Dr. Lilienthal, 230 W. 25th St., New York. THE PROPERTY OF ieal Collep sf tie Pacific. MEDICAL Cfe OF PHYSIOLOGY, BY WILLIAM SENHOUSE KIRKES, M.D. EDITED BY W. MORRANT BAKER, F.R.C.S., LECTURER ON PHYSIOLOGY, AND ASSISTANT SURGEON TO ST. BARTHOLOMEW'S HOSPITAL ; SURGEON TO THE EVELINA HOSPITAL FOR SICK CHILDREN. WITH TWO HUNDRED AND FORTY-EIGHT ILLUSTRATIONS. A NEW AMERICAN FROM THE EIGHTH ENLARGED ENGLISH EDITION. HENRY C. L E A. 1873. SHERMAN & CO., PRINTKES, PREFACE TO THE EIGHTH EDITION.' THE Eighth Edition is the result of an increased demand for this work, involving the necessity for a re- print at an earlier period after the publication of the Seventh Edition than was anticipated. The opportunity has been seized for making corrections and additions where they appeared to be most needed ; but the present issue must be regarded as, in great part, a reprint of the Edition of 1869. W. MORRANT BAKER. THE COLLEGE, ST. BARTHOLOMEW'S HOSPITAL, October, 1872. 13547 CONTENTS. CHAPTER I. PAGE THE GENERAL AND DISTINCTIVE CHARACTERS OF LIVING BEINGS, 13 CHAPTER II. CHEMICAL COMPOSITION OF THE HUMAN BODY, ... 18 CHAPTER III. STRUCTURAL COMPOSITION OF THE HUMAN BODY,. . . 26 CHAPTER IV. STRUCTURE OF THE ELEMENTARY TISSUES, .... 34 Epithelium, 34 Areolar, Cellular, or Connective Tissue, .... 38 Adipose Tissue, 40 Pigment, .......... 42 Cartilage, 43 Bones and Teeth, .46 CHAPTER V. THE BLOOD, .55 Quantity of Blood, 56 Coagulation of the Blood, ....... 58 Conditions affecting Coagulation, 62 1* VI CONTENTS. PAGE Chemical Composition of the Blood, ..... 64 The Blood-Corpuscles, or Blood-cells, ..... 65 Chemical Composition of Ked Blood-cells, .... 68 Blood-crystals, 69 The White Corpuscles, 71 The Serum, 72 Variations in the Principal Constituents of the Liquor San- guinis, .......... 73 Variations in Healthy Blood under Different Circumstances, 76 Variations in the Composition of the Blood in Different Parts of the Body, 77 Gases contained in the Blood, ...... 81 Development of the Blood, 81 Uses of the Blood, . 85 Uses of the various Constituents of the Blood, . . .85 CHAPTEK VI. CIRCULATION OF THE BLOOD, 88 The Systemic, Pulmonary, and Portal Circulations, . . 89 THE HEART, 91 Structure of the Valves of the Heart, 92 The Action of the Heart, 96 Function of the Valves of the Heart, 99 Sounds of the Heart, 105 Impulse of the Heart, ........ 107 Frequency and Force of the Heart's Action, . . . 108 Cause of the Rhythmic Action of the Heart, . . .111 Effects of the Heart's Action, . . . . . .114 THE ARTERIES, 115 Structure of the Arteries, 115 The Pulse, . . . . .123 Sphygmograph, 124 Force of the Blood in the Arteries, 129 Velocity of the Blood in the Arteries, . . . .131 THE CAPILLARIES, 131 The Structure and Arrangement of Capillaries, . . . 132 Circulation in the Capillaries, 135 CONTENTS. Vll PAGE THE VEINS, 141 Structure, 141 Agents concerned in the Circulation of the Blood, . . 145 Velocity of Blood in the Veins, 147 Velocity of the Circulation, 147 PECULIARITIES OF THE CIRCULATION IN DIFFERENT PARTS, 150 Cerebral Circulation, 150 Erectile Structures, 152 CHAPTEE VII. RESPIRATION, , 155 Position and Structure of the Lungs, ; 155 Mechanism of Eespiration, ....... 161 Respiratory Movements, ...;... 162 Kespiratory Khythm, 165 Kespiratory Movements of Glottis, 166 Quantity of Air respired, ....... 166 Movements of the Blood in Respiratory Orgaiis, . . . 172 Changes of the Air in Respiration, ..... 173 Changes produced in the Blood by Respiration, . . . 179 Mechanism of various Respiratory Actions, . . . 180 Influence of the Nervous System in Respiration, . . . 185 Effects of the Suspension and Arrest of Respiration, . . 186 CHAPTER VIII. ANIMAL HEAT, . . . . . . . . . . . 189 Variations in Temperature, ....... 190 Sources and Mode of Production of Heat in the Body, . 192 Regulation of Temperature, ....... 194 Influence of Nervous System, ...... 198 CHAPTER IX. DIGESTION, 199 Food, 199 Starvation, . . 203 Vlll CONTENTS. PAGE PASSAGE OF FOOD THROUGH THE ALIMENTARY CANAL, . . 207 The Salivary Glands and the Saliva, ..... 209 Passage of Food into the Stomach, 213 DIGESTION OF FOOD IN THE STOMACH, 214 Structure of the Stomach, 214 Secretion and Properties of the Gastric Fluid, . . . 219 Changes of the Food in the Stomach, 225 Movements of the Stomach, ....... 231 Influence of the Nervous System on Gastric Digestion, . 234 Digestion of the Stomach after Death, .... 236 DIGESTION IN THE INTESTINES, ....... 238 Structure and Secretion of the Small Intestines, . . . 238 Valvulse Conniventes, . . . . . . . . 240 Glands of the Small Intestine, 240 The Villi, 246 Structure of the Large Intestine, ...... 248 The Pancreas and its Secretion, ...... 250 Structure of the Liver, ....... 252 Functions of the Liver, ....... 259 The Bile, 259 Glycogenic Function of the Liver, ..... 267 Summary of the Changes which take place in the Food during its Passage through the small Intestine, . . 270 Summary of the Process of Digestion in the large Intestine, 272 Gases contained in the Stomach and Intestines, . . . 274 Movements of the Intestines, ...... 275 CHAPTER X. ABSORPTION, .......... 277 Structure and Office of the Lacteal and Lymphatic Vessels, and Glands, 277 Lymphatic Glands, .283 Properties of Lyrnph and Chyle, ..... 286 Absorption by the Lacteal Vessels, ..... 290 Absorption by the Lymphatics, ...... 291 Absorption by Bloodvessels, ...... 293 CHAPTER XL NUTRITION AND GROWTH, ....... 299 Nutrition, 299 Growth, . 311 CONTENTS. IX CHAPTER XII. PAGE SECRETION, 313 Secreting Membranes, ........ 314 Serous Membranes, 315 Mucous Membranes, 316 Secreting Glands, ........ 319 Process of Secretion, 321 CHAPTER XIII. VASCULAR GLANDS, OR GLANDS WITHOUT DUCTS, . . . 325 Structure of the Spleen, 327 Functions of the Vascular Glands, ..... 329 CHAPTER XIV. THE SKIN AND ITS SECRETIONS, 332 Structure of the Skin, 333 Structure of Hair and Nails, ...... 340 Excretion by the Skin, 344 Absorption by the Skin, ....... 347 CHAPTER XV. ^ THE KIDNEYS AND THEIR SECRETION, 349 Structure of the Kidneys, ....... 349 Secretion of Urine, ........ 354 The Urine ; its General Properties, . . . . . 356 Chemical Composition of the Urine, . . . . - . 357 CHAPTER XVI. THE NERVOUS SYSTEM, ........ 367 Elementary Structures of the Nervous System, . . . 368 Functions of Nerve-fibres, . . . . . . . 376 Functions of Nerve-centres, ...... 382 CEREBRO-SPINAL NERVOUS SYSTEM, ..... 386 Spinal Cord and its Nerves, 386 Functions of the Spinal Cord, 392 X CONTENTS. PAGE THE MEDULLA OBLONGATA, 402 Its Structure, 402 Distribution of the Fibres of the Medulla Oblongata, . 404 Functions of the Medulla Oblongata, 405 STRUCTURE AND PHYSIOLOGY OF THE PONS VAROLII, CRURA CEREBRI, CORPORA QUADRIGEMINA, CORPORA GENIC- ULATA, OPTIC THALAMI, AND CORPORA STRIATA, . . 409 PonsVarolii, 409 Crura Cerebri, 409 Corpora Quadrigemina, ....... 411 The Sensory Ganglia, 413 STRUCTURE AND PHYSIOLOGY OF THE CEREBELLUM, . . 414 STRUCTURE AND PHYSIOLOGY OF THE CEREBRUM, . . .419 PHYSIOLOGY OF THE CEREBRAL AND SPINAL NERVES, . . 424 Physiology of the Third, Fourth, and Sixth Cerebral or Cranial Nerves, 425 Physiology of the Fifth or Trigeminal Nerve, . . . 428 Physiology of the Facial Nerve, 433 Physiology of the Glosso-pharyngeal Nerve, . . . 435 Physiology of the Pneumogastric Nerve, .... 438 Physiology of the Spinal Accessory Nerve, . . . 443 Physiology of the IJypoglossal Nerve, .... 444 Physiology of the Spinal Nerves, 444 PHYSIOLOGY OF THE SYMPATHETIC NERVE, .... 445 CHAPTER XVII. CAUSES AND PHENOMENA OF MOTION, ..... 454 Ciliary Motion, ......... 454 Muscular Motion, ........ 456 Muscular Tissue, 456 Properties of Muscular Tissue, ...... 461 Action of the Voluntary Muscles, 467 Action of the Involuntary Muscles, ..... 473 Source of Muscular Action, 473 CONTENTS. XI CHAPTER XVIII. PAGE Or VOICE AND SPEECH, 474 Mode of Production of the Human Voice, .... 474 The Larynx, 476 Application of the Voice in Singing and Speaking, . . 483 SPEECH, 486 CHAPTER XIX. THE SENSES, 489 THE SENSE OF SMELL, 494 THE SENSE OF SIGHT, 499 Structure of the Eye, 499 Phenomena of Vision, 507 Reciprocal Action of different parts of the Retina, . . 519 Simultaneous Action of the two Eyes, . . . ' . 521 THE SENSE OF HEARING, ....... 527 Anatomy of the Organ of Hearing, ..... 527 Physiology of Hearing, ....... 534 Functions of the External Ear. 535 Functions of the Middle Ear; the Tympanum, Ossicula, and Fenestrae, 536 Functions of the Internal Ear 541 Sensibility of the Auditory Nerve, ..... 543 THE SENSE OF TASTE, 547 THE SENSE OF TOUCH, 554 CHAPTER XX. GENERATION AND DEVELOPMENT, ...... 559 Generative Organs of the Female, 560 Unimpregnated Ovum, ....... 563 Discharge of the Ovum, ....... 567 Corpus Luteum, 570 Xll CONTENTS. PAGE IMPREGNATION OF THE OVUM, 573 Male Sexual Functions, 573 DEVELOPMENT, 578 Changes of the Ovum previous to the Formation of the Embryo, 578 Changes of the Ovum within the Uterus, .... 581 The Umbilical Vesicle, 582 The Amnion and Allantois, ........ 585 The Chorion, 588 Changes of the Mucous Membrane of the Uterus and For- mation of the Placenta, 589 DEVELOPMENT OF ORGANS, 593 Development of the Vertebral Column and Cranium, . 594 Development of the Face and Visceral Arches, . . . 595 Development of the Extremities, ..... 596 Development of the Vascular System, .... 597 Circulation of Blood in the Foetus, . . . . .601 Development of the Nervous System, ..... 603 Development of the Organs of Sense, 603 Development of the Alimentary Canal, .... 606 Development of the Respiratory Apparatus, . . . 608 The Wolffian Bodies, Urinary Apparatus, and Sexual Or- gans, 609 THE MAMMARY GLANDS, 613 LIST OF WORKS REFERRED TO, 617 INDEX, . 625 HANDBOOK OF PHYSIOLOGY. CHAPTER I. ON THE GENERAL AND DISTINCTIVE CHARACTERS OF LIVING BEINGS. HUMAN PHYSIOLOGY is the science which treats of the life of man of the way in which he lives, and moves, and has his being. It teaches how man is begotten and born ; how he attains maturity ; and how he dies. Having, then, man as the object of its study, it is unneces- sary to speak here of the laws of life in general, and the means by which they are carried out, further than is requisite for the more clear understanding of those of the life of man in par- ticular. Yet it would be impossible to understand rightly the working of a complex machine without some knowledge of its motive power in the simplest form ; and it may be well to see first what are the so-called essentials of life those, namely, which are manifested by all living beings alike, by the lowest vegetable and the highest animal, before proceeding to the con- sideration of the structure and endowments of the organs and tissues belonging to man. The essentials of life are these, birth, growth and develop- ment, decline and death ; and an idea of what life is, will be best gained by sketching these events, each in succession, and their relations one to another. The term, birth, when employed in this general sense of one of the conditions essential to life, without reference to any par- ticular kind of living being, may be taken to mean, separation from a parent, with a greater or less power of independent ex- istence as a living being. Taken thus, the term, although not defining any particular stage in development, serves well enough for the expression of the fact, to which no exception has yet been proved to exist, that the capacity for life in all living beings is got by in- heritance. 14 GROWTH. Growth, or inherent power of increasing in size, although essential to our idea of life, is not a property of living beings only. A crystal of sugar or of common salt, or of any other substance, if placed under appropriate conditions for obtaining fresh material, will grow in a fashion as definitely character- istic and as easily to be foretold as that of a living creature. It is, therefore, necessary to explain the distinctions which exist in this respect between living and lifeless structures ; for the manner of growth in the two cases is widely different. First, the growth of a crystal, to use the same example as before, takes place merely by additions to its outside ; the new matter is laid on particle by particle, and layer by layer, and, when once laid on, it remains unchanged. The growth is here said to be superficial. In a living structure, on the other hand, as, for example, a brain or a muscle, where growth occurs, it is by addition of new matter, not to the surface only, but throughout every part of the mass ; the growth is not super- ficial, but interstitial. In the second place, all living structures are subject to constant decay ; and life consists, not as once supposed, in the power of preventing this never-ceasing decay, but rather in making up for the loss attendant on it by never- ceasing repair. Thus, a man's body is not composed of ex- actly the same particles day after day, although to all intents he remains the same individual. Almost every part is changed by degrees; but the change is so gradual, and the renewal of that which is lost so exact, that no difference may be noticed, except at long intervals of time. A lifeless structure, as a crystal, is subject to no such laws ; neither decay nor repair is a necessary condition of its existence. That which is true of structures which never had to do with life is true also with re- spect to those which, though they are formed by living parts, are not themselves alive. Thus, an oyster-shell is formed by the living animal which it incloses, but it is a lifeless as any other mass of saline matter; and in accordance with this cir- cumstance its growth takes place not inter stitially, but layer by layer, and it is not subject to the constant decay and recon- struction which belong to the living. The hair and nails are examples of the same fact. Thirdly. In connection with the growth of lifeless masses there is no alteration in composition or properties of the ma- terial which is taken up and added to the previously existing mass. For example, when a crystal of common salt grows on being placed in a fluid which contains the same material, the properties of the salt are not changed by being taken out of the liquid by the crystal and added to its surface in a solid form. But the case is essentially different from this in living beings, DEVELOPMENT. 15 both animal and vegetable. A plant, like a crystal, can only grow when fresh material is presented to it ; and this is ab- sorbed by its leaves and roots ; and animals for the same pur- pose of getting new matter for growth and nutrition, take food into their stomachs. But in both these cases the materials are much altered before they are finally assimilated by the struc- tures they are destined to nourish. Fourthly. The growth of all living things has a definite limit, and the law which governs this limitation of increase in size is so invariable that we should be as much astonished to find an individual plant or animal without limit as to growth as without limit to life. Development is as constant an accompaniment of life as growth. The term is used to indicate that change to which, be- fore maturity, all living parts are constantly subject, and by which they are made more and more capable of performing their several functions. For example, a full-grown man is not simply a magnified child ; his tissues and organs have not only grown, or increased in size, they have also developed, or become better in quality. No very accurate limit can be drawn between the end of de- velopment and the beginning of decline ; and the two processes may be often seen together in the same individual. But after a time all parts alike share in the tendency to degeneration, and this is at length succeeded by death. The decline of living beings is as definite in its occurrence as growth or development. Death not by disease or injury so far from being a violent interruption of the course of life, is but the fulfilment of a purpose in view from the commence- ment. It has been already said that the essential features of life are the same in all living things ; in other words, in the mem- bers of both the animal and vegetable kingdoms. It may be well now to notice briefly the distinctions which exist between the members of these two kingdoms. It may seem, indeed, a strange notion that it is possible to confound vegetables with animals, but it is true with respect to the lowest of them in which but little is manifested beyond the essentials of life, which are the same in both. I. Perhaps the most essential distinction is the presence or absence of power to live upon inorganic material; in other words, to act chemically on carbonic acid, ammonia, and water, so as to make use of their component elements as food. Indeed one ought probably to say that a question concerning the capa- bility of the lower kinds of animal to live in this way cannot be entertained; and that such a manner pf life should decide 16 ANIMALS CONTRASTED at once in favor of a vegetable nature, whatever might be the attributes which seem to point to an opposite conclusion. The power of living upon organic matter would seem to be less de- cisive of an animal nature, for some fungi appear to derive sup- port almost entirely from this source. II. There is, commonly, a marked difference in general chemical composition between vegetables and animals, even in their lowest forms ; for while the former consist mainly of a substance containing carbon, hydrogen, and oxygen only, ar- ranged so as to form a compound closely allied to starch, and called cellulose, the latter are commonly composed in great part of the three elements just named, together with a fourth, nitrogen; the proximate principles formed from these being identical, or nearly so, with albumen. It must not be supposed, however, that either of these typical compounds alone, with its allies, is confined to one kingdom of nature. Nitrogenous or albuminous compounds are freely produced by vegetable structures, although they form an infinitely smaller proportion of the whole organism than cellulose or starch. And while the presence of the latter in animals is much more rare than is that of the former in vegetables, there are many animals in which traces of it may be discovered, and some, the Ascidians, in which it is found in considerable quantity. III. Inherent power of movement is a quality which we so commonly consider an essential indication of animal nature, that it is difficult at first to conceive it existing in any other. The capability of simple motion is now known, however, to exist in so many vegetable forms, that it can no longer be held as ail essential distinction between them and animals, and ceases to be a mark by which the one can be distinguished from the other. Thus the zoospores of many of the Crypto- gamia exhibit movements of a like kind to those seen in animal- cules ; and even among the higher orders of plants, many ex- hibit such motion, either at regular times, or on the application of external irritation, as might lead one, were this fact taken by itself, to regard them as sentient beings. Inherent power of movement, then, although especially characteristic of ani- mal nature, is, when taken by itself, no proof of it. Of course, if the movement were such as to indicate any kind of purpose, whether of getting food or any other, the case would be differ- ent, and we should justly call a being exhibiting such motion, an animal. But low down in the scale of life, where alone there exists any difficulty in distinguishing the two classes, movements, although almost always more lively, are scarcely or not at all more purposive in one than the other ; and even if we decide on the animal nature of a being, it by no means WITH VEGETABLES. 17 follows that we are bound to acknowledge the presence of sen- sation or volition in the slightest degree. There may be at least no evidence of its possessing a trace of those tissues, ner- vous and muscular, by which, in the higher members of the animal kingdom, these qualities are manifested. Probably there is no more of either of them in the lowest animals than in vegetables. In both, movement is eifected by the same means ciliary action, and hence the greater value, for pur- poses of classification, of the power to live on this or that kind of food on organic or inorganic matter. As the main purpose of the lowest members of the vegetable kingdom is doubtless to bring to organic shape the elements of the inorganic world around, so the function of the lowest animals is, in like man- ner, to act on degenerating organic matter "to arrest the fugitive organized particles, and turn them back into the as- cending stream of animal life." And, because sensation and volition are accompaniments of life in somewhat higher animal forms, it is needless to suppose that these qualities exist under circumstances in which, as we may believe, they could be of no service. It is as needless as to dogmatize on the opposite side, and say that no feeling or voluntary movement is possible without the presence of those tissues which we call nervous and muscular. IV. The presence of a stomach is a very general mark by which an animal can be distinguished from a vegetable. But the lowest animals are surrounded by material that they can take as food, as a plant is surrounded by an atmosphere that it can use in like manner. And every part of their body being adapted to absorb and digest, they have no need of a special receptacle for nutrient matter, and accordingly have no stomach. This distinction then is not a cardinal one. It would be tedious as well as unnecessary to enumerate the chief distinctions between the more highly developed animals and vegetables. They are sufficiently apparent. It is neces- sary to compare, side by side, the lowest members of the two kingdoms, in order to understand rightly how faint are the boundaries between them. 18 CHEMICAL COMPOSITION OF HUMAN BODY. CHAPTER II. CHEMICAL COMPOSITION OF THE HUMAN BODY. THE following Elementary Substances may be obtained by chemical analysis from the human body : Oxygen, Hydrogen, Nitrogen, Carbon, Sulphur, Phosphorus, Silicon, Chlorine, Fluorine, Potassium, Sodium, Calcium, Magnesium, Iron, and, probably as accidental constituents, Manganesium, Alumin- ium, Copper, and Lead. Thus, of the sixty-three or more elements of which all known matter is composed, more than one fourth are present in the human body. Only one or two elements, and in very minute amount, are present in the body uncombined with others ; and even these are present much more abundantly in various states of combi- nation. The most simple compounds formed by union in various proportions of these elements are termed proximate principles ; while the latter are classified as the organic and the inorganic proximate principles. The term organic was once applied exclusively to those substances which were thought to be beyond the compass of synthetical chemistry and to be formed only by organized or living beings, animal or vegetable ; these being called organ- ized, inasmuch as they are characterized by the possession of different parts called organs. But with advancing knowledge, both distinctions have disappeared ; and while the title of Mving organism is applied to numbers of living things, having no trace of organs in the old sense of the term, and in some, so far as can be now seen, in no other sense, the term organic has long ceased to be applied to substances formed only by living tissues. In other words, substances, once thought to be formed only by living tissues, are still termed organic, al- though they can be now made in the laboratory. The term, indeed, in its old meaning, becomes year by year applicable to fewer substances, as the chemist adds to his conquests over inorganic elements and compounds, and moulds them to more complex forms. Although a large number of so-called organic compounds have long ceased to be peculiar in being formed only by living tissues, the terms organic and inorganic are still commonly used to denote distinct classes of chemical substances, and the classification of the matters of which the human body is com- CHEMICAL COMPOSITION OF HUMAN BODY. 19 posed into the organic and the inorganic is convenient, and will be here employed. No very accurate line of separation can be drawn between organic and inorganic substances, but there are certain pecu- liarities belonging to the former which may be here briefly noted. 1. Organic compounds are composed of a larger number of Elements than are present in the more common kinds of inor- ganic matter. Thus, albumen, fibrin, and gelatin, the most abundant substances of this class, in the more highly organized tissues of animals, are composed of five elements, carbon, hy- drogen, oxygen, nitrogen, and sulphur. The most abundant inorganic substance, water, has but two elements, hydrogen and oxygen. 2. Not only are a large number of elements usually com- bined in an organic compound, but a large number of equiva- lents or atoms of each of the elements are united to form an equivalent or atom of the compound. In the case of carbon- ate of ammonium, as an example among inorganic substances, one equivalent of carbonic acid is united with two of ammo- nium ; the equivalent or atom of carbonic acid consists of one of carbon with two of oxygen ; and that of ammonium of one of nitrogen with three of hydrogen. But in an equivalent or atom of fibrin, or of albumen, there are of the same elements, respectively, 72, 22, 18, and 112 equivalents. And, together with this union of large numbers of equivalents in the organic compound, it is further observable, that the several numbers stand in no simple arithmetical relation one with another, as the numbers of equivalents combining in an inorganic com- pound do. With these peculiarities in the chemical composition of or- ganic bodies we may connect two other consequent facts ; first, the large number of different compounds that are formed out of comparatively few elements ; secondly, their great proneuess to decomposition. For it is a general rule, that the greater the number of equivalents or atoms of an element that enter into the formation of an atom of a compound, the less is the stability of that compound. Thus, for example, among the various oxides of lead and other metals, the least stable in composition are those in which each equivalent has the largest number of equivalents of oxygen. So, water, composed of one equivalent of oxygen and two of hydrogen, is not changed by any slight force ; but peroxide of hydrogen, which has two equivalents of oxygen to two of hydrogen, is among the sub- stances most easily decomposed. The instability, on this ground, belonging to organic com- 20 CHEMICAL COMPOSITION OF HUMAN BODY. pounds, is, in those which are most abundant in the highly organized tissues of animals, augmented, 1st, by their contain- ing nitrogen, which, among all the elements, may be called the least decided in its affinities, and that which maintains with least tenacity its combinations with other elements; and 2dly, by the quantity of water which, in their natural state, is com- bined with them, and the presence of which furnishes a most favorable condition for the decomposition of nitrogenous com- pounds. Such, indeed, is the instability of animal compounds, arising from these several peculiarities in their constitution, that, in dead and moist animal matter, no more is requisite for the occurrence of decomposition than the presence of at- mospheric air and a moderate temperature ; conditions so com- monly present, that the decomposition of dead animal bodies appears to be, and is generally called, spontaneous. The modes of such decomposition vary according to the nature of the origi- nal compound, the temperature, the access of oxygen, the pres- ence of microscopic organisms, and other circumstances, and constitute the several processes of decay and putrefaction ; in the results of which processes the only general rule seems to be, that the several elements of the original compound finally unite to form those substances, whose composition is, under the circumstances, most stable. The organic compounds existing in the human body may be arranged in two classes, namely, the azotized, or nitrogenous, and the non-azotized, or non-nitrogenous principles. The non-azotized principles include the several fatty, oily, or oleaginous substances, as olein, stearin, cholesterin, and others. In the same category of non-nitrogenous substances may be included lactic and formic acids, animal glucose, sugar of milk, &c. The oily or fatty matter which, inclosed in minute cells, forms the essential part of the adipose or fatty tissue of the human body (p. 40), and which is mingled in minute particles in many other tissues and fluids, consists of a mixture of stearin, palmitin, and olein. The mixture forms a clear yellow oil, of which different specimens congeal at from 45 to 35. Cholesterin, a fatty matter which melts at 293 F., and is, therefore, always solid at the natural temperature of the body, may be obtained in small quantity from blood, bile, and ner- vous matter. It occurs abundantly in many biliary calculi ; the pure white crystalline specimens of these concretions being formed of it almost exclusively. Minute rhomboidal scale- like crystals of it are also often found in morbid secretions, as in cysts, the puriform matter of softening and ulcerating tumors, &c. It is soluble in ether and boiling alcohol ; but GELATINOUS SUBSTANCES. 21 alkalies do Dot change it; it is one of those fatty substances which are not saponifiable. The azotized or nitrogenous principles in the human body include what may be called the proper gelatinous and albu- minous substances, besides others of less definite rank and composition, as pepsin and ptyalin, horny matter or keratin, many coloring and extractive matters, &c. The gelatinous substances are contained in several of the tissues, especially those which serve a passive mechanical office in the economy ; as the cellular, or fibro-cellular tissue in all parts of the body, the tendons, ligaments, and other fibrous tissues, the cartilages and bones, the skin and serous membranes. These, when boiled in water, yield a material, the solution of which remains liquid while it is hot, but be- comes solid and jelly-like on cooling. Two varieties of these substances are described, gelatin and chondrin, the latter being derived from cartilages, the former from all the other tissues enumerated above, and in its purest state, from isinglass, which is the swimming-bladder of the sturgeon, and which, with the exception of about 7 per cent, of its weight, is wholly reducible into gelatin. The most char- acteristic property of gelatin is that already mentioned, of its solution being liquid when warm, and solidifying or setting when it cools. The temperature at which it becomes solid, the proportion of gelatin which must be in solution, and the firm- ness of the jelly when formed, are various, according to the source, the quantity, and the quality of the gelatin; but, as a general rule, one part of dry gelatin dissolved in 100 of water, will become solid when cooled to 60. The solidified jelly may be again made liquid by heating it, and the transitions from the solid to the liquid state by the alternate abstraction and addition of heat, may be repeated several times ; but at length the gelatin is so far altered, and, apparently, oxidized by the process, that it no longer becomes solid on cooling. Gelatin in solutions too weak to solidify when cold, is dis- tinguished by being precipitable with alcohol, ether, tannic acid, and bichloride of mercury, and not precipitable with the ferrocyanide of potassium. The most delicate and striking of these tests is the tannic acid, which is conveniently supplied in an infusion of oak-bark or gall-nuts; it will detect one part of gelatin in 5000 of water ; and if the solution of gelatin be strong it forms a singularly dense and heavy precipitate, which has been named tanno-gelatin, and is completely insoluble in water. Chondrin, the kind of gelatin obtained from cartilages, agrees with gelatin in most of its characters, but its solution 22 CHEMICAL COMPOSITION OF HUMAN BODY. solidifies on cooling much less firmly, and, unlike gelatin, it is precipitable with acetic and the mineral and other acids, and with alum, persulphate of iron, and acetate of lead. Albuminous substances, or proteids, as they are sometimes called, exist abundantly in the human body. The chief among them are albumen, fibrin, casein, syntonin, myosin,and globulin. Albumen exists in most of the tissues of the body, but es- pecially in the nervous, in the lymph, chyle, and blood, and in many morbid fluids, as the serous secretions of dropsy, pus, and others. In the human body it is most abundant, and most nearly pure, in the serum of the blood. In all the forms in which it naturally occurs, it is combined with about six per cent, of fatty matter, phosphate of lime, chloride of sodium, and other saline substances. Its most characteristic property is, that both in solution and in the half-solid state in which it exists in white of egg, it is coagulated by heat, and in thus becoming solid, becomes insoluble in water. The temperature required for the coagulation of albumen is the higher the less the proportion of albumen in the solution submitted to heat. Serum and such strong solutions will begin to coagulate at from 150 to 170, and these, when the heat is maintained, become almost solid and opaque. But weak solutions require a much higher temperature, even that of boiling, for their coagulation, and either only become milky or opaline, or pro- duce flocculi which are precipitated. Albumen, in the state in which it naturally occurs, appears to be but little soluble in pure water, but is soluble in water containing a small proportion of alkali. In such solutions it is probably combined chemically with the alkali ; it is precip- itated from them by alcohol, nitric, and other mineral acids, by ferrocyanide of potassium (if before or after adding it the alkali combined with the albumen be neutralized), by bi- chloride of mercury, acetate of lead, and most metallic salts. Coagulated albumen, i. e., albumen made solid with heat, is soluble in solutions of caustic alkali, and in acetic acid if it be long digested or boiled with it. With the aid of heat, also, strong hydrochloric acid dissolves albumen previously coag- ulated, and the solution has a beautiful purple or blue color. Fibrin is found most abundantly in the blood and the more perfect portions of the lymph and chyle. It is very doubtful, however, whether fibrin, as such, exists in these fluids whether, that is to say, it is not itself formed at the moment of coagula- tion. (See chapter on the Blood.) If a common clot of blood be pressed in fine linen while a stream of water flows upon it, the whole of the blood-color is gradually removed, and strings and various pieces remain of CASEIN SYNTONIN MYOSIN. 23 a soft, yet tough, elastic, and opaque-white substance, which consist of fibrin, impure, with a mixture of fatty matter, lymph- corpuscles, shreds of the membranes of red blood-corpuscles, and some saline substances. Fibrin somewhat purer than this may be obtained by stirring blood while it coagulates, and collecting the shreds that attach themselves to the instrument, or by retarding the coagulation, and, while the red blood- corpuscles sink, collecting the fibrin unmixed with them. But in neither of these cases is the fibrin perfectly pure. Chemically, fibrin and albumen can scarcely be distin- guished ; the only difference apparently being that fibrin con- tains 1.5 more oxygen in every 100 parts than albumen does. Mr. A. H. Smee has, indeed, apparently converted albumen into fibrin, by exposing a solution to the prolonged influence of oxygen. Nearly all the changes, produced by various agents, in coagulated albumen, may be repeated with coag- ulated fibrin, with no greater differences of result than may be reasonably ascribed to the differences in the mechanical properties of the two substances. Of such differences the prin- cipal are, that fibrin immersed in acetic acid swells up and becomes transparent like gelatin, while albumen undergoes no such apparent change ; and that deutoxide of hydrogen is decomposed when in contact with coagulated fibrin, but not with albumen. Casein, which is said to be albumen in combination with soda, exists largely in milk, and forms one of its most im- portant constituents. Syntonin is obtained from muscular tissue, both of the striated and organic kind. It differs from ordinary fibrin in several particulars, especially in being less soluble in nitrate and car- bonate of potash, and more soluble in dilute hydrochloric acid. Myosin is the substance which spontaneously coagulates in the juice of muscle. It is closely allied to syntonin ; indeed, in the act of solution in dilute acids, it is converted into it. The percentage composition of albumen, fibrin, gelatin, and chondrin, is thus given by Mulder : Albumen. Fibrin. Gelatin. Chondrin. Carbon, .... 53.5 527 50.40 49.97 Hydrogen, Nitrogen, . . 7.0 15.5 6.9 15.4 6.64 18.34 6.63 14.44 Oxygen, . . Sulphur, . . 22.0 1.6 23.5 1.2 1 24.26 \ 28.58 / 0.38 Phosphorus, . 0.4 0.3 100.0 100.0 100.00 100.00 24 CHEMICAL COMPOSITION OF HUMAN BODY. Horny Matter. The substance of the horny tissues, includ- ing the hair and nails (with whalebone, hoofs, and horns), consists of an albuminous substance, with larger proportions of sulphur than albumen and fibrin contain. Hair contains 10 per cent, and nails 6 to 8 per cent, of sulphur. The horny substances, to which Simon applied the name of keratin, are insoluble in water, alcohol, or ether; soluble in caustic alkalies, and sulphuric, nitric, and hydrochloric acids; and not precipitable from the solution in acids by ferrocyanide of potassium. Mucus, in some of its forms, is related to these horny sub- stances, consisting, in great part, of epithelium detached from the surface of mucous membrane, and floating in a peculiar clear and viscid fluid. But under the name of mucus, several various substances are included of which some are morbid albuminous secretions containing mucus and pus-corpuscles, and others consist of the fluid secretion variously altered, con- centrated, or diluted. Mucus contains an albuminous sub- stance, termed mucin. It differs from albumen chiefly in not containing sulphur. Pepsin and other albuminous ferments, as they are sometimes called, will be described in connection with the secretions of which they are the active principles. And the various color- ing matters, as of the blood, bile, &c., will be also considered with the fluids or tissues to which they belong. Besides the above-mentioned organic nitrogenous compounds, other substances are formed in the living body, chiefly by de- composition of nitrogenous materials of the food and of the tissues, which must be reckoned rather as temporary constitu- ents than essential component parts of the body; although from the continual change, which is a necessary condition of life, they are always to be found in greater or less amount. Examples of these are urea, uric and hippuric acids, creatin, creatinin, leucin, and many others. Such are the chief organic substances of which the human body is composed. It must not be supposed, however, that they exist naturally in a state approaching that of chemical purity. All the fluids and tissues of the body appear to con- sist, chemically speaking, of mixtures of several of these prin- ciples, together with saline matters. Thus, for example, a piece of muscular flesh would yield fibrin, albumen, gelatin, fatty matters, salts of soda, potash, lime, magnesia, iron, and other substances, such as creatin, which appear passing from the organic towards the inorganic state. This mixture of sub- stances may be explained in some measure by the existence of many different structures or tissues in the muscles ; the gelatin WATER POTASH SODA. 25 may be referred principally to the cellular tissue between the fibres, the fatty matter to the adipose tissue in the same posi- tion, and part of the albumen to the blood and the fluid by which the tissue is kept moist. But, beyond these general statements, little can be said of the mode in which the chemi- cal compounds are united to form an organized structure; or of how, in any organic body, the several incidental substances are combined with those which are essential. The inorganic matters which exist as such in the human body are numerous. Water forms a large proportion, probably more than two- thirds of the weight of the whole body. Phosphorus occurs in combination, as in the neutral phos- phate of sodium in the blood and saliva, the acid phosphates of the muscles and urine, the basic phosphates of calcium and magnesium in the bones and teeth. Sulphur is present chiefly in the sulphocyanide of potassium of the saliva, and in the sulphates of the urine and sweat. A very small quantity of silica exists, according to Berze- lius, in the urine, and, according to others, in the blood. Traces of it have also been found in bones, in hair, and in some other parts of the body. Chlorine is abundant in combination with sodium, potas- sium, and other bases in all parts, fluid as well as solid, of the body. A minute quantity of fluorine in combination with calcium has been found in the bones, teeth, and urine. Potassium and sodium are constituents of the blood and all the fluids, in various quantities and proportions. They exist in the form of chlorides, sulphates, and phosphates, and prob- ably, also, in combination with albumen, or certain organic acids. Liebig, in his work on the Chemistry of Food, has shown that the juice expressed from muscular flesh always contains a much larger proportion of potash-salts than of soda- salts; while in the blood and other fluids, except the milk, the latter salts always preponderate over the former ; so that, for example, for every 100 parts of soda-salts in the blood of the chicken, ox, and horse, there are only 40.8, 5.9, and 9.5 parts of potash-salts ; but for every 100 parts of soda-salts in their muscles, there are 381, 279, and 285 parts of potash-salts. The salts of calcium are by far the most abundant of the earthy salts found in the human body. They exist in the lymph, chyle, and blood, in combination with phosphoric acid, the phosphate of calcium being probably held in solution by the presence of phosphate of sodium. Perhaps no tissue is wholly void of phosphate of calcium; but its especial seats are the bones and teeth, in which, together with carbonate and 26 STRUCTURAL COMPOSITION OF HUMAN BODY. fluoride of calcium, it is deposited in minute granules, in a peculiar compound, named bone-earth, containing 51.55 parts of lime, arid 48.45 of phosphoric acid. Phosphate of calcium, probably the neutral phosphate, is also found in the saliva, milk, bile, and most other secretions, and acid phosphate in the urine, and, according to Blondlot, in the gastric fluid. Magnesium appears to be always associated with calcium, but its proportion is much smaller, except in the juice ex- pressed from muscles, in the ashes of which magnesia prepon- derates over lime. The especial place of iron is in the haemoglobin, the color- ing-matter of the blood, of which a further account will be given with the chemistry of the blood. Peroxide of iron is found, in very small quantities, in the ashes of bones, muscles, and many tissues, and in lymph and chyle, albumen of serum, fibrin, bile, and other fluids ; and a salt of iron, probably a phosphate, exists in considerable quantity in the hair, black pigment, and other deeply colored epithelial or horny sub- stances. Aluminium, Manganese, Copper, and Lead. It seems most likely that in the human body, copper, manganesium, alumin- ium, and lead are merely accidental elements, which, being taken in minute quantities with the food, and not excreted at once with the faeces, are absorbed and deposited in some tissue or organ, of which, however, they form no necessary part. In the same manner, arsenic, being absorbed, may be deposited in the liver and other parts. CHAPTER III. STRUCTURAL COMPOSITION OF THE HUMAN BODY. IN the investigation of the structural composition of the human body, it will be well to consider in the first place, what are the simplest anatomical elements which enter into its for- mation, and then proceed to examine those more complicated tissues which are produced by their union. It may be premised, that in all the living parts of all living things, animal and vegetable, there is invariably to be dis- covered, entering into the formation of their anatomical ele- ments, a greater or less amount of a substance, which, in chemical composition and general characters, is indistiuguish- PROTOPLASM. 27 able from albumen. As it exists in a living tissue or organ, it differs essentially from mere albumen in the fact of its possess- ing the power of growth, development, and the like ; but in chemical composition it is identical with it. This albuminous substance has received various names ac- cording to the structures in which it has been found, and the theory of its nature and uses which may have presented itself most strongly to the minds of its observers. In the bodies of the lowest animals, as the Rhizopoda or Gregarinida, of which it forms the greater portion, it has been called "sarcode," from its chemical resemblance to the flesh of the higher animals. When discovered in vegetable cells, and supposed to be the prime agent in their construction, it was termed "protoplasm." As the presumed formative matter in animal tissues it was called " blastema ;" and, with the belief that wherever found, it alone of all matters has to do with generation and nutrition, Dr. Beale has surnamed it " germinal matter." So far as can be discovered, there is no difference in chemical composition between the protoplasm of one part or organism and that of another. The movements which can be seen in certain vegetable cells apparently belong to a substance which is identical in composition with that which constitutes the greater portion of the bodies of the lowest animals, and which is present in greater or less quantity in all the living parts of the highest. So much appears to be a fact ; that in all living parts there exists an albuminous substance, in which in favor- able cases for observation in vegetable and the lower animal organisms, there can be noticed certain phenomena which are not to be accounted for by physical impressions from without, but are the result of inherent properties we call vital. For example, if a hair of the Tradescantia Virginica, or of many other plants, be examined under the microscope, there is seen in each individual cell a movement of the protoplasmic con- tents in a certain definite direction around the interior of the cell. Each cell is a closed sac or bag, and its contents are therefore quite cut off from the direct influence of any motive power from without. The motion of the particles, moreover, in a circuit around the interior of the cell, precludes the notion of its being due to any other than those molecular changes which we call vital. Again, in the lowest animals, whose bodies resemble more than anything else a minute mass of jelly, and which appear to be made up almost solely of this albuminous protoplasm, there are movements in correspondence with the needs of the organism, whether with respect to seiz- ing food or any other purpose, which are unaccountable accord- ing to any known physical laws, and can only be called vital. 28 STRUCTURAL COMPOSITION OF HUMAN BODY. In many, too, there is a kind of molecular current, exactly resembling that which is seen in a vegetable cell. In the higher animals, phenomena such as these are so sub- ordinate to the more complex manifestations of life that they are apt to be overlooked ; but they exist nevertheless. The mere nutrition of each part of the body in man or in the higher animals, is performed after a fashion which is strictly analogous to that which holds good in the case of a vegetable cell, or a rhizopod ; or, in other words, the life of each anatomi- cal element in a complex structure, like the human body, re- sembles very closely the life of what in the lowest organisms constitutes the whole being. For example, the thin scaly covering or epidermis, which forms the outer part of a man's skin, is made up of minute cells, which, when living, are com- posed in part of protoplasm, and which are continually wear- ing away and being replaced by new similar elements from beneath ; and this process of quick waste and repair could only take place under the very complex conditions of nutrition which exist in man. One working part of the organism of an animal is so inextricably interwoven with that of another, that any want or defect in one, is soon or immediately felt by the whole ; and the epidermis, which only subserves a mechani- cal function, would be altered very soon by any defect in the more essential parts concerned in circulation, respiration, &c. But if we take simply the life history of one of the small cells which constitute the epidermis, we find that it absorbs nour- ishment from the parts around, grows, and develops in a manner analogous to that which belongs to a cell which con- stitutes part of a vegetable structure, or even a cell which by itself forms an independent being. Remembering, however, the invariable presence of a living albuminous matter or protoplasm of apparently identical com- position in all living tissues, animal and vegetable, we must not forget that its relations to the parts with which it is in- corporated are still very doubtfully known ; and all theories concerning it must be considered only tentative and of uncer- tain stability. Among the anatomical elements of the human body, some appear, even with the help of the best microscopic apparatus, perfectly uniform and simple : they show no trace of struc- ture, i. e., of being composed of definitely arranged dissimilar parts. These are named simple, structureless, or amorphous substances. Such is the simple membrane which forms the walls of most primary cells, of the finest gland-ducts, and of the sarcolemma of muscular fibre ; and such is the membrane enveloping the vitreous humor of the eye. Such also, having NUCLEI. 29 a dimly granular appearance, but no really granular struc- ture, is the intercellular substance of the so-called hyaline car- tilage. In the parts which present determinate structure, certain primary forms may be distinguished, which, by their various modifications and modes of combination make up the tissues and organs of the body. Such are, 1. Granules or molecules, the simplest and minutest of the primary forms. They are particles of various sizes, from immeasurable minuteness to the 10,000th of an inch in diameter ; of various and generally un- certain composition, but usually so affecting light transmitted through them, that at different focal distances their centre, or margin, or whole substance, appears black. From this char- acter, as well as from their low specific gravity (for in micro- scopic examinations they always appear lighter than water), and from their solubility in ether when they can be favorably tested, it is probable that most granules are formed of fatty or oily matter ; or, since they do not coalesce as minute drops of 'oil would, that they are particles of oil coated over with albumen deposited on them from the fluid in which they float. In any fluid that is not too viscid, they exhibit the phenome- non of molecular motion, shaking and vibrating incessantly, and sometimes moving through the fluid, probably, in great measure, under the influence of external vibration. Granules may be either free, as in milk, chyle, milky serum, yolk-substance, and most tissues containing cells with granules ; or inclosed, as are the granules in nerve-corpuscles, gland-cells, and epithelium-cells, the pigment granules in the pigmentum nigrum and medullary substance of the hair ; or imbedded, as are the granules of phosphate and carbonate of lime, in bones and teeth. 2. Nuclei, or cytoblasts (Fig. 1, 6), appear to be the simplest elementary structures, next to granules. They were thus named in accordance with the hypothesis that they are always connected with cells, or tissues formed from cells, and that in the development of these, each nucleus is the germ or centre around which the cell is formed. The hypothesis is only par- tially true, but the terms based on it are too familiarly ac- cepted to make it advisable to change them till some more exact and comprehensive theory is formed. Of the corpuscles called nuclei some are minute cellules or vesicles, with walls formed of simple membrane, inclosing often one or more particles, like minute granules, called nu- cleoli (Fig, 1, c). Other nuclei, again, appear to be simply small masses of protoplasm, with no trace of vesicular struc- ture. 30 STRUCTURAL COMPOSITION OF HUMAN BODY. One of the most general characters of the nucleus, and the most useful in microscopic examinations, is, that it is neither dissolved nor made transparent by acetic acid, but acquires, when that fluid is in contact with it, a darker and more dis- tinct outline. It is commonly, too, the part of the mature cell which is capable of being stained by an ammoniacal solu- tion of carmine the test, it may be remarked, by which, ac- cording to Dr. Beale, protoplasm or germinal matter may be always known. Nuclei may be either free or attached. Free nuclei are such as either float in fluid, like those in some of the secretions, which appear to be derived from the secreting cells of the glands, or lie loosely imbedded in solid substance, as in the gray matter of the brain and spinal cord, and most abun- dantly in some quickly-growing tumors. Attached nuclei are either closely imbedded in homogeneous pellucid substance, as in rudimental cellular tissue ; or are fixed on the surface of fibres, as on those of organic muscle and organic nerve-fibres ; or are inclosed in cells, or in tissues formed by the extension or junction of cells. Nuclei inclosed in cells appear to be at- tached to the inner surface of the cell-wall, projecting into the cavity. Their position in relation to the centre or axis of the cell is uncertain ; often when the cell lies on a flat or broad surface, they appear central, as in blood-corpuscles, epithelium- cells, whether tessellated or cylindrical ; but, perhaps, more often their position has no regular relation to the centre of the cell. In most instances, each cell contains only a single nucleus; but in cartilage, especially when it is growing or ossifying, two or more nuclei in each cell are common ; and the development of new cells is often effected by a division or multiplication of nuclei in the cavity of a parent cell ; as in the primary blood-cells of the embryo, in the germinal vesicle, and others. When cells extend and coalesce, so that their walls form tubes or sheaths, the nuclei commonly remain attached to the inner surface of the wall. Thus they are seen imbedded in the walls of the minutest capillary bloodvessels of, for exam- ple,' the retina and brain ; in the sarcolemma of transversely striated muscular fibres ; and in minute gland-tubes. Nuclei are most commonly oval or round, and do not gen- erally conform themselves to the diverse shapes which the cells assume ; they are altogether less variable elements, even in regard to size, than the cells are, of which fact one may see a good example in the uniformity of the nuclei in cells so mul- tiform as those of epithelium. But sometimes they appear to be developed into filaments, elongating themselves and becom- CELLS. 31 ing solid, and uniting end to end for greater length, or by lat- eral branches to form a network. So, according to Henle, are formed the filaments of the striated and fenestrated coats of arteries ; and according to Beale, the so-called connective-tis- sue corpuscles are to be considered branched nuclei, formed of protoplasm or germinal matter. 3. Cells. The word " cell " of course implies strictly a hollow body, and the term was a sufficiently good one when all so- called cells were considered to be small bags with a membra- nous envelope, and more or less liquid contents. Many bodies, however, which are still called cells do not answer to this de- scription, and the term, therefore, if taken in its literal signifi- cation, is very apt to lead astray, and, indeed, very frequently does so. It is too widely used, however, to be given up, at least for the present, and we must therefore consider the term to indicate, either a membranous closed, bag with more or less liquid contents, and almost always a nucleus ; or a small semi- solid mass of protoplasm, with no more definite boundary-wall than such as has been formed by a condensation of its outer layers, but with, most commonly, a small granular substance in the centre, called, as in the first place, a nucleus. In both cases the nucleus may contain a nucleolus. Fat-cells (Fig. 11) are examples of the first kind of cells ; white blood-corpuscles (Fig. 26) of the second. The cell-wall, when there is one, never presents any appear- ance of structure : it appears sometimes to be an albuminous substance ; sometimes a horny matter, as in thick and dried cuticle. In almost all cases (the dry cells of horny tissue, perhaps, alone excepted) the cell-wall is made transparent by acetic acid, which also penetrates into the interior and distends it, so that it can hardly be discerned. But in such cases the cell-wall is usually not dissolved ; it may be brought into view again by merely neutralizing the acid with soda or potash. The simplest shape of cells, and that which is probably the normal shape of the primary cell, is oval or spheroidal, as in cartilage-cells and lymph-corpuscles ; but in many instances they are flattened and discoid, as in the red blood-corpuscles (Fig. 26) ; or scale-like, as in the epidermis and tessellated epithelium (Fig. 2). By mutual pressure they may become many-sided, as are most of the pigment-cells of the choroidal pigmentum nigrum (Fig. 12), and those in close-textured adipose tissue ; they may assume a conical or cylindriform or prismatic shape, as in the varieties of cylinder-epithelium (Fig. 4) ; or be caudate, as in certain bodies in the spleen ; they may send out exceedingly fine processes in the form of 32 STRUCTURAL COMPOSITION OF HUMAN BODY. vibratile cilia (Fig. 6), or larger processes, with which they become stellate, or variously caudate, as in some of the rami- fied pigment-cells of the choroid coat of the eye (Fig. 13). The contents of all living cells, including the nucleus, are formed in a greater or less degree of protoplasm less as the cell grows older. But, besides, cells contain matters almost infinitely various, according to the position, office, and age of the cell. In adipose tissue they are the oily matter of the fat ; in gland-cells, the contents are the proper substance of the secretion, bile, semen, &c., as the case may be ; in pigment-cells they are the pigment-granules that give the color ; and in the numerous instances in which the cell-contents can be neither seen because they are pellucid, nor tested because of their minute quantity, they are yet, probably, peculiar in each tissue, and constitute the greater part of the proper substance of each. Commonly, when the contents are pellucid, they contain gran- ules which float in them ; and when water is added, and the contents are diluted, the granules display an active molecular movement within the cavity of the cell. Such a movement may be seen by adding water to mucus, or granulation-corpus- cles, or to those of lymph. In a few cases, the whole cavity of the cell is filled with granules : it is so in yolk-cells and milk-corpuscles, in the large diseased corpuscles often found among the products of inflammation, and in some cells when they are the seat of extreme fatty degeneration. All cells containing abundant granules appear to be either lowly organ- ized, as for nutriment, e. g., yolk-cells, or degenerate, e. g., granule-cells of inflammation, or of mucus. The peculiar con- tents of cells may be often observed to accumulate first around or directly over the nuclei, as in the cells- of black pigment, in those of melanotic tumors, and in those of the liver during the retention of bile. Intercellular substance is the material in which, in certain tissues, the cells are imbedded. Its quantity is very variable. In the finer epithelia, especially the columnar epithelium on the mucous membrane of the intestines, it can be just seen fill- ing the interstices of the close-set cells ; here it has no appear- ance of structure. In cartilage and bone, it forms a large portion of the whole substance of the tissue, and is either hom- ogenous and finely granular (Fig. 14), or osseous, or, as in fibro-cartilage, resembles fine fibrous tissue (Fig. 15). In some cases the cells are very loosely connected with the intercellular substance, and may be nearly separated from it, as in fibro-car- tilage; but in some their walls seem amalgamated with it. The foregoing may be regarded as the simplest and the near- est to the primary forms assumed in the organization of animal TUBULES. 33 matter; as the states into which this passes in becoming a solid tissue living or capable of life. By the further development of tissue thus far organized, higher or secondary forms are pro- duced, which it will be sufficient in this place merely to enu- merate. Such are, 4. Filaments, or Fibrils. Threads of exceeding fineness, from 770-J Q0th of an inch upwards. Such filaments are cylindriform, as are those of the striated muscular and the fibro-cellular or areolar tissue (Fig. 8) ; or flattened, as are those of the organic muscles. Filaments usually lie in parallel fasciculi, as in mus- cular and tendinous tissues ; but in some instances are matted or reticular with branches and intercommunication, as are the filaments of the middle coat, and of the longitudinally-fibrous coat of arteries ; and, in other instances, are spirally wound, or very tortuous, as in the common fibro-cellular tissue (Fig. 9). 5. Fibres in the instances to which the name is commonly applied are larger than filaments or fibrils, but are by no es- sential general character distinguished from them.' The flat- tened band-like fibres of the coarser varieties of organic muscle or elastic tissue (Fig. 10) are the simplest examples of this form" ; the toothed fibres of the crystalline lens are more com- plex; and more compound, so as hardly to permit of being classed as elementary forms, are the striated muscular fibres, which consist of bundles of filaments inclosed in separate mem- branous sheaths, and the cerebro-spinal nerve-fibres, in which similar sheaths inclose apparently two varieties of nerve-sub- stance. 6. Tubules are formed of simple or structureless membrane, such as the investing sheaths of striated muscular and cerebro- spinal nerve-fibres, and the basement-membrane or proper wall of the fine ducts of secreting glands; or they may be formed, as in the case of the minute capillary lymph and bloodvessels, by the apposition, edge to edge, in a single layer, of variously shaped flattened cells (Fig. 48). With these simple materials, the various parts of the body are built up; the more elementary tissues being, so to speak, first compounded of them ; while these again are variously mixed and interwoven to form more intricate combinations. Thus are constructed epithelium and its modifications, con- nective tissue, fat, cartilage, bone, the fibres of muscle and nerve, &c. ; and these again, with the more simple structures before mentioned, are used as materials wherewith to form ar- teries, veins, and lymphatics, secreting and vascular glands, lungs, heart, liver, and other parts of the body. 34 ELEMENTARY TISSUES. CHAPTER IV. 1 STRUCTURE OF THE ELEMENTARY TISSUES. Epithelium. ONE of the simplest of the elementary structures of which the human body is made up, is that which has received the name of Epithelium. Composed of nucleated cells which are arranged most commonly in the form of a continuous mem- brane, it lines the free surfaces both of the inside and outside of the body, and its varieties, with one exception, have been named after the shapes which the individual cells in different parts assume. Classified thus, Epithelium presents itself under four principal forms, the characters of each of which are dis- tinct enough in well-marked examples; but when, as frequently happens, a continuous surface possesses at different parts two or more different epithelia, there is a very gradual transition from one to the other. 1. The first and most common variety is the squamous or tessellated epithelium (Figs. 1 and 2) which is composed of flat, FIG. 1. FIG. 2. FIG. 1. Fragment of epithelium from a serous membrane (peritoneum) ; magnified 410 diameters, a, cell; b, nucleus; c, nucleoli (Henle). FIG. 2. Epithelium scales from the inside of the mouth ; magnified 260 diameters (Henle). oval, roundish, or polygonal nucleated cells, of various size, arranged in one, or in many superposed layers. Arranged in 1 The following chapter, containing aw outline-description of the elementary tissues, has been inserted for the convenience of students. For a much fuller and better account, the reader may be referred to Dr. Sharpey's admirable descriptions in Quain's Anatomy. EPITHELIUM. 35 several superposed layers this form of epithelium covers the skin, where it is called the Epidermis, and is spread over the mouth, pharynx, and oesophagus, the conjunctiva covering the eye, the vagina, and entrance of the urethra in both sexes; while, as a single layer the same kind of epithelium lines the interior of most of the serous and synovial sacs, and of the heart, bloodvessels, and lymphvessels. 2. Another variety of epithelium named spheroidal, from the usually more or less rounded outline of the cells composing it (d, Fig. 3), is found chiefly lining the interior of the ducts of the compound glands, and more or less completely filling the small sacculations or acini, in which they terminate. It commonly indeed occupies the true secreting parts of all glands, and hence is sometimes called glandular epithelium The gastric glands of the human stomach (magnified), a, deep part of a pyloric gastric gland (from Kolliker); the cylindrical epithelium is traceable to the cseeal extremities, b and c, cardiac gastric glands (from Allen Thomson); ft, vertical sec- tion of a small portion of the mucous membrane with the glands magnified 30 diame- ters; c, deeper portion of one of the glands, magnified Go diameters, showing a slight division of the tubes, and a sacculated appearance produced by the large glandular cells within them ; d, cellular elements of the cardiac glands magnified 230 diameters. (b, c, and d, Fig. 3). Often, from mutual pressure, the cells acquire a polygonal outline. From the fact, however, of the term spheroidal epithelium being a generic one for almost all gland-cells, the shapes and sizes of the cells composing this variety of epithelium are, as might be expected, very diverse in different parts of the body. 3. The third variety is the cylindrical or columnar epithelium (Figs. 4 and 5), which extends from the cardiac orifice of the 36 ELEMENTARY TISSUES. stomach along the whole of the digestive canal to the anus, and lines the principal gland-ducts which open upon the mucous FIG. 4. Cylindrical epithelium from intestinal villus of a rabbit; magnified 300 diameters (from Kolliker). surface of this tract, sometimes throughout their whole extent (a. Fig. 3), but in some cases only at the part nearest to the orifice (b and c). It is also found in the gall-bladder and in the greater portion of the urethra, and in some other parts, as the duct of the parotid gland and of the testicle. It is com- posed of oblong cells closely packed, and placed perpendicu- larly to the surface they cover, their deeper or attached ex- tremities being most commonly smaller than those which are free. Each of such cells incloses, at nearly mid distance be- tween its base and apex, a flat nucleus with nucleoli (B, Fig. 5) ; FIG. 5. Cylinders of the intestinal epithelium (after Henle) : B, from the jejunum ; c, cyl- inders of the intestinal epithelium as seen when looking on their free extremities ; D, ditto, as seen on a transverse section of a villus. the nuclei being arranged at such heights in contiguous cells as not to interfere with each other by mutual pressure. 4. In the fourth variety of epithelium cells, usually cylin- drical, but occasionally of some other shape, are provided at their free extremities with several fine pellucid pliant processes or cilia (Figs. 6 and 7). This form of epithelium lines the whole respiratory tract of mucous membrane and its prolonga- EPITHELIUM. tions. It occurs also in some parts of the generative apparatus ; in the male, lining the vasa efferentia of the testicle, and their prolongations as far as the lower end of the epididymis; and FIG. 6. FIG. 7. FIG. 6. Spheroidal ciliated cells from the mouth of the frog ; magnified 300 diameters (Sharpoy). FIG. 7. Columnar ciliated epithelium cells from the human nasal membrane ; magnified 300 diameters (Sharpey). in the female commencing about the middle of the neck of the uterus, and extending to the fimbriated extremities of the Fallopian tubes, and for a short distance along the peritoneal surface of the latter. A tessellated epithelium, with scales partly covered with cilia, lines, in great part, the interior of the cerebral ventricles, and of the minute central canal of the spinal cord. If a portion of ciliary mucous membrane from a living or recently dead animal be moistened and examined with a micro- scope, the cilia are observed to be in constant motion, moving continually backwards and forwards, and alternately rising and falling with a lashing or fanning movement. The ap- pearance is not unlike that of the waves in a field of wheat, or swiftly running and rippling water. The general result of their movements is to produce a continuous current in a de- terminate direction, and this direction is invariably the same on the same surface, being usually in the case of a cavity towards its external orifice. Uses of Epithelium. The various kinds of epithelium serve one general purpose, namely, that of protecting, and at the same time rendering smooth, the surfaces on which they are placed. But each, also, discharges a special office in relation to the particular function of the membrane on which it is placed. In mucous and synovial membranes it is highly probable that the epithelium cells, whatever be their forms and what- ever their other functions, are the organs in which by a regular process of elaboration and secretion, such as will be afterwards 38 ELEMENTARY TISSUES. described, mucus and synovial fluid are formed and discharged. (See chapter on Secretion.) Ciliated epithelium has another superadded function. By means of the current set up by its cilia in the air or fluid in contact with them, it is enabled to propel the fluids or minute particles of solid matter, which come within the range of its influence, and aid in their expulsion from the body. In the respiratory tract of mucous membrane the current set up in the air may also assist in the diffusion and change of gases, on which the due aeration of the blood depends. In the Fallopian tube the direction of the current excited by the cilia is towards the cavity of the uterus, and may thus be of service in aiding the progress of the ovum. Of the purposes served by the cilia which line the ventricles of the brain nothing is known. The nature of ciliary motion and the circumstances by which it is influenced will be considered hereafter. (See chapter on Motion.) Epithelium is devoid of bloodvessels and lymphatics. The cells composing it are nourished by absorption of nutrient matter from the tissues on which they rest ; and as they grow old they are cast off and replaced by new cells from beneath. Areolar, Cellular, or Connective Tissue. This tissue, which has received various names according to the qualities which seemed most important to the authors who FIG. 8. Filaments of areolar tissue, in larger and smaller bundles, as seen under a magni- fying power of 400 diameters (Sharpey). AREOLAR TISSUE. 39 have described it, is met with in some form or other in every region of the body; the areolar tissue of one district being, directly or indirectly, continuous with that of all others. In most parts of the body this structure contains fat, but the quantity of the latter is very variable, and in some few re- gions it is absent altogether (p. 40). Probably no nerves are distributed to areolar tissue itself, although they pass through it to other structures ; and although bloodvessels are supplied to it, yet they are sparing in quantity, if we except those des- tined for the fat which is held in its meshes. Under the microscope areolar tissue seems composed of a meshwork of fine fibres of two kinds. The first, which makes up the greater part of the tissue, is formed of very fine white structureless fibres, arranged closely in bands and bundles, of wavelike appearance when not stretched out, and crossing and intersecting in all directions (Fig, 8). The second kind, or the yellow elastic fibre (Fig, 10), has a much sharper and FIG. 9. Magnified view of areolar tissues (from different parts) treated with acetic acid. The white filaments are no longer seen, and the yellow or elastic fibres with the nuclei come into view. At c, elastic fibres wind round a bundle of white fibres, which, by the effect of the acid, is swollen out between the turns. Some connective- tissue corpuscles are indistinctly represented in c (Sharpey). darker outline, and is not arranged in bundles, but intimately mingled with the first variety, as more or less separate and well-defined fibres, which twist among and around the bundles of white filaments (Fig. 9). Sometimes the yellow fibres divide at their ends and anastomose with each other by means of the branches. Among the fibrous parts of areolar or connective-tissue are little nuclear bodies of various shapes, 40 ELEMENTARY TISSUES. FIG. 10. called connective-tissue corpuscles (Fig. 9, c), some of which are prolonged at various points of their outline into small pro- cesses which meet and join others like them proceeding from their neighbors. The chief functions of areolar tissue seem to consist in the investment and mechanical support of various parts, and as a connecting bond between such structures as may need it. The connective-tissue corpuscles, which, according to Beale, are small branched particles of ger- minal matter or protoplasm, probably minister to the nutri- tion of the texture in which they are seated. In various parts of the body, each of the two constituents of areolar tissue which have been just mentioned, may exist sepa- rately, or nearly so. Thus ten- dons, fasciae, and the like more or less inelastic structures, are formed almost exclusively of the white fibrous tissue, arranged Elastic fibres from the ligamenta subflava, magnified about 200 diame- ters (Sharpey). according to the purpose re- quired, either in parallel bun- dles or membranous meshes ; while the yellow elastic fibres are found to make up almost alone such elastic structures as the vocal cords, the ligamenta subflava, &c., and to enter largely into the composition of the bloodvessels, the trachea, the lungs, and many other parts of the body. Adipose Tissue. In almost all regions of the human body a larger or smaller quantity of adipose or fatty tissue is present ; the chief excep- tions being the subcutaneous tissue of the eyelids, penis and scrotum, the nymphse, and the cavity of the cranium. Adipose tissue is also absent from the substance of many organs, as the lungs, liver, and others. Fatty matter, not in the form of a distinct tissue, is also widely present in the body, as the fat of the liver and brain, of the blood and chyle, &c. Adipose tissue is almost always found seated in areolar tissue, and forms in its meshes little masses of unequal size AREOLAR TISSUE. 41 and irregular shape, to which the term lobules is commonly applied. Under the microscope it is found to consist essentially FIG. 11. A small cluster of fat-cells ; magnified 150 diameters (Sharpey). of little vesicles or cells about ^J tn or TOO^ f an i llcn m diameter, each composed of a structureless and colorless mem- brane or bag, filled with fatty matter, which is liquid during life, but in part solidified after death. A nucleus is always present in some part or other of the cell-wall ; but in the ordi- nary condition of the cell it is not easily or always visible. The ultimate cells are held together by capillary bloodvessels ; while the little clusters thus formed are grouped into small masses, and held so, in most cases, by areolar tissue. The only matter contained in the cells is composed chiefly of the com- pounds of fatty acids with glycerin, which are named olein, stearin, and palmitin. It is doubtful whether lymphatics or nerves are supplied to fat, although both pass through it on their way to other structures. Among the uses of fat, these seem to be the chief: 1. It serves as a store of combustible matter which may be reabsorbed into the blood when occasion requires, and being burnt, may help to preserve the heat of the body. 2. That part of the fat which is situate beneath the skin must, by its want of conducting power, assist in preventing undue waste of the heat of the body by escape from the sur- face. 3. As a packing material, fat serves very admirably to fill up spaces, to form a soft and yielding yet elastic material wherewith to wrap tender and delicate structures, or form a bed with like qualities on which such structures may lie unen- dangered by pressure. As good examples of situations in which 42 ELEMENTARY TISSUES. fat serves such purposes may be mentioned the palms of the hands, and soles of the feet, and the orbits. 4. In the long bones, fatty tissue, in the form known as marrow, serves to fill up the medullary canal, and to support the small bloodvessels which are distributed from it to the inner part of the substance of the bone. Pigment. In various parts of the body there exists a considerable quantity of dark pigmentary matter, e. g., in the choroid coat of the eye, at the back of the iris, in the skin, &c. In all these cases the dark color is due to the presence of so-called pigment- cells. Pigment-cells are for the most part polyhedral (Fig. 12) or spheroidal, although sometimes they have irregular processes, as shown in Fig. 13. The cell-wall itself is colorless, the dark tint being produced by small dark granules heaped closely together, and more or less concealing the nucleus, itself color- FIG. 12. FIG. 1?. FIG. 12. Pigment-cells from the choroid; magnified 370 diameters (Henle). A, cells still cohering, seen on their surface ; 6, nucleus indistinctly seen. In the other cells the nucleus is concealed by the pigment-granules. FIG. 13. Ramified pigment-cells, from the tissue of the choroid coat of the eye ; magnified 350 diameters (after Kolliker). a, cells with pigment ; 6, colorless fusi- form cells. less, which each cell contains. The dark tint of the skin, in those of dark complexion and in the colored races, is seated chiefly in the epidermis, and depends on the presence of pig- ment-cells, which, except in the presence of the dark granules in their interior, closely resemble the colorless cells with which they are mingled. The pigment-cells are situate chiefly in the CARTILAGE. 43 deep layer of the epidermis, or the so-called rete mucosum. (See chapter on the Skin.) The pigmentary matter is a very insoluble compound of carbon, hydrogen, nitrogen, and oxygen, the carbon largely predominating ; besides, there is a small quantity of saline matter. The uses of pigment in most parts of the body are not clear. In the eyeball it is evidently intended for the absorption of superfluous rays of light. Cartilage. Cartilage or gristle exists in different forms in the human body, and has been classified under two chief heads, namely, temporary and permanent cartilage ; the former term being ap- plied to that kind of cartilage which, in the foetus and in young subjects, is destined to be converted into bone. The varieties of permanent cartilage have been arranged in three classes, namely, the cellular, the hyaline, and the fibrous carti- lages, the last-named, being again capable of subdivision into two kinds, namely, elastic or yellow cartilage, and the so-called fibro-cartilage. Elastic cartilage, however, contains fibres, and fibro-carti- lage is more or less elastic ; it will be well, therefore, for dis- tinction's sake to term those two kinds white fibro-cartilage and yellow fibro-cartilage respectively. The accompanying table represents the classification of the varieties of cartilage : 1. Temporary. f A Cellular. r White fibro-cartilage. C. Fibrous. fibro . cartila ge. All kinds of cartilage are composed of cells imbedded in a substance called the matrix : and the apparent differences of structure met with in the various kinds of cartilage are more due to differences in the character of the matrix than of the cells. Among the latter, however, there is also considerable diversity of form and size. With the exception of the articular variety, cartilage is in- vested by a thin but tough and firm fibrous membrane called the perichrondrium. On the surface of the articular cartilage of the foetus, the perichondrium is represented by a film of epithelium ; but this is gradually worn away up to the margin of the articular surfaces, when by use the parts begin to suffer friction. 44 ELEMENTARY TISSUES. FIG. 14. 1. Cellular cartilage may be readily obtained from the ex- ternal ear of rats, mice, or other small mammals. It is com- posed almost entirely of cells (hence its name), with little or no matrix. The latter, when present, consists of very fine fibres, which twine about the cells in various directions and inclose them in a kind of network. The cells are packed very closely together, so much so that it is not easy in all cases to make out the fine fibres often encircling them. Cellular cartilage is found in the human subject, only in early foetal life, when it constitutes the Chorda dorsalis. (See chapter on Generation.) 2. Hyaline cartilage is met with largely in the human body, investing the articular ends of bones, and forming the costal cartilages, the nasal cartilages, and those of the larynx, with the exception of the epiglottis and cornicula laryngis. Like other carti- lages it is composed of cells imbedded in a matrix (Fig. 14). The cells, which contain a nucleus with nucleoli, are irregu- lar in shape, and generally grouped together in patches. The patches are of various shapes and sizes, and placed at unequal distances apart. They generally appear flattened near the free surface of the mass of cartilage in which they are placed, and more or less perpendicular to the surface in the more deeply seated portions. The matrix in which they are imbedded has a dimly granu- lar appearance, like that of ground-glass. In the hyaline cartilage of the ribs, the cells are mostly larger than in the articular variety, and there is a tendency to the development of fibres in the matrix. The costal cartilages also frequently become ossified in old age, as also do some of those of the larynx. Temporary cartilage closely resembles the ordinary hyaline kind ; the cells, however, are not grouped together after the fashion just described, but are more uniformly distributed throughout the matrix. Articular hyaline cartilage is reckoned among the so-called A thin layer peeled off from the sur- face of the cartilage of the head of the humerus, showing flattened groups of cells. The shrunken cell-bodies are dis- tinctly seen, but the limits of the capsu- lar cavities, where they adjoin one another, are but faintly indicated. Mag- nified 400 diameters (after Sharpey). CARTILAGE. 45 non-vascular structures, no bloodvessels being supplied directly to its own substance; it is nourished by those of the bone be- neath. When hyaline cartilage is in thicker masses, as in the case of the cartilages of the ribs, a few bloodvessels traverse its substance. The distinction, however, between all so-called vascular and non-vascular parts, is at the best a very artificial one. (See chapter on Nutrition.) Nerves are probably not supplied to any variety of cartilage. Fibrous cartilage, as before mentioned, occurs under two chief forms, the yellow and the white fibre-cartilage. Yellow fibro-cartilage is found in the external ear, in the epiglottis and cornicula laryngis, and in the eyelid. The cells are rounded or oval, with well-marked nuclei and nucleoli. The matrix in which they are seated is composed almost en- tirely of fine fibres, which form an intricate interlacement about the cells, and in their general characters are allied to the yellow variety of fibrous tissue (Fig. 15). White fibro-cartilage, which is much more widely distributed throughout the body than the foregoing kind, is composed like it, of cells and a matrix ; the latter, however, being made up Fl - 15 - almost entirely of fibres close- ly resembling those of white fibrous tissue. In this kind of fibro-carti- lage it is not unusual to find a great part of its mass composed almost exclusively of fibres, and deserving the name of cartilage only from the fact that in another portion, continuous with it, cartilage-cells may be nrettv freelv distributed Sectlou of the e ^ loiiis ' magnified 380 pretty I .Ciy a ea. diameters (Dr. Baly). Ihe different situations in which white fibro-cartilage is formed have given rise to the following classification : 1. Interarticular fibro-cartilage, e. g., the semilunar carti- lages of the knee-joint. 2. Circumferential or marginal, as on the edges of the ace- tabulum and glenoid cavity of the scapula. 3. Connecting, e. g., the intervertebral fibro-cartilages. 4. Fibro-cartilage is found in the sheaths of tendons, and sometimes in their substance. In the latter situation, the nodule of fibro-cartilage is called a sesamoid fibro-cartilage, of which a specimen may be found in the tenclon of the tibialis 46 ELEMENTARY TISSUES. posticus, iii the sole of the foot, and usually in the neighboring tendon of the peroneus longus. The uses of cartilage are the following : in the joints, to form smooth surfaces ibr easy friction, and to act as a buffer, in shocks ; to bind bones together, yet to allow a certain degree of movement, as between the vertebrae ; to form a firm frame- work and protection, yet without undue stiffness or weight, as in the larynx and chest-walls ; to deepen joint-cavities, as in the acetabulum, yet not so as to restrict the movements of the bones ; to be, where such qualities are required, firm, tough, flexible, elastic, and strong. Structure of Bones and Teeth. Bone is composed of earthy and animal matter in the pro- portion of about 67 per cent, of the former to 33 per cent, of the latter. The earthy matter is composed chiefly of phos- phate of lime, but besides there is a small quantity, about 11 of the 67 per cent., of carbonate of lime, with minute quantities of some other salts. The animal matter is resolved into gela- tin by boiling. The earthy and animal constituents of bone are so intimately blended and incorporated the one with the other, that it is only by chemical action, as, for instance, by heat in one case, and by the action of acids in another, that they can be separated. Their close union, too, is further shown by the fact that when by acids the earthy matter is dis- solved out, or, on the other hand, when the animal part is burnt out, the general shape of the bone is alike preserved. To the naked eye there appear two kinds of structure in different bones, and in different parts of the same bone, namely, the dense or compact, and the cancellous tissue. Thus, in making a longitudinal section of a long bone, as the hume- rus or femur, the articular extremities are found capped on their surface by a thin shell of compact bone, while their in- terior is made up of the spongy or cancellous tissue. The shaft, on the other hand, is formed almost entirely of a thick layer of the compact bone, and this surrounds a central canal, the medullary cavity so called from its containing the medulla or marrow (p. 42). In the flat bones, as the parietal bone or the scapula, one layer of the cancellous structure lies between two layers of the compact tissue, and in the short and irregular bones, as those of the carpus and tarsus, the cancellous tissue alone fills the interior, while a thin shell of compact bone forms the outside. The spaces in the cancellous tissue are filled by a species of marrow, which differs considerably from B O N E. 47 that of the shaft of the long bones. It is more fluid, and of a reddish color, and contains very few fat-cells. The surfaces of bones, except the parts covered with articu- lar cartilage, are clothed by a tough fibrous membrane, the periosteum ; and it is from the bloodvessels which are distrib- uted first in this membrane, that the bones, especially their FIG. 16. Transverse section of compact tissue (of humerus) magnified about 150 diameters. Three of the Haversian canals are seen, with their concentric rings; also the cor- puscles or lacunae, with the canaliculi extending from them across the direction of the lamellae. The Haversian apertures had got filled with debris in grinding down the section, and therefore appear black in the figure, which represents the object as viewed with transmitted light (after Sharpey). more compact tissue, are in great part supplied with nourish- ment minute branches from the peri osteal vessels entering the little foramina on the surface of the bone, and finding their way to the Haversian canals, to be immediately de- scribed. The long bones are supplied also by a proper nutri- ent artery, which, entering at some part of the shaft so as to reach the medullary canal, breaks up into branches for the supply of the marrow, from which again small vessels are dis- tributed to the interior of the bone. Other small bloodvessels pierce the articular extremities for the supply of the cancellous tissue. Notwithstanding the differences of arrangement just men- 48 ELEMENTARY TISSUES. tioned, the structure of all bone is found, under the microscope, to be essentially the same. Examined with a rather high power its substance is found occupied by a multitude of little spaces, called lacunae, with very minute canals or canaliculi, as they are termed, leading from them, and anastomosing with similar little prolongations from other lacunae (Fig. 16). In very thin layers of bone, no other canals than these may be visible ; but on making a transverse section of the compact tissue, e. g., of a long bone, as the humerus or ulna, the arrangement shown in Fig. 16 can be seen. The bone seems mapped out into small circular districts, at or about the centre of each of which is a hole, and around this an appearance as of concentric layers ; the lacunce and canaliculi following the same concentric plan of distribution around the small hole in the centre, with which, indeed, they communicate. On making a longitudinal section, the central holes are found to be simply the cut ex- tremities of small canals which run lengthwise through the bone (Fig. 17), and are called Haversian canals, after the name of the physician, Cloptou Havers, who first accurately described them. The Haversian canals, the average diameter of which is of an inch, contain bloodvessels, and by means of them blo is conveyed to all, even the densest parts of the bone ; the mi- nute canaliculi and lacunae absorbing nutrient matter from the Haversian bloodvessels, and conveying it still more intimately to the very substance of the bone which they traverse. The bloodvessels enter the Haversian canals both from without, by traversing the small holes which exist on the surface of all bones beneath the periosteum, and from within by means of small channels which extend from the medullary cavity, or from the cancellous tissue. According to Todd and Bowman, the arteries and veins usually occupy separate canals, and the veins, which are the larger, often present, at irregular intervals, small pouch-like dilatations (Fig. 17). The lacunce are occupied by nucleated cells, or, as Dr. Beale expresses it, minute portions of protoplasm or germinal matter; and there is every reason to believe that the lacunar cells are homologous with the corpuscles of the connective tissue, each little particle of protoplasm ministering to the nutrition of the bone immediately surrounding it, and one lacunar particle communicating with another, and with its surrounding district, and with the bloodvessels of the Haversian canals, by means of the minute streams of fluid nutrient matter which occupy the canaliculi. Besides the concentric lamellce of bone-tissue which surround the Haversian canal in the shaft of a long bone, are others, es- BONE. 49 pecially near the circumference, which surround the whole bone, and are arranged concentrically with regard to the medullary canal. The ultimate structure of the lamellae appears to be reticular. If a thin film be peeled off the surface of a bone from which FIG. 17. FIG. 18. FIG. 17. Haversian canals, seen in a longitudinal section of the compact tissue of the shaft of one of the long bones. 1. Arterial canal ; 2. Venous canal ; 3. Dilatation of another venous canal. FIG. 18. Thin layer peeled off from a softened bone, as it appears under a magni- fying power of 400. This figure, which is intended to represent the reticular struc- ture of a lamella, gives a better idea of the object when held rather farther off than usual from the eye (from Sharpey). the earthy matter has been removed by acid, and examined with a high power of the microscope, it will be found composed, according to Sharpey, of a finely reticular structure, formed apparently of very slender fibres decussating obliquely, but coalescing at the points of intersection, as if here the fibres were fused rather than woven together (Fig. 18). In many places these reticular lamellae are perforated by tapering fibres, resembling in character the ordinary white or rarely the elastic fibrous tissue, which bolt the neighboring lamellae together, and may be drawn out when the latter are torn asunder (Fig. 19). Bone is developed after two different fashions. In one, the 50 ELEMENTARY TISSUES. tissue in which the earthy matter is laid down is a membrane, composed mainly of fibres and granular cells, like imperfectly developed connective-tissues. Of this kind of ossification in membrane, the flat bones of the skull are examples. In the other, and much more common case, of which a long bone may be cited as an instance, the ossification takes place in car- tilage. In most bones ossification begins at more than one point ; and from these centres of ossification, as they are called, the process of deposition of calcareous matter advances in all directions. Bones grow by constant development of the car- tilage or membrane between these centres of ossification, until by the process of calcification advancing at a quicker rate than the development of the softer structures, the bone becomes im- FlG. 19. Lamellae torn off from a decalcified human parietal bone at some depth from the surface, a, a lamella, showing reticular fibres ; ft, b, darker part, where several lamellae are superposed ; c, c, perforating fibres. Apertures through which perfor- ating fibres had passed, are seen especially ill the lower part, a, a, of the figure. Magnitude as seen under a power of 200, but not drawn to a scale (from a drawing by Dr. Allen Thomson). pregnated throughout with calcareous matter, and can grow no more. In the long bones the main centres of ossification are seated at the middle of the shaft, and at each of the ex- tremities. Increase of the length of bones, therefore, occurs at TEETH. 51 the part which intervenes between the ossifying centre in the shaft and that at each extremity ; while increase in thickness takes place by the formation of layers of osseous tissue beneath the periosteum. The former is an example of ossification in cartilage ; the latter of ossification in membrane. Teeth. A tooth is generally described as possessing a crown, neck, and fang or fangs. The crown is the portion which pro- jects beyond the level of the gum. The neck is that constricted portion just below the crown, which is embraced by the free edges of the gum, and the fang includes all below this. On making a longitudinal section through the centre of a tooth (Figs. 20 and 21), it is found to be principally composed of a hard matter, dentine or ivory ; while in the centre this dentine is hollowed out into a cavity resembling in general shape the outline of the tooth, and called the pulp-cavity, from its containing a very vascular and sensitive little mass com- posed of connective tissue, bloodvessels and nerves, which is called the tooth-pulp. The pulp is continuous below, through an opening at the end of the fang, with the mucous membrane of the gum. Capping that part of the dentine which projects FIG. 20. Sections of an Incisor and Molar Tooth. The longitudinal sections show the whole of the pulp-cavity in the incisor and molar teeth, its extension upwards within the crown, and its prolongation downwards into the fangs, with the small aperture at the point of each; these and the cross-section show the relation of the dentine and enamel. beyond the level of the gum, is a layer of very hard calcareous matter, the enamel, while sheathing the portion of dentine which is beneath the level of the gum, is a layer of true bone, called the cement or crusta petrosa. At the neck of the tooth the cement is exceedingly thin, but it gradually becomes thicker as it ap- proaches and covers the lower end or apex of the fang. Dentine or ivory in chemical composition closely resembles 52 ELEMENTARY TISSUES. FIG. 21. bone. It contains, however, rather less animal matter ; the proportion in 100 parts being about 28 of animal matter to 72 of earthy. The former, like the animal matter of bone, may be resolved into gelatin by boil- ing. The earthy matter is made up chiefly of phosphate of lime, with a small portion of the car- bonate, and traces of some other salts. Under the microscope, den- tine is seen to be finely chan- nelled by a multitude of fine tubes, which, by their inner ends, communicate with the pulp-cavity, and by their outer extremities come into contact with the under part of the en- amel and cement, and some- times even penetrate them for a greater or less distance. In their course from the pulp-cavity to the surface of the dentine, these minute tubes form gentle and nearly parallel curves, and divide and subdivide dichotom- ously, but without much lessen- ing of their calibre until they are approaching their peripheral termination. From their sides proceed other exceedingly mi- nute secondary canals, which extend into the dentine between the tubules. The tubules of the dentine, Magnified Longitudinal Section of a Bicuspid Tooth (after Retzius) 1, the ivory or dentine, showing the direc- tion and primary curves of the dental tubuli; 2, the pulp-cavity, with the small apertures of the tubuli into it; 3, the cement or crusta petrosa, cover- ing the fang as high as the border of the enamel at the neck, exhibiting lacuna? ; 4, the enamel resting on the , , f dentine; this has been worn away by the average diameter of which use from the upper part. at their inner and larger ex- tremity is 4^0 o f an i ncn > con " tain fine prolongations from the tooth-pulp, which give the den- tine a certain faint sensitiveness under ordinary circumstances, and without doubt, have to do also with its nutrition. The enamel, which is by far the hardest portion of a tooth, is composed, chemically, of the same elements that enter into the composition of dentine and bone. Its animal matter, how- ever, amounts only to about 2 or 3 per cent. TEETH. 53 FIG. 22. Examined under the microscope, enamel is found composed of fine hexagonal fibres (Figs. 22 and 23), which are set on end on the sur- face of the dentine, and fit into cor- responding depressions in the same. They radiate in such a manner from the dentine that at the top of the tooth they are more or less vertical, while towards the sides they tend to the horizontal direction. Like the dentine-tubules, they are not straight, but disposed in wavy and parallel curves. The fibres are marked by transverse lines, and are mostly solid, but some of them contain a very minute canal. The enamel itself is coated on the outside by a very thin calcified mem- brane, sometimes termed the cuticle of the enamel. The crusta petrosa, or cement, is composed of true bone, and in it are lacunae and canaliculi which some- times communicate with the outer finely-branched ends of the dentine- tubules. Development of Teeth. The teeth are developed after the following manner : Along the free edge of the toothless gum in the foetus, there ex- tends a groove, or small trench, the primitive dental groove (Goodsir), and from the bottom of this project ten small processes of mucous membrane, or papillae, containing bloodvessels and nerves. As these papillce grow up from below, the edges of the small trench begin to grow in towards each other, and overshadow them, at the same time that each papilla is cut off from its neighbor by the extension of a partition wall from the gum, which grows in from each side to separate the one from the other. Thus closed in above and all around, each dental papilla is at length contained in a separate sac, and gradually assumes the character of a tooth by deposition on its surface of the various hard matters which have been just enumerated as composing the greater part of a tooth's 7) Thin section of the enamel and a part of the dentine (from Kolliker) &%&. a, cu- ticular pellicle of the enamel ; b, enamel fibres, or columns with fissures between them and cross striae ; c, larger cavi- ties in the enamel, communi- cating with the extremities of some of the tubuli (d). 54 ELEMENTARY TISSUES. substance. The small vascular papilla is gradually encroached upon and imprisoned by the calcareous deposit, until only a small part of it is left as the tooth-pulp, which remains shut up in the harder substance, with only the before-mentioned small FIG. 23. Enamel fibres (from Kolliker) ^f fi . A, fragments and single fibres of the enamel, isolated by the action of hydrochloric acid. B, surface of a small fragment of enamel, showing the hexagonal ends of the fibres. communication with the outside, through the end of the fang. In this manner the first set of teeth, or the milk teeth, are formed ; and each tooth, by degrees developing, presses at length on the wall of the sac inclosing it, and causing its ab- sorption, is cut, to use a familiar phrase. The temporary or milk teeth, having only a very limited term of existence, gradually decay and are shed, while the per- manent teeth push their way from beneath, by gradual increase and development, so as tb succeed them. The temporary teeth are ten in each jaw, namely, four in- cisors, two canines, and four molars, and are replaced by ten permanent teeth, each of which is developed from a small sac set by, so to speak, from the sac of the temporary tooth which precedes it and called the cavity of reserve. The num- ber of the permanent teeth is, however, increased to sixteen, by the development of three others on each side of the jaw after much the same fashion as that by which the milk teeth were themselves formed. The beginning of the development THE BLOOD. 55 of the permanent teeth of course takes place long before the cutting of those which they are to succeed; one of the first acts of the newly-formed little dental sac of a milk-tooth being to set aside a portion of itself as the germ of its successor. The following formula shows, at a glance, the comparative arrangement and number of the temporary and permanent teeth : MO. CA. IN. CA. MO. f Upper, 21412 =10 Temporary Teeth,. . \ =20 (Lower, ^21412 =10 MO. BI. CA. IN. CA. BI. MO. (Upper, 321412 3 = 16 Permanent Teeth,. . \ - =32 (Lower, 3 2 1 4 1 2 3 = 16 From this formula it will be seen that the two bicuspid teeth in the adult are the successors of the two molars in the child. They differ from them, however, in some respects, the tem- porary molars having a stronger likeness to the permanent than to their immediate descendants, the so-called bicuspids. The temporary incisors and canines differ but little, except in their smaller size, from their successors. CHAPTER V. THE BLOOD. ALTHOUGH it may seem, in some respects, un advisable to describe the blood before entering upon the physiology of those subservient processes which have for their end or purpose its formation and development, yet there are many reasons for taking such a course, and we may therefore at once proceed to consider the structural and chemical composition of this fluid. Wherever blood can be seen under a moderately high micro- scopic power as it flows in the vessels of a living part, it appears a colorless fluid containing minute colored particles. The greater part of these particles are red, when seen en masse, and they are the source of the color which, so far as the naked eye can see, belongs to every part of the blood alike. The colorless fluid is named liquor sanguinis ; the particles are the 56 THE BLOOD. blood-corpuscles or blood-cells. The structural composition of the blood may be thus expressed : f Corpuscles, . . 1 Clot (containing also T- ,-j pi^,i f more or less serum). Liquid Mood, j LiqnorS anguinisC Fibrin, \ [ or Plasma. \Serum. When blood flows from the living body, it is a thickish heavy fluid, of a bright scarlet color when it comes from an artery; deep purple, or nearly black, when it flows from a vein. Its specific gravity at 60 F. is, on an average, 1055, that of water being reckoned as 1000 ; the extremes consistent with health being 1050 and 1059. Its temperature is generally about 100 F.; but it is not the same in all parts of the body. Thus, while the stream is slightly warmed by passing through the liver and some other parts, it is slightly cooled, according to Bernard, by traversing the capillaries of the skin. The temperature of blood in the left side of the heart is, again 1 or 2 higher than in the right (Savory). The blood has a slight alkaline reaction ; and emits an odor similar to that which issues from the skin or breath of the animal from which it flows, but fainter. The alkaline reac- tion appears to be a constant character of blood in all animals and under all circumstances. An exception has been supposed to exist in the case of menstrual blood ; but the acid reaction which this sometimes presents is due to the mixture of an acid mucus from the uterus and vagina. Pure menstrual blood, such as may be obtained with a speculum, or from the uteri of women who die during menstruation, is always alkaline, and resembles ordinary blood. According to Bernard, blood becomes spontaneously acid after removal from the body, owing to conversion of its sugar into lactic acid. The odor of blood is easily perceived in the watery vapor, or halitus as it is called, which rises from blood just drawn : it may also be set free, long afterwards, by adding to the blood a mixture of equal parts of sulphuric acid and water. It is said to be not difficult to tell, by the likeness of the odor to that of the body, the species of domestic animal from which any specimen of blood has been taken : the strong odor of the pig or cat, and the peculiar milky smell of the cow, are es- pecially easy to be thus discerned in their blood (Barruel). Quantity of Blood. Only an imperfect indication of the whole quantity of blood in the body is afforded by measurement of that which escapes, when an animal is rapidly bled to death, inasmuch as a cer- QUANTITY OF BLOOD. 57 tain amount always remains in the bloodvessels. In cases of less rapid bleeding, on the other hand, when life is more pro- longed, and when, therefore, sufficient time elapses before death to allow some absorption into the circulating current of the fluids of the body (p. 76), the whole quantity of blood that escapes may be greater than the whole average amount natur- ally present in the vessels. Various means have been devised, therefore, for obtaining a more accurate estimate than that which results from merely bleeding animals to death. Welcker's method is the following. An animal is rapidly bled to death, and the blood which escapes is collected and measured. The blood remaining in the smaller vessels is then removed by the injection of water through them, and the mix- ture of blood and water thus obtained, is also collected. The animal is then finely minced, and infused in water, and the infusion is mixed with the combined blood and water pre- viously obtained. Some of this fluid is then brushed on a white ground, and the color compared with that of mixtures of blood and water whose proportions have been previously determined by measurement. In this way the materials are obtained for a fairly exact estimate of the quantity of blood actually existing in the body of the animal experimented on. Another method (that of Vierordt) consists in estimating the amount of blood expelled from the ventricle, at each beat of the heart, and multiplying this quantity by the number of beats necessary for completing the "round" of the circulation. This method is ingenious, but open to various objections, the most conclusive being the uncertainty of all the premises on which the conclusion is founded. Other methods depend on the results of injecting a known quantity of water (Valentin) or of saline matters (Blake) into the bloodvessels ; the calculation being founded in the first case, on the diminution of the specific gravity which ensues, and in the other, on the quantity of the salt found diffused in a cer- tain measured amount of the blood abstracted for experiment. A nearly correct estimate was probably made by Weber and Lehmann, from the following data. A criminal was weighed before and after decapitation ; the difference in the weight representing, of course, the quantity of blood which escaped. The bloodvessels of the head and trunk, were then washed out by the injection of water, until the fluid which escaped had only a pale red or straw color. This fluid was then also weighed ; and the amount of blood which it repre- sented was calculated, by comparing the proportion of solid matter contained in it, with that of the first blood which 58 THE BLOOD. escaped on decapitation. Two experiments of this kind gave precisely similar results. The most reliable of these various means for estimating the quantity of blood in the body yield as nearly similar results as can be expected, when the sources of error unavoidably present in all, are taken into consideration; and it may be stated that in man, the weight of the whole quantity of blood, compared with that of the body, is from about 1 to 8, to 1 to 10. It must be remembered, however, that the whole quantity of blood varies, even in the same animal, very considerably, in correspondence with the different amounts of food and drink, which may have been recently taken in, and the equally vary- ing quantity of matter given out. Bernard found by experi- ment, that the quantity of blood obtainable from a fasting animal is scarcely more than half of that which is present soon after a full meal. The estimate above given, must there- fore be taken to represent only an approximate average. Coagulation of the Blood. When blood is drawn from the body, and left at rest, cer- tain changes ensue, which constitute a kind of rough analysis of it, and are instructive respecting the nature of some of its constitutents. After about ten minutes, taking a general average of many observations, it gradually clots or coagulates, becoming solid like a soft jelly. The clot thus formed has at first the same volume and appearance as the fluid blood had, and, like it, looks quite uniform ; the only change seems to be, that the blood which was fluid is now solid. But presently, drops of transparent yellowish fluid begin to ooze from the surface of the solid clot; and these gradually collecting, first on its upper surface, and then all around it, the clot or " cras- samentum" diminished in size, but firmer than it was before, floats in a quantity of yellowish fluid, which is named serum, the quantity of which may continually increase for from twenty- four to forty-eight hours after the clotting of the blood. The changes just described may be thus explained. The liquor sanguinis, or liquid part of the blood (p. 55), consists of a thin fluid called serum, holding fibrin in solution. 1 The peculiar property of fibrin, as already said, is its tendency to become solid when at rest, and in some other conditions. When, therefore, a quantity of blood is drawn from the vessels, the fibrin coagulates, and the blood-corpuscles, with part of the 1 This statement has been left unaltered in the text ; but, as will be seen farther on, it requires modification. (ED.) COAGULATION OF BLOOD. 59 serum, are held, or, as it were, entangled in the solid substance which it forms. But after healthy fibrin has thus coagulated, it always con- tracts ; and what is generally described as one process of coagu- lation should rather be regarded as consisting of two parts or stages ; namely, first, the simple act of clotting, coagulating, or becoming solid ; and, secondly, the contraction or condensa- tion of the solid clot thus formed. By this second act much of the serum which was soaked in the clot is gradually pressed out ; and this collects in the vessel around the contracted clot. Thus, by the observation of blood within the vessels, and of the changes which commonly ensue when it is drawn from them, we may distinguish in it three principal constituents, namely, 1st, the fibrin, or coagulating substance ; 2d, the serum ; 3d, the corpuscles. That the fibrin is the only spontaneously coagulable material in the blood, may be proved in many ways ; and most simply by employing any means whereby a portion of the liquor san- guinis, i. e., the serum and fibrin, can be separated from the red corpuscles before coagulation. Under ordinary circum- stances coagulation occurs before the red corpuscles have had time to subside ; and thus, from their being entangled in the meshes of the fibrin, the clot is of a deep dark red color through- out, somewhat darker, it may be, at the most dependent part, from accumulation of red cells, but not to any very marked degree. If, however, from any cause, the red cells sink more quickly than usual, or the fibrin contracts more slowly, then, in either of these cases, the red corpuscles may be observed, while the blood is yet fluid, to sink below its surface ; and the layer beneath which they have sunk, and which has usually an opaline or grayish-white tint, will coagulate without them, and form a white clot consisting of fibrin alone, or of fibrin with entangled white corpuscles; for the white corpuscles, being very light, tend upwards towards the surface of the fluid. The layer of white clot which is thus formed rests on the top of a colored clot of ordinary character, i. e., of one in which the coagulating fibrin has entangled the red corpuscles while they were sinking : and, thus placed, it constitutes what has been called a buffy coat. When a buffy coat is formed in the manner just described, it commonly contracts more than the rest of the clot does, and, drawing in at its sides, produces a cupped appearance on the top of the clot. In certain conditions of the system, and especially when there exists some local inflammation, this buffed and cupped con- dition of the clot is well marked, and there has been much dis- 60 THE BLOOD. cussion concerning its origin under these circumstances. It is now generally agreed that two causes combine to produce it. In the first place, the tendency of the red corpuscles to form rouleaux (see p. 68) is much exaggerated in inflammatory blood ; and as their rate of sinking increases with their aggre- gation, there is a ready explanation, at least in part, of the colorless condition of the top of the clot. And in the next place, inflammatory blood coagulates less rapidly than usual, and thus there is more time for the already rapidly sinking corpuscles to subside. The colorless or buffed condition of the upper part of the clot is therefore, readily accounted for; while the cupped appearance is easily explained by the greater power of contraction possessed by the fibrin of inflammatory blood arid by its contraction being now not interfered with by the presence of red corpuscles in its meshes. Although the appearance just described is commonly the re- sult of a condition of the blood in which there is an increase in the quantity of fibrin, it need not of necessity be so. For a very different state of the blood, such as that which exists in chlorosis, may give rise to the same appearance ; but in this case the pale layer is due to a relatively smaller amount of red corpuscles, not to any increase in the quantity of fibrin. It is thus evident that the coagulation of the blood is due to its fibrin. The cause of the coagulation of the fibrin, how- ever, is still a mystery. The theory of Prof. Lister, that fibrin has no natural ten- dency to clot, but that its coagulation out of the body is due to the action of foreign matter with which it happens to be brought into contact, and, in the body, to conditions of the tissues, which cause them to act towards it like foreign matter, is insufficient; because even if it be true, it still leaves unex- plained the manner in which the fibrin, fluid in the living bloodvessels, can, by foreign matter, be thus made solid. If it be a fact, it is a very important one, but it is not an expla- nation. The same remark may be applied also to another theory which differs from the last, in that while it admits a natural tendency on the part of the blood to coagulation, it supposes that this tendency in the living body is restrained by some in- hibitory power resident in the walls of the containing vessels. This also may, or may not, be true; but it is only a statement of a possible fact, and leaves unexplained the manner in which living tissue can thus restrain coagulation. Dr. Draper believes that coagulation takes place in the liv- ing body, as out of it, or as in the dead ; but in the one case the fibrin is picked out in the course of the circulation by tis- COAGULATION OF BLOOD. 61 sues which this particular constituent of the blood is destined to nourish ; in the others, it remains and becomes evident as a clot. This explanation is ingenious, but requires some kind of proof before it can be adopted. Concerning other theories, as for instance, that coagulation is due to the escape of carbonic acid, or of ammonia, it need only be said that they have been completely disproved. We must therefore, for the present, believe that the cause of the coagulation of the blood has yet to be discovered ; but some very interesting observations in connection with the sub- ject have been recently made, and seem not unlikely to lead in time to a solution of this difficult and most vexed question. The observations referred to have been made independently by Alexander Schmidt, although he was forestalled in regard to some of his experiments by Dr. Andrew Buchanan, of Glas- gow, many years ago. When blood-serum, or washed blood-clot, is added to the fluid of hydrocele, or any other serous effusion, it speedily causes coagulation, and the production of true fibrin. And this phenomenon occurs also on the admixture of serous effu- sions from different parts of the body, as that of hydrocele with that of ascites, or of either with fluid from the cavity of the pleura. Other substances also, as muscular or nervous tissue, skin, &c., have been found also able to excite coagulation in serous fluids. Thus, fluids which have little or no tendency to coagulate when left to themselves, can be made to produce a clot, apparently identical with the fibrin of blood by the addi- tion to them of matter which, on its part, was not known to have any special relation to fibrin. As may be supposed, the coagulation is not alike in extent under all these circumstances. Thus, although it occurs when apparently-few or no blood-cells exist in either constituent of the mixture, yet the addition of these very much increases the effect, and their presence evi- dently has a very close connection with the process. From the action of the buffy coat of a clot, in causing the appearance of fibrin in serous effusions, it may be inferred that the pale as well as the red corpuscles are influential in coagulation under these circumstances. Blood-crystals are also found to be effec- tive in producing a clot in serous fluids. The true explanation of these very curious phenomena is, probably, not fully known ; but Schmidt supposes that in the act of formation of fibrin there occurs the union of two substances, which he terms fibrinoplastin and fibrinogen. The substance which he terms fibrmoplastin, and which he has obtained, not only from blood, but from many other liquids 62 THE BLOOD. and solids, as the crystalline lens, chyle and lymph, connec- tive tissue, &c., which are found capable of exciting coagula- tion in serous fluids, is probably identical with the globulin of the red corpuscles. The fibrinogenous matter obtained from serous effusions dif- fers but little, chemically, from the fibrinoplastin. Thus in the experiment before mentioned, the globulin or fibrinoplastic matter of the blood-cells in the clot causes co- agulation by uniting with the fibrinogen present in the hydro- cele-fluid. And whenever there occurs coagulation with the production of fibrin, whether in ordinary bloodclotting, or in the admixture of serous effusions, or in any other way, a like union of these two substances may be supposed to occur. The main result, therefore, of these very interesting experi- ments and observations has been to make it probable that the idea of fibrin existing in a liquid state in the blood is founded on a mistaken notion of its real nature, and that, probably, it does not exist at all in solution as fibrin, but is formed at the moment of coagulation by the union of two substances which, in fluid blood, exist separately. The theories before referred to, concerning the coagulation of the blood, will therefore, if this be true, resolve themselves into theories concerning the causes of the union of fibrino- plastin and fibrinogen ; and whether, on the one hand, it is an inhibitory action of the living bloodvessels that naturally re- drains, or a catalytic action of foreign matter that excites, the union of these two substances. Conditions affecting Coagulation. Although the coagulation of fibrin appears to be sponta- neous, yet it is liable to be modified by the conditions in which it is placed; such as temperature, motion, the access of air, the substances with which it is in contact, the mode, of death, &c. All these conditions need to be considered in the study of the coagulation of the blood. The coagulation of the blood is hastened by the following means : 1. Moderate warmth, from about 100 F. to 120 F. 2. Rest is favorable to the coagulation of blood. Blood, of which the whole mass is kept in uniform motion, as when a closed vessel completely filled with it is constantly moved, co- agulates very slowly and imperfectly. But rest is not essen- tial to coagulation ; for the coagulated fibrin may be quickly CONDITIONS AFFECTING COAGULATION. 63 obtained from blood by stirring it with a bundle of small twigs ; and whenever any rough points of earthy matter or foreign bodies are introduced into the bloodvessels, the blood soon coagulates upon them. 3. Contact with foreign matter, and especially multiplica- tion of the points of contact. Thus, when all other conditions are unfavorable, the blood will coagulate upon rough bodies projecting into the vessels ; as, for example, upon threads passed through arteries or aneurismal sacs, or the heart's valves roughened by inflammatory deposits or calcareous ac- cumulations. And, perhaps, this may explain the quicker co- agulation of blood after death in the heart with walls made irregular by the fleshy columns, than in the simple smooth- walled arteries and veins. 4. The free access of air. 5. Coagulation is quicker in shallow than in tall and nar- row vessels. 6. The addition of less than twice the bulk of water. The blood last drawn is said to coagulate more quickly than that which is first let out. The coagulation of the blood is retarded by the following means : 1. Cold retards the coagulation of blood; and it is said that, so long as blood is kept at a temperature below 40 F., it will not coagulate at all. Freezing the blood, of course, prevents its coagulation ; yet it will coagulate, though not firmly, if thawed after being frozen ; and it will do so, even after it has been frozen for several months. Coagulation is accelerated, but the subsequent contraction of the clot is hindered by a temperature between 100 and 120 : a higher temperature re- tards coagulation, or, by coagulating the albumen of the serum, prevents it altogether. 2. The addition of water in greater proportion than twice the bulk of the blood. 3. Contact with living tissues, and especially with the interior of a living bloodvessel, retards coagulation, although if the blood be at rest it does not prevent it. 4. The addition of the alkaline and earthy salts in the pro- portion of 2 or 3 per cent, and upwards. When added in large proportion most of these saline substances prevent coagulation altogether. Coagulation, however, ensues on dilution with water. The time that blood can be thus preserved in a liquid state and coagulated by the addition of water, is quite in- definite. 64 THEBLOOD. 5. Imperfect aeration, as in the blood of those who die by asphyxia. 6. In inflammatory states of the system, the blood coagulates more slowly although more firmly. 7. Coagulation is retarded by exclusion of the blood from the air, as by pouring oil on the surface, &c. In vacuo, the blood coagulates quickly ; but Prof. Lister thinks that the rapidity of the process is due to the bubbling which ensues from the escape of gas, and to the blood being thus brought more freely into contact with the containing vessel. The coagulation of the blood is prevented altogether by the addition of strong acids and caustic alkalies. It has been believed, and chiefly on the authority of Mr. Hunter, that, after certain modes of death, the blood does not coagulate ; he enumerates the death by lightning, overexertion (as in animals hunted to death), blows on the stomach, fits of anger. He says, "I have seen instances of them all." Doubt- less he had done so ; but the results of such events are not con- stant. The blood has been often observed coagulated in the bodies of animals killed by lightning or an electric shock ; and Mr. Gulliver has published instances in which he found clots in the hearts of hares and stags hunted to death, and of cocks killed in fighting. Chemical Composition of the Blood. Among the many analyses of the blood that have been pub- lished, some, in which all the constituents are enumerated, are inaccurate in their statements of the proportions of those con- stituents ; others, admirably accurate in some particulars, are incomplete. The two following tables, constructed chiefly from the analyses of Denis, Lecanu, Simon, Nasse, Lehmann, Bec- querel, Rodier, and Gavarret, are designed to combine, as far as possible, the advantage of accuracy in numbers with the con- venience of presenting at one view, a list of all the constituents of the blood. Average proportions of the principal constituents of the blood in 1000 parts: Water, lied corpuscles (solid residue), Albumen of serum, Saline matters, .... Extractive, fatty, and other matters, Fibrin. . 784. 130. 70. 6.03 7.77 2.2 1000. COMPOSITION OF BLOOD. 65 Average proportions of all the constituents of the blood in 1000 parts: Water, 784. Albumen, ......... 70. Fibrin, 2.2 Ked corpuscles (dry), ...... 130. Fatty matters, 1.4 Inorganic Salts: Chloride of sodium, . . . 3.6 Chloride of potassium, . . . 0.35 Tribasic phosphate of soda, . . 0.2 Carbonate of soda, . . . . 0.28 Sulphate of soda, .... 0.28 Phosphates of lime and magnesia, 0.25 Oxide and phosphate of iron, . 0.5 Extractive matters, biliary coloring matter, gases, and accidental substances, ..... 6.40 1000. Elementary composition of the dried blood of the ox : Carbon, 57.9 Hydrogen, 7.1 Nitrogen, ......... 17.4 Oxygen, 19.2 Ashes, .......... 4.4 These results of the ultimate analysis of ox's blood afford a remarkable illustration of its general purpose, as supplying the materials for the renovation of all the tissues. For the analysts (Playfair and Boeckmann) have found that the flesh of the ox yields the same elements in so nearly the same proportions that the elementary composition of the organic constituents of the blood and flesh may be considered identical, and may be rep- resented for both by the formula C 45 H 39 N 6 O 15 . The Blood- Corpuscles or Blood- Cells. It has been already said that the clot of blood contains, with the fibrin and the portion of the serum that is soaked in it, the blood-corpuscles, or blood-cells. Of these there are two principal forms, the red and the white corpuscles. When coagulation has taken place quickly, both kinds of corpuscles may be uniformly diffused through the clot ; but, when it has been slow, the red corpuscles, being the heaviest constituent of the blood, tend by gravitation to accumulate at the bottom of the clot; and the white corpuscles, being among the lightest constituents, collect in the upper part, and contribute to the formation of the bufly coat. 66 THE BLOOD. FIG. 24. Mammals. Birds. Reptiles. Amphibia. Fish. The above illustration is somewhat altered from a drawing, by Mr. Gulliver, in the Proceed. Zool. Society, and exhibits the typical characters of the red blood-cells in the main divisions of the Vertebrata. The fractions are those of an inch, and rep- resent the average diameter. In the case of the oval cells, only the long diameter is here given. It is remarkable, that although the size of the red blood-cells varies so much in the different classes of the vertebrate kingdom, that of the white corpuscles remains comparatively uniform, and thus they are, in some animals, much greater, in others much less, than the red corpuscles existing side by side with them. It may be here remarked, that the appearance of a nucleus in the red blood-cells of birds, reptiles, amphibia, and fish, has been shown by Mr. Savory to be the result of post-mortem change; no nucleus being visible in the cells as they circulate in the living body, or in those which have just escaped from the bloodvessels. RED BLOOD-CORPUSCLES. 