Digitized by the Internet Archive in 2015 4 https://archive.org/details/handbookofphysioOObake_0 UNIVFKSiTYOF fLLINOfS LIBRARY ■;, 0 HAND-BOOK OF PHYSIOLOaY. KIRKES' HAND-BOOK OF PHYSIOLOGY. HAND-BOOK OP PHYSIOLOGY. lINIVFRSiTY OF ILLINOIS UBRARY .6 BY W. MOEEANT BAKEE, ^.E.C.S. LECTrEEE Olf PHYSIOLOGY, AlfD ASSISTAITT SUBGEON" TO ST. BARTHOLOMEW'S HOSPITAL, SUEGEOIf TO THE EVELIITA HOSPITAL FOK SICK CHILDKEIT. WITH FOUR HUNDRED ILLUSTRATIONS. LONDON: JOHN MUEEAY, ALBEMARLE STEEET. 1876. [The Might of Translation is reservcd,'\ LONDON : BRADBURY, AGNEW, & CO., PRINTERS, WHITEFRIARS. HEMOTE STORAGE PEEFACE TO THE NINTH EDITION. At the present time, the average length of life of new physiological facts may be reckoned, so it is said, at about three years ; and there is sufficient truth in the sarcasm to make the work of selection of facts for a Student's Handbook of Physiology a somewhat difficult matter. It is, indeed, impossible to do more than pick out those which seem, from various analogies, most likely to have a long term of ex- istence, or to take tlieir^ place ultimately among established truths. So much, however, I have endeavoured to do, — remembering that the present work is intended only as a student's guide to those parts of the science of Physiology which are either incontrovertible, or at least fairly esta- blished ; it makes no pretensions of being either a complete treatise or a work of reference. In the preparation of the present edition I have received great assistance from my friend Mr. Harold Schofield, more particularly in the histological portions of the work— the chapters on the Structural Composition of the Human Body, on the Elementary Tissues, and a portion of the chapter on Generation and Development having been in great part re-written by him. In other parts of the work he has also rendered me much help ; and many of the new illustrations are contributed by him from original drawings of microscopic specimens prepared by himself. Chapter II. is reprinted, almost verbatim, from an article vi PREFACE. which I contributed in 1867 to St. Bartholomew's Hospital Eeports. Many of the chapters have been in part re-cast, or re- written. Indeed, the present edition contains compara- tively little of the original work of Dr. Kirkes ; but I have preserved, as far as possible, the general plan and arrangement of the book, as being, on the whole, best adapted for the purpose for which it was written. For convenience of reference I have inserted, as an Appendix, Tables of various Anatomical Weights and ■Measures, of the Specific Gravities of some Tissues and Fluids, of the Composition of certain Foods, and of the Classification of the Animal Kingdom. To Dr. Klein I am indebted for permission to copy several histological drawings in the 'Handbook for the Physiological Laboratory' and elsewhere; and to Mr. W. Pye for origmal drawings to illustrate the subjects of Muscle, the Kidney, and the Eetina. I am desirous of expressmg my obUgations also to Dr. Allen Thomson for several illustrations, taken from the anatomical drawings which he has contributed to the later editions of Quain's Anatomy; and to Dr. John Wilhams for contributing to that part of the section on Generation which relates to "the Structure of the Mucous Membrane of the Uterus, and its periodical changes." About 150 additional illustrations appear m the present edition. They have been drawn by Mr. Godart and Mr. CoUings, and engraved by Mr. J. D. Cooper. W. MOERANT BAKER. 26, WiMPOLE Street, London, October, 1876. CONTENTS. CHAPTER I. The General and Distinctive Ciiaeacters of Living Beings . i CHAPTER TI. The Relation of Life to other Forces . „ . , . 6 CHAPTER IIL Chemical Composition of the Human Body 30 Noii-lSritrogenous Organic Principles 33 Nitrogenous Organic Principles 36 Inorganic Principles 40 CHAPTER IV. Structural Composition of the Human Body 43 Cells 43 Protoplasm 44 IlsTucleus 49 Intercellular Substance 56 Fibres ib. Tubules 57 CHAPTER V. Structure of the Elementary Tissues. . . . . * 58 Epithelium ib. Connective Tissues 68 Gelatinous ib. Eetiform 69 Fibrous . . ib. viii CONTENTS. PAOK Structure of the ElementaPwY Tissues, mitimted. Connective Tissues, continued. Adipose 7S Cartilage 7^ Bone 83 Teeth 9i CHAPTER VL The Blood Quantity of Blood Coagulation of the Blood Conditions affecting Coagulation . . . . ... 107 Chemical Composition of the Blood I09 The Blood-Corpuscles, or Blood-Cells i ic> Physical and Chemical Characters of Red Blood-Cells . . - ih. Action of Reagents .112 Blood-Crystals The White Corpuscles, or Blood-Leucocytes The Serum Yariations in the Principal Constituents of the Liquor Sanguinis . ih. Variations in Healthy Blood under Different Circumstances . .122 Yariations in the Composition of the Blood in different Parts of the Body Ill Gases contained in the Blood . . . • • • .120 Development of the Blood ^27 Uses of the Blood ' 3^ Uses of the various Constituents of the Blood CHAPTEE VIL ClKCULATION OF THE BlOOD ^34- The Systemic, Pulmonary, and Portal Circulations . . » 13^ Discovery of the Circulation The Heaut Structure of the Yalves of the Heart The Action of the Heart Function of the Yalves of the Heart H9 Sounds of the Heart ^54^ Impulse of the Heart . . • • ^57 The Cardiograph ^5 Frequency and Force of the Heart's Action . . • . . loi Influence of the Nervous System on the Action of the Heart . .164 The Rhythmic Action of the Heart ^^S Effects of the Heart's Action CONTENTS. ix TifE Artehies 169; Structure of the Arteries ........ ih. Function of tlie Arteries . . 1 72 The Pulse 178. Sphygmograph . 181 Pressure of the Blood in the Arteries, or Arterial Tension . .184. The Kymogi-aph . . 185 Influence of the Nervous System on the Arteries . . . .188. The Capillaiiies . 190. The Structure and Arrangement of Capillaries . . . . . 191 Circulation in the Capillaries . I9S Diapedesis of Blood-corpuscles 197 The A^efxs . . . 20a Structure , . . . ih. The Valves of Veins ih. Circulation in the Veins 202 Agents concerned in the Circulation of the Blood . . . . 205, Velocity of the Circulation 207 Velocity of the Blood in the Arteries ih, J J 57 5, Capillaries 209, M J, Veins ih^ Velocity of the Circulation as a whole . . . . . . .21a Peculiakities of the Cieculation m different Parts . . 213, Circulation in the Brain . ih. Circulation in Erectile Structures 214. CHAPTEE YIII. Kespiration 216 Position and Structure of the Lungs 218 Structure of the Trachea and Bronchial Tubes .... 220 Structure of the Lungs . , 223, Mechanism of Respiration 228 Respiratory Movements 229 Respiratory Rhythm 235 Respiratory Sounds 236 Respiratory Movements of Glottis . ih^ Quantity of Air respired 237 Vital or Respiratory Capacity ....... Force exerted in Respiration 239 Daily Work of the Respiratory Muscles 242 Circulation of Blood in the Respiratory Oi-gans . . . . . ib. X CONTEXTS. PAGE Bespiratiox, continued. Changes of the Air in Respiration 243 Changes produced in the Blood by Respiration 250 Effects of Vitiated Air— Ventilation 251 Mechanism of various Respiratory Actions ^ 252 Influence of the Nervous System in Respiration .... 257 Apnoea— Dyspnoea— Asxohyxia . * . . . - • • 259 CHAPTER IX. Animal Heat Variations in Temperature ^ • Sources and Mode of Production of Heat in the Body . . .266 Regulation of Temperature 267 Influence of Nervous System • • .272 CHAPTEH X. Digestion ^1.^ Food 277 Starvation . • • Necessity of Mixed Diet Passage of Food theough the Alimentaby Canal ... 282 The Salivary Glands and the Saliva 2^83 Mastication and Insalivation ^ * Structure of the Salivary Glands The Saliva Influence of the Nervous System on the Secretion of Saliva . . 2«b The Pharynx The Tonsils ^ * The (Esophagus or Gullet Swallowing or Deglutition ^94 Digestion of Food in the Stomach Structure of the Stomach ' Secretion and Properties of tlie Gastric Fluid .... 3^2 Gastric Digestion Functions of the Gastric Juice 3°^ Movements of the Stomach Vomiting . 313 Hunger and Thirst ; • • * Influence of the Nervous System on Gastric Digestion . • 3^7 Dif^estion of the Stomach after Death . . • ... 3^9 CONTENTS. xi _ PAGE Digestion in the Intestines 321 Structure and Secretions of the Small Intestine , . . . ib. Valvulixi Conniventes 324 Glands of tlie Small Intestine . . . . . . . ib. The Villi * * 331 •Structure of tlie Large Intestine 334 Succus Entericus or Intestinal Juice 337 'The Pancreas and its Secretion ib. Structure of the Liver 340 Functions of the Liver 348 , The Bile ib, Olycogenic Function of the Liver 357 Summary of the Changes which take place in the Food during its Passage through the Small Intestine 360 Summary of the Process of Digestion in the Large Intestine . 364 Defsecation ........... 365 Gases contained in the Stomach and Intestines . . . . 366 Movements of the Intestines 367 Influence of the Nervous System on Intestinal Digestion . . 369 CHAPTER XI. Absorption 369 Structure and Office of the Lacteal and Lymphatic Vessels and Glands 370 Lymphatic Glands 378 Properties of Lymph and Cliyle . 380 Absorption by the Lacteal Vessels 384 Absorption by the Lymphatic Vessels 385 Absorption by Blood-vessels 387 CHAPTER XII. l^UTRITION AND GROWTH 393 Nutrition ib. Growth 404 CHAPTER XIII. Secretion 407 Secreting Membranes 408 Serous Membranes . . ib. Mucous Membranes 411 Secreting Glands 414 Process of Secretion 417 Influence of the IsTervous System on Secretion. . . . 421 xii CONTENTS. CHAPTER XIV. PACK The YAScuLAr. Glands; oe, Glands without Ducts . . .422 Structure of tlie Spleen 424 Functions of the Vascular Glands 427 CHAPTER XV. The Skin and its Secretions • • • 43© Structure of the Skin Sudoriparous Glands 43^ Sebaceous Glands 43^ Structure of Hair and Nails Excretion b^^ the Skin 442 Absorption by the Skin 44 S CHAPTER XVI. The Kidneys and Uiune 447 Structure of the Kidneys Structure of the Ureter and Urinary Bladder 453 Secretion of Urine . 455 Micturition .... 457 The Urine ; its general Properties Chemical Composition of the Urine . . . • . . 461 CHAPTER XVII. The Income and Expenditure of the Human Body . . . 473 CHAPTER XVIII. The Nervous System • 47^ Elementary Structures of the Nervous System 479 Structm-e of Nerve-Fibres Terminations of Nerve-Fibres 4^4 Effects of Section of Nerves 4^6 Structure of Nerve-Centres Functions of Nerve-Fibres 4^^ Classification of Nerve-Fibres 490 Laws of Conduction in Nerve-Fibres . . . • • . ib. Functions of Nerve-Centres . . 496 Secondary or Acquired Reflex Actions . . • • • • S^^ CONTENTS. XIU PAGE €EiiEBKO-sriNAL Nera^ous System . . * ' 502 The Spinal Cord and its Nerves , ih. The White Matter of the Spinal Cord 505 The Grey Matter of the Spinal Cord 507 Nerves of the Spinal Cord 509 Functions of the Spinal Cord 510 The Medulla Oblongata 520 Its Structure ih. Distribution of the Fibres of the Medulla Oblongata . . . . 522 Functions of the Medulla Oblongata 523 ^TKUCTURE AND PHYSIOLOGY OF THE PoNS YAROLir, CHURA CEREBRI, Corpora Quadrigemina, Corpora Gentculata, Optic Thalami, AND Corpora Striata 526 Pons Varolii . . . ih. Crura Cerebri ih. Corpora Quadrigemina ......... The Sensory Ganglia 530 The Cerebellum 53 ^ Functions of the Cerebellum 534 The Cerebrum 537 Convolutions of the Cerebrum . . . . . . . . 538 Structure of the Cerebrum 54^ Chemical Composition of the Grey and White Matter . . . 542 Functions of the Cerebrum 543 Distinctive Characters of the Human Brain 545 Effects of the Removal of the Cerebrum 547 Electrical Stimulation of the Brain 55^ Sleep 554 Physiology of the Cranial Nerves 556 Physiology of the Third Cranial Nerve 557 Physiology of the Fourth Cranial Nerve . . . . . . 55^ Physiology of the Fifth or Trigeminal Nerve ih. Physiology of the Sixth Nerve 5^4 Physiology of the Facial Nerve 5^5 Physiology of the Glosso- Pharyngeal Nerve 567 Physiology of the Pneumogastric Nefrve . . . . . . 5^^ Physiology of the Spinal Accessory N erve . . . . '571 Physiology of the Hypoglossal Nerve . . ... 572 Physiology of the Spinal Nerves 573 Physiology of the Sympathetic Nerve ih. Functions of the Sympathetic Nervous System . . . -577 xiv CONTENTS. CHAPTER XIX. PAGE Cattses ai^d Phenomena of Motion . . . , . , . 581 Ciliary Motion Amceboid Motion . . . ■ 583 Muscular Motion Plain, or Non- Striated Muscles 5^3 Striated Muscles 5^5 Development of Muscular Tissue 590 Chemical Constitution of Muscle ih. Physiology of Muscle . . . • 59i Work done by Muscles 59^ Action of the Voluntary Muscles 603 Action of the Involuntary Muscles 607 Source of Muscular Action 608 Electric Currents in Muscle and Nerve 609 CHxiPTER XX. Of Voice and Speech Mode of Production of the Human Voice ih. The Larynx • . . 612 Application of the Voice in Singing and Speaking . . . .619 Speech • ^^3 CHAPTER XXI. The Senses • • .627 Common Sensations Special Sensations ^29 The Sense of Touch ^34 The Sense of Taste ^4 The Tongue and its Papillae • • 644 The Sense of Smell ^5 The Sense of Hearing -657 Anatomy of the Organ of Hearing Physiology of Hearing Punctions of the External Ear Functions of the Middle Ear ; the Tympanum, Ossicula, and Fenestr^e 667 Functions of the Labyrinth . . ' ^73 Sensibility of the Auditory Nerve ^75 CONTENTS. XV PAGE' The Senses, continued. The Sense of Sight 68(> The Eyelids and Ijaclirymal Apparatus 68a The Structure of tlie Eye-ball 68 1 Structure of tlie Eye '>h. Phenomena of Vision 693- Spherical Aberration 694. Chromatic Aberration 696» Accommodation of the Eye 697 Defects of Vision .......... 700 Keciprocal Action of different parts of the Ketina . . . .712 The Blind Spot 713: Simultaneous Action of the two Eyes 714. CHAPTER XXIT. Geneeation and Development 719^ Generative Organs of the Eemale ih, TJnimpregnated Ovum 723- Discharge of the Ovum 726- Relations existing between the Discharge of the Ovum and Men- struation 727 Corpus Luteum 731 Tmpeegnation of the Ovum 734 Male Sexual Functions ih. Structure of tlie Testicle . 735" The Semen 74 1 Development 742 Changes of the Ovum up to the Formation of the Blastoderm . . ib. Segmentation of the Ovum 743 Fundamental Layers of the Blastoderm : Epiblast ; Mesoblast ; Hypoblast 745 First Rudiments of the Embryo and its Chief Organs . . . 746 Foetal Membranes 754 The Umbilical Vesicle . . . . » . . . . ib. The Amnion and Allantois 755 The Chorion 758 Changes of the Mucous Membrane of the Uterus and Formation of the Placenta 759 Development of Oegans 765 Development of the Vertebral Column and Cranium . . . ih. Development of the Face and Visceral Arches 769 Development of the Extremities 771 Development of the Vascular System 772 Circulation of Blood in the Foetus 784 xvi CONTENTS. PAGE *Geneiiation axd Development, continued. Development of Organs, continued. Development of the Nervous System 7^7 Development of the Organs of Sense 79 1 Development of the Alimentary Canal . . . ... 796 Development of the Kespiratory Apparatus 800 Development of the Wolffian Bodies, Urinary Apparatus, and Sexual Organs The Mammary Glands 809 Lactation. . . . 811 Involution . • .812 Evolution 813 APPENDIX 815 Anatomical Weights and Measures ...... ih. Measures of Weight , . ib. „ Length ih. Sizes of various Histological Elements and Tissues . . . . 816 Metrical System of Weights and Measures compared with the Common Measures ih. Specific Gravity of various Fluids and Tissues 817 Table showing the per centage composition of various Articles of Food Classification of the Animal Kingdom . . . . . 818 List of Authors referred to . . . . . . . 820 INDEX . • , , 829 Cranium, - 7 Cervical Vertebra). - Clavicle. - Scapula. 12 Dorsal Vertebrae. - Humerus. 5 Lumbar Vertebrae. - Ilium. . Ulna. - Radius. ■ Pelvis. - Bones of the Carpus - Bones of the Meta- carpus. - Phalanges of Fingers - Femur. Patella. Tibia. Fibula, Bones of the Tarsus. Bones of the Meta- tarsus. Phalanges of Toes. THE SKELETON (after Holden). Anterior Su- perior iSpine of the Ilium. Sjmijhysis Pubis. DIAGRAM OF THORACIC /i. Aortic Valve. M. Mitral Valve. AND ABDOMINAL REGIONIS. P. Pulmonary Valve. T. Tricuspid Valve. HANDBOOK OF PHYSIOLOGY. CHAPTEE I. THE GENERAL AND DISTINCTIVE CHAKACTEES OF LIYINa BEINGS. Human Physiology is the science which treats of the life of man — of the way in which he lives, and moves, and has his being. It teaches how man is begotten and born; how he attains maturity ; and how he dies. Having, then, man as the object of its study, it is unnecessary to speak here of the laws of life in general, and the means by which they are carried out, further than is requisite for the more clear understanding of those of the life of man in particular. Yet it would be impossible to understand rightly the working of a complex machine without some knowledge of its motive power in the simplest form ; and it may be well to see first what are the so-called essentials of life — those, namely, which are mani- fested by all living beings alike, by the lowest vegetable and the highest animal, before proceeding to the consideration of the struc- ture and endowments of the organs and tissue belonging to man. The essentials of life are these, — birth, growth and develop- ment, decline and death. The term, hirthy when employed in this general sense of one of the conditions essential to life, without reference to any par- ticular kind of living being, may be taken to mean, separation from a parent, with a greater or less power of independent life. Taken thus, the term, although not defining any particular stage in development, serves well enough for the expression of B GROWTH. [chap. I. the fact, to which no exception has yet been proved to exist, that the capacity for life in all living beings is got by inheritance. Groicth, or inherent power of increasing in size, although essential to our idea of life, is not confined to living beings. A crystal of common salt, or of any other substance, if placed under appropriate conditions for obtaining fresh material, will grow in a fashion as definitely characteristic and as easily to be foretold as that of a living creature. It is, therefore, necessary to explain the distinctions which exist in this respect between living and lifeless structures; for the manner of growth in the two cases is widely different. First, the growth of a crystal, to use the same example as before, takes place merely by additions to its outside ; the new matter is laid on particle by particle, and layer by layer, and, when once laid on, it remains unchanged. The growth is here said to be superficial. In a living structure, on the other hand, as for example, a brain or a muscle, where growth occurs, it is by addition of new matter, not to the surface only, but through- out every part of the mass; the growth is not superficial but interstitial. In the second place, all living structures are subject to constant decay; and life consists not, as once supposed, in the power of preventing this never-ceasing decay, but rather m making up for the loss attendant on it by never-ceasing repair. Thus, I man's body is not composed of exactly the same particles day after day, although to all intents he remains the same individual. Almost every part is changed by degrees ; but the change is so gradual, and the renewal of that which is lost so exact, that no difference may be noticed, except at^ long intervals of time. A lifeless structure, as a crystal, is subject to no such laws; neither decay nor repair is a necessary condition of its existence. That which is true of structures which never had to do with life is true also with respect to those which, though they are formed by living parts, are not themselves alive. Thus, an oyster-shell is formed by the living animal which it encloses, but it is as lifeless as any other mass of inorganic matter; and in accordance with this circumstance its growth takes place not interslitialbj, but layer by layer, and it is not CHAP. I.] DEVELOPMENT, 3 subject to the constant decay and reconstruction which belong to the living. The hair and nails are examples of the same fact. Thirdly, — in connection with the growth of lifeless masses there is no alteration in the chemical constitution of the material which is taken up and added to the previously existing mass. For example, when a crystal of common salt grows on being placed in a fluid which contains the same material, the properties of the salt are not changed by being taken out of the liquid by the crystal and added to its surface in a solid form. But the case is essentially different in living beings, both animal and vegetable. A plant, like a crystal, can only grow when fresh material is presented to it ; and this is absorbed by its leaves and roots ; and animals for the same purpose of getting new matter for growth and nutrition, take food into their stomachs. But in both these cases the materials are much altered before they are finally assimilated by the structures they are destined to nourish. Fourthly. The growth of all living things has a definite limit, and the law which governs this limitation of increase in size is so invariable that we should be as much astonished to find an individual plant or animal without limit as to growth as without limit to life. Develojment is as constant an accompaniment of life as growth. The term is used to indicate that change to which, before maturity, all living parts are constantly subject, and by which they are made more and more capable of performing their several functions. For example, a full-grown man is not merely a magnified child ; his tissues and organs have not only grown, or increased in size, they have also developed, or become better in quality. No very accurate limit can be drawn between the end of development and the beginniDg of decline; and the two processes may be often seen together in the same individual. Bat after a time all parts alike share in the tendency to degeneration, and this is at length succeeded by death. It has been already said that the essential features of life are the same in all living things ; in other words, in the members B 2 4 AXniALS CO:sTEASTED [chap. I. of botli tlie animal and vegetable kingdoms. It may be well to notice briefly the distinctions wliicli exist between the mem- bers of these two kingdoms. It may seem, indeed, a strange notion that it is possible to confound vegetables with animals, but it is true with respect to the lowest of them, in which but little is manifested beyond the essentials of life, which ai-e the same in both. I. Perhaps the most essential distinction is the presence or absence of power to live upon inorganic material. By means of their green colouring matter, chlorophyll—^ substance almost exclusively confined to the vegetable kingdom, plants are capable of decomposing the carbonic acid, ammonia and water, which thev absorb by their leaves and roots, and thus utilizing them as food. The result of this chemical action, which occurs only under the influence of light, is, so far as the carbonic acid is concerned, the fixation of carbon in the plant structures, and the exhalation of oxygen. Animals are incapable of thus usmg inorganic matter, and never exhale oxygen, as a product of decomposition. The power of living upon organic as well as inorganic matter is less decisive of an auimal nature ; inasmuch as fungi and some other plants derive their nourishment in part from the • former source. II. There is, commonly, a marked difi:erence in general chemical composition between vegetables and animals, even in their lowest forms ; for while the former consists mainly of cellulose, a substance closely allied to starch and containing carbon, hydrogen, and oxygen only, the latter are composed in great part of the three elements just named, together with a foui-th, nitrogen; the chief proximate principles formed from these being identical, or nearly so, with albumen. It must not be supposed, however, that either of these typical compounds alone, with its allies, is confined to one kingdom of nature. Nitro- genous compounds are freely produced in vegetable structures, although they form a very much smaller proportion of the whole organism than cellulose or starch. And while the presence of the latter in animals is much more rare than is that CHAP. I.] WITH YEGETABLES. 5 of the former in vegetables, there are many animals in which traces of it may be discovered, and some, the Ascidians, in which it is found in considerable quantit}^ III. Inherent power of movement is a quality which we so commonly consider an essential indication of animal nature, that it is difficult at first to conceive it existing in any other. The capability of simple motion is now known, however, to exist in so many vegetable forms, that it can no longer be held as an essential distinction between them and animals, and ceases to be a mark by which the one can be distinguished from the other. Thus the zoospores of many of the Cryptogamia exhibit ciliary or amoeboid movements (p. 46) of a like kind to those seen in animalcules ; and even among the higher orders of plants, many exhibit such motion, either at regular times, or on the application of external irritation, as might lead one, were this fact taken by itself, to regard them as sentient beings. Inherent power of movement, then, although especially charac- teristic of animal nature, is, when taken by itself, no proof of it. IV. The presence of a digestive canal is a very general mark by which an animal can be distinguished from a vegetable. But the lowest animals are surrounded by material that they can take as food, as a plant is surrounded by an atmosphere that it can use in like manner. And every part of their body being adapted to absorb and digest, they have no need of a special receptacle for nutrient matter, and accordingly have no digestive canal. This distinction then is not a cardinal one. It would be tedious as well as unnecessary to enumerate the chief distinctions between the more highly developed animals and vegetables. They are sufficiently apparent. It is necessary to compare, side by side, the lowest members of the two kingdoms, in order to understand rightly how faint are the boundaries between them. CHAPTER II.* ON THE RELATION OF LIFE TO OTHER FORCES. An enumeration of theories concerning tlie nature of life would be beside the purpose of the present chapter. They are interest- ing as marks of the way in which various minds have been influenced by the mystery which has always hung about Yitahty ; their destruction is but another warning that any theory we can frame must be considered only a tie for connecting present facts, and one that must yield or break on any addition to the number ■which it is to bind together. Before attention had been drawn to the mutual convertibility of the various so-called physical forces— heat, light, electricity, and others— and until it had been shown that these, like the matter through which they act, are limited in amount, and strictly measurable; that a given quantity of one force can produce a certain quantity of another and no more ; that a given quantity of combustible material can produce only a given quantity of steam, and this again only so much motive power; it was natural that men's minds should be satisfied with the thought that vital force was some peculiar innate power, un- limited by matter, and altogether independent of structure and organisation. The comparison of life to a flame is probably as early as any thought about life at all. And so long as hght and heat were thought to be inherent qualities of certain material which perished utterly in their production, it is not strange that life also should have been reckoned some strange spirit, pent up in the germ, expending itself in growth and development, and finaUy declining and perishing with the body which it had in- habited. ,. ; 1 ■•■ With the recognition, however, of a distinct correlation between the physical forces, came as a natural consequence a revolution of the commonly ac cepted t heo ries concerning life also. ~* This chapter is a reprint, with some verbal alterations, of an essay coi^- tributed by the Editor to St. Bartholomew's Hospital Reports, 1867. CHAP. IL] THE RELATION OF LIFE TO OTHER FORCES. 7 The dictum, so long accepted, tliat life was essentially independent of physical force began to be questioned. As it is well-nigh impossible to give a definition of life that shall be short, comprehensive, and intelligible, it will be best, perhaps, to take its chief manifestations, and see how far these seem to be dependent on other forces in nature, and how con- nected with them. Life manifests itself by birth, growth, development, decline and death ; and an idea of life will most naturally arise by taking these events in succession, and studying them individually, and in relation to each other. When the embryo in a seed awakes from that state, neither life nor death, which is called dormant vitality, and, bursting its envelopes, begins to grow up and develope, it may be said that there is a birth. And so, when the chick escapes from the egg, and when any living form is, as the phrase goes, brought into the world. In each case, however, birth is not the beginning of life, but only the continuation of it under different conditions. To understand the beginning of life in any individual, whether plant or animal, existence must be traced somewhat further back, and in this way an idea gained concerning the nature of the germ, the development of which is to issue in birth. The germ may be defined as that portion of the parent which is set apart with power to grow up into the likeness of the being from which it has been derived. The manner in which the germ is separated from the parent does not here concern us. It belongs to the special subject of generation. Neither need we consider apart from others those modes of propagation, as fission and gemmation, which differ more apparently than really from the ordinary process typified in the formation of the seed or ovum. In every case alike, a new individual plant or animal is a portion of its parent ; it may be a mere outgrowth or bud, which, if separated, can maintain an independent existence ; it may be not an outgrowth but simply a portion of the parent's structure, which has been naturally or artificially cut off, as in the spontaneous or artificial cleaving of 8 THE EELATIOX OF LIFE TO OTHER FORCES, [chap. ii. a polype ; it may be tlie embryo of a seed or ovum, as in those cases^n wliicli the process of multiplication of different organs has reached the point of separation of the individual more or less completely into two sexes, the mutual conjugation of a portion of each of which, the sperm-ceU and the germ-ceU, is necessary for the production of a new being. We are so accustomed to regard the conjugation of the two sexes as necessary for what is "called generation, that we are apt to forget that it is only gradually in the upward progress of development of the vege- table and animal kingdoms, that those portions of organised matter which are to produce new beings are allotted to two separate individuals. In the least developed forms of life, almost any part of the body is capable of assuming the characters of a separate individual ; and propagation, therefore, occurs by fission or gemmation in some form or other. Then, in beings a little higher in rank, only a special part of the body can become a separate being, and only by conjugation with another special part. StiU, there is but one parent ; and this hermaphrodite- form of generation is the rule in the vegetable and least developed portion of the animal kingdom. At last, in all animals but the lowest, and in some plants, the portions of organised structure specialised for development after their mutual union into a new individual, are found on two distinct beings, which we call respectively male and female. The old idea concerning the power of growth resident in the germ of the new being, thus formed in various ways, was ex- pressed by saying that a store of dormant vitality was laid up in it, and that so long as no decomposition ensued, this was capable of manifesting itself and becoming active under the influence of certain external conditions. Thus, the dormant force supposed to be present in the seed or the egg was as- sumed to be the primary agent in effecting development and growth, and to continue in action during the whole term of life of the living being, animal or vegetable, in which it was said to reside. The influence of external forces— heat, light, and others— was noticed and appreciated; but these were thought to have no other connection with vital force than that in some CHAP. II.] THE RELATION OF LIFE TO OTHER FORCES. 9 way or other they called it into action, and that to some extent it was dependent on them for its continuance. They were not supposed to be correlated with it in any other sense than this. Now, however, we are obliged to modify considerably our notions and with them our terms of expression, when describing the origin and birth of a new being. To take, as before, the simplest case — a seed or egg. We must suppose that the heat, which in conjunction with moisture is necessary for the development of those changes which issue in the growth of a new plant or animal, is not simply an agent which so stimulates the dormant vitality in the seed or egg as to make it cause growth, but it is a force, which is itself transformed into chemical and vital power. The embryo in the seed or egg is a part which can transform heat into vital force, this term being a convenient one wherewith to express the power which particular structures possess of growing, developing, and performing other actions which we call vital.^ Of course the embryo can grow only by taking up fresh material and incorpx)rating it with its own structure, and therefore it is ' surrounded in the seed or ovum with matter sufficient for nutri- tion until it can obtain fresh supplies from without. The absorption of this nutrient matter involves an expenditure of force of some kind or other, inasmuch as it implies the raising of simple to more complicated forms. Hence the necessity for heat or some other power before the embryo can exhibit any sign of life. It would be quite as impossible for the germ to begin life without external force as without a supply of nutrient matter. Without the' force wherewith to take it, the matter would be useless. The heat, therefore, which in conjunction with moisture is necessary for the beginning of life, is partly expended as chemical power, which causes certain modifications in the nutrient material surrounding the embryo, e.g., the transforma- tion of starch into sugar in the act of germination ; partly, it is * The term ''vital force" is here employed for the sake of brevity. Whether it is strictly admissible will be discussed hereafter. The general term force is used as synonymous with what is now often termed energy. 10 THE RELATION OF LIFE TO OTHER FORCES, [chap. ii. transformed by the germ itself into vital force, ^'hereby the germ is enabled to take up the nutrient material presented to it, and arrange it in forms characteristic of Ufe. Thus the force is expended, and thus life begins— when a particle of orgamsed matter, which has itself been produced by the agency of hfe, be-ins to transform external force into vital force, or m other words into a power by which it is enabled to grow and develope. This is the true beginning of life. The time of birth is but a particular period in the process of development at which the germ, having arrived at a fit state for a more independent existence, steps forth into the outer world. The term ' dormant vitality,' must be taken to mean simply the existence of organized matter with the capacity of transform- ing heat or other force into vital or growing power, when this force is applied to it under proper conditions. The state of dormant vitality is like that of an emptj- voltaic battery, or a steam-engine in which the fuel is not yet lighted. In the former case no electric current passes, because no chemical action is going on. There is no transformation into electric force, because there is no chemical force to be transformed. Yet, we do not say, in this instance, that there is a store of electricity laid up in a dormant state in the battery; neither do we say that a store of motion is laid up in the steam-engine. And there is as little reason for saying there is a store of vitality in a dormant seed or ovum. Next to the beginning of life, we have to consider how far its continuance by growth and development is dependent on external force and to what extent correlated with it. Mere growth is not a special peculiarity of living beings. A crystal, if placed in a proper solution, will increase in size and preserve its own characteristic outline ; and even if it be injured, the flaw can be in part or wholly repaired. The manner of its growth, however, is very different from that of a living being, and the process as it occurs in the latter wiU be made more evident by a comparison of the two cases. The increase of a crystal takes place simply by the laying of material on the sur- face only, and is unaccompanied by any interstitial change. CHAP. II.] THE EELATION OF LIFE TO OTHER FOECES. II This is, however, but an accidental difference. A much greater one is to be found in the fact that with the growth of a crystal there is no decay at the same time, and proceeding with it side by side. Since there is no life there is no need of death — the one being a condition consequent on the other. During the whole life of a living being, on the other hand, there is unceasing change. At different periods of existence the relation between waste and repair is of course different. In early life the addition is greater than the loss, and so there is growth ; the reconstructed part is better than it was before, and so there is development. In the decline of life, on the contrary, the renewal is less than the destruction, and instead of development there is degeneration. But at no time is there perfect rest or stability. It must not be supposed, therefore, that life consists in the capability of resisting decay. Formerly, when but little or nothing was known about the laws which regulate the existence of living beings, it was reasonable enough to entertain such an idea ; and, indeed, life was thought to be, essentially, a myste- rious power counteracting that tendency to decay which is so evident when life has departed. Now, we know that so far from life preventing decomposition, it is absolutely dependent upon it for all its manifestations. The reason of this is very evident. Apart from the doctrine of correlation of force, it is of course plain that tissues which do work must sooner or later wear out if not constantly supplied with nourishment ; and the need of a continual supply of food, on the one hand, and, on the other, the constant excretion of matter which, having evidently discharged what was required of it, was fit only to be cast out, taught this fact very plainly. But although, to a certain extent, the dependence of vital power on supplies of piatter from without was recognised and appreciated, the true relation between the demand and supply was not until recently thoroughly grasped. The doctrine of the correlation of vital with other forces was not understood. To make this more plain, it will be well to take an instance of transformation of force more commonly known and appre- ciated. In the steam-engine a certain amount of force is 12 THE EELATIO}^ OF LIFE TO OTHER FORCES, [chap. ir. exHbited as motion, and the immediate agent in the productiou of this is steam, which again is the result of a certain expendi- ture of heat. Thus, heat is in this instance said to be trans- formed into motion, or, in other language, one— molecular- mode of motion, heat, is made to express itself by a,nother— mechanical— mode, ordinary movement. But the heat which produced the vapour is itself the product of the combustion of fuel, or, in other words, it is the correlated expression of another forcL— chemical, namely, that affinity of carbon and hydrogen for oxygen which is satisfied in the act of combustion. Again, the production of light and heat by the burning of coal and wood is only the giving out again of that heat and light of the sun which were used in their production. For, as it need scarcely be said, it is only by means of these solar forces that the leaves ■ of plants can decompose carbonic acid, &e., and thereby provide material for the construction of woody tissue. Thus, coal and wood being products of the expenditure of force, must be taken to represent a certain amount of power ; and, according to the law of the correlation of forces, must be capable of yielding, in some shape or other, just so much as was exercised in their for- mation. The amount of force requisite for rending asunder the elements of carbonic acid is exactly that amount which will again be manifested when they clash together again. °The sun, then, really, is the prime agent in the movement of the steam-engine, as it is indeed in the production of nearly aU the power manifested on this globe. In this particular instance, speaking roughly, its light and heat are manifested successively as vital and chemical force in the growth of plants, as heat and light again in the burning fuel, and lastly by the piston and wheels of the engine as motive power. We may use the term transformation of force if we will, or say that throughout the cycle of changes -there is but once force variously manifestmg itself. It matters not, so that we keep clearly in view the notion that all force, so far at least as our present knowledge extends, is but a representative, it may be in the same form or another, of some previous force, and incapable like matter of being created afresh, except by the Creator. Much of our knowledge CHAP. II.] TnE RELATION OF LIFE TO OTHER FOIiCES. I3 on this subject is of course confined to ideas and governed Ly the words with which we are compelled to express them, rather than to actual things or facts ; and probaLly the term force will soon lose the signification which we now attach to it. What is now known, however, about the relation of one force to another, is not sufiicient for the complete destruction of old ideas ; and, therefore, in applying the examples of transformation of physical force to the explanation of vital phenomena, we are compelled still to use a vocabulary which was framed for expressing many notions now obsolete. The dependence of the lowest kind of vital existence on ex- ternal force, and the manner in which this is used as a means whereby life is manifested, have been incidentally referred to more than once when describing the origin of vegetable tissues. The main functions of the vegetable kingdom are construction, and the perpetuation of the race ; and the use which is made of external physical force is more simple than in animals. The transformation indeed which is efi'ected, while much less mysterious than in the latter instance, forms an interesting link between animal and crystalline growth. The decomposition of carbonic acid or ammonia by the leaves of plants may be compared to that of water by a galvanic current. In both cases a force is applied through a special material medium, and the result is a separation of the elements of which each compound is formed. On the return of the elements to their original state of union, there will be the return also in some form or other of the force which was used to separate them. Vegetable growth, moreover, with which we are now specially concerned, resembles somewhat the increase of un- organised matter. The accidental difference of its being in one case superficial, and in the other interstitial, is but little marked in the process as it occurs in the more permanent parts of vege- table tissues. The layers of lignine are in their arrangement nearly as simple as those of a crj^stal, and almost or quite as lifeless. After their deposition, moreover, they undergo no further change than that caused by the addition of fresh matter, and hence they are not instances of that ceaseless waste and 14 THE EELATION OF LIFE TO OTHER FOECES. [chap. ii. repair wliicli have been referred to as so cliaracteristic of the higher forms of living tissue. There is, however, no contra- diction here of the axiom, that where there is life there is constant change. Those parts of a vegetable organism in which active life is going on are subject, like the tissues of animals, to constant destruction and renewal. But, in the more per- manent parts, life ceases with deposition and construction. Addition of fresh matter may occur, and so may decay also of that which is already laid down, but the two processes are not related to each other, and not, as in living parts, inter-dependent. Hence the change is not a vital one. The acquirement in growth, moreover, of a definite shape in the case of a tree, is no more admirable or mysterious than the production of a crystal. That chloride of sodium should naturally assume the form of a cube is as inexplicable as that an acorn should grow into an oak, or an ovum into a man. When we learn the cause in the one case, we shall probably in the other also. There is nothing, therefore, in the products of life's more simple forms that need make us start at the notion of their being the products of only a special transformation of ordinary/ physical force. And we cannot doubt that the growth and development of animals obey the same general laws that govern the formation of plants. The connecting links between them are too numerous for the acceptance of any other supposition. Both kingdoms alike are expressions of vital force, which is itself but a term for a special transformation of ordinary physical force. The mode of the transformation is, indeed, mysterious, but so is that of heat into light, or of either into mechanical motion or chemical affinity. All forms of life are as absolutely dependent on external physical force as a fire is dependent for its continuance on a supply of fuel ; and there is as much reason to be certain that vital force is an expression or representation of the physical forces, especially heat and light, as that these are the correlates of some force or other which has acted or is acting on the substances which, as we say, produce them. In the tissues of plants, as just said, there is but little change. II.] THE EELATION OF LIFE TO OTHER FOECES. 1 5 except such as is produced by additions of fresh matter. That which is once deposited alters but Httle ; or, if the part be transient and easily perishable, the alteration is only or chiefly one produced by the ordinary process of decay. Little or no force is manifested ; or, if it be, it is only the heat of the slow oxidation whereby the structure again returns to inorganic shape. There is no special transformation of force to which the term vital can be applied. With construction the chief end of vegetable existence has been attained, and the tissue formed represents a store of force to be used, but not by the being which laid it up. The labours of the vegetable world are not for itself but for animals. The power laid up by the one is spent by the other. Hence the reason that the constant change, which is so great a character of life, is comparatively but little marked in plants. It is present, but only in living portions of the organism, and in these it is but limited. In a tree the greater part of the tissues may be considered dead ; the only change they suffer is that fresh matter is piled on to them. They are not the seat of any transformation of force, and therefore, although their existence is the result of living action, they do not themselves live. Force is, so to speak, laid up in them, but they do not them- selves spend it. Those portions of a vegetable organism which are doing active vital work — which are using the sun's light and heat, as a means whereby to prepare building material, are, however, the seat of unceasing change. Their existence as living tissue depends upon this fact — upon their capability of perishing and being renewed. And this leads to the answer to the question. What is the cause of the constant change which occurs in the living parts of animals and vegetables, which is so invariable an accom- paniment of life, that we refuse the title of living" to parts not attended by it ? It is because all manifestations of life are exhibitions of power, and as no power can be originated by us ; as, according to the doctrine of correlation of force, all power is but the representative of some previous force in the same or another form, so, for its production, there must be 1 6 THE EELATIOX OF LIFE TO OTHER FOECES. [chap. ii. expenditure and change somewhere or other. For the vital actions of plants the light and heat of the sun are nearly or quite sufficient, and there is no need of expenditure of that store of force which is laid up in themselves ; but with animals the case is different. They cannot directly transform the solar forces into vital power, they must seek it elsewhere. The great use of the vegetable kingdom is therefore to store up power in such a form that it can be used by animals ; that so, when in the bodies of the latter, vegetable organised material returns to an inorganic condition, it may give out force in such a manner that it can be transformed by animal tissues, and manifested variously by them as vital power. Hence, then, we must consider the waste and repair attendant on living growth and development as something more than these words, taken by themselves, imply. The waste is the return to a lower from a higher form of matter ; and, in the faH, force is manifested. This force, when speciaUy transformed by organised tissues, we caU vital. In the repair, force is laid up. The analogy with ordinary transmutations of physical force is perfect. By the expenditure of heat in a particular manner a weight can be raised. By its fall heat is returned. The molecular motion is but the expression in another form of the mechanical. So with life. There is constant renewal and decay, because it is only so that vital activity can take place. The renewal must be something more than replacement, however, as the decay must be more than simple mechanical loss. The idea of life must include both storing up of force, and its transforma- tion in the expenditure. Hence we must be careful not to confound the mere preser- vation of individual form under the circumstances of concurrent waste and repair, with the essential nature of vitality. Life, in its simplest form, has been happily expressed by Mr. Savory as a state of dynamical equHibrium, since one of its most characteristic features is continual decay, yet with mainte- nance of the individual by equally constant repair. Since, then, in the preservation of the equilibrium there is ceaseless change, it is not static equilibrium but dynamical. CHAP, il] the EELATION OF LIFE TO OTHER FOECES. 1 7 Care must be taken, however, not to accept the term in too strict a sense, and not to confound that which is but a neces- sary attendant on life with life itself. For, indeed, strictly, there is no preservation of equilibrium during life. Each vital act is an advance towards death. We are accustomed to make use of the terms growth and development in the sense of pro- gress in one direction, and the words decline and decay with an opposite signification, as if, like the ebb of the tide, there were after maturity a reversal of life's current. But, to use an equally old comparison, life is really a journey always in one direction. It is an ascent, more and more gradual as the summit is ap- proached, so gradual that it is impossible to say when develop- ment ends and decline begins. But the descent is on the other side. There is no perfect equilibrium, no halting, no turning back. The term, therefore, must be used with only a limited signi- fication. There is preservation of the individual, yet, although it may seem a paradox, not of the same individual. A man at one period of his life may retain not a particle of the matter of which formerly he was composed. The preservation of a living being during growth and development is more comparable, indeed, to that of a nation, than of an individual as the term is popularly understood. The elements of which it is made up fulfil a certain work the traditions of which were handed down from their predecessors, and then pass away, leaving the same legacy to those that follow them. The individuality is preserved, but, like all things handed down by tradition, its fashion changes, until at last, perhaps, scarce any likeness to the original can be discovered. Or, as it sometimes happens, the alterations by time are so small that we wonder, not at the change, but the want of it. Yet, in both cases alike, the individuality is preserved, not by the same individual elements throughout, but by a succession of them. Again, concurrent waste and repair do not imply of necessity the existence of life. It is true that living beings are the chief instances of the simultaneous occurrence of these things. But this happens only because the conditions under which the 0 1 8 THE RELATION OF LIFE TO OTHER FORCES. [chap. ii. functions of life are discliarged are tlie principal examples of the necessity for this unceasing and mingled destruction and renewal. They are the chief, but not the only instances of this curious conjunction. A theoretical case will make this plain. Suppose an instance of some permanent structure, say a marble statue. If we imagine it to be placed under some external conditions by which each, particle of its substance should waste and be replaced, yet with maintenance of its original size and shape, we obtain no idea of life. There is waste and renewal, with preservation of the indi- vidual form, but no vitality. And the reason is plain. With the waste of a substance like carbonate of calcium whose attrac- tions are satisfied, there would be no evolution of force ; and even if there were, no structure is present with, the power to transform or manifest anew any power which might be evolved. With the repair, likewise, there would be no storing of force. The part used to make good the loss is not different from that which disappeared. There is therefore neither storing of force, nor its transformation, nor its expenditure ; and therefore there is no life. But real examples of the preservation of an individual sub- stance, under the circumstances of constant loss and renewal, may be found, yet without any semblance in tbem of life. Chemistry, perhaps, affords some of the neatest and best examples of this. One, suggested by Mr. Shepard, seems par- ticularly apposite. It is the case of trioxide of nitrogen (N^ O3) in the preparation of sulpburic acid. The gas from wbich this acid is obtained is sulphurous acid, and the addition of an equivalent of oxygen is all that is required. Sulphurous acid gas, however, cannot take the necessary oxygen directly from the atmosphere, but it can abstract it from trioxide of nitrogen (N^ O^), when the two gases are mingled. The trioxide, accordingly, by continually giving up an equivalent of oxygen to an equivalent of sulphurous acid, causes the formation of sulphuric acid, at the same time that it retains its composition by continually absorbing a fresh quantity of oxygen from the atmosphere. In this instance, then, there is constant waste and repair, yet CHAP. II.] THE EELATION OF LIFE TO OTIIEE FORCES. IQ witliout life. And here an objection cannot be raised, as it miglit be to the preceding example, that both the destruction and repair come from without, and are not dependent on any inherent qualities of the substance with which they have to do. The waste and renewal in the last-named example are strictly depen- dent on the qualities of the chemical compound which is subject to them. It has but to be placed in appropriate conditions, and destruction and repair will continue indefinitely. Force, too, is manifested, but there is nothing present which can transform it into vital shape, and so there is no life. Hence, our notion of the constant decay which, together with repair, takes place throughout life, must be not confined to any simply mechanical act. It must include the idea, as before said, of laying up of force, and its expenditure — its transformation too, in the act of being expended. The growth, then, of an animal or vegetable, implies the expenditure of physical force by organised tissue, as a means whereby fresh matter is added to and incorporated with that already existing. In the case of the plant the force used, trans- formed, and stored up, is almost entirely derived from external sources ; the material used is inorganic. The result is a tissue which is not intended for expenditure by the individual which has accumulated it. The force expended in growth by animals, on the other hand, cannot be obtained directly from without. For them a supply of force is necessary in the shape of food derived directly or indirectly from the vegetable kingdom. Part of this force-containing food is expended as fuel for the production of power; and the latter is used as a means wherewith to elaborate another portion of the food, and incorporate it as animal structure. Unlike vegetable structure, however, animal tissues are the seat of constant change, because their object is not the storing up of power, but its expenditure ; so there must be constant waste ; and if this happen, then for the continuance of life there must be equally constant repair. But, as before said, in early life the repair surpasses the loss, and so tliere is growth. The part, repaired is better than before the loss, and thus there is development. c 2 20 THE EELATION OF LIFE TO OTHER FORCES, [chap. ir. The definite limit which has been imposed on the duration of life has been abeady incidentally referred to. Like birth, growth, and development, it belongs essentially to living beings only. Dead structures and those which have never lived are subject to change and destruction, but decay in them is uncertain in its beginning and continuance. It depends almost entirely on external conditions, and differs altogether from the decline of life. The decline and death of living beings are as definite in their occurrence as growth and development. Like these they may be hastened or stayed, especially in the lower forms of life, by various influences from without ; but the putting off of decline must be the putting off also of so much life ; and, apart from disease, the reverse is true also. A living being starts on its career with a certain amount of work to do — various infinitely in different individuals, but for each well-defined. In the lowest members of both the animal and vegetable creation the progress of life in any given time seems to depend almost entirely on external circumstances ; and at first sight it seems almost as if these lowly formed organisms were but the sport of the surround- ing elements. But it is only so in appearance, not in reality. Each act of their life is so much expended of the time and work allotted to them ; and if, from absence of those surrounding conditions under which alone life is possible, their vitality is stayed for a time, it again proceeds on the renewal of the neces- sary conditions, from that point w^hich it had already attained. The amount of life to be manifested by any given individual is the same, whether it take a day or a year for its expenditure. Life may be of course at any moment interrupted altogether by disease and death. But supposing it, in any individual organ- ism, to run its natural course, it will attain but the same goal, whatever be its rate of movement. Decline and death, therefore, are but the natural terminations of life ; they form part of the conditions on which vital action begins ; they are the end towards which it naturally tends. Death, not by disease or injury, is not so much a violent interruption of the course of life, as the attainment of a distant object which was in view from the commencement. [.] THE RELATION OF LIFE TO OTHER FORCES. 