I MEMCAL SCHOOL L1IBMAMY Gift of Dr. E.F. Anderson BLQOD-5PECTRA COMPARED WITH SPECTRUM OF AR BAND- LAMR 1 Spectrum oF Ardand-iamp with Fraunhofers lines in position 2 Spectrum nP Ox/haemoglobin in diluted blood. 3 Spectrum nP Reduced Haemoglobin. 4 Spectrum oP Carbonic, oxide Hsmofllabin. 5 Spectrum oP Acid Haamatin in Ethqnal solution. 6 Spectrum aP Alkaline Hamatin. 7 Spectrum op Chloroform Extract of acidulated Dx-Bile. 8 Spectrum aP MetharnDdlDbin. 9 Spectrum oP HffimachromD^En. 10 Spectrum oF Plsmatnporphyrin. J/rtv/ oft In- tilwvr Spciim have been drawn from obsenutions l>\- WW.Lcpmik FC.S IV PREFACE TO THE TWELFTH EDITION. In the preparation of the present edition it seems only right to state, that while I am responsible with my colleague, Dr. Yincent D. Harris, for the general supervision of the work in its passage through the press, he has undertaken the labor of investigation and the arrange- ment of the details. Many parts, moreover, he has re-written. And to him has fallen in chief part the difficult task of selecting from the many new facts and observations which have been published within the last few years such as can fitly find a place in a handbook for students. W. MORRANT BAKER. September, 1888. CONTENTS. CHAPTER I. PAGE THE PHENOMENA OP LIFE, ....... 1 CHAPTER II. THE STRUCTURE OF THE ELEMENTARY TISSUES, . . . . 15 Cells, . . . . . . . . . 15 Nucleus, . . . . . • . r • ' . 16 Intercellular Substance, . . . . . . .17 Fibres, . . . , . . . . 18 Tubules, . . . . . . ' ' . . . .18 Epithelium, . . . . . . . . . 19 Connective Tissues, ........ 29 The Fibrous Tissues, . . . . ... . .32 Cartilage, ......... 40 Bone, . . ....... 44 CHAPTEB III. THE BLOOD, .......... 57 Quantity of Blood, ...*.... 57 Coagulation of the Blood, . . . . . . .58 Conditions affecting Coagulation, ...... 66 The Blood-Corpuscles, ........ 70 Physical and Chemical Characters of Red Blood-Cells, . . 70 The White Corpuscles, or Blood-Leucocytes, . . . . .74 Chemical Composition of the Blood, ..... 77 The Serum, ......... 78 Gases contained in the Blood, . . . . . . '81 Blood-Crystals, . . . . . . . .84 Derivatives of Haemoglobin, , 86 Development of the Blood, . ... 4 . . 91 Uses of the Blood, . . . . . . . . 94 CHAPTER IV. CIRCULATION OF THE BLOOD, . . . . . . .95 The Systemic, Pulmonary, and Portal Circulations, ... 96 VI CONTENTS. PAGE THE HEART, .......... 97 Structure of the Heart and its Valves, . . • . . 97 STRUCTURE OP THE ARTERIES, CAPILLARIES, AND VEINS, . . . 105 Physiology of the Heart, . . . . . . . 115 Physiology of the Arteries, . . . . . . .134 Physiology of the Capillaries, .... .153 Physiology of the Veins, . . . . . . .156 VELOCITY OF THE CIRCULATION, . . . . . .158 Velocity of the Blood in the Arteries, . . . . . .159 Capillaries, ..... 160 Veins, . . . . . .160 Velocity of the Circulation as a whole, . . . . . 161 PECULIARITIES OP THE CIRCULATION IN DIFFERENT PARTS, . . .162 Circulation in the Brain, . . . . . . . 162 Circulation in the Erectile Structures, . . . . . .163 Agents concerned in the Circulation, . . . . . 165 Discovery of the Circulation, . . . . . . .165 Proofs of the Circulation of the Blood, 165 CHAPTEE V. RESPIRATION, . . . . . . . . .167 Position and Structure of the Lungs, . . . . .168 'Structure of the Trachea and Bronchial Tubes, . . . .170 Structure of the Lungs and Pleura, . . . . .174 Mechanism of Respiration, . . . . , . .178 Respiratory Movements, . . . . . . .179 Quantity of Air respired, ....... 184 Vital or Respiratory Capacity, . . . . . .184 Force exerted in Respiration, . . . . . . . . 186 Changes of the Air in Respiration, . . . . . .187 Changes produced in the Blood by Respiration, .... 193 Mechanism of various Respiratory Actions, .... 193 Influence of the Nervous System in Respiration, .... 197 Effects of Vitiated Air— Ventilation, ..... 199 Effect of Respiration on the Circulation, . . . .200 Apnoea — Dyspnoea — Asphyxia, ...... 204 CHAPTER VI. FOOD AND DIET, ......... 208 Classification of Foods, ....... 209 Foods containing chiefly Nitrogenous Bodies, .... 210 Carbohydrate Bodies, . 212 " Fatty Bodies, ... .213 Substances supplying the Salts, ..... 213 Liquid Food, 213 CONTENTS. Vll PAGE FOOD AND DIET — continued. Effects of Cooking, ....... 213 Effects of an Insufficient Diet, . . . . . .214 Starvation, . i ...... 315 Effects of Improper Food, ....... 216 Effects of too much Food, ...... 217 Diet Scale, ......... 218 CHAPTER VII. DIGESTION, . . . . . . . . ' . .220 PASSAGE OF FOOD THROUGH THE ALIMENTARY CANAL, . . . 220 Mastication, . . . . . . . . .220 The Teeth, . . • . . . . . .221 Insalivation, . . . . . . . . .231 The Salivary Glands and the Saliva, ..... 231 Structure of the Salivary Glands, . . . . . .231 The Saliva, ......... 233 Influence of the Nervous System on the Secretion of Saliva, . . 237 The Pharynx, ........ 241 The Tonsils, ......... 241 The (Esophagus or Gullet, ....... 242 Swallowing or Deglutition, ....... 244 DIGESTION OF FOOD IN THE STOMACH, ..... 245 Structure of the Stomach, . . . . . . .246 Gastric Glands, ........ 248 The Gastric Juice, ........ 250 Functions of the Gastric Juice, ...... 252 Movements of the Stomach, ....... 255 Influence of the Nervous System on Gastric Digestion, . . , 256 Digestion of the Stomach after Death, ..... 257 Vomiting, ......... 258 DIGESTION IN THE INTESTINES, . . . . . . . 260 Structure of the Small Intestine, ...... 260 Structure of the. Large Intestine, . . . . . .265 The Pancreas and its Secretion, ...... 270 Structure and Functions of the Liver, ...... 274 The Bile, ......... 279 The Liver as a Blood-elaborating Organ, ..... 286 Succus Entericus, ........ 289 Summary of the Changes which take place in the Food during its Passage through the Small Intestine, ...... 290 Summary of the Process of Digestion in the Large Intestine, . . 292 Movements of the Intestines, ...... 293 Influence of the Nervous System on Intestinal Digestion, . . . 294 Defalcation, .....'.... 295 Gases contained in the Stomach and Intestines, .... 296 Vlll CONTENTS. CHAPTEE VIII. PAOE ABSORPTION, .......... 298 The Lacteal and Lymphatic Vessels and Glands, .... 298 Properties of Lymph and Chyle, ...... 308 Absorption by the Lacteal Vessels, ..... 309 Absorption by the Lymphatic Vessels, . . . . .310 Absorption by Blood-vessels, . . . . . .311 CHAPTEE IX. « ANIMAL HEAT, ...... . 316 Variations in Bodily Temperature, . . . . . 316 Sources of Heat, ..... . 318 Loss of Heat, ...... 320 Production of Heat, ........ 322 Inhibitory Heat-centre, ....... 323 CHAPTEE X. SECRETION, ....... . 325 CHAPTEE XL THE STRUCTURE AND FUNCTIONS OF THE SKIN, . . 340 CHAPTEE XII. THE STRUCTURE AND FUNCTIONS OF THE KIDNEYS, Structure of the Kidneys, ....... 353 Structure of the Ureter and Urinary Bladder, . . . .359 The Urine, 361 Micturition, ......... 382 CHAPTEE XIII. THE VASCULAR GLANDS, .... CHAPTEE XIV. THE MUSCULAR SYSTEM, ...... Causes and Phenomena of Motion, Plain or Unstriped Muscle, .... .394 Striated Muscle, ..... . 396 Chemical composition of Muscle, Physiology of Muscle at rest, .... • " " in activity, ...••• 406 Eigor Mortis, ..... 419 Actions of the Voluntary Muscles, . . ' • " " Involuntary Muscles, Electrical Currents in Nerves, . . . . • • .427 CONTENTS. ]'x CHAPTER XV. PAGE NUTRITION : THE INCOME AND EXPENDITURE OF THE HUMAN BODY, . 432 Nitrogenous Equilibrium and Formation of Fat, .... 435 CHAPTER XVI. THE VOICE AND SPEECH, ....... 437 CHAPTER XVII. THE NERVOUS SYSTEM, . . . . . . . .450 Elementary Structures of the Nervous System, .... 450 Functions of Nerve Fibres, . . . . . . .456 Laws of Conduction in Nerve Fibres, . . . . . 458 Functions of Nerve Centres, . . . . . . .465 CHAPTER XVIII. CEREBRO-SPINAL NERVOUS SYSTEM, . . . . . . . 472 The Spinal Cord and its Nerves, . . . . . .472 Functions of the Spinal Cord, ...... 481 The Medulla Oblongata, . . . . . . . 491 Structure and Distribution of the Fibres of the Medulla Oblangata, . 491 Functions of the Medulla Oblongata, ...... 495 PonsVarolii, ........ 499 Crura Cerebri, . . . . . . . . .499 Corpora Quadrigemma, ....... 500 THE CEREBRUM, ......... 502 Structure of the Cerebrum, ...... 503 Functions of the Cerebrum, . . . . . . • .511 The Cerebellum, ........ 524 Structure and Functions of the Cerebellum, ..... 524 CHAPTER XIX. PHYSIOLOGY OF THE CRANIAL NERVES, ..... 530 CHAPTER XX. THE SENSES, ......... 546 THE SENSE OF TOUCH, . . . . . . . 550 THE SENSE OF TASTE, ....... 556 THE SENSE OF SMELL, ... . . 56;i THE SENSE OF HEARING, . .... 567 THE SENSE OF SIGHT, . . ..... 584 CHAPTER XXI. THE SYMPATHETIC NERVOUS SYSTEM, ...... 625 X CONTENTS. CHAPTEE XXIT. PAGE THE REPRODUCTIVE ORGANS, ....... 634 CHAPTER XXITT. DEVELOPMENT, . . . . . . . . . 656 The Changes in the Ovum, . • . . . . . '656 Development of Organs, ....... 678 CHAPTER XXIV. ON THE RELATION OF LIFE TO OTHER FORCES, .... 713 APPENDIX. THE CHEMICAL BASIS OF THE HUMAN BODY, .... 732 APPENDIX B : ANATOMICAL WEIGHTS AND MEASURES, . ... 754 Measures of Weight, . . ... . . . 754 " Length, ..... 754 Sizes of various Histological Elements and Tissues, . . . 755 Metrical System of Weights and Measures compared with the Common Measures, ........ 756 CLASSIFICATION OF THE ANIMAL KINGDOM, . . . 756 INDEX, 759 XI * Table for converting Degrees of the FAHRENHEIT Ther- mometer Scale into Degrees CENTIGRADE. MEASUREMENTS. FRENCH INTO ENGLISH. LENGTH. 1 metre ^ = 39.37 English 10 decimetres inches 100 centimetres f (or 1 yard and 1,-000 millimetres J 3£ in.) FAHRENHEIT. 500° CENTIGRADE. 260° 205 200 195 190 180 175 170 165 160 155 150 140 135 130 120 115 110 100 95 90 80 . 75 60 50 45 40.54 40 37.8 401 392 383 374 356 347 1 decimetre } = 3.937 inches 10 centimetres v (or nearly 4 100 millimetres ) inches) 338 329 320 . . 311 1 centimetre "1 = .3937 or about 10 millimetres > (nearly finch.) 1 millimetre — nearly -^ - inch 302 284 275 266 248 239 230 212 203 194 CAPACITY. 1,000 cubic decimetres ) _1 rilWo mptrp 1,000,000 cubic centimetres f ~ 176 167 1 cubic decimetre ) = 1 litre or V (35i fluid oz., 1,000 cubic centimetres ) or rather less than an English quart) 140 122 ... . 113 105 104 100 98.5 95 86 36.9 35 30 25 20 10 5 0 WEIGHT. 1 gramme ] 10 decigrammes 1 - 15.432349 grs. 100 centigrammes [ (or nearly 15£) 1,000 milligrammes 77 68 50 41 32 Zero 23 14 —10 —15 —20 -25 on 1 decigramme ) 10 centigrammes [• = rather more 100 milligrammes ) than !-£ grain + 5 — 4 —13 oo —40 —76 —40 —60 1 centigramme } = rather more 10 decigrammes f than -8a0- grain 1 degree Fahr. = -10 " " .54° C. 2° G! 2.5° C. 3°C. 3.6 " " = 4.5 " " = 5.4 " " = 1 milligramme = rather more than jfo grain * Modified from Fownes1 ( Chemistry. Measure of i decimetre, or 10 centimetres, or 100 millimetres. I I I I I I I I I I I I I I 2 10 Highest point of Crest of the Ilium. Anterior Su- „ perior Spite of the Ilium. Symphysia Piibis. DIAGRAM OF THORACIC AND ABDOMINAL REGIONS. A. Aortic Valve. M. Mitral Valve. P. Pulmonary Valve. T. Tricuspid Valve. Cranium - 7 Cervical Vertebrae Clavicle. Scapula. 12 Dorsal Vertebrae. Humerus. 5 Lumbar Vertebrae. — Ilium. Ulna. Radius. Pelvis. Bones of the Carpus. Bones of the Meta- carpus. Phalanges of Fingers. Femur. Patella. Tibia. Fibula. Bones of the Tarsus. Bones of the Meta- tarsus. Phalanges of Toes. THE SKELETON (AFTER HOLDEN). HANDBOOK OF PHYSIOLOGY. CHAPTER I. THE PHENOMENA OF LIFE. HUMAK physiology is that part of animal physiology which treats 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. As, however, man is a member of the animal kingdom, although separated and specialized no doubt to a remarkable degree, he during life manifests certain characteristics — possesses certain properties and performs certain functions — in common with all living animals, even the very lowest, and these may be called essentials of animal life. If we go a step further, we find that most of these characteristic properties and functions are possessed also by the very lowest vegetable structures, and are in fact the characters by which we distinguish living from not- living matter; they are essentials or phenomena of life in general. Thus we see that as human physiology, which treats of man only, is a part of animal physiology, which treats of the functions and organization of animals in general, so is animal physiology but a part of the wider science of Biology, which embraces the organization and manifestations of all living things. Before entering upon the study of Human physiology, therefore, it is useful and even necessary to devote our attention for a little while to the investigation of what are the properties and functions common to all living matter, and how they are manifested, since it would be unwise to attempt to comprehend the working of the complex machine of the life of man without some knowledge of the motive power in its simplest form. Living matter, in its most elementary form, is found to consist of a \ jelly-like substance which is now generally known under the name of Protoplasm. This substance, in its most primitive form, and in minute masses, is found undifferentiated and perfectly homogeneous, and constitutes the lowest types both of animal and vegetable life that can be observed 1 2 HANDBOOK OF PHYSIOLOGY. under the microscope. It is this substance, too, which forms the cells, of which even the most complex organism has been proved to be made up and from which it has been developed. Thus, the human body can be shown by dissection to consist of various dissimilar parts, bones, mus- cles, brain, heart, lungs, intestines, etc., and these, on more minute examination, are found to be composed of different tissues, such as epi- thelial, connective, nervous, muscular, and the like. Each of these tis- sues is made up of cells or of their altered equivalents. Again, we are taught by Embryology, the science which treats of the growth and structure of organisms from their first coming into being, that the Imman body, made up of all these dissimilar structures, commences its life as a minute cell or ovum about one one hundred and twentieth of an inch in diameter, consisting of a spherical mass of protoplasm, in the midst of which was contained a smaller spherical body or germinal vesi- cle. The phenomena of life then are exhibited in cells, whether exist- ing alone or developed into the organs and tissues of animals and plants. It must be at once evident, therefore, 'that a correct knowledge of the nature and activities of the cell, forms the very foundation of physiology. Cells are, in fact, physiological no less than morphological units. The prime importance of the cell as an element of structure was first established by the researches of Schlejden, and his conclusions, drawn from the study of vegetable histology, were at once extended by Schwann to the animal kingdom. The earlier observers defined a cell as a more or less spherical body limited by a membrane, and containing a smaller body termed a nucleus, which iti its turn incloses one or more nucleoli. Such a definition applied admirably to most vegetable cells, but the more extended investigation of animal tissues soon showed that in many cases no limiting membrane or cell- wall could be demonstrated. The presence or absence of a cell-wall, therefore, was now regarded as quite a secondary matter, while at the same time the cell-substance came gradually to be recognized as of primary importance. Many of the lower forms of animal life, e. g , the Khizopoda, were found to con- sist almost entirely of matter very similar in appearance and chemical composition to the cell-substance of higher forms; and this from its chemical resemblance to flesh was termed Sarcode by Dujardin. When recognized in vegetable cells it was called Protoplasm by Mulder, while Remak applied the same name to the substance of animal cells. As the presumed formative matter in animal tissues it was termed Blas- tema, and in the belief that, wherever found, it alone of all substances has to do with generation and nutrition, Beale has named it Germinal matter or Bioplasm. Of these terms the one most in vogue at the pres- ent day, as we have already said, is Protoplasm, and inasmuch as all life, both in the animal and vegetable kingdoms, is associated with proto- plasm, we are justified in describing it, with Huxley, as the " physical basis of life/' or simply " living matter." A ceJJ may now be defined as a nucleaj£d_mass of protoplasm,1 of microsco-pic size, which possesses sufficient individuality to have a life- 1 In the human body the cells range from the red blood-cell foiftnr iQ-) to ganglion-cell (3£o in-)- THE PHENOMENA OF LIFE. 3 history of its own. Each cell goes through the same cycle of changes as the whole organism, though doubtless in a much shorter time. Begin- ning with its origin from some pre-existing cell, it grows, produces other cells, and finally dies. It is true that several lower forms of life consist of non-nucleated protoplasm, but the above definition holds good for all the higher plants and animals. Hence a summary of the manifestations of cell life is really an ac- count of the vital activities of protoplasm. Protoplasm. — Physically, protoplasm is viscid, varying from a semi-fluid to a strongly coherent consistency. Chemically, living proto- plasm is an extremely unstable albuminoid substance, insoluble in water. It is neutral or weakly alkaline in reaction. It undergoes heat stiffening or coagulation at about 130° F. (54.5° 0.), and hence no organism can live when its own temperature is raised beyond this point. M;iny. of course, can exist for a time in a much hotter atmosphere, since they possess the means of regulating their own temperature. Besides the coagulation produced by heat, protoplasm is coagulated and therefore killed by all the reagents which produce this change in albumen (see Appendix). If protoplasm be subjected to chemical analysis, the chief substances of which it is found to consist belong to the class of bodies called Proteids or albumins. These are bodies made up of the chemical elements 0. H. N.~0. and S., in certain slightly varying proportions. They are essential to the formation of protoplasm, for without one or more of them, protoplasm cannot exist. Indeed some would put this still more shortly, and say that protoplasm is living pro- teid. Associated with proteids as an essential, is a certain amount of water; but there are other bodies, non-essential, frequently present, and varying under different circumstances; such as glycogen, starch, cellulose, chlorophyll, fats, and the like. The protoplasmic substance of cells may undergo more or less essen- tial modifications: thus, in fat cells we may have oil, or fatty crystals, occupying nearly the whole cell; in pigment cells we find granules of pigment; in the various gland cells the elements of their secretions. Moreover, the original protoplasmic contents of the cell may undergo a gradual chemical change with advancing age; thus the protoplasmic cell-substance of the deeper layers of the epidermis becomes gradually converted into keratin as the cell approaches the surface. So, too, the original protoplasm of the embryonic blood-cells is infiltrated with the haemoglobin of the mature colored blood-corpuscle. The vital or physiological characters of protoplasm are seen in the performance of its functions. Many of these qualities are exceedingly well illustrated in the microscopic animal called the Amoeba, which is a monocellular organism found chiefly in fresh water, but also in the sea and in damp earth. Under the same term no doubt more than one kind of organism is included, but at any rate in each most of the vital properties of protoplasm may well be studied. They are as follows: — HANDBOOK OF PHYSIOLOGY. 1. TJie power of spontaneous movement. — When an amoeba is observed with a sufficiently high power of the microscope, it is found to consist of an irregular mass of protoplasm distinguished into an outer dense layer and an inner more fluid mass. If watched for a minute or two an irregular projection or pseudopodium is seen to be gradually thrust out from the main body and retracted: a second mass is then pro- truded in another direction, and gradu- ally the whole protoplasmic substance is, as it were, drawn into it. The Amoeba thus comes to occupy a new position, and when this is repeated several times we have locomotion in a definite direction, together with a continual change of form. These movements, when observed in other cells, such as the colorless blood- corpuscles of FIG. 1.— Amoebae. FIG. 2 —Human colorless blood-corpuscle, showing its successive changes of outline within ten minutes when kept moist on a warm stage. (Schofield.) higher animals (Fig. 2), in the branched cornea cells of the frog and elsewhere, are hence termed amwboid. Other illustrations of amoeboid movement. — The remarkable motions of pigment-granules observed in the branched pigment-cells of the f rog's skin by Lister are probably due to amoeboid movement. These granules are seen at one time distributed uniformly through the body and branched processes of the cell, while under the action of various stimuli (e.g., light and electricity) they collect in the central mass, leaving the branches quite colorless. Ciliary action must be regarded as only a special variety of the gen- eral motion with which all protoplasm is endowed. The grounds for this view are the following : In the case of the Infusoria, which move by the vibration of cilia (microscopic hair-like processes projecting from the surface of their bodies) it has been proved that these are simply processes of their protoplasm protruding through pores of the investing membrane, like the^oars of a galley, or the head and legs of a tortoise from its shell: certain reagents cause them to be partially retracted. Moreover, in some cases cilia have been observed to develop from, and in others to be transformed into, amoeboid pro- cesses. In the hairs of the stinging-nettle and Tradescantia and the cells of Vallisneria and Chara, the movement of protoplasm can be marked by the movement of the granules nearly always imbedded in it. For example, if part of a hair of Tradescantia (Fig. 3) be viewed under a high magnifying power, streams of protoplasm containing crowds of granules, hurrying along, like the foot passengers in a busy street, are seen flow- THE PHENOMENA OF LIFE. FIG. 3.— Cell of Tradescantia drawn at successive intervals of two minutes. The cell -contents consist of a central mass con- nected by many irregular processes to a peripheral film : the whole forms a vacuo- lated mass of protoplasm, which is continu- ally changing its shape. (Schofield.j ing steadily in definite directions, some coursing round the film which lines the interior of the cell-wall, and others flowing towards or away from the irregular mass in the centre of the cell-cavity. Many of these streams of protoplasm run together into larger ones, and are lost in the central mass, and thus ceaseless vari- ations of form are produced. 2. Irritability and the power of response to stimuli. — Although the movements of the amoeba have been described above as spontaneous, yet they may be increased under the ac- tion of various stimuli, and if the movement have ceased for a time, as is the case if. the temperature be low- ered beyond a certain point, it may be set up by raising the temperature. Again, contact with foreign bodies, gentle pressure, certain salts, and elec- tricity, if applied to the amoeba, produce or increase the movement. It is, therefore, sensitive or irritable to stimuli, and shows its irritability by movement or contraction of its mass. The effects of some of these stimuli may be thus further detailed: — 1% Changes of temperature. — Moderate heat acts as a stimulant: this is readily observed in the activity of the movements of a human colorless blood-corpuscle when placed under conditions in which its normal tem- perature and moisture are preserved. Extremes of heat and cold stop the motions entirely. 2. Mechanical stimuli. — When gently squeezed between a cover and object-glass under proper conditions, a colorless blood-corpuscle is stimu- lated to active amoeboid movement. 3. Nerve influence. — By stimulation of the nerves of the frog's cornea, contraction of certain of its branched cells has been produced. 4. Chemical stimuli. — Water generally stops amoeboid movement, and by imbibition causes great swelling and finally bursting of the cells. In some cases, however (myxomycetes), protoplasm can be almost entirely dried up, and is yet capable of renewing its motion when again moist- ened. Dilute salt-solution and many dilute acids and alkalies, stimu- late the movements temporarily. Ciliary movement is suspended in an atmosphere of hydrogen or carbonic acid, and resumed on the admission of air or oxygen. 5. Electrical. — Weak currents stimulate the movement, while strong currents cause the corpuscles to assume a spherical form and become motionless. 3. Nutritive poiuers. — The power of taking in food, modifying it, building up tissue by assimilating it, and rejecting what i* not assimi- lated. All these processes take place in the amoeba. They are effected by its simply flowing round and inclosing within itself minute organisms HANDBOOK OF PHYSIOLOGY. such as diatoms and the like, from which it extracts what it requires, and then rejects or excretes the remainder, which has never formed part of the body, by withdrawing itself from it. The assimilation which goes on in the body of the amoeba, is to replace waste of its tissue consequent upon manifestation of energy. The two processes of waste and repair, then, go on side by side, and as long as they are equal the size of the animal remains stationary. If, however, the building up exceed the waste, then the animal grows ; if the waste exceed the repair, the animal decays ; and if decay go on be- yond a certain point, life becomes impossible, so the animal dies. Growth, or inherent power of increasing in size, although essential to our idea of life, is not confined to living beings. A crystal of common salt, or of any other similar substance, if placed under appropriate con- ditions for obtaining fresh material, will grow in a fashion as definitely characteristic and as easily to be foretold as that of a living, creature. It is, therefore, necessary to explain the distinctions which exist in this re- spect between living and lifeless structures ; for the manner of growth in the two cases is widely different. Differences between living and lifeless growth. — (1.) The growth of a crystal, to use the same example as before, takes place merely by addi- tions to its outside ; the new matter is laid on particle by particle, and layer by layer, and, when once laid on, it remains unchanged. 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. (2.) All living structures are subject to constant decay ; and life con- sists 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 exactly the same particles day after day, although to all intents he remains the same in- dividual. 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 differ- ence may be noticed, except at long intervals of time. A lifeless struc- ture, 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 structure which never had to do with life is true also with respect to those which, though they are formed by living parts, are not themselves alive. Thus, an oyster-shell is formed by the living animal which it incloses, but it is as lifeless as any other mass of inorganic matter ; and in accordance with this circumstance its growth takes place layer by layer, and it is not subject to the constant decay and reconstruction which belong to the living. The hair and nails are examples of the same fact. (3.) In connection with the growth of lifeless masses there is no al- teration in the chemical composition of the material which is taken up and added to the previously existing mass. For example, when a crystal of common salt grows on being placed in a fluid which contains the same material, the properties of the salt are not changed by being taken out of the liquid by the crystal and added to its surface in a solid form. But the case is essentially different in living beings, both animal and vegetable. A plant, like a crystal, can only grow when fresh material is presented to it ; and this is absorbed by its leaves and roots ; and animals THE PHENOMENA OF LIFE. 7 for the same purpose of getting new matter for growth and nutrition, take food into their stomachs. But in both these cases the materials are much altered before they are finally assimilated by the structures they are destined to nourish. (4. ) 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 with- out limit as to growth as without limit to life. 4. Reproductive powers. — The amoeba, to return to our former illus- tration, when the growth of its protoplasm has reached a certain point, manifests the power of reproduction, by splitting up into (or in some other way producing) two or more parts, each of which is capable of in- dependent existence. The new amoebae manifest the same properties as their parent, perform the same functions, grow and reproduce in their turn. This cycle of life is being continually passed through. In more complicated structures than the amoeba, the life of indi- FIG. 4.— Diagram of an ovum (a) undergoing segmentation.— In (6) it has divided into two ; in (c) into four ; and in (d> the process has ended in the production of the so-called " mulberry mass." (Frey.) vidual protoplasmic cells is probably very short in comparison with that of the organism they compose : and their constant decay and death necessitate constant reproduction. The mode in which this takes place has long been the subject of great controversy. It is now very generally believed that every cell is descended from some pre-existing (mother-) cell. This derivation of cells from cells takes place by (1) gemmation, or (2) fission or division. (1) Gemmation. — This method has not been observed in the human body or the higher animals, and therefore requires but a passing notice. It consists essentially in the budding off and separating of a portion of the parent cell. (2) Fission or Division. — As examples of reproduction by fission, we may select the ovum, the blood cell, and cartilage cells. In the frog's ovum (in which the process can be most readily ob- served) after fertilization has taken place, there is first some amoeboid movement, the oscillation gradually increasing until a permanent dimple appears, which gradually extends into a furrow running completely round the spherical ovum, and deepening until the entire yelk-mass is divided into two hemispheres of protoplasm each containing a nucleus (Fig. 4, #). This process being repeated by the formation of a second furrow at right angles to the first, we have four cells produced (c) : this 8 HANDBOOK OF PHYSIOLOGY. subdivision is carried on till the ovum has been divided by segmentation into a mass of cells (mulberry-mass) (d) out of which the embryo is de- veloped. Segmentation is the first step in the development of all the higher animals, including man. Multiplication by fission has been observed in the colorless blood- cells of many animals. In some cases (Fig. 5), the process has been FIG. 5.— Blood-corpuscle from a young deer embryo, multiplying by fission. (Frey.) seen to commence with the nucleolus which divides within the nucleus. The nucleus then elongates, and soon a well-marked constriction occurs, rendering it hour-glass shaped, till finally it is separated into two parts, which gradually recede from each other; the same process is repeated in the cell-substance, and at length we have two cells produced which by rapid growth soon attain the size of the parent cell (direct division). In some cases there is a primary fission into three instead of the usual two cells. In cartilage (Fig. 6), a process essentially similar occurs, with the FIG. 6. -Diagram of a cartilage cell undergoing fission within its capsule. -The process of divi- rsion is represented as commencing in the nucleolus, extending to the nucleus, and at length involv- ing the body of the cell. (Frey.) exception that (as in the ovum) the cells produced by fission remain in the original capsule, and in their turn undergo division, so that a large number of cells are sometimes observed within a common envelope. This process of fission within a capsule has been by some described as a separate method, under the title " endogenous fission," but there seems Ho be no sufficient reason for drawing such a distinction. It is important to observe that fission is often accomplished with great rapidity, the whole process occupying but a few minutes, hence the comparative rarity with which cells are seen in the act of dividing. Indirect cell division. — In certain and numerous cases, the division of cells does not take place by the simple constriction of their nuclei and THE PHENOMENA OF LIFE. 8 surrounding protoplasm into two parts as above described (direct divi- sion), but is preceded by complicated changes in their nuclei (karyoki- nesis). These changes consist in a gradual re-arrangement of the intra- nuclear network of each nucleus (see p. 17), until two nuclei are formed similar in all respects to the original one. The nucleus in a resting condition, i.e., before any changes preceding division occur, consists of a very close meshworkx>f fibrils, which stain deeply in carmine, embed- ded in protoplasm, which does not possess this property, the whole nucleus being contained in an envelope. The first change consists of a slight enlargement, the disappearance of the envelope, and the increased definition and thickness of the nuclear fibrils, which are also more sepa- rated than they were, and stain better. This is the stage of convolution (Fig. 7, B, c). The next step in the process is the arrangement of the FIG. 7.— Karyokinesis. A, ordinary nucleus of a columnar epithelial cell ; B, c, the same nu- cleus in the stage of convolution ; D. the wreath or rosette form ; s, the aster or single star ; p. a nu- clear spin41e from the Descemet's endothelium of the frog's cornea ; o, H, i, diaster ; K, two daugh- ter nuclei. (Klein.) fibrils into some definite figure by an alternate looping in and out around a central space, by which means the rosette or lureath stage (Fig. 7, D) is reached. The loops of the rosette next become divided at the peri- phery, and their central points become more angular, so that the fibrils, divided into portions of about equal length, are, as it were, doubled at an acute angle, and radiate V-shaped from the centre, forming a star (aster) or wheel (Fig. 7, B), or perhaps from two centres, in which case a double star (diaster) results (Fig. 7, G, H, and i). After remaining almost unchanged for some time, the V-shaped fibres being first re-ar- ranged in the centre, side by side (angle outwards), tend to separate into two bundles, which gradually assume position at either pole. From these groups of fibrils the two nuclei of the new cells are formed (daughter nuclei) (Fig. 7, K), and the changes they pass through before reaching the resting condition are exactly those through which the^ original nu- cleus (mother nucleus) has gone, but in a reverse order, viz., the star, the rosette, and the convolution. During or shortly after the forma- tion of the daughter nuclei the cell itself becomes constricted and then divides in a line about midway between them. 5. Decay and death of cells. —There are two chief ways in which the comparatively brief existence of cells is brought to an end: (1) Me- chanical abrasion, (2) Chemical transformation. 10 HANDBOOK OF PHYSIOLOGY. 1. The various epithelia (p. 19) furnish abundant examples of mechanical abrasion. As it approaches the free surface the cell be- comes more and more flattened and scaly in form and more horny in consistence, till at length it is simply rubbed off. Hence we find epithe- lial cells in the mucus of the mouth, intestine, and genito-urinary tract. 2. In the case of chemical transformation the cell-contents undergo a degeneration which, though it may be pathological, is very often a normal process. Thus we have (a.) fatty metamorphosis pro- ducing oil-globules in the secretion of milk, fatty degeneration of the muscular fibres of the uterus after birth of the foetus, and of the cells of the Graafian follicle giving rise to the te corpus luteum." (See chapter on Generation.) (b.) Pigmentary degeneration from deposit of pig- ment, as in the epithelium of the air- vesicles of the lungs, (c.) Calca- reous degeneration, which is common in the cells of many cartilages. Differences between Plants and Animals. Having now considered somewhat at length the vital properties of protoplasm, as shown in cells of vegetable as well as animal organisms, we are now in a position to discuss the question of the differences between plants and animals. It might at the outset of our inquiry have seemed an unnecessary thing to recount the very great distinctions which exist between an animal and a vegetable, but, however great these may be between the higher animals and plants, yet in the lowest of them the distinctions are much less obvious. (1.) Perhaps the most essential distinction is the power which vege- table protoplasm possesses of being able to build up new albuminous material out of such chemical bodies as ammoniujn salts, carbonic acid gas and water, together with! mineral sulphates and phosphates. By means of their green coloring matter, Morophyl — a substance almost exclusively confined to the vegetable kingdom— plants are capable of decomposing the carbonic acid gas, which they absorb by their leaves. The result of this chemical action, which occurs only under the influence of light, is, so far as the carbonic acid is concerned, the fixation of carbon in the plant structures and the exhalation of oxygen. The carbon thus obtained becomes combined with the elements of water absorbed by the roots, to form starch. By the re-arrangement of the elements composing this body, with the addition of nitrogen and sulphur derived from nitrates and sulphates of the soil, vegetable protoplasm can con- struct albumen. Animal protoplasm is incapable of thus using such substances and never exhales oxygen as a product of decomposition. It must have ready-formed albuminous food in order to liye. The power of living upon albuminous as well as non-albuminous matter is less decisive of an animal nature; inasmuch as fungi and some other parasitic plants derive their nourishment in part from the former source. (2.) There is, commonly, a difference in general chemical composition THE PHENOMENA OF LIFE. 11 between vegetables and animals, even in their lowest forms; for associ- ated with the protoplasm of the former is a considerable amount of cellulose, a substance closely allied to starch and containing carbon, hydrogen, and oxygen only. The presence of cellulose in animals is much more rare than in vegetables, but there are many animals in which traces of it may be discovered, and some, the Ascidians, in which it is found in considerable quantity. The presence of starch in vegetable cells is very characteristic, though not distinctive, and a substance, gly- cogen, nearly allied in composition to cellulose, is very common in the organs and tissues of animals. (3.) 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 mo- tion is now known, however, to exist in so many vegetable forms, that it can no longer be held as an essential distinction between them and ani- mals, and ceases to be a mark by which the one can be distinguished from the other. Thus the zoospores of many of the Cryptogamia exhibit ciliary or amoeboid movements (p. 4) of a like kind to those seen in amoebae; and even among the higher order of plants, many, e.g., Dioncea Muscipula (Venus's fly-trap), and Mimosa sensitiva (Sensitive plant), exhibit such motion, either at regular times, or on the application of external irritation, as might lead one, were this fact taken by itself, to regard them as sentient beings. Inherent power of movement, then, although especially characteristic of animal nature, is, when taken by itself, no proof of it. (4.) The presence of a digestive canal is a very general mark by which an animal can be distinguished from a vegetable. But the lowest animals are surrounded by material that they can take as food, as a plant is surrounded by an atmosphere that it can use in like manner. And every part of their body being adapted to absorb and digest, they have no need of a special receptacle for nutrient matter, and accordingly have no digestive canal. This distinction then is not a cardinal one. It would be tedious as well as unnecessary to enumerate the chief distinctions between the more highly developed animals and vegetables. They are sufficiently apparent. In passing, it may be well to point out the main distinctions between animal and vegetable cells. It has been already mentioned that in animal cells an envelope or cell-wall is by no means always present. In adult vegetable cells, on the other hand, a well defined cellulose wall is highly characteristic; this, it should be remembered, is non-nitrogenous, and thus differs chemically as well as structurally from the contained mass. Moreover, in vegetable cells (Fig. 8, B), the protoplastic contents of the cell fall into two subdivisions: (1) a continuous film which lines the 12 HANDBOOK OF PHYSIOLOGY. interior of the cellulose wall; and (2) a reticulate mass containing the nucleus and occupying the cell-cavity; its interstices are filled with fluid. In young vegetable cells such a distinction does not exist; a finely gran- ular protoplasm occupies the whole cell-cavity (Fig. 8, A). FIG. 8.— (A) Young vegetable cells, showing cell-cavity entirely filled with granular protoplasm inclosing a large oval nucleus, with one or more nucleoli. (B) Older cells from same plant, show- ing distincc cellulose- wall and vacuolation of protoplasm. Another striking difference is the frequent presence of a large quantity of intercellular substance in animal tissues, while in vegetables it is com- paratively rare, the requisite consistency being given to their tissues by the tough cellulose walls, often thickened by deposits of lignin. As an example of the manner in which this end is attained in animal tissues, may be mentioned the deposition of lime-salts in a matrix of intercellu- lar substance in ossification. Morphological Development and Division of Functions. As we proceed upwards in the scale of life from monocellular organ- isms, we find that another phenomenon is exhibited in the life history of the higher forms, namely, that of Development. An amoeba comes into being derived from a previous amoeba; it manifests the properties and performs functions of life which have been already enumerated; it grows, it reproduces itself, whereby several amoebae result in place of one, and it dies, but it can scarcely be said to develop unless the formation of a nucleus can be so considered. In the higher organisms, however, it is different; they, indeed, begin as a single cell, but this cell on its division and subdivision does not form so many different organisms, but possesses the material from which, by development, the completed and perfected whole is to be derived. Thus, from the spherical ovum, or germ, which forms the starting-point of animal life, and which consists of a proto- plasmic cell with a nucleus and nucleolus (see Fig. 4), in a comparatively short time, by the process of segmentation which has been already men- tioned, a complete membrane of cells, polyhedral in shape from mutual pressure, called the blastoderm, is formed, and this speedily divides into two and then into three layers, chiefly from the rapid proliferation of the cells of the first single layer. These layers are called the epiblast, the mesoblast, and the hypoblast. THE PHENOMENA OF LIFE. 13 It is found in the farther development of the animal that from each of these layers is produced a very definite part of its completed body. For example, from the cells of the epiblast, are derived, among other structures, the skin and the central nervous system; from the mesoblast is derived the flesh or muscles of the body, and from the hypoblast, the epithelium of the alimentary canal and some of the chief glands. From the epiblast are ultimately developed the superficial skin or epidermis and its various appendages, also the central or cerebro-spinal nerve centres, the sensorial epithelium of the organs of special sense (the eye, the ear, the nose), and the epithelium of the mouth and salivary glands. From the JiypoUast is developed the epithelium of the whole diges- tive canal, together with that lining the ducts of all the glands which open into it ; also the glandular parenchyma of the glands (e.g., liver and pancreas) connected with it, and the "epithelium of the respiratory tract. FIG. 9.— Transverse section through embryo chick (26 hours), a, epiblast ; 6. roesoblast ; c, hy- poblast ; d, central portion of mesoblast, which is here fused with epiblast ; e, primitive groove ; /, dorsal ridge. (Klein.) From the mesoUast are derived all the tissues and organs of the body intervening between these two, the whole group of the connective tissues, the muscles and the cerebro-spinal and sympathetic nerves, with the vascular and genito-uriuary systems, and all the digestive canal, with its various appendages, with the exception of the lining epithelium above mentioned. It is obvious that these tissues and organs exhibit in a varying degree the primary properties of protoplasm. The muscles, for example, de- rived from certain cells of the mesoblast are highly contractile and re- spond to stimuli readily, but they have little to do with digestion except indirectly, and again, the cells of the liver, although doubtless contrac- tile to a certain extent, yet have secretion and digestion for their chief functions. Thus we see development in two directions going on side by side. It speedily becomes necessary for the organism to depute to different groups of cells, or their equivalents (i.e., to the tissues or organs to which they give rise), special functions, so that the various functions 14: HANDBOOK OF PHYSIOLOGY. which the original cell may be supposed to discharge, and the various properties it may be supposed to possess, are divided up among various groups of resulting cells. The work of each group is specialized. As a result of this division of labor, as it is called, these functions and prop- erties are, as might be expected, developed, and made more perfect, and the tissues and organs arising from each group of cells are developed also, with a view to the more convenient and effective exercise of their func- tions and employment of their properties. It would be out of place here to discuss the question as to the exact manner in which a property or function, rudimentary in a low form of animal life, is found to be highly developed as we pass up the series ; neither is it our province to discuss the very complicated subject of the relationship of man to other animals, and of these to one another. Having now briefly indicated the close connection which exists be- tween Human physiology and Biology in general, we are better prepared to commence the study of the former as constituting a part of a great whole. The next two chapters will be devoted to a consideration of the minute structure, or the histology (IGTOS, a tissue or web) of epithelium and the connective tissues. CHAPTER II. THE STRUCTURE OF THE ELEMENTARY TISSUES. THE cells of the body are described in various ways ; for example, ac- cording to their shape, situation, contents, origin and. functions. (a.) Their shape varies: — Starting f rom the spherical or spheroidal (Fig. 10, a) as the typical form assumed by a free cell, we find this altered to a polyhedral shape when the pressure on the cells in all directions is nearly the same (Fig. 10, b). Of this, the primitive segmentation-cells may afford an example. The discoid shape is seen in blood-cells (Fig. 10, c), and the scale-like form in superficial epithelial cells (Fig. 10, d). Some cells have a jagged outline (prickle-cells) (Fig. 27). Cylindrical, conical, or prismatic cells occur in the deeper layers of laminated epithelium, and the simple cylindrical epithelium of the in- testine and many gland ducts. Such cells may taper off at one or both ends into fine processes, in the former case being caudate, in the latter fusiform (Fig. 11). They may be greatly elongated so as to become fibres. Ciliated cells (Fig. 10, d) must be noticed as a distinct variety : FIG. 10.— Various forms of cells, a. Spheroidal, showing nucleus and nucleolus ; 6. Polyhedral ; c. Discoidal (blood cells) ; d. Scaly or squamous (epithelial cells). they possess, but only on their free surfaces, hair-like processes (cilia). These vary immensely in size, and may even exceed in length the cell itself. Finally, we have the branched or stellate cells, of which the large nerve-cells of the spinal cord, and the connective tissue corpuscle are typical examples (Fig. 11, e). In these cells the primitive branches by secondary branching may give rise to an intricate network of processes. (b.) According to their situation in the tissues cells are known as epithelial, connective tissue cells, blood cells, glandular, and the like. (c.) According to their contents, they are called fat cells when their protoplasm contains an excess of fat, pigment cells when it contains pig- ment ; colored, when their protoplasm is infiltrated with a coloring matter, as haemoglobin. 16 HANDBOOK OF PHYSIOLOGY. (d.) According to their functions, they are called secreting, protec- tive, sensitive, contractile, etc. (e.) According to their origin, they are named epiblastic, mesoblastic and hypoblastic. Nearly all cells at some period of their existence possess nuclei. As has been incidentally suggested, the origin of a nucleus in a cell is the \ FIG. 11.— Various forms of cells, a. Cylindrical or columnar ; 6. Caudate ; c. Fusiform ; A Ciliated (from trachea) ; e. Branched, stellate. first trace of the differentiation of protoplasm. The existence of nuclei was first pointed out in the year 1833 by Robert Brown, who observed them in vegetable cells. They are either small transparent vesicular bodies containing one or more smaller particles (nucleoli), or they are FIG. 12.— (A. ^ Colorless blood-corpuscle showing intra-cellular network of Heitzmann, and two nuclei with intra nuclear network (Klein and Noble Smith'. (B ) Colored blood-corpuscle showing intra-cellular network of fibrils (Heitzmann). Also oval nucleus composed of limiting membrane and fine intra-nuclear network of fibrils, x 800. vKlein and Noble Smith.) semi-solid masses of protoplasm always in the resting condition bounded by a well-defined envelope. In their relation to the life of the cell they are certainly hardly second in importance to the protoplasm itself, and thus Beale is fully justified in comprising both under the term " germi- nal matter." They exhibit their vitality by initiating, in the majority of cases, the process of division of the cell into two or more cells (fission) by first themselves dividing. Distinct observations have been made, showing that spontaneous changes of form may occur in nuclei as also in nucleoli. THE STRUCTURE OF THE ELEMENTARY TISSUES. 17 Histologists have long recognized nuclei by two important characters: (1.) Their power of resisting the action of various acids and alkalies, particularly acetic acid, by which their outline is more clearly denned, and they are rendered more easily visible. This indicates some chemi- cal difference between the protoplasm of the cell and nuclei, as the for- mer is destroyed by these reagents. (2.) Their quality of staining in solutions of carmine, haematoxylin, etc. Nuclei are most commonly oval or round, and do not generally conform to the diverse shapes of the cells; they are altogether less vari- able elements than cells, even in regard to size, of which fact one may see a good example in the uniformity of the nuclei in cells so multiform as those of epithelium. But sometimes nuclei appear to occupy the whole of the cell, as is the case in the lymph corpuscles of lymphatic glands, and in some small nerve cells. Their position in the cell is very variable. In many cells, especially where active growth is progressing, two or more nuclei are present. Minute structure of cells. — The protoplasm which forms the body as well as that which forms the nuclei of cells has been shown in many varieties of cells, e.g., the colorless blood-corpuscles, epithelial cells,, connective-tissue corpuscles, nerve-cells, to be made up of a network of very fine fibrils, the meshes of which are occupied by a hyaline intersti- tial substance (Heitzmann's network) (Fig. 12). At the nodes, where the fibrils cross, are little swellings, and these are the objects described as granules by the older observers ; but in the body of some cells, e.g., colorless blood- corpuscles, there are real granules, which appear to be quite free and unconnected with the intra-cellular network. Modes of connection. — Cells are connected together to form tissues in various ways. (1) By means of a cementing intercellular substance. This is prob- ably always present as a transparent, colorless, viscid, albuminous sub- stance, even between the closely apposed cells of epithelium, while in the case of cartilage it forms the main bulk of the tissue, and the cells only appear as imbedded in, not as cemented by, the intercellular sub- stance. This intercellular substance may be either homogeneous or fibrillated. In many cases (e.g., the cornea) it can be shown to contain a number of irregular branched cavities, which communicate with each other, and in which branched cells lie : through these branching spaces nutritive fluids can find their way into the very remotest parts of a non- vascular tissue. As a special variety of intercellular substance must be mentioned the basement membrane (mem ~brana proprid) which is found at the base of the epithelial cells in most mucous membranes, and especially as an investing tunic of gland follicles which determines their shape, and 2 18 HANDBOOK OF PHYSIOLOGY. which may persist as a hyaline saccule after the gland-cells have all been discharged. (2) By anastomosis of their processes. This is the usual way in which stellate cells, e. g., of the cornea, are united ; the individuality of each cell is thus to a great extent lost by its connection with its neighbors to form a reticulum ; as an example of a network so produced we may cite the stroma of lymphatic glands. Sometimes the branched processes breaking up into a maze of minute fibrils, adjoining cells are connected by an intermediate reticulum ; this is the case in the nerve-cells of the spinal cord. Derived tissue-elements. — Besides the Cell, which may be termed the primary tissue-element, there are materials which' may be termed secon- dary or derived tissue-elements. Such are Intercellular substance, Fibres, and Tubules. a. Intercellular substance is probably in all cases directly derived from the cells themselves. In some cases (e. g., cartilage), by the use of reagents the cementing intercellular substance is, as it were, analyzed into various masses, each arranged in concentric layers around a cell or group of cells, from which it was probably derived (Fig. 46). ft. Fibres. — In the case of the crystalline lens, and of muscle both striated and non-striated, each fibre is simply a metamorphosed cell: in the case of the striped fibre the elongation being accompanied by a mul- tiplication of the nuclei. The various fibres and fibrillae of connective tissue result from a grad- ual transformation of an originally homogeneous intercellular substance. Fibres thus formed may undergo great chemical as well as physical transformation: this is notably the case with yellow elastic tissue, in which the sharply defined elastic fibres, possessing great power of resis- tance to reagents, contrast strikingly with the homogeneous matter from which they are derived. y. Tubules, such as the capillary blood-vessels, which were originally supposed to consist of a structureless membrane, have now been proved to be composed of flat, thin cells, cohering along their edges. "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 tissues are variously mixed and interwoven to form more intricate combinations. Thus are constructed epithelium and its modifications, the connec- tive tissues, the fibres of muscle and nerve, etc. ; and these, again, with the more simple structures before mentioned, are used as materials wherewith to form arteries, veins, and lymphatics, secreting and vascu- lar glands, lungs, heart, liver, and other parts of the body. In this chapter the leading characters and chief modifications of the THE STRUCTURE OF THE ELEMENTARY TISSUES. 19 first two of the great groups of tissues — the Epithelial and Connective — will be described; while the others will be appropriately considered in the chapters treating of their physiology. Epithelium. The term epithelium is applied to the cells covering the skin, the mucous and serous membranes,, and to those forming a lining to other parts of the body as well as entering into the formation of glands. For example: — Epithelium clothes (1) the exterior surface of the body, forming the epidermis with its appendages — nails and hairs; becoming continuous at the chief orifices of the body — nose, mouth, anus, and urethra — with the (2) epithelium which lines the whole length of the (3) respiratory, ali- mentary and genito-urinary tracts, together with the ducts of their various glands. Epithelium also lines the cavities of (4) the brain, and the central canal of the spinal cord, (5) the serous aud synovial mem- branes, and (6) the interior of all blood-vessels and lymphatics. Epithelial cells possess an intracellular and an intranuclear network (p. 17). They are held together by a clear, albuminous, cement sub- stance. The viscid semi-fluid consistency, both of cells and intercellular substance, permits such changes of shape and arrangement in the indi- vidual cells as are necessary if the epithelium is to maintain its integrity in organs the area of whose free surface is so constantly changing, as the stomach, lungs, etc. Thus, if there be but a single layer of cells, as in the epithelium lining the air vesicles of the lungs, the stretching of this membrane causes such a thinning out of the cells that they change their shape from spheroidal or short columnar, to squamous, and vice versa, when the membrane shrinks. Epithelial tissues are non-vascular, but in some varieties minute channels exist between the cells of certain layers through which they may be supplied with nourishment from the subjacent blood-vessels. Nerve fibres are supplied to the cells of many epithelia. Epithelial tissue is classified according as the cells composing it are arranged in a single layer when it is simple, or in several layers when it is called stratified or laminated, or in two or three layers occupying a position between the other two forms, when it is termed transitional* Of each form, when there are several varieties, they are named according to the shape of the cells composing it. A. Simple. — (1.) Squamous, scaly, pavement or tesselated; (2.) Spheroidal or glandular; (3.) Columnar, cylindrical, conical or goblet-shaped; (4.) Ciliated. B. Transitional. 20 HANDBOOK OF PHYSIOLOGY. C. Stratified. A. Simple. — Squamous Epithelium (Fig. 13). — Arranged as a single layer, this form of epithelium is found as (a) the pigmentary layer of the retina, and forms the lining of (#) the interior of the serous and and synovial sacs, (c) the alveoli of the lungs, and (d) of the heart, blood - and lymph-vessels. It consists of cells, which are flattened and scaly, with a more or less irregular outline. J^7 FIG. 13. — Squamous epithelium scales from the inside of the mouth, x 260. (Henle.) FIG. 14.— Pigment cells from the retina. A, cells still cohering, seen on their surface; a, nucleus indistinctly seen. In the other cells the nucleus is concealed by the pigment gra- nules. B, two cells seen in profile ; a, the outer or posterior part containing scarcely any pigment. X 370. (Henle.) In the pigment cells of the retina, there is a deposit of pigment in the cell-substance. This pigment consists of minute molecules of mela- nin, imbedded in the cell-substance and almost concealing the nucleus, which is itself transparent (Fig. 14). In white rabbits and other albino animals, in which the pigment of the eye is absent, this layer is found to consist of colorless pavement epithelial cells. The squamous epithelium which is found as a single layer lining the alveoli of the lungs, the serous membranes, and the interior of blood- and lymphatic-vessels, is generally called by a distinct name — Endo- thelium. The presence of endothelium may bo demonstrated by staining the part lined by it with silver nitrate. When a small portion of a perfectly fresh serous membrane for ex- ample, as the mesentery or omentum (Fig. 15), is immersed for a few minutes in a quarter per cent solution of silver nitrate, washed with distilled water and exposed to the action of light, the silver oxide is precipitated in the intercellular cement substance and the endothelial cells are thus mapped out by fine dark and generally sinous lines of ex- treme delicacy. The cells vary in size and shape, and are as a rule irregular in outline; those lining the interior of blood-vessels and lym- phatics being spindle- shape with a very wavy outline. They inclose a clear, oval nucleus, which, when the cell is viewed in profile, is seen to project from its surface. The nuclei are not however evident unless the tissue which has been already stained in silver nitrate, is placed in an- THE STRUCTURE OF THE ELEMENTARY TISSUES. 21 other dye, such as hematoxylhi, which has the property of picking out its nuclei. Endothelial cells may be ciliated, e. g., those in the mesentery of frogs, especially about the breeding season. FIG. 15.— Part of the omentum of a cat, stained in silver nitrate, X 100. The tissue forms a " fenestrated membrane " that is to say, one which is studded with holes or windows. In the figure these are of various shapes and sizes, leaving trabeculae, the basis of which is fibrous tissue. The trabeculae are of various sizes, and are covered with endothelial cells, the nuclei of which have been made evident by staining with hgematoxylin after the silver nitrate has outlined the cells by stain- ing the intercellular substance. (T. D- Harris.) Besides the ordinary endothelial cells above described, there are found on the omentum and parts of the pleura of many animals, little FIG. 16.— Abdominal surface of centrum tendineum of diaphragm of rabbit, showing the gen- eral polygonal shape of the endothelial cells : each is nucleated. (Klein.) X 300. ~bud-like processes or nodules , consisting of small polyhedral granular cells, rounded on their free surface, whicli multiply very rapidly by division (Fig. 17). These constitute what is known as "germinating endothelium." The process of germination doubtless goes on in health, and the small cells which are thrown off in succession are carried into the lymphatics, and contribute to the number of the lymph corpuscles. 22 HANDBOOK OF PHYSIOLOGY. The buds may be enormously increased both in number and size in cer- tain diseased conditions. On those portions of the peritoneum and other serous membranes in Fio. 17.— Silver-stained preparation of great omentum of dog, which shows, amongst the flat endothelium of the surface, small and large groups of germinating endothelium between which numbers of stomata are to be seen. (Klein.) x 3JO. which lymphatics abound (Fig. 18), apertures are found surrounded by small more or less cubical cells. These apertures are called stomata. FIG. 18. -Peritoneal surf ace of septum cisternse lymphatic® magnae of frog. The stomata, some of which are open, some collapsed, are surrounded by germinating endothelium. (Klein.) They are particularly well seen in the anterior wall of the great lymph sac of the frog (Fig. 18), and in the omentum of the rabbit. These are really the open mouths of lymphatic vessels or spaces, and through them lymph-corpuscles, and the serous fluid from the serous cavity, pass into THE STRUCTURE OF THE ELEMENTARY TISSUES. 23 the lymphatic system. They should be distinguished from smaller and more numerous apertures between the cells which are not lined by small cells, although the surrounding cells seem to radiate from them, filled up by intercellular substance or by processes of the cells underneath. These are called pseudo-stomata (Fig. 16). In the neighborhood of the stomata, the cells often manifest indica- tions of germinating. They may be either large with two or more nu- clei, or about half the size of the generality of cells. Germinating cells of this kind or of the kind above described, are generally very granular. 2. Spheroidal epithelial cells are the active secreting agents in most secreting glands, and hence are often termed glandular ; they are gene- rally more or less rounded in outline : often polygonal from mutual pressure. Excellent examples are to be found in the liver, in the secreting tubes of the kidney, and in the salivary and gastric glands (Fig. 19). FIG. 19.— Glandular epithelium. A, small lobule of a mucous gland of the tongue, showing nucleated glandular spheroidal cells. B, Liver cells, x 200. CV. D. Harris.) 3. Columnar epithelium (Fig. 21, a and b) as a single layer, lines (a.) the mucous membrane of the stomach and intestines, from the car- diac orifice of the stomach to the anus, and (b.) wholly or in part the ducts of the glands opening on its free surface ; also (c.) many gland- ducts in other regions of the body, e. g., mammary, salivary, etc. Columnar epithelium consists of cefls which are cylindrical or pris- matic in form, and contain a large oval nucleus. They vary in size and also in shape to a certain extent, the outline being often irregular from pressure of neighboring cells, but speaking generally one end of the cell is narrower than the other, and by this end the cell is attached to the membrane beneath. The intercellular and internuclear network are well developed. The columnar epithelial cells of the alimentary canal possess a struc- tureless layer on their free surface : such a layer, appearing striated HANDBOOK OF PHYSIOLOGY. when viewed in section, is termed the " striated basilar border" (Fig 20, A, 0). FIG. 20.— A. Vertical section of a villus of the small intestine of a cat. a. Striated basilar bor- der of the epithelium. 6. Columnar epithelium, c. Goblet cells, d. Central lymph-vessel <•. Smooth muscular fibres. /. Adenoid stroma of the villus in which lymph corpuscles lie. B. Goblet- cells. (Klein.) Columnar cells may undergo a curious transformation, and from the alteration in their shape are termed "goblet-cells" (Fig. 20, A, c, FIG. 21.— Columnar epithelial cells from the intestinal mucous menbrane of a cat.— a and 6, small cells of the lower layer ; c, superficial layer ; d, goblet cells. (Cadiat.) FIG. 22.— Columnar ciliated cells from the human nasal membrane : magnified 300 diameters. (Sharpey.) and B). These are hardly ever seen in a perfectly fresh specimen: but if such a specimen be watched for some time, little knobs are seen FIG. 23. -A. Spheroidal ciliated cells from the mouth of the frog. X 300 diameters. (Sharpey.) B. a. Ciliated columnar epithelium lining a bronchus, b. Branched connective-tissue corpuscles. (Klein and Noble Smith.) gradually appearing on the free surface of the epithelium, and are finally detached; these consist of the cell-contents which are discharged by the THE STRUCTURE OF THE ELEMENTARY TISSUES. 25 open mouth of the globet, leaving the nucleus surrounded by the re- mains of the protoplasm in its narrow stem. This transformation is a normal process which is continually going on during life, the discharged cell-contents contributing to form mucus, the cells being supposed in many cases to recover their original shape. It is an example of secretion. 4. Ciliated cells are generally cylindrical (Fig. 23, B), but may be spheroidal or even almost squamous in shape (Fig. 23, A). This form of epithelium lines — (a.) the whole of the respiratory tract from the larynx, except over the vocal cords, to the finest sub- divisions of the bronchi, also the lower parts of the nasal passages, the nasal duct, and the lachrymal sac. In, part ofj this tract, however, the epithelium is, in several layers, of which only the most superficial is ciliated, so that it should' more accurately -be termed transitional (p. 26) pr stratified, (b. ), some portions of the generative apparatus in the male, viz., lining the " vasa efferentia" of the testicle, and their prolongations as far as the lower end of the epididymis; in the female (c.) commencing about the middle of the neck of the uterus, and ex- tending throughout the uterus and Fallopian tubes to their fimbriated extremities, and even for a short distance on the peritoneal surface of the latter, (d. ) The ventricles of the brain and the central canal of the spinal cord are clothed with ciliated epithelium in the child, but in the adult this epithelium is limited to the central canal of the cord. The Cilia, or fine hair-like processes which give the name to this variety of epithelium, vary a good deal in size in different classes of ani- mals, being very much smaller in the higher than among the lower orders, in which they sometimes exceed in length the cell itself. The number of cilia on any one cell ranges from ten to thirty, and those attached to the same cell are often of different lengths. When living ciliated epithelium, e. g., from the gill of a mussel, or oyster, or from the mouth of the frog, or from a scraping from a polypus from the human nose, is examined under the microscope, the cilia are seen to be in constant rapid motion; each cilium being fixed at one end, and swinging or lashing to and fro. The general impression given to the eye of the observer is very similar to that produced by waves in a field of corn, or swifty running and rippling water, and the result of their movement is to produce a continuous current in a definite direction, and this direction is invariably the same on the same surface, being always, in the case of a cavity, towards its external orifice. Ciliary Motion, — Ciliary, which is closely allied to amoeboid and muscular motion, is alike independent of the will, of the direct influence of the nervous system, and of muscular contraction. It continues for several hoars after death or removal from the body, provided the portion of tissue under examination be kept moist. Its independence of the ner- HANDBOOK OF PHYSIOLOGY. vous system is shown also in its occurrence in the lowest invertebrate animals apparently unprovided with anything analogous to a nervous system, in its persistence in animals killed by prussic acid, by narcotic or other poisons, and after the direct application of narcotics, such as morphia, opium, and belladonna, to the ciliary surface, or of electricity through it. The vapor of chloroform arrests the motion; but it is re- newed on the discontinuance of the application (Lister). The movement ceases when the cilia are deprived of oxygen, but is revived on the admission of this gas. Carbonic acid stops the movement. The contact of various substances, e.g., bile, strong acids, and alkalies, will stop the FIG. 24.— Epithelium of the bladder, a, one of the cells of the first row ; 6, a cell of the second row ; c, cells in situ, of first, second, and deepest layers. (Obersteiner.) FIG. 25.— Transitional epithelial cells from a scraping of the mucous membrane of the bladder of the rabbit. (V. D. Harris.) motion altogether; but this seems to depend chiefly on destruction of the delicate substance of which the cilia are composed. Temperatures above 45° C., and below 0° C., stop the movement; but moderate heat and dilute alkalies are favorable to the action and revive the movement after temporary cessation. As a special subdivision of ciliary action may be mentioned the mo- tion of spermatozoa, which may be regarded as cells with a single cil- ium. B. Transitional Epithelium. — This term has been applied to cells, which are neither arranged in a single layer, as is the case with simple epithelium, nor yet in many superimposed strata as in laminated ; in other words, it is employed when epithelial cells are found in two, three, or four superimposed layers. The upper layer may be either columnar, ciliated, or squamous. When the upper layer is columnar or ciliated, the second layer consists of smaller cells fitted into the inequalities of the cells above them, as in the trachea (Fig. 24, J). The epithelium which is met with lining the urinary bladder and ureters is, however, the transitional par excellence. In this variety there are two or three layers of cells, the upper being more or less flattened according to the full or collapsed condition of the organ, their under THE STRUCTURE OF THE ELEMENTARY TISSUES. 27 surface being marked with one or more depressions, into which the heads of the next layer of club-shaped cells fit. Between the lower and nar- rower parts of the second row of cells, are fixed the irregular cells which constitute the third row, and in like manner sometimes a fourth row (Fig. 24). It can be easily understood, therefore, that if a scraping of the mucous membrane of the bladder be teased, and examined under the microscope, cells of a great variety of forms may be made out (Fig. 25). Each cell contains a large nucleus, and the larger and superficial cells often possess two. 0. Stratified Epithelium. — This term is employed when the cells forming the epithelium are arranged in a considerable number of super- imposed layers. The shape and size of the cells of the different layers, as well as the number of the layers, vary in different situations. Thus the superficial cells are as a rule of the squamous, or scaly variety, and the deepest of the columnar form. The cells of the intermediate layers are of different shapes, but those of the middle layers are more or less rounded. The superficial cells over- lap by their edges (Fig. 26); they are broad (Fig. 13). Their chemical composition is different from that of the underlying cells, as they con- tain keratin, and are therefore horny in character. The nucleus is often not apparent. The really cellular nature of even the dry and shrivelled scales cast off from the surface of the epi- dermis, can be proved by the application of caustic potash, which causes them rapidly to swell and assume their original form. The squamous cells exist in the greatest number of layers in the epi- dermis or superficial part of the skin; and the most superficial of these FIG. 26. -Vertical section of the stratified epithelium of the Rabbit's cornea, a. Anterior epi- thelium, showing the different shapes of the cells at various depths from the free surface. 6. Por- tion of the substance of cornea. CKtem-) are being continually removed by friction, and new cells from below sup- ply the place of those cast off. The intermediate cells approach more to the flat variety the nearer they are to the surface, and to the columnar as they approach the lowest layer. There may be considerable intercellular intervals; and in many of the deeper layers of epithelium in the mouth and skin, the outline of 28 HANDBOOK OF PHYSIOLOGY. the cells is very irregular, in consequence of processes passing from cell to cell across these intervals. Such cells (Fig. 27) are termed " ridge and furrow," "cogged" or "prickle" cells. These "prickles" are prolonga- tions of the intra-cellular network which run across from cell to cell, thus joining them together (Martyn), the interstices being filled by the transparent intercel- lular cement substance. When this in- creases in quantity in inflammation, the cells are Pushed further aPart> and the ofanew' connecting fibrils or " prickles" elon- gated, and therefore more clearly visible. The columnar cells of the deepest layer are distinctly nucleated; they multiply rapidly by division; and as new cells are formed beneath, they press the older cells forwards to be in turn pressed forwards themselves towards the surface, gradually altering in shape and chemical composi- tion until they are cast off from the surface. Stratified epithelium is found in the following situations: — (1.) Forming the epidermis, covering the whole of the external surface of the body; (2.) Covering the mucous membrane of the tongue, mouth, pharynx, and oesophagus; (3.) As the conjunctival epithelium, covering the cornea; (4.) Lining the vaginal part of the cervix uteri. Functions of Epithelium. — According to function, epithelial cells may be classified as: — (1.) Protective, e.g., in the skin, mouth, blood- vessels, etc. (2.) Protective and moving — ciliated epithelium. (3.) Secreting — glandular epithelium; or, Secreting formed elements — epi- thelium of testicle secreting spermatozoa. (4.) Protective and secreting, e.g., epithelium of intestine. (5.) Sensorial, e.g., olfactory cells, rods and cones of retina, organ of Corti. Epithelium forms a continuous smooth investment over the whole body, being thickened into a hard, horny tissue at the points most ex- posed to pressure, and developing various appendages, such as hairs and nails, whose structure and functions will be considered in a future chap- ter. Epithelium lines also the sensorial surfaces of the eye, ear, nose, and mouth, and thus serves as the medium through which all impressions from the external world — touch, smell, taste, sight, hearing — reach the delicate nerve-endings, whence they are conveyed to the brain. The ciliated epithelium which lines the air-passages serves not only as a protective investment, but also by the movements of its cilia promotes currents of the air in the bronchi and bronchia, and is enabled to propel fluids and minute particles of solid matter so as to aid their expulsion from the body. In the case of the Fallopian tube, this agency assists the progress of the ovum towards the .cavity of the uterus. Of the purposes served by cilia in the ventricles of the brain nothing is known. THE STRUCTURE OF THE ELEMENTAKY 1 ISSUES. 29 The epithelium of the various glands, and of the whole intestinal tract, has the power of secretion, i.e., of chemically transforming certain materials of the blood; in the case of mucus and saliva this has been proved to involve the transformation of the epithelial cells themselves; the cell-substance of the epithelial cells of the intestine being discharged by the. rupture of their envelopes, as mucus. Epithelium is likewise concerned in the processes of transudation, diffusion, and absorption. It is constantly being shed at the free surface, and reproduced in the deeper layers. The various stages of its growth and development can be well seen in a section of any laminated epithelium such as the epi- dermis. The Connective Tissues. This group of tissues forms the Skeleton with its various connections — bones, cartilages, and ligaments — and also affords a supporting frame- work and investment to the various organs composed of nervous, muscu- lar, and glandular tissue. Its chief function is the mechanical one of Fi3. 28.— Horizontal preparation of cornea of frog, stained in gold chloride ; showing the net- work of branched cornea corpuscles. The ground substance is completely colorless, x 400. (Klein.) support, and for this purpose it is so intimately interwoven with nearly all the textures of the body, that if all other tissues could be removed, and the connective tissues left, we should have a wonderfully exact model of almost every organ and tissue in the body, correct even to the smallest minutiae of structure. Classification of Connective Tissues.— The chief varieties of con- nective tissues may be thus classified : — I. The Fibrous Connective Tissues. A. — Chief Forms. «. White fibrous. I. Elastic. c. Areolar. 30 HANDBOOK OF PHYSIOLOGY. B. — Special Varieties. a. Gelatinous. b. Adenoid or Eetiform. c. Neuroglia. d. Adipose. II. Cartilage. III. Bone. Structure of Connective Tissues. All of the varieties of connective tissue are made up of two elements, namely, cells and intercellular substance. (A.) Cells. — The cells are of two kinds. (a.) Fixed. — These are cells of a flattened shape, with branched pro- cesses, which are often united together to form a network : they can be most readily observed in the cornea, in which they are arranged, layer above layer, parallel to the free surface. They lie in spaces, in the intercellular or ground substance, which are of the same shape as the cells they contain, but rather larger, and which form by anastomosis a sys- tem of branching canals freely communicat- ing (Fig. 28). To this class of cells belong the flattened tendon corpuscles which are arranged in long lines or rows parallel to the fibres (Fig. 34) FIQ. 29.— Ramified pigment- cells, from the tissue of the choroid These branched cells, in certain situa- coat of the eye. x 350. a, cell with pigment; 6, colorless fusiform tions, contain a number of pigment-granules, cells. (Kolliker.) . . ' giving them a dark appearance : they form one variety of pigment-cell. Branched pigment-cells of this kind are found in the outer layers of the choroid (Fig. 29) . In many of the lower animals, such as the frog, they are found widely distributed, not only in the skin, but also in internal parts, e. g., the mesentery and sheaths of blood-ves- sels. In the web of the frog's foot such cells may be seen, with pigment- granules evenly distributed throughout the body of the cell, and its processes ; but under the action of light, electricity, and other stimuli, the pigment-granules become massed in the body of the cell, leaving the processes quite hyaline ; if the stimulus be removed, they will gradually be distributed again throughout the processes. Thus the skin in the frog is sometimes uniformly dusky, and sometimes quite light-colored, with isolated dark spots. In the choroid anb retina the pigment-cells absorb light. (b.) Amoeboid cells, of an approximately spherical shape: they have a great general resemblance to colorless blood-corpuscles (Fig. 2), with THE STRUCTURE OF THE ELEMENTARY TISSUES. 31 which some of them are probably identical. They consist of finely granular nucleated protoplasm, and have the property, not only of changing their form, but also moving about, whence they are termed migratory. They are readily distinguished from the branched connec- tive-tissue corpuscles by their free condition, and the absence of pro- cesses. Some are much larger than others, and are found especially in the sublingual gland ©f the dog and guinea pig, and in the mucous membrane of the intestine. A second variety of these cells called plas- ma cells (Waldeyer) are larger than the amoeboid cells, apparently granular, less active in their movements. They are chiefly to be found in the intermuscular septa, in the mucous and submucous coats of the intestine, in lymphatic glands, and in the omentum. (B.) Intercellular substance. — This may be fibrillar, as in the fibrous tissues, and in certain varie- ties of cartilage ; or homogeneous, as in hyaline cartilage. The fibres composing the former are of two kinds — (a. ) White fibres. (b.) Yellow elastic fibres. (a.) White Fibres. — These are arranged parallel to each other in wavy bundles of various sizes : such bundles may either have a parallel arrangement (Fig. 31), or may pro- duce quite a felted texture by their interlacement. The individual fibres composing these fasciculi are homogeneous, unbranched, and of the same diameter throughout. They can readily be isolated by macerating a portion of white fibrous tissue (e. g., a small piece of tendon) for a short time in lime, or baryta-water, or in a solution of common sal.t, or of potassium permanganate : these reagents possess the power of dissolving the cementing interfibrillar sub- stance (which is nearly allied to syntonin), and of thus separating the fibres from each other. By prolonged boiling the fibres yield gelatin. (b.) Yellow Elastic Fibres (Fig. 32) are of all sizes, from excessively fine fibrils up to fibres of considerable thickness : they are distinguished from white fibres by the following characters : — (1.) Their great power of resistance even to the prolonged action of chemical reagents, e.g., Caustic Soda, Acetic Acid, etc. (2.) Their well-defined outlines. (3.) Their great tendency to branch and form networks by anastomosis. (4.) They very often have a twisted corkscrew-like appearance, and FIG. 30.— Flat, pigmented, branched con- ective-tissue cells from the sheath of a large blood- vessel of frog's mesentery ; the pigment is not distributed uniformly through the substance of the larger cell, consequently some parts of the cell look blacker than others (uncontracted state). In the two smaller cells most of the pigment is withdrawn into the cell-body, so that they appear smaller blacker, and less branched, x 350. (Klein and Noble Smith.) HANDBOOK OF PHYSIOLOGY. their fi-ee ends usually curl up. (5.) They are of a yellowish tint, and very elastic. FIG. 31.— Fibrous tissue of cornea, showing bundles of fibres with a few scattered fusi- form cells lying in the inter-fasicular spaces. X 400. (Klein and Noble Smith.) FIG. 32.— Elastic fibres from the ligamenta subflava. x 200. (Sharpey.) These fibres yield a gelatinous substance called elastin. Varieties of Connective Tissue. I. FIBROUS CONNECTIVE TISSUES. A. — Chief Forms. — (a.) White Fibrous Tissue. Distribution. — Typically in tendon; in ligaments, in the periosteum, and perichondrium, the dura mater, the pericardium, the sclerotic coat of the eye, the fibrous sheath of the testicle; in the fasciae and aponeurosis of muscles, and in the sheaths of lymphatic glands. Structure. — To the naked eye, tendons, and many of the fibrous membranes, when in a fresh state, present an appearance as of watered silk. This is due to the arrangement of the fibres in wavy parallel bundles. Under the microscope, the tissue appears to consist of long, often parallel, bundles of fibres of different sizes. The fibres of the same bundle now and then intersect each other. The cells in tendons (Fig. 34) are arranged in long chains in the ground substance separating the bundles of fibres, and are more or less regularly quadrilateral with large round nuclei containing nucleoli, which are generally placed so as to be contiguous in two cells. The cells consist of a body, which is thick, from which processes pass in various directions into, and partially filling up the spaces between the bundles of fibres. The rows of cells are separated from one another by lines of cement substance. The cell spaces can be brought into view by silver nitrate. The cells are gene- THE STRUCTURE OF THE ELEMENTARY TISSUES. 33 rally marked by one or more lines or stripes when viewed longitudi- nally. This appearance is really produced by the laminar extension either projecting upwards or downwards. FIG. 33.— A. Mature white fibrous tissue of tendon, consisting mainly of fibres with a few scattered fusiform cells. (Strieker.) FIG. 34.— Caudal tendon of young rat, showing the arrangement, form, and struc- ture of the tendon cells, x 300. (Klein.) The branched character of the cells is seen in transverse section in Fig. 35. (b.) Yellow Elastic Tissue. Distribution. — In the ligamentum nuchae of the ox, horse, and many other animals; in the ligamenta subflava of man; in the arteries, con- stituting the fenestrated coat of Henle; in veins; in the lungs and trachea; in the stylo-hyoid, thyro-hyoid, and crico-thyroid ligaments; in the true vocal cords; and in areolar tissue. Structure. — Elastic tissue occurs in various forms, from a structure- less, elastic membrane to a tissue whose chief constituents are bundles of fibres, crossing each other at different angles; when seen in bundles elastic fibres are yellowish in color, but individual fibres are not so dis- tinctly colored. The varieties of the tissue may be classified as follows: — (a.) Fine elastic fibrils, which branch and anastomose to form a net- work; this variety of elastic tissue occurs chiefly in the skin and mucous membranes, in subcutaneous and submucous tissue, in the lungs and true vocal cords. (b.) Thick fibres, sometimes cylindrical, sometimes flattened like tape, which branch, anastomose and form a network: these are seen most typically in the ligamenta subflava and also in the ligamentum nuchae of such animals as the ox and horse, in which it is largely devel- oped (Fig. 32). (c.) Elastic membranes with perforations, e. g., Henle's fenestrated membrane: this variety is found chiefly in the arteries and veins. (d.) Continuous, homogeneous elastic membranes, e. g., Bowman's 3 HANDBOOK OF PHYSIOLOGY. anterior elastic lamina, and Descemet's posterior elastic lamina, both in the cornea. A certain number of flat connective tissue cells are found in the ground substance between the elastic fibres which make up this variety of con- nective tissue. (c.) Areolar Tissue. Distribution. — This variety has a very wide distribution, and constitutes the subcutaneous, subserous and submucous tissue. It is found in the mucous mem- branes, in the true skin, and in the outer sheaths of the blood-vessels. It forms sheaths for muscles, nerves, glands, and the internal organs, and, penetrating into their interior, supports and connects the finest parts. Structure. — To the naked eye it ap- pears, when stretched out, as a fleecy, white, and soft meshwork of fine fibrils, with here and there wider films joining in it, the whole tissue being evidently elastic. The openness of the meshwork varies with the locality from which the specimen is taken. Under the microscope it is found to be made up of fine white fibres, which interlace in a most irregular manner, together with FIG. 35.— Transverse section of tendon from a cross section of the tail of a rabbit, showing sheath, fibrous sep- ta, and branched connective-tissue corpuscles. The spaces left white in the drawing represent the tendinous fibres in transverse section, x 250. (Klein.) FIG. 36.— Magnified view of the areolar tissue (from different parts) treated with acetic acid. The white filaments are no longer seen, and the yellow or elastic fibres with the nuclei come into view. At c, elastic fibres wind round a bundle of white fibres, which, by the effect of the acid, is swollen out between the turns. Some connective-tissue corpuscles are indistinctly represented in c. (Sharpey.) a variable number of elastic fibres. On the addition of acetic acid, the white fibres swell up, and become gelatinous in appearance (Fig. 36); but as the elastic fibres resist the action of the acid, they may still be THE STRUCTURE OF THE ELEMENTARY TISSUES. 