QF3I M£2r (Eolumbta Httiwralg xtt % (Sixty nf $>w f nrk (Cnllrgp nf fhgsirtaua ana $itrgrmtB IFrnm llir library of (EljurrljiU (Earmalt, M. I. PrpBf ttfpo by % Extmw (dull nf Nrro $nrk Digitized by the Internet Archive in 2010 with funding from Columbia University Libraries http://www.archive.org/details/textbookofcomparOOmill A TEXT- BOOK OF COMPARATIVE PHYSIOLOGY FOR STUDENTS AND PRACTITIONERS OF COMPARATIVE (VETERINARY) MEDICINE BY WESLEY MILLS M. A., M. D., D. V. S. PROFESSOR OF PHYSIOLOGY IN THE FACULTY OF HUMAN MEDICINE AND THE FACULTY OF COMPARATIVE MEDICINE AND VETERINARY SCIENCE OF MC GILL UNIVERSITY, MONTREAL ; AUTHOR OF A TEST-BOOK OF ANIMAL PHYSIOLOGY, ETC. WITH 476 ILLUSTRATIONS NEW YORK D. APPLETON AND COMPANY LONDON: CAXTON HOUSE, PATERNOSTER SQUARE 1890 Copyright, 1890, By D. APPLETON AND COMPANY All rights reserved. PREFACE Some years of contact with students of comparative (com- monly called veterinary) medicine, and a fair knowledge of the actual needs of the practitioner of this department of the medical art, have convinced me that the time has fully come when the text-books of physiology provided for students of human medicine, and which the former classes have hitherto been compelled to use, should be replaced by works written to meet their special wants and possibilities. In fact, so dif- ferent from man are most of the animals which the veterina- rian is called upon to treat, and therefore to understand, in health as well as in disease, that only the absence of suitable works of a special character can justify the use of those that confessedly treat of man alone. Unfortunately, till within the past year the English-speak- ing student of comparative medicine has been without a single work in his own language of the special character re- quired. Within that period two have appeared — the excel- lent but ponderous Physiology of the Domestic Animals, by Prof. Smith, and my own Text-Book of Animal Physiology. It has, therefore, occurred to me that a somewhat smaller work than the latter, embodying the same plan, but with greater specialization for ' .ie domestic animals, would com- mend itself to both t> students and the practitioners of comparative medicine. In my other work I have endeavored to set before the student a short account of what has been deemed of most importance in general biology ; to furnish a full account of reproduction ; to apply these two depart- ments throughout the whole of the rest of the work ; to bring 374^ • . iv COMPARATIVE PHYSIOLOGY. before the student enough of comparative physiology in its widest sense to impress him with the importance of recog- nizing that all medicine like all science is, when at its best, comparative ; and to show that the doctrines of evolution must apply to physiology and medicine as well as to morphology. Comparative medicine is essentially broad. It will not do to measure all the animals the veterinarian is called upon to treat by the equine standard. This has been too much the case in the past for the good even of the horse himself ; while others, that fall to the practitioner's care, like the dog, have been much neglected and misunderstood. There is no more reason, theoretically, why the veterina- rian should overlook man than that the practitioner of human medicine should disregard the lessons to be learned from our domestic animals ; hence the attempt has been made to exclude references to the human subject from the volume. The stu- dent of comparative medicine may learn, by careful observa- tion on himself, to understand much that would otherwise never become realized knowledge ; and this conviction has been at the root of a large part of the advice given the stu- dent as to how to study throughout the work. All that relates to reproduction and breeding is, in these days of vast stock interests, of so much practical importance, that on this account alone the fullest treatment of the subject seems justifiable. But, apart from this, it has become clear to me that function as well as form can be much better and more easily grasped when embryology is early considered. This I have tested, with the happiest results, with my own classes. Usually those taking up physiology for the first time are, of course, not expected to master all the details of embryology, but the main outlines prove as helpful as inter- esting ; nevertheless, it is my experience that a considerable number of first-year men are not content to be confined to the merest rudiments of this or any other department of physiology. That a work written on so new a plan as my Text-Book of Animal Physiology should have met with a reception al- most universally favorable, both in Britain and America, in PREFACE, v so short a space of time, encourages me to hope for one equally favorable for this book, which is offered to a pro- fession from which I look for great things in the interests both of man and the lower animals within the next few years. The time has certainly come when medicine must leave the narrow ruts within which it has been confined, and become essentially comparative. To hasten that consumma- tion, so devoutly to be wished, has been the object with which both my earlier and the present work have been written. Un- less the student is infused with the broad comparative spirit in the earliest years of his studies, and guided accordingly, there is no sure guarantee of final success in the widest sense. My publishers again deserve my thanks for the efforts they have made to present this work in their best form. Wesley Mills. Physiological Laboratory, McGill University, Montreal, Canada, August, 1890. CONTENTS. PAGE General Biology 1 Introduction 1 Tabular statement of the subdivisions of Biology ... 4 The Cell 5 Animal and vegetable cells 5 Structure of cells .......... 5 Cell-contents . . . . 7 The nucleus 8 Tissues 8 Summary 9 Unicellular Organisms (Vegetable) ....... 9 1. Yeast .9 Morphological ......... 9 Chemical . . . . . , . . 9 Physiological . . . . . . . . .10 Conclusions 10 2. Protococcus . .11 Morphological . . . . „ . „ . . 12 Physiological „ . . .12 Conclusions . .... 12 Unicellular Animals . . . . . . . . . .13 The proteus animalcule 13 Morphological 13 Physiological . . . . . . . „ .13 Conclusions . • . . . . . ■ „ .15 Parasitic Organisms . . . . . . . . .15 Fungi 15 Mucor mucedo . . . . . . . . 17 The Bacteria 18 Unicellular Animals with Differentiation of Structure . , . 21 The bell-animalcule 21 Structure . 21 Functions 23 Vlll COMPARATIVE PHYSIOLOGY. Multicellular Organisms . . The fresh-water polyps ....... The Cell reconsidered ....... The Animal Body — an epitomized account of the functions of a mal Living and Lifeless Matter — General explanation and compari their properties ....... Classification of the Animal Kingdom .... Tabular statement ....... Man's place in the animal kingdom .... The Law of Periodicity or Rhythm in Nature — Explanation illustrations The Law of Habit . Its foundation Instincts .... The Origin of the Forms of Life Arguments from : Morphology Embryology 3 and Mimicry . Rudimentary organs . Geographical distribution Paleontology . Fossil and existing species Progression Domesticated animals Summary Reproduction General The ovum The origin and development of the ovum Changes in the ovum itself The male cell The origin of the spermatozoon Fertilization of the ovum . Segmentation and subsequent changes The gastrula . . . The hen's egg . The origin of the fowl's egg Embryonic membranes of birds The fuital (embryonic) membrane The allantoic cavity . The placenta The discoidal placenta of mammals PAGE 23 23 27 28 32 34 36 36 37 41 41 42 42 45 45 45 46 46 46 46 47 47 50 51 51 55 57 58 61 61 63 64 68 69 70 74 78 80 82 83 CONTENTS. IX The nietadiscoidal placenta The zonary placenta . The diffuse placenta . The polycotyledonary placenta . Tabulation of placentation Microscopic structure of the placenta Illustrations .... Evolution ..... Summary ..... The Development of the Embryo Itself Germ-layers ...... Origin of the vascular system . The growth of the embryo Development of the Vascular System in Vertebrates The later stages of the foetal circulation . Development of the Urogenital System The Physiological Aspects of Development Ovulation Oestrum . The nutrition of the ovum The foetal circulation Periods of Gestation . Parturition . Changes in the circulation after birth . Coitus . . • Organic Evolution reconsidered The Chemical Constitution of the Animal Body Proximate principles General characters of protcids Certain non-crystalline bodies The fats Pecilliar fats ....•• Carbohydrates ...-•• Nitrogenous metabolites .... Non-nitrogenous metabolites Physiological Research and Physiological Reason The Blood Comparative .... Corpuscles .... History of the blood-eells Chemical composition of the blood Composition of serum Composition of the corpuscles . The quantity and distribution of the blood page 84 89 89 89 90 90 91, 92 93 93 95 98 102 105 108 109 112 118 120 121 123 125 127 128 129 129 137 142 144 144 145 145 146 146 147 147 148 154 154 155 158 160 161 162 163 COMPARATIVE PHYSIOLOGY. PAGE The coagulation of the blood „ .163 Clinical and pathological 16*7 Summary 169 The Contractile Tissues ......... 171 General 171 Comparative .......... 172 Ciliary movements 173 The irritability of muscle and nerve 175 The Graphic Method and the Study of Muscle Physiology . .176 Chronographs and various kinds of apparatus .... 176 The apparatus used for the stimulation of muscle . . . . 179 A single muscular contraction . . . . . .185 Tetanic contraction 187 The muscle-tone . . . . • . . . . .189 The changes in a muscle during contraction 189 The elasticity of muscle 189 The electrical phenomena of muscle 191 Chemical changes in muscle 192 Thermal changes in the contracting muscle 195 The physiology of nerve 196 Electrotonus 196 Pathological and clinical 197 Electrical organs 197 Muscular work 198 Circumstances influencing the character of musular and nervous ac- tivity 199 The influence of blood-supply and fatigue 199 Separation of muscle from the central nervous system . . 201 The influence of temperature 201 Unstriped muscle 202 General 202 Comparative . . . 202 Special considerations ......... 203 Functional variations ........ 204 Summary of the physiology of muscle and nerve . . . 205 The Nervous System — General Considerations .... 208 Experimental . . . . . . . . . .210 Automatism . . . . . . . . . .211 Conclusions 212 Nervous inhibition . . . . . . . . .212 The Circulation oe the Blood 214 General 214 The mammalian heart ........ 215 Circulation in the mammal 219 CONTENTS. XI The action of the mammalian heart . The velocity of the blood and blood-pressure General ..... Comparative .... The circulation under the microscope The characters of the blood-flow Blood-pressure. .... The Heart The cardiac movements . The impulse of the heart . Investigation of the heart-beat from within The cardiac sounds .... Causes of the sounds Endo-cardiac pressures The work of the heart Variations in the cardiac pulsation . Comparative .... The pulse Features of an arterial pulse-tracing Venous pulse .... Pathological .... Comparative .... The beat of the heart and its modifications The nervous system in relation to the heart Influence of the vagus nerve on the heart Conclusions ..... The accelerator nerves of the heart . The heart in relation to blood-pressure The influence of the quantity of blood The capillaries Special considerations Pathological Personal observations Comparative Evolution . Summary of the physiology of the circulat Digestion of Food Foodstuffs, milk, etc Embryological . Comparative Structure, arrangement, and significance of the The digestive juices . Saliva and its action Secretion of the different elands teeth PAGE 222 223 223 224 224 226 227 231 231 232 233 234 235 236 238 239 240 241 242 244 244 244 248 249 253 257 258 260 261 264 265 266 566 267 268 269 274 274 280 286 286 297 297 298 Xll COMPARATIVE PHYSIOLOGY. Comparative Gastric juice Bile . General Pigments . Digestive action Comparative Pancreatic secretion Succus entericus Comparative Secretion as a physiological process . Secretion of the salivary glands Secretion by the stomach . The secretion of bile and pancreatic The nature of the act of secretion . Self-digestion of the digestive organs Comparative .... The alimentary canal of the vertebrate The movements of the digestive organs Deglutition Comparative The movements of the stomach Comparative Pathological The intestinal movements . Defecation Vomiting .... Comparative The removal of digestive products f Lymph and chyle The movements of the lymph Pathological Faeces . ■» . Pathological The changes produced in the food in General Comparative Pathological Special considerations Various Evolution . Summary . The Respiratory System General juice comparative om the alimentary canal the alimentary canal PAGE 298 299 301 301 302 303 304 305 307 310 311 311 315 316 318 323 324 331 332 333 336 336 337 337 337 338 339 340 341 343 344 352 352 353 354 354 357 359 359 359 363 364 366 366 CONTENTS, xin blood espiration Anatomical .... The entrance and exit of air The muscles of respiration Types of respiration . Comparative ..... Personal observation The quantity of air respired The respiratory rhythm . General ..... Pathological . . Respiratory sounds . Comparison of the inspired and the expired air Respiration in the blood . Haemoglobin and its derivatives General Blood-spectra .... Comparative .... The nitrogen and the carbon dioxide of the Foreign gases and respiration . Respiration in the tissues .... The nervous system in relation to respiration The influence of the condition of the blood on i The influence of respiration on the circulation General ...... Comparative ..... The respiration and circulation in asphyxia Pathological ..... Peculiar respiratory movements Coughing, laughing, etc. . Comparative ..... Special considerations .... Pathological and clinical . Personal observation Evolution ...... Summary of the physiology of respiration Protective and Excretorv Functions of the General ...... Comparative The excretory function of the skin . Normal sweat ..... Pathological ...... Comparative — Respiration by the skin Death from suppression of the functions of the The excretion of perspiration .... Skin skin PAGE 367 370 373 375 375 375 378 379 379 379 381 382 383 385 385 387 389 389 391 391 393 393 396 396 398 399 401 401 401 402 403 403 404 404 405 408 408 408 411 411 411 412 412 412 xiv COMPARATIVE PHYSIOLOGY. PAGE Experimental 413 Absorption by the skin 413 Comparative .......... 413 Summary ........... 413 Excretion by the Kidney - . . . . 415 Anatomical .......... 415 Comparative . 415 Urine considered physically and chemically . . . . .419 Specific gravity . . . . . . . . . .419 Color .419 Eeaction 419 Quantity . . . . . . . . . . . 419 Composition : Nitrogenous crystalline bodies .... 420 Non-nitrogenous organic bodies ...... 420 Inorganic salts . . . . . . . . , 420 Abnormal urine . . . . . . . . .421 Comparative . . . . . . . ... . 421 The secretion of urine 421 Theories of secretion . . . . ' . . . . 421 Nervous influence . . . . . . . . 423 Pathological 423 The expulsion of urine ......... 424 General .424 Facts of experiment and of experience ..... 424 Pathological ' . .426 Summary of urine and the functions of the kidneys .... 426 Comparative .......... 426 The Metabolism of the Body 428 General remarks ......... 428 The metabolism of the liver ........ 428 The glycogenic function 429 The uses of glycogen ........ 429 Metabolism of the spleen 429 Histological 430 Chemical 430 The construction of fat 432 General and experimental ....... 432 Histological . . 433 Changes in the cells of the mammary gland .... 434 Nature of fat formation 434 Milk and colostrum ......... 435 Pathological 435 Comparative .......... 436 The study of tlie metabolic processes by other methods . . . 436 CONTENTS. xv PAGE Various tabular statements ....... 437 Starvation and its lessons ....... 537 Comparative 438 Diets 439 Feeding experiments ......... 439 General 439 Nitrogeneous equilibrium ........ 440 Comparative 441 The effects of gelatine in the diet 441 Fat and carbohydrates 442 Comparative .......... 442 The effects of salts, water, etc., on the diet .... 443 Patholgical 443 Water 443 The energy of the animal body I ..... 443 Tabular statements ......... 444 Animal heat 445 General ........... 445 Comparative .......... 445 The regulation of temperature 447 Cold-blooded and warm-blooded animals compared . . . 448 Theories of heat formation and heat regulation .... 448 Pathological . . . 449 Special considerations ......... 450 Evolution 450 Hibernation .......... 451 Daily variations in temperature in man and other mammals . 452 The influence of the nervous system on metabolism (nutrition). . 452 Experimental facts ° 452 Discussion of their significance 455 General considerations, clinical and pathological . . .456 Summary of metabolism ........ 458 The Spinal Cord — General 461 General 461 Anatomical .......... 461 The reflex functions of the spinal cord 466 General and experimental ........ 466 Evolution and heredity 468 Inhibition of reflexes 469 The spinal cord as a conductor of impulses 469 Anatomical 469 Decussation .......... 471 Pathological 471 Paths of impulses 473 XVI COMPARATIVE PHYSIOLOGY. ain with another ng function ? The automatic functions of the spinal cord General . Spinal phenomena Special considerations Comparative Evolution Synoptical The Bkain .... General and anatomical Animals deprived of their cerebrum Behavior of various animals and its significance Have the semicircular canals a co-ordinat Discussion of the phenomena . Forced movements Functions of the cerebral convolutions Comparative .... Individual differences in brains The connection of one part of the br The cerebral cortex . Theories of different observers The circulation in the brain Sleep — hibernation — dreaming Hypnotism, etc. Illustrations of localization Functions of other portions of the brain The corpus striatum and the optic thalamus Corpora quadrigemina The cerebellum Pathological Crura cerebri and pons Varolii Pathological Medulla oblongata . Special considerations Embryological . Evolution .... Synoptical General Remarks on the Senses Anatomical General principles The Skin as an Organ ok Sense General .... Pressure sensations . Thermal sensations . Tactile sensibility CONTENTS. xvii PAGE The muscular sense 524 General ........... 524 Pathological 524 Comparative . . . . . . . . . .524 Synoptical .......... 525 Vision 526 Anatomical 527 Einbryological . . . . . . . . . .528 Dioptrics of vision .......... 531 Accommodation of the eye 532 Alterations in the size of the pupil 533 Phenomena and their explanations ...... 534 Pathological 536 Comparative ....... . . 536 Optical imperfections of the eye 536 Anomalies of refraction 536 Visual sensations 538 General 538 Affections of the retina ........ 540 The laws of retinal stimulation ....... 541 The visual angle 542 Color- vision , 543 Psychological aspects of vision ....... 543 The visual field 543 After-images, etc. ......... 543 Co-ordination of the two eyes in vision ...... 544 The visual axes . . . . . . . . . 544 Ocular movements ......... 544 Estimation of the size and distance of objects .... 548 Solidity 548 Protective mechanisms of the eye ....... 549 Special considerations 551 Comparative . . . . . . . . . .551 Evolution 552 Pathological 554 Brief synopsis of the physiology of vision ..... 554 Hearing 557 General ........... 557 Anatomical .......... 558 The membrana tympani ........ 558 The auditory ossicles ........ 559 Muscles of the middle ear 561 The Eustachian tube 562 Pathological 563 B XY1U COMPARATIVE PHYSIOLOGY. PAGE Auditory sensations, perceptions, judgments . 567 . 567 . 568 Synopsis of the physiology of hearing . 672 The Senses of Smell and Taste . 573 Smell . 573 Anatomical ..... . 573 . 573 . 574 , 575 , 576 . 576 Experimental ...... . 577 . 578 The Cerebro-Spinal System of Nerves . 579 . 579 . 5 SO Additional experiments . 580 . 580 . 580 . 580 The motor-oculi, or third nerve . . 581 The trochlear, or fourth nerve . . 582 The abducens, or sixth nerve . 282 The trigeminus, or fifth nerve . . 583 The glossopharyngeal, or ninth nerve . 585 The pneumogastric, or tenth nerve . . 586 The spinal accessory, or eleventh nerve . 587 The hypoglossal, or twelfth nerve . 588 Relations of the cerebrospinal and sympatheti c syst ems . 588 Recent views on this subject . 588 The Voice . 592 Physical ...... . 592 Anatomical ..... . 593 Voice-formation .... . 593 . 598 Oomparative ..... . 598 . 600 CONTENTS. XIX Evolution . Summary . Certain Tissues . Connective tissue Elastic tissue . Bone Cartilage . Locomotion . Anatomical Mechanical Standing . Walking . Running . Jumping . Hopping . Comparative : the gait of The foot of the horse Different gaits : general Walking . The amble The trot . The gallop Evolution . juadruped PAGE 600 601 602 003 604 604 604 610 610 610 610 611 613 614 614 614 616 621 623 624 624 624 627 COMPARATIVE PHYSIOLOGY. GENERAL BIOLOGY. INTRODUCTION. Biology (fiios, life ; Aoyos, a dissertation) is the science which. treats of the nature of living things ; and, since the properties of plants and animals can not be explained without some knowl- edge of their form, this science includes morphology (jioptfir), form ; Aoyo?, a dissertation) as well as physiology (<]>vial hyphse; sp, sporangium; zy, sygospore; ex, exosporium; my, mycelium; mc, mucilage; cl, columella; en, endogonidia. Fro. 17.— Spore-bearin<* hyphse of Mucor, growing from horse-dung. Fti*. 1 .—The same, teased out with needles (A, 4). Figs. 19, 20, 21.— Successive stages in the development of the sporangium. Fig. 22.— Isolated spores of Mucor. Fig. 23. — Germinating spores of the same mold. Fig. 24.— Successive stages in the germination of a single spore. Figs. 25, 26, 27.— Successive phases in the conjugative process of Mucor. FiG. 28.— Successive stages observed during ten hours in the growth of a conidiophore of Penicillium in an object-glass culture (D, 4). or tubules, filled with protoplasm and invested by cellulose walls, across which transverse partitions are found at regular intervals, and in which vacuoles are also visible. The spores, when growing thus in a liquid, gives rise to up- ward branches (aerial hyphce), and downward branches or root- lets (submerged hyphce). These multitudinous branches inter- lace in every direction, forming an intricate felt-work, which supports the green powder (spores) which may be so easily shaken off from a growing mold. In certain cases the aerial hyphae terminate in tufts of branches, which, by transverse division, become split up into spores (Conidia), each of which is similar in structure to a yeast-cell. The green coloring matter of the fungi is not chlorophyl. The Conidia germinate under the same conditions as Torula. Mucor mucedo. — The growth and development of this mold may be studied by simply inverting a glass tumbler over some horse-dung on a saucer, into which a very little water has been poured, and keeping the preparation in a warm place. Very soon whitish filaments, gradually getting stronger, ap- pear, and are finally topped by rounded heads or spore-cases {Sporangia). These filaments are the hyphce, similar in struct- ure to those of Penicillium. The spore-case is filled with a multitude of oval bodies (spores), resulting from the subdivision of the protoplasm, which are finally released by the spore-case becoming thinned to the point of rupture. The development of these spores take place in substantially the same manner as those of Penicillium. Sporangia developing spores in this fash- ion by division of the protoplasm are termed asci, and the spores ascospores. So long as nourishment is abundant and the medium of growth fluid, this asexual method of reproduction is the only one ; but, under other circumstances, a mode of increase, known as conjugation, arises. Two adjacent hyphse enlarge at the ex- tremities into somewhat globular heads, bend over toward each 2 18 COMPARATIVE PHYSIOLOGY. other, and, meeting, their opposed faces hecome thinned, and the contents intermingle. The result of this union {zygospore) undergoes now certain further changes, the cellulose coat heing separated into two — an outer, darker in color (exosporium), and an inner colorless one (endosporium) . Under favoring circumstances these coats burst, and a branch sprouts forth from which a vertical tube arises that terminates in a sporangium, in which spores arise, as before de- scribed. It will be apparent that we have in Mucor the exem- plification of what is known in biology as " alternation of gen- erations ".—that is, there is an intermediate generation be tween the original form and that in which the original is again*reached. Physiologically the molds closely resemble yeast, some of them, as Mucor, being capable of exciting a fermentation. The fungi are of special interest to the medical student, be- cause many forms of cutaneous disease are directly associated with their growth in the epithelium of the skin, as, for exam- ple, common ringworm ; and their great vitality, and the facil- ity with which their spores are widely dispersed, explain the highly contagious nature of such diseases. The media on which they flourish (feed) indicates their great physiological differ- ences in this particular from the green plants proper. They are closely related in not a few respects to an important class of vegetable organisms, known as bacteria, to be considered forth- with. The Bacteria. The bacteria include numberless varieties of organisms of extreme minuteness, many of them visible only by the help of the most powerful lenses. Then size has been estimated at from ^xiiny to i ^ 6o of an inch in diameter. They grow mostly in the longitudinal direction, and repro- duce by transverse division, forming spores from which new generations arise. Some of them have vibratile cilia, while the cause of the movements of others is quite unknown. As in many other lowly forms of life, there is a quiescent as well as an active stage. In this stage (zoogloea form) they are surrounded by a gelatinous matter, probably secreted by themselves. Bacteria grow and reproduce in Pasteur's solution, rendering it opaque, as well as in almost all fluids that abound in proteid GENERAL BIOLOGY. 19 matter. That such fluids readily putrefy is owing to the pres- ence of bacteria, the vital action of which suffices to break asun- Fig. 89. Fig. 32 Fig. 29.— Micrococcus, very like a spore, but usually much smaller. Fig. 30.— Bacterium. Fig. 31.— Bacillus. The central filament presented this segmented appearance as the result of a process of transverse division occurring during ten minutes' obser- vation. Fig. 32.— Spirillum; various forms. The first two represent vibrio, which is possibly only a stage of spirillum. Fig. 33. — A drop of the surface scum, showing a spirillum aggregate in the resting state. der complex chemical compounds and produce new ones. Some of the bacteria require oxygen, as Bacillus anthracis, while others do not, as the organism of putrefaction, Bacterium termo. Bacteria are not so sensitive to slight variations in tempera- ture as most other organisms. They can, many of them, with- stand freezing and high temperatures. All bacteria and all germs of bacteria are killed by boiling water, though the spores 20 COMPARATIVE PHYSIOLOGY. are much more resistant than the mature organisms themselves. Some spores can resist a dry heat of 140° C. The spores, like Torula and Protococcus, bear drying, with- out loss of vitality, for considerable periods. That different groups of bacteria have a somewhat different life-history is evident from tbe fact that the presence of one checks the other in the same fluid, and that successive swarms of different kinds may flourish where others have ceased to live. That these organisms are enemies of the constituent cells of the tissues of the highest mammals has now been abundantly demonstrated. That they interfere with the normal working of the organism in a great variety of ways is also clear ; and certain it is that the harm they do leads to aberration in cell- life, however that may be manifested. They rob the tissues of their nutriment and oxygen, and poison them by the products of the decompositions they produce. But apart from this, their very presence as foreign agents must hamper and derange the delicate mechanism of cell-life. These organisms seem to people the air, land, and waters with invisible hosts far more numerous than the forms of life we behold. Fortunately, they are not all dangerous to the higher forms of mammalian life ; but that a large proportion of the diseases which afflict both man and the domestic animals are directly caused by the presence of such forms of life, in the sense of being invariably associated with them, is now beyond doubt. The facts stated above explain why that should be so ; why certain maladies should be infectious ; how the germs of dis ease may be transported to a friend wrapped up in the folds of a letter. Disease thus caused, it must not be forgotten, is an illustra- tion of the struggle for existence and the survival of the fittest. If the cells of an organism are mightier than the bacteria, the latter are overwhelmed ; but if the bacteria are too great in numbers or more vigorous, the cells must yield ; the battle may waver — now dangerous disease, now improvement — but in the end the strongest in this, as in other instances, prevail. GENERAL BIOLOGY. 21 UNICELLULAR ANIMALS WITH DIFFERENTIATION OF STRUCTURE. The Bell- Animalcule (Vorticella). Amoeba is an example of a one-celled animal with little per- ceptible differentiation of structure or corresponding- division of physiological labor. This is not, however, the case with all unicellular animals, and we proceed to study one of these with considerable development of -both. The Bell - animalcule is found in both fresh and salt water, either single or in groups. It is anchored to some object by a rope-like stalk of clear pro- toplasm, that has a spiral appearance when contracted ; and which, with a certain degree of regularity, shortens and length ens alternately, suggesting that more definite movement (con- traction) of the form of protoplasm known as muscle, to be studied later. The body of the creature is bell-shaped, hence its name ; the bell being provided with a thick everted lip (peristome), covered with bristle-like extensions of the protoplasm (cilia), which are in almost constant rhythmical motion. Covering the mouth of the bell is a lid, attached by a hinge of protoplasm to the body, which may be raised or lowered A wide, funnel-like depres- sion {oesophagus) leads into the softer substance within which it ends blindly. The outer part of the animal (cuticula) is denser and more transparent than any other part of the whole creature ; next to this is a portion more granular and of inter- mediate transparency between the external and innermost por- tions (cortical layer). Below the disk is a space (contractile vesicle) filled with a thin, clear fluid, which may be seen to en- large slowly, and then to collapse suddenly. When the Vorti- cella is feeding, these vesicles may contain food-particles, and in the former, apparently, digestion goes on. Such food vacu- oles (vesicles) may circulate up one side of the body of the ani- mal and down the other. Their exact significance is not known, but it would appear as if digestion went on within them ; and possibly the clear fluid with which they are filled may be a spe- cial secretion with solvent action on food. Situated somewhat centrally is a horseshoe-shaped body, with well-defined edges, which stains more readily than the rest of the cell, indicating a different chemical composition ; and, from the prominent part it takes in the reproductive and other functions of the creature, it may be considered the nucleus (endoplast). 22 COMPARATIVE PHYSIOLOGY. Multiplication of the species is either by gemmation or by fission. In the first case the nucleus divides and the frag- Fig. 37. Fig. 40. Fig. 38. Fig. 39. Figs. 34 to 40.— In the figures d denotes disk ; p, peristome; vc, contractile vacuole; vf, food- vacuole; vs, vestibule; cf, contractile fiber; c, cyst; nc, nucleus; cl, c'ilium. Fig. 34. — A group of vorticellte showing the crea- ture in various positions (A, 3). Fig. 35. — The same, in the extended and in the retracted state. (Surface views.) Fig. 36.— Shows food-vacuoles; one in the act of ingestion. Fig. 37. — A vorticella, in which the process of multiplication by fission is begun. Fig. 38. — The results of fission; the production of two individuals of unequal size. Fig. 39.— Illustration of reproduction by conju- gation. Fig. 40.— An encysted vorticella. Fig. 35, ments are transformed into locomotive germs; in the latter the entire animal, including the nucleus, divides longitudi- nally, each half becoming a similar complete, independent or- ganism. Still another method of reproduction is known. A more or less globular body encircled with a ring of cilia and of relatively small size may sometimes be seen attached to the usual form of Vorticella, with which it finally becomes blended into one mass. This seems to foreshadow the " sexual GENERAL BIOLOGY. 23 conjugation " of higher forms, and is of great biological sig- nificance. Vorticella may pass into an encysted and quiescent stage for an indefinite period and again become active. The history of the Bell-animalcule is substantially that of a vast variety of one-celled organisms known as Infusoria, to which Amoeba itself belongs. It will be observed that the resemblance of this organism to Amoeba is very great; it is, however, introduced here to illustrate an advance in differentiation of structure ; and to show how, with the latter, there is usually a physiological advance also, since there is additional functional progress or division of labor ; but still the whole of the work is done with- in one cell. Amoeba and Vorticella are both factories in which all of the work is done in one room, but in the latter case the machinery is more complex than in the former ; there are cor- respondingly more processes, and each is performed with greater perfection. Thus, food in the case of the Bell-animalcule is swept into the gullet by the currents set up by the multitudes of vibrating arms around this opening and its immediate neigh- borhood ; the contractile vesicles play a more prominent part ; and the waste of undigested food is ejected at a more definite portion of the body, the floor of the oesophagus ; while all the movements of the animal are rhythmical to a degree not exem- plified in such simple forms as Amoeba; not to mention its various resources for multiplication and, therefore, for its perpetuation and permanence as a species. It, too, like all the unicellular organisms we have been considering, is susceptible of very wide distribution, being capable of retaining vitality in the dried f state, so that these infusoria may be carried in vari- ous directions by winds in the form of microscopic dust. MULTICELLULAR ORGANISMS. The Fresh- Water Polyps {Hydra viridis ; Hydra fused). The comparison of an animal so simple in structure, though made up of many cells, as the Polyp, with the more complex organizations with which we shall have especially to deal, may be fitly undertaken at this stage. The Polyps are easily obtain- able from ponds in which they are found attached to various kinds of weeds. To the naked eye, they resemble translucent masses of jelly with a greenish or reddish tinge. They range in size from one quarter to one half an inch ; are of an elongated COMPARATIVE PHYSIOLOGY. Fio. 40, GENERAL BIOLOGY. 25 Figs. 41 to 46.— In the figures ec denotes ectoderm; en, endoderm.; /, tentacle; hp, hypostome; /, foot; te, testes; ov, ovary; ps, pseudopodiam; ec', larger ectoderm cells: ne\ larger nematocysts before rupture; cp, Kleinenberg's fibers; <■./. sup- porting lamella; c/„ chlorophyl-forruing bodies; e, cilium. Fig. 41. — The green hydra, at the maximum of contraction and elongation of its body. The creature is represented in the act of seizing a small crustacean (A, 2). Fig. 42. — Transverse section across the body of a hydra, in the digestive cavity of which a small crustacean is represented. Fig. 43.— The leading types of thread-cells, after liberation from the body (P, 3). The cells are represented in the active and the resting conditions; in the former all the parts are more distinctly seen in consequence of the necessary eversion. Fig. 44.— Small portion of a transverse section across the bodv of a green hydra (D, 3). Fig. 45. — A large brown hydra bearing at the same time buds produced asexually and sexual organs. Fig. 4tj. — Larger cells of the ectoderm isolated. Note the processes of the cells or Kleinenberg's fibers (F, 3). All the cuts on pages 9 to 34 have been selected from Howes' Atlas of Biology. cylindrical form ; provided at the oral extremity with thread- like tenacles of considerable length, which are slowly moved about in all directions ; but they and the entire body may short- en rapidly into a globular mass. They are usually attached at the opposite (aboral) pole to some object, but may float free, or slowly crawl from place to place. It may be observed, under the microscope, that the tenacles now and then embrace some living object, convey it toward an opening (mouth) near their base, from which, from time to time, refuse material is cast out. It may be noticed, too, that a living object within the touch of these tenacles soon loses the power to struggle, which is owing to the peculiar cells (nettle-cells, urticating capsules, nemato- cysts) with which they are abundantly provided, and which se- crete a poisonovis fluid that paralyzes prey. The mouth leads into a simple cavity (coelom) in which digestion proceeds. The green color in Hydra viridis, and the red color of Hydra fusca, is owing to the presence of chlorophyl, the function of which is not known. Hydra is structurally a sac, made up of two layers of cells, an outer (ectoderm) and an inner (endoderm): the tentacles being repetitions of the scructure of the main body of the animal, and so hollow and composed of two cell layers. Speaking generally, the outer layer is devoted to obtaining information of the surroundings ; the inner to the work of preparing nutriment, and probably, also, discharging waste matters, in which latter assistance is also received from the outer layer. As digestion takes place largely within the cells themselves, or is intracellular, we are reminded of Vorticella and still more of Amoeba. There is in Hydra a general advance in development, but not very much individual cell specialization. That of the urticating capsules is one of the best examples of such specialization in this creature. 26 COMPARATIVE PHYSIOLOGY. A Polyp is like a colony of Amoebae in which some division of labor (function) has taken place ; a sort of biological state in which every individual is nearly equal to his neighbor, but somewhat more advanced than those neighbors not members of the organization. But in one respect the Polyps show an enormous advance. Ordinarily when nourishment is abundant Hydra multiplies by budding, and when cut into portions each may become a com- plete individual. However, under other circumstances, near the bases of the tentacles the body wall may protrude into little masses (tes es), in which cells of peculiar formation (sperma- tozoa) arise, and are eventually set free and unite with a cell- {ovum) formed in a similar protrusion of larger size (ovary). Here, then, is the first instance in which distinctly sexual repro- duction has been met in our studies of the lower forms of life. This is substantially the same process in Hydra as in mammals. But, as both male and female cells are produced by the same individual, the sexes are united (hermaphroditism) ; each is at once male and female. Any one watching the movements of a Polyp, and compar- ing it with those of a Bell-animalcule, will observe that the former are much less machine-like ; have greater range ; seem to be the result of a more deliberate choice ; are better adapted to the environment, and calculated to achieve higher ends. In the absence of a nervous system it is not easy to explain how one part moves in harmony with another, except by that pro- cess which seems to be of such wide application in nature, adap- tation from habitual simultaneous effects on a protoplasm capa- ble of responding to stimuli. When one process of an Amoeba is touched, it is likely to withdraw all. This we take to be due to influences radiating through molecular movement to other parts ; the same principle of action may be extended to Hydra. The oftener any molecular movement is repeated, the more it tends to become organized into regularity, to become fixed in its mode of action ; and if we are not mistaken this is a funda- mental law throughout the entire world of living things, if not of all things animate and inanimate alike. To this law we shall return. But Hydra is a creature of but very limited specializations; there are neither organs of circulation, respiration, nor excretion, if we exclude the doubtful case of the thread-cells (urticating capsules). The animal breathes by the entire surface of the GENERAL BIOLOGY. 27 body ; nourishment passes from cell to cell, and waste is dis- charged into the water surrounding the creature from all cells, though probably not quite equally. All parts are not digestive, respiratory, etc., to the same degree, and herein does it differ greatly from Amoeba or even Vorticella, though fuller knowl- edge will likely modify our views of the latter two and similar organisms in this regard. THE CELL RECONSIDERED. Having now studied certain one-celled plants and animals, and some very simple combinations of cells (molds, etc.), it will be profitable to endeavor to generalize the lessons these humble organisms convey ; for, as will be constantly seen in the study of the higher forms of life of which this work proposes to treat principally, the same laws operate as in the lowliest living creat- ures. The most complex organism is made up of tissues, which are but cells and then* products, as houses are made of bricks, mortar, wood, and a few other materials, however large or elab- orate. The student of physiology who proceeds scientifically must endeavor, in investigating the functions of each organ, to learn the exact behavior of each cell as determined by its own inherent tendencies, and modified by the action of neighboring cells. The reason why the function of one organ differs from that of another is that its cells have departed in a special direction from those properties common to all cells, or have become function- ally differentiated. But such a statement has no meaning un- less it be well understood that cells have certain properties in common. This is one of the lessons imparted by the preceding studies which we now review. Briefly stated in language now extensively used in works on biology, the common properties of cells' (protoplasm), whether animal or vegetable, whether consti- tuting in themselves entire animals or plants, or forming the elements of tissues, are these : The collective chemical processes associated with the vital activities of cells are termed its metab- olism. Metabolism is constructive when more complex com- pounds are formed from simple ones, as when the Protococcus- cell builds up its protoplasm out of the simple materials, found in rain-water, which makes up its food. Metabolism, is destructive when the reverse process takes place. The results of this process are eliminated as excreta, or useless and harmful products. 28 COMPARATIVE PHYSIOLOGY. Since all the vital activities of cells can only be manifested when supplied with food, it follows that living organisms convert po- tential or possible energy into kinetic or actual energy. When lifeless, immobile matter is taken in as food and, as a result, is converted by a process of assimilation into the protoplasm of the cell using it, we have an example of potential being converted into actual energy, for one of the properties of all protoplasm is its contractility. Assimilation implies, of course, the absorp- tion of what is to be used, with rejection of waste matters. The movements of protoplasm of whatever kind, when due to a stimulus, are said to indicate irritability ; while, if inde- pendent of any external source of excitation, they are denomi- nated automatic. Among agents that modify the action of all kinds of proto- plasm are heat, moisture, electricity, light, and others in great variety, both chemical and mechanical. It can not be too well remembered that living things are what they are, neither by virtue of their own organization alone nor through the action of their environment alone (else would they be in no sense dif- ferent from inanimate things), but because of the relation of the organization to the surroundings. Protoplasm, then, is contractile, irritable, automatic, absorp- tive, secretory (and excretory), metabolic, and reproductive. But when it is affirmed that these are the fundamental prop- erties of all protoplasm, the idea is not to be conveyed that cells exhibiting these properties are identical biologically. No two masses of protoplasm can be quite alike, else would there be no distinction in physiological demeanor — no individuality. Every cell, could we but behold its inner molecular mechanism, differs from its neighbor. When this difference reaches a certain de- gree in one direction, we have a manifest differentiation leading to physiological division of labor, which may now with advan- tage be treated in the following section. THE ANIMAL BODY. An animal, as we have learned, may be made up of a single cell in which each part performs much the same work ; or, if there be differences in function, they are ill-defined as compared with those of higher animals. The condition of tilings in such an animal as Amoeba may be compared to a civilized commu- nity in a very crude social condition. When each individual GENERAL BIOLOGY. 29 tries to perform every office for himself, he is at once carpenter, blacksmith, shoemaker, and much more, with the natural re- sult that he is not efficient in any one direction. A community may be judged in regard to its degree of advancement by the amount of division of labor existing within it. Thus is it with the animal body. We find in such a creature as the fresh-water Hydra, consisting of two layers of cells forming a simple sac, a slight amount of advancement on Amoeba. Its external surface no longer serves for inclosure of food, but it has the simplest form of mouth and tentacles. Each of the cells of the internal layer seems to act as a somewhat improved or specialized Amoe- ba, while in those of the outer layer we mark a beginning of those functions which taken collectively give the higher ani- mals information of the surrounding world. Looking to the existing state of things in the universe, it is plain that an animal to attain to high ends must have powers of rapid locomotion, capacity to perceive what makes for its in- terest, and ability to utilize means to obtain this when perceived. These considerations demand that an animal high in the scale of being should be provided with limbs sufficiently rigid to sup- port its weight, moved by strong muscles, which must act in harmony. But this implies abundance of nutriment duly pre- pared and regularly conveyed to the bones and muscles. All this would be useless unless there was a controlling and ener- gizing system capable both of being impressed and originating impressions. Such is found in the nerves and nerve- centers. Again, in order that this mechanism be kept in good running order, the waste of its own metabolism, which chokes and poi- sons, must be got rid of— hence the need of excretory apparatus. In order that the nervous system may get sufficient informa- tion of the world around, the surface of the body must be pro- vided with special message-receiving offices in the form of modified nerve-endings. In short, it is seen that an animal as high in the scale as a mammal must have muscular, osseous (aud connective), digestive, circulatory, excretory, and nervous tissues ; and to these may be added certain forms of protective tissues, as hair, nails, etc. * Assuming that the student has at least some general knowl- edge of the structure of these various tissues, we propose to tell in a simple way the whole physiological story in brief. The blood is the source of all the nourishment of the organ- ism, including its oxygen supply, and is carried to eveiy part of 30 COMPARATIVE PHYSIOLOGY. the body through elastic tubes which, continually branching and becoming gradually smaller, terminate in vessels of hair- like fineness in which the current is very slow — a condition per- mitting that interchange between the cells surrounding them and the blood which may be compared to a process of barter, the cells taking nutriment and oxygen, and giving (excreting) in return carbonic anhydride. From these minute vessels the blood is conveyed back toward the source whence it came by similar elastic tubes which gradually increase in size and be- come fewer. The force which directly propels the blood in its onward course is a muscular pump, with both a forcing and suction action, though chiefly the former. The flow of blood is maintained constant owing to the resistance in the smaller tubes on the one hand and the elastic recoil of the larger tubes on the other ; while in the returning vessels the column of blood is suppoi'ted by elastic double gates which so close as to prevent reflux. The oxygen of the blood is carried in disks of microscopic size which give it up in proportion to the needs of the tissues past which they are cai^ried. But in reality the tissues of the body are not nourished directly by the blood, but by a fluid derived from it and resem- bling it greatly in most particulars. This fluid bathes the tis- sue-cells on all sides. It also is taken up by tubes that convey it into the blood after it has passed through little factories (lymphatic glands), in which it undergoes a regeneration. Since the tissues are impoverishing the blood by withdrawal of its constituents, and adding to it what is no longer useful, and is in reality poisonous, it becomes necessary that new material be added to it and the injurious components withdrawn. The former is accomplished by the absorption of the products of food digestion, and the addition of a fresh supply of oxygen derived from without, while the poisonous ingredients that have found their way into the blood are got rid of through processes that may be, in general, compared to those of a sew- age system of a very elaborate character. To explain this re- generation of the blood in somewhat more detail, we must first consider the fate of food from the time it enters the mouth till it leaves the tract of the body in which its preparation is car- ried on. The food is in the mouth submitted to the action of a series of cutting and grinding organs worked by powerful muscles ; mixed with a fluid which changes the starchy part of it into GENERAL BIOLOGY. 31 sugar, and prepares the whole to pass further on its course : when this has heen accomplished, the food is grasped and squeezed and pushed along the tube, owing to the action of its own muscular cells, into a sac (stomach), in which it is rolled about and mixed with certain fluids of peculiar chemical com- position derived from cells on its inner surface, which trans- form the proteid part of the food into a form susceptible of ready use (absorption). When this saccular organ has done its share of the work, the food is moved on by the action of the muscles of its walls into a very long portion of the tract in which, in addition to processes carried on in the mouth and stomach, there are others which transform the food into a con- dition in which it can pass into the blood. Thus, all of the food that is susceptible of changes of the kind described is acted upon somewhere in the long tract devoted to this task. But there is usually a remnant of indigestible material which is finally evacuated. How is the prepared material conveyed into the blood ? In part, directly through the walls of the minutest blood-vessels distributed throughout the length of this tube ; and in part through special vessels with appropriate cells cov- ering them which act as minute porters (villi). The impure blood is carried periodically to an extensive sur- face, usually much folded, and there exposed in the hair-like tubes referred to before, and thus parts with its excess of car- bon dioxide and takes up fresh oxygen. But all the functions described do not go on in a fixed and invariable manner, but are modified somewhat according to circumstances. The for- cing-pump of the circulatory system does not always beat equally fast ; the smaller blood-vessels are not always of the same size, but admit more or less blood to an organ according to its needs. This is all accomplished in obedience to the commands car- ried from the brain and spinal cord along the nerves. All movements of the limbs and other parts are executed in obe- dience to its behests ; and in order that these may be in accord- ance with the best interests of each particular organ and the whole animal, the nervous centers. Which may be compared to the chief officers of, say, a telegraph or railway system, are in constant receipt of information by messages carried onward along the nerves. The command issuing is always related to the information arriving. All those parts commonly known as sense-organs — the eye, 32 COMPARATIVE PHYSIOLOGY. ear, nose, tongue, and the entire surface of the body — are faith- ful reporters of facts. They put the inner and outer worlds in communication, and without them all higher life at least must cease, for the organism, like a train directed by a conductor that disregards the danger-signals, must work its own destruction. Without going into further details, suffice it to say that the pro- cesses of the various cells are subordinated to the general good through the nervous system, and that susceptibility of proto- plasm to stimuli of a delicate kind which enables each cell to adapt to its surroundings, including the influence of remote as well as neighboring cells. Without this there could be no marked advance in organisms, no differentiation of a pro- nounced character, and so none of that physiological division of labor which will be inferred from our brief description of the functions of a mammal. The whole of physiology but illustrates this division of labor. It is hoped that the above account of the working of the ani- mal body, brief as it is, may serve to show the connection of one part functionally with another, for it is much more impor- tant that this should be kept in mind throughout, than that all the details of any one function should be known. LIVING AND LIFELESS MATTER. In order to enable the student the better to realize the na- ture of living matter or protoplasm, and to render clearer the distinction between the forms that belong to the organic and inorganic worlds respectively, we shall make some comparisons in detail which it is hoped may accomplish this object. A modern watch that keeps correct time must be regarded as a wonderful object, a marvelous triumph of human skill. That it has aroused the awe of savages, and been mistaken for a living being, is not surprising. But, admirable as is the result attained by the mechanism of a watch, it is, after all, composed of but a few metals, etc., chiefly in fact of two, brass and steel ; these are, however, made up into a great number of different parts, so adapted to one another as to work in unison and ac- complish the desired object of indicating the time of day. Now, however well constructed the watch may be, there are waste, wear and tear, which will manifest themselves more and more, until finally the machine becomes worthless for the pur- pose of its construction. If this mechanism possessed the power GENERAL BIOLOGY. 33 of adapting- from without foreign matter so as to construct it it into steel and brass, and arrange this just when required, it would imitate a living organism ; but this it can not do, nor is its waste chemically different from its component metals ; it does not break up brass and steel into something wholly differ- ent. In one particular it does closely resemble living things, in that it gradually deteriorates ; but the degradation of a liv- ing cell is tbe consequence of an actual change in its compo- nent parts, commonly a fatty degeneration. The one is a real transformation, the other mere wear. Had the watch the power to give rise to a new one like itself by any process, especially a process of division of itself into two parts, we should have a parallel with living forms ; but the watch can not even renew its own parts, much less give rise to a second mechanism like itself. Here, then, is a manifest dis- tinction between living and inanimate things. Suppose, further, that the watch was so constructed that, after the lapse of a certain time, it underwent a change in its inner machinery and perhaps its outer form, so as to be scarcely recognizable as the same ; and that as a result, instead of indi- cating the hours and minutes of a time-reckoning adapted to the inhabitants of our globe, it indicated time in a wholly dif- ferent way ; that after a series of such transformations it fell to pieces— took the original form of the metals from which it was constructed — we should then have in this succession of events a parallel with the development, decline, and death of living or- ganisms. In another particular our illustration of a watch may serve a useful purpose. Suppose a watch to exist, the works of which are so concealed as to be quite inaccessible to our vision, so that all we know of it is that it has a mechanism which when in action we can hear, and the result of which we perceive in the movements of the hands on the face ; we should then be in the exact position in reference to the watch that we now are as re- gards the molecular movements of protoplasm. On the latter the entire behavior of living matter depends ; yet it is abso- lutely hidden from us. We know, too, that variations must be produced in the mechanism of time-pieces by temperature, moisture, and other influences of the environment, resulting in altered action. The same, as will be shown in later chapters, occurs in protoplasm. Tins, too, is primarily a molecular effect. If the works of 3 34 COMPARATIVE PHYSIOLOGY. watches were beyond observation, we should not be able to state exactly how the variations observed in different kinds, or even different individuals of the same kind occurred, though these differences might be of the most marked character, such as any one could recognize. Here once more we refer the differ- ences to the mechanism. So is it with living beings : the ulti- mate molecular mechanism is unknown to us. Could we but render these molecular movements visible to our eyes, we should have a revelation of far greater scientific importance than that unfolded by the recent researches into those living forms of extreme minuteness that swarm every- where as dust in a sunbeam, and, as will be learned later, are often the source of deadly disease. Like the movements of the watch, the activities of protoplasm are ceaseless. A watch that will not run is, as such, worthless — it is mere metal — has under- gone an immense degradation in the scale of values ; so proto- plasm is no longer protoplasm when its peculiar molecular movements cease ; it is at once degraded to the rank of dead matter. The student may observe that each of the four propositions, embodying the fundamental properties of living matter, stated in the preceding chapter, have been illustrated by the simile of a watch. Such an illustration is necessarily crude, but it helps one to realize the meaning of truths which gather force with each living form studied if regarded aright ; and it is upon the realization of truth that mental growth as well as practical efficiency depends. CLASSIFICATION OF THE ANIMAL KINGDOM. There are human beings so low in the scale as not to possess such general terms as tree, while they do employ names for dif- ferent kinds of trees. The use of such a word as " tree " im- plies generalization, or the abstraction of a set of qualities from the things in which they reside, and making them the basis for the grouping of a multitude of objects by which we are sur- rounded. Manifestly without such a process knowledge must be very limited, and the world without significance ; while in proportion as generalization may be safely widened, is our progress in the unification of knowledge toward which science is tending. But it also follows that without complete knowl- edge there can be no perfect classification of objects ; hence, GENERAL BIOLOGY. 35 any classification must be regarded but as the temporary creed of science, to be modified with the extension of knowledge. As a matter of fact this has been the history of all zoological and other systems of arrangement. The only purpose of grouping is to simplify and extend knowledge ; this being the case, it fol- lows that a method of grouping that accomplishes this has value, though the system may be artificial that is based on resemblances which, though real and constant, are associated with differences so numerous and radical that the total amount of likeness between objects thus grouped is often less than the difference. Such a system was that of Linnaeus, who classified plants according to the number of stamens, etc., they bore. Seeing that animals which resemble each other are of com- mon descent from some earlier form, to establish the line of de- scent is to determine in great part the classification. Much as- sistance in this direction is derived from embryology, or the history of the development of the individual {ontogeny) • so that it may be said that the ontogeny indicates, though it does not actually determine, the line of descent (phytogeny) ; and it is owing to the importance of this truth that naturalists have in recent years given so much attention to comparative embry- ology. It will be inferred that a natural system of classification must be based both on function and structure, though chiefly on the latter, since organs of very different origin may have a similar function ; or, to express this otherwise, homologous structures may not be analogous ; and homology gives the better basis for classification. To illustrate, the wing of a bat and a bird are both homologous and analogous ; the wing of a butterfly is analogous but not homologous with these ; manifestly, to clas- sify bats and birds together would be better than to put birds and insects in the same group, thus leaving other points of re- lationship out of consideration. The broadest possible division of the animal kingdom is into groups, including respectively one-celled and many-celled forms — i. e., into Protozoa and Metazoa. As the wider the grouping the less are differences considered, it follows that the more sub- divided the groups the more complete is the information con- veyed ; thus, to say that a dog is a metazoan is to convey a cer- tain amount of information ; that he is a vertebrate, more ; that he is a mammal, a good deal more, because each of the latter terms includes the former. 36 COMPARATIVE PHYSIOLOGY. Inverte- brata. Animal Kingdom. | r Protozoa (amoeba, vorticella, etc.). Ccelenterata (sponges, jelly-fish, polyps, etc.). I Echinodermata (star-fish, sea-urchins, etc.). J Vermes (worms). 1 Ai'thropods (crabs, insects, spiders, etc.). Mollusca (oysters, snails, etc.). Molluscoidea (moss-like animals). *- Tunicata (ascidians). ( Pisces (fishes). Amphibia (frogs, menobranehus, etc.). Vertebrata. \ Keptilia (snakes, turtles, etc.). Aves (birds'). Mammalia (domestic quadrupeds, etc.). I The above classification (of Claus) is, like all such arrange- ments, but the expression of one out of many methods of view- ing the animal kingdom. For the details of classification and for the grounds of that we have presented, we refer the student to works on zoology ; but we advise those who are not familiar with this subject, when a technical term is used, to think of that animal belong- ing to the group in question with the structure of which they are best acquainted. Man's Place in the Animal Kingdom. It is no longer the custom with zoologists to place man in an entirely separate group by himself ; but he is classed with the primates, among which are also grouped the anthropoid apes (gorilla, chimpanzee, orang, and the gibbon), the monkeys of the Old and of the New World, and the lemurs. So great is the structural resemblance of man and the other primates that competent authorities declare that there is more difference be- tween the structure of the most widely separated members of the group than between certain of the anthropid apes and man. The points of greatest resemblance between man and the anthropoid apes are the following : The same number of verte- brae ; the same general shape of the pelvis ; a brain distinguish- ing them from other mammals ; and posture, being bipeds. The distinctive characters are size, rather than form of the brain, that of man being more than twice as large ; a relatively larger cranial base, by which, together with the greater size of the jaws, the face becomes prominent ; the earlier closure of the sutures of the cranium, arresting the growth of the brain ; more developed canine teeth and difference in the order of erup- tion of the permanent teeth ; the more posterior position of the foramen magnum ; the relative length of the limbs to each GENERAL BIOLOGY. 37 other and the rest of the body ; minor differences in the hands and feet, especially the greater freedom and power of apposition of the great-toe. But the greatest distinction between man and even his closest allies among the apes is to be found in the development to an incomparably higher degree of his intellectual and moral na- ture, corresponding to the differences in weight and structure of the human brain, and associated with the use of spoken and written language ; so that the experience of previous genera- tions is not only registered in the organism (heredity), but in the readily available form of books, etc. The greatest structural difference between the races of men are referable to the cranium ; but, since they all interbreed freely, they are to be considered varieties of one species. THE LAW OF PERIODICITY OR RHYTHM IN NATURE. The term rhythm to most minds suggests music, poetry, or dancing, in all of which it forms an essential part so simple, pronounced, and uncomplicated as to be recognized by all with ease. The regular division of music into bars, the recurrence of chords of the same notes at certain intervals, of forte and piano, seem to be demanded by the very nature of the human mind. The same applies to poetry. Even a child that can not under- stand the language used, or an adult listening to recitations in an unknown tongue, enjoys the flow and recurrences of the sounds. Dancing has in all ages met a want in human organi- zations, which is partly supplied in quieter moods by the regu- larity of the steps in walking and similar simple movements. But as rhythm runs through all the movements of animals, so is it also found in all literature and all art. Infinite variety wearies the mind, hence the fatigue felt by the sight-seer. Re- currence permits of repose, and gratifies an established taste or appetite. The mind delights in what it has once enjoyed, in repetition within limits. Repetition with variety is manifestly a condition of the growth and development of the mind. This seems to apply equally to the body, for every single function of each organism, however simple or complex it may be, exempli- fies this law of periodicity. The heart's action is rhythmical (beats) ; the blood flows in intermitting gushes from the central pump ; the to-and-fro movements of respiration are so regular 38 COMPARATIVE PHYSIOLOGY. that their cessation would arouse the attention of the least in- structed ; food is demanded at regular intervals ; the juices of the digestive tract are poured out, not constantly but period- ically ; the movements by which the food is urged along its path are markedly rhythmic ; the chemical processes of the body wax and wane like the fires in a furnace, giving rise to regular augmentations of the temperature of the body at fixed hours of the day, with corresponding periods of greatest bodily activity and the reverse. This principle finds perfect illustration in the nervous sys- tem. The respiratory act of the higher animals is effected through muscular movements dependent on regular waves of excitation reaching them along the nerves from the central cells which regularly discharge their forces along these channels. Were not the movements of the body periodic or rhythmical, instead of that harmony which now prevails, every muscular act would be a convulsion, though even in the movements of the latter there is a highly compounded rhythm, as a noise is made up of a variety of musical notes. The senses are subject to the same law. The eye ceases to see and the ear to hear and the hand to feel if continuously stimulated; and doubtless in all art this law is unconsciously recognized. That ceases to be art which fails to provide for the alternate repose and excita- tion of the senses. The eye will not tolerate continuously one color, the ear the same sound. Why is a breeze on a warm day so refreshing ? The answer is obvious. Looking to the world of animate nature as a whole, it is noticed that plants have their period of sprouting, flowering, seeding, and decline; animals are born, pass through various stages to maturity, diminish in vigor, and die. These events make epochs in the life-history of each species ; the recurrence of which is so constant that the agricultural and other arrange- ments even of savages are planned accordingly. That the in- dividuals of each animal group have a definite period of dura- tion is another manifestation of the same law. Superficial observation suffices to furnish facts which show that the same law of periodicity is being constantly exemplified in the world of inanimate things. The regular ebb and flow of the tides ; the rise and subsidence of rivers ; the storm and the calm; summer and winter; day and night — are all recurrent, none constant. Events apparently without any regularity, utterly beyond GENERAL BIOLOGY. 39 any law of recurrence, when sufficiently studied are found to fall under the same principle. Thus it took some time to learn that volcanic eruptions occurred with a very fair degree of regularity. In judging of this and all other rhythmical events it must be borne in mind that the time standard is for an irregularity that seems large, as in the instance just referred to, becomes small when considered in relation to the millions of years of geological time; while in the case of music a trifling irregu- larity, judged by fractions of a second, can not be tolerated by the musical organization — which is equivalent to saying that the interval of departure from exact regularity seems large. As most of the rhythms of the universe are compounded of several, it follows that they may seem, until closely studied, very far from regular recurrences. This may be observed in the interference in the regularity of the tides themselves, the daily changes of which are subject to an increase and decrease twice in each month, owing to the influence of the sun and moon being then either coincident or antagonistic. In the functions of plants and animals, rhythms must be- come very greatly compounded, doubtless often beyond recog- nition. Among the best examples of rhythm in animals are daily sleep and winter sleep, or hibernation ; yet, amid sleep, dreams or recurrences of cerebral activity are common — that is, one rhythm (of activity) overlies another (of repose). In like man- ner many hibernating animals do not remain constantly in their dormant condition throughout the winter months, but have periods of wakefulness ; the active life recurs amid the life of functional repose. To return to the world of inanimate matter, we find that the crust of the earth itself is made of layers or strata the result of periods of elevation and depression, of denudation and deposi- tion, in recurring order. The same law is illustrated by the facts of the economic and other conditions of the social state of civilized men. Periods of depression alternate with periods of revival in commercial life. There are periods when many more marriages occur and many more children are born, corresponding with changes in the material conditions which influence men as well as other animals. 40 COMPARATIVE PHYSIOLOGY. Finally, and of special interest to the medical student, are the laws of rhythm in disease. Certain fevers have their regu- lar periods of attack, as intermittent fever; while all diseases have their periods of exacerbation, however invariable the symptoms may seem to be to the ordinary observer or even to the patient himself. Doubtless the fact that certain hereditary diseases do not appear in the offspring at once, but only at the age at which they were manifested in the parents, is owing to the same cause. Let us now examine more thoroughly into the real nature of this rhythm which prevades the entire universe. If a bow be drawn across a violin-string on which some small pieces of paper have been placed, these will be seen to fly off ; and if the largest string be experimented upon, it can be ob- served to be in rapid to-and-fro motion, known as vibration, which motion is perfectly regular, a definite number of move- ments occurring within a measured period of time ; in other words the motion is rhythmical. In strings of the finest size the motion is not visible, but we judge of its existence because of the result, which is in each instance a sound. Sound is to us, however, an affection of the nerve of hearing and the brain, owing to the vibrations of the ear caused by similar vibrations of the violin-strings. The movements of the nerves and nerve- cells are invisible and molecular, and we seem to be justified in regarding molecular movements as constant and associated with all the properties of matter whether living or dead. We see, then, that all things living and lifeless are in con- stant motion, visible or invisible ; there is no such thing in the universe as stable equilibrium. Change, ceaseless change, is written on all things ; and, so far as we can judge, these changes, on the whole, tend to higher development. Neither rhythm, however, nor anything else, is perfect. Even the mo- tions of planets are subject to perturbations or irregularities in their periodicity. This subject is plainly boundless in its scope. We have introduced it at this stage to prepare for its study in detail in dealing with each function of the animal body. If we are correct as to the universality of the law of rhythm, its importance in biology deserves fuller recognition than it has yet received in works on physiology ; it will, ac- cordingly, be frequently referred to in the future chapters of this book. GENERAL BIOLOGY. 41 THE LAW OF HABIT. Every one must have observed in himself and others the tendency to fall into set ways of doing certain things, in which will and clear pui'pose do not come prominently into view. Further observation shows that the lower animals exhibit this tendency, so that, for example, the habits of the horse or the dog may be an amusing reflection of those of the master. Trees are seen to bend permanently in the direction toward which the prevailing winds blow. The violin that has experienced the vibrations, aroused by some master's hand acquires a potential musical capability not possessed by an instrument equally good originally, but the molecular movements of which never received such an educa- tion. It appears, then, that underlying what we call habit, there is some broad law not confined to living things ; indeed, the law of habit appears to be closely related to the law of rhythm we have already noticed. Certain it is that it is inseparable from all biological phenomena, though most manifest in those organ- isms provided with a nervous system, and in that system itself. What we usually call habit, however expressed, has its physical correlation in the nervous system. We may refer to it in this connection later: but the subject has relations so numerous and fundamental that it seems eminently proper to introduce it at this early stage, forming as it does one of those corner- stones of the biological building on which the superstructure must rest. When we seek to come to a final explanation of habit in this case, as in most others, in which the fundamental is involved, we are soon brought against a wall over which we ai*e unable to climb, and through which no light comes to our intellects. We must simply believe, as the result of observation, that it is a law of matter, in all the forms manifested to us, to assume accustomed modes of behavior, perhaps we may say molecular movement, in obedience to inherent tendencies. But, to recog- nize this, throws a flood of light on what would be inexplicable, even in a minor degree. We can not explain gravitation in it- self ; but, assuming its universality, replaces chaos by order in our speculations on matter. Turning to living matter, we look for the origin of habit in the apparently universal principle that primary molecular 42 COMPARATIVE PHYSIOLOGY. movement in one direction renders that movement easier after- ward, and in proportion to the frequency of repetition ; which is equivalent to saying that functional activity facilitates func- tional activity. Once accepting this as of universal application in biology, we have an explanation of the origin, the compara- tive rigidity, and the necessity of habit. There must be a phys- ical basis or correlative of all mental and moral habits, as well as those that may be manifested during sleep, and so purely in- dependent of the will and consciousness. We are brought, in fact, to the habits of cells in considering those organs, and that combination of structures which makes up the complex individ- ual mammal. It is further apparent that if the cell can trans- mit its nature as altered by its experiences at all, then habits must be hereditary, which is known to be the case. Instincts seem to be but crystallized habits, the inherited results of ages of functional activity in certain well-defined directions. To a being with a highly developed moral nature like man, the law of habit is one of great, even fearful significance. We make to-day our to-morrow, and in the present we are deciding the future of others, as our present has been made for us in part by our ancestors. We shall not pursue the subject, which is of boundless extent, further now, but these somewhat general statements will be amplified and applied in future chapters. THE ORIGIN OF THE FORMS OF LIFE. It is a matter of common observation that animals originate from like kinds, and plants from forms resembling themselves ; while most carefully conducted experiments have failed to show that living matter can under any circumstances known to us arise from other than living matter. That in a former condition of the universe such may have been the case has not been disproved, and seems to be the logical outcome of the doctrine of evolution as applied to the universe generally. By evolution is meant the derivation of more complex and differentiated forms of matter from simpler and more homogene- ous ones. When this theory is applied to organized or living forms, it is termed organic evolution. There are two views of the origin of life : the one, that each distinct group of plants and animals was independently created ; while by " creation " is GENERAL BIOLOGY. 43 simply meant that they came into being in a manner we know not how, in obedience to the will of a First Cause. The other view is denominated the theory of descent with modification, the theory of transmutation, organic evolution, etc., which teaches that all the various forms of life have been derived from one or a few primordial forms in harmony with the recog- nized principles of heredity and variability. The most widely known and most favorably received exposition of this theory is that of Charles Darwin, so that his views will be first presented in the form of a hypothetical case. Assume that one of a group of living forms varies from its fellows in some particular, and mating with another that has similarly varied, leaves progeny inheriting this characteristic of the parents, that tends to be still further increased and rendered permanent by successive pairing with forms possessing this valuation in shape, color, or whatever it may be. We may suppose that the variations may be numerous, but are always small at the beginning. Since all animals and plants tend to multiply faster than the means of support, a competition for the means of subsistence arises, in which struggle the fittest, as judged by the circumstances, al- ways is the most successful ; and if one must perish outright, it is the less fit. If any variation arises that is unfavorable in this contest, it will render the possessor a weaker competitor : hence it follows that only useful variations are preserved. The struggle for existence is, however, not alone for food, but for anything which may be an advantage to its possessor. One form of the contest is that which results from the rivalry of members of the same sex for the possession of the females ; and as the female chooses the strongest, most beautiful, most active, or the supreme in some respect, it follows that the best leave the great- est number of progeny. This has been termed sexual selection. In determining what forms shall survive, the presence of other plants or animals is quite as important as the abundance of food and the physical conditions, often more so. To illustrate this by an example : Certain kinds of clover are fertilized by the visits of the bumble-bee alone ; the numbers of bees exist- ing at any one place depends on the abundance of the field-mice which destroy the nests of these insects ; the numbers of mice will depend on the abundance of creatures that prey on the mice, as hawks and owls ; these, again, on the creatures that specially destroy them, as foxes, etc. ; and so on, the chain of connections becoming more and more lengthy. 44 COMPARATIVE PHYSIOLOGY. Fig. 47.— Shows the embryos of four mammals in the three eorresponding stages : of a hog (II), calf (C), rabbit (R), and a man (M). The conditions of the three differ- ent stages of development, which the three cross-rows (I, II, III) represent, are selected to correspond as exactly as possible. The first, or upper cross-row, I. represents a very early stage, with gill-openings, and without limbs. The second (middle) cross-row, II, shows a somewhat later stage, with the first rudiments of limbs, while the gill-openings are yet retained. The third (lowest) cross-row, III, shows a still later stage, with the limbs more developed and the gill-openings GENERAL BIOLOGY. 45 lost. The membranes and appendages of the embryonic body (the amnion, yelk- sac, allantois) are omitted. The whole twelve figures are slightly magnified, the upper ones more than the lower. To facilitate "the comparison, they are all re- duced to nearly the same size in the cuts. All the embryos are seen from the left side ; the head extremity is above, the tail extremity below ; the arched back turned to the right, The letters indicate the same parts in all the twelve figures, namely: v, fore-brain; z, twist-brain; m, mid-brain; h, hind-brain; re, after-brain; r, spinal marrow- e, nose; «, eye; o, ear; k, gill-arches; g. heart; iv, vertebral column; /, fore-limbs; b, hind-limbs; s, tail. (After Haeckel.) If a certain proportion of forms varying similarly were sep- arated by any great natural barrier, as a chain of lofty mountains or an intervening body of water of considerable extent, and so pre- vented from breeding with forms that did not vary, it is clear that -there would be greater likelihood of their differences being pre- served and augmented up to the point of their greatest usefulness. We may now inquire whether such has actually been the course of events in nature. The evidence may be arranged un- der the following heads : 1. Morphology. — Briefly, there is much that is common to entire large groups of animals ; so great, indeed, are the resem- blances throughout the whole animal kingdom that herein is found the strongest argument of all for the doctrine of descent. To illustrate by a single instance — fishes, reptiles, birds, and mammals possess in common a vertebral column bearing the same relationship to other parts of the animal. It is because of resemblances of this kind, as well as by their differences, that naturalists are enabled to classify animals. 2. Embryology.— In the stages through which animals pass in their development from the ovum to the adult, it is to be ob- served that the closer the resemblance of the mature organism in different groups, the more the embryos resemble one another. Up to a certain stage of development the similarity between groups of animals, widely separated in their post-embryonic life, is marked : thus the embryo of a reptile, a bird, and a mam- mal have much in common in their earlier stages. The embryo of the mammal passes through stages which represent condi- tions which are permanent in lower groups of animals, as for example that of the branchial arches, which are represented by the gills in fishes. It may be said that the developmental his- tory of the individual (ontogeny) is a brief recapitulation of the development of the species (phylogeny). Apart from the theory of descent, it does not seem possible to gather the true signifi- cance of such facts, which will become plainer after the study of the chapters on reproduction. 3. Mimicry may be cited as an instance of useful adaptation. 46 COMPARATIVE PHYSIOLOGY. Thus, certain beetles resemble bees and wasps, which latter are protected by stings. It is believed that such groups of beetles as these arose by a species of selection ; those escaping enemies which chanced to resemble dreaded insects most, so that birds which were accustomed to prey on beetles, yet feared bees, would likewise avoid the mimicking forms. 4. Rudimentary Organs. — Organs which were once func- tional in a more ancient form, but serve no use in the creatures in which they are now found, have reached, it is thought, their rudimentary condition through long periods of comparative disuse, in many generations. Such are the rudimentary mus- cles of the ears of man, or the undeveloped incisor teeth found in the upper jaw of ruminants. 5. Geographical Distribution.— It can not be said that ani- mals and plants are always found in . the localities where they are best fitted to flourish. This has been well illustrated within the lifetime of the present generation, for the animals intro- duced into Australia have many of them so multiplied as to displace the forms native to that country. But, if we assume that migrations of animals and transmutations of species have taken place, this difficulty is in great part removed. 6. Paleontology. — The rocks bear record to the former exist- ence of a succession of related forms; and, though all the in- termediate links that probably existed have not been found, the apparent discrepancy can be explained by the nature of the circumstances under which fossil forms are preserved ; and the "imperfection of the geological record." It is only in the sedimentary rocks arising from mud that fossils can be preserved, and those animals alone with hard parts are likely to leave a trace behind them ; while if these sedimentary rocks with their inclosed fossils should, owing to enormous pressure or heat be greatly changed (metamorphosed), all trace of fossils must disappear — so that the earliest forms of life, those that would most naturally, if preserved at all, be found in the most ancient rocks, are wanting, in consequence of the metamorphism which such formations have undergone. Moreover, our knowledge of the animal remains in the earth's crust is as yet very incomplete, though, the more it is explored, the more the evidence gathers force in favor of organic evolu- tion. But it must be remembered that those groups constitut- ing species are in geological time intermediate links. 7. Fossil and Existing Species.— If the animals and plants GENERAL BIOLOGY. 47 now peopling the earth were entirely different from those that flourished in the past, the objections to the doctrine of descent would be greatly strengthened ; but when it is found that there is in some cases a scarcely broken succession of forms, great force is added to the arguments by which we are led to infer the connection of all forms with one another. To illustrate by a single instance: the existing group of horses, with a single toe to each foot, was preceded in geological Fig. 48. — Bones of the feet of the different genera of Equidm (after Marsh), a, foot of Orohippus (Eocene); b, foot of Anchitlierium (Lower Miocene); c, foot of Hip- parion (Pliocene); d, foot of the recent genus Equus. time in America by forms with a greater number of toes, the latter increasing according to the antiquity of the group. These forms occur in succeeding geological formations. It is impossible to resist the conclusion that they are related gene- alogically (phylogenetically). 8. Progression. — Inasmuch as any form of specialization that would give an animal or plant an advantage in the struggle for existence would be preserved, and as in most cases when the competing forms are numerous such would be the case, it is possible to understand how the organisms that have appeared have tended, on the whole, toward a most pronounced pro- gression in the scale of existence. This is well illustrated in the history of civilization. Barbarous tribes give way before civilized man with the numberless subdivisions of labor he in- stitutes in the social organism. It enables greater numbers to flourish, as the competition is not so keen as if activities could be exercised in a few directions only. 9. Domesticated Animals. — Darwin studied our domestic ani- mals long and carefully, and drew many important conclusions 4S COMPARATIVE PHYSIOLOGY. from his researches. He was convinced that they had all been derived from a few wild representatives, in accordance with the principles of natural selection. Breeders have both consciously and unconsciously, formed races of animals from stocks which the new groups have now supplanted; while primitive man had tamed various species which he kept for food and to assist in the chase, or as beasts of burden. It is impossible to believe that all the different races of dogs have originated from dis- tinct wild stocks, for many of them have been formed within recent periods ; in fact, it is likely that to the jackal, wolf, and fox, must we look for the wild progenitors of our dogs. Dar- win concluded that, as man had only utilized the materials Nature provided in forming his races of domestic animals, he had availed himself of the variations that arose spontaneously, and increased and fixed them by breeding those possessing the same variation together, so the like had occurred without his aid in nature among wild forms. Evolutionists are divided as to the origin of man himself ; some, like Wallace, who are in accord with Darwin as to the 4- 3 Fia. 40.— Skeleton of hand or fore-foot of six mammals. I. man; II, dog; III, pit;; IV, ox; V, tapir; VI, horse, r, radius; u, ulna; a, scaphoid; b, semi-lunur; c, Iriquetrum (cuneiform); d, trapezium; e, trapezoid; /, capitatum (unciform pro- cess); /•/, hamatum (unciform bone); p, pisiform; ij thumb; 2, digit; 3, middle finger; 4, ring-finger; 5, little finger. (After Gegenbaur.) origin of living forms in general, believe that the theory of natural selection does not suffice to account for the intellectual GENERAL BIOLOGY. 49 50 COMPARATIVE PHYSIOLOGY. and moral nature of man. Wallace believes that man's body has been derived from lower forms, but that his higher nature is the result of some unknown law of accelerated development ; while Darwin, and those of his way of thinking, consider that mau in his entire nature is but a grand development of powers existing in minor degree in the animals below him in the scale. Summary. — Every group of animals and plants tends to in- crease in numbers in a geometrical progression, and must, if unchecked, overrun the earth. Every variety of animals and plants imparts to its offspring a general resemblance to itself, but with minute variations from the original. The variations of offsprings may be in any direction, and by accumulation constitute fixed differences by which a new group is marked off. In the determination of the variations that persist, the law of survival of the fittest operates. REPRODUCTION. As has been already noticed, protoplasm, in whatever form, after passing through certain stages in development, undergoes a decline, and finally dies and joins the world of unorganized matter ; so that the permanence of living things demands the constant formation of new individuals. Groups of animals and plants from time to time become extinct ; but the lifetime of the species is always long compared with that of the indi- vidual. Reproduction by division seems to arise from an exi- gency of a nutritive kind, best exemplified in the simpler or- ganisms. When the total mass becomes too great to be supported by absorption of pabulum from without by the surface of the body, division of the organism must take place, or death ensues. It appears to be a matter of indifference how this is accom- plished, whether by fission, endogenous division, or gemmation, so long as separate portions of protoplasm result, capable of leading an independent existence. The very undifferentiated character of these simple forms prepares us to understand how each fragment may go through the same cycle of changes as the parent form. In such cases, speaking generally, a million individuals tell the same biological story as one ; yet these must exist as individuals, if at all, and not in one great united mass. But in the case of conjugation, which takes place some- times in the same groups as also multiply by division in its various forms, there is plainly an entirely new aspect of the case presented. We have already shown that no two cells, how- ever much alike they may seem as regards form and the cir- cumstances under which they exist, can have, in the nature of the case, precisely the same history, or be the subjects of ex- actly the same experiences. We have also pointed out that all these phenomena of cell-life are known to us only as adapta- tions of internal to external conditions ; for, though we may not be always able to trace this connection, the inference is justi- 52 COMPARATIVE PHYSIOLOGY. Sable, because there are no facts known to xis that contradict such an assumption, while those that are within our knowledge bear out the generalization. We have already learned that liv- ing things are in a state of constant change, as indeed are all things ; we have observed a constant relation between certain changes in the environment, or sum total of the surrounding conditions, as, for example, temperature, and the behavior of the protoplasm of plants and animals ; so that we must believe that any one form of protoplasm, however like another it may seem to our comparatively imperfect observation, is different in some respects from every other — as different, relatively, as two human beings living in the same community during the whole of their lives ; and in many cases as unlike as individuals of very different nationality and history. "We are aware that when two such persons meet, provided the unlikeness is not so great as to prevent social intercourse, intercommunication may prove very instructive. Indeed, the latter grows out of the former; our illustration is itself explained by the law we are endeavoring to make plain. It would appear, then, that con- tinuous division of protoplasm without external aid is not pos- sible ; but that the vigor necessary for this must in some way be imparted by a particle (cell) of similar, yet not wholly like, protoplasm. This seems to furnish an explanation of the neces- sity for the conjugation of living forms, and the differentiation of sex. Very frequently conjugation in the lowest animals and plants is followed by long periods when division is the prevail- ing method of reproduction. It is worthy of note, too, that when living forms conjugate, they both become quiescent for a longer or shorter time. It is as though a period of preparation preceded one of extraordinary activity. We can at present trace only a few of the steps in this rejuvenation of life-stuff. Some of these have been already indicated, which, with others, will now be further studied in this division of our subject, both because reproduction throws so much light on cell-life, and be- cause it is so important for the understanding of the physio- logical behavior of tissues and organs. It may be said to be quite as important that the ancestral history of the cells of an organism be known as the history of the units composing a community. A, B, and C, can be much better understood if we know something alike of the history of their race, their an- cestors, and their own past; so is it with the study of any indi- vidual animal, or group of animals or plants. Accordingly, REPRODUCTION. 53 embryology, or the history of the origin and development of tissues and organs, will occupy a prominent place in the vari- ous chapters of this work. The student will, therefore, at the outset be furnished with a general account of the subject, while many details and applications of principles will be left for the chapters that treat of the functions of the various organs of animals. The more knowledge the student possesses of zoology the better, while this science will appear in a new light under the study of embryology. Animals are divisible, according to general structure, into Protozoa, or unicellular animals, and Metazoa, or multicellular forms — that is, animals composed of cell aggregates, tissues, or organs. Among the latter one form of reproduction appears for the first time in the animal kingdom, and becomes all but universal, though it is not the exclusive method ; for, as seen in Hydra, both this form of generation and the more primitive gemmation occur. It is known as sexual multiplication, which usually, though not invariably, involves conjugation of two un- like cells which may arise in the same or different individuals. That these cells, known as the male and female elements, the ovum and the spermatozoon, are not necessarily radically dif- ferent, is clear from the fact that they may arise in the one in- dividual from the same tissue and be mingled together. These cells, however, like all others, tell a story of continual progress- ive differentiation corresponding to the advancing evolution of higher from lower forms. Thus hermaphroditism, or the coex- istence of organs for the production of male and of female cells in the same individual, is confined to invertebrates, among which it is rather the exception than the rule. Moreover, in such hermaphrodite forms the union of cells with greater differ- ence in experiences is provided for by the union of different in- dividuals, so that commonly the male cell of one individual unites with (fertilizes) the female cell of a different individual. It sometimes happens that among the invertebrates the cells produced in the female organs of generation possess the power of division, and continued development wholly independently of the access of any male cell {parthenogenesis) ; such, how- ever, is almost never the exclusive method of increase for any group of animals, and is to be regarded as a retention of a more ancient method, or perhaps rather a reversion to a past biologi- cal condition. No instance of complete parthenogenesis is known among vertebrates, although in birds partial develop- 54 COMPARATIVE PHYSIOLOGY. merit of the egg may take place independently of the influence of the male sex. The best examples of parthenogenesis are to be found among insects and crustaceans. It is to be remembered that, while the cells which form tbe tissues of the body of an animal have become specialized to discharge one particular function, they have not wholly lost all others ; they do not remain characteristic amceboids, as we may term cells closely resembling Amoeba in behavior, nor do they wholly forsake their ancestral habits. They all retain the power of reproduction by division, especially when young and most vigorous ; for tissues grow chiefly by the production of new cells rather than the enlargement of already mature ones. Cells wear out and must be replaced, which is effected by the processes already described for Amoeba and similar forms. Moreover, there is retained in the blood of animals an army of cells, true amceboids, ever ready to hasten to repair tissues lost by injury. These are true remnants of an embryonic condition ; for at one period all the cells of the organism were of this un- differentiated, plastic character. But the cell {ovum) from which the individual in its entirety and with all its complexity arises mostly by the union with another cell (spermatozoon), must be considered as one that has remained unspecialized and retained, and perhaps increased its reproductive functions. They certainly have become more complex. The germ-cell may be considered unspecialized as regards other functions, but highly specialized in the one direction of exceedingly great ca- pacity for growth and complex division, if we take into account the whole chain of results ; though in considering this it must be borne in mind that after a certain stage of division each individual cell repeats its ancestral history again ; that is to say, it divides and gives rise to cells which progress in turn as well as multiply. From another point of view the ovum is a marvelous storehouse of energy, latent or potential, of course, but under proper conditions liberated in varied and unexpected forms of force. It is a sort of reservoir of biological energy in the most concentrated form, the liberation of which in sim- pler forms gives rise to that complicated chain of events which is termed by the biologist development, but which may be ex- pressed by the physiologist as the transformation of potential into kinetic energy, or the energy of motion. Viewed chemi- cally, it is the oft-repeated story of the production of forms, of greater stability and simplicity, from more unstable and com- REPRODUCTION. 55 plex ones, involving throughout the process of oxidation ; for it niust ever be kept in mind that life and oxidation are con- comitant and inseparable. The further study of reproduction in the concrete will render the meaning and force of many of the above statements clearer. THE OVUM, The typical female cell, or ovum, consists of a mass of pro- toplasm, usually globular in form, containing a nucleus and nucleolus. The ovum may or may not be invested by a membrane ; the protoplasm of the body of the cell is usually highly granular, and may have stored up within it a varying amount of proteid material (food-yelk), which has led to division of ova into classes, according to the manner of distribution of this nutri- tive reserve. It is either concentrated at one pole (telolecith- al) ; toward the center (centrolecithal) ; or evenly distributed throughout (alecithal). During development this "f 7 material is converted by x ^ the agency of the cells of the young organism (em- bryo) into active proto- plasm ; in a word, they feed upon and assimilate or build up this food-stuff into their own substance, as Amoeba does with any proteid material it appro- priates. The nucleus (germinal vesicle) is large and well defined, and contains with- in itself a highly refractive nucleolus (germinal spot). These closely resemble in general the rest of the cell, but stain more deeply and are chemically different in that they contain nucleine (nucleoplasm, chromatin >. It will be observed that the ovum differs in no essential par- ticular of structure from other cells. Its differences are hidden ones of molecular structure and functional behavior. In ac- Fig. 55. — Semi-diagrammatic representation of a mammalian ovum (Sch&fer). Highly mag- nified. z}>, zonapellucida; in, vitellus (yelk); gv, germinal vesicle; gs, germinal spot. 56 COMPARATIVE PH YSIOLOG Y, cordance with the diverse circumstances under which ova ma- ture and develop, certain variations in structure, mostly of the nature of additions, present themselves. Thus, ova may be naked, or provided with one or more cover- ings. In vertebrates there are usually two membranes around the protoplasm of the ovum : a delicate covering (Vitelline membrane) beneath which there is another, which is sieve-like from numerous perforations (zona radiata, or z. pellucida). The egg membrane may be impregnated with lime salts (shell). Between the membranes and the yelk there is a fluid albumi- nous substance secreted by the glands of the oviduct, or by other special glands, which provide proteid nutriment in different physical condition from that of the yelk. The general naked-eye appearances of the ovum may be learned from the examination of a hen's egg, which is one of 11 wy- yy eh. I Fig. 56. — Diagrammatic section of an unimpregnated fowl's egg (Foster and Balfour, after Allen Thomson), bl, blastoderm or cicatricnla; 10. y, white yelk; y. y, yel- low yelk; ch.l. chalaza; i.s.m, inner layer of shell membrane; s. m, outer layer of shell membrane; s. shell; a. c. h, air-space: w, the white of the egg; v. (, vitel- line membrane ; x, the denser albuminous layer lying next the vitelline mem- brane. the most complicated known, inasmuch as it is adapted for development outside of the body of the mother, and must, con- sequently, be capable of preserving its form and essential vital properties in a medium in which it is liable to undergo loss of water, protected as it now is with shell, etc., but which, at the REPRODUCTION. 57 same time permits the entrance of oxygen and moisture, and conducts heat, all being - essential for the development of the germ within this large food-mass. The shell serves, evidently, chiefly for protection, since the eggs of serpents (snakes, turtles, etc.) are provided only with a very tough membranous cover- ing, this answering every purpose in eggs buried in sand or otherwise protected as theirs usually are. As the hen's egg is that most readily studied and most familiar, it may be well to describe it in somewhat further detail, as illustrated in the above figure, from the examination of which it will be ap- parent that the yelk itself is made up of a white and yellow portion distributed in alternating zones, and composed of cells of different microscopical appearances. The clear albumen is structureless. The relative distribution, and the nature of the accessory or non-essential parts of the hen's egg, will be understood when it is remembered that, after leaving its seat of origin, which will be presently described, the ovum passes along a tube (oviduct) by a movement imparted to it by the muscular walls of the latter, similar to that of the gullet during the swallowing of food ; that this tube is provided with glands which secrete in turn the albumen, the membrane (outer), the lime salts of the shell, etc. The twisted appearance of the rope-like structures (chalazce) at each end is owing to the spiral rotatory movement the egg has undergone in its descent. The air-chamber at the larger end is not present from the first, but results from evaporation of the fluids of the albumen and the entrance of atmospheric air after the egg has been laid some time. THE ORIGIN AND DEVELOPMENT OF THE OVUM. Between that protrusion of cells which gives rise to the bud which develops directly into the new individual, and that which forms the ovary within which the ovum as a modified cell arises, there is not in Hydra much difference at first to be observed. In the mammal, however, the ovary is a more complex struct- ure, though, relatively to many organs, still simple. It consists, n the main, of connective tissue supplied with vessels and nerves inclosing modifications of that tissue (Graafian follicles) within which the ovum is matured. The ovum and the follicles arise from an inversion of epithelial cells, on a portion of the body 58 COMPARATIVE PHYSIOLOGY. cavity (germinal ridge), which give rise to the oviim itself, and the other cells surrounding it in the Graafian follicle. At first these inversions form tubules (egg-tubes) which latter become broken up into isolated nests of cells, the forerunners of the Graafian follicles. The Graafian follicle consists externally of a fibrous capsule (tunica fibrosa), in close relation to which is a layer of cap- illary blood-vessels (tu- nica vasculosa), the two together forming the gen- eral covering (tunica propria) for the more delicate and important cells within. Lining the Fig. 57. — Section through portion of the ovary of , . . n „ .. mammal, illustrating mode of development of tunic IS a layer Of small, the Graafian follicles (Wiedersheim). D, dis- "„__^,__-i,„+ „,,-k:„„i „„iio, cue proligerus ; El, ripe ovum; G, follicular SOmewnat CUDlcai cells cells of germinal epithelium; g, blood-vessels; /anovnhvrivtri rtvnv><)i7ni 5 Jf of 5 3 ID 5 12 0) 'S g bb.£P 3 fe it S b ? a ho: 8 CD <£' CO bo 5 "5" it ~ od 1 3 ilftiSS trJThb^&ftbb- ! fe -a +3 -I ^? is Si a "g. bo 'S ti ti "3 £ e~,5 if y~ ,3 3 B.m bo bp.j> -g S =g rt 13 CD ... "B A S ^ a a-* bhcS ^oi *- . of ^ fe o, 50" OT 2 M ^ bb _ ■" .5 J3 > -2 bo E, ; D -S bo: CD J§ = 12 "2 •- o bo' 2 "§ a" 53 2 ~ s - e S S.5 "5 « ij 5 , cr ^ co ^ C o a cd g ° o ^"ct'i a 5 -5 1 a scs^aog^-g ,xji e^ ■-, 1 a J o o CM 03 O . 60"^ 5 .$ = 5 S '^ ¥? "S "S be W "3 ,- bo H 2 =3 „co „ = s cd a t- l c a Si „s 3 s .bp > 2 . « s °"J 3 C3i "o , "ft a s » " So B ft bo iO 3 n) b0W W rg CD ^ 'Ti . » g 2 fe a -a « o « - « bo 5 I § ^ Z2 — ' 5 <3 S ^T-;' 7 '' OJ 3 ,h il biD - ffl S ^ „ 5 VJ O* CO £ p" CO I? I. r3 o 60 t? ib 5o I Si > "0 2 ^ CO ^O^H"-r-HCD la 4 ^ I ?r ^ i K C S s e , i at o cd a to 71 j, .a' P s .s j • .SP 5 2 .£P SP w fe .a" orj ^ ' cd &fe°fe £ £I^fe S .. 1* ^ sj to ~ hV a cd , d d cs ■s 5 5 3 =e '-8 "£. &( REPRODUCTION. 67 eralizecl as consisting of division and segregation, followed by aggregation. The subdivision (segmentation) of the cell, after the quies- cence referred to, again commences, but in a plane at right angles to the first, from which four spheres result, again to be followed by the resting stage. The process continues in the same way, so that there is a progressive increase in the number Fig. 05.— The cleavage of a frog's egg (10 times enlarged). A, the parent-cell; B, the first two cleavage-cells; 0.4 cells; D, 8 cells (4 animal and 4 vegetative); E. 12 cells (8 animal and 4 vegetative); F, 16 cells (8 animal and 8 vegetative); G, 24 cells (1(5 animal and 8 vegetative); H, 32 cells: /. 48 cells; A". 04 cell^: L. 96 eleav- age-cclls; .]/, mo cleavage-cells (128 animal and 32 vegetative). (Haeckel.) of segments, at least up to the point when a large number has been formed. This is rather to be considered as a type of one 68 COMPARATIVE PHYSIOLOGY. form of segmentation than as applicable to all, for even at this early stage differences are to be noted in the mode of seg- mentation wbich characterize effectually certain groups of ani- mals ; but in all there is segmentation, and that segmentation is rhythmical. Segmentation results in the formation of a multicellular aggregation which, sooner or later, incloses a central cavity (segmentation cavity, blastocele). Usually this cell aggrega- tion (blast ula, blastophere) is reduced to a single layer of invest- ing cells. The Gastrula.— Ensuing on the changes just described are C Fig. 66.— .Blastula and gastrula of amphioxus (Clans, after Hatschek). A, blastula with flattened lower pole of larger cells; B, commencing invagination; C, gastru- lation completed; the blastopore is still widely open, and one of the two hinder- pole mesoderm cells is seen at its ventral lip. The cilia of the epiblast cells are not represented. others, which result in the formation of the gastrula, a form of cell aggregation of great interest from its resemblance to the Hydra and similar forms, which constitute in themselves inde- pendent animals that never pass beyond that stage. The blas- tula becomes flattened at one pole, then depressed, the cells at this region becoming more columnar (histological differentia- tion). This depression (invagination) deepens until a cavity is formed (as when a hollow rubber ball is thrust in at one part till it meets the opposite wall), in consequence of which a two- layered embryo results, in which we recognize the primitive mouth (blastopore) and digestive cavity (archenterori), the outer layer (ectoderm) being usually separated from the inner (endo- derm) by the almost obliterated segmentation cavity. Such a form may be provided with cilia, be very actively locomotive, and bear, consequently, the greatest resemblance to the perma- nent forms of some aquatic animals. The changes by which the segmented oosperm becomes a gastrula are not always so direct and simple as in the above- REPRODUCTION. 69 described case, but the' behavior of the cells of the blastosphere may be hampered by a burden of relatively foreign matter, in the form of food-yelk, in certain in- stances ; so much so is this the case that dis- tinct modes of gastrula formation may be rec- ognized as dependent on the quantity and ar- rangement of food-yelk. These we shall pass by as being somewhat too complicated for our pur- pose, and we return to the egg of the bird. The Hen's Egg.— By far the larger part of the hen's egg is made up of yelk ; but just beneath the vitelline membrane a small, cir- cular, whitish body, about four millimetres in diameter, which al- ways floats uppermost in every portion of the egg, may be seen. This disk (blastoderm, cica- tricula) in the fertilized egg presents an outer white rim (area ojxica), within which is a trans- parent zone (area jielhi- cida), and most centrally a somewhat elongated structure, which marks off the future being itself (embryo). All Fig. 67.— Female generative organs of the fowl (after Dalton). A, ovary; B, Graafian follicle, from which the egg has just been discharged; C, yelk, entering upon extremity of oviduct: D, E, second portion of oviduct, "in which the chalaziferous membrane, chalazse. and albumen are formed; .F, third portion, in which the fibrous shell membranes are produced; Q, fourth por- tion laid open, showing the egg completely formed with its calcareous shell; //, canal through which the egg is expelled. 70 COMPARATIVE PHYSIOLOGY, these parts together constitute that portion (blastoderm) of the fowl's egg which is alone directly concerned in reproduction, all the rest serving for nutrition and protection. The appear- ance of relative opacity in some of the parts marked off as ahove is to be explained by thickening in the cell-layers of which they are composed. The Origin of the Fowl's Egg.— The ovary of a young but mature hen consists of a mass of connective tissue (stroma), Fig. 08. — Various stages in the segmentation of a fowl's Kolliker). abundantly supplied with blood-vessels, from which hang the capsules which contain the ova in all stages of development, so that the whole suggests, but for the color, a bunch of grapes in REPRODUCTION. 71 an early stage. The ovum at first, in this case as in all others, a single cell, becomes complex by addition of other cells (dis- cus proligerus, etc.), which go to make up the yelk. All the other parts of the hen's egg are additions made to it, as ex- plained before, in its passage down the oviduct. The original ovum remains as the blastoderm, the segmentation of which may now be described briefly, its character being obvious from an examination of Fig. 68, which represents a surface view of the segmenting fertilized ovum (oosperm). A segmentation cavity appears early, and is bounded above by a single layer of epiblast cells and below by a single layer of primitive hypoblast cells, which latter is soon composed of sev- eral layers, while the segmentation cavity disappears. The blastoderm of an unincubated but fertilized egg consists of a layer of epiblastic cells, and beneath this a mass of rounded cells, arranged irregularly arid lying loosely in the yelk, consti- tuting the primitive hypoblast. After incubation for a couple of hours, these cells become differentiated into a lower layer of flattened cells (hypoblast), with mesoblastic cells scattered be- Fig. 69.— Portion of section through an unincubated fowl's oOsperm (after Klein). «, epiblast composed of a single layer of columnar cells; b, irregularly disposed lower layer cells of the primitive hypoblast; c, larger formative cells resting on white yelk; f, archenteron. The segmentation cavity lies between a and b, and is nearly obliterated. tween the epiblast and hypoblast, It is noteworthy that, in the bird, segmentation will proceed up to a certain stage independ- ently of the advent of the male cell, apparently indicating a tendency to parthenogenesis. The fowl's ovum then belongs to the class, a portion of which alone segments and develops into the embryo (meroblastic), in contradistinction to what happens in the mammalian ovum, the whole of which undergoes division (holoblastic) ; a distinction which is, however, superficial rather than fundamental, for in reality in the fowl's egg the whole of the original pvum does segment. This holoblastic character of the mammalian ovum 72 COMPARATIVE PHYSIOLOGY. and its resemblance to the segmentation of those invertebrate forms previously described may become apparent from an ex- amination of the accompanying figures. Fig. 70.— Sections of ovum of a rabbit, illustrating formation of the blastodermic vesi- cle (after E. Van Beneden). A, B, C, D, are ova in successive stages of develop- ment, zp, zonapellucida; eel, ectomeres, or outer cells; ent, entomeres, or inner cells. We shall return to the development of the mammalian ovum later ; in the mean time we present the main features of develop- ment in the bird. Remembering that the development of the embryo proper takes place within the pellucid area only, we point out that the area opaca gradually extends over the entire ovum, inclosing the yelk, so that the original disk which lay like a watch-glass on the rest of the ovum, has grown into a sphere. That portion of this area nearest the pellucid zone {area vasculosa) develops blood-vessels that derive the food-supplies, which replenish the blood as it is exhausted, from the hypoblast of the area opaca. REPRODUCTION. 73 The first indications of future structural outlines in the em- bryo is the formation of the primitive streak, an opaque band Pig. 71.— Diagrammatic transverse sections through a hypothetical mammal oOsperm (Haddon). A. The yelk of the primitive mammalian oOspenn is now lost. B. Later stage; the non-embryonic epiblast has grown over the embryonic area to form the covering cells; ep. epiblast of embryo; ep'. epiblast of yelk-sac; 7uj, primitive hypoblast; y. s, yelk-sac, or blastodermic vesicle. in the long diameter of the pellucid area, opaque in consequence of cell accumulation in that region. Very soon a groove {primi- tive groove) extends through- out this band, which gradu- ally occupies a more central position. The relative thick- ness of the several parts and the arrangement of cells may be gathered from Fig 72. These structures are only temporary, and those that re- place them will be described subsequently. We have thus far spoken of cells as being arranged in- to epiblast, hypoblast, and mesoblast. The origin of the first two has been sufficiently indicated. The mesoblast forms the intermediate ger- minal layer, and is derived from the primitive hypoblast, which differentiates into a stratum of flattened cells, situated below the others, and constituting the later i Fig. T2.-Surface view of pellucid area of blastoderm of eighteen hours (Foster and Balfour). //. medullary folds ; me, me- dullary groove; pr, primitive groove. u COMPARATIVE PHYSIOLOGY. hypoblast, and intermediate less closely arranged cells, termed, from their position, mesoblast. It will be noticed that all future growth of the embryo be- gins axially, at least in the early stages of its development. As the subsequent growth and advance of the embryo de- pend on an abundant and suitable nutritive supply, we must now turn to those arrangements which are temporary and of subordinate importance, but still for the time essential to devel- opment. THE EMBRYONIC MEMBRANES OF BIRDS. It will be borne in mind throughout that the chief food-sup- ply for the embryo bird is derived from the yelk ; and, as would —pp ■vf. Vt w Pig. 73. Figs. 73-?5.— A series of diagrams intended to facilitate the comprehension of the relations of the membranes to other parts (after Foster and Balfour). A, B, C, D, E, F are vertical sections in the long axis of the embryo at different periods show- ing the stages of development of the amnion and of the yelk-sac. I, II, III, IV are transverse sections at about the same stages of development, i, ii, iii, pos- terior part of longitudinal section, to illustrate three stages in formation of the allantois. e, embryo; y, yelk; pp, pleuroperitoneal cavity; vt, vitelline mem- brane of amniotic fold;'«/, allantois; a, amnion; a', alimentary canal. be expected, the older the embryo the smaller the yelk, or, as it is now called when limited by the embryonic membranes, the REPRODUCTION, 75 \\ m n 76 COMPARATIVE PHYSIOLOGY. yelk-sac {umbilical vesicle of the mammalian embryo). The manner in which this takes place will appear npon an inspec- tion of the accompanying figures. Very early in the history of the embryo two eminences, the head and the tail folds, arise, and, curving over toward each MO. Fig. 70. — Diagrammatic longitudinal section through the axis of an embryo chick (after Foster and Balfour). N. C, Neural canal; Ch, notochord; Fg, foregnt; F. So, somatopleure; F. Sp, splanchnopleure; Sp, splanchnopleure, forming lower wall of foregut; lit, heart; pp, pleuroperitoneal cavity; Am, amniotic fold; E, epiblast; M, mesoblast; H, hypoblast. other, meet after being joined by corresponding lateral folds. Fusion and absorption result at this meeting-point, in the inclosure of one cavity and the blending of two others. These folds constitute the amniotic membranes, the inner of which Fig. 77.— Diagrammatic longitudinal section of a chick of the fourth day (after Allen Thomson), ep, epiblast; hy, hypoblast; sm, somatopleure; vm, splanchnopleure; of, pf, folds of the amnion; pp, pleuroperitoneal cavity; am, cavity of the am- nion'; at. allantois; a, position of the future anus; h, heart; i, intestine; vi, vitel- line duct; ye, yelk; x, foregut; m, position of the mouth; me, mesentery. forms the true amnion, the outer the false amnion (serous mem- brane, subzonal membrane). Within the amnion proper is the amniotic cavity filled with fluid (liquor amnii), while the space between the true and false amniotic folds, which gradually in- REPRODUCTION. 77 creases in size as the yelk-sac diminishes, forms the pleuro- peritoneal cavity, body cavity, or coclom. The amniotic cavity also extends, so that the embryo is surrounded by it or lies centrally within it. The enlargement of the ccelom and exten- sion of the false amniotic folds lead finally to a similar meeting and fusion like that which occurred in the formation of the true amniotic cavity. The yelk-sac, gradually lessening, is at last withdrawn into the body of the embryo. Fig. 76 shows how the amniotic head fold arises, from a budding out of the epiblast and mesb blast at a point where the original cell layers of the embryo have separated into two folds, the somatopleure or body fold and the splanchnoplenre or vis- ceral fold, owing to a division or cleavage of the mesoblast toward the long axis of the body. Remembering this, it is always easy to determine by a diagram the composition of any one of the membranes or folds of the embryo, for the components must be epiblast, mesoblast, or hypoblast ; thus, the splanclmopleure is made up of hypoblast internally and mesoblast externally — a principle of great sig- nificance, since, as will be learned later, all the tis- sues of the body may be classified simply, and at the same time scientifi- cally, according to their embryological origin. The allantois is a structure of much physi- ological importance. It arises at the same time as the amniotic folds are forming, by a budding or protrusion of the hind-gut into the pleuro-peritoueal cavity, and hence consists of an outgrowth of mesoblast lined by hypoblast. Fig. 78. — Diagrammatic longitudinal section through the egg of a fowl (after Duval). al, cavity of allantois; alb. albumen; a/'t. mes- enteron;" am, cavity of amnion; emb, embryo; sh. egg-shell; v. m, vitelline membrane. COMPARATIVE PHYSIOLOGY. THE FCETAL (EMBRYONIC) MEMBRANES OF MAMMALS. The differences between the development of the egg mem- branes of mammals and birds are chiefly such as result from the absence in the former of an egg-shell and its membranes, and of yelk and albumen. The mammalian ovum is inclosed by a zona radiata {zona pelluci- da) surrounded by an- other very delicate cov- ering (vitelline mem- brane). The growth of the blastodermic vesicle (yelk-sac) is rapid, and, being filled with fluid, the zona is thinned and soon disappears. The germinal area alone is made up of three layers of cells (Fig. 100), the rest of the upper part of the oosperm being lined with epiblast and hyp- oblast, while the low- er zone of the yelk-sac consists of epiblast only. Simple, non-vascular villi, serving to attach the embryo to the uterine walls, usually project from the epiblast of the subzonal membrane. In the rabbit they do not occur every- where, but only in that region of the epiblast beneath which the mesoblast does not extend, with the exception of a patch which soon appears and demarkates the site of the future pla- centa. The amnion and allantois are formed in much the same way as has been described for the chick. At about the same period as these events are transpiring the vascular yelk-sac has become smaller, and the allantois with its abundant supply of blood-vessels is becoming more promi- Fig. 79. — Diagrammatic longitudinal section oosperm of rabbit at an advanced stage of preg- nancy (KOlliker, after Bischoff). a, amnion; al, allantois with its blood-vessels; e, embryo ; ds, yelk-sac ; ed, ed' , ed", hypoblastic epithelium of the yelk-sac and its stalk (umbilical vesicle and cord); fd, vascular mesoblastic membrane of the umbilical cord and vesicle ; pi, placental villi formed by the allantois and subzonal membrane; r, space filled with fluid between the amnion, the allantois. and the yelk-sac ; ft, sinus termi- nalis (marginal vitelline blood-vessel); u, urach- us, or stalk of the allantois. REPRODUCTION. 79 ^4-^ r o'/n* nent, and extending between the amnion and subzonal mem- brane. The formation of the chorion marks an important step in the development of mammals in which it plays an important functional part. It is the result of the fusion of the allantois, which is highly vascular, with the subzonal membrane, the villi of which now become themselves vascular and more complex in other respects. An interesting re- semblance to birds has been observed (by Os- born) in the opossum (Fig. 83). When the allantois is small the blastodermic vesicle (yelk-sac) has vascular villi, which in all prob- ability not only serve the purpose of attach- ing the embryo to the uterine wall but derive nourishment, not as in birds, from the albumen of the ovum, but directly in some way from the uterine wall of the mother. It will be remembered that the opossum ranks low in the mammalian scale, so that this resemblance is the more significant from an evolutionary point of view. The term chorion is now restricted to those regions of the subzonal membrane to which either the yelk-sac or the allan- tois is attached. The former zone has been distinguished as the false chorion and the latter as the true chorion. In the rabbit the false chorion is very large (Fig. 79), and the true (placen- tal) chorion very small in comparison, but the reverse is the case in most mammals. It will be noted that in both birds and mammals the allantois is a nutritive organ. Usually the more prominent and persistent the yelk-sac, the less so Fig. 80.— Diagrammatic dorsal view of an embryo rab- bit with its membranes at the stage of nine so- mites (Hadclon, after Van Beneden and Julin). aly allantois, showing from behind the tail fold of the embryo; am, anterior border of true am- nion ; a. v, area vasculosa, the outer border of which indicates the farthest extension of the mesoblast; hi, blastoderm, here consisting only of epiblast and hypoblast; o. m. v, omphalomesen- teric or vitelline veins ; p. am, proamnion ; pi, non-vascular epiblastic villi of the future placen- ta ; s. t, sinus terminalis. 80 COMPARATIVE PHYSIOLOGY. the allantois, and vice versa ; they are plainly supplementary organs. The Allantoic Cavity.