21 In the period of decline, as during growth, life consists in continued manifestations of transformed physical force ; and there is of necessity the same series of changes by which the individual, though bit by bit perishing, yet by constant renewal retains its entity. The difference, as has been more than once said, is in the comparative extent of the loss and reproduction. In decline there is not perfect replacement of that which is lost. Eepair becomes less and less perfect. It does not of necessity happen that there is any decrease of the quantity of material added in the place of that which disappears. But although the quantity may not be lessened, and may indeed absolutely increase, it is not perfect as material for repair, and although there may be no wasting, there is degeneration. No definite period can be assigned as existing between the end of development and the beginning of decline, and chiefly because the two processes go on side by side in difi'erent parts of the same organism. The transition as a whole is therefore too gradual for appreciation. But, after some time, all parts alike share in the tendency to degeneration ; until at length, being no longer able to subdue external force to vital shape, they die ; and the elements of which they are composed simply employ what remnant of power, in the shape of chemical affinity, is still left in them, as a means whereby they may go back to the inorganic world. Of course the same process happens constantly during life ; but in death the place of the departing elements is not taken by others. Here, then, a sharp boundary line is drawn where one kind of action stops and the other begins; where physical force ceases to be manifested except as physical force, and where no further vital transformation takes place, or can in the body ever do so. For the notion of death must include the idea of impossibility of revival, as a distinction from that state of what is called dormant vitality," in which, although there is no life, there is capability of living. Hence the explanation of the difference between the effect of appliance of external force in the two cases. Take, for examples, the fertile but not yet living egg, and the barren or dead one. Every application of force to 22 THE RELATION OF LIFE TO' OTHER FORCES. [chap. ii. the one must excite movement in the direction of development ; the force, if used at all, is transformed by the germ into vital energy, or the power by which it can gather up and elaborate the materials for nutrition by which it is surrounded. Hence its freedom throughout the brooding time from putrefaction. In the other instance, the appliance of force excites only degenera- tion ; if transformed at all, it is only into chemical force, whereby the progress of destruction is hastened ; hence it soon rots. To the one, heat is the signal for development, to the other for decay. By one it is taken up and manifested anew, and in a higher form; to the other it gives the impetus for a still quicker fall. Life, then, does not stand alone. It is but a special manifes- tation of transformed force. But if this be so,'' it may be said — if the resemblance of life to other forces be great, are not the differences still greater ?" At the first glance, the distinctions between living organised tissue and inorganic matter seem so great that the difficulty is in finding a likeness. And there is no doubt that these wide differences in both outward configuration and intimate composi- tion have been mainly the causes of the delay in the recognition of the claims of life to a place among other forces. And reasonably enough. For the notion that a plant or an animal can have any kind of relationship in the discharge of its func- tions to a galvanic battery or a steam engine is sufficiently startling to the most credulous. But so it has been proved to be. Among the distinctions between living and inorganic matter, that which includes differences in structure and proximate chemical composition has been always reckoned a great one. The very terms organic and inorganic were, until quite recently, almost synonymous with those which implied the influence of life and the want of it. The science of chemistry, however, is a great leveller of artificial distinctions ; and many organic sub- stances which, it was supposed, could not be formed without the agency of life are now made directly from inorganic material. The number of organic substances so formed artificially is constantly increasing; and there seems to be no reason for CHAP. II.] THE RELATION- OF LIFE TO OTHER FORCES. 23 doubting that all organic substances, even such as albumin, gelatin, and the like, will be ultimately produced without the intermediation of living structure. The formation of the latter, such an organised structure for instance as a cell or a muscular fibre, is a different thing alto- gether. There is at present no reason for believmg that such wiU ever be formed by artificial means ; and, therefore, among the peculiarities of living force-transforming agents, must be reckoned as a great and essential one, a special intimate structure, apart from mere ultimate or proximate chemical com- position, to which there is no close likeness in any artificial apparatus, even the most complicated. This is the real distinc- tion, as regards composition, between a living tissue and an inorganic machine; namely, the difference between the structural arrangement by which force is transformed and manifested anew. The fact that one agent for transforming force is made of albumen or the like, and another of zinc or iron, is a great distinction, but not so essential or fundamental an one as the difference in mechanical structure and arrangement. In proceeding to consider the difference between what may be called the transformation-products of living tissue, and of an artificial machine, it will be well to take one of the simple cases first— the production of mechanical motion ; and especially because it is so common in both. In one we can trace the transformation. We know, as a fact, that heat produces expansion (steam), and by constructing an apparatus which provides for the application of the expansive power in opposite directions alternately, or by alternating con- traction with expansion, we are able to produce motion so as to subserve an infinite variety of purposes. For the continuance of the motion there must be a constant supply of heat, and therefore of fuel. In the production of mechanical motion by the alternate contractions of muscular fibres we cannot trace the transforma- tion of force at all. We know that the constant supply of force is as necessary in this instance as in the other ; and that the food which an animal absorbs is as necessary as the fuel in the 24 THE EELATIOX OF LIFE TO OTHER FORCES, [chap. ir. former case, and is analogous with it in function. In what exact relation, however, the latent force in the food stands to the movement in the fibre, we are at present quite ignorant. That in some way or other, however, the transformation occurs, we may feel quite certain. There is another distinction between the two exhibitions of force which must be noticed. It has been universally believed, almost up to the present time, that in the production of living force the result is obtained by an exactly corresponding waste of the tissue which produces it ; that, for instance, the power of each contraction of a muscle is the exact equivalent of the force produced by the more or less complete descent of so much muscular substance to inorganic, or less complex organic shape ; in other words, — that the immediate fuel which an animal re- quires for the production of force is derived from its own sub- stance ; and that the food taken must first be appropriated by, and enter into the very formation of living tissue before its latent force can be transformed and manifested as vital power. And here, it might be said, is a great distinction between a living structure and a simply mechanical arrangement such as that which has been used for comparison ; the fuel which is analogous to the food of a plant or animal does not, as in the case of the latter, first form part of the machine which transforms its latent energy into another variety of power. We are not, at present, in a position to deny that this is a real and great distinction between the two cases ; but modern investigations in more than one direction lead to the belief that we must hesitate before allowing such a difference to be an universal or essential one. The experiments referred to seem conclusive in regard to the production of muscular power in greater amount than can be accounted for by the products of muscular waste excreted ; and it may be said with justice, that there is no intrinsic improbability in the supposed occurrence of transformation of force, apart from equivalent nutrition and sub- sequent destruction of the transforming agent. Argument from analogy, indeed, would be in favour of the more recent theory as the likelier of the two. CHAP. II.] THE RELATION OF LIFE TO OTIIEE FOECES. 2$ Whatever may be the result of investig-ations concerning the relation of waste of living tissue to the production of power, there can be no doubt, of course, that the changes in any part which is the seat of vital action must be considerable, not only from what may be called wear and tear,'' but, also, on account of the great instability of all organised structures. Between such waste as this, however, and that of an inorganic machine there is only the difference in degree, arising necessarily from diversity of structure, of elemental arrangement, and so forth. But the repair in the two cases is different. The capability of reconstruction in a living body is an inherent quality like that which causes growth in a special shape or to a certain degree. At present we know nothing really of its nature, and we are therefore compelled to express the fact of its existence by such terms as inherent power,'' individual endowment," and the like, and wait for more facts which may ultimately explain it. This special quality is not indeed one of living things alone. The repair of a crystal in definite shape is equally an indi- vidual endowment," or inherent peculiarity," of the nature of which we are equally ignorant. In the case, however, of an inorganic machine there is nothing of the sort, not even as in a crystal. Faults of structure must be repaired by some means entirely from without. And as our notion of a living being, say a horse, would be entirely altered if flaws in his composition were repaired by external means only ; so, in like manner, would our idea of the nature of a steam engine be completely changed had it the power of absorbing and using part of its fuel as matter wherewith to repair any ordinary injury it might sustain. It is this ignorance of the nature of such an act as recon- struction which causes it to be said, with apparent reason, that so long as the term vital force " is used, so long do we beg the question at issue — What is the nature of life ? A little consideration, however, will show that the justice of this criticism depends on the manner in which the word ^Wital " is used. If by it we intend to express an idea of something which arises in a totally different manner from other forces — something which, 26 THE EELATION OF LIFE TO OTHER FOECES. [chap. ii. we know not how, depends on a special innate quality of living beings, and owns no dependence on ordinary physical force, but is simply stimulated by it, and has no correlation with it— then, indeed, it would be just to say that the whole matter is merely shelved if we retain the term vital force/' But if a distinct correlation be recognised between ordinary physical force and that which in various shapes is manifested by living beings ; if it be granted that every act — say, for example, of a brain or muscle— is the exactly correlated expression of a certain quantity of force latent in the food with which an animal is nourished ; and that the force produced either in the shape of thought or movement is but the transformed expression of external force"^ and can no more originate in a Hving organ without suppHes of force from without, than can that organ itself be formed or nourished without supplies of matter ;— if these facts be recognised, then the term used in speaking of the powers exercised by a living being is not of very much consequence. We have as much right to use the term vital" as the words galvanic and chemical. All alike are but the expressions of our ignorance concerning the nature of that power of which aU that we caU forces are various manifestations. The difference is in the apparatus by which the force is transformed. It is with this meaning that, for the present, the term ' vital force ' may still be retained when we wish shortly to name that combination of energies which we caU life. For, exult as we may at the discovery of the transformation of physical force into vital action, we must acknowledge not only that, with the excep- tion of some slight details, we are utterly ignorant of the process by which the transformation is effected ; but, as weU, that the result is in many ways altogether different from that of any other force with which we are acquainted. It is impossible to define in what respects, exactly, vital force differs from any other. For while some of its manifestations are identical with ordinary physical force, others have no paraUel whatsoever. And it is this mixed nature which has hitherto baffled all attempts to define life, and, like a Will-o'- the-wisp, has led us floundering on through one definition > II.] THE EELATION OF LIFE TO OTHER FORCES. 2/ after another only to escape our grasp and show our impotence to seize it. In examining, therefore, the distinctions between the products of transformations by a living and by an inorganic machine, we have first to recognise the fact, that while in some cases the difference is so faint as to be nearly or quite imperceptible, in others there seems not a trace of resemblance to be discovered. In discussing the nature of Hfe's manifestations— birth, growth, development, and decline— the differences which exist between them and other processes more or less resembling them, but not dependent on life, have been already briefly considered and need not be here repeated. It may be well, however, to sum up very shortly the particulars in which life as a manifestation of force differs from all others. The mere acquirement of a certain shape by growth is not a peculiarity of life. But the power of developing into so composite a mass even as a vegetable cell is a property possessed by an organised being only. In the increase of inorganic matter there is no development. The minutest crystal of any given salt has exactly the same shape and intimate structure as the largest. With the growth there is no development. There is increase of size with retention of the original shape, but nothing more. And if we consider the matter a little we shall see a reason for this. In all force-transformers, whether living or inorganic, with but few exceptions— and these are, probably, apparent only — some- thing more is required than homogeneity of structure. There seems to be a need for some mutual dependence of one part or another, some distinction of qualities, which cannot happen when all portions are exactly alike. And here Hes the resemblance between a living being and an artificial machine. Both are developments, and depend for their power of transforming force on that mutual relation of the several parts of their structure which we call organisation. But here, also, lies a great difference. The development of a living being is due to an inherent tendency to assume a certain form ; about which tendency we know abso- lutely nothing. We recognise the fact, and that is all. The de- velopment of an inorganic machine — say an electrical apparatus 2S THE EELATIOX OF LIFE TO OTHER FORCES, [chap. ii. ■ — is not due to any inherent or individual property. It is the result of a power entirely from without ; and we know exactly how to construct it. Here, then, again, we recognise the compound nature of a living being. In structure it is altogether different from a crystal in inherent capacity of growth into definite shape it resembles it. Again, in the fact of its organisation it resembles a machine made by man : in capacity of growth it entirely differs from it. In regard, therefore, to structure, growth, and development, it has combined in itself qualities which in aU other things are more or less completely separated. That modification of ordinary growth and development called • generation, which consists in the natural production and separa- tion of a portion of organised structure, with power itself to transform force so as therewith to build up an organism like the being from which it was thrown off, is another distinctive pecu- liarity of a living being. We know of nothing like it in the inorganic world. And the distinction is the greater because it is the fulfilment of a purpose, towards which life is evidently, from its very beginning, constantly tending. It is as natural a destiny to separate parts which shall form independent beings as it is to develop a limb. Hence it is another instance of that carrying out of certain projects, from the very beginning in view, which is so characteristic of things living and of no other. It is especially in the discharge of what are called the animal functions that we see vital force most strangely manifested. It is true that one of the actions included in this term — namely, mechanical movement— although one of the most striking, is by no means a distinctive one. For it must be remembered that one of the commonest transformations of physical force with which we are acquainted is that of heat into mechanical motion, and that this may be effected by an apparatus having itself nothing whatever to do with life. The peculiarity of the mani- festation in an animal or vegetable is that of the organ by which it is effected, and the manner in which the transformation takes place, not in the ultimate result. The mere fact of an animal's possessing capability of movement is not more wonderful than CHAP. II.] THE EELATION OF LIFE TO OTHER FOECES. 29 tlie possession of a similar property by a steam engine. In both cases alike, the motion is the correlative expression of force latent in the food and fuel respectively; but in one case we can trace the transformation in the arrangement of parts, in the other we cannot. The consideration of the products of the transformation of force effected by the nervous system would lead far beyond the limits of the present chapter. But although the relation of mind to matter is so little known that it is impossible to speak with any freedom concerning such correlative expressions of physical force as thought and other nerve-products, still it cannot be doubted that they are as much the results of transformation of force as the mechanical motion caused by the contraction of a muscle. But here the mystery reaches its climax. We neither know how the change is effected, nor the nature of the product, nor its analogies with other forces. It is therefore better, for the present, to confess our ignorance, than, with the knowledge which we have lately gained, to build up rash theories, serving only to cause that confusion which is worse than error. It may be said, with perfect justice, that even if the foregoing conclusions be accepted, namely, that all manifestations of force by living beings are correlative expressions of ordinary physical force, still the argument is based on the assumption of the existence of the apparatus which we call living organised matter, with power not only to use external force for its own use in growth, development, and other vital manifestations, but for that modification of these powers which consists in the separation of a part that shall grow up into the likeness of its parent, and thus continue the race. We are therefore, it may be added, as far as ever from any explanation of the origin of life. This is of course quite true. The object of the present chapter, however, is only to deal with the now commonly accepted views regarding the relations of life, as it now exists, to other forces. The manner of creation of the various kinds of organised matter, and the source of those its qualities which from our ignorance we call inherent, are different questions altogether. To say that of necessity the power to form living organised matter will never be vouciisafed to us, that it is only a mere 30 CHEMICAL COMrOSITIOX OF THE HUMAX BODY. [chap. iii. materiaHst who would believe in such a possibiHty, seems almost as absurd as the statement that such inquiries lead of necessity to the denial of any higher power than that which in various forms is manifested as force," on this smaU portion of the uni- verse. It is almost as absurd, but not quite. For, surely, he who recognises the doctrine of the mutual convertibility of all forces, vital and physical, who believes in their unity and im- perishableness, should be the last to doubt the existence of an aU-powerful Being, of whose wiH they are but the various correla- tive expressions ; from whom they all come ; to whom they return. CHAPTER III. CHEMICAL COMPOSITION OF THE HUMAN BODY. The following Elementary Substances may be obtained by chemical analysis from the human body: Oxygen, Hydrogen, Nitrogen, Carbon, Sulphur, Phosphorus, Silicon, Chlorine, Fluorine, Potassium, Sodium, Calcium, Magnesium, Iron, and, probably as accidental constituents. Lithium, Manganesium, Aluminium, Copper, and Lead. Thus of the sixty-three or more elements of which all known matter is composed, more than one fourth are present in the human body. The following table represents their relative proportion. — (Marshall). Oxygen 72'0 Carbon .... Hydrogen 9-1 Nitrogen .... . . 2-5 Calcium 1-3 Phosphorus . . . 1-15 Sulphur •1476 Sodium .... . . *i Chlorine •085 Fluorine .... . . -08 Potassium •026 Iron .... . . *oi Magnesium . •0012 Silicon .... , . '0002 lOO* :.] CHEMICAL COMrOSlTION OF THE HUMAN BODY. 3 1 Only three elements, and in very minute amount, are present in the body uncombined with others. These are Oxygen, Nitro- gen, and Hydrogen : oxygen in small amount, in the blood, the greater part, however, of this gas being chemically combined with heemoglobin (see Blood) ; nitrogen, in the blood, and other fluids of the body ; and hydrogen as well as oxygen and nitrogen, in the intestinal canal. The same elements exist, of course, abundantly in various states of combination. The compounds formed by union of the elements in various proportions are termed proximate principles ; while the latter are classified as the organic and the inorganic proximate principles. The term organic was once appUed exclusively to those substances which were thou2:ht to be beyond the compass of synthetical chemistry and to be formed only by organised or living beings, animal or vegetable ; these being called organised, inasmuch as they are characterised by the possession of different parts called organs. But with advancing knowledge, both dis- tinctions have disappeared ; and while the title of living organism is applied to numbers of living things, having no trace of organs in the strict sense of the term, the term organic has long ceased to be applied to substances formed only by living tissues. In other words, substances, once thought to be formed only by living tissues, are still termed organic, although they can be now made in the laboratory : as, for example, urea, oxalic, and tartaric acids, &c. Although a large number of so-called organic compounds have long ceased to be peculiar in being formed only by living tissues, the terms organic and inorganic are still commonly used to- denote distinct classes of chemical substances ; and the classification of the matters of which the human body is composed into organic and inorganic is convenient, and will be here employed. No very accurate distinction can be drawn between organic and inorganic substances, but there are certain peculiarities belonging to the former which may be here briefly noted. 1. Organic compounds are composed of a larger number of Elements than are present in the more common kinds of in- organic matter. Thus, albumin, the most abundant substance of this class, in the more highly organised tissues of animals, is composed of five elements, — carbon, hydrogen, oxj^gen, nitrogen, and sulphur. The most abundant inorganic substance, water, has but two elements, hydrogen and oxygen. 2. Not only are a large number of elements usually combined 32 CHEMICAL COMPOSITION OF THE HUMAN BODY. [chap. iii. in an organic compound, but a large number of atoms of each element are united to form a molecule of the compound. In carbonate of ammonium, an example among inorganic sub- stances, there are one atom of carbon, three of oxygen, two of nitrogen, and eight of hydrogen. But in a molecule of albumin, there are of the same elements, respectively, 72, 22, 1 8, and 1 12 atoms. And, together with this union of large numbers of atoms in an organic compound, it is further observable, that these numbers stand in no simple arithmetical relation one to another, as do the numbers of the atoms of an inorganic compound. With these peculiarities in the chemical composition of organic bodies we may connect two other consequent facts ; first, the large number of different compounds that are formed out of comparatively few elements ; secondly, their great proneness to decomposition. For it is a general rule, that the greater the number of equivalents or atoms of an element that enter into the formation of a molecule of a compound, the less is the stability of that compound. Thus, for example, among the various oxides of lead and other metals, the least stable in com- position are those in which each equivalent has the largest number of equivalents of oxygen. So, water, composed of one equivalent of oxygen and two of hydrogen, is not decomposed by any slight force; but peroxide of hydrogen, which has two equivalents of oxygen to two of hydrogen, is a very unstable compound. The instability, on this ground, belonging to organic com- pounds, is, in those which are most abundant in the highly organised tissues of animals, augmented, ist, by their contain- ing nitrogen, which, among aU the elements, may be called the least decided in its affinities, and that which maintains with least tenacity its combinations with other elements ; and, 2ndly, by the quantity of water which, in their natural state, is com- bined with them, and the presence of which furnishes a most favourable condition for the decomposition of nitrogenous com- pounds. Such, indeed, is the instability of animal compounds, arising from these several peculiarities in their constitution, that, in dead and moist animal matter, no more is requisite for the occurrence of decomposition than the presence of atmospheric air CHAP. III.] NON-MTEOGENOUS ORGANIC PEINCIPLES. 33 and a moderate temperature ; conditions so commonly present, that the decomposition of dead animal bodies appears to be, and is generally called, spontaneous. The modes of such decomposi- tion vary according to the nature of the original compound, the temperature, the access of oxygen, the presence of microscopic organisms, and other circumstances, and constitute the several processes of decay and putrefaction ; in the results of which processes the only general rule seems to be, that the several elements of the original compound finally unite to form those substances, whose composition is, under the circumstances, most stable. The Organic compounds existing in the human body may be arranged in two classes, namely, the Non-Nitrogenous, and the Nitrogenous principles. Non-Nitrogenous Organic Principles, The non-nitrogenous principles are comprised in the following classes : — I. Fats and oils. 2. Amyloids. 3. Certain acids. I. Fats and Oils. — (Olein C^^ H^^^ O^, Stearin C^^ H,,^ O^, Palmitin C^^ H^s ^e)- The chief example of this group found in the body is the oil or fatty matter which, enclosed in minute €ells, forms the essential part of adipose or fatty tissue, (p. 75) and which is present in greater or less amount in many other tissues and fluids. It consists of a mixture of saponifiable fats, — stearin, palmitin, and olein ; the mixture forming a clear yellow -oil, of which difi'erent specimens congeal at from 45 to 35° F. Thus, in the living body, the fat is in a liquid state, the solidity of adipose tissue being due to the microscopic cells in which the liquid oil is contained and the connective tissue, blood-vessels, &c., in the meshes of which the fat cells are held (p. 76). The fatty matter of the body is chiefly derived from the fat in the food, which is absorbed, in a manner to be hereafter considered, from the intestinal canal. But it is also indirectly derived in part from other constituents of the food — both amyloids and albuminous principles, in the course of the chemical changes which they undergo in the system. D 34 CHEMICAL COMPOSITION^ OF THE HUMAN UODY. [ciap.iii. The uses of adipose tissue will be referred to in a later chapter ^^■/rom its never-failing presence wherever active cell-growth is proceeding, it maybe inferred that fatty matter is of great importance in the processes of growth and developmen ; and that it is of not less importance to healthy nutrition at all txme. may be gathered from its wide distribution in the solids and fluids of the body. Like other combustible matters, fat scarcely appears in any of the excretions. Its force-producing properties are utilised within the system by direct and indirect combustion ; and its elements leave the body in the form of carbonic acid and ^Vholestenn (C., H,, O), a non-saponifiable fat which melts at 2Q^° F., and is, therefore, always solid at the natural tempera- tiire of the body, may be obtained in small quantity from b ood, bile and nervous matter. It occurs abundantly in many biliary calculi; the pure white crystalline specimens of these concretions heino- formed of it almost exclusively. Minute rhomboidal scale- like crystals of it are also often found in morbid secretions, as m cysts, the puriform matter of softening and ulcerating tumours, &c. It is soluble in ether and boiling alcohol ; but alkahes d» not cliange it. Cholesterin is generally regarded rather as a P-^-t "emica^^^^^^^ which is destined for rapid excretion, than a proper constituent part ot tne S;!l^^e the saponifiable fats just referred to ; the -rvous tissues hemg cspedally the site of its production, and the hver the organ ^7 -1^;* ^* ^^ seLrated from the blood, and cast into the intestine. Dr Fhnt believes Enother non-saponifiable fat, which he has discovered m th^ Ss Ts bu :tolesterin, somewhat modified in chemical constitution, m it. mssa^e through the bowel. About ten grains are excreted daily. "^X'll^nother non-saponifiable fat, was discovered, as a constituent of faces, by Dr. Marcet. II Amyloids.— Vn&QV this head are included both starch and suo-ar These substances, like the fats, contain carbon, hydrogen, and oxygen ; but the last named element is present in much larger relative amount, the hydrogen and oxygen being m the propor- tion to form water. . The following varieties of these substances are lound m health in the body. CHAP. III.] GLYCOGEN : GLUCOSE : LACTOSE. 35 {a) Ghjcogen (Cg 11^^ 0-). — This substance, wliicli is identical in composition witli starch, and like it, readily converted into sugar by ferments, is found in many embryonic tissues and in all new formations ^Yhere active cell- growth is proceeding. It is present also in the placenta. After birth it is found al- most exclusively in the liver and muscles. Glycogen is formed chiefly from the saccharine matters of the food ; but although its amount is much increased when the diet largely consists of starch and sugar, these are not its only source. It is still formed when the diet is flesh only, b}^ the decomposition, probably, of albumin into glycogen and urea. The destination of glycogen will be considered in a subsequent chapter. (See Liver). (6) Glucose or graije-sugar (C5 Og-f O) is found in minute quantities in the blood and liver, and occasionally in other j)arts of the body. It is derived directly from the starches and sugars in the food, or from the glycogen which has been formed in the body from these or other matters. However formed, it is in health quickly burnt off in the blood by union with oxygen, and thus helps in the maintenance of the body's temperature. Like other amyloids it is one source whence fat is derived. (c\ Lactosej or suga}- of milk {C^^ + is formed in large qu.antity when the mammary glands are in a condition of physiological activity, — human milk containing 5 or 6 per cent, of it. Like other sugars it is a valuable nutritive material, and hence is only discharged from the body when required for the maintenance of the offspring. The same remark is applicable to the other organic nutrient constituents of the milk, albumin and saponifiable fats, which, if we except wdiat is present in the secretions of the generative organs, are discharged from the body only under the same conditions and in the same secretion. [d) Inosite (C5 IIj, 0^-\-2 O), a variety of sugar, identical in composition with glucose, but differing in some of its properties, is found constantly in small amouDt in muscle, and occasionally in other tissues. Its origin and uses, in the economy are, presumably, similar to those of glycogen. III. Non-Nitrogenous Organic Acids. — Very few of these acids I) 2 36 CHEMICAL COMPOSITION OF THE HUMAN BODY. [chap. hi. exist in a free state in the body, and deserve enumeration among the proximate principles; almost all being in a state of combina- tion, as in the case of ordinary fat, which is a chemical com- pound of fatty acids with glycerine. Lactic acid is found in the gastric juice, and in muscle. Formic, acetic, propionic, butyric, and caproic acids occur more or less combined with bases in the sweat. Butyric acid is also found in the contents of the large intestine. Nitrogenous Organic Frinciples. The class of nitrogenous proximate principles embraces a large number of chemical compounds of various constitution and deo-ree of complexity which contain nitrogen in addition to carbon, hydrogen, and oxygen; while some of the most complex contain a minute quantity also of sulphur, and, in one or two instances, of phosphorus. They may be conveniently, though very roughly, classified as follows:— I. Proteids or albuminoids. II. Gelatinous substances. III. Ferments. IV. Colouring matters. V. Various substances which cannot be included under the previous heads. I. Proteids or Albuminoids. Under this head are included albumin, globulin, myosin, fibrin, casein, syntonin. A very large proportion of the nitrogenous matters found in the body is formed by the albuminoids ; one or more of them entering as essential parts into the formation of all living tissues. In the lymph, chyle, and blood, they also exist abundantly. Their atomic formula is not at present known. The following is the per-centage composition of albumin, which may be taken as the type of the group. Carbon 53 5 Hydrogen 7'° Nitrogen '5 5 Oxygen 22-o Sulphur I '6 Phosplioras 4 lOO'O CHAP. III.] ALBUMIN: GLOBULIN: FIBKIK 37 The albuminoids are colloid substances, and, in all the forms in which they habitually occur, are combined with phosphate of calcium, chloride of sodium, and other mineral substances, and a greater or less amount of fatty matter and water. They are derived directly from the albuminous constituents of food, and are probably not formed afresh within the body. The manner in which, as food, they are acted on by th© gastric and other digestive fluids, in order to be made fit for absorption into the blood, will be described in the chapter on Digestion. The greater part or all of the other nitrogenous substances in the body are derived directly or indirectly from the albuminoids, which are the subject of continual chemical change; and from the re- arrangement of some of their elements are also produced, it is now believed, a part of the fatty and amyloid constituents. (a) Alhumin is present abundantly in the blood and lymph^ aaid in most of the tissues of the body. (b) Ololulin and its several modifications, exist largely in the blood and in many of the tissnes. (c) Myosin^ which is closely allied to globulin, is the substance which spontaneously coagulates, after death, in the juice of muscles. (d) Fihrin is readily obtained, in a somewhat impure state, as a soft, yet tough, elastic, and opaque white, stringy substance, by washing a clot of blood in water, until all its colour has disappeared. It is almost identical in composition with albumin, — the only difference being that fibrin contains 1.5 per cent, more oxygen. Mr. A. H. Smee has, indeed, apparently converted albumin into fibrin by exposing a solution to the prolonged influence of oxygen. Fibrin is contained in the blood, and, to a less extent, in lymph and chyle. It does not exist as fibrin in the body, but is formed, in the act of coagula- tion, by the union of two albuminoids termed i es^ectiY elj fibrinojjlastin and fibrinogen (p. 105). The special distinctive character of fibrin is its spon- taneous formation by coagulation in fluids which contain these two substances. (e) Casein, an important constituent of milk, and found also, in minute- amount in the blood and some other parts of the body, is a chemical combi- nation of potash with albumin. It is coagulated by most acids. A familiar example of such coagulation is afforded by cheese, which is- made by precipitating the casein of milk by rennet (calf's stomach). (f) Syntonin, which much resembles fibrin, has been chiefly observed r.s; a product of the action of dilute acids on myosin. (g) Mucin is found in mucus ; also in synovia, the vitreous humour, and the umbilical cord. It differs in composition from albumin in not contain- ing sulphur. CHEMICAL COMPOSITIOX OF TEE HUMAN BODY, [chap hi. II. Gelatinous Substances. Gelatinous substances can be extracted in large quantity irom the connective tissues by boHing in water ; the solution remaining liquid while it is liot, and becoming soHd and jelly-like ou cooling. The composition of gelatin, the type of this group, is as follows : — Carbon 5^"i^ Hydrogen ^'^ Nitrogen 1^-3 Oxygen 24-8 ^ulpiiur fa^ Gelatin can be reaidlv extracted from fibrous connective tissue and from bone bv Ion- boiUng in water. The solution soUdifies, as a jelly, on coolin- to about 60° F., even when only one per cent, of gelatm is present \lthou-h insoluble in cold water, gelatin swells enormously when immersed in it • a^nd a mass of eelatin thus swollen is readily liquefied by heat. (b) ' Clionch'ui is a gelatinous substance obtained from cartilage. It agrees with eelatin in most of its characters. M^Ulastht is obtained, by long boiling, fi-om yellow elastic tissue. It contains no sulphur, and differs in many other respects from both gelatm and chondrin. . (d) Xeratui can be extracted from the epidermis and its appendages- hair, nails, kc. III. Ferments. Certain nitrogenous substances Q^Iledferments are found in the Tarious secretions of the alimentary canal, namely, the saliva, the gastric and the intestinal juice, the pancreatic juice, and the bile. Their function is to act on various articles of food in such a manner as to fit them for solution and absorption into the blood. Thus ptyalin is the ferment of the saliva, by which starchv ingredients of the food are converted into glucose; pepsin- that of the gastric juice, which changes albuminous ingredients into a soluble dialysable substance caUed peptone. These and other ferments will be considered further in the chapter on Digestion, with the anatomy and physiology of the glands by which they are formed. IV. Colouring Matters. Of the colouring matters contained in the human body, that of the blood, hcEmoglohln, is the most important; and, probably, all the others are derived directly or indirectly from it. -CHAiMii.] CEREBKIN: LECITHIN: PROTAGON-. 39 Colouring- matters are also present in the bile and urine ; and in some of the tissues, especially the skin, the eyeball, and other sense-organs. So far as may be necessary, the colouring matters will be further considered with the tissues and fluids in which they are present (see sections treating of Blood, Bile, Urine, etc.). V. Nitrogenous substances not included under the previous heads. There are several substances which, apparently, form an essen- tial part of the tissues in which they are found, but which do not belong to any of the preceding classes. Such are cerehriii and lecithin, and the compound formed by their union, — pro- tagon, which enters largely into the composition of nerve-tissue, - — myelin, cerebric acid, and others. Most of these contain phosphorus, in addition to carbon, hydrogen, nitrogen, and oxygen. Besides — other substances are formed, chiefly by decomposi- tion of nitrogenous materials of the food and of the tissues, which must be reckoned rather as temporary constituents than ossential component parts of the body; although from the con- tinual change, which is a necessary condition of life, they are always to be found in greater or less amount. Examples of these are urea, uric and hippuric acids, creatin, creatinin, leucin, tijrosin, and many others. Such are the chief organic substances of which the human body is composed. It must not be supposed, however, that they «xist naturally in a state approaching that of chemical purity. All the fluids and tissues of the body consist of mixtures of several of these principles, together with saline matters. Thus, a piece of muscular flesh would yield fibrin, albumin, gelatin, fatty matters, salts of sodium, potassium, calcium, magnesium, iron, and other substances, such as creatin, which are products of chemical decomposition, and, though constant, are not essential constituents of the tissue. This mixture of substances may be explained in some measure by the existence of many difi'erent structures or tissues in the muscles ; the gelatin may be referred 40 CHEMICAL COMPOSITION OF THE HUMAN BODY. [chap. m. principally to the connective tissue between the fibres, the fatty matter to the adipose tissue in the same position, and part of the albumin to the blood, and the fluid by which the tissue is kept moist. But, beyond these general statements, little can be said of the mode in which the chemical compounds are united to form an organised structure ; or of how, in any organic body, the several incidental substances are combined with those which are essential. Inorganic Principles. The inorganic proximate principles of the human body are numerous. They are derived, for the most part, directly from food and drink, and pass through the system unaltered. Some are, however, decomposed on their way, as chloride of sodium, of which only four-fifths of the quantity ingested are excreted in the same form; and some are newly formed within the body,— as for example, a part of the sulphates and carbonates, and some of the water. Much of the inorganic saline matter found in the body is a necessary constituent of its structure,— as necessary in its way as albumin or any other organic principle ; another part is important in regulating or modifying various physical processes, as absorp- tion, solution, and the like ; while a part must be reckoned only as matter, which is, so to speak, accidentaUy present, whether derived from the food or the tissues, and which wiU, at the first opportunity, be excreted from the body. The inorganic gaseous matters found in tlie body are oxygen, hydrogen, nitrogen, earhnretted and snlpMretted hydrogen, and earhome acid. The first thr;e have been referred to (p. 31). CarUiretted and sulphuretted hydrogen are found in the intestinal canal. Carbonic acid is present m the blood and other fluids, and is excreted in large quantities by the lungs, and in very minute amount by the skin. It will be specially considered m the chapter on Eespiration. Water the most abundant of the proximate principles, forms a large pio- portion,-more than two-thirds, of the weight of the whole body. Its relative amount in some of the principal solids and fluids of the body is shown in the following table (quoted by Dalton, from Eobm and ^ erdeil s table, compiled from various authors) :— CHAP. 111.] WATEE: SODIUM CIILOPJDE. 41 QUANTITY OF WATER IN lOOO PAET3. Teeth . 100 Bile Bones • . 130 MllK ..... Cartilage . • 550 Pancreatic juice Muscles . . . 750 Urine Ligaments . 768 Lymph . . , , . . . . 789 Gastric juice Blood . 795 Perspiration . . . . . . 805 . 887 900 936 960 975 995 The importance of water as a constituent of the animal body may be assumed from the preceding table, and is shown in a still more striking manner by its withdrawal. If any tissue, — as muscle, cartilage, or tendon be subjected to heat sufficient to drive off the greater part of its water, all its characteristic physical properties are destroyed ; and what was previously soft, elastic, and flexible, becomes hard, and brittle, and horny, so as to be scarcely recognisable. In all the fluids of the body— blood, lymph, &c., water acts the part of a general solvent, and by its means alone circulation of nutrient matter is possible. It is the medium also in which all fluid and solid aliments are dissolved before absorption, as well as the means by which all, except gaseous, excretory products are removed. All the various processes of secretion, transudation, and nutrition, depend of necessity on its presence for their performance. The greater part, by far, of the water present in the body is taken into it as such from without, in the food and drink. A small amount, however, is the result of the chemical union of hydrogen with oxygen in the blood and tissues. The total amount taken into the body every day is about 4J lbs. ; while an uncertain quantity (perhaps J to f lb.) is formed by chemical action within it. — (Dalton). The loss of water from the body is intimately connected with excretion from the lungs, skin, and kidneys, and, to a less extent, from the alimentary canal. The loss from these various organs may be thus apportioned (quoted by Dalton from various observers) : — From the Alimentary Canal (faeces) „ Lungs .... Skin (perspiration) Kidneys (urine) 4 per cent. 20 „ 30 „ 46 „ The chlorides of sodium and potassium are present in nearly all parts of the body. The former seems to be especially necessary, judging from the instinctive craving for it on the part of animals in whose food it is deficient, and from the diseased condition which is consequent on its with- drawal. In the blood, the quantity of chloride of sodium is greater than that of all its other saline ingredients taken together. In the muscles, on the other hand, the quantity of chloride of sodium is less than that of the chloride of potassium. / 42 CHEMICAL COMPOSITION OF THE HUMAN BODY. [chap. in. Flnoride of caloinm, in minute amount, is present in the bones and teeth, and traces have been found in the blood and some other fluids. The pJiosphafes of calcium, potassinm, sodiwn, and magnenum, ^ve iouna in nearly every tissue and fluid. In some tissues-the bones and teeth-tho phosphate of calcium exists in very large amount and the prmeipa source of that hardness of texture, on which the proper performance of then functions so much depends. The phosphate of calcium^ is mtnna ely incor- porated with the organic basis or matrix, but it can be removed by acid without destroying the general shape of the bone ; and, after the removal of its inorganic salts, a bone is left soft, tough, and flexible. . . • The phosphates of potassium and sodium ^dth the carbonates, maintain the tilkalinity of the blood. , , . t h ,. CaMo of calcium occurs in bones and teeth, but m much smaller quantity than the phosphate (pp. 83 and 93). « fonnd also m some other parts The small concretions of the internal ear (otoliths) are composed of crystalline carbonate of calcium, and form the only example of inorganic crystalline matter existing as such in the body. . , , , , , The carlonatcs of potassium and sodium are found m the blood, and some •other fluids and tissues. , The sulphates of potassium, sodium, and caleum are met with m small amount in most of the solids and fluids. _ ■ .^^t^^^a A very minute quantity of silica exists m the urme, and m the blood. Traces of it have been found also in bones, hair, and some other parts. The especial place of iron is in hemoglobin, the colouring-matter of the blood of which a further account will be given with the cheniistry of the blood. Peroxide of iron is found, in very small quantities, m the ashes of bones, muscles, and many tissues, and in lymph and chyle, albumm of serum, fibrin, bile, and other fluids ; and a salt of iron, probably a phosphate, exists in the hair, black pigment, and other deeply coloured epithelial 01- homy substances. Aluminium, Manganese, Copper, and Lead.— It seems most likely that in the human body, copper, manganesiwn, aluminium, and lead are merely accidental elements, which, being taken in minute quantities with the food, and not excreted at once with the feces, are absorbed and deposited in some tissue or organ, of which, however, they form no necessary part. In the same manner, arsenic, being absorbed, may be deposited in the liver and other parts. CHAPTER IV. STRUCTURAL COMPOSITION OF THE HUMAN BODY. By dissection, the human body can be proved to consist of various dissimilar parts, bones, muscles, brain, heart, lungs, intestines, &c., while, on more minute examination, these are found to be composed of various tissues, such as the connective, epithelial, nervous, muscular, and the like. Embryology teaches us that all this complex organisation has been developed from a microscopic body about -^-J-^ in. in diameter (ovum), which consists of a spherical mass of jelly-like matter enclosing a smaller spherical body (germinal vesicle). Further, each individual tissue can be shown largely to consist of bodies essentially similar to an ovum, though often differing from it very widely in external form. They are termed cells : and it must be at once evident that a correct knowledge of the nature and activities of the cell forms the very foundation of physiology. Cells are, in fact, physiological no less than histological units. The prime importance of the cell as an element of structure, was first established by the researches of Schleiden, and his conclusions drawn from the study of vegetable histology, were at once extended by Schwann to the animal kingdom. The earlier observers defined a cell as a more or less spherical body limited by a membrane, and containing a smaller body termed a nucleus, which in its turn encloses one or more nucleoli. Such a definition applies admirably to most vegetable cells, but the more extended investigation of animal tissues soon showed that in many cases no limiting membrane or cell-wall could be demonstrated. Its presence or absence, therefore, was now regarded as quite a secondary matter, while at the same time the cell-substance came gradually to be recognised as of primary importance. Many of the lower forms of animal life, e.g.^ the rhizopoda, were I'ound to consist almost entirely of matter very similar in ap- pearance and chemical composition to the cell-substance of higher forms : and this from its chemical resemblance to flesh was ' ^ STPXCirSAL COMPOSITIOX OF HrMAX BODY. [chap. iv. termed Sarcode bv Dujardin. VTken recognised in vegetable cells it was called Protoplasm by Mulder, while Remak appHed the same name to the substance of animal cells. As the pre- sumed formative matter in animal tissues it was termed Blastema, and in the behef that, wherever found, it alone of aU substances has to do with generation and nutrition. Dr. Beale has named it Germinal mat^ter " or Bioplasm. Of these terms the one most in vogue at the present day is Protoplasm, and inasmuch as all life, both in the animal and vegetable kingdoms, is associated with protoplasm, we are justified in describing it with Huxley, as the ''physical basis of life." A cell may now be defined as a nucleated mass of protoplasm, of microscopic size. It is true that several lower forms of life recently described by Haeckel, Huxley, and others, consist of non-nucleated protoplasm, but the above definition holds good for all the higher plants and animals. Hence a summary of the manifestations of cell-life is really an account of the vital activities of protoplasm. This must of course be- preceded by a brief review of its physical and chemical characters. Chemical characters.— ChemicaHy, protoplasm is an extremely unstable albuminoid substance, insoluble in water, but becoming gelatinous by imbibition : by analysis it cannot be distinguished from ordinary albumen, though of course its power of growth, development, &c., constitute an essential distinction. Physical characters.— PhysicaUy, protoplasm is viscid, varying in consistency from semi-fluid to strongly coherent. All protoplasm, like albumen, undergoes heat stiffening or coagulation at about 130^ Pah., and hence no organism can live Tvhen its own temperature is raised to this point, though, of course, many can exist for a time in an atmosphere much hotter than this, since they possess the means of regulating their own temperature. When it is examined under the microscope two varieties of proto^jlasm are recognised — the hyaline, and the -ranular. Both are alike transparent, but the former is perfectly C . * In the Luman body the ceUs i-ange from the red blood-ceU in.) :o the ganghon-cell in.). CHAP. IV.] PEOTOPLASM. 45 liomogeneous, while the latter (the more common variety) contains small granules or molecules of various sizes and shapes : they usually appear dark when viewed with transmitted light, they seem lighter than water, and many are soluble in ether : these latter consist probably of fatty matter. Physiological characters. — These may be conveniently treated under the three heads of motion, nutrition, and reproduction. I. Motion. — It is probable that the protoplasm of all cells is capable at some time of exhibiting movement : at any rate this phenomenon, which not long ago was regarded as quite a curiosity, has been recently observed in cells of many different kinds. It may be readily studied in the Amoebae, in the colourless blood-cells of all vertebrata, in the branched cornea-cells of the frog, in the hairs of the stinging-nettle and Tradescantia, and the cells of Vallisneria and Chara. These motions may be divided into two classes — Fluent and Ciliary. Another variety — the molecular or vibratory — ^has also been classed by some observers as vital, but it seems exceedingly probable that it is nothing more than the well-known " Brownian " molecular movement, a purely mechanical phenomenon which may be observed in any minute particles, e.g., of gamboge, suspended in a fluid of suitable density, such as water. Such particles are seen to oscillate rapidly to and fro, and not to progress in any definite direction. Fluent. — This movement of protoplasm is rendered per- ceptible (l) by the motion of the granules, which are nearly always imbedded in it, and (2) by changes in the outline of its mass. If part of a hair of Trades- cantia (fig. l) be viewed under a high magnifying power, streams of protoplasm containing' * Fig. I. Cell of Tradescantia drawn at successive intervals of two minutes. The cell-contents consist of a central mass connected by many irregular pro- cesses to a peripheral film : the whole forms a vacuolated mass of protoplasm, which is continually changing its shape (Schofield). 46 STKUCTUilAL COMPOSITION OF HUMAN EODY. [chap. iv. crowds of granules liurrying along, like the foot-passengers in a busy street, are seen flowing steadily in definite directions, some coursing round the film which lines the interior of the cell-wall, and others flowing towards or away from the irregular mass iu the centre of the cell-cavity. Many of these streams of protoplasm run together into larger ones, or are lost in the central mass, and thus ceaseless variations of form are produced. In the Amoeba, a minute animal, consisting of a shapeless and structureless mass of sarcode, an irregular mass of protoplasm is gradually thrust out from the main body and retracted : a second mass is then protruded in another direction, and gradually the Avhole protoplasmic substance is, as it were, drawn into it. The Amoeba thus comes to occupy a new position, and when this is repeated several times we have locomotion in a definite direction, together with a continual change of form. These movements, when observed in other ceUs, such as the colourless blood cor- puscles of higher animals (fig. 2) are hence termed anmboid. Colourless blood W^scles were first observed to migrate, i.e., pass throueh the walls of the bloodvessels, by Waller, whose observations were confirmed and extended to connective tissue corpuscles by the researches of Eecklinshausen, Cohnheim, and others, and thus the phenomenon of migration has been proved to play an important part in many normal, and pathological processes, especially in that of inflammation. This amffiboid movement enables many of the lower animals to capture their prey, which they accomplish by simply flowing round and enclosing it. The remarkable motions of pigment-granules observed in the branched pigment-cells of the frog's skin by Lister, are probabh- due to amffiboid movement. These granules are seen at one time * Fic^ 2 Human colourless hlood-corpusclc, showing its successive changes of outline within ten minutes when kept moist on a warm stage (Schofield>. CTTAP. IV.] rEOTOPLASMIC MOTION. 47 distributed uniformly through the body and branched processes of the cell, while under the action of various stimuli [e.g., light and electricity) they collect in the central mass, leaving the branches quite colourless. Ciliary action must be regarded as only a special variety of the general motion with which all protoplasm is endowed. The grounds for this view are the following : In the case of the Infusoria, which move by the vibration of cilia (microscopic liair-like processes projecting from the surface of their bodies) it lias been proved that these are simply processes of their proto- plasm protruding through pores of the investing membrane, like the oars of a galley, or the head and legs of a tortoise from its shell : certain reagents cause them to be partially retracted. Moreover, in some cases cilia have been observed to develop from, and in others to be transformed into, amoeboid processes. The movements of protoplasm can be very largely modified or even sns^ pended by external conditions, of which the following are the most im- portant. 1 . Changes of temperature. — Moderate heat acts as a stimulant : this is readily observed in the activity of the movements of a human colourless blood-corpuscle when placed under conditions in which its normal tempera- ture and moisture are preserved. Extremes of heat and cold stop the motions entirely. 2. Meclianieal stimnli. — When gently squeezed between a cover and object glass under proper conditions, a colourless blood-corpuscle is stimu- lated to active amoeboid movement. 3. Nerve-influence. — By stimulation of the nerves of the Frog's cornea, contraction of certain of its branched cells has been produced. 4. Chemical stimuli. — Water generally stops amoeboid movement and by imbibition causes great swelling and finally bursting of the cells. In some cases, however, (myxomycetes) protoplasm can be almost entirely dried up and is yet capable of renewing its motions when again moistened. Dilute salt-solution, and many dilute acids and alkalies, stimulate the movements temporarily. Ciliary movement is suspended in an atmosphere of hydrogen or car- bonic acid, and resumed on the admission of air or oxygen. 5. Electrical. — y^es^ currents stimulate the movement, while strong currents cause the corpuscles to assume a spherical form and become motionless. II. Nutrition. — The nutrition of cells will be more appro- priately described in the chapter on Nutrition and Secretion. 48 STEUCTUEAL COMPOSITION OF HUMAN BODY. [chap. iv. Before describing the reproduction of cells it will be necessary to consider more at length their structure. Cell-ivalL — We have seen (p. 43) that the presence of a limiting-membrane is no essential part of the definition of a cell. In nearly all cells the outer layer of the protoplasm attains a firmer consistency than the deeper portions : the individuality of the cell becoming more and more clearly marked as this cortical layer becomes more and more diff'erentiated from the deeper portions of cell- substance. Side by side with this physical, there is a gradual chemical differentiation, till at length, as in the case of the fat cells (p. 75), we have a definite limiting membrane differing chemically as well as physically from the cell-contents, and remaining as a shrivelled-up bladder when they have been removed. Such a membrane is transparent and structureless, flexible, and permeable to fluids. The cell-substance can, therefore, still be nourished by imbibi- tion through the cell-wall. In many cases (especially in fat) a membrane of some toughness is absolutely necessary to give to the tissue the requisite consistency. When these membranes attain a certain degree of thickness and independence they are termed capsules: as examples, we may cite the capsules of cartilage-cells (p. 80), and the thick, tough envelope of the ovum termed the primitive chorion/' Cell-contents. — In accordance with their respective ages, posi- tions, and functions, the contents of cells are very varied. The original protoplasmic substance may undergo many trans- formations ; thus, in fat cells we may have oil, or fatty crystals, occupying nearly the whole cell cavity : in pigment cells we find granules of pigment ; in the various gland cells the elements of their secretions. Moreover, the original protoplasmic contents of the cell may undergo a gradual chemical change with advancing age ; thus the protoplasmic cell-substance of the deeper layers of the epidermis (p. 59) becomes gradually converted into keratin as the cell approaches the surface. So, too, the original proto- plasm of the embryonic blood-cells is replaced by the haemoglobin of the mature-coloured blood corpuscle. Vegetable cells afford excellent examples of similar transfor- CHAP. IV.] NUCLEUS: NUCLEOLUS. 