35 seen arranged in various directions, sometimes appearing to pass in a more or less circular or spiral manner round a small gelatinous mass of changed white fibres. The cells of the tissue are not arranged in a very regular manner, as they are contained in the spaces (areolae) between the fibres. They communicate, however, with one another by branched processes, and also with the cells forming the walls of the capillary blood-vessels in their neighborhood. The fibres are connected together with a certain amount of albuminous cement substance. B. — Special Forms. — (a.) Gelatinous Tissue. Distribution. — Gelatinous connective tissue forms the chief part of the bodies of jelly fish ; it is found in many parts of the human embryo, FIG. 37. FIG. 38. FIG. 37.— Tissue of the jelly of Wharton from umbilical cord, a, connective-tissue corpuscles ; 6, fasciculi of connective tissue; c, spherical formative cells. (Frey.) FIG. 38.— Part of a section of a lymphatic gland, from which the corpuscles have been for the most part removed, showing the adenoid reticulum. (Klein and Noble Smith ) but remains in the adult only in the vitreous humor of the eye. It may be best seen in the last-named situation, in the " Whartonian jelly" of the umbilical cord, and in the enamel organ of developing teeth. Structure. — It consists of cells, which in the vitreous humor are rounded, and in the jelly of the enamel organ are stellate, imbedded in a soft jelly-like intercellular substance which forms the bulk of the tissue, and which contains a considerable quantity of mucin. In the umbilical cord, that part of the jelly immediately surrounding the stellate cells shows marks of obscure fibrillation (Fig. 37). (b.) Adenoid or Retiform. Distribution. — It composes the stroma of the spleen and lymphatic glands, and is found also in the thymus, in the tonsils, in the follicular 36 HANDBOOK OF PHYSIOLOGY. glands of the tongue, in Peyer's patches and in the solitary glands of the intestines, and in the mucous membranes generally. Structure. — Adenoid or retiform tissue consists of a very delicate net- work of minute fibrils, formed originally by the union of processes of branched connective-tissue corpuscles the nuclei of which, however, are visible only during the early periods of development of the tissue (Fig. 38). The nuclei found on the fibrillar meshwork do not form an essential part of it. The fibrils are neither white fibres nor elastic tissue, as they are insoluble in boiling water, although readily soluble in hot alkaline solutions. The lymphoid corpuscles found in the interstices of the tis- sue are small round cells, the protoplasm of which is practically occupied by their spherical nuclei. (c.) Neuroglia. — This tissue forms the support of the Nervous ele- Fio. 39.— Portion of submucous tissue of gravid uterus of sow. a, branched cells, more or less spindle-shaped; 6, bundles of connective tissue. (Klein.) ments in the Brain and Spinal cord. It consists of a very fine meshwork of fibrils, said to be elastic, and with nucleated plates which constitute the connective-tissue corpuscles imbedded in it. Development of Fibrous Tissues. — In the embryo the place of the fibrous tissues is at first occupied by a mass of roundish cells, derived from the " rnesoblast. v These develop either into a network of branched cells, or into groups of fusiform cells (Fig. 39). The cells are imbedded in a semi-fluid albuminous substance derived either from the cells themselves or from the neighboring blood-vessels ; this afterwards forms the cement substance. In it fibres are developed, either by part of the cells becoming fibrils, the others remaining as con- nective-tissue corpuscles, or by the fibrils being developed from the out- side layers of the protoplasm of the cells, which grow up again to their original size and remain imbedded among the fibres. The process gives rise to fibres arranged in the one case in interlacing networks (areo- lar tissue), in the other in parallel bundles (white fibrous tissue). In the mature forms of purely fibrous tissue not only the remnants of the THE STRUCTURE OF THE ELEMENTARY TISSUES. £7 cell-substance, but even the nuclei may disappear. The embryonic tis- sue, from which elastic fibres are developed, is composed of fusiform cells, and a structureless intercellular substance by the gradual fibrilla- tion of which elastic fibres are formed. The fusiform cells dwindle in size and eventually disappear so completely that in mature elastic tissue hardly a trace of them is to be found: meanwhile the elastic fibres steadily increase in size. Another theory of the development of the connective-tissue fibrils supposes that they arise from deposits in the intercellular substance and not from the cells themselves; these deposits, in the case of elastic fibres, appearing first of all in the form of rows of granules, which, joining together, form long fibrils. It seems probable that even if this view be correct, the cells themselves have a considerable influence in the produc- tion of the deposits outside them. Functions of Areolar and Fibrous Tissue. — The main function of connective tissue is mechanical rather than vital: it fulfils the subsidi- ary but important use of supporting and connecting the various tissues and organs of the body. In glands the trabeculse of connective tissue form an interstitial frame- work in which the parenchyma or secreting gland- tissue is lodged: in muscles and nerves the septa of connective tissue support the bundles of fibres which form the essential part of the structure. Elastic tissue, by virtue of its elasticity, has other important uses : these, again, are mechanical rather than vital. Thus the ligamentum nuchas of the horse or ox acts very much as an India-rubber band in the same position would. It maintains the head in a proper position with- out any muscular exertion; and when the head has been lowered by the action of the flexor muscles of the neck, and the ligamentum nuchae thus stretched, the head is brought up again to its normal position by the re- laxation of the flexor muscles which allows the elasticity of the ligamentum nuchaa to come again into play. (d.) Adipose Tissue. Distribution. — 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 nymphae, and the cavity of the cranium. Adipose tissue is also ab- sent 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 bod}T, e.g., in the liver and brain, and in the blood and chyle. Adipose tissue is almost always found seated in areolar tissue, and forms in its meshes little masses of unequal size and irregular shape, to which the term lobules is commonly applied. Structure. — Under the microscope adipose tissue is found to consist essentially of little vesicles or cells which present dark, sharply-defined edges when viewed with transmitted light: they are about ^-J-g- or -g-fg- of an inch in diameter, each composed of a structureless and colorless membrane or bag, filled with fatty matter, which is liquid during life, 38 HANDBOOK OF PHYSIOLOGY. but in part solidified after death (Fig. 40). A nucleus is always present in some part or other of the cell-protoplasm, but in the ordinary condi- tion of the cell it is not easily or always visible. FIG. 40. FIG. 41. Fia. 40.— Ordinary fat cells of a fat tract in the omentum of a rat. (Klein.) FIG. 41.— Group of fat cells (FC) with capillary vessels (O- (Noble Smith.) This membrane and the nucleus can generally be brought into view by staining the tissue; it can be still more satisfactorily demonstrated by extracting the contents of the fat-cells with ether, when the shrunken, FIG. 42. FIG. 43. FIG. 42.— Blood-vessels of adipose tissue. A. Minute flattened fat-lobule, in which the vessels only are represented, a, the terminal artery; i>, the primitive vein; 6, the fat- vesicles of one border of the lobule separately represented, x 100. B. Plan of the arrangement of the capillaries (c) on the exterior of the vesicles; more highly magnified. (Todd and Bowman.) FIG. 43.— A lobule of developing adipose tissue from an eight months1 foetus, a. Spherical or, from pressure, polyhedral cells with large central nucleus, surrounded by a finely reticulated sub- stance staining uniformly with haematoxylin. 6. Similar cells with space0 from which the fat has been removed by oil of cloves, c. Similar cells showing how the nucleus with inclosing protoplasm is being pressed towards periphery, d. Nucleus of endothelium of investigating capillaries. (Mc- Carthy.) Drawn by Treves. THE STRUCTURE OF THE ELEMENTARY TISSUES: 39 shrivelled membranes remain behind. By mutual pressure, fat-cells come to assume a polyhedral figure (Fig. 41). The ultimate cells are held together by capillary blood-vessels (Fig. 42); while the little clusters thus formed are grouped into small masses, and held so, in most cases, by areolar tissue. The oily matter contained in the cells is composed chiefly of the com- pounds of fatty acids with glycerin, which are named olein, stearin, and palmitin. Development of Adipose Tissue.— Fat-cells are developed from connective-tissue corpuscles: in the infra-orbital connective tissue cells may be found exhibiting every intermediate gradation between an ordi- nary branched connective-tissue corpuscle and a mature fat-cell. The process of development is as follows: a few small drops of oil make their appearance in the protoplasm: by their confluence a larger drop is pro- duced (Fig. 43): this gradually increases in size at the expense of the •original protoplasm of the cell, which becomes correspondingly dimin- Fi». 44.— Branched connective-tissue corpuscles, developing into fat-cells. (Klein.) ished in quantity till in the mature cell it only forms a thin crescentic film, closely pressed against the cell-wall, and with a nucleus imbedded in its substance (Figs. 43 and 44). Under certain circumstances this process may be reversed and fat- cells maybe changed back into connective-tissue corpuscles. (Kolliker, Virchow.) Vessels and Nerves. — A large number of blood-vessels are found in adipose tissue, which subdivide until each lobule of fat contains a fine mesh work of capillaries ensheathing each individual fat-globule (Fig. 42). Although nerve fibres pass through the tissue, no nerves have been demonstrated to terminate in it. The Uses of Adipose Tissue. — Among the uses of adipose tissue, these are the chief: — a. It serves as a store of combustible matter which may be re-absorbed into the blood when occasion requires, and, being burnt, may help to preserve the heat of the body. b. 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 surface. 40 HANDBOOK OF PHYSIOLOGY. c. As a packing material, fat serves very admirably to fill tip 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, not endangered by pressure. As good examples of situations in which fat serves such purposes may be mentioned the palms of the hands and soles of the feet, and the orbits. d. In the long bones, fatty tissue, in the form known as yellow marrow, fills the medullary canal, and supports the small blood-vessels which are distributed from it to the inner part of the substance of the bone. II. CARTILAGE. Structure of Cartilage. — All kinds of cartilage are composed of cells imbedded in a substance called the matrix : and the apparent differ- ences of structure met with in the various kinds of cartilage are more due jii^^mijjm •M^gMiai^jim FIG. 45. FIG. 46. FIG. 45.— Ordinary hyaline cartilage from trachea of a child. The cartilage cells are inclosed singly or in pairs in a capsule of hyaline substance, x 150 diams. (Kleia and Noble Smith.) FIG. 46.— Fresh cartilage from the Triton. (A. Rollett.) 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 invested by a thin but tough firm fibrous membrane called the perichondrium. On the surface of the articular cartilage of the foatus, 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. Nerves are probably not supplied to any variety of cartilage. Cartilage exists in three different forms in the human body, viz., 1, Hyaline cartilage, 2, Yellow elastic cartilage, and 3, White fibro-cartilage. 1. Hyaline Cartilage. Distribution. — This variety of cartilage is met with largely in the THE STRUCTURE OF THE ELEMENTARY TISSUES. 41 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, as well as those of the trachea and bronchi. Structure. — Like other cartilages it is composed of cells imbedded in a matrix. The cells, which contain a nucleus with nucleoli, are irregu- lar in shape, and generally grouped together in patches (Fig. 45). 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 of hyaline cartilage has a dimly granular appearance like that of ground glass, and in man and the higher animals has no apparent structure. In some cartilages of the frog, however, even when examined in the fresh state, it is seen to be mapped out into polygonal blocks or cell-territories, each containing a cell in the centre, and representing what is generally called the capsule of the cartilage cells (Fig. 46). Hyaline cartilage in man has really the same structure, which can be demonstrated by the use of certain reagents. If a piece of human hya- line cartilage be macerated for a long time in dilute acid or in hot water 95°-113° F. (35° to 45° C.), the matrix, which previously appeared quite homogeneous, is found to be resolved into a number of concentric lamellae, like the coats of an onion, arranged round each cell or group of cells. It is thus shown to consist of nothing but a number of large systems of capsules which have become fused with one another. The cavities in the matrix in which the cells lie are connected to- gether by a series of branching canals, very much resembling those in the cornea: through these canals fluids may make their way into the depths of the tissue. 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 (Fig. 47). The costal cartilages also frequently be- come calcified in old age, as also do some of those of the larynx. Fat- globules may also be seen in many cartilages (Fig. 47). In articular cartilage the cells are smaller, and arranged vertically in narrow lines like strings of beads. Temporary Cartilage. — In the foetus, cartilage is the material of which the bones are first constructed; the " model " of each bone being laid down, so to speak, in this substance. In such cases the cartilage is termed temporary. It 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. A variety of temporary hyaline cartilage which has scarcely any HANDBOOK OF PHYSIOLOGY. matrix is found in the human subject and in the higher animals gene- rally, in early foetal life, when it constitutes the chorda dorsalis. Nutrition. — Hyaline cartilage is reckoned among the so-called non- vascular structures, no blood-vessels being supplied directly to its own substance; it is nourished by those of the bone beneath. When hyaline cartilage is in thicker masses, as in the case of the cartilages of the ribs, a few blood-vessels traverse its substance. The distinction, however, between all so-called vascular and non-vascular parts is at the best a very artificial one. 2. Yellow Elastic Cartilage. Distribution. — In the external ear, in the epiglottis and cornicula laryngis, and in the Eustachian tube. Structure. — The cells are rounded or oval, with well-marked nuclei or nucleoli (Eig. 48). The matrix in which they are seated is composed almost entirely of fine elastic fibres, which form an intricate interlace- FIG. 47. FIG. 48. FIG. 47.— Costal cartilage from an adult dog, sho mng the fat globules in the cartilage cells. (Cadiat.) FIG. 48.— Section of the epiglottis. (Baly. ) ment about the cells, and in their general characters are allied to the yellow variety of fibrous tissue: a small and variable quantity of hya- line and intercellular substance is also usually present. A variety of elastic cartilage, sometimes called cellular, may be ob- tained from the external ear of rats, mice, or other small mammals. It is composed almost entirely of cells (hence the name), which are packed very closely, with little or no matrix. When present, the matrix consists of very fine fibres, which twine about the cells in various direc- tions and inclose them in a kind of network. Elastic cartilage seldom or never ossifies. 3. White Fibro-Cartilage. Distribution. — The different situations in which white fibro-car- tilage is found have given rise to the following classification: — THE STKTJCTUKE OF THE ELEMENTARY TISSUES. 43 1. Inter-articular fibro-cartilage, e. g., the semilunar cartilages of the knee-joint. 2. Circumferential or marginal, as on the edges of the acetabulum and glenoid cavity. 3. Connecting, e. g., the inter vertebral fibro-cartilages. 4. 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 tendon of the tibialis posticus, in the sole of the foot, and usually in the neighboring tendon of the peroneus longus. Structure. — White fibro-cartilage (Fig. 49), which is much more widely distributed throughout the body than the foregoing kind, is com- posed, like it, of cells and a matrix; the latter, however, being made Fro. Fro. 50. FIG. 49.— Transverse section through the intervertebral cartilage of tail of mouse, showing lamellae of fibrous tissue with cartilage cells arranged in rows between them. The cells ar« seen in profile, and being flattened, appear staff -shaped. Each cell lies in a capsule. X 350. (Klein and Noble Smith.) FIG. 50. White fibro-cartilage from an intervetebral ligament. (Klein and Noble Smith.) up almost entirely of fibres closely resembling those of white fibrous tissue. In this kind of fibro-cartilage it is not unusual to find a great part of its mass composed almost exclusively of fibres, and deriving the name of cartilage only from the fact that in another portion, continuous with it, cartilage cells may be pretty freely distributed. By prolonged boiling, cartilage yields a gelatinous substance called chondrin — white fibro-cartilage yields gelatin as well. Functions of Cartilage.— Cartilage not only represents in the f 03 tus the bones which are to be formed (temporary cartilage), but also offers a firm, but more or less yielding, framework for certain parts in the de- veloped body, possessing at the same time strength and elasticity. It maintains the shape of tubes as in the larynx and trachea. It affords at- tachment to muscles and ligaments; it binds bones together, yet allows a certain degree of movement, as between the vertebrae; it forms a firm 44 HANDBOOK OF PHYSIOLOGY. framework and protection, yet without undue stiffness or weight, as in the pinna, larynx, and chest-walls; it deepens joint cavities, as in the acetabulum, without unduly restricting the movements of the bones. Development of Cartilage. — Cartilage is developed out of an em- bryonal tissue, consisting of cells with a very small quantity of intercel- lular substance: the cells multiply by fission within the cell-capsules (Fig. 6); while the capsule of the parent cell becomes gradually fused with the surrounding intercellular substance. A repetition of this process in the young cells causes a rapid growth of the cartilage by the multiplication of its cellular elements and corresponding increase in its matrix. Thus we see that the matrix of cartilage is chiefly derived from the cartilage cells. III. BONE. Chemical Composition. — Bone is composed of earthy and animal mat- ter in the proportion of about 67 per cent of the former to 33 per cent of the latter. The earthy matter is composed chiefly of calcium phos- phate, but besides there is a small quantity (about 11 of the 67 percent) of calcium carbonate and fluoride, and magnesium phosphate. The animal matter is resolved into gelatin by boiling. The earthy and animal constituents of bone are so intimately blended and incorporated the one with the other, that it is only by chemical ac- tion, as, for ins-tance, by heat in one case and by the action of acids in another, that they can be separated. Their close union, too, is further shown by the fact that when by acids the earthy matter is dissolved out, or, on the other hand, when the animal part is burnt out, the shape of the bone is alike preserved. The proportion between these two constituents of bone varies in dif- ferent bones in the same individual, and in the same bone at different ages. Structure. — 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 spongy or cancellous tissue. Thus, in making a longitudinal section of a long bone, as the humerus or femur, the articular extremities are found capped on their surface by a thin shell of compact bone, while their interior is made up of the spongy or- cancellous tissue. The shaft, on the other hand, is formed almost entirely of a thick layer of the compact bone, and this surrounds a central canal, the medullary cavity — so called from its containing the medulla or marrow. 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. Marrow. — There are two distinct varieties of marrow — the red and yellow. THE STRUCTURE OF THE ELEMENTARY TISSUES. 45 Red marrow is that variety which occupies the spaces in the cancel- lous tissue; it is highly vascular, and thus maintains the nutrition of the spongy bone, the interstices of which it fills. It contains a few fat- cells and a large number of marrow-cells, many of which are undistin- guishable from lymphoid corpuscles, and has for a basis a small amount of fibrous tissue. Among the cells are some nucleated cells of very much the same tint as colored blood-corpuscles. There are also a few large cells with many nuclei, termed " giant-cells " (myeloplaxes), which are derived from over-growth of the ordinary marrow-cells (Fig. 51). Yellow marrow fills the medullary cavity of long bones, and consists chiefly of fat-cells with numerous blood-vessels; many of its cells also are in every respect similar to lymphoid corpuscles. From these marrow-cells, especially those of the red marrow, are FIG. 51.— Cells of the red marrow of the guinea pig, highly magnified, a, a large cell, the nucleus of which appears to be partly divided into three by constrictions ; 6, a cell, the nucleus of which shows an appearance of being constricted into a number of smaller nuclei; c, a so-called giant cell, or myeloplaxe, with many nuclei ; d, a smaller myeloplaxe, with three nuclei ; e— *, proper cells of the marrow. (E. A. Schafer.) derived, as we shall presently show, large quantities of red blood- corpuscles. Periosteum and Nutrient Blood-vessels.— The surfaces of the bones, except the part covered with articular cartilage, are clothed by a tough, fibrous membrane, the periosteum; and it is from the blood- vessels which are distributed in this membrane, that the bones, especially their more compact tissue, are in great part supplied with nourishment, —minute branches from the peri osteal vessels entering the little fora- mina on the surface of the bone, and finding their way to the Haver- sian canals, to be immediately described. The long bones are supplied also by a proper nutrient artery which, entering at some part of the shaft so as to reach the medullary canal, breaks up into branches for the supply of the marrow, from which again small vessels are distributed to the interior of the bone. Other small blood-vessels pierce the articular extremities for the supply of the cancellous tissue. Microscopic Structure of Bone. — Notwithstanding the differences of 4:6 HANDBOOK OF PHYSIOLOGY. arrangement just mentioned, the structure of all bone is found under the microscope to be essentially the same. Examined with a rather high power, its substance is found to contain a multitude of small irregular spaces, approximately fusiform in shape, called lacunce, with very minute canals or canaliculi, as they are termed, leading from them, and anastomosing with similar little prolongations from other Iacuns8 (Fig. 52). In very thin layers of bone, no other canals than these may be visible; but on making a transverse section of the compact tissue as of a long bone, e. g., the humerus or ulna, the arrange- ment shown in Fig. 52, 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 lacuna and canaliculi following the same con- FIG. 52.— Transverse section of compact bony tissue (of humerus \ Three of the Haversian canals are seen, with their concentric rings ; also the corpuscles or lacunae, with the canaliculi ex- tending 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. The Haversian systems are so closely packed in this section, that scarcely any interstitial lamellae are visible. X 150. (Sharpey.) centric plan of distribution around the small hole in the centre, with which, indeed, they communicate. On making a longitudinal section, the central holes are found to be simply the cut extremities of small canals which run lengthwise through the bone, anastomosing with each other by lateral branches (Fig. 53), and are called Haversian canals, after the name of the physician, Clop- ton Havers, who first accurately described them. The Haversian canals, the average diameter of which is -g-i-g- of an inch, contain blood-vessels, and by means of them blood is conveyed to all, even the densest parts of the bone; the minute canaliculi and lacunaB absorbing nutrient matter THE STRUCTURE OF THE ELEMENTARY TISSUES. 47 from the Haversian blood-vessels, and conveying it still more intimately to the very substance of the bone which they traverse. The blood-vessels enter the Haversian canals both from without, by traversing the small holes which exist on the surface of all bones beneath the periosteum, and from within by means of small channels which extend from the medullary cavity, or from the cancellous tissue. The arteries and veins usually occupy separate canals, and the veins, which are the larger, often present, at irregular intervals, small pouch-like dilata- tions. The lacunte are occupied by branched cells (bone-cells, or bone-cor- puscles) (Fig. 54), which very closely resemble the ordinary branched connective-tissue corpuscles; each of these little masses of protoplasm FIG. 53. FIG 54. FIG. 53. -Longitudinal section of human ulna, showing Haversian canal, lacunae, andcanaliculi. FIG. 54. —Bone-corpuscles with their processes as seen in a thin section of human bone. (Rollett.) ministering to the nutrition of the bone immediately surrounding it, and one lacunar corpuscle communicating with another, and with its surrounding district, and with the blood-vessels of the Haversian canals, by means of the minute streams of fluid nutrient matter which occupy the canaliculi. It will be seen from the above description that bone is essentially connective tissue impregnated with lime salts: it bears a very close resemblance to what may be termed typical connective tissue such as the substance of the cornea. The bone-corpuscles with their processes, occupying the lacunae and canaliculi, correspond exactly to the cornea- corpuscles lying in branched spaces; while the finely fibrillated structure 40 HANDBOOK OF PHYSIOLOGY. of the bone-lamellae, to be presently described, resembles the fibrillated substance of the cornea in which the branching spaces lie. Lamellae of Compact Bone.— In the shaft of a long bone three distinct sets of lamellae can be clearly recognized. (1.) General or fundamental lamellae; which are most easily traceable just beneath the periosteum, and around the medullary cavity, forming around the latter a series of concentric rings. At a little distance from the medullary and periosteal surfaces (in the deeper portions of the bone) they are more or less interrupted by (2.) Special or Haversian lamellae, which are concentrically arranged around the Haversian canals to the number of six to eighteen around each. (3.) Interstitial lamellae, which connect the systems of Haversian FIG. 55. FIG. 56. FIG. 55.— Thin layer peeled off from a softened bone. This figure, which is intended to represent the reticular structure of a lamella, gives a better idea of the object when held rather farther off than usual from the eye. X 400. (Sharpey.) FIG. 56. — Lamellae torn off from a decalcified human parietal bone at some depth from the surface, o, a lamella, showing reticular fibres ; 6, 6, darker part, where several lamellae are superposed; £, perforating fibres. Apertures though which perforating fibres had passed, are seen especially in the lower part, a, a, of the figure. (Allen Thomson.) lamellae, filling the spaces between them, and consequently attaining their greatest development where the Haversian systems are few, and vice versa. The ultimate structure of the lamellce appears to be reticular. If a thin film be peeled off the surface of a bone, from which the earthy matter has been removed by acid, and examined with a high power of the microscope, it will be found composed of a finely reticular structure, formed apparently of very slender fibres decussating obliquely, but coa- lescingrat the points of intersection, as if here the fibres were fused rather than woven together (Fig. 55). (Sharpey.) THE STRUCTURE OF THE ELEMENTARY TISSUES. 49 In many places these reticular lamellae are perforated by tapering fibres (Claviculi of Gagliardi), 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. 56). These perforating fibres originate from ingrowing processes of the periosteum, and in the adult still retain their connection with it. Development of Bone.— From the point of view of their develop- ment, all bones may be subdivided into two classes. (a.) Those which are ossified directly in membrane or fibrous tissue, e. g., the bones forming the vault of the skull, parietal, frontal. (b.) Those whose form, previous to ossification, is laid down in hy- aline cartilage, e. g., humerus, femur. The process of development, pure and simple, may be best studied in bones which are not preceded by cartilage — '' membrane-bones " (e. g., parietal) ; and without a knowledge of this process (ossification in mem- brane), it is impossible to understand the much more complex series of changes through which such a structure as the cartilaginous femur of the foetus passes in its transformation into the bony femur of the adult (ossification in cartilage). Ossification in Membrane. — The membrane, afterwards forming the periosteum, from which such a bone as the parietal is developed, consists of two layers — an external fibrous, and an internal cellular or osteo-genetic. The external one consists of ordinary connective tissue, being com- posed of layers of fibrous tissue with branched connective-tissue corpus- cles here and there between the bundles of fibres. The internal layer consists of a network of fine fibrils with a large number of nucleated cells, some of which are oval, others drawn out into a long branched process, and others branched : it is more richly supplied with capillaries than the outer layer. The relatively large number of its cellular ele- ments, which vary in size and shape, together with the abundance of its blood-vessels, clearly mark it out as the portion of the periosteum which is immediately concerned in the formation of bone. In such a bone as the parietal, the deposition of bony matter, which is preceded by increased vascularity, takes place in radiating spiculae, starting from a " centre of ossification/' and shooting out in all direc- tions towards the periphery. While the bone increases in thickness by the deposition of successive layers beneath the periosteum, in-growths of the osteogenetic layer of the periosteum take place, and it is by the action of their osteoblasts that bone is secreted at a centre of ossification. The osteoblasts, being in part retained within the primary bone trabec- ulae thus produced, forming bone corpuscles. It is doubtful what part the finely fibrillar part of the osteogenetic in-growth takes in the forma- tion of the trabeculse, probably it supplies the reticular matrix of the 4 50 HANDBOOK OF PHYSIOLOGY. new-formed bone. On the bony trabeculae first formed, fresh layers of cells (osteoblasts) from the osteogenetic layer are developed side by side, lining the irregular spaces like an epithelium (Fig. 57, #). Lime-salts are deposited in the circumferential part of each osteoblast, and thus a ring of osteoblasts give rise to a ring of bone with the remaining uncal- cified portions of the osteoblasts imbedded iu it as bone corpuscles, as in the first formation. Thus, the primitive spongy bone is formed, whose irregular branch- ing spaces are occupied by processes from the osteogenetic layer of the periosteum with numerous blood-vessels and osteoblasts. Portions of this primitive spongy bone are re-absorbed ; the osteoblasts being ar- I - ' ^ - -^r ' ^»B'U ' ; F*. ^ ^ • ~~ ' FIG. 57. FIG. 58. FIG. 57.— Osteoblasts from the parietal bone of a human embryo, thirteen weeks old septa with the cells of the lacunae ; 6, layers of osteoblasts ; c, the latter a, bony in transition to bone cor- puscles. Highly magnified. (Gegenbaur.) FIG. 58.— From a transverse section through part of the f ratal jaw near the extreme periosteum in the state of spongy bone, p, fibrous layer of periosteum , 6, osteogenetic layer of periosteum ; o, osteoblasts; c, osseous substance, containing many bone corpuscles. X 300. (Schofield.) ranged in concentric successive layers and thus giving rise to concentric Haversian lamellae of bone, until the irregular space in the centre is reduced to a well-formed Haversian canal, the portions of the primitive spongy bone between the Haversian systems remaining as interstitial, or ground lamellae (p. 48). The bulk of the primitive spongy bone is thus gradually converted into compact bony tissue with Haversian canals. Those portions of the in-growths from the deeper layer of the periosteum THE STRUCTURE OF THE ELEMENTARY TISSUES. 51 which are not converted into bone remain in the spaces of the cancellous tissue as the red marrow. Ossification in Cartilage. — Under this heading, taking the femur as a t}Tpical example, we may consider the process by which the solid cartilaginous rod which represents it in the f(etus is converted into the hollow cylinder of compact bone with expanded ends of cancellous tissue fe"M*"siWaa"" FIG. 59. FIG. 60. FIG. 59.— Ossifying cartilage showing loops of blood-vessels. FIG. 60. -Longitudinal section of ossifying cartilage from the humerus of a foetal sheep. Cal- cined trabeculae are seen extending between the the columns of cartilage cells, c, cartilage cells. X 140. (Sharpey.) which forms the adult femur; bearing in mind the fact that this foetal cartilaginous femur is many times smaller than the medullary cavity even of the shaft of the mature bone, and, therefore, that not a trace -of the original cartilage can be present in the femur of the adult. Its 52 HANDBOOK OF PHYSIOLOGY. purpose is indeed purely temporary; and, after its calcification, it is gradually and entirely absorbed as will be presently explained. The cartilaginous rod which forms the foetal femur is sheathed in a membrane termed the perichondrium, which so far resembles the peri- osteum described above, that it consists of two layers, in the deeper one of which spheroidal cells predominate and blood-vessels abound, while the outer layer consists mainly of fusiform cells which are in the mature tissue gradually transformed into fibres. Thus, the differences between the foetal perichondrium and the periosteum of the adult are such as usually exist between the embryonic and mature forms of connective tissue. FIG. 61. FIG. 62. FIG. 61.— Transverse section of a portion of raetacarpal bone of a foatus, showing— 1 , fibrous layer of periosteum; 2, osteogenetic layer of ditto; 3 periosteal bone; 4, cartilage with matrix grad- ually becoming calcified, as at 5, with cells in primary areolee; beyond 5 the calcified matrix is being entirely replaced by spongy bone, x 2 '0. (V. D. Harris.) FIG. 62.— A small isolated mass of bone next the periosteum of the lower jaw of human foetus. a, osteogenetic layer of periosteum. G, multinuclear giant cells, the one on the left acting here probably like an osteoclast. Above c, the osteoblasts are seen to become surrounded by an osse- ous matrix. (Klein and Noble Smith.) Between the hyaline cartilage of which the fcetal femur consists and the bony tissue forming the adult femur, two intermediate stages exist — viz., calcified cartilage, and embryonic spongy bone. These tissues, which successively occupy the place of the foetal cartilage, are in suc- cession entirely absorbed, and their place taken by true bone. THE STRUCTURE OF THE ELEMENTARY TISSUES. 53 The process by which the cartilaginous is transformed into the bony femur may be divided for the sake of clearness into the following six stages : — Stage z. — Vascularization of the Cartilage. — Processes from the osteogenetic or cellular layer of the perichondrium containing blood- vessels grow into the substance of the cartilage much as ivy insinuates itself into the cracks and crevices of a wall. This begins at the " centres of ossification," from which the blood-vessels spread chiefly up and down the shaft, etc. Thus the substance of the cartilage, which previ- ously contained no vessels, is traversed by a number of branched anasto- mosing channels formed by the enlargement and coalescence of the spaces in which the cartilage-cells lie, and containing loops of blood- vessels (Fig. 59) and spheroidal cells which will become osteoblasts. Stage 2. — Calcification of Cartilaginous Matrix. — Lime salts are next deposited in the form of fine granules in the hyaline matrix of the cartilage, not yet vascularized, which thus becomes gradually trans- formed into a number of calcified trabeculae (Fig. 61, 5), inclosing al- veolar spaces (primary areolce) which contain cartilage cells. By the absorption of some of the trabeculae larger spaces are developed, which contain cartilage-cells for a very short time only, their places being taken by the so-called osteogenetic layer of the perichondrium (before referred to in Stage 1) which constitutes the primary marrow. The cartilage-cells, gradually enlarging, become more transparent and finally undergo disintegration. Stage 3.— Substitution of Embryonic Spongy Bone for Carti- lage.— The cells of the primary marrow arrange themselves as a con- tinuous layer like epithelium on the calcified trabeculae and deposit a layer of bone, which ensheathes the calcified trabeculae: these calcified trabeculae, encased in their sheaths of young bone, become gradually ab- sorbed, so that finally we have trabeculae composed entirely of spongy bone, all trace of the original calcified cartilage having disappeared. It is probable that the large multinucleated giant-cells termed •' osteoclasts " by Kolliker, which are derived from the osteoblasts by the multiplication of their nuclei, are the agents by which the absorption of calcified carti- lage, and subsequently of embryonic spongy bone, is carried on (Fig. '%, G). At any rate, they are almost always found wherever absorption is in progress. Stages 2 and 3 are precisely similar to what goes on in the growing shaft of a bone which is increasing in length by the advance of the pro- cess of ossification into the intermediary cartilage between the diaphysis and epiphysis. In this case the cartilage-cells become flattened and, multiplying by division, are grouped into regular columns at right angles to the plane of calcification, while the process of calcification ex- tends into the hyaline matrix between them (Figs. 59 and 60). 54: HANDBOOK OF PHYSIOLOGY. Stage 4. — Substitution of Periosteal Bone for the Primary Embryonic Spongy Bone. — The embryonic spongy bone, formed as above described, is simply a temporary tissue occupying the place of the foetal rod of cartilage, once representing the femur; and the stages 1, 2, and 3 show the sucessive changes which, occur at the centre of the shaft. Periosteal bone is now deposited in successive layers beneath the perios- teum, i. e., at the circumference of the shaft, exactly as described in the section on " ossification in membrane," and thus a casing of periosteal B. FIG. 63.— Transverse section through the tibia of a foetal kitten, semi-diagrammatic. X 60. P. PeriosteiAn. O, osteogenetic layer of the periosteum showing the osteoblasts arranged side by side, represented as pear-shaped black dots on the surface of the newly formed bone. B, the perios- teal bone deposited in successive layers beneath the periosteum and ensheathin ? E, the spongy en- dochondralbone; represented as more deeply shaded. Within the trabeculae of endochondral spongy bone are seen the remains of the calcified cartilage trabeculae represented as dark wavy lines. C, the medulla with V, V, veins. In the lower half of the figure the endochondral spongy bone has been completely absorbed. (Klein and Noble Smith.) bone is formed around the embryonic endochondral spongy bone: this casing is thickest at the centre, where it is first formed, and thins out towards each end of the shaft. The embryonic spongy bone is absorbed, its trabeculae becoming gradually thinned and its meshes enlarging, and THE STRUCTURE OF THE ELEMENTARY TISSUES. 55 finally coalescing into one great cavity— the medullary cavity of the shaft. Stage 5.— Absorption of the Inner Layers of the Periosteal Bone — The absorption of the endochondral spongy bone is now com- plete, and the medullary cavity is bounded by periosteal bone; the inner layers of this periosteal bone are next absorbed, and the medullary cavity is thereby enlarged, while the deposition of bone beneath the periosteum continues as before. The first-formed periosteal bone is spongy in char- acter. Stage 6. — Formation of Compact Bone.— The transformation of spongy periosteal bone into compact bone is effected in a manner exactly similar to that which has been described in connection with ossification in membrane (p. 49). The irregularities in the walls of the areolae in the spongy bone are absorbed, while the osteoblasts which line them are developed in concentric layers, each layer in turn becoming ossified till the comparatively large space in the centre is reduced to a well-formed Haversian canal (Fig. 64). When once formed, bony tissue grows to some extent interstitially, as is evidenced by the fact that the lacunae are rather further apart in fully-formed than in young bone. From the foregoing description of the development of bone, it will be seen that the common terms "ossification in cartilage" and "ossifi- cation in membrane " are apt to mislead, since they seem to imply two processes radically distinct. The process of ossification, however, is in all cases one and the same, all true bony tissue being formed from mem- brane (perichondrium or periosteum); but in the development of such a bone as the femur, which may be taken as the type of so-called ' ' ossifi- cation in cartilage/' lime-salts are first of all deposited in the cartilage; this calcified cartilage, however, is gradually and entirely re-absorbed, being ultimately replaced by bone formed from the periosteum, till in the adult structure nothing but true bone is left. Thus, in the process of " ossification in cartilage/' calcification of the cartilaginous matrix precedes the real formation of bone. We must, therefore, clearly dis- tinguish between calcification and ossification. The former is simply the infiltration of an animal tissue with lime-salts, and is, therefore, a change of chemical composition rather than of structure; while ossifica- tion is the formation of true bone— a tissue more complex and more highly organized than that from which it is derived. Centres of Ossification. — In all bones ossification commences at one or more points, termed " centres of ossification." The long bones, e.g., femur, humerus, etc., have at least three such points— one for the ossifi- cation of the shaft or diaphysis, and one for each articular extremity or epipliysis. Besides these three primary centres which are always present in long bones, various secondary centres may be superadded for the ossi- fication of different processes. 56 HANDBOOK OF PHYSIOLOGY. Growth of Bone.— Bones increase in length by the advance of the process of ossification into the cartilage intermediate between the dia- physis and epiphysis. The increase in length indeed is due entirely to growth at the two ends of the shaft. This is proved by inserting two pins into the shaft of a growing bone; after some time their distance apart will be found to be unaltered though the bone has gradually in- creased in length, the growth having taken place beyond and not between them. If now one pin be placed in the shaft, and the other in the epiphysis, of a growing bone, their distance apart will increase as the bone grows in length. Thus it is that if the epiphy- ses with the intermediate car- /»tilage be removed from a young bone, growth in length is no long- er possible; while the natural ter- mination of growth of a bone in length takes place when the epi- physes become united in bony continuity with the shaft. Increase in thickness in the shaft of a long bone, occurs by the deposition of successive layers beneath the periosteum. If a thin metal plate be in- serted beneath the periosteum of a growing bone, it will soon be covered by osseous deposit, but if it be put between the fibrous and osteogenetic layers, it will never become enveloped in bone, for all the bone is formed beneath the latter. FIG. 64.— Transverse section of femur of a human embryo about eleven weeks old. a. rudi- mentary Haversian canal in cross section ; 6, in longitudinal section; c, osteoblasts; d, newly form- ed osseous substance of a lighter color ; e, that of greater age ; /, lacunae with their cells ; g, a cell still united to an osteoblast. (Frey.) Other varieties of connective tissue may become ossified, e g., the tendons in some birds. Functions of Bones. — Bones form the framework of the body; for this they are fitted by their hardness and solidity together with their comparative lightness; they serve both to protect internal organs in the trunk and skull, and as levers worked by muscles in the limbs; notwith- standing their hardness they possess a considerable degree of elasticity, which often saves them from fractures. CHAPTER III. THE BLOOD. THE blood of man, as indeed of the great majority of vertebrate animals, is a more or less viscid red fluid. The exact shade of red is variable, for whereas that taken from the arteries, from the left side of the heart, and from the pulmonary veins, is of a bright scarlet hue, that obtained from the systemic veins, from the right side of the heart, and from the pulmonary artery, is of a much darker color, and varies from bluish -red to reddish-black. At first sight, the red color appears to be- long to the whole mass of blood, but on further examination this is found not to be the case. In reality blood consists of an almost colorless fluid, called Plasma or Liquor Sanguinis, in which are suspended numerous minute rounded masses of protoplasm, called Blood Corpuscles, which are, for the most part, colored, and it is to their presence in the fluid that the red color of the blood is due. Even when examined in very thin layers blood is opaque, on account of the different refractive powers possessed by its two constituents, viz., the plasma and the corpuscles. On treatment with chloroform and other reagents, however, it becomes transparent, and assumes a lake color, in consequence of the coloring matter of the corpuscles having been dis- charged into the plasma. The average specific gravity of blood at 60° F. (15° C.) is 1055, the extremes consistent with health being 1045-1062. The reaction of blood is faintly alkaline. Its temperature varies slightly, the average being 100° F. (37.8° 0.). The blood stream is warmed by passing through the muscles, nerve centres, and glands, but is somewhat cooled on traversing the capillaries of the skin. Recently drawn blood has a distinct odor, which in many cases is characteristic of the animal from which it has been taken. It may be further developed also by adding to blood a mixture of equal parts of sulphuric acid and water. Quantity of the Blood. — The quantity of blood in any animal under normal conditions bears a pretty constant relation to the body weight. The methods employed for estimating it are not so simple as might at first sight be thought. For example, it would not be possible to get any accurate information on the point from the amount obtained by rapidly bleeding an animal to death, for then an indefinite quantity would remain in the vessels, as well -as in the tissues; nor, on the other 58 HANDBOOK OF PHYSIOLOGY. hand, would it be possible to obtain a correct estimate by less rapid bleeding, as, since life would be more prolonged, time would be allowed for the passage into the blood of lymph from the lymphatic vessels and from the tissues. In the former case, therefore, we should under-esti- mate, and in the latter over-estimate the total amount of the blood. Of the several methods which have been employed, the most accurate appears to be the following. A small quantity of blood is taken from an animal by venesection; it is defibrinated and measured, and used to make standard solutions of blood. The animal is then rapidly bled to death, and the blood which escapes is collected. The blood vessels are next washed out with water or saline solutions until the washings are no longer colored, and these are added to the previously withdrawn blood; lastly the whole animal is finely minced with water or saline solution. The fluid obtained from the mincings is carefully filtered, and added to the diluted blood previously obtained, and the whole is measured. The next step in the process is the comparison of the color of the diluted blood with that of standard solutions of blood and water of a known strength, until it is discovered to what standard solution the diluted blood corre- sponds. As the amount of blood in the corresponding standard solution is known, as well as the total quantity of diluted blood obtained from the animal, it is easy to calculate the absolute amount of blood which the latter contained, and to this is added the small amount which was withdrawn to make the standard solutions. This gives the total amount of blood which the animal contained. It is contrasted with the weight of the animal, previously known. The result of many experiments shows that the quantity of blood in various animals averages T^ to ^ of the total body weight. An estimate of the quantity in man which corresponded nearly with this proportion, was made some years ago 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 blood-vessels 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 represented was calculated by comparing the proportion of solid matter contained in it with that of "the first blood which escaped on de- capitation. Two experiments of this kind gave precisely similar results. (Weber and Lehmann.) It should be remembered, in connection with these estimations, that the quantity of the blood must vary, even in the same animal, very con- siderably with the amount of both the ingesta and egesta of the period immediately preceding the experiment; and it has been found, indeed, that the amount of blood obtainable from the body of a fasting animal rarely exceeds a half of that which is present soon after a full meal. Coagulation of the Blood. — One of the most characteristic prop- erties which the blood possesses is that of clotting or coagulating, when THE BLOOD. 59 removed from the body. This phenomenon may be observed under the most favorable conditions in blood which has been drawn into an open vessel. In about two or three minutes, at the ordinary temperature of the air, the surface of the fluid is seen to become semi-solid or jelly-like, and this change takes place, in a minute or two afterwards, at the sides of the vessel in which it is contained, and then extends throughout the entire mass. The time which is required for the blood to become solid is about eight or nine minutes. The solid mass occupies exactly the same vol- ume as the previously liquid blood, and adheres so closely to the sides of the containing vessel that if it be inverted none of its contents escape. The solid mass is the crassamentum or clot. If the clot be watched for a few minutes, drops of a light, straw-colored fluid, the serum, may be seen to make their appearan-ce on the surface and, as they become more FIG. 65.— Reticulum of fibrin, from a drop of human blood, after treatment with rosaniluu (Ranvier. > and more numerous, to run together, forming a complete superficial stratum above the solid clot. At the same time the fluid begins to transude at the sides and at the under surface of the clot, which in the course of an hour or two floats in the liquid. The first drops of serum appear on the surface about eleven or twelve minutes after the blood has been drawn; and the fluid continues to transude for from thirty-six to forty-eight hours. The clotting of blood is due to the development in it of a substance called fibrin, which appears as a mesh work (Fig. 65) of fine fibrils. This meshwork entangles and incloses within it the blood-corpuscles, as clotting takes place too quickly to allow them to sink to the bottom of the plasma. The first clot formed, therefore, includes the whole of the constituents of the blood in an apparently solid mass, but soon the fibrin- ous meshwork begins to contract, and the serum which does not belong 60 HANDBOOK OF PHYSIOLOGY. to the clot is squeezed out. When the whole of the serum has trans- uded, the clot is found to be smaller, but firmer and harder, as it is now made up of fibrin and blood-corpuscles only. It will be noticed that coagulation rearranges the constituents of the blood according to the following scheme, liquid blood being made up of plasma and blood-cor- puscles, and clotted blood of serum and clot. Liquid Blood. Plasma. Corpuscles. Serum. Fibrin. Clot. Clotted Blood. Under ordinary circumstances coagulation occurs, as we have men- tioned above, 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 red color throughout, somewhat darker, it may be, at the most dependent part, from accumulation of red corpuscles, but not to any very marked degree. When, however, coagulation is delayed from any cause, as when blood is kept at a temperature of 32° F. (0° C.), or when clotting is normally a slow process, as in the case of horse's blood, or, lastly, in certain diseased conditions of the blood in which clotting is naturally delayed, time is allowed for the colored corpuscles to sink to the bottom of the fluid. When clotting does occur, the upper layers of the blood, being free of colored corpuscles and consisting chiefly of fibrin, form a superficial stratum differing in appearance from the rest of the clot, in that it is of a grayish-yellow color. This is known as the "buffy coat." When the buffy coat has been produced in the manner just described, it commonly contracts more than the rest of the clot, on account of the absence of colored corpuscles from its meshes, and because contraction is less interfered with by adhesion to the interior of the containing vessel in the vertical than the horizontal direction. This produces a cup-like appearance of the buffy coat, and the clot is not only buffed but cupped on the surface. The buffed and cupped appearance of the clot is well marked in certain states of the system, especially in inflammation, where the fibrin-forming constituents are in excess, and it is also well marked in chlorosis where the corpuscles are deficient in quantity. Formation of Fibrin. — That the clotting of blood is due to the gradual appearance in it of fibrin is universally acknowledged. It may l>e easily demonstrated. For example, if recently drawn blood be THE BLOOD. 6} whipped with a bundle of twigs which presents numerous points of con- tact and so, as we shall presently see, facilitates coagulation, the fibrin may be withdrawn .from the blood before it can entangle the blood- corpuscles within its meshes, as it adheres to the twigs in stringy threads almost free from corpuscles; whereas the blood from which the fibrin has been withdrawn no longer exhibits the power of spontaneous coagu- lability. Although these facts have long been known, the closely asso- ciated problem as to the exact manner in which fibrin is formed is still only partially solved. It will be most convenient to treat of the question step by step. In the first place it appears that under the ordinary conditions of experiment, fibrin is chiefly, if not entirely to be obtained from plasma ; for although the colorless corpuscles may be intimately connected with the process, as will be shown later on, yet the colored corpuscles do not appear to take an active part in it. This statement does not exclude the possibility that fibrin may be derived from the colored corpuscles under certain conditions. Indeed, this is more than probable, as experiments have shown that if a little defibrinated blood be added to serum, the haemoglobin leaves the stroma of the colored corpuscles of the blood, and a substance arises from it called stroma-fibrin, indistinguishable from ordinary fibrin, which pro- duces clotting of the serum. This may be shown by experimenting with plasma free from colored corpuscles. Plasma maybe procured by delaying coagulation in blood by keeping it at a low temperature, 32° F. (0° C.), until the colored corpuscles, which are of a higher specific gravity than the other constituents of blood, have had time to sink to the bottom of the containing vessel, and to leave an upper stratum of colorless plasma, in the lower layers of which, however, are many colorless corpuscles. The blood of the horse is specially suited for the purposes of this experiment, as might have been expected from what has been stated as to its naturally slow coagu- lating power. A portion of the colorless plasma, if decanted into another vessel and exposed to the ordinary temperature of the air, will be seen to coagulate just as though it were the entire blood, producing a clot similar in all respects to blood clot, except that it is almost color- less from the absence of red corpuscles. But if some of the plasma be diluted with * neutral saline solution, coagulation is delayed, and the stages of the gradual formation of fibrin may be more conveniently watched. The viscidity which precedes the complete coagulation may be actually seen to *be due to fibrin fibrils developing in the fluid — first 1 Neutral saline solution commonly consists of a .6 to .75 solution of common salt (sodium chloride) in water. 62 HANDBOOK OF PHYSIOLOGY. of all at the circumference of the containing vessels, and gradually ex- tending throughout the mass. If a further portion of plasma be whipped with a. bundle of twigs, the fibrin may be obtained as a solid, stringy mass, just in the same way as from the entire blood, and the resulting fluid no longer retains its power of spontaneous coagulability. In these experiments, it is not necessary that the plasma shall have been obtained by the process of cooling above described, as plasma obtained in any other way. e. #., by allowing blood to flow direct from the vessels of an animal into a vessel containing a third or a fourth of its bulk of a saturated solution of a neutral salt (preferably of magne- sium sulphate) and mixing carefully, will answer the purpose and, just as in the other case, the colored corpuscles will subside leaving the clear superstratum of (salted) plasma. In order that this plasma may coagu- late, it is necessary to get rid of the salts by dialysis, or to dilute it with several times its bulk of water. Evidently, therefore, fibrin is as a rule derived from the plasma of blood The second step in the investigation is to consider from what part of the plasma fibrin is formed, and to that we shall now turn our attention. If plasma be saturated with solid magnesium sulphate or sodium chloride, a white, sticky precipitate called plasmine is thrown down, after the removal of which, by filtration, the plasma will not spontane- ously coagulate. Plasmine is soluble in dilute neutral saline solutions, and the solution of it speedily coagulates, producing a clot composed of fibrin. Blood plasma therefore contains a substance without which it cannot coagulate, and a solution of which is spontaneously coagulable. This substance is very soluble in dilute saline solutions, and is not, therefore, fibrin, which is insoluble in these fluids. We are, therefore, led to the belief that plasmine produces or is converted into fibrin, when clotting of fluids containing it takes place. There is distinct evidence that plasmine is a compound body made up of two or more substances, and that it is not mere soluble fibrin. This view is based upon the following observations: — There exists in all the serous cavities of the body in health, e. g., the pericardium, the peritoneum, and the pleura, a certain small amount of transparent fluid, generally of a pale straw color, which in diseased conditions may be greatly increased. It somewhat resembles serum in appearance, but in reality differs from it, and is probably closely allied to plasma. This serous fluid is not, as a rule, spontaneously coagulable, but may be made to clot on the addition of serum, which is also a fluid which has no ten- dency of itself to coagulate. The clot produced consists of fibrin, and the clotting is identical with the clotting of plasma. From the serous fluid (that from the inflamed tunica vaginalis testis or hydrocele fluid is THE BLOOD. 63 mostly used) we may obtain, by saturating it with solid magnesium sul- phate or sodium chloride, a white viscid substance as precipitate which is called fibrinogen^ If fibrinogen be separated by filtration, it can be dissolved in water, as a certain amount of the neutral salt used in pre- cipitating it is entangled with the precipitate, and is sufficient to pro- duce a dilute saline solution in which fibrinogen, being a body of the globulin class, is soluble. The solution of fibrinogen has no tendency to clot of itself. The same body may also be obtained as a viscid pre- cipitate from hydrocele fluid by diluting it with water, and passing a brisk stream of carbon dioxide gas through the solution. Now if blood-serum be added to a solution of fibrinogen, obtained in either of these ways, the mixture clots. On the other hand, from blood-serum may be obtained another globulin very similar in properties to fibrinogen, if it be treated in either of the ways by which fibrinogen is obtained from hydrocele fluid; this substance is called paraglobulin, and it may be separated by filtra- tion and dissolved in a dilute saline solution in a manner similar to fibrinogen. If the solutions of fibrinogen and paraglobulin be mixed, the mix- ture cannot be distinguished from a solution of plasmine, and in a great majority of cases firmly clots like that solution, whereas a mixture of the hydrocele fluid and serum, from which these bodies have been respec- tively taken, no longer manifests the like property. In addition to this evidence of the compound nature of plasmine, it may be further shown that, if sufficient care be taken, both fibrinogen and paraglobulin may be separately obtained from plasma: the one, fibri- nogen, as a flaky precipitate, by adding carefully thirteen per cent of crystalline sodium chloride to it; and the other, paraglobulin, may be precipitated, after the removal of fibrinogen by filtration, on the further addition to saturation of the same salt or of magnesium sulphate to the filtrate. It is evident, therefore, that both these substances must be thrown down together when plasma is at once saturated with sodium chloride or magnesium sulphate, and that the mixture of the two cor- responds with plasmine. So far it has been shown that plasmine, the antecedent of fibrin, to the possession of which blood owes its power of coagulating, is not a simple body, but is composed of at least two factors —viz. , fibrinogen and paraglobulin; there is reason for believing that yet another body is associated with them in plasmine to produce coagulation; this is what is known under the name of fibrin ferment (Schmidt). Let us now consider the evidence in favor of this view. It was at one time thought that the reason why hydrocele fluid coagulated, when serum was added to it, was that the latter fluid supplied the paraglobu- lin which the former lacked; this, however, is not the case, as hydrocele 64 HANDBOOK OF PHYSIOLOGY. fluid does not lack this body, and moreover, if paraglobulin, obtained from serum by the carbonic acid method, be added to it, it will not coagulate, neither will a mixture of solutions of fibrinogen and para- globulin, obtained in the same way. But if paraglobulin, obtained by the saturation method, be added to hydrocele fluid, it will clot, as will also, as we have seen above, a mixed solution of fibrinogen and para- globulin, both obtained by the saturation method. From this it is evi- dent that in plasmine there is something more than the two bodies above mentioned, and that this something is precipitated with the paraglobu- lin by the saturation method, and is not precipitated by the carbonic acid method. The following experiments show that it is of the nature of a ferment. If defibrinated blood or serum be kept in a stoppered bottle with its own bulk of alcohol for some weeks, all the proteid matter is precipitated in a coagulated form; if the precipitate be then removed by filtration, dried over sulphuric acid, finely powdered, and then suspended in water, a watery extract may be obtained by further filtration, containing ex- tremely little, if any, proteid matter. Yet a little of this watery ex- tract will produce coagulation in fluids, e.g., hydrocele fluid or diluted plasma, which are not spontaneously coagulable, or which coagulate slowly and with difficulty. It will also cause a mixture of fibrinogen and paraglobulin, both obtained by the carbonic acid method, to clot. The watery extract appears to contain the body which is precipitated with the paraglobulin by the saturation method. Its active properties are entirely destroyed by boiling. The amount of the extract added does not influence the amount of the clot formed, but only the rapidity of clotting, and moreover the active substance contained in the extract evidently does not form part of the clot, as it may be obtained from the serum after blood has clotted. So that the third factor, which is contained in the aqueous extract of blood, appears to belong to that class of bodies which promote the union of, or cause changes in, other bodies, without themselves entering into union or undergoing change, i.e., fer- ments. The third substance has, therefore, received the name fibrin ferment. This ferment is developed in blood soon after it has been shed, and its amount appears to increase for some little time afterwards (p. 65). So far we have seen that plasmine is a body composed of three sub- stances, viz., fibrinogen, paraglobulin, and fibrin ferment. The next ques- tion which presents itself is, are these three bodies actively concerned in the formation of fibrin f Here we come to a point about which two distinct opinions prevail, and which it will be necessary to mention. On the one hand Schmidt holds that fibrin is produced by the inter- action of the two proteid bodies, viz., fibrinogen and paraglobulin, brought about by the presence of a special fibrin ferment. Also, that THE BLOOD. 65 when coagulation does not occur in serum, which contains paraglobulin and the fibrin ferment, the non-coagulation is accounted for by lack of fibrinogen. and that when it does not occur in fluids which contain fibrinogen, it is due to the absence of paraglobulin, or of the ferment, or of both. It will be seen that, according to this view, paraglobulin has a very important fibrino-plastic property. On the other hand Hammersten holds that paraglobulin is not an essential in coagulation, or at any rate does not take an active part in the process. He believes that paraglobulin possesses the property in common with many other bodies of combining with — or decomposing, and so rendering inert — certain substances which have the power of pre- venting the formation or precipitation of fibrin, this power of preventing coagulation being well known to belong to the free alkalies, to the alka- line carbonates, and to certain salts ; and he looks upon fibrin as formed from fibrinogen, which is either (1) decomposed into that substance with the production of some other substances ; or (2) bodily converted into it under the action of a ferment, which is frequently precipitated with paraglobulin. Hammersten's view as to the formation of fibrin from fibrinogen by the action of a second body, possibly of the ferment class, is now very generally held. The presence of a certain but small amount of salts, especially of sodium chloride, is necessary for coagulation, and without it, clotting cannot take place. Sources of the Fibrin Generators. — It has been previously re- marked that the colorless corpuscles which are always present in smaller or greater numbers in the plasma, even when this has been freed from colored corpuscles, have an important share in the production of the clot. The proofs of this may be briefly summarized as follows : — (1) That all strongly coagulable fluids contain colorless corpuscles almost in direct proportion to their coagulability ; (2) That clots formed on for- eign bodies, such as needles projecting into the interior or lumen of living blood-vessels, are preceded by an aggregation of colorless corpus- cles ; (3) That plasma in which the colorless corpuscles happen to be scanty, clots feebly ; (4) That if horse's blood be kept in the cold, so that the corpuscles subside, it will be found that the lowest stratum, containing chiefly colored corpuscles, will, if removed, clot feebly, as it contains little of the fibrin factors ; whereas the colorless plasma, es- pecially the lower layers of it in which the colorless corpuscles are most numerous, will clot well, but if filtered in the cold will not clot so well, indicating that when filtered nearly free from colorless corpuscles even the plasma does not contain sufficient of all the fibrin factors to produce thorough coagulation ; (5) In a drop of coagulating blood, observed under the microscope, the fibrin fibrils are seen to start from the color- less corpuscles. 5 66 HANDBOOK OF PHYSIOLOGY. Although the intimate connection of the colorless corpuscles with the process of coagulation seems indubitable, for the reasons just given, the exact share which they have in contributing the various fibrin factors still remains uncertain. It is generally believed that the fibrin-ferment at any rate is contributed by them, inasmuch as the quantity of this substance obtainable from plasma bears a direct relation to the numbers of colorless corpuscles which the plasma contains. Many believe that the fibrinogen too is wholly or in part derived from them, and also that they are the usual source of the paraglobulin present in plasma. Accord- ing to this view all the fibrin factors are derived from the disintegration of the colorless corpuscles. We have seen that the colored corpuscles may also under certain circumstances take a share in producing the fibrin generators. Conditions affecting Coagulation. — The coagulation of the blood is hastened by the following means : — 1. Moderate warmth— from 100° to 120° F. (37.8-49° C.). 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 com- pletely filled with it is constantly moved, coagulates very slowly and im- perfectly. 3. Contact with foreign matter, and especially multiplication of the points of contact. Thus, as before mentioned, fibrin may be quickly ob- tained from liquid blood by stirring it with a bundle of small twigs ; and even in the living body the blood will coagulate upon rough bodies pro- jecting into the vessels. 4. The free access of air. — Coagulation is quicker in shallow than in tall and narrow vessels. 5. The addition of less than twice the bulk of water. The blood last drawn is said, from being more watery, to coagulate more quickly than the first. The coagulation of the blood is retarded, suspended, or prevented by the following means : 1. Cold retards coagulation ; and so long as blood is kept at a tem- perature, 32° F. (0° 0.), 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. A higher temperature than 120° F. (49° C.} retards coagulation, by coagulating the albumen of the serum, and a still higher one above 56° C. prevents it altogether. 2. The addition of water in greater proportion than tivice the bulk of the blood, also the addition of syrup, glycerin, and other viscid sub- stances. 3. Contact with living tissues, and especially with the interior of a living blood-vessel. THE BLOOD. 6? 4. The addition of neutral salts in the proportion 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 during which blood can be thus pre- served in a liquid state and coagulated by the addition of water, is quite indefinite. 5. Imperfect aeration — as in the blood of those who die by asphyxia. 6. In inflammatory states of the system the blood coagulates more slowly although more firmly. 7. Coagulation is retarded by exclusion of the Uoodfrom the air, as by pouring oil on the surface, etc. In vacuo, the blood coagulates quickly ; but 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. Receiving blood into a vessel, well smeared inside with oil, fat, or vaseline, is said also to retard or prevent coagulation. 8. The coagulation of the blood is prevented altogether by the addi- tion of strong acids and caustic alkalies. 9. It has been believed, and chiefly on the authority of Hunter, that after certain modes of death the blood does not coagulate ; he enumerates the death by lightning, over-exertion (as in animals hunted to death), blows on the stomach, fits of anger. He says, <( I have seen instances of them all." Doubtless he had done so ; but the results of such events are not constant. The blood has been often observed coagulated in the bodies of animals killed by lightning or an electric shock ; and 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. 10. The injection of peptones, or of various digestive ferments, e. g., trypsin or pepsin, into the vessels of an animal appears to prevent or stay coagulation of its blood if it be killed soon after. The secretion of the mouth of the leech, and possibly the blood squeezed out of its body when full, also prevents the clotting if added to blood. Cause of the fluidity of the blood within the living body.— Very closely connected with the problem of the coagulation of the blood is the question — why does the blood remain liquid within the living body? We have certain pathological and experimental facts, apparently opposed to one another, which bear upon it, and these may be, for the sake of clearness, classed under two heads: — (1) Blood will coagulate within the living body under certain condi- tions—for example, on ligaturing an artery, whereby the inner and middle coats are generally ruptured, a clot will form within it, or by passing a needle through the coats of the vessel into the blood stream a clot will gradually form upon it. Other foreign bodies, e.g., wire, thread, etc., produce the" same effect. It is a well-known fact that small clots are apt to form upon the roughened edges of the valves of the heart when 68 HANDBOOK OF PHYSIOLOGY. the roughness has been produced by inflammation, as in endocarditis, and it is also equally true that aneurysms of arteries are sometimes spon- taneously cured by the deposition within them, layer by layer, of fibrin from the blood stream, which natural cure it is the aim of the physician or surgeon to imitate. (2) Blood will remain liquid under certain conditions outside the body, without the addition of any reagent, even if exposed to the air at the ordinary temperature. It is well known that blood remains fluid in the body for some time after death, and it is only after rigor mortis has occurred that the blood is found clotted. It has been demonstrated by Hewson, and also by Lister, that if a large vein in the horse or similar animal be ligatured in two places some inches apart, and after sometime be opened, the blood contained within it will be found fluid, and that coagulation will occur only after a considerable time. But this is not due to occlusion from the air simply. Lister further showed that if the vein with the blood contained within it be removed from the body, and then be carefully opened, the blood might be poured from the vein into another similarly prepared, as from one test-tube into another, thereby suffering free exposure to the air, without coagulation occurring as long as the vessels retain their vitality. If the endothelial lining of the vein, however, be injured, the blood will not remain liquid. Again, blood will remain liquid for days in the heart of a turtle, which continues to beat for a very long time after removal from the body. Any theory which aims at explaining the normal fluidity of the blood within the living body must reconcile the above apparently contradictory facts, and must at the same time be made to include all other known facts concerning coagulation. We may therefore dismiss as insufficient the following: — that coagulation is due to exposure to the air or oxygen; that it is due to the cessation of the circulatory movement; that it is due to evolution of various gases, or to the loss of heat. Two theories, those of Lister and Briicke, remain. The former sup- poses that the blood has no natural tendency to clot, but that its coagu- lation out of the body is due to the action oi 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. The latter, on the other hand, supposes that there is a natural tendency on the part of the blood to clot, but that this is restrained in the living body by some inhibitory power resident in the walls of the containing vessels. The blood must contain all the substances from which fibrin is formed, and the re-arrangement of these substances must occur very quickly whenever the blood is shed, and so it is somewhat difficult to prevent coagulation. It seems more reasonable to hold, therefore, that the blood has a strong tendency to clot, rather than that it has no special tendency thereto. But it has been recently suggested that the reason why blood does not coagulate in the living vessels, is that the factors which are necessary for the formation of fibrin are not in the exact state required for its produc- tion, and that at any rate the fibrin ferment is not formed or is not free in the living blood, but that it is produced (or set free) at the moment THE BLOOD. 69 of coagulation by the disintegration of the colorless (and possibly of the colored) corpuscles. This supposition is certainly plausible, and if it be a true one, it must be assumed either that the living blood-vessels exert a restraining influence upon the disintegration of the corpuscles in suffi- cient numbers to form a clot, or that they render inert any small amount of fibrin ferment which may have been set free by the disintegration of a few corpuscles; as it is certain, firstly, that corpuscles of all kinds must from time to time disintegrate in the blood without causing it to clot; and, secondly, that shed and defibrinated blood which contains blood corpuscles, broken down and disintegrated, will not, when injected into the vessels of an animal, under ordinary conditions, produce clotting. There must be a distinct difference, therefore, if only in amount, between the normal disintegration of a few colorless corpuscles in the living un- injured blood-vessels and the abnormal disintegration of a large number which occurs whenever the blood is shed without suitable precaution, or when coagulation is unrestrained by the neighborhood of the living un- injured blood-vessels. The explanation of the clotting of blood which has been given in the preceding pages and which depends chiefly upon the researches of Alex. Schmidt and Hammersten, supposes that it is one of the fermentative actions, so many of which are believed to go on in the living body. Wool- dridge ably contests this view of the process. His laborious researches have led him to the belief that coagulation of the blood is a vital pro- cess, or rather that it is the last act of vitality displayed by blood plasma, which he considers to be during life, living protoplasm. Some of the results of his experiments may with advantage be here mentioned, as they correct and amplify the information as to blood-clotting which has been hitherto given and received. Firstly, he has shown that plasma itself contains everything that is necessary for coagulation. Peptone plasma obtained by injecting a solution of peptone into the veins of an animal and bleeding it immediately afterwards was experimented with. The whole of the corpuscular elements were removed by repeated treat- ment with a centrifugal machine. The plasma thus obtained was shown to clot by the use of some simple mechanical means, e.g., filtering through a clay cell, or through filter paper, or on neutralization with acetic acid, or carbonic acid, or by dilution with water or saline solution. Thus it would appear that if the colorless blood-corpuscles aid coagula- tion, their influence is only secondary. Secondly, he has shown that the important precursor of clotting in this peptone plasma may be separated from it, as a precipitate, if the plasma be kept in ice for some time, and that after its removal the plasma contains only a little fibrinogen capable of clotting by the action of fibrin ferment. If the plasma be diluted with water or slightly acid- ulated, however, the fibrin ferment is able to produce a complete clotting. In peptone plasma, Wooldridge states that three coagulable bodies exist, which he calls A, B, and 0 fibrinogen, and which are closely allied to one another. C-fibrinogen is identical with the body which has been hitherto described as fibrinogen, is present in very small amount, '70 HANDBOOK OF PHYSIOLOGY. and clots on addition of fibrin ferment. The coagulable matter present in greatest amount is B-fibrinogen, which clots on addition of lecithin, or of lymph corpuscles, but not on the addition of fibrin ferment; A- fibrinogen is separated from plasma by cooling, in minute regular rounded granules, from which rounded distinctly biconcave discs arise, if watched under the microscope, quite indistinguishable from colored blood-cor- puscles; it is not coagulated by fibrin ferment. Finally, he considers that when blood plasma dies, an action takes place between A- and B- fibrinogen which are both compounds of proteid and lecithin. The es- sential of this action is a loss of lecithin on the part of the former and a gain of lecithin on the part of the latter, with the result of the pro- duction of fibrin, a third proteid-lecithin compound, and the setting free of other substances contained in the serum, including fibrin fer- ment. Thus, fibrin ferment, a body which cm convert C-fibrinogen into fibrin, is not present in living plasma, but is a result of its disor- ganization or death. As the fibrinogen which can be clotted by the fer- ment is only present in minimal amounts in living plasma, injection of a solution of fibrin ferment or of shed blood does not produce intra- vascular clotting, whereas injection of lymph corpuscles from lymphatic glands or of lecithin, either of which will produce clotting of the other fibrinogens which form the bulk of the coagulable matter in living blood, leads to extensive intra-vascular clotting. The Blood Corpuscles. There are two principal forms of corpuscles, the red and the white, or, as they are now frequently named, the colored and the colorless. In the moist state, the red corpuscles form about 45 percent by weight of the whole mass of the blood. The proportion of colorless corpuscles is only as 1 to 500 or 600 of the colored. Red or Colored Corpuscles. — Human red blood-corpuscles are circular, biconcave discs with rounded edges, from -^Vo to :oW incn in diameter, and T?£or incn in thickness, becoming flat or convex on addi- tion of water. When viewed singly, they appear 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. They are composed of a colorless, structureless, and transparent filmy framework or stroma, in- filtrated in all parts by a red coloring matter termed licemoglobin. The stroma is tough and elastic, so that, as the corpuscles circulate, they admit of elongation and other changes of form, in adaptation to the ves- sels, yet recover their natural shape as soon as they escape from com- pression. The term cell, in the sense of a bag or sac, although sometimes ap- plied, is inapplicable to the red blood-corpuscle; and it must be consid- ered, if not solid throughout, yet as having no such variety of consis- tence 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 pervades this, and is not merely sur- THE BLOOD. 71 rounded by and mechanically inclosed within the outer wall of the corpuscle. The red corpuscles have no nuclei, although in their usual state the unequal refraction of transmitted light gives the appearance of a central spot, brighter or darker than the border, according as it is viewed in or out of focus. Their specific gravity is about 1088. Varieties. — The red corpuscles are not all alike, some being rather larger, paler, and less regular than the majority, and sometimes flat or slightly convex, with a shining part apparent like a nucleolus. In almost every specimen of blood may be also observed a certain number of corpuscles smaller than the rest. They are termed microcytes, and are probably immature corpuscles. It is necessary to take notice that much importance is attached to one form of these smaller corpuscles named blood plates by Bizzozero. They are small, more or less rounded or slightly oval granules, slightly if at all colored, and about one-third the size of ordinary colored corpuscles. From them it is supposed the fibrin ferment is specially derived. Some go so far as t» say that they are practically broken up into it alone. They rapidly under- go change in blood after it has been drawn. They may form masses by co- alescing. A peculiar property of the red ^Sl^SS'Sg^"'^ corpuscles, which is exaggerated in inflammatory blood, may be here again noticed, i. e., their great ten- dency to adhere together in rolls or columns, like piles of coins. These rolls quickly fasten together by their ends, and cluster; so that, when the blood is spread out thinly on a glass, they form a kind of irregular network, with crowds of corpuscles at the several points corresponding with the knots of the net (Fig. 66). Hence the clot formed in such a thin layer of blood looks mottled with blotches of pink upon a white ground, and in a larger quantity of such blood help, by the consequent rapid subsidence of the corpuscles, in the formation of the buffy coat already referred to. Action of Reagents. — Considerable light has been thrown on the physical and chemical constitution of red blood-cells by studying the effects produced by mechanical means and by various reagents; the fol- lowing is a brief summary of these reactions: Pressure. — If the red blood-cells of a frog or man are gently squeezed, they exhibit a wrinkling of the surface, which clearly indi- cates that there is a superficial pellicle partly differentiated from the 72 HANDBOOK OF PHYSIOLOGY. softer mass within; again, if a needle be rapidly drawn across a drop of blood, several corpuscles will be found cut in two, but this is not accom- panied by any escape of cell contents; the two halves, on the contrary, assume a rounded form, proving clearly that the corpuscles are not mere membranous sacs with fluid contents like fat-cells. Fluids, i. Water. — When water is added gradually to frog's blood, the oval disc-shaped corpuscles become spherical, and gradually discharge their haemoglobin, a pale, transparent stroma being left behind; human red blood-cells change from a discoidal to a spheroidal form, and dis- charge their cell-contents, becoming quite transparent and all but invisible. *£ & ii. S dine solution (dilute) produces no appreciable $•& effect on the red blood-cells of the frog. In the red blood-cells of man the discoid shape is exchanged for a FIG. 67. spherical one, with spinous projections, like a horse- chestnut (Fig. 67). Their orginal forms can be at once restored by the use of carbonic acid. iii. Acetic acid (dilute) causes the nucleus of the red blood-cells in the frog to become more clearly defined; if the action is prolonged, the FIG. 68.— The above illustration is somewhat altered from a drawing by 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 represent the average diameter. In the case of the oval cells, only the Ion? diameter is here given. It is remarkable, that although the siza 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 great- er, in others much less than the red corpuscles existing side by side with them. nucleus becomes strongly granulated, and all the coloring matter seems to be concentrated iu it, the surrounding cell-substance and outline of THE BLOOD. T3 the cell becoming almost invisible; after a time the cells lose their color altogether. The cells in the figure (Fig. 69) represent the successive stages of the change. A similar loss of color occurs in the red cells of human blood, which, however, from the absence of nuclei, seem to dis- appear entirely. iv. Alkalies cause the red blood-cells to swell and finally disappear. v. Chloroform added to the red blood-cells of the frog causes them to part with their haemoglobin; the stroma of the cells becomes gradually broken up. A similar effect is produced on the human red blood-cell. vi. Tannin. — When a 'I per cent solution of tannic acid is applied to frog's blood it causes the appearance of a sharply-defined little knob, projecting from the free surface (Robert's macula): the coloring matter becomes at the same time concentrated in the nucleus, which grows more distinct (Fig. 70). A somewhat similar effect is produced on the human red blood-corpuscle. vii. Magenta, when applied to the red blood-cells of the frog, pro- duces a similar little knob or knobs, at the same time staining the nu- cleus and causing the discharge of the haemoglobin. The first effect of the magenta is to cause the discharge of the haemoglobin, then the nucleus becomes suddenly stained, and lastly a finely granular matter issues through the wall of the corpuscle, becoming stained by the ma- Vf* •?