— The degree to which the various em- bryonic membranes fuse together is very variable for different groups of mammals, including our domestic species. In ruminants, but especially in solipeds, the allantois as it grows spreads itself over the inner surface of the subzonal 'Mm- Fig. 81.— Embryo of dog. twenty-five days old. opened on the ventral side. Chest and ventral walls have been removed, a, nose-pits; b, eyes; c, nnder-jaw (first gill-arch); d, second gill-arch; e,f, 0, h, heart (e, right,/, left auricle; g, right, h, left ventricle); i, aorta (origin of); kk, liver (in the middle between the two lobes is the cut yelk-vein); /, stomach; m, intestine; n, yelk-sac; o, primitive kidneys; p, allantois; q, fore-limbs; h, hind-limbs. The crooked embryo has been stretched straight. (Ilaeckel, after Bischoff.) membrane, often spoken of as the " chorion," while it also covers, though capable of easy detachment, the outer surface of the amnion ; and thus is formed the allantoic cavity. The por- tion of the allantois remaining finally within the foetus becomes the bladder, which during embryonic life communicates by its contracted portion (urachus) with the general amniotic cavity. REPRODUCTION. 81 Fig. 82. — Diagram of an embryo showing the relations of the vascular allantois to the villi of the chorion (Cadiat). e, embryo lying in the cavity of the amnion; y$, yelk-sac; al, allantois; A.Y, allantoic vessels dipping into the villi of the chorion; ch. chorion. -\-aro. In the mare especially these parts can be readily distin- guished. From the connection of the portion that ultimately forms the bladder with the main sac, as indicated above, there is ground for regarding the allantoic fluid in the later stages of gestation, at all events, as a sort of urine. This fluid is at an ear- ly period abundant and colorless, later yellowish, and finally brown. Since at one time it contains albumen and sugar, it may serve some purpose in the nutrition of the foetus. When most suggestive of urine in the latest stages of gestation, it contains 6 Fig. 83.— Diagram of the foetal membranes of the Virginian opossum (Haddon, after Os- born). Two villi are shown greatly enlarged. The processes of the cells, which have been exaggerated, doubtless correspond to the pseudopodia described by Caldwell, al, allantois; am, amnion: s.t, sinus termi- nalis; <«. z, subzonal membrane; r. villi on the subzonal membrane in the region of the yelk-sac ; ijs. yelk-sac. The vascular splanchuopleure (hypoblast and mesoblast) is indicated by the "black line. 82 COMPARATIVE PHYSIOLOGY. a characteristic body, allantoin, related to uric acid, urea, etc. Certain bodies, being probably inspissated allantoic fluid, have been termed " hippomanes. " They may either float free in the fluid or be attached to the allantois by a slender pedicle. The relation of the parts described above will become clearer after a study of the accompanying cuts and those of preceding pages, in which the allantois is figured. Fig. 84.— Exterior of chorial sac; mare. (Chauveau.) A, body; B. C. cornua. The Placenta. — This structure, which varies greatly in com- plexity, may be regarded as the result of the union of structures existing for a longer or shorter period, free and largely inde- pendent of each other. With evolution there is differentiation and complication, so that the placenta usually marks the site where structures have met and fused, differentiating a new or- gan; while corresponding atrophy, obliteration, and fusion take place in other regions. All placentas are highly vascular, all are villous, all dis- charge similar functions in providing the embryo with nourish- ment and eliminating the waste of its cell-life (metabolism). In structural details they are so different that classifications of mammals have been founded upon their resemblances and dif- ferences. They will now be briefly described. In marsupials the yelk-sac is both large and vascular; the allantois small but vascular; the former is said (Owen) to be attached to the subzonal membrane, the latter not; but no villi, and consequently no true chorion, is developed. All mammals REPRODUCTION. 83 Pig. 85. — Foetus of mare with its envelopes. (Chauveau.) A. chorion: C, amnion re- moved from allantoicl cavity and opened to expose foetus; D, infundibulum of urachus; B, allantoid portion of umbilical cord. other than the monotremes and marsupials have a true allan- toic placenta. The Discoidal Placenta. — This form of placenta is that exist- ing in the rodentia, insectivora, and cheiroptera. The condition found in the rabhit is that which has been most studied. The relation of parts is shown in Fig;. 79. The uterus of the rodent is two-homed ; so we find in gen- eral several embryos in each horn in the pregnant rabbit. They are functionally independent, each having its own set of 84 COMPARATIVE PHYSIOLOGY. membranes. It will be observed from the figure tbat tbe true villous cborion is confined to a comparatively small region; tbere is, however, in addition a false cborion witbout villi, but bigbly vascular. This blending of forms of placentation wbicb exist separately in different groups of animals is significant. In tbe rabbit at a later stage tbere is considerable interming- ling of foetal and maternal parts,, i %~ Series of diagrams representing the relations of the decidna to the ovum, at different periods, in the human subject. The decidua are dark, the ovum shaded transversely. In 4 and 5 the chorionic vascular processes are figured (after Dal- ton), 1. Ovum resting on the decidua serotina; 2. Decidua reflexa growing round th'- ovum; 3. Completion of the decidua around the ovum; 4. Villi, growing out all around the chorion; 5. The villi, specially developed at the site of the future placenta, having atrophied elsewhere. The Metadiscoidal Placenta.— This type, which, in general naked-eye appearances, greatly resembles the former, is found in man and the apes. The condition of things in man is by no means as well understood as in the lower mammals, especially in the early stages ; so that, while the following account is that REPRODUCTION. 85 usually given in works on embryology, the student may as well understand that our knowledge of human embryology in the very earliest stages is incomplete and partly conjectural. The reason of this is obvious : specimens for examination depending on accidents giving rise to abortion or sudden death, often not reaching the laboratory in a condition permitting of trust- worthy inferences. It is definitely known that the. ovum, which is usually fer- tilized in the oviduct (Fallopian tube), on entering the uterus becomes adherent to its wall and encapsuled. The mucous membrane of the uterus is known to undergo changes, its com- ponent parts increasing by cell multiplication, becoming in- tensely vascular and functionally more active. The general mucous surface shares in this, and is termed the decidua vera ; but the locality where the ovum lodges is the seat of the great- est manifestation of exalted activity, and is termed the decidua serotina ; while the part believed to have invested the ovum by Fig. 87.— Vascular system of the hnman frettis, represented diagrammatical]} - (TTnx- ley). 77. heart: TA, aortic trunk: c, common carotid artery: r'. external carotid artery: c". internal carotid artery: a, subclavian artery: v, vertebral artery: 1. '.?. 3. 4. 5. aortic arches: A', dorsal aorta: o. omphalo-mesenterie artery; '. omphalo-mesenterie vein; v 1 , umbilical vesicle; vp, portal vein: L. liver; a. v. umbilical arteries: u", v". their endincrs in the placenta: '/'. umbilical vein; Dr. ductus venosus; rh. hepatic vein: rr. inferior vena cava: vit, iliac veins; az, vena azygos; vc', posterior cardinal vein; DC, duct of Cuvier; P. lung. 86 COMPARATIVE PHYSIOLOGY. fused growths from, the junction of the decidua vera and sero- tina is the decidua reflexa. The decidua serotina and reflexa thus become the outermost of all the coverings of the ovum. These and some other devel- opments are figured below. It is to be remembered, however, that they are highly diagrammatic, and represent a mixture of inferences based, some of them, on actual observation and others on analogy, etc. The figures will convey some information, though appear- ances in all such cases must be interpreted cautiously for the reasons already mentioned. During the first fourteen days villi appear over the whole surface of the ovum ; about this fact there is no doubt. At the end of the first month of fcetal life, a complete chorion has been formed, owing, it would seem, to the growth of the allantois (its mesoblast only) beneath the whole surface of the subzonal membrane. From the chorionic surface vascular pro- cesses clothed with epithelium project like the plush of velvet. The allantois is compressed and devoid of a cavity, but abun- dantly supplied with blood-vessels by the allantoic arteries and veins, which of course terminate in capillaries in the villi. Compare the whole series of figures. Fig. 88.— Human ova during early stages of development. A and B, front and side view of an ovum supposed to be about thirteen days old; e. embryonic area (Quain, after Reichert); C, ovrnn of four to five weeks, showing the general structure of the ovum before formation of the placenta. Partof the wall or me ovum is removed to show the embryo in position (after Allen Thomson). At this stage the condition of the chorion suggests the type of the diffuse placenta which is normal for certain groups of animals, as will presently be learned. The subsequent changes are much better understood, for parts are in general no longer microscopic but of considerable size, and their real structure less readily obscured or obliterated. The amniotic cavity continues to enlarge by growth of the walls of the amnion and is kept filled with a fluid; the yelk-sac REPRODUCTION. 87 is now very small ; the decidua renexa becomes almost non- vascular, and fuses finally with the decidua vera and the cho- rion, which except at one part has ceased to be villous and vas- cular ; so that becoming thinner and thinner with the advance of pregnancy, the single membrane, arising practically from I d Fig. 89.— Human embryo, twelve weeks old, with its coverings; natural size. The navel-cord passes from the navel to the placenta, b, amnion; c, chorion; d, pla- centa; d', remains of tufts on the smooth chorion; /. decidua reflexa (inner); g, decidua vera (outer). (Haeckel after Bernhard Schu'ltze.) this fusion of several, is of a low type of structure, the result of gradual degeneration, as the role they once played was taken up by the other parts. But of paramount importance is the formation of the pla- centa. The chorion ceases to be vascular except at the spot at which the villi not only remain, but become more vascular and branch into arborescent forms of considerable complexity. It is discoidal in form, made up of a fcetal part just described and a maternal part, the decidua serotina, the two becoming blended COMPARATIVE PHYSIOLOGY. so that the removal of one involves that of more or less of the others. The connection of parts is far closer than that described Fig. 90.— Diagram illustrating the decidua, placenta, etc. (after Liegeois). e, embryo; i, intestine: p, pedicle of the umbilical vesicle; u. v, umbilical vesicle; a, amnion; eh, chorion; v. t, vascular tufts of the chorion, constituting the fcetal portion of the placenta; m.p, maternal portion of the placenta; d. v, decidua vera; d. r, de- cidua reflexa; al, allantois. for the rabbit ; and, even with the preparation that Nature makes for the final separation of the placenta from both foetus and mother, this event does not take place without some rupture of vessels and consequent haemorrhage. It is difficult to conceive of the great vascularity of the human placenta without an actual examination of this structure itself, which can be done after being cast off to great advan- tage when floating in water ; by which simple method also the thinness and other characteristics of the membranes can be well made out. The great vessels conveying the fcetal blood to and from the REPRODUCTION. 89 placenta are reduced to three, two arteries and one vein. The villi of the placenta (chorion) are usually said to hang freely in the blood of the large irregular sinuses of the decidua sero- tina; but this is so unlike what prevails in other groups of animals that we can not refrain from believing that the state- ment is not wholly true. The Zonary Placenta. — In this type the placenta is formed along a broad equatorial belt, leaving the poles free. This form of placentation is exemplified in the carnivora, hyrax, the ele- phant, etc. In the dog, for example, the yelk-sac is large, vascular, does not fuse with the chorion, and persists throughout. A rudi- mentary discoid placenta is first formed, as in the rabbit ; this gradually spreads over the whole central area, till only the ex- tremes (poles) of the ovum remain free ; villi appear, fitting into pits in the uterine surface, the maternal and foetal parts of the placenta becoming highly vascular and closely approximated. The chorionic zone remains wider than the placental. As in man there is at birth a separation of the maternal as well as foetal part of the placenta — i. e., the latter is deciduate; there is also the beginning of a decidua reflexa. The Diffuse Placenta. — As found in the horse, pig, lemur, etc., the allantois completely incloses the embryo, and it be- comes villous in all parts, except a small area at each pole. The Polycotyledonary Placenta. — This form is that met with in ruminants, in w T hich case the allantois completely covers the surface of the subzonal membrane, the placental villi being gathered into patches (chorial cotyledons), which are equivalent to so many independent placentas. The component villi fit into corresponding pits in the uterine wall (uterine cotyledons), which is specially thickened at these points. When examined in a fresh condition, under water, they constitute very beautiful objects. The pits referred to above into which the foetal villi fit are, as shown in the figures on page 91, essentially the same in structure as the villi themselves. In the cow the uterine cotyle- dons are convex ; but in the sheep and goat they are raised con- cave cups in which the openings for the foetal villi may be seen with the naked eye. The differences are not essential ones. Between the uterine cotyledons and the foetal villi which fit into them a thickish, milky-looking fluid is found, the " uterine milk " elaborated, no doubt, by the cells which line the cotyledonous pits. 90 COMPARATIVE PHYSIOLOGY. The placentation of certain of our domestic animals may be thus expressed in tabular form (Fleming-) : Simple placenta, j General - 1 s ™- ' ( Local and circular, i Bitch. ( Cow. Multiple placenta. -j Sheep. Comparing- the formation, complete development, and atro- phy (in some cases) of the various foetal appendages in mam- mals, one can not but perceive a common plan of structure, with variations in the preponderance of one part over another here arid there throughout. In birds these structures are sim- pler, chiefly because less blended and because of the presence of much food-yelk, albumen, egg-shell, etc., on the one hand, and the absence of a uterine wall, with which in the mammal the membranes are brought into close relationship, on the other ; but, as will be shown later, whatever the variations, they are adaptations to meet common needs and subserve common ends. MICROSCOPIC STRUCTURE OF THE PLACENTA. This varies somewhat for different forms, though, in that there is a supporting matrix, minute (capillary) blood-vessels, and epithelial coverings in the foetal and maternal surfaces, the several forms agree. The pig possesses the simplest form of placenta yet known. The villi fit into depressions or crypts in the maternal uterine mucous membrane. The villi, consisting of a core of connective tissue, in which capillaries abound, are covered with a flat epi- thelium; the maternal crypts correspond, being composed of a similar matrix, lined with epithelium and permeated by capillary vessels, which constitute a plexus or mesh-work. It thus results that two layers of epithelium intervene between the maternal and foetal capillaries. The arrangement is substantially the same in the diffuse and the cotyledonary placenta. In the deciduate placenta, naturally, there is greater compli- cation. In certain forms, as in the fox and cat, the maternal tissue shows a system of trabecular assuming a meshed form, in which run dilated capillaries. These, which are covered with REPRODUCTION. 91 a somewhat columnar epithelium, are everywhere in contact with the foetal villi, which are themselves covered with a flat epithelium. Fig. 94. Figs. 91 to 97. — Diagrammatic representation of the minute structure of the placenta (Foster and Balfour, after Turner). F, foetal; M, maternal placenta; e, epithelium of chorion; e\ epithelium of maternal placenta; d, foetal blood-vessels; d', mater- nal blood-vessels; v, villus. Fig. 91. — Placenta in most generalized form. Fig. 92.— Structure of placenta of a pig. Fiu. 93.— Of a cow. Fig. 94.— Of a fox. Fig. 95.— Of a cat. In the case of the sloth, with a more discoidal placenta, the dilatation of capillaries and the modification of epithelium are greater. 92 COMPARATIVE PHYSIOLOGY. In the placenta of the apes and of the human sub- ject the most marked depart- ure from simplicity is found. The maternal vessels are said to constitute large intercom- municating sinuses ; the villi may hang freely suspended in these sinuses, or be anchored to their walls by strands of tissue. There is believed to be only one layer of epithe- lial cells between the vessels of mother and foetus in the later stages of pregnancy. This, while closely investing Fig. 96.— Placenta of a sloth. Flat maternal the f oetal vessels (capillaries), epithelial cells shown in position on ... , , , ,, , right side; on left they are removed and really belongs to the mater- c£ a pu^ie^ a exposed V . eSSel wUh itB bl °° d " nal structures. The signifi- e -F. Fig 97.— Structure of human placenta; ds, decidua serotina; L trabecnlae of serotina passing to f«'lal villi; ca, curling artery; up, utero-placental vein; a;, prolongation of maternal tissue on exterior of villus, outside cellular layer e', which may repre- sent either endothelium of maternal blood-vessels or delicate connective tissue of the serotina or both; e', maternal cells of the serotina. REPRODUCTION. 93 ca'nce of this general arrangement will be explained in the chapter on the physiological aspects of the subject. It remains to inquire into the relation of these forms to one another from a phylogenetic (derivative) point of view, or to trace the evolution of the placenta. Evolution. — Passing by the lowest mammals, in which the placental relations are as yet imperfectly understood, it seems clear that the simplest condition is found in the rodentia. Thus, in the rabbit, as has been described, both yelk-sac and allantois take a nutritive part ; but the latter remains small. In forms above the rodents, the allantois assumes more and more importance, becomes larger, and sooner or later predomi- nates over the yelk-sac. The discoidal, zonary, cotyledonary, etc., are plainly evolu- tions from the diffuse, for both differentiation of structure and integration of parts are evident. Tbe human placenta seems to have arisen from the diffuse form ; and it will be remembered that it is at one period represented by the chorion with its villi distributed universally. The resemblance of the embryonic membranes at any early stage in man and other mammals to those of birds certainly suggests an evolution of some kind, though exactly along what lines that has taken place it is difficult to determine with exact- ness ; however, as before remarked, nearly all the complica- tions of the higher forms arise by concentration and fusion, on the one hand, and atrophy and disappearance of parts once functionally active, on the other. Summary. — The ovum is a typical cell ; unspecialized in most directions, but so specialized as to evolve from itself compli- cated structures of higher character. The segmentation of the ovum is usually preceded by fertilization, or the union of the nuclei of male and female cells, which is again preceded by the extrusion of polar globules. In the early changes of the ovum, including segmentation, periods of rest and activity alternate. The method of segmentation has relation to the quantity and arrangement of the food-yelk. Ova are divisible generally into completely segmenting (holoblastic), and those that under- go segmentation of only a part of their substance (meroblastic) ; but the processes are fundamentally the same. Provision is made for the nutrition, etc., of the ovum, when fertilized (oosperm) by the formation of yelk-sac and allan- tois; as development proceeds, one becomes more prominent 94 COMPARATIVE PHYSIOLOGY. than the other. The allantois may fuse with adjacent mem- branes and form at one part a condensed and hypertrophied chorion (placenta), with corresponding 1 atrophy elsewhere. The arrangement of the placenta varies in different groups of animals so constantly as to furnish a basis for classification. Whatever the variations in the structure of the placenta, it is always highly vascular ; its parts consist of villi fitting into crypts in the maternal uterine membrane — both the villi and the crypts being provided with capillaries supported by a con- nective-tissue matrix covered externally by epithelium. The placenta in its different forms would appear to have been evolved from the diffuse type. The peculiarities of the embryonic membranes in birds are owing to the presence of a large food-yelk, egg-shell, and egg- mernbranes ; but throughout, vertebrates follow in a common line of development, the differences which separate them into smaller and smaller groups appearing later and later. The same may be said of the animal kingdom as a whole. This seems to point clearly to a common origin with gradual diver- gence of type. THE DEVELOPMENT OF THE EMBRYO ITSELF. We now turn to the development of the body of the animal for which the structures we have been describing exist. It is important, however, to remember that the development of parts, though treated separately for the sake of convenience, really goes on together to a certain extent; that new structures do not appear suddenly but gradually ; and that the same law applies to the disappearance of organs which are being super- seded by others. To represent this completely would require lengthy descriptions and an unlimited number of cuts; but with the above caution it is hoped the student may be able to avoid erroneous conceptions, and form in his own mind that series of pictures which can not be well furnished in at least the space we have to devote to the subject. But, better than any abstract statements or pictorial representations, would be the examination of a setting of eggs day by day during their development under a hen. This is a very simple matter, and, while the making and mounting of sections from hardened specimens is valuable, it may require more time than the student can spare ; but it is neither so valuable nor so easily ac- complished as what we have indicated; for, while the lack of sections made by the student may be made up in part by the exhibition to him of a set of specimens permanently mounted or even by plates, nothing can, in our opinion, take the place of the examination of eggs as we have suggested. It prepares for the study of the development of the mammal, and exhibits the membranes in a simplicity, freshness, and beauty which impart a knowledge that only such direct contact with nature can supply. To proceed with great simplicity and very little apparatus, one requires but a forceps, a glass dish or two, a couple of watch-glasses, or a broad section-lifter (even a case- knife will answer), some water, containing just enough salt to be tasted, rendered lukewarm (blood-heat). 96 COMPARATIVE PHYSIOLOGY. Holding the egg longitudinally, crack it across the center transversely, gently ahd carefully pick away the shell and its Fig. 98.— Various stages in the development of the frog from the egg (after Howes). 1. The segmenting ovum, showing first cleavage furrow. 2. Section of the above at right angles to the furrow. 3. Same, on appearance of second furrow, viewed slightly from above. 4. The latter seen from beneath. 5. The same, on appear- ance of first horizontal furrow. 6. The same, seen from above. 7. Longitudinal section of 6. 8 and 9. Two phases in segmentation, on appearance of fourth and fifth furrows. 10. Longitudinal vertical section at a slightly later stage than the above. 11. Later I stage. Upper pigmented pole dividing more rapidly than lower. 12. Later phase of 11. 13. Longitudinal vertical section of 12. 14. Segmenting ovum at blastopore stage. 15. Longitudinal vertical section of same. 13 and IB •a 10 (all others x 5). 10. Longitudinal vertical section of embryo at a stage later than 14d x 10). nc, nucleus; c. c, cleavage cavity; eg, epiblast; 1. 1, yelk-Bearing lower-layer cells; bl, blastopore; al, archentcron (mid-gut); hb, hypoblast; wis, undifferentiated mesoblast; ch, notochord; n. a, neural (cerebrospinal) axis. membranes, when the blastoderm may be seen floating upward, as it always does. It should be well examined in position, THE DEVELOPMENT OF THE EMBRYO ITSELF. 97 using a hand lens, though this is knowledge ; in fact, if the exam- ination goes no further than the naked-eye appearances of a dozen eggs, selecting one every twenty- four hours during incubation, when opened and the shell and membranes well cleared away, such a knowledge will be sup- plied as can be obtained from no books or lectures however good. It will be, of course, understood that the student approaches these examinations with some ideas gained from plates and previous reading. The latter will furnish a sort of biological pabulum on which he may feed till he can supply for himself a more natu- ral and therefore more healthful one. While these remarks apply with a certain degree of force to all the departments of physiolo- gy, they are of special impor- tance to aid the constructive fac- ulty in building up correct no- tions of the successive rapid transformations that occur in the development of a bird or mammal. Fig. 99 shows the embryo of the bird at a very early period, when already, however, some of the main outlines of structure are marked out. Development in the fowl is so rapid that a few days suffice to outline all the principal organs of the body. In the mammal the process is slow- er, but in the main takes place in the same fashion. As the result of long and pa- 7 not essential to getting a fair Fig. 99.— Embryo fowl 3 mm. long, of about twenty-four hours, seen from above. 1 x 39. (Haddon, after K611iker.) Mn, union of the med- ullary folds in the region of the hind-brain; Pr, primitive streak; Pz, parietal zone ; 7?/, posterior portion of widely open neural groove; i?/'. anterior part of neu- ral groove ; Rw, neural ridge ; Stz. trunk-zone ; rAf. anterior amni otic fold ; rD, anterior umbilical sinus showing through the blasto- derm. His divides the embryonic rudiment into a central trunk-zone and a pair of lateral or parietal zones. 98 COMPARATIVE PHYSIOLOGY. tient observation, it is now settled that all the parts of the most complicated organism arise from the three-layered blastoderm previously figured ; every part may be traced back as arising in one or other of these layers of cells — the epiblast, mesoblast, or hypoblast. It frequently happens that an organ is made up of cells derived from more than one layer. Structures may, ac- cordingly, be classified as epiblastic, mesoblastic, or hypoblastic ; for, when two strata of cells unite in the formation of any part, one is always of subordinate importance to the other : thus the digestive organs are made up of mesoblast as well as hypo- blast, but the latter constitutes the essential secreting cell mech- anism. As already indicated, the embryonic membranes are also derived from the same source. The epiblast gives rise to the skin and its appendages (hair, nails, feathers, etc.), the whole of the nervous system, and the chief parts of the organs of special sense. The mesoblast originates the vascular system, the skeleton, all forms of connective tissue including the framework of glands, the muscles, and the epithelial (endothelial) structures covering serous membranes. The hypoblast furnishes the secreting cells of the digestive tract and its appendages — as the liver and pancreas— the lining epithelium of the lungs, and the cells of the secreting mucous membranes of their framework of bronchial tubes. It is difficult to overrate the importance of these morpholog- ical generalizations for the physiologist ; for, once the origin of an organ is known, its function and physiological relations gen- erally may be predicted with considerable certainty. We shall Pig. 100.— Transverse section through the medullary groove and half the blastoderm of a chick of eighteen hours (Foster and Balfour). E, epiblast; M, mesoblast; //, hypoblast; mf', medullary fold; my, medullary groove; ch, notochord. endeavor to make this prominent in the future chapters of this work. Being prepared with these generalizations, we continue our study of the development of the bird's embryo. Before the end THE DEVELOPMENT OP THE EMBRYO ITSELF. 99 of the first twenty-four hours such an appearance as that repre- sented in Fig. 100 is presented. The mounds of cells forming the medullary folds are seen coming in contact to form the medullary {neural) canal. Fig. 101.— Transverse section of embryo chick at end of first day (after KOlliker). M, mesoblast; H. hypoblast; m, medullary plate; E. epiblast; m. g, medullary groove; m.f, medullary fold; ch, chorda dorsalis ; P, protovertebral plate; d. in. division of mesoblast. The notochord, marking out the future bony axis of the body, may also be seen during the first day as a well-marked linear extension, just beneath the medullary groove. The cleav- nf fif Fig. 102. — Transverse section of chick at end of second day (KOlliker). E, epiblast; H, hypoblast; e. m, external plate of mesoblast dividing (cleavage of mesoblast): m.f, medullary fold; m. rj, medullary groove; ao, aorta; p, pleuroperitoneal cavity; P, protovertebral plate. age of the mesoblast, resulting in the commencement of the formation of somatopleure (body -fold) and the splanchnopleure (visceral fold), is also an early and important event. These give rise between them to the pleuro-peritoneal cavity . The portions of mesoblast nearest the neural canal form masses (ver- tebral plates) distinct from the thinner outer ones (lateral plates). The vertebral plates, when distinctly marked off, as represented in the figure, are termed the protovertebral (meso- blastic somites), and represent the future vertebra? and the vol- untary muscles of the trunk ; the former arising from the inner subdivisions, and the latter from the outer (muscle-plates). It will be understood that the protovertebra? are the results of 100 COMPARATIVE PHYSIOLOGY. m.b au.p, a.p. transverse division of the columns of mesoblast that formed the vertebral plates. Before the permanent vertebrae are formed, a reunion of the original pro to vertebrae takes place as one cartilaginous pillar, followed by a new segmen- tation midway between the original divisions. It is thus seen that a large number of structures either appear or are clearly outlined during the first day of incubation : the primitive streak, primitive groove, medullary plates and groove, the neural ca- nal, the head-fold, the cleavage of the mesoblast, the protovertebrae. with traces of the amnion and area opaca. During the second day nearly all the remaining important structures of the chick are marked out, while those that arose during the first day have progressed. Thus, the medullary folds close ; there is an increase in the number of protover- tebrae ; the formation of a tubular heart and the great blood-vessels ; the appear- ance of the Wolffian duct ; the progress of the head re- gion ; the appearance of the three cerebral vesicles at the anterior extremity of the neural canal ; the subdivision of the first cerebral vesicle into the optic vesicles and the beginnings of the cerebrum ; the auditory pit arising in the third cerebral vesicle (hind-brain) ; cranial flex- ure commences ; both head and tail folds become more dis- tinct ; the heart is not only formed, but its curvature becomes more marked and rudiments of auricles arise ; while outside Fig. 103. — Embryo of chick, between thirty and thirty-six hours, viewed from above as an opaque object (Foster and Balfour). /. h, forebrain; m. b, midbrain; h. b, hind- brain; op. v, optic vesicle; au. p, auditory pit ; o.f. vitelline vein ; p. v, mesoblastic somite'; m.f, line of function of medulla- ry folds abovejmedullary canal ; s.r, sinus raomboidalis; I, tail-fold; p.r, remains of primitive groove ; a.p, area pellucida. THE DEVELOPMENT OP THE EMBRYO ITSELF. 101 the embryo itself the circulation of the yelk-sac is established, the allantois originates, and the amnion makes rapid progress. It may be noticed that the cerebral vesicles, the optic vesi- cles, and the auditory pit are all derived from the epiblastic accumulations which occur in the anterior extremity of the embryo ; and their early appearance is prophetic of their physi- ological importance. The heart, too, so essential for the nutrition of the embryo, by distributing a constant blood-stream, is early formed, and becomes functionally active. It arises beneath the hind-end of the fore-gut, at the point of divergence of the folds of the B __ Fig. 104. — Diagram representing under surface of an embryo rabbit of nine days and three hours old, illustrating development of the heart (after Allen Thomson). A. view of the entire embryo; B, an enlarged outline of the heart of A; C. later stage of the development of B ; h h, ununited heart; aa, aorUe; vv, vitillme veins. splanchnopleure, and so lies within the pleuro-peritoneal cav- ity, and is derived from the mesoblast. At the beginning the heart consists of two solid columns ununited in front at first ; later, these fuse, in part, so that they have been compared with an inverted Y, in which the heart itself would correspond to the lower stem of the letter Q) and the great veins (vitel- line) to its main limbs. The solid cords of mesoblast become 102 COMPARATIVE PHYSIOLOGY. hollow prior to their coalescence, when the two tubes become one. The entire blood-vascular system originates in the mesoblast of the area opaca especially ; at first appearing in isolated spots Pro. 105.— Chick on third day, seen from beneath as a transparent object, the head being turned to one side (Foster and Balfour), a', false amnion; a, amnion; OH, cerebral hemisphere; FB, MB, HB, anterior, middle, and posterior cerebral vesi- cles; OP, optic vesicle; ot, auditory vesicle; OfV, omphalo-mesentcric veins; lit, heart; Ao, bulbus arteriosus; ch, notochord; Of.a, omphalomesenteric arteries; Pv, proto vertebra;; x, point of divergence of the splanchnopleural folds; y, ter- mination of the fore-gut, V. which come together as actual vessels are formed. The student who will pursue the plan of examining a series of incubating eggs will be struck with the early rise and rapid progress of the vascular system of the embryo, which takes, when complete, such a form as is represented diagramatically in Fig. 109. The blood and the blood-vessels arise simultaneously from THE DEVELOPMENT OF THE EMBRYO ITSELF. 103 the cells of the mesoblast by outgrowths of nuclear prolifera- tion, and in the case of vessels (Fig. 143) extension of processes, fusion, and excavation. Fig. 106.— Diagram of the heart and principal arteries of the chick (Qnain). A repre- sents an earlier and B and C later stages. 1, 1, omphalo-mesenteric reins; 2, auri- cle; 3, ventricle; 4, aortic bulb; 5,5, primitive aortse; 6,6, omphalo-mesenteric arteries; A, united aortse. The fore-gut is formed by the union of the folds of the splanchnopleure from before backward, and the hind-gut in a similar manner by fusion from behind forward. Fig. 107. — Diagrammatic outlines of the early arterial system of the mammalian em- bryo (after Allen Thomson). A. At a period corresponding to the thirty-sixth or thirty-eighth hour of incubation. B. Later stage, with two pairs of aortic arches. />. bulbus arteriosus of heart; v, vitelline arteries; 1 — 5, the aortic arches. The dotted lines indicate the position of the future arches. 104 COMPARATIVE PHYSIOLOGY. The excretory system is also foreshadowed at an early period the Wolffian duct (Fig. 110), a mass of mesoblast cells near which the cleavage of the mesoblast takes place. During the latter part of the sec- ond day the vascular system, includ- ing the heart, makes great progress. The latter, in consequence of excessive growth and the alteration of the rela- tive position of other parts, becomes bent up on itself, so that it presents a curve to the right which represents the venous part, and one to the left, answering to the arterial. The rudi- ments of the auricles also are to be seen. The arterial system is represented at this stage by the expanded portion of the heart known as the bulbus ar- teriosus, and two extensions from it, the aorta?, which, uniting above the alimentary canal, form a single poste- rior or dorsal aorta. From these great arterial vessels the lesser ones arise, and by subdivision constitute that great mesh-work represented cliagram- matically in Figs. 108, 109, from which ,. the course of the circulation may be bryonic vascular system o-athered. The beating of the heart (Wiedersheim). A, atrium; ° A', A', dorsal aorta ; Ab, commences before the corpuscles nave branchial vessels: Acd, cau- 1 , i -i ,-i„ . -i dai artery; All, allantoic (hy- become numerous, and while the tub- fSSFStSSfi l^bus ular system, through which the blood arteriosus,- c, c>. external and j s ^ be driven, is still very incomplete. internal carotids; D, ductus ' . *■ The events of the third day are of Cuvieri (precaval veins); E, external iliac arteries; H. G, posterior cardinal vein ; Ic, common iliac arteries; If. L. Kill clefts ; R. A, right and left, rootn of the aorta; S. S', branchial collecting trunks or veins ; ,S7>. subclavian ar- tery ; 8b', subclavian vein; Si, sinus venosus; V, ventri- cle ; VC, anterior cardinal vein; Vm, vitelline veins. left side the nature of the extension of parts already marked out rather than the formation of entirely new ones. The following are the principal changes : The bending of the head-end down- ward (cranial flexure) ; the turning of the embryo so that it lies on its the completion of the vitelline circulation ; the in- THE DEVELOPMENT OF THE EMBRYO ITSELF. 105 crease in the curvature of the heart and its complexity of struct- ure by divisions ; the appearance of additional aortic arches and of the cardinal veins ; the formation of four visceral clefts and five visceral arches ; a series of progressive changes in AAA Fig. 109.— Diagram of circulation of yelk-sac at end of third day (Foster and Bal- four). Blastoderm seen from below. Arteries made black. H, heart; AA, sec- ond, third, and fourth aortic arches; A 0, dorsal aorta; L.of.A, left vitelline artery; E.qf. A. right vitelline artery; 8. T. sinus terminalis; 'L. of, left vitelline vein; R. of, right vitelline vein ; S. V, sinus venosus ; D. C, ductus S. Ca. V, superior cardinal or jugular vein; V. Ca, inferior cardinal vein, Cuvicri; the organs of the special senses, such as the formation of the lens of the eye and a secondary optic vesicle ; the closing in of the optic vesicle ; and the formation of the nasal pits. In the region of the future brain, the vesicles of the cerebral hemi- spheres become distinct ; the hind-brain separates into cei'e- 106 COMPARATIVE PHYSIOLOGY. ! ; /p?^ Fio. 111. Fio. 112. ^VaWFosTe^nTttX!^ throu g h '^'^r remon of an embryo at end of fourth aay i t osier and Balfour) n. c, neural canal; pr, posterior root of spinal nerve A VT&of^hS^ r ° 0t - ; A - °- °' ! , l " ,erior ^y column of spinal cord; notochor.i I v ,r U -T ln ^urse ot ■formation; m.p, muscle-plate; c.h, vPin- w / w 7 f;r W0lhu '.' r $P,'- A0 ' dorsal aorta ; »■«■«, Posterior cardina vein; W. d, Wolffian duct; W. b, Wolffian body, consisting of tubules and Mat THE DEVELOPMENT OF THE EMBRYO ITSELF. 107 pighian corpuscles; g. e, germinal epithelium; d, alimentary canal; M, commenc- ing mesentery ; S. 0, somatopleure ; SP, splanchnopleure ; V, blood - vessels ; pp, pleuroperitoneal cavity. Fig. 111.— Diagram of portion of digestive tract of chick on fourth day (after GOtte). The black line represents hypoblast; the shaded portion, mesoblast; Iff, lung di- verticulum, expanding at bases into primary lung vesicle; st, stomach; /,hver; p, pancreas. Fig. 112.— Head of chick of third day, viewed sidewise as a transparent object (Hux- ley). /«, cerebral hemispheres; lb, vesicle of third ventricle; II, mid-brain; III, hind-brain; a, optic vesicle; cj, nasal pit; b, otic vesicle; d, infundibulum ; e, pineal body ; h, notochord ; V, fifth nerve ; VII, seventh nerve ; VIII, united glossopharyngeal and pneumogastric nerves. 1, 2, 3, 4, 5, the five visceral folds. bellum and medulla oblongata ; the nerves, both cranial and spinal, bud out from the nervous centers. The alimentary ca- nal enlarges, a fore-gut and hind-gut being formed, the former being divided into oesophagus, stomach, and duodenum ; the latter into the large intestine and the cloaca. The lungs arise from the alimentary canal in front of the stomach ; from simi- lar diverticula from the duodenum, the liver and pancreas orig- inate. Changes in the protovertebrae and muscle-plates con- tinue, while the Wolffian bodies are formed and the Wolffian duct modified. Up to the third day the embryo lies mouth downward, but now it comes to lie on its left side. See Fig. 105 with the ac- companying description, it being borne in mind that the view is from below, so that the right in the cut is the left in the em- FrB. Fig. 113.— Head of chick of fourth day. viewed from below as an opaque object (Fos- ter and Balfour). The neck is cut across between third and fourth visceral folds. C. //, cerebral hemispheres; F. B, vesicle of third ventricle: Op, eyeball; /if. naso-frontal process; M, cavity of mouth; S. J/, superior maxillary process of F. 1. the first visceral fold (mandibular arch); F. 2, F. 3, second and third visceral arches; JV, nasal pit. bryo itself. Fig. 110 gives appearances furnished by a vertical transverse section. The relations of the parts of the digestive tract and the mode of origin of the lungs may be learned from Fig. 111. 108 COMPARATIVE PHYSIOLOGY. An examination of the figures and subjoined descriptions must suffice to convey a general notion of the subsequent prog- G.Ph VII. rV.V. IF MPr Fig. 114.— Embryo at end of fourth day, seen as a transparent object (Foster and Balfour). C'H, cerebral hemisphere; F. B. fore-brain, or vesicle of third ventricle (thalamencephalon), with pineal gland (Pn) projecting; M. B, mid-brain; C.b, cerebellum; IV. V, fourth ventricle; L, lens; cJw, choroid slit; Cen. V, auditory vesicle; sm, superior maxillary process; IF, 2F, etc., first, second, etc., visceral folds ; V, fifth nerve; VII. seventh nerve; O. Ph, glossopharyngeal nerve; Pg, pneumogastric. The distribution of these nerves is also indicated ; ch, noto- chord; lit, heart; MP. muscle-plates; W, wing; H. L, hind-limb. The amnion has been removed. Al, allantois protruding from cut end of somatic stalk SS. ress of the embryo. Special points will be considered, either in a separate chapter now, or deferred for treatment in the body of the work from time to time, as they seem to throw light upon the subjects under discussion. DEVELPOMENT OF THE VASCULAR SYSTEM IN VER- TEBRATES. This subject has been incidentally considered, but it is of such importance morphological, physiological, and pathological, as to deserve special treatment. In the earliest stages of the circulation of a vertebrate the arterial system is made up of a pair of arteries derived from the single bulbus arteriosus of the heart, which, after passing for- THE DEVELOPMENT OP THE EMBRYO ITSELF. 109 ward, bends round to the dorsal side of the pharynx, each giving off at right angles to the yelk-sac a vitelline artery ; the aorta? unite dorsally, then again separate and become lost in the pos- terior end of the embryo. The so-called arches of the aorta are large branches in the anterior end of the embryo derived from the aorta itself. The venous system corresponding to the above is composed of anterior and posterior pairs of longitudinal (cardinal) veins, the former (jugular, cardinal) uniting with the posterior to form a common trunk (ductus Cuvieri) by which the venous blood is returned to the heart. The blood from the posterior part of the yelk-sac is collected by the vitelline veins, which terminate in the median sinus venosus. The Later Stages of the Foetal Circulation.— Corresponding to the number of visceral arches five pans of aortic arches arise ; but they do not exist together, the first two having undergone more or less complete atrophy before the others appear. Figs. 115, 116 convey an idea of how the permanent forms (indicated by darker shading) stand related to the entire system of vessels in different groups of animals. Thus, in birds the right (fourth) aortic arch only remains in connection with the aorta, the left forming the subclavian artery, while the reverse occurs in mammals. The fifth arch (pulmonary) always supplies the lungs. Fig. 115.— Diagrams of the aortic arches of mammal (Landois and Stirling, after Ratlike). 1. Arterial trunk with one pair of arches, and an indication where the second and third pairs will develop. 2. Ideal Btagc of five complete arches: the fourth clefts are shown on the left side. 3. The two anterior pairs of arches have disappeared. -1. Transition to the final stage. A, aortic arch; ad, dorsal aorta; ax, subclavian or axillary artery; ft. external carotid; Ci. internal carotid; dB, ductus arteriosus Botalli; P. pulmonary artery; iff, subclavian artery; la, truncua arteriosus; v, vertebral artery. The arrangement of the principal vessels in the bird, mam- mal, etc. , is represented on page 110. In mammals the two prim- 110 COMPARATIVE PHYSIOLOGY. itive anterior abdominal {allantoic) veins develop early and unite in front with the vitelline : hut the right allantoic vein and the right vitelline veins soon disappear, while the long com- mon trunk of the allantoic and vitelline veins {ductus venosus) passes through the liver, where it is said the ductus venosus gives off and receives branches. The ductus venosus Arantii persists throughout life. (Compare the various figures illustrat- ing the circulation.) A ... .. B Fro. 116.— Diagram illustrating transformations of aortic arches in a lizard, A ; a snake, B; a bird. C; a mammal, D. Seen from below, (lladdon, after Rathke.) a, internal carotid; b, external carotid; c, common carotid. A. d, ductus Botalli between the third and fourth arches; e, right aortic arch; /, subclavian; g, dorsal aorta; h, left aortic arch; i, pulmonary artery; k, rudiment of the ductus Botalli between the pulmonary artery and the aortic arches. B. d, right aortic arch; e, vertebral artery; /, left aortic arch; A, pulmonary artery; i, ductus Botalli of the latter. 0. it, origin of aorta; e, fourth arch of the right side (root of dorsal aorta); /, right subclavian; g, dorsal aorta; h, left subclavian (fourth arch of the left side); i, pulmonary artery; k and I, right and left ductus Botalli of the pulmonary arteries. D. d, origin of aorto; e, fourth arch of the left side (root of dorsal aorta); f, dorsal aorta; q, left vertebral artery; h, left subclavian; i, right sub- clavian'(fourth arch of the right side); k. right vertebral artery; I, continuation of the right subclavian; in. pulmonary artery; n, ductus Botalli of the latter (usually termed ductus arteriosus). With the development of the placenta the allantoic circula- tion renders the vitelline subordinate, the vitelline and the larger mesenteric vein forming the portal. The portal vein at a later period joins one of the vena} advehentes of the allantoic vein. At first the vena cava inferior and the ductus venosus enter the heart as a common trunk. The ductus venosus Arantii be- comes a small branch of the vena cava. THE DEVELOPMENT OF THE EMBRYO ITSELF. \\\ The allantoic vein is finally represented in its degenerated form as a solid cord {round ligament), the entire venous sup- ply of the liver being derived from the portal vein. The development of the heart has already been traced in the fowl up to a certain point. In the mammal its origin and early progress are similar and its further history may be gathered from the following series of representations. In the fowl the heart shows the commencement of a division into a right and left half on the third day, and about the fourth week in man, from which fact alone some idea may be gained as to the relative rate of development. The division is effected by the outgrowth of a septum from the ventral wall, which rap- Fig. 118. Fig. 117. ?!'•— Development of the heart in the human embryo, from the fourth to the sixth week. A. hmbryo of four weeks (KOlliker, after Coste). B, anterior C posterior views of the heart of an embryo of six weeks (KOlliker. after Eck'er)' a upper limit of buccal cavity; c, buccal cavity; b, lies between the ventral ends of second and third branchial arches; d, buds of upper limbs; e liver- f intes- tine; 1, superior vena cava: 1', left superior vena cava; 1", opening of inferior bu?b CaVa ' aM ' eft anricles ; 3 - 3 '- ri § ht and left ventricles; 4, aortic Fig. 118.— Human embryo of about three weeks (Allen Thomson), uv. yelk-sac- al allantois; am, amnion; ae, anterior extremity; pe. posterior extremity. idly reaches the dorsal side, when the double ventricle thus formed communicate by a right and a left auriculo-ventricular opening with the large and as yet undivided auricle. Later an incomplete septum forms similar divisions in the auricle ; the aperture {foramen ovale) left by the imperfect growth of this wall persisting throughout foetal life. The Eustachian valve arises on the dorsal wall of the right auricle, between the vena cava inferior and the right and left 112 COMPARATIVE PHYSIOLOGY. venae cavaa superiores ; but in many mammals, among which is man, the left vena cava superior disappears during fcetal life. For the present we may simply say that the histories of the development of the heart, the blood-vessels, and the blood itself are closely related to each other, and to the nature and changes of the various methods in which oxygen is supplied to the blood and tissues, or, in other words, to the development of the respir- atory system. THE DEVELOPMENT OF THE UROGENITAL SYSTEM. Without knowing the history of the organs, the anatomical relations of parts with uses so unlike as reproduction on the one hand and excretion on the other, can not be comprehended; nor, as will be shortly made clear, the fact that the same part may serve at one time to remove waste matters (urine) and at an- other the generative elements. The vertebrate excretory system may be divided into three parts, which result from the differentiation of the primitive kid- ney which has been effected during the slow and gradual evo- lution of vertebrate forms : 1. The head-kidney (pronephros). 2. The Wolffian body (mesonephros). 3. The kidney proper, or metanephros. But in this instance, as in others to some of which allusion has already been made, these three parts are not functional at the same time. The pronephros arises from the anterior part of the segmental duct, pronephric duct, duct of primitive kid- ney, and archinephric duct, and in the fowl is apparent on the third day ; but the pronephros is best developed in the ichthy- Pio. 110. — Diagrams illustrating development of pronephron in the fowl (Ilnddon). ao, aorta; o.c, body-cavity; ep, epiblast with its epitrichial (flattened) layer; hy, hypoblast; m. s, mesoblastic somite; n. c, neural canal; nch, notochord; p. I.., pro- nephric tubule; so, somatic, and up, splanchnic mesoblaBt. THE DEVELOPMENT OF THE EMBRYO ITSELF. H3 opsida (fishes and amphibians). A vascular process from the peritoneum (glomerulus) projects into a dilated section of the body cavity, which is in part separated from the rest of this cavity (ccelom). This process, together with the segmental duct, now coiled, and certain short tubes developed from the original duct, make up the pronephros. The segmental duct opens at length into the cloaca. The mesonephros (Wolffian body), though largely developed in all vertebrates during fcetal life, is not a persistent excretory organ of adult life. In the fowl recent investigation has shown that the Wolffian (segmental) tubes originate from outgrowths of the Wolffian Fig. 120. Fig. 181. Fig. 120. — Rudimentary primitive kidney of embryonic dog. The posterior portion of the body of the embryo is seen from the ventral side, covered by the intestinal layer of the yelk-sac. which has been torn away, and thrown back in front in order to show the primitive kidney ducts with the primitive kidney tubes (a), h, primitive vertebrae; c, dorsal medulla; d, passage into the pelvic intestinal cavity. (Hacckel. alter Bischoff.) Fig. 121. — Primitive kidney of a human embryo, v. the urine-tubes of the primitive kidney: ir. Wolffian duct; w', upper end of the latter (Morgagni's hydatid*: m, Mfillerian duct; m'. upper end of the latter (Fallopian hydatid): g, hermaphrodite > gland. (After Kobelt.) duct aud also from an intermediate cell-mass, from which latter the Malpighian bodies take rise. The ( tubes, at first not con- 114 COMPARATIVE PHYSIOLOGY. nected with the duct, finally join it. This organ is continuous with the pronephros ; in fact, all three (pronephros, rnesone- phros, and metanephros) may be regarded as largely continua- tions one of another. The metanephros, or kidney proper, arises from mesoblast at the posterior part of the Wolffian body. The ureter originates Fig. 122.— Section of the intermediate ceil-mass of fourth day (Foster and Balfour, after Waldeyer). 1 x 160. m. mesentery; L. somatopleure; «', portion of the germinal epithelium from the duct of Miiller is formed by involution; a, thick- ened portion of the germinal epithelium, in which the primitive ova, C and o. are lying; E, modified mesoblast which will form the stroma of the ovary; WK, Wolffian body; y, Wolffian duct. first from the hinder portion of the Wolffian duct. In the fowl the kidney tubules bud out from the m^eter as rounded eleva- tions. The ureter loses its connection w T ith the Wolffian duct and opens independently into the cloaca. The following account will apply especially to the higher vertebrates : The segmental (archinephric) duct is divided horizontally into a dorsal or Wolffian (mesonephric) duct and a ventral or Miillerian duct. The Wolffian duct, as we have seen, develops into both ureter and kidney proper. To carry the subject; somewhat further back, the epithelium THE DEVELOPMENT OF THE EMBRYO ITSELF. 115 lining the coelom at one region becomes differentiated into col- umnar cells {germinal epithelium) which by involution into the underlying mesoblast forms a tubule extending from before backward and in close relation with the Wolffian duct, thus forming the Mullerian duct by the process of cleavage and separation referred to previously. Fig. 123. — Diagrammatic representation of the genital organs of a human embryo pre- vious to sexual distinction (Allen Thomson!. TT". Wolffian body; gc, genital cord; m, Mlillerian duct: w, Wolffian duct: ug, urogenital sinus; cp. clitoris or penis; i, intestine; cl, cloaca; Is, part from which the scrotum or labia majora are devel- oped; ot, origin of the ovary or testicle respectively; x. part of the Wolffian body developed later into the cohi vasculosis 3, ureter: 4. bladder: 5, urachus. The future of the Mullerian and Wolffian ducts varies ac- cording to the sex of the embryo. In the male the Wolffian duct persists as the vas deferens ; in the female it remains as a rudiment in the region near the ovary (hydatid of Morgagni). In the female the Mulleriau duct becomes the oviduct and related parts (uterus and vagina) ; in the male it atrophies. One, usually the right, also atrophies in female birds. The sinus pocularis of the pi'ostate is the remnant in the male of the fused tubes. The various forms of the generative apparatus derived from the Mullerian ducts, as determined by different degrees of fu- 116 COMPARATIVE PHYSIOLOGY. Fig. 134. — Diagram of the mammalian type of male sexual organs (after Quain). Com- pare with Figs. 123, 125. C. Cowper's gland of one side; cp, corpora cavernosa penis, cut short; e, caput epididymis; g, gubernaculum; i, rectum; m, hydatid of Morgagni, the persistent anterior end of the Miillerian duct, the conjoint posterior ends of which form the uterus masculinus; pi', prostate gland; s, scrotum; sp, corpus spongiosum urethra; t, testis (testicle) in the place of its original forma- tion. The dotted line indicates the direction in which the testis and epididymis change place in their descent from the abdomen into the scrotum; vd, vas defer- ens; vh, vas aberrans; vs, vasicula seminalis; W, remnants of Wolffian body (the organ of Giraldds or paradidymis of Waldeyer); 3, 4, 5, as in Fig. 125. sion, etc., of parts, may be learned from the accompanying figures. n v C Fio. 125.— Diagram of the mammalian type of female sexual organs (after Quain). The dotted iines in one figure indicate functional organs in the other. C, gland of Bartholin (OowperV gland); <■./:, corpus cavernosum clitoridis; dQ, remains of the left Wolffian duct, which may persist as the duct of Gaertner; f, abdominal open- ing of left Fallopian tube; ground ligament (corresponding tofhegubernaculum); //.hymen; i, rectum; I, labium; m, cut Fallopian tube (oviduct, or Miillerian duct) of the right wide; //, nyinpha; o, left ovary; po. parovarium; SO, vascular bulb or corpus spongiosum; u, uterus; v, vulva; va, vagina; W, scattered remains of Wol- ffian tubes (paroophoron); w, cut end of vanished right Wolffian duct; 3, ureter; 4, •bladder passing below into tin.' urethra ; 5, urachus, or remnant, of stalk of allantois. THE DEVELOPMENT OF THE EMBRYO ITSELF. Hf In both sexes the most posterior portion of the Wolffian duct gives rise to the metanephros, or what becomes the permanent kidney and ureter ; in the male also to the vas deferens, testicle, vas aberrans, and seminal vesicle. The ovary has a similar origin to the testicle ; the germinal epithelium furnishing the cells, which are transformed into Graafian follicles, ova, etc., and the mesoblast the stroma in which these structures are imbedded. In the female the parovarium remains as the representative of the atrophied Wolffian body and duct. The bladder and urachus are both remnants of the formerly extensive allantois. The final forms of the genito-urinary or- gans arise by differentiation, fusion, and atrophy : thus, the cloaca or common cavity of the genito-urinary ducts is divided ALL, ALL Fig. 128. Fig. 129. Figs. 126 to 129.— Diagrams illustrating the evolution of the posterior passages (after Landois and Stirling). Fig. 126. — Allantois continuous with rectum. Pig. 127.— Cloaca formed. Fig. 128. — Early condition in male, before the closure of the folds of the groove on the posterior side of the penis. Fig. 129. — Early female condition. A, commencement of proctodeum; ALL, allantois; B. bladder; C. penis; CL, cloaca; M, Miillerian duct; J?, rectum; U, urethra; S, vestibule; SU, urogenital sinus: 1'. vas deferens in Fig. 128, vagina in Fig. 129. by a septum (the perineum externally) into a genito-urinary and an intestinal (anal) part ; the penis in the male and the corresponding clitoris in the female appear in the region of the cloaca, as outgrowths which are followed by extension of folds of integument that become the scrotum in the one sex and the labia in the other. The urethra arises as a groove in the under surface of the 118 COMPARATIVE PHYSIOLOGY. penis, which becomes a canal. The original opening 1 of the urethra was at the base of the penis. In certain cases development of these parts is arrested at various stages, from which result abnormalities frequently re- quiring interference by the surgeon. The accounts of the previous chapters do not complete the history of development. Certain of the remaining subjects that are of special interest, from a physiological point of view, will be referred to again ; and in the mean time we shall con- sider rather briefly some of the physiological problems of this subject to which scant reference has as yet been made. Though the physiology of reproduction is introduced here, so that ties of natural connection may not be severed, it may very well be omitted by the student who is dealing with embryology for the Pig. 130.— Various forms of mammalian uteri. A. Ornithorhynchus. B. Didelphys dorsigera. C. Phalangista vulpina. D. Double uterus and vagina; human anoma- ly. E. Lepus cuniculus (rabbit), uterus duplex. F. Uterus bicornis. G. Uterus bipartitus. H. Uterus simplex (human), a, anus; cl, cloaca; o. d, oviduct; o. I, os tincse (os uteri); ov, ovary: r, rectum; s, vaginal septum; u. b, urinary bladder; ur, ureter; ur. o, orifice of same; u. s, urogenital sinus; ut, uterus; v, vagina; v. c, vaginal caecum (Haddon). first time, and in any case should be read again after the other functions of the body have been studied. THE PHYSIOLOGICAL ASPECTS OF DEVELOPMENT. According to that law of rhythm which, as we have seen, prevails throughout the world of animated nature, there are periods of growth and progress, of quietude and arrest of devel- THE DEVELOPMENT OF THE EMBRYO ITSELF. H9 opment ; and in vertebrates one of the most pronounced epochs — in fact, the most marked of all — is that by which the young organism, through a series of rapid stages, attains to sexual ma- turity. While the growth and development of the generative organs share to the greatest degree in this progress, other parts of the body and the entire being participate. So great is the change that it is common to indicate, in the case of the human subject, the developed organism by a new name — the " boy " becomes the " man," the " girl " the " woman. 1 ' Relatively this is by far the most rapid and general of all the transformations the organism undergoes during its extra-uter- ine life. In this the entire body takes part, but very unequally. The increase in stature is not proportionate to the increase in weight, and the latter is not so great as the change in form. The modifications of the organism are localized and yet affect the whole being. The outlines become more rounded ; the pel- vis in females alters in shape ; not only do the generative organs themselves rapidly undergo increased development, but certain related glands (mammae) participate ; hair appears in certain regions of the body ; the larynx, especially in the male, under- goes enlargement and changes in the relative size of parts, re- sulting in an alteration of voice (breaking of the voice), etc. — all in conformity with that excess of nutritive energy which marks this biological epoch. Correlated with these physical changes are others belonging to the intellectual and moral (psychic) nature equally impor- tant, and, accordingly, the future being depends largely on the full and un warped developments of these few years. Sexual maturity, or the capacity to furnish ripe sexual ele- ments (cells), is from the biological standpoint the most impor- tant result of the onset of that period termed, as regards the human species, puberty. The age at which this epoch is reached varies with race, sex, climate, and the moral influences which envelop the indi- vidual. In temperate regions and with European races puberty is reached at from about the thirteenth to the eighteenth year in the female, and rather later in the male, in whom develop- ment generally is somewhat slower. Changes analogous to the above occur in all vertebrates. It is at this period that differ- ences of form, voice, disposition, and other physical and psychic characteristics first become pronounced. 120 COMPARATIVE PHYSIOLOGY. As a matter of fact, the pig, sheep, goat, cat, dog, and certain other animals may conceive when less than one year old ; and the cow and the mare when under two years. At such periods these animals are not of course mature and should not be bred. OVULATION. In all vertebrates, at periods recurring with great regularity, the generative organs of the female manifest unusual activity. This is characterized by increased vascularity of the ovary and adjacent parts ; with other changes dependent on this, and that heightened nerve influence which, in the vertebrate, seems to be inseparable from all important functional changes. Ovulation is the maturation and discharge of ova from the Graafian folli- cles. The latter, reaching the exterior zone of the ovaxy, be- coming distended and thinned, burst externally and thus free the ovum. The follicles being very vascular at this period, blood escapes, owing to this rupture, into the emptied capsule and clots ; and as a result of organization and subsequent degeneration undergoes a certain series of changes dependent on the condition of the ovary and related organs, which varies according as the ovum has been fertilized or not. When fertilization occurs the Graafian follicle undergoes changes of a more marked and lasting character, becoming a true corpus luteum of pregnancy. The number of Graafian follicles that ma- ture and the number of ripe ova that escape at about the same period varies, of course, with the species and the individual, and is not al- ways the same in the latter. In species that usually bear several young at a birth a corresponding number of ova must be ripened and fertilized at about the same time ; while the reverse holds for those that usually give birth to but one. The ovum in the fowl is fertilized in the upper part of the oviduct; in the mammal mostly in this region also, as is shown by the site of the embryos in those groups of animals with a two- horned uterus, and the occasional occurrence of tubal pregnan- Fio. 131.— Ovary of rabbit at period of oestrum, show- ing various stages of extniHion of ova. (C'hauveau.) THE DEVELOPMENT OF THE EMBRYO ITSELF. 121 cy in woman. But this is not, in the human subject at least, invariably" the site of impregnation. After the ovum has been set free, as above described, it is conveyed into the oviduct (Fallopian tube), though exactly how is still a matter of dis- pute : some holding that the current produced by the action of the ciliated cells of the Fallopian tube suffices ; others that the ovum is grasped by the fimbriated extremity of the tube as part of a co-ordinated act. It is likely, as in so many other instances, that both views are correct but partial; that is to say, both these methods are employed. The columnar ciliated cells, lining the oviduct, act so as to produce a current in the direction of the uterus, thus assisting the ovum in its passage toward its final resting place. CEstrum. — As a part of the general activity occurring at this time, the uterus manifests certain changes, chiefly in its inter- nal mucous lining, in which thickening and increased vascular- Fio. 132.— Diagram of the human uterus just before menstruation. The shaded portion represents the mucous mem- brane (Hart and Barbour, after J. Williams). Fie 133.— Uterus after menstruation has just ceased. The cavity of the body of the uterus is supposed to have been deprived of mucous membrane (J. Williams). ity are prominent. A flow of blood from the uterus in the form of a gentle oozing follows; and as the superficial parts of the mucous lining of the uterus undergo- softening and fatty degen- 122 COMPARATIVE PHYSIOLOGY. eration, they are thrown off and renewed at these periods (cata- vienia, menses, etc.), provided pregnancy does not take place. In mammals helow man, in their natural state, pregnancy does almost invariably take place at such times, hence this exalted activity of the mucous coat of the uterus, in preparation for the reception and nutrition of the ovum, is not often in vain. In the human subject the menses appear monthly ; pregnancy may or may not occur, and consequently there may be waste of na- ture's forces ; though there is a certain amount of evidence that menstruation does not wholly represent a loss; but that it is largely of that character among a certain class of women is only too evident. As can be readily understood, the catamenial flow may take place prior to, during, or after the rupture of the egg-capsule. As the uterus is well supplied with glands, during this period of increased functional activity of its lining membrane, mucus in considerable excess over the usual quantity is dis- charged ; and this phase of activity is continued for a time should pregnancy occur. All the parts of the generative organs are supplied with muscular tissue, and with nerves as well as blood-vessels, so that it is possible to understand how, by the influence of nerve- centers, the various events of ovulation, menstruation, and those that follow when pregnancy takes place, form a related series, very regular in their succession, though little prominent in the consciousness of the individual animal when normal. In all animals, without exception, the disturbance of the generative organs during the rutting season (oestrum) is accom- panied by unusual excitement and special alterations in the temper and disposition, while it may perhaps be said that the whole organism is correspondingly affected. The frequency of the season of heat or rutting is variable, as also its duration. In most of the domestic animals it lasts but a few days; though in the bitch it may be prolonged for a month. It is not common for conception to occur in the human sub- ject while the young one is being suckled, and the same remark applies to the domestic animals, though less so, and with con- siderable variation for different species. Naturally, the periods of oestrum will depend considerably on the occurrence of impregnation and the duration of gesta- tion. It is usual for the mare to be in season in spring and fall, THE DEVELOPMENT OP THE EMBRYO ITSELF. 123 but, of course, if impregnated in the spring, there will be no au- tumn oestrum on account of the prolonged period of gestation in this instance ; and, similarly, in the case of the cow and other animals. It is important to recognize that rutting is only the evidence of the maturation of the Graafian follicle within the ovary and of correlated changes. In a state of nature — i. e., in the case of wild animals — the male experiences a period of sexual excitement corresponding with an increased activity of the sexual organs and at periods answering to the rutting season of the female. In some species the testes descend into the scrotum only at this season. This may be observed in the rabbit. But in our domestic animals, as a class, the male, though capable of copulation at all times, is ex- cited only by the presence of a female in season. It is only at such periods that the approach of the male is permitted by the opposite sex. THE NUTRITION OF THE OVUM (OOSPERM). This will be best understood if it be remembered that the ovum is a cell, undifferentiated in most directions, and thus a sort of amoeboid organism. In the fowl it is known that the cells of the primitive germ devour, amoeba-like, the yelk-cells, while in the mammalian oviduct the ovum is surrounded by abundance of proteid, which is doubtless utilized in a somewhat similar fashion, as also in the uterus itself, until the embryonic membranes have formed. To speak of the ovum being nour- ished by diffusion, and especially by osmosis, is an unnecessary assumption, and, as we believe, at variance with fundamental principles; for we doubt much whether any vital process is one of pure osmosis. As soon as the yelk-sac and allantois have been formed, nutriment is derived in great part through the vessel-walls, which, it will be remembered, are differenti- ated from the cells of the mesoblast, and, it may well be as- sumed, have not at this early stage entirely lost their amoeboid character. The blood-vessels certainly have a respiratory func- tion, and suffice till the more complicated villi are formed. The latter are in the main similar in structure to the villi of the alimentary tract, and are adapted to being surrounded by sim- ilar structures of maternal origin. Both the maternal crypts and the foetal villi are, though complementary in shape, all but 124 COMPARATIVE PHYSIOLOGY. identical in minute structure in most instances. In each case the blood-vessels are covered superficially by cells which we can not help thinking are essential in nutrition. The villi are both nutritive and respiratory. It is no more difficult to under- stand their function than that of the cells of the endoderm of a polyp, or the epithelial coverings of lungs or gills. Experiment proves that there is a respiratory interchange of gases between the maternal and fcetal blood which nowhere mingle physically. The same law holds in the respiration of the foetus as in the mammals. Oxygen passes to the region where there is least of it, and likewise carbonic anhydride. If the mother be asphyxiated so is the foetus, and indeed more rapidly than if its own umbilical vessels be tied, for the mater- nal blood in the first instance abstracts the oxygen from that of the foetus when the tension of this gas becomes lower in the maternal than in the fcetal blood; the usual course of affairs is reversed, and the mother satisfies the oxygen hunger of her own blood and tissues by withdrawing that which she recently supplied to the foetus. It will be seen, then, that the embryo is from the first a parasite. This explains that exhaustion which pregnancy, and especially a series of gestations, entails. True, nature usually for the time meets the demand by an excess of nutritive energy : hence many animals are never so vigorous in appearance as when in this condition ; often, however, to be fol- lowed by corresponding emaciation and senescence. The full and frequent respirations, the bounding pulse, are succeeded by reverse conditions ; action and reaction are alike present in the animate and inanimate worlds. Moreover, it falls to the parent to eliminate not only the waste of its own organism but that of the foetus ; and not infrequently in the human subject the over- wrought excretory organs, especially the kidneys, fail, entailing disastrous consequences. The digestive functions of the embryo are naturally inact- ive, the blood being supplied with all its needful constituents through the placenta by a much shorter process ; indeed, the placental nutritive functions, so far as the foetus is concerned, may be compared with the removal of already digested ma- terial from the alimentary canal, though of course only in a general way. During fcetal life the digestive glands are developing, and at the time of birth all the digestive juices are secreted in an efficient condition, though only relatively so, necessitating a special liquid food (milk) in a form in which THE DEVELOPMENT OP THE EMBRYO ITSELF. 125 all the constituents of a normal diet are provided, easy of diges- tion. Bile, inspissated and mixed with the dead and cast-off epi- thelium of the alimentary tract, is abundant in the intestine at birth ; but bile is to be regarded perhaps rather in the light of an excretion than as a digestive fluid. The skin and kidneys, though not functionless, are rendered unnecessary in great part by the fact that waste can be and is withdrawn by the placenta, which proves to be a nutritive, respiratory, and excretory organ ; it is in itself a sort of abstract and brief chronicle of the whole physiological story of foetal life. All the foetal organs, especially the muscles, abound in an animal starch (glycogen), which in some way, not well under- stood, forms a reserve fund of nutritive energy which is pretty well used up in the earlier months of pregnancy. We may sup- pose that the amceboid cells — all the undifferentiated cells of the body — feed on it in primitive fashion; and it will not be forgotten that the older the cells become, the more do they de- part from the simpler habits of their earlier, cruder existence ; hence the disappearance of this substance in the later months of foetal life. In one respect the foetus closely resembles the adult : it draws the pabulum for all its various tissues from blood which it- self may, perhaps, be regarded as the first completed tissue. We are, accordingly, led to inquire how this river of life is distrib- uted ; in a word, into the nature of the foetal circulation. Foetal Circulation. — The blood leaves the placenta by the umbilical vein, reaches the inferior vena cava, either directly (by the ductus venosus), or, after first passing to the liver (by the venae advehentes, and returning by the venae, revehent.es), and proceeds, mingled with the blood returning from the lower extremities, to the right auricle. This blood, though far from being as arterial in character as the blood after bii'th, is the best that reaches the heart or any part of the organism. After arriv- ing at the right auricle.being dammed back by the Eustachian valve, it avoids the right ventricle, and shoots on into the left auricle, passing thence into the left ventricle, from which it is sent into the aorta, and is tben carried by the great trunks of this arch to the head and iipper extremities. The blood returning from these parts passes into the right auricle, then to the corre- sponding ventricle, and thence into the pulmonary artery; but, finding the branches of this vessel unopened, it takes the line of Pulmonary Art Foramen Ovale Eustachian Valve. Bight Auric. -Vent. Opening. fc\%l./i!. / *%. Bladder Pulmonary Art. Left Auricle. .Left Auric. ■ Vent. Opening, Hepatic Vein. Branches of the Umbilical Vein, . to the Liver. Ductus Venoms. Internal Iliac Arteries. Via. 13-1.— Diagram of the foetal circulation (Flint). THE DEVELOPMENT OP THE EMBRYO ITSELF. 127 least resistance through the ductus arteriosus into the aortic arch beyond the point "where its great branches emerge. It will be seen that the blood going to the head and upper parts of the body is greatly more valuable as nutritive pabulum than the rest, especially in the quantity of oxygen it contains ; that the blood of the foetus, at best, is relatively ill-supplied with its vital essen- tial ; and as a result we find the upper (anterior in quadrupeds) parts of the foetus best developed, and a decided resemblance be- tween the mammalian foetus functionally and the adult forms of reptiles and kindred groups of lower vertebrates. But this con- dition is well enough adapted to the general ends to be attained at this period — the nourishment of structures on the way to a higher path of progress. As embryonic maturity is being reached, preparation is made for a new form of existence ; so it is found that the Eustachian valve is less prominent and the foramen ovale smaller. PERIODS OF GESTATION. As a rule, the shorter the period of gestation the more nu- merous the offspring at a single birth and the greater the num- ber produced within the lifetime of the animal relatively to its duration. Thus, on account of the number of young produced by the rabbit at one birth, the short period of gestation, and the frequency with which impregnation occurs, there is a much larger number of progeny, short as is the animal's life usually, than in the case of the cow, for example, that may bear young for a much longer period. The following table gives approximately the duration of the period of gestation of some of our domestic animals and their wild allies : Guinea-pig (cavy) 3 weeks. Rabbit, squirrel, rat 4 " Ferret 6 " Cat S " Dog, fox 9 " Lion 4 months. Sow 4 Sheep, goat 5 " Bear 7 " Reindeer 8 " Cow, buffalo 10 " 128 COMPARATIVE PHYSIOLOGY. Mare, ass, zebra 11 months. Camel 12 " Giraffe 14 Elephant 22 to 25 " The period of gestation in the human subject is nine months; in the monkeys and apes somewhat less. The incubation pe- riod of certain of our domestic birds and related species is about as follows : Canary 11 days. Pigeon 18 " Hen 21 " Duck, goose, pea-hen 28 " Guinea-hen 25 Turkey : 28 '• It is interesting to note that the smaller varieties of fowls hatch out sooner than the larger ; and that the period of incu- bation of all of the above varies with the weather, the steadi- ness of the incubating bird, the date on which the eggs selected were laid, etc. With very recent eggs, an attentive sitter, and warm weather, the incubation period is shortened. PARTURITION. All the efforts that have hitherto been made to determine the exact cause of the result of that series of events which make up parturition have failed. This has probably been owing to an attempt at too simple a solution. The foetus lies surrounded (protected) by fluid contained in the amniotic sac. For its ex- pulsion there is required, on the one hand, a dilatation of the uterine opening (os uteri), and, on the other, an expulsive force. The latter is furnished by the contractions of the uterus itself, aided by the simultaneous action of the abdominal muscles. Throughout the greater part of gestation the uterus experiences somewhat rhythmical contractions, feeble as compared with the final ones which lead to expulsion of the foetus, but to be re- garded as of the same character. With the growth and func- tional development of other organs, the placenta becomes of less consequence, and a fatty degeneration sets in, most marked at the periphery, usually where it is thinnest and of least use. It does not seem rational to believe that the onset of labor is referable to any one cause, as has been so often taught ; but rather that it is the final issue to a series of processes long ex- THE DEVELOPMENT OP THE EMBRYO ITSELF. 129 isting and gradually, though at last rapidly, reaching that climax which seerus like a vital storm. The law of rhythm affects the nervous system as others, and upon this depends the direction and co-ordination of those many activities which make up parturition. We have seen that. throughout the whole of foetal life changes in one part are accompanied by correspond- ing changes in others ; and in the final chapter of this history it is not to be expected that this connection should be severed, though it is not at present possible to give the evolution of this process with any more than a general approach to probable correctness. CHANGES IN THE CIRCULATION AFTER BIRTH. When the new-born mammal takes the first breath, effected by the harmonious action of the respiratory muscles, excited to action by stimuli reaching them from the nerve-center (or centers) which preside over respiration, owing to its being roused into action by the lack of its accustomed supply of oxygen, the hitherto solid lungs are expanded ; the pulmonary vessels are rendered permeable, hence the blood now takes the path of least resistance along them, as it formerly did through the ductus arteriosus. The latter, from lack of use, atrophies in most instances. The blood, returning to the left auricle of the heart from the lungs in increased volume, so raises the pressure in this chamber that the stream that formerly flowed through the foramen ovale from the right auricle is opposed by a force equal to its own, if not greater, and hence passes by an easier route into the right ventricle. The fold that tends to close the foramen ovale grows gradually over the latter, so that it usually ceases to exist in a few days after birth. At birth, ligature of the umbilical cord cuts off the placental circulation ; hence the ductus venosus atrophies and becomes a mere ligament. The placenta, being now a foreign body in the uterus, is ex- pelled, and this organ, by the contractions of its walls, closes the ruptured and gaping vessels, thus providing against haemor- rhage. COITUS. In all the higher vertebrates congress of the sexes is essential to bring the male sexual product into contact with the ovum 9 130 COMPARATIVE PHYSIOLOGY. Erection of the penis results from the conveyance of an ex- cess of blood to the organ, owing- to dilatation of its arteries, and the retention of this blood within its caverns. Fig. 135.— Section of erectile tissue (Cadiat). a, trabecule of connective tissue, with elastic fibers, and bundles of plain muscular tissue (c); b. venous spaces (Schafer). The structure of the penis is peculiar, and, for the details of the anatomy of both the male and female generative organs, the student is referred to works on this subject ; suffice it to say that it consists of erectile tissue, the chief characteristic of which is the opening of the capillaries into cavernous venous spaces (sinuses) from which the veinlets arise ; with such an arrangement the circulation must be very slow — the inflow being greatly in excess of the outflow — apart altogether from the compressive action of certain muscles connected with the organ. The manner and duration of the act of copulation in the domestic animals varies with the structure of the penis, the animal's nervous excitability, etc. In the stallion the act is of moderate duration, the penis long, and the glans penis highly sensitive. In the bull the penis is of a different shape. During erec- tion it is believed that the S-shaped curve disappears, and that the extremity of the organ enters the mouth of the uterus itself. ( !<»l>ulation is of very brief duration. In the dog the penis is of very peculiar formation. Its an- THE DEVELOPMENT OF THE EMBRYO ITSELF. 131 terior part contains a bone, while there are two erectile portions independent of each other. During copulation the largest (pos- Fig. 136.— Section of parts of three seminiferous tubules of the rat (Schfifer). a, with the spermatozoa least advanced in development; b, more advanced; c. containing fully developed spermatozoa. Between the tubules are seen strands of interstitial cells, with blood-vessels and lymph-spaces. terior) erectile region is spasmodically (reflexly) grasped by the sphincter cunni of the female, which is the analogu e of the bulbo-cavernosus, ischio-cavernosus, and deep transverse mus- cle of the perinaeum, so that the penis can not be withdrawn till the erection subsides, an advantage, considering that the seminal vesicles are wanting in the dog, as well as Cowper's glands. In the cat tribe there is also an incomplete penial bone. The free poi*tion of the organ is provided with rigid papillae capable of erection during copulation. As previously explained, the spermatozoa originate in the seminal tubes, from which they find their "way to the seminal vesicles or receptacles for semen till required to be discharged. The spermatozoa as they mature are forced on by fresh addi- tions from behind and by the action of the ciliated cells of the epididymis, together with the wave-like (peristaltic) action of the vas deferens. Discharge of semen during coitus is effected by more vigorous peristaltic action of the vas deferens and the seminal vesicles, followed by a similar rhythmical action of the bulbo-cavernosus and ischio-cavernosus muscles, by which the fluid is forcibly ejaculated. 132 COMPARATIVE PHYSIOLOGY. PlG. 137. — Generative organs of the mare, isolated and partly opened (Chanvean). 1, 1. ovaries; 2, Si, Fallopian tubes; 3, pavilion of tube, external face; 4, the same, in- ner face, showing opening in the middle; 5, ligament of ovary; o, intact horn of uterus; ',', a born thrown open; 8, body of uterus, upper face; !), broad ligament; 10, cervix, with its mucous folds; 11, cul-de-sac of vagina with its folds of mucous membrane; 13, urinary meatus and its valve, 14; 15, mucous fold, a vestige of hy- men; 10, interior of vulva; 17, clitoris; 18, 18, labia of vulva; 19, inferior commis- sure of vulva. THE DEVELOPMENT OF THE EMBRYO ITSELF. 133 Semen itself, though composed essentially of spermatozoa, is mixed with the secretions of the vas deferens, of the seminal vesicles, of Cowper's glands, and of the prostate. Chemically it is neutral or alkaline in reaction, highly albuminous, and con- tains nuclein, lecithin, cholesterin, fats, and salts. The movements of the male cell, owing to the action of the tail (cilium), suffice of themselves to convey them to the ovi- ducts ; but there is little doubt that during or after sexual con- gress there is in the female, even in the human subject, at least Fig. 138 —Uterus and ovaries of the sow. semi-diagrammatic (after Dalton). 0, ovary; II, Fallopian tube; h, horn of the uterus; 5, body of the uterus; v, vagina. in many cases, a retrograde peristalsis of the uterus and ovi- ducts which would tend to overcome the results of the activity of the ciliated cells lining the oviduct. It is known that the male cell can survive in the female organs of generation for several days, a fact not difficult to understand, from the method of nutrition of the female cell (ovum) ; for we may suppose that both elements are not a little alike, as they are both slightly modified amoeboid organisms. Nervous Mechanism — Incidental reference has been made to the directing influence of the nervous system over the events of reproduction ; especially their subordination one to another to bring about the general result. These may now be consid- ered in greater detail. Most of the processes in which the nervous system takes part are of the nature of reflexes, or the result of the automa- ticity (independent action) of the nerve-centers, increased by some afferent (ingoing) impressions along a nerve-path. It is not always possible to estimate the exact share each factor takes, which must be highly variable. Certain experiments have assisted in making the matter clear. It has been found 134 COMPARATIVE PHYSIOLOGY. that if, in a female dog, the spinal cord he divided when the animal is still a puppy, menstruation and impregnation may occur. If the same experiment he performed on a male dog, erection of the penis and ejaculation of semen may he caused by stimulation of the penis. As the section of the cord has left the hinder part of the animal's hody severed from the brain, the creature is, of course, unconscious of anything happening in all the parts below the section, of whatever nature. If the nervi erigentes (from the lower part of the spinal cord) be stimulated, the penis is erected ; and if they be cut this act be- comes impossible, either reflexly by experiment or otherwise. Seminal emissions, it is well known, may occur during sleep, and may be associated, either as result or cause, with voluptu- ous dreams. Putting all these facts together, it seems reason- able to conclude that the lower part of the spinal cord contains the nervous machinery requisite to initiate those influences ("impulses) which, passing along the nerves to the generative organs, excite and regulate the processes which take place in them. In these, vascular changes, as we have seen, always play a prominent part. Usually we can recognize some afferent influence, either from the brain (psychical), from the surface — at all events from without that part of the nervous system (center) which functions directly in the various sexual processes. It is com- mon to speak of a number of sexual centers — as the erection center, the ejaculatory center, etc.— but we much doubt whether there is such sharp division of physiological labor as these terms imply, and they are liable to lead to misconception ; ac- cordingly, in the present state of our knowledge, we prefer to speak of the sexual center, using even that term in a somewhat broad sense. The effects of stimulation of the sexual organs are not con- fined to the parts themselves, but the ingoing impulses set up radiating outgoing ones, which affect widely remote areas of the body, as is evident, especially in the vascular changes:, the central current of nerve influence breaks up into many streams as a result of the rapid and extensive rise of the outflowing current, which breaks over ordinary barriers, and takes paths which are not properly its own. Bearing this fact in mind, the chemical composition of semen, so rich in proteid and other material valuable from a nutritive point of view, and consid- ering how the sexual appetites may engross the mind, it is not THE DEVELOPMENT OF THE EMBRYO ITSELF. 135 difficult to understand that nothing so quickly disorganizes the whole man, physical, mental, and moral, as sexual excesses, whether by the use of the organs in a natural way, or from masturbation. Nature has protected the lower animals by the strong bar- rier of instinct, so that habitual sexual excess is with them an impossibility, since the females do not permit of the approaches of the male except during the rutting period, which occurs only at stated, comparatively distant periods in most of the higher mammals. When man keeps his sexual functions in subjection to his higher nature, they likewise tend to advance his whole development. Summary. — Certain changes, commencing with the ripening of ova, followed by their discharge and conveyance into the uterus, accompanied by simultaneous and subsequent modifica- tions of the uterine mucous membrane, constitute, when preg- nancy occurs, an unbroken chain of biological events, though usually described separately for the sake of convenience. When impregnation does not result, there is a retrogression in the uterus (menstruation) and a return to general quiescence in all the reproductive organs. Parturition is to be regarded as the climax of a variety of rhythmic occurrences which have been gradually gathering head for a long period. The changes which take place in the placenta of a degenerative character fit it for being cast off, and may render this structure to some extent a foreign body before it and the foetus are finally expelled, so that these changes may constitute one of a number of exciting causes of the increased uterine action of parturition. But it is important to regard the whole of the occurrences of pregnancy as a connected series of processes co-ordinated by the central nervous system so as to accomplish one great end. the development of a new individual. The nutrition of the ovum in its earliest stages is effected by means in harmony with its nature as an amoeboid organism ; nutrition by the cells of blood-vessels is similar, while that by villi may be compared to what takes place through the agency of similar structures in the alimentary canal of the adult mammal. The circulation of the foetus puts it on a par physiologically with the lower vertebrates. Before birth there is a gradual though somewhat rapid preparation, resulting in changes which speedily culminate after birth on the establishment of the per- manent condition of the circulation of extra-uterine life. 136 COMPARATIVE PHYSIOLOGY. The blood of the foetus (as in the adult) is the great store- house of nutriment and the common receptacle of all waste products ; these latter are in the main transferred to the moth- er's blood indirectly in the placenta; in a similar way nutri- ment is imported from the mother's blood to that of the foetus. The placenta takes the place of digestive, respiratory, and excre- tory organs. Coitus is essential to bring the male and female elements together in the higher vertebrates. The erection of the penis is owing to vascular changes taking place in an organ composed of erectile tissue ; ejaculation of semen is the result of the peristaltic action of the various parts of the sexual tract, aided by rhythmical action of certain striped muscles. The sperma- tozoa, which are unicellular, flagellated (ciliated) cells, make up the essential part of semen ; though the latter is complicated by the addition of the secretions of several glands in connection with the seminal tract. Though competent by their own move- ments of reaching the ovum in the oviduct, it is probable that the uterus and oviduct experience peristaltic actions in a direc- tion toward the ovary, at least in a number of mammals. The lower part of the spinal cord is the seat in the higher mammals of a sexual center or collection of cells that l^eceives afferent impulses and sends out efferent impulses to the sexual organs. This, like all the lower centers, is under the control of the higher centers in the brain, so that its action may be either initiated or inhibited by the cerebrum. ORGANIC EVOLUTION RECONSIDERED. Admitting that the theories of the leading writers on the subject have advanced us on the way to more complete views of the mode of origin of the forms of the organic world, it must still be felt that all theories yet propounded fall short of being entirely satisfactory. It seems to us unfortunate that the sub- ject has not received more attention from physiologists, as without doubt, the final solution must come through that sci- ence which deals with the properties rather than the forms of protoplasm ; or, in other words, the fundamental principles underlying organic evolution are physiological. But, in the unraveling of a subject of such extreme complexity, all sciences must probably contribute their quota to make up the truth, as many rays of different colors compounded form white light. As with other theories of the inductive sciences, none can be more than temporary ; there must be constant modification to meet increasing knowledge. Conscious that any views we our- selves advance must sooner or later be modified as all others, even if acceptable now, we venture to lay before the reader the opinions we have formed upon this subject as the result of con- siderable thought. All vital phenomena may be regarded as the resultant of the action of external conditions and internal tendencies. Amid the constant change which like involves we recognize two things : the tendency to retain old modes of behavior, and the tendency to modification or variatiou. Since those impulses originally bestowed on matter when it became living, must, in order to prevail against the forces from without, which tend to destroy it, have considerable potency, the tendency to modifica- tion is naturally and necessarily less than to permanence of form and function. From these principles it follows that when an Amoeba or kindred organism divides after a longer or shorter period, it is 138 COMPARATIVE PHYSIOLOGY. not in reality the same in all respects as when its existence be- gan, though we ruay be quite unable to detect the changes ; and when two infusorians conjugate, the one brings to the other protoplasm different in molecular behavior, of necessity, from having had different experiences. We attach great importance to these principles, as they seem to us to lie at the root of the whole matter. What has been said of these lower but inde- pendent forms of life applies to the higher. All organisms are made up of cells or aggregations of cells and their products. For the present we may disregard the latter. When a muscle- cell by division gives rise to a new cell, the latter is not identi- cally the same in every particular as the .parent cell was origi- nally. It is what its parent has become by virtue of those ex- periences it has had as a muscle-cell per se, and as a member of a populous biological community, of the complexities of which we can scarcely conceive. Now, as a body at rest may remain so, or may move in a certain direction according to the forces acting upon it exactly counterbalance one another, or produce a resultant effect in the direction in which the body moves, so in the case of heredity, whether a certain quality in the parent appears in the offspring, depends on whether this quality is neutralized, augmented, or otherwise modified by any corresponding quality in the other parent, or by some opposite quality, taken in connection with the direct influence of the environment during development. This assumption explains among other things why acquired peculiarities (the results of accident, habit, etc.) may or may not be inherited. These are not usually inherited because, as is to be expected, those forces of the organism which have been gathering head for ages are naturally not easily turned aside. Again, we urge, heredity must be more pronounced than variation. The ovum and sperm-cell, like all other cells of the body, are microcosms representing the whole to a certain extent in themselves — that is to say, cell A is what it is by reason of what all the other millions of its fellows in the biological republic are ; so that it is possible to understand why sexual cells repre- sent, embody, and repeat the whole biological story, though it is not yet possible to indicate exactly how they more than others have this power. This falls under the laws of specialization and the physiological division of labor ; but along what paths they have reached this we can not determine. ORGANIC EVOLUTION RECONSIDERED. 139 Strong- evidence is furnished for the above views by the his- tory of disease. Scar-tissue, for example, continues to repro- duce itself as such ; like produces like, though in this instance the like is in the first instance a departure from the normal. Gout is well known to be a hereditary disease ; not only so, but it arises in the offspring at about the same age as in the parent, which is equivalent to saying that in the rhythmical life of certain cells a period is reached when they display the behavior, physiologically, of their parents. Yet gout is a disease that can be traced to peculiar habits of living and may be eventually escaped by radical changes in this respect — that is to say, the behavior of the cells leading to gout can be induced and can be altered ; gout is hereditary, yet eradicable. Just as gout may be set up by the formation of certain modes of action of the cells of the body, so may a mode of behavior, in the nervous system, for example, become organized or fixed, become a habit, and so be transmitted to offspring. It will pass to the descendants or not according to the principles already noticed. If so fixed in the individual in which it arises as to predominate over more ancient methods of cell behavior, and not neutralized by the strength of the normal physiological ac- tion of the corresponding parts in the other parent, it will reap- pear. We can never determine whether this is so or not before- hand ; hence the fact that it is impossible, especially in the case of man, whose vital processes are so modified by his psychic life, to predict whether acquired variations shall become heredi- tary ; hence also the irregulai'ity which characteiizes heredity in such cases ; they may reappear in offspring or they may not. In viewing heredity and modification it is impossible to get a true insight into the matter without taking into the account both the original natural tendencies of living matter and the influence of environment. We only know of vital manifes- tations in some environment ; and, so far as our experience goes, life is impossible apart from the reaction of surround- ings. With these general principles to guide us, we shall at- tempt a brief examination of the leading theories of organic evolution. First of all, Spencer seems to be correct in regarding evolu- tion as universal, and organic evolution but one part of a whole. No one who looks at the facts presented in every field of nature can doubt that struggle (opposition, action and reac- tion) is universal, and that in the organic world the fittest to a 140 COMPARATIVE PHYSIOLOGY. given environment survives. But Darwin has probably fixed his attention too closely on this principle and attempted to ex- plain too much by it, as well as failed to see that there are other deeper facts underlying- it. Variation, which this author scarcely attempted to explain, seems to us to be the natural re- sult of the very conditions under which living things have an existence. Stable equilibrium is an idea incompatible with our fundamental conceptions of life. Altered function implies al- tered molecular action, which sometimes leads to appreciable structural change. From our conceptions of the nature of liv- ing matter, it naturally follows that variation should be great- est, as has been observed, under the greatest alteration in the surroundings. We are but very imperfectly acquainted as yet with the conditions under which life existed in the earlier epochs of the earth's history. Of late, deep-sea soundings and arctic explo- rations have brought surprising facts to light, showing that living matter can exist under a greater variety of conditions than was previously supposed. Thus it turns out that light is not an essential for life everywhere. We think these recent revelations of unexpected facts should make us cautious in assuming that life always manifested itself under conditions closely similar to those we know. Variation may at one period have been more sudden and marked than Darwin supposes; and there does seem to be rooni for such a conception as the "extraordinary births'" of Mivart implies; though we would not have it understood that we think Darwin's view of slow modification inadequate to produce a new species, we simply venture to think that he was not justified in insisting so strongly that this was the only method of Nature; or, to put it more justly for the great author of the Origin of Species, with the facts that have accumulated since his time he would scarcely be warranted in maintaining so rigidly his conviction that new forms arose almost exclusively by the slow process he has so ably described. We must allow a great deal to use and effort, doubtless, and they explain the origin of variations up to a certain point, but the solution is only partial. Variations must arise as we have attempted to explain, and use and disuse are only two of the factors amid many. Correlated growth, or the changes in one part induced by changes in another, is a principle which, though recognized by Darwin, Cope, and others, has not, we ORGANIC EVOLUTION RECONSIDERED. 141 think, received the attention it deserves. To the mind of the physiologist, all changes must be correlated -with others. In what sense has the line that evolution has taken been predetermined ? In the sense that all things in the universe are unstable, are undergoing change, leading to new forms and qualities of such a character that they result in a gradual prog- ress toward what our minds can not but consider higher mani- festations of being. The secondary methods according to which this takes place constitute the laws of nature, and as w y e learn from the prog- ress of science are very numerous. The unity of nature is a reality toward which our conceptions are constantly leading vis. Evolution is a necessity of living matter (indeed, all matter) as we view it. THE CHEMICAL CONSTITUTION OF THE ANIMAL BODY. One visiting the ruins of a vast and elaborate building-, which had been entirely pulled to pieces, would get an amount of information relative to the original structure and uses of the various parts of the edifice largely in proportion to his fa- miliarity with architecture and the various trades which make that art a practical success. The study of the chemistry of the animal body is illustrated by such a case. Any attempt to determine the exact chemical composition of living matter must result in its destruction ; and the amount of information conveyed by the examination of the chemical ruins, so to speak, will depend a great deal on the knowledge already possessed of chemical and vital processes. It is in all probability true that the nature of any vital pro- cess is at all events closely bound up with the chemical changes involved ; but we must not go too far in this direction. We are not yet prepared to say that life is only the manifestation of certain chemical and physical processes, meaning thereby such chemistry and physics as are known to us ; nor are we prepared to go the length of those who regard life as but the equivalent of some other force or forces ; as electricity may be considered as the transformed representative of so much heat and vice versa. It may be so, but we do not consider that this view is warranted in the present state of our knowledge. On the other hand, vital phenomena, when our investiga- tions are pushed far enough, always seem to be closely asso- ciated with chemical action : hence the importance to the stu- dent of physiology of a sound knowledge of chemical princi- ples. We think the most satisfactory method of studying the functions of an organ will be found to be that which takes into consideration the totality of the operations of which it is the seat, together with its structure and chemical composition ; CHEMICAL CONSTITUTION OP THE ANIMAL BODY. 143 hence we shall treat chemical details in the chapters devoted to special physiology, and here give only such an outline as will bring before the view the chemical composition of the body in its main outlines ; and even many of these will gather a signifi- cance, as the study of physiology progresses, that they can not possibly have at the present. Fewer than one third of the chemical elements enter into the composition of the mammalian body ; in fact, the great bulk of the organism is composed of carbon, hydrogen, nitro- gen, and oxygen ; sodium, potassium, magnesium, calcium, sulphur, phosphorus, chlorine, iron, fluorine, silicou, though occurring in very small quantity, seem to be indispensable to the living body ; while certain others are evidently only pres- ent as foreign bodies or impurities to be thrown out sooner or later. It need scarcely be said that the elements do not occur as such in the living body, but in combination form- ing salts, which latter are usually united with albuminous compounds. As previously mentioned, the various parts which make up the entire body of an animal are composed of living matter in very different degrees ; hence we find in such parts as the bones abundance of salts, relative to the proportion of proteid matter; a condition demanded by that rigidity without which an internal skeleton would be useless, a defect well illus- trated by that disease of the bones known as rickets, in which the lime-salts are insufficient. It is manifest that there may be a very great variety of classifications of the compounds found in the animal body according as we regard it from a chemical, physical, or physiological point of view, or combine many aspects in one whole. The latter is, of course, the most correct and profitable method, and as such is impossible at this stage of the student's progress ; we shall simply present him with the following outline, which will be found both simple and com- prehensive. * CHEMICAL CONSTITUTION OF THE BODY. Such food as supplies energy directly must contain carbon compounds. Living matter or protoplasm always contains nitrogenous carbon compounds. * Taken from the author's Outlines of Lectures on Physiology, W. Drys- dale & Co., Montreal. 144 COMPARATIVE PHYSIOLOGY. In consequence, C, H, O, N, are the elements found in great- est abundance in the body. The elements S and P are associated with the nitrogenous carbon compounds ; they also form metallic sulphates and phos- phates. CI and F form salts with the alkaline metals Na, K, and the earthy metals Ca and Mg. Fe is found in hcemoglobin and its derivatives. Protoplasm, when submitted to chemical examination, is killed. It is then found to consist of proteids, fats, carbohy- drates, salines, and extractives. It is probable that when living it has a very complex mole- cule consisting of C, H, O, N, S, and P chiefly. Proximate Principles. (a) Nitrogenous. -j Certain crystalliue bodies> (b) Non-nitrogenous, j ^btfhydrates. ~ T ( Mineral salts. 2. Inorganic, j Water _ Salts. — In general, the salts of sodium are more characteris- tic of animal tissues and those of potassium of vegetable tissues. Na CI is more abundant in the fluids of animals ; K and phosphates more abundant in the tissues. ■ Earthy salts are most abundant in the harder tissues. The salts are probably not much, if at all, changed in their passage through the body. In some cases there is a change from acid to neutral or alkaline. The salts are essential to preserve the balance of the nutritive processes. Their absence leads to disease, e. g., scurvy. GENERAL CHARACTERISTICS OF PROTEIDS. They are the chief constituents of most living tissues, includ- ing blood and lymph. The molecule consists of a great number of atoms (complex constitution), and is formed of the elements C, !!,• N, 0, S, and P. All proteids are amorphous. All are non-diffusible, the peptones excepted. They are soluble in strong acids and alkalies, with change of properties or constitution. In general, they are coagulated by alcohol, ether, and heating. CHEMICAL CONSTITUTION OF THE ANIMAL BODY. 145 Coagulated proteids are soluble only in strong acids and alkalies. Classification and Distinguishing Characters of Proteids. 1. Native albumins : Serum albumin ; egg albumin ; solu- ble in water. 2. Derived albumins (albuminates) : Acid and alkali albu- min ; casein ; soluble in dilute acids and alkalies, insoluble in water. Not precipitated by boiling. 3. Globulins: Globulin (globin) ; paraglobulin ] myosin; fibrinogen. Soluble in dilute saline solutions, and precipitated by stronger saline solutions. 4. Peptones : Soluble in water ; diffusible through anima membranes; not precipitated by acids, alkalies, or heat. De- rived from the digestion (peptic, pancreatic) of all proteids. 5. Fibrin : Insoluble in water and dilute saline solutions. Soluble, but not readily, in strong saline solutions and in dilute acids and alkalies. CERTAIN NON-CRYSTALLINE BODIES. The following bodies are allied to proteids, but are not the equivalents of the latter in the food. They are all composed of C, H, N, O. Chondrin, gelatin, keratin have, in addition, S. Chondrin : The organic basis of cartilage. Its solutions set into a firm jelly on cooling. Gelatin : The organic basis of bone, teeth, tendon, etc. Its solutions set (glue) on cooling. Elastin : The basis of elastic tissue. Its solutions do not set jelly-like (gelatinize). Mucin : From the secretion of mucous membranes ; precipi- tated by acetic acid, and insoluble in excess. Keratin : Derived from hair, nails, epidermis, horn, feathers. Highly insoluble. Nuclein: Derived from the nuclei of cells. Not digested by pepsin ; contains P but no S. THE FATS. The fats are hydrocarbons ; are less oxidized than the carbo- hydrates ; are inflammable ; possess latent energy in a high degree. Chemically, the neutral fats are glycerides or ethers of the 10 146 COMPARATIVE PHYSIOLOGY. fatty acids, i. e., the acid radicles of the fatty acids of the oleic and acetic series replace the exchangeable atoms of H in the triatomic alcohol glycerine, e. g. : Glycerine. Palmitic acid. Glycerine tripalmitate or palmitin. i OH HO.OC.Ci6H 3l l O.CO.C 16 H 31 C 3 H 6 \ OH + HO.OC.C15H31 = C3H5 \ O.CO.C 15 H 31 + 3H 2 ( OH HO.OC.Ci & H 31 ( O.CO.Ci 5 H 31 A soap is formed by the action of caustic alkalies on fats, e. g. : Tripalmitin. Potassium palmitate. (c,?h% \ °> + 3 < K0H > = 3 j (g'* Hl,0 )o f + C, I; f o, The soap may be decomposed by a strong acid into a fatty acid and a salt, e. g. : ClHslCCK + HC1 = Ci 6 H,i.CO s H + KC1. Potassium palmitate. Palmitic acid. The fats are insoluble in water, but soluble in hot alcohol, ether, chloroform, etc. The alkaline soaps are soluble in water. Most animal fats are mixtures of several kinds in varying proportions ; hence the melting-point for the fat of each species of animal is different. PECULIAR FATS. Lecithin, Protagon, Cerebrin : They consist of C, H, N, O, and the first two of P in addi- tion. They occur in the nervous tissues. CARBOHYDRATES. General formula, C m (H 2 0)„. 1. The Sugars : Dextrose, or grape-sugar, C H 12 O 6 readily undergoes alcoholic fermentation ; less readily lactic fermen- tation. Lactose, milk-sugar, Ci 2 H M On ; susceptible of the lactic acid fermentation. Inosit, or muscle-sugar, C H,nO ; capable of the lactic fer- mentation. Maltose, C 12 H M 0„, capable of the alcoholic fermentation. The chief sugar of the digestive process. All the above are much less sweet and soluble than ordinary cane-sugar. CHEMICAL CONSTITUTION OF THE ANIMAL BODY. 147 2. The Starches : Glycogen, C c H 10 O 5 , convertible into dex- trose. Occurs abundantly in many foetal tissues and in the liver, especially of the adult animal. Dextrin, C 6 Hjo0 6 , convertible into dextrose. Soluble in water ; intermediate between starch and dextrose ; a product of digestion. Pathological : Grape-sugar occurs in the urine in diabetes mellitus. Certain substances formed within the body may be regarded as chiefly waste-products, the result of metabolism or tissue- changes. They are divisible into nitrogenous metabolites and non- nitrogenous metabolites. Nitrogenous Metabolites. 1. Urea, uric acid and compounds, kreatinin, xanthin, hypo- xanthin (sarkin), hippuric acid, all occuring in urine. 2. Leucin, tyrosin, taurocholic, and glycocholic acids, which occur in the digestive tract. 3. Kreatin, constantly found in muscle, and a few others of less constant occurrence. The above consists of C, H, N, O. Taurocholic acid contains also S. The molecule in most instances is complex. Non-Nitrogenous Metabolites. These occur in small quantity, and some of them are secreted in an altered form. They included lactic and sarcolactic acid, oxalic acid, succinic acid, etc. PHYSIOLOGICAL RESEAKCH AND PHYSIOLOGICAL SEASONING. We propose in this chapter to examine into the methods employed in physiologcial investigation and teaching, and the character of conclusions arrived at hy physiologists as depend- ent on a certain method of reasoning. The first step toward a legitimate conclusion in any one of the inductive sciences to which physiology belongs is the col- lection of facts which are to constitute the foundation on which the inference is to be based. If there be any error in these, a correct conclusion can not be drawn by any reliable logical process. On the other hand, facts may abound in thou- sands and yet the correct conclusion never be reached, because the method of interpretation is faulty, which is equivalent to saying that the process of inference is either incomplete or in- correct. The conclusions of the ancients in regard to nature were usually faulty from errors in both these directions ; they neither had the requisite facts, nor did they correctly interpret those with which they were conversant. Let us first examine into the methods employed by modern physiologists, and determine in how far they are reliable. First, there is the method of direct observation, in which no appara- tus whatever or only the simplest kind is employed ; thus, the student may count his own respirations, feel his own heart- beats, count his pulse, and do a very great deal more that will be pointed out hereafter; or he may examine in like manner an- other fellow-being or one of the lower animals. This method is simple, easy of application, and is that usually employed by the physician even at the present day, especially in private practice. The value of the results obviously depends on the reliability of the observer in two respects : First, as to the ac- curacy, extent, and delicacy of his perceptions ; and, secondly, on the inferences based on these sense-observations. Much PHYSIOLOGICAL RESEARCH AND REASONING. 149 must depend on practice — that is to say, the education of the senses. The hand may become a most delicate instrument of observation ; the eye may learn to see what it once could not ; the ear to detect and discriminate what is quite beyond the uncultured hearing of the many. But it is one of the most convincing evidences of man's superiority that in every field of observation he has risen above the lower animals, some of which by their unaided senses naturally excel him. So in this science, instruments have opened up mines of facts that must have other- wise remained hidden ; they have, as it were, provided man with additional senses, so much have the natural powers of those he already possessed been sharpened. But the chief value of the results reached by instruments * consists in the fact that the movements of the living tissues can be registered ; i. e., the great characteristic of modern physiol- ogy is the extensive employment of the graphic method, which has been most largely developed by the distinguished French experimenter Marey. Usually the movements of the point of lever are impressed on a smoked surface, either of glazed paper or glass, and rendered permanent by a coating of some material applied in solution and drying quickly, as shellac in alcohol. The surface on which the tracing is written may be stationary, though this is rarely the case, as the object is to get a succession of records for comparison ; hence the most used form of writing surface is a cylinder which may be raised or lowered, and which is moved around regularly by some sort of clock-work. It follows that the lever point, which is moved by the physiological effect, describes curves of varying complexity. That tracings of this or any other character should be of any value for the purposes of physiology, they must be susceptible of relative measurement both for time and space. This can be accomplished only when there is a known base-line or abscissa from which the lever begins its rise, and a time record which is usually in seconds or portions of a second. The first is easily obtained by simply allowing the lever to write a straight line before the physiological effect proper is recorded. Time inter- vals are usually indicated by the interruptions of an electric current, or by the vibrations of a tuning-fork, a pen or writer of some kind being in each instance attached to the apparatus so as to record its movements. * Illustrated in the sections on muscle physiology and others. 150 COMPARATIVE PHYSIOLOGY. As levers, in proportion to their length, exaggerate all the movements imparted to them, a constant process of correction must be carried on in the mind in reading the records of the graphic method, as in interpreting the field of view presented by the microscope. The student is epecially warned to carry on this process, otherwise highly distorted views of the reality will become fixed in his own mind ; and certainly a condition of ignorance is to be preferred to such false knowledge as this may become. But it is likewise apparent that movements that would without such mechanism be quite unrecognized may be rendered visible and utilized for inference. There is another source of possible misconception in the use of the graphic method. The lever is sometimes used to record the movements of a column of fluid (manometer, Fig. 197), as water or mercury, the inertia of which is considerable, so that the record is not that of the lever as affected by the physiological (tissue) movement, but that move- ment conveyed through a fluid of the kind indicated. Again, all points, however delicate, write with some friction, and the question always arises, In how far is that friction sufficient to be a source of inaccuracy in the record ? When organs are di- rectly connected with levers or apparatus in mechanical rela- tion with them, one must be sure that the natural action of the organ under investigation is in no way modified by this con- nection. From these remarks it will be obvious that in the graphic method physiologists possess a means of investigation at once valuable and liable to mislead. Already electricity has been extensively used in the researches of physiologists, and it is to this and the employment of photography that we look in the near future for methods that are less open to the objections we have noticed. However important the methods of physiology, the results are vastly more so. We next notice, then, the progress from methods and observations to inferences, which we shall en- deavor to make clear by certain cases of a hypothetical charac- ter. Proceeding from the brain and entering the substance of the heart, there is in vertebrates a nerve known as the vagus. Suppose that, on stimulating this nerve by electricity in a rab- bit, the heart ceases to beat, what is the legitimate inference ? Apparently that the effect has been due to the action of the nerve on the heart, an action excited by the use of electricity. PHYSIOLOGICAL RESEARCH AND REASONING. 151 This does not, however, according to the principles of a rigid logic, follow. The heart may have ceased beating from some cause wholly unconnected with this experiment, or from the electric current escaping along the nerve and affecting some nervous mechanism within the heart, which is not a part of the vagus nerve ; or it may have been due to the action of the cur- rent on the muscular tissue of the heart directly, or in some other way. But suppose that invariably, whenever this experiment is repeated, the one result (arrest of the beat) follows, then it is clear that the vagus nerve is in some way a factor in the causa- tion. Now, if it could be ascertained that certain branches of the nerve were distributed to the heart-muscle directly, and that stimulation of these gave rise to arrest of the cardiac pulsation, then would it be highly probable, though not certain, that there was in the first instance no intermediate mechanism ; while this inference would become still more probable if in hearts totally without any such nervous apparatus whatever, such a result followed on stimulation of the vagus. Suppose, further, that the application of some drug or poison to the heart pro- vided with special nervous elements besides the vagus termi- nals prevented the effect before noticed on stimulating the vagus, while a like result followed under similar circumstances in those forms of heart unprovided with such nervous struct- ures, there would be additional evidence in favor of the view that the result we are considering was due solely to some action of the vagus nerve ; while, if arrest of the heart followed in the first case but not in the second, and this result were invariable, there would be roused the suspicion that the action of the vagus was not direct, but through the nervous structures with- in the heart other than vagus endings. And if, again, there were a portion of the rabbit's heart to which there were distributed this intrinsic nervous supply, which on stimulation directly was arrested in its pulsation, it would be still more probable that the effect in the first instance we have considered was clue to these structures, and only indirectly to the vagus. But be it observed, in all these cases there is only probability. The con- clusions of physiology never rise above probability, though this may be so strong as to be practically equal in value to absolute certainty. Would it be correct, from any or all the experi- ments we have supposed to have been made, to assert that the vagus was the arresting (inhibitory) nerve of the heart ? All hearts thus far examined have much in common in structure 152 COMPARATIVE PHYSIOLOGY. and function, and in so far is the above generalization probable. Such a statement would, however, be far from that degree of probability which is possible, and should therefore not be ac- cepted till more evidence has been gathered. The mere resem- blance in form and general function does not suffice to meet the demands of a critical logic. Such a statement as the above would not necessarily apply to the hearts of all vertebrates or even all rabbits, if the experiments had been conducted on one animal alone, for the result might be owing to a mere idiosyncrasy of the rabbit under observation. The further we depart from the group of animals to which the creature under experiment be- longs, the less is the probability that our generalizations for the one class will apply to another. It will, therefore, be seen that wide generalizations can not be made with that amount of certainty which is attainable until experiments shall have be- come very numerous and widely extended. A really broad and sound j)hysiology can only be constructed when this science has become much more comparative — that is, extended to many more groups and sub-groups of animals than at present. We have incidently alluded throughout the work to the teaching of disease. " Disease " is but a name for disordered function. One viewing a piece of machinery for the first time in improper action might draw conclusions with comparative safety, provided he had a knowledge of the correct action of similar machines. Our experience gives us a certain knowl- edge of the functions of our own bodies. By ordinary observa- tion and by experiment on other animals we get additional data, which, taken with the disordered action resulting from gross or molecular injury (disease), gives a basis for certain conclusions as to the normal functions of the human body or those of lower animals. This information is especially valu- able in the case of man, since he can report with a fair de- gree of reliability, in most diseased conditions, his own sensa- tions. It is hoped that this brief treatment of the methods and logic of physiology will suffice for the present. Throughout the work they will be illustrated in every chapter, though not always with distinct references to the nature of the intellectual process followed. Summary. — There are two methods of physiological observa- tion, the direct and the indirect. The first is the simplest, and is valuable in proportion to the accuracy and delicacy and PHYSIOLOGICAL RESEARCH AND REASONING. 