49 mations in accordance with the age of the cell and its functions in the economy of the plant : thus we have starch, sugar, gum, and various acids produced and stored up. So, too, by the deposition of successive layers of lignin on the inner surface of the cell- wall the primitive cavity is oblite- rated and the cell replaced by a laminated woody material. Nucleus. — Nuclei (fig. 7, a) were first pointed out in the year by Dr. llobert Brown, who observed them in vegetable cells. They are either small transparent vesicular bodies con- taining one or more smaller particles (nucleoli), or they are semi-solid masses of protoplasm. In their relation to the life of the cell they are certainly hardly second in importance to the protoplasm itself, and thus Dr. Beale is fully justified in com- prising both under the term germinal matter." They exhibit their vitality, not in amoeboid movements, but by initiating the process of division of the cell into two or more cells (fission) by first themselves dividing. Amoeboid movement has, however, in one case been observed in nucleoli (Kidd) . Histologists have long recognised nuclei by two important characters : — ( I .) Their power of resisting the action of various acids and alkalies, particularly acetic acid, by which their outline is more clearly defined, and they are rendered more easily visible. (2.) Their quality of staining in solutions of carmine, haema- toxylin, &c. Nuclei are most commonly oval or round, and do not generally conform to the diverse shapes of the cells ; they are altogether less variable elements than cells, even in regard to size, of which fact one may see a good example in the uni- formity of the nuclei in cells so multiform as those of epithelium. Their position in the cell is very variable. In many cells, especially where active growth is progressing, two or more nuclei are present. III. Beproduction. The life of individual cells is probably very short in comparison with that of the organism they compose : and their constant decay and death necessitate constant reproduction. The mode in which this takes place has long been the subject of great controversy. 50 STEUCTURAL COMPOSITION OF HUMAN BODY. [chap. iv. In the case of plants, all of whose tissues are either cellular, or composed of cells which are modified or have coalesced in various ways, the theory that all new cells are derived from pre-existing ones was early advanced and very generally accepted. But in the case of animal tissues Schwann and others maintained a theory of spontaneous or free cell formation. According to this view a minute corpuscle (the future nucleolus) springs up spontaneously in a structureless inter- cellular substance (blastema) very much as a crystal is formed in a solution. This nucleolus attracts the surrounding molecules of matter to form the nucleus, and by a repetition of the process the substance and wall of the cell are produced. This theory, once almost universally current, was first disputed and finally overthrown by Eemak and Virchow, whose researches established the truth expressed in the words Omnis cellula e cellula.'^ It will be seen that this view is in strict accordance with the truth established much earlier in Vegetable Histology that every cell is descended from some pre-existing (mother-) cell. This derivation of cells from cells takes place by (l) fission or (2) gemmation. The latter method has not been observed in the human body or the higher animals, and therefore requires but a passing notice. It consists essentially in the budding off and separating of a portion of the parent cell. The former must now be briefly described. As typical examples we may select the ovum, the blood cell, and cartilage cells. In the frog's ovum (in which the process can be most readily observed) after fertilization has taken place, there is first some amoeboid movement, the oscillation gradually increasing until a permanent dimple appears, which gradually extends into a furrow running completely round the spherical ovum, and deepening until the entire yelk-mass is divided into two hemi- spheres of protoplasm each containing a nucleus (fig. 3, b). This process being repeated by the formation of a second furrow at right angles to the first, we have four cells produced (c) : this CHAP. IV.] REPRODUCTION OF CELLS. 51 subdivision is carried on till the ovum has been divided by seg- mentation into a mass of cells (mulberry-mass) (d) out of v^hicli the embryo is developed. Segmentation is the first step in the development of most animals, and doubtless takes place in man. Fig. 3.* Blood-cells. — Multiplication by fission has been observed in the colourless blood-cells of many animals. In some cases (fig. 4), the process has been seen to commence v^ith the nucleolus which divides within the nucleus. The nucleus then elongates, Fig. 4.t and soon a well-marked constriction occurs, rendering it hour- glass shaped, till finally it is separated into two parts, which gradually recede from each other : the same process is repeated in the cell- substance, and at length we have two cells produced which by rapid growth soon attain the size of the parent-cell. In some cases there is a primary fission into three instead of the usual two cells. Cartilage. — In cartilage (fig. 5), a process essentially similar occurs with the exception that (as in the ovum) the cells produced by fission remain in the original capsule, and in their turn undergo division, so that a large number of cells are sometimes observed within a common envelope. This process of fission within a capsule has been by some described as a separate Fig. 3. Diagram of an ovum (a) undergoing segmentation. In (6) it has divided into two ; in (c) into four ; and in (d) the process has ended in the production of the so-called " mulberry mass" (Frey). t Fig. 4. Blood- corpuscle from a young deer embryo, multiplying by fission (Frey). ' E 2 52 STRUCTUEAL COMrOSITIOX OF HUMAN BODY. [chap. iv. metliod, under the title "endogenous fission," but there seems to be no siitfieient reason for drawing such a distinction. It is important to observe that fission is often accomplished -svith great rapidity, the -svhole process occupying but a few minutes, hence the comparative rarity with which cells are seen in the act of dividing. Funrtions of Cells.— The functions of cells are almost infinitely varied and make up nearly the whole of Physiology. They wiU be more appropriately considered In the chapters treating of the several organs and systems of organs which the ceHs compose. Decay and Death of Cells.— There are two chief ways in which the comparatively brief existence of cells is brought to an end. (l) Mechanical abrasion, (2) Chemical transformation. 2Iechanical abrasion.— The various epithelia furnish abundant examples. As it approaches the free surface the cell becomes more and more flattened and scaly in form and more horny in consistency, till at length It is simply rubbed oS as in the epidermis. Hence we find epithelial cells in the mucus of the mouth, intestine and genito-uiinary tract. In the secretion of mucus the epithelial ceUs generaUy discharge their contents (mucin) and then the ceU-membrane is broken iip. Chemical transformation.— In this case the cell-contents undergo * Fic S. Diagram of a cartilage cell undergoing fission withm its capsule. Tlie process of division is represented as commencing in the nucleolus, ex- tendincr to the nucleus, and at length involving the body of the ceU (Frey). CHAl'. IV.] DEGENERATION OF CELLS. S3 a degeneration which, though it may be pathological, is very often a normal process. Thus we have (a) fatty metamorphosis producing oil-globules in the secretion of milk, fatty degeneration of the muscular fibres of the uterus after the birth of the foetus, and of the cells of the Graafian follicle giving rise to the corpus luteum." (^See chapter on Generation.) (b) Pigmentary degeneration from deposit of pigment, as in the epithelium of the air-vesicles of the lungs. (c) Calcareous degeneration which is common in the cells of many cartilages. Having thus reviewed the life-history of cells in general, we may now discuss the leading varieties of form which they present. In passing, it may be well to point out the main distinctions between animal and vegetable cells. It has been already mentioned that in animal cells an envelope or cell- wall is by no means always present. In adult vegetable cells, on the other Fig. 6.* A B hand, a well-defined cellulose wall is highly characteristic : this, it should be observed, is non-nitrogenous, and thus differs chemically as well as structurally from the contained mass. Moreover, in vegetable cells (fig. 6, B), the protoplasmic contents of the cell fall into two subdivisions : (i) a continuous film which lines the interior of the cellulose wall ; and (2) a reticulate mass containing the nucleus and * Fig. 6(a). Young vegetable cells, showing cell-cavity entirely filled with granular protoplasm enclosing a large oval nucleus, with one or more nucleoli. (b.) Older cells from same plant, showing distinct cellulose- wall and vacuo- lation of protoplasm. 54 STEUCTUEAL COMPOSITION OF HUMAN BODY. [chap, iv. occupying the cell-cavity ; its interstices are filled with fluid. In young vegetable cells such a distinction does not exist ; a finely granular proto- plasm occupies the whole cell-cavity (fig. 6, A). Another striking difference is the frequent presence of a large quantity of intercellular substance in animal tissues, while in vegetables it is com- paratively rare, the requisite consistency being given to their tissues by the tough cellulose walls, often thickened by deposits of lignin. In animal cells this end is attained by the deposition of lime-salts in a matrix of inter- cellular substance, as in the process of ossification. Forms of Cells. — Starting with tlie spherical or spheroidal (fig. 7, a) as the typical form assumed by a free cell, we find this altered to a polyhedral shape when the pressure on the cells in all directions is nearly the same (fig. 7, />). Fig. 7.* a, if c d Of this, the primitive segmentation- cells may afford an example. The discoid shape is seen in blood-cells (fig. 7, c), and the scale-like form in superficial epithelial cells (fig. 7, d). In some squamous cells (ridge and furrow cells) the cell- wall becomes, as it were, cogged, the processes of one cell ^iiivi^ into corresponding depressions in an adjoining one, like bristles of two brushes which are pressed together (fig. 11). Cylindrical, conical, or prismatic cells occur in the deeper layers of laminated epithelium, and the simple cylindrical epithelium of the intestine and many gland ducts. Such cells may taper off at one or both ends into fine processes, in the former case being caudate, in the latter fusiform (fig. 8). They may be greatly elongated so as to become fibres. Ciliated cells (figs. 19, 20) must be noticed as a distinct variety : they possess, but only on their free surfaces, hair-like processes (cilia). These vary im- mensely in size, and may even exceed in length the cell itself. * Fig. 7. "Various forms of cells, a. Spheroidal, showing nucleus and i^ucleolus. h. Polyhedral, c. Discoidal (blood cells), d. Scaly or squamous (epithelial cells). CHAP. IV.] CLASSIFICATION OF CELLS. 55 Finally we have the branched or stellate cells, of which the large nerve-cells of the spinal cord, and the connective tissue corpuscle are typical examples (fig. 8, e). In these cells the primitive Flcj. 8.* branches by secondary branching may give rise to an intricate network of processes. Cells may be classified in many ways. According to Form, they may be classified infco spheroidal or polyhedral, discoidal, flat or scaly, cylindrical, caudate, fusiform, ciliated and stellate. According to Situation, we may divide them into blood cells, gland cells, connective tissue cells, &c. According to Contents, into fat and pigment cells and the like. According to Function, into secreting, protective, contractile, &c. According to their Origin into hypoblastic, mesoblastic, and epiblastic cells. (See chapter on Generation.) It remains only to consider the various ways in which cells are connected together to form tissues, and the transformations by which intercellular substance, fibres and tubules are produced. Modes of connection. — Cells are connected : — (l) By a cementing intercellular substance. This is probably always present even between the closely apposed cells of cylin- * Fig. 8. Various forms of cells, a. Cylindrical or columnar, b. Caudate. c. Fusiform, d. Ciliated (from trachea), e. Branched, stellate. 56 STEUCTUEAL COMPOSITION OF HUMAN BODY. [chap. iv. drical epitlielium, while in tlie case of cartilage it forms the main bulk of the tissue, and the cells only appear as imbedded in, not as cemented by, the intercellular substance. Thia intercellular substance may be either homogeneous or fibrillated. In many cases {e.g. the cornea) it can be shown to contain a number of irregular branched cavities, which communicate with each other, and in which the branched cells lie : through these branching spaces nutritive fluids can find their way into the very remotest parts of a non-vascular tissue. As a special variety of intercellular substance must be men- tioned the basement membrane (jnemhrana loropria) which is found at the base of the epithelial cells in most mucous mem- branes, and especially as an investing tunic of gland follicles which determines their shape, and which may persist as a hyaline saccule after the gland-cells have all been discharged. (2) By anastomosis of their processes, This is the usual way in which stellate cells, e.g. of the cornea, are united : the individuality of each cell is thus to a great extent lost by its connection with its neighbours to form a reticulum : as an example of a network so produced, we may cite the stroma of lymphatic glands. Sometimes the branched processes breaking up into a maze of minute fibrils, adjoining cells are connected by an intermediate reticulum : this is the case in the nerve-cells of the spinal cord. Besides the Cell, which may be termed the primary tissue- element, there are materials which may be termed secondary or derived tissue-elements. Such are Intercellular substance, Fibres and Tubules. Intercellular substance is probably in all cases directly derived from the cells themselves. In some cases {e.g. cartilage), by the use of re-agents the cementing intercellular substance is, as it were, analysed into various masses, each arranged in concentric layers around a cell or group of cells, from which it was probably derived (fig. 35). Pihres, — In the case of the crystalline lens, and of muscle CHAP. IV.] riBr.ES: TUBULES. 57 Loth striated and non-striated, eacli fibre is simply a meta- morphosed cell : in the case of striped fibre the elongation being accompanied by a multiplication of the nuclei. The various fibres and fibrilla) of connective tissue result from a gradual transformation of an originally homogeneous inter- cellular substance. Fibres thus formed may undergo great chemical as well as physical transformation : this is notably the case with yellow elastic tissue, in which the sharply defined elastic fibres, possessing great power of resistance to re- agents, contrast strikingly with the homogeneous matter from Y\-hich they are derived. Tubules which were originally supposed to consist of struc- tureless membrane, have now, by the action of nitrate of silver, been proved in many cases to be composed of fiat, thin cells, cohering along their edges. (See Capillaries.) The boundaries between the cells are marked out by the precipitation of oxide of silver under the action of light. In this way the composite structure of blood- and lymph -capillaries has been clearly demonstrated. With these simple materials the various parts of the body are built up ; the more elementary tissues being, so to speak, first compounded of them; while these again are variously mixed and interwoven to form more intricate combinations. Thus are constructed epithelium and its modifications, connec- tive tissue, fat, cartilage, bone, the fibres of muscle and nerve, &c. ; and these, again, with the more simple structures before mentioned, are used as materials wherewith to form arteries, veins, and lymphatics, secreting and vascular glands, lungs, heart, liver and other parts of the body. CHAPTER V. STRUCTURE OF THE ELEMENTARY TISSUES. In this chapter the leading characters and chief modifications of two great groups of tissues — the Epithelial and Connective — will be briefly described; while the Nervous and Muscular, together with several other more highly specialized tissues, will be appropriately considered in the chapters treating of their l^hysiology. Epithelium, Epithelium is composed of cells of various shapes held together by a small quantity of cementing intercellular substance. Epithelium clothes the whole exterior surface of the body, forming the epidermis with its appendages — nails and hairs ; becoming continuous at the chief orifices of the body — nose, mouth, anus, and urethra — with the epithelium which lines the whole length of the alimentary and genito-urinary tracts, together with the ducts of their various glands. Epithelium also lines the cavities of the brain and the central canal of the spinal cord, the serous and synovial membranes, and the interior of all blood-vessels and lymphatics. The cells composing it may be arranged in either one or more layers, so that it may be sub divided into {a) Simple, and {h) Laminated epithelium. A simple epithelium, for example, lines the whole intestinal mucous membrane from the stomach to the anus : the epidermis on the other hand is laminated throughout its entire extent. Epithelial cells may be conveniently classified as : — Squa- mous, scaly, pavement, or tessellated. 2. — Spheroidal, glandular, or polyhedral. 3. — Columnar, cylindrical, conical, or goblet- shaped. 4. — Ciliated. Although, for convenience, epithelial cells are thus classified, yet the first three forms of cells are sometimes met with at different depths in the same membrane. As an example of such CHAP, v.] SQUAMOUS EPITHELIUM. ^59 a latninaled epitlielium showing these different cell-forms at various depths, we may select the anterior epithelium of the cornea (%. 9). Fig. Q. * I. Squamous Epithelium (fig. 10). — Arranged in several superposed layers, this form of epithelium covers the skin, where it is called the Epidermis, and lines the mouth, pharynx, and oesophagus, the conjunctiva covering the eye, the vagina, and entrance of the urethra in both sexes ; while, as a single layer, the same kind of epithelium forms the innermost stratum of the choroid, and lines the interior of the serous and synovial sacs, and of the heart, blood- and lymph-vessels. It consists of cells, which are . flattened and scaly, with an irregular outline : and, when laminated, may form a dense horny investment, as on parts of the palms of the hands and soles of the feet. The nucleus is often not apparent, though it can generally be rendered visible by the use of caustic potash. The really cellular nature of even the dry and shrivelled scales cast off from the surface of the epidermis, can be proved by the application of this re-agent, which causes them rapidly to swell and assume their originally spheroidal form. * Fig. 9. Vertical section of Eahbit's cornea, a. Anterior epithelium, showing the different shapes of the cells at various depths from the free sur- face, h. Portion of the substance of cornea ( Klein ^. t Fig. 10. Epithelium scales from the inside of the mouth, x 260. (Henle. ) Fig. 10. t 6o STRUCTURE OF ELEMENTARY TISSUES. [chap. v. Squamous cells are generally united by an intercellular sub- stance ; but in many of the deeiDer layers of epithelium in the mouth and skin, the outline of F'^g- II-* the cells is very irregular, (fig. Il) and the cells are as it were interlocked — the processes of one cell fitting into depres- sions in the adjoining ones. The ^\^ay in which these ''ridge and furrow," or cogged" cells are held together, has been differently explained by Mr. Martin, who maintains that the interlocking is only apparent, and that the processes meet end to end and are fused together, and that consequently the cells can only be separated by breaking across these processes. Squamous epithelium, e.g., the cells of the choroid, may have a deposit of pigment in the cell-substance. This pigment consists of minute molecules of melanin, imbedded in the cell- substance and almost concealing the nucleus, which is itself trans- parent (fig. 12). In albino rabbits, in which the pigment of the choroid is absent, this layer is found to consist of colourless pavement epithelial cells. The squamous epithelium lining and the interior of blood-vessels, presents so many special features as to demand a special description ; by some histologists it is even called by a distinct name — Endo- thelium. the serous membrt * Fig. II. Jagged cells of tlie middle layers of pavement epithehum, from a vertical section of the gum of a newborn infant (Klein). t Fig. 12. Pigmem-cells from the choroid. A, cells still cohering, seen on their snrface ; a, nucleus indistinctly seen. In the other cells the nucleus is concealed by the pigment gi^annles. B, two cells seen in profile ; a, the outer or posterior part containing scarcely any pigment. x 370. (Henle.) CHAP, v.] ENDOTHELIUM. 6i The main points of distinction above alluded to are, I. tlie very flattened form of these cells ; 2. their constant occurrence in only a single layer ; 3. the fact that they are developed from the mesoblast," while all other epithelial cells are derived from the ^^epiblast," or ^'hypoblast.'' (See chapter on Generation.) Endothelial cells form an important and well-defined subdivision of squamous epithelial cells, which has been especially studied during the last few years. Their examination has been much facilitated by the adoption of the method of staining serous membranes with nitrate of silver. When a small portion of a perfectly fresh serous membrane, as the mesentery or omentum, is immersed for a few minutes in a quarter per cent, solution of this re- agent, washed with water and exposed to the action of light, the silver oxide is precipitated along the boundaries of the cells, and the whole surface is found to be marked out with exquisite delicacy, by fine dark lines, into a number of polygonal spaces (endothelial cells) (fig. 13). Fig. 13.* Endothelium lines all the serous cavities of the body, including the anterior chamber of the eye, also the synovial membranes of joints, and the interior of the heart and of all blood-vessels and lymphatics. It forms also a delicate investing sheath for nerve-fibres and peripheral ganglion- cells. * Fig. 13. Abdominal surface of centrum tendineum of rabbit, showing the general polygonal shape of the endothelial cells : each is nucleated (Klein). X 300. 62 STRITCTUEE OF ELEMENTAEY TISSUES. [chap. v. Endotlielial cells are scaly in form, and irregular in outline ; tliose lining the interior of blood-vessels and lymphatics having a spindle-shape with a very wavy outline. They enclose a clear, oval nucleus, which, when the cell is viewed in profile, is seen to project from its surface. Endothelial cells may be ciliated, e.g., those in the mesentery of frogs, especially about the breeding season. Besides the ordinary endothelial cells above described, there are found on the omentum and parts of the pleura of many animals, little bud-like processes or nodules, consisting of small Fig. 14.* polyhedral granular cells, rounded on their free surface, v^hich multiply very rapidly by division (fig. 1 4). These constitute what is known as germinating endothelium." The process of germination doubtless goes on in health, and the small * Fig. 14. Silver-stained preparation of great omentum of dog, which shows, amongst the flat endothelium of the surface, small and large groups of germinatiDg endothelium, between which numbers of stomata are to be seen (Klein)- x 300. CHAP, v.] ENDOTHELIUM : STOMATA. 63 cells T\'hich are tlirown off in succession are carried into the lymphatics. The buds may be enormously increased both in Fig. 15.* number and size, in certain diseased conditions. (Klein, Burdon- Sanderson.) On those portions of the peritoneum and other serous mem- Fig. 1 6.+ branes where lymphatics abound, there are numerous small orifices — stomota — (fig. 15) between the endothelial cells : these * Fig. 15. Peritoneal surface of septum cisternse lymphaticse magnee of frog. The stomata,. some of which are open, some collapsed, are surrounded by germinating endotheUum (Klein). x 160. t Fig. 16. Section of submaxillary gland of dog. a. Salivary duct, with columnar epithelium, h. Spheroidal or glandular epithelium lining follicle. iKoUiker.) 64 STEUCTURE OF ELEMENTARY TISSUES. [chap. v. are really the open mouths of lymphatic vessels, and through them lymph- corpuscles, and the serous fluid from the serous cavity, pass into the lymphatic system. 2. Spheroidal epithelial cells are the active secreting agents in most secreting glands, and thence are often termed glandular : they are generally more or less rounded in outline : often poly- gonal from mutual pressure. Excellent examples are to be found in the secreting tubes cf the kidney, and in the salivary and peptic glands ^Ag- 1 6). 3. Columnar epithelium (fig. 17, h) lines the mucous mem- brane of the stomach and intestines, ^ from the cardiac orifice of the sto- mach to the anus, and wholly or in part the ducts of the glands open- ing on its free surface ; also many gland-ducts in other regions of the bod}', e.g., mammary, salivary, &c. ; fui^ther, it lines the uterine mucous membrane, and forms the deeper layers of the epithelial lining of the trachea and oviducts. It consists of cells which are approximately cylindrical or pris- matic in form, and contain a large oval nucleus. When evenly packed side by side as a single layer, the ceUs are uni- formly columnar; but when occurring in several layers as m the deeper strata of the epithelial lining of the trachea, their shape is very variable, and often departs very widely from tiie typical columnar form. GoUet-celh.-^l^nj cylindrical epitheHal cells undergo a curious traasformation, and from tlie alteration in their shape are termed goblet-cells (fig. 17, c, and 1 8). These are never seen in a perfectly fresh specimen : but if sucii ' Fic^ 17. Vertical section of a villus of the small intestiue of a eat. a Striated basal border of the epithelium. I. Columnar epithehum. Goble cells d. Central lymph-Yessel. e. Smooth muscular fibre.. /. AdenoM stroma of the villus in which lymph corpuscles lie (Klein). > CHAP. Y.] GOBLET-CELLS : CILIATED EPITHELIUM. 6$ a specimen be watched for some time, little knobs are seen gradually appearing on the free surface of the epithelium, and are finally detached ; these consist of the cell-contents which are discharged by the open mouth of the goblet, leaving the nucleus surrounded by the remains of the protoplasm in its narrow stem. Some regard this transformation as a normal process which is continually going on during life, the discharged cell-contents contributing to form the mucus of the alimentary canal, the cells being supposed in many cases to recover their original shape. Some epithelia possess a structureless layer on their free surface, which may form a definite cuticular membrane : such a layer is present in the intestine, and appearing striated when viewed in section, is termed the striated basilar border'' (fig. 17). 4. Ciliated cells are generally cylindrical (fig. 20), but may be spheroidal or even almost squamous in shape (fig. 19). ^^9' I9.t Ficf, 20. t This form of epithelium lines the whole of the respiratory tract from the larynx to the finest sub-divisions of the bronchi, also the lower parts of the nasal passages, and some portions of the generative apparatus— in the male, lining the vasa efi'erentia'' of the testicle, and their prolongations as far as the lower end of * Fig. 18. Goblet-cells (Klein). + Fig. 19. Spheroidal ciliated cells from the mouth of the frog ; magnified 300 diameters (Sharpey). I Fig. 20. Columnar ciHated epithelium cells from the human nasal membrane ; magnified 300 diameters (Sharpey). F 66 STRTJCXrEE OF ELEMEXTAEY TISSUES. [chap. v. the epididymis; iu the female commencing about the middle of Se ne k of the uterus, and extending throughout the uterus and Fdlopian tubes to their fimbriated extremities, and even for a short distance on the peritoneal sui-face of the latter. The ventricles of the brain and the central canal of the spmal cord are clothed .vith ciliated epitheUum in the child, but m the adult it is limited to the central canal of the cord. The Cma, or fine hair-Uke processes ^vhich give the name to this variety of epithelium, vary a good deal in si.e m different d ses of animals, being very much smaUer in the higher than among the lower orders, in which they sometimes exceed m length the cell itself. , • ^ The number of cilia on any one ceU ranges from ten to thirty, and those attached to the same ceU are often of different lengths. When examined in a portion of living ciUated epithehum im- mersed in some indifferent fluid, they are seen to be m constant rapid motion ; each cilium being fixed at one end, and swmging or lashing to and fro. The general impression given to the eye of the observer is very similar to that produced by waves in a field of corn, or swiftly running and rippUng water, and the result of their movement is to produce a continuous current in a definite direction, and this direction is invariably the same on the same surface, being always, in the case of a cavity, towards its external orifice. In addition to the above kinds of epitheUum, certain highly specialized forms of epithelial ceUs are found in the organs of smell, sight, and hearing, viz., olfactory cells, retinal rods and cones, auditory cells; they wiU be described in the chapters ^vHch deal with their functions. (See Index.) Functions of epithelium. -Accor^g to function, epithelial ceUs may be classified as : — (I.) Protective, e.g., in the skin, mouth, blood-vessels, &c. (2.) Protective and moving — ciliated epithelium. CHAP, v.] FUNCTIONS OF EPITHELIUM. 67 (3.) Secreting — glandular epithelium; or, Secreting formed elements — epithelium of testicle secreting spermatozoa. (4.) Protective and secreting y e.g., epithelium of intestine. (5.) Sensorial, e.g., olfactory cells, rods and cones, organ of Corti. Epithelium forms a continuous smooth investment over the whole body, being thickened into a hard, horny tissue at the points most exposed to pressure, and developing various appen- dages, such as hairs and nails, whose structure and functions will be considered in a future chapter. Epithelium lines also the sensorial surfaces of the eye, ear, nose, and mouth, and thus serves as the medium through which all impressions from the external world — touch, smell, taste, sight, hearing — reach the delicate nerve-endings, whence they are conveyed to the brain. The ciliated epithelium which lines the air-passages serves not only as a protective investment, but also by the movements of its cilia is enabled to propel fluids and minute particles of solid matter so as to aid their expulsion from the body. In the case of the Fallopian tube, this agency doubtless assists the progress of the ovum towards the cavity of the uterus. Of the purposes served by cilia in the ventricles of the brain, nothing is known. (For an account of the nature and conditions of ciliary motion, see chapter on Motion.) The epithelium of the various glands, and of the whole intestinal tract, has the power of secretion, i.e., of chemically transforming certain materials of the blood in the case of mucus and saliva this has been proved to involve the transformation of the epithelial cells themselves ; the cell substance of the epithelial cells of the intestine being discharged by the rupture of their envelopes, as mucus. Epithelium is likewise concerned in the processes of transuda- tion, diffusion, and absorption. It is constantly being shed at the free surface, and reproduced in the deeper layers. The various stages of its growth and de- velopment can be well seen in a section of any laminated epithelium, such as the epidermis. 68 STEUCTUEE OF ELEMENTARY TISSUES. [chap. v. Connective Tissues, This group of tissues forms the skeleton ^vith its various con- nections—bones, cartiLages, ligaments, &c.— and also affords a supporting framework and investment to various organs composed of nervous, muscular, and glandular tissue. Its chief function is the mechanical one of support, and for this purpose it is so intimately interwoven with nearly aU the textures of the body, that if all other tissues could be removed, and the connective tissues left, we should have a wonderfully exact model of almost every organ and tissue in the body, correct even to the smallest minutise of structure. The chief varieties of connective tissue may be conveniently represented in the following tabular view : — Gelatinous Reticular Connective Tissues < r White fibrous. Fibrous ( Yellow elastic. 1 Areolar. Adipose Cartilage Bone Connective tissue consists essentiaUy of cells and intercellular substance. The ceUs are of various shapes, and the intercellular substance may be homogeneous, as in hyahne c^ntilage, fibrillar, as in white fibrous tissue, or calcified, as in bone. These tissue elements combined in different arrangements, give us the above varieties, which will be now considered in order. I. Gelatinous, OT mucoid. This, which is the simplest form of connective tissue, constitutes the chi'ef part of the bodies of jelly-fish; it is found in many parts of the human embryo, but remains in the adult only in the vitreous humour of the eye. It may be best seen in the vitreous humour, the Whartonian jeUy'^ of the umbilical cord, and the enamel organ of developing teeth. It consists of cells, which in the vitreous humour are rounded, CHAP, v.] CONNECTIVE TISSUES: RETIFORM : FIBROUS. 69 tig. 21. in tlie jelly of the umbilical cord and in the enamel organ are stellate, imbedded in a soft semi-diffluent jelly-like intercellular substance which forms the bulk of the tissue, and which contains a considerable quan- tity of mucin (fig. 21). In the umbilical cord, that part of the jelly immediately surrounding the stellate cells shows marks of obscure fibril- lation. 2. Retiform. This is a special variety of connective tissue, consist- ing of a very delicate net- work of minute fibrils, formed by the union of processes of branched connective-tissue corpuscles (fig. 22). It composes the stroma of the spleen and lymphatic glands. A very delicate variety of connective tissue, allied to the retiform, and sometimes termed neu- roglia, forms the supporting tissue in the brain, spinal cord, and retina. 3. Fibrous tissue forms the periosteum and perichond- rium, the aponeuroses, fasciae, ligaments and tendons, the stroma of serous and mucous membranes, of the true skin, of the subserous and submu- cous tissues ; it also occurs in the blood- and lymph-vessels and their sheaths, and in the endocardium, in the tunics of the * Fig. 21. Tissue of the jelly of Wharton from umbilical cord, a, con- nective-tissue corpuscles ; h, fasciculi of connective tissue ; c, spherical formative cells (Frey). t Fig. 22. Transverse section of mucous membrane of intestine, a, Lieberkiihn's gland ; c, and retiform tissue (Frey). STEUCTURE OF ELEMENTAEY TISSUES. [chap. v. eye-baU, and as the interstitial connective tissue of most other organs. The elements of fibrous tissue are Cells and Fibres. (j^lls^ — The cells are of two kinds — (a.) Branched cells.— These are fixed cells of a flattened shape, with branched processes, which are often united together to form a network : they can be most readily observed in the cornea in which they are arranged, layer above layer, parallel to the free surface. They lie in spaces, which they accurately fill, and which form by anastomosis a system of branching canals freely com- municating ffig. 23). These branched cells, in certain situa- Fig. 23.* tions, contain a number of pigment-granules, giving them a dark appearance : they form one variety of pigment- cells. Branched pigment-cells of this kind are found in the outer layers of the choroid (fig. 24). In many lower animals, such as the frog, they are found widely distributed, not only in the skin, but also in many internal parts, e.g.y the mesentery and sheaths of blood-vessels. In the web of the frog's foot such pigment cells may be seen, with pigment evenly distributed through the body of the cell and * Eig. 23. Horizontal preparation of cornea of frog ; showing the network of branched cornea corpuscles. The ground-substance is completely colour- less. X400. (Klein.) cii-vr. v.] FIBROUS TISSUE. 7^ its processes ; but under the action of light, electricity, and other stimuli, the pigment-granules become massed in the body of the cell, leaving the processes quite hyaline ; if the stimulus be removed, they will gradually be distributed again aU over the processes. Thus the skin in the frog is sometimes uniformly dusky, and some- times quite light-coloured, with isolated dark spots. In the choroid the pigment- cells absorb stray light. (b.) Amcehoid cells, of an approximately spherical shape : they have a great general resemblance to colourless blood corpuscles (fig. 2), with which some of them are probably identical. They consist of finely granular nucleated protoplasm, and have the property, not only of changing their form, but also of moving about, whence they are termed migratory. They are readily distinguished from the branched connective-tissue corpuscles by their free condition, and the absence of processes. Fibres. — These also are of two kinds — (a.) White fibres. (6.) Yellow elastic fibres. (a.) White Fibres. — These are arranged parallel to each other in wavy bundles of various sizes : such bundles may either have a parallel arrangement (fig. 26, a), or may produce quite a felted texture by their interlacement. The individual fibres composing these fasciculi are homogeneous, unbranched, and of the same diameter throughout. They can readily be isolated by macerating a portion of white fibrous tissue {e.g., sl small piece of tendon) for a short time in lime, or baryta-water, or in a solution of common salt, or permanganate of potash : these reagents possessing the power of dissolving the cementing inter- fibrillar substance (which is nearly allied to syntonin), and thus separating the fibres from each other. (b.) Yelloiv elastic fibres (fig. 26, b) are of all sizes, from exces- * Fig. 24. Ramified pigment-cells, from the tissue of the choroid coat of the eye ; magnified 350 diameters, a, cells with pigment ; b, colourless fusiform cells (Kolliker). 72 STEQCTURE OF ELEMENTARY TISSUES. [chap. v. sively fine fibrils up to fibres of considerable thickness : they are distinguished from white fibres by the following characters: — (l.) Their great power of resist- ance even to the pro- longed action of chemical reagents, e.g., Caustic Soda, Acetic Acid, &c. (2.) Their well-defined outlines. (3.) Their great tendency to branch and form networks by anastomosis. (4.) They very often have a twisted corkscrew-like appearance, and their Fig. 26. t * Fig. 25. Magnified view of areolar tissue (from different parts) treated with acetic acid. The white filaments are no longer seen, and the yellow or elastic fibres with the nuclei come into view. At c, elastic fibres wind round a bundle of white fibres, which, by the eftect of the acid, is swollen out between the turns. Some connective-tissue corpuscles are indistinctly represented in c (Sharpey). . - ^ t + Fig. 26. A. Mature white fibrous tissue of tendon, consistmg mainly of fibres with a few scattered fusiform cells (Strieker). B. Elastic fibres from the ligamenta subflava, magnified about 200 diameters (Sharpey). CHAP, v.] AREOLAR TISSUE. 73 free ends usually curl up. (5.) They are of a yellowish tint, and very elastic. Areolar tissue consists of cells, and white and yellow fibres in vari- ous proportions; its elasticity depending, of course, upon the elastic fibres which it contains. When treated with acetic acid, the fasci- culi of white fibres in areolar tissue swell up and lose their fibrillar appearance, becoming clear and transparent ; while the nuclei and yellow elastic fibres come more plainly into view (fig. 25). White fibrous tissue (fig. 26, a) occurs typically developed in tendons. A tendon consists essentially of bundles of white fibres, with chains of cells among them. In a very young tendon these cells are of a quadrangular shape, and are arranged end to end, forming a chain of cells in the long axis of the tendon (fig. 27): these chains of cells partially ensheath the bundles of fibres. ^^H- 27*. Fi^, 28.t In a mature tendon the cells become branched, and though no longer in such close apposition as before, they remain connected by a network of branched processes: this appearance is well shown in a transverse section of mature tendon (fig. 28). Fig. 27. Caudal tendon of young rat, showing the arrangement, form, and structure of the tendon cells. x 300. (Klein.) + Fig. 28. Transverse section of tendon from a cross section of the tail of a rabbit, x 250. (Klein.) 74 STKUCTURE OF ELEMENTARY TISSUES. [cii.' Yellow Elastic Tissue. , • We have seen that while tendons, fascia, and other inelastic structures consist almost exclusively of elastic fibres are present in greater or less proportion in aU foims of areolar connective tissue which have any appreciah le degree of lasticity If now the proportion of elastic fibres be increased ts to form the bvdl. of the tissue -^-e a. i.portan variety of connective tissue termed " yellow elastic tissue _ This occurs in the ligamentum nuchce of lower animals (not m man) :r true vocal cords, in the ligamenta subflava, m arteries and veins, especially the larger arteries, in the lungs, trachea, and manv other parts of the body. .... eLic tisL as it occurs in the inner coats of arteries is, as a rule no longer divisible into individual fibres, but consists of broad anastomosing elastic bands united so as to form a fenestrated membrane. Development of Fibrous Tissue. _ In the embryo the place of the fibrous tissues is at first occupied by amass of roundish ceUs derived fi:om the "meso- blast " (See chapter on Generation.) These develop either into a network of branched cells, or into groups of fusiform cells (fig. 29). Fig. 29.* These branched and fusiform ceUs alike undergo a process of splitting^jiving^is^^ * T7i. 10 Portion of suTDmucous tissue of gravid uterus of sow a branSfd ok m- or less sprndle-sHaped ; 6. bundles of connective tissue (Klein). CHAl'. Y.] ADIPOSE TISSUE. 75 interlacing networks (areolar tissue), in the otlier in parallel bundles (white fibrous tissue) : the nuclei, surrounded by more or less of the protoplasm of the original cell, remain imbedded among the fibres. In the mature forms of purely fibrous tissue not only the remnants of the cell- substance, but even the nuclei may disappear. The embryonic tissue, from which elastic fibres are developed, is composed of fusiform cells, and a structureless intercellular substance by the gradual fibrillation of which elastic fibres are formed. The fusiform cells dwindle in size and eventually disappear so completely that in mature elastic tissue not a trace of them is to be found : meanwhile the elastic fibres steadily increase in size. 4. Adipose Tissue. In almost all regions of the human body a larger or smaller quantity of adipose or fatty tissue is present ; the chief excep- tions being the subcutaneous tissue of the eyelids, penis, and scrotum, the nymphse and the cavity of the cranium. Adipose tissue is also absent from the substance of many organs, as the lungs, liver, and others. Fatty matter, not in the form of a distinct tissue, is also widely present in the body, as the fat of the liver and brain, of the blood and chyle, &c. Adipose tissue is almost always found seated in areolar tissue, and forms in its meshes little masses of unequal size and irregular shape, to which the term, lobules, is commonly applied. Under the microscope it is found to consist essentially of little vesicles or cells which present dark, sharply de- fined edges when viewed with trans- mitted light : they are about 4- ^-^ or of an inch in diameter, each com- posed of a structureless and colourless membrane or bag, filled with fatty matter, which is liquid during life, but in part solidified after death (fig. 30). A nucleus is always present in some part or * Fig. 30. Ordinary fat-cells of a fat tract in the omentum of a rat (Klein). 76 STRUCTURE OF ELEMENTARY TISSUES. [chap. v. other of the cell-wall, but in the ordinary condition of the cell it is not easily or always visible. This membrane and the nucleus can generally be brought into view by staining the tissue : it can be still more satisfactorily de- monstrated by extracting the con- tents of the fat-cells by ether, when the shrunken, shrivelled membranes remain behind. By mutual pressure, fat- cells come to assume a polyhedral figure (fig. 31). The ultimate cells are held together by capillary blood-vessels (fig. 32) ; while the little clusters thus formed are grouped into small masses, and held so, in most cases, by areolar tissue. Fig, 32. t The oily matter contained in the cells is composed chiefly of the compounds of fatty acids with glycerin, which are named A. Minute flattened fat-lobule, in which a, the terminal artery ; % the primitive * Fig. 31. Adipose tissue. f Fig. 32. Blood-vessels of fat, the vessels only are represented, t., ^^^^ — j ^ - - vein • &, the fat-vesicles of one border of the lobule separately represented^ X 100. B. Plan of the arrangement of the capillaries (c) on the exterior ot the vesicles : more highly magnified (Todd and Bowman). CHAP. Y.'] DEVELOPMENT OF FAT. 77 olein, stearin, and palmitin. It is doubtful whether lympliatics or nerves are supplied to fat, although, both pass through it on their way to other structures. Development of Fat. Fat-cells are developed from connective-tissue corpuscles : in the infra- orbital connective-tissue cells may be found exhibiting every intermediate gradation between an ordinary branched connective-tissue corpuscle and a mature fat-cell. The process of development is as follows : a few small drops of oil make their appearance in the protoplasm : by their confluence a larger drop is produced (fig. 33) : this gradually increases in size at the expense of the original protoplasm of the cell, which becomes correspondingly diminished in quantity till in the mature cell it only forms a thin crescentic film, closely pressed against the cell- wall, and with a nucleus imbedded in its substance (fig. 30). F^g- 33-* Under certain circumstances this process may be reversed and fat-cells may be changed back into connective-tissue corpuscles (Kolliker, Virchow). Among the uses of fat, these seem to be the chief: — 1. It serves as a store of combustible matter which may be re-absorbed into the blood when occasion requires, and being burnt, may help to preserve the heat of the body. 2. That part of the fat which is situate beneath the skin must. * Fig. 33. Branched connective-tissue corpuscles, developing into fat-cells (Klein). 78 STRUCTURE OF ELEMENTARY TISSUES. [chap. v. by its want of conducting power, assist in preventing undue waste of tlie heat of the body by escape from the surface. 3. As a packing material, fat serves very admirably to fill up spaces, to form a soft and yielding yet elastic material wherewith to wrap tender and delicate structures, or form a bed with like qualities on which such structures may lie, unendangered by pressure. As good examples of situations in which fat serves such purposes may be mentioned the palms of the hands, and soles of the feet, and the orbits. 4. In the long bones, fatty tissue, in the form known as marrow, serves to fill up the medullary canal, and to support the small blood-vessels which are distributed from it to the inner part of the substance of the bone. 5. Cartilage.— Csiitilsige or gristle exists in different forms in the human body, and has been classified under two chief heads, namely, temporary and permanent cartilage; the former term being applied to that kind of cartilage which, in the foetus and in young subjects, is destined to be converted into bone. It may also be classified according to its histological characters under three heads, cellular, hyaline, and fibrous, the last being again capable of subdivision into two kinds— elastic or yeUow cartilage, and the so-caUed fibre -cartilage. Elastic cartilage, however,' contains fibres, and fibre -cartilage is more or less elastic ; it will be well, therefore, for distinction's sake, to term those two kinds white fibro-cartilage and yellow fibre -cartilage respectively. The accompanying table represents the classification of the varieties of cartilage : — II. Temporary ... (Either Cellular or Hyaline.) ( A. Cellular (not present in man). 2. Permanent ... B. Hyaline. ^.^^ fi^.o-cartilage. iC. Fibrous . I Yellow fibro-cartilage. AH kinds of cartilage are composed of cells imbedded in a substance called the matrix: and the apparent differences ot structure met with in the various kinds of cartilage are more •CHAP, v.] CARTILAGE: CELLULAE: IIYALmE. 79 due to differences in the cliaracter of the matrix than of the cells. Among the latter, however, there is also considerable diversity of form and size. With the exception of the articular variety, cartilage is invested by a thin, but tough firm fibrous membrane called the j)ericliondTiiim. On the surface of the articular cartilage of the foetus, the perichondrium is represented by a film of epithelium ; but this is gradually worn away up to the margin of the articular surfaces, when by use the parts begin to suffer friction. A. Cellular or parenchymatous cartilage may be readily ob- tained from the external ear of rats, mice, or other small mammals. It is composed almost entirely of cells (hence its name), with little or no matrix. The latter, when present, consists of very fine fibres, which twine about the cells in various directions, and enclose them in a kind of network. The cells are packed very closely together — so much so that it is not easy in all cases to make out the fine fibres often en- circling them. Cellular cartilage is found in the human subject, only in early foetal life, when it constitutes the Chorda dorsalis. (See chapter on Generation.) B. Hyaline cartilage is met with largely in the human body ; — investing the articular ends of bones, and forming the costal cartilages, the nasal and those of the larynx, with the exception of the epiglottis and cornicula laryngis. The cartilages of the trachea and bronchi are also hyaline. Like other cartilages it is composed of cells imbedded in a matrix. The cells which contain a nucleus with nucleoli, are irregular in shape, and generally grouped together in patches (fig. 34). The patches are of various shapes and sizes, and placed at unequal distances apart. They generally appear flattened near the free surface of the mass of cartilage in which they are placed. ''^ Fig. 34. Hyaline cartilage. 80 STRUCTURE OF ELEMENTARY TISSUES. [chap. v. and more or less perpendicular to the surface in the more-deeplj seated portions. The matrix of hyaline cartilage may have a dimly granular appearance like thafc of ground glass, but in man and the higher animals it has no apparent structure. In some cartilages of the frog, however, even when examined in the fresh state, the matrix is seen to be mapped out into polygonal blocks or cell territories, each containing a cell in the centre, and represent- ing what is generally called the capsule of the cartilage cells (fig. 35). Hyaline cartilage in man has really the same struc- ture, which can be demonstrated by the use of certain reagents. If a piece of human hyaline cartilage be macerated for a long time in dilute acid or in hot water 35° — 40° C, the matrix, which previously appeared quite homogeneous, is found to be resolved into a number of concentric lamellae, like the coats of an onion, arranged round each cell or group of cells. It is thus shown to consist of nothing but a number of large systems of capsules which have become fused with one another. The cavities in the matrix in which the cells lie are connected together by a series of branching canals, very much resembling those in the cornea : through these canals fluids may make their way into the depths of the tissue. * I'ig- 35- Fresh cartilage from the Triton (A. Rollett). CHAP. Y.] HYALINE CARTILAGE : riBEO-CAETILAGE. 8 1 In the hyaline cartilage of the ribs, the cells are mostly larger than in the articular variety, and there is a tendency to the development of fibres in the matrix. The costal cartilages also frequently become calcified in old age, as also do some of those of the larynx. Temporary hyaline cartilage closely resembles the ordinary lyaline kind ; the cells, however, are not grouped together after the fashion just described, but are more uniformly distributed throughout the matrix. Articular hyaline cartilage is reckoned among the so-called non-vascular structures, no blood-vessels being supplied directly to its own substance ; it is nourished by those of the bone beneath. When hyaline cartilage is in thicker masses, as in the case of the cartilages of the ribs, a few blood-vessels traverse its substance. The distinction, however, between all so-called vascular and non-vascular parts, is at the best a very artificial one. (See chapter on Nutrition.) Nerves are probably not supplied to any variety of cartilage. C. Fibrous cartilage, as before mentioned, occurs under two chief forms, (a), the yellow, and (h) the white, fibro- cartilage. {a.) Yellow Jihro- cartilage is found in the external ear, in the epiglottis and cornicula laryngis, and in the eyelid. The ceUs are rounded or oval, vrith well-marked nuclei and nucleoli (fig. 36). The matrix in which they are seated is composed almost entirely of fine elastic fibres, which form an intricate interlace- ment about the cells, and in their general characters are allied to the yellow variety of fibrous tissue : a small and variable quantity of hyaline intercellular substance is also usually present. {b,) White fibro-cartilage, which is much more widely dis- tributed throughout the bod}' than the foregoing kind, is com- * Fig. 36. Section of the Epiglottis (Baly). x 380. 82 STRUCTURE OF ELEMENTARY TISSUES. [chap. v. posed, like it, of cells and a matrix ; the latter, however, being^ made' up almost entirely of fibres closely resembling those of white fibrous tissue (fig. 37). In this kind of fibro-cartilage it is not unusual to find a great part of its mass composed almost exclusively of fibres, and deriving the name of cartilage only from the fact that in another portion, continuous with it, cartilage cells may be pretty freely distributed. The different situations in which white fibro-cartilage is formed have given rise to the following classification :— 1. Inter-articular fibro-cartilage, e.g., the semilunar cartilages of the knee-joint. 2. Circumferential or marginal, as on the edges of the aceta- bulum and glenoid cavity. 3. Connecting, e.g., the inter-vertebral fibro-cartilages. 4. Fibro-cartilage is found in the sheaths of tendons and some- times in their substance. In the latter situation, the nodule of fibro-cartilage is called a sesamoid fibro-cartilage, of which a specimen may be found in the tendon of the tibialis posticus, in the sole of the foot, and usually in the neighbouring tendon of the peroneus longus. The uses of cartilage are the following :— in the joints to form smooth surfaces, reducing friction to a minimum, and to- act' as a hufer in shocks ; to bind bones together, yet to aUow a certain degree of movement, as between the vertebrae; to form a firm framework and protection, yet without undue stiffness or weight, as in the pinna, larynx and chest walls ; to deepen joint cavities, as in the acetabulum, yet not so as to restrict the movements of the bones; to be, where such qualities are required, firm, tough, flexible, elastic, and strong. Development of cartilage. It is developed out of an embryonal tissue, consisting of cells with a very smaU quantity of inter- ceUular substance : the cells multiply by fission within the ceU- capsules (fig. 5) ; while the capsule of the parent ceil becomes gradually fused with the surrounding inter cellular substance: a * Fig. 37. White fibro-cartilage. ciiAr. v."| COMPOSITION OF BONE. 83 repetition of this process in the young cells causes a rapid growth of the cartilage by the multiplication of its cellular elements and the corresponding increase in its matrix. Bones and Teeth, Bone is composed of earthy and animal matter in the propor- tion of about 67 per cent, of the former to 33 per cent, of the latter. The earthy matter is composed chiefly of calcium phosphate, but besides there is a small quantity, about 1 1 of the 67 per cent., of carbonate and fluoride of calcium, and magnesium phosphate. The animal matter is resolved into gelatin by boiling. The earthy and animal constituents of bone are so intimately blended and incorporated the one with the other, that it is only by chemical action, as, for instance, by heat in one case, and by the action of acids in another, that they can be separated. Their close union, too, is further shown by the fact that when by acids the earthy matter is dissolved out, or, on the other hand, when the animal part is burnt out, the general shape of the bone is alike preserved. The proportion between these two constituents of bone, varies in different bones in the same individual, and in the same bone at different ages. Thus, the petrous portion of the temporal bone contains about the largest, and the sternum and scapula about the smallest proportion of earthy or inorganic matter : while the comparatively flexible bones of a child contain a much smaller proportion of earthy matter than the relatively brittle bones of the old man. To the naked eye there appear two kinds of structure in different bones, and in different parts of the same bone, namely, the dense or compact, and the cancellous tissue. Thus, in making a longitudinal section of a long bone, as the humerus or femur, the articular extremities are found capped on their surface by a thin shell of compact bone, while their interior is made up of the spongy or cancellous tissue. The shaft, on the other hand, is formed almost entirely of a thick layer of the compact bone, and this surrounds a central canal, the G 2 84 STRUCTURE OF ELEMENTARY TISSUES. [chap. v. medullary cavity— so called from its containing tlie medulla or marrow (p. 78)- In the flat bones, as the parietal tone, or the scapula, one layer of the cancellous structui'e lies bet\yeen two layers of the compact tissue, and in the short and irregular bones, as those of the carpus and tarsus, the cancellous tissue alone fills the interior, while a thin shell of compact bone forms the outside. The spaces in the cancellous tissue are fiUed by a species of marrow, which differs ^ considerably from that of the shaft of the long bones. It is more . fluid, and of a reddish colour, and contains very few fat cells. The surfaces of bones, except the parts covered with articular , cartilage, are clothed by a tough, fibrous membrane, the perios- Fig. 3S.* teum : and it is from the blood-vessels which are distributed first in this membrane, that the bones, especially their more compact tissue, are in great part sup plied with nourishment,— minu te * Yi^ 38. Transverse section of compact tissue (of humenis). Three of the Hrversian canals are seen, with their concentric rings ; also the cor- puscles or lacunee, ^Yiththe canaliculi extending from them across the direction of the lamell^^. The Haversian apertures had got filled with debris m grind- ing down the section, and therefore appear black in the figure, which repre- .sents the object as viewed with transmitted Hght. x 150 (Sharpey). CHAP. V.J STEUCTUIIE OF BONE. 85 branches from the periosteal vessels entering the little foramina on the surface of the bone, and finding their way to the Haversian canals, to be immediately described. The long bones are supplied also by a proper nutrient artery, which, entering at some part of the shaft so as to reach the medullary canal, breaks up into branches for the supply of the marrow, from which again small vessels are distributed to the interior of the bone. Other small blood-vessels pierce the articular extremities for the supply of the cancellous tissue. ^ Notwithstanding the differences of arrangement just men- tioned, the structure of all bone is found, under the microscope to be essentially the same. Examined with a rather high power its substance is found to contain a multitude of little irregular spaces, approximately fusiform in shape, caUed lacunae, with very minute canals or canaliculi, as they are termed, leading from them, and anasto- mosing with similar little prolongations from other lacunae (fig. 38). In very thin layers of bone, no other canals than these may be visible; but on making a transverse sec- tion of the compact tissue, e,g., of a long bone, as the humerus or ulna, the arrangement shown in fig. 38 can be seen. The bone seems mapped out into small circular districts, at or about the centre of each of which is a hole, and around this an appearance as of con- centric l^yeT^—ihe lacmm^^ following the same con-- * Fig. 39 LongitTi^dinal section of hum^i^^di^i^Tsl^^ lacunae, and canaliculi (Rollett). 