*» FIG. 69. FIG. 70. FIG. 71. FIG. 72 FIG. 73. genta, and a macula is formed at the point of escape. A similar macula is produced in the human red blood-cell. viii. Boracic acid. — A 2 per cent solution applied to nucleated red blood-cells (frog) will cause the concentration of all the coloring matter in. the nucleus; the colored body thus formed gradually quits its central position, and comes to be partly, sometimes entirely, protruded from the surface of the now colorless cell (Fig. 71). The result of this ex- periment led Briicke to distinguish the colored contents of the cell (zooid) from its colorless stroma (oecoid). When applied to the non- nucleated mammalian corpuscle its effect merely resembles that of other dilute acids. ix. Ammonia. — Its effects seem to vary according to the degree of concentration. Sometimes the outline of the corpuscles becomes dis- tinctly crenated; at other times the effect resembles that of boracicacid, while in other cases the edges of the corpuscles begin to break up. Gases. Carbonic acid. - Ii the red blood-cells of a frog be first ex- posed to the action of water-vapor (which renders their outer pellicle more readily permeable to gases), and then acted on by carbonic acid, the nuclei immediately become clearly defined and strongly granulated; when air or oxygen is admitted the original appearance is at once re- stored. The upper and lower cell in Fig. 72 show the effect of carbonic acid; the middle one the effect of the re-admission of air. These effects 74 HANDBOOK OF PHYSIOLOGY. can be reproduced five or six times in succession. If, however, the ac- tion of the carbonic acid be much prolonged, the granulation of the nucleus becomes permanent; it appears to depend on a coagulation of the paraglobulin. Heat.— The effect of heat up to 120°-140° F. (50°-60° 0.) is to cause the formation of a number of bud-like processes (Fig. 73). Electricity causes the red blood-corpuscles to become crenated, and at length mulberry-like. Finally they recover their round form and become quite pale. The Colorless Corpuscles. — In human blood the white or colorless corpuscles or leucocytes are nearly spherical masses of granular proto- plasm without cell. wall. The granular appearance more marked in some than in others (vide infra), is due to the presence of particles probably of a fatty nature. In all cases one or more nuclei exist in each corpuscle. The size of the corpuscle averages -y-^ of an inch in diameter. In health, the proportion of red to white corpuscles, which, taking an FIG. 74.— A. Three colored blood-corpuscles. B. Three colorless blood-corpuscles acted on by acetic acid ; the nuclei are veiy clearly visible, x 900. average, is about 1 to 500 or 600, 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 increased by a meal, and diminished again on fasting. Also in }*oung persons, during pregnancy, and after great loss of blood, there is a larger proportion of colorless blood-corpus- cles, which probably shows that they are more rapidly formed under these circumstances. In old age, on the other hand, their proportion is diminished. Varieties. — The colorless corpuscles present greater diversities of form than the red ones. Two chief varieties are to be seen in human blood; one which contains a considerable number of granules, and the other which is paler and less granular. In size the variations are great, for in most specimens of blood it is possible to make out, in addition to the full-sized varieties, a number of smaller corpuscles, consisting of a large spherical nucleus surrounded by a variable amount of more or less granular protoplasm. The small corpuscles are, in all probability, the undeveloped forms of the others, and are derived from the cells of the lymph. Besides the above-mentioned varieties, Schmidt describes another THE BLOOD, 75 form which he looks upon as intermediate between the colored and the colorless forms, viz., certain corpuscles which contain red granules of haemoglobin in their protoplasm. The different varieties of colorless corpuscles are especially well seen in the blood of frogs, newts, and other cold-blooded animals. Amoeboid movement. — The remarkable property of the colorless corpuscles of spontaneously changing their shape was first demonstrated by Wharton Jones in the blood of the skate. If a drop of blood be examined with a high power of the microscope on a warm stage, or, in other words, under conditions by which loss of moisture is prevented, and at the same time the temperature is maintained at about that of the blood within the walls of the living vessels, 100° F. (37.8° 0.), the- colorless corpuscles will be observed slowly to alter their shapes, and to send out processes at various parts of their circumference. The amoeboid movement can be most conveniently studied in the newt's blood. The processes which are sent out from the corpuscle are either lengthened or withdrawn. If lengthened, the protoplasm of the whole corpuscle flows as it were into its process, and the corpuscle changes its position; if withdrawn, protrusion of another process at a different point of the cir- FIG. 75.— Human colorless blood-corpuscle, showing its successive changes of outline within ten minutes when kept moist on a warm stage. (.Schofield.) cumference speedily follows. The change of position of the corpuscle can also take place by a flowing movement of the whole mass, and in this case the locomotion is comparatively rapid. The activity both in the processes of change of shape and also of change in position, is much more marked in some corpuscles, viz., in the granular variety than in others. Klein states that in the newt's blood the changes are especially likely to occur in a variety of the colorless corpuscle, which consists of masses of finely granular protoplasm with jagged outline, containing three or four nuclei, or of large irregular masses of protoplasm contain- ing from five to twenty nuclei. Another phenomenon may be observed in such a specimen of blood, viz., the division of the corpuscles, which occurs in the following way. A cleft takes place in the protoplasm at one point, which becomes deeper and deeper, and then by the lengthen- ing out and attenuation of the connection, and finally by its rupture, two corpuscles result. The nuclei have previously undergone division. The cells so formed are remarkably active in their movements. Thus we see that the rounded form which the colorless corpuscles present in ordinary microscopic specimens must be looked upon as the shape natural to a dead corpuscle or to one whose vitality is dormant rather than as the shape proper to one living and active. 76 HANDBOOK OF PHYSIOLOGY. Action of reagents upon the colorless corpuscles.— Feeding the corpuscles. — If some fine pigment granules, e.g., powdered vermilion, be added to a fluid containing colorless blood-corpuscles, on a glass slide, these will be observed, under the microscope, to take up the pigment. In some cases colorless corpuscles have been seen with fragments of colored ones thus imbedded in their substance. This property of the •colorless corpuscles is especially interesting as helping still further to connect them with the lowest forms of animal life, and to connect both with the organized cells of which the higher animals are composed. The property which the colorless corpuscles possess of passing through the walls of the blood-vessels will be described later on. Enumeration of the blood-corpuscles.— Several methods are em- ployed for counting the blood-corpuscles, most of them depending upon the same principle, i.e., the dilution of a minute volume of blood with SL given volume of a colorless solution similar in specific gravity to blood plasma, so that the size and shape of the corpuscles is altered as little as possible. A minute quantity of the well-mixed solution is then taken, •examined under the microscope, either in a flattened capillary tube (Malassez) or in a cell (Hayem & Cachet, Gowers) of known capacity, and the number of corpuscles in a measured length of the tube, or in a given area of the cell is counted. The length of the tube and the area of the cell are ascertained by means of a micrometer scale in the micro- scope ocular; or in the case of Gowers' modification, by the division of the cell area into squares of known size. Having ascertained the number of corpuscles in the diluted blood, it is easy to find out the number in a given volume of normal blood. Gowers' modification of Hayem & Nachet's instrument, called by him '* Hcemacytometer," appears to be the most convenient form of instrument for counting the corpuscles, and as such will alone be described (Fig. 76). It consists of a small pipette (A), which, when filled up to a mark on its stem, holds 995 cubic milli- metres. It is furnished with an india-rubber tube and glass mouth-piece to facilitate filling and emptying; a capillary tube (B) marked to hold 5 cubic millimetres, and also furnished with an india-rubber tube and mouth-piece; a small glass jar (D) in which the dilution of the blood is performed; a glass stirrer (B) for mixing the blood thoroughly, (F) a needle, the length of which can be regulated by a screw; a brass stage plate (c) carrying a glass slide, on which is a cell one-fifth of a millimetre deep, and the bottom of which is divided into one-tenth millimetre squares. On the top of the cell rests the cover-glass, which is kept in its place by the pressure of two springs proceeding from the stage plate. A standard saline solution of sodium sulphate, or similar salt, of specific gravity 1025, is made, and 995 cubic millimetres are measured by means of the pipette into the glass jar, and with this five cubic millimetres of blood, obtained by pricking the linger with a needle, and measured in the capillary pipette (B), are thoroughly mixed by the glass stirring-rod. A drop of this diluted blood is then placed in the cell and covered with a cover-glass, which is fixed in position by means of the two lateral springs. The preparation is then examined under a microscope with a power of about 400 diameters, and focussed until the lines dividing the cell into .squares are visible. THE BLOOD. 77 After a short delay, the red corpuscles which have sunk to the bottom of the cell, and are resting on the squares, are counted in ten squares, and the number of white corpuscles noted. By adding together the numbers counted in ten (one-tenth millimetre) squares, and multiplying FIG. 76.— Heemacytometer. by ten thousand, the number of corpuscles in one cubic millimetre of blood is obtained. The average number of corpuscles per each cubic millimetre of healthy blood, according to Vierordt and Welcker, is 5,000,000 in adult men, and rather fewer in women. CHEMICAL COMPOSITION OF THE BLOOD. Before considering the chemical composition of the blood as a whole, it will be convenient to take in order the composition of the various chief factors which have been set out in the table on p. 58, into which the blood may be separated, viz. : — (1.) The Plasma ; (2.) The Serum; (3.) The Corpuscles ; (4.) The Fibrin. (1.) Chemical Composition of Plasma.— The Plasma, or liquid part of the blood, in which the corpuscles float, may be obtained free from colored corpuscles in either of the ways mentioned below. In it are the fibrin factors, inasmuch as when exposed to the ordinary temperature of the air it undergoes coagulation and splits up into fibrin and serum. It differs from the serum in containing fibrinogen, but in appearance and in reaction it closely resembles that fluid; its alkalinity, however, is less than that of the serum obtained from it. It may be freed from white corpuscles by filtration at a temperature below 41° F. (5° C.), or by the centrifugal machine. 78 HANDBOOK OF PHYSIOLOGY. The chief methods of obtaining plasma free from corpuscles are : (1) by cold, the temperature should be about 0° C. and may be two or three degrees higher, but not lower. (2) The addition of neutral salts, in certain proportions, either solid or in solution, e. g.9 of sodium sul- phate, if solid, 1 part to 12 parts of blood; if a saturated solution, 1 part to 6 parts of blood; of magnesium sulphate, of a 23$, or if saturated .solution 1 part to 4 of blood. (3) A third way is to mix frog's blood with an equal part of a 5f0 of cane sugar, and to get rid of the corpuscles by nitration; or (4) by the injection of peptone into the veins of mam- mals, previous to bleeding them to death, and afterwards subjecting the plasma thus obtained to the action of a centrifugal machine. Salts of the plasma. — In 1000 parts of the plasma there are: — Sodium chloride, 5.546 Soda, 1.532 Sodium phosphate, ...... .271 Potassium chloride, . .... .359 " sulphate, . . . . . .281 Calcium phosphate, .298 Magnesium phosphate, 218 8.505 (2.) Chemical Composition of Serum.— The serum is the liquid part of the blood or of the plasma remaining after the separation of the clot. It is an alkaline, yellowish, transparent fluid, with a specific gravity of from 1025 to 1032. In the usual mode of coagulation, part of the serum remains in the clot, and the rest, squeezed from the clot by its contraction, lies around it. 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. There is nearly as much, by weight, of serum as there is clot in coagu- lated blood. Serum may be obtained from blood corpuscles by allowing blood to clot in large test tubes, and subjecting the test tubes to the action of a centrifugal machine for some time. In tabular form the composition may be thus summarized. In 1000 parts of serum there are: — Water, about 900 Proteids : a. Serum-albumin, . . . . . • I en ft. Paraglobulin, f Salts. Fats — including fatty acids, cholesterin, lecithin ; and ^ some soaps, ....... Grape sugar in small amount, ..... Extractives — kreatin, kreatinin, urea, etc., . Yellow pigment, which is independent of haemoglobin, Gases — small amounts of oxygen, nitrogen, and car- bonic acid, 1000 THE BLOOD. 79 a. Water. — The water of the serum varies in amount according to the amount of food, drink, and exercise, and with many other circum- stances. b. Proteids. — a. Serum-albumin is the chief proteid found in serum. The proportion which it bears to paraglobulin, the other proteid, is as 1.011 to 1. in human blood. Serum-albumin has been shown by Halliburton to be a compound body, which may be called serine, made up of three proteid s, which co- agulate at different temperatures, <*at 73° C., fi at 77° C., and y at 85° 0. The serine is entirely coagulated at 94° C., and also by the addition of strong acids, such as nitric and hydrochloric ; by long contact with al- 'cohol it is precipitated. It is not precipitated on addition of ether, and so differs from the other native albumin, viz., egg-albumin. When dried at 104° F. (40° C.) serum-albumin is a brittle, yellowish substance, solu- ble in water, possessing a laevorotary power of —56°. It is with great difficulty freed from its salts, and is precipitated by solutions of metallic salts, e.g., of mercuric chloride, copper sulphate, lead acetate, sodium tungstate, etc. If dried at a temperature over 35° C. the residue is insol- uble in water, having been changed into coagulated proteid. Serum- albumin may be precipitated from serum, from which the paraglobulin has been previously separated by saturation with magnesium sulphate, and removed by filtration, by further saturation with sodium sulphate, sodium nitrate, or iodide of potassium. fi. Paraglobulin can be obtained as a white precipitate from cold serum by adding a considerable excess of water, and passing through the mix- ture a current of carbonic acid gas or by the cautious addition of dilute acetic acid. It can also be obtained by saturating serum with either crystallized magnesium sulphate, or sodium chloride, nitrate, acetate, or carbonate. When obtained in the latter way, precipitation seems to be much more complete than. by means of the former method. Paraglobu- lin belongs to the class of proteids called globulins. c. The salts of sodium predominate in serum as in plasma, and of these the chloride generally forms by far the largest proportion. d. Fats are present partly as fatty acids and partly emulsified. The fats are tri-olein, tri-stearin, tri-palmitin. The amount of fatty matter varies according to the time after, and the ingredients of, a meal. Of cholesterin and lecithin there are mere traces. e. Grape sugar is found principally in the blood of the hepatic vein, about one part in a thousand. f. The extractives vary from time to time ; sometimes uric and hip- puric acids are found in addition to urea, kreatin, and kreatinin. Urea exists in proportion from .02 to .04 per cent. g. The yellow pigment of the serum and the odorous matter which gives the blood of each particular animal a peculiar smell, have not yet been exactly differentiated. The former is probably choletelin {MacMunn). 80 HANDBOOK OF PHYSIOLOGY. (3.) Chemical Composition of the Corpuscles. — a. Colored. — Analysis of a thousand parts of moist blood-corpuscles shows the follow- ing result: — Water, 688 Solids— j Organic, 303.88 (Mineral,. .'.... • • 8.12—312=1000. Of the solids the most important is Hcemoglobin, the substance to which the blood owes its color. It constitutes, as will be seen from the appended Table, more than 90 per cent of the organic matter of the corpuscles. Besides haemoglobin there are proteid : and fatty matters, the former chiefly consisting of globulins, and the latter of cholesterin and lecithin. In 1000 parts organic matter are found : — Haemoglobin, ...... 905.4 Proteids, . . . . . . .86.7 Fats, 7-9=1000 Of the inorganic salts of the corpuscles, with the iron omitted — In 1000 parts corpuscles (Schmidt) are found : — Potassium Chloride, 3.679 Potassium Phosphate, . . . • . 2.343 Potassium sulphate, 132 Sodium, 633 Calcium, . 094 Magnesium, . .060 Soda, 341 = 7.282 The properties of haemoglobin will be considered in relation to the Gases of the blood (p. 83). b. Colorless. — The corpuscles may be said also to contain flbrinogen, paraglobulin, and the ferment. In consequence of the difficulty of ob- taining colorless corpuscles in sufficient number to make an analysis, little is accurately known of their chemical composition; in all proba- bility, however, the stroma of the corpuscles is made up of proteid mat- ter, and the nucleus of nuclein, a nitrogenous, phosphorus-containing body akin to mucin, capable of resisting the action of the gastric juice. The proteid matter, chiefly globulins, soluble in a ten per cent solution of sodium chloride, the solution being precipitated on the addition of water, by heat and by the mineral acids. The stroma contains fatty granules, and in it also the presence of glycogen has been demonstrated. The salts of the corpuscles are chiefly potassium, and of these the phos- phate is in greatest amount. 1 An account of the proteid bodies, etc., will be found in the Appendix, and should be referred to for explanation of the terms employed in the text. THE BLOOD. 0-L (4.) Chemical Composition of Fibrin.— The part played by fibrin in the formation of a clot has been already described (p. 58), and it is only necessary to consider here its general properties. It is a stringy elastic substance belonging to the proteid class of bodies. It is insoluble in water and in weak saline solutions ; soluble in ten per cent solu- tion of sodium chloride, it swells up into a transparent jelly when placed in dilute hydrochloric acid, but does not dissolve, but in strong acid it dissolves, producing acid-albumin x ; it is also soluble in strong saline solutions. Blood contains only .2 per cent of fibrin. It can be converted by the gastric or pancreatic juice into peptone. It possesses the power of liberating the oxygen from solutions of hydric peroxide, H20, or ozonic ether. This may be shown by dipping a few shreds of fibrin in tincture of guaiacum, and then immersing them in a solution of hydric peroxide. The fibrin becomes of a bluish color, from its hav- ing liberated from the solution oxygen, which oxidizes the resin of guai- acum contained in the tincture, and thus produces the coloration. The Gases of the Blood. The gases contained in the blood are Carbonic acid, Oxygen, and Nitrogen, 100 volumes of blood containing from 50 to 60 volumes of these gases collectively. Arterial blood contains relatively more oxygen and less carbonic acid than venous. But the absolute quantity of carbonic acid is in both kinds of blood greater than that of the oxygen. Oxygen. Carbonic Acid. Nitrogen. Arterial Blood, . . 20 vol. per cent 39 vol. per cent 1 to 2 vols. Venous " (from muscles at rest) 8 to 12 " " " 40 " " " 1 to 2 vols. The Extraction of the Gases from the Blood. — As the ordinary air- pumps are not sufficiently powerful for the purpose, the extraction of the gases from the blood is accomplished by means of a mercurial air- pump, of which there are many varieties, those of Ludwig, Alvergnidt, Geissler, and Sprengel being the chief. The principle of action in all is much the same. Lud wig's pump, which may be taken as a type, is rep- resented in Fig. 77. It consists of two fixed glass globes, C'and F, the upper one communicating by means of the stop-cock D, and a stout in- dia-rubber tube with another glass globe, L, which can be raised or lowered by means of a pulley ; it also communicates by means of a stop- cock, B, and a bent glass tube, A, with a gas receiver (not represented in the figure), A, dipping into a bowl of mercury, so that the gas maybe received over mercury. The lower globe, F, communicates with 0 by 1 The use of the two words albumen and albumin may need explanation. The former is the generic word, which may include several albuminous or proteid bodies, e. g., albumen of blood; the latter which requires to be qualified by another word is the specific form, and is applied to varieties, e. g.. egg-albumin, serum-albumin. 6 HANDBOOK OF PHYSIOLOGY. means of the stopcock, E, with /in which the blood is contained by the stopcock, 6r, and with a movable glass globe, M, similar to />, by means of the stopcock, H, and the stout india-rubber tube, K. In order to work the pump, L and M are filled with mercury, the blood from which the gases are to be extracted is placed in the bulb /, the stopcocks, H, E, D, and B, being open, and G closed. Mis raised by means of the pulFey until F is full of mercury, and the air is driven out. E is then closed, and L is raised so that (7 becomes full of mer- cury, and the air driven off. B is then closed. On lowering L the mercury runs into it from C, and a vacuum is established in C. On opening E and lowering J/, a vacuum is similarly established in F; if G. be now opened, the blood in / will enter into ebullition, and the gases will pass off into F and C, and on raising M and then L, the stopcock B being opened, the gas is driven through A, and is received into the receiver over mercury. By repeat- ing the experiment several times the whole of the gases of the specimen of blood is obtained, and may be estimated. A. The Oxygen of the Blood.— It has been found that a very small proportion of the oxygen which can be obtained, by the aid of the mercurial pump, from the blood, exists in a state of simple solution in the plasma. If the gas were in simple solution, the amount of oxygen in any given quantity of blood exposed to any given atmosphere ought to vary with the amount of oxygen contained in the atmosphere. Since, speak- ing generally, the amount of any gas ab- sorbed by a liquid such as plasma would de- pend upon the proportion of the gas in the atmosphere to which the liquid is exposed— if the proportion is great, the absorption will be great; if small, the absorption will be similarly small. The absorption continues until the proportions of the gas in the liquid and in the atmosphere are equal. Other things will, of course, influence the absorption, such as the nature of the gas employed, the nature of the liquid, and the temperature, but cceteris paribus, the amount of a gas which a liquid absorbs depends upon the proportion — the so-called par- tial pressure — of the gas in the atmosphere to which the liquid is subjected. And conversely, if a liquid containing a gas in solution be exposed to an atmosphere containing none of the gas, the gas will be given up to the atmosphere until the amount in the liquid and in the FIG. 77.— Ludwig's Mercurial Pump. THE BLOOD. 83 atmosphere becomes equal. This condition is called a condition of equal tensions. The condition may be understood by a simple illustration. A large amount of carbonic acid gas is dissolved in a bottle of water by exposing tha liquid to extreme pressure of the gas, and a cork is placed in the bottle and wired down. The gas exists in the water in a condition of extreme tension, and therefore exhibits a tendency to escape into the atmosphere, in order to relieve the tension; this produces the violent expulsion of the cork when the wire is removed, arid if the aerated water be placed in a glass the gas will continue to be evolved until it has almost entirely passed into the atmosphere, and the tension of the gas in the water approximates to that of the atmosphere in which, it should be remembered, the carbon dioxide is, naturally, in very small amount, viz., .04 per cent. The oxygen of the blood does not obey this law of pressure. For if blood which contains little or no oxygen be exposed to a succession of atmospheres containing more and more of that gas, we find that the absorption is at first very great, but soon becomes relatively very small, not being therefore regularly in proportion to the increased amount (or tension) of the oxygen of the atmospheres, and that conversely, if arte- rial blood be submitted to regularly diminishing pressures of oxygen, at first very little of the contained oxygen is given oif to the atmosphere, then suddenly the gas escapes with great rapidity, and again disobeys the law of pressures. Very little oxygen can be obtained from serum freed from blood- corpu;des, even by the strongest mercurial air-pump, neither can serum be made to absorb a large quantity of that gas; but the small quantity which is so given up or so absorbed follows the laws of absorption according to pressure. It must be, therefore, evident that the chief part of the oxygen is contained in the corpuscles, and not in a state of simple solution. The chief solid constituent of the colored corpuscles is hemoglobin, which constitutes more than 90 per cent of their bulk. This body has a very remarkable affinity for oxygen, absorbing it to a very definite extent under favorable circumstances, and giving it up when subjected to the action of reducing agents, or to a sufficiently low oxygen pressure. From these facts it is inferred that the oxygen of the blood is combined with hcemoglobin, and not simply dissolved; but inasmuch as it is com- paratively easy to cause the haemoglobin to give up its oxygen, it is believed that the oxygen is but loosely combined with the substance. Haemoglobin. — Haemoglobin is a crystallizable body which consti- tutes by far the largest portion of the colored corpuscles. It is inti- mately distributed throughout their stroma, and must be dissolved out before it will undergo crystallization. Its percentage composition is 84 HANDBOOK OF PHYSIOLOGY. C. 53.85; H. 7.32; K 16.17; 0. 21.84; S. .63; Fe. .42; and if the molecule be supposed to contain one atom of iron the formula would be C60f, H960, N154, FeS3 0]79. The most interesting of the properties of haemoglobin are its powers of crystallizing and its attraction for oxygen and other gases. Crystals. — The haemoglobin of the blood of various animals pos- sesses the power of crystallizing to very different extents (blood- crystals). In some animals the formation of crystals is almost spon- taneous, whereas in others it takes place either with great difficulty or not at all. Among the animals whose blood coloring-matter crystallizes most readily are the guinea-pig, rat, squirrel, and dog; and in these cases to obtain crystals it is generally sufficient to dilute a drop of recently-drawn blood with water and expose it for a few minutes to the air. Light seems to favor the formation of the crystals. In many in- Fia. 78.— Crystals of oxy-haemoglobin— prismatic from human blood. stances other means must be adopted, e. g., the addition of alcohol, ether, or chloroform, rapid freezing, and then thawing, an electric cur- rent, a temperature of 140° F. (60° C.), or the addition of sodium sulphate. The haemoglobin of human blood crystallizes with difficulty, as does also that of the ox, the pig, the sheep, and the rabbit. The forms of haemoglobin crystals, as will be seen from the appended figures, differ greatly. Haemoglobin crystals are soluble in water. Both the crystals them- selves and also their solutions have the characteristic color of arterial blood. A dilute solution of haemoglobin gives a characteristic appearance with the spectroscope. Two absorption bands are seen between the solar lines D (which is the sodium band in the yellow) and E (see plate), one in the yellow, with its middle line some little way to the right of D, is very THE BLOOD. 85 intense, but narrower than the other, which lies in the green near to the left of E. Each band is darkest in the middle and fades away at the sides. As the strength of the solution increases, the bands become broader and deeper and both the red and the blue ends of the spectrum become encroached upon until the bands coalesce to form one very broad band, and only a slight amount of the green remains unabsorbed, and part of the red; on still further increase of the strength the former disappears. If the crystals of oxy haemoglobin be subjected to a mercurial air- pump, they give off a definite amount of oxygen (1 gramme giving off 1.59 c. cm. of oxygen), and they become of a purple color; and a solution of oxy-haemoglobin may be made to give up oxygen, and to be- come purple in a similar manner. This change may be also effected by passing through it hydrogen or FIG. 79. FIG. 80. FIG. 79.-Oxy-h8emoglobin crystals— tetrahedral, from blood of the guinea-pig. FIG. 80.— Hexagonal oxy-haemoglobin crystals, from blood of squirrel. On these hexagonal plates prismatic crystals grouped in a stellate manner not unfrequently occur (after Funke). nitrogen gas, or by the action of reducing agents, of which Stokes' fluid ' or ammonium sulphide is the most convenient. With the spectroscope, a solution of deoxidized or reduced haemo- globin is found to give an entirely different appearance from that of oxidized haemoglobin. Instead of the two bands at D and E we find' a single broader but fainter band occupying a position midway between the two, and at the same time less of the blue end of the spectrum is ab- sorbed. Even in strong solutions this latter appearance is found, thereby differing from the strong solution of oxidized haemoglobin 1 Stokes' Fluid consists of a solution of ferrous sulphate, to which ammonia has been added and sufficient tartaric acid to prevent precipitation. Another reducing agent is a solution of stannous chloride, treated in a way similar to the ferrous sulphate, and a third reagent of like nature is an aqueous solution of ammonium sulphide. N H4 H S. 86 HANDBOOK OF PHYSIOLOGY. which lets through only the red and orange rays; accordingly to the naked eye, the one (reduced haemoglobin solution) appears purple, the other (oxy-haemoglobin solution) red. The deoxidized crystals or their solu- tions quickly absorb oxygen on exposure to the air, becoming scarlet. It solutions of blood be taken instead of solutions of haemoglobin, results similar to the whole of the foregoing can be obtained. Venous blood never, except in the last stages of asphyxia, fails to show the oxy-haemoglobin bands, inasmuch as the greater part of the haemo- globin even in venous blood exists in the more highly oxidized condition. Action of Gases on Haemoglobin. — Carbonic oxide gas, passed through a solution of haemoglobin, causes it to assume a bluish color, and its spectrum to be slightly altered; two bands are still visible, but are slightly nearer the blue end than those of oxy-haemoglobin (see plate). The amount of carbonic oxide taken up is equal to the amount of the oxygen displaced. Although the carbonic oxide gas readily dis- places oxygen, the reverse is not the case, and upon this property depends the dangerous effect of coal-gas poisoning. Coal gas contains much carbonic oxide, and when breathed, the gas combines with the haemo- globin of the blood, and produces a compound which cannot easily be reduced. This compound (carboxy-haemoglobin) is by no means an oxygen carrier, and death may result from suffocation due to the want of oxygen notwithstanding the free entry of pure air into the lungs. Crystals of carbonic-oxide haemoglobin closely resemble those of oxy- haemoglobin. Nitric oxide produces a similar compound to the carbonic-oxide haemoglobin, which is even less easily reduced. Nitrous oxide reduces oxy-haemoglobin. and therefore leaves the reduced haemoglobin in a condition to actively take up oxygen. Sulphuretted Hydrogen. — If this gas be passed through a solution of oxy-haemoglobin, the haemoglobin is reduced and an additional band appears in the red. If the solution be then shaken with air, the two bands of oxy-haemoglobin replace that of reduced haemoglobin, but the band in the red persists. Derivatives of Haemoglobin. Methaemoglobin. — If an aqueous solution of oxy-haemoglobin is ex- posed to the air for some time, its spectrum undergoes a change; the two D and E bands become faint, and a new line in the red at c is developed. The solution, too, becomes brown and acid in reaction, and is precipitable by basic lead acetate. This change is due to the decom- position of oxy-haemoglobin, and to the production of methc&moglMn On adding ammonium sulphide, reduced haemoglobin is produced, and on shaking this up with air, oxy-haemoglobin is reproduced. Methaemo- globin is probably a stage in the deoxidation of oxy-haemoglobin. It appears to contain less oxygen than oxy-haemoglobin, but more than reduced haemoglobin. Its oxygen is in more stable combination, how- ever, than is the case with the former compound. Haematin. — By the action of heat, or of acids or alkalies in the presence of oxygen, haemoglobin can be split up into a substance called Hcematin, which contains ^11 the iron of the haemoglobin from which it THE BLOOD. was derived, and a proteid residue. Of the latter it is impossible to say more than that it probably consists of one or more bodies of the globulin class. If there be no oxygen present, instead of haematin a body called haemochrpmogen is produced, which, however, will speedily undergo oxidation into ha3matin. Ha3matin is a dark brownish or black non-crystallizable substance of metallic lustre. Its percentage composition is C. 64.30; H. 5.50; N. 9.06; Fe. 8.82; 0, 12.32; which gives the formula C68, H70, N"8, Fef, 010 (Hoppe-Seyler). It is insoluble in water, alcohol, and ether; soluble in the caustic alkalies; soluble with difficulty in hot alcohol to which is added sulphuric acid. The iron may be removed from haematin by heating it with fuming hydrochloric acid to 320° F. (160° C.), and a new body, haematoporphyrin, is produced. Hagmatoporphyrin (C6X8, H74, N8, 0,,, Hoppe-Seyler) may also be obtained by adding 'blood to strong sulphuric acid, and if necessary filtering the fluid through asbestos. It forms a fine crimson solution, which has a distinct spec- trum, viz., a dark band just beyond D, and a second all but midway between D and E. It may be precipitated from its acid solution by adding water or by neutralization, and when redissolved in alkalies presents I FIG. 81. FIG. 82. FIG. 81.— Haematoidin crystals. (Frey.) FIG. 82.— Haemin crystals. (Frey.) four bands, a pale band between c and D, a second between D and E, nearer D, another nearer E, and a fourth occupying the chief part of the space between b and F. Hcematin in acid solution. — If an excess o"f acetic acid be added to blood, and the solution is boiled, the color alters to brown from decom- position of haemoglobin and the setting free of haematin; by shaking this solution with ether, solution of the haematin in acid solution is obtained. The spectrum of the ethereal solution (colored plate) shows no less than four absorption bands, viz., one in the red between c and D, one faint and narrow close to D, and then two broader bands, one between D and E, and another nearly midway between b and F. The first band is by far the most distinct, and the acid aqueous solution of haematin shows it plainly. Hcematin in alkaline solution. — If an alkali be added to blood and the solution is boiled, alkaline haematin is produced, and the solution becomes olive green in color, the absorption band of which is still in the red, but nearer to D, and the blue end of the spectrum is partially absorbed to a considerable extent. If a reducing agent be added, two bands resembling those of oxy-haemoglobin, but nearer to the blue, appear; this is the spectrum of reduced hcematin, or haemochromogen. 88 HANDBOOK OF PHYSIOLOGY. On shaking the reduced haematin with air or oxygen the two bands are replaced by the single band of alkaline haematin. Haematoidin.— This substance is found in the form of yellowish crystals in old blood extravasations, and is derived from the haemoglobin. Their crystalline form and the reaction they give with nitric acid seem to show them to be identical with BiliruUn, the chief coloring matter of the Bile. Haemin. — One of the most important derivatives of haematin is has- min. It is usually called Hydrochlornte of Hcematin (or hydrochloride), but its exact chemical composition is uncertain. Its formula is C68, H70, N8, Fe2, 010, 2 HC1, and it contains 5.18 per cent of chlorine, but by some it is looked upon as simply crystallized haematin. Although diffi- cult to obtain in bulk, a specimen may be easily made for the microscope in the following way : — A small drop of dried blood is finely powdered with a few crystals of common salt on a glass slide, and spread out ; a cover glass is then placed upon it, and glacial acetic acid added by means of a capillary pipette. The blood at once turns of a brownish color. The slide is then heated, and the acid mixture evaporated to dryness at a high temperature. The excess of salt is washed away with water from the dried residue, and the specimen may then be mounted. A large number of small, dark, reddish black crystals of a rhombic shape, some- times arranged in bundles, will be seen if the slide be subjected to micro- scopic examination. The formation of these haemin crystals is of great interest and impor- tance from a medico-legal point of view, as it constitutes the most certain and delicate test we have for the presence of blood (not of necessity the blood of man) in a stain on clothes, etc. It exceeds in delicacy even the spectroscopic test. Compounds similar in composition to haemin, but containing hydrobromicand hydriodic acids, instead of hydrochloric, may be also readily obtained. Estimation of Haemoglobin. — The most exact method is by the estimation of the amount of iron in a given specimen of blood, but as this is a somewhat complicated process, a method has been proposed which, though not so exact, has the advantage of simplicity. This con- sists in comparing the color of a given small amount of diluted blood with glycerin jelly tinted with carmine and picrocarmine to represent a standard solution of blood diluted one hundred times. The amount of dilution which the given blood requires will thus approximately repn- sent the quantity of haemoglobin it contains. (Gowers. ) Distribution of Haemoglobin. — Haemoglobin occurs not only in the red blood-cells of all Vertebrata (except one fis'h [leptocephalus] whose blood cells are all colorless), but also in similar cells in many Worms ; moreover, it is found diffused in the vascular fluid of some other worms and certain Crustacea ; it also occurs in all the striated muscles of Mam- mals and Birds. It is generally absent from unstriated muscle except that of the rectum. It has also been found in Mollusca in certain rnus- THE BLOOD. 89 cles which are specially active, viz., those which work the rasp-like tongue. B. The Carbon Dioxide Gas in the Blood. — Of this gas in the blood, part exists in a state of simple solution in the serum, and and the rest in a state of weak chemical combination. It is believed that the latter is combined with the sodium carbonate in a condition of bicar- bonate. Some observers consider that part of the gas is associated with the corpuscles. C. The Nitrogen in the Blood. — The whole of the small quantity of the nitrogen contained in the blood is simply dissolved in the fluid plasma. Chemical Composition of the Blood in Bulk.— Analyses of the blood as a whole differ slightly, but the following table may be taken to- represent the average composition : Water, 784 Solids- Corpuscles, 130 Proteids (of serum), 70 Fibrin (of clot), . , . . . . 2.2 Fatty matters (of serum), .... 1.4 Inorganic salts (of serum), ... 6 Gases, kreatin, urea and other extractive ) 6.4 — matter, glucose and accidental substances, j 216 ' 1000 Variations in the Composition of healthy Blood. The conditions which appear most to influence the composition of the blood in health are these : Sex, Pregnancy, Age, and Temperament. The composition of the blood is also, of course, much influenced by diet. 1. Sex. — The blood of men diifers from that of women, chiefly in being of somewhat higher specific gravity, from its containing a rela- tively larger quantity of red corpuscles. 2. Pregnancy. — The blood of pregnant women is rather lower than the average specific gravity, from deficiency of colored corpuscles. The quantity of the uncolored corpuscles, on the other hand, and of fibrin, is increased. 3. Age. — The blood of the foetus is very rich in solid matter, and es- pecially in colored corpuscles ; and this condition, gradually diminishing, continues for some weeks after birth. The quantity of solid matter then falls during childhood below the average, rises during adult life, and in old age falls again. 4. Temperament. — There appears to be a relatively larger quantity of solid matter, and particularly of colored corpuscles, in those of a ple- thoric or sanguineous temperament. 5. Diet. — Such differences in the composition of the blood as are due 90 HANDBOOK OF PHYSIOLOGY. to the temporary presence of various matters absorbed with the food and drink, as well as the more lasting changes which must result from gener- ous or poor diet respectively, need be here only referred to. 6. Effects of Bleeding. — The result of bleeding is to diminish the specific gravity of the blood ; and so quickly, that in a single venesection, the portion of blood last drawn has often a less specific gravity than that of the blood that flowed first. 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 colored corpuscles is well marked, but with this exception, no considerable alteration seems to be produced in the composition of the blood for more than a very short time ; the loss of the other constituents, including the colored corpuscles, being very quickly repaired. Variations 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 system — the blood contained in some veins differing remarkably from that in others. Differences between Arterial and Venous Blood. — The differences between arterial and venous blood are these: — (a.) Arterial blood is bright red, from the fact that almost all its haemoglobin is combined with oxygen (Oxy-hsemoglobin, or scarlet haemoglobin), while the purple tint of venous blood is due to the deoxi- dation of a certain quantity of its oxy-hsemoglobin, and its consequent reduction to the purple variety (Deoxidized, or purple haemoglobin). (b.) Arterial blood coagulates somewhat more quickly. (c.) Arterial blood contains more oxygen than venous, and less car- Tronic acid. Some of the veins 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, from the gastric and mesen- teric veins, which contains the soluble elements of food absorbed from the stomach and intestines during digestion, and from the splenic vein; it must, therefore, 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, and other soluble matters which may have been THE BLOOD. 91 absorbed from the alimentary canal, this blood appears to be deficient in solid matters, especially in colored corpuscles, owing to dilution by the quantity of water absorbed, to contain an excess of proteid matter, and to yielof a less tenacious kind of fibrin than that of blood generally. The blood from the splenic vein is generally deficient in colored cor- puscles, and contains an unusually large proportion of preteids. The fibrin obtainable from the blood seems to vary in relative amount, but to be almost always above the average. The proportion of colorless corpuscles is also unusually large. The whole quantity of solid matter is decreased, the diminution appearing to be of colored 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 colored corpuscles, more proteids, and yields a less firm clot than that yielded by other blood, owing to the deficient tenacity of its fibrin. Guarding (by ligature of the portal vein) against the possibility of an error in the analysis from regurgitation of hepatic blood into the portal vein, recent observers have determined that hepatic venous blood contains less water, proteids, and salts than the blood of the portal vein; but that it yields a much larger amount of extractive matter, in which is one constant element, namely, grape-sugar, which is found, whether saccha- rine or farinaceous matter have been present in the food or not. Development of the Blood-Corpuscles. The first formed blood-corpuscles of the human embryo differ much in their general characters from those which belong to the later periods FIG. 83.— Part of the network of developing blood-vessels in the vascular area of a guinea-pig. bl, blood-corpuscles becoming free in an enlarged and hollowed out part of the network ; a, process of protoplasm. (E. A. Schafer.) of intra-uterine, and to all periods of extra-uterine life. Their manner of origin is at first very simple. Surrounding the early embryo is a circular area, called the vascular area, in which the first rudiments of the blood-vessels and blood-corpus- cles are developed. Hare the nucleated embryonal cells of the inesoblast, from which the blood-vessels and corpuscles are to be formed, send out processes in various directions, and these joining together, form an irreg- 92 HANDBOOK OF PHYSIOLOGY. ular meshwork. The nuclei increase in number, and collect chiefly in the larger masses of protoplasm, but partly also in the processes. These nuclei gather around them a certain amount of the protoplasm, and be- coming colored, form the red blood-corpuscles. The protoplasm of the cells and their branched network in which these corpuscles lie then be- come hollowed out into a system of canals inclosing fluid, in which the red nucleated corpuscles float. The corpuscles at first are from about mhro to TtW °^ an incn in diameter, mostly spherical, and with granular contents, and a well-marked nucleus. Their nuclei, which, are about. WOT °^ an incn in diameter, are central, circular, very little prominent on the surfaces of the corpuscle, and apparently slightly granular or tuberculated. The corpuscles then strongly resemble the colorless corpuscles of the fully developed blood, but are colored. They are capable of amoeboid movement and multiply by division. 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, and new blood-cells are produced by this organ, and also by the lymphatic glands, thymus and spleen. These are at first colorless arid nucleated, but afterwards acquire the ordinary blood-tinge, and resemble very much those of the first set. They also multiply by division. In whichever way produced, however, whether from the original formative cells of the embryo, or by the liver and the other organs mentioned above, these colored nucleated cells begin very early in fcetal life to be mingled with colored non-nucleated, corpuscles resembling those of the adult, and at about the fourth or fifth month of embryonic existence are completely replaced by them. Origin of the Mature Colored Corpuscles. — The non-nucleated red corpuscles may possibly be derived from the nucleated, but in all probability are an entirely new formation, and the methods of their ori- FIG. 84.— Development of red corpuscles in connective-tissue cells. From the subcutaneous tissue of a new-born rat. h, a cell containing haemoglobin in a diffused form in the protoplasm ; h', one containing colored globules of varying size and vacuoles ; h", a cell filled with colored globules of nearly uniform size ; /, /', developing fat cells. (E. A. Schafer.) gin are the following : — (1.) During foetal life and possibly in some ani- mals, e.g., the rat, which are born in an immature condition, for some little time after birth, the blood discs arise in the connective-tissue cells THE BLOOD. 93 in the following way. Small globules, of varying size, of coloring matter arise in the protoplasm of the cells, and the cells Uiemselves be- come branched, their branches joining tbe branches of similar cells. The cells next become vacuolated, and the red globules are free in a cavity filled with fluid (Fig. 85) ; by the extension of the cavity of the cells into their processes anastomosing vessels are produced, which ulti- mately join with the previously existing vessels, and the globules, now having the size and appearance of the ordinary red corpuscles, are passed into the general circulation. This method of formation is called intra- cellular (Schafer). (2.) From the ivliitc, corpuscles. — The belief that the red corpuscles are derived from the white is still very general, although no new evidence has been recently advanced in favor of this view. It is, however, uncer- tain whether the nucleus of the white corpuscle becomes the red cor- puscle, or whether the whole white corpuscle is bodily converted into the red by the gradual clearing up of its contents with a disappearance of the nucleus. Probably the latter view is the correct one. (3.) From the medulla of hones. — Colored corpuscles are to a very large extent derived during adult life from the large pale cells in the red mar- row of bones, especially of the ribs (Figs. 83, 84). These cells become colored from the formation of hgemoglobin chiefly in one part of their protoplasm. This colored part becomes separated from the rest of the cell and forms a red corpuscle, being at first cup-shaped, but soon taking on the normal appearance of the mature corpuscle. It is supposed that the protoplasm may grow up again and form a number of red corpuscles in a similar way. (4.) From the tissue of the spleen. — It is probable that colored as well as colorless corpuscles may be produced in the spleen. (5.) From Microcytes. — Hayem describes the small particles (micro- cytes), previously mentioned as contained in the blood (p. 71), and which he calls haematoblasts, as the precursors of the red corpuscles. They ac- quire color, and enlarge to the normal size of red corpuscles. Without doubt, the red corpuscles 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. It is generally believed that a certain number of the colored corpuscles undergo disintegration in the spleen ; and indeed corpuscles in various degrees of degeneration have been observed in that organ. Origin of the Colorless Corpuscles.— The colorless corpuscles of the blood are derived from the lymph corpuscles, being, indeed, indis- tinguishable from them ; and these come chiefly from the lymphatic glands. Their number is increased by division. HANDBOOK OF PHYSIOLOGY. Colorless corpuscles are also in all probability derived from the spieen and thymus, and also from the germinating endothelium of serous mem- Fio. 85.— Further development of blood-corpuscles in connective-tissue cells and transformation of the latter into capillary blood-vessels, a, an elongated cell with a cavity in the protoplasm oc- cupied by fluid and by blood -corpuscles which are still globular; 6, a hollow cell, the nucleus of which has multiplied. The new nuclei are arranged around the wall of the cavity, the corpuscles in which have now become discoid; c, shows the mode of union of a " hsemapoietic " cell, which in this instance contains only one corpuscle, with the prolongation (bt) of a previously existing vessel; a and c, from the new-born rat; 6, from the foatal sheep. (E. A. Schafer.) branes, and from connective tissue. The corpuscles are carried into the blood either with the lymph and chyle, or pass directly from the lym- FIG. 86.— Colored nucleated corpuscles, from the red marrow of the guinea-pig. (E. A. Schafer.) phatic tissue in which they have been formed into the neighboring blood-vessels. 1. Uses of the Blood.— To be a medium for the reception and storing of matter (ordinary food, drink, and oxygen) from the outer world, and for its conveyance to all parts of the body. 2. To be a source whence the various tissues of the body may take the materials necessary for their nutrition and maintenance ; and whence the secreting organs may take the constituents of their various secretions. 3. To be a medium for the absorption of refuse matters from all the tissues, and for their conveyance to those organs whose function it is to separate them and cast them out of the body. 4. To warm and moisten all parts of the body. CHAPTER IV. THE CIRCULATION OF THE BLOOD. THE Heart is a hollow muscular organ consisting of four chambers, two auricles and two ventricles, arranged in pairs. On the right and left sides of the heart is an auricle joined to and communicating with a ventricle, but the chambers on the right side do not directly communi- cate with those on the left side. The circulation of the blood is chiefly FIG. 87.— Diagram of the circulation.— The unshaded part of the figure to the right indicates the district of the circulation of arterial blood; the dark part to the left the district of venous blood. carried on by the contraction or systole of the muscular walls of the chambers of the heart : the auricles contracting simultaneously, and their contraction being followed by the simultaneous contraction of the ventricles. The blood is conveyed away from the left side of the heart (as in the diagram, Fig. 87) by the arteries, and returned to the right 96 HANDBOOK OF PHYSIOLOGY. side of the heart 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 capillaries. The blood, therefore, in its passage from the heart passes first into the arteries, then into the ca- pillaries, and lastly into the veins, by which it is conveyed back again to the heart, thus completing a revolution or circulation. As the right side of the heart, however, does not directly communi- cate with the left, in order to complete the circulation it is necessary that the blood should pass from the right side to the lungs, through the pulmonary artery, then through the pulmonary capillary-vessels, and through the pulmonary veins to the left side of the heart (Fig, 87). Thus there are two circulations by which the blood must pass ; the one. Pulmonary Artery. Diaphragm. FIG. 88. —View of heart and lungs in situ. The front portion of the chest wall, and the outer or parietal layers of the pleurae and pericardium have been removed. The lungs are partly col- lapsed. a shorter circuit from the right side of the heart to the lungs and back again to the left side of the heart ; the other and larger circuit, from the left side of the heart to all parts of the body and back again to the right side ; but. more strictly speaking, there is only one complete circu- ation, which may be diagrammatically represented by a double loop, as in the accompanying figure (Fig. 87). 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 principal circulations, the Pulmonary and Systemic, as they are THE CIRCULATION OF THE BLOOD. 97 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 liver, before it finally reaches the heart and com- pletes a revolution. This subordinate stream through the liver is called the Portal circulation. As a necessary step towards the consideration of the method by which the circulation is maintained, it will be advisable in the first place to de- vote some time to the description of various important points in the anatomy and minute structure of — I. The Heart; II. The Arteries; III. The Capillaries; IV. The Veins. We shall then be in a better po- sition to discuss the problems in the physiology of the circulation. (I.) The Heart. The heart is contained in the chest or thorax, and lies between the right and left lungs (Fig. 88), enclosed in a membranous sac — the peri- cardium, which is made up of two distinct parts, an external fibrous membrane, composed of closely interlacing fibres, which has its base at- tached to the diaphragm or midriff, the great muscle which forms the floor of the chest and divides it from the abdomen — both to the central tendon and to the adjoining muscular fibres, while the smaller and upper end is lost on the large blood-vessels by mingling its fibres with that of their external coats; and an internal serous layer, which not only lines the fibrous sac, but also is reflected on to the heart, which it completely invests. The part which lines the fibrous membrane is called the parietal layer, and that enclosing the heart, the visceral layer, and these being continuous for a short distance along the great vessels of the base of the heart, form a closed sac, the cavity of which in health contains just enough fluid to lubricate the two surfaces, and thus enable them to glide smoothly over each other during the movements of the heart. Most of the vessels passing in and out of the heart receive more or less invest- ment from this sac. The heart in the chest is situated behind the sternum and costal car- tilages, being placed obliquely from right to left, quite two-thirds to the left of the mid-sternal line. It is of pyramidal shape, with the apex pointing downwards, outwards, and towards the left, and the base back- wards, inwards, and towards the right. It rests upon the diaphragm, and its pointed apex, formed exclusively of the left side of the heart, is in contact with the chest wall, and during life beats' against it at a point called the apex beat, situated in the fifth intercostal space, about two inches below the left nipple, and an inch and a half to the sternal side. The heart is suspended in the chest by the large vessels which proceed 7 98 HANDBOOK OF PHYSIOLOGY. from its base, but, excepting the base, the organ itself lies free in the sac of the pericardium. The part which rests upon the diaphragm is flattened, and is known as the posterior surface, whilst the free upper part is called the anterior surface. The margin towards the left is thick and obtuse, whilst the lower margin towards the right is thin and acute. On examination of the external surface the division of the heart into parts which correspond to the chambers inside of it may be traced, for a deep transverse groove called the auriculo-ventricular groove divides FIG. 89.— The right auricle and ventricle opened, and a part of their right and anterior walls removed, so as to show their interior. %.-!, superior vena cava; 2, inferior vena cava; 2', he- patic 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 ven- tricular 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 commencement 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 outside of the left ventricle, the lower figure near the apex. (Allen Thomson.) the auricles which form the base of the heart from the ventricles which form the remainder, including the apex, the ventricular portion being by far the greater; and, again, the inter-ventricular groove runs between THE CIRCULATION OF THE BLOOD. 99 the ventricles both front and back, and separates the one from the other. The anterior groove is nearer the left margin and the posterior nearer the right, as the front surface of the heart is made up chiefly of the right ventricle and the posterior surface of the left ventricle. In the furrows run the coronary vessels, which supply the tissue of the heart with blood, as well as nerves and lymphatics imbedded in more or less fatty material. The Chambers of the Heart. — The interior of the heart 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 portion, called respectively, as already incidentally mentioned, auricle and ventricle, which freely communicate one with the other; the aperture of communication, however, is guarded by valves, 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 — the auricle and ventricle of one side being quite separate from those of the other (Fig. 89). (1.) Right auricle. — The right auricle is situated at the right part of the base of the heart as viewed from the front. It is a thin-walled cavity of more or less quadrilateral shape, prolonged at one corner into a tongue-shaped portion, the right auricular appendix, which slightly overlaps the exit of the great artery, the aorta, from the heart. The interior is smooth, being lined with the general lining of the heart, the endocardium, and into it open the superior and inferior venae cavas, or great veins, which convey the blood from all parts of the body to the heart. The former is directed downwards and forwards, the latter upwards and inwards; between the entrances of these vessels is a slight tubercle called tubercle of Lower. The opening of the inferior cava is protected and partly covered by a membrane called the Eustachian valve. In the posterior wall of the auricle is a slight depression called the fossa ovalis which corresponds to an opening between the right and left auricles which exists in foetal life. The right auricular appendix is of oval form, and admits three fingers. Various veins, including the coronary sinus, or the dilated portion of the right coronary vein, open into this chamber. In the appendix are closely set elevations of the muscular tissue covered with endocardium, and on the anterior wall of the auricle are similar elevations arranged parallel to one another, called musculi pectinati. (2.) Right Ventricle. — The right ventricle occupies the chief part of the anterior surface of the heart, as well as a small part of the poste- rior surface : it forms the right margin of the heart. It takes no part in the formation of the apex. On section its cavity, in consequence of the encroachment upon it of the septum ventriculorum, in semilunar or crescentic (Fig. 91) ; into it are two openings, the auriculo-ventricular at the base, and the opening of the pulmonary artery alsp_at the.base, , 100 HANDBOOK OF PHYSIOLOGY. but more to the left ; the part of the ventricle leading to it is called the conus arteriosus or infundibulum ; both orifices are guarded by valves, the former called tricuspid and the latter semilunar or sigmoid. In this ventricle are also the projections of the muscular tissue called columnce carnew (described at length p. 103). (3.) Left Auricle. — The left auricle is situated at the left and poste- rior part of the base of the heart, and is best seen from behind. It is quadrilateral, and receives on either side two pulmonary veins. The auricular appendix is the only part of the auricle seen from the front, and corresponds with that on the right side, but is thicker, and the in- terior is more smooth. The left auricle is only slightly thicker than the right, the difference being as 1^- lines to 1 line. The left auriculo-ven- tricular orifice is oval, and a little smaller than that on the right side of the heart. There is a slight vestige of the foramen between the auri- cles, which exists in fcetal life, on the septum between them. (4.) Left Ventricle. — Though taking part to a comparatively slight extent in the anterior surface, the left ventricle occupies the chief part of the posterior surface. In it are two openings very close together, viz., the auriculo-ventricular and the aortic, guarded by the valves cor- responding to those of the right side of the heart, viz., the bicuspid or mitral and the semilunar or sigmoid. The first opening is at the left and back part of the base of the ventricle, and the aortic in front and towards the right. In this ventricle, as in the right, are the columns carneae, which are smaller but more closely reticulated. They are chiefly found near the apex and along the posterior wall. They will be again referred to in the 'description of the valves. The walls of the left ven- tricle, which are nearly half an inch in thickness, are, with the excep- tion of the apex, twice or three times as thick as those of the right. Capacity of the Chambers. — The capacity of the two ventricles is about four to six ounces of blood, the whole of which is impelled into their respective arteries at each contraction. The capacity of the auri- cles is rather less than that of the ventricles : the thickness of their walls is considerably less. The latter is adapted to the small amount of force which the auricles require in order to empty themselves into their adjoining ventricles ; the former to the circumstance of the ventricles being partly filled with the blood before the auricles contract. Size and Weight of the Heart. — The heart is about 5 inches long, 3£ inches greatest width, and 2J inches in its extreme thickness. The average weight of the heart in the adult is from 9 to 10 ounces ; its weight gradually increasing throughout life till middle age ; it dimin- ishes in old age. Structure. — The walls of the heart are constructed almost entirely of layers of muscular fibres ; but a ring of connective tissue, to which some of the muscular fibres are attached, is inserted between each auricle and THE CIRCULATION OF THE BLOOD. 101 ventricle, and forms the boundary of the auricula-ventricular opening. Fibrous tissue also exists at the origins of the pulmonary artery and aorta. The muscular fibres of each auricle are in part continuous with those of the other, and partly separate ; and the same remark holds true for the ventricles. The fibres of the auricles are, however, quite separate from those of the ventricles, the bond of connection between them be- ing only the fibrous tissue of the auriculo-ventricular openings. FIG. 90. --The left auricle and ventricle opened and a part of their anterior and left walls re- moved. %.— The pulmonary artery has been divided at its commencement; the opening into the left ventricle is carried a short distance into the aorta between two of the segments of the semi- lunar valves; and the left part of the auricle with its appendix has been removed. The right auri- cle is 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 remains 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 preserved round the auriculo-ventrirular orifice; 3, 3', the cut surface of the walls of the ventricle, seen to be- come 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 principal anterior columna carnea or musculous papil- laris attached to it; 5, 5, musculi papillares; .V, the left side of the septum, between the two ventri- cles, 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 hang- ing loosely together; 7', the exterior of the great aortic sinus; 8, the root of the pulmonary artery and its semilunar valves; 8', the separated portion 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. (Allen Thomson.) 102 HANDBOOK OF PHYSIOLOGY. The muscular fibres of the heart, unlike those of most of the invol- untary muscles, are striated ; but although, in this respect, they TC- semble the skeletal muscles, they have distinguishing characteristics of their own. The fibres which lie side by side are united at frequent intervals by short branches (Fig. 92). The fibres are smaller than those of the ordinary striated muscles, and their striation is less marked. No sarcolemma can be discerned. The muscle-corpuscles are situate in the middle of the substance of the fibre ; and in correspondence with these the fibres appear under cer- tain conditions subdivided into oblong portions or "cells," the offsets from which are the means by which the fibres anastomose one with another (Fig. 93). Endocardium. — As the heart is clothed on the outside by a thin transparent layer of pericardium, so its cavities are lined by a smooth and shining membrane, or endocardium, which is directly continuous FIG. 91.— Transverse section of bul- lock's heart in a state of cadaveric rigid- ity, a, cavity of left ventricle, b, cavity of right ventricle. (Dalton.) Fio. 92. FIG. 93. Fio. 92.— Network of muscular fibres (striated) from the heart of a pig. The nuclei of the muscle-corpuscles are well shown, x 450. (Klein and Noble Smith.) FIG. 93. -Muscular fibre cells from the heart. (E.A. Schafer.) with the internal lining of the arteries and veins. The endocardium is composed of connective tissue with a large admixture of elastic fibres ; and on its inner surface is laid down a single tesselated layer of flattened THE CIRCULATION OF THE BLOOD. 103 endothelial cells. Here and there unstriped muscular fibres are some- times found m the tissue of the endocardium. Valves of the Heart. — The arrangement of the heart's valves is such that the blood can pass only in one direction (Fig. 94). The tricuspid valve (5, Fig. 89) presents three principal cusps or subdivisions, and mitral or bicuspid valve, because it has two such por- tions (6, Fig. 90). But in both valves there is between each two prin- cipal portions a smaller one ; so that more properly, the tricuspid may be described as consisting of six, and the mitral of four, portions. Each portion is of triangular form, its base is continuous with the bases of the neighboring portions, so as to form an annular membrane around the auriculo-ventricular opening, and is fixed to a tendinous ring which en- FIG. 94. -Diagram of the circulation through the heart (Dalton). circles the orifice between the auricle and ventricle and receives the insertions of the muscular fibres of both. In each principal cusp may be distinguished a central part, extending from base to apex, and includ- ing about half its width. It is thicker, and much tougher than the border-pieces or edges. While the bases of the cusps of the valves are fixed to the tendinous rings, their ventricular surface and borders are fastened by slender ten- dinous fibres, the chordcB tendinece, to the internal surface walls of the ventricles, the muscular fibres of which project into the ventricular cavity in the form of bundles or columns — the columnce carnece. These columns are not all alike, for while some are attached along their whole length on one side, and by their extremities, others are attached only by their 104 HANDBOOK OF PHYSIOLOGY. extremities ; and a third set, to which the name musculi papillares has been given, are attached to the wall of the ventricle by one extremity only, the other projecting, papilla-like, into the cavity of the ventricle (5, Fig. 89), and having attached to it chordae tendineae. Of the tendi- nous chords, besides those which pass from the walls of the ventricle and the musculi papillares to the margins of the valves, there are some of especial strength, which pass from the same parts to the edges of the middle and thicker portions of the cusps before referred to. 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 surface 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 princi- pal ones. Moreover, the musculi papillares are so placed that, from the summit of each, tendinous cords proceed to the adjacent halves of two of the principal divisions, and to one intermediate or smaller division, of the valve. The preceding description applies equally to the mitral and tricuspid valve ; but it should be added that the mitral is considerably thicker and stronger than the tricuspid, in accordance with the greater force which it is called upon to resist. The semilunar valves, three in number, guard the orifices of the pul- monary artery and of the aorta. They are nearly alike on both sides of the heart ; but the aortic valves are altogether thicker and more strongly constructed than the pulmonary valves, in accordance with the greater pressure which they have to withstand. 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, so that each valve forms a little pouch like a watch- pocket (7, Fig. 90). In the centre of the free edge of the valve, which contains a fine cord of fibrous tissue, is a small fibrous nodule, I\\Q -cor- pus Arantii, and from this and from the attached border fine fibres ex- tend into every part of the mid substance of the valve, except a small lunated space just within the free edge, on each side of tine corpus Aran- tii. 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 separated by the blood passing out of the ventricle, but which immediately afterwards are pressed together so as to prevent any return (7, Fig. 89, and 7, Fig. 90). This will be again referred to. Opposite each of the semilunar cusps, both in the aorta and pulmonary artery, there is a bulging outwards of the wall of the vessel : these bulgings are called the sinuses of Valsalva. THE CIRCULATION OF THE BLOOD. 105 Structure of the Valves. — The valves of the heart are formed essen- tially of thick layers of closely woven connective and elastic tissue, over which, on every part, is reflected the endocardium. II. The Arteries. Distribution. — The arterial system begins at the left ventricle in a single large trunk, the aorta, which almost immediately after its origin gives off in the thorax three large branches for. the supply of the head, neck, and upper extremities; it then traverses the thorax and abdomen, giving off branches, some large and some small, for the supply of the FIG. 95. FIG. 96. FIG. 95 —Minute artery viewed in longitudinal section, e. Nucleated endothelial membrane, with faint nuclei in lumen, looked at from above, i. Thin elastic tunica intirna. m. Muscular coat or tunica media, a. Tunica adventitia. (Klein and Noble Smith.) x 250. FIG. 96.— Transverse section through a large branch of the interior mesenteric artery of a pig. e. endothelial membrane ; i, tunica elastica interna, no subendothelial layer is sesn ; m, muscular tunica media, containing only a few wavy elastic fibres ; e, e, tunica elastica externa, dividing the media from the connective-tissue adventitia, a. (Klein, and Noble Smith.) X 350. various organs and tissues it passes on its way. In the abdomen it di- vides into two chief branches, for the supply of the lower extremities. The arterial branches wherever given off divide and subdivide, until the calibre of each subdivision becomes very minute, and these minute ves- sels pass into capillaries. Arteries are, as a rule, placed in situations protected from pressure and other dangers, and are, with few exceptions, straight in their course, and frequently communicate (anastomose or in- osculate) with other arteries. The branches are usually given off at an acute angle, and the area of the branches of an artery generally exceeds that of the parent trunk; and as the distance from the origin is increased, the area of the combined branches is increased also. After death, arteries are usually found dilated (not collapsed as the veins are) and empty, and it was to this fact that their name was given them, as the ancients believed that they conveyed air to the various parts 106 HANDBOOK OF PHYSIOLOGY. of the body. As regards the arterial, system of the lungs (pulmonary system) it begins at the right ventricle in the pulmonary artery, and is distributed much as the arteries belonging to the general systemic circu- lation Structure. — The walls of the arteries are composed of three principal coats, termed (a) the external or tunica adventitia, (b) the middle or tunica media, and (c) the internal or tunica intima. (a) The external coat or tunica adventitia (Figs. 95 and 96 a), the strongest and toughest part of the wall of the artery, is formed of areo- lar tissue, with which is mingled throughout a network of elastic fibres. At the inner part of this outer coat the elastic network forms in most arteries so distinct a layer as to be sometimes called the external elastic coat (Fig. 99, e e). FIG. 97. FIG. 98. FIG. 97.— Portion of a fenestrated membrane from the femoral artery, x 200. a, 6, c, Perfora- tions. (Henle.) FIG. 98.— Muscular fibre-cells from human arteries, magnified 350 diameters. (KoUiker.) a. Nucleus. 6. A fibre-cell treated with acetic acid. (b) The middle coat (Fig. 95, m) is composed of both muscular and elastic fibres, with a certain proportion of areolar tissue. In the larger arteries (Fig. 99) its thickness is comparatively as well as absolutely much greater than in the small, constituting, as it does, the greater part of the arterial wall. The muscular fibres, which are of the unstriped variety (Fig. 98), are arranged for the most part transversely to the long axis of the artery (Fig. 95, m)\ while the elastic element, taking also a transverse direc- tion, is disposed in the form of closely interwoven and branching fibres, which intersect in all parts the layers of muscular fibre. In arteries of various size there is a difference in the proportion of the muscular and elastic element, elastic tissue preponderating in the largest arteries, while this condition is reversed in those of medium and small size. THE CIRCULATION OF THE BLOOD. 107 (c) The internal coal is formed by layers of elastic tissue, consisting in part of coarse longitudinal branching fibres, and in part of a very thin and brittle membrane which possesses little elasticity, and is thrown « « j FIG. 99. —Transverse section of aorta through internal and about half the middle coat. a. Lining endothelium with the nuclei of the cells only shown. 6. Subepithelial layer of connective tissue, c, d. Elastic tunica intima proper, with fibrils running circularly or longitudinally, e, f. Middle coat, consisting of elastic fibres arranged longitudinally, with muscular fibres, cut obliquely or longitu- dinally. (Klein.) into folds or wrinkles when the artery contracts. This latter mem- brane, the striated or fenestrated coat of Henle (Fig. 97), is peculiar in FIG. 100.— Transverse section of small artery from soft palate, e, endothelial lining, the nuclei of the cells are shown ; t, elastic tissue of the intima. which is a goo.t deal folded ; c, m, circular muscular coat, showing nuclei of the muscle cells ; t.a, tunica adventitia. X 300. (Schofleld.) its tendency to curl up, when peeled off from the artery, and in the per- forated and streaked appearance which it presents under the microscope. Its inner surface is lined with a delicate layer of elongated endothelial 108 HANDBOOK OF PHYSIOLOGY. cells (Fig. 101, «), which make it smooth and polished, and furnish a nearly impermeable surface, along which the blood may flow with the smallest possible amount of resistance from friction. FIG. 101.— Two blood-vessels from a frog's mesentery, injected with nitrate of silver, showingthe outlines of the endothelial cells, a. Artery, The endpthelial cells are long and narrow; the trans- verse markings indicate the muscular coat. t.a. Tunica adventitia. v. Vein, showing the shorter and wider endothelial cells with which it is lined; c, c. two capillaries entering the vein. (Scho- field.) tr.n. Ln. FIG 102. -Blood-vessels from mesocolon of rabbit, a. Artery, with two branches, showing tr. n. nuclei of transverse muscular fibres; Ln. nuclei of endothelial lining; t.a. tunica adventitia. v. Vein Here the transverse nuclei are more oval than those of the artery. The vein receives a small branch at the lower end of the drawing; it is distinguished from the artery among other things by its straighter course and larger calibre, c. Capillary, showing nuclei of endothelial cells, x 300, (Schofield.) THE CIKCULATION OF THE BLOOD. 109 Immediately external to the endothelial lining of the artery is fine connective tissue, sub-endothelial layer, with branched corpuscles. Thus the internal coat consists of three parts (a) an endothelial lining, (#) the sub-en dothelial layer, and (c) elastic layers. Vasa Vasorum. — The walls of the arteries, with the possible excep- tion of the endothelial lining and the layers of the internal coat imme- diately outside it, are not nourished by the blood which they convey, but are, like other parts of the body, supplied with little arteries, ending in capillaries and veins, which, branching throughout the external coat, FIG. 103.— Ramification of nerves and termination in the muscular coat of a small artery of the frog (Arnold). extend for some distance into the middle, but do not reach the internal coat. These nutrient vessels are called vasa vasorum. Nerves. — Most of the arteries are surrounded by a plexus of sympa- thetic nerves, which twine around the vessel very much like ivy round a tree: and gangli are found at frequent intervals. The smallest arte- ries and capillaries are also surrounded by a very delicate network of similar nerve-fibres, many of which appear to end in the nuclei of the transverse muscular fibres (Fig. 103). III. The Capillaries. Distribution. — In all vascular textures except some parts of the cor- pora cavernosa of the penis, and of the uterine placenta, and of the spleen, the transmission of the blood from the minute branches of the arteries to the minute veins is effected through a network of capillaries. They may be seen in all minutely injected preparations. The point at which the arteries terminate and the minute veins com- 110 HANDBOOK OF PHYSIOLOGY. mence cannot be exactly denned, for the transition is gradual; but the capillary network has, nevertheless, this peculiarity, that the .small vessels which compose it maintain the same diameter throughout: they do not diminish in diameter in one direction, like arteries and veins; and the meshes of the network that they compose are more uniform in shape and size than those formed by the anastomoses of the minute ar- teries and veins. Structure. — This is much more simple than that of the arteries or veins. Their walls are composed of a single layer of elongated or radi- FIG. 104. FIG. 105. FIG. 104.— Blood-vessels of an intestinal villus, representing the arrangement of capillaries be- tween the ultimate venous and arterial branches; a, a, the arteries; b, the vein. FIG. 105.— Capillary blood-vessels from the omentum of rabbit, showing the nucleated endothe- lial membrane of which they are composed. (Klein and Noble Smith.) ate, flattened and nucleated cells, so joined and dovetailed together as to form a continuous transparent membrane (Fig. 105). Outside these cells, in the larger capillaries, there is a structureless, or very finely fibrillated membrane, on the inner surface of which they are laid down. In some cases this external membrane is nucleated, and may then be regarded as a miniature representative of the tunica adventitia of arteries. Here and there, at the junction of two or more of the delicate endo- thelial cells which compose the capillary wall, pseudo-stomata may be seen (p. 22). The endothelial cells are often continuous at various points with processes of adjacent connective-tissue corpuscles. Capillaries are surrounded by a delicate nerve-plexus resembling, in miniature, that of the larger blood-vessels. The diameter of the capillary vessels varies somewhat in the differ- ent textures of the body, the most common size being about -g-oV^th of THE CIRCULATION OF THE BLOOD. Ill an inch. Among the smallest may be mentioned those of the brain, and of the follicles of the mucous membrane of the intestines; among the largest, those of the skin, and especially those of the medulla of bones. The size of capillaries varies necessarily in different animals in rela- tion to the size of their blood-corpuscles: thus, in the Proteus, the capillary circulation, can just be discerned with the naked eye. The form of the capillary network presents considerable variety in the different textures of the body: the varieties consisting principally of modifications of two chief kinds of mesh, the rounded and the elongated. That kind in which the meshes or interspaces have a roundish form is the most common, and prevails in those parts in which the capillary net- work is most dense, such as the lungs (Fig, 106), most glands, and mucous membranes, and the cutis. The meshes of this kind of network FIG. 106. Fio. 107. FIG. 106.— Network of capillary vessels of the air-cells of the horse's lung magnified. capillaries proceeding from 6, 6, terminal branches of the pulmonary artery. < Frey .) FIG. 107.— Injected capillary vessels of muscle seen with a low magnifying a, a, (Sharpey.) are not quite circular, but more or less angular, sometimes presenting a nearly regular quadrangular or polygonal form, but being more fre- quently irregular. The capillary network with elongated meshes (Fig. 107) is observed in parts in which the vessels are arranged among bundles of fine tubes or fibres, as in muscles and nerves. In such parts, the meshes usually have the form of a parallelogram, the short sides of which may be from three to eight or ten times less than the long ones; the long sides always corresponding to the axis of the fibre or tube, by which it is placed. The appearance of both the rounded and elongated meshes is much varied according as the vessels composing them have a straight or tortuous form. Sometimes the capillaries have a looped arrangement, a single capillary projecting from the common network HANDBOOK OF PHYSIOLOGY. into some prominent organ, and returning after forming one or more loops, as in the papillae of the tongue and skin. The number of the capillaries and the size of the meshes in different parts determine in general the degree of vascularity of those parts. The parts in which the network of capillaries is closest, that is, in which the meshes or interspaces are the smallest, are the lungs and the choroid membrane of the eye. In the iris and ciliary body, the interspaces are somewhat wider, }?et very small. In the human liver the interspaces are of the same size, or even smaller than the capillary vessels themselves. In the human lung they are smaller than the vessels; in the human kid- ney, and in the kidney of a dog, the diameter of the injected capillaries, compared with that of the interspaces, is in the proportion of one to four, or of one to three. The brain receives a very large quantity of blood; but the capillaries in which the blood is distributed through its substance are very minute, and less numerous than in some other parts. Their diameter, according to E. H. Weber, compared with the long diameter of the meshes, being in the proportion of one to eight or ten; compared with the transverse diameter, in the proportion of one to four or six. In the mucous membranes — for example in the conjunctiva and in the cutis vera, the capillary vessels are much larger than in the brain, and the interspaces narrower — namely, not more than three or four times wider than the vessels. In the periosteum the meshes are much larger. In the external coat of arteries, the width of the meshes is ten times that of the vessels (Henle). It may be held as a general rule, that the more active the functions of an organ are, the more vascular it is. Hence the narrowness of the interspaces in all glandular organs, in mucous membranes, and in grow- ing parts; their much greater width in bones, ligaments, and other very tough and comparatively inactive tissues; and the usually complete absence of vessels in cartilage, and such parts as those in which, prob- ably, very little vital change occurs after they are once formed. IV. The Veins. Distribution. — The venous system begins in small vessels which are slightly larger than the capillaries from which they spring. These ves- sels are gathered up into larger and larger trunks until they terminate (as regards the systemic circulation) in the two venas cavas and the coro- nary veins, which enter the right auricle, and (as regards the pulmonary circulation) in four pulmonary veins, which enter the left auricle. The total capacity of the veins diminishes as they approach the heart ; but, as a rule, their capacity exceeds by twice or three times that of their corresponding arteries. The pulmonary veins, however, are an excep- tion to this rule, as they do not exceed in capacity the pulmonary arte- ries. The veins are found after death as a rule to be more or less col- THE CIRCULATION" OF THE BLOOD. 113 lapsed, and often to contain blood. The veins are usually distributed in a superficial and a deep set which communicate frequently in their course. Fio 108 -Transverse section through a small artery and vein of the mucous membane of a ch!ld%^^?S?SSSrt between the thick- walle/artery and the thin^aUed vein is well shown A Artery the letter is placed in the lumen of the vessel, e. Endothehal cells with nuclei clearly vis- ible-these cells appear very thick from the contracted state of the vessel. Outside it a double wavy linl martS the efaPstfc tunica intima. m. Tunica media forming the chief part of artenal wall and consisting of unstriped muscular fibres circularly arranged: their nuclei are weU [seen, a Part of the tunica adventitia showing bundles of connective-tissue fibre in section, with the circular nuclei of the connective tissue corpuscles. This coat gradually merges into the surroundmg connective tissue, V. In the lumen of the vein. The other letters indicate the same as in the artery The muscular coat of the vein (m) is seen to be much thinner than that of the artery. X 350. (Klein and Noble Smith.) FIG. 109.— Diagram showing valves of veins. A, part of a vein laid open and spread out, with two pairs of valves. B, longitudinal section of a vein, showing the apposition of the edges of the valves in their closed state, c, portion of a distended vein, exhibiting a swelling in the situation of a pair of valves. /Structure. — In structure the coats of veins bear a general resemblance to those of arteries (Fig. 108). Thus, they possess an outer, middle, and internal coat. The outer coat is constructed of areolar tissue like that of the arteries, but is thicker. In some veins it contains muscular fibre- cells, which are arranged longitudinally. 8 114 HANDBOOK OF PHYSIOLOGY. The middle coat is considerably thinner than that of the arteries ; and, although it contains circular unstriped muscular fibres or fibre-cells, these are mingled with a larger proportion of yellow elastic and white fibrous tissue. In the large veins, near the heart, namely the vence cavcB and pulmonary veins, the middle coat is replaced, for some distance from the heart, by circularly arranged striped muscular fibres, continuous with those of the auricles. The internal coat of veins is less brittle than the corresponding coat of an artery, but in other respects resembles it closely. Valves. — The chief influence which the veins have in the circulation, is effected with the help of the valves, which are placed in all veins sub- ject to local pressure from the muscles between or near which they run. The general construction of these valves is similar to that of the semi- FIG. 110.— A, vein with valves open. B, vein with valves closed: stream of blood passing off by lateral channel. (Dalton.) lunar valves of the aorta and pulmonary artery, already described ; but their free margins are turned in the opposite direction, i.e., towards the heart, so as to stop any movement of blood backward in the veins. They are commonly placed in pairs, at various distances in different veins, but almost uniformly in each (Fig. 109). In the smaller veins, single valves are often met with ; and three or four are sometimes placed together, or near one another, in the largest veins, such as the subclavian, and at their junction with the jugular veins. The valves are semilunar; the unattached edge being in some examples concave, in others straight. They are composed of inextensile fibrous tissue, and are covered with endothelium like that lining the veins. During the period of their in- action, when the venous blood is flowing in its proper direction, they lie by the sides of the veins ; but when in action, they close together like the valves of the arteries, and offer a complete barrier to any backward THE CIRCULATION OF THE BLOOD. 