fat-cells distributed within the red marrow, although the amount of adipose tissue is very meagre in localities farthest removed from the medulla of the long bones. The varieties of leucocytes usually seen in the blood are also encountered within the red marrow in consequence of the intimate relations between the latter tissue and the blood-stream conveyed by the medullary capillaries. Yellow Marrow. Since the appearance of the yellow marrow is due to the preponderating accumulation of fat-cells which have replaced the typical elements of the marrow contained within the shaft of certain bones, the formation of this variety is secondary and must be regarded as a regression. Examined in section, yellow marrow resembles ordinary adipose tissue, since it consists chiefly of the large oval fat-cells supported by a delicate reticulum of connective tissue. In localities in which the latter exists in considerable quantity, numerous lymphoid cells represent the remaining elements of the originally typical marrow-tissue. After prolonged fasting the yellow marrow loses much of its oily material and becomes converted into a gelatinous substance containing compara- tively few fat-cells ; upon the re-establishment of normal nutrition this tissue may again assume the usual appearance of yellow marrow. Blood-Vessels. The generous blood-supply of bones is arranged as two sets of vessels, the periosteal and the medullary. The former constitutes an external net-work within the periosteum, from which, on the one hand, minute twigs enter the subjacent compact substance through channels ( Volkmanri s canals} communi- cating with the Haversian canals, within which they anastomose with the branches derived from the medullary system ; additional vessels, on the other hand, pass to the cancellated tissue occupying the ends of the long bones. The medullary artery is often, as in the case of the long bones, a vessel of con- siderable size, which, accompanied by companion veins, traverses the compact sub- stance through the obliquely directed medullary canal to gain the central part of the marrow. On reaching this position the medullary artery usually divides into ascend- ing and descending branches, from which radiating twigs pass towards the periphery. The latter terminate in relatively narrow arterial capillaries, which, in turn, expand somewhat abruptly into the larger venous capillaries. Such arrangement results in diminished rapidity of the blood-stream, the blood slowly passing through the net- work formed by the venous capillaries. The latter vessels, within the red marrow, possess thin walls and an imperfect endothelial lining in consequence of which the blood comes into close relation with the elements of the medullary tissue. During its sluggish course within the blood-spaces of the red marrow, the blood takes up the newly formed red cells, which thus gain entrance into the circulation to replace the effete corpuscles which are continually undergoing destruction within the spleen. It is probable that leucocytes also originate in the bone-marrow. After thus coming into intimate relations with the marrow-tissue, the blood is collected by capillaries which form small veins. In addition to the companion veins accompanying the nutrient artery along the medullary canal, in many instances the larger veins pursue a course independent of the arteries and emerge from the can- cellous tissue by means of the canals piercing the compact substance at the ends of the bones. Although destitute of valves within the medulla, the veins possess an unusual number of such folds immediately after escaping from the bone. Lymphatics. The definite lymphatic channels of the bones are principally associated with the blood-vessels of the periosteum and the marrow as perivascular channels, although it is probable that lymphatic spaces exist within the deeper layers of the periosteum, in close relation to the osseous tissue. The perivascular lym : phatics follow the blood-vessels into the Haversian canals, where, as well as on other surfaces upon which the canaliculi open, the system of intercommunicating juice- channels represented by the lacunae and the canaliculi is closely related with the lymphatic trunks. 94 HUMAN ANATOMY. Nerves. The periosteum contains a considerable number of nerves, the ma- jority of which, however, are destined for the supply of the underlying osseous tissue, since those distributed to the fibrous envelope of the bone are few. The periosteal nerves follow the larger blood-vessels, in the walls of which they chiefly terminate. Medullary nerves accompany the corresponding blood-vessels through the medullary canal, and within the marrow break up into fibrillae to be, probably, distributed to the walls of the vascular branches along which they lie. Regarding the ultimate endings and arrangement of the sensory fibres little is known ; in view of the low degree of sensibility possessed by healthy bones and their periosteum, the number of such nerves present in osseous structures must be very small. FIG. 124. DEVELOPMENT OF BONE. The bones composing the human skeleton, with few exceptions, are preceded by masses of embryonal cartilage, which indicate, in a general way, the forms and relations of the subsequent osseous segments, although many details of the model- ling seen in the mature bones appear only after completed development and the pro- longed exercise of the powerful modifying influences exerted by the action of the attached muscles. Since the primary formation of such bones takes place within the cartilage, the process is appropriately termed endochondral development. Certain other bones, notably those forming the vault of the skull and almost all those of the face, are not preceded by cartilage, but, on the contrary, are produced within sheets of connective tis- sue ; such bones are said, therefore, to arise by intra- membranous development. It will be seen, however, that the greater part of the bone formed by endochondral de- velopment undergoes absorption, the spongy substance within the ends of the long and the bodies of the irregu- lar bones representing the persistent contribution of this process of bone-production. Even in those cases in which the intracartilaginous mode is conspicuous, as in the de- velopment of the humerus, femur, and other long bones, the important parts consisting of compact substance are the product of the periosteal connective tissue, and hence ge- netically resemble the intramembranous group. Although both methods of bone-formation in many instances proceed coincidently and are closely related, as a matter of con- venience they may be described as independent processes. Endochondral Bone Development. The pri- mary cartilage, formed by the proliferation and condensa- tion of the elements of the young mesoblastic tissue, grad- ually assumes the characteristics of embryonal cartilage, which by the end of the second month of intra-uterine life maps out the principal segments of the fcetal cartilaginous skeleton. These segments are invested by an immature form of perichondrium, or primary periosteum, from which proceed the elements actively engaged in the production of the osseous tissue. The primary periosteum consists of a compact outer fibrous and a looser inner osteogenetic layer; the latter is rich in cells and delicate intercellular fibres. The initial changes appear within the cartilage at points known as centres of ossification, which in the long bones are situated about the middle of the future shaft. These early changes (Fig. 125) involve both cells and matrix, which exhibit con- spicuous increase in size and amount respectively. As a further consequence of this activity, the cartilage-cells become larger and more vesicular, and encroach upon the intervening matrix, in which deposition of lime salts now takes place, as evidenced by the gritty resistance offered to the knife when carried through such ossific centres. On acquiring their maximum growth the cartilage-cells soon exhibit indications of impaired vitality, as suggested by their shrinking protoplasm and degenerating Clarified human foetus of about three and one-half months, show- ing the partially ossified skeleton. Two-thirds natural size. DEVELOPMENT OF BONE. 95 Embryonal cartilage nuclei. The enlarged spaces enclosing these cells are sometimes designated as the primary areolce. Coincidently with these intracartilaginous changes, a thin peripheral layer of bone has been formed beneath the young periosteum ; from the latter bud-like processes of the osteogenetic layer grow inward from the periphery and invade the embryonal cartilage, by absorption of the cartilage-matrix gaining the centre of ossification and there effecting a destruction of the less resistant cells and inter- vening matrix. In consequence of the penetration of the periosteal processes and the accompanying absorption of the cartilage, a space, the primary marrow-cavity -, now occupies the centre of ossification and contains the direct continuation of the osteogenetic layer. This tissue, the primary marrow, which has thus gained access to the interior of the cartilage, contributes the cellular elements upon which a double r61e devolves, to produce osseous tissue and to remove the embryonal cartilage. The cartilage-matrix closing the enlarged cell-spaces next the pri- FIG. 125. mary. marrow-cavity suffers absorp- tion, whereby the cartilage-cells are liberated and the opened spaces are converted into the secondary areoltz, and directly communicate with the growing medullary cavity. After the establishment of this communi- cation, the cartilage-cells escape from their former homes and undergo dis- integration, taking no part in the direct production of the osseoiis tissiie. Beyond the immediate limits of the primary marrow- cavity the car- tilage-cells, in turn, repeat the pre- paratory stages of increased size and impaired vitality already described, but in addition they often exhibit a conspicuous rearrangement, where- by they form columnar groups sepa- rated by intervening tracts of calci- fied matrix (Figs. 126, 129). This characteristic belt, or zone of calci- fication, surrounds the medullary cavity and marks the area in which the destruction of the cartilage ele- ments is progressing with greatest energy. In consequence of the columnar grouping of the enlarged cartilage-cells and the intervening septa of calcified matrix, an arrange- ment particularly well marked in the ends of the diaphysis of the long bones, a less and a more resistant portion of the cartilage are offered to the attacks of the marrow- tissue by the cell- and matrix-columns respectively ; as a result of this difference, the cells and the immediately surrounding partitions are first absorbed, while the intervening trabeculae of calcified cartilage- matrix remain for a time as irregular and indented processes, often deeply tinted in sections stained with haematoxylin, which extend beyond the last cartilage-cells into the medullary cavity. These trabeculae of calcified cartilage-matrix serve as supports for the marrow-cells assigned to pro- duce the true bone, since these elements, the osteoblasts, become arranged along these trabecuke, upon which, through the influence of the cells, the osseous tissue is formed. Simultaneously with the destructive phase attending the absorption of the car- tilage, the constructive process is instituted by the osteoblasts by which the bone- tissue is formed. These specialized connective tissue elements, resting upon the Cartilage-cells be- coming enlarged and regrouped Enlarged cartilage- cells at centre of ossification Periosteum Section of tarsal bone of foetal sheep, showing centre of ossifi- cation. X 50. 9 6 HUMAN ANATOMY. irregular trabeculae of the calcified cartilage, bring about, through the influence of their protoplasm, the deposition of a layer of bone-matrix upon the surface of the FIG. 126. Emb Cartilage-cells becoming en- larged and grouped zo n .o,calci fi ca.,o n ,,eo 8 , ti c, ay ero,pHos-_^^W;:iVffefff ; ^ teum '-> I'S'^.A'F. ;.i : -;.*"v.-.^.*.' 'jf;? ."^.'. '.';j! Central spongy bone en- closing remains of carti- lage Longitudinal section of metatarsal bone of foetal sheep, showing stages of endochondral bone-development. X 40. trabeculae, which thus becomes enclosed within the new bone. After the latter has attained a thickness of at least the diameter of the osteoblasts, some of the cells in closest apposition are gradually surrounded by the osseous matrix (Fig. 127), until, ENDOCHONDRAL BONE. 97 FIG. 127. Osteoblasts Bone-cell Section of a portion of osseous tra- becula and foetal marrow. X 375- finally, they lie isolated within the newly formed bone as its cells ; the bone-cells are therefore imprisoned osteoblasts, which, in turn, are specialized connective-tissue elements. The bone-cells occupy minute lenticular spaces, the primary lacunce, at this immature stage the canaliculi being still unformed. The early bone-matrix is at first soft, since the deposition of the calcareous materials takes place subsequently. The increase in the thickness of the new bone is attended by the gradual disap- pearance of the enclosed remains of the calcified cartilage, the last traces of which, however, can be seen for some considerable time as irregular patches within the osseous trabeculae (Fig. 131), somewhat removed from the zone of calcification. The cartilage and the bone of the trabeculae stand, therefore, in inverse relations, since the stratum of bone is thinnest where the cartilage is thickest, and, con- versely, the calcified matrix disappears within the robust bony trabeculae. A number of the latter, together with the enclosed remains of the calcified cartilage, soon undergo absorption, with a corresponding enlargement of the intervening marrow-spaces. The remaining tra- beculae increase by the addition of new lamellae on the surface covered by the osteoblasts, and at some distance from the zone of calcification form a trabecular reticulum, the primary central spongy bone. In the case of the irregular bones, the central spongy bone is represented by the cancellated substance forming the internal frame- work ; in the long bones, on the contrary, the primary cancellated tissue undergoes further absorption within the middle of the shaft simul- taneously with its continued development at the ends of the diaphysis from the car- tilage. As the result of this absorption, a large space the central marrow- cavity is formed (Fig. 129), the growth of which keeps pace with the general expansion of the bone. The absorption of the young osseous tissue to which reference has been made is effected through the agency of large polymorphonucleated elements, the osteo- clasts. These are specialized marrow- cells whose particular role is the break- ing up and absorption of bone-matrix. They are relatively very large, their irregularly oval bodies measuring from .050 to .100 millimetre in length and from .030 to .040 millimetre in breadth. The osteoclasts (Fig. 128), singly or in groups, lie in close relation to the surface of the bone which they are at- tacking within depressions, or How- ship'' 's lacuna ', produced in consequence of the erosion and absorption of the osseous matrix which they effect. When not engaged in the destruction of bone, these cells occupy the more central portions of the marrow-tissue, where, in the later stages, they are probably identical with the myeloplaxes or giant cells encountered in the red marrow. The only part of the central spongy bone which persists after the completed development and growth of the long bones is that constituting the cancellated tissue occupying their ends. It will be seen, therefore, in many cases, that the product of the endochondral bone-formation, the primary central osseous tissue, is to a large extent absorbed, and constitutes only a small part of the mature skeleton. The early marrow-cavity, as well as all its ramifications between the trabeculae, is filled with the young marrow-tissue ; the latter gives rise to the permanent red marrow 7 FIG. 128. ?.f|L-Osteoclast Osteoblasts Portion of trabecula of spongy bone undergoing absorp- clast. tion by osteocl X5- 9 8 HUMAN ANATOMY. FIG. 129. in the limited situations where the central spongy bone persists, as in the vertebrae, the ribs, the sternum, and the ends of the long bones. The important fact may be here emphasized that the process sometimes spoken of as the " ossification of cartilage" is really a substitution of osseous tissue for car- tilage, and that even in the endochondral mode of formation cartilage is never directly converted into bone. The ossification of the epiphyses (Fig. 130), which in the majority of cases does not begin until some time after birth, the cartilage capping the diaphysis mean- while retaining its embryonal character, repeats in the essential features the details already described in connection with endochondral bone-formation of the shaft. After the establishment of the primary marrow-cavity and the surrounding spongy bone, ossification extends in two directions, towards the periphery and towards the adjacent end of the diaphysis. As this process continues, the layer of cartilage in- terposed between the central spongy bone and the free surface on the one hand, and between the central bone of the epiphysis and the diaphysis on the other, is gradually reduced until in places it entirely disappears. Over the areas which correspond to the later joint-surfaces the cartilage persists and be- comes the articular cartilage covering the free ends of the bone. With the final ab- sorption of the plates separating the epiphyses from the shaft the osseous tissue of the seg- ments becomes continuous, i( bony union" being thus accomplished. Intramembranous Bone-Develop- ment. The foregoing consideration of the formation of bone within cartilage renders it evident that the true osteogenetic elements are contributed by the periosteum when the latter membrane sends its processes into the ossific centre ; the distinction, therefore, be- tween endochondral and membranous bone is one of situation rather than of inherent difference, since in both the active agents in the production of the osseous tissue are the osteoblasts, and in essential features the pro- cesses are identical. Since in the produc- tion of membrane-bone the changes within pre-existing cartilage do not come into ac- count, the development is less complicated and concerns primarily only a formative pro- Embryonal cartilage Zone of calci- fication Endochondral spongy bone Periosteum Subperiostea bone Young mar- row Endochondral spongy bone Zone of calci- fication Embryonal cartilage Longitudinal section of phalanx of foetus of five months. X 23. cess. Although the development of all osseous tissue outside of cartilage may be grouped under the general heading of intramembranous, two phases of this mode of bone- formation must be recognized ; the one, the intramembranous, in the more literal sense, applying to the development of such bones as those of the vault of the skull and of the face, in which the osseous tissue is formed within the mesoblastic sheets, and the other, the subpcriosteal, contributing with few exceptions to the production of every skeletal segment, in which the bone is deposited beneath rather than within the connective-tissue matrix. In consideration of its almost universal participation, the periosteal mode of development will be regarded as the representative of the intramembranous formation. Subperiosteal Bone. The young periosteum, it will be recalled, consists of an outer and more compact fibrous and an inner looser osteogenetic layer. The latter, in addition to numerous blood-vessels, contains young connective-tissue ele- ments and delicate bundles of fibrous tissue. These cells, or osteoblasts, become more regularly and closely arranged along the fibrillae, about which is deposited the SUBPERIOSTEAL BONE. 99 new bone-tissue, the osteoblasts becoming enclosed within the homogeneous matrix to constitute the bone-cells. The osseous trabecula thus begins to increase not only FIG. 130. . ffiJbffl :3:i5, $ $ * ..* .% i erf &-SSga Remains of carti- lage separating bone of epiphysis and diaphysis Longitudinal section including epiphysis and upper end of diaphysis of long bone of cat, just before osseous union of the head and shaft takes place. X 50. in length, by the addition of the last-formed matrix upon the supporting fibres, but also in width, by the deposition of new layers of osseous material by the osteoblasts. 100 HUMAN ANATOMY. These cells cover the exterior of the trabeculae as they lie surrounded by the young marrow-tissue which extends from the osteogenetic layer of the periosteum into the intertrabecular spaces. The union of the young trabeculae results in the production of a subperiosteal net-work of osseous tissue, the peripheral spongy bone. The latter forms a shell surrounding the central endochondral bone, or, where the latter has already disappeared, the central marrow-cavity of the shaft. The two processes, central and peripheral bone-formation, progress simultaneously, so that the produc- tions of both lie side by side, often in the same microscopical field (Fig. 131). FIG. 131. Fibrous layer of periosteum Reniains of car- tilage Portion of developing humerus of foetal sheep, showing periosteal and central spongy bone. X 160. The development of compact bone involves the partial absorption of the subperiosteal net- work of osseous trabeculae and the secondary deposition of new bone-tissue. The initial phase in the conversion of the peripheral spongy bone into compact substance is the partial absorption of the trabeculae by the osteoclasts of the primary marrow-tissue ; in consequence of this process the close reticulum of perios- teal bone is reduced to a delicate framework, in which the comparatively thin remains of the trabeculae separate the greatly enlarged primary marrow-cavities, which, now known as the Haversian spaces, are of round or oval form. After the destructive work of the osteoclasts has progressed to the required extent, the osteoblastic elements of the young marrow contained within the Haver- INTRAMEMBRANOUS BONE. 101 FIG. 132. sian spaces institute a secondary formative process, by which new bone is deposited on the walls of the Haversian spaces. This process is continued until, layer after layer, almost the entire Haversian space is filled with the resulting concentrically disposed osseous lamellae ; the cavity remaining at the centre of the new bone per- sists as the Haversian canal, while the concentrically arranged layers are the lamellae of the Haversian system, the extent of the latter corresponding to the form and size of the Haversian space in which the secondary deposit of bone occurs. It is evident from the development of the compact substance that the interstitial or ground- lamellae of the adult tissue correspond to the remains of the trabeculae of the primary spongy bone ; these lamellae are, therefore, genetically older than those constituting the Haversian systems. The details of the formation of the Haversian lamellae, in- cluding the deposition of the matrix and the inclusion of the osteoblasts to form the bone-cells, are identical with those of the production of the trabeculae of the earlier bone. Intramembranous Bone. The development of certain bones, as those con- stituting the vault of the skull and the greater part of the skeleton of the face, differs in its earliest details from that of the subperiosteal bone, although the essen- tial features of the processes are identi- cal. The mode by which these mem- brane-bones are formed may claim, therefore, a brief consideration. The early roof of the skull consists, except where developing muscle occurs, only of the integument, the dura mater, and an intervening connective-tissue layer in which the membranous bones are formed. The earliest evidences of ossification usually appear about the middle of the area corresponding to the later bone, delicate spicules of the new bone radiating from the ossific centre towards the periphery. As the tra- beculae increase in size and number they join to form a bony net-work (Fig. 132), close and robust at the centre and wide-meshed and delicate towards the margin where the reticulum fades into the connective tissue. With the con- tinued growth of the bony tissue the net-work becomes more and more compact until it forms an osseous plate, which gradually expands towards the limits of the area devoted to the future bone. For a time, however, until the completion of the earliest growth, the young bones are separated from their neighbors by an intervening tract of unossified connective tissue. Subsequent to the earlier stages of the formation of the tabular bones, the continued growth takes place beneath the periosteum in the manner already described for other bones. On examining microscopically the connective tissue in which the formation of membrane-bone has begun, this layer is seen to contain numerous osteogenetic fibres around and upon which are grouped many irregularly oval or stellate cells ; the latter correspond to the osteoblasts in other locations, since through the agency of these elements the osseous matrix is deposited upon the fibres. As the stratum of bony material increases some of the cells are enclosed to form the future bone-cor- puscles. Although the osteogenetic fibres correspond to delicate bundles of fibrous tissue, they are stiffer, straighter, and present less indication of fibrillar structure. Since the fibres forming the ends of the bony spicules generally spread out, they fre- quently unite and interlace with the fibres of adjacent spicules, thus early suggesting the production of the bony net-work which later appears. Growth of Bone. It is evident, since the new bone is deposited beneath the periosteum, that the growth of the subperiosteal bone results in an increased diame- Parietal bone of human foetus of three months, showing trabecular net-work of intramembranous bone. X 5. 102 HUMAN ANATOMY. ter of the shaft as well as in thickening of the osseous wall separating the medullary cavity from the surface. In order, therefore, to maintain the balance between the longitudinal growth of the medullary cavity, effected by the absorption of the endo- chondral bone, and its lateral expansion, the removal of the innermost portions of the subperiosteal bone soon becomes necessary. Absorption of the older internal trabeculae thus accompanies the deposition of new osseous tissue at the periphery ; by this combination of destructive and formative processes the thickness of the cylindrical wall of the compact substance of the diaphysis is kept within the proper limits and the increased diameter of the medullary cavity insured. Throughout the period of early growth the increase in length of the bone is due to the addition of new cartilage at the ends ; later, the cartilaginous increments, contributed by the chondrogenetic layer of the perichondrium, are supplemented by interstitial expansion following the multiplication of the existing cartilage-cells. On attaining the maximum growth and the completion of epiphyseal ossification, a por- tion of the cartilage may persist to form the articular surfaces. After the cessation of peripheral growth and the completion of the investing layer of compact substance, the osteogenetic layer of the periosteum becomes more condensed and less rich in cellular elements, retaining, however, an intimate connection with the last-formed subjacent bone by means of the vascular processes of its tissue, which are in con- tinuity with the marrow-tissue within the intraosseous canals. In addition to being the most important structure for the nutrition of the bone, on account of the blood- vessels which it supports, the periosteum responds to demands for the production of new osseous tissue, whether for renewed growth or repair, and again becomes active as a bone-forming tissue, its elements assuming the role of osteoblasts in imitation of their predecessors. THE SKELETON: INCLUDING THE BONES AND THE JOINTS. FIG. 133. THE skeleton forms the framework of the body. In the widest sense it includes, besides the bones, certain cartilages and the joints by which the different parts are held together. The skeleton of vertebrates is divided into the axial and the appendic- ular ; the former constitutes the support- ing framework of the trunk and head ; the latter, that of the extremities. The Axial Skeleton. The general plan of the axial skeleton of the trunk is as follows : a rod composed of many bony disks (the vertebral bodies) connected by fibro-cartilage separates two canals, a dorsal and a ventral. In most vertebrates the rod is in the main horizontal, with the dorsal canal above and the ventral below ; but in man the rod is practically vertical, with the dorsal canal behind and the ventral in front. The former is called the neural, because it encloses the central nervous system ; the latter, the visceral. The vertebral cohimn has developed about the primary axis, the notochord. The neural canal is enclosed by a series of separate arches springing one from each vertebra. The skeletal parts of the anterior, or ventral, canal are less nu- merous ; they are the ribs, the costal carti- lages, and the breast bone. Above is the bony framework of the head, or the skull. This also is divided into a dorsal and a ven- tral portion by a bony element which is apparently a continuation of the bodies of the vertebrae, and, indeed, is actually de- veloped, in part, around the front of the notochord. The cephalic axis, however, is bent at an angle with the vertebral bodies, so that the neural arches, which here en- close the brain, are chiefly no longer be- hind but above. Below and in front of the brain-case is the face, which contains the beginning of the digestive tube, of which the jaws and teeth are special organs. In the head we do not find the separation of the parts enclosing the brain into a series of vertebrae, but they are clearly a continuation of the vertebral arches, the posterior, or occipital, division strongly suggesting a vertebra. The face is far more complicated, the vertebral plan being lost. In short, the axial skeleton consists of a 103 The tinted portions constitute the axial skeleton ; the untinted, the appendicular skeleton. 104 HUMAN ANATOMY. central, many-jointed rod bent forward near the top, with very perfect bony walls behind and above it, enclosing the central nervous system, and very imperfect bony and cartilaginous walls before and below it, enclosing the digestive apparatus and its associates, the circulatory, respiratory, urinary, and reproductive organs. The Appendicular Skeleton has an entirely distinct origin ; it is the frame- work of the limbs. It consists of two girdles, a thoracic and a pelvic, to each of which is attached a series of segments, the terminal one of which expands into five rays, -fingers and toes. According to some anatomists, the true vertebrate plan is of seven terminal rays, but, the question being still undecided, the more usual sys- tem is followed. Each of these rays consists of three or four bones. Proximal to this comes a series of short bones, wrist and ankle ; still nearer, a pair of bones, -fore- arm and leg ; then a single bone, arm and thigh; and lastly a bony arch, the girdle. In man, the thoracic girdle, made up of collar-bone and shoulder-blade, lies external to the chest, while the pelvic girdle fuses on each side into one bone, meets its fellow in front, and unites with the bodies of certain vertebrae. Thus, besides bearing a limb, the pelvic girdle forms a part of the wall of the abdominal and the pelvic cavities and would seem to belong to the axial skeleton, but embryology and comparative anatomy show that it does not. GENERAL CONSIDERATION OF THE BONES. The bones have the physiological function of bearing weight, of affording pro- tection, and especially, by the systems of levers composing the limbs, of effecting movements through the action of the muscles. They must, therefore, be capable of resisting pressure, accidental violence, and the strain caused by the pull of the muscles. The size of the bones must be such that besides serving the obvious needs of support and protection they may be sufficiently large to offer adequate surface for the origin and insertion of muscles, and the shape must be such as to allow this without undue weight. Shapes of Bones. Bones are divided, according to their form, into long, flat, and irregular ; such classification, however, is of little value, since many bones might be variously placed. Long bones form the best-defined group. They consist of a shaft and two extremities, each of which takes part in the formation of a joint, or, as in the case of the last phalanges, is terminal. Flat bones, where very thin, consist of a single plate ; where thicker, they con- sist of two plates separated by spongy substance called diploe. Irregular bones may be regarded as embracing all others. The group of the so-called short bones has no significance. Sesamoid Bones, with the exception of the patella, are not usually included in the description of the skeleton. With the above exception, they are small rounded bones developed, for the most part, in the capsules of joints, but sometimes in ten- dons. Usually one surface is cartilage-covered, and either enters into the formation of a joint or, separated by a bursa, plays on another bone, or on cartilage or liga- ment. Their function is to obviate friction, and, in some cases, to change the direc- tion of the pull of a muscle. The number of sesamoid bones is very variable ; but the usual idea that they are, so to speak, accidental, depending on the mechanics of a certain joint or tendon, must probably be abandoned. They are rather to be con- sidered as real parts of the skeleton, 1 all of which have their places in certain animals, but all of which either are not developed, or, if they do appear, are again lost in others. Thus, certain sesamoid bones of the fingers are very frequent in the foetus and very rare in the adult. Growth of Bones. The microscopical details of bone-growth are given else- where (page 94). Suffice it to say here that each bone has certain so-called centres of ossification from which the formation of the new bone spreads. In the long bones there is one main centre in the shaft, or diaphysis, which appears in the first half of tcetal life. Other centres appear, usually some time after birth, in the ends of the 1 Thilenius : Morpholog. Arbeiten, Bd. vi., 1896. MECHANICS OF BONE. 105 bones. There may be one or several in each end. The part formed around each of these secondary centres is called an epiphysis. Growth takes place chiefly in the cartilage between the epiphyses and the shaft. When, therefore, a joint is resected in childhood the surgeon tries to leave a part of the epiphysis in place. A curious relation exists between the course of the chief medullary artery of the shaft of a long bone and the behavior of the epiphyses. The epiphysis towards which the vessel is di- rected is the last to appear and the first to unite. (The fibula furnishes an exception. ) As a rule, also, the largest epiphyses appear first and unite last. In long bones with an epiphysis at one end only, the nutrient canal leads towards the opposite extremity. Mechanics of Bone. A long bone has a hollow shaft containing marrow, the wall being of compact bone. The hollowness of the shaft takes from the weight, and, moreover, conforms to the well-known law that a given quantity of matter is much stronger, both lengthwise and crosswise, when disposed as a hollow cylinder than as a solid one of equal length. The proportion of the central or medullary cavity is not the same in all bones. Perhaps, as an average, its diameter may be said to equal one-third of that of the bone. In the shaft this cavity is crossed by a few bony trabeculae, almost all of which are destroyed in maceration. Towards the ends, as the outer wall becomes thinner, large numbers of thin plates spring from its inner surface and incline towards one another in graceful curves, until at last the expanded end of the bone consists of spongy or cancellated tissue enclosed within a delicate wall of compact substance. The arrangement of these plates is distinctly pur- poseful, since it has been shown that they are so disposed as to correspond with the stress-lines an engineer would construct for the special purpose served by the end of the bone. None the less, it would be unwarranted to maintain that mathematical correctness is always to be found, or that there are not other modifying influences. The internal structure of all bones, excepting, perhaps, those of the skull, is of this nature, so that the following remarks apply to spongy bone in general. The delicate cancellated structure is for the most part in thin plates. The sim- plest arrangement occurs in a short bone exposed to pressure only at two opposite surfaces ; in such cases the plates run between these surfaces with few and insignifi- cant cross-pieces. Where severe pressure may come in almost any direction, as in the case of the globular heads of the humerus and femur, the round-meshed pattern predominates, producing a very dense spongy structure which may be represented diagrammatically by drawing lines crossing at right angles and by enlarging every point of intersection. In the midst of this round-meshed type there is very fre- quently a central core with stronger plates and larger spaces. The va^dted system is found at the projecting ends of bones, and between the round-meshed cancellated substance and the shaft. Several special arrangements will be described in connec- tion with the bones in which they occur. An epiphysis, until it has fused, shows the mechanical structure of a separate bone. A process tor the attachment of muscles or ligaments generally contains a very light internal -structure, the surface of the shaft of the bone being rarely continued under it. The continuation of the fibres of attached tendons is not represented by internal plates of bone, although the oppo- site opinion has supporters. Certain of the bones of the cranium and the face are in parts hollowed out into mere shells bounding a cavity lined with mucous membrane continuous with that of the nose or the pharynx. The elasticity of bones is enhanced by curves. The long bones very usually present a double curve. It has been maintained that these curves form a spiral structure. There are striking instances of it, but the universality of the law is not proved ; although shocks are thus lessened, the passage of one curve to another is a weak point in the bone. The ends of the long bones are enlarged for articulation with their neighbors. The greater part of this enlargement forms the joint, the various shapes of which will be discussed later. Besides this, there are usually at the ends prominences for muscles. The shaft generally bears ridges, which in some cases are made of dense bone and materially add to the strength of the bone. A ridge or prominence usually implies the insertion of a fibrous aponeurosis or a tendon. Muscular fibres, however, may spring from the periosteum over a flat surface. 