67 The human red blood-cell* or blood-corpuscles (Figs. 25 and 29) are circular flattened disks of different sizes, the majority varying in diameter from 3^00 to 4$^ of an inch, and about TOtWfi f an i ncn i n thickness. When viewed singly, they ap- pear of a pale yellowish tinge; the deep red color which they give to the blood being observable in them only when they are seen en masse. Their borders are rounded ; their surfaces, in the perfect and most usual state, slightly concave; but they readily acquire flat or convex surfaces when, the liquor san- guinis being diluted, they are swollen by absorption of fluid. They are composed of a colorless, structureless, and transparent filmy framework or stroma, infiltrated in all parts by a red coloring-matter termed hcemoglobin. The stroma is tough and elastic, so that, as the cells circulate, they admit of elongation and other changes of form, in adaptation to the vessels, yet recover their natural shape as soon as they escape from com- pression. The term cell, in the sense of a bag or sac, is inap- plicable to the red blood-corpuscle ; and it must be considered, if not solid throughout, yet as having no such variety of con- sistence in different parts as to justify the notion of its being a membranous sac with fluid contents. The stroma exists in all parts of its substance, and the coloring matter uniformly per- vades this, and is not merely surrounded by and mechanically inclosed within the outer wall of the corpuscle. The red cor- puscles have no nuclei, although in their usual state, the un- equal refraction of transmitted light gives the appearance of a central spot, brighter or darker than the border, according as it is viewed in or out of focus. Their specific gravity is about 1088. In examining a number of red corpuscles with the micro- scope, it is easy to observe certain natural diversities among them, though they may have been all taken from the same part. The great majority, indeed, are very uniform; but some are rather larger, and the larger ones generally appear paler and less exactly circular than the rest ; their surfaces also are usually flat or slightly convex, they often contain a minute shining particle like a nucleolus, and they are lighter than the rest, floating higher in the fluid in which they are placed. Other deviations from the general characters assigned to the corpuscles depend on changes that occur after they are taken from the body. Very commonly they assume a granulated or mulberry-like form, in consequence, apparently, of a peculiar corrugation of their cell-walls. Sometimes, from the same cause, they present a very irregular, jagged, indented, or star- like appearance. The larger cells are much less liable to this change than the smaller, and the natural shape may be restored by diluting the fluid in which the corpuscles float ; by such 68 THE BLOOD. dilution the corpuscles, as already said, may be made to swell up, by absorbing the fluid ; and, if much water be added, they will become spherical and pellucid, their coloring-matter being dissolved, and, as it were, washed out of them. Some of them may thus be burst; the others are made obscure; but many of these latter may be brought into view again by evaporating, or adding saline matter to, the fluid, so as to restore it to its previous density. The changes thus produced by water are more quickly effected by weak acetic acid, which immediately makes the corpuscles pellucid, but dissolves few or none of them, for the addition of an alkali, so as to neutralize the acid, will restore their form though not their color. A peculiar property of the red corpuscles, which is exag- gerated in inflammatory blood, and which appears to exist in a marked degree in the blood of horses, may be here noticed. It gives them a great tendency to adhere together in rolls or columns, like piles of coin, and then, very quickly, these rolls fasten together by their ends, and cluster ; so that, when the blood is spread out thinly on a glass, they form a kind of ir- regular network, with crowds of corpuscles at the several points corresponding with the knots of the FIG. 25. net (Fig. 25). Hence, the clot formed in such a thin layer of blood looks mottled with blotches of pink upon a white ground ; in a larger quantity of such blood, as soon as the corpuscles have clustered and collected in rolls (that is, generally in two or three minutes after the blood is drawn), they begin to sink very quickly; for in the aggregate Red corpuscles collected into they present less surface to the re- roils (after Henie). sistance of the liquor sanguinis than they would if sinking separately. Thus quickly sinking, they leave above them a layer of liquor sanguinis, and this coagulating, forms a buffy coat, as before described, the volume of which is augmented by the white corpuscles, which have no tendency to adhere to the red ones, and by their lightness float up clear of them. Chemical Composition of Red Blood-cells. It has been before remarked, that the red blood-corpuscles are formed of a colorless stroma, infiltrated with a coloring matter termed hcemoglobin. As they exist in the blood, they contain about three-fourths of their weight of water. The stroma appears to be composed of a nitrogenous prox- BLOOD-CRYSTALS. 69 imate principle termed protagon, combined with albuminous matter (paraglobulin or fibrinoplastin), fatty matters includ- ing cholesterin, and salts, chiefly phosphates, of potash, soda, and lime. Haemoglobin, which enters far more largely into the compo- sition of the red corpuscles than any other of their constituents, is allied to albumen in some respects, but differs remarkably from it in others. One of its most marked distinctive charac- ters is its tendency under certain artificial conditions to crys- tallize ; the so-called blood-crystals being but the natural crys- talline forms assumed by this substance. Haemoglobin can be obtained in a crystalline form, with various degrees of difficulty, from the blood of different ani- mals, that of man holding an intermediate place in this re- spect. Among the animals whose blood-coloring matter crys- tallizes most readily are the guinea-pig and the dog ; and in these cases, to obtain crystals, it is generally sufficient to dilute a drop of recently drawn blood with water, and expose it for a few minutes to the air. In many instances, however, a some- what less simple process must be adopted ; as the addition of chloroform or ether, rapid freezing and then thawing, or other means which separate the coloring matter from the other con- stituents of the corpuscles. Different forms of blood-crystals are shown in the accom- panying figures. Prismatic, from human blood. Another and most important character of haemoglobin is its attraction for oxygen, and some other gases, as carbonic and 1 Figs. 26, 27, and 28, illustrate some of the principal forms of blood-crj'stals. 70 THE BLOOD. nitrous oxides, with all of which it appears to form definite chemical combinations. The combination with oxygen is that which is of most physiological inportance. During the passage of the blood through the lungs, it is constantly formed; while it is as constantly decomposed, in consequence of the readiness with which haemoglobin parts with oxygen, when the latter is FIG. 27. Tetrahedral, from blood of the guinea-pig. exposed to other attractions in its circulation through the sys- temic capillaries. Thus, the red corpuscles, in virtue of their coloring matter, which readily absorbs oxygen and as readily FIG. 28. Hexagonal crystals, from blood of squirrel. On these six-sided plates, prismatic crystals, grouped in a stellate manner, not unfrequently occur (after Funke). WHITE COEPUSCLES. 71 gives it up again, are the chief means by which oxygen is carried in the blood (see also p. 75). By heat, mineral and other acids, alkalies, &c., haemoglobin is decomposed into an albuminous matter (resembling glob- ulin) 'and hcematin. The latter, now known to be a product of the decomposition of haemoglobin, was once thought to be the natural coloring matter of the blood. The White Corpuscles of the Blood or Blood Leucocytes. The white corpuscles are much less numerous than the red. On an average, in health, there may be one white to 400 or 500 red corpuscles ; but in disease, the proportion is often as high as one to ten, and sometimes even much higher. In health, the proportion varies considerably even in the course of the same day. The variations appear to depend chiefly on the amount, and probably also on. the kind of food taken ; the number of leucocytes being very considerably in- creased by a meal, and diminished again on fasting. They present greater diversities of form than the red ones do ; but the gradations between the extreme forms are so regular, that no sufficient reason can be found for supposing that there is in healthy blood more than one species of white corpuscles. In their most general appearance they are circular and nearly spherical, about ^^ of an inch in diameter (Fig. 29). They have a grayish, pearly look, appearing variously shaded or nebulous, the shading being much darker in some than in others. They seem to be formed of protoplasm (p. 26), containing granules which are in some specimens few and very distinct, in others (though rarely) so numerous that the whole corpuscle looks like a mass of granules. These corpuscles cannot be said to have any true cell- wall. In a few instances an apparent cell-membrane can be traced around them ; but, much more commonly, even this is not discernible till after the addition of water or di- lute acetic acid, which pen- etrates the corpuscle, and lifts up and distends what looks * ed and wh * te Wood-corpuscles A, ,.V 11 11 ,1 White corpuscle of natural aspect; B, like a Cell-Wall, tO the m- Three white corpuscles acted on by weak terior of which the material, acetic acid, c, Red blood-corpuscles. FIG. 29. 72 THE BLOOD. that before appeared to form the whole corpuscle, remains attached as the nucleus of the cell (Fig. 29). A remarkable property of the white corpuscles, first observed by Mr. Wharton Jones, consists in their capability of assuming different forms, irrespective of any external influence. If a drop of blood be examined with a high microscope power under conditions by which loss of moisture is prevented, at the same time that the temperature is maintained at about the degree natural to the blood as it circulates in the living body, the leucocytes can be seen alternately contracting and dilating very slowly at various parts of their circumference shooting out irregular processes, and again withdrawing them partially or completely, and thus in succession assuming various irreg- ular forms. These movements, called amoeboid, from their resemblance to the movements exhibited by an animal called the Amoeba, the structure of which is as simple as that of a white blood-cor- puscle, are characteristic of the living leucocyte, and form a good example of the contractile property of protoplasm, before referred to. Indeed, the unchanging rounded form which the corpuscles present in specimens of blood examined in the ordinary manner under the microscope, must be looked upon as the shape natural to a dead corpuscle, or one whose vitality is dormant, rather than as the proper shape of one living and active. Besides the red and white corpuscles, the microscope reveals numerous minute molecules or granules in the blood, circular or spherical, and varying in size from the most minute visible speck to the -g^ 1 ^ of an inch (Gulliver). These molecules are very similar to those found in the lymph and chyle, and are, some of them, fatty, being soluble in ether, others prob- ably albuminous, being soluble in acetic acid. Generally, also, there may be detected in the blood, especially during the height of digestion, very minute equal-sized fatty particles, similar to those of which the molecular base of chyle is con- stituted (Gulliver). The Serum. The serum is the liquid part of the blood remaining after the coagulation of the fibrin. In the usual mode of coagula- tion, part of the serum remains soaked in the clot, and the rest, squeezed from the clot by its contraction, lies around and over it. The quantity of serum that appears around the clot "de- pends partly on the total quantity in the blood, but partly also on the degree to which the clot contracts. This is affected by many circumstances : generally, the faster the coagulation SERUM OF BLOOD. 73 the less is the amount of contraction ; and, therefore, when blood coagulates quickly, it will appear to contain a small proportion of serum. Hence, the serum always appears de- ficient in blood drawn slowly into a shallow vessel, abundant in inflammatory blood drawn into a tall vessel. In all cases, too, it should be remembered, that since the contraction of the clot may continue for thirty-six or more hours, the quantity of serum in the blood cannot be even roughly estimated till this period has elapsed. The serum is an alkaline, slimy or viscid, yellowish fluid, often presenting a slight greenish, or grayish hue, and with a specific gravity of from 1025 to 1030. It is composed of a mixture of various substances dissolved in about nine times their weight of water. It contains, indeed, the greater part of all the substances enumerated as existing in the blood, with the exception of the fibrin and the red corpuscles. Its prin- cipal constituent is albumen, of which it contains about 8 per cent., and the coagulation of which, when heated, converts nearly the whole of the serum into a solid mass. The liquid which remains uncoagulated, and which is often inclosed in little cavities in the coagulated serum, is called serosity; it con- tains, dissolved in water, fatty, extractive, and saline matters. Variations in the principal Constituents of the Liquor Sanguinis. The water of the blood is subject to hourly variations in its quantity, according to the period since the taking of food, the amount of bodily exercise, the state of the atmosphere, and all the other events that may affect either the ingestion or the excretion of fluids. According to these conditions, it may vary from 700 to 790 parts in the thousand. Yet uniformity is on the whole maintained ; because nearly all those things which tend to lower the proportion of water in the blood, such as active exercise, or the addition of saline and other solid matter, excite thirst ; while, on the other hand, the addition of an ex- cess of water to the blood is quickly followed by its more copious excretion in sweat and urine. And these means for adjusting the proportion of the water find their purpose in maintaining certain important physical conditions in the blood ; such as its proper viscidity, and the degree of its ad- hesion to the vessels through which it ought to flow with the least possible resistance from friction. On this also depends, in great measure, the activity of absorption by the bloodves- sels, into which no fluids will quickly penetrate, but such as are of less density than the blood. Again, the quantity of water in the blood determines chiefly its volume, and thereby 74 THE BLOOD. the fulness and tension of the vessels and the quantity of fluid that will exude from them to keep the tissues moist. Finally, the water is the general solvent of all the other materials of the liquor sanguinis. It is remarkable, that the proportion of water in the blood may be sometimes increased even during its abstraction from an artery or vein. Thus Dr. Zimmerman, in bleeding dogs, found the last drawn portion of blood contain 12 or 13 parts more of water in 1000 than the blood first drawn ; and Polli noticed a corresponding diminution in the specific gravity of the human blood during venesection, and suggested the only probable explanation of the fact, namely, that, during bleed- ing, the bloodvessels absorb very quickly a part of the serous fluid with which all the tissues are moistened. The albumen may vary, consistently with health, from 60 to 70 parts in the 1000 of blood. The form in which it exists in the blood is not yet certain. It may be that of simple solution as pure albumen ; but it is, more probably, in combin- ation with soda, as an albuminate of soda ; for, if serum be much diluted with water, and then neutralized with acetic acid, pure albumen is deposited. Another view entertained by En- derlin is that the albumen is dissolved in the solution of the neutral phosphate of sodium, to which he considers the alkaline reaction of the blood to be due, and solutions of which can dissolve large quantities of albumen and phosphate of lime. The proportion of fibrin in healthy blood may vary between 2 and 3 parts in 1000. In some diseases, such as typhus, and others of low type, it may be as little as 1.034; in other dis- eases, it is said, it may be increased to as much as 7.528 parts in 1000. But, in estimating the quantity of fibrin, chemists have not taken account of the white corpuscles of the blood. These cannot, by any mode of analysis yet invented, be sepa- rated from the fibrin of mammalian blood : their composition is unknown, but their weight is always included in the estimate of the fibrin. In health they may, perhaps, add too little to its weight to merit consideration ; but in many diseases, espe- cially in inflammatory and other blood diseases in which the fibrin is said to be increased, these corpuscles become so numer- ous that a large proportion of the supposed increase of the fibrin must be due to their being weighed with it. On this account all the statements respecting the increase of fibrin in certain diseases need revision. The enumeration of the fatty matters of the blood makes it probable that most of those which are found in the tissues or secretions exist also ready-formed in the blood; for it contains the cholesterin of the bile, the cerebrin and phosphorized fat FATTY MATTERS IN THE BLOOD. 75 of the brain, and the ordinary saponifiable fats, stearin, olein, and palmitin. A volatile fatty acid is that on which the odor of the blood mainly depends; and it is supposed that when sulphuric acid is added (see p. 56), it evolves the odor by com- bining with the base, with which, naturally, this acid is neu- tralized. According to Lehmann, much of the fatty matter of the blood is accumulated in the red corpuscles. These fatty matters are subject to much variation in quan- tity, being commonly increased after every meal in which fat, or starch, or saccharine substances have been taken. At such times, the fatty particles of the chyle, added quickly to the blood, are only gradually assimilated ; and their quantity may be sufficient to make the serum of the blood opaque, or even milk-like. As regards the inorganic constituents of the blood the sub- stances which remain as ashes after its complete burning one may observe in general their small quantity in proportion to that of the animal matter contained in it. Those among them of peculiar interest are the phosphate and carbonate of sodium, and the phosphate of calcium. It appears most probable that the blood owes its alkaline reaction to both these salts of sodium. The existence of the neutral phosphate (Na 2 H,PO 4 ) was proved by Enderlin: the presence of carbonate of sodium has been proved by Lehmann and others. In illustration of the characters which the blood may derive from the phosphate of sodium, Liebig points out the large ca- pacity which solutions of that salt have of absorbing carbonic acid gas, and then very readily giving it off again when agitated in atmospheric air, and when the atmospheric pressure is di- minished. It is probably, also, by means of this salt, that the phosphate of calcium is held in solution in the blood in a form in which it is not soluble in water, or in a solution of albu- men. Of the remaining inorganic constituents of the blood, the oxide and phosphate of iron referred to, exist in the liquor sanguinis, independently of the iron in the corpuscles. Schmidt's investigations have shown that the inorganic con- stituents of the blood-cells somewhat differ from those con- tained in the serum ; the former possessing a considerable pre- ponderance of phosphates and of the salts of potassium, while the chlorides, especially of sodium, with phosphate of sodium, are particularly abundant in the latter. Among the extractive matters of the blood, the most note- worthy are Oreatin and Creatinin. Besides these, other or- ganic principles have been found either constantly or gen- erally in the blood, including casein, especially in women during lactation : glucose, or grape-sugar, found in the blood 76 THE BLOOD. of the hepatic vein, but disappearing during its transit through the lungs (Bernard) ; urea, and in very minute quantities, uric add (Gar rod); hippuric and lactic acids; ammonia (Rich- ardson); and, lastly, certain coloring and odoriferous matters. Variations in healthy Blood under different Circumstances. As the general condition of the body depends so much on the condition of the blood, and as, on the other hand, any- thing that affects the body must sooner or later, and to a greater or less degree, affect the blood also, it might be ex- pected that considerable variations in the qualities of this fluid would be found under different circumstances of disease ; and such is found to be the case. Even in health, however, the general composition of the blood varies considerably. The conditions which appear most to influence the compo- sition of the blood in health, are these : sex, pregnancy, age, and temperament. The composition of the blood is also, of course, much influenced by diet. 1. Sex. The blood of men differs from that of women, chiefly in being of somewhat higher specific gravity, from its containing a relatively larger quantity of red corpuscles. 2. Pregnancy. The blood of pregnant women has a rather lower specific gravity than the average, from deficiency of red corpuscles. The quantity of white corpuscles, on the other hand, and of fibrin, is increased. 3. Age. From the analysis of Denis it appears that the blood of the foetus is very rich in solid matter, and especially in red corpuscles; and this condition, gradually diminishing, continues for some weeks after birth. The quantity of solid matter then falls during childhood below the average, again rises during adult life, and in old age falls again. 4. Temperament. But little more is known concerning the connection of this with the condition of the blood, than that there appears to be a relatively larger quantity of solid matter, and particularly of red corpuscles, in those of a plethoric or sanguineous temperament. 5. Diet. Such differences in the composition of the blood as are due to the temporary presence of various matters ab- sorbed with the food and drink, as well as the more lasting changes which must result from generous or poor diet respect- ively, need be here only referred to. Effects of Bleeding. The result of bleeding is to diminish the specific gravity of the blood ; and so quickly, that in a single venesection, the portion of blood last draw r n has often a less specific gravity than that of the blood that flowed first VAEIATIONS IN COMPOSITION. 77 (J. Davy and Polli). This is, of course, due to absorption of fluid from the tissues of the body. The physiological import of this fact, namely, the instant absorption of liquid from the tissues, is the same as that of the intense thirst which is so common after either loss of blood, or the abstraction from it of watery fluid, as in cholera, diabetes, and the like. For some little time after bleeding, the want of red blood- cells is well marked ; but with this exception, no considerable alteration seems to be produced in the composition of the blood for more than a very short time, the loss of the other constitu- ents, including the pale corpuscles, being v.ery quickly repaired. Variations in the Composition of the Blood, in different Parts of the Body. The composition of the blood, as might be expected, is found to vary in different parts of the body. Thus, arterial blood differs from venous ; and although its composition and general characters are uniform throughout the whole course of the systemic arteries, they are not so throughout the venous sys- tem the blood contained in some veins differing remarkably from that in others. 1. Differences between Arterial and Venous Blood. These maybe arranged under two heads, differences in color, and in general composition. a. Color. Concerning the cause of the difference in color between arterial and venous blood, there has been much doubt, not to say confusion. For while the scarlet color of the ar- terial blood has been supposed by so me observers, and for some reasons, to be due to the chemical action of oxygen, and the purple tint of that in the veins to the action of carbonic acid, there are facts which made it seem probable that the cause was a mechanical one rather than a chemical, and that it de- pended on a difference in the shape of the red corpuscles, by which their power of transmitting and reflecting light was al- tered. Thus, carbonic acid was thought to make the blood dark by causing the red cells to assume a biconvex outline, and oxygen was supposed to reverse the effect by contracting them and rendering them biconcave. We may believe, how- ever, that, at least for the present, this vexed question has, by the results of investigations undertaken by Professor Stokes and others, been now set at rest. The coloring matter of the blood, or haemoglobin (p. 69), is capable of existing in two different states of oxidation, and the respective colors of arterial and venous blood are caused by differences in tint between these two varieties oxidized or scar- 78 THE BLOOD. let haemoglobin and deoxidized or purple haemoglobin. The change of color produced by the passage of the blood through the lungs, and its consequent exposure to oxygen, is due, prob- ably, to the oxidation of purple, and its conversion into scarlet haemoglobin; while the readiness with which the latter is de- oxidized offers a reasonable explanation of the change, in re- gard to tint, of arterial into venous blood, the transformation being effected by the delivering up of oxygen to the tissues, by the scarlet haemoglobin, during the blood's passage through the capillaries. The changes of color are more probably due to this cause, namely, a varying quantity of oxygen chemically combined with the haemoglobin, than to any mechanical effect of this gas, or to the influence of carbonic acid, either chemi- cally, on the coloring matter, or mechanically, on the corpuscles which contain it. We are not, perhaps, in a position to deny altogether the possible influence of mechanical conditions of the red corpuscles on the color of arterial and venous blood respectively ; but it is probable that this cause alone would be quite insufficient to explain the differences in the color of the two kinds of blood, and therefore if it be an element at all in the change, it must be allowed to take only a subordinate position. The distinction between the two kinds of haemoglobin nat- urally present in the blood, or in other words, the proof that the addition or subtraction of oxygen involves the production respectively of two substances having fundamental differences of chemical constitution, has been made out chiefly by spectrum- analysis, the effects produced by placing oxidized and de- oxidized solutions of haemoglobin in the path of a ray of light traversing a spectroscope being different. For while the oxi- dized solution causes the appearance of two absorption bands in the yellow and the green part of the spectrum, these are re- placed by a single band intermediate in position, when the ox- idized or scarlet solution is darkened by deoxidizing agencies, or, in other words, when the change which naturally ensues in the conversion of arterial into venous blood is artificially pro- duced. 1 The greater part of the haemoglobin in both arterial and venous blood probably exists in the scarlet or more highly ox- idized condition, and only a small part is deoxidized and made purple in its passage from the arteries into the veins. The differences in regard to color between arterial and 1 The student to whom the terms employed in connection with spectrum analysis are not familiar, is advised to consult, with ref- erence to the preceding paragraph, an elementary treatise on Physics. BLOOD OF PORTAL VEIN. 79 venous blood are sometimes not to be observed. If blood runs very slowly from an artery, as from the bottom of a deep and devious wound, it is often as dark as venous blood. In persons nearly asphyxiated also, and sometimes, under the influence of chloroform or ether, the arterial blood becomes like the venous. In the foetus also both kinds of blood are dark. But, in all these cases, the dark blood becomes bright on exposure to the air. Bernard has shown that venous blood returning from a gland in active secretion is almost as bright as arterial blood. b. General Composition. The chief differences between ar- terial and ordinary venous blood are these. Arterial blood contains rather more fibrin, and rather less albumen and fat. It coagulates somewhat more quickly. Also, it contains more oxygen, and less carbonic acid. According to Denis, the fibrin of venous blood differs from arterial, in that when it is fresh and has not been much exposed to the air, it may be dissolved in a slightly heated solution of nitrate of potassium. Some of the veins, however, contain blood which differs from the ordinary standard considerably. These are the portal, the hepatic, and the splenic veins. Portal Vein. The blood which the portal vein conveys to the liver is supplied from two chief sources ; namely, that in the gastric and mesenteric veins, which contains the soluble elements of food absorbed from the stomach and intestines during digestion, and that in the splenic vein ; it must, there- fore, combine the qualities of the blood from each of these sources. The blood in the gastric and mesenteric veins will vary much according to the stage of digestion and the nature of the food taken, and can therefore be seldom exactly the same. Speaking generally, and without considering the sugar, dex- trin, and other soluble matters which may have been absorbed from the alimentary canal, this blood appears to be deficient in solid matters, especially in red corpuscles, owing to dilution by the quantity of water absorbed, to contain an excess of al- bumen, though chiefly of a lower kind than usual, resulting from the digestion of nitrogenized substances, and termed al- buminose, and to yield a less tenacious kind of fibrin than that of blood generally. The blood from the splenic vein is probably more definite in composition, though also liable to alterations according to the stage of the digestive process, and other circumstances. It seems generally to be deficient in red corpuscles, and to con- tain an unusually large proportion of albumen. The fibrin seems to vary in relative amount, but to be almost always above the average. The proportion of colorless corpuscles ap- 80 THE BLOOD. pears also to be unusually large. The whole quantity of solid matter is decreased, the diminution appearing to be chiefly in the proportion of red corpuscles. The blood of the portal vein, combining the peculiarities of its two factors, the splenic and mesenteric venous blood, is usually of lower specific gravity than blood generally, is more watery, contains fewer red corpuscles, more albumen, chiefly in the form of alburninose, and yields a less firm clot than that yielded by other blood, owing to the deficient tenacity of its fibrin. These characteristics of portal blood refer to the com- position of the blood itself, and have no reference to the ex- traneous substances, such as the absorbed materials of the food, which it may contain ; neither, indeed, has any complete analysis of these been given. Comparative analyses of blood in the portal vein and blood in the hepatic veins have also been frequently made, with the view of determining the changes which this fluid undergoes in its transit through the liver. Great diversity, however, is ob- servable in the analyses of these two kinds of blood by dif- ferent chemists. Part of this diversity is no doubt attributable to the fact pointed out by Bernard, that unless the portal vein is tied before the liver is removed from the body, hepatic venous blood is very liable to regurgitate into the portal vein, and thus vitiate the result of the analysis. Guarding against this source of error, recent observers seemed to have deter- mined that hepatic venous blood contains less water, albumen, and salts, than the blood of the portal vein ; but that it yields a much larger amount of extractive matter, in which, accord- ing to Bernard and others, is one constant element, namely, grape-sugar, which is found, whether saccharine or farinaceous matter have been present in the food or not. Besides the rather wide difference between the composition of the blood of these veins and of others, it must not be for- gotten that in its passage through every organ and tissue of the body, the blood's composition must be varying constantly, as each part takes from it or adds to it such matter as it, roughly speaking, wishes either to have or to throw away. Thus the blood of the renal vein has been proved by experi- ment to contain less water than does the blood of the artery, and doubtless its salts are diminished also. The blood in the renal vein is said, moreover, by Bernard and Brown-Sequard not to coagulate. This then is an example of the change produced in the blood by its passage through a special excretory organ. But all parts of the body, bones, muscles, nerves, &c., must act on the blood as it passes through them, and leave in it some mark DEVELOPMENT OF BLOOD. 81 of their action, too slight though it may be, at any given mo- ment, for analysis by means now at our disposal. On the Gases contained in the Blood. The gases contained in the blood are carbonic acid, oxygen, and nitrogen, 100 volumes of blood containing from 40 to 50 volumes of these gases collectively. Arterial blood contains relatively more oxygen and less carbonic acid than venous. But the absolute quantity of car- bonic acid is in both kinds of blood greater than that of the oxygen. The proportion of nitrogen is in both very small. It is most probable that the carbonic acid of the blood is partly in a state of simple solution, and partly in a state of weak chemical combination. That portion of the carbonic acid which is chemically combined, is contained partly in a bicarbonate of soda, and partly is united with phosphate of the same base. The oxygen is combined chemically with the haemoglobin of the red corpuscles (pp. 69 and 77). That the oxygen is absorbed chiefly by the red corpuscles is proved by the fact that while blood is capable of absorbing oxygen in considerable quantity, the serum alone has little or no more power of absorbing this gas than pure water. Development of the Blood. In the development of the blood little more can be traced than the processes by which the corpuscles are formed. The first formed blood-cells of the human embryo differ much in their general characters from those which belong to the latter periods of intra-uterine, and to all periods of extra- uterine life. Their manner of origin differs also, and it will be well perhaps to consider this first. In the process of development of the embryo, the plan, so to speak, of the heart and chief bloodvessels is first laid out in cells. Thus the heart is at first but a solid mass of cells, resembling those which constitute all other parts of the em- bryo ; and continuous with this are tracts of similar cells the rudiments of the chief bloodvessels. The formation of the first blood-corpuscles is very simple. While the outermost of the embryonic cells, of which the ru- dimentary heart and its attendant vessels are composed, gradu- ally develop into the muscular and other tissues which form the walls of the heart and bloodvessels, the inner cells simply separate from each other, and form blood-cells ; some fluid plasma being at the same time secreted. Thus, by the same 82 DEVELOPMENT OF BLOOD. process, blood is formed, and the originally solid heart and bloodvessels are hollowed out. The blood-cells produced in this way, are from about ^-^ to y-g-ftfl of an inch in diameter, mostly spherical, pellucid, and colorless, with granular contents, and a well-marked nucleus. Gradually, they acquire a red color, at the same time that the nucleus becomes more defined, and the granular matter clears away. Mr. Paget describes them as, at this period, circular, thickly disk-shaped, full-colored, and, on an average, about 2~5"0 f an i ncn m diameter ; their nuclei, which are about WOTT f an i ncn m diameter, are central, circular, very little prominent on the surfaces of the cell, and apparently slightly granular or tuberculated. Before the occurrence, however, of this change from the colorless to the colored state in many instances, probably, during it, and in many afterwards, a process of multiplication takes place by division of the nucleus and subsequently of the cell, into two, and much more rarely, three or four new cells, which gradually acquire the characters of the original cell from which they sprang. Fig. 30 (B, c, D, E). FIG. 30. D E ^^^ F Development of the first setof blood-corpuscles in the mammalian embryo. A. A dotted, nucleated embryo-cell in process of conversion into a blood-corpuscle: the nucleus provided with a nucleolus. B. A similar cell with a dividing nucleus ; at c, the division of the nucleus is complete ; at D, the cell also is dividing. E. A blood- corpuscle almost complete, but still containing a few granules. F. Perfect blood- corpuscle. When, in the progress of embryonic development, the liver begins to be formed, the multiplication of blood-cells in the whole mass of blood ceases, according to Kolliker, and new blood-cells are produced by this organ. Like those just de- scribed, they are at first colorless and nucleated, but afterwards acquire the ordinary blood tinge, and resemble very much those of the first set. Like them they may also multiply by DEVELOPMENT OF BLOOD; S3 division. In whichever way produced, however, whether from the original formative cells of the embryo, or by the liver, these colored nucleated cells begin very early in foetal life to be mingled with colored non-nucleated corpuscles resembling those of the adult, and about the fourth or fifth month of embry- onic existence are completely replaced by them. The manner of origin of these perfect non-nucleated cor- puscles must be now considered. I. Concerning the Cells from which they arise. a. Before Birth. It is uncertain whether they are derived only from the cells of the lymph, which, at about the period of their appearance, begins to be poured into the blood ; or whether they are derived also from the nucleated red cells, which they replace, or also from similar nucleated cells, which Kolliker thinks are produced by the liver during the whole time of fcetal existence. b. After Birth. It is generally agreed that after birth the red corpuscles are derived from the smaller of the nucleated lymph or chyle-corpuscles, the white corpuscles of the blood. II. Concerning the Manner of their Development. There is not perfect agreement among physiologists concern- ing the process by which lymph-globules or white corpuscles (and in the foetus, perhaps the red nucleated cells) are trans- formed into red non-nucleated blood-cells. For while some maintain that the whole cell is changed into a red one by the gradual clearing up of the contents, including the nucleus, it is believed by Mr. Wharton Jones and many others, that only the nucleus becomes the red blood-cell, by escaping from its envelope and acquiring the ordinary blood-tint. Of these two theories, that which supposes the nucleus of the lymph or chyle globule to be the germ of the future red blood- corpuscle is the theory now generally adopted. The development of red blood-cells from the corpuscles of the lymph and chyle continues throughout life, and there is no reason for supposing that after birth they have any other origin. Without doubt, these little bodies have, like all other parts of the organism, a tolerably definite term of existence, and in a like manner die and waste away when the portion of work allotted to them has been performed. Neither the length of their life, however, nor the fashion of their decay, has been yet clearly made out, and we can only surmise that in these things 84 DEVELOPMENT OF BLOOD. they resemble more or less closely those parts of the body which lie more plainly within our observation. From what has been said, it will have appeared that when the blood is once formed, its growth and maintenance are ef- fected by the constant repetition of the development of new portions. In the same proportion that the blood yields its materials for the maintenance and repair of the several solid tissues, and for secretions, so are new materials supplied to it in the lymph and chyle, and by development made like it. The part of the process which relates to the formation of new corpuscles has been described, but it is probably only a small portion of the whole process ; for the assimilation of the new materials to the blood must be perfect, in regard to all those immeasurable minute particulars by which the blood is adapted for the nutrition of every tissue, and the maintenance of every peculiarity of each. How precise the assimilation must be for such an adaptation, may be conceived from some of the cases in which the blood is altered by disease, and by assimilation is maintained in its altered state. For example, by the inser- tion of vaccine matter, the blood is for a short time manifestly diseased ; however minute the portion of virus, it affects and alters, in some way, the whole of the blood. And the alteration thus produced, inconceivably slight as it must be, is long main- tained ; for even very long after a successful vaccination, a second insertion of the virus may have no effect, the blood being no longer amenable to its influence, because the new blood, formed after the vaccination, is made like the blood as altered by the vaccine virus ; in other words, the blood exactly assimilates to its altered self the materials derived from the lymph and chyle. In health we cannot see the precision of the adjustment of the blood to the tissues ; but we may imagine it from the small influences by which, as in vaccination, it is disturbed ; and we may be sure that the new blood is as per- fectly assimilated to the healthy standard as in disease it is as- similated to the most minutely altered standard. 1 How far the assimilation of the blood is affected by any for- mative power which it may possess in common with the solid tissues, we know not. That this possible formative power is, however, if present, greatly ministered to and assisted by the actions of other parts there can be no doubt; as 1st, by the di- gestive and absorbent systems, and probably by the liver, and all of the so-called vascular glands ; and, 2dly, by the excre- tory organs, which separate from the blood refuse materials, 1 Corresponding facts in relation to the maintenance of the tissues by assimilation will be mentioned in the chapter on NUTRITION. USES OF THE BLOOD. 85 including in this term not only the waste substance of the tissues, but also such matters as, having been taken with food arid drink, may have been absorbed from the digestive canal, and have been subsequently found unfit to remain in the cir- culating current. And, 3dly, the precise constitution of the blood is adjusted by the balance of the nutritive processes for maintaining the several tissues^so that none of the materials appropriate for the maintenance of any part may remain in excess in the blood. Each part, by taking from the blood the materials it requires for its maintenance, is, as has been ob- served, in the relation of an excretory organ to all the rest; inasmuch as by abstracting the matters proper for its nutrition, it prevents excess of such matters as effectually as if they were separated from the blood and cast out altogether by the ex- creting organs specially present for such a purpose. Uses of the Blood. The purposes of the blood, thus developed and maintained, appear, in the perfect state, to be these : 1st, to be a source whence the various parts of the body may abstract the ma- terials necessary for their nutrition and maintenance ; and whence the secreting organs may take the materials for their various secretions ; 2d, to be a constantly replenished store- house of latent chemical force, which in its expenditure will maintain the heat of the body, or be transformed by the living tissues, and manifested by them in various forms as vital power ; 3d, to convey oxygen to the several tissues which may need it, either for the discharge of their functions, or for combination with their refuse matter ; 4th, to bring from all parts refuse matters, and convey them to places whence they may be dis- charged ; 5th, to warm and moisten all parts of the body. Uses of the various Constituents of the Blood. Regarding the uses of the various constituents of the blood, it may be said that the matter almost resolves itself into an analysis of the different parts of the body, and of the food and drink which are taken for their nutrition, with a subsequent consideration of how far any given constituent of the blood may be supposed to be on its way to the living tissues, to be incorporated with and nourish them ; or, having fulfilled its purpose, to be on its way, in a more or less changed condition, to the excretory organs to be cast out. It must be remem- bered, however, that the blood contains also matters which serve by their combustion to produce heat, and, again, others which possibly subserve pnly a mechanical, although most im- 86 USES OF THE BLOOD. portant purpose ; as, for instance, the preservation of the due specific gravity of the blood, or some other quality by which it is enabled to maintain its proper relation to the vessels con- taining it, and to the tissues through which it passes. Lastly, among the constituents of the blood, are the gases, oxygen, and carbonic acid, and the substances specially adapted to carry them, which can scarcely be aid to take part in the nutrition of the body, but are rather the means and evidence of the combustion before referred to, on which, to a great extent, directly or indirectly, all vitality depends. Albumen. The albumen, which exists in so large a propor- tion among the chief constituents of the blood, is without doubt mainly for the nourishment of those textures which contain it or other compounds nearly allied to it. Besides its purpose in nutrition, the albumen of the liquor sanguinis is doubtless of importance also in the maintenance of those essen- tial physical properties of the blood to which reference has been already made. Fibrin. It has been mentioned in a previous part of this chapter, that the idea of fibrin existing in the blood, as fibrin, is probably founded in error ; and that it is formed, in the act of coagulation, by the union of two substances, which before existed separately (p. 61). In considering, therefore, the func- tions of fibrin, we may exclude the notion of its existence, as such, in the blood, in a fluid state, and of its use in the nutri- tion of certain special textures, and look for the explanation of its functions to those circumstances, whether of health or disease, under which it is produced. In hemorrhage, for ex- ample, the formation of fibrin in the clotting of blood, is the means by which, at least for a time, the bleeding is restrained or stopped ; and the material which is produced for the per- manent healing of the injured part, contains a coagulable ma- terial probably identical, or very nearly so, with the fibrin of clotted blood. Fatty Matters. The fatty matters of the blood subserve more than one purpose. For while they are the means, at least in part, by which the fat of the body, so widely distributed in the proper adipose and other textures, is replenished, they also, by their union with oxygen, assist in maintaining the temperature of the body. In certain secretions also, notably the milk and bile, fat is an important constituent. Saline Matter, The uses of the- saline constituents of the blood are, first, to enter into the composition of such textures and secretions as naturally contain them, and, secondly, to assist in preserving the due specific gravity and alkalinity of the blood and, perhaps, also in preventing its decomposition. USES OF THE BLOOD. 87 The phosphate and carbonate of sodium, besides maintaining the alkalinity of the blood, are said especially to preserve the liquidity of its albumen, and to favor its circulation through the capillaries, at the same time that they increase the absorp- tive power of the serum for gases. But although, from the constant presence of a certain quantity of saline matter in the blood, we may believe that it has these last-mentioned impor- tant functions in connection with the blood itself, apart from the nutrition of the body, yet, from the amount which is daily separated by the different excretory organs, and especially by the kidneys, w r e must also believe that a considerable quantity simply passes through the blood, both from the food and from the tissues, as a temporary and useless constituent, to be ex- creted when opportunity offers. Corpuscles. The uses of the red corpuscles are probably not yet fully known, but they may be inferred, at least in part, from the composition and properties of their contents. The affinity of haemoglobin for oxygen has been already mentioned ; and the main function of the red corpuscles seems to be the absorption of oxygen in the lungs by means of this constitu- ent, and its conveyance to all parts of the body, especially to those tissues, the nervous and muscular, the discharge of whose functions depends in so great a degree upon a rapid and full supply of this element. The readiness with which haemoglo- bin absorbs oxygen, and delivers it up again to a reducing agent, so well shown by the experiments of Prof. Stokes, ad- mirably adapts it for this purpose. How far the red corpus- cles are concerned in the nutrition of the tissues is quite un- known. The relation of the white to the red corpuscles of the blood has been already considered (p. 83); of the functions of the former, other than are concerned in this relationship, nothing is positively known. Recent observations of the migration of the white corpuscles from the interior of the bloodvessels into the surrounding tissues (see section, On the Circulation in the Capillaries) have, however, opened out a large field for inves- tigation of their probable functions in connection with the nu- trition of the textures, in which, even in health, they appear to wander. 88 THE CIRCULATION. CHAPTER VI. CIRCULATION OF THE BLOOD. THE body is divided into two chief cavities the chest or thorax and abdomen, by a curved muscular partition, called the diaphragm (Fig. 31). The chest is almost entirely filled by the lungs and heart ; the latter being fitted in, so to speak, between the two lungs, nearer the front than the back of the chest, and partly overlapped by them (Fig. 31). Each of these organs is contained in a distinct bag, called respectively the right and left pleura and the pericardium, the latter being fibrous in the main, but lined on the inner aspect by a smooth shining epithelial covering, on which can glide, with but little friction, the equally smooth surface of the heart enveloped by it. In Fig. 31 the containing bags of pleura and pericardium are supposed to have been removed. Entering the chest from above is a large and long air-tube, called the trachea, which divides into two branches, one for each lung, and through which air passes and repasses in respiration. Springing from the upper part or base of the heart may be seen the large ves- sels, arteries, and veins, which convey blood either to or from this organ. In the living body the heart and lungs are in constant rhythmic movement, the result of which is an unceasing stream of air through the trachea alternately into and out of the lungs, and an unceasing stream of blood into and out of the heart. It is with this last event that we are concerned especially in this chapter, with the means, that is to say, by which the blood which at one moment is forced out of the heart, is in a few moments more returned to it, again to depart, and again pass through the body in course of what is technically called the circulation. The purposes for which this unceasing cur- rent is maintained, are indicated in the uses of the blood enu- merated in the preceding chapter. The blood is conveyed away from the heart by the arteries, and returned to it by the veins; the arteries and veins being continuous with each* other, at one end by means of the heart, and at the other by a fine network of vessels called the capil- laries. The blood, therefore, in its passage from the heart THE CIRCULATION. 89 passes first into the arteries, then into the capillaries, and lastly into the veins, by which it is conveyed back again to the heart, thus completing a revolution, or circulation. FIG. 31. Lnrynx. Trachea. Aorta. - Pnlmonar. Artery. Diaphragm. Heart. View of heart and lungs in situ. The froct portion of the chest-wall, and the outer or parietal layers of the pleurae and pericardium, have been removed. The lungs are partly collapsed. As generally described there are two circulations by which all the blood must pass ; the one, a shorter circuit from the heart to the lungs and back again ; the other and larger cir- cuit, from the heart to all parts of the body and back again ; but more strictly speaking, there is only one complete circula- tion, which may be diagrammatically represented by a double loop, as in Fig. 32. On reference to this figure and noticing the direction of the arrows which represent the course of the stream of blood, it will be observed that while there is a smaller and a larger circle, both of which pass through the heart, yet that these are not distinct, one from the other, but are formed really by one continuous stream, the whole of which must, at one part of its course, pass through the lungs. Subordinate to the two prin- cipal circulations, the pulmonary and systemic, as they are 90 THE CIRCULATION. named, it will be noticed also in the same figure, that there is another, by which a portion of the stream of blood having been diverted once into the capillaries of the intestinal canal, and some other organs, and gathered up again into a single stream, is a second time divided in its passage through the FIG. 32. Diagram of the circulation. liver, before it finally reaches the heart and completes a revo- lution. This subordinate stream through the liver is called the portal circulation. The principal force provided for constantly moving the blood through this course is that of the muscular substance of the heart ; other assistant forces are, (2) those of the elastic walls of the arteries, (3) the pressure of the muscles among which some of the veins run, (4) the movements of the walls of the chest in respiration, and probably, to some extent, (5) THE HEART. 91 the interchange of relations between the blood and the tissues which ensues in the capillary system during the nutritive pro- cesses. The right direction of the blood's course is determined and maintained by the valves of the heart to be immediately described ; which valves open to permit the movement of the blood in the course described, but close when any force tends to move it in the contrary direction. We shall consider separately each member of the system of organs for the circulation : and first The Heart. The heart is a hollow muscular organ, the interior of which is divided by a partition in such a manner as to form two chief chambers or cavities right and left. Each of these chambers is again subdivided into an upper and a lower por- tion called respectively the auricle and ventricle, which freely communicate one with the other ; the aperture of communica- tion, however, being guarded by valvular curtains, so disposed as to allow blood to pass freely from the auricle into the ven- tricle, but not in the opposite direction. There are thus four cavities altogether in the heart two auricles and two ventri- cles ; the auricle and ventricle of one side being quite sepa- rate from those of the other. The right auricle communicates, on the one hand, with the veins of the general system, and, on the other, with the right ventricle, while the latter leads directly into the pulmonary artery, the orifice of which is guarded by valves. The left auricle again communicates, on the one hand, with the pulmonary veins, and, on the other, with the left ventricle, while the latter leads directly into the aorta a large artery which conveys blood to the general sys- tem, the orifice of which, like that of the pulmonary artery, is guarded by valves. The arrangement of the heart's valves is such that the blood can pass only in one definite direction, and this is as follows (Fig. 33): From the right auricle the blood passes into the right ventricle, and thence into the pulmonary artery, by which it is conveyed to the capillaries of the lungs. From the lungs the blood, which is now purified and altered in color, is ga- thered by the pulmonary veins and taken to the left auricle. From the left auricle it passes into the left ventricle, and thence into the aorta, by which it is distributed to the capil- laries of every portion of the body. The branches of the aorta, from being distributed to the general system, are called sys- temic arteries ; and from these the blood passes into the systemic 92 THE CIRCULATION. capillaries, where it again becomes dark and impure, and thence into the branches of the systemic veins, which, forming by their union two large trunks, called the superior and in- ferior vena cava, discharge their contents into the right auricle, whence we supposed the blood to start (Fig. 33). FIG. 33. Diagram of the circulation through the heart (after Dalton). a, a. Vena cava, su- I erior and inferior, b. Right ventricle, c. Pulmonary artery, d. Pulmonary vein. e. Left ventricle. /. Aorta. Structure of the Valves of the Heart. It will be well now to consider the structure of the valves of the heart, and the manner in which they perform their func- tion of directing the stream of blood in the course which has been just described. The valve between the right auricle and ventricle is named tricuspid (Fig. 34), because it presents three principal cusps or pointed portions, and that between the left auricle and ventricle bicuspid or mitral, because it has two such portions (Fig. 35). But in both valves there is between each two principal portions a smaller one; so that more properly, the tricuspid may be described as consisting of six, and the STRUCTURE OF HEARTHS VALVES. 93 mitral of four, portions. Each portion is of triangular form, its apex and sides lying free in the cavity of the ventricle, and its base, which is continuous with the bases of the neighboring FIG. 34. The right auricle and ventricle opened, and a part of their right and anterior walls removed, so as to show their interior. %. 1, superior vena cava; 2, inferior vena cava ; 2', hepatic veins cut short ; 3, right auricle ; 3', placed in the fossa ovalis, below which is the Eustachian valve ; 3", is placed close to the aperture of the coronary vein; +, +, placed in the auriculo-ventricular groove, where a narrow portion of the adjacent walls of the auricle and ventricle has been preserved ; 4, 4, cavity of the right ventricle, the upper figure is immediately below the semilunar valves; 4', large columna carnea or musculus papillaris ; 5, 5', 5", tricuspid valve ; 6, placed in the interior of the pulmonary artery, a part of the anterior wall of that vessel having been removed, and a narrow portion of it preserved at its commencement where the semilunar valves are attached ; 7, concavity of the aortic arch close to the cord of the ductus arteriosus ; 8, ascending part or sinus of the arch covered at its com- mencement by the auricular appendix and pulmonary artery ; 9, placed between the innominate and left carotid arteries , 10, appendix of the left auricle ; 11, 11, the out- side of the left ventricle, the lower figure near the apex. (From Quain's Anatmy.) 94 THE CIRCULATION. portions, so as to form an annular membrane around the auric- ulo- ventricular opening, being fixed to a tendinous ring, which encircles the orifice between the auricle and ventricle, and receives the insertions of the muscular fibres of both. In each principal portion of the valve may be distinguished a middle- piece, extending from its base to its apex, and including about half its width; this piece is thicker, and much tougher and tighter than the border-pieces, which are attached loose and flapping at its sides. While the bases of the several portions of the valves are fixed to the tendinous rings, their ventricular surfaces and borders are fastened by slender tendinous fibres, the chorda tendinece, to the walls of the ventricles, the muscular fibres of which project into the ventricular cavity in the form of bun- dles or columns the columnce carnece. These columns are not all of them alike, for while some of them are attached along their whole length on one side, and by their extremities, others are attached only by their extremities ; and a third set, to which the name musculi papillares has been given, are at- tached to the wall of the ventricle by one extremity only, the other projecting, papilla-like, into the cavity of the ventricle (5, Fig. 35), and having attached to it chordce tendinece. Of the tendinous cords, besides those which pass from the walls of the ventricle and the musculi papillares, to the margins of the valves both free and attached, there are some of especial strength, which pass from the same parts to the edges of the mid- dle pieces of the several chief portions of the valve. The ends of these cords are spread out in the substance of the valve, giving its middle-piece its peculiar strength and toughness ; and from the sides numerous other more slender and branching cords are given off, which are attached all over the ventricular sur- face of the adjacent border-pieces of the principal portions of the valves, as well as to those smaller portions which have been mentioned as lying between each two principal ones. Moreover, the musculi papillares are so placed that from the summit of each tendinous cords may proceed to the adjacent halves of two of the principal divisions, and to one interme- diate or smaller division, of the valve. It has been already said that while the ventricles communi- cate, on the one hand, with the auricles, they communicate, on the other, with the large arteries which convey the blood away from the heart ; the right ventricle with the pulmonary artery (6, Fig. 34), which conveys blood to the lungs, and the left ventricle with the aorta, which distributes it to the general system (7, Fig. 35). And as the auriculo-ventricular orifice STRUCTURE OF HEARTHS VALVES. 95 is guarded by valves, so are also the mouths of the pulmonary artery and aorta (Figs. 34, 35). FIG. 35. The left auricle and ventricle opened and a part of their anterior and left walls removed so as to show their interior. %. The pulmonary artery has been divided at its commencement so as to show the aorta ; the opening into the left ventricle has been carried a short distance into the aorta between two of the segments of the semilunar valves ; the left part of the auricle with its appendix has been removed. The right auricle has been thrown out of view. 1, the two right pulmonary veins cut short ; their openings are seen within the auricle ; 1', placed within the cavity of the auricle on the left side of the septum and on the part which forms the re- mains of the valve of the foramen ovale, of which the crescentic fold is seen towards the left hand of 1' ; 2, a narrow portion of the wall of the auricle and ventricle pre- served round the auriculo-ventricular orifice; 3, 3', the cut surface of the walls of the ventricle, seen to become very much thinner towards 3", at the apex ; 4, a small part of the anterior wall of the left ventricle which has been preserved with the 96 THE CIRCULATION. The valves, three in number, which guard the orifice of each of these two arteries, are called the semilunar valves. They are nearly alike on both sides of the heart ; but those of the aorta are altogether thicker and more strongly constructed than those of the pulmonary artery. Like the tricuspid and mitral valves, they are formed by a duplicature of the lining membrane of the heart, strengthened by fibrous tissue. Each valve is of semilunar shape, its convex margin being attached to a fibrous ring at the place of junction of the artery to the ventricle, and the concave or nearly straight border being free (Fig. 35). In the centre of the free edge of the valve, which contains a fine cord of fibrous tissue, is a small fibrous nodule, the corpus Arantii, and from this and from the attached bor- der, fine fibres extend into every part of the mid substance of the valve, except a small lunated space just within the free edge, on each side of the corpus Arantii. Here the valve is thinnest, and composed of little more than the endocardium. Thus constructed and attached, the three semilunar valves are placed side by side around the arterial orifice of each ventricle, so as to form three little pouches, which can be thrown back and flattened by the blood passing out of the ventricle, but which belly out immediately so as to prevent any return (6, Fig. 34). This will be again referred to immediately. The muscular fibres of the heart, unlike those of most in- voluntary muscles, present a striated appearance under the microscope. (See chapter on Motion.) THE ACTION OF THE HEART. The heart's action in propelling the blood consists in the successive alternate contractions and dilatations of the muscu- lar walls of its two auricles and two ventricles. The auricles contract simultaneously; so do the ventricles; their dilatations also are severally simultaneous ; and the contractions of the one pair of cavities are synchronous with the dilatations of the other. The description of the action of the heart may best be corn- principal anterior columna carnea or musculus papillaris attached to it; 5, 5, mus- culi papillares; 5', the left side of the septum, between the two ventricles, within the cavity of the left ventricle ; 6, 6', the mitral valve ; 7, placed in the interior of the aorta near its commencement and above the three segments of its semilunar valve, which are hanging loosely together ; 7', the exterior of the great aortic sinus; 8, the root of the pulmonary artery and its semilunar valves ; 8', the separated por- tion of the pulmonary artery remaining attached to the aorta by 9, the cord of the ductus arteriosus ; 10, the arteries rising from the summit of the aortic arch. (From Quain's Anatomy.) ACTION OF THE HEART. 97 menced at that period in each action which immediately pre- cedes the beat of the heart against the side of the chest, and, by a very small interval more, precedes the pulse at the wrist. For at this time the whole heart is in a passive state, the walls of both auricles and ventricles are relaxed, and their cavities are being dilated. The auricles are gradually filling with blood flowing into them from the veins ; and a portion of this blood passes at once through them into the ventricles, the opening between the cavity of each auricle and that of its cor- respond ing ventricle being, during all the pause, free and patent. The auricles, however, receiving more blood than at once passes through them to the ventricles, become, near the end of the pause, fully distended ; then, in the end of the pause, they contract and empty their contents into the ventricles. The contraction of the auricles is sudden and very quick ; it com- mences at the entrance of the great veins into them, and is thence propagated towards the auriculo- ventricular opening ; but the last part which contracts is the auricular appendix. The effect of this contraction of the auricles is to propel nearly the whole of their blood into the ventricles. The reflux of blood into the great veins is resisted not only by the mass of blood in the veins and the force with which it streams into the auricles, but also by the simultaneous contraction of the mus- cular coats with which the large veins are provided for some distance before their entrance into the auricles ; a resistance which, however, is not so complete but that a small quantity of blood does regurgitate, i. e., flow backwards into the veins, at each auricular contraction. The effect of this regurgitation from the right auricle is limited by the valves at the junction of the subclavian and internal jugular veins, beyond which the blood cannot move backwards ; and the coronary vein, or vein which brings back to the right auricle the blood which has circulated in the substance of the heart, is preserved from it by a valve at its mouth. The blood which is thus driven, by the contraction of the auricles, into the corresponding ventricles, being added to that which had already flowed into them during the heart's pause, is sufficient to complete the dilatation or diastole of the ven- tricles. Thus distended, they immediately contract : so im- mediately, indeed, that their contraction, or systole, looks as if it were continuous with that of the auricles. This has been graphically described by Harvey in the following passage : " These two motions, one of the ventricles, another of the au- ricles, take place consecutively, but in such a manner that there is a kind of harmony, or rhythm, present between them, the two concurring in such wise that but one motion is appar- 98 T H E C I R C U L A T I O N. ent ; especially in the warmer-blooded animals, in which the movements in question are rapid. Nor is this for any other reason than it is in a piece of machinery, in which, though one wheel gives motion to another, yet all the wheels seem to move simultaneously ; or in that mechanical contrivance which is adapted to fire-arms, where the trigger being touched, down comes the flint, strikes against the steel, elicits a spark which, falling among the powder, it is ignited, upon which the flame extends, enters the barrel, causes the explosion, propels the ball, and the mark is attained all of which incidents by reason of the celerity with which they happen, seem to take place in the twinkling of an eye." The ventricles contract much more slowly than the auricles, and in their contraction, probably always thoroughly empty themselves, differing in this respect from the auricles, in which, even after their complete contrac- tion, a small quantity of blood remains. The form and position of the fleshy columns on the internal walls of the ventricle ap- pear, indeed, especially adapted to produce this obliteration of their cavities during their contraction ; and the completeness of the closure may often be observed on making a transverse section of a heart shortly after death, in any case in which the contraction of the rigor mortis is very marked. In such a case, only a central fissure may be discernible to the eye in the place of the cavity of each ventricle. At the same time that the walls of the ventricles contract, the fleshy columns," and especially those of them called the musculi papillares, contract also, and assist in bringing the margins of the auriculo-ventricular valves into apposition, so that they close the auriculo-ventricular openings, and prevent the backward passage of the blood into the auricles (p. 100). The whole force of the ventricular contraction is thus directed to the propulsion of the blood through their arterial orifices. During the time which elapses between the end of one con- traction of the ventricles, and the commencement of another, the communication between them and the great arteries the aorta on the left side, the pulmonary artery on the right is closed by the three semilunar valves situated at the orifice of each vessel. But the force with which the current of blood is propelled by the contraction of the ventricle separates these valves from contact with each other, and presses them back against the sides of the artery, making a free passage for the stream of blood. Then, as soon as the ventricular contraction ceases, the elastic walls of the distended artery recoil, and by pressing the blood behind the valves, force them down towards the centre of the vessel, and spread them out so as to close the FUNCTION OF THE VALVES. 99 orifice, and prevent any of the blood flowing back into the ventricles (p. 104). As soon as the auricles have completed their contraction they begin again to dilate, and to be refilled with blood, which flows into them in a steady stream through the great venous trunks. They are thus filling during all the time in which the ventricles are contracting; and the contraction of the ventricles being ended, these also again dilate, and receive again the blood that flows into them from the auricles. By the time that the ventricles are thus from one-third to two-thirds full, the auricles are distended ; these, then suddenly contracting, fill up the ventricles, as already described. If we suppose a cardiac revolution, which includes the con- traction of the auricles, the contraction of the ventricles, and their repose, to occupy rather more than a second, the following table will represent, in tenths of a second, the time occupied by the various events we have considered. Contraction of Auricles, . . . 1 -f Repose of Auricles, . . 10 = 11 " Ventricles, . . 4 -j- " Ventricles, . 7 = 11 Repose (no contraction of either auricles or ventricles), . . (' -f Contraction of either auricles or ventricles, 5=11 11 Action of the Valves of the Heart. The periods in which the several valves of the heart are in action may be connected with the foregoing table ; for the auriculo-ventricular valves are closed, and the arterial valves are open during the whole time of the ventricular contraction, while, during the dilatation and distension of the ventricles the latter valves are shut, the former open. Each half or side of the heart, through the action of its valves, may be compared with a kind of forcing-pump, like the common enema-syringe with two valves, of which one admits the fluid on raising the piston, but is closed again when the piston is forced down ; while the other opens for the escape of the fluid, but closes when the piston is raised, so as to prevent the regurgitation of the fluid already forced through it. The ventricular dilata- tion is here represented by the raising up of the piston ; the valve thus admitting fluid represents the auriculo-veutricular valve, which is closed a^aft) whenUae" piston Js;' foi;cd down, i. e., when the ventricle contracts^ and ,tlye" ;opier, i. e., the arterial, valye^opecs: , Tfy<^ diagrams, ,pn^ the following page illustrate this 9 13.85 2882 Solids. f Ferment, Pepsin (with a trace of Ammonia), 3.19 4.20 17.50 Hydrochloric Acid, . 0.20 1.55 2.70 Chloride of Calcium, . 0.06 0.1 1 1.66 Sodium, . 1.46 4.36 314 " Potassium, 0.55 1.51 1.07 Phosphate of Lime, [ Magnesia, and Iron, 0.12 2.09 2.73 Iii all the above analyses the amount of water given must be reckoned as rather too much, inasmuch as a certain quan- tity of saliva was mixed with the gastric fluid. The allow- ance, however, to be made on this account is only very small. Considerable difference of opinion has existed concerning the nature of the free acid contained in the gastric juice, chiefly whether it is hydrochloric or lactic. The weight of evidence, however, is in favor of free hydrochloric acid, being that to which, in the human subject, the acidity of the gastric fluid is mainly due ; although there is no doubt that others, as lactic, acetic, butyric, are not unfrequently to be found therein. The animal matter mentioned in the analysis of the gastric fluid is named pepsin, from its power in the process of diges- tion. It is an azotized substance, and is best procured by di- gesting portions of the mucous membrane of the stomach in cold water, after they have been macerated for some time in water at a temperature between 80 and 100 F. The warm water dissolves various substances as well as some of the pepsin, but the cold water takes up little else than pepsin, which, on evaporating the cold solution, is obtained in a grayish-brown viscid fluid. The addition of alcohol throws down the pepsin in grayish-white flocculi ; and one part of the principle thus prepared, if dissolved in even 60,000 parts of water, will digest meat and other alimentary substances. The digestive power of the gastric fluid is manifested in its softening, reducing into pulp, and partially or completely dis- solving various articles of food placed in it at a temperature of from 90 to 100. This, its peculiar property, requires the presence of both the pepsin and the acid ; neither of them can digest alone, and when they are mixed, either the decomposi- tion of the pepsin, or the neutralization of the acid, at once DIGESTIVE POWER OF GASTRIC FLUID. 223 destroys the digestive property of the fluid. For the perfec- tion of the process also, certain conditions are required, which are all found in the stomach ; namely (1), a temperature of about 100 F. ; (2), such movements as the food is subjected to by the muscular actions of the stomach, which bring in suc- cession every part of it in contact with the mucous membrane, whence the fresh gastric fluid is being secreted ; (3), the con- stant removal of those portions of food which are already digested, so that what remains undigested may be brought more completely into contact with the solvent fluid ; and (4) a state of softness and minute division, such as that to which the food is reduced by mastication previous to its introduction into the stomach. The chief circumstances connected with the mode in which the gastric fluid acts upon food during natural digestion, have been determined by watching its operations when removed from the stomach and placed in conditions as nearly as possi- ble like those under which it acts while within that viscus. The fact that solid food, immersed in gastric fluid out of the body, and kept at a temperature of about 100, is gradually converted into a thick fluid similar to chyme, was shown by Spallanzani, Dr. Stevens, Tiedemann and Gmelin and others. They used the gastric fluid of dogs, obtained by causing the animals to swallow small pieces of sponge, which were subse- quently withdrawn, soaked with the fluid and proved nearly as much as the latter experiments of the same kind of gastric fluid by Blondlot, Bernard and others. But these need not be particularly referred to, while we have the more satisfac- tory and instructive observations which Dr. Beaumont made with the fluid obtained from the stomach of St. Martin. After the man had fasted seventeen hours, Dr. Beaumont took one ounce of gastric fluid, put into it a solid piece of boiled recently salted beef weighing three drachms, and placed the vessel which contained them in a water-bath heated to 100. " In forty minutes digestion had distinctly commenced over the surface of the meat ; in fifty minutes, the fluid had become quite opaque and cloudy, the external texture began to separate and become loose ; and in sixty minutes chyme began to form. At 1 P.M." (two hours after the commencement of the experi- ment) " the cellular texture seemed to be entirely destroyed, leaving the muscular fibres loose and unconnected, floating about in small fine shreds, very tender and soft." In six hours, they were nearly all digested a few fibres only remaining. After the lapse of ten hours, every part of the meat was com- pletely digested. The gastric juice, which was at first trans-; parent, was now about the color of whey, and deposited a fine 224 DIGESTION. sediment of the color of meat. A similar piece of beef was, at the time of the commencement of this experiment, suspended in the stomach by means of a thread : at the expiration of the first hour it was changed in about the same degree as the meat digested artificially ; but at the end of the second hour, it was completely digested and gone. In other experiments, Dr. Beaumont withdrew through the opening of the stomach some of the food which had been taken twenty minutes previously, and which was completely mixed with the gastric juice. He continued the digestion, which had already commenced, by means of artificial heat in a water-bath. In a few hours the food thus treated was completely chymified ; and the artificial seemed in this, as in several other experi- ments, to be exactly similar to, though a little slower than, the natural digestion. The apparent identity of the process in- and outside of the stomach thus manifested, while it shows that we may regard digestion as essentially a chemical process, when once the gas- tric fluid is formed, justifies the belief that Dr. Beaumont's other experiments with the digestive fluid may exactly repre- sent the modifications to which, under similar conditions, its action in the stomach would be liable. He found that, if the mixture of food and gastric fluid were exposed to a temperature of 34 F., the process of digestion was completely arrested. In another experiment, a piece of meat which had been macerated in water at a temperature of 100 for several days, till it ac- quired a strong putrid odor, lost, on the addition of some fresh gastric juice, all signs of putrefaction, and soon began to be digested. From other experiments he obtained the data for estimates of the degrees of digestibility of various articles of food, and of the ways in which the digestion is liable to be af- fected, to which reference will again be made. When natural gastric juice cannot be obtained, many of these experiments may be performed with an artificial digestive fluid, the action of which, probably, very closely resembles that of the fluid secreted by the stomach. It is made by macerat- ing in water portions of fresh or recently dried mucous mem- brane of the stomach of a pig 1 or other omnivorous animal, or of the fourth stomach of the calf, and adding to the in- fusion a few drops of hydrochloric acid about 3.3 grains to half an ounce of the mixture, according to Schwann. Por- tions of food placed in such fluid, and maintained with it 1 The best portion of the stomach of the pig for this purpose is that between the cardiac and pyloric orifices ; the cardiac portion appears to furnish the least active digestive fluid. CHYME. 225 at a temperature of about 100, are, in an hour or more, according to the toughness of the substance, softened and changed in just the same manner as they would be in the stomach. The nature of the action by which the mucous membrane of the stomach and its secretion work these changes in organic matter is exceedingly obscure. The action of the pepsin may be compared with that of a ferment, which at the same time that it undergoes change itself, induces certain changes also in the organic matters with which it is in contact. Or its mode of action may belong to that class of chemical processes termed " catalytic," in which a substance excites, by its mere presence, and without itself undergoing change as ordinary ferments do, some chemical action in the substances with which it is in con- tact. So, for example, spongy platinum, or charcoal, placed in a mixture, however voluminous, of oxygen and hydrogen, makes them combine to form water ; and diastase makes the starch in grains undergo transformation, and sugar is produced. And that pepsin acts in some such manner appears probable from the very minute quantity capable of exerting the peculiar digestive action on a large quantity of food, and apparently with little diminution in its active power. The process differs from ordinary fermentation, in being unattended with the for- mation of carbonic acid, in not requiring the presence of oxygen, and in being unaccompanied by the production of new quan- tities of the active principle, or ferment. It agrees with the processes of both fermentation and organic catalysis, in that whatever alters the composition of the pepsin (such as heat above 100, strong alcohol, or strong acids), destroys the diges- tive power of the fluid. Changes of the Food in the Stomach. The general effect of digestion in the stomach is the conver- sion of the food into chyme, a substance of various composition according to the nature of the food, yet always presenting a characteristic thick, pultaceous, grumous consistence, with the undigested portions of the food mixed in a more fluid substance, and a strong, disagreeable acid odor and taste. Its color de- pends on the nature of the food, or on the admixture of yellow or green bile which may, apparently, even in health, pass into the stomach. Reduced into such a substance, all the various materials of a meal may be mingled together, and near the end of the diges- tive process hardly admit of recognition ; but the experiments of artificial digestion, and the examination of stomachs with 226 DIGESTION. fistulse, have illustrated many of the changes through which the chief alimentary principles pass, and the times and modes in which they are severally disposed of. These must now be traced. The readiness with which the gastric fluid acts on the several articles of food is, in some measure, determined by the state of division, and the tenderness and moisture of the substance pre- sented to it. By minute division of the food, the extent of surface with which the digestive fluid can come in contact is increased, and its action proportionably accelerated. Tender and moist substances offer less resistance to the action of the gastric juice than tough, hard, and dry ones do, because they may be thoroughly penetrated by it, and thus be attacked not only at the surface, but at every part at once. The readiness with which a substance is acted upon by the gastric fluid does not, however, necessarily imply the degree of its nutritive property ; for a substance may be nutritious, yet, on account of its toughness and other qualities, hard to digest ; and many soft, easily digested substances contain comparatively a small amount of nutriment. But for a substance to be nutritive, it must be capable of being assimilated to the blood ; and to h'nd its way into the blood, it must, if insoluble, be digestible by the gastric fluid or some other secretion in the intestinal canal. There is, therefore, thus far, a necessary connection between the digestibility of a substance and its power of affording nutri- ment. Those portions of food which are liquid when taken into the stomach, or which are easily soluble in the fluids therein, are probably at once absorbed by the bloodvessels in the mucous membrane of the stomach. Magendie's experiments, and better still, those of Dr. Beaumont, have proved this quick absorption of water, wine, weak saline solutions, and the like ; that they are absorbed without manifest change by the diges- tive fluid, and that, generally, the water of such liquid food as soups is absorbed at once, so that the substances suspended in it are concentrated into a thicker material, like the chyme from solid food, before the digestive fluid acts upon them. The action of the gastric fluid on the several kinds of solid food has been studied in various ways. In the earliest experi- ments, perforated metallic and glass tubes, filled with the ali- mentary substances, were introduced into the stomachs of ani- mals, and after the lapse of a certain time withdrawn, to ob- serve the condition of the contained substances ; but such ex- periments are fallacious, because gastric fluid has not ready access to the food. A better method was practiced in a series of experiments by Tiedemann and Gmelin, who fed dogs with DIGESTION OF FOOD IN THE STOMACH. 227 different substances, and killed them in a certain number of hours afterwards. But the results they obtained are of less interest than those of the experiments of Dr. Beaumont on his patient, St. Martin, and of Dr. Gosse, who had the power of vomiting at will. Dr. Beaumont's observations show, that the process of di- gestion in the stomach, during health, takes place so rapidly, that a full meal, consisting of animal and vegetable substances, may nearly all be converted into chyme in about an hour, and the stomach left empty in two hours and a half. The details of two days' experiments will be sufficient examples : Exp. 42. April 7th, 8 A.M. St. Martin breakfasted on three hard-boiled eggs, pancakes, and coffee. At half-past eight o'clock, Dr. Beaumont examined the stomach, and found a heterogeneous mixture of the several articles slightly digested At a quarter past ten, no part of the break- fast remained in the stomach. Exp. 43. At eleven o'clock the same day, he ate two roasted eggs and three ripe apples. In half an hour they were in an incipient state of digestion ; and a quarter past twelve no vestige of them remained. Exp. 44. At two o'clock P.M. the same day, he dined on roasted pig and vegetables. At three o'clock they were half chymified, and at half-past four nothing remained but a very little gastric juice. Again, Exp. 46. April 9th. At three o'clock P.M. he dined on boiled dried codfish, potatoes, parsnips, bread, and drawn butter. At half-past three o'clock examined, and took out a portion about half digested ; the potatoes the least so. The fish was broken down into small filaments ; the bread and parsnips were not to be distinguished. At four o'clock, ex- amined another portion. Very few particles of fish remained entire. Some of the few potatoes were distinctly to be seen. At half-past four o'clock, he took out and examined another portion ; all completely chymified. At five o'clock stomach empty. Many circumstances besides the nature of the food are apt to influence the process of chymification. Among them are, the quantity of food taken ; the stomach should be fairly filled, not distended : the time that has elapsed since the last meal, which should be at least enough for the stomach to be quite clear of food : the amount of exercise previous and subsequent to the meal, gentle exercise being favorable, overexertion in- jurious to digestion ; the state of mind tranquillity of temper being apparently essential to a quick and due digestion : the bodily health : the state of the weather. But under ordinary 228 DIGESTION. circumstances, from three to four hours may be taken as the average time occupied by the digestion of a meal in the stom- ach. Dr. Beaumont constructed a table showing the times required for the digestion of all usual articles of food in St. Martin's stomach, and in his gastric fluid taken from the stomach. Among the substances most quickly digested were rice and tripe, both of which were chymified in an hour; eggs, salmon, trout, apples, and venison, were digested in an hour and a half; tapioca, barley, milk, liver, fish, in two hours; turkey, lamb, potatoes, pig, in two hours and a half; beef and mutton re- quired from three hours to three and a half, and both were more digestible than veal ; fowls were like mutton in their de- gree of digestibility. Animal substances were, in general, con- verted into chyme more rapidly than vegetables. Dr. Beaumont's experiments were all made on ordinary arti- cles of food. A minuter examination of the changes produced by gastric digestion on various tissues has been made by Dr. Rawitz, who examined microscopically the product of the arti- ficial digestion of different kinds of food, and the contents of the faeces after eating the same kinds of food. The general results of his examinations, as regards animal food, show that muscular tissue breaks up into its constituent fasciculi, and that these again are divided transversely; gradually the trans- verse strife become indistinct, and then disappear; and finally, the sarcolemma seems to be dissolved, and no trace of the tissue can be found in the chyme, except a few fragments of fibres. These changes ensue most rapidly in the flesh of fish and hares, less rapidly in that of poultry and other animals. The cells of cartilage and fibro-cartilage, except those of fish, pass unchanged through the stomach and intestines, and may be found in the faeces. The interstitial tissues of these structures are converted into pulpy textureless substances in the artificial digestive fluid, and are not discoverable in the faeces. Elastic fibres are un- changed in the digestive fluid. Fat-cells are sometimes found quite unaltered in the faeces; and crystals of cholesterin may usually be obtained from faeces, especially after the use of pork fat. As regards vegetable substances, Dr. Rawitz states, that he frequently found large quantities of cell-membranes unchanged in the faeces ; also starch-cells, commonly deprived of only part of their contents. The green coloring principle, chlorophyll, was usually unchanged. The walls of the sap-vessels and spiral-vessels were quite unaltered by the digestive fluid, and were usually found in large quantities in the faeces; their con- tents, probably, were removed. DIGESTION IN THE STOMACH. 229 From these experiments, we may understand the structural changes which the chief alimentary substances undergo in their conversion into chyme; and the proportions of each which are not reducible to chyme, nor capable of any further act of di- gestion. The chemical changes undergone in and by the proxi- mate principles are less easily traced. Of the albuminous principles, some, as the casein of milk, are coagulated by the acid of the gastric fluid; and thus, be- fore they are digested, come into the condition of the other solid principles of the food. These, including solid albumen and fibrin, in the same proportion that they are broken up and anatomically disorganized by the gastric fluid, appear to be reduced or lowered in their chemical composition. This chemi- cal change is probaby produced, as suggested by Dr. Prout, by the principles entering into combination with water. It is suf- ficient to conceal nearly all their characteristic properties ; the albumen is rendered scarcely coagulable by heat; the gelatin, even when its solution is evaporated, does not congeal in cool- ing; the fibrin and casein cannot be found by their character- istic tests. It would seem, indeed, that all these various sub- stances are converted into one and the same principle, a low form of albumen, not precipitable by nitric acid or heat, and now generally termed albuminose or peptone, from which, after being absorbed, they are again raised, in the elaboration of the blood, to which they are ultimately assimilated. The change of molecular constitution suffered by the albu- minous parts of the food, in consequence of the action of the gastric juice, has an important relation to their absorption by the bloodvessels of the stomach. From the condition of " col- loids," or substances, so named by Professor Graham, which are absorbed with extreme difficulty, they appear, from ex- periments of Funke, to assume to a great degree the char- acter of " crystalloids," which can pass through animal mem- branes with ease. 1 Whatever be the mode in which the gastric secretion affects these principles, it, or something like it, appears essen- tial, in order that they may be assimilated to the blood and tissues. For, when Bernard and Barreswil injected albumen dissolved in water into the jugular veins of dogs, they always in about three hours after, found it in the urine. But if, pre- vious to injection, it was mixed with gastric fluid, no trace of it could be detected in the urine. The influence of the liver seems to be almost as efficacious as that of the gastric fluid, in 1 These terms will be further explained and illustrated in the chapter on Absorption. 20 230 DIGESTION. rendering albumen assimilable; for Bernard found that, if diluted egg-albumen, unmixed with gastric fluid, is injected into the portal vein, it no longer makes its appearance in the urine, and is, therefore, no doubt, assimilated by the blood. Probably, most of the albuminose, with other soluble and fluid materials, is absorbed directly from the stomach by the minute bloodvessels with which the mucous membrane is so abundantly supplied. The saccharine including the amylaceous principles are at first, probably, only mechanically separated from the vege- table substances within which they are contained, by the action of the gastric fluid. The soluble portions, viz., dextrin and sugar, are probably at once absorbed. The insoluble ones, viz., starch and lignin (or some parts of them), are ren- dered soluble and capable of absorption, by being converted into dextrin or grape-sugar. It is probable that this change is carried on to some extent in the stomach ; but this conver- sion of starch into sugar is effected, not by the gastric fluid, but by the saliva introduced with the food, or subsequently swallowed. The transformation of starch is continued in the intestinal canal, as will be shown, by the secretion of the pan- creas, and perhaps by that of the intestinal glands and mu- cous membrane. The power of digesting uncooked starch is, however, very limited in man and Carnivora, for when starch has been taken raw, as in corn and rice, large quantities of the granules are passed unaltered with the excrements. Cook- ing, by expanding or bursting the envelopes of the granules, renders their interior more amenable to the action of the di- gestive organs ; and the abundant nutriment furnished by bread, and the large proportion that is absorbed of the weight consumed, afford proof of the completeness of their power to make its starch soluble and prepare it for absorption. Of the oleaginous principles, as to their changes in the stomach, no more can be said than that they appear to be reduced to minute particles, and pass into the intestines min- gled with the other constituents of the chyme. In the case of the solid fats, this effect is probably produced by the sol- vent action of the gastric juice on the areolar tissue, albumin- ous cell -walls, &c., which enter into their composition, and by the solution of which the true fat is able to mingle more uni- formly with the other constituents of the chyme. Being fur- ther changed in the intestinal canal, fat is rendered capable of absorption by the lacteals. MOVEMENTS OF THE STOMACH. 231 Movements of the Stomach. It has been already said, that the gastric fluid is assisted in accomplishing its share in digestion by the movements of the stomach. In granivorous birds, for example, the contraction of the strong muscular gizzard affords a necessary aid to di- gestion, by grinding and triturating the hard seeds which con- stitute part of the food. But in the stomachs of man and Mammalia the motions of the muscular coat are too feeble to exercise any such mechanical force on the food ; neither are they needed, for mastication has already done the mechanical work of a gizzard ; and the experiments of Reaumur and Spallanzani have demonstrated that substances inclosed in perforated tubes, and consequently protected from mechanical influence, are yet digested. The normal actions of the muscular fibres of the human stomach appear to have a threefold purpose : first, to adapt the stomach to the quantity of food in it, so that its walls may be in contact with the food on all sides, and, at the same time, may exercise a certain amount of compression upon it ; secondly, to keep the orifices of the stomach closed until the food is digested ; and, thirdly, to perform certain peristaltic movements, whereby the food, as it becomes chymified, is gradually propelled towards, and ultimately through, the py- lorus. In accomplishing this latter end, the movements with- out doubt materially contribute towards effecting a thorough intermingling of the food and the gastric fluid. When digestion is not going on, the stomach is uniformly contracted, its orifices not more firmly than the rest of its walls ; but, if examined shortly after the introduction of food, it is found closely encircling its contents, and its orifices are firmly closed like sphincters. The cardiac orifice, every time food is swallowed, opens to admit its passage to the stom- ach, and immediately again closes. The pyloric orifice, during the first part of gastric digestion, is usually so com- pletely closed, that even when the stomach is separated from the intestines, none of its contents escape. But towards the termination of the digestive process, the pylorus seems to offer less resistance to the passage of substances from the stomach ; first it yields to allow the successively digested portions to go through it ; and then it allows the transit of even undigested substances. From the observations of Dr. Beaumont on the man St. Martin, it appears that food, so soon as it enters the stomach, is subjected to a kind of peristaltic action of the muscular coat, whereby the digested portions are gradually approxi- 232 DIGESTION. mated towards the pylorus. The movements were observed to increase in rapidity as the process of 'chymification advanced, and were continued until it was completed. The contraction of the fibres situated towards the pyloric end of the stomach seems to be more energetic and more de- cidedly peristaltic than those of the cardiac portion. Thus, Dr. Beaumont found that when the bulb of the thermometer was placed about three inches from the pylorus, it was tightly embraced from time to time and drawn towards the pyloric orifice for a distance of three or four inches. The object of this movement appears to be, as just said, to carry the food to- wards the pylorus as fast as it is formed into chyme, and to propel the chyme into the duodenum ; the undigested portions of food being kept back until they are also reduced into chyme, or until all that is digestible has passed out. The ac- tion of these fibres is often seen in the contracted state of the pyloric portion of the stomach after death, when it alone is contracted and firm, while the cardiac portion forms a dilated sac. Sometimes, by a predominant action of strong circular fibres placed between the cardia and pylorus, the two por- tions, or ends as they are called, of the stomach, are separated from each other by a kind of hour-glass contraction. The interesting researches of Dr. Brinton have clearly es- tablished that, by means of this peristaltic action of the mus- cular coats of the stomach, not merely is chymified food gradually propelled through the pylorus, but a kind of double current is continually kept up among the contents of the stom- ach, the circumferential parts of the mass being gradually moved onward towards the pylorus by the peristaltic contrac- tion of the muscular fibres, while the central portions are pro- pelled in the opposite direction, namely, towards the cardiac orifice ; in this way is kept up a constant circulation of the contents of the viscus, highly conducive to their free mixture with the gastric fluid and to their ready digestion. These actions of the stomach are peculiar to it and indepen- dent. But it is, also, adapted to act in concert with the ab- dominal muscles, in certain circumstances which can hardly be called abnormal, as in vomiting and eructation. It has indeed been frequently stated that the stomach itself is quite passive during vomiting, and that the expulsion of its contents is effected solely by the pressure exerted upon it when the ca- pacity of the abdomen is diminished by the contraction of the diaphragm, and subsequently of the abdominal muscles. The experiments and observations, however, which are supposed to confirm this statement, only show that the contraction of the abdominal muscles alone is sufficient to expel matters, from an MOVEMENTS OF THE STOMACH. 233 unresisting bag through the oesophagus ; and that, under very abnormal circumstances, the stomach, by itself, cannot or rather does not expel its contents. They by no means show that in ordinary vomiting the stomach is passive ; and, on the other hand, there are good reasons for believing the contrary. It is true that facts are wanting to demonstrate with cer- tainty this action of the stomach in vomiting ; but some of the cases of fistulous opening into the organ appear to support the belief that it does take place ; l and the analogy of the case of the stomach with that of the other hollow viscera, as the rec- tum and bladder, may be also cited in confirmation. Besides the influence which it may thus have by its contrac- tion, the stomach also essentially contributes to the act of vomiting, by the contraction of its pyloric orifice at the same time that the oblique fibres around the cardiac orifice are re- laxed. For, until the relaxation of these fibres, no vomiting can ensue ; when contracted, they can as well resist all the force of the contracting abdominal and other muscles, as the muscles by which the glottis is closed can resist the same force in the act of straining. Doubtless we may refer many of the acts of retching and ineffectual attempts to vomit, to the want of concord between the relaxation of these muscles and the con- traction of the others. The muscles with which the stomach co-operates in contrac- tion during vomiting, are chiefly and primarily those of the abdomen ; the diaphragm also acts, but not as the muscles of the abdominal walls do. They contract and compress the stomach more and more towards the back and upper parts of the diaphragm ; and the diaphragm (which is usually drawn down in the deep inspiration that precedes each act of vomit- ing) holds itself fixed in contraction, and presents an unyield- ing surface against which the stomach may be pressed. It is enabled to act thus, and probably only thus, because the in- spiration which precedes the act of vomiting is terminated by the closure of the glottis; after which the diaphragm can neither descend further, except by expanding the air in the lungs, nor, except by compressing the air, ascend again until, the act of vomiting having ceased, the glottis is opened again (see diagram, p. 181 ; see also p. 183). Some persons possess the power of vomiting at will, without applying any undue irritation to the'stomach, but simply by a voluntary effort, It seems also, that this piower may be ac- 1 A collodion of cases of fistulous communication with the stomach, through the abdominal parietes, has been given by Dr. Murchison in vol, $lj of the Medico-Chirurgical Transactions, 234 DIGESTION. quired by those who do not naturally possess it, and by con- tinual practice may become a habit. There are cases also of rare occurrence in which persons habitually swallow their food hastily, and nearly unmasticated, and then at their leisure re- gurgitate it, piece by piece, into their mouth, remasticate, and again swallow it, exactly as is done by the ruminant order of Mammalia. Influence of the Nervous System on Gastric Digestion. This influence is manifold; and is evidenced, 1st, in the sen- sations which induce to the taking of food ; 2d, in the secretion of the gastric fluid; 3d, in the movements of the food in and from the stomach. The sensation of hunger is manifested in consequence of de- ficiency of food in the system. The mind refers the sensation to the stomach ; yet since the sensation is relieved by the in- troduction of food either into the stomach itself, or into the blood through other channels than the stomach, it would ap- pear not to depend on the state of the stomach alone. This view is confirmed by the fact, that the division of both pneu- mogastric nerves, which are the principal channels by which the mind is cognizant of the condition of the stomach, does not appear to allay the sensations of hunger. But that the stomach has some share in this sensation is proved by the relief afforded, though only temporarily, by the introduction of even non-alimentary substances into this organ. It may, therefore, be said that the sensation of hunger is de- rived from the system generally, but chiefly from the condition of the stomach, the nerves of which, we may suppose, are more affected by the state of the insufficiently replenished blood than those of other organs are. The sensation of thirst, indicating the want of fluid, is re- ferred to the fauces, although, as in hunger, this is merely the local declaration of a general condition existing in the system. For thirst is relieved for only a very short time by moistening the dry fauces ; but may be relieved completely by the intro- duction of liquids into the blood, either through the stomach, or by injections into the bloodvessels, or by absorption from the surface of the skin or the intestines. The sensation of thirst is perceived most naturally whenever there is a dispro- portionately small quantity of water in the blood; as well, therefore, when water has been abstracted from the blood, as when saline or any solid matters have been abundantly added to it. We can express the fact (even if it be not an explana- tion of it), by saying that the nerves of the mouth and i'auces, INFLUENCE OF THE NERVOUS SYSTEM. 235 through which the sense of thirst is chiefly derived, are more sen- sitive to this condition of the blood than other nerves are. And the cases of hunger and thirst are not the only ones in which the mind derives, from certain organs, a peculiar predominant sensation of some condition affecting the whole body. Thus, the sensation of the " necessity of breathing," is referred es- pecially to the lungs ; but, as Volkmann's experiments show, it depends on the condition of the blood which circulates every- where, and is felt even after the lungs of animals are removed ; for they continue, even then, to gasp and manifest the sensa- tion of want of breath. And, as with respiration when the lungs are removed, the mind may still feel the body's want of breath ; so in hunger and thirst, even when the stomach has been filled with innutritions substances, or the pneumogastric nerves have been divided, and the mouth and fauces are kept moist, the mind is still aware, by the more obscure sensations in other parts, of the whole body's need of food and water. The influence of the nervous system on the secretion of gastric fluid, is shown plainly enough in the influence of the mind upon digestion in the stomach ; and is, in this regard, well illustrated by several of Dr. Beaumont's observations. M. Bernard also, watching the act of gastric digestion in dogs which had fistulous openings into their stomachs, saw that on the instant of dividing their pneumogastric nerves, the process of digestion was stopped, and the mucous membrane of the stomach, previously turgid with blood, became pale, and ceased to secrete. These, however, and the like experiments showing the instant effect of division of the pneumogastric nerves, may prove no more than the effect of a severe shock, and the fact that influences affecting digestion may be conveyed to the stomach through those nerves. From other experiments it may be gathered, that although, as in M. Bernard's, the division of both pneumogastric nerves always temporarily suspends the secretion of gastric fluid, and so arrests the process of digestion, and is occasionally followed by death from inanition ; yet the digestive powers of the stomach may be completely restored after the operation, and the formation of chyme and the nutri- tion of the animal may be carried on almost as perfectly as in health. In thirty experiments on Mammalia, which M. Wernscheidt performed under Miiller's direction, not the least difference could be perceived in the action of narcotic poisons introduced into the stomach, whether the pneumogastric had been divided on both sides or not, provided the animals were of the same species and size. It appears, however, that such poisons as are capable of being rendered inert by the action of the gastric 236 DIGESTION. fluid, may, if taken into the stomach shortly after division of both pneumogastric nerves, produce their poisonous effects; in consequence, apparently, of the temporary suspension of the secretion of gastric fluid. Thus, in one of his experiments, M. Bernard gave to each of two dogs, in one of which he had di- vided the pneumogastric nerves, a dose of emulsin, and half an hour afterwards a dose of amygdalin, substances which are innocent alone, but when mixed produce hydrocyanic acid. The dog whose nerves were cut, died in a quarter of an hour, the substances being absorbed unaltered and mixing in the blood; in the other, the emulsin was decomposed by the gas- tric fluid before the amygdalin was administered; therefore, hydrocyanic acid was not formed in the blood, and the dog survived. The influence of the pneumogastric nerves over the secretion of gastric fluid has been of late even more decidedly shown by M. Bernard, who found that galvanic stimulus of these nerves excited an active secretion of the fluid, while a like stimulus applied to the sympathetic nerves issuing from the semilunar ganglia, caused a diminution and even complete arrest of the secretion. The influence of the nervous system on the movements of the stomach has been often seen in the retardation or arrest of these movements after division of the pneumogastric nerves. The results of irritating the same nerves were ambiguous ; but the experiments of Longet and Bischoff have shown that the dif- ferent results depended on whether the stomach were digesting or not at the time of the experiment. In the act of digestion, the nervous system of the stomach appears to participate in the excitement which prevails through the rest of its organiza- tion, and a stimulus applied to the pneumogastric nerves is felt intensely, and active movements of the muscular fibres of the stomach follow; but in the inaction of fasting, the same stimu- lus produces no effect. So, while the stomach is digesting, the pylorus is too irritable to allow anything but chyme to pass; but when digestion is ended, the undigested parts of the food, and even large bodies, coins, and the like, may pass through it. Digestion of the Stomach after Death. If an animal die during the process of gastric digestion, and when, therefore, a quantity of gastric juice is present in the interior of the stomach, the walls of this organ itself are fre- quently themselves acted on by their own secretion, and to such an extent, that a perforation of considerable size may be produced, and the contents of the stomach may in part escape POST-MORTEM DIGESTION. 237 into the cavity of the abdomen. This phenomenon is not un- frequently observed in post-mortem examinations of the human body; but, as Dr. Pavy observes, the effect may be rendered, by experiment, more strikingly manifest. "If, for instance," he remarks, "an animal, as a rabbit, be killed at a period of digestion, and afterwards exposed to artificial warmth to pre- vent its temperature from falling, not only the stomach, but many of the surrounding parts will be found to have been dis- solved. With a rabbit killed in the evening, and placed in a warm situation (100 to 110 Fahr.) during the night, I have seen in the morning, the stomach, diaphragm, part of the liver and lungs, and the intercostal muscles of the side upon which the animal was laid all digested away, with the muscles and skin of the neck and upper extremity on the same side also in a semi-digested state." From these facts, it becomes an interesting question why, during life, the stomach is free from liability to injury from a secretion, which, after death, is capable of such destructive effects ? John Hunter, who particularly drew attention to the phenomena of post-mortem digestion, explained the immunity from injury of the living stomach, by referring it to the pro- tective influence of the "vital principle." But this dictum has been called in question by subsequent observers. It is, indeed, rather a statement of a fact, than an explanation of its cause. It must be confessed, however, that no entirely satisfactory theory has been yet stated as a substitute. It is only necessary to refer to the idea of Bernard, that the living stomach finds protection from its secretion in the pres- ence of epithelium and mucus, which are constantly renewed in the same degree that they are constantly dissolved, in order to remark that this theory has been disproved by experiments of Pavy's, in which the mucous membrane of the stomachs of dogs was dissected off for a small space, and, on killing the animals some days afterwards, no sign of digestion of the stomach was visible. " Upon one occasion, after removing the mucous membrane and exposing the muscular fibres over a space of about an inch and a half in diameter, the animal was allowed to live for ten days. It ate food every day, and seemed scarcely affected by the operation. Life was destroyed whilst digestion was being carried on, and the lesion in the stomach was found very nearly repaired : new matter had been deposited in the place of what had been removed, and the denuded spot had contracted to much less than its original dimensions." Dr. Pavy believes that the natural alkalinity of the blood, which circulates so freely during life in the walls of the stom- 238 DIGESTION. ach, is sufficient to neutralize the acidity of the gastric juice, were it, so to speak, to make an attempt at digesting parts with which it has no business ; and as may be gathered from what has been previously said (p. 228), the neutralization of the acidity of the gastric secretion is quite sufficient to destroy its digestive powers. He also very ingeniously argues that this very alkalinity must, from the conditions of the circula- tion naturally existing in the walls of the stomach, be in- creased in proportion to the need of its protective influence. " In the arrangement of the vascular supply," he remarks, " a doubly effective barrier is, as it were, provided. The vessels pass from below upwards towards the surface : capillaries having this direction ramify between the tubules by which the acid of the gastric juice is secreted ; and being separated by secretion below, must leave the blood that is proceeding upwards correspondingly increased in alkalinity ; and thus, at the period when the largest amount of acid is flowing into the stomach, and the greatest protection is required, then is the provision afforded in its highest state of efficiency." Dr. Pavy's theory is the best and most ingenious hitherto framed in connection with this subject; but the experiments adduced in its favor are open to many objections, and afford only a negative support to the conclusions they are intended to prove. The matter, therefore, can scarcely be considered finally settled. DIGESTION IN THE INTESTINES. The intestinal canal is divided into two chief portions, named, from their differences in diameter, the small and large intestine. These are continuous with each other, and com- municate by means of an opening guarded by a valve, the ileo-ccecal valve, which allows the passage of the products of digestion from the small into the large bowel, but not, under ordinary circumstances, in the opposite direction. The structure and functions of each organ or tissue con- cerned in intestinal digestion will be first described in detail, and afterwards a summary will be given of the changes which the food undergoes in its passage through the intestines, 1st, from the pylorus to the ileo-csecal valve ; and, 2d, from the ileo-csecal valve to the anus. Structure arid Secretions of the Small Intestine. The small intestine, the average length of which in an adult is about twenty feet, has been divided, for convenience DIGESTION IN THE INTESTINES. 239 of description, into three portions, viz., the duodenum, which extends for eight or ten inches beyond the pylorus ; the jeju- num, which occupies two-fifths, and the ileum, which occupies three-fifths of the rest of the canal. The small intestine, like the stomach, is constructed of three principal coats, viz., the serous, muscular, and mucous. The serous coat, formed by the visceral layer of the peritoneum, need not be here specially described. The fibres of the mus- cular coat of the small intestine are arranged in two layers ; those of the outer layer being disposed longitudinally ; those of the inner layer transversely, or in portions of circles encom- passing the canal. They are composed of the unstriped kind of muscular fibre. Between the mucous and muscular coats, there is a layer of submucous tissue, in which numerous bloodvessels and a rich plexus of nerves and ganglia are imbedded (Meissner). The mucous membrane is the most important coat in relation to the function of digestion. The following structures which enter into the composition of the mucous membrane may be now successively described : the valvulce conniventes ; the wlli ; and the glands. The general structure of the mucous mem- brane of the intestines resembles that of the stomach (p. 215), and, like it, is lined on its inner surface by columnar epithe- lium. Lymphoid or Retiform tissue (Fig. 72) enters largely FIG. 72. The figure represents a cross-section of a small fragment of the mucous mem- brane, including one entire crypt of Lieberkiihn and parts of several others; a, cavity of the tubular glands or crypts ; b, one of the lining epithelial cells ; c, the lymphoid or retiforni spaces, of which some are empty, and others occupied by lymph cells, as at d. into its construction ; and on its deep surface is a layer of the muscular is mucosce (p. 216). 240 DIGESTION. FIG. 73. Valvulce Conniventes. The valvulse conniventes commence in the duodenum, about one or two inches beyond the pylorus, and becoming larger and more numerous immediately beyond the entrance of the bile-duct, continue thickly arranged and well developed throughout the jejunum ; then, gradually diminishing in size and number, they cease near the middle of the ileum. In structure they are formed by a doubling inwards of the mu- cous membrane, the crescentic, nearly circular, folds thus formed being arranged transversely with regard to the axis of the intestine, and each individual fold seldom extending around more than or f of the bowel's circumference. Un- like the rugae in the stomach, they do not disappear on dis- tension. Only an imperfect notion of their natural position and function can be obtained by looking at them after the intestine has been laid open in the usual manner. To understand them aright, a piece of gut should be distended either with air or alcohol, and not opened until the tissues have become hardened. On then making a section, it may be seen that instead of disap- pearing, as the rugae in the stomach would under similar circumstances, they stand out at right angles to the general surface of the mucous membrane (Fig. 73). Their functions are probably these: Besides (1) offering a largely increased surface for secretion and ab- sorption, they probably (2) prevent the too rapid passage of the very liquid products of gastric digestion, immedi- ately after their escape from the stom- hardeued by alcohol) laid b -. /ON -, .-, . ,. j open to show the normal ach > and ( 3 )> b 7 their Projection, and position of the vaivuise con- consequent interference with a uni- niventes. form and untroubled current of the intestinal contents, probably assist in the more perfect mingling of the latter with the secretions poured out to act on them. Glands of the Small Intestine. The glands are of three prin- cipal kinds, named after their describers, the glands of Lieber- kiihn, of Peyer, and of Brunn. The glands or follicles of I/ieberkilhn are simple tubular depressions of the intestinal Piece of small intestine (previously distended and PEYERS GLANDS. 241 FIG 74. mucous membrane, thickly distributed over the whole surface both of the large and small intestines. In the small intestine they are visible only with the aid of a lens ; and their orifices appear as mi- nute dots scattered between the villi. They are larger in the large intestine, and increase in size the nearer they approach the anal end of the intestinal tube ; and in the rectum their orifices may be visible to the naked eye. In length they vary from ^ to ^ of a line. Each tubule (Fig. 74) is constructed of the same es- sential parts as the intestinal mucous mem- brane, viz., a fine structureless membrana pro- pria, or basement-membrane, a layer of cylin- drical epithelium lining it, and capillary blood- vessels covering its exterior. Their contents appear to vary, even in health ; the varieties being dependent, probably, on the period of time in relation to digestion at which they are examined. At the bottom of the follicle, the contents usually consist of a granular material, in which a few cytoblasts or nuclei are imbedded; these cytoblasts, as they ascend towards the surface, are supposed to be gradually developed into nucleated cells, some of which are discharged into the intestinal cavity. The purpose served by the material secreted by these glands is still doubtful. Their large number and the extent of surface occupied by them, seem, however, to indicate that they are concerned in other and higher offices than the mere production of fluid to moisten the surface of the mucous membrane, although, doubtless, this is one of their functions. The glands of Peyer occur exclusively in the small intestine. They are found in greatest abundance in the lower part of the ileum near to the ileo-csecal valve. They are met with in two conditions, viz., either scattered singly, in which case they are termed glandulce solitaries, or aggregated in groups varying from one to three inches in length and about half an inch in width, chiefly of an oval form, their long axis parallel with that of the intestine. In this state they are named glandulce agininatce, the groups being commonly called Peyer's patches (Fig. 75). The latter are placed almost always opposite the attachment of the mesentery. In structure, and probably in function, there is no essential difference between the solitary glands and the individual bodies of which each group or patch is made up; but the surface of the solitary glands (Fig. 76) is beset with villi, from which those forming the agminate 242 DIGESTION. patches (Fig. 77) are usually free. In the condition in which they have been most commonly examined, each gland appears as a circular opaque-white sacculus, from half a line to a line Agminate follicles, or Peyer's patch, in a state of distension : magnified about 5 diameters (after Boehm). in diameter, and, according to the degree in which it is de- veloped, either sunk beneath, or more or less prominently raised on, the surface of a depression or fossa in the mucous FIG. 77. FIG. 76. Solitary gland of small intestine Rafter Boehm). FIG. 77. Part of a patch of the so-called Peyer's glands magnified, showing the various forms of the sacculi, with their zone of foramina. The rest of the mem- brane marked with Lieberkuhn's follicles, and sprinkled with villi (after Boehm). 243 membrane. Each gland is surrounded by openings like those of Lieberkiihn's follicles (see Fig. 77) except that they are more elongated ; and the direction of the long diameter of each opening is such that the whole produce a radiated ap- pearance around the white sacculus. These openings appear to belong to tubules identical with Lieberkiihn's follicles : they have no communication with the sacculus, and none of its contents escape through them on pressure. Neither can any permanent opening be detected in the sacculus or Peyer's gland itself (see Fig. 78). Each gland is an imperfectly closed sac or follicle formed of a tolerably firm membranous capsule of fine connective tissue, imbedded in a rich plexus of minute bloodvessels, many fine branches from which pass through the capsule and enter, chiefly loopwise, the interior of the follicle (Fig. 79). Entering into the formation of the sacculus, moreover, and forming a stroma or supporting framework throughout its in- terior, is lymphoid or adenoid tissue (Fig. 72), continuous with that which forms a great part of the mucous membrane out- side of it. The contents of each sac consist of a pale grayish FIG. 78. a- /,' Side view of a portion of intestinal mucous membrane of a cat, showing a Peyer's gland (a) : it is imbedded in the submucous tissue (/), the line of separation between which and the mucous membrane passes across the gland ; 6, one of the tubular fol- licles, the orifices of which form the zone of openings around the gland ; c, the fossa in the mucous membrane ; d, villi ; e, follicles of Lieberkuhn (after Bendz). opalescent pulp, formed of albuminous and fatty matter, and a multitude of nucleated corpuscles of various sizes, resembling exactly those found in lymphatic glands. The real office of these Peyerian glands or follicles is still unknown. It was formerly believed that each follicle was a 244 DIGESTION. kind of secreting cell, which, when its contents were fully ma- tured, formed a communication with the cavity of the intes- tine by the absorption or bursting of its own cell -wall, and of the portion of mucous membrane over it, and thus discharged its secretion into the intestinal canal. A small shallow cavity or space was thought to remain, for a time, after this absorp- tion or dehiscence, but shortly to disappear, together with all trace of the previous gland. More recent acquaintance with the real structure of these bodies seems, however, to prove that they are not mere tempo- rary gland-cells which thus discharge their elaborated con- tents into the intestine and then disappear, but that they are rather to be regarded as structures analogous to lymphatic or FIG. 79. Transverse section of injected Peyer's glands (from Kolliker). The drawing was taken from a preparation made by Frey : it represents the fine capillary looped net- work spreading from the surrounding bloodvessels into the interior of three of Peyer's capsules from the intestine of the rabbit. absorbent glands, and that their office is to take up certain materials from the chyle, elaborate and subsequently discharge them into the lacteals, with which vessels they appear to be closely connected, although no direct communication has been proved to exist between them. Moreover, it has been lately suggested that since the mo- 245^ lecular and cellular contents of the glands are so abundantly traversed by minute bloodvessels, important changes may mu- tually take place between these contents and the blood in the vessels, material being abstracted from the latter, elaborated by the cells, and then restored to the blood, much in the same manner as is believed to be the case in the so-called vascular glands, such as the spleen, thymus, and others ; and that thus Peyer's glands should also be regarded as closely analogous to these vascular glands. Possibly they may combine the func- tions both of lymphatic and vascular glands, absorbing and elaborating material both from the chyle and from the blood within their minute vessels, and transmitting part to the lac- teal system and part direct to the blood. Brunrfs glands (Fig. 80) are confined to the duodenum; they are most abundant and thickly set at the commencement FIG. 80. Enlarged view of one of Brunn's glands from the human duodenum (from Frey). The main duct is seen superiorly ; its branches are elsewhere hidden by the bunches of opaque glandular vesicles. of this portion of the intestine, diminishing gradually as the duodenum advances. Situated beneath the mucous membrane, and imbedded in thesubmucous tissue, they are minutely lobu- lated bodies, visible to the naked eye, like detached small por- tions of pancreas, and provided with permanent gland-ducts, which pass through the mucous membrane and open on the internal surface of the intestine. As in structure, so probably in function, they resemble the pancreas; or at least stand to it in a similar relation to that which the small labial and buccal 21 246 DIGESTION. glands occupy in relation to the larger salivary glands, the" parotid, and submaxillary. The Villi (Figs. 81, 82) are confined exclusively to the mu- cous membrane of the small intestine. They are minute vas- cular processes, from a quarter of a line to a line arid two-thirds in length, covering the surface of the mucous membrane, and giving it a peculiar velvety, fleecy appearance. Krauss esti- FIG. 81. (Slightly altered from Teiehmann.) A. Villus of sheep. B. Villi of man. mates them at fifty to ninety in number in a square line, at the upper part of the small intestine, and at forty to seventy in the same area at the lower part. They vary in form even in the same animal, and differ according as the lymphatic ves- sels they contain are empty or full of chyle; being usually, in the former case, flat and pointed at their summits, in the latter cylindrical or clavate. Each villus consists of a small projection of mucous mem- brane, and its interior is therefore supported throughout by THE VILLI. 