115 movement of the blood (Figs. 109 and 110). Their situation in the su- perficial veins of the fore-arm is readily discovered by pressing along its surface, in a direction opposite to the venous current, i.e., from the el- bow towards the wrist ; when little swell- ings (Fig. 109 c) appear in the position of each pair of valves. These swellings at once disappear when the pressure is re- laxed. Valves are not equally numerous in all veins, and in many they are absent alto- gether. They are most numerous in the veins of the extremities, and more so in those of the leg than the arm. They are commonly absent in veins of less than a line in diameter, and, as a general rule, there are few or none in those which are not subject to muscular pressure. Among those veins which have no valves may be mentioned the superior and inferior vena cava, the trunk and branches of the portal vein, the hepatic and renal veins and the pulmonary veins; those in the interior of the cranium and vertebral col- umn, those of the bones, and the trunk and branches of the umbilical vein are also destitute of valves. Lymphatics of Arteries and Veins. — Lymphatic spaces are present in the coats of both arteries and veins; but in the tunica adventitia or external coat of large vessels they form a distinct plexus of more or less tubular vessels. In smaller vessels they appear as sinous spaces lined by endothelium. Sometimes, as in the arteries of the omentum, mesentery, and membranes of the brain, in the pulmonary, hepatic, and splenic arteries, the spaces are continuous with vessels which distinctly ensheath them — perivascular lymphatic sheaths (Fig. 111). Lymph channels are said to be present also in the tunica media. The Action of the Heart. The heart's action in propelling the blood consists in the successive alternate contraction (systole) and relaxation (diastole) of the muscular walls of its two auricles and two ventricles. 1. Action of the Auricles. — The description of the action of the heart may be commenced at that period in each action which immediately FIG. 111.— Surface view of an artery from the mesentery of a frog, en- sheathed in a perivascular lymphatic vessel, a. The artery, with its circular muscular coat (media) indicated by broad transverse markings, with an indication of the adventitia outside. I. Lymphatic vessel ; its wall is a simple endothelial membrane. (Klein and Noble Smith.) llfi HANDBOOK OF PHYSIOLOGY. precedes the beat of the heart against the side of the chest. At this period the whole heart is in a passive state, the walls of both auricles and ventricles are relaxed, and their cavities are becoming 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 corresponding 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; and at the end of the pause, they contract and expel 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 pro- pagated 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 quicken the flow of blood from them into the ven- tricles; the force of their contraction not being sufficient under ordinary circumstances to cause any back-flow in the veins. The reflux of blood into the great veins is moreover 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 muscular coats with which the large veins are provided near their entrance into the auricles. Any slight regurgitation from the right auricle is limited also by the valves at the junction of the subclavian and internal jugular veins, beyond which the blood cannot move backwards; and the coronary vein is preserved from it by a valve at its mouth. In birds and reptiles regurgitation from the right auricle is prevented by valves placed at the entrance of the great veins. During the auricular contraction the force of the blood propelled into the ventricle is transmitted in all directions, but being insufficient to separate the semilunar valves, it is expended in distending the ventricle, and, by a reflux of the current, in raising and gradually closing the au- riculo-ventricular valves, which, when the ventricle is full, form a com- plete septum between it and the auricle. 2. Action of the Ventricles.— 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 their diastole. Thus distended, they immediately contract: so immediately, indeed, that their systole looks as if it were continuous with that of the auricles. 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 contraction, a small quan- THE CIRCULATION OF THE BLOOD. 117 tity of blood remains. The shape of both ventricles during systole undergoes an alteration, the left probably not altering in length but to a certain degree in breadth, the diameters in the plane of the base being diminished. The right ventricle does actually shorten to a small extent. The systole has the effect of diminishing the diameter of the base, espe- cially in the plane of the auriculo-ventricular valves; but the length of the heart as a whole is not altered. (Ludwig. ) During the systole of the ventricles, too, the aorta and pulmonary artery, being filled with blood by the force of the ventricular action against considerable resist- ance, elongate as well as expand, and the whole heart moves slightly to- wards the right and forwards, twisting on its long axis, and exposing more of the left ventricle anteriorly than is usually in front. When the systole ends the heart resumes its former position, rotating to the left again as the aorta and pulmonary artery contract. Functions of the Valves of the Heart. — (1) The Auricula- Ventric- ular.— The distention of the ventricles with blood continues throughout the whole period of their diastole. The auriculo-ventricular valves are gradually brought into place by some of the blood getting behind the cusps and floating them up; and by the time that the diastole is complete, the valves are no doubt in apposition, the completion of this being brought about by the reflux current caused by the systole of the auricles. This elevation of the auriculo-ventricular valves is materially aided by the action of the elastic tissue which has been shown to exist so largely in their structure, especially on the auricular surface. At any rate at the commencement of the ventricular systole they are completely closed. It should be recollected that the diminution in the breadth of the base of the heart in its transverse diameters during ventricular systole is es- pecially marked in the neighborhood of the auriculo-ventricular rings, and this aids in rendering the auriculo-ventricular valves competent to close the openings, by greatly diminishing their diameter. The margins of the cusps of the valves are still more secured in apposition with an- other, by the simultaneous contraction of the musculi papillares, whose chordae tendineaB have a special mode of attachment for this object (p. 104). The cusps of the auriculo-ventricular valves meet not by their edges only, but by the opposed surfaces of their thin outer borders. The form and position of the fleshy columns of the internal walls of the ventricle no doubt help to produce the obliteration of the ventric- ular cavity during 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 rigor mortis is very marked (Fig. 91). In such a case only a central fissure may be discernible to the eye in the place of the cavity of each ventricle. If there were only circular fibres forming the ventricular wall, it is evident that on systole the ventricle would elongate ; if there were only 118 HANDBOOK OF PHYSIOLOGY. longitudinal fibres the ventricle would shorten on systole ; but there are both. The tendency to alter in length is thus counter-balanced, and the whole force of the contraction is expended in diminishing the cavity of the ventricle ; or, in other words, in expelling its contents. On the conclusion of the systole the ventricular walls tend to expand by virtue of their elasticity, and a negative pressure is set up, which tends to suck in the blood. This negative or suctional pressure on the left side of the heart is of the highest importance in helping the pul- monary circulation. It has been found to be equal to 23 mm. of mer- cury, and is quite independent of the aspiration or suction power of the thorax, which will be described in the chapter on Kespiration. Function of the Musculi Papillares. — The special function of the musculi papillares is to prevent the auriculo-ventricular valves from be- ing everted into the auricle. For the chordas tendineae might allow" the valves to be pressed back into the auricle, were it not that when the wall of the ventricle is brought by its contraction nearer the auriculo-ventric- ular orifice, the musculi papillares more than compensate for this by their own contraction — holding the cords tight, and, by pulling down the valves, adding slightly to the force with which the blood is expelled. What has been said applies equally to the auriculo-ventricular valves on both sides of the heart, and of both alike the closure is generally complete every time the ventricles contract. But in some circumstances the closure of the tricuspid valve is not complete, and a certain quantity of blood is forced back into the auricle. This has been called the safe- ty-valve action of this valve. The circumstances in which it usually happens are those in which the vessels of the lung are already full enough when the right ventricle contracts, as e. g., in certain pulmonary diseases, in very active exertions, and in great efforts. In these cases, the tricuspid valve does not completely close, and the regurgitation of the blood may be indicated by a pulsation in the jugular veins synchro- nous with that in the carotid arteries, (2) Of the Semilunar Valves.— The arterial or semilunar valves are forced apart by the out-streaming blood, with which the contracting ventricle dilates the large arteries. The dilatation of the arteries is, in a peculiar manner, adapted to bring the valves into action. The lower borders of the semilunar valves are attached to the inner surface of the tendinous ring, which is, as it were, inlaid at the orifice of the artery, between the muscular fibres of the ventricle and the elastic fibres of the walls of the artery. The tissue of this ring is tough, and does not admit of extension under such pressure as it is commonly exposed to ; the valves are equally inextensile, being, as already mentioned, formed mainly of tough, close-textured, fibrous tissue, with strong interwoven cords. Hence, when the ventricle propels blood through the orifice and into the canal of the artery, the lateral pressure which it exercises is THE CIRCULATION OF THE BLOOD. 119 sufficient to dilate the walls of the artery, but not enough to stretch in an equal degree, if at all, the unyielding valves and the ring to which their lower borders are attached. The effect, therefore, of each such propulsion of blood fr jm the ventricle is, thafc the wall of the first por_ FIG. 112. -Sections of aorta, to show the action of the semilunar valves. A is intended to show the valves, represented by the dotted lines, lying near the arterial walls, represented by the continuous outer line. B (after Hunter) shows the arterial wall distended into three pouches (a), and drawu away from the valves, which are straightened into the form of an equilateral triangle, as represent- ed by the dotted lines. tion of the artery is dilated into three pouches behind the valves, while the free margins of the valves are drawn inward towards its centre (Fig. 112 B). Their positions may be explained by the diagrams, in which FIG. 113.— View of the base of the ventricular part of the heart, showing the relative position of the arterial and auriculo- ventricular orifices.— %. The muscular fibres of the ventricles are exposed by the removal of the pericardium, fat, blood-vessels, etc. ; the pulmonary artery and aorta have been removed by a section made immediately beyond the attachment of the semilunar v'ves, and the auricles have been removed immediately above the auriculo-ventricular orifices. The semilunar and auriculo-ventricular valves are in the nearly closed condition. 1, 1, the base of the right ven- tricle; 1', the conus arteriosus; 2, 2, the base of the left ventricle; 3, 3, the divided wall of the right auricle; 4, thatof the left; 5, 5'. 5", the tricuspid valve; 6, 6', the mitral valve. In the angles be- tween these segments are seen the smaller fringes frequently observed; 7, the anterior part of the pulmonary artery; 8, placed upon the posterior part or the root of the aorta; 9, the right, 9', the left coronary artery. (Allen Thomson.) the continuous lines represent a transverse section of the arterial walls, the dotted ones the edges of the valves, firstly, when the valves are near- est to the walls (A), as in the dead heart, and, secondly, when, the walls being dilated, the valves are drawn away from them (B). 120 HANDBOOK OF PHYSIOLOGY. This position of the valves and arterial walls is retained so long as the ventricle continues in contraction : but, as soon as it relaxes, and the dilated arterial walls can recoil by their elasticity, the blood is forced backwards towards the ventricles as onwards in the course of the circu- lation. Part of the blood thus forced back lies in the pouches (sinuses of Valsalva) (a, Fig. 112, B) between the valves and the arterial walls ; and the valves are by it pressed together till their thin lunated margins meet in three lines radiating from the centre to the circumference of the artery (7 and 8, Fig. 113). The contact of the valves in this position,' and the complete closure of the arterial orifice, are secured by the peculiar construction of their borders before mentioned. Among the cords which are interwoven in the substance of the valve, are two of greater strength and prominence than the rest ; of which one extends along the free border of each valve, and the other forms a double curve or festoon just below the free border. Each of these cords is attached by its outer extremities to the outer end of the free margin of its valve, and in the middle to the corpus Arantii; they thus inclose a lunated space ,_ j from a line to a line and a half in width, in which space the substance of the valve is much thinner and more pliant than elsewhere. FIG. 114. -vertical section When the valves are pressed down, all these through the aorta at its junction r . with the left ventricle, a, Sec- parts or spaces of their surfaces come into tion of aorta. 66, Section of two r valves, c, section of wall of ven- contact, and the closure of the arterial orifice tricle. d, Internal surface of ventricle. is thus secured by the apposition not of the mere edges of the valves, but of all those thin lunated parts of each which lie between the free edges and the cords next below them. These parts are firmly pressed together, and the greater the pressure that falls on them the closer and more secure is their apposition. The corpora Arantii meet at the centre of the arterial orifice when the valves are down, and they probably assist in the closure ; but they are not essential to it, for, not unfrequently, they are wanting in the valves of the pulmonary artery, which are then extended in larger, thin, flapping margins. In valves of this form, also, the inlaid cords are less distinct than in those with corpora Arantii ; yet the closure by contact of their surfaces is not less secure. It has been clearly shown that this pressure of the blood is not en- tirely sustained by the valves alone, but in part by the muscular substance of the ventricle (Savory). By making vertical sections (Fig. 114) through various parts of the tendinous rings it is possible to show clearly that the aorta and pulmonary artery, expanding towards their termina- THE CIRCULATION OF THE BLOOD. 121 tion, are situated upon the outer edge of the thick upper border of the ventricles, and that consequently the portion of each seinilunar valve adjacent to the vessel passes over and rests upon the muscular substance — being thus supported, as it were, on a kind of muscular floor formed by the upper border of the ventricle. The result of this arrangement is that the reflux of the blood is most efficiently sustained by the ventricu- lar wall. 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 the great venous trunks. Indeed, a chief func- tion of the auricles is to form a receptacle for the on-streaming blood during the ventricular contraction. 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 ven- tricles are thus from one-third to two-thirds full, the auricles are dis- tended : these, then suddenly contracting, fill up the ventricles, as already described (p. 116). Cardiac Cycle. — If we suppose a cardiac cycle divided into five parts, one of these will be occupied by the contraction of the auricles, two by that of the ventricles, and two by repose of both auricles and ventricles. Contraction of Auricles, . . . 1 + Eepose of Auricles, . 4 = 5 " Ventricles, . . 2 + " " Ventricles, . 3 = 5 Eepose (no contraction of either auricles or ventricles), . . . 2 + Contraction (of either au- ricles or ventricles), . 3 = 5 5 If the speed of the heart be quickened, the time occupied by each cardiac revolution is of course diminished, but the diminution affects only the diastole and pause. The systole of the ventricles occupies very much the same time, about T4F sec., whatever the pulse-rate. 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 dilation and distention of the ventricles, the latter valves are shut, the former open. Thus whenever the auriculo-ventricular valves are open, the arterial valves are closed and vice versa. The Sounds of the Heart. When the ear is placed over the region of the heart, two sounds may be heard at every beat of the heart, which follow in quick succession, and are succeeded by a pause or period of silence. The first sound is 122 HANDBOOK OF PHYSIOLOGY. dull and prolonged; its commencement coincides with the movement or impulse of the heart against the chest wall, and just precedes the pulse at the wrist. The second is a shorter and sharper sound, with a some- what napping character; and follows close after the arterial pulse. The period of time occupied respectively by the two sounds taken together, and by the pause, are almost exactly equal. The relative length of time occupied by each sound, as compared with the other, is a little uncer- tain. The difference may be best appreciated by considering the differ- ent forces concerned in the production of the two sounds. In one case there is a strong, comparatively slow, contraction of a large mass of muscular fibres, urging forward a certain quantity of fluid against con- siderable resistance; while in the other it is a strong but shorter and sharper recoil of the elastic coat of the large arteries — shorter because there is no resistance to the flapping back of the semilunar valves, as there was to their opening. The sounds may be expressed by saying the words lilbb — diip (C. J. B. Williams). The events which correspond, in point of time, with the first sound are (1) the contraction of the ventricles, (2) the first part of the dilata- tion of the auricles, (3) the tension of the auriculo- ventricular valves, (4) the opening of the semilunar valves, and (5) the propulsion of blood into the arteries. The sound is succeeded, in about one-thirtieth of a second, by the pulsation of the facial arteries, and in about one-sixth of a second, by the pulsation of the arteries at the wrist. The second sound, in point of time, immediately follows the cessation of the ventricular contraction, and corresponds with (a) the tension of the semi- lunar valves, (b) the continued dilatation of the auricles, (c) the com- mencing dilatation of the ventricles, and (d) the opening of the auriculo- ventricular valves. The pause immediately follows the second sound, and corresponds in its first part with the completed distention of the auricles, and in its second with their contraction, and the completed distention of the ventricles; the auriculo-ventricular valves being, all the time of the pause, open, and the arterial valves closed. Causes. — The exact causes of the first sound of the heart are not exactly known. Two factors probably enter into it, viz., the vibration of the auriculo-ventricular valves and chordae tendinese, due to their stretching, and also, but to a less extent, of the ventricular walls, and coats of the aorta and pulmonary artery, all of which parts are suddenly put into a state of tension at the moment of ventricular contraction; and secondly the muscular sound produced by contraction of the mass of muscular fibres which form the ventricle. The first factor is probably the more important. The cause of the second sound is more simple than that of the first. It is probably due entirely to the vibration consequent on the sudden closure of the semilunar valves when they are pressed down across the THE CIRCULATION OF THE BLOOD. 123 orifices of the aorta and pulmonary artery. The influence of the valves in producing the sound is illustrated by the experiment performed on large animals, such as calves, in which the results could be fully appre- ciated. In thesa experiments two delicate curved needles were inserted, one into the aorta, and another into the pulmonary artery, below the line of attachment of the semilunar valves, and, after being carried upwards about half an inch, were brought out again through the coats of the respective vessels, so that in each vessel one valve was included between the arterial walls and the wire. Upon applying the stethoscope to the vessels, after such an operation, the second sound had ceased to be audi- ble. Disease of these valves, when so extensive as to interfere with their efficient action, also often demonstrates the same fact by modifying or destroying the distinctness of the second sound. One reason for the second sound being a clearer and sharper one than the first may be, that the semilunar valves are not covered in by the thick layer of fibres composing the walls of the heart to such an extent as are the auricula-ventricular. It might be expected therefore that their vibration would be more easily heard through a stethoscope applied to the walls of the chest. The contraction of the auricles which takes place in the end of the pause is inaudible outside the chest, but may be heard, when the heart is exposed and the stethoscope placed on it, as a slight sound preceding and continued into the louder sound of the ventricular contraction. The Impulse of the Heart. At the commencement of each ventricular contraction, the heart may be felt to beat with a slight shock or impulse against the walls of the chest. The force of the impulse, and the extent to which it may be perceived beyond this point, vary considerably in different individuals, and in the same individual under different circumstances. It is felt more distinctly, and over a larger extent of surface, in emaciated than in fat and robust persons, and more during a forced expiration than in a deep inspiration; for, in the one case, the intervention of a thick layer of fat or muscle between the heart and the surface of the chest, and in the other the inflation of the portion of lung which overlaps the heart, prevents the impulse from being fully transmitted to the surface. An excited action of the heart, and especially a hypertrophied condition of the ventricles, will increase the impulse; while a depressed condition, or an atrophied state of the ventricular walls, will diminish it. Cause of the Impulse. — During the period which precedes the ven- tricular systole, the apex of the heart is situated upon the diaphragm and against the chest-wall in the fifth intercostal space. When the ven- tricles contract, their walls become hard and tense, since to expel their HANDBOOK OF PHYSIOLOGY. contents into the arteries is a distinctly laborious action, as it is resisted by the elasticity of the vessels. It is to this sudden hardening that the impulse of the heart against the chest- wall is due, and the shock of the sudden tension may be felt not only externally, but also internally, if the abdomen of an animal be opened and the finger be placed upon the under surface of the diaphragm, at a point corresponding to the under surface of the ventricle. The shock is felt, and possibly seen more dis- tinctly because of the partial rotation of the heart, already spoken of, along its long axis towards the right. The movement produced by the ventricular contraction against the chest-wall may be registered by means of an instrument called the cardiograph, and it will be found to corre- spond almost exactly with a tracing obtained by the same instrument applied over the contracting ventricle itself. The Cardiograph (Fig. 115) consists of a cup-shaped metal box over the open front of which is stretched an elastic india-rubber mem- brane, upon which is fixed a small knob of hard wood or ivory. This knob, however, may be attached instead, as in the figure, to the side of the bt>x by means of a spring, and may be made to act upon a metal disc attached to the elastic membrane. The knob (A) is for application to the chest-wall over the place of the greatest im- pulse of the heart. The box or tympanum communicates by means of an air-tight elastic tube (/) with the interior of a second tym- panum (Fig. 116, b), in connection with which is a long and light lever (a). The shock of the heart's impulse being communi- cated to the ivory knob, and through it to the first tympanum", the effect is, of course, at once transmitted by the column of air in the elastic tube to the interior of the second tympanum, also closed, and through the elastic and movable lid of the latter to the lever, which is placed in connection with a registering apparatus. This generally consists of a cylinder or drum covered with smoked paper, Tevolving according to a definite velocity by clock-work. The point of FIG. 115. -Cardiograph. (San derson's.) Fia. 116 — Marey's Tambour (6), to which the movement of the column of air in the first tympa- num is conducted by the tube, /, and from which it is communicated by the lever a, to a revolving •cylinder, so that the tracing of the movement of the impulse beat is obtained. THE CIRCULATION OF THE BLOOD. 125 the lever writes upon the paper, and a tracing of the heart's impulse or cardiogram, is thus obtained. By placing three small india-rubber air-bags or cardiac sounds in the interior respectively of the right auricle, the right ventricle, and in an intercostal space in front of the heart of living animals (horse), and placing these bags, by means of long narrow tubes, in communication with three levers, arranged one over the other in connection with a re- gistering apparatus (Fig. 117), MM. Chauveau and Marey have been able FIG. 117.— Apparatus of MM. Chauveau and Marey for estimating the variations of endocardia! pressure, and production of impulse of the heart. to record and measure with much accuracy the variations of the endocar- dial pressure and the comparative duration of the contractions of the auricles and ventricles. By means of the same apparatus, the synchron- ism of the impulse with the con- traction of the ventricles, is also well shown; and the causes of the several vibrations of which it is really com- posed, have been demonstrated. In the tracing (Fig. 118), the inter- vals between the vertical lines repre- sent periods of a tenth of a second. The parts on which any given vertical line falls represent simultaneous events. It will be seen that the contraction of the auricle, indicated by the marked curve at A in first tracing, causes a slight increase of pressure in the ven- tricle, which is shown at A' in the sec- ond tracing, and produces also a slight impulse, which is indicated by A" in the third tracing. The closure of the semilunar valves causes a momentarily increased pressure in the ventricle at D' affects the pressure in the auri- cle D, and is also shown in the tracing of the impulse also, D". The large curve of the ventricular and the impulse tracings, between A' and D', and A" and D", are caused by the ventricular contraction, while the smaller undulations, between B and c, B' and c', B" andc", are FIG. 118.— Tracings of (1), Intra-auri- cular, and (2), Intra- ventricular pressures, and (3), of the impulse of the heart, to be read from left to right, obtained by Chau- veau and Marey's apparatus. 126 HANDBOOK OF PHYSIOLOGY. caused by the vibrations consequent on the tightening and closure of the auriculo-ventricular valves. The method thus described may, as a rule, demonstrate quite cor- rectly the variations of endocardial pressure, and these variations only, but there is a danger lest the muscular walls should grip the air-bags, even after the complete expulsion of the fluid contents of the chamber, and if so the lever would remain at its highest point for too long a time. The highest curve under such circumstances would represent on the tracing not only, as it ought to do, the endocardiac pressure, but also in addition the muscular pressure exerted upon the cardiac sound itself. (M. Foster.) Frequency and Force of the Heart's Action. The heart of a healthy adult man contracts from seventy to seventy- five times in a minute ; but many circumstances cause this rate, which of course corresponds with that of the arterial pulse, to vary even in health. The chief are age, temperament, sex, food and drink, exercise, time of day, posture, atmospheric pressure, temperature. (1.) Age. — The frequency of the heart's action gradually diminishes from the commencement to near the end of life, but is said to rise again somewhat in extreme old age, thus :— Before birth the average number of pulsations per minute is 150 Just after birth, from 140 to 130 During the first year, , . . . 130 to 115 During the second year, .... 115 to 100 During the third year, 100 to 90 About the seventh year, .... 90 to 85 About the fourteenth year, the average number of pulses in a minute is from . . . 85 to 80 In adult age, 80 to 70 In old age, 70 to 60 In decrepitude, 75 to 65 (2.) Temperament and Sex. — In persons of sanguine temperament, the heart acts somewhat more frequently than in those of the phleg- matic ; and in a female sex more frequently than in the male. (3 and 4.) Food and Drink, Exercise. — After a meal the heart's ac- tion is accelerated, and still more so, during bodily exertion or mental excitement ; it is slower during sleep. (5.) Diurnal Variation. — In the state of health, the pulse is most frequent in the morning, and becomes gradually slower as the day ad- vances : and that this diminution of frequency is both more regular and more rapid in the evening than in the morning. (6.) Posture. — The pulse, as a general rule, especially in the adult male, is more frequent in the standing than in the sitting posture, and in the latter than in the recumbent position ; the difference being great- est between the standing and the sitting postures. The effect of change of posture is greater as the frequency of the pulse is greater, and, ac- cordingly, is more marked in the morning than in the evening. By supporting the body in different positions, without the aid of muscular THE CIRCULATION OF THE BLOOD. 127 effort of the individual, it has beeu proved that the increased frequency of the pulse in the sitting and standing positions is dependent upon the muscular exertion engaged in maintaining them ; the usual effect of these postures on the pulse being almost entirely prevented when the usually attendant muscular exertion was rendered unnecessary. (Gruy.) (7.) Atmospheric Pressure. — The frequency of the pulse increases in a corresponding ratio with the elevation above the sea. (8. ) Temperature. — The rapidity and force of the heart's contractions are largely influenced by variations of temperature. The frog's heart, when excised, ceases to beat if the temperature be reduced to 32°F. (o° C.). When heat is gradually applied to it, both the speed and force of the contractions increase till they reach a maximum. If the tempera- ture is still further raised, the beats become irregular and feeble, and the heart at length stands still in a condition of '* heat rigor." Similar effects are produced in warm-blooded animals. In the rabbit, the number of heart-beats is more than doubled when the temperature of the air was maintained at 105° F. (40°.5 C.). At 113°-L14° F. (45° C.), the rabbit's heart ceases to beat. Relative Frequency of the Heart's Contractions to the number of Res- pirations.— In health there is observed a nearly uniform relation between the frequency of the beats of the heart and of the respirations ; the pro- portion being, on an average, one respiration to three or four beats. The same relation is generally maintained in the cases in which the ac- tion of the heart is naturally accelerated, as after food or exercise; but in disease this relation usually ceases. In many affections accompanied with increased frequency of the heart's contraction, the respiration is, indeed, also accelerated, yet the degree of its acceleration may bear no definite proportion to the increased number of the heart's actions : and in many other cases, the heart's contraction becomes more frequent with- out any accompanying increase in the number of respirations ; or, the respiration alone may be accelerated, the number of pulsations remain- ing stationary, or even falling below the ordinary standard. The Force of the Ventricular Action. — The force of the left ven- tricular systole is more than double that exerted by the contraction of the right ventricle : this difference results from the walls of the left ventricle being about twice or three times as thick as those of the right, And the difference is adapted to the greater degree of resistance which the left ventricle has to overcome, compared with that to be overcome by the right : the former having to propel blood through every part of the body, the latter only through the lungs. The actual amount of the intra-ventricular pressures during systole in the dog has been found to be 2.4 inches (60 mm.) of mercury in the right ventricle, and 6 inches (150 mm.) in the left. During diastole there is in the right ventricle a negative or suction pressure of about f of an inch ( — 17 to —16 mm.), and in the left ven- tricle from 2 inches to f of an inch ( — 52 to —20 mm.). Part of this 128 HANDBOOK OF PHYSIOLOGY. fall in pressure, and possibly the greater part, is to be referred to the influence of respiration ; but without this the negative pressure of the left ventricle caused by its active dilatation is about equal to f of an inch (23 mm.) of mercury. The right ventricle is undoubtedly aided by this suction power of the left, so that the whole of the work of conducting the pulmonary circu- lation does not fall upon the right side of the heart, but is assisted by the left side. The Force of the Auricular Contractions. — The maximum pressure within the right auricle is about £ of an inch (20 mm. ) of mer- cury, and is probably somewhat less in the left. It has been found that during diastole the pressure within both auricles sinks considerably be- low that of the atmosphere ; and as some fall in pressure takes place, even when the thorax of the animal operated upon has been opened,, a certain proportion of the fall must be due to active auricular dilatation independent of respiration. In the right auricle, this negative pressure is about —10 mm. Work Done by the Heart. — In estimating the work done by any machine it is usual to express it in terms of the " unit work/' In Eng- land, the unit of work is the "foot-pound," and is defined to be the energy expended in raising a unit of weight (1 Ib.) through a unit of height (1 ft.) : in France, the " kilogram-metre." The work done by the heart at each contraction can be readily found by multiplying the weight of the blood expelled by the ventricles by the height to which the blood rises in a tube tied into an artery. This height was found to be about 9 ft. in the horse, and this estimate is nearly correct for a large artery in man. Taking the weight of blood expelled from the left ventricle at each systole at 6 oz., i. e., f Ib., we have 9 X |=3.375 foot pounds as the work done by the left ventricle at each systole ; and adding to this the work done by the right ventricle (about one-third that of the left) we have 3.375 x 1.125=4.5 foot-pounds as the work done by the heart at each contraction. Other estimates give •J kilogram-metre, or about 3£ foot-pounds. Haughton estimates the total work of the heart in 24 hours as about 124 foot-tons. Influence of the Nervous System on the Action of the Heart. The hearts of warm-blooded animals cease to beat very soon after removal from the body, and are, therefore, unfavorable for the study of the nervous mechanism which regulates their action. The hearts of cold-blooded animals, therefore, e. g., the frog, tortoise, and snake, which will continue to beat under favorable conditions for many hours after removal from the body, are generally employed, as more conveni- ent for the purpose. Of these animals, the frog is the one most fre- THE CIRCULATION OF THE BLOOD. 129 quently used, and, indeed, until recently, it was from the study of the frog's heart that the chief part of our information on the subject was obtained. If removed from the body entire, the frog's heart will con- tinue to beat for many hours and even days, and the beat has no appa- rent difference from the beat of the heart before removal from the body; it will take place without the presence of blood or other fluid within its chambers. If the beats have become infrequent, an additional one may be induced by mechanically stimulating the heart by means of a blunt needle ; but the time before the stimulus applied produces its results (the latent period) is very prolonged, and as in this way the cardiac beat is like the contraction of unstriped muscle, it has been likened to a peri- staltic contraction. There is much uncertainty about the nervous mechanism of the beat FIG. 119A. FIG. 119B. FIG. 1 19A.— The Heart of a Frog CRana esculenta '» from the front. V, ventricle ; Ad, right auricle ; As, left auricle; B, bulbus arteriosus dividing into right and left aortae. (Ecker. FIG. 119B. - The Heart of a Frog (Rana esculenta) from the back, s.v., sinus venosus opened; c.s.s., left vena cava superior; c.s.d., right vena cava superior; c.t., vena cava inferior; v.p., vena pulmonales; A.d., right auricle; A.S., left auricle; A .p., opening of communication between the right auricle and the sinus venosus. x 2/4 — 3. (Ecker j of the frog's heart, but what has just been said shows, at any rate, two things : firstly, that as the heart will beat when removed from the body in a way differing not all from the normal, it must contain within itself the mechanism of rhythmical contraction ; and, secondly, that as it can beat without the presence of fluid with its chambers, the movement can- not depend solely on reflex excitation ly the entrance of blood. The nervous apparatus existing in the heart itself has been found to consist of collections of microscopic ganglia, and of nerve-fibres proceed- ing from them. These ganglia are demonstrable as being collected chiefly into three groups: one is in the wall of the sinus veuosus at the 9 130 HANDBOOK OF PHYSIOLOGY. junction of the sinus with the auricles (RemaVs); a second, -near the junction between the auricles and ventricle (Bidder's)', and the third in the septum between the auricles. It is generally believed that the rhythmical contractions of the frog's heart are, under ordinary circum- stances, closely associated with the ganglia. Thus, (1) if the heart be removed entire from the body, the sequence of the contraction of its sev- eral beats will take place with rhyth- mical regularity, viz., of the sinus venosus, the auricles, the ventricle, and bulbuw ^rteriosus, in order. (2) If the heart be removed at the junction of the sinus and auricle, the former, remaining in situ, will continue to beat, but the removed portion will for a short variable time stop beating, and when it resumes its beats, it will be with a different rhythm to that of the sinus; and, further, (3) if the ventricle only be removed, it will take a still long- er time before recommencing its pul- sation after its removal than the larger "portion consisting of the auricles and ventricle does in experiment (2), and its rhythm is different from that of the unremoved portion, and not so regular. It will not continue to pulsate so long; but during the period of stoppage a contraction will occur if it be mechanically or otherwise stimulated. (4) If the lower two-thirds or apex of the vetftricle be removed, the remainder of the heart will go on beating regularly in the body, but the part removed will remain motionless and will not beat spontaneously, although it will re- spond to stimuli by a single beat for each stimulus. (5) If the heart be divided lengthwise, its parts will continue to pulsate rhythmically, and the auricles may be cut up into pieces, and the pieces will continue their movements of rhythmical contraction. It will be thus seen that the rhythmical movements appear to be more marked in the parts supplied by the ganglia, and that the apical portion of the ventricle, in which the ganglia are not found, does not, under ordinary circumstances, possess the power of automatic movement. It has, however, been shown by Graskell that the extreme apex of the ventricle of the heart of the tortoise, which contains no ganglia, may under appropriate stimuli be made to contract rhythmically. This proves that the muscular tissue of the heart is capable of rhythmical contraction, but it does not prove that in the living animal the muscular FIG. 120.— Course of the nerves in the auricular partition wall of the heart of a frog, d, dorsal branch; v, ventral branch. (Ecker.) THE CIRCULATION OF THE BLOOD. 131 rhythm occurs without nervous stimulation, nor indeed is this at all likely. Inhibition of the Heart's Action. — Although, under ordinary con- ditions, the apparatus of ganglia and nerve-fibres in the substance of the heart forms the medium through which its action is excited and rhythmi- cally maintained, yet they, and through them, the heart's contractions, are regulated by nerves which pass to them from the higher nerve-cen- tres. These nerves are branches from the pneumogastric or vagus and the sympathetic. The influence of the vagi nerves over the heart beat may be shown by stimulating one (especially the right), or both of the nerves, when a record is being taken of the beats of the frog's heart. If a single induc- tion shock be sent into the nerve, the heart, as a rule after a short inter- val, ceases beating, but after the suppression of several beats resumes its action. As already mentioned, the effect of the stimulus is not immediately seen, and one beat may occur before the heart stops after the applica- tion of the electric current. The stoppage of the heart may occur apparently in one of two ways, either by diminishing the strength of the, systole or by increasing the length of the diastole (Figs. 121, 122) The stoppage of the heart may be brought about by the application of the electrodes to any part of the vagus, but FIG. 121.— Tracing showing the actions of the va- gus on the heart. Aur., auricular; Vent., ventricu- lar tracing. The part between perpendicular lines indicates period of vagus stimulation. C.8 indicates that the secondary coil was 8 c.ra. from the primary. The part of tracing to the left shows the regular con- tractions of moderate height before stimulation. Dur- ing stimulation and for some time after the beats of auricle and ventricle are arrested. After they com- mence again they are single at first, but soon acquire a much greater amplitude than before the application of the stimulus. (.From Brunton, after Gaskell.) most " effectually if lihey are applied near the position of Kemak's ganglia. It is supposed that the fibres of the vagi, therefore, terminate there in the ganglia in the heart - walls, and that the inhibition of the heart's beats by means of the vagus is not a direct action, but that it is brought about indirectly by stimu- lating these centres in the heart itself. If this idea be correct, it may be supposed that the inhibitory centres are paralyzed by injection of atropine, as after this has been done no amount of stimulation of the vagus, or of the heart itself, will produce any effect upon the cardiac beats. Also that urari in large doses paralyzes the vagus fibres, but as the inhibitory action can be produced by direct stimulation of the heart, it is inferred that this drug does not paralyze the ganglia themselves. Muscarin and pilocarpine appear to produce effects similar to those ob- tained by stimulating the vagus fibres. They stimulate the inhibitory ganglia. " The remarkable effects of ligaturing the heart at various parts (Stan- 132 HANDBOOK OF PHYSIOLOGY. nius' experiments) however, complicate if they do not contradict the above explanation. If a ligature be tightly tied round the heart over the situation of the ganglia between the sinus and the auricles, the heart below the ligature stops beating. The ligature might be supposed to stimulate the inhibitory ganglia, but for the remarkable fact that the ex- hibition of atropin does not interfere with the success of the experiment. Section of the heart at the f™ situation ™ ha™ sf n ventricle without complete stoppage during irritation has (experiment xf, p. loO) a of the vagus. (From Brunton, after Gaskell.) similar effect to ligature. Again, if the ventricle be separated from the auricles by ligature or by section, it will recommence its pulsation and continue to beat rhythmically, but the auricles will con- tinue at a standstill. It has been suggested as an alternative explanation of these further experiments that the sinus contains the chief motor ganglia of the heart, and that from it as a rule proceed the impulses which cause the sequence of contraction of the other parts ; that the auricles contain inhibitory ganglia which are not sufficiently power- ful to prevent the motor impulses from the sinus ganglia, but that when their influence is removed by section, by ligature, or by excessive stimu- lation that the inhibitory ganglia are able to prevent the rhythmical con- traction of the auricles and ventricle, but that the ventricle contains in- dependent motor ganglia, since when it is removed from the influence of the inhibitory ganglia of the auricles, it recommences rhythmical pul- sation. Even if this theory cannot be absolutely maintained, yet it is evident that the power of spontaneous contraction is strongest in the siuus, less strong in the auricles, and less so still in the ventricle, and that, there- fore, the sinus ganglia are important in exciting the rhythmical contrac- tion of the whole heart. So far, the effect of the terminal apparatus of the vagi only has been considered; there is, however, no doubt that the vagi nerves are simply the media of an inhibitory or restraining influence over the action of the heart, which is conveyed through them from a centre in the medulla ob- longata which is always in operation, and, because of its restraining the heart's action, is called the cardio-inhibitory centre. For, on dividing these nerves, the pulsations of the heart are increased in frequency, an effect opposite to that produced by stimulation of their divided (peri- pheral) ends. The restraining influence of the centre in the medulla may be reflexly increased, so as to produce slowing or stoppage of the heart, through impulses from it passing down the vagi. As an example of the latter, the well-known effect on the heart of a violent blow on the THE CIRCULATION OF THE BLOOD. 133 •epigastrium may be referred to. The stoppage of the heart's action in this case, is due to the conveyance of the stimulus by fibres of the sym- pathetic (afferent) to the medulla oblongata, and its subsequent reflection through the vagi (afferent) to the inhibitory ganglia of the heart. It is also believed that the power of the medullary inhibitory centre may in a similar manner be reflexly lessened so as to produce accelerated action of the heart. Acceleration of the Heart's Action. — The heart receives an ac- celerating influence from the medulla oblongata through certain fibres of the sympathetic. These accelerating nerve-fibres, issuing from the spinal cord in the lower cervical and upper dorsal regions, reach the inferior cervical ganglion of the sympathetic, and pass thence to the cardiac plexus, and so to the heart. Their function is shown in the quickened pulsation which follows stimulation of the spinal cord, when the latter has been cut off from all connection with the heart, excepting by these accelerating filaments. Unlike the inhibitory fibres of the pneumogas- tric, the accelerating fibres are not continuously in action. The accelerator nerves must not, however, be considered as direct an- tagonists of the vagus ; for if at the moment of their maximum stimula- tion, the vagus be stimulated with minimum currents, inhibition is produced with the same readiness as if these were not acting. Nor is there any evidence that these fibres are constantly in action like those of the vagus. The connection of the heart with other organs by means of the ner- vous system, and the influence to which it is subject through them, are shown in a striking manner by the phenomena of disease. The influence of mental shock in arresting or modifying the action of the heart, the slow pulsation which accompanies compression of the brain, the irregu- larities and palpitations caused by dyspepsia or hysteria, are good evi- dence of the connection of the heart with other organs through the ner- vous system. Other Influences affecting the Action of the Heart. The healthy action of the heart no doubt very materially depends (1) upon a due supply of healthy Uood to its muscular tissue. It is not un- likely that the apparently contradictory effect of poisons may be ex- plained by supposing that the influence of some of them is either partially or entirely directed to the muscular tissue itself, and not to the nervous apparatus alone. As will be explained presently, the heart exercises a considerable influence upon the condition of the pressure of blood within the arteries but in its turn (2) the blood pressure within the arteries reacts upon the heart, and has a distinct effect upon its contractions, increasing by its 134 HANDBOOK OF PHYSIOLOGY. increase, and vice versa, the force of the cardiac beat, although the fre- quency is diminished as the blood-pressure rises. (3) The quantity (and quality?) of the blood contained in its chambers, too, has an influence upon its systole, and within normal limits the larger the quantity the stronger the contraction, Eapidity of systole does not of necessity indi- cate strength, as two weak contractions often do no more work than a strong and prolonged one. (4) In order that the heart may do its maxi- mum work, it must be allowed free space to act; for if obstructed in its action by mechanical outside pressure, as by an excess of fluid within the pericardium, such as is produced by inflammation, or by an over- loaded stomach, or the like, the pulsations become irregular and feeble. Functions of the Arteries. The External Coat. — The external coat forms a strong and tough investment, which, though capable of extension, appears principally de- signed to strengthen the arteries and to guard against their excessive distention by the force of the heart's action. It is this coat which alone prevents the complete severance of an artery when a ligature is tightly applied ; the internal and middle coats being divided. In it, too, the little vasa vasorum (p. 109) find a suitable tissue in which to subdivide for the supply of the arterial coats. The Elastic Tissue. — The purpose of the elastic tissue, which enters so largely into the formation of all the coats of the arteries, is, (a) to guard the arteries from the suddenly exerted pressure to which they are subjected at each contraction of the ventricles. In every such contraction, the contents of the ventricles are forced into the arteries more quickly than they can be discharged into and through the capilla- ries. The blood therefore, being, for an instant, resisted in its onward course, a part of the force with which it was impelled is directed against the sides of the arteries ; under this force their elastic walls dilate, stretching enough to receive the blood, and as they stretch, becoming more tense and more resisting. Thus, by yielding, they break the shock of the force impelling the blood. On the subsidence of the pressure, when the ventricles cease contracting, the arteries are able, by the same elas- ticity, to resume their former calibre, (b.) It equalizes the current of the blood by maintaining pressure on it in the arteries during the periods at which the ventricles are at rest or dilating. If the arteries had been rigid tubes, the blood, instead of flowing, as it does, in a con- stant stream, would have been propelled through the arterial system in a series of jerks corresponding to the ventricular contractions, with in- tervals of almost complete rest during the inaction of the ventricles. But in the actual condition of the arteries, the force of the successive contractions of the ventricles is expended partly in the direct propulsion THE CIRCULATION OF THE BLOOD. 135 of the blood, and partly in the dilatation of the elastic arteries ; and in the intervals between the contractions of the ventricles, the force of the recoil is employed in continuing the same direct propulsion. Of course the pressure they exercise is equally diffused in every direction, and the blood tends to move backwards as well as onwards, but all movement backwards is prevented by the closure of the aortic semi-lunar valves (p. 104), which takes place at the very commencement of the recoil of the arterial walls. By this exercise of the elasticity of the arteries, all the force of the ventricles is expended upon the circulation ; for that part of their force which is used in dilating the arteries, is restored in full when they recoil. There is thus no loss of force ; but neither is there any gain, for the elastic walls of the artery cannot originate any force for the propulsion of the blood — they only restore that which they received from the ven- tricles. The force with which the arteries are dilated every time the ventricles contract, might be said to be received by them in store, to be all given out again in the next succeeding period of dilatation of the ventricles. It is by this equalizing influence of the successive branches of every artery that at length the intermittent accelerations produced in the arterial current by the action of the heart, cease to be observable, and the jetting stream is converted into the continuous and equable movement of the blood which we see in the capillaries and veins. In the production of a continuous stream of blood in the smaller arteries and capillaries, the resistance which is offered to the blood-stream in these vessels, is a necessary agent. Were there no greater obstacle to the es- cape of blood from the larger arteries than exists to its entrance into them from the heart, the stream would be intermittent, notwithstand- ing the elasticity of walls of the arteries. (c.) By means of the elastic and muscular tissue in their walls the arteries are enabled to dilate and contract readily in correspondence with any temporary increase or diminution of the total quantity of blood in the body ; and within a certain range of diminution of the quantity, still to exercise due pressure on their contents; (d.) The elastic tissue assists in restoring the normal state after diminution of its calibre, whether this has been caused by a contraction of the muscular coat, or the temporary application of a compressing force from without. This action is well shown in arteries which, having contracted by means of their muscular element, after death, regain their average patency on the cessation of post-mortem rigidity, (e.) By means of their elastic coat the arteries are enabled to adapt themselves to the different movements of the several parts of the body. The natural state of all arteries, in regard at least to their length, is one of tension — they are always more or less stretched, and ever ready to recoil by virtue of their elasticity, whenever the opposing force is re- 136 HANDBOOK OF PHYSIOLOGY. moved. The extent to which the divided extremities of arteries retract is a measure of this tension, not of their elasticity. (Savory.) The Muscular Coat. — The most important office of the muscular coat is, (1) that of regulating the quantity of blood to be received by each part or organ, and of adjusting it to the requirements of each, ac- cording to various circumstances, but, chiefly, according to the activity with which the functions of each are at different times performed. The amount of work done by each organ of the body varies at different times, and the variations often quickly succeed each other, so that, as in the brain, for example, during sleep and waking, within the same hour a part may be now very active and then inactive. In all its active exer- cise of function, such a part requires a larger supply of blood than is sufficient for it during the times when it is comparatively inactive. It is evident that the heart cannot regulate the supply to each part at dif- ferent periods ; neither could this be regulated by any general and uni- form contraction of the arteries ; but it may be regulated by the power which the arteries of each part have, in their muscular tissue, of con- tracting so as to diminish, and of passively dilating or yielding so as to permit an increase of the supply of blood, according to the requirements of the part to which they are distributed. And thus, while the ventricles of the heart determine the total quantity of blood, to be sent onwards at each contraction, and the force of its propulsion, and while the large and merely elastic arteries distribute it and equalize its stream, the smaller arteries, in addition, regulate and determine, by means of their muscular tissue, the proportion of the whole quantity of blood which shall be distributed to each part. It must be remembered, however, that this regulating function of the arteries is itself governed and directed by the nervous system i^see p. 145). Another function of the muscular element of the middle coat of ar- teries is (2), to co-operate with the elastic in adapting the calibre of the vessels to the quantity of blood which they contain. For the amount of fluid in the blood-vessels varies very considerably even from hour to hour, and can never be quite constant; and were the elastic tissue only pres- ent, the pressure exercised by the walls of the containing vessels on the contained blood would be sometimes very small, and sometimes inordi- nately great. The presence of a muscular element, however, provides for a certain uniformity in the amount of pressure exercised; and it is by this adaptive, uniform, gentle, muscular contraction, that the normal tone of the blood-vessels is maintained. Deficiency of this tone is the cause of the soft and yielding pulse, and its unnatural excess, of the hard and tense one. The elastic and muscular contraction of an artery may also be regarded as fulfilling a natural purpose when (3), the artery being cut, it first limits and then, in conjunction with the coagulated fibrin, arrests the THE CIRCULATION OF THE BLOOD. 137 escape of blood. It is only in consequence of such contraction and coagulation that we are free from danger through even very slight wounds; for it is only when the artery is closed that the processes for the more permanent and secure prevention of bleeding are established. (4) There appears no reason for supposing that the muscular coat as- sists, to more than a very small degree, in propelling the onward current of blood. (1.) "When a small artery in the living subject is exposed to the air or cold, it gradually but manifestly contracts. Hunter observed that the posterior tibial artery of a dog when laid bare, became in a short time so much contracted as almost to prevent the transmission of blood; and the observation has been often and variously confirmed. Simply elasticity could not effect this. (2.) When an artery is cut across, its divided ends contract, and the orifices may be completely closed. The rapidity and completeness of this contraction vary in different animals; they are generally greater in young than in old animals; and less, apparently, in man than in the lower animals. This contraction is due in part to elasticity, but in part, also, to muscular action; for it is generally increased by the application of cold, or of any simple stimulating substances, or by mechanically irri- tating the cut ends of the artery, as by picking or twisting them. (3.) The contractile property of arteries continues many hours after death, and thus affords an opportunity of distinguishing it from their elasticity. When a portion of an artery of a recently killed animal is exposed, it gradually contracts, and its canal may be thus completely closed; in this contracted state it remains for a time, varying from a few hours to two days; then it dilates again, and permanently retains the same size. The Pulse. If we place our fingers upon the radial artery at the wrist, or upon any artery of the body which is sufficiently superficial, we experience a sensation as if our fingers were alternately lifted or raised up from the artery and allowed to fall again, and this action is repeated very fre- quently in the course of a minute. In other words we feel the pulse of the artery. The pulse is generally described as an expansion of the artery pro- duced by the wave of blood set in motion by the injection of blood into the already full aorta at each ventricular systole. As the force of the left ventricle, however, is not expended in dilat- ing the aorta only, the wave of blood passes on, expanding the arteries as it goes, running as it were on the surface of the more slowly travelling blood already contained in them, and producing the pulse as it pro- ceeds. The distention of each artery increases both its length and its diam- eter. In their elongation, the arteries change their form, the straight ones becoming slightly curved, and those already curved becoming more 138 HANDBOOK OF PHYSIOLOGY. so; but they recover their previous form as well as their diameter when the ventricular contraction ceases, and their elastic walls recoil. The increase of their curves which accompanies thedistention of arteries, and the succeed ing recoil, maybe well seen in the prominent temporal artery of an old person. In feeling the pulse, the finger cannot distinguish the sensation produced by the dilatation from that produced by the elongation and curving; that which it perceives most plainly, however, is the dila- tation, or return, more or less, to the cylindrical form, of the artery which has been partially flattened by the finger. The pulse — due to any given beat of the heart — is not perceptible at the same moment in all the arteries of the body. Thus, it can be felt in the carotid a very short time before it is perceptible in the radial ar- tery, and in this vessel again before the dorsal artery of the foot. The delay in the beat is in proportion to the distance of the artery from the heart, but the difference in time between the beat of any two arteries never exceeds probably £ to -J of a second. A distinction must be carefully made between the passage of the wave along the arteries and the velocity of the stream (p. 156) of blood. Both wave and current are present; but the rates at which they travel are very different, that of the wave 16.5 to 33 feet per second (5 to 10 metres), being twenty or thirty times as great as that of the current. The Sphygmograph. — A great deal of light has been thrown on spRina ;§>- Fia. 123.— Diagram of the mode of action of the Sphygmograph. what may be called the form of the pulse wave by the Sphygmograph (Figs. 123 and 124). The principle on which it acts is very simple (see Fig. 123). The small button replaces the finger in the act of taking the pulse, and is made to rest lightly on the artery, the pulsations of which it is desired to investigate. The up-and-down movement of the button is communicated to the lever, to the hinder end of which is attached a slight spring, which allows the lever to move up, at the same that time it is just strong enough to resist its making any sudden jerk, and in the in- terval of the beats also to assist in bringing it back to its original position. For ordinary purposes the instrument is bound on the wrist (Fig. 124). It is evident that the beating of the pulse with the reaction of the spring will cause an up-and-down movement of the lever, the pen of THE CIRCULATION OF THE BLOOD. 139 which will write the effect on a smoked card, which is made to move by clockwork in the direction of the arrow. Thus a tracing of the pulse is FIG. 124.— The Sphygmograph applied to the arm. obtained, and in this way much more delicate effects can be seen than can be felt on the application of the finger. The tracing of the pulse (sphygmogram), obtained by the use of the sphygmograph, differs somewhat according to the artery upon which it is applied, but its general characters are much the same in all cases. It consists of: — A sudden upstroke (Fig. 125, A), which is somewhat higher and more abrupt in the pulse of the carotid and of other arteries near the heart than in the radial and other arteries more remote; and a gradual decline (B), less abrupt, and therefore taking a longer time than (A). It is seldom, however, that the decline is an uninterrupted fall; it is usually marked about half-way by a dis- tinct notch (c), called the dicrotic notch, which is caused by a second more or less marked ascent of the lever at that point by a second wave called the dicrotic wave (D); not unfrequently (in which case the tracing is said to have a double apex) there is also soon after the com- mencement of the descent a slight ascent previous to the dicrotic notch: this is called the pre- dicrotic ivave (c), and in addition there may be one or more slight ascents after the dicrotic, called post-dicrotic (E). The explanation of these tracings presents some difficulties, not, how- ever, as regards the two prirnary factors, viz., the Upstroke and down- stroke, because they are universally taken to mean the sudden injection of blood into the already full arteries, and that this passes through the artery as a wave and expands them, the gradual fall of the lever signi- fying the recovery of the arteries by their recoil. It may be demonstrated on a system of elastic tubes, where a syringe pumps in water at regular intervals, just as well as on the radial artery, or on a more complicated system of tubes in which the heart, the arteries, the capillaries and veins are represented, which is known as an arterial schema. If we place two FIG. 1 5 Diagram of pulse-tracing. A, upstroke; B, downstroke; c, pre-di- crotic wave; D, dicrotic; E, post-dicrotic wave. 140 HANDBOOK OF PHYSIOLOGY. or more sphygmographs upon such a system of tubes at increasing dis- tances from the pump, we may demonstrate that the rise of the lever commences first in that nearest the pump, and is higher and more sud- den, while at a longer distance from the pump the wave is less marked, and a little later. So in the arteries of the body the wave of blood gradually gets less and less as we approach the periphery of the arterial system, and is lost in the capillaries. By the sudden injection of blood two distinct waves are produced, which are called the tidal and percus- Fio. 126.— Diagram of the formation of the pulse-tracing. A, percussion wave; B, tidal wave; c, dicrotic wave. (Mahomed.) sion waves. The tidal wave occurs whenever fluid is injected into an elastic tube (Fig. 126, B), and is due to the expansion of the tube and its more gradual collapse. The percussion wave occurs (Fig. 126, A) when the impulse imparted to the fluid is more sudden; this causes an abrupt FIG. 127. -Pulse-tracing of radial artery, somewhat deficient in tone. (Sanderson.) upstroke of the lever, which then falls until it is again caught up perhaps by the tidal wave which begins at the same time, but is not so quick. In this way, generally speaking, the apex of the upstroke is double; the second upstroke, the so-called pre-dicrotic elevation of the lever, representing the tidal wave. The double apex is most marked in trac- ing from large arteries, especially when their tone is deficient. In tracings, on the other hand, from arteries of medium size, e. g., the THE CIRCULATION OF THE BLOOD. 141 radial, the upstroke is usually single. In this case the percussion-im- pulse is not sufficiently strong to jerk up the lever and produce an effect distinct from that of the systolic wave which immediately follows it, and which continues and completes the distention. In cases of feeble arterial tension, however, the percussion-impulse may be traced by the FIG. 128.~Pulse-tracing of radial artery, with double apex. (Sanderson.) sphygmograph, not only in the carotid pulse, but to a less extent in the radial also (Fig. 128). The interruptions in the downstroke are called the katacrotic waves, to distinguish them from an interruption in the upstroke, called the FIG. 129.— Anacrotic pulse from a case of aortic aneurism. A, anacrotic wave (or percussion wave). B, tidal or pre-dicrotic wave, continued rise in tension (or higher tidal wave). anacrotic wave, which is occasionally met with in cases in which the pre-dicrotic or tidal wave is higher than the percussion wave. There is considerable difference of opinion as to whether the dicrotic. wave is generally present in health, and also as to its cause. The balance of opinion, however, appears to be in favor of the belief that the dicrotic wave is present in health, although it may be very faint;, while in certain conditions not necessarily diseased, it becomes so marked as to be quite plain to the unaided finger. Such a pulse is called dicro- tic. Sometimes the dicrotic rise exceeds the initial upstroke, and the pulse is then called liy per dicrotic. As to the cause of dicrotism, one opinion (1) is that it is due to a re- covery of pressure during the elastic recoil, in consequence of a rebound from the periphery. It may indeed be produced on a schema by ob- structing the tube at a little distance beyond the spot where the sphygmo- graph is placed. Against this view, however, is the fact that the notch appears at about the same point in the downstroke in tracings from the carotid and from the radial, and not first in the radial tracing, as it should do, if this theory was correct, since that artery is nearer the periphery than the carotid, and as it does in the corresponding experiment with the arterial schema when the tube is obstructed. (2) The generally ac- 142 HANDBOOK OF PHYSIOLOGY. ccpted notion among clinical observers, is that the dicrotic wave is due to the rebound from the aortic valves which causes a second wave; but the question cannot be considered settled, and the presence of marked dicrotism in cases of hemorrhage, of anaemia, and of other weaken- ing conditions, as well as its pres- ence in cases of diminished pres- sure within the arteries, would imply that it might, at any rate sometimes, be due to the altered specific gravity of the blood within the vessels, either directly or through the indirect effect of these conditions on the tone of the arte- rial walls. Waves may be produced in any elastic tube when a fluid is being driven through it with an inter- mittent force, such waves being called waves of oscillation (M. Fos- ter). Their origin has received various explanations. In an arte- rial schema they vary with the spe- cific gravity of the fluid used, and with the kind of tubing, and may be therefore supposed to vary in the body with the condition of the blood and of the arteries. Some consider the secondary waves in the downstroke of a nor- mal tracing to be oscillation waves; but, as just mentioned, even if this be the case, as is most likely with post-dicrotic waves, the dicro- tic wave itself is almost certainly due to the rebound from the aortic valves. The anacrotic notch is usually associated with disease of the arteries, e. g., in atheroma and aneurism. The dicrotic notch is called diastolic or aortic, and in point of time indicates the closure of the aortic valves. Of the three main parts then of a pulse tracing, viz., the percussion wave, the tidal, and the dicrotic, the percussion wave is produced by sudden and forcible contraction of the heart, perhaps exaggerated by an excited action, and may be transmitted much more rapidly than the tidal wave, and so the two may be distinct ; frequently, however, they FIG. 130.— Diagrams of pulse curves with exaggeration of one or other of the three waves. A, percussion; B, tidal; c, dicrotic. 1, percus- sion wave very marked; 2, tidal wave sudden; 3, dicrotic pulse curve; 4, and 5, the tidal wave very exaggerated, from high tension. (Maho- med.) THE CIRCULATION OF THE BLOOD. 14°> are inseparable. The dicrotic wave may be as great or greater than the other two. According to Mahomed, the distinctness of the three waves depends upon the following conditions : — The percussion wave is increased by : — 1. Forcible contraction of the Heart ; 2. Sudden contraction of the Heart ; 3. Large volume of blood; 4. Fulness of vessel ; and diminished by the reversed conditions. The tidal wave is increased by: — 1. Slow and prolonged contraction of the Heart ; 2. Large volume of blood ; 3. Comparative emptiness of vessels; 4. Diminished outflow or slow capillary circulation ; and dimin- ished by the reverse conditions. The dicrotic wave is increased by :— 1. Sudden contraction of the Heart ; 2. Low blood pressure ; 3. Increased outflow or rapid capillary circulation ; 4. Elasticity of the aorta; 5. Eelaxationof muscular coat ; and diminished by the reversed conditions. One very important precaution in the use of the sphygmograph lies in the careful regulation of the pressure. If the pressure be too great, the characters of the pulse may be almost entirely obscured, or the ar- tery may be entirely obstructed, and no tracing is obtained ; and on the other hand, if the pressure be too slight, a very small part of the charac- ers may be represented on the tracing. The Pressure of the Blood within the Arteries (producing arterial tension). It will be understood from all that has been said about the ar- teries in a normal condition (a) that they are during life continually "on the stretch," even during the cardiac diastole, and that in con- sequence of the injection of more blood at each systole of the ven- tricle into the elastic aorta, that this stretched condition is exagge- rated each time the ventricle empties itself. This state of distention of the arteries is due to the pressure of blood within them, and arises in consequence of the resistance presented by the smaller arteries and capillaries (peripheral resistance) to the sudden emptying of the ar- terial system between the contractions of the ventricle. It is called the condition of arterial tension. It will be further understood (b) that, as the blood is forcibly injected into the already full arteries against their elasticity, it must be subjected to the pressure of the arterial walls, so that, when an artery is cut across, the blood is projected forwards by this force for a considerable distance. Thus, although the blood distends the arteries and produces tension, yet the elasticity of the arteries reacts upon the blood, and subjects it to pressure. We have therefore to remem- ber that we have to do with two things related but not identical, viz., the pressure which the blood exerts upon the arterial walls tending to stretch them, and the pressure to which the blood is subject by the arteries tend- 144 HANDBOOK OF PHYSIOLOGY. / ing to drive it on in the direction of least resistance. The only direction in which it can be driven is onwards towards the capillaries, and so the blood-pressure in the arteries is one of the great agents in maintaining the circulation. The relations which exist between the arteries and their contained blood are thus so obviously 's& importance to the carrying on of the qjjeulation, that it becomes neces- sary to be able to gauge the alterations in blood- pressure very accurately. This may be done by means of a mercurial manometer in the following way: — The short horizontal limb of this (Fig. 131) is connected, by means of an elastic tube and canula, with the interior of an artery; a solution of sodium or potassium carbonate being previously in- troduced into this part of the apparatus to prevent coagulation of the blood. The blood-pressure is thus communicated to the upper part of the mercurial column; and the depth to which the latter sinks, added to the height to which it rises in the other, will give the height of the mercurial column which the blood-pressure balances; the weight of the soda solution being subtracted. For the estimation of the arterial tension at any given moment, no further apparatus than this, which is called Poiseuilles's Jmmadynamo- meter, is necessary; but for noting the variations of pressure in the ar- terial system, as well as its absolute amount, the instrument is usually combined with a registering apparatus, and in this form is called a kymo- graph. The kymograph, invented by Ludwig, is composed of a haemadyna- mometer, the open mercurial column of which supports a floating piston and vertical rod, with short horizontal pen (Fig. 132). The pen is adjusted in contact with a sheet of paper, which is caused to move at a uniform rate by clockwork; and thus the up-and-down movements of the mercurial column, which are communicated to the rod and pen, are marked or registered on the moving paper, as in the registering appa- ratus of the sphygmograph, and minute variations are graphically re- corded (Fig. 134). For some purposes the spring kymograph of Fick (Fig. 135) is pref- erable to the mercurial kymograph. It consists of a hollow C-shaped spring, filled with fluid, the interior of which is brought into connection with the interior of an artery, by means of a flexible metallic tube and FIG. 131.— Diagram of mercu rial manometer. THE CIRCULATION OF THE BLOOD. canula. In response to the pressure transmitted to its interior, the spring, x %. FIG. 145.— Outline showing the general form of the larnyx, 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, epiglottis; a. points to the back of both the arytenoid cartilages, which are surmounted by thecornicula; c, the middle ridge on the back of the cricoid cartilage; tr, the posterior mem- branous part of the trachea; 6, 6', right and left bronchi. (Allen Thomson.) %. vical vertebra, to a point opposite the third dorsal vertebra, where it divides into the two bronchi, one for each lung (Fig. 144). It measures, on an average, four or four and a half inches in length, and from three- quarters of an inch to an inch in diameter. RESPIRATION. 171 Structure. — The trachea is essentially a tube of fibro-elastic mem- brane, within the layers of which are inclosed a series of cartilaginous rings, from sixteen to twenty in number. These rings extend only around the front and sides of the trachea (about two-thirds of its cir- cumference), and are deficient behind; the interval between their pos- terior extremities being bridged over by a continuation of the fibrous membrane in which they are inclosed (Fig. 144). The cartilages of the trachea and bronchial tubes are of the hyaline variety. Immediately within this tube, at the back, is a layer of unstriped FIG. 146.— Section of the trachea, a, columnar ciliated epithelium; 6 and c, proper structure of the mucous membrane, containing elastic fibres cut across transversely; d, submucous tissue containing mucous glands, e, separated from the hyaline cartilage, g, by a fine fibrous tissue, /; ft, external investment of fine fibrous tissue. (S. K. Alcock.) muscular fibres, which extends, transversely, between the ends of the cartilaginous rings to which they are attached, and opposite the intervals between them, also; their evident function being to diminish, when re- quired, the calibre of the trachea by approximating the ends of the car- tilages. Outside these are a few longitudinal bundles of muscular tissue, which, like the preceding, are attached both to the fibrous and cartila- ginous framework. 172 HANDBOOK OF PHYSIOLOGY. The mucous membrane consists of adenoid tissue, separated from the stratified columnar epithelium which lines it by a homogeneous base- ment membrane. This is penetrated here and there by channels which connect the adenoid tissue of the mucosa with the intercellular substance of the epithelium. The stratified columnar epithelium is formed of several layers of cells (Fig. 146), of which the most superficial layer is ciliated, and is often branched downwards to join connective-tissue cor- puscles, while between these branched cells are smaller elongated cells prolonged up towards the surface and down to the basement membrane. Beneath these are one or more layers of more irregularly shaped cells. In the deeper part of the mucosa are many elastic fibres between which lie connective-tissue corpuscles and capillary blood-vessels. Numerous mucous glands are situate on the exterior and in the sub- stance of the fibrous framework of the trachea ; their ducts perforating FIG 147.— Transverse section of a bronchus, about yf inch in diameter, e, Epithelium (ciliated^; immediately beneath il is the mucous membrane or internal fibrous layer, of varying thickness; m, muscular layer; s, m, submucous tissue; /, fibrous tissue; c, cartilage inclosed within the layers of fibrous tissue; g, mucous gland. (F. E. Schultze.) the various structures which form the wall of the trachea, and opening through the mucous membrane into the interior. The two bronchi into which the trachea divides, of which the right is shorter, broader, and more horizontal than the left (Fig. 144), resem- ble the trachea exactly in structure, and in the arrangement of their car- tilaginous rings. On entering the substance of the lungs, however, the rings, although they still form only larger or smaller segments of a circle, are no longer confined to the front and sides of the tubes, but are distributed impartially to all parts of their circumference. The bronchi divide and subdivide, in the substance of the lungs, into a number of smaller and smaller branches, which penetrate into every part of the organ, until at length they end in the smaller subdivisions of the lungs, called lobules. All the larger branches still have walls formed of tough membrane, containing portions of cartilaginous rings, by which they are held open, RESPIRATION. 173 and unstriped muscular fibres, as well as longitudinal bundles of elastic tissue. They are lined by mucous membrane, the surface of which, like that of the larynx and trachea, is covered with ciliated epithelium (Fig. 146). The mucous .membrane is abundantly provided with mucous glands. As the bronchi become smaller and smaller, and their walls thinner, the cartilaginous rings become scarcer and more irregular, until, in the smaller bronchial tubes, they are represented only by minute and scat- tered cartilaginous flakes. And when the bronchi, by successive branches are reduced to about ^ of an inch in diameter, they lose their cartilagi- nous element altogether, and their walls are formed only of a tough fibrous elastic membrane with circular muscular fibres ; they are still lined, however, by a thin mucous membrane, with ciliated epithelium, th& length of the cells bearing the cilia having become so far diminished that the cells are now almost cubical. In the smaller bronchi the circu- FIG, 148.— Transverse section of the chest. lar muscular fibres are more abundant than in the trachea and larger bronchi, and form a distinct circular coat. The Lungs and Pleurae. — The Lungs occupy the greater portion of the thorax. They are of a spongy elastic texture, and on section ap- pear to the naked eye as if they were in great part solid organs, except here and there, at certain points, where branches of the bronchi or air- tubes may have been cut across, and show, on the surface of the section, their tubular structure. In fact, however, the lungs are hollow organs, each of which communicates by a separate orifice with a common air- tube, the trachea. The Pleura. — Each lung is enveloped by a serous membrane — the vleura, one layer of which adheres closely to the surface, and provides it with its smooth and slippery covering, while the other adheres to the in- ner surface of the chest-wall. The continuity of the two layers, which 174 HANDBOOK OF PHYSIOLOGY. form a closed sac, as in the case of other serous membranes, will be best understood by reference to Fig. 148. The appearance of a space, how- ever, between the pleura which covers the lung (visceral layer), and that which lines the inner surface of the chest (parietal layer), is inserted in the drawing only for the sake of distinctness. These layers are, in health, everywhere in contact, one with the other ; and between them is only just so much fluid as will insure the lungs gliding easily, in their expansion and contraction, on the inner surface of the parietal layer, which lines the chest-wall. While considering the subject of normal respiration, we may discard altogether the notion of the existence of any space or cavity between the lungs and the wall of the chest. If, however, an opening be made so as to permit air or fluid to enter the pleural sac, the lung, in virtue of its elasticity, recoils, and a con- siderable space is left between it and the chest-wall. In other words, the natural elasticity of the lungs would cause them at all times to con- FKJ. 149.— Ciliary epithelium of the human trachea, o, Layer of longitudinally arranged elastic fibres; 6, basement membrane; r, deepest cells, circular inform; d, intermediate elongated cells; e, outermost layer of cells fully developed and bearing cilia. X 350. (Kolliker.) tract away from the ribs, were it not that t-he contraction is resisted by atmospheric pressure which bears only on the inner surface of the air-tubes and air-cells. On the admission of air into the pleural sac, at- mospheric pressure bears alike on the inner and outer surfaces of the lung, and their elastic recoil is thus no longer prevented. Structure of the Pleura and Lung. — The pulmonary pleura consists of an outer or denser layer and an inner looser tissue. The former or pleura proper consists of dense fibrous tissue with elastic fibres, covered by endothelium, the cells of which are large, flat, hyaline, and transparent when the lung is expanded, but become smaller, thicker, and granular when the lung collapses. In the pleura is a lymph-canalicular system ; and connective-tissue corpuscles are found in the fibrous tissue which forms its groundwork. The inner, looser, or subpleural tissue contains lamellae of fibrous connective tissue and connective-tissue corpuscles be- tween them. Numerous lymphatics are to be met with, which form a dense plexus of vessels, many of which contain valves. They are simple RESPIRATION. 175 endothelial tubes, and take origin in the lymph-canalicular system of the pleura proper. Scattered bundles of unstriped muscular fibre occur in the pulmonary pleura. They are are especially strongly developed on the anterior and internal surfaces of the lungs, the parts which move most freely in respiration : their function is doubtless to aid in expira- tion. The structure of the parietal portion of the pleura is very similar to that of the visceral layer. Each lung is partially subdivided into separate portions called lobes; the right lung into three lobes, and the left into two. Each of these lobes, again, is composed of a large- number of minute parts, called lob- ules. Each pulmonary lobule may be considered a lung in miniature, consisting, as it does, of a branch of the bronchial tube, of air-cells, blood-vessels, nerves, and lymphatics, with a sparing amount of areolar tissue. On entering a lobule, the small bronchial tube, the structure of FIG. 150. FIG. 151. FIG. 150.— Terminal branch of a bronchial tube, with its inf undibula and air cells, from the mar- gin of the lung of a monkey, injected with quicksilver, a, terminal bronchial twig; 6 6, inf undib- ula and air-cells, x 10. (F. E. Schulze.) FIG. 151. — Two small infundibula or groups of air-cells, a a, with air-cells, 6 6, and the ultimate bronchial tubes, c c, with which the air-cells communicate. From a new-born child. (Kolliker.) which has been just described (a, Fig. 151), divides and subdivides ; its walls at the same time becoming thinner and thinner, until at length they are formed only of a thin membrane of areolar and elastic tissue, lined by a layer of squamous epithelium, not provided with cilia. At the same time, they are altered in shape ; each of the minute terminal branches widening out funnel-wise, and its walls being pouched out ir- regularly info small saccular dilatations, called air-cells (Fig. 151, b). Such a funnel-shaped terminal branch of the bronchial tube, with its group of pouches or air-cells, has been called an infundibulum (Figs. 150, 151), and the irregular oblong space in its centre, with which the air-cells communicate, an intercellular passage. 170 HANDBOOK OF PHYSIOLOGY. The air-cells, or air vesicles, may be placed singly, like recesses from the intercellular passage, but more often they are arranged in groups or even in rows, like minute sacculated tubes ; so that a short series of ves- icles, all communicating with one another, open by a common orifice into the tube. The vesicles are of various forms, according to the mu- tual pressure to which they are subject ; their walls are nearly in contact, and they vary from ^ to -fa of an inch in diameter. Their walls are formed of fine membrane, similar to that of the intercellular passages, and continuous with it, which membrane is folded on itself so as to form a sharp-edged border at each circular orifice of communication between contiguous air-vesicles, orbetween the vesicles and the bronchial pas- FIG. 152.