106 HUMAN ANATOMY. Parts of Bones. The following are some of the names applied to features of bone : A process is a general term for a projection. A spine or spinous process is a sharp projection. A tuberosity is a large rounded one, a tubercle is a small one, either rounded or pointed. A crest is a prominent ridge. A head is an enlargement at the end of a bone, in part articular. A neck is a constriction below a head. A condyle is a rounded articular eminence, generally a modification of a cylinder. A fossa is a pit. A glenoid cavity is a shallow articular depression. A cotyloid cavity is a deep one. A sulcus is a furrow. A foramen is a hole, in the sense of a perforation. A sinus is the cavity of a hollow bone, equivalent to antrum. It is used also to designate certain grooves for veins in the cavity of the cranium. In addition to the cartilage-covered articular surfaces proper, the fresh bones show in some places a plate of cartilage quite like one for a joint; such plates serve to lessen the friction of a tendon playing over the bone. In other places a look of pecul- iar smoothness is conferred by the presence of a bursa, although cartilage is wanting. Sex of Bones. Female bones are characterized in general by: (a) a greater slenderness; () a smaller development of processes and ridges for muscular attach- ment 1 ; (c} and, most important of all, the small size of the articular surfaces. These guides usually suffice to determine the sex of the chief bones; some, especially those of the pelvis, possess characteristic sexual differences of form. Age of Bones. At birth the long bones have cartilaginous ends in which, with one or two exceptions, the centres of ossification have not yet appeared. Many bones at this period still consist of several pieces which ultimately fuse. The shape and proportions are in some cases different from those of the adult. Sexual differ- ences cannot in most cases be determined. During the first years new centres of ossification appear, distinct pieces unite, and the proportions change from the type of the infant to that of the child. Towards puberty important further changes in proportion occur, and sexual differences develop. After puberty the bones present three stages, adolescence, maturity, and senility. In the first the union of the epiphyscs is going on ; after this has taken place the line of separation is visible for a time, but gradually disappears. Our knowledge of the time at which these changes occur enables us to determine the age of the skel- eton. The long period of maturity presents little that allows of a precise estimate of age. The separate bones of the vault of the cranium gradually fuse into one. The senile skeleton in its extreme stage is very striking. There is a general atrophy of the bones both within and without, those of the face becoming in parts of papery thinness ; not only the cavities within the cranial bones become larger, but also the spaces within the cancellous tissue inside the bones, due to the partial absorption of the spongy substance. The only bones, however, which show a distinct change of form are the jaws, and this is a secondary result of the loss of the teeth. In many cases, however, senile absorption and atrophy do not occur, except, perhaps, in the head ; it may be, therefore, absolutely impossible to distinguish a long bone of an old subject from one of an individual in early maturity. The periods at which the age of bones is most often a matter of medico-legal inquiry are at the time of birth and in childhood and youth. The dates of the first appearance of ossification in the various bones are the criteria for the first. These are to be used, however, with great caution, since variation is considerable. The information to be derived from consideration of the general development of the body is perhaps of equal value. The same holds good for childhood and adolescence. The particular point on which the writer holds strong views, based on his own observations, differing from those generally accepted, is as to the time of union of the epiphyses at the end of ado- 1 Dwight : American Journal of Anatomy, Vol. iv., 1904. GENERAL CONSIDERATION OF THE JOINTS. 107 lescence. He is convinced, as his statements will show, that this union occurs earlier than is generally taught. Relation of the Bones to the Figure. While it may be said that power- ful muscles leave their imprint on the bones in strong, rough ridges, yet it is impos- sible to give a trustworthy description of the figure from the size and shape of the bones, since these are determined chiefly by prenatal influences. Very delicate, even puny, bodies may have large and strong bones, and great muscular develop- ment may coexist with a light framework. Variations. Besides the great range of individual variation, without departure from the usual type, bones occasionally show greater peculiarities. These may occur through either excess or defect of ossification. .Structures which are normally car- tilaginous or fibrous may become replaced by bone, and abnormal foramina may occur in consequence, or to accommodate the aberrant course of blood-vessels or nerves. The most interesting of these variations are such as present an arrangement which is normal in some of the lower animals. Many variations may be plausibly accounted for as reversions, but others cannot be explained in this way according to any conceivable scheme of descent. By speaking of these variations as animal analogies we avoid theories and keep to scientific truth. Number of Bones. The usual enumeration of the bones composing the human skeleton is misleading, for while it is customary in some parts, as the head, to count each bone, in others, like the sacrum and the hyoid, only the ultimate condition, after union of the component segments, is considered. In other cases, like the sternum, there may be grave doubt which course is the proper one to follow ; and finally, as in the coccyx, the number is variable. Bearing these impor- tant facts in mind, it may be stated that the human skeleton in middle life usually comprises, as conventionally reckoned, two hundred separate bones, excluding the sesamoids within the tendons of the short flexor of the thumb and of the great toe and the ear-ossicles, but including the patella and the hyoid bone. Of this number, seventy-four bones belong to the axial and one hundred and twenty-six to the appen- dicular skeleton. The skeleton is advantageously described in the following order : the spine, the thorax, the head, the shoulder-girdle and the arm, the pelvic girdle and the leg. The account of the bones of each region is succeeded by that of the joints and the ligaments holding them together, followed by a consideration of the region as a whole and of its relation to the surface. The applications of anatomical details of the skeleton to the requirements of medicine and surgery are pointed out in appro- priate places. GENERAL CONSIDERATION OF THE JOINTS. A JOINT or articulation implies the union of two or more bones. Joints may be divided, according to their mobility, into three great classes : the FIXED JOINT (Syn- arthrosis), the HALF-JOINT (Amphiarthrosis), and the TRUE JOINT (Diarthrosis} . Fixed Joints. These allow no motion in the mature condition, and are rep- resented by two subdivisions, the Suture and the Synchondrosis. The suture is the direct union of two bones which at first may be separated by membrane or by fibrous tissue, but which eventually become firmly united. Several varieties of this form of union are recognized ; thus a serrated suture is one in which the edges are interlocked, as the teeth of two saws ; conspicuous examples are seen in the interparietal and the parieto-occipital junctures. Frequently one bone tends to overlap at one end of the suture and to be overlapped at the other. A squamous suture is one in which a scale-like bone very much overlaps another, as in the relation between the temporal and the parietal bone. An harmonic suture is one in which two approximately plane surfaces are apposed, as in the case of the vertical plate of the palate and the maxillary bone. The term grooved suture is sometimes employed to designate a form of union in which one bone is received within the grooved sur- face of another, as the rostrum of the sphenoid and the vomer. Wormian bones are small, irregular ossifications which appear as bony islands in the course of a suture. Familiar examples of these are seen in the line of the parieto-occipital suture. io8 HUMAN ANATOMY. Synchondrosis is the union of two bones by an intervening strip of cartilage, which usually ultimately becomes replaced by bone. Such is the union between the pieces of the body of the sternum and between certain bones of the base of the skull. The term is also applied to the union of the shaft and the epiphyses of long bones. Half-Joints, including Symphysis and Syndesmosis. From the stand-point of development, there is no fundamental difference between symphyses and the true joints. In both cases a small cavity appears within the intervening mesoblastic tissue connecting the ends of the embryonal bones. This small cavity, in the case of the true joints, rapidly increases, and later is lined by the flattened mesoblastic cells investing the subsequently differentiated synovial membrane. When, on the con- trary, the bones are to become united by dense fibrous and nbro-cartilaginous tissue, as in the case of a symphysis, the interarticular space is always a mere cleft sur- rounded by the interlacing and robust bundles of the dense tissue forming the union in the mature joint. A symphysis implies great strength and very limited and indefinite motion, there being no arrangement of surfaces to determine its nature. The chief function of this form of union seems rather to be to break shocks. The central cavity is not always found. The symphysis pubis (Fig. 361) is a typical half -joint. Those con- necting the bodies of the vertebrae are usually so classed, but it is not certain that they quite agree either in structure or development with the description. A transi- FIG. 134. Diagrams of various forms of suture. A, serrated ; B, squamous ; C, harmonic ; D, grooved. tional form leading from the symphysis to the true joint is one in which the limited synovial cavity, instead of being in the centre of a mass of fibro-cartilage, lies between two cartilaginous surfaces, somewhat like that of a true joint, but so interlocked and surrounded by short, tense fibres as to preclude more than very slight motion. This arrangement is often seen in the articulation between the sacrum and ilium, some- times improperly called the sacro-iliac synchondrosis. Syndesmosis is to be included among the half-joints. It is the binding together of bones by fibres, either in bundles or as a membrane, without any inter- vening cartilage ; an example of this arrangement is seen in the union effected by the interosseous ligament in the lower tibio-fibular articulation. True Joints. These articulations develop in a similar manner to the half- joints, except that the opposed ends of the developing bones are of hyaline carti- lage, fibro-cartilage being present only at the sides, except in the case of a compound joint, where it forms the intervening plate. The tissue at the sides of the articular cleft differentiates into two layers, the inner, which is the synovial membrane, consist- ing of a layer of cells continuous with the superficial layer of the cartilage-cells and secreting a viscid fluid, the synovia, which lubricates the joint ; and the outer part, which becomes a fibrous bag called the capsular ligament. The latter, in its simplest form, consists of only enough fibrous tissue to support the synovial membrane. The capsular ligament is strengthened by accessory ligaments developing in or around it, the arrangement of which depends on the needs of the joint. During development, STRUCTURE OF JOINTS. 109 independent of the influence of motility or of muscular action, the articular ends of the bones assume definite shapes such as will allow the motion peculiar to that joint, and (barring the frequent want of perfect coaptation) no other. The common char- acteristics of true joints are articular surfaces covered by hyaline cartilage, so shaped as to determine the nature of the movement, enclosed by a capsule lined with a synovial membrane. The articular surfaces are not necessarily formed wholly of bone, since very often increased concavity is secured by the addition of a lip of fibro- cartilage to the margin of the bone ; in other cases ligaments coated with carti- lage complete a socket ; or again, disks of fibro-cartilage loosely attached to the periphery may project into a joint and partially subdivide it, following one bone in certain movements and the other in others. Compoiind joints result from the persistence and differentiation of a portion of the tissue uniting the ends of the embryonal bones into a partition which, in the complete compound joint, separates the two synovial cavities developed, one on either side of the septum. The tissue between the bones becomes a fibre-cartilagi- nous disk, 1 which partially or completely subdivides the cavity. In such a joint, when typical, there are two ends of bone covered with articular cartilage, separated FIG. 135. Diagrams illustrating formation of joints. A, bones are united by young connective tissue; B, appearance of joint-cavity; C, differentiation of joint-cavity and capsule; D, development of two joint-cavities separated by fibrous septum, resulting in a compound joint. by a fibro-cartilaginous disk or meniscus, and two distinct synovial membranes. The movements are, however, still determined to a considerable extent by the shape of the bones, so that these articulations may be classed as true joints. The fibro- cartilaginous meniscus may be replaced by a row of bones as in the wrist. Structure of True Joints. The opposed ends of the bones, and sometimes other tissues, are coated with hyaline articular cartilage, which gives a greater smoothness to the articulating surfaces than is found on the macerated bones. Though following in the main the bony contours, the cartilage does not do so accu- rately ; details are found on the cartilage that are obscure on the bones. It dimin- ishes the force of shocks. Although, as already stated, the shape of the articular ends determines the nature of the motion, it is important to recognize that, as in the case of saddle-joints, the opposed surfaces are not so accurately in apposition that irreg- ular movements cannot and do not occur. Failure to appreciate this fact has given rise to much difficulty in accounting for motions that undoubtedly take place, but which, according to the mathematical conception of the joint, are impossible. Further, the range of individual variation is great ; just as a man may have a long or a short head, so any of the articular ends of his bones may depart considerably from the average proportions. It is even possible in some of the smaller joints that 1 Discus articularis. 110 HUMAN ANATOMY. FIG. 136. the articular surface of a certain bone may be plane, convex, or concave in different persons. The capsule. Every joint, with possibly some exceptions in the carpus and the tarsus, is enclosed by a capsule, 1 or capsular ligament, which arises from the peri- osteum near the borders of the articular cartilage and surrounds the joint. This envelope consists of a membrane, often containing fat within its meshes, composed of two layers, the inner delicate synovial membrane and the external fibrous layer. The latter, while in some places very thin, is usually strengthened by the incorpora- tion of fibrous bands which, from their position, are known as lateral, anterior, or posterior ligaments. These bands are of strong, non-elastic fibrous tissue which under ordinary circumstances do not admit of stretching. The strength and security of the joint are often materially increased through thickenings of fasciae and expansion of tendons which blend with the underlying capsule. The capsule must be large enough to allow the characteristic movements of the joint ; consequently, when the bone is moved in any particular direction that side of the capsule is relaxed and thrown into folds. These folds are drawn out of the way either by small special muscles situated beneath those causing the chief movement or by fibres from the deeper surfaces of these latter muscles. In the joints of the arches of the vertebrae, there being no muscles inside the spinal canal, a dif- ferent arrangement exists for the inner side of the capsule, elastic tissue there taking the place of muscle. The relation of the insertion of the capsule to the line of the epiphysis is important. Although this point is fully con- sidered in the description of the individual joints, it may be here stated that, as a rule, in the long bones, the capsule arises very near the line of the epiphysis. The synovial membrane which lines the interior of the capsule and other portions of the joint, except the surfaces of the articular cartilages, consists of a delicate connective- tissue sheet, containing many branched and flattened connective-tissue cells. The latter, where numerous, as is the case except at points subjected to considerable pressure, are ar- ranged on the free surface of the synovial mem- brane as a more or less continuous layer, often spoken of as the endothelium of the synovial sac. Since in many places the layer of connective-tissue elements is imperfect and the component cells retain their stellate form, the cellular investment of the joint- cavity is at best endothelioid, suggesting, rather than constituting, an endothelium. The synovial membrane is in certain places pushed inward by accumulations of fat of definite shape between it and the capsule. It is also prolonged, as the synovial fringes? into any space that might otherwise be left vacant in the various movements. They are alternately drawn in or thrust out, according to circumstances. Some- times pieces of them, or of fibro-cartilage, become detached in the joint, giving rise to much trouble. The cavity which is found when a joint is opened on the cadaver, with the tissues dead and relaxed, easily suggests a false impression. It is to be remembered that the synovial fluid normally is present in quantity little more than sufficient to lubricate the joint, and that in life all the parts are strongly pressed together so that no true cavity exists. This is well shown by frozen sections. Certain so-called intra- articular ligaments, as the ligamentum teres of the hip, or the crucial ligaments of the knee-joint, are found in the adult, roughly speaking, inside the joint. The sketch of development given above shows that they cannot be truly within the articular cavity. In fact, either they wander in from the capsule, carrying with them a reflection of synovial membrane, or they are the remnants of 1 Capsula articularis . " Plicae synoviales. Capsule Synovial membrane Articular cartilage Joint-cavity Reflection of syno- vial membrane Epiphyseal bone Diagram showing the parts of a typical joint. Ill the capsules separating two distinct joints which have broken down so as to make a common articular cavity. Such ligaments retain their synovial covering and really lie without the joint-cavity. Vessels and Nerves. Important arterial anastomoses surround all the larger joints ; from the larger vessels small branches pass inward to the ends of the bones, to the periphery of the articular cartilages, and to the capsule. The margins of the cartilages are surrounded by vascular loops ; the articulating surfaces are, however, free from blood-vessels. The synovial membrane is usually well supplied with minute branches, a rich net-work being described at the bases of the synovial fringes. The veins form strong plexuses. Lymphatics are found well developed directly beneath the inner surface of the synovial membrane ; while it is certain that they absorb from the joint, direct open- ings into the articular cavity have not been demonstrated. Nerves^ presumably sensory and vasomotor, end in the tissues around the syno- vial membrane. In addition to the Pacinian bodies, which are sometimes very numerous, Krause has described special articular end-bulbs outside the synovial membrane surrounding the finger-joints in man. . Blood-vessel Free surface of articular car- tilage Bone Marrow-cavity ':/s* ,'A "Synovial mem- ' brane Union of carti- lage and syno- V ' a ^ mem brane Section through margin of joint, showing articular cartilage and capsule. X 135. Bursae J are sacs filled with fluid found in various places where friction occurs between different layers or structures. They are sometimes divided into synovia/ and mucous bursae. These varieties are distinct in typical instances, but, since the one passes insensibly into the other, it is doubtful whether this subdivision is war- ranted. Some bursae, especially those around the tendons of the fingers, have a true synovial lining reflected over the tendons, and are surrounded by strong fibrous sheaths known as the thecce synoviales. 2 Other bursae are placed as capsules around a cartilage-coated facet over which a tendon plays. Both the vaginal and capsular varieties may be classed as synovial bursae; Representatives of the mucous bursae are those within the subcutaneous tissue where the skin is exposed to friction, as at the elbow and the knee. These bursae seem little more than exaggerations of the spaces between layers of areolar tissue. The same may be said of some of those among the muscles. The mucous bursat are provided with more or less of a cellu- lar lining, but the latter is less perfect than in the synovial class. A bursa may be simple or composed of several cavities communicating more or less freely. They often communicate with joints. Their number is uncertain. Many, perhaps most, are present at birth, but new ones may appear in situations exposed in certain 1 Bursae synoviales. " Vaginae mucosae tendinum. ii2 HUMAN ANATOMY. individuals to uncommon pressure or friction, and, under these circumstances, the ones usually present may be enormously enlarged. Modes of Fixation in Joints. Ligaments, muscles, atmospheric pressure, and cohesion are the agents for fixation. Ligaments. A capsular ligament, pure and simple, has little retaining strength. The accessory ligaments, on the contrary, have great influence. Their arrangement differs with the nature of the joint. Thus, a ball-and-socket joint has thickenings at such parts of the capsule as the particular needs of that joint require. A hinge-joint implies strong lateral ligaments ; a rotary joint, some kind of a retain- ing-band that shall not arrest motion. Sometimes certain ligaments are tense, or nearly so, in every position of the joint, as the lateral ligaments of a hinge-joint. Often a ligament is tense only when a joint is in a particular position, as the ilio- femoral ligament of the hip when the thigh is extended. A strong ligament like the one just mentioned is, when tense, the greatest protection against displace- ment. Muscles. The action of the muscles is of great importance in maintaining the joints in position, in certain instances being the most efficient agency. The con- stant pull of the muscles keeps the more movable bone closely applied to the more fixed in all positions. Muscles which are nowhere in contact with the joint may exercise this function. The tendons of muscles sometimes act as ligaments, which differ from the ordinary ligamentous bands in that they may be made tense or relaxed by muscular action. Sometimes they are intimately connected with the capsule, at other times distinct from it. Some muscles, whose tendons cross several joints, exercise, by their tonicity, an influence on them all. Thus, the peroneus longus is essential to the maintenance of the transverse arch of the foot. Certain muscles passing over more than one joint exert a ligamentous action on one joint determined by the position of the other. This, however, is more properly dis- cussed in connection with the action of muscles. Atmospheric Pressure. Much has been written about the action of this agency in holding joints in place. The atmosphere exerts a certain pressure on all bodies, animate or inanimate, and thus tends to compress them. The joints, as parts of the body, are subject to this general influence. It is by no means very effi- cacious. The shoulder-joint has a capsule long enough to allow very free motion, and consequently too long to hold the humerus in place. This is done chiefly by the muscles. When these are paralyzed the arm falls out of place, atmospheric pressure being inadequate to resist the weight. The most important action of atmospheric pressure is to keep the soft parts closely applied to the bones. Cohesion is the action of the viscid synovial fluid which tends to hold the surfaces together. It is very feeble, but probably has an appreciable influence in the smaller joints. Limitation of Motion. The shape of the joint determines the nature of the movement ; its range depends in part on other factors, such as the tension of liga- ments or of the tendons of muscles and the resistance of the soft parts. Motion in True Joints. It is easy to conceive that an upright rod on the highest point of rather less than half a sphere may FlG - : 3 8 - slide to the periphery along an indefinite number of OTATION ^ > ^A/ (?(/< lines. This is angular motion. The rod on reach- ^ -" ~"<^ ing the periphery, or at any point on the way, may travel round in a circle describing the surface of a cone. This is circumduction. Finally, without any change of position, the rod may revolve on its own axis. This is rotation. Changes of Position of Parts of the Body. Diagram illustrating different kinds of -Assuming that the palms are looking forward, an- motion. gular motion of a limb, or of a part of one, towards the median plane of the body is called adduction ; the opposite movement, abduction. A motion bringing the distal end of a limb bone nearer to the head is called flexion ; the opposite movement, extension. The move- ments of the ankle and the foot, however, present a difficulty, although the above VARIETIES OF TRUE JOINTS. 113 nomenclature is generally accepted, since the digital extensor muscles flex and the flexors extend. It is best with reference to the ankle-joint to speak of plantar flexion and dorsal flexion. Pronation in the arm is turning the front of a limb downward ; supination, the converse. Thus, when the palm rests upon a table the arm is pronated ; when the back of the hand rests upon the same support the arm is supinated. Reference to the skeleton during these movements will show that pro- nation is associated with crossing of the bones of the forearm, while during supina- tion they are parallel. These terms should not be applied to motions of the leg. Rotation is inward or outward, according as it is towards or away from the median line of the body. Varieties of True Joints. The following are the chief kinds of true joints, the nature of the motion being determined by the articular surfaces : Arthrodia, 1 a gliding joint permitting merely a sliding between two nearly plane surfaces, as between the articular processes of the vertebrae. Knarthrosis/ a ball-and-socket joint permitting angular motion in any direc- tion, circumduction and rotation. The shoulder- and hip-joints are conspicuous examples. Condylarthrosis, 3 an egg-shaped joint permitting angular motions more freely on the long axis than on the short one, circumduction but (theoretically, at least) no rotation, as in the radio-carpal articulations. The imaginary axes for the angular motions lie in the convex bone. The Saddle-Joint, 4 is a modification of the above, the end of one bone being convex in one plane and concave in another, at right angles to the first, while the other bone is the converse ; thus in one plane one bone is the receiver and in the other the received. The articulation of the trapezium with the first metacarpel bone is an example. The motions in such joints are precisely the same as those of the preceding form. The two imaginary axes are, however, on opposite sides of the joint, each being at right angles to the convex plane of its own bone. It is clear that if the reciprocal curves of the two bones of a saddle-joint coincide, and that if they fit closely, rotation is out of the question ; but, in point of fact, that is not the case, for there is no very accurate agreement of the surfaces, and the contained curve is smaller than the containing, so that a certain amount of rotation is possible. 5 Ginglymus, 6 a hinge-joint permitting motion only on a single axis approxi- mately transverse to the long axis of the bone, consequently the moving bone keeps in one plane. The ankle-joint is an example. The inclination of the transverse axis may vary, and one end of the joint be larger than the other. If the course of the revolving bone is that of a spiral around the transverse cylinder the articulation constiutes a screw-joint? as the humero-ulnar articulation. Trochoides, 8 a pivot-joint permitting motion only on one axis coincident with at least a part of the long axis of the bone, namely, rotation, as in the atlanto-axial articulation. Should a part of the bone be so bent as to lie outside of the axis, as in the radius, this part undoubtedly changes position ; nevertheless, there is merely rotation, for the change of position is accidental, depending on the shape of the bone, not on the nature of the motion. Certain complicated joints may combine several of the above forms. 5 Ren6du Bois-Reymond. Archiv fiir Anat. u. Phys., Phys. Abtheil., 1895. 'Arthrodia. - Enarthrosis. 3 Articulatio ellipsoidea. 4 Artlculatio sellaris. 