153 range of the observer ; the latter implies the use of apparatus, and is more complex, more extended, more delicate, and precise. It is usually employed with the graphic method, which has the advantage of recording and thus preserving movements which correspond with more or less exactness to the movements of tissues or organs. It is valuable, but liable to errors in record- ing and in interpretation. The logic of physiology is that of the inductive sciences. It proceeds from the special to the general. The conclusions of physiology never pass beyond extreme probability, which, in some cases, is practically equal to certainty. It is especially important not to make generalizations that are too wide. THE BLOOD. It is a matter of common observation that the loss of the whole, or a very large part, of the blood of the body entails death ; while an abundant haemorrhage, or blood-disease in any of its forms, causes great general weakness. The student of embryology is led to inquire as to the neces- sity for the very early appearance and the rapid development of the blood- vascular system so prominent in all vertebrates. An examination of the means of transit of the blood, as already intimated, reveals a complicated system of tubes dis- tributed to every organ and tissue of the body. These facts would lead one to suppose that the blood must have a tran- scendent importance in the economy, and such, upon the most minute investigation, proves to be the case. The blood has been aptly compared to an internal world for the tissues, an- swering to the external world for the organism as a whole. This fluid is the great storehouse containing all that the most exacting cell can demand ; and, further, is the temporary re- ceptacle of all the waste that the most busy cell requires to dis- charge. Should such a life-stream cease to flow, the whole vital machinery must stop — death must ensue. Comparative. — It will prove more scientific and generally satisfactory to regard the blood as a tissue having a fluid and flowing matrix, in which flow cellular elements or corpuscles — a view of the subject that is less startling when it is remem- bered that the greater part of the protoplasm which makes up the other tissues of the body is of a semifluid consistence. In all animals possessing blood, the matrix is a clear, usually more or less colored fluid. Among invertebrates the color may be pronounced : thus, in cephalopods and some crustaceans it is blue, but in most groups of animals and all vertebrates the matrix is either colorless or more commonly of some slight tinge of yellow. Invertebrates with few exceptions possess THE BLOOD. 155 only colorless corpuscles, but all vertebrates have colored cells which invariably outnumber the other variety, and display forms and sizes which are sufficient- ly constant to be characteristic. In all groups below mam- mals the colored cor- puscles are oval, mostly biconvex, and nucleated dur- ing all periods of the animal's existence ; in mammals they are circular biconcave disks (except in the camel tribe, the cor- puscles of which are oval), and in post- embryonic life with- out a nucleus ; nor do they possess a cell-wall. The red cells vary in size in different groups and sub-groups of animals, being smaller the higher the place the animal occupies, as a general rule ; thus, they are very large in verte- brates below mammals, in some cases being al- most visible to the un- aided eye, while in the whole class of mam- mals they are very mi- nute ; their numbers also in this group are vastly greater than in others lower in the scale. The average size in man is ^5 inch ('0077 mm.) and the nnmber in a cubic millimetre Fig. 139. — Leucocytes of human blood, showing amoe- boid movements (Landois). These movements are not normally in the blood-vessels so marked as pic- tured here, so that the figure represents an extreme case. Fia. 140.- -Photograph of colored corpuscles of frog. 1 x"370. (After Flint.). 156 COMPARATIVE PHYSIOLOGY. of the blood about 5,000,000 for the male and 500,000 less for the female, which would furnish about 250,000,000,000 in a pound of blood. It will be understood that averages only are spoken of, as all kinds of variations occur, some of which will be referred to later, and their significance explained. The size of the corpuscles in the domestic animals is variable — a matter of importance when transfusion of blood is under consideration. Under the microscope the blood of vertebrates is seen to owe its color to the cells chiefly, and, so far as the red goes, almost wholly. Corpuscles when seen singly are never of the deep red, however, of the blood as a whole, but rather a yellowish red, the tinge varying some- what with the class of animals from which the specimen has been taken. Certain other mor- phological elements found in mammalian blood deserve brief mention, though their significance is as yet a matter of much dis- pute. 1. The blood-plates (plaques, hcematoblasts, third element), very small, colorless, biconcave disks, which are deposited in great numbers on any thread or similar foreign body introduced into the circulation, and rapidly break up when blood is shed. 2. On a slide of blood that has been prepared for some little time, aggregations of very minute granules (elementary gran- ules) may be seen. These are supposed to represent the disin- tegrating protoplasm of the corpuscles. The pale or colorless corpuscles are very few in number in mammals compared with the red, there being on the average only about 1 in 400 to GOO, though they become much more numerous after a meal. They are granular in appearance, and possess one or more nuclei, which are not, however, readily Fig. 141.— Corpuscles from human subject (Pnnke). A few colorless corpuscles are seen among the col- ored disks, which are many of them arranged in rouleaux. THE BLOOD. 157 seen in all cases without the use of reagents. They are charac- terized by greater size, a globular form, the lack of pigment, © s Fia. 142.— Blood-plaques and their derivatives (Landois, after Bizzozero and Laker). 1, red blood-corpuscles on the flat; 2, from the side; 3, unchanged blood-plaques; 4, lymph-corpuscle surrounded with blood-plaques; 5, blood-plaques variously altered; 6, lymph-corpuscle with two masses of fused blood-plaques and threads of fibrin; 7, group of blood-plaques fused or run together; 8, similar small mass of partially dissolved blood-plaques with fibrils of fibrin. and the tendency to amoeboid movements, which latter may be exaggerated in disordered conditions of the blood, or when the blood is withdrawn and observed under artificial conditions. It will be understood that these cells (leucocytes) are not con- fined to the blood, but abound in lymph and other fluids. They are the representatives of the primitive cells of the em- bryo, as is shown by their tendency (like ova) to throw out processes, develop into higher forms, etc. In behavior they strongly suggest Amoeba and kindred forms. We may, then, say that in all invertebrates the blood, when it exists, consists of a plasma (liquor sanguinis), in which float the cellular elements which are colorless ; and that in verte- brates in addition there are colored cells which are always nu- cleated at some period of their existence. The colorless cells are globular masses of protoplasm, containing one or more nuclei, and with the general character of amoeboid organisms. 158 COMPARATIVE PHYSIOLOGY. bx: The History of the Blood-Cells. We have already seen that the blood and the vessels in which it flows have a common origin in the mesoblastic cells of the embryo chick ; the same applies to mammals and lower groups. The main facts may be grouped under two head- ings : 1. Development of tbe blood-corpuscles during embry- onic life. 2. Development of the corpuscles in post-embryonic life. The origin and fate of the corpuscles, especially of the colored variety, have been the subject of much discussion. The best established facts are stated in the summary below, while they are illustrated by the accompanying fig- ures. The colorless cells of the blood first arise as migrated uu differen- tiated remnants of the eai'ly embryonic cell colonies. That they re- main such is seen by their physiological be- havior, to be considered a little later. Afterward they are chiefly pro- duced from a peculiar form of connective tis- sue known as leucocy- tenic, and which is gathered into organs (lymphatic glands), the chief function of which is to produce these cells, though this tissue is rather widely distributed in the mammalian body in other forms than these. Summary. — The student may, with considerable certainty, consider the colorless corpuscle of the blood as the most primi- tive; the red, derived either from the white or some form of more specialized cell ; the nucleated, as the earlier and more youthful form of the colored corpuscle, which may in some groups of vertebrates be replaced by a more specialized (or de- Fig. 143.— Surface view from below of a small por- tion of posterior end of pellucid area of a cliick of thirty-six hours, 1 x 400 (Foster and Balfour). b. c, blood-corpuscles ; a, nuclei, which subse- quently become nuclei of cells forming walls of blood-vessels ; p. pr, protoplasmic processes, containing nuclei with large nucleoli, n. THE BLOOD. 159 graded ?) non-nucleated form mostly derived directly from the former ; that in the first instance the blood-vessels and blood © © | Fig. 144. Fig. 145. Fig. 146. a Fig. 147. Fig. 148. Fig. 144.— Cell elements of red marrow, a, large granular marrow cells; b, smaller, more vesicular cells; c. free nuclei, or small lymphoid cells, some of which may be even surrounded with a delicate rim of protoplasm; d, nucleated red corpuscles of the bone marrow. Fig. 145. — Nucleated red cells of marrow, illustrating mode of development into the ordinary non-nucleated red corpuscles, a. common forms of the colored nucleated cells of red marrow; b, 1. 2, 3. gradual disappearance of the nucleus; c, large non- nucleated red corpuscle resembling 2 and 3 of b in all respects save in the absence of any trace of nucleus. Fig. 146.— Nucleated red corpuscles, illustrating the migration of the nucleus from the cell, a process not unfrequently seen in the red marrow. Fig. 147.— Blood of human embryo of four months. «, 1, 2, 3. 4, nucleated red corpus- cles. In 4 the same granular disintegrated appearance of the nucleus as is noted in marrow cells, b. 1. microcyte; 2, megalocyte; 3, ordinary red corpuscle. Fig. 148.— From spleen. 1, blood-plaques, colorless and varying a little in size; 2, two microcytcs of a deep-red color; 3. two ordinary red corpuscles; 4, a solid, translu- cent, lymphoid cell or free nucleus. (Figs. 144-148 after Osier.) arise simultaneously in the mesohlastic embryonic tissue ; that such an organ may exist after birth, either normally in some mammals or under unusual functional need ; that the red mar- row is the chief birthplace of colored cells in adult life ; that 160 COMPARATIVE PHYSIOLOGY. the spleen, liver, lymphatic glands, and other tissues of similar structure contribute in a less degree to the development of the red corpuscles ; and that the last mentioned organs are the chief producers of the colorless amoeboid blood-cells. Finally, it is well to remember that Nature's resources in this, as in many other cases, are numerous, and that her mode of procedure is not invariable ; and that, if one road to an end is blocked, another is taken. The Decline and Death of the Blood-Cells.— The blood cor- puscles, like other cells, have a limited duration, with the usual chapters in a biological history of rise, maturity, and decay. There is reason to believe that the red cells do not live longer than a few weeks at most. The red cells, in various degrees of disorganization, have been seen within the white cells {phagocy- tes), and the related cells of the spleen, liver, bone-marrow, etc. In fact, these cells, by virtue of retained ancestral {amoeboid) qualities, have devoured the weakened, dying red cells. It seems to be a case of survival of the fittest. It is further known that abundance of pigment containing iron is found in both spleen and liver ; and there seems to be no good reason for doubting that the various pigments of the secretions of the body {urine, bile, etc.) are derived from the universal pigment of the blood. These coloring matters, then, are to be regarded as the excreta in the first instance of cells behaving like amceboids, and later as the elaborations of certain others in the kidney and else- where, the special function of which is to get rid of waste prod- ucts. The birth-rate and the death-rate of the blood-cells must be in close relation to each other in health ; and some of the gravest disturbances arise from decided changes in the normal proportions of the cells {anaimia, leucocyt hernia). Both the red and white corpuscles show, like all other cells of the organism, alterations corresponding to changes in the surrounding conditions. The blood may be withdrawn and its cells more readily observed than those of most tissues ; so that the study of the influence of temperature, feeding of the leuco- cytes, and the action of reagents in both classes of cells is both of practical importance and theoretic interest, and will well re- pay the student for the outlay in time and labor, if attention is directed chiefly to the results and the lessons they convey, and not, as too commonly happens, principally to the methods of manipulation. The Chemical Composition of the Blood,— Blood has a decided THE BLOOD. 161 but faint alkaline reaction, owing 1 chiefly to the presence of sodium salts, a saline taste, and a faint odor characteristic of the animal group to which it belongs, owing probably to volatile fatty acids. The specific gravity of human blood varies between 1045 and 1075, with a mean of 1055 ; the specific gravity of the corpuscles being about 1105 and of the plasma 1027. This dif- ference explains the sinking of the corpuscles in blood with- drawn from the vessels and kept quiet. Much the same diffi- culties are encountered in attempts at the exact determination of the chemical composition of the blood, as in the case of other living tissues. Plasma alters its physical and its chemical com- position, to what extent is not exactly known, when removed from the body. Composition of Serum.— The fluid remaining after coagula- tion of the blood can, of course, be examined chemically with considerable thoroughness and confidence. By far the greater part of serum consists of water ; thus, it has been estimated that of 100 parts the following statement will represent fairly well the proportional composition : Water 90 parts ; Proteids 8 to 9 " Salines, fats, and extractives (small in quantity and not readily obtained free) 1 to 2 parts. The proteids are made up of two substances which can be distinguished by solubility, temperature at which coagulation occurs, etc., known as paraglobulin and serum-albumen, and which may exist in equal amount. It is not possible, of course, to say whether these substances exist as such in the living blood-plasma or not. The fats are very variable in quantity iu serum, depend- ing on a corresponding variability in the plasma, in which they would be naturally found in greatest abundance after a meal. They exist as neutral stearin, palmitin, olein, and as soaps. The principal extractives found are urea, creatin and allied bodies, sugar, and lactic acid. Serum in most animals contains more of sodium salts than the corpuscles, while the latter in man and some other mammals contain a preponderating quan- tity of potassium compounds. The principal salts of serum are sodium chloride, sodium bi- carbonate, sodium sulphate and phosphate ; in smaller quantity, 11 162 COMPARATIVE PHYSIOLOGY. also phosphate of calcium and magnesium, with rather more of potassium chloride. It is highly prohahle that this proportion also represents moderately well the composition of plasma, which is, of course, from a physiological point of view, the important matter. The Composition of the Corpuscles.— Taken together, the dif ferent forms of blood-cells make up from one third to nearly one half the weight of the blood, and of this the red corpuscles may be considered as constituting nearly the whole. The colorless cells are known to contain fats and glycogen, which, with salts, we may believe exist in the living cells, and, in addition to the proteids, into which protoplasm resolves it- self upon the disorganization that constitutes its dying, lecithin, protagon, and other extractives. The prominent chemical fact connected with the red corpus- cles is their being composed in great part of a peculiar colored proteid compound containing iron. This will be fully considered later: but, in the mean time we may state that the haemoglobin is itself infiltrated into the meshes or framework (stroma) of the corpuscle, which latter seems to be composed of a member of the globulin class, so well characterized by solubility in weak saline solutions. The following tabular statement represents the relative pro- portions in 100 parts of the dried organic matter of the red cor- puscles : Haemoglobin 90 54 Proteids 8-67 Lecithin. 0"54 Cholesterin 0-25 100-00 The quantity of salts is very small, less than one per cent (inorganic). So much for the results of our analyses ; but when we con- sider the part the blood plays in the economy of the body, it must appear that, since the life-work of every cell expresses it- self through this fluid, both as to what it removes and what it adds, the blood can not for any two successive moments be of precisely the same composition ; yet the departures from a nor- mal standard must be kept within very narrow limits, other- wise derangement or possibly death results. We think that, before we have concluded the study of the various organs of THE BLOOD. 163 the body, it will appear to the student, as it does to the writer, that it is highly probable that there are great numbers of com- pounds hi the blood, either of a character unknown as yet to our chemistry, or in such small quantity that they elude detec- tion by our methods ; and we may add that we believe the same holds for all the fluids of the body. The complexity of vital processes is great beyond our comprehension. It must be especially borne in mind that all the pabulum for every cell, however varied its needs, can be derived from the blood alone ; or, as we shall show presently, strictly speak- ing from the lymph, a sort of middle-man between the blood and the tissues. The Quantity and the Distribution of the Blood.— The rela- tive quantities of blood in different parts of the body have been estimated to be as follows: Liver one fourth. Skeletal muscles " " Heart, lungs, large arteries, and veins. " " Other structures ... " " The significance of this distribution will appear later. The Coagulation of the Blood.— When blood is removed from its accustomed channels, it undergoes a marked chemical and physical change, termed clotting or coagulation. In the case of most vertebrates, almost as soon as the blood leaves the vessels it begins to thicken, and gradually acquh'es a consistence that may be compared to that of jelly, so that it can no longer be poured from the containing vessel. Though some have rec- ognized different stages as distinct, and named them, we think that an unprejudiced observer might fail to see that there were any well-marked appearances occurring invariably at the same moment, or with resting stages in the process, as with the devel- opment of ova. After coagulation has reduced the blood to a condition in which it is no longer diffluent, minute drops of a thin fluid gradually show themselves, exuding from the main mass, faintly colored, but never red, if the vessel in which the clot has formed has been kept quiet so that the red corpuscles have not been disturbed ; and later it may be noticed that the main mass is beginning to sink in the center {cupping) ; and in the blood of certain animals, as the horse, which clots slowly, the upper part of the coagulum (cr assume ntum) appears of a lighter color, owing, as microscopic examination shows, to the relative 164 COMPARATIVE PHYSIOLOGY. fewness of red corpuscles. This is the huffy -coat, or, as it oc- curs in inflammatory conditions of the hlood, was termed by older writers, the crusta plilogistica. It is to be distinguished from the lighter red of certain parts of a clot, often the result of greater exposure to the air and more complete oxidation in con- sequence. The white blood-cells, being lighter than the red, are also more abundant in the upper part of the clot (buffy-coat). If the coagulation of a drop of blood withdrawn from one's own finger be watched under the microscope, the red corpuscles may be seen to run into heaps, like rows of coins lying against each other (rouleaux, Fig. 141), and threads of the greatest fineness are observed to radiate throughout the mass, gradually increasing in number, and, at last, including the whole in a meshwork which slowly contracts. It is the formation of this fibrin which is the essential factor in clotting; the inclusion of the blood-cells and the extrusion of the serum naturally result- ing from its formation and contraction. The great mass of every clot consists, however, of corpuscles ; the quantity of fibrin, though variable, not amounting to more usually than about '2 per cent in mammals. Tbe formation of the clot does not occupy more than a few minutes (two to seven) in most mammals, including man, but its contraction lasts a very considerable time, so that serum may continue to exude from the clot for hours. It is thus seen that, instead of the plasma and corpuscles of the blood a%it exists within the living body, coagulation has resulted in the formation of two new products — serum and fibrin — differing botli physically and chemically. These facts may be put in tabular form thus: Blood as it flows ) Liquor sanguinis (plasma). in tbe vessels. ( Corpuscles. Blood after co- \ Coagulum | £^ cles< agulation. j Serum. As fibrin may be seen to arise in the form of threads, under the microscope, in coagulating blood, and since no trace of it in any form has been detected in the plasma, and the process can be accounted for otherwise, it seems unjustifiable to assume that fibrivi exists preformed in the blood, or arises in any way prior to actual coagulation. Fibrin belongs to the class of bodies known as proteids, and can be distinguished from the other subdivisions of this group of substances by certain chemical as well as physical character- THE BLOOD. 165 istics. It is insoluble in water and in solutions of sodium chlo- ride; insoluble in hydrochloric acid, though it swells in this menstruum. It may be whipped out from the freshly shed blood by a bundle of twigs, wires, or other similar arrangement presenting a considerable extent of surface ; and when washed free from red blood-cells presents itself as a white, stringy, tough sub- stance, admirably adapted to retain anything entangled in its meshes. If fibrin does not exist in the plasma, or does not arise directly as such in the clot, it must have some antecedents al- ready existing as its immediate factors in the plasma, either before or after it is shed. The principal theories of coagulation are these : 1. Coagu- lation results from the action of a fibrin-ferment on fibrinogen and paraglobulin. 2. Coagulation results from the action of a fibrin-ferment on fibrinogen alone. Fibrinogen and para- globulin (see sections on " The Chemistry of the Animal Body") are proteids originating from the plasma, during clotting in all probability. Fibrin-ferment loses its properties on boiling, and a very small quantity suffices in most cases to induce the result. For these and other reasons this agent has been classed among bodies known as unorganized ferments, which are distinguished by the following properties : They exert their influence only under well-defined circum- stances, among which is' a certain narrow range of tempera- ture, about blood-heat being most favorable for their action. They do not seem to enter themselves into the resulting prod- uct, but act from without, as it were (catalytic action), hence a very small quantity suffices to effect the result. In all cases they are destroyed by boiling, though they bear exposure for a limited period to a freezing temperature. From observations, microscopic and other, it has been con- cluded that the corpuscles play an important part in coagula- tion by furnishing the fibrin-ferment ; but the greatest diver- sity of opinion prevails as to which one of the morphological elements of the blood furnishes the ferment, for each one of them has been advocated as the exclusive source of this fer- ment bj r different observers. We do not favor the current theories of the coagulation of the blood. We would explain the whole matter somewhat thus: What the blood is in chemical composition and other properties from moment to moment is the result of the complicated inter- 166 COMPARATIVE PHYSIOLOGY. action of all the various cells and tissues of the body. Any one of these, departing from its normal behavior, at once affects the blood ; but health implies a constant effort toward a certain equilibrium, never actually reached but always being striven after by the whole organism. The blood can no more maintain its vital equilibrium, or exist as a living tissue out of its usual environment, than any other tissue. But the exact circum- stances under which it may become disorganized, or die, are legion ; hence, it is not likely that the blood always clots in the same way in all groups of animals, or even in the same group. The normal disorganization or death of the tissue re- sults in clotting ; but there may be death without clotting, as when the blood is frozen, in various diseases, etc. To say that fibrin is formed during coagulation expresses in a crude way a certain fact, or rather the resultant of many facts. To explain : When gunpowder and certain other ex- plosives are decomposed, the result is the production of cer- tain gases. If we knew these gases and their mode of com- position but in the vaguest way, we should be in much the same position as we are in regard to the cogulation of the blood. There is no difficulty in understanding why the blood does not clot in the vessels after death so long as they live, nor why it does coagulate upon foreign bodies introduced into the blood- stream. So long as it exists under the very conditions under which it began its being, there is no reason why the blood should become disorganized (clot). It would be marvelous if it did clot, for then we could not understand how it could ever have been developed as a tissue at all. It is just as reasonable to ask, Why does not a muscle-cell become rigid (clot) in the body during life ? Probably in no field in physiology has so much work been done with so little profit as in the one we are now discussing ; and. as we venture to think, owing to a misconception of the real nature of the problem. We can understand the practical im- portance of determining what circumstances favor coagulation or retard it, both within the vessels and without them ; but from a theoretical point of view the subject has been exalted out of all proportion to its importance. Coagulation is favored by gentle movement, contact with foreign bodies, a temperature of about 38° to 40° C, addi- tion of a small quantity of water, free access of oxygen, etc. THE BLOOD. 167 The process is retarded by a low temperature, addition of abundance of neutral salts, extract of the mouth of the leech, peptone, much water, alkalies, and many other substances. The excess of carbonic anhydride and diminution of oxygen seem to be the cause of the slower coagulation of venous blood, , hence the blood long remains fluid in animals asphyxiated. A little reflection suffices to explain the action of most of the fac- tors enumerated. Any cause which hastens the disintegration of the blood-cells must accelerate coagulation ; chemical changes underlie the changes in this as in all other cases of vital action. Slowing of the blood-stream to any appreciable extent likewise favors clotting, hence the explanation of the success of the treatment of aneurisms by pressure. It is plain that in all such cases the normal relations between the blood and the tis- sues are disturbed, and, when this reaches a certain point, death (coagulation) ensues, as with any other tissue. Clinical and Pathological. — The changes in the blood that characterize certain abnormal states are highly instructive. If blood from an animal be injected into the veins of one of an- other species, the death of the latter often results, owing to non- adaptation of the blood already in the vessels, and to the tissues of the creature generally. The corpuscles break up — the change of conditions has been too great. Deficiency in the quantity of the blood as a whole (oligcemia) causes serious change in the functions of the body ; but that a haemorrhage of considerable extent can be so quickly recovered from speaks much for the recuperative power of the blood-forming tissues. Various kinds of disturbances in these blood-forming organs result in either deficiency or excess of the blood-cells, and in some cases the appearance of unusual forms of corpuscles. Ancemia may arise from a deficiency either in the numbers or the quality of the red cells ; they may be too few, deficient in size, or lacking in the normal quantity of haemoglobin. In one form (pernicious ancemia), which often proves fatal in man a variety of forms in the red blood-cells may appear in the blood-stream ; some may be very small, some larger than usual, others nucleated, etc. Again, the white cells may be so multi- plied that the blood may bear in extreme cases a resemblance to milk. In these cases there has been found associated an unusual condition of the bone-marrow, the lymphatic glands, the spleen, and, some have thought, of other parts. 168 COMPARATIVE PHYSIOLOGY, The excessive action of these organs results in the production and discharge into the blood-current of cells that are immature and embryonic in character. This seems to us an example of Fig. 153. Fig. 149.— Outlines of red corpuscles in a case of profound annemia. 1, 1, normal cor- puscles; 2, large red corpuscle— megalocyte; 3, 3, very irregular forms— poikilo- cyt.es; 4, very small, deep-red corpuscles— microcytes. Fig. 150.— Origin of microcytes from red corpuscles by process of budding and fission. Specimen from red marrow. Fig. 151. — Nucleated red blood-corpuscles from blood in case of leukssmia. Fig. 152.— Corpuscles containing red blood-corpuscles. 1, from blood of child at term; 2, from blood of a leukemic patient. Fig. 153. — a, 1. 2, 3, spleen-cells containing red blood-corpuscles, b, from marrow; 1, cell containing nine red corpuscles; 2, cell with reddish granular pigment; 3, fusi- form cell containing a single red corpuscle, c, connective-tissue corpuscle from subcutaneous tissue of young rat, showing the intracellular development of red blood-corpuscles. (Figs. 149-153, after Osier.) a reversion to an earlier condition. It is instructive also in that the facts point to a possible seat of origin of the cells in the adult, and, taken in connection with otber facts, we may say, to their normal source. These blood-producing organs, having too much to do in disease, do their work badly — it is incom- plete. Althougb the evidence, from experiment, to show that the THE BLOOD. 169 nervous system in mammals, and especially in man, has an in- fluence over the formation and fate of the blood generally, is scanty, there can be little doubt that such is the case, when we take into account instances that frequently fall under the notice of physicians. Certain forms of anaemia have followed so di- rectly upon emotional shocks, excessive mental work and worry, as to leave no uncertainty of a connection between these and the changes in the blood ; and the former must, of course, have acted chiefly if not solely through the nervous system. It will thus be apparent that the facts of disease are in har- mony with the views we have been enforcing in regard to the blood, which we may now briefly recapitulate. Summary. — Blood may be regarded as a tissue, with a fluid matrix, in which float cell-contents. Like other tissues, it has its phases of development, including origin, maturity, and death. The colorless cells of the blood may be considered as original imdifferentiated embryo cells, which retain their primi- tive character ; the non-nucleated red cells of the adult are the mature form of nucleated cells that in the first instance are colorless, and arise from a variety of tissues, and which in certain diseases do not mature, but remain, as they originally were at first, nucleated. When the red cells are no longer fitted to discharge their functions, they are in some instances taken up by amoeboid organisms (cells) of the spleen, liver, etc. The chief function of the red corpuscles is to convey oxy- gen ; of the white, to develop as required into some more differ- entiated form of tissue, act as porters of food-material, and probably to take up the work of many other kinds of cells when the needs of the economy demand it. The fluid matrix or plasma furnishes the lymph by which the tissues are directly nourished, and serves as a means of transport for the cells of the blood. The chemical composition of the blood is highly complex, in accordance with the function it discharges as the reservoir whence the varied needs of the tissues are supplied ; and the immediate receptacle (together with the lymph) of the entire waste of the body ; but the greater number of substances exist in very minute quantities. The blood must be maintained of a certain composition, varying only within narrow limits, in order that neither the other tissues nor itself may suffer. The normal disorganization of the blood results in coagula- 170 COMPARATIVE PHYSIOLOGY. tion, by which, a substance, proteid in nature, known as fibrin, is formed, the antecedents of which are probably very variable throughout the animal kingdom, and are likely so even in the same group of animals, under different circumstances ; and a substance abounding in proteids (as does also plasma), known as serum, squeezed from the clot by the contracting fibrin. It represents the altered plasma. Certain well-known inorganic salts enter into the composi- tion of the blood — both plasma and corpuscles — but the princi- pal constituent of the red corpuscles is a pigmented, ferrugi- nous proteid capable of crystallization, termed haemoglobin. It is respiratory in function. THE CONTEACTILE TISSUES. That contractility, which is a fundamental property in some degree of all protoplasm, becoming' pronounced and definite, giving rise to movements the character of which can be pre- dicted with certainty once the form of the tissue is known, finds its highest manifestation in muscular tissue. Very briefly, this tissue is made up of cells which may be either elongated, fusiform, nucleated, finally striated lengthwise, Fig. 154. Fig. 155. Fig. 154.— Muscular fibers from the urinary bladder of the human subject. 1 x 200 (Sappey.) 1,1,1, nuclei; 2,2,2. borders of some of the fibers; 3,3, isolated fibers; 4, 4, two fibers joined together at 5. Fig. 155.— Muscular fibers from the aorta of the calf. 1 x 200. (Sappey.) 1, 1, fibers joined with each other; 2, 2, 2, isolated fibers. but non-striped transversely, united by a homogeneous cement substance, the whole constituting non-striped or involuntary 172 COMPARATIVE PHYSIOLOGY. muscle ; or, long nucleated fibers transversely striped, covered with an elastic sheath of extreme thinness, bound together into small bundles by a delicate connective tissue, these again into larger ones, till what is commonly known as a " muscle " is formed. This, in the higher vertebrates, ends in tough, ine- lastic extremities suitable for attachment to the level's it may be required to move (bones). Certain of the tissues will be found briefly described in the sections preceding " Locomotion." Comparative. — The lowest animal forms possess the power of movement, which, as we have seen in Amoeba, is a result rather of a groping after food ; and takes place in a direction it is impossible to predict, though no doubt regulated by laws definite enough, if our knowledge were equal to the task of de- fining them. Those ciliary movements among the infusorians, connected with locomotion and the capture of food, are examples of a protoplasmic rhythm of wonderful beauty and simplicity. Muscular tissue proper first appears in the Coelenterata, but not as a wholly independent tissue in all cases. In many ccelente rates cells exist, the lower part of which alone forms a delicate muscular fibei', while the superficial portion (myoblast), composing the body of the cell, may be ciliated and is not con- tractile in any special sense. The non-striped muscle-cells are most abundant among the in- ..-—"•-••,.,-. . ..,-•; :"■'■>'' vertebrates, though found in ,.^/ the viscera and a few other parts of vertebrates. This : - - J "^ / ; 'jg_2,_ form is plainly the simpler and more primitive. The / voluntary muscles are of the ri ^^r^£SfaS aJelly - fiSh ' the ^ Griped variety in articulates and some other invertebrate gi*oups and in all vertebrates ; and there seems to be some re- lation between the size of the muscle-fiber and the functional power of the tissue — the finer they are and the better supplied with blood, two constant relations, the greater the contrac- tility. Whether a single smooth muscle-cell, a striped fiber (cell), or a collection of the latter (muscle) be observed the invariable result of contraction is a change of shape which is perfectly definite, the long diameter of the cell or muscle becoming shorter, and the short diameter longer. THE CONTRACTILE TISSUES. 173 Ciliary Movements. — This subject has been already con- sidered briefly in connection with some of the lower forms of life presented for study. It is to be noted that there is a gradual replacement of this form of action by that of muscle as we ascend the animal scale ; it is, however, retained even in the highest animals in the discharge of functions analogous to those it fulfills in the invertebrates. Thus, in Vorticella, we saw that the ciliary movements of the peristome caused currents that carried in all sorts of parti- cles, including food. In a creature so high in the scale as the frog we find the alimentary tract ciliated ; and in man himself a portion of the respiratory tract is provided with ciliated cells concerned with assisting gaseous interchange, a matter of the highest importance to the well-being of the mammal. As be- fore indicated, ciliated cells are found in the female generative organs, where they play a part already explained. It is a matter of no little significance from an evolutionary point of view, that cil- iated cells are more widely distributed in the foetus than in the fully developed ani- mal. As would be ex- pected the movements of cilia are affected by a variety of circum- stances and reagents ; thus, they are quick- ened by bile, acids, alkalies, alcohol, ele- vation of temperature up to about 40° C, etc. ; retarded by cold, ' carbonic anhydride, Fk ether, chloroform, etc. In some cases their action may be arrested and re-established by treatment with rea- gents, or it may recommence without such assistance. . 157.— Nodes of Ranvier and lines of Fromann (Kanvier). A. Intercostal nerve of the mouse, treated with silver nitrate. B. Nerve-fiber from the sciatic nerve of a full-grown rabbit. A, node of Ranvier ; J/, medullary substance rendered transparent by the action of glycerin: CY, axis- cylinder presenting the lines of Fromann, which are very distinct near the node. The lines are less marked at a distance from the node. All this 174 COMPARATIVE PHYSIOLOGY. seems to point to ciliary action as falling under the laws gov- erning the movements of protoplasm in general. It is impor- tant to bear in mind that ciliary action may go on in the cells of a tissue completely isolated from the animal to which it he- longs, and though influenced, as just explained, by the sur- roundings, that the movement is essentially automatic, that is, independent of any special stimulus, in which respect it differs a good deal from voluntary muscle, which usually, if not al- ways, contracts only when stimulated. The lines along which the evolution of the contractile tissues has proceeded from the indefinite out do wings and withdrawals of the substance of Amoeba up to the highly specialized movements of a striped muscle-cell are not all clearly marked out ; but even the few facts mentioned above suffice to show gra- dation, intermedi- ate forms. A sim- ilar law is involved in the muscular contractility mani- fested by cells with other functions. The automatic (self - Fig. 158.— Mode of termination of the motor-nerves (Flint, after Rouget). A. Primitive fasciculus of the thyro- hyoid muscle of the human subject, and its nerve- tube : 1,1, primitive muscular fasciculus; 2, nerve- tube ; 3, medullary substance of the tube, which is seen extending to the terminal plate, where it disap- pears ; 4, terminal plate situated beneath the sarco- femma— that is to say, between it and the elementary fibrilhe; 5, 5, sarcolemma. B. Primitive fasciculus of the intercostal muscle of the lizard, in which a nerve- Originated, mcle- tube terminates: 1,1, sheath of the nerve-tube: 2, T , „.i„„ + l.j,,™^!^ ^t nucleus of the sheath; 3, 3, sarcolemma becoming peuueiii/ iciigei^ ui continuous with the sheath; 4, medullary substance „ cti*miln pm-isnstiTiP' of a balls for regulating the velocity of the vals or time > consisting 01 d cylinder; e, marker recording a line on simple mechanism for pro- ducing the movement of a cylinder, which may be covered with smoked paper, or other- THE STUDY OF MUSCLE PHYSIOLOGY. 177 wise prepared to receive impressions made upon it by a point and capable of being raised or lowered, and its movements reg- C. ,1 I. 1 J. Fig. 161. — Myographic tracing, such as is obtained when the cylinder on which it is written does not revolve during the contraction of the muscle (after McKendrlck). ulated. The cylinder is ruled vertically into a certain number of spaces, so that, if its rate of revolution is known and is con- stant (very important), the length of time of any event recorded on the sensitive surface may be accurately known. This whole apparatus may be considered a chronograph in a rough form. But a tuning-fork is the most reliable form of chronograph, provided it can be kept in coustant action so long as required ; Fig. 162. — Marty's chronograph as applied to revolving cylinder (after McKendrick). a, galvanic element; b, wooden stand bearing tuning-fork (two hundred vibrations per second); c, electro-magnet between limbs of tuning-fork: il. e. positions for tuning-forks of one hundred and fifty vibrations per second; /. tuning-fork lying loose, which may be applied to d\ g, revolving cylinder: h. electric chronograph kept in vibration synchronous with the tuning-fork interrupter. The current working the electro-magnet from a, is interrupted at i. Foucaulfs regulator is seen over the clock-work of the cylinder, a little to the right of cj. 12 178 COMPARATIVE PHYSIOLOGY. and is provided with a recording apparatus that does not cause enough friction to interfere with its vibrations. Fig. 162 illustrates one arrangement that answers these con- ditions fairly well. The marker, or chronograph, in the more limited sense, is kept in automatic action by the fork interrupting the current from a battery at a certain definite rate answering to its own proper note. Marey's chronograph, which is represented at h above, and in more detail below, in Fig. 163, consists of two electro-mag- nets armed with keepers, between which is the writer, which Fio. 163.— Side view of Marey's chronograph (after McKendriek). a, a, coils of wire; b, b, keepers of electro-magnets; c, vibrating style fixed to the steel plate e; d, binding screws for attachment of wires; + from interrupting tuning-fork; — to the battery. has a little mass of steel attached to it, the whole working in unison with the tuning-fork, so that an interruption of the cur- rent implies a like change of position of the writing-style, which is always kept in contact with the recording surface. Fig. 173 shows the arrangements for recording a single muscle contraction, and Fig. 174 the character of the tracing obtained. A muscle-nerve prepa- ration, which usually con- sists of the gastrocnemius of the frog with the sciatic nerve attached, clamped by Fig. 164.— Muscle-nerve preparation, showing a portion of the femur cut gastrocnemius muscle, sciatic nerve, and „. ... ,, . portion of femur of frog, for attachment Oil With the muscle, IS to a vise (after Rosenthal). made> Qn stimulatioilj to THE STUDY OF MUSCLE PHYSIOLOGY. 179 raise a weighted lever which is attached to a point writing on a cylinder moved by some sort of clock-work. In this case the cylinder is kept stationary during the contraction of the mus- cle ; hence the records appear as straight vertical lines. For recording movements of great rapidity, so that the in- tervals between them may be apparent, such an apparatus as is Fig. 165. — Spring myograph of Du Bois-Rcymond (after Rosenthal). The arrange- ments for registering various details are similar to those for pendulum myograph (Fig. 173). figured here (Fig. 165) answers well, the vibrations of a tuning- fork being written on a blackened glass plate, shot before a chro- nograph by releasing a spring. Several records may be made successively by more compli- cated arrangements, as will be explained by another figure later. THE APPARATUS USED FOR THE STIMULATION OF MUSCLE. It is not only important that there should be accurate and delicate methods of recording muscular contractions, but that there be equally exact methods of applying, regulating, and measuring the stimulus that induces the contraction. Fig. 166 gives a representation of the inductorium of Du Bois-Reymond, by which either a single brief stimulation or a series of such repeated with great regularity and frequency 180 COMPARATIVE PHYSIOLOGY. Fig. 166. — Du Bois-Reymond's inductorium (after Rosenthal), i, secondary coil; c, primary coil; b, electro-magnet; A, armature of hammer; /, small movable screw. The current from battery, ascending metal pillar, passes along hammer, and by 6crew gets into primary coil, thus inducing current in secondary coil. By con- nection between primary coil and wires around soft iron of b, iron becomes a mag- net, hammer is attracted from screw/, and current thus broken; but when this occurs, soft iron ceases to be a magnet necessarily, and, hammer springing back, the whole course of events is repeated. This may occur several hundred times in a second. The above may be clearer from diagram, Fig. 167. By sliding second- ary coil up and down, strength of induced current can be graduated. may be effected. The apparatus consists essentially of a pri- mary coil, secondary coil, magnetic interrupter, and a scale Fig. 167.— Diagrammatic representation of the working of Fig. 166 (after Rosenthal) THE STUDY OF MUSCLE PHYSIOLOGY. 181 to determine the relative strength of the current employed. The instrument is put into action by one or more of the various well-known galvanic cells, of which Daniell's are suitable for most experiments. Fig. 169. Fig. 168. Fig. 168.— Pflfiger's myograph. The muscle may be fixed to the vise C in the moist chamber, the vise connecting with the lever E E, the point of which touches the plate of smoked glass G. The lever is held in equipoise by //. When weights are placed in scale-pan F, the lever writes the degree of extension effected (after Ro- senthal). Fig. 169. — Tetanizing key of Du Bois-Iteymond (after Rosenthal). Wires may be attached at b and c. When d is down the current is "short-circuited," i. e.. does not pass through the wires, but direct from c through d to b. or the reverse, since 6, c, d are of metal, and, on account of their greater cross-section, conduct so much more readily than the wires, a is an insulating plate of ebonite. This form of key is adapted for attachment to a table, etc. The access to, or exclusion of the current from, the indue- torium is effected by some of the forms of keys, a specimen of which is illustrated in Fig. 169. The moist chamber, or some other means of preventing the drying of the preparation, which would soon result in impaired 182 COMPARATIVE PHYSIOLOGY. action, followed by death, is essential. A moist chamber con- sists essentially of an inclosed cavity, in which is placed some wet blotting-paper, etc., and is usually made with glass sides. The air in such a chamber must remain saturated with moisture. A good knowledge of the subject of electricity is especially valuable to the student of physiology. But there are a few ele- mentary facts it is absolutely necessary to bear in mind : 1. An induced current exists only at the moment of making or break- ing a primary (battery) current. 2. At the moment of making, the induced current is in the opposite direction to that of the primary current, and the reverse at breaking. 3. The strength of the induced current varies with the strength of the primary current. 4. The more removed the secondary coil from the primary the weaker the current (induced) becomes. The clock-work mechanism and its associated parts, as seen in Fig. 170, on the right, is usually termed a myograph. Fig. 170.— Arrangement of apparatus for transmission of muscular movement by tam- bours (after McKendrick). a, galvanic element; b, primary coil; c, secondary coil of inductorium; d, metronome for interrupting primary circuit when induction current is sent to electrodes k\ h, forceps for femur; the muscle, which is not here represented, is attached to the receiving tambour g, by which movement is transmitted to recording tambour e, which writes on cylinder/. Instead of muscular or other movements being communi- cated directly to levers, the contact may be through columns of air, which, it will be apparent, must be capable of communi- cating very slight changes if the apparatus responds readily to the alterations in volume of the inclosed air. Fig. 171 represents a Marey's tambour, which consists essen- THE STUDY OF MUSCLE PHYSIOLOGY. 183 Fig. 171. — Tambour of Marey (after McKendrick). a, metallic case; b, thin India-rub- ber membrane; c, thin disk of aluminium supporting lever d, a small portion of which only is represented; e, screw for placing support of lever vertically over'c; /j metallic tube communicating with cavity of tambour for attachment to an In- dia-rubber tube. tially of a rigid metallic case provided with an elastic top, to which a lever is attached, the whole being brought into com- munication with a column of air in an elastic tube. The work- ing of such a mechanism will be evident from Figs 170 and 172. 1 Fig. 172.— Tambours of Marey arranged for transmission of movement (after McKen- drick). a, receiving tambour; b, India-rubber tube; c, registering tambour; d, spiral of wire, owing to elasticity of which, when tension is removed from a, the lever ascends. The greatest danger in the use of such apparatus is not fric- tion but oscillation, so that it is possible that the original move- ment may not be expressed alone or simply exaggerated, but also complicated by additions, for which the apparatus itself is responsible. 184 COMPARATIVE PHYSIOLOGY. Fih. 173. THE STUDY OF MUSCLE PHYSIOLOGY. 185 Fig. 173.— Diagrammatic representation of the pendulum myograph. The smoked- glass plate. A, swings with a pendulum, B. Before an experiment is commenced the pendulum is raised up to the right and kept in position by the tooth, a. catch- ing on the spring-catch, b. On depressing the catch, b, the glass plate being set free swings into the new position indicated by the dotted lines, and is held there by the tooth, «', meeting the catch, b' . In the course of its swing the tooth, a, coming into contact with the projecting steel rod, c, knocks it to one side, into the position indicated by the dotted line, c'. The rod, c, is in electric continuity with the wire, x, of the"primary coil of an induction machine. In like manner the screw, d, is in electric continuity with the wire, y, of the same primary coil. The screw, d, and the rod, c. are provided with platinum points, and both are in- sulated by means of the ebonite block, e. The circuit of the primary coil to which .rand y belong is closed as long as c and d are in contact. When in its swing the tooth. «', knocks c away from d, the circuit is immediately broken, and a •' breaking" shock is sent through the electrodes connected with the secondary coil of the machine, and so through the nerve. A lever is brought to bear on the glass plate, and when at rest describes an arc of a circle of large radius. The tun- ing-fork, /(ends only seen), serves to mark the time (after Foster). Apparatus of this kind is not usually employed much for experiments with muscle ; such an arrangement is, however, showm in Fig. 170, in which also will be seen a metronome, the pendulum of wdiich, by dipping into cups containing mercury, makes the circuit. Such or a simple clock may be utilized for indicating the longer intervals of time, as seconds. A SINGLE SIMPLE MUSCULAR CONTRACTION. Experimental Facts.— The phases in a single twitch or mus- cular contraction may be studied by means of the pendulum myograph (Fig. 173). It consists of a heavy pendulum, which swings from a position on the right to a corresponding one on the left, where it is secured by a catch. During the swing of the pendulum, which carries a smoked-glass plate (by means of arrangements more minutely described below the figure), a tuning-fork writes its vibrations on the plate, on which is in- scribed the marking indicating the exact moment of the break- ing of an electric current, which gives rise to a muscle contrac- tion that is also recorded on the plate. The tracing on analysis presents : 1. The record of a tuning- fork making one hundred and eighty vibrations in a second. 2. The parallel marking of the lever attached to the muscle before it began to rise. 3. A curve, at first rising slowly, and then rapidly to a maximum. 4. A cmwe of descent similar in character, but somewhat more lengthened. We may interpret this record somewhat thus : 1. A rise of the lever answering to the shortening of the muscle to which it is attached following upon the momentary induction shock, as the entrance of the current into the nerve, the stimulation of which causes the contraction, may be called. 2. A period before 186 COMPx\RATIVE PHYSIOLOGY. the contraction begins, which, as shown by the time marking, occupies in this case — - , or about T V of a second. In the tracing the upward curve indicates that the contraction is at first rela- tively slow, then more rapid, and again slower, till a brief sta- a b Fig. 174.— Muscle-curve obtained by the pendulum myograph (Foster). Read from left to right. The latent period is indicated by the space between a and b, the length of which is measured by the waves of a tuning-fork, making one hundred and eighty double vibrations in a second; and in like manner the duration of the other phases of the contraction may be estimated. tionary period is reached, when the muscle gradually but rap- idly returns to its previous condition, passing through the same phases as during contraction proper. In other words, there is a period of rising and of falling energy, or of contraction and relaxation. 4. A period during which invisible changes, as will be explained later, are going on, answering to those in the nerve that cause the molecular commotion in muscle which precedes the visible contraction — the latent period, or the period of latent stimulation. The facts may be briefly stated as follows : The stimulation of a muscle either directly or through its nerve causes contrac- tion, followed by relaxation, both of which are preceded by a latent period, during which no visible but highly important molecular changes are taking place. The whole change of events is of the briefest duration, and is termed a muscle contraction. The tracing shows that the latent period occupied rather more than T fs second, the period of contraction proper about T ^, and of relaxation ^ second, so that the whole is usually begun and ended within T V second ; yet, as will be learned later, many chemical and electrical phenomena, the concomitants of vital change, are to be observed. In the case just considered it was assumed that the muscle THE STUDY OF MUSCLE PHYSIOLOGY. 1S7 was stimulated through its nerve. Precisely the same results would have followed had the muscle been caused to contract by the momentary application of a chemical, thermal, or mechanical stimulus. If the length of nerve between the point of stimulation and the muscle was considerable, some difference would be observed Fig. 175. — Diagrammatic representation of the measurement of velocity of nervous impulse (Foster). Tracing taken by pendulum myograph (Fig. 173). The nerve of same muscle-nerve preparation is stimulated in one case as far as possible from muscle, in the other as near to it as possible. Latent period is ab, ab'. respect- ively. Difference between ab and ab' indicates, of course, length of time occu- pied by nervous impulse in traveling along nerve from distant to near point. in the latent period if in a second case the nerve were stimu- lated, say, close to the muscle. This is represented in Fig. 175, in which it is seen that the latent period in* the latter case is shortened by the distance from b' to &, which must be owing to the time required for those molecular changes which, occur- ring in a nerve, give rise to a contraction in the muscle to which it belongs ; in fact, we have in this method the means of estimat- ing the rate at which these changes pass along the nerve — in other words we have a means of measimng the speed of the propagation of a nervous impulse. The estimated rate is for the frog twenty-eight metres per second, and for man about thirty- three metres. As the latter has been estimated for the nerve, with its muscle in position in the living body, it must be re- garded rather as a close approximation than as exact as the other measurements referred to in this chapter. It will be borne in mind that the numbers given as repre- senting the relative duration of the events vary with the ani- mal, the kind of muscle, and a variety of conditions affecting the same animal. TETANIC CONTRACTION. It is well known that a weight may be held by the out- stretched arm with apparently perfect steadiness for a few 188 COMPARATIVE PHYSIOLOGY. seconds, but that presently the arm begins to tremble or vi- brate, and soon the weight must be dropped. The arm was maintained in its position by the joint contraction of several mus- cles, the action of which might be described (traced) by a writer attached to the hand and recording on a moving surface. Such a record would indicate roughly what had happened ; but the exact nature of a muscular contraction in such a case can best be learned by laying bare a single muscle, say in the thigh of a frog, and arranging the experiment so that a graphic record shall be made. Using the apparatus previously described (Fig. 173), a series of induction shocks may be sent into the muscle with the result indicated in Figs. 176 and 177, according to the rate of interrup- tion of the current. j* 1** ^^ w »*''* iw ~* 1 * ' — \ Fig. 176.— Curve of imperfect, tetanic contraction (Foster). Uppermost tracing indi- cates contractions of muscle; intermediate, when the shocks were given; lower, time-markings of intervals of one second. Curve to be read, like others, from left to right, and illustrates at the end a ."'contraction remainder." If the stimuli follow each other with a certain rapidity, such a tracing as that represented in Fig. 176 is obtained ; and if the rapidity of the stimulation exceeds a fixed rate, the result is that seen in Fig. 177. Fro. li'T.— Curve of complete tetanic contraction (Foster). THE STUDY OP MUSCLE PHYSIOLOGY. 189 It is possible to see in these tracings a genetic relation, the second figure being evidently derivable from the first, and the third from the second, by the fusion of all the curves into one straight line. The Muscle Tone. — There are a number of experimental facts from which the conclusion has been drawn that tetanic contrac- tion is accompanied by a muscle tone whioii is in itself evidence of the nature of the contraction. We may safely conclude that, at all events, most of the mus- cular contractions occurring within the living body are tetanic — i. e., the muscle is in a condition of shortening, with only very brief and slight phases of relaxation ; and that a comparatively small number of individual contractions suffice for tetanus when caused by the action of the central nervous system; though, as proved by experiments on muscle removed from the body, they may be enormously increased. While a few stimu- lations per second suffice to cause tetanus, it will also persist though thousands be employed. THE CHANGES IN A MUSCLE DURING CONTRACTION. Though the change in form is very great during the con- traction of a muscle, the change in bulk is almost inappreci- able, amounting to a diminution of not more than about T -gVo of the volume. In fact, according to the latest investigator, there is no diminution whatever. Since the fibers of striped muscle are of very limited length (30 to 40 mm.), it would seem that a contraction originating in one fiber must be capable of initiating a similar action in its neighbor ; and, as the ends of the fibers lie in contact, it is easy to understand how the wave of contraction spreads. Normally, the contraction must pass from about the center of the muscle- cell where the nerve terminates in the end-plate. THE ELASTICITY OF MUSCLE. In proportion as bodies tend to resume their original form when altered by mechanical force are they elastic, and the ex- tent to which they do this marks the limit of their elasticity. If a muscle (best one with bundles of fibers of about equal length and parallel arrangement) be stretched by a weight attached to one end, it will, on removal of the extending force, 190 COMPARATIVE PHYSIOLOGY. return to its original length ; and if a series of weights which differ by a common increment be applied in succession and the degrees of extensions compared, as may be done by the graphic method, it will be ap- parent that the increase in the extension does not exactly correspond with incre- mnent in the weight, but is proportionally less. With an inorganic body, as a watch- spring, this is not the case. Further, the recoil of the muscle after the removal of the weight is not perfect for all weights ; but within certain narrow limits this is the case, i. e., the elasticity of muscle, though slight (for it is easily over-extended), is perfect. When once a muscle is over-extended, so weighted that it can not reach its original length almost at once, it is very slow to recover, which explains the well-known duration of the effects of sprains, no doubt owing to some profound molecular change associated with the stretching. The tracings below show at a glance the difference between the elasticity of muscle and of ordinary bodies. It is a curious fact that a muscle during the act of contraction is more extensible than when passive ; a disadvantage from a purely physical point of view, but prob- ably a real advantage as tending to obviate tey . sprain by preventing too sudden an appli- mond's apparatus for cation of the extending force, tension in muscle (after It will be borne in mind that the limbs SrSd ) " at?a h chld ad t U o are held together as by elastic bands slight m itn C a e ien8° be observed ly on the stretch, owing to the elasticity of the muscles. Now, as seen in many tracings of muscular contraction, there is a tendency to imper- fect relaxation after contraction — the contraction remainder or elastic after-effect, which can be overcome by gentle trac- tion. In the living body, the weight of the limbs and the action of the stretched muscles on the side of the limb opposite to that on which the muscles in actual contraction are situated, com- THE STUDY OF MUSCLE PHYSIOLOGY. 191 bine to make the action of the muscle more perfect by over- coming this tendency to imperfect relaxation, which is proba- Fig. 179.— Illustrations of the difference in elasticity of inanimate and living matter (after Yeo). 1. Shows graphically behavior of a steel spring under equal incre- ments of weight. 2. A similar tracing obtained from an India-rubber band. 3. The same from a frog's muscle. Note that the extension decreases with equal in- crements of weight, and that the muscle fails to return to the original position (abscissa) after removal of the weight. bly less marked, independent of these considerations, in the living body. This elasticity of living muscles, which is com- pletely lost on death, is a fair measure of their state of health or organic perfection. Hence that hard (elastic recoil) feeling of the muscles in young and vigorous persons, especially ath- letes, in whom muscle is brought to the highest degree of per- fection. This property is then essentially the outcome of vitality, which is in a word the foundation of the differences noted be- tween the elasticity of inorganic and organic bodies. A mus- cle, the nutrition of which is suffering from whatever cause, whether deficient blood-supply, fatigue, or actual disease, is deficient in elasticity. We wish to emphasize these relations, for we consider it very important to avoid regarding vital phe- nomena in the light of physics merely, which the employment of the graphic method (and indeed all methods by which we re- move living things out of their normal relations) fosters. Electrical Phenomena of Mnscle. — The contraction and probably the resting stage of muscle are attended by the gen- eration of electrical currents, the direction of which is indicated in Fig. 180. It will be observed that the diagram indicates that between no current and the strongest obtainable there are all shades of 192 COMPARATIVE PHYSIOLOGY. Fig. 180.— Representation of electrical currents in a muscle-rhombus (after Rosenthal). strength, according to the parts of the muscle connected by the electrodes. The strongest is that resulting when the superfi- cial equator and the transverse center are connected ; and it is found that the nearer these points are approached the stronger the current becomes. It is important to note that the electric current of muscle, however viewed, is associated with the chemical and all the other molecular changes of which the actual contraction is but the outward and visible sign ; and since the currents have an appreciable duration, wane with the vitality of the tissue, and wholly disappear at death, they must be associated with the fundamental facts of organic life ; for it is to be remembered that electrical currents are not confined to muscle, but have been detected in the developing embryo, and even in vegetable protoplasm. Though the evidence is not yet complete, it seems likely that electrical phenomena may prove to be associated with (we designedly avoid any more definite expression) all vital phenomena. Chemical Changes in Muscle.— At a variable period after death the muscles become rigid, producing that stiffness {rigor mortis) so characteristic of a recent cadaver. THE STUDY OP MUSCLE PHYSIOLOGY. 193 The subject can be studied in some of its aspects to great ad- vantage in an isolated individual muscle. Three changes in a muscle that has passed into death rigor are constant and pronounced. The living muscle, either alka- line or neutral in reaction, has become decidedly acid ; an abundance of carbonic anhydride is suddenly given off ; and myosin, a specific proteid, has been formed. That these phe- nomena have some indissoluble connection with each other so far as the first two at least are concerned, while not absolutely certain, seems probable, as will be learned shortly. It will be borne in mind that muscle-fibers are tubes con- taining semifluid protoplasm, and that a coagulation of the lat- ter must give rise to general rigor. This protoplasmic substance can be extracted at a low temperature from the muscles of the frog, and, as the temperature rises, coagulates like blood, giving rise to a clot (myosin) and muscle-serum, a fluid not very unlike the serum of blood. This myosin can also be extracted from dead rigid muscles by ammonium chloride, etc. It resembles the globulins gen- erally, but is less soluble in saline solutions than the globulin of blood (paraglobulin) ; is less tough than fibrin ; has a very low coagulating point (55° to 60° C.) ; and is somewhat jelly- like in appearance. The clotting of blood and of muscle is thus analogous, myosin answering to fibrin, and there being a serum in each case, both processes marking the permanent disorgani- zation of the tissue. The reaction seems to be due to the forma- tion of a kind of lactic acid, probably sarcolactic ; though whether due to excessive production of this acid, on the death of the muscle, which for some reason does not remain free in the living muscle, or whether sarcolactic acid arises as a new product, is uncertain. It is certain that the acid reaction of dead muscle is not owing to carbonic acid, for the reddened litmus does not change color on drying. That a muscle in action does use up oxygen and give off carbonic anhydride can be definitely proved ; though it is equally clear that the life of a muscle is not dependent on a constant supply of oxygen as is that of the individual, for a muscle can live, even contract long and vigorously, in an atmos- phere free from this gas, as in nitrogen. From the suddenness of the increase of carbonic anhydride, the onset of death and rigor mortis has been compared to an explosion. 13 194 COMPARATIVE PHYSIOLOGY. After this the muscle becomes greatly changed physically ; its elasticity and translucency are lost ; there is absence of muscle-currents ; it is wholly unheritable, is less extensible — it is, as before stated, firmer — it is dead. But these fundamental phenomena, the increase of carbonic anhydride and the acid reaction, are observable after prolonged tetanus. It was, therefore — putting all the facts together that we now refer to and others, not forgetting that a muscle is always respiring, inhaling oxygen, and exhaling carbonic an- hydride — not um^easonable to conclude that normal tetanus and rigor mortis were but exaggerated conditions of a natural state. The coagulation of the muscle protoplasm {plasma), giving rise to myosin, was, however, a serious obstacle to the adoption of this view. But it has very recently been urged with great plausibility that an old view is correct, viz., that rigor mortis (contracture) is the last act of muscle-life ; it is, in fact, a prolonged tetanus or contracture, ending in most cases, though not all, in coagulation of the myosin. This state can be induced and recovered from in favorable cases by cutting off the blood from a part by ligature, and later readmitting it to the starving region. It has been suggested that the prod- ucts of the muscle-waste, usually washed away by the blood- stream, in such an experiment and after death, collect and act as a stimulant to the muscle, causing it to remain in permanent contraction. The other constituents of dead muscle and their relative properties may be learned from the following table (Von Bibra): Water 744'5 Solids : Myosin, elastic substance, etc., in- soluble in water 155*4 Soluble proteids 19*3 Gelatin 207 Extractives and salts 37'1 Fats 230 255-5— 255-5 Total 1,000 Among the extractives of muscle very important is creatin ( - 2 to '3 per cent), a nitrogenous crystalline body. Certain allied forms, as xanthin, hypoxanthin (sarkin), carnin, taurin, and uric acid, are also found. Glycogen (animal starch), very abundant in all the tissues, THE STUDY OF MUSCLE PHYSIOLOGY. 195 including the muscles of the embryo, is found in small quantity in the muscles of the adult; and in the heart-muscle a peculiar sugar (inosit) is present. It is, of course, very difficult to say to what extent the bodies known as extractives exist in living muscle, though that glyco- gen, fats, and certain salts are normally present admits of little doubt. There is a coloring matter in muscle, more abundant in the red muscles of certain animals than the pale, allied to haemo- globin, if not identical with that body. It may be stated as a fact, the exact significance of which is unknown, that during contraction the extractives soluble in water decrease, while those soluble in alcohol increase. It will, however, be very plain, from what has been stated in this section, that life processes and chemical changes are closely associated, and to realize this is worth much to the student of Nature. THERMAL CHANGES IN THE CONTRACTING MUSCLE. Since very marked chemical changes accompany muscular contraction, it might be expected that there would be some modification in temperature, and probably in the direction of elevation. Experiment proves this to be the case. But why should a muscle when at rest, as may be shown, maintain a certain temperature, unless chemical changes are constantly taking place ? As already stated, such is the case, and the rise on passing into tetanus is simply an expression of increased chemical action. No machine known to us resembles muscle except super- ficially. The steam-engine changes fuel into heat and mechani- cal motion, but there the resemblance ends. Muscle changes its food, or fuel, not directly either into heat or motion, but into itself ; yet as a machine it is more effective than the steam- engine, for more work and less heat are the outcome of its activity than is the case with the steam-engine. THE PHYSIOLOGY OF NERVE. Muscle and nerve are constantly associated functionally, and have so much in common that it becomes desirable to study them together. Much that has been established for muscle 196 COMPARATIVE PHYSIOLOGY. holds equally well for nerve ; and the latter, though apparently wholly different in structure at first sight, is really not so. Nerve has its protoplasmic part (axis-cylinder), which is the essential structure, its protective sheaths, and its nuclei (nerve- corpuscles). As already indicated, a nerve possesses irritability. It is found that when the constant (polarizing) current is passing from above downward — that is, when the cathode (negative-pole) is on the side toward the muscle — the irritability of the nerve is increased, and the reverse when the opposite conditions prevail. This altered condition is known as electrotonus. It has been found as the result of many experiments that profound modifications of the irritability of a nerve do take place during the passage of a constant current. These are diagrammatically represented in Fig. 181. Fig. 181. — Diagrammatic representation of variations in electrotonus according to strength of current employed (after Prliiger). nn', a section of nerve; a, anode (+ pole); k, cathode (—pole). Curves above the horizontal denote cateleetroto- nus; below, the opposite. Briefly stated, they are these : 1. The nature of the change depends on the direction of the polarizing (constant) current ; hence, if the current is descending, there is an increase of irri- tability (catelectrotonus) in the portion of the nerve nearest the muscle, and vice versa. 2. The extent of the change of irrita- bility is dependent on the strength of the polarizing current. . 3. This change is most marked close to the electrodes, spreads to a considerable extent beyond this point without the elec- trodes (extra-polar regions), and also exists within the region of contact of the electrodes (intra-po]ar regions). 4. It follows that there must be a point at which it is not experienced (indif- ferent point or neutral point). Now, it is possible to understand why a sudden change in THE STUDY OF MUSCLE PHYSIOLOGY. 197 the current should cause a muscular contraction. An equally sudden alteration, a profound molecular effect, has been caused, and this we must believe essential to the causation of a muscu- lar contraction through the influence of a nerve. To use an illustration which may serve a good purpose if not taken too literally, it is a well-known experience that one sitting in a room in which a clock is ticking soon fails to no- tice this regular sound : but should the clock stop suddenly or as suddenly commence to tick very rapidly, the attention is aroused, while a very gradual slowing to cessation or the re- verse would have escaped notice. The explanation of such facts takes us down to the very foundations of biology ; but just now we wish only to elucidate by our own experience how it is possible to conceive of a muscle being stimulated by the molecular movements of nerve, or rather a change in these. There are important practical aspects to this question. One may understand why it is that electricity proves so ready a stimulus, and is so valuable a therapeutic agent. It seems, in fact, as will be learned later, to be capable of taking the place to some extent of that constant nerve influence which we be- lieve is being exerted in the higher animals toward the mainte- nance of the regularity of their cell-life (metabolism). Pathological and Clinical.— It is believed that in the nerves of a living animal body, the electrotonic condition can be in- duced as in an isolated piece of nerve. Hence, the value of the constant current in diminishing nerve irritability in neu- ralgia and allied conditions. Apparatus of great nicety of con- struction and capable of generating, accurately measuring, and conveniently applying electrical currents of different kinds, now adds to the resources of the practitioner. But we are probably as yet only on the threshold of electro-therapeutics. Electrical Organs. — Electrical properties can be manifested by a large number of fishes ; and the subject is of special theo- retical interest. It is now established that the development of electrical organs points to their being specially modified mus- cles — tissues, in fact, in which the contractile substance has disap- peared and the nervous elements become predominant and peculiar. No work is done, but the whole of the chemical energy is represented by electi*icity. Functionally an electric organ (which usually is some form of cell, on the w T alls of which nerves are distributed, inclosing a gelatinous substance, 198 COMPARATIVE PHYSIOLOGY. the whole being very suggestive of a galvanic battery) closely resembles a muscle-nerve preparation or its equivalent in the normal body. The electric organs experience fatigue ; have a latent period ; their dis- charge is tetanic (interrupted) ; is excited by mechanical, ther- mal, or electrical stimuli ; and the effectiveness of the organs is heightened by elevation of temperature, and the reverse by cooling, etc. MUSCULAR WORK. V % ^ m Fig. 182.— Tlie electric-fish torpedo, dissect- ed to show electric apparatus (Huxley). b, branchiae; c, brain; e, electric organ; ff, cranium; me, spinal cord; //,, nerves to pectoral fins ; nl, nervi laterales ; np, branches of pneumpgastric nerves to electric organs; o, eye. If during a given period one of two persons raises a weight through the same height but twice as frequent- ly as the other, it is plain that he does twice the work ; from such a case we may deduce the rule for calculating work, viz., to multiply the weight and height together. The effectiveness of a given muscle must, of course, depend on the degree to which it shortens, which is from one half to three fifths of its length; and the number of fibers it contains — i. e., upon its length and the area of its cross-section, taking into account in connection with the first factor the arrangement of the fibers ; those muscles in which the fibers run longitudinal- ly being capable of the greatest total shortening. There is, as shown by actual experimental trial, a relation between the work done and the load to be lifted. With double the weight tbe contraction may be as great as at first, or even greater ; but a limit is soon reached beyond which contraction is impossible. This principle may be stated thus: The contrac- tion is a function of the stimulus, and is illustrated by the diagram below (Fig. 183). It has been shown experimentally that the chemical inter- changes in a muscle, acting against a considerable resistance, THE STUDY OF MUSCLE PHYSIOLOGY. 199 are increased — i. e., the metabolism and the working tension are related. These experimental facts harmonize with our experience of a sense of satisfaction and effectiveness in the use of the muscles "i — r T~l-T 10 £0 30 40 45 50 55 60 65 70 80 90 100 Fig. 183.— Diagram of muscular contractions with same stimulus and increasing weights. The numbers represent grammes (McKendrick). when weights are held in the hands ; and it must be a matter of practical importance that each person should, in taking sys- tematic exercise, keep to that kind which does not either over- weight or underweight the muscles. CIRCUMSTANCES INFLUENCING THE CHARACTER OF MUSCULAR AND NERVOUS ACTIVITY. The Influence of Blood-Supply. Fatigue.— Fig. 184 shows at a glance differences in the curves made by a contracting muscle suffering from increasing fatigue. ISO D V. Fig. 184.— Curves of a muscle contraction in different stages of fatigue (after Yeo). A, curve when muscle was fresh: B, C, D, E, each just after muscle had already- contracted two hundred times. The alteration iu length of latent period is not well brought out in these tracings. Suppose that in such a case the blood had been withheld from the muscle, and that it is now admitted, an almost im- madiate effect is seen in the nature of the contractions ; but even if only saline solution had been sent through the vessels of the muscle, a similar change would have been noticeable. We may fairly conclude that the blood and saline removed some- thing which had been exercising a depressing effect on the vitality of the muscle. In a working muscle, like all living tissues, there are products of vital action (metabolism) that are poisonous. We have already learned tbat a working muscle generates an excess of carbonic anhydride, and something which gives it an acid reaction ; and that it uses up oxygen as well as other matters derivable from the blood. 200 COMPARATIVE PHYSIOLOGY. Fatigue will occur, it is well known, if the muscles are used for an indefinitely long period, no matter how favorable the blood-supply— another evidence that there is, in all probability, some chemical product, the result of their own activity, depress- ing them; and this is rendered all the more likely when it is learned that the injection of lactic acid, to take one example, produces effects like ordinary fatigue. It is also a matter of common experience that exercise, while beneficial to the whole body, the muscles included, as shown by their enlargement under it, becomes injurious when carried to the point of fatigue. Why the use of the muscles is conducive to their welfare is but a part of a larger question, Why does the use of any tissue improve it ? When the nerve which supplies a muscle is stimulated its blood-vessels dilate, and it has been assumed that the same happens when a muscle contracts normally in the body ; and when muscular action is increased there is a corresponding augmentation in the quantity of blood driven through the muscles in a given period, even if there be no actual increase in the caliber of the blood-vessels, for the heart-beat is greatly accelerated. But repose is as necessary as exercise for the greatest effect- iveness of the muscles, as the experience of all, and especially athletes, proves. That the nervous system plays a great part in the nutrition of muscles is evident from the fact, among countless others, that it is not possible to use the brain to its greatest capacity and the muscles to their fullest at the same time ; the individual engaged in physical " training " must forego severe mental ap- plication. Nervous energy is required for the muscles, and all questions of blood-supply are, though important, subordinate. But it would be premature to enter into a full discussion of this interesting topic now. The sense of fatigue experienced after prolonged muscular action is complex, though there can be no doubt that the nerve- centers must be taken into account, since any muscular work that, from being unusual, requires closer attention and a more direct influence of the will, is well known to be more fatigu- ing. On the other hand, the accumulation of products of fatigue doubtless reports itself through the local nervous mech- anism. THE STUDY OF MUSCLE PHYSIOLOGY. 201 Separation of Muscle from the Central Nervous System.— When the nerve belonging to a muscle is divided, certain his- tological changes ensue, which may be briefly described as fatty degeneration, followed by absorption ; and when regener- ation of the nerve-fibers takes place on apposition of the cut ends, a more or less complete restoration of the functions of the nerve follows, but the exact nature of the process of repair is not yet fully agreed upon ; it seems, in fact, to vary in differ- ent cases as to details, though it is likely that, in instances in which there is a complete return to the normal functionally, the axis-cylinders, at all events, are reproduced. The degeneration downward is complete ; upward, only to the first node of Ranvier. Immediately after the section the irritability of the nerve is increased, but rapidly disappears, from the center toward the periphery (Ritter-Valli law). In the mean time the muscle has been suffering. Its nota- bility at first diminishes, then becomes greater than usual to shocks from the make or break of the constant current ; but finally all irritability is lost, and fatty degeneration and disap- pearance of true muscular structure complete the history. It is theoretically interesting, as well as of practical importance, that degeneration may be delayed by the use of the constant current, the significance of which we have already endeavored to explain. The Influence of Temperature,— If a decapitated frog be placed in water of the ordinary temperature, and heat be gradually applied, the animal does not move (proving that the spinal cord alone is not conscious), but the muscles, when 43° to 50° C. is reached, contract and become rigid, a condition known as " heat-rigor. 1 ' There are some advantages in investigating changes in tem- perature by the graphic method. Curves from a muscle-nerve preparation show that elevation of temperature shortens the latent period and the curve of contraction. Lowering the tem- perature has an exactly opposite effect, as might be supposed, and these changes take place in the muscles of both cold- blooded and warm-blooded animals, though more marked in the latter. The modifications evident to the eye are accompanied by others, chemical in nature, and a comparison of these shows that the rapidity and force of the muscular contraction 202 COMPARATIVE PHYSIOLOGY. run parallel with the rapidity and extent of the chemical changes. Certain drugs also modify the form of the muscle-curve very greatly, so that it appears that the' molecular action which un- derlies all the phenomena of muscle and nerve (for what has been said of muscle applies also to nerve, if we substitute nerv- ous impulse for contraction) can go on only within those nar- row bounds wbich, one realizes more and more in the study of physiology, are set to the activities of living things. UNSTRIPED MUSCLE. This form of muscular tissue is characterized by its long latent period, its slow wave of contraction, and the prog- ress of the contraction being in either a transverse or longi- tudinal direction, a wave of contraction in one cell being cap- able of setting up a corresponding wave in adjoining cells even when no nerve-fibers are distributed to them. It is ex- cited, though less readily, by all the kinds of stimuli that act upon striped muscle. In the higher groups of animals this tissue is chiefly confined to the viscera of the chest and abdo- men, constituting in the case of some of them the greater part of the whole organ. The slow but powerful and rhythmical contraction of this form of muscle adapts it well to the part such organs play in the economy. There are variations, however, in the rapidity, force, regularity, and other qualities of the contraction in dif- ferent parts ; thus, it is comparatively rapid in the iris, and ex- tremely powerful and regular in the uterus, serving to produce that prolonged yet ' intermittent pressm^e essential under the circumstances (expulsion of the foetus). Comparative. — Muscular contraction is relatively sluggish and prolonged among the invertebrates, to which, however, the movement of the wings of insects is a marked exception, some of them having been shown by the graphic method to vibrate some hundreds of times in a second. The slow movements of the snail are proverbial. As a rule, the strength of the muscles of the invertebrates is incomparably greater than that of vertebrates, as witness the powerful grasp of a crab's claw or a beetle's jaws. These facts are in harmony with the generally slow metab- olism of most invertebrates and the lower vertebrates. THE STUDY OF MUSCLE PHYSIOLOGY. 203 The muscles of the tortoise contract tardily but with great power, resist fatigue well, retain their vitality under unfavor- able conditions, and after death for a very long period (days). Without resorting to elaborate experiments, the student may convince himself of the truth of most of the above statements by observing the movements of a water-snail attached to a glass vessel ; the note made by the buzzing of an insect, and compar- ing it with one approaching it in pitch sounded by some instru- ment of music ; the force necessary to withdraw the foot or tail of a tortoise ; the peristaltic movements of the intestine and other organs in a freshly killed animal ; or the action of a bee, wasp, or wood-boring beetle on the cork of a bottle in which one of them may be inclosed. • SPECIAL CONSIDERATIONS. In the case of weakly tuberculous animals a sharp tap on the chest will often produce a contraction of the muscles thus stimulated; but, in addition, a local contraction lasting some little time, known as a tvheal or iclio-muscular contraction, fol- lows. This phenomenon seems to be the result of a special irritability in such muscles. Cramp may arise under a great variety of circumstances, but it seems to be in all cases either a complete prolonged teta- nus, in which there is unusual muscular shortening in severe cases, at least, or the persistence of a contraction remainder. The great differences known to exist between individuals of the same species in strength, endurance, fleetness, and other particulars in which the muscles are concerned, raise numer- ous interesting inquiries. The build of the greyhound or race- horse suggests in itself part of the explanation on mechanical principles, lung capacity, etc. But when it is found that one dog, horse, deer, or man excels another of the same race in swiftness or endurance, and there is nothing in the form to furnish a solution, we are prompted to ask whether the muscles may not contract more energetically, experience a shortening of the latent period or other phase of contraction ; or whether they produce less of waste-products or get rid of them more rapidly. The whole subject is extremely complicated, and we may say here that there is some evidence to show that in races of dogs and other animals which surpass their fellows the nerve regulating the heart and lungs (vagus) has greater power ; 204 COMPARATIVE PHYSIOLOGY. but, leaving 1 this and much more out of the account, it is likely there are individual differences in the functional nature of the muscle. Of equal or more importance is the energizing- influ- ence of the nervous system, which probably under great excite- ment (public boat-races, etc.) acts to produce in man those supermaximal contractions which seem to leave the muscle long the worse of its unusual action. The nerve-centers, it is likely, suffer still more from excessive discharge of nerve-force (as we may speak of it for the present) necessary to originate the muscular work. Hence the importance of training in all animals to minimize the non-effective expenditure, ascertain the capacity possessed, learn the direction in which weaknesses lie ; and equally important the much neglected-period of rest before actual contests — if such are to be undertaken at ail- so that all the activities of the body may gather head, and thus be prepared to meet the unusual demand upon them. The law of rhythm in organic nature is beautifully illus- trated by the behavior of nerve and especially muscle; at least it is more obvious in the case of muscle, at this stage of our progress. The regularity with which one phase succeeds another in a single contraction ; the essentially rhythmic (vibratory) char- acter of tetanus, fatigue and recovery ; the recurrence of in- crease and decrease in the muscle and nerve currents — in fact, the whole history of muscle is an admirable commentary on the truth of the law of rhythm, into which in further detail space will not permit us to enter. It is a remarkable fact that the endurance of man, especially civilized man, seems to be greater than that of any other mam- mal. It may be hazardous to express a dogmatic opinion as to the reason of this, but the influence of the mind over the body is unquestionably greater in man than in any other animal ; and, if we are correct in assigning so much importance to the influence of the nervous system in maintaining the proper molecular balance which is at the foundation of the highest good of an organism, we certainly think that it is in this direc- tion we must look for the explanation of the above-mentioned fact, and much more that would otherwise be obscure in man's functional life. Functional Variations.— We have endeavored, in treating this subject of muscle, to point out how the phenomena vary with the animal, the kind of muscle, and the circumstances THE STUDY OF MUSCLE PHYSIOLOGY. 205 under which they are manifested. It may be shown that every one of the qualities which a muscle possesses varies with the temperature, the blood-supply, the duration of its action, the character of the stimulus, and other modifying agents. Not only are there great variations for different groups of animals, but lesser ones for individuals ; though the latter are made more evident indirectly than when tested by the usual laboratory methods ; but they must be taken account of if we would un- derstand animals as they are. Some of these will be referred to later. If a muscle-cell be regarded in the aspect that we are now emphasizing, its study will tend to impress those fundamental biological laws, the comprehension of which is of more impor- tance than the acquisition of any number of facts, winch, how- ever interesting, can, when isolated, profit little. Summary of the Physiology of Muscle and Nerve.— The movements of a muscle are distinguished from those of other forms of protoplasm by their marked definiteness and limit- ation. The contraction of a muscle-fiber (cell) results in an increase in its short transverse diameter, and a diminution of its long diameter, without appreciable change in its total bulk. Muscle and nerve are not automatic, but are irritable. Though muscle normally receives its stimulus through a nerve, it possesses independent irritability. Stimuli may be mechanical, chemical, thermal, electrical, and in the case of muscle, nervous ; and to be effective they must be applied suddenly and last for a brief but appreciable time. Electrical stimulation, especially, is only effective when there is a sudden change in the force or direction of the cur- rents. This applies to both muscle and nerve. A muscular contraction consists of three phases : the latent period, the period of rising, and the period of falling energy, or of contraction and relaxation. When the phase of relaxation is minimal and that of con- traction approaches continuity, a tetanus results. The contrac- tions of the muscles in situ are tetanic, and are accompanied by a low sound, evidence in itself of their vibratory character. The prolonged contraction of a muscle leads to fatigue ; owing in part, at least, to the accumulation of waste-products within the muscle which depress its energies. This is a necessary consequence of the fact that all proto- 206 COMPARATIVE PHYSIOLOGY. plasmic activity is accompanied by chemical change, and that some of these processes result in the formation of products which are hurtful and are usually rapidly expelled. Muscular contraction is accompanied by chemical changes, in which the formation of carbon dioxide, and some substance that causes an acid reaction to take the place of an alkaline or neutral one. Since free oxygen is not required for the act of contraction, but is still used up by a contracting muscle, it may be assumed that the oxygen that plays a part in actual contrac- tion is intra-molecular. Chemical changes are inseparable from the vital processes of all protoplasm, and the phenomena of muscle show that they are constantly in operation, but exalted during ordinary contraction and that tetanic condition which precedes and may end in coagidation of muscle plasma and the formation of myosin. The latter is a result of the disorganization of muscle, and has points of resemblance to the coagulation of the blood. The contraction of a muscle and the passage of a nervous impulse are accompanied by electrical changes. Whether cur- rents exist in uninjured muscle and nerve is a matter of contro- versy. Ml physiologists agree that they exist in muscle (and nerve) duiing functional activity. During the passage of a constant (polarizing) current from a battery through a nerve, it undergoes a change in its irrita- bility and shows a variation in the electro-motive force of the ordinary nerve-current (electrotonus). This fact is of thera- peutic importance. The electrical phenomena of nerve are altogether more prominent than the chemical, the reverse of which is true of muscle. The activity of a muscle (and nerve probably) is accompanied by the generation of heat, an exalta- tion of which takes place during muscular contraction. Rigor mortis causes an increase in temperature and the chemical interchanges which accompany the other phenomena. A muscle may also become rigid by passing into rigor caloris. Living muscle is translucent, alkaline or neutral in reaction, and elastic ; dead muscle, opaque, acid in reaction, and devoid of elasticity, but firmer than living muscle, owing to coagula- tion of the muscle-plasma. Dead nerve undergoes similar changes. The elasticity of muscle is restricted but perfect within its own limits. It differs from that of inorganic bodies in that the increments of extension are not directly proportional to the THE STUDY OF MUSCLE PHYSIOLOGY. 207 increments of the weight. When overstretched, muscle does not return to its original length (loss of elasticity), hence the serious nature of sprains. It is important to regard muscular elasticity as an expression of vital properties. The work done by a muscle is ascertained by multiplying the load lifted by the height; and the capacity of an individual muscle will vary with its length, the arrangement of its fibers, and the area of its cross-section (i. e., the number of fibers). The work done may be regarded as a function of the resist- ance (load), as the contraction is also a function of the stimulus. The sepai'ation of a muscle from its nerve by section of the lat- ter leads to certain changes, most rapid in the nerve, which show that the two are so related that prolonged independent vitality of the muscle is impossible, and make it highly proba- ble that muscle is constantly receiving some beneficial stimulus from nerve, which is exalted and manifest when contraction takes place. The study of the development of the electrical cells of cer- tain fishes shows that they are greatly modified muscles in which contractility, etc., has been exchanged for a very decided exaltation of electrical properties. It is likely, though not demonstrated, that all forms of protoplasm undergo electrical changes — that these, in fact, like chemical phenomena, are vital constants. The phases of the contraction of smooth muscular tissue are all of longer duration; the contraction- wave passes in different directions, and may spread into cells devoid of nerves, which we think not unlikely also to be the case, though less so, for all forms of muscle. The smooth muscle-cell must be regarded as a more primi- tive, less specialized, form of tissue. Variations in all the phe- nomena of muscle with the animal and the circumstances are clear and impressive. Finally, muscle illustrates an evolution of structure and function, and the law of rhythm. THE NERVOUS SYSTEM.— GENERAL CONSIDER- ATIONS. Since in the higher vertebrates the nervous system is domi- nant, regulating apparently every process in the organism, it will be well before proceeding further to treat of some of its functions in a general way to a greater extent than we have yet done. Manifestly, it must be highly important that an animal shall be able to place itself so in relation to its surroundings that it may adapt itself to them. Prominent among these adaptations are certain movements by which food is secured and dangers avoided. The movements having a central origin, a peripheral mechanism of some kind must exist so as to place the centers in connection with the outer world. Passing by the evolution of the nervous system for the present, it is found that in verte- brates generally there is externally a modification of the epi- thelial covering of the body {end-organ) in which a nerve ter- minates, which latter may be traced to a cell or cells removed from the surface (center), and from which in most cases other nerves proceed. The nervous system, we may remind the student, consists in vertebrates of centers in which nerve-cells abound, united by nerve-fibers and by the most delicate form of connective tissue known, in connection with which there are incased strands of protoplasm or nerves as outgrowths. The main centers are, of course, aggregated in the brain and spinal cord. It is possible to conceive of the work of a nervous system carried on by a single cell and an afferent and efferent nerve ; but inasmuch as such an arrangement would imply that the central cell should act the part of both receiving and origi- nating impulses (except it were a mere conductor, in which case there would be no advantage whatever in the existence of a cell at all), according to tbe principle of the physiological division NERVOUS SYSTEM.— GENERAL CONSIDERATIONS. 209 of labor, we might expect that there would be at least two cen- tral cells — one to receive and the other to transmit impulses — or at least that there should be some specialization among the central cells ; and we shall have good reason later to believe that this has reached a surprising degree in the highest ani- mals. Moreover, it would be a great advantage if the termination of the ingoing (afferent) nerve should not lie exposed on the surface, but be protected by some form of ce)l that had also the power to transmit to it the impressions received from without, in a form suitable to the nature of the nerve and the needs of the organism. So that a complete mechanism in its simplest form would furnish : 1. A periphei'al cell or nerve end-organ. 2. An affer- ent or sensory nerve. 3. Two or more central cells. 4. An efferent nerve, usually connected with — 5. A muscle or other form of cell, the action of which may be modified by the out- going nerve, or, as we should prefer to say, by the central nervous cells through the efferent nerve. The advantages of the principal cells being within and protected are obvious. When, then, an impression made on the peripheral cell is carried inward, there modified, and results in an outgoing nerv- ous impulse answering to the afferent one, giving rise to a mus- cular contraction or other effect not confined to the recipient cells, the process is termed reflex action. Tbe great size, the multiplicity of forms, the distinct out- line and large nuclei of nerve-cells, suggest the probability that they play a very important part, and such is found to be the case. Indeed, in some sense the rest of the nervous system may be said to exist for them. Probably nerve-cells do sometimes act as mere conductors of nervous impulses originating elsewhere, but such is their lowest function. Accordingly, it is found that the nature of any reflex action depends most of all on the behavior of the central cells. It can not be too well borne in mind that nerves are con- ductors and such only. They never originate impulses. The properties considered in the last chapter are common to all kinds of nerves known; and though we must conceive that there are some differences in the form of impulses, these are to be traced, not to the nerve primarily, but to the organ in which it ends peripherally or to the central cells. 14 210 COMPARATIVE PHYSIOLOGY. To return to reflex action, it is found that the muscular re- sponse to a peripheral irritation vai'ies with the point stimu- lated, the intensity of the stimulus, etc., but is, above all, deter- mined by the central cells. Nerve influence may be considered as following lines of least resistance, and there is much evidence to show that an im- pulse having once taken a certain path, it is easier for it to pass in this direction a second time, so that we have the foundation of the laws of habit and a host of interesting phenomena in this simple principle. It is found that, in a frog deprived of its brain and sus- pended by the under jaw, there is no movement unless some stimulus be applied ; but if this be done under suitable condi- tions, instructive results follow, which we now proceed to indi- cate briefly. The experiments are of a simple character, which any student may carry out for himself. Experimental. — Preparing a frog by cutting off the whole of the upper jaw and brain-case after momentary anaesthesia, suspend the animal by the lower jaw and wait till it is perfectly quiet. Add to water in a beaker sulphuric acid till it tastes distinctly but not strongly sour, to be used as a stimulus. 1. Apply a small piece of bibulous paper, moistened with the acid, to the inner part of the thigh of the animal. The leg will be drawn up and the paper probably removed. Remove the paper and cleanse the spot. 2. Apply a similar piece of paper to the middle of the abdomen ; one or both legs will probably be drawn up, and wipe off the offending body. 3. Let the foot of the frog hang in the liquid ; after a few moments it will be withdrawn. 4. Repeat, holding the leg ; probably the other leg will be drawn up. 5. Apply stronger acid to the inside of the right thigh ; the whole frog may be convulsed, or the left leg may be put in action after the right. Even if the stimulat- ing paper be applied near the anus, it will be removed by the hind-legs. 6. Beneath the skin Df the back, (posterior lymph- sac) inject a few drops of liquor strychnia? of the pharama- copocia; after a few minutes apply the same sort of stimulus to the thigh as before. The effects follow more quickly and are much more marked-- the animal, it may be, passing into a gen- eral tetanic spasm. These experiments may be varied, but suffice to establish the following conclusions : 1. The stimulus is not immediately effective, but requires to act for a certain variable period, de- NERVOUS SYSTEM.— GENERAL CONSIDERATIONS. 211 6ENSORY CENTRE INHIBITORY CENTRE C-^V SENSORY CELL AND AFFERENT NERVE Fig. 187. — Diagram intended to illustrate nervous mechanism of— 1. automatism; 2, reflex action; and 3. how nervous impulses in the latter case may pass into the higher parts of brain and become part of consciousness, or be wholly inhibited. A reflex or automatic center may, for the sake of simplicity, be reduced to a sin- gle cell, as above on the left. pending 1 chiefly on the condition of the central nervous sys- tem. 2. The movements of the muscles harmonize (are co-ordi- nated), and tend to accomplish some end — are purposive. If the nerve alone and not the skin be stimulated, there may be a spasm only and not adaptive movement. 3. Nervous impulses, when very abundant, niay pass along unaccustomed or less ac- customed paths (experiments 4 and 5). This is sometimes spoken of as the radiation of nervous impulses. The sixth experiment is very important, for it shows that the result varies far more with the condition of the nervous centers (cells) than the stimulus, the part excited, or any other factor. Automatism. — But, seeing that these central cells have such independence and controlling power, the question arises. Are 212 COMPARATIVE PHYSIOLOGY. these, or are there any such cells, capable of originating im- pulses in nerves wholly independent of any stimulus from without ? In other words, have the nerve-centers any true automatism ? Apparently this quality is manifested by uni- cellular organisms of the rank of Amceba. Has it been lost, or has it become a special characteristic developed to a high degree in nerve-cells ? We shall present the facts and the opinions based on them as held by the majority of physiologists, reserving our own criticisms for another occasion : 1. The medulla oblongata is supposed to be the seat of numerous small groups of cells, to a large extent independent of each other, that are constantly sending out nervous impulses which, proceeding to certain sets of muscles, maintain them in rhythmical action. One of the best known of these centers is the respiratory. 2. The poste- rior lymph hearts of the frog are supplied by nerves (tenth pair), which are connected, of course, with the spinal cord. When these nerves are cut, the hearts for a time cease to beat, but later resume their action. 3. The heart beats after all its nerves are cut, and it is removed from the body, for many hours, in cold-blooded animals. 4. The contractions of the intestine take place in the absence of food, and in an isolated piece of the gut. The intestine, it will be remembered, is abundantly supplied with nerve-elements. 5. In a portion of the ureters, from which it is believed nerve-cells are absent, rhythmical ac- tion takes place. Conclusions. — 1. Whether the action of the respiratory and similar centers could continue in the absence of all stimuli can not be considered as determined. 2. That there are regular rhythmical discharges from the spinal nerve-cells along the nerves to the lymph hearts seems also doubtfbl. 3. Later in- vestigations render the automaticity of the heart more uncer- tain than ever, so that the result stated above (3) must not be interpreted too rigidly. Similar doubts hang about the other cases of apparent au- tomatism. As regards the various comparatively isolated collections of cells known as ganglia, the evidence, so far as it goes, is against their possessing either automatic or reflex action ; and new views of their nature will be presented in due course. Nervous Inhibition. — If the pneumogastric nerve passing from the medulla to the heart of vertebrates be divided and the NERVOUS SYSTEM.— GENERAL CONSIDERATIONS. 213 lower (peripheral) end stimulated, a decided change in the ac- tion of the heart follows, which may be in the direction of" weakening- or slowing, or positive arrest of its action. Assuming, for the present, that the cells (center) of the me- dulla have the power to bring about the same result, it is seen that such nervous influence is preventive or inhibitory of the normal cardiac beat, so that the vagus is termed an inhibitory nerve. Such inhibition plays a very important part in the economy of the higher animals, as will become rnore and more evident as we proceed. The nature of the influences that pro- duce such remarkable results will be discussed when we treat of the heart. An illustration will probably serve in the mean time to make the meaning of what has been presented in this chapter more clear and readily grasped. In the management of railroads a very great variety of com- plicated results are brought about, owing to system and orderly arrangement, by which the wishes of the chief manager are carried out. Telegraphing is of necessity extensively employed. Sup- pose a message to be conveyed from one office to another, this may (1) simply pass through an intermediate office, without special cognizance from the operator in charge ; (2) the operator may receive and transmit it unaltered ; (3) he may be required to send a message that shall vary from the one he receives in a greater or less degree ; or (4) he may arrest the command alto- gether, owing to the facts which he alone knows and upon which he is empowered always to act according to his best dis- cretion. In the first instance, we have an analogy with the passage of a nervous impulse through central fibers, or, at all events, unaffected by cells ; in the second, the resemblance is to cells acting as conductors merely ; in the third, to the usual behavior of the cells in reflex action; and, in the fourth, we have an in- stance of inhibition. The latter may also be rendered clear by the case of a horse and its rider. The horse is controlled by the rider, who may be compared to the center, through the reins answering to the nerves, though it is not possible for either rider or reins to originate the movements of the animal, except as they 'may be stimuli, which latter are only effective when there are suitable conditions— when, in fact, the subject is irritable in the physiological sense. THE CIRCULATION OF THE BLOOD. Every tissue, every cell, requiring constant nourishment, some means must necessarily have been provided for the con- veyance of the blood to all parts of the organism. We now enter upon the consideration of the mechanisms by which this is accomplished and the method of their regulation. Let us consider possible mechanisms, and then inquire into their defects and the extent to which they are found embodied in nature. That there must be a central pump of some kind is evident. Assume that it is one-chambered, and with an outflow-pipe which is continued to form an inflow-pipe. This might be pro- vided with valves at the openings, by which energy would be saved by the prevention of regurgitation. In such a system things must go from bad to worse, as the tissues, by constantly using up the prepared material of the blood, and adding to it their waste products, would effect their own gradual starvation and poisoning. It might be conceived, however, that waste at all events was got rid of by the blood being conducted through some elimi- nating organs ; and assume that one such at least is set aside for respiratory work. If the blood in its course anywhere passed through such organs, the end would be attained in some degree ; but if the division of labor were considerable, we should suppose that, gaseous interchange being so very impor- tant as we bave been led to see from the study of the chapters on general biology, and on muscle, organs to accomplish this work might receive the blood in due course and return it to the central pump in a condition eminently fit from a respiratory point of view. Such, however, would necessarily be associated with a more complicated pump ; and, if this were so constructed as to pre- vent the mixture of blood of different degrees of functional value, higher ends would be attained. THE CIRCULATION OP THE BLOOD. 215 Turning to the channels themselves in which the blood flows, a little consideration will convince one that rigid tubes are wholly unfit for the purpose. Somewhere in the course of the circulation the blood must flow sufficiently slowly, and through vessels thin enough to permit of that interchange be- tween the blood and the tissues, through the medium of the lymph, which is essential from every point of view. The main vessels must have a strength sufficient to resist the force with which the blood is driven into them. Now, it is possible to conceive of this being accomplished with an intermittent flow ; but manifestly it would be a great advantage, from a nutritive aspect, that the flow and therefore the supply of tissue pabulum be constant. With a pump regu- larly intermittent in action, provided with valves, elastic tubes having a resistance in them somewhere sufficient to keep them constantly over-distended, and a collection of small vessels with walls of extreme thinness, in which the blood-current is greatly slackened, a steady blood-flow would be maintained, as the student may readily convince himself, by a few experiments of a very simple kind : 1. To show the difference between rigid tubes and elastic ones, let a piece of glass-rod, drawn out at one end to a small diameter, have attached to the other end a Higginson's (two- bulb) syringe, communicating with a vessel containing water. Every time the bulb is squeezed, water flows from the end of the glass rod, but the outflow is perfectly intermittent. 2. On the other hand, with a long elastic tube of India-rub- ber, ending in a piece of glass rod drawn out to a point as be- fore, if the action of the pump (bulb) be rapid the outflow will be continuous. An apparatus that every practitioner of medi- cine requires to use answers perhaps still better to illustrate these and other principles of the circulation, such as the pulse, the influence of the force and frequency of the heart-beat on the blood-pressure, etc. We refer to a two-bulb atomizer, the bulb nearer the outflow serving to maintain a constant air-pressure. We may now examine the most perfect form of heart known, that of the mammal, in order to ascertain how far it and its adjunct tubes answer to a priori expectations. The Mammalian Heart.— In order that the student may gain a correct and thorough knowledge of the anatomy of the heart and the, workings of its various parts, we recommend him to pursue some such course as the following : 216 COMPARATIVE PHYSIOLOGY. 1. To consult a number of plates, such as are usually fur- nished in works on anatomy, in order to ascertain in a general way the relations of the heart to other organs, and to the chest wall, as well as to become familiar with its own structure. 2. To supplement this with reading the anatomical descrip- tions, without too great attention to details at first, but with the ob- ject of getting his ideas clear so far as they go. 3. Then, with plates and descriptions before him, to examine sever- al dead specimens of the heart of the sheep, ox, pig, or other mam- mal, first somewhat generally, then syste- matically, with the purpose of getting a more exact knowledge of the various struct- ures and their anatom- ical as well as physi- ological relations. We would not have the student confine his attention to any single form of heart, for each shows some one struct- ure better than the others ; and the addi- tional advantages of comparison are very great. The heart of the ox, from its size, is excellent for the study of valvular action, and the framework with which the muscles, valves, and vessels are connected ; while the heart of the pig (and dog) resemble the human organ more closely than most others that can be obtained. I'n;. 186.— The left auricle and ventricle opened and pari of their walls removed to show their cavities (Allen Thomson). 1, right pulmonary vein cut short; V, cavity of left auricle; 3, thick wall of left ventricle ; 4, portion of the same with pap- illary muscle attached ; 5, 5', the other papillary muscles; 0, one segment of the mitral valve; 7, in aorta is placed over the semilunar valves. THE CIRCULATION OF TI1E BLOOD. 217 It will be found very helpful to perform some of the dissec- tions under water, and by the use of this or some other fluid the action of the valves may be learned as it can in no other way. By a little manipulation the heart may be so held that water may be poured into the orifices, prepared by a removal of a portion of the blood-vessels or the auricles, when the valves may be seen closing together, and thus revealing their action in a way which no verbal or pictorial representation can do at all adequately. A heart thoroughly boiled and allowed to get cold shows, on being pulled somewhat apart, the course, attachment, and other ■It T^m.v.1 JRAV Fio. 187. -View of the orifices of the heart from below, the whole of the ventricles having been cut away (after Huxley). JRAV, right auriculo-ventricular orifice, surrounded bv the three flaps, t. v. 1, t. v. 2. t. v. 3, of the tricuspid valve, which are stretched by weights attached to the chorda tendinece. LAV. left auriculo-ven- tricular orifice, etc. PA. orifice of the pulmonary artery, the semilunar valves represented as having met and closed together. A 0, orifice of the aorta. features of the fibers very well, as also the skeleton of the organ, which may be readily separated. When this has all been done, the half is not yet accom- plished. A visit to an abattoir will now repay amply for the time spent. Animals are there killed and eviscerated so rapidly that an observer may not only gain a good practical acquaint- ance with the relations of the heart to other parts, but may often see the organ still living and exemplifying that action 218 COMPARATIVE PHYSIOLOGY. peculiar to it as it gradually approaches quiescence and death — a matter of the utmost importance. If the student will then compare what he has learned of the mammalian heart in this way with the behavior of the heart of a frog - , snake, fish, turtle, or other animal that may be killed after brief ether narcosis, without cessation of the heart's ac- tion, he will have a broader basis for his cardiac physiology than is usual ; and we think we may promise the medical stu- dent, who will in this and other ways that may occur to him supplement the usual work on the human cadaver, a pleasure and profit in the study of heart-disease which come in no other way. With the view of assisting the observation of the student as regards the heart of the mammal, we would call special atten tion to the following points among others : Its method of sus- pension, chiefly by its great vessels ; the strong fibrous frame- work for the attachment of valves, vessels, and muscle-fibers; the great complexity of the arrangement of the latter; the various lengths, mode of attachment, and the strength of the PA Fig. 188.— Orifices of the heart seen from above, after the auricles and great vessels had been cut awav (after Huxley). PA . pulmonary artery with its semilunar valves. Ao, aorta in a similar condition. RAV, right auriculo-ventricular orifice, with m. v. 1 and 2 flaps of mitral valve: b. style passed into coronary vein. On the left part of LA Fthe section of the auricle is carried through the auricular appendage, hence the toothed appearance due to the portions in relief cut.across. inelastic chordae tendinese; the papillary muscles, which doubt- less act at the moment the valves flap back, thus preventing THE CIRCULATION OF THE BLOOD. 219 the latter being carried too far toward the auricles, the pocket- ing action of the semilunar valves with their strong margin and meeting nodules {corpora Arantii) ; the relative thickness of auricles and ventricles, and the much greater thickness of the walls of the left than of the right ventricle— differences which are related to the work these parts perform. The latter may be well seen by making transverse sections of the heart of an animal, especially one that has been bled to death, which specimen also shows how the contraction of the heart obliterates the ventricular cavity. It will also be well worth while to follow up the course of the coronary arteries, noting especially their point of origin. The examination of the valves of the smaller hearts of cold- blooded animals is a matter of greater difficulty and is facili- tated by dissection under water with the help of a lens or dis- secting microscope ; but even without these instruments much may be learned, and certainly that the valves are relatively to those of the mammalian heart imperfectly developed, will be- come very clear. CIRCULATION OF THE BLOOD IN THE MAMMAL. It is highly important and quite possible in studying the circulation to form a series of mental pictures of what is trans- piring. It will be borne in mind that there is a set of elastic tubes of relatively thick walls, standing open when cut across, dividing into smaller and smaller branches, and finally ending in vessels of more than cobweb fineness, and opening out into others, that become larger and larger and fewer and fewer, till they are gathered up into two of great size which form the right auricle. The larger pipes consist everywhei'e of elastic tissue proper, muscular tissue (itself elastic), fibrous tissue, and a flat epithelial lining, so smooth that the friction therefrom must be minimal as the blood flows over it. The return tubes or veins are like the arteries, but so thin that their walls fall together when cut across. They are differ- ent from all the other blood-tubes in that they possess valves opening toward the heart throughout their course. The veins are at least twice as numerous as the arteries, and their capacity many times greater. The small vessels or capillaries are so abundant and wide-spread that, as is well known, the smallest 220 COMPARATIVE PHYSIOLOGY. cut anywhere gives rise to a flow of blood, owing to sec- tion of some of these tubes, which, it will be remembered, are not visible to the unaided eye. It is estimated that their united area is several hun- dred (500 to 800) times that of the arteries. If we suppose the epithe- lial lining pushed out of a small artery we have, so far as structure alone goes, a good idea of a capillary — i. e., its walls are but one cell thick, and these cells though loug are extremely thin, so that it is quite easy to under- stand how it is that the amoe- boid corpuscles can, under certain circumstances, push tbeir way through its proba- bly semi-fluid walls. From what has been said, it will be seen that the whole collection of vascular tubes may be compared to two inverted funnels or cones with the Fig. 189.— Various layers of the walls of a small artery (Landois). e, endothelium; i. e, internal elastic lamina; c. m, circu- lar muscular fibers of the middle coat; c. t, connective tissue of the outer coat, or T. adventitia. Fig. 190. Fio. 191. Fig. 190.— Vein with valves lying open (Dalton). In. 191.— Vein with valves closed, the blood passing on by a lateral branch below (Dalton). THE CIRCULATION OF THE BLOOD. 221 w Ifi iqo _CaDillarv blood-vessels (Landois). The cement substance between the en- dothelium h'^Len rendered dark by silver nitrate, and the nuclei made prominent by staining. smaller end toward the heart aud the widest portions repre- senting the capillaries. Pig. 193—Diagram to illustrate the relative proportions of the ^egate sectional we.; of the different parts of the vascular system (after \eo). A, aorta, C, capillars . V, veins. 222 COMPARATIVE PHYSIOLOGY, THE ACTION OF THE MAMMALIAN HEART. What takes place may be thus very briefly stated : The right auricle contracting- squeezes the blood through the au- ricular-ventricular opening into the right ventricle, never quite Superior Vena Cava. Inferior Vena Cava. Capillaries of the Head, etc. Pulmonary Ca- pillaries. Capillaries of Trunk and Lower Ex- tremities. Fk;. 194.— Diagram of the circulation. The arrows indicate the course of the blood. Though the pulmonary, the lower and the upper parts of the systemic circulation are represented so as to show the distinctness of each, it will be also apparent that they are not independent. Relative size of different parts of the system is only very generally indicated. emptying itself probably; immediately after the right ventricle contracts, by which its valves are brought into sudden tension and opposition, thus preventing reflux into the auricle ; while the blood within it takes the path of least resistance, and the THE CIRCULATION OP THE BLOOD. 