86 STRUCTURE OF ELEMENTARY TISSUES. [chap. v. centric plan of distribution around the small hole in the centre, with which, indeed, they communicate. On making a longitudinal section, the central holes are found to be simply the cut extremities of smaU canals which run lengthwise through the bone, anastomosing with each other by lateral branches (fig. 39). ^^^^^^ Haversian canals, after the name of the physician, Clopton Havers, who first accurately described them. The Haversian canals, the average diameter of which is of an inch, contain blood-vessels, and by means of them, blood' 'is conveyed to all, even the densest parts of the bonei the minute canaliculi and lacunae absorbing nutrient matter from the Haversian blood-vessels, and conveying it stiU more intimately to the very substance of the bone which they traverse. The blood-vessels enter the Haversian canals both from without, by traversing the small holes which exist on the surface of aU bones beneath the periosteum, and from within by means of Fig. 40-' small channels, which extend from the medullary cavity, or from the cancellous tissue. According to Todd and Bowman, the arteries and veins usually occupy separate canals, and the veins, which are the larger, often present, at irregular intervals, small pouchlike dilatations. The lacwm are occupied by branched cells (bone-cells, or bone-corpuscles), (fig. 40), which very closely resemble the ordinary branched con- nective tissue corpuscles ; each of these little masses of pro- toplasm ministering to the nutrition of the bone immediately * Fig. 40. Bone corpuscles with their processes as seen in a thin section of human bone (Rollett). n STRUCTURE OF BONE. ^7 CIIAl'. V.J JTuid nutrieat matter which occupy the canaliculi. ttt aft of a long bone two distinct sets of lamell. can he ^'^ar« -fundamental Wl. : which are most easily Jeil!;. heneath the ^^^^^^ iZ^o^i::^ S^,r^I'a S= rrmeX la periosteal surfa^ (Xe dtper portions of the bone) they are more or less mter- 'T)l'-^-HaversianlamelHwhichsurroundtheHaversian canals to the number of six to eighteen around each. The ultimate structure of the lanell. appears to be reticular. If a thin film be peeled off the surface of pig, a bone from which the earthy matter has ^ been removed by acid, and exammed with a high power of the microscope, it •wiU be found composed, according to Sharpey, of a finely reticular structure, formed apparently of very slender fibres decussating obliquely, but coalescing at the points of intersection, as if here the fibres were fused rather than woven toge- ther (fig. 41)- . , , „ -v..>^,.,.xv... In many places these reticular lamella are perforated by tapering fibres, resembling m character th ordinary white or rarely the elastic fibrous tissue, which bolt the nlghbouring lamella together, and may be drawn out when the latter are torn asunder (fig. 42)- Development of 6on..-From the point of view of their develop- ment, all bones may be subdivided into two classes. .Fi. 41. Thin layer peeled off from a softened bone. f gje ^1^^^^^^^ (Sharpey). STRUCTUEE OF ELEMENTARY TISSUES. [chap. v. (a.) Those whose form, previous to ossification, is laid dowa in hyaline cartilage, e.g.^ humerus, femur, &c. (b.) Those which are not preformed in cartilage, but are ossified directly in membrane, e.g., the bones forming the vault, of the skull, parietal, frontal, &c. The true process of ossification is really the same in both, onljr in (a) it is preceded by a calcifica- tion of the cartilage matrix. The former method may be considered first. (a.) Ossification in Cartilage, — If a section be taken through a carti- Fig. 42.* Fig. 43.t lage in which calcification is going on (fig. 43), as, e.g., the ex^ tremity of the shaft of a long bone, the cartilage- cells are seen. * Fig. 42. Lamellae torn oJBf from a decalcified human parietal bone at some depth from the surface, a, a lamella, showing reticular fibres • b h darker part, where several lamellae are superposed ; c, perforatincr fibres' Apertures through which perforating fibres had passed, are seen especially iii- the lower part, a, a, of the figure (Allen ^'hompson). t Fig. 43 Longitudinal section of ossifyin- cartilage from the humerus, ot a f^tal sheep. Spiculse of bone are seen extending between the columns, ol cartilage cells, c, cartilage cells, x 140 (Sharpey). OHAr. v.] DEVELOPMENT OF EONE. 89 to be collected into regular columns arranged perpendicular to the plane of calcification, the individual cells being flattened from above downwards. Shooting up into tlie matrix of the cartilage intervening between the columns of cells are seen deh- cate calcified spiculce, the calcareous matter being deposited in small granules from the blood-vessels which are arranged in loops perpendicular to the calcifying surface. As the spiculoe shoot further and further up into the cartilage, most of the cartilage-cells disappear; the larger part of the hyaline matrix becoming replaced by calcareous spiculse, and the process of calcification is thus com- ^^.^ pleted. Between these spiculae are irregular spaces originally occupied by the cartilage-cells, many of which have now become liquefied and dis- appeared. These spaces are further enlarged and rendered more irregular by the absorption of the remains of the cartila- ginous matrix surround- ing the spiculee. These irregular spaces become lined as by an epithelium, with sphe- roidal cells (osteoblasts) y derived partly from the remaining cartilage-cells, but chiefly from ingrowing processes of periosteum (fig. 44). The true process of ossification, as distinct from the preceding * Fig. 44. Transverse section of femur of a human embryo of about eleven weeks old. a, rudimentary Haversian canal in cross section ; b, in longitudinal section ; c, osteoblasts ; d, newly formed osseous substance of a lighter colour ; e, that of greater age ; /, lacuna with their cells ; g, a cell still united to an osteoblast (Frey). 90 STUUCTUEE of elementary tissues. [chap. v. calcification of the cartilage, consists in the gradual deposition, around this layer of osteoblasts, of a lamella of bone. The individual osteoblasts are imbedded in it and, becoming branched, persist as bone-corpuscles. The inner surface of this lamella is lined by a fresh layer of osteoblasts, and a fresh layer of bone is deposited concentric with the first; this process continuing till the large irregular space is reduced to a small Haversian canal (fig. 44). (h.) Ossification in Membrane, — The membrane or periosteum, from which such a bone as the parietal is developed consists of two layers — an external fibrous, and an internal cellular or osteo- {genetic. The external one consists of ordinary connective tissue, being composed largely of fusiform cells and some fibres ; the internal layer consists of rounded cells quite un- distinguishable from the osteoblasts above men- tioned. The process of ossifica- tion in membrane, as seen, e.g.j in the parietal bone (fig. 45), is precisely simi- lar to that which takes place in cartilage, if, as before said, we except the previous calcification; the osteoblasts being doubtless derived from the osteogenetic layer of the periosteum. In all bones ossification commences at one or more points, termed centres of ossification.'' The long bones, e.g., femur, numerus, &c., have at least three such points — one for the ossifi- cation of the shaft or diapliysis, and one for each articular extremity or ejnphysis. Besides these three primary centres which are always present in long bones, various secondary centres may be superadded for the ossification of difierent processes. * Fig. 45. Osteoblasts from the parietal bone of a human embryo, thirteen weeks old. a, bony septa with the cells of the lacunae ; b, layers of osteo- blasts ; c, the latter in transition to bone corpuscles (Gegenbaur). Fig. 45.* CHAP, v.] GIlO^YTII OF EONE. 91 Sucli bones increase in length by tlie advance of tbe process of ossification into tlie cartilage intermediate between the diaphysis and epiphysis. The increase in length indeed is due entirely to growth at the two ends of the shaft. This is proved by inserting two pins into the shaft of a growing bone : after some time their distance apart will be found to be unaltered though the bone has gradually increased in length, the growth having taken place beyond and not between them. If now one pin be placed in the shaft, and the other in an epiphysis of a growing bone, their distance apart will increase as the bone grows in length. Thus it is that if the epiphyses with the intermediate cartilage be removed from a young bone, growth in length is no longer possible. Increase in thickness in the shaft of a long bone, occurs by the deposition of successive layers beneath the periosteum. If a thin metal plate be inserted beneath the periosteum of a growing bone, it will soon be covered by osseous deposit, but if it be put between the fibrous and osteogenetic layers, it will never become enveloped in bone, for all the bone is formed beneath the latter. Side by side with the increase in length and thickness above-mentioned, there goes on a hollowing out of the shaft of long bones by absorption, producing in the mature bone a large cavity— medullary cavity. This cavity in the long bone of the adult is much larger than the cartilaginous mould of the bone in the foetus, and thus it is obvious that not a trace of the original embyronic cartilaginous mould can be present in the adult bone. Other varieties of connective tissue may become ossified, e.g., the tendons in some birds. Functions of hones. — Bones form the framework of the body ; for this they are fitted by their hardness and solidity together with their comparative lightness ; they serve both to protect internal organs in the trunk and skull, and as levers worked by muscles in the limbs ; notwithstanding their hardness they possess a consider- iible degree of elasticity, which often saves them from fractures. Teeth. A tooth is generally described as possessing a crown, neckj and fang ox fangs. 92 STRUCTURE OF ELEMENTARY- TISSUES. [chap. y. The croivn is the portion which projects beyond the level of the gum. The neck is that constricted portion just below the crown which is embraced by the free edges of the gum, and the/an^ includes all below this. On making a longitudinal section through the centre of a tooth (figs. 46, 47), it is found to be principally composed of a Flj. 46.* hard matter, dentine or ivory ; while in the centre this dentine is hollowed out into a cavity resembling in general shape the outline of the tooth, and called the pulp-cavity , from its containing a very vascular and sensitive little mass, composed of connective-tissue, blood-vessels, and nerves, which is called the tooth-jmlp. The blood-vessels and nerves enter the pulp through a small opening at the extremity of the fang. Capping that part of the dentine which projects beyond the level of the gum, is a layer of very hard calcareous matter, the enamel ; while sheathing the portion of dentine which is beneath the level of the gum, is a layer of true bone, called the cement or crust a petrosa. At the neck of the tooth, where the enamel and cement come * Fig. 46. A. Longitudinal section of a human molar tooth ; c, cement ; d, dentine ; e, enamel ; v, pulp cavity (Owen). B. Transverse section. The letters indicate the same as in k. CHAr. v.] STRUCTURE OF TEETH. 93 into contact, eacli is reduced to an exceedingly thin layer. Tlie covering of enamel becomes thicker as we approach the crown, and the cement as we approach the lower end or apex of the fang. Dentine or ivol-y in chemical composition closely resembles bone. It contains, however, rather less animal matter ; the proportion in hundred parts being about twenty-eight animal to seventy-two of earthy. The former, like the animal matter of bone, may be resolved into gelatin by boiling. The earthy matter is made up chiefly of calcium phosphate, with a small portion of the carbonate, and traces of fluoride of calcium and phosphate of magnesium. Under the microscope dentine is seen to be finely channelled by a multitude of delicate tubes, which, by their inner ends, communicate with the pulp-cavity, and by their outer ex- tremities come into contact with the under part of the enamel and cement and sometimes even penetrate them for a greater or less distance (fig. 48). In their course from the pulp-cavity to the surface of the dentine, the minute tubes form gentle and nearly parallel curves and divide and subdivide dichotomously, but without much lessening their calibre until they are approaching their peripheral termination. * Fig. 47. Premolar tooth of cat in situ. Vertical section, i. Enamel with decussating and parallel strice. 2. Dentine with Schreger's lines. 3. Cement. 4. Periosteum of the alveolus. 5. Inferior maxillary bone (Waldeyer). 94 STPcUCTUEE OF ELEMEXTARY TISSUES. [ck From tLeir sides proceed other exceedingly minute secondary canals, which extend into the dentine between the tubules, and anastomose with each other. The tubules of the dentine, the average diameter of which at their inner and largv^r extremity is -4 tVo an inch, contain fine prolongations from the tooth-pulp, which give the dentine a certain faint sensitiveness under ordi- nary circumstances, and without doubt, have to do also with its nutrition. These prolongations from the tooth-pulp are really processes of the dentine-cells or odontoblasts, which are branched cells lining the pulp^cavity ; the relation of these processes to the tubules in which they lie, is precisely similar to that of the pro- cesses of the bone-corpuscles to the canaliculi of bone. The outer portion of the dentine underlying both the cement and enamel, forms a more or less distinct layer termed the granular or inter- globular layer. It is characterised by the presence of a number of minute cell-like cavities, much more closely packed than the lacunae in the cement, and communicating with one another, and with the ends of the dentine-tubes (fig. 48). ^h. 48.* The enamel which is by far the hardest portion of a tooth, is composed, chemically, of the same elements that enter into the composition of dentine and bone. Its animal matter, however, amounts only to about 2 or 3 per cent. It contains a larger * Fig. 48. Section of a portion of the dentine and cement from the middle of the root of an incisor tooth, cf, dental tubuH ramifying and terminating, some of them in the interglobular spaces h and c, which somewhat resemble bone lacunae ; d, inner layer of the cement with numerous closely set canal hcuh ; e, outer layer of cement ; /, lacun£e ; g, canahcuh. x 350. \Koliiker.) CHAP, v.] TEETH: ENAMEL. 95 proportion of inorganic matter, and is harder than any other tissue in the body. Examined under the microscope, enamel is found composed of fine hexagonal fibres (figs. 49, 50) tt>Vo ^^^^ diameter^ Fig. 49.* Fig, 50. t which are set on end on the surface of the dentine, and fit into corresponding depressions in the same. They radiate in such a manner from the dentine that at the top of the tooth they are more or less vertical, while towards the sides they tend to the horizontal direction. Like the dentine- * Fig. 49. Thin section of the enamel and a part of the dentine, a, cuti- cular pellicle of the enamel ; b, enamel fibres, or columns with fissures between them and cross striae ; c, larger cavities in the enamel, communicating with the extremities of some of the tubuli (d). x 350. (Kolliker.) t Fig. 50. Enamel fibres. A, fragments and single fibres of the enamel, isolated by the action of hydrochloric acid. B, surface of a small fragment of enamel, showing the hexagonal ends of the fibres, x 350. (Kolliker.) 96 STPXCTUP.E OF ELEMENTARY TISSUES. [chap. v. tubules, they are not straight, but disposed in Tvavy and parallel curves. The fibres are marked by transverse lines, and are mostly solid, but some of them contain a very minute canal. The enamel itself is coated on the outside by a very thin calcified membrane, sometimes termed the cuticle of the enamel. The crusta petrosa, or, cement, is composed of true bone, and in it are lacunae and canaliculi -which sometimes communicate with the outer finely branched ends of the dentine tubules. Its laminae are as it were bolted together by perforating fibres like those of ordi- nary bone (see fig. 42), but it difi'ers in not possessing Haversian canals. Developnent of teeth. The first step in the development of the teeth, consists in a thickening of the epi- thelium which covers the free border of the jaw, and^in the forma- tion of a shallow groove in the subjacent tissue (primitive dental groove of Goodsir) in which it is contained The deeper layer of this epithelium begins Fi^ 51. Development of the teeth. Vertical transverse sections of upper jaw. i, 2. From a small embryo ; a, dental ridge ; h, younger layers of epirlielium ; c, the deepest ; d, enamel germ ; e, enamel organ ; /, dental £Cerm • a, inner ; and h, outer laver of the growing tooth sac. 3. From an older embryo ; rf, the style of the enamel organ ; i, blood-vessel severed ; k, bony substance. The remaining letters as in i and 2 (Thiersch). CHAP, v.] DEVELOPMENT OF TEETH. 97 to grow down into the substance of the mucous membrane, forming an ingrowing process which is met and indented by an upwardly growing papilla : the papilla, in its growth towards the free surface, indents this epithelial process (fig. 51) more and more till the latter forms, as it were, a cap for the dental papilla (enamel organ), consisting of two layers of cylindrical epithelium, which are in close apposition towards the apex of the papilla, but elsewhere are separated by a mass of loosely arranged stellate cells. The pedicle or stalk of cells by which the enamel organ communicates with the free surface gradually disappears : and the em- bryonic tooth becomes ^^'^ completely enveloped in its dental sac (see fig. 52). A glance at the accompa- nying figures (51 and 52) will render all these points clear. It is to be observed that the papilla and the surrounding dental sac are both well-supplied with blood-vessels, while the enamel-organ, though now quite separated from the epithelium, shows its epi- thelial character by the entire absence of vessels. The papilla gradually becomes moulded into the shape of the crown of the future tooth, while a cap of dentine is slowly deposited on it, increasing in extent by additions to i ts edges, and in thickness b y additions to its interior. * Fig. 52. Yertical transverse section of the dental sac, pulp, &c., of a kitten X 14 (Thiersch.) a, dental papilla or pulp ; ^, the^a? of d nt ne ormed upon the summit ; its covering of enamel ; inner layer of epithe- 1mm of the enamel organ ; e, gelatinous tissue ; /, outer epithelial layer of the enamel organ ; g, inner layer, and h, outer layer of dental sac. H 98 STRUCTURE OF ELEMENTARY TISSUES. [chap. v. The substance of the papilla undergoes -a corresponding de- crease, and its remains finaUy persist as the pulp of the mature tooth. At the same time that layer of the enamel organ which is in immediate contact with the dentinal cap, becomes transformed into enamel by the direct calcification of the long cylindrical epithelial cells of which it was originally composed : the layers of the enamel organ external to this remain as the cuticle above mentioned (sometimes termed Nasmyth's membrane). In this manner the first set of teeth, or the milk-teeth, are formed; and each tooth, by degrees developing, presses at length on the wall of the sac enclosing it, and causing its absorption, is cut, to use a familiar phrase. As the tooth grows upwards the fang is gradually calcified, and the cement is deposited on it from the inner layer of the tooth-sac. The temporary or milk-teeth, have only a very limited term of existence : this is due to the growth of the permanent teeth, wbicli push their way up from beneath, absorbing in their Fig. 53. * YicT C3 Part of the lower jaw of a child of three or four years old, showin°' the relations of the temporary and permanent teeth. The specimen contain! all the milk-teeth of the right side, together with the mcisors of the left • the inner plate of the jaw has been removed, so as to expose the sacs of all the permanent teeth of the right side, except the eighth or wisdom tooth v-hich is not yet formed. The large sac near the ramus of the jaw is that ot the first permanent molar, and above and behind it is the commencing rudi- ment of the second molar. (Quain.) ciiAr. v.] EEUPTION OF THE TEETH. 99 progress the whole of the fang of each milk-tooth, and leaving at length only the crown as a mere shell, whicli is shed to make way for the eruption of the permanent teeth (fig. 53). The temporary teeth are ten in each jaw, mamely, four incisors, two canines, and four molars, and are replaced by ten permanent teeth, each of which is developed from a small sac set by, so to speak, from the sac of the temporary tooth which precedes it, and called the cavity of reserve. The number of permanent teeth is, however, increased to sixteen, by the development of three others on each side of the jaw after much the same fashion as that by which the milk-teeth were themselves formed. The beginning of the development of the permanent teeth of course takes place long before the cutting of those which they are to succeed ; one of the first acts of the newly- formed little dental sac of a milk-tooth being to set aside a portion of itself as the germ of its successor. The following formula shows, at a glance, the comparative arrangement and number of the temporary and permanent teeth ; — Temporary Teeth . j Upper 2 I 4 Lower 2 i 4 i MO. CA. IN. CA. MO. I 2 =^IO = 20 = 10 Permanent Teeth , MO. BI. CA. IN. CA. BI. MO. j Upper 3 2 I 4 I 2 3=16 . . < =32 (Lower 3 2 i 4 i 2 3=16 From this formula it will be seen that the two bicuspid teeth in the adult are the successors of the two molars in the child. They differ from them, however, in some respects, the temporary molars having a stronger likeness to the permanent than to their immediate descendants, the so-called bicuspids. The temporary incisors and canines differ from their successors but little except in their smaller size. The following tables show the average times of eruption of the Temporary and Permanent teeth. In both cases, the eruption of any given tooth of the lower jaw precedes, as a rule, that of the corresponding tooth of the upper. H 2 30 STEUCTUEE OF ELEMENTARY TISSUES. [chap. v. Teuqwrary or JMilh Teeth. The figures indicate in months the age at which each tooth appears. MOLARS. CANINES. INCISORS. CANINES. MOLARS. 24 12 18 9 7 7 9 18 12 24 Permanent Teeth. The age at which each tooth is cut is indicated in this table in years. MOLARS. BICUSPID. CANINES. INCISORS. CANINES. BICUSPID. MOLARS. 17 12 to to 6 25 13 10 9 II to 12 8778 II to 12 9 10 12 17 6 to to 13 25 The times of eruption put down in the above tables are only approximate : the limits of variation being tolerably wide. Some children may cut their first teeth before the age of six months and others not till nearly the twelfth month. In nearly all cases the two central incisors of the lower jaw are cut first; these being succeeded after a short interval by the four incisors of the upper jaw ; next follow the lateral incisors of the lower jaw, and so on as indicated in the table till the completion of the milk dentition at about the age of two years. The milk-teeth usually come through in batches, each period of eruption being succeeded by one of quiescence lasting some- times several months. The milk-teeth are in use from the age of two up to five and a haK years : at about this age the first permanent molars (four in number) make their appearance hehincl the milk molars, and for a short time the child has four permanent and twenty temporary teeth in position at once. It is worthy of note that from the age of five years to the shedding of the first milk-tooth the child has no fewer than forty- eight teeth, twenty milk-teeth and twenty-eight calcified germs of permanent teeth (all in fact except the four wisdom teeth). CHAPTER VI. THE BLOOD. As blood flows from the living body, it is seen to be a thickish heayy fluid, of a bright scarlet colour when it comes from an artery ; deep purple or nearly black when flowing from a vein. Although to the naked eye, however, it seems uniformly tinted, it is found by the microscope to be really a colourless fluid, containing minute coloured cells or corpuscles ; and these cells, which are red, when seen en masse, are the real source of the colour which seems to the naked eye to belong to every part of the blood alike. The colourless fluid portion of the blood is termed liquor sanguinis, or 'plasma ; the coloured cells are termed blood-cells, or blood-corpuscles. The blood is, 'even in very thin layers, opaque, on account of the different refractive powers of the corpuscles and the plasma in which they are suspended ; but it assumes a lake tint, and becomes transparent on the employment of means by which the colouring matter is dissolved out of the corpuscles by the plasma (p. Il6). Its specific gravity at 60° F. is on an average 1055 ; the extremes consistent with health being 1050 and 1059. has a faint alkaline re-action. Its temperature is generally about 100° F. ; but this is not the same in all parts of the body. Thus, while the stream is slightly warmed by passing through the muscles, nerve-centres, and glands, most notably the liver, it is slightly cooled on traversing the capillaries of the skin. (Bernard.) The odour of blood is easily perceived in the watery vapour which rises from blood just drawn : and it may also be set free, afterwards, by adding to the blood a mixture of equal parts of sulphuric acid and water. It is said not to be difficult to tell, by the likeness of the odour to that of the body, the species of domestic animal from which any specimen of blood has been 102 THE BLOOD. [chap. VI. taken. Tlie strong odour of the pig; or cat, and the peculiar milky smell of the cow, are especially easy to be detected. (Barruel.) Quantity of Blood, Only an imperfect indication of the whole quantity of blood in the body is afforded by measurement of that which escapes when an animal is rapidly bled to death, inasmuch as a certain amount always remains in the blood-vessels. In cases of less rapid bleeding, on the other hand, when life is more prolonged, and when, therefore, sufficient time elapses before death to allow some absorption into the circulating current of the fluids of the body (p. 122), the whole quantity of blood that escapes may be greater than the whole average amount naturally present in the vessels. Various means have been devised for obtaining a more accurate estimate than that which results from merely bleeding animals to death. Welcker's method is the following. An animal is rapidly bled to death, and the blood which escapes is collected and measm^ed. The blood remain- ing in the smaller vessels is then removed by the injection of water through them, and the mixture of blood and water thus obtained, is also collected. The animal is then finely minced, and infused in water, and the infusion is mixed with the combined blood and water previously obtained. Some of this fluid is then brushed on a white ground, and the colour compared with that of mixtures of blood and water whose proportions have been previously determined by measurement. In this way the materials are obtained for a fairly exact estimate of the quantity of blood actually existing in the body of the animal experimented on. Another method (that of Vierordt) consists in estimating the amount of blood expelled from the ventricle, at each beat of the heart, and multiplying this quantity by the number of beats necessary for completing the " round" of the circulation. This method is ingenious, but open to various objections, the most conclusive being the uncertainty of all the premisses on which the conclusion is founded. Other methods depend on the results of injecting a known quantity of water (Valentin)- or of saline matters (Blake) into the blood-vessels ; the calculation being founded, in the first case, on the diminution of the specific gravity which ensues, and in the other, on the quantity of the salt found diffused in a certain measured amount of the blood abstracted for experiment. A nearly correct estimate was probably made by Weber and Ijchmann, from the following data. A criminal was weighed before and after decapi- tation ; the difference in the weight representiug, of course, the quantity of blood which escaped. The blood-vessels of the head and trunk were then washed out by the injection of water, until the fluid which escaped had only CHAP. VI.1 COAGULATION^ OF THE BLOOD. IO3 a pale red or straw colour. This fluid was then also weighed ; and the amount of blood which it represented was calculated, by comparing the proportion of solid matter contained in it, with that of the first blood which escaped on decapitation. Two experiments of this kind gave precisely similar results. The most reliable of the various means for estimating the quantity of blood in the body yield as nearly similar results as can be expected, when the sources of error unavoidably present in all, are taken into consideration ; and it may be stated that in man, the weight of the whole quantity of blood, compared with that of the body, is from about i to 8, to i to 10. It must be remembered, however, that the whole quantity of blood varies, even in the same animal, very considerably, in correspondence with the different amounts of food and drink, which may have been recently taken in, and the equally varying quantity of matter given out. Bernard found by experiment, that the quantity of blood obtainable from a fasting animal is scarcely more than a half of that which is present soon after a full meal. The estimate above given must therefore be taken to represent only an approximate average. Coagulation of the Blood, In a very few minutes after removal from the living body, blood becomes semi-solid and jelly-like by the formation through- out its whole substance of what is called the crassamentum or clot. The clot thus formed has at first the same volume and appear- ance as the fluid blood, and, like ifc, looks quite uniform; the only change seems to be, that the blood which was fluid is now solid. But presently, drops of transparent yellowish fluid begin to ooze from the surface of the solid clot ; and these gradually collecting, first on its upper surface, and then all around it, the clot, diminished in size, but firmer than ifc was before, floats in a quantity of yellowish fluid, which is named serum, the quantity of which may continually increase on account of its being gradually squeezed out of the meshes of the clot in the course of its contraction, for from twenty-four to forty-eight hours after coagulation. 104 THE BLOOD. [chap. vt. Blood clot is composed of red corpuscles, held together as a solid mass, in the meshes of a substance termed fibrin ; the latter being formed, at the moment of coagulation, in the liquor sanguinis. A rough analysis of the blood is thus spontaneously made. Blood. ( * ^ Liq. Sanguinis or Plasma. Corpuscles. ( ^ Serum. Fibrin. | V ^ ; Clot (containing also more or less serum). That the fibrin is formed in the plasma may be proved by employing means by which, before coagulation, the plasma and corpuscles are separated, the one from the other. In the case of the blood of animals, as the frog, which have large corpuscles, this separation can be effected simply by filtration ; the colourless liquor sanguinis passing through and spontaneously coagulating as a colourless jelly, while the corpuscles remain on the filter. The same thing can be effected in the blood of mammalia by exposing it to cold of about 32° F. By this means, coagulation is prevented ; and the corpuscles, having now time to subside, leave the clear supernatant plasma, which spontaneously forms a colourless clot as soon as the temperature is allowed to rise. Under ordinary circumstances, however, coagulation occurs before the red corpuscles have had time to subside ; and thus, from their being entangled in the meshes of the fibrin, the clot is of a deep dark red colour throughout, — somewhat darker, it may be, at the most dependent part, from accumulation of red cells, but not to any very marked degree. If, however, from any cause, the red corpuscles sink more quickly than usual, or the fibrin contracts more slowly, then, in either of these cases, the red corpuscles may be observed, while the blood is yet fluid, to sink below its surface ; and the layer beneath which they have sunk, and which has usually an opaline or greyish white tint, will coagulate without them, and form a colourless or buff- coloured clot consisting of fibrin alone, or of fibrin with entangled white corpuscles ; for the white corpuscles, being very light, tend CHAP. VI.] PARAGLOBULIN : FIBEINOGEN. IO5 upwards towards the surface of the fluid. The layer of clot which is thus formed rests on the top of a coloured clot of ordinary character, i.e., of one in which the coagulating fibrin has entangled the red corpuscles while they were sinking : and, thus placed, it constitutes what has been called a huffy coat. When a bufly coat is formed in the manner just described, it commonly contracts more than the rest of the clot, on account of the absence of red corpuscles from its meshes, and contraction being less interfered with by adhesion to the interior of the con- taining vessel in the vertical than the horizontal direction (Burden- Sanderson), a cupped appearance is produced on the top of the clot. In certain conditions of the system, and especially when there •exists some local inflammation, this bufied and cupped condition of the clot is well marked, because the tendency of the red cor- puscles to form rouleaux (see p. 1 15) is much exaggerated in inflammatory blood; and their rate of sinking increases with their aggregation. Inflammatory blood coagulates also less rapidly, although more firmly, than healthy blood. Although the appearance just described is commonly the result of a condition of the blood in which there is an increase in the quantity of fibrin, it need not of necessity be so. For a very dif- ferent state of the blood, such as that which exists in chlorosis, may give rise to the same appearance ; but in this case the pale layer is due to a relatively smaller amount of red corpuscles. It is a curious fact that in the case of the horse, the buffed and cupped appearance of the blood is a natural phenomenon, and has no connection with those conditions of disease under which alone it appears in man. Fibrin does not exist, as fibrin, in liquid blood. It is always formed, in the act of coagulation, by the union of two albu- minous substances, which, by some means yet unknown, exist separately in the blood, as it circulates. These fibrin-forming substances are termed paraglohulin (fibrino-plastic substance) and fibrinogen. Experiments made many years ago by Dr. Andrew Buchanan of Glasgow, and confirmed by more recent independent obser- vations of Alexander Schmidt, have led chiefly to this belief. io6 THE BLOOD. [cHAr. vr. When blood-serum or blood clot is added to the fluid of hydro- cele, or any other serous effusion, it speedily causes coagulation with the production of fibrin. And this phenomenon may occur also on the admixture of serous effusions from different parts of the body, as that of hydrocele with that of ascites, or of either with fluid from the pleui^al cavity. Other substances also, as muscular or nervous tissue, skin, &c., have been found to excite coagulation in serous fluids. Thus, fluids which have no tendency to coagulate sponta- neously can be made to produce a clot identical with blood- fibrin, by the addition to them of some other albuminous fluids and substances. Fibrino-plastic matter (paraglobuUn) can be obtained as a grannlar pre- cipitate by passin^r a ciuTent of carbonic acid gas through a mixture of ice- cold plasma and water, or dilute serum. From the former mixture, a second precipitate (fibrinoc^en) can be obtained by passing carbonic acid gas through the clear liquid left by the subsidence of the paraglobulin, after diluting it ^vith t^-ice its bulk of ice-cold water. Fibrinogen maybe obtained also from hydrocele fiuid by satm^ating it with chloride_ of sodium ; while a similar treatment of serum will precipitate paraglobulin. The fact that the fluid part (plasma) of the blood contains in itself the factors required for the formation of fibrin must not be taken as a proof that the corpuscles have nothing to do under ordinary circumstances with the process of coagulation. The reverse appears to be the case. Serum to which coloured blood corpuscles, which have been separated by subsidence and decantation from a knoTO quantity of blood, are added, acquires the property of coagulation ; and that the colourless corpuscles may have also a large share in the formation of fibrin, may be inferred from several facts. Vaccine and blister fiuid are both coagulable ; they contain no coloured blood-corpuscles, but always many colourless corpuscles. If the process of coa2:ulation is watched in either of these liquids under the microscope, it is seen, not merely that it begins from these elements, but that it occurs nowhere in the liquid excepting where they are present. Again, if a ligature is drawn through a vein in which blood is circulating, as, e.g., through the external jugular of a rabbit or guinea-pig, and allowed to remain there ciiAr. VI.] CONDITIO]S^S AFFECTING COAGULATION. I07 for a time, and then removed and examined microscopically, it is found that the threads of the ligature are crowded, and its surface encrusted with colourless corpuscles. These bodies are held together by fibrin, which appears to grow from their surface into the blood-stream.'' (Burdon-Sanderson.) The share, however, taken in ordinary blood-coagulation by the coloured and colourless corpuscles, either comparatively or absolutely, is still unknown. The immediate cause of the coagulation of the blood, is still a mystery. Prof. Lister supposes that blood has no natural tendency to clot, but that its coagulation out of the body is due to the action of foreign matter with which it happens to be brought into contact, and, in the body, to conditions of the tissues, which cause them to act towards it like foreign matter. Another theory (Briicke's) differs from the last, in that while it admits a natural tendency on the part of the blood to coagula- tion, it supposes that this tendency in the living body is restrained by some inhibitory power resident in the walls of the containing vessels. Support was once thought to be given to this and like theories by cases of injury, in which blood extravasated in the living body has seemed to remain uncoagulated for weeks, or even months, on account of its contact with living tissues. But the supposed facts have been shown to be without foundation. The blood-like fluid in sucli cases is not uncoagulated blood, but a mixture of serum and blood-corpuscles, with a certain proportion of clot in various stages of disintegration. (Morrant Baker.) Conditions affecting Coagulation, The coagulation of the blood is hastened by the following means : — 1. Moderate warmth, — from about ioo° F. to 120° F. 2. Rest is favourable to the coagulation of blood. Blood, of which the whole mass is kept in uniform motion, as when a closed vessel completely filled with it is constantly moved, coagu- lates very slowly and imperfectly. 3. Contact with foreign matter, and especially multiplication of the points of contact. Thus, coagulated fibrin may be quickly io8 THE BLOOD. [chap. VI. obtained from liquid blood by stirring it with a bundle of small twigs ; and even in tbe living body the blood will coagulate upon rough bodies projecting into the vessels ; as, for example, upon threads passed through them, or upon the heart's valves roughened by inflammatory deposits or calcareous accumulations. 4. The free access of air. 5 . Coagulation is quicker in shallow than in tall and narrow vessels. 6. The addition of less than twice the bulk of water. The blood last drawn is said to coagulate more quickly than the first. The coagulation of the blood is retarded by the following means : — 1. Cold retards coagulation ; and so long as blood is kept at a temperature below 40° F., it wiU not coagulate at all. Freezing the blood, of course, prevents its coagulation ; yet it will coagu- late, though not firmly, if thawed after being frozen ; and it will do so, even after it has been frozen for several months. A higher temperature than 120° F. retards coagulation, or, by coagulating the albumen of the serum, prevents it altogether. 2. The addition of water in greater proportion than twice the bulk of the blood. 3. Contact with living tissues, and especially with the interior of a living blood-vessel, retards coagulation. 4. The addition of alkaline and earthy salts in the proportion of 2 or 3 per cent, and upwards. When added in large propor- tion most of these saline substances prevent coagulation alto- gether. Coagulation, however, ensues on dilution with water. The time that blood can be thus preserved in a liquid state and coagulated by the addition of water, is quite indefinite. 5. Imperfect aeration, — as in the blood of those who die by asphyxia. 6. In inflammatory states of the system, the blood coagulates more slowly although more firmly. 7. Coagulation is retarded by exclusion of the blood from the air, as by pouring oil on the surface, etc. In vacuo, the blood coagulates quickly ; but Prof. Lister thinks that the rapidity of ciiAr. VI.] CHEMICAL COMPOSITION OF THE BLOOD. IO9 tlie process is due to tlie bubbling wliich ensues from the escape of gas, and to the blood being thus brought more freely into contact with the containing vessel. The coagulation of the blood is prevented altogether by the addition of strong acids and caustic alkalies. It has been believed, and chiefly on the authority of Mr. Hunter, that after certain modes of death, the blood does not coagulate ; he enumerates the death by lightning, over-exertion (as in animals hunted to death), blows on the stomach, fits of anger. He says, " I have seen instances of them all." Doubtless he had done so ; but the results of such events are not constant. The blood has been often observed coagulated in the bodies of animals killed by lightning or an electric shock ; and Mr. Gulliver has published instances in which he found clots in the hearts of hares and stags hunted to death, and of cocks killed in fighting. Chemical ComiDOsition of the Blood. Average proportions of the constituents of the blood in I,000 parts : — Water 784- Albumen (of serum) 70* Fibrin .......... 2*2 Ked corpuscles (dry) ....... 130* Fatty matters Inorganic Salts : Chloride of sodium . . . .3*6 Chloride of potassium . . . . 0*35 Tribasic phosphate of sodium . . 0*2 Carbonate of sodium . . . . 0*28 Sulphate of sodium . . . .0*28 Phosphates of calcium and magnesium 0*25 Oxide and phosphate of iron . . . 0*5 Odoriferous and colouring matter, gases, creatin, urea, and other extractive matters, glucose, and acci- dental substances 5-40 Elementary composition of the dried blood of the ox Carbon Hydrogen Nitrogen Oxygen Ashes These results of the ultimate analysis of ox's blood afford a remark- able illustration of its general purpose, as supplying the materials for the lOOO* 57*9 71 17-4 19-2 4'4 110 THE BLOOD. [chap. VI. renovation of all the tissues. For the analysts (Playfair and Bocckmann) have found that the flesh of the ox yields the same elements in eo nearly the same proportions, that the elementary composition of the organic constituents of the blood and flesh may be considered identical, and may be represented for both by the formula C^gHsgNgOj^. The Blood- Corpuscles or Blood-Cells, It has been already said, that the clot of blood contains, with the fibrin and the portion of the serum that is soaked in it, the hlood-corpuscles or hlood-cells. Of these there are two principal forms, the red and the icJiite corpuscles, or, as they are now frequently named, the coloured and the colourless. AVhen coagu- lation has taken place quickly, both kinds of corpuscles may be uniformly diffused through the clot ; but when it has been slow, the red corpuscles, being the heaviest constituent of the blood, tend by gravitation to accumulate at the bottom of the clot ; and the white corpuscles, being among the lightest constituents, collect in the upper part, and contribute to the formation of the buffy coat. In the moist state, the red corpuscles form 45 per oent. by weight, of the whole mass of the blood. (Robin.) Physical and Chemical Characters of Bed Blood- Corpuscles. The human red hlood-cells or hlood-corpuscles (figs. 63 and 67) are circular or coin-shaped flattened disks, varying in diameter from to ■4^Vo inch, and about i^^^ thick- ness. In other words, if placed flat, edge to edge, about ten millions would lie on a square inch of surface. Their borders are rounded ; their surfaces, in the perfect and most usual state, slightly concave ; but they readily acquire flat or convex surfaces when, the liquor sanguinis being diluted, they are swollen by absorption of fluid. When viewed singly, they appear of a pale yellowish tinge ; the deep red colour which they give to the blood being observable in them only when they are seen en masse. They are composed of a colourless, structureless, and transparent filmy framework or stroma, infiltrated in all parts by a xed colouring-matter termed hcemoglohin. The stroma is tough and elastic, so that, as the cells circulate, they admit of elongation ciiAr. VI.] THE EED BLOOD-COEPUSCLES. Ill Fig. 54.* Mammals. Birds. Eeptilcs. Amphibia. Fish. * The above illustration is somewhat altered from a drawing, by Mr. Gulliver, in the Proceed. Zool. Society, and exhibits the typical characters of the red blood-cells in the main divisions of the Yertebrata. The fractions are those of an inch, and represent the average diameter. In the case of the oval cells, only the long diameter is here given. It is remarkable, that although the size of the red blood-cells varies so much in the different classes of the vertebrate kingdom, that of the white corpuscles remains comparatively uniform, and thus they are, in some animals, much greater, in others much less than the red corpuscles existing side by side with them. 112 THE BLOOD. [chap. VI. and other changes of form, in adaptation to tlie vessels, yet recover their natural shape as soon as they escape from compres- sion. The term cell, in the sense of a bag or sac, is inapplicable to the red blood-corpuscle ; and it must be considered, if not solid throughout, yet as having no such variety of consistence in different parts as to justify the notion of its being a membranous sac with fluid contents. The stroma exists in all parts of its substance, and the colouring-matter uniformly pervades this, and is not merely surrounded by and mechanically enclosed within the outer waU of the corpuscle. The red corpuscles have no nuclei, although, in their usual state, the unequal refraction of transmitted light gives the appearance of a central spot, brighter or darker than the border, according as it is viewed in or out of focus. Their specific gravity is abou.t 1088. In examining a number of red corpuscles with the microscope, it is easy to observe certain natural diversities among them, though they may have been all taken from the same part. The great majority, indeed, are very uniform ; but some are rather larger, and the larger ones generaUy appear paler and less exactly circular than the rest ; their surfaces also are, usuaUy, flat or slighty convex, they often contain a minute shining particle like a nucleolus, and they are lighter than the rest, floating higher in the fluid in which they are placed. Other deviations from the general characters assigned to the corpuscles, depend on changes that occur after they are taken from the body. Very commonly they assume a granulated or mulberry-like form, in consequence, apparently, of a peculiar corrugation of their ceU-walls. Sometimes, from the same cause, they present a very irregular, jagged, indented, or star-like appearance. The larger ceUs are much less liable to this change than the smaller, and the natural shape may be restored by diluting the fluid in which the corpuscles float. Action of i?m^^/.t9.-Considerable light has been thro^Yn on the physical and chemical constitution of red blood cells by stndying the effects pro- dnced by various reagents : the following is a brief snmmaiy ot these Prm?!/'^^^ the red blood-cells of a frog or man are gently squeezed, they exhibit a wrinkling of the surface, which clearly indicates that there is CHAP. YI.] ACTION OF EE-AGENTS. ji superficial pellicle partly differentiated from the softer mass within ; ixgain, if a needle be rapidly drawn across a drop of blood several corpuscles will be found cut in two ; but this is not accompanied by any escape of cell-contents ; the two halves, on the contrary, assume a rounded form, proving clearly that the corpuscles are not mere membranous sacs with fluid contents like fat-cells. Fluids — Water. — When water is added gradually to frog's blood, the oval disc-shaped corpuscles become spherical, and gradually discharge their hsBmoglobin, a pale, transparent stroma being left behind ; human red blood-cells change from a discoidal to a spheroidal form, and discharge their cell-contents, becoming quite transparent and all but invisible. Solution of common salt (dilute) produces no appreciable 55- effect on the red blood -cells of the frog. In the red blood-cells ^ of man the discoid shape is exchanged for a spherical one^ with spinous projections, like a horse-chestnut. Their original ^ forms can be at once restored by the use of carbonic acid. Acetic acid (dilute) causes the nucleus of the red blood-cells in the frog to become more clearly defined ; if the action is prolonged, the nucleus becomes strongly granulated, and all the colouring matter seems to be concentrated in it, the surrounding cell-substance and outline of the cell becoming almost invisible ; after a time the cells lose their colour altogether. The cells in the figure represent the successive stages of the change. A similar loss of colour occurs in the red cells of human blood. Alkalies cause the red blood-cells to swell and finally dis- appear. Chloroform added to the red blood-cells of the frog causes them to part with their hemoglobin ; the stroma of the cells becomes gradually broken up, the nucleus resisting disintegration longest. A similar effect is produced on the human red blood-cell. Taiinin. — When a 2 per cent, solution of tannic acid is applied to frogs' blood it causes the appearance of a sharply-defined little knob, projecting from the free surface : the colouring matter becomes at the same time concentrated in the nucleus, which grows Fig. 57. more distinct. A similar effect is produced on the human red blood-cell. (Eoberts.) Magenta^ when applied to the red blood-cells of the frog, produces a similar little knob or knobs, at the same time staining the nucleus and causing the discharge of the hemoglobin. (Roberts.) The first effect of the magenta is to cause the discharge of the hemoglobin, then the nucleus becomes suddenly stained, and lastly a finely granular matter issues through the wall of the corpuscle, becoming- stained by the magenta, and a macula is formed at the point of escape. A similar macula is produced in the human red blood-cell. Boracic Acid. — A 2 per cent, solution applied to nucleated Fig. 58. red blood-cells (frog) will cause the concentration of all the -,/r>v /r>^ colouring matter in the nucleus ; the coloured body thus formed ^ \ r W gradually quits its central position and comes to be partly, ^ sometimes entirely, protruded from the surface of the now colourless cell. The result of this experiment led Brucke to distinguish the I 114 THE BLOOD. [chap. VI. coloured contents of the cell (zooid) from its colourless stroma (oecoid). When applied to the non-nucleated mammalian corpuscle, its effect merely resembles that of other dilute acids. Gases— Carhonic Acid,— It the red blood-cells of a frog be first exposed pig^ 59. action of water-vapour (which renders their outer pellicle more readily permeable to gases), and then acted on by car- bonic acid, the nuclei immediately become clearly defined and strongly granulated ; when air or oxygen is admitted the original appearance is at once restored. The upper and lower cell in the figure show the effect of carbonic acid ; the middle one the effect of the re-admission of air. These effects can be repro- duced five or six times in succession. If, however, the action of the carbonic acid be much prolonged, the granulation of the nucleus be- Fig, 60. comes permanent ; it appears to depend upon a coagula- tion of the paraglobulin. (Strieker.) Aimnonia. — Its effects seem to vary according to the degree of concentration. Sometimes the outline of the corpuscles becomes distinctly crenated ; at other times the effect resembles that of boracic acid, while in other cases the edges of the corpuscles begin to break up. (Lankester.) Heat, — The effect of heat up to 50 — 60° C. is to cause the formation of Fia 61 ^ number of bud-like processes. ^' Electricity causes the red blood-corpuscles to become P crenated, and at length mulberry-like. Finally they recover ®^ I «^ their round form and become quite pale. The general conclusions to be drawn from these observations have been summed up as follows by Mr. Ray Lankester : The red blood-corpuscle of the vertebrata is a viscid, and at the same time elastic disc, oval, or round, in outline, its surface being differentiated somewhat from the underlying material, and forming a pellicle or mem- brane of great tenuity, r^ot distinguishable with the highest powers (whilst the corpuscle is normal and living), and having no pronounced inner limi- tation. The viscid mass consists of (or rather yields, since the state of com- bination of the components is not known) a variety of albuminoid and other bodies, the most easily separable of which is haemoglobin ; secondly, the matter which segregates to form Eoberts's macula; and ±ig. 02. thirdly, a residuary stroma, apparently homogeneous in the mammalia (excepting as far as the outer surface or pellicle may be of a different chemical nature), but containing in the other vertebrata a sharply definable nucleus, this nucleus being already differentiated, but not sharply delineated during life, and consisting of (or separable into) at least two com- ponents, one (paraglobulin) precipitable by C 0^, and removable by the action of weak N H3 ; the other pellucid, and not granulated by acids.'* A peculiar property of the red corpuscles, which is exaggerated in inflammatory blood, may be here noticed. It gives them a great tendency to adhere together in rolls or columns, like piles of coins, and then, very quickly, these rolls fasten together by CHAP. VI.] RED COEPUSCLES. 115 their ends, and cluster ; so tliat, when the blood is spread out thinly on a glass, they form a kind of irregular network, with crowds of corpuscles at the several points corresponding with the knots of the net (fig. 63). Hence, the clot formed in such a thin layer of blood looks mottled with blotches of pink upon a white ground : in a larger quantity of such blood, as soon as the corpuscles have clus- tered and collected in rolls (that is, generally in two or three minutes after the blood is drawn), they begin to sink very quickly ; for in the ag- gregate they present less sur- face to the resistance of the liquor sanguinis than they would if sinking separately. Thus quickly sinking, they leave above them a layer of liquor sanguinis, and this coagulating, forms a buffy coat, as before described, the volume of which is augmented by the white cor- puscles, which have no tendency to adhere to the red ones, and by their lightness float up clear of them. This tendency, on the part of the red corpuscles, to form rouleaux, is pro- bably, as Dr. Norris suggests, only a physical phenomenon, comparable to the collection into somewhat similar rouleaux of discs of cork when they are partially immersed in water. Chemical Characters. — As they exist in the blood, the coloured corpuscles contain three-fourths of their weight of water. The stroma is composed of globulin, protagon, fatty matters, • including cholesterin, and salts, chiefly phosphates of potassium, sodium, and calcium. The stroma is infiltrated, as before men- tioned (p. no), with a red colouring matter termed hcemoglohin, Hcemoglobin, which enters far more largely into the composi- tion of the coloured corpuscles than any other ingredient, is an albuminous compound with the following composition : — * Fig. 63. Ked corpuscles in rouleaux. At a, a, are two white corpuscles. I 2 ii6 THE BLOOD. [chap. VI. Carbon . Hydrogen 54-0 7-25 16-25 2 1 45 0-63 042 Nitrogen Oxygen Sulphur Iron . lOO'OO Allied as it is in chemical composition to albumin, haemoglobin differs remarkably from it in many of its properties. The most interesting and important of these, physiologically considered, are (a) its power of crystallizing, the so-called hlood-crystals being the natural crystalline forms of haemoglobin ; and (h) its attraction for oxygen and some other gases. Hsemoglobin can be obtained in a crystalline form with various degrees of ■diflS-culty from the blood of different animals, that of man holding an inter- mediate place in this respect. Among the animals whose blood colouring- matter crystallizes most readily are the guinea-pig and the dog ; and in these cases to obtain crystals it is generally sufficient to dilute a drop of recently drawn blood with water and expose it for a few minutes to the air. Light seems to favour the formation of crystals. In many instances,' however, other means must be adopted, e.(/., the addition of alcohol, ether, or chloroform, rapid freezing and then thawing, an electric current, a tem- perature of 60° C, or the addition of sodium sulphate. Hemoglobin, though soluble in water, and, as we have seen, crystallisable, is not diffusible, i.e., its solution cannot pass through the pores of an animal membrane. When heated, its solution coagulates, the haemoglobin being decomposed into an albuminous substance, glohuUn, and a colouring matter, liceuiatin. A similar separation can be effected by the action of some acids and alkalies. Hsematin was once thought to be the natural colouring matter of the blood, but is now known to be a product of the decomposition of haemoglobin. Another very important derivative of hemoglobin is Hcemin or Hydro- chlorate of Hcematin, which may be prepared as follows : — A small portion of a dried drop of blood is placed on a glass slide, to- gether with a few small crystals of common salt. A thin glass cover is put on, and a drop of glacial acetic acid introduced beneath it : heat is gradually applied, and the excess of salt washed away with water. A number of small brownish crystals of a rhombic shape are thus formed. The formation of these haemin crystals is of great interest and importance in a medico-legal point of view, as it constitutes the most certain and deli- cate test we have for the presence of blood in a stain on clothes, &c. It exceeds in delicacy even the spectroscopic test to be mentioned further on. Different forms of blood-crystals are shown in the accompany- ing figures (Figs. 64, 65 and 66), CHAP. VI.] BLOOD-CRYSTALS. 117 Another most important character of haemoglobin is its at- traction for oxygen, with which it enters into definite chemi- cal combination (oxyhaemo- globin). Oxyhsemoglobin readily parts with its combined oxygen in the presence of reducicg agents, or even in vacuo ; and on this, not less than on its readiness to combine with oxygen, depends its most important physiological properties. During the passage of the blood through the lungs, the compound with oxygen is constantly formed ; while it is as constantly decomposed, in consequence of the readiness with which haemoglobin parts with oxygen, when: the latter is exposed to other attractions in its ^'^9- 65- 1 Figs. 64, 65, 66, illustrate some of the principal forms of blood-crystals :r— * Fig. 64. Prismatic, from human blood. t Fig. 65. Tetrahedral, from blood of the guinea-pig. Ii8 THE BLOOD. [chap. VI. share in the alteration of the colour of the blood in its veins and vice versa (see p. 124). Xitrous oxide forms a com- bination with haemoglobin, which gives two absorption bands, very similar to those of cxvhaemoglobin (p. 124). Car- bonic oxide possesses the pro- perty of entirely replacing the oxygen in oxyhaemoglobin, and its combination with haemo- globin has a spectrum very closely resembling that of oxy- hsemoglobin. Distribution of HcpinogloMn. — In connection with the ascer- tained function of haemoglobin as the great oxygen-carrier, the following facts with regard to its distribution are of importance. It occurs not only in the red blood-cells of all vertebrata except one fish (leptocephalus) whose blood-cells are all colourless, but also in similar cells in many worms : moreover it is found diffused in the vascular fluid of some other worms and certain Crustacea ; it also occurs in all the striated muscles of mammals and birds. It Is generally absent from unstriated muscle except that of the rectum. It has also been found in moUusca in certain muscles which are specially active, viz., those which work the rasp-like tongue. In the muscles of fish it has hitherto only been met with in the very active muscle which moves the dorsal fin of the Hippocampus (Kay Lankester) The White Corpuscles of the Blood or Blood-Leucocytes. The li'hite or colourless corpuscles of the blood, ^vhich are identical with lymph- corpuscles, are much less numerous than the red. On an average, in health, there may be one white to 400 or 500 red corpuscles ; but in disease, the proportion is often as high as one to ten, and sometimes even much higher. In health, the proportion varies considerably even in the course of the same day. The variations appear to depend chiefly * Fig. 66. Hexagonal crystals, from blood of squirrel. On these six-sided plates, prismatic crystals, grouped in a stellate manner, not unfrequently occur (after Funke). passage from the arteries to the Fig. 66^ €HAr. VI.] COLOURLESS CORPUSCLES. on the amount and probably also on the kind of food taken ; the number of leucocytes being very considerably increased by a meal, and diminished again on fasting. Also in young persons, •during pregnancy, and after great loss of blood, there is a larger proportion of colourless blood-corpuscles, which probably shows that they are more rapidly formed under these circumstances. In old age, on the other hand, their proportion is diminished. They present greater diversities of form than the red ones do ; Idut the gradations between the extreme forms are so regular, that no sufficient reason can be found for supposing that there is in healthy blood more than one species of white corpuscles. In -their most general appearance, ^ they are globular, and are about 2-5V0" i^ch in ■diameter (a, fig. 63 ) , They have a greyish, pearly look, appear- ing variously shaded or nebu- lous, the shading being much darker in some than in others. They consist of protoplasm •containing granules which are in some specimens few and very distinct, in others (though Tarely) so numerous that the whole corpuscle looks like a mass of ^anules. These corpuscles cannot be said to nave any true cell-wall. In a few instances an apparent cell-membrane can be traced around them ; but, much more commonly, even this is not dis- ■cernible till after the addition of water or dilute acetic acid (Fig. 67). A remarkable property of the colourless corpuscles, first observed by Mr. Wharton Jones, consists in their capability of spontaneously changing their shape. If a drop of blood bo examined with a high microscope-power under conditions by which loss of moisture is prevented, at the same time that the * Fig. 67. Red and white blood-corpuscles, Three white corpuscles acted on by weak acetic acid, c, Red blood-corpuscles. 