247 fine retiform or adenoid tissue, which forms the framework or stroma in which the other constituents are contained. The surface of the villus is clothed by columnar epithelium, which rests on a fine basement-membrane; while within this (From Teichiminn.) A, lacteals in villi. p, Payer's glands. Band D, superficial and deep network of lacteals in sulmiucous tissue. L, Lieberkiihn's glands. K, small branch of lacteal vessel on its way to mesenteric gland. 11 and o, muscular fibres of intestine, s, peritoneum. are found, reckoning from without inwards, bloodvessels, fibres of the muscularu mucosce, and a single lymphatic or lacteal 248 DIGESTION. vessel rarely looped or branched (Fig. 81); besides granular matter, fat-globules, &c. The epithelium is of the columnar kind, and continuous with that lining the other parts of the mucous membrane. The cells are arranged with their long axis radiating from the sur- face of the villus (Fig. 81), and their smaller ends resting on the basement-membrane. Some doubt exists concerning the minute structure of these cells, and their relation to the deeper parts of the villus. Beneath the basement or limiting membrane there is a rich supply of bloodvessels. Two or more minute arteries are dis- tributed within each villus ; and from their capillaries, which form a dense network, proceed one or two small veins, which pass out at the base of the villus. The layer of the muscularis mucosce in the villus forms a kind of thin hollow cone immediately around the central lacteal, and, is therefore, situate beneath the bloodvessels. The ad- dition of acetic acid to the villus brings out the characteristic nuclei of the muscular fibres, and shows the size and position of the layer most distinctly. Its use is still unknown, although it is impossible to resist the belief, that it is instrumental in the propulsion of chyle along the lacteal. The lacteal vessel enters the base of each villus, and passing up in the middle of it, extends nearly to the tip, where it ends commonly by a closed and somewhat dilated extremity. In the larger villi there may be two small lacteal vessels which end by a loop (Fig. 81.), or the lacteals may form a kind of network in the villus. The last method of ending, however, is rarely or never seen in the human subject, although com- mon in some of the lower animals (A, Fig. 81). The office of the villi is the absorption of chyle from the completely digested food in the intestine. The mode in which they effect this will be considered in the chapter on Absorp- tion. Structure of the Large Intestine. The large intestine, which in an adult is from about 4 to 6 feet long, is subdivided for descriptive purposes into three portions, viz. : the ccecum, a short wide pouch, communicating with the lower end of the small intestine through an opening, guarded by the ileo-ccecal valve ; the colon, continuous with the caecum, which forms the principal part of the large intestine, and is divided into an ascending, transverse, and descending portion ; and the rectum, which, after dilating at its lower part, again contracts, and immediately afterwards opens externally THE LARGE IXTESTIXE. 249 through the anus. Attached to the caecum is the small appen- dix vermiformis. Like the small intestine, the large is constructed of three principal coats, viz., the serous, muscular, and mucous. The serous coat need not be here particularly described. Connected with it are the small processes of peritoneum containing fat, called appendices epiploicce. The fibres of the muscular coat, like those of the small intestine, are arranged in two layers the outer longitudinally, the inner circularly. In the caecum and colon, the longitudinal fibres, besides being, as in the small intestine, thinly disposed in all parts of the wall of the bowel, are collected, for the most part, into three strong bands, which being shorter, from end to end, than the other coats of the in- testine, hold the canal in folds, bounding intermediate sacculi. On the division of these bands, the intestine can be drawn out to its full length, and it then assumes, of course, a uniformly cylindrical form. In the rectum, the fasciculi of these longi- tudinal bands spread out and mingle with the other longitudi- nal fibres, forming with them a thicker layer of fibres than exists on any other part of the intestinal canal. The circular muscular fibres are spread over the whole surface of the bowel, but are somewhat more marked in the intervals between the sacculi. Towards the lower end of the rectum they become more numerous, and at the anus they form a strong band called the internal sphincter muscle. The mucous membrane of the large, like that of the small intestine, is lined throughout by columnar epithelium, but, unlike it, is quite smooth and destitute of villi, and is not pro- jected in the form of valvulse conniveutes. Its general micro- scopic structure resembles that of the small intestine. Glands of the Large Intestine. The glands with which the large intestine is provided are of two kinds, the tubular and. lenticular. The tubular glands, or glands of Lieberkiihn, resemble those of the small intestine, but are somewhat larger and more numerous. They are also more uniformly distributed. The lenticular glands are most numerous in the caecum and vermiform appendix. They resemble in shape and structure, almost exactly, the solitary glands of the small intestine, and, like them, have no opening. Just over them, however, there is commonly a small depression in the mucous membrane, which has led to the erroneous belief that some of them open on the surface. The functions discharged by the glands found in the large intestine are not known with any certainty, but there is no 250 DIGESTION. reason to doubt that they resemble very nearly those discharged by the glands of like structure in the small intestine. The difficulty of determining the function of any single set of the intestinal glands seems indeed almost insuperable, so many fluids being discharged together into the intestine ; for all acting, probably, at once, produce a combined effect upon the food, so that it is almost impossible to discern the share of any one of them in digestion. Ileo-ccecal valve. The ileo-csecal valve is situate at the place of junction of the small with the large intestine, and guards against any reflux of the contents of the latter into the ileum. It is composed of two semilunar folds of mucous membrane. Each fold is formed by a doubling inwards of the mucous mem- brane, and is strengthened on the outside by some of the circu- lar muscular fibres of the intestine, which are contained be- tween the outer surfaces of the two layers of which each fold is composed. The inner surface of the folds is smooth ; the mucous membrane of the ileum being continuous with that of the csecum. That surface of each fold which looks towards the small intestine is covered with villi, while that which looks to the csecum has none. When the csecum is distended, the margins of the folds are stretched, and thus are brought into firm apposition one with the other. While the circular muscular fibres of the bowel at the junc- tion of the ileum with the csecum are contained between the outer opposed surfaces of the folds of mucous membrane which form the valve, the longitudinal muscular fibres and the peri- toneum of the small and large intestine respectively are con- tinuous with each other, without dipping in to follow the cir- cular fibres and the mucous membrane. In this manner, therefore, the folding inwards of these two last-named structures is preserved, while on the other hand, .by dividing the longi- tudinal muscular fibres and the peritoneum, the valve can be made to disappear, just as the constrictions between the sacculi of the large intestine can be made to disappear by performing a similar operation. The Pancreas, and its Secretion. The pancreas is situated within the curve formed by the duodenum ; ami its main duct opens into that part of the in- testine, either through a small opening or through a duct com- mon to itself and to the liver. The pancreas, in its minute anatomy, closely resembles the salivary glands ; and the fluid elaborated by it appears almost identical with saliva. When obtained pure, in all the different animals in which it has been THE PA Is CREATIC SECRETION. 251 hitherto examined, it has been found colorless, transparent, and slightly viscid. It is alkaline when fresh, and contains a pecu- liar animal matter named pancreatin and certain salts, both of which are very similar to those found in saliva. In pancreatic secretion, however, there is no sulpho-cyanogen. Pancreatin is a substance coagulable by heat, and in many other respects very like albumen : to it the peculiar digestive power of the pancreatic secretion is probably due-. Like saliva, the pan- creatic fluid, shortly after its escape, becomes neutral and then acid. The following is the mean of three analyses by Schmidt : Composition of Pancreatic Secretion. Water, 980.45 Solids, 19.55 Panerealin, . . . . . .1271 Inorganic bases and salt?, . . . . 6 84 19.55 The functions of the pancreas are probably as follows : 1. Numerous experiments have shown, that starch is acted upon by the pancreatic secretion, or by portions of pancreas put in starch-paste, in the same manner that it is by saliva and portions of the salivary glands. And although, as before stated (p. 212), many substances besides those glands can ex- cite the transformation of starch into dextrin and grape-sugar, yet it appears probable that the pancreatic fluid, exercising this power of transformation, is largely subservient to the pur- pose of digesting starch. MM. Bouchardat and Sandras have shown that the raw starch-granules which have passed un- changed through the crops and gizzards of granivorous birds, or through the stomachs of herbivorous Mammalia, are, in the small intestine, disorganized, eroded, and finally dissolved, as they are when exposed, in experiment, to the action of the pancreatic fluid. The bile cannot effect such a change m starch ; and it is most probable that the pancreatic secretion is the principal agent in the transformation, though it is by no means clear that the office may not be shared by the secretion of the intestinal mucous membrane, whicli alsf> seems to possess the power of converting starch into sugar. 2. The existence of a pancreas in Carnivora, which have little or no starch in their food, and the results of various ob- servations and experiments, leave very little doubt that the pancreatic secretion also assists largely in the digestion of 252 DIGESTION. fatty matters, by transforming them into a kind of emulsion, and thus rendering them capable of absorption by the lacteals. Several cases have been recorded in which the pancreatic duct being obstructed, so that the secretion could not be discharged, fatty or oily matter was abundantly discharged from the in- testines. In nearly all these cases, indeed, the liver was eoin- cidently diseased, and the change or absence of the bile might appear to contribute to the result ; yet the frequency of exten- sive disease of the liver, unaccompanied by fatty discharges from the intestines, favors the view that, in these cases, it is to the absence of the pancreatic fluid from the intestines that the excretion or non-absorption of fatty matter should be ascribed. In Bernard's experiments too, fat always appeared in the evacuations when the pancreas was destroyed or its duct tied. Bernard, indeed, is of opinion that to emulsify fat is the ex- press office of the pancreas, and the evidence that he and others have brought forward in support of this view is very weighty. The power of emulsifying fat, however, although perhaps mainly exercised by the secretion of the pancreas, is evidently possessed to some extent by other secretions poured into the intestines, and especially by the bile. 3. The pancreatic secretion discharges a third function also, namely, that of dissolving albuminous substances ; the peptone produced by the action of the pancreatic secretion on proteids not differing essentially from that formed by the action of the gastric juice (see p. 229). Structure of the Liver. The liver is an extremely vascular organ, and receives its supply of blood from two distinct vessels, the portal vein and hepatic artery, while the blood is returned from it into the vena cava inferior by the hepatic vein. Its secretion, the bile, is conveyed from it by the hepatic duct, either directly into the intestine, or, when digestion is not going on, into the cystic duct, and thence into the gall-bladder, where it accumulates until required. The portal vein, hepatic artery, and hepatic duct branch together throughout the liver, while the hepatic vein and its tributaries run by themselves. On the outside the liver has an incomplete covering of peri- toneum, and beneath this is a very fine coat of areolar tissue, continuous over the whole surface of the organ. It is thickest where the peritoneum is absent, and is continuous on the general surface of the liver with the fine, and, in the human subject, almost imperceptible, areolar tissue investing the STRUCTURE OF THE LIVER. 253 lobules. At the transverse fissure it is merged in the areolar investment called Glisson's capsule, which surrounding the FIG. 83. Under surface of the liver (from Bonamy). R, right lobe ; L, left lobe ; Q. lobus quadratus ; S, lobus Spigelii ; C, lobus caudatus ; 1, umbilical vein in longitudinal fissure; 2, gall-bladder in its fissure; 8, hepatic ar- tery in transverse fissure ; 4, hepatic duct in transverse fissure ; 5, portal vein in transverse fissure; 6, line of reflexion of peritoneum; 7, vena cava; 8, obliterated ductus venosus ; 9, ductus commuuis choledochus. FIG. 84. portal vein, hepatic artery, and hepatic duct, as they enter at this part, accompanies them in their branchings through the sub- stance of the liver. The liver is made up of small roundish or oval portions called lobules, each of which is about ^ of an inch in diameter, and com- posed of the minute branches of the portal vein, hepatic artery, he- patic duct, and hepatic vein ; while the interstices of these vessels are filled by the liver cells. These cells (Fig. 84), which make up a great portion of the substance of the organ, are rounded or polygonal, from about g ^ to j^ of an inch in diameter, containing well-marked nuclei and granules, and having sometimes a yellowish tinge, especially about their nuclei ; fre- quently, they contain also various sized particles of fat (Fig. 84 A). Each lobule is very sparingly invested by areolar tis- sue. To understand the distribution of the bloodvessels in the 22 254 DIGESTION. liver, it will be well to trace, first, the two bloodvessels and the duct which enter the organ on the under surface at the transverse fissure, viz., the portal vein, hepatic artery, and he- patic duct. As before remarked, all three run in company, and their appearance on longitudinal section is shown in Fig. 85. Running together through the substance of the liver, FIG. 85. Longitudinal section of a portal canal, containing a portal vein, hepatic artery, and hepatic duct, from the pig (after Kiernan) ^. p, branch of vena portse, situated in a portal canal, formed amongst the lobules of the liver, and giving off vaginal branches; there are also seen within the large portal vein numerous orifices of the smallest interlobular veins arising directly from it ; A, hepatic artery ; D, hepatic duct. they are contained in small channels, called portal canals, their immediate investment being a sheath of areolar tissue, called Glisson's capsule. To take the distribution of the portal vein first: In its course through the liver this vessel gives off small branches, which divide and subdivide between the lobules surrounding them and limiting them, and from this circumstance called inter- lobular veins; From these small vessels a dense capillary net- work is prolonged into the substance of the lobule, and this network gradually gathering itself up, so to speak, into larger vessels, converges finally to a single small vein, occupying the centre of the lobule, and hence called wJralobular. This ar- STRUCTURE OF THE LIVER. 255 rangement is well seen in Fig. 86, which represents a trans- verse section of a lobule. The smaller branches of the portal vein being closely surrounded by the lobules, give off directly FIG. 86. Cross-section of a lobule of the human liver, in which the capillary network be- tween the portal and hepatic veins has been fully injected (from Sappey) 60 j. Section of the iw/ralobular vein; 2, its smaller branches collecting blood from the capillary network ; 3, tnterlobular branches of the vena portse with their smaller ramifications passing inwards towards the capillary network in the substance of the lobule. veins (see Fig. 85) ; but here and there, especially where the hepatic artery and duct intervene, branches called vaginal first arise, and breaking up in the sheath are subse- quently distributed like the others around the lobules and be- come mferlobular. The larger trunks of the portal vein being more separated from the lobules by a thicker sheath of Glisson's capsule, give off vaginal branches alone, which, however, after breaking up in the sheath, are distributed like the others be- tween the lobules, and become iwferlobular veins. The small mralobular veins discharge their contents into veins called sitMobular (Fig. 88), while these again, by their union, form the main branches of the hepatic vein, which leaves the posterior border of the liver to end by two or three prin- cipal trunks in the inferior vena cava, just before its passage through the diaphragm. The s-u61obular and hepatic veins, unlike the portal vein and its companions, have little or no areolar tissue around them, and their coats being very thin, 256 DIGESTION. they form little more thaD mere channels in the liver sub- stance which closely surrounds them. FIG. 87. Section of a portion of liver passing longitudinally through a considerable hepatic vein, from the pig (after Kiernan) 5.. H, hepatic venous trunk, against which the sides of the lobules are applied ; b, sublobular hepatic veins, on which the bases of the lobules rest, and through the coats of which they are seen as polygonal figures ; a, a, walls of the hepatic venous canal, formed by the polygonal bases of the lobules. The manner in which the lobules are connected with the sublobular veins by means of the small intralobular veins is well seen in the diagram, Fig. 88, and in Fig. 87, which rep- resent the parts as seen in a longitudinal section. The ap- pearance has been likened to a twig having leaves without footstalks the lobules representing the leaves, and the sub- lobular vein the small branch from which it springs. On a transverse section, the appearance of the intralobular veins is that of 1, Fig. 86, while both a transverse and longitudinal section are exhibited in Fig. 89. The hepatic artery, the function of which is to distribute blood for nutrition to Glisson's capsule, the walls of the ducts and bloodvessels, and other parts of the liver, is distributed in a very similar manner to the portal vein, its blood being re- turned by small branches either into the ramifications of the STRUCTURE OF THE LIVER. 257 FIG. 88. Lobu.Ii portal vein, or into the capillary plexus of the lobules which connects the inter- and mfralobular veins. The hepatic duct divides and subdivides in a manner very like that of the portal vein and hepatic artery, the larger branches being lined by cylindrical, and the smaller by small polygonal epi- thelium. The exact arrangement of its terminal branches, however, and their relation to the liver-cells have not been clearly made out, or, at least, have not been agreed upon by different observers. The chief theories on the subject are three in number: 1. That the terminal branches of the hepatic duct form an inter- Loin lobular network, which abuts on the outermost cells of a lobule, but does not enter the inside of the lob- ule, or only for a little way. 2. That minute branches begin on the sublobular veins (after K ier- in the lobules between the cells, not na n). inclosing them. 3. That the ultimate branches begin in the lobules and in- close hepatic cells. FIG. 89. Diagram showing the manner in which the lobules of the liver rest Capillary network of the lobules of the rabbit's liver (from Kolliker) 4 T 5 . The figure is taken from a very successful injection of the hepatic veins, made by Hart- ing: it shows nearly the whole of two lobules, and parts of three others; p, portal branches running in the interlobular spaces; h, hepatic veins penetrating and radi- ating from the centre of the lobules. 258 DIGESTION. The illustrations below will show the conflicting theories at a glance. FIG. 90. Diagrams showing the arrangement of the radicles of the hepatic duct, according to different observers. 1. 2, 2, are two branches of the hepatic duct, which is supposed to commence in a plexus situated towards the circumference of the lobule marked 4, 4, called by Kier- nan the biliary plexus. Within this is seen the central part of the lobule, contain- ing- branches of the intralobular vein, 1, 1. 2. A small fragment of an hepatic lobule, of which the smallest intercellular bili- ary ducts were filled with coloring matter during life, highly magnified (from Chrzonszczewsky). 3. View of some of the smallest biliary ducts illustrating Beale's view of their relation to the biliary cells (from Kolliker after Beale), 1 |A. The drawing is taken from an injected preparation of the pig's liver ; a, small branch of an interlobular hepatic duct ; c, smallest biliary ducts ; b, portions of the cellular part of the lobule in which the cells are seen within tubes which commu- nicate with the finest ducts. THE BILE. 259 Functions of the Liver. The Secretion of Bile is the most obvious, and one of the chief functions which the liver has to perform ; but, as will be presently shown, it is not the only one ; for important changes are effected in certain constituents of the blood in its transit through this gland, whereby they are rendered more fit for their subsequent purposes in the animal economy. The Bile. Composition of the Bile. The bile is a somewhat viscid fluid, of a yellow or greenish-yellow color, a strongly bitter taste, and when fresh with a scarcely perceptible odor ; it has a neutral or slightly alkaline reaction, and its specific gravity is about 1020. Its color and degree of consistence vary much, apparently independent of disease ; but, as a rule, it becomes gradually more deeply colored and thicker as it advances along its ducts, or when it remains long in the gall-bladder, wherein, at the same time, it becomes more viscid and ropy, of a darker color, and more bitter taste, mainly from its greater degree of concentration, on account of partial absorp- tion of its water, but partly also from being mixed with mucus. The following analysis is by Frerichs : Composition of Human Bile. Water, 859.2 Solids, 140.8 1000.0 Biliary acids combined "I D . r nl K with alkalies, } Bllin ' * Fat, 9.2 Cholesterin, 2.6 Mucus and coloring matters, .... 29.8 Salts, 7.7 140.S The bilin or biliary matter when freed by ether from the fat with which it is combined, is a resinoid substance, soluble in water, alcohol, and alkaline solutions, and giving to the watery solution the taste and general character of bile. It is a com- pound of soda, with two resinous acids, named glycocholic and taurocholic acids. The -former consists of cholic acid conjugated with glycin (or sugar of gelatin), the latter of the same acid conjugated with taurin. Fatty substances are found in variable proportions. Besides 260 DIGESTION. FIG. 91. Crystalline scales of c-holcsterin. the ordinary saponifiable fats, there is a small quantity of cholesterin (p. 20), which, with the other free fats, is probably held in solution by the tauro-cholate of soda. A peculiar substance, which Dr. Flint has discovered in the faeces, and named stercorin (p. 274), is closely allied to choles- terin ; and Dr. Flint believes that while one great function of the liver is to excrete cholesterin from the blood, as the kidney excretes urea, the stercorin of fseces is the modified form in which cholesterin finally leaves the body. Ten grains and a half of stercorin, he reckons, are excreted daily. The coloring matter of the bile has not yet been obtained pure, owing to the facility with which it is decomposed. It occasionally deposits itself in the gall-bladder as a yellow substance mixed with mucus, and in this state has been frequently examined. It is composed of two coloring matters, called biliverdin and bilifalvin. By oxidizing agencies, as exposure to the air, or the addition of nitric acid, it assumes a dark green color. In cases of biliary obstruction, it is often reabsorbed, circulates with the blood, and gives to the tissues the yellow tint charac- teristic of jaundice. There seems to be some relationship between the coloring matters of the blood and bile, and, it may be added, between these and that of the urine also, so that it is possible they may be, all of them, varieties of the same pigment, or derived from the same source. Nothing, however, is at present certainly known regarding the relation in which one of them stands to the other. The mucus in bile is derived chiefly from the mucous mem- brane of the gall-bladder, but in part also from the hepatic ducts and their branches. It constitutes the residue after bile is treated with alcohol. The epithelium with which it is mixed may be detected in the bile with the microscope in the form of cylindrical cells, either scattered or still held together in layers. To the presence of this mucus is probably to be ascribed the rapid decomposition undergone by the bilin ; for, according to Berzelius, if the mucus be separated, bile will remain unchanged for many days. The saline or inorganic constituents of the bile are similar to THE BILE. 261 those found in most other secreted fluids. It is possible that the carbonate and neutral phosphate of sodium and potassium, found in the ashes of bile, are formed in the incineration, and do not exist as such in the fluid. Oxide of iron is said to be a common constituent of the ashes of bile, and copper is gen- erally found in healthy bile, and constantly in biliary calculi. Such are the principal chemical constituents of bile ; but its physiology is, perhaps, better illustrated by its ultimate ele- mentary composition. According to Liebig's analysis, the biliary matter, consisting of bilin and the products of its spontaneous decomposition yields, on analysis, 76 atoms of carbon, 66 of hydrogen, 22 of oxygen, 2 of nitrogen, and a cer- tain quantity of sulphur. 1 Comparing this with the ultimate composition of the organic parts of blood, which may be stated at C 4S H 36 N 6 O 14 , with sulphur and phosphorus it is evident that bile contains a large preponderance of carbon and hydrogen, and a deficiency of nitrogen. The import of this will pres- ently appear. TESTS FOR BILE. A common test for the presence of bile consists of the addition of a small quantity of nitric acid, when, if bile be present, a play of colors is produced, beginning with green and passing through various tints to red. This test will detect only the coloring matter of the bile. The best test for the bilin is Pettenkofer's. To the liquid suspected to contain bile must be added, first, a drop or two of a strong solution of cane-sugar (one part of sugar to four parts of water), and immediately afterwards sulphuric acid, to the extent of about two-thirds of the liquid. On first adding the acid, a whitish precipitate falls; but this redissolves with a slight excess of the acid, and on the further addition of the latter there appears a bright cherry-red color, gradually chang- ing through a lake tint to a dark purple. The process of secreting bile is probably continually going on, but appears to be retarded during fasting, and accelerated on taking food. This was shown by Blondlot, who, having tied the common bile-duct of a dog, and established a fistulous opening between the skin and gall-bladder, whereby all the bile secreted was discharged at the surface, noticed that when the animal was fasting, sometimes not a drop of bile was discharged for several hours ; but that, in about ten minutes after the 1 The sulphur is combined with the taurin one of the substances yielded by the decomposition of bilin. According to Dr. Kemp, the sulphur in the bile of the ox, dried and freed from mucus, coloring matter, and salts, constitutes about 3 per cent. 262 DIGESTION. introduction of food into the stomach, the bile began to flow abundantly, and continued to do so during the whole period of digestion. Bidder and Schmidt's observations are quite in accordance with this. The bile is probably formed first in the hepatic cells ; then, being discharged into the minute hepatic ducts, it passes into the larger trunks, and from the main hepatic duct may be carried at once into the duodenum. But, probably, this hap- pens only while digestion is going on ; during fasting it flows from the common bile-duct into the cystic-duct, and thence into the gall-bladder, where it accumulates till, in the next period of digestion, it is discharged into the intestine. The gall-bladder thus fulfils what appears to be its chief or only office, that of a reservoir ; for its presence enables bile to be constantly secreted for the purification of the blood, yet insures that it shall all be employed in the service of digestion, although digestion is periodic, and the secretion of bile constant. The mechanism by which the bile passes into the gall-bladder is simple. The orifice through which the common bile-duct communicates with the duodenum is narrower than the duct, and appears to be closed, except when there is sufficient pres- sure behind to force the bile through it. The pressure exer- cised upon the bile secreted during the intervals of digestion appears insufficient to overcome the force with which the orifice of the duct is closed ; and the bile in the common duct, finding no exit in the intestine, traverses the cystic-duct, and so passes into the gall-bladder, being probably aided in this retrograde course by the peristaltic action of the ducts. The bile is dis- charged from the gall-bladder, and enters the duodenum on the introduction of food into the small intestine : being pressed on by the contraction of the coats of the gall-bladder, and probably of the common bile-duct also ; for both these organs contain organic muscular fibre-cells. Their contraction is ex- cited by the stimulus of the food in the duodenum acting so as to produce a reflex movement, the force of which is sufficient to open the orifice of the common bile-duct. Various estimates have been made of the quantity of bile dis- charged in the intestines in twenty-four hours : the quantity doubtless varying, like that of the gastric fluid, in proportion to the amount of food taken. A fair average of several com- putations would give thirty to forty ounces as the quantity daily secreted by man. The purposes served by the secretion of bile may be considered to be of two principal kinds, viz., excrementitious and digestive. As an excrementitious substance, the bile serves especially as a medium for the separation of excess of carbon and hydro- THE BILE MECONIUM. 263 gen from the blood ; and its adaptation to this purpose is well illustrated by the peculiarities attending its secretion and dis- posal in the foetus. During intra-uterine life, the lungs and the intestinal canal are almost inactive ; there is no respiration of open air or digestion of food ; these are unnecessary, because of the supply of well-elaborated nutriment received by the vessels of the foetus at the placenta. The liver, during the same time, is proportionally larger than it is after birth, and the secretion of bile is active, although there is no food in the intestinal canal upon which it can exercise any digestive property. At birth, the intestinal canal is full of thick bile, mixed with intestinal secretion ; for the meconium, or faeces of the fcetus, are shown by the analyses of Simon and of Frerichs to contain all the essential principles of bile. Composition of Meconium (Frerichs) : Biliary resin, ....... 15.6 Common fat and cholesterin, .... 15.4 Epithelium, mucus, pigment, and salts, . . 69 100. In the foetus, therefore, the main purpose of the secretion of bile must be the purification of the blood by direct excretion, i. e. y by separation from the blood, and ejection from the body without further change. Probably all the bile secreted in foetal life is incorporated in the meconium, and with it dis- charged, and thus the liver may be said to discharge a function in some sense vicarious of that of the lungs. For, in the foetus, nearly all the blood coming from the placenta passes through the liver, previous to its distribution to the several organs of the body ; and the abstraction of carbon, hydrogen, and other elements of bile will purify it, as in extra-uterine life it is purified by the separation of carbonic acid and water at the lungs. The evident disposal of the foetal bile by excretion, makes it highly probable that the bile in extra-uterine life is also, at least in part, destined to be discharged as excrementitious. But the analysis of the faeces of both children and adults shows that (except when rapidly discharged in purgation) they con- tain very little of the bile secreted, probably not more than one-sixteenth part of its weight, and that this portion includes only its coloring, and some of its fatty matters, but none of its essential principle, the bilin. All the bilin is again absorbed from the intestines into the blood. But the elementary com- position of bilin (see p. 261) shows such a preponderance of carbon and hydrogen, that it cannot be appropriated to the nutrition of the tissues; therefore, it maybe presumed that 264 DIGESTION. after absorption, the carbon and hydrogen of the bilin com- bining with oxygen, are excreted as carbonic acid and water. The destination of the bile is, on this theory, essentially the same in both foetal and extra-uterine life; only, in the former, it is directly excreted, in the latter for the most part indirectly, being, before final ejection, modified in its absorption from the intestines, and mingled with the blood. The change from the direct to the indirect mode of excre- tion of the bile may, with much probability, be connected with a purpose in relation to the development of heat. The tem- perature of the foetus is maintained by that of the parent, and needs no source of heat within the body of the foetus itself; but, in extra-uterine life, there is (as one may say) a waste of material for heat when any excretion is discharged unoxid- ized ; the carbon and hydrogen of the bilin, therefore, instead of being ejected in the faeces, are reabsorbed, in order that they may be combined with oxygen, and that in the combina- tion, heat may be generated. From the peculiar manner in which the liver is supplied with much of the blood that flows through it, it is probable, as Dr. Budd suggest, that this organ is excretory, not only for such hydro-carbonaceous matters as may need expulsion from any portion of the blood, but that it serves for the direct purification of the stream which, arriving by the portal vein, has just gathered up various substances in its course through the digestive organs substances which may need to be ex- pelled, almost immediately after their absorption. For it is easily conceivable that many things may be taken up during digestion, which not only are unfit for purposes of nutrition, but which would be positively injurious if allowed to mingle with the general mass of the blood. The liver, therefore, may be supposed placed in the only road by which such matters can pass into the general current, jealously to guard against their further progress, and turn them back again into an excretory channel. The frequency with which metallic poisons are either excreted by the liver or intercepted and retained, often for a considerable time, in its own substance, may be adduced as evidence for the probable truth of this supposition. Though one chief purpose of the secretion of bile may thus appear to be the purification of the blood by ultimate excre- tion, yet there are many reasons for believing that, while it is in the intestines, it performs an important part in the process of digestion. In nearly all animals, for example, the bile is discharged, not through an excretory duct communicating with the external surface or with a simple reservoir, as most secretions are, but is made to pass into the intestinal canal, so FUNCTIONS OF THE LIVER. 265 as to be mingled with the chyme directly after it leaves the stomach ; an arrangement, the constancy of which clearly in- dicates that the bile has some important relations to the food with which it is thus mixed. A similar indication is furnished also by the fact that the secretion of bile is most active, and the quantity discharged into the intestines much greater, during digestion than at any other time ; although, without doubt, this activity of secretion during digestion may, how- ever, be in part ascribed to the fact that a greater quantity of blood is sent through the portal vein to the liver at this time, and that this blood contains some of the materials of the food absorbed from the stomach and intestines, which may need to be excreted, either temporarily, to be reabsorbed, or per- manently. Respecting the functions discharged by the bile in diges- tion, there is little doubt that it assists in emulsifying the fatty portions of the food, and thus rendering them capable of being absorbed by the lacteals. For it has appear in some experiments in which the common bile-duct was tied, that although the process of digestion in the stomach was un- affected, chyle was no longer well-formed ; the contents of the lacteals consisting of clear, colorless fluid, instead of being opaque and white, as they ordinarily are, after feeding. (2.) It is probable, also, from the result of some experiments by Wistinghausen and Hoffmann, that the moistening of the mucous membrane of the intestines by bile may facilitate ab- sorption of fatty matters through it. (3.) The bile, like the gastric fluid, has a strongly antisep- tic power, and may serve to prevent the decomposition of food during the time of its sojourn in the intestines. The experi- ments of Tiedemaun and Gmelin show that the contents of the intestines are much more fetid after the common bile-duct has been tied than at other times ; and the experiments of Bidder and Schmidt on animals with an artificial biliary fistula, con- firm this observation ; moreover, it is found that the mixture of bile with a fermenting fluid stops or spoils the process of fermentation. (4.) The bile has also been considered to act as a kind of natural purgative, by promoting an increased secretion of the intestinal glands, and by stimulating the intestines to the pro- pulsion of their contents. This view receives support from the constipation which ordinarily exists in jaundice, from the diarrhrea which accompanies excessive secretion of bile, and from the purgative properties of ox-gall. Nothing is known with certainty respecting the changes which the reabsorbed portions of the bile undergo, either in 266 DIGESTION. the intestines or in the absorbent vessels. That they are much changed appears from the impossibility of detecting them in the blood ; and that part of this change is effected in the liver is probable from an experiment of Magendie, who found that when he injected bile into the portal vein a dog was unharmed, but was killed when he injected the bile into one of the sys- temic vessels. The secretion of bile, as already observed, is only one of the purposes fulfilled by the liver. Another very important func- tion appears to be that of so acting upon certain constituents of the blood passing through it, as to render some of them capable of assimilation with blood generally, and to prepare others for being duly eliminated in the process of respiration. From the labors of M. Bernard, to whom we owe most of what we know on this subject, it appears that the low form of albu- minous matter, or albuminose, conveyed from the alimentary canal by the blood of the portal vein, requires to be submitted to the influence of the liver before it can be assimilated by the blood ; for if such albuminous matter is injected into the jugu- lar vein, it speedily appears in the urine ; but if introduced into the portal vein, and thus allowed to traverse the liver, it is no longer ejected as a foreign substance, but is probably incorporated with the albuminous part of the blood. An important influence seems also to be exerted by the liver upon the saccharine matters derived from the alimentary canal. The chief purpose of the saccharine and amylaceous princi- ples of food is, probably, in relation to respiration and the production of animal heat ; but in order that they may fulfil this, their main office, it seems to be essential that they should undergo some intermediate change, which is effected in the liver, and which consists in their conversion into a peculiar form of saccharine matter, very similar to glucose, or diabetic sugar. That such influence is exerted by . the liver seems proved by the fact that when cane sugar is injected into the jugular vein it is speedily thrown out of the system, and ap- pears in the urine ; but when injected into the portal vein, and thus enabled to traverse the liver, it ceases to be excreted at the kidneys ; and, what is still uiore to the point, a very large quantity of glucose may be injected into the venous system without any trace of it appearing in the urine. So that it may be concluded, that the saccharine principles of the food un- dergo, in their passage through the liver, some transformation necessary to the subsequent purpose they have to fulfil in rela- tion to the respiratory process, and without which, such pur- pose probably could not be properly accomplished, and the FORMATION OF SUGAR IN THE LIVER. 267 substances themselves would be eliminated as foreign matters by the kidneys. Then, again, it was discovered by Bernard, and the dis- covery has been amply confirmed, that the liver possesses the remarkable property of forming glucose or grape-sugar (C 6 H 13 O 6 ), or a substance readily convertible into sugar, even out of principles in the blood which contain no trace of saccharine or amylaceous matter. In Herbivora and in animals living on mixed diet, a large part of the sugar is derived from the sac- charine and amylaceous principles introduced in their food. But in animals fed exclusively on flesh, and deprived therefore of this source of sugar, the liver furnishes the means whereby it may be obtained. Not only in Carnivora, however, but ap- parently in all classes of animals, the liver is continually en- gaged, during health, in forming sugar, or a substance closely allied to it, in large amount. This substance may always be found in the liver, even when absent from -all other parts of the body. To demonstrate the presence of sugar in the liver, a. portion of this organ, after being cut into small pieces, is bruised in a mortar to a pulp with a small quantity of water, and the pulp is boiled with sulphate of soda in order to precipitate albu- minous and coloring matters. The decoction is then filtered and may be tested for glucose. The most usual test is Trom- mer's. To the filtered solution an equal quantity of liquor potassse is added, with a few drops of a solution of sulphate of copper. The mixture is then boiled, when the presence of sugar is indicated by a reddish-brown precipitate of the sub- oxide of copper. The researches of Bernard and others, however, have shown that the sugar is not formed at once at the liver, but that this organ has the power of producing a peculiar substance allied to starch, which is readily convertible into glucose when in contact with any animal ferment. This substance has received the different names of glycogen, glycogenic substance, animal starch, hepatin. Glycogen (C^H^C)^) is obtained by taking a portion of liver from a recently^ killed animal, and, after cutting it into small pieces, placing it for a short time in boiling water. It is then bruised in a mortar, until it forms a pulpy mass, and subsequently boiled in distilled water for about a quarter of an hour. The glycogen is precipitated from the filtered decoc- tion by the addition of alcohol. When purified, glycogen is a white, amorphous, starch-like substance, odorless and tasteless, soluble in water, but insoluble 268 DIGESTION. in alcohol. It is converted into glucose by boiling with dilute acids, or by contact with any animal ferment. There are two chief theories concerning the immediate desti- nation of glycogen. (1.) According to Bernard and most other physiologists, its conversion into sugar takes place rapidly during life, and the sugar is conveyed away by the blood of the hepatic veins to be consumed in respiration at the lungs. (2.) Pavy and others believe that the conversion into sugar only occurs after death, and that during life no sugar exists in healthy livers, the amyloid substance or glycogen being pre- vented by some force from undergoing the transformation. The chief arguments advanced by Pavy in support of this view are, first, that scarcely a trace of sugar is found in blood drawn during life from the right ventricle, or in blood collected from the right side of the heart immediately after an animal has been suddenly deprived of life, while if the examination be delayed for a little while after death, sugar in abundance may be found in such blood ; secondly, that the liver, like the venous,blood in the heart, is, at the moment of death, almost completely free from sugar, although afterwards its tissue speedily becomes saccharine, unless the formation of sugar be prevented by freezing, boiling, or other means calculated to interfere with the action of a ferment on the amyloid substance of the organ. Instead of adopting Bernard's view, that nor- mally, during life, glycogen passes as sugar into the hepatic venous blood, and thereby is conveyed to the lungs to be further disposed of, Pavy inclines to believe that it may repre- sent an intermediate stage in the formation of fat from ma- terials absorbed from the alimentary canal. For the present we must remain uncertain as to which of these theories contains most truth in it. Whatever be the destination of this peculiar amyloid sub- stance formed at the liver, most recent observers agree that it is formed at, and exists within, the hepatic cells, from which it may be extracted by the process just described. Much doubt exists also respecting the mode in which gly- cogen is formed in the liver, and the materials which furnish its source. Since its quantity is increased after feeding, espe- cially on substances containing much sugar or starch, it is probable that part of it is derived from saccharine principles absorbed from the digestive canal ; but since its formation con- tinues even when there is no starch or sugar in the food, the albuminous or fatty principles also have been thought capable of furnishing part of it. Numerous experiments, however, having proved that the liver continues to form sugar in animals after prolonged starvation, and during hibernation, and even FORMATION OF SUGAR IN THE LIVER. 269 after death, its production is clearly independent of the ele- ments of food. One of Bernard's experiments may be quoted in proof of this : Having fed a healthy dog for many days ex- clusively on flesh, he killed it, removed the liver at once, and before the contained blood could have coagulated, he thor- oughly washed out its tissue by passing a stream of cold water through the portal vein. He continued the injection until the liver was completely exsanguined, until the issuing water con- tained not a trace of sugar or albumen, and until no sugar was yielded by portions of the organ cut into slices and boiled in water. Having thus deprived the liver of all saccharine mat- ter, he left it for twenty-four hours, and on then examining it, found in its tissue a large quantity of soluble sugar, which must clearly have been formed subsequently to the organ being washed, and out of some previously insoluble and non-sac- charine substance. This and other experiments led him and others to the conclusion that the formation of the amyloid sub- stance by the liver is the result of a kind of secretion or elabo- ration out of materials in the solid tissues of the gland such secretion being probably effected by the hepatic cells, in which, indeed, as already observed, the substance has been detected. According to this view, then, the liver may be regarded as an organ engaged in forming two kinds of secretion, namely, bile and sugar, or rather, glycogen readily convertible into sugar. The former, chiefly excrementitious, passes along the bile-ducts into the intestines, where it may subserve some pur- poses in relation to digestion, and is then for the most part re- absorbed, and ultimately eliminated during the processes con- cerned in the production of animal heat. The latter, namely sugar, being soluble, is, unless Pavy's view be correct, taken up by the blood in the hepatic vein, conveyed through the right side of the heart to the lungs, where it is probably con- sumed in the respiratory process, and thus contributes to the production of animal heat. The formation of glycogen or of sugar is, like all other pro- cesses in the living body, under the control of the nervous sys- tem. Bernard discovered that by pricking the floor of the fourth ventricle, the quantity of sugar formed was so much in excess of the normal quantity, as to be excreted by the kidney, and thus produce the leading symptom of diabetes. Section of the inferior cervical ganglion of the sympathetic nerve also produces diabetes. The channel by which the influence of the nervous system, is conducted in the preceding and similar experiments is not accurately known ; no theory having been permanently estab- lished, which explains all the facts hitherto observed in con- 23 270 DIGESTION. nection with the influence of the nervous system on the pro- duction of glucose. Summary of the Changes which take place in the Food during its Passage through the Small Intestine. In order to understand the changes in the food which occur during its passage through the small intestine, it will be well to refer briefly to the state in which it leaves the stomach through the pylorus. It has been said before, that the chief office of the stomach is not only to mix into a uniform mass all the varieties of food that reach it through the oesophagus, but especially to dissolve the nitrogenous portion by means of the gastric juice. The fatty matters, during their sojourn in the stomach, become more thoroughly mingled with the other constituents of the food taken, but are not yet in a state fit for absorption. The conversion of starch into sugar, which began in the mouth, has been interfered with, although not stopped altogether. The soluble matters both those which were so from the first, as sugar and saline matter, and those which have been made so by the action of the saliva and gastric juice have begun to disappear by absorption into the blood- vessels, and the same thing has befallen such fluids as may have been swallowed, wine, water, &c. The thin pultaceous chyme, therefore, which during the whole period of gastric digestion, is being constantly squeezed or strained through the pyloric orifice into the duodenum, con- sists of albuminous matter, broken down, dissolving and half dissolved, fatty matter, broken down, but not dissolved at all, starch very slowly in process of conversion into sugar, and as it becomes sugar, also dissolving in the fluids with which it is mixed ; while with these are mingled gastric fluid, and fluid that has been swallowed, together with such portions of the food as are not digestible and will be finally expelled as part of the fseces. On the entrance of the chyme into the duodenum, it is sub- jected to the influence of the fluid secreted by Lieberkiihn's and Brunn's glands, before described, and to that of the bile and pancreatic juice, which are poured into this part of the intestine. Without doubt, that part of digestion which it is a chief duty of the small intestine to perform, is the alteration of the fat in such a manner as to make it fit for absorption. And there is no doubt that this change is chiefly effected in the upper part of the small intestine. What is the exact share of the process, however, allotted respectively to the bile, pancreatic DIGESTION IN SMALL INTESTINE. 271 secretion, and the secretion of the intestinal glands, is still un- certain. It is most probable, however, that the pancreatic secretion and the bile are the main agents in emulsifying the fat, and that they do this by direct admixture with it. They also promote its absorption by moistening the surface of the villi (p. 265). During digestion in the small intestine, the villi become turgid with blood, their epithelial cells become filled, by ab- sorption, with fat-globules, which, after minute division, trans- ude into the granular basis of the villus, and thence into the lacteal vessel in the centre, by which they are conveyed along the mesentery to the lymphatic glands, and thence into the thoracic duct. A part of the fat is also absorbed by the blood- vessels of the intestine. The term chyle is sometimes applied to the emulsified contents of the intestine after their admixture with the bile and pancreatic juice; but more strictly to the fluid contained in the lacteal vessels during digestion, which differs from ordinary lymph contained in the same vessels at other times, chiefly in the greatly increased quantity of fat particles which have been absorbed from the small intestine. Although the most evident function of the small intestine is the digestion of fat, it must not be forgotten that a great part of the other constituents of the food is by no means completely digested when it leaves the stomach. Indeed, its leaving it unabsorbed would, alone, be proof of this fact. The albuminous substances which have been partly dissolved in the stomach continue to be acted on by the gastric juice which passes into the duodenum with them, and the effect of the last-named secretion is assisted or complemented by that of the pancreas and intestinal glands. As the albuminous matters are dissolved, they are absorbed chiefly by the blood- vessels, and only to a small extent, probably, by the lacteals. The starchy, or amylaceous portion of the food, the conver- sion of which into dextrin and sugar was more or less inter- rupted during its stay in the stomach, is now acted on briskly by the secretion of the pancreas, and of Brunn's glands, and perhaps of Lieberkuhn's glands also, and the sugar as it is formed dissolves in the intestinal fluids, and afterwards, like the albumen, is absorbed chiefly by the bloodvessels. The liquids, swallowed as such, which may have escaped absorption in the stomach, are absorbed probably very soon after their entrance into the intestine ; the fluidity of the con- tents of the latter being preserved more by the constant secre- tion of fluid by the intestinal glands, pancreas, and liver, than by any given portion of fluid, whether swallowed or secreted, remaining long unabsorbed. From this fact, therefore, it may 272 DIGESTION. be gathered that there is a kind of circulation constantly pro- ceeding from the intestines into the blood, and from the blood into the intestines again ; for, as all the fluid, probably a very large amount, secreted by the intestinal glands, must come from the blood, the latter would be too much drained, were it not that the same fluid after secretion is again reabsorbed into the current of blood going into the blood charged with nutrient products of digestion, coming out again by secretion through the glands in a comparatively uncharged condition. It has been said before that the contents of the stomach dur- ing gastric digestion have a strongly acid reaction. On the entrance of the chyme into the small intestine, this is gradu- ally neutralized to a greater or less degree by admixture with the bile and other secretions with which it is mixed, and the acid reaction becomes less and less strongly marked as the chyme passes along the canal towards the ileo-csecal valve. Thus, all the materials of the food are acted on in the small intestine, and a great portion of the nutrient matter is absorbed, the fat chiefly by the lacteals, the other principles, when in a state of solution, chiefly by the bloodvessels, but neither, prob- ably, exclusively by one set of vessels. At the lower end of the small intestine, the chyme, still thin and pultaceous, is of a light yellow color, and has a distinctly fecal odor. In this state it passes through the ileo-csecal opening into the large intestine. Summary of the Process of Digestion in the Large Intestine. The changes which take place in the chyme after its passage from the small into the large intestine are probably only the continuation of the same changes that occur in the course of the food's passage through the upper part of the intestinal canal. From the absence of villi, however, we may conclude that absorption, especially of fatty matter, is in great part com- pleted in the small intestine, while, from the still half-liquid, pultaceous consistence of the chyme when it first enters the caecum, there can be no doubt that the absorption of liquid is not by any means concluded. The peculiar odor, moreover, which is acquired after a short time by the contents of the large bowel, would seem to indicate the addition to them, in this region, of some special matter, probably excretory. The acid reaction, which had become less and less distinct in the small bowel, again becomes very manifest in the caecum prob- ably from acid fermentation processes in some of the materials of the food. DIGESTION IN LARGE INTESTINE. 273 There seems no reason, however, to conclude that any special, " secondary," digestive process occurs in the caecum or in any other part of the large intestine. Probably any constituent of the food which has escaped digestion and absorption in the small bowel may be digested in the large intestine; and the power of this part of the intestinal canal to digest fatty, albu- minous, or other matters, may be gathered from the good effects of nutrient enemata, so frequently given when from any cause there is difficulty in introducing food into the stom- ach. In ordinary healthy digestion, however, the changes which ensue in the chyme after its passage into the large intestine, are mainly the absorption of the more liquid parts, and the addition of the special excretory products which give it the characteristic odor. At the same time, as before said, it is probable that a certain quantity of nutrient matter always escapes digestion in the small intestine, and that this happens more especially when food has been taken in excess, or when it is of such a kind as to be difficult of digestion. Under these circumstances there is no doubt that such changes as were proceeding in it at the lower part of the ileum may go on unchecked in the large bowel, the process being as- sisted by the secretion of the numerous tubular glands therein present. By these means the contents of the large intestine, as they proceed towards the rectum, become more and more solid, and losing their more liquid and nutrient parts, gradually acquire the odor and consistence characteristic of faeces. After a sojourn of uncertain duration in the rectum, they are finally expelled by the contraction of its muscular coat, aided, under ordinary circumstances, by the contraction of the abdominal muscles. For a description of the mechanism by which the act of defecation is accomplished, see p. 183. The average quantity of solid fecal matter evacuated by the human adult in twenty-four hours is about five ounces ; an uncertain proportion of which consists simply of the undi- gested or chemically modified residue of the food and the re- mainder of certain matters which are excreted in the intesti- nal canal. 274 DIGESTION. Composition of Fences. Water, 733 00 Solids, 267.00 Special excreientitious constituents : Excretin, excretoleic acid (Marcet), and steroorin (Austin Flint). Salts : Chiefly phosphate of magnesia and phos- phate of lime, with small quantities of iron, | soda, lime, and silica. Insoluble residue of the food (chiefly starch, \ 267.00 grains, woody tissue, particles of cartilage, and fibrous tissue, undigested muscular fibres or fat, and the like, with insoluble substances accidentally introduced with the food). Mucus, epithelium, altered coloring matter of bile, fatty acids, &c. j The time occupied by the journey of a given portion of food from the stomach to the anus, varies considerably even in health, and on this account, probably, it is that such different opinions have been expressed in regard to the subject. Dr. Brinton supposes twelve hours to be occupied by the journey of an ordinary meal through the small intestine, and twenty- four to thirty-six hours by the passage through the large bowel. On the Gases contained in the Stomach and Intestines. It need scarcely be remarked that, under ordinary circum- stances, the alimentary canal contains a considerable quantity of gaseous matter. Any one who has had occasion, in a post- mortem examination, either to lay open the intestines, or to let out the gas which they contain, must have been struck by the small space afterwards occupied by the bowels, and by the large degree, therefore, in which the gas, which naturally dis- tends them, contributes to fill the cavity of the abdomen. In- deed, the presence of air in the intestines is so constant, and, within certain limits, the amount in health so uniform, that there can be no doubt that its existence here is not a mere ac- cident, but intended to serve a definite and important purpose, although, probably, a mechanical one. The sources of the gas contained in the stomach and bowels may be thus enumerated : 1. Air introduced in the act of swallowing either food or saliva. 2. Gases developed by the decomposition of alimentary MOVEMENTS OF THE INTESTINES. 275 matter, or of the secretions and excretions mingled with it in the stomach and intestines. 3. It is probable that a certain mutual interchange occurs between the gases contained in the alimentary canal, and those present in the blood of the gastric and intestinal bloodvessels ; but the conditions of the exchange are not known, and it is very doubtful whether anything like a true and definite secre- tion of gas from the blood into the intestines or stomach ever takes place. There can be no doubt, however, that the intes- tines may be the proper excretory organs for many odorous and other substances, either absorbed from the air taken into the lungs in inspiration, or absorbed in the upper part of the alimentary canal, again to be excreted at a portion of the same tract lower down in either case assuming rapidly a gaseous form after their excretion, and in this way, perhaps, obtaining a more ready egress from the body. It is probable that, under ordinary circumstances, the gases of the stomach and intestines are derived chiefly from the second of the sources which have been enumerated. Tabular Analysis of Gases contained in the Alimentary Canal. Whence obtained. Composition by Volume. Oxygen Nitrog. Carbon. Acid. Hydrog. Carburet. Hydrogen. Sulphuret. Hydrogen. Stomach, . . . Small Intestine, . Caecum, .... Colon, .... 11 71 32 66 35 46 22 14 30 12 57 43 41 4 38 8 6 19 13 8 11 19 y trace. J Rectum, .... Expelled per anum The above tabular analysis of the gases contained in the alimentary canal has been quoted from the analyses of Jurine, Magendie, Marchand, and Chevreul, by Dr. Brinton, from whose work the above enumeration of the sources of the gas has been also taken. Movements of the Intestines. It remains only to consider the manner in which the food and the several secretions mingled with it are moved through the intestinal canal, so as to be slowly subjected to the in- fluence of fresh portions of intestinal secretion, and as slowly 276 DIGESTION. exposed to the absorbent power of all the villi and blood- vessels of the mucous membrane. The movement of the intes- tines is peristaltic or vermicular, and is effected by the alternate contractions and dilatations of successive portions of the intes- tinal coats. The contractions, which may commence at any point of the intestine, extend in a wave-like manner along the tube. In any given portion, the longitudinal muscular fibres contract first, or more than the circular ; they draw a portion of the intestine upwards, or, as it were, backwards, over the sub- stance to be propelled, and then the circular fibres of the same portion contracting in succession from above downwards, or, as it were, from behind forwards, press on the substance into the portion next below, in which at once the same succession of actions next ensues. These movements take place slowly, and, in health, are commonly unperceived by the mind ; but they are perceptible when they are accelerated under the in- fluence of any irritant. The movements of the intestines are sometimes retrograde ; and there is no hindrance to the backward movement of the contents of the small intestine. But almost complete security is afforded against the passage of the contents of the large into the small intestine by the ileo-csecal valve. Besides, the orifice of communication between the ileum and caecum (at the bor- ders of which orifice are the folds of mucous membrane which form the valve) is encircled with muscular fibres, the contrac- tion of which prevents the undue dilatation of the orifice. Proceeding from above downwards, the muscular fibres of the large intestine become, on the whole, stronger in direct proportion to the greater strength required for the onward moving of the fseces, which are gradually becoming firmer. The greatest strength is in the rectum, at the termination of which the circular unstriped muscular fibres form a strong band called the internal sphincter, while an external sphincter muscle with striped fibres is placed rather lower down, and more externally, and holds the orifice close by a constant slight contraction under the influence of the spinal cord. The peculiar condition of the sphincter, in relation to the nervous system, will be again referred to. The remaining portion of the intestinal canal is under the direct influence of the sympathetic or ganglionic system, and, indirectly, or more distantly, is subject to the influence of the brain and spinal cord, which influence appears to be, in some degree, transmitted through the vagus nerve. Experimental irritation of the brain or cord produces no evident or constant effect on the move- ments of the intestines during life ; yet in consequence of cer- tain conditions of the mind, the movements are accelerated or LYMPHATIC VESSELS AND GLANDS. 277 retarded ; and in paraplegia the intestines appear after a time much weakened in their power, and costiveness, with a tym- panitic condition, ensues. Immediately after death, irritation of both the sympathetic and pneumogastric nerves, if not too strong, induces genuine peristaltic movements of the intestines. Violent irritation stops the movements. These stimuli act, no doubt, not directly on the muscular tissue of the intestine, but on the rich ganglionic structure shown by Meissner to exist in the submucous tissue. This regulates and controls the move- ments, and gives to them their peculiar slow, orderly, rhyth- mic, and peristaltic character, both naturally, and when arti- ficiallv excited. CHAPTER X. ABSORPTION. THE process of absorption has, for one of its objects, the in- troduction into the blood of fresh materials from the food and air, and of whatever comes into contact with the external or internal surfaces of the body ; and, for another, the taking away of parts of the body itself, when, having fulfilled their office, or otherwise requiring removal, they need to be re- newed. In both these offices, i. e., in both absorption from without and absorption from within, the process manifests some variety, and a very wide range of action ; and in both it is probable that two sets of vessels are, or may be, concerned, namely, the bloodvessels, and the lacteals or lymphatics, to which the term absorbents has been especially applied. Structure and Office of the Lacteal and Lymphatic Vessels and Glands. Besides the system of arteries and veins, with their inter- mediate vessels, the capillaries, there is another system of canals in man and other vertebrata, called the lymphatic sys- tem, which contains a fluid called lymph. Both these systems of vessels are concerned in absorption. The principal vessels of the lymphatic system are, in struc- ture and general appearance, like very small and thin-walled veins, and like them are provided with valves. By one ex- tremity they commence by fine microscopic branches, the lym- phatic capillaries or lymph-capillaries, in the organs and tissues 24 278 ABSORPTION. of nearly every part of the body, and by their other extremi- ties they end directly or indirectly in two trunks which open into the large veins near the heart (Fig. 92). Their contents, the lymph and chyle, unlike the blood, pass only in one direc- FIG. 92. Lymphatics of head and neck, right. Right internal jug- ular vein. Right subclavian vein. Lymphatics of right arm. Receptaculum chyli. Lymphatics of Icn er extremities Lymphatics of head and neck, left. Thoracic duct. Left subclavian vein. Thoracic duct. Lacteals. Lymphatics of low- er extremities. Diagram of the principal groups of lymphatic vessels (from Quain). tion, namely, from the fine branches to the trunk and so to the large veins, on entering which they are mingled with the stream of blood, and form part of its constituents. Remem- bering the course of the fluid in the lymphatic vessels, viz., its passage in the direction only towards the large veins in the neighborhood of the heart, it will be readily seen from Fig. 92 that the greater part of the contents of the lymphatic system of vessels passes through a comparatively large trunk called COURSE OF THE LYMPHATICS. 279 the thoracic duct, which finally empties its contents into the blood-stream at the junction of the internal jugular and sub- clavian veins of the left side. There is a smaller duct on the right side. The lymphatic vessels of the intestinal canal are called lacteals, because, during digestion, the fluid contained in them resembles milk in appearance ; and the lymph in the lacteals during the period of digestion is called chyle. There FIG. 93. Lymphatic vessels of the head and neck of the upper part of the trunk (from Mas- oagni) l r T ne chest and pericardium have been opened on the left side, and the left mamma detached and thrown outwards over the left arm, so as to expose a great part of its deep surface. The principal lymphatic vessels and glands are shown on the side of the head and face, and in the neck, axiila, and mediastinum. Between the left internal jugular vein and the common carotid artery, the upper ascending part of the thoracic duct marked 1, and above this, and descending to 2, the arch and last part of tin- duct. The termination of the upper lymphatics of the diaphragm in the mediastinal glands as well as the cardiac and the deep mammary lymphatics, are also shown. 280 ABSORPTION. is no essential distinction, however, between lacteals and lym- phatics. In some part of their course all lymphatic vessels pass through certain bodies called lymphatic glands. Lymphatic vessels are distributed in nearly all parts of the body. Their existence, however, has not yet been determined in the placenta, the umbilical cord, the membranes of the ovum, or in any of the non-vascular parts, as the nails, cuticle, hair, and the like. The lymphatic capillaries commence most commonly either in closely-meshed networks, or in irregular lacunar spaces between the various structures of which the different organs are composed. The former is the rule of origin with those lymphatics which are placed most superficially, as, for instance, immediately beneath the skin, or under the mucous and serous membranes ; while the latter is most common with those which arise in the substance of organs. In the former instance, their walls are composed of but little more than homogeneous mem- brane, lined by a single layer of epithelial cells, very similar to those which line the blood-capillaries (Fig. 49). In the latter instance the small irregular channels and spaces from which the lymphatics take their origin, although they are formed mostly by the chinks and crannies between the blood- vessels, secreting ducts, and other parts which may happen to form the framework of the organ in which they exist, yet have also a layer of epithelial cells to define and bound them. The lacteals apear to offer an illustration of another mode of origin, namely, in blind dilated extremities (Figs. 81, 82) ; but there is no essential difference in structure between these and the lymphatic capillaries of other parts. Recent discoveries seem likely to put an end soon to the long-standing discussion whether any direct communications exist between the lymph-capillaries and blood-capillaries ; the need for any special intercommunicating channels seeming to disappear in the light of more accurate knowledge of the struc- ture and endowments of the parts concerned. For while, on the one hand, the fluid part of the blood constantly exudes or is strained through the walls of the blood-capillaries, so as to moisten all the surround ing tissues, and occupy the interspaces which exist among their different elements, these same inter- spaces have been shown, as just stated, to form the beginnings of the lymph-capillaries. And while, for many years, the no- tion of the existence of any such channels between the blood- vessels and lymphvessels, as would admit blood-corpuscles, has been given up, recent observations have proved that, for the passage of such corpuscles, it is not necessary to assume the ORIGIN OF LYMPHATICS. 281 presence of any special channels at all, inasmuch as blood- corpuscles can pass bodily, without much difficulty, through FIG. 94. FIG. 95. FIG. 94. Superficial lymphatics of the forearm and palm of the hand, (after Mascagni). 5. Two small glands at the bend of the arm. 6. Radial lymphatic ves- sels. 7. Dinar lymphatic vessels. 8, 8. Palmar arch of lymphatics. 9, 9'. Outer and inner sets of vessels, b. Cephalic vein. d. Radial vein. e. Median vein. /. Dinar vein. The lymphatics are represented as lying on the deep fascia. FIG. 95. Superficial lymphatics of right groin and upper part of thigh, 1 (after Mascagni). 1. Dpper inguinal glands. 2'. Lower inguinal or femoral glands. 3, 3. Plexus of lymphatics in the course of the long saphenous vein. 282 ABSORPTION. the walls of the blood-capillaries and small veins (p. 138), and could pass with still less trouble, probably, through the com- paratively ill-defined w r alls of the capillaries which contain lymph. Observations of Recklinghausen have led to the discovery that in certain parts of the body openings exist by which lym- phatic capillaries directly communicate with parts hitherto supposed to be closed cavities. If the peritoneal cavity be in- jected with milk, an injection is obtained of the plexus of lym- phatic vessels of the central tendon of the diaphragm ; and on removing a small portion of the central tendon, with its peri- toneal surface uninjured, and examining the process of absorp- tion under the microscope, Recklinghausen noticed that the milk-globules ran towards small natural openings or stomata between the epithelial cells, and disappeared by passing vortex- like through them. The stomata, which had a roundish out- line, were only wide enough to admit two or three milk-glob- ules abreast, and never exceeded the size of an epithelial cell. Openings of a similar kind have been found by Dybskowsky in the pleura ; and as they may be presumed to exist in other serous membranes, it would seem as if the serous cavities, hitherto supposed closed, form but a large widening out, so to speak, of the lymph-capillary system with which they directly communicate. In structure, the medium-sized and larger lymphatic vessels are very like veins ; having, according to Kolliker, an exter- nal coat of fibro-cellular tissue, with elastic filaments ; within this, a thin layer of fibro-cellular tissue, with organic muscu- lar fibres, which have, principally, a circular direction, and are much more abundant in the small than in the larger vessels ; and again, within this, an inner elastic layer of longitudinal fibres, and a lining of epithelium, and numerous valves. The valves, constructed like those of veins, and with the free edges turned towards the heart, are usually arranged in pairs, and, in the small vessels, are so closely placed, that when the vessels are full, the valves constricting them where their edges are attached, give them a peculiar braided or knotted appearance (Fig. 99). With the help of the valvular mechanism, all occasional pressure on the exterior of the lymphatic and lacteal vessels propels the lymph towards the heart: thus muscular and other external pressure accelerates the flow of the lymph as it does that of the blood in the veins (see p. 143). The actions of the muscular fibres of the small intestine, and probably the layer of organic muscle present in each intesti- nal villus (p. 246), seem to assist in propelling the chyle : for, LYMPHATIC GLANDS. 283 in the small intestine of a mouse, Poiseuille saw the chyle moving with intermittent propulsions that appeared to corre- spond with the peristaltic movements of the intestine. But for the general propulsion of the lymph and chyle, it is probable that, together with the vis a tergo resulting from absorption (as in the ascent of sap in a tree), and from external pressure, some of the force may be derived from the contractility of the vessel's own walls. Kolliker, after watching the lymphatics in the transparent tail of the tadpole, states that no distinct movements of their walls can ever be seen, but as they are emptied after death they gradually contract, and then, after some time, again dilate to their former size, exactly as the small arteries do under the like circumstances. Thus, also, the larger vessels in the human subject commonly empty themselves after death ; so that, although absorption is proba- bly usually going on just before the time of death, it is not common to see the lymphatic or lacteal vessels full. Their power of contraction under the influence of stimuli has been demonstrated by Kolliker, who applied the wire of an electro- magnetic apparatus to some well-filled lymphatics on the skin of a boy's foot, just after the removal of his leg by am- putation, and noticed that the calibre of the vessels diminished at least one half. It is most probable that this contraction of the vessels occurs during life, and that it consists, not in peristaltic or undulatory movements, but in a uniform con- traction of the successive portions of the vessels, by which pressure is steadily exercised upon their contents, and which alternates with their relaxation. Lymphatic Glands. Almost all lymphatic and lacteal vessels in some part of their course pass through one or more small bodies called lym- phatic glands (Fig. 99). A lymphatic gland is covered externally by a capsule of connective tissue, which invests and supports the glandular structure within ; while prolonged from its inner surface are processes QYtrabeculw which, entering the gland from all sides, and freely communicating, form a fibrous scaffolding or strorna in all parts of the interior. Thus are formed in the outer or cortical part of the glands (Fig. 96), in the intervals of the trabeculse, certain intercommunicating spaces termed alveoli; while a finer meshwork is formed in the more central or medullary part. In the alveoli and the trabecular meshwork the proper gland-substance is contained ; in the form of nod- 284 ABSORPTION. ules in the cortical alveoli, and of rounded cords in the medullary part (Fig. 97). The gland-substance of one part is continuous directly or indirectly with that of all others. FIG. 96. Section of a mesenteric gland from the ox, slightly magnified, a, hilus ; b (in the central part of the figure), medullary substance ; c, cortical substance with indis- tinct alveoli ; d, capsule (after Kolliker). The essential structure of lymphatic gland-substance resem- bles that which was described as existing, in a simple form, in the interior of the solitary and agminated intestinal follicles FIG. 97. Section of medullary substance of an inguinal gland of an ox (magnified 90 diameters), a, a, glandular substance or pulp forming rounded cords joining in a continuous net (dark in the figure) ; c, c, trabeculse ; the space, b, b, between these and the glandular substance is the lymph-sinus, washed clear of corpuscles and traversed by filaments of retiform connective tissue (after Kolliker). LYMPHATIC GLANDS. 285 (p. 242). Pervading all parts of it, and occupying the alveoli and trabecular spaces before referred to, is a network of the variety of connective tissue termed retiform tissue (Fig. 98), the interspaces of which are occupied by lymph-corpuscles. The corpuscles are arranged in such a way, that while in the centre of the alveoli and of each mesh they are so crowded together as to be, with the retiform tissue pervading them, a consistent gland-pulp, continuous in the form of the nodules and cords, before referred to, throughout the whole gland, they are in comparatively small numbers in the outer part of the alveoli and meshes, and leave this portion, as it were, open. FIG. A small portion of medullary substance from a mesenteric gland of the ox (mag- nified 300 diameters), d, d, trabeculse ; a, part of a cord of glandular substances from which all but a few of the lymph-corpuscles have been washed out to show its sup- porting meshwork of retiform tissue and its capillary bloodvessels (which have been injected, and are dark in the figure) ; b, b, lymph-sinus, of which the retiform tissue is represented only at c, c (after Kolliker). (See Figs. 97, 98.) This free space between the gland-pulp and the trabecular stroma, occupied only by retiform tissue, is called the lymph-channel or lymph-path, because it is traversed 286 ABSORPTION. by the lymph, which is continually brought to the gland and conveyed away from it by lymphatic vessels ; those which bring it being termed afferent vessels, and those which take it away efferent vessels. The former enter the cortical part of the gland and open into its alveoli, at the same time that they lay aside all their coats except the epithelial lining, which may be said to continue to line the lymph-path into which the contents of the afferent vessels now pass. The efferent vessels begin in the medullary part of the gland, and are continuous with the lymph-path here as the afferent vessels were with the cortical portion ; the epithelium of one is continuous with that of the other. Bloodvessels are freely distributed to the trabecular tissue and to the gland-pulp (Fig. 98). Properties of Lymph and Chyle. The fluid, or lymph, contained in the lymphatic vessels is, under ordinary circumstances, clear, transparent, and color- less, or of a pale yellow tint. It is devoid of smell, is slightly alkaline, and has a saline taste. As seen with the micro- scope in the small transparent vessels of the tail of the tad- pole, the lymph usually contains no corpuscles or particles of any kind ; and it is probably only in the larger trunks in which, by a process similar to that to be described in the chyle, the lymph is more elaborated, that any corpuscles are formed. These corpuscles are similar to those in the chyle, but less numerous. The fluid in which the corpuscles float is commonly and in health albuminous, and contains no fatty particles or molecular base ; but it is liable to variations ac- cording to the general state of the blood, and that of the organ from which the lymph is derived. As it advances towards the thoracic duct, and passes through the lymphatic glands, it be- comes, like chyle, spontaneously coagulable from the formation of fibrin, and the number of corpuscles is much increased. The fluid contained in the lacteals, or lymphatic vessels of the intestine, is clear and transparent during fasting, and differs in no respect from ordinary lymph ; but during diges- tion, it becomes milky, and is termed chyle. Chyle is an opaque, whitish fluid, resembling milk in ap- pearance, and having a neutral or slightly alkaline reaction. Its whiteness and opacity are due to the presence of innumer- able particles of oily or fatty matter, of exceedingly minute though nearly uniform size, measuring on the average about 00077 of an inch (Gulliver). These constitute what Mr. Gul- CHYLE. 287 liver appropriately terms the molecular base of chyle. Their number, and consequently the opac- ity of the chyle, are dependent upon the quantity of fatty matter con- tained in the food. Hence, as a rule, the chyle is whitish and most turbid in carnivorous animals ; less so in Herbivora ; while in birds it is usually transparent. The fatty na- ture of the molecules is made mani- fest by their solubility in ether, and, when the ether evaporates, by their being deposited in various-sized drops of oil. 1 Yet, since they do not run together and form a larger drop, as particles of oil would, it appears very probable that each molecule consists of oil coated over with albumen, in the manner in which, as Ascherson observed, oil always becomes covered when set free in minute drops in an albuminous solution. And this view is supported by the fact, that when water or dilute acetic acid is added to chyle, many of the molecules are lost sight of, and oil-drops appear in P hatic s land - 3 < with its com - their place, as if the investments of ponent cellsfilled with mercury r , , 111 j- i j and having three sets of afferent the molecules had been dissolved, vessels ^ ^ ^ leading into it and their oily Contents had run to- and one set of efferent vessels, 2, gether. passing out from it. The arrows Except these molecules, the chyle indicate the course of the lymph taken from the villi Or from lacteals in these vessels. The varicose or near them contains no other solid or SS7^T? Organized bodies. 1 lie fluid in Which phatic vesse l somewhat enlarged, the molecules float is albuminOUS, and cut through, to show the and does not Spontaneously COagU- Httle double valves in its interior. , , . i i 11 v IT. jj- c, lymph-corpuscles, one granu- late, though coagulable by the addi- ^ J three fcreated wlth dilute tion of ether. But as the chyle passes acet i c acid, showing the envelope On towards the thoracic duct, and and the pale nucleus ; also some especially while it traverses One Or finer granules and oil-particles more of the mesenteric glands (pro- free - Magnified 400 diameters. pelled by forces which have been described with the structure of the vessels), it is elaborated. (Mascagni), a, plan of a lyi 1 Some of the molecules may remain undissolved by the ether; but this appears to bo due to their being defended from the action of the ether by bein^ entangled within the albumen which it coagulates. 288 ABSORPTION. The quantity of molecules and oily particles gradually di- minishes; cells, to which the name of chyle-corpuscles is given, are developed in it; and by the formation of fibrin, it acquires the property of coagulating spontaneously. The higher in the thoracic duct the chyle advances, the more is it, in all these respects, developed ; the greater is the number of chyle-cor- puscles, and the large? and firmer is the clot which forms in it when withdrawn and left at rest. Such a clot is like one of blood, without the red corpuscles, having the chyle-corpuscles entangled in it, and the fatty matter forming a white creamy film on the surface of the serum. But the clot of chyle is softer and moister than that of blood. Like blood, also, the chyle often remains for a long time in its vessels without coagu- lating, but coagulates rapidly on being removed from them (Bouissou). The existence of fibrin, or of the materials which by their union form it (p. 62 et seq.\ is, therefore, certain ; its increase appears to be commensurate with that of the corpus- cles ; and, like them, it is not absorbed as such from the chyme (for no fibrin exists in the chyle in the villi), but is gradually elaborated out of the albumen which chyle in its earliest con- dition contains. The structure of the chyle-corpuscles was described when speaking of the white corpuscles of the blood, with which they are identical. From what has been said, is will appear that perfect chyle and lymph are, in essential characters, nearly similar, and scarcely differ, except in the preponderance of fatty matter in the chyle. The comparative analysis of the two fluids obtained from the lacteals and the lymphatics of a donkey is thus given by Dr. Owen Rees : Chyle. Lymph. Water, . . . 90.237 96.536 Albumen, . Fibrin. Animal extractive, Fatty matter, Salts, . 3.516 1.200 0.370 0.120 1.565 1.559 3 601 a trace. 0.711 0.585 100000 100.000 The analyses of Nasse afford an estimate of the relative com- positions of the lymph, chyle, and blood of the horse. 1 1 The analysis of the blood differs rathtr widely from that given at psige 72 ; but if it be erroneous, it is probable that corresponding errors exist in the analysis of the lymph and chyle; and that therefore the tables in the text may represent accurately enough the relation in. which the three fluids stand to each other. COMPOSITION OF LYMPH AND CHYLE. 289 Lymph. Chyle. Blood. Water, 950. 935. 810. Corpuscles, ... 4. 92.8 Albumen, . 39 11 31. 80 Fibrin, . 0.75 2.8 Extractive matter, 4.88 6.25 5.2 Fatty matter, 0.09 15. 1.55 Alkaline salts, 5.61 7. 6.7. Phosphateof lime and magnesia, oxide ) A 01 i n nr of iron, &c., j 1000. 1000. 1000. The contents of the thoracic duct, including both the lymph and chyle mixed, in an executed criminal, were examined by Dr. Rees, who found them to consist of Water, Albumen and fibrin. Extractive matter, . Fatty " Saline " 90.48 7.08 0.108 0.92 0.44 From all these analyses of lymph and chyle, it appears that they contain essentially the same organic constituents that are found in the blood, viz., albumen, fibrin, and fatty matter, the same saline substances, and iron. Their composition differs from that of the blood in degree rather than in kind ; they contain a less proportion of all the substances dissolved in the water (see Nasse's analyses, just quoted), and much less fibrin. The fibrin 1 of lymph, besides being less in quantity, appears to be in a less elaborated state than that of the blood, coagulating less rapidly and less firmly. According to Virchow, it never coagulates, under ordinary circumstances, within the lymphatic vessels, either during life or after death. These differences gradually diminish, while the lymph and chyle, passing to- wards and through the thoracic duct, gradually approach the place at which they are to be mingled with the blood. For, in the thoracic duct, besides the higher and more abundant de- velopment of the fibrin, the lymph and chyle-corpuscles are found more advanced towards their development into red blood-corpuscles; sometimes even that development is com- pleted, and the lymph has a pinkish tinge from the number of red blood-corpuscles that it contains. The general result, therefore, of both the microscopic and the chemical examinations of the lymph and chyle, demon~ strate that they are rudimental blood ; their fluid part being, 1 For observations on the nature of fibrin, see p. 62. 290 ABSORPTION. like the liquor sanguinis, diluted, but gradually becoming more concentrated ; and their corpuscles being in process of development into red blood-corpuscles. Thus, in quality, the lymph and chyle are adapted to replenish the blood ; and their quantity, so far as it can be estimated, appears ample for this purpose. In one of Mageudie's experiments, half an ounce of chyle was collected in five minutes from the thoracic duct of a middle-sized dog ; Collard de Martigny obtained nine grains of lymph, in ten minutes, from the thoracic duct of a rabbit which had taken no food for twenty-four hours ; and Gieger, from three to five pounds of lymph daily from the foot of a horse, from whom the same quantity had been flowing several years without injury to health. Bidder found, on opening the tho- racic duct in cats, immediately after death, that the mingled lymph and chyle continued to flow from one to six minutes ; and, from the quantity thus obtained, he estimated that if the contents of the thoracic duct continued to move at the same rate, the quantity which would pass into a cat's blood in twenty- four hours would be equal to about one-sixth of the weight of the whole body. And, since the estimated weight of the blood in cats is to the weight of their bodies as 1.7, the quantity of lymph daily traversing the thoracic duct would appear to be about equal to the quantity of blood at any time contained in the animals. Schmidt's observations on foals have yielded very similar results. By another series of experiments, Bidder estimated that the quantity of lymph traversing the thoracic duct of a dog in twenty-four hours is about equal to two-thirds of the blood in the body. If we take these estimates, it will not follow from them that the whole of an animal's blood is daily replaced by the development of lymph and chyle; for even if the quantity of lymph and chyle daily formed be equal to that of the blood, the solid contents of the blood will be much too great to be replaced by those of the lymph and chyle. According to Nasse's analyses, the solid matter of a given quantity of blood could not be replaced out of less than three or four times the quantity of lymph and chyle. Absorption by the Lacteal Vessels. During the passage of the chyme along the whole tract of the intestinal canal, its completely digested parts are absorbed by the bloodvessels and lacteals distributed in the mucous membrane. The bloodvessels appear to absorb chiefly the dissolve^ portions of the fqod, and these, including especially the albuminous and saccharine, they imbibe without choice ; whateyer can mix with the blood passes into the vessels, as ABSORPTION BY LYMPHATICS. 291 will be presently described. But the lacteals appear to absorb only certain constituents of the food, including particularly the fatty portions. The absorption by both sets of vessels is carried on most actively, but not exclusively, in the villi of the small intestine ; for in these minute processes, both the capillary bloodvessels and the lacteals are brought almost into contact with the intestinal contents. It has been already stated that the villi of the small intestine (Figs. 81 and 82), are minute vascular processes of mucous membrane, each containing a delicate network of bloodvessels and one or more lacteals, and are invested by a sheath of cylin- drical epithelium. In the interspaces of the mucous mem- brane between the villi, as well as over all the rest of the intestinal canal, the lacteals and bloodvessels are also densely distributed in a close network, the lacteals, however, being more sparingly supplied to the large than to the small in- testine. There seems to be no doubt that absorption of fatty matters during digestion, from the contents of the intestines, is effected chiefly by the epithelial cells which line the intestinal tract, and especially by those which clothe the surface of the villi (Fig. 81). From these epithelial cells, again, the fatty parti- cles are passed on into the interior of the lacteal vessels (Figs. 81 and 82), but how they pass, and what laws govern their so doing, are not at present exactly known. It is probable that the process of absorption by the epithe- lial cells, is assisted by the pressure exercised on the contents of the intestines by their contractile walls ; and that the ab- sorption of fatty particles is also facilitated by the presence of the bile, the pancreatic and intestinal secretions, which moisten the absorbing surface. For it has been found by experiment, that the passage of oil through an animal membrane is made much easier when the latter is impregnated with an alkaline fluid. Absorption by the Lymphatic Vessels. The real source of the lymph, and the mode in which its absorption is effected by the lymphatic vessels, were long mat- ters of discussion. But the problem has been much simplified by more accurate knowledge of the anatomical relations of the lymphatic capillaries. It is most probable that the lymph is derived, in great part, from the liquor snnguinis, which, as be- fore remarked, is always exuding from the blood-capillaries into the interstices of the tissues in which they lie; and changes in the character of the lymph correspond very closely with changes in the character of either the whole mass of 292 ABSORPTION. blood, or of that in the vessels of the part from which the lymph is examined. Thus Herbst found that the coagula- bility of the lymph is directly proportionate to that of the blood ; and that when fluids are injected into the bloodvessels in sufficient quantity to distend them, the injected substance may be almost directly afterwards found in the lymphatics. It is not improbable, however, that some other matters than those originally contained in the exuded liquor sanguinis may find their way with it into the lymphatic vessels. Parts which, having entered into the composition of a tissue, and, having fulfilled their purpose, require to be removed, may not be altogether excrementitious, but may admit of being reorganized and adapted again for nutrition ; and these may be absorbed by the lymphatics, and elaborated with the other contents of the lymph in passing through the glands. Lymph- Hearts. In reptiles and some birds, an important auxiliary to the movement of the lymph and chyle is supplied in certain muscular sacs, named lymph-hearts (Fig. 100), and Mr. Wharton Jones has lately shown that the caudal heart of the eel is a lymph-heart also. The number and position of FIG. 100. Lymphatic heart (9 lines long, 4 lines broad) of a large species of serpent, the Python bivittatus (after E. Weber). 4. The external cellular coat. 5. The thick muscular coat. Four muscular columns run across its cavity, which communicates with three lymphatics (1 only one is seen here), with two veins (2,2). 6. The smooth lining membrane of the cavity. 7. A small appendage, or auricle, the cavity of which is continuous with that of the rjst of the organ. these organs vary. In frogs and toads there are usually four, two anterior and two posterior ; in the frog, the posterior lymph- heart on each side is situated in the ischiatic region, just be- neath the skin ; the anterior lies deeper, just over the transverse process of the third vertebra. Into each of these cavities several lymphatics open, the orifices of the vessels being guarded ABSORPTION BY BLOODVESSELS. 293 by valves, which prevent the retrograde passage of the lymph. From each heart a single vein proceeds and conveys the lymph directly into the venous system. In the frog, the inferior lym- phatic heart, on each side, pours its lymph into a branch of the ischiatic vein ; by the superior, the lymph is forced into a branch of the jugular vein, which issues from its anterior sur- face, and which becomes turgid each time that the sac contracts. Blood is prevented from passing from the vein into the lym- phatic heart by a valve at its orifice. The muscular coat of these hearts is of variable thickness ; in some cases it can only be discovered by means of the micro- scope ; but in every case it is composed of transversely-striated fibres. The contractions of the hearts are rhythmical, occur- ring about sixty times in a minute, slowly, and, in comparison with those of the blood-hearts, feebly. The pulsations of the cervical pair are not always synchronous with those of the pair in the ischiatic region, and even the corresponding sacs of opposite sides are not always synchronous in their action. Unlike the contractions of the blood-heart, those of the lymph-heart appear to be directly dependent upon a certain limited portion of the spinal cord. For Volkmann found that so long as the portion of spinal cord corresponding to the third vertebra of the frog was uninjured, the cervical pair of lym- phatic hearts continued pulsating after all the rest of the spinal cord and the brain was destroyed ; while destruction of this portion, even though all other parts of the nervous centres were uninjured, instantly arrested the heart's movements. The posterior or ischiatic pair of lymph-hearts were found to be governed, in like manner, by the portion of spinal cord cor- responding to the eighth vertebra. Division of the posterior spinal roots did not arrest the movements ; but division of the anterior roots caused them to cease at once. Absorption by Bloodvessels. The process thus named is that which has been commonly called absorption by the veins; but the term here employed seems preferable, since, though the materials absorbed are commonly found in the veins, this is only because they are carried into them with the circulating blood, after being ab- sorbed by all the bloodvessels (but chiefly by the capillaries) with which they were placed in contact. There is nothing in the mode of absorption by bloodvessels, or in the structure of veins, which can make the latter more active than arteries of the same size, or so active as the capillaries, in the process. In the absorption by the lymphatics or lacteal vessels just 25 294 ABSORPTION, FIG. 101. described, there appears something like the exercise of choice in the materials admitted into them. But the absorption by bloodvessels presents no such appearance of selection of ma- terials; rather, it appears, that every substance, whether gaseous, liquid, or a soluble or minutely divided solid, may be absorbed by the bloodvessels, provided it is capable of per- meating their walls, and of mixing with the blood ; and that of all such substances, the mode and measure of absorption are determined solely by their physical or chemical properties and conditions, and by those of the blood and the walls of the bloodvessels. The phenomena are, indeed, exactly comparable to that passage of fluids through membrane, which occurs quite inde- pendently of vital conditions, and the earliest and best scien- tific investigation of which was made by Dutrochet. The in- strument which he employed in his experiments was named an endosmometer. It may consist of a graduated tube expanded into an open-mouthed bell at one end, over which a portion of mem- brane is tied (Fig. 101). If now the bell be filled with a solution of a salt, say chloride of sodium, and be immersed in water, the water will pass into the solution, and part of the salt will pass out into the water ; the water will pass into the solution much more rapidly than the salt will pass out into the water, and the diluted solution will rise in the tube. To this passage of fluids through membrane the term Osmosis is ap- plied. The nature of the membrane used as a septum, and its affinity for the fluids subjected to ex- periment, have an important influence, as might be anticipated, on the rapidity and duration of the osmotic current. Thus, if a piece of ordinary bladder be used as the septum between water and alcohol, the current is almost solely from the water to the alcohol, on account of the much greater affinity of water for this kind of mem- brane ; while, on the other hand, in the case of a membrane of caoutchouc, the alcohol, from its greater affinity for this sub- stance, would pass freely into the water. Various opinions have been advanced in regard to the na- ture of the force by which fluids of different chemical compo- sition thus tend to mix through an intervening membrane. According to some, this power is the result of the different de- grees of capillary attraction exerted by the pores of the mem- COLLOIDS AND CRYSTALLOIDS. 295 brane upon the two fluids. Prof. Graham, however, believes that the passage or osmose of water through membrane may be explained by supposing that it combines with the membra- nous septum, which thus becomes hydrated, and that on reach- ing the other side it partly leaves the membrane, which thus becomes to a certain degree dehydrated. For example, a membrane such as that used in the endosmometer, is hydrated to a higher degree if placed in pure water than in a neutral saline solution. Hence, in the case of the endosmometer filled with the saline solution and placed in water, the equilibrium of hydration is different on the two sides ; the outer surface being in contact with pure water tends to hydrate itself in a higher degree than the inner surface does. " When the full hydration of the outer surface extends through the thickness of the membrane, and reaches the inner surface, it there re- ceives a check. The degree of hydration is lowered, and water must be given up by the inner layer of the membrane." Thus the osmose or current of water through the membrane is caused. The passage outwards of the saline solution, on the other hand, is not due, probably, to any actual fluid current ; but to a solution of the salt in successive layers of the water contained in the pores of the membrane, until it reaches the outer surface and diffuses in the water there situate. Thus, " the water movement in osmose is an affair of hydra- tion and of dehydration in the substance of the membrane or other colloid septum, and the diffusion of the saline solution placed within the osmometer has little or nothing to do with the osmotic result, otherwise than as it affects the state of hy- dration of the septum." Prof. Graham has classed various substances according to the degree in which they possess this property of passing, when in a state of solution in water, through membrane ; those which pass freely being termed crystalloids, and those which pass with difficulty, colloids. This distinction, however, between colloids and crystalloids, which is made the basis of their classification, is by no means the only difference between them. The colloids, besides the absence of power to assume a crystalline form, are character- ized by their inertness as acids or bases, and feebleness in all ordinary chemical relations. Examples of them are found in albumen, gelatin, starch, hydrated alumina, hydrated silicic acid, &c. ; while the crystalloids are characterized by qualities the reverse of those just mentioned as belonging to colloids. Alcohol, sugar, and ordinary saline substances are examples of crystalloids. Absorption by bloodvessels is the consequence of their walls 296 ABSORPTION. being, like the membranous septum of the eudosmometer, por- ous and capable of imbibing fluids, and of the blood being so composed that most fluids will mingle with it. The process of absorption, in an instructive, though very imperfect degree, may be observed in any portion of vascular tissue removed from the body. If such a one be placed in a vessel of water, it will shortly swell, and become heavier and moister, through the quantity of water imbibed or soaked into it; and if now, the blood contained in any of its vessels be let out, it will be found diluted with water, which has been absorbed by the bloodvessels and mingled with the blood. The water round the piece of tissue also will become bloodstained ; and if all be kept at perfect rest, the stain derived from the solution of the coloring matter of the blood (together with which chemistry would detect some of the albumen and other parts of the liquor sanguinis) will spread more widely every day. The same will happen if the piece of tissue be placed in a saline solution in- stead of water, or in a solution of coloring or odorous matter, either of which will give their tinge or smell to the blood, and receive, in exchange, the color of the blood. Even so simple an experiment will illustrate the absorption by bloodvessels during life ; the process it shows is imitated, but with these differences : that, during life, as soon as water or any other substance is admitted into the blood, it is carried from the place at which it was absorbed into the general cur- rent of the circulation, and that the coloring matter of the blood is not dissolved so as to ooze out of the bloodvessels into the fluid which they are absorbing. The absorption of gases by the blood may be thus simply imitated. If venous blood be suspended in a moist bladder in the air, its surface will be reddened by the contact of oxygen, which is first dissolved in the fluid that moistens the bladder, and is then carried in the fluid to the surface of the blood : while, on the other hand, watery vapor and carbonic acid will pass through the membrane, and be exhaled into the air. In all these cases alike there is a mutual interchange be- tween the substances ; while the blood is receiving water, it is giving out its coloring matter and other constituents : or, while it is receiving oxygen, it is giving out carbonic acid and water; so that, at the end of the experiment, the two substances em- ployed in it are mixed ; and if, instead of a piece of tissue, one had taken a single bloodvessel full of blood and placed it in the water, both blood and water would, after a time, have been found both inside and outside the vessel. In such a case, more- over, if one were to determine accurately the quantity of water that passed to the blood, and of blood -that passed to the water, RAPIDITY OF ABSORPTION. 297 it would be found that the former was always greater than the latter. And so with other substances ; it almost always hap- pens, that if the two liquids placed on opposite sides of a mem- brane be of different densities or specific gravities, a larger quantity of the less dense fluid passes into the more dense, than of the latter into the former. The rapidity with which matters may be absorbed from the stomach probably by the bloodvessels chiefly, and diffused through the textures of the body, may be gathered from the history of some experiments by Dr. Bence Jones. From these it appears that even in a quarter of an hour, after being given on an empty stomach, chloride of lithium maybe diffused into all the vascular textures of the body, and into some of the non- vascular, as the cartilage of the hip-joint, as well as into the aqueous humor of the eye. Into the outer part of the crystal- line lens it may pass after a time, varying from half an hour to an hour and a half. Carbonate of lithia, when taken in five or ten-grain doses on an empty stomach, may be detected in the urine in 5 or 10 minutes; or, if the stomach be full at the time of taking the dose, in 20 minutes. It may sometimes be detected in the urine, moreover, for six, seven, or even eight days. Some experiments on the absorption of various mineral and vegetable poisons, by Mr. Savory, have brought to light the singular fact, that, in some cases, absorption takes place more rapidly from the rectum than from the stomach. Strychnia, for example, when in solution, produces its poisonous effects much more speedily when introduced into the rectum than into the stomach. When introduced in the solid form, how- ever, it is absorbed more rapidly from the stomach than from the rectum, doubtless because of the greater solvent property of the secretion of the former than of that of the latter. With regard to the degree of absorption by living blood- vessels, much depends on the facility with which the substance to be absorbed can penetrate the membrane or tissue which lies between it and the bloodvessels ; for, naturally, the blood- vessels are not bare to absorb. Thus absorption will hardly take place through the epidermis, but is quick when the epi- dermis is removed, and the same vessels are covered with only the surface of the cutis, or with granulations. In general, the absorption through membranes is in an inverse proportion to the thickness of their epithelia ; so Miiller found the urinary bladder of a frog traversed in less than a second ; and the ab- sorption of poisons by the stomach or lungs appears sometimes accomplished in an immeasurably small time. The substance to be absorbed must, as a general rule, be in 298 ABSOKPTION. the liquid or gaseous state, or, if a solid, must be soluble in the fluids with which it is brought in contact. Hence the marks of tattooing, and the discoloration produced by nitrate of silver taken internally, remain. Mercury may be absorbed even in the metallic state ; and in that state may pass into and remain in the bloodvessels, or be deposited from them (Oesterlen) ; and such substances as exceedingly finely-divided charcoal, when taken into the alimentary canal, have been found in the raesenteric veins (Oesterlen) ; the insoluble materials of oint- ments may also be rubbed into the bloodvessels ; but there are no facts to determine how these various substances effect their passage. Oil, minutely divided, as in an emulsion, will pass slowly into bloodvessels, as it will through a filter moistened with water (Vogel) ; and, without doubt, fatty matters find their way into the bloodvessels as well as the lymphvessels of the intestinal canal, although the latter seem to be specially intended for their absorption. As in the experiments before referred to, the less dense the fluid to be absorbed, the more speedy, as a general rule, is its absorption by the living bloodvessels. Hence the rapid ab- sorption of water from the stomach ; also of weak saline solu- tions ; but with strong solutions, there appears less absorption into, than effusion from, the bloodvessels. The absorption is the less rapid the fuller and tenser the bloodvessels are ; and the tension may be so great as to hinder altogether the entrance of more fluid. Thus, Magendie found that when he injected water into a dog's veins to repletion, poison was absorbed very slowly ; but when he diminished the tension of the vessels by bleeding, the poison acted quickly. So, when cupping-glasses are placed over a poisoned wound, they retard the absorption of the poison, not only by diminish- ing the velocity of the circulation in the part, but by filling all its vessels too full to admit more. On the same ground, absorption is the quicker the more rapid the circulation of the blood ; not because the fluid to be absorbed is more quickly imbibed into the tissues, or mingled with the blood, but because as fast as it enters the blood, it is carried away from the part, and the blood, being constantly renewed, is constantly as fit as at the first for the reception of the substance to be absorbed. NUTRITION. 299 CHAPTER XI. NUTRITION AND GROWTH. NUTRITION or nutritive assimilation is that modification of the formative process peculiar to living bodies by which tissues and organs already formed maintain their integrity. By the incorporation of fresh nutritive principles into their substance, the loss consequent on the waste and natural decay of the com- ponent particles of the tissues is repaired ; and each elementary particle seems to have the power not only of attracting ma- terials from the blood, but of causing them to assume its struc- ture, and participate in its vital properties. The relations between development and growth have been already stated (Chap. I) ; under the head of Nutrition will be now considered the process by which parts are maintained in the same general conditions of form, size, and composition, which they have already, by development and growth, at- tained ; and this, notwithstanding continual changes in their component particles. It is by this process that an adult per- son, in health, is maintained, through a series of some years, with the same general outline of features, the same size and form, and perhaps even the same weight ; although, during all this time, the several portions of his body are continually changing: their particles decaying and being removed, and then replaced by the formation of new ones, which, in their turn, also die and pass away. Neither is it only a general similarity of the whole body which is thus maintained. Every organ or part of the body, as much as the whole, exactly main- tains its form and composition, as the issue of the changes con- tinually taking place among its particles. The change of component particles, in which the nutrition of organs consists, is most evidently shown when, in growth, they maintain their form and other general characters, but increase in size. When, for example, a long bone increases in circumference, and in the thickness of its walls, while, at the same time, its medullary cavity enlarges, it can only be by the addition of materials to its exterior, and a coincident removal of them from the interior of its wall ; and so it must be with the growth of even the minutest portions of a tissue. And that a similar change of particles takes place, even while 300 NUTRITION. parts retain a perfect uniformity, may be proved, if it can be shown that all the parts of the body are subject to waste and impairment. In many parts, the removal of particles is evident. Thus, as will be shown when speaking of secretion, the elementary structures composing glands are the parts of which the secre- tions are composed : each gland is constantly casting off its cells, or their contents, in the secretion which it forms : yet each gland maintains its size and proper composition, because for every cell cast off a new one is produced. So also the epidermis and all such tissues are maintained. In the mus- cles, it seems nearly certain, that each act of contraction is accompanied with a change in the composition of the con- tracting tissue, although the change from this cause is less rapid and extensive than was once supposed. Thence, the development of heat in acting muscles, and then the discharge of urea, carbonic acid, and water the ordinary products of the decomposition of the animal tissues which follows all active muscular exercise. Indeed, the researches of Helm- holtz almost demonstrate the chemical change that muscles undergo after long-repeated contractions; yet the muscles retain their structure and composition, because the particles thus changed are replaced by new ones resembling those which preceded them. So again, the increase of alkaline phosphates discharged with the urine after great mental exertion, seems to prove that the various acts of the nervous system are at- tended with change in the composition of the nervous tissue ; yet the condition of that tissue is maintained. In short, for every tissue there is sufficient evidence of impairment in the discharge of its functions : without such change, the produc- tion or resistance of physical force is hardly conceivable : and the proof as well as the purpose of the nutritive process ap- pears in the repair or replacement of the changed particles ; so that, notwithstanding its losses, each tissue is maintained unchanged. But besides the impairment and change of composition to which all parts are subject in the discharge of their natural functions, an amount of impairment which will be in direct proportion to their activity, they are all liable to decay and degeneration of their particles, even while their natural actions are not called forth. It may be proved, as Dr. Carpenter first clearly showed, that every particle of the body is formed for a certain period of existence in the ordinary condition of active life ; at the end of which period, if not previously destroyed by outward force or exercise, it degenerates and is absorbed, or dies and is cast out. NUTRITION OF HAIR. 301 The simplest examples that can be adduced of this are in the hair and teeth ; and it may be observed, that, in the pro- cess which will now be described, all the great features of the process of nutrition seem to be represented. 1 An eyelash which naturally falls, or which can be drawn out without pain, is one that has lived its natural time, and has died, and been separated from the living parts. In its bulb such a one will be found different from those that are Intended to represent the changes undergone by a hair towards the close of its period of existence. At A, its activity of growth is diminishing, as shown by the small quantity of pigment contained in the cells of the pulp, and by the interrupted line of dark medullary substance. At B, provision is being made for the formation of a new hair, by the growth of a new pulp connected with the pulp or capsule of the old hair. c. A hair at the end of its period of life, deprived of its sheath and of the mass of cells composing the pulp of a living hair. still living in any period of their age. In the early period of the growth of a dark eyelash, the medullary substance appears like an interior cylinder of darker granular substance, con- 1 These and other instances are related more in detail in Mr. Paget's Lectures on Surgical Pathology, from which this chapter was originally written. 26 302 NUTRITION. tinned down to the deepest part, where the hair enlarges to form the bulb. This enlargement, which is of nearly cup- like form, appears to depend on the accumulation of nucleated cells, whose nuclei, according to their position, are either, by narrowing and elongation, to form the fibrous substance of the outer part of the growing and further protruding hair, or are to be transformed into the granular matter of its medullary portion. At the time of early and most active growth, all the cells and nuclei contain abundant pigment-matter, and the whole bulb looks nearly black. The sources of the material out of which the cells form themselves are at least two ; the inner surface of the sheath or capsule, which dips into the skin, enveloping the hair, and the surface of a vascular pulp which fits in a conical cavity in the bottom of the hair-bulb. Such is the state of parts so long as the growing hair is all dark. But as the hair approaches the end of its existence, instead of the almost sudden enlargement at its bulb, it only swells a little, and then tapers nearly to a point ; the conical cavity in its base is contracted ; and the cells produced on the inner surface of the capsule contain no pigment. Still, for some time, it continues thus to live and grow ; and the vigor of the pulp lasts rather longer than that of the sheath or capsule, for it continues to produce pigment-matter for the medullary sub- stance of the hair after the cortical substance has become white. Thus the column of dark medullary substance appears paler and more slender, and perhaps interrupted, down to the point of the conical pulp, which, though smaller, is still dis- tinct, because of the pigment-cells covering its surface. At length the pulp can be no longer discerned, and un- colored cells are alone produced, and maintain the latest growth of the hair. With these it appears to grow yet some further distance ; for traces of the elongation of their nuclei into fibres appear in lines running from the inner surface of the capsule inwards and along the surface of the hair ; and the column of dark medullary substance ceases at some distance above the lower end of the contracted hair-bulb. The end of all is the complete closure of the conical cavity in which the hair-pulp was lodged, the cessation of the production of new cells from the inner surface of the capsule, and the detach- ment of the hair, which, as a dead part, is separated and falls. Such is the life of a hair, and such its death ; which death is spontaneous, independent of exercise, or of any mechanical external force the natural termination of a certain period of life. Yet, before the hair dies, provision is made for its suc- cessor : for when its growth is failing, there appears below its base a dark spot, the germ or young pulp of the new hair MAINTENANCE BY NUTRITION. 303 FIG. 103. covered with cells containing pigment, and often connected by a series of pigment-cells with the old pulp or capsule (Fig. 102, B). Probably there is an intimate analogy between the process of successive life and death, and life communicated to a suc- cessor, which is here shown, and that which constitutes the ordinary nutrition of a part. It may be objected, that the death and casting out of the hair cannot be imitated in inter- nal parts ; therefore, for an example in which the assumed absorption of the worn-out or degenerate internal particles is imitated in larger organs at the end of their appointed period of life, the instance of the deciduous or milk-teeth may be adduced. Each milk-tooth is developed from its germ ; and in the course of its own development, separates a portion of itself to be the germ of its successor; and each, having reached its perfec- tion, retains for a time its perfect state, and still lives, though it does not grow. But at length, as the new tooth comes, the de- ciduous tooth dies ; or rather its crown dies, and is cast out like the dead hair, while its fang, with its bony sheathing, and vas- cular and nervous pulp, degen- erates and is absorbed (Fig. 103). The degeneration is accompanied by some unknown spontaneous decomposition of the fang ; for it could not be absorbed unless it was first so changed as to be solu- ble. And it is degeneration, not death, which precedes its re- moval; for when a tooth-fang dies, as that of the second tooth does in old age, then it is not absorbed, but cast out entire, as a dead part. Such, or generally such, it seems almost certain, is the pro- cess of maintenance by nutrition ; the hair and teeth may be fairly taken as types of what occurs in other parts, for they are parts of complex organic structure and composition, and the teeth-pulps, which are absorbed as well as the fangs, are very vascular and sensitive. Nor are they the only instances that might be adduced. The like development, persistence for a time in the perfect state, death, and discharge, appear in all the varieties of cuti- Section of a portion of the upper jaw of a child, showing a new tooth in process of formation, the fang of the corresponding deciduous tooth being absorbed. 304 NUTRITION. cles and glaDd-cells ; and in the epidermis, as in the teeth, there is evidence of decomposition of the old cells, in the fact of the different influence which acetic acid and potash exer- cise on them and on the young cells. Seeing, then, that the process of nutrition, as thus displayed, both in active organs and in elementary cells, appears in these respects similar, the general conclusion may be that, in nutrition, the ordinary course of each complete elementary organ in the body, after the attainment of its perfect state by development and growth, is to remain in that state for a time ; then, independently of the death or decay of the whole body, and in some measure, independently of its own exercise, or exposure to external violence, to die or to degenerate ; and then, being cast out or absorbed, to make way for its successor. It appears, moreover, that the length of life which each part is to enjoy is fixed and determinate, though in some degree subject to accidents and to the expenditure of life in exercise. It is not likely that all parts are made to last a certain and equal time, and then all need to be changed. The bones, for instance, when once completely formed, must last longer than the muscles and other softer tissues. But when we see that the life of certain parts is of determined length, whether they be used or not, we may assume, from analogy, the same of nearly all. Now, the deciduous human teeth have an appointed average duration of life. So have the deciduous teeth of all other animals; and in all the numerous instances of moulting, shed- ding of antlers, of desquamation, change of plumage in birds, and of hair in Mammalia, the only explanation is that these organs have their severally appointed times of living, at the ends of which they degenerate, die, are cast away, and in due time are replaced by others, which, in their turn, are to be de- veloped to perfection, to live their life in the mature state, and in their turn to be cast off. So also, in some elementary struc- tures, we may discern the same laws of determinate period of life, death, or degeneration, and replacement. They are evi- dent in the history of the blood-corpuscles, both in the super- seding of the first set of them by the second at a definite period in the life of the embryo, and in the replacement of those that degenerate by others new-formed from lymph -corpuscles. (See p. 83.) And if we could suppose the blood-corpuscles grouped together in a tissue instead of floating, we might have in the changes they present an image of the nutrition of the elements of the tissues. The duration of life in each particle is, however, liable to be modified ; especially by the exercise of the function of the PROCESS OF NUTRITION. 