223 only one open to it into the pulmonary artery, and by its branches is conveyed to the capillaries of the lungs, from which it is returned freed from much of its carbonic anhydride and replenished with oxygen, to the left auricle, whence it proceeds in a similar manner into the great arterial main, the aorta, for general distribution throughout the smaller arteries and the capillaries to the most remote as well as the nearest parts, from which it is gathered up and returned laden with many impuri- ties, and robbed of a large proportion of its useful matters, to the right side of the heart. It will be remembered that corresponding subdivisions of each side of the heart act simultaneously, and that any decided departure from this harmony of rhythm would lead to serious disturbance. THE VELOCITY OF THE BLOOD AND BLOOD-PRESSURE. If the relative capacity and arrangement of the various parts of the circulatory system be as has been represented, it follows that we may predict with some confidence, apart from experi- ment, what the speed of the flow and the vascular tension must be in different parts of the course of the circulation. We should suppose that, in the nature of the case, the veloc- ity would be greatest in the large arteries, gradually diminish to the capillaries, in which it would be much the slowest and, getting by degrees faster, would reach a speed in the largest veins approaching that of the corresponding arteries. The methods of determining the velocity of the blood-stream have not entirely surmounted the difficulties, but they do give results in harmony with the above-noted anticipations. The area of the great aortic trunk being so much less than that of the capillaries, the flow in that vessel we should expect to be very much swifter than in the arterioles or the capillaries. Moreover there must be a great difference in the velocity during cardiac systole and diastole, and according as the beat of the heart is forcible or otherwise. But apart from these more ob- vious differences, there are variations depending on complex changes in the peripheral circulation, owing to the frequent variations in the diameter of the arterioles in different parts, as well as differences in the resistance offered by the capillaries, the causes of which are but ill understood, though less obscure, we think, than they are often represented to be. Since for the 224 COMPARATIVE PHYSIOLOGY. maintenance of the circulation, the quantity of blood entering and leaving the heart must be equal, in consequence of the sec- tional area of the great veins that enter the heart being greater than that of the aorta, it follows that the venous flow even at its quickest is necessarily slower than the arterial. Comparative. — There must be great variations in velocity in different animals, as such measurements as have been made demonstrate. Thus, in the carotid of the horse, the speed of the blood-current is calculated as about 306 mm., in the dog at from 205 to 357 mm. These results can not be considered as more than fair approximations. Highly important is it to note that the rate of flow in the capillaries of all animals is very slow indeed, not being as much as 1 mm. in a second in the larger mammals. The time occupied by the circulation is also, of course, variable, being as a rule shorter the smaller the animals. As the result of a number of calculations, though by methods that are more or less faulty, the following law may be laid down as meeting approximately the facts so far as warm-blooded animals are concerned. The circulation is effected by 27 beart-beats ; thus for a man with a pulse of 81, the time occupied in the completion of the course of the blood from and to the heart would be f| = 3 ; i. e., the circulation is completed three times in one minute, or its period is twenty seconds ; and it is to be well borne in mind that by far the greater part of this time is occupied in traversing the capillaries. THE CIRCULATION UNDER THE MICROSCOPE. There are few pictures more instructive and impressive than a view of the circulation of the blood under the microscope. It is well to have similar preparations, one under a low power and another under a magnification of 300 to 500 diameters. With the former a view of arterioles, veins, and capillaries may be obtained at once. Many different parts of animals may be used, as the web of the frog's foot, its tongue, lung, or mesen- tery ; the gill or tail of a small fish, tadpole, etc. The relative size of the vessels ; the speed of the blood flow; the greater velocity of the central part of the stream ; the aggre- gation of colorless corpuscles at the sides of the vessels, and the occasional passage of one through a capillary wall, when the exposure has lasted some time; the crowding of the red cells; THE CIRCULATION OF THE BLOOD. 225 their plasticity ; the small size of some of the capillaries, barely allowing the corpuscles to be squeezed through; the changes in the velocity of the current, especially in the capillaries ; its pos- sible arrest or retrocession ; the velocity in one so much greater than in its neighbor, without very obvious cause — all this and Fig. 195. — Portion of the web of a frog's foot as seen under a low magnifying power, showing the blood-vessels, and in one corner the pigment-spots (after Huxley), a, small arteries (arterioles); v, small veins. The smaller vessels are the capillaries. The course of the blood is indicated by arrows. much more forms, as we have said, a remarkable lesson for the thinking student. This, like all microscopic views, especially if motion is represented, has its fallacies. It is to be remem- bered that the movements are all magnified, or else one is apt to suppose the capillary circulation, extremely rapid, whereas it is like that of the most sluggish part of a stream, and very irregular. 15 226 COMPARATIVE PHYSIOLOGY. Fig. 196.— Circulation in the web of a frog's foot (Wagner). V, venous trunk com- posed of the three principal branches (v. v, v) covered with a plexus of smaller ves- sels. The whole is dotted over with pigment masses. THE CHARACTERS OF THE BLOOD-FLOW. If an artery be opened, the blood is seen to flow from it in a constant stream, with periodic exaggerations, which, it is found, answer to the heart-beats ; in the case of veins and capillaries the flow is also constant, but shows none of the spurting of the arterial stream, nor has the cardiac beat appar- ently an equal modifying effect upon it. We have already explained why the flow should be constant, though it would be well to be clearer as to the peripheral re- sistance. The amount of friction from linings so smooth as those of the blood-vessels can not be considerable. Whence, then, arises that friction which keeps the arterial vessels always distended by its backward influence ? The microscopic study of the circulation helps to answer this question. The plas- ticity of the corpuscles and of the vessel walls themselves must be taken into account, in consequence of which a dragging influence is exerted whenever the corpuscles touch the wall, which must constantly happen with vast numbers of them in the smallest vessels and especially in the capillaries. The arrangement of capillaries into a mesh-work, must also, in THE CIRCULATION OF THE BLOOD. 227 consequence of so many angles, be a source of much fric- tion. The action of the corpuscles on one another may be com- pared to a crowd of people hurrying along a narrow passage — the obstruction comes from interaction of a variety of forces, owing to the crowd itself rather than the nature of the thor- oughfare. We must set down a great deal to the influence of the corpuscles on one another, as they are carried along accord- ing to mechanical principles ; but, as we shall see later, other and more subtle factors play a part in the capillary circulation. Owing to the peripheral resistance and the pumping force of the heart, the arteries become distended, so that, during cardiac diastole, their recoil, owing to the closure of the semilunar valves, forces on the blood in a steady stream. It follows, then, that the main force of the heart is spent in distending the arteries, and that the immediate propelling force of the circu- lation is the elasticity of the arteries hi which the heart stores up the energy of its systole for the moment. BLOOD-PRESSURE. Keeping in mind our schematic representation of the circu- lation, we should expect that the blood must exercise a certain pressure everywhere throughout the vascular system ; that this blood-pressure would be highest in the heart itself ; considera- ble in the whole arterial system, though gradually diminishing toward the capillaries, in which it would be feeble ; lower still in the smaller veins ; and at its minimum where the great veins enter the heart. Actual experiments confirm the truth of these views ; and, as the subject is one of considerable importance, we shall direct attention to the methods of estimating and record- ing an animal's blood-pressure. First of all, the well-known fact that, when an artery is cut, the issuing stream spurts a certain distance, as when a water- main, fed from an elevated reservoir, bursts, or a hj T drant is opened, is itself a proof of the existence of blood-pressure, and is a crude measure of the amount of the pressure. One of the simplest and most impressive ways of demon- strating blood-pressure is to connect the carotid, femoral, or other large artery of an animal by means of a small glass tube (drawn out in a peculiar manner to favor insertion and reten- tion by ligature in the vessel), known as a cannula, by rubber 228 COMPARATIVE PHYSIOLOGY. Fig. 197. THE CIRCULATION OB" THE BLOOD. 229 Fi«. 19?.— Apparatus used in making a blood-pressure experiment (after Foster) //. b. pressure-bottle, elevated so as to raise the pressure several inches of mercury, as seen in the manometer (m) below. It contains a saturated solution of sodium car- bonate; r. t, rubber tube connecting the pb with the leaden tube: /. t, tube made of lead, so as to be pliable, yet have rigid walls; t «. c, a stop-cock, the top of which is removable, to allow escape of bubbles of air; p, the pen, writing on the roll of paper, r. The former floats on the mercury; m, the manometer, the shaded por- tion of the bent tube denoting the mercury, the rest is tilled with a fluid unfavor- able to the coagulation of the Dlood, and derived from the pressure-bottle; ca, the carotid, in which is placed the cannula, and below the latter a forceps, which may be removed when the blood-pressure is to be actually measured. The registration of the height, variation, etc., of blood-pressure, is best made on a continuous roll of paper, as seen in Fig. 198. tubing, with a long glass rod of bore approaching that of the artery opened, into which the blood is allowed to flow through the above-mentioned connections, while it is maintained in a vertical position. To prevent the rapid coagulation of the blood in such ex- periments, it is customary to fill the cannula and other tubes to a certain extent, at least, with a solution of some salt that tends to retard coagulation, such as sodium carbonate or bicar- bonate, magnesium sulphate, etc. If other connections are made in a similar way with smaller arteries and veins, it may be seen that the height of the respective columns representing the blood-pressure, varies in each and in accordance with ex- pectations. While all the essential facts of blood-pressure and many others may be illustrated by the above simple methods, it is inad- equate when exact measurements are to be made or the results to be recorded for permanent preservation ; hence apparatus of a somewhat elaborate kind has been devised to accomplish these . purposes. The graphic methods are substantially those already ex- plained in connection with the physiology of muscle ; but, since it is often desirable to maintain blood-pressure experiments for a considerable time, instead of a single cylinder, a series so connected as to provide a practically endless roll of paper (Fig. 198) is employed. When, in the sort of experiments referred to above, the height of the fluid used in the glass tube to prevent coagula- tion just suffices to prevent outflow from the artery into the connections, we have, of course, in this a measure of the blood- pressure; however, it is convenient in most instances to use mercury, contained in a glass tube bent in the form of a U. for a measure, as shown in the subjoined illustration. It is also desirable, in order to prevent outflow of the blood into the apparatus, to get up a pressure in the U-tube or manometer as 230 COMPARATIVE PHYSIOLOGY. near as may be equal to that of the animal to be employed in the experiment. This may be effected in a variety of ways, one of the most convenient of which is by means of a vessel con- taining some saturated sodium carbonate or similar solution in connection with the manometer. It is important that the pressure should express itself as directly and truthfully on the mercury of the manometer as possible, hence the employment of a tube with rigid walls, yet capable of being bent readily in different directions for the sake of convenience. Mercury, on account of its inertia, is not free from objec- tion ; and when very delicate variations in the blood-pressure — Pia. 198.— Large kymograph, with continuous roll of paper (Foster). The clock-work machinery unrolls the paper from the roll C, carries it smoothly over the cylinder B, and then winds it up into the roll A. Two electro-magnetic markers are seen in position recording intervals of time on the moving roll of paper. A manometer may !><• fixed in any convenient position. e. g., feeble pulse-beats — are to be indicated, it fails to express them, in which case other fluids may be employed. It will be noted that when an ordinary cannula is used, inserted as it is lengthwise into the blood-vessel, the pressure recorded is not that on the side of the vessel into which it is inserted as when a —\ - piece is used, but of the vessel, of which the one in question is a branch. The blood-pressure, in the main arterial trunk, for example, must depend largely on the THE CIRCULATION OF THE BLOOD. 231 force of the heart-beat ; consequently it would be expected, and it is actually found, that the pressure varies for different ani- mals, size having, of course, in most instances a relation to the result. It has been estimated that in the carotid of the horse the arterial pressure is 150 to 200 mm. of mercury, of the dog 100 to 175, of the rabbit 50 to 90. Man's blood-pressure is not known, but is probably high, we may suppose not less than 150 to 200 mm. After the fact that there is a certain considerable blood- pressure, the other most important one to notice is that this blood-pressure is constantly varying during the experiment, and, as we shall give reason to believe, in the normal animal ; and to these variations and their causes we shall presently turn our attention. THE HEART. The heart, being one of the great centers of life, to speak figuratively, it demands an unusually close study. THE CARDIAC MOVEMENTS. There is no special difficulty in ascertaining the outlines of the heart by means of percussion on either the dead or the living subject. Quite otherwise is it with the changes in form which accompany cardiac action. Attempts have been made to ascertain the alterations in position of the heart with respect to other parts, and especially its own alterations in shape dur- ing a systole, the chest being unopened, by the use of needles thrust into its substance through the thoracic walls ; but the re- sults have proved fallacious. Again, casts have been made of the heart after death, in a condition of moderate extension, prior to rigor mortis ; and also when contracted by a hardening fluid. These methods, like all others as yet employed, are open to seri- ous objections. Following the rapidly beating heart of the mammal with the eye produces uncertainty and confusion of mind. It may be very confidently said that the mode of contrac- tion of the hearts of different groups of vertebrates is variable, though it seems highly probable that the divergences in mam- mals are slight. The most that can be certainly affirmed of the mammalian heart is, that during contraction of the ventricles 232 COMPARATIVE PHYSIOLOGY. they become more conical ; that the long diameter is not appre- ciably altered ; that the antero-posterior diameter is lengthened ; and that the left ventricle at least turns on its own axis from left to right. This latter may be distinctly made out by the eye in watching the heart in the opened chest. THE IMPULSE OF THE HEART. When one places his hand over the region of the heart in man and other mammals, he experiences a sense of pressure varying with the part touched, and from moment to moment. Instruments constructed to convey this movement to recording Fig,, 199.— Marey's cardiac sound, which may be used to explore the chambers of the heart (after Foster), a is made of rubber stretched over a wire framework, with metallic supports above and below; b is a long tube. levers also teach that certain movements of the chest wall cor- respond with the propagation of the pulse, and therefore to the systole of the heart. It can be recognized, whether the hand or an instrument be used, that all parts of the chest wall over the heart are not equally raised at the one instant. If the beat- ing heart be held in the hand, it will be noticed that during systole there is a sudden hardening. The relation of the apex to the chest wall is variable for different mammals, and with different positions of the body in man. As a result of the investigation which this subject has re- ceived, it may be inferred that the sudden tension of the heart, owing to the ventricle contracting over its fluid contents, causes in those cases in which during diastole the ventricle lies against the chest wall, a sense of pressure beneath the hand, which is usually accompanied by a visible movement upward in some part of the thoracic wall, and downward in adjacent parts. It will not be forgotten that the heart lies in a pericardial sac, moistened with a small quantity of albuminous fluid ; and that by this sac the organ is tethered to the walls of the chest by its mediastinal fastenings ; so that in receding from the chest wall the latter may be drawn after it ; though this might THE CIRCULATION OF THE BLOOD. 233 Cardiac impulse. Fig. 200.— Simultaneous tracings from the interior of the right auricle, from the inte- rior of the right ventricle, and of the cardiac impulse in the horse (after Chauveau and Marey). Tracings to be read from left to right, and the references above are in the order from top to bottom. A complete cardiac cycle is included between the thick vertical lines I and II. The thin vertical lines indicate tenths of a sec- ond. The gradual rise of pressure within the ventricle (middle tracing) during diastole, the sudden rise with the systole, its maintenance with oscillations for an appreciable time, its sudden fall, etc., are all well shown. There is disagreement as to the exact meaning of the minor curves in the larger ones. also follow from the intercostal muscles being simply unsup- ported when the heart recedes. INVESTIGATION OP THE HEART-BEAT FROM WITHIN. By the use of apparatus, introduced within the heart of the mammal and reporting those changes susceptible of graphic record, certain tracings have been obtained about the details of which there are uncertainty and disagreement, though they seem to establish the nature of the main features of the cardiac beat clearly enough. An interpretation of such tracings in tbe light of our general and special knowledge warrants the follow- ing statement. 1. Both auricular and ventricular systole are sudden, but the latter is of very much greater duration. 2. While the chest wall feels the ventricular systole, the auriculo-ventricular valves shield the auricle from its shock. 3. During diastole in both chambers tbe pressure rises gradu- 234 COMPARATIVE PHYSIOLOGY. ally from, the inflow of "blood ; and the auricular contraction produces a brief, decided, though but slight rise of pressure in the ventricles. 4. The onset of the ventricular systole is rapid, its maximum pressure suddenly reached, and its duration considerable. The relations of these various events, their duration and the corresponding movements of the chest wall, may be learned by a study of the above tracing which the student will find worthy of his close attention. THE CARDIAC SOUNDS. Two sounds, differing in pitch, duration, and intensity, may be heard over the heart when the chest is opened and the heart listened to by means of a stethoscope. These sounds may also be heard, and present the same characters when the heart is auscultated through the chest wall ; hence the cardiac impulse can take no essential part in their production. The sounds are thought to be fairly well represented, so far as the human heart is concerned, by the syllables lub, dup ; the first sound being longer, louder, lower-pitched, and '" booming " in quality ; the second short, sharp, and high-pitched. In the exposed heart, the first sound is heard most distinctly over the base of the organ or a little below it ; while the sec- ond is communicated most distinctly over the roots of the great vessels — that is to say, both sounds are heard best over the auriculo-ventricular and semilunar valves respectively. When the chest Wall intervenes between the heart and the ear, it is found that the second sound is usually heard most distinctly over the second costal cartilage on the right ; and the first in the fifth costal interspace where the heart's impulse is also often most distinct. In these situations the arch of the aorta in the one case, and the ventricular walls in the other, are close to the situations referred to during the cardiac systole ; hence it is inferred that, though the sounds do not originate directly be- neath these spots, they are best propagated to the chest wall at these points. Prior to the study of the heart in our domestic animals the student is recommended to investigate the subject on himself by means of a double stethoscope or on another per- son with or without any instruments. There are, however, individual differences, owing to a va- riety of causes, which it is not always possible to explain fully THE CIRCULATION OP THE BLOOD. 235 in each case, but owing doubtless in great part to variations in anatomical relations. The Causes of the Sounds of the Heart.— There is general agreement in the view that the second sound is owing to the closure of the semilunar valves of the aortic and pulmonary vessels ; the former, owing to their greater tension in conse- quence of the higher blood-pressure in the aorta, taking much the larger share in the production of the sound, as may be ascertained by listening over these vessels in the exposed heart. When these valves are hooked back, the second sound disap- pears, so that there can be no doubt that they bear some impor- tant relation to the causation of the sound. In regard to the first sound of the heart the greatest diversity of opinion has prevailed and still continues to exist. The fol- lowing among other views have been advocated by physiolo- gists : 1. The first sound is caused by the tension and vibration of the auriculo- ventricular valves. 2. The first sound is owing to the contractions of the large mass of muscle composing the ventricles. 3. The sound is directly traceable to eddies in the blood. Fig. 201. Fig. 201.— Microscopic appearance of fibers from the heart. The cross-strite, divisions (branching), and junctures are visible (Landois). Fig. 202.— Muscular fiber-cells from the heart. (1 x 425.) a, line of juncture between two cells; b. c, branching cells. But, looking at the whole question broadly, is it not unrea- sonable to explain the sound resulting from such a complex act 236 COMPARATIVE PHYSIOLOGY. as the contraction of the heart and what it implies in the light of any single factor ? That such narrow and exclusive views should have been propagated, even by eminent physiologists, should admonish the student to receive with great caution ex- planations of the working of complex organs, based on a single experiment, observation, or argument of any kind. The view we recommend the student to adopt in the light of our present knowledge is, that the first sound is the result of several causative factors, prominent among which are the sud- den tension of the auriculo-ventricular valves, and the contrac- tion of the cardiac muscle, not leaving out of the account the possible and probable influence of the blood itself through eddies or otherwise; nor would we ridicule the idea that in some cases, at all events, the sound may be modified in quality and intensity by the shock given to the chest wall during sys- tole. ENDO-CARDIAC PRESSURES. Bearing in mind the relative extent of the pulmonary and systemic portions of the circulation, we should suppose that the resistance to be overcome in opening the aortic valves and lift- ing the column of blood that keeps them pressed together, would be much greater in the left ventricle than in the right ; or, in other words, that the intra-ventricular pressure of the left side of the heart would greatly exceed that of the right, and this is confirmed by actual experiment. That there should be a negative pressure in, say, the left ventricle, follows naturally enough from the fact that not only are the contents of the ventricle expelled with great sudden- ness, but that its walls remain (see Figs. 200 and 204) pressed together for a considerable portion of the time occupied by the whole systole ; so that in relaxation it follows that there must be an empty cavity to fill, or that there must be an aspiratory effect toward the ventricle; hence also one factor in the closure of the semilunar valves. It thus appears that the heart is not only a force-pump but also to some extent a suction-pump ; and, if so, the aspirating effect must express itself on the great veins, lacking valves as they do, at their entrance into the heart ; hence, with each diastole the blood would be sucked on into the auricles, a result that is intensified by the respiratory movements of the thorax. THE CIRCULATION OF THE BLOOD. 237 Fig. 203. — Diagram showing the relative height of the blood-pressure in different parts of the vascular system (after Yeo). h, heart; a, arterioles ; v, small veins; A, arteries ; C, capillaries ; V, large veins; H, V, representing the zero-line, i. e., at- mospheric pressure ; the blood-pressure is indicated by the height of the curve. The numbers on the left give the pressure approximately, in mm. of mercury. Relative Time occupied by the Various Phases of the Cardiac Cycle. — The old and valuable diagram reproduced below is meant to convey through the eye the relations of the main events in a complete beat of the heart or cardiac cycle. The relative length of the sounds; the long peri- od occupied by the pause ; the duration of the ventricular sys- tole, which it is to be observed is in excess of that of the first sound, are among the chief facts to be noted. The tracings of Chauveau and Marey, obtained from the heart Of the horse, Fig. 204.— Diagram representing the movements and , . , , , sounds of the heart during a cardiac cycle (after which has a very slow sharpey). 238 COMPARATIVE PHYSIOLOGY. rhythm, show that of the whole period, the auricular systole occupies £ or T a o of a second ; the ventricular systole, f or T \ of a second ; and the diastole, f or T % of a second. With the more rapid beat in man (70 to 80 per minute), the duration of the cardiac cycle may be estimated at about ■& of a second, and the probable proportions for each event are about these : The auricular systole, T V of a second ; the ventricular systole, T 3 o of a second ; and the pause, T *o of a second. It will be noted that the pause of the heart is equal in dura- tion to the other events put together ; and even assuming that there is some expenditure of energy in the return (relaxation) of the heart to its passive form, there still remains a consider- able interval for rest, so that this organ, the very type of cease- less activity, has its periods of complete repose. THE WORK OF THE HEART. Since the pressure against which the heart works must, as we shall see, vary from moment to moment, and sometimes very considerably, the work of the heart must also vary within wide limits, even making allowance for large adaptability to the burden to be lifted ; for it will be borne in mind that the de- gree to which the heart empties its chambers is also variable. If one knew the quantity of blood ejected by the left ven. tricle, and the rate of the beat, the calculation of the work done would be an easy matter, since the former multiplied by the latter would represent, as in the case of a skeletal muscle, the work of the muscles of the left ventricle ; from which the work of the other chambers might be approximately calculated. The work of the auricles must be slight. The right ven- tricle, it is estimated, does from one fourth to one third the work of the left. When we calculate the work done by the heart for certain in- tervals, as the day, the week, month, year, and especially for a moderate lifetime, and compare this with that of any machine it is within the highest modern skill to construct, the great superi- ority of the vital pump in endurance and working capacity will be very apparent ; not to take into the account at all its wonder- ful adaptations to the countless vicissitudes of life, without which it would be absolutely useless, even destructive to the organism. Some of these variations in the working of the heart we may now to advantage consider. THE CIRCULATION OF THE BLOOD. 239 VARIATIONS IN THE CARDIAC PULSATION. These may be ascertained either by the investigation of the arteries or of the heart, for every considerable alteration in the working of the heart expresses itself also through the arterial system. In speaking of the pulse, the reference is principally to the arteries, but in each case we may equally well think of the heai-t primarily as acting upon the arteries. 1. The frequency of the heart-beat varies, as might be sup- posed, with a great multitude of conditions, the principal of which are : age, being most frequent at birth, gradually slow- ing to old age, while in feeble old age the heart-beat may, like many other of the functions of the body, approximate the con- dition at birth, being very frequent, small, feeble, and easily disturbed in its rhythm ; sex, the cardiac beat being more fre- quent in females ; posture, most rapid in the standing position, slower when sitting, and slowest in the recumbent attitude; season, more frequent in summer ; period of the day, more frequent in the afternoon and evening. Elevation of tem- perature, the inspiratory act, emotions, and mental activity, eating, muscular exercise, etc., render the heart-beats more frequent. 2. The length of the systole, though variable, is more con- stant than that of the diastole. 3. Tlie force of the pulsation varies very greatly and exer- cises an important influence on the blood-pressure and the velocity of the blood-stream. As a rule, when the heart beats rapidly, especially for any considerable length of time, the force of the individual pulsations is diminished. 4. The heart-beat may vary much and in ways it is quite possible to estimate, either directly by the hand placed over the organ on the chest, by the modifications of the cardiac sounds, or by the use of instruments. It is wonderful how much in- formation may be conveyed, without the employment of any instruments, through palpation and auscultation, to one who has long investigated tbe heart and the arteries with an intelli- gent, inquiring mind; and we strongly recommend the student to commence personal observations early and to maintain them persistently. Practitioners recognize the pulse (and heart) as " slow " as dis- tinguished from " infrequent," " slapping," " heaving," " thrill- ing," u bounding," etc. 240 COMPARATIVE PHYSIOLOGY. Now, if with these terms there arise in the mind correspond- ing mental pictures of the action of the heart under the cir- cumstances, well ; if not, there is a very undesirable blank. How the student may be helped to a knowledge of the actual behavior of the heart under a variety of conditions we shall endeavor to explain later. Apart from all the above peculiarities, the heart may cease its action at regular intervals, or at intervals which seem to possess no definite relations to each other — that is, the heart may be irregular in its action, which may be made evident either to the hand or the ear. There are certain deviations from the quicker rhythm whicb occur with such regularity and are so dependent on events that take place in other parts of the body that they may be con- sidered normal. Comparative. — We strongly recommend the student to verify all the statements made in these sections by direct observation for himself. Such is invaluable to the practitioner. The fol- lowing table gives the mean number of cardiac pulsations per minute (after Gamgee) : SPECIES. Adult. Horse 36- 40 Ass and mule 46- 50 Ox 45-50 Sheep and goat 70-80 Pig. 70- 80 Dog 90-100 Cat 120-140 Youth. 60- 72 65- 75 60- 70 85- 95 100-110 110-120 120-140 Old age. 32- 38 55- 60 40- 45 55- 60 55- 60 60- 70 100-120 The variations with age, for the horse and the ox, are as fol- lows, according to Kreutzer : Horse. Ox. At birth 100-120 When 14 days old 80-96 When 3 months old 68- 76 When 6 months old 64- 72 When 1 year old 48-56 When 2 years old 40-48 When 3 years old 38-48 When 4 years old 38-50 When aged 32- 40 At birth 92-132 When 4-5 days old 100-120 When 14 days old 68 When 4-6 weeks old 64 When 6-12 months old . . 56- 68 For the young cow 46 For the four-year-old ox . 40 THE CIRCULATION OF THE BLOOD. 241 THE PULSE. Naturally the intermittent action of the heart gives rise to corresponding phenomena in the elastic tubes into which it may he said to be continued, for it is very desirable to keep in mind tbe complete continuity of the vascular system. The following phenomena are easy of observation : When a finger- tip is laid on any artery, an interrupted pressure is felt ; if the vessel be laid bare (or observed in an old man), it may be seen to be moved in its bed forward and upward ; the press- ure is less the farther the artery from the heart ; if the vessel be opened, blood flows from it continuously, but in spurts ; if one finger be laid on the carotid and another on a distant ves- sel, as one of the arteries of the foot, it may be observed (though it is not easy from difficulty in attending to two events hap- pening so very close together) that the beat in the nearer ves- sel precedes by a slight interval that in the more distant. Investigating the latter phenomenon with instruments, it is found tbat an appreciable interval, depending on the distance apart of the points observed, intervenes. What is the explanation of these facts ? The student may get at this by a few additional observa- tions that can be easily made. Fig. 205. — Marey's apparatus for showing the mode in which the pulse is propagated in the arteries. B. a rubber pump, with valves to prevent regurgitation. The working of the apparatus will be apparent from the inspection of the figure. If water be sent through a long elastic tube (so coiled that points near and remote may be felt at the same time) by a bulb 1(3 242 COMPARATIVE PHYSIOLOGY. syringe, imitating the heart, and against a resistance made by drawing out a glass tube to a fine point and inserting it into the terminal end of the rubber tube, an intermittent pi'essure like that occurring in the artery may be observed ; and further that it does not occur at precisely the same moment at the two points tested. Information more exact, though possibly open to error, may be obtained by the use of more elaborate apparatus and the graphic method. By measurement it has been ascertained that in man the pulse-wave travels at the rate of from five to ten metres per sec- ond, being of course very variable in velocity. It would seem that the more rigid the arteries the more rapid the rate, for in children with their more elastic arteries the speed is slower ; and the same principle is supposed to explain the higher veloci- ty noticed in the arteries of the lower extremities. But with such a speed as even five metres a second it is evident that with a systole of moderate duration (say '3 second) the most distant arteriole will have been reached by the pulse-wave before that systole is completed. It is known that the blood-current at its swiftest has no such speed as this, never perhaps exceeding in man half a metre per second, so that the pulse and the blood-current must be two totally distinct things. When the left ventricle throws its blood into vessels already full to distention, there must be considerable concussion in con- sequence of the rapid and forcible nature of the cardiac systole, and this gives rise to a wave in the blood which, as it passes along its surface, causes each part of every artery in succession to respond by an elevation above the general level, and it is this which the finger feels when laid upon an artery. That there is considerable distention of the arterial system with each pulse may be realized in various ways, as by watch- ing and feeling an artery laid bare in its course, or in very thin or very old people, and by noticing the jerking of one leg crossed over the other, by which method in fact the pulse-rate may be ascertained. And that not only the whole body but the entire room in which a person sits is thrown into vibra- tion by the heart's beat, may be learned by the use of a tele- scope to observe objects in the room, which may thus be seen to be in motion. Features of an Arterial Pulse-Tracing.— In order to judge of THE CIRCULATION OF THE BLOOD. 243 the nature of arterial tracings, it is important that the circum- stances under which they are obtained should be known. ;. »uo. — luarey s improveu spnygmograpn arranged lor iaK spring; B, first lever; C, writing lever; 6", its free writing end; D\ screw for bringing B in contact with C\ G, slide with smoked paper; H, clock-work; L, screw for increasing the pressure; M, dial indicating the amount of pressure, K, A", straps for fixing the instrument to the arm, and the latter to the double- inclined plane or support (Byrom Bramwell). The movements of the vessel wall in most mammals suitable for experiment aud in man is so slight that it becomes necessary to exaggerate them in the tracing, hence long levers are used to accomplish this. The sphygmograph is the usual form of instrument em- ployed for the purpose. It consists, essentially, of a clock-work for moving a smoked surface (mica plate commonly) on which the movements of a lever-tip, answering to those of a button placed on the artery, are recorded. We shall do well to in- quire whether there are any features in common in trac- ings obtained in various ways, and which have there- fore in all probability a real foundation in nature. An inspection of a large number of pulse- tracings, taken un- der diverse conditions, seems to show that in all of them there occurs, more or less marked, the following : 1. An upward Fig. 207.— Diagrammatic schema showing the essential part of the instrument when in working order. The knife-edge B" of the short lever is in contact with the writing-lever C. Every movement of the steel spring at A", communicated by the arteries, will be imparted to the writing- lever (Byrom Bramwell). 244 COMPARATIVE PHYSIOLOGY. curve. 2. A downward curve, rendered irregular by the occur- rence of peaks or crests and notches. The first of these are Fig, 208.— Pulse tracing from carotid artery of healthy man (after Moens). x, com- mencement of expansion of artery; A, summit of first rise; C, dicrotic secondary wave; B, predicrotic secondary wave; p, notch preceding this; D, succeeding sec- ondary wave. Curve above is that made by a tuning-fork with ten double vibra- tions in a second. termed the predicrotic notch and crest, and the succeeding ones the dicrotic notch and crest. The latter seem to be the more constant. Venous Pulse. — Apart from the variations in the caliber of the great veins near the heart, constituting a sort of pulse, though due to variations in intra-cardiac pressure, a venous pulse proper is rare as a normal feature. One of the best-known examples of such occurs in the salivary gland. When, during secretion, the arterioles are greatly dilated, a pulse may be wit- nessed in the veins into which the capillaries open out, owing to diminution in the resistance which usually is sufficiently great to obliterate the pulse-wave. Pathological. — In severe cases of heart-disease, owing to cardiac dilatation or other conditions, giving rise to incompe- tency of the tricuspid valves, there may be with each ventricu- lar systole a back-flow, visible in the veins of the neck. A venous pulse is a phenomenon, it will be evident, that always demands special investigation. It means that the usual bounds of nature are for some good reason being overstepped. Comparative. — Before entering on the consideration of phe- nomena that all are agreed are purely vital, we call attention to the circulation in forms lower than the mammal, in order to give breadth to the student's views and prepare him for the special investigations, which must be referred to in subsequent chapters; and which, owing to the previous narrow limits (re- searches upon the frog and a few well-known mammals) having at last been overleaped, have opened up entirely new aspects of THE CIRCULATION OP THE BLOOD. 215 cardiac physiology — one might almost say revolutionized the subject. Owing to the limitations of our space, the references to lower forms must be brief. We recommend the student, however, to push the subject further, and especially to carry out some of the experiments to which attention will be directed very shortly. In the lowest organisms {Infusorians) represented by Amoe- ba, Vorticella, etc., there are, of course, no circulatory organs, unless the pulsating vacuoles of some forms mark the crude beginnings of a heart. It will be borne in mind, bowever, that there is a constant streaming of the protoplasm itself within the organism. The heart is first represented, as in worms, by a pulsatile tube, which may, as in the earth-worm, extend throughout the greater part of the length of the animal, and has usually dorsal and ventral and transverse connections The dilatations of the transverse portions in one division (metamere) of the animal seem to foreshadow the appearance of auricles. The pulsation of the dorsal vessel in a large earth-worm is easy of observation. In amphioxus, which is often instanced as the lowest verte- brate, the blood-vessels, including the portal vein, are pulsatile, while there is no distinct and separate heart. Although the respiratory system will be treated from the comparative point of view, the student will do well to note now Ab Ao 4' Ba V ^7F Fig. 209.— Diagram of the circulation of a Teleostean fish (Clans). V. ventricle; Ba, bulbus arteriosus, with the arterial arches which carry the blood to the gills; Ab, arterial arches: Jo, aorta descendens, into which the epibranchiaJ arteries passing out from the gills unite; K, kidneys; /, intestine; Pc. portal circulation. (in the figures) the close relation between the organs for dis- tributing and aerating the blood. Passing on to the vertebrates, in the lowest group, the fishes, 246 COMPARATIVE PHYSIOLOGY. the heart consists of two chambers, an auricle and a ventricle, the latter being supplemented by an extension (bulbus arterio- sus) pulsatile in certain species ; and an examination of the Fig. 210. Pig. 211. Fig. 210. — The arterial trunks and their main branches in the frog {Rana esculenta). 1 x 1|. (Howes.) I, lingual vessel; c. c, common carotid artery; p.cu, pulmo- cutaneous artery; c. gl, carotid gland; aw', right auricle; aw", left auricle; v, ven- tricle; tr.a, truncus arteriosus; pul', pulmonary; Ig, left lung; ao, left aortic arch; br, brachial; cu ) cutaneous; d. ao, dorsal aorta; cm, coeliaco-mesenteric; cm', coeliac; hn, hepatic vessels; 17, gastric; j>c', pancreas; m, mesenteric; sp, splenic; du', duodenal; h, hajmorrhoidal; W, ileal; h,y, hypogastric; c. II, com- mon iliac; re, renal; k, kidney; Is, spermatic. Fig. 211.— Venous trunks and their main branches in the frog (Rana esculenta). 1 x 1J. (Howes.) I, lingual vein; e.j, external jugular; in, innominate; i.j, internal jugu- lar; s. sc, subscapular; pr. c, vena cava superior; s. v, sinus venosus; hp, hepatic; I11', right lobe of liver; Iv", left lobe of liver; pt. c, vena cava inferior; ov, ovarian; d. I, dorso-lumbar; od, oviducal; r.p, renal-portal; fm, femoral; sc, sciatic; a, femoro-sciatic anastomosis; pv' , right pelvic; vs, vesical; ant. ab, anterior ab- dominal; a', abdominal-portal anastomosis; W, ileal; sp, splenic; du', duodenal; I. int, lieno-intestinal; g, gastric; p, portal; Ig", left lung; pul, pulmonary; m. cu, musculo-cutancous; br, brachial. course of the circulation will show that the heart is throughout venous, the blood being oxidized in the gills after leaving the former. Among the amphibians, represented by the frog, there are two auricles separated by an almost complete septum, and one THE CIRCULATION OF THE BLOOD. 247 ventricle characterized by a spongy arrangement of the muscle- fibers of its walls. In the reptiles the division between the auricles is complete, and there is one ventricle which shows imperfect subdivisions. Fig. 212. Fig 213. Fig. 212.— The frog's heart, seen from the front, the aortic arches of the left side hav- ing been removed. (1 x 4.) ca, carotid; c. ffl, carotid gland; ao, aorta; an', right auricle; au", left auricle; pr. c, vena cava superior; pt. c, vena cava inferior; p. cu, pulmo-cutaneous trunk; tr, truncus arteriosus: v, ventricle (Howes). Fig. 213.— The same, seen from behind, the sinus venosus having been opened up 10 show the sinu-auricular valves. (1 x 4.) p.v, pulmonary vein ; s. v, sinus veno- bus; va", sinu-auricular valve. Other lettering as in Fig. 212 (Howes). In the crocodile, however, the heart consists of four per- fectly divided chambers. Of the two aortic arches, one arises together with the pulmonary artery from the right ventricle, and, as it crosses over, the left communicates with it by a small opening, so that, although the arterial and the venous blood are completely separated in the heart, they intermingle outside of this organ. In birds the circulatory system is substantially the same as in mammals ; but in all vertebrate forms below birds the blood distributed to the tissues is imperfectly oxidized or is partially venous. As a result of the entire vascular arrangements in the frog, etc., the least oxidized blood passes to the lungs, and the most aerated to the head and anterior parts of the animal. Whatever ground for differences of opinion there may be as to the extent to which the phenomena we have as yet been describing are mechanical in their nature, all are agreed that 24S COMPARATIVE PHYSIOLOGY. such explanations are insufficient when applied to the facts with which we have yet to deal. They, at all events, can be regarded only as the result of vitality. When one reflects upon the vicissitudes through which an animal must pass daily and hourly, necessitating either that they be met by modified action of the organs of the body or that the destruction of the organism ensue, it becomes clear that the varying nutritive needs of each part must be answered by changes in the circulatory system. These changes may affect any part of the entire arrangement, and it rarely happens, as will appear, that one part is modified without a corresponding one, very frequently of a different kind, taking place in some other. What these various correlated modifications are, and how they are brought about, we shall now attempt to describe, and it will greatly assist in the comprehension of the whole if the student will endeavor to keep a clear mental picture of the parts before his mind throughout, using the figures and verbal descriptions only to assist in the construction of such a mental image. We shall begin with the vital pump — the heart. THE BEAT OF THE HEART AND ITS MODIFICATIONS. As has been already noted, the cardiac muscle has features peculiar to itself, and occupies histologically an intermediate place between the plain and the striped muscle-cells, and that the contraction of the heart is also intermediate in character, and is best seen in those fox*ms of the organ which are somewhat tubular and beat slowly. But the contraction, though peristal- tic, is more rapid than is usually the case in organs with the smooth form of muscle-fiber. The heart behaves under a stimulus in a peculiar manner, the effect of a single induction shock depends on the phase of contraction hi which the heart happens to be at the moment of its application. Thus at the commencement of a systole there is no visible effect, while beats of unusual character result at other times. But tetanus can not be induced by any form or method of stimulation. The latent period of cardiac muscle is long. In a heart at rest a single stimulus (as the prick of a needle) usually calls forth but one contraction. THE CIRCULATION OF THE BLOOD. 249 THE NERVOUS SYSTEM IN RELATION TO THE HEART. The attempts to determine just why the heart heats at all, and especially the share taken by the nervous system, if any direct one, are beset with great difficulty ; though, as we shall attempt to show later, this subject also has been cramped within too narrow limits, and hence regarded in a false light. Till comparatively recently the frog's heart alone received much attention, if we except those of certain well-known mam- mals. In the heart of the frog there are ganglion-cells in vari- ous parts, especially numerous in the sinus venosus (or expan- sion of the great veins where they meet the auricles) ; also in the auricles, more especially in the septum (ganglia of Remak), while they are absent from the greater part of the ventricle, though found in the auriculo-ventricular groove (ganglia of Bidder). Recently it has been found that ganglion-cells occur in the ventricles of warm-blooded animals. In the hearts of the dog, calf, sheep, and pig, which are those lately subjected to investi- gation, it is found that the nerve-cells do not occur near the apex of the ventricles, but mainly in the middle and basal por- tions, being most abundant in the anterior and posterior inter- ventricular furrows and in the left ventricle. But there are differences for each group of animals ; thus, these ganglion- cells are most abundant, so far as the mammals as yet inves- tigated ai'e concerned, in the ventricles of the pig, and least so in those of the dog. In tbe cat they ai'e also scanty. Ganglion- cells occur in the auricles, and are especially abundant near the terminations of the great veins. It has long been known that the heart of a frog removed from the body will pulsate for hours, especially if fed with serum, blood, or similar fluids ; and that it may be divided in almost any conceivable way, even when teased up into minute particles, and still continue to beat. The apex, however, when separated does not beat. Yet even this quiescent apex may be set pulsating if tied upon the end of a tube, through which it may be fed under pressure. We may here point out that the whole heart or a part of it may be made to describe its action by the graphic method in various ways, the principles underlying which are either that the heart pulls upon a recording lever (lifts it) ; acts against the fluid of a manometer ; or, inclosed in a vessel containing oil or similar fluid, moves a piston in a cylinder. 250 COMPARATIVE PHYSIOLOGY. It lias also long been known that a ligature drawn around the sinus venosus (in the frog) at its junction with the auricles stopped the heart for a certain period, and this experiment (of Stannius) was thought to demonstrate that the heart was ar- rested because the nervous impulses proceeding to the ganglion- cells along the cardiac nerves or ganglia of this region were cut off by the ligature ; in other words, the heart ceased to beat because the outside machinery on which the action of the inner depended was suddenly disconnected. Other explanations have been offered of this fact. Within the last few years great light has been thrown upon the whole subject of cardiac physiology in consequence of in- vestigators having studied the hearts of various cold-blooded animals and of several invertebrates. The hearts of the Che- lonians (tortoises, turtles) have received special attention, and their investigation has been fruitful of results, to the general outcome of which, as well as those accruing from recent com- parative studies as a whole, we can alone refer. Since in other parts of the work the limits of space will not always allow us to give the evidence on which conclusions rest, attention is especially called to what here follows, as an example of the methods of physiological research, and the nature of the reason- ing employed. Very briefly the following are some of the main facts : 1. In all cold-blooded animals the order in which the sub- divisions of the heart ceases to pulsate when kept under the same conditions is invariable, viz., ventricle, auricles, sinus. 2. The sinus and auricles, when separated by section, liga- ture, or otherwise, either together or singly, continue to beat, whether amply provided with or surrounded by blood. 3. The ventricle thus separated displays less tendency to beat independent of some stimulus (as feeding under pressure), though a very weak one usually suffices — i. e., its tendency to spontaneous rhythm is less marked than is the case with the other parts of the heart. These remarks apply to the hearts of Chelonians — fishes, snakes, and some other cold-blooded animals. 4. In certain fishes (skate, ray, shark) the beat may be re- versed by stimulation, as a prick of the ventricle. This is accomplished with more difficulty in other cold-blooded ani- mals, and still more so in the mammal. 5. In certain invertebrates, notably the Poulpe (Octopus), a THE CIRCULATION OF THE BLOOD. 251 careful search has revealed no nerve-cells, yet their hearts con- tinue to beat when their nerves are severed, on section of parts of the organ, etc. 6. A strip of the muscle from the ventricle of the tortoise, when placed in a moist chamber and a current of electricity passed through it for some hours, will commence to pulsate and continue to do so after the current has been withdrawn ; and this holds when the strip is wholly free from nerve-cells. From the above facts certain inferences have been drawn : 1. It has been concluded that the sinus is the originator and director of the movements of the rest of the heart. 2. That this is owing to the ganglia in its walls. While all recognize the importance of the sinus, some physiologists hold to the gangli- onic influence as essential to the heart-beat still ; while others, influenced by the facts mentioned above, are disposed to regard tbem as of very doubtful importance — at all events, as origina- tors of the movements of the heart. The tendency now seems to be to attach undue importance to the spontaneous contractility of the heart-muscle ; for it by no means follows logically that, because a muscle treated by electricity, when cut off from the usual nerve influence that we believe is being constantly exerted on the heart like other or- gans, will contract and continue to do so in the absence of the stimulus, it does so normally.; or, because some hearts beat in the absence of nerve-cells, that therefore nerve-cells are of no account in any case. Such views, when pressed to the extreme, lead to as narrow conceptions as those they are intended to re- place. Taking into account the facts mentioned and others we have not space to enumerate, we submit the following as a safe view to entertain of the beat of the heart in the light of our present knowledge : Recent investigations show clearly that there are great dif- ferences in the hearts of animals of diverse groups, so that it is not possible to speak of "the heart" as though our remarks applied equally to this organ in all groups of animals. It must be admitted that our understanding of the hearts of the cold-blooded animals is greater than of the mammalian heart ; while, so far as exact or experimental knowledge is con- cerned, the human heart is the least understood of all, though there is evidence of a pathological and clinical kind and subject- ive experience on which to base conclusions possessing a certain 252 COMPARATIVE PHYSIOLOGY. value ; but it is clear to those who have devoted attention to comparative physiology that the more this subject is extended the better prepared we shall be for taking a broad and sound view of the physiology of the human heart and man's other organs. Whatever may be said of the invertebrates, among which greater simplicity of mechanism doubtless prevails, there can be no doubt that the execution of a cardiac cycle of the heart in all vertebrates, and especially in the higher, is a very com- plex process from the number of the factors involved, their in- teraction, and their normal variation with circumstances ; and we must therefore be suspicious of any theory of excessive sim- plicity in this as well as other parts of physiology. We submit, then, the following as a safe provisional view of the causation of the heart-beat : 1. The factors entering into the causation of the heart-beat of all vertebrates as yet examined are : (a) A tendency to spon- taneous contraction of the muscle-cells composing the organ: (6) intra-carcliac blood-pressure ; (c) condition of nutrition as determined directly by the nervous supply of the organ and in- directly by the blood. 2. The tendency to spontaneous contraction of muscle-cells is most marked in the oldest parts of the heart (e.g., sinus), ancestrally (phylogenetically) considered. 3. Inti*a-carcliac pressure exercises an influence in determin- ing the origin of pulsation in probably all hearts, though like other factors its influence varies with the animal group. In the mollusk (and allied forms) and in the fish it seems to be the controlling factor. 4. We must recognize the power one cell has to excite, when in action, neighboring heart-cells \o contraction. The ability that one protoplasmic cell-mass has to initiate in others, under certain circumstances, like conditions with its own, is worthy of more serious consideration in health and disease than it has yet received. 5. The influence of the cardiac nerves becomes more pro- nounced as we ascend the animal scale. Their share in the heart's beat will be considered later. 6. Apparently in all hearts there is a functional connection leading to a regular sequence of beat in the different parts, in which the sinus or its representatives (the terminations of great veins in the heart) always takes the initiative. One part having THE CIRCULATION OF THE BLOOD. 253 contracted, the others must necessarily follow ; hence the rapid onset of the ventricular after the auricular contraction in the mammal, and the long wave of contraction that seems to pass evenly over the whole organ in cold-blooded animals. The basis of all these factors is to be sought finally in the natural contractility of protoplasm. A heart in its most de- veloped form still retains, so to speak, the inherited but modified Amoeba in its every cell. Whether the intrinsic nerve-cells of the heart take any share directly in the cardiac beat must be considered as yet undeter- mined. Possibly they do modify motor impulses from nerves, while again it may be that they have an influence over nutri- tive processes only. The subject requires further study, both anatomical and physiological. INFLUENCE OF THE VAGUS NERVE UPON THE HEART. The principal facts in this connection may be stated as fol- lows, and apply to all the animals thus far examined : 1. In all cases the action of the heart is modified by stimu- lation of the medulla oblongata or the vagus nerve. 2. The modification may consist in prompt arrest of the heart, in slowing, in enfeeblement of the beat, or a combination of the two latter effects. 3. After the application of the stimulation there is a latent Fig. 214.— Inhibition of frog's heart by stimulation of the vagus nerve. To be read from right to left. The contractions of the ventricle are registered by a simple lever resting on it. The interrupted current was thrown in at a. Note that one beat occurred before arrest (latent period), and that when standstill of the heart did take place it lasted for a considerable period (.Foster). period before the effect is manifest, and the latter may outlast the stimulation by a considerable period. 254 COMPARATIVE PHYSIOLOGY. 4. In most animals the sinus venosus and auricles are af- fected before the ventricles, and the vagus may influence these parts when it is powerless over the ventricle. 5. After vagus inhibition, the action of the heart is (almost unexceptionally) different, the precise result being variable, but generally the beat is both accelerated and increased in force. We may say that the werking capacity of the heart is tem- porarily increased. 6. The improvement in the efficiency of the heart is in pro- portion to its previous working power, and in cases when the Stimulation Vagus. Fig. 915.— Effects of vagus stimulation, illustrated by a form of sphygmograpliic curve derived from the carotid of a rabbit (Foster). action is feeble and irregular (abnormal) it might be said to be in proportion to its needs. This is a very important law that deserves to receive a general recognition. 7. Section of both vagi nerves results in histological altera- tions in the heart's structure, chiefly fatty degeneration, which must, of course, impair its working capacity and expose it to rupture or other accidents under the frequently recurring strains of life. 8. In the cold-blooded animals the heart may be kept at a standstill by vagus stimulation till it dies, a period of hours (one case of six hours reported for the sea-turtle). 9. Certain drugs (as atropine), applied directly to the heart, or injected into the blood, prevent the usual action of the vagus. 10. During vagus arrest the heart substance undergoes a change, resulting in an unusual dilatation of the organ. This may be witnessed whether the heart contains blood or not. THE CIRCULATION OP THE BLOOD. 255 11. The heart may he arrested by direct stimulation, espe- cially of the sinus, and at the points at which the electrodes are applied there is apparently a temporary paralysis. The same alteration in the beat may he noticed as when the main trunk of the vagus is stimulated. 12. The heart may he inhibited through stimulation of vari- ous parts of the body, both of the surface and internal organs (reflex inhibition). 13. One vagus being divided, stimulation of its upper end may cause arrest of the heart. 14. Stimulation of a small part of the medulla oblongata will produce the same result, provided one or both vagi be intact. 15. Section of both vagi in some animals (the dog notably) increases the rate of the cardiac beat. The result of section of one pneumogastric nerve is variable. The heart's rhythm is usually to some extent quickened. 16. During vagus inhibition from any cause in mammals and many other animals, the heart responds to a single stimu- lus, as the prick of a needle, by at least one beat. An observer studying for himself the behavior of the heart in several groups of animals with an open mind, for the purpose of observing all he can rather than proving or disproving some one point, be- comes strongly impressed with the variety in unity that runs through cardiac physiology, including the influence of nerve- cells (centers) through nerves ; for it will not be forgotten that normally nerves originate nothing, being conductors only, so that when the vagus is stimulated by us we are at the most but imitating in a rough way the work of central nerve-cells. We can only mention a few points to illustrate this. In the frog a succession of light taps, or a single sharp one (" Klopf versuch " of Goltz), will usually arrest the heart reflexly ; though sometimes it is very difficult to accomplish. But in the fish the ease with which the heart may be reflexly inhibited by gentle stimulation of almost any portion of the animal is won- derful. Again, in some animals the vagus arrests the heart for only a brief period, when it breaks away into its usual (but in- creased) action. In the fish, menobranchus, and probably other animals, the irritability of some subdivision of the heart is lost during the vagus inhibition — i. e., it does not x-espond to a mechanical stimulus. 256 COMPARATIVE PHYSIOLOGY. There is usually a certain order in which the heart recom- mences after inhibition (viz. , sinus, auricles, ventricles) ; but there are variations in this, also, for different animals. It is also a fact that in most of the cold-blooded animals the right S. Vagus, Heart. Brain above Medulla. Cardie-inhibitory Cen- ter in Medulla Ob- Afferent Nerve. Outlying Area with its Nerves. Fio. 310. — Diagram of the inhibitory mechanism of the heart. The arrows indicate in all cases the path the nervous impulses take. I. Path of afferent impulses from I he heart itself. II. Path from parts of the brain above (or anterior to) the vaso-motor center. A similar one might, of course, be mapped out along the spinal cord. III. Path from some peripheral region. The downward arrows in- dicate the course of efferent impulses, which probably usually pass by both vagi. vagus is more efficient than the left, owing, we think, not to the nerves themselves so much as to their manner of distribu- tion in the heart—the greater portion of the driving part of the THE CIRCULATION OP THE BLOOD. 257' organ,so to speak, being supplied by tbe right nerve ; for, when even a small part of the heart is arrested, it may be overcome by the action of a larger portion of the same, or a more dominant region (the sinus mostly). Conclusions. — The inferences from tbe facts stated in the above paragraphs are these : 1. There is in the medulla a col- lection of cells (center) which can generate impulses that reach the heart by the vagi nerves and influence its muscular tissue, though whether directly or through the intermediation of nerve-cells in its substance is uncertain. It may possibly be in both ways. 2. This center (cardio-inhibitory) may be influ- enced reflexly by influences ascending by a variety of nerves from the periphery, including paths in the brain itself, as shown by the influence of emotions or the behavior of the heart. 3. The cardio-inhibitory center is the agent, in part, through which the rhythm of the heart is adapted to the needs of the body. 4. The arrest, on direct stimulation of the heart, is owing to the effect produced on the terminal fibers of the vagi, as shown by the dilatation, etc., corresponding to what takes place when the trunk of the nerve or the center is stimu- lated. 5. The quickening of the heart, following section of the vagi, seems to show that in some animals the inhibitory center exercises a constant regulative influence over the rhythm of the heart. 6. The irritability and dilatability of the cardiac tissue may be greatly modified dui'ing vagus inhibition. Some- times this is evident before the rhythm itself is appreciably altered. 7. The heart-muscle has a latent period, like other kinds of muscle ; and cardiac effects, when initiated, last a vari- able period. There are many other obvious conclusions, which the stu- dent will draw for himself. But a question arises in regard to the significance of the cardiac arrest under these circumstances, and the altered action that follows. The fact that, when the heart is severed from the central nervous system by section of its nerves, profound changes in the minute structure of its cells ensue, points un- mistakably to some nutritive influence that must have operated through the vagi nerves. That stimulation of the vagus re- stores regularity of rhythm and strengthens the beat of the failing heart, is also very suggestive. That many disorders of the heart are coincident with periods of mental anguish or worry, and that in certain cases of severe mental application 17 258 COMPARATIVE PHYSIOLOGY. the heart's rhythm lias become very slow, also point to influ- ences of a central origin as greatly affecting the life-processes of this organ. It has been shown that the vagus nerve in some cold-blooded animals, as is probable also in the higher vertebrates, consists of two sets of fibers — those which are inhibitory proper and those which are not, but belong to the sympathetic system. Sepai'ate stimulation of the former favors nutritive processes, is preservative ; of the latter, destructive. This has been ex- pressed by saying that the former favors constrictive (anabolic) metabolism ; the latter destructive (katabolic) metabolism. It is assumed that all the metabolism of the body may be repre- sented as made up of katabolic following anabolic processes. Whether such a view of metabolism expresses any more than a sort of general tendency of the chemistry of the body is doubtful. It is a very simple representation of what in all probability is extremely complex ; and if it be implied that throughout the body certain steps are always taken upward in construction to be always afterward followed by certain down- ward destructive changes, we must reject it as too rigid and artificial a representation of natural processes. We think, however, that, upon all the evidence, pathological and clinical as well as physiological, the student may believe that the vagus nerve, like the other nerves of the body, accord- ing to our own theory, exercises a constant beneficial, guiding — let us say determining — influence over the metabolism of the organ it supplies ; and we here suggest that, if this view were applied to the origin and course of cardiac disease, it would result in a gain to the science and art of medicine. THE ACCELERATOR (AUGMENTOR) NERVES OF THE HEART. It has been known for many years that in the dog, cat, rabbit, and some other mammals, there are nerves proceeding from certain of the ganglia of the sympathetic chain high up, stimulation of which lead to an acceleration of the heart-beat. Very recently these nerves have been traced in a number of cold-blooded animals, and the whole subject placed on a broader and sounder basis. There are variations in the distribution of these nerves for different groups of animals, but it will suffice if we indicate THE CIRCULATION OF THE BLOOD. 259 their course in a general way, without special reference to the variations for each animal group: 1. These nerves emerge from the spinal cord (upper dorsal region), and proceed upward Spinal Cord. Accelerator Center in Me- dulla. Superior Cervical Ganglion. Middle Cervical Ganglion. Inferior Cervical Ganglion. Basal Ganglion in Region of First Rib. Accelerator Nerves. Heart. Fig. 217.^Diagrani to illustrate the origin, course, etc., of accelerator impulses. It will be understood that this is intended to indicate the general plan, and not pre- cisely what takes place in any one animal. Thus, while the accelerator nerves may arise in this way, it is not meant to be implied that the heart is actually sup- plied by three nerves of such origin in any case. The arrows, as before, indicate the path of the impulses. before being distributed to the heart. 2. They may leave for their cardiac destination either at (a) the first thoracic (or basal cardiac ganglion, as it might be named in this case), (6) the in- ferior cervical ganglion, (c) the annulus of Vieussens, or (d) the middle cervical ganglion. It follows that the heart may be made to do increased work in three ways : First, the relaxation of a normal inhibitory 260 COMPARATIVE PHYSIOLOGY. control through the vagus nerve by the cardio-inhibitory cen- ter ; second, through the sympathetic (motor) fibers in the vagus itself : and, finally, through fibers with similar action in the sympathetic system, usually so called. The share taken by these factors is certainly variable in dif- ferent species of animals, and it is likely that this is true of the same animals on different occasions. It is also conceivable, and indeed probable, that they act together at times, the inhibi- tory action being diminished and the augmentor influence in- creased. THE HEART IN RELATION TO BLOOD-PRESSURE. It is plain that all the other conditions throughout the cir- culatory system remaining the same, an increase in either the force or the frequency of the heart-beat must raise the blood- pressure. But, if the pressure were generally raised when the heart beats rapidly, it would fare ill with the aged, the elasticity of their arteries being usually greatly impaired. As a matter of fact any marked rise of pressure that would thus occur is pre- > w~- s Fig. 218 —Tracing from a rabbit, showing the influence of cardiac inhibition on blood- pressure. The fall in this case was very rapid, owing to sudden cessation of the heart-beat. The relative emptiness of the vessels accounts for the peculiar char- acter of the curve of rising blood-pressure (Foster). vented as a rule, and in different ways, as will be seen ; but, so far as the heart is concerned, its beat is usually the weaker the more rapid it is, so that the cardiac rhythm and the blood-press- ure are in inverse proportion to each other. By what method is the heart's action tempered to the condi- THE CIRCULATION OF THE BLOOD. 261 tions prevailing at the time in the other parts of the vascular system ? The matter is complex. The effect of vagus stimulation on the blood-pressure is always very marked, as would be supposed. As seen in the tracing, the beats, when the heart commences its action again tell on the comparatively slack walls of the arteries, distending them greatly, and this may be made evident by the sphymograph as well as the manometer ; indeed, may be evident to the finger, the pulse resembling in some features that following excessive loss of blood, If the heart has been merely slowed, or its pulsation weak- ened, the effects will of course be less marked. The Quantity of Blood.— The blood-pressure may also be augmented, the cardiac frequency remaining the same, by the quantity of blood ejected from the ventricles, which again depends on the quantity entering them, a factor determined by the condition of the vessels, and to this we shall presently turn. In consequence of changes in different parts of the system by way of compensation, results follow in an animal which might not have been anticipated. Thus, bleeding, unless to a dangerous extreme, does not lower the blood-pressure except temporarily. It is estimated that the body can adapt itself to a loss of as much as 3 per cent of the body-weight. The adaptation is probably not through absorption chiefly, but through constriction of the vessels by the vaso- motor nerves. Again, an injection of fluid into the blood does not cause an appreciable rise of blood-pressure, so long as the nervous svs- tem is intact ; but, if by section of the spinal cord the vaso- motor influences are cut off, then a rise may take place to the extent of 2 to 3 per cent of the body -weight, the extra quan- tity of fluid seeming to be accommodated in the capillaries and smaller veins. These facts are highly significant in illustrat- ing the adaptive power of the circulatory system (protective in its nature), and are of practical importance in the treatment of disease. We think the benefit that sometimes follows bleeding has not as yet received an adequate explanation, but we shall not attempt to tackle the problem now. Changes in the circulation depend on variations in the size of the blood-vessels. 262 COMPARATIVE PHYSIOLOGY. It is important in considering this subject to have clear no- tions of the structure of the blood-vessels. It will be borne in mind that, while muscular elements are perhaps not wholly lacking in any of the arteries, they are most abundant in the smallest, the arterioles, which by their variations in size are best fitted to determine the quantity of blood reaching any organ. It is well known that nerves derived chiefly from the sympa- thetic system pass to blood-vessels, though their exact mode of termination is obscure. As the result of the section and stimu- lation of certain nerves the following inferences have been drawn in regard to the nerves supplying blood-vessels. 1. There are vaso-motor nerves of two kinds — vaso-constrict- ors and vaso-dilators — which may exist in nerve-trunks either separately or mingled. Examples of the former are found in the cervical sympathetic, splanchnic, etc., of the latter in the chorda tympani, nerves of the muscles and nervi erigentes (from the first, second, and third sacral nerves), while the sci- atic seems to contain both. 2. Impulses are constantly passing from the medullary vaso- motor center along the nerves to the blood-vessels, hence their dilatation after section of the nerves. The nerves are traceable to the spinal cord, and in some part of their course run, as a rule, in the sympathetic system. 3. Impulses pass at intervals to the areas of distribution of vaso-dilators along these nerves, the effect of which is to dilate the vessels through their influence, as in other cases, on the muscular coat. It is inferred that there are vaso-motor centers in the spinal cord which are usually subordinated to the main center in the medulla, but which in the absence of the control of the chief center in the medulla assume an independent regulating influence. There is a nerve with variable origin, course, etc., in differ- ent mammals, but in the rabbit given off from either the vagus, the superior laryngeal, or by a branch from each, which, run- ning near the sympathetic nerve and the carotid artery, reaches the heart, to which it is distributed. This is known as the de- pressor nerve. From stimulation of the central end of this nerve results follow which warrant the conclusion that impulses can by it reach the vaso-motor center in the medulla, and interfere with (inhibit) the outflow of efferent, constrictive, or tonic impulses, THE CIRCULATION OP THE BLOOD. 263 which start from the vasomotor center, descend the cord, and find their way to the organs of a definite region, in consequence of which the muscular coats of the arterioles relax, more blood flows to this area which is very large, and the general blood- pressure is lowered. Again, if the central end of one of the main nerves — e. g., sciatic — be stimulated, a marked change in the blood-pressure iso-motor Center in Medulla. Spinal Cord Efferent Vaso-mo tor Nerve. Outlying Vascularl Area. ~ v *-^!ti|ST Afferent Nerve from Periphery. Fig. 219.— Diagram of nervous vasomotor mechanism. I. Course of afferent impulses from the heart itself along the depressor nerve. II. Course from some other part of the brain. III. Course from some peripheral region along a nerve joining the spinal cord. The efferent impulses are represented as passing to a vascular area, • reduced for the sake of simplicity to a single arteriole. results, but whether in the direction of rise or fall seems to de- pend upon the condition of the central nervous system, for, with the animal under the influence of chloral, there is a fall; if under urari, a rise, 264 COMPARATIVE PHYSIOLOGY. It is not to be supposed that the change in any of these cases is confined, to any one vascular area invariably, but that it is this or that, according to the nerve stimulated, the condi- J fUUUUUUUUUVAJUUUUlAJlAAJLJLAJUUUULA^ Fig. 220.— Curve of blood-pressure resulting from stimulation of the central end of the depressor nerve. To be read from right to left. T indicates the rate at which the recording surface moved, the intervals denoting seconds. At V the current was thrown into the nerve, and shut off at 0. The result appears after a period of latency, and outlasts the stimulus (Poster) tion of the centers, and a number of other circumstances. Moreover, it is importaut to bear in mind that with a fall of blood-pressure in one region there may be a corresponding rise in another. With these considerations in mind, it will be ap- parent that the changes in the vascular system during the course of a single hour are of the most complex and variable character. The question of the distribution of vaso-motor nerves to veins is one to which a definite answer can not be given. THE CAPILLARIES. The cells of which the capillaries are composed have a con- tractility of their own, and hence the caliber of the capillaries is not determined merely by the arterial pressure or any similar mechanical effect. Certain abnormal conditions, induced in these vessels by the application of irritants, cause changes in the blood-flow, which can not be explained apart from the vitality of the ves- sels themselves. Watched through the microscope under such circumstances, THE CIRCULATION OP THE BLOOD. 265 the blood-corpuscles no longer pursue their usual course in the mid-stream, but seem to be generally distributed and to hug the walls, one result of which is a slowing of the stream, wholly independent of events taking place in other vessels. It is thus seen that in this condition (stasis) the capillaries have an in- dependent influence essentially vital. We say independent, for it is still an open question whether nerves are distributed to capillaries or not. That inflammation, in which also the walls undergo such serious changes that white and even red blood- cells may pass through them {diapedesis), is not uninfluenced by the nervous system, possibly induced through it in certain cases, if not all, seems more than probable. But when we consider the lymphatic system new light will, it is hoped, be thrown upon the subject of the nature and the influences which modify the capillaries. One thing will be clear from what has been said, that even normally the capil- laries must exert an influence of the nature of a resistance, owing to their peculiar vital properties ; and, as we have already intimated such considerations should not be excluded from any conclusions we may draw in regard to tubes that are made up of living cells, whether arteries, veins, or capillaries, though manifestly the applicability to capillaries, with their less modi- fied or more primitive structure, is stronger. It has now become clear that the circulation may he modi- fied either centrally or peripherally; that a change is never purely local, but is correlated with other changes ; that the whole is, in the higher animals, directly under the dominion of the central nervous system ; and that it is through this part chiefly that harmony in the vascular as in other systems and with other systems is established. To have adequately grasped this conception is worth more than a knowledge of countless details. SPECIAL CONSIDERATIONS. Pathological —Changes may take place either in the sub- stance of the cardiac muscle, in the valves, or in the blood-ves- sels, of a nature unfavorable to the welfare of the body. Some of these have been incidentally referred to already. Hypertrophy, or an increase in the tissue of the heart, is generally dependent on increased resistance, either within or without the heart, in the region of the arterioles or capillaries. Imperfections of the aortic valves may permit of regurgitation 266 COMPARATIVE PHYSIOLOGY. of blood, entailing an extra effort if it is to be expelled in addi- tion to the usual quantity, which again leads to hypertrophy ; but this is often suceeded by dilatation of the chambers of the heart one after the other, and a host of evils growing out of this, largely dependent on imperfect venous circulation, and increased venous pressure. And it may be here noticed that arterial and venous pressures are, as a general rule, in inverse proportion to each other. If the quantity of blood in the ventricle, in consequence of regurgitation, should prove to be greater than it can lift (eject), the heart ceases to beat in diastole ; hence some of the sudden deaths from disease of the aortic valves. As a result of fatty, or other forms of degeneration, the heart may suddenly rupture under strains. Actual experiment on the arteries of animals recently dead, including men, shows that the elasticity of the arteries of even adult mammals is as perfect as that of the vessels of the child, so that man ranks lower than other animals in this respect. After a certain period of life the loss of arterial elasticity is considerable and progressive. The arteries may undergo a de- generation from fatty changes or deposit of lime ; such vessels are, of course, liable to rupture ; hence one of the modes of death among old animals is from paralysis traceable to rupture of vessels in the brain. These and other changes also cause the heart more work, and may lead to hypertrophy. Even in young animals the strain of a prolonged racing career may entail hypertrophy or some other form of heart-disease. We mention such facts as these to show the more clearly how important is balance and the power of ready adaptation in all parts of the circulation to the maintenance of a healthy condition of body. The heart is itself nourished through the coronary arteries ; so that morbid alterations in these vessels cause, if not sudden and painful death, at least nutritive changes in the heart-sub- stance, which may lead to a dramatic end or to a slow impair- ment of cardiac power, etc. Personal Observation. — The circulation is one of those de- partments of physiology in which the student may verify much upon his own person. The cardiac impulse, the heart's sounds (with a double stethoscope), the pulse— its nature and changes with circumstances, the venous circulation, and many other THE CIRCULATION OF THE BLOOD, 267 subjects, are all easy of observation, and after a little practice without liability of causing those aberrations due to the atten- tion being drawn to one's self. The observations need not, of course, be confined to the stu- dent's own person ; it is, however, very important that the nor- mal should be known before the observer is introduced to cases of disease. Frequent comparison of the natural and the dis- eased condition renders physiology, pathology, and clinical medicine much good service. We again urge upon the student to try to form increasingly vivid and correct mental pictures of the circulation under its many changes. Comparative,— An interesting arrangement of blood-vessels, known as a rete mirabile, occurs in every main group of verte- Fig 221 —Rete mirabile of sheep, seen in profile (after Chauveau). The larger rete is in connection with the encephalic arteries; the smaller, the ophthalmic. The large artery is the carotid. brates. An artery breaks up into a great number of vessels of nearly the same size, which terminate, abruptly and without capillaries, in another arterial trunk. They are found in a variety of situations, as on the carotid and vertebrate arteries of animals that naturally feed from the ground for long periods together, as the ruminants ; in the sloth, that hangs from trees ; in the legs of swans, geese, etc. ; in the horse's foot, in which the arteries break up into many small divisions. It has been suggested that these arrangements 268 COMPARATIVE PHYSIOLOGY. permit of a supply of arterial blood being maintained without congestion of the parts. Very marked tortuosity of vessels, as in the seal, the carotid of which is said to be forty times as long Fig. 222. — Section of a lymphatic rete mirabile, from the popliteal space (after Chan- veau). a, a, afferent vessels, b, b, efferent vessels. The whole very strongly sug- gests a crude form of lymphatic gland. as the space it traverses, in all probability serves the same pur- pose. Evolution. — The comparative sketch we have given of the vascular system will doubtless suggest a gradual evolution. We observe throughout a dependence and resemblance which we think can not be otherwise explained. The similarity of the fcetal circulation in the mammal to the permanent circula- tion of lower groups has much meaning. Even in the highest form of heart the original pulsatile tube is not lost. The great veins still contract in the mammal ; the sinus venosus is proba- bly the result of blending and expansion. The later differentia- tions of the parts of the heart are clearly related to the adapta- tion to altered surroundings. Such is seen in the fcetal heart and circulation, and has probably been the determining cause of the forms which the circulatory organs have assumed. It is a fact that the part of the heart that survives the long- est under adverse conditions is that which bears the stamp of greatest ancestral antiquity. It (the sinus venosus) may not THE CIRCULATION OF THE BLOOD. 269 be less under nervous control, but it certainly is least dependent on tbe nervous system, and bas tbe greatest automaticity. The law of rhythm in organic nature finds some of its most evident exemplifications in tbe circulation. Most of tbe rhytbms are com- pound, one being |illifl|HflJf blended with or su- perimposed on an- other. Even tbe ap- parent irregularities of tbe normal heart are rhythmical, such as the very marked slowing and other changes accompany- ing expiration, espe- cially in some ani- mals. We trust we have made it evident that the greatest allow- ance must be made for the animal group, and some even for the individual, in es- timating any one of the factors of the cir- culation. We know a good deal at present of cardiac physiology, but we do not know a physiology of " the heart " in the sense in which we understand that term to have been used till recently — i. e., we are not in a position to state tbe laws that apply to all forms of heart. Summary of the Physiology of the Circulation.— In the mammal the circulatory apparatus forms a closed system con- sisting of a central pump or heart, arteries, capillaries, and veins. All the parts of the vascular system are elastic, but this property is most developed in the arteries. Since the tissue-lymph is prepared from the blood in the capillaries, it may be said that the whole circulatory system exists for these vessels. As a result of the action of an intermittent pump on elastic Fig. 283.— Veins of the foot of the horse (after Chau- veau). 270 COMPARATIVE PHYSIOLOGY. vessels against peripheral resistance, in consequence of which the arteries are always kept more than full (distended), the flow through the capillaries and veins is constant — a very great advantage, enabling the capillaries to accomplish their work of feeding the ever-hungry tissues. While physical forces play a very prominent part in the circulation of the blood, vital ones must not be ignored. They lie at the foundation of the whole, here as elsewhere, and must be taken into the account in every explanation. As a consequence of the anatomical, physical, and vital char- acters of the circulatory system, it follows that the velocity of the blood is greatest in the arteries, least in the capillaries, and intermediate in the veins. The veins with their valves, their superficial position and thinner walls, make up a set of conditions favoring the onflow of the blood, especially under muscular exercise. In the mammal the circulatory system, by reason of its con- nections with the digestive, respiratory, and lymphatic systems, and in a lesser degree with all parts of the body, especially the glandular organs, maintains at once the usefulness and the fit- ness of the blood. The arterioles, by virtue of their highly developed muscular coat, are enabled to regulate the blood-supply to every part, in obedience to the nervous system. The blood exercises a certain pressure on the walls of all parts of the vascular system, which is greatest in the heart it- self, high in the arteries, lower in the capillaries, and lowest in the veins, in the largest of which it may be less than the atmos- pheric pressure, or negative. The heart in the mammal consists of four perfectly separated chambers, each upper and each lower pair working synchronously, intermixture of arterial and venous blood being prevented by septa and interference in working by valves. The heart is a force-pump chiefly, but, to some extent, a suction-pump also, though its power as such purely from its own action and independent of the respiratory movements of the chest is slight under ordinary circumstances. In consequence of the lesser resistance in the pulmonary divis- ion of the circulation, the blood-pressure within the heart is much less in the right than in the left ventricle — a fact in har- mony with and causative of the greater thickness of the walls of the latter; for in the foetus, in which the conditions are dif- ferent, this distinction does not hold. THE CIRCULATION OF THE BLOOD. 271 The ventricles usually completely empty themselves of blood and maintain their systolic contraction even after this has been effected. The contraction of the heart, which really begins in the great veins near their junction with the auri- cles (that do not fully empty themselves), is at once fol- lowed up by the auricular and ventricular contraction, the whole constituting one long peristaltic wave. Then follows the cardiac pause, which is of longer duration than the entire systole. When the heart contracts it hardens, owing to closing on a non-compressible fluid dammed back within its walls by resist- ance a f route. At the same time the hand placed on the chest- walls over the heart is sensible of the cardiac impulse, owing to what has just been mentioned. The systole of the chambers of the heart gives rise to a first and a second sound, so called, caused by several events combined, in which, however, the ten- sion of the valves must take a prominent share. The work of the heart is dependent on the quantity of blood it ejects and the pressure against which it acts. The pulse is an elevation of the arterial wall, occurring with each heart-beat, in conse- quence of the passage of a wave over the general blood- stream. There is a distention of the entire arterial system in every direc- tion. The pulse travels with extreme velocity as compared with the blood-current. The heart-beat varies in force, frequency, duration, etc., and with age, sex, posture, and numerous other circumstances. The whole of the circulatory system is regulated by the cen- tral nervous system through nerves. There is in the medulla oblongata a small collection of nerve-cells making up the cardio-inhibitory center. This center, with varying degrees of constancy, depending on the group of animals and the needs of the organism, sends forth impulses (which modify the beat of the heart in force and frequency) through the vagi nerves. There are nerves of the sympathetic system with a center in the cervical spinal cord, and possibly another in the medulla, which are capable of originating either an acceleration of the heart rhythm or an increase of the force of the beat, or both together, known as accelerators or augmentors. In the verte- brates thus far examined the vagus is in reality a vago-sympa- thetic nerve, containing inhibitory fibers proper, and sympa- thetic, accelerator, or motor fibers. The inhibitory fibers can arrest, slow, or weaken the cardiac 272 COMPARATIVE PHYSIOLOGY. beat ; the sympathetic accelerate it or augment its force. When both are stimulated together, the inhibitory prevail. These nerves, as also the accelerators, exercise a profound influence upon the nutrition of the heart, and affect its electri- cal condition when stimulated, and we may believe when influ- enced by their own centers. The inhibitory fibers tend to preserve and restore cardiac energy ; the sympathetic, whether in the vagus or as the aug- mentors, the reverse. The vagus nerve (and probably the de- pressor) acts as an afferent, cardiac sensory nerve reporting on the intra-cardiac pressure, etc., and so enabling the vaso- motor and cardio-inhibitory centers, which are, it would seem, capable of related and harmonious action to act for the general good. The arterioles must be conceived as undergoing very fre- quent changes of caliber. They are governed by the vaso-motor center, situated in the medulla, and possibly certain subordi- nate centers in the spinal cord, through vaso-motor nerves. These are (a) vaso-constrictors, which maintain a constant but variable degree of contraction of the muscle-cells of the vessels ; (b) vaso-dilators, which are not in constant functional activity ; and (c) mixed nerves, with both kinds. An inherited tendency to rhythmical contraction throughout the entire vascular sys- tem, including the vessels, must be taken into account. The depressor nerve acts by lessening the tonic contraction of (dilating) the vessels of the splanchnic area especially. It is important to remember that all the changes of the vascular system, so long as the nervous system is intact — i. e., so long as an animal is normal— are correlated ; and that the action of such nerves as the depressor is to be taken rather as an example of how some of these changes are brought about, mere chapters in an incomplete but voluminous history, if we could but write it all. The changes in blood-pressure, by the addition or removal of a considerable quantity of blood, are slight, owing to the sort of adaptation referred to above, effected through the nervous system. Finally, the capillary circulation, when studied microscopically, and especially in disordered con- ditions, shows clearly that the vital properties of these vessels have an important share in determining the character of the circulation in themselves directly and elsewhere indirectly. The study of the circulation , in other groups shows that below birds the arterial and venous blood undergoes mixture THE CIRCULATION OF THE BLOOD. 273 somewhere, usually in the heart, but that in all the vertebrates the best blood is invariably that which passes to the head and upper regions of the body. The deficiencies in the heart, owing to the imperfections of valves, septa, etc., are in part counter- acted in some groups by pressure relations, the blood always flowing in the direction of least resistance, so that the above- mentioned result is achieved. Capillaries are wanting in most of the invertebrates, the • blood flowing from the arteries into spaces (sinuses) in the tis- sues. It is to be noted that a modified blood (lymph) is also found in the interspaces of the cells of organs. Indeed, the circulatory system of lower forms is in many respects analogous to the lymphatic system of higher ones. 18 DIGESTION OF FOOD. The processes of digestion may be considered as having for their end the preparation of food for entrance into the blood. This is in part attained when the insoluble parts have been rendered soluble. At this stage it becomes necessary to inquire as to what constitutes food or a food. Inasmuch as animals, unlike plants, derive none of their food from the atmosphere, it is manifest that what they take in by the mouth must contain every chemical element, in some form, that enters into the composition of the body. But actual experience demonstrates that the food of animals must, if we except certain salts and water, be in organized form — i. e., it must approximate to the condition of the tissues of the body in a large degree. Plants, in fact, are necessary to animals in working up the elements of the earth and air into form suitable for them. Foodstuffs are divisible into : I. Organic. 1. Nitrogenous, (a) Albumins; (b) Albuminoids (as gelatin). 2. Non-nitrogenous, (a) Carbohydrates (sugars, starches) ; (b) Fats. II. Inorganic. 1. Water. 2. Salts. Animals may derive the whole of their food from the bodies of other animals (carnivora) ; from vegetable matter exclusively (herbivora) ; or from a mixture of the animal and vegetable, as in the case of the pig, bear, and man himself (omnivora). It has been found by feeding experiments, carried out mostly on dogs, that animals die when they lack any one of the con- stituents of food, though they live longer on the nitrogenous than any other kind. In some instances, as when fed on gela- tin and water, or sugar and water, the animals died almost as DIGESTION OP FOOD. 275 soon as if they had been wholly deprived of food. But it has also been observed that some animals will all but starve rather than eat certain kinds of food, though chemically sufficient. We must thus recognize something more in an animal than merely the mechanical and chemical processes which suffice to accomplish digestion in the laboratory. A food must be not only sufficient from the chemical and physical point of view, but be capable of being acted on by the digestive juices, and of such a nature as to suit the particular animal that eats it. To illustrate, bones may be masticated and readily digested by a hyena, but not by an ox or by man, though they meet the conditions of a food in containing all the requisite constituents. Further, the food that one man digests readily is scarcely di- gestible at all by another ; and it is within the experience of every one that a frequent change of diet is absolutely necessary. Since all mammals, for a considerable period of their exist- ence, feed upon milk exclusively, this must represent a perfect or typical food. It will be worth while to examine the compo- sition of milk. The various substances composing it, and their relative proportions for different animals, may be seen from the following table, which is based on a total of 1,000 parts : CONSTITUENTS. Human. Cow. Goat. Ass. Water 889-08 857-05 863-58 910-24 Casein j- 39-24 26-66 43-64 1-38 j 48-28 33-60 } 5-76 12-99 4305 43-57 40-37 40-04 5-48 622 j- 20-18 12-56 Albumin Butter Milk-sugar Salts j- 57-02 Total solids 110-92 142 95 136-42 89 76 The following table, giving the percentage composition of the milk of different animals, may prove instructive. Woman. Cow. Mare. Bitch. Pats 2-00 2-75 0-25 5-00 4-00 4-00 0-60 4-40 2-50 2-00 0-50 5-00 10-00 10-00 Salts Sugar 0-50 3-50 Total solids 10-00 90-00 13-00 87-00 10-00 90-00 24-00 Water 76-00 276 COMPARATIVE PHYSIOLOGY. 1. The proteids of milk are : (a.) An albumin very like serum-albumin. (p.) Casein, normally in suspension, in the form of extremely minute particles, "which contributes to the opacity of milk. It can be removed by filtration through porcelain ; and pre- cipitated or coagulated by acids and by rennet, an extract of the mucus membrane of the calf's stomach. After this coagu- lation, whey, a fluid more or less clear, separates, which con- tains the salts and sugar of milk and most of the water. Much of the fat is entangled with the casein. Casein, with some fat, makes up the greater part of cheese. 2. Fats. — Milk is an emulsion — i. e., contains fat suspended in a fine state of division. The globules, which vary greatly in size, are surrounded by an envelope of proteid matter. This covering is broken up by churning, allowing the fatty globules to run together and form butter. Butter consists chiefly of olein, palmitin, and stearin, but contains in smaller quantity a variety of other fats. The ran- cidity of butter is due to the presence of free fatty acids, espe- cially butyric. The fat of milk usually rises to the surface as cream when milk is allowed to stand. 3. Milk-sugar, which is converted into lactic acid, probably by the agency of some form of micro-organism, thus furnish- ing acid sufficient to cause the precipitation or coagulation of the casein. Milk, when fresh, should be neutral or faintly alkaline. 4. Salts (and other extractives), consisting of phosphates of calcium, potassium, and magnesium, potassium chloride, with traces of iron and other substances. It can be readily understood why animals fed on milk rarely suffer from that deficiency of calcium salts in the bones leading to rickets, so common in the ill-fed. It thus appears that milk contains all the constituents requisite for the building up of the healthy mammalian body; and experiments prove that these exist in proper proportions and in a readily digestible form. The author has found that a lai'ge number of animals, into the usual food of which, in the adult form, milk does not enter, like most of our wild mammals, as well as most birds, will not only take milk but soon learn to like it, and thrive well upon it. Since the embryo chick lives upon the egg, it might have been supposed that eggs would form excellent food for DIGESTION OF FOOD. 277 adult animals, and common experience proves this to be the case ; while chemical analysis shows that they, like milk, con- tain all the necessary food constituents. Meat (muscle, with fat chiefly) is also, of course, a valuable food, abounding in Animal Foods. Explanation of the signs. Proteids. Albuminoids. N-free org. bodies. Salts. I2 M^jwsraiiiiii 55 73.5 1' I" ■».. 0.4 Vegetable Foods. Explanation of the signs. Tl'heaten-bread. Peas. Rice. Potatoes. Wliite Turnip. Cauliflower. Beer. 41.3 13 Digestible Non-digestible N-free organ bodies. 90.5 90 Fig. 234 (Landois). Salts. Illllllllllllllllllll 1» 2.5 i-lllllllill 0.5: I 0.2 10.5. I 1 I 0,5 proteids. Cereals contain starch in large proportion, but also a mixture of proteids. Green vegetables contain little actual nu- 278 COMPARATIVE PHYSIOLOGY. tritive material, but are useful in furnishing salts and special substances, as certain compounds of sulphur which, in some ill- understood way, act beneficially on the metabolism of the body. They also seem to stimulate the flow of healthy digestive fluids. Condiments act chiefly, perhaps, in the latter way. Tea, coffee, etc., contain alkaloids, which it is likely have a conservative effect on tissue waste, but we really know very little as to how it is that they prove so beneficial. Though they are recognized to have a powerful effect on the nervous system as stimulants, nevertheless it would be erroneous to suppose that their action was confined to this alone. The accompanying diagrams will serve to represent to the eye the relative proportions of the food-essentials in various articles of diet. Pig. 225. — Alimentary canal of embryo while the rudimentary mid-gut is still in con- tinuity with yelk-sac (KQlliker, after Bischoff). A. View from before, a, pharyn- geal plates; b, pharynx; c, c, diverticula forming the lungs; d, stomach; /, diver- ticula of liver; g, membrane torn from yelk-sac; A, hind gut. B. Longitudinal section, a, diverticulum of a lung; b, stomach; c, liver; d, yelk-sac. It is plain that if, in the digestive tract, foods are changed in solubility and actual chemical constitution, this must have been brought about by chemical agencies. That food is broken up at the very commencement of the alimentary tract is a matter of common observation; and that there should be a gradual movement of the food from one part of the canal to another, where a different fluid is secreted, would be expected. As a matter of fact, mechanical and chemical forces play a DIGESTION OF FOOD. 279 large part in the actual preparation of the food for absorption. Behind these lie, of course, the vital properties of the glands, which prepare the active fluids from the blood, so that a study of diges- tion naturally divides itself into the consideration of — 1. The digestive juices; 2. The secretory processes; and, 3. The muscular and nervous mechanism by which the food is carried from one part of the digest- ive tract to another, and the waste matter finally expelled. Embryological— The alimentary tract, as we have seen, is formed by an infolding of the splanchnopleure, and, according as the growth is more or less marked, does the canal become tortuous or remain some- what straight. The alimentary tract of a mammal passes through stages of development which correspond with the permanent form of other groups of vertebrates, according to a general law of evo- lution. Inasmuch as the embryonic gut is formed of mesoblast Fig. 226. — Diagram of alimentary canal of chick at fourth day (Foster and Balfour, after GOt- te). Ig, diverticulum of one lung; St, stomach; I, liver; p, pancreas. Fie. 227.— Position of various parts of alimentary canal at different stages. A. Em- bryo of five weeks. B. Of eight weeks. C. Of ten weeks (Allen "Thomson). I, pharynx with the lungs; s, stomach; i, small intestine; i', large intestine; g, geni- tal duct; u, bladder; cl, cloaca; c, caecum; vi, ductus vitello-intestinalis; si, uro- genital sinus; v, yelk-sac. 280 COMPARATIVE PHYSIOLOGY. and hypoblast, it is easy to understand why the developed tract should so invariaably consist of glandular structures and mus- cular tissue disposed in a certain regular arrangement. The fact that all the organs that pour digestive juices into the ali- mentary tract are outgrowths from it serves to explain why there should remain a physiological connection with an ana- tomical isolation. The general resemblance of the epithelium throughout, even in parts widely separated, also becomes clear, as well as many other points we can not now refer to in detail, to one who realizes the significance of the laws of descent (evo- lution). Comparative. — Amoeba ingests and digests apparently by every part of its body ; though exact studies have shown that it neither accepts nor retains without considerable power of discrimination ; and it is also possible that some sort of digest- ive fluid may be secreted from the part of the body with which the food-particles come in contact. It has been shown, too, that there are differences in the digestive capacity of closely allied forms among Infusorians. The ciliated Infusorians have a permanent mouth, which may also serve as an anus ; or, there may be an anus, though usually less distinct from the rest of the body than the mouth. Among the Ccelenterates intra-cellular digestion is found. Certain cells of the endoderm (as in Hydra) take up food-parti- cles Amceba-like, digest them, and thus provide material for other cells as well as themselves, in a form suitable for assimi- lation. This is a beginning of that differentiation of function which is carried so far among the higher vertebrates. But, as recent investigations have shown, such intra-cellular digestion exists to some extent in the alimentary canal of the highest members of the vertebrate group (see page 345). The means for grasping and triturating food among in- vertebrates are very complicated and varied, as are also those adapted for sucking the juices of prey. Examples to hand are to be found in the crab, crayfish, spider, grasshopper, beetle, etc., on the one hand, and the butterfly, housefly, leech, etc., on the other. Before passing on to higher groups, it will be well to bear in mind that the digestive organs are to be regarded as the out- come both of heredity and adaptation to circumstances. We find parts of the intestine, e. g., retained in some animals in whose economy they seem to serve little if any good purpose, as DIGESTION OF FOOD. 281 the vermiform appendix of man. Adaptation has been illus- trated in the lifetime of a single individual in a remarkable Fig. 228. — Diagram illustrating arrangement of intestine, nervous system, etc., in com- mon snail, Helix (after Huxley), m, mouth; t, tooth; od, odontophore; g, gullet; c, crop; a', stomach; r, rectum; a, anus; r. s, renal sac; h, heart; I, lung (modified pallia) chamber); n. its external aperture; em. thick edge of mantle united with sides of body; /, foot; cpg, cerebral, pedal, and parieto-splanchnic ganglia aggre- gated round gullet. manner ; thus, a seagull, by being fed on grain, has had its stomach, naturally thin and soft-walled, converted into a mus- cular gizzard. Since digestion is a process in which the mechanical and chemical are both involved, and the food of animals differs so widely, great variety in the alimentary tract, both anatomical and physiological, must be expected. Vegetable food must usually be eaten in much larger bulk to furnish the needed elements; hence the great length of intestine habitually found in herbivorous animals, associated often with a capacious and chambered stomach, furnishing a larger laboratory in which Nature may carry on her processes. To illustrate, the stomach of the ruminants consists of four parts (rumen, reticu- lum, omasum or psalterium, abomasum). The food when cropped is immediately swallowed; so that the paunch (rumen) is a mere storehouse in which it is softened, though but little changed otherwise ; and it would seem that real gastric digestion is almost confined to the last division, which may be compared 282 COMPARATIVE PHYSIOLOGY. to the simple stomach, of the Carnivora or of man ; and, before the food reaches this region, it has been thoroughly masticated and mixed with saliva. The stomach of the horse is small, though the intestine, Fig. 229.— The viscera of a rabbit, as seen upon simply opening the cavities of the thorax and abdomen without any further dissection. A, cavity of the thorax, pleural cavity on either side; fi, diaphragm; C, ventricles of the heart; Z>, auri- cles; E, pulmonary artery; F, aorta; G. lungs collapsed, and occupying only back part, of chest; //, lateral portions of pleural membranes; /.cartilage at. the end of sternum (ensiform cartilage); K, portion of the wall of body left between thorax and abdomen; «, cut ends of the ribs; L, the liver, in this case lying more to the left than to the right of the body; M, the stomach, a large part of the greater curvature beting shown; IV, duodenum; O. small intestine; P, the cfecum, so largely developed in this and other herbivorous animals; Q, the large intestine. 'Huxley.) DIGESTION OF FOOD. 283 especially the lai'ge gut is capacious. The stomach is divisible into a cardiac region, of a light color internally, and lined with epithelium, like that of the oesophagus, and a redder Pig. 230. Fig. 232. Fig. 230. — General and lateral view of dog's teeth (after Chauveau). Fig. 231.— Anterior view of incisors and canine teeth in a year-old dog (Chauveau). Fig. 232.— Dentition of inferior jaw of horse (after Chauveau). pyloric area, in which the greater part of the digestive process goes on (Fig. 266). 284 COMPARATIVE PHYSIOLOGY. The mouth parts, even in some of the higher vertebrates, as the Carnivora, serve a prehensile rather than a digestive pur- pose. This is well seen in the dog, that bolts his food ; but in this and allied groups of mammals gastric digestion is very active. Tbe teeth as triturating organs find their highest develop- ment in ruminants, the combined side-to-side and forward-and- backward motion of the jaws rendering them very effective. In Carnivora the teeth serve for grasping and tearing, while in the Insectivora the tongue, as also in certain birds (wood- peckers), is an important organ for securing food. Fig. 333. — Profile of upper teeth of the horse, more especially intended to show the molars, the fangs having been exposed (Chauveau). a, molar teeth; b, supple- mentary molar; c, tusk; d. incisors. It is to be noted, too, that, while the horse crops grass by biting it off, the ox uses the tongue, as well as the teeth and lips, to secure the mouthful. Man's teeth are somewhat intermediate in form between the carnivorous and the herbivorous type. Birds lack teeth, but the strong muscular gizzard suffices to grind the food against the small pebbles that are habitually swallowed. The crop, well developed in granivorous birds, is a dilatation of the oesophagus, serving to store and soften the food. In the pigeon a glandular epithelium in the crop secretes a Fig. 234.— General view of digestive apparatus of fowl (after Chauveau). 1, tongue; 2 pharynx; 3, first portion of oesophagus; 4, crop; 5, second portion of oesopha- gus; 6, succentric ventricle (proventricuniB); 7, gizzard; 8, origin of duodenum; !), first branch of duodenal flexure; 10, second branch of same; 11, origin of floating portion of small intestine; 12, small intestine; 12', terminal portion of this intes- tine, Hanked on each side by the two coeca (regarded as the analogue of colon of mammals); 18, 13, free extremities of csecums; 11. insertion of these two culs-de- sac into intestinal tube; 15, rectum; lfi, cloaca; 17', anus; 18, mesentery; 19. left lobe ol' liver; 20, right, lobe; 21, gall-bladder; 22, insertion of pancreatic and biliary ducts; the two pancreatic ducts are the most anterior, the choledic or hepatic is in the middle, and the cystic duct is posterior; 23, pancreas; 24, diaphragmatic aspect of lung; 25, ovary (in a state of atrophy); 2G, oviduct. Pia. 234. 286 COMPARATIVE PHYSIOLOGY. milky-looking substance that is regurgitated into the mouth of the young one, which is inserted within that of the parent bird. The proventriculus — an enlargement just above the gizzard —is relatively to the latter very thin -walled, but provides the true gastric juices. Certain plants digest proteid matter, like animals ; thus the sun-dew (Drosera), by the closure of its leaves, captures insects, which are digested and the products absorbed. The digestive fluid consists of a pepsin-containing secretion, together with formic acid. STRUCTURE, ARRANGEMENT, AND SIGNIFICANCE OF THE TEETH. In a tooth we recognize a portion imbedded in the jaw (fang, root), a free portion (crown), and a constricted region (neck). B Ki... ;r,:, Fig. 237. Pia 835— Magnified section of a canine tooth, showing its intimate structure. 1, crown- 2 -i neck- 3 fang, or root; 4, cavitas pulpae; 5, opening by which the ves- sels and nerves communicate with the pulp; 6, 6, ivory, showing fibrous structure; 7,7, enamel; 8, 8, cement. , . Pio. 236.— A, transverse section of enamel, Showing its hexagonal prisms; B, sepa- rated prisms (Cliauveau). Kir. !.':{7 Section through fang of molar tooth (Cliauveau). a, a. dentine traversed by its tubuli; b, b, interglobular or nodular layer; c, c, cementum. DIGESTION OF POOD. 287 Pig. 238.— Incisor teeth of the horse. Details of structure (Chauveau). 1, a tooth in which is indicated general shape of a permanent incisor and the particular forms successively assumed by dental table in consequence of friction and the continued pushing outward of these teeth; 2. a virgin tooth, anterior and posterior faces; 3, longitudinal section of a virgin tooth, intended to show the internal conformation and' structure. Not to complicate the figure, the external cement and that amassed in the infundibulum have not been exhibited; 4, transverse section for the same purpose; a, encircling enamel; b, central enamel; c, dental star; d, dentine; 5, de- ciduous tooth. A tooth is made up of enamel, dentine or " ivory," and ce- ment (crusta petrosa). The relative distribution of these is shown in Fig. 235. B D Fig. 239.— Transverse section of a horse's upper molar tooth (Chauveau). A, external cement; JB, external enamel; C, dentine; D, internal enamel; E, internal cement. 288 COMPARATIVE PHYSIOLOGY. Enamel is made up of elongated hexagonal prisms set almost vertically in the dentine (Fig. 236). It is the hardest substance known in the animal body, con- sisting almost entirely of inorganic material ; and when lost is but indifferently if at all replaced. Fio. 240.— Tooth of cat in situ (Waldeyer). 1, enamel; 2, dentine; 3, cement; 4, peri- osteum of alveolar cavity; 5, bone of jaw; 6, pulp cavity. Dentine is traversed by the dentine tubules (Fig. 237), which radiate outward from the pulp cavity. DIGESTION OF FOOD. 289 The latter is filled by the tooth-pulp, which consists of a delicate connective tissue supporting blood-vessels and nerves which ramify in it after entering by the openings in the fang of the tooth. From the pulp protoplasmic fibers extend into the dentine tubules. The crusta petrosa is very similar to bone, but is usually without Haversian canals, and, like bone, is covered with peri- osteum. Fig. 241.— The teeth of the ox (Chauveau). 1, upper jaw, with a, friction surface and b, external surface; 2, lower jaw, with 0, dental tables, and b, external face. ' Teeth are simple and compound. In the former (carnivora) the entire crown is covered with enamel ; in the latter, owing 19 290 COMPARATIVE PHYSIOLOGY. to wear, the other constituents appear on the upper surface of the crown (Figs. 238, 239, 240). It follows that the former are better adapted for tearing, the latter for grinding, as the different components wear un- equally and leave the top of the crown rough, so that the upper and lower jaws of a ruminant are like two millstones, (Fig. 241). It also follows that in the horse and in ruminants the age may be learned with considerable accuracy from the condition of wear of the teeth and as the incisors are most readily ex- amined they are taken as the chief indicators of the age of the animal. In nearly all animals are found the deciduous or milk teeth succeeded by the permanent teeth. This arises as a necessity from the growth of the jaw and the need of stronger teeth, either as weapons of defense and attack or in order the more effectu- ally to secure and prepare food. The permanent teeth are also more numerous than the milk teeth. The dentition of our domestic animals may be expressed thus : Dos. Cat. Man. Pig- Ox. Horse. . 3-3 Incisors, o~~q 1-1 canines, y~. r 3-3 1-1 3-3 1-1 2-2 1-1 2-2 1-1 3-3 1-1 3-3 1-1 0-0 0-0 3-3 1-1 3-3 1-1 3-3 1-1 3-3 0-0 3-3 0-0 4-4 premolars, t^7 3-3 2-2 2-2 2-2 3-3 3-3 3-3 3-3 3-3 3-3 0-0 0-0 2-2 molars, 5—5 = 42. 1-1 1-1 3-3 3-3 4-4 4-4 3-3 3-3 3-3 3-3 3-3 3-3 = 30. = 32. = 44. 32. = 40. = 24. The latter is the representation of the milk dentition. The mare is without canines ("tushes"). It will be noticed that in the ox incisors and canines do not appear in the upper jaw, though they are represented by embry- onic rudiments. The table above and that on page 296 (after Leyh) give a large amount of information in a small space, and are illus- trated by the accompanying figures : DIGESTION OF FOOD. 291 Fig. 242.— The teeth of the pig (Chauveau). 1, upper teeth, table surface; 2, lower teeth, table aspect; 3, lateral view of jaws. 2$ years. 4 years. (6 broad incisors.) (8 broad incisors.) Over 7 years, broad incisors.) 2 months. (Milk-teeth.) 1± years. (2 broad incisors.) If years. (4 broad incisors.) Fig. 243. — Changes in incisor teeth of the sheep (WilckensX 292 COMPARATIVE PHYSIOLOGY. New-born. 3 months. 6 months JSwSl^ ■"■'■"■■ " ■ ' ■ 1 year. 2 years. 2£ years. 3 years. 4 years. 5 years. (i years. 7 years. Fig. 244 (1).— Changes in incisor teeth of horse with age (Wilckens). DIGESTION OF POOD 293 8 years. it years. 12 years. 15 years. 18 years. 24 years. Fig. 244 (2). Changes in incisor teeth of horse with age (Wilckens). 294 COMPARATIVE PHYSIOLOGY. 1 New-born. JSD 4 weeks. 1 year. 2 years. 2| years. 3 years. •3J years. 4 years. 5 years. Fig. 245 (1).— Changes in incisor teeth of ox with age (Wilckens). DIGESTION OF POOD. 295 6 years. ' years. 8 years. 10 years. 12 years. 14 years. 16 years. 18 years. 20 years. Fig. 245 (2).— Changes in incisor teeth of ox with age (Wilckens). 296 COMPARATIVE PHYSIOLOGY. a o a a a ft 5 a a 'A a o a 3 2 S i . cj-^> CO El ■as CD M Pi a _o P. 3 & g a ag a _o ft g H CO « c K cs a as oj a a' O p. ! t i i 6 i co c/: c Jp £ p -r O o g — eo JO a & — 3 3 O += "# i^> CO to o o +3 He ?> CO 03 05 c-' o Ej ■J; ;-°2 O - O OJ - oj4h -* o 1 o o r O It PQ co CO CO - - =3 c3 Oj OJ oj aj ^ >-> J-. ^ H* hcnhwh=» t— I ea CO -— ? a CO CO K o3 CO OJ OJ OJ fc-> ^ ^ -*" H" -ts C4 CO -* E o CO o OJ co 03 OJ CO o a co «0 os c ■■—• O c OJ cq "* o . hepatic duct; c, opening of second duct of pancreas; 3 Ml «SB ! ao ' 1 If '^ ofi, I Fig. 849.— Micro-organisms of large intestine (after Landois), 1. bacterium coli com- mune; 2, bacterium lactis aerogenes; 3, 4, large bacilli of Bienstock, with partial endogenous spore-formation; 5, various stages of development of bacillus which causes fermentation of albumen. in the laboratory without their occurrence, and they vary in degree with the animal, the individual, the food, and other con- ditions. It is not, however, to be inferred that micro-organisms serve no useful purpose in the alimentary canal ; the subject, in fact, requires further investigation. Succus Entericus.— The difficulties of collecting the secre- tions of Lieberkuhn's, Briinner's, and other intestinal glands will be at once apparent. But by dividing the intestine in two places, so as to isolate a loop of the gut, joining the sundered Fig. 250.— Portion of one of Briinner's glands (Chauveau). ends by ligatures, thus making the continuity of the main gut as complete as before, closing one end of the isolated loop, and 308 COMPARATIVE PHYSIOLOGY. bringing the other to the exterior, as a fistulous opening, the secretions could be collected, food introduced, etc. But it seems highly improbable that information approxi- mately correct at best, and possibly highly misleading, could "m •i)-,y) Pig. 251.— Intestinal tubules (follicles of Lieberkiihn) 1 x 100 (Sappey). A, from dog; B, ox; C, sheep; D, pig; E, rabbit. be obtained in such manner. Moreover, the greatest diversity of opinion prevails as to the facts themselves, so that it seems scarcely worth while to state the contradictory conclusions ar- rived at. It is, however, on the face of it, probable that the intestine — even the large intestine — does secrete juices that in herbivora, at all events, play no unimportant part in the digestion of their Fig. 252. — General view of horse's intestines; animal is placed on its back, and intes- tinal mass spread out (after Ohaiiveau). A, duodenum as it passes behind great mesenteric artery; B, free portion of small intestine; 0, ileocaecal portion; D, creciim; E, I<\ G, loop formed by large colon; G, pelvic flexure; F, F, point where colic loop is doubled to constitute suprasternal and diaphragmatic flexures. Fig. 352. 310 COMPARATIVE PHYSIOLOGY. bulky food ; and it is also probable, as in so many otber in- stances, that, when the other parts of the digestive tract fail when the usual secretions are not prepared or do not act on the food, glands that normally play a possibly insignificant part may function excessively — we may almost say vicariously — and that such glands must be sought in the email intestine. There are facts in clinical medicine that seem to point strongly in this direction, though the subject has not yet been reduced to scientific form. Comparative.— Within the last few years the study of vege- table assimilation from the comparative aspect has been fruit- ful in results which, together with many other facts of vegeta- ble metabolism, show that even plants ranking high in the organic plane are not in many of their functions so different from animals as has been supposed. It has been known for a longer period that certain plants are carnivorous ; but it was somewhat of a surprise to find, as has been done within the past few years, that digestive ferments are widely distributed in the vegetable kingdom and are found in many different parts of plants. What purpose they may serve in the vegetable economy is as yet not well known. At present it would seem as though, from their presence in so many cases in the seed, they might have something to do with changing the cruder forms of nutriment into such as are better adapted for the nour- ishment of the embryo. Thus far, then, not only diastase but pepsin, a body with action similar to trypsin, and a rennet ferment, rank among the vegetable ferments best known. A ferment has been extracted from the stem, leaves, and un- ripe fruit of Carica papaya, found in the East and West Indies and elsewhere, which has a marked proteolytic action. It is effective in a neutoal, most so in an alkaline medium ; and, though its action is suspended in a feeble acid menstruum, it does not appear to be destroyed under such circumstances, as is trypsin. This body is attracting a good deal of attention, and its use has been recently introduced into medical prac- tice. Very lately also a vegetable rennet has been found in sev- eral species of plants. The subject is highly promising and suggestive. DIGESTION OF FOOD. 311 SECRETION AS A PHYSIOLOGICAL PROCESS. Secretion of the Salivary Glands.— We shall treat this subject at more length because of the light it throws on the nervous phenomena of vital process ; and, since the salivary glands have been studied more thoroughly and successfully than any other, they will receive greater attention. Fig. 253. Fig. 254. Fig. 253.— Lobule of parotid gland, injected with mercury, and magnified 50 diame- ters. Fig. 254.— Capillary network around the follicles of the parotid gland. The main facts, ascertained experimentally and otherwise, are the following : Assuming that the student is familiar with the general ana- tomical relations of the salivary glands in some mammal, we would further remind him that the submaxillary gland has a double nervous supply : 1. From the cervical sympathetic by branches passing to tbe gland along its arteries. 2. From the chorda tympani nerve, which after leaving the facial makes connection with the lingual, whence it proceeds to its desti- nation. The following facts are of importance as a basis for conclu- sions: 1. It is a matter of common observation that a flow of saliva may be excited by the smell, taste, sight, or even thought of food. 2. It is also a matter of experience that emotions, as fear, anxiety, etc., may parch the mouth — i. e., ari'est the flow of saliva. The excited speaker thus suffers in his early efforts. 3. If a glass tube be placed in the duct of the gland and any substance that naturally causes a flow of saliva be placed on the tongue, saliva may be seen to rise rapidly in the tube. 4. The same may be observed if the lingual nerve, the glossopha- 312 COMPARATIVE PHYSIOLOGY. ryngeal, and many other nerves be stimulated ; also if food be introduced into the stomach through a fistula. 5. If the pe- Fig. 255.— Maxillary and sublingual gland (Chauveau). R, maxillary gland; S, Whar- ton's duct; T, sublingual gland. ripheral end of the chorda tympani be stimulated, two results follow : (a) There is an abundant flow of saliva, and (6) the arterioles of the gland become dilated ; the blood may pass through with such rapidity that the venous blood may be bright red in color and there may be a venous pulse. 7. Stimu- lation of the medulla oblongata gives rise to a flow of saliva, which is not possible when the nerves of the gland, especially the chorda tympani, are divided; nor can a flow be then excited by any sort of nervous stimulation, excepting that of the ter- minal branches of the nerves of the gland itself. 