120 THE BLOOD. [chap. VI. temperature is maintained at about the degree natural to the blood as it circulates in Hving body, the leucocytes can be seen alternately contracting and dilating very slowly at various parts of their circumference,— shooting out irregular processes, and again withdrawing them partially or completely, and thus in succession assuming various irregular forms (p. 46). Fig. 6S.* These amcehoid movements (p. 46) are characteristic of the living leucocyte, and form a good example of the contractile^ property of protoplasm, before referred to. Indeed, the un- changing rounded form which the corpuscles present in ordinary microscopic specimens must be looked upon as the shape natural to a dead corpuscle, or one whose vitality is dormant, rather than as the proper shape of one living and active. In many lower vertebrata, such as the newt, two or three distinct kinds of colourless blood corpuscles may be distinguished, and their movements are both more rapid, and the resulting changes of form more extreme, than in human colourless corpuscles. ActioR of Reagents. — Water checks the amoe'boid movements and causes the corpuscle to become globular : the nuclei, when multiple, coalesce into one, and the cell suddenly bursts, discharging its contents. Acetic Acid (dilute) causes the cessation of the amoeboid movements and the clear definition of the nucleus or nuclei, together with the appearance of granules. (Fig. 67.) If some fine pigment-granules be added to a fluid containing^ colourless blood-corpuscles, on a glass slide, these will be observed, under the microscope, to take up the pigment. In some cases colourless blood-corpuscles have been seen with fragments of coloured ones thus imbedded in their substance. Colourless blood-corpuscles have been observed to multiply by fission. The locomotion of leucocytes has been already referred ta (P» 70- * Fig. 68. Human colourless blood-corpuscle, showing its successive changes of outline within ten minutes when kept moist on a warm stage (Schofield). CHAr. VI.] SERUM. 121 Besides tlie red and white corpuscles, the inicroscox^e reveals numerous minute molecules or granules in the blood, circular or spherical, and varying in size from the most minute visible speck to the i^Vo-o ^^^^ (Gulliver). These molecules are very similar to those found in the lymph and chyle, and are some of them, fatty (being soluble in ether), others probably albuminous. Generally, also, there may be detected in the blood, especially during the time of active digestion, very minute equal-sized fatty particles, similar to those of which the mole- cular base of chyle is constituted (Gulliver). The Serum. The serum is the liquid part of the blood remaining after the coagulation of the fibrin. In the usual mode of coagulation, part of the serum remains in the clot, and the rest, squeezed from the clot by its contraction, lies around it. The quantity of serum that appears around the clot depends partly on the total quantity in the blood, but partly also on the degree to which the clot contracts. This is affected by many circumstances : gene- rally, the faster the coagulation the less is the amount of con- traction ; and, therefore, when blood coagulates quickly, it will appear to contain a small proportion of serum. In all cases, too,, it should be remembered, that, since the contraction of the clot may continue for thirty-six or more hours, the quantity of serum in the blood cannot be even roughly estimated till this period has elapsed. The serum is an alkaline, yellowish fluid, with a specific gravity of from 1 025 to IO30. It is composed mainly of water^ in which are dissolved all the substances enumerated in the table (p. 109), excepting the fibrin and corpuscles. The water of the Uood is subject to hourly variations in its quantity, ac- cording to the period since the taking of food, the amount of bodily exercise the state of the atmosphere, and all the other events that may affect either the ingestion or the excretion of fluids. According to these conditions, it may vary from 700 to 790 parts in the thousand. Yet uniformity is on the whole maintained ; because nearly all those things which tend to lower the proportion of water in the blood, such as active exercise, or the addition of saline or other solid matter, excite thirst ; while, on the other hand the addition of an excess of water to the blood is quickly followed by its more 122 THE BLOOD. [chap. VI. copious excretion in sweat and urine. And these means for adjusting the proportion of the water find their purpose in maintaining certain important physical conditions in the blood ; such as its proper viscidity, and the degree of its adhesion to the vessels through which it ought to flow with the least possible resistance from friction. On this also depends, in great measure, the activity of absorption by the blood-vessels, into which no fluids will quickly penetrate, but such as are of less density than the blood. Again, the quantity of water in the blood determines chiefly its volume, and thereby the fulness and tension of the vessels and the quantity of fluid that will exude from them to keep the tissues moist. Finally, the water is the general solvent of all the other materials of the liquor sanguinis. It is remarkable, that the proportion of water in the blood may be some- times increased even during its abstraction from an artery or vein. Thus Dr. Zimmermann in bleeding dogs, found the last drawn portion of blood •contain 12 or 13 parts more of water in 1000 than the blood first drawn ; and Polli noticed a corresponding diminution in the specific gravity of the human blood during venesection, and suggested the only probable explana- tion of the fact, namely, that during bleeding, the blood-vessels absorb very quickly a part of the serous fluid with which all the tissues are moistened. The albumen may vary, consistently with health, from 60 to 70 parts in the 1000 of blood. It is, probably, in combination with soda, as an albuminate of soda ; for, if serum be much diluted with water, and then neutralized with acetic acid, pure albumen is deposited. Another view entertained by Enderlin is that the albumen is dissolved in the solution of the neutral phosphate of sodium, to which he considers the alkaline reaction of the blood to be due, and solutions of which can dissolve large quantities of al- bumen and phosphate of calcium. The proportion oi fibrin in healthy blood may vary between 2 and 3 parts in 1000. In some diseases, such as typhus, and others of low type, it may 1)6 as little as i '034 ; in other diseases, it is said, it may be increased to as much as 7*528 parts in 1000. But in all these analyses it must be remembered that the white corpuscles are also included, inasmuch as it is impossible to separate them from the fibrin. The fatty matters are subject to much variation in quantity, being com- monly increased after every meal in which fat has been taken. At such times, the fatty particles of the chyle, added quickly to the blood, are only gradually assimilated ; and their quantity may be sufficient to make the serum of the blood opaque, or even milk-like. Variations in healthy Blood under different Circumstances. The conditions which appear most to influence the composition of the blood in health, are these : sex, pregnancy, age, and temperament. The composition of the blood is also, of course, much influenced by diet. I. Sex. — The blood of men differs from that of women, chiefly in being of somewhat higher speciflc gravity, from its containing a relatively larger quantity of red corpuscles. CHAP. VI.] VARIATIONS m BLOOD. 123 2. Pregnancy. — The blood of pregnant women has a rather lower specific gravity than the average, from deficiency of red corpuscles. The quantity of white corpuscles, on the other hand, and of fibrin, is increased. 3. Age. — From the analysis of Denis it appears that the blood of the foetus is very rich in solid matter, and especially in red corpuscles ; and this con- dition, gradually diminishing, continues for some weeks after birth. The quantity of solid matter then falls during childhood below the average, again rises during adult life, and in old age falls again. 4. Temperament. — But little more is known concerning the connection of this with the condition of the blood, than that there appears to be a relatively larger quantity of solid matter, and particularly of red corpuscles, in those of a plethoric or sanguineous temperament. 5. Diet. — Such differences in the composition of the blood as are due to the temporary presence of various matters absorbed with the food and drink, as well as the more lasting changes which must result from generous or poor diet respectively, need be here only referred to. Effects of Bleeding. — The result of bleeding is to diminish the specific gravity of the blood ; and so quickly, that in a single venesection, the portion of blood last drawn has often a less specific gravity than that of the blood that flowed first (J. Davy and Polli). This is, of course, due to ab- sorption of fluid from the tissues of the body. The physiological import of this fact, namely, the instant absorption of liquid from the tissues, is the same as that of the intense thirst which is so common after either loss of blood, or the abstraction from it of watery fluid, as in cholera, diabetes, and the like. For some little time after bleeding, the want of red blood-cells is well marked ; but with this exception, no considerable alteration seems to be produced in the composition of the blood for more than a very short time ; the loss of the other constituents, including the pale corpuscles, being very quickly repaired. ' Variations in the Composition of the Blood, in different Parts of the Body. The composition of the blood, as might be expected, is found io vary in different parts of the body. Thus arterial blood differs from venous ; and although its composition and general characters are uniform throughout the whole course of the systemic arteries, they are not so throughout the venous system, — the blood contained in some veins differing remarkably from that in others. I. Differences between arterial and venous Mood. — These may be arranged under two heads, — differences in colour, and in general composition. The colouring matter of the blood, or haemoglobin (p. 115), is capable of existing in two different states of oxidation, and the respective colours of 124 THE BLOOD. [chap, arterial and venous blood are caused by differences in tint bt^tween these two varieties — scarUt hcemogloUn or oxy-hcemoglohiTh and deoscidised or purple haemoglobin. The change of colour produced by the passage of the blood through the lungs, and its consequent exposure to oxygen, is due, chiefly, to the oxidation of purple, and its conversion into scarlet haemoglobin ; while the readiness with which the latter is de-oxidised offers a reasonablt^ explanation of the change, in regard to tint, of arterial into venous blood,- - the transformation being effected by the delivering up of oxygen to oxidis- able matters, by the scarlet hsemoglobin, during the blood's passage through the capillaries. Formerly carbonic acid was believed to make blood dark by causing the red corpuscles to assume a bi-convex shape, while oxygen was thought to reverse the effect by contracting them and rendering them bi-concave. But although we are not in a position to deny altogether the possible influence of mechanical conditions of the red corpuscles on the colour of arterial and venous blood respectively, it is probable that this cause alone would be quite insufficient to explain the differences in the colour of the two kinds of blood, and therefore if it be an element at all in the change, it must be allowed to take only a subordinate position. The distinction between the two kinds of hemoglobin naturally present in the blood, or in other words, the proof that the addition or subtraction of oxygen involves the production of two substances having fundamental differences of chemical constitution, has been made out chiefly by sjpectnaip- Fig. 69.* analysis * For while a solution of oxy -hemoglobin, causes the appearance of two absorption bands in the yellow and the green part of the sjpectinim, be- tween D and E, these are replaced by a single band intermediate in position. * The student to whom the terms employed in connection with spectrum- analysis are not familiar, is advised to consult, with reference to this para- graph, an elementary treatise on Physics. CHAP, yi.] VARIATIONS IN BLOOD. 125 when the oxidised or scarlet solution is darkened by de-oxidising agencies, — or, in other words, when the change which naturally ensues in the conversion of arterial into venous blood is artificially produced. (Stokes.) The greater part of the haemoglobin in both arterial and venous blood exists in the scarlet or more highly oxidised condition, and only a small part is de-oxidised and made purple in its passage from the arteries into the veins. The differences in regard to colour between arterial and venous blood are sometimes not to be observed. If blood runs very slowly from an artery, as from the bottom of a deep and devious wound, it is often as dark as venous blood. In persons nearly asphyxiated also, and sometimes, under the in- fluence of chloroform or ether, the arterial blood becomes like the venous. In the foetus also both kinds of blood are dark. But, in all these cases, the dark blood becomes bright on exposure to the air. Bernard has shown that venous blood returning from a gland in active secretion is almost as bright as arterial blood. 1), General Comiws%txon.—Ti\Q chief differences between arterial and ordinary venous blood are these. Arterial blood contains rather more fibrin, and rather less albumen and fat. It coagulates somewhat more quickly. Also, it contains more oxygen, and less carbonic acid. According to Denis, the fibrin of venous blood differs from arterial, in that when it is fresh and has not been much exposed to the air, it may be dissolved in a slightly heated solution of nitrate of potassium. Some of the veins contain blood whicli differs from the ordinary standard considerably. These are the portal, the hepatic, and the splenic veins. Portal vein. — The blood which the portal vein conveys to the liver is supplied from two chief sources ; namely, that in the gastric and mesenteric veins, w^hich contains the soluble elements of food absorbed from the stomach and intestines during digestion, and that in the splenic vein ; it must, there- fore, combine the qualities of the blood from each of these sources. The blood in the gastric and mesenteric veins will vary much according to the stage of digestion and the nature of the food taken, and can therefore be seldom exactly the same. Speaking generally, and without considering the sugar, dextrin, and other soluble matters which may have been absorbed from the alimentary canal, this blood appears to be deficient in solid matters, especially in red corpuscles, owing to dilution by the quantity of water absorbed, to contain an excess of albumen, and to yield a less tenacious kind of fibrin than that of blood generally. The blood from the splenic vein is generally deficient in red corpuscles, and contains an unusually large proportion of albumen. The fibrin seems to vary in relative amount, but to be almost always above the average. The proportion of colourless corpuscles is also unusually large. The w^hole quantity of solid matter is decreased, the diminution appearing to be chiefly in the proportion of red corpuscles. The blood of the portal vein, combining the peculiarities of its two factors, the splenic and mesenteric venous blood, is usually of lower specific gravity than blood generally, is more watery, contains fewer red corpuscles, more 126 THE BLOOD. [chap. VI. albumen, chiefly in the form of albuminose, and yields a less firm clot than that yielded by other blood, owing to the deficient tenacity of its fibrin. These characteristics of portal blood refer to the composition of the blood itself, and have no reference to the extraneous substances, such as the ab- sorbed materials of the food, which it may contain ; neither, indeed, has any complete analysis of these been given. Comparative analyses of blood in the portal vein and blood in the hepatic veins have also been frequently made, with the view of determining the changes which this fluid undergoes in its transit through the liver. Great diversity, however, is observable in the analyses of these two kinds of blood by different chemists. Part of this diversity is no doubt attributable to the fact pointed out by Bernard, that unless the portal vein is tied before the liver is removed from the body, hepatic venous blood is very liable to re- gurgitate into the portal vein, and thus vitiate the result of the analysis. Guarding against this source of error, recent observers have determined that hepatic venous blood contains less water, albumen, and salts, than the blood of the portal vein ; but that it yields a much larger amount of extractive matter, in which, according to Bernard and others, is one constant element, namely, grape-sugar, which is found, w^hether saccharine or farinaceous matter have been present in the food or not. Besides the rather wide difference between the composition of the blood of these veins and of others, it must not be forgotten that in its passage through every organ and tissue of the body, the blood's composition must be varying constantly, as each part takes from it or adds to it such matter as it, roughly speaking, wishes either to have or to throw away. Thus the blood of the renal vein has been prove Fig. 85. t Fig. 84/ * Fig. 84. Dr. Burdon-Sanderson's Cardiograph, t Fig. 85. Registering apparatus of Cardiograph. CHAP. Yir.] TRACINGS OF IMPULSE in connection with wliicli is a long and ligHt lever (a) . The shock of the heart's impulse being communicated to the ivory knob, and through it to the first tympanum, the effect is, of course, at once transmitted by the column of air in the elastic tube to the interior of the second tympanum, also closed, and through the elastic and movable lid of the latter to the lever, which is placed in connection with a registering apparatus,. (For explanation of Registering apparatus see figs. 8/ and 102,. with accompanying descriptions in the text.) A tracing of the heart's impulse is thus obtained. (Fig. 86.) Fig. 86.^ Its interpretation will be best understood by reference to Figs. 8/ and 88, with the accompanying text. Ficf. 87.+ * Fig. 86. Tracing of heart's impulse of man (Marey). t Fig. 87. Apparatus of MM. Cliaiiveau and Marey for estimating the variations of endocardial pressure, and production of impulse of the heart. i6o CIECULATIOX OF THE BLOOD. [chap. vit. By placing three small india-rubber air-bags in the interior re- spectively of°the right auricle, the right ventricle, and in an intercostal space in front of the heart of living animals (horse), and placing these bags, by means of long narrow tubes, in communication with three levers,' arranged one over the other in connection with a registering apparatus (fig. 87), MM. Chauveau and Marey have been able to measure with much accuracy the variations of the endocardial pressure and the <}omparative duration of the contractions of the auricles and ventricles. By means of the same apparatus, the synchronism of the impulse with the contraction of the ventricles, is also well shown ; and the causes of the several vibrations of which it is really composed, have been dis- covered. In the tracing (fig. 88), the intervals between the vertical lines represent periods of a tenth of a second. The parts on which any given vertical line 1. Auricular tracing. ■2. Ventricular tracing. Impulse tracing. falls represent, of course, simultaneous events. Thus, — it will be seen that the contraction of the auricle, indicated by the upheaval of the tracing at A in first tracing, causes a slight increase of pressure in the ventricle (A' in second tracing), and produces a tiny impulse (A" in third tracing). 80 also, the closure of the semilunar valves, while it causes a momentarily increased pressure in the ventricle at T>', does not fail to affect the pres- sure in the auricle D, and to leave its mark in the tracing of the impulse also, d". The large upheaval of the ventricular and the impulse tracings, between A' and D', and A" and d", are caused by the ventricular contraction, while the smaller undulations, between B and c, B' and c', b" and c", are caused by the vibrations consequent on the tightening and closure of the aunculo- ventricular valves. * Fig. 88. Tracings obtained by Chauveau and Marey's apparatus (Fig. 87). CHAP. Yii.] PULSE-EATE. l6l Frequency and Force of the Heart's Action, The heart of a healthy adult man in the middle period of life, contracts from seventy to seventy -five times in a minute; but many circumstances cause this rate, which of course corresponds with that of the arterial pulse (p. 179), to vary even in health. The chief are age, temperament, sex, food and drink, exercise, time of day, posture, atmospheric pressure, temperature. — The frequency of the heart's action gradually diminishes from the commencement to near the end of life, but is said to rise again somewhat in extreme old age, thus Before birth the average number of pulses in a minute is 150 Just after birth from 140 to 130 During the first year 130 to 115 During the second year 1 1 5 to 100 During the third year 100 to 90 About the seventh year 90 to 85 About the fourteenth year, the average number of pulses in a minute is from . . . . 85 to 80 In adult age 80 to 70 In old age 70 to 60 In decrepitude 75 to 65 Temperament and Sex, — In persons of sanguine temperament, the heart acts somewhat more frequently than in those of the phlegmatic ; and in the female sex more frequently than in the male. Food and Drink. Exercise. — After a meal its action is accelerated, and still more so during bodily exertion or mental excitement ; it is slower during sleep. Diurnal Variation. — From the observation of several experimenters, it appears that, in the state of health, the pulse is most frequent in the morning, and becomes gradually slower as the day advances : and that this diminution of frequency is both more regular and more rapid in the evening than in the morning. Posture. — It is found that, as a general rule, the pulse, especially in the adult male, is more frequent in the standing than in the sitting posture, and in the latter than in the recumbent position ; the difference being greatest between the standing and the sitting posture. The effect of change of posture is greater as the frequency of the pulse is greater, and, accordingly, is more marked in the morning than in the evening. Dr. Guy, by supporting the body in different postures, without the aid of muscular effort of the individual, has proved that the increased frequency of the palse in the sitting and standing positions is dependent upon the muscular exertion engaged in maintaining them ; the usual effect of these postures on the M l62 CIRCrLATIOX OF THE BLOOD. [chap. vii. pulse being almost entirely prevented when the usually attendant muscular exertion was rendered unnecessary. Atmosj)Jieric Pressure. — According to Parrot, the frequency of the pulse increases in a corresponding ratio with the elevation above the sea ; and Dr. Frankland informed the author, that at the summit of Mont Blanc his pulse was about double its ordinary rate. After six hours' perfect rest and sleep at the top, it was 120, on descending to the corridor it fell to 108, at the Grands Mulcts it was 88, at Chamounix 56 ; normally, his pulse is 60. Tenqjerature. — The rapidity and force of the heart's contractions are largely influenced by variations of temperature. The frog's heart, when exc^ised, ceases to beat if the temperature be reduced to 32°. When heat is gradually applied to it, both the speed and force of the heart's contractions increase till they reach a maximum. If the temperature is still further raised the beats become irregular and feeble, and the heart at length stands still in a condition of heat rigor." Similar effects are produced in warm-blooded animals. In the rabbit, Dr. Brunton found that the number of heart-beats was more than doubled when the temperature of the au' was maintained at 105° F. At 113° — 114° F. the rabbit's heart ceases to beat. In health there is observed a nearly uniform relation between the frequency of the pulse and of the respirations ; the propor- tion being, on an average, one of the latter to three or four of the former. The same relation is generally maintained in the eases in which the pulse is naturally accelerated, as after food or exercise ; but in disease this relation usually ceases to exist. In many affections accompanied with increased frequency of the pulse, the respiration is, indeed, also accelerated, yet the degree of its acceleration may bear no definite proportion to the increased number of the heart's actions : and in many other cases, the pulse becomes more frequent without any accompanying increase in the number of respirations ; or, the respiration alone may be accelerated, the number of pulsations remaining stationary, or even falling below the ordinary standard. - The force with which the left ventricle of the heart contracts is about double that exerted by the contraction of the right : being equal (according to Valentin) to about -V^h of the weight of the whole body, that of the right being equal only to -^-o th of the same. This difference in the amount of force exerted by the contraction of the two ventricles, results from the walls of the left ventricle being about twice as thick as those of the right. And the difference is adapted to the greater degree of resistance CHAP, til] CArACITY OF HEART. 163 which the left ventricle has to overcome, compared with tliat to be overcome by the rig-ht : the former having to propel blood through every part of the body, the latter only through the lungs. The force exercised by the auricles in their contraction has not been determined. Neither is it known with what amount of force either the auricles or the ventricles dilate ; but there is no evidence for the opinion, that in their dilatation they can materially assist the circulation by any such action as that of p sucking-pump, or a caoutchouc bag, in drawing blood into their cavities. That the force which the ventricles exercise in dilatation is very slight, lias been proved by Oesterreicher. He removed the heart of a frog from the body, and laid upon it a substance sufficiently heavy to press it flat, and yet so small as not to conceal the heart from view ; he then observed that during the contraction of the heart, the weight was raised ; but that during its dilatation, the heart remained flat. And the same was shown by Dr. Clen- dinning, who, applying the points of a pair of spring callipers to the heart of a live ass, found that their points were separated as often as the heart swelled up in the contraction of the ventricles, but approached each other by the force of the spring when the ventricles dilated. Seeing how slight the force exerted in the dilatation of the ventricles is, it has been supposed that they are only dilated by the pressure of the blood impelled from the auricles ; but that both ventricles and auricles dilate spontaneously is proved by their continuing their successive contractions and dilatations when the heart is removed, or even when they are separated from one another, and when therefore no such force as the pressure of blood can be exercised to dilate them. The capacity of the two ventricles is probably the same. It is difficult to determine with certainty how much this may be ; but, taking the mean of various estimates, it may be inferred that each ventricle is able to contain on an average, about three ounces of blood, the whole of which is impelled into their re- spective arteries at each contraction. The capacity of the auricles is rather less than that of the ventricles : the thickness of their walls is considerably less. The latter condition is adapted to the small amount of force which the auricles require in order to empty themselves into their adjoining ventricles ; the former to the circumstance of the ventricles being partly filled with blood before the auricles contract. 164 CIECULATION OF THE ELOOD. [chap. yii. Work done ly tlie Heart.— In estimating the work done by any machine it is nsnal to express it in terms of the " unit of work." The unit of work is defined to be the energy expended in raising a unit of weight (i lb.) through a unit of height (i ft.). In England, the unit of work is the " foot-po\mdr in France, the " Ulogrammetrer The work done by the heart at each contraction can be readily found by multiplying the weight of blood expelled by the ventricles by the height to which the blood rises in a tube tied into an artery. This height Dr. Hales found to be about 9 ft. in the horse, and Dr. Haughton has shown that his estimate is nearly correct for a large artery in man. Taking the weight of blood expelled from the left yentricle at each systole as 4 oz., %.e., ilb., we have 9xi = 2i foot pounds as the work done by the left ventricle at each systole ; and adding to this the work done by the right ventricle (about \ that of the left) we have 2^ + 1 = 3 foot pounds as the work done by the heart at each contraction. Other estimates give \ kilogrammetre, or about 3^- foot Bounds. Dr. Haughton calculates that the total work of the heart m 24 hrs. is about 124 ""foot tons, and to give a more definite idea of this wonderful energy, exhibits it by contrast : " Let us suppose that the heart expends its entire force in lifting its own weight vertically, then the height through which it could lift itself in one Jiour is found to be 20,250 ft. (Helmholtz). It has been frequently stated that an active climber can ascend 9,000 feet in nine hours, which is only at the rate of 1,000 feet per hour, or ^th part of the energy of the heart. •'When the railway was constructed from Trieste to Vienna, a prize was offered for the locomotive Alp-engine that could lift its own weight through the greatest height in one hour. The prize locomotive was the ' Bavaria,' which lifted herself through 2,700 feet in one hour : the greatest feat yet accomplished on steep gradients. This result, remarkable as it is, is only ^th part of the energy of the human heart." N.B. In making these comparisons we must not forget that the work done by the climber and the engine in foot pounds far exceeds that of the heart, thouo'h the height to which the heart would raise its oivn (small) weight is much greater than in the other two cases. Influence of the Nervous System on the Action of the Heart, The heart contains in its own waUs microscopic gangHa or nerve-centres, and inter- communicating nerve -fibres, by which its action is immediately governed. Under ordinary conditions, the contact of blood with the endocardium and the accompanying distension of the heart's cavities are the stimuH, which, by reflex action through these ganglia and nerve-fibres, excite the heart's contraction. The mo- mentary exhaustion of the nerve and muscle-apparatus, which CHAP. Yii.] RHYTHM OF HEART. 1 65 of necessity foHows the contraction, provides tlie condition of relaxation, under which the heart's cavities can be again dis- tended by the in-flowing blood. This alternation of contraction and dilatation, which is repeated at regular intervals of rather less than a second, is called the rhythm of the heart. The functions of the microscopic ganglia present in the heart have been made the subject of physiological experiments only in cold-blooded animals. In the frog, ganglia (Remak's) lie in the wall of the sinus venosus ; another ganglion (Bidder's) is situate near the junction of the auricles with the ventricle, and a third group of ganglionic corpuscles is situate in the septum between the two auricles. There can be little doubt that these ganglia, or, at least, some of them, are the nerve-centres through which is reflected the stimulus which excites the heart's contraction ; for when the heart is divided into two or more parts, only those parts which contain ganglionic corpuscles are capable of pulsating rhythmically. When the heart is divided or ligatured at the line of junction of the sinns venosus with the right auricle, the sinus continues to pulsate, while the rest of the heart is, for a time, motionless : (Stannius.) although, after a time, it again begins to act ; but its rhythm is now different from that of the sinus. If the ventricle be cut off from the auricles it will continue to pulsate ; and its rhythmic pulsation will at once re-commence, if this be done during the time in which the heart is lying motionless on account of the separation, as just mentioned, of the sinus venosus. The heart's rhythmic contraction is, under ordinary circum- stances, sufficiently intelligible, and is what might be expected of any muscle under analogous conditions of regularly repeated * Fig. 89* Heart of frog (Burdon-Sanderson after Fritsche). Front view to the left, back view to the right. A A, Aortoc. V. c. s. Vence cavce su- periores. At. s. left auricle. At. d. right auricle. Yen. Ventricle. B. ar, Bulhiis arteriosus. S. v. Sinus venosus. Y. c. i. Vena cava inferior. Y. h. Vcnoe hepaticce. Y. p. VencB puhmnales. cincuLATiOiSr of the blood. [chap. VII. stimulation. It is less easy to understand the apparently strange plienomenon of continuance of rhythmic action in a heart which has been removed from the body— a xohenomenon which, although lasting only for a minute or two in a warm-blooded animal, may continue for many hours in a cold-blooded, if the precautions as to temperature, moisture, and the presence of oxygen be observed. The best interpretation yet given of it, and of rhythmic processes in general, is that by Sir James Paget, who regards them as dependent on rhythmic nutrition, i.e. on a method of nutrition in which, after the exhaustion produced by action, the acting parts are gradually raised, with time-regulated progress, to a certain state of instability of composition, which then issues in the discharge of their functions. Thus, in the present case, nerve-force issues, or is liberated from the cardiac ganglia, so soon as it reaches a certain degree of tension, and the effect of its transmission is the contraction of the muscular fibres to which branches from the ganglia are distributed, and whose irritability has been also simultaneously raised. The comparative frequency of the heart's rhythmic movements depends on the original constitution of the parts concerned, and introduces no fresh difficulty in the way of understanding the matter. All muscles and nerve-centres have a tendency to rhythm, when there is uniformity in the stimulus w^hich excites them to action. And the difficulty in comprehending the fact of the heart being ^set' to act sixty or seventy times in a minute, is neither more nor less than that which attends the compre- hension of the rhythm of those muscles which act at longer intervals, as e.g. the diaphragm, the eyelids, or, during gastric digestion, the stomach. From what has been said, it will be noticed that there is no exception to the rule, that, in the case of nerve and muscle, rest must alternate with work. We are apt to speak of the heart constantly acting, and to forget that it would be equally true to say that it is constantly resting. The difference from other muscles is only that tlie alternations of work and rest occur at shorter intervals. ciiAr. VII.] EIIYTIIM OF HEART. The comiiaratively long-continued maintenance of tlie power of contracting in the case of the heart of a cold-blooded animal, introduces, moreover, no fresh difficulty in the com- prehension of the subject. It is but an example of the rule that tissues which live and act at a slow rate, die at a slow rate also. Although, under ordinary conditions, the apparatus of ganglia and nerve-fibres in the substance of the heart forms the medium through which its action is excited and rhythmically maintained, yet they, and, through them, the heart's contractions are re- gulated by nerves which pass to them from the higher nerve- centres. These nerves are branches from the pneumogastric and sympathetic. The pneumogastric nerves are the media of an inhibitory or restraining influence over the action of the heart which is conveyed through them from the medulla oblongata, and which is always in operation. For, on dividing these nerves, the pulsations of the heart are increased in frequency; while an opposite effect is produced by stimulating them, — the transmis- sion of a galvanic current of even moderate strength diminishing the number of pulsations or stopping the action of the heart altogether (in diastole). This inhibitory influence may originate in the medulla oblongata, or may be merely reflected by it. As an example of the latter, the well-known effect on the heart of a violent blow on the epigastrium may be referred to. The stoppage of the heart's action is due to the conveyance of the stimulus by fibres of the sympathetic to the medulla oblongata, and its subsequent reflection through the pneumogastric to the heart's ganglia. Through certain fibres of the sympathetic, the heart receives an accelerating influence from the medulla oblongata. These accelerating nerve-fibres, issuing from the spinal cord in the neck, reach the inferior cervical ganglion, and pass thence to the cardiac plexus, and so to the heart. Their function is shown in the quickened pulsation which follows stimulation of the spinal cord, when the latter has been cut off from all connection with the CIHCrLATIOX OF THE BLOOD. [chap. VII. heart, excepting that which is formed by the accelerating fila- ments from the inferior cervical ganglion. Unlike the inhibitory fibres of the pneumogastric, of which they may be considered the antagonists, the accelerating fibres are not continuously in action. The connection of the heart with other organs by means of the nervous system, and the iniiuences to which it is subject through them, are shown in a striking manner by the phenomena of disease. The influence of mental shock in arresting or modifying the action of the heart, the slow pulsation which accompanies compression of the brain, the iiTCgularities and palpitations caused by dyspepsia or hysteria, are as good evidence of the connection of the heart with other organs through the nervous system, as any results obtained by direct experiment. Effects of the Heart* s Action, That the contractions of the heart supply alone a sufficient force for the circulation of the blood, is established by the results of several experiments, of which the following is one of the most conclusive : — Dr. Sharpey injected bullock's blood into the thoracic aorta of a dog recently killed, after tying the abdominal aorta above the renal aiteries, and found that, with a force just equal to that by which the ventricle commonly impels the blood in the dog, the blood which he injected into the aorta passed in a free stream out of the trunk of the vena cava inferior. It thus traversed both the systemic and hepatic capillaries ; and when the aorta was not tied above the renals, blood injected imder the same pressure fiowed freely through the vessels of the lower extremities. A pressure equal to that of one and a half or two inches of mercury was, in the same way, found sufficient to propel blood through the vessels of the lungs. But although it is true that the heart's action alone is sufficient to ensure the ciixulation, yet there exist several other forces which are, as it were, supplementary to the action of the heart, and assist it in maintaining the circulation. The principal of these CHAP, vir.l STRIJCTUIIE OF AETEPJES. 169 Biipplementcal forces have been already alluded to, and will now be more fully pointed out. Fig. 90.* THE ARTERIES. The walls of the arteries are composed of three principal coats, termed the external or tunica adventitia, the middle, and the ijiternal coat or tunica intiyna, while the latter is lined within by a single layer of tessellated epithelium. The external coat or tunica adventitia (figs. 91 and 92, t. a.), the strongest and toughest part of the wall of the artery, is formed of areolar tissue, with which is mingled throughout a network of elastic fibres. At the inner part of this outer coat the elastic network forms in most arteries so distinct a layer as to be sometimes called the external elastic coat. The middle coat (fig. 92, cm.) is composed of both muscular and elastic fibres, with a certain proportion of areolar tissue. In the larger arteries its thickness is comparatively as well as abso- lutely much greater than in the small, constituting, as it does, the greater part of the arterial wall. The muscular fibres, which are of the pale or unstriped variety (Fig. 90) (see Chapter on Motion), are arranged for the most part transversely to the long axis of the artery (fig. 91) ; while the elastic element, taking also a trans- verse direction, is disposed in the form of closely interwoven and branching fibres, which intersect in all parts the layers of muscular fibre. In arteries of various size there is a difference in the proportion of the muscular and elastic element, elastic tissue preponderating in the largest * Fig. 90. Muscular fibre-cells from liuman arteries, magnified 350 dia- meters (Kolliker). a, nucleus ; 5, a fibre-cell treated with, acetic acid. 170 CIECULATION OF THE BLOOD. [chap. vii. arteries, wliile this condition is reversed in those of medium and small size. Fig. 91.* The internal arterial coat is formed by layers of elastic tissue, consisting in part of coarse longitudinal branching fibres, and in part of a very thin and brittle membrane which possesses little elasticity, and is thrown into folds or wrinkles when the artery contracts. This latter membrane, the striated or fenestrated coat of Henle (fig. 94), is peculiar in its tendency to curl up, when peeled ofi" from the artery, and in the perforated and streaked appearance which it presents under the microscope. Its inner * Fig. 91. Blood-vessels from mesocolon of rabbit, a. Artery, witb two branches, showing tr. n. nuclei of transverse muscular fibres ; I. n. nuclei of endothelial lining ; t. a, tunica adventitia. v. Vein. Here the transverse nuclei are more oval than those of the artery. The vein receives a small branch at the lower end of the drawing ; it is distinguished from the artery among other things by its straighter course and larger calibre, c. Capillary, showing nuclei of endothelial cells, x 300. (Schofield). CHAr. VII.] STUUCTUUE OF AllTERIES. I?! surf\xco is lined with a delicate layer of epitlielium, composed of tliin squamous elongated cells (fig. 93, a.), wliicli make it smootli and polished, and furnish a nearly impermeable surface, along which the blood may flow with the smallest possible amount of resistance from friction. Fig. 93 -t c Immediately external to the epithelial lining of the artery is a fine connective tissue, sub -epithelial layer, with branched cor- puscles. Thus the internal coat consists of three parts, (a) an epithelial lining, (b) the sub-epithelial layer just mentioned, (c) elastic layers. The walls of the arteries, with the possible exception of the epithelial lining and the layers of the internal coat immediately * Fig. 92. Transverse section of small artery from soft palate, e, endo- thelial lining, the nuclei of the cells are shown; i, elastic tissue of the intima, which is a good deal folded ; c. m. circular muscular coat, showing nuclei of the muscle cells ; t. a. tunica adventitia. x 300. (Schofield). t Fig. 93. Two blood-vessels from a frogs mesentery, injected with nitrate of silver, showing the outlines of the endothelial cells, a. Artery. The endothelial cells are long and narrow ; the transverse markings indicate the m.uscular coat. t. a. Tunica adventitia. v. Vein. Showing the shorter and wider endothelial cells with which it is lined. c, c. Two capillaries entering the vein. (Schofield). 1/2 CIECULATIO^ST OF THE BLOOD. [chap. VII. Fig. 94.'* outside it, are not nourished by tlie blood which they convey, but are, like other parts of the body, supplied with little arteries, ending in capillaries and veins, which, branching throughout the external coat, extend for some distance into the middle, but do not reach the internal coat. These nutrient vessels are called vasa vasorum. Nerve- fibres are also supplied to the walls of the arteries. Most arteries are surrounded by a plexus of sympathetic nerves, which twine around the vessel very much like ivy round a tree ; and ganglia are found at frequent intervals. The smallest arteries and capillaries are similarly surrounded by a J^ig-. 95.t very delicate network of non- medullated nerve -fibres, many of which appear to end in the nuclei of the transverse muscu- lar fibres (fig. 95 ) . It is doubt- less through these plexuses that the calibre of the vessels is regulated by the nervous system (p. 188). The function of the arteries is to convey blood from the heart to all parts of the body, and each tissue which enters into the construction of an artery -has a special purpose to serve in this distribution. ^ Fig. 94. Portion of fenestrated membrane from the femoral artery. X 200. a, c, perforations (Henle). f Fig. 95. Eamification of nerves and termination in the muscular coat of . small artery of the frog. (Arnold). CHAP. VII.] FUNCTIONS OF AUTERIAL COATS. 173 (i.) The external coat forms a strong and tougli investment, which, though capable of extension, appears principally designed to strengthen the arteries and to guard against their excessive distension from the force of the heart's action. It is this ooat whiclL ak)ne prevents the complete severance of an artery when a ligature is tightly applied ; the internal and middle coats being usually divided. In it, too, the little vasa vasorum (p. 172) find a suitable tissue in which to subdivide for the supply of the arterial coats. (2.) The purpose of the elastic tissue, which enters so largely into the formation of all the coats of the arteries, is, (a). To guard the arteries from the suddenly exerted pressure to which they are subjected at each, contraction of the ventricles. In every such contraction, the contents of the ventricles are forced into the arteries more quickly than they can be discharged into and through the capillaries. The blood therefore being, for an instant, resisted in its onward course, a part of the force with which it was impelled is directed against the sides of the arteries; under this force their elastic walls dilate, stretching enough to receive the blood, and as they stretch, becoming more tense and more resisting. Thus, by yielding, they, as it were, break the shock of the force impelling the blood. On the subsidence of the pressure, when the ventricles cease contracting, the arteries are able, by the same elasticity, to resume their former calibre ; and in thus doing, they manifest {b) another chief purpose of their elasticity, that, namely, of equalizing the current of the blood by maintaining pressure on the blood in the arteries during the periods at which the ven- tricles are at rest or dilating. If some such method as this had not been adopted — if for example the arteries had been rigid tubes, the blood, instead of flowing as it does, in a constant stream, w^ould have been propelled through the arterial system in a series of jerks corresponding to the ventricular contractions, with intervals of almost complete rest during the inaction of the ventricles. But in the actual condition of the arteries, the force of the successive contractions of the ventricles is expended partly in the direct propulsion of the blood, and partly in the 174 CIRCULATION OF THE BLOOD. [chap. VI t. diLatation of tlie elastic arteries ; and in the intervals between the contractions of the ventricles, the force of the recoiling and contracting arteries is employed in continuing the same direct propulsion. Of course, the pressure exercised by the recoiling arteries is equally diffused in every direction through the blood, and the blood would tend to move backwards as well as onwards, but all movement backw^ards is prevented by the closure of the semi-lunar arterial valves (p. 1 51), which takes place at the very commencement of the recoil of the arterial walls. By this exercise of the elasticity of the arteries, all the force of the ventricles is made advantageous to the circulation ; for that part of their force which is expended in dilating the arteries, is restored in full when they recoil. There is thus no loss of force ; but neither is there any gain, for the elastic walls of the artery cannot originate any force for the propulsion of the blood — they only restore that which they received from the ventricles. The force with which the arteries are dilated every time the ventri- cles contrpot, might be said to be received by them in store, to be all given out again in the next succeeding period of dilatation of the ventricles. It is by this equalizing influence of the successive branches of every artery that, at length, the inter- mittent accelerations produced in the arterial current by the action of the heart, cease to be observable, and the jetting stream is converted into the continuous and equable movement of the blood which we see in the capillaries and veins. In the production of a continuous stream of blood in the smaller arteries and capillaries, the resistance which is offered to the blood-stream in the capillaries (p. 196), is a necessary agent. Were there no greater obstacle to the escape of blood from the arteries than exists to its entrance into them from the heart, the stream w^ould be intermittent, notwithstanding the elasticity of the walls of the arteries. (c.) By means of the elastic tissue in their w^alls (and of the muscular tissue also), the arteries are enabled to dilate and contract readily in correspondence w^ith any temporary increase or diminution of the total quantity of blood in the body ; and CHAP. Yii.] FUNCTIONS OF AETEEIAL COATS. within a certain range of diminution of the quantity, still to exercise due pressure on their contents. The elastic coat, however, not only assists in restoring the normal calibre of an artery after temporary dilatation^ but also, (d.) may assist in restoring it after diminution of the calibre, whether this be caused by a temporary contraction of the muscular coat, or the application of a compressing force from without. This action of the elastic tissue in arteries, is well shown in arteries which contract after death, but regain their average patency on the cessation of post-mortem rigidity (p. 177). (e.) By mcixns of their elastic coat the arteries are enabled to adapt themselves to the different movements of the several parts of the body. With regard to the purpose served by the muscular coat of the arteries, there appears no sufficient reason for supposing that it assists, to more than a very small degree, in propelling the onward current of blood. That it contributes, however, in some degree, to the forces concerned in the circulation of the blood, may be fairly inferred not only from the presence of muscular fibres, but from the actual observations of contractions of the arteries during life, in some of the lower animals, (rabbit, bat, frog,) the rhythm of which is quite different from that of the heart. (Wharton Jones, Schiff, Ludwig, Brunton.) The most important office of the muscular coat, is (2) that of regulating the quantity of blood to be received by each part, and of adjusting it to the requirements of each, according to various circumstances, but, chiefly, according to the activity with which the functions of each are at different times performed. The amount of work done by each organ of the body varies at dif- ferent times, and the variations often quickly succeed each other, so that, as in the brain for example, during sleep and waking, within the same hour a part may be now very active and then inactive. In all its active exercise of function, such a part requires a larger supply of blood than is sufficient for it during the times when it is comparatively inactive. It is evident that the heart cannot regulate the supply to each part at different periods \ neither could this be regulated by any general and uni- 176 CIRCULATION OF THE BLOOD. [chap. yii. form contraction of the arteries ; but it may be regulated by tlie power which, the arteries of each part have, in their muscular tissue, of contracting so as to diminish, and of passively dilating or yielding so as to permit an increase of, the supply of blood, according to the requirements of the part to which they are distributed. And thus, while the ventricles of the heart deter- mine the total quantity of blood, to be sent onwards at each con- traction, and the force of its propulsion, and while the large and merely elastic arteries distribute it and equalise its stream, the smaller arteries, in addition, regulate and determine, by means of their muscular tissue, the proportion of the whole quantity of blood which shall be distributed to each part. It must be remembered, however, that this regulating function of the arteries is itself governed and directed by the nervous system (p. 1 88). Another function of the muscular element of the middle coat of arteries is, doubtless (3), to co-operate with the elastic in adapting the calibre of the vessels to the quantity of blood which they contain. For the amount of fluid in the blood-vessels varies very considerably even from hour to hour, and can never be quite constant; and were the elastic tissue only present, the pres- sure exercised by the walls of the containing vessels on the contained blood would be sometimes very small, and sometimes inordinately great. The presence of a muscular element, however, provides for a certain uniformity in the amount of pressure exer- cised ; and it is by this adaptive, uniform, gentle, muscular con- traction, that the tone of the blood-vessels is maintained. Defi- ciency of this tone is the cause of the soft and yielding pulse, and its unnatural excess, of the hard and tense one. The elastic and muscular contraction of an artery may also be regarded as fulfilling a natural purpose when (4), the artery being cut, it first limits and then, in conjunction with the coagu- lated fibrin, arrests the escape of blood. It is only in consequence of such contraction and coagulation that we are free from danger through even very slight wounds ; for it is only when the artery is closed that the processes for the more permanent and secure prevention of bleeding are established. CHAP. VII.] MUSCULARITY OF ARTERIES. 177 (i.) When a small artery in the living subject is exposed to the air or cold, it gradually but manifestly contracts. Hunter observed that the posterior tibial artery of a dog when laid bare, became in a short time so much contracted as almost to prevent the transmission of blood ; and the observation has been often and variously confirmed. Simple elasticity could not effect this ; for after death, when the vital muscular power has ceased, and the mechanical elastic one alone operates, the contracted artery dilates again. (2.) When an artery is cut across, its divided ends contract, and the orifices may be completely closed. The rapidity and completeness of this contraction vary in dilierent animals ; they are generally greater in young than in old animals ; and less, apparently, in man than in animals. In part this contraction is due to elasticity, but in part, no doubt, to muscular action ; for it is generally increased by the application of cold, or of any simple stimulating substances, or by mechanically irritating the cut ends of the artery, as by picking or twisting them. Such irritation would not be followed by these effects, if the arteries had no other power of contracting than that depending upon elasticity. (3.) The contractile property of arteries continues many hours after death, and thus affords an opportunity of distinguishing it from elasticity. When a portion of an artery, the splenic, for example, of a recently killed animal, is exposed, it gradually contracts, and its canal may be thus completely closed : in this contracted state it remains for a time, varying from a few hours to two days : then it dilates again, and permanently retains the same size. If, while contracted, the artery be forcibly distended, its contractility is destroyed, and it holds a middle or natural size. This persistence of the contractile property after death was well shown in an observation of Hunter, which may be mentioned as proving, also, the greater degree of contractility possessed by the smaller than by the larger arteries. Having injected the uterus of a cow, which had been removed from the animal upwards of twenty-four hours, he found, after the lapse of another day, that the larger vessels had become much more turgid than when he injected them, and that the smaller arteries had contracted so as to force the injection back into the larger ones. The results of an experiment which Hunter made with the vessels of an umbilical cord prove still more strikingly the long continuance of the con- tractile power of arteries after death. In a woman delivered on a Thursday afternoon, the umbilical cord was separated from the foetus, having been first tied in two places, and then cut between, so that the blood contained in the cord and placenta was confined in them. On the following morning, Hunter tied a string round the cord, about an inch below the other ligature, that the blood might still be confined in the placenta and remaining cord. Having cut off this piece, and allowed all the blood to escape from its vessels, he attentively observed to what size the ends of the cut arteries were brought by the elasticity of their coats, and then laid aside the piece of cord to see the influence of the contractile power of its vessels. On Saturday morning, the day after, the mouths of the arteries were completely closed up. He repeated the experiment the same day with another portion of the same cord, and on the following morning found the results to be precisely similar. On the Sunday, he performed the experiment the third N 178 CIRCULATION OF THE BLOOD. [chap. VII. time, but the artery then seemed to have lost its contractility, for on the Monday morning, the mouths of the cut arteries were found open. In each of these experiments there was but little alteration perceived in the orifices of the veins. (4.) The influence of cold in increasing the contraction of a divided artery has been referred to : it has been shown, also, by Schwann, in an experi- ment on the mesentery of a living toad. Having extended the mesentery under the microscope, he placed upon it a few drops of water, the tempe- rature of which was some degrees lower than that of the atmosphere. The contraction of the vessels soon commenced, and gradually increased until, at the expiration of ten or fifteen minutes, the diameter of the canal of an artery, which at first was 0*0724 of an English line, was reduced to 0-0276, The arteries then dilated again, and at the expiration of half an hour had acquired nearly their original size. By renewing the application of the . water, the contraction was reproduced : in this way the experiment could be performed several times on the same artery. It is thus proved, that cold will excite contraction in the walls of very small, as well as of comparatively larse arteries : it could not produce such contraction in a merely elastic substance ; but it is a stimulus to the organic muscular fibres in many other parts, as well as in the arterial coat ; as, e.g., in the skin, the dartos, and the walls of the bronchi. (5.) Evidence of the muscular contractility of the arterial coats is furnished by the experiments of Ed. and E. H. Weber, and of Professor Kolliker, in which they applied the stimulus of electro-magnetism to small arteries. The experiments of the Webers were performed on the small mesenteric arteries of frees : and the most striking results were obtained when the diameter of the vessels examined did not exceed from i to i of a Paris line. When a vessel of this size was exposed to the electric current, its diameter in from five to ten seconds, became one-third less, and the area of its section about one-half. On continuing the stimulus, the narrowing gradually increased,, until the calibre of the tube became from three to six times smaller than it was at first, so that only a single row of blood-corpuscles could pass along it at once ; and eventually the vessel was closed and the current of blood arrested. ]Mr. Savory lias sliown tliat tlie natural state of all arteries, in regard at least to th.eir length, is one of tension — that they are always more or less stretched, and ever ready to recoil by virtue of their elasticity, whenever the opposing force is removed. The extent to which the divided extremities of arteries retract is a measure of this tension, not of their elasticity. The Pulse. The jetting movement of the blood, due to the intermittent action of the heart, which the elasticity of the arteries converts CHAP, yii.] THE PULSE. 