305 part. The less a part is exercised the longer do its component particles appear to live : the more active its functions are, the less prolonged is the existence of its individual particles. So in the case of single cells ; if the general development of the tadpole be retarded by keeping it in a cold, dark place, and if hereby the function of the blood-corpuscles be slowly and im- perfectly discharged, they will maintain their embryonic state for even several weeks later than usual, the development of the second set of corpuscles will be proportionally postponed, and the individual life of the corpuscles of the first set will be, by the same time, prolonged. Such being the mode in which the necessity for the process of nutritive maintenance is created, such the sources of impair- ment and waste of the tissues, the next consideration may be the manner in which the perfect state of a part is maintained by the insertion of new particles in the place of those that are absorbed or cast off. The process by which a new particle is formed in the place of the old one is probably always a process of development ; that is, the cell or fibre, or other element of tissue, passes in its formation through the same stages of development as those elements of the same tissue did which were first formed in the embryo. This is probable from the analogy of the hair, the teeth, the epidermis, and all the tissues that can be observed : in all, the process of repair or replacement is effected through development of the new parts. The existence of nuclei or cyto- blasts in nearly all parts that are the seats of active nutrition makes the same probable. For these nuclei, such as are seen so abundant in strong, active muscles, are not remnants of the embryonic tissue, but germs or organs of power for new forma- tion, and their abundance often appears directly proportionate to the activity of growth. Thus, they are always abundant in the foetal tissues, and those of the young animal ; and they are peculiarly numerous in the muscles and the brain, and their disappearance from a part in which they usually exist is a sure accompaniment and sign of degeneration. A difference may be drawn between what may be called nutritive reproduction and nutritive repetition. The former is shown in the case of the human teeth. As the deciduous tooth is being developed, a part of its productive capsule is detached, and serves as a germ for the formation of the second tooth in which second tooth, therefore, the first may be said to be re? produced, in the same sonse as that in which we speak of the organs by which new individuals are formed, as the reproduc- tive organs. But in the shark's jaws, and others, in which we see row after row of teeth succeeding each other ? the row be- 306 NUTRITION. hind is not formed of germs derived from the row before : the front row is simply repeated in the second one, the second in the third, and so on. So, in cuticle, the deepest layer of epi- dermis-cells derives no germs from the layer above : their de- velopment is not like a reproduction of the cells that have gone on towards the surface before them : it is only a repetition. It is not improbable that much of the difference in the degree of repair, of which the several tissues are capable after injuries or diseases, may be connected with these differences in their ordinary mode of nutrition. In order that the process of nutrition may be perfectly ac- complished, certain conditions are necessary. Of these, the most important are : 1. A right state and composition of the blood, from which the materials for nutrition are derived. 2. A regular and not far distant supply of such blood. 3. A cer- tain influence of the nervous system. 4. A natural state of the part to be nourished. 1. This right condition of the blood does not necessarily im- ply its accordance with any known standard of composition, common to all kinds of healthy blood, but rather the existence of a certain adaptation between the blood and the tissues, and even the several portions of each tissue. Such an adaptation, peculiar to each individual, is determined in its first formation, and is maintained in the concurrent development and increase of both blood and tissues ; and upon its maintenance in adult life appears to depend the continuance of a healthy process of nutrition, or, at least, the preservation of that exact sameness of the whole body and its parts, which constitutes the perfec- tion of nutrition. Some notice of the maintenance of this sameness in the blood has been given already (p. 84), in speaking of the power of assimilation which the blood exer- cises, a power exactly comparable with this of maintenance by nutrition in the tissues. And evidence of the adaptation be- tween the blood and the tissues, and of the exceeding fineness of the adjustment by which it is maintained, is afforded by the phenomena of diseases, in which, after the introduction of cer- tain animal poisons, even in very minute quantities, the whole mass of the blood is altered in composition, and the solid tis- sues are perverted in their nutrition. It is necessary to refer only to such diseases as syphilis, small -pox, and other erup- tive fevers, in illustration. And when the absolute dependence of all the tissues on the blood for their very existence is re- membered, on the one hand, and, on the other, the rapidity with which substances introduced into the blood are diffused into all, even non-vascular textures (p. 297), it need be no source of wonder that any, even the slightest alteration, from CONDITIONS NECESSARY FOR NUTRITION. 307 the normal constitution of the blood, should be immediately reflected, so to speak, as a change in the nutrition of the solid tissues and organs which it is destined to nourish. 2. The necessity of an adequate supply of appropriate blood in or near the part to be nourished, in order that its nutrition may be perfect, is shown in the frequent examples of atrophy of parts to which too little blood is sent, of mortification or ar- rested nutrition when the supply of blood is entirely cut off, and of defective nutrition when the blood is stagnant in a part. That the nutrition of a part may be perfect, it is also neces- sary that the blood should be brought sufficiently near to it for the elements of the tissue to imbibe, through the walls of the bloodvessels, the nutritive materials which they require. The bloodvessels themselves take no share in the process of nutrition, except as carriers of the nutritive matter. There- fore, provided they come so near that this nutritive matter may pass by imbibition into the part to be nourished, it is comparatively immaterial whether they ramify within the substance of the tissue, or are distributed only on its surface or border. The bloodvessels serve alike for the nutrition of the vascular and the non-vascular parts, the difference between which, in regard to nutrition, is less than it may seem. For the vascu- lar, the nutritive fluid is carried in streams into the interior ; for the non-vascular, it flows on the surface ; but in both alike, the parts themselves imbibe the fluid ; and although the pas- sage through the walls of the bloodvessels may effect some change in the materials, yet all the process of formation is, in both alike, outside the vessels. Thus, in muscular tissue, the fibrils in the very centre of the fibre nourish themselves : yet these are distant from all bloodvessels, and can only by imbibition receive their nutriment. So, in bones, the spaces between the bloodvessels are wider than in muscle ; yet the parts in the meshes nourish themselves, imbibing materials from the nearest source. The non-vascular epidermis, though no vessels pass into its substance, yet imbibes nutritive matter from the vessels of the immediately subjacent cutis, and main- tains itself, and grows. The instances of the cornea and vitre- ous humor are stronger, yet similar ; and sometimes even the same tissue is in one case vascular, in the other not, as the osseous tissue, which, when it is in masses or thick layers, has bloodvessels running into it ; but when it is in thin layers, as in the lachrymal and turbinated bones, has not. . These bones subsist on the blood flowing in the minute vessels of the mucous membrane, from which the epithelium derives nutriment on one side, the bone on the other, and the tissue of the membrane 308 NUTRITION. itself on every side : a striking instance how, from the same source, many tissues maintain themselves, each exercising its peculiar assimilative and self-formative power. 3. The third condition said to be essential to a healthy nu- trition, is a certain influence of the nervous system. It has been held that the nervous system cannot be essential to a healthy course of nutrition, because in plants and the early embryo, and in the lowest animals, in which no nervous system is developed, nutrition goes on without it. But this is no proof that in animals which have a nervous system, nutri- tion may be independent of it ; rather it may be assumed, that in ascending development, as one system after another is added or increased, so the highest (and, highest of all, the nervous system) will always be inserted and blended in a more and more intimate relation with all the rest: according to the general law, that the interdependence of parts augments with their development. The reasonableness of this assumption is proved by many facts showing the influence of the nervous system on nutrition, and by the most striking of these facts being observed in the higher animals, and especially in man. The influence of the mind in the production, aggravation, and cure of organic dis- eases is matter of daily observation, and a sufficient proof of influence exercised on nutrition through the nervous system. Independently of mental influence, injuries either to por- tions of the nervous centres, or to individual nerves, are fre- quently followed by defective nutrition of the parts supplied by the injured nerves, or deriving their nervous influence from the damaged portions of the nervous centres. Thus, lesions of the spinal cord are sometimes followed by mortification of por- tions of the paralyzed parts ; and this may take place very quickly, as in a case by Sir B. C. Brodie, in which the ankle sloughed within twenty -four hours after an injury of the spine. After such lesions also, the repair of injuries in the paralyzed parts may take place less completely than in others ; so, Mr. Travers mentions a case in which paraplegia was produced by fracture of the lumbar vertebrae, and, in the same accident, the humerus and tibia were fractured. The former in due time united ; the latter did not. The same fact was illustrated by some experiments of Dr. Baly, in which having, in salaman- ders, cut off the end of the tail, and then thrust a thin wire some distance up the spinal canal, so as to destroy the cord, he found that the end of the tail was reproduced more slowly than in other salamanders in whom the spinal cord was left unin- jured above the point at which the tail was amputated. Illus- trations of the same kind are furnished by the several cases in INFLUENCE OF NERVOUS SYSTEM. 309 which division or destruction of the trunk of the trigeminal nerve has been followed by incomplete and morbid nutrition of the corresponding side of the face; ulceration of the cornea being often directly or indirectly one of the consequences of such imperfect nutrition. Part of the wasting and slow de- generation of tissue in paralyzed limbs is probably referable also to the withdrawal of nervous influence from them; though, perhaps, more is due to the want of use of the tissues. Undue irritation of the trunks of nerves, as well as their division or destruction, is sometimes followed by defective or morbid nutrition. To this may be referred the cases in which ulceration of the parts supplied by the irritated nerves occurs frequently, and continues so long as the irritation lasts. Further evidence of the influence of the nervous system upon nutrition is furnished by those cases in which, from mental an- guish, or in severe neuralgic headaches, the hair becomes gray very quickly, or even in a few hours. So many and various facts leave little doubt that the ner- vous system exercises an influence over nutrition as over other organic processes ; and they cannot be explained by supposing that the changes in the nutritive processes are only due to the variations in the size of the bloodvessels supplying the affected parts. The question remains, through what class of nerves is the influence exerted? When defective nutrition occurs in parts rendered inactive by injury of the motor nerve alone, as in the muscles and other tissues of a paralyzed face or limb, it may appear as if the atrophy were the direct consequence of the loss of power in the motor nerves ; but it is more probable that the atrophy is the consequence of the want of exercise of the parts ; for if the muscles be exercised by artificial irritation of their nerves their nutrition will be less defective (J. Reid). The defect of the nutritive process which ensues in the face and other parts, moreover, in consequence of destruction of the trigeminal nerve, cannot be referred to loss of influence of any motor nerves; for the motor nerves of the face and eye, as well as the olfactory and optic, have no share in the defective nu- trition which follows injury of the trigeminal nerve ; and one or all of them may be destroyed without any direct disturbance of the nutrition of the parts they severally supply. It must be concluded, therefore, that the influence which is exercised by nerves over the nutrition of parts to which they are distributed is to be referred either to those among their branches which conduct impressions to the brain and spinal cord, namely, the nerves of common sensation, or, as it is by some supposed, by nerve-fibres which preside specially over 310 NUTRITION. the nutrition of the tissues and organs to which they are sup- plied. Such special nerves are called trophic nerves (see chap- ter on the Nervous System). It is not at present possible to say whether the influence on nutrition is exercised through the cerebro-spinal or through the sympathetic nerves, which, in the parts on which the observa- tion has been made, are generally combined in the same sheath. The truth perhaps is, that it may be exerted through either or both of these nerves. The defect of nutrition which ensues after lesion of the spinal cord alone, the sympathetic nerves being uninjured, and the general atrophy which sometimes occurs in consequence of diseases of the brain, seem to prove the influence of the cerebro-spinal system : while the observa- tion of Magendie and Mayer, that inflammation of the eye is a constant result of ligature of the sympathetic nerve in the neck, and many other observations of a similar kind, exhibit very well the influence of the latter nerve in nutrition. 4. The fourth condition necessary to healthy nutrition is a healthy state of the part to be nourished. This seems proved by the very nature of the process, which consists in the forma- tion of new parts like those already existing ; for, unless the latter are healthy, the former cannot be so. Whatever be the condition of a part, it is apt to be perpetuated by assimilating exactly to itself, and endowing with all its peculiarities, the new particles which it forms to replace those that degenerate. So long as a part is healthy, and the other conditions of healthy nutrition exist, it maintains its healthy condition. But, ac- cording to the same law, if the structure of a part be diseased or in any way altered from its natural condition, the alteration is maintained ; the altered, like the healthy structure, is per- petuated. The same exactness of the assimilation of the new parts to the old, which is seen in the nutrition of the healthy tissues, may be observed also in those that are formed in disease. By it, the exact form and relative size of a cicatrix are preserved from year to year ; by it, the thickening and induration to which inflammation gives rise are kept up, and the various morbid states of the blood in struma, syphilis, and other chronic diseases are maintained, notwithstanding all diversities of diet. By this precision of the assimilating process, may be explained the law that certain diseases occur only once in the same per- son, and that certain others are apt to recur frequently ; because in both cases alike, the alteration produced by the first attack of the disease is maintained by the exact likeness which the new parts bear to the old ones. The period, however, during which an alteration of structure GROWTH. 311 may be exactly maintained by nutrition, is not unlimited; for in nearly all altered parts there appears to exist a tendency to recover the perfect state ; and, in many cases, this state is, in time, attained. To this we may attribute the possibility of re- vaccination after the lapse of some years ; the occasional recur- rence of small-pox, scarlet-fever, and the like diseases, in the same person ; the wearing out of scars, and the complete restor- ation of tissues that have been altered by injury or disease. Such are some of the more important conditions which ap- pear to be essential to healthy nutrition. Absence or defect of any one of them is liable to be followed by disarrangement of the process ; and the various diseases resulting from defective nutrition appear to be due to the failure of these conditions, more often than to imperfection of the process itself. GROWTH. Growth, as has been already observed, consists in the increase of a part in bulk and weight by the addition to its substance of particles similar to its own, but more than sufficient to re- place those which it loses by the waste or natural decay of its tissue. The structure and composition of the part remain the same ; but the increase of healthy tissue which it receives is attended with the capability of discharging a larger amount of its ordinary function. While development is in progress, growth frequently pro- ceeds with it in the same part, as in the formation of the various organs and tissues of the embryo, in which parts, while they grow larger, are also gradually more developed until they attain their perfect state. But, commonly, growth continues after development is completed, and in some parts, continues even after the full stature of the body is attained, and after nearly every portion of it has gained its perfect state in both size and composition. In certain conditions, this continuance or a renewal of growth may be observed in nearly every part of the body. When parts have attained the full size which in the ordinary process of growth they reach, and are then kept in a moderate exercise of their functions, they commonly (as already stated) retain almost exactly the same dimensions through the adult period of life. But when, from any cause, a part already full- grown in proportion to the rest of the body, is called upon to discharge an unusual amount of its ordinary function, the demand is met by a corresponding increase or growth of the part. Illustrations of this are afforded by the increased thick- ening of cuticle at parts where it is subjected to an unusual 312 NUTRITION. degree of occasional pressure or friction, as in the palms of the hands of persons employed in rough manual labor ; by the enlargement and increased hardness of muscles that are largely exercised ; and by many other facts of a like kind. The increased power of nutrition put forth in such growth is greater than might be supposed ; for the immediate effect of increased exercise of a part must be a greater using of its tissue, and might be expected to entail a permanent thinning or diminution of the substance of the part. But the energy with which fresh particles are formed is sufficient not only to replace completely those that are worn away, but to cause an increase in the substance of the part the amount of this in- crease being proportioned to the more than usual degree in which its functions are exercised. The growth of a part from undue exercise of its functions is always, in itself, a healthy process ; and the increased size which results from it must be distinguished from the various kind of enlargement to which the same part may be subject from disease. In the former case, the enlargement is due to an increased quantity of healthy tissue, providing more than the previous power to meet a particular emergency ; the other may be the result of a deposit of morbid material within the natural structure of the part, diminishing, instead of augment- ing, its fitness for its office. Such a healthy process of growth in a part, attended with increased power and activity of its functions, may, however, occur as the consequence of disease in some other part; in which case it is commonly called Hy- pertrophy, i. e., excess of nutrition. The most familiar ex- amples of this are in the increased thickness and robustness of the muscular walls of the cavities of the heart in cases of continued obstruction to the circulation ; and in the increased development of the muscular coat of the urinary bladder when, from any cause, the free discharge of urine from it is interfered with. In both these cases, though the origin of the growth is the consequence of disease, yet the growth itself is natural, and its end is the benefit of the economy ; it is only common growth renewed or exercised in a part which had attained its size in due proportion to the rest of the body. It may be further mentioned, in relation to the physiology of this subject, that when the increase of function, which is requisite in the cases from which hypertrophy results, cannot be efficiently discharged by mere increase of the ordinary tissue of the part, the development of a new and higher kind of tissue is frequently combined with this growth. An exam- ple of this is furnished by the uterus, in the walls of which, whei| it becomes enlarged by pregnancy, or by the growth of SECRETION. 313 fibrous tumors, organic muscular fibres, found in a very ill- developed condition in its quiescent state, are then enormously developed, and provide for the expulsion of the foetus or- the foreign body. Other examples of the same kind are furnished by cases in which, from obstruction to the discharge of their contents and a consequently increased necessity for propulsive power, the coats of reservoirs and of ducts become the seat of development of organic muscular fibres, which could be said only just to exist in them before, or were present in a very imperfectly developed condition. Respecting the mode and conditions of the process of growth, it need only be said, that its mode seems to differ only in de- gree from that of common maintenance of a part ; more par- ticles are removed from, and many more added to a growing tissue, than to one which only maintains itself. But so far as can be ascertained, the mode of removal, the disposition of the removed parts, and the insertion of the new particles, are as in simple maintenance. The conditions also of growth are the same as those of com- mon nutrition, and are equally or more necessary to its occur- rence. When they are very favorable or in excess, growth may occur in the place of common nutrition. Thus hair may grow profusely in the neighborhood of old ulcers, in consequence, apparently, of the excessive supply of blood to the hair-bulbs and pulps ; bones may increase in length when disease brings much blood to them ; and cocks' spurs transplanted from their legs into their combs grow to an unnatural length ; the conditions common to all these cases being both an increased supply of blood, and the capability, on the part of the growing tissue, of availing itself of the opportunity of increased absorption and nutrition thus afforded to it. In the absence of the last-named condition, increased supply of blood will not lead to increased nutrition. CHAPTER XII. SECRETION. SECRETION is the process by which materials are separated from the blood, and from the organs in which they are formed, for the purpose either of serving some ulterior office in the economy, or being discharged from the body as excrement. In the former case, both the separated materials and the processes 314 SECKETION. for their separation are termed secretions; in the latter, they are named excretions. Most of the secretions consist of substances which, probably, do not pre-exist in the same form in the blood, but require special organs and a process of elaboration for their formation, e. g., the liver for the formation of bile, the mammary gland for the formation of milk. The excretions, on the other hand, commonly or chiefly consist of substances which, as urea, car- bonic acid, and probably uric acid, exist ready-formed in the blood, and are merely abstracted therefrom. If from any cause, such as extensive disease or extirpation of an excretory organ, the separation of an excretion is prevented, and an ac- cumulation of it in the blood ensues, it frequently escapes through other organs, and may be detected in various fluids of the body. But this is never the case with secretions ; at least with those that are most elaborated ; for after the removal of the special organs by which any of them is elaborated, it is no longer formed. Cases sometimes occur in which the secretion con- tinues to be formed by the natural organ, but not being able to escape towards the exterior, on account of some obstruction, is reabsorbed into the blood, and afterwards discharged from it by exudation in other ways ; but these are not instances of true vicarious secretion, and must not be thus regarded. These circumstances, and their final destination, are, how- ever, the only particulars in which secretions and excretions can be distinguished ; for, in general, the structure of the parts engaged in eliminating excretions, e. g., the kidneys, is as com- plex as that of the parts concerned in the formation of secre- tions. And since the diiferences of the two processes of sepa- ration, corresponding with those in the several purposes and destinations of the fluids, are not yet ascertained, it will be sufficient to speak in general terms of the process of separation or secretion. Every secreting apparatus possesses, as essential parts of its structure, a simple and apparently textureless membrane, named the primary or basement-membrane ; certain cells ; and bloodvessels. These three structural elements are arranged together in various ways ; but all the varieties may be classed under one or other of two principal divisions, namely, mem- branes and glands. SECRETING MEMBRANES. The principal secreting membranes are the serous and syno- vial membranes, the mucous membranes, and the skin. 1 1 The skin will be described in a subsequent chapter. SEROUS MEMBRANES. 315 The serous membranes are formed of fibro-cellular tissue, interwoven so as to constitute a membrane, the free surface of which is covered with a single layer of flattened cells, forming, in most instances, a simple tessellated epithelium. Between the epithelium and the subjacent layer of fibro-cellular tissue, is situated the primary or basement-membrane (Bowman). FIG. 104. Plan of a secreting membrane : a, membrana propria, or basement-membrane ; 6, epithelium composed of secreting nucleated cells ; c, layer of capillary bloodvessels (after Sharpey). In relation to the process of secretion, the layer of fibro- cellular tissue serves as a groundwork for the ramification of bloodvessels, lymphatics, and nerves. But in its usual form it is absent in some instances, as in the arachnoid covering the dura mater, and in the interior of the ventricles of the brain. The primary membrane and epithelium are probably always present, and are concerned in the formation of the fluid by which the free surface of the membrane is moistened. The serous membranes are of two principal kinds: 1st. Those which line visceral cavities, the arachnoid, pericar- dium, pleurae, peritoneum, and tunicse vaginales. 2d. The synovial membranes lining the joints, and the sheaths of ten- dons and ligaments, with which, also, are usually included the synovial bursse, or bursce mucosce, whether these be subcuta- neous, or situated beneath tendons that glide over bones. The serous membranes form closed sacs, and exist wherever the free surfaces of viscera come into contact with each other, or lie in cavities unattached to surrounding parts. The viscera, which are invested by a serous membrane, are, as it were, pressed into the shut sac which it forms, carrying before them a portion of the membrane, which serves as their investment. To the law that serous membranes form shut sacs, there is, in the human subject, one exception, viz. : the opening of the Fallopian tubes into the abdominal cavity, an arrangement which exists in man and all Vertebrata, with the exception of a few fishes. The principal purpose of the serous and synovial membranes is to furnish a smooth, moist surface, to facilitate the move- ments of the invested organ, and to prevent the injurious effects of friction. This purpose is especially manifested in 316 SECRETION. joints, in which free and extensive movements take place ; and in the stomach and intestines, which, from the varying quan- tity and movements of their contents, are in almost constant motion upon one another and the walls of the abdomen. The fluid secreted from the free surface of the serous mem- branes is, in health, rarely more than sufficient to insure the maintenance of their moisture. The opposed surfaces of each serous sac, are at every point in contact with each other, and leave no space in which fluid can collect. After death, a larger quantity of fluid is usually found in each serous sac ; but this, if not the product of manifest disease, is probably such as has transuded after death, or in the last hours of life. An excess of such fluid in any of the serous sacs constitutes dropsy of the sac. The fluid naturally secreted by the serous membranes appears to be identical, in general and chemical characters, with the serum of the blood, or with very dilute liquor sanguinis. It is of a pale yellow or straw-color, slightly viscid, alkaline, and, because of the presence of albumen, coagulable by heat. The presence of a minute quantity of fibrin, at least in the dropsical fluids effused into the serous cavities, is shown by their partial coagulation into a jelly-like mass, on the addition of certain animal substances, or on mixture with certain fluids, especially such as contain cells (p. 70 et seq.}. This similarity of the serous fluid to the liquid part of blood, and to the fluid with which most animal tissues are moistened, renders it probable that it is, in great measure, separated by simple transudation through the walls of the bloodvessels. The probability is increased by the fact that, in jaundice, the fluid in the serous sacs is, equally with the serum of the blood, colored with the bile. But there is reason for supposing that the fluid of the cerebral ventricles and of the arachnoid sac are exceptions to this rule ; for they differ from the fluids of the other serous sacs not only in being pellucid, colorless, and of much less specific gravity, but in that they seldom receive the tinge of bile in the blood, and are not colored by madder, or other similar substances introduced abundantly into the blood. It is also probable that the formation of synovial fluid is a process of more genuine and elaborate secretion, by means of the epithelial cells on the surface of the membrane, and espe- cially of those which are accumulated on the edges and pro- cesses of the synovial fringes ; for, in its peculiar density, vis- cidity, and abundance of albumen, synovia differs alike from the serum of blood and from the fluid of any of the serous cavities. The mucous membranes line all those passages by which in- MUCOUS MEMBRANES. 317 ternal parts communicate with the exterior, and by which either matters are eliminated from the body or foreign substances taken into it. They are soft and velvety, and extremely vas- cular. Their general structure resembles that of serous mem- branes. It consists of epithelium, basement-membrane, and fibro-cellular or areolar tissue containing bloodvessels, lym- phatics, and nerves. The structure of mucous membranes is less uniform, especially as regards their epithelium, than that of serous membranes ; but the varieties of structure in different parts are described in connection with the organs in which mucous membranes are present, and need not be here noticed in detail. The external surfaces of mucous membranes are attached to various other tissues ; in the tongue, for example, to muscle ; on cartilaginous parts, to perichondrium ; in the cells of the ethmoid bone, in the frontal and sphenoid sinuses, as well as in the tympanum, to periosteum ; in the intestinal canal, it is connected with a firm submucous membrane, which on its exterior gives attachment to the fibres of the muscular coat. The mucous membranes are described as lining certain prin- cipal tracts. 1. The digestive trad commences in the cavity of the mouth, from which prolongations pass into the ducts of the salivary glands. From the mouth it passes through the fauces, pharynx, and oesophagus, to the stomach, and is thence con- tinued along the whole tract of the intestinal canal to the ter- mination of the rectum, being in its course arranged in the various folds and depressions already described, and prolonged into the ducts of the pancreas and liver and into the gall-blad- der. 2. The respiratory tract includes the mucous membrane lining the cavity of the nose, and the various sinuses commu- nicating with it, the lachrymal canal and sac, the conjunctiva of the eye and eyelids, and the prolongation which passes along the Eustachian tubes and lines the tympanum and the inner surface of the membrana tympani. Crossing the pharynx, and lining that part of it which is above the soft palate, the respi- ratory tract leads into the glottis, whence it is continued, through the larynx and trachea, to the bronchi and their divisions, which it lines as far as the branches of about -$ of an inch in diameter, and continuous with it is a layer of delicate epithelial membrane which extends into the pulmonary cells. 3. The genito-urinary tract, which lines the whole of the urinary pas- sages, from their external orifice to the termination of the tubuli uriniferi of the kidneys, extends into and through the organs of generation in both sexes, into the ducts of the glands connected with them ; and in the female becomes continuous 27 318 SECRETION. with the serous membrane of the abdomen at the fimbrise of the Fallopian tubes. Along each of the above tracts, and in different portions of each of them, the mucous membrane pres'ents certain struc- tural peculiarities adapted to the functions which each part has to discharge; yet in some essential characters mucous membrane is the same, from whatever part it is obtained. In all the principal and larger parts of the several tracts, it pre- sents, as just remarked, an external layer of epithelium, situated upon basement-membrane, and beneath this, a stratum of vascular tissue of variable thickness, which in different cases presents either outgrowths in the form of papillae and villi, or depressions or involutions in the form of glands. But in the prolongations of the tracts, where they pass into gland-ducts, these constituents are reduced in the finest branches of the ducts to the epithelium, the primary or basement-membrane, and the capillary bloodvessels spread over the outer surface of the latter in a single layer. The primary or basement-membrane is a thin transparent layer, simple, homogeneous, and with no discernible structure, which on the larger mucous membranes that have a layer of vascular fibro-cellular tissue, may appear to be only the blastema or formative substance, out of which successive layers of epithelium-cells are formed. But in the minuter di- visions of the mucous membranes, and in the ducts of glands, it is the layer continuous and correspondent with this basement- membrane that forms the proper walls of the tubes. The cells also which, lining the larger and coarser mucous membranes, constitute their epithelium, are continuous with and often similar to those which, lining the gland-ducts, are called gland-cells, rather than epithelium. Indeed, no certain dis- tinction can be drawn between the epithelium-cells of mucous membranes and gland-cells. In reference to their position, as covering surfaces, they might all be called epithelium-cells, whether they lie on open mucous membranes, or in gland- ducts ; and in reference to the process of secretion, they might all be called gland-cells, or at least secreting-cells, since they probably all fulfil a secretory office by separating certain definite materials from the blood and from the part on which they are seated. It is only an artificial distinction which makes them epithelial cells in one place, and gland-cells in another. It thus appears, that the tissues essential to the production of a secretion are, in their simplest form, a simple membrane, having on one surface bloodvessels, and on the other a layer of cells, which may be called either epithelium-cells or gland- SECRETING GLANDS. 319 cells. Glands are provided also with lymphatic vessels and nerves. The distribution of the former is not peculiar, and need not be here considered. Nerve-fibres are distributed both to the bloodvessels of the gland and to its ducts ; and, in some glands, it is said, to the secreting cells also. The structure of the elementary portions of a secreting ap- paratus, namely, epithelium, simple membrane, and blood- vessels, having been already described in this and previous chapters, we may proceed to consider the manner in which they are arranged to form the varieties of secreting glands. SECRETING GLANDS. The secreting glands are the organs to which the office of secreting is more especially ascribed : for they appear to be occupied with it alone. They present, amid manifold diversi- ties of form and composition, a general plan of structure, by which they are distinguished from all other textures of the body; especially, all contain, and appear constructed with particular regard to, the arrangement of the cells, which as already expressed, both line their tubes or cavities as an epi- thelium, and elaborate, as secreting cells, the substances to be discharged from them. For convenience of description, they may be divided into three principal groups, the characters of each of which are de- termined by the different modes in which the sacculi or tubes containing the secreting cells are grouped : 1. The simple tubule or tubular gland (A, Fig. 105), exam- ples of which are furnished by the several tubular follicles in mucous membranes : especially by the follicles of Lieberkiihn in the mucous membrane of the intestinal canal (p. 241), and the tubular or gastric glands of the stomach (p. 217). These appear to be simple tubular depressions of the mucous mem- brane on which they open, each consisting of an elongated gland- vesicle, the wall of which is formed of primary mem- brane, and is lined with secreting cells arranged as an epithe- lium. To the same class may be referred the elongated and tortuous sudoriparous glands of the skin (p. 338), and the Meibomian follicles beneath the palpebral conjunctiva ; though the latter are made more complex by the presence of small pouches along their sides (B, Fig. 105), and form a connecting link between the members of this division and the next, as the former by their length and tortuosity do between the first di- vision and the third (D, Fig. 105). 2. The aggregated glands, including those that used to be called conglomerate, in which a number of vesicles or acini are 320 SECKETION. arranged in groups or lobules (c, Fig. 105). Such are all those commonly called mucous glands, as those of the tra- chea, vagina, and the minute salivary glands. Such, also, are Plans of extension of secreting membrane by inversion or recession in form of cavities. A, simple glands, viz., g, straight tube ; h, sac ; i, coiled tube. B, multi- Jocular crypts ; k, of tubular form ; I, saccular. C, racemose, or saccular compound gland ; TO, entire gland, showing branched duct and lobular structure ; , a lobule, detached with o, branch of duct proceeding from it. D, compound tubular gland (after Sharpey). the lachrymal, the large salivary and mammary glands, Brunn's, Cowper's, and fiuverney's glands, the pancreas and PROCESS OF SECRETION. 321 prostate. These various organs differ from each other only in secondary points of structure ; such as, chiefly, the arrange- ment of their excretory ducts, the grouping of the acini and lobules, their connection by fibro-cellular tissue, and supply of bloodvessels. The acini commonly appear to be formed by a kind of fusion of the walls of several vesicles, which thus com- bine to form one cavity lined or filled with secreting cells which also occupy recesses from the main cavity. The small- est branches of the gland-ducts sometimes open into the cen- tres of these cavities ; sometimes the acini are clustered round the extremities, or by the sides of the ducts : but, whatever secondary arrangement there may be, all have the same essen- tial character of rounded groups of vesicles containing gland- cells, and opening, either occasionally or permanently, by a common central cavity into minute ducts, which ducts in the large glands converge and unite to form larger and larger branches, and at length, by one common trunk, open on a free surface of membrane. 3. The convoluted tubular glands (D, Fig. 105), such as the kidney and testis, form another division. These consist of tu- bules of membrane, lined with secreting cells arranged like an epithelium. Through nearly the whole of their long course, the tubules present an almost uniform size and structure; ultimately they terminate either in a cul-de-sac, or by dilating, as in the Malpighian capsules of the kidney, or by forming a simple loop and returning, as in the testicle. Among these varieties of structure, all the permanent glands are alike in some essential points, besides those which they have in common with all truly secreting structures. They agree in presenting a large extent of secreting surface within a comparatively small space ; in the circumstance that while one end of the gland-duct opens on a free surface, the oppo- site end is always closed, having no direct communication with bloodvessels, or any other canal ; and in uniform arrange- ment of capillary bloodvessels, ramifying and forming a net- work around the walls and in the interstices of the ducts and acini. PROCESS OF SECRETION. From what has been said, it will have already appeared that the modes in which secretions are produced are at least two. Some fluids, such as the secretions of serous membranes, appear to be simply exudations or oozings from the bloodves- sels, whose qualities are determined by those of the liquor san- guinis, while the quantities are liable to variation, or are chiefly dependent on the pressure of the blood on the interior 322 SECRETION. of the bloodvessels. But, in the production of the other se- cretions, such as those of mucous membranes and all glands, other besides these mechanical forces are in operation. Most of the secretions are indeed liable to be modified by the cir- cumstances which affect the simple exudation from the blood- vessels, and the products of such exudations, when excessive, are apt to be mixed with the more proper products of all the se- creting organs. But the act of secretion in all glands is the result of the vital processes of cells or nuclei, which, as they develop themselves and grow, form in their interior the proper materials of the secretion, and then discharge them. The best evidence for this view is : 1st. That cells and nuclei are constituents of all glands, however diverse their outer forms and other characters, and are in all glands placed on the surface or in the cavity whence the secretion is poured. 2d. That many secretions which are visible with the micro- scope may be seen in the cells of their glands before they are discharged. Thus, bile may be often discerned by its yellow tinge in the gland-cells of the liver ; spermatozoids in the cells of the tubules of the testicles ; granules of uric acid in those of the kidneys of fish ; fatty particles, like those of milk, in the cells of the mammary gland. The process of secretion might, therefore, be said to be accomplished in, and by the life of, these gland-cells. They appear, like the cells or other elements of any other organ, to develop themselves, grow, and attain their individual perfec- tion by appropriating the nutriment from the adjacent blood- vessels and elaborating it into the materials of their walls and the contents of their cavities. In this perfected state, they subsist for some brief time, and when that period is over they appear to dissolve or burst and yield themselves and their con- tents to the peculiar material of the secretion. And this ap- pears to be the case in every part of the gland that contains the appropriate gland-cells ; therefore not in the extremities of the ducts or in the acini alone, but in great part of their length. In these things there is the closest resemblance between secretion and nutrition ; for if the purpose which the secreting glands are to serve in the economy be disregarded, their for- mation might be considered as only the process of nutrition of organs, whose size and other conditions are maintained in, and by means of, the continual succession of cells developing themselves and passing away. In other words, glands are maintained by the development of the cells, and their con- tinuance in the perfect state ; and the secretions are discharged as the constituent gland-cells degenerate and are set free. The processes of nutrition and secretion are similar, also, in DISCHARGE OF SECRETIONS. 323 their obscurity : there is the same difficulty in saying why, out of apparently the same materials, the cells of one gland elaborate the components of bile, while those of another form the components of milk, and of a third those of saliva, as there is in determining why one tissue forms cartilage, another bone, a third muscle, or any other tissue. In nutrition, also, as in secretion, some elements of tissues, such as the gelatinous tis- sues, are different in their chemical properties from any of the constituents ready-formed in the blood. Of these differences, also, no account can be rendered ; but, obscure as the cause of these diversities may be, they are not objections to the ex- planation of secretion as a process similar to nutrition ; an explanation with which all the facts of the case are recon- cilable. It may be observed that the diversities presented by the other constituents of glands afford no explanation of the dif- ferences or peculiarities of their several products. There are many differences in the arrangements of the bloodvessels in different glands and mucous membranes ; and, in accordance with these, much diversity in the rapidity with which the blood traverses them. But there is no reason for believing that these things do more than influence the rate of the pro- cess and the quantity of the material secreted. Cceteris pari- bus, the greater the vascularity of a secreting organ, and the larger the supply of blood traversing its vessels in a given time, the larger is the amount of secretion ; but there is no evidence that the quantity or mode of movement of the blood can directly determine the quality of the secretion. The discharge of secretions from glands may take place as soon as they are formed; or the secretion may be long re- tained within the gland or its ducts. The secretion of glands which are continually in active function for the purification of the blood, such as the kidneys, are generally discharged from the gland as rapidly as they are formed. But the secre- tions of those whose activity of function is only occasional, such as the testicle, are usually retained in the ducts during the periods of the gland's inaction. And there are glands which are like both these classes, such as the lachrymal and salivary, which constantly secrete small portions of fluid, and on occa- sions of greater excitement discharge it more abundantly. When discharged into the ducts, the further course of secre- tions is effected partly by the pressure from behind ; the fresh quantities of secretion propelling those that were formed before. In the larger ducts, its propulsion is assisted by the contraction of their walls. All the larger ducts, such as the ureter and common bile-duct, possess in their coats organic muscular 324 SECBETION. fibres ; they contract when irritated, and sometimes manifest peristaltic movements. Bernard and Brown-Sequard, indeed, have observed rhythmic contractions in the pancreatic and bile-ducts, and also in the ureters and vasa deferentia. It is probable that the contractile power extends along the ducts to a considerable distance within the substance of the glands whose secretions can be rapidly expelled. Saliva and milk, for in- stance, are sometimes ejected with much force ; doubtless by the energetic and simultaneous contraction of many of the ducts of their respective glands. The contraction of the ducts can only expel the fluid they contain through their main trunk ; for at their opposite ends all the ducts are closed. Circumstances influencing Secretion. The influence of exter- nal conditions on the functions of glands, is manifested chiefly in alterations of the quantity of secretion ; and among the prin- cipal of these conditions are variations in the quantity of blood, in the quantity of the peculiar materials for any secretion that it may contain, and in the conditions of the nerves of the glands. In general, an increase in the quantity of blood traversing a gland, coincides with an augmentation of its secretion. Thus, the mucous membrane of the stomach becomes florid when, on the introduction of food, its glands begin to secrete ; the mam- mary gland becomes much more vascular during lactation ; and it appears that all circumstances which give rise to an in- crease in the quantity of material secreted by an organ, pro- duce, coincidently, an increased supply of blood. In most cases, the increased supply of blood rather follows than pre- cedes the increase of secretion ; as, in the nutritive processes, the increased nutrition of a part just precedes and determines the increased supply of blood ; but, as also in the nutritive process, an increased supply of blood may have, for a conse- quence, an increased secretion from the glands to which it is sent. Glands also secrete with increased activity when the blood contains more than usual of the materials they are designed to separate. Thus, when an excess of urea is in the blood, whether from excessive exercise, or from destruction of one kidney, a healthy kidney will excrete more than it did before. It will, at the same time, grow larger : an interesting fact, as proving both that secretion and nutrition in glands are identical, and that the presence of certain materials in the blood may lead to the formation of structures in which they may be incorporated. The process of secretion is, also, largely influenced by the condition of the nervous system. The exact mode in which the nervous system influences THE DUCTLESS GLANDS. 325 secretion must be still regarded as somewhat obscure. In part, it exerts its influence by increasing or diminishing the quantity of blood supplied to the secreting gland, in virtue of the power which it exercises over the contractility of the smaller blood- vessels ; while it also has a more direct influence analogous to the trophic influence referred to in the chapter on Nutrition. Its influence over secretion, as well as over other functions of the body, may be excited by causes acting directly upon the nervous centres, upon the nerves going to the secreting organ, or upon the nerves of other parts. In the latter case, a reflex action is produced : thus the impression produced upon the nervous centres by the contact of food in the mouth, is reflected upon the nerves supplying the salivary glands, and produces, through these, a more abundant secretion of saliva. Through the nerves, various conditions of the mind also in- fluence the secretions. Thus, the thought of food may be suf- ficient to excite an abundant flow of saliva. And, probably, it is the mental state which excites the abundant secretion of urine in hysterical paroxysms, as well as the perspirations and, occasionally, diarrhoea, which ensue under the influence of terror, and the tears excited by sorrow or excess of joy. The quality of a secretion may also be affected by the mind ; as in the cases in which, through grief or passion, the secretion of milk is altered, and is sometimes so changed as to produce irritation in the alimentary canal of the child, or even death (Carpenter). The secretions of some of the glands seem to bear a certain relation or antagonism to each other, by which an increased activity of one is usually followed by diminished activity of one or more of the others ; and a deranged condition of one is apt to entail a disordered state in the others. Such relations appear to exist among the various mucous membranes : and the close relation between the secretion of the kidney and that of the skin is a subject of constant observation. CHAPTER XIII. THE VASCULAR GLANDS; OR GLANDS WITHOUT DUCTS. THE materials separated from the blood by the ordinary process of secretion by glands, are always discharged from the organ in which they are formed, and either straightway ex- 28 326 THE DUCTLESS GLANDS. pelled from the body, or if they are again received into the blood, it is only after they have been altered from their original condition, as in the cases of the saliva and bile. There ap- pears, however, to be a modification of the process of secre- tion, in which certain materials are abstracted from the blood, undergo some change, and are added to the lymph or restored to the blood, without being previously discharged from the secreting organ, or made use of for any secondary purpose. The bodies in which this modified form of secretion takes place, are usually described as vascular glands, or glands without ducts, and include the spleen, the thymus and thyroid glands, the supra-renal capsules, and, according to (Esterlin and Ecker and Gull, the pineal gland and pituitary body ; possibly, also the tonsils. The solitary and agminate glands of the intestine (p. 242), and lymph-glands in.general, also closely resemble them ; in- deed, both in structure and function, the vascular glands bear a close relation, on the one hand, to the true secreting glands, and on the other, to the lymphatic glands. The evidence in favor of the view that these organs exercise a function analogous to that of secreting glands, has been FIG. 106. Vesicles from the thyroid gland of a child (from Kolliker) 25. o _ a, connective tissue between the vesicles ; b, capsule of the vesicles ; c, their epithelial lining. chiefly obtained from investigations into their structure, which have shown that most of the glands without ducts contain the same essential structures as the secreting glands, except the ducts. They are mainly composed of vesicles, or sacculi, either simple and closed, as in the thyroid (Fig. 106), and supra- THE DUCTLESS GLANDS. 327 renal capsules, or variously branched, and with the cavities of the several branches communicating in and by common canals, as in the thy m us (Fig. 107). These vesicles, like the acini of secreting glands, are formed of a delicate homogeneous mem- brane, are surrounded with and often traversed by a vascular plexus, and are filled with finely molecular albuminous fluid, suspended in which are either granules of fat, or cytoblasts, or nuclei, or nucleated cells, or a mixture of all these. Structure of the Spleen. The spleen is covered externally almost completely by a serous coat derived from the peri- toneum, while within this is the proper fibrous coat or capsule of the organ. The latter, composed of connective tissue, with FIG. 107. Transverse section of a lobule of an injected infantile thyinus gland (after K61- liker) (magnified 30 diameters), a, capsule of connective tissue surrounding the lobule ; ft, membrane of the glandular vesicles ; c, cavity of the lobule, from which the larger bloodvessels are seen to extend towards and ramify in the spheroidal masses of the lobule. a large preponderance of elastic fibres, forms the immediate investment of the spleen. Prolonged from its inner surface are fibrous processes or trabeculce, which enter the interior of the organ, and, dividing and anastomosing in all parts, form a kind of supporting framework or stroma, in the interstices of which the proper substance of the spleen, or the spleen-pulp, is contained. At the hilus of the spleen, or the part at which 328 THE DUCTLESS GLANDS. the bloodvessels, nerves, and lymphatics enter, the fibrous coat is prolonged into the spleen-substance in the form of investing sheaths for the arteries and veins, which sheaths again are con- nected with the trabeculcv before referred to. The spleen-pulp, which is of a dark red or reddish-brown color, is composed chiefly of cells. Of these, some are granular corpuscles resembling the lymph-corpuscles, both in general appearance and in being able to perform amoeboid movements; others are red blood-corpuscles of normal appearance or vari- ously changed ; while there are also large cells containing either pigment allied to the coloring matter of the blood, or rounded corpuscles like red blood-cells. The splenic artery, which enters the spleen by its concave surface or hilus, divides and subdivides, with but little anas- tomosis between its branches, in the midst of the spleen-pulp, at the same time that its branches are sheathed, as before said, by the fibrous coat, which they, so to speak, carry into the spleen with them. Ending in capillaries, they either com- municate, as in other parts of the body, with the radicles of the veins, or end in lacunar spaces in the spleen-pulp, from which veins arise (Gray). On the face of a section of the spleen can be usually seen, readily with the naked eye, minute, scattered, rounded or oval whitish spots, mostly from ^ to g 1 ^ inch in diameter. These are the Malpighian corpuscles of the spleen, and are situated on the sheaths of the minute splenic arteries, of which, indeed, they may be said to be outgrowths (Fig. 108). For while the sheaths of the larger arteries are constructed of ordinary con- nective tissue, this has become modified where it forms an in- vestment for the smaller vessels, so as to be a fine retiform tissue, with abundance of corpuscles, like lymph-corpuscles, contained in its meshes ; and the Malpighian corpuscles are but small outgrowths of this cytogenous or cell-bearing connec- tive tissue. They are composed of masses of corpuscles, inter- sected iu all parts by a delicate fibrillar tissue, which, though it invests the Malpighian bodies, does not form a complete capsule. Blood-capillaries traverse the Malpighian corpuscles and form a plexus in their interior. The structure of a Mal- pighian corpuscle of the spleen is, therefore, very similar to that of lymphatic-gland substance (p. 284). The general resemblances in structure between certain of the vascular glands and the true glands lead to the supposition that both sets of organs pursue, up to a certain point, a similar course in the discharge of their functions. It is assumed that certain principles in an inferior state of organization are effused FUNCTIONS OF DUCTLESS GLANDS. 329 from the vessels into the sacculi, arid gradually develop into nuclei or cytoblasts, which may be further developed into cells ; that in the growth of these nuclei and cells, the materials de- FIG. 108. The figure shows a portion of a small artery, to one of the twigs of which the Malpighian corpuscles are attached. rived from the blood are elaborated into a higher condition of organization ; and that when liberated by the dissolution of these cells, they pass into the lymphatics, or are again received into the blood, whose aptness for nutrition they contribute to maintain. The opinion that the vascular glands thus serve for the higher organization of the blood, is supported by their being all especially active in the discharge of their functions during foetal life and childhood, when, for the development and growth of the body, the most abundant supply of highly or- ganized blood is necessary. The bulk of the thymus gland, in proportion to that of the body, appears to bear almost a direct proportion to the activity of the body's development and growth, and when, at the period of puberty, the development of the body may be said to be complete,*the gland wastes, and finally disappears. The thyroid gland and supra-renal cap- sules, also, though they probably never cease to discharge some 330 THE DUCTLESS GLANDS. amount of function, yet are proportionally much smaller in childhood than in foetal life and infancy ; and with the years advancing to the adult period, they diminish yet more in pro- portionate size and apparent activity of function. The spleen more nearly retains its proportionate size, and enlarges nearly as the whole body does. The function of the vascular glands seems not essential to life, at least not in the adult. The thymus wastes and dis- appears ; no signs of illness attend some of the diseases which wholly destroy the structure of the thyroid gland ; and the spleen has been often removed in animals, and in a few in- stances in men, without any evident ill-consequence. It is possible that, in such cases, some compensation for the loss of one of the organs may be afforded by an increased activity of function in those that remain. The experiment, to be com- plete, should include the removal of all these organs, an opera tion of course not possible without immediate danger to life. Nor, indeed, would this be certainly sufficient, since there is reason to suppose that the duties of the spleen, after its re- moval, might be performed by lymphatic glands, between whose structure and that of the vascular glands there is much resemblance, and which, it is said, have been found peculiarly enlarged when the spleen has been removed (Meyer). Although the functions of all the vascular glands may be similar, in so far as they may all alike serve for the elabora- tion and maintenance of the blood, yet each of them probably discharges a peculiar office, in relation either to the whole economy, or to that of some other organ. Respecting the special office of the thyroid gland, nothing reasonable can be suggested ; nor is there any certain evidence concerning that of the supra-renal capsules. 1 Respecting the thymus gland, the observations of Mr. Simon, confirmed by those of Friedle- ben and others, have shown that in the hibernating animals, in which it exists throughout life, as each successive period of hibernation approaches, the thymus greatly enlarges and be- comes laden with fat, which accumulates in it and in fat- glands connected with it, in even larger proportions than it does in the ordinary seats of adipose tissue. Hence it appears 1 Mr. J. Hutchinson, and more recently, Dr. Wilks, following out Dr. Addison's discovery, have, by the collection of a large and valua- ble series of cases in which the supra-renal capsules were diseased, demonstrated most sati>f'actorily the very close relation subsisting be- tween disease of these organs and brown discoloration of the skin; but the explanation of this relation is still involved in obscurity, and consequently does not aid much in determining the functions of the supra-renal capsules. FUNCTIONS OF SPLEEN. 331 to serve for the storing up of materials which, being reabsorbed in the inactivity of the hibernating period, may maintain the respiration and the temperature of the body in the reduced state to which they fall during that time. With respect to the office of the spleen, we have somewhat more definite information. In the first place, the large size which it gradually acquires towards the termination of the digestive process, and the great increase observed about this period in the amount of the finely-granular albuminous plasma within its parenchyma, and the subsequent gradual decrease of this material, seem to indicate that this organ is concerned in elaborating the albuminous or formative materials of food, and for a time storing them up, to be gradually introduced iiito the blood, according to the demands of the general system. The small amount of fatty matter in such plasma, leads to the inference that the gland has little to do in regard to the prepa- ration of material for the respiratory process. Then again, it seems not improbable that, as Hewson origi- nally suggested, the spleen, and perhaps to some extent the other vascular glands, are, like the lymphatic glands, engaged in the formation of the germs of subsequent blood-corpuscles. For it seems quite certain, that the blood of the splenic vein contains an unusually large amount of white corpuscles ; and in the disease termed leucocythsemia, in which the pale cor- puscles of the blood are remarkably increased in number, there is almost always found an hypertrophied state of the spleen or thyroid body, or some of the lymphatic glands. Accordingly there seems to be a close analogy in function be- tween the so-called vascular and the lymphatic glands : the former elaborating albuminous principles, and forming the germs of new blood-corpuscles out of alimentary materials absorbed by the bloodvessels ; the latter discharging the like office on nutritive materials taken up by the general absorbent system. In Kolliker's opinion, the development of colorless and also colored corpuscles of the blood is one of the essential functions of the spleen, into the veins of which the new-formed corpuscles pass, and are thus conveyed into the general cur- rent of the circulation. There is reason to believe, too, that in the spleen many of the red corpuscles of the blood, those probably which have discharged their office and are worn out, undergo disintegra- tion ; for in the colored portion of the spleen-pulp an abun- dance of such corpuscles, in various stages of degeneration, are found, while the red corpuscles in the splenic venous blood are said to be relatively diminished. According to Kolliker's description of this process of disintegration, the blood-corpus- 332 THE SKIN. cles, becoming smaller and darker, collect together in roundish heaps, which may remain in this condition, or become each surrounded by a cell-wall. The cells thus produced may con- tain from one to twenty blood-corpuscles in their interior. These corpuscles become smaller and smaller ; exchange their red for a golden yellow, brown, or black color ; and, at length are converted into pigment-granules, which by degrees become paler and paler, until all color is lost. The corpuscles undergo these changes whether the heaps of them are enveloped by a cell-wall or not. Besides these, its supposed direct offices, the spleen is be- lieved to fulfil some purpose in regard to the portal circula- tion, with which it is in close connection. From the readiness with which it admits of being distended, and from the fact that it is generally small while gastric digestion is going on, and enlarges when that act is concluded, it is supposed to act as a kind of vascular reservoir, or diverticulum to the portal system, or more particularly to the vessels of the stomach. That it may serve such a purpose is also made probable by the enlargement which it -undergoes in certain affections of the heart and liver, attended with obstruction to the passage of blood through the latter organ, and by its diminution when the congestion of the portal system is relieved by discharges from the bowels, or by the effusion of blood into the stomach. This mechanical influence on the circulation, however, can hardly be supposed to be more than a very subordinate part of the office of an organ of so great complexity as the spleen, and containing so many other structures besides bloodvessels. The same may also be said with regard to the opinion that the thyroid gland is important as a diverticulum for the cerebral circulation, or the thymus for the pulmonary in childhood. These, like the spleen, must have peculiar and higher, though as yet ill-understood, offices. CHAPTER XIV. THE SKIN AND ITS SECRETIONS. To complete the consideration of the processes of organic life, an4 especially of those which, by separating materials from the blood, maintain it in the state necessary for the nutrition of the body, the structure and fuuctions of the skin must be now considered : for besides the purposes which it serves (1), as an internal integument for the protection of EPIDERMIS. 333 the deeper tissues, and (2), as a sensitive organ in the exercise of touch, it is also (3), an important excretory, and (4) an absorbing organ ; while it plays a most important part in (5) the regulation of the temperature of the body. Structure of the Skin. The skin consists, principally, of a layer of vascular tissue, named the corium, derma, or cutis vera, and an external cover- ing of epithelium termed the cuticle or epidermis. Within and beneath the corium are imbedded several organs with special functions, namely, sudoriparous glands, sebaceous glands, and hair-follicles ; and on its surface are sensitive papillae. The so-called appendages of the skin the hair and noils are modi- fications of the epidermis. Epidermis. The epidermis is composed of several layers of epithelial cells of the squamous kind (p. 34), the deeper cells, however, being rounded or elongated, and in the latter in- stance having their long axis arranged vertically as regards the general surface of the skin, while the more superficial cells are flattened and scaly (Fig. 109). The deeper FIG. 109. part of the epidermis, which is softer and more opaque than the super- ficial, is called the rete mucosum. Many of the epidermal cells contain pigment, and the varying quantity of this is the source of the different shades of tint in the skin, both of individuals and races. The coloring mat- ter is contained chiefly in the deeper cells composing the rete mucosum, and be- comes less evident in them as they are gradually pushed up by those under L j J -t 1-1 Skin of the negro, in a vertical section, mag- them, and become, like nified 250diameters . ,, cutaneous papiii* ; their predecessors, flat- b> um i e rmost and dark-colored layer of oblong tened and Scale-like (Fig. vertical epidermis cells; c, mucous or Malpig- 109). It is by this pro- Wan layer ; d, horny layer (from Sharpey). cess of production from beneath, to make up for the waste at the surface, that the growth of the cuticle is effected. 334 THE SKIN. The thickness of the epidermis on different portions of the skin is directly proportioned to the friction, pressure, and other sources of injury to which it is exposed; and the more it is subjected to such injury, within certain limits, the more does it grow, and the thicker and more horny does it become ; for it serves as well to protect the sensitive and vascular cutis from injury from without, as to limit the evaporation of fluid from the bloodvessels. The adaptation of the epidermis to the latter purposes may be well shown by exposing to the air two dead hands or feet, of which one has its epidermis perfect, and the other is deprived of it ; in a day, the skin of the lat- ter will become brown, dry, and horn-like, while that of the former will almost retain its natural moisture. Cutis vera. The corium or cutis, which rests upon a layer of adipose and cellular tissue of varying thickness, is a dense and tough, but yielding and highly elastic structure, composed of fasciculi of fibre-cellular tissue, interwoven in all directions, and forming, by their interlacements, numerous spaces or areolae. These areolae are large in the deeper layers of the cutis, and are there usually filled with little masses of fat (Fig. 112) : but, in the more superficial parts, they are exceedingly small or entirely obliterated. By means of its toughness, flexibility, and elasticity, the skin is eminently qualified to serve as the general integument of the body, for defending the internal parts from external violence, and readily yielding and adapting itself to their various move- ments and changes of position. But, from the abundant sup- ply of sensitive nerve-fibres which it receives, it is enabled to fulfil a not less important purpose in serving as the principal organ of the sense of touch. The entire surface of the skin is extremely sensitive, but its tactile properties are due chiefly to the abundant papillae with which it is studded. These papillae are conical elevations of the corium, with a single or divided free extremity, more prominent and more densely set at some parts than at others (Figs. 110 and 111). The parts on which they are most abundant and most prominent are the palmar surface of the hands and fingers, and the soles of the feet parts, therefore, in which the sense of touch is most acute. On these parts they are disposed in double rows, in parallel curved lines, separated from each other by depressions (Fig. 112). Thus they may be seen easily on the palm, whereon each raised line is composeed of a double row of papillae, and is intersected by short transverse lines or furrows corresponding with the interspaces between the successive pairs of papillae. Over other parts of the skin they are more or less thinly scat- tered, and are scarcely elevated above the surface. Their THE CORIUM OK CUTIS VERA. 335 average length is about T -J th of an inch, and at their base they measure about ^th of an inch in diameter. Each pa- FlG. Ill FIG. 110. Papilla, as seen with a microscope, on a portion of the true skin, from which the cuticle has been removed (after Breschet). FIG. 111. Compound papillse from the palm of the hand, magnified 60 diameters ; a. basis of a papilla ; ft, b, divisions or branches of the same ; c, c, branches belonging to papillse, of which the bases are hidden from view (after Kolliker). FIG. 112. Vertical section of the skin and subcutaneous tissue, from end of the thumb, across the ridges and furrows, magnified 20 diameters (from Kolliker) : a, horny, and b, mucous layer of the epidermis ; c, corium; d, panniculus adiposus; e, papilla; on the ridges ; /, fat clusters ; g, sweat-glands ; h, sweat-ducts ; i, their openings on the surface. 336 THE SKIN. pilla is abundantly supplied with blood, receiving from the vascular plexus in the cutis one or more minute arterial twigs, which divide into capillary loops in its substance, and then reunite into a minute vein, which passes out at its base. The abundant supply of blood which the papillae thus receive ex- plains the turgescence or kind of erection which they undergo when the circulation through the skin is active. The majority, but not all, of the papillae contain also one or more terminal nerve-fibres, from the ultimate ramifications of the cutaneous plexus on which their exquisite sensibility depends. The exact mode in which these nerve-fibres terminate is not yet satisfac- torily determined. In some parts, especially those in which the sense of touch is highly developed, as, for example, the palm of the hand and the lips, the fibres appear to terminate, in many of the papillae, by one or more free ends in the sub- stance of a dilated oval-shaped body, not unlike a Paciniau corpuscle (Figs. 136, 137), occupying the principal part of the interior of the papillae, and termed a touch-corpuscle (Fig; 113). FIG. 113. Papillse from the skin of the hand, freed from the cuticle and exhibiting the tac- tile corpuscles. Magnified 350 diameters. A. Simple papilla with four nerve-fibres: a, tactile corpuscle ; b, nerves. B. Papilla treated with acetic acid : a, cortical layer with cells and fine elastic filaments ; b, tactile corpuscle with transverse nuclei ; c, entering nerve with neurilemma or perineurium ; d, nerve-fibres winding round the corpuscle, c. Papilla viewed from above so as to appear as a cross-section : o, corti- cal layer; b, nerve-fibre; c, sheath of the tactile corpuscle containing nuclei; d, core (after Kolliker). The nature of this body is obscure. Kolliker, Huxley, and others, regard it as little else than a mass of fibrous or con- nective tissue, surrounded by elastic fibres, and formed, accord- ing to Huxley, by an increased development of the neurilemma TOUCH-CORPUSCLES END-BULBS. * 337 of the nerve-fibres entering the papillae. Wagner, however, to whom seems to belong the merit of first fully describing these bodies, believes that, instead of thus consisting of a homogene- ous mass of connective-tissue, they are special and peculiar bodies of laminated structure, directly concerned in the sense of touch. They do not occur in all the papillae of the parts where they are found, and, as a rule, in the papillae in which they are present there are no bloodvessels. Since these pecu- liar bodies in which the nerve-fibres end are only met with in the papillae of highly sensitive parts, it may be inferred that they are specially concerned in the sense of touch, yet their absence from the papillae of other tactile parts shows that they are not essential to this sense. Closely allied in structure to the Pacinian corpuscles and touch-corpuscles are some little bodies about g ^ of an inch in diameter, first particularly described by Krause, and named by him " end-bulbs." They are generally oval or spheroidal, and composed externally of a coat of connective tissue inclos- ing a softer matter, in which the extremity of a nerve termin- ates. These bodies have been found chiefly in the lips, tongue, palate, and the skin of the glans penis (Fig. 114). FIG. 114. End-bulbs in papillae (magnified) treated with acetic acid. A, from the lips ; the white loops in one of them are capillaries. B, from the tongue. Two end-bulbs seen in the midst of the simple papillae: a, a, nerves (from Kolliker). Although destined especially for the sense of touch, the papillae are not so placed as to come into direct contact with external objects ; but, like the rest of the surface of the skin, 338 THE SKIN. are covered by one or more layers of epithelium, forming the cuticle or epidermis. The papillae adhere very intimately to the cuticle, which is thickest in the spaces between them, but tolerably level on its outer surface : hence, when stripped off from the cutis, as after maceration, its internal surface presents a series of pits and elevations corresponding to the papillae and their interspaces, of which it thus forms a kind of mould. Besides affording by its impermeability a check to undue evaporation from the skin, and providing the sensitive cutis with a protecting investment, the cuticle is of service in rela- tion to the sense of touch. For, by being thickest in the spaces between the papilla, and only thinly spread over the summits of these processes, it may serve to subdivide the sen- tient surface of the skin into a number of isolated points, each of which is capable of receiving a distinct impression from an external bodies. By covering the papillae it renders the sensa- tion produced by external bodies more obtuse, and in this manner also is subservient to touch : for unless the very sensi- tive papillae were thus defended, the contact of substances would give rise to pain, instead of the ordinary impressions of touch. This is shown in the extreme sensitiveness and loss of tactile power in a part of the skin when deprived of its epi- dermis. If the cuticle is very thick, however, as on the heel, touch becomes imperfect, or is lost, through the inability of the tactile papillae to receive impressions through the dense and horny layer covering them. Sudoriparous Glands. In the middle of each of the trans- verse furrows between the papillae, and irregularly scattered between the bases of the papillae in those parts of the surface of the body in which there are no furrows between them, are the orifices of ducts of the sudoriparous or sweat glands, by which it is probable that a large portion of the aqueous and gaseous materials excreted by the skin are separated. Each of these glands consists of a small lobular mass, which appears formed of a coil of tubular gland-duct, surrounded by blood- vessels and imbedded in the subcutaneous adipose tissue (Fig. 112). From this mass, the duct ascends, for a short distance, in a spiral manner through the deeper part of the cutis, then pass- ing straight, and then sometimes again becoming spiral, it passes through the cuticle and opens by an oblique valve- like aperture. In the parts where the epidermis is thin, the ducts themselves are thinner and more nearly straight in their course (Fig. 115). The duct, which maintains nearly the same diameter throughout, is lined with a layer of epithelium con- tinuous with the epidermis ; while the part which passes through the epidermis is composed of the latter structure only ; the SEBACEOUS GLANDS. 339 cells which immediately form the boundary of the canal in this part being somewhat differently arranged from those of the adjacent cuticle. The sudoriparous glands are abundantly distributed over the whole surface of the body ; but are especially numerous, as well as very large, in the skin of the palm of the hand, where, according to Krause, they amount to 2736 in each su- perficial square inch, and according to Mr. Erasmus Wilson, to as many as 3528. They are almost equally abundant and large in the skin of the sole. The glands by which the pecu- liar odorous matter of the axillae is secreted form a nearly complete layer under the cutis, and are like the ordinary su- doriparous glands, except in being larger and having very short ducts. In the neck and back, where they are least numerous, the glands amount to 417 on the square inch (Krause). Their total number Krause estimates at 2,381,248 ; and, supposing the orifice of each gland to present a surface of s'gth of a line in diameter (and regarding a line as equal to T ! fl th of an inch), he reckons that the whole of the glands would present an evaporating surface of about eight square inches. 1 Sebaceous Glands. Besides the perspiration, the skin se- cretes a peculiar fatty matter, and for this purpose is provided with another set of special organs, termed sebaceous glands (Fig. 115), which, like the sudoriparous glands, are abun- dantly distributed over most parts of the body. They are most numerous in parts largely supplied with hair, as the scalp and face, and are thickly distributed about the entrances of the various passages into the body, as the anus, nose, lips, and external ear. They are entirely absent from the palmar surface of the hands and the plantar surfaces of the feet. They are minutely lobulated glands, composed of an aggregate of small vesicles or sacculi filled with opaque white substances, like soft ointment. Minute capillary vessels overspread them ; and their ducts, which have a bearded appearance, as if formed of rows of shells, open either on the surface of the skin, close to a hair, or, which is more usual, directly into the follicle of the hair. In the latter case, there are generally two glands to each hair (Fig. 115). 1 The peculiar bitter yellow substance secreted by the skin of the external auditory passage is named cerumen, and the glands them- selves ceruminous glands ; but they do not much differ in structure from the ordinary sudoriparous glands. 340 THE SKIN. Structure of Hair and Nails. Hair. A hair is produced by a peculiar growth and modi- fication of the epidermis. Externally it is covered by a layer FIG- 115a. FIG. 115. Sebaceous glands of the skin, after Gurlt : or, a, sebaceous glands opening into the follicle of the hair by efferent ducts ; b, a hair on its follicle. FIG. 11 5a. Sweat-gland and the commencement of its duct. a. Venous radicles on the wall of the cell in which the gland rests. This vein anastomoses with others in the vicinity, b. Capillaries of the gland separately represented, arising from their arteries, which also anastomose. The bloodvessels are all situated on the out- side or deep surface of the tube, in contact with the basement-membrane. Magn. 35 diam. of fine scales closely imbricated, or overlapping like the tiles of a house, but with the free edges turned upwards (Fig. 116, A). It is called the cuticle of the hair. Beneath this is a much thicker layer of elongated horny cells, closely packed together so as to resemble a fibrous structure. This, very commonly, in the human subject, occupies the whole of the inside of the hair ; but in some cases there is left a small cen- tral space filled by a substance called the medulla or pith, composed of small collections of irregularly shaped cells, con- taining fat- and pigment-granules. The follicle, in which the root of each hair is contained (Fig. 117), forms a tubular depression from the surface of the STRUCTURE OF HAIR. 341 skin, descending into the subcutaneous fat, generally to a greater depth than the sudoriparous glands, and at its deepest part enlarging in a bulbous form, and often curving from its FIG. 116. A, surface of a white hair, magnified 160 diameters. The wave lines mark the upper or free edges of the cortical scales. , separated scales, magnified 350 diame- ters (after Kolliker). previous rectilinear course. It is lined throughout by cells of epithelium, continuous with those of the epidermis, and its walls are formed of pellucid membrane, which commonly, in the follicles of the largest hairs, has the structure of vascular fibro-cellular tissue. At the bottom of the follicle is a small papilla, or projection of true skin, and it is by the production and outgrowth of epidermal cells from the surface of this pa- pilla that the hair is formed. The inner wall of the follicle is lined by epidermal cells continuous with those covering the general surface of the skin ; as if indeed the follicle had been formed by a simple thrusting in of the surface of the integu- ment (Figs. 117, 118). This epidermal lining of the hair- follicle, or root-sheath of the hair, is composed of two layers, the inner one of which is so moulded on the imbricated scaly cuticle of the hair, that its inner surface becomes imbricated also, but of course in the opposite direction. When a hair is pulled out, the inner layer of the root-sheath and part of the outer layer also are commonly pulled out with it. Nails. A nail, like a hair, is a peculiar arrangement of epidermal cells, the undermost of which, like those of the general surface of the integument, are rounded or elongated, while the superficial are flattened, and of more horny consist- ence. That specially modified portion of the corium, or true skin, by which the nail is secreted, is called the matrix. The back edge of the nail, or the root as it is termed, is received into a shallow crescentic groove in the matrix, while the front part is free, and projects beyond the extremity of the digit. The intermediate portion of the nail rests by its broad 29 342 THE under surface on the front part of the matrix, which is here called the bed of the nail. This part of the matrix is not uni- FIG. 117. FIG. 118. a be FIG. 117. Medium-sized hair in its follicle, magnified 50 diameters (from Kolli- ker). a, stem cut short ; b, root ; c, knob ; d, hair cuticle ; e, internal, and/, external root-sheath ; g, h, dermic coat of follicle ; i, papilla ; k, k, ducts of sebaceous glands ; I, corium ; m, mucous layer of epidermis ; o, upper limit of internal root-sheath (from Kolliker). FIG. 118 Magnified view of the root of a hair (after Kohlrausch). a, stem or shaft of hair cut across ; 6, inner, and c, outer layer of the epidermal lining of the hair-follicle, called also the inner and outer root-sheath ; d, dermal or external coat of the hair-follicle, shown in part; e, imbricated scales about to form a cortical layer on the surface of the hair. The adjacent cuticle of the root-sheath is not repre- sented, and the papilla is hidden in the lower part of the knob where that is rep- resented lighter. formly smooth on the surface, but is raised in the form of lon- gitudinal and nearly parallel ridges or laminae, on which are STRUCTURE OF NAILS. 343 moulded the epidermal cells of which the nail is made up (Fig. 119). The growth of the nail, like that of a hair, or of the epi- dermis generally, is effected by a constant production of cells from beneath and behind, to take the place of those which are worn or cut away. Inasmuch, however, as the posterior edge of the nail, from its being lodged in a groove of the skin, can- no. 119. Vertical transverse section through a small portion of the nail and matrix largely magnified (after Kolliker). A, corium of the nail-bed, raised into ridges or laminie a, fitting in between cor- responding laminae ft, of the nail. B, Malpighian, and C, horny layer of nail; d, deepest and vertical cells ; e, upper flattened cells of Malpighian layer. not grow backwards, on additions being made to it, so easily as it can pass in the opposite direction, any growth at its hinder part pushes the whole forwards. At the same time fresh cells are added to its under surface, and thus each por- tion of the nail becomes gradually thicker as it moves to the front, until, projecting beyond the surface of the matrix, it can receive no fresh addition from beneath, and is simply moved forwards by the growth at its root, to be at last worn away or cut off. 344 THE SKIN. Excretion by the Skin. The skin, as already stated, is the seat of a twofold excre- tioD ; of that formed by the sebaceous glands and hair-follicles, and of the more watery fluid, the sweat or perspiration, elimi- nated by the sudoriparous glands. The secretion of the sebaceous glands and hair-follicles (for their products cannot be separated) consists of cast-off epithe- lium-cells, with nuclei and granules, together with an oily matter, extractive matter, and stearin ; in certain parts, also, it is mixed with a peculiar odorous principle, which is said by Dr. Fischer to contain caproic, butyric, and rutic acids. It is, perhaps, nearly similar in composition to the unctuous coat- ing, or vernix caseosa, which is formed on the body of the foetus while in the uterus, and which contains large quantities both of olein and margariu ( J. Davy). Its purpose seems to be that of keeping the skin moist and supple, and, by its oily nature, of both hindering the evaporation from the surface, and guarding the skin from the effects of the long-continued ac- tion of moisture. But while it thus serves local purposes, its removal from the body entitles it to be reckoned among the excretions of the skin ; though the share it has in the purify- ing of the blood cannot be discerned. The fluid secreted by the sudoriparous glands is usually formed so gradually, that the watery portion of it escapes by evaporation as fast as it reaches the surface. But, during strong exercise, exposure to great external warmth, in some diseases, and when evaporation is prevented by the application of oiled silk or plaster, the secretion becomes more sensible and collects on the skin in the form of drops of fluid. A good analysis of the secretion of these glands, unmixed with other fluids secreted from the skin, can scarcely be made ; for the quantity that can be collected pure is very small. Krause in a few drops from the palm of the hand, found an acid reac- tion, oily matter, and margarin, with water. The perspiration of the skin, as the term is sometimes em- ployed in physiology, includes all that portion of the secre- tions and exudations from the skin which passes off by evap- oration ; the sweat includes that which may be collected only in drops of fluid on the surface of the skin. The two terms are, however, most often used synonymously ; and for distinc- tion, the former is called insensible perspiration : the latter, sensible perspiration. The fluids are the same, except that the sweat is commonly mingled w T ith various substances lying on the surface of the skin. The contents of the sweat are, in part, matters capable of assuming the form of vapor, such as car- THE SWEAT. 345 bonic acid and water, and in part, other matters which are deposited on the skin, and mixed with the sebaceous secretion. Thenard collected the perspiration in a flannel shirt which had been washed in distilled water, and found in it chloride of sodium, acetic acid, some phosphate of soda, traces of phos- phate of lime, and oxide of iron, together with an animal sub- stance. In sweat which had run from the forehead in drops, Berzelius found lactic acid, chloride of sodium, and chloride of ammonium. Anselmino placed his arm in a glass cylinder, and closed the opening around it with oiled silk, taking care that the arm touched the glass at no point. The cutaneous exhalations collected on the interior of the glass, and ran down as a fluid : on analyzing this, he found water, acetate of ammonia, and carbonic acid ; and in the ashes of the dried residue of sweat he found carbonate, sulphate, and phosphate of soda, and some potash, with chloride of sodium, phosphate and carbonate of lime, and traces of oxide of iron. Urea has also been shown to be an ordinary constituent of the fluid of perspiration. The ordinary constituents of perspiration, may, therefore, according to Gorup-Besanez, be thus summed up : water, fat, acetic, butyric and formic acid, urea, and salts. The princi- pal salts are the chlorides of sodium and potassium, together with, in small quantity, alkaline and earthy phosphates and sulphates ; and, lastly, some oxide of iron. Of these several substances, none, however, need particular consideration, ex- cept the carbonic acid and water. The quantity of watery vapor excreted from the skin was estimated very carefully by Lavoisier and Sequin. The latter chemist inclosed his body in an air-tight bag, with a mouth- piece. The bag being closed by a strong band above, and the mouth-piece adjusted and gummed to the skin around the mouth, he was weighed, and then remained quiet for several hours, after which time he was again weighed. The differ- ence in the two weights indicated the amount of loss by pul- monary exhalation. Having taken off the air-tight dress, he was immediately weighed again, and a fourth time after a cer- tain interval. The difference between the two weights last ascertained gave the amount of the cutaneous and pulmonary exhalation together ; by subtracting from this the loss by pul- monary exhalation alone, while he was in the air-tight dress, he ascertained the amount of cutaneous transpiration. The repetition of these experiments during a long period, showed that, during a state of rest, the average loss by cutaneous and pulmonary exhalation in a minute, is from seventeen to eigh- teen grains, the minimum eleven grains, the maximum 346 THE SKIN. thirty-two grains ; and that of the eighteen grains, eleven pass off by the ski a, and seven by the lungs. The maximum loss by exhalation, cutaneous and pulmonary, in twenty-four hours, is about 3f lb.; the minimum about H lb. Valentin found the whole quantity lost by exhalation from the cutaneous and respiratory surfaces of a healthy man who consumed daily 40,000 grains of food and drink, to be 19,000 grains or 2f lb. Subtracting from this, for the pulmonary exhalation, 5000 grains, and, for the excess of the weight of the exhaled car- bonic acid over that of the equal volume of the inspired oxy- gen, 2256 grains, the remainder, 11,744 grains, or nearly if lb., may represent an average amount of cutaneous exhalation in the day. The large quantity of watery vapor thus exhaled from the skin, will prove that the amount excreted by simple transuda- tion through the cuticle must be very large, if we may take Krause's estimate of about eight square inches for the total evaporating surface of the sudoriparous glands ; for not more than about 3365 grains could be evaporated from such a sur- face in twenty-four hours, under the ordinary circumstances in which the surface of the skin is placed. This estimate is not an improbable one, for it agrees very closely with that of Milne-Edwards, who calculated that when the temperature of the atmosphere is not above 68 F., the glandular secretion of the skin contributes only Jth to the total sum of cutaneous exhalation. The quantity of watery vapor lost by transpiration, is of course influenced by all external circumstances which affect the exhalation irom other evaporating surfaces, such as the temperature, the hygrometric state, and the stillness of the atmosphere. But, of the variations to which it is subject un- der the influence of these conditions, no calculation has been exactly made. Neither, until recently, has there been any estimate of the quantity of carbonic acid exhaled by the skin on an average, or in various circumstances. Regnault and Reiset attempted to supply this defect, and concluded, from some careful exper- iments, that the quantity of carbonic acid exhaled from the skin of a warm-blooded animal is about -^th of that furnished by the pulmonary respiration. Dr. Edward Smith's calcula- tion is somewhat less than this. The cutaneous exhalation is most abundant in the lower classes of animals, more particu- larly the naked Amphibia, as frogs and toads, whose skin is thin and moist, and readily permits an interchange of gases between the blood circulating in it and the surrounding atmos- phere. Bischoff found that, after the lungs of frogs had been ABSORPTION BY THE SKIN. 347 tied and cut out, about a quarter of a cubic inch of carbonic acid gas was exhaled by the skin in eight hours. And this quantity is very large, when it is remembered that a full-sized frog will generate only about half a cubic inch of carbonic acid by his lungs and skin together in six hours (Milne- Edwards and Miiller). That the respiratory function of the skin is, perhaps, even more considerable in the higher animals than appears to be the case from the experiments of Regnault and Reiset just alluded to, seemed probable by the fact ob- served by Magendie and others, that if the skin of animals is covered with an impermeable varnish, or the body inclosed, all but the head, in a caoutchouc dress, animals soon die, as if asphyxiated ; their heart and lungs being gorged with blood, and their temperatures, during life, gradually falling many degrees, and sometimes as much as 36 F. below the ordinary standard (Magendie). Some recent experiments of Lashke- witzch appear, however, to confirm the opinion of Valentin, that loss of temperature is the immediate cause of death in these cases. A varnished animal is said to have suffered no harm when surrounded by cotton wadding, but it died when the wadding was removed. Absorption by the skin has been already mentioned, as an instance in which that process is most actively accomplished. Metallic preparations rubbed into the skin have the same action as when given internally, only in a less degree. Mer- cury applied in this manner exerts its specific influence upon syphilis, and excites salivation ; potassio-tartrate of antimony may excite vomiting, or an eruption extending over the whole body ; and arsenic may produce poisonous effects. Vegetable matters, also, if soluble, or already in solution, give rise to their peculiar effects, as cathartics, narcotics, and the like, when rubbed into the skin. The effect of rubbing is probably to convey the particles of the matter into the orifices of the glands whence they are more readily absorbed than they would be through the epidermis. When simply left in con- tact with the skin, substances, unless in a fluid state, are sel- dom absorbed. It has long been a contested question whether the skin covered with the epidermis has the power of absorbing water ; and it is a point the more difficult to determine because the skin loses water by evaporation. But, from the result of many experiments, it may now be regarded as a well-ascer- tained fact that such absorption really occurs. M. Edwards has proved that the absorption of water by the surface of the body may take place in the lower animals very rapidly. Not only frogs, which have a thin skin, but lizards, in which the 348 THE SKIN. cuticle is thicker than in man, after having lust weight by being kept for some time in a dry atmosphere, were found to recover both their weight and plumpness very rapidily when immersed in water. When merely the tail, posterior extremi- ties, and posterior part of the body of the lizard were im- mersed, the water absorbed was distributed throughout the system. And a like absorption through the skin, though to a less extent, may take place also in man. Dr. Madden, having ascertained the loss of weight, by cutaneous and pulmonary transpiration, that occurred during half an hour in the air, entered the bath, and remained im- mersed during the same period of time breathing through a tube which communicated with the air exterior to the room. He was then carefully dried and again weighed. Twelve experiments were performed in this manner ; and in ten there was a gain of weight, varying from 2 scruples to 5 drachms and 4 scruples, or a mean gain of 1 drachm 2 scruples and 13 grains. The loss in the air during the same length of time (half an hour) varied in ten experiments from 2J drachms to 1 ounce 2J scruples, or in the mean was about 6J drachms. So that, admitting the supposition that the cutaneous trans- piration was entirely suspended, and estimating the loss by pulmonary exhalation at 3 drachms, there was, in these ten experiments of Dr. Madden, an average absorption of 4 drachms 1 scruple, and 3 grains, by the surface of the body, during half an hour. In four experiments performed by M. Berthold, the gain in weight was greater than in those of Dr. Madden. In severe cases of dysphagia, when not even fluids can be taken into the stomach, immersion in a bath of warm water or of milk and water may assuage the thirst ; and it has been found in such cases that the weight of the body is increased by the immersion. Sailors also, when destitute of fresh water, find their urgent thirst allayed by soaking their clothes in salt water and wearing them in that state ; but these effects may be in part due to the hindrance to the evaporation of water from the skin. The absorption, also, of different kinds of gas by the skin is proved by the experiments of Abernethy, Cruikshank, Beddoes, and others. In these cases, of course, the absorbed gases com- bine with the fluids, and lose the gaseous form. Several phys- iologists have observed an absorption of nitrogen by the skin. Beddoes says, that he saw the arm of a negro become pale for a short time when immersed in chlorine ; and Abernethy ob- served that when he held his hands in oxygen, nitrogen, car- STRUCTURE OF THE KIDNEY. 349 bonic acid, and other gases contained in jars, over mercury, the volume of the gases became considerably diminished. The share which the evaporation from the skin has in the maintenance of the uniform temperature of the body, and the necessary adaptation thereto of the production of heat, have been already mentioned (p. 195). CHAPTER XV. THE KIDNEYS AND THEIR SECRETION. Structure of the Kidney. THE kidney is covered on the outside by a rather tough fibrous capsule, which is slightly attached by its inner surface to the proper substance of the organ by means of very fine fibres of areolar tissue and minute bloodvessels. From the healthy kidney, therefore, it may be easily torn off without FIG. 120. Plan of a longitudinal section through the pelvis and substance of the right kid- ney, i/; a, the cortical substance; b, b, broad part of the pyramids of Malpighi; c, c, the divisions of the pelvis named calyces, laid open ; c', one of these unopened ; d, summit of the pyramids or papillae projecting into calyces ; e, e, section of the narrow part of two pyramids near the calyces ; p, pelvis or enlarged divisions of the ureter within the kidney ; u, the ureter ; s, the sinus ; h, the hilus. 30 350 THE KIDNEYS AND THEIR SECRETION. injury to the subjacent cortical portion of the organ. At the hilus or notch of the kidney, it becomes continuous with the external coat of the upper and dilated part of the ureter. On making a section lengthwise through the kidney (Fig. 120) the main part of its substance is seen to be composed of two chief portions, called respectively the cortical and the medullary portion, the latter being also sometimes called the pyramidal portion, from the fact of its being composed of about a dozen conical bundles of urine-tubes, each bundle being called a pyramid. The upper part of the duct of the organ, or the ureter, is dilated into what is called the pelvis of the kidney ; and this, again, after separating into two or three principal divisions, is finally subdivided into still smaller portions, vary- ing in number from about 8 to 12, or even more, and called calyces. Each of these little calyces or cups, again receives the pointed extremity or papilla of a pyramid. Sometimes, how- ever, more than one papilla is received by a calyx. The kidney is a gland of the class called tubular, and both its cortical and medullary portions are composed essentially of secreting tubes, the tubuli uriniferi, which by one extremity, in the cortical portion, end commonly in little saccules con- taining bloodvessels, called Malpighian bodies, and by the other open through the papillae into the pelvis of the kidney, and thus discharge the urine which flows through them. In the pyramids they are chiefly straight dividing and diverging as they ascend through these into the cortical por- tion ; while in the latter region they spread out more irregu- larly, and become much branched and convoluted. The tubuli uriniferi (Fig. 121) are composed of a nearly homogeneous membrane, lined internally by spheroidal epithe- lium, and for the greater part of their extent are about g j^ of an inch in diameter, becoming somewhat larger than this immediately before they open through the papillae. On trac- ing these tubules upwards from the papillse, they are found to divide dichotomously as they ascend through the pyramids, and on reaching the bases of the latter, they begin to branch and diverge more widely, and to form by their branches and convolutions the essential part of the cortical portion of the organ. At their extremities they become dilated into the Malpighian capsules. Until recently, it was believed that the straight tubules in the pyramids branch out and become con- voluted immediately on reaching the bases of the pyramids ; but between the straight tubes in the pyramids and the convo- luted tubes in the cortical portion, there has been shown to be a system of tubules of smaller diameter than either, which form intercommunications between the two varieties formerly STRUCTURE OF THE KIDNEY. 351 recognized. These intervening tubules, called the looped tubes of Henle, arising from the straight tubes in some part of their course, or being continued from their extremities at the bases of the pyramids, pass down loopwise in the pyramids for a FIG. 122. FIG. 121. FIG. 121. A. Portion of a secreting canal from the cortical substance of the kid- ney. B. The epithelium or gland-cells, more highly magnified (700 times). FIG. 122. Diagram of the looped uriniferous tubes and their connection with the capsules of the glomeruli (from Southey, after Ludwig). In the lower part of the figure one of the large branching tubes is shown opening on a papilla; in the mid- dle part two of the looped small tubes are seen descending to form their loops, and reascending in the medullary substance ; while in the upper or cortical part, these tubes, after some enlargement, are represented as becoming convoluted and dilated in the capsules of glomeruli. longer or shorter distance, and then, again turning up, end in the convoluted tubes whose extremities are dilated into the Malpighian capsules before referred to (Fig. 122). On a transverse section of a pyramid (Fig. 123), these looped tubes 352 THE KIDNEYS AND THEIR SECRETION. are seen to be of much smaller calibre than the straight ones, which are passing down to open through the papillse. The Malpighian bodies are found only in the cortical part of the kidney. On a section of the organ, some of them are just visible to the naked eye as minute red points ; others are too small to be thus seen. Their average diameter is about T ^ a of an inch. Each of them is composed of the dilated extremity of a urinary tube, or Malpighian capsule, inclosing a tuft of bloodvessels. In connection with these little bodies the general distribu- tion of bloodvessels to the kidney may be here considered. The renal artery divides into several branches, which, pass- ing in at the hilus of the kidney, and covered by a fine sheath of areolar tissue derived from the capsule, enter the substance of the organ chiefly in the intervals between the papillse, and penetrate the cortical substance, where this dips down between the bases of the pyramids. Here they form a tolerably dense FIG. 123. Transverse section of a renal papilla (from Kolhker) -y^- a, larger tubes or papil- lary ducts ; b, smaller tubes of Henle ; c, bloodvessels, distinguished by their natter epithelium , d, nuclei of the stroma. plexus of an arched form, and from this are given off smaller arteries which ultimately supply the Malpighian bodies. The small afferent artery (Fig. 124), which enters the Mal- pighian body by perforating the capsule, breaks up in the in- terior into a dense and convoluted and looped capillary plexus, which is ultimately gathered up again into a single small effer- STRUCTURE OF THE KIDNEY. 353 ent vessel, comparable to a minute vein, which leaves the Malpighian capsule just by the point at which the afferent artery enters it. On leaving, it does not immediately join other small veins as might have been expected, but again breaking up into a network of capillary vessels, is distributed on the exterior of the tubule, from whose dilated end it had just emerged. After this second breaking up it is finally col- lected into a small vein, which, by union with others like it, helps to form the radicles of the renal vein. The Malpighian capsule is lined by a layer of fine squamous epithelial cells ; but whether the small glomerulus or tuft of capillaries in the interior is covered by a similar layer is un- FIG. 124. FIG. 125. FIG. 124. Plan of the renal circulation in man and the Mammalia, a, terminal branch of the artery, giving the terminal twig 1, to the Malpighian tuft m, from which emerges the efferent or portal vessel, 2. Other efferent vessels, 2, are seen entering the plexus of capillaries, surrounding the uriniferous tube, /. From the plexus, the emulgent vein, v, springs. FIG. 125. Semidiagrammatic representation of a Malpighian body in its relation to the uriniferous tube (from Kolliker) -y-. a, capsule of the Malpighian body; d, epithelium of the uriniferous tube ; e, detached epithelium ; /, afferent vessel ; g, efferent vessel ; k, convoluted vessels of the glomerulus. certain. Kolliker believes that such a covering, although ex- ceedingly thin, is present, and has delineated the appearance in the accompanying diagram (Fig. 125). Besides the small afferent arteries of the Malpighian bodies, there are, of course, others which are distributed in the ordi- nary manner, for nutrition's sake, to the different parts of the organ ; and in the pyramids, between the tubes, there are nu- 354 THE KIDNEYS AND THEIR SECRETION. merous straight vessels, the vasa recta, supposed by some ob- servers to be branches of vasa efferentia from Malpighian bodies, and therefore comparable to the venous plexus around the tubules in the cortical portion, while others think that they arise directly from small branches of the renal arteries. Between the tubes, vessels, &c., which make up the main substance of the kidney, there exists in small quantity a fine matrix of areolar tissue. The nerves of the kidney are derived from the renal plexus. 1 Secretion of Urine. The separation from the blood of the solids in a state of so- lution in the urine is probably effected, like other secretions, by the agency of the gland-cells, and equally in all parts of the urine-tubes. The urea and uric acid, and perhaps some of the other constituents existing ready formed in the blood, may need only separation, that is, they may pass from the blood to the urine without further elaboration ; but this is not the case with some of the other principles of the urine, such as the acid phosphates and the sulphates, for these salts do not exist as such in the blood, and must be formed by the chemi- cal agency of the cells. The watery part of the urine is probably in part separated by the same structures that secrete the solids, but the ingeni- ous suggestion of Mr. Bowman that the water of the urine is mainly strained off, so to speak, by the Malpighiau bodies, from the blood which circulates in their capillary tufts, is ex- ceedingly probable ; although if, as Kolliker and others main- tain, there is an epithelial covering to these tufts or glomeruli, it is very likely that the solids of the urine may be in part se- creted here also. We may, therefore, conclude that all parts of the tubular system of the kidney take part in the secretion of the urine as a whole, but that there is a provision also in the arrangement of the vessels in the Malpighiau bodies for a more simple draining off of water from the blood when re- quired. The large size of the renal arteries and veins permits so rapid a transit of the blood through the kidneys, that the whole of the blood is purified by them. The secretion of urine is rapid in comparison with other secretions, and as each por- 1 For a more detailed account of the structure of the kidney and a summary of the various opinions on the subject, the student may be referred especially to Quain's Anatomy, 7th ed., and to a paper by Dr. Reginald Southey, in vol. i of the St. Bartholomew's Hospital Reports. PASSAGE OF URINE INTO THE BLADDER. 355 tion is secreted, it propels that which is already in the tubes onwards into the pelvis of the kidney. Thence through the ureter the urine passes into the bladder, into which its rate and mode of entrance has been watched in cases of ectopia vesicse, i. e., of such fissures in the anterior and lower part of the walls of the abdomen, and of the front wall of the bladder, as exposed to view its hinder wall together with the orifices of the ureters. Some good observations on such cases were made by Mr. Erichsen. The urine does not enter the bladder at any regular rate, nor is there a synchronism in its movement through the two ureters. During fasting, two or three drops enter the bladder every minute, each drop as it enters first raising up the little papilla on which, in these cases, the ureter opens, and then passing slowly through its orifice, which at once again closes like a sphincter. In the recumbent posture, the urine collects for a little time in the ureters, then flows gently, and, if the body be raised, runs from them in a stream till they are empty. Its flow is increased in deep inspiration, or straining, and in active exercise, and in fifteen or twenty minutes after a meal. The same observations, also, showed how fast some substances pass from the stomach through the circulation, and through the vessels of the kidneys. Ferrocyanide of potassium so passed on one occasion in a minute : vegetable substances, such as rhubarb, occupied from sixteen to thirty-five minutes ; neutral alkaline salts with vegetable acids, which were generally de- composed in transitu, made the urine alkaline in from twenty- eight to forty-seven minutes. But the times of passage varied much ; and the transit was always slow when the substances were taken during digestion. The urine collecting in the urinary bladder is prevented from regurgitation into the ureters by the mode in which these pass through the walls of the bladder, namely, by their lying for between half and three-quarters of an inch between the muscular and mucous coats, and then turning rather abruptly forwards, and opening through the latter, it collects till the distension of the bladder is felt either by direct sensation, or, in ordinary cases, by a transferred sensation at and near the orifice of the urethra. Then, the effort of the will being di- rected primarily to the muscles of the abdomen, and through them (by reason of its tendency to act with them) to the urinary bladder, the latter, though its muscular walls are really com- posed of involuntary muscle, contracts, and expels the urine. (See also p. 183.) 356 THE URINE. The Urine : its General Properties. Healthy urine is a clear limpid fluid, of a pale yellow or amber color, with a peculiar faint aromatic odor, which be- comes pungent and ammoniacal when decomposition takes place. The urine, though usually clear and transparent at first, often becomes as it cools opaque and turbid from the de- position of part of its constituents previously held in solution ; and this may be consistent with health, though it is only in disease that, in the temperature of 98 or 100, at which it is voided, the urine is turbid even when first expelled. Although ordinarily of pale amber color, yet, consistently with health, the urine may be nearly colorless, or of a brownish or deep orange tint, and, between these extremes, it may present every shade of color. When secreted, and most commonly when first voided, the urine has a distinctly acid reaction in man and all carnivorous animals, and it thus remains till it is neutralized or made alka- line by the ammonia developed in it by decomposition. In most herbivorous animals, on the contrary, the urine is alka- line and turbid. The difference depends, not on any peculi- arity in the mode of secretion, but on the differences in the food on which the two classes subsist: for when carnivorous animals, such as dogs, are restricted to a vegetable diet, their urine becomes pale, turbid, and alkaline, like that of an her- bivorous animal, but resumes its former acidity on the return to an animal diet; while the urine voided by herbivorous ani- mals, e. g., rabbits, fed for some time exclusively upon animal substances, presents the acid reaction and other qualities of the urine of Carnivora, its ordinary alkalinity being restored only on the substitution of a vegetable for the animal diet (Bernard). Human urine is not usually rendered alkaline by vegetable diet, but it becomes so after the free use of alkaline medicines, or of the alkaline salts with carbonic or vegetable acids ; for these latter are changed into alkaline carbonates previous to elimination by the kidneys. Except in these cases, it is very rarely alkaline, unless ammonia has been developed in it by decomposition commencing before it is evacuated from the bladder. The average specific gravity of the human urine is about 1020. Probably no other animal fluid presents so many va- rieties in density within twenty-four hours as the urine does ; for the relative quantity of water and of solid constituents of which it is composed is materially influenced by the condition and occupation of the body during the time at which it is se- creted, by the length of time which has elapsed since the last COMPOSITION OF URINE. 357 meal, and by several other accidental circumstances. The ex- istence of these causes of difference in the composition of the urine has led to the secretion being described under the three heads of urina sanguinis, urina potus, and urina cibi. The first of these names signifies the urine, or that part of it which is secreted from the blood at times in which neither food nor drink has been recently taken, and is applied especially to the urine which is evacuated in the morning before breakfast. The urina potus indicates the urine secreted shortly after the intro- duction of any considerable quantity of fluid into the body ; and the urina cibi the portions secreted during the period im- mediately succeeding a meal of solid food. The last kind con- tains a larger quantity of solid matter than either of the others ; the first or second, being largely diluted with water, possesses a comparatively low specific gravity. Of these three kinds, the morning urine is the best calculated for analysis, since it represents the simple secretion unmixed with the ele- ments of food or drink ; if it be not used, the whole of the urine passed during a period of twenty -four hours should be taken. In accordance with the various circumstances above- mentioned, the specific gravity of the urine may, consistently with health, range widely on both sides of the usual average. The average healthy range may be stated at from 1015 in the winter to 1025 in the summer, and variations of diet and ex- ercise may make as great a difference. In disease, the varia- tion may be greater ; sometimes descending, in albumin uria, to 1004, and frequently ascending in diabetes, when the urine is loaded with sugar, to 1050, or even to 1060. The whole quantity of urine secreted in twenty-four hours is subject to variation according to the amount of fluid drunk, and the proportion of the latter passing off from the skin, lungs, and alimentary canal. It is because the secretion of the skin is more active in summer than in winter, that the quantity of urine is smaller, and its specific gravity propor- tionately higher. On taking the mean of numerous observa- tions by several experimenters, Dr. Parkes found that the average quantity voided in twenty-four hours by healthy male adults from twenty to forty years of age, amounted to fluid ounces. Chemical Composition of the Urine. The urine consists of water, holding in solution certain ani- mal and saline matters as its ordinary constituents, and occa- sionally various matters taken into the stomach as food salts, coloring matter, and the like. The quantities of the several natural and constant ingredients of the urine are 358 THE URINE. stated somewhat differently by the different chemists who have analyzed it ; but many of the differences are not important, and the well-known accuracy of the several chemists renders it almost immaterial which of the analyses is adopted. The analyses by A. Becquerel being adopted by Dr. Prout, and by Dr. Golding Bird, will be here employed. (Table I.) Table II has been compiled from the observations of Dr. Parkes, and of numerous other authors quoted in his admira- ble work on the urine. TABLE I. Average quantity of each constituent of the Urine in 1000 parts. Water, ........... 967. Urea, ........... 14/230 Uric acid, ........... 468 Coloring matter, | inseparable from ) 10 lr? Mucus, and animal extractive matter, j each other, J Sulphates, {^ {Lime, Magnesia, Ammonia, J Salts, Hippurate of soda, Fluoride of potassium, 8.13-3 Silica, ........... traces. TABLE II. 1000.000 Average quantity of the chief constituents of the Urine excreted in 24 hours by healthy male adults. Water, ........ 52. fluid ounces. Urea, . . . . . . . . . 512.4 grains. Uric acid, Hippuric acid, uncertain, Sulphuric acid, . Phosphoric acid, . Chlorine, Chloride of Ammonium, Potash, Soda, . Lime, . Magnesia, Mucus, Extractives, Creatin, Creatinin, Pigment, Xanthin, [ Hypoxanthin, Resinous matter, &c. . 52. . 512.4 8.5 probablv 10 to 15. 31.11 45. 105.0 35.25 58. 125. 3.5 3. 7. 