8. If the sym- pathetic nerves of the gland be divided, there is no immediate flow of saliva, though there may be some dilatation of its ves- DIGESTION OF FOOD. 313 sels. 9. Stimulation of the terminal ends of the sympathetic and chorda nerves causes a flow of saliva, differing as to total quantity and the amount of contained solids; hut the nerve that produces the more abundant watery secretion, or the re- verse, varies with the animal, e. g., in the cat chorda saliva is Part of brain above medulla Afferent nerves from tongue- Fig. 256. -Diagram intended to indicate the nervous mechanism of salivary secretion. more viscid, in the dog less so ; though in all animals as yet examined it seems that the secretion as a result of stimulation of the chorda tympani nerve is the most abundant ; and in the 314 COMPARATIVE PHYSIOLOGY. case of stimulation of the chorda the vessels of the gland are dilated, while in the case of the sympathetic they are con- stricted. 10. If atropin be injected into the blood, it is impos- sible to induce salivary secretion by any form of stimulation, though excitation of the chorda nerve still causes arterial dila- tation. Conclusions. — 1. There is a center in the medulla presiding over salivary secretion. 2. The influence of this center is ren- dered effective through the chorda tympani nerve at all events, if not also by the sympathetic. 3. The chorda tympani nerve contains both secretory and vaso-dilator fibers; the sympathetic secretory and vaso-constrictor fibers. 4. Arterial change is not essential to secretion, though doubtless it usually accompanies it. Secretion may be induced in the glands of an animal after decapitation by stimulation of its chorda tympani nerve, analo- gous to the secretion of sweat in the foot of a recently dead animal, under stimulation of the sciatic nerve. 5. The char- acter of the saliva secreted varies with the nerve stimulated, so that it seems likely that the nervous centers normally in the intact animal regulate the quality of the saliva through the degree to which one or the other kind of nerves is called into action. 6. Secretion of saliva may be induced reflexly by ex- periment, and such is probably the normal course of events. 7. The action of the medullary center may be inhibited by the cerebrum (emotions). Some have located a center in the cerebral cortex (taste cen- ter), to which it is assumed impulses first travel from the tongue and which then rouses the proper secreting centers in the medulla into activity. It seems more likely that the corti- cal center, if there be one, completes the physiological processes by which taste sensations are elaborated. From the influence of drugs (atropin and its antagonist pilocarpin) it is plain that the gland can be effected through the blood, though whether wholly by direct action on the cen- ter, on any local nervous mechanism or directly, on the cells, is as yet undetermined. It is found that pilocarpin can act long after section of the nerves. This does not, however, prove that in the intact animal such is the usual modus operandi of this or other drugs, any more than the so-called paralytic secretion after the section of nerves proves that the latter are not con- cerned in secretion. We look upon paralytic secretion as the work of the cells DIGESTION OP FOOD. 315 when gone wrong — passed from under the dominion of the nerve-centers. Secretion is a part of the natural life-processes of gland-cells — we may say a series in the long chain of pro- cesses which are indispensable for the health of these cells. They must be either secreting cells, or have no place in the nat- ural order of things. It is to be especially noted that the secre- tion of saliva continues when the pressure in the ducts of the gland is greater than that of the blood in its vessels or even of the carotid ; so that it seems possible that over-importance has been attached to blood-pressure in secretory processes gen- erally. It may, then, be safely assumed that formation of saliva re- sults in consequence of the natural activity of certain cells, the processes of which are correlated and harmonized by the nerv- ous system ; their activity being accompanied by an abundant supply of blood. The actual outpouring of saliva depends usu- ally on the establishment of a nervous reflex arc. The other glands have been less carefully studied, but the parotid is known to have a double nervous supply from the cerebro- spinal and the sympathetic systems. It would appear that, as the vaso-motor changes run paral- lel with the secretory ones, the vaso-motor and the proper secretory centers act in concert, as we have, seen holds of the former and the respiratory center. But it is to our own mind very doubtful whether the doctrine of so sharp a demarkation of independent centers, prominently recognized in the physi- ology of the day, will be that ultimately accepted. Secretion by the Stomach.— The mucous membrane of St. Martin's stomach was observed (through an accidental fistulous opening) to be pale in the intervals of digestion, but flushed when secreting, which resembled sweating, so far as the flow of the fluid is concerned. When the man was irritated, the gastric membrane became pale, and secretion was lessened or arrested, and it is a common experience that emotions may help, hinder, or even render aberrant the digestive processes. While the evidence is thus clear that gastric secretion is regulated by the nervous system, the way in which this is ac- complished is very obscure. We know little of either the cen- ters or nerves concerned, and what we do know helps but doubtfully to an understanding of the matter, if, indeed, it does not actually confuse and puzzle. Digestion can proceed in a fashion after section of the nerves 316 COMPARATIVE PHYSIOLOGY. going to the stomach, though this has little force as an argu- ment against nerve influence. We may conclude the subject by stating that, while the influence of the nervous system over gastric secretion is undoubted as a fact, the method is not un- derstood ; and the same remark applies to the secreting activity of the liver and pancreas. The Secretion of Bile and Pancreatic Juice. — When the con- tents of the stomach have reached the orifice of the discharging bile-duct, a large flow of the biliary secretion takes place, prob- ably as the result of the emptying of the gall-bladder by the contraction of its walls and those of its ducts. This is probably a reflex act, and the augmented flow of bile when digestion is proceeding is also to be traced chiefly to nervous influences reaching the gland, though by what nerves or under the gov- ernment of what part of the nervous centers is unknown. Very similar statements apply to the secretion of the pancre- Fig. 257.— Diagram to show influence of food in secretion of pancreatic juice (after N. O. Bernstein). The abscissa? represent hours after taking food ; ordinates amount in cubic centigrammes of secretion in ten minutes. Food was taken at • Ji und C. This diagram very nearly also represents the secretion of bile. atic glands, though this is not constant, as in the case of bile — at all events in most animals. It is known that after food has been taken there is a sudden DIGESTION OP POOD. 317 increase in the quantity of bile secreted, followed by a sudden diminution, then a more gradual rise, with a subsequent fall. Almost the same holds for the pancreas. It seems impossible to explain tbese facts, especially the first rapid discharge of fluid apart from the direct influence of the nervous system. Upon the whole, the evidence seems to show that the press- ure in the bile-ducts is greater than in the veins that unite to make up the portal system; but there are difficulties in the investigation of such and kindred subjects as regards the liver, owing to its peculiar vascular supply. It will be borne in miud that the liver in mammals consists of a mass of blood-vessels, between the meshes of which are packed innumerable cells, and that around the latter meander the bile capillaries; that the portal vein breaks up into the intralobular, from which capil- laries arise, that terminate in the central interlobular veins, which make up the hepatic veinlets or terminate in these vessels. But the structure is complicated by the branches of the hepatic artery, which, as arterioles and capillaries, enters to some extent into the formation of the lobular vessels. A question of interest, though difficult to answer, is the extent to which the various constituents of bile are manufact- ured in the liver. Taurin, for example, is present in some of - wo \-. r T- "Mil "-TT *f .-• '". Pig. 258.— Lobules of liver, interlobular vessels, and intralobular veins (Sappey). 1, 1, 1, 1, 3, 4, lobules; 2,2, 2, 2, intralobular veins injected with white; 5, 5, 5, 5,5, in- tralobular vessels tilled with a dark injection. 318 COMPARATIVE PHYSIOLOGY. the tissues, but whether this is used in the manufacture of taurocholic acid or whether the latter is made entirely anew, and possibly by a method in which taurin never appears as such, is an open question. It is highly probable that a portion of the bile poured into the intestine is absorbed either as such or after partial decomposition, the products to be used in some way in the econo- §||| ';v A my and presumably in the construction of bile by the liver. There are many facts, including some pathological phe- nomena, that point clearly to the formation of the pigments of bile from haemoglobin in some of its stages of de- generation. Pathological.— When the liver fails to act, either from derange- ment of its cells prima- rily or owing to obstruc- tion to the outflow of bile leading to reabsorp- tion by the liver, bile acids and bile pigments appear in the urine or may stain the tissues, indicating their presence in ex- cess in the blood. This action of one gland (kidneys) for another is highly suggestive, and especially important to bear in mind in medical practice, both in treatment and prognosis. The chances of re- covery when only one excreting gland is diseased are much greater evidently than when several are involved. Such facts as we have cited show, moreover, that there are certain common fundamental principles underlying secretion everywhere— a statement which will be soon mbre fully illustrated. Fig. 259. —Portion of transverse section of hepatic lobule of rabbit magnified 400 diameters (K51H- ker). b,\b, b, capillary blood-vessels; g, g, g, cap- ■ illary bile-ducts; I, 1,1, liver-cells. THE NATURE OF THE ACT OF SECRETION. We are now about to consider some investigations, more particularly their results, which are of extraordinary interest. The secreting cells of the salivary, the pancreatic glands, DIGESTION OF FOOD. 310 and the stomach have been studied by a combination of histo- logical and, more strictly, physiological methods, to which we shall now refer. Specimens of these glands, both before and after prolonged secretion, under stimulation of these nerves, Fig. 260.— Portion of pancreas of rabbit (after Kiihne and Lea), at rest; B, during secretion. A represents gland were hardened, stained, and sections prepared. As was to be expected, the results were not entirely satisfactory under these methods ; however, the pancreas of a living rabbit has been viewed with the microscope in its natural condition ; and by this plan, especially when supplemented by the more involved and artificial method first referred to, results have been reached which may be ranked among the greatest triumphs of modern physiology. Some of these we now proceed to state briefly. To begin with the pancreas, it has been shown that, when the gland is not secreting — i. e., not discharging its prepared fluid — or dur- ing the so-called resting stage, the appearances are strikingly different from what they are during activity. The cell pre- sents during rest an inner granular zone and an outer clearer zone, which stains more readily, and is relatively small in size. The lumen of the alveolus is almost obliterated, and the in- dividual cells very indistinct. After a period of secreting activity, the lumen is easily perceived, the granules have dis- appeared in great part, the cells as a whole are smaller, and have a clear appearance throughout. Coincident with the changes in the gland's cells it is to be noticed that more blood passes through it, owing to dilatation of the arterioles. Again, the course of the changes in the salivary glands, whether of the mucous or serous variety, is very similar. In 320 COMPARATIVE PHYSIOLOGY. the mucous gland in the resting stage the cells are large, and hold much clear matter in the interspaces of the cell network ; Pig. 261. — Section of mucous gland (after Lavdowsky). In A, gland at rest; in B, after secreting for some time. and, as this does not stain readily, it can not be ordinary protoplasm. This, when the gland is stimulated through its nerves, disappears, leaving the containing cells smaller. It has become mucin, and may itself be called mucinogen. It is to be noted that, as the cells become more protoplasmic, less burdened with the products of their activity, the nucleus becomes more prominent, suggestive of its having a probable directive influence over these manufacturing processes. Substantially the same chain of events has been established for the serous salivary glands and the stomach, so that we may safely generalize upon these well-established facts. It seems clear that a series of changes constructive and, from one point of view, destructive, following the former are con- Fig. 202.— ( 'hanges in parotid (serous) gland during secretion (after Langley). A, dur- ing rest; B, after moderate, C, after prolonged stimulation. Figures partly dia- grammatic. stantly going on in the glands of the digestive organs. Proto- plasm under nerve influence constructs a certain substance, DIGESTION OF FOOD. 321 which is an antecedent of the final product, which we term a ferment. It is now customary to speak of these changes as constructive (anabolic) and destructive (katabolic), though we have already pointed out (page 258) that this view is, at best, only one way of looking at the matter, and we doubt if it may not be cramping and misleading. We must also urge caution in regard to the conception to be associated with the use of the terms " resting " and " active " stage. It is not to be forgotten that strictly in living cells there is no absolute rest — such means death ; but, if these terms be understood as denoting but degrees of activity, they need not mislead. It is also more than probable that in certain of the glands, or in some animals, the processes go on simultane- ously ; the protoplasm being renewed, the zymogen, or mother- ferment, being formed, and the latter converted into actual fer- ment, all at the same time. The nature of secretion is now tolerably clear as a wbole ; though it is to be remembered that this account is but general, and that there are many minor differences for each gland and variations that can scarcely be denominated minor for different animals. Evidently no theory of filtration, no process depend- ing solely on blood-pressure, will apply here. And if in this, the best-studied case, mechanical theories of vital processes utterly fail, why attempt to fasten them upon other glands, as the kidneys and the lungs, or, indeed, apply such crude concep- tions to the subtle processes of living protoplasm anywhere or in any form \ It is somewhat remarkable that an extract of a perfectly fresh pancreas is not proteolytic ; yet the gland yields such an extract when it has stood some hours or been treated with a weak acid. These facts, together with the microscopic appear- ances, suggested that there is formed a forerunner to the actual ferment — a zymogen, or mother-ferment, which at the moment of discharge of the completed secretion is converted into the actual ferment. We might, therefore, speak of a pepsinogen, trypsinogen, etc., and, though there may be a cessation in the series of processes, and no doubt there is in some animals, this may not be the case in all, or in all glands. Secretion by the Stomach.— The glands of the stomach differ in most animals in the cardiac and pyloric regions. In those of the former zone, both central (columnar) and parietal (ovoid) cells are to be recognized. It was thought that possibly the lat- 21 #s Mm -ft 1 .*) .vt;«l 5;. "1 Fig. 263. Pig. 364. Fio. 265. Pig, 268.— Piii- m mucous membrane of stomach in which arc openings of tubular glands, 1 x 20 (Sappey). Pig. 364.- Glands of stomach with both central and parietal (ovoid) cells (Heidenhain). Fi<;. 865.- - Pyloric, glands (Ebstein). DIGESTION OF FOOD. 323 ter were concerned in the secretion of the acid of the stomach, hut this is by no means certain. Possibly these, like the demi- lune cells of the pancreas, may be the progenitors of the central (chief) cells. The latter certainly secrete pepsin, and probably also rennet. Mucus is secreted by the cells lining the neck of glands and covering the mucous membrane intervening be- tween their mouths. The production of hydrochloric acid by any act of secretion is not believed in by all writers, some hold- ing that it is derived from decomposition of sodium chloride, possibly by lactic acid. So simple an origin is not probable, not being in keeping with what we know of chemical processes within the animal body. Self-Digestion of the Digestive Organs.— It has been found, both in man and other mammals, that when death follows in a healthy subject while gastric digestion is in active progress and the body is kept warm, a part of the stomach itself and often adjacent organs are digested, and the question is con- stantly being raised, Why does not the stomach digest itself during life ? To this it has been answered that the gastric juice is constantly being neutralized by the alkaline blood ; and, again, that the very vitality of a tissue gives it the neces- sary resisting powers, a view contradicted by an experiment which is conclusive. If the legs of a living frog be allowed to hang against the inner walls of the stomach of a mammal when gastric digestion is going on, they will be digested. The first view (the alkalinity of the blood) would not suffice to explain why the pancreas, the secretion of which acts best in an alkaline medium, should not be digested. It seems to us there is a good deal of misconception about the facts of the case. Observation on St. Martin shows that the secretion of gastric juice runs parallel with the need of it, as dependent on the introduction of food, its quantity, quality, etc. Now, there can be little doubt that, if the stomach were abundantly bathed when empty with a large quantity of its own acid secretion, it would suffer to some extent at least. But this is never the case ; the juice is carried off and mixed with the food. This food is in constant motion and doubtless the inner portions of the cells, which may be regarded as the dis- charging region (the outer, next tbe blood capillaries, being the chief manufacturing region of the digestive ferment), are frequently renewed. Such considerations, though they seem to have been some- 324 COMPARATIVE PHYSIOLOGY. what left out of the case, do not go to the bottom of the matter. Amoeba and kindred organisms do not digest themselves. Some believe that the little pulsatile vacuoles of the Infusorians are a sort of temporary digestive cavities. But, to one who sees in the light of evolution, it must be clear that a structure could not have been evolved that would be self -destructive. The difficulty here is that which lies at the very basis of all life. We might ask, Why do living things live, since they are constantly threatened with destruction from within as from without ? Why do not the liver, kidney, and other glands that secrete noxious substances, poison themselves ? We can not in detail explain these things ; but we wish to make it clear that the difficulty as regards the stomach is not peculiar to that gland, and that even from the ordinary point of view it has been exaggerated. Comparative. — More careful examination of the stomachs of some mammals has revealed the fact that in several animals, in which the stomach appears to be simple, it is in reality com- pound. There are different grades, however, which may be regarded as transition forms be- tween the true simple stomach and that highly compound form of the organ met with in the ruminants. It has been shown recently that the stomach of the hog has an oesophageal dilatation ; and that the entire organ may be divided into several zones with different kinds of glandular epi- thelium, etc. These portions differ in digestive power, in the characteristics of the fluid se- creted, and other details beyond those which a superficial exam- ination of this organ would lead one to suspect. The stomach of the horse represents a more advanced form of compound stomach than that of the hog, which is not evi- dent, however, until its glandular structure is examined closely. The entire left portion of the stomach represents an oesophageal dilatation lined with an epithelium that closely resembles that Fig. 266.— Interior of horse's stomach (after Chauveau). A. left sac; B, right sac; 0, duodenal dilatation. DIGESTION OF FOOD. 325 of the oesophagus, and with little if auy digestive function. It thus appears that the stomach of the horse is in reality smaller, as a true digestive gland, than it seems, so that a great part of the work of digestion must he done in the intestine ; though in this animal, if the food be retained as long as it is in the hog. which is not, however, the general opinion as regards the stomach of the horse, salivary digestion may continue for a considerable period after the food has left the mouth. The secretion of mucus by the stomach in herbivora is abundant. As has been already explained, the stomach of ruminants consists of several compartments which are supplementary to one another, though genuine gastric digestion does not take place except in the fourth stomach. The first and second stomachs being destitute of other than mucous glands, and lined with a horny epithelium, are to be con- sidered rather as dilatations of the oesophagus. They answer admirably the purpose of storehouses for the bulky food in which the softening process preparatory to mastication goes on. Pig. 267. — Stomach of the ox scon on its right upper face, tiie aboniasum being de- pressed (Chauveau). A. rumen, left hemisphere; B, rumen, right hemisphere; ('. termination of the oesophagus; D, reticulum; E. omasum; P, abomasnm. 326 COMPARATIVE PHYSIOLOGY. Fig. 268. — Interior of stomach in ruminants; the upper plane of the rumen and reticu- lum, with the oesophageal furrow (Chauveau). A, left sac of the rumen; B, ante- rior extremity of that sac turned back on right sac; 0, its posterior extremity, or left conical cyst; G, section of anterior pillar of rumen; g, g, its two superior branches; H, posterior pillar of same; h, h, h, its three inferior branches; I, cells of reticulum; J. oesophageal furrow; K, oesophagus; L, abomasum. Fig. 269. Stomach of llama (Colin). A, lower extremity of gullet; B, single pillar ot oesophageal canal: C, superior opening of the psalter; D, reticulum; E, right or anterior water-cells; W, inferior water-cells; G, fleshy column separating the two cell groups. DIGESTION OF FOOD. 327 The reticulum, so called from the peculiar arrangement of the mucus membrane, is usually regarded as a receptacle for water more especially ; however, this stomach is to be regarded both anatomically and lihysiologically as a subdivision of the first, or at all events as equivalent to that. The quantity of food that it can hold in the ox is enormous, (150 to 200 pounds), a condition of things advantageous in an Fig. 270. — Omasum and abomasum of ox cut open (Smith). A. psalterium, with open- ing between it and the reticulum at B; P. foldings (plicse) of mucous membrane at C. fourth stomach. animal feeding upon substances so poor in nutritive material in proportion to their bulk and requiring so much mastication to fit them to be acted on by the digestive juices. The reaction of tbe first two stomachs is alkaline. In the camel tribe, water cells are arranged in parallel order in the rumen. The edges of these are provided with muscular 328 COMPARATIVE PHYSIOLOGY. fibers constituting sphincters by which their openings inward may be closed. These cells number several hundred, and are capable of containing some quarts of water. Pig. 271.— A. Stomach. of sheep. B. Stomach of musk-deer, m, oesophagus; Rn, ru- men; Ret. reticulum; Ps, psaltesium; A, Ab, abomasum; Bit, duodenum; Pij, pylorus (Huxley). The manyplies is so named from the arrangement of its mucus membrane in folds, a condition, however, not equally well marked in all ruminants. A structure known as the oesophageal canal, (furrow, groove) communicates with the first three stomachs. During swallow- ing, its lower portion is raised above the level of the third stomach, so that it is likely that this is a barrier against the entrance of all except liquids or soft foods into the manyplies. It is difficult to make any positive statement as to what other part it may take in determining the direction of food when en- tering or leaving the various stomachs. It does not seem to be C'ssential to return of the cud. DIGESTION OF FOOD. 329 The abomasum or rennet resembles other forms of true digestive stomachs in all essential particulars. While the opening between the first and second stomachs is large enough to allow of free intercommunication, the reverse applies to the entrance into the third stomach. The rumen is nearly always tolerably well filled with food, a condition of things favorable to its return for remastication. Pig. 272. — Stomach of horse (after Chauveau). A, cardiac extremity of oesophagus; B, pyloric ring. We may conclude that only food in a proper form for the action of the fourth stomach passes to any extent beyond the first two. After the food has been duly softened and has undergone some fermentative changes in the rumen, leading to the evolution of gases (C0 2 , H2S) and certain organic acids (acetic, butyric), it is regurgitated by a process that closely resembles vomiting. In this the diaphragm and the abdominal muscles, as well 330 COMPARATIVE PHYSIOLOGY. as the stomach itself and the gullet, take part. Probably as a result of the descent of the diaphragm and consequent diminu- tion of the intrathoracic pressure, the asceut of the cud is as- sisted by an aspiratory process. The returning food is pre- vented from passing into the nasal chambers by co-ordinated movements analogous to those of swallowing. The whole pro- cess is reflex in the same sense as is deglutition. Normally the rumen always contains considerable liquid, a portion of which passes up with the cud, but is in great part returned at once. A ruminant given dry food without water can not return the cud. In the second mastication the process is in most ruminants unilateral ; and as hundreds of cuds are to be chewed, a con- siderable proportion of the whole day is occupied with rumina- tion. When a single cud is sufficiently masticated it is swallowed, Pig. 273.— Stomach of dog (after Chaveau). A, oesophagus; B, pylorus. and being finely comminuted passes at once through the small opening between the reticulum and manyplies into the third stomach, and thence into the abomasum, though possibly on the way a little may pass into the first two stomachs. Pathological. — While moderate fullness of the paunch is DIGESTION OF FOOD. 331 favorable to rumination, extreme distention tends to paralysis of the muscular coat of the organ, allowing- of the accumulation of the gases of fermentation which may lead, if not artificially relieved, to rupture of the organ. THE ALIMENTARY CANAL OF THE VERTEBRATE. Amid all variations in this great group, the alimentary canal has common features, both of structure and function. Through- out the entire tract muscle cells of the unstriped (involuntary) kind, arranged in two layers, constitute the motor mechanism for the transportation of food from one part to another. Out- side of these is the serous coat, consisting of fibrous and elastic tissue, and admirably adapted to preserve organs from undue distention, at the same time providing a smooth external cover- ing which lessens the friction of one organ against another in the abdominal cavity ; while folds of such tissue constitute the omentum for supporting the various organs. Between the muscular and mucous coats of the organs that constitute the alimentary canal there is a submucous coat of loose connective tissue in which ramify blood-vessels, nerves, etc. It is the mucous coat, however, that is of paramount impor- tance, and for which all other parts may in some sense be con- sidered to exist ; for it is from the glands with which it is sup- plied that the digestive juices are derived, as well as that mucus which keeps the tract moist and its delicate structures shielded under all circumstances. The amount of surface provided by the mucous membrane is increased by its various foldings (rugce, valvidce conniventes, etc.), so generally present, and which also allow of distention ; and if the secreting glands are regarded as minute induplications of this coat, it will be evident that its total area is much greater than at first ap- pears. While each part has glands with structure peculiar to them- selves, it may be noticed that all the essential epithelium has a tendency to assume a somewhat cubical form. The secreting glands of the stomach and intestines are tubu- lar ; while the salivary glands, the pancreas, and the liver are masses of cells so packed together as to form great colonies of cells with lesser subdivisions (lobules), the whole being bound together by some form of connective tissue, and well supplied with blood-vessels and nerves, thus constituting organs with a 332 COMPARATIVE PHYSIOLOGY. covering (capsule) in structure allied to the serous covering of the stomach and intestines. Details will be referred to in various parts of the sections devoted to this subject as far as may be necessary to render function clear, but we think these few generalizations may tend to widen the student's field of view, and at the same time lessen his labor and render it more effective. THE MOVEMENTS OF THE DIGESTIVE ORGANS. As with other parts of the body, so in the alimentary tract, the slower kind of movement is carried out by plain muscular fibers ; and the movements, as a whole, belong to the class known as peristaltic ; in fact, it is only at the beginning of the digestive tract that voluntary (striped) muscle is to be found and to a limited extent in the part next to this— i. e., in the oesophagus. Teeth in the highly organized mammal are remarkable in being to the least degree living structures of any in the entire animal, thus being in marked contrast to other organs. The enamel covering their exposed surfaces is the hardest of all the tissues, and is necessarily of low vitality. We have already alluded to the difference in the teeth of different animals, and their relation to customary food and digestive functions. In fact, it is clear that the teeth and all the parts of the digestive system are correlated to one another. The compound stomach of the ruminants, Avith its slow digestion of a bulky mass of food which must be softened and thoroughly masticated be- fore the' digestive juices can attack it successfully, harmonizes with the powerful jaws, strong muscles of mastication, and grinding teeth ; and all these in marked contrast with the teeth of a carnivorous animal with its simple but highly effective stomach. Compare figures in earlier pages. Mastication in man is of that intermediate character befit- ting an omnivorous animal. The jaws have a lateral and for- ward-and-backward movement, as well as a vertical one, though the latter is predominant. The upper jaw is like a fixed mill- stone, against which the lower jaw works as a nether millstone. The elevation of the jaw is effected by the masseter, temporal, and internal pterygoid muscles ; depressed by the mylohyoid and geniohyoid, though principally by the digastric. The jaw is advanced by the external pterygoids; unilateral contraction DIGESTION OF FOOD. 333 of these muscles also produces lateral movement of the inferior maxilla, which is retracted by the more horizontal fibers of the temporal. The movements of mastication are, of course, very pronounced in ruminants. The cheeks and tongue likewise take part in preparing the food for the work of the stomach, nor must the lips be over- looked even in man. The importance of these parts is well illustrated, by the imperfect mastication, etc., when there is paralysis of the muscles of which they are formed. Even when there is loss of sensation only, the work of the mouth is done in a clumsy way, showing the importance of common sensation, as well as the muscular sense. Nervous Supply. — The muscles of the tongue are governed by the hypoglossal nerve ; the other muscles of mastication chiefly by the fifth. The afferent nerves are branches of the fifth and glossopharyngeal. It is, of course, important that the food should be rolled about and thoroughly mixed with saliva (in- salivation). Deglutition. — The transportation of the food from the mouth to the stomach involves a series of co-ordinated muscular acts of a 'complicated character, by which difficulties are overcome with marvelous success. It will be remembered that the respiratory and digestive tracts are both developed from a common simple tube — a fact which makes the close anatomical relation between these two physiologically distinct systems intelligible ; but it also involves difficulties and dangers. It is well known that a small quantity of food or drink entering the windpipe produces a perfect storm of excitement in the respiratory system. The food, there- fore, when it reaches the oesophagus, must be kept, on the one hand, from entering the nasal, and on the other, the laryngeal openings. This is accomplished as follows: When the food has been gathered into a bolus on tbe back of the tongue, the tip of this organ is pressed against the hard palate, by which the mass is prevented from passing forward, and, at the same time, forced back into the pharynx, the soft palate being raised and the edges of the pillars of the fauces made to approach the uvula, which fills up the gap remaining, so that the posterior nares are closed and an inclined plane provided, over which the morsel glides. The after-result is said to depend on the size of the bolus. When considerable, the constrictors of the pharynx seize it and press it on into the gullet ; when the mor- 334 COMPARATIVE PHYSIOLOGY. sel is small or liquid is swallowed, it is rapidly propelled on- ward by the tongue, the oesophagus and pharynx being largely passive at the time, though contracting slowly afterward; at -■'■•'/ ■,■ ■'■■: - ■*/;* ;A* -a \\ '7£&: . /O Fig. 274.— Cavities of mouth and pharynx, etc.. in man (after Sappey). Section, in median line, of face and superior portion of neck, designed to show the mouth in its relations to the nasal fossae, pharynx, and larynx: 1, sphenoidal sinuses; 2, in- ternal orifice of Eustachian tube; 3, palatine arch; 4, velum pendulum palati; 5. anterior pillar of soft palate; 6, posterior pillar of soft palate; 7, tonsil; 8, lingual portion of cavity of pharynx; 9, epiglottis; 10, section of hyoid bone; 11, laryn- geal portion of cavity of pharynx; 12, cavity of larynx. the same time the larynx as a whole is raised, the epiglottis pressed down, chiefly by the meeting of the tongue and itself, while its cushion lies over the rima glottidis, which is closed or all but closed by the action of the sphincter muscles of the larynx, so that the food passes over and by this avenue of life, not only closed but covered by the glottic lid. The latter is not so essential as might be supposed, for persons in whom it DIGESTION OF FOOD. 335 was absent have been known to swallow fairly well. The ascent of the larynx any one may feel for himself ; and the be- havior of the pharynx and larynx, especially the latter, may be viewed by the laryngoscope. The grip of the pharyngeal muscles and the oesophagus may be made clear by attaching a piece of food (meat) to a string and allowing it to be partially swallowed. The upward movement of food under the action of the constrictors of the pharynx is anticipated by the closure of the passage by the palato-glossi of the anterior pillors of the fauces. The circular muscular fibers of the gullet are probably the most important in squeezing on the food by a peristaltic move- ment, passing progressively over the whole tube, though the longitudinal also take part in swallowing, perhaps, by steady- ing the organ. Deglutition can take place in an animal so long as the medulla oblongata remains intact ; and the center seems to lie higher than that for respiration, as the latter act is possible when, from slicing away the medulla, the former is not. An- encephalous monsters lacking the cerebrum can swallow, suck, and breathe. Food placed in the pharynx of animals when unconscious is swallowed, proving that volition is not essential to the act ; but our own consciousness declares that the first stage, or the removal of the food from the mouth to the pharynx, is volun- tary. When we seem to swallow voluntarily there is in reality a stimulus applied to the fauces, in the absence of food and drink, either by the back of the tongue or by a little saliva. It thus appears that deglutition is an act in the main reflex, though initiated by volition. The afferent nerves concerned are usually the glossopharyngeal, some branches of the fifth, and of the vagus. The efferent nerves are those of the numer- ous muscles concerned. When food has reached the gullet it is, of course, no longer under the control of the will. Section of the vagus or stimulation of this nerve modifies the action of the oesophagus, though it is known that contrac- tions may be excited in the excised organ ; but no doubt nor- mally the movements of the gullet arise in response to natural nerve stimulation. 336 comparative; physiology. Comparative. — That swallowing is independent of gravity is evident from the fact that long-necked animals (horse, giraffe) can and do usually swallow with the head and neck down, so that the fluid is rolled up an inclined plane. The peristaltic nature of the contractions of the gullet can also be well seen in such animals. In the frog the gullet, as well as the mouth, is lined with ciliated epithelium, so that in a recently killed ani- mal one may watch a slice of moistened cork disappear from the mouth, to be found shortly afterward in the stomach. The rate of the descent is surprising — in fact, the movement is plainly visible to the unaided eye. The Movements of the Stomach. — The stomach of mammals, including man, is provided with three layers of muscular fibers ; 1. External longitudinal, a continuation of those of the oesopha- gus. 2. Middle circular. 3. Internal oblique. The latter are the least perfect, viewed as an investing coat. The pyloric end of the stomach is best supplied with muscles ; where also there is a thick muscular ring or sphincter, as compared with which the cardiac sphincter is weak and ill-developed. Fig. 275. Pig. 276. Pig 275 —Muscular fibers of the stomach of horse; external and middle layers (Chau- veau). A, fillers of external layer enveloping left sac; B, fibers of middle plane in right sac: C, fibers of oesophagus. F„ ; 270 —Deep and muscular layers exposed by removing mucous membrane from an everted stomach (Chau'veau). A, deep layer of libers enveloping left sac; B, fibers of middle plane which alone form the muscular layer of right sac; C, fibers of oesophagus. The movements of the stomach begin shortly after a meal has been taken, and, as shown by observations on St. Martin, continue for hours, not constantly, but periodically. The effect DIGESTION OF FOOD. 337 of the conjoint action of the different sets of muscular fibers is to move the food from the cardiac toward the pyloric end of the stomach, along 1 the greater curvature and back by the lesser curvature, while there is also, probably, a series of in-and-out currents to and from the center of the food-mass. The quantity of food is constantly being lessened by the removal of digested portions, either by the blood-vessels of the organ or by its passing through the pyloric sphincter. The empty stomach is quiescent and contracted, its mucous membrane being thrown into folds. The movements of the stomach may be regarded as reflex, the presence of food being an exciting cause, though probably not the only one ; and so largely automatic is the central mech- anisms concerned that but a feeble stimulus suffices to arouse them, especially at the accustomed time. Of the paths of the impulses, either afferent or efferent, little is known. Certain effects follow section or stimulation of the vagi or splanchnics, but these can not be predicted with certainty, or the exact relation of events indicated. It is said that the movements of the stomach cease, even when it is full, during sleep, from which it is argued that gas- tric movements do normally depend on the influence of the nervous system. However, the subject is too obscure at present for further discussion. Comparative. — Recent investigations on the stomach of the pig indicate that in this animal the contents of the two ends of the stomach may long remain but little mingled ; and such is certainly the case in this organ among ruminants. Pathological.— Distention of the stomach, either from excess of food or gas arising from fermentative changes, or by secre- tion from the blood, may cause, by upward pressure on the diaphragm, etc., uneasiness from hampered respiration and irregularity of the heart, possibly, also, in part traceable to the physical interference with its movements. After gx*eat and prolonged distention there may be weakened digestion for a considerable interval. It seems not improbable that this is to be explained, not alone by the impaired elasticity (vitality) of the muscular tissue, but also by defective secreting power. It is not necessary to impress the lesson such facts convey. The Intestinal Movements.— The circular fibers play a much more important part than the longitudinal, being, in fact, much more developed. It is also to be remembered that nerves in 338 COMPARATIVE PHYSIOLOGY. the form of plexuses (of Auerbach and Meissner) abound in its walls. Normally the movement, slowly progressive, with occasional haltings, is from above downward, stopping at the ileo-caecal valve ; the movements of the large gut being apparently mostly independent. Movements may be excited by external or internal stimula- tion, and may be regarded as reflex; in which, however, the tendency for the central cells to discharge themselves is so great (automatic) that only a feeble stimulus is required, the normal one being the presence of food. It is noticeable in a recently killed animal, or in one in the last stages of asphyxia, that the intestines contract vigorously. Whether this is due to the action of blood overcharged with carbonic anhydride and deficient in oxygen on the centers pre- siding over the movements, on the nerves in the intestinal walls, or on the muscle-cells directly, is not wholly clear, but it is probable that all of these may enter into the result. The vagus nerve, when stimulated, gives rise to movements of the intestines, while the splanchnic seems to have the reverse effect ; but the cerebrum itself has an influence over the movements of the gut, as is plain from the diarrhoea traceable to unusual fear or anxiety. There is little to add in regard to the movements of the large intestine. They are, no doubt, of considerable im- portance in animals in which it is extensive. Normally they begin at the ileo-caecal valve. Defecation. — The removal of the waste matter from the ali- mentary tract is a complicated process, in which both smooth and striped muscle, the spinal cord, and the brain take part. Defecation may take place during the unconsciousness of sleep or of disease, and so be wholly independent of the will ; but, as we all know, this is not usually the case. Against ac- cidental discharge of faeces there is a provision in the sphinc- ter ani, the tone of which is lost when the lower part of the spinal cord is destroyed. We are conscious of being able, by an effort of will, to prevent tbe relaxation of the sphincter or to increase its holding power, though the latter is probably almost wholly due to the action of extrinsic muscles ; at all events any one may convince himself that the latter may be made to take a great part in preventing faecal discharge, though whether the tone of the sphincter can be increased or not by volition it is difficult to say. DIGESTION OF FOOD. 339 What happens during an ordinary act of defecation is about as follows : After a long inspiration the glottis is closed ; the diaphragm, which has descended, remains low, affording, with the obstructed laryngeal outlet, a firm basis of support for the action of the abdominal muscles, which, bearing on the intes- tine, forces on their contents, which, before the act has been called for, have been lodged mostly in the large intestine ; at the same time the sphincter ani is relaxed and peristaltic move- ments accompany and in some instances precede the action of the abdominal muscles. The latter may contract vigorously on a full gut without success in the absence of the intestinal peri- stalsis, as too many cases of obstinate constipation bear witness. Like deglutition, and unlike vomiting, there is usually both a voluntary and involuntary part to the act. Though the will, through the cerebrum, can inhibit defeca- tion, it is likely that it does so through the influence of the cerebrum on some center in the cord ; for in a dog, the lumbar cord of which has been divided from the dorsal, the act is, like micturition, erection of the penis, and others which are under the control of the will, still possible, though, of course, per- formed entirely unconsciously. Vomiting. — If we consult our own consciousness and observe to the best of our ability, supplementing information thus gained by observations on others and on the lower animals, it will become apparent that vomiting implies a series of co-ordi- nated movements into which volition does not enter either necessarily or habitually. There is usually a preceding nausea, with a temporary flow of saliva to excess. The act is initiated by a deep inspiration, followed by closure of the glottis. Whether the glottis is closed during or prior to the entrance of air is a matter of disagreement. At all events, the dia- phragm descends and remains fixed, the lower ribs being re- tracted. The abdominal muscles then acting against this sup- port, force out the contents of the stomach, in which they are assisted by the essential relaxation of the cardiac sphincter, the shortening of the oesophagus by its longitudinal fibers, and the extension and straightening of the neck, together with the open- ing of the mouth. As the expulsive effort takes place, it is accompanied by an expiratory act which tends to keep the egesta out of the larynx and carry them onward, though it may also contribute to over- come the resistance of the elevated soft palate, which serves to 340 COMPARATIVE PHYSIOLOGY. protect the nasal passages. The stomach and oesophagus are not wholly passive, though the part they take actively in vom- iting is variable in different animals. Retching may be very violent and yet ineffectual when the cardiac sphincter is not fully relaxed. The pyloric outlet is usually closed, though in severe and long-continued vomiting bile is often ejected, which must have reached the stomach through the pylorus. Comparative. — The ease with which some animals vomit in comparison with others is extraordinary, as in carnivora like our dogs and cats ; a matter of importance to an animal accus- tomed in the wild state to eat entire carcasses of animals — hair, bones, etc., included. The readiness with which an animal vomits depends in great part on the conformation and relations of the parts of its digest- ive tract. The stomach of the human being during infantile life is less pouched than in the adult, which in part explains the ease with which very young children vomit. It is well known that the horse vomits rarely and with great difficulty. This has been attributed by different writers to va- rious conditions of a structural kind, such as the length of the gullet ; the manner in which it enters the stomach (centrally) ; the pressure of a tightly closing sphincter at this point ; the valve-like foldings of the mucous membrane at the cardiac opening ; the small size of the stomach and its sheltered posi- tion, so that the abdominal muscles can not readily act on it ; the existence of a considerable length of the oesophagus be- tween the stomach and diaphragm which is against dilatation of the orifice by the longitudinal fibers of the gullet ; the open pylorus, permitting of the gastric contents being driven into the intestines rather than upward. But in the ox these peculiarities do not exist; in fact, from a mechanical point of view, the stracture and illation of parts is favorable, yet this animal seldom vomits, and never with ease. Why does the horse vomit after rupture of the stomach when conditions are less favorable from a mechanical point of view ? There is the greatest difference as to the readiness with which different human beings vomit ; moreover, persons that vomit usually with difficulty may do so very perfectly when suffi- ciently prepared, as by sea-sickness. These and many other considerations have led us to conclude DIGESTION OF FOOD. 341 that, while there is a certain amount of force in the various views stated briefly above, they do not go to the root of the matter. Vomiting is a very complex act, implying numerous muscu- lar and nervous co-oi'dinations. In the natural wild state the horse can have but rare necessity to vomit (unlike the carnivora), hence these co-ordinations have not been organized by habit and use ; they are foreign to the nature of the animal. After rupture of the stomach in the horse, and in sea-sickness in man, the nervous system is profoundly affected and the unusual hap- pens ; in other words, the necessary muscular and nervous co-ordinations take place. At all events, we are satisfied that it lies with the nervous system chiefly. Similarly, the habit of regurgitating the food is intelligible in the light of evolution. The fact that mammals are descended from lower forms in which unstriped muscle-cells go to form organs that have a rhythmically contractile function, renders it clear why this function may become, as in ruminants, spe- cialized in certain parts of the digestive tract ; why carnivora should vomit readily, and why human subjects should learn to regurgitate food. There is, so to speak, a latent inherited ca- pacity which may be developed into actual function. Apart from this it is difficult to understand such cases at all. The vomiting center is usually located in the medulla, and is represented as working in concert with the respiratory center. But when we consider that there is usually an increased flow of saliva and other phenomena involving additional central nervous influence, we see reason to believe in co-ordinated action implying the use of parts of the central nervous system not so closely connected anatomically as the respiratory and vomiting centers are assumed to be. THE REMOVAL OF DIGESTED PRODUCTS FROM THE ALIMENTARY CANAL. The glands of the stomach are simply seci-etive, and all ab- sorption from this organ is either by blood-vessels directly or by lymphatics ; at least, such is the ordinary view of the subject — whether it is not too narrow a one remains to be seen. It is important to remember that the intestinal mucous membrane is supplied not only with secreting glands but lym- phatic tissue, m the form of the solitary and agmiiiated glands 3±2 COMPARATIVE PHYSIOLOGY. (Peyer's patches) and thickly studded with villi, giving the small gut that velvety appearance appreciable even by the naked eye. It will not be forgotten that the capillaries of the digestive organs terminate in the veins of the portal system, and that the blood from these parts is conducted through the liver before it reaches the general circulation. Main venous trunk Right auricle Vena cava Hepatic vein Lymph, gland Portal system W @0 Blood vessel, tissue cells. ilimentary tract Fig. 277.— Diagram intended to illustrate the general relations of blood and lymph to metabolism (nutrition), and the method by which the portal, lymphatic, and gen- eral venous systems are related to the alimentary tract. The lymphatics of these organs form a part of the general lymphatic system of the body ; but the peculiar way in which absorption is effected by villi, and the fact that the lymphatics of the intestine, etc., at one time (fasting) contain ordinary lymph and at another (after meals) the products of digestion, imparts to them a physiological character of their own. Absorption will be the better understood if we treat now of lymph and chyle and the lymph vascular system, which were purposely postponed till the present; though its connection with the vascular system is as close and important as with the digestive organs. The lymphatic system, as a whole, more closely resembles the venous than the arterial vessels. We may speak of lym- phatic capillaries, which are, in essential points of structure, like the arterial capillaries; while the larger vessels may be compared to veins, though thinner, being provided with valves and having very numerous anastomoses. These lymphatic DIGESTION OF FOOD, 343 capillaries begin in spaces between tbe tissue- cells, from wbicb they take up the effete lymph. It is interesting to note that there are also perivascular lymphatics, the exist- ence of which again shows how close is the relation between the blood vascular and lym- phatic systems, and as we would suppose, and as is actually found to be the case, between the contents of each. Lymph and Chyle.— If one compares the mesentery in a kitten when fasting with the same part in an animal that was killed some hours after a full meal of milk, it may be seen that the formerly clear lines indicating the course of the lymphatics and ending in glands have in the latter case become whitish (hence their name, lacteals), owing to the absorp- tion of the emulsified fat of the milk. Microscopic examination shows the chyle to contain (when coagulated) fibrin, many Fig. 278.— Valves of lymphatics (Sappey). Fig. 279.— Origin of lymphatics (after Landois). I. From central tendon of diaphragm of rabbit (semi-diagrammatic) ; 6'. lymph-canals communicating by X with lym- phatic vessel L\ A, origin of lymphatic by union of lymph-canals; E, E. endothe- lium. II. Perivascular canal. 344 COMPARATIVE PHYSIOLOGY. leucocytes, a few developing red corpuscles, an abundance of fat in the form both of very minute oil-globules and particles smaller still. Fig. 280. — Epithelium from duodenum of rabbit, two hours after having been fed with melted butter (Funke). Fig. 281.— Villi filled with fat, from small intestine of an executed crim inal, one hour after death (Funke). There are also present fatty acids, soaps small in quantity as compared with the neutral fats, also a little cholesterin and lecithin. But chyle varies very widely even in the same animal at different times. To the above must be added proteids (fibrin, serum-albumin, and globulin) ; extractives (sugar, urea, leucin) ; and salts in which sodium chloride is abundant. The composition of lymph is so similar to that of chyle, and both to blood, that lymph might, 'though only roughly, be regarded as blood without its red cor- puscles, and chyle as lymph with much neutral fat in a very fine state of division. The Movements of the Lymph — comparative. — In some fishes, some birds, and :. 882.— Chyle taken from the lacteals and amphibians, there are lymph thoracic duct, of a criminal executed dur- ing digestion (Funke). shown leucocytes hearts, and excessively fine granules of fatty In the frog there are two Fir DIGESTION OP FOOD. 345 axillary and two sacral lymph hearts. The latter are, espe- cially, easily seen, and. there is no doubt that they are under the control of the nervous system. In the mammals no such special helps for the propulsion of lymph exist. There is little doubt that the blood-pressure is always higher than the lymph-pressure, and when the blood-vessels are dilated the fluid within the perivascular lymph-channels is likely com- pressed; muscular exercise must act on the lymph-channels as on veins, both being provided with valves, though themselves readily compressible; the inspiratory efforts, especially when forcible, assist in two ways: by the compressing effect of the respiratory muscles, and by the aspirating effect of the negative pressure within the thorax, producing a similar aspirating effect within the great veins, into which the large lymphatic trunks empty. The latter are provided at this point with valves, so that there is no back-flow; and, with the positive pressure within the large lymphatic trunks (thoracic duct, etc.), the physical conditions are favorable to the outflow of lymph or chyle. Our knowledge of the nature of the passage of the chyle from the intestines into the blood is now clearer than it was till recently, though still incomplete. The exact structure of a villus is to be carefully considered. If we assume that the muscular cells in its structure have a rhythmically contractile function, the blind terminal portion of the lacteal inclosed within the villus must, after being emptied, act as a suction -pump to some extent; at all events, the conditions as to pressure would be favorable to inflow of any material, especially fluid without the lacteal. The great difficulty hitherto was to understand how the fat found its way through the villus into the blood, for, that most of it passes in this direction there is little doubt. It is now known that leucocytes (amceboids, phagocytes) migrate from within the villus outward, and may even reach its surface, that they take up (eat) fat-particles from the epi- thelium of the villus, and, independently themselves, carry them inward, reach the central lacteal and break up, thus re- leasing the fat. How the fat gets into the covering epithelium is not yet so fully known — possibly by a similar inceptive pro- cess; nor is it ascertained what constructive or other chemical processes it may perform; though it is not at all likely that Pig. 288 DIGESTION OF FOOD. 347 Fig. 283.— Lymphatic system of horse (Chauveau). A, facial and nasal plexus whose branches pass to subglossal glands; B, C, parotid lymphatic gland, sending ves- sels to pharyngeal gland; D. E, large trunks passing toward thorax; P. G, H, glands receiving superficial lymphatics of neck, a portion of those of limbs, and those of pectoral parietes; I, junction of jugulars; J, axillary veins; K, summit of anterior vena cava; L, thoracic duct; M, lymphatics of spleen; in', of stomach; O, of large colon; S, of small colon; R, lacteals of small intestine, all going to form two trunks, P. Q, which open directly into receptaculum chyli; T, trunk which receives branches of sub lumbar glands. U, to which vessels of internal iliac glands, V, the receptacles of lymphatics of abdominal parietes, pass; W, precrural glands receiving lymphatics of posterior limb, and which arrive independently in the abdomen; S, superficial inguinal glands into which lymphatics of the mam- mce, external generative organs, some superficial trunks of posterior limb, etc., pass; Z, deep inguinal glands receiving the superficial lymphatics, Z, of posterior limbs. the work of the amoeboid cells is confined to the transport of fat alone, but that other matters are also thus removed inward to the lacteal. When a multitude of facts are taken into account, thei^e Fig. 284.— Perpendicular section through one of Peyer's patches in the lower part of the ileum of the sheep (Chauveau). a, a, lacteal vessels in villi; b, b, superficial layer of lacteal vessels; c, c, deep layer of lacteals; d. cl, efferent vessels provided with valves; /, Peyer's glands; g, circular muscular layer of wall of intestine; /;, longitudinal layer. seems little reason to doubt that so important a process as ab- sorption can not fail to be regulated by the nervous centers. 348 COMPARATIVE PHYSIOLOGY. There are two points that are very far from being deter- mined : the one the fate of the products of digestion ; the other the exact limit to which digestion is carried. How much — e. g., of proteid matter — does actually undergo conversion into peptone; how much is converted into leucin and tyrosin; or, again, what proportion of the albuminous matters are dealt with as such by the in- testine without conversion into peptone at all, either as soluble proteid or in the form of solid particles ? 1. It is generally believed that solu- ble sugars are absorbed, usually after conversion into maltose or glucose, by the capillaries of the stomach and intes- tine. 2. There is some positive evidence of the presence of fats, soaps, and sugars in unusual amount after a meal in the por- tal vein, which implies removal from the intestinal contents by the capillaries, though, so far as experiment goes, the fat is chiefly in the form of soaps, Certain experiments have been made by ligating the pyloric end of the stom- ach, by introducing a cannula into the thoracic duct, so as to continually remove its contents, etc. But we are surprised that serious conclusions should have been drawn under such circumstances, seeing that the natural condi- tions are so altered. What we wish to get at in physiology is the normal function of parts, and not the possible results after our interference. Under such circumstances the phenomena may have a suggestive but certainly can not have a conclusive value. It is a very striking fact that little peptone (none, according to some observers) can be detected even in the portal blood. True it is, the circulation is rapid and constant, and a small quantity might escape detection, yet a considerable amount be removed from the intestine in the space of a few hours by the capillaries alone. Peptone is not found in the contents of the thoracic duct. For a considerable period it has been customary to use the Fig . 285. — Intestinal villus (after Leydig). a, a, a, epithelial covering; b,b, capillary network; c, c, longitudinal muscular fibers; d, lacteal. DIGESTION OF FOOD. 349 terms osmosis and diffusion in connection with the functions of the alimentary canal, and especially the intestinal tract, as if this thin-walled but complicated organ, or rather collec- Fig. 286. — A. Villi of man. showing blood-vessels and lacteals; B. Villus of sheep (after Chauveau). tion of organs, were little more, so far as absorption is con- cerned, than a moist membrane, leaving the process of the re- moval of digested food products to be explained almost wholly on physical principles. From such views we dissent. We believe they are opposed to what we know of living tissue everywhere, and are not sup- ported by the special facts of digestion. When certain foreign bodies (as purgatives) are introduced into the blood or the ali- mentaiy canal, that diffusion takes place, according to physical laws, may indicate the manner in which the intestine can act; but even admitting that under such circumstances physical principles actually do explain the whole, which we do not grant, it would by no means follow that such was the natural behav- ior of this organ in the discharge of its ordinary functions. 350 COMPARATIVE PHYSIOLOGY. When we consider that the blood tends to maintain an equi- librium, it must be evident that the removal of substances from the alimentary canal, unless there is to be excessive activity of str Fig. 287.— A. Section of villus of rat killed during fat absorption (Schafer). ep, epi- thelium; str, striated border; c, lymph-cells; c', lymph-cells in epithelium; /, cen- tral lacteal containing disintegrating corpuscles. B. Mucous membrane of frog's intestine during fat absorption (Schafer). ep, epithelium; str, striated border; C, lymph-corpuscles; I, lacteal. the excretory organs and waste of energy both by them and the digestive tract, must in some degree depend on the demand for the products of digestion by the tissues. That there is to some extent a corrective action of the excretory organs always going on is no doubt true, and that it may in cases of emergency be great is also true ; but that this is minimized in ways too complex for us to follow in every detail is equally true. Diges- tion waits on appetite, and the latter is an expression of the needs of the tissues. We believe it is literally true that in a healthy organism the rate and character of digestion and of the removal of prepared products are largely dependent on the condition of the tissues of the body. Why is digestion more perfect in overfed animals after a short fast ? The whole matter is very complex, but we think DIGESTION OF FOOD. 351 it is infinitely better to admit ignorance than attempt to ex- plain by principles that do violence to our fundamental con- ceptions of life processes. To introduce " ferments " to explain so many obscure points in physiology, as the conversion of peptone in the blood, for example, is taking refuge in a way that does no credit to science. Without denying that endosmosis, etc., may play a part in the vital processes we are considering, we believe a truer view of the whole matter will be ultimately reached. In the mean time we think it best to express our belief that we are ignorant of the real nature of absorption in great part ; but we think that, if the alimentary tract were regarded as doing for the digested food (chyle, etc.) some such work as certain other glands do for the blood, we would be on the way to a truer con- ception of the real nature of the processes. It would then be possible to understand that proteids, either in the form of soluble or insoluble substances, including pep- tone, might be taken in hand and converted by a true vital process into the constituents of the blood. If we were to regard the kidney as manufacturing useful instead of harmful products, the resemblance in behavior would in many points be parallel. We have seen that physical expla- nations of the functions of the kidney have failed, and that it must be regarded even in those parts that eliminate most water as a genuine secreting mechanism. We wish to present a somewhat truer conception of the lymph that is separated from the capillaries and bathes the tissues. We would regard its separation as a true secretion, and not a mere diffusion dependent wholly on blood-pressure. The mere ligature of a vein does not suffice to cause an excess of diffusion, but the vaso-motor nerves have been shown to be concerned. The effusions that result from pathological pro- cesses do not correspond with the lymph — that is, the nutrient material — provided by the capillaries for the tissues. These vessels are more than mere carriers ; they are secretors — in a sense they are glands. We have seen that in the foetus they function both as respiratory and nutrient organs in the allan- tois and yelk-sac, and, in our opinion, they never wholly lose this function. The kind of lymph that bathes a tissue, we believe, depends on its nature and its condition at the time, so that, as we view 352 COMPARATIVE PHYSIOLOGY. tissue-lymph, it is not a mere effusion with which the tissues, for which it is provided, have nothing- to do. The differences may be beyond our chemistry to determine, but to assume that all lymph poured out is alike is too crude a conception to meet the facts of the case. Glands, too, it will be remembered, derive their materials, like all other tissues, not directly from the blood, but from the lymph. We believe that the cells of the capillaries, like all others, are influenced by the nervous system, notwithstanding that nerves have not been traced terminating in them. It is to be borne in mind that the lymph, like the blood, receives tissue waste-products — in fact, it is very important to realize that the lymph is, in the first instance, a sort of better blood — an improved, selected material, so far as any tissue is concerned, which becomes gradually deteriorated. We have not the space to give all the reasons on which the opinions expressed above are founded ; but, if the student has become imbued with the principles that pervade this work thus far, he will be prepared for the attitude we have taken, and sympathize with our departures from the mechanical (physical) physiology. We think it would be a great gain for physiology if the use of the term " absorption." as applied to the alimentary tract, were given up altogether, as it is sure to lead to the substitu- tion of the gross conceptions of physical processes instead of the subtle though at present rather indefinite ideas of vital processes. We prefer ignorance to narrow, artificial, and er- roneous views. Pathological.— Under certain circumstances, of which oue is obstruction to the venous circulation or the lymphatics, fluid may be poured out or effused into the neighboring tissues or the serous cavities. This is of very variable composition, but always contains enough salts and proteids to remind one of the blood. • Such fluids are often spoken of as "lymph," though the resemblance to normal tissue-lymph is but of the crudest kind; and the condition of the vessels when it is secreted, if such a term is here appropriate, is not to be compared to the natural separation of the normal lymph— in fact, were this not so, it would be identical with the latter, which it is not. When such effusions take place they are in themselves evidence of altered (and not merely increased) function. The Faeces.— The fseces may be regarded in at least a three- DIGESTION OF FOOD. 353 fold aspect. They contain undigested and indigestible rem- nants, the ferments and certain decomposition products of the digestive fluids, and true excretory matters. In carnivorous and omnivorous animals, including man, the undigested materials are those that have escaped the action of the secretions — such as starch and fats — together with those substances that the digestive juices are powerless to attack, as horny matter, hairs, elastic tissue, etc. In vegetable feeders a larger proportion of chlorophyl, cel- lulose, and starch will, of course, be found. These, naturally, are variable with the individual, the spe- cies, and the vigor of the digestive organs at the time. Besides the above, certain products are to be detected in the faeces plainly traceable to the digestive fluids, and showing that they have tmdergone chemical decomposition in the ali- mentary tract, such as cholalic acid, altered coloring-matters like urobilin, derivable probably from bilirubin; also ckoles- terin, fatty acids, insoluble soaps (calcium, magnesium), to- gether with ferments, having the properties of pepsin and amylopsin. Mucus is also abundant in the faeces. We know little of the excretory products proper, as they probably normally exist in small quantity, and it is not impos- sible that some of the products of the decomposition of the digestive juices may be reabsorbed and worked over or excreted by the kidneys, etc. There is, however, a recognized non-nitrogenous crystalline body known as excretin, which contains sulphur, salts, and pigments, and that may rank perhaps as a true excretion of the intestine. It is well known that bacteria abound in the alimentary tract, though their number is dependent on a variety of circum- stances, including the kind of food and the condition in which it is eaten. These minute organisms feed, of course, and to get their food produce chemical decompositions. Skatol and indol are possibly thus produced, and give the faecal odor to the con- tents of the intestine. But as yet our ignorance of these matters is greater than our knowledge — a remark which ap- plies to the excretory functions of the alimentary tract gen- erally. Pathological.— The facts revealed by clinical and pathologi- cal study leave no doubt in the mind that the intestine at all events may, when other glands, like the kidney, are at fault, 23 354 COMPARATIVE PHYSIOLOGY. undertake an unusual share of excretory work, probably even to the length of discharging urea. Obscure as the subject is, and long as it may be before we know exactly what and how matter is thus excreted, we think that it will greatly advance us toward a true conception of the vital processes of the mammalian body if we regard the ali- mentary tract as a collection of organs with both a secreting and excreting function ; that what we have been terming ab- sorption is in the main, at least, essentially secretion or an allied process ; and that the parts of this long train of organs are mutually dependent and work in concert, so that when one is lacking in vigor or resting to a greater or less degree, the others make up for its diminished activity ; and that the whole must work in harmony with the various excretory organs, as an excretor itself, and in unison with the general state of the economy. We are convinced that even as an excretory mech- anism one part may act (vicariously) for another. Of course, in disease the condition of the fasces is an indica- tion of the state of the digestive organs ; thus color, consistence, the presence of food in lumps, the odor, and many other points tell a plain story of work left undone, ill-done, or disordered by influences operating from within or from without the tract. The intelligent physician acts the part of a qualified inspector, surveying the output of a great factory, and drawing conclu- sions in regard to the kind of work which the operatives have performed. THE CHANGES PRODUCED IN THE FOOD IN THE ALIMENTARY CANAL. We have now considered the method of secretion, the secre- tions themselves, and the movements of the various parts of the digestive tract, so that a brief statement of the results of all this mechanism, as represented by changes in the food, will be appropriate. We shall assume for the present that the effects of the digestive juices are substantially the same in the body as in artificial digestion. Among mammals food is, in the mouth, comminuted (except in the case of the carnivora, that bolt it almost whole, and the ruminants, that simply swallow it to be regurgitated for fresh and complete mastication), insalivated, and, in most species, chemically changed, but only in so far as starch is concerned. DIGESTION OF POOD. 355 Deglutition is the result of the co-ordinated action of niany muscular mechanisms, and is reflex in nature. The oesophagus secretes mucus, which lubricates its walls, and aids mechan- ically in the transport of the food from the mouth to the stom- ach. In the stomach, by the action of the gastric juice, food is further broken up, the proteid covering of fat-cells is digested, and the structure of muscle, etc., disappears. Proteid matters become peptone, and in some animals fat is split up into free fatty acid and glycerin ; but the digestion of fat in the stom- ach is very limited at best and probably does not go on to emulsification or saponification. The digestion of starch con- Fig. 288. — Matters taken from pyloric portion of stomach of dog during digestion of mixed food (after Bernard), a. disintegrated muscular fibers, striae having disap- peared; b, c, muscular fibers in which stria; have partly disappeared; d, d, d, glob- ules of fat; e, e, starch: g, molecular granules. tinues in the stomach until the reaction of the food-mass be- comes acid. This in the hog may not be far from one to two hours, and the amylolytic ferment acts with great rapidity even without the body. The food is moved about to a certain ex- tent, so as to expose every part freely to the mucous mem- brane and its secretions. It is likely that the sugar resulting from the digestion of starch, the peptones, and, to some ex- tent, the fat formed (if any), is received into the blood from the stomach. 356 ' COMPARATIVE PHYSIOLOGY. As the partially digested mass (chyme) is passed on into the intestine as a result of the action of the alkaline hile, the para- peptone, pepsin, and bile-salts are deposited. Certain of the constituents of digestion are thus delayed, a portion of the pep- sin is probably absorbed, either altered or unaltered, and pep- sin is thus got rid of, making the way clear, so to speak, for the action of trypsin. At all events, digestion in one part of the tract is antagonized by digestion in another, but we must also add supplemented. The fat, which had been but little altered, is emulsified by the joint action of the bile and pancreatic secretion ; a portion is saponified, which again helps in emulsification, while an addi- tional part, in form but little changed, is probably dealt with by the absorbents. Proteid digestion is continued, and, besides peptones, nitro- genous crystalline bodies are formed (leucin and tyrosinj, but under what conditions or to what extent is not known; though the quantity is likely very variable, both with the species of animal and the circumstances, such as quantity and quality of food ; and it is likely also dependent not a little on the rate of absorption. It seems altogether probable that in those that use an excess of nitrogenous food more of these bodies are formed, and thus give an additional work to the excreting organs, in- cluding the liver. But the absence of albumin from healthy faeces points to the complete digestion of proteids in the ali- mentary canal. Plainly the chief work of intestinal digestion is begun and carried on in the upper part of the tract, where the ducts of the main glands are to be found. The contents of the intestine swarm with bacteria, though these are probably kept under control, to some extent, by the bile, the functions of which as an antiseptic we have already considered. The removal of fats by the villi will be shortly considered. The other products of digestion probably find their way into the general circulation by the portal blood, passing through the liver, which organ modifies some of them in ways to be examined later. The valvulce conniventes greatly increase the surface of the intestine, and retard the movements of the partially digested mass, both of which are favorable. The peristaltic movements of the small gut serve the obvious purpose of moving on the digesting mass, thus making way for fresh additions of chyme DIGESTION OP FOOD. 357 from the stomach, and carrying on the more elaborated con- tents to points where they can receive fresh attention, both digestive and absorptive. Comparative. — -In man, the carnivora, and some other groups, it is likely that digestion in the large intestine is slight, the work being mostly completed — at all events, so far as the action of the secretions is concerned — before this division of the tract is reached, though doubtless absorption goes on there also. The muscular strength of this gut is important in the act of defe- cation. But the great size of the large intestine in ruminants — in the horse, etc. — together with the bulky character of the food of such animals, points to the existence of possibly extensive processes of which we are ignorant. It is generally believed that food remains but a short time in the stomach of the horse, and that the caecum is a sort of reservoir in which digestive processes are in progress, and also for water. Fermentations go on in the intestine, and probably among ruminants they are numerous and essential, though our actual knowledge of the subject is very limited. The gases found in the stomach are atmospheric air (swal- lowed) and carbon dioxide, derived from the blood. Those of the intestine are nitrogen, hydrogen, carbonic anhydride, sul- phuretted hydrogen, and marsh-gas, the quantity varying con- siderably with the diet. In herbivora the quantity of C0 2 and CH 4 is large. Although our knowledge of the actual processes by which food is digested in the domestic animals is meager, there are certain considerations to which it may be well to give promi- nence at this point. The whole subject becomes clearer and the way is paved for more exact and comprehensive knowledge if it be borne in mind that the entire alimentary tract has a common embryo- logical origin from the splanchnopleure (Fig. 225, etc.), consist- ing of outer mesoblast and lining hypoblast, the former giving rise to the muscular and other less essential structures, the lat- ter to the all-important glandular epithelium. But of all re- gions the alimentary tract has been modified in relation to the development and habits of the animal group. It can not be too well remembered that digestion is highly complex, with one organ and one process supplementary to another. If mastication is imperfect, as in the camivora, gastric diges- 358 COMPARATIVE PHYSIOLOGY. tion is unusually active, as is well seen in the dog ; if the stom- ach is capacious the intestine is shorter, also exemplified in this group. The stomach may be small and the small intes- tines not lengthy, but the large intestine of enormous size, as in the horse. When the quantity of starchy matters found in the food of the animal is large, provision is made for its digestion in sev- eral parts of the alimentary tract. This is seen in the horse and other herbivora. Mastication is fairly complete in these animals, yet a part of the small stomach of the horse is a sort of oesophageal dilatation (Fig. 266) in which amylolytic diges- tion goes on by the action of the swallowed saliva and possibly by a ferment provided in this region of the organ. The gastric juice of the horse has been proved capable of digesting starch, possibly because mixed with the swallowed saliva. The stomach of the pig is large, and both proteid and starchy digestion exceedingly active. In the intestines the pro- cesses are of brief duration, but very effective. Digestion in the upper part of the small intestines is, in some animals, as the horse, really a continuation of that in the stomach ; or, at all events, the contents of the duodenum and jejunum are usually acid in reaction, so that the digestion peculiar to one region of the tract does not always abruptly end when food has left that part. The readiness with which food passes from the stomach into the intestines is very vari- able in different animals, and even in the same animal under different circumstances. In the horse the pyloric orifice seems never to be very tightly closed, though in most of our domestic animals the reverse is the case ; and with them the quantity of undigested material, as fat, that passes into the small intestine depends on the rate of digestion and absorption in the latter. In the horse, if water, or even hay, be given after oats a por- tion of the latter is soon carried on into the intestines, so that the obvious rule for feeding such an animal is to give the water and hay before the oats, or, at least, the water and no hay im- mediately after the oats. Digestion in the large intestine is of great importance in the monogastric herbivora, as the horse. The caecum is of enor- mous size — about twice that of the stomach — and has communi- cation with the colon by a small opening, so that it furnishes a sort of supplementary reservoir for digestion as well as for water. As the results of experiments, it has been concluded DIGESTION OF FOOD. 359 that food is found in the stomach twelve hours after feeding ; in the caecum after twenty-four hours, with a residue in the jejunum ; after forty-eight hours, in the ventral colon, with re- mains in the caecum; after seventy-two hours, in the dorsal colon ; and after ninety hours in the dorsal colon and rectum. The caecum appears to digest large quantities of cellulose, which does not seem to be affected by either the saliva, gastric, or pancreatic juices. The pi'ocess is ill understood. In her- bivora the large intestine takes some very important part — in digestion and absorption — and we would again remind the stu- dent that the latter term has been used in a very vague if not unwarrantable sense. It is important for the practitioner to bear in mind that nutrient enemata can be utilized for the gen- eral good of the economy when passed into either the large or small intestine. During the suckling period digestion in all the various groups of animals is probably closely analogous. At this time, in ruminants, the first three divisions of the stomach are but slightly developed. Pathological. — In subjects of a highly neurotic temperament and unstable nervous system it sometimes happens that im- mense quantities of gas are belched from an empty stomach or distend the intestines. It is known that the oxygen swallowed is absorbed into the blood, and the carbonic anhydride found in the stomach de- rived from that fluid. It will thus be seen that the alimentary tract has not lost its respiratory functions even in man, and that these may in cer- tain instances be inordinately developed (reversion). SPECIAL CONSIDERATIONS. It is a matter well recognized by those of much experience in breeding and keeping animals with restricted freedom and under other conditions differing widely from the natural ones — i. e., those under which the animals exist in a wild state — that the nature of the food must vary from that which the untamed ancestors of our domestic animals used. Food may often with advantage be cooked for the tame and confined animal. The digestive and the assimilative powers have varied with other changes in the organism brought about by the new surround- ings. So much is this the case, that it is necessary to resort to 360 COMPARATIVE PHYSIOLOGY. common experience and to more exact experiments to ascertain the best methods of feeding animals for fattening, for work, or for breeding. Inferences drawn from the feeding habits of wild animals allied to the tame to be valuable must always, before being applied to the latter, be subjected to correction by the results of experience. It is now well established by experience that animals kept in confinement must have, in order to escape disease and attain the best results on the whole, a diet which not only imitates that of the corresponding wild forms generally, but even in details, with, it may be, altered proportions or added constitu- ents, in consequence of the difference in the environment. To illustrate: poultry can not be kept healthy confined in a shed without sand, gravel, old mortar, or some similar preparation ; and for the best results they must have green food also, as lettuce, cabbage, chopped green clover, grass, etc. They must not be provided with as much food as if they had the exercise afforded by running hither and thither over a large field. We have chosen this case because it is not commonly recognized that our domesticated birds have been so modified that special study must be made of the environment in all cases if they are not to degenerate. The facts in regard to horned cattle, horses, and dogs are perhaps better known. Cooking greatly alters the chemical composition, the me- chanical condition, and, in consequence, the flavor, the digesti- bility, and the nutritive value of foods. To illustrate: meat in its raw condition would present mechanical difficulties, the di- gestive fluids permeating it less completely; an obstacle, how- ever, of far greater magnitude in the case of most vegetable foods. By cooking certain chemical compounds are replaced by others, while some may be wholly removed. As a rule, boiling is not a good form of preparing meat, because it with- draws not only salts of importance, but proteids and the ex- tractives — nitrogenous and other. Beef-tea is valuable chiefly because of these extractives, though it also contains a little gelatin, albumin, and fats. Meat, according to the heat employed, may be so cooked as to retain the greater part of its juices within it or the reverse. With a high temperature (65° to 70° C.) the outside in roasting may bo so quickly hardened as to retain the juices. In feeding dogs it is both physiological and economical to give the animal the broth as well as the meat itself. DIGESTION OF FOOD. 361 It is remarkable in the highest degree that man's appetite, or the instinctive choice of food, has proved wiser than our science. It would he impossible even yet to match, by calcula- tions based on any data we can obtain, a diet for each man equal upon the whole to what his instincts prompt. With the lower mammals we can prescribe with greater success. At the same time chemical and physiological science can lay down general principles based on actual experience, which may serve to cor- rect some artificialities acquired by perseverance in habits that were not based on the true instincts of a sound body and a healthy mental and moral nature; for the influence of the latter can not be safely ignored even in such discussions as the present. These remarks, however, are meant to be suggestive rather than exhaustive. We may with advantage inquire into the nature of hunger and thirst. These, as we know, are safe guides usually in eat- ing and drinking. After a long walk on a warm day one feels thirsty, the mouth is usually dry; at all events, moistening the mouth, especially the back of it (pharynx), will of itself partially re- lieve thirst. But if we remain quiet for a little time the thirst grows less, even if no fluid be taken. The dryness has been relieved by the natural secretions. If, however, fluid be intro- duced into the blood either directly or through the alimentary canal, the thirst is also relieved speedily. The fact that we know when to stop drinking water shows of itself that thei'e must be local sensations that guide us, for it is not possible to believe that the whole of the fluid taken can at once have en- tered the blood, Hunger, like thirst, may be mitigated by injections into the intestines or the blood. It is, therefore, clear that, while in the case of hunger and thirst there is a local expression of a need, a peculiar sensation, more pronounced in certain parts (the fauces in the case of thirst, the stomach in that of hunger), yet these may be appeased from within through the medium of the blood, as well as from without by the contact of food or water, as the case may be. Up to the present we have assumed that the changes wrought in the food in the alimentary tract were identical with those produced by the digestive ferments as obtained by extracts of the organs naturally producing them. But for many reasons it seems probable that artificial digestion can not be regarded as 362 COMPARATIVE PHYSIOLOGY. parallel with the natural processes except in a very general way. When we take into account the absence of muscular movements, regulated according to no rigid principles, hut vary- ing with innumerable circumstances in all probability ; the ab- sence of the influence of the nervous system determining the variations in the quantity and composition of the outflow of the secretions; the changes in the rate of so-called absorption, which doubtless influences also the act of the secretion of the juices — by these and a host of other considerations we are led to hesitate before we commit ourselves too unreservedly to the belief that the processes of natural digestion can be exactly imitated in the laboratory. What is it which enables one animal to digest habitually what may be almost a poison to another ? How is it that each one can dispose readily of a food at one time that at another is quite indigestible ? To reply that in the one case, the digestive fluids are poured out and in the other not, is to go little below the surface, for one asks the reason of this, if it be a fact, as it no doubt is. When we look further into the peculiarities of digestion, etc., we recognize the influence of race as such, and in the race and the individual that obtrusive though ill-under- stood fact — the force of habit — operative here as elsewhere. And there can be little doubt that the habits of animals, as to food eaten and digestive peculiarities established, become or- ganized, fixed, and transmitted to posterity. It is probably in this way that, in the course of the evolu- tion of the various groups of animals, they have come to vary so much in their choice of diet and in their digestive processes, did we but know them thoroughly as they are; for to assume that even the digestion of mammals can be summed up in the simple way now prevalent seems to us too broad an assump- tion. The field is very wide, and as yet but little explored. The law of rhythm is illustrated, both in health and disease, in striking ways in the digestive tract. An animal long accus- tomed to eat at a certain hour of the day will experience at that time not only hunger, but other sensations, probably referable to secretion of a certain quantity of the digestive juices and to the movements that usually accompany the presence of food in the alimentary tract. Hence that '* colic " so common in horses fed at irregular times and unwisely, after excessive work, etc. It is well known that defecation at periods fixed, even within a few minutes, has become an established habit Avith hosts of DIGESTION OP FOOD. 363 people; and the same is to a degree true of dogs, etc., kept in confinement, that are taught cleanly habits, and encouraged therein by regular attention to their needs. This tendency (rhythm) is important in preserving energy for higher ends, for such is the result of the operation of this law everywhere. The law of correlation, or mutual dependence, is well illustrated in the series of organs composing the alimentary tract. The condition of the stomach has its counterpart in the rest of the tract ; thus, when St. Martin had a disordered stomach, the epithelium of his tongue showed corresponding changes. We have already referred to the fact that one part may do extra work to make up for the deficiencies in another. It is confidently asserted of late that, in the case of persons long vmable to take food by the mouth, nutritive substances given by enemata find their way up to the duodenum by anti- peristalsis. Here, then, is an example of an acquired adaptive arrangement under the stress of circumstances. It can not be too much impressed on the mind that in the complicated body of the mammal the work of any one organ is constantly varying with the changes elsewhere. It is this mutual dependence and adaptation — an old doctrine too much left out of sight in modern physiology — which makes the at- tempt to completely un ravel vital processes well-nigh hopeless; though each accumulating true observation gives a better in- sight into this kaleidoscopic mechanism. We have not attempted to make any statements as to the quantity of the various secretions discharged. This is large, doubtless, but much is probably reabsorbed, either altered or unaltered, and used over again. In the case of fistula?, the con- ditions are so unnatural that any conclusions as to the normal quantity from the data they afford must be highly unsatisfac- tory. Moreover, the quantity must be very variable, accord- ing to the law we are now considering. It is well known that dry food provokes a more abundant discharge of saliva, and this is doubtless but one example of many other relations be- tween the character of the food and the quantity of secretion provided. Evolution. — We have from time to time either distinctly pointed out or hinted at the evolutionary implications of the 364 COMPARATIVE PHYSIOLOGY. facts of this department of physiology. The structure of the digestive organs, plainly indicating a rising scale of complexity with greater and greater differentiation of function, is, beyond question, an evidence of evolution. The law of natural selection and the law of adaptation, giving rise to new forms, have both operated, we may believe, from what can be observed going on around us and in our- selves. The occurrence of transitional forms, as in the epi- thelium of the digestive tract of the frog, is also in harmony with the conception of a progressive evolution of structure and function. But the limits of space will not permit of the enu- meration of details. Summary. — A very brief resume of the subject of digestion will probably suffice. Food is either organic or inorganic and comprises proteids, fats, carbohydrates, salts, and water ; and each of these must enter into the diet of all known animals. They must also be in a form that is digestible. Digestion is the reduction of food to such a form that it may be further dealt with by the aliment- ary tract prior to being introduced into the blood (absorption). This is effected in different parts of the tract, the various con- stituents of food being differently modified, according to the secretions there provided, etc. The digestive juices contain essentially ferments which act only under definite conditions of chemical reaction, temperature, etc. The changes wrought in the food are the following : starches are converted into sugars, proteids into peptones, and fats into fatty acids, soaps, and emulsion ; which alterations are effected by ptyalin and amylopsin, pepsin and trypsin, and bile and pan- creatic steapsin, respectively. Outside the mucous membrane containing the glands are muscular coats, serving to bring about the movements of the food along the digestive tract and to expel the faeces, the circu- lar fibers being the more important. These movements and the processes of secretion and so-called absorption are under the control of the nervous system. The preparation of the digestive secretions involves a series of changes in the epithelial cells concerned, which can be dis- tinctly traced, and take place in response to nervous stimula- tion. These we regard as inseparably bound up with the healthy life of the cell. To be natural, it must secrete. DIGESTION OP FOOD. 365 The blood-vessels of the stomach and intestine and the villi of the latter receive the digested food for further elaboration (absorption). The undigested remnant of food and the excre- tions of the intestine make up the faeces, the latter being ex- pelled by a series of co-ordinated muscular movements essen- tially reflex in origin. THE RESPIRATORY SYSTEM. In the mammal the breathing 1 organs are lodged in a closed cavity, separated by a muscular partition from that in which the digestive and certain other organs are contained. This thoracic chamber may be said to be reserved for circulatory and respiratory organs which, we again point out, are so related that they really form parts of one system. The mammal's blood requires so much aeration (ventilation) that the lungs are very large and the respiratory system has become greatly specialized. We no longer find the skin or ali- mentary canal taking any large share in the process; and the lungs and the mechanisms by which they are made to move the gases with which the blood and tissues are concerned become very complicated. Our studies of muscle physiology should have made clear the fact that tissue-life implies the constant consumption of oxygen and discharge of carbonic anhydride, and that the pro- cesses which give rise to this are going on at a rapid rate ; so that the demands of the animal for oxygen constantly may be readily understood if one assumes, what can be shown, though less readily than in the case of muscle, that all the tissues are constantly craving, as it were, for this essential oxygen — well called '"vital air." Respiration may, then, be regarded from a physical and chemical point of view, though in this as in other instances we must be on our guard against regarding physiological processes as ever purely physical or purely chemical. The respiratory process in the mammal, unlike the frog, consists of an active and a (largely) passive phase. The air is not pumped into the lungs, but sucked in. So great is the complexity of the lungs in the mammal, that the frog's lung (which may be readily understood by blowing it up by inserting a small pipe in the glottic opening of the animal and then ligaturing the distended TPIE RESPIRATORY SYSTEM. 367 organ) may be compared to a single infundibulum of the mam- malian lung. Assuming that the student is somewhat conversant with the coarse and fine anatomy of the respiratory organs, we call at- Fi8. Fig. 299. Fig. 298. — Vertical transverse section of fresh-water mussel (Anodon) through heart (after Huxley). V. ventricle; a, auricles; r, rectum; p, pericardium; i. inner, o, outer gill; o', vestibule of organ of Bojanus, fi; f. foot; m.tn, mantle lobes. Fig. 299.— Gill of fish (perch), to illustrate relations of different blood-vessels, etc., concerned in respiration (after Bell). A, branchial artery; B, branchial arch seen in cross-section; V. branchial vein; a, V, branches of artery and vein re- spectively. widening during inspiration and narrowing during expiration, in accordance with the action of the muscles attached to the arytenoid cartilages, the action of which may be studied in man by means of the laryngscope. The abdominal muscles have a powerful rhythmical action during forced respiration, though whether they function dur- THE RESPIRATORY SYSTEM, 375 h\g ordinary quiet breathing is undetermined ; if at all, prob- ably but slightly. Though the removal of the external inter- costals in the dog and some other animals reveals the fact that the internal intercostals contract alternately with the dia- IV V VI VII VIII IX X XL XJJ f XII) XIV Fig. 300. — Diagram of scorpion, most of the appendages having been removed (after Huxley), a, mouth; b, alimentary tract; c, anus; d, heart; e. pulmonary sac: /, position of ventral ganglionated cord; q, cerebral ganglia; T, telson. VII — XX, seventh to twentieth somite. IV, V, VI, basal joints of pedipalpi and two fol- lowing pairs of limbs. phragm, it must not be regarded as absolutely certain that such is their action when their companion muscles are present, for Nature has more ways than one of accomplishing the same purpose — a fact that seems often to be forgotten in reasoning from experiments. This result, however, carries some weight with it. Types of Respiration. — There are among mammals two prin- cipal types of breathing recognizable — the costal (thoracic) and abdominal — according as the movements of the chest or the abdomen (diaphragm) are the more pronounced. Personal Observation.— The student would do well at this stage to test the statements we have made in regard to the respira- tory movements on the human subject especially. This he can very well do in his own person when stripped to the waist be- fore a mirror. Many of the abnormalities of the forced respira- ation of disease may be imitated — in fact, this is one of the departments of physiology in w T hich the human aspects may be 376 COMPARATIVE PHYSIOLOGY, examined into by a species of experiment on one's self that is as simple as it is valuable. Comparative. — It is hoped that the various figures accompa- nied by descriptions, introduced in this and other chapters, will #'&■ Fig. 303. Fig. 301. Fig. 301.— A. Pulmonary sac. B. Respiratory leaflets of Scorpio occitanus (after Blancharch. Fig. 302.— Left pulmonary sac. viewed from dorsal aspect, of a spider (after Duges). Pm. pulmonary lamella?; Stg. stigma, or opening to former. make the relations of the circulation and respiration in the va- rious classes of animals, whether terrestrial or aquatic, evident Fig. 303 -A. B. Tadpoles with external branchiae (after Huxley), n. nasai sacs; a, eye; o, ear: /.'. '>. branchiae; m, mouth; z. horny jaws; s, suckers; d, opercular (or gill) fold. C. More advanced frosts larva, y. rudiment of hind-limb; k. s, single branchial aperture. Owing to figure not having been reversed, this aper- ture seems to lie on right instead of left side. without extended treatment of the subject in the text. What we are desirous of impressing is that throughout the entire animal kingdom respiration is essentially the same process: that THE RESPIRATORY SYSTEM. 377 finally it resolves itself into tissue-breathing — the appropriation of oxygen and the excretion of carbon dioxide. Since the man- ner in which oxy- gen is intro into the lungs and foul gases expelled from them in some reptiles and amphib- ians is largely dif- ferent from the method of respira- tion in the mam- mal, we call atten- tion to this process in an animal readily watched — the com- mon frog. This creature, by depress- ing the floor of the mouth, enlarges his air-space in this re- gion and conse- quently the air free- ly enters through the nostrils ; where- upon the latter are closed by a sort of valve, the glottis opened and the air forced into the lungs by the elevation of the floor of the mouth. By a series of flank movements the elasticity of the lungs is aided in expelling the air through the now open nostrils. The inspiration of the turtle and some oth- er reptiles is somewhat similar Fir,. 304.— General view of air-reservoirs of duck, opened interiorly, also their relations with principal viscera of trunk (after Sappey). 1, 1, anterior extremity of cervical reservoirs ; 2, thoracic reservoir; 3, anterior diaphragmatic reservoir; 4, posterior ditto; 5, abdom- inal reservoir; a, membrane forming anterior dia- phragmatic reservoir; b, membrane forming posterior ditto; c, section of thoracoabdominal diaphragm: d, subpectoral prolongation of thoracic reservoir; ■. pericardium; f, liver; tj. gizzard: //. intestines; in, lieart; n, n, section of great pectoral muscle above its insertion into the humerus; o. anterior clavicle; p, pos- terior clavicle of right side cut and turned outward. In the case of aquatic animals, 378 COMPARATIVE PHYSIOLOGY. both invertebrate and vertebrate, excepting mammals, the blood is freely exposed in the gills to oxygen dissolved in the water as it is to the same gas mixed with nitrogen in terrestrial animals. In the land-snail, land-crab, etc., we have a sort of intermediate condition, the gills being kept moist. It is not to be forgotten, however, that normally the respiratory tract of mammals is never other than slightly moist. THE QUANTITY OF AIR RESPIRED. We distinguish between the quantity of air that usually is moved by the thorax and that which may be respired under special effort, which, of course, can never exceed the capacity of the respiratory organs. Accordingly, we recognize: 1. Tidal air, or that which passes in and out of the respiratory passages in ordinary quiet breathing, amounting to about 500 cc, or thirty cubic inches. 2. Complemental air, which may be voluntarily inhaled by a forced inspiration in addition to the tidal air, amounting to 1,500 cc, or about 100 cubic inches. 3. Supplemental {reserve) air, which may be expelled at the end of a normal respiration — i. e., after the expulsion of the tidal air, and which represents the quantity usually left in the lungs after a normal quiet ex- piration, amounting to 1,500 cc. 4. Residual air, which can not be voluntarily expelled at all, amounting to about 2,000 cc, or 120 cubic inches. Although these quantities have been esti- mated for man, probably a similar relation (proportion) between them holds for the domestic animals. The vital capacity is estimated by the quantity of air that may be expired after the most forcible inspiration. This will, of course, vary with the age, which determines largely the elas- ticity of the thorax, together with sex, position, height, and a variety of other circumstances. But, inasmuch as the result may be greatly modified by practice, like the power to expand the chest, the vital capacity is not so valuable an indication as might at first be supposed. It is important to bear in mind that the tidal air is scarcely more than sufficient to fill the upper air-passages and larger bronchi, so that it requires from five to ten respirations to re- move a quantity of air inspired by an ordinary act. Very much must, therefore, depend on diffusion, the quantity of air remain- ing in the lungs after each breath being the sum of the residual THE RESPIRATORY SYSTEM. 379 and reserve air, or about 3,500 cc. (220 cubic iucbes). Consider- ing the creeping slowness of the capillary circulation, it would not be supposed that the respiratory process in its essential parts should be the rapid one that a greater movement of the air would imply. THE RESPIRATORY RHYTHM. In man, and most of our domestic mammals, a definite though somewhat different relation between the cardiac and respiratory movements obtains, there being about three to five heart-beats to one respiration, which would make the rate of breathing in man about sixteen to eighteen per minute. Usual- ly, of course, the largest animals have the slower pulse and res- piration ; and this is an invariable rule for the varieties of a species, as observable in the canine race, to mention a well- known instance. The horse breathes 9 to 12 times in a minute; the ox 15 to 20 ; the sheep 13 to 17 ; and the dog 15 to 20. The rate of the respiratory movements is to some extent a measure of the rapidity of the oxidative processes in the body, as witness the slow and intermittent breathing of cold-blooded animals as compared with the more rapid respiration of birds and mammals (Fig. 305). Pathological. — Any condition that lessens the amount of re- spiratory surface, or diminishes the mobility of the chest-walls, is usually accompanied by accelerated movements, but beneath this is the demand for oxygen, part of the avenues by which this gas usually enters having been closed or obstructed by the disease. So that it is not surprising that, in consequence of the effusion of fluid into the thoracic cavity, leading to the compression of the lung, the opposite one should be called into more frequent use, and even enlarge to meet the demand. These facts show how urgent is the need for constant ventila- tion of the blood, and at the same time how great is the power of adaptation to meet the emergency. The difference between the inspiratory and the expiratory rhythm may be gathered by watching the movements of the bared chest, or more accurately from a graphic record. It is usually considered that expiration is only slightly longer than inspiration, and that any marked deviation from this relation should arouse suspicion of disease. Normally the respiratory pause is very slight, so that inspiration seems to follow directly 380 COMPARATIVE PHYSIOLOGY. on expiration ; though the latter act reminds us of the pro- longation of the ventricular systole after the blood is expelled. 10 Fig 305 —Tracings of respiratory movements of individuals belonging to different groups Of the animal kingdom (after Thanhoffer). Differences in depth, frequency, and especially regularity, are very noticeable. 1, fish; 2, tortoise; 3, adder (nv winter); 4, boa-constrictor (in summer); 5, frog; 0, alligator; 7, lizard; 8, canary- bird; 0. adult dog; 10, rabbit; 11, man; 12, dog; 13, horse. Compare these, and note that in nl respiration is shallow, and in ml deep. THE RESPIRATORY SYSTEM. 381 If, in the tracing, the small waves on the upper part of the expiratory curve really represent the effect of the heart-beat, it makes it easier to understand how such might assist in venti- lating the blood when the respirations occur only once in a considerable interval and very feebly then, as in hibernating animals or individuals that have fainted ; though it must be remembered that diffusion is a ceaseless process in all living vertebrates. FrG. 306. — Tracings of respiration of horse when at rest and after exercise (after Than- hoffer). /, inspiration; E, expiration. Spaces between vertical lines indicate time periods of one second each. 1, animal standing at rest; 3, after walk of few minutes; 7 and 8, after trotting; 9, after a brief rest; 11, after trotting and run- ning for some minutes; 17, after resting from last for a short time; 51, tracing at end of experiment. It is scarcely necessary to point out that the respiratory movements are increased by exercise, emotions, position, sea- son, hour of the day, taking meals, etc. Respiratory Sounds.— The entrance and exit of air are ac- companied by certain sounds, which vary with eacli part of the 382 COMPARATIVE PHYSIOLOGY. respiratory tract. To these sounds names have been given, but as they are somewhat inconstant in their application, or at least have several synonyms, we pass them by, recommending the student to actually learn the nature of the respiratory murmurs by listening to the normal chest in both man and the lower ani- mals. With the use of a double stethoscope he may practice upon himself, though not so advantageously as in the case of the heart. The sounds are caused in part by the friction of the air, though they are probably complex, several factors entering into their causation. COMPARISON OF THE INSPIRED AND EXPIRED AIR. The changes that take place in the air respired may be brief- ly stated as follows : 1. Whatever the condition of the inspired air, that expired is about saturated with aqueous vapor — i. e., it contains all that it is capable of holding at the existing temperature. 2. The temperature of the expired air is about that of the blood itself, so that if the air is very cold when breathed, the body loses a great deal of its heat in warming it. The expired air of the nasal passages is slightly warmer than that of the mouth. 3. Experiment shows that the expired air is really dimin- ished in volume to the extent of from one fortieth to one fif- tieth of the whole. Since two volumes of carbonic anhydride require for their composition two volumes of oxygen, if the amount of the former gas expired be not equal to the amount of oxygen inspired, some of the latter must have been used to CO form other combinations. -=-^, amounting to rather less than 1, is called the respiratory coefficient. 4. The difference between inspired and expired air in man may be gathered from the following : Inspired air. Expired air. Oxygen. Nitrogen. Carbonic dioxide. 20-810 79-150 0-040 10-083 79-587 4-380 From which the most important conclusions to be drawn are, that the expired air is poorer in oxygen to the extent of 4 to 5 per cent, and richer in carbonic anhydride to somewhat THE RESPIRATORY SYSTEM. 333 less than this amount. A similar relationship may be considered to hold for the domestic animals, the quantities varying, of course. From experiment it has been ascertained that the amount of carbonic dioxide is for the average man 800 grammes (406 litres, equivalent to 218*1 grammes carbon) daily, the oxygen actually used for the same period being 700 grammes. But the variations in such cases are very great, so that these numbers must not be interpreted too rigidly. Experience proves that, while chemists often work in laboratories in which the per- centage of carbonic anhydride (from chemical decompositions) reaches 5 per cent, an ordinary room in which the amount of this gas reaches 1 per cent is entirely unfit for occupation. This is not because of the amount of the carbon dioxide present, but of other impurities which seem to be excreted in proportion to the amount of this gas, so that the latter may be taken as a measure of these poisons. What these are is as yet almost entirely unknown, but that they are poisons is beyond doubt. Small effete particles of once living protoplasm are carried out with the breath, but these other substances are got rid of from the blood by a vital process of secretion (excretion), we must believe; which shows that the lungs to some degree play the part of glands, and that their whole action is not to be explained as if they were merely moistened bladders acting in accordance with ordinary physi- cal laws. An estimation of the amount of atmospheric air required may be calculated from data already given. Thus, assuming that a man gives up at each breath 4 per cent of carbon dioxide to the 500 cc. of tidal air he expires, and breathes, say, seventeen times a minute, we get for the amount of air thus charged in one hour to the extent of 1 per cent : 500 x 4 x 17 x 60 = 2,040,000 cc, or 2,040 litres. But if the air is to be contaminated to the extent of only T V per cent of carbonic anhydride, the amount should equal at least 2,040 x 10 hourly. A very much larger quantity would, of course, be required for a horse or an ox. RESPIRATION IN THE BLOOD. It may be noticed that arterial blood kept in a confined space grows gi'adually darker in color, and that the original bright- scarlet hue may be restored by shaking it up with air. When 384 COMPARATIVE PHYSIOLOGY. the blood has passed through the capillaries and reached the veins, the color has changed to a sort of purple, characteristic of venous blood. Putting these two facts together, we are led to suspect that the change has been caused in some way by oxygen. Exact experiments with an appropriate form of blood- pump show that from one hundred volumes of blood, whether arterial or venous, about sixty volumes of gas may be obtained : that this gas consists chiefly of oxygen and carbonic anhydride, but that the proportions of each present depends upon whether the blood is arterial or venous. The following table will make this clear : Arterial blood Venous blood. :ygen. Cai ■bonic anhydride. Nitrogen. 20 40 1-2 8-12 46 1-2 from 100 volumes of blood at 0° C. and 760 millimetre pressure. Arterial blood, then, contains 8 to 12 per cent more oxygen and about 6 per cent more carbonic dioxide than venous blood. It is not, of course, true, as is sometimes supposed, that arterial blood is " pure blood " in the sense that it contains no carbonic anhydride, as in reality it always carries a large percentage of this gas. The Conditions under which the Gases exist in the Blood — If a fluid, as water, be exposed to a mixture of gases which it can absorb under pressure, it is found that the amount taken up depends on the quantity of the particular gas present independ- ent of the presence or quantity of the others ; thus, if water be exposed to a mixture of oxygen and nitrogen, the quantity of oxygen absorbed will be the same as if no nitrogen were pres- ent — i. e. , the absorption of a gas varies with the partial press- ure of that gas in the atmosphere to which it is exposed. But whether blood, deprived of its gases, be thus exposed to oxygen under pressure, or whether the attempt be made to remove this gas from arterial blood, it is found that the above-stated law does not apply. When blood is placed under the exhaustion-pump, at first very little oxygen is given off; then, when the pressure is con- siderably reduced, the gas is suddenly liberated in large quan- tity, and after this comparatively little. A precisely analogous course of events takes place when blood deprived of its oxygen is submitted to this gas under pressure. On the other hand, if these experiments be made with serum, absorption follows THE RESPIRATORY SYSTEM. 385 according to the law of pressures. Evidently, then, if the oxy- gen is merely dissolved in the hlood, such solution is peculiar, and we shall presently see that this supposition is neither ne- cessary nor reasonable. HEMOGLOBIN AND ITS DERIVATIVES. Haemoglobin constitutes about -^\ of the corpuscles, and, though amorphous in the living blood-cells, may be obtained in crystals, the form of which varies with the animal ; indeed, in many animals this substance crystallizes spontaneously on the death of the red cells. It is unique among albuminous compounds in being the only one found in the animal body that is susceptible of crystallization. Its estimated composi- tion is: Carbon 53'85 Hydrogen 7"32 Nitrogen 16 '17 Oxygen 2184 Iron -43 Sulphur -39 together with 3 to 4 per cent of water of crystallization. The formula assigned is: CeooHgeoOivgN^FeSs. The molecu- lar constitution is not known, and the above formula is merely an approximation, which will, however, serve to convey an idea of the great complexity of this compound. The presence of iron seems to be of great importance. If not the essential re- spiratory constituent, certainly the administration of this metal in some form proves very valuable when the blood is deficient in haemoglobin. This substance can be recognized most certainly by the spec- troscope. The appearances vary with the strength of the solu- tion, and, as this test for blood (haemoglobin) is of much prac- tical importance, it will be necessary to dwell a little upon the subject; though, after a student has once recognized clearly the differences of the spectrum appearances, he has a sort of knowl- edge that no verbal description can convey. This is easily ac- quired. One only needs a small, flat-sided bottle and a pocket- spectroscope. Filling the bottle half full of water, and getting the spectroscope so focused that the Fraunhofer lines appear distinctly, blood, blood-stained serum, a solution of haemoglo- bin crystals, or the essential substance in any form of dilute 25 386 COMPARATIVE PHYSIOLOGY. solution, may be added drop by drop till changes in the spec- trum in the form of dark bands appear. By gradually increas- ing the quantity, ap- pearances like those fig- ured below may be ob- served, though, of course, much will depend on the thickness of the layer of fluid as to the quantity to be added before a par- ticular band comes into view. When wishing to be precise, we speak of the most highly oxidized form of haemoglobin as oxy-haemoglobin (O-H), and the reduced form as haemoglobin simply, or reduced haemoglobin (H) . By a comparison of the spectra it will be seen that the bands of oxy- haemoglobin lie between the D and E lines; that the left band near D is always the most definite in outline and the most pronounced in every re- spect except breadth ; that it is in weak solutions the first to ap- pear, and the last to disappear on reduction ; that there are two instances in which there may be a single band from haemoglo- bin — in the one case when the solution is very dilute and when it is very concentrated. These need never be mistaken for each other nor for the band of reduced haemoglobin. The latter is a hazy broad band with comparatively indistinct outlines, and darkest in the middle. It will be further noticed that in all these instances, apart from the bands, the spectrum is otherwise modified at each end, so that the darker the more centrally placed characteristic bands, the more is the light at the same time cut off at each end of the spectrum. Fig. 307.— Crystallized haemoglobin (Gautier). a, b, crystals from venous blood of man; c, from blood of cat; d, of Guinea-pig; e, of marmot; f, of squirrel. THE RESPIRATORY SYSTEM. 387 388 COMPARATIVE PHYSIOLOGY. If, now, to a specimen showing the two bands of oxy-haemo- globin distinctly a few drops of ammonium sulphide or other reducing agent be added, a change in the color of the solution will result, and the single hazy band characteristic of hemo- globin will appear. It is not to be supposed, however, that venous blood gives this spectrum. Even after asphyxia it will be difficult to see this band, for usually some of the oxy-ha^moglobin remains reduced ; but it is worthy of note, as showing that the appear- ances are normal, that the blood, viewed through thin tissues when actually circulating, whether arterial or venous, gives the spectrum of oxy-bsemoglobin. At the same time there can be no doubt that the changes in color which the blood under- goes in passing through the capillaries is due chiefly to loss of oxygen, as evidenced by the experiments before referred to ; and the reason that tbe two bands are always to be seen in venous blood is simply that enough oxy-haamoglobin remains to give the two-band spectrum which prevails over that of (reduced) haemoglobin. We are thus led by many paths to the important conclusion that the red corpuscles are oxygen-carriers, and, though this may not be and probably is not their only func- tion, it is without doubt their principal one. Of their oxygen they are being constantly relieved by the tissues; hence the necessity of a circulation of the blood from a respiratory point of view. There are other gases that can replace oxygen and form compounds with haemoglobin ; hence we have CO-hasmoglobin and NO-haemoglobin, which in turn are replaced by oxygen with no little difficulty — a fact which explains why carbonic oxide is so fatal when respired, and, as it is a constituent of illuminat- ing gas, the cause of the death of those inhaling the latter is often not far to seek. Blood may, in fact, be saturated with carbonic oxide by allowing illuminating gas to pass through it, when a change of color to a cherry red may be observed, and which will remain in spite of prolonged shaking up with air or attempts at reduction with the usual reagents. Haemoglobin may be resolved into a proteid (globin) not well understood, and hcematin. This happens when the blood is boiled (perhaps also in certain cases of lightning-stroke), and when strong acids are added. Haematin is soluble in dilute acids and alkalies, and has then characteristic spectra. Alkaline haematin may be re- duced ; and, as the iron can be separated, resulting in a change THE RESPIRATORY SYSTEM. 389 of color to brownish red, after which there are no longer any reducing- effects, it would seem that the oxygen-carrying power and iron are associated. This iron-free haernatin is named hcematorporphyrin or hcematoin. Hcemin is hydrochlorate of haernatin (Teichinanms crystals), and may be formed by adding glacial acetic acid and common salt to blood, dried blood-clot, etc., and heating to boiling. This is one of the best tests for blood, valuable in medico-legal and other cases. When oxy-haemoglobin stands exposed to the air, or when diffused in urine, it changes color and becomes, in fact, another substance — methcemoglobin, irreducible by other gases (CO, etc.), and not surrendering its oxygen in vacuo, though giving it up to ammonium sulphide, becoming again oxy-haemoglobin, when shaken up with atmospheric air. Its spectrum differs from that of oxy-haemoglobin in that it has a band in the red end of the spectrum between the C and D lines. Hcematoidin is some- times found in the body as a remnant of old blood-clots. It is probably closely allied to if not identical with the bilirubin of bile. Comparative. — "While haemoglobin is the respiratory agent in all the groups of vertebrates, this is not true of the inverte- brates. Red blood-cells have as yet been found in but a few species, though haemoglobin does exist in the blood plasma of several groups, to one of which the earth-worm and several other annelids belong. It is interesting to note that the respir- atory compound in certain families of crustaceans, as the com- mon crab, horseshoe-crab (limulus), etc., is blue, and that in this substance copper seems to take the place of iron. The Nitrogen, and the Carbon Dioxide of the Blood. -The little nitrogen which is found in about equal quantity in venous and arterial blood seems to be simply dissolved. The relations of carbonic anhydride are much more complex arid obscure. The main facts known are that — 1. The quantity of tin's gas is as great in serum as in blood, or, at all events, the quantity in serum is very large. 2. The greater part may be extracted by an exhaustion-pump ; but a small percentage (2 to 5 volumes per cent) does not yield to this method, but is given off when an acid is added to the serum. 3. If the entire blood be sub- jected to a vacuum, the whole of the C0 2 is given off. From these facts it has been concluded that the greater part of the C0 2 , exists in the plasma, associated probably with sodium 390 COMPARATIVE PHYSIOLOGY. salts, as sodium bicarbonate, but that the corpuscles in some way determine its relations of association and disassociation. Some think a good deal of this gas is actually united with the red cor- puscles. We may now inquire into the more intimate nature of respi- ration in the blood. From the facts we have stated it is obvious that respiration can not be wholly explained by the Henry-Dal- ton law of pressures or any other physical law. It is also plain that any explanation which leaves out the principle of pressure must be incomplete. While there is in oxy-haemoglobin a certain quantity of oxygen, which is intra-molecular and incapable of removal by reduction of pressure, there is also a portion which is subject to this law, though in a peculiar way ; nor is tbe question of temperature to be excluded, for experiment shows that less oxygen is taken up by blood at a high than at a low tempera- ture. We have learned that in ordinary respiration, the propor- tion of carbonic dioxide and oxygen in different parts of the respiratory tract must vary greatly ; the air of necessity being much less pure in the alveoli than in the larger bronchi. It is customary to speak of the oxygen of oxy-hsemoglobin as being in a state of u loose chemical combination." The en- tire truth seems to lie in neither view, though both are partially correct. The view entertained by some physiologists, to the effect that diffusion explains the whole matter, so far, at least, as carbonic anhydride is concerned, and that the epithelial cells of the lung have no share in the respiratory process, does not seem to be in harmony either with the facts of respiration or with the laws of biology in general. Why not say at once that the facts of respiration show that, here as in other parts of the economy, while physical and chemical laws, as we know them, stand related to the vital processes, yet, by reason of being vital processes, we can not explain them according to the theories of either physics or chemistry ? Surely this very subject shows that neither chemistry nor physics is at present adequate to explain such processes. It is, of course, of value to know the circumstances of tension, temperature, etc., under which respi- ration takes place. We, however, maintain that these are con- ditions only — essential no doubt, but though important, that they do not make up the process of respiration. But, because we do not know the real explanation, let us not exalt a few THE RESPIRATORY SYSTEM. ' 39 1 facts or theories of chemistry or physics into a solution of a complex problem. Besides, some of the experiments on which the conclusions have been based are questionable, inasmuch as they seem to induce artificial conditions in the animals oper- ated upon ; and we have already insisted on the blood being regarded as a living tissue, behaving differently in the body and when isolated from it, so that even in so-called blood-gas experiments there may be sources of fallacy inherent in the nature of the case. Foreign Gases and Respiration.