6 Ginglymus. ' Articulatio cochlearis. s Articulntio trochoidea. THE SPINAL COLUMN. THE spinal column is the central part of the skeleton. It supports the head, bears the ribs, thus indirectly supporting the arms, and encloses the spinal cord. It gives origin to many muscles, some passing between different parts of the spine, others connecting it with the body. These purposes demand great strength and flexibility. The spine is composed of many pieces united by tough fibre-cartilagi- nous disks, by which the force of shocks is broken and the great range of move- ment is distributed among many joints. It is convex behind in the regions of the thorax and pelvis, so as to enlarge those cavities, and has forward convexities in the neck and loins. The numerous prominences which it presents serve for the support of the ribs, the attachment of muscles, and the interlocking of the various pieces. The spinal column is firmly fixed near the lower end between the bones of the pelvis. The bones composing this column are called vertebrce, of which in the adult there are thirty-three or thirty-four in all. They are divided into five groups. The FIG. 139. Spinous process Facet for tubercle of rib Transverse proces Superior articular process Demi-facet for head of rib Body Sixth thoracic vertebra from above. first seven are the cervical ; the next twelve, which bear ribs, are the thoracic ; the next five are the lumbar, making twenty-four above the pelvis. These are known as the presacral vertebrae. The remainder are in the adult united into two bones, the first five forming the sacrum, the last four or five the coccyx. As many as thirty-eight are seen in the young embryo, but some disappear or are fused. With the exception of the first two, the atlas and the axis, which require a separate description (page 119), the vertebrae above the sacrum present the following features, which are common to all, but which are modified in the different regions : (i) a body 1 or centrum ; (2) ^pedicle* springing from the back of the body on either side, supporting (3) the lamina? a plate which meets its fellow in the middle line to form an arch bounding the spinal or -vertebral foramen*' for the spinal cord. Each vertebra gives origin to several processes. namely, (4) a spinous process? springing from the point of union of the laminae ; (5) a transverse process on each side, pro- jecting outward from the junction of the pedicle and lamina ; (6) two articidating 1 Corpus. - Radix arcus vertebrae. 3 Arcus. 4 Foramen vertebrale. 6 Processus spinosus. 6 Processus transversus. 114 THORACIC VERTEBRA. Processes 1 on each side, one above and one below the lamina, forming true joints with the opposed processes of the neighboring vertebrae ; (7) a rib or costal element, which in the thoracic region is a separate bone, in the cervical region is a part of the vertebra, and in the lumbar FIG. 140. Pedicle Superior articular process- and facet Articular facet on transverse process Superior demi-facet for rib region mingles with the trans- verse process. The costal ele- ment is also represented in the sacrum. Thoracic Vertebrae. A vertebra from the middle of the thoracic region is de- scribed first as intermediate in several respects to the others. The body is but a little broader transversely than from before backward. It is a little deeper behind than in front, thereby helping to form the curve of the spine. The upper and lower borders pro- ject a little anteriorly. The upper and lower surfaces, as in all the vertebrae, are rough where the intervertebral disks join them. The posterior surface is concave from side to side, and presents in the middle one or two foramina for the escape of the veins. At the back of the side of the body there is half an articular facet both above and below, which, with the intervening disk, forms an oval, shallow socket for the head of the rib belonging to the lower vertebra. The spinal foramen, en- FIG. 141. Body Inferior demi-facet for rib Inferior articular process Spinous process Sixth thoracic vertebra from the side. Superior articu- lar process and facet closed by the arch, is circular. The pedicles, which are much deeper than thick, arise from the upper half of the body. The supe- rior border rises gradually to the articular process. The inferior bor- der is concave, forming the top of the notch? which, when the succeed- ing vertebra is in place, forms the top of the intervertebral foramen? which is wholly behind the lower half of the body. The laminae are broad, each reaching to the level of those of the next vertebra. The spinous process is long, and points strongly downward, over- lapping the one below. It has a narrow under surface which is grooved, and two lateral ones meet- ing above in a ridge continued from the laminae. This arrangement of the laminae and spines completely closes the cavity of the spinal canal. The spinous processes are slightly enlarged at the end for the supraspinous liga- ment and muscles. The transverse processes are strong, having to support the ribs. They pro- 1 Processus articularis. ~ Incisura vertebralis. a Foramen intervertebrale. Spinous process Sixth thoracic vertebra from behind. n6 HUMAN ANATOMY. ject outward and backward, and enlarge at the tip, which anteriorly presents a con- cave articular surface for the tubercle of the rib, and is rough behind for muscles. The articular surfaces are in two pairs above and below, each pair facing in opposite directions, so that the lower ones of one vertebra meet the upper ones of FIG. 142. Spinous process Inferior articular process Superior articular process Transverse foramen Transverse process Posterior limb of transverse process Posterior tubercle Costal element Anterior tubercle -j Body Anterior limb of transverse process Fourth cervical vertebra from above. the next. Each presents a smooth, roughly oval articular surface. The superior ones face backward, a little outward, and a very little upward ; the inferior, con- versely, look forward, inward, and slightly downward. FIG. 143. Groove for spinal nerve Superior articular process Anterior tubercle .Posterior tubercle Fourth cervical vertebra from in front. Cervical Vertebrae. A typical cervical vertebra is much smaller than the thoracic. The body is decidedly longer from side to side than from before backward. The upper surface is raised at the sides FIG. 144. so as to embrace the body next above :icular process and facet and hag ^ frQnt border rounded for the latter to descend over it ; for this pur- -Anterior pose the lower anterior border is pro- tubercie longed downward. The height of the P tublrck body is about the same before and be- hind. The spinal foramen is triangular, with the greatest diameter transverse. The pedicles are short and light, and extend backward and outward from Fourth cervical vertebra from the side. the body. The notches above and be- low them are about equal. The intervertebral foramen is opposite the intervertebral disk, and a part of the bodies of two vertebrae. Inferior articular pro- cess and facet Intervertebral Groove for spinal notch nerve LUMBAR VERTEBRA. 117 The laminae are smooth and do not quite meet those of the next vertebra, unless the head be bent backward. The spinous process projects backward and a little downward. It is short and forked at the end, very often unevenly. The transverse processes are often described as double. The posterior limb, which is the true transverse process, projects outward and somewhat forward from the junction of the pedicle and lamina, and ends in a flattened, nearly vertical pro- jection, the posterior transverse tubercle. The anterior limb, a vertical plate spring- ing from the side of the body and extending outward, ends in the anterior transverse tubercle. This limb is the shorter of the two and its tubercle the larger. The limbs are connected by a concave plate or bone, slanting slightly outward, which forms the floor of a gutter l in which the spinal nerve lies, and which represents the costal element. A round hole, the transverse foramen, for the vertebral artery and veins, lies internal to this plate ; the artery usually does not pass through the foramen of the seventh vertebra. Since the scalenus anticus muscle springs from the anterior FIG. 145. Spinous process Superior articular facet Mammillary proces: Third lumbar vertebra from above. * tubercles and the scalenus medius from the posterior ones, on leaving the spine the spinal nerves pass between these muscles. The articular processes are placed at the outer ends of the laminae ; the upper face upward and backward, the lower forward and downward. Lumbar Vertebrae. A typical lumbar vertebra is very much larger than the others. The body is broad from side to side, the upper and lower borders projecting especially at the sides. The posterior surface is slightly concave and presents two large venous openings. The spinal foramen is three-sided, with a transverse diameter but slightly exceeding the antero-posterior. The pedicles are short and strong, diverging only slightly. They are very nearly on a level above with the top of the body, so that there is a small notch above and a large one below. The laminae are broad at the sides, but less so near the mid-line, so that in this 1 Sulcus n. spinalis. n8 HUMAN ANATOMY. region there is a large opening into the spinal canal. A considerable part of the arch is lower than the body. The spinous process is a flat projection extending nearly straight backward, with two lateral surfaces and a superior, inferior, and posterior border. The last is rough and thickened below, with occasionally a tendency to become bifid. The transverse processes, which are solely for muscular attachments, and FIG. 146. Superior articular process Mammillary process* Transverse process Accessory process -Inferior articular process and facet Third lumbar vertebra from the side. therefore not heavy, project outward and somewhat backward. They are thin, having an anterior and a posterior surface and a blunt end. The articular processes are large, very nearly vertical, and curved. The superior, facing somewhat backward but chiefly inward, are concave and embrace the inferior ones of the vertebra above, which are convex, and face in the opposite direction. Superior articular process and facet Mammillary process Accessory process Inferior articular process Spinous process Lamina Third lumbar vertebra from behind and the side. The mammillary processes form on either side a rounded lateral projection on the posterior border of the superior articular process. Additional tubercles, the accessory processes, appear as inconspicuous elevations at the junction of the posterior border of the transverse with the superior articular processes. The details and the morphological significance of the mammillary and the accessory processes are discussed later (page 123). PECULIAR VERTEBRAE. The chief points of difference between typical vertebrae of groups may be tabulated as follows : 119 the three presacral BODY. CERVICAL. 1. Broad. 2. Upper surface with raised sides and rounded anterior bor- der. 3. No facets. THORACIC. Diameters nearly equal; concave behind. Plane. Costal semifacets. LUMBAR. SPINAL FORAMEN. Triangular, with great- Nearly circular, est diameter trans- verse. PEDICLES. LAMINJE. Notches above and be- Rising from top of body; low nearly equal. great notch below. Narrow, with spaces be- Broad ; no spaces be- tween, tween. TRANSVERSE PRO- Double foramen at root ; Strong, with articular two tubercles. facet. CESSES. SUPERIOR ARTICU- Nearly plane ; face up- Plane, vertical ; face LAR SURFACES. ward and backward. nearly backward. Broad. Plane. No facets. Triangular, with diam- eters nearly equal. Small notch above, great one below. Extending downward; large spaces be- tween. Slender. Concave, vertical ; face chiefly inward. PECULIAR VERTEBRA. Certain vertebrae differ more or less markedly from the type of their respective groups ; in some cases, as the upper two cervical vertebrae, these variations result in conspicuous modifications ; in others, as the lower thoracic, the peculiarities are less pronounced. Although the most noteworthy differences are here given, the reader FIG. 148. Posterior tubercle Posterior arch Lateral mass roove for vertebral artery Superior articular facet Transverse pr cess Transverse foramen Facet for odontoid process of axis Anterior arch Anterior tubercle The atlas from above. is referred to the discussion of the gradual changes which occur in passing from one region to the other (page 122) for a more complete account of the modifications to be observed. The first and second cervical vertebrae, known as the atlas and axis, consti- tute a special apparatus for the security and movements of the head. The key to 120 HUMAN ANATOMY. the arrangement is that the part which in the ordinary process of development should become the body of the atlas is instead fused with the body of the axis. The atlas, having no body, consists of two lateral masses, connected by a short anterior arch and a long posterior one. The lateral masses present the articular facets on their lower and upper surfaces. The inferior look downward and slightly inward, and are very slightly concave from before backward. The superior facets are oval concavities the backs of which are strongly raised from the surface. Their long axis runs forward and inward, the outer wall being decidedly higher than the inner. The articular facet narrows at the middle, and is often marked by a trans- verse ridge at this point ; rarely it is divided into two parts. The articular surfaces of the two sides sometimes very nearly correspond with parts of the surface of a single imaginary sphere. Their variation in all respects is great. Thus, Macalister J finds in one hundred bones that the distance between the front ends of the two facets varies from ten to twenty millimetres, being usually from fifteen to twenty millimetres, and that the hind ends are from thirty-two to fifty millimetres apart, the greater number being separated from thirty-five to forty millimetres. The angle formed by the intersec- FIG. 149. Posterior tubercle Posterior arch Transverse foramen Anterior ar nferior articular facet Position of transverse ligament (dotted lines) Facet for odontoid process Anterior tubercle The atlas from below. tion of the prolonged axes of the articular facets ranges from thirty-two to sixty- three degrees. Each lateral mass presents a rough tubercle on the inner side between which passes the transverse ligament holding the odontoid process close against the anterior arch. The anterior arch is compressed from before backward It presents the anterior tubercle in front in the median line, and behind has a slightly concave articular facet for the odontoid process. Tte posterior arch bounds the spinal canal behind. I he transverse ligament, confining the odontoid process, bounds the spinal canal in front, and, being in place, the transverse diameter of the canal is the longer. 1 he place of the spinous process is taken by the posterior tubercle. The transverse processes extend farther out than any in the cervical region. Each ends in a single flattened knob with a surface slanting downward and forward. Bifurcation is rare The transverse foramen is at its base ; from the foramen a groove for the vertebral artery crosses the root of the posterior arch and winds round behind the raised border of the articular surface. This groove is occasionally bridged over by a little arch of bone extending from the edge of the articular surface either to the trans- verse process or to the posterior arch. Variations. The atlas may be fused with the occipital bone in various ways; this may occur by the pathological destruction of the joint, or the arch, or a 1 Journal of Anatomy and Physiology, vol. xxvii., 1893. THE AXIS. 121 FIG. 150. Superior articular facet Articular facet on front of odontoid process The axis from in front. Inferior articu- lar facet part of it, may be fused with the skull around the foramen magnum. Such union may be partial or complete, and is usually associated with an imperfect development of the atlas, especially on one side. There is reason to regard such cases as con- genital. The transverse process and the paroccipital process of the occipital bone may be connected by bone. The axis ! differs less from the other cervical vertebrae ; seen from below it pre- sents no essential peculiarity. The body is very long even without the odontoid process (the separated body of the atlas) which surmounts it. The odontoid? a cylindrical process lower behind than in front, ends above in a median ridge, on either side of which is a rough, slant- ing surface for the origin of the check ligaments connecting it with the skull. It bears an oval articular facet in front, resting against one on the atlas, and a smaller facet behind at a lower level which forms part of a joint with the transverse ligament. The lamina, in- stead of being plates, are heavy and prismatic, each with a rather sharp upper edge, which, meeting its fellow, forms a ridge on the spine. The spinous process is heavy, projecting considerably beyond the third. It varies greatly in length and in degree of bifurcation. The transverse process is small ; the anterior tubercle is a mere point or altogether wanting. The transverse foramen is replaced by a short canal, so curved that its upper opening looks almost outward. The superior articular surfaces are approxi- mately circular facets on the upper surface of the body instead of on the arch, as are all below ; they look upward and a little outward. Although nearly plane, they present a very slight antero-posterior convexity. The seventh cervical vertebra, called vertebra prominens on account of its long, knobbed spine, rather resembles the upper thoracics. The transverse foramen is smaller than those above it, FIG. 151. and the anterior tubercle of the transverse process is particu- larly small and near the body. The first thoracic ver- tebra has the sides of the upper surface somewhat raised at the roots of the pedicles. It has a complete facet for the head of the first rib and a half- facet at the lower border of the body. Sometimes the former is imperfect, being completed on the intervertebral disk. The facet on the transverse process is smaller and less con- cave than the ones following ; sometimes it is even convex. The ninth thoracic vertebra has no half-facet below. The tenth thoracic vertebra has a nearly complete facet above and none below. The eleventh thoracic vertebra has a complete facet on the body and none on the transverse process, which is small. The twelfth thoracic vertebra has a complete facet a little above the middle of the body. The transverse process is broken up into the three tubercles. The lower articular facets face outward. The spine is of the lumbar type. 1 Epistropheus. '-' Dens. Odontoid process Articular facet for transverse ligament Spinous process Inferior articular process and facet The axis from the side. Body Transverse foramen 122 HUMAN ANATOMY. The fifth lumbar vertebra is much higher in front than behind. The trans- verse process is broad at the base, springing in part from the body ; the spine is relatively small. DIMENSIONS OF VERTEBRA. (The measurements are given in centimetres. ) Vertebrae. Height of Front of Bodies. (Dwight.) Height of Front of Bodies. (Anderson. 1 ) Height of Back of Bodies. (Anderson.) Transverse Diameter. (Anderson.) Antero- Posterior Diameter. (Anderson.) Spread of Transverse Processes. (Dwight.) Twenty Thirty Thirty Fifty-three Twenty-eight Fourteen spines. spines. spines. spines. spines. spines. Cervical . . 2 i-9 i-9 1.5 5-5 3 1.2 1.2 i-9 i-5 5-4 4 1.2 1.2 2.1 i-5 5-4 5 1.2 1.2 2-3 1.6 5-7 6 I.I I.I 2-5 i-7 5-9 7 i-3 1-3 2.7 1.8 7.2 Thoracic . . . i 1-5 1-4 1-5 2.7 i-7 7-6 2 i-7 1.6 i-7 2.8 i-7 7-i 3 1-7 i-7 1.8 2.6 i-9 6-3 4 i-7 i-7 1.9 2.6 2.2 6-3 5 1-7 1-7 2.0 2-5 2-4 6.4 6 1.8 1.8 !-9 2.7 2-5 6.4 7 1.8 1.8 2.O 2.8 2.6 6-3 8 1.8 1.8 2.1 3-o 2.8 6-3 9 i-9 1.9 2.1 3-i 2.9 6.2 10 2.1 2.1 2.2 3-4 2.9 5-8 ii 2.1 2.1 2.4 3-6 2-9 5-2 12 2-3 2-3 2-5 4.0 3-o 4-7 Lumbar . . I 2.4 2-4 2.6 4-2 2.9 7-3 2 2-5 2-5 2-7 4-4 3-1 8.0 3 2-5 2.6 2.7 4-7 3-6 9.0 4 2-5 2.6 2.6 4.8 ' 3-3 8.5 5 2.6 2-7 2.2 5-2 3-6 9.1 GRADUAL CHANGES FROM ONE REGION TO ANOTHER. Bodies. The height of the bodies increases as we descend the spine, very gradually in each region but rather rapidly at the junction of two regions, as shown in the table. The first two lumbars, like those above them, are rather deeper behind than in front, but the reverse is true of the last two, and especially of the fifth, in which the difference is considerable. The breadth of the bodies increases to the first or second thoracic, then dwindles to the fourth or fifth, and then again increases to the sacrum. The elevation at the sides of the upper surfaces of the bodies of the cervical vertebrae diminishes in the lower part of that region ; in the seventh it is limited to near the root of the pedicle. The same condition is found in the first thoracic vertebra and to a slight extent in the next two. The downward prolonga- tion of the front of the body of a cervical vertebra is slight in the lower part of the neck. The first thoracic has an entire facet for the head of the first rib near the top of the body and a part of one at the lower border for a portion of the head of the second. As a rule, in the thoracic region the head of each rib rests in a facet on two vertebrae and the intervening disk, the lower vertebra contributing more of the joint than the upper, and corresponding with the rib in name. Thus, the head of the fourth rib lies between the third and fourth thoracic vertebrae, and its tubercle rests on the transverse process of the fourth. Towards the lower part of the region the heads have a tendency to take a lower relative position on the column coinci- dently with the increase in size of the bodies. The head of the tenth rib usually rests wholly on the body of the tenth vertebra or on it and the disk above, conse- 1 Journal of Anatomy and Physiology, vol. xvii., 1883. Anderson states that the vertical diameters of the front and back of the cervical vertebrae are generally the same ; hence, prob- ably, he thought it needless to give the posterior measurements. The close correspondence of his anterior measurements with those of the author is very striking. REGIONAL CHANGES. 123 quently the ninth vertebra has no half-facet below. The tenth has a nearly or quite complete facet at its upper border, the eleventh has a complete one rather below the top of the body, and the twelfth has a complete facet nearly half-way down. At the ninth or tenth the facet begins to leave the body and to travel backward onto the root of the pedicle. When the body is seen from above or below in certain parts of the thoracic region the front curve is flattened on the left by the pressure of the aorta. This com- pression usually is first seen at the top of the fifth thoracic, and is traceable down- ward for a few vertebrae, sometimes as far as the lumbar region. The depression gradually passes from the side to the front as it descends the spine. The Transverse Processes. As shown by the table, the spread of the transverse processes increases greatly at the junction of the cervical and the thoracic regions, falls rapidly to the third thoracic, remains stationary to the tenth, falls to the last thoracic, the narrowest point, and then gains at once, reaching the maximum at the third lumbar. The anterior tubercles of the transverse processes of the cervical region increase to the sixth, which is the tubercle of Chassaignac, who taught that the carotid artery can be compressed against it, the force being directed backward and a little inward. The anterior limb of the transverse process of the seventh ver- tebra is very short, and its tubercle is usually rudimentary. It is distinctly in series with the slight elevation of the socket for the head of the first rib often seen on the first thoracic vertebra. The piece of bone between the tubercles, forming the floor of the gutter for the spinal nerve, is much longer and more anteriorly placed in the seventh than in those above it. It is this piece connecting the two tubercles that is the true costal element in the neck. The so-called anterior limb of the transverse process with the tubercle on it is in line, not with the ribs but with the anterior tubercle called the processus costarius. The articular facet on the transverse process of the first thoracic is shallow, often convex, and faces a little downward. That of the second, at which point the processes slant more backward, is concave and some- what overhung above ; this is seen in the two or three following, after which the facets grow smaller, more shallow, and look upward as well as forward. As the eleventh rib has but a rudimentary tubercle and the twelfth none at all, there is no facet on the transverse process of the last two thoracic vertebrae. The latter process of the eleventh is small, and that of the last broken up into three tubercles, ( i ) the superior or mammillary, rising from the posterior surface ; (2) the accessory or infe- rior, pointing downward ; (3) the external, a knob, the smallest of the three. The latter two represent the transverse process of the upper thoracic vertebrae. All three tubercles are usually to be recognized on the eleventh thoracic, although the acces- sory tubercle is usually not seen higher up. The knobs for muscular attachment on the backs of the thoracic transverse processes are evidently in line with the mammil- lary tubercles, rudiments of which are found in a large part of the thoracic region. In the lumbar region they are found on the side of the superior articular processes, growing smaller in the lower vertebrae, and being lost in the fifth. The lumbar transverse processes increase in length to the third, which is the longest, unless it be equalled by the fifth. That of the fourth is peculiar in being shorter and lighter than its neighbors. It usually has a rather triangular outline, owing to the lower border approaching the upper near the tip, and also arises farther forward, i.e., nearer the side of the pedicle than those above it. The fifth is much heavier and arises from the side of the body as well as from the pedicle, so that its ante- rior portion is evidently in series with the costal element developed in the sacrum, described in connection with that bone. The process which, in accordance with gen- eral usage, has been called the lumbar transverse process, is clearly in direct continua- tion with the line of the ribs. This is particularly striking in certain cases in which it is not easy to determine whether there is a thirteenth rib, or whether this process is to be considered as free in the first lumbar. The accessory tubercle, which can be inade out in the lumbar region, and is particularly large in the lower vertebrae, is in line with the ends of the transverse processes of the thorax. Thus the so-called lumbar transverse process represents at its root both a rib and the accessory and transverse tubercles, and beyond its root a rib only. This is especially marked in the broad process of the fifth lumbar, which springs from the side of the body I2 4 HUMAN ANATOMY. as well as from the pedicle. The homologies of the costal elements are shown in Fig. 158. The Spinous Processes. These are short and bifid in the third, fourth, and fifth cervical vertebrae ; longer and usually not forked in the sixth ; and longer, larger, and knobbed in the seventh. The type is that of the last mentioned in the upper thoracic, only the spine is a little longer, stronger, and more slanting. At about the fourth a sudden change occurs : the process becomes longer, sharper, and more descending. At about the tenth it shortens again, points more backward, and approaches the lumbar type, which is generally reached in the last thoracic. The spine of the last lumbar is usually much smaller than those above it. The Articular Processes. The change from the cervical type to the thoracic is gradual, but that from the thoracic to the lumbar occurs suddenly at the junction of those regions. The inferior processes of the last thoracic face outward. Not infre- quently the change occurs a space higher, but rarely one lower. Occasionally the facets between the regions face in an intermediate direction. Sometimes the change is normal on one side and not on the other. Superior articular process FIG. 152. Promontory Transverse process Lines of union between fused sacral vertebrae Iliacus Anterior sacral foramina Pyriformis Coccygeus Notch for fifth sacral nerve Apex The sacrum, anterior surface. THE SACRUM. This bone * is composed of five fused and modified vertebrae, of which the three upper support the pelvis laterally. The vertebras decrease very much in size from above downward, the lower being bent strongly forward. The first vertebra is com- paratively but little changed ; the last consists of little more than the body. The 1 Os sacrum. THE SACRUM. 125 essential modification, besides the fusion, is the occurrence of the lateral masses, 1 representing transverse processes and ribs, which, springing from the bodies and arches, are connected with the innominate bones by joints and ligaments. The sacrum has an upper surface, or base, a lower, or apex, and a front, back, and two lateral surfaces. The base has above a rough space representing the end of the body of a vertebra to which the last lumbar disk is attached. It is raised a little from the bone and forms an acute projecting angle with the front surface, known as the promontory of the sacrum, an important landmark in midwifery. Behind the body of the first sacral vertebra is the triangular orifice of the sacral canal, the Articular process FIG. 153. Lamina Sacral canal Transverse process of first sacral vertebra Spinous process Glutens maximus Sacral cornu Sacral canal The sacrum, posterior surface. transverse diameter of which is the greater. The articular process, springing from the side of the arch, is vertical, the concave facet facing backward and inward. The upper surface of the lateral mass, the a/a, springs from the side of the body and the pedicle, expanding into a broad area, and is bounded in front by an ill-marked, rounded border which separates it from the anterior surface and curves forward ; behind by a shorter border curving backward, on which the auricular process rests ; and outside by an irregular convex border. The latter may often be subdivided into two parts : an anterior, running pretty nearly forward and backward and cor- responding to the top of the auricular surface, and a posterior, running backward 1 Partes laterales. 126 HUMAN ANATOMY. Articular process Rudimentary transverse processes and inward. Thus the sacrum is broader before than behind. The apex is nothing but the under side of the body of the very small fifth sacral vertebra. The anterior surface is a triangular concavity formed by the bodies and lat- eral masses of the five sacral vertebrae. It has a double row of four openings, the anterior sacral foramina, one on each side of the ridges, representing the ossified disks connecting the bodies of the fused sacral vertebrae. The sacral nerves, like the other spinal nerves, divide into an anterior and a posterior division on leaving the spinal canal ; in the case of the sacral nerves, however, this takes place inside the bone, the anterior divisions escaping by these foramina. The bodies and the foramina grow smaller from above downward, and the latter are nearer together. A transverse depression across the body of the third vertebra usually marks a rather sudden change in the FIG. 154. curvature of the anterior Transverse process surface. The irregular outline of the lateral bor- ders may be divided into two parts : the upper, rather concave, ends be- low in a little point on a level with the third verte- bral body, and represents the extent of the articu- lar surface. Below this the border slants down- ward and inward until opposite the lower part of the fifth sacral seg- ment, when it suddenly turns inward, forming a notch over the anterior division of the fifth sacral nerve, which emerges be- tween it and the coccyx. The posterior sur- face is composed of the fused lamina and their modifications. The up- per borders of the first laminae slant downward, and below their junction is a well-marked spinel Below this the laminae of the sacral vertebrae are fused and the spines small. The laminae of the fifth sacral never join, and those of the fourth frequently do not, thus leaving the lower end of the canal uncovered. The laminae that do not meet end in tubercles each representing one-half of a spinous process. The lowest two project downward at the sides of the open canal, and are called the sacral cormia. Four posterior sacral foramina for the exit of the posterior divisions of the nerves appear on each side of the laminae. Outside of these are some irregu- lar tubercles representing the transverse processes, 2 and internal to the first three foramina are tubercles in line with the articular processes? The lateral surface begins just outside of the transverse tubercles. It is broad above, but below the third vertebra is merely a line. The upper part is divided into two portions : the front one is the auricular surface, from a slight resemblance to an ear, which joins, by fibro-cartilage, the corresponding surface on the ilium. It is broader above than below, convex in front, indented behind, with 1 Crista media. " Cristac lateralcs. 3 Cristae articulates. Auricular (articular) surface Fourth posterior sacral foramen Notch Sacral cornu The sacrum, lateral view. THE COCCYX. 127 slightly raised edges and a rough, irregular surface. The auricular surface is formed chiefly by the lateral mass of the first sacral (vertebra fulcr alls, as having the most to do in supporting the pelvis), to a less extent by that of the second, and very little by that of the third. Behind this articular portion lies the rough ligamentous sur- face, which slants backward and inward, and affords origin for the posterior sacro- iliac ligaments. Differences depending upon Sex. The female sacrum is relatively broader than the male. The sacral index, or the ratio of the breadth to the length ( I00 ^"^ dth ) > is 112 for the white male and 116 for the female. Such a rule is, however, not abso- lute, there being many doubtful cases, but a narrow sacrum is almost invariably male. Another, and very reliable, guide, especially in conjunction with the first, is the curve. There are contradictory statements among authors, but the truth is, as originally shown by Ward, that the male sacrum is the more regularly curved, while the anterior surface of the female bone runs in nearly a straight line from the prom- ontory to the middle of the third piece and then suddenly changes its direction. Variations. The sacrum often consists of six vertebrae. Such a one may be recognized even when the lower part is wanting, so that the vertebrae cannot be counted. If a line across the front, connecting the lowest points of the auricular surfaces, passes below the middle of the third sacral, the sacrum is of six pieces ; if above, of five. 1 Sacra consisting of only four vertebrae are rare. THE COCCYX. This bone is composed of four or five 2 flattened plates representing vertebral bodies. It is an elongated triangle with the apex below. The base, joined by fibro- cartilage to the apex of the sacrum, is oblique, the posterior border being higher than the front, so that the coccyx slants forward from the sacrum. The anterior surface of the coccyx is, moreover, very slightly concave. The_/?r^/ vertebra consists of a thin body, about twice as broad as long, from the back of which on each side the rudiment of an arch extends upward as a straight process, the coccygeal cornu, which FIG. 155. Surface for sacr Transverse process' Coccygeus Cornu Transverse process Gluteus maximus Sphincter ani Posterior The coccyx. overlaps the back of the body of the last sacral vertebra and joins the sacral cornu. A short lateral projection from the side of the body represents the transverse process; perhaps the costal element also. On the upper border of this process, at its origin, is a notch, which usually forms a foramen with the sacrum for the anterior division of the fifth sacral nerve. Very faint rudiments of these two pairs of processes are sometimes to be made out on the second vertebra, which is much smaller than the first, but also broad and flat. The succeeding ones are much smaller and ill-defined. Constrictions on the surfaces and notches on the edges mark the outlines of the 1 Bacarisse : Le sacrum suivant le sexe et suivant les races. Thse, Paris, 1873. 