— From a section of the lung of a cat, stained with silver nitrate. A. D. Alveolar duct or intercellular passage. S. Alveolar septa. N. Alveoli or air-cells, lined with large flat, nucleated cells, with some smaller polyhedral nucleated cells. U. Unstriped muscular fibres. Circular mus- cular fibres are seen surrounding the interior of the alveolar duct, and at one part is seen a group of small polyhedral cells continued from the bronchus. (Klein and Noble Smith.) sages. Numerous fibres of elastic tissue are spread out between contig- uous air-cells, and many of these are attached to the outer surface of the fine membrane of which each cell is composed, imparting to it addi- tional strength, and the power of recoil after distention. The cells are lined by a layer of epithelium (Fig. 152), not provided with cilia. Out- side the cells, a network of pulmonary capillaries is spread out so dense- ly (Fig. 153), that the interspaces or meshes are even narrower than the vessels, which are, on an average, -^fan of an inch in diameter. Between the atmospheric air in the cells and the blood in these vessels, nothing intervenes but the thin walls of the cells and capillaries ; and the ex- RESPIRATION. posure of the blood to the air is the more complete, because the folds of membrane between contiguous cells, and often the spaces between the walls of the same, contain only a single layer of capillaries, both sides of which are thus at once exposed to the air. The air-vesicles situated nearest to the centre of the lung are smaller and their networks of capillaries are closer than those nearer to the cir- cumference. The vesicles of adjacent lobules do not communicate; and those of the same lobule or proceeding from the same intercellular pas- sage, do so as a general rule only near angles of bifurcation; so that, when any bronchial tube is closed or obstructed, the supply of air is lost for all the cells opening into it or its branches. Blood-supply. — The lungs receive blood from two sources, (a) the pulmonary artery, (t>) the bronchial arteries. The former conveys ven-< ous blood to the lungs for its arterialization, and this blood takes no FIG. 153.— Capillary network of the pulmonary blood-vessels in the human lung, x 60. (Kol- liker.) share in the nutrition of the pulmonary tissues through which it passes. (b) The branches of the bronchial arteries ramify for nutrition's sake in the walls of the bronchi, of the larger pulmonary vessels, in the inter- lobular connective tissue, etc. ; the blood of the bronchial vessels .being returned chiefly through the bronchial and partly through the pulmo- nary veins. Lywphatics.—The lymphatics are arranged in three sets :— 1. Irreg- ular lacunae in the walls of the alveoli or air-cells. The lymphatic ves- sels which lead from these accompany the pulmonary vessels towards the root of the lung. 2. Irregular anastomosing spaces in the walls of the bronchi. 3. Lymph-spaces in the pulmonary pleura. The lymphatic vessels from all these irregular sinuses pass in towards the root of the lung to reach the bronchial glands. 12 ITS HANDBOOK OF PHYSIOLOGY. Nerves. — The nerves of the lung are to be traced from the anterior and posterior pulmonary plexuses, which are formed by branches of the vagus and sympathetic. The nerves follow the course of the vessels and bronchi, and in the walls of the latter many small ganglia are situated. Mechanism of Respiration. Respiration consists of the alternate expansion and contraction of the thorax, by means of which air is drawn into or expelled from the lungs. These acts are called Inspiration and Expiration respectively. For the inspiration of air into the lungs it is evident that all that is necessary is such a movement of the side- walls or floor of the chest, or of both, that the capacity of the interior shall be enlarged. By such in- crease of capacity there will be of course a diminution of the pressure of FIG. 154.— Diagram of axes of movement of ribs. the air in the lungs, and a fresh quantity will enter through the larynx and trachea to equalize the pressure on the inside and outside of the chest. For the expiration of air, on the other hand, it is also evident that, by an opposite movement which shall diminish the capacity of the chest, the pressure in the interior will be increased, and air will be expelled, until the pressure within and without the chest are again equal. In both cases the air passes through the trachea and larynx, whether in en- tering or leaving the lungs, there being no other communication with the exterior of the body ; and the lung, for the same reason, remains under all the circumstances described closely in contact with the walls and floor of the chest. To speak of expansion of the chest, is to speak also of expansion of the lung. KESPIEA.TION. 179 We have now to consider the means by which the respiratory move- ments are effected. Respiratory Movements. A. Inspiration. — The enlargement of the chest in inspiration is a muscular act; the effect of the action of the inspiratory muscles being an increase in the size of the chest-cavity (a) in the vertical, and (b) in the lateral and antero-posterior diameters.. The muscles engaged in ordinary inspiration are the diaphragm; the external intercostals; parts of the internal intercostals; the levatores costarum; and serratus posti- cus superior. (a. ) The vertical diameter of the chest is increased by the contraction and consequent descent of the diaphragm, — the sides of the muscle descending most, and the central tendon remaining comparatively un- moved; while the intercostal and other muscles, by acting at the same FIG. 155.— Diagram of movement of a rib in inspiration. time, prevent the diaphragm, during its contraction, from drawing in the sides of the chest. (b.) The increase in the lateral and antero-posterior diameters of the chest is effected by the raising of the ribs, the greater number of which are attached very obliquely to the spine and sternum . (see Figure of Skeleton in frontispiece). The elevation of the ribs takes place both in front and at the sides — the hinder ends being prevented from performing any upward movement by their attachment to the spine. The movement of the front extremi- ties of the ribs is of necessity accompanied by an upward and forward movement of the sternum to which they are attached, the movement being greater at the lower end than at the upper end of the latter bone. The axes of rotation in these movements are two: one corresponding with a line drawn through the two articulations which the rib forms 180 HANDBOOK OF PHYSIOLOGY. with the spine (a b, Fig. 154); and the other, with a line drawn from one of these (head of rib) to the sternum (A B, Fig. 154 and Fig. 155); the motion of the rib around the latter axis being somewhat after the fashion of raising the handle of a bucket. The elevation of the ribs is accompanied by a slight opening out of the angle which the bony part forms with its cartilage (Fig. 15^ A); and thus an additional means is provided for increasing the antero-poste- rior diameter of the chest. . The muscles by which the ribs are raised, in ordinary quiet inspira- tion, are the external intercostals , and that portion of the internal inter - costals which is situate between the costal cartilages; and these are assisted by the levatores costarum, and the serratus posticus superior. The action of the levatores and the serratus is very simple. Their fibres, Ns. FIG. 156. FIG. 157. FIG. 156.— Diagram of apparatus showing the action of the external intercostal muscles. FIG. 157. — Diagram of apparatus showing the action of the internal intercostal muscles. arising from the spine as a fixed point, pass obliquely downwards and forwards to the ribs, and necessarily raise the latter when they contract. The action of the intercostal muscles is not quite so simple, inasmuch as, passing merely from rib to rib, they seem at first sight to have no filed point towards which they can pull the bones to which they are attached. A very simple apparatus will make their action plain. Such an apparatus is shown in Fig. 156. A B is an upright bar, representing the spine, with which are jointed two parallel bars, 0 and D, which repre- sent two of the ribs, and are connected in front by movable joints with another upright, representing the sternum. If with such an apparatus elastic bands be connected in imitation of the intercostal muscles, it will be found that when stretched on the bars after the fashion of the external intercostal fibres (Fig. 156, C D), i. e., passing downwards and forwards, they raise them (Fig. 156, C' D'); RESPIRATION. 181 while on the other hand, if placed in imitation of the position of the internal intercostals (Fig. 157, E F), i. e., passing downwards and back- wards, they depress them (Fig. 157, E' F'). The explanation of the foregoing facts is very simple. The intercos- tal muscles, in contracting, merely do that which all other contracting fibres do, viz., bring nearer together the points to which they are attached; and in order to do this, the external intercostals must raise the ribs, the points C and D (Fig. 156) being nearer to each other when the parallel bars are in the position of the dotted lines. The limit of the movement in the apparatus is reached when the elastic band extends at right angles to the two bars which it connects — the points of attach- ment 0' and D' being then at the smallest possible distance one from the other. The internal intercostals (excepting those fibres which are attached to the cartilages of the ribs) have an opposite action to that of the external. In contracting they must pull down the ribs, because the points E and F (Fig. 157) can only be brought nearer one to another (Fig. 157, E' F') by such an alteration in their position. On account of the oblique position of the cartilages of the ribs with reference to the sternum, the action of the inter-cartilaginous fibres of the internal intercostals must, of course, on the foregoing principles, resemble that of the external intercostals. In tranquil breathing, the expansive movements of the lower part of the chest are greater than those of the upper. In forced inspiration, on the other hand, the greatest extent of movement appears to be in the upper antero-posterior diameter. Muscles of Extraordinary Inspiration. — In extraordinary or forced inspiration, as in violent exercise, or in cases in which there is some interference with the due entrance of air into the chest, and in which, therefore, strong efforts are necessary, other muscles than those just enumerated, are pressed into the service. It is very difficult or impossi- ble to separate by a hard and fast line, the so-called muscles of ordinary from those of extraordinary inspiration; but there is no doubt that the following are but little used as respiratory agents, except in cases in which unusual efforts are required — the scaleni muscles, the sternomas- toid, the serratus magnus, the pector ales, and the trapezius. Types of Respiration. — The expansion of the chest in inspiration presents some peculiarities in different persons. In young children, it is effected chiefly by the diaphragm, which, being highly arched in ex- piration, becomes flatter as it contracts, and, descending, presses on the abdominal viscera, and pushes forward the front walls of the abdomen. The movement of the abdominal wall being here more manifest than that of any other part, it is usual to call this the abdominal type of respira- tion. In men, together with the descent of the diaphragm, and the pushing forward of the front wall of the abdomen, the chest and the sternum are subject to a wide movement in inspiration (inferior costal type). In women, the movement appears less extensive in the lower, and 182 HANDBOOK OF PHYSIOLOGY. more so in the upper, part of the chest (superior costal type). (See Figs. 158, 159.) B. Expiration. — From the enlargement produced in inspiration, the chest and lungs return in ordinary tranquil expiration, by their elas- ticity; the force employed by the inspiratory muscles in distending the chest and overcoming the elastic resistance of the lungs and chest-walls, being returned as an expiratory effort when the muscles are relaxed. This elastic recoil of the chest and lungs is sufficient, in ordinary quiet breathing, to expel air from the lungs in the intervals of inspiration, and no muscular power is required. In all voluntary expiratory efforts, however, as in speaking, singing, blowing, and the like, and in many involuntary actions also, as sneezing, coughing, etc., something more than merely passive elastic power is necessary, and the proper expiratory Fia. 158. FIG. 159. Fm. 158.— The changes of the thoracic and abdominal walls of the male during respiration. The back is supposed to be fixed, in order to throw forward the respiratory movement as much as possible. The outer black continuous line in front represents the ordinary breathing movement: the anterior margin of it being the boundary of inspiration, the posterior margin the limit of expi- ration. The line is thicker over the abdomen, since the ordinary respiratory movement is chiefly ab- dominal: thin over the chest, for there is less movement over that region The dotted line indicates the movement on deep inspiration, during which the sternum advances while the abdomen recedes. FIG. 159.— The respiratory movement in the female. The lines indicate the same changes as in the last figure. The thickness of the continuous line over the sternum shows the larger extent of the ordinary breathing movement over that region in the female than in the male. (John Hutchin- son.) The posterior continuous line represents in both figures the limit of forced expiration. muscles are brought into action. By far the chief of these are the ab- dominal muscles, which, by pressing on the viscera of the abdomen, push up the floor of the chest formed by the diaphragm, and by thus making pressure on the lungs, expel air from them through the trachea and larynx. All muscles, however, which depress the ribs, must act also as muscles of expiration, and therefore we must conclude that the abdom- inal muscles are assisted in their action by the greater part of the internal RESPIRATION. 183 intercostals, the triangularis sterni, the serratus posticus inferior, and quadratus lumborum. When by the efforts of the expiratory muscles, the chest has been squeezed to less than its average diameter, it again, on relaxation of the muscles, returns to the normal dimensions by virtue of its elasticity. The construction of the chest-walls, therefore, admirably adapts them for recoiling against and resisting as well undue contraction as undue dilatation. In the natural condition of the parts, the lungs can never contract to the utmost, but are always more or less <{ on the stretch," being kept closely in contact with the inner surface of the walls of the chest By cohesion as well as by atmospheric pressure, and can contract away from these only when, by some means or other, as by making an opening into the pleural cavity, or by the effusion of fluid there, the pressure on the exterior and interior of the lungs becomes equal. Thus, under ordinary circumstances, the degree of contraction or dilatation of the lungs is dependent on that of the boundary walls of the chest, the outer surface of the one being in close contact with the inner surface of the other, and obliged to follow it in all its movements. Respiratory Rhythm. — The acts of expansion and contraction of the chest, take up, under ordinary circumstances, a nearly equal time. The act of inspiring air, however, especially in women and children, is a little shorter than that of expelling it, and there is commonly a very slight pause between the end of expiration and the beginning of the next inspiration. The respiratory rhythm may be thus expressed: — Inspiration, 6 Expiration, . . . . . 7 or 8 A very slight pause. Respiratory Sounds. — If the ear be placed in contact with the wall of the chest, or be separated from, it only by a good conductor of sound or stethoscope, a faint respiratory murmur is heard during inspiration. This sound varies somewhat in different parts — being loudest or coarsest in the neighborhood of the trachea and large bronchi (tracheal and bronchial breathing), and fading off into a faint sighing as the ear is placed at a distance from these (vesicular breathing). It is best heard in children, and in them a faint murmur is heard in expiration also. The cause of the vesicular murmur has received various explanations. Most observers hold that the sound is produced by the friction of the air against the walls of the alveoli of the lungs when they are undergoing distention (Laennec, Skoda), others that it is due to an oscillation of the current of air as it enters the alveoli (Chauveau), whilst others believe that the sound is produced in the glottis, but that it is modified in its passage to the pulmonary alveoli (Beau, Gee). Respiratory Movements of the Nostrils and of the Glottis.— 184 HANDBOOK OF PHYSIOLOGY, During the action of the muscles which directly draw air into the chest, those which guard the opening through which it enters are not passive. In hurried breathing the instinctive dilation of the nostrils is well seen, although under ordinary conditions it may not be noticeable. The open- ing at the upper part of the larynx, however, or rima glottidis (Fig. 143), is dilated at each inspiration, for the more ready passage of air, and becomes smaller at each expiration; its condition, therefore, correspond- ing during respiration with that of the walls of the chest. There is a further likeness between the two acts in that, under ordinary circum- stances, the dilatation of the rima glottidis is a muscular act, and its contraction chiefly an elastic recoil; although, under various conditions, to be hereafter mentioned, there may be, in the latter, considerable mus- cular power exercised. Terms used to express Quantity of Air breathed.— a. Breathing or tidal air, is the quantity of air which is habitually and almost uni- formly changed in each act of breathing. In a healthy adult man it is about 30 cubic inches. b. Complemental air, is the quantity over and above this which can be drawn into the lungs in the deepest inspiration; its amount is various, as will be presently shown. c. Reserve air. — After ordinary expiration, such as that which expels the breathing or tidal air, a certain quantity of air remains in the lungs, which may be expelled by a forcible and deeper expiration. This is termed reserve air. d. Residual air is the quantity which still remains in the lungs after the most violent expiratory effort. Its amount depends in great measure on the absolute size of the chest, but may be estimated at about 100 cubic inches. The total quantity of air which passes into and out of the lungs of an adult, at rest, in 24 hours, is about 686,000 cubic inches. This quan- tity, however, is largely increased by exertion; the average amount for a hard-working laborer in the same time being 1.568,390 cubic inches. e. Respiratory Capacity. — The greatest respiratory capacity of the chest is indicated by the quantity of air which a person can expel from his lungs by a forcible expiration after the deepest inspiration that he can make; it expresses the power which a person has of breathing in the emergencies of active exercise, violence, and disease. The average capacity of an adult (at 60° F. or 15.4° C.) is about 225 cubic inches. The respiratory capacity, or as Hutchinson called it, vital capacity, is usually measured by a modified gasometer (spirometer of Hutchinson), into which the experimenter breathes — making the most prolonged ex- piration possible after the deepest possible inspiration. The quantity of air which is thus expelled from the lungs is indicated by the height to which the air chamber of the spirometer rises; and by means of a KESPIKATION. 185 scale placed in connection with this, the number of cubic inches is read off. In healthy men, the respiratory capacity varies chiefly with the sta- ture, weight, and age. It was found by Hutchmson, from whom most of our information on this subject is derived, that at a temperature of 60° F., 225 cubic inches is the average vital or respiratory capacity of a healthy person, five feet seven inches in height. Circumstances affecting the amount of respiratory capacity. — For every inch of height above this standard the capacity is increased, on an average, by eight cubic inches; and for every inch below, it is diminished by the same amount. The influence of weight on the capacity of respiration is less manifest and considerable than that of height : and it is difficult to arrive at any definite conclusions on this point, because the natural average weight of a healthy man in relation to stature has not yet been determined. As a general statement, however, it may be said that the capacity of respira- tion is not affected by weights under 161 pounds, or 1H stones ; but that, above this point, it is diminished at the rate of one cubic inch for every additional pound up to 196 pounds, or 14 stones. By age, the capacity appears to be increased from about the fifteenth to the thirty-fifth year, at the rate of five cubic inches per year ; from thirty-five to sixty-five it diminishes at the rate of about one and a half cubic inch per year ; so that the capacity of respiration of a man of sixty years old would be about 30 cubic inches less than that of a man forty years old, of the same height and weight. (John Hutchinson.) Number of Respirations, and Relation to the Pulse. — The number of respirations in a healthy adult person usually ranges from fourteen to eighteen per minute. It is greater in infancy and childhood. It varies also much according to different circumstances, such as exer- cise or rest, health or disease, etc. Variations in the number of respira- tions correspond ordinarily with similar variations in the pulsations of the heart. In health the proportion is about 1 to 4, or 1 to 5, and when the rapidity of the heart's action is increased, that of the chest move- ment is commonly increased also ; but not in every case in equal propor- tion. It happens occasionally in disease, especially of the lungs or air-passages, that the number of respiratory acts increases in quicker pro- portion than the beats of ihe pulse; and, in other affections, much more commonly, that the number of the pulses is greater in proportion than that of the respirations. There can be no doubt that the number of respirations of any given animal is largely affected by its size. Thus, comparing animals of the same kind, in a tiger (lying quietly) the number of respirations was 20 per minute, while in a small leopard (lying quietly) the number was 30. In a small monkey 40 per minute ; in a large baboon, 20. The rapid, panting respiration of mice, even when quite still, is fa- miliar, and contrasts strongly with the slow breathing of a large animal 186 HANDBOOK OF PHYSIOLOGY. such as the elephant (eight or nine times per minute). These facts may be explained as follows : — The heat-producing power of any given animal depends largely on its bulk, while its loss of heat depends to a great ex- tent upon the surface area of its body. If of two animals of similar shape, one be ten times as long as the other, the area of the large animal (representing its loss of heat) is 100 times that of the small one, while its bulk (representing production of heat) is about 1000 times as great. Thus in order to balance its much greater relative loss of heat, the smaller animal must have all its vital functions, circulation, respiration, etc., carried on much more rapidly. Force of Inspiratory and Expiratory Muscles. — The force with which the inspiratory muscles are capable of acting is greatest in indivi- duals of the height of from five feet seven inches to five feet eight inches, and will elevate a column of three inches of mercury. Above this height, the force decreases as the stature increases ; so that the average of men of six feet can elevate only about two and a half inches of mercury. The force manifested in the strongest expiratory acts is, on the average, one-third greater than that exercised in inspiration. But this difference is in great measure due to the power exerted by the elastic reaction of the walls of the chest ; and it is also much influenced by the dispropor- tionate strength which the expiratory muscles attain, from their being called into use for other purposes than that of simple expiration. The force of the inspiratory act is, therefore, better adapted than that of the expiratory for testing the muscular strength of the body. (John Hut- chinson. ) The instrument used by Hutchinson to gauge the inspiratory and ex- piratory power was a mercurial manometer, to which was attached a tube fitting the nostrils, and through which the inspiratory or expiratory ef- fort was made. The following table represents the results of numerous experiments : Power of Power of Inspiratory Muscles. Expiratory Muscles. 1.5 in. . . . Weak, . . .2.0 in. 2.0 " . . Ordinary, . . 2,5 " 2.5 " . . . Strong, . . . 3.5 " 3.5" . . Very strong, . 4.5" 4.5 " . . . Eemarkable, . . 5.8 " 5.5 " . . Very remarkable, 7.0 " 6.0 " . . . Extraordinary, . 8.5 " 7.0" . . Very extraordinary, 10.0 " The greater part of the force exerted in deep inspiration is employed in overcoming the resistance offered by the elasticity of the walls of the chest and of the lungs. The amount of this elastic resistance was estimated by observing the elevation of a column of mercury raised by the return of air forced, after death, into the lungs, in quantity equal to the known capacity of res- RESPIRATION. 1ST piration during life ; and Hutchinson calculated, according to the well- known hydrostatic law of equality of pressures (as shown in the Bramah press), that the total force to be overcome by the muscles in the act of inspiring 200 cubic inches of air is more than 450 Ibs. The elastic force overcome in ordinary inspiration is, according to the same authority, equal to about 170 Ibs. Douglas Powell has shown that within the limits of ordinary tranquil respiration, the elastic resilience of the walls of the chest favors inspira- tion; and that it is only in deep inspiration that the ribs and rib-carti- lages offer an opposing force to their dilatation. In other words, the elastic resilience of the lungs, at the end of an act of ordinary breathing, has drawn the chest-walls within the limits of their normal degree of ex- pansion. Under all circumstances, of course, the elastic tissue of the lungs opposes inspiration, and favors expiration. Functions of Muscular Tissue of Lungs. — It is possible that the contractile power which the bronchial tubes and air-vesicles possess, by means of their muscular fibres may (1) assist in expiration ; but it is more likely that its chief purpose is (2) to regulate and adapt, in some measure, the quantity of air admitted to the lungs, and to each part of them, according to the supply of blood ; (3) the muscular tissue con- tracts upon and gradually expels collections of mucus, which may have accumulated within the tubes, and which cannot be ejected by forced expiratory efforts, owing to collapse or other morbid conditions of the portion of lung connected with the obstructed tubes (Gairdner). (4) Apart from any of the before-mentioned functions, the presence of mus- cular fibre in the walls of a hollow viscus, such as a lung, is only what might be expected from analogy with other organs. Subject as the lungs are to such great variation in size it might be anticipated that the elastic tissue, which enters so largely into their composition, would be supplemented by the presence of much muscular fibre also. Respiratory Changes in the Air and in the Blood. A. In the Air. Composition of the Atmosphere. — The atmosphere we breathe has, in every situation in which it has been examined in its natural state, a nearly uniform composition. It is a mixture of oxygen, nitrogen, car- bonic acid, and watery vapor, with, commonly, traces of other gases, as ammonia, sulphuretted hydrogen, etc. Of every 100 volumes of pure atmospheric air, 79 volumes (on an average) consist of nitrogen, the re- maining 21 of oxygen. By weight the proportion is N. 75, 0. 25. The proportion of carbonic acid is extremely small; 10,000 volumes of atmospheric air contain only about 4 or 5 of carbonic acid. The quantity of watery vapor varies greatly according to the tempera- ture and other circumstances, but the atmosphere is never without 188 HANDBOOK OF PHYSIOLOGY. some. In this country, the average quantity of watery vapor in the atmosphere is 1.40 per cent. Composition of Air ivhich has been breathed. — The changes effected by respiration in the atmospheric air are : 1, an increase of temperature; 2, an increase in the quantity of carbonic acid ; 3, a diminution in the quantity of oxygen ; 4, a diminution of volume ; 5, an increase in the amount of watery vapor ; 6, the addition of a minute amount of organic matter and of free ammonia. 1. The expired air, heated by its contact with the interior -of the lungs, is (at least in most climates) hotter than the inspired air. Its temperature varies between 97° and 99.5° F. (36°-37.5° C.), the lower temperature being observed when the air has remained but a short time in the lungs. Whatever may be the temperature of the air when in- haled, it nearly acquires that of the blood before it is expelled from the chest. 2. The Carbonic Acid is always increased; but the quantity exhaled in a given time is subject to change from various circumstances. From every volume of air inspired, about 4.8 per cent of oxygen is abstracted; while a rather smaller quantity, 4.3 of carbonic acid is added in its place : the air will contain, therefore, 434 vols. of carbonic acid in 10,000. Under ordinary circumstances, the quantity of carbonic acid exhaled into the air breathed by a healthy adult man amounts to 1346 cubic inches, or about 636 grains per hour. According to this estimate, the weight of carbon excreted from the lungs is about 173 grains per hour, or rather more than 8 ounces in twenty-four hours. These quan- tities must be considered approximate only, inasmuch as various circum- stances, even in health, influence the amount of carbonic acid excreted, and, correlatively, the amount of oxygen absorbed. Circumstances influencing the amount of carbonic acid excreted. — The following are the chief : — Age and sex. Eespiratory movements. External temperature. Season of year. Condition of respired air. Atmospheric conditions. Period of the day. Food and drink. Exer- cise and sleep. a. Age and Sex. — The quantity of carbonic acid exhaled into the air breathed by males, regularly increases from eight to thirty years of age ; from thirty to fifty the quantity, after remaining stationary for a while, gradually diminishes, and from fifty to extreme age it goes on diminish- ing, till it scarcely exceeds the quantity exhaled at ten years old. In females (in whom the quantity exhaled is always less than in males of the same age) the same regular increase in quantity goes on from the eighth year to the age of puberty, when the quantity abruptly ceases to increase, and remains stationary so long as they continue to menstruate. When menstruation has ceased, it soon decreases at the same rate as it does in old men. b. Respiratory Movements. — The more quickly the movements of respiration are performed, the smaller is the proportionate quantity of carbonic acid contained in each volume of the expired air. Although, RESPIRATION. 1 89 however, the proportionate quantity of carbonic acid is thus diminished during frequent respiration, yet the absolute amount exhaled into the air within a given time is increased thereby, owing to the larger quan- tity of air which is breathed in the time. The last half of a volume of expired air contains more carbonic acid than the half first expired ; a circumstance which is explained by the one portion of air coming from the remote part of the lungs, where it has been in more immediate and prolonged contact with the blood than the other has, which comes chiefly from the larger bronchial tubes. c. External temperature. — The observation made by Vierordt at vari- ous temperatures between 38° F. and 75° F. (3.4°-23.8° C.) show, for warm-blooded animals, that within this range, every rise equal to 10° F., causes a diminution of about two cubic inches in the quantity of carbonic acid exhaled per minute. d. Season of the Year. — The season of the year, independently of temperature, materially influences the respiratory phenomena; spring being the season of the greatest, and autumn of the least activity of the respiratory and other functions. (Edward Smith.) e. Purity of the Respired Air. — The average quantity of carbonic acid given out by the lungs constitutes about 4.3 per cent of the expired air; but if the air which is breathed be previously impregnated with carbonic acid (as is the case when the same air is frequently respired), then the quantity of carbonic acid exhaled becomes much less. /. Hygrometric State of Atmosphere. — The amount of carbonic acid exhaled is considerably influenced by the degree of moisture of the atmosphere, much more being given off when the air is moist than when it is dry. (Lehmann.) ff. Period of the Day. — During the day-time more carbonic acid is exhaled than corresponds to the oxygen absorbed; while, on the other hand, at night very much more oxygen is absorbed than is exhaled in carbonic acid. There is, thus, a reserve fund of oxygen absorbed by night to meet the requirements of the day. If the total quantity of carbonic acid exhaled in 24 hours be represented by 100, 52 parts are ex- haled during the day, and 48 at night. While, similarly, 33 parts of the oxygen are absorbed during the day, and the remaining 67 by night. (Pette'nkofer and Voit.) h. Food and Drink. — By the use of food the quantity is increased, whilst by fasting it is diminished; it is greater when animals are fed on farinaceous food than when fed on meat. The effects produced by spirituous drinks depend much on the kind of drink taken. Pure alco- hol tends rather to increase than to lessen respiratory changes, and the amount therefore of carbonic acid expired; rum, ale, and porter, also sherry, have very similar effects. On the other hand, brandy, whiskey, and gin, particularly the latter, almost always lessened the respiratory changes, and consequently the amount of carbonic acid exhaled. (Ed- ward Smith.) i. Exercise. — Bodily exercise, in moderation, increases the quantity to about one-third more than it is during rest: and for about an hour after exercise the volume of the air expired in the minute is increased about 118 cubic inches: and the quantity of carbonic acid about 7.8 cubic inches per minute. Violent exercise, such as full labor on the tread - wheel, still further increases the amount of the acid exhaled. (Edward. Smith.) 190 HANDBOOK OF PHYSIOLOGY. A larger quantity is exhaled when the barometer is low than when it is high. 3. The oxygen is diminished, and its diminution is generally propor- tionate to the increase of the carbonic acid. For every volume of carbonic acid exhaled into the air, 1.17421 volumes of oxygen are absorbed from it, and 1346 cubic inches, or 630 grains being exhaled in the hour the quantity of oxygen absorbed in the same time is 1584 cubic inches or 542 grains. According to this esti- mate, there is more oxygen absorbed than is exhaled with carbon to form carbonic acid. 4. The volume of air expired in a given time is less than that of the air inspired (allowance being made for the expansion in being heated), and that the loss is due to a portion of oxygen absorbed and not returned in the exhaled carbonic acid, all observers agree, though as to the actual quantity of oxygen so absorbed, they differ even widely. The amount of oxygen absorbed is on an average of 4.8 per cent, so that the expired air contains 16.2 volumes per cent of that gas. The quantity of oxygen that does not combine with the carbon given off in carbonic acid from the lungs is probably disposed off in forming some of the carbonic acid and water given off from the skin, and in combining with sulphur and phosphorus to form part of the acids of the sulphates and phosphates excreted in the urine, and probably also with the nitrogen of the decomposing nitrogenous tissues. The quantity of oxygen in the atmosphere surrounding animals appears to have very little influence on the amount of this gas absorbed by them, for the quantity consumed is not greater even though an excess of oxygen be added to the atmosphere experimented with. It has often been discussed whether Nitrogen is absorbed by or ex- haled from the lungs during respiration. At present, all that can be said on the subject is that, under most circumstances, animals appear to expire a very small quantity above that which exists in the inspired air. During prolonged fasting, on the contrary, a small quantity appears to be absorbed. 5. The watery vapor is increased. — The quantity emitted is, as a general rule, sufficient to saturate the expired air, or very nearly so. Its absolute amount is, therefore, influenced by the following circum- stances (1), by the quantity of air respired; for the greater this is, the greater also will be the quantity of moisture exhaled; (2) by the quantity of watery vapor contained in the air previous to its being inspired; be- cause the greater this is, the less will be the amount required to complete the saturation of the air; (3) by the temperature of the expired air; for the higher this is, the greater will be the quantity of watery vapor re- quired to saturate the air; (4) by the length of time which each volume of RESPIRATION. 191 inspired air is allowed to remain in the lungs; for although, during ordinary respiration, the expired air is always saturated with watery vapor, yet when respiration is performed very rapidly the air has scarcely time to be raised to the highest temperature, or be fully charged with moisture ere it is expelled. The quantity of water exhaled from the lungs in twenty-four hours ranges (according to the various modifying circumstances already men- tioned) from about 6 to 27 ounces, the ordinary quantity being about 9 or 10 ounces. Some of this is probably formed by the chemical combi- nation of oxygen with hydrogen in the system ; but the far larger pro- portion of it is water which has been absorbed, as such, into the blood from the alimentary canal, and which is exhaled from the surface of the air-passages and cells, as it is from the free surfaces of all moist animal membranes, particularly at the high temperature of warm-blooded ani- mals. 6. A small quantity of ammonia is added to the ordinary constitu- ents of expired air. It seems probable, however, both from the fact that this substance cannot be always detected, and from its minute amount when present, that the whole of it may be derived from decom- posing particles of food left in the mouth, or from carious teeth or the like ; and that it is, therefore, only an accidental constituent of expired air. 7. The quantity of organic matter in the breath is increased and is about 3 grains in about twenty-four hours. (Ransome.) Method of Experiment. — The following represents the kind of experi- ment by which the foregoing facts regarding the excretion of carbonic acid, water, and organic matter, have been established. A bird or mouse is placed in a large bottle, through the stopper of which two tubes pass, one to supply fresh air, and the other to carry off that which has been expired. Before entering the bottle, the air is made to bubble through a strong solution of caustic potash, which absorbs the carbonic acid, and then through lime-water, which, by remaining limpid, proves the absence of carbonic acid. The air which has been breathed by the animal is made to bubble through lime-water, which at once be- comes turbid and soon quite milky from the precipitation of calcium carbonate ; and it finally passes through strong sulphuric acid, which, by turning brown, indicates the presence of organic matter. The watery vapor in the expired air will condense inside the bottle if the surface be kept cool. By means of an apparatus sufficiently large and well-constructed, experiments of the kind have been made extensively on man. Methods by which the Respiratory Changes in the Air are effected. The method by which fresh air is inhaled and expelled from the lungs has been explained. It remains to consider how it is that the 192 HANDBOOK OF PHYSIOLOGY. blood absorbs oxygen from, and gives up carbonic acid to, the air of the alveoli. In the first place, it must be remembered that the tidal air only amounts to about 25-30 cubic inches at each inspiration, and that this is of course insufficient to fill the lungs, but it mixes with the sta- tionary air by diffusion, and so supplies to it new oxygen. The amount of oxygen in expired air, which may be taken as the average composi- tion of the mixed air in the lungs, is about 16 to 17 per cent ; in the pulmonary alveoli it may be rather less than this. From this air the venous blood has to take up oxygen in the proportion of 8 to 12 vols. in every hundred volumes of blood, as the difference between the amount of oxygen in arterial and venous blood is no less than that. It seems therefore somewhat diffcult to understand how this can be accomplished at the low oxygen tension of the pulmonary air. But as was pointed out in a previous Chapter (IV. ), the oxygen is not simply dissolved in the blood, but is to a great extent chemically combined with the haemoglobin of the red corpuscles ; and when a fluid contains a body which enters into loose chemical combination in this Way with a gas, the tension of the gas in the fluid is not directly proportional to the total quantity of the gas taken up by the fluid, but to the excess above the total quantity which the substance dissolved in the fluid is capable of taking up (a known quantity in the case of haemoglobin, viz., 1.59 cm. for one gm. haemoglobin). On the other hand, if the sub- stance be not saturated, i. e., if it be not combined with as much of the gas as it is capable of taking up, further combination leads to no increase of its tension. However, there is a point at which the haemoglobin gives up its oxygen when it is exposed to a low partial pressure of oxygen, and there is also a point at which it neither takes up nor gives out oxy- gen ; in the case of arterial blood of the dog, this is found to be when the oxygen tension of the atmosphere is equal to 3.9 per cent (29.6 mm. of mercury), which is equivalent to saying that the oxygen tension of arterial blood is 3. 9 per cent; venous blood, in a similar manner, has been found to have an oxygen tension of 2.8 per cent. At a higher tem- perature, the tension is raised, as there is a greater tendency at a high temperature for the chemical compound to undergo dissociation. It is therefore easy to see that the oxygen tension of the air of the pulmonary alveoli is quite sufficient, even supposing it much less than that of the expired air, to enable the venous blood to take up oxygen, and what is more, it will take it up until the haemoglobin is very nearly saturated with the gas. As regards the elimination of carbonic acid from the blood, there is evidence to show that it is given up by a process of simple diffusion, the only condition necessary for the process being that the tension of the carbonic acid of the air in the pulmonary alveoli should be less than the tension of the carbonic acid in venous blood. The carbonic acid RESPIRATION. 193 tension of the alveolar air probably does not exceed in the dog 3 or 4 per cent, while that of the venous blood is 5.4 per cent, or equal to 41 mm. of mercury. B. In the Blood. Circulation of Blood in the Respiratory Organs. — To be exposed to the air thus alternately moved into and out of the air-cells and minute bronchial tubes, the blood is propelled from the right ventricle through the pulmonary capillaries in steady streams, and slowly enough to per- mit every minute portion of it to be for a few seconds exposed to the air with only the thin walls of the capillary vessels and the air-cells inter- vening. The pulmonary circulation is of the simplest kind : for the pulmonary artery branches regularly ; its successive branches run in straight lines, and do not anastomose : the capillary plexus is uniformly spread over the air-cells and intercellular passages ; and the veins derived from it proceed in a course as simple and uniform as that of the arteries, their branches converging but not anastomosing. The veins have no valves, or only small imperfect ones prolonged from their angles of junc- tion, and incapable of closing the orifice of either of the veins between which they are placed. The pulmonary circulation also is unaffected by changes of atmospheric pressure, and is not exposed to the influence of the pressure of muscles : the force by which it is accomplished, and the course of the blood are alike, simple. Changes in the Blood. — The most obvious change which the blood of the pulmonary artery undergoes in its passage through the lungs is 1st, that of color, the dark crimson of venous blood being exchanged for the bright scarlet of arterial blood ; %d, and in connection with the pre- ceding change, it gains oxygen ; 3d, it loses carbonic acid ; ±th, it be- comes slightly cooler ; 5th, it coagulates sooner and more firmly, appar- ently containing more fibrin. The oxygen absorbed into the blood from the atmospheric air in the lungs is combined chemically with the haemo- globin of the red-corpuscles. In this condition it is carried in the arterial blood to the various parts of the body, and brought into near relation or contact with the tissues. In these tissues, and in the blood which cir- culates in them, a certain portion of the oxygen, which the arterial blood contains, disappears, and a proportionate quantity of carbonic acid and water is formed. The venous blood, containing the new-formed carbonic acid returns to the lungs, where a portion of the carbonic acid is exhaled, and a fresh supply of oxygen is taken in. Mechanism of Various Respiratory Actions. It will be well here, perhaps, to explain some respiratory acts, which appear at first sight somewhat complicated, but cease to be so when the mechanism by which they are performed is clearly understood. The ac- 13 194 HANDBOOK OF PHYSIOLOGY. companying diagram (Fig. 160) shows that the cavity of the chest is separated from that of the abdomen by the diaphragm, which, when acting, will lessen its curve, and thus descending, will push downwards and forwards the abdominal viscera ; while the abdominal muscles have the opposite effect, and in acting will push the viscera upwards and backwards, and with them the diaphragm, supposing its ascent to be not from any cause interfered with. From the same diagram it will be seen that the lungs communicate with the exterior of the body through FIG. 160. the glottis, and further on through the mouth and nostrils— through either of them separately, or through both at the same time, according to the position as the soft palate. The stomach communicates with the exterior of the body through the oesophagus, pharynx, and mouth ; while below the rectum opens at the anus, and the bladder through the ure- thra. All these openings, through which the hollow viscera communi- cate with the exterior of the body, are guarded by muscles, called sphinc- ters, which can act independently of each other. The position of the latter is indicated in the diagram. RESPIRATION. 195 Sighing. — In sighing there is a rather prolonged inspiration ; the air almost noiselessly passing in through the glottis, and by the elastic recoil of the lungs and chest-walls, and probably also of the abdominal walls, being rather suddenly expelled again. Now, in the first, or inspirator^ part of this act, the descent of the diaphragm presses the abdominal viscera downwards, and of course this pressure tends to evacuate the contents of such as communicate with the exterior of the body. Inasmuch, however, as their various openings are guarded by sphincter muscles, in a state of constant tonic contraction, there is no escape of their contents, and air simply enters the lungs. In the second, or expiratory part of the act of sighing, there is also pressure made on the abdominal viscera in the opposite direction, by the elastic or muscular recoil of the abdominal walls ; but the pressure is relieved by the escape of air through the open glottis, and the relaxed diaphragm is pushed up again into its original position. The sphincters of the stomach, rectum, and bladder, act in the same manner as before. Hiccough resembles sighing in that it is an inspiratory act ; but the inspiration is sudden instead of gradual, in consequence of the diaphragm acting suddenly and spasmodically ; and the air, therefore suddenly rush- ing through the unprepared rima glottidis, causes vibration of the vocal cords, and the peculiar sound. Coughing. — In the act of coughing, there is most often first of all a deep inspiration, followed by an expiration ; but the latter, instead of being easy and uninterrupted, as in normal breathing, is obstructed, in consequence of the glottis being momentarily closed by the approxima- tion of the vocal cords. The abdominal muscles, then strongly acting, push up the viscera against the diaphragm, and thus make pressure on the air in the lungs until its tension is sufficient to noisily burst open the vocal cords which oppose its outward passage. In this way considerable force is exercised, and mucus or any other matter that may need expul- sion from the air-passages is quickly and sharply expelled by the out- streaming current of air. It will be evident on reference to the diagram (Fig. 160), that pres- sure exercised by the abdominal muscles in the act of coughing, acts as forcibly on the abdominal viscera as on the lungs, inasmuch as the viscera form the medium by which the upward pressure on the diaphragm is made, and there is of necessity quite as great a tendency to the expulsion of their contents as of the air in the lungs. The instinctive, and if necessary, voluntarily increased contraction of the sphincters, however, prevents any escape at the openings guarded by them, and the pressure is effective at one part only, at the rima glottidis. Sneezing. — The same remarks that apply to coughing, are almost exactly applicable to the act of sneezing ; but in this instance the blast of air, on escaping from the lungs, is directed, by an instinctive contrac- 196 HANDBOOK OF PHYSIOLOGY. tion of the pillars of the fauces and descent of the soft palate, chiefly through the nose, and any offending matter is thence expelled. Speaking. — In speaking, there is a voluntary expulsion of air through the glottis by means of the expiratory muscles. The vocal cords are put, by the muscles of the larynx, in a proper position and state of tension for vibrating as the air passes over them, and thus sound is produced. The sound is moulded into articulate speech by the tongue, teeth, lips, etc. — the vocal cords producing the sound only, and having nothing to do with articulation. Singing. — Singing resembles speaking in the manner of its produc- tion ; the laryngeal muscles, by variously altering the position and de- gree of tension of the vocal cords, producing the different notes. Words used in the act of singing are of course framed, as in speaking, by the tongue, teeth, lips, etc. Sniffing. — Sniffing is produced by a rapidly repeated but incomplete action of the diaphragm and other inspiratory muscles. The mouth is closed, and the whole stream of air is made to enter the air-passages through the nostrils. The alae nasi are, commonly, at the same time, instinctively dilated. Sobbing. — Sobbing consists of a series of convulsive inspirations, at the moment of which the glottis is usually more or less closed. Laughing. — Laughing is made up of a series of short and rapid ex- pirations. Yawning. — Yawning is an act of inspiration, but is unlike most of the preceding actions, as it is always more or less involuntary. It is attended by a stretching of various muscles about the palate and lower jaw, which is probably analogous to the stretching of the muscles of the limbs in which a weary man finds relief, as a voluntary act, when they have been some time out of action. The involuntary and reflex char- acter of yawning probably depends on the fact that the muscles con- cerned are themselves at all times more or less used involuntarily, and require, therefore, something beyond the exercise of the will to set them in action. For the same reason, yawning, like sneezing, cannot be well performed voluntarily. Sucking.— Sucking is not properly a respiratory act, but it may be most conveniently considered in this place. It is caused chiefly by the depressor muscles of the os hyoides. These, by drawing downwards and backwards the tongue and floor of the mouth, produce a partial vacuum in the latter : and the weight of the atmosphere then acting on all sides tends to produce equilibrium on the inside and outside of the mouth as best it may. The communication between the mouth and pharynx is completely shut off by the contraction of the pillars of the soft palate and descent of the latter so as to touch the back of the tongue ; and the equilibrium, therefore, can be restored only by the entrance of some- RESPIRATION. 19 T thing through the mouth. The action, indeed, of the tongue and floor of the mouth in sucking may be compared to that of the piston in a syringe, and the muscles which pull down the os hyoides and tongue, to the power which draws the handle. Influence of the Nervous System in Respiration. Like all other functions of the body, the discharge of which is neces- sary to life, respiration is essentially an involuntary act. Unless this were the case, life would be in constant danger, and would cease on the loss of consciousness for a few moments, as in sleep. It is, however, also necessary that respiration should be to some extent under the con- trol of the will. For were it not so, it would be impossible to perform those voluntary respiratory acts which have been just discussed, such as speaking, singing, and the like. The respiratory movements and their rhythm, so far as they are in- voluntary and independent of consciousness, as they are on all ordinary occasions, are under the governance of a nerve-centre in the medulla oUongata which corresponds in position with the origin of the pneumo- gastric nerves ; that is to say, the muscles concerned in the respiratory movements, are excited by stimuli which issue from this part of the nervous system, and which are conveyed by the various motor nerves supplying the muscles. These nerves are the phrenics and intercostals chiefly. On division of one phrenic, for example, the corresponding half of the diaphragm supplied by it ceases to take part in the respira- tory movement, and on division of both nerves, the whole muscle ceases to act. Similarly, division of the intercostal nerves one by one produces cessation of action of the muscles supplied by them. To what extent the medullary centre acts automatically, i. e.. how far the stimulus orig- inates in it, or how far it is merely a nerve-centre for reflex action, is not certainly known. It is clear, however, that the medullary centre is bilateral or double, since the respiratory movements continue after the medulla at this point is bisected in the middle line. There is considerable evidence in favor of its automatic action. Thus it has been shown that if the spinal cord be divided below the medulla, so that no afferent impulses can reach the centre from below, that the nasal and laryngeal respiration continues. The only possible course of the afferent impulses would, under such circumstances, be through the cranial nerves; and when the cord and medulla are intact the division of these nerves produces no effect upon respiration, and indicates that they are not used for the transmission of afferent impulses to the medul- lary centre. It appears evident, therefore, that afferent stimuli are not absolutely necessary for maintaining the respiratory movements. The 198 HANDBOOK OF PHYSIOLOGY. respiratory centre, although automatic in its action, may, however, be reflexly excited. The chief channel of this reflex influence is the vagus nerve, for when the nerve of one side is divided, respiration is slowed, and if both vagi are cut it becomes still slower. The influence of the vagus trunk upon the centre may be twofold, for if the nerve is divided below the origin of the superior laryngeal branch and the central end is stimulated, respiratory movements are in- creased in rapidity, and indeed follow one another so quickly if the stimuli be increased in number, that after a time cessation of respiration in inspiration takes place in consequence of a tetanus of the respiratory muscles (diaphragm). Whereas if the superior laryngeal branch is di- vided, although no effect, or scarcely any, follows the mere division, on stimulation of the central end respiration is slowed, and after a time, if the stimulus is sufficiently increased, stops, not in inspiration as in the other case, but in expiration. Thus the vagus trunk contains fibres which are capable of slowing and fibres which are capable of accelerating respiration. The theory that the respiratory centre in the floor of the medulla consists of two parts, one of which tends to produce inspiration and the other to produce expiration, is very plausible. The inspiratory part of the centre is complementary to the expiratory, and the two parts: send out impulses alternately. If we adopt this th^y, we must look upon the main trunk of the vagus as aiding the inlpjj^ory, and upon the superior laryngeal as aiding the expiratory part of the centre, the first nerve possibly inhibiting the action of the expiratory centre, whilst it aids the inspiratory, and the latter nerve having the very opposite effect. But inasmuch as the respiration is slowed on division of the vagi, and not quickened or manifestly aifected at all on simple division of the su- perior laryngeal, it must be supposed that the vagi fibres are always in action, but that the superior laryngeal fibres are not. It appears that there are, in some animals at all events, subordinate centres in the spinal cord which are able, under certain conditions, to discharge the function of the chief respiratory centre in the medulla. The centre in the medulla may be influenced not only by afferent im- pulses proceeding along the vagus and laryngeal nerves but also by im- pulses passing downward from the cerebrum ; by impressions made upon the nerves of the skin, or upon part of the fifth nerve distributed to the nasal mucous membrane ; or upon other sensory nerves. Such afferent influences are exemplified in the deep inspiration excited by the applica- tion of cold to the surface of the skin, and by the production of sneezing on the slightest irritation of the nasal mucous membrane. At the time of birth, the separation of the placenta, and the conse- quent non-oxygenation of the foetal blood, are the circumstances which immediately lead to the issue of automatic impulses from the respiratory centre in the medulla oblongata. RESPIRATION. 199 Methods of Stimulation of Respiratory Centre. — The means by which the respiratory centre or centres are stimulated must now be con- sidered. It is well known that the more venous the blood, the more marked are the insprator^ impulses, and that if the air is prevented from enter- ing the chest, that the respiration in a short time becomes very labored. The obstruction to the entrance of air, whether partial or complete, is followed by an abnormal rapidity of the inspiratory acts, which make up even in depth for the previous stoppage. The condition caused by the obstruction, or by any circumstance in consequence of which the oxygen of the blood is used up in an abnormally quick manner, is known as dyspnwa, and as the aeration of the blood becomes more and more inter- fered with, not only are the ordinary respiratory muscles employed, but also those extraordinary muscles which have been previously enumerated (p. 181). As the blood becomes more and more venous the action of the medullary centre becomes more and more active. The question arises as to what quality of the venous blood it is which causes this increased ac- tivity ; whether it is its deficiency of oxygen or its excess of carbonic acid. This question has been answered by the experiments, which show on the one hand that dyspnoea occurs when there is no obstruction to the exit of carbonic acid, as when an animal is placed in an atmosphere of nitrogen, and that it cannot therefore be due to the accumulation of car- bonic acid ; and on the other, that if plenty of oxygen is supplied, true dvspnoea does not occur, although the carbonic acid of the blood is in ex- cess. It is highly probable, therefore, that the respiratory centre is stimulated to action by the absence of sufficient oxygen in the blood cir- culating in it, and not by the presence of an excess of carbonic acid. The means by which the vagus is excited to increase the activity of the respiratory centre, appears to be that the venous blood circulating in the lungs, or the air in the pulmonary alveoli, stimulates the peri- pheral fibres of the nerve. If these be the stimuli it will be evident that the vagus action must help to increase the activity of the centre, when the blood in the lungs becomes more and more venous. No doubt the venous condition of the blood affects all the sensory nerves in a similar manner. It has been shown that the circulation of too little blood through the centre, as when its blood supply is cut off, greatly increases its inspiratory action. Effects of Vitiated Air.— Ventilation. — As the air expired from the lungs contains a large proportion of carbonic acid and a minute amount of organic putrescible matter, it is obvious that if the same air be breathed again and again, the proportion of carbonic acid and organic matter will constantly increase till it becomes unfit to be breathed, but long before this point is reached, uneasy sensations occur, such as headache, languor, and a sense of oppression. It is a remarkable fact, 200 HANDBOOK OF PHYSIOLOGY. however, that the organism after a time adapts itself to such a vitiated atmosphere, and that a person soon comes to breathe, without sensible in- convenience, an atmosphere which, when he first entered it, felt intoler- able. Such an adaptation, however, can only take place at the expense of a depression of all the vital functions, which must be injurious if long continued or often repeated. This power of adaptation is well illustrated by the experiments of Claude Bernard. A sparrow is placed under a bell-glass of such a size that it will live for three hours. If now at the end of the second hour (when it could have survived another hour) it be taken out and a fresh healthy sparrow introduced, the latter will perish instantly. It must be evident that provision for a constant and plentiful supply of fresh air, and the removal of that which is vitiated, is of far greater importance than the actual cubic space per head of occupants. Not less than 2000 cubic feet per head should be allowed in sleeping apartments (barracks, hospitals, etc.), and with this allowance the air can only be maintained at the proper standard of purity by such a system of venti- lation as provides for the supply of 1500 to 2000 cubic feet of fresh air per head per hour. (Parkes.) The Effect of Respiration on the Circulation. The heart and great vessels being situated in the air-tight thorax, are exposed to a certain alteration of pressure when the capacity of the latter is increased ; for although the expansion of the lungs during in- spiration tends to counterbalance this increase of area, it never does so entirely, since part of the pressure of the air which is drawn into the chest through the trachea is expended in overcoming the elasticity of the lungs themselves. The amount thus used up increases as the lungs become more and more expanded, so that the pressure inside the thorax during inspiration, as far as the heart and great vessels are concerned, never quite equals that outside, and at the conclusion of inspiration -is considerably less than the atmospheric pressure. It has been ascertained that the amount of the pressure used up in the way above described, varies from 5 to 7 mm. of mercury during the pause, and to 30 mm. of mer- cury when the lungs are expanded at the end of a deep inspiration, so that it will be understood that the pressure to which the heart and great vessels are subjected diminishes as inspiration progresses. It will be understood from the accompanying diagram how, if there were no lungs in the chest, but if its capacity were increased, the effect of the increase would be expended in pumping blood into the heart from the veins, but even with the lungs placed as they are, during inspiration the pressure outside the heart and great vessels is diminished, and they have therefore a tendency to expand and to diminish the intra-vascnlar pressure. The diminution of pressure within the veins passing to the right auricle and RESPIRATION. 201 within the right auricle itself, will draw the blood into the thorax, and so assist the circulation. This suction action is independent of the suc- tion power of the diastole of the auricle about which we have previously spoken (p. 127). The effect of sucking more blood into the right auricle will, cceteris paribus, increase the amount passing through the right ventricle, which also exerts a similar suction action, and through the lungs into the left auricle and ventricle and thus into the aorta. This all tends to increase the arterial tension. The effect of the diminished pressure upon the pulmonary vessels will also help towards the same end, i. e.. an increased flow through the lungs, so that, as far as the FIG. 161.— Diagram of an apparatus illustrating the effect of inspiration upon the heart and , , and A, the aorta; nZ, -LI, the right and left lung; T, the trachea; M, mercurial manometer in connec- tion with the pleura. The increase in the capacity of the box representing the thorax is seen to dilate the heart as well as the lungs, and so to pump in blood through v, whereas the valve prevents reflex through A. The position of the mercury in M shows also the suction which is taking place. heart and its veins are concerned, inspiration increases the blood pres- sure in the arteries. The effect of inspiration upon the aorta and its branches within the thorax would be, however, contrary; for as the pres- sure outside is diminished the vessels would tend to expand, and thus to diminish the tension of the blood within them, but inasmuch as the large arteries are capable of little expansion beyond their natural calibre, the diminution of the arterial tension caused by this means would be insuf- 202 HANDBOOK OF PHYSIOLOGY. ficient to counteract the increase of arterial tension produced by the ef- fect of inspiration upon the veins of the chest, and the balance of the whole action would be in favor of an increase of arterial tension during the inspiratory period. But if a tracing of the variation be taken at the same time that the respiratory movements are being recorded, it will be found that, although speaking generally, the arterial tension is increased during inspiration, the maximum of arterial tension does not correspond with the acme of inspiration (Fig. 162). As regards the effect of expiration, the capacity of the chest is dimin- ished, and the intra-thoracic pressure returns to the normal, which is not exactly equal to the atmospheric, pressure. The effect of this on the veins is to increase their intra-vascular pressure, and so to diminish the flow of blood into the left side of the heart, and with it the arterial tension, but this is almost exactly balanced by the necessary increase of FIG. 162.— Comparison of blood-pressure curve with curve of intra-thoracic pressure. (To be read from left to right.) a is the curve of blood-pressure with its respiratory undulations, the slow- er beats on the descent being very marked ; 6 is the curve of intra-thoracic pressure obtained by connecting one limb of a manometer with the pleural cavity. Inspiration begins at i and expiration at e. The intra-thoracic pressure rises very rapidly after the cessation of the inspiratory effort, and then slowly falls as the air issues from the chest; at the beginning of the inspiratory effort the fall becomes more rapid. (M. Foster.) arterial tension caused by the increase of the extra-vascular pressure of the aorta and large arteries, so that the arterial tension is not much affected during expiration either way. Thus, ordinary expiration does not produce a distinct obstruction to the circulation, as even when the expiration is at an end the intra-thoracic pressure is less than the extra- thoracic. The effect of violent expiratory efforts, however, has a distinct action in preventing the current of blood through the lungs, as seen in the blueness of the face from congestion in straining; this condition- being produced by pressure on the small pulmonary vessels. We may summarize this mechanical effect of respiration on the blood- pressure therefore, and say that inspiration aids the circulation and so increases the arterial tension, and that although expiration does not KESPIKATION. 203 materially aid the circulation, yet under ordinary conditions neither does it obstruct it. Under extraordinary conditions, however, as in violent expirations, the circulation is decidedly obstructed. But we have seen that there is no exact correspondence between the points of extreme arterial tension and the end of inspiration, and we must look to the ner- vous system for an explanation of this apparently contradictory result. The effect of the nervous system in producing a rhythmical alteration of the blood-pressure is twofold. In the first place the car die-inhibitory centre is believed to be stimulated during the fall of blood-pressure, pro- ducing a slower rate of heart-beats during expiration, which will be FIG. 163.— Traube-Hering's curves. (To be read from left to right.) The curves 1, 2, 3, 4, and 5 are portions selected from one continuous tracing forming the record 9f a prolonged observation, so that the several curves represent successive stages of the same experiment. Each curve is placed in its proper position relative to the base line, which is omitted; the blood-pressure rises in stages from 1, to 2, 3, and 4, but falls again in stage 5. Curve 1 is taken from a period when artificial res- piration was being kept up, but the vagi having been divided, the pulsations on the ascent and de- scent of the undulations do not differ; when artificial respiration ceased these undulations fora while disappeared, and the blood-pressure rose steadily while the heart-beats became slower. Soon, as at 2, new undulations appeared; a little later, the blood-pressure was still rising, the heart- beats still slower, but the undulations still more obvious (3) ; still later (4), the pressure was still higher, but the heart-beats were quicker and the undulations flatter, the pressure then began to fall rapidly (5), and continued to fall until some time after artificial respiration was resumed. (M. Fos- noticed in the tracing (Fig. 162). The undulations during the decline of blood-pressure being longer but less frequent, this effect disappears when, by section of the vagi, the effect of the centre is cut off from the heart; and in the second place, the vaso-motor centre is also believed to '204 HANDBOOK OF PHYSIOLOGY. send out rhythmical impulses, by which undulation of blood -pressure fe produced independently of the mechanical effects of respiration. The action of the vaso-motor centre in taking part in producing rhythmical changes of blood-pressure which are called respiratory, is shown in the following way: — In an animal under the influence of urari, .a record of whose blood-pressure is being taken, and where artificial res- piration has been stopped, and both vagi cut, the blood-pressure curve rises at first almost in a straight line, but after a time new rhythmical undulations occur very like the original respiratory undulations, only somewhat larger. These are called Traube's or Traiibe-Hering* s curves. These continue whilst the blood-pressure continues to rise and only cease when the vaso-motor centre and the heart are exhausted, when the pressure speedily falls. These curves must be dependent upon the vaso- motor centre, as the mechanical effects of respiration have been elimi- nated by the poison and by the cessation of artificial respiration, and the •effect of the cardio-inhibitory centre by the division of the vagi. It may be presumed therefore that the vaso-motor centre, as well as the cardio- inhibitory, must be considered to take part with the mechanical changes of inspiration and expiration in producing the so-called respiratory undulations of blood-pressure. Cheyne- Stokes' breathing. — This is a rhythmical irregularity in res- pirations which has been observed in various diseases, and is especially connected with fatty degeneration of the heart. Respirations occur in groups, at the beginning of each group the inspirations are very shallow, but each successive breath is deeper than the preceding until a climax is reached, after which the inspirations become less and less deep, until they cease after a slight pause altogether. Apnoea.— Dyspnoea. — Asphyxia. As blood which contains a normal proportion of oxygen sufficiently excites the respiratory centre (p. 199) to produce normal respiration, and, as the excitement and consequent respiratory muscular movements are greater (dyspnoea) in proportion to the deficiency of this gas, so an abnormally large proportion of oxygen in the blood leads to diminished breathing movements, and, if the proportion be large enough, to their temporary cessation. This condition of absence of breathing is termed Apncea,1 and it can be demonstrated, in one of the lower animals, by performing artificial respiration to the extent of saturating the blood ivith oxygen. When, on the other hand, the respiration is stopped, by, e.g., inter- ference with the passage of air to the lungs, or by supplying air devoid 1 This term lias been, unfortunately, often applied to conditions of dyspnoea or as- phyxia ; but the modern application of the term, as in the text, is the more convenient. RESPIRATION. 205 of oxygen, a condition ensues, which passes rapidly from HYPERPXCEA (excessive breathing) to the state of DYSPNCEA (difficult breathing), and afterwards to ASPHYXIA ; and the latter quickly ends in death. The ways by which this condition of asphyxia may be produced are very numerous. As, for example, by the prevention of the due entry of oxygen into the blood, either by direct obstruction of the trachea or other part of the respiratory passages, or by introducing instead of ordinary air a gas devoid of oxygen, or by interference with the due interchange of gases between the air and the blood. Symptoms, — The symptoms of asphyxia may be divided into three groups, which correspond with the stages of the condition which are usually recognized, these are (1), the stage of exaggerated breathing ; (2), the stage of convulsions ; (3), the stage of exhaustion. In the first stage the patient breathes more rapidly and at the same time more deeply than usual, the inspirations at first being especially ex- aggerated and prolonged. The muscles of extraordinary inspiration are called into action and the effort to respire is labored and painful. This is soon followed by a similar increase in the expiratory efforts, which be- come excessively prolonged, being aided by all the muscles of extraordi- nary expiration. During this stage, which lasts a varying time, from a minute upwards, according as the deprivation of oxygen is sudden or gradual, the patient's face and lips become blue, his eyes are prominent, and his expression intensely anxious. The prolonged respirations are accompanied by a distinctly audible sound ; the muscles attached to the chest stand out as distinct cords. The stage includes the two conditions hyperpnoea and dyspnoea already spoken of. It is due to the increasingly powerful stimulation of the respiratory centres by the increasingly venous blood. In the second stage, which is not marked out by any distinct line of demarcation from the first, the violent expiratory efforts give way to general convulsions (in men and other warm-blooded animals at any rate), which arise from the further stimulation of the centres. The spasms of the muscles are those of the body in general, and not of the respiratory muscles only. The convulsive stage is a short one, and soon passes into the third stage, of exhaustion. In it, the respirations all but cease, the spasms give way to flaccidity of the muscles, the patient is insensible, the conjunctivas are insensitive and the pupils are widely dilated. Every now and then a prolonged sighing inspiration takes place, at longer and longer intervals until they cease altogether, and the patient dies. During this stage the pulse is scarcely to be felt, but the heart may beat for some seconds after respirations have quite ceased. The condition is due to the gradual paralysis of the respiratory centre by the prolonged action of the increasingly venous blood. As with the first stage, the duration of the second and third stages 206 HANDBOOK OF PHYSIOLOGY. depends upon the manner of the deprivation of oxygen, whether sudden or gradual. The convulsive stage is short, lasting, it may be, only one minute. The third stage may last three minutes and upwards. The circulatory conditions which accompany these symptoms are — (1) More or less interference with the passage of the blood through the pulmonary blood-vessels. (2) Accumulation of blood in the right side of the heart and in the systemic veins. (3) Circulation of impure (non-aerated) blood in all parts of the body. Cause of death. — The causes of these conditions and the manner in which they act, so as to be incompatible with life, may be here briefly considered. (1) The obstruction to the passage of blood through the lungs is not very great ; and such as there is occurs chiefly in the later stages of asphyxia, when, by the violent and convulsive action of the expiratory muscles, pressure is indirectly made upon the lungs, and the circulation through them is proportionately interfered with. ('2) Accumulation of blood, with consequent distention of the right side of the heart and of the systemic veins, is the direct result, at least in part, of the obstruction to the pulmonary circulation just referred to. Other causes, however, are in operation, (a) The vaso-motor centres stimulated by blood deficient in oxygen, causes contraction of all the small arteries with increase of arterial tension, and as an immediate consequence the filling of the systemic veins, (b) The increased arte- rial tension is followed by inhibition of the action of the heart, and, the heart, contracting less frequently, and also gradually enfeebled by defi- cient supply of oxygen, becomes over-distended with blood which it cannot expel. At this stage the left as well as the right cavities are over-distended. The ill effects of these conditions are to be looked for partly in the heart, the muscular fibres of which, like those of the urinary bladder or any other hollow muscular organ, may be paralyzed by over-stretching ; and partly in the venous congestion, and consequent interference with the function of the higher nerve-centres, especially the medulla oblongata. (3) The passage of non-aerated blood through the lungs and its distri- bution over the body are events incompatible with life in one of the higher animals, for more than a few minutes ; the rapidity with which death ensues in asphyxia being due, more particularly, to the effect of non-oxygenized blood on the medulla oblongata, and, through the coro- nary arteries, on the muscular substance of the heart. The excitability of both nervous and muscular tissue is dependent on a constant and large supply of oxygen, and, when this is interfered with, excitability is RESPIRATION. 207 rapidly lost. The diminution of oxygen has a more direct influence in the production of the usual symptoms of asphyxia than the increased amount of carbonic acid. Indeed, the fatal effect of a gradul accumu- lation of the latter in the blood, if a due supply of ox \ gen is maintained, resembles rather that of a narcotic poison, and not of asphyxia. In some experiments performed by a committee appointed by the Medico-Chirurgical Society to investigate the subject of Suspended Ani- mation, it was found that, in the dog, during simple asphyxia, i. e., by simple privation of air, as by plugging the trachea, the average duration of the respiratory movements after the animal had been deprived of air, was 4 minutes and 5 seconds ; the extremes being 3 minutes 30 seconds, and 4 minutes 40 seconds. The average duration of the heart's action, on the other hand, was 7 minutes 11 seconds ; the extremes being 6 minutes 40 seconds, and 7 minutes 45 seconds. It would seem, there- fore, that on an average, the heart's action continues for 3 minutes 15 seconds after the animal had ceased to make respiratory efforts. A very similar relation was observed in the rabbit. Kecovery never took place after the heart's action had ceased. The results obtained by the committee on the subject of droivning were very remarkable, especially in this respect, that whereas an animal may recover, after simple deprivation of air for nearly four minutes, yet, after submersion in water for 1-j- minutes, recovery seems to be impos- sible. This remarkable difference was found to be due, not to the mere submersion, nor directly to the struggles of the animal, nor to depres- sion of temperature, but to the two facts, that in drowning, a free pas- sage is allowed to air out of the lungs, and a free entrance of water into them. It is probably to the entrance of water into the lungs that the speedy death in drowning is mainly due. The results of post-mortem examination strongly support this view. On examination the lungs of animals deprived of air by plugging the trachea, they were found simply congested ; but in the animals drowned, not only was the con- gestion much more intense, accompanied with ecchymosed points on the surface and in the substance of the lung, but the air tubes were com- pletely choked up with a sanious foam, consisting of blood, water, and mucus, churned up with the air in the lungs by the respiratory efforts of the animal. The lung-substance, too, appeared to be saturated and sodden with water, which, stained slightly with blood, poured out at any point where a section was made. The lung thus sodden with water was heavy (though it floated), doughy, pitted on pressure, and was incapable of collapsing. It is not difficult to understand how, by such infarction of the tubes, air is debarred from reaching the pulmonary cells ; indeed the inability of the lungs to collapse on opening the chest is a proof of the obstruction which the froth occupying the air-tubes offers to the transit of air. We must carefully distinguish the asphyxiating effect of an insuffi- cient supply of oxygen from the directly poisonous action of such gases as carbonic oxide, which is contained to a considerable amount in com- mon coal-gas. The fatal effects often produced by this gas (as in acci- dents from burning charcoal stoves in small, close rooms), are due to its entering into combination with the haemoglobin of the blood-corpuscles (p. 87). and thus expelling the oxven. CHAPTER VI. FOODS AND DIET. IN order that life of the individual may be maintained it is neces- sary that his body should be supplied with food in proper quality and quantity. The food taken in by the animal body is used for the purpose of re- placing the waste of the tissues. In order to arrive, therefore, at a reasonable estimation of the proper diet required in the twenty-four hours, it is essential that we should know the amount and composition of the excreta daily eliminated from the body. Careful analysis of the excreta shows that they are made up chiefly of the chemical elements, carbon, hydrogen, oxygen, and nitrogen, but that they also contain to a less extent, sulphur, phosphorus, chlorine, potassium, sodium, and certain other of the elements. Since this is the case it must be evident that, to balance this waste, foods must be supplied containing all these elements to a certain degree, and some of them, viz., those which take a principal part in forming the excreta, in large amount. Of the excreta we have seen in the last Chapter that carbonic acid and ammonia, which are made up of the elements, carbon, oxygen, nitrogen, hydrogen, are given off from the lungs. By the excretion of the kidneys — the urine — many elements are eliminated from the blood, especially nitrogen, hydrogen, and oxygen. In the sweat, the elements chiefly represented are carbon, hydrogen, and oxygen, and also in the faeces. By all the excretions large quantities of water are got rid of daily, but chiefly by the urine. The relations between the amounts of the chief elements contained in these various excreta in twenty-four hours may be thus summarized: Water. C. H. N. O. By the lungs 330 248.8 .... ? 651.15 By the skin. ... 660 2.6 .... 7.2 By the urine By the fasces 1700 128 9 8 20. 3 3 3 15.8 3. 11.1 12. Grammes 2818 281.2 6 3 18.8 681.41 FOODS AND DIET. 209 To this should be added 296. grammes water, which are produced by the union of hydrogen and oxygen in the body during the process of oxidation (i. e., 32.89 hydrogen and 263.11 oxygen). There are twenty- six grammes of salts got rid of by the urine and six by the fasces. The quantity of carbon daily lost from the body amounts to about 281.2 grammes or nearly 4,500 grains, and of nitrogen 18.8 grammes or nearly 300 grains; and if a man could be fed by these elements, as such, the problem would be a very simple one; a corresponding weight of charcoal, and, allowing for the oxygen in it, of atmospheric air, would be all that is necessary. But an animal can live only upon these ele- ments when they are arranged in a particular manner with others, in the form of an organic compound, as albumen, starch, and the like; and the relative proportion of carbon to nitrogen in either of these compounds alone, is, by no means, the proportion required in the diet of man. Thus, in albumen, the proportion of carbon to nitrogen is only as 3.5 to 1. If, therefore, a man took into his body, as food, sufficient albumen to supply him with the needful amount of carbon, he would receive more than four times as much nitrogen as he wanted; and if he took only sufficient to supply him with nitrogen, he would be starved for want of carbon. It is plain, therefore, that he should take with the albuminous part of his food, which contains so large a relative amount of nitrogen in proportion to the carbon he needs, substances in which the nitrogen exists in much smaller quantities relatively to the carbon. It is therefore evident that the diet must consist of several sub- stances, not of one alone, and we must therefore turn to the available food- stuffs. For the sake of convenience they may be classified as under: A. ORGANIC. I. Nitrogenous, consisting of Proteids, e.g., albumen, casein, syntonin, gluten, legumin and their allies ; and Gelatins, which include gelatin, elastin, and chondrin. All of these contain carbon, hydrogen, oxygen, and nitrogen and some in addition, P. and S. II. Non-Nitrogenous, comprising : (1.) Amyloid or saccharine bodies, chemically known as carbo-hy- drates, since they contain carbon, hydrogen, and oxygen, with the last two elements in the proportion to form water, i.e., H^nOn. To this class belongs starch and sugar. (2.) Oils and fats. — These contain carbon, hydrogen, and oxygen, but the oxygen is less in amount than in the amyloids and saccharine bodies. B. INORGANIC. I. Mineral and saline matter. II. Water. 14 210 HANDBOOK OF PHYSIOLOGY. To supply the loss of nitrogen and carbon, it is found by experience that it is necessary to combine substances which contain a large amount of nitrogen with others in which carbon is in considerable amount ; and although, without doubt, if it were possible to relish and digest one or other of the above-mentioned proteids when combined with a due quan- tity of an amyloid to supply the carbon, such a diet, together with salt and water, ought to support life ; yet we find that for the purposes of ordinary life this system does not answer, and instead of confining our nitrogenous foods to one variety of substance we obtain it in a large number of allied substances, for example, in flesh, of bird, beast, or fish; in eggs ; in milk ; and in vegetables. And, again, we are not content with one kind of material to supply the carbon necessary for maintaining life, but seek more, in bread, in fats, in vegetables, in fruits. Again, the fluid diet is seldom supplied in the form of pure water, but in beer, in wines, in tea and coffee, as well as in fruits and succulent vegetables. Man requires that his food should be cooked. Very few organic sub- stances can be properly digested without previous exposure to heat and to other manipulations which constitute the process of cooking. A. — Foods containing nitrogenous principles chiefly. I. — Flesh of Animals, of the ox (beef, veal), sheep (mutton, lamb), pig (pork, bacon, ham). Of these, beef is richest in nitrogenous matters, containing about 20 per cent, whereas mutton contains about 18 per cent, veal, 16.5, and pork, 10 ; the flesh is also firmer, more satisfying, and is supposed to be more strengthening than mutton, whereas the latter is more digestible. The flesh of young animals, such as lamb and veal, is less digestible and less nutritious. Pork is comparatively indigestible and contains a large amount of fat. Flesh contains : — (1) Nitrogenous bodies : myosin, serum-albumin, gelatin (from the interstitial fibrous connective tissue); elastin (from the elastic tissue), as well as hcemoglobin. (2) Fatty matters, including lecithin and cholesterin. (3) Extractive matters, some of which are agreeable to the palate, e.g., osmazome, and others, which are weakly stimulating, e.g., Jcreatin. Besides, there are sarcolactic and inositic acids, taurin, xanthin, and others. (4) Salts, chiefly of potassium, cal- cium, and magnesium. (5) Water, the amount of which varies from 15 per cent in dried bacon to 39 in pork, 51 to 53 in fat beef and mutton, to 72 per cent in lean beef and mutton. (6) A certain amount of carbo- hydrate material is found in the flesh of some animals, in the form of inosite, dextrin, grape sugar, and (in young animals) glycogen. FOODS AND DIET. 211 TABLE OF PERCENTAGE COMPOSITION OF BEEF, MUTTON, PORK AND VEAL — (LETHEBY.) Water. Albumen. Fats. Salts. Beef.