— These are divided into: 1. Indifferent gases, as N, H, CH 4 , which though not in themselves injurious, are entirely useless to the economy. 2. Poisonous gases, fatal, no matter how abundant the nor- mal respiratory food may be. They are divisible into : (a) those that kill by displacing oxygen, as NO, CO, HCN ; (b) narcotic gases, as C0 2 , N 2 0, producing asphyxia when present in large quantities; (c) reducing gases, as H 2 S, (NH 4 ) 2 S, PH 3 , AsH 3 , C 2 N 2 , which rob the haemoglobin of its oxygen. There are probably a number of poisonous products, some of them possibly gases, produced by the tissues themselves and eliminated normally by the respiratory tract ; and these are doubtless greatly augmented, either in number or quantity, or both, when other excreting organs are disordered. RESPIRATION IN THE TISSUES. We first direct attention to certain striking facts : 1. An isolated (frog's) muscle will continue to contract for a considerable period and to exhale carbon dioxide in the total absence of oxygen, as in an atmosphere of hydrogen ; though, of course, there is a limit to this, and a muscle to which either no blood flows, or only venous blood, soon shows signs of fatigue. 2. In a frog, in which physiological saline solution has been substituted for blood, the metabolism will continue, carbonic anhydride being exhaled as usual. 3. Substances, which are readily oxidized, when introduced into the blood of a living animal or into that blood when withdrawn uudergo but little oxidative change. 4. An entire frog will respire car- bonic dioxide for hours in an atmosphere of nitrogen. Such facts as these seem to teach certain lessons clearly. It is evident, first of all, that the oxidative processes that give rise to carbon dioxide occur chiefly in the tissues and not in the 392 COMPARATIVE PHYSIOLOGY. blood ; that in the case of muscle the oxygen that is used is first laid by, banked as it were against a time of need, in the form of infra-molecular oxygen, which is again set free in the form of carbon dioxide, but by what series of changes we are quite un- able to say. Though our knowledge of the respiratory processes of muscle is greater than for any other tissue, there seems to be no reason to believe that they are essentially different else- where. The advantages of this banking of oxygen are, of course, obvious ; were it otherwise, the life of every cell must be at the mercy of the slightest interruption of the flow of blood, the entrance of air, etc. Even as it is, the need of a constant supply of oxygen in warm-blooded animals is much greater than in cold-blooded creatures, which can long endure almost entire cessation of both respiration and circulation, owing to the comparatively slow rate of speed of the vital machinery. If one were to rely on mere appearances he might suppose that in the more active condition of certain organs there was less chemical interchange (respiration) between the blood aud the tissues than in the resting stage, or, properly speaking, more tranquil stage, for it must be borne in mind that a living cell is never wholly at rest ; its molecular changes are cease- less. It happens, e. g., that when certain glands (salivary) are secreting actively, the blood flowing from them is less venous in appearance than when not functionally active. This is not because less oxygen is used or less abstracted from the blood, but because of the greatly increased speed of the blood-flow, so that the total supply to draw from is so much larger that, though more oxygen is actually used, it is not so much missed, nor do the greater additions of carbon dioxide so rapidly pol- lute this rapid stream. It is thus seen that throughout the animal kingdom respira- tion is fundamentally the same process. It is in every case finally a consumption of oxygen and production of carbonic anhydride by the individual cell, whether that be an Amoeba or an element of man's brain. These are, however, but the beginning and end of a very complicated biological history of by far the greater part of which nothing is yet known ; and it must be admitted that diffusion or any physical explanation carries us but a little way on toward the understanding of it. THE RESPIRATORY SYSTEM. 393 THE NERVOUS SYSTEM IN RELATION TO RESPIRA- TION. We have considered the muscular movements by which the air is made to enter and leave the lungs in consequence of changes in the diameters of the air-inclosing case, the thorax. It remains to examine into the means by which these muscles were set into harmonious action so as to accomplish the pur- pose. The nerves supplying the muscles of respiration are de- rived from the spinal cord, so that they must be under the do- minion of central nerve-cells situated either in the cord or the brain. Is the influence that proceeds outward generated within the cells independently of any afferent impulses, or is it depend- ent on such causes ? A host of facts, experimental and other, show that the cen- tral impulses are modified by afferent impulses reaching the center through appropriate nerves. Moreover, drugs seem to act directly on the center through the blood. The vagus is without doubt the afferent respiratory nerve, though how it is affected, whether by the mechanical movement of the lungs, merely, by the condition of the blood as regards its contained gases, or, as seems most likely, by a combination of circumstances into which these enter and are probably the principal, is not demonstrably clear. When others function as afferent nerves, capable of modifying the action of the respira- tory center, they are probably influenced by the respiratory condition of the blood, though not necessarily exclusively. But when all the principal afferent impulses are cut off by division of the nerves reaching the respiratory center directly or indirectly, respiration will still continue, provided the motor nerves and the medulla remain intact. The center, then, is not mainly at least, a reflex but an auto- matic one, though its action is modified by afferent impulses reaching it from every quarter. It has been argued that there are both inspiratory and ex- piratory centers in the spinal cord, but this can not yet be re- garded as established. But, as we have pointed out, on more than one occasion, we must always be on our guard in inter- preting the behavior of one part when another is out of gear. The Influence of the Condition of the Blood in Respiration. — If for any reason the tissues are not receiving a due supply of oxygen, they manifest their disapproval, to speak figuratively, 394 COMPARATIVE PHYSIOLOGY. Brain above medulla from which impulses modifying respiration may proceed. 'acial muscles. Respiratory centre in the medulla. Cutaneous surface from which afferent impulses proceed cZn rectly to brain. ---I — Thoracic resp. muscles. Spinal cord.— r — Respiratory tract. Diaphragm with phrenic nerve. Cutaneous sur- face from which impulses reach res- piratory centre by spinal cord. Fig. 809.— Diagram intended to illustrate nervous mechanism of respiration. Arrows indicate course of impulses. THE RESPIRATORY SYSTEM. 395 by reports to the responsible center in the medulla, and if the medulla is a sharer in the lack, as it naturally would be, it takes action independently. One of the most obvious instances in which there is oxygen starvation is when there is hindrance to the entrance of air, owing to obstruction in the respiratory tract. At first the breathing is merely accelerated, with perhaps some increase in the depth of the inspirations (hyperpnoea), a stage which is soon succeeded by labored breathing (dyspnoea), which, after the medulla has called all the muscles usually em- ployed in respiration into violent action, passes into convul- sions, in which every muscle may take part. In other words, the respiratory impulses not only pass along their usual paths as energetically as possible, but radiate into unusual ones and pass by nerves not commonly thus set into functional activity. It would be more correct, perhaps, to assume that the vari- ous parts of the nervous system are so linked together that ex- cessive activity of one set of connections acts like a stimulus to rouse another set into action, the order in which this happens depending on the law of habit — habit personal and especially ancestral. An opposite condition to that described, known as apnoea, may be induced by pumping air into an amimal's chest very rapidly by a bellows ; or in one's self by a succession of rapid, deep respirations. After ceasing, the breathing may be entirely interrupted for a brief interval, then commence very quietly, gradually in- creasing to the normal. Apnoea has been interpreted in two ways. Some think that it is due to fatigue of the muscles of respiration or the respira- tory center; others that the blood has under these circum- stances an excess of oxygen, which so influences the respiratory center that it is quieted (inhibited) for a time. The latter view is that usualty adopted ; but considering that apnoea results from the sobbing of children following a pro- longed fit of crying, also in Cheyne-Stokes and other abnormal forms of breathing, and that the blood is normally almost satu- rated with oxygen, it will be agreed that there is a good deal to be said for the first view, especially that part of it which repre- sents the cessation of breathing as owing to excessive activity and exhaustion of the respiratory center. We find such a calm in asphyxia after the convulsive storm. Perhaps if we regard the respiratory center as double, half being situated on each side 396 COMPARATIVE PHYSIOLOGY. of the middle Hue ; also as made up of au inspiratory and ex- piratory part; automatic essentially, but greatly modified by afferent impulses, especially those ascending the vagi nerves; while the latter may be considered as containing both inhib- itory and augmenting fibers for the center, the whole process will be clearer. Respiration on this view would be self -regula- tive ; the deeper the inspiration, the stronger the inhibitory in- fluence, so the greater the tendency to arrest of inspiration; hence either expiration or apncea. Is it, then, the excessive accumulation of carbon dioxide or the deficiency of oxygen that induces dyspnoea ? Considering that the former gas acts as a narcotic, and does not induce con- vulsions, even when it constitutes a large percentage of the atmosphere breathed, and that the need of oxygen for the tis- sues is constant, it certainly seems most reasonable to conclude that the phenomena of dyspnoea are owing to the lack of oxy- gen, chiefly at least; though the presence of an excess of car- bonic anhydride may take some share in arousing that vigorous effort on the part of the nervous system, to restore the func- tional equilibrium, so evident under the circumstances. THE INFLUENCE OF RESPIRATION ON THE CIRCULATION. An examination of tracings of the intra-thoracic and blood- pressure, taken simultaneously, shows (1) that during inspira- tion the blood-pressure rises and the intra-thoracic pressure falls ; (2) that during expiration the reverse is true ; and (3) that the heart-beat is slowed, and has a decided effect on the form of the pulse. But it also appears that the period of high- est blood-pressure is just after expiration has begun. We must now attempt to explain how these changes are brought about. By intra-thoracic pressure is meant the press- ure the lungs exert on the costal pleura or any organ within the chest, which must differ from intra-pulmonary pressure and the pressure of the atmosphere, because of the resistance of the lungs by virtue of their own elasticity. It has been noted that even in death the lungs remain par- tially distended ; and that when the thorax is opened the pul- monary collapse wbich follows demonstrates that their elas- ticity amounts to about five millimetres of mercury, which must, of course, represent but a small portion of that elasticity THE RESPIRATORY SYSTEM. 397 which may he brought into play when these organs are greatly distended, so that they never press on the costal walls, heart, Fig. 310. — Tracings of blood-pressure and intrathoracic pressure in the dog (after Foster), a, blood-pressure tracing showing irregularities due to respiration and pulse; b, curve of intrathoracic pressure; i, beginning of inspiration; e, of expira- tion. Intrathoracic pressure is seen to rise rapidly after inspiration ceases, and then slowly sinks as the expiratory blast continues, to become a rapid fall when inspiration begins. etc., with a pressure equal to that of the atmosphere. It follows that the deeper the inspiration the greater the difference be- tween the intra-thoracic and the atmospheric pressure. Even in expiration, except when forced, the intra-thoracic pressure remains less, for the same reason. These conditions must have an influence on the heart and blood-vessels. Bearing in mind that the pressure without is practically constant and always greater than that within the thorax, the conditions are favorable to the flow of blood toward the heart. As in inspiration, the pressure on the great veins and the heart is diminished, and, as these organs are not rigid, they tend to expand within the thorax, thus favoring an on- ward flow. But the opposite effect would follow as regards the large arteries. Their expansion must tend to withdraw blood. During expiration the conditions are reversed. The effects on the great veins can be observed by laying them bare in the neck of an animal, when it may be seen that during inspiration they become partially collapsed, and refilled during expiration. In consequence of the marked thickness of the coats of the great arteries, the effect of changes in intra-thoracic pressure must be slight. The comparatively thin-walled auricles act somewhat as the veins, and it is likely that the increase of pressure during expiration must favor, so far as it goes, the car- diac systole. 398 COMPARATIVE PHYSIOLOGY. More blood, then, entering the right side of the heart dur- ing inspiration, more will be thrown into tbe systemic circula- tion, unless it be retained in the lungs, and, unless the effect be counteracted, the arterial pressure will rise, and, as all the con- ditions are reversed during expiration, we look for and find exactly opposite results. The lungs themselves, however, must be taken into the account. During inspiration room is pro- vided for an increased quantity of blood, the resistance to its flow is lessened, hence more blood reaches the left side of the heart. The immediate effect would be, notwithstanding, some diminution in the quantity flowing to the left heart, in conse- quence of the sudden widening of the pulmonary vessels, the reverse of which would follow during expiration ; hence the period of highest intra-thoracic pressure is after the onset of the expiratory act. During inspiration the descent of the diaphragm compressing the abdominal organs is thought to force on blood from the abdominal veins into the thoracic vena cava. That the respiratory movements do exert in some way a pronounced effect on the circulation the student may demon- strate to himself in the following ways : 1. After a full inspira- tion, close the glottis and attempt to expire forcibly, keeping the fingers on the radial artery. It may be noticed that the pulse is modified or possibly for a moment disappears. 2. Re- verse the experiment by trying to inspire forcibly with closed glottis after a strong expiration, when the pulse will again be found to vary. In the first instance, the heart is comparatively empty and hampered in its action, intra-thoracic pressure being so great as to prevent the entrance of venous blood by com- pression of the heart and veins, while that already within the organ and returning to it from the lungs soon passes on into the general system, hence the pulseless condition. The expla- nation is to be reversed for the second case. The heart's beat is modified, probably reflexly, through the cardio-inhibitory cen- ter, for the changes in the pulse-rate do not occur when the vagi nerves are cut, at least not to nearly the same extent. Comparative.— It may be stated that the cardiac phenomena referred to in this section are much more marked in some ani- mals than in others. Very little change may be observed in the pulse-rate in man, while in the dog it is so decided that one observing it for the first time might suppose that such pro- nounced irregularity of the heart was the result of disease ; THE RESPIRATORY SYSTEM. 399 though even in this animal there are variations in this respect with the breed, age, etc. The Respiration and Circulation in Asphyxia.— A most in- structive experiment may be arranged thus : Let an anaesthetized rabbit, cat, or such-like animal, have the carotid of one side connected with a glass tube as before desci'ibed (pages 228, 229), by which the blood-pressure and its changes may be indicated, and, when the normal respiratory acts have been carefully observed, proceed to notice the effects on the blood-pressure, etc., of pumping air into the chest by a bellows, of hindering the ingress of air to a moderate degree, and of struggling. "With a small animal it will be difficult to observe the respiratory effects on the blood-pressure by simply watching the oscillations of the fluid in the glass tube, but this is readily enough made out if more elaborate arrangements be made, so that a graphic tracing may be obtained. But the main events of asphyxia may be well (perhaps best) studied in this manner: Let the trachea be occluded (ligatured). At once the blood- pressure will be seen to rise and remain elevated for some time, then gradually fall to zero. These changes are contemporane- ous with a series of remarkable manifestations of disturbance in the respiratory system as it at first appears, but in reality due to wide-spread and profound nutritive disturbance. So far as the breathing is concerned, it may be seen to become more rapid, deeper, and labored, in which the expiratory phase be- comes more than proportionably marked (dyspnoea) ; this is fol- lowed by the gradual action of other muscles than those usually employed in respiration, until the whole body passes into a ter- rible convulsion — a muscle-storm in consequence of a nerve- storm. When this has lasted a variable time, but usually about one minute, there follows a period of exhaustion, during which the subject of the experiment is in a motionless condi- tion, interrupted by an occasional respiration, in which inspi- ration is more pronounced than expiration; and, finally, the animal quietly stretches every limb, the sphincters are relaxed, there may be a discharge of urine or faeces from peristaltic movements of the bladder or intestines, and death ends a strik- ing scene. These events may be classified in three stages, though the first and second especially merge into one another: 1. Stage of dyspnoea. 2. Stage of convulsions. 3. Stage of exhaustion. 400 COMPARATIVE PHYSIOLOGY. • It is during the first two stages that the blood-pressui'e rises, and during the third that it sinks, due in the first instance chiefly to excessive activity of the vaso-motor center, and in the second to its exhaustion and the weakening of the heart- beat. These violent movements are owing, we repeat, to the action of blood deficient in oxygen on the respiratory center (or cen- ters), leading to inordinate action followed by exhaustion. The duration of the stages of asphyxia varies with the ani- mal, but rarely exceeds five minutes. In this connection it may be noted that newly born animals (kittens, puppies) bear im- mersion in water for as much as from thirty to fifty minutes, while an adult dog dies within four or five minutes. This is to be explained by the feeble metabolism of new-born mam- mals, which so slowly uses up the vital air (oxygen). If the chest of an animal be opened, though the respiratory muscles contract as usual there is, of course, no ventilation of the lungs which lie collapsed in the chest ; and the animal dies about as quickly as if its trachea were occluded. It passes through all the phases of asphyxia as in the former case ; but additional information may be gained. The heart is seen to beat at first more quickly and forcibly, later vigorously though slower, and finally both feebly and irregularly, till the ventri- cles, then the left auricle, and finally tho right auricle cease to beat at all or only at long intervals. The terminations of the great veins (representing the sinus venosus) beat last of all. At death the heart and great veins are much distended with blood, the arteries comparatively empty. Even after rigor mortis has set in, the right heart is still much engorged. These phenomena are the result of the operation of several causes. The increasingly venous blood at first stimulates the heart probably directly, in part at least, but later has the con- trary effect. The nutrition of the organ suffers from the de- graded blood, from which it must needs derive its supplies. The cardio-inhibitory center probably has a large share in the slowing of the heart, if not also in quickening it. Whether the accelerator fibers of the vagus or sympathetic play any part is uncertain. The increase of peripheral resistance caused by the action of the vaso-motor center makes it more difficult for the heart to empty its left side and thus receive the venous blood as it pours on. At the same time the deep inspirations (when the chest is unopened) favor the onflow of venous blood; THE RESPIRATORY SYSTEM. 401 and in any case the whole venous system, including the right heart, tends to become engorged from these several causes act- ing together. The heart gives up the struggle, unable to main- tain it, but not so long as it can beat in any part. The share which the elasticity of the arteries takes in forcing on the blood when the heart ceases, and the contraction of the muscular coat of these vessels, especially the smaller, must not be left out of the account in explaining the phenom- ena of asphyxia and the post-mortem appearances. Pathological.— The importance of being practically as well as theoretically acquainted with the facts of asphyxia is very great. The appearance of the heart and venous system gives une- quivocal evidence as to the mode of death in any case of as- phyxia; and the contrast between the heart of an animal bled to death, or that has died of a lingering disease, and one drowned, hanged, or otherwise asphyxiated, is extreme. We strongly recommend the student to asphyxiate some small mammal placed under the influence of an anaesthetic, and to note the phenomena, preferably with the chest opened ; and to follow up these observations by others after the onset of rigor mortis. PECULIAR RESPIRATORY MOVEMENTS. Though at first sight these seem so different, and are so as regards acts of expression, yet from the respiratory point of view they resemble each other closely; they are all reflex, and, of course, involuntary. Many of them have a common pur- pose, either the better to ventilate the lungs, to clear them of foreign bodies, or to prevent their ingress. Coughing, in which such a purpose is evident, is made up of several expiratory efforts preceded by an inspiratory act. The afferent nerve is usually the vagus or laryngeal, but may be one or more of several others. The glottis presents characteristic appearances, being closed and then opened suddenly, the mouth being kept open. Coughing is often induced in attempting to examine the ear with instruments. (Reflex act). Laughing is very similar to the last, so far as the behavior of the glottis is concerned, though it usually acts more rapidly, of course. Several expirations follow a deep inspiration. 26 402 COMPARATIVE PHYSIOLOGY. Crying is essentially the same as laughing, hut the facial expression is different, and the lachrymal gland functions ex- cessively, though with some persons this occurs during laughter also. Sobbing is made up of a series of inspirations, in which the glottis is partially closed, followed by a deep expiration. Yawning involves a deep-drawn, slow inspiration, followed by a more sudden expiration, with a well-known depression of the lower jaw and usually stretching movements. Sighing is much like the preceding, though the mouth is not opened widely if at all, nor do the stretching movements com- monly occur. Hiccough is produced by a sudden inspiratory effort, though fruitless, inasmuch as the glottis is suddenly closed. It is spoken of as spasm of the diaphragm, and when long continued is very exhaustive. Sneezing is the result of a powerful and sudden expiratory act following a deep inspiration, the mouth being usually closed by the anterior pillars of the fauces against the outgoing cur- rent of ah", which then makes its exit through the nose, while the glottis is forcibly opened after sudden closure. It will be noticed that in most of these acts the glottis is momentarily closed, which is never the case in mammals during quiet res- piration. This temporary occlusion of the respiratory passages per- mits of a higher intrapulmonary pressure, which is very effect- ive in clearing the passages of excess of mucus, etc., when the glottis is suddenly opened. Though the acts described are all involuntary, they may most of them be imitated and thus studied deliberately by the student. It will also appear, con- sidering the many ways in which some if not all of them may be brought about, that if the medullary center is responsible for the initiation of them it must be accessible by numberless paths. Comparative. — Few of the lower animals cough with the same facility as man, while laughing is all but unknown, cry- ing and sobbing rare, though the whining of dogs is allied to the crying of human beings. Sneezing seems to be voluntary in some animals, as squir- rels, when engaged in toilet operations, etc. Barking is voluntary, and in mechanism resembles cough- ing, the vocal cords being, however, more definitely employed, as also in growling. THE RESPIRATORY SYSTEM. 403 Balding, neighing, braying, etc., are made up of long ex- piratory acts, preceded by one or more inspirations. The vocal cords are also rendered tense. SPECIAL CONSIDERATIONS. Pathological and Clinical. —The number of diseases that lessen the amount of available pulmonary tissue, or hamper the move- ments of the chest, are many, and only the briefest reference can be made to a few of them. Inflammation of the lungs may render a greater or less por- tion of one or both lungs solid; inflammation of the pleura (pleuritis, pleurisy) by the dryness, pain, etc., may restrict the thoracic movements; phthisis may solidify or excavate the lungs, or by pleuritic inflammation glue the costal and pulmo- nary pleural surfaces together; bronchitis may clog the tubes and other air-passages with altered secretions ; emphysema (dis- tention of air-cells) may destroy elasticity of parts of the lung ; pneuma-thorax from rupture of the lung-tissue and consequent accumulation of gases in the pleural cavity, or pleurisy with effusion render one lung all but useless from pressure. In all such cases Nature attempts to make up what is lost in amplitude by increase in rapidity of the respiratory movements. It is interesting to note too how the other lung, in diseased condi- tions, if it remain unaffected, enlarges to compensate for the loss on the opposite side. When the muscles are weak, espe- cially if there be hindrance to the entrance of air while the thoracic movements are marked, there may be bulging inward of the intercostal spaces. Normally, this would also occur, as the intra-thoracic press- ure is less than the atmospheric, were it not for the fact that the intercostal muscles when contracting have a certain resisting power. The imperfect respiration of animals when dying, permitting the accumulation of carbonic anhydride with its soporific effects, smooths the way leading to the end ; so that there may be to the uninitiated the appearance of a suffering which does not exist, consciousness itself being either wholly or partially absent. The dyspnoea of anaemic animals, whether from sud- den loss of blood or from imperfect renewal of the haemo- globin, shows that this substance has a respiratory function; while in forms of cardiac disease with regurgitation, etc., the 404 COMPARATIVE PHYSIOLOGY. blood may be imperfectly oxidized, giving rise to labored res- piration. Personal Observation. — As hinted from time to time during the treatment of this subject, there is a large number of facts the student may verify for himself. A simple way of proving that CO2 is exhaled is to breathe (blow) into a vessel containing some clear solution of quick- lime (CaO), the turbidity showing that an insoluble salt of lime (CaC0 3 ) has been formed by the addition of this gas. The functions of most of the respiratory muscles, the phe- nomena of dyspnoea, apncea (by a series of long breaths), par- tial asphyxia by holding the breath, and many other experi- ments, simple but convincing, will occur to the student who is willing to learn in this way. The observation of respiration in a dreaming animal (dog) will show how mental occurrences affect the respiratory center in the absence of all the usual outward influences. The respira- tion of the domestic animals, and of the frog, turtle, snake, and fish, is easily watched if these cold-blooded animals be placed for observation beneath a glass vessel. Their study will teach how manifold are the ways by which the one end is attained. Compare the tracings of Fig. 305. Evolution. — A study of embryology shows that the respira- tory and circulatory systems develop together ; that the vascu- lar system functions largely as a respiratory system also in cer. tain stages, and remains such, from a physiological point of view, throughout embryonic life. The changes that take place in the vascular system — the heart, especially — of the mammal when the lungs have become functionally active at birth, show how one set of organs modi- fies the other. When one considers, in addition to these facts, that the digestive as well as the vascular and respiratory organs are represented in one group of structures in a jelly-fish, and that the lungs of the mammal are derived from the same mesoblast as gives rise to the digestive and circulatory organs, many of the relations of these systems in the highest groups of animals become intelligible ; but unless there be descent with modifica- tion, these facts, clear enough from an evolutionary standpoint, are isolated and out of joint, bound together by no common principle that satisfies a philosophical biology. It has been found that in hunting-dogs and wild rabbits the THE RESPIRATORY SYSTEM. 405 vagus is more efficient than in other races of dogs and in rab- bits kept in confinement ; and possibly this may in part account for the greater speed and especially the endurance of the former. The very conformation of some animals, as the grey- hound, with his deep chest and capacious lungs, indicates an unusual respiratory capacity. The law of habit is well illustrated in the case of divers, who can bear deprivation of air longer than those unaccustomed to such submersion in water. Greater toleration on the part of the respiratory center has probably much to do with the case, though doubtless many other departures from the normal occur, either independently or correlated to the changes in the respira- tory center. Some mammals, like the whale, can long remain under water. Summary of the Physiology of Respiration.— The purpose of respiration in all animals is to furnish oxygen for the tissues and remove the carbonic anhydride they produce, which in all vertebrates is accomplished by the exposure of the blood in capillaries to the atmospheric air, either free or dissolved in water. A membrane lined with cells always intervenes between the capillaries and the air. The air may be pumped in and out, or sucked in and forced out. Respiration in the Mammal.— The air enters the lungs, owing to the enlargement of the chest in three directions by the action of certain muscles. It leaves the lungs because of their own elastic recoil and that of the chest- wall chiefly. Inspiration is active, expiration chiefly passive. The diaphragm is the principal muscle of respiration. In some animals there is a well-marked facial and laryngeal as well as thoracic respiration. Respiration is rhythmical, con- sisting of inspiration, succeeded without appreciable pause by expiration, the latter being in health of only slightly longer duration. There is also a definite relation between the number of respirations and of heart-beats. According as respiration is normal, hurried, labored, or interrupted, we describe it as eupnoe, hyperpnoea, dyspnoea, and apnoea. The intra-thoracic pressure is never equal to the atmospheric — i. e., it is always negative — except in forced expiration ; and the lungs are never collapsed so long as the chest is unopened. The expired air diffei'S from that inspired in being of the temperature of the body, saturated with moisture, and containing about 4 to 5 per 406 COMPARATIVE PHYSIOLOGY. cent less oxygen and 4 per cent more carbonic anhydride, be- sides certain indifferently known bodies, the result of tissue metabolism, excreted by the lungs. The quantity of air actually moved by a respiratory act, as compared with the total capacity of the respiratory organs, is small ; hence a great part must be played by diffusion. The portion of air that can not be removed from the lungs by any respiratory effort is relatively large. It is customary to distinguish tidal, complementary, supple- mentary, and residual air. The vital capacity is estimated by the quantity of air the respiratory organs can move, and is very variable. The blood is the respiratory tissue, through the mediation of its red cells, by the haemoglobin they contain. This sub- stance is a ferruginous proteid, capable of crystallization, and assuming under chemical treatment many modifications. When it contains all the oxygen it can retain, it is said to be saturated and is called oxy-haemoglobin, in which form it exists (with some reduced haemoglobin) in arterial blood, and to a lesser extent in venous blood, which differs from arterial in the rela- tive proportions of haemoglobin (reduced) it contains, as viewed from the respiratory standpoint. Oxy-haemoglobin does not assume or part with its oxygen, according to the Henry -Dalton law of pressures, nor is this gas in a state of ordinary chemical combination. It is found that the oxygen tension of the blood is lower and that of carbonic anhydride higher than in the air of the alveoli of the lungs, while the same may be said of the tissues and the blood re- spectively. This has been, however, recently again denied. Respiration is a vital process, though certain physical con- ditions (temperature and pressure) must be rigidly maintained in order that the gaseous interchanges shall take place. Res- piration is always fundamentally bound up with the metabo- lism of the tissues themselves. All animal cells, whether they exist as unicellular animals (Amoeba) or as the components of complex organs, use np oxygen and produce carbonic dioxide. Respiratory organs, usually so called, and the respiratory tissue par excellence (the blood) are only supplementary mechanisms to facilitate tissue respiration. Carbonic anhydride exists in blood probably in combination with sodium salts, though the whole matter is very obscure. Respiration, like all the other functions of the body, is con- THE RESPIRATORY SYSTEM. 4Q7 trolled by the central nervous system through nerves. The medulla oblongata is chiefly concerned, and especially one small part of it known as the respiratory center. It is possible, even probable, that there are subordinate centers in the cord, which, under peculiar circumstances, assume importance ; but how far they act in concert with the medullary center, or whether they act at all when normal conditions prevail, is an open question. The vagus is the principal afferent respiratory nerve. The efferent nerves are the phrenies. intercostals, and others supply- ing the various muscles used in moving the chest-walls, etc. The respiratory center is automatic, but its action is sus- ceptible of modification through afferent influences taking a variety of paths, the principal of which is along the vagi nerves. The respiratory, vaso-motor, and cardio-inhibitory centers seem to act somewhat in concert. Blood-pressure is being constantly modified by the respira- tory act, rising with inspiration and sinking with expiration. In some animals the heart-beat also varies with these phases of respiration, becoming slow and irregular during expiration. Into the causation of these changes both mechanical and nerv- ous factors enter, and make a very complex mesh, which we can at present but imperfectly unravel. When the access of air to the tissues is prevented, a series of stages of respiratory activity and decline, accompanied by pronounced changes in the vascular system, are passed through, known as asphyxia. Three stages are distinguishable : one of dyspnoea, one of convulsions, and one of exhaustion — while at the same time there is a rise of blood-pressure during the first two, and a decline during the third, accompanied by marked alterations in the cardiac rhythm. PROTECTIVE AND EXCRETORY FUNCTIONS OF THE SKIN. As lias been intimated from time to time, thus far, as a result of the metabolism of the tissues, certain products require constant removal from the blood to prevent poisonous effects. These substances are in all probability much more numerous than physiological chemistry has as yet distinctly recognized or, at all events, isolated. Quantitatively considered, the most important are carbonic anhydride, water, urea, and, of less im- portance, perhaps, certain salts. In many invertebrates and in all vertebrates several organs take part in this work of elimination of waste products or puri- fication of the blood, one set of which — the respiratory — we have just studied ; and we now continue the consideration of the subject of excretion, this term being reserved for the pro- cess of separating harmful products from the blood and dis- charging them from the body. We strongly recommend the student to make the study of excretion comparative in the sense of noting how one organ engaged in the process supplements another. A clear under- standing of this relation even to details makes the practice of medicine more scientific and practically effective, and gives physiology greater breadth. The skin has a triple function : it is protective, excretory, sensory, and, we may add, nutritive (absorptive) and respira- tory, especially in some groups of animals. As a sensory organ, the skin will receive attention later. Protective Function of the Skin. — Comparative. — Among many groups of invertebrates the principal use of the exterior covering of the body is manifestly protection. Among these forms, an internal skeleton being absent, the exo-skeleton is developed externally, and serves not only for protection, but for the attachment of muscles, as seen in crustaceans and in- PROTECTIVE FUNCTION OP THE SKIN. 409 sects. But this part of the subject is too large for detailed treatment in such a work as this. Turning to the vertebrates, we see scales, bony plates, feathers, spines, hair, etc., most of them to be regarded as modifications of the epidermis, always useful, and frequently also ornamental. Primitive man was probably much more hirsute than his modern representative ; and, though the human subject is at present provided with a skin in which protective functions are at their lowest, still the epidermis does serve such a purpose, as all have some time realized when it has been accidentally removed by blistering, etc. Taking the structure of the skin of man as representing that of mammals generally, certain points claim attention from the physiologist. Its elasticity, the failure of which in old age ac- counts for wrinkles; its epider- E> ■jrftSfea^y •"?}..- -*3?' ,v^ Fig. 311. Fig. 312. Fig. 311.— Sudoriparous gland?. 1x20. (Af ter Sappey.) 1. 1. epidermis; 2. 2, mucous layer; 3, 3, papillae; 4. 4, derma; 5. 5, subcutaneous areola tissue; 6. 0. 6, 6, sudo- riparous glands; 7 7, adipose vesicles; 8, 8, excretory ducts in derma; 9, 9, excre- tory ducts divided. Fig. 312. — Portion of skin of palm of hand about one-half an inch (127 mm.) square. 1x4. (After Sappey.) 1, 1, 1, 1, openings of sudoriferous ducts; 2, 2, 2, 2, grooves between papillse of skin. mal covering, made up of numerous layers of cells; its coiled and spirally twisted sudoriferous glands, permitting of move- ments of the skin without harm to these structures; its hair- follicles and associated sebaceous glands, the fatty secretion of which keeps the hair and the skin generally soft and pliable. 410 COMPARATIVE PHYSIOLOGY. Tlie muscles of the skin, which either move it as a whole or erect individual hairs, play an important part in. modifying" ex- pression, well seen in the whole canine tribe and many others. There are several modifications of the sebaceous glands that furnish highly odor- iferous secretions as in the civet cat, the skunk, the musk- deer, and many low- er vertebrates. In some, these are pro- tective (skunk) ; in others, though they may not be agreeable to the senses of man, they are doubtless at- tractive to the fe- males of the same tribe, and are to be regarded as impor- tant in " sexual se- lection," being often confined to the males alone. Ear-wax and the Meibomian secretion are the work of modified sebaceous glands ; as also the oil-glands so highly developed in birds, especially aquatic forms, and of which these creatures make great use in preserv- Fig. 313.— Hair and hair-follicle (after Sappey). 1, root ing their feathers of hair; 2, bulb of hair; 3, internal root-sheath; 5, f rom wettin 0- membrane of hair-follicle; 6, external membrane of & follicle; 7, 7, muscular bands attached to follicle; I n our ClomOSUC 8, 8, extremities of bands passing to skin; 9, com- . ™„, T ac! pound sebaceous K iand, with duct (10) opening into animals we may es- W^gd$^^'* amnB *" i pecially notice a cu THE EXCRETORY FUNCTION OF THE SKIN. 4H taneous gland in the pig, placed at the posterior inner aspect of the knee and of considerable size. In the sheep, the interungulate gland is an inversion of the integu- ment forming an elongated sac, which is supplied with secreting structures analogous to the seba- ceous glands. The importance of protective structures of this kind in such situations is obvious. THE EXCRETORY FUNCTION OF THE SKIN. The quantity of matter dis- Fig- 314. — Interungulate gland of , , ., i,i i • • i sheep (Chauveau). «, inner as- charged through the skm is large pectof first phalanx; b, hoof or -greater in man than by the lungs Se Viteffi" 1 ^ * tad; * (about as 7 to 11), though the amount is very variable, depending on the degree of activ- ity of other related excreting organs, as the lungs and kidneys, and largely upon the temperature as a physical condition; and so in other animals. When the watery vapor is carried off, before it can condense, the perspiration is said to be insensible ; when small droplets become visible, sensible. As to whether the one or the other is predominant will, of course, depend on the rapidity of renewal of the air, its humidity, and its temperature. Apart from the temperature, the amount of sweat is influenced by the quality and quantity of food and, especially of drink taken, the amount of exercise, and psychic conditions ; not to speak of the effect of drugs, poisons, or disease. Perspiration in man is a clear fluid, mostly colorless, with a characteristic odor, devoid of morphological elements (ex- cept epidermal scales), and alkaline in reaction. It may be acid from the admixture of the secretion of the sebaceous glands. Its solids (less than 2 per cent) consist of sodium salts, mostly chlorides, cholesterin, neutral fats, and traces of urea. The acids of the sweat belong to the fatty series (acetic, butyric, formic, propionic, caprylic, cax^roic, etc.). Pathological. — The sweat may contain blood, proteids, abun- dance of urea (in cholera), uric acid, oxalates, sugar, lactic acid, 412 COMPARATIVE PHYSIOLOGY. bile, indigo, and other pigments. Many medicines are elimi- nated in part through the skin. Respiration by the Skin.— Comparative.— In i-eptiles and batrachians, with smooth, moist skin, the respiratory functions of this organ are of great importance ; hence these animals can live long under water. It is estimated that in the frog the greater part of the car- bonic anhydride of the body -waste is eliminated by the skin. Certainly frogs can live for days immersed in a tank supplied with running water ; and it is a significant fact that in this animal the vessel that gives rise to the pulmonary artery sup- plies also a cutaneous branch. The respiratory capacity of the skin in man and most mammals is comparatively small under ordinary circum- stances. The amount of carbonic anhydride thus eliminated in twenty-four hours in man is estimated at not more than 10 grammes. It varies greatly, however, with temperature, exer- cise, etc. The skin is highly vascular in mammals, and its importance as a heat regulator is thus very great. When an animal is varnished over, its temperature rapidly falls, though heat production is in excess. From the fact that life may be prolonged by diminishing loss of heat through wrapping up the animal in cotton-wool, it is inferred that depression of the temperature is, at all events, one of the causes of death. Though the subject is obscure, it is likely that the retention of poisonous products so acts as to derange metabo- lism, as well as poison directly, which might thus lead to the disorganization of the machinery of life to the point of disrup- tion or death. It is also possible that the reduction of the tem- perature from dilatation of the cutaneous vessels may be so great that the animal is cooled below that point at which the vital functions can continue. THE EXCRETION OF PERSPIRATION. In secretion in the wider sense we find usually certain nerv- ous and vascular effects associated. The vessels supplying the gland are dilated during the most active phase, and at the same time nervous impulses are conveyed to the secreting cells which stimulate them to action. There is a certain proportion of water given off by transpiration ; but the sweat, as a whole, THE EXCRETION OP PERSPIRATION. 413 even the major part of the water, is a genuine secretion, the result of the metaholism of the cells. From experiments it is clear that nervous influences alone, in the absence of any vascular changes, or in the total depriva- tion of blood, suffice to induce the secretion of perspiration. If the central stump of the divided sciatic be stimulated, sweating of the other limbs follows, showing that perspiration may be a reflex act. It is found that stimulation of the peripheral end of the divided cervical sympathetic leads to sweating on the cor- responding side of tbe face. Sweating during dyspnoea and from fear, when the cutane- ous surfaces are pale, as well as in the dying animal, shows also the independent influence over the sudorific glands of the nerv- ous system. Heat induces sweating by acting both reflexly and directly on the sweat-centers we may suppose. Unilateral sweating is known as a pathological as well as experimental phenomenon. Perspiration may be either increased or dimin- ished in paralyzed limbs, according to circumstances. It is possible that there is a paralytic secretion of sweat as of saliva. The subject is very intricate, and will be referred to again on account of the light it throws on metabolic pi'ocesses generally. Absorption by the skin in man and other mammals is, under natural conditions probably very slight, as would be expected when it is borne in mind that the true skin is covered by sev- eral layers of cells, the outer of which are hardened. Ointments may unquestionably be forced in by rubbing ; and perhaps absorption may take place when an animal's tis- sues are starving, and food can not be made available through the usual channels. It is certain that abraded surfaces are a source of danger, from affording a means of entrance for dis- ease-producing substances or for germs. Comparative. — It is usually stated in works on physiology that the horse sweats profusely, the ox less so ; the pig in the snout: and the dog, cat, rabbit, rat, and mouse, either not at all or in the feet (between the toes) only. That a closer observa- tion of these animals will convince any one that the latter statements are not strictly correct, we have no doubt. These animals, it is true, do not perspire sensibly to any great extent ; but to maintain that their skin has no excretory function is an error. Summary.— The skin of the mammal has protective, sensory, respiratory, and excretory functions. The respiratory are in- 414 COMPARATIVE PHYSIOLOGY. significant under ordinary circumstances in this group, though well marked in reptiles and especially in hatrachians (frog, menobranchus). Sweating is probably dependent on the action of centers situated in the brain and spinal cord, through nerves that run generally in sympathetic tracts during some part of their course. While the function of sweating may go on inde- pendently of abundant blood-supply, it is usually associated with increased vascularity. Sweat contains a very small quantity of solids, is alkaline in reaction when pure, but liable to be acid from the admixture of sebaceous matter that has undergone decomposition. Sebum consists chiefly of olein, palmitin, soaps, cholesterin, and ex- tractives of little known composition. The salty taste of the perspiration is due chiefly to sodium chloride, and its smell to volatile fatty acids ; especially is this so of the sweat of certain parts of the body of man and other mammals. The functional activity of the skin varies with the tempera- ture, moisture, etc., of the air and certain internal conditions; especially is it important to remember that it is one of a series of excretory organs which act in harmony to eliminate the waste of the body, so that when one functions more the other may and usually does function less. The protective function of the skin and its modified epithe- lium (hair, horns, nails, feathers, etc.) is in man slight, but very important in many other vertebrates, among which provision against undue loss of temperature is one of the most constantly operative, and enables a vast number of groups of animals to adapt successfully to their varying surroundings. EXCEETION BY THE KIDNEY. The kidney in man and other mammals may be described as a very complex arrangement of tubes lined with many differ- ent forms of secreting- cells, surrounded by a great mesh work of capillaries, bound together by connective tissue, the quantity Fig. 315.— Vertical longitudinal section of horse's kidney (Chauveau). a, cortical por- tion; b, medullary portion; c, peripheral portion of latter; d, interior of pelvis; d', d', arms of pelvis; e, border of crest; /, infundibulum; g, ureter. varying with the animal, and the whole inclosed in a capsule. The organ is well supplied with lymphatics and nerves. Though the tubes are so complex, the kidney may be divided into zones which contain mostly but one kind of tubule. Among vertebrates, till the reptiles are reached, the kidney is a persistent Wolffian body, hence its more simple form. In most fishes the kidney is a very elongated organ, though 416 COMPARATIVE PHYSIOLOGY. T?,n sip. stricture of kidnev (after Landois). I. Blood-vessels and tubes (semi-dia- SimS A Cap Ss of cortical substance. B.. Capillaries of medu lary gSSESS?" 1, ^impenetrating Malpighian body; 2, van _.„£ rom a Ma pig- hem hortv It arterioles rectee: C, venm rectse 7, F, interlobular veins, ^.stciiaie ve ns ) / cansu «•« of Mullur X, A', convoluted tubes; T, T, T, tubes of Henle; N NNN ■ diiimui.i.-utiiiK tubes 0, 0, straight tubes; O, opening into pelvis of kidney ' II. Malpighian body. A, artery; ti, vein; 0, capilfanes; if, epithelium EXCRETION BY THE KIDNEY. 41? of capsule; H, beginning of convoluted tube. III. Rodded cells from convoluted tube. 1, view from surface; 2, side view {G, granular zone). IV. Cells lining tubes of Henle. V. Cells lining communicating tubes. VI. Section of straight tube. in the lowest it consists of little more than tubules, coiling but slightly, ending by one extremity in a glomerulus and by the Fig. 317 • 317.— Blood-vessels of MalpigTuan bodies and convoluted tubes of kidney (after Sappey). 1, 1, Malpighian bodies surrounded by capsules; 2. 2, 2. convoluted tubes connected with Malpighian bodies; 3. artery branching to go to Malpighian bodies; 4, 4, 4. branches of artery; 6, 6, Malpighian bodies from" which a portion of capsules has been removed; 7. 7, 7, vessels passing out of Malpighian bodies; 8. vessel, branches of which (9) pass to capillary plexus (10). 27 418 COMPARATIVE PHYSIOLOGY. other opening- into a long- common efferent tube or duct. The glomerulus is, however, peculiar to the vertebrate kidney. The graded complexity in arrangement, etc., of the tubes is represented well in the figure below. It is a significant fact Pig. 318.— Diagrammatic representation of distribution of tubules of kidney (after Huxley). 0, cortical region; B. boundary zone, containing large part of Henle's , loops; P, papillary zone, in which are the main outflow tubules. that the kidney of the human subject is lobulated in the embryo, which condition is persistent in some mammals (ruminants, etc.). As the lungs are the organs employed especially for the elimination of carbonic anhydride, so the kidneys are above all others the excretors of the nitrogenous waste products of the body chiefly in the form of uric acid or urea. Before treating of secretion by the kidney it will be well to examine into the phys- ical and chemical properties of urine with some detail, especially on account of its great importance in the diagnosis of disease. EXCRETION BY THE KIDNEY. 419 URINE CONSIDERED PHYSICALLY AND CHEMI- CALLY. Urine is naturally a fluid of very variable composition, espe- cially regarded quantitatively — a fact to be borne in mind in considering' all statements of the constitution of this fluid. Specific Gravity. — Urine must needs be heavier than water, on account of the large variety of solids it contains. The aver- age specific gravity of the urine for the twenty -four hours is in man 1015 to 1020 ; in the horse, 1030 to 1060 ; in the ox, 1020 to 1030 ; in the sheep and goat, 1005 to 1015 ; in the pig, 1010 to 1015 ; in the dog, 1030 to 1050. It is lowest in the morning and varies greatly with the quantity and kind of food eaten, the ac- tivity of the lungs and especially of the skin, etc. Color. — Some shade of yellow, which is also very variable, being increased in depth either by the presence of an excess of pigment or a diminution of water. In herbivora the urine is turbid, and may darken on exposure to the air. The reaction of human urine is acid, owing to acid salts, especially acid sodium phosphate (NafLPQ,). In the carnivora it is strongly acid ; in the herbivora, alkaline. The reaction of urine depends largely though not wholly on the character of the food. Quantity. — This is, of course, like the specific gravity, highly variable, and frequently they run parallel with each other. The following tabular statement will prove useful for refer- ence: Composition of the Urine (Boussingault). Horse.* Cow.t Pigt Urea 31-0 4-7 20-1 15-5 4-2 10-8 1-2 0-7 1-0 o-o 910-0 18-5 16-5 17-2 16-1 4-7 06 3-6 1-5 trace. o-o 921-3 4-9 Potassium hippurate o-o Alkaline lactates. Potassium bicarbonate 10-7 Magnesium carbonate 0-9 Calcium carbonate Potassium sulphate 20 Sodium chloride 1-3 01 1-0 Water and substances undetermined. 979-1 Total 1000-0 1000-0 1000-0 * Diet of clover, grass, and oats. X Diet of potatoes, cooked. f Diet of hay and potatoes. 420 COMPARATIVE PHYSIOLOGY. Nitrogenous Crystalline Bodies.— These are the derivatives of the metabolism of the body, and not in any appreciable de- gree drawn froni the food itself. Besides urea, and of much less importance, occurring in small quantities, are bodies that may be regarded as less oxidized forms of nitrogenous metabolism, such as creatinin, xanthin, hypoxanthin (sarkin), hippuric acid, ammonium oxalurate, and urea, CO > tvttt 2 The latter was first prepared artificially from ammonium cyanate, |jtt I O, with which it is isomeric. The quantity of urea is generally in inverse proportion to that of hippuric acid, and varies much with the diet in the herbivora. The richer in proteids the diet, as when oats are fed, the greater the quantity of urea. In the horse this proportion varies with the ordinary diet between 2*5 and 4"0 per cent. When air has free access to urine for some time in a warm room, the urea becomes ammonium carbonate by hydration, probably owing to the influence of micro-organisms, thus: CO (NH 2 )o+ 2 H,0 = (NH 4 ) 2 C0 3 ; hence the strong ammoniacal smell of old urine, urinals, etc. Uric acid (C5H4N4O3) occurs sparingly (see table), combined with sodium and ammonium chiefly as acid salts. Non-nitrogenous Organic Bodies.— A series of well-known aromatic bodies occurs in urine, especially in that of the horse, cow, etc. These are phenol, cresol, pyrocatechin, etc., which occur not free, but united with sulphuric acid. Inorganic Salts. — These are mostly in simple solution, in urine, and not as in some other fluids of the body, united with proteid bodies. The salts are chlorides, phosphates, sulphates, nitrates, and carbonates ; the bases being sodium, potassium, calcium, magnesium. The imosphates are to be traced to the food, to the phosphorus of proteids, and to phosphorized fats (lecithin). The sulphates are derived from those of the food and from the sulphur of the proteids of the body. The greater part of the carbonates is supplied directly by the food. In the horse the salts of potassium and calcium (CaCOa), are abundant; while in the dog magnesium and calcium salts abound as sul- phates and phosphates. Doubtless many bodies appear either regularly or occasion- ally in urine that have so far escaped detection, which are, like the poisonous exhalations of the lungs, not the less important because unknown to science. EXCRETION BY THE KIDNEY. 421 Abnormal Urine. — There is not a substance in the urine that does not vary under disease, while the possible additions act- ually known are legion. These may be derived either from the blood or from the kidneys and other parts of the urinary tract. The kidneys seem to take upon themselves more readily than any other organ the duty of eliminating foreign matters from the body. But this aspect of the subject is too wide for detailed consideration in this work. The student of medicine should be thoroughly familiar with the urine in its normal condition before he enters upon the examination of the variations produced by disease. This is not difficult, and much of it may be carried out with but a meager supply of apparatus. For this purpose, however, we recom- mend some work devoted to the chemical and microscopic study of the urine. It greatly assists to remember a few points in regard to solu- bilities. From a physiological point of view, the urine and its variations, as the result of changes in the organism, may be ob- served with advantage in one's own person — eg., the influence of food and drink, temperature, emotions, etc. Comparative. — In fishes, reptiles, and birds, uric acid re- places urea, and is very abundant. In these animals most of this substance is white. The urine is passed with the fasces. In certain groups of invertebrates uric acid seems to be a normal excretion. THE SECRETION OF URINE. By means of apparatus adapted to register the changes of volume the kidney undergoes, it is found that this organ not only responds to every general change in blood-pressure, but to each heart-beat — that is, its volume varies momentarily. This shows how sensitive it is to variations in blood-pressure. Theories regarding the secretion of urine may be divided into those that are almost wholly physical, partly physical, and purely secretory: 1. To the first class belongs that of Ludwig, which teaches that very dilute urine is separated from the blood in the glomeruli, and by a process of osmosis and absorption of water by the tubular capillaries is gradually concentrated to the normal. 2. As an example of the second class is that of Bowman, who maintained that the greater part of the water and some of the more soluble and diffusible salts 422 COMPARATIVE PHYSIOLOGY. are separated by the glomeruli but the characteristic constitu- ents of the urine by the epithelium of the renal tubules. 3. As an example of the third is the theory of Heidenhain, who attrib- uted little to blood-pressure in itself, and much, if not the whole, to the secreting activity of the epithelium of the tubules more particularly. This physiologist showed that while liga- BLOOD PRESSURE CURVE VVWvVWA^ Fig. 319. — Blood-pressure curve and curve of the volume of the kidney; T, time- curve, intervals indicate a quarter of a minute; A, abscissa (Stirling, after Koy). ture of a vein raised the blood-pressure within a glomerulus, it was not followed by any increase in the quantity of the secretion, but by its actual arrest. He also showed that injection of a col- ored substance (sodium sulphindigodate) into the blood, after the pressure had been greatly lowered by section of the spinal cord, led to its appearance in the urine ; and microscopic examination showed that it had passed through the epithelial cells of the tubules, not of the glomeruli. It is found, however, that after the removal of a ligature applied to the renal artery the urine is albuminous, showing plainly that the cells have been injured by the operation ; hence Heidenhain's experiment described above is not valid against the blood-pressure theory. Moreover, too much must not be inferred from the action of foreign substances under the ab- normal conditions of such an experiment. While some physi- ologists claim that the glomeruli are filtering mechanisms, they explain that filtration is not to be understood in its ordinary laboratory acceptation, but that the glomeruli discriminate as to what they allow to pass, yet they in no way explain how this is done. They make the whole process depend on blood- pressure, and attribute little special action to the flat epithelium of the Malpighian capsules. Though we can not admit the full force of Heidenhain's ex- periments as he interprets them, we still believe that his views are most in harmony with the general laws of biology and the EXCRETION BY THE KIDNEY. 423 special facts of renal secretion. More recently it lias been ren- dered clear that physical theories of the work of the kidney can not hold, even of the glomeruli, which are shown to be, as we should have expected, true secreting organs. Now, there can be no doubt that blood-pressure is a most important determin- ing condition here as in other secreting processes, in the mam- mal at all events ; but whether of itself or because of the influ- ence it has on the rapidity of blood-flow, it is difficult to deter- mine ; or rather whether solely to the latter, for that the con- stant supply of fresh blood is a regular condition of normal secretion there can be no doubt. Further, it seems probable that blood-pressure has more to do with the secretion of water than any other constituent of urine. But we maintain that it should be called a genuine secretion, and that nothing is gained by using the term " filtration " — on the contrary, that it is mislead- ing, and tends to divert attention from the real though often hidden nature of vital processes. The facts of disease and the evidence of therapeutics, we think, all favor such a view of the work of the kidneys. Nerves having an influence over the secretion of urine simi- lar to those acting on the digestive glands have not yet been determined. The powerful influence of emotion, especially well seen in the dog, over the secretion of urine shows that there must be nervous channels through which the nerve- centers act on the kidneys ; though whether the results are not wholly dependent upon vaso-motor effects may be considered as an open question by many. We think such a view im- probable in the highest degree. The most recent investigations would seem to show that the vaso-motor fibers run in the dor- sal nerves, especially the eleventh, twelfth, and thirteenth, in the dog, and that of these the vaso-constrictors are the best de- veloped. Pathological.— When the kidneys are excised, the ureters ligatured, or when the former are so diseased as to be inca- pable of performing their functions, death is the result, being preceded by marked depression of the brain-centers, passing into coma. Exactly which of the retained products brings about these results is not known. They are likely due to sev- eral, and it impresses on the mind the importance of those pro- cesses by which the constantly accumulating waste is elimi- nated. 424 COMPARATIVE PHYSIOLOGY. THE EXPULSION OF URINE. We now present in concise form certain facts on which to base opinions as to the nature of the processes by which the bladder is emptied. It will be borne in mind that the secretion of urine is con- stant, though of course very variable, that the urine is con- veyed in minute quantities by rhythmically contractile tubes (ureters) which open into the bladder obliquely ; and that the bladder itself is highly muscular, the cells being arranged both circularly and obliquely, with a special accumulation of the circular fibers around the neck of the organ to form the sphinc- ter vesicae. 1. It is found that the pressure which the sphincter of the bladder can withstand in the dead is much less (about one third) than in the living subject. 2. We are conscious of being able to empty the bladder, whether it contains much or little fluid. 3. We are equally conscious of an urgency to evacuation of the vesical contents, according to the fullness of the organ, the quality of the urine, and a variety of other conitions. 4. Emotions may either retard or render micturition urgent. 5. In a dog in which the cord has been divided in the dorsal region some months previously, micturition may be induced reflexly, as by sponging the anus. 6. In the paralyzed there may be retention or dribbling of urine. 7. In cases of urethral obstruction from a calculus, stricture, etc., there may be excess- ive activity of the muscular tissue of the bladder walls. 8. Evacuation of the bladder may occur in the absence of con- sciousness (sleep). The most obvious conclusions from these facts are that — 1. The urine finds its way to the bladder partly through muscular (peristaltic) contractions of the ureters, partly through gravity, in man at all events, and partly from the pressure within the tubules of the kidneys themselves. 2. The evacuation of urine may take place independently of the will (see 8), and is a reflex (5) act. 3. Micturition may be initiated by the will, which is usually the case, when by the action of the abdominal muscles a little urine is squeezed into the urethra, upon which afferent impulses set up contractions of the bladder by acting on the detrusor center of the cord and at the same time inhibit the center presiding over the sphincter (if such there be), permit- ting of its relaxation. 4. Emotions seem to interfere with the EXCRETION BY THE KIDNEY. 425 Fig. &0.— Superior and general view of the genito-urinary apparatus in the stallion with the axtenesr,(Chauveau). A, left kidney; B, right kiduev a b ureters- C C suprarenal capsules; D, bladder; E. E. testicles; e, head of ep'ididimis; ',' tail of epididimis; *, deferent canal; G, pelvic dilatation of deferent canal- H left seminal vesicle; the right has been removed, along with the deferent canal of 426 COMPARATIVE PHYSIOLOGY. same side, to show insertion of ureters into bladder; I, prostate; J, Cowper's glands; K, membranous or intra-pelvic portion of urethral canal; L, its bulbous portion; M, cavernous body of penis; in, m, its roots; N, head of penis; 1, ab- dominal aorta; 2,2, arteries (renal) giving off principal capsular artery; 3, sper- matic artery; 4, common origin of umbilical and arteries of bulb; 5, umbilical artery; 6, its vesical branch; 7, internal artery of bulb; 8, its vesico-prostatic branch. ordinary control of the brain-centers over those in the spinal cord. 5. It may be assumed that the normal tone of the sphincter of the bladder is maintained reflexly by the spinal cord. The unwonted muscular contraction associated with an obstruction to the outflow of urine may be in part of nervous origin, but is also, in all probability, owing in some degree to the muscle-cells resuming an independent contractility, due to what we recognize as the principle of reversion. The same is seen in the heart, ureters, and similar structures. Pathological. — There may be incontinence of urine from pa- ralysis, the cerebral centers being unable to control those in the spinal cord. Dribbling of urine may be due to retention in the first instance, the tone of the sphincter being finally over- come, owing to increase of pressure within the bladder. Over- distention of the bladder may arise in consequence of lack of tone in the muscular walls, though this is rare. Strangury is due to excessive action of the walls of the bladder and the sphincter, brought about reflexly, when the organ is unduly irritable, as in inflammation, after the abuse of certain drugs (cantharides), etc. Comparative. — In man the last drops of urine are expelled by the action of the bulbo-cavernosus muscle and perhaps some others. In the dog and many other animals the regulated and voluntary use of this muscle, marked in a high degree, produces that interrupted flow so characteristic of the micturition of these animals. Summary. — Urine is in mammals a fluid of variable specific gravity and reaction, yellow in color, and containing certain salts, pigments, and nitrogenous bodies. The chief of the latter is urea. The kidneys and skin especially supplement one another, and normally great activity of the one implies lessened ac- activity of the other. This is availed of in the treatment of dis- ease. Both the Malpighian capsules and the renal tubules have a true secretory function, though the larger pai^t of the water of urine is secreted by the former. Blood-pressure is an important EXCRETION BY THE KIDNEY. 427 condition of secretion, though it is likely that this is so chiefly because it favors a rapid renewal of the blood circulating through the organ. Whether there are nerves that influence secretion directly, as in the case of the skin, is not determined. Suppression of the renal functions leads to symptoms in which the nervous system is recognized as suffering to the extent often of coma, ending in death. The urine of most other animals is more concentrated than that of man ; this secretion in carnivora being acid, and in herbivora alkaline in reaction. THE METABOLISM OF THE BODY. In the widest sense the term metabolism may he conven- iently applied to all the numerous changes of a chemical kind, resulting from the activity of the protoplasm of any tissue or oi*gan. In a more restricted meaning it is confined to changes undergone hy the food from the time it enters till it leaves the body, in so far as these are not the result of obvious mechanical causes. The sense in which it is employed in the present chapter will be plain from the context, though usually we shall be concerned with those changes effected in the as yet compara- tively unprepared products of digestion, by which they are ele- vated to a higher rank and brought some steps nearer to the final goal toward which they have been tending from the first. As yet our attempts to trace out these steps have been little better than the fruitless efforts of a lost traveler to find a road, the general direction of which he knows, but the ways by which it is reached only the subject of plausible conjecture. We shall therefore not discuss the subject at length from this point of view. THE METABOLISM OF THE LIVER. This organ has two well-recognized functions : 1. The for- mation of bile. 2. The formation of glycogen. We have already considered the first. Glycogen may be obtained from the liver of mammals as a whitish amorphous powder, having the chemical composition of starch, and has in fact been termed animal starch. By appropriate treatment it may be converted into sugar by a process of hydration (CoHioOc + H2O = CoHiaOo). The principal facts as to the storage of glycogen in the liver may be briefly stated thus : 1. Glycogen has been found in the liver of a large number THE METABOLISM OF THE BODY. 429 of groups of animals including some invertebrates. 2. Among mammals it is most abundant wben the animal feeds largely on carbohydrates. 3. It is found in the liver of the carnivora, and in those of omnivora, when feeding exclusively on flesh. 4. When an animal starves (does not feed), the glycogen grad- ually disappears. 5. A fat-diet does not give rise to glycogen. 6. During early foetal life glycogen is found in all the tissues, but later it is restricted more and more to the liver, though even in adixlts it is to be found in various tissues, especially the muscles, from which it is almost never absent. From the facts the inference is plain that glycogen is formed from carbohydrate materials ; or, to be rather more cautious, that the formation of this substance is dependent on the pres- ence of such material in the food. The Uses of Glycogen. — No positive statement can be made on this subject. It is generally believed to be transformed into sugar. What is the fate of the transformed glycogen ? What be- comes of the sugar ? We can answer, negatively, that it is not used up in the blood, it is not oxidized there ; but by what tissues it is used or how it is made available in the economy is a subject on which we are profoundly ignorant. The presence of so much glycogen in the partially developed tissues of the foetus points to its importance, and suggests its being a crude material which is laid up to be further elaborated, as in vege- tables, by the growing protoplasm. METABOLISM OF THE SPLEEN. The physiological significance of the peculiar structure of this organ, though not yet fully understood, is much plainer than it was till recently. The student is recommended to look carefully into the histology of the spleen, especially the dis- tribution of its muscular tissue and the peculiarities of its blood-vascular system. It has already been pointed out that there is little doubt that leucocytes are manufactured here even in the adult, possibly also red cells ; and that the latter are dis- integrated, and the resulting substances worked over, possibly by this organ itself. This view is rendered probable, not only by microscopic study of the organ, but by a chemical examina- tion of the splenic pulp; for a ferruginous proteid, and numer- ous pigments, of a character such as harmonizes with this con- ception, are found, 430 COMPARATIVE PHYSIOLOGY. The fact that the spleen-pulp does not agree in composition with either blood or serum ; that it abounds in extractives such Fig. 321. — Vertical section of a small superficial portion of human spleen, seen with low power (Schafer). A, peritoneal and fibrous covering; b, trabecules; c, c, Mal- pighian corpuscles, in one of which an artery is seen cut transversely, in the other, longitudinally; d, injected arterial twigs; e, spleen-pulp. as lactic, butyric, formic, and acetic acids, together with inosit, xanthin, hypoxanthin, leuciu and uric acid — points to its being Pio. 322.— Thin section of spleen-palp, highly magnified, showing mode of origin of a small vein in the interstices of pulp (Scnafer). v, vein filled with blood-corpus- cles, which are in continuity with others, hi. filling up interstices of retiform tissue of pulp; w, wull of vein. The shaded bodies among red corpuscles are pale cor- puscles. THE METABOLISM OF THE BODY. 431 the seat of a complex metabolism, though neither the changes themselves nor their purpose are well understood. Nevertheless, it must be admitted that to recognize this was a great advance upon the view that the spleen had no impor- tant function, and that this was shown by the removal of the organ without change in the ani- mal's economy. But to believe that there are no such changes, and to have clear proof of it, are two different things. As a matter of fact, closer study does show that in some animals there are alterations in the lymphatic glands and bone - marrow, which organs are undoubtedly manu- facturers of blood- cells. These changes Fig. 323. — Portion of spleen of cat, showing Malpighian (lymphatic) corpuscle (after Cadiat). A, artery around which corpuscle is placed; B, meshes of spleen-pnip, injected; C, artery of corpuscle ramify- ing in lymphatic tissue. The clear space around corpuscle represents a lymphatic sinus. are unquestionably compensatory, and that other similar ones corresponding to comparatively unknown functions of the spleen have not as yet been discovered is owing likely to our failures rather than their real absence. We dwell for a mo- ment on this, because it illustrates the danger of the sort of rea- soning that has been applied in the case of this and other or- gans; and it shows the importance of recognizing the force of the general pi'inciples of biology, and also the desirability of refraining from drawing conclusions that are too wide for the premises. In every department of physiology it must be more and more recognized that what is true of one group of animals is not necessarily true of another, or even of other individuals, though the differences in the latter case are of course usually less marked. We have referred to this be- fore, and shall do so again, for it is as yet but too little con- sidered. 432 COMPARATIVE PHYSIOLOGY. THE CONSTRUCTION OF FAT. It is a well-known fact that, speaking generally, a diet rich in carbohydrates favors fat formation, hoth in man and other animals; though it is not to be forgotten that many persons seem to be unable to digest such food, or, at all events, to as- similate it so as to form fat to any great extent. Persons given to excessive fat production are as frequently as not sparing users of fat itself. It is possible in man and probable in ruminants that fer- mentations may occur in the intestines giving rise to fatty acids which are possibly converted into fats by the cells of the villi or elsewhere. Certain feeding experiments favor the view that carbobydrates may be converted into fat or in some way give rise to an increase in this substance ; for it is to be borne in mind that fat may arise from a certain diet in various ways other than its direct transformation into this substance itself. There are certain facts that make it clear that fat can be formed from proteids: 1. A cow will produce more butter than can be accounted for by the fat in her food alone. 2„ A bitch which had been fed on meat produced more fat in her milk than could have been derived directly from her food, and this, when the animal was gaining in weight, which is usually to be traced to the addition of fat ; so that the fat of the milk was not, in all probability, derived from that of the dog's body ; and, as will be seen presently, can be accounted for without such a supposition. 3. It has been shown by analysis that 472 parts of fat were deposited in the body of a pig for every 100 in its food. These facts of themselves suffice to show that fat can be formed from proteid, or at least that proteid food can of itself give rise to a metabolism, resulting in fat formation; and the latter is probably the better way to state the case in the present condition of knowledge. That fat is a real formation, dependent for its composition on the work of living tissues, is clear from the well-known fact that the fat of one animal differs from that of another, and that it preserves its identity, no matter what the food may be, or. in what form fat itself may be provided. Certain constituents of the animal's fat may be wholly absent from the fat of its food, yet they appear just the same in the fat produced under such THE METABOLISM OF THE BODY. 433 diet. Even bees can construct their wax from proteicl, or use unlike substances, as sealing-wax. Pig. 324. — Section of mammary gland (udder and nipplel of cow (after Thanhoffcr)- Ma, substance of gland; N nipple; A, acini of gland; m. d, milk-ducts; C, milk- cisterns;/, folds in wide milk-ducts; S, section of sphincter muscle; s, external skin; n. m. d, narrow milk-duct in nipple. But histological examination of forming adipose tissue itself throws much light upon the subject. Fat-cells are those in which the protoplasm has been largely replaced by fat. The 28 434 COMPARATIVE PHYSIOLOG-Y. latter is seen to arise in the former as very small globules which run together more and more till they may wholly re- place the original protoplasm. The history of the mammary gland is, perhaps, still more instructive. In this case, the appearance of the cells during lactation and at other periods is entirely different. Fat may he seen to arise within these cells and be extruded, perhaps in the same way as an Amoeba gets rid of the waste of its food. So far as the animal is concerned, milk is an excretion in a limited sense. It is, in the nature of the case, impossible to follow with the eye the formation and separation of milk-sugar, casein, etc. Fig. 325. — I. Acinus from mamma of a bitch when inactive (after Heidenhain). II. During secretion of milk, a, b, milk-globules; c, d, e, colostrum-corpuscles; /, pale cells. But the whole process is plainly the work of the cells, and in no mechanical sense a mere deposition of fat, etc., from the blood ; and the same view applies to the construction of fat by connective (adipose) tissue. Whether fat, as such, or fatty acid, is dealt with without being built up into the protoplasm of the cell, is not known ; but, taking all the facts into the account, and considering the behavior of cells generally, it seems most natural to regard the construction of fat as a sort of secretion or excretion. To sup- pose that a living cell acts upon material in the blood as a workman in a factory on his raw material, or even as a chemist does in the laboratory, seems to be too crude a conception of vital processes. Until it can be rendered very much clearer than at present, it is not safe to assume that their chemistry is our chemistry, or their methods our methods. It maybe so; but let us not, in our desire for simple explanations or undue haste to get some sort of theory that apparently fits into our THE METABOLISM OF THE BODY. 435 own knowledge, assume it gratuitously, in the absence of the clearest proofs, especially when our failures on this supposition are so numerous. We may say, then, that fat is not merely selected from the blood,' but formed in the animal tissues ; that fat formation Fig. 326 —Microscopic appearances of— I. milk; II, cream; III, butter; IV, colostrum of mare; V, colostrum of cow (after Thanhoffer). may take place when the food consists largely of carbohydrates, when it is chiefly proteid, or when proteid and fatty. In other words, fat results from the metabolism of certain cells, which is facilitated by the consumption of carbohydrate and fatty food, but is possible when the food is chiefly nitrogenous. We must, however, recognize differences both of the species and the individual in this respect, as to the extent to which one kind of food or the otber most favors fat formation (excretion). The use of the adipose tissue as a packing to prevent undue escape of heat is evident ; but more important purposes are probably served, as will appear from later considerations. Pathological.— Excessive fat formation, leading to the ham- pering of respiration, the action of the muscles, and, to a certain extent, many other functions of the body, does not arise in man 436 COMPARATIVE PHYSIOLOGY. usually till after middle life, when the organism has seen its best days. It seems to indicate, if we judge by the frequency of fatty degeneration after disease, that the protoplasm stops short of its best metabolism, and becomes degraded to a lower rank ; for certainly adipose tissue does not occupy a high place in the histological scale. Such pathological facts throw a good deal of light upon the general nature of fat excre- tion, as it would be better to term it, perhaps, and seem to warrant the view that we have presented of the metabolic pro- cesses. Although the nerves governing the secretion of milk have not been traced, there can be no doubt that the nervous system controls this gland also. The influence of the emotions on both the quantity and quality of the milk in the human subject and in lower animals is well known. Comparative. — While breeders recognize certain foods as tending to fat formation and others to milk production, it is interesting to note that their experience shows that race and individuality, even on the male side, tell. The same conditions being in all respects observed, one breed of cows gives more and better milk than another, and the bull is himself able to transmit this peculiarity ; for, when crossed with inferior breeds, he improves the milking qualities of the latter. Individual differences are also well known. THE STUDY OF THE METABOLIC PROCESSES BY OTHER METHODS. It will be abundantly evident that our attempts to follow the changes which the food undergoes from the time of its introduction into the blood until it is removed in altered form from the body has not been as yet attended with great success. It is possible to establish relations between the ingesta and the egesta, or the income and output which have a certain value. It is important, however, to remember that, when quantitive estimations have to be made, a small error in the data becomes a large error in the final estimate ; one untrue assumption may vitiate completely all the conclusions. In discussing the subject we shall introduce a number of tables, but it will be remembered that the results obtained by one investigator differ from those obtained by another ; and tbat in all of tbem there are some deviations from strict ac- THE METABOLISM OP THE BODY. 437 curacy, so that the results must he regarded as only approxi- mately correct. It is, however, we think, better to examine such statistical tables of analyses, etc., than to rely on the mere verbal statement of certain results, as it leaves more room for individual judgment and the assimilation of such ideas as they may suggest outside of the subject in hand. The subject of diet is a very large one ; but it will be evi- dent on reflection that, before an average diet can be prescribed on any scientific grounds, the composition of the body and the nature of those processes on which nutrition generally depends must be known. . Not a little may be learned by an examina- tion of the behavior of the body in the absence of all diet, when it may be said to feed on itself, one tissue supplying another. All starving animals are in the nature of the case carnivorous. For the cat an analysis has yielded the following : Muscle and tendons. 45 "0 per cent. Bones 147 Skin 12-0 Mesentery and adipose tissue 3*8 Liver 4*8 Blood (escaping at death) 6'0 Other organs and tissues 13'7 The large proportional weight of the muscles, the similarly large amount of blood they receive, which is striking in the case* of the liver, also suggest that the metabolism of these structures is very active, and we should expect that they would lose greatly during a starvation period. It is a matter of common observation that animals do lose weight and grow thin under such circumstances, which means that they must lose in the muscles and the adipose tissue. Attempts have been made to determine exactly the extent to which the various tissues do suffer during complete abstinence from food, and this may be gathered from the table given below. It will not be forgotten that about three fourths of the body is made up of water, so that the loss of a lai'ge amount of the latter during starvation is to be expected. In the case of a cat during a starvation period of thirteen days 734 grammes of solids were lost, of which 248 grammes were fat and 118 muscle — i. e., about one half of the total loss was referable to these two tissues alone. The other tissues lost as follows, estimated as dry solids : 438 COMPARATIVE PHYSIOLOGY. Adipose tissues 97 - per cent. Spleen 63 - l " Liver 56-6 Muscles 30-2 Blood 17-6 Brain and spinal cord - " It will be observed (a) that the loss of the fatty tissue was greatest, nearly all disappearing ; (b) that the grandular struct- ures were next in order the greatest sufferers ; (c) that after them come the skeletal muscles. Now, it has been already seen that these tissues all engage in an active metabolism with the exception of adipose tissue. The small loss on the part of the heart, which is still less for the nervous system, is especially noteworthy. The loss of adi- pose tissue is so striking that we must regard it as an especially valuble storehouse of energy, available as required. When we turn to the urine for information, it is found that in the above case 27 grammes of nitrogen were excreted and almost entirely, of course, in the form of urea; and since the loss of nitrogen from the muscles amounted to 15 grammes, it will appear that more than one half of the nitrogenous excreta is traceable to the metabolism of muscular tissue. It has been customary to account for the urea in two ways : first, as derived from the metabolism of the tissues as stich, and continuously throughout the whole starvation period ; and, secondly, from a stored surplus of proteid which was assumed to be used up rapidly during the early clays of the fasting, and was the luxus consumption of certain investigators. Comparative. — Experiment has shown that the length of time during which different groups of animals can endure com- plete withdrawal of food is very variable, and this applies to individuals as well as species. That such differences hold for the human subject is well illustrated by the history of the sur- vivors of wrecks. Making great allowances for such devia- tions from any such results as can be established by a limited number of experiments, it may be stated that the human being succumbs in from twenty-one to twenty-four days ; dogs in good condition at the outset in from twenty-eight to thirty days; small mammals and birds in nine days, and frogs in nine months. Very much depends on whether water is allowed or not — life lasting much longer in the former case. The very young and the very old yield sooner than persons of middle THE METABOLISM OF THE BODY. 439 age. It has been estimated that strong adults die when they lose t 4 q of the body-weight. Well-fed animals lose weight more rapidly at first than afterward. Diet. — All experiments and observations tend to show that an animal can not remain in health for any considerable period without having in its food proteids, fats, carbohydrates, and salts; indeed, sooner or later deprivation of any one of these will result in death. Estimates based on many observations have been made of the proportion in which these substances should enter into a normal diet. For the herbivora from 1 to 8-9 (some claim 1 to 5|) is the estimated ratio of nitrogenous to non-nitrogenous foods ; and 2 of the former to 1 of fat. One conclusion that is obvious from analysis of foods is that, in order to obtain the amount of proteids needed from certain kinds, enormous quantities must be eaten and digested ; and as there would be in such cases an excess of carbohydrates, fats, etc., unnecessary work is entailed upon the organism in order to dispose of this ; so that to feed a working horse entirely on grass, a dog wholly on porridge, or a man on bread would be very unwise. FEEDING EXPERIMENTS (Ingesta and Egesta). If all that enters the body in any form be known, and all that leaves it be equally well known, conclusions may be drawn in regard to the metabolism the food has undergone. The pos- sible sources of fallacy will appear as we proceed. The ingesta, in the widest sense, include the respired air as well as the food ; though from the latter must be subtracted the waste (undigested) matters that appear in the fasces. The ingesta when analyzed include carbon, hydrogen, oxygen, ni- trogen, sulphur, phosphorus, water, and salts, their source being the atmosphere and the food-stuffs. The egesta, the same, and chiefly in the form of carbonic an- hydride, of water from the lungs, skin, alimentary canal, and kidneys, of salts and water from the skin and kidneys, and of nitrogen, chiefly as urea almost wholly from the kidneys. Usu- ally in experimental determinations the total quantity of the nitrogen of the urine is estimated. If free nitrogen plays any part in the metabolic processes it is unknown. 440 COMPARATIVE PHYSIOLOGY. A large number of feeding experiments have been made by different investigators, chiefly, though. not exclusively, on the lower animals. Some such method as the following has usu- ally been pursued: 1. The food used is carefully weighed and a sample of it analyzed, so that more exact data may be obtained. 2. The amount of oxygen used and carbonic anhydride exhaled, as well as the amount of water given off in any form is esti- mated. 3. The amount of the nitrogenous excreta is calculated, chiefly from an analysis of the urine, though any loss by hair, etc., is also to be taken into account. It has been generally assumed that the nitrogen of the ex- creta represents practically the whole of that element entering the body. This has been denied by some investigators. The respiratory products have been estimated in various ways. One consists in measuring the quantity of oxygen sup- plied to the chamber in which the animal under observation is inclosed, and analyzing from time to time samples of the air as it is drawn through the chamber; and on these results the total estimates are based. It will appear that even errors in calculating the composi- tion of the food — and this is very variable in different samples, e. g., of flesh; or any errors in the analysis of the urine, or in the more difficult task of estimating the respiratory products, may, when multiplying to get the totals, amount to serious de- partures from accuracy in the end ; so that all conclusions in such a complicated case must be drawn with the greatest cau lion. But it can not be doubted that such investigations have proved of much practical and some scientific value. The labor they entail is enormous. Nitrogenous Equilibrium.— It is possible to so feed an ani- mal, say a dog, that the total nitrogen of the ingesta and egesta shall be equal; and this may be accomplished without the ani- mal losing or gaining weight appreciably or again while he is gaining. If there be a gain, it can usually be traced to the formation of fat, so that the proteid, we may suppose, has been split up into a part that is constructed into fat and a part which is represented by the urea, the fat being either used up or stored in the body. Moreover, an analysis of a pig that had been fed on a fixed diet, and a comparison made with one of the same litter killed at the commencement of the experiment, showed that of the dry nitrogenous food only about seven per cent in this animal, and four per cent in the sheep had been laid away THE METABOLISM OF THE BODY. 441 as dry proteid. It is perfectly plain, then, that proteid diet does not involve only proteid construction within the body. Comparative. — The amount of flesh which a dog, being a carnivorous animal, can digest and use for the maintenance of his metabolic processes is enormous; though it lias been learned that ill-nourished dogs can not even at the outset of a feeding experiment of this kind maintain the equilibrium of their body weight on a purely flesh diet (fat being excluded). They at once commence to lose weight — i. e., they draw upon their own limited store of fat. The digestion of herbivora being essentially adapted to a vegetable diet, they can not live at all upon flesh, while a dog can consume for a time without manifest harm ? 1 5 to ^ of its body-weight of this food. Man, when fed exclusively on meat soon shows failure, he being unable to digest enough to supply the needed cai'bohy- drates, etc. But the large amount of urea in the urine of car- nivorous animals generally, and the excess found in the urine of man when feeding largely on a flesh diet, show that the pro- teid metabolism is under such circumstances very active. It is also a well-known observation that carnivorous ani- mals (dogs) are more active and display to a greater extent their latent ferocity, evidence of their descent from wild car- nivorous progenitors, when like them they feed very largely on flesh. The evidence seems to point pretty clearly to the con- clusion that a nitrogenous (flesh) diet increases the activity of the vital processs of the body, and especially the proteid me- tabolism. But in all these considerations it must be borne in mind that the metabolic processes go on in the tissues and not in the blood, and probably not in the lymph. Not that these fluids (tissues) are without their own metabolic processes for and by themselves; but what is meant to be conveyed is that the met- abolic processes of the body generally do not take place in the blood. The Effects of Gelatin in the Diet.— Actual experiment shows that this substance can not take the place of proteid, though it also makes it evident that less of the latter suffices when mixed with a certain proportion of gelatin. It will be borne in mind that ordinary flesh contains, as we find it naturally in the car- cass, not only some fat, but a good deal of fibrous tissue, which can be converted by heating into gelatin. 442 COMPARATIVE PHYSIOLOGY. Fats and Carbohydrates.— It is a matter of common observa- tion and of more exact experiment tbat even a carnivorous ani- mal thrives better on a diet of fat and lean meat than on lean flesh alone. Thus, it has been found that nitrogenous equi- librium was as readily established by a due mixture of fat and lean as upon twice the quantity of lean flesh alone. It is plain, then, that the metabolism is actually slowed by a fatty diet. When an animal is given but little fat, none whatever is laid up, but all the carbon of the fat can be accounted for in the excreta, chiefly as carbonic anhydride. Again, the fatty por- tion remaining constant, it has been found that increasing the proteid leads not to a storage of the carbon of the proteid ex- cess, but to an increased consumption of this element. It is then possible to understand how excessive consumption of pro- teids may lead, as seems to be the case, to the disappearance of fat and loss of weight, so that a proteid diet increases not only nitrogenous but non-nitrogenous metabolism. That carbohy- drates mixed with a due proportion of the other constituents of a diet do increase fat formation is well established ; though there is no equally well-grounded explanation of how this is accomplished. Upon the whole, it seems most likely that fat can be directly formed from carbohydrates, or, at all events, that they directly give rise to fat if they are not converted themselves into that substance. Comparative, — It is found that there are relations between the food used and the quantity of carbonic dioxide expelled which are instructive. The formula following show the amount of oxygen necessary to convert a starch and a fat into carbonic anhydride and water : 1. CoH 10 05 + 12 =6(C0 2 )+5(H 2 0). 2. CeiH.cOe + O 10u = 57(C0 2 ) + 52(H 2 0). It will be observed that in the first case the oxygen used to oxidize the starch has all reappeared as C0 2 , while in the sec- ond only 114 parts out of 160 so reappear. As a matter of fact, more of the oxygen used does in herbivora reappear as C0 2 , and less as water, while the reverse holds for the carnivora, the proportion being, it is estimated, as ninety to sixty per cent. This is to be explained by the character of the food in each instance, for this relation no longer holds during fasting, when the herbivorous animal becomes carnivorous in the sense that it consumes its own tissues. THE METABOLISM OF THE BODY. 443 The Effects of Salts, Water, etc., in the Diet.— When we come to inquire as to the part salts play when introduced into the blood, we soon find that our knowledge is very limited. Sulphur, and especially phosphorus, seem to have some im- portant use which quite eludes detection. It is important to remember that certain salts are combined with proteids in the body, possibly to a greater extent than we can learn from the mere analysis of dead tissues. Pathological. — The withdrawal of any of the important salts of the body soon leads to disease, clear evidence in itself of their great importance. This is notably the case in scurvy, in which disease the blood seems to be so disordered and the nutrition of the vessel- walls so altered that the former (even some of the blood-cells) passes through the latter. Water. — The use of water certainly has a great influence over the metabolic processes of the body. The temporary ad- dition or withdrawal of even a few ounces of water from the regular supply of a dog in the course of a feeding experiment greatly modfies the results obtained for the time. It is well known that increase of water in the diet leads to a correspond- ing increase in the amount of urea excreted. It is likely that even yet we fail to appreciate fully the great part which water plays in the animal economy. THE ENERGY OF THE ANIMAL BODY. As already explained, we distinguish between potential or latent and actual energy. All the energy of the body is to be traced to the influence of the tissues upon the food. Energy may be estimated as mechanical work or as heat, and the one may be converted into the other. All the processes of the organism involve chemical changes, and a large proportion of these are of the nature of oxidations ; so that speaking broadly, the oxidations of the animal body are the sources of its energy ; and in estimating the quantity of energy, either as heat or work, that a given food-stuff will produce, one must consider whether the oxidative processes are complete or partial ; thus, in the case of proteid food, if we suppose that the urea excreted represents the form in which the oxidative processes end or are arrested, we must, in estimating the actual energy of the proteid, sub- tract the amount of energy that would be produced were the urea itself completely oxidized (burned.) u± COMPARATIVE PHYSIOLOGY. If the amount of heat that a body will produce in its com- bustion be known, then by the law of the conversion and equiv- alence of energy the mechanical equivalent can be estimated in that particular case. The heat-producing power of different substances can be directly learned by ascertaining the extent to which, when fully burned (to water and carbonic anhydride), they elevate the temperature to a given volume of water ; and this can at once be translated into its mechanical equivalent of work, so that we may say that one gramme of dry proteid would give rise to a certain number of gramme-degrees of heat or kilogramme- metres of work. A few figures will now show the relative values of certain food -stuffs : 1 gramme proteid 1 gramme urea Available energy of the proteid Gram.-deg. 5,103 735 4,308 Kilomet. 2,161 311 1,850 The reason of the subtraction has been explained above. Taking another diet in regard to whicb the estimates differ somewhat from those given previously, but convenient now as showing how equal weights of substances produce very dif- ferent amounts of energy, we find that—. 100 grammes proteid yield 100 grammes fat yield . . . 240 grammes starch yield Total Gram.-deg. 430,800 900,900 938,880 2,281,580 Kilomet. 185,000 384,100 397,080 900,780 In other words nearly a million kilogramme-metres of en- ergy are available from the above diet for one day, provided it be all oxidized in the body. Food-stuffs, then, with the oxygen of the air, are the body's sources of energy. What are the forms in which its expendi- ture appears % We may answer at once heat and mechanical work ; for it is assumed that internal movements as those of the viscera, and all the friction of the body, all its molcular motion, all secretive processes, are to be regarded as finally THE METABOLISM OF THE BODY. 445 augmenting the heat of the hody. Heat is lost by the skin, lungs, urine, and faeces. The division of foods into heat-producers and tissue-builders is unjustifiable, as will appear from what has just been stated, as well as from such facts as the production of fat from proteid food, thus showing that the latter is indirectly a producer of carbonic anhydride, assuming that fat is oxidized into that substance. ANIMAL HEAT. Though a large part of the heat generated within the body is traceable to oxidations taking place in the tissues, it is better to speak of the heat as being the outcome of all the chemical processes of the organism ; and though heat may be rendered latent in certain organs for a time, in the end it must appear. While all the tissues are heat-producers (thermogenic), the ex- tent to which they are such would depend, we should suppose, upon the degree to which they were the seat of metabolic pro- cesses ; and actual tests establish this fact. Thus, among glands the liver is the greatest heat-producer ; hence the blood from this organ is the warmest of the whole body. The muscles also are especially the thermogenic tissue. The temperature of the blood in the hepatic vein is wanner than that in the portal, a clear evidence that the metabolism of this organ has elevated the temperature of the blood flowing through it. The temperature of the blood (its own metabolism being slight) is a pretty fair indication of the resultant effect of the production and the loss of heat. For obvious reasons, the temperature of different parts of the body of man and other animals varies. The statements of observers in regard to the temperature of various animals and of different parts of the body disagree in a way that would be puzzling, were it not known how difficult it is to procure perfectly accurate thermometers, not to mention individual differences. The axillary temperature is in man about 37° C. (98 6 F.); that of the mouth a little higher, and of the rectum or vagina slightly more elevated. The mean temperature of the blood is placed at 39° C. (102'2 F.). Comparative. — The temperature of various groups of animals has been stated to be as follows: Hen and pigeon, 42° (107'6 F.) ; swallow, 4403° (111-25 F.) ; dolphin, 35'5 (95 "9 F.) ; mouse, 41-1° 446 COMPARATIVE PHYSIOLOGY. (106 F.); snakes, 10° to 12° (50 to 53"6 F.) ; but higher in large specimens (python). Cold-blooded animals have a tempera- ture a little higher (less than 1° C. usually) than the surround- ing air. During the swarming of bees the hive temperature may rise from 32° to 40° (89 -6 to 104 F.). All cold-blooded animals have probably a higher temperature in the breeding- season. In our domestic mammals the normal temperature is not widely different from that of man. In the horse the aver- age is 37-5° to 38° (99 "5 to 100-4 F.) ; in the ass, 38° to 39-5° (100-4 to 103 F) ; in the ox, 38° to 38-5° (100*4 to 101 "3 F.) ; in the sheep and pig, 39° to 40° (102-2 to 104 F.) ; in the cat, 38'5° to 39° (101*3 to 102-2 P.); in the dog, 38-5° (101-3 P.). Variations in the average temperature are dependent on numerous causes which may affect either the heat production or heat loss : 1. Change of climate has a very slight but real influence, the temperature being elevated a fraction of a degree when an individual travels from the poles toward the equator, and the same may be said of the effect of the temperature of a warm summer day as compared with a cold winter one. The wonder is that, considering the external temperature, the vari- ation is so light. 2. Starvation lowers the temperature, and the ingestion of food raises it slightly, the latter increasing, the former decreasing, the rate of the metabolic processes. 3. Age has its influence, the very young and the very old, in whom metabolism (oxidation) is feeble, having a lower temperature. This especially applies to the newly born, both among man- kind and the lower mammals; and, as might be supposed, the temperature falls during sleep, when all the vital activi- ties are diminished. The same remark applies with greater force to the hibernating state of animals. The temperature of man does not vary more than about 1° C. during the twenty- four hours. It will be inferred, from the facts and figures already cited, that different kinds of food have considerably different capacity for heat production. It is well known that an animal when working not only feels warmer, but actually produces more heat. It appears from a multitude of considerations that the body is like a steam-engine, producing beat and doing work ; but it is found that while a very good steam-engine, as a result of the chemical processes going on within it, converts £ of the poten- tial energy of its supplies into mechanical work, the other I THE METABOLISM OP THE BODY. 447 appearing as heat, the animal hody produces } as work and | as heat, from its income of food and oxygen. While it is perfectly clear that it is in the metabolic pro- cesses of the body that we must seek for the final cause of the heat produced, it is incumbent on the physiologist to explain the remarkable fact that the mammalian body maintains, under a changing external temperature and other climatic conditions, and with a varying diet, during rest and labor, a temperature varying within, usually, no more than a fraction of a degree centigrade. This we shall now endeavor to explain in part. The Regulation of Temperature.— It is manifest from the facts adduced that so long as life lasts heat is being of necessity constantly produced. If there were no provision for getting rid of a portion of this heat, it is plain that the body would soon be consumed as effectually as if it were placed in a furnace. We observe, however, that heat is being constantly lost by the breath, by perspiration (insensible), by conduction and radia- tion from the surface of the body, and periodically by the urine and faeces. We have seen that, while heat is being pro- duced in all the tissues and organs of the body, some are es- specially thermogenic, as the glands and muscles. The skin presents an extensive surface, abundantly supplied with blood- vessels, which when dilated may receive a large quantity of blood, and when contracted may necessitate a much larger in- ternal supply, in the splanchnic region especially. It is a mat- ter of common observation that, when an individual exercises, the skin becomes flushed, and so with the increased production of heat, especially in the muscles (see page 195), there is a pro- vision for unusual escape of the surplus ; at the same time sweat breaks out visibly, or if not, the insensible perspiration is generally increased ; and this accounts for an additional in- crement of loss ; while the lungs do extra work and exhale an increased quantity of aqueous vapor, so that in these various ways the body is cooled. Manifestly there is some sort of rela- tion between the processes of heat production and heat expendi- ture. The vaso-motor, secretory, and respiratory functions are modified. We may suppose that the various co-ordinations effected, chiefly at all events through the central nervous sys- tem, and not by the direct action of the heat upon local nerv- ous mechanisms, or the tissues themselves directly, are re- flexes. 448 COMPARATIVE PHYSIOLOGY. The 'production of heat, however, seems to be equally under the influence of the nervous system, though we know less about the details of the matter. A cold-blooded animal differs from a warm-blooded one in that its temperature varies more with the surrounding medium: hence the terms poikilothermer and homoiothermer for cold- blooded and warm-blooded, would be appropriate. Such an animal, as a frog or turtle, may have its chemical processes slowed or quickened, almost like those going on in a test-tube or crucible, by altering the temperature. Very differ- ent is it, as we have seen, in the normal state of the animal with any mammal. Hence hibernation or an allied state has be- come a necessary protection for poikilothermers, otherwise they would perish outright, and the groups become extinct in north- ern latitudes. It is plain that vaso-motor changes alone can not explain these effects ; and, though possibly a part of the rise of tem- perature, following exposure of the naked body in a cool air, may be accounted for by the increased metabolism of internal organs, accompanying the influx of blood caused by constric- tion of the cutaneous capillaries, it is probable that in this as in so many other instances the blood and circulation have been credited with too much, and the direct influence of the nervous system on nutrition and heat production overlooked or under- estimated. The thermogenic center has not yet been definitely located, though some recent investigations seem to favor a spot in or near the corpus striatum for certain mammals. Some in- vestigators also recognize a cortical heat-center. It has been suggested that we may to advantage speak of a thermotaxic (regulative of loss) and a thermogenic mechanism (regulative of production), and even a thermolytic or discharging mechan- ism. It has been further suggested that different nerve-fibers may be concerned in the actual work of conveying the different impulses of these respective mechanisms to the tissues ; and the whole theory has been framed in accordance with the prevalent conception of metabolism as consisting of anabolism and ca- tabolism, or constructive and destructive processes. But these theories have not yet been confirmed by experiments on ani- mals, though they are, in the opinion of their authors, in har- mony with the facts of fever. Certainly, any theory that will imply that vital processes are more under the control of the nervous system than has hitherto been taught, will, we think, THE METABOLISM OF THE BODY. 449 advance physiology, as will shortly appear from our discussion of the influence of the nervous system on the various metaholic processes generally. The phenomena observable in an animal gradually freezing to death point strongly to the direct influence of the nervous system on the production as well as the regulation of heat. The circulation must of course be largely concerned, but it ap- pears as though the nervous system refused to act when the temperature falls below a certain point. A low temperature favors hibernation, ' in which we believe the nervous system plays the chief part, though the temperature in itself is not the determining cause, as we have ourselves proved. The fact that the whole metabolism of a hibernating animal is lowered, that with this there is loss of consciousness much more profound than in ordinary sleep, of itself seems to indicate that the nerv- ous system is at the bottom of the whole matter. Pathological, — It is found that many drugs and poisons lower temperature, acting in a variety of ways. In certain dis- eases, as cholera, the temperature may sink to 23° C. in extreme cases before death supervenes. When the temperatui'e of the blood is raised 6° to 8° C (as in sunstroke, etc.), death occurs ; and it is well known that prolonged high tempei*ature leads to fatty degeneration of the tissues generally. All the evidence goes to show that in fever both the heat production and the heat expenditure are interfered with ; or, at least, if not always, that there may be in certain cases such a double disturbance. In fever excessive consumption of oxygen and production of carbon dioxide occur, the metabolism is quickened, hence its wasting (consuming) effects ; the rapid respiration tends to in- crease the thirst, from the extra amount of aqueous vapor ex- haled. The body is actually warmest during the " cold stage " of fever, when the vessels of the skin ai*e constricted and the patient feels cold, because the internal metabolism is heightened ; while the " sweating stage " is marked by a natural fall of tem- perature. The fact that the skin may be dry and pale in fever shows that the thermotaxic nervous mechanism is at fault; but the chemical facts cited above (excess of CO2 etc.) indicate that the thermogenic mechanism is also deranged. 20 450 COMPARATIVE PHYSIOLOGY. SPECIAL CONSIDERATIONS. If the student will now read afresh what has been written under the above heading in relation to the subject of digestion, it will probably appear in a new light. We endeavored to show that, according to that general principle of correlation which holds throughout the entire organism, and in harmony with certain facts, we were bound to believe that digestion and as- similation, or, to speak in other terms, the metabolic processes of the various tissues, in a somewhat restricted sense, were closely related. Beneath the common observation that " diges- tion waits on appetite " lies the deeper truth that food is not prepared in the alimentary canal (digested) without some rela- tion to the needs of the system generally. In other words, the voice of the tissues elsewhere is heard in the councils of the digestive tract, and is regarded ; and this is effected chiefly through the nervous system. Excess in eating may lead to vomiting or diarrhoea — plain ways of getting rid of what can not be digested. Evolution. — We have already alluded to some of those modi- fications in the form of the digestive organs that indicate an unexpected plasticity, and impress the fact of the close rela- tion of form and function. The conversion of a sea-gull into a graminivorous bird, with a corresponding alteration in the na- ture of the form of the stomach (it becoming a gizzard), with doubtless modifications in the digestive processes, when re- garded more closely, implies coadaptations of a very varied kind. These are as yet but imperfectly known or understood, and the subject is a wide and inviting field for the physiologist. Darwin and others have indicated, though but imperfectly, some of the changes that are to be regarded in animals as cor- relations ; but in physiology the subject has received but little attention as yet. We have in several parts of this work called attention to it ; but the limits of space prevent us doing little more than attempting to widen the student's field of vision by introducing such considerations. The influence of climate on metabolism, an undoubted fact, has many implications. Any one who keeps a few wild animals in confinement un- der close observation, and endeavors to ascertain how their natural, self-chosen diet may be varied when confined, will be astonished at the plasticity of their instincts, usually con- sidered as so rigid in regard to feeding. These facts help one THE METABOLISM OF THE BODY. 451 to understand how by the law of habit and heredity each group of animals has come to prefer and flourish best upon a certain diet. But habit itself implies an original deviation some time, in which is involved, again, plasticity of nature and power to adapt as well as to organize. Without this, evolution of func- tion is incomprehensible ; but with this principle, and the tendency for what has once been done to be easier of repetition, and, finally, to become organized, a flood of light is thrown upon the subject of diet, digestion, and metabolism generally. On these principles it is possible to understand those race differ- ences, even individual differences, which as facts must be patent to all observers. The principle of natural selection has clearly played a great part in determining the diet of a species ; the surviving immi- grants to a new district must be those that can adapt to the local environment best, including the food which the region supplies. The greater capability of resisting hunger and thirst in some individuals of a species implies great differences in the meta- bolic processes, though these are mostly unknown to us; and the same remark applies to heat and cold It seems clear that hibernation is an acquired habit of the whole metabolism, with great changes in the functional condi- tion of the nervous system recurring periodically, and, in fact, dependent on these, by which certain large divisions, as the reptiles, amphibians, and certain mammals among vertebrates, are enabled to escape individual death and extinction as groups. We may suppose that, for example, among invertebrates, by a process of natural selection, those survived that could thus adapt themselves to the environment ; while, among mammals, hiber- nation may be considered as a process of reversion, perhaps, for the homoiothermer becomes very much a poikilothermer during hibernation, the latter reverting to a condition existing in lower forms, and not wholly unlike that of plants in winter. This can be understood on the principle of the origin of higher from lower forms; otherwise it is difficult to understand why similar states of the metabolism should prevail in groups widely sepa- rated in form and function. If all higher groups bear a deriva- tive relation to the lower, what is common in their nature, as we usually find them, as well as the peculiar resemblances of the metabolism of higher to lower forms in sleep, hibernation, etc., can be understood in the light of physiological reversion. The origin of a homoiothermic (warm-blooded) condition 452 COMPARATIVE PHYSIOLOGY. itself is to be sought for in the principle of natural selection. It was open to certain organisms, we may assume, either to adapt to a temperature much below that of their blood, or to hibernate; failing to make either adaptation would result in death; and gradually, no doubt, involving the death of num- berless individuals or species, the resisting power attained the marvelous degree that we are constantly witnessing in all homoiothermers. The daily variations of the bodily temperature in homoio- thermers is a beautiful example of the law of rhythm evident in the metabolism. Hibernation is another such. While these are clear cases, it is without doubt true that, did we but know more of the subject, a host of examples of the operation of this law might be instanced. We can but touch on these subjects enough to show that they deserve an attention not as yet bestowed on them ; and to the thoughtful it will be evident that their influence on prac- tical life might be made very great were they but rightly ap- prehended. THE INFLUENCE OF THE NERVOUS SYSTEM ON METABOLISM (NUTRITION). This subject is of the utmost importance, and has not re- ceived the attention hitherto, in works on physiology, to which we believe it is entitled, so that we must discuss it at some length. We may first mention a number of facts on which to base conclusions : 1. Section of the nerves of bones is said to be fol- lowed by a diminution of their constituents, indicating an alteration in their metabolism. 2. Section of the nerves sup- plying a cock's comb interferes with the growth of that ap- pendage. 3. Section of the spermatic nerves is followed by de- generation of the testicle. 4. After injury to a nerve or its center in the brain or spinal cord, certain affections of the skin may appear in regions corresponding to the distribution of that nerve ; thus, herpes zoster is an eruption that follows frequently the distribution of the intercostal nerve. 5. When the motor cells of the anterior horn of the spinal cord or cer- tain cells in the pons, medulla, or crus cerebri are disordered, there is a form of muscular atrophy which has been termed " active," inasmuch as the muscle does not waste merely, but THE METABOLISM OE THE BODY. 453 the dwindling is accompanied by proliferation of the muscle nuclei. 6. After neurotomy for navicular disease a form of de- generation of the structures of the foot is not uncommon. 7. After section of both vagi, death results after a period, varying in time, as do also the symptoms with the animal. In some animals pneumonia seems to account for death, since it is found that, if this disease be prevented, life may, at all events, be greatly prolonged. The pneumonia has been attributed to paralyses of the muscles of the larynx, together with loss of sensibility of the larynx, trachea, bronchi, and the lungs, so that the glottis is not closed during deglutition, and the food, finding its way into the lungs, has excited the disease by irrita- tion. The possibility of vaso-motor changes is not to be over- looked. In birds, death may be subsequent to pneumonia or to inanition from paralysis of the oesophagus, food not being swallowed. It is noticed that in these creatures there is fatty (and sometimes other) degeneration of the heart, liver, stomach, and muscles. 8. Section of the trigeminus nerve within the skull has led to disease of the corresponding eye. This opera- tion renders the whole eye insensible, so that the presence of offending bodies is not recognized; and it has been both as- serted and denied that protection of the eye from these pre- vents the destructive inflammation. With the loss of sensi- bility there is also vaso-motor paralysis, the intra-ocular ten- sion is diminished, and the relations of the nutritive lymph to the ocular tissues are altered. But all disturbances of the eye in which there are vaso-motor alterations are not followed by degenerative changes. 9. Degeneration of the salivary glands follows suture of their nerves. 