179 into an uniform motion, in the arterioles (smallest arteries) and capillaries, is the cause of the pidse. As the blood is not able to pass througli the arteries so quickly as it is forced into them by the ventricle, on account of the resistance it experiences in the small arteries and capillaries, a part of the force with which the heart impels the blood is exercised upon the walls of the vessels which it distends — thus producing the arterial tension or blood- pressure, to be afterwards referred to (p. 1 84). The maximum of that tension, which follows each beat of the heart, is called the pulse. The distension of each artery increases both its length and its diameter. In their elongation, the arteries change their form, the straight ones becoming slightly curved, or having such a tendency, and those already curved becoming more so but they recover their previous form as well as their diameter when the ventricular contraction ceases, and their elastic walls recoil. The increase of their curves which accompanies the distension of arteries, and the succeeding recoil, may be well seen in the pro- minent temporal artery of an old person. The elongation of the artery is in such a case quite manifest. The mind cannot dis- tinguish the sensation produced by the dilatation from that produced by the elongation and curving ; that which it perceives most plainly, however, is the dilatation, or return, more or less, to the cylindrical form, of the artery which has been partially flattened by the finger. The pulse — due to any given beat of the heart — is not per- ceptible at the same moment in all the arteries of the body. Thus,— it can be felt in the carotid a very short time before it is perceptible in the radial artery, and in this vessel again before the dorsal artery of the foot. The delay in the beat is in pro- portion to the distance of the artery from the heart, but the difference in time between the beat of any two arteries never exceeds probably ^ to J of a second. * There is, perhaps, an exception to this in the case of the aorta, of which the "curve is by some supposed to be diminished when it is elongated ; hut if this be so, it is because only one end of the arch is immoveable ; the other end, with the heart, may mcve forward slightly when the ventricles contract. N 2 i8o CIBCULATION OF THE BLOOD. [chap. vii. It was formerly supposed that the pulse was caused, not by the direct action of the ventricle, but by the propagation of a wave in consequence of the elastic recoil of the large arteries, after their distension ; and successive acts of dilatation and recoil, extending along the arteries in the direction of the circulation, were supposed to account for the later appearance of the pulse in the vessels most distant from the heart. The fact, however, that the pulse is perceptible in every part of the arterial system previous to the occurrence of the second sound of the heart, that is, previous to the closure of the aortic valves, is a fatal objection to this theory. For, if the pulse were the effect of a wave propagated by the alternate dilatation and con- traction of successive portions of the arterial tube, it ought, in all the arteries except those nearest to the heart, to follow or coincide with, but could never precede, the second sound of the heart ; for the first effect of the elastic recoil of the arteries first dilated is the closure of the aortic valves ; and their closure produces the second sound. The theory which seems to reconcile all the facts of the case, and especially those two which appear most opposed, namely, that the pulse always pre- cedes the second sound of the heart, and yet is later in the arteries far from the heart than in those near it, may be thus stated :— It supposes that the blood which is impelled onwards by the left ventricle does not so impart its pressure to that which the arteries already contain, as to dilate the whole arterial system at once ; but that it enters the arteries, it displaces and propels that which they before contained, and flows on with what may be called a Jiead-wave, like that which is formed when a rapid stream of water overtakes another moving more slowly. The slower stream offers resistance to the more rapid one, till their velocities are equalized : and, because of such resistance, some of the force of the more rapid stream of blood just expelled from the ventricle, is diverted laterally, and with the rising of the wave the arteries nearest the heart are dilated and elongated. They do not at once recoil, but continue to be distended so long as blood is entering them from the ventricle. The wave at the head of the more rapid stream of blood Tuns on, propelled and maintained in its velocity by the continuous con- traction of the ventricle : and it thus dilates in succession every portion of the arterial system, and produces the pulse in all. At length, the whole arterial system (wherein a pulse can be felt) is dilated ; and at this time when the wave we have supposed has reached all the smaller arteries, the entire system may be said to be simultaneously dilated ; then it begms to contract, and the contractions of its several parts ensue in the same suc- cession as the dilatations, commencing at the heart. The contraction of the first portion produces the closure of the valves and the second sound of the heart : and both it and the progressive contractions of all the more distant parts 'maintain, as already said, that pressure on the blood during the inaction of the ventricle, by which the stream of the arterial blood is sus- tained between the jets, and is finally equalized by the time it reaches the capillaries. It may seem an objection to this theory, that it would probably reqmre a larger quantity of blood to dilate all the arteries that can be discharged by the ventricle at each contraction. But the quantity necessary for such a purpose is less than might be supposed. Injections of the arteries prove that, including all down to those of about one-eighth of a line m diameter, CEAP. YII.] THE SriIYGMOGRArH. l8l they do not contain on an average more than one and a half pints of fluid, even when distendetl. There can be no doubt, therefore, that the three or four ounces which the ventricle discharges at each contraction, being added to that which already fills the arteries, is sufficient to distend them all, A distinction mu^t be carefully made between the passage of the 7vave along the arteries, and the velocity of the stream (p. 207) of blood. Both wave and current are present ; but the rates at which they travel are very different ; thatof the wave, 28'5 feet per second (E. H. Weber), being twenty or thirty times as great as that of the current. The Sphygmograph. A great deal of light has been thrown on what may be called the form of the pulse by the sphygmograph (figs. 96 and 97). Fig. 96.* BUTTOM. The principle on which the sphygmograph acts is very simple (see fig. 96). The small button replaces the finger in the act of Fig. 97.* taking the pulse, and is made to rest lightly on the artery, the pulsations of which it is desired to investigate. The up-and- * Fig. 96. Diagram of the mode of action of the Sphygmograph. t Fig. 97. The Sphygmograph applied to the arm. 1 82 CIRCULATIOX OF THE BLOOD. [chap. vii. down movement of the button is communicated to the lever, to tlie hinder end of which is attached a slight spring, which allows the lever to move up, at the same time that it is just strong enough to resist its making any sudden jerk, and in the interval of the beats also to assist in bringing it back to its original position. For ordinary pui'poses, the instrument is bound on the wrist (fig. 97}. It is evident that the beating of the pulse with the reaction of the spring will cause an up-and-down movement of the lever, .and if the extremity of the latter be inked, it will write the effect on the card, which is made to move by clockwork in the direction •of the arrow. Thus a tracing of the pulse is obtained, and in ithis way much more delicate effects can be seen, than can be felt on the application of the finger. Fig. 98 represents a healthy pulse-tracing of the radial artery, but somewhat deficient in tone. On examination, we see that Fia. 9S.* -> ^ the up-stroke which represents the beat of the pulse is a nearly vertical line, while the down-stroke is very slanting, and inter- rupted by a slight re-ascent. The more vigorous the pulse, if it be healthy, the less is this re-ascent, and vice versa. Fig. 99 Fig. 99. t represents the tracing of a healthy pulse in which the tone of the vessel is better than in the last instance, and tlie down- stroke is therefore less interrupted. In the large arteries, when at least there is much loss of tone, the up-stroke is double, the almost instantaneous i)ro- * Fig. 98. Pulse-tracing of radial artery, somewhat deficient in tone. + Fig. 99. Firm, and long pulse of vigorous health. CHAP. VII.] PULSE-TRACINGS. 183 pagation of tiie force of contraction of tlio left ventricle along the column of blood in the arteries, or the j)ercussio7i-impulse Fig. 100.* as it is termed by Dr. Burden- Sanderson, being siiiBciently strong to jerk up the lever for an instant, while the wave of blood, rather more slowly propagated from the ventricle, catches it, so to speak, as it begins to fall, and again slightly raises it. In the radial artery tracings, on the other hand, we see that the up-stroke is single. In this case the percussion-impulse is not sufficiently strong to jerk up the lever and produce an effect distinct from that of the systolic wave which immediately follows it, and which continues and completes the distension. In cases ^f feeble arterial tension, however, the percussion-impulse may be traced by the sphygmograph, not only in the carotid pulse, tut to a less extent in the radial also (fig. lOO). In looking now at the down-stroke (fig. 98) in the tracings, we see that in the case of an artery with deficient tone, it is interrupted by a well-marked notch, or, in other words, that the descent is interrupted by a slight uprising. There are indica- tions also of slighter irregularities or vibrations during the fall of the lever ; while these alone are to be seen in the pulse of health, or, in other words, when the walls of the artery are of good tone (fig. 99) . In some cases of disease the re-ascent is so considerable as to be perceptible to the finger, and this double beat has received the technical name of dicrotous " pulse. As a diseased condition this has long been recognised, but it is only since the invention of the sphygmograph that it has been found to belong in a certain degree to the normal pulse also. Various theories have been framed to account for the dicrotism * Fig. ICQ. Pulse-tracing of radial artery, with double apex. The above tracings are taken from Dr. Sanderson's work, *' On the Sphyg- mograph. " 1 84 CIRCULATION OF THE BLOOD. [chap. yii. of the pulse. By some, it is supposed to be due to the aortic valves, the sudden closure of which stops the incipient regurgita- tion of blood into the ventricle, and causes a momentary rebound throughout the arterial system; while Dr. Burden- Sanderson considers it to be caused by a rebound from the periphery rather than from the central part of the circulating apparatus. Pressure of the blood in the Aiteries, or Arterial Tension, From what has been previously said, it will have become evident that the blood in the arteries is always the subject of a certain amount of pressure, both during the Fig. loi. action of the ventricle and in the intervals. In the former case this is the direct result of the force exercised by the contracting ventricle, and, in the latter, by the force with which the walls of the arteries recoil after distension ; another necessary condi- tion being the comparative diflGlculty with which the blood escapes into the veins through the arterioles and capillaries (p. 179). The instrument employed for the purpose of gauging the amount of the blood- pressure or arterial tension is a mercurial manometer (fig. loi), of which the short horizontal limb (i) is connected, by me-3.ns of an elastic tube and canula, with the interior of an artery ; a solution of sodium or potassium carbonate being previously introduced into this part of the apparatus to prevent coagula- tion of the blood. The blood-pressure is thus com- municated to the upper part of the mercurial column (2) ; and the depth to which the latter sinks, added to the height to which it rises in the other (3), will give the height of the mercurial column which the blood-pressure balances ; the weight of the soda solution being subtracted. For the estimation of the arterial tension at any given moment, no further apparatus than this, which is called Poiseuille's TicBmadynamometer^ is neces- sary ; but for noting the 'variations of pressure in the arterial system, as well as its absolute amount, the instrument is usually combined with a registering apparatus and in this form is called a Tiymogra^h. CHAP. YII.] THE KYMOGRAPH. The kymograph, invented by Ludwie, is composed of a haemadynamo- meter, the open mercurial column of which supports a floating piston and Fig. 1 02.* vertical rod, with short horizontal pen (fig. 102). The pen is adjusted in contact with a sheet of paper, which is caused to move at an uniform rate Fig, 103. t hy clockwork; and thus the up-and-down movements of the mercunal column, which are communicated to the rod and pen, are marked or * Fig. 102. Diagram of Mercurial Kymograph, a, Floating rod and pen. The arrow is placed on the revolving paper cylinder, on which are inscribed the movements of the pen in contact with it. h, tube, which communicates with a bottle containing an alkaline solution, c, elastic tube and canula, the latter being intended for insertion in an artery. t Fig. 103. Normal tracing of arterial pressure obtained with the mercurial kymograph in the rabbit. (Burdon-Sanderson). 1 86 CIECULATIOX OF THE BLOOD [cha.p. vii. registered on the moving paper, as in the registering apparatus of the sphygmograph, and minute variations are graphically recorded (fig. 103). For some purposes sioring-liymograpli of Professor Fick (fig. 104) is pre- Fig. 104.* -ferable to the mercurial kymograph. It consists of a hollow C-shaped spring, iilled with fluid, the interior of which is brought into connection with the i^?*/;. 1 05.+ interior of an artery, by means of an elastic tube and cannla, as in the last case (fig. 102, 6^.). In response to the pressure transmitted to its interior, the * Fig. 104. Fick's Spring Kymograph. A. Hollow C-spring, fLxed at one end, I, to a piece of board, l, which is connected by an upright, H, to the wooden support, J. The other end a, is freely moveable, and its movements -are communicated by the rod, c, to the lever, a e, and thus to the writing needle, g. The C-spring is filled with alcohol, and its interior communicates with the artery through the tube k, which is tilled with a soda-solution, and to which is attached an elastic tube and canula. t Fig. 105. Normal arterial tracing obtained with Fick's kymograph in the dog. (Burdon-Sanderson). CHAr. VII.] ULOOD-PRESSUKE. 187 spring tends to straighten itself, and the movement thus produced is commu- nicated hj means of a lever to a writing-needle, and registering apparatus. Fig. 105 exhibits an ordinary arterial pulse-tracing, as obtained by the spring-kymograph. Poiseuille calculated, from the mean result of several observations on horses and dogs, that the blood-pressure in any large artery is capable of sujjporting a mercurial column of rather more than six inches in height ; and that to measure the absolute amount of this pressure in any artery, it is necessary merely to multiply the area of its transverse section by the height of the column of mercury which is already known to be. supported by the blood-pressure in any part of the arterial system. The weight of a column of mercury thus found will represent the pressure of the blood. Calculated in this way, he supposed that the blood-pressure in the human aorta is equal to 4 lb. 4 oz. avoirdupois ; that in the aorta of the horse being 1 1 lb. 9 oz. ; and that in the radial artery at the human wrists only 4 drs. Supposing the muscular power of the right ventricle to be only one-half that of the left, the blood-pressure in the pulmonary artery will be only 2 lb. 2 oz. avoirdupois. The amounts above stated represent the arterial tension at the time of the ventricular contraction. Many circumstances cause considerable variations in tlie amount of the blood-pressure. The following are the chief: — 1. The alternating systole and diastole of the heart; the arterial tension increasing during systole and diminishing during diastole. The greater the frequency, moreover, of the hearths contractions, the greater is the blood-pressure, cceteris paribus; although this effect is not constant, as it may be compensated for by the delivery into the arteries at each beat of a comparatively small quantity of blood. The greater the quantity of blood 'expelled from the heart at each contraction the greater is the blood-pressure. 2. The respiratory movements. Arterial tension is increased ■during inspiration, and falls during expiration. (Burdon-Sander- son.) 3. Variations in the degree of contraction of the smaller arteries modify the blood-pressure by favouring or impeding the accumulation of blood in the arterial system which follows -every contraction of the heart (p. 179); the contraction of the arterial walls increasing the blood-pressure, while their relaxa- tion lowers it. 4. The greater the total quantity of blood, the greater, cceteris paribus, is the blood-pressure. iS8 CIECULATION OF THE BLOOD. [chap. VII. Acting indirectly, that is, by influencing one or more of the above mentioned conditions which act directly, the nervous system powerfully affects in various ways the blood-pressure. (See Sections on the Influence of the Nervous System on the Heart and Bloodvessels.) A due amount of blood-pressure is, in the higher animals, one of the conditions of life ; inasmuch as it is only under such cir- cumstances that the blood is supplied to the various organs and tissues with constancy and force sufficient for the maintenance of their functions. This is best shown by the effect of its absence on the higher organs of the nervous system ; lessening of the blood-pressure below a certain amount being invariably accom- panied by a temporary or permanent cessation of their functions. Thus, syncope or fainting is caused by diminished blood-pressure in the cerebral arteries, depending either upon feebleness of the heart's action, or upon some other cause which di min ishes the arterial tension, as haemorrhage or the like. Influence of the Nervous System on the Arteries. The arteries of all parts of the body are supplied with nerve- fibres by the sympathetic system. Thus, the blood-vessels of the head and neck receive fibres from the superior cervical ganglion, those of the thorax from the cervical and upper dorsal, prsevertebral ganglia, and so forth ; the fibres, however, being frequently bound up in cerebro-spinal nerve bundles, and distri- buted as offsets from them. The tone of the arteries, or, in other words, the amount of contraction of the muscular fibres of the arterial coats (p. 1 76), which is ever varying, depends entirely on the influence which is exercised through these vasomotor branches of the sympathetic. If one of them be stimulated — as, for example, by applying aa electric current, the arteries to which its branches are distributed contract, and diminish the stream of blood which is flowing^ through them. If, on the other hand, the nerve be divided, the arteries are paralysed, that is, they lose their muscular tone altogether, and become dilated. (Brown-Sequard.) The most usual experiment in illustration of these facts is performed by CHAP. VII.] INFLUENCE OF NERVES ON ARTEHIES. 1 89 exposing in a rabbit the cervical sympathetic and dividing it. The blood- vessels of the corresponding side of the head and neck, thus paralysed, and unable to contract on the stream of blood in their interior, become dilated. The effect is best seen in the ear, the blood-vessels of which become mani- festly larger than those of the opposite side ; while the part becomes both redder and warmer from the increased quantity of blood which circulates in it. On galvanizing the distal cut end of the nerve, the muscular fibres of the blood vessels are caused to contract again; and while the stimulus lasts, the car and other parts become paler, colder, and less sensitive than natural. A familiar example of similar physiological conditions, arising from a different cause, is the act of blushing which is produced by a temporary paralysis of blood-vessels, and consequently enlarged stream of blood. Experiments by Ludwig and others show that the vasomotor nerves come primarily from grey matter (vasomotor centre) in the interior of the medulla oblongata, between the calamus scriptorius and the corpora qiiadrigemina. Thence the vasomotor fibres pass down in the interior of the spinal cord, and issuing with the anterior roots of the spinal nerves, traverse the various ganglia on the prse-vertebral cord of the sympathetic, and, accompanied by branches from these ganglia, pass to their destination. By the vasomotor nerve-centre in the medulla, which is always in action, more or less, the tone of all the blood- vessels is regulated ; but secondary or subordinate centres probably exist in the ganglia of various regions of the body, and through these, directly, under ordinary circumstances, vaso- motor changes are also effected. The nerve-impulses which issue from the vasomotor nerve- centres are for the most part the results of reflex action, and may lead to either contraction or dilatation of the blood-vessels. Thus, — on stimulating the sensory nerve of a part, the stimulus, if sufficiently strong, leads to contraction of all the blood-vessels of the body, except those which are situate in the region to which the sensory nerve in question is distributed ; and here the blood-vessels become dilated. In the former case (contraction) the action is called excito-motorj and in the latter inhibitory. A familiar example of such inhibitory action is afforded by the redness of the skin, which follows scratching or other slight injury. Cyon and Ludwig discovered that a remarkable power is 190 CIECULATION OF THE BLOOD. [chap. vii. exercised on the dilatation of the blood-vessels by a small nerve which arises, in the rabbit, from the superior laryngeal branchy or from this and the trunk of the pneumogastric nerve, and after communicating with filaments of the inferior cervical ganglion proceeds to the heart. If this nerve be divided, and its upper extremity feebly galvanised, an inhibitory influence is conveyed to the vasomotor centre in the medulla oblongata, so as to cause, by reflex action, dilatation of the principal blood-vessels, with diminution of the force and frequency of the heart's action. From the remarkable lowering of the blood pressure thus produced, this branch of the vagus is called the depressor nerve ; and it is presumed to be a means of conveying to the vasomotor centre indications of such conditions of the heart as require a diminu- tion of the tension in the blood-vessels ; as, for example, when the heart cannot, with sufiicient ease, propel blood into the already too full or too tense arteries. The influence of vasomotor changes in one part or region in relation to other parts of the body, is most notably shown by experiments on the function of the splanchnic nerves. These nerves contain the greater part of the vasomotor fibres of the blood-vessels of the abdominal viscera ; and, as the blood supply of the latter is normally very large, variations in its quantity will largely affect the blood pressure of all parts. On stimulating the splanchnics and thus causing contraction of the abdominal vessels, the general blood- pressure rises very considerably. On dividing these nerves, on the other hand, the abdominal blood-vessels dilate, and the blood-pressure falls ; and so large and numerous are th6 vessels of the abdominal viscera that, when fully dilated in consequence of the division of their nerves, they contain a great part of the whole mass of blood, and as a consequence other parts are drained of their due proportion. The effect of such a condition of the abdominal system of blood-vessels on other parts has, indeed, been compared to that of a large internal hemorrhage ; and the symptoms pro- duced in a living animal by division of the splanchnics, prove the justice of the comparison. THE CAPILLARIES. In all organic textures, exept some parts of the corpora cavernosa of the penis, and of the uterine placenta, and of the spleen, the transmission of the blood from the minute branches of the arteries to the minute veins is efiected through a network of microscopic vessels, called capillaries. These may be seen in all minutely injected preparations ; and during life, by the aid CHAP. VII.] CAPILLARIES. 191 of the microscope, in any transparent vascular parts, — sucli as the web of the frog's foot, the tail or external branchiae of the tadpole, or the wing of the bat. The branches of the minute arteries form repeated anastomoses with each other and give off the capillaries which, by their anas- tomoses, compose a continuous and uni- form network, from which the venous radicles, on the other hand, take their rise (fig. 106). The reticulated vessels connect- ing the arteries and veins are called capil- lary, on account of their minute size ; and intermediate vessels, on account of their position. The point at which the arteries terminate and the minute veins commence, cannot be exactly defined, for the transition is gradual ; but the capil- lary network has, nevertheless, this pe- culiarity, that the small vessels which compose it maintain the same diameter throughout ; they do not diminish in diameter in one direction, like arteries and veins; and the meshes of the net- work that they compose are more uni- form in shape and size than those formed by the anastomoses of the minute arteries and veins. The structure of the capillaries is much more simple than that of the arteries or veins. Their walls are composed of a single layer of elongated or radiate, flattened and nucleated cells, so joined and dovetailed together as to form a continuous transparent membrane (fig. 107). Outside these cells, in the larger capillaries, there is a structureless, or very finely fibrillated membrane, on the inner surface of which they are laid down. * Fig. 106. Blood-vessels of an intestinal villus, representing the arrange- ment of capillaries between the ultimate venous and arterial branches ; a, a, the arteries ; h, the vein. 192 CIRCULATION OF THE BLOOD. [chap, til In some cases this membrane is nucleated, and may then be regarded as a miniature representative of the tunica adventitia of arteries. Fig. 107.* Here and there at the junction of t^vo or more of the delicate endothelial cells Tvhich compose the capillary wall, stomata may be seen resembling those in serous membranes (p. 63). The endothelial cells are often continuous at various points with processes of adjacent connective tissue corpuscles. (An explana- tion of this latter appearance will be found in the Chapter on Development.) Capillaries are surrounded by a delicate nerve-plexus re- sembling, in miniature, that of the larger blood-vessels. The diameter of the capillary vessels varies somewhat in the ♦ Fig. 107. Magnified view of capillary vessels from the bladder of the cat. —A, T, an artery and a vein ; i, transitional vessel between them and c, c, the capillaries. The muscular coat of the larger vessels is left out in the figure to allow the epithehum to be seen : at c', a radiate epithelium scale with four pointed processes, running out upon the four adjoining capillaries (after Chrzonszczewesky, Yiixh. Arch. 1866). €11 AP. VII.] SIZE AND FORM OF CAPILLARIES. different textures of tlie body, the most common size being about ^i_th of an inch. Among the smallest may be mentioned those of the brain, and of the follicles of the mucous membrane of the intestines ; among the largest, those of the skin, and especially those of the medulla of bones. The size of capillaries varies also in different animals in relation to the size of their blood-corpuscles : thus, in the Proteus, the capillary circulation can just be discerned with the naked eye. The form of the capillary network presents considerable variety in the different textures of the body : the varieties consisting principally of modifications of two chief kinds of mesh, the rounded and the elongated. That kind in which the meshes or interspaces have a roundish form is the most common, and prevails in those parts in which the capillary network is most dense, such as the lungs (fig. 1 08), most glands, and mucous Fig. 108.* Fig. 109. f membranes, and the cutis. The meshes of this kind of network are not quite circular, but more or less angular, sometimes presenting a nearly regular quadrangular or polygonal form, * Fig. 108. Network of capillary vessels of the air-cells of the horse's lung, magnified, a, a, capillaries proceeding from b, terminal brandies of the pulmonary artery (Frey). t Fig. 109. Injected capillary vessels of muscle, seen with a low magnifying power (Sharpey). o 194 CIRCULATION OF THE BLOOD. [chap. vir. but being more frequently irregular. The capillary networlc with elongated meshes (fig. 109) is observed in parts in which the vessels are arranged among bundles of fine tubes or fibres^ as in muscles and nerves. In such parts, the meshes usually have the form of a parallelogram, the short sides of which may be from three to eight or ten times less than the long ones ; the long sides always corresponding to the axis of the fibre or tube, by which it is placed. The appearance of both the rounded and elongated meshes is much varied according as the vessels com- posing them have a straight or tortuous form. Sometimes the capillaries have a looped arrangement, a single capillary project- ing from the common network into some prominent organ, and returning after forming one or more loops, as in the papillae of the tongue and skin. The number of the capillaries and the size of the meshes in different parts determine in general the degree of vascularity of those parts. The parts in which the net-work of capillaries is closest, that is, in which the meshes or interspaces are the smallest, are the lungs and the choroid membrane of the eye. In the iris and ciliary body, the interspaces are somewhat wider, yet very small. In the human liver, the interspaces are of the same size, or even smaller than the capillary vessels themselves. In the human lung they are smaller than the vessels ; in the human kidney, and in the kidney of the dog, the diameter of the injected capillaries, compared with that of the interspaces, is in the proportion of one to four, or of one to three. The brain receives a very large quantity of blood ; but the capillaries in which the blood is distributed through its substance are very minute, and less numerous than in some other parts. Their diameter, according to E. H. Weber, compared with the long diameter of the meshes, being in the proportion of one to eight or ten ; compared with the transverse diameter, in the proportion of one to four or six. In the mucous membranes — for example, in the conjunctiva — and in the cutis vera, the capillary vessels are much larger than in the brain, and the interspaces narrower, — namely, not more than three or four times wider than the vessels. In the periosteum the meshes are much larger. In the ciTAP. VII.] CIECULATION IN CAPILLARIES. external coat of arteries, the width of the meshes is ten times that of the vessels (Henle). It may be held as a general rule, that the more active the functions of an organ are, the more vascular it is ; that is, the closer is its capillary network and the larger its supply of blood. Hence the narrowness of the interspaces in all glandular organ?, in mucous membranes, and in growing parts; their much greater width in bones, ligaments, and other very tough and comparatively inactive tissues; and the complete absence of vessels in cartilage, and such parts as those in which, probably, very little organic change occurs after they are once formed. But the general rule must be modified by the consideration, that some organs, such as the brain, though they have smaU and not very closely arranged capillaries, may receive large supplies of blood by reason of its more rapid movement. When an organ has large arterial trunks and a comparatively small supply of capillaries, the movement of the blood through it will be so quick, that it may, in a given time, receive as much fresh blood as a more vascular part with smaller trunks, though at any given instant the less vascular part will have in it a smaller quantity of blood. In the Capillary Circulation, as seen in any transparent part of a living adult animal by means of the microscope (fig. 110), the blood flows with a constant equable motion; the red blood-corpuscles moving along, mostly in single file, and bending in various ways to accommodate themselves to the tortuous course of the capillary, but instantly recovering their normal outline on reaching a wider vessel. In very young animals, the motion, though continuous, is accelerated at intervals correspond- ing to the pulse in the larger arteries, and a similar motion ol the blood is also seen in the capillaries of adult animals when they * Fig. HQ. Capillaries in the web of the frog's foot connecting a small artery' with a small vein. 0 2 196 CIRCULATION OF THE BLOOD. [chap. VII. are feeble : if tlieir exhaustion is so great that the power of the heart is still more diminished, the red corpuscles are observed to have merely the periodic motion, and to remain stationary in the intervals ; while, if the debility of the animal is extreme, they even recede somewhat after each impulse, apparently because of the elasticity of the capillaries, and the tissues around them. These observations may be added to those already advanced (p. 168) to prove that, even in the state of great debility, the action of the heart is sufficient to impel the blood through the capillary vessels. Moreover, Dr. Marshall Hall having placed the pectoral fin of an eel in the field of the microscope and com- pressed it by the weight of a heavy probe, observed that the movement of the blood in the capillaries became obviously pul- satory, the pulsations being synchronous with the contractions of the ventricle. The pulsatory motion of the blood in the capil- laries cannot be attributed to an action in these vessels ; for, when the animal is tranquil, they present not the slightest change in their diameter. It is in the capillaries, that the chief resistance is offered to the progress of the blood ; for in them the friction of the blood is greatly increased by the enormous multiplication of the surface with which it is brought in contact. At the circumference of the stream, in contact with the walls of the vessel, and adhering to them, there is a layer of liquor sanguinis which appears to be motionless. The existence of this still layer, as it is termed, is inferred both from the general fact that such an one exists in all fine tubes traversed by fluid, and from what can be seen in watching the movements of the blood- corpuscles. The red corpuscles occupy the middle of the stream and move with comparative rapidity; the colourless lymph - corpuscles run much more slowly by the walls of the vessel ; while next to the wall there is often a transparent space in which the fluid appears to be at rest; for if any of the corpuscles happen to be forced within it, they move more slowly than before, rolling lazily along the side of the vessej, and often adhering to its wall. Part of this slow movement of the pale corpuscles and their occasional stoppage may be due, as E. H. CHAP. VII.] DIAPEDESIS. 197 Weber has suggested, to their having a natural tendency to adhere to the walls of the vessels. Sometimes, indeed, when the motion of the blood is not strong, many of the white corpuscles collect in a capillary vessel, and for a time entirely prevent the passage of the red corpuscles. Diapedesis of Blood- Corpuscles. Until within the last few years it has been generally supposed that the occurrence of any transudation from the interior of the- capillaries into the midst of the surrounding tissues was confined., in the absence of injury, strictly to the fluid part of the blood in other words, that the corpuscles could not escape from the- circulating stream, unless the wall of the containing blood-vessel were ruptured. It is true that an English physiologist. Dr. Augustus Waller, afiirmed in 1 846, that he had seen blood- corpuscles, both red and white, pass bodily through the wall of the capillary vessel in which they were contained (thus con-- firming what had been stated a short time pre- viously by Dr. Addison) ; and that, as no open- ing could be seen before their escape, so none could be observed afterwards — so rapidly was the part healed. But these observations did not attract much notice until the phenomena of escape of the blood-corpuscles firom the capillaries and minute veins, apart from mechanical injury, were re-discovered by Professor Cohnheim in 1867. Professor Cohnheim's experiment demonstrating the passage of the corpuscles through the wall of the blood- vessel, is performed in the following manner. A frog is curarized, that is to say, paralysis is produced by injecting under the skin a minute quantity of the poison called curare ; and the abdomen having been opened, a portion of small intestine is drawn out, and its trans- parent mesentery spread out under a microscope. After a variable time, occupied by dilatation, following con- traction, of the minute vessels, and accompanying quickening of the bloody * Fig. III. A large capillary from the frog's mesentery eight hours after irritation had been set up, showing emigration of leucocytes, a, cells in the act of traversing the capillary wall ; 6, some already escaped (Frey). CIECULATION OF THE BLOOD. [chap. VI r. stream, there ensues a retardation of the current ; and blood-corpuscles, both red and white, begin to make their way through the capillaries and small veins. The diajjedesis of the white corpuscles is thus described by Dr. Burdon- Sanderson : — Simultaneously with the retardation, the leucocytes, instead of loitering here and there at the edge of the axial current, begin to crowd in numbers against the vascular wall, as was long ago described by Dr. Williams. In this way the vein becomes lined with a continuous pavement of these bodies, which remain almost motionless, notwithstanding that the axial current sweeps by them as continuously as before, though with abated velocity. Now is the moment at which the eye must be fixed on the outer contour of the vessel, from which (to quote Professor Cohnheim's words) here and there minute, colourless, button-shaped eleva- tions spring, just as if they were produced by budding out of the wall of the vessel itself. The buds increase gradually and slowly in size, until each assumes the form of a hemispherical projection, of width corresponding to that of a leucocyte. Eventually the hemisphere is converted into a pear-shaped body, the small end of which is still attached to the surface of the vein, while the round part projects freely. Gradually the little mass of proto- plasm removes itself further and further away, and, as it does so, begins to shoot out delicate prongs of transparent protoplasm from its surface, in nowise differing in their aspect from the slender thread by which it is still moored to the vessel. Finally the thread is severed and the process is complete.'^ The process of diapedesis of the red corpuscles, which occurs under circumstances of impeded venous circulation, and conse- quently increased blood-pressure, resembles closely that of the leucocytes, with the exception that they are squeezed through the wall of the vessel and do not, like the colourless corpuscles, work their way through by active amseboid movement. Various explanations of these remarkable phenomena have been suggested. Dr. Norris happily compares the phenomenon to the passage of a solid through a soap-bubble film, which closes up afterwards unbroken ; while others believe that minute o]3en- CHAP. VII.] DIAPEDESIS. 199 ings or stomata between contiguous endothelial cells (p. 1 92), provide the means of escape for the blood corpuscles. But the chief share in the process is to be found probably in the vital endowments with respect to mobility and contraction of the parts concerned — both of the corpuscles (Bastian) and the capillary wall (Strieker). Dr. Burdon-Sanderson remarks, *^the capillary is not a dead conduit, but a tube of living protoplasm. There is no difficulty in understanding how the membrane may open to allow the escape of leucocytes, and close again after they have passed out; for it is one of the most striking peculiarities of ►contractile substance that when two parts of the same mass are separated, and again brought into contact, they melt together as if they had not been severed." Hitherto, the escape of the corpuscles from the interior of the blood-vessels into the surrounding tissues has been studied chiefly in connection with pathology. But it is impossible to say, at present, to what degree the discovery may not influence all present notions regarding the nutrition of the tissues^ even in health. The circulation through the capillaries must, of necessity, be ■largely influenced by that which occurs in the vessels on either «ide of them — in the arteries or the veins ; their intermediate position causing them to feel at once, so to speak, any alteration in the size or rate of the arterial or venous blood-stream. Thus, the apparent contraction of the capillaries, on the application of certain irritating substances, and during fear, and their dilatation in blushing, may be referred to the action of the small arteries, rather than to that of the capillaries themselves. But largely as the capillaries are influenced by these, and by the conditions of the parts which surround and support them, their own endow- ments must not be disregarded. They must be looked upon, not as mere passive channels for the passage of blood, but as possessing endowments of their own, in relation to the circula- tion. The capillary wall is, according to Strieker, actively living and contractile ; and there is no reason to doubt that, as such, it must have an important influence in connection with the blood- current. 200 CIRCULATION OF THE BLOOD. [chap. "VII. The results of morbid action, as well as the phenomena of health, strongly support the notion of the existence of a force in the capillaries, which aids the circulation of the blood, after the same manner that nutritive fluids circulate in plants and lowly organised animals, which have no central propelling organ com- parable to a heart. But this so-called vital capillary force- occupies, in the higher animals, an entirely subordinate position. THE VEINS. In Structure the coats of veins bear a general resemblance to- those of arteries. Thus, they possess an outer, middle, and inter- nal coat. The outer coat is constructed of areolar tissue like that, of the arteries, but is thicker. In some veins it contains muscu- lar fibre-cells, which are arranged longitudinally. The middle coat is considerably thinner than that of the arteries ; and, although it contains circular unstriped muscular fibres or fibre-cells, these are mingled with a larger proportion of yellow elastic and white fibrous tissue. In the large veins- near the heart, namely the vencB cavcB and pulmonary veins, the middle coat is replaced, for some distance from the heart, by cir- cularly arranged striped muscular fibres, continuous with those' of the auricles. The internal coat of veins is less brittle than the corresponding- coat of an artery, but in other respects resembles it closely. The chief influence which the veins have in the circulation, is- effected with the help of the valves, which are placed in all veins subject to local pressure from the muscles between or near which they run. The general construction of these valves is similar to- that of the semilunar valves of the aorta and pulmonary artery, already described (p. 145) ; but their free margins are turned in the opposite direction, i.e, towards the heart, so as to stop any movement of blood backward in the veins. They are commonly placed in pairs, at various distances in different veins, but almost uniformly in each (fig. 1 1 2). In the smaller veins, single valves- are often met with; and three or four are sometimes placed CHAP. VII.] VALYES OF VEINS. 201 together, or n^r one another, in the largest veins, such as the subclavian, and at their junction with the jugular veins. The valves are semilunar; the un- attached edge being in some examples concave, in others straight. They are composed of inextensile fibrous tissue, and are covered with epithelium like that lining the veins. During the period of their inaction, when the venous blood is flowing in its proper direction, they lie by the sides of the veins ; but when in action, they close together like the valves of the arteries, and offer a complete barrier to any backward movement of the blood (figs. 113 and 1 14). Their situa- tion in the superficial veins of the fore- arm is readily discovered by pressing along its surface, in a direction opposite to the venous current, i.e., from the elbow towards the wrist ; when little swellings (fig. 1 12 c) appear in the position of each pair of valves. These swellings at once disappear when the pressure is relaxed. Valves are not equally numerous in all veins, and in many they are absent altogether. They are most numerous in the veins of the extremities, and more so in those of the leg than the arm. They are commonly absent in veins of less than a line in diameter, and, as a general rule, there are few or none in those which are not subject to muscular pressure. Among those veins which have no valves may be mentioned the superior and inferior vena cava, the trunk and branches of the portal vein, the hepatic and renal veins, and the pulinonary veins ; those in the interior of the cranium and vertebral column, those of the bones, and the * Fig. 112. Diagrams showing valves of veins. A. Part of a vein laid open and spread out, with two pairs of valves. B. Longitudinal section of a vein, showing the apposition of the edges of the valves in their closed state. C. Portion of a distended vein, exhibiting a swelling in the situation of a pair of valves. 202 CIRCULATION OF THE BLOOD. [GHAP. VII. trunk and branches of the umbilical vein are also destitute of valves. The principal obstacle to the circulation is already overcome when the blood has traversed the capillaries ; and the force of the heart which is not yet consumed, is sufficient to complete itR passage through the veins, in which the obstructions to its move- ment are very slight. For the formidable obstacle supposed to be presented by the gravitation of the blood, has no real existence, since the pressure exercised by the column of blood in the arteries, will be always sufficient to support a column of venous blood of the same height as itself: the two columns mutually balancing each other. Indeed, so long as both arteries and veins contain continuous columns of blood, the force of gravitation, whatever be the position of the body, can have no power to move or resist the motion of any part of the blood in any direction. The lowest blood-vessels have, of course, to bear the greatest amount of pressure ; the pressure on each part being directly proportionate to the height of the column •of blood above it : hence their liability to distension. But this pressure bears equally on both arteries and veins, and cannot either move, or resist the motion of, the fluid they con- tain, so long as the columns of fluid are of equal height in both, and continuous. In experiments to determine what proportion of the force of the left ventricle remains to propel the blood in the veins, Yalentin found that the pressure of the blood in the jugular vein of a dog, as estimated by the haemadynamometer, did not amount to more than or -^-^ of that in the carotid artery of the same animal. In the upper part of the inferior vena cava, Valentin €Ould scarcely detect the existence of any pressure, nearly the whole force received from the heart having been, apparently, consumed during the passage of the blood through the capillaries. But slight as this remaining force might be (and the experiment in which it was estimated would reduce the force of the heart below its natiu-al standard), it would be enough to complete the circulation of the blood ; for, as already stated, the spontaneous dilatation of the auricles and ventricles, though it may not be CHAP. Vll.j circulatioint in veins. 203 forcible enough to assist the movement of blood into them, is adapted to ofTer to that movement no obstacle. Very effectual assistance to the flow of blood in the veins is afforded by the action of the muscles capable of pressing on such veins as have valves. The effect of muscular pressure on such veins may be thus explained. When pressure is applied to any part of a vein, and the current of blood in it is obstructed, the portion behind the seat of pressure becomes swollen and distended as far back as to the next pair of valves. These, acting like the arterial valves, and being, like them, inextensible both in themselves and at their margins of attachment, do not follow the vein in its disten- sion, but are drawn out towards the axis of the canal. Then, if the pressure continues on the vein, the compressed blood, tending to move equally in all directions, presses the valves down into contact at their free edges, and they close the vein and prevent regurgitation of the blood. Thus, whatever force is exercised by the pressure of the muscles on the veins, is distributed partly in pressing the blood onwards in the proper course of the circula- tion, and partly in pressing it backwards and closing the valves hehind. The circulation might lose as much as it gains by such compression of the veins, if it were not for the numerous anasto- moses by which they communicate, one with another; for through these, the closing up of the venous channel by the backward pressure is prevented from being any serious hindrance to the •circulation, since the blood, of which the onward course is arrested by the closed valves, can at once pass through some anastomosing channel, and proceed on its way by another vein «?/.— During the day-time more carbonic acid is exhaled than corresponds to the oxygen absorbed ; while, on the other hand, at night very much more oxygen is absorbed than is exhaled in carbonic acid. There is, thus, a reserve fund of oxygen absorbed by night, to meet the- requirements of the day. If the total quantity of carbonic acid exhaled in 24 hours be represented by 100, 52 parts are exhaled during the day and 48 at night. While, similarly, 33 parts of the oxygen are absorbed during the day, and the remaining 67 by night. (Pettenkofer and Voit.) The ])eriod of day seems to exercise a slight influence on the amount of carbonic acid exhaled in a given time, though beyond the fact that the quantity exhaled is much less by night, we are scarcely in a position to state that variations in the amount exhaled occur at uniform periods of the day, independently of the influence of other circumstances. />. Food and Drinli.—Bj the use of food the quantity is increased, whilst by fasting it is diminished : and, according to Eegnault and Eeiset. it is greater when animals are fed on farinaceous food than when fed on meat. Dr. Edward Smith found that the effects produced by spirituous drinks depend much on the kind of drink taken. Pure alcohol tended rather to increase than to lessen respiratory changes, and the amount therefore of carbonic acid expired : rum, ale and porter, also sherry, had very similar effects. On the other hand, brandy, whisky and gin, particularly the latter, almost always lessened the respiratory changes, and consequently the amount of carbonic acid exhaled. i. Exercise and Sleej),— Bodily exercise, in moderation, increases the quantity to about one-thii'd more than it is during rest : and for about an hour after exercise, the volume of the air expii^ed in the minute is increased ciiAr. VIII.] ABSORPTION OF OXYGEN. 247 about 118 cubic inches : and the quantity of oarbonic acid about 7*8 cubic inches per minute. Violent exercise, such as full labour on the treadwheel, still further increases the amount of the acid exhaled. (Edward Smith.) During slccj^^ on the other hand, there is a considerable diminution in the quantity of this gas evolved ; a result probably in great measure dependent on the tranquillity of breathing. A larger quantity is exhaled when the barometer is low than when it is high. 3. The Oxygen in Respired Air is alivays less than in the same air before respiration, and its diminution is generally proportion- ate to the increase of the carbonic acid. The absorption of oxygen from inspired air would appear to depend not so much on a difference of tension of the gas in the air and venous blood, as on the strong chemical affinity which haemoglobin has for it (p. 11 7). Since the oxygen enters into chemical combination with the haemoglobin, its tension in the blood is very small, and hence an animal breathing in a closed space will consume almost all the oxygen in the contained air, though its tension constantly diminishes, if provision be made for the constant removal of the carbonic acid. For every volume of carbonic acid exhaled into the air, I • 1 742 1 volumes of oxygen are absorbed from it : and when the average quantity of carbonic acid, i.e., 1 346 cubic inches, or 636 grains, is exhaled in the hour, the quantity of oxygen absorbed in the same time is 1584 cubic inches or 542 grains (Valentin and Brunner). According to this estimate, there is more oxygen absorbed than is exhaled with carbon to form carbonic acid without change of volume; and to this general conclusion, namely, that the volume of air expired in a given time is less than that of the air inspired (allowance being made for the expansion in being heated), and that the loss is due to a portion of oxygen absorbed and not returned in the exhaled car- bonic acid, all observers agree, though as to the actual quantity of oxygen so absorbed, they differ even widely. The quantity of oxygen that does not combine with the carbon given off in carbonic acid from the lungs, is probably disposed of in forming some of the carbonic acid and water given off from the skin, and in combining with sulphur and phosphorus to form part of the acids of the sulphates and phosphates excreted in the urine, and probably also, from the experiments of Dr. Bence Jones, with the nitrogen of the decomposing nitrogenous tissues. The quantity of oxygen in the atmosphere surrounding 248 EESPIRATIOiNT. [chap. VIII. animals, appears to have very little influence on the amount of this gas absorbed by them, for the quantity consumed is not greater even though an excess of oxygen be added to the atmos- phere experimented with (Regnault and E-eiset). The Nitrogen of the Atmosphere, in relation to the respiratory process, is supposed to serve only mechanically, by diluting the oxygen, and moderating its action upon the system. This purpose, or the mode of expressing it. has been denied by Liebig, on the ground that if we suppose the nitrogen removed, the amount of oxygen in a given space would not be altered. But, although it be true that, if all the nitrogen of the atmosphere were removed and not replaced by any other gas, the oxygen might still extend over the whole space at present occupied by the mixture of which the atmosphere is composed ; yet since, under ordinary circumstances, oxygen and nitrogen, when mixed together in the ratio of one volume to four, produce a mixture which occupies precisely five volumes, with all the properties of atmospheric air, it must result that a given volume of atmosphere drawn into the lungs contains four-fifths less weight of oxygen than an equal volume composed entirely of oxygen. The greater rapidity and brilliancy with which combustion goes on in an atmosphere of oxygen than in one of common air, and the increased rapidity with which the ordinary effects of respiration are produced when oxygen instead of atmospheric air is breathed, leave no doubt that the nitrogen with which the oxygen of the atmosphere is mixed, has the effect of diluting this gas, under the present conditions of atmospheric pressure, in the same sense and degree as one part of alcohol is diluted when mixed with four parts of water. It has been often discussed whether nitrogen is absorbed by or exhaled from the lungs during respiration. At present, all that can be said on the subject is that, under most circumstances, animals appear to expire a very small quantity above that which exists in the inspired air. During prolonged fasting, on the contrary, a small quantity appears to be absorbed. 4. Watery Vapour is, under ordinary circumstances, always ex- haled from the lungs in breathing. The quantity emitted is, as a general rule, sufficient to saturate the expired air, or very nearly so. Its absolute amount is, therefore, influenced by the following circumstances, (i), by the quantity of air respired; for the greater this is, the greater also will be the quantity of moisture exhaled. (2), by the quantity of watery vapour contained in the air previous to its being inspired ; because the CHAr. YIII.] EXHALATION OF WATER. 249 greater this is, the less will be the amount required to complete the saturation of the air ; (3) by the temperature of the expired air ; for the higher this is, the greater will be the quantity of watery vapour required to saturate the air ; (4), by the length ' of time which each volume of inspired air is allowed to remain in the lungs ; for although, during ordinary respiration, the expired air is always saturated with watery vapour, yet when respiration is performed very rapidly the air has scarcely time to be raised to the highest temperature, or be fully charged with moisture ere it is expelled. The quantity of water exhaled from the lungs in twenty -four hours ranges (according to the various modifying circumstances already mentioned) from about 6 to 27 ounces, the ordinary quantity being about 9 or 10 ounces. Some of this is probably formed by the combination of the excess of oxygen absorbed in the lungs with the hydrogen of the blood; but the far larger proportion of it is water which has been absorbed, as such, into the blood from the alimentary canal, and which is exhaled from the surfaces of the air-passages and cells, as it is from the free surfaces of all moist animal membranes, particularly at the high temperature of warm-blooded animals. 