2 According to Steinbach, there are five in man and four or five in woman. Die Zahl der Caudalwirbel beim Menschen. Inaugural Dissertation, Berlin, 1899. 128 HUMAN ANATOMY. original pieces, which become less and less flat and more and more rounded. It is rare to see more than four distinct segments, but very often the last is somewhat elongated and shows signs of subdivision. It is not uncommon for the first piece to remain separate, neither fusing with the sacrum nor the next coccygeal plate. STRUCTURE OF THE VERTEBRAE. The shell of compact bone forming the surface is everywhere very thin. The general plan of the internal spongy bone is one of vertical plates which in a frontal section (Fig. 156, A) are bowed somewhat outward from the middle of the bone, and of transverse plates connecting them near together at the ends and farther apart in the FIG. 156. B Frontal (A) and sagittal (B) sections of body of lumbar vertebra, showing the arrangement of the bony lamellae. Natural size. middle third where larger spaces occur. The strongest plates spring from the pedi- cles and diverge through the bone, joining, probably, for the most part the hori- zontal system. In the sacrum the same general plan prevails, but in addition there are series of plates, mainly horizontal, in the lateral parts ; those from the first sacral are the most important. DEVELOPMENT OF THE VERTEBRAE. Presacral Vertebrae. These vertebrae ossify from three chief centres and at least five accessory ones. The median one of the three chief centres forms the greater part of the body ; while the other two, one appearing in each pedicle, form the postero-lateral part of the body, the arch, and the greater part of the processes. The oblique neuro-central sutures separate the regions of these centres. The lat- eral centres of the upper thoracic and the cervical vertebras appear first. It is usually taught that they appear in the sixth or seventh week of foetal life, but Bade * with the Rontgen rays found no sign of them at eight weeks. The point is unset- tled. The first median centres to appear are those of the lower thoracic and the upper lumbar vertebrae. In this region and below it the median centres precede the lateral ones ; in the upper part of the spine the growth is much more vigorous in the lateral centres. The median centres of the cervical vertebrae appear in order from below upward. The upper ones (judging from Rontgen-ray work and from transparent foetuses) sometimes have not appeared as late as the sixth month, although we have seen them towards the close of the third. At birth the upper and lower ends of the bodies are still cartilaginous, but the arches are well advanced in ossification, although bone does not cross the median line until some months later. The transverse processes of the thoracic vertebrae are farther advanced than those in other regions. The spines are still cartilaginous. The neuro-central suture is lost at from four to six years, disappearing first in the 1 Arch, fur Mikros. Anat., Bd. lv,, 1899. DEVELOPMENT OF THE VERTEBRA. 129 lumbar region. The tips of the spinous and transverse processes develop from cen- tres which appear about puberty and fuse about the eighteenth year. A thin epi- physeal disk, covering the upper and lower surfaces of each body, grows from a centre seen about the seventeenth year, and joins by the twentieth, the line of union per- sisting a year or two longer. The mammillary processes of the lumbar region arise from separate centres ; so do also the costal elements of the sixth and seventh cer- vicals, and sometimes that of the first lumbar. In cases in which this costal element of the seventh cervical remains free there is a cervical rib and no transverse fora- men ; exceptionally in these cases a foramen persists. According to Leboucq, 1 the development of the anterior limb of the transverse process of the cervical vertebrae is more complicated than is usually taught. There is a slight outward projection from the ventral side of the body rep- resenting the prominence for the head FIG. 157. of the rib to rest upon ; this grows out- ward and meets a growth from the transverse process that grows inward like a hook. This inward growth rep- resents what we commonly call the costal element of a cervical vertebra, but there may be also a separate ossi- fication representing an actual rib, namely, a small piece of bone on the ventral aspect of the tip of the trans- verse process of the seventh cervical vertebra. When a separate ossifica- tion occurs in this region in the fifth or sixth vertebra, it is situated still more externally than in the seventh, and forms the floor of the gutter be- tween the anterior and the posterior tubercles, which is the true costal ele- ment. It is probable that in certain cases of cervical ribs accompanied by a transverse foramen, the latter is en- closed by the hook-like process from the transverse process meeting the growth from the body of the vertebra, and that the rib coming from the separate ossification lies anteriorly to it and distinct from it. At birth the Ossification of the vertebrae. A, cervical vertebra at birth ; centres for body (a), neural arches (6), and costal ele- ment (c). B, dorsal vertebra at two years; cartilaginous tips of transverse (a) and spinous (b) processes ; d, centre for body. C, lumbar vertebra at two years ; position of ad- ditional later centres for various processes indicated (a, b, c) ; d, centre for body. lumbar articular processes resemble the thoracic. The type changes in early childhood. The Sacrum. Each sacral ver- tebra has the three primary centres of the others, the median ones appearing before the lateral of the same vertebra. Proba- bly the median centres of the first three appear first and then the lateral ones of the first vertebra ; data, however, are wanting for a definite statement. The time of the first appearance of ossification in the sacral vertebrae is very variable ; probably the earliest median centres appear about the beginning of the fourth month and the lateral ones some weeks later. In a skiagraph of a foetus estimated to be about three and a half months old the median centres of the upper three vertebrae and the lateral ones of the first are visible. This is, perhaps, earlier than the rule. Little progress in ossification of the last two sacrals takes place before birth. The lateral centres join the median, in the lower vertebrae, during the second year ; in the upper ones, three or four years later. In the upper three vertebrae a centre appears out- side the anterior sacral foramen, from which a part of the lateral mass is developed. j Me'moires couronne's, etc., Acad. Royale des Sciences de Belgique, tome lv., 1896. 9 130 HUMAN ANATOMY. This represents a costal element which fuses with the front of the pedicle. Those of the first two sacrals appear shortly before birth (Bade). The line of union can still be seen at seven years on the top of the first vertebra. The time at which the lamina FIG. 158. c. e (rib) F Costal element Illustrating homology of costal element (c. e.). A, sixth cervical vertebra; B, seventh cervical; C", fifth thoracic; D, second lumbar ; E, fifth lumbar ; f, sacrum in transverse section. meet in the middle is uncertain ; the arch of the first vertebra is sometimes complete at seven, those below it being still open. The five distinct sacral vertebrae which are thus formed remain separate for some time, the bodies being separated by interver- FIG. 159. FIG. 160. Superior and anterior surfaces of young sacrum of about five years. Sacrum and coccyx of about seventeen years. tebral disks. A thin plate appears in the upper and lower parts of these disks which fuses with the bodies before the latter unite. The union of the vertebrae begins below and proceeds upward in a very irregular manner. Probably union generally occurs first in the lateral masses, between the laminae VARIATIONS OF THE VERTEBRA. 131 sooner than between the bodies. By the fifteenth year the lower three vertebrae are generally fused, the second joining them from eighteen to nineteen. The five pieces are united by the twentieth year. In some cases several of the sutures are still to be seen, but all may have disappeared. The union of the bodies, as shown by sections, in the case of the upper ones, may not be complete internally till a much later period. Two thin epiphyses appear on each side of the sacrum about the eighteenth year, one for the auricular surface and the other below it. The lines of union of these plates may be visible after twenty-one. The Coccyx. Our data concerning the ossification of the coccyx are very un- satisfactory. Each segment has one centre, but the first may have two, one on each side, and, according to some, secondary centres for the cornua. Ossification begins in the first piece at about birth, and successively in the others, from above down- ward, until puberty. The lower three or four pieces fuse within two or three years after birth, and join the first at perhaps about twenty ; there is, however, great diversity, and frequently the first unites with the sacrum instead of with the others. The Atlas. The atlas is almost wholly formed from two centres which ^ IG- x z> appear in the seventh week of foetal life in the root of the posterior arches ; from these points ossification spreads most rapidly backward. In the course of the first year a centre is found in the middle of the anterior arch. The lateral masses meet behind in the fourth year and join the median anterior nucleus in the fifth. Sometimes the union of the posterior arches does not OCCUr. The anterior nu- Unique case of absence of the anterior arch of the atlas. cleus may be absent, and the front arch may show a median suture or be represented by ligament or cartilage. In one in- stance the anterior arch was wholly wanting, the lateral masses being fastened to the odontoid by ligament 1 (Fig. 161). The Axis. The ossification of the axis begins by two lateral points appearing by the eleventh week. The median one, which does not come till the fifth month, is at first double, but the two points speedily fuse. At about the same time two nuclei appear side by side in the odontoid process, which join together before birth, leaving a space between them at the tip. This may be closed by the extension of ossification, or a centre may appear in it at the second year, which fuses by the twelfth. The piece thus formed has been held to represent the epiphyseal plate for the top of the atlas. The odontoid process joins the body at the periphery, the union beginning in the third year and being complete a year or two later ; a piece of car- tilage in the middle of the juncture is said to persist under the odontoid until old age. Very rarely the odontoid remains distinct. The arches join the body in the third year, and usually meet behind at the same time ; the latter union, however, may be delayed. Variations of the Vertebrae. The commonest and most interesting variations are those of number. These are very frequent in the coccyx, since there are originally more elements than persist, and indeed we are not sure even of the normal number in this bone. Numerical varia- tions are also often observed in the sacral, less so in the lumbar, still less so in the thoracic, and extremely rarely in the cervical region. The number of vertebrae above the sacrum (twenty- four) is usually unchanged, but, owing to differences in development of the costal element, one region is not rarely increased or diminished at the expense of the next one. Thus the very com- mon condition of six lumbar vertebrae is due to the want of development of the costal element (the rib) of the last thoracic, and implies only eleven vertebrae in that region. Conversely, thir- teen thoracics imply an undue development of the costal element of the first lumbar, and con- sequently only four lumbar vertebrae. Often the costal element of the last cervical is free and over-developed, making a cervical rib. But even if this be large enough to reach the sternum, which is exceedingly rare, the number of cervical vertebrae is usually considered unchanged. Other changes are due to variations in development of the costal element in the last lumbar and the first sacral. Transitional forms are here very frequently met with. The last lumbar 1 Dwight : Journal of Anatomy and Physiology, vol. xxi., 1887. 132 HUMAN ANATOMY. may, by an excessive growth of these elements, become sacralized, articulating more or less per- fectly with the ilium, and, conversely, the first sacral may have almost freed itself from those below it. Thus we may find a partially sacralized vertebra, which may be either the twenty-fifth or the twenty-fourth. It often happens, particularly in the latter case, that a vertebra appears to be a first sacral on superficial examination, which is found to have little or nothing to do in form- ing the articular surface, in which case it is not a true sacral, for the first sacral is the fulcralis which has the largest surface for the joint with the ilium. A false promontory may coexist with the normal one. This is probably most frequent when the twenty-fourth vertebra is partly sacralized. Any of the preceding peculiarities may be unilateral, so that sometimes a vertebra may seem from one side to belong surely to one region, and equally surely to the other region when seen from the opposite side. There is, however, another set of variations in which the number of presacral vertebrae is increased or diminished. There may be, for instance, one thoracic or one lumbar vertebra too many or too tew, without any compensatory change in the next region. In these cases, more- over, the terminal vertebrae of the region may be very nearly typical ones, and sometimes even the size of the vertebrae will be modified so as to give the region its approximate relative length. Similar changes may be found in the neck, but they are exceedingly rare. Variations of either kind are likely to have an effect on the column as a whole ; thus, ii there be a large cervical rib the last thoracic rib is likely to be small, or if the first rib is rudi- mentary the last is apt to be large. It follows that the thorax seems to be in certain cases moved upward or downward ; this change may occur on one side only. Rosenberg's theory, formerly much in vogue, is that there are opposite tendencies at the two ends of the spine. At the upper there is a tendency for the cervical region to encroach on the thoracic, and at the lower for each of the regions to encroach on the one above it. Such changes he considers progressive. On the other hand, the opposite movement by which the thorax encroaches on the neck or loins is considered reversive. Rosenberg has described a spine which he considers archaic, in which there are two extra presacral vertebrae and fifteen pairs of ribs, the first being cervical. There are two spines in the Warren Museum with a simi- lar number of presacrals in which the last is sacralized on one side. As to the way in which anomalies of the lower part of the spine come about, Rosenberg x thinks he has shown that in the course of development the sacrum is composed of vertebrae placed farther back than the permanent ones, and that the ilium enters into connection with vertebrae more and more ante- rior. As new ones join it above former ones become detached from it below. If it does not make the usual progress the spine is archaic, having too many presacrals ; if it goes too far the spine is of the future. Rosenberg's theory has been overthrown by Bardeen, 2 who has shown that the original position of the ilium is opposite the superior part of the lumbar region and that it travels tailwards. Having joined a vertebra at the fifth week, it never leaves it. At this early time the thoracic vertebrae are differentiated. The author 3 and Fischel* believe that numerical variation is the result of an error in segmentation. A want of development of the bodies, which may be only half the normal height, is found almost exclusively in the lumbar region. We have seen (apparently congenital) fusion of the lumbar bodies while all the arches were present, but three of them crowded together. The separation of the pedicles of the fifth lumbar from the body is a very rare anomaly among whites, but not among American aborigines. ARTICULATIONS OF THE VERTEBRAL COLUMN. The ligaments connecting the segments of the spine may be divided, according to the parts of the vertebrae which they unite, into two groups : 1. Those connecting the Bodies of the Vertebras ; 2. Those connecting the Laminae and the Processes. LIGAMENTS CONNECTING THE BODIES. Intervertebral Disks 5 (Figs. 162, 163). These form a series of fibre-carti- lages interposed between the bodies of the vertebrae, forming about one-fourth of the movable part of the spine and adding greatly to its strength. They are developed, like the bodies, around the notochord, persisting parts of this structure forming a central core to each disk. The outer part of the disks consists of oblique layers of fibres, slanting alternately in opposite directions, some almost horizontal, which hold the vertebral bodies firmly together ; the centre of the disks is occupied by a space containing fluid in the meshes of a yellowish pulp. 6 This central core is strongly compressed, so as practically to be a resistant ball within the more yielding fibro- cartilaginous socket. The proportion of the disks to the vertebral bodies varies in the different parts of the spine. They are absolutely largest in the lumbar region, but relatively in the cervical. For many reasons it is difficult to reckon the per- x Morph. Jahrbuch, Bd. i. and xxvii. 4 Anatomische Hefte, No. 95, 1906. 2 Anat. Anzeiger, Bd. xxv., 1904, and American Journal of Anatomy, vol. iv., 1905. 3 Dwight : Memoirs Boston Society of Nat. Hist., vol. v., 1901. 5 Fibrocartilagines intcrvertebrales. 6 Nucleus pulposus. LIGAMENTS OF THE SPINE. 133 Posterior at- lanto-axial ligament centage very accurately, and there is much variation. The following proportions are, therefore, only approximate. The disks form in the cervical region forty per cent., in the thoracic, twenty per cent., and in the lumbar, thirty-three per cent, of the length of the spine. Anterior and Posterior FIG. 162. Common Ligaments. The Odontoid process of axis Transverse ligament bodies are connected by short fibres surrounding the disks, and by long bands which are only partially separable from the general envelope. The an- terior common ligament^ (Figs. 163, 165) begins at the axis and extends to the sacrum. It consists of shorter and longer fibres blending with the peri- osteum and springing from the edges of the vertebrae and from the disks, to end at similar points on the next vertebra, or on the second, third, fourth, or fifth. The borders are not sharply defined. The posterior common ligament' 1 ( Fig. 164) is a much more distinct struc- ture. It arises from the back of the body of the axis, re- ceiving fibres from the occipito- axial ligament, and runs to the sacrum. It also is attached to the disks and the edges of the bodies, but possesses a dis- tinct margin, which, except in the neck, expands laterally into a series of points at the intervertebral disks. It stands well out from the middle of the bodies, bridging over the veins of the larger ones. LIGAMENTS CONNECT- ING THE LAMINA AND THE PROCESSES. The articular processes (Fig. 165) are coated with hyaline articular cartilage and surrounded by loose capsules, with which, especially in the thorax, the ligamenta subflava are inseparably connected, pre- venting by their tension the occurrence of folds. The ligamenta subflava* (Fig. 163) are elastic membranes of considerable strength connecting the laminae from the axis to the sacrum. They are particularly developed in the lumbar region. As just mentioned, they encroach on the side of the capsules towards the canal. They also extend a short distance under the spinous processes. The supraspinous ligament ( Figs. 162, 163) extends as a well-marked cord 1 Lig. longitudinale anterius. - Lig. longltudinalc posterius. 3 Ligg. flava. Tenth tho- racic ver- tebra Median section of upper half of spine. 134 HUMAN ANATOMY. along the tips of the spines from the last cervical to the sacrum. The interspinous ligaments are membranes connecting the spinous processes between the tips and the laminae, extending from the ligamenta subflava to the supraspinous ligament. FIG. 163. Tenth thoracic vertebra Ligamentum subflavum Intervertebral foramen First lumbar vertebra Posterior common ligament Intervertebral disk Anterior common ligament. Fifth lumbar vertebra First sacral vertebra Supraspinous ligament Fifth lumbar spine Median section of lower half of spine. The ligamentum nuchae (Fig. 166) represents in the neck a modification of the two last-mentioned ligaments. It is a vertical curtain reaching from the exter- LIGAMENTS OF THE SPINE. 135 FIG. nal occipital protuberance to the spine of the muscles of the two sides. The free border is continuous with the supraspinous ligament, but, instead of touching the cervical spines, it lies in the superficial layer of muscles, and is rein- forced below by radiating fibres from each of the spinous processes of the cervical region. It is inseparably blended with the origin of the trapezii and with the fasciae between the muscular layers, especially with that covering the semispinalis and the short suboccipital muscles. In the region of the axis it is a thick median membrane ; in the lower cervical region it is . of little importance. In man it contains but a small proportion of elas- tic fibres, in marked contrast to what is found in many quadrupeds in which the structure con- sists principally of elastic tissue, since in these animals the ligamentum nuchae forms an important organ for the support of the head at the end of the horizontal vertebral axis. The intertransverse ligaments (Fig. 162) are trifling collections of fibres between the transverse processes, although occasionally distinct round cords region. FIG. 165. Anterior occipito-atlantal ligament Mastoid process Lateral occipito-atlantal ligament Anterior tubercle of a seventh cervical, separating the Posterior common ligament Posterior surface of bodies of vertebrae shown after removal of arches by cutting through the pedicles. in the thoracic Atlanto-axial ligament and joint Anterior common ligament Anterior ligaments of upper end of spine. ARTICULATIONS OF THE OCCIPITAL BONE, THE ATLAS, AND THE AXIS. The arrangement here differs in some points considerably from that of the rest of the spine in order to provide for the security and the free movement of the head. The ligaments effecting this union consist of three groups : i. Those connecting the Atlas and the Axis, including the Anterior Atlanto-Axial ; Transverse ; Posterior Atlanto-Axial ; Two Capsular. 136 HUMAN ANATOMY. 2. Those connecting the Occipital Bone and the Atlas, including the Anterior Occipito-Atlantal ; Posterior Occipito-Atlantal ; Accessory Occipito-Atlantal ; Two Capsular. 3. Those connecting the Occipital Bone and the Axis, including the Lateral Odontoid or Check ; Middle Odontoid ; Occipito- Axial. The important peculiarities are the odontoid and the transverse ligaments. The odontoid, or check ligaments 1 ( Fig. 168), are two strong, symmetrical bundles of fibres extending from the slanting surface on each side of the top of the odontoid process outward and a little upward to a roughness on the inner side of each occipital condyle. Some fibres pass directly across from one condyle to the FIG. 166. Ligamentum nuchae Trapezius muscle Ligamentum nuchae Posterior occipito-atlantai ligament Posterior atlanto-axial ligament Ligaments of back of neck. other. These are occasionally collected into a distinct round, glistening bundle. The space above the odontoid process, between it and the basilar process, is oc- cupied by a mass of dense fibrous tissue reaching to the anterior occipito-atloid ligament, in the midst of which is a more or less distinct median band connecting these parts, the middle odontoid ligament. 2 A supra-odontoid bursa may be developed in this tissue. 3 The transverse ligament 4 (Figs. 167, 168) of the atlas is a strong band passing between the tubercles on the inner side of each lateral mass of the atlas. It does not run straight, but curves backward around the odontoid, from which it is separated by a bursa. A band from the middle of the transverse ligament passes upward to the cerebral side of the basilar process, and another downward to the body of the axis, so that the whole structure is called the cruciform ligament. 5 3 Trolard : Journ. de 1'Anat. et de la Physiol., 1897. 1 Ligg. alaria. - Lig. apicis dentis. 4 Lig. transversum atlantis. 5 Lig cruciatum atlantis. OCCIPITO-SPINAL LIGAMENTS. 137 Another bursa lies between the odontoid and the anterior arch of the atlas. The transverse ligament and the two check ligaments are in series with the interarticular ligaments of the heads of the ribs. The other ligaments of this region are in the main simple membranes connect- FIG. 167. Upper end of occipito-axial ligament Lateral odontoid ligament Occipito-atlantal joint Cruciform ligament Atlas Atlanto-axial joint Axis Occipito-axial ligament, fused with dura, turned down Dura FIG. 168. Middle odontoid ligament Back of occiput and arches removed ; occipito-axial ligament cut and turned down. ing neighboring parts. The anterior occipito-atlantal ligament l ( Fig. 165 ) extends between the front of the foramen magnum and the anterior arch of the atlas ; the anterior atlanto-axial (Fig. 165) is in serial continuation with it. A distinct rounded, raised band, the accessory occipito-atlantal, passes in the median line from the under side of the occiput to the front tubercle of the atlas (Fig. 165), and thence to the body of the axis, where it joins the anterior common ligament of the spine. The occipito-axial ligament 2 {appa- ratus ligamentosus} (Fig. 167) descends in- side the spinal canal from the basilar process to the body of the axis, where it joins the posterior common ligament and completely conceals the odontoid process and its special ligaments. The posterior occipito-atlantal 3 and the posterior atlanto-axial ligaments 4 lie in the region of the arches (Fig. 166). The former extends between the posterior border removal ^"middle "of "transverse ligament; basilar of the foramen magnum and the arch of the process is thrown strongly upward. atlas ; the latter between the arch of the atlas and that of the axis. These are in series with the ligamenta subflava, but differ from them in being non-elastic. In the former of these membranes there is an opening just behind the facets on the atlas for the condyles, bridged over by a band, for the entrance of the vertebral artery. 1 Membrana atlantooccipitalis anterior. 2 Membrana tectoria. 3 Membrana atlantooccipitalis posterior. 4 Membrana atlantoepistrophica. Lateral odontoid ligament Bursa on back of odontoid =- Atlas Transverse liga- ment, cut Articular facet of axis Posterior surface of odontoid process shown by 138 HUMAN ANATOMY. Synovial joints, the shapes of which are described with the bones, exist be- tween the occipital bone and the atlas and between the atlas and the axis. The capsule of the upper joint is very thick, especially behind, where it is continuous with the posterior occipito-atloid ligament. The capsule surrounding the articular surfaces of the atlas and axis is strengthened posteriorly by a bundle running upward and outward from the axis. FIG. 169. osterior tubercle of atlas Spinal cord Posterior burs Transverse process of atlas Vertebral artery cut obliquely Dura Apparatus ligamentosus Vertebral artery cut in transverse foramen Anterior bursa Transverse ligament Anterior tubercle of atlas Section of odontoid process Transverse section of spine passing through atlas and odontoid process. THE SPINE AS A WHOLE. Anterior Aspect (Fig. 170). The bodies enlarge, in the main, regularly from above downward. This progression is interrupted only by a slight decrease from the first to the fourth thoracic. In the cervical region the origin of the costal elements from the sides of the bodies gives the latter a false appearance of breadth. The middle of the thoracic region is particularly prominent in front, owing in part to the aortic depression on the left. A slight curve to the right in this region is generally seen ; it is probably attributable to this cause. Posterior Aspect (Fig. 170). A deep gutter extends on each side of the spinous processes, bounded externally in the neck and loins by the articular pro- cesses and in the back by the transverse. In the latter region the spines which are subcutaneous are often deflected from the median line, and may be arranged in zig- zag. The laminae completely close the spinal canal in the convex thoracic and sacral regions, while it is left open in the neck and loins, except during extension of the former. Lateral Aspect (Fig. 171). The profile view shows best of all the increase in the importance of the bodies from above downward, and coincidently with this the gradual moving backward of the intervertebral foramina. These increase greatly in size from the lower part of the thoracic region. The Curves. The curve of the spine is necessarily an arbitrary one, since it varies not only in individuals and according to age, sex, and occupation, but also with position and the time of day, being longer when lying than standing, and after a night's rest than after a day's work. The difference occasioned by position occurs especially in youth, when it may amount to half an inch or more. It is of little consequence after middle age. Bearing these variations in mind, the following guide to the curve, suggested by Humphry, may be accepted : a line dropped from the middle of the odontoid process passes through the middle of the body of the second thoracic, that of the twelfth thoracic, and the anterior inferior angle of the fifth lumbar. Henle divides the spine into four quarters ; and although this method has the defect of using the unreliable pelvic section, it very often proves remarkably correct. Thus, if we continue Humphry's line to the level of the tip of the coccyx, the middle point is opposite the eleventh thoracic, the end of the first quarter oppo- site the lower border of the third thoracic, and that of the third quarter opposite the lower edge of the fourth lumbar. The development of the curves can hardly be said to have begun at birth. At THE SPINE AS A WHOLE. Anterior FIG. 170. I. Cervical I. Thoracic m Posterior 139 I. Lumbar Sacrum Coccyx Anterior and posterior views of adult spine. 140 HUMAN ANATOMY. FIG. 171. -I. Thoracic -I. Lumbar W- I. Sacral > I. Coccygeal Lateral view of adult spine. that age the infant's spine presents in front one general concavity, slightly interrupted by the promontory of the sacrum. The liga- mentous spine, containing little bone, is ex- ceedingly flexible in any direction : the atlas can be made to touch the sacrum. It is more accurate to say that the general axis of the spine is a curved one than that any per- manent or fixed curve exists. The cervical curve appears as the infant grows strong enough to hold up its head ; it is never, properly speaking, consolidated (Syming- ton), since it is always obliterated by a change in the position of the head. The lumbar curve appears at from one to two years when the child begins to walk. The mechanism of its production is explained as follows. When an infant lies on its back the thighs are flexed and fall apart. If these be held together and pressed forcibly down, the lumbar region will spring upward, owing to the shortness of the ilio-femoral ligaments, which bend the pelvis and, indirectly, the spine. The psoas muscles, moreover, act directly on the spine. When the child first stands, the body is inclined forward ; when the muscles of the back straighten it, the lumbar curve is produced by the same mech- anism, since it is immaterial whether the legs are extended on the trunk or the trunk on the legs. How or when these curves be- come consolidated is very difficult to deter- mine. The influence of differences in thick- ness of the front and back of the various bodies and disks is inappreciable in the neck ; in the lower part of the back and in the first, and perhaps the second, lumbar vertebrae the height is greater behind. In the loins the fifth vertebra is much thicker in front and, above it, the fourth and third in a less degree. The intervertebral disks are also much thicker in front. How soon actual difference in the diameters of the vertebrae appears is un- certain. A child of about three shows little of it, except in the last lumbar, and, accord- ing to Symington's plates, there is not much more difference at five or even thirteen years. It is certain that throughout the period of growth the curves can be nearly or quite effaced. The restraining influences are the gradually developing differences in the verte- brae and the disks, the effect of the sternum and the ribs on the thoracic region, the pull of the elastic ligaments of the arches, and perhaps, above all, muscular tonicity. In the latter part of middle age the curves of the back and loins become consolidated ; this is, however, distinctly a degenerative process. LENGTH OF PRESACRAL REGIONS. 