154.0 UREA. 359 From these proportions, however, most of the constituents are, even in health, liable to variations. Especially the water is so. Its variations in different seasons, and according to the quantity of drink and exercise, have already been mentioned. It is also liable to be influenced by the condition of the ner- vous system, being sometimes greatly increased in hysteria, and some other nervous affections ; and at other times diminished. In some diseases it is enormously increased ; and its increase may be either attended with an augmented quantity of solid matter, as in ordinary diabetes, or may be nearly the sole change, as in the affection termed diabetes iusipidus. In other diseases, e. g., the various forms of albumiuuria, the quantity may be considerably diminished. A febrile condition almost always diminishes the quantity of water ; and a like diminu- tion is caused by any affection which draws off a large quantity of fluid from the body through any other channel than that of the kidneys, e. g., the bowels and the skin. Urea. Urea is the principal solid constituent of the urine, forming nearly one-half of the whole quantity of solid matter. It is also the most important ingredient, since it is the chief substance by which the nitrogen of decomposed tissue and superfluous food is excreted from the body. For its re- FlG - 126 moval, the secretion of urine seems especially provided ; and by its retention in the blood the most pernicious effects are produced. Urea, like the other solid constituents of the urine, ex- ists in a state of solution. But it may be procured in the solid state, and then appears in the form of delicate silvery acicu- lar crystals, which under the microscope, appear as four- sided prisms (Fig. 126). It 18 Crystals of urea. obtained in this state by evapo- rating urine carefully to the consistence of honey, acting on the inspissated mass with four parts of alcohol, then evaporat- ing the alcoholic solution, and purifying the residue by repeated solution in water or alcohol, and finally allowing it to crystallize. It readily combines with an acid, like a weak base ; and may thus be conveniently procured in the form of a nitrate, by add- ing about half a drachm of pure nitric acid to double that quan- tity of urine in a watch-glass. The crystals of nitrate of urea are 360 THE URINE. formed more rapidly if the urine have been previously concen- trated by evaporation. Urea is colorless when pure ; when impure, yellow or brown ; without smell, and of a cooling, nitre-like taste ; has neither an acid nor an alkaline reaction, and deliquesces in a moist and warm atmosphere. At 59 F. it requires for its solution less than its weight of water ; it is dissolved in all proportions by boiling water ; but it requires five times its weight of cold alcohol for its solution. At 248 F. it melts without under- going decomposition ; at a still higher temperature ebullition takes place, and carbonate of ammonia sublimes ; the melting mass gradually acquires a pulpy consistence ; and, if the heat is carefully regulated, leaves a gray-white powder, cyanic acid. Urea is identical in composition with cyanate of ammonia, and was first artificially produced by Wohler from this sub- stance. Thus : Cyanate of Ammonia. Urea. CHNO. H 3 N = CH 4 N 2 0. The action of heat upon urea in evolving carbonate of am- monia, and leaving cyanic acid, is thus explained. A similar decomposition of the urea with development of carbonate of ammonia ensues spontaneously when urine is kept for some days after being voided, and explains the ammoniacal odor then evolved. It is probable that this spontaneous decom- position is accelerated by the mucus and other animal matters in the urine, which, by becoming putrid, act the part of a ferment and excite a change of composition in the surrounding compounds. It is chiefly thus that the urea is sometimes de- composed before it leaves the bladder, when the mucous mem- brane is diseased, and the mucus secreted by it is both more abundant and, probably, more prone than usual to become putrid. The same occurs also in some affections of the nervous system, particularly in paraplegia. The quantity of urea excreted is, like that of the urine itself, subject to considerable variation. It is materially influenced by diet, being greater when animal food is exclusively used, less when the diet is mixed, and least of all with a vegetable diet. As a rule, men excrete a larger quantity than women, and persons in the middle periods of life a larger quantity than infants or old people (Lecanu). The quantity of urea does not necessarily increase and decrease with that of the urine, though on the whole it would seem that whenever the amount of urine is much augmented, the quantity of urea also is usually in- creased (Becquerel) ; and it appears from observations of Genth, that the quantity of urea, as of urine, may be especially in- UREA. 361 creased by drinking large quantities of water. In various dis- eases, as albuminuria, the quantity is reduced considerably be- low the healthy standard, while in other affections it is above it. The urea appears to be derived from two different sources. That it is derived in part from the uuassimilated elements of nitrogenous food, circulating with the blood, is shown in the increase which ensues on substituting an animal or highly nitrogenous for a vegetable diet ; in the much larger amount, nearly double, excreted by Carnivora than Herbivora, inde- pendent of exercise ; and in its diminution to about one-half during starvation, or during the exclusion of non-nitrogenous principles of food. But that it is in larger part derived from the disintegration of the azotized animal tissues, is shown by the fact that it continues to be excreted, though in smaller quantity than usual, when all nitrogenous substances are strictly excluded from the food, as when the diet consists for several days of sugar, starch, gum, oil, and similar non-azotized vegetable substances (Lehmann). It is excreted, also, even though no food at all be taken for a considerable time ; thus it is found in the urine of reptiles which have fasted for months ; and in the urine of a madman, who had fasted eigh- teen days, Lassaigne found both urea and all the components of healthy urine. Probably all the nitrogenous tissues furnish a share of urea by their decomposition. It has been commonly taken for granted that the quantity of urea in the urine is greatly increased by active exercise ; but numerous observers have failed to detect more than a slight increase under such circumstances ; and our notions concern- ing the relation of this excretory product to the destruction of muscular fibre, consequent on the exercise of the latter, have lately undergone considerable modification. There is no doubt, of course, that like all parts of the body, the muscles have but a limited term of existence, and are being constantly renewed, at the same time that a part of the products of their disintegration appears in the urine in the form of urea. But the waste is not so fast as it has been frequently supposed to be ; and the theory that the amount of w r ork done by the muscle is expressed by the quantity of urea excreted in the urine, and that each act of contraction corresponds to an equivalent waste of muscle-structure, is founded on error. (See also chapter on Motion.) Urea exists ready-formed in the blood, and is simply ab- stracted therefrom by the kidneys. It may be detected in small quantity in the blood, and in some other parts of the body, e. g., the humors of the eye (Millon), even while the func- tions of the kidneys are unimpaired : but when from any cause, 362 THE URINE. especially extensive disease or extirpation of the kidneys, the separation of urine is imperfect, the urea is found largely in the blood and in most other fluids of the body. Uric Add. This, which is another nitrogenous animal sub- stance, with the formula C 5 N 4 FlG - 127 - H 4 O 3 , and was formerly termed lithic acid, on account of its existence in many forms of urinary calculi, is rarely ab- sent from the urine of man or animals, though in the feline tribe it seems to be sometimes entirely replaced by urea (G. Bird). Its proportionate quan- tity varies considerably in dif- ferent animals. In man, and Mammalia generally, especially the Herbivora, it is compara- tively small. In the whole Various forms of uric acid crystals. , .1 /? > -i i ( tribe oi birds and 01 serpents. on the other hand, the quantity is very large, greatly exceed- ing that of the urea. In the urine of granivorous birds, in- deed, urea is rarely if ever found, its place being entirely sup- plied by uric acid. The quantity of uric acid, like that of urea, in human urine, is increased by the use of animal food, and decreased by the use of food free from nitrogen, or by an exclusively vegetable diet. In most febrile diseases, and in plethora, it is formed in unnaturally large quantities ; and in gout it is deposited in, and in the tissues around, joints, in the form of urate of soda, of which the so-called chalkstones of this disease are principally composed. The condition in which uric acid exists in solution in the urine has formed the subject of some discussion, because of its difficult solubility in water. According to Liebig the uric acid exists as urate of soda, produced, he supposes, by the uric acid, as soon as it is formed, combining with part of the base of the alkaline phosphate of soda of the blood. Hippuric acid, which exists in human urine also, he believes, acts upon the alkaline phosphate in the same way, and increases still more the quantity of acid phosphate, on the presence of which it is probable that a part of the natural acidity of the urine depends. It is scarcely possible to say whether the union of uric acid with the base soda and probably ammonia, takes place in the blood, or in the act of secretion in the kidney ; the latter is the more probable opinion ; but the quantity of either uric acid or HIPPURIC ACID. 363 urates in the blood is probably too small to allow of this ques- tion being solved. The source of uric acid is probably in the disintegrated ele- ments of albuminous tissues. The relation which uric acid and urea bear to each other is, however, still obscure. The fact that they often exist together in the same urine, makes it seem probable that they have different origins or different offices to perform ; but the entire replacement of either by the other, as of urea by uric acid in the urine of birds, serpents, and many insects, and of uric acid by urea, in the urine of the feline tribe of Mammalia, shows that each alone may dis- charge all the important functions of the two. Owing to its existence in combination in healthy urine, uric acid for examination must generally be precipitated from its bases by a stronger acid. Frequently, however, when ex- creted in excess, it is deposited in a crystalline form (Fig. 127), mixed with large quantities of urate of ammonia or soda (Fig. 130). In such cases it may be procured for microscopic exam- ination, by gently warming the portion of urine containing the sediment ; this dissolves urate of ammonia and soda, while the comparatively insoluble crystals of uric acid subside to the bottom. The most common form in which uric acid is deposited in urine, is that of a brownish or yellowish powdery substance, consisting of granules of urate of ammonia or soda. When deposited in crystals, it is most frequently in rhombic or dia- mond-shaped laminae, but other forms are not uncommon (Fig. 127). When deposited from urine, the crystals are generally more or less deeply colored, by being combined with the color- ing principles of the urine. Hippuric Add has long been known to exist in the urine of herbivorous animals in combi- nation with soda. Liebig has shown that it also exists nat- urally in the urine of man, in quantity equal to the uric acid, and Weismann's obser- vations agree with this. It is a nitrogenous compound with the formula CgHgNOg. It is closely allied to ben zoic acid ; and this substance when in- troduced into the system, is excreted by the kidneys as hippuric acid (Ure). * Its source is not satisfactorily de- Crystals of hippuric acid. 364 THE URINE. FIG. 129. termined : in part it is probably derived from some constitu- ents of vegetable diet, though man has no hippuric acid in his food, nor, commonly, any benzoic acid that might be converted into it ; in part from the natural disintegration of tissues, inde- pendent of vegetable food, for Weismann constantly found an appreciable quantity, even when living on an exclusively ani- mal diet. The nature and composition of the coloring matter of urine are involved in some obscurity. It is probably closely related to the coloring matter of the blood. The mucus in the urine consists principally of the epithelial debris of the mucous surface of the urinary passages. Particles of epithelium, in greater or less abundance, may be detected in most samples of urine, especi- ally if it has remained at rest for some time, and the lower strata are then examined (Fig. 129). As urine cools, the mu- cus is sometimes seen suspended in it as a delicate opaque cloud, but generally it falls. In in- flammatory affections of the urinary passages, especially of the bladder, mucus in large quantities is poured forth, and speedily undergoes decomposi- tion. The presence of the de- composing mucus excites (as already stated) chemical changes in the urea, whereby ammonia, or carbonate of ammonia, is formed, which, combining with the excess of acid in the super- phosphates in the urine, produces insoluble neutral or alkaline phosphates of lime and magnesia, and phosphate of ammonia and magnesia. These, mixing with the mucus, constitute the peculiar white, viscid, mortar-like substance which collects upon the mucous surface of the bladder, and is often passed with the urine, forming a thick, tenacious sediment. Besides mucus and coloring matter, urine contains a consid- erable quantity of animal matter, usually described under the obscure name of animal extractive. The investigations of Lie- big, Heintz, and others, have shown that some of this ill-defined substance consists of Creatin and Creatinin, two crystallizable substances derived, probably, from the metamorphosis of mus- cular tissue. These substances appear to be intermediate be- tween the proper elements of the muscles, and, perhaps, of other azotized tissues and urea :. the first products of the dis- Mucus deposited from urine. ALKALINE AND EARTHY PHOSPHATES. 365 integrating tissues probably consisting not of urea, but of cre- atin and creatinin, which subsequently are partly resolved into urea, partly discharged, without change, in the urine. The names of some other substances of which there are commonly traces in the urine, will be found in Table II, p. 358. It has been shown by Scherer that much of the substance classed as extractive matter of the urine, is the peculiar coloring matter, probably derived from the haemoglobin of the blood. Saline Matter. The sulphuric acid in the urine is combined chiefly or entirely with soda and potash : forming salts which are taken in very small quantity with the food, and are scarcely found in other fluids or tissues of the body; for the sulphates commonly enumerated among the constituents of the ashes of the tissues and fluids are, for the most part or entirely, pro- duced by the changes that take place in the burning. Dr. Parkes, indeed, considers that only about one-third of the sul- phuric acid found in the urine is derived directly from the food. Hence the greater part of the sulphuric acid which the sulphates in the urine contain, must be formed in the blood, or in the act of secretion of urine; the sulphur of which the acid is formed, being probably derived from the decomposing nitro- genous tissues, the other elements of which are resolved into urea and uric acid. It may be in part derived also, as Dr. Parkes observes, from the sulphur-holding taurin and cystin which can be found in the liver, lungs, and other parts of the body, but not generally in the excretions ; and which, therefore, must be broken up. The oxygen is supplied through the lungs, and the heat generated during combination with the sulphur, is one of the subordinate means by which the animal tempera- ture is maintained. Besides the sulphur in these salts, some also appears to be in the urine, uncombined with oxygen ; for after all the sul- phates have been removed from urine, sulphuric acid may be formed by drying and burning it with nitre. Mr. Ronalds believes that from three to five grains of sulphur are thus daily excreted. The combination in which it exists is certain : possibly it is in some compound analogous to cystin or cystic oxide (p. 367). The phosphoric acid in the urine is combined partly with the alkalies, partly with the alkaline earths about four or five times as much with the former as with the latter. In blood, saliva, and other alkaline fluids of the body, phosphates exist in the form of alkaline or neutral acid salts. In the urine they are acid salts, viz., the phosphates of sodium, am- monium, calcium, and magnesium, the excess of acid being, according to Liebig, due to the appropriation of the alkali 31 366 THE URINE. with which the phosphoric acid in the blood is combined, by the several new acids which are formed or discharged at the kidneys, namely, the uric, hippuric, and sulphuric acids, all of which he supposes to be neutralized with soda. The presence of the acid phosphates accounts, in great measure, or, according to Liebig, entirely, for the acidity of the urine. The phosphates are taken largely in both vege- table and animal food ; some thus taken, are excreted at once ; others, after being transformed and incorporated with the tis- sues. Phosphate of calcium forms the principal earthy con- stituent of bone, and from the decomposition of the osseous tissue the urine derives a large quantity of this salt. The de- composition of other tissues also, but especially of the brain and nerve-substance, furnishes large supplies of phosphorus to the urine, which phosphorus is supposed, like the sulphur, to be united with oxygen, and then combined with bases. This quantity is, however, liable to FIG. 130. considerable variation. Any undue exercise of the mind, and all circumstances produc- ing nervous exhaustion, in- crease it. The earthy phos- phates are more abundant af- ter meals, whether on animal or vegetable food, and are diminished after long fasting. The alkaline phosphates are increased after animal food, diminished after vegetable food. Exercise increases the alkaline, but not the earthy phosphates (Bence Jones). Phosphorus uncombined with oxygen appears, like sulphur, to be excreted in the urine (Ronalds). When the urine undergoes alkaline fermentation, phosphates are deposited in the form of a urinary sediment consisting chiefly of phosphate of ammonia and magnesia (triple phosphate) (Fig. 130.) This compound does not, as such, exist in healthy urine. The ammonia is chiefly or wholly derived from the decomposition of urea (p. 360). The chlorine of the urine occurs chiefly in combination with sodium, but slightly also with ammonium, and, perhaps, potassium. As the chlorides exist largely in food, and in most of the animal fluids, their occurrence in the urine is easily understood. Urinary sediment of triple phosphates (large prismatic crystals) and urate of ammonia, from urine which had under- gone alkaline fermentation. THE NERVOUS SYSTEM. 367 Cystin (Fig. 132) is an occasional constituent of urine. It resembles taurin in containing a large quantity of sulphur- more than 25 per cent. It does not exist in healthy urine. Another common morbid qonstituent of the urine is oxalic acid, which is frequently deposited in combination with lime FIG. 181. FIG. 132. Crystals of oxalate of lime. Crystals of cystin. (Fig. 131) as a urinary sediment. Like cystin, but much more commonly, it is the chief constituent of certain calculi. A small quantity of gas is naturally present in the urine in a state of solution. It consists chiefly of carbonic acid and nitrogen. CHAPTER XVI. THE NERVOUS SYSTEM. THE nervous system consists of two portions or systems, the cerebro-spinal and the sympathetic or ganglionic, each of which (though they have many things in common) possesses certain peculiarities in structure, mode of action, and range of influ- ence. The cerebro-spinal system includes the brain and spinal cord, with the nerves proceeding from them, and the several ganglia seated upon these nerves, or forming part of the sub- stance of the brain. It was denominated by Bichat the ner- vous system of animal life; and includes all the nervous organs in and through which are performed the several functions 368 THE NERVOUS SYSTEM. with which the mind is more immediately connected, namely, those relating to sensation and volition, and the mental acts connected with sensible things. The sympathetic or ganglionic portion of the nervous system, which Bichat named the nervous system of organic 1 life, con- sists essentially of a chain of ganglia connected by nervous cords, which extend from the cranium to the pelvis, along each side of the vertebral column, and from which, nerves with ganglia proceed to the viscera in the thoracic, abdomi- nal, and pelvic cavities. By its distribution, as well as by its peculiar mode of action, this system is less immediately con- nected with the mind, either as conducting sensations or the impulses of the will ; it is more closely connected than the cerebro-spinal system is with the processes of organic life. The differences however, between these two systems, are not essential : their actions differ in degree and object more than in kind or mode. Elementary Structures of the Nervous System. The organs of the nervous system or systems are composed essentially of two kinds of structure, vesicular and fibrous ; both of which appear esssential to the construction of even the simplest nervous system. The vesicular structure is usually collected in masses, and mingled with the fibrous structure, as in the brain, spinal cord, and the several ganglia ; and these masses constitute what are termed nerve-centres, being the organs in which it is supposed that nervous force may be gen- erated, and in which are accomplished all the various reflec- tions and other modes of disposing of impressions when they are not simply conducted along nerve-fibres. The fibrous nerve- substance, besides entering into the composition of the nervous centres, forms alone the nerves, or cords of communication, which connect the various nervous centres, and are distributed in the several parts of the body, for the purpose of conveying nervous force to them, or of transmitting to the nervous cen- tres the impressions made by stimuli. 1 The term organic is often used in connection with a function, such as digestion or secretion, which belongs to all organized beings alike; while the term animal function, or animal life, is used in con- nection with such qualities as volition or motion, which seem alto- gether or in great part to belong only to animals. The terms which have been thus used in this general way, are often loosely applied to special tissues. Thus organic nerve-fibres are those which are dis- tributed especially to organs concerned in the discharge of the func- tions of organic, as distinguished from animal life ; and the term is still more commonly applied to one kind of muscular fibre. STRUCTURE OF NERVE- FIB RES. 369 FIG. 133. B c Along the nerve-fibres impressions or conditions of excite- ment are simply conducted : in the nervous centres they may be made to deviate from their direct course, and be variously diffused, reflected, or otherwise disposed of. Nerves are constructed of minute fibres or tubules full of nervous matter, arranged in parallel or interlacing bundles, which bundles are connected by intervening connective tissue, in which their principal bloodvessels ramify. A layer of the areolar, or of strong fibrous tissue, also surrounds the whole nerve, and forms a sheath or neurilemma for it. In most nerves, two kinds of fibres are mingled ; those of one kind being most numerous in, and charac- teristic of, nerves of the cerebro- spinal system ; those of the other, most numerous in nerves of the sympathetic system. The fibres of the first kind appear to consist of tubules of a pellucid simple membrane, within which is contained the proper nerve sub- stance, consisting of transparent oil- like, and apparently homogeneous material, which gives to each fibre the appearance of a fine glass tube filled with a clear transparent fluid (Fig. 133, A). This simplicity of composition is, however, only ap- parent in the fibres of a perfectly fresh nerve ; for shortly after death, they undergo changes which make it probable that their contents are composed of two different materials. The internal or central part, occu- pying the axis of the tube, becomes grayish, while the outer, or cortical portion, becomes opaque and dimly granular or grumous, as if from a kind of coagulation. At the same time, the fine outline of the pre- viously transparent cylindrical tube is exchanged for a dark double con- tour (Fig. 133, B), the outer line being formed by the sheath of the fibre, the inner by the margin of curdled or coagulated medullary substance. The granular Primitive nerve-tubules. A. A perfectly fresh tubule with a single dark outline. B. A tubule or fibre with a double contour from commencing post-mortem change, c. The changes further advanced, producing a varicose or beaded appearance. D. A tubule or fibre, the central part of which, in consequence of still further changes, has accumulated in separate portions within the sheath (after Wagner). 370 THE NERVOUS SYSTEM. material shortly collects into little masses, which distend por- tions of the tubular membrane, while the intermediate spaces collapse, giving the fibres a varicose, or beaded appearance (Fig. 133 c and D), instead of the previous cylindrical form. The difference produced in the contents of the nerve-fibres when exposed to the same conditions, has, with other facts, led to the opinion now generally adopted, that the central part or axis-cylinder of each nerve-fibre differs from the outer portion. The outer portion is usually called the medullary or white sub- stance of Schwann, being that to which the peculiar white aspect of cerebro-spinal nerves is principally due. The whole contents of the nerve-tubules appear to be extremely soft, for when subjected to pressure they readily pass from one part of the tubular sheath to another, and often cause a bulging at the side of the membrane. They also readily escape, on pressure, from the extremities of the tubule, in the form of a grumous or granular material. That there is an essential difference in chemical composition between the central and circumferential parts of the nerve- fibre, i. e., between the axis-cylinder and the medullary sheath, has of late been clearly shown by Messrs. Lister and Turner. Their observations, founded on Mr. Lockhart Clarke's method of investigating nervous substance by means of chromic acid and carmine, have shown that the axis-cylinder of the nerve- fibre is unaffected by chromic acid, but imbibes carmine with great facility, while the medullary sheath is rendered opaque and brown and laminated by chromic acid, but is entirely un- tinged by the carmine. From this difference in their chemi- cal behavior, the central and circumferential portions of the nerve-fibres are readily distinguished on microscopic examina- tion, the former being indicated by a bright red carmine- colored point, the latter by a pale ring surrounding it. The laminated character of the medullary sheath after treatment with chromic acid is believed by Mr. Lockhart Clarke to be due to corrugations effected by the acid, and not to its having a fibrous structure, as maintained by Stilling. The size of the nerve-fibres varies, and the same fibres do not preserve the same diameter through their whole length, being largest in their course within the trunks and branches of the nerves, in which the majority measure from ^oo to sf/ou of an inch in diameter. As they approach the brain or spinal cord, and generally also in the tissues in which they are dis- tributed, they gradually become smaller. In the gray or vesic- ular substance of the brain or spinal cord, they generally do not measure more than from y^Joo to Tfi, groove in the middle of the fourth ven- tricle, ending below in the calamus scriptorius; 7, 7, roots of the auditory nerves. the lateral, continuous with the lateral columns of the cord, are named simply from their position. On the fibres of the lateral column of each side, near its upper part, is a small oval mass, containing gray matter, and named the olivary body ; and at the posterior part of the restiform column, immediately on each side of the posterior median groove, a small tract is marked off by a slight groove from the remainder of the resti- 404 THE NERVOUS SYSTEM. form body, and called the posterior pyramid. The restiform columns, instead of remaining parallel with each other through- out the whole of the medulla oblongata, diverge near its upper part, and by thus diverging, lay open, so to speak, a space called the fourth ventricle, the floor of which is formed by the gray matter of the interior of the medulla, by this divergence exposed. On separating the anterior pyramids, and looking into the groove between them, some decussating fibres can be plainly seen. Distribution of the Fibres of the Medulla Oblongata. The anterior pyramid of each side, although mainly com- posed of continuations of the fibres of the anterior columns of the spinal cord, receives fibres from the lateral columns, both of its own and the opposite side ; the latter fibres forming al- most entirely those decussating strands before mentioned, which are seen in the groove between the anterior pyramids. Thus composed, the anterior pyramidal fibres proceeding onwards to the brain are distributed in the following manner : 1. The greater part pass on through the poiis to the cerebrum. 1 A portion of the fibres, however, running apart from the others, joins some fibres from the olivary body, and unites with them to form what is called the olivary fasciculus or fillet. 2. A small tract of fibres proceeds to the cerebellum. The lateral column on each side of the medulla, in proceed- ing upwards, divides into three parts, outer, inner, and middle, which are thus disposed of : 1. The outer fibres go with the restiform tract to the cerebellum. 2. The middle decussate across the middle line with their fellows, and form a part of the anterior pyramid of the opposite side. 3. The inner pass on to the cerebrum along the floor of the fourth ventricle, on each side, under the name of the fasciculus teres. The fibres of the restiform body receive some small contribu- tions from both the lateral and anterior columns of the me- 1 The expressions "continuous fibres," and the like, appear to be usually understood as meaning that certain primitive nerve-fibres pas* without interruption from one part to another. But such con- tinuity of primitive fibres through long distances in the nervous centres is very far from proved. The apparent continuity of fasciculi (which is all that dissection can yet trace) is explicable on the suppo- sition that many comparatively short fibres lie parallel, with the ends of each inlaid among many others. In such a case, th^re would be an apparent continuity of fibres; just as there is, for example, when one untwists and picks out a long cord of silk or wool, in which each fibre is short, and yet each fasciculus appears to be continued through the whole cord. FUNCTIONS OF THE MEDULLA OBLONGATA. 405 dulla, and proceed chiefly to the cerebellum, but that small part behind, called posterior pyramid, is continued on with the fasciculus teres of each side along the floor of the fourth ven- tricle to the cerebrum. As in structure, so also in the general endowments of their several parts, there is, probably, the closest analogy between the medulla oblongata and the spinal cord. The difference between them in size and form appears due, chiefly, first, to the divergence, enlargement, and decussation of the several columns, as they pass to be connected with the cerebellum or the cerebrum; and, secondly, to the insertion of new quantities of gray matter in the olivary bodies and other parts, in adap- tation to the higher office and wider range of influence which the medulla oblongata as a nervous centre exercises. Functions of the Medulla Oblongata. In its functions the medulla oblongata differs from the spinal cord chiefly in the importance and extent of the actions that it governs. Like the cord, it may be regarded, first, as conducting impressions, in which office it has a wider extent of function than any other part of the nervous system, since it is obvious that all impressions passing to and fro between the brain and the spinal cord and all nerves arising below the pons, must be transmitted through it. The decussation of part of the fibres of the anterior pyramids of the medulla oblongata explains the phenomena of cross-paralysis, as it is termed, i. e., of the loss of motion in cerebral apoplexy, being always on the side opposite to that on which the effusion of blood has taken place. Looking only to the anatomy of the medulla oblongata, it was not possible to explain why the loss of sensation also is on the side opposite the injury or disease of the brain ; for there is no evidence of a decussation of posterior fibres like that which ensues among the anterior fibres of the medulla oblongata. But the discoveries of Brown-SSquard have shown that the crossing of sensitive impressions occurs in the spinal cord (see p. 393). The functions of the medulla oblongata as a nerve-centre seem to be more immediately important to the maintenance of life than those of any other part of the nervous system, since from it alone, or in chief measure, appears to be reflected the nervous force necessary for the performance of respiration and deglutition. It has been proved by repeated experiments on the lower animals that the entire brain may be gradually cut away in successive pprtions, and yet life may continue for a considerable time, ancj the respiratory movements be uninter- 406 THE NERVOUS SYSTEM. rupted. Life may also continue when the spinal cord is cut away in successive portions from below upwards as high as the point of origin of the phrenic nerve, or in animals without a diaphragm, such as birds or reptiles, even as high as the me- dulla oblongata. In Amphibia, these two experiments have been combined ; the brain being all removed from above, and the cord from below ; and so long as the medulla oblongata was intact, respiration and life were maintained. But if, in any animal, the medulla oblongata is wounded, particularly if it is w r ounded in its central part, opposite the origin of the pneumogastric nerves, the respiratory movements cease, and the animal dies as if asphyxiated. And this effect ensues even when all parts of the nervous system, except the medulla ob- longata, are left intact. Injury and disease in men prove the same as these experi- ments on animals. Numerous instances are recorded in which injury to the human medulla oblongata has produced instanta- neous death ; and, indeed, it is through injury of it, or of the part of the cord connecting it with the origin of the phrenic nerve, that death is commonly produced in fractures and diseases with sudden displacement of the upper cervical vertebrae. The centre whence the nervous force for the production of combined respiratory movements appears to issue is in the interior of that part of the medulla oblongata from which the pneumogastric nerves arise; for with care the medulla ob- longata may be divided to within a few lines of this part, and its exterior may be removed without the stoppage of respira- tion; but it immediately ceases when this part is invaded. This is not because the integrity of the pneumogastric nerves is essential to the respiratory movements ; for both these nerves may be divided without more immediate effect than a retarda- tion of these movements. The conclusion, therefore, may safely be, that this part of the medulla oblongata is the nervous centre whereby the impulses producing the respiratory move- ments are reflected. The power by which the medulla oblongata governs and combines the action of various muscles for the respiratory movements, is an instance of the power of reflexion, which it possesses in common with all nervous centres. Its general mode of action, as well as the degree to which the mind may take part in respiration, and the number of nerves and mus- cles which, under the governance of the medulla oblongata, may be combined in the forcible respiratory movements, have been already briefly described (see p. 184, et seq.). That which seems most peculiar in this centre of respiratory action is its wide range of connection, the number of nerves by which the FUNCTIONS OF THE MEDULLA OBLONGATA. 407 centripetal impression to excite motion may be conducted, and the number and distance of those through which the motor impulse may be directed. The principal centripetal nerves engaged in respiration are the pneumogastic, whose branches supplying the lungs appear to convey the most acute impres- sion of the " necessity of breathing." When they are both divided, the respiration becomes slower (J. Reid), as if the necessity were less acutely felt : but it does not cease, and therefore other nerves besides them must have the power of conducting the like impression. The experiments of Volk- mann make it probable that all centripetal nerves possess it in some degree, and that the existence of imperfectly aerated blood in contact with any of them acts as a stimulus, which, being conveyed to the medulla oblongata, is reflected to the nerves of the respiratory muscles : so that respiratory move- ments do not wholly cease so long as any centripetal nerves, and any nerve supplying muscles of respiration, are both in continuous connection with the respiratory centre of the medulla oblongata. The circulation of imperfectly aerated blood in the medulla oblongata itself may also act as a stimu- lus, and react through this nerve-centre on the nerves which supply the inspiratory muscles. The wide extent of connection which belongs to the medulla oblongata as the centre of the respiratory movements, is further shown by the fact that impressions by mechanical and other ordinary stimuli, made on many parts of the external or inter- nal surface of the body, may induce respiratory movements. Thus involuntary respirations are induced by the sudden con- tact of cold with any part of the skin, as in dashing cold water into the face. Irritation of the mucous membrane of the nose produces sneezing. Irritation in the pharynx, oesophagus, stom- ach, or intestines, excites the concurrence of the respiratory movements to produce vomiting. Violent irritation in the rec- tum, bladder, or uterus, gives rise to a concurrent action of the respiratory muscles, so as to effect the expulsion of the faeces, urine, or foetus. The medulla oblongata appears to be the centre whence are derived the motor impulses enabling the muscles of the palate, pharynx, and oesophagus, to produce the successive co-ordinate and adapted movements necessary to the act of deglutition (see p. 213). This is proved by the persistence of swallowing in some of the lower animals after destruction of the cerebral hemispheres and cerebellum ; its existence in anencephalous monsters ; the power of swallowing possessed by marsupial embryos before the brain is developed ; and by the complete arrest of the power of swallowing when the medulla oblongata 408 THE NERVOUS SYSTEM. is injured in experiments. But the reflecting power herein exercised by the medulla oblongata is of a much simpler and more restricted kind than that exercised in respiration ; it is, indeed, not more than a simple instance of reflex action by a segment of the spinal axis, receiving impressions for this pui> pose from only a few centripetal nerves, and reflecting them to the motor nerves of the same organ. The incident or cen- tripetal nerves in this case are the branches of the glosso- pharyngeal, and, in a subordinate degree, those of the fifth nerve, some of the branches of the superior laryngeal nerve, which are distributed to the pharynx ; and the nerves through which the motor impressions to the fauces and pharynx are reflected, are the pharyngeal branches of the vagus, and, in sub- ordinate degrees, or as supplying muscles accessory to the move- ments of the pharynx, the branches of the hypoglossal, facial, cervical, recurrent, and fifth nerves. For the oesophageal move- ments, so far as they are connected with the medulla oblon- gata, the filaments of the pneumogastric nerve alone, which contain both afferent and efferent fibres, appear to be sufficient (John Reid). Though respiration and life continue while the medulla oblongata is perfect and in connection with respiratory nerves, yet, when all the brain above it is removed, there is no more appearance of sensation, or will, or of any mental act in the animal, the subject of the experiment, than there is when only a spinal cord is left. The movements are all involuntary and unfelt ; and the medulla oblongata has, therefore, no claim to be considered as an organ of the mind, or as the seat of sensa- tion or voluntary power. These are connected with parts next to be described. It would appear that much of the reflecting power of the medulla oblongata may be destroyed ; and yet its power in the respiratory movements may remain. Thus, in patients completely affected with chloroform, the winking of the eye- lids ceases, and irritation of the pharynx will not produce the usual movements of swallowing, or the closure of the glottis (so that blood may run quietly into the stomach, or even into the lungs) ; yet, with all this, they may breathe steadily, and show that the power of the medulla oblougata to combine in action all the nerves of the respiratory muscles is perfect. In addition to its influence over the functions of respiration and deglutition, the medulla oblongata appears to be largely concerned also in the faculty of speech. In the medulla oblongata appears to be seated also the chief vaso-motor nerve-centre (p. 452). From this arise fibres which, passing down the spinal cord, issue with the anterior THE PONS VAROLII. 409 roots of the spinal nerves, and enter the ganglia and branches of the sympathetic, by which they are conducted to the blood- vessels. The influence which is exercised by the medulla oblongata, or, at least, by its irritation, on the formation of sugar in the liver, has been referred to (p. 269). STRUCTURE AND PHYSIOLOGY OF THE PONS VAROLII, CRURA CEREBRI, CORPORA QUADRIGEMINA, CORPORA GENICU- LATA, OPTIC THALAMI, AND CORPORA STRIATA. Pom Varolii. The mesocephalon, or pons (o, Fig. 145), is composed principally of transverse fibres connecting the two hemispheres of the cerebellum, and forming its principal com- missure. But it includes, interlacing with these, numerous longitudinal fibres which connect the medulla oblongata with the cerebrum, and transverse fibres which connect it with the cerebellum. Among the fasciculi of nerve-fibres by which these several parts are connected, the pons also contains abun- dant gray or vesicular substance, which appears irregularly placed among the fibres, and fills up all the interstices. The anatomical distribution of the fibres, both transverse and longitudinal, of which the pons is composed, is sufficient evidence of its functions as a conductor of impressions from one part of the cerebro-spinal axis to another. Concerning its functions as a nerve-centre, little or nothing is certainly known. Crura Cerebri. The crura cerebri (I, Fig. 145), are prin- cipally formed of nerve-fibres, of which the inferior or more superficial are continuous with those of the anterior py- ramidal tracts of the medulla oblongata, and the superior or deeper fibres with the lateral and posterior pyramidal tracts, and with the olivary fasciculus. Besides these fibres from the medulla oblongata, are others from the cerebellum ; and some of the latter as well as a part of the fibres derived from the lateral tract of the medulla oblongata, decussate across the middle line. On their upper part, the crura cerebri bear three pairs of small ganglia, or masses of mingled gray and white nerve- substance, namely, the corpora geniculata externa and internet, and the corpora quadrigemina, or nates and testes. And in their onward course to the cerebrum, the fibres of each crus cerebri pass through two large ganglia, the optic thalamus and corpus striatum, and in their substance come into connection with variously-shaped masses and layers of gray substance. Whether all the fibres of the crura cerebri end in the gray 410 THE NERVOUS SYSTEM. matter of these two ganglia, while others start afresh from them to enter the cerebral hemispheres ; or whether some of the fibres of the crura pass through them, while only a portion FIG. 145. Shows the under surface or base of the encephalon freed from its membranes A, anterior, B, middle, and c, posterior lobe of cerebrum. a. The fore part of the great longitudinal fissure. 6. Notch between hemispheres of the cerebellum, c. Optic commissure, d. Left peduncle of cerebrum, e. Posterior perforated space, e to i. Interpeduncular space. //'. Convolution of Sylvian fissure, h. Termination of gyrus fornicatus behind the Sylvian fissure, i. Infundibulum. I. Right middle crus or peduncle of cerebellum, m m. Hemispheres of cerebellum, n. Corpora albicantia. o. Pons varolii, continuous at each side with middle crura of cerebel- lum, p. Anterior perforated space, q. Horizontal fissure of cerebellum, r. Tuber cinereum. s s'. Sylvian fissure, t. Left peduncle or crus of cerebrum, u u. Optic tracts, v. Medulla oblougata. x. Marginal convolution of the longitudinal fissure. 1 to 9 indicate the several pairs of cerebral nerves, numbered according to the usual notation, viz. 1. Olfactory nerve. 2. Optic. 3. Motor nerve of eye. 4. Pathetic. 5. Trifacial. 6. Abducent nerve of eye. 7. Auditory, and 7'. Facial. 8. Glosso- pharyngeal, 8'. Vagus, and 8". Spinal accessory nerve. can be strictly said to have their termination there, must re- main at present undecided, the difficulties in the way of solv- ing such an anatomical doubt being at present insuperable. CORPORA QUADRIGEMINA. 411 Each cms cerebri contains among its fibres a mass of vesic- ular substance, the locus niger, the nerve-corpuscles of which abound in pigment-granules, and afford some of the best in- stances of the caudate structure. With regard to their functions, the crura cerebri may be regarded as, principally, conducting organs. As nerve-centres they are probably connected with the functions of the third cerebral nerve, which arises from the locus niger, and through which are directed the chief of the numerous and complicated movements of the eyeball and iris. From the result of vivisection it appears that when one of the crura cerebri is cut across, the animal moves round and round, rotating around a vertical axis from the injured towards the sound side. Such movements, however, attend the sections of other parts than the crura cerebri; and as indications of the functions of these parts, the results of such experiments have been hitherto almost valueless. Corpora Quadrigemiua. The corpora quadrigemiua (from which, in function, the corpora geniculata are not distinguished), are the homologues of the optic lobes in birds, amphibia, and fishes, and may be regarded as the principal nervous centres for the sense of sight. The experiments of Flourens, Longet, and Hertwig, show that removal of the corpora quadrigemina wholly destroys the power of seeing ; and diseases in which they are disorganized are usually accompanied with blindness. Atrophy of them is also often a consequence of atrophy of the eyes. Destruction of one of the corpora quadrigemina (or of one optic lobe in birds), produces blindness of the opposite eye. This loss of sight is the only apparent injury of sensibility sustained by the removal of the corpora quadrigemina. The removal of one of them affects the movements of the body, so that animals rotate, as after division of the crus cerebri, only more slowly : but this is probably due to giddiness and partial loss of sight. The more evident and direct influence is that produced on the iris. It contracts when the corpora quadri- gemina are irritated : it is always dilated when they are re- moved : so that they may be regarded, in some measure at least, as the nervous centres governing its movements, and adapting them to the impressions derived from the retina through the optic nerves and tracts. Concerning the functions, taken as a whole, discharged by the olfactory and optic lobes, the gray substance of the pons, the corpora striata and optic thalami (b, d, Fig. 146), with 412 THE NERVOUS SYSTEM. some other centres of gray matter not so distinct, such as the gray matter on the floor of the fourth ventricle with which the auditory nerve is connected, the most philosophical theory is FIG. 146. Dissection of brain, from above, exposing the lateral, fourth, and fifth ventricles, with the surrounding parts (from Hirschfeld and Leveill6). %. a, anterior part, or genu of corpus callosum; 6, corpus striatum; &', the corpus striatum of left side, dis- sected so as to expose its gray substance ; c, points by a line to the tsenia semicircu- laris ; d, optic thalamus; e, anterior pillars of fornix divided; below they are seen descending in front of the third ventricle, and between them is seen part of the an- terior commissure ; in front of the letter e is seen the slit-like fifth ventricle, between the two laminae of the septum lucidum; /, soft or middle commissure; g is placed in the posterior part of the third ventricle; immediately behind the latter are the posterior commissure (just visible) and the pineal gland, the two crura of which extend forwards along the inner and upper margins of the optic thalami ; h and i, the corpora quadrigemina ; k, superior crus of cerebellum ; close to k is the valve of Vieussens, which has been divided so as to expose the fourth ventricle ; /, hippo- campus major and corpus fimbriatum, or tsenia hippocampi ; m, hippocampus minor ; n, eminentia collateralis ; o, fourth ventricle ; p, posterior surface of medulla oblon- gata ; r, section of cerebellum ; s, upper part of left hemisphere of cerebellum exposed by the removal of part of the posterior cerebral lobe. undoubtedly that which has been so ably enunciated by Dr. Carpenter. He supposes these ganglia to constitute the real SENSORY GANGLIA. 413 sensorium ; that is to say, it is by means of them that the mind becomes conscious of impressions made on the organs or tissues with which (by means of nerve-fibres) they are in com- munication. Thus impressions made on the optic nerve, or its expansion in the retina, are conducted by the fibres of the optic nerve to the corpora quadrigemina, and through the medium of these ganglia the mind becomes conscious of the impression made. And impressions on the filaments of the olfactory or auditory nerve are in the same way perceived through the medium of the olfactory or auditory ganglia, to which they are first conveyed. The optic thalami and corpora striata probably have some function of a like kind perhaps in relation to ordinary sensation, but nothing is certainly known regarding them. Besides their functions, however, as media of communica- tion between the mind and external objects, these sensory ganglia, as they are termed, are probably the nerve-centres by means of which those reflex acts are performed which require either a higher combination of muscular acts than can be directed by means of the medulla oblongata or spinal cord alone, or, on the other hand, such reflex actions as require for their right performance the guidance of sensation. Under this head are included various acts, as walking, reading, writ- ing, and the like, which we are accustomed to consider volun- tary, but which really are as incapable of being performed by distinct and definite acts of the will as are those more simple movements of which we are not conscious, and which, per- formed under the guidance of the spinal cord or medulla oblongata alone, we call simple reflex actions. It is true that, in the performance of such acts as those just mentioned, a certain exercise of the will is required at the commencement, but that the carrying out of its mandates is essentially reflex and involuntary, any one may convince himself by trying to perform each individual movement concerned, strictly as a voluntary act. That such movements are reflex and essentially independent as regards their mere production of the will, there is no doubt ; that the nerve-centres through which such reflex actions are performed are the so-called sensory ganglia, is, of course, only a theory which may or not be confirmed by future investigations. Besides their possible functions in the manner just men- tioned, it is supposed that these sensory ganglia may be the means of transmitting the impulses of the will to the muscles, which act in obedience to it, and thus be the centres of reflex action as well for impressions conveyed downwards to them 35 414 THE NERVOUS SYSTEM. from the cerebral hemispheres, as for impressions carried up- wards to them by the different nerves which preserve their connection with the organs of the various senses. STRUCTURE AND PHYSIOLOGY OF THE CEREBELLUM. The cerebellum (7, 8, 9, 10, Fig. 147) is composed of an elongated central portion called the vermiform processes, and two hemispheres. Each hemisphere is connected with its fel- low, not only by means of the vermiform processes, but also by a bundle of fibres called the middle crus or peduncle (the latter forming the greater part of the pons Varolii), while a superior crus with the valve of Vieussens, connects it with the cerebrum (Fig. 147, 5), and an inferior crus (formed by the FIG. 147. "* View of cerebellum in section and of fourth ventricle, with the neighboring parts (from Sappey after Hirschfeld and Leveille). 1, median groove of fourth ventricle, ending below in the calamus scriptorius, with the longitudinal eminences formed by the fasciculi teretes, one on each side ; 2, the same groove, at the place where the white streaks of the auditory nerve emerge from it to cross the floor of the ventricle ; 3, inferior crus or peduncle of the cerebellum, formed by the restiform body ; 4 , posterior pyramid ; above this is the calamus scriptorius ; 5, superior crus of cere- bellum, or processus a cerebello ad cerebrum (or ad testes) ; 6, 6, fillet to the side of the cruracerebri ; 7, 7, lateral grooves of the crura cerebri ; 8, corpora quadrigemina. prolonged restiform body) connects it with the medulla ob- longata (3, Fig. 147). The cerebellum is composed of white and gray matter like FUNCTIONS OF THE CEREBELLUM. 415 that of the cerebrum, but arranged after a different fashion, as shown in Fig. 147. Besides the gray substances on the surface, however, there is near the centre of the white substance of each hemisphere, a small capsule of gray matter called the corpus dentatum (Fig. 148, c d), resembling very closely the corpus dentatum of the olivary body of the medulla oblongata (Fig. 148, o). The physiology of the cerebellum may be considered in its relation to sensation, voluntary motion, and the instincts or higher faculties of the mind. It is itself insensible to irrita- tion, and may be all cut away without eliciting signs of pain (Longet). Yet, if any of its crura be touched, pain is indi- cated ; and, if the restiform tracts of the medulla oblongata be irritated, the most acute suffering appears to be produced. Its removal or disorganization by disease is also generally un- accompanied with loss or disorder of sensibility ; animals from which it is removed can smell, see, hear, and feel pain, to all appearance, as perfectly as before (Flourens ; Magendie). So FIG. 148. Outline sketch of a section of the cerebellum showing the corpus dentatum (from Quain). %. The section has been carried through the left lateral part of the pons, so as to divide the superior peduncle and pass nearly through the middle of the left cerebellar hemisphere. The olivary body has also been divided longitudinally so as to expose in section its corpus dentatum. c r, crus cerebri ; /, fillet ; q, corpora quadrigemina ; s p, superior peduncle of the cerebellum divided ; m p, middle pe- duncle or lateral part of the pons Varolii, with fibres passing from it into the white stem ; a v, continuation of the white stem radiating towards the arbor vitse of the folia ; c d, corpus dentatum ; o, olivary body with its corpus dentatum ; p, anterior pyramid. that, although the restiform tracts of the medulla oblongata, which themselves appear so sensitive, enter the cerebellum, it cannot be regarded as a principal organ of sensibility. In reference to motion, the experiments of Longet and most others agree that no irritation of the cerebellum produces movement of any kind. Remarkable results, however, are produced by removing parts of its substance. Flourens 416 . THE NERVOUS SYSTEM. (whose experiments have been abundantly confirmed by those of Bouillaud, Longet, and others) extirpated the cerebellum in birds by successive layers. Feebleness and want of har- mony of the movements were the consequence of removing the superficial layers. When he reached the middle layers, the animals became restless without being convulsed ; their move- ments were violent and irregular, but their sight and hearing were perfect. By the time that the last portion of the organ was cut away, the animals had entirely lost the powers of springing, flying, walking, standing, and preserving their equi- librium. When an animal in this state was laid upon its back, it could not recover its former posture ; but it fluttered its wings, and did not lie in a state of stupor ; it saw the blow that threatened it, and endeavored to avoid it. Volition, sen- sation, and memory, therefore, were not lost, but merely the faculty of combining the actions of the muscles ; and the en- deavors of the animal to rnaintan its balance were like those of a drunken man. The experiments afforded the same results when repeated on all classes of animals ; and, from them and the others be- fore referred to, Flourens inferred that the cerebellum belongs neither to the sensitive nor the intellectual apparatus; and that it is not the source of voluntary movements, although it belongs to the motor-apparatus ; but is the organ for the co- ordination of the voluntary movements, or for the excitement of the combined action of muscles. Such evidence as can be obtained from cases of disease of this organ confirms the view taken by Flourens ; and, on the whole, it gains support from comparative anatomy ; animals whose natural movements require most frequent and exact combinations of muscular actions being those whose cerebella are most developed in proportion to the spinal cord. M. Foville holds that the cerebellum is the organ of muscu- lar sense, i. e., the organ by which the mind acquires that knowledge of the actual state and position of the muscles which is essential to the exercise of the will upon them ; and it must be admitted that all the facts just referred to are as well ex- plained on this hypothesis as on that of the cerebellum being the organ for combining movements. A harmonious combina- tion of muscular actions must depend as much on the capa- bility of appreciating the condition of the muscles with regard to their tension, and to the force with which they are con- tracting, as on the power which any special nerve-centre may possess of exciting them to contraction. And it is because the power of such harmonious movement would be equally lost, whether the injury to the cerebellum involved injury to FUNCTIONS OF THE CEREBELLUM. 417 the seat of muscular sense, or to the centre for combining mus- cular actions, that experiments on the subject afford no proof in one direction more than the other. Gall was led to believe, that the cerebellum is the organ of physical love, or, as Spurzheim called it, of amativeness ; and this view is generally received by phrenologists. The facts favoring it are, first, several cases in which atrophy of the testes and loss of sexual passion have been the consequence of blows over the cerebellum, or wounds of its substance ; sec- ondly, cases in which disease of the cerebellum has been at- tended with almost constant erection of the penis, and frequent seminal emissions; and thirdly, that it has seemed possible to estimate the degree of sexual passion in different persons by an external examination of the region of the cerebellum. The cases of disease of the cerebellum do not prove much ; for the same affections of the genital organs are more gener- ally observed in diseases, and in experimental irritations of the medulla oblongata and upper part of the spinal cord (Longet). The facts drawn from craniological examination will receive the credit given to the system of which they are a principal evidence. But, in opposition to them, it must be stated that there has been a case of complete disorganization or absence of the cerebellum without loss of sexual passion (Combiette, Longet, and Cruveilhier) ; that the cocks from whom M. Flourens removed the cerebellum showed sexual desire, though they were incapable of gratifying it ; and that among animals there is no proportion observable between the size of the cere- bellum and the development of the sexual passion. On the contrary, many instances may be mentioned in which a larger sexual appetite coexists with a smaller cerebellum ; as e. g. y that rays and eels, which are among the fish that copulate, have not laminse on their almost rudimental cerebella ; and that cod-fish, which do not copulate, but deposit their genera- tive fluids in the water, have comparatively well-developed cerebella. Among the Amphibia, the sexual passion is ap- parently very strong in frogs and toads ; yet the cerebellum is only a narrow bar of nervous substance. Among birds there is no enlargement of the cerebellum in the males that are polyg- amous ; the domestic cock's cerebellum is not larger than the hen's, though his sexual passion must be estimated at many times greater than hers. Among Mammalia the same rule holds; and in this class the experiments of M. Lassaigne have plainly shown that the abolition of the sexual passion by re- moval of the testes in early life is not followed by any diminu- tion of the cerebellum ; for in mares and stallions the average 418 THE NERVOUS SYSTEM. absolute weight of the cerebellum is 61 grains, and in geldings 70 grains ; and its proportionate weight, compared with that of the cerebrum, is, on average, as 1 : 6.59 in mares ; as 1 : 5.97 in geldings, and only as 1 : 7.07 in stallions. On the whole, therefore, it appears advisable to wait for more evidence before concluding that there is any peculiar and direct connection between the cerebellum and the sexual in- stinct or sexual passion. From all that has been observed, no other office is manifest in it than that of regulating and com- bining muscular movements, or of enabling them to be regu- lated and combined by so informing the mind of the state and position of the muscles that the will may be definitely and aptly directed to them. The influence of each half of the cerebellum is directed to muscles on the opposite side of the body ; and it would appear that for the right ordering of movements, the actions of its two halves must be always mutually balanced and adjusted. For if one of its crura, or if the pons on either side of the middle line, be divided, so as to cut off from the medulla oblongata and spinal cord the influence of one of the hemispheres of the cerebellum, strangely disordered movements ensue. The ani- mals fall down on the side opposite to that on which the crus cerebelli has been divided, and then roll over continuously and repeatedly ; the rotation being always round the long axis of their bodies, and from the side on which the injury has been inflicted. 1 The rotations sometimes take place with much ra- pidity ; as often, according to M. Magendie, as sixty times in a minute, and may last for several days. Similar movements have been observed in men ; as by M. Serres in a man in whom there was apoplectic effusion in the right crus cerebelli ; and by M. Belhomme in a woman, in whom an exostosis pressed on the left crus. 3 They may, perhaps, be explained by assuming that the division or injury of the crus cerebelli produces paral- ysis or imperfect and disorderly movements of the opposite side of the body ; the animal falls, and then, struggling with the disordered side on the ground, and striving to rise with the 1 Magendie and Miiller, and others following them, say the rotation is towards the injured side ; but Longet and others more correctly give the statement as in the text. The difference has probably arisen from using the words right and left, without saying whose right and left are meant, whether those of the observer or those of the observed. When, for example, an animal's right crus cerebelli is divided, he rolls from his own right to his own left, but from the left to the right of one who is standing in front of him. 8 See such cases collected and recorded by Dr. Paget in the Ed. Med. and Surg. Journal for 1847. STRUCTURE OF THE CEREBRUM. 419 other, pushes itself over; and so, again and again, with the same act, rotates itself. Such movements cease when the other crus cerebelli is divided ; but probably only because the paral- ysis of the body is thus made almost complete. STRUCTURE AND PHYSIOLOGY OF THE CEREBRUM. The cerebrum is placed in connection with the pons and medulla oblongata by its two crura or peduncles (Fig. 149) : it is connected with the cerebellum, by the processes called su- FlG. 149. Plan in outline of the encephalon, as seen from the right side. y& (From Quain.) The parts are represented as separated from one another somewhat more than natural, so as to show their connections. A, cerebrum ; /, g, h, its anterior, middle, and posterior lobes; e, fissure of Sylvius; B, cerebellum; C, pons varolii ; D, me- dulla oblongata ; a, peduncles of the cerebrum ; b, c, d, superior, middle, and inferior peduncles of the cerebellum. perior crura of the cerebellum, or proeessus a cerebello ad testes, and by a layer of gray matter called the valve of Vieussens, which lies between these processes, and extends from the in- ferior vermiform process of the cerebellum to the corpora quad- rigemina of the cerebrum. These parts, which thus connect the cerebrum with the other principal divisions of the cerebro- spinal nervous centre, form parts of the walls of a cavity (the fourth ventricle) and a canal (the iter a tertio ad quartum ven- 420 THE NERVOUS SYSTEM. triculum), which are the continuation of the canal that in the foetus extended through the whole length of the spinal cord and brain. They may, therefore, be regarded as the contin- uation of the cerebro-spinal axis or column ; on which, as a de- velopment from the simple type, the cerebellum is placed; and, on the further continuation of which, structures both larger and more numerous are raised, to form the cerebrum (Fig. 142). The cerebral convolutions appear to be formed of nearly parallel plates of fibres, the ends of which are turned towards the surface of the brain, and are overlaid and mingled with successive layers of gray nerve-substance. The external gray matter is so arranged in layers, that a vertical section of a convolution, according to Mr. Lockhart Clarke, generally presents the appearance of seven layers of pale and dark ner- vous substance. The structure of the gray matter is that which belongs to vesicular nervous substance (p. 375). It is nearly certain that the cerebral hemispheres are the organ by which, 1st, we perceive those clear and more im- pressive sensations which we can retain, and according to which we can judge ; 2dly, by which are performed those acts of will, each of which requires a deliberate, however quick, de- termination ; 3dly, they are the means of retaining impressions of sensible things, and reproducing them in subjective sensa- tions and ideas; 4thly, they are the medium of the higher emotions and feelings, and of the faculties of judgment, under- standing, memory, reflection, induction, and imagination, and others of a like class. The evidences that the cerebral hemispheres have the func- tions indicated above, are chiefly these : 1. That any severe injury of them, such as a general concussion, or sudden pres- sure by apoplexy, may instantly deprive a man of all power of manifesting externally any mental faculty. 2. That in the same general proportion as the higher sensuous mental facul- ties are developed in the vertebrate animals, and in man at different ages, the more is the size of the cerebral hemispheres developed in comparison with the rest of the cerebro-spinal system. 3. That no other part of the nervous system bears a corresponding proportion to the development of the mental faculties. 4. That congenital and other morbid defects of the cerebral hemisphere are, in general, accompanied with corre- sponding deficiency in the range or power of the intellectual faculties and the higher instincts. Respecting the mode in which the brain discharges its func- tions, there is no evidence whatever. But it appears that, for all but its highest intellectual acts, one of the cerebral hemi- spheres is sufficient. For numerous cases are recorded in FUNCTIONS OF THE CEREBRUM. 421 which no mental defect was observed, although one cerebral hemisphere was so disorganized or atrophied that it could not be supposed capable of discharging its functions. The re- maining hemisphere was, in these cases, adequate to the func- tions generally discharged by both; but the mind does not seem in any of these cases to have been tested in very high intellectual exercises ; so that it is not certain that one hemi- sphere will suffice for these. In general, the mind combines, as one sensation, the impressions which it derives from one object, through both hemispheres, and the ideas to which the two such impressions give rise are single. In relation to common sensation and the effort of the will, the impressions .to and from the hemispheres of the brain are carried across the middle line : so that in destruction or com- pression of either hemisphere, whatever effects are produced in loss of sensation or voluntary motion, are observed on the side of the body opposite to that on which the brain is injured. In speaking of the cerebral hemispheres as the so-called organs of the mind, they have been regarded as if they were single organs, of which all parts are equally appropriate for the exercise of each of the mental faculties. But it is pos- sible that each faculty has a special portion of the brain ap- propriated to it as its proper organ. For this theory the principal evidences are as follows : 1. That it is in accordance with the physiology of the other compound organs or systems in the body, in which each part has its special function ; as, for example, of the digestive system, in which the stomach, liver, and other organs perform each their separate share in the general process of the digestion of the food. 2. That in different individuals the several mental functions are mani- fested in very different degrees. Even in early childhood, before education can be imagined to have exercised any in- fluence on the mind, children exhibit various dispositions each presents some predominant propensity, or evinces a sin- gular aptness in some study or pursuit ; and it is a matter of daily observation that every one has his peculiar talent or pro- pensity. But it is difficult to imagine how this could be the case, if the manifestation of each faculty depended on the whole of the brain : different conditions of the whole mass might affect the mind generally, depressing or exalting all its functions in an equal degree, but could not permit one faculty to be strongly and another weakly manifested. 3. The plu- rality of organs in the brain is supported by the phenomena of some forms of mental derangement. It is not usual for all the mental faculties in an insane person to be equally disor- dered ; it often happens that the strength of some is increased, 36 422 THE NERVOUS SYSTEM. while that of others is diminished ; and in many cases one function only of the mind is deranged, while all the rest are performed in a natural manner. 4. The same opinion is sup- ported by the fact that the several mental faculties are devel- oped to their greatest strength at different periods of life, some being exercised with great energy in childhood, others only in adult age; and that, as their energy decreases in old age, there is not a gradual and equal diminution of power in all of them at once, but, on the contrary, a diminution in one or more, while others retain their full strength, or even increase in power. 5. The plurality of cerebral organs appears to be indicated by the phenomena of dreams, in which only a part of the mental faculties are at rest or asleep, while the others are awake, and, it is presumed, are exercised through the me- dium of the parts of the brain appropriated to them. These facts have been so illustrated and adapted by phren- ologists, that the theory of the plurality of organs in the cere- brum, thus made probable, has been commonly regarded as peculiar to phrenology, and as so essentially connected with it, that if the system of Gall and Spurzheim be untrue, this theory cannot be maintained. But it is plain that all the system of phrenology built upon the theory may be false, and the theory itself true ; for phrenologists assume not only this theory, but also that they have determined all the primitive faculties, of which the mind consists, i. e., all the faculties to which special organs must be assigned, and the places of all those organs in the cerebral hemispheres and the cerebellum. That this is a system of error there need be no doubt, but it is possibly founded on a true theory : the cerebrum may have many organs, and the mind as many faculties ; but what are the faculties that require separate organs, and where those organs are situate, are subjects of which only the most general and rudimentary knowledge has been yet attained. From the apparently greater frequency of interference with the faculty of speech in disease of the left than of the right half of the cerebrum, it has been thought that the nerve-centre for language, including in this term all intellectual expression of ideas, is situated in the left cerebral hemisphere. It cannot be said, however, that the existing evidence for this theory is at present sufficient to have established it. Of the physiology of the other parts of the brain, little or nothing can be said. Of the offices of the corpus callosum, or great transverse and oblique commissure of the brain, nothing positive is known. But instances in which it was absent, or very deficient, either without any evident mental defect, or with only such as might THE CORPUS CALLOSUM. 423 be ascribed to coincident affections of other parts, make it probable that the office which is commonly assigned to it, of enabling the two sides of the brain to act in concord, is exer- cised only in the highest acts of which the mind is capable. And this view is confirmed by the very late period of its de- velopment, and by its absence in all but the placental Mam- malia. 1 FIG. 150. View of the corpus callosum from above (from Sappey after Foville). ^. The upper surface of the corpus callosum has been fully exposed by separating the cere- bral hemispheres and throwing them to the side ; the gyms fornicatus has been detached, and the transverse fibres of the corpus callosum traced for some distance into the cerebral medullary substance. 1, the upper surface of the corpus callosum ; 2, median furrow" or raphe ; 3, longitudinal striae bounding the furrow ; 4, swelling formed by the transverse bands as they pass into the cerebrum ; 5, anterior extrem- ity or knee of the corpus callosum ; 8, posterior extremity ; 7, anterior, and, 8, pos- terior part of the mass of fibres proceeding from the corpus callosum ; 9, margin of the swelling ; 10, anterior part of the convolution of the corpus callosum ; 11, hem or band of union of this convolution; 12, internal convolutions of the parietal lobe; 13, upper surface of the cerebellum. 1 See oases of congenital deficiency of the corpus callosum, by Mr. Paget and Mr. Henry, in the twenty-ninth and thirty-first volumes of the Medico-Chirurgical Transactions. 424 THE NERVOUS SYSTEM. To the fornix and other commissures no special function can be assigned ; but it is a reasonable hypothesis that they con- nect the action of the parts between which they are severally placed. As little is known of the function of the pineal and pitu- itary glands. The latter has been supposed, from its micro- scopic structure, to be rather a ductless gland (p. 325) than a nervous organ. PHYSIOLOGY OF THE CEREBRAL AND SPINAL NERVES. The cerebral nerves are commonly enumerated as nine pairs ; but the number is in reality twelve, the seventh nerve consist- ing, as it does, of two nerves, and the eighth of three. These and the spinal nerves, of which there are thirty-one pairs, symmetrically arranged on each side of what, reduced to its simplest form, may be regarded as a column or axis of nervous matter, extending from the olfactory bulbs on the ethmoid bone to the filum terminate of the spinal cord in the lumbar and sacral portions of the vertebral canal. The spinal nerves all present certain characters in common, such as their double roots ; the isolation of the fibres of sensation in the posterior roots, and those of motion in the anterior roots ; the formation of the ganglia on the posterior root ; and the subsequent min- gling of the fibres in trunks and branches of mixed functions. Similar characters probably belong essentially to the cerebral nerves ; but even when one includes the nerves of special sense, it is not possible to discern a conformity of arrangement in any besides the fifth, or trifacial, which, from its many anal- ogies to the spinal nerves, Sir Charles Bell designated as a spinal nerve of the head. According to their several functions, the cerebral or cranial nerves may be thus arranged: Nerves of special sense, . . Olfactory, optic, auditory, part of the glosso-pharyngeal, and the lingual branch of the fifth. Nerves of common sensation, Tho greater portion of the fifth, and part of the glosso-pharyngeal. Nerves of motion, .... Third, fourth, lesser division of the fifth, sixth, facial, and hypoglossal. Mixed nerves, Pneumogastric and accessory. The physiology of the several nerves of the special senses will be considered with the organs of those senses. THE CEREBRAL NERVES. 425 Physiology of the Third, Fourth, and Sixth Cerebral or Cranial Nerves. The physiology of these nerves may be in some degree com- bined, because of their intimate connection with each other in the actions of the muscles of the eyeball, which they supply. They are probably all formed exclusively of motor fibres : some pain is indicated when the trunk of the third nerve is ir- ritated near its origin ; but this may be because of some fila- ments of the fifth nerve running backwards to the brain in .the trunk of the third, or because adjacent sensitive parts are in- volved in the irritation. The third nerve, or motor oculi, supplies the levator palpe- brse superioris muscle, and, of the muscles of the eyeball, all but the superior oblique or trochlearis, to which the fourth nerve is appropriated, and the rectus externus, which receives the sixth nerve. Through the medium of the ophthalmic or lenticular ganglion, of which it forms what is called the short root, it also supplies the motor filaments to the iris. When the third nerve is irritated within the skull, all those muscles to which it is distributed are convulsed. When it is paralyzed or divided, the following effects ensue : first, the upper eyelid can be no longer raised by the levator palpebrse, but drops and remains gently closed over the eye, under the unbalanced influence of the orbicularis palpebrarum, which is supplied by the facial nerve : secondly, the eye is turned out- wards by the unbalanced action of the rectus externus, to which the sixth nerve is appropriated : and hence, from the irregularity of the axes of the eyes, double-sight is often ex- perienced when a single object is within view of both the eyes : thirdly, the eye cannot be moved either upwards, downwards, or inwards ; fourthly, the pupil is dilated. The relation of the third nerve to the iris is of peculiar in- terest. In ordinary circumstances the contraction of the iris is a reflex action, which may be explained as produced by the stimulus of light on the retina being conveyed by the optic nerve to the brain (probably to the corpora quadrigemina), and thence reflected through the third nerve to the iris. Hence the iris ceases to act when either the optic or the third nerve is divided or destroyed, or when the corpora quadri- gemina are destroyed or much compressed. But when the optic nerve is divided, the contraction of the iris may be excited by irritating that portion of the nerve which is connected with the brain ; and when the third nerve is divided, the irritation of its distal portion will still excite contraction of the iris, in which its fibres are distributed. 426 THE NERVOUS SYSTEM. The contraction of the iris thus shows all the character of a reflex act, and in ordinary cases requires the concurrent ac- tion of the optic nerve, corpora quadrigemina, and third nerve ; and, probably also, considering the peculiarities of its perfect mode of action, the ophthalmic ganglion. But, besides, both irides will contract their pupils under the reflected stimulus of light falling only on one retina or under irritation of one optic nerve. Thus, in amaurosis of one eye, its pupil may contract when the other eye is exposed to a stronger light : and gener- ally the contraction of each of the pupils appears to be in di- rect proportion to the total quantity of light which stimulates either one or both retinae, according as one or both eyes are open. The iris acts also in association with certain other muscles supplied by the third nerve: thus, when the eye is directed inwards, or upwards and inwards, by the action of the third nerve distributed in the rectus internus and rectus superior, the iris contracts, as if under direct voluntary influence. The will cannot, however, act on the iris alone through the third nerve ; but this aptness to contract in association with the other muscles supplied by the third, may be sufficient to make it act even in total blindness and insensibility of the retina, whenever these muscles are contracted. The contraction of the pupils, when the eyes are moved inwards, as in looking at a near object, has probably the purpose of excluding those outermost rays of light which would be too far divergent to be refracted to a clear image on the retina ; and the dilatation in looking straight forwards, as in looking at a distant object, per- mits the admission of the largest number of rays, of which none are too divergent to be so refracted. The fourth nerve, or Nervus trochlearis or patheticus, is ex- clusively motor, and supplies only the trochlearis or obliquus superior muscle of the eyeball. The sixth nerve, Nervus abducens or ocularis extermis is also, like the fourth, exclusively motor, and supplies only the rectus extern us muscle. 1 The rectus externus is, therefore, convulsed, and the eye is turned outwards, when the sixth nerve is irri- tated ; and the muscle paralyzed when the nerve is disorganized, compressed, or divided. In all such cases of paralysis, the eye squints inwards, and cannot be moved outwards. 1 In several animals it sends filaments to the iris (Radctyffe Hall) ; and it has probably done so in man, in some instances in which the iris has not been paralyzed, while all the other parts supplied by the third nerve were (Grant). THE CEREBRAL NERVES. 427 In its course through the cavernous sinus, the sixth nerve forms larger communications with the sympathetic nerve than any other nerve within the cavity of the skull does. But the import of these communications with the sympathetic, and the subsequent distribution of its filaments after joining the sixth nerve, are quite unknown; and there is no reason to believe that the sixth nerve is, in function, more closely connected with the sympathetic than any other cerebral nerve is. The question has often suggested itself why the six muscles of the eyeball should be supplied by three motor nerves when all of them are within reach of the branches of one nerve; and the true explanation would have more interest than attaches to the movements of the eye alone; since it is probable that we have, in this instance, within a small space, an example of some general rule according to which associate or antagonist muscles are supplied with motor nerves. Now, in the several movements of the eyes, we sometimes have to act with symmetrically placed muscles, as when both eyes are turned upwards or downwards, inwards or outwards. 1 All the symmetrically placed muscles are supplied with sym- metrical nerves, i. e., with corresponding branches of the same nerves on the two sides; and the action of these symmetrical muscles is easy and natural, as we have a natural tendency to symmetrical movement in most parts. But because of this tendency to symmetrical movements of muscles supplied by symmetrical nerves, it would appear as if, when the two eyes are to be moved otherwise than symmetrically, the muscles to effect such a movement must be supplied with different nerves. So, when the two eyes are to be turned towards one side, say the right, by the action of the rectus extern us of the right eye and the rectus internus of the left, it appears as if the tendency to action through the similar branches of corresponding nerves (which would move both eyes inwards or outwards) were cor- rected by one of these muscles being supplied by the sixth, and the other by the third nerve. So with the oblique muscles : the simplest and easiest actions would be through branches of the corresponding nerves, acting similarly as symmetrical muscles ; but the necessary movements of the two eyes require the contraction of the superior oblique of one side, to be asso- ciated with the contraction of the inferior oblique, and the re- laxation of the superior oblique, of the opposite side. For this, the fourth nerve of one side is made to act with a branch of 1 It is sometimes said that the external reeti cannot be put in action simultaneously: yet they are so when the eyes, having been both di- rected inwards, are restored to the position which they have in looking straight forwards. 428 THE NERVOUS SYSTEM. the third nerve of the other; as if thus the tendency to simul- taneous action through the similar nerves of the two sides were prevented. At any rate, the rule of distribution of nerves here seems to be, that when in frequent and necessary move- ments any muscle has to act with the antagonist of its fellow on the opposite side, it and its fellow's antagonist are supplied from different nerves. Physiology of the Fifth or Trigeminal Nerve. The fifth or trigeininal nerve resembles, as already stated, the spinal nerves, in that its branches are derived through two roots ; namely, the larger or sensitive, in connection with which is the Gasserian ganglion, and the smaller or motor root, which has no ganglion, and which passes under the gan- glion of the sensitive root to join the third branch or division which issues from it. The first and second divisions of the nerve, which arise wholly from the larger root, are purely sensitive. The third division being joined, as before said, by the motor root of the nerve, is of course both motor and sen- sitive. Through the branches of the greater or ganglionic portion of the fifth nerve, all the anterior and antero-lateral parts of the face and head, with the exception of the skin of the parotid region (which derives branches from the cervical spinal nerves), acquire common sensibility ; and among these parts may be in- cluded the organs of special sense, from which common sensa- tions are conveyed through the fifth nerve, and their peculiar sensation through their several nerves of special sense. The muscles, also, of the face and lower jaw acquire muscular sen- sibility through the filaments of the ganglionic portion of the fifth nerve distributed to them with their proper motor nerves. Through branches of the lesser or non-ganglionic portion of the fifth, the muscles of mastication, namely, the temporal, masseter, two pterygoid, anterior part of the digastric, and mylo-hyoid, derive their motor nerves. The motor function of these branches is proved by the violent contraction of all the muscles of mastication in experimental irritation of the third, or inferior maxillary, division of the nerve; by paralysis of the same muscles, when it is divided or disorganized, or from any reason deprived of power ; and by the retention of the power of these muscles, when all those supplied by the facial nerve lose their power through paralysis of that nerve. The last instance proves best, that though the buccinator mus- cle gives passage to, and receives some filaments from, a buccal branch of the inferior division of the fifth nerve, yet it derives THE FIFTH NERVE. 429 its motor power from the facial, for it is paralyzed together with the other muscles that are supplied by the facial, but retaius its power when the other muscles of mastication are paralyzed. Whether, however, the branch of the fifth nerve which is supplied to the buccinator muscle is entirely sensi- tive, or in part motor also, must remain for the present doubt- ful. From the fact that this muscle, besides its other func- tions, acts in concert or harmony with the muscles of mastica- tion, in keeping the food between the teeth, it might be sup- posed from analogy, that it would have a motor branch from the same nerve that supplies them. There can be no doubt, however, that the so-called buccal branch of the fifth, is, in the main, sensitive ; although it is not quite certain that it may not give a few motor filaments to the buccinator muscle. The sensitive function of the branches of the greater divi- sion of the fifth nerve is proved by all the usual evidences, such as their distribution in parts that are sensitive and not capable of muscular contraction, the exceeding sensibility of some of these parts, their loss of sensation when the nerve is paralyzed or divided, the pain without convulsions produced by morbid or experimental irritation of the trunk or branches of the nerve, and the analogy of this portion of the fifth to the posterior root of the spinal nerve. But although formed of sensitive filaments exclusively, the branches of the greater or ganglionic portion of the fifth nerve exercise a manifold influence on the movements of the mus- cles of the head and face, and other parts in which they are distributed. They do so, in the first place, by providing the muscles themselves with that sensibility without which the mind, being unconscious of their position and state, cannot voluntarily exercise them. It is, probably, for conferring this sensibility on the muscles, that the branches of the fifth nerve communicate so frequently with those of the facial and hypoglossal, and the nerves of the muscles of the eye ; and it is because of the loss of this sensibility that when the fifth nerve is divided, animals are always slow and awkward in the movement of the muscles of the face and head, or hold them still, or guide their movements by the sight of the objects towards which they wish to move. Again, the fifth nerve has an indirect influence on the mus- cular movements, by conveying sensations of the state and position of the skin and other parts : which the mind perceiv- ing, is enabled to determine appropriate acts. Thus, when the fifth nerve or its infra-orbital branch is divided, the move- ments of the lips in feeding may cease, or be imperfect ; a fact which led Sir Charles Bell into one of the verv few errors of his 430 THE NERVOUS SYSTEM. physiology of the nerves. He supposed that the motion of the upper lip, in grasping food, depended directly on the infra-orbital nerve ; for he found that, after he had divided that nerve on both sides in an ass, it no longer seized the food with its lips, but merely pressed them against the ground, and used the tongue for the prehension of the food. Mr. Mayo cor- rected this error. He found, indeed, that after the infra-or- bital nerve had been divided, the animal did not seize its food with the lip, and could not use it well during mastication, but that it could open the lips. He, therefore, justly attributed the phenomena in Sir C. Bell's experiments to the loss of sen- sation in the lips ; the animal not being able to feel the food, and, therefore, although it had the power to seize it, not know- ing how or where to use that power. Lastly, the fifth nerve has an intimate connection with muscular movements through the many reflex acts of muscles of which it is the necessary excitant. Hence, when it is divided, and can no longer convey impressions to the nervous centres to be thence reflected, the irritation of the conjunctiva produces no closure of the eye, the mechanical irritation of the nose excites no sneezing, that of the tongue no flowing of saliva ; and although tears and saliva may flow naturally, their afflux is not increased by the mechanical or chemical or other stimuli, to the indirect or reflected influence of which it is liable in the perfect state of this nerve. The fifth nerve, through its ciliary branches and the branch which forms the long root of the ciliary or ophthalmic gan- glion, exercises also some influence on the movement of the iris. When the trunk of the ophthalmic portion is divided, the pupil becomes, according to Valentin, contracted in men and rabbits, and dilated in cats and dogs ; but in all cases, becomes immovable, even under all the varieties of the stimu- lus of light. How the fifth nerve thus affects the iris is unex- plained ; the same effects are produced by destruction of the superior cervical ganglion of the sympathetic, so that, possibly, they are due to the injury of those filaments of the sympathetic which, after joining the trunk of the fifth, at and beyond the Gasserian ganglion, proceed with the branches of its oph- thalmic division to the iris ; or, as Dr. R. Hall ingeniously suggests, the influence of the fifth nerve on the movements of the iris may be ascribed to the affection of vision in conse- quence of the disturbed circulation or nutrition in the retina, when the normal influence of the fifth nerve and ciliary gan- glion is disturbed. In such disturbance, increased circulation making the retina more irritable might induce extreme con- traction of the iris ; or, under moderate stimulus of light, pro- THE FIFTH NERVE. 431 ducing partial blindness, might induce dilatation : but it does not appear why, if this be the true explanation, the iris should in either case be immovable and unaffected by the various degrees of light. Furthermore, the morbid effects which division of the fifth nerve produces in the organs of special sense, make it prob- able that, in the normal state, the fifth nerve exercises some indirect influence on all these organs or their functions. Thus, after such division, within a period varying from twenty-four hours to a week, the cornea begins to be opaque ; then it grows completely white; a low destructive inflammatory process en- sues in the conjunctiva, sclerotica, and interior parts of the eye ; and within one or a few weeks, the whole eye may be quite disorganized, and the cornea may slough or be penetrated by a large ulcer. The sense of smell (and not merely that of mechanical irritation of the nose), may be at the same time lost, or gravely impaired ; so may the hearing, and commonly, whenever the fifth nerve is paralyzed, the tongue loses the sense of taste in its anterior and lateral parts, i. e., in the por- tion in which the lingual or gustatory branch of the inferior maxillary division of the fifth is distributed. 1 The loss of the sense of taste is no doubt chiefly due to the lingual branch of the fifth nerve being a nerve of special sense ; partly, also, perhaps, it is due to the fact that this branch supplies, in the anterior and lateral parts of the tongue, a nec- essary condition for the proper nutrition of that part. But, deferring this question until the glosso-pharyngeal nerve is to be considered, it may be observed that in some brief time after complete paralysis or division of the fifth nerve, the power of all the organs of the special senses may be lost ; they may lose not merely their sensibility to common impressions, for which they all depend directly on the fifth nerve, but also their sen- sibility to the several peculiar impressions for the reception and conduction of which they are purposely constructed and sup- plied with special nerves besides the fifth. The facts observed in these cases 2 can, perhaps, be only explained by the influence which the fifth nerve exercises on the nutritive processes in the organs of the special senses. It is not unreasonable to believe, that, in paralysis of the fifth nerve, their tissues may be the 1 That complete paralysis of the fifth nerve may, however, be un- accompanied, at least, for a considerable period, by injury to the or- gans of special sense, with the exception of that portion of the tongue which is supplied by its gustatory branch, is well illustrated by a valu- able case lately recorded by Dr. Althaus. 2 Two of the best cases are published, with analyses of others, by Mr. Dixon, in the Medico-Chirurgical Transactions, vol. xxviii. 432 THE NERVOUS SYSTEM. seats of such changes as are seen in the laxity, the vascular congestion, oedema, and other affections of the skin of the face and other tegumentary parts which also accompany the pa- ralysis ; and that these changes, which may appear unimpor- tant when they affect external parts, are sufficient to destroy that refinement of structure by which the organs of the special senses are adapted to their functions. According to Magendie and Longet, destruction of the eye ensues more quickly after division of the trunk of the fifth beyond the Gasserian ganglion, or after division of the oph- thalmic branch, than after division of the roots of the fifth between the brain and the ganglion. Hence it would appear as if the influence on nutrition were conveyed through the fila- ments of the sympathetic, which join the branches of the fifth nerve at and beyond the Gasserian ganglion, rather than through the filaments of the fifth itself; and this is confirmed by experiments in which extirpation of the superior cervical ganglion of the sympathetic produced the same destructive disease of the eye that commonly follows the division of the fifth nerve. And yet, that the filaments of the fifth nerve, as well as those of the sympathetic, may conduct such influence, appears certain from the cases, including that by Mr. Stanley, in which the source of the paralysis of the fifth nerve was near the brain, or at its very origin, before it receives any commu- nication from the sympathetic nerve. The existence of gan- glia of the sympathetic in connection with all the principal divisions of the fifth nerve where it gives off those branches which supply the organs of special sense for example, the connection of the ophthalmic ganglion with the ophthalmic nerve at the origin of the ciliary nerves ; of the spheno-pala- tine ganglion with the superior maxillary division, where it gives its branches to the nose and the palate ; of the otic gan- glion with the inferior maxillary near the giving off of fila- ments to the internal ear; and of the submaxillary ganglion with the lingual branch of the fifth all these connections suggest that a peculiar and probably conjoint influence of the sympathetic and fifth nerves is exercised in the nutrition of the organs of the special senses ; and the results of experiment and disease confirm this, by showing that the nutrition of the organs may be impaired in consequence of impairment of the power of either of the nerves. A possible connection between the fifth nerve and the sense of sight, is shown in cases of no unfrequent occurrence, in which blows or other injuries implicating the frontal nerve as it passes over the brow, are followed by total blindness in the THE FACIAL NERVE. 433 corresponding eye. The blindness appears to be the conse- quence of defective nutrition of the retina ; for although, in some cases, it has ensued immediately, as if from concussion of the retina, yet in some it has come on gradually like slowly progressive amaurosis, and in some with inflammatory disor- ganization, followed by atrophy of the whole eye. 1 Physiology of the Facial Nerve. The facial, or portio dura of the seventh pair of nerves, is the motor nerve of all the muscles of the face, including the platysma, but not including any of the muscles of mastication already enumerated (p. 428); it supplies, also, the parotid gland, and through the connection of its trunk with the Vidian nerve, by the petrosal nerves, some of the muscles of the soft palate, most probably the levator palati and azygos uvulae ; by its tympanic branches it supplies the stapedius and laxator tympani, and, through the otic ganglion, the ten- sor tympani ; through the chorda tympani it sends branches to the submaxillary gland and to the lingualis and some other muscular fibres of the tongue ; and by branches given off be- fore it comes upon the face, it supplies the muscles of the external ear, the posterior part of the digastricus, and the stylo-hyoideus. To the greater number of the muscles to which it is dis- tributed it is the sole motor nerve. No pain is produced by irritating it near its origin (Valentin), and the indications of pain which are elicited when any of its branches are irritated may be explained by the abundant communications which, in all parts of its course, it forms with sensitive nerves, whose filaments being mingled with its own are the true source of the pain. Besides its motor influence, the facial is also, by means of the fibres which are supplied to the submaxillary and parotid glands, a so-called secretory nerve (p. 377). For through the last-named branches impressions may be conveyed which excite increased secretion of saliva. For example, if, in a dog, the submaxillary gland be exposed, and the chorda tympani be divided, it will be seen that on stimulating the distal end of the nerve by a weak electric current, the gland becomes ex- ceedingly vascular, and saliva is secreted in largely increased amount. Under ordinary circumstances of increased secretion of saliva by the submaxillary gland, as from the presence of 1 Such H case is recorded by Snabilie in the Nederlandsch Lancet, August, 1846. 434 THE NERVOUS SYSTEM. food in the mouth, the stimulus is conveyed by the same channel, the chorda tympani being the efferent nerve in a reflex action, in which the afferent fibres are branches of the fifth and glosso-pharyngeal nerves. When the facial nerve is divided, or in any other way par- alyzed, the loss of power in the muscles which it supplies, while proving the nature and extent of its functions, displays also the necessity of its perfection for the perfect exercise of all the organs of the special senses. Thus, in paralysis of the facial nerve, the orbicularis palpebrarum being powerless, the eye remains open through the unbalanced action of the levator palpebrse; and the conjunctiva, thus continually exposed to the air and the contact of dust, is liable to repeated inflamma- tion, which may end in thickening and opacity of both its own tissue and that of the cornea. These changes, however, ensue much more slowly than those which follow paralysis of the fifth nerve, and never bear the same destructive character. In paralysis of the facial nerve, also, tears are apt to flow con- stantly over the face, apparently because of the paralysis of the tensor tarsi muscle, and the loss of the proper direction and form of the orifices of the puncta lachrymalia. From these circumstances, the sense of sight is impaired. The sense of hearing, also, is impaired in many cases of paralysis of the facial nerve; not only in such as are instances of simultaneous disease in the auditory nerves, but in such as may be explained by the loss of power in the muscles of the internal ear. The sense of smell is commonly at the same time impaired through the inability to draw air briskly to- wards the upper part of the nasal cavities, in which part alone the olfactory nerve is distributed ; because, to draw the air per- fectly in this direction, the action of the dilators and com- pressors of the nostrils should be perfect. Lastly, the sense of taste is impaired, or may be wholly lost, in paralysis of the facial nerve, provided the source of the paralysis be in some part of the nerve between its origin and the giving off of the chorda tympani. This result, which has been observed in many instances of disease of the facial nerve in man, appears explicable only by the influence which, through the chorda tympani, it exercises on the movements of the lingualis and the adjacent muscular fibres of the tongue ; and, according to some, or probably in some animals, on the move- ments of the stylo-glossus. We may therefore suppose that the accurate movement of these muscles in the tongue is in some way connected with the proper exercise of taste. Together with these effects of paralysis of the facial nerve the muscles of the face being all powerless, the countenance THE GLOSSO-PHARYNGEAL NERVE. 435 acquires on the paralyzed side a characteristic, vacant look, from the absence of all expression : the angle of the mouth ig lower, and the paralyzed half of the mouth looks longer than that on the other side : the eye has an unmeaning stare. All these peculiarities increase, the longer the paralysis lasts ; and their appearance is exaggerated when at any time the muscles of the opposite side of the face are made active in any expres- sion, or in any of their ordinary functions. In an attempt to blow or whistle, one side of the mouth and cheek acts prop- erly, but the other side is motionless, or flaps loosely at the impulse of the expired air; so in trying to suck, one side only of the mouth acts; in feeding, the lips and cheek are powerless, and food lodges between the cheek and gum. As a nerve of expression, the seventh nerve must not be considered independent of the fifth nerve, with which it forms so many communications ; for, although it is through the facial nerve alone that all the muscles of the face are put into their naturally expressive actions, yet the power which the mind has of suppressing or controlling all these expressions can only be exercised by voluntary and well-educated actions directed through the facial nerve with the guidance of the knowledge of the state and position of every muscle, and this knowledge is acquired only through the fifth nerve, which confers sensi- bility on the muscles, and appears, for this purpose, to be more abundantly supplied to the muscles of the face than any other sensitive nerve is to those of other parts. Physiology of the Olosso-Pharyngeal Nerve. The glosso-pharyngeal nerves (4, Fig. 151), in the enume- ration of the cerebral nerves by numbers according to the po- sition in which they leave the cranium, are considered as di- visions of the eighth pair of nerves, in which term are included with them the pneumogastric and accessory nerves. But the union of the nerves under one term is inconvenient, although in some parts the glosso-pharyngeal and pneumogastric are so combined in their distribution that it is impossible to separate them in either anatomy or physiology. The glosso-pharyngeal nerve appears to give filaments through its tympanic branch (Jacobson's nerve), to the fenestra ovalis, and feuestra rotunda, and the Eustachian tube ; also, to the carotid plexus, and, through the petrosal nerve, to the spheno-palatine ganglion. After communicating, either within or without the cranium, with the pneumogastric, and soon after it leaves the cranium, with the sympathetic, digastric branch of the facial, and the accessory nerve, the glosso-pharyngeal 436 THE NERVOUS SYSTEM. nerve parts into the two principal divisions indicated by its name, and supplies the mucous membrane of the posterior and lateral walls of the upper part of the pharynx, the Eustachian tube, the arches of the palate, the tonsils and their mucous membrane, and the tongue as far forwards as the foramen caecum in the middle line, and to near the tip at the sides and inferior part. Some experiments make it probable that the glosso-pharyn- geal nerve contains, even at its origin, some motor fibres, to- gether with those of common sensation and the sense of taste. Whatever motor influence, however, is conveyed directly through the branches of the glosso-pharyngeal, may be as- cribed to the filaments of the pneumogastric or accessory that are mingled with it. The experiments of Dr. John Reid, confirming those of Panizza and Longet, tend to the same conclusions ; and their results probably express nearly all the truth regarding the part of the glosso-pharyngeal nerve which is distributed to the pharynx. These results were that, 1. Pain was produced when the nerve, particularly its pharyngeal branch, was irri- tated. 2. Irritation of the nerve before the origin of its pharyngeal, or of any of these branches, gave rise to extensive muscular motions of the throat and lower part of the face: but when the nerve was divided, these motions were excited by irritating the upper or cranial portion, while irritation of the lower end, or that in connection with the muscles, was followed by no movement ; so that these motions must have depended on a reflex influence transmitted to the muscles through other nerves by the intervention of the nervous centres. 3. When the functions of the brain and medulla oblongata were arrested by poisoning the animal with prussic acid, irritation of the glosso-pharyngeal nerve, before it was joined by any branches of the poeumogastric, gave rise to no movements of the muscles of the pharynx or other parts to which it was distributed ; while, on irritating the pharyngeal branch of the pneumogastric, or the glosso-pharyngeal nerve, after it had received the com- municating branches just alluded to, vigorous movements of all the pharyngeal muscles and of the upper part of the oesoph- agus followed. The most probable conclusion, therefore, may be that what motor influence the glosso-pharyngeal nerve may seem to exer- cise, is due either to the filaments of the pneumogastric or ac- cessory that are mingled with it, or to impressions conveyed through it to the medulla oblongata, and thence reflected to muscles through motor nerves, especially the pneumogastric, accessory, and facial. Thus, the glosso-pharyngeal nerve ex- THE GLOSSO-PHARYNGEAL NERVE. 437 cites, through the medium of the medulla obloDgata, -the ac- tions of the muscles of deglutition. It is the chief centripetal nerve engaged in these actions ; yet not the only one, for, as Dr. John Reid has shown, the acts are scarcely disturbed or retarded when both the glosso-pharyngeal nerves are divided. But besides being thus a nerve of common sensation in the parts which it supplies, and a centripetal nerve through which impressions are conveyed to be reflected to the adjacent muscles, the glosso-pharyngeal is also a nerve of special sensation ; being the gustatory nerve, or nerve of taste, in all the parts of the tongue to which it is distributed. After many discussions, the question, which is the nerve of taste ? the lingual branch of the fifth, or the glosso-pharyngeal ? may be most probably answered by stating that they are both nerves of this special function. For very numerous experiments and cases have shown that when the trunk of the fifth nerve or its lingual branch is paralyzed or divided, the sense of taste is completely lost in the superior surface of the anterior and lateral parts of the tongue. The loss is instantaneous after division of the nerve ; and, therefore, cannot be ascribed to the defective nu- trition of the part, though to this, perhaps, may be ascribed the more complete and general loss of the sense of taste when the whole of the fifth nerve has been paralyzed. But, on the other hand, while the loss of taste in the part of the tongue to which the lingual branch of the fifth nerve is distributed proves that to be a gustatory nerve, the fact that the sense of taste is at the same time retained in the posterior and postero-lateral parts of the tongue, and in the soft palate and its anterior arch, to which (and to some parts of which exclusively) the glosso-pharyngeal is distributed, proves that this also must be a gustatory nerve. In a female patient at St. Bartholomew's Hospital, the left lingual branch of the fifth nerve was divided in removing a portion of the lower jaw : she lost both common sensation and the sensation of taste in the tip and the anterior parts of the left half of the tongue, but retained both in all the rest of the tongue. M. Lisfranc and others have noted similar cases ; and the phenomena in them are so simple and clear, that there can scarcely be any fallacy in the conclusion that the lingual branches of both the fifth and the glosso-pharyngeal nerves are gustatory nerves in the parts of the tongue which they severally supply. This conclusion is confirmed by some experiments on ani- mals, and, perhaps, more satisfactorily as concerns the sense of taste in man, by observation of the parts of the tongue and fauces, in which the sense is most acute. According to Valen- tin's experiments made on thirty students, the parts of the 37 438 THE NERVOUS SYSTEM. tongue from which the clearest sensations of taste are derived, are the base, as far as the foramen caecum and lines diverging forwards on each side from it ; the posterior palatine arches down to the epiglottis; the tonsils and upper part of the pharynx over the root of the tongue. These are the seats of the distribution of the glosso-pharyngeal nerve. The anterior dorsal surface, and a portion of the anterior and inferior sur- face of the tongue, in which the lingual branch of the fifth is alone distributed, conveyed no sense of taste in the majority of the subjects of Valentin's experiments ; but even if this were generally the case, it would not invalidate the conclusion that, in those who have the sense of taste in the anterior and upper part of the tongue, the lingual branch of the fifth is the nerve by which it is exercised. Physiology of the Pneumogastric Nerve. The pneumogastrie nerve, nervus vagus, or par vagwn (Fig. 151), has, of all the cranial and spinal nerves, the most various distribution, and influences the most various functions, either through its own filaments, or those which, derived from other nerves, are mingled in its branches. The parts supplied by the branches of the pneumogastrie nerve are as follows : By its pharyngeal branches, which enter the pharyngeal plexus, a large portion of the mucous mem- brane, and, probably, all the muscles of the pharynx ; by the superior laryugeal nerve, the mucous membrane of the under surface of the epiglottis, the glottis, and the greater part of the larynx, and the crico-thyroid muscle ; by the inferior laryngeal nerve, the mucous membrane and muscular fibres of the trachea, the lower part of the pharynx and larynx, and all the muscles of the larynx, except the crico-thyroid ; by cesophageal branches, the mucous membrane and muscular coats of the oesophagus. Moreover, the branches of the pneu- mogastrie nerve form a large portion of the supply of nerves to the heart and the great arteries through the cardiac nerves, derived from both the trunk and the recurrent nerve ; to the lungs, through both the anterior and the posterior pulmonary plexuses ; and to the stomach, by its terminal branches pass- ing over the walls of that organ ; while branches are also dis- tributed to the liver and to the spleen. From the parts thus enumerated as receiving nerves from the pneumogastrie, it might be assumed that this latter is a nerve of mixed function, both sensitive and motor. Experi- ments prove that it is so from its origin, for the irritation of its roots, even within the cranial cavity, produces both pain THE PNEUMOGASTRIC NERVE. 439 and convulsive movements of the larynx and pharynx ; and when it is divided within the skull, the same movements follow the irritation of the distal portion, showing that they are not due to reflex action. Similar experiments prove that, through its whole course, it contains both sensitive and motor fibres, but after it has emerged from the skull, and, in some instances even sooner, it enters into so many anastomoses that it is hard to say whether the filaments it contains are, from their origin, its own, or whether they are derived from other nerves com- bining with it. This is particularly the case with the filaments of the sympathetic nerve, which are abundantly added to nearly all the branches of the pneumogastric. The likeness to the sympathetic which it thus acquires is further increased by its containing many filaments derived, not from the brain, but from its own petrosal ganglia, in which filaments originate, in the same manner as in the ganglia of the sympathetic, so abundantly that the trunk of the nerve is visibly larger below the ganglia than above them (Bidder and Volkmann). Next to the sympathetic nerve, that which most importantly commu- nicates with the pneumogastric is the accessory nerve, whose internal branch joins its trunk, and is lost in it. Properly, therefore, the pneumogastric might be regarded as a triple-mixed nerve, having out of its own sources, motor, sensitive, and sympathetic or ganglionic nerve-fibres ; and to this natural complexity it adds that which it derives from the reception of filaments from the sympathetic, accessory, and cervical nerves, and, probably, the glosso-pharyngeal and facial. The most probable account of the particular functions which the branches of the pneumogastric nerve discharge in the sev- eral parts to which they are distributed, may be drawn from Dr. John Reid's experiments on dogs. They show that: 1. The pharyngeal branch is the principal, if not the sole motor nerve of the pharynx and soft palate, and is most probably wholly motor ; a part of its motor fibres being derived from the internal branch of the accessory nerve. 2. The inferior laryngeal nerve is the motor nerve of the larynx, irritation of it producing vigorous movements of the arytenoid cartilages ; while irritation of the superior laryngeal nerve gives rise to no action in any of the muscles attached to the arytenoid carti- lages, but merely to contractions of the crico-thyroid muscle. 3. The superior laryngeal nerve is chiefly sensitive; the in- ferior, for the most part, motor ; for division of the recurrent nerves puts an end to the motions of the glottis, but without lessening the sensibility of the mucous membrane ; and division of the superior laryngeal nerves leaves the movements of the 440 THE NERVOUS SYSTEM. glottis unaffected, but deprives it of its sensibility. 4. The motions of the oasophagus are dependent on motor fibres of the pneumogastric, and are probably excited by impressions made upon sensitive fibres of the same ; for irritation of its trunk excites motions of the oesophagus, which extend over the cardiac portions of the stomach ; and division of the trunk paralyzes the ossophagus, which then becomes distended with the food. 5. The cardiac branches of the pneumogastric nerve are one, but not the sole channel through which the in- fluence of the central organs and of mental emotions is trans- mitted to the heart. 6. The pulmonary branches form the principal, but not the sole channel by which the impressions on the mucous surface of the lungs that excite respiration, are transmitted to the medulla oblongata. Dr. Keid was unable to determine whether they contain motor fibres. From these results, and by referring to what has been said in former chapters, the share which the pneumogastric nerve takes in the functions of the several parts to which it sends branches may be understood : 1. In deglutition, the motions of the pharynx are of the reflex kind. The stimulus of the food or other substance to be swallowed, acting on the filaments of the glosso-pharyngeal nerve as well as the filaments of the superior laryngeal given to the pharynx, and of some other nerves, perhaps, with which these communicate, is conducted to the medulla oblongata, whence it is reflected, chiefly through the pneumogastric, to the muscles of the pharynx. 2. In the functions of the larynx, the sensitive filaments of the pneumogastric supply that acute sensibility by which the glottis is guarded against the ingress of foreign bodies, or of irrespirable gases. The contact of these stimulates the fila- ments of the superior laryngeal branch of the pneumogastric ; and the impression conveyed to the medulla oblongata, whe- ther it produce sensation or not, is reflected to the filaments of the recurrent or inferior laryngeal branch, and excites con- traction of the muscles that close the glottis. Both these branches of the pneumogastric co-operate also in the produc- tion and regulation of the voice ; the inferior laryngeal deter- mining the contraction of the muscles that vary the tension of the vocal cords, and the superior laryngeal conveying to the mind the sensations of the state of these muscles necessary for their continuous guidance. And both the branches co-operate in the actions of the larynx in the ordinary slight dilatation and contraction of the glottis in the acts of expiration and inspiration, and more evidently in those of coughing and other forcible respiratory movements (p. 182). THE PNEUMOGASTRIC NERVE. 441 3. It is partly through their influence on the sensibility and muscular movements in the larynx, that the pneumogastric nerves exercise so great an influence on the respiratory pro- cess, and that the division of both the nerves is commonly fatal. To determine how death is in these cases produced, has been the object of innumerable, and often contradictory, ex- periments. It is probably produced differently in different cases, and in many is the result of several co-operating causes. Thus, after division of both the nerves, the respiration at once becomes slower, the number of respirations in a given time being commonly diminished to one-half, probably because the pneumogastric nerves are the principal conductors of the im- pression of the necessity of breathing to the medulla oblon- gata. Respiration does not cease ; for it is probable that the impression may be conveyed to the medulla oblongata through the sensitive nerves of all parts in which the imperfectly aerated blood flows (see p. 407): yet the respiration being re- tarded, adds to the other injurious effects of division of the nerves. Again, division of both pneumogastric trunks, or of both their recurrent branches, is often very quickly fatal in young animals ; but in old animals the division of the recurrent nerve is not generally fatal, and that of both the pneumogastric trunks is not always fatal (J. Reid), and, when it is so, the death ensues slowly. This difference is, probably, because the yielding of the cartilages of the larynx in young animals per- mits the glottis to be closed by the atmospheric pressure in in- spiration, and they are thus quickly suffocated unless trache- otomy be performed (Legallois). In old animals, the rigidity and prominence of the arytenoid cartilages prevent the glottis from being completely closed by the atmospheric pressure ; even when all the muscles are paralyzed, a portion at its posterior part remains open, and through this the animal continues to breathe. Yet the diminution of the orifice for respiration may add to the difficulty of maintaining life. In the case of slower death, after division of both the pneu- mogastric nerves, the lungs are commonly found gorged with blood, oadematous, or nearly solid, or with a kind of low pneu^ monia, and with their bronchial tubes full of frothy bloody fluid and mucus, changes to which, in general, the death may be proximately ascribed. These changes are due, perhaps in part, to the influence which the pneumogastric nerves exercise on the movements of the air-cells and bronchi ; yet, since they are not always produced in one lung when its pneumogastric nerve is divided, they cannot be ascribed wholly to the suspension of organic nervous influence (J. Reid). Rather, they may be 442 THE NERVOUS SYSTEM. ascribed to the hindrance to the passage of blood through the lungs, in consequence of the diminished supply of air and the excess of carbonic acid in the air-cells and in the pulmonary capillaries (see p. 187) ; in part, perhaps, to paralysis of the bloodvessels, leading to congestion ; and in part, also, as the experiments of Traube especially show, they appear due to the passage of food and of the various secretions of the mouth and fauces through the glottis, which, being deprived of its sensi- bility, is no longer stimulated or closed in consequence of their contact. He says, that if the trachea be divided and separated from the oesophagus, or if only the oesophagus be tied, so that no food or secretion from above can pass down the trachea, no degeneration of the tissue of the lungs will follow the division of the pneumogastric nerves. So that, on the whole, death after division of the pneumogastric nerves may be ascribed, when it occurs quickly in young animals, to suffocation through mechanical closure of the paralyzed glottis : and, when it occurs more slowly, to the congestion and pneumonia produced by the diminished supply of air, by paralysis of the bloodvessels, and by the passage of foreign fluids into the bronchi ; and ag- gravated by the diminished frequency of respiration, the in- sensibility to the diseased state of the lungs, the diminished aperture of the glottis, and the loss of the due nervous influ- ence upon the process of respiration. 4. Respecting the influence of the pueumogastric nerves on the movements of the oesophagus and stomach, the secretion of gastric fluid, the sensation of hunger, absorption by the stomach, and the action of the heart, 1 former pages may be referred to. Cyon and Ludwig have discovered that a remarkable power appears to be exercised on the dilatation of the bloodvessels by a small nerve, which arises, in the rabbit, from the superior laryngeal branch, or from this and the trunk of the pneumo- gastric nerve, and after communicating with filaments of the inferior cervical ganglion proceeds to the heart. If this nerve be divided, and its upper extremity be stimulated by a weak interrupted current, an inhibitory influence is conveyed to the vaso-motor centre in the medulla oblongata (p. 452), so as to cause, by reflex action, dilatation of the principal bloodvessels, with diminution of the force and frequency of the heart's action. From the remarkable lowering of the blood-pressure in the vessels, thus produced, this branch of the vagus is called the depressor nerve ; and it is presumed, as an afferent nerve of the heart, to be the means of conveying to the vaso-motor 1 See foot-note, p. 453. THE SPINAL ACCESSORY NERVE. 443 centre in the medulla indications of such conditions of the heart as require a lowering of the blood pressure in the vessels; as, for example, when the heart cannot, with sufficient ease, propel blood into the already too full or too tense arteries. Physiology of the Spinal Accessory Nerve. In the preceding pages it is implied that all the motor in- fluence which the pneumogastric nerves exercise, is conveyed through filaments, which, from their origin, belong to them; and this is, perhaps, true. Yet a question, which has been often discussed, may still be entertained, whether a part of the motor filaments that appear to belong to the pneumogastric nerves are not given to them from the accessory nerves ? The principal branch of the accessory nerve, its external branch, supplies the sterno-mastoid and trapezius muscles; and though pain is produced by irritating it, is composed almost exclusively of motor fibres. It might appear very probable, therefore, that the internal branch, which is added to the trunk of the pneumogastric just before the giving off of the pharyngeal branch, is also motor; and that through it the pneumogastric nerve derives part of the motor fibres which it supplies to the muscles enumerated above. And further, since the pneumogastric nerve has a ganglion just above the part at which the internal branch of the accessory nerve joins its trunk, a close analogy may seem to exist between these two nerves and the spinal nerves with their anterior and posterior roots. In this view, Arnold and several later physiologists have regarded the accessory nerve as constituting a motor root of the vagus nerve ; and although this view cannot now be maintained, yet it is very probable that the accessory nerve gives some motor filaments to the pneumogastric. For, among the experiments made on this point, many have shown that when the accessory nerve is irritated within the skull, convul- sive movements ensue in some of the muscles of the larynx ; all of which, as already stated, are supplied, apparently, by branches of the pneumogastric ; and (which is a very signifi- cant fact) Vrolik states that in the chimpanzee the internal branch of the accessory does not join the pneumogastric at all, but goes direct to the larynx. On the whole, therefore, al- though in some of the experiments no movements in the larynx followed irritation of the accessory nerve, yet it may be con- cluded that this nerve gives to the pneumogastric some of the motor filaments which pass, with the laryngeal branches, to the muscles of the larynx, especially to the crico-thyroid (Ber- nard) ; although it is certain that the accessory nerve does not 444 THE NERVOUS SYSTEM. supply all the motor filaments which the branches of the pneu- mogastric contain. Among the roots of the accessory nerve, the lower, arising from the spinal cord, appear to be composed exclusively of motor fibres, and to be destined entirely to the trapezius and sterno-mastoid muscles ; the upper fibres, arising from the medulla oblongata, contain many sensitive as well as motor fibres. Physiology of the Hypoglossal Nerve. The hypoglossal or ninth nerve, or motor linguce, has a pe- culiar relation to the muscles connected with the hyoid bone, including those of the tongue. It supplies through its de- scending branch (descendens noni\ the sterno-hyoid, sterno- thyroid, and omo-hyoid ; through a special branch the thyro- hyoid, and through its lingual branches the genio-hyoid, stylo-glossus, hyo-glossus, and genio-hyoglossus and linguales. It contributes, also, to the supply of the submaxillary gland. The function of the hypoglossal is, probably, exclusively motor. As a motor nerve, its influence on all the muscles enumerated above is shown by their convulsions when it is irritated, and by their loss of power when it is paralyzed. The effects of the paralysis of one hypoglossal nerve are, how- ever, not very striking in the tongue. Often, in cases of hemi- plegia involving the functions of the hypoglossal nerve, it is not possible to observe any deviation in the direction of the protruded tongue ; probably because the tongue is so compact and firm that the muscles on either side, their insertion being nearly parallel to the median line, can push it straight for- wards or turn it for some distance towards either side. Physiology of the Spinal Nerves. Little need be added to what has been already said of these nerves (pp. 390 to 392). The anterior roots of the spinal nerves are formed exclusively of motor fibres ; the posterior roots exclusively of sensitive fibres. Beyond the ganglia all the spinal nerves appear to be mixed nerves, and to contain as well sympathetic filaments. Of the functions of the ganglia of the spinal nerves nothing very definite is known. That they are not the reflectors of any of the ascertained reflex actions through the spinal nerves, is shown by the reflex movements ceasing when the posterior roots are divided between the ganglia and the spinal cord. THE SYMPATHETIC NERVE. 445 PHYSIOLOGY OF THE SYMPATHETIC NERVE. The sympathetic nerve, or sympathetic system of nerves, obtained its name from the opinion that it is the means through which are effected the several sympathies in morbid action which distant organs manifest. It has also been called the nervous system of organic life, upon the supposition, now proved erroneous, that it alone, as a nervous system, influences the organic processes. Both terms are defective ; but, since the title sympathetic nerve has the advantage of long and most general custom in its favor, and is not more inaccurate than the other, it will be here employed. The general differences between the fibres of the cerebro- spinal and sympathetic nerves have been already stated (p. 371) ; and it has been said, that although such general differ- ences exist, and are sufficiently discernible in selected filaments of each system of nerves, yet they are neither so constant, nor of such a kind, as to warrant the supposition, that the different modes of action of the two systems can be referred to the dif- ferent structures of their fibres. Kather, it is probable, that the laws of conduction by the fibres are in both systems the same, and that the differences manifest in the modes of action of the systems are due to the multiplication and separation of the nervous centres of the sympathetic : ganglia, or nerve- centres, being placed in connection with the fibres of the sym- pathetic in nearly all parts of their course. According to the most general view, the sympathetic system may be described as arranged in two principal divisions, each of which consists of ganglia and connecting fibres. The first division may include those ganglia which are seated on and involve the main trunks or branches of cerebral and spinal nerves. This division will include the large Gasserian gan- glion on the sensitive trunk of the fifth cerebral nerve (Fig. 152), the ganglia on the glosso-pharyngeal and pneumogastric nerves, and the ganglia on the posterior or sensitive branches of the spinal nerves (Fig. 141). To the second division belong the double chain of praeverte- bral ganglia (24, 30, Fig. 151) and their branches, extend- ing from the interior and base of the skull to the coccyx ; the various sympathetic visceral plexuses and their ganglia, as the cardiac, the solar, the renal and hypogastric plexuses ; and in the same division may be included the ganglia in the neigh- borhood of the head and neck, namely, the ophthalmic or len- ticular, the spheno-palatine, the otic, and the submaxillary ganglia (Fig. 152). 38 446 THE NERVOUS SYSTEM. FIG. 151. Distribution of the eighth pair of nerves on the left side (from Hirschfeld and Leveill6). 1, Gasserian ganglion of fifth nerve; 2, internal carotid artery; 3, pharyngeal branch of pneumogastric ; 4, glosso-pharyngeal nerve ; 5, lingual nerve (fifth) ; 6, spinal-accessory nerve ; 7, middle constrictor of pharynx ; 8, internal jugular vein (cut); 9, superior laryngeal nerve; 10, ganglion of trunk of pneumogastric nerve; 11, hypoglossal nerve (cut) on hyoglossus ; 12, ditto (cut) communicating with eighth THE SYMPATHETIC NERVE. 