5. The Rev. J. B. Eeade showed, some years ago, and Dr. Kichardson's experiments confirm the fact, that ammonia is among the ordinary constituents of expired air. It seems probable, however, both from the fact that this substance cannot be always detected, and from its minute amount when present, that the whole of it may be derived from decomposing particles of food left in the mouth, or from carious teeth or the like ; and that it is, therefore, only an accidental constituent of expired air. The quantity of organic matter in the breath has been inves- tigated by Dr. A. Ransome, who calculates that about 3 grains are given off from the lungs of an adult in twenty-four hours. The following represents the kind of experiment by which the foregoing facts regarding the excretion of carbonic acid, water, and organic matter, have been established. A bird or mouse is placed in a large bottle, through the stopper of which two tubes pass, one to supply fresh air, and the other to carry ofE that which 250 RESPIRATION. [chap. yiir. has been expired. Before entering the bottle, the air is made to bubble through a strong solution of caustic potash, which absorbs the carbonic acid, and then through lime-water, which by remaining limpid, proves the absence of carbonic acid. The air which has been breathed by the animal is made to bubble through lime-water, which at once becomes turbid, and soon quite milky from the precipitation of calcium carbonate ; and it finally passes through strong sulphuric acid, which, by turning brown, indicates the presence of organic matter. The watery vapour in the expii-ed air will condense inside the bottle if the surface be kept cool. By means of an apparatus, sufficiently large and well constructed, experi- ments of the kind have been made extensively on man. Changes produced in the Blood by Respiration. The most obvious change which the blood of the pulmonary artery undergoes in its passage through the lungs is that of colour y the dark crimson of venous blood being exchanged for the bright scarlet of arterial blood. (The circumstances which give rise to this change, and some other differences between arterial and venous blood, were discussed in the chapter on BiiOOD, pp. 123-5) : — 2ndy and in connection with the preced- ing change, it gains oxygen ; 2>^'d, it loses carbonic acid ; ^thy it becomes 1° or 2° F. warmer; 5^/1, it coagulates sooner and more firmly, and, apparently, contains more fibrin. The oxygen absorbed into the blood from the atmospheric air in the lungs is combined chemically with the haemoglobin of the red blood- corpuscles. In this condition it is carried in the arterial blood to the various parts of the body, and with it is, in the capillary system of vessels, brought into near relation or contact with the elementary parts of the tissues. In these tissues, and in the blood which circulates in them, a certain portion of the oxygen, which the arterial blood contains, dis- appears, and a proportionate quantity of carbonic acid and water is formed. The venous blood, containing the new-formed carbonic acid, returns to the lungs, where a portion of the carbonic acid is ex- haled, and a fresh supply of oxygen is again taken in. The process of respiration has often been compared to that of combustion. When a candle is burnt in a closed space, oxygen CHAP. YIII.] EFFECTS OF VITIATED AIR. 251 is abstracted, and the air becomes warmer and loaded with carbonic acid and watery vapour. TJie same changes take place when an animal is confined in a closed space, and in a short time, if no fresh air be admitted, the candle goes out, and the animal dies. But though the resemblance appears to be so close, it is really only a superficial comparison, for respiration is essentially a process of exchange, of elimination, and absorption, in which oxygen is ab- sorbed, and carbonic acid, watery vapour, and heat are given out. The process of oxidation, which may fairly be compared ta the burning of a candle or the rusting of iron, takes place in the tissues all over the body, and necessarily precedes the elimination of the waste products by the lungs. The experiments of Claude Bernard prove clearly the difference between respiration and combustion. A candle was placed in an atmosphere com- posed half of oxygen and half of carbonic acid : it was found to burn bril- liantly for some time, owing to the large proportion (50 per cent.) of oxygen present. A bird placed in the same atmosphere dies almost at once, for the great tension of carbonic acid in the atmosphere prevents any elimination of carbonic acid from the animal's lungs. That this is the explanation is proved as follows : — A candle and a small bird are placed each under a bell-glass containing air. After a certain time, the candle will go out and the bird expire. But if ^ just before this happens, a strong solution of potash be introduced into each to absorb the carbonic acid, the bird will quickly recover, while the candl&- will go out just as quickly as if no potash had been introduced. If a small bird be placed in tliis atmosphere, in which the candle has gone out, it will breathe easily for some time. Such an atmosphere contains 15 per cent, of oxygen (the rest having combined with the carbon and hydrogen of the candle to form carbonic acid and water) and 2 per cent, of carbonic acid (the rest having been absorbed by the potash). Thus we can make an artificial atmosphere in which a candle will bum while an animal will die, and Tice versa. The candle goes out from deficiency of oxygen, the animal expires mainly because of the excess of carbonic acid^ Effects of Vitiated Air, — Ventilation, We have seen that the air expired from the lungs contains a large proportion of carbonic acid and some organic putrescible matter. Hence it is obvious that if the same air be breathed again and again, the proportion of carbonic acid and organic matter will constantly increase till fatal results are produced ; but long 252 EESPIliATIOX. [chap. yiii. before tins point is reached, uneasy sensations occur, sucli as lieadache, languor, and a sense of oppression. It is a remark- able fact that the organism after a time adapts itself to such a vitiated atmosphere, and that a person soon comes to breathe, -v^'ithout felt inconvenience, an atmosphere which, when he first •entered it, seemed intolerable. But such an adaptation can only take place at the expense of .viii.] MECHANISM OF IlESPIRATOr.Y ACTIONS. 253 be so when the mechanism by which they are performed is clearly understood. The accompanying diagram (fig. 133) shows that the cavity of the chest is separated from that of the abdomen by the diaphragm, which, when acting, will lessen its curve, and ^'''r/- 133- thus descending, will push downwards and forwards the abdomi^ nal viscera; while the abdominal muscles have the opposite effect, and in acting will push the viscera upiuards and backwards, and with them the diaphragm, supposing its ascent to be not from any cause interfered with. From the same diagram it will be seen that the lungs communicate with the exterior of the body through the glottis, and further on through the mouth and 254 EESPIRATIOX. [ciiAr. virr. nostrils — tlirougli either of them separately, or through both at the same time, according to the position of the soft palate. The stomach communicates with the exterior of the body through the oesophagus, pharynx, and mouth ; while below, the rectum opens at the anus, and the bladder through the urethra. All these openings, through which the hollow viscera communicate with the exterior of the body, are guarded by muscles, called sphincters, which can act independently of each other. The position of the latter is indicated in the diagram. Let us take first the simple act of sighing. In this case there is a rather prolonged inspiratory effort by the diaphragm and other muscles concerned in inspiration; the air almost noiselessly passing in through the glottis, and by the elastic recoil of the lungs and chest- walls, and probably also of the abdominal walls, being rather suddenly expelled again. Now, in the first, or inspiratory part of this act, the descent of the diaphragm presses the abdominal viscera downwards, and of oourse this pressure tends to evacuate the contents of such as communicate with the exterior of the body. Inasmuch, how- ever, as their various openings are guarded by sphincter muscles, in a state of constant tonic contraction, there is no escape of their contents, and air simply enters the lungs. In the second, or expiratory part of the act of sighing, there is also pressure made on the abdominal viscera in the opposite direction, by the elastic or muscular recoil of the abdominal walls ; but the pres- sure is relieved by the escape of air through the open glottis, and the relaxed diaphragm is pushed up again into its original position. The sphincters of the stomach, rectum, and bladder, act as before. A familiar illustration of the physiological import of sighing is the well-known fact that when the mind is intensely concen- trated on any subject the respirations become very shallow (hence the expression breathless attention"). This shallow respiration is compensated for by the occurrence of a long sighing inspiration at frequent intervals. Hiccough resembles sighing in that it is an inspiratory act, but the inspiration is sudden instead of gradual, from the CHAP. VIII.] COUGHING: SNEEZING: SPEAKING. 255 diaphragm acting suddenly and spasmodically; and the air, therefore, suddenly rushing through the unprepared rima glottidis, causes vibration of the vocal cords, and the peculiar sound. In the act of coughing, there is most often first an inspiration, and this is followed by an expiration ; but when the lungs have been filled by the preliminary inspiration, instead of the air being easily let out again through the glottis, the latter is momentarily closed by the approximation of the vocal cords; and then the abdominal muscles, strongly acting, push up the viscera against the diaphragm, and thus make pressure on the air in the lungs until its tension is sufiicient to burst open noisily the vocal corda which oppose its outward passage. In this way a considerable force is exercised, and mucus or any other matter that may need expulsion from the lungs or trachea is quickly and sharply expelled by the outstreaming current of air. Now it is evident on reference to the diagram (fig. 1 3 3), that pressure exercised by the abdominal muscles in the act of cough- ing, acts as forcibly on the abdominal viscera as on the lungs, inasmuch as the viscera form the medium by which the upward pressure on the diaphragm is made, and of necessity there is quite as great a tendency to the expulsion of their contents as of the air in the lungs. The instinctive, and if necessary, volun- tarily increased contraction of the sphincters, however, prevents any escape at the openings guarded by them, and the pressure is effective at one part only, namely, the rima glottidis. The same remarks that apply to coughing, are almost exactly applicable to the act of sneezing ; but in this instance the blast of air, on escaping from the lungs, is directed by an instinctive contraction of the pillars of the fauces and descent of the soft palate, chiefly through the nose, and any offending matter is thence expelled. In speaJdng, there is a voluntary expulsion of air through the glottis by means of the abdominal muscles ; and the vocal cords are put, by the muscles of the larynx, in a proper position and state of tension for vibrating as the air passes over them, and thus producing sound. The sound is moulded into words by EESPIRATION. [chap. VIII. the tongue, teetli, lips, etc. — tlie vocal cords producing the sound only, and having nothing to do with articulation. Singing resembles speaking in the manner of its production ; the laryngeal muscles, by variously altering the position and degree of tension of the vocal cords, producing the different notes. Words used in the act of singing are of course framed, as in speaking, by the tongue, teeth, lips, etc. Sniffing is produced by a somewhat quick action of the diaphragm and other inspiratory muscles. The mouth is, how- ever, closed, and by these means the whole stream of air is made to enter by the nostrils. The alse nasi are, commonly, at the same time, instinctively dilated. Sucking is not properly a respiratory act, but it may be most conveniently considered in this place. It is caused chiefly by the depressor muscles of the os hyoides. These, by drawing downwards and backwards the tongue and floor of the mouth, produce a partial vacuum in the latter ; and the weight of the atmosphere then acting on all sides tends to produce equilibrium on the inside and outside of the mouth as best it may. The communication between the mouth and pharynx is completely shut off, probably by the contraction of the pillars of the soft palate and descent of the latter so as to touch the back of the tongue ; and the equilibrium, therefore, can be restored only by the entrance of something through the mouth. The action, indeed, of the tongue and floor of the mouth in sucking may be compared to that of the piston in a syringe, and the muscles which pull down the os hyoides and tongue, to the power which draws the handle. Sobbing consists in a series of convulsive inspirations, at the- moment of which the glottis is usually more or less closed. Laughing is a series of short and rapid expirations. Yawning is an act of inspiration, but is unlike most of the* preceding actions in being always more or less involuntary. It is attended by a stretching of various muscles about the palate' and lower jaw, which is probably analogous to the stretching of the muscles of the limbs in which a weary man finds relief, as a voluntary act, when they have been some time out of action. CHAP. VIII.] INFLUENCE OF NEEYOUS SYSTEM. 257 The involuntary and reflex character of yawning depends probably on the fact that the muscles concerned are themselves at all times more or less involuntary, and require, therefore, something beyond the exercise of the will to set them in action. For the same reason, yawning, like sneezing, cannot be well performed voluntarily. In the preceding account of respiratory actions, the diaphragm and abdominal muscles have been, as the chief muscles engaged and for the sake of clearness, almost alone referred to. But, of course, in all inspiratory actions, the other muscles of inspiration (p. 233) are also more or less engaged; and in expiration, the abdominal muscles are assisted by others, previously enumerated (p. 235) as grouped in action with them. Influence of the Nervous System in Respiration, Like all other functions of the body, the discharge of which is necessary to life, respiration must be essentially an involuntary act. Else, life would be in constant danger, and would cease on the loss of consciousness for a few moments, as in sleep. But it is also necessary that respiration should be to some extent under the control of the will. For were it not so, it would be impossible to perform those voluntary respiratory acts which have been just enumerated and explained, as speaking, singing, and the like. The respiratory movements and their rhythm, so far as they are involuntary and independent of consciousness (as on all ordinary occasions) are under the governance of a nerve-centre in the medulla oblongata corresponding with the origin of the pneumogastric nerves; that is to say, the motor nerves and, through them, the muscles concerned in the respiratory move- ments, are excited by a stimulus which issues from this part of the nervous system. How far the medulla acts automatically, i.e. how far the stimulus originates in it, or how far it is merely a nerve-centre for reflex action, is not certainly known. Probabl}-, both events happen ; and, in both cases, the stimulus is the result of the condition of the blood. 258 EESPIEATION. [chap. VI I r. On tlie latter (reflex) theory, the venous blood which the right ventricle propels to the lungs, is the direct excitant of the pneumogastric filaments distributed in these organs; and the stimulus is conveyed by these filaments to the medulla oblongata, and thence reflected to the respiratory muscles. In so far, on the other hand, as the medulla acts automati- cally, it is by virtue of the condition of the blood which circulates in it. So long as the blood is normal, it is a sufficient stimulus to the sensitive nerve-centre through which it circulates; and rhythmic impulses to respiratory action issue from the medulla. When the relative quantities of carbonic acid and oxygen in the blood are changed, the respiratory movements are changed also. If the oxygen be diminished or the carbonic acid increased, the respiratory movements are proportionally more frequent, and a greater number of muscles are engaged in their performance ; while an opposite effect is the result of an excess of oxygen with diminution of carbonic acid. The rhythm of the respiratory movements is best explained in the theory of rhythmic nutrition of the nerve-centres and muscles, as in the case of the heart (p. i66). Of the circumstances which cause the circulatory apparatus to act four or five times as fre- quently as the respiratory, we know nothing. Unlike the cardiac rhythm, that of respiration can be for a short time interfered with by the exercise of the will. But the need of breath respiratory sense, *'besoin de respirer'') becomes soon so urgent as to overcome the strongest opposition ; and no one has ever committed suicide by simply holding his breath, although, it is said, many have attempted to do so. The respiratory nerve-centre in ,the medulla oblongata is very sensitive to impressions other than those which are connected directly or by means of the pulmonary branches of the pneumo- gastric, with the condition of the blood. The effect on the respiratory movements of the sudden application of cold to the skin (as from a shower bath) and of various mental emotions is well known ; and many other examples might be quoted. At the time of birth, the separation of the placenta, and the consequent non-oxygenation of the foetal blood, axe the circumstances which immediately CHAP. VIII.] APN(EA: DYSPN(EA: ASPHYXIA. 259 lead to the issue of automatic impulses to action from the respiratory centre in the medulla oblongata. But the quickened action which ensues on the application of cold air or water, or other sudden stimulus, to the skin, shows well the intimate connection which exists between this centre and other parts which are not ordinarily connected with the function of respiration. The dependence of the function of respiration on the medulla oblongata is shown by the cessation of the respiratory move- ments and, therefore, instant death, which follows an injury to the respiratory nerve-centre, although every other part of the nervous system remain intact. Division of the spinal cord will affect respiration in different degrees according to the place of section. These facts are frequently illustrated by the effects of accidental injury in man. Thus, if the spinal cord be injured in the lower part of the cervical region, inspiration is performed by the diaphragm only, and the chest is almost motionless ; because there is an interruption to all communication between the medulla oblongata and the intercostal and many other respira- tory muscles. If the injury be somewhat higher in the neck, that is, above the origin of the phrenic nerves, death occurs immediately ; the respiratory centre in the medulla being now cut off from the diaphragm also. In the performance of voluntary respiratory acts, the brain, as well as the medulla oblongata, is engaged. But even when the brain is thus in action, it is the medulla oblongata which com- bines the several respiratory muscles, so that they act harmoniously together; while frequently the same nerve-centre brings into adapted combination of action many other muscles than those commonly exerted in respiration. Ajmcea. — Dyspnoea. — Asphyxia. As blood which contains a normal proportion of oxygen excites the respiratory centre (p. 258), and, as the excitement and consequent respiratory muscular movements are greater (dyspnoea) in proportion to the deficiency of this gas, so an abnormally large proportion of oxygen in the blood leads to diminished breathing movements, and, if large enough, to their temporary cessation. This condition of absence of breathing is termed s 2 26o EESPIEATIOX. [chap. VIII. cqmceaj"^' and it can be demonstrated, in one of the lower animals, by performing artificial respiration to the extent of saturating the blood with oxygen. When, on the other hand, the respiration is stopped, by, e.c/., interference with the passage of air to the lungs, or by supplying air devoid of oxygen, a condition ensues, which passes rapidly from the state of dyspncea (difficult breathing) to what is termed asphyxia; and the latter quickly ends in death. The most evident symptoms of asphyxia or suffocation are well-known. Violent action of the respiratory muscles and, more or less, of all the muscles of the body ; lividity of the skin and all other vascular parts, while the veins are also distended, and the tissues seem generally gorged with blood; convulsions, quickly followed by insensibility, and death. The conditions which accompany these symptoms are — (1) More or less interference with the passage of the blood through the pulmonary blood-vessels. (2) Accumulation of blood in the right side of the heart and in the systemic veins. (3) Circulation of impure (non-aerated) blood in all parts of the body. The causes of these conditions and the manner in which they act, so as to be incompatible with life, may be here briefly con- sidered. (1) The obstruction to the passage of blood through the lungs is not so great as it was once supposed to be ; and such as there is occurs chiefly in the later stages of asphyxia, when, by the violent and convulsive action of the expiratory muscles, pressure is indirectly made on the lungs, and the circulation through them is proportionally interfered with. (2) Accumulation of blood, with consequent distension of the right side of the heart and systemic veins, is the direct result, at least in part, of the obstruction to the pulmonary circulation just referred to. . Other causes, however, are in operation, (a) The * This term is, unfortunately, often apphed to conditions of clyHimcea or o.sx>hyxia; but the modern application of the term, as in the text, is the more convenient. CHAP. VITI.] ASPHYXIA. 261 vaso-motor centre (p. iSg) stimulated by blood deficient in oxygen, causes contraction of all the small arteries with increase of arterial tension, and as an immediate consequence tlie filling of the systemic veins, {h) The increased arterial tension is fol- lowed by inhibition of the action of the heart, and, thus, the latter, contracting less frequently, and gradually enfeebled also by deficient supply of oxygen, becomes over-distended by blood which it cannot expel. At this stage the left as well as the right cavities are distended with blood.* The ill effects of these conditions are to be looked for partly in the heart, the muscular fibres of which, like those of the urinary bladder or any other hollow muscular organ, are paralysed by over -stretching; and partly in the venous congestion, and consequent interference with the function of the higher nerve centres, especially the medulla oblongata. (3) The passage of non-aerated blood through the lungs and its distribution over the body are events incompatible with life, in one of the higher animals, for more than a few minutes ; the rapidity with w^hich death ensues in asphyxia being due, more particularly, to the effect of non-oxygenised blood on the medulla oblongata, and, through the coronary arteries, on the muscular substance of the heart. The excitability of both nervous and muscular tissue is dependent on a constant and large supply of oxygen, and, when this is interfered with, is rapidly lost. In some experiments performed by a committee appointed by the Medico- Chirurgical Society to investigate the subject of Suspended Animation, it was found that, in the dog, during simple asphyxia, i.e., by simple privation of air, as by plugging the trachea, the average duration of the respiratory movements after the animal had been deprived of air, was 4 minutes 5 seconds ; the extremes being 3 minutes 30 seconds, and 4 minutes 40 seconds. The average duration of the heart's action, on the other hand, was 7 minutes II seconds; the extremes being 6 minutes 40 seconds, and 7 minutes 45 seconds. It would seem, therefore, that on an average, the heart's action continues for 3 minutes 15 seconds after the animal has ceased to make respiratory efforts. A very similar relation was observed in the rabbit. Recovery never took place after the heart's action had ceased. * See "Handbook for the Physiological Laboratory," by Dr. Burdon- Sanderson, p. 322. 262 EESPIEATION. [chap. Yiir. The results obtained by the committee on the subject of drowning were very remarkable, especially in this respect, that whereas an animal may recover, after simple deprivation of air for nearly four minutes, yet, after submersion in water for ij minute, recovery seems to be impossible. This remarkable difference was found to be due, not to the mere submersion, nor directly to the struggles of the animal, nor to depression of temperature, but to the two facts, that in drowning, a free passage is allowed to air out of the lungs, and a free entrance of water into them. In proof of the correctness of this explanation it was found that when two dogs of the same size, one, however, having his windpipe plugged, the other not, were submerged at the same moment, and taken out after being under water for 2 minutes, the former recovered on removal of the plug, the latter did not. It is probably to the entrance of water into the lungs that the speedy death in drowning is mainly due. The results of jpost-mortem examination strongly support this view. On examining the lungs of animals deprived of air by plugging the trachea, they were found simply congested ; but in the animals drowned, not only was the congestion much more intense, accompanied with ecchy- mosed points on the surface and in the substance of the lung, but the air tubes were completely choked up with a sanious foam, consisting of blood, water, and mucus, churned up with the air in the lungs by the respiratory efforts of the animal. The lung-substance, too, appeared to be saturated and sodden with water, which, stained slightly with blood, poured out at any point where a section was made. The lung thus sodden with water was heavy (though it floated), doughy, pitted on pressure, and was incapable of collapsing. It is not difficult to understand how, by such infarction of the tubes, air is debarred from reaching the pulmonary cells : indeed the inability of the lungs to collapse on opening the chest is a proof of the ob- struction which the froth occupying the air- tubes offers to the transit of air. The entire dependence of the early fatal issue, in asphyxia by drowning, upon the open condition of the windpipe, and its results, was also strikingly shown by the following experiment. A strong dog had its windpipe plugged, and was then submerged in water for four minutes ; in three quarters of a minute after its release it began to breathe, and in four minutes had fully recovered. This experiment was repeated with similar results on other dogs. When the entrance of water into the iungs, and its drawing up with the air into the bronchial tubes by means of the respiratory efforts, were diminished, as by rendering the animal insensible by chloroform previously to immersion, and thus depriving it of the power of making violent respiratory efforts, it was found that it could bear immersion for a longer period without dying than when not thus rendered insensible. Probably to a like diminution in the respiratory efforts, may also be ascribed the greater length of time per- sons have been found to bear submersion without being killed, when in a state of intoxication, poisoning by narcotics, or during insensibility from syncope. We must carefully distinguish, the asphyxiating effect of carbonic acid from the directly poisonous action of such gases as carbonic oxide or common coal-gas. The fatal effects often produced by carbonic oxide (as in accidents from burning char- CHAP. IX.] ANIMAL HEAT. 263 coal stoves in small, close rooms), are due to its entering into combination witli the hsemoglobin of tlie blood-corpuscles (p. 118), and thus expelling the oxygen. CHAPTER IX. ANIMAL HEAT. The average temperature of the human body in those internal parts which are most easily accessible, as the mouth and rectum, is from 98-5° to 99 5° F. In different parts of the external surface of the human body the temperature varies only to the extent of two or three degrees, when all are alike protected from cooling influences; and the difference which under these circumstances exists, depends chiefly upon the different degrees of blood- supply. In the arm-pit — the most convenient situation, under ordinary circumstances, for examination by the thermometer — the average temperature is 98 -6° F. The temperature varies in different internal parts, by one or two degrees ; those parts and organs being warmest which contain most blood, and in which there occurs the greatest amount of chemical change. Thus the glands and the muscles are the warmest for this reason, and their temperature is highest, of course, when they are most actively working : while those tissues which, subserving only a mechanical function, are the seat of least active circulation and chemical change, are the coolest. These differences of temperature, however, are actually but slight, on account of the provisions which exist for maintaining uni- formity of temperature in different parts (p. 268). The chief circumstances by which the temperature of the healthy body is influenced are the following : — Age ; Sex ; Period of the day ; Exercise ; Climate and Season ; Food and Drink. Age. — The average temperature of the new-born child is only about 1° F. above that proper to the adult ; and the difference becomes still more trifling during infancy and early childhood. According to Wunderlich, the tempe- 264 AXIMAL HEAT. [ciiAr. IX. ratnre falls to the extent of about 4-° to h° F. from early infancy to puberty, and by about the same amount from puberty to fifty or sixty years of age. In old age the temperature again rises, and approaches that of infancy. Although the average temperature of the body, however, is not lower than that of younger persons, yet the power of resisting cold is less in them — exposure to a low temperature causing a greater reduction of heat than in young persons. The same rapid diminution of temperature was observed by M. Edwards in the new-born young of most carnivorous and rodent animals when they were removed from the parent, the temperature of the atmosphere being between 50° and o3J° F, ; whereas, while lying close to the body of the mother, their temperature was only 2 or 3 degrees lower than hers. The same law applies to the young of birds. Young sparrows, a week after they were hatched, had a temperature of 95° to 97°, while in the nest ; but when taken from it, their temperature fell in one hour to 66J°, the temperature of the atmosphere being at the time 62J°. It appears fi'om liis investigations, that in respect of the power of generating heat, some mammalia are born in a less developed condition than others ; and that the young of dogs, cats, and rabbits, for example, are inferior to the young of those animals which are not born blind. The need of external warmth to keep np the tempe- rature of new-born children is well known ; the researches of M. Edwards show, that the want of it is, as Hunter suggested, a much more frequent cause of death in new-born children than is generally supposed, and fm-nish a strong argument against the idea, that childi'en. by early exposui'e to cold, can soon be hardened into resisting its injurious influence. Sex. — The average temperature of the female would appear from observa- tions by Dr. Ogle to be very slightly higher than that of the male. Period of the Da?/. — The temperature undergoes a gradual alteration, to the extent of about 1° to 1 J° F. in the course of the day and night ; the mifiimuni being at night or in the early morning, the maximum late in the afternoon. Exercise. — Active exercise raises the temperature of the body from 1° to 2° F. (J. Davy, Clifford AUbutt). This may be partly ascribed to generally increased combustion-processes, and partly to the fact, that every muscular contraction is attended by the development of one or two degrees of heat in the acting muscle ; and that the heat is increased according to the number and rapidity of these contractions, and is quickly diffused by the blood circu- lating from the heated muscles. Possibly, also, some heat may be generated in the various movements, stretchings, and recoilings of the other tissues, as the arteries, whose elastic walls, alternately dilated and contracted, may give out some heat, just as caoutchouc alternately stretched and recoiling becomes hot. But the heat thus developed cannot be great. The great apparent increase of heat during exercise depends, in a great measure, on the increased ciiTulation and quantity of blood, and, therefore, greater heat, in parts of the body (as the skin, and especially the skin of the extremities), which, at the same time that they feel more acutely than others any changes of temperature, are. under ordinary conditions, by some degrees colder than organs more centrally situated. Climate and Season. — In passing fi-om a temperate to a hot climate the temperatui'e of the human body rises slightly, the increase rarely exceeding CHAP. IX.] YAHIATIONS OF TEMrERATUEE. 265 2° to 3° F. In summer the temperature of the body is a little hidier than in winter ; the difference amounting to from ^ to F. (Wunderlich.) The same effects are observable in alterations of temperature not depending on season or climate. Food and Drink. — The effect of a meal upon the temperature of a body is but small. A very slight rise usually occurs. Cold alcoholic drinks depress the temperature somewhat (4° to 1° F.). Warm alcoholic drinks, as well as warm tea and coffee, raise the temperature (about 4° F.). In disease the temperature of the body deviates from the normal standard to a greater extent than would be anticipated from the slight effect of external conditions during health. Thus, in some diseases, as pneumonia and typhus, it occasionally rises as high as 106° or 107° F. ; and consider- ably higher temperatures have been noted. In a case of malignant fever recorded by Dr. Norman Moore, the temperature in the axilla rapidly rose to 111° F., when the patient died. A temperature of 112*5° F. was observed by Wunderlich, in a case of idiopathic tetanus, at the time of death. In Asiatic cholera a thermometer placed in the mouth sometimes rises only to 77° or 79° ; and in a case of tubercular meningitis, observed by Dr. Gee, the temperature of the rectum remained for hours at 794° F. The temperature maintained by Mammalia in an active state of life, according to the tables of Tiedemann and Eudolphi, averages 101°. The extremes recorded by them were 96° and 106°, the former in the narwhal, the latter in a bat (Yespertilio Pipistrella). In Birds, the average is as high as 107° ; the highest temperature, 111-25°, being in the small species, the linnets, &c. Among Eeptiles, Dr. John Davy found, that while the medium they were in was 75°, their average temperature was 82*5°. As a general rule, their temperature, though it falls with that of the surrounding medium, is, in temperate media, two or more degrees higher ; and though it rises also with that of the medium, yet at very high degrees it ceases to do so, and remains even lower than that of the medium. Fish and Invertebrata present, as a general rule, the same temperature as the medium in which they live, whether that be high or low ; only among fish, the tunny tribe, with strong hearts and red meat-like muscles, and more blood than the average of fish have, are generally 7° warmer than the water around them. The difference, therefore, between what are commonly called the warm- and the cold-blooded animals, is not one of absolutely higher or lower tem- perature ; for the animals which to us, in a temperate climate, feel cold (being like the air or water, colder than the surface of our bodies), would, in an external temperature of 100°, have nearly the same temperature and feel hot to us. The real difference is, as Mr. Hunter expressed it, that what we call warm-blooded animals (Birds and Mammalia), have a certain '* per- manent heat in all atmospheres," while the temperature of the others, which we call cold-blooded, is "variable with every atmosphere." The power of maintaining a uniform temperature, which Mammalia and Birds possess, is combined with the want of power to endure such changes of temperature of their bodies as are harmless to the other classes : and when their power of resisting change of temperature ceases, they suffer serious disturbances or die. 266 ANIMAL HEAT. [chap. IX. Sources and Mode of Production of Heat in the Body, Id explaining the chemical changes effected in the process of respiration (p. 251), it was stated that the oxygen of the atmos- phere taken into the blood is combined, in the course of the circulation, with the carbon and the hydrogen of disintegrated and absorbed tissues, and of certain elements of food which have not been converted into tissues. That such a combination between the oxygen of the atmosphere and the carbon and hydrogen in the blood, is continually taking place, is made certain by the fact, that a larger amount of carbon and hydrogen is CDnstantly being added to the blood from the food than is required for the ordinary purposes of nutrition, and that a quantity of oxygen is also constantly being absorbed from the air in the lungs, of the disposal of which no account can be given except by regarding it as combining, for the most part, with the excess of carbon and hydrogen, and being excreted in the form of carbonic acid and water. In other words, the blood of warm- blooded animals appears to be always receiving from the digestive canal and the lungs more carbon, hydrogen, and oxygen than are consumed in the repair of the tissues, and to be always emitting carbonic acid and water, for which there is no other know^n source than the combination of these elements.^' By such combination, heat is continually produced in the animal body. It is not, indeed, necessary to assume that the combustion processes, which ultimately issue in the production of carbonic acid and water, are as simple as the bare statement of the fact might seem to indicate. But complicated as the various stages of combustion of organic matter in the blood and tissues may be, the ultimate result is as simple as in ordinary combustion outside the body, and the products are the same. The same amount of heat will be evolved in the union of any given quantities of carbon and oxygen, and of hydrogen and oxygen, whether the combination be rapid and evident, as in ordinary combustion, or slow and imperceptible, as in the changes which occur in the * Some "heat will also be generated in the combination of sulphur and phos- phorus with oxygen, to which reference has been made (p. 247) ; but the amount thus produced is but small. CHAP. IX.] EEGULATION OF TEMrERATURE. 26/ living body. And since the heat thus arising will be generated wherever the blood is carried, every part of the body will be heated equally, or nearly so. This theory, that the maintenance of the temperature of the living body depends on continual chemical change, chiefly by oxidation, of combustible materials existing in the tissues and in the blood, has long been established by the demonstration that the quantity of carbon and hydrogen which, in a given time, unites in the body with oxygen, is sufficient to account for the amount of heat generated in the animal w^ithin the same time : an amount capable of maintaining the temperature of the body at from 98° to IOO°, notwithstanding a large loss by radiation and evaporation. Many things observed in the economy and habits of animals are explicable by this theory, and may here briefly be quoted, although no longer required as additional evidence for its truth. Thus, as a general rule, in the various classes of animals, as well as in individual examples of each class, the quan- tity of heat generated in the body is in direct proportion to the activity of the respiratory process. The highest animal temperature, for example, is found in birds, in whom the function of respiration is most actively per- formed. In mammalia, the process of respiration is less active, and the average temperature of the body less, than in birds. In reptiles, both the respiration and the heat are at a much lower standard ; while in animals below them, in which the function of respiration is at the lowest point, a power of producing heat is, in ordinary circumstances, hardly discernible. Among these lower animals, however, the observations of Mr. Newport supply confirmatory evidence. He shows that the larva, in which the respiratory organs are smaller in comparison with the size of the body, has a lower temperature than the perfect insect. Volant insects have the highest temperature, and they have always the largest respiratory organs and breathe the greatest quantity of air : while among terrestrial insects, those also produce the most heat which have the largest respiratory organs and breathe the most air. During sleep, hybernation, and other states of inaction, respiration is slower or suspended, and the temperature is proportionately diminished ; while, on the other hand, when the insect is most active and respiring most voluminously, its amount of temperature is at its maximum, and corresponds with the quantity of respiration. Neither the rapidity of the circulation, nor the size of the nervous system, according to Mr. Newport, presents such a constant relation to the evolution of heat. On the Hegulatioyi of the Temperature of the Human Body. The continual production of heat in the body has been already referred to. There is also, of necessity, a continual loss. But 268 ANIMAL HEAT. [chap. IX. in healtliy warm-blooded animals the loss and gain of heat are so nearly balanced one by the other, that under all ordinary circumstances, an uniform temperature, within two or three degrees, is preserved. The loss of heat from the human body takes place chiefly by radiation and conduction from its surface, and by means of the constant evaporation of water from the same part, and from the air-passages. In each act of respiration, heat is also lost by so much warmth as the expired air acquires (p. 244). All food and drink which enter the body at a lower temperature than itself abstract also a small measure of heat : while the urine and faeces which leave the body at about its own temperature are also means by which a small amount is lost. By far the most important loss of heat from the body,- — probably 80 or 90 per cent, of the whole amount, is that which takes place by radiation, conduction, and evaporation from the skin. And it is to this part especially, and in a smaller measure to the air-passages, that we must look for the means by which the temperature is regulated; in other words, by which it is prevented from rising beyond the normal point on the one hand, or sinking below it on the other. The chief indirect means for accomplishing the same end are, variations in the amount and quality of the food and drink taken, variations in clothing, and in exposure to external heat or cold. In order to understand the means by which the heat of the body is regulated, it is necessary to take into consideration the following facts : First, the immediate source of heat in the body is the presence of a large quantity of a warm fluid — the blood, the temperature of which is, in health, about 100° F. In the second place, the blood, while constantly moving in a multitude of different streams, is every minute or so, gathered up in the heart, into one large stream, before being again dispersed to all parts of the body. In this way, the temperature of the blood remains almost exactly the same in all parts ; for while a portion of it in passing through one organ, as the skin, may become cooler, and through another organ, as the liver, may become warmer, tlie effect on each separate stream is more CHAP. IX.] EEGULATION OF TEMPEEATUEE. 269 or less neutralized when it mingles with, another, and an average is struck, so to speak, for all the streams when they form one, in passing through the heart. Uniformity of temperature is maintained also by the contiguity and continuity of the various organs and tissues one with another. The means by which the skin is able to act as one of the most important organs for regulating the temperature of the blood, are — (i), that it offers a large surface for radiation, conduction, and evaporation ; (2), that it contains a large amount of blood ; (3), that the quantity of blood contained in it is the greater under those circumstances which demand a loss of heat from the body, and vice versa. For the circumstance which directly determines the quantity of blood" in the skin, is that which governs the supply of blood to all the tissues and organs of the Dody, namely, the power of the vaso-motor nerves to cause a greater or less tension of the muscular element in the walls of the arteries (seep. 188), and, in correspondence with this, a lessening or increase of the calibre of the vessel, accompanied by a less or greater current of blood. A warm or hot atmosphere so acts on the nerve fibres of the skin, as to lead them to cause in turn a relaxation of the muscular fibre of the blood-vessels ; and, as a result, the skin becomes full-blooded, hot, and sweat- ing ; and much heat is lost. With a low temperature, on the other hand, the blood-vessels shrink, and in accordance with the consequently diminished blood-supply, the skin becomes pale, and cold, and dry. Thus, by means of a self-regulating appa- ratus, the skin becomes the most important of the means by which the temperature of the body is regulated. In connection with loss of heat by the skin, reference has been made to that which occurs both by radiation and conduction, and by evaporation ; and the subject of animal heat has been considered almost solely with regard to the ordinary case of man living in a medium colder than his body, and therefore losing heat in all the ways mentioned. The importance of the means, however, adopted, so to speak, by the skin for regulating the temperature of the body, will depend on the conditions by which it is surrounded; an inverse proportion existing in most cases 2/0 ANIMAL HEAT. [chap. IX. between tlie loss by radiation and conduction on the one hand, and by evaporation on the other. Indeed, the small loss of heat by evaporation in cold climates may go far to compensate for the greater loss by radiation; as, on the other hand, the great amount of fluid evaporated in hot air may remove nearly as much heat as is commonly lost by both radiation and evapora- tion in ordinary temperatures ; and thus, it is possible, that the quantities of heat required for the maintenance of an uniform proper temperature in various climates and seasons are not so different as they, at first thought, seem. Many examples might be given of the power which the body possesses of resisting the effects of a high temperature, in virtue of evaporation from the skin. Sir Charles Blagden and others supported a temperature varying between 198° and 211° F. in diy air for several minutes ; and in a subsequent experi- ment he remained eight minutes in a temperature of 260°. The workmen of Sir F. Chantrey were accustomed to enter a furnace, in which his moulds were dried, whilst the floor was red-hot, and a ther- mometer in the air stood at 350° ; and Chabert, the fire- king, was in the habit of entering an oven the temperature of which was from 400° to 600°." (Carpenter). But such heats are not tolerable when the air is moist as well as hot, so as to prevent evaporation from the body. Mr. C. James states, that in the vapour baths of Nero he was almost suffocated in a temperature of 112° while in the caves of Testaccio, in which the air is dry, he was but little incommoded by a temperature of 176°. In the former, evaporation from the skin was impossible ; in the latter it was abundant, and the layer of vapour which would rise from all the surface of the body would, by its very slowly conducting power, defend it for a time from the full action of the external heat. (The glandular apparatus, by which secretion of fluid from the skin is effected, will be considered in the Section on the Skin.) The ways by which the skin may be rendered more efficient as a cooling-apparatus by exposure, by baths, and by other means which man instinctively adopts for lowering his tempera- ture when necessary, are too well known to need more than to be mentioned. Although, under ordinary circumstances, the external application of cold only temporarily depresses the temperature to a slight extent, it is otherwise in cases of high temperature (107—108°) in fever. In these cases a tepid CHAP. IX.] REGULATION OF TEMPERATURE. 271 bath (80°) may reduce the temperature several degrees, and the effect so produced lasts for many hours. As a means for lowering the temperature, the lungs and air- passages are very inferior to the skin ; although, by giving heat to the air we breathe, they stand next to the skin in importance. As a regulating power, the inferiority is still more marked. The air which is expelled from the lungs leaves the body at about the temperature of the blood, and is always saturated with mois- ture. No inverse proportion, therefore, exists between the loss of heat by radiation and conduction on the one hand, and by evaporation on the other. The colder the air, for example, the greater will be the loss in all ways. Neither is the quantity of blood which is exposed to the cooling influence of the air di- minished or increased, so far as is known, in accordance with any need in relation to temperature. It is true that by varying the number and deptb of the respirations, the quantity of heat given off by the lungs may be made, to some extent, to vary also. But the respiratory passages, while they must be considered important means by which heat is lost, are altogether subordinate in the power of regulating the temperature, to the skin. It may seem to have been assumed, in the foregoing pages, that the only regulating apparatus for temperature required by the human body is one that shall, more or less, produce a cooling effect; and as if the amount of heat produced were always, therefore, in excess of that which is required. Such an assump- tion would be incorrect. We have the power of regulating the production of heat, as well as its loss. In food we have a means for elevating our temperature. It is the fuel, indeed, on which animal heat ultimately depends alto- gether. Thus, when more heat is wanted, we instinctively take more food, and take such kinds of it as are good for combustion ; while every-day experience shows the different power of resisting cold possessed respectively, by the well-fed and by the starved. In northern regions, again, and in the colder seasons of more southern climes, the quantity of food consumed is (speaking very generally) greater than that consumed by the same men or animals in opposite conditions of climate and season. And the food 272 AXIMAL HEAT. [chap. IX. which appears naturally adapted to the inhabitants of the coldest climates, such as the several fattv and oily substances, abounds in carbon and hydrogen, and is fitted to combine with the large Cjuantities of oxygen which, breathing cold dense air, they absorb from their lungs. In exercise, again, we have an important means of raising the temperature of our bodies (p. 264). The influence of external coverings for the body must not be unnoticed. In warm-blooded animals, they are always adapted, among other purposes, to the maintenance of uniform tempera- ture ; and man adapts for himself such as are, for the same purpose, fitted to the various climates to which he is exposed. By their means, and by his command over food and fire, he maintains his temperature on all accessible parts of the surface of the earth. The influence of the nervous system in modifying the production of heat has been already referred to. The experiments and observations which best illustrate it are those showing, first, that when the supply of nervous infiuence to a part is cut off, the temperature of that part falls below its ordinary degree ; and, secondly, that when death is caused by severe injury to, or removal of, the nervous centres, the temperatui^e of the body rapidly falls, even though artificial respiration be performed, the ciix-ulation maintained, and to all appearance the ordinary chemical changes of the body be completely efi'ected. It has been repeatedly noticed, that after division of the nerves of a limb its temperature falls ; and this diminution of heat has been remarked still more plainly in limbs deprived of nervous in- fiuence by paralysis. For example, Mr. Earle found the tempera- ture of the hand of a paralysed arm to be 70^, while the hand of the sound side had a temperature of 92^ F. On electrifying the paralysed limb, the temperature rose to yj^. In another case, the temperature of the paralysed finger was 56" F., while that of the unafiected hand was 62^. With equal certainty, though less definitely, the infiuence of the nervous system on the production of heat, is shown in the rapid and momentary increase of temperature, sometimes general, OHAr. IX.] SUN-STEOKE. at other times quite local, which is observed in states of nervous excitement; in the general increase of warmth of the body, sometimes amounting to perspiration, which is excited by passions of the mind; in the sudden rush of heat to. the face, which is not a mere sensation ; and in the equally rapid diminution of temperature in the depressing passions. But none of these instances suffice to prove that heat is generated by mere nervous action, independent of any chemical change ; all are explicable, on the supposition that the nervous system alters, by its power of controlling the calibre of the blood-vessels (p. 188), the quantity of blood supplied to a part; while any influence which the nervous system may have in the production of heat, apart from this influence on the blood-vessels, is an indirect one, and is derived from its power of causing nutritive change in the tissues, which may, by involving the necessity of chemical action, involve the production of heat. The existence of nerves which regulate animal heat otherwise than by their influence in trophic (nutritive) or vaso-motor changes, although by many considered probable, is not yet proven. In connection with the regulation of animal temperature, and its maintenance in health at the normal height, may be noted the result of circumstances too powerful, either in raising or lowering the heat of the body, to be controlled by the proper regulating apparatus. Walther found that rabbits and dogs, when tied to a board and exposed to a hot sun, reached a temperature of 114-8'' F., and then died. Cases of sunstroke furnish us with similar examples in the case of man; for it would seem that here death ensues chiefly or solely from eleva- tion of the temperature. In a case related by Dr. Gee, the temperature in the axilla was 109*5° F.; and in many febrile diseases the immediate cause of death appears to be the elevation of the temperature to a point inconsistent with the continuance of life. The efiect of mere loss of bodily temperature in man is less well known than the efi'ect of heat. From experiments by Walther, it appears that rabbits can be cooled down to 48° F. before they die, if artificial respiration bo 274 DIGESTION [chap. X. kept up. Cooled down to 64° F., they cannot recover unless external warmth be applied together with the employment of artificial respiration. Rabbits not cooled below 77"" F. recover by external warmth alone. CHAPTER X. DIGESTION. Digestion is the process by which the materials of our food are so changed as to be made fit for absorption and addition to the blood. Food. The following is a convenient tabular classification of the usual and necessary kinds of food : — r Nitrogenous :— Proteids, as Albumen, Casein, Sjoitonin, Gluten, and their allies, and Gelatin (containing Carbon, Hydrogen, Oxygen, and Nitrogen ; 6 some of them, also Sulphur and Phosphorus), g J g) 1 NON-NlTROGENOUS :— ^ (i). Amyloids — Starch, Sugar, and their allies (containing Carbon^ Hydrogen, and Oxygen). (2). Oils and Fats (containing Carbon, Hydrogen, and Oxygen ; ^the Oxygen in much smaller proportion than in starch or sugar). » 6 ( (3)- Mineral or Saline Matters ; as Chloride of Sodium, Phosphate 0*3 -J of Calcium, &c. ^ bi3 f (4). Water. Animals require, for food, both organic and inorganic sub- stances; the apparent sustenance of life and health on a diet composed of the first-named group only being due to the fact that inorganic substances are contained in all the natural organic foods. Pure fibrin, pure gelatin, and other organic principles purified from the inorganic substances naturally mingled with them, are incapable of supporting life for more than a brief time. Moreover, health cannot be maintained by any number of substances derived exclusively from one only of the two groups of CHAP. X.] COMPOSITION OF MILK AND EGGS. 275 organic alimentary principles mentioned above. A mixture of nitrogenous and non-nitrogenous organic substances, together with the inorganic principles which are severally contained in them, is essential to the well-being, and, generally, even to the existence of an animal. The truth of this has been demonstrated by experiments, and is illustrated also by the composition of those foods which are sufficient by themselves for the maintenance of life. Milk and eggs are good examples of this. Composition of Milk. Human. Cows'. Water .... 890 .... 858 Solids no 142 I5O00 1,000 Casein .... 35 .... 68 Butter 25 38 Sugar (with extractives) . 48 . . , .30 Salts 2 6 no 142 In milk, as will be seen from the preceding table, the albu- minous group of aliments is represented by the casein, the oleaginous by the butter, the aqueous by the water, the saccharine by the sugar of milk. Among the salts of milk are likewise phos- phate of calcium, alkaline and other salts, and a trace of iron ; so that it may be briefly said to include all the substances which the tissues of the growing animal need for their nutrition, and which are required for the production of animal heat. The yelk and albumen of eggs are in the same relation as food for the embryoes of oviparous animals, that milk is to the young of Mammalia, and afford another example of the necessity for a mixture of various alimentary principles. Composition of Fowls' Eggs. White. Water .... 78 . . Nitrogenous matter . . 20*4 , Fatty matter ... — . , Salts 1-6 Yelk. . 16 • 307 . 1*3 loo-o 1000 T 2 2/6 DIGESTION. [crrAi'. X. Experiments illustrating the same principle have been performed by Magendie and others. Dogs were fed exclusively on sugar and distilled water. During the first seven or eight days they were brisk and active, and took their food and drink as usual ; but in the course of the second week, they began to get thin, although their appetite continued good, and they took daily between six and eight ounces of sugar. The emaciation increased during the third week, and they became feeble, and lost their activity and appetite. At the same time an ulcer formed on each cornea, followed by an escape of the humours of the eye : this took place in repeated experiments. The animals still continued to eat three or four ounces of sugar daily ; but became^ at length so feeble as to be incapable of motion, and died on a day varying from the thirty-first to the thirty -fourth. On dissection, their bodies pre- sented all the appearances produced by death from starvation ; indeed, dogs will live almost the same length of time without any food at all. When dogs were fed exclusively on gum, results almost similar to the above ensued. When they were kept on olive-oil and water, all the pheno- mena produced were the same, except that no ulceration of the cornea took place : the effects were also the same with butter. The experiments of Chossat and Letellier prove the same ; and in men, the same is shown by the various diseases to which they who consume but little nitrogenous food are liable, and especially, as Dr. Budd has shown, by the affection of the cornea which is observed in Hindus feeding almost exclusively on rice. But it is not only the non-nitrogenous substances, which, taken alone, are in- sufiicient for the maintenance of health. The experiments of the Academies of France and Amsterdam were equally conclusive that gelatin alone soon ceases to be nutritive. Mr. Savory's observations on food confirm and extend the results obtained by Magendie, Chossat, and others. They show that animals fed exclusively on non-nitrogenous diet speedily emaciate and die, as if from starvation ; that life is much more prolonged in those fed with nitrogenous than by those with non-nitrogenous food ; and that animal heat is maintained a& well by the former as by the latter— a fact which proves that nitrogenous elements of food, as well as non-nitrogenous, may be regarded as calori- facient. Man is supported as well by food constituted wholly of animal substances, as by tliat wbich. is formed entirely of vegetable matters, on the condition, of course, that it contain a mixture of tlie various nitrogenous and non-nitrogenous substances just sboY/n to be essential for healthy nutrition. In the case of carnivorous animals, the food upon which they exist, con- sisting as it does of the flesh and blood of other animals, not only contains all the elements of which their own blood and tissues are composed, but contains them combined, probably, in the same forms. Therefore, little more may seem requisite, in the j)reparation of this kind of food for the nutrition of the body, than that it should be dissolved and conveyed into the blood in a condition capable of being re-organised. But in the case of CHAP. X.] STARVATIO^^. 