141 Dimensions and Proportions. The length and the proportions of the dif- ferent presacral regions (including the intervertebral disks), measured along the an- terior surface of the spine, have, in fifty males and twenty-three female bodies, been found by us as stated below. We give for comparison Ravenel' s 1 and Aeby's 2 propor- tions combined. The former measured eleven and the latter eight spines of each sex. Cunningham's 3 proportions, from six male and five female spines, are also added. In the proportions, one hundred represents the total presacral length along the curves. ACTUAL LENGTH OF PRESACRAL REGIONS OF SPINE. Male. Centimetres. (Inches.) Neck 13.3 ( 5.25) Back 28.7 (11.31) Loins 19.9 ( 7.82) 61.9 (24-38) Female. Centimetres. (Inches.) I2-I ( 4-75) 26.5 (10.44) I8. 7 ( 7.38) 57-3 (22.57) PROPORTIONS OF PRESACRAL REGIONS OF SPINE. Neck Back Loins (Dwight.) 21-5 46.3 32.2 Male. (R. & A.) 21.7 46.7 31-4 Female. (Cunningham.) (Dwight.) (R. & A.) (Cunningham.) 21.8 21.2 21-7 21.6 46.5 46-1 46-5 45-8 31-7 32.7 32-4 . 32.8 IOO.O IOO.O IOO.6 IOO.2 Thus, while it is true that the lumbar region is relatively longer in woman, the difference is trifling. ABSOLUTE AND RELATIVE LENGTH OF PRESACRAL REGIONS DURING GROWTH. AGE. OBSERVER. ABSOLUTE LENGTH. (In Millimetres.) RELATIVE LENGTH. (Total = loo.) Neck. Back. Loins. Total. Neck. Back. Loins. At birth Ravenel. Ravenel. Ravenel. Chipault. 4 Chipault. Ravenel. Aeby. Aeby. Dwight. Chipault. Chipault. Chipault. Chipault. Chipault. Ravenel. Aeby. Dwight. Aeby. Chipault. Symington. 5 Ravenel. Symington. Ravenel. Aeby. Symington. Dwight. Aeby. Aeby. Dwight. 50 40 40 40 42 50 52.5 53-5 61 60 69 67 62 68 70 79-5 78 79-9 81 80 80 80 85 9i 95 1 20 IOO 107.5 "3 93 IOO 95 80 80 IOO 103 107 125 121 129 118 130 132 140 153-5 162 162 174 170 180 175 195 218.7 220 265 221.8 229.5 250 50 50 50 45 44 58 60 61 77 72 83 79 69 79 90 98 101 103-3 102.8 104 135 106 150 153-5 136 183 151 152-5 161 193 190 185 165 1 66 208 215-5 221.5 263 253 281 264 261 279 300 33i 34i 345-2 357-8 354 395 36i 430 463-2 45i 568 472.8 489-5 524 25-9 21 21.6 24.2 25-3 24 24-3 24.1 23.2 23-7 24-5 25.2 23-7 24-3 23-3 24 22-9 23.1 22.6 22.5 20.3 22.2 19.8 19.7 21-5 21. 1 21. 1 21.9 21-5 48.2 52.6 51-3 48.4 48.1 48.1 47-5 48.6 47-5 47-8 45-9 44-8 49-7 47-4 46.7 46.4 47-5 46.9 48.9 48 45-6 48-5 45-4 47-2 48.7 46.6 46.9 46-9 47-7 25-9 26.3 27 27.4 26.6 27-9 2 7 .8 27-5 29.2 28.5 29.6 30 26.6 28.3 30 29.6 29.6 29-9 28.5 29-4 34-2 29-3 34-9 33-1 29.1 32-2 31-9 3i-i 30-7 At birth At birth One month One month . Three months Six months Six months Ten months . One year, boy One year boy . . One year and one month, boy . One and a half years, girl . . . One and a half years, boy . . Two years boy .... Two years boy .... Three years girl .... Four years girl .... Four and a half years, boy . . Five years boy . . . . Five years boy Six years boy . . Nine years girl . . Eleven years boy . ... Thirteen years girl . . Fifteen years boy Sixteen years girl Sixteen years girl Seventeen years girl 1 Zeitschrift fur Anat. und Entwicklng., 1876. 3 Cunningham : Memoirs, 1886. 6 The Anatomy of the Child. 1 Arch, fur Anat. und Entwicklng., 1879. 4 Revue d' Orthopedic, 1895. 142 HUMAN ANATOMY. It appears from the above that in the adult the neck is a little more than one- fifth of the movable part of the spine and the loins a little less than one-third. In the young embryo these proportions are reversed, but by the time of birth these two parts are nearly equal. Movements of the Head. Those between the occiput and atlas are almost wholly limited to flexion and extension, of which the latter is much the greater. This is in part due to the reception of the posterior pointed extremities of the articu- lar processes of the atlas into the inner parts of the posterior condyloid fossae. The anterior occipito-atlantal ligament and the odontoid ligaments are tense in extreme extension, Inflexion the tip of the odontoid is very close to, if it does not touch, the basilar process. The range of both these motions is much increased by the participation of the cervical region. There may be a little lateral motion between the atlas and head, and there is some slight rotation. The great variation of the shape of the articular facets makes it clear that both the nature and extent of the motions must vary considerably. The joint between the atlas and axis is devoted almost wholly to rotation. The transverse ligament keeps the odontoid in place, and the very strong odontoid liga- ments check rotation alternately. The head is highest when directed straight forward, but the joints are in more perfect adaptation if one condyle be a little anterior to the other, and if the atlas be slightly rotated on the axis. This position, though entail- ing a slight loss of height, is the one naturally chosen as that of greatest stability. Movements of the Spine. The very extensive range of motion of the whole spine is the sum of many small movements occurring at the intervertebral disks. The whole column is a flexible rod, but this conception is modified by the following peculiarities : ( i ) the motion is not equally distributed, owing to the vary- ing distances between the disks and the differences of thickness of the disks them- selves ; (2) the bodies, which form the essential part of the rod, are not circular, so that motion is easier in one direction than in another ; (3) the rod is not straight but curved ; (4) the kind of motion is influenced by the articular processes, and varies in the different regions. Other modifying circumstances exist, but these suf- fice to show that, while certain general principles may be laid down, an accurate analysis of the spinal movements is absolutely impossible. The incompressible semifluid centre of each disk has been compared to a ball on which the rest of the disk plays. This would, therefore, be a universal joint were there no restraining apparatus. The motions are flexion and extension, i.e. , angular movements on a transverse axis ; lateral motion, i.e. , the same on an antero-posterior axis, and rotation on a vertical axis. It is unlikely that any single one of these motions ever occurs without some mingling of another. Flexion and extension are greatest in the neck and loins. Extension is more free than flexion in the neck, where it is limited by the locking of the laminae, which, when the head is thrown as far back as possible, gives great rigidity to the neck. In the loins and in the region of the last two thoracic vertebrae flexion is the more exten- sive. Before the spine is consolidated, slight flexion is possible throughout the back, but extension is very quickly checked by the locking of the laminae and spines. Lateral motion is greatest in the neck, considerable in the back and least in the loins. Such motion is always associated with rotation, which is most free in the neck, considerable in the back, and very slight, at most, in the loins. It is to be remembered that motions both in the antero-posterior and in the transverse plane are checked by the tension of the ligaments on the side of the body of the vertebra opposite to the direction of the motion, and also by the resistance to compression of that side of the intervertebral disk towards which the motion occurs. The liga- menta subflava, being elastic, tend continually to bring the bones back into position from the innumerable slight displacements to which they are subject. That this replacement is effected by a purely physical property of the tissue instead of by muscular action implies a great saving of energy. The amount of all motions, and of rotation in particular, decreases throughout life and varies much in individuals. According to Keen, the rotary motion between the atlas and the axis amounts to twenty-five degrees, that in the rest of the neck to forty-five degrees, and that of the thoracic and lumbar regions to about thirty degrees on each side. PRACTICAL CONSIDERATIONS: THE SPINE. 143 PRACTICAL CONSIDERATIONS. While the number of vertebrae in the neck is almost invariable in man (and indeed in all the mammalia except the sloth and the sea-cow), the length of the cervical region varies greatly in individuals. As it is apparently shortened during full inspiration and lengthened during full expiration, so an actual change in its length is associated with the types of thorax that correspond to these conditions. The long neck is therefore found in persons with chests that are flat above the mammae, with wide upper intercostal spaces and narrow lower ones, and with lack of prominence of the sternum. These conditions are often associated with phthisical tendencies. The short neck is found in persons with chests of the reverse type. Its theoretical association with apoplectic tendencies is very doubtful. The remaining variations both in the length and in the shape of the vertebral column are closely connected with corresponding variations in its curves. The normal curves of the spine are four : the cervical, thoracic, lumbar, and pelvic (or sacro-coccygeal). The cervical and lumbar are concave backward, the thoracic and pelvic convex backward (Fig. 171). These curves are produced and kept up partly by the twenty-three intervertebral disks. They are altered by disease. An additional curve not uncommon in absolutely healthy persons consists in a slight deflection of the thoracic spine to the right ; this asymmetry is usually ascribed to the greater use of the right arm, but it is due to the position of the heart and the aorta. All the vertebral bodies are composed of cancellous tissue, which is more spongy in direct proportion to the size of the vertebrae, and therefore is least so in the neck and most spongy in the lumbar region. This corresponds with the greater succu- lence and elasticity of the lower intervertebral disks and aids in minimizing the effect of jars and shocks such as are received in alighting from a height upon the feet, the lower portion of the column of course receiving the greater weight. If in such falls the calcaneum or tibia is broken, the spine usually escapes injury. If the lower extremity remains intact, the safety of the spine depends largely upon the elasticity given by its curves and by the disks. The fact that the bodies have to bear the chief strain of such shocks and of extreme flexion and extension, the most usual forms of spinal injury, serves, together with their comparative vascularity, to make them the seat of tuberculous infection when it invades the spine. Their spongy texture, once they are softened by inflam- mation, leads to their ready disintegration under the superincumbent weight. In the neck and in the loins the process may at first merely cause a straightening of the column, the normal curves being concave backward. In the thoracic region the most common situation it soon produces kyphosis, an exaggerated backward curve, the sharp projection of the spinous processes of the affected vertebrae causing it to be known as ' ' angular curvature. ' ' The abscesses which result from caries of the vertebrae are governed as to their position and course by the fasciae and muscles that surround them. They will, therefore, be described later (page 643). The suspension of the whole body from the chin and occiput separates the indi- vidual vertebrae so that they are held together mainly by their ligaments. This obviously relieves or removes the pressure of the superincumbent weight on the bodies of diseased vertebrae. The relief of pressure in cases of thoracic caries is continued by the use of appliances which transfer the weight of the head and shoulders to the pelvis. The simplest of these is the plaster jacket. For cervical caries, the weight of the head is transferred to the trunk beneath the level of disease by means of an apparatus extending from above the head to a band (of leather or plaster) encircling the chest. In cases of kyphosis corrected by the method of "forcible straightening" it is obvious that a gap proportionate to the amount of bone which has previously been destroyed must be left between the bodies of the diseased vertebrae. The ultimate integrity of the spinal column will depend upon the extent and character of the ankylosis which takes place between the separated vertebrae. It is asserted (Calot) that such consolidation does occur between the bodies in moderately severe cases, and between the laminae, transverse processes, and spines in the more serious 144 HUMAN ANATOMY. ones. It has been shown (Wullstein) that injury to the dura and cord and even fracture of the arches and processes are possible concomitants of forcible rectifica- tion of kyphosis. If the curve forward of the lumbar spine is exaggerated, constituting lordosis, it is usually compensatory, and is acquired in an effort to maintain the erect position, as in cases of high caries, great obesity, pregnancy, ascites, abdominal tumors, etc. Scoliosis or lateral curvature commonly results from faulty positions in young, undeveloped persons with weak muscles, as school-girls, who sit or stand in such atti- tudes that the muscles are relieved and the strain is borne by insensitive structures, like ligaments and fasciae. This results in a deflection of one part of the column generally the thoracic to one side, usually the right, and the formation of a compen- satory curve below, and occasionally of one above also. The bodies of the affected vertebrae are at the same time rotated, partly by the action of the slips of the longis- simus dorsi which are attached to the ribs near the angles and to the tips of the trans- verse processes (Fig. 520), so that in advanced cases the tips of the spinous pro- cesses of the affected segments turn towards the concavity of the curves, while the transverse processes of the vertebrae involved tend to lie in an antero-posterior plane and can often be felt projecting backward. A further explanation of the causes of the rotation may be found in the behavior of a straight flexible rod under similar conditions. Torsion results from any motion in which all particles of a straight flexible rod do not move in parallel columns. Therefore, if it be bent in two planes at the same time torsion must inevitably occur. The vertebral column being bent in the antero-posterior plane by a series of gentle curves, lateral bending must, therefore, inevitably lead to torsion, since it means bending in two planes. A little consideration of the relations of the spine to the ribs, scapula, and pelvis will show that lateral flexion and rotation cannot take place without causirig (a) sep- aration of the ribs on the convex side; (b*) change in the costal angles, making the ribs more horizontal on the convex and more oblique on the opposite side ; (V) undue prominence of their angles on the convex side, the scapula being carried upon them so that it also is more prominent ; (a?) diminution of the ilio-costal space on the concave side ; (^) elevation of the shoulder on the convex side ; (/") flatten- ing of the chest in front on the convex and undue prominence of the chest on the opposite side ; (g) projection of the ilium on the concave side. Lateral curvature with these secondary deformities may also be produced by unequal length of the lower limbs, one-sided muscular atrophy, hypertrophy, or spasm, sacro-iliac disease, empyema, and asymmetry of either the pelvis or the head. The latter factor is especially interesting from an anatomical stand-point. From what has been said (page 142) of the position of greatest- stability of the joints be- tween the head and the atlas and the latter and the axis, it is evident that the position of greatest ease is with the head slightly turned to one side, the condyles of the occiput not being in their best contact with the superior articular surfaces of the atlas when the head is held straight, but rather when the head is slightly twisted (Dwight). The effects of this are far-reaching. First, there is an instinctive effort to get the eyes on the same plane in looking forward, which is presumably the primary cause of the asymmetry of the face that is usually found. It is also easier to support the weight in standing chiefly on one leg, hence the other side of the pelvis is allowed to fall so that the lumbar region slants away from the supporting leg. This must be corrected by a lateral motion of the spine above it, and as this is not pure but mixed with rotation, there occurs a twist in the spine ; one shoulder is higher than the other as well as farther forward. In healthy persons such positions, if not maintained too long, do little harm ; but there is likely to be some spinal asymmetry in all, and there is the danger that it may become pronounced and fixed in the weak. Sprains of the spine are most common in the cervical and lumbar regions : in the former because of the greater mobility of the articulation with the cranium, and in both because of their own mobility, the greatest degree of bending in an antero- posterior direction being possible in those two segments of the spine. The thoracic and pelvic curves are primary, form part of the walls of the thorax and pelvis, PRACTICAL CONSIDERATIONS: THE SPINE. 145 appear early, and are chiefly due to the shape of the vertebral bodies. The cervical and lumbar curves are secondary, develop after birth, and depend mainly on the shape of the disks. Greater mobility would naturally be expected under the latter circumstances. The close articulation between the separate vertebrae throughout the whole column, while it renders a slight degree of sprain not uncommon, tends at the same time to diffuse forces applied to the spine and to concentrate them within certain areas. These areas are the points at which fixed and movable portions of the spine join each other, as in the neighborhood of the atlanto-axial, the cervico- thoracic, and the thoracico-lumbar regions. If the force is sufficient to cause an injury of greater severity than a sprain it is apt to be a dislocation or a fracture with dislocation at one or other of these localities. The latter accident is usually caused by extreme flexion of the spine, and of the three points mentioned is most often found in the segment including the lower two thoracic and the upper one or two lumbar vertebrae. This is due to the fact that ( i ) this segment has to bear almost as much weight as the lumbar spine, and yet its vertebrae are smaller and weaker. (2) The transverse processes are short, while the longer ones below, together with the crest of the ilium and the ribs above, give a powerful leverage to the muscles that move the region in question. (3) It is the region at which the most concave part of the thoracico-lumbar curve is found, making the ' ' hollow of the back' ' and corresponding to the ' ' waist' ' where the circumference of the trunk is smallest. (4) Its nearness to the middle of the column enables a greater length of leverage to be brought to bear against it than against any other part. (5) The different segments of the spine above it are com- paratively fixed (Humphry). These anatomical facts account for the frequency and severity of the injury known as " fracture-dislocation" in this region as a result of extreme flexion. A view of the vertebral column from behind (Fig. 170) serves well to illustrate some of these points. Pure dislocations are rare, but are more frequent in the upper than in the lower part of the spine, because the bodies of the cervical vertebrae are small, and the interlocking of the articular processes is less firm than it is lower in the column. The vertebra most commonly dislocated is the fifth cervical, which might be expected from the fact that in the neck flexion and extension are freest between the third and sixth vertebrae. The dislocation is usually anterior, that is, the articular process of one vertebra slips forward and falls down on the pedicle of the vertebra below, resting in the intervertebral notch, this accident being rendered easy by the com- paratively horizontal position of the articular processes in the cervical region. Such dislocation is practically impossible in the thoracic or lumbar region without fracture, while fracture is comparatively rare in the cervical region. The lumen of the spinal canal may be but little, if at all, invaded. As to reduction, experiments show (Walton) that no moderate amount of exten- sion in a direct line would raise the displaced articular processes in the least degree. It was, however, found easy to unlock these processes by retro-lateral flexion, bend- ing the head towards the side to which the face was already turned, an inappreciable amount of force being necessary. Rotation into place completed the reduction. All pure dislocations are really subluxations, as without extensive fracture of the processes and great laceration of ligaments a complete separation of the articu- lar surfaces of two adjoining vertebrae is practically impossible. Pure fracture, not the result of a gunshot wound, is rare. If from flexion, the fracture involves the body ; if from direct violence, usually the laminae. These facts require no explanation. Dislocations and fractures of the upper two cervical vertebrae are especially serious on account of the proximity of the medulla and of their position above the roots of the phrenic nerve and of the nerves supplying the external muscles of respiration. If the accident is from overflexion, it may be a dislocation between the occiput and the atlas, as it is there that the movements of flexion and extension of the head take place. If it arises from extreme rotation, and especially if there is rupture of the check ligaments, it may be a dislocation of the atlas from the axis, as it is there that the rotary movements of the head occur. " A dumb person expresses 'yes' at the 146 HUMAN ANATOMY. occipito-atloid joint and 'no' at the atlo-axoid" (Owen). Painless nodding and rotation of the head aid, therefore, in the exclusion of the occipito-atlantal and atlanto-axial regions in obscure cases of high caries. The axis is more spongy than the atlas, and is weakest about one centimetre below the neck of the odontoid process, and this is one of the most frequent seats of fracture. In fracture-dislocations, which constitute from seventy to eighty per cent, of se- vere spinal injuries, the thoracico-lumbar region suffers most commonly for the reasons above stated. The almost vertical direction of the articular processes of the thoracic vertebrae causes them, when flexion is extreme, as when a weight has fallen on the back, to be frequently fractured, which, together with the accompanying crushing of the vertebral body and rupture of the supra- and interspinous ligaments and the ligamenta subflava, permits the immediate sliding forward of the vertebrae above the crushed one and the compression of the cord often its practical severance between the anterior edge of the posterior arch of the upper vertebra and the poste- rior edge of the body of the lower one. (For the resulting symptoms, see section on Nervous System, page 1053.) It may be mentioned here that the spinal nerves do not arise from the cord opposite the vertebrae after which they are named. Their regions of origin may briefly be stated as follows : 1 i ) Occiput to sixth cervical spine, eight cervical nerves. (2) Seventh cervical to fourth thoracic spine, upper six thoracic nerves. (3) Fifth to tenth thoracic spine, lower six thoracic nerves. (4) Eleventh and twelfth thoracic spines, five lumbar nerves. (5) First lumbar spine, five sacral nerves. Landmarks. To fix the limits of the spine in the living, draw a horizontal line from the anterior nasal spine to the lower edge of the external occipital pro- tuberance and another backward from the top of the symphysis pubis. Seen from the side, the top of the spine is in a line connecting the front of the lobe of each ear, passing behind the neck of the lower jaw. Frozen sections show that the front of the vertebral bodies is much nearer the centre of the body than one is prepared to expect. A vertical transverse, or frontal, plane through the thorax at its greatest breadth strikes the angle of the jaw, the front of the cervical convexity of the spine, and cuts the body of the fourth lumbar (Langer). The relations of the spine anteriorly are considered with the parts in front of it. The parts felt from the surface are the spinous processes and some few of the trans- verse ones. The line of the spines is a good example of the general rule that prominences on the skeleton lie in hollows in the flesh ; a deep furrow between the muscular masses marks their position. Palpation of the normal spine with the soft parts in place gives the following information. The spine of the second cervical can be felt by deep pressure a little below the occiput. The short spines of the succeeding vertebrae are made out with great difficulty. The fifth is longer than those just above it. The sixth is much longer and nearly as* long as that of the seventh. The name vertebra prominens conferred on the seventh is misleading, for the spine of the first thoracic is the most prominent in this region. The third, fourth, and fifth cervical spines recede from the surface by reason of the forward curve of the cervical segment and on account of their shortness. This permits of free extension of the head and neck. The liga- mentum nuchse also prevents them from being felt distinctly. The sixth and seventh cervical and first thoracic are easily felt. The remainder, lying in the groove caused by the prominence of the erector spinae muscles, can usually be palpated without much difficulty. The relative sizes vary so much that it is not safe to identify any spine in this way. If the whole series from the second cannot be counted, it is best to start from the fourth lumbar, which is on a level with the highest points of the ilia. Vertebrae can also be identified from the lower ribs by the relations of the heads to the bodies. The relations of the spinous processes to the body vary. Thus, in the cervical region the first five spines pass nearly straight backward. The sixth and seventh, like the upper two or three thoracic spines, descend a little, so that the tip is opposite LANDMARKS OF THE SPINE. FIG. 172. '47 Pons Anterior boundary of foramen magnum Superior laryngeal opening Cricoid cartilage Thyroid gland Upper border of manubrium Left innominate vein Innominate artery Ascending portion of aortic arch Upper border of body of sternum Section of right lung Right auricular appendage Right auricle Lower border of body of sternum Diaphragm Lower end of ensiform cartilage Liver Stomach Pancreas Duodenum Transverse colon Sigmoid flexure Bladder Symphysis pubis Medulla Posterior boundary of foramen magnum Odontoid process of axis (Esophagus Division of trachea Right pulmonary artery Left auricle Aorta End of abdominal aorta Left common iliac vein Sacrum Rectum Coccyx Seminal vesicles Prostate Median section of the body of a man aged twenty-one years. (After Braune.) i 4 8 HUMAN ANATOMY. to the body next below it. With the fourth or fifth thoracic they point much more strongly downward, so as to be opposite the disk below the succeeding body. This continues to the tenth, where they are opposite the body below. In the loins the spines have a considerable posterior surface, which is opposite the disk and the upper part of the body below it. The tips of the spines are not always in a straight line, but sometimes describe a zigzag. The transverse process of the atlas can be felt below the tip of the mastoid process, moving with the head when the latter is turned. The transverse processes below this are felt with great difficulty through the muscles of the side of the neck. Those of the back and loins are too thickly covered to be felt. The laminae are also thickly covered with muscles, so that the operation of laminectomy necessarily involves a deep wound, and in the thoracic region this difficulty is increased by the backward projection of the ribs. As landmarks the spines of the vertebrae, on account of their accessibility, have great value. These spines have the following relations. The fourth cervical spine corresponds to (i) the opening of the larynx ; (2) the bifurcation of the carotid artery, and hence the point of origin of both the external and internal carotid arteries. The sixth cervical indicates the level of the carotid tubercle (transverse process of the sixth vertebra) and the entrance of the vertebral artery into the bony canal. The seventh cervical spine is a guide to ( i ) the lower border of the cricoid cartilage ; the lower opening of the larynx and the beginning of the trachea ; (2) the lower end of the pharynx and the upper opening of the oesophagus ; (3) the crossing of the omo-hyoid over the common carotid ; (4) the level of the apex of the lung and to the summit- of the arch of the subclavian artery. The fourth thoracic spine corresponds to the level at which the aorta reaches the spinal column, the trachea bifurcates, and posteriorly the apex of the lower lobe of the lung is found. It is on the same level as the root of the spine of the scapula. The seventh thoracic lies on a level with the inferior angle of the scapula. The eighth thoracic indicates the lower level of the heart and that of the central tendon of the diaphragm and the level at which the inferior vena cava passes through the diaphragm. The ninth tho- racic marks the level at which the upper edge of the spleen is found in health, and at which also the oesophagus pierces the diaphragm. The tenth thoracic corresponds to the lower edge of the lung, the spot at which the liver comes to the surface poste- riorly. The spines of the third to the ninth thoracic correspond to the heads of the fourth to the tenth ribs respectively. The eleventh thoracic is a guide to the normal situation of the lower border of the spleen and to the upper part of the kidney. The twelfth thoracic marks the lower limit of the pleura, the passage of the aorta through the diaphragm, and the situation of the pyloric end of the stomach, and is on a level with the head of the last rib. The first lumbar spine is on the line of the renal arteries and the pelvis of the kidney. The second lumbar spine corre- sponds to (i) the termination of the duodenum and the commencement of the jejunum ; (2) the opening of the ductus communis choledochus into the intestine ; (3) the lower border of the kidney ; (4) the lower border of the pancreas ; (5) the upper end of the root of the mesentery ; (6) the point of origin of the superior mesenteric artery ; (7) the commencement of the thoracic duct ; (8) the commence- ment of the vena porta ; (9) the termination of the spinal cord and the origin of the cauda equina ; (10) the upper end of the receptaculum chyli. The third lumbar corresponds to the level of the umbilicus and the origin of the inferior mesenteric artery ; the fourth lumbar spine marks the point of bifurcation of the abdominal aorta into the two common iliac arteries, and lies on a level with the highest part of the ilium ; and, finally, the fifth lumbar spine is a little below the beginning of the inferior vena cava. Direct cocainization of the spinal cord has recently been employed in surgery in operations on the lower abdomen, pelvis, and lower extremities. The injection into the subarachnoid space surrounding the cord is made through the space between the fourth and fifth lumbar vertebrae. To find this space, draw a line connecting the highest points of the crest of the ilium posteriorly. This will pass through the spine of the fourth lumbar vertebra. The point for injection is one centimetre below and one centimetre to the outer side of the point at which the transverse line crosses the vertebral spine in the median line. THE THORAX. THE thorax is that part of the body-cavity separated by the diaphragm from the abdomen below, but without complete separation from the neck above. Its bony walls are formed behind by the thoracic vertebrae, at the sides by the ribs, and in front by their continuations, the costal cartilages, and the sternum. FIG. 173. The bony thorax, anterior view. THE RIBS. The ribs, arranged as twelve pairs, are flat bars of bone, curved and twisted, which are attached behind to the spine and continued in front by the costal cartilages ; they form the greater part of the bony walls of the thorax. The first seven pairs, 149 150 HUMAN ANATOMY. exceptionally eight, reach the sternum through their cartilages ; hence they are called sternal ribs, 1 as distinguished from the remaining five pairs of asternal ribs. 2 Each cartilage of the next three joins that of the rib above it. The last two pairs have the cartilages ending free, and are termed floating ribs. Their complicated curves are best understood by studying them in place. Each rib (with certain exceptions to be detailed later) has an articular surface, the head, at the posterior end ; followed by a narrower neck, succeeded by an articular facet on the tubercle which rests on the transverse process of the vertebra. The first rib has an upper and a lower surface, an outer and an inner border ; the second faces in a direction intermediate to this and the following, which have an outer and an inner surface, an upper and a lower border. They are placed obliquely, the front end being lower than the hind one. The outline of the ribs is irregular, so that their declination is not due wholly to their position, but in part also to their shape. Thus, one in the middle of the series slants a little downward as far as the tubercle, then declines more sharply to a roughness near the tubercle known as the angle, and thence more gradually to the end. The main curve of such a rib is backward, outward, and downward as far as the angle, which marks a rather sudden change of direction, the course changing to one forward, slightly outward, and downward, until, as it reaches the front of the chest, it runs forward, downward, and inward. The external surface is vertical at the back and side and slants slightly upward in front. Bearing the declination of the rib in mind, it is evident that to accomplish this the rib must be twisted on itself, otherwise the upper edge would project in front. FIG. 174. Articular facets for bodies of vertebrae . Articular facet on tubercle for transverse process Right fifth rib from behind. The head :i is an enlargement at the posterior end and on the outer surface, t.e. , the one farthest from the cavity of the chest. It has an articular surface at the end facing inward and backward, divided into an upper and a loiver facet, each for the body of a vertebra, by a transverse ridge, whence a ligament passes to the inter- vertebral disk. The lower facet is the larger, and is generally concave ; the upper is nearly plane. The head increases in size to the ninth rib and then lessens. The neck 4 is compressed from before backward, smooth in front and rough for ligaments behind. The upper aspect has a sharp border, the crest, 5 for the superior costo-transverse ligament. The neck grows slightly longer in descending the series to the same level. The crest on the top of the neck is most developed in the sixth, seventh, and eighth ribs. The tubercle 6 is an elevation beyond the neck on the posterior surface of the rib, bearing internally a round articular surface facing backward and, in most cases, downward, to rest on the transverse process ; beyond the articular facet is a rough knob for the external costo-transverse ligament. The shaft 7 is smooth inside, the surface being continuous with that of the neck. The subcostal groove* for the intercostal vein is best marked in the middle ribs, begin- ning at the tubercle and running forward, growing fainter, along three-quarters of the rib, just under cover of the lower border. The outer surface is rather irregular. The angle 9 at which the shaft changes its direction is marked by a rough line on the posterior surface, some distance beyond the tubercle, receiving muscles from the system of the erector spinae. The angle, which is not found in the first rib, is Costae verae. ~ Costae spuriae. 