447 The structure of all these ganglia appears to be essentially similar, all containing 1st, nerve-fibres traversing them ; 2dly, nerve-fibres originating in them ; 3dly, nerve- or ganglion- corpuscles, giving origin to these fibres ; and 4thly, other cor- puscles that appear free. And in the trunk, and thence pro- ceeding branches of the sympathetic, there appear to be al- ways 1st, fibres which arise in its own ganglia ; 2dly, fibres derived from the ganglia of the cerebral and spinal nerves ; 3dly, fibres derived from the brain and spinal cord and trans- mitted through the roots of their nerves. The spinal cord, indeed, appears to furnish a large source of the fibres of the sympathetic nerve. Respecting the course of the filaments belonging to the sym- pathetic, the following appears to have been determined. Of the filaments derived from the ganglia on the cerebral nerves, some may pass towards the brain ; for, in the trunks of the nerves, between the ganglia and the brain, fine filaments like those of the sympathetic are found. But these may be pro- ceeding from the brain to the ganglia ; and, on the whole, it is probable that nearly all the filaments originating in the ganglia or cerebral nerves, go out towards the tissues and or- gans to be supplied, some of them being centrifugal, some cen- tripetal ; so that each ganglion with its outgoing filaments may form a kind of special nervous system appropriated to the part in which its filaments are placed. Such, for example, may be the ophthalmic ganglion with the ciliary nerves, con- nected with the brain and the rest of the sympathetic system by the branches of the third, fifth, and sympathetic nerves that form its roots, yet, by filaments of its own, controlling in some mode and degree, the processes in the interior of the eye. Of the fibres that arise in the spinal ganglia, some appear to pass into the posterior branches of the spinal nerves, and to be distributed with them ; the rest pass through the branches by which the spinal nerves communicate with the trunks of the sympathetic, and then entering the sympathetic are disr tributed with its branches to the viscera. With these, also a and first cervical nerve ; 13, external laryngeal nerve ; 14, Second cervical nerve loop- ing with first; 15, pharyngeal plexus on inferior constrictor; 16, superior cervical ganglion of sympathetic ; 17, superior cardiac nerve of pneumogastric ; 18, third cer- vical nerve ; 19, thyroid body; 20, fourth cervical nerve ; 21, 21, left recurrent laryn- geal nerve; 22, spinal-accessory, communicating with cervical nerves; 23, trachea; 24, middle cervical ganglion of sympathetic ; 25, middle cardiac nerve of pneumo- gastric ; 26, phrenic nerve (cut) ; 27, left carotid artery (cut) ; 28, brachial plexus ; 29, phrenic nerve (cut); 30, inferior cervical ganglion of sympathetic; 31, pulmonary plexus of pneuraogastric ; 32, arch of the thoracic aorta ; 33, cesophageal plexus ; 34, vena azygos superior; 35, vena azygos minor; 36, gangliated cord of sympathetic. 448 THE NERVOUS SYSTEM. certain number of the large ordinary cerebro-spinal nerve- fibres, after traversing the ganglia, pass into the sympathetic. FIG. 152. General plan of the branches of the fifth pair (after a sketch by Charles Bell). y z . 1, lesser root of the fifth pair ; 2, greater root passing forwards into the Gasserian ganglion ; 3, placed on the bone above the ophthalmic nerve, which is seen dividing into the supra-orbital, lachrymal, and nasal branches, the latter connected with the ophthalmic ganglion ; 4, placed on the bone close to the foramen rotundum, marks the superior maxillary division, which is connected below with the spheno-palatine ganglion, and passes forwards to the infra-orbital foramen ; 5, placed on the bone over the foramen ovale, marks the submaxillary nerve, giving off the anterior au- ricular and muscular branches, and continued by the inferior dental to the lower jaw, and by the gustatory to the tongue ; a, the submaxillary gland, the submaxillary ganglion placed above it in connection with the gustatory nerve ; 6, the chorda tympani ; 7, the facial nerve issuing from the stylo-mastoid foramen. Of the fibres derived from the ganglia of the sympathetic itself, some go straightway towards the viscera, the rest pass through the branches of communication between the sympa- thetic and the branches of the spinal nerves, and joining these spinal nerves, proceed with them to their respective seats of distribution, especially to the more sensitive parts. THE SYMPATHETIC NERVE. 449 Thus, through these communicating branches, which have been generally called roots or origins of the 'sympathetic nerve, an interchange is effected between all the spinal nerves and the sympathetic trunks ; all the ganglia, also, which are seated on the cerebral nerves, have roots (as they are called) through which filaments of the cerebral nerves are added to their own. So that, probably, all sympathetic nerves contain some inter- mingled cerebral or spinal nerve-fibres ; and all cerebral and spinal nerves some filaments derived from the sympathetic system or from ganglia. But the proportions in which these filaments are mingled are not uniform. The nerves which arise from the brain and spinal cord retain throughout their course and distribution a preponderance of cerebro-spinal fibres, while the nerves immediately arising from the so-called sym- pathetic ganglia probably contain a majority of sympathetic fibres. But inasmuch as there is no certainty that in struc- ture the branches of cerebral or spinal nerves differ always from those of the sympathetic system, it is impossible in the present state of our knowledge to be sure of the source of fibres which from their structure might lead the observer to believe that they arose from the brain or spinal cord on the one hand, or from the sympathetic ganglia on the other. In other words, although the large white tubular fibres are espe- cially characteristic of cerebro-spinal nerves, and the pale or gelatinous fibres of a sympathetic nerve, in which they largely preponderate, there is no certainty to be obtained in a doubt- ful case, of whether the nerve-fibre is derived from one or the other, from mere examination of its structure. It may be de- rived from either source. With respect to the functions of the sympathetic nervous system, it may be stated generally that the sympathetic nerve- fibres are simple conductors of impressions, as those of the cerebro-spinal system are, and that the ganglionic centres have (each in its appropriate sphere) the like powers both of con- ducting and of communicating impressions. Their power of conducting impressions is sufficiently proved in ordinary dis- eases, as when any of the viscera, usually unfelt, give rise to sensations of pain, or when a part not commonly subject to mental influence is excited or retarded in its actions by the various conditions of the mind ; for in all these cases impres- sions must be conducted to and fro through the whole distance between the part and the spinal cord and brain. So, also, in experiments, now more than sufficiently numerous, irritations of the semiluuar ganglia, the splanchnic nerves, the thoracic, hepatic, and other ganglia and nerves, have elicited expres- 450 THE NERVOUS SYSTEM. sions of pain, and have excited movements in the muscular organs supplied from the irritated part. In the case of pain excited, or movements affected by the mind, it may be supposed that the conduction of impressions is effected through the cerebro-spinal fibres which are mingled in all, or nearly all, parts of the sympathetic nerves. There are no means of deciding this ; but if it be admitted that the conduction is effected through the cerebro-spinal nerve-fibres, then, whether or not they pass uninterruptedly between the brain or spinal cord and the part affected, it must be assumed that their mode of conduction is modified by the ganglia. For, if such cerebro-spinal fibres are conducted in the ordinary manner, the parts should be always sensible and liable to the influence of the will, and impressions should be conveyed to and fro instantaneously. But this is not the case ; on the con- trary, through the branches of the sympathetic nerve and its ganglia, none but intense impressions, or impressions exagger- ated by the morbid excitability of the nerves or ganglia, can be conveyed. Respecting the general action of the ganglia of the sympa- thetic nerve, little need be said here, since they may be taken as examples by which to illustrate the common modes of action of all nerve-centres (see p. 382). Indeed, complex as the sym- pathetic system, taken as a whole, is, it presents in each of its parts a simplicity not to be found in the cerebro-spinal system : for each ganglion with afferent and efferent nerves forms a simple nervous system, and might serve for the illustration of all the nervous actions with which the mind is unconnected. But it will be more convenient to consider the ganglia now in connection with the functions that they may be supposed to control, in the several organs supplied by the sympathetic sys- tem alone, or in conjunction with the cerebro-spinal. The general processes which the sympathetic appears to in- fluence, are those of involuntary motion, secretion, and nutri- tion. Many movements take place involuntarily in parts supplied with cerebro-spinal nerves, as the respiratory and other spinal reflex motions ; but the parts principally supplied with sym- pathetic nerves are usually capable of none but involuntary movements, and when the mind acts on them at all, it is only through the strong excitement or depressing influence of some passion, or through some voluntary movement with which the actions of the involuntary part are commonly associated. The heart, stomach, and intestines are examples of these state- ments ; for the heart and stomach, though supplied in large THE SYMPATHETIC XERVE. 451 measure from the pneumogastric nerves, yet probably derive through them few filaments except such as have arisen from their ganglia, and are therefore of the nature of sympathetic fibres. The parts which are supplied with motor power by the sym- pathetic nerve continue to move, though more feebly than be- fore, when they are separated from their natural connections with the rest of the sympathetic system, and wholly removed from the body. Thus, the heart, after it is taken from the body, continues to beat in Mammalia for one or two minutes, in reptiles and Amphibia for hours ; and the peristaltic motions of the intestine continue under the same circumstances. Hence the motion of the parts supplied with nerves from the sympa- thetic are shown to be, in a measure, independent of the brain and spinal cord. It seems to be a general rule, at least in animals that have both cerebro-spinal and sympathetic nerves much developed, that the involuntary movements excited by stimuli conveyed through ganglia are orderly and like natural movements, while those excited through nerves without ganglia are convulsive and disorderly ; and the probability is that, in the natural state, it is through the same ganglia that natural stimuli, im- pressing centripetal nerves, are reflected through centrifugal nerves to the involuntary muscles. As the muscles of respira- tion are maintained in uniform rhythmic action chiefly by the reflecting and combining power of the medulla oblongata, so, probably, are those of the heart, stomach, and intestines, by their several ganglia. And as with the ganglia of the sympa- thetic and their nerves, so with the medulla oblongata and its nerves distributed to respiratory muscles, if these nerves or the medulla oblougata itself be directly stimulated, the move- ments that follow are convulsive and disorderly ; but if the medulla be stimulated through a centripetal nerve, as when cold is applied to the skin, then the impressions are reflected so as to produce movements which, though they may be very quick and almost convulsive, are yet combined in the plan of the proper respiratory acts. Among the ganglia of the sympathetic nerves to which this co-ordination of movements is to be ascribed, must be reckoned, not those alone which are on the principal trunks and branches of the sympathetic external to any organ, but those also which lie in the very substance of the organs ; such as those dis- covered in the heart by Remak. Those also may be included which have been found in the mesentery close by the intestines, as well as in the submucous tissue of the stomach and intestinal canal (Meissner), and in other parts. The extension of dis- 452 THE NERVOUS SYSTEM. coveries of such ganglia will probably diminish yet further the number of instances in which the involuntary movements appear to be effected independently of central nervous in- fluence. Respecting the influence of the sympathetic nerve in nutri- tion and secretion, we may refer to the chapters on those pro- cesses. The influence of the sympathetic nerves on the bloodvessels has been already referred to in the section on the Arteries. It was stated that the muscular tissue of the bloodvessels was supplied by sympathetic nerve-branches, called from their dis- tribution and function vaso-motor nerves ; and that by these the condition of the vessels with respect to contraction or relax- ation, and therefore to the stream of blood which flowed through them in a given time, is governed. When these vaso-motor nerves are intact, the muscular tissue of the arteries is always in a state of tonic contraction, which varies in degree at dif- ferent times. When they are divided, the muscular fibres in which they are distributed are paralyzed, and the bloodvessels become dilated. The most usual experiment in illustration of these facts is performed by exposing in a rabbit the cervical sympathetic, from which vaso-motor branches are given to the bloodvessels of the head and neck. On dividing the nerve, the bloodvessels of the same side are paralyzed, and the stream of blood, now uncontrolled, dilates them. The effect is best seen in the ear, the bloodvessels of which become manifestly larger than those of the opposite side ; while the part becomes redder and warmer from the increased quantity of blood circulating through it. On galvanizing the upper divided extremity of the nerve, the muscular fibres of the bloodvessels respond to the stimulus by again contracting, and the parts become paler, colder, and less sensitive than natural. The vaso-motor nerves arise directly from the sympathetic. Thus the bloodvessels of the head and neck are supplied by branches from the superior cervical ganglion, those of the thorax from the cervical and upper dorsal ganglia, those of the abdomen chiefly by the splanchnic nerves, and so forth. But it is now generally agreed, from the results of experi- ments by Ludwig and others, that the principal vaso-motor nerve-centre, with which all these nerves communicate, and by which their action is regulated, is situate in the medulla ob- longata or, in other words, that the vaso-motor fibres, aris- ing from this nerve-centre, pass down the spinal cord, and issuing by the anterior roots of the spinal nerves, enter the various ganglia on the prsevertebral cord of the sympathetic, and thence reach their destination, probably taking with them THE SYMPATHETIC NERVE. 453 fibres which arise in the ganglia through which they pass. The vaso-motor centre in the medulla appears to have a regu- lating power over the whole of the vaso-motor nerves ; but it seems likely that other secondary vaso-motor centres may exist in ganglia in different parts of the body, and may be the centres by which, under ordinary circumstances, vaso-motor changes are regulated in the territory in which they are placed. The vaso-motor nerve-centres are not only centres from which influences are directly transmitted to the bloodvessels, but, like other nerve-centres, may be the means by which impulses are reflected (p. 385). And reflex actions occur in connection with the muscular fibres of bloodvessels, as with those of the vol- untary muscles. Such reflected impressions may lead either to contraction or to dilatation of bloodvessels ; or, in other words, the action may be excito-vaso-motor, or vaso-inhibitory. The most remarkable instance at present known of a nerve, the stimulation of which leads by reflex action through the vaso-motor centre in the medulla oblongata, to dilatation of bloodvessels, is the depressor branch of the vagus (p. 442) ; but similar effects have been observed in a less degree, on stimu- lating other afferent spinal nerves. 1 It is, of course, very difficult to determine the relative share exercised by the true sympathetic and the ordinary cerebro- spinal fibres in the contraction of bloodvessels, and in the general processes of nutrition and secretion, since both kinds of fibres appear to be distributed to most parts, and there seems to be no possibility of isolating them. Probably the safest view of the question at present is, still to regard all the pro- cesses of organic life, in man, as liable to the combined influ- ences of the cerebro-spinal and the sympathetic systems ; to consider that those influences may be so combined as that the sympathetic nerves and ganglia may be in man, as in the lower animals, the parts through which the ordinary and constant influence of nervous force is exercised on the organic processes ; while the cerebro-spinal nervous centres and their ganglia are so closely connected with the proper sympathetic ganglia, that neither of them can be said to be independent of the other ; each, as a rule, and under ordinary circumstances, governing its own domain, but always liable to be influenced by the other. _, 1 For an admirable summary of what is at present known regard- ing the Innervation of the Heart and Bloodvessels, see Lectures by Dr. Eutherford, in the " Lancet," December 16th, 1871, and January 20th, 1872. 454 MOTION. CHAPTER XVII. CAUSES AND PHENOMENA OF MOTION. THE most evident vital motions observable in the bodies of animals, are performed in one or other of the following ways : first, by means of the oscillatory motion or vibration of micro- scopic cilia, with which the surfaces of certain membranes are beset ; and secondly, by the contraction of fibres which either have a longitudinal direction and are fixed at both extremities, or form circular bands ; the contraction or shortening of the fibres bringing the parts to which they are fixed nearer to each other. There are, besides, various molecular movements allied to those which need not here be considered. CILIARY MOTION. Ciliary motion consists in the incessant vibration of fine, pellucid, blunt processes, about ^^^ of an inch long, termed cilia (Figs. 153, 154), situated on the free extremities of the cells of epithelium covering certain surfaces of the body. The distribution and structure of ciliary epithelium and the microscopic appearances of cilia in motion have been already described (p. 37). Ciliary motion seems to be alike independent of the will, of the direct influence of the nervous system, and of muscular contraction, for it is involuntary ; there is no nervous or mus- cular tissue in the immediate neighborhood of the cilia, and it continues for several hours after death or removal from the body, provided the portion of tissue under examination be kept moist. Its independence of the nervous system is shown also in its occurrence in the lowest invertebrate animals appar- ently unprovided with anything analogous to a nervous sys- tem, in its persistence in animals killed by prussic acid, by narcotic or other poisons, and after the direct application of narcotics to the ciliary surface, or the discharge of a Leyden jar, or of a galvanic shock through it. The vapor of chloro- form arrests the motion ; but it is renewed on the discontinu- ance of the application (Lister). According to Kuhne, the movement ceases in an atmosphere deprived of oxygen, but is revived on the admission of this gas. Carbonic acid stops the CILIARY MOTION. 455 movement. The contact of various substances will stop the motion altogether ; but this seems to depend chiefly on destruc- tion of the delicate substance of which the cilia are composed. FIG. 153. FIG. 154. FIG. 153. Spheroidal ciliated cells from the mouth of the frog; magnified 300 diameters (Sharpey). FIG. 154. Columnar ciliated epithelium cells from the human nasal membrane ; magnified 300 diameters (Sharpey). Little or nothing is known with certainty regarding the na- ture of ciliary action. As Dr. Sharpey observes, however, it is a special manifestation of a similar property to that by which the other motions of animals are effected, namely, by what we term vital contractility. The fact of the more evident movements of the larger animals being effected by a structure apparently different from that of cilia, is no argument against such a supposition. For, if we consider the matter, it will be plain that our prejudices against admitting a relationship to exist between the two structures, muscles and cilia, rests on no definite ground ; and for the simple reason, that we know so little of the manner of production of movement in either case. The mere difference of structure is not an argument in point ; neither is the presence or absence of nerves. The movements of both muscles and cilia are manifestations of force, by cer- tain special structures, which we call respectively muscles and cilia. We know nothing more about the means by which the manifestation is effected by one of these structures than by the other ; and the mere fact that one has nerves and the other has not, is no more argument against cilia having what we call a vital power of contraction, than the presence or absence of stripes from voluntary or involuntary muscles respectively, is an argument for or against the contraction of one of them being vital and the other not so. Inasmuch then as cilia are found in living structures only, and inasmuch as they are a means whereby force is transformed (see Chap. II), their pe- culiar properties have as much right to be invested with the term vital as have those of muscular fibres. The term may be 456 MOTION. in both instances a bad one, it certainly is an unsatisfactory one, but it is as good for one case as the other. MUSCULAR MOTION. There are two chief kinds of muscular tissue, the striped, and the plain or unstriped, and they are distinguished by struc- tural peculiarities and mode of action. The striped form of muscular fibre is sometimes called voluntary muscle, because all muscles under the control of the will are constructed of it. The plain or unstriped variety is often termed involuntary, be- cause it alone is found in the greater number of the muscles over which the will has no power. The involuntary or unstriped muscles are made up, accord- ing to Kolliker, of elongated, spindle-shaped, nucleated fibre- cells (Fig. 155), which in their most perfect form are flat, from about ^^ to -3-3^0 of an inch broad, and about g ^ to 8 -Jn of an inch in length very clear, granular, and brittle, so that FIG. 155. FIG. 156. FIG. 155. Muscular fibre-cells from human arteries, magnified 350 diameters (Kolliker). a, natural state; b, treated with acetic acid. FIG. 156. Plain muscular fibres from the human bladder, magnified 250 diameters, a, in their natural state ; 6, treated with acetic acid to show the nuclei. when they break, they often have abruptly rounded or square extremities. Each fibre-cell possesses an elongated nucleus, and many are marked along the middle, or, more rarely, along MUSCULAR MOTION. 457 one of the edges, either by a fine continuous dark streak, or by short isolated dark lines, or by dark points arranged in a row, or scattered. These fibre-cells, by their union, form fibres and bundles of fibres (Fig. 156). The fibres have no distinct sheath. The fibres of involuntary muscle, such as are here described, form the proper muscular coats of the digestive canal from the middle of the O3sophagus to the internal sphincter ani, of the ureters and urinary bladder, the trachea and bronchi, the ducts of glands, the gall-bladder, the vesiculaB seminales, the pregnant uterus, of bloodvessels and lymphatics, the iris, and some other parts. This form of tissue also enters largely into the composition of the tunica dartos, and is the principal cause of the wrin- kling and contraction of the scrotum on exposure to cold. The fibres of the cremaster assist in some measure in producing this effect, but they are chiefly concerned in drawing up the FIG. 157. Perpendicular section through the scalp, with two hair-sacs; a, epidermis; 6, cutis ; c, muscles of the hair-follicles (after Kolliker). testis and its coverings towards the inguinal opening. Un- striped muscular tissue occurs largely also in the cutis (p. 334), being especially abundant at the interspaces between the bases of the papillae. Hence, when it contracts under the influence of cold, fear, electricity, or any other stimulus, the papillae are made unusually prominent, and give rise to the peculiar roughness of the skin termed cutis anserina, or goose-skin. It occurs also in the superficial portion of the cutis, in all parts where hairs occur, in the form of flattened roundish bundles, which lie alongside the hair-follicles and sebaceous glands. They pass obliquely from without inwards, embrace the seba- ceous glands, and are attached to the hair-follicles near their base (Fig. 157). 458 MOTION. To this kind of muscular fibre the term organic is often ap- plied, from the fact that it enters especially into the construc- tion of such parts as are concerned in what has been called organic life (see note, p. 368). The muscles of animal life, or striped muscles, include the whole class of voluntary muscles, the heart, and those muscles neither completely volun- FlG - 158 - tary nor involuntary, which form part of the walls of the pharynx, and exist in many other parts of the body, as the internal ear, urethra, &c. All these muscles are composed of fleshy bundles called fasciculi, inclosed in coverings of fibro-cellular tissue, by which each is at once connected with, and A small portion of muscle natural sue, igolated from thoge adjacent consisting of larger and smaller fasciculi, /-n. -txo\ 17 r, seen in a transverse section, and the same to lt V* 1 *''. .''. . C magnified 5 diameters (after Sharpey). bundle is again divided into smaller ones, similarly en- sheathed and similarly divisible ; and so on, through an un- certain number of gradations, till one arrives at the primitive fasciculi, or the muscular fibres peculiarly so called. Muscular fibres consist, each of them, of a tube or sheath of delicate, structureless membrane, called the sarcolemma, inclosing a number of filaments or fibrils. They are cylindri- form or prismatic, with five or more sides, according to the manner in which they are compressed by adjacent fibres. Their average diameter is about -5^ of an inch, and their length never exceeds an inch and a half. Each muscular fibre is thus constructed : Externally is a fine, transparent, structureless membrane, called the sarco- lemma, which in the form of a tubular investing sheath forms the outer wall of the fibre, and is filled by the contractile material of which the fibre is chiefly made up. Sometimes, from its comparative toughness, the sarcolemma will remain untorn, when by extension the contained part can be broken (Fig. 159), and its presence is in this way best demonstrated. The fibres, which are cylindriform or prismatic, with an aver- age diameter of about ^Q of an inch, are of a pale yellow color, and apparently marked by fine striae, which pass trans- versely round them, in slightly curved or wholly parallel lines. Other, but generally more obscure striae, also pass lon- gitudinally over the tubes, and indicate the direction of the STRUCTURE OF STRIPED MUSCLE. 459 filaments or primitive fibrils of which the substance of each fibre is composed (Fig. 160). The whole substance of the fibre contained within the sarco- lemma may be thus supposed to be constructed of longitudinal FIG. 159. FIG. 160. FIG. 159. Muscular fibre torn across ; the sarcolemma still connecting the two parts of the fibre (after Todd and Bowman). FIG. 160. A few muscular fibres, being part of a small fasciculus, highly magni- fied, showing the transverse striae, a, end view of b, b, fibres ; c, a fibre split into its fibrils (after Sharpey). fibrils a- bundle of fibrils surrounded by the sarcolemma con- stituting a fibre. There is still some doubt regarding the nature of the fibrils. Each of them appears to be composed of a single row of minute dark quadrangular particles called sarcous elements, which are separated from each other by a bright space formed of a pel- lucid substance continuous with them. A fine streak can be sometimes discerned passing across the bright interval between the sarcous elements. Dr. Sharpey believes that, even in a fibril so constituted, the ultimate anatomical element of the fibre has not been isolated. He believes that each fibril with quadrangular sarcous elements is composed of a number of other fibrils still finer, so that the sarcous element of an ulti- mate fibril would be not quadrangular but as a streak, and the dark transverse streak on the bright space but a row of dots. In either case the appearance of striation in the whole fibre would be produced by the arrangement, side by side, of the dark and light portions respectively of the fibrils (Fig. 161). 460 FIG. 162. A. Portion of a medium-sized human muscular fibre, magnified nearly 800 diam- eters. B. Separated bundles offilsrils equally magnified ; a, a, larger, and 6, 6, smaller collections ; c, still smaller ; d, d, the smallest which could be detached, possibly rep- resenting a single series of sarcous elements (after Sharpey). Although each muscular fibre may be considered to be formed of a number of longitudinal fibrils, ar- ranged side by side, it is also true that they are not naturally separate from each other, there being lateral cohesion, if not fusion, of each sarcous element with those around and in contact with it ; so that it happens that there is a tendency for a fibre to split, not only into separate fibrils, but also occa- sionally into plates or disks, each of which is composed of sarcous elements laterally adherent one to another. The muscular fibres of the heart, al- though striped and resembling closely those of the voluntary muscles in their Muscular fibres from general structure, present these distinctions : the heart magnified, rp, fi d f j ^ striated showing their cross- ', J .,, stria;, divisions, and tne 7 branch and anastomose one with an- junctions (from KoiH- other, and no sarcolemma can be usually ker). discerned (Fig. 162). PROPERTIES OF MUSCULAR TISSUE. 461 The voluntary muscles are freely supplied with bloodves- sels ; the capillaries form a network with oblong meshes around the fibres on the outside of the sarcolemma. No vessels pene- trate the sarcolemma to enter the interior of the fibre. Nerves also are supplied freely to muscles; the voluntary muscles receiving chiefly nerves from the cerebro-spinal system, and the unstriped muscles from the sympathetic or ganglionic system. Properties of Muscular Tissue. The property of muscular tissue, by which its peculiar functions are exercised, is its contractility, which, in the con- traction or shortening of muscle, is excited by all kinds of stimuli, applied either directly to the muscles, or indirectly to them through the medium of their motor nerves. This prop- erty, although commonly brought into action through the nervous system, is inherent in the muscular tissue. For 1st, it may be manifested in a muscle which is isolated from the influence of the nervous system by division of the nerves sup- plying it, so long as the natural tissue of the muscle is duly nourished; and 2dly, it is manifest in a portion of muscular fibre, in which, under the microscope, no nerve-fibre can be traced. If the removal of nervous influence be long continued, as by division of the nerve supplying a muscle, or in cases of paral- ysis of long standing, the irritability, i. e., the power of both perceiving and responding to a stimulus, may be lost ; but probably this is chiefly due to the impaired nutrition of the muscular tissue, which ensues through its inaction (J. Reid). The irritability of muscles is also of course soon lost, unless a supply of arterial blood to them is kept up. Thus, after liga- ture of the main arterial trunk of a limb, the power of moving the muscles is partially or wholly lost, until the collateral cir- culation is established; and when, in animals, the abdominal aorta is tied, the hind legs are rendered almost powerless (Se- galas). So, also, it is to the imperfect supply of arterial blood to tile muscular tissue of the heart, that the cessation of the action of this organ in asphyxia is in some measure due (p. 189). ( Besides the property of contractility, the muscles, especially the striated or those of animal life, possess sensibility by means of the sensitive nerve-fibres distributed to them. The amount of common sensibility in muscles is not great; for they may be cut or pricked without giving rise to severe pain, at least in their healthy condition. But they have a peculiar sensibility, or at least a peculiar modification of common sensibility, which 39 462 MOTION. is shown in that their nerves can communicate to the mind an accurate knowledge of their states and positions when in action. By this sensibility, we are not only made conscious of the mor- bid sensations of fatigue and cramp in muscles, but acquire, through muscular action, a knowledge of the distance of bodies and their relation to each other, and are enabled to estimate and compare their weight and resistance by the effort of which we are conscious in measuring, moving, or raising them. Ex- cept with such knowledge of the position and state of each muscle, we could not tell how or when to move it for any re- quired action ; nor without such a sensation of effort could we maintain the muscles in contraction for any prolonged exer- tion. The mode of contraction in the transversely striated muscular tissue, has been much disputed. The most probable account, which has been especially illustrated by Mr. Bowman, is that the contraction is effected by an approximation of the constitu- ent parts of the fibrils, which, at the instant of contraction, without any alteration in their general direction, become closer, flatter, and wider ; a condition which is rendered evident by the approximation of the transverse striae seen on the surface of the fasciculus, and by its increased breadth and thickness. The appearance of the zigzag lines into which it was supposed the fibres are thrown in contraction, is due to the relaxation of a fibre which has been recently contracted, and is not at once stretched again by some antagonist fibre, or whose extremities are kept close together by the contractions of other fibres. The contraction is therefore a simple, and according to Ed. Weber, a uniform, simultaneous, and steady shortening of each fibre and its contents. What each fibril or fibre loses in length, it gains in thickness: the contraction is a change of form, not of size ; it is, therefore, not attended with any dimi- nution in bulk, from condensation of the tissue. This has been proved for entire muscles, by making a mass of muscle, or many fibres together, contract in a vessel full of water, with which a fine, perpendicular, graduated tube communicates. Any diminution of the bulk of the contracting muscle would be attended by a fall of fluid in the tube ; but when the ex- periment is carefully performed, the level of the water in the tube remains the same, whether the muscle be contracted or not. 1 In thus shortening, muscles appear to swell up, becoming 1 Edward Weber, however, states that a very slight diminution does take place in the bulk of a contracting muscle ; but it is so slight as to be practical^ of no moment. SOUND OF MUSCULAR CONTRACTION. 46& rounder, more prominent, harder, and apparently tougher. But this hardness of muscle in the state of contraction, is not due to increased firmness or condensation of the muscular tissue, but to the increased tension to which the fibres, as well as their tendons and other tissues, are subjected from the re- sistance ordinarily opposed to their contraction. When no resistance is offered, as when a muscle is cut off from its ten- don, not only is no hardness perceived during contraction, but the muscular tissue is even softer, more extensile, and less elastic than in its ordinary uncontracted state (Ed. Weber). Heat is developed in the contraction of muscles. Becquerel and Breschet found, with the thermo-multiplier, about 1 of heat produced by each forcible contraction of a man's biceps ; and when the actions were long continued, the temperature of the muscle increased 2. It is not known whether this devel- opment of heat is due to chemical changes ensuing in the muscle, or to the friction of its fibres vigorously acting : in either case, we may refer to it a part of the heat developed in active exercise (p. 190). And Nasse suspects that to it is due the higher temperature of the blood in the left ventricle ; for he says that this fluid is always warmer in the left ventricle than in the left auricle, and that the blood in the latter is but little warmer than that on the right side of the heart. But these experiments need confirmation. Sound is said to be produced when muscles contract forcibly. Dr. Wollaston showed that this sound might be easily heard by placing the tip of the little finger in the ear, and then making some muscles contract, as those of the ball of the thumb, whose sound may be conducted to the ear through the substance of the hand and finger. A low shaking or rum- bling sound is heard, the height and loudness of the note being in direct proportion to the force and quickness of the muscular action, and to the number of fibres that act together, or, as it were, in time. The two kinds of fibres, the striped and unstriped, have characteristic differences in the mode in which they act on the application of the same stimulus ; differences which may be ascribed in great part to their respective differences of struc- ture, but to some degree possibly, to their respective modes of connection with the nervous system. When irritation is ap- plied directly to a muscle with striated fibres, or to the motor nerve supplying it, contraction of the part irritated, and of that only, ensues ; and this contraction is instantaneous, and ceases on the instant of withdrawing the irritation. But when any part with unstriped muscular fibres, e. g., the intes- tines or bladder, is irritated, the subsequent contraction ensues 464 MOTION. more slowly, extends beyond the part irritated, and with alter- nating relaxation, continues for some time after the withdrawal of the irritation. Ed. Weber particularly illustrated the dif- ference in the modes of contraction of the two kinds of mus- cular fibres by the effects of the electro-magnetic stimulus. The rapidly succeeding shocks given by this means to the nerves of muscles excite in all the transversely-striated muscles a fixed state of tetanic contraction, which lasts as long as the stimulus is continued, and on its withdrawal instantly ceases : but in the muscles with smooth fibres they excite, if any move- ment, only one that ensues slowly, is comparatively slight, alternates with rest, and continues for a time after the stim- ulus is withdrawn. In their mode of responding to these stimuli, all the volun- tary muscles, or those with transverse striae, are alike; but among those with plain or unstriped fibres there are many dif- ferences a fact which tends to confirm the opinion that their peculiarity depends as well on their connection with nerves and ganglia as on their own properties. According to Weber, the ureters and gall-bladder are the parts least excited by stimuli ; they do not act at all till the stimulus has been long applied, aud then contract feebly, and to a small extent. The contractions of the caecum and stomach are quicker and wider- spread ; still quicker those of the iris, and of the urinary bladder, if it be not too full. The actions of the small and large intestines, of the vas deferens, and pregnant uterus, are yet more vivid, more regular, and more sustained ; and they require no more stimulus than that of the air to excite them. The heart is the quickest and most vigorous of all the muscles of organic life in contracting upon irritation, and appears in this as in nearly all other respects, to be the connecting mem- ber of the two classes of muscles. All the muscles retain their property of contracting under the influence of stimuli applied to them or to their nerves for some time after death, the period being longer in cold-blooded than in warm-blooded Vertebrata, and shorter in birds than in Mammalia. It would seem as if the more active the respi- ratory process in the living animal, the shorter is the time of duration of the irritability in the muscles after death ; and this is confirmed by the comparison of different species in the same order of Vertebrata. But the period during which this irritability lasts, is not the same in all persons, nor in all the muscles of the same persons. In a man it ceases, according to Nysten, in the following order : First in the left ventricle, then in the intestines and stomach, the urinary bladder, right ventricle, oesophagus, iris ; then in the voluntary muscles of the EIGOR MORTIS. 465 trunk, lower and upper extremities ; lastly in the right and left auricle of the heart. After the muscles of the dead body have lost their irrita- bility or capability of being excited to contraction by the ap- plication of a stimulus, they spontaneously pass into a state of contraction, apparently identical with that which ensues during life. 1 It affects all the muscles of the body ; and, where ex- ternal circumstances do not prevent it, commonly fixes the limbs in that which is their natural posture of equilibrium or rest. Hence, and from the simultaneous contraction of all the muscles of the trunk, is produced a general stiffening of the body, constituting the rigor mortis or post-mortem rigidity} The muscles are not affected exactly simultaneously by the post-mortem contraction, but rather in succession. It affects the neck and lower jaw first ; next, the upper extremities, ex- tending from above downwards ; and lastly, reaches the lower limbs ; in some rare instances only, it affects the lower ex- tremities before, or simultaneously with, the upper extremities. It usually ceases in the order in which it began ; first at the head, then in the upper extremities, and lastly in the lower extremities. According to Sommer, it never commences earlier than ten minutes, and never later than seven hours, after death ; and its duration is greater in proportion to the lateness of its accession. According to Schiffer, and others have confirmed the truth of his observation, heat is developed during the passage of a muscular fibre into the condition of rigor mortis. Since rigidity does not ensue until muscles have lost the ca- pacity of being excited by external stimuli, it follows that all circumstances which cause a speedy exhaustion of muscular irritability, induce an early occurrence of the rigidity, while conditions by which the disappearance of the irritability is delayed, are succeeded by a tardy onset of this rigidity. Hence its speedy occurrence, and equally speedy departure in 1 If, however, arterial blood be made to circulate through the body or through a limb, the post-mortem contraction of the muscles thus supplied with blood, may, as Dr. Brown-Sequard has shown, be suspended, and the muscles again admit of contracting on the appli- cation of a stimulus. 2 It should be stated here, however, that the generally accepted ex- planation of the state of the muscles during rigor mortis, namely, that it is due to contraction of the fibres, as in strong action during life, is denied by some physiologists, who maintain that the condition of the muscles is not due to contraction at all, but is caused by a kind of coagulation of the interfibrillar juices This idea has been of late especially supported by Dr. Norris (see Camb. Journal of Anat- omy and Physiology, Part I). 466 MOTION. the bodies of persons exhausted by chronic diseases ; and its tardy onset and long continuance after sudden death from acute diseases. In some cases of sudden death from lightning, violent injuries, or paroxysms of passion, rigor mortis has been said not to occur at all ; but this is not always the case. It may, indeed, be doubted whether there is really a complete absence of the post-mortem rigidity in any such cases ; for the experiments of M. Brown-Sequard with electro-magnetism make it probable that the rigidity may supervene immediately after death, and then pass away with such rapidity as to be scarcely observable. Thus, he took five rabbits, and killed them by removing their hearts. In the first, rigidity came on in ten hours, and lasted 192 hours ; in the second, which was feebly electrified, it commenced in seven hours, and lasted 144; in the third, which was more strongly electrified, it came on in two, and lasted 72 hours ; in the fourth, which was still more strongly electrified, it came on in one hour, and lasted 20 ; while, in the last rabbit, which was submitted to a powerful electro-galvanic current, the rigidity ensued in seven minutes after death, and passed away in 25 minutes. From this it appears that the more powerful the electric current, the sooner does the rigidity ensue, and the shorter is its duration ; and as the lightning shock is so much more powerful than any ordi- nary electric discharge, the rigidity may ensue so early after death and pass away so rapidly as to escape detection. The influence exercised upon the onset and duration of post-mortem rigidity by causes which exhaust the irritability of the mus- cles, was well illustrated in further experiments by the same physiologist, in which he found that the rigor mortis ensued far more rapidly and lasted for a shorter period in those mus- cles which had been powerfully electrified just before death than in those which had not been thus acted upon. The occurrence of rigor mortis is not prevented by the pre- vious existence of paralysis in a part, provided the paralysis has not been attended with very imperfect nutrition of the muscular tissue. The rigidity affects the involuntary as well as the voluntary muscles, whether they be constructed of striped or unstriped fibres. The rigidity of involuntary muscles with striped fibres is shown in the contraction of the heart after death. The con- traction of the muscles with unstriped fibres is shown by an experiment of Valentin, who found that if a graduated tube connected with a portion of intestine taken from a recently- slain animal, be filled with water, and tied at the opposite end, the water will in a few hours rise to a considerable height in the tube, owing to the contraction of the intestinal walls. ACTIONS OF VOLUNTAEY MUSCLES. 467 It is still better shown in the arteries, of which all that have muscular coats contract after death, and thus present the roundness and cord-like feel of the arteries of a limb lately re- moved, or those of a body recently dead. Subsequently they relax, as do all the other muscles, and feel lax and flabby, and lie as if flattened, and with their walls nearly in contact. 1 Actions of the Voluntary Muscles. The greater part of the voluntary muscles of the body act as sources of power for moving levers, the latter consisting of the various bones to which the muscles are attached. All levers have been divided into three kinds, according to the relative position of the power, the weight to be moved, and the axis of motion or fulcrum. In a lever of the first kind the power is at one extremity of the lever, the weight at the other, and the fulcrum between the two. If the initial letters only of the power, weight, and fulcrum be used, the arrangement will stand thus: P.F.W. A poker, as ordinarily used, or the bar in Fig. 164, may be cited as an example of this variety of lever ; while as an instance in which the bones of the human skeleton are used as a lever of the same kind, may be men- tioned the act of raising the body from the stooping posture by means of the hamstring muscles attached to the tuberosity of the ischium (Fig. 163). In a lever of the second kind, the arrangement is thus : P.W.F. ; and this leverage is employed in the act of raising the handles of a wheelbarrow, or in stretching an elastic band, as in Fig. 164. In the human body the act of opening the 1 Although the preceding remarks represent the views generally entertained in regard to muscular action, yet it must be observed that a new and very different theory on the subject has been lately advanced by several writers, and especially developed by Dr. Rad- cliffe, who has also made it the basis of new views on the pathology of various convulsive affections. According to this doctrine, the ordi- nary relaxed or elongated state of a muscle is due to a certain " state of polarity" in which the muscle is maintained, and contraction is brought about by anything (such as an effort of the will) which lib- erates the muscle from this influence, and thus leaves it to the opera- tion of the attractive force inherent in the muscular molecules. Ac- cording to this doctrine, also, the stage of rigor mortis is readily explicable : death depriving the muscles of the " state of polarity'" whereby they had hitherto been kept relaxed, and thus allowing the attractive force of the muscular particles to come into play. For facts and arguments in support of this view, and for references and confirmatory opinions, Dr. Radcliffe's work on epileptic and other convulsive affections may be consulted. 468 M O T I O N. mouth by depressing the lower jaw, is an example of the same kind, the tension of the muscles which close the jaw repre- senting the weight (Fig. 164). In a lever of the third kind the arrangement is F.P.W., and the act of raising a pole, as in Fig. 165, is an example. In the human body there are numerous examples of the em- ployment of this kind of leverage. The act of bending the forearm may be mentioned as an instance (Fig. 165). In the human body, levers are most frequently used at a disadvantage as regards power, the latter being sacrificed for the sake of a greater range of motion. Thus in the diagrams of the first and third kinds it is evident that the power is so close to the fulcrum, that great force must be exercised in order VARIETIES OF LEVERS. 469 to produce motion. It is also evident, however, from the same diagrams, that by the closeness of the power to the fulcrum a great range of movement can be obtained by means of a com- paratively slight shortening of the muscular fibres. FIG. 165. The greater number of the more important muscular actions of the human body those, namely, which are arranged har- moniously so as to subserve some definite purpose or other in the animal economy are described in various parts of this work, in the sections which treat of the physiology of the pro- cesses by which these muscular actions are resisted or carried out. The combined action of the respiratory muscles, for in- stance, will be found described in the chapter on " Respira- tion ;" the action of the heart and bloodvessels, under the head of " Circulation ;" while the movements of the stomach and intestines are too intimately associated with the function of " Digestion," to be described apart from it. There are, however, one or two very important and somewhat complicated muscular acts which may be best described in this place. Walking. In the act of walking, almost every voluntary muscle in the body is brought into play, either directly for purposes of progression, or indirectly for the proper balancing of the head and trunk. The muscles of the arms are least concerned ; but even these are for the most part instinctively in action also to some extent. Among the chief muscles engaged directly in the act of walking are those of the calf, which, by pulling up the heel, pull up also the astragalus, and with it, of course, the whole body, the weight of which is transmitted through the tibia to this bone (Fig. 166). When starting to walk, say with the left leg, this raising of the body is not left entirely to the muscles of the left calf, but the trunk is thrown forward in 40 470 MOTION. such a way that it would fall prostrate were it Dot that the right foot is brought forward and planted on the ground to support it. Thus the muscles of the left calf are assisted in their action by those muscles on the front of the trunk and legs which, by their contraction, pull the body forwards ; and of course, if the trunk form a slanting line, with the inclina- FlG. 166. tion forwards, it is plain that when the heel is raised by the calf-muscles, the whole body will be raised, and pushed ob- liquely forwards and upwards. The successive acts in taking the first step in walking are represented in Fig. 166, 1, 2, 3. Now it is evident that by the time the body has assumed the position No. 3, it is time that the right leg should be brought forward to support it and prevent it from falling pros- trate. This advance of the other leg (in this case the righfy is effected partly by its mechanically swinging forwards, pen- dulum-wise, and partly by muscular action ; the muscles used being, 1st, those on the front of the thigh, which bend the thigh forwards on the pelvis, especially the rectus femoris, with the psoas and the iliacus ; 2dly, the hamstring muscles, which slightly bend the leg on the thigh ; and 3dly, the muscles on the front of the leg, which raise the front of the foot and toes, and so prevent the latter in swinging forwards from hitching in the ground. Anybody who has attentively watched the helpless flapping action of the foot and leg in cases of partial paralysis affecting the muscles of the leg, or who will, in his own case, note the act of bringing the leg forward in walking, will be convinced of the large share which the muscles take in the act in question ; although, of course, their work is ren- dered much easier by the pendulum-like swinging forward of the leg by its own weight. The second part of the act of walking, which has been just described, is shown in the diagram (4, Fig. 166). When the right foot has reached the ground the action of WALKING. 471 the left leg has not ceased. The calf-muscles of the latter con- tinue to act, and by pulling up the heel, throw the body still more forwards over the right leg, now bearing nearly the whole weight, until it is time that in its turn the left leg should swing forwards, and the left foot be planted on the ground to prevent the body from falling prostrate. As at first, while the calf- muscles of one leg and foot are preparing, so to speak, to push the body forward and upward from behind by raising the heel, the muscles on the/r0?^of the trunk and of the same leg (and of the other leg, except when it is swinging forwards) are helping the act by pulling the legs and trunk, so as to make them incline forward, the rotation in the inclining forwards being effected mainly at the ankle-joint. Two main kinds of leverage are, therefore, employed in the act of walking, and if this idea be firmly grasped, the detail will be understood with comparative ease. One kind of leverage employed in walking is essentially the same with that employed in pulling forward the pole, as" in Fig. 165. And the other, less exactly, is that employed in raising the handles of a wheelbarrow. Now, sup- posing the lower end of the pole to be placed in the barrow, we should have a very rough and inelegant, but not altogether bad representation of the two main levers employed in the act of walking. The body is pulled forward by the muscles in front, much in the same way that the pole might be by the force applied at P, Fig. 165, while the raising of the heel and pushing forwards of the trunk by the calf-muscles is roughly represented on raising the handles of the barrow. The man- ner in which these actions are performed alternately by each leg, so that one after the other is swung forwards to support the trunk, which is at the same time pushed and pulled for- wards by the muscles of the other, may be gathered from the previous description. There is one more thing to be noticed especially in the act of walking. Inasmuch as the body is being constantly sup- ported and balanced on each leg alternately, and therefore on only one at the same moment, it is evident that there must be some provision made for throwing the centre of gravity over the line of support formed by the bones of each leg, as, in its turn, it supports the weight of the body. This may be done in various ways, and the manner in which it is effected is one element in the differences which exist in the walking of differ- ent people. Thus it may be done by an instinctive slight ro- tation of the pelvis on the head of each femur in turn, in such a manner that the centre of gravity of the body shall fall over the foot of this side. Thus when the body is pushed onwards and upwards by the raising, say, of the right heel, as in Fig. 472 MOTION. 166, 3, the pelvis is instinctively, by various muscles, made to rotate on the head of the left fernur at the acetabulum, to the left side, so that the weight may fall over the line of support formed by the left leg at the time that the right leg is swing- ing forwards, and leaving all the work of support to fall on its fellow. Such a " rocking " movement of the trunk and pelvis, however, is but an awkward manner of doing what can be done more gracefully by combining a slight " rocking" with a movement of the whole trunk and leg over the foot which is being planted on the ground (Fig. 167); the action being FIG. 167. accompanied with a compensatory outward movement at the hip, more easily appreciated by looking at the figure (167) than described. Thus the body in walking is continually rising and swaying alternately from one side to the other, as its centre of gravity has to be brought alternately over one or other leg ; and the curvatures of the spine are altered in correspondence with the varying position of the weight which it has to support. The extent to which the body is raised or swayed differs much in different people. In walking, one foot or the other is always on the ground. The act of leaping, or jumping, consists in so sudden a raising of the heels by the sharp and strong contraction of the calf- SOURCE OF MUSCULAR ACTION. 473 muscles, that the body is jerked off the ground. At the same time the effect is much increased by first bending the thighs on the pelvis, and the legs on the thighs, and then suddenly straightening out the angles thus formed. The share which this action has in producing the effect may be easily known by attempting to leap in the upright posture, with the legs quite straight. Running is performed by a series of rapid low jumps with each leg alternately ; so that, during each complete muscular act concerned, there is a moment when both feet are off the ground. In all these cases, however, the description of the manner in which any given effect is produced, can give but a very im- perfect idea of the infinite number of combined and harmoni- ously arranged muscular contractions which are necessary for even the simplest acts of locomotion. Actions of the Involuntary Muscles. The involuntary mus- cles are for the most part not attached to bones arranged to act as levers, but enter into the formation of such hollow parts as require a diminution of their calibre by muscular action, under particular circumstances. Examples of this action are to be found in the intestines, urinary bladder, heart and blood- vessels, gall-bladder, gland-ducts, &c. The difference in the manner of contraction of the striated and non-striated fibres has been already referred to (p. 463) ; and the peculiar vermicular or peristaltic action of the latter fibres in some regions of the body has been described at p. 276. Source of Muscular Action. It was formerly supposed that each act of contraction on the part of a muscle was accompanied by a correlative waste or destruction of its own substance ; and that the quantity of the nitrogenous excreta, especially of urea, presumably the ex- pression of this waste, was in exact proportion to the amount of muscular work performed. It has been found, however, both that the theory itself is erroneous, and that the supposed facts on which it was founded do not exist. It is true that in the action of muscles, as of all other parts, there is a certain destruction of tissue or, in other words, a certain "wear and tear," which may be represented by a slight increase in the quantity of urea excreted : but it is not the cor relative expression or only source of the power manifested. The increase in the amount of urea which is excreted after muscular exertion is by no means so great as was formerly supposed ; indeed, it is very slight. And as there is no reason 474 VOICE AND SPEECH. to believe that the waste of muscle-substance can be expressed, with unimportant exceptions, in any other way than by an increased excretion of urea, it is evident that we must look elsewhere than in destruction of muscle, for the source of muscular action. For, it need scarcely be said, all force manifested in the living body must be the correlative expres- sion of force previously latent in the food eaten or the tissue formed ; and evidences of force expended in the body must be found in the excreta. If, therefore, the nitrogenous excreta, represented chiefly by urea, are not in sufficient quantity to account for the work done, we must look to the non-nitrogenous excreta as carbonic acid and water, which presumably, cannot be the expression of wasted muscle-substance. The quantity of these non-nitrogenous excreta is undoubtedly increased by active muscular efforts, and to a considerable extent ; and whatever may be the source of the water, the car- bonic acid, at least, is the result of chemical action in the system, and especially of the combustion of non-nitrogenous food, although, doubtless, of nitrogenous food also. We are, therefore, driven to the conclusion, that the substance of muscles is not wasted in proportion to the work they perform ; and that the non-nitrogenous as well as the nitrogenous foods may, in their combustion, afford the requisite conditions for muscular action. The urgent necessity for nitrogenous food, especially after exercise, is probably due more to the need of nutrition by the exhausted muscles and other tissues for which, of course, nitrogen is essential, than to such food being superior to non-nitrogenous substances as a source of muscular power. CHAPTER XVIII. OF VOICE AND SPEECH. IN nearly all air-breathing vertebrate animals there are arrangements for the production of sound, or voice, in some part of the respiratory apparatus. In many animals the sound admits of being variously modified and altered during and after its . production ; and, in man, one of the results of such modification is speech. Mode of Production of the Human Voice. It has been proved by observations on living subjects, by VOICE AND SPEECH. 475 means of the laryngoscope, as well as by experiments on the larynx taken from the dead body, that the sound of the human voice is the result of the inferior laryngeal ligaments, or true vocal cords (A, cv, Fig. 172) which bound the glottis, being FIG. 168. Outline showing the general form of the larynx, trachea, and bronchi, as seen from before. %. h, the great cornu of the hyoid bone ; e, epiglottis ; t, superior, and /', inferior cornu of the thyroid cartilage ; c, middle of the cricoid cartilage ; tr, the trachea, showing sixteen cartilaginous rings ; ft, the right, and b', the left bronchus. thrown into vibration by currents of expired air impelled over their edges. Thus, if a free opening exists in the trachea, the 476 VOICE AND SPEECH. sound of the voice ceases, but returns on the opening being closed. An opening into the air-passages above the glottis, on the contrary, does not prevent the voice being formed. Injury of the laryngeal nerves supplying the muscles which move the vocal cords puts an end to the formation of vocal sounds ; and when these nerves are divided on both sides, the loss of voice is complete. Moreover, by forcing a current of air through the larynx in the dead subject, clear vocal sounds are pro- duced, though the epiglottis, the upper ligaments of the larynx or false vocal cords, the ventricles between them, and the inferior ligaments or true vocal cords, and the upper part of the arytenoid cartilages, be all removed ; provided the true vocal cords remain entire, with their points of attachment, and be kept tense and so approximated that the fissure of the glot- tis may be narrow. The vocal ligaments or cord, therefore, may be regarded as the proper organs of the mere voice : the modifications of the voice are effected by other parts as well as by them. Their structure is adapted to enable them to vibrate like tense mem- branes, for they are essentially composed of elastic tissue ; and they are so attached to the cartilaginous parts of the larynx that their position and tension can be variously altered by the contraction of the muscles which act on these parts. The Larynx. The larynx, or organ of voice, consists essentially of two elastic lips called the vocal cords, which are so attached to certain cartilages, and so under the control of certain muscles, that they can be made the means not only of closing the larynx against the entrance and exit of air to or from the lungs, but also can be stretched or relaxed, shortened or lengthened, in accordance with the conditions that may be necessary for the air in passing over them, to set them vibrating and produce various sounds. Their action in respiration has been already referred to (p. 166), in connection with ordinary tranquil res- piration, and also (p. 182, et seq.) with other respiratory acts, in which the opening or closing of the glottis, or, in other words, the close apposition or separation of the vocal cords, is an essential part of the performance. In these respiratory acts, however, any sound that may be produced, as in cough- ing, is, so to speak, an accident, and not performed with pur- pose. In the present chapter the sound produced by the vibra- tion of the vocal cords is the only part of their function with which we have to deal. It will be well, perhaps, to refer to a few points in the auat- THE LARYNX. 477 omy of the larynx, before considering its physiology in con- nection with voice and speech. The principal parts entering into the formation of the larynx (Figs. 169 and 170) are (t) the thyroid cartilage; (c) the FIG. 169. Outline showing the general form of the larynx, trachea, and bronchi as seen from behind. %. h, great cornu of the hyoid bone ; t, superior, and t', the inferior cornu of the thyroid cartilage ; e, the epiglottis ; a, points to the back of both the arytenoid cartilages, which are surmounted by the cornicula ; c, the middle ridge on the back of the cricoid cartilage ; tr, the posterior membranous part of the trachea ; b, b', right and left bronchi. 478 VOICE AND SPEECH. cricoid cartilage ; (a) the two arytenoid cartilages ; and the two true vocal cords (A, cv, Fig. 172). The epiglottis (Fig. 170, e) has but little to do with the voice, and is chiefly useful in falling down as a " lid " over the upper part of the larynx, to prevent the entrance of food and drink in deglutition. The false vocal cords (cvs, Fig. 172), and the ventricle of the larynx, which is a space between the false and the true cord of either side, need be here only referred to. The thyroid cartilage (Fig. 170, 1 to 4) does not form a complete ring around the larynx, but only covers the front portion. The cricoid cartilage (Fig. 170, 5, 6), on the other hand, is a complete ring ; the back part of the ring being much broader than the front. On the top of this broad portion of the cricoid are the arytenoid cartilages (Fig. 169, a) the con- nection between the cricoid below and arytenoid cartilages above being a joint with sy no vial membrane and ligaments, the latter permitting tolerably free motion between them. But, although the arytenoid carti- FlG 170 lages can move on the cricoid, they of course accompany the latter in all their movements, just as the head may nod or turn on the top of the spinal column, but must ac- company it in all its movements as a whole. The thyroid cartilage is also con- nected with the cricoid, not only by ligaments, but by two joints with synovial membrane (if, Figs. 168 and 169) ; the lower cortma of the thyroid clasping, or nipping, as it were, the cricoid between them, but not so tightly but that the thyroid can revolve, within a certain range, around an axis passing transversely through the two joints at which the cricoid is clasped. The vocal cords are attached (behind) to the front portion of the base of the arytenoid cartilages, and (in front) to the re- entering angle at the back part of the thyroid; it is evident, therefore, that all movements of either of these cartilages must produce an effect on them of some kind or other. Inasmuch, too, as the arytenoid car- Cartilages of the larynx seen from before. %. 1 to 4, thyroid cartilage ; 1, vertical ridge or po- mum Adami ; 2, right ala ; 3, superior, and 4, inferior cornu of the right side ; 5, 6, cricoid carti- lage; 5, inside of the posterior part; 6, anterior narrow part of the ring ; 7, arytenoid cartilages. THE LARYNX. 479 FIG. 171. tilages rest on the top of the back portion of the cricoid car- tilage (a, Fig. 169), and are connected with it by capsular and other ligaments, all movements of the cricoid cartilage must move the arytenoid cartilages, and also produce an effect on the vocal cords. The so-called intrinsic muscles of the larynx, or those which, in their action, have a direct action on the vocal cords, are nine in number four pairs, and a single muscle; namely, two crico-thyroid muscles, two thyro-arytenoid, two posterior crico- arytenoid, two lateral crico-arytenoid, and one arytenoid muscle. Their actions are as follows : When the crico-thyroid muscles (10, Fig. 171) contract, they rotate the cricoid on the thyroid cartilage in such a manner that the upper and back part of the former, and of necessity the aryt- enoid cartilages on the top of it, are tipped backwards, while the thyroid is inclined forward ; and thus, of course, the vocal cords being attached in front to one, and behind to the other, are "put on the stretch." The thyro-arytenoid muscles (7, Fig. 174), on the other hand, have an opposite action pull- ing the thyroid backwards, and the arytenoid and upper and back part of the cricoid carti- lages forwards, and thus relaxing the vocal cords. The crico-arytenoidei postici muscles (Fig. 173, b) dilate the glottis, and separate the vocal cords, the one from the other, by an action on the arytenoid cartilage, which will be plain on C hea. reference to B' and c', Fig. 172. By their contraction they tend to pull together the outer angles of the arytenoid cartilages in such a fashion as to rotate the latter at their joint with the cricoid, and of course to throw asunder their anterior angles to which the vocal cords are at- tached. These posterior crico-arytenoid muscles are opposed by the crico-arytenoidei laterales, which, pulling in the opposite direc- tion from the other side of the axis of rotation, have of course Lateral view of exterior of the larynx, after Mr. Willis. 8, thyroid cartilage ; 9, cricoid cartilage ; 10, crico-thyroid muscle; 11, crico-thy- roid ligament; 12, first rings of tra- 480 VOICE AND SPEECH. exactly the opposite effect, and close the glottis (Fig. 174, 4 and 5). The aperture of the glottis can be also contracted by the arytenoid muscle (, Fig. 173, and 6, Fig. 174), which, in its FIG. 172. Three laryngoscopic views of the superior aperture of the larynx and surrounding parts and different states of the glottis during life (from Czermak). A, the glottis during the emission of a high note in singing ; B, in easy and quiet inhalation of air; C, in the state of widest possible dilatation, as in inhaling a very deep breath. The diagrams A', B', and C', have been added to Czermak's figures, to show in horizontal sections of the glottis the position of the vocal ligaments and aryt- enoid cartilages in the three several states represented in the other figures. In all the figures, so far as marked, the letters indicate the parts as follows, viz. : I, the base of the tongue; e, the upper free part of the epiglottis; e', the tubercle or cushion of the epiglottis ; ph, part of the anterior wall of the pharynx behind the larynx ; in the margin of the aryteno-epiglottidean fold, w, the swelling of the membrane caused by the cartilages of Wrisberg ; s, that of the cartilages of Santorini ; a, the tip or summit of the arytenoid cartilages; c v, the true vocal cords or lips of the rima glottidis ; c vs, the superior or false vocal cords; between them the ventricle of the larynx ; in C, tr is placed on the anterior wall of the receding trachea, and b indicates the commence- ment of the two bronchi beyond the bifurcation which may be brought into view in this state of extreme dilatation (from Quain's Anatomy). PRODUCTION OF VOCAL SOUNDS. 481 contraction, pulls together the upper parts of the arytenoid cartilages between which it extends. The placing of the vocal cords in a position parallel one with the other, is effected by a combined action of the various little muscles which act on them the thyro-arytenoidei having, without much reason, the credit of taking the largest share in the production of this effect. Fig. 172 is intended to show the various positions of the vocal cords under different circum- stances. Thus, in ordinary tranquil breathing, the opening of the glottis is wide and triangular, becoming a little wider at each inspiration, and a little narrower at each expiration (Fig. 172, see also p. 166). On making a rapid and deep inspiration the opening of the glottis is widely dilated, as in c, Fig. 172, and somewhat lozenge-shaped. At the moment of the emission of sound, it is more narrowed, the margins of the aryteuoid cartilages being brought into contact, and the edges of the vocal cords approximated and made parallel, at the same time that their tension is much increased. The higher the note produced, the tenser do the cords become (Fig. 172, A); and the range of a voice depends, of course, in the main, on the extent to which the degree of tension of the vocal cords can be thus altered. In the production of a high note, the vocal cords are brought well within sight, so as to be plainly visible with the help of the laryngoscope. In the ut- terance of grave tones, on the other hand, the epiglottis is depressed and brought over them, and the aryteuoid cartilages look as if they were trying to hide themselves under it (Fig. 175). The epiglottis, by being somewhat pressed down so as to cover the superior cavity of the larynx, serves to render the notes deeper in tone, and at the same time somewhat duller, just as covering the end of a short tube placed in front of caoutchouc tongues lowers the tone. In no other respect does the epiglottis appear to have any effect in modifying the vocal sounds. The degree of approximation of the vocal cords also usually corresponds with the height of the note produced ; but probably not always, for the width of the aperture has no essential in- fluence on the height of the note, as long as the vocal cords have the same tension ; only with a wide aperture, the tone is more difficult to produce, and is less perfect, the rushing of the air through the aperture being heard at the same time. No true vocal sound is produced at the posterior part of the aperture of the glottis, that, viz., which is formed by the space between the arytenoid cartilages, for, as Miiller's experi- ments showed, if the arytenoid cartilages be approximated in such a manner that their anterior processes tpucfy each other, 482 VOICE AND SPEECH. but yet leave an opening behind them as well as in front, no second vocal tone is produced by the passage of the air through the posterior opening, but merely a rustling or bubbling sound ; FIG. 173. FIG. 174. FIG. 175. FIG. 173. View of the larynx and part of the trachea from behind, with the muscles dissected ; h, the body of the hyoid bone; e, epiglottis; t, the posterior borders of the thyroid cartilage; c, the median ridge of the cricoid; a, upper part of the arytenoid; s, placed on one of the oblique fasciculi of the arytenoid muscle ; b, left posterior crico-arytenoid muscle ; ends of the incomplete cartilaginous rings of the trachea; /, fibrous membrane crossing the back of the trachea; n, muscular fibres exposed in a part ('from Quain's Anatomy). FIG. 174. View of the larynx from above. 1, aperture of glottis; 2, arytenoid car- tilages ; 3, vocal cords ; 4, posterior crico-arytenoid muscles ; 5, lateral crico-arytenoid muscle of right side, that of left side removed ; 6, arytenoid muscle ; 7, thyro-aryte- iioid muscle of left side, that of right side removed ; 8, thyroid cartilage ; 9, cricoid cartilage; 13, posterior crico-arytenoid ligament. With the exception of the aryte- noid muscle, this diagram is a copy from Mr. Willis's figure. FIG. 175. View of the upper part of the larynx as seen by means of the laryngo- scope during the utterance of a grave note, c, epiglottis ; s, tubercles of the cartilages of Santorini; a, arytenoid cartilages; z, base of the tongue; ph, the posterior wall of the pharynx. and the height or pitch of the note produced is the same whe- ther the posterior part of the glottis be open or not, provided the vocal cords maintain the same degree of tension. COMPASS OF THE VOICE. 483 Application of the Voice in Singing and Speaking. The notes of the voice thus produced may observe three dif- ferent kinds of sequence. The first is the monotonous, in which the notes have nearly all the same pitch as in ordinary speak- ing ; the variety of the sounds of speech being due to articula- tion in the mouth. In speaking, however, occasional syllables generally receive a higher intonation for the sake of accent. The second mode of sequence is the successive transition from high to low notes, and vice versa, without intervals; such as is heard in the sounds, which as expressions of passion accompany crying in men, and in the howling and whining of dogs. The third mode of sequence of the vocal sounds is the musical, in which each sound has a determinate number of vibrations, and the numbers of the vibrations in the successive sounds have the same relative proportions that characterize the notes of the musical scale. The compass of the voice in different individuals comprehends one, two, or three octaves. In singers that is, in persons apt for singing it extends to two or three octaves. But the male and female voices commence and end at different points of the musical scale. The lowest note of the female voice is about an octave higher than the lowest of the male voice ; the highest note of the female voice about an octave higher than the highest of the male. The compass of the male and female voices taken together, or the entire scale of the human voice, includes about four octaves. The principal difference between the male and female voice is, therefore, in their pitch ; but they are also dis- tinguished by their tone the male voice is not so soft. The voice presents other varieties besides that of male and female ; there are two kinds of male voice, technically called the bass and tenor, and two kinds of female voice, the contralto and soprano, all differing from each other in tone. The bass voice usually reaches lower than the tenor, and its strength lies in the low notes ; while the tenor voice extends higher than the bass. The contralto voice has generally lower notes than the soprano, and is strongest in the lower notes of the female voice; while the soprano voice reaches higher in the scale. But the difference of compass, and of power in different parts of the scale, is not the essential distinction between the different voices ; for bass singers can sometimes go very high, and the contralto frequently sings the high notes like soprano singers. The essential difference between the bass and tenor voices, and between the contralto and soprano, consists in their tone or "timbre," which distinguishes them even when they are sing- ing the same note. The qualities of the baritone and mezzo- 484 VOICE AND SPEECH. soprano voices are less marked ; the baritone being intermedi- ate between the bass and tenor, the mezzosoprano between the contralto and soprano. They have also a middle position as to pitch in the scale of the male and female voices. The different pitch of the male and the female voice depends on the different length of the vocal cords in the two sexes ; their relative length in men and women being as three to two. The difference of the two voices in tone or " timbre," is owing to the different nature and form of the resounding walls, which in the male larynx are much more extensive, and form a more acute angle anteriorly. The different qualities of the tenor and bass, and of the alto and soprano voices, probably depend on some peculiarities of the ligaments, and the membranous and cartilaginous parietes of the laryngeal cavity, which are not at present understood, but of which we may form some idea, by recollecting that musical instruments made of different materials, e. g., metallic and gut strings, may be tuned to the same note, but that each will give it with a peculiar tone or " timbre." The larynx of boys resembles the female larynx ; their vocal cords before puberty have not two-thirds the length which they acquire at that period ; and the angle of their thyroid cartilage is as little prominent as in the female larynx. Boys' voices are alto and soprano, resembling in pitch those of women, but louder, and differing somewhat from them in tone. But, after the larynx has undergone the change produced during the period of development at puberty, the boy's voice becomes bass or tenor. While the change of form is taking place, the voice is said to " crack ;" it becomes imperfect, frequently hoarse and crowing, and is unfitted for singing until the new tones are brought under command by practice. In eunuchs, who have been deprived of the testes before puberty, the voice does not undergo this change. The voice of most old people is deficient in tone, unsteady, and more restricted in extent : the first de- fect is owing to the ossification of the cartilages of the larynx and the altered condition of the vocal cord ; the want of steadi- ness arises from the loss of nervous power and command over the muscles ; the result of which is here, as in other parts, a tremulous motion. These two causes combined render the voices of old people void of tone, unsteady, bleating, and weak. In any class of persons arranged, as in an orchestra, accord- ing to the characters of voices, each would possess, with the general characteristics of a bass, or tenor, or any other kind of voice, some peculiar character by which his voice would be recognized from all the rest. The conditions that determine these distinctions are, however, quite unknown. They are VARIETIES OF VOCAL TONES. 485 probably inherent in the tissues of the larynx, and are as in- discernible as the minute differences that characterize men's features ; one often observes, in like manner, hereditary and family peculiarities of voice as well marked as those of the limbs or face. Most persons, particularly men, have the power, if at all capable of singing, of modulating their voices through a double series of notes of different character : namely, the notes of the natural voice, or chest-notes, and the falsetto notes. The natural voice, which alone has been hitherto considered, is fuller, and excites a distinct sensation of much stronger vibration and resonance than the falsetto voice, which has more a flute- like character. The deeper notes of the male voice can be produced only with the natural voice, the highest with the falsetto only; the notes of middle pitch can be produced either with the natural or falsetto voice ; the two registers of the voice are therefore not limited in such a manner as that one ends when the other begins, but they run in part side by side. The natural, or chest notes, are produced by the ordinary vibrations of the vocal cords. The mode of production of the falsetto notes is still obscure. By Miiller they are thought to be due to vibrations of only the inner borders of the vocal cords. In the opinion of Petrequin and Diday, they do not result from vibrations of the vocal cords at all, but from vibrations of the air passing through the aperture of the glottis, which they be- lieve assumes, at such times, the contour of the embouchure of a flute. Others (considering some degree of similarity which ex- ists between the falsetto notes, and the peculiar tones called harmonic, which are produced when, by touching or stopping a harp-string at a particular point, only a portion of its length is allowed to vibrate) have supposed that, in the falsetto notes, portions of the vocal ligaments are thus isolated, and made to vibrate while the rest are held still. The question cannot yet be settled ; but any one in the habit of singing may assure himself, both by the difficulty of passing smoothly from one set of notes to the other, and by the necessity of exercising himself in both registers, lest he should become very deficient in one, that there must be some great difference in the modes in which their respective notes are produced. The strength of the voice depends partly on the degree to which the vocal cords can be made to vibrate ; and partly on the fitness for resonance of the membranes and cartilages of the larynx, of the parietes of the thorax, lungs, and cavities of the mouth, nostrils, and communicating sinuses. It is di- minished by anything which interferes with such capability of vibration. The intensity or loudness of a given note with 41 486 VOICE AND SPEECH. maintenance of the same " pitch," cannot be rendered greater by merely increasing the force of the current of air through the glottis; for increase of the force of the current of air cceteris paribus, raises the pitch both of the natural and the falsetto notes. Yet, since a singer possesses the power of in- creasing the loudness of a note from the faintest " piano " to " fortissimo " without its pitch being altered, there must be some means of compensating the tendency of the vocal cords to emit a higher note when the force of the current of air is increased. This means evidently consists in modifying the tension of the vocal cords. When a note is rendered louder and more intense, the vocal cords must be relaxed by remission of the muscular action, in proportion as the force of the cur- rent of the breath through the glottis is increased. When a note is rendered fainter, the reverse of this must occur. The arches of the palate and the uvula become contracted during the formation of the higher notes ; but their contraction is the same for a note of given height, whether it be falsetto or not ; and in either case the arches of the palate may be touched with the finger, without the note being altered. Their action, therefore in the production of the higher notes seems to be merely the result of involuntary associate nervous action, excited by the voluntarily increased exertion of the muscles of the larynx. If the palatine arches contribute at all to the production of the higher notes of the natural voice and the falsetto, it can only be by their increased tension strengthen- ing the resonance. The office of the ventricles of the larynx is evidently to afford a free space for the vibrations of the lips of the glottis ; they may be compared with the cavity at the commencement of the mouth-piece of trumpets, which allows the free vibration of the lips. SPEECH. Besides the musical tones formed in the larynx, a great number of other sounds can be produced in the vocal tubes, between the glottis and the external apertures of the air-pas- sages, the combination of which sounds into different groups to designate objects, properties, actions, &c., constitutes lan- guage. The languages do not employ all the sounds which can be produced in this manner, the combination of some with others being often difficult. Those sounds which are easy of combination enter, for the most part, into the forma- tion of the greater number of languages. Each language con- tains a certain number of such sounds, but in no one are all brought together. On the contrary, different languages are VARIETIES OF VOCAL TONES. 487 characterized by the prevalence in them of certain classes of these sounds, while others are less frequent or altogether ab- sent. The sounds produced in speech, or articulate sounds, are com- monly divided into vowels and consonants ; the distinction be- tween which is, that the sounds for the former are generated by the larynx, while those for the latter are produced by interruption of the current of air in some part of the air-pas- sages above the larynx. The term consonant has been given to these because several of them are not properly sounded, except consonantly with a vowel. Thus, if it be attempted to pronounce aloud the consonants b, d, and g, or their modifica- tions, p, t, k, the intonation only follows them in their combi- nation with a vowel. To recognize the essential properties of the articulate sounds, we must, according to Muller, first examine them as they are produced in whispering, and then investigate which of them can also be uttered in a modified character conjoined with local tone. By this procedure we find two series of sounds : in one the sounds are mute, and cannot be uttered with a vocal tone ; the sounds of the other series can be formed inde- pendently of voice, but are also capable of being uttered in conjunction with it. All the vowels can be expressed in a whisper without vocal tone, that is, mutely. These mute vowel-sounds differ, how- ever, in some measure, as to their mode of production, from the consonants. All the mute consonants are formed in the vocal-tube above the glottis, or in the cavity of the mouth or nose, by the mere rushing of the air between the surfaces differently modified in disposition. But the sound of the vowels, even when mute, has its source in the glottis though its vocal cords are not thrown into the vibrations necessary for the production of voice ; and the sound seems to be produced by the passage of the current of air between the relaxed vocal cords. The same vowel-sound can be produced in the larynx when the mouth is closed, the nostrils being open, and the utterance of all vocal tone avoided. This sound, when the mouth is open, is so modified by varied forms of the oral cavity, as to assume the characters of the vowels a, i, o, u, in all their modifications. The cavity of the mouth assumes the same form for the articulation of each of the mute vowels as for the correspond- ing vowel when vocalized ; the only difference in the two cases lies in the kind of sound emitted by the larynx. Krantzen- stein and Kempelen have pointed out that the conditions necessary for changing one and the same sound into the differ- 488 VOICE AND SPEECH. ent vowels, are differences in the size of two parts the oral canal and the oral opening ; and the same is the case with regard to the mute vowels. By oral canal, Kempeleu means here the space between the tongue and palate : for the pro- nunciation of certain vowels both the opening of the mouth and the space just mentioned are widened ; for the pronuncia- tion of other vowels both are contracted ; and for others one is wide, the other contracted. Admitting five degrees of size, both of the opening of the mouth and of the space between the tongue and palate, Kempelen thus states the dimensions of these parts for the following vowel-sounds : / Vowel. Sound. Size of oral Size of oral opening. canal. a as in a " e " o " oo " 'far,". ' name," ' theme," 'go," . 'cool," 5 3 4 2 3 1 2 4 1 5 Another important distinction in articulate sounds is, that the utterance of some is only of momentary duration, taking place during a sudden change in the conformation of the mouth, and being incapable of prolongation by a continued expiration. To this class belong 6, p, d, and the hard g. In the utterance of other consonants the sounds may be continu- ous; they may be prolonged, ad libitum, as long as a particular disposition of the mouth and a constant expiration are main- tained. Among these consonants are h, m, n, /, s, r, I. Cor- responding differences in respect to the time that may be occupied in their utterance exist in the vowel-sounds, and prin- cipally constitute the differences of long and short syllables. Thus, the a as in "far" and "fate," the o as in "go" and " fort," may be indefinitely prolonged ; but the same vowels (or more properly different vowels expressed by the same let- ters), as in " can" and " fact," in " dog" and " rotten," cannot be prolonged. All sounds of the first or explosive kind are insusceptible of combination with vocal tone (" intonation "), and are abso- lutely mute ; nearly all the consonants of the second or con- tinuous kind may be attended with " intonation." The peculiarity of speaking, to which the term ventriloquism is applied, appears to consist merely in the varied modification of the sounds produced in the larynx, in imitation of the modi- fications which voice ordinarily suffers from distance, &c. From the observations of Miiller and Colombat, it seems that the essential mechanical parts of the process of ventriloquism con- sist in taking a full inspiration, then keeping the muscles of THE SENSES. 489 the chest and neck fixed, and speaking with the mouth almost closed, and the lips and lower jaw as motionless as possible, while air is very slowly expired through a very narrow glottis ; care being taken also, that none of the expired air passes through the nose. But, as observed by Mu'ller, much of the ventriloquist's skill in imitating the voices coming from par- ticular directions, consists in deceiving other senses than hear- ing. We never distinguish very readily the direction in which sounds reach our ear ; and, when our attention is directed to a particular point, our imagination is very apt to refer to that point whatever sounds we may hear. The tongue, which is usually credited with the power of speech language and speech being often employed as synony- mous terms plays only a subordinate, although very impor- tant part. This is well shown by cases in which nearly the whole organ has been removed on account of disease. Patients who recover from this operation talk imperfectly, and their voice is considerably modified ; but the loss of speech is con- fined to those letters, in the pronunciation of which the tongue is concerned. CHAPTER XIX. THE SENSES. SENSATION consists in the mind receiving, through the me- dium of the nervous system, and, usually as the result of the action of an external cause, a knowledge of certain qualities or conditions, not of external bodies but of the nerves of sense themselves ; and these qualities of the nerves of sense are in all different, the nerve of each sense having its own peculiar quality. There are two principal kinds of sensation, named common and special. The first is the consequence of the ordinary sen- sibility or feeling possessed by most parts of the body, and is manifested when a part is touched, or in any ordinary manner is stimulated. According to the stimulus, the mind perceives a sensation of heat, or cold, of pain, of the contact of hard, soft, smooth, or rough objects, &c. From this, also, in morbid states, the mind perceives itching, tingling, burning, aching, and the like sensations. In its greatest perfection, common sensibility constitutes touch or tact. Touch is, indeed, usually classed with the special senses, and will be considered in the same group with them ; yet it differs from them in being a property common to many nerves, e. g., all the sensitive spinal 490 THE SENSES. nerves, the pneumogastric, glosso-pharyngeal, and fifth cerebral nerves, and in its impressions being communicable through many organs. Including the sense of touch, the special senses are five in number, the senses of sight, hearing, smell, taste, and touch. The manifestation of each of the first three depends on the existence of a special nerve ; the optic for the sense of sight, the auditory for that of hearing, and the olfactory for that of smell. The sense of taste appears to be a property common to branches of the fifth and of the glosso-pharyngeal nerves. The senses, by virtue of the peculiar properties of their sev- eral nerves, make us acquainted with the states of our own body ; and thus indirectly inform us of such qualities and changes of external matter as can give rise to changes in the condition of the nerves. That which through the medium of our senses is actually perceived by the mind is, indeed, merely a property or change of condition of our nerves ; but the mind is accustomed to interpret these modifications in the state of the nerves produced by external influences as proper- ties of the external bodies themselves. This mode of regard- ing sensations is so habitual in the case of the senses which are more rarely affected by internal causes, that it is only on reflection that we perceive it to be erroneous. In the case of the sense of feeling, on the contrary, where many of the pecu- liar sensations of the nerves perceived by the sensorium are excited as frequently by internal as by external causes, we more readily apprehend the truth. For it is easily conceived that the feeling of pain or pleasure, for example, is due to a condition of the nerves, and is not a property of the things which excite it. What is true of these is true of all other sensations ; the mind perceives conditions of the optic, olfac- tory, and other nerves specifically different from that of their state of rest ; these conditions may be excited by the contact of external objects, but they may also be the consequence of internal changes : in the former case the mind, having knowl- edge of the object through either instinct or instruction, rec- ognizes it by the appropriate changes which it produces in the state of the nerves. The special susceptibility of the different nerves of sense for certain influences, as of the optic nerve, or rather its centre, for light ; of the auditory nerve, or centre, for vibrations of the air, &c., and so on, is not due entirely to those nerves having each a specific irritability for such influences exclu- sively. For although, in the ordinary events of life, the optic nerve is excited only by the undulations or emanations of which light may consist, the auditory only by vibrations of the air, THE SENSES. 491 and the olfactory only by odorous particles yet each of these nerves may have its peculiar properties called forth by other conditions. In fact, in whatever way and to whatever degree a nerve of special sense is stimulated, the sensation produced is essentially of the same kind ; irritation of the optic nerve invariably producing a sensation of light, of the auditory nerve a sensation of some modification of sound. The phenomenon must, therefore, be ascribed to a peculiar quality belonging to each nerve of special sense. It has been supposed, indeed, that irritation of a nerve of special sense, when excessive, may produce pain ; but experiments seem to have proved that none of these nerves possess the faculty of common sensibility. Thus Magendie observed that when the olfactory nerves laid bare in a dog were pricked, no signs of pain were manifested ; and other experiments of his seemed to show that both the retina and optic nerve are insusceptible of pain. External impressions on a nerve can give rise to no kind of sensation which cannot also be produced by internal causes, exciting changes in the condition of the same nerve. In the case of the sense of touch, this is at once evident. The sensa- tions of the nerves of touch (or common sensibility), excited by causes acting from without, are those of cold and heat, pain and pleasure, and innumerable modifications of these, which have the same kind of sensation as their element. All these sensations are constantly being produced by internal causes, in all parts of our body endowed with sensitive nerves. The sensations of the nerves of touch are therefore states or qualities proper to themselves, and merely rendered manifest by exciting causes, whether external or internal. The sensa- tion of smell, also, may be perceived independently of the ap- plication of any odorous substance from without, through the influence of some internal condition of the nerve of smell. The sensations of the sense of vision, namely, color, light, and darkness, are also often perceived independently of all external exciting causes. So, also, whenever the auditory nerve is in a state of excitement, the sensations peculiar to it, as the sounds of ringing, humming, &c., are perceived. The same cause, whether internal or external, excites in the different senses different sensations ; in each sense the sensa- tions peculiar to it. For instance, one uniform internal cause, which may act on all the nerves of the senses in the same manner, is the accumulation of blood in their capillary vessels, as in congestion and inflammation. This one cause excites in the retina, while the eyes are closed, the sensations of light and luminous flashes ; in the auditory nerve, the sensation of humming and ringing sounds ; in the olfactory nerve, the 492 THE SENSES. sense of odors ; and in the nerves of feeling, the sensation of pain. In the same way, also, a narcotic substance introduced into the blood, excites in the nerves of each sense peculiar symptoms ; in the optic nerves, the appearance of luminous sparks before the eyes; in the auditory nerves, "tinnitus au- rium ;" and in the common sensitive nerves, the sensation of creeping over the surface. So, also, among external causes, the stimulus of electricity, or the mechanical influence of a blow, concussion, or pressure, excites in the eye the sensation of light and colors ; in the ear, a sense of a loud sound or of ringing ; in the tongue, a saline or acid taste ; and at the other parts of the body, a perception of peculiar jarring or of me- chanical impression, or a shock like it. Although, in the cases just referred to, and in all ordinary conditions, sensations are derived from peculiar conditions of the nerves of sense, whether excited by external or by internal causes, yet the mind may have the same sensations independ- ently of changes in the conditions of at least the peripheral portions of the several nerves, and even independently of any connection with the external organs of the senses. The causes of such sensations are seated in the parts of the brain in which the several nerves of sense terminate. Thus pressure on the brain has been observed to cause the sensation of light: lumi- nous spectra may be excited by internal causes after complete amaurosis of the retina ; and Humboldt states, that, in a man who had lost one eye, he produced by means of galvanism, luminous appearances on the blind side. Many of the various morbid sensations attending diseases of the brain, the vision of spectra, and the like, are of the same kind. Again, although the immediate objects of the perception of our senses are merely particular states induced in the nerves, and felt as sensations, yet, inasmuch as the nerves of the senses are material bodies, and therefore participate in the properties of matter generally, occupying space, being susceptible of vi- bratory motion, and capable of being variously changed chem- ically, as well as by the action of heat and electricity, they make known to the mind, by virtue of the different changes thus produced in them by external causes, not merely their own condition, but also some of the different properties and changes of condition of external bodies ; as, e. g., progressive and tremulous motion, chemical change, &c. The information concerning external nature thus obtained by the senses, varies in each sense, having a relation to the peculiar qualities or energies of the nerve. The sensation of motion is, like motion itself, of two kinds progressive and vibratory. The faculty of the perception of THE SENSES. 493 progressive motion is possessed chiefly by the senses of vision, touch, and taste. Thus an impression is perceived travelling from one part of the retina to another, and the movement of the image is interpreted by the mind as the motion of the object. The same is the case in the sense of touch ; so also the movement of a sensation of taste over the surface of the organ of taste, can be recognized. The motion of tremors, or vibra- tions, is perceived by several senses, but especially by those of hearing and touch. For the sense of hearing, vibrations con- stitute the ordinary stimulus, and so give rise to the perception of sound. By the sense of touch, vibrations are perceived as tremors, ocasionally attended with the general impression of tickling; for instance, when a vibrating body, such as a tuning- fork, is approximated to a very sensible part of the surface, the eye can communicate to the mind the image of a vibrating body, and can distinguish the vibrations when they are very slow ; it may be also that the vibrations are communicated to the optic as to the auditory nerve in such a manner that it re- peats them, or receives their impulses. We are made acquainted with chemical actions principally by taste, smell, and touch, and by each of these senses in the mode proper to it. Volatile bodies disturbing the conditions of the nerves by a chemical action, exert the greatest influence upon the organ of smell ; and many matters act on that sense which produce no impression upon the organs of taste and touch ; for example, many odorous substances, as the vapor of metals, such as lead, and the vapor of many minerals. Some volatile substances, however, are perceived not only by the sense of smell, but also by the senses of touch and taste, pro- vided they are of a nature adapted to disturb chemically the condition of those organs, and in case of the organ of taste, to be dissolved by the fluids covering it. Thus, the vapors of horseradish and mustard, and acrid suffocating gases, act upon the conjunctiva and the mucous membrane of the lungs, exciting through the common sensitive nerves, merely modi- fications of common feeling; and at the same time they excite the sensations of smell and of taste. Sensations are referred from their proper seat towards the exterior ; but this is owing, not to anything in the nature of the nerves themselves, but to the accompanying idea derived from experience. For in the perception of sensations, there is a combined action both of the mind and of the nerves of sense; and the mind, by education or experience, has learned to refer the impressions it receives to objects external to the body. Even when it derives impressions from internal causes, it com- monly refers them to external objects. The light perceived 42 494 THE SENSES. in congestion of the retina seems external to the body ; the ringing of the ears in disease is felt as if the sound came from some distance ; the mind referring it to the outer world from which it is in the habit of receiving the like impression. Moreover, the mind not only perceives the sensations, and interprets them according to ideas previously obtained, but it has a direct influence upon them, imparting to them intensity by its faculty of attention. Without simultaneous attention, all sensations are only obscurely, if at all, perceived. If the mind be torpid in indolence, or if the attention be withdrawn from the nerves of sense in intellectual contemplation, deep speculations, or an intense passion, the sensations of the nerves make no impression upon the mind ; they are not perceived, that is to say, they are not communicated to the conscious " self," or with so little intensity, that the mind is unable to retain the impression, or only recollects it some time after, when it is freed from the preponderating influence of the idea which had occupied it. This power of attention to the sensations derived from a single organ, may also be exercised in a single portion of a sentient organ, and thus enable one to discern the detail of what would otherwise be a single sensation. For example, by well-directed attention, one can distinguish each of the many tones simultaneously emitted by an orchestra, and can even follow the weaker tones of one instrument apart from the other sounds, of which the impressions being not attended to are less vividly perceived. So, also if one endeavors to direct atten- tion to the whole field of vision at the same time, nothing is seen distinctly ; but when the attention is directed first to this, then to that part, and analyzes the detail of the sensation, the part to which the mind is directed is perceived with more dis- tinctness than the rest of the same sensation. THE SENSE OF SMELL. The sense of smell ordinarily requires, for its excitement to a state of activity, the action of external matters, which action produces certain changes in the olfactory nerve ; and this nerve is susceptible of an infinite variety of states dependent on the nature of the external stimulus. The first condition essential to the sense of smell is the exis- tence of a special nerve, the changes in whose condition are perceived as sensations of odor ; for no other nerve is capable of these sensations, even though acted on by the same causes. The same substance which excites the sensation of smell in the olfactory nerves may cause another peculiar sensation through the nerves of taste, and may produce an irritating and burn- THE SENSE OF SMELL. 495 ing sensation on the nerves of touch ; but the sensation of odor is yet separate and distinct from these, though it may be sim- ultaneously perceived. The second condition of smell is a pe- culiar state of the olfactory nerve, or a peculiar change pro- duced in it by the stimulus or odorous substance. The material causes of odors are, usually, in the case of ani- mals living in the air, either solids suspended in a state of ex- tremely fine division in the atmosphere ; or gaseous exhalations often of so subtile a nature that they can be detected by no other reagent than the sense of smell itself. The matters of odor must, in all cases, be dissolved in the mucus of the mu- cous membrane before they can be immediately applied to, or affect the olfactory nerves ; therefore a further condition neces- sary for the perception of odors is, that the mucous membrane of the nasal cavity be moist. When the Schneiderian mem- brane is dry, the sense of smell is impaired or lost ; in the first stage of catarrh, when the secretion of mucus within the nos- trils is lessened, the faculty of perceiving odor is either lost, or rendered very imperfect. In animals living in the air, it is also requisite that the odorous matter should be transmitted in a current through the nostrils. This is effected by an inspiratory movement, the mouth being closed ; hence we have voluntary influence over the sense of smell ; for by interrupting respiration we prevent the perception of odors, and by repeated quick inspiration, as- sisted, as in the act of sniffing, by the action of the nostrils, we render the impression more intense (see p. 184). The human organ of smell is essentially formed by the fila- ments of the olfactory nerves, distributed in minute arrange- ment, in the mucous membrane covering the upper third of the septum of the nose, the superior turbinated or spongy bone, the upper part of the middle turbinated bone, and the upper wall of the nasal cavities beneath the cribriform plates of the ethmoid bones (Figs. 176 and 177). This olfactory region is covered by cells of cylindrical epithe- lium not provided with cilia ; and interspersed with these are peculiar fusiform cells with fine processes, called olfactory cells. They are supposed to have some connection with the terminal filaments of the olfactory nerve. The lower, or respiratory part, as it is called, of the nasal fossae is lined by cyliindrical ciliated epithelium, except in the region of the nostrils, where it is squamous. In all the distribution, the branches of the olfactory nerves retain much of the same soft and grayish texture which dis- tinguishes their trunks (as the olfactory lobes of the brain are called) within the cranium. Their individual filaments, also, 496 THE SEXSE OF SMELL. are peculiar, more resembling those of the sympathetic nerve than the filaments of the other cerebral nerves do, containing Fw. 176. Nerves of the septum nasi, seen from the right side (from Sappey after Hirschfeld and Leveille). %. I, the olfactory bulb; 1, the olfactory nerves passing through the foramina of the cribriform plate, and descending to be distributed on the septum ; 2, the internal or septal twig of the nasal branch of the ophthalmic nerve ; 3, naso- palatine nerves. no outer white substance, and being finely granular and nu- cleated. The branches are distributed principally in close plexuses ; but the mode of termination of the filaments is not yet satisfactorily determined. The sense of smell is derived exclusively through those parts of the nasal cavities in which the olfactory nerves are dis- tributed ; the accessory cavities or sinuses communicating with the nostrils seem to have no relation to it. Air impregnated with the vapor of camphor was injected by Deschamps into the frontal sinus through a fistulous opening, and Richerand injected odorous substances into the antrum of Highmore; but in neither case was any odor perceived by the patient. The purposes of these sinuses appear to be, that the bones, necessarily large for the action of the muscles and other parts connected with them, may be as light as possible, and that there may be more room for the resonance of the air in vocalizing. The former purpose, which is in other bones ob- tained by filling their cavities with fat, is here attained, as it is in many bones of birds, by their being filled with air. All parts of the nasal cavities, whether or not they can be the seats of the sense of smell, are endowed with common sen- sibility by the nasal branches of the first and second divisions THE SENSE OF SMELL. 497 of the fifth nerve. Hence the sensations of cold, heat, itching, tickling, and pain ; and the sensation of tension or pressure in the nostrils. That these nerves cannot perform the function of the olfactory nerves is proved by cases in which the sense FIG. 177. Left olfactory nerve, with its distribution on the septum narium (from Wilson). 1, Frontal sinus ; 2, nasal bone ; 3, crista galli of ethmoid bone ; 4, sphenoidal sinus of left side ; 5, sella turcica; 6, basilar process of sphenoid and occipital bone ; 7, pos- terior opening of the right naris ; 8, Opening of the Eustachian tube in the upper part of the pharynx ; 9, soft palate divided through its middle ; 10, cut surface of the hard palate ; a, olfactory nerve; b, its three roots of origin ; c, the olfactory bulb; d, nasal nerve (ophthalmic division of 5th) ; e, naso-palatine nerve (from the spheno- palatine ganglion); /, the anterior palatine foramen ; g, branches of the naso-pala- tine nerve to the palate ; A, anterior and posterior palatine nerves; i, septum narium. of smell is lost, while the mucous membrane of the nose re- mains susceptible of the various modifications of common sen- sation or touch. But it is often difficult to distinguish the sensation of smell from that of mere feeling, and to ascertain what belongs to each separately. This is the case particularly with the sensations excited in the nose by acrid vapors, as of ammonia, horseradish, mustard, &c., which resemble much the sensations of the nerves of touch ; and the difficulty is the greater, when it is remembered that these acrid vapors have nearly the same action upon the mucous membrane of the eye- lids. It was because the common sensibility of the nose to these irritating substances remained after the destruction of the olfactory nerves, that Magendie was led to believe the fifth nerve might exercise the special sense. Animals do not all equally perceive the same odors; the 498 THE SENSE OF SMELL. odors most plainly perceived by an herbivorous animal and by a carnivorous animal are different. The carnivora have the power of detecting most accurately by the smell the special peculiarities of animal matters, and of tracking other animals by the scent ; but have apparently very little sensibility to the odors of plants and flowers. Herbivorous animals are pecu- liarly sensitive to the latter, and have a narrower sensibility to animal odors, especially to such as proceed from other in- dividuals than their own species. Man is far inferior to many animals of both classes in respect of the acuteness of smell ; but his sphere of susceptibility to various odors is more uni- form and extended. The cause of this difference lies probably in the endowments of the cerebral parts of the olfactory ap- paratus. Opposed to the sensation of an agreeable odor, is that of a disagreeable or disgusting odor, which corresponds to the sen- sations of pain, dazzling and disharmony of colors, and disso- nance in the other senses. The cause of this difference in the effect of different odors is unknown ; but this much is certain, that odors are pleasant or offensive in a relative sense only, for many animals pass their existence in the midst of odors which to us are highly disagreeable. A great difference in this respect is, indeed, observed amongst men : many odors, generally thought agreeable, are to some persons intolerable ; and different persons describe differently the sensations that they severally derive from the same odorous substances. There seems also to be in some persons an insensibility to certain odors, comparable with that of the eye to certain colors ; and among different persons, as great a difference in the acuteuess of the sense of smell as among others in the acuteness of sight. We have no exact proof that a relation of harmony and dis- harmony exists between odors as between colors and sounds ; though it is probable that such is the case, since it certainly is so with regard to the sense of taste ; and since such a relation would account in some measure for the- different degrees of per- ceptive power in different persons ; for as some have no ear for music (as it is said) so others have no clear appreciation of the relation of odors, and therefore little pleasure in them. The sensations of the olfactory nerves, independent of the external application of odorous substances, have hitherto been little studied. It has been found that solutions of inodorous substances, such as salts, excite no sensation of odor when in- jected into the nostrils. The friction of the electric machine is, however, known to produce a smell like that of phosphorus. Hitter, too, has observed, that when galvanism is applied to the organ of smell, besides the impulse to sneeze, and the tick- THE SENSE OF SIGHT. 499 ling sensation excited in the filaments of the fifth nerve, a smell like that of ammonia was excited by the negative pole, and an acid odor by the positive pole ; whichever of these sen- sations was produced, it remained constant as long as the circle was closed, and changed to the other at the moment of the circle being opened. Frequently a person smells something which is not present, and which other persons cannot smell ; this is very frequent with nervous people, but it occasionally happens to every one. In a man who was constantly conscious of a bad odor, the arachnoid was found after death, by MM. Cullerier and Maignault, to be beset with deposits of bone ; and in the middle of the cerebral hemispheres were scrofulous cysts in a state of suppuration. Dubois was acquainted with a man who, ever after a fall from his horse, which occurred several years before his death, believed that he smelt a bad odor. THE SENSE OF SIGHT. The eyeball or the organ of vision (Fig. 178) consists of a variety of structures which may be thus enumerated : FIG. 178. Ciliary muscle. Ciliary process. Canal of Petit. Cornea. Anterior chamber. Lens. Iris. Ciliary process. Ciliarv muscle. The sclerotic, or outermost coat, envelops about five-sixths of the eyeball : continuous with it, in front, and occupying the remaining sixth, is the cornea . The cornea and front portion 500 THE SENSE OF SIGHT. FIG. 179. of the sclerotic are covered by mucous membrane, the con- junctiva; that which covers the front of the cornea being little more than squamous epithelium. Immediately within the sclerotic is the choroid coat, and within the choroid is the retina. The interior of the eyeball is well- nigh filled by the aqueous and vitreous humors and the crystal- line lens; but also, there is sus- pended in the interior a contrac- tile and perforated curtain, the iris, for regulating the ad- mission of light, and behind the junction of the sclerotic and cor- nea is the ciliary muscles, the function of which is to adapt the eye for seeing objects at various distances. These structures may be now examined rather more in detail. The sclerotic coat is composed of connective tissue, arranged in variously disposed and inter- communicating .layers. It is strong, tough, and opaque, and not very elastic. The cornea (Fig. 179) is, like the sclerotic, with which it is continuous, chiefly of a fibrous structure, but the fibres are so modified and arranged as to form a transparent membrane for the passage of light. Both in front of and behind the fib- __ rous tissue of the cornea is a structure of the cornea (after BOW- structureless elastic membrane man). A 80 ; B and O soo. A, small w ith epithelium. pwrtionofaTerti f al8ecttoIlof i ^ '' The choroid, which is the ^ a r in . f a ' a> conjunc " next tunic of the eye within the d/fibrouViaminrwUh'rucir^bod'ies sclerotic and immediately out- interspersed between them ; d, posterior side the retina, consists of a thin elastic lamina or membrane of De- an( J highly vascular membrane, mours; e, internal epithelium of d. B, of which the i uterna l surface is epithelium of the membrane of De- -, , i f 11 i mours, as seen looking towards its sur- C ?red by a layer of black face. O, the same seen in section. pigment-Cells. I he principal THE RETINA. 501 use of the choroid is to absorb, by means of its pigment, those rays of light which pass through the transparent retina, and thus to prevent their being thrown again upon the retina, so as to interfere with the distinctness of the images there formed. Hence animals in which the choroid is destitute of pigment, and human albinos, are dazzled by daylight, and see best in the twilight. The choroid coat ends in front in what are called the ciliary processes (Fig. 180*). FIG. 180. Ciliary processes as seen from behind. 1, posterior surface of the iris with the sphincter muscle of the pupil ; 2, anterior part of the choroid coat ; 3, one of the ciliary processes, of which about seventy are represented. The retina (Fig. 181) is a delicate membrane, concave, with the concavity directed forwards and ending in front, near the outer part of the ciliary processes in a finely notched edge the ora serrata. Semi-transparent when fresh, it soon becomes clouded and opaque, with a pinkish tint from the blood in its minute vessels. It results from the sudden spreading out or expansion of the optic nerve, of whose terminal fibres, ap- parently deprived of their external white substance, together with nerve-cells, it is essentially composed. Exactly in the centre of the retina, and at a point thus cor- responding to the axis of the eye in which the sense of vision is most perfect, is a round yellowish elevated spot, about ^4 of an inch in diameter, having a minute aperture at its summit, and called, after its discoverer, the yellow spot of Scemmering. It is not covered by the fibrous part of the retina, but a layer of closely set cells passes over it, and in its centre is a minute depression called fovea ceniralis. About y 1 ^ of an inch to the inner side of the yellow spot, and consequently of the axis of the eye, is the point at which the optic nerve spreads out its 502 THE SENSE OF SIGHT. fibres to form the retina. This is the only point of the surface of the retina from which the power of vision is absent. FIG. 181. C7i The posterior half of the retina of the left eye viewed from before (after Henle); *, the cut edge of the sclerotic coat ; ck, the choroid ; r, the retina ; in the interior at the middle, the macula lutea with the depression of the fovea centralis is represented by a slight oval shade ; towards the left side the light spot indicates the colliculus or eminence at the entrance of the optic nerve, from the. centre of which the arteria centralis is seen spreading its branches into the retina, leaving the part occupied by the macula comparatively free. On making a vertical section of the retina, it is seen, under the microscope, to be composed of several layers, which differ from each other in structure and arrangement, while besides these there are fibres, the so-called fibres of Muller, which ex- tend through the different layers, and perforate them, so to speak. Fig. 182 represents a vertical section of a small piece of the retina. On examination it will be seen that there are three principal layers, bounded on the inner aspect by a mem- brana limitams, and on the outer by the choroid coat. 1. The outermost is the membrane of Jacob, or the columnar layer. 2. In the middle is the granular layer. 3. The innermost is the nervous layer. Each of these layers, again, is composed of different strata, after the fashion shown in the figure. The columnar layer (Jacob's membrane) is composed of cyl- indrical or staff-shaped transparent and highly refractive bodies, arranged perpendicularly to the surface of the retina, with their outer extremities imbedded, to a greater or less depth, in a layer of black pigment of the choroid coat. Re- THE RETINA. 503 cent researches seem to have determined that this membrane, instead of being, as was formerly considered, an independent covering, is intimately associated, both in structure and func- FIG. 182. FIG. 182a. FIG. 182. Vertical section of retina of human eye. 1, bacillar layer ; 2, outer layer granular; 3, intermediate fibrous layer; 4, inner granular layer; 5, finely granular gray layer; 6, layer of nerve-cells ; 7, layer of fibre of optic nerve; 8, limitary mem- brane. FIG. 18'2a. Elements of human retina. 1, large fibre of optic nerve; 2, very fine fibre of the same; 3, rod with a granule,/, attached; 4, a similar rod with a fine fibrous prolongation, connecting it with the granule ; 5, portions of rods altered by the action of water; 6, 7, two cones, b b, with their nuclei, c c, their bacillar portions, d d, and their fine fibrous prolongations, ee; 8, radiating fibre, ee, with granule of outer layer, y, and subdividing in the bacillar layer, as well as in the optic layer, h; 9, connec- tion of rods, a, with granules of inner layer,/, granule of outer layer, g, and expan- sion of the fibre proceeding from the latter in the optic layer at h; 10, similar con- nection of cone, b, c, with granule, g, and with nerve-cell, I, which has another librous prolongation, ni. tion, with the sensitive part of the retina; for the conical and staff-shaped bodies, of which it is composed, appear to be con- 504 THE SENSE OF SIGHT. nected, by means of delicate fibres issuing from them, with the nerve-vesicles of the retina, and even to become continuous with the radiating processes which some of these vesicles pre- sent. Concerning the use of these bodies, the discovery of their connection with the sensitive part of the retina supports the opinion entertained by Kolliker and H. Miiller, that their special office is to receive and transmit impressions of light. The structures of which the granular layer is composed are indicated in the figure. The nervous layer is composed of nerve-corpuscles and nerve- fibres. The nerve-corpuscles are the outermost, and are most numerous over the yellow spot, and absent altogether from the point of entrance of the optic nerve. They are imbedded in fine molecular matter, which also forms a layer outside them. The nerve-fibres radiate as a fine membranous network from the point of entrance of the optic nerve, of whose fibres they are the continuation. They end probably in the nerve-corpus- cles. The fibres are absent from the yellow spot. Two of the fibres of Mutter are, for the sake of illustration, arranged in the figure separately on each side of the layer which they perforate. About the connection of the fibres of Miiller there is some uncertainty. They are supposed to be connected by their outer ends with the rods and cones; and by their inner, which are thought to be modifications of con- nective tissue, they rest on the membrana limitans. Between these points they are supposed to have connections also with some of the other structures through which they pass, espe- cially with the inner layer of nuclei. The retinal bloodvessels ramify chiefly in the nervous layer. The structures which have been just described are modified in their distribution over the yellow spot in the following man- ner: Of the columnar layer, or membrani Jacobi, the cones greatly predominate; of the nervous layers the cells are nu- merous, while the nerve-fibres are absent. There are capilla- ries here, but none of the larger branches of the retinal ar- teries. Opposite the fovea centralis, there are, moreover, neither the granular, nor the fine molecular layer, nor the fibres of Miiller. By means of the retina and the other parts just described, a provision is afforded for enabling the terminal fibres of the optic nerve to receive the impression of rays of light, and to communicate them to the brain, in which they excite the sen- sation of vision. But that light should produce in the retina images of the objects from which it comes, it is necessary that, REFRACTION BY THE CORNEA. 