277 the herbivorous animals, which feed exclusively upon vegetable substances, it might seem as if there would be greater difficulty in procuring food capable of assimilation into their blood and tissues. But the chief ordinary articles of vegetable food contain substances identical in composition with the albumen, fibrin, and casein, which constitute the principal nutritive materials in animal food. Albumen is abundant in the juices and seeds of nearly all vegetables ; the gluten which exists, especially in corn and other seeds of grasses as well as in their juices, is identical in composition with fibrin, and is often named vegetable fibrin ; and the substance named legumen, which is obtained especially fi'om peas, beans, and other seeds of leguminous plants, and from the potato, is identical with the casein of milk. All these vegetable substances are, equally with the corresponding animal principles, and in the same manner, capable of conversion into blood and tissue ; and as the blood and tissues in both classes of animals are alike, so also the nitrogenous food of both may be regarded as, in essential respects, similar. It is in the relative quantities of the nitrogenovTS and non-nitrogenous compounds in these different foods that the difference lies, rather than in the presence of substances in one of them which do not exist in the other. The only non-nitrogenous compounds in ordinary animal food are the fat, the saline matters, and water, and, in some instances, the vegetable matters which may chance to be in the digestive canals of such animals as are eaten whole. The amount of these, however, is altogether much less than that of the non-nitrogenous substances represented by the starch, sugar, gum, oil, etc., in the vegetable food of herbivorous animals. Starvation, The effects of total deprivation of food have been made the subject of experiments on the lower animals, and have been but too frequently illustrated in man. (i). One of the most notable effects of starvation, as might be expected, is loss of weight;, the loss being greatest at first, as a rule, but afterwards not varying very much, day by day, until death ensues. Chossat found that the ultimate propor- tional loss was, in different animals experimented on, almost exactly the same ; death occurring when the body had lost two- fifths (forty per cent.) of its original weight. Different parts of the body lose weight in very different pro- portions. The following results are taken, in round numbers, from the table given by M. Chossat : — Fat loses Blood Spleen . Pancreas 93 per cent. 75 71 64 DIGESTION. [CHAP. X. Liver loses 52 per cent. Heart 44 Intestines 42 „ Muscles of locomotion , . . . 42 „ Stomach loses 39 Pharynx, (Esophagus . , . . 34 „ Skin 33 „ Kidneys 31 Kespiratory apparatus . , , .22 „ Bones 16 „ Eyes 10 „ Kervous system 2 (nearly (2) . The effect of starvation on the temperature of the various animals experimented on by Chossat was very marked. For some time the variation in the daily temperature was more marked than its absolute and continuous diminution, the daily fluctuation amounting to 5° or 6° F., instead of 1° or 2° F., as in health. But a short time before death, the temperature fell very rapidly, and death ensued when the loss had amounted to about 30'' F. It has been often said, and with truth, although the statement requires some qualification, that death by star- vation is really death by cold ; for not only has it been found that differences of time with regard to the period of the fatal result are attended by the same ultimate loss of heat, but the effect of the application of external warmth to animals cold and dying from starvation, is more effectual in reviving them than the administration of food. In other words, an animal exhausted by deprivation of nourishment is unable so to digest food as to use it as fuel, and therefore is dependent for heat on its supply from without. Similar facts are often observed in the treatment of exhaustive diseases in man. (3) . The symptoms produced by starvation in the human subject are hunger, accompanied, or it may be replaced by pain^, referred to the region of the stomach ; insatiable thirst ; sleepless- ness ; general weakness and emaciation. The exhalations both from the lungs and skin are foetid, indicating the tendency to decomposition which belongs to badly-nourished tissues; and death occurs, sometimes after the additional exhaustion caused by diarrhoea, often with symptoms of nervous disorder, delirium, or convulsions. CHAP. X.] VARIETIES OF FOOD. 279 (4) . In the human subject death commonly occurs within six to ten days after total deprivation of food. But this period may be considerably prolonged by taking a very small quantity of food, or even water only. The cases so frequently related of survival after many days, or even some weeks, of abstinence, have been due either to the last-mentioned circumstances, or to others less effectual, which prevented the loss of heat and moisture. Cases in which life has continued after total absti- nence from food and drink for many weeks, or months, exist only in the imagination of the vulgar, (5) . The appearances presented after death from starvation are those of general wasting and bloodlessness, the latter con- dition being least noticeable in the brain. The stomach and intestines are empty and contracted, and the walls of the latter appear remarkably thinned and almost transparent. The various secretions are scanty or absent, with the exception of the bile, which, somewhat concentrated, usually fills the gall-bladder. All parts of the body readily decompose. It has just been remarked that man can live upon animal matters alone, or upon vegetables. The structure of his teeth, however, as well as experience, seems to declare that he is best fitted for a mixed diet ; and the same inference may be readily gathered from other facts and considerations. The food a man takes into his body daily, represents or ought to represent, the quantity and kind of matter necessary for replacing that which is daily cast out by the way of lungs, skin, kidneys, and other organs. To find out, therefore, the quantity and kind of food necessary for a healthy man, it will, evidently, be the best plan to consider in the first place what he loses by excretion. For the sake of example, we may now take only two elements, carbon and nitrogen, and, if we discover what amount of these is respectively dis- charged in a given time from the body, we shall be in a position to judge what kind of food will most readily and economically replace their loss. The quantity of carbon daily lost from the body amounts to about 4,500 grains, and of nitrogen 300 grains ; and if a man could be fed by these elements, as such, the problem would be a very simple one ; a corresponding weight of charcoal, and, allowiDg for the oxygen in it, of atmospheric air, would be all that is necessary. But, as before remarked, an animal can live only upon these elements when they are arranged in a particular manner with others, in the form of an organic compound, albumen, starch, and 280 DIGESTIOJ^". [chap. X. the like ; and the relative proportion of carbon to nitrogen in either of these compounds alone, is, by no means, the proportion required in the diet of man. Thus, in albumen, the proportion of carbon to nitrogen is only as 3'5 to I. If, therefore, a man took into his body, as food, sufficient albumen to supply him with the needful amount of carbon, he would receive more than four times as much nitrogen as he wanted ; and if he took only suffi- cient to supply him with nitrogen, he would be starved for want of carbon. It is plain, therefore, that he should take with the albuminous part of his food, which contains so large a relative amount of nitrogen in proportion to the carbon he needs, substances in which the nitrogen exists in much smaller quantities. Food of the latter kind is provided in such compounds as starch and fat. The latter indeed as it exists for the most part in considerable amount mingled with the flesh of animals, removes to a great extent, in a diet of aninial food, the difficulty which would otherwise arise from a deficiency of cai-bon— fat containing a large relative proportion of this element, and no nitrogen. To take another example ; the proportion of carbon to nitrogen in bread is about 30 to I. If a man's diet were confined to bread, he would eat, therefore, in order to obtain the requisite quantity of nitrogen, t^\ice as much carbon as is necessary ; and it is evident, that, in this instance, a certain quantity of a substance with a large relative amount of nitrogen is the kind of food necessary for redressing the balance. To place the preceding facts in a tabular form, and taking meat as an example instead of pm-e albumen : — meat contains about 10 per cent, of carbon, and rather more than 3 per cent, of nitrogen. Supposing a man to take meat for the supply of the needful carbon, he would require 45,000 grains, or nearly 6^ lbs. containing : — Carbon 46^ grains Nitrogen i?35<^ ^ Excess of Nitrogen above the amount required . 1,050 Bread contains about 30 per cent, of carbon and i per cent, of nitrogen. If bread alone, therefore, were taken as food, a man would require, in order to obtain the requisite nitrogen, 30,000 grains, containing — Carbon 9-000 grains Nitrogen 300 „ Excess of Carbon above the amount required . 4,500 But a combination of bread and meat would supply much more economi- cally what was necessary. Thus — Carbon. Nitrogen. 15,000 grains of bread (or rather more than 2lb.) contain 4-500 grs. 150 grs. 5,000 grains of meat (or about fib.) contain 500 ,, 150 5,000 ,, 300 „ So that f lb. of meat, and less than 2 lbs. of bread would supply all the needful carbon and nitrogen with but little waste. CHAr. X.] NECESSARY AMOUNT OF FOOD. 281 From these facts it will be plain that a mixed diet is the best and most economical food for man ; and the result of experience entirely coincides with what might have been anticipated on theoretical grounds only. It must not be forgotten that the value of certain foods may depend quite as much on their digestibility as on the relative quantities of the necessary elements which they contain. In ac^ual practice, moreover, the quantity and kind of food to be taken with most economy and advantage cannot be settled for each individual, only by considerations of the exact quantities of certain elements that are required. Much will of necessity depend on the habits and digestive powers of the individual, on the state of his excretory organs, and on many other circum- stances. Food which to one person is appropriate enough, may be quite unfit for another ; and the changes of diet so instinc- tively practised by all to whom they are possible, have much more reliable grounds of justification than any which could be framed on theoretical considerations only. In many of the experiments on the digestibility of various articles of food, disgust at the sameness of the diet may have had as much to do with inability to consume and digest it, as the want of nutritious properties in the substances which were experimented on. And that disease may occur from the want of particular food, is well shown by the occurrence of scurvy when fresh vegetables are deficient, and its rapid cure when they are again eaten : and the disease which is here so remarkably evident in its symptoms, causes, and cure, is matched by number- less other ailments, the causes of which, however, although analogous, are less exactly known, and therefore less easily combated. With regard to the quantity, too, as well as the kind of food necessary, there will be much diversity in difi'erent individuals. Dr. Dalton believed, from some experiments which he performed, that the quantity of food necessary for a healthy man, taking free exercise in the open air, is as follows : — Meat . . . . .16 ounces, or i 'oo lb. avoird. Bread . . . ... 19 „ „ ng „ „ Butter or Fat .... 3J „ 0-22 „ „ Water 52 fluid ozs. „ 3-38 „ „ 282 DIGESTION. [chap. X. The quantity of meat, however, here given is probably more in proportion to the other articles of diet enumerated than is needful for the majority of individuals under the circumstances stated. PASSAGE or FOOD THROUGH THE ALIMENTARY CANAL. The course of the food through the alimentary canal of man will be readily seen from the accompanying diagram (fig. 134). 134.* * Fig. 134. Diagram of the Alimentary Canal. The small intestine of man is from about 3 to 4 times as long as the large intestine. CHAP. X.] SALIVAllY GLANDS : MASTICATION. 283 The food taken into the mouth passes thence through the oeso- phagus into the stomach, and from this into the small and large intestine successively ; gradually losing, by absorption, the greater portion of its nutritive constituents. The residue, together with such matters as may have been added to it in its passage, is discharged from the rectum through the anus. We shall now consider, in detail, the process of digestion, as it takes place in each stage of this journey of the food through the alimentary canal. The Salivary Glands and the Saliva, The first of a series of changes to which the food is subjected in the digestive canal, takes place in the cavity of the mouth. The solid articles of food are here submitted to the action of the teeth (p. 92), whereby they are divided and crushed, and by being at the same time mixed with the fluids of the mouth, are reduced to a soft pulp, capable of being easily swallowed. The fluids with which the food is mixed in the mouth consist of the secretion of the salivary glands, and the mucous glands which line the mouth. Mastication and Insalivation, The act of chewing or mastication is performed by the biting and grinding movement of the lower range of teeth against the upper. The simultaneous movements of the tongue and cheeks, assist partly by crushing the softer portions of the food against the hard palate, gums, &c., and thus supplementing the action of the teeth, and partly by returning the morsels of food to the action of the teeth, again and again, as they are squeezed out jBrom between them, until they have been sufficiently chewed. The simple up and down, or biting movements of the lower jaw, are performed by the temporal, masseter, and internal ptery- goid muscles, the action of which in closing the jaws alternates with that of the digastric and other muscles passing from the^ OS hyoides to the lower jaw, which open them. The grinding or side to side movements of the lower jaw are performed mainly by the external pterygoid muscles, the muscle of one side acting^ 284 DIGESTION. [chap. X. alternately with the other. When both external pterygoids act together, the lower jaw is pulled directly forwards, so that the lower incisor teeth are brought in front of the level of the upper. The act of mastication is much assisted by the saliva which is secreted in largely increased amount during the process, and the intimate incorporation of which with the food, as it is being shewed, is termed insalivation. As in the case of so many other actions, that of mastication is partly voluntary, and partly reflex and involuntary. The con- sideration of such sensori'inotor actions will come hereafter (see Chapter on the Nervous System). It wiU suffice here to state that the nerves chiefly concerned are the sensory branches of the fifth, and the glosso -pharyngeal, and the motor branches of the fifth and ninth (hypoglossal) cerebral nerves. The nerve-centre by which the reflex action occurs, and by which the movements of the various muscles are harmonised, is situate in the medulla oblongata. In so far as mastication is voluntary or mentally perceived, it becomes so under the influence, in addition to the medulla oblongata, of the cerebral hemispheres. The function of the inter-articular fibro-cartilage of the tem- poro-maxillary joint in mastication may be here mentioned, (l) As an elastic pad it serves well to distribute the pressure €aused by the exceedingly powerful action of the masticatory muscles. (2) It also serves as a joint-surface or socket for the condyle of the lower jaw, when the latter has been partially drawn forward out of the glenoid cavity of the temporal bone by the external pterygoid muscle, some of the fibres of the latter being attached to its front surface, and consequently drawing it forward with the condyle which moves on it. The glands concerned in the production of saliva, are very extensive, and, in man and Mammalia generally, are presented in the form of four pairs of large glands, the parotid, submaxil- lary, sublingual, and numerous smaller bodies, of similar structure and with separate ducts, which are scattered thickly beneath the mucous membrane of the lips, cheeks, soft palate, and root of the tongue. The structure of aU these glands is essentially the same. Each is composed of several parts, caUed CHAr. X.] STRUCTUEE OF SALIVAEY GLANDS. 285 lobes, which are connected by areolar tissue ; and each of these ]obes, again, is made up of a number of smaller parts called Fig. 135.* lobules, bound together as before by areolar tissue. Each of these lobules is a miniature representation of the whole gland. It contains a small branch of the duct, which, subdividing, ends in small vesicular pouches, called acini, a group of which may be considered the dilated end of one of the smaller ducts (fig. 1 3 5 v^) ^ Fig. I36.t Each of the acini is about of an inch in diameter, and is formed of a fine structureless membrane, lined on the inner surface and often filled by spheroidal or glandular epithelium while on the outside is a plexus of capillary blood-vessels (% 136) . * F''g. 135. Diagram of a racemose or saccular compound gland ; on, entire gland, showing branched duct and lobular structure ; n, a lobule detached, with 0, branch of duct proceeding from it (Sharpey). t Fig. 136. Section of Submaxillary gland of dog. Showing gland-cells (h) and a duct, a, in section (Kolliker) 286 DIGESTION. [chap. X. In the salivaiy glands of many animals besides man two distinct kinds of gland-cells — the " mucous " and " protoplasmic " — may be distinguished. The mucous cells are closely packed in the acini while the protoplasmic cells, which are darker and strongly granulated, occupy the peripheral part of the acini like the peptic cells of the gastric glands (p. 300), which they resemble both in position and general appearance. After prolonged irritation of the secreting nerves remarkable changes occur : the mucous cells discharge their contents in the saliva, and the gland is now found to be filled with protoplasmic cells ; these multiply rapidly by -division and many of them by a transformation of their cell- contents into mucin become mucous cells, thus restoring the gland to its original condition (Heidenhain). According to Pfliiger's researches some of the terminal filaments of the nerves which enter the submaxillary gland actually enter the substance of the secreting cells and end in their nuclei ; but these observations require further confirmation before they can be finally accepted. Independently of them there seem strong grounds for accepting the view that the cliorda tympani contains " secreting fibres" which directly influence the gland cells, in addition to the vaso-inhibitory fibres whose stimulation •causes an increased blood supply and consequently an increased secretion (p. 289). Saliva, as it commonly flows from the mouth, is mixed with the secretion of the mucous glands, and often with air bubbles, which, being retained by its viscidity, make it frothy. When obtained from the parotid ducts, and free from mucus, saliva is a transparent watery fluid, the specific gravity of which varies from i -004 to i 'OOS, and in which, when examined with the microscope, are found floating a number of minute particles, derived from the secreting ducts and vesicles of the glands. In the impure or mixed saliva are found, besides these particles, numerous epithelial scales separated from the surface of the mucous membrane of the mouth and tongue, and mucus- corpuscles, discharged probably from the mucous glands of the mouth and the tonsils, which, when the saliva is collected in a deep vessel, and left at rest, subside in the form of a white opaque matter, leaving the supernatant salivary fluid transparent and colourless, or with a pale bluish-grey tint. In reaction, the saliva, when first secreted appears to be always alkaline; and that from the parotid gland is said to be more strongly alkaline than that from the other salivary glands. This alkaline con- dition is most evident when digestion is going on, and according to Dr. Wright, the degree of alkalinity of the saliva bears a CHAP. X.] THE SALIVA. 28;- direct proportion to the acidity of the gastric fluid secreted at the same time. During fasting, the saliva, although secreted alkaline, shortly becomes neutral; and it does so especially when secreted slowly and allowed to mix with the acid mucous of the mouth, by which its alkaline reaction is neutralised. The following analysis of the saliva is by Frerichs : — Composition of Saliva. Water 994-* lo Solids 5-90 Ptyalin 1-41 Fat 0*07 Epithelium and Mucus Salts :— Sulpho-Cyanide of Potassium Sodium Phosphate Calcium Phosphate . Magnesium Phosphate . Sodium Chloride Potassium Chloride 5-90 The rate at which saliva is secreted is subject to considerable variation. "WTien the tongue and muscles concerned in masti- €ation are at rest, and the nerves of the mouth are subject to no unusual stimulus, the quantity secreted is not more than suffi- cient, with the mucus, to keep the mouth moist. But the flow is much accelerated when the movements of mastication take place, and especially when they are combined with the presence of food in the mouth. It may be excited also, even when the mouth is at rest, by the mental impressions produced by the sight or thought of food ; also by the introduction of food into the stomach. The influence of the latter circumstance was well shown in a case mentioned by Dr. Gairdner, of a man whose pharynx had been divided : the injection of a meal of broth into the stomach was followed by the secretion of from six to eight ounces of saliva. Under these varying circumstances, the quantity of saliva secreted in twenty-four hours varies also ; its average amount is probably from I to 2 lbs. (Harley). 288 DIGESTION. [chap. X. The process of secretion in the salivary glands is identical with that of s^lands in sreneral (see chapter on Secretion) ; the cells which line the ulti- mate branches of the ducts {h, fig. 136) being the agents by which the special constituents of the saliva are formed. The process may be compared with that of growth, inasmuch as these cells grow and develope as they take up into their substance the materials of the secretion they are destined to form. Their most highly developed condition, however, is but transitory. The materials which they have incorporated with themselves are almost at once given up again, in the form of a fluid (secretion), which escapes from the ducts of the gland ; and the cells, themselves, to what extent it is not known, undergo disintegration, — again to be renewed, in the intervals of the active exercise of their functions. The source whence the cells obtain the materials of their secretion, is the blood, or, to speak more accurately, the plasma which is filtered off from the circulating blood into the interstices of all living textures. The secretion of saliva is probably continuous, but it is very largely increased during the period of digestion ; and the con- dition of the glands corresponds with the difference. During digestion the process of secretion is in excess ; while in the intervals the growth of the cells is in excess of their disinte- gration. These facts have been confirmed by microscopic exami- nation (p. 286). The increased secretion and discharge of saliva, which occur on the introduction of food into the mouth, are due to reJieoiT nervous action, the afferent or sensory filaments concerned being branches of the fifth cerebral and glossopharyngeal nerves, and the efferent fibres, the facial and sympathetic (Bernard). The chief nerve-centre concerned is situate in the medulla oblongata ; but the submaxillary ganglion is also engaged, when the stimulus is other than gustatory (Bernard). The influence of the nervous system on the secretion from the salivary glands has been made the subject of direct experiment on some of the lower animals, especially the dog : the submaxillary being the gland chiefly operated on, fi'om the comparative facility with which it is exposed, with its vessels and nerves. Nerve-fibres are supplied to the gland from the facial (chorda tympani), from the superior cervical ganglion of the sympathetic, and from the sub- maxillary gangbon. After exposure of the parts, if the cliorda tympani be stimulated by a galvanic current, the ai'teries dilate, the stream of blood thi'ough the gland becomes larger and more rapid : even the veins may pulsate, and the blood in them is more arterial than venous. At the same time an abundance of watery saliva is secreted, and flows from the duct. A similar, but less striking effect is produced when the gustatory nerve CHAP. X.] USES OF SALIYA. 289 having been divided, its central end is galvanized. In tliis case, the stimulus conveyed to the medulla oblongata by the fibres of the gustatory is reflected to the submaxillary gland by the chorda tympani filaments of the facial. AVhen, on the other hand, the stimulus is applied to the symjmtliGtic filaments, the arteries contract, the blood stream is in consequence much diminished, and from the veins, when opened, there escapes only a sluggish stream of dark blood. The saliva, instead of being abundant and watery, becomes, as one would expect, scanty and tenacious. If both chorda tympani and sym- })athetic branches be divided, the gland, released from nervous control, secretes continuously and abundantly {paralytic secretion). The abundant secretion of saliva, which follows stimulation of the chorda tympani, is not merely the result of a filtration of fluid from the blood- vessels, in consequence of the largely increased circulation through them. This is proved by the fact that, when the main duct is obstructed, the pressure within may considerably exceed the blood-pressure in the arteries (Ludwig). At the same time, the altered state of the blood-current is the chief factor, indirectly, in the production of the increased secretion, in- asmuch as, with it, there is necessarily a greater supply of the materials, whence the gland-cells may take the constituents of saliva. It is quite possible that the secreting cells are also directly influenced by the nervous system ; and it may be fairly concluded that this is so, if Pfluger's observations on the termination of nerve-filaments in the cells (p. 286) are confirmed by future investigations. The nerves which influence secretion in the parotid gland are branches of the facial (lesser superficial petrosal) and of the sympathetic. The former nerve, after passing through the otic ganglion, joins the auriculo-temporal branch of the fifth cerebral nerve, and, with it, is distributed to the gland. The nerves by which the stimulus exciting to secretion is conveyed to the medulla oblongata, are, as in the case of the submaxillary gland, the fifth, and the glossopharyngeal. The pneumogastric nerves convey a further stimulus to the secretion of saliva, when food has entered the stomach ; the reflection occurring at the medulla oblongata. The purposes served hy saliva are of several kinds. In the first place, acting mechanically in conjunction with mucus, it keeps the mouth in a due condition of moisture, facilitating the move- ments of the tongue in speaking, and the mastication of food. (2.) It serves also in dissolving sapid substances, and rendering them capable of exciting the nerves of taste. But the principal mechanical purpose of the saliva is, (3) that by mixing with the food during mastication, it makes it a soft pulpy mass, such as may be easily swallowed. To this purpose the saliva is adapted both by quantity and quality. For, speaking generally, the quantity secreted during feeding is in direct proportion to the dryness and hardness of the food. The quality of saliva is equally adapted to this end. It is easy to see how much more u 290 DIGESTION. [chap. X. readily it mixes witli most kinds of food than water alone does ; and M. Bernard has shown that the saliva from the parotid, labial, and other small glands, being more aqueous than the rest, is that which is chiefly braided and mixed with the food in mastication ; while the more viscid mucoid secretion of the submaxillary, palatine, and tonsillitic glands is spread over the surface of the softened mass, to enable it to slide more easily through the fauces and oesophagus. This view obtains confirma- tion from the interesting fact pointed out by Professor Owen, that in the great ant-eater, whose enormously elongated tongue is kept moist by a large quantity of viscid saliva, the sub- m.axillary glands are remarkably developed, while the parotids are not of unusual size. Beyond these, its mechanical purposes, saliva performs (4) a chemical part in the digestion of the food. When saliva, or a portion of a salivary gland, or even a portion of dried j^tyalin, is added to starch paste in a test-tube and the mixture kept at a temperature of 100° F., the starch is very rapidly transformed into dextrin and grape-sugar. In such an experiment the presence of sugar is at once discovered by the application of Trommer's test, which consists in the addition of a drop or two of a sokition of sulphate of copper, followed by a larger quantity of caustic potash. When the liquid is boiled an orange red precipitate of sub- oxide of copper indicates the presence of sugar ; and when common raw starch is masticated and mingled with saliva, and kept with it at a tempera- ture of 90° or 100°, the starch-grains are cracked or eroded, and their con- tents are transformed in the same manner as the starch-paste. Changes similar to these are effected on the starch of farina- ceous food (especially after cooking) in the stomach; and it is reasonable to refer them to the action of the saliva, because the acid of the gastric fluid tends to retard or prevent, rather than favour, the transformation of the starch. It may there- fore be held, that one purpose served by the saliva in the digestive process is that of assisting in the transformation of the starch, which enters so largely into the composition of most articles of vegetable food, and which (being naturally insoluble) is converted into soluble dextrin and grape-sugar, and made fit for absorption. CHAP. X.] FERMENTS. 291 The power of converting starch into sugar appears in man to belong alike to the saliva secreted by the parotid, submaxillary, and sublingual glands, while in many of the lower animals, e.g., dogs, it is only present in the parotid secretion. The salivary glands of children do not become functionally active till tlie age of 4 to 6 months, and hence the bad effect of feeding them before this age on starchy food, corn flour, &c., which they are unable to render soluble and capable of absorption. The important class of bodies, known as Ferments, exist chiefly in the digestive fluids. They are nitrogenous bodies, which have the power, under suitable conditions, of initiating and carrying on chemical changes in various organic substances ; a very small quantity of the ferment serving for the transformation of very large quantities, e.g., of starch into sugar or albu- minoids into peptones (p. 307). They have, further, the following characters in common. Their action is retarded, or even quite prevented, by cold ; a moderate warmth (100° F.) greatly facilitates it, while a high temperature (above 140° F.) completely prevents their action. In this case the action is not merely temporarily suspended, but irrecoverably destroyed. In many cases their action appears to depend upon taking up water : thus the trans- formation of starch, H.^ O5, into sugar, H,^ 0 involves the addition of one molecule of water, H.^ 0^ + H^O^ Cg H,^ Og. These ferments are termed " Hydrolytic." The chief classes of ferments are, — 1. Sugar-forming, e.g., ptyalin, xjanereatin. 2. Those wliicli transform albuminoids into j^epf ones ; e.g., j^ejmn. Besides saliva, many azotised substances, especially if in a state of in- cipient decomposition, may excite the transformation of starch, such as pieces of the mucous membrane of the mouth, bladder, rectum, and other parts, various animal and vegetable tissues, and even morbid products ; but the gastric fluid will not produce the same effect. The transformation in question is effected much more rapidly by saliva, however, than by any of the othei fluids or substances experimented with, except the pancreatic secretion, which, as will be presently shown, is very analogous to saliva. The majority of observers agree that the transformation of starch into sugar, on the entrance of the food into the stomach, is retarded, but not stopped. It is at least certain that the addi- tion of a small quantity of hydrochloric acid (the strength of the acid being the same as that in the gastric juice) to a solution of starch and saliva does not stop the transformation into sugar. Starch appears to be the only principle of food upon which saliva acts chemically : it has no apparent influence on any of the other ternary princi- ples, such as sugar, gum, cellulose, or (according to Bernard) on fat, and seems to be equally destitute of power over albuminous and gelatinous substances. u 2 292 DIGESTION. [chap. X. The Pharynx. That portion of tlie alimentary canal whicli intervenes between the mouth and the oesophagus is termed the Pharynx (fig. 1 34). It will suffice here to mention that it is constructed of a series of three muscles with striated fibres (constrictors) , which are covered by a thin fascia externally, and are lined internally by a strong fascia (pharyngeal aponeurosis), on the inner aspect of which is areolar (submucous) tissue and mucous membrane, continuous with that of the mouth, and, in so far as the part concerned iu swallowing is concerned, identical with it in general structure. The epithelium of this part of the pharynx, like that of the mouth, is laminated and squamous. The pharynx is well supplied with mucous glands (Fig. 138). The Tonsils, Between the anterior and posterior arches of the soft palate are situated the Tonsils, one on each side. They consist essentially of masses of lymphoid tissue closely re- Fig. 137.* sembling Peyer's glands (p. 326), invested with a laminated epithelium. Their sur- face is indented by numerous depressions of various shapes (fig. 137), and into many of these crypts open the ducts of mucous glands (fig. 138), which are abundantly distributed through the proper adenoid or lymphoid tissue of the tonsils : here and there this lymphoid tissue comes right up to the free surface, replacing the usual epithelial investment. The viscid secretion which exudes from the tonsils may serve to lubricate the bolus of food as it passes them in the second part of the act of deglutition ; and possibly the * Fig. 137. Lingual foUicle or crypt, a, Invohition of mucous membrane with its papillae ; h, lymphoid tissue, with several lymphoid sacs (Frey). The tonsil is constructed essentially of a mass of follicles or crypts, more or less similar to the above, together with mucous glands, the ducts of which open into the bottom of the follicles. CHAP. X.] THE (ESOniAGUS. lymphoid cells of tlie tonsils are the source of the so-called saliva-corpuscles. Bodies similar to the tonsils have been described by Kolliker under the name of pharyngeal tonsils : they lie in the posterior part of the pharynx between the orifices of the Eustachian tubes. (For the structure of Mu- cous Glands, see Chapter on Secretion.) The (Esophagus or Gullet. The (Esophagus or Gullet (Fig. 134), the narrowest portion of the alimentary canal, is a muscular and mucous tube, nine or ten inches in length, which extends from the lower end of the pharynx to the cardiac orifice of the stomach. It is made up of three chief layers or coats — the outer muscular, the middle areolar or sub-mucous, and the innermost mucous. The muscular coat, which is covered externally by connective tissue, consists of two layers of fibres — the outer being longitu- dinal, and the inner transverse. At the upper end of the oeso- phagus, the fibres of both these layers are, for the most part, striated ; but, as we descend, the proportions of striated and plain fibres are gradually reversed, and only the latter are found in the lower half of the tube. The mucous membrane, which, when the oesophagus is not distended, is thrown into numerous longitudinal folds or rugce, * Fig. 138. Mucous gland, from -tongue of dog. e, epithelium, showing different shapes of nuclei at various depths ; m, mucus discharged from orifice of d m, duct of mucous gland lined by epithelium, and containing a mass of mucus; a, areolar tissue of submucous layer; m /, muscular fibres of tongue; g c, gland cells of the various contorted tubes and acini of which the gland consists (Schoheld). Fig. 138.* 294 DIGESTION. [chap. x. is provided with minute 'papilla and mucous glands (fig. 138). The former are buried beneath the thick laminated squamous epithelium, with which the tube is lined. In newly-born children the mucous membrane exhibits, in many parts, the structure of lymphoid tissue (Klein.) Between the mucous membrane and the submucous coat is a well-defined layer of plain muscular fibres (quite distinct from the layers, before mentioned, of the proper muscular coat), termed the vnisciilaris vuicoscb. Blood- and lymph -vessels, and nerves, are distributed in the walls of the oesophagus. Between the outer and inner layers of the muscular coat, ganglia of Auerbach are also found. Swallowing or Deglutition. When properly masticated, the food is transmitted in succes- sive portions to the stomach by the act of deglutition or swallow- ing. This act, for the purpose of description, may be divided into three parts. In the first, particles of food collected to a morsel glide between the surface of the tongue and the palatine arch, till they have passed the anterior arch of the fauces ; in the second, the morsel is carried through the pharynx ; and in the third, it reaches the stomach through the oesophagus. These three acts follow each other rapidly. (I.) The first part of the act of deglutition may be voluntary, although it is usually performed unconsciously; the morsel of food, when sufficiently masticated, being pressed between the tongue and palate, by the agency of the muscles of the former, in such a manner as to force it back to the entrance of the pharynx. (2.) The second act of deglutition is the most complicated, because the food must pass by the posterior orifice of the nose and the upper opening of the larynx without touching them. When it has been brought, by the first act, between the anterior arches of the palate, it is moved onwards by the tongue being carried backwards, and by the muscles of the anterior arches contracting on it and then beliind it. The root of the tongue being retracted, and the larynx being raised with the pharynx and carried for- CHAP. X.] DEGLUTITIO]N\ 295 wards under tlie tongue, the epiglottis is pressed over the upper opening of the larynx, and the morsel glides past it ; the closure of the glottis being additionally secured by the simultaneous con- traction of its own muscles : so that, even when the epiglottis is destroyed, there is little danger of food or drink passing into the larynx so long as its muscles can act freely. At the same time the raising of the soft palate, so that its posterior edge touches the back part of the pharynx, and the approximation of the sides of the posterior palatine arch, which move quickly inwards like side curtains, close the passage into the upper part of the pharynx and the posterior nares, and form an inclined plane, along the under surface of which the morsel descends ; then the pharynx, raised up to receive it, in its turn contracts, and forces it onwards into the oesophagus. (3.) In the third act, in which the food passes through the oeso- phagus, every part of that tube, as it receives the morsel and is dilated by it, is stimulated to contract: hence an undulatory contraction of the oesophagus, which is easily observable in horses while drinking, proceeds rapidly along the tube. It is only when the morsels swallowed are large, or taken too quickly in succession, that the progressive contraction of the oesophagus is slow, and attended with pain. Division of both pneumogastric nerves paralyses the contractile power of the oesophagus, and food accordingly accumulates in the tube (Bernard). The second and third parts of the act of deglutition are involuntary. The nerves engaged in the reflex act of deglutition are, mainly, sensory branches of the fifth cerebral, glosso-pharyngeal, and pneumo-gastric nerves ; while the motor fibres concerned are branches of the fifth, the facial, the hypoglossal, the pneumo- gastric, and spinal accessory. The nerve-centre by which the muscles are harmonised in their action, is situate in the medulla oblongata. In the movements of the oesophagus, the ganglia contained in its walls, with the pneumo-gastrics, are the nerve- structures chiefly concerned. It is important to note that the swallowing both of food and drink is a muscular act, and can, therefore, take place in opposi- 296 DIGESTION. [chap. X. tion to the force of gravity. Thus, horses and many other animals habitually drink up-hill, and the same feat can be per- formed by jugglers. DIGESTION OF FOOD IN THE STOMACH. Structure of the Stomach. In man and those Mammalia which are provided with a single stomach, its walls consist of four distinct layers or coats, viz., an external peritoneal, a muscular, a submucous, and a mucous coat; with blood-vessels, lymphatics, and nerves distributed in and between them. The 'peritoneal coat has the structure of serous membranes in general. (See Serous Membranes.) The muscular coat of the stomach consists of three separate layers or sets of fibres, which, according to their several direc- tions, are named the longitudinal, circular, and oblique. The longitudinal set are the most superficial: they are continuous with the longitudinal fibres of the oesophagus, and spread out in a diverging manner over the great end and sides of the stomach. They extend as far as the pylorus, being especially distinct at the lesser or upper curvature of the stomach, along which they pass in several strong bands. The next set are the circular or transverse fibres, which more or less completely encircle all parts of the stomach ; they are most abundant at the middle and in the pyloric portion of the organ, and form the chief part of the thick projecting ring of the pylorus. According to Pettigrew, these fibres are not simple circles, but form double or figure-of-8 loops, the fibres intersecting very obliquely. The next, and con- sequently deepest set of fibres, are the oblique, continuous with the circular muscular fibres of the oesophagus, and, according to Pettigrew, with the same double-looped arrangement that pre- vails in the preceding layer : they are comparatively few in number, and are placed only at the cardiac orifice and portion of the stomach, over both surfaces of which they are spread, some passing obliquely from left to right, others from right to left, around the cardiac orifice, to which^ by their interlacing, CHAP. X.] STRUCTURE OF STOMACH. 297 they form a kind of spliincter, continuous with that around the lower end of the oesophagus. The muscular fibres of the stomach and of the intestinal canal are unstriped, being composed of elongated, spindle-shaped fibre-cells. (See Section on Muscle.) The mucous membrane of the stomach, which rests upon a layer of loose cellular membrane, or submucous tissue, is smooth, level, soft, and velvety ; of a pale pink colour during life, and in the contracted state is thrown into numerous, chiefly longitudinal, folds or rugae, which disappear when the organ is distended. In its general structure the mucous membrane of the stomach resembles that of other parts. (See Structure of ^lucous Mem- brane.) But there are certain peculiarities shared with the mucous membrane of the small and large intestines, which, doubtless, are connected with the peculiar functions, especially those relating to absorption, which these parts of the alimentary canal perform. Entering largely into the construction of the mucous mem- brane, especially in the superficial part of the corium, is a cjuantity of a very delicate kind of connective tissue, called reti- form tissue (fig. 1 47), or sometimes lymi)]ioid or adenoid tissue, because it so closely resembles that which forms the stroma, or supporting framework of lymphatic glands (see Section on Lym- phatic Glands) ; the resemblance being made much closer by the fact that the interspaces of this retiform tissue are filled with corpuscles not to be distinguished from lymph-corpuscles. At the deepest part of the mucous membrane, is a layer of unstriped muscular fibres, called the mtcscularis mucosce (fig. 140), which must not be confounded with the layers of muscle constitut- ing the proper muscular coat, and from which it is separated by the submucous tissue. When examined with a lens, the internal or free surface of the stomach presents a peculiar honeycomb appearance, produced by shallow polygonal depressions (fig. 1 39), the diameter of which varies generally from ^Icr^^ -sto^^ i^^^ j ^^^^ pylorus is as much as --^ o-th of an inch. They are separated by slightly elevated ridges, which sometimes, especially in certain morbid states of the stomach, bear minute, narrow, vascular pro- 298 DIGESTION. [chap. x. cesses, whicli look like villi, and have given rise to the erroneous supposition that the stomach has absorbing villi, like those of the small intestines. In the bottom of Fig. 139.* these little pits, and to some extent between them, minute openings are visible (fig. 1 39), which are the orifices of perpendicularly arranged tubular glands (fig. 140), im- bedded side by side in sets or bundles, in the substance of the mucous membrane, and composing nearly the whole structure. The glands found in the human stomach may be divided into two classes, the tubular and lenticular. Tubular glands.— The tubular glands may be escribed as a collection of cylinders with blind extremities, about -^^th of an Fig. 140- + * Fig. 139. Small portion of the surface of the mucous membrane of the stomach. The specimen shows the shallow depressions, in each of which the smaller dark spots indicate the orifices of a variable number of the gastric tubular glands, x 30 (Ecker). t Fig. 140. Portion of human stomach (magnified 30 diameters) cut verticafly, both in a direction parallel to its long axis, and across it (altered from Brinton). CHAP, X.] STEUCTURE OF STOMACH. 299 inch in length, and ^th in diameter, packed closely together, with their long axis at right angles to the surface of the mucous membrane on which they open, their blind ends resting on the submucous tissue. (See fig. 140). They are all composed of basement membrane, and lined by epithelial cells, but they are not all of exactly similar shape ; for while some are simple straight tubes, open at one end and closed at the other (fig. 141,5), others are at their deeper extremities branched (fig. 141, c). Fig. 141.* In the stomach of man, the simple undivided tubes are the rule, and the branched the exception (Brinton). According to recent observations, three distinct kinds of cells may be distinguished in the peptic glands, (i) Ordinary columnar epithelium, which lines the upper fourth of the gland. (2) Smaller epithelial cells, approaching to a spheroidal form (these line the succeeding fourth of the tube, and also partly the lower end). (3) Large strongly granulated spheroidal 'peptic' * Fig. 141. The gastric glands of the human stomach (magnified), a, deep part of a pyloric gastric gland (KoUiker) ; the cylindrical epithelium is traceable to the csecal extremities, b, and c, cardiac gastric glands (Allen Thomson) ; b^ vertical section of a small portion, of the mucous mem- brane with the glands magnified 30 diameters ; c, deeper portion of one of the glands, magnified 65 diameters, shoAving a slight division of the tubes, and a sacculated appearance, produced by the large glandular cells within them ; dy cellular elements of the cardiac glands magnified 250 diameters. 300 DIGESTION. [chap. X. cells, wliich form a continuous lining in some parts of tlie tube, while in other parts they are scattered irregularly, adhering to its outer part. They are readily distinguished by their large relative size and prominence. (See figs. 142 and 1 43.) The peptic cells are especially prominent during digestion, while during fasting they are much less conspi- Fig. 142.* cuous, and the whole tabule appears shrunken (Heidenhain) . In the greater number of the glands which are branched at their deeper extremities, the peptic and small spheroidal cells exist in the divisions, while the main duct and the upper part of the branches are lined by the cylin- drical variety (fig. 141, c). The varieties in the epithelial cells lining the different parts of the tubes, correspond probably with differences in the fluid secreted by their agency — the cylinder -epithelium, like that on the free surface of the stomach, being engaged in separating the thin alkaline mucus 5 Mi Fig. 143. t which is always present in greater or less quantity, while the larger glandular cells secrete the proper gastric juice. * Fig. 142. A gastric gland of cat in side view (Frey). a, cyhndrical epi- thelium ; h, small spheroidal epithelium ; c, peptic cells ; the part of the gland-tubules which is lined by two kinds of cells (peptic and small spheroidal). t Fig. 143. Transverse section through lower part of peptic glands of a cat (Frey). a peptic cells ; small spheroidal cells ; c, transverse section of capillaries. CHAP. X. ] BLOOD-VESSELS OF STOMACH. 301 Near the pylorus there exist glands branched at thoir deep extremities, which are lined throughout by cylinder- epithelium (fig. 141, a)j and probably serve only for the secretion of mucus. Lenticular glands. — Besides the cylindrical glands, there are also small closed lymphoid sacs beneath the surface of the mucous membrane, resembling exactly the solitary glands of the intestine, to be described hereafter. Their number is very variable, and they are found chiefly along the lesser curvature of the stomach, and in the pyloric region, but they may be present in any part of the organ. According to Dr. Brinton they are rarely absent in children. Their function doubtless resembles that of the intestinal solitary glands. The blood-vessels of the stomach, which first break up in the submucous tissue, send branches upward between the closely Pig- i44-* packed glandular tubes, anasto- mosing around them by means of a fine capillary network with oblong meshes. Continuous with this deeper plexus, or pro- longed upwards from it, so to speak, is a more superficial net- work of larger capillaries, which branch densely around the ori- fices of the tubes, and form the framework on which are moulded the small elevated ridges of mucous membrane bounding the minute, polygonal pits before referred to. From this superficial network the veins chiefly take their origin. Thence passing down between the tubes, with no very free connection with the deeper inter-tubular Fig. 144. Plan of the bloodvessels of the stomach, as they would be seen in a vertical section, a, arteries, passing up from the vessels of sub-mucous coat ; b, capillaries branching between and around the tubes ; c, superficial plexus of capillaries occupying the ridges of the mucous membrane ; d, vein formed by the union of veins which, having collected the blood of the super- ficial capillary plexus, are seen passing down between the tubes. (Brinton. ) 302 DIGESTIOX. [chap. X. capillary plexus, they open finally into the venous network in the submucous tissue. The stomach possesses a highly developed lymphatic system, the radicles of which ascend between the tubules, nearly to the free surface, while large lymphatic sinuses exist in the submucous tissue. The nerves of the stomach are derived from the pneumo- gastric and sympathetic, and form a plexus in the submucous and muscular coats, containing many ganglia (Kern ah, Meissner). Secretion and Properties of the Gastric Fluid. While the stomach contains no food, and is inactive, no gastric fluid is secreted ; and mucus, which is either neutral or slightly alkaline, covers its surface. But immediately on the introduction of food or other substance into the stomach, the mucous membrane, previously quite pale, becomes slightly turgid and reddened with the influx of a larger quantity of blood; the gastric glands commence secreting actively, and an acid fluid is poured out in minute drops, which gradually run together and flow down the walls of the stomach, or soak into the substances introduced. The first accurate analysis of the gastric fluid was made by Dr. Prout : but it does not appear that it was collected in any large quantity, or pure and separate from food, until the time when Dr. Beaumont was enabled, by a fortunate circumstance, to obtain it from the stomach of a man named St. Martin, in whom there existed, as the result of a gunshot wound, an opening leading directly into the stomach, near the upper extremity of the great curvature, and three inches from the cardiac orifice. The external opening was situate two inches below the left mamma, in a line drawn from that part to the spine of the left ilium. The borders of the opening into the stomach, which was of considerable size, had united, in healing, with the margins of the external wound, but the cavity of the stomach was at last separated from the exterior by a fold of mucous membrane, which projected from the upper and back part of the opening, and closed it like a valve, but could be pushed back with the finger. The introduction of any mechanical irritant, such as the bulb of a thermometer, into the stomach, excited at once the secretion of gastric fluid. This could be drawn oS with a caoutchouc tube, and could often be obtained to the extent of nearly an ounce. The introduction of alimentary substances caused a much more rapid and abun- dant secretion of pure gastric fluid than the presence of other mechanical irritants did. No increase of temperature could be detected dm^ing the most active secretion : the thermometer introduced into the stomach always CTIAr. X.] THE GASTEIC JUICE. stood at 100° Fahr., except during muscular exertion, when the temperature of the stomach, like that of other parts of the body, rose one or two degrees higher. M. Blondlot, and subsequently M. Bernard, and several others, by main- taining fistulous openings into the stomachs of dogs, have confirmed most of the facts discovered by Dr. Beaumont. And the man St. Martin has frequently submitted to renewed experiments on his stomach, by various physiologists. From all these observations it appears, that pepper, salt, and other soluble stimulants, excite a more rapid discharge of gastric fluid than mechanical irritation does ; so do alkalies generally, but acids have a con- trary effect. When mechanical irritation is carried beyond certain limits so as to produce pain, the secretion, instead of being more abundant, diminishes or ceases entirely, and a ropy mucus is poured out instead. Very cold water, or small pieces of ice, at first render the mucous membrane pallid, but soon a kind of reaction ensues, the membrane becomes turgid with blood, and a larger quantity of gastric juice is poured out. The application of too much ice is attended by diminution in the quantity of fluid secreted, and by consequent retardation of the process of digestion. The quantity of the secretion seems to be influenced also by impressions made on the mouth ; for Blondlot found that when sugar was introduced into the dog's stomach, either alone, or mixed with human saliva, a very small secretion ensued : but when the dog had himself masticated and swallowed it the secretion was abundant. Dr. Beaumont described tlie secretion of the human stomach as ''a clear transparent fluid, inodorous, a little saltish, and very perceptibly acid. Its taste is similar to that of thin muci- laginous water, slightly acidulated with muriatic acid. It is readily diffusible in water, wine, or spirits. It possesses the property of coagulating albumen in an eminent degree." The chemical composition of the gastric juice of the human subject has been particularly investigated by Schmidt ; a favour- able case for his doing so occurring in the person of a peasant named Catherine Kiitt, aged 35, who for three years had had a gastric fistula under the left mammary gland, between the carti- lages of the ninth and tenth ribs. The fluid was obtained by putting into the stomach some hard indigestible matter, as dry peas, and a little water, by which means the stomach was excited to secretion, at the same time that the matter introduced did not complicate the analysis by being digested in the fluid secreted. The gastric juice was drawn off through an elastic tube inserted into the fistula. The fluid thus obtained was acid, limpid, and odourless, with 304 DIGESTIO^^. [chap. X. a mawkisli taste. Its density varied from 1*0022 to 1*0024. Under the microscope a few cells from the gastric glands and some fine granular matter were observable. The following table gives the mean of two analyses of the above-mentioned fluid ; and arranged by the side of it, for pur- poses of comparison, is an analysis of gastric juice from the sheep and dog. Composition of Gastric Juice. Human Sheep's Dog's Gastric Juice. Gastric Juice. Gastric Juice. Water 994'40 986-14 97i*i7 Solid Constituents . . 5'59 iS'^S 28-82 f^Ferment, Pepsin (witli a trace of Ammonia) . 3.19 4*20 IT'S^ Hydrochloric Acid . . 0*20 1-55 270 Chloride of Calcium . . o-o6 O'li i'66 \ „ Sodium . . 1-46 4.36 3-14 „ Potassium .0-55 1-51 1*07 Phosphate of Calcium, ^ Magnesium, and Iron . 0.12 2*09 273 The quantity of gastric juice secreted daily has been variously estimated ; but the average for a healthy adult may be assumed to range from ten to twenty pints in the twenty-four hours (Brinton). Considerable difference of opinion has existed concerning the nature of the free acid contained in the gastric juice, chiefly whether it is liydrocliloric or lactic. The weight of evidence, however, is in favour of free hydrochloric acid, being that to which, in the human subject, the acidity of the gastric fluid is mainly due ; although there is no doubt that others, as lactic, acetic, butyric, are not unfrequently to be found therein. Fepsin is a nitrogenous ferment (p. 291 ) which can be procured by digesting portions of the mucous membrane of the stomach in cold water, after they have been macerated for some time in water at a temperature between 80° and 100° F. The warm water dissolves various substances as well as some of the pepsin, but the cold water takes up little else than pepsin, which, on evaporating the cold solution, is obtained in a greyish-brown viscid fluid. The addition of alcohol throws down the pepsin in greyish-white flocculi. Gastric Digestion. The digestive power of the gastric juice depends on the pepsin and acid contained in it, both of which are necessary for the CHAP. X.] GASTRIC DIGESTION: CHYME. process. Neither of them can digest alone ; and when they are mixed, either the decomposition of the pepsin, or the neutraliza- tion of the acid at once destroys the digestive property of the lliiid. The same fact is well shown by experimenting with an artificial gastric juice, prepared by dissolving pepsin in water, to which hydrochloric acid (l part in looo) has been added. A solution so made will digest portions of food placed in it if due precautions as to temperature be observed ; while separate solutions of similar amounts of pepsin and hydrochloric acid respectively, are inert. For the perfection of the process of digestion, the following conditions are necessary; which are all present in the case of normal digestion — namely, (i) a temperature of about lOO° F. ; (2) such movements as the food is subjected to by the muscular contraction of the stomach, which bring in succession every part of it in contact with the mucous membrane, whence the fresh gastric juice is being secreted ; (3) the constant removal of those portions which are already digested, so that what remains may be brought more completely into contact with the solvent fluid ; and (4) a state of softness and minute subdivision, such as that to which the food is reduced by mastication before its introduction into the stomach. The general effect of digestion in the stomach is the conversion of the food into chyme, a substance of various composition ac- cording to the nature of the food, yet always presenting a characteristic thick, pultaceous, grumous consistence, with the undigested portions of the food mixed in a more fluid substance, and a strong, disagreeable acid odour and taste. Reduced into such a substance, all the various materials of a meal may be mingled together, and near the end of the digestive process hardly admit of recognition; but the experiments of artificial digestion, and the examination of stomachs with fistulae, have illustrated many of the changes through which the chief alimentary principles pass, and the times and modes in which they are severally disposed of. The functions of the gastric fluid may be, perhaps, best arranged under the following heads, {a) Its action on albu- 3o6 DIGESTION. [ciiAr. X. minous and other nitrogenous suLstances. {l>) Its action on other varieties of food ; and (c) as an antiseptic fluid. The Acticvi of Gastric Juice on Xitro