3 Capitulum. 4 Collum. 5 Crista colli. 6 Tuberculum. ~ Corpus costae. 8 Sulcus costal is. '' Angulus costae. THE RIBS. very near (one centimetre beyond) the tubercle in the second ; it gradually recedes from the tubercle, being in the ninth and tenth about five centimetres distant. The angle is a little nearer in the eleventh, and is wanting in the last. The twist is greatest from the sixth to the ninth rib. Several of the upper ribs present near FIG. 175. Tuberositv Angle Head Inferior border (external intercostal] Articular facets for bodies of vertebrae Right fifth rib: A, under surface; , postero-lateral aspect. the middle a rough impression for a point of the serratus magnus. The upper border of the shaft is thick and rounded behind, but thin near the front. The lower border is sharp where it overhangs the subcostal groove ; less so in front. The anterior end of each rib is cupped to receive the costal cartilage. 152 HUMAN ANATOMY. The ribs increase in length from the first to the seventh or eighth, after which they decrease to the last, which is usually the shortest. The length of the last rib is, however, very uncertain, varying from one centimetre to perhaps fifteen centime- tres or more. It often is longer than the first. The curve is comparatively regular in the first rib, after which the difference between the two ends becomes more marked, the curve being very pronounced behind and less so in front. The curve is much less throughout in the lower ribs ; in fact, it decreases continually. The first rib is the broadest of all at the anterior end. There is a general, but not regular, increase from the second to the seventh rib, and a subsequent decrease. The fourth rib is relatively broad, the fifth narrow. 1 Exceptional Ribs. Certain of the ribs the first, second, tenth, eleventh, and twelfth present peculiarities which claim mention. FIG. 176. Cervicalis ascendens Serratus posticus superior External Angle Second digitation of serratus magnus -Third digitation of serratus magnus First and second ribs of right side, upper surface. The first rib is flat, not twisted, with an outer and an inner border. The head is small and has but one facet, resting as it does on the first thoracic vertebra. The neck is small and flat like the body. The tubercle is very prominent. The scalene tubercle is a very small but, from its relations, important elevation on the inner margin of the upper surface, at about the middle, for the insertion of the scalenus anticus. It separates two grooves crossing the bone for the subclavian artery and vein. The posterior one for the artery is the more marked. There is a rough impression behind the latter near the outer border for the scalenus medius. There is no subcostal groove. The second rib is intermediate in shape between the first and the rest. The roughness for the serratus magnus is very marked about the middle of the shaft. 1 Anderson : Journal of Anatomy and Physiology, vol. xviii., 1884. EXCEPTIONAL RIBS. 153 Tenth rib. Single facet The tenth rib has usually only a single articular facet on the head ; it may or may not have a facet on the tubercle. The eleventh rib has a single articular facet on the head ; the tubercle is rudi- mentary and non-articular ; the angle and the subcostal groove are slightly marked. The twelfth rib has also a single articular facet on the head ; the tubercle is at most a faint roughness ; the angle and the subcostal groove are wanting. Development. The first centre for the shaft appears in the ninth week of fetal life, and spreads so rapidly that by the end of the fourth month the perma- nent proportion of bone has been formed. At an uncertain period, probably before puberty, a centre appears for the head and another, except in the last two or three ribs, for the tubercle ; these unite presumably by the twentieth year. Variations. The num- ber of ribs is often increased or FIG. 177. diminished by one, generally by a change at the end of a re- gion, as explained in varia- tions of the spine (page 131). Cervical ribs occur by the cos- tal element of the seventh cer- vical becoming free. In the lowest and most common grade it consists of a head, a neck, a tubercle, and a rudimentary shaft one or two centimetres long, ending free. In the next grade it is longer, and its end, perhaps continued in cartilage, rests on the first rib. Some- times it fuses with the first rib, which then becomes bicipital, as is normal in certain whales. In the third grade, which is very uncommon, it resembles a small first rib, reaching the ster- num. A cervical rib has been seen more than once with the transverse foramen persisting. The explanation of this condition is given under ossification of the vertebrae. When a cervical rib reaches the sternum, the next rib is usually attached to the side of the manubrium by a broad cartilage, fusing with that of the cervical rib. The rib of the eighth vertebra has been seen to end like an ordinary second rib. It is also very rare to have only twelve pairs of ribs, of which the first is cervical. There may be thirteen ribs by the addition of the costal element of the first lumbar. This may be so small as to present no rib-like feature, or it may resemble an ordinary twelfth rib. In cases of an extra rib from this source the twelfth rib is usually uncom- monly long. Very rarely the first true thoracic rib is imperfect, being continued in ligament to the sternum, joining the shaft of the second rib, or even ending free. A bicipital rib may occur also by the fusion of the first thoracic with the second be- yond the tubercles. The resulting plate later subdivides, to be continued by two normal costal cartilages. Ribs sometimes divide, generally near the front. The parts formed by such cleavage are continued by costal cartilages which usually re- unite, so that a foramen is formed which is bounded laterally or externally by bone, mesially by cartilage. This occurs most commonly in the third and fourth ribs, espe- cially in the latter. THE COSTAL CARTILAGES. The costal cartilages 1 continue the ribs, the first seven going directly to the ster- num, the next three each to the one above it, and the last two ending free. They grow longer from the first to the seventh, sometimes to the eighth. The last two 1 Cartilagincs costales. Singl facet Vertebral ends of tenth, eleventh, and twelfth ribs of right side from below. 154 HUMAN ANATOMY. cartilages are short and pointed. There is occasionally a projection downward from the fifth, at its most dependent point, which articulates with the sixth. Usually there is a similar projection on the latter for the seventh. The eighth, ninth, and FIG. 178. . Interclavicular notch Rectus abdominis Obliquus externus and transversalis Posterior Anterior Surfaces of sternum with coossified ensiform cartilage. tenth cartilages have usually their chief connection with the one above, not through their ends, but through similar facets. As to direction : the first cartilage descends, the second is horizontal, the third rises very slightly, and the fourth is the first to fall and then rise. This change of direction occurs in each to the ninth or tenth carti- THE STERNUM. 155 lage, the falling portion becoming always relatively shorter and the rising longer. The last twp cartilages continue the line of their ribs, having no rising portion. It is not uncommon to find eight cartilages joining the sternum. Tredgold found this condition in ten per cent, of white men. It is very much more frequent in negroes and in other dark FIG. 179. It is said to occur more often on the right side. races. THE STERNUM. First rib-- cartilage MANUBRIUM Second Third" BODY Fourth- The adult sternum consists of three flat median plates, the two former being bone, the last largely car- tilage, namely, the presternum or manubrium, the mesosternum, gladiolus, or body, and the metasternum or ensiform cartilage. The manubrium 2 is broad in mammals having clavicles, to which it gives support at the upper angles. In man it is irregularly quadrilateral, with the angles cut off, broad above, narrower below, the greatest breadth equalling or exceeding the length. It is con- cave behind, but in front it is convex from side to side and slightly concave from above down. The upper border is concave in the middle, forming the bottom of the interclavicular notch* On each side of this, in the place of a corner, is a concavity for the sternal end of the clavicle. This depression 4 is more on the top than on the side of the sternum, and usually encroaches more on the back of the bone. It is concave from within outward and may, or may not, be slightly con- cave from before backward. The facet is coated with articular cartilage. Just below the joint, the side of the manubrium projects outward to meet the cartilage of the first rib. This is the widest part of the first piece, the border then slanting inward to the lower angle, which also is cut off by a notch for the second costal cartilage, which is received between it and the body. The lower border, separated from the meso- sternum by fibro-cartilage, projects a little forward into a transverse ridge, always to be felt in life, which in- dicates the level of the second costal cartilage. The oblong body, or gladiolus, 5 ossifying origi- nally in four pieces, one above another, varies con- siderably in shape. It is generally slightly concave behind and nearly plane in front, but it may be convex or even concave. The greatest breadth is below the middle, whence the borders slant inward to the lower end, the narrowest part, where it joins the ensiform cartilage. The sides of the body present alternately smooth concavities opposite the spaces between the costal cartilages and articular facets for the latter. To understand the position of these articular facets, we must recall the composition of the mesosternum as consisting of four pieces. The second cartilage reaches the junction of the manubrium and the body ; the third, that of the first and second pieces of the body ; the fourth, that of the second and third pieces ; the fifth, that of the third and fourth pieces. The two remaining sternal ribs send their cartilages to this fourth piece of the body ; the sixth to the side, and the seventh to the lower angle, or even the 1 Journal of Anatomy and Physiology, vol. xxxi, 1897. Lamb : Nature, 1888. 2 Manubrium sterni. 3 Incisura jugularis. 4 Incisura clavicularis. 5 Corpus sterni. Fifth- Si xth- Seventh- ENSIFORM CARTILAGE Right side of sternum. 156 HUMAN ANATOMY. lower edge. The first and second pieces of the body are about equal in length ; the third is shorter, and the fourth still more so ; hence the fifth, sixth, ajid seventh cartilages end very close together, especially the two last. The ensiform cartilage, 1 or xiphoid process, more or less bony in middle life, is a flat plate with a rounded end, not rarely bifid. It is fastened to the lower end of the body in such a way that their posterior surfaces are continuous, but that the ensiform, being thinner, is overlapped by the ends of the seventh cartilages ; its front is therefore at a deeper level than that of the body. The size and shape of the ensiform cartilage are very uncertain ; usually the tip projects somewhat forward. Differences due to Sex. The body of the male sternum is both absolutely and relatively longer than that of the female. This is in accordance with the greater development of the male thorax. The following table gives the actual size, accord- ing to the writer 2 and to Strauch. 3 DWIGHT. Men. Centimetres. Manubrium 5.37 Body . . 11.04 Total 16.41 IT. STRAUCH. Women. Men. Women. Centimetres. Centimetres. Centimetres. 4-94 5-049 5-056 9.19 11.014 9-059 14.13 16.063 14.115 FIG. 180. Hyrtl gave a rule for determining the sex, that the manubrium of the female exceeds half the length of the body, while the latter in the male is at least twice as long as the manubrium. A study of 342 sterna, of which 222 were male and 120 female, confirmed Hyrtl's law for the mean ; since, however, approximately forty per cent, of the cases were exceptions, it is clearly worthless to determine the sex in any given case. Probably the law would be correct if we had to do only with well-formed sterna, but the body varies greatly. It is easy to recognize a typical male or female sternum. The former has a long, regular body, the lower pieces of which are well developed, sepa- rating the lower cartilages of the true ribs. The latter has a shorter and relatively broader body, the lower parts of which are poorly developed, so that the carti- lages are near together, and the seventh ones of the two sides almost, or quite, meet below the body in front of the base of the ensiform. Variations. The very rare cases of fissure of the sternum, and the not uncommon ones of perfora- tion in the median line, represent different degrees of arrest of development. The lower half of the sternum is sometimes imperfectly developed. We have de- scribed a case in a negress in which there was but little and irregular ossification below the fourth costal carti- lage. A very rare anomaly is that of the manubrium being prolonged to the insertion of the third costal cartilages, as occurs usually in the gibbons and occa- sionally in other anthropoid apes. The suprasternal bones, very rarely seen in the adult, are a pair of rounded bones compressed later- ally, about the size of peas, placed on the top of the manubrium at the posterior border just internal to the sterno-clavicular joint. They are presumably the tops of the lateral cartilaginous strips forming the sternum, in which they are normally lost. They are regarded as representing the episternum of lower vertebrates. 2 Journal of Anatomy and Physiology, vol. xxiv., 1890. 3 Inaug. Disser., Dorpat, 1881. 1 Proccssus xiphoideus. ..Foramen Sternum, showing foramen due to im- perfect union of lateral parts. DEVELOPMENT OF THE STERNUM. 157 FIG. 181. B Development and Subsequent Changes. The cartilaginous bars repre- senting the ribs in the early embryo end in front in a strip connecting them from the first to the ninth, which approaches its fellow above and recedes from it below. The union of these two strips, which begins above, forms the future sternum as far as the ensiform cartilage. Thus at this early stage there are nine sternal ribs. While the mesosternum is forming by the union of the lower part, a portion of the ninth strip separates itself from the rest to fuse with its fellow for the ensiform cartilage, and the remainder of the ninth joins the eighth, which, as a rule, itself later recedes from the sternum. The original cartilaginous strips having fused, points of ossification first appear in the manubrium about the sixth month of foetal life. There is one chief one and a varying number of small ones variously disposed. Sometimes it ossifies in a larger upper and a smaller lower piece. In the latter months, before birth, several points appear in the mesosternum. The first piece generally has a single centre, those below two in pairs. At birth one usually finds ossification begun in the first three pieces of the body. The centre for the last piece of the body begins to ossify at a very variable time. We have seen bone in it at thirteen days and have found none at seven years. Perhaps three years is not far from the average. The centre, or cen- tres, for this last piece of the body are placed in its upper part. Its cartilage is directly continuous with that of the ensiform, the line of demarcation being determined by the difference in thickness, the ensiform being thinner and continuing the plane of the posterior surface. Thus, the lower part of the last piece may continue cartilaginous for a con- siderable time. A centre in the ensiform is sometimes seen at three, but may not come for several years later. The four pieces of the meso- sternum join one another from be- low upward, the union being com- pleted on the posterior surface first. The process is extremely variable. The only points regarding which we are certain are that it is more rapid than is usually stated and that the body is almost always in one piece at twenty. The fourth piece of the body joins the third at about eight, the third joins the second at about fifteen, and the second unites with the first usually at eighteen or nineteen. We once saw all four pieces distinct at eighteen, but in one or two instances only have we found the body incomplete after twenty. The amount of bone in the ensiform at twenty is still small. The adult condition, except that the ensiform gradually becomes wholly bone, may persist to extreme old age. The ensiform often joins the body after middle age, rarely before thirty. The union of the manubrium and the body is rare, and appears to be the result of a con- stitutional tendency rather than of age, as in our observations we have repeatedly found it under fifty, and have seen all three pieces united at twenty-five. The different pieces are more apt to fuse in man than in woman. ARTICULATIONS OF THE THORAX. The joints uniting the bones taking part in the formation of the bony thorax constitute two general groups, the Anterior and the Posterior Thoracic Articula- tions. The former include the joints between the pieces of the sternum, those be- tween the sternum and the costal cartilages, and those between the costal cartilages ; the latter, or the costo-vertebral articulations, include those between the vertebrae and the ribs. Ossification of the sternum. A, at sixth foetal month ; a, centre for manubrium. B, at birth ; a, for manubrium ; b, c, d, for seg- ments of body. C, at about ten years ; a, manubrium ; 6, c, d, seg- ments of body ; e, ensiform cartilage. 158 HUMAN ANATOMY. THE ANTERIOR THORACIC ARTICULATIONS. These include three sets : i. The Intersternal Joints, or those uniting the segments of the sternum ; FIG. 182. Sterno-clavicular joint Anterior intersternal ligament Chondro-sternal ligament Costo-xiphoid ligament Interchondral ligament The sternum and costal cartilages from before. 2. The Costo-Sternal Joints, or those uniting the ribs by means of their cartilaginous extensions with the sternum ; 3. The Interchondral Joints, or those uniting certain of the costal cartilages with one another. INTERSTITIAL ARTICULATIONS. 159 THE INTERSTERNAL JOINTS. While the manubrium and the four pieces of the body, or sternebrse, are still separate ossifications in a common strip of cartilage, the structure is greatly strength- FIG. 183. First rib. MANUBRIUM Interarticular ligament Chondro-sternal joint BODY Interchondral joint - ENSIFORM CARTILAGE Interchondral ligament Longitudinal section through sternum and costal cartilages. ened by the thick periosteum, reinforced by the radiating bands from the costal joints and longitudinal fibres before and behind. When the body has become one piece it is separated from the manubrium by the persisting cartilaginous strip. The 160 HUMAN ANATOMY. strengthening bands require no further description. A cavity is often found in the cartilage, making a typical half-joint. At what time it appears is unknown. Some- times it is so developed that the joint is practically a true one, with articular carti- lage ; this exceptional arrangement is more common in women than in men, being especially adapted to the female type of respiration. The cartilage persisting between body and ensiform is strengthened in a similar manner. A cavity rarely occurs in the cartilage, which, on the contrary, often undergoes ossification. THE COSTO-STERNAL JOINTS. The first costal cartilage joins directly, without interruption, the lateral expan- sion of the sternum ; the following costal cartilages articulate at the points already mentioned by synovial joints. Those that come between different sternebrae that is, from the second to the fifth often have the joint subdivided by a band into an upper and a lower half. This is usual in the joint of the second cartilage ; progres- sively rare as we descend. The sixth and seventh cartilages frequently have no true joint. 1 Each of these joints is enclosed by a capsule, the front and back fibres of which radiate over the sternum. THE INTERCHONDRAL JOINTS. The seventh, eighth, ninth, and tenth costal cartilages have each an articulation by a true joint on the projections above described with the one above it. There is a connection between the fifth and sixth cartilages ; usually on the right, very frequently on the left. 2 This is, as a rule, also a true joint, but the cartilages may be merely bound together by bands of fibres. The joint on the right side is almost always a true one. The ends of the eighth, ninth, and tenth cartilages are joined by fibrous tissue to the cartilage above. The costo-xiphoid ligament is a band extending from either side of the base of the ensiform to the lower border and, perhaps, the front of the seventh cartilage near its end. THE COSTO-VERTEBRAL ARTICULATIONS. The joints between the ribs and the spine are in two series : an inner, or Costo- Central, between the heads of the ribs and the bodies of the vertebrae ; an outer, or Costo- Transverse, between the tubercles and the transverse processes. The Costo-Central Joints. The head of the rib is received in a hollow articular fossa formed by a part of two bodies and the disk between them. Although as a whole concave, it may in a typical case be further analyzed. The lower half of the socket is convex from above downward, fitting into the hollow at the lower part of the joint of the rib ; the upper part is about plane, looking downward and out- ward, with the upper border considerably overhanging the joint. These two facets have each a synovial capsule and are separated by an inlerarticular ligament? a band running from the ridge on the head of the rib to the posterior part of the inter- vertebral disk. In the foetus before term it extends across the back of the disk to the head of the opposite rib. The front of the capsules is strengthened by the anterior costo-vertebral ligament? which is a series of radiating fibres from the head to both vertebrae and the interven- ing disk, not clearly separable into three bands. These stellate ligaments (Fig. 184) are least developed in the upper part of the thorax. The strongest collection of fibres is to the lower vertebra. The joint of the first and last two ribs is not sub- divided ; that of the tenth is uncertain. Strong fibres pass from the head of the first rib to the seventh cervical vertebra. Few or no fibres from the last rib reach the body of the eleventh thoracic. The lower fibres are made tense when the rib is raised and the upper when it is depressed. The Costo-Transverse Joints. The articular surfaces of the tubercles, 'Musgrove: Journal of Anatomy and Physiology, vol. xxvii., 1893. 2 Fawcett: Anat. Anzeiger, Bd. xv. Bardeleben : ibid. 3 Llg. capituli costae interarticulare. 4 Lig. capituli costae radiatum. COSTO-TRANSVERSE ARTICULATIONS. 161 convex vertically, are received into the hollows on the facets of the transverse pro- cesses. The cavities are deepest in the upper part of the thoracic region, but the facet on the first transverse process is nearly plane. In the lower part of the region FIG. 184. VII rib Superior costo-trans verse ligament VIII ri Posterior costo-trans verse ligament IX rib Intervertebral foramen Upper part of stellate ligament Lower part of same Body of ninth thoracic vertebra Ligaments uniting ribs with spine, from before. these cavities are smaller and less concave, allowing freer motion. There is none for the twelfth rib, and but a poor one, if any, for the eleventh. There are three costo-transverse ligaments : the posterior, the middle, and the superior. The pos- FIG. 185. Transverse process , Posterior costo- Lamina of vertebra above of vertebra below transverse ligament Costo-transverse joint Rib Middle costo- transverse liga- ment Middle costo-trans- verse ligament Costo-vertebral joint Interarticular liga- ment Intervertebral disk Transverse section through intervertebral disk and ribs. terior 1 are strong bands running outward from the tips of the transverse processes to the rough part of the tubercle beyond the joint. The middle 2 are strong short fibres connecting the front of the transverse process and the back of the neck of the 1 Llg. costotransversarium posterius. 2 Lig. colli costae. l62 HUMAN ANATOMY. rib between the head and the tubercle. Those for the last two ribs are small, that for the twelfth springing from the accessory tubercle. The superior costo-transverse ligaments 1 are thin bands, passing downward and a little inward from the under side of the transverse processes to the crest on the upper edge of the neck of the rib below. Those of the first and last two ribs are of little account. This band becomes tense when the rib is depressed and carried inward ; the inner fibres are tense when the rib is raised. The outer fibres fuse with the front surface of the posterior inter- costal aponeurosis. Weaker and inconstant bands of the same general direction are described behind these. The fibres of the aponeurosis are particularly strong between the last two ribs. A special band of the same series runs from the transverse process of the first lumbar upward and outward to the last rib. The movements of the ribs are described with those of the thorax (page 165). THE THORAX AS A WHOLE. The thorax is a cage with movable walls capable of expansion. In shape it is an irregular truncated cone, much deeper behind than in front and broader from side to side than from before backward. The thoracic vertebrae form the posterior FIG. 186. Tubercle Lamina of VII thoracic vertebra Middle costo-transverse ligament Posterior costo-transverse ligament Superior costo-transverse perior co ligament Ligamentum subflavum Intertransverse ligament VII thoracic rib a. VIII rib IX rib Ligaments uniting ribs with spine, from behind. boundary ; the sternum, including the very beginning of the ensiform cartilage, the anterior. The inlet, or upper boundary, is an imaginary plane slanting downward and forward from the top of the first thoracic vertebra to that of the sternum, and bounded laterally by the inner borders of the first rib. The inferior boundary, made by the diaphragm, does not exist in the skeleton. Suffice it to say that the dome- like disposition of the diaphragm makes the abdomen much larger and the thorax much smaller than one would expect from the skeleton alone. The thorax of the living presents a fairly well-defined posterior surface, while the lateral ones pass in- sensibly into the anterior ; the upper part is hidden by the shoulder-girdle and arm. The line of the angles of the ribs marks the limits of the back and sides. The inside of the thorax is heart-shaped in horizontal section. The spine projects into it behind, and the ribs recede from this on either side. As the bodies of the vertebrae are larger in the lower part, the projection into the thorax is greater ; but as the area of the section is much larger, the effect is less striking. The distance from front to 1 Lig. costotransversarium anterius. THE THORAX AS A WHOLE. 163 back in the median line is least at the top. It increases at once, owing to the back- ward bend of the spine and the forward slant of the sternum, reaching the maxi- mum at about the middle of the thorax. It decreases slightly below, owing to the forward sweep of the spine, but the position of the lower end of the sternum is so uncertain that this is very variable. The breadth of the thorax increases very rapidly, reaching nearly the maximum where the third rib crosses the axillary line. Below this it increases a little, being greatest where the fifth rib crosses the same FIG. 187. The bony thorax, lateral view. line. It then continues very nearly the same with some slight diminution below. The greatest length of the thoracic framework is in the axillary line, the lowest point being the cartilage of the tenth or eleventh rib, which in the male may nearly reach the crest of the ilium. The downward slant of the ribs and the rise of most of the cartilages make the study of horizontal sections at first very confusing. The relations at certain levels must be somewhat conventional, for the variations are very great, depending on figure, age, health, position, and the stage of the respiratory movements. Two levels must be taken as standards, subject to these corrections. 164 HUMAN ANATOMY. The top of the sternum is on a level with the disk between the second and third thoracic vertebrae ; the junction of manubrium and body of sternum is on a level FIG. 188. FIG. 189. \ Transverse section through thorax at level of third thoracic vertebra. (Braune.) Transverse section at level of fourth thoracic vertebra. (Braune.) Transverse section at level of eighth thoracic vertebra. (Braune.) with the top of the fifth thoracic vertebra. Less accurate, but still useful, is a third level : the lower end of the body of the sternum is opposite the ninth thoracic vertebra. Accompanying diagrams, FIG. 190. taken from Braune, show the varia- tions of size, form, and relations at different levels (Figs. 188 to 191). The breadth of the intercostal spaces is very different in diverse parts. Between the tubercles and angles it is pretty nearly the same throughout, butthe last two spaces are a little broader. The first two spaces are much the broader at the sides and in front. They are broad near the sternum as far down as the fifth cartilage. At the sides the ribs are very close together, from the fourth to the ninth often almost in contact. The lowest spaces are again broader. The Thorax in Infancy and Childhood. At birth the thorax is relatively insignificant. The sternum is small and undeveloped in the lower part. The ribs are more horizontal. The top of the sternum is opposite the body FIG. 191. of the first thoracic vertebra. In 7 the course of the first year it lies opposite the upper part of the second, and at five or six has reached its definite level opposite the disk between the second and third thoracic vertebrae. The lower part of the sternum is undeveloped, and the ribs do not fall so low at the sides. The want of breadth is very striking, while in the adult, throughout the chest below the level of the second costal cartilage, the antero-posterior diameter is to the transverse as i to 2^, or as i to 3 ; at birth it is as 2 to 3. '0 '0 D Transverse section at level of eleventh vertebra. Shaded areas (6, 7) are sections of costal cartilages. (Braune.) We have found it at probably three years as i to 2 ; at five or six the thorax has nearly reached its permanent shape. Differences due to Sex. The whole structure is lighter in women, but the MOVEMENTS OF THE THORAX. 165 chief differences in the proportions appear below the third rib. The manubrium is as large, relatively to the height, in one sex as the other, although the mesosternum in women, especially its lower part, is less developed ; hence the ends of the car- tilages of the lower sternal ribs are crowded together, and those of the seventh often meet below the sternum, in front of the ensiform, thus practically lengthening the body. The effect of this is that the relations of the viscera to the walls are not so different in the sexes as one would expect. 1 The floating ribs are small in women and do not approach the pelvis so closely as in the male. The antero-posterior diameter of the female chest is to the transverse as i to 2^ (subject to variation), thus more resembling the proportions of the child. THE MOVEMENTS OF THE THORAX. The motions permitted by the following joints are to be considered separately, although their interdependence is to be remembered. First, the joints of the verte- bral ends of the ribs, the costo-central and the costo-transverse being taken together ; second, those between the manubrium and gladiolus ; third, the costo-sternal and interchondral joints ; fourth, as modifying these, flexion and extension of the spine ; and fifth, the elasticity of the ribs and cartilages. Motions in the Costo-Vertebral Joints. These vary greatly in different parts of the column. The first rib moves as a hinge on a fixed axis running out- ward, backward, and a little upward through the joint on the body of the verte- bra and that on the transverse process. If this axis were strictly transverse, the rising of the front of the rib would increase only the antero-posterior diameter of the thorax, as the motion occurs in a plane at right angles to the axis. Since, however, the axis is oblique, a plane at right angles to it extends forward and outward, and motion in it thus increases also the transverse diameter of the chest. The shape of the first rib is such that this transverse increase amounts to little or nothing, but this principle comes into play with the longer ribs. The joint of the second rib is prac- tically similar, except that the outer end of the axis at the tubercle is farther back, so that the plane of motion slants more outward and the lateral expansion gained by raising the second rib is more marked independently of the greater length of that rib. With the third rib, usually, an important modification begins ; the outer end of the axis is not fixed, for the tubercle slides on the transverse process. The changes in the facets on the transverse processes have been described ; it appears that, as we descend the spine, they are so placed and so shaped as to allow this movement more and more freely. Thus, in the middle of the thoracic region the outer end of the axis of rotation is so movable that the motion is to be decomposed into two, namely, one on the axis already described through the head and the tubercle, and another on an antero-posterior axis passing through the head of the rib and the joint between its costal cartilage and the sternum. At the eighth rib of the dissected spine a new motion appears, which becomes much more extensive in the succeeding ones. The ligaments connecting the tubercle and neck to the transverse process are less tense, and it is possible to move the tubercle a little forward from its socket ; in the lower joints the rib can be moved upward, down- ward, forward and backward, and circumducted. These motions are particu- larly free at the last two thoracic vertebrae. Motion backward is checked by contact with the transverse process ; forward, by the posterior and middle costo- transverse ligaments ; upward motion of the last two ribs by the particularly strong bands of fascial origin described with the ligaments ; downward motion by the in- tercostal structures. An important deduction from this is that the last ribs can be pulled downward and backward, so as to fix the posterior costal origin of the diaphragm. Motions in the Intersternal Joints. The joint between the manubrium and the body of the sternum admits of motion on a transverse axis, which is free in the young, but much restricted or abolished in the old. At rest, the two parts form a slight angle open behind. This is effaced by the forward motion of the body on 1 Henke : Arch, fur Anat. u. Phys., Anat. Abtheil, 1883. 