10. After suture of long-di- vided nerves, indolent ulcers have been known to heal with great rapidity. This last fact especially calls for explanation. It will be observed, when one comes to examine nearly all such instances as those referred to above, that they are complex. Undoubtedly, in such a case as the trigeminus or the vagi, many factors contribute to the destructive issue; but the fact that many symptoms and lesions are concomitants does not, of itself, negative the view that there may be lesions directly dependent on the absence of the functional influence of nerve- Abel's. We prefer, however, to discuss the subject on a broader basis, and to found opinions on a wider survey of the facts of physiology. After a little time (a few hours), when the nerves of the sub- 45i COMPARATIVE PHYSIOLOGY. maxillary gland have been divided, a flow of saliva begins and is continuous till the secreting cells become altered in a way visible by the microscope. Now, we have learned that proto- plasm can discharge all its functions in the lowest forms oi animals and in plants independently of nerves altogether. What, then, is the explanation of this so-called u paralytic se- cretion " of saliva ? The evidence that the various functions of the body as a whole are discharged as individual acts or series of acts correlated to other functions has been abundantly shown; and, looking at the matter closely, it must seem un- reasonable to suppose that this would be the case if there was not a close supervision by the nervous system over even the details of the processes. We should ask that the contrary be proved, rather than that the burden of proof should rest on the other side. Let us assume that such is the case ; that the entire behavior of every cell of the body is directly or indirectly con- trolled by the nervous system in the higher animals, especially mammals, and ask, What facts, if any, are opposed to such a view ? We must suppose that a secretory cell is one that has been, in the course of evolution, specialized for this end. What- ever may have been the case with protoplasm in its unspecialized form, it has been shown that gland-cells can secrete independ- ently of blood-supply (page 314, etc.) when the nerves going to the gland are stimulated. Now, if these nerves have learned, in the course of evolution, to secrete, then in order that they shall remain natural (not degenerate) they must of necessity secrete; which means that they must be the subject of a chain of meta- bolic processes, of which the final link only is the expulsion of formed products. Too much attention was at one time directed to the latter. It was forgotten, or rather perhaps unknown, that the so-called secretion was only the last of a long series of acts of the cell. True, when the cells are left to themselves, when no influences reach them from the stimulating nervous centers, their metabolism does not at once cease. As we view it, they revert to an original ancestral state, when they pei'- formed their work, lived their peculiar individual life as less specialized forms wholly or partially independent of a nervous system. But such divorced cells fail; they do not produce normal saliva, their molecular condition goes wrong at once, and this is soon followed by departures visible by means of the microscope. But just as secretion is usually accompanied by excess of blood, so most functional conditions, if not all, de- THE METABOLISM OF THE BODY. 455 mand an unusual supply of pabulum. This is, however, do more a cause of the functional condition than food is a cause of a man's working - . It may hamper, if not digested and assimi- lated. It becomes, then, apparent that the essential for metab- olism is a vital connection with the dominant nervous system. It has been objected that the nervous system has a metab- olism of its own independent of other regulative influences; but in this objection it seems to be forgotten that the nervous system is itself made up of parts which are related as higher and lower, or at all events which intercommunicate and ener- gize one another. We have learned that one muscle-cell has power to rouse another to activity when an impulse has reached it from a nervous center. Doubtless this phenomenon has many parallels in the body, and explains how remotely a nerv- ous center may exert its power. It enables one to understand to some extent many of those wonderful co-ordinations (obscure in detail) that are constantly taking place in the body. We think the facts as they accumulate will more and more show, as has been already urged, that the influence of blood-pressure on the metabolic (nutritive) processes has been much over- estimated. They are not essential but concomitant in the highest animals. Turning to the case of muscle we find that when a skeletal muscle is tetanized the essential chemical and electrical phenomena are to be regarded as changes differing in degree only from those of the so-called resting state. There is more oxygen used, more carbonic anhydride excreted, etc. The change in form seems to be the least important from a physio- logical point of view. Now, while all this can go on in the absence of blood or even of oxygen, it can not take place with- out nerve influence or something simulating it. Cut the nerve of a muscle, and it undergoes fatty degeneration, and atrophies. True, this may be deferred, but not indefinitely, by the applica- tion of electricity, acting somewhat like a nerve itself, and in- ducing the approximately normal series of metabolic changes. If, then, the condition when not in contraction (rest) differs from the latter in all the essential metabolic changes in rate or degree only ; and if the functional condition or accelerated metabolism is dependent on nerve influence, it seems reason- able to believe that in the resting condition the latter is not withheld. The recent investigations on the heart make such views as we are urging' clearer still. It is known that section of the 456 COMPARATIVE PHYSIOLOGY. vagi leads to degeneration of the cardiac structure. We now know that this nerve contains fibers which have a diverse action on the metabolism of the heart, and that, according as the one or the other set is stimulated, so does the electri- cal condition vary ; and everywhere, so far as known, a differ- ence in electrical conditions seems to be associated with a difference in metabolism, which may be one of degree only, perhaps, in many instances — still a difference. The facts as brought to light by experimental stimulation harmonize with the facts of degeneration of the cardiac tissue on section of the vagi ; but this is only clear on the view we are now presenting, that the action of the nervous system is not only universal, but that it is constant ; that function is not an isolated and independent condition of an organ or tissue, but a part of a long series of metabolic changes. It is true that one or more of such changes may be arrested, just as all of them may go on at a less rate, if this actual outpouring of pancreatic secre- tion is not constant ; but secretion is not summed up in dis- charge merely ; and, on the other hand, it would seem that in some animals the granules of the digestive glands are being renewed while they are being used up, in secreting cells. The processes may be simultaneous or successive. Nor do we wish to imply that the nervous system merely holds in check or in a very general sense co-ordinates processes that go on unorigi- nated "by it. We think the facts warrant the view that they are in the highest mammals either directly (mostly) or indirectly originated by it, that they would not take place in the absence of this constant nervous influence. The facts of common ob- servation, as well as the facts of disease, point in the strongest way to such a conclusion. Every one has observed the in- fluence, on not one but many functions of the animal, we might say the entire metabolism, of depressing or exalting emotions. The failure of appetite and loss of flesh under the influence of grief or worry, tell a plain story. Such broad facts are of infi- nitely more value in settling such a question as that now dis- cussed than any single experiment. The best test of any theory is the extent to which it will explain the whole round of facts. Take another instance of the influence over metabolism of the nervous system. Every trainer of race-horses knows that he may overwork his beast — i. e. , he may use his muscles so much as to disturb the balance of his powers somewhere — very frequently his di- THE METABOLISM OF THE BODY. 457 gestion ; but often there seems to be a general break — the whole metabolism of the body seems to be out of gear ; and the same applies to our domestic animals. If we assume a constant nervcus influence over the metabolic processes, this is compre- hensible. The centers can produce only so much of what we may call nervous force, using the term in the sense of directive power; and if this be unduly diverted to the muscles, other parts must suffer. On this view also the value of rest or change of work becomes clear. The neiwous centers are not without some re- semblance to a battery ; at most, the latter can generate only a definite quantity of electricity, and, if a portion of this be di- verted along one conductor, less must remain to pass by any other. It is of practical importance to recognize that under great excitement unusual discharges from a nerve-center may lead to unwonted functional activity; thus, under the stimulus of the occasion an animal may in a race originate muscular con- tractions that he could not call forth under other circumstances. Such are always dangerous. We might speak of a reserve or residualnerve force, the expenditure of which results iu serious disability. It seems that the usually taught views of secretion and nutrition have been partial rather than erroneous in themselves, and it is a question whether it would not be well to substitute some other terms for them, or at least to recognize them more clearly as phases of a universal metabolism. We appear to be warranted in making a wider generalization. To regard pro- cesses concerned in building up a tissue as apart from those that are recognized as constituting its function, seems with the knowl- edge we at present possess, to be illogical and unwise. Whether, in the course of evolution, certain nerves, or, as seems more likely, certain nerve-fibers in the body of nerve-trunks, have become the medium of impulses that are restricted to regulat- ing certain phases of metabolism — as e. g., expulsion of formed products in gland-cells — is not, from a general point of view, improbable, and is a fitting subject for fui'ther investigation. But it will be seen that we should regard all nerves as " tro- phic " in the wider sense. What is most needed, apparently, is a more just estimation of the relative parts played by blood and blood- pressure, and the direct influence of the nervous system on the life-work of the cell. 458 COMPARATIVE PHYSIOLOGY. We must regard the nervous centers as the source of cease- less impulses that operate upon all parts, originating and con- trolling the entire metabolism, of which what we term func- tions are but certain phases, parts of a whole, but essential for the health or normal condition of the tissues. Against such a view we know no facts, either of the healthy or disordered or- ganism. Summary of Metabolism.— Very briefly and somewhat in- completely, we may sum up the chief results of our present knowledge (and ignorance) as follows: Glycogen is found in the livers of all vertebrate and some invertebrate animals. The quantity varies with the diet, being greatest with an excess of carbohydrates. Glycogen may be regarded as stored material to be convert- ed into sugar, as required by the organism ; though the exact use of the sugar and the method of its disposal are unknown. Fat is not stored up in the body as the result of being merely picked out from the blood ready made ; but is a genuine product of the metabolism of the tissues, and may be formed from fatty, carbohydrate, or proteid food. This becomes es- pecially clear when the difference in the fat of animals from that on which they feed is considered, as well as the direct re- sults of feeding experiments, and the nature of the secretion of milk. The liver seems to be engaged in a very varied round of meta- bolic processes ; the manufacture of bile, of glycogen, of urea, and probably of many other substances, some known and others unknown, as chemical individuals. Urea is in great part probably only appropriated by tbe kidney-cells (Amoeba- like) from the blood in which it is found ready made ; though it may be that a part is formed in these cells, either from bodies some steps on the way toward urea, or out of their pro- toplasm, as fat seems to be by the cells of the mammary gland. The leucin (and tyrosin ?) of the digestive canal sustains some relation to the manufacture of urea by the liver, and pos- sibly by tbe spleen and other organs ; for a proteid diet increases these products, and also the urea excreted. Creatin, one of the products of proteid metabolism, and possibly allied bodies, may be considered as in a certain sense antecedents of urea ; uric-acid, however, does not seem to be such, nor is it to be regarded as a body that has some of it escaped complete oxidation, but rather as a result of a distinct departure of the metabolism ; and there THE METABOLISM OF THE BODY. 459 are facts which seem to indicate that the uric-acid metabolism is the older, from an evolutionary point of view, and that in mammals, and especially in man, as the results of certain errors there may be a physiological (or pathological) reversion. Hip- puric acid, as replacing uric acid in the herbivora, maybe re- garded in a similar light. Our knowledge of the metabolism of the spleen, beyond its relations to the formation of blood-cells and their disintegra- tion, is in the suggestive rather than the positive stage. It seems highly probable that this organ plays a very important part, the exact nature of which is as yet unknown. When an animal starves, it may be considered as feeding on its own tissues, the more active and important utilizing the others. Notwithstanding, organs with a very active metabo- lism, as the muscles and glands, lose weight to a large extent. The presence of urea to an amount not very greatly below the average in health, shows that there is an active proteid metabo- lism then as at all times in progress. General experience and exact experiments prove that, while an animal's diet may be supplied with special regard to fatten- ing, to increase working power, or simply to maintain it in health, as evidenced by breeding capacity, form, etc., in all cases there must be at least a certain minimum quantity of each of the food-stuffs. No one food can be said to be exclusively fattening, heat-forming, or muscle-forming. A carbohydrate diet tends to production of fat ; proteid food to supply muscular energy, but the latter also produces fat, and a diet of proteid mixed with fat or gelatin will serve the pur- poses of the economy better than one containing a very much larger quantity of proteid alone. Muscular energy, as is to be inferred from the excreta, is not the result of nitrogenous me- tabolism alone; and in arranging any diet for man or beast the race and the individual must be considered. Animals can not be treated as machines, like engines using similar quantities of fuel ; though this holds far more of man than the lower ani- mals — i.e., the results may be predicted from the diet with far more certainty in their case than for man. Food is related to excreta in a definite way, so that all that enters as food must sooner or later appear as urea, salts, car- bonic anhydride, water, etc. These are individually to be re- garded as the final links in a long chain of metabolic processes, or rather a series of these. Fats and carbohydrates are repre- 460 COMPARATIVE PHYSIOLOGY. sented finally as carbonic anhydride and water principally, proteids as urea. Nitrogenous foods may be regarded as accelerating the metabolic processes generally and proteid metabolism in par- ticular, while fats have the reverse effect ; hence fat in the diet renders a less quantity of proteid sufficient. Gelatin seems to act when mixed with proteid food either like an additional quantity of proteid, or possibly like fat, at all events under such circumstances less proteid suffices. These facts have a bearing not only on health but on econ- omy, in the expenditure for food. Salts hold a very important place in every diet, though their exact influence is in great part unknown. The heat of the body is the resultant of all 'the metabolic processes of the organism, especially the oxidative ones. Certain food-stuffs have greater potential capacity for heat formation than others ; but, finally, the result depends on whether the organism can best utilize one or the other. A certain body temperature, varying only within narrow limits, is maintained, partly by regulation of the supply and partly by the regulation of the loss. Both these are, in health, under the direction of the nervous system, and both are co-ordinated by the same. Loss is chiefly through the skin and lungs ; gain chiefly through the organs of most active metabolism, as the muscles and glands. Vaso-motor effects play a great part in the escape of heat. Animals may be divided into poikilothermers and homoio- thermers, or cold-blooded and warm-blooded animals, accord- ing as their body heat varies with or is independent of the ex- ternal changes of temperature. All the facts go to show that in mammals the processes of the body (metabolism) can con- tinue only within a slight range of variations in temperature, though the upward limit is narrower than the downward. Upon the whole, the evidence justifies the conclusion that the nervous system is concerned in all the metabolic processes of the body in mammals including man, and that, as we descend the scale, the dominion of the nervous system becomes less till we reach a point when protoplasm goes through the whole cycle of its changes by virtue of its own properties uninfluenced by any modification of itself in the form of a nervous system. THE SPINAL CORD.— GENERAL. Among the higher vertebrates the spinal cord is found to consist of nerve-cells, nerve-fibers, and a delicate connective tis- sue binding them together ; while these different structures are arranged in definite forms, so that a cross-section anywhere pre- sents a characteristic appearance, the more important gangli- onic nerve-cells being internal and forming a large part of the gray matter of the cord. All the various regions of this organ or series of organs are connected with one another, white with white and gray matter, as well as white with gray substance. While we do not attempt to furnish a complete and detailed account of the anatomy of the cord or other parts of the nervous system, for which the student is referred to works on anatomy, we would remind him that the spinal cord is situated within- a bony case with joints permitting of a certain amount of move- ment, variable in different regions. Inasmuch as the cord itself does not fill its bony covering, but floats in fluid and tethered to the walls by bands of connective tissue, it is well protected from laceration, bruising, or concussion. Like the brain, it has a protective tough outer membrane (dura mater) with a closer- fitting iuner covering abounding in blood-vessels (pia mater). The white matter of the cord invests the horns of gray matter and is made up of nerve-fibers wanting the outer sheath. Here, as elsewhere, these fibers have only a conducting func- tion ; they do not originate nervous impulses. The gray matter, on the other hand, abounds in cells, some of them with many processes, that can originate, modify, and conduct impulses. Certain well-recognized groups of these cells are arranged in columns throughout the cord, as shown in the accompany- ing figures. The supporting basis for these cells (neuroglia) is the most delicate form of connective tissue known. The cord may be regarded either as an instrument for the Fio. 32' Fio. 328. THE SPINAL CORD.— GENERAL. 463 Fig. 327.— General view of spinal cord (Chauveau). A, cervical bulb; B, lumbar bulb; C, cauda equina. Fig. 328.— Segment of spinal cord at the cervical bulb, or brachial plexus, showing its upper face and the roots of the spinal nerves (Chauveau). A, superior roots; J5, inferior roots; C, multiple ganglia of superior roots; D, single ganglion on an exceptional pair; E, £, upper roots passing through the envelopes. reception and generation of impulses independent of the brain ; or as a conductor of afferent and efferent impulses destined for the brain or originating in that organ. As a matter of fact, however, it is better to bear in mind that the cord and brain constitute one organ or chain of organs, which, as we have learned from our studies in development, are differentiations of one common track, originating from the epiblast. While the brain and the cord may act independently to a Fig. 329.— Transverse section of spinal cord of child six months old, at middle of lum- bar region, showing especially the fibers of gray substance. 1 x 20. (After Ger- lach.) a, anterior columns; b. posterior columns; c, lateral columns: , medulla oblongata; I—VIIT, nerves in usual order; K, lateral branch of vagns; I, upper twig of same; m, dorsal branch of trigeminus, joined by n, dor- sal branch of vagus; o, /3, y, three branches of trigeminus; Se, facial nerve; \, branchial branches of vagus. THE BRAIN. 487 in the largest specimens, the brain of man is even absolutely heavier than that of any other animal, which is in great part due to the preponderating development of the cerebrum. While the cerebral surface is smooth in all the lower verte- brates, and but little convoluted until the higher mammals are reached, the brain of the primates, and especially of man. has its surface enormously increased, owing to its numerous fis- sures and convolutions, which, in fact, arise from the growth Fig. 341. — Brain and spinal cord of frog (Bastian). A, olfactory lobes; B, cerebral lobes; R, pineal body; C, D, optic lobes; E, cerebellum; H, spinal cord. The cerebellum is notably small. of the organ being out of proportion to that of the bony case in which it is contained ; and since those cells which go to make up the gray matter and are devoted to the highest func- tions, are disposed over the surface, the importance of the fact in accounting for the superior intelligence of the primates, Fig. 343. Fh.o.-? Fig. 343. Fig. 342.— Brain of the pike, viewed from above (Huxley). ^4, the olfactory nerves or lobes, and beneath them the optic nerves; B, the cerebral hemispheres; C, the optic lobes; T). the cerebellum. Fig. 343.— The t>rain of edible frog (Eana esculenta). 1x4. (After Huxley.) L.ol, the rhinencephalon, or olfactory lobes, with 7, the olfactory nerves; He. the cere- bral hemispheres; Fh.o, the thalamencephalon with the pineal gland, P/r, L.op, optic lobes; 0, cerebellum; 8. rh, the fourth ventricle; Mo, medulla oblongata. 488 COMPARATIVE PHYSIOLOGY. A Fig. 344. — A, C, the brain of a lizard (Psammosaurus Bengalensis), and B, D, of a bird (Meleagris gallopavo, the turkey), drawn as if they were of equal lengths (after Huxley). A. B, viewed from above; C, D, from the left side. Olf, olfactory lobes; Pn, pineal gland; Hmp, cerebral hemispheres; Mb, optic lobes of the mid- brain; Cb, cerebellum; M. O, medulla oblongata; ii, iv, vi, second fourth, and sixth pairs of cerebral nerves; Py, pituitary body. Fri;. 8 1"). —Brains of a lizard (Psammosaurns Benr/alensifs) and of a bird {Meleagris gallopava) in longitudinal and vertical section. The upper figure represents the THE BRAIN. 489 lizard's brain; the lower, that of the bird (after Huxley and Carus). Letters as in the preceding figure, except L. t, lamina terminaUs, or anterior wall of the third ventricle; /. .)/, foramen of Munro; a, anterior commissure; Th. E, thalamen- cephalon; s, soft commissure; p, posterior commissure; iv, indicates the exact point of exit of the fourth pair from that part of the brain which answers to the value of Vieussens. and especially of man, becomes apparent. Depth of Assuring is, however, of more importance than multiplicity of furrows ; and it may be observed that intelligence is not always in pro- portion to the extent to which the cerebral surface is broken up into fissures and convolutions. Tbe depth of the gray mat- ter is also very variable, and seems to bear an important rela- tion to psychic development. Man's brain, then, is character- ized by its great size and complexity ; while those parts treated elsewhere, concerned in co-ordination, vision, etc., are well developed, the cerebrum, especially its convolutions as distin- guished from its basal ganglia, is, out of all proportion, greater than in any other animal. The gray matter of the brains of the higher vertebrates is distributed as masses of ganglionic cells internally, and as a fairly uniform layer over its surface. The brain of man weighs about three pounds on the average, that of the male being a few ounces (four to six) heavier than that of the female. Fig. 346. Fig. 347. Fig. 348. Fig. 346.— Brain of pigeon (after Ferrierl. A, cerebral hemispheres; B, optic lobe; C, cerebellum, the lateral lobes of which are very small. Fig. 347. — Brain and spinal cord of chick at sixteen days old; optic lobes, b, are still in contact (after Owen and Anderson). Fig. 348.— Brain and part of spinal cord of chick twenty days old, showing optic lobes widely separated and cerebellum, c, largely developed. The individual and race differences, though considerable, are not comparable ha degree to those that distinguish man from even the highest apes, the brain of the latter weighing not more than about one third as much as that of the human sub- ject. While it has been shown that individual men and women, having brains of average or even sub-medium weight, may reach 490 COMPARATIVE PHYSIOLOGY. even distinction in the intellectual world; and though idiots have been known to possess brains abnormally heavy, it is Pig. 350. Fig. 349. — Outer surface of brain of horse (after Solly and Leuret). e, olfactory lobe; h, hippocampal lobe (processus pyriformis); 1,2,3, lobes of cerebellum; o, optic nerve; m, motor oculi; p, fourth nerve; t, fifth nerve; u, sixth nerve; /, facial; I, auditory; r/, glossopharyngeal; v, vagus; s, spinal accessory; n, hypoglossal; X, pons Varolii. Pig. 350. — Longitudinal section through center of brain of horse, presenting view of internal surface (after Solly and Leuret). c.c, corpus callosum; p, thalamus; co, middle commissure; t. q, corpora quadrigemina, in front of which is the pineal body. The cerebellum lias been cut through. nevertheless true that brain-weight and the higher powers of man bear a close though not invariable relationship. The apparent discrepancies are susceptible of explanation. Besides the gray matter, with its cells of highest functional value from the standpoint now taken, the brain consists, and in large part, of neuroglia and nerve-fibers, with probably chiefly, and in the case of the fibers solely, a conducting func- tion. THE BRAIN. 491 The Connection of one Part of the Brain with another - Though it has long been known that the different parts of the suDra-orbit-il- 7 p \r j? t tr ,, „ ■ eyjviannssure, /». the insula; S. Or, S. Oc, M. Oc, I. Oc, the three occipUal'gyri. ' ' ^ ^ three tem P oraI > and 492 COMPARATIVE PHYSIOLOGY. brain were connected by bridges of fibers (commissures, etc.), tbe physiological significance of the fact seems to have been largely ignored, and even at the present day is too little con- PlG. 352.— Inner views of cerebral hemispheres of the rabbit, pig, and chimpanzee, drawn as before, and placed in the same order (Huxley). 01, olfactory lobe; C.c, corpus callosum ; A. C, anterior commissure; H, hippocampal sulcus; Vh, unci- nate; M, marginal; 6', callosal gyri; /. P, internal perpendicular; C'a, calcarine; Coll, collateral sulci; /'', fornix. sidered. 1. Cerebral fibers pass between the convolutions of this part of the brain and the cerebellum ; between the former t-pr= "Y^y \"<^f 494 COMPARATIVE PHYSIOLOGY. mr. J Fig. 354.— Diagrammatic horizontal section of a vertebrate brain (Huxley). The follow- ing letters serve for both this figure and the one following. Mb, mid-brain. What lies in front of this is the fore-brain, and what lies behind, the hind-brain. L. t, the lamina terminalis; Olf, olfactory lobes; limp, hemispheres; T/i. E, thala- mencephalon; Pn, pineal gland; Py, pituitary body; FM, foramen of Munro; CS, corpus striatum; Th, optic thalamus; CQ, corpora quadrigemina; CC, crura cere- bri; Cb, cerebellum; PV, pons Varolii; MO, medulla oblongata; /, olf ac tori i; II, optici; III, point of exit from brain of motores oculorum; IV, of pathetici; VI, of abducentes; V—XII, origins of the other cerebral nerves. 1, olfactory ven- tricle; 2, lateral ventricle; 3, third ventricle; 4, fourth ventricle; +, iter a tertio ad quartum ventriculum. and the main basal ganglia; between the gray matter of the convolutions on the same side, and between the latter and those Fio. 355.— A longitudinal and vertical section of a vertebrate brain (Huxley). Letters as above. The lamina terminalis is represented by the strong black line between FMimA Z. THE BRAIN. 495 on the opposite halves; between the gray matter of the cortex and the internal capsule, the corpora striata, optic thalami, pons Varolii, the medulla oblongata, and so to the spinal cord. The course of the latter tracts of fibers have been, especially by the help of pathology, definitely followed. Some of these connec- tions are given in more detail below. 1. Cerebro-cerebellar fibers, (a.) From the cortical cells of the anterior cerebral lobe to tbe pons Varolii, passing through the internal" capsule and thence through the lower and outer part of the crus cerebri (crusta). (b.) Fibers from the occipital and temporo-sphenoidal lobes, passing by the crusta, reach the upper surface of the cerebellum. 2. Fibers bridging the tivo sides of the cerebrum, (a.) By means of the corpus callosum chiefly, passing from the gray matter in the first instance. (6.) From the temporo-sphenoidal lobe on each side through the corpora striata and anterior com- missure, (c.) Fibers from the upper part of the crus cerebri (tegmentum) to the optic thalamus of each side and onward to the temporo-sphenoidal lobes, forming the posterior com- missure. 3. Fibers connecting different parts of the cerebral convolu- tions on the same side. These are exceedingly numerous and belong to such tracts as the "arcuate fibers," passing from one gyrus to another; "collateral fibers," forming distant convo- lutions; fibers of the fornix between the uncinate gyrus, hip- pocampus major, and optic thalamus; longitudinal fibers of the corpus callosum; fibers of the taenia semicircularis, uncinate fasciculus, etc. 4. Fibers forming the cerebrum and the spinal cord. Ac- cording as they pass downward or upward do they converge or diverge, and the most important seem to pass through the in- ternal capsule ; and while the majority do perhaps form some connection either with the corpora striata and optic thalami, some seem to pass directly downward through the internal cap- sule. It is held by many that the fibers passing through the posterior portion of the internal capsule are derived from the posterior lobe of the cerebrum, and are the paths of sensory im- pulses upward ; while the rest of the internal capsule is made up of fibers from the anterior, and especially the middle portion of the cerebral cortex (motor area), and these fibers are the paths of motor (efferent) impulses. It now becomes clearer that the brain is constituted a whole 496 COMPARATIVE PHYSIOLOGY. by such connections ; and that, apart from the multiplicity of cells with different functions to perform, situated in different Fig. 356.— Diagrammatic representation of the course of some of the fibers in the cere- brum of man (after Le Bon). areas, the complexity and at the same time the unity of the encephalon becomes increasingly evident, merely upon anatomi- cal grounds; but we shall find such a view still further strength- ened by study of the functions of the various parts. While the tracts enumerated are anatomical and have been clearly traced, there can be little doubt that many others yet remain to be marked out ; and that, apart from such collections of fibers, we must recognize functional paths by the neuroglia, and possibly others still. It is not to be forgotten that in the brain, as in the spinal cord, nerve-cells are themselves conductors, and while THE BRAIX. 497 there may be certain areas within which the resistance is such that impulses are usually confined to them, it is also true that, as in the cord, there may be a kind of overflow. Adjacent cells, possibly widely separated cells, may become involved. We shall return to tbis important subject again, however, as, without recognizing such relationships, it seems to us quite impossible to understand the facts as we find them in the working of the body and the mind. The Cerebral Cortex. — We may now proceed to inquire what are the functions of the cells of the gray matter covering the surface of the cerebrum. Before the birth of physiology as a 32 49S COMPARATIVE PHYSIOLOGY. science, Gall recognized and taught that the encephalon is a col- lection of organs ; that these have separate functions ; that the relative size of each determines the degree of its functional ac- tivity ; and that the cranium developing in proportion to the growth of the brain, the former might give information as to the probable size of what lay beneath it in different regions. It will be seen that, as thus interpreted, phrenology is a very different thing from what usually passes under that name, and is paraded before wondering audiences by ignorant charlatans. In the main the doctrines of Gall are not without a certain foundation in facts ; and the modern theory of localization of function bears some resemblance to what Gall taught, though with greater limitations. FlG. 358.— Outer surface of cerebrum (after Exner). The shaded portion represents the motor area in man and the monkey— i. e., the area which most observers be- lieve to be associated with certain voluntary movements of the limbs, etc. In the mean time it has been found that in many cases it was possible to locate the site of a brain-lesion (tumor, etc.) by the symptoms, chiefly motor, of the patient ; and brain-surgery THE BRAIN. 499 has in consequence entered upon a new era of development. Tumors thus localized have been removed successfully, and the patients restored to health. As a result of the various kinds of observations and discussions on this subject of late years, the localizationists are willing to admit that the areas of the cortex can not be marked off mathematically — that, in fact, they "overlap." This is in itself an important concession. Again, there is less confidence in the location of the various sensory centers than of the motor centers. Most investigators are be- lievers in a " motor area " par excellence (for the arm, leg, etc.) around the fissure of Eolando (Fig. 358). This view is now, so far as man is concerned, widely accepted. There is agreement in placing the sensory centers behind the above-mentioned motor area, and especially in the occipital lobes. The tendency to locate a visual center in this region is growing stronger. There is much disagreement as to the other sensory centers formerly placed in the angular gyrus and tem- poro-sphenoidal lobes. The intellectual faculties have not been located in any such sense as Gall and his followers attempted to establish. The first two frontal convolutions are those, per- haps, to which localization has as yet been least applied. Chiefly on clinical and pathological grounds a center for speech has long been located in the third (left) frontal convolu- tion (Broca's) and parts immediately behind it. It has been ob- served that when disease attacks this area speech is interfered with in some way. We may say then, generally, that the tendency at the pres- ent time, both on the part of physiologists and clinical ob- servers, is to admit localization to some degree and in some sense. This has been the result in part of experiments on the dog and especially on the monkey, combined with the discus- sion of clinical cases which resulted in death (followed by an autopsy), or of others marked by a successful diagnosis and re- moval of lesions or other treatment. In other words, the truth, if it is to be reached at all, must be sought by the plan we have advocated throughout this work — the discussion of the re- sult's of as inany different methods as can be brought to bear on this or any other subject. Neither the experimental nor the pathological method alone can settle such complex questions. Although localization of function has not been established for the cerebral cortex in the case of those animals with which the practitioner of veterinary medicine has to deal as it has for man 500 COMPARATIVE PHYSIOLOGY. and the monkey, we have thought it well to bring the subject before the student of comparative medicine, since it can not be doubted that future research will put the physiology of the brains of the domesticated animals in a new light, in doing which guidance will naturally be sought from what has been already clone, more especially in the case of the human subject and his nearest allies. Some would maintain that in the case of the dog, motor and sensory localization has been established ; that in this animal there is a motor area in the region of the crucial sulcus corresponding to that around the fissure of Rolando in man. The subject is, however, far from finally settled even in the case of the dog, the brain of which has been more thoroughly investigated than that of any other of our domestic animals. Very little can as yet be said in regard to cortical localization in the horse, ox, etc. It seems highly probable that investiga- tion will show that cortical localization in the primates (man and the monkey tribe) exists in a far higher degree than in any other animals. The Circulation in the Brain. — The brain, being inclosed within an air-tight bony case, its circulation is of necessity peculiar. Since any undue compression of the encephalon may lead to even a fatal stupor, it is clear that there must exist some provision to permit of the excess of arterial blood that is re- quired for unusual activity of the brain. It is to be borne in mind that the fluid within the ventricles is continuous, through the foramen of Magendie in the roof of the fourth ventricle, with that surrounding the spinal cord (spinal cavity) ; so that an increase in the volume of the encephalon in consequence of an afflux of blood might be in some degree compensated by an efflux of the cerebro-spinal fluid. The part played by this ar- rangement has, however, been probably overestimated. But the peculiar venous sinuses do, it is likely, serve to regulate the blood-supply ; being very large, they may answer as temporary overflow receptacles. An inspection of the fontanelles of an infant reveals a beating corresponding with the pulse; and, when a large part of the cranium is removed in an animal, a plethysmograph shows a rise in volume corresponding with the pulse and the respiratory movements, as in the case of the fontanelles. But, besides these, periodic waves of contraction are now known to pass over the cerebral arteries. Whether the latter is part of a general wave traversing the whole arterial system is as yet uncertain. Though there is THE BRAIN. 501 considerable anastomosis of vessels in the encephalon, it is not equal to what takes place in many other organs. It is well known that a clot or other plug within a cerebral vessel is more serious than in many other regions, which is partly to be ex- plained by the lack of sufficient anastomosis for the vascular needs of the parts. It is also well known that, in organs which constitute parts of a related series, as the different divisions of the alimentary tract, all are not usually at the same time vas^ cular to the same extent. While they act functionally in rela- tion to each other, they exemplify also a certain degree of inde- pendence. Such a condition of things is now known to exist in the brain — i. e., certain areas may be abundantly supplied with blood as compared with others : and it seems highly probable that a condition of equal arterial tension throughout is scarcely a normal condition. Though the quantity of blood contained within the vessels of the whole brain at any one time is not so large as in some other organs (glands), yet the foregoing facts and the rapidity of the flow must be taken into account. The capillaries are very close and abundant, in the gray matter es- pecially ; and it is to be borne in mind that it is chiefly these vessels which are concerned in the actual metabolism (nutri- tion) of parts. However, the chemical changes in the nervous system being feeble, it would appear probable that it does its work with less consumption of pabulum than other parts of the body. We wish to lay stress on the local nature of vascular dilatation in the brain, as it greatly assists in explaining certain phenomena about to be considered. Sleep. — Observations upon animals from which portions of the cranium have been removed, so that the brain was visible, show that during sleep the blood-vessels are much less promi- nent than usual ; and it is well known that means calculated to diminish the circulation in the brain, as cold and pressure, favor sleep. It is also well established by general experience that withdrawal of the usual afferent impulses through the various senses favors sleep. A remarkable case is on record of a youth whose avenues for sensory impressions were limited to one eye and a single ear, and who could be sent to sleep by closing these against the outer world. Yet this subject after a long sleep would awake of his own accord, showing that, while affer- ent impulses have undoubtedly much to do with maintaining the activity of the cerebral centers, yet their automaticity (in- dependence) must also be recognized. 502 COMPARATIVE PHYSIOLOGY. It is a matter of common experience that weariness, or the exhaustion following on pain, mental anxiety, etc., is favorable to sleep. A good deal of light is thrown on this subject by hiberna- tion, particularly in mammals. From special study of the subject we have ourselves learned that, however temperature, and certain other conditions may influence this state, it will appear at definite periods in defiance, to a large extent, of the conditions prevailing. Hibernation, we are convinced, is marked by a general slowing of all of the vital processes in which the nervous system takes a prominent part. Sleep and hibernation are closely related. In both there is a diminution of the rate of the vital processes, as shown by the income and output, measured by chemical standards, with of course obvious physical signs, as slowed respiration, circula- tion, etc. While sleep, then, is primarily the result of a rhyth- mical retardation of the vital processes, especially within the nervous system, it is like hibernation in some degree (in the lowest creatures, without a nerve system) the outcome of that rhythm impressed on every cell of the organism and the influ- ence of which is felt in a thousand ways, that no doubt we are quite unable to recognize. Hypnotism. — By the help of the above principles the sub- ject of hypnotism, now of absorbing interest, may be in great part explained. This condition is characterized by loss of vo- lition and judgment. It may be induced in man and certain other animals by prolonged staring at a bright object, assisted by a concentration of the attention on that alone, as far as pos- sible, combined with a condition of mental passivity in other respects. The individual gradually becomes drowsy, and finally falls into a state in many respects strongly resembling sleep. Hypnotism proper may be combined with catalepsy, a con- dition in which the limbs remain rigid in whatever condition tbey may be placed. Modifications of the vascular and respira- tory systems occur. Various animals have been hypnotized, as the fowl, rabbit, Guinea-pig, crayfish, frog, etc. This condi- tion is readily induced in the common fowl, more especially tbe wilder individuals, by holding the creature with the bill down on a table and the whole animal perfectly quiet for a short time. Upon the removal of the pressure the bird re- mains perfectly passive and apparently asleep for some little time. THE BRAIN, 503 Fig. 359.— Lateral surface of brain of monkey, displaying motor areas (after Horsley and Schafer). Fk.. 860.- Median surface of brain of monkey (after Horsley and Schafer). Figs. 359 and 300 may be said to embody the views of Horsley and Schafer more espe- cially in regard to motor localization. 50A COMPARATIVE PHYSIOLOGY. FUNCTIONS OF OTHER PORTIONS OF THE BRAIN. Certain parts of the encephalon are spoken of as the basal ganglia, prominent among which are the corpus striatum and the optic thalamus. The Corpus Striatum and the Optic Thalamus.— The corpus striatum consists of several parts, the main divisions being an intra- ventricular portion or caudate nucleus, and an extra-ven- tricular part or lenticular nucleus. Fio. 361. — Transverse section of cerebral hemispheres of man at level of cerebral gan- glia (after Dalton). 1, great longitudinal fissure; 2, part of same between occipital lobes; 3, anterior part of corpus callosum; 4, fissure of Sylvius; 5, convolutions of island of Rei) (insula); 6, caudate nucleus of corpus striatum; 7, lenticular nu- cleus of corpus striatum; 8, optic thalamus; 9, internal capsule; 10, external cap- sule; 11, claustrum. THE BRAIN. 505 Between these lies the internal capsule, through which pass fibers that spread out toward the cortex, as the corona radiata. Pathology, especially, has shown that a lesion of the intra- ventricular portion of the corpus striatum, and, above all, of the internal capsule, is followed by failure of voluntary move- ment (akinesia). It would appear that a great part of the fibers from the motor area around the fissure of Rolando, pass through the intra-ventricular parts of the corpus striatum, and especially its internal capsule. But it is also to be borne in mind that a large part of the fibers passing from the cortex make connection with the cells of the corpus striatum before reaching the cord. These facts render the occurrence of loss of voluntary motor power comprehensible. The fibers of the peduncles of the brain may be divided into an interior or lower division (cmista), going mostly to the Fig. 362. — Transverse section of human brain (after Dalton). This and the preceding figure are somewhat diagrammatic. 1, pons Varolii; -2,2. crura cerebri; 3,3, in- ternal capsule; 4, 4, corona radiata; 5, optic thalamus; (j, lenticular nucleus; 7. corpus callosum. corpus striatum, and a posterior division (tegmentum), passing principally to the optic thalami ; many, possibly most of them, ultimately reach the cortex. Many clinical observers do not hesitate to speak of the optic thalamus as sensory in function. 506 COMPARATIVE PHYSIOLOGY. and the corpus striatum, as inotor ; but the clinical and patho- logical evidence is conflicting — all lesions of these parts not being followed by loss of sensation and motion respectively ; though an injury to the internal capsule generally results in paralysis. All are agreed that the symptoms are manifested on the side of the body opposite to the side of the lesion, so that a decussation must take place somewhere between the ganglion and the periphery of the body. There is no doubt that the optic thalamus, especially its posterior part, is concerned with vision, for injury to it is fol- lowed by a greater or less degree of disturbance of this func- tion. As has been already pointed out, unilateral injury of either of these ganglia leads to inco-ordination or to forced movements. That these regions act some intermediate part in the transmission of impulses to and from the brain cortex, and that the anterior one is concerned with motor, and the pos- terior possibly with sensory (tactile, etc.), and certainly with visual impulses, may be stated with some confidence, though further details are not yet a subject of general agreement. Corpora Quadrigemina. — The function of these parts in vis- ion, as in the co-ordination of the movements of the ocular muscles, and their relations to the movements of the pupil, will be considered later. However, the actual centers for these func- tions seem to lie in the anterior portion of the floor of the aqueduct of Sylvius, and are indirectly affected by stimulation of the corpora quadrigemina. Extirpation of these parts on one side produces blindness of the opposite eye, and in birds, etc., the same result follows when their homologues — the optic lobes — are similarily treated. There can be no doubt, therefore, that they are a part of the central nervous machinery of vision, and it seems to be probable that the anterior parts of the cor- pora quadrigemina alone have this visual function. But, since it is the opposite eye that is affected, and in some animals (rabbits) that alone, we are led to infer a decussation of the optic fibers, or at least of impulses. In dogs, on the other hand, the crossing seems to be but partial. It begins to appear that there are several parts of the brain concerned with vision. After removal of almost any part of the cerebral cortex, if of sufficient extent, vision is impaired. We may say, then, that before an object is " seen " in the high- est sense, processes beginning in the retina undergo further elaboration in the corpora quadrigemina, optic thalami, and, THE BRAIN. 507 finally in the cerebral cortex. We may safely assume that the part played by the latter is of very great importance, making the perception assume that highest completeness which is of Pig. 303. — Diagrammatic representation of brain on transverse section to illustrate course of fibers (after Landois). C, C, cortex cerebri; C.s, corpus striatum: X.I, lenticular nucleus; T.o, optic thalamus; P. peduncle; H, tegmentum; j>. crusta; V, corpora quadrigemina; 1. 1, corona radiata of corpus striatum; 2, 2, of 1cm icu- lar nucleus; 3, 8, of optic thalamus; 4, 4, of corpora quadrigemina; 5, direct fibers to cortex cerebri (Fleehsig); 6, 6, fibers from corpora quadrigemina to tegmentum; m, further course of these fillers; S. s. tillers from corpus striatum and lenticular nucleus to crusta of peduncle of cerebrum; M, further course of these; ■'\ 3, course of sensory fibers; Ii, transverse section of spinal cord; v. W, anterior, and h. H". posterior roots; a, a, system of association fibers; c. c, commissural fibers. 508 COMPARATIVE PHYSIOLOGY. very varying character, no doubt, with different groups of ani- mals. In a sense, all mammals may see alike, and, in another sense, they may see things very differently ; for, if we may judge by the differences in this respect between educated and unedu- cated men, the great dissimilarity lies in the interpretation of what is seen ; in a word, the cortex has to do with the perfect- ing of visual impulses. Nevertheless, a break anywhere in the long and complicated chain of processes must lead to some serious impairment of vision. Much of the same sort of reason- ing applies to the other senses and also to speech. To speak, therefore, of a visual center or a speech center in any very restricted sense is unjustifiable ; at the same time, it is becoming clearer that there is in the occipital lobe, rather than in other parts of the cortex, an area which takes a pecul- iar and special share in elaborating visual impulses into visual sensations and perceptions ; and there can be little doubt that the other senses are represented similiarly in the cerebral cortex. The Cerebellum. — Both physiological and pathological re- search point to the conclusion that the cerebellum has an im- portant share in the co-ordination of muscular movements. Ablation of parts of the organ leads to disordered movements ; and, when the whole is removed in the bird, co-ordination is all but impossible, and the same holds for mammals. Section of the middle peduncle of one side is liable to give rise to roll- ing forced movements. In fact, injury to the cerebellum causes symptoms very similar to those following section of the semi- circular canals, so that many have thought that in the latter case the cerebellum had itself been injured. Pathological. — Tumors and other lesions frequently, though not invariably, give rise to unsteadiness of gait, much like that affecting an intoxicated person. It may safely be said that the cerebellum takes a very prominent share in the work of the muscular co-ordination of the body. As has already been pointed out, several tracts of the spinal cord make connection with the cerebellum, and it is not to be forgotten that this part of the brain has, in general, most ex- tensive connections with other regions. Insufficient study has as yet been given to the cerebellum, and it is likely that the part it takes in the functions of the encephalon is greater than has yet been rendered clear. The old notion that this organ bears any direct relation to the sexual functions seems to be THE BRAIN. 509 without foundation. It has now been clearly demonstrated that the lower region of the spinal cord is, in the dog and prob- ably most mammals, the part of the nerve-centers essential for the sexual processes. Crura Cerebri and Pons Varolii.— As has been already noted, the peduncles (crura) are the paths of impulses from certain parts of the cerebral cortex, the basal ganglia, and the spinal cord. The functions of the gray matter of the crura are un- known. But, since forced movements ensue on unilateral sec- tion, it is plain that they also have to do with muscular co- ordination. The transverse fibers of the pons Varolii connect the two halves of the cerebellum. Its longitudinal fibers have extensive connections— the anterior pyramids and olivary bodies of the medulla, the lateral, and perhaps also a part of the posterior columns of the cord, while upward these fibers connect with the crura cerebri and so with the cortex. Pathological. — Paralysis of the face usually occurs on the same side as that of the rest of the body ; hence it must be inferred that there is a decussation somewhere of the fibers of the facial nerve ; but there is much still to be learned about this subject. Medulla Oblongata. — In some animals (frogs) it is certainly known that this region of the brain has a co-ordinating func- tion, and it is probable that it is concerned with such uses in all animals that possess the organ, or rather collection of organs, seeing that this part of the brain must be regarded as especially a mass of centers, the functions of which have been already considered at length. So long as the medulla is intact, life may continue ; but, except under special circumstances, which do not invalidate this general statement, its destruction is followed by the death of the animal. We may simply enumerate the centers that are usually located in the medulla : The respiratory (and convulsive), car- dio-inhibitory, vaso-motor, center for deglutition, center for the movements of the gullet, stomach, etc., and the vomiting center ; center for the seci'etion of saliva and possibly other of the digestive fluids. Some add a diabetic and other centers. 510 COMPARATIVE PHYSIOLOGY. SPECIAL CONSIDERATIONS, Embryological. — The further we progress in the study of the nervous system, the greater the significance of the facts of its Fig. 364.— Vertical longitudinal section of brain of human embryo of fourteen weeks. 1x3. (After Sharpey and Reichert.) c, cerebral hemisphere; cc, corpus callosum beginning to pass back; /, foramen of Munro; p, membrane over third ventricle and the pineal body; th, thalamus; 3, third ventricle; /. olfactory bulb ; cq, corpora quadrigemina; cr, crura cerebri, and above them, aqueduct of Sylvius, still wide; c', cerebellum, and below it the fourth ventricle; jw, pons Varolii; m, medulla oblongata. early development becomes. It will be remembered that from that uppermost epiblastic layer of cells, so early marked off in Fig. 366. PlG. 365.— Outer surface of human frctal brain at six months, showing origin of prin- cipal fissures (after Sharpey and R. Wagner). F, frontal lobe; P, parietal; 0, occipital; T, temporal; a, a, a, faint appearance of several frontal convolutions; g, 8, Sylvan fissure; *', anterior division of same; C, central lobe of island of Reil; r, fissure of Rolando; p, external perpendicular fissure. Pig. 366. Upper surface of bruin represented in Fig. 304 (after Sharpey and R. Wag- ner). the blastoderm, is formed *the entire nervous system, including centers, nerves, and end organs. The brain may be regarded as a specially differentiated part of the anterior region of the THE BRAIN. 511 medullary groove and its subdivisions ; and the close relation of the eye, ear, etc., to the brain in their early origin, is not without special meaning, while the more diffused sensory de- velopments in the skin connect the higher animals closely with the lower — even the lowest, in which sensation is almost wholly referable to the surface of the body. Without some knowledge of the mode of development of tbe encephalon, it is scarcely possible to appreciate that rising grade of complexity met with as we pass from lower to higher groups of animals, especially noticeable in vertebrates ; nor is it possible to recognize fully the evidence found in the nervous system for the doctrine that higher are derived from lower forms by a process of evolution. Evolution. — The same law applies to the nervous system as to other parts of the organism, viz., that tbe individual devel- opment (ontogeny) is a synoptical representation, in a general way. of the development of the group (phylogeny). A com- parison of the development of even man's brain reveals the fact that, in its earliest stage, it is scarcely, if at all, distinguishable from that of any of the lower vertebrates. There is a period when even this, the most convoluted of all brains, is as smooth and devoid of gyri as the brain of a frog. The exti'erne com- PlG. 367.— A. brain of aye-aye {Lemur); B. of marmoset; C. of squirrel monkey (Cal- lithri.c); D, of macaque monkey; E, of gibbon; F, of a fifth-month human foetus (after ( >wen i. Although naturalists are agreed that tbe monkey.-, apes, and lemurs are related, considerable differences are to be observed in their brains. These fig- ures also illustrate the remark made after those following. plexity of the human brain is referable to excessive growth of certain parts, crowding and alteration of shape, owing to the influence of its bony case, its membranes, etc. 512 COMPARATIVE PHYSIOLOGY. It is evident, from an inspection of the cranial cavities of those enormous fossil forms that preceded the higher verte- brates, that their brains, in proportion to their bodies, were very small, so that any variation in the direction of increase in the encephalon — especially the cerebrum — must have given the creatures, the subject of such variation, a decided advan- tage in the struggle for existence, and one which may partly account, perhaps, for the extinction of those animals of vast proportions but limited intelligence. That the size of the brain Fig. 368. — A, brain of a chelonian; B, of a foetal calf; C, of a cat. (All after Gegen- baur.) /, indicates cerebral hemispheres ; //, thalamus ; III, corpora quadri- gemina; IV, cerebellum; V, medulla; st, corpus striatum; /, fornix; h, hippocam- pus; sr, fourth ventricle; g, geniculate body; ol, olfactory lobe. It will be observed (1) how the foetal brain in a higher animal form resembles the developed brain in a lower form, and (2) how certain parts become crowded together and covered over by more prominent regions, e. g., the cerebrum, as we ascend the animal scale. as well as its quality can be increased by use, seems to have been established by the measurements, at different periods of development, of the heads of those engaged in intellectual pur- suits, and comparing the results with those obtained by similar measurement of the heads of those not thus specially employed. Of course, it must be assumed that the head measurement is a gauge of the size of the brain, which is approximately true, if not entirely so. Recent investigations seem to show that the development of the ganglion cells of the brain takes place first in the me- dulla, next in the cerebellum, after that in the mid-brain, and finally in the cerebral cortex. Animals most helpless at birth are those with the least development of such cells. The me- THE BRAIN. 513 dulla may be regarded in some sense as the oldest (phylogeneti- cally) part of the brain. In it are lodged those cells (centers) which are required for the maintenance of the functions essen- tial to somatic life. This may serve to explain how it is that so many centers are there crowded together. It is remarkable that so small a part of the brain should preside over many im- portant functions ; but the principle of concentration with pro- gressive development, and the law of habit making automatism prominent, throw some light upon these facts, and especially the one otherwise not easy to understand, that so much impor- tant work should be done by relatively so few cells. Possibly, however, if localization is established as fully as it may eventu- ally be, this also will not be so astonishing. The law of habit has, in connection with our psychic life and that of other mammals, some of its most striking develop- ments. This has long been recognized, though that the same law is of universal application to the functions of the body has as yet received but the scantiest acknowledgment. We shall not dwell upon the subject beyond stating that in our opinion the psychic life of animals can be but indifferently understood unless this great factor is taken into the account ; and when it is, much that is apparently quite inexplicable be- comes plain. That anything that has happened once any- where in the vital economy is liable to repetition under a Fig. 369. Fig. 370. Fig. 369.- Fig. 370.- -Brnin of cat, seen from above (after Tiedemann). -Brain of dog, seen from above (after Tiedemann). slighter stimulus, is a law of the utmost importance in physiol- ogy, psychology, and pathology. The practical importance of this, especially to the young animal, is of the highest kind. Synoptical.— There is as yet no systematized clear physiology 514 COMPARATIVE PHYSIOLOGY. of "the brain." We are conversant with certain phenomena referable to this organ in a number of animals, chiefly the higher mammals ; but our knowledge is as yet insufficient to generalize, except in the broadest way, regarding the functions of the brain — i.e., to determine what is common to the brains of all vertebrates and what is peculiar to each group. Referring, then, to the higher mammals, especially to the dog, the cat, the monkey, and man, we may make the following statements : The medulla oblongata is functionally the ruler of vegeta- tive life — the lower functions ; and so may be regarded as the seat of a great number of " centers," or collections of cells with functions to a large degree distinct, but like close neighbors, with a mutual dependence. Phylogenetically (ancestrally) the medulla is a very ancient region, hence the explanation apparently of so many of its functions being common to the whole vertebrate group. Parts of the mesencephalon, the pons Varolii, the optic lobes or corpora quadrigemina, the crura cerebri, etc., are not only connecting paths between the cord and cerebrum, but seem to preside over the co-ordination of muscular movements, and to take some share in the elaboration of visual and perhaps other sensory impulses. The cerebellum may have many functions unknown to us. Its connections with other parts of the nerve-centers are numer- ous, though their significance is in great part unknown. Both pathological and physiological investigation point to its having a large share in muscular co-ordination. It is certain that the cerebrum is the part of the brain essen- tial for all the higher psychic manifestations in the most ad- vanced mammals and in man. The preponderating development of man's cerebrum ex- plains at once his domination in the animal world, his power over the inanimate forces of Nature, and his peculiar infirmities, tendencies to a certain class of diseases, etc., — in a word, man is man, largely by virtue of the size and peculiarities of this part of his brain. Modern research has made it clear also that there is a " pro- jection " of sensory and motor phenomena in the cerebral cor- tex ; in other words, that there are sensory and motor centers in the sense that in the cortex there are certain cells which have an important share in the initiation of motor impulses, and others employed in the final elaboration of sensory ones. THE BRAIN. 515 It is even yet premature to dogmatize in regard to the site of these centers; especially are we not ready for large generali- zations. In man the convolutions around the fissure of Rolando constitute the motor area hest determined. The whole subject of cortical localization requires much ad- ditional study, especially by the comparative method in the widest sense — i. e., by a comparison of the results of operative procedure in a variety of groups of animals, and the results of clinical, pathological, physiological, and psychological investi- gation. Especially must allowance be made for differences to be observed, both for the group and the individual ; and also for the influence which one region exerts over another. Be- tween the weight of the cerebrum, the extent of its cortical surface, and psychic power, there is a general relationship. GENERAL REMARKS ON THE SENSES. Oue studies in embryology have taught us that all the vari- ous forms of end-organs are developed from the epiblast, and so may be regarded as modified epithelial cells, with which are associated a vascular and nervous supply. These end-organs are at once protective to the delicate nerves which terminate in Fig. 371.— Papillae of skin of palm of hand (after Sappey). A vascular network in all cases, and in some nerves and tactile corpuscles, enter the papillae. them, and serve to convey to the latter peculiar impressions which are widely different in most instances from those result- ing from the direct contact of the nerve with the foreign body, All are acquainted with the fact that, when the epithelium is removed, as by a blister, we no longer possess tactile sensibility of the usual kind, and experience pain on contact with objects ; in a word, the series of connections necessary to a sense-percep- tion is broken at the commencement. Seeing that all the end-organs on the surface of the body have a common origin morphologically, it would be reasonable to expect that the senses would have much in common, espe- cially when these organs are all alike connected with central nervous cells by nerves. As a matter of fact, such is the case, GENERAL REMARKS ON THE SENSES. 517 and in every instance we can distinguish between sensory im- pulses generated in the end-organ, conveyed by a nerve inward, and those in the cells of these central nervous systems, giving rise to certain molecular changes which enable the mind or the ego to have a perception proper ; which, when taken in connec- tion with numerous past experi- ences of this and other senses, furnishes the material for a sen- sory judgment. The chief events are, after all, internal, and hence it is Fig. 372. Fig 373. Fig. 372.— Corpuscle of Vater (after Sappey). Fig. 373. — End-bulbs (corpuscles) of Krause (after Luddon). A. from conjunctiva ot man; B, from conjunctiva of calf. It may be noticed that in all these cases the nerve loses its non-essential parts before entering the corpuscle. found that the higher in the scale the animal ranks, the more de- veloped its nervous centers, especially its brain, and the more it is able to capitalize its sensory impulses ; also the greater the de- gree of possible improvement by experience, a difference well seen in blind men whose ability to succeed in life without vision is largely in proportion to their innate and acquired mental powers. Inasmuch as all cells require rest, one would expect 518 COMPARATIVE PHYSIOLO -Y. that under eonstaut stimulation fatigue w. raid soon result ana perceptions be imperfect Hence it happens that all the senses fail when exercised. G \ even for but a short period, without change of stimulus lead:: _ alteration of condition in the central cells, The change need not be one of entire rest. but merely a new form of exercise. Hence the freshness experienced by a change of view on passing through beau- tiful scenery. Exhaustion may not he confined wholly to the central nerve- Fig. 374.— Nerves with sranglion cells {G) beneath a cells, but there Can be $&?Jw*'- fr ° m Skin ° f " m arthropod little doubt that they are the most affected. Since also there must be a certain momentum, so to speak, to molecular activity, it is not surprising that we find that the sensation outlasts the stimulus for a brief period: and this ap- plies to all the senses, and necessarily determines the rapidity with which the successive stimuli may follow each other with- out causing a blending of tne sensations. Thus, then, in every sense organ in which the chain of pi nerve through which (3 the central near and we may speak, therefore, of (1) senso sensations, when these give rise t<-> atf'ect nervous cells resulting in 1 perceptions when we take into account the psychic i r nature of cell-life generally, we must reco£ sity of the stimulus ry to arouse a sensation and a limit within which alone we have power to discriminate (range of stimulation and perception): and also a limit to the ra with which stimuli may succeed each other to any advantage, to give ris rsens is; and a limit to the endur- ance of the apparatus in good working condition corresponding must recognize 1< an end- ses begins; (2) a conducting e-cells are affected ; y impulses and 2 ons of the central and •„' jui a. e-^e> : and. from the a certain inten- GENERAL REMARKS ON THE SENSES. 519 to clear mental perceptions, together with the value of past ex- perience in the interpretation of our sensations. A man can necessarily have positive knowledge only of his own conscious- ness ; but he infers similarity of conscious states by likeness in action and expression in his fellows. It is by an analogous process and by such alone that we can draw any conclusions in regard to the sensations of the lower animals. The presence of structures, undoubtedly sensory, in them is fairly good evidence that their sensations resemble ours when similar organs are em- ployed. However, this does not absolutely follow; and the whole subject of the senses of animals incapable of articulate speech is beset witb great difficulties. It only remains for us to set forth what is known regarding man, assuming that at least much of it applies to our domestic animals. Patient thoughtful observation will in time place the subject in a better position. THE SKIN AS AN ORGAN OF SENSE. Bearing in mind that all the sensory organs originate in the ectoderm, we find in the skin even of the highest animals the power to give the central nervous system such sense-im- pressions as bear a relation to the original undifferentiated sensations of lower forms as derived from the general surface of the body, but with less of specialization than is met with in the sense of hearing and vision, so that it is possible to under- stand how it is that the skin must be regarded not only as the original source of sensory impulses for the animal kingdom, but why it still remains perhaps the most important source of information in regard to the external world, and the condition of our own bodies; for it must be remembered that the data afforded for sensory judgments by all the other senses must be interpreted in the light of information supplied by the skin. We really perceive by the eye only retinal images. The dis- tance, position, shape, etc., of objects are largely determined by feeling them, and thus associating with a certain visual sensa- tion others derived from the skin and the muscles, which latter are, however, generally also associated with tactile sensations. It is recorded of those blind from birth that, when restored to sight by surgical operations, they find themselves quite un- able to interpret their visual sensations; or, in other words, seeing they do not understand, but must leaim by the other senses, especially tactile sensibility, what is the real nature of the objects that form images on their retinae. All objects seen appear to be against the eyes, and any idea of distance is out of the question. Special forms of end-organs are found scattered over the skin, mucous and serous surfaces of the body, such as Pacinian corpuscles, touch-corpuscles, end-bulbs, etc. ; while in lower forms of vertebrates many others are found in parts where sen- sibility is acute. There seems to be little doubt that these are THE SKIN AS AN ORGAN OF SENSE. 521 all concerned with the various sensory impulses that originate in the parts where they are found, but it is not possible at pres- ent to assign definitely to each form its specific function. It has been contended that the various specific sensations of taste, as bitter, sweet, etc., are the result of impulses con- veyed to the central nervous system by fibers that have this function, and no other; and a like view has been maintained for those different sensations that originate from the skin. For such a doctrine there is a certain amount of support from experiment as well as analogy ; but the more closely the subject is investigated the more it appears that the complexity of our sensations is scarcely to be explained in so simple a way as many of these theories would lead us to believe. Whether there are nerve-fibers with functions so specific, must be re- garded as at least not yet demonstrated. Let us now examine into the facts. What are the different sensations, the origin of which must be in the first instance, sought in the skin, as the impulses aroused in some form of end-organ or nerve-termination ? Suppose that one blindfolded lays his left hand and arm on a table, and a piece of iron be placed on the palm of his hand, he may be said to be conscious of the nature of the sur- face, whether rough or smooth, of the form, of the size, of the weight, and of the temperature of the body: in other words, the subject of the experiment has sensations of pressure, of tactile sensibility, and of temperature at least, if not also to some extent of muscular sensibility. But if the right hand be used to feel the object its form and surface characters can be much better appreciated ; while, if the body be poised in the hand, a judgment as to its weight can be formed with much greater accuracy. The reason of the former is to be sought in the fact that the finger-tips are relatively very sensitive in man, and that from experience the mind has the better learned to interpret the sensory impulses originating in this quarter; which again resolves itself into the particular condition of the central nerve-cells associated with the nerve-fibers that convey inward the impulses from those regions of the skin. Mani- festly if there be a sense referable to the muscles (muscular sense) at all, when they are contracted at will the impression must be clearer than when they but feebly respond to the mere pressure of some body. 522 COMPARATIVE PHYSIOLOGY. PRESSURE SENSATIONS. 1. There is a relation between the intensity of the stimulus and the sensation resulting, and this limit is narrow. The greater the stimulus the more pronounced the sensation, though ordinary sensibility soon passes into pain. 2. The law of con- trast may be illustrated by passing the finger up and down in a vessel containing mercury, when the pressure will be felt most distinctly at the point of contact of the fluid. 3. Pressure is much better estimated by some parts than others ; bence the use of the employment of those to so large an extent. THERMAL SENSATIONS. 1. The law of contrast is well illustrated by this sense ; in fact, the temperature of a body exactly the same as that of the part of the skin applied to it can scarcely be estimated at all. The first plunge into a cold bath gives the impression that the water is much colder than it seems in a few seconds after, when the temperature has in reality changed but little ; or, perhaps, the subject may be better illustrated by dipping one hand into wanner and the other into colder water than that to be ad- judged. The sample feels colder than it really is to the hand that has been in the warm water, and warmer than it is to the other. 2. The limit within which we can discriminate is at most small, and the nicest determinations are made within about 27° and 33° C. — i. e., not far from the normal temperature of the body. 3. Variations for the different parts of the skin are easily ascertained, though they do not always correspond to those most sensitive to changes in pressure. The cheeks, lips, and eyelids are very sensitive to pressure. Recent investigations have revealed the fact that there are in the human skin " pressure-spots,'' and " cold-spots," and " heat- spots" — i. e., the skin may be mapped out into very minute areas which give when touched a sensation of pressure different from that produced by the same stimulus in the intermediate regions ; and in like manner are there areas which are sensitive to warm and to cold bodies respectively, but not to both ; and these do not correspond with the pressure-spots, nor to those that give rise when touched to the sensation of pain. It has been shown, also, that the extent of the area of skin stimulated detennines to a large degree the quality of the re- THE SKIN AS AN ORGAN OF SENSE. 523 suiting sensation. Thus, the temperature of a fluid does not seem the same to a finger and the entire hand. This fact is not irreconcilable with the existence of the various kinds of ther- mal spots, referred to above, but it does re-enforce the view we are urging of the complexity of those sensations which seem to us to form simple wholes — as, indeed, they do — just as a piece of cloth may be made up of an unlimited number of different kinds of threads. TACTILE SENSIBILITY. As a matter of fact, one may learn, by using a pair of com- passes, that the different parts of the surface of our bodies are not equally sensitive in the discrimination between the contact of objects — i. e., the judgment formed as to whether at a given instant the skin is being touched by one or two points is de- pendent on the part of the body with which the points are brought into contact. Certain it is that exercise of these and all the senses greatly improves them, though it is likely that such advance must be referred rather to the central nerve-cells than to the peripheral mechanism. We practically distinguish between a great many sensations that we can neither analyze nor describe, though the very variety of 'names suffices to show how much our interpretation of sense depends on past experience. Mammals are always able to define the part of their bodies touched, and with great accuracy, no doubt, owing to the simul- taneous use in the early months and years of life of vision and the senses resident in the skin. An impression made on the trunk of a nerve is referred to the peripheral distribution of that nerve in the skin ; thus, if the elbow be dipped in a freezing mixture, the skin around the joint will experience the sensation of cold, but a feeling of pain will be referred to the distribution of the ulnar nerve in the hand and arm. The same principle is illustrated by the com- mon experience of the effects of a blow over the ulnar nerve, the pain being referred to the peripheral distribution ; also by the fact that pain in the stump of an amputated limb is thought to arise in the missing toes, etc. 524 COMPARATIVE PHYSIOLOGY. THE MUSCULAR SENSE. Every one must be aware how difficult it is to regulate his movements when the limbs are cold or otherwise deadened in sensibility. We know too that, in judging of the muscular effort necessary to be put forth to accomplish a feat, as throw- ing a ball or lifting a weight, we judge by our past experience. It is ludicrous to witness the failure of an individual to pick up a mass of metal which was mistaken for wood. In these facts we recognize that in the successful use of the muscles we are dependent, not alone on the sensations derived from the skin, but also from the muscles themselves. True, the muscles are not very sensitive to pain when cut; it does riot, however, fol- low that they may not be sensitive to that different effect, their own contraction. Whether the numerous Pacinian bodies around joints, or the end-organs of the nerves of muscles are directly concerned, is not determined. Pathological. — The teaching of disease is plainly indicative of the importance of sensations derived both from the skin and the muscles for co-ordination of muscular movements. In locomotor ataxy, in which the power of muscular co- ordination is lost to a large extent, the lesions are in the pos- terior columns of the spinal cord, or the posterior roots of the nerves, or both, and these are the parts involved in the trans- mission of afferent impulses. Comparative. — The more closely the higher vertebrates are observed, the more convinced does one become that those sen- sory judgments, based upon the information derived from the skin and muscles, which they are constantly called upon to form are in extent, variety, and perfection scarcely if at all surpassed by those of man. Of course, sensory data in man, with his ex- cessive cerebral development, may by associations in his expe- rience be worked up into elaborate judgments impossible to the brutes, but we now refer to the judgments of sense in them- selves. The lips, the ears, the vibrissa? or stiff hairs, especially about the lips, the nose, in some cases the paws, all afford delicate and extensive sensory data. It is a remarkable fact that the most intelligent of the groups of animals have these sensory surfaces well developed, as witness the elephant with his wonderful trunk, the band of the monkey, and the paws and vibrissa? of the cat and dog tribe. THE SKIN AS AN ORGAN OF SENSE. 525 On the other hand, the groups with hoofs are notably inferior in the mental scale. When we pass to the lower forms of in- vertebrates the appreciation of vibrations of the air or water in which they live, of its temperature, of its pressure, etc., must be considerable to enable them to adapt themselves to a suitable environment. We have not spoken of sensations derived from the internal organs and surfaces. These are ill-defined, and we know them mostly either as a vague sense of comfort or discomfort, or as actual pain. We are quite unable to refer them at present to special forms of end-organs. They are valuable as reports and warnings of the animal's own conditions. After-impressions (" after-images") of all the senses referred to exist, mostly positive in nature— i. e., the sensation remains when the stimulus is withdrawn. Synoptical. — The information derived from the skin in man and the other higher vertebrates relates to sensations of press- ure, temperature, touch, and pain. The muscles also supply information of their condition. In how far these are referable to certain end-organs in the skin is uncertain. There are der- mal areas that give rise to the sensations of heat, cold, pressure, and pain. Whether these are connected with nerve-fibei's that convey no other forms of impulses than those thus arising is undetermined . In all these senses the laws of contrast, duration of the im- pression, limit of discrimination, etc., hold. The judgments based on sensations derived from the skin are syntheses or the result of the blending of many component sensations simultaneous in origin. All our sensory judgments are very largely dependent on our past experience. VISION. Light and vision are to some degree correlatives of each other. Light is supposed to have as its physical basis the vibra- tions of an imponderable ether. Such is, however, to a non- seeing animal as good as non-existent, so that we may look at 15 13 16 Fig. 37.").— Eye partially dissected (after Sappeyl. 1. optic nerve; 2, 3, 4, sclerotic dis- sected back so as to uncover the choroid coat; 5, cornea, divided and folded back with sclerotic coat; 6, canal of Schlemm; 7, external surface of choroid, traversed by one of the long ciliary arteries and by ciliary nerves; 8. central vessel, into which the vam vorticoea empty; 9, 10. choroid zone; 11, ciliary nerves; 12, long ciliary artery; 13, anterior ciliary arteries; 14, iris; 15, vascular circle of iris; 16, pupil. this subject either with the eyes of the physiologist or the phys- icist, according as we regard the cause of the effects or the latter and their relations to one another. It is, however, im- possible to understand the physiology of vision without a sound knowledge of the anatomy of the eye, and an apprehen- VISION. 527 sion of at least some of the laws of the science of optics. The student is, therefore, recommended to learn practically the SUPERIOR RECTUS CHOROID OPTIC NERVE -INFERIOR RECTUS Fig. 376.— Section of human eye, somewhat diagrammatic (after Flint) coarse and microscopic structure of the eye in detail. The eyes of mammals are sufficiently alike to make the dissection of any of them profitable. Bullocks' eyes are readily obtainable, and from their large size may be used to advantage. We recom- mend one to be boiled hard, another to be frozen, and sections in different meridians to be made, especially one axial vertical longitudinal section. Other specimens may be dissected with and without the use of water. Assuming that some such work has been done, and that the student has become quite familiar with the general structure of the eye, we call attention specially to the strength of the sclerotic coat ; the great vascularity of the choroid coat and its terminal ciliary processes, its pigmented character adapting it for the absorption of light; the complicated structure and pro- tected position of the retinal expansion. It may be said that 528 COMPARATIVE PHYSIOLOGY. the whole eye exists for the retina, and that the entire mechan- ism besides is subordinated to the formation of images on this Fig. 377. — Certain parts of eye. 1 x 10. (After Sappey.) 1, 1, crystalline lens; 2, hyaloid membrane; 3, zonule of Zinn; 4, iris; 5, a ciliary process; 6, radiating fibers of ciliary muscle; 7, section of circular portion of ciliary muscle; 8, venous plexus of ciliary muscle; 9, 10, sclerotic coat; 11, 12, cornea; 13, epithelial layer of cornea; 14. Descemet's membrane; 15, pectinate ligament of iris; 16, epithelium of membrane of Descemet; 17, union of sclerotic coat with cornea; 18, section of canal of Schlemm. nervous expansion. The eye of the mammal may be regarded as an arrangement of refracting media, protected by coverings, with a window for the admission of light, a curtain regulating the quantity admitted; a sensitive screen on which the images are thrown ; surfaces for the absorption of superfluous light ; apparatus for the protection of the eye as a whole, and for pre- serving exposed parts moist and clean. Embryological. — We have already learned that the first indi- cation of the eye is the formation of the optic vesicle, an out- growth from the first cerebral vesicle. This optic vesicle be- VISION. 329 comes more contracted at the base, and the optic stalk remains as the optic nerve. At an early stage of development (second or third day in the chick) the outer portion of the optic vesicle is pushed inward, Fig. 379. Fig. 378. Fig. 378. — Section through head of chick on third day, showing origin of eye (after Yeo). a, epiblast undergoing thickening to form lens; o, optic vesicle; V x , first cerebral vesicle; V 2 , posterior cerebral vesicle. It will be observed that the retina is already distinctly indicated. Fig. 379. — Later stages in development of eye (after Cardiat). a, epiblast; c, develop- ing lens; o, optic vesicle. so that the cavity is almost obliterated; the anterior portion, becoming thickened, ultimately forms the retina proper, while the posterior is represented by the tesselated pigment layer of the choroid. As this retinal portion breaks away from the superficial epithelium, the latter foi'ms an elliptical mass of cells, the future lens, the changes of which in the formation of the cells peculiar to the lens illustrate to how great lengths differentiation in structue is carried in the development of a single organ. It will thus be seen that the most essential parts of the eye, the optic nerve, the retina, and the crystalline lens, are, according to a general law, the earliest marked out. The cornea, the iris, the choroid, the vascular supply, the sclerotic, etc., are all sec- ondary in importance and in formation to these, and are derived from the mesoblast, while the essential structures are traceable, like the nervous system itself, to the epiblastic layer. Any act of perfect vision in a mammal may be shown to consist of the following: (1) The focusing of rays of light from 34 530 COMPARATIVE PHYSIOLOGY. an object on the retina, so as to form a well-defined image; (2) the conduction of the sensory impulses thus generated in the Fig. 380.— More advanced stage of development of eye (after Cardiat). «, epithelial cells forming lens, now much altered; b, lens capsule; c, cutaneous tissue about to form conjunctiva; d, e, two layers of optic vesicle, now folded back and form- ing retina; /, mucous tissue forming vitreous humors; g, intercellular substance; h, developing optic nerve; i, nerve-fibers entering retina. retina by the optic nerve inward to certain centers ; and (3) the elaboration of these data in consciousness. We thus have the formation of an image — a physical pro- cess; sensation, perception, and judgment— physiological and psychical processes. In the natural order of things we must discuss first those arrangements which are concerned with the focusing of light — i. e., the formation of the image on the retinal screen. VISION. 531 DIOPTRICS OF VISION. One of the most satisfactory methods of ascertaining that the eye does form images of the objects in the field of vision is to remove the eye of a recently killed albino rabbit. On holding up before such an eye any small object, as a pair of forceps, it may be readily observed that an inverted image of the object is formed on the back of the eye {fundus). If, however, the lens be removed from such an eye, no image is formed. If the lens be itself held behind the object, an in- verted image will be thrown upon a piece of paper held at a suitable (its focal) distance. By substituting an ordinary bicon- vex lens, the same effect follows. It thus appears, then, that the lens is the essential part of the refracting media, though the aqueous and vitreous humors and the cornea are also focus- ing mechanisms. In the actual human eye the focus must correspond with the fovea of the retina if a distinct image is to be formed. I II Fkj. 381.— Refraction by convex lenses (after Flint and Weinhokl). The lens may be assumed to consist of a series of lenses (II in figure), for the sake of simplicity. though, of course, this is not strictly accurate. It will appear that we may represent the eye as reduced to the lens and the retina. The experiments referred to above will convince the student that such is the case. 532 COMPARATIVE PHYSIOLOGY. ACCOMMODATION OF THE EYE. Using the material already referred to, the student may observe that, with the natural eye of the albino rabbit, its lens (or better that of a bullock's eye, being larger), or a bi- convex lens of glass, there is only one position of the instru- ments and objects which will produce a perfectly distinct image. If either the eye (retina), the lens, or the object be shifted, in- stead of a distinct image, a blurred one, or simply diffusion- circles, appear. A photographer must alter either the position of the object or the position of his lens when the focus is not perfect. The eye may be compared to a camera, and since the retina and lens can not change position, either the shape of the lens must change or the object assume a different position in space. As a matter of fact, any one may observe that he can not see objects distinctly within a certain limit of nearness to the eye, known as the near point (punctum proximum) ■; while he be- comes conscious of no effect referable to the eye until objects approach within about sixty-five to seventy yards. Beyond the latter distance objects are seen clearly without any effort. There are many ways in which we may be led to realize these truths : 1. When one is reading a printed page it is only the very few words to which the eyes are then specially di- rected that are seen clearly, the rest of the page appearing blurred ; and the same holds for the objects in any small room. We speak of picking out an acquaintance in an audience or crowd, which implies that each of the individuals composing the throng is not distinctly seen at the same time. 2. If an ob- server hold up a finger before his eyes, and direct bis gaze into the distance (relax his accommodation), presently he will be- hold a second shadowy finger beside the real one — i. e., he sees double; his eyes being accommodated for the distant objects, can not adapt themselves at the same time for near ones. In what does accommodation consist ? From experiments it has been concluded that accommodation consists essentially in an alteration of the convexity of the anterior surface of the lens. This change in the shape of the lens is accomplished as follows : The lens is naturally very elastic and is kept in a par- tially compressed condition by its capsule, to which is attached the suspensory ligament which has a posterior attachment to VISION. 533 the choroid and ciliary processes. When the ciliary muscle, which operates from a fixed point the corneo-sclerotic junction, Fig. 382. — Illustrates mechanism of accommodation (after Fick). The left side de- picts the relation of parts during the passive condition of the eye (negative accom- modation, or accommodation for long distances); the right side, that for near ob- jects. pulls upon the choroid, etc., it relaxes the suspensory ligament; hence the lens, not being pressed upon in front as it is from behind by the vitreous humor (invested by its hyaloid mem- brane), is free to bulge and so increase its refractive power. The nearer an object approaches the eye, the greater the diver- gence of the rays of light proceeding from it, and hence the necessity for greater focusing power in the lens. If an animal be observed closely when looking from a remote to a near object, it may be noticed that the eyes turn inward — i.e., the visual axes converge and the pupils contract. These are not, however, essential in the sense in which the changes in the lens are ; for, as before stated, in the absence of the lens distinct vision is quite impossible. ALTERATIONS IN THE SIZE OF THE PUPIL. The pupil varies in size according as the iris is in a greater or less degree active. All observers are agreed that the circu- lar fibers around the pupillary margin are muscular, forming the so-called sphincter of the iris; but great differences of opin- ion still exist in regard to the radiating fibers. It is thought by many that all the changes in the iris may be explained by the elasticity of its structure without assuming the existence of muscular fibers other than those of the sphincter ; thus a contraction of the latter would result in diminution of the pu- 53J: COMPARATIVE PHYSIOLOGY. pillary aperture, its relaxation to an enlargement, provided the rest of the iris were highly elastic. The conclusions in regard to the innervation of the iris rest largely upon the results of certain experiments which we shall Brain above medulla Optic centre Retina Sympathetic nerve to radiating fibres Spinal dilator centre — Fig. 383. — Diagram to illustrate innervation of the iris. Dotted lines indicate general functional connection (correlation). Course of impulses indicated by arrows. now hriefly detail : 1. When the third nerve is divided, stimu- lation of the optic nerve (or retina) does not cause contraction of the pupil as usual. 2. When the optic nerve is divided, light no longer causes a contraction of the pupil, though stimulation of the third nerve or its center in the anterior portion of the floor of the aqueduct of Sylvius does hring about this result. 3. Section of the cervical sympathetic is followed by contrac- VISION. 535 tion and stimulation of its peripheral end by dilatation of the pupil. From such experiments it has been concluded that — 1. The optic is the afferent nerve and the third nerve the efferent nerve concerned in the contraction of the pupil ; and that the center in the brain is situated as indicated above, so that the act is or- dinarily a reflex. 2. That the cervical sympathetic is the path of the efferent impulses regulating the action of the radiating fibers of the iris. Its center has been located near that for the contraction of the pupil, and it may be assumed to exert a tonic action over the iris comparable to that of the vaso-motor center over the blood-vessels. The impulses may be traced through the cervical sympa- thetic and its ganglia back to the first thoracic ganglion, and thence to the spinal cord and brain. There may be subsidiary centers in the cervical spinal cord. It is to be remembered that, although the dilating center is automatic in action, it may also act reflexly, or be modified by unusual afferent impulses — as, e. g., the strong stimulation of any sensory nerve which causes enlargement of the pupil through inhibition of the center. To render the paths of impulses affecting the iris somewhat clearer, it is well to bear in mind the nervous supply of the part : 1. The third nerve, through the ciliary (ophthalmic, lenticular) ganglion, supplies short ciliary nerves to the iris, ciliary muscle, and choroid. 2. The cervical sympathetic reaches the iris chiefly through the long ciliary nerves and the ophthalmic division of the fifth. 3. There are sensory fibers from the fifth nerve ; and. according to some observers, also dilating Abel's from this nerve inde- pendent of the sympathetic, as well as those that may reach the eye by the long ciliary nerves without entering the ciliarv ganglion. 4. The centers from which both the contracting and dilating impulses proceed are situated near to each other in the floor of the aqueduct of Sylvius. It is of practical im- portance to remember the various circumstances under which the pupil contracts and dilates. Contraction (Myosis).—l. Access of strong light to the retina. 2. Associated contraction on accommodation for near objects. 3. Similar associated contraction when the visual axes converge, as in accommodation for near objects. 4. Reflex stimulation of afferent nerves, as the nasal or ophthalmic divis- 536 COMPARATIVE PHYSIOLOGY. ion of the fifth nerve. 5. During sleep. 6. Upon stimulation of the optic or the third nerve, and the corpora quadrigemina or adjacent parts of the brain. 7. Under the effects of certain drugs, as physostignhn, morphia, etc. Dilatation {Mydriasis). — 1. In darkness. 2. On stimulation of the cervical sympathetic. 3. During asphyxia or dyspnoea. 4. By painful sensations from irritation of peripheral parts. 5. From the action of certain drugs, as atropin, etc. The student may impress most of these facts upon his mil id by making the necessary observations, which can be readily done. Pathological. — As showing the importance of such connec- tions, we may instance the fact that, in certain forms of nervous disease (e. g., locomotor ataxia), the pupil contracts when the eye is accommodated to near objects, but not to light (the Argyll-Robertson pupil). In other cases, owing to brain-dis- ease, the pupils may be constantly dilated or the reverse ; or one may be dilated and the other contracted. Comparative. — The iris varies in color in different groups of animals, and even in individuals of the same group ; while the color in early life is not always the permanent one. In shape the pupil is elliptical in solipeds and most rumi- nants. In the pig and dog it is circular, as also in the cat when dilated ; but when greatly contracted in the latter animal, it may become a mere perpendicular slit. The iris is covered posteriorly with a layer of pigment (uvea), portions of which may project through the pupil into the an- terior chamber, and constitute the " sootballs " (corpora nigra) well seen in horses, and very suggestive of inflammatory growths, though, of course, perfectly normal. OPTICAL IMPERFECTIONS OF THE EYE. Anomalies of Refraction.— 1. We may speak of an eye in which the refractive power is such that, under the limitations referred to before (page 531), images are focused on the retina, as the emmetropic eye. The latter is illustrated by Fig. 384. In the upper figure, in which the eye is represented as passive (negatively accommodated), parallel rays— i. e., rays from ob- jects distant more than about seventy yards (according to some writers much less) — are focused on the retina ; but those from objects near at hand, the rays from which are divergent, are focused behind the retina. In the lower figure the lens is rep- VISION. 537 resented as more bulging, from accommodation, as such diver- gent rays are properly focused. 2. In the myopic (near-sighted) eye the parallel rays cross within the vitreous humor, and diffusion-circles being formed on the retina, the image of the object is necessarily blurred, Vs^r. Fig. 384. — Diagrams to illustrate conditions of refraction in normal eye when unac- commodated (passive, or nearly accommodated), and when accommodated for "near" objects (after Landois). so that an object must, in the case of such an eye, be brought unusually near, in order to be seen distinctly — i. e., the near ■point is abnormally near and the far point also, for parallel Fig. 385.— Anomalies of refraction in a myopic eye (after Landois). rays can not be focused ; so that objects must be near enough for the rays from them that enter the eye to be divergent. The myopic eye is usually a long eye, and, though the mechanism of accommodation may be normal, it is not so usually, the ciliary muscle being frequently defective in some of its fibers, which may be either hypertrophied or atrophied, or with some affected one way and others in the opposite. More- 538 COMPARATIVE PHYSIOLOGY. over, there is also generally, in bad cases, " spasm of accommo- dation" (i. e., of the ciliary muscle), with increased ocular ten- sion, etc. The remedies are, rest of the accommodation mechan- ism and the use of concave glasses. 3. The opposite defect is hypermetropia. The hypermetropic eye (Fig. 386), being too short, parallel rays are focused be- hind the retina ; hence no distinct image of distant objects can Fig. 386.— Anomalies of refraction in the hypermetropic eye (after Landois). be formed, and they can only be seen clearly by the use of con- vex glasses, except by the strongest efforts at accommodation. When the eye is passive, no objects are seen distinctly beyond a certain distance — i.e., the near point is abnormally distant (eight to eighty inches). The defect is to be remedied by the use of convex glasses. 4. Presbyopia, resulting from the presbyopic eye of the old, is owing to defective focusing power, partly from diminished elasticity (and hence flattening) of the lens, but chiefly, proba- bly, to weakness of the ciliary muscle, so that the changes required in the shape of the lens, that near objects may be dis- tinctly seen, can not be made. The obvious remedy is to aid the weakened refractive power by convex glasses. It is practi- cally important to bear in mind that, as soon as any of these defects in refractive power (though the same remark applies to all ocular abnormalities) are recognized, the remedy should be at once applied, otherwise complications that may be to a large extent irremediable may ensue. VISUAL SENSATIONS. We have thus far considered merely what takes place in the eye itself or the physical causes of vision, without reference to those nervous changes which are essential to the perception of VISION. 539 an object. It is true that an image of the object is formed on the retina, but it would be a very crude conception of nervous processes, indeed, to assume that anything resembling that image were formed on the cells of the brain, not to speak of the superposition of images inconsistent with that clear mem- ory of objects we retain. Before an object is " seen," not only must there be a clear image formed on the retina, but impulses generated in that nerve expansion must be conducted to the brain, and rouse in certain cells there peculiar molecular condi- tions, upon which the perception finally depends. For the sake of clearness, we may speak of the changes Fig. 387. Fig. 388. Fig. 387.— Vertical section of retina (after H. Mflller). 1, layer of rods and cones; 2 rods; 8, cones; 4, 5, 6, external granule layer; 7, interna! granule layer; !), 10 fine- ly granular gray layer; 11, layer of nerve-cells; 1:2,14, fibers of optic nerve; 13 membrana hmitans. Fio. 388.— Connection of rods and cones of retina with nervous elements (after Sap- |>ey) 1,2,3, rods and cones seen from in front; 4, 5,0, side view. The rest will be clear from the preceding figure. 540 COMPARATIVE PHYSIOLOGY. effected in the retina as sensory impressions or impulses, which, when completed by corresponding changes in the brain, develop into sensations, which are represented psychically by percep- tions ; hence, though all these have a natural connection, they may for the moment be considered separately. It is as yet beyond our power to explain how they are related to each other except in the most general way, and the manner in which a mental perception grows out of a physical alteration in the molecules of the brain is at present entirely beyond human comprehension . Affections of the Eetina.— There is no doubt that the fibers of the optic nerves can not of themselves be directly affected by light. This may be experimentally demonstrated to one's self by a variety of methods, of which the following is readily car- ried out : Look at the circle (Fig. 389) on the left hand with the Fig. 389 (after Bernstein). right eye, the left being closed, and, with the page about twelve to fifteen inches distant, gradually approximate it to the eye, when suddenly the cross will disappear, its image at that dis- j:<)j^ry:;: t ;:l;r,!r';''^. ...;■■'.■'../■;, "" :i " ...j, ■•■■.,■':■■' ■■';:i , ',\' ui /i l , , ! , "; , / l j,i! 1 , m/m S-e- Fig. 390.— Diagrammatic section of macula lutea in man (after Huxley), a, a, pigment of choroid; b, c, b, c, rods and cones; d, d, outer granular or nuclear layer; /,/, inner granular layer; g, ij, molecular layer; h, h, layer of nerve-cells; i, i, fibers of optic nerve. VISION. 511 tance having fallen on the blind-spot, or the point of entrance of the optic nerve. It remains, then, to determine what part of the retina is affected by light. The evidence that it is the layers of rods and cones is convincing. It has been shown that parts of the retina itself internal to these layers cast perceptible, shadows, the con- clusion that the rods and cones are the essential parts of the sensory organ would be inevitable. The Laws of Retinal Stimulation. — It may be noticed that, when a circular saw in a mill is rotated with extreme rapidity, it seems to be at rest. If a stick on fire at one end be rapidly moved about, there seems to be a continuous fiery circle. If a top painted in sections with various colors be spun, the different colors can not be distinguished, but there is a color resulting from the blending of the sensations from them all, which will be white if the spectral colors be employed. When, on a dark night, a moving animal is illuminated by a flash of lightning, it seems to be at rest, though the attitude is one we know to be appropriate for it during locomotion. It becomes necessary to explain these and similar phe- nomena. Another observation or two will furnish the data for the solution. If on awakening in the morning, when the eyes have been well rested and the retina is therefore not so readily fatigued, one looks at the window for a few seconds and then closes the eyes, he may perceive that the picture still remains visible as a positive after-image ; while, if a light be gazed upon at night and the eyes suddenly closed, an after-image of the light may be observed. It thus appears, then, that the impression or sensation out- lasts the stimulus in these cases, and this is the explanation into which all the above-mentioned facts fit. When the fiery point passing before the eyes hi the case of the fire-brand stimu- lates the same parts of the retina more frequently than is con- sistent with the time required for the previous impression to fade, there is, of necessity, a continuous sensation, which is in- terpreted by the mind as referable to one object. In like man- ner, in the case of a moving object seen by an electric flash, the duration of the latter is so brief that the object illuminated can not make any appreciable change of position while it lasts; a second flash would show au alteration, another part of the 542 COMPARATIVE PHYSIOLOGY. retina being stimulated, or the original impression having faded, etc. In the case of a top or (better seen) color-disk, painted into black and white sectors, it may be observed that with a faint light the different colors cease to appear distinct with a slower rotation than when a bright light is used. The variation is between about T V and -5V of a second, according to the intensity of the light used. Fusion is also readier with some colors than others. It is a remarkable fact that one can distinguish as readily between the quantity of light emanating from 10 and 11 candles as between 100 and 110. The Visual Angle. — If two points be marked out with ink on a sheet of white paper, so close together that they can be just distinguished as two at the distance of 12 to 20 inches, then on removing them a little farther away they seem to merge into one. The principle involved may be stated thus : When the dis- tance between two points is such that they subtend a less visual angle than 60 seconds, they cease to be distinguished as two. Fig. 391 illustrates the visual angle. It will be noticed that a larger object at a greater distance subtends the same visual angle as a smaller one much nearer. The size of the retinal Fig. 391.— The visual angle. The object at A" appears no larger than the one at .4 (Le Conte). image corresponding to 60 seconds is "004 mm. (4 n), and this is about the diameter of a single rod or cone. It is not, how- ever, true that when two cones are stimulated two objects are inferred to exist in every case by the mind ; for the retina va- ries in different parts very greatly in general sensibility and in sensibility to color. It is noticeable that visual discriminative power can be greatly improved by culture, a remark which applies especially to colors. It seems altogether probable that the change is cen- tral in the nerve-cells of the part or parts of the brain con- VISION. 543 cerned, especially of the cortical region, where the cell processes involved in vision are finally completed. Color- Vision, — As we are aware by experience the range and accuracy of color perception in man is very great, though vari- able for different persons, a good deal being dependent on culti- vation. However, there are also pronounced natural differ- ences, some individuals being unable to differentiate between certain primary colors as red and green, and so are " color- blind." It is of course difficult to determine in how far the lower animals can discriminate between colors ; but in certain groups, as the birds, it would seem to be reasonable to conclude that their color-perceptions are highly developed. It is further probable that in this group, and possibly some others with the eyes placed more in the lateral than the anterior portion of the head, the retinal area for the most distinct vision, including that for colors is larger than in man, at all events. PSYCHOLOGICAL ASPECTS OF VISION. It is impossible to ignore entirely, in treating of the physi- ology of the senses, the mind, or perceiving ego. By virtue of our mental constitution, we refer what we " see " to the external world, though it is plain that all that we per- ceive is made up of certain sensations. We recognize the " visual field " as that part of the outer world within which alone our vision can act at any one time ; and this is, of course, smaller for one than for both eyes. If one takes a large sheet of paper and marks on its center a spot on which one or both eyes are fixed, by moving a point up or down, to the right or the left, he may ascertain the limits of the visual field for a plane surface. The visual field for both eyes measures about 180° in the horizontal meridian ; for one eye about 145°; and in the vertical meridian 100°. After-images, etc. — Positive after-images have already been referred to ; but an entirely different result, owing to exhaus- tion of the retina, may follow when the eye is turned from the object. If, after gazing some seconds at the sun, one turns away or merely closes the eyes, he may see black suns. In like manner, when one turns to a gray surface after keeping the eyes fixed on a black spot on a white ground, he will see a light spot. Such are termed negative after-images, and these may themselves be colored, as when one turns from a red to a 544 COMPARATIVE PHYSIOLOGY. white surface and sees the latter green, siclered as the results of exhaustion. They may be con- CO-ORBINATION OF THE TWO EYES IN VISION. As a matter of fact, we are aware that an object may be seen as one either with a single eye or with both. For bi- nocular vision it may be shown that the images formed on the two retinas must fall invariably on corresponding points. The position of the latter may be gathered from Fig. 392. It will be no- ticed that the malar side of one eye corresponds to the nasal side of the other, though upper always answers to upper and lower to lower. This may also be made evident if two saucers (representing the fundus of each eye) be laid over each other and marked off, as in the figure. That such corresponding points do Fig. 392.— Diagram to illus- actually exist maybe shown by turniug (after C Foster) 11 z?j?°leit one eye so that the image shall not plttt te o?, e e 8; eye''ire e - &11, *> indicated in the figure. Only eponding to a x , b u c x , m now au( j then, however, is a person to the other. The lower fig- ' A ures are projections of the be found w T ho can voluntarily acco.n- tffitaft (l?ey" S Itmay be plish this, but it occurs in all kinds of side'oTone'reti^ fovrl natural or induced squint, as inalcohol- ■ sponds to the nasal side of { sm owing 1 to partial paralysis of some tliG other —Z. of the ocular muscles. We are thus naturally led to consider the action of these muscles. Ocular Movements. — Upon observing the movements of an individual's eyes, the head being kept stationary, it may be noticed that (1) both eyes may converge ; (2) one diverge and the other turn inward ; (3) both move upward or downward ; (4) these movements may be accompanied by a certain degree of rotation of the eyeball. The eye can not be rotated around a horizontal axis without combining this movement with others. To accomplish the above movements it is obvious that certain muscles of the six with which the eye is provided must work in harmony, both as VISION. 545 to the direction and degree of the movement — i. e., the move- ments of the eyes are affected hy very nice muscular co-ordina- tions. Fig. 303.— View of the two eyes and related parts (after Helmholtz.) Fig-. 394 is meant to illustrate diagrammatically the move- ments of the eyeball. While the several recti muscles elevate or depress the eye, and turn it inward or outward, and the oblique muscles rotate it, the movements produced by the superior and inferior recti always corrected by the assistance of the oblique muscles, since the former tend of themselves to turn the eye somewhat in- ward. In like manner the oblique muscles are corrected by the recti. The following tabular statement will expi*ess the conditions of muscular contraction for the various movements of the eye in man : Straight move- ments;. Elevation Rectus superior and obliquus in- ferior. Depression Rectus inferior and obliquus su- perior. | Adduction to nasal side. . .Rectus interims, i. Adduction to malar side. . . Rectus externus. 35 546 COMPARATIVE PHYSIOLOGY. Oblique move- - ments. r Elevation with adduction. . Rectus superior and internus, with obliquus inferior. Depression with adduction.Reetus inferior and internus, with obliquus superior. Elevation with abduction. . Rectus superior and externus, with obliquus inferior. Depression with abduction.Rectus inferior and externus, with obliquus superior. "What is the nervous mechanism by which these " associ- ated " movements of the eyes are accomplished ? It has been found, experimental- ly, that when different parts of the corpo- ra quaclrigemina are stimulated, certain movements of»the eyes follow. Thus stimu- lation of the right side of the nates leads to movements of both eyes to the left, and the reverse when the opposite side is stimu- lated ; also, stimula- tion in the middle line causes convergence and downward move- ment, etc., with the corresponding move- ments of the iris. Since section of the nates in the middle line leads to move- ments confined to the eye of the same side, the center would ap- pear to be double. However, it may be that the cells actually concerned do not lie in the corpora quadrigemina, but below or outside of them. The localization is as yet incomplete. In many groups of animals, including the solipeds, ruminants, and Carnivoi^a, there is a posterior rectus or retractor oculi by which the eye may be Fig. 304.— Diagram intended to illustrate action of extrinsic ocular muscles (after Pick). The heavy lines represent the muscles of the eyeball, and the fine lines the axes of movement. VISION. 547 Fig. 395. — Diagrammatic section of the eye of the horse (Chauvean). a, optic nerve; b. sclerotic coat; c, choroid; d, retina; e, cornea; /, iris; g, h, ciliary ligament and processes of choroid represented as separated from it, the better to define its lim- its; i, insertion of ciliary processes on crystalline lens; j, crystalline lens; k\ lens capsule; /, vitreous body; m,m, anterior and posterior chambers; o. theoretical indication of aqueous humor; p, p. tarsi (eyelids); q, q, fibrous membrane of eye- lids; r, elevator muscle of upper eyelid; s. s, orbicularis muscle of lids; t, t, skin of eyelids; u, conjunctiva; ?>, epidermic layer of the latter covering cornea; x, posterior rectus muscle; y, superior rectus; z, inferior rectus; w, fibrous sheath of orbit (orbital membrane). drawn inward and thus protected the more effectually against blows and obstacles. It seenis to be of special importance in animals that feed with the head down for long- periods, as in the ruminants, in which class it is most highly developed. The macula lutea is believed to exist only in man, the quadra- man a. and certain of the lizard tribe — i. e., in animals in which the axes of the eyeballs are parallel to each other. Nevertheless, there is no reason to doubt that the cen- tral part of the retina is more sensitive than the periphery or that there is a central retinal zone for distinct vision in all vertebrates, though not so limited in all cases as in man. Fig. 896. — Diagram to illustrate de- cussation of fibers in the optic commissure of man (after Flint). 548 COMPARATIVE PHYSIOLOGY. Estimation of the Size and Distance of Objects.— The pro- cesses by which we form a judgment of the size and distance of objects are closely related. As we have already shown (page 542), the visual angle varies both with the size and the distance of an object. Knowing that two objects are at the same distance from the eye, we esti- mate that the one is larger than the other when the image one forms on the retina is larger, or when the visual angle it sub- tends is greater than in the other case, and conversely. Thus, knowing that two persons are at the distance of half a mile away, if one is judged by us to be smaller than the other, it will be because the retinal image corresponding to the object is smaller, other things being equal. But the subject is more complex than might be inferred from these statements. Objects of a certain color seem nearer than others ; also those that are brighter, as in the case of mountains on a clear day. And not only do all the qualities of the image itself enter as data into the construction of the judgment, but numerous mus- cular sensations. The eyes accommodating and converging for near objects, from the law of association, give rise to the idea of nearness, for habitually such takes place when near objects are viewed, so that the subject becomes very complex. That we judge imperfectly of the position of an object with but one eye is realized on attempting to stick a pin into a certain small spot, thread a needle, cork a small bottle, etc., when one eye is closed. Solidity. — By the use of one eye alone we can form an idea of the shape of a solid body ; though, in the case of such as are very complex, this process is felt to be both laborious and im- perfect. From the limited nature of the visual field for distinct vision, it follows that we can not with one eye see equally dis- tinctly all the parts of a solid that is turned toward us. After a little practice one may learn to define for himself what he actually does see. Such a figure as that following results from the combina- tion, mentally, of two others, which answer to the images fall- ing on the right and on the left eye respectively. In order that such fusion shall take place, the respective images must fall on identical (corresponding) parts of the retina. As is well known, the pictures used for stereoscopes give different views of the one object, as represented on a flat sur- VISION. 549 face. These are thrown upon corresponding points of the retina by the use either of prisms or mirrors, when the idea of solidity is produced. As to whether movements of the eyes (conver- gence) are necessary for stereoscopic vision is disputed. It has d/ a Fig. 397. — Illustrates binocular vision. If the truncated pyramid, P, be looked at with the head held perpendicularly over the figure, the image formed in the right eye when the left is closed is figured on the right, and that seen when the right eye is closed is represented by the figure in the middle. No superposition of these figures will give P, yet by a pyschic process they are combined into P, the figure as it appears to both eyes (after Bernstein). been inferred, from the fact that objects appear solid during an electric flash, the duration of which is far too short to per- mit of movements of the ocular muscles, that such movements are not essential. The truth seems to lie midway ; for while simple figures may not require them, the more complex do, or, at all events, the judgment is very greatly assisted thereby. It is of the utmost importance to bear in mind that all visual judgments are the result of many processes, in which, not the sense of vision alone, but others, are concerned; and the mutual dependence of one sense on another is great, probably beyond our powers to estimate. Reference has been made to this sub- ject previously. PROTECTIVE MECHANISMS OF THE EYE. The eyelids have been appropriately compared to the shut- ters of a window. They are, however, not impervious to light, as any one may convince himself by noticing that he can locate the position of a bright light with the eyes shut; also that a sensitive person (child) will turn away (reflexly) from a light when sleeping if it be suddenly brought near the head. The Meibomian glands, a modification of the sebaceous, secrete an oily substance that seems to protect the lids against the lachry- mal fluid, and prevents the latter running over their edges as oil would on the margins of a vessel. The lachrymal gland is 550 COMPARATIVE PHYSIOLOGY. not in structure unlike the parotid, the secretion of which its own somewhat resembles. The saltness of the tears, owing to abundance of sodium chloride, is well known to all. The nervous mechanism of se- cretion of tears is usually reflex, the stimulus coming from the action of the air against the eyeball or from partial desiccation owing to evaporation. When the eyeball itself, or the nose, is irritated, the afferent nerves are the branches of the fifth, to which also belong the efferent nerves. Tbe latter include also the cervical sympathetic. But it will, of course, be understood that the afferent impulses may be derived through a large num- ber of nerves, and that the secreting center may be acted upon directly by the cerebrum (emotions). The excess of lachrymal secretion is carried away by the nasal duct into which tbe lach- rymal canals empty. While it is well known that closure of the lids by the orbicularis muscle favors the removal of the fluid, the method by which the latter is accomplished is not agreed upon. Some believe that the closure of the lids forces the fluid on through the tubes, when they suck in a fresh quantity ; others that the orbicularis drives the fluid directly through the tubes, kept open by muscular arrangements ; and there are several other divergent opinions. The prevention of winking leads to irrita- tion of the eye, which may assume a serious character, so that the obvious use of the secretion of tears is to keep the eye both moist and clean. Though rudimentary in man, there is in all our domestic animals a third eyelid {membrana nictitans) which may be made to sweep over the eye and thus cleanse it. It is especially well developed in those groups of mammals that can not derive assist- ance in wiping the eyes from their fore- limbs, hence is found in perfection in solipeds and ruminants. It is made up of a fibro - cartilage, prismatic at its base, and thus anteriorly where it is covered by the conjunctiva. It is most attached at the inner can thus of the eye, from which region it can spread over the whole globe anteriorly. The fibro-cartilage is con- tinued backward by a fatty cushion which is loosely attached Fig. 398.— Lachrymal canals, lachrymal sac, and nasal canal in man opened from the front (after Sappey.). VISION. 551 to all the ocular muscles. When the globe of the eye is with- drawn by its muscles, the third eyelid is pushed out in a me- chanical way with little or no direct assistance from muscles. In this connection may also be mentioned the gland of Harder, a yellowish red glandular structure situated about the middle of the outer surface of the third eyelid, which furnishes a thick unctuous secretion, also of a protective character. These structures are all the more necessary, as in few animals is the globe of the eye so well protected by bony walls as in man. SPECIAL CONSIDERATIONS* Comparative. — It seems to be established that some animals devoid of eyes, as certain myriopods, are able to perceive the presence of light, even when the heat-rays are cut off. The most rudimentary beginning of a visual apparatus appears to be a mass of pigment with a nerve attached, as in certain worms ; though it is questionable whether mere collections of pigment without nerves may not in some instances represent still earlier rudiments of our eyes. The eye of the fish is characterized by flatness of the cornea ; spherical form of the lens, the anterior surface of which pro- jects far beyond the pupillary opening ; the presence of a pro- cess of the choroid {processus falciformis) ; and usually the ab- sence of eyelids, the cornea being covered with transparent skin. The eye of the bird, in some respects the most perfect visual organ known, is of peculiar shape as a whole, presenting a large posterior surface for retinal expansion ; a very convex cornea, a highly developed lens, an extremely movable iris ; eyelids and a nictitating membrane (third eyelid), which may be made to cover the whole of the exposed part of the eye, and thus shield screen -like from excess of light ; ossifications of the scle- rotic ; a structure which is a peculiar modification of the choroid, of which it is a sort of offshoot and like it very vascular, answering to the falciform process of the eye of the fish and the reptile. From its appearance it is termed the pecten. Birds, on account of a highly developed ciliary muscle, possess wonderful powers of accommodation, rendered important on account of their rapid mode of progression. They also seem to be able to alter the size of the pupil at will. Their iris is composed of striped muscular fibers. A layer of fibrous tissue outside of the choroidal epithelium 552 COMPARATIVE PHYSIOLOGY. forms the tapetum. It is most pronounced in the carnivora and gives the glare to their eyes as well seen in the cat tribe at night. It has been supposed to act as a reflector and thus assist in vision in the same way as a pair of carriage lamps light up the roadway. Evolution. — From the above brief account of the eye in dif- ferent grades of animals, it will ap- pear that its modifications answer to differences in the environment. Adaptation is evident. Darwin believes this to have been effected partly by natural selection — i. e., the survival of the animal in which the form of eye appeared best adapted to its needs — and partly by the use or disuse of certain parts. The latter is illustrated by the blind fishes, insects, etc., of certain caves, as those of Kentucky; and it is of extreme interest to note that various grades of transition toward complete blindness are observable, CM .ciliary muscle. Birds have according to the degree of darkness usually keen vision, great pow- , ° ° er of accommodation, and ex- in which the animal lives, whether treme mobility of the iris. , ,, .., . ,, , wholly within the cave or where there is still some light. A crab has been found with the eye- stalk still present, but the eye itself atrophied. Again, ani- mals that burrow seem to be in process of losing their eyes, through inflammation from obvious causes ; and some of them, as the moles, have the eye still existing, though well-nigh or wholly covered with skin. Internal parasites are often with- out eyes. It is not difficult to understand how one bird of prey, with eyes superior to those of its fellows, would gain supremacy, and, in periods of scarcity, survive and leave offspring when others would perish. It is, of course, impossible to trace each step by which the vertebrate eye has been developed from more rudimentary forms, though the data for such an attempt have greatly accu- mulated within the last few years; and it is not to be forgotten that even the vertebrate eye has many imperfections, so that no doctrine of complete adaptation, according to the argument from design as usually understood, can apply. Fig. 399.— Eye of nocturnal bird of prey (after Wiedersheim). Co, cornea; L, lens; Rt, retina; P, pecten; No, optic nerve; Sc, ossification of sclerotic coat; VISION. 