— Lean, ... 72 19.3 3.6 5.1 Fat, . . . 51 14.8 29.8 4.4 Mutlon.—Lean, . .72 18.3 4.9 4.8 Fat, ... 53 12.4 31.1 3.5 Veal, . . . . 63 16.5 15.8 4.7 Pork.- Fat, . . .39 9.8 48.9 2.3 Together with the flesh of the above-mentioned animals, that of the deer, hare, rabbit, and birds, constituting venison, game, and poultry, should be added as taking part in the supply of nitrogenous substances, and also fish — salmon, eels, etc., and shell-fish, e.g., lobster, crab, mus- sels, oysters, shrimps, scollops, cockles, etc. TABLE OF PERCENTAGE COMPOSITION' OF POULTRY AND FISH. — (LETHEBY.) Water. Albumen. Fats. Salts. Poultry, .... 74 21 3.8 1.2 (Singularly devoid of fat, and is therefore generally eaten with bacon or pork. ) Water. Albumen. Fats. Salts. White Fish, . . . 78 18.1 2.9 1. Salmon, ... 77 16.1 5.5 1.4 Eels (very rich in fat), . 75 9.9 13.8 1.3 Oysters, . 75.74 11.72 2.42 2.73 (7.39 consist of non-nitrogenous matter and loss.) (Payen.) Even now the list of fleshy foods is not complete, as the flesh of nearly all animals has been occasionally eaten, and we may presume that except for difference of flavor, etc., the average composition is nearly the same in every case. II. Milk. — Is intended as the entire food of young animals, and as such contains, when pure, all the elements of a typical diet. (1) Albu- minous substances in the form of casein and serum-albumin. (2) Fats in the cream. (3) Carbohydrates in the form of lactose or milk sugar. (4) Salts, chiefly calcium phosphate ; and (5) Water. From it we obtain (a) cheese, which is the casein precipitated with more or less of fat according as the cheese is made of skim milk (skim cheese), or of fresh milk with its cream (Cheddar and Cheshire), or of fresh milk plus cream (Stilton and double Gloucester). The precipitated casein is allowed to ripen, by which process some of the albumen is split up, with formation of fat. (0) Cream, consists of the fatty globules incased in casein, and which being of low specific gravity float to the surface, (y) Butter, or the fatty matter deprived of its casein envelope by the process 212 HANDBOOK OF PHYSIOLOGY. of churning. (6) Butter-milk, or the fluid obtained from cream after butter has been formed; very rich therefore in nitrogen, (f) Whey, or the fluid which remains after the precipitation of casein; it contains sugar, salt, and a small quantity of albumen. TABLE OF COMPOSITION OF MILK, BUTTER-MILK, CKEAM, AND CHEESE. — (LETHEBY AND PA YEN.) Nitrogenous matters. Fats. Lactose. Salts. Water. Milk (Cow), . .4.1 3.9 5.2 - .8 86 Butter-milk, . . 4.1 .7 6.4 .8 88 Cream, . . .2.7 26.7 2.8 1.8 66 Cheese.— Skim, . 44.8 6.3 — 4.9 44 " Cheddar, . 28.4 31.1 4.5 36 Non-nitrogenous matter and loss. " Neufchatel (fresh), 8. 40.71 36.58 .51 36.58 III. Eggs. — The yelk and albumen of eggs are in the same relation as food for the ernbryoes of oviparous animals that milk is to the young of mammalia, and afford another example of the natural admixture of the various alimentary principles. TABLE OF THE PERCENTAGE COMPOSITION OF FOWLS' EGGS. Nitrogenous substances. Fats. Salts. Water. White, . . . . 20.4 — 1.6 78 Yelk, ... 16. 30.7 1.3 52 IV. Leguminous fruits are used by vegetarians, as the chief source of the nitrogen of the food. Those chiefly used are peas, beans, lentils, etc., they contain a nitrogenous substance called legumin, allied to albu- men. They contain about 25.30 per cent of this nitrogenous body, and twice as much nitrogen as wheat. B. Foods containing carbohydrate bodies chiefly. I. Bread, made from the ground grain obtained from various so- called cereals, viz., wheat, rye, maize, barley, rice, oats, etc., is the direct form in which the carbohydrate is supplied in an ordinary diet. Flour, however, besides the starch, contains gluten, a nitrogenous body, and a small amount of fat. TABLE OF PERCENTAGE COMPOSITION OF BREAD AND FLOUR. Nitrogenous matters. Carbohydrates. Fats. Salts. Water. Bread, . . 8.1 51. 1.6 2.3 37 Flour, . . 10.8 70.85 2. 1.7 15 Various articles of course besides bread are made from flour, e. g., sago, macaroni, biscuits, etc. FOODS AND DIET. 213 II. Vegetables, especially potatoes. They contain starch and sugar. III. Fruits contain sugar, and organic acids, tartarie, malic, citric, and others. C. Substances supplying fatty bodies principally. The chief are butter, lard (pig's fat), suet (beef and mutton fat). D. Substances supplying the salts of the food. Nearly all the foregoing substances in A, B, and C, contain a greater or less amount of the salts required in food, but green vegetables and fruits supply certain salts, without which the normal health of the body cannot be maintained. E. Liquid foods. Water is consumed alone, or together with certain other substances used to flavor it, e.g., tea, coffee, etc. Tea in moderation is a stimulant, and contains an aromatic oil to which it owes its peculiar aroma, an as- tringent of the nature of tannin, and an alkaloid, theine. The compo- sition of coffee is very nearly similar to that of tea. Cocoa, in addition to similar substances contained in tea and coffee, contains fat, albuminous matter, and starch, and must be looked upon more as a food. Beer, in various forms, is an infusion of malt (barley which has sprouted, and in which its starch is converted in great part into sugar), boiled with hops and allowed to ferment. Beer contains from 1.2 to 8.8 per cent of alcohol. Cider and Perry, the fermented juice of the apple and pear. Wine, the fermented juice of the grape, contains from 6 or 7 (Rhine wines, and white and red Bordeaux) to 24-25 (ports and sherries) per cent of alcohol. Spirits, obtained from the distillation of fermented liquors. They contain upwards of 40-70 per cent of absolute alcohol. Effects of cooking upon Food. In general terms this may be said to make food more easily digestible; this usually implies two alterations — food is made more agreeable to the palate and also more pleasing to the eye. Cooking consists in exposing the food to various degrees of heat, either to the direct heat of the fire, as in roasting, or to the indirect heat of the fire, as in broiling, baking, or frying, or to hot water, as in boiling or stewing. The effect of heat upon (a) flesh is to coagulate the albumen and coloring matter, to solid- ify fibrin, and to gelatinize tendons and fibrous connective tissue. Pre- vious beating or bruising (as with steaks and chops), or keeping (as in the case of game), renders the meat more tender. Prolonged exposure HANDBOOK OF PHYSIOLOGY, to heat also develops on the surface certain empyreumatic bodies, which are agreeable both to the taste and smell. By placing meat in hot water, the external coating of albumen is coagulated, and very little, if any, of the constituents of the meat are lost afterwards if boiling be prolonged; but if the constituents of the meat are to be extracted, it should be ex- posed to prolonged simmering at a much lower temperature, and the "broth" will then contain the gelatin and extractive matters of the meat, as well as a certain amount of albumen. The addition of salt will help to extract myosin. The effect of boiling upon (Z») an egg is to coagulate the albumen, and this helps to render the article of food more suitable for adult dietary. Upon (c) milk, the effect of heat is to produce a scum composed of al- bumen and a little casein (the greater part of the casein being uncoag- ulated) with some fat. Upon (d) vegetables, the cooking produces the necessary effect of rendering them softer, so that they can be more readily broken up in the mouth; it also causes the starch grains to swell up and burst, and so aids the digestive fluids in penetrating into their substance. The albuminous matters are coagulated, and the gummy, saccharine and saline matters are removed. The conversion of flour into dough is effected by mixing it with water, and adding a little salt and a certain amount of yeast. Yeast consists of the cells of an organized ferment ( Torula cerevisice), and it is by -the growth of this plant, which lives upon the sugar produced from the starch of the flour, that a quan- tity of carbonic acid gas and alcohol is formed. By means of the former the dough rises. Another method of making dough consists in mixing the flour with water containing a large quantity of carbonic acid gas in solution. By the action of heat during baking (d) the dough continues to ex- pand, and the gluten being coagulated, the bread sets as a permanently vesiculated mass. I. — Effects of an insufficent diet. Hunger and Thirst. — The sensation of hunger is manifested in con- sequence of deficiency of food supplied to the system. The mind refers the sensation to the stomach; yet since the sensation is relieved by the introduction of food either into the stomach itself, or into the blood through other channels than the stomach, it would appear not to depend on the state of the stomach alone. This view is confirmed by the fact, that the division of both pneumogastric nerves, which are the principal channels by which the brain 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 FOODS AND DIET. 215 into this organ. It may, therefore, be said that the sensation of hunger is caused both by a want in the system generally, and also by the condi- tion of the stomach itself, by which condition, of course, its own nerves are more directly affected. The sensation of thirst, indicating the want of fluid, is referred to the fauces, although, as in hunger, this is, in great part, only the local declaration of a general condition. For thirst is relieved for only a very short time by moistening the dry fauces ; but may be relieved completely by the introduction of liquids into the blood, either through the stomach, by injections into the blood-vessels, or by absorption from the surface of the skin or the intestines. The sensation of thirst is perceived most naturally whenever there is a disproportionately 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. 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 sensa- tion of the " necessity of breathing," is referred especially to the air- passages ; but, as Volkmann's experiments show, it depends on the con- dition of the blood which circulates everywhere, and is felt even after the lungs of animals are removed ; for they continue, even then, to gasp and manifest the sensation of want of breath. Starvation. — The effects of total deprivation of food have been made the subject of experiments on the lower animals, and have been but too frequently illustrated in man. (1) One of the most notable effects of starvation, as might be expected, is loss of weight ; the loss being great- est at first, as a rule, but afterwards not varying very much, day by day, until death ensues. Chossat found that the ultimate proportional loss was, in different animals experimented on, almost exactly the same ; death occurring when the body had lost two-fifths (forty per cent) of its original weight. Different parts of the body lose weight in very differ- ent proportions. The following results are taken, in round numbers, from the table given by M. Chossat : — Fat loses, .... 93 per cent. Blood, 75 " " Spleen, .... 71 " " Pancreas, .... 64 " " Stomach loses, . . 39 " " Pharynx, (Esophagus, 34 " " Skin, 33 " " Kidneys lose, . . 31 " " (2.) The effect of starvation on the temperature of the various ani- mals experimented on by Chossat was very marked. For some time the variation in the daily temperature was more marked than its absolute and continuous diminution, the daily fluctuation amounting to 5° or 6° Liver loses, ... 52 per cent. Heart, 44 " " Intestines, . . . 42 " " Muscles of locomotion, 42 " " Respiratory apparatuses " " Bones, 16 " " Eyes, 10 " " Nervous System, . . 2 (nearly.) 2 It) HANDBOOK OF PHYSIOLOGY. F. (3° C.), instead of 1° or 2° F. (.5° to 1° C.), as in health. But a. short time before death, the temperature fell very rapidly, and death ensued when the loss had amounted to about 30° F. (16.2° C.). It has been often said, and with truth, although the statement requires some qualification, that death by starvation is really death by cold ; for not only has it been found that difference of time with regard to the period of the fatal result are attended by the same ultimate loss of heat, but the effect of the application of external warmth to animals cold, and dy- ing from starvation, is more effectual in reviving them than the admin- istration of food. In other words, an animal exhausted by deprivation of nourishment is unable so to digest food as to use it a? fuel, and there- fore is dependent for heat on its supply from without. (3.) The symptoms produced by starvation in the human subject are hunger, accompanied, or it may be replaced, by pain, referred to the region of the stomach ; insatiable thirst ; sleeplessness ; general weak- ness and emaciation. The exhalations both from the lungs and skin are foetid, indicating the tendency to decomposition which belongs to badly- nourished tissues ; and death occurs, sometimes after the additional ex- haustion caused by diarrhoea, often with symptoms of nervous disorder, delirium or convulsions. (4.) In the human subject death commonly occurs within six to ten days after total deprivation of food. But this period may be consider- ably prolonged by taking a very small quantity of food, or even water only. The cases so frequently related of survival after many days, or even some weeks, of abstinence, have been due either to the last-men- tioned circumstances, or to others no less effectual, which prevented the loss of heat and moisture. Cases in which life has continued after total abstinence from food and drink for many weeks, or months, exist only in the imagination of the vulgar. (5. ) The appearances presented after death from starvation are those of general wasting and bloodlessness, the latter condition being least no- ticeable in the brain. The stomach and intestines are empty and con- tracted, and the walls of the latter appear remarkably thinned and almost transparent. The various secretions are scanty or absent, with the exception of the bile, which, somewhat concentrated, usually fills the gall-bladder. All parts of the body readily decompose. II. — Effects of Improper Diet. Experiments on Feeding. — Experiments illustrating the ill-effects produced by feeding animals upon one or two alimentary substances only have been often performed. Dogs were fed exclusively on sugar and distilled water. During the first seven or eight days they were brisk and active, and took their food FOODS AND DIET. 217 and drink as usual ; but in the course of the second week they began to get thin, although their appetite continued good, and they took daily between seven and eight ounces of sugar. The emaciation increased during the third week, and they became feeble, and lost their activity and appetite. At the same time an ulcer formed on each cornea, followed by an escape of the humors of the eye : this took place in repeated ex- periments. The animals still continued to eat three or four ounces of sugar daily ; but became at length so feeble as to be incapable of motion and died on a day varying from the thirty-first to the thirty-fourth. On dissection, their bodies presented all the appearances produced by death from starvation'; indeed, dogs will live almost the same length of time without any food at all. When dogs were fed exclusively on gum, results almost similar to.the above ensued. When they were kept on olive oil and water, all the phe- nomena produced were the same, except that no ulceration of the cornea took place ; the effects were also the same with butter. The experi- ments of Ohossat and Letellier prove the same ; and in men, the same is shown by the various diseases to which those who consume but little nitrogenous food are liable, and especially by the affection of the cornea which is observed in Hindus feeding almost exclusively on rice. But it is not only the non-nitrogenous substances, which, taken alone, are in- sufficient for the maintenance of health. The experiments of the Acad- emies of France and Amsterdam were equally conclusive that gelatin alone soon ceases to be nutritive. III.— Effect of Too Much Food. Sometimes the excess of food is so great that it passes through the alimentary canal, and is at once got rid of by increased peristaltic action of the intestines. In other cases, the unabsorbed portions undergo putre- factive changes in the intestines, which are accompanied by the produc- tion of gases, such as carbonic acid, carbu retted and sulphuretted hydrogen, and a distended condition of the bowels, together with symp- toms of indigestion, is the result. An excess of the substances required as food may undergo absorption. It is a well-known fact that numbers of people habitually eat too much, and especially of nitrogenous food. Dogs can digest an immense amount of meat if fed of ten, and the amount of meat taken by some men would supply not only the nitrogen, but also the carbon which is requisite for an ordinary natural diet. A method of getting rid of an excess of nitrogen is provided by the digestive pro- cesses in the duodenum, to be presently described, whereby the excess of the albuminous food is capable of being changed before absorption into nitrogenous crystalline matters easily converted into urea and so easily excreted by the kidneys, affording one variety of what is called luxus 218 HANDBOOK OF PHYSIOLOGY. consumption ; but no doubt after a time the organs, especially the liver upon which the extra amount of the ingested diet throws most of the stress, will yield to the strain of the over-work, and will not reduce the excess of nitrogenous material brought to it into urea, but into other less oxidized products, such as uric acid ; general plethora, and gout being the result. This state of things, however, is delayed for a long time, if not altogether obviated, when large meat-eaters take a considera- ble amount of exercise. Excess of carbohydrate food produces an accumulation of fat, which may not only be an inconvenience by causing obesity, but may interfere with the proper nutrition of muscles, causing a feebleness of the action of the heart, and other troubles. The accumulation of fat is due to the excess of carbohydrate being stored up by the protoplasm in the form of fat. Starches when taken in great excess are almost certain to give rise to dyspepsia, with acidity and flatulence. Excess of starch or of sugar in the food may, however, be got rid of by the urine in the form of gly- cosuria. There is evidently a limit to the absorption of starch and of fat, as, if taken beyond a certain amount, they appear unchanged in the faeces. Requisites of a Normal Diet. It will have been understood that it is necessary that a normal diet should be made up of various articles, that they should be well cooked, and that they should contain about the same amount of carbon and ni- trogen as are got rid of by the excreta. No doubt these desiderata may be satisfied in many ways, and it would be unreasonable to expect that the diet of every adult should be unvarying. The age, sex, strength, and circumstances of each individual must ultimately determine his diet. A dinner of bread and hard cheese with an onion contains all the requisites for a meal, but such diet would be suitable only for those possessing strong digestive powers. It is a well-known fact that the diet of the continental nations differs from that of our own country, and that of cold from that of hot climates, but the same principle underlies them all, viz., the replacement of the loss of the excreta in the most convenient and economical way possible. Without going into detail in the matter^ it may be said that any one in active work requires more nitrogenous matter than one at rest, and that children and women require less than adult men. The quantity of food for a healthy adult man of average height and weight may be stated in the following table: — FOODS AND DIET. 219 TABLE or FOOD REQUIRED FOR A HEALTHY ADULT. (PARKES.) For laborious Af f occupation. Nitrogenous substances, e.g. , flesh, . 6 to 7 oz. av. 2.5 oz. Fats, . . . . . . 3. 5 to 4. 5 oz. 1 oz. Carbo-hydrates, . . . . 10 to 18 oz. 12 o Salts, .... . 1.2 to 1.5 oz. .5 oz. 26.7 to ;31 oz. 10 oz. The above table contains the weights of dry or solid food required. Such food is found in practice to be nearly always combined with 50 to 60 per cent of water, and so the above numbers should be correspond- ingly increased. The amount of liquids required in addition is about three pints per diem. Full diet scale for an adult male in hospital (St. Bartholomew's Hospital) . Breakfast. — 1 pint of tea (with milk and sugar), bread and butter. Dinner. — Jib. of cooked meat, -Jib. potatoes, bread and beer. Tea. — 1 pint of tea, bread and butter. Supper. — Bread and butter, beer. Daily alloivance to each patient. — 'I pints of tea, with milk and sugar; 14 oz. bread; ^lb. cooked meat; -Jib. potatoes; 2 pints of beer; loz. but- ter; 31 oz. solid, and 4 pints (80 oz.), liquid. CHAPTER VII. DIGESTION. THE object of digestion is to prepare the food to supply the waste of the tissues, which we have seen is its proper function in the economy. Few of the articles of diet are taken in the exact condition in which it is possible for them to be absorbed into the system by the blood-vessels and lymphatics, without which they would be useless fcr the purposes they have to fulfil. Almost the whole of the food, therefore, undergoes various digestive changes before it is fit for absorption. Having been received into the mouth, it is subjected to the action of the teeth and tongue, and is mixed with the first of the digestive juices — the saliva. It is then swallowed, and, passing through the pharynx and oesophagus into the stomach, is subjected to the action of the gastric juice — the second digestive juice. Thence it passes into the intestines, where it meets with the bile, i\iQ pancreatic juice, and the intestinal juices, all of which exer- cise an influence upon the portion of the food not absorbed in the stomach. By this time most of the food is capable of absorption, and the residue of undigested matter leaves the body in the form offices by the anus. The course of the food through the alimentary canal of man will be readily seen from the accompanying diagram (Fig. 1G4). The Mouth is the cavity contained between the jaws and inclosed by the cheeks laterally, the lips anteriorly; behind it opens into the pha- rynx by the fauces, and is separated from the nasal cavity above, by the hard palate in front, and the soft palate behind, which forms its roof. The tongue forms the lower part or floor. In the jaws are contained the teeth, and when the mouth is shut these form its anterior bounda- ries. The whole of the mouth is lined with mucous membrane, covered with stratified squamous epithelium, which is continuous in front along the lips with the epithelium and the skin, and posteriorly with that of the pharynx. The mucous membrane is provided with numerous glands (small tubular), called mucous glands, and into it open the ducts of the salivary glands, three chief glands on each side. The tongue is not only a prehensile organ, but is also the chief seat of the sense of taste. We shall first* of all devote some little space to the consideration of the structure and development of the teeth, and then shall proceed to- DIGESTION. 221 discuss, in detail, the process of digestion, as it takes place in. each stage of the journey of food through the alimentary canal. FIG. 164.— Diagram of the Alimentary Canal. The small intestine of man is from about 3 to 4 times as long as the large intestine. The Teeth. During the course of his life, man in common with other mammals, is provided with two sets of teeth, the first set is called the temporary or milk teeth, which makes its appearance in infancy, and is in the course of a few years shed and replaced hy the second or permanent set. The temporary or milk teeth have only a very limited term of existence. They are ten in number in each jaw, namely, on either side from the middle line two incisors, one canine, and two deciduous molars, and are replaced by ten permanent teeth. The number of permanent teeth in each jaw is, however, increased to sixteen, by the development of three others on each side of the law. 222 HANDBOOK OF PHYSIOLOGY. The following formula shows, at a glance, the comparative arrange- ment and number of the temporary and permanent teeth : — Dec. Mo. Ca. In. Ca. Mo. (Upper 21412 =10 J Temporary Teeth. ( Lower =20 = 10 Bicuspids True or Prse- ( Upper 3 ( Lower 3 Molars. Ca. In. Ca, Bi. Mo . 2 1 4 1 2 3 = 16 - — 22 Permanent Teeth. 21412 3 = 16 From this formula it will be seen that the two bicuspid or praemolar teeth in the adult are the successors of the two deciduous molars in the child. They differ from them, however, in some respects, the temporary molars having a stronger likeness to the permanent than to their imme- diate descendants, the so-called bicuspids. FIG. 165. -Part of the lower jaw of a child of three or four years old, showing the relations of the temporary and permanent teeth. The specimen contains all the milk-teeth of the right side, together with the incisors of the left; the inner plate of the jaw has been removed, so as to expose the sacs of all the permanent teeth of the right side, except the eighth or wisdom tooth, which is not yet formed. The large sac near the ascending ramus of the jaw is that of the first permanent molar, and above and behind it is the commencing rudiment of the second molar. (Quam.) The temporary incisors and canines differ from their successors but little except in their smaller size. The following tables show the average times of eruption of the Tem- porary and Permanent teeth. In both cases, the eruption of any given tooth of the lower jaw precedes, as a rule, that of the corresponding tooth of the upper. Temporary or Milk Teeth. The figures indicate in months the age at which each tooth appears. Deciduous Deciduous Molars. Canines. Incisors. Canines. Molars. 24 12 18 9779 18 12 24 DIGESTION. 223 Permanent Teeth. The age at which each tooth is cut is indicated in this table in years. Biscupid or Biscupid or Molars. Prsemolars. Canines. Incisors, Canines. Praemolars. Molars. 17 12 to to 6 25 13 10 9 11 to 12 8778 11 to 12 9 10 12 17 6 to to 13 25 The times of eruption given in the above tables are only approximate, the limits of variation being tolerably wide. Some children may cut their first teeth before the age of six months, and others not till nearly the twelfth month. In nearly all cases the two central incisors of the lower jaw are cut first ; these being succeeded after a short interval by the four incisors of the upper jaw, next follow the lateral incisors of the lower jaw, and so on as indicated in the table till the completion of the milk dentition at about the age of two years. The milk-teeth usually come through in batches, each period of eruption being succeeded by one of quiescence lasting sometimes several months. The milk-teeth are in use from the age of two up to five and a half years; at about this age the first permanent molars (four in num- ber) make their appearance behind the milk-molars, and for a short time the child has four permanent and twenty temporary teeth in posi- tion at once. It is worthy of note that from the age of five years to the shedding of the first milk-tooth the child has no fewer than forty-eight teeth, twenty milk teeth and twenty-eight calcified germs of permanent teeth (all in fact except the four wisdom teeth). Structure of a Tooth. A tooth is generally described as possessing a crown, neck, and fang or fangs. The crown is the portion which projects beyond the level of the gum. The neck is that constricted portion just below the crown which is em- braced by the free edges of the gum, and the fang includes all below this. On making a longitudinal section through its centre (Figs. 166, 167), a tooth is found to be principally composed of a hard material, dentine or ivory, which is hollowed out into a central cavity which resembles in general shape the outline of the tooth, and is called the pulp cavity, from its containing the very vascular and sensitive tooth pulp which is composed of connective tissue, blood-vessels, and nerves. The blood-vessels and nerves enter the pulp through a small opening at the extremity of the fang. 224 HANDBOOK OF PHYSIOLOGY. A layer of very hard calcareous matter, the enamel, caps that part of the dentine which projects beyond the level of the gum. ; while sheath- ing the portion of dentine which is beneath the level of the gum, is a layer of true bone, called the cement or crusta petrosa. Fio. 166.— A. Longitudinal section of a human molar tooth: c, cement; d, dentine; e, enamel; v, pulp cavity. (Owen.) B. Transverse section. The letters indicate the same as in A. At the neck of the tooth, where the enamel and cement come into contact, each is reduced to an exceedingly thin layer. The covering of enamel becomes thicker towards the crown, and the cement towards the lower end or apex of the fang. I. — Dentine. Chemical composition. — Dentine closely resembles bone in chemical composition. It contains, however, rather less animal matter ; the pro- portion in a hundred parts being about twenty-eight animal to seventy- two of earthy. The former, like the animal matter of bone, may be re- solved into gelatin by boiling. The earthy matter is made up chiefly of calcium phosphate, with a small portion of the carbonate, and traces of calcium fluoride and magnesium phosphate. Structure. — Under the microscope dentine is seen to be finely chan- nelled by a multitude of delicate 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 enamel and cement, and some- times even penetrate them for a greater or less distance (Fig. 168). In their course from the pulp-cavity to the surface, the minute tubes form gentle and nearly parallel curves and divide and subdivide dicho- tomously, but without much lessening of their calibre until they are ap- proaching their peripheral termination. From their sides proceed other exceedingly minute secondary canals, DIGESTION. 225 which extend into the dentine between the tubules, and anastomose with each other. The tubules of the dentine, the average diameter of which at their inner and larger extremity is ^^ir of an inch, contain fine pro- longations from the tooth-pulp, which give the dentine a certain faint sensitive- ness under ordinary circumstances and, without doubt, have to do also with its nutrition. These prolongations from the tooth-pulp are really processes of the den- tine-cells or odontoUasts, which are branched cells lining the pulp-cavity ; the relation of these processes to the tubules in which they lie being precisely similar to that of the processes of the bone-cor- puscles to the canaliculi of bone. The outer portion of the dentine, underlying both the cement and enamel, forms a more or less distinct layer termed the granular or inter globular layer. It is characterized by the presence of a number of minute cell-like cavities, much more closely packed than the lacunas in the cement, and communicating with one an- other and with the ends of the dentine- tubes (Fig. 168), and containing cells like bone-corpuscles. FIG. 167.— Prernolar tooth of cat in situ. Vertical section. 1. Enamel with decussating and parallel striae. 2. Den- tine with Schreger's lines. 3. Cement. 4. Periosteum of the alveolus. 5. In- ferior maxillary bone, showing canal for the inferior dental nerve and ves- sels, which appears nearly circular in transverse section. (Waldeyer.) II. — Enamel. Chemical composition. — The enamel, which is by far the hardest portion of a tooth, is composed, chemically, of the same elements that enter into the composition of dentine and bone. Its animal matter, however, amounts FIG. 168.-Section of a portion of the dentine and cement from the middle of the root of an incisor 15 226 HANDBOOK OF PHYSIOLOGY. only to about 2 or 3 per cent. It contains a larger proportion of inor- ganic matter and is harder than any other tissue in the body. Structure. — Examined under the microscope, enamel is found com- posed of fine hexagonal fibres (Figs. 169, 170) WOT °^ an inch in diameter, which are set on end on the surface of the dentine, and fit into corresponding depressions in the same. They radiate in such a manner from the den- tine 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 tu- bules, 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-prisms are connected together by a very minute quantity of hyaline cement-sub- stance. In the deeper part of the enamel, between the prisms, are small lacunce, which communicate with the " interglobular spaces " on the surface of the dentine. The enamel itself is coated on the outside by a very thin calcified membrane, sometimes termed the cuticle of the enamel. FIG. 169.— Thin section of the enamel and a part of the dentine, a, cuticular pellicle of the enamel; &, enamel fibres, or columns III. — Crust a Petrosa. cavities in the enamel, com- municating with the extrem- ities of some of the tubuli (d). X350. (Kolliker.) The crusta petrosa, or cement (Fig. 168, c, d), is composed of true bone, and in it are lacuna? (f) and canaliculi (g), which sometimes com- municate with the outer finely branched ends of the dentine tubules. Its laminae are as it were bolted together by perforating fibres like those of ordinary bone, but it differs from ordinary bone in possessing Haversian canals only in the thickest part. Development of the Teeth. Development of the Teeth. — The first step in the development of the teeth consists in a downward growth (Fig. 171, A, 1) from the stratified epithelium of the mucous membrane of the mouth, which first becomes thickened in the neighborhood of the jaws or maxillae which are in the course of formation. This process passes downward into a recess (enamel groove) of the imperfectly developed tissue of the embryonic jaw. The downward epithelial growth forms the primary enamel organ or enamel germ, and its position is indicated by a slight groove in the mucous DIGESTION. membrane of the jaw. The next step in the process consists in the elongation downward of the enamel groove and of the enamel germ and the inclination outward of the deeper part (Fig. 171, B, f), which is now inclined at an angle with the upper portion or neck (/), and has become bulbous. After this, there is an increased development at certain points corresponding to the situations of the future milk-teeth, and the enamel germ or (fommon enamel germ, as it may be called, becomes divided at its deeper portion, or extended by further growth, into a number of special enamel germs corresponding to each of the above-mentioned milk- teeth, and connected to the common germ by a narrow neck, each tooth being placed in its own special recess in the embryonic jaw (Fig. 171, B, As these changes proceed, there grows up from the underlying tissue into each enamel germ (Fig. 171, c, p), a distinct vascular papilla (den- Fia. 170.— Enamel fibres. A, fragments and single fibres of the enamel, isolated by the action of hydrochloric acid. B, surface of a small fragment of enamel, showing the hexagonal ends of the fibres. X 350. (Kolliker. tal papilla), and upon it the enamel germ becomes moulded, and pre- sents the appearance of a cap of two layers of epithelium separated by an interval (Fig. 171, c, f). Whilst part of the sub-epithelial tissue is ele- vated to form the dental papillae, the part which bounds the embryonic teeth forms the dental sacs (Fig. 171, c, s) ; and the rudiment of the jaw, at first a bony gutter in which the teeth germs lie, sends up processes forming partitions between the teeth. In this way small chambers are produced in which the dental sacs are contained, and thus the sockets of the teeth are formed. The papilla, which is really part of the dental sac (if one thinks of this as the whole of the sub-epithelial tissue surround- ing the enamel organ and interposed between the enamel germ and the developing bony jaw), is composed of nucleated cells arranged in a mesh- work, the outer or peripheral j>art being covered with a layer of colum- 228 HANDBOOK OF PHYSIOLOGY. nar nucleated cells called odontoblasts. The odontoblasts form the den- tine, while the remainder of the papilla forms the tooth-pulp. The method of the formation of the dentine from the odontoblasts is as follows : — The cells elongate at their outer part, and these processes are directly converted into the tubules of dentine (Fig. 172). The continued formation of dentine proceeds by the elongation of the odon- toblasts, and their subsequent conversion by a process of cal- cification into dentine tubules. The most recently formed tu- bules are not immediately cal- cified. The dentine fibres con- tained in the tubules are said to be formed from processes of the deeper layer of odonto- blasts, which are wedged in be- tween the cells of the super- ficial layer (Fig. 172) which form the tubules only. Since the papillae are to form the main portion of each tooth, i.e., the dentine, each of them early takes the shape of the crown of the tooth to which it corresponds. As the dentine increases in thickness, the papillae diminish, and at last when the tooth is cut, only a small amount of the papilla remains as the dental pulp, and is supplied by vessels and nerves which enter at the end of the fang. The shape of the crown of the tooth is taken by the corresponding papilla, and that of the single or double fang by the subsequent con- striction below the crown, or by division of the lower part of the papilla. The enamel cap is found later on to consist (Fig. 173) of three parts; (a) an inner membrane, composed of a layer of columnar epithelium in contact with the dentine, called enamel cells, and outside of these one or more layers of small polyhedral nucleated cells (stratum intermedium of Hannover); (J) an outer membrane of several layers of epithelium; (c) FIG. 171. -Section of the upper jaw of a foetal sheep. A. —1, common enamel-germ dipping down into the mucous membrane; 2, palatine process of jaw. B.- Section similar to A, but passing through one of the special enamel-germs here be- coming flask-shaped; c, c', epithelium of mouth; /, neck; /', body of special enamel germ. C.— A later stage; c, outline of epithelium of gum ; /, neck of enamel germ;/', enamel organ; p, papilla; s, dental sacformine:; fp, the enamel germ of permanent tooth. (Waldeyer and Kolliker.) Copied from Quain's Anatomy. DIGESTION. 229 a middle membrane formed of a matrix of non-vascular gelatinous tissue containing a hyaline interstitial substance. The enamel is formed by the enamel cells of the inner membrane, by the elongation of their distal FIG. 172.— Part of section of developing tooth of a young rat, showing the mode of desposition •of the dentine. Highly magnified, a, outer layer of fully formed dentine; 6, uncalcified matrix with one or two nodules of calcareous matter near the calcified parts; c, odontoblasts sending pro- cesses into the dentine; d, pulp. The section is stained in carmine, which colors the uncalcified matrix but not the calcified part. (E. A. Schiif er.) extremities, and the diroct conversion of these processes into enamel. The calcification of the enamel processes or prisms takes place first at the periphery, the centre remaining for a time transparent. The cells of the stratum intermedium are used for the regeneration of the enamel cells, but these and the middle membrane after a time disappear. The cells of the outer membrane give origin to the cuticle of the enamel. The cement or crusta petrosa is formed from the tissue of the tooth sac, the structure and function of which are identical with those of the osteogenetic layer of the perios- teum. In this manner the first set of teeth, or the milk-teeth, are formed; and each tooth, by degrees develop- ing, presses at length on the wall of the sac inclosing it, and, caus- ing its absorption, is cut, to use a familiar phrase. The temporary or milk-teeth, are speedily replaced by the growth of the permanent teeth, which push their way up from beneath them, ab- .sorbing in their progress the whole of the fang of each milk-tooth, and FIG. 173.— Vertical transverse section of the dental sac, pulp, etc., of a kitten, a, dental pa- pilla or pulp; 6, the cap of dentine formed upon the summit; c, its covering of enamel; d, inner layer of epithelium of the enamel organ ; e, gelatinous tissue; /, outer epithelial layer of the enamel organ; g, inner layer, and ft, outer layer of dental sac. x 14. (Thiersch.) 230 HANDBOOK OF PHYSIOLOGY. leaving at length only the crown as a mere shell, which is shed to make way for the eruption of the permanent teeth (Fig. 165). Each temporary tooth is replaced by a corresponding tooth of the permanent set 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. MASTICATION. The act of chewing or mastication is performed by the biting and grinding movement of the lower range of teeth against the upper. The simultaneous movements of the tongue and cheeks assist partly by crush- ing the softer portions of the food against the hard palate and gums, and thus supplementing the action of the teeth, and partly by returning the morsels of food to the action of the teeth, again and again, as they are squeezed out from between them, until they have been sufficiently chewed. Muscles. — The simple up and down, or liting movements of the lower jaw, are performed by the temporal, masseter, and internal pterygoid muscles, the action of which in closing the jaws alternates with that of the digastric and other muscles passing from the os hyoides to the lower jaw, which open them. The grinding or side to side movements of the lower jaw are performed mainly by the external pterygoid muscles, the muscle of one side acting alternately with the other. When both exter- nal pterygoids act together, the lower jaw is pulled directly forwards, so that the lower incisor teeth are brought in front of the level of the upper. Temporo-maxillary Fibro-cartilage. — The function of the inter-artic- ular fibro-cartilage of the temporo-maxillary joint in mastication is to serve : (1) As an elastic pad to distribute the pressure caused by the ex- ceedingly powerful action of the masticatory muscles. (2) As a joint- surface or socket for the condyle of the lower jaw, when the latter has been partially drawn forward out of the glenoid cavity of the temporal bone by the external pterygoid muscle, some of the fibres of the latter being attached to its front surface, and consequently drawing it forward with the condyle which moves on it. Nervous Mechanism. — The act of mastication is partly voluntary and partly reflex and involuntary. The consideration of such sensori-motor actions will come hereafter (see Chapter on the Nervous System). It will suffice here to state that the afferent nerves chiefly concerned are the sensory branches of the fifth and the glosso-pharyngeal, and the ef- ferent are the motor branches of the fifth and the ninth (hypoglossal) cerebral nerves. The nerve-centre through which the reflex action oc- curs, and by which the movements of the various muscles are harmo- nized, is situated in the medulla oblongata. In so far as mastication is voluntary or mentally perceived, it becomes so under the influence, in addition to the medulla oblongata, of the cerebral hemispheres. DIGESTION. 231 INSALIVATIOST. The act of mastication is much assisted by the saliva which is secreted by the salivary glands in largely increased amount during the process, and the intimate incorporation of which with the food, as it is being chewed, is termed insalivation. The Salivary Glands. The human salivary glands are the parotid, the subm axillary, and the sublingual, and numerous smaller bodies of similar structure, and with separate ducts, which are scattered thickly beneath the mucous membrane of the lips, cheeks, soft palate, and root of the tongue. Structure. — The salivary glands are compound tubular glands. They are made up of lobules. Each lobule consists of the branchings of a Fia. 174.— Section of submaxillary gland of dog. Showing gland cells, 6, and a duct, a. in sec- tion. (Kolliker.) subdivision of the main duct of the gland, which are generally more or less convoluted towards their extremities, and sometimes, according to some observers, sacculated or pouched. The convoluted or pouched portions form the alveoli, or proper secreting parts of the gland. The alveoli are composed of a basement membrane of flattened cells joined together by processes to produce a fenestrated membrane, the spaces of which are occupied by a homogeneous ground-substance. Within, upon this membrane, which forms the tube, the nucleated salivary secreting cells, of cubical or columnar form, are arranged parallel to one another inclosing a central canal. The granular appearance frequently seen in the salivary cells is due to the very dense network of fibrils which they contain. "When isolated, the cells not unfrequently are found to be branched. Connecting the alveoli into lobules is a considerable amount of fibrous connective tissue, which contains both flattened and granular protoplasmic cells, lymph corpuscles, and in some cases fat cells. The lobules are connected to form larger lobules (lobes), in a similar man- ner. The alveoli pass into the intralobular ducts by a narrowed portion 232 HANDBOOK OF PHYSIOLOGY. (intercalary), lined with flattened epithelium, with elongated nuclei. The intercalary ducts pass into the intralobular ducts by a narrowed neck, lined with cubical cells with small nuclei, The intralobular duct is larger in size, and is lined with large columnar nucleated cells, the parts of which, towards the lumen of the tube, present a fine longitudi- nal striation, due to the arrangement of the cell network. It is most marked in the submaxillary gland. The intralobular ducts pass into the larger ducts, and these into the main duct of the gland. As these ducts become larger they acquire an outside coating of connective tissue, and later on some unstriped muscular fibres. The lining of the larger ducts consist of one or more layers of columnar epithelium, the cells of which contain an intracellular network of fibres arranged longitudinally. Varieties. — Certain differences in the structure of salivary glands FIG. 175. FIG. 176. FIG. 175.— From a section through a true salivary gland, a, the gland alveoli, lined with albu- minous " salivary cells;11 6, intralobular duct cut transversely. (Klein and Nob e Smith.) FIG. 176.— From a section through a mucous gland in a quiescent state. The alveoli are lined with transparent mucous cells, and outside these are the semilunes of Heidenhain. The cells should have been represented as more or less granular. (Heidenhain.) may be observed according as the glands secrete pure saliva, or saliva mixed with mucus, or pure mucus, and therefore the glands have been classified as : — (1) True salivary glands (called most unfortunately by some serous glands), e. g., the parotid of man and other animals, and the submaxil- lary of the rabbit and the guinea-pig (Fig. 175). In this kind the alve- olar lumen is small, and the cells lining the tubule are short granular columnar cells, with nuclei presenting the intranuclear network. Dur- ing rest the cells become larger, highly granular, with obscured nuclei, and the lumen becomes smaller. During activity, and after stimulation of the sympathetic, the cells become smaller and their contents more opaque ; the granules first of all disappearing from the outer part of the cells, and then being found only at the extreme inner part and contigu- ous border of the cell. The nuclei reappear, as does also the lumen. (2) In the true mucous-secreting glands, as the sublingual of man and DIGESTION. 233 other animals, and in the submaxillary of the dog, the tubes are larger, contain a larger lumen and also have larger cells lining them. The cells are of two kinds, (a) mucous or central cells, which are transparent col- umnar cells with nuclei near the basement membrane. The cell sub- stance is made up of a fine network, which in the resting state contains a transparent substance called mucigen, during which the cell does not stain well with logwood (Fig. 176). When the gland is secreting, mu- cigen is converted into mucin, and the cells swell up, appear more trans- parent and stain deeply in logwood (Fig. 177). During rest, the cells become smaller and more granular from having discharged their con- tents. The nuclei appear more distinct, (b) Demilunes of Heidenhain (Fig. 176), which are crescentic masses of granular parietal cells found here and there between the basement membrane and the central cells. The cells composing the mass are small and have a very dense reticulum, the nuclei are spherical, and increase in size during secretion. In the mu- cous gland there are some large tubes, lined with large transparent central cells, and having besides a few granu- lar parietal cells ; other small tubes are lined with small granular parietal cells alone ; and a third variety are lined equally with each kind of cell. (3) In the muco-salivary or mixed qlands, as the human submaxillary FIG. 177. -A part of a section through J a mucous gland after prolonged electrical gland, part Of the gland presents the stimulation. The alveoli are lined with small granular cells. (Lavdovski. ) structure of the mucous gland whilst the remainder has that of the salivary glands proper. Nerves and Blood-vessels. — Nerves of large size are found in the sali- vary glands, they are principally contained in the connective tissue of the alveoli, and in certain glands, especially in the dog, are provided with ganglia. Some nerves have special endings in Pacinian corpuscles, some supply the blood-vessels, and others, according to Pfliiger, pene- trate the basement membrane of the alveoli and enter the salivary cells. The blood-vessels form a dense capillary network around the ducts of the alveoli, being carried in by the fibrous trabeculse between the al- veoli, in which also begin the lymphatics by lacunar spaces. Saliva. Saliva, as it commonly flows from the mouth, is mixed with the secretion of the mucous glands, and often with air bubble, which, being retained by its viscidity, make it frothy. When obtained from the parotid ducts, and free from mucus, saliva is a transparent watery fluid, the specific gravity of which varies from 1004 to 1008, and in which, 234 HANDBOOK OF PHYSIOLOGY. when examined with the microscope, are found floating a number of mi- nute particles, derived from the secreting ducts and vesicles of the glands. In the impure or mixed saliva are found, besides these particles, numer- ous epithelial scales separated from the surface of the mucous mem- brane of the mouth and tongue, and the so-called salivary corpuscles, discharged probably from the mucous glands of the mouth and the ton- sils, which, when the saliva is collected in a deep vessel, and left at rest, subside in the form of a white opaque matter, leaving the supernatant salivary fluid transparent and colorless, or with a pale bluish-gray tint. In reaction, the saliva, when first secreted, appears to be always alka- line. During fasting, the saliva, although secreted alkaline, shortly be- comes neutral; especially when it is secreted slowly and is allowed to mix with the acid mucus of the mouth, by which its alkaline reaction is neutralized. Chemical Composition of Mixed Saliva (Frerichs). Water, 994.10 Solids :— Ptyalin, 1.41 Fat, ^ 0.07 Epithelium and Proteids (including Serum- Albumin, Globulin, Mucin, etc.), 2.13 Salts :— Potassium Sulpho-Cyanate, . . ") Sodium Phosphate, . Calcium Phosphate. . . * I 2 29 Magnesium Phosphate, . . j Sodium Chloride, . . . | Potassium Chloride, . . . . J 5.9 1000 The presence of potassium sulphocyanate (or thiocyanate) (C N K S) in saliva, may be shown by the blood-red coloration which the fluid gives with a solution of ferric. chloride (Fe.2 01. 6), and which is bleached on the addition of a solution of mercuric chloride (Hg Cla), but not by hydrochloric acid. Rate of Secretion and Quantity. — The rate at which saliva is secreted is subject to considerable variation. When the tongue and muscles con- cerned in mastication are at rest, and the nerves of the mouth are sub- ject to no unusual stimulus, the quantity secreted is not more than sufficient, with the mucus, to keep the mouth moist. During actual secretion the flow is much accelerated. The quantity secreted' in twenty-four hours varies : its average amount is probably from 1 to 3 pints (1 to 2 litres). DIGESTION. 235 Uses of Saliva. — The purposes served by saliva are (1) mechanical and (2) chemical. I. Mechanical. — (1) It keeps the mouth in a due condition of mois- ture, facilitating the movements of the tongue in speaking, and the mastication of food. (2) It serves also in dissolving sapid substances, and rendering them capable of exciting the nerves of taste. But the principal mechanical purpose of the saliva is, (3) that by mixing with the food during mastication, it makes it a soft pulpy mass, such as may be easily swallowed. To -this purpose the saliva is adapted both by quantity and quality. For, speaking generally, the quantity secreted during feeding is in direct proportion to the dryness and hardness of the food. The quality of saliva is equally adapted to this end. It is easy to see how much more readily it mixes with most kind of foods than water alone does ; and the saliva from the parotid, labial, and other small glands, being more aqueous than the rest, is that which is chiefly braided and mixed with the food in mastication ; while the more viscid mucous secretion of the submaxillary, palatine, and tonsillitic glands is spread over the surface of the softened mass, to enable it to slide more easily through the fauces and oesophagus. II. Chemical. — The chemical action which the saliva exerts upon the food in the mouth is to convert the starchy materials which it contains into some kind of sugar. This power the saliva owes to one of its con- stituents ptyalin, which is a nitrogenous body of uncertain composition. It is classed among the unorganized ferments, which are substances of uncertain composition capable of producing changes in the composition of other bodies with which they come into contact, without themselves undergoing change of suffering diminution. The conversion of the starch under the influence of the ferment into sugar takes place in sev- eral stages, and in order to understand it, a knowledge of the structure and composition of starch granules is necessary. A starch granule con- sists of two parts : an envelope of cellulose, which does not give a blue color with iodine except on addition of sulphuric acid, and of yranulose, which is contained within, and which gives a blue with iodine alone. Briike states that a third body is contained in the granule, which gives a red with iodine, viz., erythro-granulose. On boiling, the granulose swells up, bursts the envelope, and the whole granule is more or less com- pletely converted into a paste or gruel, which is called gelatinous starch. When ptyalin or other amylolytic ferment is added to boiled starch, sugar almost at once makes its appearance in small quantities, but in ad- dition there is another body, intermediate between starch and sugar, called erythro-dextrin, which gives a reddish-brown coloration with iodine. As the sugar increases in amount, the erythro-dextrin disappears, hut its place is taken in part by another dextrin, achroo-dextrin, which 236 HANDBOOK OF PHYSIOLOGY. gives no color with iodine. However long the reaction goes on, it is un- likely that all the dextrin becomes sugar. Next with regard to the kind of sugar formed, it is, at first at any rate, not glucose but maltose, the formula for which is C^H.,,,0^ Mal- tose is allied to saccharose or cane-sugar more nearly than to glucose ; it is crystalline ; its solution lias the property of polarizing light to a greater degree than solutions of glucose ; is not so sweet, and reduces copper sulphate le'ss easily. It can be converted into glucose by boiling with dilute acids, and by the further action of the ferment. According to Brown and Heron the reactions may be represented thus : — One molecule of gelatinous starch is converted by the action of an amy- lolytic ferment into n molecules of soluble starch. One molecule of soluble starch = 10 (C12H20010) 4- 8 (HaO), which is further converted by the ferment into 1. Erythro-dextrin (giving red with iodine) 4- Maltose. MC.A.O,.) (0,^,0,,) then into 2. Erythro-dextrin (giving yellow with iodine) 4- Maltose. 8'(0,.H1.0I.) next into 3. Achroo-dextrin 4- Maltose. 7 (Ol,HMOl.) 3 (ClaHS2Ou) And so on ; the resultant being : — 10 (C,,H!0OJ + 8 (H,0) = 8 (0,.HM011) + 2 .,. Soluble starch Water Maltose Achroo-dextrm. Test for Sugar. — In such an experiment the presence of sugar is at once discovered by the application of Trommer's test, which consists in the addition of a drop or two of a solution of copper sulphate, followed "by a larger quantity of caustic potash. When the liquid is boiled, an orange-red precipitate of copper suboxide indicates the presence of sugar. The action of saliva on starch is facilitated by: (a) Moderate heat, about 100° F. (37.8° 0.). (b) A slightly alkaline medium, (c) Kemoval of the changed material from time to time. Its action is retarded ~by (a) Cold ; a temperature of 32° F. (0° C.) stops it for a time, but does not destroy it, whereas a high temperature above 140° F. (60° C.) destroys it. (b) Acids or strong alkalies either delay or stop the action altogether. (c) Presence of too much of the changed material. Ptyalin, in that it converts starch into sugar, is an amylolytic ferment. Starch appears to be the only principle of food upon which saliva acts chemically : the secretion has no apparent influence on any of the other ternary principles, such as sugar, gum, cellulose, or on fat, and seems to be equally destitute of power over albuminous and gelatinous substances. Saliva from the parotid is less viscid, less alkine, clearer, and more watery than that from the submaxillary. It has moreover a less power- ful action on starch. Sublingual saliva is the most viscid, and contains DIGESTION. 237 more solids than either of the other two, but does not appear to be so powerful in its action. The salivary glands of children do not become functionally active till the age of 4 to 6 months, and hence the bad effect of feeding them be- fore this age on starchy food, corn-flour, etc., which they are unable to render soluble and capable of absorption. Influence of the Nervous System. The secretion of saliva is under the control of the nervous system. It is a reflex action. Under ordinary conditions it is excited by the stimulation of the peripheral branches of two nerves, viz., the gustatory or lingual branch of the inferior maxillary division of the fifth nerve, and the glosso-pharyngeal part of the eighth pair of nerves, which are distributed to the mucous membrane of the tongue and pharynx con- jointly. The stimulation occurs on the introduction of sapid substances into the mouth, and the secretion is brought about in the following way. From the terminations of the above-mentioned sensory nerves distributed in the mucous membrane an impression is conveyed upwards (afferent) to the special nerve-centre situated in the medulla, which controls the process, and by it is reflected to certain nerves supplied to the salivary glands, which will be presently indicated. In other words, the centre, stimulated to action by the sensory impressions carried to it, sends out impulses along efferent or secretory nerves supplied to the salivary glands, which cause the saliva to be secreted by and discharged from the gland cells. Other stimuli, however, besides that of the food, and other sen- sory nerves besides those mentioned, may produce reflexly the same ef- fects. For example, saliva may be caused to flow by irritation of the mucous membrane of the mouth with mechanical, chemical, electrical, or thermal stimuli, also by the irritation of the mucous membrane of the stomach in some way, as in nausea, which precedes vomiting, when some of the peripheral fibres of the vagi are irritated. Stimulation of the olfactory nerves by smell of food, of the optic nerves by the sight of it, and of the auditory nerves by the sounds which are known by expe- rience to accompany the preparation of a meal, may also, in the hungry, stimulate the nerve-centre to action. In addition to these, as a secretion of saliva follows the movement of the muscles of mastication, it may be assumed that this movement stimulates the secreting nerve-fibres of the gland, directly or reflexly. From the fact that the flow of saliva may be increased or diminished by mental emotions, it is evident that impres- sions from the cerebrum also are capable of stimulating the centre to ac- tion or of inhibiting its action. Salivary secretion may also be excited by direct stimulation of the centre in the medulla. 238 HANDBOOK OF PHYSIOLOGY. A. On the Submaxillary Gland. — The submaxillary gland has been the gland chiefly employed for the purpose of experimentally demon- strating the influence of the nervous system upon the secretion of saliva, because of the comparative facility with which, with its blood-vessels and nerves, it may be exposed to view in the dog, rabbit, and other animals. The chief nerves supplied to the gland are: (I) the chorda tympani, a branch given off from the facial (or portio dura of the seventh pair of nerves), in the canal through which it passes in the temporal bone, in its passage from the interior of the skull to the face; and (2) branches of the sympathetic nerve from the plexus around the facial artery and its branches to the gland. The chorda (Fig. 178, ch. t.), after quitting the temporal bone, passes downwards and forwards, under cover of the ex- ternal pterygoid muscle, and joins at an acute angle the lingual or gus- tatory nerve, proceeds with it for a short distance, and then passes along the submaxillary gland duct (Fig. 178, sm. d.), to which it is distributed, giving branches to the submaxillary ganglion (Fig. 178, sm. gl.)\ and sending others to terminate in the superficial muscles of the tongue. If this nerve be exposed and divided anywhere in its course from its exit from the skull to the gland, the secretion, if the gland be in action, is arrested, and no stimulation either of the lingual or of the glosso-pharyn- geal will produce a flow of saliva. But if the peripheral end of the di- vided nerve be stimulated, an abundant secretion of saliva ensues, and the blood-supply is enormously increased, the arteries being dilated. The veins even pulsate, and the blood contained within them is more arterial than venous in character. When, on the other hand, the stimulus is applied to the sympathetic filaments (mere division producing no apparent effect), the arteries con- tract, and the blood stream is in consequence much diminished ; and from the veins, when opened, there escapes only a sluggish stream of dark blood. The saliva, instead of being abundant and watery, becomes scanty and tenacious. If both chorda tympani and sympathetic branches be divided, the gland, released from nervous control, secretes continuously and abundantly (paralytic secretion). The abundant secretion of saliva, which follows stimulation of the chorda tympani, is not merely the result of a filtration of fluid from the blood-vessels, in consequence of the largely increased circulation through them. This is proved by the fact that, when the main duct is obstructed the pressure within may considerably exceed the blood-pressure in the arteries, and also that when into the veins of the animal experimented upon some atropin has been previously injected, stimulation of the peri- pheral end of the divided chorda produces all the vascular effects as be- fore, without any secretion of saliva accompanying them. Again, if an animal's head be cut off, and the chorda be rapidly exposed and stimu- lated with an interrupted current, a secretion of saliva ensues for a short DIGESTION. 239 time, although the blood-supply is necessarily absent. These experi- ments serve to prove that the chorda contains two sets of nerve-fibres, one set (vaso-dilator) which, when stimulated, act upon a local vaso- motor centre for regulating the blood-supply, inhibiting its action, and causing the vessels to dilate, and so producing an increased supply of blood to the gland; while another set, which are paralyzed by injection of atropin, directly stimulate the cells themselves to activity, whereby they secrete and discharge the constituents of the saliva which they produce. These latter fibres very possibly terminate in the salivary cells themselves. If, on the other hand, the sympathetic fibres be di- yided, stimulation of the tongue by sapid substances, or of the trunk of the lingual, or of the glosso-pharyngeal continues to produce a flow of saliva. From these experiments it is evident that the chorda tympani FIG. 178. -Diagrammatic representation of submaxillary gland of the dog with its nerves and blood-vessels. (This is not intended to illustrate the exact anatomical relations of the several structures.) sm. gld., the submaxillary gland into the duct (sm. d.) of which a canula has been tied. The sublingual gland and duct are not shown; n. 1., n. I'., the lingual or gustatory nerve; ch. t., ch. t'., the chorda tympani proceeding from the facial nerve, becoming conjoined with the lingual at n. I'., and afterwards diverging and passing to the gland along the duct; sm. gl., sub- maxillary ganglion with its roots; n. L, the lingual nerve proceeding to the tongue; a. car., the cartoid artery, two branches of which, a. sm. a. and r. sm. p. pass to the anterior and posterior parts of the gland; v. sm., the anterior and posterior veins from the gland ending in v. j., the jugu- lar vein; v. fsym , the conjoined vagus and sympathetic trunks; gl. cer. s, the superior-cervical ganglion, two branches of which forming a plexus, a. /., over the facial artery, are distributed (n. sym. sm.) along the two glandular arteries to the anterior and posterior portion of the gland. The arrows indicate the direction taken by the nervous impulses; during reflex stimulations of the gland they ascend to the brain by the lingual and descend by the chorda tympani. CM. Foster.) nerve is the principal nerve through which efferent impulses proceed from the centre to excite the secretion of this gland. The sympathetic fibres appear to act principally as a vaso-constrictor nerve; and to exalt the action of the local vaso-motor centres. The .sympathetic is more powerful in this direction than the chorda. There is not sufficient evidence in favor of the belief that the submaxillary 240 HANDBOOK OF PHYSIOLOGY. ganglion is ever the nerve-centre which controls the secretion of the sub- maxillary gland. B. On the Parotid Gland. — The nerves which influence secretion in the parotid gland are branches of the facial (lesser superficial petrosal) and of the sympathetic. The former nerve, after passing through the otic ganglion, joins the auriculo-temporal branch of the fifth cerebral nerve, and, with it, is distributed to the gland. The nerves by which the stimulus ordinarily exciting secretion is conveyed to the medulla oblon- gata, are, as in the case of the submaxillary gland, the fifth, and the glosso-pharyngeal. The pneumogastric nerves convey a further stimulus to the secretion of saliva, when food has entered the stomach; the nerve centre is the same as in the case of the submaxillary gland. Changes in the Gland Cells. — The method by which the salivary cells produce the secretion of saliva appears to be divided into two stages, which differ somewhat according to the class to which the gland belongs, viz., whether to (1) the true salivary, or (2) to the mucous type. In the FIG. 179.— Alveoli of true salivary gland. A, at rest; B, in the first stage cf secretion; c, after prolonged secretion. (Langley.) former case, it has been noticed, as has been already described (p. 232), that during the rest which follows an active secretion the lumen of the alveolus becomes smaller, the gland cells larger, and very granular. During secretion the alveoli and their cells become smaller, and the granular appearance in the latter to a considerable extent disappears, and at the end of secretion, the granules are confined to the inner part of the cell nearest to the lumen, which is now quite distinct (Fig. 179). It is supposed from these appearances that the first stage in the act of secretion consists in the protoplasm of the salivary cell taking up from the lymph certain materials from which it manufactures the elements of its own secretion, and which are stored up in the form of granules in the cell during rest, the second stage consisting of the actual discharge of these granules, with or without previous change. The granules are taken to represent the chief substance of the salivary secretion, i. e., the ferment ptyalin. In the case of the submaxillary gland of the dog, at any rate, the sympathetic nerve-fibres appear to have to do with the first stage of the process, and when stimulated the protoplasm is extremely active in manufacturing the granules, whereas the chorda tympani is DIGESTION. 241 concerned in the production of the second act, the actual discharge of the materials of secretion, together with a considerable amount of fluid, the latter being an actual secretion by the protoplasm, as it ceases to oc- cur when atropine has been subcutaneously injected. In the mucous-secreting gland, the changes in the cells during secre- tion have been already spoken of (p. 233). They consist in the gradual secretion by the protoplasm of the cell of a substance called mucigen, which is converted into mucin. and discharged on secretion into the canal of the alveoli. The mucigen is, for the most part, collected into the inner part of the cells during rest, pressing the nucleus and the small portion of the protoplasm which remains, against the limiting membrane of the alveoli. The process of secretion in the salivary glands is identical with that of glands in general; the cells which line the ultimate branches of the ducts being the agents by which the special constituents of the saliva are formed. The materials which they have incorporated with themselves are almost at once given up again, in the form of a fluid (secretion), which escapes from the ducts of the gland; and the cells, themselves, undergo disintegration — again to be renewed, in the intervals of the ac- tive exercise of their functions. The source whence the cells obtain the materials of their secretion, is the blood, or, to speak more accurately, the plasma, which is filtered off from the circulating blood into the in- terstices of the glands as of all living textures. THE PHARYNX. That portion of the alimentary canal which intervenes between the mouth and the oesophagus is termed the Pharynx (Fig. 164). It will suffice here to mention that it is constructed of a series of three muscles with striated fibres (con- strictors), which are covered by a thin fascia ex- ternally, and are lined internally by a strong fas- cia (pharyngeal aponeurosis), on the inner aspect of which is areolar (submucous) tissue and mu- cous membrane, continuous with that of the mouth, and, as regards the part concerned in swal- lowing, is identical with it in general structure. Fia. ^Lingual foi- The epithelium of this part of the pharynx, like \\cle or crypt, a, invoiu- * a * » tion of mucous mem- that of the mouth, is stratified and squamous. £rane with its papillae; 6, lymphoid tissues, with The pharynx IS Well Supplied With muCOUS several lymphoid sacs. glands (Fig. 182). The Tonsils. Between the anterior and posterior arches of the soft palate are situ- ated the Tonsils, one on each side. A tonsil consists of an elevation of 16 242 HANDBOOK OF PHYSIOLOGY. the mucous membrane presenting 12 to 15 orifices, which lead into crypts or recesses, in the walls of which are placed nodules of adenoid or lym- phoid tissue (Fig. 181). These nodules are enveloped in a less dense adenoid tissue which reaches the mucous surface. The surface is covered with stratified squamous epithelium, and the subepithelial or mucous membrane proper may present rudimentary papillae formed of adenoid tissue. The tonsil is bounded by a fibrous capsule (Fig. 181, e). Into the crypts open the ducts of numerous mucous glands. FIG. 181. — Vertical section through a crypt of the human tonsil, a., entrance to the crypt" which is divided below by the elevation which does not quite reach the surface; 6, stratified epithe lium; c, masses of adenoid tissue ; d, mucous glands cut across; e, fibrous capsule. Semidiagram matic. (V. D. Harris.) The viscid secretion which exudes from the tonsils serves to lubricate the bolus of food as it passes them in the second part of the act of deglu- tition. THE (ESOPHAGUS OR GULLET. The (Esophagus or G-ullet (Fig. 164), the narrowest portion of the alimentary canal, is a muscular and mucous tube, nine or ten inches in length, which extends from the lower end of the pharynx to the cardiac orifice of the stomach. Structure. — The oesophagus is made up of three coats — viz., the outer, muscular; the middle, submucous; and the inner, mucous. The mus- cular coat (Fig. 183, g and i), is covered externally by a varying amount of loose fibrous tissue. It is composed of two layers of fibres, the outer being arranged longitudinally, and the inner circularly. At the upper part of the oesophagus this coat is made up principally of striated muscle fibres, as they are continuous with the constrictor muscles of the pharynx; but lower down the unstriated fibres become more and more numerous, DIGESTION. 243 and towards the end of the tube form the entire coat. The muscular coat is connected with the mucous coat by a more or less developed layer of areolar tissue, which forms the submucous coat (Fig. 183, 7), in which are contained in the lower half or third of the tube many mucous glands, the ducts of which, passing through the mucous membrane (Fig. 183, c) open on its surface. Separating this coat from the mucous membrane proper is a well-developed layer of longitudinal, unstriated muscle (d), called the muscularis mucosce. The mucous membrane is composed of a closely felted meshwork of fine connective tissue, which, towards the sur- FIQ. 182. FIG. 183. FIG. 182.— Section of a mucous gland from the tongue. A, opening of the duct on the free sur- face; c, basement membrane with nuclei; B, flattened epithelial cells lining duct. The duct divides into several branches, which are convoluted and end blindly, being lined throughout by columnar epithelium. D, lumen of one of the tubuli of the gland, x 90. (Klein and Noble Smith.) FIG. 183.— Longitudinal section of the oesophagus of a dog towards the lower end. a, stratinea epithelium of the mucous membrane; 6, mucous membrane proper; c, duct of mucous gland; a, muscularis muscosse; e, mucous glands; /, submucous coat; 0, circular muscular layer; h, inter- muscular layer, in which are contained the ganglion cells of Auerbach; t, longitudinal muscular layer; fc, outside investment of fibrous tissue. Semidiagrammatic. (V. D. Harris.) face, is elevated into rudimentary papillae. It is covered with a stratified epithelium, of which the most superficial layers are squamous. The epithelium is arranged upon a basement membrane. In newly-born children the mucous membrane exhibits, in many parts, the structure of lymphoid tissue (Klein). Blood- and lymph-vessels, and nerves, are distributed in the.walls of 24:4: HANDBOOK OF PHYOLOGY. the oesophagus. Between the outer and inner layers of the muscular coat, nerve-ganglia of Auerbach are also found. DEGLUTITION" OR SWALLOWING. When properly masticated, the food is transmitted in successive por- tions to the stomach by the act of deglutition or swallowing". This, for the purpose of description, may be divided into three acts. In the first, particles of food collected to a morsel are made to glide between the surface of the tongue and the palatine arch, till they have passed the anterior arch of the fauces; in the second, the morsel is carried through the pharynx; and in the third, it reaches the stomach through the oesoph- agus. These three acts follow each other rapidly. (1.) The first act may be voluntary, although it is usually performed unconsciously; the morsel of food, when sufficiently masticated, being pressed between the tongue and palate, by the agency of the muscles of the former, in such a manner as to force it back to the entrance of the pharynx. (2.) The second act is the most complicated, because the food must pass by the posterior orifice of the nose and the upper opening of the larynx without touching them. When it has been brought, by the first act, between the anterior arches of the palate, it is moved onwards by the movement of the tongue backwards, and by the muscles of the anterior arches contract- ing on it and then behind it. The root of the tongue being retracted, and the larynx being raised with the pharynx and carried forwards under the base of the tongue, the epiglottis is pressed over the upper opening" of the larynx, and the morsel glides past it; the closure of the glottis being additionally secured by the simultaneous contraction of its own muscles, so that, even when the epiglottis is destroyed, there is little danger of food or drink passing into the larynx so long as its muscles can act freely. At the same time, the raising of the soft palate, so that its posterior edge touches the back part of the pharynx, and the approx- imation of the sides of the posterior palatine arch, which move quickly inwards like side curtains, close the passage into the upper part of the pharynx and the posterior nares, and form an inclined plane, along the under surface of which the morsel descends; then the pharynx, raised up to receive it, in its turn contracts, and forces it onwards into the oesoph- agus. (3.) In the third act, in which the food passes through the oesophagus, every part of that tube, as it receives the morsel, and is di- lated by it, is stimulated to contract; hence an undulatory contraction of the oesophagus, which is easily observable in horses while drinking, proceeds rapidly along the tube. It is only when the morsels swallowed are large, or taken too quickly in succession, that the progressive con- traction of the oesophagus is slow, and attended with pain. Division of both pneumogastric nerves paralyzes the contractile power of the cesoph- DIGESTION. 245 agus, and food accordingly accumulates in the tube. The second and third parts of the act of deglutition are involuntary. Nerve Mechanism. — The nerves engaged in the reflex act of degluti- tion are: — sensory, branches of the fifth cerebral supplying the soft pal- ate; glosso-pharyngeal, supplying the tongue and pharynx; the superior laryngeal branch of the vagus, supplying the epiglottis and the glottis; while the motor fibres concerned are: — branches of the fifth, supplying part of the digastric and mylo-hyoid muscles, and the muscles of masti- cation; the facial, supplying the levator palati; the glosso-pharyngeal, supplying the muscles of the pharynx; the vagus, supplying the muscles of the larynx through the inferior laryngeal branch, and the hypoglossal, the muscles of the tongue. The nerve-centre by which the muscles are harmonized in their action, is situated in the medulla oblongata. In the movements of the oesophagus, the ganglia contained in its walls, with the pneumo-gastrics, are the nerve-structures chiefly concerned. It is important to note that the swallowing both of food and drink is a muscular act, and can, therefore, take place in opposition to the force of gravity. Thus, horses and many other animals habitually drink up- hill, and the same feat can be performed by jugglers. THE STOMACH. In man and those Mammalia which are provided with a single stom- ach, it consists of a dilatation of the alimentary canal placed between FIG. 184.— Stomach of a sheep, ce, oesophagus; Ru, rumen; Ret, reticulum; Ps, psalterium, or manyplies; A, abomasum; Z>u, duodenum; g, groove from oesophagus to psalterium. (Huxley.) and continuous with the oesophagus, which enters its larger or cardiac end on the one hand, and the small intestine, which commences at its narrowed end or pylorus, on the other. It varies in shape and size ac- cording to its state of distention. The Ruminants (ox, sheep, deer, etc.) possess very complex stom- achs; in most of them four distinct cavities are to be distinguished (Fig. 184). 1. The Paunch or Rumen, a very large cavity which occupies the cardiac end, and into which large quantities of food are in the first in stance swallowed with little or no mastication. 2. The Reticulum, or 246 HANDBOOK OF PHYSIOLOGY. Honeycomb stomach, so called from the fact that its mucous membrane is disposed in a number of folds inclosing hexagonal cells. 3. The Psaltenum, or Manyplies, in which the mucous membrane is arranged in very prominent longitudinal folds. 4. Abomasum, Reed, or Rennet, narrow and elongated, its mucous membrane being much more highly vascular than that of the other divisions. In the process of rumination small portions of the contents of the rumen and reticulum are succes- sively regurgitated into the mouth, and there thoroughly masticated and insalivated (chewing the cud): they are then again swallowed, being this time directed by a groove (which in the figure is seen running from the lower end of the oesophagus) into the manyplies, and thence into the abomasum. It will thus be seen that the first two stomachs (paunch and reticulum) have chiefly the mechanical functions of storing and moisten- ing the fodder: the third (manyplies) probably acts as a strainer, only allowing the finely divided portions of food to pass on into the fourth stomach, where the gastric juice is secreted and the process of digestion carried on. The mucous membrane of the first three stomachs is lowly vascular, while that of the fourth is pulpy, glandular, and highly vascular. In some other animals, as the pig, a similar distinction obtains be- tween the mucous membrane in different parts of the stomach. In the pig the glands in the cardiac end are few and small, while to- wards the pylorus they are abundant and large. A similar division of the stomach into a cardiac (receptive) and a pyloric (digestive) part, foreshadowing the complex stomach of rumi- nants, is seen in the common rat, in which these two divisions of the stomach are distinguished, not only by the characters of their lining membrane, but also by a well-marked constriction. In birds the function of mastication is performed by the stomach (gizzard) which in granivorous orders, e.g., the common fowl, possesses very powerful muscular walls and a dense horny epithelium. Structure. — The stomach is composed of four coats, called respec- tively— an external or (1) peritoneal, (2) muscular, (3) submucous, and (4) mucous coat ; with blood-vessels, lymphatics, and nerves distributed in and between them. (1) The peritoneal coat has the structure of serous membranes in general. (2) The muscular coat consists of three separate layers or sets of fibres, which, according to their several directions, are named the longi- tudinal, circular, and oblique. The longitudinal set are the most superfi- cial : they are continuous with the longitudinal fibres of the oesophagus, and spread out in a diverging manner over the cardiac end and sides of the stomach. They extend as far as the pylorus, being especially dis- tinct at the lesser or upper curvature of the stomach, along which they pass in several strong bands. The next set are the circular or transverse fibres, which more or less completely encircle all parts of the stomach ; they are most abundant at the middle and in the pyloric portion of the organ, and form the chief part of the thick projecting ring of the py- lorus. These fibres are not simple circles, but form double or figure- DIGESTION. of-S loops, the fibres intersecting very obliquely. The next, and conse- quently deepest set cf fibres, are the oblique, continuous with the circular muscular fibres of the oesophagus, and having the same double-looped arrangement that prevails in the preceding layer : they are comparatively few in number, and are placed only at the cardiac orifice and portion of the stomach, over both surfaces of which they are spread, some passing ob- liquely from left to right, others from right to left, around the cardiac ori- fice, to which, by their interlacing, they form a kind of sphincter, con- tinuous with that around the lower end of the oesophagus. The mus- cular fibres of the stomach and of the intestinal canal are unstriated, being composed of elongated, spindle-shaped fibre-cells. (3) and (4) The mucous membrane of the stomach, which rests upon a layer of loose cellular membrane, or submucous tissue, is smooth, level, soft, and velvety; of a pale pink color during life, and in the contracted state thrown into numerous, chiefly longitudinal, folds or rugae, which disappear when the organ is dis- tended. The basis of the mucous mem- brane is a fine connective tissue, which approaches closely in structure to adenoid tissue ; this tissue supports the tubular glands of which the su- perficial and chief part of the mucous membrane is composed, and passing up between them assists in binding them together. Here and there are to be found in this coat, immedi- ately underneath the glands, masses of adenoid tissue sufficiently marked to be termed by some lymphoid follicles. The glands are separated from the rest of the mucous membrane by a very fine homogeneous basement membrane. At the deepest part of the mucous membrane are two layers (circular FIG. 185.— From a vertical section through the mucous membrane of the cardiac end of stomach. Two peptic glands are shown with a duct common to both, one gland only in part, a, duct with columnar epithelium becoming shorter as the cells are traced downward; n, neck of gland tubes, with central and parietal or so-called peptic cells; 6, fundus with curved caecal extremity— the parietal cells are not so numerous here. X 400. (Klein and Noble Smith.) 248 HANDBOOK OF PHYSIOLOGY-. and longitudinal) of unstriped muscular fibres, called the muscularis mucoscB, which separate the mucous membrane from the scanty submu- eous tissue. When examined with a lens, the internal or free surface of the stomach presents a peculiar honeycomb appearance, produced by shallow polyg- onal depressions, the diameter of which varies generally from yfoth to •3-l^th of an inch; but nearer the pylorus is as much as -j^th of an inch. They are separated by slightly elevated ridges, which sometimes, espe- cially in certain morbid states of the stomach, bear minute, narrow vas- cular processes, which look like villi, and have given rise to the errone- ous supposition that the stomach has absorbing villi, like those of the small intestines. In the bottom of these little pits, and to some extent between them, minute openings are visible, which are the orifices of the ducts of perpendicularly arranged tubular glands (Fig. 185), imbedded side by side in sets or bundles, on the surface of the mucous membrane, and composing nearly the whole structure. Gastric Glands.— Of these there are two varieties, (a) Peptic, (b) Pyloric or Mucous. (a) Peptic glands are found throughout the whole of the stomach except at the pylorus. They are arranged in groups of four or five, which are sepa- rated by a fine connective tissue. Two or three tubes often open into one duct, which forms about a third of the whole length of the tube and opens on the sur- face. The ducts are lined with columnar D — epithelium. Of the gland tube proper, FIG. 186. -Transverse section *'<>'> the Part °f the gland bel°W the duct, the ^Per third is the neck and the rest the tody- The neck is narr°wer than the body, and is lined with granular cubical cells which are continuous with the columnar cells of the duct. Between these cells and the membrana propria of the tubes, are large oval or spherical cells, opaque or granular in appearance, with clear oval nuclei, bulging out the membraua propria; these cells are called peptic or parie- tal cells. They do not form a continuous layer. The body, which is broader than the neck and terminates in a blind extremity or fundus near the muscularis mucosae, is lined by cells continuous with the cu- bical or central cells of the neck, but longer, more columnar and more transparent. In this part are a few parietal cells of the same kind as in the neck (Fig. 185). As the pylorus is approached the gland ducts become longer, and the tube proper becomes shorter, and occasionally branched at the fun- dus. DIGESTION. 249 (£) Pyloric Glands. — These glands (Fig. 187), have much longer ducts than the peptic glands. Into each duct two or three tubes open by very short and narrow necks, and the body of each tube is branched, wavy, and convoluted. The lumen is very large. The ducts are lined with columnar epithelium, and the neck and body with shorter and more granular cubical cells, which correspond with the central cells of the peptic glands. During secretion the cells become, as in the case of the peptic glands, larger, and the granules restricted to the inner zone of the cell. As they approach the duodenum the pyloric glands become larger, more convoluted, and more deeply situated. They are directly continuous with Brunner's glands in the duodenum. (Watney.) •mm. FIG 187. FIG. 188. FIG. 187.— Section showing the pyloric glands, s, free surface; d, ducts of pyloric glands; w, neck of same; m, the gland alveoli; mm, muscularis mucosae. (Klein and Noble Smith.) FIG. 188.