1 66 HUMAN ANATOMY. the manubrium, but in no case is an entering angle formed in front. A slight twisting may also occur in this joint in the young. In these motions the second costal cartilages follow the manubrium. The motions at the inconstant joint between the sternal body and the ensiform process are necessarily indefinite ; they appear to consist chiefly of a drawing in of the ensiform. Motions in the Costo-Sternal and the Interchondral Joints. On the dissected preparation the second cartilage can be moved up and down, forward and backward, and circumducted ; these motions, however, are very slight. In the succeeding joints the same motions are more and more free as we descend. The lower cartilages of the true ribs from the fifth to the seventh, or to the eighth, inclu- sive, should the latter meet the sternum, move in a somewhat similar manner, but nearly as in one piece. The motion on an antero-posterior axis is most free. The joints between the costal cartilages are very lax, and the surfaces are so placed that the lower one slides forward on the upper. The advantage of these joints is that the lower ribs and the thorax give and receive support, while greater freedom of motion is possible than would be the case were they of one piece. Flexion and extension of the spine modify these motions. The more the spine is flexed the more the upper ribs in particular are depressed, and the more it is extended the more they are raised, independently of any motion in the joints. Thus, when the chest is fully inflated the spine is always strongly extended. The elasticity of the ribs and cartilages, particularly of the latter, exercises an important, but indefinite, influence on all motions which does not admit of accurate analysis. Even the ribs (except in the old) are not rigid bars, and, especially in forced inspiration, there is a pull upon them increasing their convexity. Moreover, the walls of the chest adapt themselves to the surface of the lungs and to abnormal contents of the thorax, so that certain conditions are marked by particular forms of thorax. It follows from the above that the nature of the respiratory movements cannot be deduced solely from the movements of each set of joints considered separately. The soft parts connecting them alone modify greatly the freedom of motion. Braune has shown that the motion of the ribs is much limited by the sternum, and that if the gladiolus be divided into its original pieces and the cartilage above it cut through, the thorax can be more fully inflated. Beyond question in forced inspiration the sternum is raised, thus increasing the antero-posterior diameter ; since the ribs at the same time swing upward and outward, the transverse diameter is likewise increased. Surface Anatomy. The sternum is always to be felt in the middle line. The suprasternal notch is filled up to a large extent by the interclavicular ligament. The angle between the manubrium and the body varies considerably, but it is always easily recognized by a cross-ridge. The ensiform cartilage is at a deeper level and overhung on each side by the costal arch. The front of the chest on each side is covered by the pectoralis major, making it hard to feel the ribs, except at the borders of the sternum. At the side they are easily felt to near the top of the axilla, where the third can be recognized. The upper ribs are concealed by thick muscles, especially between the spine and the angles. The scapula covers them from the second to the seventh, with considerable variations. The first rib cannot be felt except where its cartilage joins the sternum. To count the ribs, begin with the second at the junction of the manubrium and body of the sternum. There is no possibility of error, for the rare cases of the manubrium reaching to the third cartilage may be disregarded ; feel the third and fourth cartilages below it, and then carry the finger downward and out- ward across the chest. The twelfth rib may be too small to be made out. It is not safe to begin counting from below, for the error of mistaking the eleventh rib for the twelfth has led to opening the pleural cavity in an operation in the lumbar region. The nipple is said to be usually over the fourth intercostal space some two centimetres external to the cartilage, but it is very variable, especially in women, and should never be used as a starting-point for counting the ribs. The width of the intercostal spaces at different parts is of obvious importance, but has been described elsewhere (page 164). PRACTICAL CONSIDERATIONS : THE THORAX. 167 PRACTICAL CONSIDERATIONS. The bony and cartilaginous thorax is made up of the ribs, sternum, costal car- tilages, and thoracic vertebrae, and varies in shape as a result of several influences. The slightly larger circumference of the right side of the chest as compared with the left side is probably due to the greater use of the right upper limb, and may be accepted as physiological. Increased circumference of the left side, therefore (in a right-handed person), should indicate careful examination of the spine (for lateral curvature) and of the thoracic viscera. In pigeon-breast the sternum protrudes together with the costal cartilages, while the line of the costo-chondral junction becomes a deep groove. The sides of the chest are flattened, and a transverse section would be almost triangular in shape. There are three modes of production of this very common deformity : . i. In rickety children it is favored by the softening of the bones and cartilages, which are thus of diminished resiliency, the actual exciting cause being often some form of respiratory obstruction, e.g. , enlarged pharyngeal and faucial tonsils, bronchitis, nasal obstructions, etc. In ordinary breathing, on inspiration, air enters the chest freely to prevent the production of a vacuum, and at the end of the act the external atmospheric pressure is balanced by the pressure within. If an impedi- ment to the free ingress of air exists, the external pressure during at least part of the act is in excess, and in young children, particularly rickety children, this is followed by the bending inward along the weakest part of the thorax (the costo- chondral line) and the relative projection of the sternum. 2. The lowest five costal cartilages form an especially weak portion of the chest- wall. They are the most distant from the fulcrum (the spine) on which the ribs move in respiration, and hence the expansive forces act with the greatest disadvan- tage of leverage (Humphry). At the same time the diaphragm, during its contrac- tion, tends to draw them inward. If, however, its central arch cannot descend during inspiration on account of an engorged liver, enlarged abdominal lymphatics, persistent flatulence, etc. (as in a poorly nourished child), it becomes the fixed point, and the lateral walls are pulled in and the sternum correspondingly protruded. 3. Some cases of " pigeon -breast " are seen at or soon after birth in otherwise healthy children. It is probable that these are cases of arrest of development. The so-called " keeled chest " (in which the antero- posterior diameter is increased at the expense of the transverse diameter) is characteristic of the quadrupedal class of mammals, and is necessitated by, and correlated with, the backward and forward swing of the anterior limbs in walking. 1 In the foetus the antero-posterior diameter is relatively greater than in the adult. Attention has already been called (page 164) to the varying ratio between the antero-posterior and transverse diameters of the chest, the transverse diameter in the adult exceeding the anterior in the proportion of 2.5 to i. If this change stops short of full completion, a greater or less degree of relative prominence of the ster- num results. The " bellows chest " 2 is found among mammals almost exclusively in the bats, the anthropoid apes, and man, that have in common simply the disuse of the anterior limbs as a means of support. In them the chief movements of these limbs tend to pull the sternum towards the vertebral column. The exaggeration of this type results in the so-called "flat chest," which is, however, within proper limits, the type of vigor, as it results from the full contraction of normal muscles. Emphysema produces a rotund configuration of the chest-walls, affecting chiefly the upper portion, throwing out the ribs, effacing the intercostal spaces, and making the thorax "barrel-shaped." Old age, owing to an increased bowing of the thoracic spine under the weight of the head and shoulders and to a slipping forward of the shoulder-girdle with its mass of muscles, often causes a depression of the sternum and its approximation to the spine, a common form of flat chest. 1 Woods Hutchinson : Journal of the American Medical Association, vol. xxix., 1897. 2 Ibid. 168 HUMAN ANATOMY. The pulmonary capacity is but roughly indicated by the circumference of the chest, as the vertical diameter is also obviously an important determining factor. Chest measurements, to be of value, should therefore be supplemented by investiga- tion into the amount of air which can be inhaled and exhaled. The resulting information is often of great value as a basis for prognosis and for advice as to exer- cise and hygiene, especially in persons with a predisposition to pulmonary disease. In the infant the thorax is relatively smaller than in the adult. In the female the upper portion of the thorax is less compressed from before backward and is more capacious than in the male. The upper aperture is larger and the range of movement between the upper ribs and the sternum and vertebrae is greater. These circumstances account both for the fulness of the upper portion of the chest in the female and for the character of the respiratory movement, which is known as thoracic ; while that of the male, in which the lower ribs and abdominal walls move more freely, is known as the abdominal type of respiration. The sternum may be entirely wanting, or may be divided into two portions by a fissure down the middle, the result of developmental failure, which, when it exposes the thoracic cavity and the heart, is known as ectopia cordis. Its subcutaneous position makes it the subject of slight but frequent traumatisms, which often serve to localize the bone lesions of syphilis, tuberculosis, and other infections ; and this fact, in conjunction with its cancellous structure, accounts for the frequency with which it is the seat of gummatous periostitis and tuberculous caries. There are sometimes little circular defects in the body of the sternum, through which an abscess may pass from the mediastinum outward, or infections from without may find their way within the thorax. They are congenital defects due to a failure of the two halves of the body of the sternum to unite. The seven depressions on each side of the sternum for the reception of the cartilages of the seven true ribs are so shaped that the upper and anterior edges of each notch are more prominent and larger than the lower and posterior edges. This accounts for the rarity of luxation forward of these cartilages and their ribs by the forces which so constantly pull the ribs upward and forward, as the action of the scaleni and intercostals in violent inspiratory efforts, that of the pectorals in swinging by the hands or on parallel bars, etc. Backward dislocation at the chondro-sternal junction is even rarer ; but this is because, owing to the elastic curves of the ribs, the sternum and the anterior extremities of the ribs move backward together on the application of direct force to the front of the chest. As it is thus movable, and is supported on the ends of elastic levers or springs, the sternum is rarely fractured. When the fracture is the result of indirect violence, it is often associated with injuries to the spine, as the extreme extension or extreme flexion, which is the common cause of a sternal fracture, must necessarily put a severe strain on the thoracic spine. In extension the sternum is fixed between the sterno-mastoids and sterno- hyoids and thyroids above and the recti and diaphragm below. In flexion the force may be transmitted through the chin. In either case the most common seat of fracture is at or about on a line with the second costal cartilage, because (a) the bone there is narrowest (Fig. 173), and () at that level lies the junction between the manubrium and body. As the various portions of the bone are not united until about twenty years of age, fracture is almost unknown before that time. Moreover, during that period the symphysis between the manubrium and the body is so shaped that, together with the natural curve forward of the bone, it increases the elasticity of the sternum and enables it to resist both direct violence and tensile strain. The projection * at the union between the manubrium and body (angulus Ludovici) is sometimes exceptionally prominent, and when this is noticed for the first time after an accident or an illness, may give rise to the erroneous diagnosis of fracture or of bone disease. This angle is increased in phthisis, owing to the reces- sion of the manubrium ; it is increased in emphysema, as the second ribs carry for- ward the lower border of the manubrium. The greater thickness and strength of the layer of fibrous tissue that covers 1 Angulus sterni. PRACTICAL CONSIDERATIONS: THE THORAX. 169 the posterior surface of the sternum, as compared with that on the anterior surface, account for the rarity with which effusions of blood or collections of purulent fluid find their way to the anterior mediastinum. The ribs, in addition to the already described classification into sternal, asternal, and floating, are sometimes designated as upper and lower. It may be well to mention that the term ' ' upper' ' includes the first six ribs, which have convex lower borders, give origin to the pectoralis major (an elevator of the ribs), and move upward in inspiration ; while the term "lower" applies to the last six ribs, which have concave lower borders, give origin to the diaphragm (a depressor of the ribs), and move downward in inspiration. The obliquity of the ribs adds greatly to their range of movement in respiration. The most oblique rib, the longest, and the most movable the seventh is a part of the wall of that portion of the thorax that contains the largest amount of pulmo- nary tissue. The most fixed and most nearly horizontal of the ribs (and the shortest of the sternal ribs) the first is a part of the wall where the least lung tissue is to be found. The ribs below the eighth have less and less relation to the lungs, and become both shorter and more horizontal. They have increased mobility as regards their anterior ends, but lessened rotation on a line drawn between their two extrem- ities, the movement most important in respiration. These facts have relation to the distribution of acute and chronic disease in the lungs : the acute affecting particularly the area of greatest movement and vas- cularity, the bases ; the chronic, the area of lessened mobility and expansion, the apices. The involuntary partial immobilization of the chest-wall after injury and in inflammatory affections of the pleura is of some diagnostic value, as is also the permanent restriction of its movements following the contraction of old adhesions, as after a pleurisy, or pleuro-pneumonia, or fibroid phthisis. The obliquity of the ribs serves also the purpose of securing the necessary expansion of the chest with the least possible motion in the joints between the ribs and the spine and between the cartilages and the sternum. They are thus but little liable to strain, and, in spite of their unceasing movement during life, are very rarely the seat of either dislocation or disease. At the articulation of the ribs with the spine the provision for preventing the ascent of the ribs during the action of the inspiratory muscles (similar to that at the costo-sternal junction) is seen in the fact that the articulating surface of the upper vertebra entering into the joint stands out more boldly than that of the lower one. The participation of the intervertebral disks in the costo-vertebral articulation gives greater safety to those joints and adds to the elasticity of the whole thorax by furnishing a resilient buffer which takes up and distributes forces directed against the chest-wall. Variation in the development of the costal element of the seventh cervical vertebra (page 129) may result in the production of a cervical rib. This, growing beyond its ordinary limits, sometimes reaches half-way to the sternum, running parallel to the first rib, with which its anterior end is sometimes joined. Occasion- ally a process grows up from the first rib to meet it. This, or the cervical rib itself, may raise the subclavian artery and give rise to a mistaken diagnosis of aneurism, or may be thought to indicate chronic (tuberculous or syphilitic) infection of bone, and lead to unnecessary operation or treatment. As a result of rickets, changes often take place at the chondro-costal junctions, causing beaded ribs when a few bones only are affected, or the "rickety rosary" when the enlargements are bilateral and numerous. The ribs most frequently broken are the sixth, seventh, and eighth ; the first and second are protected by the clavicle ; the lower two by their small size and great mobility. The most common form of muscular action causing fracture is coughing ; sneezing and lifting heavy weights have had the same effect. The lower ribs are most frequently broken in this way. When the first rib is broken, a character- istic symptom is said to be pain behind the upper part of the sternum on lifting with the hand on the injured side. This may be due to the fact that the first thoracic nerve lies for about two inches in contact with the under surface of the first 170 HUMAN ANATOMY. rib, and ends at or near the region mentioned, pain being often referred to the peripheral ends of sensory nerves. In fractures by indirect violence (when the sternum and spine are forced together), the theoretical point of fracture would be at or about the summit of the arch ; but practically it is often found very near the point at which the force is apt to be received, i.e., an inch or two outside of the sternal extremity. Unless the force has been great, there is but little displacement in fracture of a rib, owing to the splinting of the bone between the two sets of intercostal muscles above and below it. Shortening is absent, unless an extensive crush of the whole side of the chest has occurred, because the two ends of the bone are fixed, and because of the unbroken bones above and below the fractured one. The complica- tions are those obviously due to the proximity of the pleura and lung on the inner surface of the fracture, the common results of wounds of those structures being various degrees of haemothorax, or pneumothorax, or sometimes (by valvular action) emphysema of the cellular tissue of the trunk (page 1865). Broken ribs always unite with a considerable amount of ensheathing or pro- visional callus, due to the motion which to some degree must be present between the fragments during the process of union. Rupture of an intercostal artery (unless associated with a wound of the pleura) is not usually a serious complication ; but occasionally it is necessary to arrest hemorrhage from this vessel. It lies between the inner and outer intercostal muscles in the groove running along the lower part of the inner surface of each rib. The collateral branch runs near the upper surface of the ribs. Midway between the ribs is, therefore, the safest place to introduce a trocar or to make an incision in opening the chest. The intercostal spaces are wider in the antero-lateral parts of the chest than they are more posteriorly, especially in the neighborhood of the seventh rib ; they are narrowest in close proximity to the sternum and spine. They can be widened by bending the body to the opposite side. For paracentesis of the thorax the centre of the sixth or seventh space should be selected in the mid-axillary line. The lower spaces are in too close proximity to the diaphragm, especially on the right side. More anteriorly it is also in danger ; farther posteriorly the intercostal artery (which runs more horizontally than the ribs) crosses the space obliquely, and behind the angles the ribs are covered by the thick muscles of the back. The ribs are frequently subject to infectious disease. Syphilis and tubercu- losis often produce periostitis or caries, and they are more often the seat of post- typhoidal osteitis than any other bones of the skeleton. This is due to their subcutaneous position exposing them to frequent traumatisms and to the similar effects produced by the numerous strains through muscular action in coughing and sneezing and in lifting or straining. Pus is very apt to travel along the loose connective tissue between the two planes of intercostal muscles, and it is therefore unusual to find suppurative disease confined to one rib, or even to the immediate vicinity of its point of origin. No instance of traumatic separation of the epiphysis of either the head or the tuberosity of a rib has been recorded. The internal mammary artery runs from above downward beneath the cartilages about half an inch from the sternum. Landmarks. The oblique elevations formed by the ribs can usually be seen extending downward from the axillary region. The upper ribs are covered by the great pectoral, but beneath its lower border the ribs from the sixth to the tenth can often be seen. The lower border of the great pectoral follows the direction of the fifth costal cartilage. The curved arch of the costal cartilages is frequently plainly visible, and is accentuated during forced expiration and when a superincumbent weight is held up by the trunk and arms. In short persons the arch is commonly flatter than in tall ones. In counting the ribs it is well to begin with the second, which is easily identified by its relation to the ridge between the manubrium and body of the sternum. The nipple is usually over the fourth intercostal space, somewhat less than 2.5 PRACTICAL CONSIDERATIONS: THE THORAX. 171 centimetres (one inch) external to the costo-chondral junction, or about ten centi- metres (four inches) from the middle line. Its position is variable, and is much lower in fat persons, especially females. In emphysema the nipple may remain stationary, while the upper ribs ascend, and it may be opposite the fifth, sixth, seventh, or even the eighth rib. In phthisis with a shallow depressed chest it may be opposite the fourth rib. A line drawn horizontally from the nipple around the chest is on a level with the sixth intercostal space at the mid-axillary line. A horizontal line around the trunk on the level of the angle of the scapula (the arms hanging down) would traverse the sternum between the fourth and fifth ribs, the fifth rib at the nipple line, and the ninth rib at the vertebral column (Treves). The sternum is subcutaneous in the groove between the pectoral muscles. Near the upper third the ridge between the manubrium and body may be seen or felt. It is on a level with the second costal cartilage. This cartilage projects for- ward more than the others. As the origins of the pectoral muscles diverge the sternal groove becomes broader. It ends at the lower portion of the body of the sternum in a slight projection usually seen and easily felt. This marks the upper limit of the " infrasternal depression" (epigastric fossa, scrobiculus cordis}, the floor of which is over the ensiform process, and which is bounded laterally by the seventh costal cartilages and inferiorly by the upper ends of the recti muscles. In many abdominal diseases, and sometimes after laparotomies, the obliteration of this depression (by the occurrence of tympany) is an important clinical symptom. When the arm is raised, the highest visible digitation of the serratus corre- sponds to the fifth rib ; the largest is that attached to the sixth rib. During expiration the upper end of the sternum is on a level with the second dorsal intervertebral disk ; the line between the manubrium and body is on a level with the fifth thoracic vertebra ; the junction of the sternal body and the ensiform process is opposite the lower part of the ninth thoracic vertebra. The eleventh and twelfth ribs can be felt as blunt bony projections directed downward and outward just outside the erector spinae muscles. (The relations of the various thoracic viscera to the chest-wall will be con- sidered in connection with the anatomy of the former. ) THE SKULL. THE head consists of the cranium and the face. The former is the brain-case ; the latter is chiefly concerned in forming the jaws. The head also contains the terminal organs of four special senses. That of hearing is entirely inside one of the cranial bones, while the organs of sight and of smell lie in cavities formed partly by cranial and partly by facial bones. The special organ of taste, a paVt of the surface of the tongue, is in the mouth, bounded wholly by facial bones. Thus, while the cranial bones have a share in forming the face, no facial bone has any part in forming the brain-case. The latter is an egg-shaped cavity which communicates by a large opening the foramen magnum with the spinal canal, through which the spinal cord passes down from the brain. The brain-case has many smaller openings in the base, through which nerves escape both to the face and to a large part of the body and blood-vessels pass for the nutrition of the brain and its membranes and the walls of the skull. As the bones of the head can be separated in a young subject, it is customary to describe every bone by itself. It is too often forgotten that this knowledge is merely a means to an end, namely, the understanding of the skull as a whole. In the following account this end is kept constantly in view. THE CRANIUM. The cranial cavity is formed by eight bones : the occipital, the sphenoid, the two temporals, the ethmoid, the frontal, and the two parietals. The cranium consists of the vault and the base. The vault is formed by the parietals, the greater part of \\iefrontal, and a part of the sphenoid, of the temporals, and of the occipital. The base of the cranium is divided into three fossae extending across the skull. The posterior fossa is the lowest ; it opens by the foramen magnum into the spinal canal, and contains the cerebellum, the medulla, and the pons. The middle one is narrow at the centre and expands laterally into the temporal regions. The anterior is the highest, lying above the orbits and the nose. The anterior fossa transmits the olfactory nerves, the middle the optic, the posterior the auditory and the glosso- pharyngeal, the nerve of taste. THE OCCIPITAL BONE. The occipital bone 1 is divided for description into an anterior part, the basilar ; two lateral ones, the condylar ; and a posterior one, the tabular or squamous portion. These correspond to the basi-occipital, the exoccipital, and the supra-occipital of comparative anatomy. They all develop from separate centres and bound the foramen magnum? a nearly circular opening, transmitting the spinal cord with its enveloping membranes. The spinal accessory nerves and the vertebral arteries ascend within the latter from the cavity of the spine to that of the cranium. The basilar portion 3 bounding the foramen magnum in front is originally rough anteriorly, but shortly after puberty it coossifies with the body of the sphenoid. Its superior surface is smooth and concave and supports the medulla oblongata. Just internal to the edges is a very shallow groove for the inferior petrosal sinus. The inferior surface is smooth for about one centimetre in front of the foramen magnum, and rough in front of this for the rectus capitis anticus major and minor. In the mid- dle line at the junction of the rough and smooth surfaces is the pharyngeal tubercle,*' Very rarely this aspect presents a depression, \hepharyngealfossa. Sometimes there is a facet near the edge of the foramen for the anterior arch of the atlas. Also, there may be a tubercle on the posterior part of the basilar portion against which the odontoid process may rest, called the third condyle. Laterally, the basilar portion 1 Os occipitale. 2 Foramen occipitale magnum. 3 Pars basilaris. 4 Tuberculum pharyngeum. 172 THE OCCIPITAL BONE. 173 is separated by a suture, the petro-occipital, containing cartilage, from the petrous portion of the temporal. Each condylar portion * {exocdpitai) presents on the inferior surface an oval articular swelling, the condyle, which rests in the hollow on the atlas. They are placed on each side of the front half of the foramen magnum. The hind ends reach almost precisely to the middle of the aperture, and anteriorly they extend to the line of the anterior border, their long axes converging in front. The articular surface, which is convex in the line of the long axis, faces downward and outward. The curve it presents varies greatly. In some cases it is nearly regular, in others the front and back halves almost meet at an angle. There is usually a constriction of the articular surface at the middle, where it may be crossed by a groove or a ridge. On the thick inner border of each condyle is a tubercle for the odontoid ligament. Behind the condyle is a. fossa, into which usually opens the inconstant posterior condyloid fora- men? transmitting a vein. In front of the base of the condyle at its outer border is the constant anterior condyloid foramen? the termination of a canal, from five to ten millimetres long, which pierces the bone above the condyle and transmits the hypo- FIG. 192. Highest curved line Superior curved line Inferior curved line Oc External occipital protuberance Trapezius Complexus Jugular process Jugular notch Pharyngeal tubercle cipitahs Sterno-mastoideus 'Reel, capit.post. minor Splenius Rect. capit. post. major Obliquus superior Posterior condyloid fora- men Rect. capit. lateralis Anterior condyloid foramen, probe in canal Rect. capit. antic, minor Superior constrictor Rect. capit. antic, major Occipital bone, external surface, from below. glossal nerve and, usually, a branch from the ascending pharyngeal artery and vein or veins. It is sometimes divided into two. The bone projects outward from the condyle as the jugular process? which is enlarged at its outer end where it coossifies with the petrous portion of the temporal. This enlargement, moreover, extends downward as the paroccipital process, which shows its greatest development in odd- toed ungulates. In man it is usually very small, but it may be large and, very rarely, join the atlas. The concave front of the jugular process and the bone extending forward on its inner side form the jugular notch? which bounds the posterior lacer- ated foramen 6 behind and internally. This is completed by the temporal bone. A very small point, the anterior jugular process, marks the front of the foramen. A little behind this a larger though very delicate spine, the intrajugular process, reaches across, marking off a small anterior part of the jugular foramen for the passage of the ninth, tenth, and eleventh nerves from the larger one behind for the lateral sinus. Sometimes the front of the jugular process is a smooth surface bounded below by a ridge to which is attached the rectus capitis lateralis, and above by a short border marking off a fossa on the upper surface of the bone ; occasionally jugulare 1 Pars lateralis. 2 Canalis condyloideus. 3 Canalis bypoglossi. 4 Processus jugularis. 5 Incisura jugularis. 6 Foramen lare. 174 HUMAN ANATOMY. the latter ridge is wanting, the groove of the lateral sinus curving over the jugular process. The upper surface of the lateral portion of the process shows on its inner side the entrance of the anterior condyloid foramen, which is really a short canal. Above and anterior to this is a slight swelling, the jugular tubercle. The upper surface of the jugular process is marked by the termination of the groove of the lateral sinus, which curves round an upward projection of the process. In some cases, as just mentioned, the groove is depressed into a deep hollow. The inner opening of the posterior condyloid foramen, when present, is connected with the lateral sinus. The squamous portion ' forms the lower and back part of the skull. Below it contributes the posterior boundary of the foramen magnum and joins the exoccipitals. The lateral borders meet above at a sharp angle. These borders may be subdivided into a lower part, which ascends nearly vertically in articulation with the mastoid part of the temporal, and into a higher part, very serrated and joining the parietal. A slight angle lies on either side at the junction of these two divisions. FIG. 193. Internal occipital protuber- ance Superior occipita fossa Groove for right lateral sinus Inferior occipital fossa roove for left lateral sinus Jugular process 1 Jugular notch Groove for lateral sinus Jugular tubercle Anterior condyloid foramen, probe in canal Occipital bone, internal surface, from before. The posterior surface is marked by a prominence, somewhat below the middle, the external occipital protuberance? to which is attached the ligamentum nuchae. This tuberosity varies greatly in development. From it the superior curved line' extends laterally to the above-mentioned angle. To this line are attached a series of muscles which form the contour of the back of the neck, chiefly the trapezius and part of the sterno-cleido-mastoid. A short and varying distance above the supe- rior ridge is often seen the so-called highest curved line.' It is usually very faint, and may curve down to the external occipital protuberance, or pass above it. The epicramal aponeurosis and part of the occipitalis spring from this line. The surface of the bone above the level of the protuberance is smooth ; below it is rather rough and irregular. The torus occipitalis trans-versus is an occasional prominence in- volving the protuberance and extending laterally along the superior curved line. It sometimes involves the space between that line and the highest one. The upper border of the swelling may have a median concavity. In the mid-line a slight ridge, 1 Squamosa occipitalis. 