553 It is of great importance to recognize that what we really see depends more upon the brain and the mind than the eye. If any one will observe how frequent are his incipient errors of vision speedily corrected, he will realize the truth of the Brain above medulla Centre in region of- corp. quadrigemina -Cortical centre Centre in optic thalamus etina Fig. 400. — Diagram intended to illustrate the elaboration of visual impulses, beginning in retina and culminating in the cerebral cortex. Course of impulses is indicated by arrows. Knowledge of auditory centers is not yet exact enough to permit of the construction of a diagram, though doubtless eventually the central processes will be localized as with vision. The latter remark applies" to the other senses to nearly the same extent, possibly quite as much. above remark. Precisely the same data furnished by the eye are in one mind wox\ked up in virtue of past experience (edu- cation) into an elaborate conception, while to another they an- swer only to certain vague forms and colors. And herein lies the great superiority of man's vision over that of all other animals. Within the limits of their mental vision do all creatures see. Man has not the keen ocular discriminating power of the hawk; he can neither see so far nor so clearly ; nor has he the wide field of vision of the gazelle; but he has the mental resource which enables him to make more out of the materials with which his eyes furnish him. It is by virtue of his higher cere- bral development that he has added to his natural eyes others 554 COMPARATIVE PHYSIOLOGY. in the microscope and telescope, which none of Nature's forms can approach. Pathological. — There may be ulceration of the cornea, in- flammation of this part, or various other disorders which lead to opacity. The low vitality of this region, probably owing to absence of blood-vessels, is evidenced by the slowness with which small ulcers heal. Opacity of the lens (cataract) when complete causes blindness, which can be only partially reme- died by removal of the former. Inflammations of any part of the eye are serious, from possible adhesions, opacities, etc., fol- lowing. Should such be accompanied by great excess of intra- ocular tension, serious damage to the retina may result. Of course, atrophy of the optic nerve (due to lesions in the brain, etc.) is irremediable, and involves blindness. Inspection of the internal parts of the eye (fundus oculi) often reveals the first evidence of disease in remote parts as the kidneys. From what has been said of the movements of the two eyes in harmony, etc. , the student might be led to infer that disease of one organ, in consequence of an evident close connection of the nervous mechanism of the eyes, would be likely to set up a corresponding condition in the other unless speedily checked. Such is the case, and is at once instructive and of great practi- cal moment. Paralysis of the various ocular muscles leads to squinting, as already noticed. Brief Synopsis of the Physiology of Vision.— All the other parts of the eye may be said to exist for the retina, since all are .related to the formation of a distinct image on this nervous ex- pansion. The principal refractive body is the crystalline lens. The iris serves to regulate the quantity of light admitted to the eye, and to cut off too divergent rays. In order that objects at different distances may be seen distinctly, the lens alters in shape in response to the actions of the ciliary muscle on the suspensory ligament, the anterior surface becoming more con- vex. Accommodation is associated with convergence of the visual axes and contraction of the pupil. The latter has circular and radiating plain muscular fibers (striped in birds, that seem to be able to alter the size of the pupil at will), governed by the third, fifth, and sympathetic nerves. Contraction of the pupil is a reflex act, the nervous center lying in the front part of the Hoor of the aqueduct of Sylvius, while the action of the other center (near this one) through the sympathetic nerve is tonic. VISION. 555 Accommodation through the ciliary muscle is governed hy a center situated in the hind part of the floor of the third ven- tricle near the anterior bundles of the third nerve, which latter is the medium of the change. When rays of light are focused anterior to the retina, the eye is myopic ; when posterior to it, hypermetropic. The presbyopic eye is one in which the mechanism of accom- modation is at fault, chiefly the ciliary muscle. The point of entrance of the optic nerve (blind-spot) is insensible to light; and visual impulses can be shown to originate in the layers of rods and cones, probably through stimulation from chemical changes effected by light acting on the retina. The sensation outlasts the stimulus ; hence positive after-images occur. Nega- tive after-images occur in consequence of excessive stimulation and exhaustion of the retina, or disorder of the chemical pro- cesses that excite visual impulses. When stimuli succeed one another with a certain degree of rapidity, sensation is continu- ous. The eye can distinguish degrees of light within certain limits, varying by about -j^-g- of the total. Objects become fused or are seen as one when the rays from them falling on the retina approximate too closely on that surface. The brain, as well as the eye itself, is concerned in such discriminations, the former probably more than the latter. The macula lutea, and especially the fovea centralis, are in man the points of greatest retinal sensitiveness. When the images of objects are thrown on these parts, they are seen with complete distinctness; and it is to effect this result that the movements of the two eyes in concert take place. An object is seen as one when the position of the eyes (visual axes) is such that the images formed fall on corresponding parts of the retina. Binocular vision is necessary to supply the sensory data for the idea of solidity. It is important to remember that, before an .ob- ject is " seen " at all, the sensory impressions furnished by the retina and conveyed inward by the optic nerve are elaborated in the brain and brought under the cognizance of the perceiving ego. We recognize many visual illusions and imperfections of various kinds, the course of which it is difficult to locate in any one part of the visual tract, such as are referred to " irradiation," " contrast," etc. Tbere may also be visual phe- nomena that are purely subjective, and others that result from suggestion rather than any definite sensory basis of retinal 556 COMPARATIVE PHYSIOLOGY. images. Hence what one sees depends on his state of mind at the time. This applies to appreciation of size and distance also, though in such cases we have the visual angle, certain muscular move- ments (muscular sense), the strain of accommodation etc., as guides. HEARING. As the end organ oi vision is protected both without and within, so is the still more complicated end-organ of the sense of hearing more perfectly guarded against injury, being in- closed within a membranous as well as bony covering and sur- rounded by fluid, which must shield it from stimulation, except through this medium. Hearing proper, as distinguished from the mere recognition of jars to the tissues, can, in fact, only be attained through the impulses conveyed to the auditory brain-centers, as originated in the end-organ by the vibrations of the fluid with which it is bathed. It will be assumed that the student has made himself famil- iar with the general anatomy of the ear. The essential points in regard to sound are considered in the chapter on The Voice. It will be remembered that what we term a musical tone, as distinguished from a noise, is characterized by the regularity of vibrations of the air that reach the ear ; and that just as ethereal vibrations of a certain wave-length give rise to the sensation of a particular color, so do aerial vibrations of a definite wave-length originate a certain tone. In each case must we take into account a physical cause for the physiological effect, and these bear a very exact relationship to one another. As will be seen later, while in all animals that have a well- defined sense of hearing the process is essentially such as we have indicated above, the means leading up to the final stimu- lation of the end-organ are very various. At present we shall consider the acoustic mechanism in mammals, with special ref- erence to man. There are in fact three sets of apparatus : (1) one for collecting the aerial vibrations; (2) one for transmit- ting them ; and (3) one for receiving the impression through a fluid medium; in other words, an external, middle, and in- ternal ear. 558 COMPARATIVE PHYSIOLOGY. The external ear in man being- practically immovable, owing to the feeble development of its muscles, has, as compared with ;:i ttl- n Ftg. 401. —Section through auditory organ (after Sappey). 1, pinna; 2, 4, 5, cavity of concha, external and auditory meatus with opening of ceruminous glands; 6, membrana tympani; 7. anterior part of incus; 8, malleus; 9, long handle of mal- leus, attached to internal surface of tympanic membrane— it is here represented as strongly indrawn; 10, tensor tympani muscle; 11, tympanic cavity; 12, Eustachian tube; 13, superior semicircular canal; 14, posterior semicircular canal; 15, exter- nal semicircular canal; 16, cochlea; 17, internal auditory meatus; 18, facial nerve; 19. large petrosal nerve; 20, vestibular branch of auditory nerve; 21, cochlear branch of same. such animals as the horse or cow, but little use as a collecting' organ for the vibrations of the air. The meatus or auditory canal may be regarded both as a conductor of vibrations and as protective to the middle ear, especially the delicate drum- head, since it is provided with hairs externally in particular, and with glands that secrete a bitter substance of an unctuous nature. The Membrana Tympani is concavo-convex in form, and having attached to it the chain of bones shortly to be noticed, is well adapted to take up the vibrations communicated to it from the air; though it also enters into sympathetic vibration when the bones of the head are the medium, as when a tuning- fork is held between the teeth. Ordinary stretched membranes have a fundamental (self-tone, proper tone) tone of their own, to which they respond more readily than to others. HEARING. 559 If such held for the membrana tympani, it is evident that certain tones would be heard better than others, and that when Fig. 402.— Photographic representation of right membrana tympani, viewed from within (after Flint and Uiidinger). 1. divided head of malleus; 2. neck; 3. handle, with attachment of tendon of tensor tympani; 4, divided tendon; 5. 6, long handle of malleus; 7, outer radiating and inner circular fibers of tympanic membrane; 8, fibrous ring encircling membrana tympani; 9, 14, 15, dentated fibers of Gruber; 10, 11. posterior pocket connecting with malleus; 12, anterior pocket; 13, chorda tym- pani nerve. the fundamental one was produced the result might be a sensa- tion unpleasant from its intensity. This is partially obviated by the damping- effect of the auditory ossicles, which also pre- vent after-vibrations. Some suppose that what we denominate shrill or harsh sounds are, in part at least, owing- to the auditory meatus hav- ing a corresponding fundamental note of its own. The Auditory Ossicles.— Though these small bones are con- nected by perfect joints, permitting a certain amount of play 560 COMPARATIVE PHYSIOLOGY. upon one another, experiment has shown that they vibrate in response to the movements of the drum-head en masse ; though the stapes has by no means the range of movement of the han- dle of the malleus ; in other words, there is loss in amplitude, Fig. 403.— Section of auditory organ of horse (after Chauveau). A, auditory canal; B, membrana tympani; C, malleus; D, incus; F, stapes; O, mastoid cells; //, fenestra ovalis; I, vestibule; ./, K, L, outline of semicircular canals; M, cochlea; N, commencement of scala tympani. but gain in intensity. A glance at Fig. 404 will show that the end attained by this arrangement of membrane and bony levers, which may be virtually reduced to one (as it is in the frog, etc.), is the transmission of the vibrations to the membrane of the fenestra ovalis, through the stapes finally, and so to the fluids within the internal car. But it might be supposed that, for the avoidance of shocks and the better adaptation of the apparatus HEARING. 561 to its work, some regulative apparatus, in the form of a nerv- ous and muscular mechanism, would have been evolved in the Fig. 404.— Diagrammatic representation illustrating auditory processes (after Beaunis). A, external ear; B, middle ear: C, internal ear; 1, auricle; 2. external auditory meatus; 3, tympanum; 4, membrana tympani; 5, Eustachian tube; 6. mastoid cells; 10, foramen rotundum; 11, foramen ovale; 12. vestibule; 13, cochlea; 14, scala tympani; 15, scala vestibuli; 16, semicircular canals. N. B. — The ear is so complicated an organ that it is almost impossible to give a dia- grammatic representation of it at once simple and complete in a single figure. A comparison of the whole series of cuts is therefore desirable. It is essential to understand how the end-organ within the scala media is stimulated. higher groups of animals. Such is found in the tensor tym- pani, laxator tympani, and stapedius muscles, as well as the Eustachian tube. Muscles of the Middle Ear. — The tensor tympani regulates the degree of tension of the drum-head, and hence its ampli- tude of vibration, having a damping effect, and thus preventing the ill results of very loud sounds. Ordinarily, this is. doubtless, a reflex act, in which the fifth is usually the afferent nerve concerned. It is well-known that, when we are aware that an explosion is about to take place, we are not as much affected by it, which would seem to argue a voluntary power of accommodation ; but of this we must speak with caution. According to some authorities the laxator tympani is not a 36 562 COMPARATIVE PHYSIOLOGY. muscle, but a supporting ligament for the malleus. The stape- dius, however, has the important function of regulating the movements of the stapes, so that it shall not be too violently driven against the membrane covering the fenestra ovalis. The two muscles, stapedius and tensor, suggest the accom- modative mechanism of the iris. The motor nerve of the sta- pedius is derived from the facial; of the tensor, from the tri- geminus through the otic ganglion. The Eustachian Tube. — Manifestly, if the middle ear were closed permanently, its air would gradually be absorbed. The drum-head would be thrust in by atmospheric pressure, and become useless for its vibrating function. The Eustachian tube, by communicating with the throat, keeps the external and internal pressure of the middle ear balanced. Whether this canal is permanently open, or only during swallowing, is as yet undetermined. ■■:-:-^:^%-" : ' ■>-.. PlG. 405.— Diagram intended to illustrate the processes of hearing (after Landois) A G, external auditory meatus; 7\ tympanic membrane: A", malleus; a, incus; P, mid- dle ear; o, fenestra ovalis; r, fenestra rotunda; pi, seala tympani; vt, scala vesti- Imli; I'. vestibule; 8, saccnle; U, utricle; H, semicircular canals; TE, Eustachian tube. Long arrow indicates line of traction of tensor tympani; short curved one that of Stapedius. One may satisfy himself that the middle ear and pharynx communicate, by closing the nostrils and then distending the upper air-passages by a forced expiratory effort, when a sense of distention within the ears is experienced, owing to the rise of atmospheric pressure in the tympanum. HEARING, 563 Fig. 406. — Section through one of the coils of cochlea (after Chauveau). ST. scala tym- pani; SV, scaja vestibuli; CC, cochlear canal (scala media); Co, organ of Corti: 11, membrane of Reissner; b, membrana basilaris; feo, lamina spiralis ossea: I, membrana tectoria; 1, 2, rods of Corti; nc, cochlear nerve with its ganglion, gs. Pathological. — Inflammation of the tympanum may result in adhesions of the small bones to other parts or to each other, Fid. 407.— I. Transverse section of a turn of cochlea. TI. Ampulla of a semicircular canal and its crista acoustica; ap, auditory cells, one of which is a hair-cell. 111. Diagram of labyrinth of man. IV. Of bird. V. Of fish. (After Landois.i 564 COMPARATIVE PHYSIOLOGY. Fig. 408.— Diagrammatic representation of ductus cochlearis and organs of Corti (after Landois). N, nerve of cochlea;- A', inner, and P, outer, hair-cells; n, nerve-fibrils terminating in P; a, a, supporting cells; d, cells of sulcus spiralis; z. inner rod of Corti; y, outer rod of Corti; mb, membrane of Corti (membrana tectoria); o, membrana reticularis; H, G, cells of area toward outer wall. A.N. Coch. Fig. 410. Fio. 409. Fig. 409. — Auditory epithelium from macula acoustica of saccule of alligator, much magnified (after Schafer). c. c. columnar hair-cells; f,f, fiber cells; n, nerve-fiber losing its medullary sheath and about to terminate in "columnar auditory cells; h, auditory hair; h' , base of auditory hairs split up into fibrils. I'ii;. 410. — Diagrammatic representation of distribution of auditory nerve in membra- nous labyrinth and cochlea (after Huxley). HEARING. 565 •or to occlusion of the Eustachian tube from excess of secretion, cicatrices, etc., in consequence of which the relations of atmos- pheric pressure become altered, the membrana tympani being indrawn, and the whole series of conditions on which the nor- mal transmission of vibrations depends disturbed, with the natural result, partial deafness. The hardness of hearing- ex- perienced during a severe cold in the head (catarrh, etc.) is owing in great part to the occlusion of the Eustachian tube, which may be either partial or complete. By filling one or both of the ears external to the mem- brana tympani with cotton-wool, one may satisfy himself how essential for hearing is the vibratory mechanism, which is, of course, under such circumstances inactive or nearly so ; hence the deafness. When the middle ear is not functionally active, it is still possible, so long as the auditory nerve is normal, to hear vibra- tions of a body (as a tuning-fork) held against the head; though, as would be expected, discrimination as to pitch is very imperfect. Auditory impulses originate within the inner ear — that is to say, in the vestibule and possibly the semicircular canals, but especially in the cochlea. It is to he remembered that the Fig. til.— Diagram intended to illustrate relative position of various parts of ear (after Huxley). K. M, external auditory meatus; Tij. .1/. tympanic membrane; /'>/. tvm- panum; Mall, mallens; Tnc, incus; Stp, stapes; F. o, fenestra ovalis; F.r, fenes- tra rotunda; Si*, Eustachian tube; M. L, membranous labyrinth, only one of the semicircular canals and its ampulla being represented; Sea. V,Sca. T. Sea. \f, scahc of cochlea, represented as straight umcoiled). 566 COMPARATIVE PHYSIOLOGY. whole of the end-organ concerned in hearing is bathed by endo- lymph ; and that the vibrations of the latter are originated by corresponding vibrations of the perilymph, which again is sent Fig. 412.— Photographic diagram of labyrinth (after Flint and Rudinger). Upper fig- ure: 1, utricle; 2, saccule; 3,5, membranous cochlea; 4, canalis reuniens; 6, semi- circular canals. Lower figure: 1, utricle; 2, saccule; 3,4,6, ampulla?; 5,7.8,9, semicircular canals; 10, auditory nerve (partly diagrammatic); 11, 12, 13, 14, 15, dis- tribution of branches of nerve to vestibule and semicircular canals. into oscillation by the movements of the stapes against the membrane covering the fenestra ovalis ; so that the vibrations thus set up without the membranous labyrinth are ti*ans- formed into similar ones within the vestibule and the scala vestibuli, and end, after passing over the scala tympani, against the membrane of the fenestra rotunda. The cochlear canal may be regarded as the seat of the most important part of the HEARING. 567 organ of hearing, and answers to the macula lutea of the eye in many respects. The function of the organ of Corti is unknown. The structure of the ampullae of the semicircular canals, and other parts of the lahyrinth besides those specially con- Fig. 413.— Distribution of cochlear nerve in spiral lamina of anteroinferior part of cochlea of right ear (after Sappey). 1, trunk of cochlea nerve; 2, membranous zone of spiral lamina; 3, terminal expansion of cochlear nerve exposed through- out by removal of superior plate of lamina spiralis; 4, orifice of communication between scala tympani and scala vestibuli. sidered, with their peculiar hair-cells, suggests an auditory function ; but what that may be is as yet quite undetermined. It has been thought that the parts, other than the cochlea, are concerned with the appreciation of noise, or perhaps the in- tensity of sounds ; but this is a matter of pure speculation. AUDITORY SENSATIONS, PERCEPTIONS, AND JUDGMENTS. We have thus far been concerned with the conduction of the aerial vibrations that are the physical cause of hearing ; but before we can claim to have " heard " a word in the highest sense, certain processes, some of them physiological and some psychical, take place, as in the case of vision ; hence we may speak of the affection of the end-organ or of auditory impulses, and of the processes by which these become, by the mediation of the brain, auditory sensations, and when brought under the cognizance of the mind as auditory perceptions and judg- ments. 568 COMPARATIVE PHYSIOLOGY. Auditory Judgments. — Such are much more frequently erro- neous than are our visual judgments, whether the direction or the distance of the sound he considered. As in the case of the eye, the muscular sense, from accommodation of the vibratory mechanism, may assist our judgments, being aided by our stored past experiences (memory) according to the law of asso- ciation. Sounds are, however, always referred to the world without us. The animals with movable ears greatly excel man in estimating the direction, if not the distance, of sounds. There are few physiological experiments more amusing than those performed on a person blindfolded, when attempting to deter- mine either the distance or the direction of a sounding tuning- fork, so gross are the errors made. One who makes such observations on others may notice that most persons move the ears slightly when attempting to make the necessary discriminations, which of itself tends to show how valuable mobility of these organs must be to those animals that have it highly developed. SPECIAL CONSIDERATIONS. Comparative. — Among invertebrates steps of progressive de- velopment can be traced. Thus, in certain of the jelly-fishes we find an auditory vesicle (Fig. 414) inclosing fluid provided with one or more otoliths or calcareous nodules and auditory cells with at- tached cilia, the whole making up an end-organ connected with the auditory nerve. A not very dis- similar arrangement of parts exists in certain mollusks (Fig. 415). The vesicle may lie on a ganglion of the central nervous system. On the other hand, the vesicle may be open to the exterior, as in decapod crustaceans ; and the otoliths be re- placed by grains of sand from with- out. It is difficult to decide what the function of otoliths may be in Fie. 414.— Auditory vesicle of Gery- onia (Garmarin a) seen from the surface (after O. and R. Hert- wigV N and N', the auditory nerves: Ot, otolith; llz, audito- ry cells; ////, auditory cilia (type mammals ; but there seems to be of the auditory organ of the TrachymedUBdi) . little reason to doubt that they com- HEARING. 569 municate vibrations in the invertebrates. When the cephal- opod mollusks, with their highly developed nervous system, are reached, we find a membranous and cartilaginous labyrinth. Among vertebrates the different parts of the mammalian ear are found in all stages of development. The outer ear may be wholly wanting, as in the frog, or it may exist as a meatus only, as in birds. The tympanic cavity is wanting in snakes. Most fishes have a utricle and three semicircular canals, but some Fig. 415. — Auditory vesicle of a heteropod mollusk (Pterotrachea) (after Claus). V, auditory nerve; Of, otolith in fluid of vesicle; Wz, ciliated cells on inner wall of vesicle; Hz, auditory cells; Cz, central cells. have only one ; and the lowest of this group have an ear not greatly removed from the invertebrate type, as may be seen in the lamprey, which has a saccule with auditory hairs and oto- liths, in communication with two semicircular canals. Most of the amphibia are without a membrana tympani. The frog has (1) a membrana tympani communicating with the inner ear by (2) a bony and cartilaginous lever (columella auris), and (3) an inner ear consisting of three semicircular canals, a saccule and utricle containing many otoliths, and a small dilatation of the vestibule, which may indicate an undeveloped cochlea. The membranous labyrinth is contained in a periotic capsule, partly bony and partly cartilaginous, which is supplied with 570 COMPARATIVE PHYSIOLOGY. Fig. 416.— Otoliths from various animals (after Eiidinger). 1, from goat; 2, herring; 3, devil-fish; 4, mackerel; 5, flying-fish; 6, pike; 7, carp; 8, ray; 9, shark; 10, grouse. G.C. Fio. 417.— Transverse section through head of foetal sheep, in region of hind-brain, to illustrate development of ear (after BOtteher). //. B, hind-brain; N, auditory nerve; V. JB, vertical semicircular canal; (!(', canal of cochlea; Ii. V, recessus vestibuli; G, C, auditory ganglion; (/', terminal portion of auditory nerve. HEARING. 571 perilymph. There is a fenestra ovalis, but not a fenestra ro- tunda, though the latter is present in reptiles. In crocodiles and birds the cochlea is tubular, straight, and divided into a scala tympani and a scala vestibuli. The columella of lower forms still persists. In birds and mammals the bone back of the ear is hollowed out to some extent and communicates with the tympanum. Except among the very lowest mammals (Echidna), the ear is such as has been described in detail already. Evolution. — The above brief description of the auditory organ in different groups of the animal kingdom will suffice to show that there has been a progressive development or increasing differentiation of structure, while the facts of physiology point to a corresponding progress in function — in other words, there has been an evolution. No doubt natural selection has played a great part. It has been suggested that this is illustrated by cats, that can hear the high tones produced by mice, which would be inaudible to most mammals ; and, as the very exist- ence of such animals must depend on their detecting their prey, it is possible to understand how this principle has operated to determine even what cats shall survive. The author has noticed that terrier dogs also have a very acute sense of hearing, and they also kill rats, etc. But, unless it be denied that the im- provement from use and the reverse can be inherited, this factor must also be taken into the account. There seem to be great differences between hearing as it exists in man and in lower forms. Birds, and at least some horses, possibly some cats and dogs, like music, and give evidence of the possession of a sense of rhythm, as evidenced by the conduct of the steed of the soldier. On the other hand, some dogs seem to greatly dislike music. Certain animals that appear to be devoid of true hearing, as spiders, are nevertheless sensitive to aerial vibrations ; whether by some special undiscovered organ or through the general cutaneous or other kind of sensibility is unknown. It also seems to be more than probable that some groups of insects can hear sounds quite inaudible to us, though by what organs is in great measure unknown. The so-called musical ear differs from the non-musical in the ability to discriminate differences in pitch rather thau in quality; in fact, that one defective in the former power may possess the latter in a high degree is a fact that has been some- what lost sight of, both theoretically and practically. It does 572 COMPARATIVE PHYSIOLOGY. not at all follow that one with little capacity for tune may not have the qualifications of ear requisite to make a first-rate elo- cutionist. Following custom, we have spoken as though certain defects and their opposites depended on the ear, but in reality we can not, in the case of man at all events, affirm that such is the case ; indeed, it seems, on the whole more likely that they are cerebraF or mental. Auditory discriminations seem to be equally if not more susceptible of improvement by culture than visual ones, especially in the case of the young. A " good ear " seems to depend in no small degree on mem- ory of sounds, though the latter may again have its basis in the auditory end-organs or in the cerebral cortex, as concerned in hearing. The necessity for the close connection between the co-ordinations of the laryngeal apparatus in singing and speak- ing and the ear might be inferred from the fact that many ex- cellent musicians are themselves unable to vocalize the music they perfectly appreciate. Synopsis of the Physiology of Hearing.— The ear can appre- ciate differences in pitch, loudness, and quality of sounds, though whether different parts of the inner ear are concerned in these discriminations is unknown. Hearing is the result of a series of processes, having their physical counterpart in aerial vibrations, which begin in the end-organ in the labyrinth and terminate in the cerebral cortex. We recognize conducting apparatus which is membranous, bony, and fluid. The auditory nerve conveys the auditory impulses to the brain, though ex- actly what terminal cells are concerned and how in originating them must be regarded as undetermined. The essential part of the organ of hearing is bathed by endolymph, and the princi- pal part (in mammals) is within the cochlear canal. Man's power to locate sounds is very imperfect. The auditory brain center (or centers) has not been definitely located. Compara- tive anatomy and physiology point clearly to a progressive development of the sense of hearing. THE SENSES OF SMELL AND TASTE. SMELL. The nose internally may be divided into a respiratory and an olfactory region. The latter, which corresponds, of course, with the distribution of the olfactory nerve, embraces the upper and part of the middle turbinated bone and the upper part of the septum, all of which differ in microscopic structure from the respiratory region. Among the ordinary cylindrical epi- thelium of the olfactory region are found peculiar hair-cells highly suggestive of those of the labyrinth of the ear, and Fig. 418.— Parts concerned in smell (after Hirschfeld). 1, olfactory ganglion and nerves; 2, branch of nasal nerve, distributed over the turbinated bones." which are to be regarded as the en d- organs of smell. If arom atic bodies be held before the nose, and respiration suspended, they will not be recognized as such, and it is well known that sniff- 574 COMPARATIVE PHYSIOLOGY. ing greatly assists the sense of smell. Again, if fluids, such, as eau de Cologne, he held in the nose, their aroma is not detected ; and immediately after water has heen kept in the nostrils for a few seconds, it may he noticed that smell is greatly hlunted. Such is the case also when the mucous membrane is much swollen from a cold. There can be no doubt that the presence of fluid in the above cases is injurious to the del- icate hair-cells, and that smell is dependent upon the excitation of these cells by extremely minute particles emanating from aromatic bodies. When ammonia is held before the nose, a powerful sensation is Fig. 419.— End-organs concerned m ' * smell (after koiiiker). i, from experienced ; but this is not smell frog — a, epithelial cell of the _, . . -. olfactory area; b, olfactory cell, proper, but an affection ot orcti- i"?SSS5S*Sf5?SSSY£SSS *ary sensation, owing to stimula- of varicose fibers. 3, olfactory ^ f ^ terminals of the fifth cell of sheep. nerve. It is possible that the audi- tory nerve may also participate, though certainly not so as to produce a pure sensation of smell. Like the other sense-organs, that of smell is readily fa- tigued ; and perhaps the satisfaction from smelling a bouquet of mixed flowers is comparable to viewing the same, one scent after another being perceived, and no one remaining predomi- nant. Our judgment of the position of bodies possessing smell is less perfect even than for those emitting sounds ; but we always project our sensations into the outer world, never referring the object to the nose itself. Subjective sensations of smell are rare in the normal subject, though common enough among the diseased, as is complete or partial loss of smell. It has been found that injury to the fifth nerve interferes with smell, which is probably due to trophic changes in the olfactory region. Comparative.— The investigation of the senses in the lower forms of life is extremely difficult, and in the lowest presents almost insurmountable obstacles to the physiologist because SENSES OF SMELL AXD TASTE. 575 their psychic life is so far removed from our own in terms of which we must interpret, if at all. The earliest form of olfactory organ appears to be a depres- sion lined with special cells in connection with a nerve, which, indeed, suggests the embryonic beginnings of the olfactory organ in vertebrates, as an involution (pit) on the epithelium of the head region. It would appear that we must believe that in some of the lower forms of invertebrates the senses of smell and taste are blended, or possibly that a perception results which is totally different from anything known to us. The close relation of smell and taste, even in man, will be referred to presently, There are, perhaps, greater individual differences in sensitiveness of the nasal organ among mankind than of any other of the sense-organs. Women usually have a much keener perception of odors than men. The sense of smell in the dog is well known to be of extraordinary acuteness ; but there are not only great differences among the various breeds of dogs, but among individuals of the same breeds; and this sense is being constantly improved by a process of " artificial selection " on the part of man, owing to the institution of field trials for setters and pointers, the best dogs for hunting (largely deter- mined by the sense of smell) being used to breed from, to the exclusion of the inferior in great part. Our own power to think in terms of smell is very feeble, and in this respect the dog and kindred animals probably have a world of their own to no small extent. Their memory of smells is also immeasur- ably better than our own. A dog has been known to detect an old hat, the property of his master, that had been given away two years before, as evidenced by his recovering it from a re- mote place. The importance of smell as a guide in the selection of food, in detecting the presence of prey or of enemies, etc., is very obvious. By culture some persons have learned to distinguish individuals by smell alone, like the dog, though to a less degree. TASTE. The tongue is provided with peculiar modifications of epi- thelial cells, etc., known as papillae and taste-buds which may be regarded as the end-organs of the glosso-pharyngeaJ and lingual nerves ; though that these all, especially the taste-buds, are concerned with taste alone seems more than doubtful. In 576 COMPARATIVE PHYSIOLOGY, certain animals with rough, tongues, the papillae, certain of them at least, answer to the hairs of a brush for the cleansing and general preservation of the coat of the animal in good con- dition. We may, perhaps, speak of certain fundamental taste- perceptions, such as siceet, bitter, acid, and saline. Certainly the natural power of gustatory discrimination is considerable; Fig. 420.— Papillae of tongue (after Sappey). 1, circumvallate papillae; 3, fungiform papillae; I, filiform papillae; 0, glands at base of tongue; 7, tonsils. and. as in the case of tea-tasters, capable of extraordinary culti- vation. All parts of the tongue are not equally sensitive, nor SENSES OF SMELL AND TASTE. 577 is taste-sensation confined entirely to the tongue. It can be shown that the back edges and tip of the tongue, the soft palate, the anterior pillars of the fauces, and a limited portion of the back part of the hard palate, are concerned in tasting. Making allowances for individual differences, it may be said that the back of the tongue appreciates best bitter substances, the tip, sweet ones, and the edges acids. If any substance with a decided taste be placed upon the tongue when wiped quite dry, it can not be tasted at all, show- ing that solution is essential. If a piece of apple, another of potato, and a third of onion, be placed upon the tongue of a person blindfolded, and with the nostrils closed, he will not be able to distinguish them, showing that the senses of smell and of taste are related ; or, perhaps, it may be said that much that we call tasting is in large part smelling. When the electrodes from a battery are placed on the tongue, a sensation of taste is aroused, described differently by different persons ; also when the tongue is quick- ly tapped, showing that, though taste is usually the result of chemical stimulation, it may be excited by such as are electrical or mechanical. But it is not to be forgotten that we have usually no pure gustatory sensations, but that these are necessarily blended Fig. 421. Fig. 483. Fig. 421.— Medium-sized circumvallate papilla (after Sappcv). Fig. 422.— Various kinds of papilhe cafter Sappey). 1. fungiform; 2. 3, 4. 5 6 filiform- 7, hemispherical papillae. with those of common sensation, temperature, etc.. and that our .-judgments must, in the nature of the case, be based upon highly 37 578 COMPARATIVE PHYSIOLOGY. complex data, even leaving out of account other senses, such as vision. The glosso-pharyngeal is the principal nerve for the back of the tongue, and for the tip the lingual ; or according to some special fibers in this nerve, derived from the chord tympani. It is worthy of note that temperature has much to do with gustatory sensations, a very low or a very high temperature Fig. 423. — Taste-buds from tongue of rabbit (after Engelmann). being fatal to nice discrimination, and, as would be expected, a temperature not far removed from " body-heat " (40° C) is the most suitable. A certain amount of pressure is favorable to tasting, as any one may easily determine by simply allowing some solution of quinine to rest on the tongue, and comparing the sensation with that resulting when the same is rubbed into the organ ; hence the importance of the movements of the tongue in appreciating the sapid qualities of food. Comparative. — Among the lowest forms of life it is extremely difficult to determine to what extent taste and smell exist sepa- rately or at all, as we can conceive of them. The differentia- tion between ordinary tactile sensibility and these senses has no doubt been very gradually effected. Observations on our domestic animals show that their power of discrimination by taste as well as by smell is very pronounced, though their likes and dislikes are so different from our own in many instances. At the same time we find that they often coincide, and it is not unlikely that a dog's power of discriminating between a good beefsteak and a poor one is quite equal if not superior to man's, and certainly so if his sense of taste, as in the human subject, is developed in proportion to his smelling power. THE CEREBRO-SPINAL SYSTEM OF XEEVES. I. SPINAL NERVES. These (thirty-one pairs), which leave the spinal cord through the intervertebral foramina, are mixed nerves — i. e., their mam trunks consist of motor and sensory fibers. But before they enter the spinal cord they separate into two groups, which are Fig. 424. — Diagram of roots of spinal nerve illustrating: effects of section (after Dal- ton). The dark regions indicate the degenerated parts. known as the anterior or motor and the posterior or sensory roots, which make connection with the anterior and posterior gray horns respectively. These facts have been established by a few simple but im- portant physiological experiments, which will now be briefly described : 1. Stimulation of the peripheral end of a spinal nerve gives rise to muscular movements ; while stimulation of its central end causes pain. 2. Upon section of the anterior root, stimulation of its central end gives negative results ; but of its peripheral end causes muscular movements. 3. After section of the posterior root stimulation of the distal end is fol- lowed by no sensory or motor effects ; of its central end, by sensory effects (pain). These experiments show clearly that the anterior roots are motor, the posterior sensory, and the main trunk of the nerve made up of mixed motor and sensory fibers. 580 COMPARATIVE PHYSIOLOGY. Exception. — It has been found that sometimes stimulation of the peripheral end of the anterior root has given rise to pain, an effect which disappears if the posterior root be cut. From this it is inferred that certain sensory fibers turn up into the anterior root a certain distance. Such are termed " recurrent sensory fibers." Additional Experiments.— 1. It is found that if the anterior root be cut, the fibers below the point of section degenerate, while those above it do not. 2. On the other hand, when the posterior root is divided above the ganglion, the fibers toward the cord degenerate, while those on either side of the ganglion do not. From these experiments it is inferred that the cells of the posterior ganglion are essential to the nutrition of the sen- sory fibers, and those of the anterior horn of the cord to the motor fibers. Pathological. — Pathology teaches the same lesson, for it is observed that, when there is disease of the anterior gray cornua, degeneration of motor fibers is almost sure to follow. These cells, whether in the ganglion or the anterior horn, have been termed "trophic." It is true, the functions of the ganglia on the posterior roots, other than those just indicated, are un- known ; on the other band, the cells of the anterior horn are distinctly motor in function. To assume, then, that the cells of the ganglion are exclusively trophic, with the evidence now before us, would be premature. The view we have presented of the relation of the nervous system makes all cells trophic in a certain sense ; and we think the view that certain cells or certain fibers are exclusively tro- phic must, as yet, be regarded as an open question. It is important, however, to recognize that certain connec- tions between the parts of the nervous system, and indeed all of the tissues, are essential for perfect "nutrition," if we are to continue the use of that term at all. II. THE CRANIAL NERVES. These nerves have been divided into nerves of special sense, motor, and mixed nerves. The first class has already been considered, with the senses to which they belong. The physiology of the cranial nerves has been worked out by means of sections and clinico-pathological investigations. THE CEREBRO-SPINAL SYSTEM OP NERVES. 581 Speaking generally, a good knowledge of the anatomy of these nerves is a great step toward the mastery of what is known of Corpus (anticum quadri- \ geminum(poshcum Locus cocruleus Eminentia teres Crus cercbt'lli ad pontem Ala cinerea Acccssorius nucleus Brachinm conjunctivum anticum Brachium conjunctivum. poslicum Corpus rjeiiiculatum mediate Pedunculus cerebri ad corpora qua-} drigemina I Crus ad mediillam I cerebelli oblongatam ] VII Obex Clavd Funiculus cuneatus Funiculus gracilis Fig. 425. — Intended .0 show especially the origin of both deep and superficial cranial nerves (after Landois). Roman characters are used to indicate the nerves as they emerge, and Arabic figures their nuclei or deep origin. their functions, and such will he assumed in this chapter, so that the student may expect to find the treatment of the siibject somewhat condensed. The Motor-Oculi or Third Nerve. — With a deep origin in the gray matter of the floor and roof of the aqueduct of Sylvius, branches of distribution pass to the following muscles : 1. All of the muscles attached to the eyeball, with the exception of the external rectus and the superior oblique. 2. The levator pal- pebrae. 3. The circular muscle of the iris. 4. The ciliary muscle. Both the latter branches reach the muscles by the ciliary nerves, as they pass from the lenticular (ciliary, ophthal- mic) ganglion. The relation of the third nerve, as seen in the 582 COMPARATIVE PHYSIOLOGY. changes of the pupil with the movements of the eyeballs, has already been noticed. Pathological. — It follows that section or lesion of the third nerve must give rise to the following symptoms : 1. Drooping of the upper lid (ptosis). 2. Fixed position of the eye in the outer angle of the orbit (luscitas). 3. Immobility, with the dila- tation of the pupil (mydriasis). 4. Loss of accommodation. The Trochlear or Fourth Nerve. — This nerve, arising in the aqueduct of Sylvius, passes to the superior oblique muscle. Pathological. — Lesion of this nerve leads to peculiar changes. As there is double vision, some alteration must have occurred in the usual position of the globe of the eye, though this is not easily seen on looking at a subject thus affected. The double image appears when the eyes are directed downward, and ap- pears oblique and lower than that seen by the unaffected eye. The Abductor or Sixth Nerve. — Arising on the floor of the fourth ventricle, it passes to the external rectus of the eyeball, thus with the third and fourth nerve completing the innerva- tion of the external ocular muscles (extrinsic muscles). Pathological. — Lesion of this nerve causes paralysis of the above-mentioned muscle, and consequently internal squint (strabismus). The Facial, Portia Dura, or Seventh Nerve.— It arises in a gray nucleus in the floor of the fourth ventricle, and has an extensive distribution to the muscles of the face, and may be regarded, in fact, as the nerve of the facial muscles, since it sup- plies (1) the muscles of expression, as those of the forehead, eyelids, nose, cheek, mouth, chin, outer ear, etc., and (2) certain muscles of mastication, as the buccinator, posterior belly of the digastric, the stylohyoid, and also (3) to the stapedius, with branches to the soft palate and uvula. Pathological. — It follows that paralysis of this nerve must give rise to marked facial distortion, loss of expression, and flattening of the features, as well as possibly some deficiency in hearing, smelling, and swallowing. Mastication is difficult, and the food not readily retained in the mouth. Speech is affected from paralysis of the lips, etc. Secretory fibers proceed (1) to the parotoid gland by the superficial petrosal nerve, thence (2) to the otic ganglion, from which the fibers pass by the auriculotemporal nerve to the gland. Gustatory Fibers. — According to some, the chorda tympani THE CEREBROSPINAL SYSTEM OF NERVES. 583 really supplies the fibers to the lingual nerve that are concerned with taste. It will thus be seen that the facial nerve has a great variety of important functions, and that paralysis may be more or less serious, according to the number of fibers involved. The Trigeminus, Trifacial, or Fifth Nerve. — This nerve has very extensive functions. It is the sensory nerve of the face : but, as will be seen, it is peculiar, being a combination of the motor and sensory ; or, in other words, has paths for both afferent and efferent impulses. The motor and less extensive division arises from a nucleus in the floor of the fourth ventricle. The sensory, much the larger, seems to have a very wide origin. The nerve-fibers may be traced from the pons Varolii through the medulla oblongata to the lower boundary of the olivary body and to the posterior horn of the spinal cord. This origin sug- gests a resemblance to a spinal nerve, the motor root corre- sponding to the anterior, and the sensory to a posterior root, the more so as there is a large ganglion connected with the sensory part of the nerve within the brain-case. Efferent Fibers.— 1. Motor. — To certain muscles (1) of mas- tication — temporal, masseter, pterygoid, mylohyoid, and the anterior part of the digastric. 2. Secretory. — To the lachrymal gland of the ophthalmic division of this nerve. 3. Vaso-motor. — Probably to the ocular vessels, those of the mucous mem- brane of the cheek and gums, etc, 4. Trophic. — From the re- sults ensuing on section of this nerve, it has been maintained that special trophic fibers pass in it. We have discussed this subject in an earlier chapter. Afferent Fibers. — 1. Sensory. — To the entire face. To par- ticularize regions : 1. The whole of the skin of the face and that of the anterior surface of the external ear. 2. The external auditory meatus. 3. The mucous lining of the cheeks, the floor of the mouth, and the anterior region of the tongue. 4. The teeth and periosteum of the jaws. 5. The lining membrane of the entire nasal cavity. 6. The conjunctiva, globe of the eye, and orbit. 7. The dura mater throughout. Many of these afferent fibers are, of course, intimately con- cerned with reflexes, as sneezing, winking, etc. Certain secre- tory acts are often excited through this nerve, as lachrymation, when the nasal mucous membrane is stimulated : indeed, the paths for afferent impulses giving rise to reflexes, including secretion, are very numerous. 584 COMPARATIVE PHYSIOLOGY. Gustatory impulses from the anterior end and lateral edges of the tongue are conveyed hy the lingual (gustatory) branch of this nerve. Many are of opinion, however, that the fibers of the chorda tympani, which afterward leave the lingual to unite with the facial nerve, alone con- vey such impressions. The subject can not be regarded as quite settled. Tactile sensibility in the tongue is very pronounced, as we have all experi- enced when a tooth, etc., has for some reason presented an unusual surface quality, and become a source of con- stant offense to the tongue. The ganglia of the fifth nerve, so far as the functions of their cells are concerned, are enigmatical at present. They are doubtless in some sense tro- phic at least. With each of these are nerve connections (" roots " of the gan- glia), which seem to contain different kinds of fibers. These ganglia are connected with the main nerve-centers by both afferent and efferent nerves, and also with the sympathetic nerves themselves. Some regard the ganglia as the representatives of the sympa- thetic system within the cranium. I. The Ciliary (Ophthalmic, Len- ticular) Ganglion. — Its three roots are : 1. From the branch of the third nerve to the inferior oblique muscle (motor root). 2. From the nasal branch of the ophthalmic division of the fifth. 3. From the carotid plexus of the sympathetic. The efferent branches pass to the iris, are derived chiefly from the sympathetic, and cause dilatation of the pupil. There are also vaso-motor fibers to the choroid, iris, and retina. The afferent fibers are sensory, passing from the conjunctiva, cor- nea, etc. II. The Nasal or Spheno-Palatine Ganglion.— The motor Fk;. 426.— Unipolar cell from Gaeserian ganglion (after Schwalbe). N, N, N, nuclei of sheath; 7', liber branch- ing at a node of Ranvier. THE CEREBRO-SPINAL SYSTEM OF NERVES. 585 root is dei-ived from the facial through the great superficial petrosal nerve; its sympathetic root from the carotid plexus. Both together constitute the vidian nerve. It would seem that afferent impulses from the nasal chambers pass through this ganglion. The efferent paths are : 1. Motor to the levator pa- lati and azygos uvulae. 2. Vaso-motor, derived from the sym- pathetic. 3. Secretory to the glands of the cheek, etc. III. The Otic Ganglion.— Its roots are : 1. Motor, from the third division. 2. Sensory, from the inferior division of the fifth. 3. Sympathetic, from the plexus around the meningeal artery. It makes communication with the chorda tympani and seventh, and supplies the parotid gland with some fine fila- ments. Motor fibers mixed with sensory ones pass to the tensor tympani and tensor palati. IV. The Submaxillary Ganglion. — Its roots are: 1. Branch- es of the chorda tympani, from which pass (a) secretory fibers to the submaxillary and sublingual glands, (b) vaso-motor (dilator) fibers to the vessels of the same glands. 2. The sympathetic, derived from the supei'ior cervical ganglion, passing to the sub- maxillary gland. It is also thought to be the path of vaso-con- strictor fibers to the gland. 3. The sensory, from the lingual nerve, supplying the gland substance, its ducts, etc. Pathological. — 1. The motor division of the nerve, when the medium of efferent impulses, owing to central disorder, may cause trismus (locked-jaw) from tonic tetanic action of the mus- cles of mastication supplied by this nerve. 2. Paralysis of the same muscles may ensue from degeneration of the motor nuclei or pressure on the nerve in its course. 3. Neuralgia of any of the sensory branches may occur from a great variety of causes, and often maps out very exactly the course and distribution of the branches of the nerve. 4. Vaso-motor distui'bances are not infrequently associated with neuralgia. Blushing is an evi- dence of the normal action of the vaso motor fibers of the fifth nerve. 5. A variety of trophic (metabolic) disturbances may arise from disorder of this nerve, its nuclei of origin or its gan- glia, such as grayness and loss of hair (imperfect nutrition), eruptions of the skin along the course of the nerves, etc. Atro- phy of the face, on one or both sides, gradual and progressive, may occur. Such affections as well as others, point in the most forcible manner to the influence of the nervous system over the metabolism of the body. The Glosso-pharyngeal or Ninth Nerve. — This nerve, to- 586 COMPARATIVE PHYSIOLOGY. gether with, the vagus and spinal accessory, constitutes the eighth pair, or rather trio. Functionally, however, they are quite distinct. The glosso-pharyngeal arises in the floor of the fourth ven- tricle ahove the nucleus for the vagus. It is a mixed nerve with efferent and afferent fibers : Efferent fibers, furnishing motor fibers to the middle constrictor of the pharynx, stylo- pharyngeus, levator palati, and azygos uvulae. 2. Afferent fibers, which are the paths of sensory impulses from the base of the tongue, the soft palate, the tonsils, the Eustachian tube, tympanum, and anterior portion of the epiglottis. Stimulation of the regions just mentioued gives rise reflexly to the move- ments of swallowing and to reflex secretion of saliva. This nerve is also the special nerve of taste to the back of the tongue. The Pneumogastric, Vagus, or Tenth Nerve.— Most of the functions of this nerve have already been considered in previous chapters. In some of the lower vertebrates (sharks) the nerve arises by a series of distinct roots, some of which remain separate throughout. This fact explains its peculiarities, anatomical and functional, in the higher vertebrates. In these there have been concentration and blending, so that what seems to be one nerve is really made up of several distinct bundles of fibers, many of which leave the main trunk later. It may be regarded as the most complicated nerve-trunk in the body, and the distribution of its fibers is of the most exten- sive character. Following our classification of efferent and afferent, we recognize : 1. Efferent fibers, which are motor to an extensive tract in the respiratory and alimentary regions. Thus the constrictors of the pharynx, certain muscles of the palate, the oesophagus, the stomach, and the intestine, receive an abundant supply from this source. By the laryngeal nerves, probably derived originally from the spinal accessory, the mus- cles of the larynx are innervated. The muscles of the trachea, bronchi, etc., are also supplied by the pneumogastric. It is probable that vaso-motor fibers derived from the sympathetic run in branches of the vagus. The relations of this nerve to the heart and lungs have already been explained. 2. Afferent Fibers. — It may be said that afferent impulses from all the regions to which efferent fibers are supplied pass THE CEREBRO-SPINAL SYSTEM OF NERVES. 587 inward by the vagus. One of the widest tracts in the body for afferent impulses giving- rise to reflexes is connected with the nerve-centers by the branches of this nerve, as evidenced by the niany well-known phenomena of this character referable to the pharynx, larynx, lungs, stomach, etc., as vomiting, sneez- ing, coughing, etc. This nerve plays some important part in secretion, no doubt, but what that is has not been as yet well established. Pathological. — Section of both vagi, as might be expected, leads to death, which may take place from a combination of pathological changes, the factors in which vary a good deal with the class of animals the subject of experiment. Thus, the heart in some animals (dog) beats with great rapidity and tends to exhaust itself. In birds especially is fatty degeneration of heart, stomach, intestines, etc., liable to follow. Paralysis of the muscles of the larynx renders breathing laborious. From loss of sensibility food accumulates in the pharynx and finds its way into the larynx, favoring, if riot actually exciting, inflammation of the air-passages. But it is not to be forgotten that upon the views we advocate as to the constant influence of the nervous system over all parts of the bodily metabolism, it is plain that after section of the trunk of a nerve with fibers of such wide distribution and va- ried functions the most profound changes in so-called nutrition must be expected, as well as the more obvious functional de- rangements ; or, to put it otherwise, the results that follow are in themselves evidence of the strongest kind for the doctrine of a constant neuro-metabolic influence which we advocate. It will not be forgotten that the depressor nerve, which exerts re- flexly so important an influence over blood-pressure, is itself derived from the vagus. The Spinal Accessory or Eleventh Nerve!— This nerve arises from the medulla oblongata somewhat far back, and from the spinal cord in the region of the fifth to the seventh vertebra. Leaving the lateral columns, its fibers run upward between the denticulate ligament and the posterior roots of the spinal nerve to enter the cranial cavity, which as they issue from the cra- nium subdivide into two bundles, one of which unites with the vagus, while the other pursues an independent course to reach the sterno-mastoid and trapezius muscles, to which they furnish the motor supply; so that it may be considered functionally equivalent to the anterior root of a spinal nerve. The portion 588 COMPARATIVE PHYSIOLOGY. joining the vagus seems to supply a large part of the motor fibers of that nerve. Pathological. — Tonic contraction of the flexors of the head causes wry -neck, and when they are paralyzed the head is drawn to the sound side. The Hypoglossal or Twelfth Nerve.— It arises from the low- est part of the calamus scriptorius and perhaps from the olivary body. The manner of its emergence between the anterior pyra- mid and the olivary body, on a line with the anterior spinal roots, suggests that it corresponds to the latter ; the more so as it, is motor in function, though also containing some vaso-motor fibers, in all probability destined for the tongue. Such sensory fibers as it may contain are derived from other sources (vagus, trigeminus). It supplies motor fibers to the tongue and the muscles, attached to the hyoid bone. Pathological. — Unilateral section of the nerve gives rise to a corresponding lingual paralysis, so that when the tongue is protruded it points to the injured side ; when being drawn in, the reverse. Speech, singing, deglutition, and taste may also be abnormal, owing to the subject being unable to make the usual co-ordinated movements of the tongue essential for these acts. RELATIONS OF THE CEREBRO-SPINAL AND SYMPA- THETIC SYSTEMS. No division of the nervous system has been so unsatisfac- tory, because so out of relation with other parts, as the sympa- thetic. It was also desirable to attempt to co-ordinate the cere- bral and spinal nerves in a better fashion ; and various attempts in that direction have been made. Very recently a plan, by which the whole of the nerves issuing from the brain and cord may be brought into a unity of conception, has been proposed; and, though it would be premature to pronounce definitely as yet upon the scheme, yet it does seem to be worth while to lay it before the student, as at all events better than the isolation implied in the three divisions of the nerves which has been taught hitherto. Instead of the classification of nerves into efferent and affer- ent, connected with the anterior and the posterior horns of the gray matter of the spinal cord, another division has been pro- posed, viz., a division of nerve-fibers and their centers of origin THE CEREBRO-SPINAL SYSTEM OF NERVES. 589 in the gray matter for the supply of the internal and the exter- nal parts of the hody — i. e., into splanchnic and somatic nerves. The centers of origin of the splanchnic nerves are referred to groups of cells in the gray matter of the cord around the cen- Fig. 427. Fig. 428. Fig. 427. — Ganglion cell from sympathetic ganglion of frog; greatly magnified, and showing both straight and coiled fibers (after Quain). Fig. 428. — Multipolar ganglion cells from sympathetic system of man, highly magni- fied (after Max Scnuftze). a, cell freed from capsule; b, inclosed within a "nu- cleated capsule. In both the processes have been broken away. tral canal ; while the somatic nerves spring from the gray cor- nua and supply the integument and the ordinary muscles of locomotion, etc. The splanchnic nerves supply certain muscles of respiration and deglutition, derived from the embryonic lateral plates of the mesoblast; the somatic nerves, muscles formed from the muscle-plates of the same region. It is assumed that the segmentation of the vertebrate and invertebrate animal is related; and that segmentation is pre- 590 COMPARATIVE PHYSIOLOGY. served in the cranial region of the vertebrate, as shown by the nerves themselves. The afferent fibers of both splanchnic and somatic nerves pass into the spinal ganglion, situated in the nerve-root, which may be regarded as stationary. It is different with the anterior roots. Some of the fibers are not connected with ganglia at all ;' others with ganglia not fixed in position, but occurring at variable distances from the central nervous system (these being the so-called sympathetic ganglia) : thus, the anterior root-fibers are divisible into two groups, both of which are efferent, viz., ganglionated and non- ganglionated. The ganglionated belong to the splanchnic sys- tem, and have relatively small fibers; the non-ganglionated include both somatic and splanchnic nerves, composing the ordinary nerve-fibers of the voluntary striped muscles of res- piration, deglutition, and locomotion. It would appear that these now isolated ganglia have been themselves derived from a primitive ganglion mass situated on the spinal nerves; so that the distinction usually made of gan- glionated and non-ganglionated roots is not fundamental. A spinal nerve is, then, formed of — 1. A posterior root, the ganglion of which is stationary in position, and connected with splanchnic and somatic nerves, both of which are afferent. 2. An anterior root, the ganglion of which is vagrant, and con- nected with the efferent small-fibered splanchnic nerves. Among the lower vertebrates both anterior and posterior roots pass into the same stationary ganglion. Such is also the case in the first two cervical nerves of the dog. Does the above-mentioned plan of distribution, etc., hold for the cranial nerves ? Leaving out the nerves of special sense (olfactory, optic, and auditory), the other cranial nerves maybe thus divided: 1. A foremost group of nerves, wholly efferent in man, viz., the third, fourth, motor division of the fifth, the sixth, and seventh. 2. A hindmost group of nerves of mixed character, viz., the ninth, tenth, eleventh, and twelfth. The nerves of the first group, since they have both large- fibered, non-ganglionated motor nerves, and also small-fibered splanchnic efferent nerves, with vagrant ganglia (ganglion oculomotorii, ganglion geniculatum, etc.), resemble a spinal nerve in respect to their anterior roots. They also resemble spinal nerves as to their posterior roots, for at their exit from THE CEREBRO-SPINAL SYSTEM OP NERVES. 591 the brain tliey pass a gang-lion corresponding- to the stationary- posterior ganglion of the posterior root of a spinal nerve. These being, however, neither in roots nor ganglion functional, are to be regarded as the pbylogenetically (ancestrally) degen- erated remnants of what were once functional ganglia and nerve-fibers ; in other words, the afferent roots of these nerves and their ganglia have degenerated. The hindmost group of cranial nerves also answers to the spinal nerves. They arise from nuclei of origin in the medulla and in the cervical region of the spinal cord, directly continu- ous with corresponding groups of nerve-cells in other parts of the spinal cord ; but in these nerves there is a scattering of the components of the corresponding spinal nerves. Cei'tain pecul- iarities of these cranial nerves seem to become clearer if it be assumed that, in the development of the vertebrate, degenera- tion of some region once functional has occurred, in conse- quence of which certain portions of nerves, etc., have disap- peared or become functionless. It is also to be remembered thafsa double segmentation ex- ists in the body, viz., a somatic, represented by vertebras and their related muscles, and a splanchnic represented by visceral and branchial clefts, and that these two have not followed the same lines of development ; so that in comparing spinal nerves arranged in regard to somatic segments with cranial nerves, the relations of the latter to the somatic muscles of the head must be considered; in other words, like must be compared with like. THE VOICE, It is convenient to speak, in the case of man, of the singing voice and the speaking voice, though there is no fundamental difference in their production. The voice of the lower animals approximates the former leather than the latter. It is to be remembered that sound is an affection of the nervous centers through the ear, as the result of aerial vibra- tions. We are now to explain how such vibrations are caused by the vocal mechanisms of animals and especially of man. The toues of a piano or violin are demonstrably due to the vibrations of the strings ; of a clarionet to the vibration of its reed. But, however musical tones may be produced, we distin- guish in them differences in pitch, quantity, and quality. The pitch is dependent solely upon the number of vibrations within a given time, as one second; the quantity or loudness upon the amplitude of the vibrations, and the quality upon the form of the vibrations. The first two scarcely require any fur- ther notice ; but it is rather important for our purpose to under- stand clearly the nature of quality or timbre, which is a more complex matter. If a note be sounded near an open piano, it may be observed that not only the string capable of giving out the correspond- ing note passes into feeble vibration, but that several others also respond. These latter produce the overtones or partials which enter into notes and determine the quality by which one instrument or one voice differs from another. In other words, every tone is in reality compound, being composed of a funda- mental tone and overtones. These vary in number and in rela- tive strength with each form of instrument and each voice; and it is now customary to explain the differences in quality of voices solely in this way ; and this is, no doubt, correct in the main. THE VOICE, 593 What are the mechanisms by which voice is produced in man ? Observation proves that the following are essential : 1. Fig. 429. Fig. 430. Fig. 429. — Longitudinal section of human larynx (after Sappey). 1, ventricle of lar- ynx; 2, superior vocal cord; 3, inferior vocal cord; 4. arytenoid cartilage; 5, sec- tion of arytenoid muscle; 6. 6. inferior portion of cavity of larynx: ',. section Of posterior part of cricoid cartilage: 8. section of anterior part of same; 9, superior border of cricoid cartilage; 10, section of thyroid cartilage; 11.11. superior portion of cavity of larynx: 12, 13, arytenoid gland; 14, 16, epiglottis; 15, 17, adipose tissue: 18. section of liyoid bone; 19, 19. SO, trachea. Pig. 430. — Posterior aspect of muscles of human larynx (after Sappey). 1, posterior crico-arytenoid muscle; 2, 3, 4, different fasciculi' of arytenoid muscle; 5, aryteno- epiglottidean muscle. A certain amount of tension of the vocal cords (bands). 2. A certain degree of approximation of their edges. 3. An expira- tory blast of air. It will be noted that these are all conditions favorable to the vibration of the vocal bauds. The greater the tension the higher the pitch ; and the more occluded the glottic orifice the more effective the expiratory blast of air. The principle on which the vocal bands act may be illus- 38 594 COMPARATIVE PHYSIOLOGY. tratecl in the simplest way by a well-known toy, consisting of an elastic bag tied upon a hollow stem of wood, across which rubber bands are stretched, and the vibration of which caused by the air within the distended bag gives rise to the note, It is especially important to recognize the nature, extent, and Fig. 431. Fig. 432. Fig. 431.— Lateral view of laryngeal muscles (after Sappey). 1, body of hyoid hone; 2, vertical section of thyroid cartilage; 3, horizontal section of thyroid cartilage, turned downward to show deep attachment of crico-thyroid muscle; 4, facet of the articulation of small cornu of thyroid cartilage with cricoid cartilage; 5, facet on cricoid cartilage; 6, superior attachment of crico-thyroid muscle; 7, posterior crico-arytenoid muscle; 8, lateral crico-arytenoid muscle; 9, thyro-arytenoid mus- cle; 10, arytenoid muscle proper; 11, aryteno-epiglottidean muscle; 12, middle thyro-hyoid ligament; 13, lateral thyro-hyoid ligament. Fig. 432. — Distribution of nerves in larynx of horse (Chauvean, after Toussaint). a, base of tongue; b, epiglottis; c, arytenoid muscles; d, section of thyroid cartilage to show pails it, covers; e, cricoid cartilage; /, trachea; g, (esophagus; h, thyro- arytenoid muscle; i, lateral crico-arytenoid muscle; j, posterior crico-arytenoid muscle; k, arytenoid muscle; 1, superior laryngeal nerve; 2, inferior laryngeal; 3, branches of superior laryngeal passing to epiglottis and tongue; 4, branches of superior laryngeal passing to oesophagus; 5, very fine multiple anastomoses be- tween two laryngeals; fi, tracheal branches; 7, branch to posterior crico-arytenoid muscle; a portion is distributed, through the muscles, to subjacent mucous mem- brane; lo, branch passing to arytenoid muscle; 11, oesophageal branch to aryte- noid muscle; 11, (esophageal branch of pharyngeal nerve; it sometimes comes from external laryngeal. THE VOICE. 595 effect on the vocal bands of the movements of the arytenoid cartilages. These are most marked around a vertical axis, giv- ing rise to an inward and outward movement of rotation, but Fig. 433. — Diagrammatic section of larynx to illustrate action of Posterior crico-aryte noid muscle (after Landois). In this and the two following figures the dotted lines indicate the new position of the parts owing to the action of the muscles concerned. there are also movements of less extent in all directions. It is in fact through the movements of these cartilages to which the Fir. 434.— Diagrammatic section of larynx to illustrate action of Arytenoids* jirc- privs musbb (after Landois). 596 COMPARATIVE PHYSIOLOGY. Fig. 435. — Illustrates action of thyro-arytenoideus interims. vocal bands are attached posteriorly, that most of the important changes in the tension, approximation, etc., of the latter are produced. The lungs are to he regarded as the bellows furnish- ing the necessary wind-power to set the vocal bands vibrating, while the larynx has respiratory as well as vocal functions, as has been already learned. Assuming that the student has a good knowledge of the general anatomy of the larynx, we call attention briefly to the following : Widening of the glottis is effected by the crico-arytenoideus posticus pulling outward the processus vocalis or attachment posteriorly of the vocal band, and a similar effect is produced by the arytenoideus posticus acting alone. Narrowing of the glottis is accomplished by the crico-aryt- enoideus lateralis, and the following when acting either singly (except the arytenoideus posticus), or in concert, as the sphinc- ter of the larynx, viz., the thyro-arytenoideus externus, thyro- arytenoideus internus, thyro-aryepiglotticus arytenoideus pos- ticus. Tension of the vocal bands is brought about by the sphincter group, and especially by the external and internal thyro-aryte- noid muscles. Nerve Supply. — The superior laryngeal contains the motor fibers for the crico-thyroid (possibly also the arytenoideus pos- ticus) and also supplies the mucous membrane. The inferior laryngeal supplies all the other muscles. "While both of these nerves are derived from the vagus, their fibers really belong to the spinal accessory. It is worthy of note that the entire group THE VOICE. 597 of muscles making up the sphincter of the larynx is contracted when the inferior laryngeal is stimulated. Superior Face. Inferior Face. Fig. 436.— Cartilaginous pieces of the larynx of horse, maintained in their natural position by the articular ligaments (Chauveau). a, cricoid cartilage; b. b, aryte- noid cartilages; c, body of the thyroid; c', c', lateral plates of the thyroid; d. epi- glottis; e, body of the hyoid; /, trachea. 1, crico-arytenoid articulation; 2, capsule of the crico-thyroid articulation; 3, crico-thyroid membrane; 4, thyro-hyoid mem- brane; 5, crico-trachealis ligament. Above the true vocal bands composed of elastic fibers lie the so-called false vocal bands (cords) to be regarded as folds of the mucous membrane which take no essential part in voice-produc- tion. Between these two pairs of bands are the ventricles of Morgagni, which, as well as the adjacent parts, secrete mucus and allow of the movements of both sets of bands and in so far only assist in phonation. The whole of the supra-laryngeal cavities, the trachea and bronchial tubes, may be regarded as resonance-chambers, the former of which are of the most importance, so far as the quality of the voice is concerned. There seems to be little doubt that they have much to do with determining the differ- ences by which one individual's voice at the same pitch differs from another ; nor is the view that they may have a slight in- fluence on the pitch of the voice, or even its intensity, to be ignored. '598 COMPARATIVE PHYSIOLOGY. The epiglottis, in so far as it has any effect, in all probability modifies the voice in the direction of quality. Pathological.— Paralysis of the laryngeal muscles, owing to pressure on nerves and conse- quent narrowing of the glottic opening, explains " roaring " in the horse, in certain instances at all events. Comparative.— Much more is known of the sounds emanat- ing from the lower animals than of the mechanisms by which they are produced. This applies, of course, especially to such sounds as are not pro- duced by external parts of the body, it being very difficult to investigate these experimental- ly or to observe the animal closely enough when produc- ing the various vocal effects naturally. All our domestic mammals have a larynx, not as widely different from that of man as might be supposed from the feeble range of their vocal powers. There are structural differences in the larynx of the domestic animals, some of which are more readily appreciated by the eye than described. The false (superior) vocal bands are rudimentary or want- ing in many mammals, including the horse, ass, etc. In ruminants the larynx is proportionately ill-developed ; the glottis is short, the vocal bands rudimentary, and the ven- tricles wanting. The larnyx of the pig is peculiar in that the ventricles are deep, though their opening is only a narrow slit; there is, how- ever, a large membranous sac below the epiglottis, which, acting as a resonator, explains the great intensity of the voice of this animal. The actual behavior of the vocal bands has been studied experimentally, in the dog when growling, barking, etc. And, so far as it goes, this animal's mechanism of voice-production Fig. 437.— Posterolateral view of the lar- ynx of the horse (Chauveau). 1, epi- glottis ; 2, arytenoid cartilages ; 3, thyroid cartilage ; 4, arytenoideus muscle; 5, crico-arytenoideus latera- lis; 6, thyro-arytenoideus; 7, crico- arytenoideus posticus: 8, crico-thy- roidens; 9, ligament between the cri- coid cartilage and first ring of trachea; 10. 11, infero-posterior extremities of crico-thyroid cartilages. THE VOICE. 599 is not essentially different from that of man. Growling is the result of a functional activity of the vocal mechanism, not un- like that of man when singing a bass note; barking, of that analogous to coughing or laughing, when the vocal bands are rapidly approximated and separated. The grunting of hogs and the lowing and bawling of horned cattle are probably very similar in production, so far as the larynx is concerned, to the above. The cat has plainly very great command over the larynx, and can produce a wide range of tones. The peculiarities of the bray of the ass are owing to voice production both during inspiration and expiration. The quality of the voice of most animals appears harsh to our ears, owing probably "to a great preponderance of over- tones, in consequence of an im- perfect and unequal tension of the vocal bands; but the influ- ence of the supra-laryngeal cavi- ties, often very large, must also be taken into account. In certain of the primates, and especially in the howling mon- keys, large cheek-pouches can be Fig. 438-Lower larynx (Syrinx) of inflated with air from the larynx, and so add to the intensity of the note produced by the vocal bands that then* voice may be heard for miles. Song-birds produce their notes, as may be seen, by exter- nal movements low down at the bifurcation of the trachea (syrinx). The notes are owing to the vibration of two folds of the mucous membrane, which project into each bronchus, and are regulated in their movements by muscles, the bronchial rings in this region being correspondingly modified. A large number of species of fishes produce sounds and in a variety of ways, in which the air-bladder, stomach, intestines. etc., take part. Most reptiles are voiceless, in the proper sense, though there are few that can not produce a sort of hissing sound, caused by the forcible emission of air through the upper respiratory passages. Frogs, as is well known, produce sounds of great variety in crow (after Gegenbaurl." A, seen from side: B, seen from in front. a—f, muscles concerned in move- ments of lower larynx; rj. mem- brana tympaniformis interna, stretching from median surface of either bronchus to a bony ridge (.pessulus) which projects at the angle of bifurcation of trachea. (500 COMPARATIVE PHYSIOLOGY. pitch, quality, and intensity, some species croaking so as to be heard at the distance of at least a mile. It is a matter of easy observation that when frogs croak the capacity of the mouth cavity is greatly increased, owing to the distention of resonat- ing sacs situated at each angle of the jaws. When tree-frogs croak, their throats are greatly distended, apparently in suc- cessive waves. SPECIAL CONSIDERATIONS AND SUMMARY. Evolution. — The very lowest forms, and in fact most inverte- brate groups, seem to be voiceless. Darwin has shown that voice is, in a large number of groups, confined either entirely to the male, or that it is so much more developed in him as to become what he terms a " sexual character." There is abundant evidence that males are chosen as mates by the females, among birds especially, not alone for superiority in beauty of plumage, but also for their song. Thus, by a process of natural selection (sexual selection), the voice would tend to improve with the lapse of time, if we admit heredity, which is an undeniable fact, even among men — whole families for generations, as the Bachs, having been musicians. One can also understand why on these principles voice should be especially developed in certain groups (birds), while among others (mammals) form and strength should determine sexual selection, the strongest winning in the contests for the possession of the females, and so propagating their species under the more favorable circumstance of choice of the most desira- ble females. Pathology teaches that, when certain parts of the brain (speech-centers) of man are injured by accident or disease, the power of speech may be lost. From this it is evident that the vocal appai-atus may be perfect and yet speech be wanting; so that it becomes comprehensible that the vocal powers of, e. g., a dog, are so limited, notwithstanding his comparatively highly developed larynx. He lacks the energizing and directive ma- chinery situated in the brain. Some believe that there was a period when man did not pos- sess the power of speech at all ; and many are convinced that the human race have undergone a gradual development in this as in other respects. Certain it is that races differ still very widely in capacity to express ideas by spoken words. THE VOICE 601 We may regard the development of a race of speaking ani- mals as dependent upon a corresponding advance in brain- structure, whether that was acquired by a sudden and pro- nounced variation, or by gradual additions of increase in cer- tain regions of the brain, or whether to the first there was then added the second. Apart from speech proper, there is a language of the face and body generally, in which there is much that we share with lower forms, especially lower mammals. Darwin, noticing this resemblance, regarded it as evidence strengthening the belief that man is derived from lower forms. Why should the forms of facial expression associated so generally with certain emotions among different races of men be so similar to each other and to those which the lower animals employ, if there is not some community of origin ? This is Darwin's query, and he con- sidered, as has been stated, that the answer to be given was in harmony with his views of man's origin, as based on an alto- gether different sort of testimony. The high functional development of the hand and arm in man, and the use of these parts in writing, are suggestive. Summary. — The musical tones of the voice are caused by the vibrations of the vocal bands, owing to the action on them of an expiratory blast of air from the lungs. In order that the bands may act effectively, they must be rendered tense and ap- proximated, which is accomplished by the action of the laryn- geal muscles, especially those attached to the arytenoid carti- lages. We may speak of the respiratory glottis and the vocal- izing glottis, according as we consider the position and move- ments of the vocal bands in respiration or in phonation. The pitch of the voice is determined by the length and the tension of the vocal bands, and frequently both shortening and increased tension are combined; perhaps we may say that al- tered (not necessarily increased) tension and length are always combined. The quality of the voice depends chiefly upon the supi-a- laryngeal cavities. It is important to remember that in all phonation, in the case of man at least, many parts combine to produce the result: so that voice-production is complex and variable in mechanism, beyond what would be inferred from the apparent simplicity of the mechanism involved ; while the central nervous processes are, when comparison is made with phonation in lower ani- 602 COMPARATIVE PHYSIOLOGY. mals, seen to be the most involved and important of the whole — a fact which the results of disease of the brain are well calcu- lated to impress, inasmuch as interruptions anywhere among a class of cerebral connections, now known to be very extensive, suffice to abolish voice, and especially speech -production. Among mammals below man the vocal bands and laryngeal and thoracic mechanism are very similar, but less perfectly and complexly co-ordinated ; so that their vocalization is more limited in range, and their tones characterized by a quality which to the human ear is less agreeable. Man's superiority as a speaking animal is to be traced chiefly to the special develop- ment of his cerebrum, both generally and in certain definite regions. CERTAIN TISSUES. Prior to considering the subject of the next chapter, it may be well to give a short account of certain tissues specially con- cerned. Connective Tissue.— This is the most widely distributed tissue in the body, since it binds together all other forms of tissue, and, in some of its many varieties, enters into the forma- tion of every organ. As connective tissue proper, its function is subordinate ; but when it becomes the aponeuroses of mus- Fig. 439.— Fibers of tendon of man (Rollettt. cles, and especially tendons, by which, from its inextensibility, the muscles are rendered effective in moving the levers (bones) to which they are attached, its importance is more pronounced. In structure, this fibrous tissue consists of bundles of fine fibrils, among which, especially in the younger stage, connective-tissue cells may be found, and from which the Abel's themselves are formed. 604 COMPARATIVE PHYSIOLOGY. It is owing- to differences in the shape and size of these cells chiefly that the structural variations of connective tissue in dif- ferent regions of the body are due. 1 1 I Fig. 440.— Loose network of connective tissue from man, in which are connective-tis- sue corpuscles among the fibers (Rollet). a, a, capillary with blood-cells. Elastic Tissue. — This form of tissue is also of very wide distri- bution and of great importance in the economy of a complicated living organism that must constantly adapt itself to the stress and strains of existence. In its purest form it occurs, e. g., in the ligamentum nuclese of the ox, as a somewhat yellow, tough, elastic structure easily fibrillated when boiled, but with diffi- culty torn asunder when fresh. Under the microscope it ap- pears as fibers with a very distinct outline and of varying size. In the arteries, as already referred to, it forms a sort of elastic membrane of the utmost importance in the functions of these organs. Bone. — In a long bone, as the femur, in the dried state, we recognize a compact shaft and two extremities of a more porous nature, while the central portion of the former presents a more or less circular cavity, the medullary canal. By treatment with hydrochloric acid abundance of lime salts may be ob- CERTAIN TISSUES. 605 ^■^{^"y.n.; k Fig. 433. Fig. 441. Fig. 442. Fig. 441. — Fine elastic fibers from peritonaeum, 1 x 350 (Kolliker). Fig. 442. — Larger elastic fibers (Robin). Fig. 443. — Elastic network (fenestrated membrane) from middle coat of carotid of horse, 1 x 350 (Kolliker). ym^miMm ill* 8 n ip**;*tsiv, ' : ty\W Fig. 444.— Longitudinal section of humerus, showing Haversian canals and lacunre, 1 x 200 (Sappey). 606 COMPARATIVE PHYSIOLOGY. Fig. 445. — Transverse section of humerus. 1 x 200 (Sappey). 1, section of vascular (Haversian) cells; 2, longitudinal canal at point of junction with transverse canal. Lacunae and canaliculi arranged in concentric rings. tained. A microscopic transverse section shows the substance of the shaft to be penetrated by longitudinal channels (Haver- r r> Pig. 446. — Bone-corpuscles and their processes which fill the lacunae and canaliculi (Rollett). CERTAIN TISSUES. 601 sian canals), while the intermediate space is occupied by cavi- ties (lacunae) connected with one another by very fine canals thl Fig. 447. — Vertical section of articular cartilage resting on bone, and showing cells and capsules arranged in layers as indicated by numerals (Sappey). (Fig. 444). A vertical cross - section exhibits the lamella? of which it is made up and the vascular channels cut across (Fig. 445). All this is, however, only the framework of osseous tissue. If a bone from an animal freshly killed, without bleeding, be examined, a very different state of things will be found. The bone is heavier ; its surface is covered with a closely adherent, tough, fibrous structure, the periosteum : and its medullary cavity filled with marrow. If the bone be broken across, its section looks red, and blood flows from the surface. Investiga- tion proves that the covering periosteum is a bed in which blood-vessels and nerves ramify, and from which they enter 608 COMPARATIVE PHYSIOLOGY- the openings to be seen on the surface of the dead bone. The Haversian canals are vascular channels, and the lacunae filled with bone corpuscles (Fig. 446). The marrow in the extremi- ties of the bone is of a red color in consequence of its great vas- cularity; and in the young animal a similar marrow fills the Pia. 448.— Section of cartilage of ear of man (Rollett). a, fibre-cartilage; b, connect- ive tissue. The cartilage had been boiled and dried prior to cutting. medullary canal, but later it is less vascular, and abounds in fat. Blood-vessels pass from it into the compact tissue of the CERTAIN TISSUES. 609 bone. The main artery, whence the others are derived, for the shaft of the hone, enters by the nutrient foramen on the sur- face, and toward the center. The bone-corpuscles (Fig. 446), answering to the connective- tissue cells, are nutritive and formative after a considerable portion of the tissue has become the seat of the deposit of lime- salts. Bone is a living tissue, though in a less degree than most others ; but it is only by bearing these relations in mind that its function in the support of the soft parts of an animal, and especially as constituting the essential levers of its locomo- tive mechanism, can be understood. Cartilage. — In the earliest stages of an animal's existence the bones are represented by cartilage, and at all periods of its existence this structure forms those elastic pads that cover its articular surfaces, and shield the bones and the entire animal from, undue concussion. The kind of cartilage that covers the extremities of the long bones, known as articular, is character- ized by abundance of cells lying in a homogeneous bed or ma- trix (Fig. 447). Fibro-cartilage (Fig. 448) abounds in fibrous tissue, some elastic fibers, characteristic cells, etc., and is found between the bodies of the vertebrae and in similar situations, as well as in the epiglottis, the ear, etc. 39 LOCOMOTION. The entire locomotor system of tissues is derived from the embryonic mesoblast. These include the muscles, hones, carti- lage, and connective and fibrous tissues ; and the tissues that make up the vascular system or the motor apparatus for the circulation of the blood. Locomotion in the mammal is effected by the movement of certain bony levers, while the equilibrium of the body is maintained. The whole series of levers is bound together by muscles, tendons, ligaments, etc., and play over one another at cer- tain points where they are in- vested with cartilage, and kept moist by a secretion from the cells covering the syno- vial membranes that form the inner linings of joints. Cartilage, a very low form of tissue destitute of blood- vessels, and hence badly re- paired when lost by injury or disease, forms a series of smooth surfaces admirably adapted for joints, and espe- cially fitted to act as a series of elastic buffers, and thus prevent shocks. Bone, though brittle in the dried state, possesses, when alive, a favorable de- gree of elasticity, while sufficiently rigid. Provision is made by its vascular periosteum and central marrow (in the case of the long bones), as Avell as by the blood-supply derived from the nutrient artery and its ramifications throughout the osseous LOCOMOTION. 611 tissue, for abundant nourishment, growth, and repair after in- jury- We find in the body of mammals, including man, examples of all three kinds of levers. It sometimes happens that there is an apparent sacrifice of energy, the best leverage not being exemplified ; but on closer examination it will be seen that the weight must either be moved with nice precision or through large distances, and these objects can not be accomplished al- ways by the arrangements that would simply furnish the most powerful lever. This is illustrated by the action of the biceps on the forearm. It is to be remembered that, while the flexors and extensors of a limb act in a certain degree the opposite of one another, Fig. 452. — Skeleton of deer. The bone.* in the extremities of this the fleetest of quad- rupeds are inclined very obliquely toward each other and toward the scapular and iliac bones. This arrangement increases the leverage of the muscular system and confers great rapidity on the moving parts. It augments elasticity, diminishes shock, and indirectly begets continuity of movement, n. angle formed by femur with ilinm; 6, angle formed by tibia and fibula with femur; c, angle formed by phalanges with cannon-bone; e, angle formed by humerus with scapula: /. angle formed by radius and ulna with humerus (Pettigrew). there is also, in all cases perhaps, a united action ; the one set, however, preponderating over the other, and usually sev- eral muscles, whether of the same or different classes, act to- gether. Standing itself requires the exercise of a large number of 612 COMPARATIVE PHYSIOLOGY. similar and antagonistic muscles so co-ordinated that the line of gravity falls within the area of the feet. An unconscious animal falls, which is itself an evidence of the truth of the above remarks. The folio-wing statements in regard to the direction of the line of gravity in man may prove useful : 1. That for the head falls in front of the occipital articulation, as exemplified by the nodding of the head in a drowsy person occupying the sitting attitude. 2. That for the head and trunk together passes behind a line joining the centers of the two hip-joints, hence the uncor- rected tendency of the erect body of man is to fall backward. 3. That for the head, trunk, and thighs falls behind the knee- joints somewhat, which would also favor falling backward (bending of the knees). 4. The line of gravity of the whole body passes in front of a line joining the two ankle-joints, so Fig. 453.— Shows the simultaneous positions of both legs during a step, divided into four groups (after Weber). First group (,4), 4 to 7, gives the different positions which the legs simultaneously assume while both are on the ground; second group (B), 8 to 11, shows the various positions of both legs at the time when the poste- rior leg is elevated from the ground, but behind the supported one; third group (C), 12 to 14, shows the positions which the legs assume when the swinging leg overtakes the standing one; and the fourth group (/)), 1 to 3. the positions during the time when the swinging leg is propelled in advance of the resting one. The letters it. h, and c indicate the angles formed by (he bones of the right leg when engaged in making a step; the letters m, n, and o, the positions assumed by the right foot when the trunk is rolling over it; {/, shows the rotating forward of the trunk upon the left foot (/) as an axis; h, shows the rotating forward of the left leg and foot upon the trunk ((/) as an axis. that the body would tend, but for the contraction of the mus- cles of the calves of the legs, to fall forward. Taking these different facts into consideration explains the LOCOMOTION. 013 various directions in which an individual, when erect, may fall according 1 as one or the other line (centei'j of gravity is dis- placed for a long enough time. Walking (man) implies the alternate movement of each leg forward, pendulum-like, so that for a moment the entire body must be supported on one foot. "When the right foot is lifted or swung forward, the left must support the weight of the body. It becomes oblique, the heel being raised, the toe still resting on the ground ; and it is upon this as a fulcrum that the body -weight is moved forward, when a similar action is taken up by the opposite leg. It follows that to prevent a fall there must be a leaning of the body to one side, so that the line of gravity may pass through each stationary foot ; hence a person walking describes a series of vertical curves with the head and of horizontal ones with the body, the resulting total being complex. Fig?. -154 and 455. — Showing the more or less perpendicular direction of the stroke of the wing in the flight of the bird (gull); how the wing is gradually extended as it is elevated (e,f, g)\ how it descends as a long lever until it assumes the position indicated by A : how it is flexed toward thetermination of the down-stroke, as shown at h'i.j. to convert it into a short lever («. ti) and prepare it for making the lip-Stroke. The difference in the length of the w ing during flexion and extension is indicated by the short and Jong levers a, b and c. d. The sudden conversion of the wing froni a Ion;; into a short lever at the end of the down-stroke is of great importance, as it robs the wing of its momentum and prepares it for reversing its movements (Pettigrew). The peculiarities of the gait of different persons are naturally determined by their height, length of leg. and a variety of other factors, which are often inherited with great exactness. We instinctively adopt that gait which economizes energy, both physical and mental. Running differs from walking, in that both feet are for a 614 COMPARATIVE PHYSIOLOGY. Figs. 450 and 457 show that when the wings are elevated («,/, g) the body falls («); and that when the wings are depressed (h, i,j) the body is elevated (r). Fig. 456 shows that the wings are elevated as short levers (e) until toward the termination of the up-stroke, when they are gradually expanded (/, g) to prepare them for making the down-stroke. Fig. 457 shows that the wings 'descend as long levers (A) until toward the termination of the down-stroke, when they are gradually folded or flexed (i,j) to rob them of their momentum and prepare them for mak- ing the up-stroke. (Compare with Figs. 454 and 455.) By this means the air be- neath the wings is vigorously seized during the down-stroke, while that above it is avoided during the up-stroke. The concavo-convex form of the wings and the forward travel of the body contributes to this result. The wings, it will be ob- served, act as a parachute both during the up and down strokes. Fig. 457 shows also the compound rotation of the wing, how it rotates upon a, as a center, with a radius m, b, n, and upon a, c, b as a center, with a radius k, I (Pettigrew). period of the cycle off the ground at the same time, owing to a very energetic action of the foot acting as a fulcrum. Jumping implies the propulsion of the body by the impulse given by both feet at the same moment. Hopping is the same act accomplished by the use of one leg. Comparative. — Tbe movements of quadrupeds are naturally very complicated, but have now been well worked out by the use of instantaneous photography. Even the bird's flight is no longer a wholly unsolved problem, but is fairly well under- stood. The movements of centipedes and and other many- legged invertebrates are highly complicated, while their rapid movements are to be accounted for by the multiplicity of their levers rather than the rapidity with which they are moved. LOCOMOTION. 615 The length and flexibility of their bodies must also be taken into account, rendering 1 many legs necessary for support. The subject of locomotion is of such great importance in the practice of comparative medicine that we shall now enter upon it in somewhat more detail, especially as regards the horse. This, of all our domestic animals, has become specialized as a locomotive mechanism. All the parts of his whole economy have been co-ordinated to that end ; and, except the horse be viewed in this light, the significance of much in his nature sJfe^- r — Fig. 458.— Chillingham bull (Bos Scoticus). Shows powerful, heavy body, and the small extremities adapted for land transit. Also the figure-of-8 movements made by the feet and limbs in walking and running. ?/, t. curves made by right and left anterior extremities; r, s, curves made by right and left posterior extremities. The right fore and the left hind foot move together to form the waved line (s, w); the left fore and the right hind foot move together to form the waved line (/•, t). The curves formed by the anterior (t, u) and posterior (,/■, s) extremities form ellipses (Pettigrew;. will be missed. But, however well his other parts might be suited to tbis purpose, unless the feet were adapted to rapid movements and great and frequently repeated concussions, the animal must soon break down. As it is, under the unnatural conditions of our artificially constructed roads, faulty shoeing, housing, and feeding, lamenesses of the feet constitute a large proportion of the cases that fall under the care of the practitioner. It may be well at the outset to give a little consideration to the feet of the horse, in order to learn to what extent they are adapted to natural conditions. The feet of all mammals illustrate how the soft and yielding tissues are combined with the rigid, to adapt to conditions of the surface over which they are re- quired to move. In the carnivora, beneath the outer tough skin covering the sole, there is the fatty cushion protective to the bones and more delicate soft parts ; while the claws, nails, etc. 616 COMPARATIVE PHYSIOLOGY. in which the toes end, are not only weapons of offense and de- fense, but protective against injury from contact with hard sur- faces, as well as directly helpful in locomotion. These princ pies are admirably exemplified in the foot of solipeds. The foot of the horse may be said to consist of terminal bones incased in soft structures adapted to shield the animal from the effects of excessive concussion and for nutrition, the Fig. 459. Fig. 460. Fig. 430.— Longitudinal median section of foot. 1. anterior extensor of phalanges, or extensor pedis; 2, lateral extensor, or extensor suffraginis; 3, capsule of meta- carpophalangeal articulation; 4. large metacarpal bone; 5. superficial flexor of phalanges, or perforatns; 0. deep flexor, or perforans; 7, sheath; 8, bursa; 9, sesa- moid bone; 10, ergot and fatty cushion of fetlock; 11, crucial ligament; 12, short sesamoid ligament; 13. first phalanx; 14, bursa; 15, second phalanx; 16, navicu- lar bone; 17. plantar cushion; 18, third phalanx; 19, plantar surface of hoof ; 20, sensitive or keratosrenous membrane of third phalanx. Fig. 460.— Horizontal section of horse's foot. 1. front or toe of hoof; 2, thickness of wall: 3. laminae; 4, insertion of extensor pedis; 5, os pedis; 6, navicular bone; 7, u -JiiL's of os pedis; 8, lateral cartilage; 9, flexor pedis tendon; 10, plantar cushion; 1 1 , inflexion of wall or " bar "; 12, horny frog. whole being incased in a protective covering which in a state of nature is constantly being worn away and renewed. The hoof is the homologue of the nails and claws of other mammals, and so may be regarded as a modification of the epidermis ; and thus viewed, its structure is at once* more readily under- stood and more interesting. To speak from an anatomical standpoint, the foot of the horse is made up of the terminal LOCOMOTION. 617 i|\ Fig. 461. Fig. 461.— Lower face of horse's foot, hoof being removed. 1, heel; 2, coronary cush- ion; 3, branch of plantar cushion; 4, median lacuna; 5, lamina of the bars; 6, velvety tissue of sole. Pig. 462.— Lateral view of horse's foot after removal of hoof. 1, perioplic ring, divided by a narrow groove from coronary cushion, 2, which is continuous with plantar cushion, 4, and joins vascular lamina, 3, through medium of white zone. phalanx, the navicular bone, and the lower part of the second phalanx; certain ligaments entering into the articulations ; the Fig. 464. Fig. 463.— Hoof just removed from foot; side view. a. inner surface of periople, or coronary frog-band, with some hairs passing through: «'. outer surface of same at posterior part of foot; a", a section through the wall to show its thickness; b toe, quarter of hoof; from b to front is outside (or inside) toe, from c to d the outside (or inside) heel; e, frog; /, bevel, or upper niargiu of wall for reception of coronary cushion; g, keraphylla, or horny laminae. Fig. 464.— Hoof, with outer portion of wall removed to show its interior, a, a, peri- ople, or coronary frog-band; b, cavity in upper part of wall for coronary cush- ion; c, upper or 'inner surface of •'liar"; d, vertical section of wall: /". lateral laminellaa; e,e, arteries of vascular lamina which have been in- jected. cortical parts. In the horse the fore-limbs are not attached to the trunk by osseous connections, but the animal may be said to be slung between its fore-limbs, all connections with the trunk being by soft parts, as muscles, tendons, and ligaments. 620 COMPARATIVE PHYSIOLOGY. The advantages of such an ar- l'angenient, to an animal in which a great deal of forward-pitching movement occurs, in breaking shocks are evident. The length- ened metatarsals and phalanges are accompanied by a very per- fect bracing of joints by liga- ments and tendons below, while the shoulder is strengthened and bound to the trunk by numerous muscles, so that the whole, in neatness, strength, and other qualities required in a fleet ani- mal, is, especially when taken in connection with the feet, an ex- ample of marvelous adaptation to conditions to be constantly met, aided in the wild species by natural selection, and in our do- mestic varieties by artificial se- lection. An examination of Fig. 470 will show the several levers (bones) and the muscles acting on them in one main movement of the fore-limb. The hind-limbs are in all gaits of the animal its main propellers, and these are in bony connection with the pelvis. Fig. 400. — Extern.il muscles of right anterior limb (Chauveau), 1, 1, long abductor of arm; V. its humeral insertion: 2, super- spinatus ; 3, subspinatus; 3', its tendon of insertion ; 4, short abductor of arm; 5, biceps; 6, anterior brachialis; 7, large extensor of forearm; 8, short extensor of forearm; 0, anconeus; 11, anterior ex- tensor of metacarpus; W, its tendon; 12, aponeurosis, separating that muscle from anterior brachialis; 13, oblique extensor of metacarpus; 14, anterior extensor of phalanges; II', its principal tendon; 15, small tendinous branch it furnishes to lateral extensor; 16, lateral extensor of phalanges; 16', its tendon; 17, fibrous band it receives from carpus; in, external flexor of metacarpus; 19, its metacar- pal tendon; 2D. its supracarpal tendon; 21. ulnar portion of perforans; 22, tendon of perforans; 23, its carpal ligament; 21, its re-enforcing phalangeal sheath; 25, tendon of the perforans, LOCOMOTION. 621 It will not be forgotten that in joints the insheaihing carti- lages (sometimes others more or less free), the synovial fluid, etc., all tend to diminish friction and lessen concussion. We shall now describe the principal gaits of the horse in a somewhat synoptical way. In each gait we have to consider the relative position of the four limbs, the duration of each phase in the move- ment, the length of the stride, its rate, etc. Much that applies to the horse holds good, of course, of other quadrupeds. In every gait each leg passes from a condition of flexion to one of extension, the degree be- ing dependent on the speed or, more correctly, the effort of the animal to attain high speed or reverse. Pig. 470. — Internal aspect of left an- terior limb (Chauveau). 1, pro- longing cartilage of scapula ; 2, inner surface of scapula; 3, sub- scapulars; 4. adductor of fore- arm, or portion of caput mag- num; 7. large extensor of fore- arm, other portion of caput mag- num; 8, middle extensor, or ca- put medium: 11. humeralis exter- nns, or short flexor of forearm; 10, coraco-humeralis; 11, upper extremity of humerus ; 12, co- raco-radialis. or flexor brachii ; 13, lower extremity of humerus; 14. brachial fascia ; 15. anterior extensor of metacarpus, or ex- tensor metacarpi magnus : 16, belly and aponeurotic termina- tion of flexor brachii; 17. ulna: 18, ulnaris accessorius. or oblique flexor of metacarpus: 11), inter- na! flexor of metacarpus, or epicondylo-metacarpus; 20. ther set is wholly radius: 21, tendon of oblique extensor: 22. large meta- carpal-bone : 33, flexor tendons of foot : 24, suspensory relaxed. xhe ligament; 25, internal rudimentary metacarpal bone; 26. ,-■ -i i extensor tendon of foot; 27, metucarpo - phalangeal more UlOl'OUgim sheath; 28, lateral cartilages of foot; 29, podophylhe. muscular move- ments are studied the more complex, so far as the use of muscles is concerned, are they found to be, a fact which is illustrated when even a single muscle is weakened or paralyzed. When the foot rests upon the ground before the limb is re- moved, it de- scribes the arc of a circle, or os- cillates like a pendulum so that the flexors and extensors are used alternately more and less ; ==^=-* though in all movements it is likely that nei- 622 COMPARATIVE PHYSIOLOGY. Walking. — In tliis gait the body rests on diagonal feet alter- nately with the two of the same side ; the center of gravity being shifted to one side, then returned to its original position, to be moved next to the opposite side. In drawing heavy loads the body is supported on three limbs. The rate of movement is one to two metres per second. Amble. — In this mode of progression, most common in the Fig. 471— Movements (oscillation) of an extended hind-leg (Colin). The hip-jomt describes the arc of a circle, ABC, while the foot is on the ground, the lines A D, B D, and C D representing the changing axis of the hind-leg. giraffe and camel tribe, occasional in ruminants and solipeds, the body is supported by the two legs on the same side, as in the walk, but the two opposite legs are elevated simultaneously and not separately. In horses this gait is often termed pacing, and is frequently very fast. Only two strokes of the feet are heard in this gait. In racking the hind-leg leaves the ground sooner than the corresponding fore-leg, hence four strokes of the feet are heard. The Trot.— The diagonal feet act together, two strokes of the feet being heard at each complete step. In the fast trot there is an interval in which all four feet are in the air. The hind- feet strike the ground in front of the fore-feet. The speed in LOCOMOTION. 623 the fast trot may reach from eight to twelve metres per sec- ond. The Gallop. — The gallop may be regarded as a series of Fig. 472. — Movements of fore-limbs of horse (Colin). While one fore-leg is describine the movements figured above the other acts as a support. While the right fore- foot describes the arc gh, the left shoulder describes the arc a' b' c', owing to the impulse from extension of the hind legs. The center of gravity is advanced from m to n, the left leg in one complete step occupying the six positions indicated at abed ef. jumps in which the hind-legs take the greater part, though as in all gaits the fore-legs are not only supporters but propellers. Fig. 473.— Various positions of the limbs in the trot (Colin). In the perfect gallop only two strokes of the feet are heard; in the canter or slow gallop four, in the ordinary gallop three. According as the one or other hind-leg is extended farthest behind the body the gallop is termed right-handed or left- handed. 624 COMPARATIVE PHYSIOLOGY. In the fastest gallop the length of stride may amount to six to seven metres, and the speed to twelve to fifteen metres per second. In such a rapid gait the contact of the one hind foot produces a sound lengthened by the rapid impact of the fellow- foot. The same applies to the fore-feet, hence only two sounds, while in the other varieties of this gait the interval between the impacts is sufficient to allow of three, or it may be four sounds. The accompanying plate, constructed by the help of instan- taneous photography, illustrates the different positions of a horse in the gallop. Sloping shoulder-blades and well-bent stifle-joints are gener- ally recognized as of great importance to an animal intended for high speed, and these are commonly to be met with in the Fig. 474.— Various positions in the trot (Colin) fleetest of horses, dogs, and other quadrupeds (Fig. 452). It may be seen that such an arrangement permits of a length- ened stride being taken with ease, tends to reduce concussion, and adds to beauty of form. To this must, in part at all events, be attributed the grace of form and fleetness of the race-horse and the greyhound, not to mention wild animals. A horse for heavy-draught purposes requires great muscular power, which in turn implies a strongly developed osseous sys- tem; and in order that this may be attained some of those principles on which speed depends must be subordinated to those involved in strength. As is well known, the cart-horse and race-horse, the mastiff and the greyhound, are opposites in build and capacity for speed. However, between these extreme forms there are many others of an intermediate character, as the hunter, roadster, etc. When famous race-horses are studied, 626 COMPARATIVE PHYSIOLOGY. while the form of the animal generally agrees with what would have been expected on mechanical principles it is a fact that some of the fleetest horses that have ever run on the course have not in all respects been built in conformity with them. But it is to be remembered that a vital mechanism differs from all others in that the whole consists of parts dependent not only as one portion of any machine is on the other, but that every part is energized and directed by a governing nervous system ; that every cell is being in a sense constantly renewed, so that the comparison between any ordinary mechanism and the body of a living animal holds only to a limited extent. Moreover, apart from peculiarities in the muscles of animals, to which atten- tion has been drawn (page 205), it is well to bear in mind that not only every animal, but every tissue has its own functional indi- viduality ; and to this especially (as exemplified in the most im- portant of all the tissues, the nervous) must we attribute the undoubted fact that the speed, endurance, etc., of animals can not be explained on mechanical principles alone — a truth to which too little attention has hitherto been drawn. These principles have, however, been unconsciously recognized prac- tically, hence the great attention paid by breeders to using ani- mals for stock purposes that have actually shown merit by their performances. Evolution. — It is noteworthy that with almost all quadru- peds the gallop is the natural method for rapid propulsion. In all animals, either bred by man to attain great speed, as the race-horse and greyhound, or those that have become so by the process of natural selection, the entire conformation of the body has been modified in harmony with the changes that have taken place in the legs and feet. This is seen in the greyhound among domestic animals, and in the wild deer of the plain and forest. Such instances illustrate not only the principle of natural selection as a whole, but the subordinate one of corre- lated growth. Any one observing the modes of locomotion of quadrupeds, especially horses and dogs, will perceive the advantages of the four-legged arrangement. Not only is there a variety of modes of progression, as walking, trotting, galloping, cantering, the alternations of which permit of rest to certain groups of mus- cles, with their corresponding nervous connections, etc., but on occasion some of these animals can progress fairly well with three legs. Sometimes it may also be noticed that a horse that LOCOMOTION. 627 prefers one gait, as pacing, for his easy, slow movements, will break into a trot when pushed to a higher rate of speed. Trotting can not be considered the natural gait for high speed in the horse, yet, by a process of "artificial selection" (by man) from horses that have shown capacity for great speed by this mode of progression, strains of racers have been bred, showing that even an acquired mode of locomotion may be hereditary ; while that galloping is the more natural mode of locomotion of the horse is evident, among other things, by the tendency of even the best trotting racers to break into a gallop when unduly pushed — an instance also of an hereditary tend- ency of more ancient origin prevailing over one more recent. The bipedal modes of progression of birds are naturally very like those of man. INDEX Abductor or sixth nerve, 582. Abnormal urine, 421. Accelerator nerves of heart, 258. Accommodation of eye, 532. Action of mammalian heart, 222. Affections of retina, 540. Afferent fibers, 583. After-images, etc., 543. Alimentary canal of vertebrate, 331. Allantoic, 77. Allantoic cavity, 80. Albumins, 145. derived, 145. Alterations in size of pupil, 533. Amble, 624. Amnion, 76. Amoeba, 13. Amylolytic action of saliva, 297. Animal body, 28. Animal foods (table), 277. Animal heat, 445. Animals deprived of cerebrum, 482. Anaemia, 117. Anomalies of refraction, 536. Apncca, 395. Apparatus used for stimulation of muscle, 179. for transmission of muscular move- ments by tambours, 182. Asphyxia, respiration and circulation in, 399. Auditory ossicles, 559. Auditory impulses, 565. sensations, etc., 567. Automatism, nervous system, 211. Automatic functions of spinal cord, 475. Bacteria, 18. Barking, 402. Bawling, neighing, braying. 403. Beat of the heart and its modifica- tions, 248. Bell-animalcule, 21. Bile, digestive action of, 303 . salts, 302. pigments, 302. Biology, general, 1. table, 4. Blastodermic vesicle, 78. Blood, 154. cells, 158. cells, decline, and death, 1 60. chemical composition of, 160. pressure, 223-227. flow, 227. Bone, 604. Botany, 4. Brain. 481. Capillaries. 264. Carbon-dioxide of blood, 389. Carbohydrates, 146. Cardiac movements, 231. sounds, 234. 630 COMPARATIVE PHYSIOLOGY. Cartilage, 609. Causes of the sounds of the heart, 235. Cell, 5. the male, 61. Cellulose, 9. Cerebellum, 508. Cerebral cortex, 497. Cerebro-spinal system of nerves, 580. and sympathetic systems, relations of, 588. Certain tissues, 603. Characteristics of proteids, general, 144. of blood-flow, 226. of secretion of different glands, 298. Changes in muscle during contrac- tion, 189. produced in food in alimentary canal, 354. in circulation after birth, 129. Chemical constitution of animal body, 142. changes in muscle, 189. composition of blood, 160. Chemistry of unicellular plants, 9. Chondrin, 145. Chorion, 79. Chronographs, 176. Ciliary movements, 179. (ophthalmic lenticular), ganglion, 584. Circulation of blood, 214. in mammal, 219, under microscope, 224. in brain, 500. Circumstances influencing character of muscular and nervous activ- ity, 199. Classification of animal kingdom, 34. of proteids, 145. Clinical and pathological re blood, 167. Coagulation of blood, 163. Coitus, 129. Color-vision, 543. Comparative re blood, 154, 172. unstriped muscle, 202. blood-pr,essure, 224. cardiac pulsation, 240. circulation, 244, 267. digestion, 280, 310, 357. metabolism, 436. diet, 438. digestive juices, 298. digestive organs, 324, 337. bile, 303. feeding experiments, 441. fats and carbohydrates, 442. animal heat, 445. spinal cord, 477. cerebral convolutions, 485. muscular sense, 524. vision, 536-551. hearing, 568. senses of smell and taste, 674, 578. voice, 598. locomotion, 614. swallowing, 336. vomiting, 340. movements of lymph, 344. respiration, 376, 398. haemoglobin, 389. respiratory movements, 402. respiration by skin, 412. perspiration, 413. expulsion of urine, 426. Comparison of inspired and expired air, 382. Composition of serum, 131. of corpuscles, 162. of milk, 275. Conclusions re unicellular plants, 10, protococcus, 12. unicellular animals, 15. nervous system, 212. heart, 257. salivary secretion, 314. INDEX, 631 Condiments, 278. Connective tissue, 603. Contractile tissues, 1*71. Connection of one part of brain with another, 491. Construction of fat, 432. Conditions under which gases exist in blood, 384. Co-ordination of two eyes in vision, 544. Coughing, 401. Corpuscles, 156. action of the, 22Y. Corpora quadrigemina, 506. Corpus striatum and optic thalamus, 504. Cranial nerves, 580. Crying, 402. Decussation, 4*71. Defecation, 338. Deglutition, 333. Dentition of domestic animals (table), 290-296. Development of embryo, 95. of vascular system in vertebrates, 108. of urogenital system, 112. Dextrin, 146. Diet, 437-439. effects of gelatin in, 441. effects of salts, water, etc., 443. Digestion of food, 274. Digestive juices, 297. action of bile, 303. organs, movements of, 332. Dioptrics of vision, 531. Discoidal placenta, 83. Domesticated animals, 47. Dyspnoea, 396. Effects of gelatin in diet, 411. Efferent nerve-fibers, 583. Entrance and exit of air, 370. Elasticity of muscle, 189. Elastin, 145. Elastic tissues, 604. Electrical phenomena of muscle, 191. organs, 197. Embryological re digestion, 279. brain, 510. vision, 528. Embryo, development of, 95. Embryology, applied to evolution, 45. Embryonic membrane of birds, 74. Endocardiac pressure, 236. Energy of animal body, 443. Epiblast, 98. Epithelium, 7. Evolution, 42. re reproduction, 93. circulation, 268. digestion, 363. respiration, 404. metabolism, 450. spinal cord, 478. brain, 511. vision, 552. hearing, 571. voice, 600. locomotion, 627. Estimation of size, etc., of objects, 548. Excretory function of skin, 411. Excretion of perspiration, 412. by the kidney, 415. Experimental facts, 185. re nervous system, 210. digestion, 305. spinal nerves, 580. Eustachian tube, 562. Eye, accommodation of, 532. optical imperfections of, 536. protective mechanisms of, 549. Facial nerve, 581. and laryngeal respiration, 374. Fat, construction of, 432. 632 COMPARATIVE PHYSIOLOGY. Fats, 145. . and carbohydrates, 442. Fatigue, 199. Feeding experiments, 439. Features of an arterial pulse tracing, 242. Fertilization of ovum, 63. Fibrin, 145, 164. Faeces, 352. Foetal circulation, 125. later stages of, 109. membranes of mammals, 78. Food, digestion of, 274. stuffs, 274. Foods, animal, table of, 277. vegetable, table of, 277. Foreign gases in respiration, 391. Forced movements, 484. Fossil and existing species, 46. Fresh-water polyps, 23. Fungi, 15. Functional variations, 204. Functions of cerebral convolutions, 485. of other portions of brain, 504. Gastrula, 68. Gastric juice, 299. Gelatin, 145. Geographical distribution, 46. Globulins, 145. Glosso-pharyngeal or ninth nerve, 585. Glycogen, 428. uses of, 429. Glycocholic acid, 302. Gout, 139. Graafian follicle, 58. Graphic method and study of muscle physiology, 176. Gustatory fibers, 582. Haemoglobin and its derivatives, 385. Hearing, 557. Heart, 231. of various animals, 245, 246. beat in cold-blooded animals, 250. causation of beat of, 253. influence of vagus nerve on, 253. accelerator nerves of, 258. in relation to blood-pressure, 260. Hen's egg, 69. Hiccough, 402. History of blood-cells, 158. Hydra, 26. Hypoblast, 98. Hypertrophy, 265. Hypnotism, 502. Hypoglossal or twelfth nerve, 588. Impulse of heart, 232. Influence of blood-supply, 199. of temperature, 201. of vagus nerve upon heart, 253. of condition of blood in respira- tion, 393. of respiration on circulation, 396. of nervous system on metabolism, 452. Inhibition of reflexes, 469. Inorganic food-stuffs, 144, 274. Inosit, 145. Inorganic salts, 420. Instincts, 42. Investigation of heart - beat from within, 233. Intestinal movements, 337. Irritability of muscle and nerve, 175. Juices, digestive, 297. Keratin, 145. Lactose, 146. Law of periodicity or rhythm in na- ture, 37. of habit, 41,405. of rhythm, 269. INDEX. 633 Laws of retinal stimulation, 541. Laughing, 401. Living things, 2. Living and lifeless matter, 32. Lymphatic system, 342. Lymph and chyle, 343. Locomotion, 610. Maltose, 146. Mammalian heart, 215, 222. Man's place in animal kingdom, 36. Medulla oblongata, 509. Membrana tympani, 558. Mesoblast, 98. Metadiseoidal placenta, 84. Metabolism, 27, 428. of liver, 428. of spleen, 429. influence of nervous system on, 452. summary of, 45S. Metazoa, 5, 53. Milk, composition of, 275. sugar, 276. Mimicry, 45. Molds, 15. Morphology of unicellular plants, 9. of protococcus, 12. of unicellular animals, 13. applied to evolution, 45. Motor oculi nerve, 581. Movements of digestive organs, 332. stomach, 336. lymph, 344. Mucin, 145. Mucor mucedo, 17. Mullerian duct, 115. Multicellular organisms, 23. Muscular contraction, 185. Muscle tone, 189. Muscular work, 198. Muscles of respiration, 373. of middle ear, 561. Muscular sense, 524. Nasal or spheno-palatine ganglion, 584. Nature of act of secretion, 318. Nerve-cells, 209. supply (voice), 596. Nervous mechanism, 134. system, 208. inhibition, 212. system in relation to heart, 249. supply — digestion, 333. system in relation to respiration, 393. Nitrogen of blood, 389. Nitrogenous metabolites, 147. crystalline bodies, 420. equilibrium, 440. Non-nitrogenous metabolites, 147. organic bodies, 420. Non-crystalline bodies, 145. Notochord, 99. Nucleus, 5. i ' Nucleolus, 5. Nuclear division, 6. Nuclein, 145. Nutrition of ovum, 123. Ocular movements, 544. (Estrum, 121. Optical imperfections of the eye, 536. Origin of forms of life, 42. of spermatozoon, 61. of fowl's egg, 70. Organic evolution reconsidered, 137. food-stuffs, 144-274. Otic ganglion, 585. Ovulation, 120. Ovum, 55. origin and development of, 57. changes in, 58. Oxy-ha3moglobin, 386. Paleontology, 46. Pancreatic juice, 305. Parasitic organisms, 15. 634 COMPARATIVE PHYSIOLOGY. Parturition, 128. Pathological re food-stuffs, 147. muscle, 197. circulation, 244, 265. bile, 318. stomach, 330, 337. lymphatics, 352. faeces, 354. digestion, 359. respiration, 379, 401, 403. skin, 411. urine, 423. expulsion of urine, 426. metabolism, 435, 443. temperature, 449. spinal cord, 471. brain, 508, 509. muscular sense, 524. vision, 536, 554. hearing, 563. spinal nerves, 580. third and other nerves, 582, 585, 587, 588. voice, 598. Peculiar respiratory movements, 401. Peptones, 145. Pendulum myograph, 184. Perspiration, excretion of, 412. Periods of gestation, 127. Pfliigcr's monograph, 181. Physiology of unicellular plants, 10. of protococcus, 12. unicellular auimals, 13. nerves, 195. Physiological aspects of development, 118. research and reasoning, 148. Placenta, 82. discoidal, 83. metadiscoidal, 84. zonary, 89. diffuse, 89. polycotyledonary, 89. Simple, 90. Placenta multiple, 90. microscopic structure of, 90. Pneumogastric nerve, 586. Polyps, 23. Pressure, endocardiac, 236. Pressure sensations, 522. Proteids, general characteristics of, 144. of milk, 276. Proteus animalcule, 13. Protococcus, 11. Protozoa, 5, 53. Protective and excretory function of skin, 40S. Protective mechanism of the eye, 549. Protoplasm, 3. Proximate principles, 144. Pulse, 241. the venous, 244. Psychological aspects of vision, 543. Quantity and distribution of blood, 163. of blood, influence on blood-press- ure, 261. of air respired, 378. Reflex functions of the spinal cord, 466. Regulation of temperature, 447. Relations of cerebro-spinal and sym- pathetic symptoms, 588. Relative value of food-stuffs, 444. time occupied by cardiac cycle, 237. Removal of digested products from the alimentary canal, 341. Reproduction, 51. Retinal stimulation, laws of, 541. Respiratory system, 366. rhythm, 379. sounds, 81. Respiration, muscles of, 373. facial and laryngeal, 374. types of, 375. in blood, 383. INDEX. 635 Respiration in tissues, 391. Respiration and circulation in as- phyxia, 399. in mammal, 405. by skin, 412. Rigor mortis, 206. Rhythm, 37. law of, 269. Rudimentary organs, 46. Salts, 144, 276. inorganic, 420. Saliva, 297. amylolytic action of, 297. Scar-tissue, 139. Serum, composition of, 161. Secretion as a physiological process, of salivary glands, 311. by stomach, 315, 321. of bile and pancreatic juice, 316. nature of the act, 318. of mine, 421. Secretory fibers, 582. Segmentationand subsequent changes, 64. Self-digestion of digestive organs, 323. Sense organs, 31. Sexual selection, 43. Separation of muscle from central nervous system, 201. Semicircular canals, function of, 484. Sighing, 402. Senses, general remarks, 516. of smell and taste, 573. Skin as an organ of sense, 520. Smell, 573. Sleep, 501. Sobbing, 402. Solidity, 548. Soap, 146. Sneezing, 402. Special considerations re muscle, 208. circulation, 265. Special considerations re digestion, 359. metabolism, 450. spinal cord, 477. brain, 510. vision, 551. hearing, 568. voice, 600. Specific gravity of urine, 419. Spermatozoa, 61. Spbygmograph, 243. Spinal cord, general, 461. reflex functions of, 466. as a conductor of impulses, 469. automatic functions of, 475. nerves, 579. accessory, 587. Sporangia, 17. Starches, 147. Study of metabolic processes, 436. Submaxillary ganglion, 585. Succus entericus, 307. Sugars, 146. Summary of biology, 9. of evolution, 50. of reproduction, 93. development of the embryo, 135. physiological research, etc., 152. blood, 169. muscle and nerve, 205. circulation, 269. blood-cells, 158. digestion, 364. respiration, 405. perspiration, 413. urine, 426. metabolism, 458. voice, 001. Synoptical re spinal cord, 479. brain, 513. skin, 525. vision, 564. hearing, 572. Swallowing, 335. 636 COMPARATIVE PHYSIOLOGY. Tactile sensibility, 523. Tambour of Marey, 183. Taste, 575. Teeth, 2S4. structure and arrangement of the, 286. Temperature, regulation of, 447. Tetanizing key of Du Bois-Reymond, 181. Tetanic contraction, 187. Thermal changes in contracting mus- cle, 195. sensations, 522. Tissues, 8. Trigeminus, trifacial or fifth nerve, 583. Trochlear or fourth nerve, 582. Trot, 624. Types of respiration, 375. Unicellular plants, 9. animals, 13. with differentiation of structure, 21. Unstriped muscle, 202. Urea, 147. Urine considered physically and chem- ically, 419. abnormal, 421. Urine, secretion of, 421. expulsion of, 424. Variations in cardiac pulsation, 239. of average tempei'ature, 446. Vasomotor nerves, 262. Vegetable foods (table), 277. Velocity of blood and blood-press- ure, 223. Venous pulse, 244. Vision, 526. dioptrics of, 531. psychological aspects, 543. Visual sensations, 538. angle, 542. Voice, 593. Vomiting, 339. Vorticella, 21. Walking, 623. Water, 443. Wolffian duct, LI 5. Work of the heart, 238. Yeast, 9. cells, 10. Yawning, 402. Zoology, 4. 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