505 when emitted or reflected from determinate parts of the exter- nal objects, it should stimulate only corresponding parts of the retina. For as light radiates from a luminous body in all di- rections, when the media offer no impediment to its transmis- sion, a luminous point will necessarily illuminate all parts of a surface, such as the retina opposed to it, and not merely one single point. A retina, therefore, without any optical appa- ratus placed in front of it to separate the light of different objects, would see nothing distinctly, but would merely per- ceive the general impression of daylight, and distinguish it from the night. Accordingly, we find that in man, and all vertebrate animals, certain transparent refracting media are placed in front of the retina for the purpose of collecting to- gether into one point, the different divergent rays emitted by each point of the external body, and of giving them such di- rections that they shall fall on corresponding points of the retina, and thus produce an exact image of the object from which they proceed. These refracting media are, in the order of succession from without inwards, the cornea, the aqueous humor, the crystalline lens, and the vitreous humor (Fig. 178). The cornea, the structure of which has been already referred to (p. 500), is in a twofold manner capable of refracting and causing convergence of the rays of light that fall upon and traverse it. It thus affects them first, by its density ; for it is a law in optics that when rays of light pass from a rarer into a denser medium, if they impinge upon the surface in a direc- tion removed from the perpendicular, they are bent out of their former direction towards that of a line perpendicular to the surface of the denser medium ; and, secondly, by its convexity; for it is another law in optics that rays of light impinging upon a convex transparent surface, are refracted towards the centre, those being most refracted which are farthest from the centre of the convex surface. Behind the cornea is a space containing a thin watery fluid, the aqueous humor, holding in solution a small quantity of chloride of sodium and extractive matter. The space con- taining the aqueous humor is divided into an anterior and posterior chamber by a membranous partition, the iris, to be presently again mentioned. The effect produced by the aque- ous humor on the rays of light traversing it, is not yet fully ascertained. Its chief use, probably, is to assist in filling the eyeball, so as to maintain its proper convexity, and at the same time to furnish a medium in which the movements of the iris can take place. Behind the aqueous humor and the iris, and imbedded in 506 THE SENSE OF SIGHT. the anterior part of the medium next to be described, viz., the vitreous humor, is seated a doubly- FlG - 183 - convex body, the crystalline lens, which is the most important refract- ing structure of the eye. The struc- ture of the lens is very complex. It consists essentially of fibres united side by side to each other, and ar- ranged together in very numerous laminae, which are so placed upon one another, that when hardened in spirit the lens splits into three por- tions, in the form of sectors, each of Laminated structure of the , . ; IP crystalline Ions f -The lamin* whlch 1S . Composed of superimposed are split up after hardening in Concentric laming. The 1DS Ill- alcohol, creases in density and, consequently, in power of refraction, from without inwards ; the central part, usually termed the nucleus, being the most dense. The density of the lens increases with age ; it is comparatively soft in infancy, but very firm in advanced life ; it is also more spherical at an early period of life than in old age. The vitreous humor constitutes nearly four-fifths of the whole globe of the eye. It fills up the space between the retina and the lens, and its soft jelly-like substance consists essentially of numerous layers, formed of delicate, simple membrane, the spaces between which are filled with a watery, pellucid fluid. It probably exercises some share in refracting the rays of light to the retina; but its principal use appears to be that of giv- ing the proper distension to the globe of the eye, and of keep- ing the surface of the retina at a proper distance from the lens. As already observed, the space occupied by the aqueous humor is divided into two portions by a vertically-placed mem- branous diaphragm, termed the iris, provided with a central aperture, the pupil, for the transmission of light. The iris is composed of organic muscular fibres imbedded in ordinary fibro-cellular or connective tissue. The muscular fibres of the iris have a direction, for the most part, radiating from the cir- cumference towards the pupil ; but as they approach the pu- pillary margin, they assume a circular direction, and at the very edge form a complete ring. By the contraction of the radiating fibres, the size of the pupil is enlarged : by the con- traction of the circular ones, which resemble a kind of sphinc- ter, it is diminished. The object effected by the movements of the iris, is the regulation of the quantity of light transmitted THE VITREOUS HUMOR AND IRIS. 507 to the retina; the quantity of which is, cceteris paribus, directly proportioned to the size of the pupillary aperture. The pos- terior surface of the iris is coated with a layer of dark pig- ment, so that no rays of light can pass to the retina, except such as are admitted through the aperture of the pupil. The ciliary muscle is composed of organic muscular fibres, which form a narrow zone around the interior of the eyeball, near the line of junction of the cornea with the sclerotic, and just behind the outer border of the iris (Fig. 178). The outer- most fibres of this muscle are attached in front to the inner part of the sclerotic and cornea at their line of junction, and, diverging somewhat, are fixed to the ciliary processes, and a small portion of the choroid immediately behind them. The inner fibres, immediately within the preceding, form a circular zone around the interior of the eyeball, outside the ciliary processes. They compose the ring formerly called the ciliary ligament. The function of this muscle is to adapt the eye for seeing objects at various distances. The manner in which it effects this object will be considered afterwards (p. 511). The contents of the ball of the eye are surrounded and kept in position by the cornea, and the dense, fibrous membrane before referred to as the sclerotic, which, besides thus incasing the contents of the eye, serves to give attachment to the va- rious muscles by which the movements of the eyeball are effected. These muscles, and the nerves supplying them, have been already considered (p. 425, et seq.}. Of the Phenomena of Vision. The essential constituents of the optical apparatus of the eye may be thus enumerated : A nervous structure to receive and transmit to the brain the impressions of light ; certain refract- ing media for the purpose of so disposing of the rays of light traversing them as to throw a correct image of an external body on the retina ; a contractile diaphragm with a central aperture for regulating the quantity of light admitted into the eye ; and a contractile structure by which the chief refracting medium shall be so controlled as to enable objects to be seen at various distances. With the help of the diagram below (Fig. 184), represent- ing a vertical section of the eye from before backwards, the mode in which, by means of the refracting media of the eye, an image of an object of sight is thrown on the retina, may be rendered intelligible. The rays of the cones of light emitted by the points A B, and every other point of an object placed 508 THE SENSE OF SIGHT. before the eye are first refracted, that is, are bent towards the axis of the cone, by the cornea c c, and the aqueous humor contained between it and the lens. The rays of each cone are FIG. 184. again refracted and bent still more towards its central ray or axis by the anterior surface of the lens E E ; and again, as they pass out through its posterior surface into the less dense medium of the vitreous humor. For a lens has the power of refracting and causing the convergence of the rays of a cone of light, not only on their entrance from a rarer medium into its anterior convex surface, but also at their exit from its pos- terior convex surface into the rarer medium. In this manner the rays of the cones of light issuing from the points A and B are again collected to points at a and b; and if the retina F be situated at a and 6, perfect, though reversed, images of the points A and B will be perceived ; but if the ret- ina be not at a and b, but either before or behind that situa- tion for instance, at H or o circular luminous spots c, and /, or e and o, instead of points, will be seen; for at H the rays have not yet met, and at G they have already intersected each other, and are again diverging. The retina must therefore be situated at the proper focal distance from the lens, otherwise a defined image will not be formed ; or, in other words, the rays emitted by a given point of the object will not be collected into a corresponding point of focus upon the retina. The means by which distinct and correct images of objects are formed in the retina, in the various conditions in which the eye is placed in relation to external objects, may be sepa- rately considered under the following heads: 1, the means for preventing indistinctness from aberration ; 2, the means for preventing it when objects are viewed at different distances; 3, the means by which the reversed image of an object on the retina is perceived as in its right position by the mind. S P H E R I C A L A B E R II A T I () N. 509 1. Since the retina is concave, and from its centre towards its margins gradually approaches the lens, it follows that the images of objects situated at the sides cannot be so distinct as those of objects nearer to the middle of the field of vision, and of which the images are formed at a distance beyond the lens exactly corresponding to the situation of the retina. More- over, the rays of a cone of light from an object situated at the side of the field of vision do not meet all in the same point, owing to their unequal refraction ; for the refraction of the rays which pass through the circumference of a lens is greater than that of those traversing its central portion. The concur- rence of these two circumstances would cause indistinctness of vision, unless corrected by some contrivance. Such correction is effected, in both cases, by the iris, which forms a kind of annular diaphragm to cover the circumference of the lens, and to prevent the rays from passing through any part of the lens but its centre, which corresponds to the pupil. The image of an object will be most defined and distinct when the pupil is narrow, the object at the proper distance for vision, and the light abundant; so that, while a sufficient number of rays are admitted, the narrowness of the pupil may prevent the production of indistinctness of the image by this spherical aberration or unequal refraction just mentioned. But even the image formed by the rays passing through the circumference of the lens, when the pupil is much dilated, as in the dark, or in a feeble light, may, under certain circum- stances, be well defined ; the image formed by the central rays being then indistinct or invisible, in consequence of the retina not receiving these rays where they are concentrated to a focus. - Distinctness of vision, is further secured by the inner sur- face of the choroid, immediately external to the retina itself, as well as the posterior surface of the iris and the ciliary pro- cesses, being coated with black pigment, which absorbs any rays of light that may be reflected within the eye, and pre- vents their being thrown again upon the retina so as to inter- fere with the images there formed. The pigment of the cho- roid is especially important in this respect ; for the retina is very transparent, and if the surface behind it were not of a dark color, but capable of reflecting the light, the luminous rays which had already acted on the retina would be reflected again through it, and would fall upon other parts of the same membrane, producing both dazzling from excessive light, and indistinctness of the images. In the passage of light through an ordinary convex lens, decomposition of each ray into its elementary colored parts 43 510 THE SENSE OP SIGHT. commonly ensues, and a colored margin appears around the image, owing to the unequal refraction which the elementary colors undergo. In the optical instruments this, which is termed chromatic aberration, is corrected by the use of two or more lenses, differing in shape and density, the second of which continues or increases the refraction of the rays produced by the first, but by recombining the individual parts of each ray into its original white light, corrects any chromatic aberration which may have resulted from the first. It is probable that the unequal refractive power of the transparent media in front of the retina may be the means by which the eye is enabled to guard against the effect of chromatic aberration. The human eye is achromatic, however, only so long as the image is re- ceived at its focal distance upon the retina, or so long as the eye adapts itself to the different distances of sight. If either of these conditions be interfered with, a more er less distinct appearance of colors is produced. 2. The distinctness of the image formed upon the retina is mainly dependent on the rays emitted by each luminous point of the object being brought to a perfect focus upon the retina. If this focus occur at a point either in front of, or behind the retina, indistinctness of vision ensues, with the production of a halo. The focal distance, i. e., the distance of the point at which the luminous rays from a lens are collected, besides being regulated by the degree of convexity and density of the lens, varies with the distance of the object from the lens, being greater as this is shorter, and vice versa. Hence, since objects placed at various distances from the eye can, within a certain range, different in different persons, be seen with almost equal distinctness, there must be some provision by which the eye is enabled to adapt itself, so that whatever length the focal dis- tance may be, the focal point may always fall exactly upon the retina. This power of adaptation of the eye to vision at different dis- tances has received the most varied explanations. It is ob- vious that the effect might be produced in either of two ways, viz., by altering the convexity or density, and thus the refract- ing power, either of the cornea or lens ; or, by changing the position either of the retina or of the lens, so that whether the object viewed be near or distant, and the focal distance thus increased or diminished, the focal point to which the rays are converged by the lens may always be at the place occupied by the retina. The amount of either of these changes required in even the widest range of vision, is extremely small. For, from the refractive powers of the media of the eye, it has been cal- culated by Olbers, that the difference between the focal dis- ADAPTATION TO VARIOUS DISTANCES. 511 tances of the images of an object at such a distance that the rays are parallel, and of one at the distance of four inches, is only about 0.143 of an inch. On this calculation, the change in the distance of the retina from the lens required for vision at all distances, supposing the cornea and lens to maintain the same form, would not be more than about one line. It is now almost universally believed that Helmholtz is right in his statement that the immediate cause of the adaptation of the eye for objects at different distances is a varying shape of the lens, its front surface becoming more or less convex, accord- ing to the distance of the object looked at. The nearer the object, the more convex does the front surface of the lens be- come, and vice versd; the back surface taking little or no share in the production of the effect required. Of course, the lens has no inherent power of contraction, and therefore its changes of outline must be produced by some power from without ; and there seems no reason to doubt that this power is supplied by the ciliary muscle. The exact manner, however, in which, by its contraction, the ciliary muscle effects a change in the shape of the crystalline lens is doubtful. The most probable expla- nation of the phenomenon, however, is that in adapting the eye for viewing near objects the ciliary muscle contracts, and, by such contraction, diminishes the force with which the elastic suspensory ligament of the lens is tending to flatten it. On the latter supposition, the lens may be supposed to be always in-a state of tension and partial flattening from the ac- tion of the suspensory ligament ; while the ciliary muscle, by diminishing the tension of this ligament, diminishes to a pro- portional degree, the flattening of which it is the cause. On diminution or cessation of the action of the ciliary muscle, the lens returns, in a corresponding degree, to its former shape, by virtue of the elasticity of its suspensory ligament. In view- ing near objects, the iris contracts, so that its pupillary edge is mbved a very little forwards, and the pupil itself is con- tracted the opposite effect taking place on withdrawal of the attention from near objects, and fixing it on those distant. The range of distances through which persons can adapt their power of vision is not in all cases the same. Some per- sons possess scarcely any power of adaptation, and of this de- fect of vision there are two kinds ; one, in which the person can see objects distinctly only when brought close to the eye, having little power to discern distant objects; another, in which distant objects alone can be distinctly perceived, a small body being almost invisible except when held at a considerable distance from the eye. In the one case the person is said to be short-sighted or myopic : in the other, long-sighted or 512 THE SENSE OF SIGHT. presbyopic. Myopia is caused by anything, such as undue con- vexity of the lens, which increases the refracting power of the eye, and so causes the image of the object to be formed at a point anterior to the retina : the defect is remedied by the use of concave glasses. Presbyopia, or long-sightedness, is the result of conditions the reverse of the above, and is remedied by the use of convex glasses, which diminish the focal distance of an image formed in the eye. 1 3. The direction given to the rays by their refraction is regu- lated by that of the central ray, or axis of the cone, towards which the rays are bent. The image of any point of an object is, therefore, as a rule (the exceptions to which need not here be stated), always formed in a line identical with the axis of the cone of light, as in the line of B a, or A b, Fig. 185 : so that the spot where the image of any point will be formed upon the ret- ina may be determined by prolonging the central ray of the cone of light, or that ray which traverses the centre of the FIG. 185. pupil. Thus A b is the axis or central ray of the cone of light issuing from A ; B a, the central ray of the cone of light issuing from B ; the image of A is formed at b, the image of B at a, in the inverted position ; therefore what in the object was above is in the image below, and vice versd, the right hand part of the object is in the image to the left, the left-hand to the right. If an opening be made in an eye at its superior surface, so that the retina can be seen through the vitreous humor, this re- versed image of any bright object, such as the windows of the room, may be perceived at the bottom of the eye. Or still better, if the eye of any albino animal, such as a white rabbit, in which the coats, from the absence of pigment, are transpar- ent, is dissected clean, and held with the cornea towards a window, a very distinct image of the window completely in- verted is seen depicted on the posterior translucent wall of the 1 For details on this subject, consult the various treatises on the Physiology and Defects of Vision. REVERSION OF IMAGE ON RETINA. 513 eye. Volkrnann has also shown that a similar experiment may be successfully performed in a living person possessed of large prominent eyes, and an unusually transparent sclerotica. No completely satisfactory explanation has yet been offered to account for the mind being able to form a correct idea of the erect position of an object of which an inverted image is formed on the retina. Miiller and Volkmann are of opinion that the mind really perceives an object as inverted, but needs no correction, since everything is seen alike inverted, and the relative position of the objects therefore remains unchanged ; and the only proof we can possibly have of the inversion is by experiment and the study of the laws of optics. It is the same thing as the daily inversion of objects consequent on the revo- lution of the entire earth, which we know only by observing the position of the stars ; and yet it is certain that, within twenty-four hours, that which was below in relation to the stars, comes to be above. Hence it is, also, that no discordance arises between the sensations of inverted vision and those of touch, which perceives everything in its erect position ; for the images of all objects, even of our own limbs, in the retina, are equally inverted, and therefore maintain the same relative position. Even the image of our hand, while used in touch, is seen inverted. The position in which we see objects, we call therefore the erect position. A mere lateral inversion of our body in a mirror, where the right hand occupies the left of the image, is indeed scarcely remarked : and there is but little discordance between the sensations acquired by touch in regu- lating our movements by the image in the mirror, and those of sight, as, for example, in tying a knot in the cravat. There is some want of harmony here, on account of the inversion being only lateral, and not complete in all directions. The perception of the erect position of objects appears, there- fore, to be the result of an act of the mind. And this leads us to a consideration of the several other properties of the retina, and of the co-operation of the mind in the several other parts of the act of vision. To these belong not merely the act of sensation itself, and the perception of the changes produced in the retina, as light and colors, but also the conversion of the mere images depicted in the retina into ideas of an ex- tended field of vision, of proximity and distance, of the form and. size of objects, of the reciprocal influence of different parts of the retina upon each other, the simultaneous action of the two eyes, and some other phenomena. To speak first of the ideal size of the field of vision : The actual size of the field of vision depends on the extent of the retina, for only so many images can be seen at any one time 514 THE SENSE OF SIGHT. as can occupy the retina, at the same time ; and thus consid- ered, the retina, of which the affections are perceived by the mind, is itself the field of vision. But to the mind of the individual the size of the field of vision has no determinate limits ; sometimes it appears very small, at another time very large ; for the mind has the power of projecting the images on the retina towards the exterior. Hence the mental field of vision is very small when the sphere of the action of the mind is limited to impediments near the eye : on the contrary, it is very extensive when the projection of the images on the retina towards the exterior, by the influence of the mind, is not im- peded. It is very small when we look into a hollow body of small capacity held before the eyes ; large when we look out upon the landscape through a small opening ; more extensive when we look at the landscape through a window ; and most so when our view is not confined by any near object. In all these cases the idea which we receive of the size of the field of vision is very different, although its absolute size is in all the same, being dependent on the extent of the retina. Hence it follows, that the mind is constantly co-operating in the acts of vision, so that at last it becomes difficult to say what belongs to mere sensation, and what to the influence of the mind. By a mental operation of this kind, we obtain a correct idea of the size of individual objects, as well as of the extent of the field of vision. To understand this, it will be necessary to refer again to Fig. 185, p. 512. The angle x, included between the decussating central rays of two cones of light issuing from different points of an object, is called the optical angle angulus opticus sen visorim. This angle becomes larger, the greater the distance between the points A and B ; and since the angles x and y are equal, the distance between the points a and 6 in the image on the retina increases as the angle x becomes larger. Objects at different distances from the eye, but having the same optical angle, x for example, the objects, c, d, and e, must also throw images of equal size upon the retina ; and if they occupy the same angle of the field of vision, their image must occupy the same spot in the retina. Nevertheless, these images appear to the mind to be of very unequal size when the ideas of distance and proximity come into play ; for from the image a b, the mind forms the concep- tion of a visual space extending to e, d, or c, and of an object of the size which that represented by the image on the retina appears to have when viewed close to the eye, or under the most usual circumstances. A landscape depicted on the retina, as a b, and viewed under the angle x, is therefore con- VISUAL DIRECTION. 515 ceived by the mind to have an extent of two miles perhaps, if we know that its extent is such, or if we infer it to be so from the number of known objects seen at the same time. And in the same way that the images of several different objects, viewed under the same angle, thus appears to the mind to have a different size in the field of vision, so the whole field of vision which has always the same absolute size, is interpreted by the mind as of extremely various extent ; and for this reason also, the image viewed in the camera obscura is regarded as a real landscape as the true field of vision although only a small image depicted upon paper. The same mental process gives rise to the idea of depth in the field of vision ; this idea being fixed in our mind principally by the circumstance that, as we ourselves move forward, different images in succession become depicted on our retina, so that we seem to pass be- tween these images, which to the mind is the same thing as passing between the objects themselves. The action of the sense of vision in relation to external objects is, therefore, quite different from that of the sense of touch. The objects of the latter sense are immediately pres- ent to it ; and our own body, with which they come into con- tact, is the measure of their size. The part of a table touched by the hand appears as large as the part of the hand receiv- ing an impression from it, for a part of our body in which a sensation is excited is here the measure by which we judge of the magnitude of the object. In the sense of vision, on the contrary, the images of objects are mere fractions of the objects themselves realized upon the retina, the extent of which re- mains constantly the same. But the imagination, which ana- lyzes the sensations of vision, invests the images of objects, together with the whole field of vision in the retina, with very varying dimensions ; the relative size of the images in propor- tion to the whole field of vision, or of the affected parts of the retina to the whole retina, alone remaining unaltered. The direction in which an object is seen, the direction of vision, or visual direction, depends on the part of the retina which receives the image, and on the distance of this part from, and its relation to, the central point of the retina. Thus, objects of which the images fall upon the same parts of the retina lie in the same visual direction ; and when, by the action of the mind, the images or affections of the retina are projected into the exterior world, the relation of the images to each other remains the same. The estimation of the form of bodies by sight is the . result partly of the mere sensation, and partly of the association of ideas. Since the form of the images perceived by the retina 516 THE SENSE OF SIGHT. depends wholly on the outline of the part of the retina affected, the sensation alone is adequate to the distinction of only su- perficial forms of each other, as of a square from a circle. But the idea of a solid body, as a sphere, or a body of three or more dimensions, e. g., a cube, can only be attained by the ac- tion of the mind constructing it from the different superficial images seen in different positions of the eye with regard to the object; and, as shown by Mr. Wheatstone and illustrated in the stereoscope, from two different perspective projections of the body being presented simultaneously to the mind by the two eyes. Hence, when, in adult age, sight is suddenly re- stored to persons blind from infancy, all objects in the field of vision appear at first as if painted flat on one surface ; and no idea of solidity is formed until after long exercise of the sense of vision combined with that of touch. We judge of the motion of an object, partly from the motion of its image over the surface of the retina, and partly from the motion of our eyes following it. If the image upon the retina moves while our eyes and our body are at rest, we con- clude that the object is changing its relative position with re- gard to ourselves. In such a case the movement of the object may be apparent only, as when we are standing upon a body which is in motion, such as a ship. If, on the other hand, the image- does not move with regard to the retina, but remains fixed upon the same spot of that membrane, while our eyes follow the moving body, we judge of the motion of the object by the sensation of the muscles in action to move the eye. If the image moves over the surface of the retina while the mus- cles of the eye are acting at the same time in a manner cor- responding to this motion, as in reading, we infer that the ob- ject is stationary, and we know that we are merely altering the relations of our eyes to the object. Sometimes the object appears to move when both object and eye are fixed, as in vertigo. The mind can, by the faculty of attention, concentrate its ac- tivity more or less exclusively upon the senses of sight, hear- ing, and touch alternately. When exclusively occupied with the action of one sense, it is scarcely conscious of the sensations of the others. The mind, when deeply immersed in contem- plations of another nature, is indifferent to the actions of the sense of sight, as of every other sense. We often, when deep in thought, have our eyes open and fixed, but see nothing, be- cause of the stimulus of ordinary light being unable to excite the mind to perception when otherwise engaged. The atten- tion which is thus necessary for vision, is necessary also to ANALYSIS OF FIELD OF VISION. 517 analyze what the field of vision presents. The mind does not perceive all the objects presented by the field of vision at the same time with equal acuteness, but directs itself first to one and then to another. The sensation becomes more intense, according as the particular object is at the time the principal object of mental contemplation. Any compound mathematical figure produces a different impression according as the attention is directed exclusively to one or the other part of it. Thus, in Fig. 186, we may in succession FlG - 186 - have a vivid perception of the whole, or of dis- tinct parts only ; of the six triangles near the outer circle, of the hexagon in the middle, or of the three large triangles. The more nume- rous and varied the parts of which a figure is composed, the more scope does it afford lor the play of the attention. Hence it is that architectural orna- ments have an enlivening effect on the sense of vision, since they afford constantly fresh subject for the action of the mind. The duration of the sensation produced by a luminous impression on the retina is always greater than that of the impression which produces it. However brief the luminous impression, the effect on the retina always lasts for about one- eighth of a second. Thus, supposing an object in motion, say a horse, to be revealed on a dark night by a flash of lightning. The object would be seen apparently for an eighth of a second, but it would not appear in motion ; because, although the image remained on the retina for this time, it was really re- vealed for such an extremely short period (the duration of a flash of lightning being almost instantaneous), that no appre- ciable movement on the part of the object could have taken place in the period during which it was revealed to the retina of the observer. And the same fact is proved in a reverse w r ay. The spokes of a rapidly revolving wheel are not seen as distinct objects, because at every point of the field of vision over which the revolving spokes pass, a given impression has not faded before another comes to replace it. Thus every part of the interior of the wheel appears occupied. The duration of the after sensation or spectrum, produced by an object, is greater in a direct ratio with the duration of the impression which caused it. Hence the image of a bright object, as of the panes of a window through which the light is shining, may be perceived in the retina for a considerable period, if we have previously kept our eye fixed for some time on it. The color of the spectrum varies with that of the object which produced it. The spectra left by the images of white or 44 518 THE SENSE OF SIGHT. luminous objects, are ordinarily white or luminous ; those left by dark objects are dark. Sometimes, however, the relation of the light and dark parts in the image may, under certain circumstances, be reversed in the spectrum ; what was bright may be dark, and what was dark may appear light. This occurs whenever the eye, which is the seat of the spectrum of FIG. 187. A circle showing the various simple and compound colors of light, and those which are complemental of each other, i.e., which, when mixed, produce a neutral gray tint. The three simple colors, red, yellow, and blue, are placed at the angles of an equilateral triangle ; which are connected together by means of a circle ; the mixed colors, green, orange, and violet, are placed intermediate between the cor- responding simple or homogeneous colors ; and the complemental colors, of which the pigments, when mixed, would constitute a gray, and of which the prismatic spectra would together produce a white light, will be found to b3 placed in each case opposite to each other, but connected by a line passing through the centre of the circle. The figure is also useful in showing the further shades of color which are complementary of each other. If the circle be supposed to contain every transition of color between the six marked down, those which, when united, yield a white or gray color, will always be found directly opposite to each other; thus, for example, the intermediate tint between orange or red is complemental of the middle tint between green and blue. a luminous object, is not closed, but fixed upon another bright or white surface, as a white wall, or a sheet of white paper. Hence the spectrum of the sun, which, while light is excluded from the eye is luminous, appears black or gray when the eye is directed upon a white surface. The explanation of this is, that the part of the retina which has received the luminous image remains for a certain period afterwards in an exhausted or less sensitive state, while that which has received a dark image is in an unexhausted, and therefore much more excitable condition. The ocular spectra which remain after the impression of colored objects upon the retina are always colored ; and their color is not that of the object, or of the image produced di- rectly by the object, but the opposite, or complemental color. The spectrum of a red object is, therefore, green ; that of a COMPLEMENTARY COLORS. 519 green object, red ; that of violet, yellow ; that of yellow, violet, and so on. The reason of this is obvious. The part of the retina which receives, say, a red image, is wearied by that particular color, but remains sensitive to the other rays which with red make up white light ; and, therefore, these by them- selves reflected from a white object produce a green hue. If, on the other hand, the first object looked at be green, the retina, being tired of green rays, receives a red image when the eye is turned to a white object. And so with the other colors; the retina while fatigued by yellow rays will suppose an object to be violet, and vice versd; the size and shape of the spectrum corresponding with the size and shape of the original object looked at. The colors which thus reciprocally excite each other in the retina are those placed at opposite points of the circle in Fig. 187. Of the Reciprocal Action of different Parts of the Retina on each other. Although each elementary part of the retina represents a distinct portion of the field of vision, yet the different elemen- tary parts, or sensitive points, of that membrane have a certain influence on each other ; the particular condition of one in- fluencing that of another, so that the image perceived by one part is modified by the image depicted in the other. The phenomena, which result from this relation between the dif- ferent parts of the retina, may be arranged in two classes ; the one including those where the condition existing in the greater extent of the retina is imparted to the remainder of that mem- brane ; the other, consisting of those in which the condition of the larger portion of the retina excites, in the less extensive portion, the opposite condition. 1. When two opposite impressions occur in contiguous parts of an image on the retina, the one impression is, under certain circumstances, modified by the other. If the impressions oc- cupy each one-half of the image, this does not take place ; for in that case, their actions are equally balanced. But if one of the impressions occupies only a small part of the retina, and the other the greater part of its surface, the latter may, if long continued, extend its influence over the whole retina, so that the opposite less extensive impression is no longer perceived, and its place becomes occupied by the same sensation as the rest of the field of vision. Thus, if we fix the eye for some time upon a strip of colored paper lying upon a white surface, the image of the colored object, especially when it falls on the 520 THE SENSE OF SIGHT. lateral parts of the retina, will gradually disappear, and the white surface be seen in its place. 2. In the second class of phenomena, the affection of one part of the retina influences that of another part, not in such a manner as to obliterate it, but so as to cause it to be- come the contrast or opposite of itself. Thus a gray spot upon a white ground appears darker than the same tint of gray would do if it alone occupied the whole field of vision, and a shadow is always rendered deeper when the light which gives rise to it becomes more intense, owing to the greater contrast. The former phenomena ensue gradually, and only after the images have been long fixed on the retina; the latter are instantaneous in their production, and are permanent. In the same way, also, colors may be produced by contrast. Thus, a very small dull-gray strip of paper, lying upon an ex- tensive surface of any bright color, does not appear gray, but has a faint tint of the color which is the complement of that of the surrounding surface (seepage 519). A strip of gray paper upon a green field, for example, often appears to have a tint of red, and when lying upon a red surface, a greenish tint ; it has an orange-colored tint upon a bright blue surface, and a bluish tint upon an orange-colored surface ; a yellowish color upon a bright violet, and a violet tint upon a bright yellow surface. The color excited thus, as a contrast to the exciting color, being wholly independent of any rays of the corresponding color acting from without upon the retina, must arise as an opposite or antagonistic condition of that mem- brane ; and the opposite conditions of which the retina thus becomes the subject would seem to balance each other by their reciprocal reaction. A necessary condition for the production of the contrasted colors is, that the part of the retina in which the new color is to be excited, shall be in a state of compara- tive repose ; hence the small object itself must be gray. A second condition is, that the color of the surrounding surface shall be very bright, that is, it shall contain much white light. The retina corresponding to the point of entrance of the optic nerve is completely insensible to the impressions of light. The phenomenon itself is very readily shown. If we direct one eye, the other being closed, upon a point at such a distance to the side of any object, that the image of the latter must fall upon the retina at the point of entrance of the optic nerve, this image is lost either instantaneously, or very soon. If, for SINGLE VISION. 521 example, we close the left eye, and direct the axis of the right eye steadily towards the circular spot above represented, while the page is held at a distance of about six inches from the eye, both dot and cross are visible. On gradually increasing the distance between the eye and the object, by removing the book farther and farther from the face, and still keeping the right eye steadily on the dot, it will be found that suddenly the cross disappears from view, while on removing the book still farther, it suddenly comes in sight again. The cause of this phenomenon is simply that the portion of retina which is occupied by the entrance of the optic nerve, is quite blind ; and therefore that when it alone occupies the field of vision, objects cease to be visible. Of the Simultaneous Action of the two Eyes. Although the sense of sight is exercised by two organs, yet the impression of an object conveyed to the mind is single. Various theories have been advanced to account for this phe- nomenon. By Gall, it was supposed that we do not really employ both eyes simultaneously in vision, but always see with only one at a time. This especial employment of one eye in vision certainly occurs in persons whose eyes are of very un- equal focal distance, but in the majority of individuals both eyes are simultaneously in action in the perception of the same object ; this is shown by the double images seen under certain conditions. If two fingers be held up before the eyes, one in front of the other, and vision be directed to the more distant, so that it is seen singly, the nearer will appear double ; while, if the nearer one be regarded, the most distant will be seen double ; and one of the double images in each case will be found to belong to one eye, the other to the other eye. Single vision results only when certain parts of the two retinae are affected simultaneously ; if different parts of the retinae receive the image of the object, it is seen double. The parts of the retinae in the two eyes which thus correspond to each other in the property of referring the images which affect them simultaneously to the same spot in the field of vision are, in man, just those parts which would correspond to each other, if one retina were placed exactly in front of, and over the other (as in Fig. 188, c). Thus, the outer lateral portion of one eye corresponds to, or, to use a better term, is identical with the inner portion of the other eye ; or a of the eye A (Fig. 188) with a' of the eye B. The upper part of one retina is also identical with the upper part of the other ; and the lower parts of the two eyes are identical with each other. 522 THE SENSE OF SIGHT. This is proved by a single experiment. Pressure upon any part of the ball of the eye, so as to affect the retina, produces a luminous circle, seen at the FIG. IBS. opposite side of the field of vision to that on which the pressure is made. If, now, in a dark room, we press with the finger at the upper part of one eye, and at the lower part of the other, two lumin- ous circles are seen, one above the other; so, also, two figures are seen when pressure is made simultaneously on the two outer or the two inner sides of both eyes. It is certain, therefore, that neither the upper part of one retina and the lower part of the other are identical, nor the outer lateral parts of the two retinae, nor their inner lateral portions. But if pressure be made with the fingers upon both eyes simultaneously at their lower part, one luminous ring is seen at the middle of the upper part of the field of vision ; if the pressure be applied to the upper part of both eyes, a single luminous circle is seen in the middle of the field of vision below. So, also, if we press upon the outer side a of the eye A, and upon the inner side a' of the eye B, a single spectrum is produced, and is apparent at the extreme right of the field of vision ; if upon the point b of one eye, and the point b' of the other, a single spectrum is seen to the extreme left. The spheres of the two retinae may, therefore, be regarded as lying one over the other, as in c, Fig. 188 ; so that the left portion of one eye lies over the identical left portion of the other eye, the right portion of one eye over the identical right portion of the other eye ; and with the upper and lower por- tions of the two eyes, a lies over a', b over b', and c over c'. The points of the one retina intermediate between a and c, are again identical w r ith the corresponding points of the other retina between a' and c' ; those between b and c of the one retina, with those between b' and c' of the other. In short, all other parts are non-identical : and, when they are excited to action, the effect is the same as if the impressions were made on different parts of the same retina : and the double images belonging to the eyes A and B, are seen at exactly the same distance from each other as exists between the image of the eye A and the part of the retina of the eye A which corre- sponds to, or is identical with, the seat of the second image in the eye B ; or, to return to the figure already used in illustra- tion (Fig. 188), if a of one eye be affected, and b' of the other, SINGLE VISION. 523 the distances of the two images a and b' will, inasmuch as a is identical with a', and b' with b, lie at exactly the same distance from each other as images produced by impressions on the points a b of the one eye, or a' b' of the other. In application of these results to the phenomena of vision, if the position of the eyes with regard to a luminous object be such that similar images of the same object fall on identi- cal parts of the two retinse, as occurs when the axes meet in some one point, the object is seen single ; if otherwise, as in the various forms of squinting, two images are formed, and double vision results. If the axes of the eyes, A and B (Fig. 189), be so directed that they meet at a, an object at a will be seen singly, for the point a of the one retina, and a' of the other, are identical. So, also, if the object /? be so situated that its image falls in both eyes at the same distance from the. central point of the retina, namely, at b in the one eye, and at b' in the -other, /9will be seen single, for it affects identical parts of the two retinse. The same will apply to the object /. In quadrupeds, the relation between the identical and non- identical parts of the retinse cannot be the same as in man ; FIG. 189. for the axes of their eyes generally diverge, and can never be made to meet in one point of an object. When an animal re- gards an object situated directly in front of it, the image of the object must fall, in both eyes, on the outer portion of the retiuse. Thus the image of the object a (Fig. 191) will fall at a' in one, and at a" in the other : and these points a' and a" must be identical. So, also, for distinct and single vision of objects, b or c, the points b' and b", or c' c", in the two retinse, 524 THE SENSE OF SIGHT. on which the images of these objects fall, must be identical. All points of the retina in each eye which receive rays of light from lateral objects only, can have no corresponding identi- cal points in the retina of the other eye ; for otherwise two ob- jects, one situated to the right and the other to the left, would appear to lie in the same spot of the field of vision. It is probable, therefore, that there are, in the eyes of animals, parts of the retinse which are identical, and parts which are not identical, i. e., parts in one which have no corresponding parts in the other eye. And the relation of the retinse to each other in the field of vision may be represented as in Fig. 190. The cause of the impressions on the identical points of the two retinse giving rise to but one sensation, and the perception of a single image, must either lie in the structural organization of the deeper or cerebral portion of the visual apparatus, or be the result of a mental operation ; for in no other case is it the property of the corresponding nerves of the two sides of the body to refer their sensations as one to one spot. FIG. 190. Many attempts have been made to explain this remarkable relation between the eyes, by referring it to anatomical rela- tion between the optic nerves. The circumstance of the inner portion of the fibres of the two optic nerves decussating at the commissure, and passing to the eye of the opposite side, while the outer portion of the fibres continue their course to the eye of the same side, so that the left side of both retinae is formed from one root of the nerves, and the right side of both retinse from the other root, naturally lead to an attempt to explain SINGLE VISION. 525 the phenomenon by this distribution of the fibres of the nerves. And this explanation is favored by cases in which the entire of one side of the retina, as far as the central point in both eyes, sometimes becomes insensible. But Miiller shows the inadequateness of this theory to explain the phenomenon, un- less it be supposed that each fibre in each cerebral portion of the optic nerves divides in the optic commissure into two branches for the identical points of the two retinae, as is shown in Fig. 192. But there is no foundation for such supposition. By another theory it is assumed that each optic nerve con- tains exactly the same number of fibres as the other, and that the corresponding fibres of the two nerves are united in the sensorium (as in Fig. 193). But in this theory no account is FIG. 192. FIG. 193. FIG. 194. taken of the partial decussation of the fibres of the nerves in the optic commissure. According to a third theory, the fibres a and a', Fig. 194, coming from identical points of the two retinse, are in the optic commissure brought into one optic nerve, and in the brain either are united by a loop, or spring from the same point. The same disposition prevails in the case of the iden- tical fibres b and b'. According to this theory, the left half of each retina would be represented in the left hemisphere of the brain, and the right half of each retina in the right hem- isphere. Another explanation is founded on the fact, that at the anterior part of the commissure of the optic nerve, certain fibres pass across from the distal portion of one nerve to the corresponding portion of the other nerves, as if they were com- missural fibres forming a connection between the retinae of the two eyes. It is supposed, indeed, that these fibres may con- nect the corresponding parts of the two retinse, and may thus explain their unity of action ; in the same way that corre- sponding parts of the cerebral hemispheres are believed to be 526 THE SENSE OF SIGHT. connected together by the commissural fibres of the corpus callosum, and so enabled to exercise unity of function. But, on the whole, it is more probable, that the power of forming a single idea of an object from a double impression conveyed by it to the eye is the result of a mental act. This view is supported by the same facts as those employed by Professor Wheatstone to show that this power is subservient to the purpose of obtaining a right perception of bodies raised in FIG. 195. relief. When an object is placed so near the eyes that to view it the optic axes must converge, a different perspective pro- jection of it is seen by each eye, these perspectives being more dissimilar as the convergence of the optic axes becomes greater. Thus, if any figure of three dimensions, an outline cube, for example, be held at a moderate distance before the eyes, and viewed with each eye successively, while the head is kept per- fectly steady, A (Fig. 195) will be the picture presented to the right eye, and B that seen by the left eye. Mr. Wheatstone has shown that on this circumstance depends in a great meas- ure our conviction of the solidity of an object, or of its pro- jection hi relief. If different perspective drawings of a solid body, one representing the image seen by the right eye, the other that seen by the left (for example, the drawing of a cube A, B, Fig. 195), be presented to corresponding parts of the two retinae, as may be readily done by means of the stereo- scope, an instrument invented by Professor Wheatstone for the purpose, the mind will perceive not merely a single rep- resentation of the object, but a body projecting in relief, the exact counterpart of that from which the drawings were made. THE SENSE OF HEARING. 527 FIG. 196. SENSE OF HEARING. Anatomy of the Organ of Hearing. For descriptive purposes, the ear, or organ of hearing, is divided into three parts, the external, the middle, and the in- ternal ear. The two first are only ac- cessory to the third or internal ear, which contains the essential parts of an organ of hearing. The accompanying figure shows very well the relation of these divisions one to the other (Fig. 196). The external ear consists of the pinna or auricle, and the external auditory canal or meatm. The principal parts of the pinna are two prominent rims inclosed one within the other (helix and antihelix}, and inclosing a central hol- low named the concha ; in front of the concha, a prominence directed back- wards, the tragus, and opposite to this, one directed forwards, the antitragus. From the concha, the auditory canal, with a slight arch directed upwards, passes inwards and a little forwards to the membrani tympani, to which it thus serves to convey the vibrating air. Its outer part consists of fibro-cartilage continued from the concha ; its inner part of bone. Both are lined by skin continuous with that of the pinna, and ex- tending over the outer part of the membrana tympani. Towards the outer part of the canal are fine hairs and sebaceous glands, while deeper in the canal are small glands, resembling the sweat-glands in structure, which secrete a peculiar yellow sub- stance called cerumen, or ear-wax. The middle ear, or tympanum (b, Fig. 197) is separated by the membrana tympani from the external auditory canal. It is a cavity in the temporal bone, opening through its anterior and inner wall into the Eustachian tube, a cylindriform flattened canal, dilated at both ends, composed partly of bone and partly of cartilage, lined with mucous membrane, and forming a communication between the tympanum and the pharynx. It opens into the cavity of the pharynx just behind Outer surface of the pin- na of the right auricle. %. 1, helix; 2, fossa of the helix ; 3, antihelix ; 4, fossa of the antihelix; 5, anti- tragus; 6, tragus; 7, con- cha ; 8, lobule. 528 THE SENSE OF HEARING. the posterior aperture of the nostrils. The cavity of the tym- panum communicates posteriorly with air-cavities, the mastoid cells in the mastoid process of the temporal bone ; but its only opening to the external air is through the Eustachian tube (c, Fig. 197). The walls of the tympanum are osseous, except where apertures in them are closed with membrane, as at the FIG. 197. General view of the external, middle, and internal ear, as seen in a prepared sec- tion through a, the auditory canal, b. The typanura or middle ear. c. Eustachian tube, leading to the pharynx, d. Cochlea; and e. Semicircular canals and vestibule, seen on their exterior, as brought into view by dissecting away the surrounding petrous bone. The styloid process projects below, and the inner surface of the carotid canal is seen above the Eustachian tubes (from Scarpa). fenestra rotunda, and fenestra ovalis, and at the outer part where the bone is replaced by the membrana tympani. The cavity of the tympanum is lined with mucous membrane, the epithelium of which is ciliated and continuous with that of the pharynx. It contains a chain of small bones (ossicula auditus), which extends from the membrana tympani to the fenestra ovalis. The membrana tympani is placed in a slanting direction at the bottom of the external auditory canal, its plane being at an angle of about 45 with the lower wall of the canal. It is THE LABYRINTH. 529 formed chiefly of a tough and tense fibrous membrane, the edges of which are set in a bony groove ; its outer surface is covered with a continuation of the cutaneous lining of the auditory canal, its inner surface with part of the ciliated mucous membrane of the tympanum. The small bones or ossicles of the ear are three, named malleus, incus, and stapes. The malleus, or hammer-bone, is attached by a long slightly-curved process, called its handle, to the membrani tympani ; the line of attachment being verti- cal, including the whole length of the handle, and extending from the upper border to the centre of the membrane. The head of the malleus is irregularly rounded ; its neck, or the line of boundary between it and the handle, supports two pro- cesses ; a short conical one, which receives the insertion, of the tensor tympani, and a slender one, processus gracilis, which ex- tends forwards, and to which the laxator tympani muscle is at- tached. The incus, or anvil-bone, shaped like a bicuspid molar tooth, is articulated by its broader part, corresponding with the surface of the crown of a tooth, to the malleus. Of its two fang-like processes, one, directed backwards, has a free end, the other, curved downwards and more pointed, articulates by means of a roundish tubercle, formerly called os orbiculare, with the stapes, a little bone shaped exactly like a stirrup, of which the base or bar fits into the fenestra ovalis. To the neck of the stapes, a short process, corresponding with the loop of the stirrup, is attached the stapedius muscle. The bones of the ear are covered with mucous membrane reflected over them from the wall of the tympanum; and are movable both altogether and one upon the other. The malleus moves and vibrates with every movement and vibration of the membrana tympani, and its movements are communicated through the incus to the stapes, and through it to the mem- brane closing the fenestra ovalis. The malleus, also, is mova- ble in its articulation with the incus ; and the membrana tympani moving with it is altered in its degree of tension by the laxator and tensor tympani muscles. The stapes is mov- able on the process of the incus, when the stapedius muscle acting draws it backwards. The proper organ of hearing is formed by the distribution of the auditory nerve within the internal ear, or labyrinth of the ear, a set* of cavities within the petrous portion of the temporal bone. The bone which forms the walls of these cavities is denser than that around it, and forms the osseous labyrinth ; the membrane within the cavities forms the mem- branous labyrinth. The membranous labyrinth contains a 530 THE SENSE OF HEARING. fluid called endolymph ; while outside it, between it and the osseous, labyrinth, is a fluid called perilymph (see p. 533). The osseous labyrinth consists of three principal parts, namely, the vestibule, the cochlea, and the semicircular canals. The vestibule is the middle cavity of the labyrinth, and the central organ of the whole auditory apparatus. It presents, in its inner wall, several openings for the entrance of the divisions of the auditory nerve; in its outer wall, the fenestra ovalis (5, Fig. 198), an opening filled by the base of the stapes, FIG. 198. FIG. 198a. FIG. 198. A view of the labyrinth of the left ear of a foetus of 8 months, as seen from above. Magnified 4 diameters. 1,2, 3. The cochlea. 1, 1. Its first turn. 2, 2. Its second turn. 3, 3. Its third or half turn, and apex or cupola. 4. The fenestra ro- tunda. 5. The fenestra ovalis. 6. The groove around it. 7, 7. The vestibule. 8, 9, 10. The posterior semicircular canal, with its ampulla at 8. 11, 11. The superior semi- circular canal. 12. The external semicircular canal. (S. & H.) FIG. 198a. An outline, of the natural size, of figure 198. one of the small bones of the ear ; in its posterior and superior walls, five openings by which the semicircular canals communi- cate with it : in its anterior wall, an opening leading into the cochlea. The hinder part of the inner wall of the vestibule also presents an opening, the orifice of the aquceductus vestibuli, a canal leading to the posterior margin of the petrous bone, with uncertain contents and unknown purpose. THE LABYRINTH. 531 The semicircular canals (Figs. 198, 199) are three arched cylindriform bony canals, set in the substance of the petrous bone. They all open at both ends into the vestibule (two of them first coalescing). The ends of each are dilated just be- FlG. 199. Interior of the osseous labyrinth. V. Vestibule, av. Aqueduct of the vestibule, o. Fovea hemielliptica. r. Fovea hemispherica. S. Semicircular canals. *. Su- perior, p. Posterior, i. Inferior, a, a, a. The ampullar extremity of each. C. Cochlea, ac. Aqueduct of the cochlea, sv. Osseous zone of the lamina spiralis, above which is the scala vestibuli, communicating with the vestibule, st. Scala tympani below the spiral lamina. From Scemmerring. fore opening into the vestibule ; and one end of each being more dilated than the other is called an ampulla. Two of the canals form nearly vertical arches ; of these the superior is also anterior; the posterior is inferior; the third canal is horizontal, and lower and shorter than the others. The cochlea (1, 2, 3, Fig. 198, and Fig. 200), a small organ, shaped like a common snail shell, is seated in front of the ves- tibule, its base resting on the bottom of the internal meatus, where some apertures transmit to it the cochlear filaments of the auditory nerve. In its axis, the cochlea is traversed by a conical column, the modiolus, around which a spiral canal winds with about two turns and a half from the base to the apex. At the apex of the cochlea the canal is closed ; at the base it presents three openings, of which one, already men- 532 THE SENSE OF HEARING. tioued, communicates with the vestibule ; another, called fenes- tra rotunda, is separated by a membrane from the cavity of the tympanum ; the third is the orifice of the aquceductus cochleae, a canal leading to the jugular fossa of the petrous bone, and corresponding, at FlG - 20 - least in obscurity of pur- pose and origin, to the aquseductus vestibuli. The spiral canal is divided into two passages or scalse by a partition of bone and mem- brane, the lamina spiralis. The osseous part or zone of this lamina is connected View of the osseous cochlea divided through w [fa t ^ e mo diolus; the the middle (from Arnold). 5. 1, central mpmhranrms nart with a canal of the modiolus; 2, lamina spiralis r ' P art ' W . 8 ossea ; 3, scala tympani ; 4, scala vestibuli ; 5, mUSCU Jar zone, according porous substance of the modiolus near one of to Todd and Bowman, the sections of the canalis spiralis modioli. forming its OUter margin, is attached to the outer wall of the canal. Commencing at the base of the cochlea, between its vestibular and tympanic openings, they form a partition between these apertures ; the two scalse are, therefore, in correspondence with this arrangement, named scala ves- tibuli and scala tympani. At the apex of the cochlea, the lamina spiralis ends in a small hamulus, the inner and concave part of which, being detached from the summit of the modio- lus, leaves a small aperture named helicotrema, by which the two scalse, separated in all the rest of their length, communi- cate. Besides the scala vestibuli and scala tympani, there is a third space between them, in the substance of the lamina spiralis, called the scala media, or canalis membranacea, and in this are some peculiar club-shaped little bodies called the rods of Corti, set up on end, with their big extremities upwards, and leaning against each other at the top a section, therefore having the appearance of the gable-end of a house. On their outer part are numerous cells of various shapes. The regularity with which the little rods of Corti are arranged has caused them to be compared to rows of keys in a piano. In close relation with these rods and the cells outside them, and probably projecting also by free ends into the little tri- angular canal containing fluid which is between the rods, are filaments of the auditory nerve. The membranous labyrinth corresponds generally with the form of the osseous labyrinth, so far as regards the vestibule THE LABYRINTH. 533 and semicircular canals, but is separated from the walls of these parts by fluid, except where the nerves enter into con- nection within it. In the cochlea, the membranous labyrinth completes the septum between the two scalce, and incloses a separate spiral canal, the canalis membranacea. As already mentioned, the membranous labyrinth contains a fluid called endolyrnph ; and between its outer surface and the inner sur- face of the walls of the vestibule and semicircular canals is another collection of similar fluid called perilymph: so that all the sonorous vibrations impressing the auditory nerves on these parts of the internal ear are conducted through fluid to a membrane suspended in and containing fluid. The fluid in the scales of the cochlea is continuous with the perilymph in the vestibule and semicircular canals, and there is no fluid exter- nal to its lining membrane. The vestibular portion of the membranous labyrinth com- prises two, probably communicating cavities, of which the larger and upper is named the utriculus ; the lower, the saccu- lus. Into the former open the orifices of the membranous semicircular canals ; into the latter the canalis membranacea of the cochlea. The membranous labyrinth of all these parts is laminated, transparent, very vascular, and covered on the inner surface with nucleated cells, of which those that line the ampullae are prolonged into stiff hair-like processes; the same appearance, but to a much less degree, being visible in the utricle and saccule. In the cavities of the utriculus and saccu- lus are small masses of calcareous particles, otoconia or otolithes; and the same, although in more minute quantities, are to be found in the interior of other parts of the membranous laby- rinth. The auditory nerve, for the appropriate exposure of whose filaments to sonorous vibrations all the organs now described are provided, is characterized as a nerve of special sense by its softness (whence it derived its name of portio mollis of the seventh pair), and by the fineness of its component fibres. It enters the labyrinth of the ear in two divisions ; one for the vestibule and semicircular canals, and the other for the coch- lea. The branches for the vestibule spread out and radiate on the inner surface of the membranous labyrinth : their exact determination is unknown. Those for the semicircular canals pass into the ampullae, and form, within each of them, a forked projection which corresponds with a septum in the interior of the ampulla. The branches for the cochlea enter it through orifices at the base of the modiolus, which they ascend, and thence successively pass into canals in the osseous part of the lamina spiralis. In the canals of this osseous part or zone, 45 534 THE SENSE OF HEARING. the nerves are arranged in a plexus, containing ganglion cells. Their ultimate termination is not known with certainty ; but some of them, without doubt, end in the organ of Corti, prob- ably in cells. Physiology of Hearing. The acoustic portion of the physiology of hearing is thus il- lustrated by Miiller: chiefly in applications of the results of his experiments on the conduction of sonorous vibrations through various combinations of air, water, and solid sub- stances, especially membrane. All the acoustic contrivances of the organ of hearing are means for conducting the sound, just as the optical apparatus of the eye are media for conducting the light. Since all mat- ter is capable of propagating sonorous vibrations, the simplest conditions must be sufficient for mere hearing ; for all sub- stances surrounding the auditory nerve would communicate sound to it. In the eye a certain construction was required for directing the rays or undulations of light in such a man- ner that they should fall upon the optic nerve with the same relative disposition as that with which they issued from the object. In the sense of hearing this is not requisite. Sonorous vibrations, having the most various direction and the most unequal rate of succession, are transmitted by all media with- out modification, however manifold their decussations ; and, wherever these vibrations or undulations fall upon the organ of hearing and the auditory nerves, they must cause the sensa- tion of corresponding sounds. The whole development of the organ' of hearing, therefore, can have for its object merely the rendering more perfect the propagation of the sonorous vibra- tions, and their multiplication by resonance ; and, in fact, all the acoustic apparatus of the organ may be shown to have reference to these two principals. Functions of the External Ear. The external auditory passage influences the propagation of sound to the tympanum in three ways : 1, by causing the sonorous undulations, entering directly from the atmosphere, to be transmitted by the air in the passage immediately to the membrana tympani, and thus preventing them from being dispersed ; 2, by the walls of the passage conducting the so- norous undulations imparted to the external ear itself, by the shortest path to the attachment of the membrana tympani, and so to this membrane ; 3, by the resonance of the column of air contained within the passage. FUNCTIONS OF THE EXTERNAL EAR. 535 As a conductor of undulations of air, the external auditory passage receives the direct undulations of the atmosphere, of which those that enter in the direction of its axis produce the strongest impressions. The undulations which enter the pas- sage obliquely are reflected by its parietes, and thus by reflec- tion reach the membrana tympani. By reflection, also, the external meatus receives the undulations which impinge upon the concha of the external ear, when their angle of reflection is such that they are thrown towards the tragus. Other sonor- ous undulations again, which could not enter the meatus from the external air either directly or by reflection, may still be brought into it by inflection ; undulations, for instance, whose direction is that of the long axis of the head, and which pass over the surface of the ear, must, in accordance with the laws of inflection, be bent into the external meatus by its margins. But the action of those undulations which enter the meatus directly are most intense ; and hence we are enabled to judge of the point whence sound comes, by turning one ear in differ- ent directions, till it is directed to the point whence the vibra- tions may pass directly into the meatus, and produce the strong- est impressions. The walls of the meatus are also solid conductors of sound ; for those vibrations which are communicated to the cartilage of the external ear, and not reflected from it, are propagated by the shortest path through the parietes of the passage to the membrana tympani. Hence, both ears, being close stopped, the sound of a pipe is heard more distinctly when its lower extremity, covered with a membrane, is applied to the carti- lage of the external ear itself, than when it is placed in con- tact with the surface of the head. Lastly, the external auditory passage is important, inasmuch as the air which it contains, like all insulated masses of air, increases the intensity of sounds by resonance. To convince ourselves of this, we need only lengthen the passage by affix- ing to it another tube : every sound that is heard, even the sound of our own voice, is then much increased in intensity. The action of the cartilage of the external ear upon sonor- ous vibrations is partly to reflect them, and partly to condense and conduct them to the parietes of the external passage. With respect to its reflecting action, the concha is the most important part, since it directs the reflected undulations to- wards the tragus, whence they are reflected into the auditory passage. The other inequalities of the external ear do not promote hearing by reflection ; and, if the conducting power of the cartilage of the ear were left out of consideration, they might be regarded as destined for no particular use ; but re- 536 THE SENSE OF HEARING. ceiving the impulses of the air, the cartilage of the external ear, while it reflects a part of them, propagates within itself and condenses the rest, as all other solid and elastic bodies would do. Thus, the sonorous vibrations which it receives by an ex- tended surface, are conducted by it to its place of attachment. In consequence of the connection of the parietes of the audi- tory passage with the solid parts of the whole head, some dis- persion of the undulations will result ; but the points of at- tachment of the membrana tympani will receive them by the shortest path, and will as certainly communicate them to that membrane, as the solid sides of a drum communicate sonorous undulations to the parchment head, or the bridge of a musical string, its vibrations to the string. Regarding the cartilage of the external ear, therefore, as a conductor of sonorous vibrations, all its inequalities, elevations, and depressions, which are useless with regard to reflection, become of evident importance ; for those elevations and de- pressions upon which the undulations fall perpendicularly, will be affected by them in the most intense degree ; and, in conse- quence of the various form and position of these inequalities, sonorous undulations, in whatever direction they may come, must fall perpendicularly upon the tangent of some one of them. This affords an explanation of the extraordinary form given to this part. Functions of the Middle Ear : the Tympanum, Ossicula, and Fenestrce. In animals living in the atmosphere, the sonorous vibrations are conveyed to the auditory nerve by three different media in succession ; namely, the air, the solid parts of the body of the animal and of the auditory apparatus, and the fluid of the labyrinth. Sonorous vibrations are imparted too imperfectly from air to solid bodies, for the propagation of sound to the internal ear to be adequately effected by that means alone ; yet already an instance of its being thus propagated has been mentioned. In passing from air directly into water, sonorous vibrations suffer also a considerable diminution of their strength ; but if a tense membrane exists between the air and the water, the sonorous vibrations are communicated from the former to the latter medium with very great intensity. This fact, of which Mu'ller gives experimental proof, furnishes at once an expla- nation of the use of the fenestra rotunda, and of the membrane closing it. They are the means of communicating, in full in- tensity, the vibrations of the air in the tympanum to the fluid FUNCTIONS OF THE MIDDLE EAR. 537 of the labyrinth. This peculiar property of membranes is the result, not of their tenuity alone, but of the elasticity and ca- pability of displacement of their particles ; and it is not im- paired when, like the membrane of the fenestra rotunda, they are not impregnated with moisture. Sonorous vibrations are also communicated without any per- ceptible loss of intensity from the air to the water, when to the membrane forming the medium of communication, there is attached a short, solid body, which occupies the greater part of its surface, and is alone in contact with the water. This fact elucidates the action of the fenestra ovalis, and of the plate of the stapes which occupies it, and, with the preceding fact, shows that both fenestrse that closed by membrane only, and that with which the movable stapes is connected trans- mit very freely the sonorous vibrations from the air to the fluid of the labyrinth. A small, solid body, fixed in an opening by means of a border of membrane, so as to be movable, communicates sonor- ous vibrations from air on the one side, to water, or the fluid of the labyrinth, on the other side, much better than solid media not so constructed. But the propagation of sound to the fluid is rendered much more perfect if the solid conductor thus occupying the opening, or feiiestra ovalis, is by its other end fixed to the middle of a tense membrane, which has atmos- pheric air on both sides. A tense membrane is a much better conductor of the vibra- tions of air than any other solid body bounded by definite surfaces: and the vibrations are also communicated very readily by tense membranes to solid bodies in contact with them. Thus, then, the membrana tympani serves for the trans- mission of sound from the air to the chain of auditory bones. Stretched tightly in its osseous ring, it vibrates with the air in the auditory passage, as any thin tense membrane will when the air near it is thrown into vibrations by the sounding of a tuning-fork or a musical string. And, from such a tense vi- brating membrane, the vibrations are communicated with great intensity to solid bodies which touch it at any point. If, for example, one end of a flat piece of wood be applied to the membrane of a drum while the other end is held in the hand, vibrations are felt distinctly when the vibrating tuning-fork is held over the membrane without touching it ; but the wood alone, isolated from the membrane, will only very feebly propa- gate the vibrations of the air to the hand. The ossicula of the ear, which are represented in this experi- ment by a piece of wood, are the better conductors of the so- norous vibrations communicated to them, on account of being 538 THE SENSE OF HEARING. isolated by an atmosphere of air, and not continuous with the bones of the cranium; for every solid body thus isolated by a different medium propagates vibrations with more intensity through its own substance than it communicates them to the surrounding medium, which thus prevents a dispersion of the sound ; just as the vibrations of the air in the tubes used for conducting the voice from one apartment to another are pre- vented from being dispersed by the solid walls of the tube. The vibrations of the membraua tympani are transmitted, therefore, by the chain of ossicula to the fenestra ovalis and fluid of the labyrinth, their dispersion in the tympanum being prevented by the difficulty of the transition of vibrations from solid to gaseous bodies. The mernbrana tympani being a tense, solid body, bounded by free surfaces, the sonorous undu- lations will be partially reflected at its surfaces, so as to cause a meeting of undulations from opposite directions within it; it will, therefore, by resonance, increase the intensity of the vi- brations communicated to it, and the undulations, thus ren- dered more intense, will act, in their turn, upon the chain of auditory bones. The necessity of the presence of air on the inner side of the membrana tympani, in order to enable it and the ossicula au- ditus to fulfil the objects just described, is obvious. Without this provision, neither would the vibrations of the membrane be free, nor the chain of bones isolated, so as to propagate the sonorous undulations with concentration of their intensity. But while the oscillations of the membrana tympani are readily communicated to the air in the cavity of the tympanum, those of the solid ossicula will not be conducted away by the air, but will be propagated to the labyrinth without being dispersed in the tympanum. Equally necessary is the communication of the air in the tympanum with the external air, through the medium of the Eustachian tube, for the maintenance of the equilibrium of pressure and temperature between them. The propagation of sound through the ossicula of the tym- panum to the labyrinth must be effected either by oscillations of the bones, or by a kind of molecular vibration of their par- ticles, or, most probably, by both these kinds of motion. 1 1 Edouard Weber has shown that the existence of the membrane over the fenestra rotunda will permit approximation and removal of the stapes to and from the labyrinth. When by the stapes the mem- brane of the fenestra ovalis is pressed towards the labyrinth, the mem- brane of the fenestra rotunda may, by the pressure communicated through the fluid of the labyrinth, be pressed towards the cavity of the tympanum. FUNCTIONS OF THE MIDDLE EAR. 539 The long process of the malleus receives the undulations of the membrana tympani (Fig. 201, a, a) and of the air in a direction indicated by the arrows, FlG - 201 - nearly perpendicular to itself. From the long process of the malleus they are propa- gated to its head (6) ; thence into the incus (e), the long process of which is parallel with the long process of the malleus. From the long process of the incus the undulations are communicated to the stapes (d), which is united to the incus at right angles. The several changes in the direction of the chain of bones have, however, no influence on that of the undulations, which remains the same as it was in the meatus externus and long process of the malleus, so that the undula- tions are communicated by the stapes to the fenestra ovalis in a perpendicular direction. Increasing tension of the membrana tympani diminishes the facility of transition of sonorous undulations from the air to it. Mr. Savart observed that the dry membrana tympani, on the approach of a body emitting a loud sound, rejected particles of sand strewn upon it more strongly when lax than when very tense ; and inferred, therefore, that hearing is rendered less acute by increasing the tension of the membrana tympani. Mu'ller has confirmed this by experiments with small mem- branes arranged so as to imitate the membrana tympani ; and it may be confirmed also by observations on one's self. For the membrana tympani on one's own person may be rendered tense at will in two ways, namely, by a strong and continued effort of expiration or of inspiration while the mouth and nos- trils are closed. In the first case, the compressed air is forced with a whizzing sound into the tympanum, the membrana tym- pani is made tense, and immediately hearing becomes indis- tinct. The same temporary imperfection of hearing is pro- duced by rendering the membrana tympani tense, and convex towards the interior, by the effort of inspiration. The imper- fection of hearing, produced by the last-mentioned method, may continue for a time even after the mouth is opened, in consequence of the previous effort at inspiration having in- duced collapse of the walls of the Eustachian tube, which pre- vents the restoration of equilibrium of pressure between the air within the tympanum and that without : hence we have the opportunity of observing that even our own voice is heard with less intensity when the tension of the membrana tympani is great. 540 THE SENSE OF HEARING. If the pressure of the external air or atmosphere be very great, while on account of collapse of the walls of the Eusta- chian tube, the air in the interior of the tympanum fails to exert an equal counter-pressure, the membrana tympani will of course be forced inwards, and imperfect deafness be pro- duced. Thus it may be explained why, in a diving-bell, voices sound faintly. In all cases, the effect of the increased tension of the membrana tympani is not to render both grave and acute sounds equally fainter than before. On the contrary, as observed by Dr. Wollaston, the increased tension of the mem- brana tympani, produced by exhausting the cavity of the tympanum, makes one deaf to grave sounds only. The principal office of the Eustachian tube, in Muller's opinion, has relation to the prevention of these effects of in- creased tension of the membrana tympani. Its existence and openness will provide for the maintenance of the equilibrium between the air within the tympanum and the external air, so as to prevent the inordinate tension of the membrana tympani which would be produced by too great or too little pressure on either side. While discharging this office, however, it will serve to render sounds clearer, as (Henle suggests) the aper- tures in violins do ; to supply the tympanum with air ; and to be an outlet for mucus : and the ill effects of its obstruction may be referred to the hindrance of all these its offices, as well as of that one ascribed to it as its principal use. The influence of the tensor tympani muscle in modifying hearing may also be probably explained in connection with the regulation of the tension of the membrana tympani. If, through reflex nervous action, it can be excited to contraction by a very loud sound, just as the iris and orbicularis palpe- brarum muscle are by a very intense light, then it is manifest that a very intense sound would, through the action of this muscle, induce a deafening or muffling of the ears. In favor of this supposition we have the fact that a loud sound excites, by reflection, nervous action, winking of the eyelids, and, in persons of irritable nervous system, a sudden contraction of many muscles. The influence of the stapedius muscle in hearing is unknown. It acts upon the stapes in such a manner as to make it rest obliquely in the fenestra ovalis, depressing that side of it on which it acts, and elevating the other side to the same extent. When the fenestra ovalis and fenestra rotunda exist together with a tympanum, the sound is transmitted to the fluid of the internal ear in two ways, namely, by solid bodies and by membrane ; by both of which conducting media sonorous vibra- tions are communicated to water with considerable intensity. FUNCTIONS OF THE LABYRINTH. 541 The sound being conducted to the labyrinth by two paths, will, of course, produce so much the stronger impression ; for undu- lations will be thus excited in the fluid of the labyrinth from two different though contiguous points ; and by the crossing of these undulations stationary waves of increased intensity will be produced in the fluid. Miiller's experiments show that the same vibrations of the air act upon the fluid of the labyrinth with much greater intensity through the medium of the chain of auditory bones and the fenestra oval is than through the medium of the air of the tympanum and the membrane closing the fenestra rotunda : but the cases of disease in which the ossicula have been lost without loss of hearing, prove that sound may also be well conducted through the air of the tympanum and the membrane of the fenestra rotunda. Functions of the Labyrinth. The fluid of the labyrinth is the most general and constant of the acoustic provisions of the labyrinth. In all forms of organs of hearing, the sonorous vibrations affect the auditory nerve through the medium of liquid the most convenient medium, on many accounts, for such a purpose. The function usually ascribed to the semicircular canals is the collecting in their fluid contents, the sonorous undulations from the bones of the cranium. They have probably, also, in some degree, the power of conducting sounds in the direction of their curved cavities more easily than the sounds are carried off by the surrounding hard parts in the original direction of the undulations, though this conducting power is in them much less perfect than in tubes containing air. Admitting that they have these powers, the increased inten- sity of the sonorous vibrations thus attained will be of advan- tage in acting on the auditory nerve where it is expanded in the ampullae of the canals, and in the utriculus. Where the membranous canals are in contact with the solid parietes of the tubes, this action must be much more intense. But the membranous semicircular canals must have a function inde- pendent of the surrounding hard parts ; for in the Petromyzon they are not separately inclosed in solid substance, but lie in one common cavity with the utriculus. The crystalline pulverulent masses in the labyrinth would re- inforce the sonorous vibrations by their resonance, even if they did not actually touch the membranes upon which tjie nerves are expanded ; but, inasmuch as these bodies lie in contact with the membranous parts of the labyrinth, and the vestibur lar nerve-fibres are imbedded in them, they commupicate to 40 542 THE SENSE OF HEARING. these membranes and the nerves vibratory impulses of greater intensity than the fluid of the labyrinth can impart. This ap- pears to be the office of the otoconia. Sonorous undulations in water are not perceived by the hand itself immersed in the water, but are felt distinctly through the medium of a rod held in the hand. The fine hair-like prolongations from the epi- thelial cells of the ampullae have, probably, the same function. The cochlea seems to be constructed for the spreading out of the nerve fibres over a wide extent of surface, upon a solid lamina which communicates with the solid walls of the laby- rinth and cranium, at the same time that it is in contact with the fluid of the labyrinth, and which, besides exposing the nerve-fibres to the influence of sonorous undulations by two media, is itself insulated by fluid on either side. The connection of the lamina spiralis with the solid walls of the labyrinth, adapts the cochlea for the perception of the sonorous undulations propagated by the solid parts of the head and the walls of the labyrinth. The membranous labyrinth of the vestibule and semicircular canals is suspended free in the perilymph, and is destined more particularly for the per- ception of sounds through the medium of that fluid, whether the sonorous undulations be imparted to the fluid through the fenestrse, or by the intervention of the cranial bones, as when sounding bodies are brought into communication with the head or teeth. The spiral lamina on which the nervous fibres are expanded in the cochlea, is, on the contrary, continuous with the solid walls of the labyrinth, and receives directly from them the impulses which they transmit. This is an important advan- tage; for the impulses imparted by solid bodies, have, cceteris paribus, a greater absolute intensity than those communicated by water. And, even when a sound is excited in the water, the sonorous undulations are more intense in the water near the surface of the vessel containing it, than in other parts of the water equally distant from the point of origin of the sound : thus we may conclude that, cceteris paribus, the sonorous undu- lations of solid bodies act with greater intensity than those of water. Hence we perceive at once an important use of the cochlea. This is not, however, the sole office of the cochlea ; the spiral lamina, as well as the membranous labyrinth, receives sonor- ous impulses through the medium of the fluid of the labyrinth from the cavity of the vestibule and from the fenestra rotunda. The lamina spiralis is, indeed, much better calculated to render the action of these undulations upon the auditory nerve effi- cient, than the membranous labyrinth is ; for, as a solid body insulated by a different medium, it is capable of resonance. SENSIBILITY OF THE AUDITORY NERVE. 543 The rods of Corti are probably arranged so that each is set to vibrate in unison with a particular tone, and thus strike a particular note, the sensation of which is carried to the brain by those filaments of the auditory nerve with which the little vibrating rod is connected. The distinctive function therefore of these minute bodies is, probably, to render sensible to the brain the various musical notes and tones, one of them answering to one tone, and one to another ; while perhaps the other parts of the organ of hearing discriminate between the intensities of different sounds, rather than their qualities. Sensibility of the Auditory Nerve. Most frequently, several undulations or impulses on the auditory nerve concur in the production of the impressions of sound. By the rapid succession of several impulses at unequal inter- vals, a noise or rattle is produced ; from a rapid succession of several impulses at equal intervals, a musical sound results, the height or acuteness of which increases with the number of the impulses communicated to the ear within a given time. A sound of definite musical value is also produced when each one of the impulses, succeeding another thus at regular intervals, is itself compounded of several undulations, in such a way that, heard alone, it would give the impression of an unmusical sound; that is to say, by a sufficiently rapid succession of short unmusical sounds at regular intervals, a musical sound is generated. It would appear that two impulses, which are equivalent to four single or half vibrations, are sufficient to produce a definite note, audible as such through the auditory nerve. The note produced by the shocks of the teeth of a revolving wheel, at regular intervals upon a solid body, is still heard when the teeth of the wheel are removed in succession, until two only are left ; the sound produced by the impulse of these two teeth has still the same definite value in the scale of music. The maximum and minimum of the intervals of successive impulses still appreciable through the auditory nerve as de- terminate sounds, have been determined by M. Savart. If their intensity is sufficiently great, sounds are still audible which result from the succession of 48,000 half vibrations, or 24,000 impulses in a second ; and this, probably, is not the extreme limit in acuteness of sounds perceptible by the ear. For the opposite extreme, he has succeeded in rendering sounds audible which were produced by only fourteen or eighteen half 544 THE SENSE OF HEARING. vibrations, or seven or eight impulses in a second ; and sounds still deeper might probably be heard, if the individual im- pulses could be sufficiently prolonged. By removing one or several teeth from the toothed wheel before mentioned, M. Savart was also enabled to satisfy himself of the fact, that in the case of the auditory nerve, as in that of the optic nerve, the sensation continues longer than the impression which causes it ; for the removal of a tooth from the wheel produced no interruption of the sound. The gradual cessation of the sensation of sound renders it difficult, how- ever, to determine its exact duration beyond that of the im- pression of the sonorous impulses. The power of perceiving the direction of sounds is not a faculty of the sense of hearing itself, but is an act of the mind judging on experience previously acquired. From the modifi- cations which the sensation of sound undergoes according to the direction in which the sound reaches us, the mind infers the position of the sounding body. The only true guide for this inference is the more intense action of the sound upon one than upon the other ear. But even here there is room for much deception, by the influence of reflection or resonance, and by the propagation of sound from a distance, without loss of intensity, through curved conducting-tubes filled with air. By means of such tubes, or of solid conductors, which convey the sonorous vibrations from their source to a distant resonant body, sounds may be made to appear to originate in a new situation. The direction of sound may also be judged of by means of one ear only ; the position of the ear and head being varied, so that the sonorous undulations at one moment fall upon the ear in a perpendicular direction, at another moment obliquely. But when neither of these circumstances can guide us in dis- tinguishing the direction of sound, as when it falls equally upon both ears, its source being, for example, either directly in front or behind us, it becomes impossible to determine whence the sound comes. Ventriloquists take advantage of the difficulty with which the direction of sound is recognized, and also the influence of the imagination over our judgment, when they direct their voice in a certain direction, and at the same time pretend themselves to hear the sounds as coming from thence. The distance of the source of sounds is not recognized by the sense itself, but is inferred from their intensity. The sound itself is always seated but in one place, namely, in our ear ; but it is interpreted as coming from an exterior soniferous body. When the intensity of the voice is modified in imita- DIRECTION AND DISTANCE OF SOUNDS. 545 tion of the effect of distance, it excites the idea of its originat- ing at a distance; and this is also taken advantage of by ven- triloquists. The experiments of Savart, already referred to, prove that the effect of the action of sonorous undulations upon the nerve of hearing, endures somewhat longer than the period during which the undulations are passing through the ear. If, how- ever, the impression of the same sound be very long continued, or constantly repeated for a long time, then the sensation pro- duced may continue for a very long time, more than twelve or twenty-four hours even, after the original cause of the sound has ceased. This must have been experienced by every one who has travelled several days continuously ; for some time after the journey, the rattling noises are heard when the ear is not acted on by other sounds. We have here a proof that the perception of sound, as sound, is not essentially connected with the existence of undu- latory pulses ; and that the sensation of sound is a state of the auditory nerve, which, though it may be excited by a succes- sion of impulses, may also be produced by other causes. Even if it be supposed that undulations excited by the impulse are kept up in the auditory nerve for a certain time, they must be undulations of the nervous principle itself, which, being ex- cited, continue until the equilibrium is restored. Corresponding to the double vision of the same object with the two eyes, is the double hearing with the two ears ; and analogous to the double vision with one eye, dependent on unequal refraction, is the double hearing of a single sound with one ear, owing to the sound coming to the ear through media of unequal conducting power. The first kind of double hearing is very rare ; instances of it are recorded, however, by Sauvages and Itard. The second kind, which depends on the unequal conducting power of two media through which the same sound is transmitted to the ear, may easily be experi- enced. If a small bell be sounded in water, while the ears are closed by plugs, and a solid conductor be interposed be- tween the water and the ear, two sounds will be heard differ- ing in tensity and tone ; one being conveyed to the ear through the medium of the atmosphere, the other through the conduct- ing-rod. The sense of vision may vary in its degree of perfection as regards either the faculty of adjustment to different distances, the power of distinguishing accurately the particles of the retina affected, sensibility to light and darkness, or the per- ception of the different shades of color. In the sense of hear- ing, there is no parallel to the faculty by which the eye is 546 THE SENSE OF HEARING. accommodated to distance, nor to the perception of the particu- lar part of the nerve affected ; but just as one person sees dis- tinctly only in a bright light, and another only in a moderate light, so in different individuals the sense of hearing is more perfect for sounds of different pitch ; and just as a person whose vision for the forms of objects, &c., is acute, neverthe- less distinguishes colors with difficulty, and has no perception of the harmony and disharmony of colors, so one, whose hear- ing is good as far as regards the sensibility to feeble sounds, is sometimes deficient in the power of recognizing the musical relation of sounds, and in the sense of harmony and discord ; while another individual, whose hearing is in other respects imperfect, has these endowments. The causes of these differ- ences are unknown. Subjective sounds are the result of a state of irritaton or excitement of the auditory nerve produced by other causes than sonorous impulses. A state of excitement of this nerve, however induced, gives rise to the sensation of sound. Hence the ringing and buzzing in the ears heard by persons of irrita- ble and exhausted nervous system, and by patients with cerebral disease, or disease of the auditory nerve itself; hence also the noise in the ears heard for some time after a long journey in a rattling noisy vehicle. Hitter found that electricity also excites a sound in the ears. From the above truly subjective sound we must distinguish those dependent, not on a state of the audi- tory nerve itself merely, but on sonorous vibrations excited in the auditory apparatus. Such are the buzzing sounds atten- dant on vascular congestion of the head and ear, or on aneu- rismal dilatation of the vessels. Frequently even the simple pulsatory circulation of the blood in the ear is heard. To the sounds of this class belong also the snapping sound in the ear produced by a voluntary effort, and the buzz or hum heard during the contraction of the palatine muscles in the act of yawning ; during the forcing of air into the tympanum, so as to make tense the membrana tympani ; and in the act of blow- ing the nose, as well as during the forcible depression of the lower jaw. Irritation or excitement of the auditory nerve is capable of giving rise to movements in the body, and to sensations in other organs of sense. In both cases it is probable that the laws of reflex action, through the medium of the brain, come into play. An intense and sudden noise excites, in every person, closure of the eyelids, and, in nervous individuals, a start of the whole body or an unpleasant sensation, like that produced by an electric shock, throughout the body, and sometimes a particular feeling in the external ear. Various THE SENSE OF TASTE. 547 sounds cause in many people a disagreeable feeling in the teeth, or a sensation of cold trickling through the body, and, in some people, intense sounds are said to make the saliva col- lect. The sense of hearing may in its turn be affected by impres- sions on many other parts of the body ; especially in diseases of the abdominal viscera, and in febrile affections. Here, also, it is probable that the central organs of the nervous system are the media through which the impression is trans- mitted. SENSE OF TASTE. The conditions for the perception of taste are : 1, the pres- ence of a nerve with special endowments ; 2, the excitation of the nerves by the sapid matters, which for this purpose must be in a state of solution. The nerves concerned in the produc- tion of the sense of taste have been already considered (pp. 431 and 437). The mode of action of the substances which excite taste probably consists in the production of a change in the internal condition of the gustatory nerves ; and, according to the dif- ference of the substances, an infinite variety of changes of con- dition, and consequently of tastes, may be induced. It is not, however, necessary for the manifestation of taste that sapid substances in solution should be brought into contact with its nerves. For the nerves of taste, like the nerves of other special senses, may have their peculiar properties excited by various other kinds of irritation, such as electricity and me- chanical impressions. Thus Henle observed that a small cur- rent of air directed upon the tongue gives rise to a cool saline taste, like that of saltpetre ; and Dr. Baly has shown that a distinct sensation of taste, similar to that caused by electricity, may be produced by a smart tap applied to the papillae of the tongue. Moreover, the mechanical irritation of the fauces and palate produces the sensation of nausea, which is probably only a modification of taste. The matters to be tasted must either be in solution or be soluble in the moisture covering the tongue ; hence insoluble substances are usually tasteless, and produce merely sensations of touch. Moreover, for the perfect action of a sapid, as of an odorous substance, it is necessary that the sentient surface should be moist. Hence, when the tongne and fauces are dry, sapid substances, even in solution, are with difficulty tasted. The principal, but not exclusive seat of the sense of taste is the fauces and tongue. 548 THE SENSE OF TASTE. The tongue is a muscular organ covered by mucous mem- brane ; the latter resembling other mucous membranes (p. 316) in essential points of structure, but containing certain parts, the papillae, more or less peculiar to itself; peculiar, however, in details of structure and arrangement, not in their nature. The tongue is beset with numerous mucous follicles and glands. Its use in relation to mastication and deglutition has already been considered (p. 213). Besides other functions, the mucous membrane of the tongue serves as a groundwork for the ramification of the abundant bloodvessels and nerves which the tongue receives, and affords insertion to the extremities of the muscular fibres of which the chief substance of the organ is composed. The larger papillae of the tongue are thickly set over the an- terior two-thirds of its upper surface, or dor sum (Fig. 202), and give to it its characteristic roughness. Their greater promi- nence than those of the skin is due to their interspaces not being filled up with epithelium, as the interspaces of the papillae of the skin are. The papillae of the tongue present several di- versities of form ; but three principal varieties, differing both in seat and general characters, may usually be distinguished, namely, the circumvallate or calyciform, the funpiform, and the filiform papillae. Essentially these have all of them the same structure, that is to say, they are all formed by a projection of the mucous membrane, and contain special branches of bloodvessels and nerves. In details of structure, however, they differ considerably one from another. All the three varieties of papillae just described have been commonly regarded as simple processes, like the papillae of the skin; but Todd and Bowman have shown that the surface of each kind is studded by minute conical processes of mucous membrane, which thus form secondary papillae. These secon- dary papillae also occur over most other parts of the tongue, not occupied by the compound papillae, and extend for some dis- tance behind the papillae circumvallatae. The mucous mem- brane immediately in front of the epiglottis is, however, free from them. They are commonly buried beneath the epithe- lium ; hence they had been previously overlooked. Circumvallate or Calyciform Papillce. These papillae (Fig. 203), eight or ten in number, are situate in two V-shaped lines at the base of the tongue (1, 1, Fig. 202). They are circular elevations, from o^th to y^th of an inch wide, each with a central depression, and surrounded by a circular fissure, at the outside of which again is a slightly elevated ring, both the central elevation and the ring being formed of close-set simple papillae (Fig. 203). STRUCTURE OF THE TONGUE. 549 Fungiform Papillae. The fungiform papilla (Fig. 204) are scattered chiefly over the sides and tip, and sparingly over the FIG. 202. Papillar surface of the tongue, with the fauces and tonsils (from Sappey). 1, 1, circuruvallate papillae, in front of 2, the foramen caecum; 3, fungiform papillae; 4, filiform and conical papillae; 5, transverse and oblique rugae ; 6, mucous glands at the hase of the tongue and in the fauces; 7, tonsils; 8, part of the epiglottis; 9, median glosso-epiglottidean fold, frsenum epiglottidis. middle of the dorsum, of the tongue; their name is derived from their being usually narrower at their base than at their 550 THE SENSE OF TASTE. summit. They also consist of groups of simple papillae, each of which contains in its interior a loop of capillary blood- vessels, and a nerve-fibre. FIG. 203. Vertical section of the circumvallate papillae (from Kolliker). ij> . A, the papillae ; B, the surrounding wall ; a, the epithelial covering ; &, the nerves of the papilla and wall spreading towards the surface ; c, the secondary papillae. Conical or Filiform Papillae. These, which are the most abundant papillae, are scattered over the whole surface of the tongue, but especially over the middle of the dorsum. FIG. 204. a Surface and section of the fungiform papillae (from Kolliker, after Todd and Bow- man.) A, the surface of a fungiform papilla, partially denuded of its epithelium, 3^5. a, epithelium. B, section of a fungiform papilla with the bloodvessels injected ; a, artery ; v, vein ; c, capillary loops of simple papillae in the neighboring structure of the tongue. They vary in shape somewhat, but for the most part are conical or filiform, and covered by a thick layer of epidermis, which is arranged over them, either in an imbricated manner, or is prolonged from their surface in the form of fine, stiff projections, hair-like in appearance, and in some instances in structure also (Fig. 205). From their peculiar structure, it seems likely that these papillae have a mechanical function, or PAPILLJE OF THE TONGUE. 551 one allied to that of touch, rather than of taste ; the latter sense being probably seated especially in the other two va- rieties of papillae, the circumvallate and the fungiform. FIG. 205. ft 7, A c' a A. Vertical section near the middle of the dorsal surface of the tongue : a, a. Fungiform papillae. 6. Filiform papillae, with their hair-like processes, c. Similar ones deprived of their epithelium, magnified 2 diameters. B. Filiform compound papillae: a. Artery, v. Vein. c. Capillary loops of the secondary papillae, b. Line of basement-membrane, d. Secondary papillae, deprived of e, e, the epithelium. /. Hair-like processes of epithelium capping the simple papillae, magnified 25 diameters, g. Separated nucleated particles of epithelium, magnified 800 diameters. 1, 2. Hairs found on the surface of the tongue. 3, 4, 5. Ends of hair-like epithelial processes, showing varieties in the imbricated arrangement of the particles, but in all a coalescence of the particles towards the point. 5 incloses a soft hair, magnified 160 diameters. (Todd and Bowman.) The epithelium of the tongue is of the squamous or tessel- lated kind (p. 34). It covers every part of the surface ; but 552 THE SENSE OF TASTE. over the fungiform papillae forms a thinner layer than else- where, so that these papillae stand out more prominently than the rest. The epithelium covering the filiform papillae has been shown by Todd and Bowman to have a singular arrange- ment ; being extremely dense and thick, and, as before-men- tioned, projecting from their sides and summits in the form of long, stiff, hair-like processes. Many of these processes bear a close resemblance to hairs, and some actually contain minute hair-tubes. Bloodvessels and nerves are supplied freely to the papillae. The nerves in the fungiform and circurnvallate pa- pillae form a kind of plexus, spreading out brush-wise (Fig. 203), but the exact mode of termination of the nerve filaments is not certainly known. Such, in outline, is the structure of the sensitive surface of the tongue. But the tongue is not the only seat of the sense of taste ; for the results of experiments as well as ordinary ex- perience show that the soft palate and its arches, the uvula, tonsils, and probably the upper part of the pharynx, are en- dowed with taste. These parts, together with the base and posterior parts of the tongue, are supplied with branches of the glosso-pharyngeal nerve, and evidence has been already ad- duced (p. 437 et seq.) that the sense of taste is conferred upon them by this nerve. In most, though not in all persons, the anterior part of the tongue, especially the edges and tip, are endowed with the sense of taste. The middle of the dorsuni is only feebly en- dowed with the sense, probably because of the density and thickness of the epithelium covering the filiform papillae of this part of the tongue, which will prevent the sapid substances from penetrating to their sensitive parts. The gustatory prop- erty of the anterior part of the tongue is due, as already said (p. 431 ), to the lingual branches of the fifth nerve. Besides the sense of taste, the tongue, by means also of its papillae, is endued, especially at its sides and tip, with a very delicate and accurate sense of touch, which renders it sensible of the impressions of heat and cold, pain and mechanical pres- sure, and consequently of the form of surfaces. The tongue may lose its common sensibility, and still retain the sense of taste, and vice versa. This fact renders it probable that, al- though the senses of taste and of touch may be exercised by the same papillae supplied by the same nerves, yet the nervous conductors for these two different sensations are distinct, just as the nerves for smell and common sensibility in the nostrils are distinct ; and it is quite conceivable that the same nervous trunk may contain fibres differing essentially in their specific properties. Facts already detailed (p. 430) seem to prove that THE SENSE OF TASTE. 553 the lingual branch of the fifth nerve is the seat of sensations of taste in the anterior part of the tongue: and it is also certain, from the marked manifestations of pain to which its division in animals gives rise, that it is likewise a nerve of common sen- sibility. The glosso-pharyngeal also seems to contain fibres both of common sensation and of the special sense of taste. The concurrence of common and special sensibility in the same part makes it sometimes difficult to determine whether the impression produced by a substance is perceived through the ordinary sensitive fibres, or through those of the sense of taste. In many cases, indeed, it is probable that both sets of nerve-fibres are concerned, as when irritating acrid substances are introduced into the mouth. The impressions on the mind leading to the perception of taste seem to result, as already said, from certain changes in the internal condition of the nerves produced by the contact of sapid substances with the papillae in which the fibres of these nerves are distributed. This explanation, obscure though it be, may account generally for the sense ; but the variations of taste produced by different substances are as yet inexplicable. In the case of hearing, we know that sounds differ from one another according to the differences in the number of undula- tions producing them ; and in the case of vision, it is reasonably inferred that different colors result from differences in the num- ber of undulations, or in the rate of transit, of the principle of light. But, in the cases of taste and smell, no such probable explanation has yet been offered. It would appear, indeed, from the experiments of Horn, that while some substances taste alike in all regions of the tongue's surface, others excite different tastes, according as they are applied to different pa- pillae of the tongue. This observation, if confirmed, would seem to show that, in some cases at least, different fibres are capable of receiving different impressions from the same sapid substance. Much of the perfection of the sense of taste is often due to the sapid substances being also odorous, and exciting the sim- ultaneous action of the sense of smell. This is shown by the imperfection of the taste of such substances when their action on the olfactory nerves is prevented by closing the nostrils. Many fine wines lose much of their apparent excellence if the nostrils are held close while they are drunk. Very distinct sensations of taste are frequently left after the substances which excited them have ceased to act on the nerve ; and such sensations often endure for a long time, and modify the taste of other substances applied to the tongue afterwards. Thus, the taste of sweet substances spoils the flavor of wine, 554 THE SENSE OF TOUCH. the taste of cheese improves it. There appears, therefore, to exist the same relation between tastes as between colors, of which those that are opposed or complementary render each other more vivid, though no general principles governing this relation have been discovered in the case of tastes. In the art of cooking, however, attention has at all times been paid to the consonance or harmony of flavors in their combination or order of succession, just as in painting and music the fundamental principles of harmony have been employed empirically while the theoretical laws were unknown. Frequent and continued repetitions of the same taste render the perception of it less and less distinct, in the same way that a color becomes more and more dull and indistinct the longer the eye is fixed upon it. Thus, after frequently tasting first one and then the other of two kinds of wine, it becomes im- possible to discriminate between them. The simple contact of a sapid substance with the surface of the gustatory organ seldom gives rise to a distinct sensation of taste ; it needs to be diffused over the surface, and brought into intimate contact with the sensitive parts by compression, fric- tion, and motion between the tongue and palate. The sense of taste seems capable of being excited also by in- ternal causes, such as changes in the conditions of the nerves or nerve-centres, produced by congestion or other causes, which excite subjective sensations in the other organs of sense. But little is known of the subjective sensations of taste ; for it is difficult to distinguish the phenomena from the effects of ex- ternal causes, such as changes in the nature of the secretions of the mouth. SENSE OF TOUCH. The sense of touch is not confined to particular parts of the body of small extent, like the other senses ; on the contrary, all parts capable of perceiving the presence of a stimulus by ordinary sensation are, in certain degrees, the seat of this sense ; for touch is simply a modification or exaltation of common sensation or sensibility. The nerves on which the sense of touch depends are, therefore, the same as those which confer ordinary sensation on the different parts of the body, viz., those derived from the posterior roots of the nerves of the spinal cord, and the sensitive cerebral nerves. But, although all parts of the body supplied with sensitive nerves are thus, in some degree, organs of touch, yet the sense is exercised in perfection only in those parts the sensibility of which is extremely delicate, e.g., the skin, the tongue, and the THE SENSE OF TOUCH. 555 lips, which are provided with abundant papillae. (See chapter on SKIN, and section on TASTE.) The sensations of the common sensitive nerves have as pe- culiar a character as those of any other organ of sense. The sense of touch renders us conscious of the presence of a stimu- lus, from the slightest to the most intense degree of its action, neither by sound, nor by light, nor by color, but by that inde- scribable something which we call feeling, or common sensa- tion. The modifications of this sense often depend on the extent of the parts affected. The sensation of pricking, for example, informs us that the sensitive particles are intensely affected in a small extent ; the sensation of pressure indicates a slighter affection of the parts in a greater extent, and to a greater depth. It is by the depth to which the parts are affected that the feeling of pressure is distinguished from that of mere contact. Schiff and Brown-Sequard are of opinion that common sensibility and tactile sensibility manifest them- selves to the individual by the aid of different sets of fibres. Dr. Sieveking has arrived at the same conclusion from patho- logical observation. By the sense of touch the mind is made acquainted with the size, form, and other external characters of bodies. And in order that these characters may be easily ascertained, the sense of touch is especially developed in those parts which can be readily moved over the surface of bodies. Touch, in its more limited sense, or the act of examining a body by the touch, consists merely in a voluntary employment of this sense combined with movement, and stands in the same relation to the sense of touch, or common sensibility, generally, as the act of seeking, following, or examining odors, does to the sense of smell. Every sensitive part of the body which can, by means of movement, be brought into different relations of contact with external bodies, is an organ of "touch." No one part, consequently, has exclusively this function. The hand, how- ever, is best adapted for it, by reason of its peculiarities of structure, namely, its capability of pronation and supination, which enables it, by the movement of rotation, to examine the whole circumference of a body ; the power it possesses of op- posing the thumb to the rest of the hand ; and the relative mobility of the fingers. Besides the hand, and especially the fingers, are abundantly endowed with papillce and touch- eorpuscles (pp. 336, 337) which are specially necessary for the perfect employment of this sense. In forming a conception of the figure and extent of a sur- face, the mind multiplies the size of the hand or fingers used in the inquiry by the number of times which it is contained 556 THE SENSE OF TOUCH. in the surface traversed ; and by repeating this process with regard to the different dimensions of a solid body, acquires a notion of its cubical extent. The perfection of the sense of touch on different parts of the surface is proportioned to the power which such parts pos- sess of distinguishing and isolating the sensations produced by two points placed closed together. This power depends, at least in part, on the number of primitive nerve-fibres distributed to the part ; for the fewer the primitive fibres which an organ receives, the more likely is it that several impressions on differ- ent contiguous points will act on only one nervous fibre, and hence be confounded, and perhaps produce but one sensation. Experiments to determine the tactile properties of different parts of the skin, as measured by this power of distinguishing distances, were made by E. H. Weber. One experiment con- sisted in touching the skin, while the eyes were closed, with the points of a pair of compasses sheathed with cork, and in ascertaining how close the points of the compasses might be brought to each other, and still be felt as two bodies. He ex- amined in this manner nearly every part of the surface of the body, and has given tables showing the relative degrees of sen- sibility of different parts. Experiments of a similar kind have been performed also by Valentin ; and, among the nu- merous results obtained by both these investigators, it appears that the extremity of the third finger, and the point of the tongue are the parts most sensitive : a distance of as little as half a line being here distinguished. Next in sensitiveness to these is the mucous surface of the lips, which can perceive the two points of the compass when separated to the distance of about a line and a half: on the dorsum of the tongue they re- quire to be separated two lines. The parts in which the sense of touch is least acute are the neck, the middle of the back, the middle of the arm, and the middle of the thigh, on which the points of the compass have to be separated to the distance of thirty lines to be perceived as distinct points (Weber). Other parts of the body possess various degrees of sensibility intermediate between the above extremes. A sensation in a part endowed with touch appears to the mind to be, cceteris paribus, more intense when it is excited in a large extent of surface than when it is confined to a small space. The temperature of water into which he dipped his whole hand, appeared to Weber to be higher than that of water of really higher temperature, in which he immersed only one finger of the other hand. Similar observations may be made by persons bathing in warm or cold water. Part of the ideas which we obtain of the conditions of ex- THE SENSE OF TOUCH. 557 ternal bodies is derived through the peculiar sensibility with which muscles are endowed -the sensibility by which we are made acquainted with their position, and the degree of their contraction. By this sensation, we are enabled to estimate the degree of force exerted in resisting pressure or in raising weights. The estimate of weight by muscular effort is more accurate than that by pressure on the skin, according to Weber, who states that by the former a difference between two weights may be detected when one is only one-twentieth or one-fifteenth less than the other. It is not the absolute, but the relative, amount of the difference of weight which we have thus the faculty of perceiving. It is not, however, certain, that our idea of the amount of muscular force used is derived solely from sensation in the muscles. We have the power of estimating very accurately beforehand, and of regulating, the amount of nervous influ- ence necessary for the production of a certain degree of move- ment. When we raise a vessel, with the contents of which we are not acquainted, the force we employ is determined by the idea we have conceived of its weight. If it should happen to contain some very heavy substance, as quicksilver, we shall probably let it fall ; the amount of muscular action, or of ner- vous energy, which we had exerted, being insufficient. The same thing occurs sometimes to a person descending stairs in the dark ; he makes the movement for the descent of a step which does not exist. It is possible that in the same way the idea of weight and pressure in raising bodies, or in resisting forces, may in part arise from a consciousness of the amount of nervous energy transmitted from the brain rather than from a sensation in the muscles themselves. The mental conviction of the inability longer to support a weight must also be dis- tinguished from the actual sensation of fatigue in the muscles. So, with regard to the ideas derived from sensation of touch combined with movements, it is doubtful how far the conscious- ness of the extent of muscular movement is obtained from sen- sations in the muscles themselves. The sensation of movement attending the motions of the hand is very slight ; and persons who do not know that the action of particular muscles is nec- essary for the production of given movements, do not suspect that the movement of the fingers, for example, depends on an action in the forearm. The mind has, nevertheless, a very definite knowledge of the changes of position produced by movements ; and it is on this that the ideas which it conceives of the extension and form of a body are in great measure founded. In order that an impression made on a sensitive surface 47 558 THE SENSE OF TOUCH. may be perceived, it is necessary that there should exist a reciprocal influence between the mind and the sense of touch ; for, if the mind does not thus co-operate, the organic condi- tions for the sensation may be fulfilled, but it remains unper- ceived. Moreover, the distinctness and intensity of a sensa- tion in the nerves of touch depend, in great measure, on the degree in which the mind co-operates for its perception. A painful sensation becomes more intolerable the more the at- tention is directed to it : thus, a sensation in itself inconsider- able, as an itching in a very small spot of the skin, may be rendered very troublesome and enduring. As every sensation is attended with an idea, and leaves behind it an idea in the mind which can be reproduced at will, we are enabled to compare the idea of a past sensation with another sensation really present. Thus we can compare the weight of one body with another which we had previously felt, of which the idea is retained in our mind. Weber was indeed able to distinguish in this manner between tempera- tures, experienced one after the other, better than between temperatures to which the two hands were simultaneously subjected. This power of comparing present with past sensa- tions diminishes, however, in proportion to the time which has elapsed between them. The after-sensations left by impressions on nerves of common sensibility or touch are very vivid and durable. As long as the condition into which the stimulus has thrown the organ endures, the sensation also remains, though the exciting cause should have long ceased to act. Both painful and pleasurable sensations afford many examples of this fact. The law of contrast, which we have shown modifies the sen- sations of vision, prevails here also. After the body has been exposed to a warm atmosphere, a degree of temperature a very little lower, which would under other circumstances appear warm, produces the sensation of cold; and a sudden change to the extent of a few degrees from a cold temperature to one less severe, will produce the sensation of warmth. Heat and cold are, therefore, relative terms ; for a particular state of the sentient organs causes what would otherwise be warmth to appear cold. So, also a diminution in the intensity of a long- continued pain gives pleasure, even though the degree of pain that remains would in the healthy state have seemed intoler- able. Subjective sensations, or sensations dependent on internal causes, are in no sense more frequent than in the sense of touch. All the sensations of pleasure and pain, of heat and cold, of lightness and weight, of fatigue, &c., may be produced GENERATION AND DEVELOPMENT. 