2 Protuberantia occipitalis externa. 3 Linea nuchae superior. 4 Linea nuchae suprema. DEVELOPMENT OF THE OCCIPITAL BONE. 175 FIG. 194. Superior median fissure the external occipital crest? runs from the protuberance to the foramen magnum. Above the middle of this crest the inferior curved line' 1 ' leaves it to extend outward and downward to the border of the bone. The inner part of this line is rough, the outer indistinct. Below this line there is usually a depression on either side of the crest. The internal surface of the squamous portion is divided into four depressions or fossa ; the upper two lodge the occipital lobes of the cerebrum and the lower two the lateral lobes of the cerebellum. Below the middle is the internal occipital pro- tuberance* approximately opposite to the outer. A ridge runs from the apex of the bone to the protuberance, and is continued as the internal occipital crest* to the foramen magnum. Very often the second part of this ridge divides shortly after its origin, so as to enclose a depression, the vermian fossa, so called because it is below the middle lobe, or vermis, of the cerebellum. A ridge runs transversely from the protuberance to the lateral angle of the bone. The superior vertical ridge may be grooved for the superior longitudinal sinus and the transverse ridge for the lateral sinus. More frequently the longitudinal sinus lies to one side of the vertical ridge and is continued into one of the lateral ones, much larger than its fellow, and usually the right, which lies above the transverse ridge, and shows in the bone no communication with the smaller, which lies in or above the other ridge. There are many variations in this arrangement, of which the rarest is a symmetrical course and division of the supe- rior groove. A single or a bifur- cated groove is sometimes found on the internal crest. Development. Four cen- tres appearin the cartilage around the foramen magnum about the eighth week of foetal life : one for the basilar, one for each exoccipi- tal, and one (or more probably a pair that speedily fuse) for the lower part of the squamous por- tion, the supra-occipital. A week or so later two nuclei appear in the membrane above the latter, from which a strip of bone de- velops which soon joins it. From this upper ossification, the siipe- rior occipital, is developed all the upper part of the squamous por- tion, including the external occipital protuberance and the superior curved line. 5 Occasionally still another nucleus appears on each side, anterior and external to the preceding, which probably accounts for certain separate ossifications often found in the lambdoidal suture. The squamous part shows a median cleft above, which quickly disappears, two lateral ones between the ossifications, which persist till birth, and a notch at the posterior border of the foramen magnum. The squa- mous portion joins the exoccipitals in the course of the second or third year. The latter begin to unite with the basilar a year or so later. None of these sutures, es- pecially the latter, is completely closed before the seventh year, or even later. The front parts of the condyles are formed from the basilar, which joins the ex- occipitals at the anterior condyloid foramina. Separate ossifications, large Woimian bones* are found in the suture between the squamous portion and the parietals. Sometimes there is a large median triangular one which is interpreted as the result of a want of union of the usual superior centre of the squamous portion, and said to 5 Consult Stieda : Anatomische Hefte, iv., 1892, and Debierre : Journ. de 1'Anat. et de la Phys ., 1895. 1 Linia nuchae medtana. 2 L. nuchae inferior. 3 Protub. occip. interna. 4 Crista occipltalis interim. 6 0ssa suturarum. 'superior oc- cipital Squamous por- tion Fissure be- tween upper and lower portions Supra-occipital Exoccipital Basi-occipital Posterior condyloid foramen Occipital bone at birth, from before. 176 HUMAN ANATOMY. be the homologue of the interparietal bone. This interpretation is inconsistent with the history of ossification. Kerkring has described an occasional triangular minute piece of bone which appears during the fifth month in the notch at the back of the foramen magnum, and is fused before birth. We have specimens which imply that it is, or may be, originally double. Improved methods of investigation will prob- ably show that this bone is not uncommon. The cerebral side of the basilar is fused with the sphenoid by seventeen ; the lower side unites later, probably before twenty. THE TEMPORAL BONE. The plan of the organ of hearing must be known to understand the temporal bone. 1 The external ear, besides the auricle, consists of a cartilaginous and bony tube, the external auditory meatus? leading to the membrane of the tympanum which closes it. The middle ear, the cavity of the tympanum, is a space internal to the FIG. 195. SQUAMOUS PORTION Supramastoid crest Occipitalis Spina suprameatum Splenius capitis Squamo-mastoid suture Sterno-mastoid- Mastoid foramen- Auricularis posterior Trachelo-mastoid MASTOID PORTION Tympano-mastoid fissure Mastoid process External auditory meatus' TYMPANIC PORTIO.. Vaginal process Zygoma Masseter Anterior root of zygoma Glenoid fossa APEX OF PETROUS PORTION Glaserian fissure 'Stylo-glossus Stylo-hyoid Styloid process Right temporal bone, external aspect. membrane, opening through the Eustachian tube into the throat, and communicating behind with cavities in the bone. It is lined with mucous membrane and is crossed by a chain of small bones, the ear ossicles, the embryological importance of which is explained elsewhere. The internal ear is a complicated system of cavities in the substance of the bone containing the organ of hearing connected with the brain by the auditory nerve, which leaves the bone through a canal, the internal auditory meatus. Development shows that the bone consists of the following three parts (i) The petro-mastoid, the petrous part of which is first found surrounding the special apparatus of the organ of hearing, constituting the internal ear, while the mastoid process is a much later outgrowth. (2) The tympanic portion, which at birth is a ring, incomplete above, encloses the membrane of the tympanum as a frame holds a glass. This ring grows out later into a cylinder, still open above, which forms the external auditory meatus. Not all its growth, however, is outward, since a part 1 Os temporale. 2 Meatus acusticus e.vternus THE TEMPORAL BONE. 177 expands forward and deeper than the original ring, making the front part of the tympanic plate, bounding the cavity of the tympanum and the Eustachian tube externally. The tympanic cavity, or the middle ear, lies between the petro-mastoid and the tympanic portion, the roof and floor being developed from the former. (3) The squamou s portion is external and above. It forms a part of the side of the skull, the roof of the external meatus where the tympanic portion is deficient, the articulating surface for the jaw, and a part of the mastoid process. There is also the long, slender styloid process, which is a part of the hyoid bar of the second visceral arch of the embryo. It begins as an ossification of a distinct piece of cartilage, but joins the petro-mastoid. The following description is that of the adult bone. The Squamous Portion. 1 Most of this is a thin vertical layer forming part of the wall of the skull, joined below by a horizontal one which forms a small part of the base of the skull, the articulating surface for the jaw, and the roof of the external FIG. 196. Eminentia articularis Zygoma Glenoid fossa Postglenoid tubercle Fissure of Glaser Tympanic plate External auditory meatus Eustachian tube Carotid canal Cochlea Semicircular canal Facial canal Antrum Groove for lateral sinus Horizontal section through right temporal bone, seen from below. auditory meatus. The edge of the vertical part is convex except below. The upper and posterior borders overlap the parietal bone by a broad bevelled surface. The anterior border joins the great wing of the sphenoid, overlapping above and over- lapped below, where it passes into the horizontal part. The posterior angle of the vertical portion sends downward the postauditory process, from which the upper part of the mastoid, including some of the mastoid cells, is developed. The squamo- mastoid suture, separating this from the mastoid portion, is usually lost in the second year. When it persists, it shows that the anterior portion of the mastoid down to the lower border of the external meatus, or even lower, is formed from the squamosal. Its surface is smoother than that of the mastojd proper. A small, particularly smooth, but inconstant patch situated on the level of the upper part of the meatus, one centimetre or more behind it, marks the position of the antrum. The thick- ness of the bone at this place, which is that of note-paper in the infant reaches 1 Pars squamosa. 12 178 HUMAN ANATOMY. six millimetres in the adult. A small, sharp prominence, the spina suprameatum, is found just behind the upper part of the meatus. It is an important landmark in the surgery of the region. Just posterior to it is usually a minute venous foramen. The inner side of the squamous portion, besides the large bevelled articular surface, presents a smooth one, forming part of the wall and floor of the cranial cavity. This is separated from the petrous portion by the petro-squamous suture, which is closed early. Two grooves for branches of the middle meningeal artery diverge from its lower border, one running upward and the other backward. The front of the hori- zontal part forming the floor is rough and thick, joining the great wing of the sphenoid. The zygomatic process l projects forward from the outer surface of the squamosal to complete the zygomatic arch with the malar, which it joins by a serrated end. The free part has an external and an internal surface, a rounded bor- der below and a sharp edge above. The latter, which receives the insertion of the temporal fascia, can be followed back to the origin of the process. The zygoma has two roots. The posterior root passes directly backward above the auditory FIG. 197. SQUAMOUS PORTION Zygoma Groove for meningeal artery PETROUS PORTION ~ " - for lateral sinus Styloid process ir Aquseductus cochleae Right temporal bone, internal aspect. meatus, crosses the squamous portion above the postauditory process, and, curving slightly upward, is lost at the notch between the squamous and mastoid portions: Its hind part is the supramastoid crest, which joins the inferior temporal ridge on the parietal. The anterior root bends sharply inward. It is grooved above for the passage of the fibres of the temporal muscle. Its lower surface forms a semi- cylin- drical transverse elevation, the eminentia articularis? the front part of the articular cavity of the lower jaw. Near its outer end is a t^lbercle for the external lateral ligament. Just in front of the auditory meatus, on the under side of the bone, is the smaller postglenoid tubercle, sometimes described as a third root. The glenoid fossa 3 is a deep hollow on the under side of the squamous portion, with its greatest diameter nearly transverse, but passing somewhat forward and outward, bounded externally by the posterior root of the zygoma ; behind, by the Assure of G las erf which separates it from the tympanic portion ; and extends forward and inward to meet the inner end of the eminentia articularis. Both glenoid fossa and articular eminence are covered with cartilage. The bone separating the glenoid fossa from 1 Processus zygoma ticus. 2 Tubercu1um articulate. 3 Fossa mandibularis. 4 Fissura petrotympanica. THE TEMPORAL BONE. 179 the interior of the cranium is very thin. Behind the glenoid fossa the horizontal part of the squamosal forms the roof of the external auditory meatus. The Tympanic Portion. 1 The tympanic portion of the temporal bone appears as a trumpet-shaped layer of bone, forming all but the roof of the external auditory meatus. Its edge is thin in front, thick below, and very thin behind, where it curls up before the mastoid to meet the postauricular process of the squamosal. It is separated from the mastoid by the minute tympano-mastoid fissure. The ante- rior part of the tympanic portion, called the tympanic plate, runs obliquely forward, concealing the petrosal. It is separated from the glenoid fossa and from the thick anterior edge of the squamosal by the fissure of Glaser, which opens into the tym- panic cavity. The outer end of the fissure is closed ; the inner part is double, since a thin piece of the petrous, the tegmen tympani, bends- down between the squamous and tympanic portions. The lower edge of the tympanic plate ends free. A part covering the base of the styloid process is the vaginal process? which sometimes splits to enclose it. FIG. 198. SQUAMOUS PORTION Eustacbiantube Carotid canal Aquaeductus cochleae PETRO-MASTOID PORTION Jugular fossa Joining occipital Zygoma Articular eminence Glenoid fossa Tegmen tympani Glaserian fissure TYMPANIC PORTION Styloid process Stylo-mastoid foramen Mastoid process Digastric groove Occipital groove Right temporal bone from below. The Petro-Mastoid Portion. 3 This part of the temporal bone may for convenience of description be subdivided into the mastoid and the petrous. The mastoid subdivision forms a part of the wall of the skull behind the tympanic. It is prolonged downward into a nipple-shaped process, the outside of which is rough and slightly prominent. On its lower surface, under cover of the apex, is the digastric groove*' for the origin of the posterior belly of the digastric muscle. Just internal to this, at the very edge of the bone, is the much smaller occipital groove for the occipital artery. The ridge between the two may be developed into a para- mastoid process. The greater part of the internal surface is occupied by a broad and &&QQ groove? running obliquely downward, forward, and inward for the lateral sinus on its way to the jugular foramen. The direction of this groove is very uncertain. Sometimes it descends gradually ; at others it turns far forward and descends nearly vertically. In the latter case it approaches closer than otherwise to the outer wall of the skull, but the distance in all cases is very variable (Figs. 199, 200). It may be only a few millimetres. As it descends it reaches the inner side of the antrum and the mastoid cells. It is separated from the antrum by a plate some six 1 Pars tympanica. - Vagina processus styloideus. 3 Pars petrosa et mastoidea. 4 Incisura mastoidea. 5 Sulcus sigmoideus. i8o HUMAN ANATOMY. millimetres thick in early childhood, and from the antrum or upper mastoid cells by a very thin one in adult life. 1 Behind the groove a small, smooth surface forms a part of the cerebellar fossa. FIG. 199. A B Carotid canal Tympanic cavity Jugular fossa Facial canal - External auditory meatus Groove for lateral sinus .Tympanic cavity Facial canal External auditory meatus roove for lateral sinus Mastoid canal Horizontal sections through a right temporal bone with slight development of the mastoid cells. A, just above the floor of the external auditory meatus ; S, near the roof of the same canal. FIG. 200. Tympanic cavity roove for lateral sinus Similar sections of a right temporal bone with considerable development of the mastoid cells and consequent removal of the lateral sinus from the surface. A small canal, the mastoid foramen, 2 transmitting a vein, runs from the sinus to the outside of the bone, which it sometimes reaches as far back as the suture between Clarke : Journal of Anatomy and Physiology, vol. xxvii, 1893. 2 Fora men mastoideum. THE TEMPORAL BONE. 181 Facial canal Area cribrosa superior -^IP-Cut wall of in- ternal meatus Area cribrosa media Foramen singu- lare Bottom of right internal auditory meatus. X 5. ihe -temporal and the occipital. The interior of the mastoid process contains spaces, the mastoid cells, to be described later. The size and shape of the mastoid process are very variable. The rough upper border of the mastoid subdivision forms an entering angle with the squamosal, into which fits a sharp point from the lower bor- der of the parietal, which rests on it above. Behind and below the FIG. 201. mastoid joins the occipital bone. The petrous subdivision is an elongated pyramid running for- ward and inward, presenting four surfaces (besides the base covered by the mastoid), four borders, and an apex. The surfaces are the supe- rior, posterior, inferior, and anterior. The superior surface slants forward and downward in the floor of the middle cerebral fossa. It has the following features. Above the apex there is a depression x for the Gasserian ganglion. Just external to this the bone is excessively thin and often deficient, so as to leave the end of the carotid canal uncovered. Behind the middle of the pyramid is an elevation, nearly at right angles to its long axis, caused by the superior semicircular canal. External to this the surface is made of a very thin plate of bone, the tegmen tympani, which, extending outward from the petrous, forms the roof of the tympanum and of its continuation, the Eustachian tube. Externally, this plate bends down into the Glaserian fissure, so that its edge may appear between the squamosal and tympanic portions (Fig. 198). At the inner border of the tegmen tympani near FIG. 202. its front is a groove leading to a little rent in the bone, the hiatus Fallopii, 2 through which passes the great su- perficial petrosal nerve. A minute opening, more external, transmits the smaller superficial petrosal nerve. In youth the outer side of the teg- men is bounded by the petro-squa- mous suture. The posterior surface forms a part of the posterior cranial fossa. The chief feature is the internal auditory meatus? a nearly round canal with a slight groove leading to it from the front. Its shorter posterior wall is about five milli- metres long. The canal is closed by a plate of bone, the lamina cribrosa (Fig. 201), which is divided by the falciform crest into a smaller fossa above and a larger one below. The former has an opening by which the facial nerve enters its canal, the aqueduct of Fallopius. Branches of the auditory nerve pass through minute openings in both fossae. About one centi- metre behind the meatus is a little cleft, the aqu&ductus vestibrdi^ entering the bone obliquely from below. Higher and nearer to the meatus is a minute depression, the remnant of the floccular fossa," which is large in some animals and in the infant. It receives a fold of the dura. The inferior surface of the petrous presents in front a large rough surface for Petro-squamous suture Squamous por- tion Internal audi- tory meatus Internal ear- Aquaeductus cochleae xternal audi- tory canal Tympanic cavity Tympanic ring Styloid process Frontal section through temporal bone, showing the cavities of the outer, middle, and inner ear and the four sides of the petrous. 1 Imprcssio tegmenti. - Hiatus canalis facialis. 6 Fossa subarcuata. ! Meatus acusticus inu-rnus. 4 Apertura externa aquaeductus vestibuli. 182 HUMAN ANATOMY. the origin of the levator palati and tensor tympani muscles. External to the back of this is the round orifice of the carotid canal 1 ; back of this, and more internal, is \)\^ jugular fossa. This presents two extreme types, entirely different, with inter- mediate forms. It may be a large thimble-shaped hollow, the edge of which bounds the venous part of the jugular foramen internally, forming a large reservoir for the blood of the lateral sinus as it leaves the skull. On the other hand, it may be a small flat surface. A minute, but very constant, foramen in the ridge between it and the Carotid canal transmits the tympanic branch of the glosso-pharyngeal nerve. A minute foramen, usually found in the jugular fossa, transmits the auricular branch of the vagus. The aquceductus cochlea ends at a small triangular opening 2 in front of the jugular fossa, close to the inner edge. Behind the fossa is a small surface where the temporal bone is united to the occipital, first by cartilage and then by bone. The stylo-mastoid fora-men, the orifice of the facial canal for the facial nerve, is near the outer edge of this surface. The stylo-mastoid branch of the posterior auricular artery enters it. FIG. 203. SQUAMOUS PORTION Zygoma Zygomatic tubercle Groove for meningeal artery Foramen for lesser superficial pe- trosal nerve Hiatus Fallopii Depression for Gasserian ganglion Eustachian tube Carotid canal APEX OF PETROUS Carotid canal (lower end) Tympanic plate Vaginal process Styloid process Right temporal bone from before. The anterior surface of the petrous is nearly all hidden by the tympanic plate. It forms the inner wall of the cavity of the tympanum and of the bony part of the Eustachian tube, which leaves the bone in the entering angle between this surface of the petrous and the tympanic. The features of this surface are treated in the section on the ear. The processu s cochleariformis? attached like a shelf to this outer wall, divides the canal for the tensor tympani muscle from the Eustachian tube below it. The front of this plate can be seen at the entering angle, where the bony tube ends. The small portion of the outer surface of the petrous which is visible is in front of this point, and rests against the inner edge of the great wing of the sphenoid. The superior internal border of the petrous is a prominent ridge in the base of the skull, separating the middle and the posterior fossae. The tentorium is attached to it. The superior petrosal sinus runs along it in a shallow groove within the attached border of the tentorium. Near the front a groove by which the fifth nerve reaches the Gasserian ganglion crosses this border. The inferior internal border articulates anteriorly with the basilar process of 1 Cannlis caroticus. " Apcrtura externa aquaeductos cochleae. 3 Septum canalis musculotubarli. THE TEMPORAL BONE. 183 the occipital bone, and is separated posteriorly from the occipital by the jugular foramen. A little spine on the edge of the thimble-shaped fossa, or on the plane surface that may take its place, the intrajugular process, joins the corresponding process of the occipital either directly or by ligament, so as to divide the foramen into two parts, the posterior for the vein, the anterior for nerves. In front of the foramen a small groove on the cerebral edge of this border marks the position of the inferior petrosal sinus. The superior and the inferior external borders are concealed by the other elements of the temporal, except near the front, where they bound the surface which touches the sphenoid. The apex of the petrous is mostly occupied by the opening of the carotid canal. The styloid process is a part of the hyoid bar (from the second branchial arch), which joins the temporal under cover of the vaginal process. It is thick at its origin, but presently becomes thinner and ends in a sharp point. It is usually about an inch long, but varies greatly. It runs downward, forward, and inward, and is con- tinued as the stylo-hyoid ligament to the lesser horn of the hyoid. Three muscles, the stylo-glossus, stylo-hyoid, and stylo-pharyngeus, diverge from it to the tongue, the hyoid bone, and the pharynx. An ill-defined process of the cervical fascia, the stylo-maxillary ligament, passes from it to the back of the ramus of the lower jaw. Canal for tensor tympani CAVITIES AND PASSAGES WITHIN THE TEMPORAL BONE. The Cavity of the Tympanum. 1 The tympanic cavity is a narrow cleft about five millimetres broad at the top, narrowing to a mere line below. It measures about fifteen millimetres vertically and from be- FIG. 204. fore backward. It is bounded internally by the petrous ; above by a projection from it, the legmen tympani ; below by the jugular fossa, or, if this be very small, by the bone external to it ; externally by the tym- panic portion of the bone and the membrane, except at the top, where the squamosal is ex- ternal to it. The part above the level of the membrane is the supra- tympanic space, the attic, or the epitympanum. This is separated from the cranial cavity by a very thin plate, which is sometimes imperfect. In front, the cavity of the tympanum narrows to the Eusta- chian tube. It opens behind through the antrum, which serves as a vestibule, into the mastoid cells. The antrum. is a cavity of irregular size and shape, compressed somewhat from side to side, with an antero-posterior diameter of from ten to fifteen millimetres, situated behind the epitympanum in the backward projection of the squamosal, which forms the superficial part of what appears to be the mastoid, and contains some of the so-called mastoid cells. The communication with the tympanum is a narrow one, and a certain number of cells open into the latter independently. The antrum and the cells nearest it are lined with mucous membrane continued from the middle ear. The inside of the mastoid varies greatly. Sometimes it con- 1 The detailed description of this space is given in connection with the ear. Carotid canal (inferior end) Sagittal section through righ temporal bone, seen from outer side. 1 84 HUMAN ANATOMY. tains large pneumatic cavities, sometimes diploe instead of air-cells, and, again, it may be almost solid ; the latter condition is, however, probably always pathological. According to Zuckerkandl's 1 investigations of 250 temporal bones, the mastoid is entirely pneumatic in 36.8 per cent, and wholly diploetic in 20 per cent. The re- maining 43.2 per cent, were mixed, the diploe being at the point of the mastoid and the cells above. Neither size nor shape indicates its internal structure. The relation of the cells to the lateral sinus has been already mentioned. The Facial Canal. The course of the canal 2 for the facial nerve is important. It runs outward from the superior fossa of the internal auditory meatus for some three millimetres, until joined by the canal from the hiatus Fallopii. It then makes a sharp turn (the getm) backward, passing internal to the attic of the tympanum just below the external semicircular canal, which almost always projects a little farther outward. It then curves backward to descend to the stylo-mastoid foramen, passing just above the fenestra ovalis. The descending portion is rarely strictly vertical. Below the genu the facial canal may make a bend either outward or inward, but its general line of descent usually inclines outward, sometimes very strongly. Rarely the descent is tortuous. The lower part may incline forward. The genu is opposite a point on the surface above the external meatus, and the subsequent course of the canal can be indicated in general by a line following the posterior border of the auditory opening. An instrument introduced straight into the front of the mastoid will pass behind the facial canal. 3 The diameter of the latter is about one and one- half millimetres. Just before its lower end a very minute canal, transmitting the chorda tympani nerve, runs upward and forward from it to the cavity of the tympa- num. From the front of the cavity this nerve escapes by the minute canal of Huguier, which opens near the inner end of the fissure of Glaser, passing between the tym- panic plate and the tegmen tympani. The facial canal has several other minute openings. There are also minute canals for Jacob son' s nerve from the glosso- pharyngeal, leading to the tympanum, and for Arnold' s branch of the vagus, which enters the jugular fossa and leaves by the fissure between the mastoid and tympanic portions. The carotid canal 4 is close to the front of the tympanum and just before the cochlea of the internal ear. The internal auditory meatus is almost behind the canal, and the Eustachian tube lies to the outer side of its horizontal portion. The temporal bone is porous in structure, except about the internal ear, where it is very dense. A transverse section, either vertical or horizontal, through the external and internal meatus (the middle and internal ears) shows how nearly the entire bone is pierced (Fig. 202). The carotid canal and the jugular fossa, when deep, are further sources of weakness. The fossa sometimes opens into the middle ear by a small rent. Articulations. The temporal bone joins the occipital by the petro-mastoid portion. These two bones form the entire posterior fossa of the skull, except at the extreme front, in the middle, where it extends along the back of the sphenoid, and at the side, where a smaJJ portion of the lateral sinus is made by the posterior inferior angle of the parietal. V This latter bone articulates with the squamous and the top of the mastoid. The L -eat wing of the sphenoid fits into the angle between the squamous and petrous portions, articulating at the side of the skull with the front of the foramen. These two bones the sphenoid and the temporal form the entire middle fossa. The malar bone joins the zygoma, completing the arch. The lower jaw articulates with the glenoid fossa by a true joint Development. The squamous portion is ossified in membrane from one centre, appearing near the end of the second month of fcetal life. In the course of the third month a centre appears in the lower part of the future tympanic ring. The ossification of the petro-mastoid portion comes from several nuclei, the number of which probably varies. The process begins towards the end of the fifth month about the membranous labyrinth. The opisthotic nucleus lies at the inner side of the tympanic cavity and spreads to the lower part of the bone. The/rabVzV is near the superior semicircular canal. The epiotic, arising near the posterior canal, 1 Monatsschrift fur Ohrenheilkunde, Bd. xiii, 1879. 3 Joyce : Journal of Anatomy and Physiology, vol. xxxiv., 1900. 2 Canalis facialis. 4 Canalis caroticus. DEVELOPMENT OF THE TEMPORAL BONE. 185 FIG. 205. Squamous portion spreads into the mastoid portion. This one is sometimes double. There is also a separate nucleus for the tegmen, but this is not constant. When present, it seems to be the last to fuse with the others, which become one by the end of the sixth month. The carotid artery passes at first along the base of the skull in a groove which is made into a canal by the opisthotic. The separated petrous portion, when ossification has made some progress, shows a very promi- nent superior semicircular canal, and a deep cavity under it, extending back- ward from the inner surface. This is the floccular fossa, which, however, is completely hidden by the dura. The mastoid process becomes fairly distinct in the course of the second year. It Tympanic ring Tegmen tympani in nnerwall Glaserian fissure of tympanum Temporal bone at about birth, outer aspect. FIG. 206. Petro-squamous suture Position of superior semicircular canal develops greatly about the time of puberty, when it becomes pneumatic. This may occur much earlier. J. J. Clarke has seen it wholly pneumatic several times before the tenth year ; once at three and a half. 1 The squamosal joins the petrous in the course of the first year. At birth the tympanic por- tion consists solely of the im- perfect ring open above. This enlarges trumpet-like from the edges, the front one forming the tympanic plate. The growth is of unequal rapidity, so that the lower part is left behind, presenting a deep notch the outer edges of which meet by the end of the second year, leaving a foramen below, which usually closes two or three years later, but exceptionally persists. The tympanic plate fuses almost at once with the petrous, but the Glaserian fissure remains ; the groove showing the line of union of the tympanic and mastoid processes generally disappears in the second year, but occasionally persists through life. Kircher 2 found it present on both sides in five per cent, of 300 skulls. The styloid process consists of two parts. The first joins the petrous at about birth. The second, which represents all but the base, is an ossification of the stylo-hyoid ligament, and does not join till puberty or later. In very early foetal life the chief vein returning the blood from the brain passes through the membrane that is to become the squamosal. This open- ing the foramen jugulare spurium is later of less importance, and is finally Posterior semicir- cular canal Floccular fossa Carotid canal internal auditory canal Temporal bone at about birth, from above and within. Tympanic ring Tegmen tympani in Glaserian fissure Tympanic portion of temporal bone in the second year. closed. In the skull, at birth, a pin-hole representing it may be found at the postglenoid tubercle. later. 1 Journal of Anatomy and Physiology, vol. xxvii., 2 Archiv fur Ohrenheilkunde, Bd. xiv., 1879. It is sometimes seen 1 86 HUMAN ANATOMY. THE SPHENOID BONE. In the adult this bone * consists of a cubical body, from the sides of which arise the great wings, from its front the lesser wings, and from below the pterygoid pro- cesses. Both development and comparative anatomy show that these parts represent several bones. The body consists of two parts, a posterior and an anterior. The posterior, the basisphenoid, is the centre of the middle fossa of the base of the skull ; from its sides spread the great wings, or alisphenoids. These with the temporal bones complete the middle fossa. The anterior part, the presphenoid, inseparably connected with the basisphenoid, is in both the middle and the anterior fossae. The lesser wings, the orbito-sphenoids , spread out from the presphenoid and cover the apices of the orbits. The pterygoid processes consist each of two plates, the inner of which represents a separate bone of the face, the outer being an expansion from the alisphenoid. Two bones called the cornua sphenoidalia, or sphenoidal turbinates, of independent origin, ultimately form a part of the body of the sphenoid. Optic foramen FIG. 208. Sphenoidal turbinate Sphenoidal foramen External ptery- goid plate Hamular process Pterygoid notch Internal pterygoid plate The sphenoid bone from before. The Body. It is necessary to describe the basisphenoid and the presphenoid together, since they form the roughly cubical body. The superior surface con- tains the deep pituitary fossa* or sella turcica, in which hangs the pituitary body from the brain. Behind it is the dorsum sell