— Plan of the blood-vessels of the stomach, as they would be seen in a vertical section, a, arteries, passing up from the vessels of submucous coat; 6, capillaries branching between and around the tubes; c, superficial plexus of capillaries occupying the ridges of the mucous membrane; d. veins formed by the union of veins which, having collected the blood of the superficial capillary plexus, are seen passing down between the tubes. (Brinton.) Changes in the gland cells during secretion, — The chief or cubical cells of the peptic glands, and the corresponding cells of the pyloric glands during the early stage of digestion, if hardened in alcohol, appear swollen and granular, and stain readily. At a later stage the cells be- come smaller, but more granular and stain even more readily. The parietal cells swell up, but are otherwise not altered during digestion. The granules, however, in the alcohol-hardened specimen, are believed not fco exist in the living cells, but to have been precipitated by the hard- 250 HANDBOOK OF PHYSIOLOGY. ening reagent; for if examined during life they appear to be confined to the inner zone of the cells, and the outer zone is free from granules, whereas during rest the cell is granular throughout. These granules are thought to be pepsin, or the substance from which pepsin is formed, pepsinogen, which is during rest stored chiefly in the inner zone of the cells and discharged into the lumen of the tube during secretion. (Lang- ley.) Lymphatics. — Lymphatic vessels surround the gland tubes to a greater or less extent. Towards the fundus of the peptic glands are found masses of lymph oid tissue, which may appear as distinct follicles, somewhat like the solitary glands of the small intestine. Blood-vessels. — The blood-vessels of the stomach, which first break up in the submucous tissue, send branches upward between the closely packed glandular tubes, anastomosing around them by means of a fine capillary network, with oblong meshes. Continuous with this deeper plexus, or prolonged upwards from it, so to speak, is a more superficial network of larger capillaries, which branch densely around the orifices of the tubes, and form the framework on which are moulded the small elevated ridges of mucous membrane bounding the minute, polygonal pits before referred to. From this superficial network the veins chiefly take their origin. Thence passing down between the tubes with no very free connection with the deeper intertubular capillary plexus, they open finally into the venous network in the submucous tissue. Nerves. — The nerves of the stomach are derived from the pneu mo- gastric and sympathetic, and form a plexus in the submucous and mus- cular coats, containing many ganglia (Eemak, Meissner). Gastric Juice. Gastric Juice. — The functions of the stomach are to secrete a digest- ive fluid (gastric juice), to the action of which the food is subjected after it has entered the cavity of the stomach from the oesophagus; to thor- oughly incorporate the fluid with the food by means of its muscular movements; and to absorb such substances as are ready for absorption. While the stomach contains no food, and is inactive, no gastric fluid is secreted; and mucus, which is either neutral or slightly alkaline, covers its surface. But immediately on the introduction of food or other sub- stance the mucous membrane, previously quite pale, becomes slightly turgid and reddened with the influx of a larger quantity of blood; the gastric glands commence secreting actively, and an acid fluid is poured out in minute drops, which gradually run together and flow down the walls of the stomach, or soak into the substances within it. Chemical Composition. — The first accurate analysis of gastric juice was made by Prout: but it does not appear to have been collected in any large quantity, or pure and separate from food, until the time when DIGESTION. 251 Beaumont was enabled, by a fortunate circumstance, to obtain it from the stomach of a man named St. Martin, in whom there existed, as the result of a gunshot wound, an opening leading directly into the stomach, near the upper extremity of the great curvature, and three inches from the cardiac orifice. The introduction of any mechanical irritant, such as the bulb of a thermometer, into the stomach, through this artificial opening, excited at once the secretion of gastric fluid. This was drawn off, and was often obtained to the extent of nearly an ounce. The in- troduction of alimentary substances caused a much more rapid and abundant secretion than did other mechanical irritants. No increase of temperature could be detected during the most active secretion; the ther- mometer introduced into the stomach always stood at 100° F. (37.8° C.) except during muscular exertion, when the temperature of the stomach, like that of other parts of the body, rose one or two degrees higher. The chemical composition of human gastric juice has been also in- vestigated by Schmidt. The fluid in this case was obtained by means of an accidental gastric fistula, which existed for several years below the left mammary region of a patient between the cartilages of the ninth and tenth ribs. The mucous membrane was excited to action by the intro- duction of some hard matter, such as dry peas, and the secretion was re- moved by means of an elastic tube. The fluid thus obtained was found to be acid, limpid, odorless, with a mawkish taste — with a specific gravity of 1002, or a little more. It contained a few cells, seen with the micro- scope, and some fine granular matter. The analysis of the fluid obtained in this way is given below. The gastric juice of dogs and other animals obtained by the introduction into the stomach of a clean sponge through an artifically made gastric fistula, shows a decided difference in compo- sition, but possibly this is due, at least in part, to admixture with food, Chemical Composition of Gastric Juice. Dogs. Human. Water, 971.17 994.4 Solids, 28.82 5.39 Solids- Ferment— Pepsin, .... 17.5 3.19 Hydrochloric acid (free), . . . 2.7 .2 Salts- Calcium, sodium, and potassium, chlor- ides; and calcium, magnesium, and iron, phosphates, . . . . 8.57 2,18 The quantity of gastric juice secreted daily has been variously esti- mated ; but the average for a healthy adult may be assumed to range from ten to twenty pints in the twenty-four hours. The acidity of the 252 HANDBOOK OF PHYSIOLOGY. fluid is due to free hydrochloric acid, although other acids, e.g., lactic, acetic, butyric, are not unfrequently to be found therein as products of gastric digestion or abnormal fermentation. The amount of hydro- chloric acid varies from 2 to .2 per 1000 parts. In healthy gastric juice the amount of free acid may be as much as .2 per cent. As regards the formation of pepsin and acid, the former is produced "by the central or chief cells of the peptic glands, and also most likely by the similar cells in the pyloric glands ; the acid is chiefly found at the surface of the mucous membrane, but is in all probability formed by the secreting action of the parietal cells of the peptic glands, as no acid is formed by the pyloric glands in which this variety of cell is absent. The ferment Pepsin can be prepared by digesting portions of the mu- cous membrane of the stomach in cold water, after they have been mace- rated for some time in water at a temperature 80°-100° F. (27.°-37.8° C.). The warm water dissolves various substances as well as some of the pepsin, but the cold water takes up little else than pepsin, which is con- tained in a grayish-brown viscid fluid, on evaporating the cold solution. The addition of alcohol throws down the pepsin in grayish-white flocculi. Glycerin also has the property of dissolving out the ferment ; and if the mucous membrane be finely minced, and the moisture removed by absolute alcohol, a powerful extract may be obtained by throwing into glycerin. Functions — The digestive power of the gastric juice depends on the pepsin and acid contained in it, both of which are, under ordinary cir- cumstances, necessary for the process. The general effect of digestion in the stomach is the conversion of the food into chyme, a substance of various composition according to the na- ture 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. The chief function of the gastric juice is to convert proteids into pep- tones. This action maybe shown by adding a little gastric juice (natural or artificial) to some diluted egg-albumin, and keeping the mixture at a temperature of about 100° F. (37.8° C.) ; it is soon found that the albu- min cannot be precipitated on boiling, but that if the solution be neu- tralized with an alkali, a precipitate of acid-albumin is thrown down. After a while the proportion of acid-albumin gradually diminishes, so that at last scarcely any precipitate results on neutralization, and finally it is found that all the albumin has been changed into another proteid substance which is not precipitated on boiling or on neutralization. This is called peptone. Characteristics of Peptones. — Peptones have certain characteristics which distinguish them from other proteids. 1. They are diffusible, i.e., they possess the property of passing through animal membranes. 2. DIGESTION. 253 They cannot be precipitated by heat, by nitric, or acetic acid, or by po- tassium ferrocyanide and acetic acid. They are, however, thrown down by tannic acid, by mercuric chloride and by picric acid. 3. They are very soluble in water and in neutral saline solutions. In their diffusibility peptones differ remarkably from egg-albumin, and on this diffusibility depends one of their chief uses. Egg-albumin as such, even in a state of solution, would be of little service as food, in- asmuch as its indiffusibility would effectually prevent its passing by ab- sorption into the blood-vessels of the stomach and intestinal canal. Changed, however, by the action of the gastric juice into peptones, albu- minous matters diffuse readily, and are thus quickly absorbed. After entering the blood th.e peptones are very soon again modified, so as to re-assume the chemical characters of albumin, a change as neces- sary for preventing their diffusing out of the blood-vessels, as the pre- vious change was for enabling them to pass in. This is effected, proba- bly, in great part by the agency of the liver. Products of Gastric Digestion. — The chief product of gastric diges- tion is undoubtedly peptone. We have seen, however, in the above experiment that there is a by-product, and this is almost identical with syntonin or acid albumin. This body is probably not exactly identical, however, with syntonin, and its old name of parapeptone had better be retained. The conversion of native albumin into acid-albumin may be effected by the hydrochloric acid alone, but the further action is un- doubtedly due to the ferment and the acid together, as although under high pressure any acid solution may, it is said, if strong enough, produce the entire conversion into peptone, under the condition of digestion in the stomach this would be quite impossible ; and, on the other hand, pepsin will not act without the presence of acid. The production of two forms of peptone is usually recognized, called respectively anti-peptone and hemi-peptone. Their differences in chemical properties have not yet been made out, but they are distinguished by this remarkable fact, that the pancreatic juice, while possessing no action over the former, is able to convert the latter into leucin and tyrosin. Pepsin acts the part of a hydrolytic ferment (proteolytic), and appears to cause hydration of al- bumin, peptone being a highly hydrated form of albumin. Circumstances favoring Gastric Digestion. 1. A temperature of about 100° F. (37.8° 0.) ; at 32° F. (0° C.) it is delayed, and by boiling is altogether stopped. 2. An acid medium, is necessary. Hydrochloric is the best acid for the purpose. Excess of acid or neutralization stops the process. 3. The removal of the products of digestion. Excess of peptone delays the action. Action of the Gastric Juice on Bodies other than Proteids. — All pro- teids are converted by the gastric juice into peptones, and, therefore, 254: HANDBOOK OF PHYSIOLOGY. whether they be taken into the body in meat, eggs, milk, bread, or other foods, the resultant still is peptone. Milk is curdled, the casein being precipitated, and then dissolved. The curdling is due to a special ferment of the gastric juice (curdling or rennet ferment), and is not due to the action of the free acid only. The effect of rennet, which is a decoction of the fourth stomach of a calf in brine, has long been known, as it is used extensively to cause precipita- tion of casein in cheese manufacture. The ferment which produces this curdling action is distinct from pepsin. Gelatin is dissolved and changed into peptone, as are also chondrin and elastin; but Mucin, and the Horny tissues, which contain keratin generally are unaffected. On the Amylaceous articles of food, and upon pure Oleaginous prin- ciples, the gastric juice has no action. In the case of adipose tissue, its effect is to dissolve the areolar tissue, albuminous cell-walls, etc., which enter into its composition, by which means the fat is able to mingle more uniformly with the other constituents of the chyme. The gastric fluid acts as a general solvent for some of the saline con- stituents of the food, as, for example, particles of common salt, which may happen to have escaped solution in the saliva ; while its acid may enable it to dissolve some other salts which are insoluble in the latter or in water. It also dissolves cane sugar, and by the aid of its mucus causes its conversion in part into grape sugar. The action of the gastric juice in preventing and checking putrefac- tion has been often directly demonstrated. Indeed, that the secretions which the food meets with in the alimentary canal are antiseptic in their action, is what might be anticipated, not only from the proneness to decomposition of organic matters, such as those used as food, espe- cially under the influence of warmth and moisture, but also from the well-known fact that decomposing flesh (e. g., high game) may be eaten with impunity, while it would certainly cause disease were it allowed to enter the blood by any other route than that formed by the organs of digestion. Time occupied in Gastric Digestion. — Under ordinary conditions, from three to four hours may be taken as the average time occupied by the digestion of a meal in the stomach. But many circumstances will modify the rate of gastric digestion. The chief are : the nature of the food taken and its quantity (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 a meal (gentle exercise being fa- vorable, over-exertion injurious to digestion) ; the state of mind (tran- quillity of temper being essential, in most cases, to a quick and due di- gestion) ; the bodily health ; and some others. DIGESTION. 255 Movements of the Stomach. — The gastric fluid is assisted in accom- plishing 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 digestion, by grinding and triturating the hard seeds which constitute part of the food. But in the stomachs of man and other Mammalia the movements 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 experiments have demonstrated that substances are digested even inclosed in perforated tubes, and consequently protected from mechanical influence. The normal actions of the muscular fibres of the human stomach ap- pear to have a three-fold purpose : (1) to adapt the stomach to the quan- tity 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 com- pression upon it; (2) to keep the orifices of the stomach closed until the food is digested ; and (3) to perform certain peristaltic movements, whereby the food, as it becomes chymified, is gradually propelled to- wards, and ultimately through, the pylorus. In accomplishing this lat- ter end, the movements without 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 stomach, and immediately again closes. The pyloric orifice, during the first part of gastric digestion, is usually so completely 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 py- lorus 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. 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 moved towards the pylorus. The movements were observed to increase in rapidity as the process of chymification ad- vanced, 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 decidedly peristaltic than those of the cardiac portion. Thus, it was found in the case of St. Mar- tin, that when the bulb of the thermometer was placed about three inches from the pylorus, through the gastric fistula, it was tightly em- braced from time to time, and drawn towards the pyloric orifice for a 256 HANDBOOK OF PHYSIOLOGY. distance of three or four inches. The object of this movement appears to be, as just said, to carry the food towards 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, until all that is digestible has passed out. The action 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 ac- tion of strong circular fibres placed between the cardia and pylorus, the two portions, or ends as they are called, of the stomach, are partially separated from each other by a kind of hour-glass contraction. By means of the peristaltic action of the muscular 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 stomach, the circumferential parts of the mass being gradually moved onward towards the pylorus by the contraction of the muscular fibres, while the central portions are propelled in the opposite direction, name- ly, towards the cardiac orifice ; in this way is kept up a constant circu- lation of the contents of the viscus, highly conducive to their free mixture with the gastric fluid and to their ready digestion. Influence of the Nervous System on Gastric Digestion.— The normal movements of the stomach during gastric digestion are directly connected with the plexus of nerves and ganglia contained in its walls, the presence of food acting as a stimulus which is conveyed to the gang- lia and reflected to the muscular fibres. The stomach is, however, also directly connected with the higher nerve-centres by means of branches of the vagus and solar plexus of the sympathetic. The vaso-motor fibres of the latter are derived, probably, from the splanchnic nerves. The exact function of the vagi in connection with the movements of the stomach is not certainly known. Irritation of the vagi produces contraction of the stomach, if digestion is proceeding; while, on the other hand, peristaltic action is retarded or stopped, when these nerves are divided. Bernard, watching the act of gastric digestion in dogs which had fistulous openings into their stomachs, saw that on the instant of divid- ing their vagi 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 facts may be explained by the theory that the vagi are the media by which, during digestion, an inhibitory impulse is conducted to the vaso-motor centre in the medulla; such im- pulse being reflected along the splanchnic nerves to the blood-vessels of the stomach, and causing their dilatation (Rutherford). From other experiments it may be gathered, that although division of both vagi always temporarily suspends the secretion of gastric fluid, and so arrests DIGESTION. 257 the process of digestion, being occasionally followed by death from ina- nition; 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. This would indicate the existence of a special local nervous mechanism which controls the secretion. Bernard found that galvanic stimulus of these nerves excited an active secretion of the fluid, while a like stimulus applied to the sympa- thetic nerves issuing from the semilunar ganglia, caused a diminution and even complete arrest of the secretion. The influence of the higher nerve-centres on gastric digestion, as in the case of mental emotion, is too well known to need more than a reference. 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 frequently 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 into the cavity of the abdomen. This phenomenon is not unfrequently observed in post- mortem examinations of the human body. If a rabbit be killed during 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 dissolved (Pavy). 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? It is only necessary to refer to the idea of Bernard, that the living stomach finds protection from its secretion in the presence of epithelium and mucus, which are constantly renewed in the same degree that they are constantly dissolved, in order to remark that, although the gastric mucus is probably protective, this theory, so far as the epithelium is concerned, 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 remov- ing the mucous me'mbrane, 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 dimen- sions/' Pavy believes that the natural alkalinity of the blood, which circu- lates so freely during life in the walls of the stomach, is sufficient to neutralize the acidity of the gastric juice; and as may be gathered from what has been previously said, the neutralization of the acidity of the gastric secretion is quite sufficient to destroy its digestive powers; but 258 HANDBOOK OF PHYSIOLOGY. the experiments adduced in favor of this theory are open to many objec- tions, and afford only a negative support to the conclusions they are intended to prove. Again, the pancreatic secretion acts best on proteids in an alkaline medium; but it has no digestive action on the living in- FIG. 189. Auerbach's nerve-plexus in small intestine. The plexus consists of fibrillated sub- stance, and is made up of trabeculae of various thicknesses. Nucleus-like elements and ganglion- cells are imbedded in the plexus, the whole of which is inclosed in a nucleated sheath. (Klein.) testine. It must be confessed that no entirely satisfactory theory has been yet stated. VOMITING. The expulsion of the contents of the stomach in vomiting, like that of mucus or other matter from the lungs in coughing, is preceded by an inspiration; the glottis is then closed, and immediately afterwards the abdominal muscles strongly act; but here occurs the difference in the two actions. Instead of the vocal cords yielding to the action of the abdominal muscles, they remain tightly closed. Thus the diaphragm being unable to go up, forms an unyielding surface against which the stomach can be pressed. In this way, as well as by its own contraction, the diaphragm is -fixed, to use a technical phrase. At the same time the cardiac sphincter muscle being relaxed, and the orifice which it naturally guards being actively dilated, while the pylorus is closed, and the stomach itself also contracting, the action of the abdominal muscles, by these means assisted, expels the contents of the organ through the oesophagus, pharynx, and mouth. The reversed peristaltic action of the oesophagus probably increases the effect. It has 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 capacity of the abdomen is DIGESTION. 259 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 con- traction of the abdominal muscles alone is sufficient to expel matters from an unresisting bag through the oesophagus; and that, under very abnormal circumstances, the stomach, by itself, cannot expel its con- tents. 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 the facts are wanting to demonstrate with certainty this action of the stomach in vomiting; but some of the cases of fistu- lous opening into the organ appear to support the belief that it does take place; and the analogy of the case of the stomach with that of the other hollow viscera, as the rectum and bladder, may be also cited in confirm- ation. The muscles concerned in the act of vomiting are chiefly and prima- rily those of the abdomen; the diaphragm also acts, but usually not as the muscles of the abdominal walls do. They contract and compress the stomach more and more towards the diaphragm; and the diaphragm (which is usually drawn down in the deep inspiration that precedes each act of vomiting) is fixed, and presents an unyielding surface against which the stomach may be pressed. The diaphragm is, therefore, as a rule passive, during the actual expulsion of the contents of the stomach. But there are grounds for believing that sometimes this muscle actively contracts, so that the stomach is, so to speak, squeezed between the de- scending diaphragm and the retracting abdominal walls. 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 power may be acquired by those who do not natu- rally possess it, and by continual 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 regurgi- tate it, piece by piece, into their mouth, remasticate, and. again swallow it, like members of the ruminant order of Mammalia. The various nerve-actions concerned in vomiting are governed by a nerve-centre situate in the medulla oblongata. The sensory nerves are the fifth, glosso-pharyngeal and vagus prin- cipally; but, as well, vomiting may occur from stimulation of sensory nerves from many organs, e. g., kidney, testicle, etc. The centre may also be stimulated by impressions from the cerebrum and cerebellum, so called central vomiting occurring in disease of those parts. The efferent impulses are carried by the phrenics and other spinal nerves. 260 HANDBOOK OF PHYSIOLOGY. THE INTESTINES. The Intestinal canal is divided into two chief portions, named from their differences in diameter, the (I.) small and (II.) large intestine (Fig. 164). These are continuous with each other, and communicate 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. I. The Small Intestine. — The Small Intestine, the average length of which in an adult is about twenty feet, has been divided, for conveni- ence of description, into three portions, viz., the duodenum, which extends for eight or ten inches beyond the pylorus; the jejunum, which forms two-fifths, and the ileum, which forms three-fifths of the rest of the canal. Structure. — The small intestine, like the stomach, is constructed of four principal coats, viz., the serous, muscular, submucous, and mucous. (1. ) The serous coat, formed by the visceral layer of the peritoneum, and has the structure of serous membranes in general. (2.) The muscular coats consist of an internal circular and an exter- nal longitudinal layer; the former is usually considerably the thicker. Both alike consist of bundles of unstriped muscular tissue supported by connective tissue. They are well provided with lymphatic vessels, which form a set distinct from those of the mucous membrane. Between the two muscular coats is a nerve-plexus (Auerbaoh/s plexus, plexus myentericus) (Fig. 189), similar in structure to Meissner's (in the submucous tissue), but with more numerous ganglia. This plexus regu- lates the peristaltic movements of the muscular coats of the intestines. (3.) Between the mucous and muscular coats is the submucous coat, which consists of connective tissue, in which numerous blood-vessels and lymphatics ramify. A fine plexus, consisting mainly of non-medul- lated nerve-fibres, Meissner's plexus, with ganglion cells at its nodes, occurs in the submucous tissue from the stomach to the anus. From the position of this plexus and the distribution of its branches, it seems highly probable that it is the local centre for regulating the calibre of the blood-vessels supplying the intestinal mucous membrane, and pre- siding over the processes of secretion and absorption. (4.) The mucous membrane is the most important coat in relation to the function of digestion. The following structures, which enter into its composition, may now be successively described: — the valvulcv conni- ventes; the villi; and the glands. The general structure of the mucous membrane of the intestines resembles that of the stomach (p. 245), and, like it, is lined on its inner surface by columnar epithelium. Adenoid tissue (Fig. 190, c and d) enters largely into its construction; and on its DIGESTION. 281 deep surface is the musculans mucoscz (m m, Fig. 191), the fibres of which are arranged in two layers: the outer longitudinal and the inner circular. ValvulcB Conniventes. — The valvulae conniventes(Fig. 192) 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 through- out the jejunum; then, gradually diminishing in size and number, they cease near the middle of the ileum. They are formed by a doubling inwards of the mucous membrane; the crescentic, nearly circular, folds thus formed being arranged transversely to the axis of the intestine, and FIG. 190. FIG. 191. FIG. 190.— Horizontal section of a small fragment of the mucous membrane, including one en- tire crypt of Lieberkiihn and parts of several others; a, cavity of the tubular glands or crypts; 6, one of the lining epithelial cells; c, the lymphoid or retiform spaces, o: which some are empty, and others occupied by lymph cells, as at d. FIG. 191. —Vertical section through portion of small intestine of dog. v, two villi showing e, epithelium; g, goblet cells. The free surface is seen to be formed by the v' striated basilar border." while inside the villus the adenoid tissue and unstriped muscle-cells are seen; If. Lieberkuhn's folli- cles; m m, muscularis mucosse, sending up fibres between the follicles into the villi; sm, submucous tissue; containing (grri), ganglion cells of Meissner's plexus. (Schofield.) each individual fold seldom extending around more than ^ or f of the bowel's circumference. Unlike the ruga? in the oesophagus and stomach, they do not disappear on distention of the canal. 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. 262 HANDBOOK OF PHYSIOLOGY. On then making a section it will be seen that, instead of disappearing, they stand out at right angles to the general surface of the mucous membrane (Fig. 192). Their functions are (1) that they offer a largely increased surface for secretion and absorption, and (2) that they prevent the too rapid passage of tho very liquid products of gastric digest-ion, immediately after their escape from the stomach, and (3), by their pro- jection, and consequent interference with an uniform and untroubled current of the intestinal contents, that they assist in the more perfect mingling of the latter with the secretions poured out to act on them. Glands. — -The glands are of three principal kinds: — viz., those of (1) Lieberkiihn, (2) Brunner, and (3) Peyer. (1.) The glands or crypts of LieberlcuJin are simple tubular depres- sions of the intestinal mucous membrane, thickly distributed over the whole surface both of the large and small intestines. In the small in- testine they are visible only with the aid of a lens; and their orifices ap- pear as minute dots scattered between the villi. They are larger in the FIG. 192. FIG. 193. FIG. 194. FIG. 192.— Piece of small intestine (previously distended and hardened by alcohol) laid open to show the normal position of the valvulae conniventes. FIG. 193.— Transverse section through four crypts of Lieberkuhn from the large intestine of the Smith.) FIG. 194.— A rrland of Lieberkuhn hi longitudinal section. (Brinton.) 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 vis- ible to the naked eye. In length they vary from -^ to TV of a line. Each tubule (Fig. 194) is constructed of the same essential parts as the intestinal mucous membrane, viz., of a fine membrana propria, or base- ment membrane, a layer of cylindrical epithelium lining it, and capil- lary blood-vessals covering its exterior, the free surface of the columnar DIGESTION. 263 cells presenting an appearance precisely similar to the " striated basilar border " which covers the villi. 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. Among the columnar cells of Lieberkiihn's follicles, goblet cells fre- quently occur (Fig. 193). (2.) Brunner's glands (Fig. 196) are confined to the duodenum; they are most abundant and thickly set at the commencement of this portion of the intestine, diminishing gradually as the duodenum advances. They are situated beneath the mucous membrane, and imbedded in the sub- mucous tissue, each gland is a branched and convoluted tube, lined with columnar epithelium. As before said, in structure they are very similar to the pyloric glands of the stomach, and their epithelium undergoes a similar change during secretion; but they are more branched and con- voluted and their ducts are longer. (Watney.) The duct of each gland passes through the muscularis mucosae, and opens on the surface of the mucous membrane. (3. ) The glands of Peyer occur chiefly but not exclusively in the small intestine. They are found in greatest abundance in the lower part of the ileum near to the ileo-caecal valve. They are met with in two condi- tions, viz., either scattered singly, in which case they are termed glan- dulce 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 glandules agminatcv, the groups being commonly called Peyer's patches (Fig. 197), and almost always placed opposite the attachment of the mesentery. In structure, and in function, there is no essential dif- ference between the solitary glands and the individual bodies of which each group or patch is made up. They are really single or aggregated masses of adenoid tissue forming lymph-follicles. In the condition in which they have been most commonly examined, each gland appears as a circular opaque-white rounded body, from -fa to -fa inch in diameter, according to the degree in which it is developed. They are principally contained in the submucous coat, but sometimes project through the muscularis mucosce into the mucous membrane. In the agminate glands, each follicle reaches the free surface of the intestine, and is covered with columnar epithelium. Each gland is surrounded by the openings of Lieberkiihn's follicles. The adjacent glands of a Peyer's patch are connected together by adenoid tissue. Sometimes the lymphoid tissue reaches the free surface, replacing the epithelium, as is also the case with some of the lymphoid follicles of the tonsil (p. 242). Peyer's glands are surrounded by lymphatic sinuses which do not penetrate into their interior ; the interior is, however, traversed by a 264 HANDBOOK OF PHYSIOLOGY. very rich blood capillary plexus. If the vermiform appendix of a rabbit which consists largely of Peyer's glands be injected with blue by press- ing the point of a fine syringe into one of the lymphatic sinuses, the Peyer's glands will appear as grayish-white spaces surrounded by blue ; if now the arteries of the same be injected with red, the grayish patches will change to red, thus proving that they are surrounded by lymphatic spaces but penetrated by blood-vessels. The lacteals passing out of the villi communicate with the lymph sinuses round Peyer's glands. It is to be noted that they are largest and most prominent in children and young persons. Villi.— The Villi (Figs. 191, 196, 198, and 199), are confined ex- Fio. 195.— 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 network spreading from the surrounding blood-vessels into the interior of three of Peyer's capsules from the intestine of the rabbit. clusively to the mucous membrane of the small intestine. They are mi- nute vascular processes, from a quarter of a line to a line and two-thirds in length, covering the surface of the mucous membrane, and giving it a peculiar velvety, fleecy appearance. Krause estimates them at fifty to ninety in number in a square line at the upper part of the small intes- tine, 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 lym- phatic vessels they contain are empty or full of chyle; being usually, in the former case, flat and pointed at their summits, in the latter cylindri- cal or cleavate. Each villus consists of a small projection of mucous membrane, and its interior is therefore supported throughout by fine adenoid tissue, which forms the framework or stroma in which the other constituents are contained. DIGESTION. 265 The surface of the villas is clothed by columnar epithelium, which rests on a fine basement membrane; while within this are found, reck- oning from without inwards, blood-vessels, fibres of the muscularis mu- cosw, and a single lymphatic or lacteal vessel rarely looped or branched (Fig. 200); besides granular matter, fat-globules, etc. 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 surface of the villus (Fig. 199), and their smaller ends resting on the basement membrane. The free surface of the epithelial cells of the villi, like that of the cells which cover the gene- ral surface of the mucous membrane, is covered by a fine border which exhibits very delicate striations, whence it derives its name, " striated basilar border." Beneath the basement or limiting membrane there is a rich supply of Mood- vessels. Two or more minute arteries are distributed within each villus ; and from their capillaries, which form a dense net- work, 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 blood- vessels. It is without doubt instrumental in the propulsion of ch}Tle along the lacteal. The lacteal vessel enters the base of each villus, and passing up in the mid- dle 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. 200), 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 common in some of the lower animals (A, Fig. 201). The office of the villi is the absorption of chyle and other liquids from the intestine. The mode in which they effect this will be considered in the next Chapter. II. The Large Intestine. — The Large Intestine, which in an adult f FIG. 196.— Vertical section of duo- denum showing a, villi: b crypts of Lieberkuhn, and c, Brunner's glands in the submucosa s, with ducts, d; muscularis mucosae, m; and circular muscular coat /. (Schofleld.) 266 HANDBOOK OF PHYSIOLOGY. is from about 4 to 6 feet long, is subdivided for descriptive purposes into three portions (Fig. 164) viz. : — thecacum, a short wide pouch, commu- nicating with the lower end of the small intestine through an opening, FIG. 197.— Agminate follicles, or Peyer's patch, in the state of distention. x 5. (Boehm.) guarded by the ileo-ccBcal valve ; the colon, continuous with the caecum, which forms the principal part of the large intestine, and is divided into ascending, transverse, and descending portions ; and the rectum, which, after dilating at its lower part, again contracts, and immediately after- m, tn FIG. 198. FIG. 198.— Section of small intestine, showing villi, Lieberkuhn's glands and a Peyer's solitary gland, m, m, muscularis mucosae CKlein and Noble Smith.) FIG. 199.— Vertical section of a villus of the small intestine of a cat. a, striated basilar border of the epithelium; 6, columnar epithelium; c, goblet cells; d, central lymph -vessel; e, smooth muscular fibres; /, adenoid stroma of the villus in which lymph corpuscles lie. (Klein.) wards opens externally through the anus. Attached to the caecum is the small appendix vermiformis. Structure. — Like the small intestine, the large intestine is constructed of four principal coats, viz., the serous, muscular, submucous, and mu- cous. The serous coat need not be here particularly described. Con- nected with it are the small processes of peritoneum containing fat, DIGESTION. 20 7 called appendices epiploicce. The fibres of the muscular coat, like those- of the small intestine, are arranged in two layers— the outer longitudinal, the inner circular. In thecajcum and colon, the longitudinal fibres, be- sides 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 intestine, 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, an uniformly cylindrical form. In the rectum, the fasciculi of these longitudinal bands spread out and FIG. 200.— A. Villus of sheep. B. Villi of man. (Slightly altered from Teichmann.) mingle with the other longitudinal fibres, forming \vith 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 projected in the form of valvulcB conniventes. Its general microscopic structure resembles that of the small intestine : and it is bounded below by the muscularis mucosce. 288 HANDBOOK OF PHYSIOLOGY. The general arrangement of ganglia and nerve-fibres in the large in- testine resembles that in the small (p. 260). Glands. — The glands with which the large intestine is provided are of two kinds, (1) the tubular and (2) the lymphoid. (1.) 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. (2.) Follicles of adenoid or lymphoid tissue are most numerous in the caecum and vermiform appendix. They resemble in shape and structure, FIG. 201.— Diagram of lacteal vessels in small intestine. A, lacteals inyilli; p, Peyer's glands; B and D, superficial and deep network of lacteals in submucous tissue; L, Lieberkuhn's glands: E, small branch of lacteal vessel on its way to mesenteric gland; H and o, muscular fibres of intes- tine ; s, peritoneum. (Teichmann.) almost exactly, the solitary glands of the small intestine. Peyer's patches are not found in the large intestine. Ileo-caecal Valve. — The ileo-caecal valve is situate at the place of junction of the small with the large intestine, and guards against any re- flux 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 dou- oling inwards of the mucous membrane, and is strengthened on the out- side by some of the circular muscular fibres of the intestine, which are contained between the outer surfaces of the two layers of which each fold is composed. "While the circular muscular fibres, however, of the bowel DIGESTION. at the junction of the ileum with the caecum are contained between the outer opposed surfaces of the folds of mucous membrane which form the valve, the longitudinal muscular fibres and the peritoneum of the small and large intestine respectively are continuous with each other, without FIG. 202. — Horizontal section through a portion of the mucous membrane of the large intes- tine, showing Lieberkiihn's glands in transverse section, a, lumen of gland— lining of columnar cells with c, goblet cells, 6, supporting connective tissue. Highly magnified. (V. D. Harris.; dipping in to follow the circular 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 longitu- dinal muscular fibres and the peritoneum, the valve can be made to dis- appear, just as the constrictions between the sacculi of the large intestine can be made to disappear by performing a similar operation. The inner surface of the folds is smooth : the mucous membrane of the ileum being continuous with that of the caacum. That surface of each fold which looks towards the small intestine is covered with villi, while that which looks to the caecum has none. When the caacum is distended, the margins of the folds are stretched, and thus are brought into firm apposi- tion one with the other. DIGESTION IN THE I After the food has been duly acted upon by the stomach, such as has not been absorbed passes into the duodenum, and is there subjected to the action of the secretions of the pancreas and liver which enter that portion of the small intestine. Before considering the changes which the food undergoes in consequence, attention should be directed to the HANDBOOK OF PHYSIOLOGY. structure and secretion of these glands, and to the secretion (succus •entericus) which is poured out into the intestines from the glands lining them. o \l j THE PANCREAS, AND ITS SECRETION. The Pancreas is situated within the curve formed by the duode- num; and its main duct opens into that part of the small intestine, through a small opening, or through a duct common to it and to the liver, about two and a half inches from- the pylorus. Structure. — In structure the pancreas bears some resemblance to the salivary glands. Its capsule and septa, as well as the blood-vessels and lymphatics, are similarly distributed. It is, however, looser and softer, the lobes and lobules being less compactly arranged. The main duct FIG. 203.— Section of the pancreas of a dog during digestion, a, alveoli lined with cells, the outer zone of which is well stained with haematoxylin; d, intermediary duct lined with squamous epithelium, x 350. (Klein and Noble Smith.) divides into branches (lobar ducts), one for each lobe, and the-e branches subdivide into intralobular ducts, and these again by their division and branching form the gland tissue proper. The intralobular ducts corre- spond to a lobule, while between them and the secreting tubes or alveoli are longer or shorter intermediary ducts. The larger ducts possess a very distinct lumen and a membrana propria lined with columnar epi- thelium, the cells 'of which are longitudinally striated, but are shorter than those found in the ducts of the salivary glands. In the intralobu- lar ducts the epithelium is short and the lumen is smaller. The inter- mediary ducts opening into the alveoli possess, a distinct lumen, with a membrana propia lined with a single layer of flattened elongated cells. The alveoli are branched and convoluted tubes, with a membrana pro- pria lined with a single layer of columnar cells. They have no distinct lumen, the centre portion of the tube being occupied by fusiform or branched cells. Heidenhain has observed that the alveolar cells in DIGESTION. 271 the pancreas of a fasting dog consist of two zones, an inner or central zone which is finely granular, and which stains feebly, and a smaller parietal zone of finely striated protoplasm which stains easily. The nucleus is partly in one, partly in the other zone. During digestion, it is found that the outer zone increases in size, and the central zone diminishes ; the cell itself becoming smaller from the discharge of the secretion. At the end of digestion the first condition again appears, the inner zone enlarging at the expense of the outer. It appears that the granules are formed by the protoplasm of the cells, from material sup- plied to it by the blood. The granules are thought to be not the fer- ment itself, but material from which, under certain conditions, the fer- ments of the gland are made, and therefore called Zymogen. The special form of nerve terminations, called Pacinian corpuscles, are often found in the pancreas. Pancreatic Secretion. — The secretion of the pancreas has been ob- tained for purposes of experiment from the lower animals, especially the dog, by opening the abdomen and exposing the duct of the gland, which is then made to communicate with the exterior. A pancreatic fistula is thus established. An extract of pancreas made from the gland which has been re- moved from an animal killed during digestion possesses the active prop- erties of pancreatic secretion. It. is made by first dehydrating the gland, which has been cut up into small pieces, by keeping it for some days in absolute alcohol, and then, after the entire removal of the alcohol, plac- ing it in strong glycerin. A glycerin extract is thus obtained. It is a remarkable fact, however, that the amount of the ferment trypsin greatly increases if the gland be exposed to the air for twenty-four hours before placing in alcohol; indeed, a glycerin extract made from the gland immediately upon removal from the body often appears to contain none of the ferment. This seems to indicate that the conversion of zymogen in the gland into the ferment only takes place during the act of secretion, and that the gland, although it always contains in its cells the materials (trypsinogen) out of which trypsin is formed, yet the con- version of the one into the other only takes place by degrees. Dilute acid appears to assist and accelerate the conversion.and if a recent pan- creas be rubbed up with dilute acid, before de@clration, a glycerin extract made afterwards, even though the gland ma^have been onlv re- cently removed from the body, is very active. Properties. — Pancreatic juice is colorless, transparent, and slightly viscid, alkaline in reaction. It varies in specific gravity from 1010 to 1015, according as it is obtained from a permanent fisiula — then more watery — or from a newly-opened duct. The solids varylin a temporary fistula from 80 to 100 parts per thousand, and in a permanent one from 16 to 50 per thousand. 272 HANDBOOK OF PHYSIOLOGY. Chemical Composition of the Pancreatic Secretion. From a permanent fistula. (Bernstein.) Water, 975 Solids — Ferments (including trypsin, amylop- sin, rennet, and ? steapsin): Proteids, including Serum- Albumin ) and Casein, v 17 Leucin and Tyrosin; Fats and Soaps. ) Inorganic resfdue, especially Sodium [ 8 Carbonate, j 25 1000 r~^? Functions. — (1.) By the aid of its proteolytic ferment trypsin, it converts proteids into peptones, the intermediate product being not akin to syntonin or acid-albumin as in gastric digestion, but to alkali-albu- min. Kuhne calls the intermediate products, both in the peptic and pancreatic digestion of proteids, anti-albumose and herni-albumose, and states that the peptones formed correspond to these products, which he therefore terms anti-peptone and hemi-peptone. The hemipeptone is capable of being converted by the action of the pancreatic ferment— trypsin — into leucin or amido-caproic acid (C6H12N02) and tyrosin (C9HUN03), but is not so changed by pepsin: the antipeptone cannot be further split up. The products of pancreatic digestion are sometimes further complicated by the appearance of certain faecal substances of which indot (C3H2N), skatol (C9H9N), phenol (C6H60), and napthila- mine are the most important. (Kiihue.) When the digestion goes on for a long time the indol is formed in considerable quantities, and emits a most disagreeable faecal odor. These further products are produced by the presence of numerous micro-organ- isms in the pancreatic digestion fluid. All the albuminous or proteid substances which have not been con- verted into peptone and absorbed in the stomach, and the partially changed substances, i. e., the para-peptones, are converted into peptone by the pancreatic juice, and then in part into leucin and tyrosin. (2.) The action of the pancreatic juice upon the gelatins, or nitrog- enous bodies other than proteids, is not so distinct. Mucin can, however, be dissolved, but not keratin in horny tissues. Gelatin itself is formed into peptone (gelatin-peptone). (3.) Starch is converted into maltose and then into glucose in an ex- actly similar manner to that which happens with the saliva; erythro- and achroo-dextrine being intermediate products. If the sugar which is at first formed is maltose, the ferment of the pancreatic juice after a time completes the whole change of starch into glucose. This distinct amy- DIGESTION. 273 lolytic ferment in the pancreatic juice which cannot be distinguished from ptyalin, is called Amylopsin. (4. ) Pancreatic juice possesses the property of curdling milk, contain- ing a special (rennet) ferment for that purpose. The ferment is distinct from trypsin, and will act in* the presence of an acid (W. Roberts). It is best extracted by brine. (5.) Oils and fats are emulsified and saponified ~by pancreatic secre- tion. The terms emulsification and saponification may need a little ex- planation. The former is used to signify an important mechanical change in oils or fats, whereby they are made into an emulsion, or in other words are minutely subdivided into small particles. If a small drop of an emulsion be looked at under the microscope it will be seen to be made up of an immense number of minute rounded particles of oil or fat, of varying sizes. The more complete the emulsion the smaller are these particles. An emulsion is formed at once if oil or fat, which nearly always is slightly acid from the presence of free fatty acid, is mixed with an alkaline solution. Saponification signifies a distinct chemical change in the composition of oils and fats. An oil or a fat is made up chemically of glycerin, a triatomic alcohol (see Appendix), and one or more fatty acid radicles. When an alkali is added to a fat and heat is applied, two changes take place, firstly, the oil or fat is split up into glycerin and its corresponding fatty acid; secondly, the fatty acid combines with the alkali to form a soap which is chemically known as stearate, oleate, or palmitate of potassium or sodium. Thus saponifica- tion means a chemical splitting up of oils or fats into new compounds, and emulsification means merely a mechanical splitting of them up into minute particles. The pancreatic juice has been for many years credited with the possession of a special ferment, which was called by Claude Bernard steapsin, and which was supposed to aid in one or both of these processes. It appears very doubtful, however, if either the mechanical or the chemical splitting up of fats by the alkaline pancreatic juice is a ferment action at all. Several cases have been recorded in which the pancreatic duct being obstructed, so that its secretion could not be discharged, fatty or oily matter was abundantly discharged from the intestines. In nearly all these cases, indeed, the liver was coinci dentally diseased, and the change or absence of the bile might appear to contribute to the result, yet the frequency of extensive disease of the liver, unaccompanied by fatty dis- charges 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. Conditions favorable to the Action. — These are similar to those which are favorable to the action of the saliva, and the reverse (p. 18 274 /j, ' HANDBOOK OF PHYSIOLOGY: ' (0 THE LIVER. The Liver, the largest gland in the body, situated in the abdomen on the right side chiefly, is an extremely vascular organ, and receives its sup- ply of blood from two distinct sources, viz., from the portal vein and from the hepatic artery, while the blood is returned from it into the vena cava inferior by the hepatic veins. 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 veins and their tributaries run by themselves. On the outside, the liver has an incomplete covering of peritoneum, and beneath this is a very fine coat of areolar tissue, continuous over the Fio. 204.— The under surface of the liver, o. B., gall-bladder; H. D., common bile-duct; H. A., hepatic artery; v. p., portal vein; L. Q., lobulus quadratus; L. s., lobulus spigelii; L. c., lobulus caudatus; D. v., ductus venosus; u. v., umbilical vein. (Noble Smith.) whole surface of the organ. It is thickest when the peritoneum is ab- sent, and is continuous on the general surface of the liver with the fine and, in the human subject, almost imperceptible areolar tissue investing the lobules. At the transverse fissure it is merged in the areolar invest^ ment called Glisson's capsule, which, surrounding the portal vein, he- patic artery, and hepatic duct, as they enter at this part, accompanies them in their branchings through the substance of the liver. Structure. — The liver is made up of small roundish or oval portions called lobules, each of which is about -fa of an inch in diameter, and composed 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. The hepatic cells (Fig. 205), which form the glandular or secreting part of the liver, are of a spheroidal form, some- what polygonal from mutual pressure, about -g-Jo-to y^-g-inch in diameter, possessing one, sometimes two nuclei. The cell-substance contains nu- DIGESTION. 275 merous fatty molecules, and some yellowish-brown grannies of bile-pig- ment. The cells sometimes exhibit slow amoeboid movements. They are held together by a very delicate sustentacular tissue, continuous with the interlobular connective tissue. To understand the distribution of the blood-vessels in the liver, it will be well to trace, first, the two blood-vessels and the duct which en- ter the organ on the under surface at the transverse fissure, viz., the portal vein, hepatic artery, and hepatic duct. As before remarked, all three run in company, and their appearance on longitudinal section is shown in Fig. 206. Kunning together through the substance of the liver, they are contained in small channels called portal canals, their immediate investment being a sheath of areolar tissue (Glisson's capsule). d a FIG. 205. FIG. 206. FIG. 205.— A. Liver-cells. B. Ditto, containing various-sized particles of fat. FIG. 206.— Longitudinal section of a portal canal, containing a portal vein, hepatic artery and hepatic duct, from the pig. p, branch of vena portae, situate in a portal canal formed amongst the lobules of the liver, I Z, 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; fc, hepatic duct. X 5. (Kiernan.) To take the distribution of the portal vein first : — In its course through the liver this vessel gives off small branches which divide and and subdivide between the lobules surrounding them and limiting them, and from this circumstance called ^Wer-lobular veins. From these small vessels a dense capillary network 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 aWra-lobular. This arrangement is well seen in Fig. 207, which represents a transverse section of a lobule. The small intra-lobul&r veins discharge their contents into veins HANDBOOK OF PHYSIOLOGY. called sMft-lobular (h h li, Fig. 208); while these again, by their union, form the main branches of the hepatic veins, which leave the posterior border of the liver to end by two or three principal trunks in the infe- Fio. 207.— Cross section of a lobule of the human liver, in which the capillary network between the portal and hepatic veins has been fully injected 1, section of the tnfra-lobular vein; 2, its smaller branches collecting blood from the capillary network; 3, iwter-lobular branches of the vena portae with their smaller ramifications passing inwards towards the capillary network in the sub- stance of the lobule, x 60. (Sappey.) FIG. 208.— Section of a portion of liver passing longitudinally through a considerable hepatic vein, from the pig. H, hepatic venous trunk, against which the sides of the lobules (0 are applied ; h, h, h, sublobular hepatic veins, on which the bases of the lobules rest, and through the coats of which they are seen as polygonal figures ; i, mouth of the intralobular veins, opening into the sublobular veins ; i', intralobular veins shown passing up the centre of some divided lobules ; I, I, cut surface of the liver ; c, c, walls of the hepatic venous canal, formed by the polygonal bases of the lobules. X 5, (Kiernan.) DIGESTION. 277 rior vena cava, just before its passage through the diaphragm. The sw#-lobular and hepatic veins, unlike the portal vein and its companions, have little or no areolar tissue around them, and their coats being very thin, they form little more than mere channels in the liver substance which closely surrounds them. The manner in which the lobules are connected with the sublobular veins by means of the small intralobu lar veins is well seen in the diagram (Fig. 209 and in Fig. 208), which represent the parts as seen in a longi- tudinal section. The appearance has been likened to a twig 'having leaves without footstalks — the lobules representing the leaves, and the sublobular vein the small branch from which it springs. On a trans- verse section, the appearance of the intralobular veins is that of 1, Fig. 207, while both a transverse and longitudinal section are exhibited in Fig. 208. The hepatic artery, the function of which is to distribute blood for -Lobule* Lobulcw FIG. 2C9. FIG. 210. FIG. 209.— Diagram showing the manner in which the lobules of the liver rest on the sublobular veins. (After Kiernan.) FIG. 210. -Capillary network of the lobules of the rabbit's liver. The figure is taken from a very successful injection of the hepatic veins, made by Harting ; it shows nearly the whole of two lobules, and parts of three others ; p, portal branches running in the interlobular spaces ; ft, hepatic veins penetrating and radiating from the centre of the lobules. X 45. (Kolliker.) nutrition to G-lisson's capsule, the walls of the ducts and blood-vessels, and other parts of the liver, is distributed in a very similar manner to the portal vein, its blood being returned by small branches either into the ramifications of the portal vein, or into the capillary plexus of the lobules which connects the inter- and twtfrfl-lobular 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 epithelium. The bile-capillaries commence between the hepatic cells, and are bounded by a delicate membranous wall of their own. They appear to 278 HANDBOOK OF PHYSIOLOGY. be always bounded by hepatic cells on all sides, aiid are thus separated, from the nearest blood-capillary by at least the breadth of one cell (Figs. 211 and 212). THE GALL-BLADDER. The Gall-bladder (G, B, Fig. 204) is a pyriform bag, attached to the under surface of the liver, and supported also by the peritoneum, which passes below it. The larger end or fundus, projects beyond the front margin of the liver ; while the smaller end contracts into the cystic duct. Structure. — The walls of the gall-bladder are constructed of three principal coats. (1) Externally (excepting that part which is in con- tact with the liver), is the serous coat, which has the same structure as FIG. 211. FIG. 212. FIG. 211.— Portion of a lobule of liver, a, bile capillaries between liver-cells, the network in which is well seen ; 6, blood capillaries. X 350. (Klein and Noble Smith.) FIG. 212.— Hepatic cells and bile capillaries, from the liver of a child three months old. Both figures represent fragments of a section carried through the periphery of a lobule. The red cor- mds to an interlpbular :ts, to which, at. Hering.) the peritoneum with which it is continuous. Within this is (2) the fibrous or areolar coat, constructed of tough fibrous and elastic tissue, with which is mingled a considerable number of plain muscular fibres, both longitudinal and circular. (3) Internally the gall-bladder is lined by mucous membrane, and a layer of columnar epithelium. The surface of the mucous membrane presents to the naked eye a minutely honey- combed appearance from a number of tiny polygonal depressions with intervening ridges, by which its surface is mapped out. In the cystic duct the mucous membrane is raised up in the form of crescentic folds, which together appear like a spiral valve, and which minister to the- function of the gall-bladder in retaining the bile during the intervals, of digestion. DIGESTION. 279 The gall-bladder and all the main biliary ducts are provided with mucous glands, which open on their internal surface. Functions of the Liver. — The functions of the Liver may be classified under the following heads : — 1. The Secretion of Bile. 2. The Elaboration of Blood ; under this head may be included the Glyco- genic Function. 1. THE SECRETION OF BILE. The Bile. — Properties. — The bile is a somewhat viscid fluid, of a yellow or a reddish-yellow color, a strongly bitter taste, and, when fresh, with a scarcely perceptible odor: it has a neutral or slightly alka- line reaction, and its specific gravity is about 1020. Its color and degree of consistence vary much, quite independent of disease; but, as a rule, it becomes gradually more deeply colored and thicker as it ad- vances 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 absorption of its water, but partly also from being mixed with mucus. Chemical Composition of Human Bile. (Frerichs.) Water, . . . . . . . .859.2 Solids — Bile salts or Bilin, .... 91.5 Fat, ... .... 9.2 Cholesterin, . .... 2.6 Mucus and coloring matters, . . .29.8 Salts, 7.7 140.8 1000.0 (a) Bile salts, or Bilin, can be obtained as colorless, exceedingly del- iquescent crystals, soluble in water, alcohol, and alkaline solutions, giving to the watery solution the taste and general characters of bile. They consist of sodium salts of glycocholic and taurocholic acids. The former salt is composed of cholic acid combined with glycin (see Ap- pendix), the latter of the same acid combined with taurin. The pro- portion of these two salts in the bile of different animals varies, e. g., in ox bile the glycocholate is in great excess, whereas the bile of the dog, cat, bear, and other carnivora contains taurocholate alone ; in human bile both are present in about the same amount (glycocholate in excess?). Preparation of Bile Salts. — Bile salts may be prepared in the fol- lowing manner: mix bile which has been evaporated to a quarter of its bulk with animal charcoal, and evaporate to perfect dryness in a water bath. Next extract the mass whilst still warm with absolute alcohol. 280 HANDBOOK OF PHYSIOLOGY. Separate the alcoholic extract by filtration, and to it add perfectly an- hydrous ether as long as a precipitate is thrown down. The solution and precipitate should be set aside in a closely stoppered bottle for some days, when crystals of the bile salts or bilin will have separated out. The glycocholate may be separated from the taurocholate by dissolving bilin in water, and adding to it a solution of neutral lead acetate, and then a little basic lead acetate, when lead glycocholate separates out. Filter and add to the filtrate lead acetate and ammonia, a precipitate of lead taurocholate will be formed, which may be filtered off. In both cases, the lead may be got rid of by suspending or dissolving in hot alcohol, adding hydrogen sulphate, filtering and allowing the acids to separate out by the addition of water. The Test for bile salts is known as Pettenkofer's. If to an aqueous solution of the salts strong sulphuric acid be added, the bile acids are first of all precipitated, but on the further addition of the acid are re- dissolved. If to the solution a drop of solution of cane sugar be added? a fine deep cherry red to purple color is developed. The reaction will also occur on the addition of grape or fruit sugar instead of cane sugar, slowly with the first, quickly with the last; and a color similar to the above is produced by the action of sulphuric acid and sugar on albumen, the crystalline lens, nerve tissue, oleic acid, pure ether, cholesterin, morphia, codeia and amylic alcohol. The spectrum of Pettenkofer's reaction, when the fluid is moderately diluted, shows four bands — the most marked and largest at E; and a little to the left; another at F; a third between D and E, nearer to D; and a fourth near D. (b) The yellow coloring matter of the bile of man and the Carnivora is termed Biliriibin or Bilifulvin (C16H18N203) crystallizable and insolu- ble in water, soluble in chloroform or carbon disulphide; a green color- ing matter, Biliverdin (016H20N205), which always exists in large amount in the bile of Herbivora, being formed from bilirubin on expo- sure to the air, or by subjecting the bile to any other oxydizing agency, as by adding nitric acid. When the bile has been long in the gall- bladder, a third pigment, Biliprasin, may be also found in small amount. In cases of biliary obstruction, the coloring matter of the bile is re- absorbed, and circulates with the blood, giving to the tissues the yellow tint characteristic of jaundice. The coloring matters of human bile do not appear to give character- istic absorption spectra; but the bile of the guinea pig, rabbit, mouse, sheep, ox, and crow do so, the most constant of which appears to be a band at F. The bile of the sheep and ox give three bands in a thick layer, and four or five bands with a thinner layer, one on each side of D, one near E, and a faint line at F. (McMunn.) DIGESTION. 281 There seems to be a close relationship between the color-matters of the blood and of the bile, and it may be added, between these and that of the urine (uroMlin), and of the faeces (stercoUliri) also; .it is probable they are, all of them, varieties of the same pigment, or derived from the same source. Indeed it is maintained that UroMlin is identical with HydroMUrubin, a substance which is obtained from bilirubin by the action of sodium amalgam, or by the action of sodium amalgam on alka_ line haamatin; both urobilin and hydrobilirubin giving a characteristic absorption band between b and F. They are also identical with sterco- bilin, which is formed in the alimentary canal from bile pigments. The Test (Gmelin's) for the presence of 'bile-pigment consists of the addition of a small quantity of nitric acid, yellow with nitrous acid; if bile be present, a play of colors is produced, beginning with green and passing through blue and violet to red, and lastly to yellow. The spec- Fio. 213. — Crystalline scales of cholesterin. trum of Gmelin's test gives a black band extending from near b to beyond F. (c) Fatty substances are found in variable proportions in the bile. Besides the ordinary saponifiable fats, there is a small quantity of Cho- lesterin, a so-called non-saponifiaUefat, which is really an alcohol, and, with the free fats, is probably held in solution by the bile salts. It is a body belonging to the class of monatomic alcohols (C26H440), and crystal- lizes in rhombic plates (Fig. 213). It is insoluble in water and cold alcohol, but dissolves easily in boiling alcohol or ether. It gives a red color with strong sulphuric acid, and with nitric acid and ammonia; also a play of colors beginning with blood red and ending with green on the addition of sulphuric acid and chloroform. Lecithin (C44H90NPOH), a phosphorus-containing body and Neurin (C5H15N02), are also found in bile, the latter probably as a decomposition product of the former. (d) The Mucus in bile is derived from the mucous membrane and glands of the gall-bladder, and of the hepatic ducts. 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 282 HANDBOOK OF PHYSIOLOGY. cylindrical cells, either scattered or still held together in layers. To the presence of the mucus is probably to be ascribed the rapid decomposition of the bile; for, according to Berzelius, if the mucus be separated, it will remain unchanged for many days. (e) The /Saline or inorganic constituents of the bile are similar to those found in most other secreted fluids. It is possible that the carbo- nate 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 generally found in healthy bile, and constantly in biliary calculi. (/) Gas. — Small amounts of carbonic acid, oxygen, and nitrogen gases may be extracted from bile. Mode of Secretion and Discharge. — The secretion of bile is continu- ally going on, but it appears to be retarded during fasting, and accele- rated on taking food. This has been shown by tying the common bile- duct of a dog, and establishing a fistulous opening between the skin and gall-bladder, whereby all the bile secreted was discharged at the surface. It was 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 introduction of food into the stomach, the bile began to flow abundantly, and continued to do so during the whole period of digestion. The bile is formed in the hepatic cells; thence, 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 happens only while digestion is going on; during fasting, it regurgitates from the common bile-duct through the cystic duct, into the gall-bladder, where it accumulates till, in the next period of diges- tion, 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, yet insures its employ- ment 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 pressure behind to force the bile through it. The pressure exercised 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 peri- staltic action of the ducts. The bile is discharged from the gall-bladder and enters the duodenum on the introduction of food into the small in- DIGESTION. testine: being pressed on by the contraction of the coats of the gall- bladder, and of the common bile-duct also; for both these organs con- tain unstriped muscular fibre-cells. Their contraction is excited 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 com- mon bile-duct. Bile, as such, is not pre-formed in the blood. As just observed, it is formed or secreted by the hepatic cells, although some of the material may be brought to them almost in the condition for immediate secretion. When it is, however, prevented by an obstruction of some kind, from escaping into the intestine (as by the passage of a gall-stone along the hepatic duct) it is absorbed in great excess into the blood, and, circulat- ing with it, gives rise to the well-known phenomena of jaundice. This is explained by the fact that the pressure of secretion in the ducts is normally very low, and if it exceeds f inch of mercury (16 mm.) the se- cretion ceases to be poured out, and if the opposing force be increased, the bile finds its way into the blood. Quantity. — Various estimates have been made of the quantity of bile discharged into the intestines in twenty-four hours; the quantity doubt- less varying, like that of the gastric fluid, in proportion to the amount of food taken. A fair average of several computations would give 20 to 40 oz. (600-900 cc.) as the quantity daily secreted by man. Functions. — (1) As an excrementitious substance, the bile may serve especially as a medium for the separation of excess of carbon and hydro- gen from the blood; and its adaptation to this purpose is well illustrated by the peculiarities attending its secretion and disposal ia 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, on account of the supply of well elaborated nutriment received by the vessels of the foetus at the placenta. The- liver, during the same time, is proportionately 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 se- cretion; the meconium, or faeces of the foetus, containing all the essential principles of bile. Composition of Meconium (Frerichs): Biliary resin, 15.6 Common fat and cholesterin, . . . .15.4 Epithelium, mucous, pigment, and salts, . 69.0 100.0 In the foetus, therefore, the main purpose of the secretion of bile must }>e the purification of blood by direct excretion, i.e., by separation from HANDBOOK OF PHYOLOGY. 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 discharged, and thus the liver may be said to discharge a func- tion 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. Disposal of the Bile. — The evident disposal of the foetal bile by excre- tion, makes it highly probable that the bile in extra-uterine life is also, at least in part, destined to be discharged as excrement] tious. The analysis of the faeces of both children and adults shows, however, that (except when rapidly discharged in purgation) they contain very little of the bile secreted, probably not more than one-sixteenth part of its weight, and that this portion includes chiefly its coloring matter in the form of stercobilin, and some of its fatty matters, and to only a very slight de- gree, its salts, almost all of which have been re-absorbed from the intes- tines into the blood. The elementary composition of bile-salts shows such a preponderance of carbon and hydrogen, that probably, after absorption, it combines with oxygen, and is excreted in the form of carbonic acid and water. The change after birth, 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 temperature of the foetus is maintained by that of the parent, and needs no source of heat within itself; but, in extra-uterine life, there is (as one may say) a waste of material for heat when any excretion is discharged unoxidized ; the car- bon and hydrogen of the bilin, therefore, instead of being ejected in the faeces, are re-absorbed, in order that they may be combined with oxygen, and that in the combination heat may be generated. It appears that taurocholic acid may easily be split up in the intestine into taurin and cholalic acid. The former does not appear in the faeces, but the latter has been found there. So that in part it is excreted, but part is re-ab- sorbed in the intestine and returned to the liver. It is probable that al- though part of this may unite to re-form glycocholic or taurocholic acid, the remainder is united with oxygen, and is burnt off in the form of car- bonic acid and water. A substance, which has been discovered in the fasces, and named ster- corin is closely allied to cholesterin ; and it has been suggested that while one great function of the liver is to excrete cholesterin from the blood, as the kidney excretes urea, the stercorin of faeces is the modified form in which cholesterin finally leaves the body. Ten grains and a half •of stercorin are excreted daily (A. Flint). DIGESTION. 285 From the peculiar manner in which the liver is supplied with much of the blood that flows through it, it is probable that this organ is excre- tory, not only for such hydro-carbonaceous matters as may need expul- sion 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 expelled, 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 nu- trition, but which would be positively injurious if allowed to mingle with the general mass of the blood. The liver, therefore, may be sup- posed placed in the only road by which such matters can pass unchanged into the general current, jealously to guard against their further pro- gress, and turn them back again into an excretory channel. The fre- quency with which metallic poisons are either excreted by the liver, or intercepted and retained, often for a considerable time, in its own sub- stance, may be adduced as evidence for the probable truth of this sup- position. (2.) As a digestive fluid. — Though one chief purpose of the secretion of bile may thus appear to be the purification of the blood by ultimate ex- cretion, 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 excretions are, but is made to pass into the in- testinal canal, so as to be mingled with the chyme directly after it leaves the stomach ; an arrangement, the constancy of which clearly indicates 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 in- testines much greater, during digestion than at any other time ; al- though, without doubt, this activity of secretion during digestion may, however, 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 tempora- rily (to be afterwards re-absorbed), or permanently. Eespecting the functions discharged by the bile in digestion, there is little doubt that it (a.) assists in emulsifying the fatty portions of the food, and thus rendering them capable of being absorbed by the lacteals. For it has appeared 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 forme.d ; the contents of the lacteals 286 HANDBOOK OF PHYSIOLOGY. consisting of clear, colorless fluid, instead of being opaque and white, as they ordinarily are, after feeding. (b.) It is probable, also, that the moistening of the mucous membrane of the intestines by bile facilitates absorption of fatty matters through it. (c.) The bile, like the gastric fluid, has a considerable antiseptic power, and may serve to prevent the decomposition of food during the time of its sojourn in the intestines. Experiments show that the con- tents of the intestines are much more foetid after the common bile-duct has been tied than at other times : moreover, it is found that the mix- ture of bile with a fermenting fluid stops or spoils the process of fermen- tation. (d.) The bile has also been considered to act as a natural purgative, by promoting an increased secretion of the intestinal glands, and by stimulating the intestines to the propulsion of their contents. This view receives support from the constipation which ordinarily exists in jaundice, from the diarrhoea which accompanies excessive secretion of bile, and from the purgative properties of ox-gall. (e.) The bile appears to have the power of precipitating the gastric parapeptones and peptones, together with the pepsin, which is mixed up with them, as soon as the contents of the stomach meet it in the duode- num. The purpose of this operation is probably both to delay any change in the parapeptones until the pancreatic juice can act upon them, and also to prevent the pepsin from exercising its solvent action on the ferments of the pancreatic juice. II. BLOOD-ELABOKATIOX. The secretion of bile, as already observed, is only one of the purposes fulfilled by the liver. Another very important function 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 the blood generally, and to prepare others for being duly eliminated in the process of respiration. It appears that the peptones, conveyed from the alimen- tary canal by the blood of the portal vein, require to be submitted to the influence of the liver before they can be assimilated by the blood; for if such albuminous matter is injected into the jugular 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 incorporated with the albuminous part of the blood. Glycogenic Function. One of the chief uses of the liver in connection with that elaboration of the blood is known as its glycogenic function. The important fact that the liver normally forms glucose, or a substance readily convertible DIGESTION. 28 T into it, was discovered by Claude Bernard in the following way: he fed a dog for seven days with food containing a large quantity of sugar and starch; and, as might be expected, found sugar in both the portal and hepatic veins. And this dog was fed with meat only, and, to his sur- prise, sugar was still found in the hepatic veins. Repeated experiments gave invariably the same result; no sugar being found, under a meat diet, in the portal vein, if care were taken, by applying a ligature on it at the transverse fissure, to prevent reflux of blood from the hepatic ve- nous system. Bernard found sugar also in the substance of the liver. It thus seemed certain that the liver formed sugar, even when, from the absence of saccharine and amyloid matters in the food, none could be brought directly to it from the stomach or intestines. Excepting cases in which large quantities of starch and sugar were taken as food, no sugar was found in the blood after it had passed through the lungs; the sugar formed by the liver, having presumably disappeared by combustion, in the course of the pulmonary circulation. Bernard found, subsequently to the before-mentioned experiments, that a liver, removed from the body, and from which all sugar had been completely washed away by injecting a stream of water through its blood- vessels, will be found, after the lapse of a few hours, to contain sugar in abundance. This post-mortem production of sugar was a fact which could only be explained in the supposition that the liver contained a substance, readily convertible into sugar in the course merely of post- mortem decomposition; and this theory was proved correct by the dis- covery of a substance in the liver allied to starch, and now generally termed glycogen. We may believe, therefore, that the liver does not form sugar directly from the materials brought to it by the blood, but that glycogen is first formed and stored in its substance, and that the sugar, when present, is the result of the transformation of the latter. Quantity of Glycogen formed. — Although, as before mentioned, gly- cogen is produced by the liver when neither starch nor sugar is present in the food, its amount is much leas under such a diet. Average amount of Glycogen in the Liver of Dogs under various Diets (Pavy). Diet. Amount of Glycogen in Liver. Animal food, 7. 19 per cent. Animal food with sugar (about J Ib. of sugar daily), . 14.5 Vegetable diet (potatoes, with bread or barley-meal), . 17.23 " The dependence of the formation of glycogen on the food taken is also well shown by the following results, obtained by the same experi- menter:— 288 HANDBOOK OF PHYSIOLOGY. Average quantity of Glycogen found in the Liver of Rabbits after Fast- ing, and after a diet of Starch and Sugar respectively. Average Amount of Glycogen in Liver. After fasting for three days, . . . Practically absent. " diet of starch and grape-sugar, . 15.4 per cent. " " cane-sugar, . . . 16.9 " Regarding these facts there is no dispute. All are agreed that gly- cogen is formed, and laid up in store, temporarily, by the liver-cells; and that it is not formed exclusively from saccharine and amylaceous foods, but from albuminous substances also; the albumen, in the latter case, being probably split np into glycogen, which is temporarily stored in the liver, and urea, which is excreted by the kidneys. Destination of Glycogen. — There are two chief theories on the subject of the destination of glycogen. (1.) That the conversion of glycogen into sugar takes place rapidly during life by the agency of a ferment (liver diastase) also formed in the liver: and the sugar is conveyed away by the blood of the hepatic veins, and soon undergoes combustion. (2.) That the conversion into sugar only occurs after death, and that during life no sugar exists in healthy livers; glycogen not undergoing this trans- formation. The chief arguments advanced in support of this view are, (a) 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 killed; while if the examina- tion be delayed for a very short time after death, sugar in abundance may be found in such blood; (#), that the liver, like the venous blood in the heart, is, at the moment of death, completely free from sugar, al- though 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 sub- stance of the organ. Instead of adopting Bernard's view, that normally, 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 the belief that it may represent an intermediate stage in the formation of fat from materials absorbed from the alimentary canal. Liver-Sugar. — 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 sodium-sulphate in order to precipitate albuminous and coloring matters. The decoction is then filtered and may be tested for glucose. Glycogen (C6 H10 06) is an amorphous, starch-like substance, odorless and tasteless, soluble in water, insoluble in alcohol. It is converted into glucose by boiling with dilute acids, or by contact with any animal fer- ment. It may be obtained by taking a portion of liver from a recently killed rabbit, 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 DIGESTION. 289 pulpy mass, and subsequently boiled in distilled water for about a quarter of an hour. The glycogen is precipitated from the filtered decoction by the addition of alcohol. Glycogen has been found in many other struc- tures than the liver (See Appendix). Glycosuria. — The facility with which the glycogen of the liver is transformed into sugar would lead to the expectation that this chemical change, under many circumstances, would occur to such an extent that sugar would be present not only in the hepatic veins, but in the blood generally. Such is frequently the case; the sugar when in excess in the blood being secreted by the kidneys, and thus appearing in variable quantities in the urine (G-lycosuria). Influence of the Nervous System. — Glycosuria maybe experimentally produced by puncture of the medulla oblongata in the region of the vaso-motor centre. The better fed the animal the larger is the amount of sugar found in the urine; whereas in the case of a starving animal no sugar appears. It is, therefore, highly probable that the sugar comes from the hepatic glycogen, since in the one case glycogen is in excess, and in the other it is almost absent. The nature of the influence is uncertain. It may be exercised in dilating the hepatic vessels, or pos- sibly may be exerted on the liver cells themselves. The whole course of the nervous stimulus cannot be traced to the liver, but at first it passes from the lower part of the floor of the fourth ventricle and medulla down the spinal cord as far as — in rabbits— the fourth dorsal vertebra, and thence to the first thoracic ganglion. Many other circumstances will cause glycosuria. It has been ob- served after the administration of various drugs, after the injection of urari, poisoning with carbonic oxide gas, the inhalation of ether, chloro- form, etc., the injection of oxygenated blood into the portal venous sys- tem. It has been observed in man after injuries to the head, and in the course of various diseases. The well-known disease, diabetes mellitus, in which a large quantity of sugar is persistently secreted daily with the urine, has, doubtless, some close relation to the normal glycogenic function of the liver; but the nature of the relationship is at present quite unknown. THE INTESTINAL SECRETION, OR Succus ENTERICUS. On account of the difficulty in isolating the secretion of the glands in the wall of the intestine (Brunner's and Lieberkiihn's) from other secretions poured into the canal (gastric juice, bile, and pancreatic secre- tion), but little is known regarding the composition of the former fluid (intestinal juice, succus entericus). It is said to be a yellowish alkaline fluid with a specific gravity of 1011, and to contain about 2.5 per cent of solid matters (Thiry). 19 290 HANDBOOK OF PHYSIOLOGY. Functions. — The secretion of Brunner's glands is said to be able to convert proteids into peptones, and that of Lieberkiihn's is believed to convert starch into sugar. To these functions of the succus entericus the powers of converting cane into grape sugar, and of turning grape sugar into lactic, and afterwards into butyric acid, are added by some physiologists. It also probably contains a milk-curdling ferment (W. Roberts). The reaction which represents the conversion of cane sugar into grape sugar may be represented thus: — 80,.H1,011 + 2H,0 = C15H,40,, + C,,H,,012 Saccharose water Dextrose Laevulose The conversion is effected probably by means of a hydrolytic fer- ment. (Inversive ferment, Bernard.) The length and complexity of the digestive tract seem to be closely connected with the character of the food on which an animal lives. Thus in all carnivorous animals, such as the cat and dog, and pre-emi- nently in carnivorous birds, as hawks and herons, it is exceedingly short. The seals, which, though carnivorous, possess a very long intestine, appear to furnish an exception; but this is doubtless to be explained as an adaptation to their aquatic habits, their constant exposure to cold re- quiring that they should absorb as much as possible from their intestines. Herbivorous animals, on the other hand, and the ruminants espe- cially, have very long intestines (in the sheep 30 times the length of the body), which is no doubt to be connected with their lowly nutritious diet. In others, such as the rabbit, though the intestines are not ex- cessively long, this is compensated by the great length and capacity of the caecum. In man, the length of the intestines is intermediate be- tween the extremes of the carnivora and herbivora, and his diet also is intermediate. Summary of the Digestive Changes in 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 an 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 in- terfered with, if not altogether stopped. The soluble matters — both those which were so from the first, as sugar and saline matter, and the gastric peptones — 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, etc. DIGESTION. 