559 by internal causes. Neuralgic pains, the sensation of rigor, formication or the creeping of ants, and the states of the sexual organs occurring during sleep, afford striking examples of sub- jective sensations. The mind, also, has a remarkable power of exciting sensa- tions in the nerves of common sensibility ; just as the thought of the nauseous excites sometimes the sensation of nausea, so the idea of pain gives rise to the actual sensation of pain in a part predisposed to it. The thought of anything horrid excites the sensation of shuddering; the feelings of eager expectation, of pathetic emotion, of enthusiasm, excite in some persons a sensation of " concentration" at the top of the head, and of cold trickling through the body ; fright causes sensations to be felt in many parts of the body ; and even the thought of tickling excites that sensation in individuals very susceptible of it, when they are threatened with it by the movements of another person. These sensations from internal causes are most fre- quent in persons of excitable nervous systems, such as the hypochondriacal and the hysterical, of whom it is usual to say that their pains are imaginary. If by this is meant that their pains exist in their imagination merely, it is certainly quite in- correct. Pain is never imaginary in this sense ; but is as truly pain when arising from internal as from external causes ; the idea of pain only can be unattended with sensation, but of the mere idea no one will complain. Still, it is quite certain that the imagination can render pain that already exists more in- tense and can excite it when there is a disposition to it. CHAPTER XX. GENERATION AND DEVELOPMENT. THE several organs and functions of the human body which have been considered in the previous chapters, have relation to the individual being. We have now to consider those organs and functions which are destined for the propagation of the species. These comprise the several provisions made for the formation, impregnation, and development of the ovum, from which the embryo or foetus is produced and gradu- ally perfected into a fully-formed human being. The organs concerned in effecting these objects are named 560 GENERATION AND DEVELOPMENT. the geDerative organs, or sexual apparatus, since part belong to the male and part to the female sex. Generative Organs of the Female. The female organs of generation consist of two ovaries, whose function is the formation of ova ; of a Fallopian tube, or oviduct, connected with each ovary, for the purpose of con- ducting the ovum from the ovary to the uterus or cavity in which, if impregnated, it is retained until the embryo is fully developed, and fitted to maintain its existence, independently of internal connection with the parent; and, lastly, of a canal, of vagina, with its appendages, for the reception of the male generative organ in the act of copulation, and for the subse- quent discharge of the foetus. FIG. 206. Diagrammatic view of the uterus and its appendages, as seen from behind (from Quain). %. The uterus and upper part of the vagina have been laid open by re- moving the posterior wall ; the Fallopian tube, round ligament, and ovarian liga- ment have been cut short, and the broad ligament removed on the left side ; u. the upper part of the uterus; c, the cervix opposite the os internum ; the triangular shape of the uterine cavity is shown, and the dilatation of the cervical cavity with the rugae termed arbor vitse ; v, upper part of the vagina; od, Fallopian tube or ovi- duct ; the narrow communication of its cavity with that of the cornu of the uterus on each side is seen ; /, round ligament ; lo, ligament of the ovary ; o, ovary ; t, wide outer part of the right Fallopian tube ;fi, its fimbriated extremity; po, parovarium; A, one of the hydatids frequently found connected with the broad ligament. The ovaries are two oval compressed bodies, situated in the cavity of the pelvis, one on each side, inclosed in the folds of the broad ligament. Each ovary is attached to the uterus by FEMALE ORGANS OF GENERATION. 561 a narrow fibrous cord (the ligament of the ovary), and, more slightly, to the Fallopian tube by one of the fimbrise, into which the walls of the extremity of the tube expand. The ovary is enveloped by a capsule of dense nbro-cellular tissue, which again is surrounded by peritoneum. The internal structure of the organ consists of a peculiar soft fibrous tissue, or stroma, abundantly supplied with bloodvessels, and having imbedded in it, in various stages of development, numerous minute follicles or vesicles, the Graafian vesicles,- or sacculi, containing the ova (Fig. 207). A further account of the FIG. 207. View of a section of the prepared ovary of the cat (from Schron) 6. 1, outer cover- ing and free border of the ovary ; 1', attached border ; 2, the ovarian stroma, present- ing a fibrous and vascular structure ; 3, granular substance lying external to the fibrous stroma; 4, bloodvessels ; 5, ovigerms in their earliest stages occupying a part of the granular layer near the surface ; 6, ovigerms which have begun to enlarge and to pass more deeply into the ovary; 7, ovigerms round which the Graafian follicle and tunica granulosa are now formed, and which have passed somewhat deeper into the ovary and are surrounded by the fibrous stroma ; 8, more advanced Graafian follicle with the ovum imbedded in the layer of cells constituting the pro- ligerous disk; 9, the most advanced follicle containing the ovum, &c. ; 9', a follicle from which the ovum has accidentally escaped ; 10, corpus luteum. Graafian vesicles and of their contained ova will be presently given. The Fallopian tubes are about four inches in length, and ex- tend between the ovaries and the upper angles of the uterus. At the point of attachment to the uterus, the Fallopian tube is very narrow ; but in its course to the ovary it increases to about a line and a half in thickness ; at its distal extremity, which is free and floating, it bears a number of fimbrice, one of which, longer than the rest, is attached to the ovary. The 562 GENERATION AND DEVELOPMENT. canal by which each Fallopian tube is traversed is narrow, especially at its point of entrance into the uterus, at which it will scarcely admit a bristle ; its other extremity is wider, and opens into the cavity of the abdomen, surrounded by the zone of fimbrise. Externally, the Fallopian tube is invested with peritoneum ; internally, its canal is lined with mucous mem- brane, covered with ciliary epithelium (p. 37) : between the peritoneal and mucous coats, the walls are composed like those of the uterus, of fibrous tissue and organic muscular fibres (pp. 456-7). The uterus (u, c, Fig. 206) is a somewhat pyriform, fibrous organ, with a central cavity lined with mucous membrane. In the unimpregnated state it is about three inches in length, two in breadth at its upper part, or fundus, but at its lower pointed part or neck, only about half an inch. The part between the fundus and neck is termed the body of the uterus; it is about an inch in thickness. The walls of the organ are composed of dense fibro-cellular tissue, with which are intermingled fibres of organic muscle : in the impregnated state the latter are much developed and increased in number. The cavity of the uterus corresponds in form to that of the organ itself: it is very small in the unimpregnated state ; the sides of its mucous surface being almost in contact, and probably only separated from each other by mucus. Into its upper part, at each side, opens the canal of the corresponding Fallopian tube: below, it com- municates with the vagina by a fissure-like opening in its neck, the os uteri, the margins of which are distinguished into two lips, an anterior and posterior. In the mucous membrane of the cervix are found several mucous follicles, termed ovula or glaudulse Nabothi: they probably form the jelly-like substance by which the os uteri is usually found closed. The vagina is a membranous canal, six or eight inches long, extending obliquely downwards arid forwards from the neck of the uterus, which it embraces, to the external organs of generation. It is lined with mucous membrane, which, in the ordinary contracted state of the canal, is thrown into trans- verse folds. External to the mucous membrane, the walls of the vagina are constructed of fibro-cellular tissue, within which, especially around the lower part of the tube, is a layer of erectile tissue. The lower extremity of the vagina is embraced by an orbicular muscle, the constrictor vaginae ; its external orifice, in the virgin, is partially closed by a fold or ring of mucous membrane, termed the hymen. The external organs of generation consist of the clitoris, a small elongated body, situated above and in the middle line, and constructed, like the male penis, of two erectile corpora cavernosa, but unlike THE UNIMPREGNATED OVUM. 563 it, without a corpus spongiosum, and not perforated by the urethra ; of two folds of mucous membrane, termed labia in- terna, or nymphce; and, in front of these, of two other folds, the labia externa, or pudenda, formed of the external integu- ment, and lined internally by mucous membrane. Between the nymphse and beneath the clitoris is an angular space, termed the vestibule, at the centre of whose base is the orifice of the meatus urinarius. Numerous mucous follicles are scattered beneath the mucous membrane composing these parts of the external organs of generation ; and at the side of the fore part of the vagina, are two large lobulated glands, named vulvo-vaginal, or Duverney's glands, which are anal- ogous to Cowper's glands in the male. Having given this general outline of the several parts which, in the female, contribute to the reproduction of the species, it will now be necessary to examine successively the formation, discharge, impregnation, and development of the ovum, to which these several parts are subservient. Unimpregnated Ovum. If the structure and formation of the human ovary be ex- amined at any period between early infancy and advanced age, but especially during that period of life in which the power of conception exists, it will be found to contain, on an average, from fifteen to twenty small vesicles or membranous sacs of various sizes; these have been already alluded to as the follicles or vesicles of De Graaf, the anatomist who first accurately de- scribed them ; they are also sometimes called ovisacs. At their first formation, the Graafian vesicles, according to Schron, are near the surface of the stroma of the ovary, but subsequently become more deeply placed ; and again, as they increase in size, make their way towards the surface. When mature, they form little prominences on the exterior of the ovary, covered only by the peritoneum. Each follicle has an external mem- branous envelope, composed of fine fibro-cellular tissue, and connected with the surrounding stroma of the ovary by net- works of bloodvessels (Fig. 208). This envelope or tunic is lined with a layer of nucleated cells, forming a kind of epi- thelium or internal tunic, and named membrana granulosa. The cavity of the follicle is filled with an albuminous fluid in which microscopic granules float; and it contains also the ovum or ovule. The ovum is a minute spherical body situated, in immature follicles, near the centre ; but in those nearer maturity, in con- tact with the membrana granulosa at that part of the follicle 564 GENERATION AND DEVELOPMENT. which forms a prominence on the surface of the ovary. The cells of the membrana granulosa are at that point more numer- ous than elsewhere, and are heaped around the ovum, forming a kind of granular zone, the discus proligerus (Fig. 208.) In order to examine an ovum, one of the Graafian vesicles, it matters not whether it be of small size or arrived at maturity, should be pricked, and the contained fluid received upon a piece of glass. The ovum then, being found in the midst of the fluid by means of a simple lens, .may be further examined with higher microscopic powers. Owing to its globular form, however, its structure cannot be seen until it is subjected to gentle pressure. The human ovum is extremely small, measuring according to Bischoff, from 5 |(j to ^-Q- of an inch. Its external invest- ment is a transparent membrane, about ^QQ of an inch in thickness, which under the microscope, appears as a bright ring (Fig. 209), bounded externally and internally by a dark FIG. 208. Section of the Graafian vesicle of a Mammal, after Von Baer. 1. Stroma of the ovary with bloodvessels. 2. Peritoneum. 3 and 4. Layers of the external coat of the Graafian vesicle. 5. Membrana granulosa. 6. Fluid of the Graafian vesicle. 7. Granular zone, or discus proligerus, containing the ovum (8). Fitt. 209. Ovum of the sow, after Barry. 1. Germinal spot. 2. Germinal vesicle. 3. Yelk. 4. Zona pellucida. 5. Discus proligerus. 6. Adherent granules or cells. outline : it is called the zona pellucida, or vitelline membrane. It adheres externally to the heap of cells constituting the dis- cus proligerus. Within this transparent investment or zona pellucida, and usually in close contact with it, lies the yelk or vitellus, which is composed of granules and globules of various sizes, imbed- ded in a more or less fluid substance. The smaller granules, which are the more numerous, resemble in their appearance, as well as their constant motion, pigment-granules. The larger granules or globules, which have the aspect of fat-globules, are in greatest number at the periphery of the yelk. The number of the granules is, according to Bischoff, greatest in the ova of DEVELOPMENT OF OVUM. 565 carnivorous animals. In the human ovum their quantity is comparatively small. In the substance of the yelk is imbedded the germinal vesicle, or vesicula germinativa (Figs. 209, 210). This vesicle is of greatest relative size in the smallest ova, and is in them sur- rounded closely by the yelk, nearly in the centre of which it lies. During the development of the ovum, the germinal vesicle increases in size much less rapidly than the yelk, and comes to be placed near to its surface. Its size in the human ovum has not yet been ascertained, owing to the difficulty of isolat- ing it; but it is probably about ^^ of an inch in diameter. It consists of a fine, transparent, structureless membrane, con- Diagram of a Graafian vesicle, containing an ovum. 1. Stroma or tissue of the ovary. 2 and 3. External and internal tunics of the Graafian vesicle. 4. Cavity of the vesicle. 5. Thick tunic of the ovum or yelk-sac. 6. The yelk. 7. The germinal vesicle. 8. The germinal spot. taining a clear, watery fluid, in which are sometimes a few granules ; and at that part of the periphery of the germinal vesicle which is nearest to the periphery of the yelk is situated the germinal spot (macula germinativa}, a finely granulated sub- stance, of a yellowish color, strongly refracting the rays of light, and measuring, in the Mammalia generally, from ^g 1 ^ to ^7770 f an i ucn (Wagner). Such are the parts of which the Graafian follicle and its contents, including the ovum, are composed. The diagram (Fig. 210) represents them in their relative positions when mature. With regard to the mode and order of development of these parts there is considerable uncertainty; but it seems most likely that the ovum is formed before the Graafian vesi- cle or ovisac. 48 566 GENERATION AND DEVELOPMENT. With regard to the parts of the ovum first formed, it appears certain that the formation of the germinal vesicle precedes that of the yelk and zona pellucida, or vitelline membrane. Whether the germinal spot is formed first, and the germinal vesicle afterwards developed around it, cannot be decided in the case of vertebrate animals ; but the observations of Kol- liker and Bagge on the development of the ova of intestinal worms show that in these animals, the first step in the process is the production of round bodies resembling the germinal spots of ova, the germinal vesicles being subsequently devel- oped around these in the form of transparent membranous cells. The more important changes that take place in the ovum next to the formation of these its essential component parts, consist in alterations of the size and position of these parts with relation to each other, and of the ovum itself with rela- tion to the Graafian vesicle, and in the more complete elabora- tion of the yelk. The earlier the stage of development the larger is the germinal vesicle in relation to the whole ovum, and of the ovum in relation to the Graafian vesicle. For, as the ovum becomes mature, although all these parts increase in size, the Graafian vesicle enlarges most, and the germinal vesicle least. Changes take place also in the position of the parts. The ovum at first occupies the centre of the Graafian vesicle, but subsequently is removed to its periphery. The germinal vesicle, too, which in young ova is in the centre of the yelk, is in mature ova found at the periphery. The change of position of the ovum, from the centre to the periphery of the Graafian vesicle, is possibly connected with the formation of the membrana granulosa which lines the vesicle. For, according to Valentin, at a very early period, the contents of the vesicle between its wall and the ovum are almost wholly formed of granules ; but in the process of growth a clear fluid collects in the centre of the vesicle, and the granules, which from the first have a regular arrangement, are pushed outwards, and form the membrana granulosa. Now, as the mature ovum lies imbedded in a thickened por- tion of the membrana granulosa, it is possible that when the elementary parts of this membrane are pushed outwards, in the way just described, the ovum is carried with them from the centre to the periphery of the follicle. While the changes here described take place, the zona pellucida increases in thick- ness. According to Bischoff, the number of the granules of the yelk is greater the more mature the ovum, consequently the the yelk is more opaque in the mature, and more transparent DISCHARGE OF THE OVUM. 567 in the immature ova. The matter in which the granules are contained is fluid in the immature ova of all animals ; in some it remains so ; but in others, as the human ovum, it subse- quently becomes a consistent gelatinous substance. From the earliest infancy, and through the whole fruitful period of life, there appears to be a constant formation, devel- opment, and maturation of Graafian vesicles, with their con- tained ova. Until the period of puberty, however, the pro- cess is comparatively inactive ; for previous to this period, the ovaries are small and pale, the Graafian vesicles in them are very minute, few in number, and probably never attain full development, but soon shrivel and disappear, instead of burst- ing, as matured follicles do ; the contained ova are also inca- pable of being impregnated. But, coincident with the other changes which occur in the body at the time of puberty, the ovaries enlarge, and become very vascular, the formation of Graafian vesicles is more abundant, the size and degree of development attained by them are greater, and the ova are capable of being fecundated. Discharge of the Ovum. In the process of development of individual vesicles, it has been already observed, that as each increases in size, it gradu- ally approaches the surface of the ovary, and when fully ripe or mature, forms a little projection on the exterior. Coinci- dent with the increase of size, caused by the augmentation of its liquid contents, the external envelope of the distended vesicle becomes very thin and eventually bursts. By this means, the ovum and fluid contents of the Graafian vesicle are liberated, and escape on the exterior of the ovary, whence they pass into the Fallopian tube, the fimbriated processes of the extremity of which are supposed coincidentally to grasp the ovary, while the aperture of the tube is applied to the part corresponding to the matured and bursting vesicle. In animals whose capability of being impregnated occurs at regular periods, as in the human subject, and most Mammalia, the Graafian vesicles and their contained ova appear to arrive at maturity, and the latter to be discharged at such periods only. But in other animals, e. g., the common fowl, the formation, maturation, and discharge of ova appear to take place almost constantly. It has long been known, that in the so-called oviparous animals, the separation of ova from the ovary may take place independently of impregnation by the male, or even of sexual union. And it is now established that a like maturation and 568 GENERATION AND DEVELOPMENT. discharge of ova, independently of coition, occurs in Mamma- lia, the periods at which the matured ova are separated from the ovaries and received into the Fallopian tubes being indi- cated in the lower Mammalia, by the phenomena of heat or rut; in the human female by the phenomena of menstruation. Sexual desire manifests itself in the human female to a greater degree at these periods, and in the female of mammiferous animals at no other time. If the union of the sexes take place, the ovum may be fecundated, and if no union occur it perishes. That this maturation and discharge occur periodically, and only during the phenomena of heat in the lower Mammalia, is made probable by the facts that, in all instances in which Graafian vesicles have been found presenting the appearance of recent rupture, the animals were at the time, or had recently been, in heat; that on the other hand, there is no authentic and detailed account of Graafian vesicles being found ruptured in the intervals of the periods of heat; and that female animals do not admit the males, and never become impregnated, ex- cept at those periods. Many circumstances make it probable that the human female is subject, in these respects, to the same law as the females of other mammiferous animals ; namely, that in her as in them, ova are matured and discharged from the ovary independent of sexual union, and that this maturation and discharge occur periodically at the epochs of menstruation. Thus Graafian vesicles recently ruptured have been frequently seen in ovaries of virgins or women who could not have been recently impregnated, and although it is true that the ova dis- charged under these circumstances have rarely been discovered in the Fallopian tube, 1 partly on account of their minute size, and partly because the search has seldom been prosecuted with much care, yet analogy forbids us to doubt that in the human female, as in the domestic quadrupeds, the result and purpose of the rupture of the follicles is the discharge of the ova. The evidence of the periodical discharge of ova at the epochs of menstruation is, first, that nearly all authors who have touched on the point, agree that no traces of follicles having burst are ever seen in the ovaries before puberty or the first menstruation ; secondly, that in all cases in which ovarian follicles have been found burst, independently of sexual inter- course, the women were at the time menstruating, or had very recently passed through the menstrual state ; thirdly, that although conception is not confined to the periods of menstru- 1 See, however, the record of two such cases by Dr. Letheby, in the Philosophical Transactions, 1861. MENSTRUATION. 569 ation, yet it is more likely to occur within a few days after the cessation of the menstrual flux than at other times ; and, lastly, that the ovaries of the human female become turgid and vas- cular at the menstrual periods, as those of animals do at the time of heat. From what has been said, it may, therefore, be concluded that the two states, heat and menstruation, are analogous, and that the essential accompaniment of both, is the maturation and extrusion of ova. In both there is a state of active con- gestion of the sexual organs, sympathizing with the ovaries at the time of the highest degree of development of the Graafian vesicles; and, in both, the crisis of this state of congestion is attended by a discharge of blood or mucus, or both, from the external organs of generation. The occurrence of a menstrual discharge is one of the most prominent indications of the commencement of puberty in the female sex ; though its absence even for several years is not necessarily attended with arrest of the other characters of this period of life, or with inaptness for sexual union, or incapabil- ity of impregnation. The average time of its first appearance in females of this country and others of about the same latitude, is from fourteen to fifteen; but it is much influenced by the kind of life to which girls are subjected, being accelerated by habits of luxury and indolence, and retarded by contrary conditions. On the whole, its appearance is earlier in persons dwelling in warm climes than in those inhabiting colder lati- tudes; though the extensive investigations of Mr. Roberton show that the influence of temperature on the development of puberty has been exaggerated. Much of the influence attrib- uted to climate appears due to the custom prevalent in many hot countries, as in Hindostan, of giving girls in marriage at a very early age, and inducing sexual excitement previous to the proper menstrual time. The menstrual functions continue through the whole fruitful period of a woman's life, and usually cease between the forty-fifth and fiftieth years. The several menstrual periods usually occur at intervals of a lunar month, the duration of each being from three to six days. In some women the intervals are as short as three weeks or even less; while in others they are longer than a month. The periodical return is usually attended by pain in the loins, a sense of fatigue in the lower limbs, and other symptoms, which are different in different individuals. Menstruation does not usually occur in pregnant women, or in those who are suckling ; but instances of its occurrence in both these con- ditions are by no means rare. The menstrual discharge consists of blood effused from the 570 GENERATION AND DEVELOPMENT. inner surface of the uterus, and mixed with mucus from the uterus, vagina, and external parts of the generative apparatus. Being diluted by this admixture, the menstrual blood coagu- lates less perfectly than ordinary blood ; and the frequent acidity of the vaginal mucus tends still further to diminish its coagulability. This has led to the erroneous supposition that the menstrual blood contains an unusually small quantity of fibrin, or none at all. The blood-corpuscles exists in it in their natural state : mixed with them may also be found numerous scales of epithelium derived from the mucous passages along which the discharge flows. Corpus Luteum. Immediately before, as well as subsequent to, the rupture of a Graafian vesicle, and the escape of its ovum, certain changes ensue in the interior of the vesicle, which result in the produc- tion of a yellowish mass, termed a corpus luteum. When fully formed the corpus luteum of mammiferous animals is a roundish solid body, of a yellowish or orange color, and composed of a number of lobules, which surround, some- times a small cavity, but more frequently a small stelliform mass of white substance, from which delicate processes pass as septa between the several lobules. Very often, in the cow and sheep, there is no white substance in the centre of the corpus luteum ; and the lobules projecting from the opposite walls of the Graafian vesicle appear in a section to be separated by the thinnest possible lamina of semi-transparent tissue. When a Graafian vesicle is about to burst and expel the ovum, it becomes highly vascular and opaque ; and, immedi- ately before the rupture takes place, its walls appear thickened on the interior by a reddish glutinous or fleshy-looking sub- stance. Immediately after the rupture, the inner layer of the wall of the vesicle appears pulpy and flocculent. It is thrown into wrinkles by the contraction of the outer layer, and, soon, red fleshy mammillary processes grow from it, and gradually enlarge till they nearly fill the vesicle, and even protrude from the orifice in the external covering of the ovary. Subsequently this orifice closes, but the fleshy growth within still increases during the earlier period of pregnancy, the color of the sub- stance gradually changing from red to yellow, and its con- sistence becoming firmer. The corpus luteum of the human female (Fig. 211) differs from that of the domestic quadruped in being of a firmer tex- ture, and having more frequently a persistent cavity at its centre, and in the stelliform cicatrix, which remains in the CORPUS LUTEUM. 571 cases where the cavity is obliterated, being proportionately of much larger bulk. The quantity of yellow substance formed is also much less : and, although the deposit increases after the vesicle has burst, yet it does not usually form mammillary growths projecting into the cavity of the vesicle, and never protrudes from the orifice, as is the case in other Mammalia. It maintains the character of a uniform, or nearly uniform, layer, which is thrown into wrinkles, In consequence of the contraction of the external tunic of the vesicle. After the orifice of the vesicle has closed, the growth of the yellow sub- stance continues during the first half of pregnancy, till the cavity is reduced to a comparatively small size, or is obliterated ; in the latter case, nearly a white stelliform cicatrix remains in the centre of the corpus luteum. A Corpora lutea of different periods. B. Corpus luteum of about the sixth week after impregnation, showing its plicated form at that period. 1. Substance of the ovary. '2. Substance of the corpus luteum. 3. A grayish coagulum in its cavity. After Dr. Paterson. A. Corpus luteum, two days after delivery. D. In the twelfth week after delivery. After Dr. Montgomery. An effusion of blood generally takes place into the cavity of the Graah'an vesicle at the time of its rupture, especially in the human subject ; but it has no share in forming the yellow body; it gradually loses its coloring matter, and acquires the character of a mass of fibrin. The serum of the blood some- times remains included within a cavity in the centre of the coagulum, and then the decolorized fibrin forms a membrani- form sac, lining the corpus luteum. At other times the serum is removed, and the fibrin constitutes a solid stelliform mass. The yellow substance of which the corpus luteum consists, both in the human subject and in the domestic animals, is a growth from the inner surface of the Graafian vesicle, the re- sult of an increased development of the cells forming the mem- 572 GENERATION AND DEVELOPMENT. brana granulosa, which naturally Hues the internal tunic of the vesicle. The first changes of the internal coat of the Graafian vesicle in the process of formation of a corpus luteura, seem to occur in every case in which an ovum escapes ; as well in the human subject as in the domestic quadrupeds. If the ovum is im- pregnated, the growth of the yellow substance grows on during nearly the whole period of gestation, and forms the large cor- pus luteum commonly described as a characteristic mark of impregnation. If the ovum is not impregnated, the growth of yellow substance on the internal surface of the vesicle proceeds, in the human ovary, no further than the formation of a thin layer, which shortly disappears ; but in the domestic animals it continues for some time after the ovum has perished, and forms a corpus luteum of considerable size. The fact, that a structure, in its essential characters similar to, though smaller than, a corpus luteum observed during pregnancy, is formed in the human subject, independent of impregnation or of sexual union, coupled with the varieties in size of corpora lutea formed during pregnancy, necessarily renders unsafe all evidence of previous impregnation founded on the existence of a corpus luteum in the ovary. The following table by Daltou, expresses well the differences between the corpus luteum of the pregnant and uuimpregnated condition respectively. At the end of three weeks, Gne month, Two months, Six months, Nine months, CORPUS LUTEUM OF MEN- STRUATION. Three-quarters of an inch reddish ; convoluted wall Smaller; convoluted wall bright yellow; clot still reddish. Reduced to the condition of an insignificant cica- trix. Absent. Absent. CORPUS LUTEUM OP PREG- NANCY. in diameter; central clot pale. Larger; convoluted wall bright yollow ; clot still reddish. Seven-eighths of an inch in diameter; convoluted wall bright yellow ; clot perfectly decolorized. Still as large as at end of second mouth ; clot fib- rinous; convoluted wall paler. One-half an inch in diam- eter ; central clot con- verted into a radiating cicatrix ; the external wall tolerably thick and convoluted, but without anj 7 bright yellow color. IMPREGNATION OF THE OVUM. 573 IMPREGNATION OF THE OVUM. Male Sexual Functions. The fluid of the male, by which the ovum is impregnated, consists essentially of the semen secreted by the testicles ; and to this are added, as necessary, perhaps, to its perfection, a material secreted by the vesiculae serainales, in which, as in reservoirs, the semen lies before its discharge, as well as the secretion of the prostate gland, and of Cowper's glands. Por- tions of these several fluids are, probably, all discharged, to- gether with the proper secretion of the testicles. The secreting structure of the testicle is disposed in two contiguous parts (1), the body of the testicle inclosed within a tough fibrous membrane, the tunica albuginea, on the outer surface of which is the serous covering formed by the tunica vaginalis, and (2) the epididymw. The vas deferens, the main trunk of the secreting tube, when followed back to its origin, is found to pass to the lower part of the epididymis, and as- sumes there a much less diameter with a very tortuous course ; with its various convolutions it forms first the mass named globus minor, then the body, and then the globus major of the epididymis. At the last-named part, the duct divides into ten or twelve small branches, the convolutions of which form coniform masses, named coni vasculosi ; and the vessels con tinned from these, the vasa efferentia, after anastomosing, one with another, in what is called the rete testis, lead finally through the tubuli recti or vasa recta to the tubules which form the proper substance of the testicle, wherein they are arranged in lobules, closely packed, and all attached to the tough fibrous tissue at the back of the testicle. The seminal tubes, or tubuli seminiferi, which compose the proper substance of the testicle, are fine thread-like tubules, formed of simple homogeneous membrane, measuring on an average T i T th to ^^h of an inch in diameter, and lined with epithelium or gland-C3lls. Rarely branching, they extend as simple tubes through a great length, with the same uniform structure, and probably terminate either in free closed extremi- ties or in loops. Their walls are covered with fine capillary bloodvessels, through which, reckoning their great extent in 574 GENERATION AND DEVELOPMENT. comparison with the size of the spermatic artery, the blood must move very slowly. The seminal fluid secreted by the testicle is one of those se- cretions in which a process of development is continued after its formation by the secreting cells, and its discharge from them into the tubes. The principal part of this development con- sists in the formation of the peculiar bodies named seminal fila- ments, spermatozoa or spermatozoids (Fig. 213), the complete development of which, in their full proportion of number, is not achieved till the semen has reached, or has for some time lain in, the vesiculse seminales. Earlier, after its first secretion, the semen contains none of these bodies, but granules and round corpuscles (seminal corpuscles), like large nuclei, in- closed within parent-cells (Fig. 213). Within reach of these corpuscles, or nuclei, a seminal filament is developed, by a similar process in nearly all animals. Each corpuscle, or nu- cleus, is filled with granular matter ; this is gradually con- verted into a spermatozoid, which is at first coiled up, and in contact with the inner surface of the wall of the corpuscle CFig. 213, C, 1). Thus developed, the human seminal filaments consist of a long, slender, tapering portion, called the body or tail, to dis- tinguish it from the head, an oval or pyriform portion of larger diameter, flattened, and sometimes pointed. They are from 5- j- th to gj^th of an inch in length, the length of the head alone being from 70 Vi) ta to ^oVo tn f an i ncn > an( * i ts width about half as much. They present no trace of structure, or dissimilar organs ; a dark spot often observed in the head, is probably due to its being concave, like a blood-corpuscle. They move about in the fluid like so many minute corpuscles, with each a ciliary process, lashing their tails, and propelling their heads forwards in various lines. Their movement, which is probably essentially, as well as apparently, similar to that of ciliary processes, appears nearly independent of external conditions, provided the natural density of the fluid is pre- served ; disturbing this condition, by either evaporating the semen or diluting it, will stop the movement. It may continue within the body of the female for seven or eight days, and out of the body for at least nearly twenty-four hours. The direc- tion of the movement is quite uncertain ; but in general, the current that each excites keeps it from the contact of others. The rate of motion, according to Valentin, is about one inch in thirteen minutes. Respecting the purpose served by these seminal filaments, or concerning their exact nature, little that is certain can be said. Their occurrence in the impregnating fluid of nearly all SEMINAL FILAMENTS. 575 classes of animals, proves that they are essential to the process of impregnation ; but beyond this, and that their contact with the ovum is necessary for its development, nothing is known. A, spermatic filaments from the human vas deferens (from Kolliker). 1, magnified 850 diameters ; 2, magnified 800 diameters ; a, from the side ; 6, from above. B, sper- matic cells and spermatozoa of the bull undergoing development (from Kolliker) 15.0.. i ( spermatic cells, with one or two nuclei, one of them clear; 2, 3, free nuclei, with spermatic filaments forming; 4, the filaments elongated and the body widened ; 5, filaments nearly fully developed. C, es- cape of the spermatozoa from their cells in the same animal. 1, spermatic cell contain- ing the spermatozoon coiled up within it; 2, the cells elongated by the partial uncoil- ing of the spermatic filament ; 3, a cell from which the filament has in part become free ; 4, the same with the body also partially free ; 5, spermatozoon from the epididymis with vestiges of the cell adherent; 6, sper- matozoon from the vas deferens, showing the small enlargement, b, on the filament. The seminal fluid is, probably, after the period o puberty, secreted constantly, though, except under excitement, very slowly, in the tubules of the testicles. From these it passes 576 GENERATION AND DEVELOPMENT. along the vasa deferentia into the vesiculse seminales, whence, if not expelled in emission, it may be discharged, as slowly as it enters them, either with the urine, which may remove mi- nute quantities, mingled with the mucus of the bladder and the secretion of the prostate, or from the urethra in the act of defecation. The vcsiculce seminales have the appearance of outgrowths from the vasa deferentia. Each vas deferens, just before it enters the prostate gland, through part of which it passes to terminate in the urethra, gives off a side-branch, which bends back from it at an acute angle; and this branch dilating, variously branching, and pursuing in both itself and its branches a tortuous course, constructs the vesicula serninalis. Each of the vesiculse, therefore, might be unravelled into a single branching tube, sacculated, convoluted, and folded up. The mucous membrane lining the vesiculse seminales, like that of the gall-bladder, is- minutely wrinkled and set with folds and ridges arranged so as to give it a finely reticulated appearance. The rest of their walls is formed, chiefly of a FIG. 214. The base of the male bladder, with the .vesiculae semiuales and prostate gland. 1. The urinary bladder. 2. The longitudinal layer of muscular fibres. 3. The pro- state gland. 4 Membranous portion of the urethra. 5. The ureters. 6. Bloodves- sels. 7. Left; 8. Right vas deferens. 9. Left seminal vesicle in its natural posi- tion. 10. Dnctus ejaculatorius of the left side traversing the prostate gl and. 11. Right seminal vesicle injected and unravelled. 12, 13. Blind pouches of vesiculse. 14 Right ductus ejaculatorius traversing the prostate. (Haller.) layer of organic muscular fibres, from which they derive con- tractile power for the expulsion of their contents. To the vesiculse seminales a double function may be as- signed ; for they both secrete some fluid to be added to that of THE VESICUL^: SEMINALES. 577 the testicles, and serve as reservoirs for the seminal fluid. The former is their most constant and probably most important office ; for in the horse, bear, guinea-pig, and several other animals, in whom the vesiculse seminales are large and of ap- parently active function, they do not communicate with the vasa deferentia, but pour their secretions, separately, though it may be simultaneously, into the urethra. In man, also, when one testicle is lost, the corresponding vesicula seminalis suffers no atrophy, though its function as a reservoir is abro- gated. But how the vesiculse seminales act as secreting or- gans is unknown ; the peculiar brownish fluid which they con- tain after death does not properly represent their secretion, for it is different in appearance from anything discharged during life, and is mixed with semen. It is nearly certain, however, that their secretion contributes to the proper composition of the impregnating fluid ; for in all the animals in whom they exist, and in whom the generative functions are exercised at only one season of the year, the vesiculse seminales, whether they communicate with the vasa deferentia or not, enlarge commensurately with the testicles at the approach of that season. That the vesiculse are also reservoirs in which the seminal fluid may lie for a time previous to its discharge, is shown by their commonly containing the seminal filaments in larger abundance than any portion of the seminal ducts themselves do. The fluid-like mucus, also, which is often discharged from the vesiculse in straining during defecation, commonly contains seminal filaments. But no reason can be given why this office of the vesiculse should not be equally necessary to all the animals whose testicles are organized like those of man, or why in many animals the vesiculse are wholly absent. There is an equally complete want of information respecting the secretions of the prostate and Cowper's glands, their nature and purposes. That they contribute to the right composition of the impregnating fluid, is shown both by the position of the glands and by their enlarging with the testicles at the ap- proach of an animal's breeding-time. But that they contribute only a subordinate part is shown by the fact, that, when the testicles are lost, though these other organs be perfect, all procreative power ceases. The mingled secretions of all the organs just described, form the semen or seminal fluid. Its corpuscles have been already described (p. 574) ; its fluid part has not been satis- factorily analyzed; but Henle says it contains fibrin, because, shortly after being discharged, flocculi form in it by sponta- 578 GENERATION AND DEVELOPMENT. neous coagulation, and leave the rest of it thinner and more liquid, so that the filaments move in it more actively. Nothing has shown what it is that makes this fluid with its corpuscles capable of impregnating the ovum, or (what is yet more remarkable) of giving to the developing offspring all the characters, in features, size, mental disposition, and liability to disease, which belong to the father. This is a fact wholly in- explicable ; and is, perhaps, only exceeded in strangeness by those facts which show that the seminal fluid may exert such an influence, not only on the ovum which it impregnates, but, through the medium of the mother, on many which are sub- sequently impregnated by the seminal fluid of another male. It has been often observed, for example, that a well-bred bitch, if she have been once impregnated by a mongrel dog, will not bear thorough-bred puppies in the next two or three litters after that succeeding the copulation with the mongrel. But the best instance of the kind was in the case of a mare belong- ing to Lord Morton, who, while he was in India, wished to obtain a cross-breed between the horse and quagga, and caused this mare to be covered by a male quagga. The foal that she next bore had distinct marks of the quagga, in the shape of its head, black bars on the legs and shoulders, and other char- acters. After this time she was thrice covered by horses, and every time the foal she bore had still distinct, though decreas- ing marks of the quagga ; the peculiar characters of the quagga being thus impressed not only on the ovum then impregnated, but on the three following ova impregnated by horses. It would appear, therefore, that the constitution of an impreg- nated female may become so altered and tainted with the peculiarities of the impregnating male, through the medium of the foetus, that she necessarily imparts such peculiarities to any offspring she may subsequently bear by other males. Of the direct means by which a peculiarity of structure on the part of a male is thus transmitted, nothing whatever is known. DEVELOPMENT. Changes in the Ovum previous to the Formation of the Embryo. Of the changes which the ovum undergoes previous to the formation of the embryo, some occur while it is still in the ovary, and are apparently independent of impregnation : others take place after it has reached the Fallopian tube. The knowledge we possess of these changes is derived almost ex- clusively from observations on the ova of mammiferous ani- CLEAVAGE OF THE YELK. 579 mals, especially the bitch and rabbit : but it may be inferred that analogous changes ensue in the human ovum. Bischoff describes the yelk of an ovarian ovum after coitus as being unchanged in its characters, with the single exception of being fuller and more dense ; it is still granular, as before, and does not possess any of the cells subsequently found in it. The germinal vesicle always disappears, sometimes before the ovum leaves the ovary, at other times not until it has entered the Fallopian tube ; but always before the commencement of the metamorphosis of the yelk. As the ovum approaches the middle of the Fallopian tube, it begins to receive a new investment, consisting of a layer of transparent albuminous or glutinous substance, which forms upon the exterior of the zona pellucida. It is at first exceed- ingly fine, and, owing to this, and to its transparency, is not easily recognized : but at the lower part of the Fallopian tube it acquires considerable thickness. About this time, that is to say, during its passage through the Fallopian tube, a very remarkable change takes place in the interior of the ovum. The whole yelk becomes constricted in the middle, and surrounded by a furrow, which, gradually deepening, at length cuts the yelk in half, while the same process begins almost immediately in each half of the yelk, and cuts it also in two. The same process is repeated in each of the quarters, and so on, until at last by continual cleavings the whole yelk is changed into a mulberry-like mass of small and more or less rounded bodies, sometimes called "vitelline spheres," the whole still inclosed by the zona pellucida or vitel- line membrane (Fig. 215). Each of these little spherules con- tains a transparent vesicle, like an oil-globule, which is seen with difficulty, on account of its being enveloped by the yelk- granules which adhere closely to its surface. The cause of this singular subdivision of the yelk is quite obscure : though the immediate agent in its production seems so be the central vesicle contained in each division of the yelk. Originally there was probably but one vesicle, situated in the centre of the entire granular mass of the yelk, and probably derived from the germinal vesicle. This, by some process of multiplication, divides and subdivides : then each division and subdivision attracts around itself, as a centre, a certain portion of the substance of the yelk. About the time at which the manimiferous ovum reaches the uterus, the process of division and subdivision of the yelk appears to have ceased, its substance having been resolved into its ultimate and smallest divisions, while its surface pre- 580 GENERATION AND DEVELOPMENT. FIG. 215. sents a imiform finely-granular aspect, instead of its late mul- berry-like appearance. The ovum, indeed, appears at fii\ of umbilical vessel (after Goodsir). andei \ Harvey s experiments were very decisive on this point, ihe view has also received abundant support of late from Mr. Hutchinson's important observations on the communication of syphilis from the father to the mother, through the instrumentality of the foetus ; and still more from Mr. Savory's experimental researches, which prove quite clearly that the female parent may be directly inoculated through the foetus. Having opened the abdomen and uterus of a pregnant bitch, Mr. Savory injected a solution of strychnia into the abdominal cavity of one foetus, and into the thoracic cavity of another, and then replaced all the parts, every precaution being taken to prevent escape of the poison. In less than THE PLACENTA. 593 half an hour, the bitch died from tetanic spasms ; the foetuses operated on were also found dead, while the others were alive and active. The experiments, repeated on other animals with like results, leave no doubt of the rapid and direct transmis- sion of matter from the foetus to the mother, through the blood of the placenta. The placenta, therefore, of the human subject is composed of a, fatal part and a maternal part, the term, placenta, prop- erly including all that entanglement of foetal villi and mater- nal sinuses, by means of which the blood of the foetus is en- riched and purified after the fashion necessary for the proper growth and development of those parts which it is destined to nourish. The whole of this structure is not, as might be imagined, thrown off immediately after birth. The greater part, indeed, comes away at that time, as the afterbirth, and the separation of this portion takes place by a rending or crushing through of that part at which its cohesion is least strong, namely, where it is most burrowed and undermined by the cavernous spaces before referred to. In this way it is cast off with the foetal membranes and the decidua vera and reflexa, together with a part of the decidua serotina. The remaining portion withers, and disappears by being gradually either absorbed, or thrown off in the uterine discharges or the lochia, which occur at this period. A new mucous membrane is of course gradually developed, as the old one, by its peculiar transformation into what is called the decidua, ceases to perform its original functions. The umbilical cord, which in the latter part of foetal life is almost solely composed of the two arteries and the single vein which respectively convey foetal blood to and from the pla- centa, contains the remnants of other structures which in the early stages of the development of the embryo were, as already related, of great comparative importance. Thus, in early foetal life, it is composed of the following parts : (1.) Exter- nally, a layer of the amnion, reflected over it from the um- bilicus. (2.) The umbilical vesicle with its duct and apper- taining omphalo-mesenteric bloodvessels. (3.) The remains of the allantois, and continuous with it the urachus. (4.) The umbilical vessels, which as just remarked, ultimately form the greater part of the cord. DEVELOPMENT OF ORGANS. It remains now to consider in succession the development of the several organs and systems of organs in the further prog- ress of the embryo. 594 GENERATION AND DEVELOPMENT. Development of the Vertebral Column and Cranium. The primitive part of the vertebral column in all the Ver- tebrata is the gelatinous chorda dorsalis, which consists entirely of cells. This cord tapers to a point at the cranial and caudal extremities of the animal. In the progress of its development, it is found to become inclosed in a membranous sheath, which at length acquires a fibrous structure, composed of transverse annular fibres. The chorda dorsalis is to be regarded as the azygos axis of the spinal column, and, in particular, of the future bodies of the vertebrae, although it never itself passes into the cartilaginous or osseous state, but remains inclosed as in a case within the persistent parts of the vertebral column which are developed around it. It is permanent, however, only in a few animals: in the majority it disappears at an early period. The cartilaginous or osseous vertebrae are always first de- veloped in pairs of lateral elements at the sides of the chorda dorsalis. From these lateral elements are formed the bodies and the arches of the vertebrae. In some animals, as the stur- geon, however, the lateral elements of the vertebrae undergo no further development, and it is here that the chorda dorsalis is persistent through life. In the myxinoid fishes the spinal column presents no vertebral segments, and there exists merely the chorda dorsalis with the fibrous layer surrounding its sheath, which is the layer in which the skeleton originates. This fibrous layer also forms superiorly the membranous cover- ing of the vertebral canal. In reptiles, birds, and mammals, the mode in which the vertebrae are formed around the chorda dorsalis seems to be different. When the formation of these parts from the blastema commences, there appears at each side of the chorda dorsalis a series of quadrangular figures, the rudiments of the future vertebrae. These gradually increase in number and size, so as to surround the chorda both above and below, sending out, at the same time, superiorly, processes to form the arches des- tined to inclose the spinal cord. In this primitive condition the body and arches of each vertebra are formed by one piece on each side. At a certain period these two primary elements, which have become cartilaginous, unite iuferiorly by a suture. The chorda is now inclosed in a case, formed by the bodies of the vertebrae, but it gradually wastes and disappears. Before the disappearance of the chorda, the ossification of the bodies and arches of the vertebrae begins at distinct points. The ossification of the body of a vertebra is first observed at the point where the two primitive elements of the vertebrae FACE AND VISCERAL ARCHES. 595 have united inferiorly. Those vertebrae which do not bear ribs, such as the cervical vertebrae, have generally an additional centre of ossification in the transverse process, which is to be regarded as an abortive rudiment of a rib. In the foetal bird, these additional ossified portions exist in all the cervical ver- tebrae, and gradually become so much developed in the lower part of the cervical region as to form the upper false ribs of this class of animals. The same parts exist in mammalia and man ; those of the last cervical vertebrae are the most developed, and in children may, for a considerable period, be distinguished as a separate part on each side, like the root or head of a rib. The true cranium is a prolongation of the vertebral column, and is developed at a much earlier period than the facial bones. Originally, it is formed of but one mass, a cerebral capsule, the chorda dorsalis being continued into its base, and ending there with a tapering point. This relation of the chorda dor- salis to the basis of the cranium is persistent through life in some fish, e. g., the sturgeon. The first appearance of a solid support at the base of the cranium observed by Miiller in fish, consists of two elongated bands of cartilage, one on the right and the other on the left side, which are connected with the cartilaginous capsule of the auditory apparatus, and united with each other in an arched manner anteriorly beneath the anterior end of the cerebral capsule. Hence, in the cranium, as in the spinal column, there are at first developed at the sides of the chorda dorsalis two symmetrical elements, which subsequently coalesce, and may wholly inclose the chorda. 1 Development of the Face and Visceral Arches. It has been said before that at an early period of develop- ment of the embryo, there grow up on the sides of the primi- tive groove the so-called dorsal lamince, which at length coalesce, and complete by their union the spinal canal. The same process essentially takes place in the head, so as to in- close the cranial cavity. The so-called visceral lamince have been also described as passing forwards, and gradually coalescing in front, as the dorsal laminae do behind, and thus inclosing the thoracic and abdominal cavity. An analogous process occurs in the facial and cervical regions, but the inclosing laminae, instead of being simple, as in the former instances, are cleft. 1 For much new and original matter relating to the development of the cranium, the reader is referred to the important lectures on Comparative Anatomy, delivered at the College of Surgeons by Pro- fessor Huxley. 596 GENERATION AND DEVELOPMENT. In this way the so-called visceral arches and clefts are formed, four on each side (Fig. 230, A), and from or in connection with these arches the following parts are developed : From the first arch, and its maxillary process, the superior maxillary, the palate bone, and the internal pteryyoid plate of the sphenoid bone, the incus and malleus and the lower jaw. FlG. 230. A, magnified view from before of the head and neck of a human embryo of about three weeks (from Ecker) 1, anterior cerebral vesicle or cerebrum ; 2, middle ditto ; 3, middle or fronto-nasal process ; 4, superior maxillary process ; 5, eye ; 6, inferior maxillary process, or first visceral arch, and below it the first cleft; 7, 8, 9, second, third, and fourth arches and clefts. B, anterior view of the head of a human foatus of about the fifth week (from Ecker, as before, Fig. IV). 1, 2, 3, 5, the same parts as in A; 4, the external nasal or lateral frontal process; 6, the superior maxillary pro- cess ; 7, the lower jaw ; X , the tongue ; 8, first branchial cleft becoming the meatus auditorius externus. The upper part of the face in the middle line is developed from the so-called fronto-nasal process (A, 3, Fig. 230). From the second arch are developed the stapes, the stapedius muscle, the styloid process of the temporal bone, the stylo-hyoid liga- ment, and the smaller cornu of the hyoid bone. From the third visceral arch, the greater cornu and body of the hyoid bone. In man and other mammalia the fourth visceral arch is indis- tinct. Development of the Extremities. The extremities are developed in a uniform manner in all vertebrate animals. They appear in the form of leaflike ele- vations from the parietes of the trunk (see Fig. 231), at points where more or less of an arch will be produced for them within. The primitive form of the extremity is nearly the same in all Vertebrata, whether it be destined for swimming, crawling, walking, or flying. In the human foetus the fingers are at first DEVELOPMENT OF VASCULAR SYSTEM. 597 united, as if webbed for swimming ; but this is to be regarded not so much as an approximation to the form of aquatic ani- FlG. 231. 'if A human embryo of the fourth week, 3^ lines in length. 1, the chorion ; 3, part of the amnion ; 4, umbilical vesicle with its long pedicle passing into the abdomen ; 7, the heart ; 8, the liver ; 9, the visceral arch destined to form the lower jaw, be- neath which are two other visceral arches separated by the branchial clefts ; 10, ru- diment of the upper extremity; 11, that of the lower extremity; 12, the umbilical cord ; 15, the eye ; 16, the ear ; 17, the cerebral hemispheres ; 18, the optic lobes or corpora quadrigemina. mals, as the primitive form of the hand, the individual parts of which subsequently become more completely isolated. Development of the Vascular System. The first development of the vascular system and heart in the germinal membrane has been already alluded to (p. 588). The earliest form of the heart presents itself as a solid com- pact mass of embryonic cells, similar to those of which the other organs of the body are constituted. It is at first un- provided with a cavity ; but this shortly makes its appearance, resulting apparently from the separation from each other of the cells of the central portion. A liquid is now formed in the still closed cavity, and the central cells may be seen float- ing within it. These contents of the cavity are soon observed to be propelled to and fro with a tolerable degree of regularity, owing to the commencing pulsations of the heart. These pul- sations take place even before the appearance of a cavity, and immediately after the first " laying down" of the cells from which the heart is formed. At first they seldom exceed from fifteen to eighteen in the minute. The fluid within the cavity 598 GENERATION AND DEVELOPMENT. of the heart shortly assumes the characters of blood. At the same time the cavity itself forms a communication with the great vessels in contact with it, and the cells of which its wall FIG. 232. Capillary bloodvessels of the tail of a young larval frog. Magnified 350 times (after Kolliker). a, capillaries permeable to blood ; b, fat-granules attached to the walls of the vessels, and concealing the nuclei ; c, hollow prolongation of a capillary ending in a point ; d, a branching cell with nucleus and fat-granules ; it communi- cates by three branches with prolongation of capillaries already formed ; e, e, blood- corpuscles still containing granules of fat. are composed are transformed into fibrous and muscular tis- sues, and into epithelium. Bloodvessels appear to be developed in two ways, according to the size of the vessels. In the formation of large bloodves- DEVELOPMENT OF VASCULAR SYSTEM. 599 sels, masses of embryonic cells similar to those from which the heart and other structures of the embryo are developed, ar- range themselves in the position, form, and thickness of the developing vessel. Shortly afterwards the cells in the interior of a column of this kind seem to be developed into blood-cor- puscles, while the external layer of cells is converted into the walls of the vessel. In the development of capillaries another plan is pursued. This has been well illustrated by Kolliker, as observed in the tails of tadpoles. The first lateral vessels of the tail have the form of simple arches, passing between the main artery and vein, and are produced by the junction of prolongations, sent from both the artery and vein, with certain elongated or star-shaped cells, in the substance of the tail. When these arches are formed and are permeable to blood, new prolonga- tions pass from them, join other radiated cells, and thus form secondary arches. In this manner, the capillary network ex- tends in proportion as the tail increases in length and breadth, and it, at the same time, becomes more dense by the formation, according to the same plan, of fresh vessels within its meshes. The prolongations by which the vessels communicate with the star-shaped cells, consist at first of narrow-pointed projections from the side of the vessels, which gradually elongate until they come in contact with the radiated processes of the cells. The thickness of such a prolongation often does not exceed that of a fibril of fibrous tissue, and at first it is perfectly solid ; but, by degrees, especially after its junction with a cell, or with another prolongation, or with a vessel already per- meable to blood, it enlarges, and a cavity then forms in its in- terior (see Fig. 232). With Kolliker's account, our own ob- servations, made on the fine gelatinous tissue conveying the umbilical vessels of a sheep's embryo to the uterine cotyledons, completely accord. This tissue is well calculated to illustrate the various steps in the development of bloodvessels from elongating and branching cells. About the time that the heart at its lowest extremity re- ceives the venous trunks, and at its upper extremity gives off the large arterial trunk, it becomes curved from a straight into a horseshoe form, and shortly divides into three cavities (Fig. 233). Of these three cavities, which are developed in all Vertebrata, the most posterior is the simple auricle; themidde one the simple ventricle ; and the most anterior the bulbus arteriosus. These three parts of the heart contract in succes- sion. The auricle and the bulbus arteriosus at this period lie at the extremities of the horseshoe. The bulging out of the middle portion inferiorly gives the first indication of the future 600 GENERATION AND DEVELOPMENT. form of the ventricle (see Fig. 233). The great curvature of the horseshoe by the same means becomes much more developed than the smaller curvature between the auricle and bulbus; FIG. 233. Heart of the chick at the 45th, 65th, and 85th hours, of incubation. 1, the venous trunks; 2, the auricle; 3, the ventricle; 4, the bulbus arteriosus (after Dr. Allen Thomson). and the two extremities, the auricle and bulb, approach each other superiorly, so as to produce a greater resemblance to the latter form of the heart, whilst the ventricle becomes more and more developed inferiorly. The heart of fishes retains these three cavities, no further division by internal septa into right and left chambers taking place. In Amphibia, also, the heart throughout life consists of the three muscular divi- sions which are so early formed in the embryo ; but the auricle is divided internally by a septum into a pulmonary and sys- temic auricle. In reptiles, not merely the auricle is thus divided into two cavities, but a similar septum is more or less developed in the ventricle. In birds, mammals, and the human subject, both auricle and ventricle undergo complete division by septa ; whilst in these animals as well as in reptiles, the bulbus aortse is not permanent, but becomes lost in the ven- tricles. The septum dividing the ventricle commences at the apex and extends upwards. When it is complete, a septum is developed in the bulbus aortse, separating the roots of the proper aorta and the pulmonary artery. The septum of the auricles is developed from a semilunar fold, which extends from above downwards. In man, the septum between the ventricles, according to Meckel, begins to be formed about the fourth week, and at the end of eight weeks is complete. The septum of the auricles, in man and all animals which possess it, remains imperfect throughout foetal life. When the par- tition of the auricles is first commencing, the two venae cavse have different relations to the two cavities. The superior cava enters, as in the adult, into the right auricle ; but the inferior cava is so placed that it appears to enter the left auricle ; and the posterior part of the septum of the auricles is formed by the Eustachian valve, which extends from the point of en- THE FCETAL CIRCULATION. 601 trance of the inferior cava. Subsequently, however, the septum, growing from above downwards, becomes directed more and more to the left of the vena cava inferior. During the entire period of foetal life, there remains an opening in the septum, which the valve of the foramen ovale, developed in the third month, imperfectly closes. Circulation of Blood in the Fcetus. The circulation of blood in the foetus is peculiar, and differs considerably from that of the adult. It will be well, perhaps, to begin its description by tracing the course of the blood, which, after being carried out to the placenta by the two um- bilical arteries, has returned, cleansed and replenished, to the foetus by the umbilical vein. It is at first conveyed to the under surface of the liver, and there the stream is divided, a part of the blood passing straight on to the inferior vena cava, through a venous canal called the ductus venosus, while the remainder passes into the portal vein, and reaches the inferior vena cava only after cir- culating through the liver. Whether, however, by the direct route through the ductus venosus or by the roundabout way through the liver, all the blood which is returned from the placenta by the umbilical vein reaches the inferior vena cava at last, and is carried by it to the right auricle of the heart, into which cavity is also pouring the blood that has circulated in the head and neck and arms, and has been brought to .the auricle by the superior vena cava. It might be naturally ex- pected that the two streams of blood would be mingled in the right auricle, but such is not the case, or only to a slight ex- tent. The blood from the superior vena cava the less pure fluid of the two passes almost exclusively into the right ven- tricle, through the auriculo-ventricular opening, just as it does in the adult ; while the blood of the inferior vena cava is di- rected by a fold of the lining membrane of the heart, called the Eustachian valve, through the foramen ovale into the left auricle, whence it passes into the left ventricle, and out of this into the aorta, and thence to all the body. The blood of the superior vena cava, which, as before said, passes into the right ventricle, is sent out thence in small amount through the pul- monary artery to the lungs, and thence to the left auricle, as in the adult. The greater part, however, by far, does not go to the lungs, but instead, passes through a canal, the ductus arteriosus, leading from the pulmonary artery into the aorta just below the origin of the three great vessels which supply the upper parts of the body ; and there meeting that part of 602 GENERATION AND DEVELOPMENT. the blood of the inferior vena cava which has not gone into these large vessels, it is distributed with it to the trunk and lower parts, a portion passing out by way of the two umbili- FIG. 234. cal arteries to the placenta. From the placenta it is returned by the umbilical vein to the under surface of the liver, from which the description started. After birth the foramen ovale closes, and so do the ductus DEVELOPMENT OF ORGANS OF SENSE. 603 arteriosus and ductus venosus, as well as the umbilical vessels ; so that the two streams of blood which arrive at the right auricle by the superior and inferior vena cava respectively, thenceforth mingle in this cavity of the heart, and passing into the right ventricle, go byway of the pulmonary artery to the lungs, and through these, after purification, to the left auricle and ventricle, to be distributed over the body. (See chapter on Circulation.) Development of the Nervous System. The mode in which the rudimentary structures of the cere- bro-spinal nervous system are formed, has been already stated (p. 582). The dorsal laminae, the inner borders of which close in and form the canal of the spinal cord, seem to leave a fis- sure in the situation of the medulla oblongata. Between this and the most anterior extremity of the canal, three vesicular enlargements, the vesicles of the brain, are developed (see Fig. 217), and from these again are developed the following parts : From the anterior primary vesicle the optic thalami, cor- pora striata, the third ventricle, and the cerebral hemispheres, together with some other parts in connection with those above named, as the corpus callosum, fornix, &c. From the middle primary vesicle the corpora quadrigem- ina and crura cerebri, with the aqueduct of Sylvius. From the posterior primary vesicle the cerebellum, pons Varolii, medulla oblongata, &c. Development of the Organs of Sense. The eye is in part developed as a protruded portion of the first primary cerebral vesicle ; while passing backwards, and Diagram of development of the lens. ABC. Different stages of development. 1. Epidermic layer. 2. Thickening of this layer. 3. Crystalline depression. 4. Primitive ocular vesicle, its anterior part pushed back by the crystalline depression. 5. Posterior part of the primitive ocular vesicle, forming the external layer of the secondary ocular vesicle. 6. Point of separation between the lens and the epidermic layer. 7. Cavity of the secondary ocular vesicle, occupied by the vitreous. 604 GENERATION AND DEVELOPMENT. pressing on the front of this process or primary optic vesicle, is a pouch of the common integument, which subsequently be- comes a shut sac, and in which is developed the lens and its capsule (Fig. 236). Subsequently there is protruded from below upwards, between the lens in front and the primary optic vesicle behind, another process or pouch, remaining for some time imperfect below, and called the secondary optic vesicle. The deficiency below contracts into what is called the ocular cleft, which subsequently becomes entirely obliterated. In connection with the primary optic vesicle are developed the FIG. 237. op FIG. 236. Diagrammatic sketch of a vertical longitudinal section through the eye- ball of a human foetus of fourweeks (after Kolliker) !!. The section is alittle to the side, so as to avoid passing through the ocular cleft ; c, the cuticle where it becomes later the cornea; I, the lens; op, optic nerve formed by the pedicle of the pri- mary optic vesicle ; vp, primary medullary cavity or optic vesicle ; p, the pigment layer of the choroid coat of the outer wall ; r, the inner wall forming the retina; vs, secondary optic vesicle containing the rudiment of the vitreous humor. FIG. 237. Transverse vertical section of the eyeball of a human embryo of four weeks (from Kolliker) 1 o o. The anterior half of the section is represented : pr, the remains of the cavity of the primary optic vesicle ; p, the inner part of the outer layer forming the choroidal pigment; r, the thickened inner part giving rise to the columnar and other structures of the retina; v, the commencing vitreous humor within the secondary optic vesicle ; v', the ocular cleft through which the loop of the central bloodvessel, a, projects from below ; I, the lens with a central cavity. retina from the invaginated portion, and the pigmentary portion of the choroid in connection with the outer part (Fig. 236). In the secondary optic vesicle the vitreous humor is formed. The outer walls of the eyeball, the sclerotic and cornea, are developed from the tissues immediately around those which have been just described. The iris is formed rather late, as a circular septum project- DEVELOPMENT OF THE EYEBALL. 605 ing inwards, from the fore part of the choroid, between the lens and the cornea. In the eye of the foetus of Mammalia, the pupil is closed by a delicate membrane, the membrana pupillaris, which forms the front portion of a highly vascular membrane that, in the foetus, surrounds the lens, and is named the membrana capsulo-pupillaris. It is supplied with blood by FIG. 238. Bloodvessels of the capsulo-pupillary membrane of a new-born kitten, magnified (from Kolliker). The drawing is taken from a preparation injected by Tiersch, and shows in the central part the convergence of the network of vessels in the pupil- lary membrane. a branch of the arteria centralis retince, which, passing for- wards to the back of the lens, there subdivides. The mem- brana capsulo-pupillaris withers and disappears in the human subject a short time before birth. The eyelids of the human subject and mammiferous animals, like those of birds, are first developed in the form of a ring. They then extend over the globe of the eye until they meet and become firmly agglutinated to each other. But before birth, or in the Garni vora after biHh, they again separate. The ear likewise, according to Huschke, consists of a part developed from within, and of one formed externally. The labyrinth is developed upon the hollow protruded part of the brain which forms the auditory nerve. It appears first in the form of an elongated vesicle at the hinder part of the head of very young embryos above the second so-named branchial cleft. From it is developed a second vesicle, the rudiment of the cochlea, the convolutions of which are then formed. The 51 606 GENERATION AND DEVELOPMENT. semicircular canals are produced as diverticula of the vestibule, which terminate by again communicating with the same cavity. The Eustachian tube, the cavity of the tympanum, and the external auditory passage, are remains of the first branchial cleft. The membrana tympani divides the cavity of this cleft into an internal space, the tympanum, and the external meatus. The mucous membrane of the mouth, which is prolonged in the form of a diverticulum through the Eustachian tube into the tympanum, and the external cutaneous system, come into relation with each other at this point ; the two membranes being separated only by the proper membrane of the tym- panum. Development of the Alimentary Canal. The alimentary canal, the early stage of whose development has been already referred to (p. 585), is at first a uniform straight tube, which gradually becomes divided into its special parts, stomach, small intestine, and large intestine (Fig. 239). Outlines of the form and position of the alimentary canal in successive stages of its development (from Quain). A, alimentary canal, &c., in an embryo of four weeks ; B, at six weeks ; C, at eight weeks ; D, at ten weeks ; I, the primitive lungs connected with the pharynx ; s, the stomach ; d, duodenum ; i, the small intestine ; i', the large ; c, the caecum and vermiform appendage ; r, the rectum ; cl, in A, the cloaca ; a, in B, the anus, distinct from si, the sinus uro-genitalis ; v, the yelk-sac ; vi, the vitello-intestinal duct ; u, the urinary bladder and urachus leading to the allantois; g, genital ducts. DEVELOPMENT OF THE LIVER. 607 The stomach originally has the same direction as the rest of the canal ; its cardiac extremity being superior, its pylorus in- ferior. The changes of position which the alimentary canal undergoes may be readily gathered from the accompanying figures. The principal glands in connection with the intestinal canal are the salivary, pancreas, and the liver. In Mammalia, each salivary gland first appears as a simple canal with bud-like processes (Fig. 240), lying in a gelatinous nidus or blastema, and communicating with the cavity of the mouth. As the development of the gland advances, the canal becomes more and more rainitied, increasing at the expense of the blastema FIG. 241. FIG. 240. FIG. 240. First appearaace of the parotid gland in the embryo of a sheep. FIG. 241. Lobules of the parotid, with the salivary ducts, in the embryo of the sheep, at a more advanced stage. in which it is still inclosed. The branches or salivary ducts constitute an independent system of closed tubes (Fig. 241). The pancreas is developed exactly as the salivary glands. The liver in the embryo of the bird is developed by the protrusion, as it were, of a part of the walls of the intestinal canal, in the form of two conical hollow branches, which em- brace the common venous stem (Fig. 242). The outer part of these cones involves the omphalo-meseuteric vein, which breaks up in its interior into a plexus of capillaries, ending in venous 608 GENERATION AND DEVELOPMENT. trunks for the conveyance of the blood to the heart. The inner portion of the cones forms the cellular structure of the organ into which the bloodvessels extend, and in which they FIG. 242. Rudiments of the liver on th ' intestine of a chick at the fifth day of incubation, o, heart; b, intestine ; c, diverticulura of the intestine on which the liver (d) is de- veloped; e, part of the mucous layer of the germinal membrane. are, with the ducts, gradually developed. The gall-bladder is developed as a diverticulum from the hepatic duct. Development of the Respiratory Apparatus. The lungs, at their first development, appear as small tuber- cles, or diverticula from the abdominal surface of the oesoph- FIG. 243. A B C I u Illustrating the development of the respiratory organs. A, is the oesophagus of a chick on the fourth day of incubation, with the rudiments of the trachea on the lung of the left side, viewed laterally: 1, the inferior wall of the resophagus; 2, the upper wall of the same tube; 3, the rudimentary lung; 4, the stomach. B, is the same object seen from below, so that both lungs are visible, c, shows the tongue and respiratory organs of the embryo of a horse: 1, the tongue; 2, the larynx; 3, the trachea; 4, the lungs viewed from the upper side. (After Rathke.) agus. They are united at the anterior part of their circum- ference ; and here a pedicle is formed which becomes elongated into the trachea (see Fig. 243, A, B). Soon afterwards, the THE WOLFFIAN BODIES. 609 lung is seen to consist of a mass of csecal tubes issuing from the branches of the trachea. (Fig. 243, c.) The diaphragm is early developed. The Wolffian Bodies, Urinary Apparatus, and Sexual Organs. The Wolffian bodies are organs peculiar to the embryonic state, and may be regarded as temporary, rather than rudi- mental, kidneys; for although they seem to discharge the functions of these latter organs, they are not developed into them. They probably bear the same relation to the persistent kidneys that the branchiae of Amphibia do to the lungs which succeed them. In Mammalia, the Wolffian bodies (Fig. 244, W) are bean- shaped, and are composed of transverse csecal canals, united by an excretory duct (to), which leads from the lower extrem- ity of the organ to the sinus urogenitalis of the foetus (Fig. 244, ug\ The kidneys (r) and suprarenal capsules (sr) are developed behind them. Their size is at first so great that they entirely conceal the kidneys ; but in proportion as the latter bodies increase in size, they grow relatively smaller, and come to be placed more inferiorly. At length, towards the end of foetal life, only an atrophied remnant of them is left. Their ducts, in the male, are ultimately developed to form the vas deferens and ejaculatory duct of each side ; the vesiculse semi- nales forming diverticula from their lower part. In the female, the ducts of the Wolffian bodies disappear. The testicles or ovaries are formed independently at the internal excavated border of these organs; and at first it is not Cible to say which of them the testicle or ovary the new lation is to become. Gradually, however, the special char- acters belonging to one of them are developed ; and in either case the organ soon begins to assume a relatively lower posi- tion in the body ; the ovaries being ultimately placed in the pelvis ; while towards the end of foetal existence the testicles descend into the scrotum, the testicle entering the internal inguinal ring in the seventh month of foetal life, and complet- ing its descent through the inguinal canal and external ring into the scrotum by the end of the eighth month. A pouch of peritoneum, the processus vaginalis, precedes it in its descent, and ultimately forms the tunica vaginalis or serous covering of the organ ; the communication between the tunica vaginalis and the cavity of the peritoneum being closed only a short time before birth. In its descent, the testicle or ovary of course retains the bloodvessels, nerves, and lymphatics, which were supplied to it while in the lumbar region, and which are 610 GENERATION AND DEVELOPMENT. compelled to follow it, so to speak, as it assumes a lower posi- tion in the body. Hence the explanation of the otherwise strange fact of the origin of these parts at so considerable a distance from the organ to which they are distributed. The means by which the descent of the testicles into the scrotum is effected are not fully and exactly known. It was formerly believed that a membranous and partly muscular cord, called the gubernaculum testis, which extends while the testicle is yet high in the abdomen, from its lower part, through the abdominal wall (in the situation of the inguinal canal) to the front of the pubes and lower part of the scrotum, was the agent by the contraction of which the descent was effected. It is now generally believed, however, that such is not the case ; and that the descent of the testicle and ovary is rather the result of a general process of development in these and neighboring parts, the tendency of which is to produce this change in the relative position of these organs. In other words, the descent is not the result of a mere mechanical action, by which the organ is dragged down to a lower position, but rather one change out of many which attend the gradual development and rearrangement of these organs. It may be repeated, however, that the details of the process by which the descent of the testicle into the scrotum is effected are not accurately known. The homologue, in the female, of the gubernaculum testis, is a structure called the round ligament of the uterus, which ex- tends through the inguinal canal, from the outer and upper part of the uterus to the subcutaneous tissue in front of the symphysis pubis. At a very early stage of fostal life, the efferent ducts of the Wolffian bodies of the kidneys and of the ovaries or testes, open into a receptacle formed by the lower end of the allan- tois, or rudimentary bladder ; and as this communicates with the lower extremity of the intestine, there is for the time, a common receptacle or cloaca for all these parts, which opens to the exterior of the body through a part corresponding with the future anus. In a short time, however, the intestinal por- tion of the cloaca is cut off from that which belongs to the urinary and generative organs ; a separate passage or canal to the exterior of the body, belonging to these parts, being called the sinus urogenitalis. Subsequently, this canal is divided, by a process of division extending from before backwards or from above downwards, into a " pars urinaria" and a " pars genita- lis." The former, continuous with the urachus (p. 587), is converted into the urinary bladder. The Fallopian tubes, the uterus, and the vagina are de- THE MULLERIAN DUCTS. 611 veloped from two threads of blastema, called the Mullerian ducts (Fig. 244, w), which appear in front of the Wolffian bodies at about the time that these begin to change their rela- tive position to neighboring parts, and to decrease in size. The two Mullerian ducts are united below into a single cord, called, the genital cord, and, from this are developed the vagina, as well as the cervix and the lower portion of the body of the uterus ; while the ununited portion of the duct on each side forms the upper part of the uterus, and the Fallopian tube. In certain cases of arrested or abnormal development, these portions of the Mullerian ducts may not become fused FIG. 244. Diagram of the Wolffian bodies, Mullerian ducts and adjacent parts previous to sexual distinction, as seen from before (from Quain). sr, the suprarenal bodies; r, the kidneys ; ol, common blastema of ovaries or testicles ; W, Wolffian bodies ; w, Wolffian ducts: m, m, Miillerian ducts; gc, genital cord; ug, sinus urogenitalis ; i, intestine ; cl, cloaca. together at their lower extremities, and there is left a cleft or horned condition of the upper part of the uterus, resembling a condition which is permanent in certain of the lower ani- mals. 612 GENERATION AND DEVELOPMENT. In the male, the Mullerian ducts have no special function, and are but slightly developed : the small prostatic pouch, or sinus pocularis, forms the atrophied remnant of the genital cord, and is, of course, therefore, the homologue, in the male, of the vagina and uterus in the female. FIG. 245. Urinary and generative organs of a human female embryo, measuring 3% inches in length. A, general view of these parts ; 1, suprarenal capsules ; 2, kidneys ; 3, ovary ; 4, Fallopian tuba ; 5, uterus ; 6, intestine ; 7, the bladder. B, Bladder and generative organs of the same embryo viewed from the side ; a, the urinary bladder (at the upper part is a portion of the urachus) ; 2, urethra ; 3, uterus (with two cor- nua) ; 4, vagina ; 5, part as yet common to the vagina and urethra ; 6, common ori- fice of the urinary and generative organs ; 7, the clitoris, c, Internal generative or- gans of the same embryo ; 1, the uterus ; 2, the round ligaments ; 3, the Fallopian tubes (formed by the Mullerian duets) ; 4, the ovaries ; 5, the remains of the Wolffian bodies. r>, External generative organs of the same embryo ; 1, the labia majora ; 2 the nymphse ; 3, the clitoris. After Miiller. The external parts of generation are at first the same in both sexes. The opening of the genito-urinary apparatus is, in both sexes, bounded by two folds of skin, whilst in front of it there is formed a penis-like body surmounted by a glans, and cleft or furrowed along its under surface. The borders of THE MAMMARY GLANDS. 613 the furrow diverge posteriorly, running at the sides of the genito-urinary orifice internally to the cutaneous folds just mentioned (see Fig. 245, B, D). In the female, this body be- coming retracted, forms the clitoris, and the margins of the furrow on its under surface are converted into the nymphse, or labia minora, the labia majora pudendse being constituted by the great cutaneous folds. In the male foetus, the margins of the furrow at the under surface of the penis unite at about the fourteenth week, and form that part of the urethra which is included in the penis. The large cutaneous folds form the scrotum, and at a later period, namely, in the eighth month of development, receive the testicles, which descend into them from the abdominal cavity. Sometimes the urethra is not closed, and the deformity called hypospadias then results. The appearance of hermaphroditism may, in these cases, be in- creased by the retention of the testes within the abdomen. The Mammary Glands. The mammary glands, which may be considered as organs superadded to the reproductive system in man and other mem- bers of the class (Mammalia) which derives its name from them, are, in the essential details of their structure, very simi- lar to other compound glands, as the pancreas and salivary glands ; that is to say, they are composed of larger divisions or lobes, and these are again divisible into lobules, the lob- ules being composed of the follicular extremities of ducts, lined by glandular epithelium. The lobes and lobules are bound together by areolar tissue ; while, penetrating between the lobes, and covering the general surface of the gland, with the exception of the nipple, is a considerable quantity of yellow fat, itself lobulated by sheaths and processes of tough areolar tissue (Fig. 246) connected both with the skin in front and the gland behind ; the same bond of connection extending also from the under surface of the gland to the sheathing connec- tive tissue of the great pectoral muscle on which it lies. The main ducts of the gland, fifteen to twenty in number, called the lactiferous or galactophorous ducts, are formed by the union of the smaller ducts, and open by small separate orifices through the nipple. Just before they enter the base of the nipple, these ducts are dilated (6, Fig. 246) ; and, during lactation, the period of active secretion by the gland, they form reservoirs for the milk, which collects in them and distends them. The walls of the gland-ducts are formed of areolar and elastic tissue, and are lined, internally by a fine mucous membrane, the surface of which is covered by squamous or spheroidal epithelium. 52 614 GENERATION AND DEVELOPMENT. The nipple, which contains the terminations of the lactifer- ous ducts, is composed also of areolar tissue, and contains un- striped muscular fibres. Bloodvessels are also freely supplied FIG. 246. Dissection of the lower half of the female mamma during the period of lactation (from Luschka). %. In the left-hand side of the dissected part the glandular lobes are exposed and partially unravelled; and on the right-hand side, the glandular substance has been removed to show the reticular loculi of the connective tissue in which the glandular lobules are placed : 1, upper part of the mammilla or nipple ; 2, areola ; 3, subcutaneous masses of fat ; 4, reticular loculi of the connective tissue which support the glandular substance and contain the fatty masses ; 5, one of three lactiferous ducts shown passing towards the mammilla where they open ; 6, one of the sinus lactei or reservoirs ; 7, some of the glandular lobules which have been un- ravelled ; 7', others massed together. to it, so as to give it a species of erectile structure. On its surface are very sensitive papillae ; and around it is a small area or areola of pink or dark-tinted skin, on which are to be seen small projections formed by minute secreting glands. Bloodvessels, nerves, and lymphatics are plentifully supplied to the mammary glands ; the calibre of the bloodvessels, as well as the size of the glands, varying very greatly under cer- tain conditions, especially those of pregnancy and lactation. The secretion of milk, which under ordinary healthy cir- COMPOSITION OF MILK. 615 cumstances only occurs after parturition, if we except the slight secretion which takes place in the latter months of preg- nancy, is effected by the epithelial cells lining the ultimate follicles of the mammary gland. The process does not differ from secretion in glands generally (see p. 321), and need not here be particularly described. Under the microscope, milk is found to contain a number of globules of various size (Fig. 247), the majority about $* Microscopic appearance of human milk with an intermixture of colostric corpuscles. an nc n diameter. They are composed of oily matter, probably coated by a fine layer of albuminous material, and are called milk-globules; while, accompanying these, are numerous minute particles, both oily and albuminous, which exhibit ordinary molecular movements. The milk which is secreted in the first few days after parturition, and which is called the colostrum, differs from ordinary milk in containing a larger quantity of solid matter ; and under the microscope are to be seen certain granular masses called colostrum-cor- puscles. These, which appear to be small masses of albumin- ous and oily matter, are probably secreting cells of the gland, either in a state of fatty degeneration, or, as Dr. Gedge re- marks, old cells which in their attempts at secretion under the new circumstances of active need of milk, are filled with oily matter ; which, however, being unable to discharge, they are themselves shed bodily to make room for their successors. The specific gravity of human milk is about 1030. 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May, 1851. u C. J. B. WILLIAMS. Carpenter: Princ. of Human Phys. 3d ed., p. 588. " VOLKMANN. R.Wagner: Handworterbuch der Phys. Braun- schweig Art. Nervenphysiologie, p. 586. 173. VALENTIN UND BRUNNER. Lehrbuch der Phys. des Menschen. Braunschweig, 1844. Vol. i, p. 547. 174. ED. SMITH. Philosoph Trans. 1859. ' ; ANDRAL ET GAVARRET. Recherches sur la quantite d'Acide Carbonique exhale par le Poumon. Paris, 1843. " VIERORDT. Phys. des Athmens. 1845. 175. LETTELLIER. Annales de Chimie et de Physique. 1845. " ED. SMITH Philosoph Trans. 1859. " ALLEN AND PEPYS. Philos. Trans. 1808-9. 176. LEHMANN. Dr. Geo. E. Day: Chemistry in Relation to Phys. and Med. 1860. Balliere. p. 469. " ED. SMITH. Philosoph. Trans. 1859 177. VALENTIN UND BRUNNER. Lehrbuch der Phys. des Menschen. Braunschweig, 1844. Bd. i. " BENCE JONES. Chem. Gaz. April, 1851. " BENCE JONES. The Med. Times. August 30th, 1851. " REQNAULT AND REISET. Brit, and For. Med.-Chir. Rev. July, 1850, p 252. 178. LIEBIG. Animal Chem. Trans, by Dr. Gregory. 3d ed., p. 184. 179. RANSOME. Cambr. Jour, of Anat and Phys., May, 1870. 187. ECCLES London Med. Gaz. Vol. xliv, p. 657. " Report of Med.-Chir. Committee. Trans, of the Royal Med.- Chirur. Soc., 1862. 189. MARSHALL HALL. The Cyclopaedia of Anat. and Phys. Ed. by Dr. Todd. Vol. ii, p. 771. " WUNDERLICH. Med. Thermometry. Transl. by B. Woodman, M.D. New Syd. Soc. Trans. 1871. 620 LIST OF AUTHORS. PAGK 190. OGLE. St. George's Hosp. Keports. Vol. i. 191. GEE. Gulstonian Lectures. Brit. Med. Jour., 1871. " DAVY. Proceedings of the Royal Soc., June, 1845. " TIEDEMANN AND RuDOLPHl. Tiedemann : Phys., translated bv Gully and Lane, p. 234. 192. JOHN" HUNTER. Works of, Ed. by Mr. Palmer. 1835. Vol. iii, p. 16, and vol. iv, p. 131, et seq. 194. NEWPORT. Philosoph. Trans. 1837. 196. C. JAMES. Gazette Medicale de Paris, April, 1844. 198. EARLE. Trans, of the Royal Med.-Chir. Soc. Vol. vii, p. 173. 201. CHOSSAT. Gaz. He'd, de Paris, Oct., 1843 " LETTELLIER. Annales de Chimie et de Physique. 1844. 210. WRIGHT. The Lancet. 1842-3. 212. OWEN. Dr. Carpenter : Princ. of Human Phys., 5th ed. 1855, p. 76, note. 213. BERNARD. The Med. Times and Gaz., July 7th, 1860. 218. BRINTON. On Food and its Digestion, Churchill, 1861. 220. BEAUMONT. Experiments and Observations on the Gastric Juice, and the Phys. of Digestion, by W. Beaumont. US, 1834.- Reprinted, with notes, by Dr A. Combe. Edinburgh, 1838. " BLONDLOT. Traite" Analytiquede Digestion. 8vo., Paris, 1844. " BERNARD. Gaz. Med de Paris, June, 1844. 221. Chem in relation to Phys. and Mod. by Dr. G. E. Day. 1860. Bailliere. p. 158. " GEO. E. DAY. Chem., &c., p. 159. 223. BEAUMONT. Experiments and Observations on the Gastric Juice, and the Phys. of Digestion, by W. Beaumont. U. S., 1834. Reprinted, with notes, by Dr. A. Combe. Edinburgh, 1838, p. 120. 227. GOSSE. Spallanzani : Versuch iiber das Verdauungsgeschaft. Leipzig, 1785. 228. RAWITZ. Weber : die Einfachen Nahrungsmittel. Breslau, 1846. 239. BERNARD. Gaz. Med. de Paris, 1850, p. 889. 232. BRINTON. London Med. Gaz , 18J9. " BRINTON. On Food and its Digestion. Churchill, 1861, p. 100. 236. BERNARD. The Med. Times and Gaz., Aug., 1860. 11 LONGET. Anat. et Phys duSysteme Nerveux, &c. Paris, 1842. Vol. i, p. 323. " BISCHOFF Muller: Archiv. far Anat, Phys. und wissenschaft- liche Medecin. Berlin, 1848. Jahresbericht, p. 140. 239. MEISSNER. Henleund Pfeufer: Zeitschrift fur Rationelle Medi- zin. Heidelberg, 2d Ser., vol. viii. 240. LIEBERKUHN, J. N. Diss. de Fabrica et Actione Villorum In- testinorum tenuium. 1782, " PEYER. De Glanduris Intestinorum. 1682. " BRUNN, J. C. Glandulas Duodeni seu Pancreas secundarium. 4to. 1715. 251 MM. BOUCHARDAT ET SANDRAS. Gaz. Mdd. de Paris, Jan., 1845. 252. DISCHARGE OF FATTY MATTERS FROM INTESTINE. Trans, of the Royal Med.-Chir. Soc. Vol. xviii, p. 57. f BERNARD. Quarterly Jour of Microscop. Science. Churchill. 259. TABULAR COMPOSITION OF BILE, by Frerichs. V. Gorup-Besa- nez. Physiologic Chemie. 1862, p. 469. 26,1. PRESENCE OF COPPER IN BILE AND BILIARY CALCULI. Gorup- Besanez. Untersuchungen iibcr Galli. Erlangen, 1846. LIST OF AUTHORS. 621 VAGE 261. BLONDLOT. Essai sur les Fonctions du Foie. Paris, 1846, p. 62. " KEMP. Chemical Gaz. No. 99, 1846. 263. SIMON. Animal Chem. Trans, by Dr, Day for the Sydenham Soc. Vol. ii. p. 367. " FRERICHS Ranking: Half-yearly Abst. of the Med. Sciences. Churchill. Vol. iii, 314. 268. PAVY. Phil. Trans. 1800, p. 595. " PAVY. On the Nature and Treatment of Diabetes. Churchill. 1862. 274. BRINTON On Food and its Digestion, Churchill. 1861. 282. KOLLIKER. Annales des Sciences Naturelles. Geologie. 1846, p. 99. " RECKLINGHAUSEN. Art. Lymphatic System. Strieker's His- tology, translated by H. Power. Vol. i. 283. KOLLIKER. Brit, and For. Med.-Chir Rev. July, 1850. 286. GULLIVER Howson : Works, Ed. for the Syd. Soc. by Mr. Gul- liver, 1846-7, p. 82, note. 287. ASCHERSON. Miiller: Archiv. fur Anat. Phys. und wissensehaft- liche Medecin. Berlin, 1840. 288. BOUISSON. Gaz. Med. de Paris. 1844. 289. OWEN REES. London Med. Gaz Jan., 1841. " R. VIRCHOW. Die Cellular Pathologic (since translated by Dr. Chance). Berlin, 1858, s. 143. 290. BIDDER. Miiller : Archiv. fur Anat. Phys. und wissenschaftliche Medecin. Berlin, 1845. " SCHMIDT. New Syd. Soc.'s Year-Book of Med., &c. London, 1863, p. 24. 292. HERBST. Das Lymphageiass system und seine Verrichtungen. Gottingen, 1844 " LYMPH-HEARTS. J. MULLER Elements of Phys., trans, by Dr. Baly. 2d ed., 1840, p. 293. 293. VOLKMANN. Miiller: Archiv. fur Anat. Phys. und wissen- schaftliche Medecin. Berlin, 1844. 297. BENCE JONES. Proceedings of the Royal Soc. Vol. xiv. " SAVORY. The Lancet. 1863, May 9 and 16. 298 OESTERLEN. Oesterreichische Medecinische Wochenschrift. Wien, Feb., 1844. " OESTERLEN. Archiv. fiir Phys. und Pathol. Chemie und Mikro- scopie. Von J. F. Heller. Wien, 1847, p. 56. 300. HELMHOLTZ. Miiller: Archiv. fiir Anat. Phys. und wissen- schaftliche Medecin. Berlin, 1845. " CARPENTER. Princ. of Human Phys. 3d ed., p. 623. 308. BRODIE. Lectures on Pathol. and Surg. 1846, p. 309. u TRAVERS. Further Inquiry concerning Constitutional Irrita- tion, p. 436. " BALY. J. Miiller : Elements of Phys. Transl. by Dr. Baly. 2d ed., 1840, p. 396. 309. Defective Nutrition from Irritation of Nerves r London Med. , Gaz., vol xxxix, p. 1022. 315. BOWMAN. The Cyclopaedia of Anat. and Phys., Edited by Dr. Todd. Art. Mucous Membrane. 324. PERISTALTIC MOVEMENTS or LARGE GLAND-DUCTS. J. Muller : Elements of Phys. Translated by Dr. Baly. 2d ed., 1840, p. 521. " DR. BROWN-SKQUARD. Journ. de Phys. 1858. 622 LIST OF AUTHORS. 325. CARPENTER. Princ. of Human Phys. 3d ed., p. 476. " SIMON. A Physiological Essay on the Thymus Gland. London, 1845. 4to. 326. ECKER. Der feinere Bau der Neben-nieren beim Menschen und den Vier Wirbelthierclassen. Braunschweig, 1846. 330. MEYER. Boehm, L. De Glandularum intestinalium Structura penitiori. Berol., 1835. March, 1845. 11 SIMON. A Physiological Essay on the Thymus Gland. London, 1845. 4to" 11 FRIEDLEBEN. Die Phys. der Thymusdriise. Frankfurt. 1858. u HUTCHINSON. Funke : Atlas der Phys. Cheniie. Leipzig, 1853-6. " WILKS. Guy's Hosp. Kep. 1862. 331. KOLLIKER. Manual of Human Microscop. Anat. Parker, 1860, p. 374. " KOLLIKER. Manual, &c. p. 365. 339. KRAUSE. K. Wagner : Handworterbueh der Phys. Braun- schweig. Article Haut. " ERASMUS WILSON. A Practical Treatise on Healthy Skin. Churchill, 1846. 344. J.DAVY. Trans, of the Royal Med.-Chir.Soc. Vol. xxvii, p. 189. " KRAUSE. Bulletin de 1' Academic Royale de Medecine. 345. BERZELIUS. Trait4 de Chimie, tradint par Esslinger. 8 vols. 8vo. Paris. Vol. vii contains the Chemistry of Animal Structures. " ANSELMINO. Zoochemie, by D. Lehmann. Heidelberg, 1858, p. 301. " GORUP-BESANEZ. Lehrbuch der Phys. Chemie. 1862, p. 504. " LAVOISIER ET SEQUIN. Memoires de 1'Acad des Sciences de Paris. 1790. 346. KRAUSE. Paget : Reports on the use of the Microscope, and on the Progress of Human Anat. and Phys. ; in the Brit, and For. Med. Rev. 1843-4, p. 40. " MILNE-EDWARDS. Influence des Agens Physiques sur la Vie. Trans, by Dr. Hodgkin. " REQNAULT ET REISET. Annales de Chimie et de Pharmacie. 1849. " EDWARD SMITH. Dr. Carpenter : Princ. of Human Phys. 5th ed., 1855. 6th ed., p. 293. 347. MILNE-EDWARDS AND MULLER. J. Mviller: Elements of Phys. Trans, by Dr. Baly. 2d ed., 1840, p. 328. " MAGENDIE. Gaz. Med. de Paris, Dec., 1843. " MILNE-EDWARDS. Influence des Agens Physiques sur la Vie. Translated by Dr. Hodgkin. 348. MADDEN. Experimental Inquiry into the Phys. of Cutaneous Absorption. Edinburgh, 1835. " BERTHOLD. Miiller: Archiv. for Anat., Phys., und wissen- gchaftliche Medecin. Berlin, 1838, p. 177. 355. ERICHSEN. London Med. Gaz. 1845. 356. BERNARD. Comptes Reridus des Seances de 1' Academic Royale des Sciences de Paris. 1846. 358. PROUT. On the Nature, &c. " GOLDINQ BIRD. Urinary Deposits. 1844. p. 31. " PARKES. On the Composition of the Urine. 1866. 360. WOHLER. Annales de Chirnie et de Pharmacie. xxvii, 196. LIST OF AUTHORS. 623 PAGE 360. LECANU. Bulletin de 1'Acad. Royale de Med. T. xxv, p. 261. 361. LEHMANN. F. Simon: Animal Chem. Trans, by Dr. Davy for the Syd. Soc. Vol. ii, p. 163. u LASSIGNE. Journal de Chimie Medicale. p. 272. " MILLON. Comptes Rendus des Seances de 1'Acad. Royale des Sciences de Paris. 1843. 362. G. BIRD. London Med. Gaz. Vol. xli, p. 1106. " LIEBIG. The Lancet, June, 1844. 363. LIEBIG. The Lancet, June, 1844. " WEISMANN. Henle und Pfeufer: Zeitschrift fur Rationelle Medizin. Heidelberg. 3 ser., p. 837. " URE. Transac. of the Royal Med. -Chir. Soc. Vol. xxiv. 364. LIKBIG. Chem. of Food. Walton and Maberly, 1847. " HEINTZ. Canstatt: Jahresbericht uber die Fortschritte in der Biologie. Erlangen, 1847, p. 105. 365. RONALDS. Philosophical Mag. 1846. 370. LISTER AND TURNER. Quar. Jour, of Microscop. Science. 1850. " LOCKHART CLARKE. Philosoph. Transac. 1859. " STILLING. Ueber den Bau der Nerve n-primitivfaser und der Nerven-zelle. 1856. 873. GULL. The Med. Times. 1849. " KOLLIKER. Manual of Human Microscopic Anat. Parker, 1860, p. 248. PACINI. AnnaliUniversalidi Medicini. Luglio. 1845, p. 208. 379. HELMHOLTZ AND BAXT. Camb. Journ. of Anat. and Phys. P. i, new series, p. 190. " RUTHERFORD. Lancet, April 1st, 1871. 382. SAVORY. Lancet, August 1st, 1868. 389. LOCKHART CLARKE. Philosoph. Transac. 1851 to 1859. 391. KOLLIKER. Manual of Human Microscopic Anat. Parker. 1860. p. 244. 393. BROWN-SEQUARD. On the Phys. and Pathol. of the Cerebral Nervous System. Philadelphia, 1860. 401. VOLKMANN. Miiller: Archiv. fur Anat., Phys., und wissen- schaftliche Medecin. Berlin, 1844. 408. J. REID. Edinburgh Med. and Surg. Journ. 1838. 415. LONGET. Anat. et Phys. du Systeme Nerveux, &c. Paris, 1842. T. i, p. 733, and others. " FLOURENS. Recherches Experimentales sur les Fonc. du Sys- teme Nerveux, &c. Paris. " MAGENDIE. Le9ons sur les Functions du Systeme Nerveux. 416. BOUILLAUD. Recherches Cliniques et Experimentales sur le Cervelet. Referred to by. " LONGET. Anat. et Phys. du Systeme Nerveux, &c. Paris, 1842. T. i, p. 740. 417. LONGET. Anat. et Phys. du Systeme Nerveux, &c. Paris, 1842. T. i, p. 762. " COMBIETTE. Revue Medicale. 426. GRANT. See Longet, 1. c. T. ii, p. 388. 430. VALENTIN. Lehrbuch der Phys. des Menschen. Braunschweig, 1844. Vol. ii, p. 666. 436. RKID. Edinburgh Med. and Surg. Journ. 1838. 439. BIDDER UND VOLKMANN. R. Wagner: Handworterbuch der Phys. Braunschweig. Art. Nervenphysiologie " REID. Edinburgh Med. and Surg. Journ. Vols. xlix and Ii. 624 LIST OF AUTHORS. PAGE 441. REID. Edinburgh Med. and Surg. Journ. Vols. xlix and li. 11 LEGALLIOS. (Euvres Completes, Edited by M. Poriset. Paris, 1830 u TRAUBE. Beitrage zur Experimentellen Pathologie und Phys. Berlin, 1840. 443. BERNARD. Archives Generales de Me"decine. 1844. 454. KUHNE. Canib. Journ. of Anat. and Phys. Part ii. 456. KOLLIKER. Manual of Human Microscopic Anat. Parker, 1860. p. 63. 459. SHARPEY. Quain : Anat. 7th ed. 461. SEGALAS. J. Muller: Elements of Phys. Trans, by Dr. Baly. 2d ed , 1840, p. 895. 462. BOWMAN. Phil. Trans. 1840, 1841. " No DIMINUTION IN BULK or CONTRACTING MUSCLE. Mayo, J. Muller: Elements of Phys.; Trans, by Dr. Baly. 2d ed., 1840, p. 886. Valentin: Lehrbuch der Phys. des Menschen. Braunschweig, 1844. Matteucci : Erdmann's Jour. 463. ED. WEBER. R. Wagner : Handworterbuch der Phys. Braun- schweig. Art. Muskelbewegung. 464. ED. WEBER. K. Wagner : Handworterbuch der Phys. Braun- schweig. Art. Muskelbewegung. 465. SCHIFFER: Camb. Journ. Part ii, new series, p. 416 ; Part iii, new series, p. 236. 466. BROWN-SEQUARD. Proc. of the Royal Soc. 1862, p. 204. " BROWN-SQUARD. London Med. Gaz. May 16, 1851. " VALENTIN. Lehrbuch der Phys. des Menschen. Braunschweig, 1844. Bd. ii, p. 36. 485. PETREQUIN ET DIDAY. Gaz. M6d. de Paris. 488. MULLER UND COLOMBAT. Froriep : Neue Notizen aus dem Gebiete der Natur. Weimar, 1840. 491. MAGENDIE. Journ. de Phys. T. iv, p. 180. 513. VOLKMANN. R. Wagner : flandworterbuch der Phys. Braun- schweig. Art. Sehen, p. 286. 538. Note. ED. WEBER. Archives d'Anat. Generale et de Phys. 1846. 555. SCHIFF AND BROWN-S^QUARD. The Lancet, 1858. " SIEVEKIVG. Brit, and For. Med.-Chir. Rev., 1858. Vol. ii, p. 501. 556. VALENTIN. Lohrbuch der Phys. des Menschen. Braunschweig, 1844. Vol. ii, p. 566. * 566. VALENTIN. Muller: Archiv. fur Anat. Phys., und wissen- schaftliche Medecin. Berlin, 1838. 572, DALTON. Phys. 1864, p. 585. 579. BISCHOFF. Entwickelungs-Geschichte der Saugethierc und des Menschen. 1842. 589. H. MULLER. Brit, and For. Med.-Chir. Rev. Vol. xiii, p. 546. 592. HARVEY. On a remarkable Effect of Cross-Breeding. By Dr. Alex. Harvey. Blackwood & Sons, 1851. " HUTCHINSON. Med. Times and Gaz , Dec., 1856. " SAVORY. On Effects upon the Mother of Poisoning the Foetus. Pamphlet, 1858. 599. KOLLIKER. Annales des Sciences Naturelles : Zoologie. Aug., 1846. INDEX. Abdominal muscles, action of in res- piration, 165 type of respiration, 163 Aberration, chromatic, 510 spherical, 509 Absorbents. See Lymphatics. Absorption, 277 by bloodvessels, 293 of gases by blood, 296 by lacteal vessels, 248, 290 by lymphatics, 291 of oxygen by lungs, 177 process of by osmosis, 294 purposes of, 277 rapidity of, 297 from rectum, rapidity of, 297 by the skin, 347 from stomach, preparation of food for, 229 See Chyle, Lymph, Lymphatics, Lacteals. Accessory nerve, 443 distribution of, 443 roots of, 444 Accidental elements in human body, 26 Accidents, involuntary movements in, 400 Acetic acid in gastric fluid, 222 Acids, strong, prevent coagulation, 64 Acini of secreting glands, 319 Adaptation of the eye to distances, 510 Adenoid tissue. See Retiform Tissue. Adipose tissue, 40 situations of, 40 structure of, 41 See Fat. Afferent arteries of kidney, 352 lymphatics, 286 nerve-fibres, 376 After-birth, 593 After-sensations of taste, 553 of touch, 558 of vision, 517 Age, in relation to blood, 76 to capacity of chest, 168 to excretion of urea, 360 to exhalation of carbonic acid, 173 to heat of body, 189 to mental faculties, 422 to pulse, 109 to respiration, 168 to voice, 484 Aggregated glands, 319 Agminate glands, 241 Air, atmospheric, composition of, 173 changes by breathing, 173 favors coagulation of blood, 63 quantity breathed, 166 states of influencing production of carbonic acid, 175, 176 transmissions of sonorous vibra- tions through, 536 in tympanum, necessary for hear- ing, 538 undulations of, conducted by ex- ternal ear, 534 by tympanum, 537 Air-cells. 158 Air-tubes. See Bronchi. Albinos, imperfect vision in, 501 Albumen, action of gastric fluid on, 229 characters of, 22 chemical composition of, 23 coagulated, properties of, 22 coating oily matter, 287 relation to fibrin, 22 tissues and secretion in which it exists, 22 of blood, 74 uses of, 86 vegetable, 202 Albuminose, 229 action of liver on, 266 Albuminous substances, 21 absorption of, 271 action of gastric fluid on, 229, 271 of liver on, 229 of pancreas on, 252, 271 626 INDEX. Alcoholic drinks, effect on respiratory changes, 176 Aliments. See Food. Alimentary canal, development of, 606. See Stomach, Intestines, Ac. Alkalies, caustic, prevent coagulation, 64 Alkaline and earthy salts, influence of on coagulation, 63 Allantois, 585, 586 Aluminium, an accidental element in tissues, 26 Amoeba, 72 Amoeboid movements of white corpus- cles, 72 Amaurosis, action of iris in, 425 after injury of the fifth nerve, 432 Ammonia in blood, 76 cyanate of, identical with urea, 360 exhaled from lungs, 179 from skin, 345 urate of, 363 Amnion, 585 Ampulla, 531 Amputations, sensations after, 381 Amylaceous principles, action of gas- tric fluid on, 230 of pancreas and intestinal glands on, 250,251, 271 of saliva on, 213 Amyloid substance in liver, 267 Amyloids, 200 Anastomoses of muscular fibres of heart, 460 of nerves, 372 of veins, 144 in erectile tissues, 153 Anatomical elements of human body, 26-33 Angle, optical, 514 Angulus opticus seu visorius, 514 Ani sphincter. See Sphincter. Animal fats, 20 food, digestion of, 228 in relation to urea, 361 in relation to uric acid, 362 in relation to reaction of urine, 356 heat, 189. See Heat and Tem- perature, life, muscles of, 458 nervous system of, 368 starch. 267 Animals, their distinction from plants, 15 Anterior pyramids, 403 roots of spinal nerves. 391 Antihelix, 527 Antitragus, 527 Anus, 249 Aorta, 91 development of, 599 elasticity of, 117 pressure of blood in. 131 valves of, 96 action of, 101 Apnoea, force of inspiratory efforts in, 170. See Asphyxia. Apoplexy, effects of, 420 with cross paralysis, 405 Appendices epiploicae, 249 Appendix vermiformis, 249 Aqugeductus cochleae, 532 vestibuli, 530 Aqueous humor, 505 Arches, visceral, 596 Area germinativa, 581 pellucida, 580 vasculosa, 587 Areola of nipple, 614 Areolar tissue, 38 functions of, 40 situations where found, 39 Arteries, 88, 115 calibre of, how regulated, 118, 121 coats of, 115 muscular contraction of, 118 effect of cold on, 119 effect of division, 119 of electro-magnetism, 120 elasticity of, 116, 117 purposes of, 1 16 elongation and dilatation in the pulse, 123 force of blood in, 130, 131 muscularity of, 115 evidence of, 118 governed by nervous system, 121, 122, 451 purpose of, 121 nerves of, 121 office of, 116 pulse in, 123. See Pulse, structure of, 115 distinction in large and small arteries, 115 systemic, 91 velocity of blood in, 131 Articular cartilage, 43 Articulate sounds, classification of, 487. See Vowels and Consonants Artificial digestive fluid, 224 Arytenoid cartilages, 478 effect of approximation, 481 movements of, 478 muscle, 478, 479 Asphyxia, 186 cause of death in, 186 experiments on, 186 essential cause of, 188 INDEX. 627 Assimilation or maintenance of blood, 84 nutritive, 299 Atmospheric air. See. Air. pressure in relation to respiration, 161 Atrophy, from deficient blood, 307 from disensed nerves, 309 Attention, influence of, on sensations, 493 on special senses, 516 Auditory canal, 527 function of, 534 Auditory nerve, 533 distribution of, 533, 541 effects of irritation of, 546 fibres of. 377 sensibility of, 543 Auricle of ear, 527 Auricles of heart, 91 action of, 96 capacity of, 111 development of, 599 dilatation of, 99 force of contraction of, 110 Axis-cylinder of nerve-fibre, 370 Azote. See Nitrogen. Azotized principles, 21 Baritone voice, 483, 484 Basement-membrane, of mucous mem- branes, 318 of secreting membranes, 314 Bass voice, 483 Benzoic acid, relation to hippuric acid, 363 Bicuspid valve, 92 Bile, 259 antiseptic power of, 265 coloring matter of, 260 coloring serous secretions, 316 composition of. 259 elementary, 261 digestive properties of, 265 excrementitious, 262 fat made capable of absorption by, 265 functions of in digestion, 265 mixture with chyme, 270 mucus in, 260 a natural purgative, 265 process of secretion of, 262 purposes of, 262 in relation to animal heat, 264 quantity secreted, 262 reabsorption of, 263, 265 saline constituents of, 260 secretion and flow of, 262 secretion of in foetus, 263 Bile, tests for. 261 Bilin, 259 , reabsorption of, 263 Biliverdin and bilifulvin, 260 Bipolar nerve-corpuscles, 375 Birds, their high temperature, 191, 192 Birth, 13 Bladder, urinary. See Urinary Blad- der. Blastema, 27 Blastoderuiic membrane, 581 Bleeding, effects of on blood, 76 Blood, 55-87 adaptation of to tissues, 306 adequate supply necessary for nutrition, 307 albumen of, 74 use of, 86 alteration of by disease, 84 ammonia in. 76 animal poisons, how affected by, 306 arterial and venous, differences between, 77, 81, 180 assimilation of, 84 casein in, 75 changes in by respiration, 179 chemical composition of, 64 circulation of, 88. See Circula- tion, coagulation of, 58-62 circumstances influencing, 62 color of, 56 changed by respiration, 179 differences in, 77 coloring matters in, 76 coloring matter, relation to that of bile, 260 compared with lymph and chyle, 286 composition of, chemical, 64 physical, 56 ' variations in, 76-80 conditions necessary to nutrition, 306 corpuscles or cells of, 65-72. See Blood-corpuscles, red, 67 white, 71 creatin and creatinin in, 75 crystals of, 69 development of, 81 from lymph or chyle, 83 exposure to air in lungs, 159 extractive matters of, 75 fatty matters in, 74 use of. 86 fibrin of, 74 separation of, 22, 23, 62 use of, 86 628 INDEX. Blood, force of in arteries, 131 formation of in liver, 83 in spleen, 327 gases in, 81 changed by respiration, 180 of gastric and mesenteric veins, 79 globulin of, 68, 69 glucose or grape-sugar in, 75 growth and maintenance of, 85 haematin or cruorin of. 71 hepatic, characters of, 80 hippuric acid in, 76 inorganic constituents of, 75 lactic acid in, 76 menstrual, 56, 569 molecules or granules in, 72 movement of, in capillaries, 137 in lungs, 172 odoriferous matters in, 76 odor or halitus of, 56 portal, characters of, 80 purification of by liver, 264 quantity of, 56 reabsorption of bile into, 263 reaction of, 56 relation of to lymph, 289 to secretions, 319, 321, 323 of rennl vein, 80 saline constituents of, 75 uses of, 86 serum of, 72 compared with secretion of serous membrane, 316 specific gravity of, 56 splenic, characters of, 80 structural composition of, 56 supply of, adapted to each part, 122 to brain. 151 necessary for nutrition, 307 necessary to secretion, 323 temperature of, 56 urea in, 76 uric acid in, 76 uses of, 85 variations of in different circum- stances, 76 in different parts of body, 77 water in, 73 Blood-corpuscles,' red, characters of, 65 chemical composition of, 68 development of, 81-83, 304 in liver, 83 in spleen, 327, 328 disintegration and removal of, 329 diversities of, 67 movement of in capillaries, 138 sinking of, 59 tendency to adhere, 60, 68 uses of, 87 Blood-corpuscles, white, 71 amoeboid movements of, 72 formation of in spleen, 328 Blood-crystals, 69 Bloodvessels absorption by, 29,3-298 circumstances influencing, 297, 298 difference from lymphatic absorp- tion, 293, 294 osmotic character of, 294, 296 rapidity of, 297 area of, 137 communication with lymphatics, 282 development of, 598 influence of nervous system on, 452 of placenta, 589 relation to secretion, 324 share in nutrition, 306 Bone, 46 canaliouli of, 48 cancellous structure of, 46 composition of, 46 development of, 49 Haversian canals of, 48 lacunae of, 48 lamellae of, 48 periosteum of, 47 structure of, 46 Bone-earth, composition of, 26 Bones or ossicles of ear, 529 Bones, growth of, 299 nutrition of, 305 Boys, voice of, 484 Brain. See Cerebellum, Cerebrum, Pons, &c. capillaries of, 134 circulation of blood in, 150 development of, 582, 605 disease of, with atrophy, 307 influence on heart's action. Ill quantity of blood in. 152 Breathing-air, 166 Breathing. See Respiration. Bronchi, arrangement and structure of, 157 muscularity of, 171 Bronchial arteries and veins, 172, 173 Brunn's glands, 245 Buccinator muscle, nervous supply of, 428 Buflfy coat, formation of, 59, 68 Bulbus arteriosus, 599 Bursae mucosse, 315 Caecum, 250 changes of food in, 273 Calcium, salts of in human body, 25 INDEX. 629 Calculi, biliary, containing choles- terin, 20 containing copper, 261 Calculus, radiation of sensation from, 385, 395 Calorifaeient food, 202 Calyces of the kidney, 350 Calyciform papillae of tongue, 548 Canal, alimentary. See Stomach, In- testine, &G. external auditory, 527 function of, 534 oral, 488 of spinal cord, 390 spiral, of cochlea, 531 Canaliculi of bone. 48 Canals, Haversian, 48 portal, 254 semicircular, 530 function of, 541 Cancellous texture of bone, 46 Capacity of chest, vital, 167 how increased or diminished, 162- 165 of heart, 109 Capillaries, 88, 131 circulation in, 135 rate of, 137 contraction of, 138 development of, 599 diameter of, 132 influence of on circulation, 139 lymphatic, 280 network of, 133 number of, 134 passage of corpuscles through walls of, 138 resistance to flow of blood in, 136 still layer in, 137 structure of, 132 systemic, 91 of lungs, 159, 160, 172 of muscle, 460 of stomach, 219 Capsule of Qlisson, 254 Capsules, Malpighian, 351 Carbon, union of with oxygen, pro- ducing heat, 192, 193 Carbonic acid in atmosphere, 173 in blood, 79, 80, 180 effect of in producing asphyxia, 188, 189 exhaled from skin, 345 increase of in breathed air, 173 in lungs, 171 in relation to heat of body, 192, 193 Cardiac orifice, action of, 231 sphincter of, 183 relaxation in vomiting, 233 Cardiac branches of pneumogastric nerve, 439 Cardiograph, 108 Jarnivorous animals, food of, 202 sense of smell in, 498 !artilage, 43 articular, 44 cellular, 44 chondrin obtained from, 21 elastic, 43 fibrous, 45. See Fibro-cartilage. hyaline, 44 matrix of, 44 ossification in, 50 perichondrium of, 43 permanent, 43 structure of, 43 temporary, 43, 44 uses of, 46 varieties of, 43 Cartilage of external ear, use in hear- ing, 535 Cartilages of larynx. 477, 478 of ribs, elasticity of, 164 Casein in blood, 76 Catalytic process, 225 Cauda equina, 388 Caudate ganglion-corpuscles, 375 Cells, primary or elementary, defini- tion of, 31 contents of, 32 shape of, 31 structure of, 31 blood, 65. See Blood-corpuscles, cartilage, 43 embryonic, 81 epithelium, 34, 38. See Epithe- lium, of glands, 35, 322 action of in secretion, 323 lacunar of bone, 48 mastoid, 528 nerves ending in, 373 olfactory, 495 pigment, 42 of stomach, 216 Cellular cartilage, 44 Cellular tissue, 38. See Areolar Tissue. Cement of teeth, 51, 53 Centres, nervous. See Nervous Cen- tres. of ossification, 50 Centrifugal nerve-fibres, 377 Centripetal nerve-fibres, 376 Cerebellum, 414 co-ordinate function of, 415 cross-action of, 418 effects of injury of crura, 418 of removal of, 416 functions of, 415 53 630 INDEX. Cerebellum, in relation to sensation, 415 to motion, 415 to muscular sense, 416 to sexual passion, 417 structure of, 414 Cerebral circulation, 150 ganglia, function of. 412, 413 hemispheres. See Cerebrum. Cerebral nerves, 424 arrangement of, 424 third, 425 effects of irritation and injury of, 425 relation of to iris, 425 fourth, 426 fifth, 428 a conductor of reflex impres- sions, 430 distribution of, 428, 429 effect of division of, 309, 431 influence of on iris, 430 on muscles of mastica- tion, 428 on muscular movements, 430 on organs of special sense, 431, 433 relation of to nutrition, 431 resemblance to spinal nerves, 428 sensitive function of greater division of, 429 sixth, 426 communication of, with sym- pathetic, 427 seventh. See Auditory Nerve and Facial Nerve. eighth. See Glosso-pharyngeal, Pneumogastric, and Spinal Accessory Nerves, ninth, 444 Cerebro-spinal nervous system, 637, 386 influence on organic life, 453. See Brain, Spinal Cord, Ac. Cerebro-spinal fluid, relation to cir- culation, 152 Cerebrum, its structure, 419 convolutions of, 420 crura of, 409 development of, 603 effects of injury of, 420 functions of, 420 in relation to speech, 422 relation to mental faculties, 421 Cerumen, or ear-wax, 339. 527 Chalkstones, 362 Chambers of eye, 505 Charcoal, absorption of, 298 Chemical actions, how perceived, 493 Chemical composition of the human body, 18 distinctions between animals and vegetables, 15 sources of heat in the body, 192, 193 stimuli, action of nerves excited by, 378 Chest, its capacity, 167 contents of, 88, 162 contraction of in expiration, 163 enlargement of in inspiration, 162 Chest-notes, 485 Children, respiration in, 163 Chlorine, action on negro's skin, 348 in human body, 25 in urine, 366 Chloroform, effects of, 408 Cholesterin, properties of, 20 in bile, 260 in blood, 74 Chondrin, properties of, 21 Chorda dorsalis, 582 Chorda tyrapani, 433 Chordae tendineae, 94 action of. 101 Chorion, 588 villi of, 588 Choroid coat of eye, 500. 501 use of pigment of, 501 Chromatic aberration, 510 Chyle, 279, 286 absorption of, 290 analysis of, 289 bile essential to, 265 coagulation of, 287 compared with lymph, 289 corpuscles of, 288. See Chyle- corpuscles. course of, 278 fibrin of, 287 forces propelling, 282 molecular base of, 287 properties of, 286 quantity found, 290 relation of to blood, 289, 290 Chyle-corpuscles, 288 development into blood-corpus- cles. 83, 288 Chyme, 225. 270 absorption of digested parts of, 270 changes of in intestines, 270 Cicatrix, effect of nutrition on, 310 Cilia, 36, 37. 454 Ciliary epithelium, 36 of air-passages, 157 function of, 38 Ciliary motion, 37, 454 action of in bronchial tubes, 172 INDEX. 631 Ciliary motion, independent of ner- vous system, 454 nature of, 455 Ciliary muscle, 507 action of in adaptation to dis- tances, 511 Ciliary processes, 501 Circulation of blood, 83 action of heart on, 96-104 agents concerned in, 145 in arteries, 115 force of, 129 velocity of, 131 in brain. 150 in capillaries, 135 rate of, 137 course of, 89, 91 in erectile structures, 152 in foetus, 601 forces acting in, 90, 91, 145 influence of respiration on, 145 peculiarities of in different parts, 150 portal, 90 pulmonary, 89, 172 systemic, 89., 92 in veins, 141 affected by muscular pres- sure, 143 by respiratory movements, 146 velocity of, 147 velocity of, 147 Circulus venosus, 588 Circumferential fibro-cartilages, 45 Circumvallate papillee, 548 Cleaving of yelk, process of, 579 Cleft, ocular, 604 Clefts, visceral, 596 Climate, relation of to heat of body, 191 Clitoris, 562 development of, 612 an erectile structure, 153 Clot or coagulum of blood, 58 contraction of, 59. See Coagula- tion. of chyle, 287 Coagulation of albumen, 22 of blood, 58 conditions affecting, 62 influence of respiration on, 179, 180 theories of, 61 of chyle, 288 of lymph, 289 Cochlea of the ear, 530, 531 office of, 542 Cold-blooded animals, 192 extent of reflex movements in, 397 Cold-blooded animals, retention of muscular irritability in, 464 Collateral circulation in veins, 143 Colloids, 295 Colon, 248 Colostrum, 615 Coloring matters, 24 Coloring matter of bile, 260 of blood, 68, 69 of urine. 364 Colors, optical phenomena of, 517 Columnao carnese, 94 action of, 98 Columnar epithelium, 35 layer of retina, 502 Columns of medulla oblongata, 402 Columns of spinal cord, 388 functions of, 393 Combined movements, office of cere- bellum in, 416 Commissure of spinal cord, 388 Complemental air, 166 colors, 518 Concha, 527 use of, 534 Conduction of impressions, in medulla oblongata, 405 in or through nerve-centres, 383 in nerve-fibres, 376 in spinal cord, 392 in sympathetic nerve, 449 Conductors, nerve-fibres as, 376 Conglomerate glands, 319 Coni vasculosi, 573 Conical papillae. 550 Conjunctiva, 500 Connective tissue, 38. See Areolar Tissue. corpuscles, 40 Consonants, 487 varieties of, 488 Contractility, of arteries, 118 of bronchial tubes, 171 of muscular tissue, 461 of influence of nerves on, 461 Contraction, of coagulated fibrin, 59 of muscular tissue, mode of. 462 Contralto voice, 483 Convoluted glands, 321 Convolutions, cerebral, 420 Co-ordination of movements, office of cerebellum in, 416 office of sympathetic ganglia in, 451 Copper, an accidental element in the body, 26 in bile, 261 Cord, spinal. See Spinal Cord. umbilical, 593 Cords, tendinous, in heart, 94 vocal. See Vocal Cords. 632 INDEX. Corium, 323, 324 Cornea, 500 action of on rays of light, 505 nutrition of, 309 protective function of, 507 ulceration of, in imperfect nutri- tion, 309 after injury of fifth nerve, 309, 431 Corpora Arantii, 96, 104, 105 geniculata, 409 quadrigeinina, 409 their function, 411 stria ta, 409 their function, 411 Corpus callosum, office of, 422 cavernosum penis, 153 dentatum, 415 luteum, 570 of human female, 570 of mammalian animals, 571 of menstruation and preg- nancy compared, 572 spongiosuui urethrae, 153 Corpuscles of blood. See Blood-cor- puscles, of chyle, 288 of connective tissue, 40 of lymph, 286 nerve. See Nerve-corpuscles. Pacinian, 373 Cortical substance of kidney, 350 of lymphatic glands, 283 Corti's rods, 532 office of, 543 Costal types of respiration, 163 Coughing, influence on circulation in veins, 146 mechanism of, 182 sensation in larynx before, 384 Cowper's glands, 573 office uncertain, 577 Cracked voice, 484 Cramp, 383, 392 Cranium, development of, 595 Crassamentum, 58 Creatin and Creatinin, 24 in blood, 75 in urine, 364 Crico-arytenoid muscles, 479, 480 Cricoid cartilages, 478 Cross paralysis, 405 Crura cerebelli, 414 effect of dividing, 418 of irritating, 415 cerebri, 409 effects of dividing, 411 their office, 411 Crusta petrosa, 51, 53 Cryptogamio plants, movements of spores of, 16 Crystalline lens, 506 Crystalline lens, in relation to vision at different distances, 511 masses in ear, 541 Crystalloids, 295 Crystals, growth of, 14 in blood, 69 Cupped appearance of blood-clot, 59 Curves of arteries, 123 Cuticle. See Epidermis, Epithelium. of hair, 340 thickening of, 311 Cutis anserina, 457 vera, 333, 334 Cyanate of ammonia, 360 Cylindrical epithelium, 36 Cystic duct, 252, 262 Cystin in urine, 367 Cytoblasts, 29 in developing and growing parts, 305 Day, time of, influence on exhalation of carbonic acid, 176 Decapitated animals, reflex acts in, 396, 397 Decay of blood-corpuscles, 83 Decidua, 591 reflexa, 591 serotina, 591 vera, 591 Decomposition, tendency of animal compounds to, 20 Decussation of fibres in medulla ob- longata, 404, 405 in spinal cord, 394 of optic nerves, 524 Defecation, mechanism of, 183 influence of spinal cord on, 401 Degeneration of tooth-fangs, 303 Deglutition, 213 connection with medulla oblon- gata, 408 a reflex act, 396 relation of pneumogastric nerve to, 439 Dental groove, primitive, 53 Dentine, 51 Depressor nerve, 442 Derma, 333 Descendens noni nerve, 444 Development, 15, 578 relation to growth, 311 of organs, 593 of alimentary canal, 606 of blood, 81 of bone, 49 of embryo, 578, e. s. of extremities, 596 of face and visceral arches, 595 of heart and vessels, 597 INDEX. 633 Development of nervous system, 603 of organs of sense, 603 of respiratory apparatus, 608 of teeth, 53 of vascular system, 597 of vertebral column and cranium, 594 of Wolffian bodies, urinary appa- ratus and sexual organs, 609 Dextrin, formation of in digestion, 230 Diabetes, 269, 359 Diaphragm, 88 action of on abdominal viscera, 182 in inspiration, 162 in various respiratory acts, 180, 184 in vomiting, 232, 233 Dicrotous pulse, 129 Diet, influence of on blood, 76 Diffusion of gases in respiration, 171 of impressions, 385 Digestion, general nature of, 199 of food in the intestines, 238 of food in the stomach, 214, 222, 223 influence of nervous system on, 234 of stomach after death, 236 See Gastric Fluid, Food, Stomach. Digestive fluid. See Gastric Fluid, artificial, 224 tract of mucous membrane, 317 Direction of sounds, perception of, 544 of vision, 515 Discus proligerus, 564 Disease in relation to assimilation and nutrition, 307 in relation to beat of body, 191 Diseased parts, assimilation in, 310 Diseases, alteration of blood produced by, 307 maintenance of alterations by, 310 reflex acts in, 400 Distance, adaptation of eye to, 510 512 of sounds, how judged of, 544 Distinctness of vision, how secured, 508 Dorsal laminse, 582, 595 Dorsum of tongue, 548 Double hearing, 545 vision, 521 Dreams, phenomena of, 422 Dropsy, serous fluid of, contains albu- men, 22 Drowning, cause of death in, 187 Duct, cystic, 252, 262 hepatic, 252, 257 thoracic, 279, 286 Duct, vitelline, 584 Ductless glands, 325 Ducts of glands, arrangement of, 319 contraction of, 324 lactiferous, 613 Ductus arteriosus, 601 venosus, 601 closure of, 602, 603 Duodenum, 239 Duration of impressions on retina, 517 Duverney's glands, 563 Dysphagia, absorption from nutritive baths in, 348 Ear, 527 bones or ossicles of, 529 function of, 538 development of, 605 external, 527 function of, 534 internal, 529 function of, 541 middle, 527 function of, 536 Ectopia vesicse, observations on, 355 Efferent nerve-fibres, 376 lymphatics, 286 vessels of kidney, 352 Eggs as articles of food, 201 Eighth cerebral nerve, 435 Elastic cartilage, 43 coat of arteries, 115 fibres, 39 recoil of chest and lungs, 165 tissue in arteries, 115 in bronchi, 156 tissues, heat developed in, 190 Elasticity, of arteries, 116 employed in expiration, 165 Electricity, effect on nerves, 378 Electro-magnetism, effect on arteries, 120 on rigor mortis, 466 on voluntary muscles, 464 Elementary substances in the human body, 18 accidental, 26 Embryo. See Development and Foetus. formation of blood in, 81 Emission of semen a reflex act, 399 Emotions, connection of with cerebral hemispheres, 420 Enamel of teeth, 51, 52 End-bulbs, 337, 373 End-plates, motorial, 373 Endolyrnph, 530, 533 function of, 541 Endosmometer, 294 Epidermis, 35, 333 development,