291 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, consists of albuminous matter, broken down, dissolving and half dissolved; fatty matter broken down and melted, 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 faBces. On the entrance of the chyme into the duodenum, it is subjected to the influence of the bile and pancreatic juice, which are then poured out, and also to that of the succus entericus. All these secretions have a more or less alkaline reaction, and by their admixture with the gastric chyme, its acidity become less and less until at length, at about the middle of the small intestine, the reaction becomes alkaline and con- tinues so as far as the ileo-caecal valve. The special digestive functions of the small intestine may be taken in the following order: — (1.) One important duty of the small intestine 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 re- spectively to the bile, to the pancreatic secretion, and to the intestinal juice, is still uncertain. The fat is changed in two ways, (a.) To a slight extent it is chemically decomposed by the alkaline secretions with which it is mingled, and a soap is the result, (b.) It is emulsionized, i. e., its particles are minutely subdivided and diffused, so that the mix- ture assumes the condition of a milky fluid or emulsion. As will be seen in the next Chapter, most of the fat is absorbed by the lacteals of the intestine, but a small part, which is saponified, is also absorbed by the blood-vessels. (2.) The albuminous substances which have been partly dissolved in the stomach, and have not been absorbed, are subjected to the action of the pancreatic and intestinal secretions. The pepsin is rendered inert by being precipitated together with the gastric peptones and parapep- tones, as soon as the chyme meets with bile. By these means the pan- creatic ferment trypsin is enabled to proceed with the further conversion of the parapeptones into peptones, and of part of the peptones (hemipep- tone, Kiihne) intoleucin and tyrosin. Albuminous substances, which are chemically altered in the process of digestion (peptones) and gelatinous matters similarly changed, are absorbed by the blood-vessels and lym- phatics of the intestinal mucous membrane. Albuminous matters, in state of solution, which have not undergone the peptonic change, are 292 HANDBOOK OF PHYSIOLOGY. probably, from the difficulty with which they diffuse, absorbed, if at all, almost solely by the lymphatics. (3.) The starchy, or amyloid portions of the food, the conversion of which into dextrin and sugar was more or less interrupted during its stay in the stomach, is now acted on briskly by the pancreatic juice and the succus entericus; and the sugar as it is formed, is dissolved in the intestinal fluids, and is absorbed chiefly by the blood-vessels. (4.) Saline and saccharine matters, as common salt, or cane sugar, if not in a state of solution beforehand in the saliva or other fluids which may have been swallowed with them, are at once dissolved in the stomach, and if not here absorbed, are soon taken up in the small intes- tine; the blood-vessels, as in the last case, being chiefly concerned in the absorption. Cane sugar is in part or wholly converted into grape-sugar before its absorption. This is accomplished partially in the stomach, but also by a ferment in the succus entericus. (5.) The liquids, including in this term the ordinary drinks, as water, wine, ale, tea, etc., which may have escaped absorption in the stomach, are absorbed probably very soon after their entrance into the intestine; the fluidity of the contents of the latter being preserved more by the constant secretion of fluid by the intestinal glands, pancreas, and liver, than by any given portion of fluid, whether swallowed or secreted, re- maining long unabsorbed. From this fact, therefore, it may be gathered that there is a kind of circulation constantly proceeding from the intes- tines into the blood, and from the blood into the intestines again; for as all the fluid — 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 re-absorbed 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. At the lower end of the small intestine, the chyme, still thin and pultaceous, is of a light yellow color, and has a distinctly fascal odor. This odpr depends upon the formation of indol and its allies. In this state it passes through the ileo-caacal opening into the large in- testine. Summary of the Digestive Changes in the Large Intestine. The changes which take place in the chyme in 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 completed in the small intes- tine; while, from the still half-liquid, pultaceous consistence of the DIGESTION. 293 •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 a further chemical change in the alimentary matters or in the digestive fluids, or both. The acid reac- tion, which had disappeared in the small bowel, again becomes very manifest in the caecum — probably from acid fermentation-processes in some of the materials of the food. There seems no reason to conclude that any special ' secondary diges- tive' process occurs in the caecum or in any other part of the large in- testine. Probably any constituent of the food which has escaped diges- tion 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, albuminous, 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 stomach. 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; the chief function of the large intestine being to act .as a reservoir for the residues of digestion before their expulsion from the body. 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 influence of fresh portions of intes- tinal secretion, and as slowly exposed to the absorbent power of all the villi and blood-vessels of the mucous membrane. The movement of the intestines is peristaltic or vermicular, and is effected by the alternate contractions and dilatations of successive portions of the intestinal 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 musjcular fibres contract first, or more than the circular ; they draw a portion of the intestine upwards, or, as it were, backwards, over the substance 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 action next ensues. These movements take place slowly, and, in health, commonly give rise to no sensation; but they are perceptible when they are accelerated under the influence 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 294 HANDBOOK OF PHYSIOLOGY. 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 borders of which orifice are the folds of mucous mem- brane which form the valve) is encircled with muscular fibres, the con- traction 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 fasces, 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 exter- nally, and as we have seen above, holds the orifice close by a constant slight tonic contraction. Experimental irritation of the brain or cord produces no evident or constant effect on the movements of the intestines during life ; yet in consequence of certain mental conditions the movements are accelerated or retarded ; and in paraplegia the intestines appear after a time much weakened in their power, and costiveness, with a tympanitic 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 gan-glionic plexus before referred to. Influence of the Nervous System on Intestinal Digestion. As in the case of the oesophagus and stomach, the peristaltic move- ments of the intestines are directly due to reflex action through the gang- lia and nerve fibres distributed so abundantly in their walls (p. 258) ; the presence of chyme acting as the stimulus, and few or no movements occurring when the intestines are empty. The intestines are, moreover, connected with the higher nerve-centres by the splanchnic nerves, as well as other branches of the sympathetic which come to them from the cceliac and other abdominal plexuses. The splanchnic nerves are in relation to the intestinal movements, inhibitory — these movements being retarded or stopped when the splanchnics are irritated. As the vaso-motor nerves of the intestines, the splanchnics are also much concerned in intestinal digestion. Duration of Intestinal Digestive Period.— 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 expresed in regard to the subject. About twelve hours are occupied by the journey of an ordinary meal DIGESTION. 295 through the small intestine, and twenty-four to thirty-six hours by the passage through the large bowel. The contents of the large intestine, as they proceed towards the rec- tum, become more and more solid, and losing their more liquid and nu- trient parts, gradually acquire the odor and consistence characteristic of fwces. After a sojourn of uncertain duration in the sigmoid flexure of the colon, or in the rectum, they are finally expelled by the act of defae- cation. The average quantity of solid faecal matter evacuated by the human adult in twenty-four hours is about six or eight ounces. Composition of Faeces. Water, 733.00 Solids : Special excrementitious constituents : — Excretin, ex- cretoleic acid (Marcet), and stercorin (Austin Flint). Salts : — Chiefly phosphate of magnesium and phos- phate of calcium, with small quantities of iron, soda, lime, and silica. Insoluble residue of the food (chiefly starch grains, woody tissue, particles of cartilage and fibrous tissue, undigested muscular fibres or fat, and the like, with insoluble substances accidentally in- troduced with the food. Mucus, epithelium, altered coloring matter of bile, fatty acids, etc. Varying quantities of other constituents of bile, and derivatives from them. 267.00 1000 Defalcation. — The act of the expulsion of faeces is in part due to an increased reflex peristaltic action of the lower part of the large intes- tine, namely of the sigrnoid flexure and rectum, and in part to the more or less voluntary action of the abdominal muscles. In the case of active voluntary eiforts, there is usually, first an inspiration, as in the case of coughing, sneezing, and vomiting ; the glottis is then closed, and the diaphragm fixed. The abdominal muscles are contracted as in expira- tion ; but as the glottis is closed, the whole of their pressure is exercised on the abdominal contents. The sphincter of the rectum being relaxed, the evacuation of its contents takes place accordingly ; the effect being, of course, increased by the peristaltic action of the intestine. As in the other actions just referred to, there is as much tendency to the escape of the contents of the lungs or stomach as of the rectum ; but the pres- sure is relieved only at the orifice, the sphincter of which instinctively or involuntarily yields. Nervous Mechanism. — The anal sphincter muscle is normally in a 296 HANDBOOK OF PHYSIOLOGY. state of tonic contraction. The nervous centre which governs this con- traction is probably situated in the lumbar region of the spinal cord, in- asmuch as in cases of division of the cord above this region the sphincter regains, after a time, to some extent the tonicity which is lost imme- diately after the operation. By an effort of the will, acting through the centre, the contraction may be relaxed or increased. In ordinary cases the apparatus is set in action by the gradual accumulation of fasces in the sigmoid flexure and rectum, pressing by the peristaltic action of these parts of the large intestine against the sphincter, and causing by reflex action its relaxation ; this sensory impulse acting through the brain and reflexly through the spinal centre. The Gases contained in the Stomach and Intestines.— Under ordinary circumstances, 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 after- wards occupied by the bowels, and by the large degree, therefore, in which the gas, which naturally distends them, contributes to fill the cavity of the abdomen. Indeed, 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 accident, but intended to serve a definite and important purpose, although, proba- bly, a mechanical one. Sources. — The sources of the gas contained in the stomach and bowels may be thus enumerated i — 1. Air introduced in the act of swallowing either food or saliva; 2. Gases developed by the decomposition of alimentary matter, or of the se- cretions 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 these gastric and intestinal blood-vessels ; but the conditions of the exchange are not known, and it is very doubtful whether anything like a true and definite secretion of gas from the blood into the intestines or stomach ever takes place. There can be no doubt, however, that the in- testines may be the proper excretory organs for many odorous and other substances, either absorbed from the air taken into the lungs in inspira- tion, 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 as- suming 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 intes- tines are derived chiefly from the second of the sources which have been enumerated. It is now very generally admitted that the decompositions of food in the alimentary canal are partially the result of the growth of various kinds of micro-organisms, some of which have been already mentioned, and that these decompositions are independent of as well as distinct from DIGESTION. 297 the action of the digestive fluids. It is to these special fermentative changes that the gases in the intestines are chiefly due. Composition of Gases contained in the Alimentary Canal. (TABULATED FROM VARIOUS AUTHORITIES BY BRINTON.) Whence obtained. Composition by Volume. Oxygen. Nitrog. Carbon. Acid. Hydrog. Carburet. Hydrogen. Sulphuret. Hydrogen. Stomach 11 71 32 66 35 46 22 14 30 12 57 43 41 4 38 8 6 19 13 8 11 19 V trace. * Small Intestines Caecum Colon. ... Expelled per anutn . CHAPTER VIII. ABSORPTION. THE process of Absorption has, for one of its objects, the introduc- tion 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 gradual removal of parts of the body itself, when they need to be renewed. In absorption from without and absorp- tion from within, the process manifests some variety, and a very wide range of action; and in both two sets of vessels are, or may be, con- cerned, namely, the Blood-vessels, and the Lymph-vessels or Lymphatics to which the term Absorbents has been specially applied. Lymphatic Vessels. Distribution.— The principal vessels of the lymphatic system are, in structure and general appearance, like very small and thin-walled veins. They are provided with valves."* They commence in fine microscopic lymph-capillaries, in the organs and tissues of the body, and they end directly or indirectly in two trunks which open into the large veins near the heart (Fig. 314). —The lymph and chyle which they contain, unlike the blood, pass only in one direction, 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. Kemem- 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 readily be seen from Fig. 214 that the greater part of the contents of the lymphatic system of vessels passes through a compara- tively large trunk called the thoracic duct, which finally empties its contents into the blood-stream, at the junction of the internal jugular and subclavian veins of the left side. There is a smaller duct on the right side. The lymphatic vessels of the intestinal canal are called lac- teals, 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 is no essential distinction, however, between lacteals and lymphatics. In some parts of their course all lymphatic vessels pass through certain bodies called lymphatic glands. ABSORPTION. 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 so-called non-vascular parts, as the nails, cuticle, hair, and the like. Origin of Lymph Capillaries. — The lymphatic capillaries commence most commonly either (a) in closely-meshed networks, or (#) in irregu- lar lacunar spaces between the various structures of which the different Lymphatics of head and I Vfltgil ! Lymphatics of head and neck, right. KraMM WfwKwl neck' left- Right internal jugular vein, j ^^SHIi^M^^^g 8 Thoracic duct- Right subclavian vein. 1 Left subclavian vein Lymphatics of right arm. Thoracic duct. Receptaculum chyli. ^^••^^^•B&jftflBggnggLiiuaiHi Lacteals. • BUS of Iower ex- 'WMMKBHPIKw'*K»*BMi^»**«54iIia»«B^^^^^B FIG. 214.— Diagram of the principal groups of Lymphatic vessels (from Quain). organs are composed. Such irregular spaces, forming what is now termed the lymph-canalicular system, have been shown to exist in many tissues. In serous membranes, such as the omentum and mesentery, they occur as a connected system of very irregular branched spaces partly occupied by connective-tissue corpuscles, and both in these and in many other tissues are found to communicate freely with regular lymphatic vessels. In many cases, though 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, they are lined by a distinct layer of endothelium. 300 HANDBOOK OF PHYSIOLOGY. The lacteals offer an illustration of another mode of origin, namely, (c) in blind dilated extremities; but there is no essential difference in structure between these and the lymphatic capillaries of other parts. /Structure of Lymph Capillaries. — The structure of lymphatic capil- laries is very similar to that of blood-capillaries: their walls consist of a single layer of endothelial cells of an elongated form and sinuous out- line, which cohere along their edges to form a delicate membrane. They differ from blood-capillaries mainly in their larger and very vari- able calibre, and in their numerous communications with the spaces of the lymph-canalicular system. Communications of the Lymphatics. — The fluid part of the blood constantly exudes from or is strained through the walls of the blood- FIG, 215.— Lymphatics of central tendon of rabbit's diaphragm, stained with silver nitrate. The ground substance has been shaded diagrammatically to bring out the lymphatics clearly. Z. Lymphatics lined by long narrow endothelial cells, and showing v. valves at frequent intervals. (Schofield.) capillaries, so as to moisten all the surrounding tissues, and occupies the interspaces which exist among their different elements, which form the beginnings of the lymph-capillaries; and the latter, therefore, are the means of collecting the exuded blood plasma, and returning that part which is not directly absorbed by the tissues into the blood -stream. It is not necessary to assume the presence of any special channels between the blood and lymphatic vessels, inasmuch as even blood-corpuscles can pass bodily, without much difficulty, through the walls of the blood- capillaries and small veins, and could pass with still less trouble, prob- ably, through the comparatively ill-defined walls of the capillaries which contain lymph. It has been already mentioned that in certain parts of the body, open- ABSORPTION. 301 ings or STOMATA exist, by which lymphatic capillaries directly communi- cate with parts hitherto supposed to be closed cavities. When absorption into the lymphatic system takes place in membranes covered by epithelium or endothelium through the interstitial or inter- cellular cement-substance, it is said to take place through pseudostomata, already alluded to. FIG. 216.— Lymphatic vessels of the head and neck and the upper part of the trunk (Mascagni). 1/6. — The 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 prin- cipal lymphatic vessels and glands are shown on the side of the head and race, and in the neck axilla, 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 the duct. The termination of the upper lymphatics of the diaphragm in the mediastmal glands, as well as the cardiac and the deep mammary lymphatics, is also shown. Demonstration of Lymphatics of Diaphragm. — The stomuta on the peritoneal surf ace, of the diaphragm are the openings of short vertical canals which leacfup into the lymphatics, and are lined by cells like those of germinating endothelium. By introducing a solution of Berlin blue into the peritoneal cavity of an animal shortly after death, and suspend- ing it, head downwards, an injection of the lymphatic vessels of the diaphragm, through the stomata oh its peritoneal surface, may readily be obtained, if artificial respiration be carried on for about half an hour. In this way it has been found that in the rabbit the lymphatics are ar- ranged between the tendon bundles of the centrum tendineum; and they are hence termed interfascicular. The centrum tendineum is coated by 302 HANDBOOK OF PHYSIOLOGY. endothelium on its pleural and peritoneal surfaces, and its substance consists of tendon bundles arranged in concentric rings towards the pleural side and in radiating bundles towards the peritoneal side. The lymphatics of the anterior half of the diaphragm open into those of the anterior mediastinum, while those of the posterior half pass into a lymphatic vessel in the posterior mediastinum, which soon enters the thoracic duct. Both these sets of vessels, and the glands into which they pass, are readily injected by the method above described; and there Fio. 217. FIG. 218. FIG. 217.— Superficial lymphatics of the forearm and palm of the hand, 1/5.— 5. Two small glands at the bend of the arm. 6. Radial lymphatic vessels. 7. Ulnar lymphatic vessels. 8, 8. Palmar arch of lymphatics. 9, 9'. Outer and inner sets of vessels. 6. Cephalic vein. d. Radial vein. e. Median vein. /, Ulnar vein. The lymphatics are represented as lying on the deep fascia. (Mascagni.; FIG. 218.— Superficial lymphatics of right groin and upper part of thigh, 1/6. 1. Upper inguinal ands. 2, 2'. Lower inguinal or femoral glands. 3, 3'. Plexus of lymphatics in the course of the glan long saphenous vein. (Mascagni.) can be little doubt that during life the flow of lymph along these chan- nels is chiefly caused by the action of the diaphragm during respiration. As it descends in inspiration, the spaces between the radiating tendon ABSOEPTfON. 303 bundles dilate, and lymph is sucked from the peritoneal cavity, through the widely open stomata, into the interfascicular lymphatics. During expiration, the spaces between the concentric tendon bundles dilate, and the lymph is squeezed into the lymphatics towards the pleura! surface (Klein). It thus appears probable that during health there is a con- tinued sucking in of lymph from the peritoneum into the lymphatics by the " pumping " action of the diaphragm; and there is doubtless an equally continuous exudation of fluid from the general serous surface of the peritoneum. When this balance of transudation and absorption is disturbed either by increased transudation or some impediment to absorp- tion, an accumulation of fluid necessarily take place (ascites). Stomata have been found in the pleura; and as they maybe presumed to exist in other serous membranes, it would seem as if the serous cavi- ties, hitherto supposed closed, form but a large lymph-sinus or widening out, so to speak, of the lymph-capillary system with which they directly communicate. FIG. 219.— Peritoneal surface of septum cisternae lymphatic® magnae of frog. The storaata, some of which are open, some collapsed, are surrounded by germinating endothelium. x 160. (Klein.) Structure of Lymphatic Vessels. — The larger vessels are very like veins, having an external coat of fibre-cellular tissue, with elastic fila- ments; within this, a thin layer of fibro-cellular tissue, with plain 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 epithe- lium; 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 at- tached, give them a peculiar beaded or knotted appearance. Current of the Lymph. — With the help of the valvular mechanism (1) all occasional pressure on the exterior of the lymphatic and lacteal vessels propels the lymph towards the heart: thus muscular and other 304 HANDBOOK OF PHYSIOLOGY. external pressure accelerates the flow of the lymph as it does that of the blood in the veins. The actions of (2) the muscular fibres of the small intestine, and probably the layer of unstriped muscle present in each in- testinal villus, seem to assist in propelling the chyle: for, in the small intestine of a mouse, the chyle has been seen moving with intermittent propulsions that appeared to correspond with the peristaltic movements of the intestine. But for the general propulsion of the lymph and chyle, it is probable that, together with (3) the visa/ tcrgo resulting from absorption (as in the ascent of sap in a tree), and from external pressure, some of the force may be derived (4) from the contractility of the ves- seFs own walls. The respiratory movements, also, (5) favor the current of lymph through the thoracic duct as they do the current of blood in the thoracic veins. 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. 220), and it has been shown that the FIG. 220.— Lymphatic heart (9 lines long, 4 lines broad) of a large species of serpent, the Python bivittatus. 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), and 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 rest of the organ (after E. Weber). caudal heart of the eel is a lymph-heart also. The number and position of these organs vary. In frogs and toads there are usually four, two an- terior and two posterior; in the frog, the posterior lymph-heart on each side is situated in the ischiatic region, just beneath 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 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 lymphatic 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 surface, and which becomes turgid each time that the sac contracts. Blood is prevented from passing from the vein into the lymphatic 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 microscope; but in every case it is composed of striped fibres. The contractions of the hearts are ABSORPTION. 305 rhythmical, occurring 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 lymphatic hearts continued pulsating after all the rest of the spinal cord and the brain were destroyed; while destruction of this portion, even though all other parts of the nervous centres were un- injured, instantly arrested the heart's movements. The posterior, or ischiatic, pair of lymph-hearts were found to be governed, in like man- ner, by the portion of spinal cord corresponding 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. Lymphatic Glands. Lymphatic glands are small round or oval compact bodies varying in size from a hempseed to a bean, interposed in the course of lymphatic FIG. 221. FIG. 222. FIG. 221.— Section of a mesenteric gland from the ox, slightly magnified, o, Hilus; 6 (in the cen- tral part of the figure), medullary substance; c, cortical substance with indistinct alveoli; d, cap- sule (Kolliker.) FIG. 222.— Section of medullary substance of an inguinal gland of an ox; a, a, glandular sub- stance or pulp forming rounded cords joining in a continuous net (dark in the figure) ; c, c, trabec- ulae; the space, 6, 6, between these and the glandular substance is the lymph sinus, washed clear of corpuscles and traversed by filaments of retif orm connective tissue. X 90. (Kolliker.) vessels, and through which the chief part of the lymph passes in its course to be discharged into the blood-vessels. They are found in great numbers in the mesentery, and along the great vessels of the abdomen, thorax, and neck; in the axilla and groin; a few in the popliteal space, but not further down the leg, and in the arm as far as the elbow. Some ' 306 HANDBOOK OF PHYSIOLOGY. lymphatics do not, however, pass through glands before entering the thoracic duct. Structure. — A lymphatic gland is covered externally by a capsule of connective tissue, generally containing some unstriped muscle. At the inner side of the gland, which is somewhat concave (hilus), (Fig. 221 «), the capsule sends inwards processes called trabeculcB in which the blood- vessels are contained, and these join with other processes prolonged from the inner surface of the part of the capsule covering the convex or outer part of the gland; they have a structure similar to that of the capsule, and entering the gland from all sides, and freely communicating, form a fibrous supporting stroma. The interior of the gland is seen on sec- tion, even when examined with the naked eye, to be made up of two FIG. 223.— Diagrammatic section of lymphatic gland, a. Z., afferent; el., efferent lymphatics; C, cortical substance; l.h., reticulating cords of medullnry substance; l.s., lymph-sinus; c., fibrous coat sending in trabeculae, t.r., into the substance of the gland. (Sharpey.) parts, an outer or cortical (Fig. 221, c, c), which is light colored, and an inner of redder appearance, the medullary portion (Fig. 221). In the outer or cortical part of the gland (Fig. 223) the intervals between the trabeculae are comparatively large, and form more or less triangular in- tercommunicating spaces termed alveoli ; whilst in the more central or medullary part is a finer meshwork formed by the more free anastomosis of the trabecular processes. Within the alveoli of the cortex and in the meshwork formed by the trabeculas in the medulla, is contained the proper gland structure. In the former it is arranged as follows: occu- pying the central and chief part of each alveolus, is a more or less wedge-shaped mass of adenoid tissue, densely packed with lymph cor- puscles ; but at the periphery surrounding the central portion and im- ABSORPTION. 307 mediately next the capsule and trabeculae, is a more open mesh work of adenoid tissue constituting the lymph sinus or channel, and containing fewer lymph corpuscles. The central mass is inclosed in endothelium. the cells of which join by their processes, the processes of the adenoid framework of the lymph sinus. The trabeculas are also covered with endothelium. The lining of the central mass does not prevent the pas- sage of fluids and even of corpuscles into the lymph sinus. The frame- work of the adenoid tissue of the lymph sinus is nucleated, that of the central mass is non-nucleated. At the inner part of the alveolus, the wedge-shaped central mass divides into two or more smaller rounded or cord-like masses which joining with those from the other alveoli, form a much closer arrangement of the gland tissue than in the cortex; spaces FIG. 224.— A small portion of medullary substance from a mesenteric gland of the ox. d, d, trabeculae; a, part of a cord of glandular substances from which all but a few of the lymph-corpus- cles have been washed out to show its supporting meshwork of retif orm tissue and its capillary blood-vessels c which have been injected, and are dark in the figure) ; 6, 6, lymph-sinus, of which the retiform tissue is represented only at c, c. X 300. (Kolliker.) (Fig. 223 #), are left within those anastomosing cords, in which are found portions of the trabecular meshwork and the continuation of the lymph sinus. The essential structure of lymphatic-gland substance resembles that which was described as existing, in a simple form, in the interior of the solitary and agminated intestinal follicles. The lymph enters the gland by several afferent vessels, which open beneath the capsule into the lymph-channel or lymph-path; at the same time they lay aside all their coats except the endothelial lining, which is continuous with the lining of the lymph-path. The efferent vessels 308 HANDBOOK OF PHYSIOLOGY. 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 endothelium of one is continuous with that of the other. The efferent vessels leave the gland at the hilus, the more or less concave inner side of the gland, and generally either at once or very soon after join together to form a single vessel. Blood-vessels which enter and leave the gland at the hilus are freely distributed to the trabecular tissue and to the gland-pulp. The Lymph and Chyle. Lymph is, under ordinary circumstances, a clear, transparent, and yellowish fluid. It is devoid of smell, is slightly alkaline, and has a saline taste. As seen with the microscope in the small transparent ves- sels of the tail of the tadpole, it usually contains no corpuscles or par- ticles of any kind; and it is only in the larger trunks that any corpuscles are to be found. These corpuscles are similar to colorless blood-cor- puscles. The fluid in which the corpuscles float is albuminous, and con- tains no fatty particles; but is liable to variations according to the general state of the blood, and to that of the organ from which the lymph is derived. As it advances towards the thoracic duct, after pass- ing through the lymphatic glands, it becomes spontaneously coagulable and the number of corpuscles is much increased. Chyle, found in the lacteals after a meal, is an opaque, whitish, milky fluid, neutral or slightly alkaline in reaction. Its whiteness and opacity are due to the presence of innumerable particles of oily or fatty matter, of exceedingly minute though nearly uniform size, measuring on the average about -3-0^015- °^ an incn- These constitute what is termed the molecular base of chyle. Their number, and consequently the opacity of the chyle, are dependent upon the quantity of fatter matter contained in the food. The fatty nature of the molecules is made mani- fest by their solubility in ether. Each molecule probably consists of a droplet of oil coated over with albumen, in the manner in which minute drops of oil always become covered in an albuminous solution. This is proved when water or dilute acetic acid is added to chyle, many of the molecules are lost sight of, and oil-drops appear in their place, as the investments of the molecules have been dissolved, and their oily contents have run together. Except these molecules, the chyle taken from the villi or from lac- teals near them, contains no other solid or organized bodies. The fluid in which the molecules float is albuminous and does not spontaneously coagulate. 'But as the chyle passes on towards the thoracic duct, and especially whilst traversing one or more of the mesenteric glands, it is elaborated. The quantity of molecules and oily particles gradually di- minishes; cells, to which the name of chyle-corpuscles is given, appear in ABSORPTION. 309 it; and it acquires the property of coagulating spontaneously. The higher in the thoracic duct the chyle advances, the greater is the num- ber of chyle-corpuscles, and the larger 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 coagulating, but coagulates rapidly on being removed from them. The existence of the materials which, by their union form fibrin, is there- fore, certain; and their increase appears to be commensurate with that of the corpuscles. The structure of the chyle-corpuscles was described when speaking of the white corpuscles of the blood, with which they are identical. The lymph, in chemical composition, resembles diluted plasma, and from what has been said, it will appear that perfect chyle and lymph are, in essential characters, nearly similar, and scarcely differ, except in the preponderance of fatty and proteid matter in the chyle. Chemical Composition of Lymph and Chyle (Owen Kees). I. II. III. Lymph Chyle Mixed Lymph & (Donkey.) (Donkey). Chyle (Human). Water, 9fi.536 690.23? 90.48 Solids, 3.454 9.763 9.52 Solids— Proteids, including Serum- Al bu- ) 1 32Q 3>8g6 7>og mm, Fibrmogen, and Globulin. [ Extractives, including in (i and n) [ -, ^Q -, KO.K in« TT T- O /-N1 1 ' T J-.OOy J-.OOt) .J.VO Sugar, Urea, Leucm & Cholesterm, j Fatty matter, .... atrace 3.601 .92 Salts, 58'5 .711 .44 Quantity. — The quantity which would pass into a cat's blood in twenty-four hours has been estimated to 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 to 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. By another series of experiments, the quantity of lymph traversing the tho- racic duct of a dog in twenty-four hours was found to be about equal to two-thirds of the blood in the body. THE PROCESS OF ABSOEPTIOK. (a.) By the Lacteals. — During the passage of the chyme along the intestinal canal, its completely digested parts are absorbed by the blood- 310 HANDBOOK OF PHYSIOLOGY: vessels and lacteals distributed in the mucous membrane. The lacteals appear to absorb only certain constituents of the digested food, includ- ing 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 blood- vessels and the lacteals are brought almost into contact with the intes- tinal contents. There seems to be no doubt that absorption of fatty matters during digestion, from the contents of the intestines, is effected chiefly between the epithelial cells which line the intestinal tract (Wat- ney), and especially those which clothe the surface of the villi. Thence, the fatty particles are passed on into the interior of the lacteal vessels, but how they pass, and what laws govern their passage, are not at pres- ent exactly known. The process of absorption is assisted by the pressure exercised on the contents of the intestines by their contractile walls ; and the absorption of fatty particles is also facilitated by the presence of the bile, and the pancreatic and intestinal secretions, which moisten the absorbing sur- face. For it has been found by experiment, that the passage of oil through an animal membrane is made much easier when the latter is im- pregnated with an alkaline fluid. (b.) By the Lymphatics. — The real source of the lymph, and the mode in which its absorption is effected by the lymphatic vessels, were long matters of discussion. But the problem has been much simplified by more accurate knowledge of the anatomical relations of the lymph- atic capillaries. The lymph is, as has been pointed out, diluted liquor sanguinis, which is always exuding from the blood-capillaries into the interstices of the tissues in which they lie ; and as these interstices form in most parts of the body tho beginnings of the lymphatics, the source of the lymph is sufficiently obvious. In connection with this may be mentioned the fact that changes in the character of the lymph corre- spond very closely with changes in the character of either the whole mass of blood, or of that in the vessels of the part from which the lymph is exuded. Thus it appears that the coagulability of the lymph, although always less than, is directly proportionate to that of the blood ; and that when fluids are injected into the blood-vessels in sufficient quantity to distend them, the injected substance may be almost directly after- wards found in the lymphatics. Some other matters than those originally contained in the e-xuded liquor sanguinis may, however, 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 lymph- ABSORPTION. 311 atics, and elaborated with the other contents of the lymph in passing through the glands. (c.) By Blood- Vessels. — In the absorption by the lymphatic or lacteal vessels just described, there appears something like the exercise of choice in the materials admitted into them. But the absorption by blood-vessels presents no such appearance of selection of materials ; rather, it appears, that every substance, whether gaseous, liquid, or a soluble, or minutely divided solid, may be absorbed by the blood-vessels, provided it is capable of permeating their walls, and of mixing with the blood ; and that of all such substances, the mode and measure of ab- sorption are determined almost solely by their physical or chemical properties and conditions, and by those of the blood and the walls of the blood-vessels. Method of Absorption. («.) Osmosis. — The phenomena of absorption of all the materials of the food except the fats are, to a great extent, comparable to that passage of fluids through membrane, which occurs quite independently of vital conditions, and the earliest and best scientific investigation of which was made by Dutrochet. The instrument 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 membrane is tied (Fig. 226). If now the bell be filled with a solution of a salt — say sodium chlo- ride, 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, however, 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 applied. The nature of the membrane used as a septum, and its affinity for the fluids subjected to experiment have an im- portant influence, as might be anticipated, on the rapidity FIO. 225.-En- 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 membrane ; while, on the other hand, in the case of a membrane of caoutchouc, the alcohol, from its greater affinity for this substance, would pass freely into the water. Absorption by blood-vessels is the consequence of their walls being, like the membranous septum of the endosmometer, porous and capable 312 HANDBOOK OF PHYSIOLOGY. of imbibing fluids, and of the blood being so composed that most fluids will mingle with it. Thus the relation of the chyme in the stomach and intestines to the blood circulating in the vessels of the gastric and i intestinal mucous membrane is evidently just that which is required for : osmosis. The chyme contains substances which have been so acted upon by the digestive juices as to have become quite able to pass through an animal membrane, or to dialyze as it is called. The thin animal mem- brane is the coat of the blood-vessels with the intervening mucous mem- brane. The nature of the fluid within the vessels, the very feeble power of dialyzation which the albuminous blood possesses, determines the di- rection of the osmotic current, viz., into and not out of the blood-ves- sels. The current is of course aided by the fact of the constant change in the blood presented to the osmotic surface, as it rapidly circulates within the vessels. As a rule the current is from the stomach or intes- tine into the blood, but the reversed action may occur, when, for ex- ample, a certain salt, e. g., sulphate of magnesia, is taken into the stomach, in which case there is a rapid discharge of water from the blood-vessels into the alimentary canal resulting in purgation. The presence of various substances in the food has the power of diminishing the rate of absorption, their influence is probably exerted upon the mem- brane, diminishing its power of permitting osmosis. Whereas the pres- ence of a little hydrochloric acid in the contents of the stomach appears to determine the direction of the osmosis, or at any rate to diminish or prevent exosmosis. The conditions for osmosis exist not only in the alimentary mucous membrane, but also in the serous cavities and the tissues elsewhere. 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 blood-vessels and mingled with the blood. The water round the piece of tissue also will become blood-stained; and if all be kept at perfect rest, the stain derived from the solution of the coloring matter of the blood, together with 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 instead 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. Various substances have been classified according to the degree in which they possess the property of passing, when in a state of solution in water, through membrane; those which pass freely, inasmuch as they are usually capable of crystallization, being termed crystalloids, and ABSORPTION. 313 those which pass with difficulty, on account of their, physically, glue- like character, 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 characterized by their inertness as acids or bases, and feebleness in all ordinary chemical relations. Examples of them are found in albumin, gelatin, starch, hydrated alumina, hydrated silicic acid, etc. ; while the crystalloids are characterized by qualities the re- verse of those just mentioned as belonging to colloids. Alcohol, sugar, and ordinary saline substances are examples of crystalloids. (b.) Filtration, or transudation. A distinction must be drawn be- tween osmosis and filtration. The latter means the passage of fluids through the pores of a membrane under pressure. The greater the pressure the greater the amount which passes through the membrane. Collojds will filter, although less easily than crystalloids. The nature of the substance to be filtered and the nature of the membrane which acts as the filter materially affect the activity of the process. No doubt both osmosis and filtration go on together in the process of absorption. An excellent example of filtration or transudation occurs in the pathological condition known as dropsy, in which the connective tissues become in- filtrated with serous fluid. The fluid passes out of the vein when the intra-venous pressure passes a certain point, the fluid is, as it were, squeezed through the walls of the vessels by this excess of pressure. Rapidity of Absorption. — The rapidity with which matters may be absorbed from the stomach, probably by the blood-vessels chiefly, and diffused through the textures of the body, has been found by experiment. It appears that lithium chloride may be 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, in a quar- ter of an hour after being given on an empty stomach. Into the outer part of the crystalline lens it may pass after a time, varying from half an hour to an hour and a half. Lithium carbonate, 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 six7 seven, or eight days. Some experiments on the absorption of various mineral and vegetable poisons have brought to light the singular fact that, in some cases, ab- sorption takes place more rapidly from the rectum than from the stomach. Strychnia, for example/ when in solution, produces its poi- sonous effects much more speedily when introduced into the rectum than into the stomach. When introduced in the solid form, however, it is. absorbed more rapidly from the stomach than from the rectum, doubt- 314 HANDBOOK OF PHYSIOLOGY. less because of the greater solvent property of the secretion of the former than 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 blood- vessels. Thus, absorption will hardly take place through the epidermis, but is quick when the epidermis 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 o-f their epithelia^ so that the urinary bladder of a frog is traversed in less than a second; and the absorption of poisons by the stomach or lungs appears sometimes accomplished in an immeasurably small time. \ Conditions for Absorption. — 1. The substance to be absorbed must, as a general rule, be in the liquid or gaseous state, or, if a solid, must be soluble in the fluids ivith which it is brought into contact. Hence the marks of tattooing, and the discoloration produced by silver nitrate taken internally, remain. Mercury may be absorbed even in the metallic state; and in that state may pass into and remain in the blood-vessels, or be deposited from them; and such substances as exceedingly finely- divided charcoal, when taken into the alimentary canal, have been found in the mesenteric veins; the insoluble materials of ointments may also be rubbed into the blood-vessels; but there are no facts to determine how these various substances effect their passage. Oil, minutely divided, in an emulsion, will pass slowly into blood-vessels, as it will through a\ filter moistened with water ; and, without doubt, fatty matters find their way into the blood-vessels as well as the lymph-vessels of the in- testinal canal, although the latter seem to be specially intended for their .absorption. 2. The less dense the fluid to be absorbed, the more speedy, as a gen- eral rule, is its absorption by the living blood-vessels. Hence the rapid absorption of water from the stomach ; also of weak saline solutions ; but with strong solutions, there appears less absorption into, than effu- sion from, the blood-vessels. 3. The absorption is the less rapid the fuller and tenser the blood- vessels are ; and the tension may be so great as to hinder altogether the entrance of more fluid. Thus, if water is injected into a dog's veins to repletion, poison is absorbed very slowly ; but when the tension of the vessels is diminished by bleeding, the poison acts quickly. So, when cupping-glasses are placed over a poisoned wound, they retard the ab- sorption of the poison not only by diminishing the velocity of the circu- lation in the part, but by filling all its vessels too full to admit more. 4. On the same ground, absorption is the quicker the more rapid the ABSORPTION. 315 circulation of the Hood; 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. CHAPTEE IX. ANIMAL HEAT. THE Average Temperature of the human body in those internal parts which are most easily accessible, as the mouth and rectum, is from 98.5° to 99.5° F. (36.9°-37.4° C.). In different parts of the external surface of the human body the temperature varies only to the extent of two or three degrees (F.), when all are alike protected from cooling influences; and the difference which under these circumstances exists, depends chiefly upon the different degrees of blood-supply. In the armpit — the most convenient situation, under ordinary circumstances, for examina- tion by the thermometer — the average temperature is 98.6° F, (36.9° 0.). In different internal parts, the variation is one or two degrees ; those parts and organs being warmest which contain most blood, and in which there occurs the greatest amount of chemical change, e.g., the glands and the muscles ; and the temperature is highest, of course, when they are most actively working : while those tissues which, subserving only a mechanical function, are the seat of least active circulation and chemical change, are the coolest. These differences of temperature, however, are actually but slight, on account of the provisions which exist for main- taining uniformity of temperature in different parts. Circumstances causing Variations in Temperature. — The chief circumstances by which the temperature of the healthy body is influ- enced are the following: — Age; Sex; Period of the day; Exercise; Climate and Season ; Food and Drink. Age. — The average temperature of the new-born child is only about 1° F. (.54° 0.) above that of the adult ; and the difference becomes still more trifling during infancy and early childhood. The temperature falls to the extent of about .2°-. 5° F. from early infancy to puberty, and by about the same amount from puberty to fifty or sixty years of age. In old age the temperature again rises, and approaches that of infancy ; but although this is the case, yet the power of resisting cold is less in them — exposure to a low temperature causing a greater reduction of heat than in young persons. Sex. — The average temperature of the female is very slightly higher than that of the male. Period of the Day. — The temperature undergoes a gradual alteration, ANIMAL HEAT. 31 1 to the extent of about 1° to 1.5° F. (.54-.80 C.) in the course of the day and night ; the minimum being at night or in the early morning, the maximum late in the afternoon. Exercise. — Active exercise raises the temperature of the body from 1° to 2° F. (.54-1.08° 0.). This maybe partly ascribed to generally in- creased combustion processes, and partly to the fact that every muscular contraction is attended by the development of one or two degrees of heat in the acting muscle; and that the heat is increased according to the number and rapidity of these contractions, and is quickly diffused by the blood circulating from the heated muscles. Possibly, also, some small amount of heat may be generated in the various movements, stretchings* and recoilings of the other tissues, as the arteries, whose elastic walls, alternately dilated and contracted, may give out some heat, just as caout- chouc alternately stretched and recoiling becomes hot. Climate and Season. — The temperature of the human body is the same in temperate and tropical climates (Furnell). In summer the temperature of the body is a little higher than in winter ; the difference amounting to about a third of a degree F. Food and Drink. — The effect of a meal upon the temperature of a body is but small. A very slight rise usually occurs. Cold alcoholic drinks depress the temperature somewhat (.5° to 1° F.). Warm alco- holic drinks, as well as warm tea and coffee, raise the temperature (about .5° F.). In disease the temperature of the body deviates from the normal stand- ard to a greater extent than would be anticipated from the slight effect of external conditions during health. Thus, in some diseases, as pneu- monia and typhus, it occasionally rises as high as 106° or 107° F. (41°- 41.6° 0.) ; and considerably higher temperatures have been noted. In Asiatic cholera, on the other hand, a thermometer placed in the mouth may sometimes rise only to 77° or 79° F. (25°-26.2 C.). The temperature maintained by Mammalia in an active state of life, according to the tables of Tiedemann and Rudolphi, averages 101° (38.3° C.). The extremes recorded by them were 96° and 106°, the former in the narwhal, the latter in a bat (Vespertilio pipistrella). In Birds, the average is as high as 107° (41.2° C.) ; the highest temperature, 111.25° (46.2° C.) ; being in the small species, the linnets, etc. Among Eep- tiles, while the medium they were in was 75° (23.9° C.), their average temperature was 82.5° (31.2° 0.). As a general rule, their temperature, though it falls with that of the surrounding medium, is, in temperate media, two or more degrees higher; and though it rises also with that of the medium, yet at very high degrees it ceases to do so, and remains even lower than that of the medium. Fish and invertebrata present, as a general rule, the same temperature as the medium in which they live, whether that be high or low; only among fish, the tunny tribe, with strong hearts and red meat-like muscles, and more blood than the average fish have, are generally 7° (3.8° C.) warmer than the water around them. 318 HANDBOOK OF PHYSIOLOGY. The difference, therefore, between what are commonly called the warm and the cold-blooded animals is not one of absolutely higher or lower temperature ; for the animals which to us in a temperate climate, feel cold (being like the air or water, colder than the surface of our bodies), would in an external temperature of 100° (37.8° C.) have nearly the same temperature and feel hot to us. The real difference is that what we call warm-blooded animals (Birds and Mammalia), have a certain "permanent heat in all atmospheres/' while the temperature of the others, which we call cold-blooded, is " variable with every atmosphere." (Hunter.) The power of maintaining a uniform temperature, which Mammalia and Birds possess, is combined with the want of power to endure such changes of body temperature as are harmless to the other classes; and when their power of resisting change of temperature ceases, they suffer serious disturbance or die. Sources and Mode of Production of Heat in the Body.— Tho heat which is produced in the body arises from combustion, and is due to the fact that the oxygen of the atmosphere taken into the system is ultimately combined with carbon and hydrogen, and discharged from the body as carbonic acid and water. Any changes, indeed, which occur in the protoplasm of the tissues, resulting in an exhibition of their func- tion, are attended by the evolution of heat and the formation of carbonic acid and water. The more active the changes, the greater is the heat produced and the greater is the amount of the carbonic acid and water formed. But in order that the protoplasm may perform its function, the waste of its own tissue (destructive metabolism) must be repaired by the due supply of food material and therefore for the production of heat food is necessary. In the tissues, therefore, two processes are continu- ally going on: the building up of the protoplasm from the food (con- structive metabolism), which is not accompanied by the evolution of heat but possibly by the reverse, and the oxidation of the protoplastic materials, resulting in the production of energy, by which heat is pro- duced and carbonic acid and water are evolved. Some heat also is generated in the combination of sulphur and phosphorus with oxygen, but the amount thus produced is but small. It is not necessary to assume that the combustion processes, wliich ultimately issue in the production of carbonic acid and water, are as simple as the bare statement of the fact might seem to indicate. But complicated as the various stages may be, the ultimate result is as simple as in ordinary combustion outside of the body, and the products are the same. The same amount of heat will be evolved in the union of any given quantities of carbon and oxygen, and of hydrogen and oxygen, whether the combination be rapid and direct, as in ordinary combustion, or slow and almost imperceptible, as in the changes which occur in the living body. And since the heat thus arising will be distributed wherever ANIMAL HEAT. 319 the blood is carried, every part of the body will be heated equally, or nearly so. This theory, that the maintenance of the temperature of the living body depends on continual chemical change, chiefly by oxidation of com- bustible materials existing in the tissues, has long been established by the demonstration that the quantity of carbon and hydrogen which, in a given time, unites in the body with oxygen, is sufficient to account for the amount of heat generated in the animal within the same period : an amount capable of maintaining the temperature of the body at from 98°-100° F. (36.8°-37.8° C.), notwithstanding a large loss by radiation and evaporation. It should be remembered that some heat may be introduced into the body by means of warm drinks and foods, and, again, that it is possible for the preliminary digestive changes to be accompanied by the evolution of heat. Chief Heat-producing Tissues. — The chemical changes which produce the body-heat appear to be especially active in certain tissues: — (1) In the Muscles, which form so large a part of the organism. The fact that the manifestation of muscular energy is always attended by the evolution of heat and the production of carbonic acid has been demon- strated by actual experiment; and when not actually in a condition of active contraction, a metabolism, not so active but still actual, goes on, which is accompanied by the manifestation of heat. The total amount set free by the muscles, therefore, must be very great; and it has been calculated in a way which will be referred to later on, that even neglect- ing the heat produced by the quiet metabolism of muscular tissue, the amount of heat generated by muscular activity supplies the principal part of the total heat produced within the body. (2) In the Secreting glands, and principally in the liver as being the largest and most active. It has been found by experiment that the blood leaving the glands is considerably warmer than that entering them. The metabolism in the glands is very active and, as we have seen, the more active the metabo- lism the greater the heat produced. (3) In the Brain ; the venous blood having a higher temperature than the arterial. It must be re- membered, however, that although the organs above mentioned are the chief heat-producing parts of the body, all living tissues contribute their quota, and this in direct proportion to their activity. The blood itself is also the seat of metabolism, and, therefore, of the production of heat; but the share which it takes in this respect, apart from the tissues in which it circulates, is very inconsiderable. - EEGULATION' OF THE TEMPERATURE OF THE HUMAN BODY. The average temperature of the body is maintained under different conditions of external circumstances by mechanisms which permit of (1) 320 HANDBOOK OF PHYSIOLOGY. variation in the amount of heat got rid of, and (2) variations in the amoujfc of heat produced or introduced into the body. In healthy warm- blooded animals the loss and gain of heat are so nearly balanced one by the other that, under all ordinary circumstances, an uniform tempera- ture, within two or three degrees, is preserved. I. Methods of Variation in the amount of Heat got rid of. — The loss of heat from the human body is principally regulated by the amount lost by radiation and conduction from its surface, and by means of the constant evaporation of water from the same part, and (2) to a much less degree from the air-passages; in each act of respiration, heat is lost to a greater or less extent according to the temperature of the at- mosphere; unless indeed the temperature of the surrounding air exceed that of the blood. We must remember too that all food and drink which enter the body at a lower temperature than itself abstract a small measure of heat ; while the urine and faeces which leave the body at about its own temperature are also means by which a small amount is lost. (a.) Loss of Heat from the Surf ace of the Body: the Skin. — By far the most important loss of heat from the body — probably 70 or 80 per cent of the whole amount, is that which takes place by radiation, con- duction, and evaporation from the skin. The means by which the skin is able to act as one of the most important organs for regulating the temperature of the blood are — (1), that it offers a large surface for radi- ation, conduction, and evaporation ; (2), that it contains large amount of blood ; (3), that the quantity of blood contained in it is the greater under those circumstances which demand a loss of heat from the body, and vice versa. For the circumstance which directly determines the quantity of blood in the skin, is that which governs the supply of blood to all the tissue and organs of the body, namely, the power of the vaso- motor nerves to cause a greater or less tension of the muscular element in the walls of the arteries, and, in correspondence with this, a lessen- ing or increase of the calibre of the vessel, accompanied by a less or greater current of blood. A warm or hot atmosphere so acts on the nerve-fibres of the skin, as to lead them to cause in turn a relaxation of the mucular fibre of the blood-vessels; and, as a result, the skin becomes full-blooded, hot, and sweating; and much heat is lost. With a low temperature, on the other hand, the blood-vessels shrink, and in accord- ance with the consequently diminished blood-supply, the skin becomes pale, and cold, and dry; and no doubt a similar effect may be produced through the vaso-motor centre in the medulla and spinal cord. Thus, by means of a self -regulating apparatus, the skin becomes the most im- portant of the means by which the temperature of the body is regulated. In connection with loss of heat by the skin, reference has been made to that which occurs both by radiation and conduction, and by evapora- ANIMAL HEAT. 321 tion ; and the subject of animal heat has been considered almost solely with regard to the ordinary case of man living in a medium colder than his body, and therefore losing heat in all the ways mentioned. The im- portance of the means however, adopted, so to speak, by the skin for reg- ulating the temperature of the body, will depend on the conditions by which it is surrounded; an inverse proportion existing in most cases be- tween the loss by radiation and conduction on the one hand, and by evaporation on the other. Indeed, the small loss of heat by evaporation in cold climates may go far to compensate for the greater loss by radia- tion; as, on the other hand, the great amount of fluid evaporated in hot air may remove nearly as much heat as is commonly lost by both radiation and evaporation in ordinary temperatures; and thus, it is pos- sible that the quantities of heat required for the maintenance of an uni- form proper temperature in various climates and seasons are not so different as they, at first thought, seem. Many examples may be given of the power which the body possesses of resisting the effects of a high temperature, in virtue of evaporation from the skin. Blagden and others supported a temperature varying between 198°-211° F. (92°-100° 0.) in dry air for several minutes, and in a subsequent experiment he remained eight minutes in a temper- ature of 260° K. (126.5° C.) "The workmen of Sir F. Chantrey were accustomed to enter a furnace, in which his moulds were dried, whilst the floor was red-hot, and a thermometer in the air stood at 350° F. (177.8° 0.), and Chabert, the fire-king, was in the habit of entering an oven, the temperature of which was from 400° to 600° F. (205°-315° C.)." (Carpenter.) But such heats are not tolerable when the air is moist as well as hot, so as to prevent evaporation from the body. C. James states, that in the vapor baths of Nero he was almost suffocated in a temperature of 112° F. (44.5° C.), while in the caves of Testaccio, in which the air is dry, he was but little incommoded by a temperature of 176° F. (80° 0.). In the former, evaporation from the skin was impossible; in the latter it was abundant, and the layer of vapor which would rise from all the surface of the body would, by its very slowly conducting power, defend it for a time from the full action of the external heat. (The glandular apparatus, by which secretion of fluid from the skin is effected, will be considered in the Section on the Skin.) The ways by which the skin may be rendered more efficient as a cool- ing*apparatus by exposure, by baths, and by other means which man in- stinctively adopts for lowering his temperature when necessary, are too well known to need more than to be mentioned. Although under any ordinary circumstances, the external applica- tion of cold only temporarily depresses the temperature to a slight ex- tent, it is otherwise in cases of high temperature in fever. In these cases a tepid bath may reduce the temperature several degrees, and the effect so produced lasts in some cases for many hours. 21 322 HANDBOOK OF PHYSIOLOGY. (b.) Loss of Heat from the Lungs. — As a means for lowering the tem- perature, the lungs and air-passages are very inferior to the skin ; al- though, by giving heat to the air we breathe, they stand next to the skin in importance. As a regulating power, the inferiority is still more marked. The air which is expelled from the lungs leaves the body at about the temperature of the blood, and is always saturated with moist- ure. No inverse proportion, therefore, exists, as in the case of the skin, between the loss of heat by radiation and conduction on the one hand, and by evaporation on the other. The colder the air, for example, the greater will be the loss in all ways. Neither is the quantity of blood which is exposed to the cooling influence of the air diminished or in- creased, so far as is known, in accordance with any need in relation to temperature. It is true that by varying the number and depth of the respirations, the quantity of heat given off by the lungs may be made, to some extent, to vary also. But the respiratory passages, while they must be considered important means by which heat is lost, are altogether subordinate, in the power of regulating the temperature, to the skin. (c.) By Clothing. — The influence of external coverings for the body must not be unnoticed. In warm-blooded animals, they are always adapted, among other purposes, to the maintenance of uniform temper- ature ; and man adapts for himself such as are, for the same purpose, fitted to the various climates to which he is exposed. By their means, and by his command over food and fire, he maintains his temperature on all accessible parts of the surface of the earth. II. Methods of Variation in the Amount of Heat Produced. — It may seem to have been assumed, in the foregoing pages, that the only regulating apparatus for temperature required by the human body is one that shall, more or less, produce a cooling effect ; and as if the amount of heat produced were always, therefore, in excess of that which is required. Such an assumption would be incorrect. "We have the power of regulating the production of heat, as well as its loss. (a.) By Regulating the Quantity and Quality of the Food taken.— In food we have a means for elevating our temperature. It is the fuel, indeed, on which animal heat ultimately depends altogether. Thus, when more heat is wanted, we instinctively take more food, and take such kinds of it as are good for combustion ; while every-day experience shows the different power of resisting cold possessed, respectively, by the well-fed and by the starved. In northern regions, again, and in the colder seasons of more southern climes, the quantity of food consumed is (speaking very generally) greater than that consumed by the same men or animals in opposite conditions of climate and season. And the food which appears naturally adapted to the inhabitants of the coldest cli- mates, such as the several fatty and oily substances, abounds in carbon and hydrogen, and is fitted to combine ultimately with the large quanti- ANIMAL HEAT. 323 ties of oxygen which, breathing cold dense air, they absorb from their lungs. (£.) By Exercise. — In exercise, we have an important means of rais- ing the temperature of onr bodies. (c.) By Influence of the Nervous System. — The influence of the nervous system in modifying the production of heat must be very important, as upon nervous influence depends the amount of the metabo- lism of the tissues. The experiments and observations which best illus- trate it are those showing, first, that when the supply of nervous influ- ence to a part is cut off, the temperature of that part after a time falls below its ordinary degree ; and, secondly, that when death is caused by severe injury to, or removal of, the nervous centres, the temperature of the body rapidly falls, even though artificial respiration be performed, the circulation maintained and to all appearance the ordinary chemical changes of the body be completely effected. It has been repeatedly no- ticed, that after division of the nerves of a limb its temperature ulti- mately falls ; and this diminution of heat has been remarked still more plainly in limbs deprived of nervous influence by paralysis. With equal certainty, though less definitely, the influence of the nervous system on the 'production of heat, is shown in the rapid and momentary increase of temperature, sometimes general, at other times quite local, which is observed in states of nervous excitement; in the general increase of warmth of the body, sometimes amounting to perspi- ration, which is excited by passions of the mind; in the sudden rush of heat to the face, which is not a mere sensation; and in the equally rapid diminution of temperature in the depressing passions. But none of these instances suffice to prove that heat is generated by mere nervous action, independent of any chemical change; all are explicable, on the supposition that the nervous system alters, by it power of controlling the calibre of the blood-vessels (p. 147), the quantity of blood supplied to a part; while any influence which the nervous system may have in the production of heat, apart from this influence on the blood-vessels, is an indirect one, and is derived from its power of causing such nutritive change in the tissues as may, by involving the necessity of chemical action, involve the production of heat. The existence of nerve-centres and nerves which regulate animal heat (thermogenic) otherwise than by their influence in trophic (nutritive) or vaso-motor changes, although by many considered probable, is not yet proven. Inhibitory heat-centre. — Whether a centre exists which regulates the production of heat in warm-blooded animals, is still undecided. Ex- periments have shown that exposure to cold at once increases the oxygen taken in, and the carbonic acid given out, indicating an increase in the activity of the metabolism of the tissues, but that in animals poisoned by urari, exposure to cold diminishes both the metabolism and the 324 HANDBOOK OF PHYSIOLOGY. temperature, and warm-blooded animals then react to variations of the external temperature just in the same way as cold-blooded. These ex- periments seem to suggest that there is a centre, to which, under nor- mal circumstances, the impression of cold is conveyed, and from which by efferent nerves impulses pass to the muscles, whereby an increased metabolism is induced, and so an increased amount of heat is generated. The centre is probably situated above the medulla. Thus in urarized animals, as the nerves to the muscles, the metabolism of which is so im- portant in the production of heat, are paralyzed, efferent impulses from the centre cannot induce the necessary metabolism for the production of heat, even though afferent impulses from the skin, stimulated by the alteration of temperature, have conveyed to it the necessity of altering the amount of heat to be produced. The same effect is produced when the medulla is cut. Influence of Extreme Heat and Cold. — In connection with the regulation of animal temperature, and its maintenance in health at the normal height, may be noted the result of circumstances too powerful, either in raising or lowering the heat of the body, to be controlled by the proper regulating apparatus. Walther found that rabbits and dogs kept exposed to a hot sun, reached a temperature of 114. 8° F., and then died. Cases of sunstroke furnish us with several examples in the case of man; for it would seem that here death ensues chiefly or solely from elevation of the temperature. In many febrile diseases the immediate cause of death appears to be the elevation of the temperature to a point incon- sistent with the continuance of life. The effect of mere loss of bodily temperature in man is less well known than the effect of heat. From experiments by Walther, it ap- pears that rabbits can be cooled down to 48° F. (8.9° C.), before they die, if artificial respiration be kept up. Cooled down to 64° F. (17.8° C.), they cannot recover unless external warmth be applied together with the employment of artificial respiration. Eabbits not cooled below 77° F. (25° C.) recover by external warmth alone. CHAPTER X. SECRETION. Secretion is the process by which materials are separated from the blood by the cells of secreting glands and membranes, and are either elaborated for the purpose of serving some ulterior office in the economy, or are discharged from the body as useless or injurious. In the former case, the separated materials are termed secretions; in the latter, they are termed excretions. Most of the secretions consist of substances which, probably, do not pre-exist in the same form in the blood, but require special cells and a process of elaboration for their formation,, e. g., the liver cells for the formation of bile, the mammary gland-cells for the formation of milk. The excretions, on the other hand, commonly or chiefly consist of sub- stances which 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 accumulation 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 organ by which each of them is elaborated, the secretion is no longer formed. Cases sometimes occur in which the secretion continues to be formed by the natural or- gan, but not being able to escape towards the exterior, on account of some obstruction, is re-absorbed 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, however, the only particulars in which secretions and excretions can be distinguished; for, in general, the structure of the parts engaged in eliminating excre- tions is as complex as that of the parts concerned in the formation of secretions. And since the differences of the two processes of separation, 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 almost textureless membrane, named the primary or base- 326 HANDBOOK OF PHYSIOLOGY. merit-membrane; certain cells; and blood-vessels. 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. ORGANS AND TISSUES OF SECRETION. The principal secreting membranes are (1) the Serons and Synovial membranes; (2) the Mucous membranes; (3) the Mammary gland; (4) the Lachrymal gland; and (5) the Skin. (1) Serous Membranes. The serous membranes are especially distinguished by the characters of the endothelium covering their free surface: it always consists of a single layer of polygonal cells. The ground substance of most serous membranes consists of connective-tissue corpuscles of various forms lying in the branching spaces which constitute the " lymph canalicular sys- tem " (p. 299), and interwoven with bundles of white fibrous tissue, and numerous delicate elastic fibrillae, together with blood-vessels, nerves, and lymphatics. In relation to the process of secretion, the layer of connec- tive tissue serves as a ground-work for the ramification of blood-vessels, lymphatics, and nerves. But in its usual form it is absent in some in- stances, as in the arachnoid covering the dura mater, and in the interior of the ventricles of the brain. The primary membrane and epithelium are always present, and are concerned in the formation of the fluid by which the free surface of the membrane is moistened. Serous membranes are of two principal kinds: 1st. Those which line visceral cavities — the arachnoid, pericardium, pleurae, peritoneum, and tunicce vaginales. 2d. The synovlal membranes lining the joints, and the sheaths of tendons and ligaments, with which, also, are usually in- cluded the synovial bursce, or bursce mucosce, whether these be subcuta- neous, or situated beneath tendons and 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 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. Functions.— The principal purpose of the serous and synovial mem- branes is to furnish a smooth, moist surface, to facilitate the movements of the invested organ, and to prevent the injurious effects of friction. SECRETION. 327 This purpose is especially manifested in joints, in which free and exten- sive movements take place; and in the stomach and intestines, which, from the varying quantity and movements of their contents, are in al- most constant motion upon one another and the walls of the abdomen. Fluid.— 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. After death, a larger quantity of fluid is usually found in each serous sac; but this, if not the product of mani- fest 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 consti- tutes dropsy of the sac. The fluid naturally secreted by the serous membranes appears to be FIG. 226.— Section of synovial membrane, a, endothelial covering of the elevations of the mem- rane; 6, subserous tissue containing fat and blood-vessels; c, ligament covered by the synovial membrane. (Cadiat.) identical, in general and chemical characters, with very dilute liquor sanguinis. It is of a pale yellow or straw color, slightly viscid, alkaline, and on account of the presence of albumen, coaguable by heat. 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 blood-vessels. 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 ex- ceptions to this rule; for they differ from the fluids of the other serous 328 HANDBOOK OF PHYSIOLOGY. sacs not only in being pellucid, colorless, and of much, less specific gravity, but in that they seldom receive the tinge of bile when pres- ent in the blood, and are not colored by madder, or other similar sub- stances 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 especially of those which are accumu- lated on the edges and processes of the synovial fringes; for, in its pecu- liar density, viscidity, and abundance of albumen, synovia differs alike from the serum of blood and from the fluid of any of the serous cavities. (2) Mucous Membranes. The mucous membranes line all those passages by which internal 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 vascular. 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 sphenoidal sinuses, as well in the tympanum, to periosteum; in the intestinal canal, it is connected with a firm sub mucous membrane, which on its exterior gives attach- ment to the fibres of the muscular coat. The mucous membranes line certain principal tracts — Gastro-Pulmonary and Genito-Urinary; the former being subdivided into the Digestive and Eespiratory tracts. 1. The Digestive tract 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 continued along the whole tract of the intestinal canal to the termination of the rectum, being in its course arranged in the various folds and depressions already described, and prolonged into the ducts of the intestinal glands, the pancreas and liver, and into the gall-bladder. 2. The Respiratory tract includes the mucous membrane lining the cavity of the nose, and the various sinuses communicating 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 respiratory 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. SECRETION. 329 3. The Genito-urinary tract, which lines the whole of the urinary passages, from their external orifice to the termination of the tuhuli Jr O * uriniferi of the kidneys, extends also into the organs of generation in both sexes, and into the ducts of the glands connected with them; and in the female becomes continuous with the serous membrane of the ab- domen at the fimbrise of the Fallopian tubes. Structure. — These mucous tracts, and different portions of each of them, present certain structural peculiarities of the mucous membrane, adapted to the functions which 'each part has to discharge; yet in some essential characters the mucous membrane is the same, from whatever part it is obtained. In all the principal and larger parts of the several tracts, it presents, as just remarked, an external layer of epithelium, situated upon a basement membrane, and beneath this, a stratum of vas- cular tissue of variable thickness, containing lymphatic vessels and nerves. The vascular stratum or corium, together with the basement membrane and epithelium, in different cases, is elevated into minute papillae and villi, or depressed into 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 blood- vessels spread over the outer surface of the latter in a single layer. The primary or basement membrane is a thin, transparent layer, sim- ple, homogeneous, or composed of endothelial cells. In the minuter divisions 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. No certain distinction can be drawn be- tween the epithelium-cells of mucous membranes and gland-cells. Mucous Fluid : Mucus. — From all mucous membranes there is se- creted either from the surface or from certain special glands, or from both, a more or less viscid, grayish, or semi-transparent fluid, of alka- line reaction and high specific gravity, named mucus. It mixes imper- fectly with water, but, rapidly absorbing liquid, it swells considerably when water is added. Under the microscope it is found to contain epi- thelium and leucocytes. It is found to be made up, chemically, of a nitrogenous principle called mucin, which forms its chief bulk, of a little albumen, of salts chiefly chlorides and phosphates, and water with traces of fats and extractives. SECRETING GLAXDS. The structure of the elementary portions of a secreting apparatus, namely epithelium, simple membrane, and blood-vessels having been al- 330 HANDBOOK OF PHYSIOLOGY. ready described in this and previous chapters, we may proceed to con- sider the manner in which they are arranged to form the varieties of secreting glands. The secreting glands are the organs to which the function of secretion is more especially ascribed; for they appear to be occupied with it alone. They present, amid manifold diversities of form and composition, a gen- eral 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 epithelium, and elabo- rate, as secreting cells, the substances to be discharged from them. Glands are provided also with lymphatic vessels and nerves. The distri- bution of the former is not peculiar, and need not be here considered. Nerve-fibres are distributed both to the blood-vessels of the gland and to its ducts; and to the secreting cells also in some glands. Varieties. — 1. The simple tubule or tubular gland (A, Fig. 227), ex- amples of which are furnished by some mucous glands, the follicles of Lieberkuhn, and the tubular glands of the stomach. These appear to be simple tubular depressions of the mucous membrane, the wall of which is formed of primary membrane, is lined with secreting cells arranged as an epithelium. To the same class may be referred the elongated and tortuous sudoriferous glands. 2. The compound tubular glands (D, Fig. 227) form another division. These consist of main gland-tubes, which divide and subdivide. Each gland may consist of the subdivisions of one or more main tubes. The ultimate subdivisions of the tubes are generally highly convoluted. They are formed of a basement-membrane, lined by epithelium of various forms. The larger tubes may have an outside coating of fibrous, areolar, or muscular tissue. The Kidney, Testis, Salivary glands, Pancreas, Brunner's glands with the Lachrymal and Mammary glands, and some Mucous glands are examples of this type, but present more or less marked variations among themselves 3. The aggregate or racemose glands, in which a number of vesicles or acini are arranged in groups or lobules (c, Fig. 227). The Meibomian follicles are examples of this kind of gland. These various organs differ from each other only in secondary points of structure ; such as, chiefly, the arrangement of their excretory ducts, the grouping of the acini and lobules, their connection by areolar tissue, and supply of blood-vessels. The acini commonly appear to be formed by a kind of fusion of the walls of several vesicles, which thus combine to form one cavity lined or filled with secreting cells which also occupy recesses from the main cavity. The smallest branches of the gland-ducts sometimes open into the centres of these cavities ; sometimes the acini are clustered round the extremities, or by the sides of the ducts : but, SECRETION. 331 whatever secondary arrangement there may be, all have the same essen- tial character of rounded groups of vesicles containing gland-cells, and opening 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. Among these varieties of structure, all the secreting glands are alike in some essential points, besides those which they have in common with FIG. 227. -Plans of extension of secreting membrane by inversion or recession in form of cavi- ties. A, simple glands, viz. q, straight tube ; 7i, sac; i, coiled tube. B, multilocular crypts; k, of tubular form; I, saccular. C. racemose, or saccular compound gland; m, entire gland, showing branched duct and lobular structure; n, a lobule, detached with o, branch of duct proceeding from it. D, compound tubular gland (Sharpey). all truly secreting structures. They agree in presenting a large extent of secreting surface within a comparatively small space; in the circum- stance that while one end of the gland-duct opens on a free surface, the opposite end is always closed, having no direct communication with blood-vessels, or any other canal; and in a uniform arrangement of ca- 332 HANDBOOK OF PHYSIOLOGY. pillary blood-vessels, ramifying and forming a network around the walls and in the interstices of the ducts and acini. Process of Secretion. — In secretion two distinct processes are con- cerned which may be spoken of as I. Physical, and II. Chemical. 1. Physical processes. — These, already discussed in the last chapter, are such as can be closely imitated in the laboratory, inasmuch as they consist in the operation of well-known physical laws ; they are — (a) Fil- tration; (b) Dialysis. (a) Filtration is, as we have already mentioned, simply the passage of a fluid through a porous membrane under the influence of pressure. If two fluids be separated by a porous membrane, and the pressure on one side is greater than on the other, it is evident that in the absence of counteracting osmotic influences (see below), there will be a filtration through the membrane until the pressure on the two sides is equalized. Of course there may be fluid only on one side of the membrane, as in the ordinary process of filtering through blotting-paper, and then the filtra- tion will continue as long as the pressure (in this case, the weight of the fluid) is sufficient to force it through the pores of the filter. The neces- sary inequality of pressure may be obtained either by diminishing it on one side, as in the case of cupping; or increasing it on the other, as in the case of the increased blood-pressure, and consequent increased flow of urine resulting from copious drinking. By filtration, not merely water, but various salts in solution, and even colloids of all kinds, may transude from the blood-vessels. The amount of a liquid which will pass through a filter in a given time depends not only upon the amount of pressure to which it is subjected, but also upon the natuye of the fluid filtered, and upon the kind of membrane employed as the filter. It seems probable that some fluids, such as the secretions of serous mem- branes, are simply exudations or oozings (filtration) from the blood- vessels, whose qualities are determined by those of the liquor sanguinis, while the quantities are liable to variation, and are chiefly dependent upon the blood-pressure. (b) Dialysis is the passage of fluids through a moist animal mem- brane independent of pressure, and sometimes actually in opposition to it. There must always be in this process two fluids differing in composition, one or both possessing an affinity for the intervening membrane, and the fluids must be capable of mixing one with the other ; the osmotic current continuing in each direction (when both fluids have an affinity for the membrane) until the chemical composition of the fluid on each side of the septum becomes the same. 2. Chemical processes. — The chemical processes constitute the process of secretion, properly so called, as distinguished from mere transudation .spoken of above. In the chemical process of secretion various materials "which do not exist as such in the blood are elaborated by the agency of SECRETION. 333 the gland-cells from the blood, or to speak more accurately, from the plasma which exudes from the blood-vessels into the interstices of the gland-textures. The best evidence in favor of this view is : 1st. That cells and nuclei are constituents of all glands, nowever diverse their outer forms and other characters, and that they are in all glands placed on the surface or in the cavity whence the secretion is poured. 2d. That many secre- tions which are visible with the microscope may be seen in the gland- cells before they are discharged. Thus, bile may be often discerned by its yellow tinge in the 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. Secreting cells, like the cells or other elements of any other organ, appear to develop, grow, and attain their individual perfection by appro- priating nutriment from the fluid exuded by adjacent blood-vessels and elaborating it, so that it shall form part of their substance. In this per- fected state, the cells subsist for some brief time, and when that period is over they appear to dissolve, wholly or in part, and yield their con- tents to the peculiar material of the secretion. And this appears 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. We have described elsewhere the changes which have been noticed from actual experiment in the cells of the salivary glands, pancreas, and peptic gland. Discharge of Secretions from glands may either take place as soon as they are formed; or the secretion may be long retained within the glands or its ducts. The former is the case with the sweat glands. But the secretions of those glands whose activity of function is only occasional are usually retained in the cells in an undeveloped form during the periods of the gland's inaction. And there are glands which are like both these classes, such as the lachrymal, which constantly secrete small portions of fluid, and on occasions of greater excitement discharge it more abundantly. When discharged into the ducts, the further course of secretions is effected (1) partly by the pressure from behind; the fresh quantities of secretion propelling those that were formed before. In the larger ducts, its propulsion is (2) assisted by the contraction of their walls. All the larger ducts, such as the ureter and common bile-duct, possess in their coats plain muscular fibres; they contract when irritated, and sometimes manifest peristaltic movements. Ehythmic contractions in the pancre- atic and bile-ducts have been observed, and also in the ureters and vasa deferentia. It is probable that the contractile power extends along the 334 HANDBOOK OF PHYSIOLOGY. ducts to u considerable distance within the substance of the glands whose secretions can be rapidly expelled. Saliva and milk, for instance, are sometimes ejected with much force. Circumstances Influencing Secretion. — The principal conditions which influence secretion are (1) variations in the quantity of blood, (2) variations in the quantity of the peculiar materials for any secretion that the blood may contain, and (3) variations in the condition of the nerves of the glands. (1.) An increase in the quantity of blood traversing a gland, as in nearly all the instances before quoted, coincides generally with an aug- mentation of its secretion. Thus, the mucous membrane of the stomach becomes florid when, on the introduction of food, its glands begin to se- crete; the mammary gland becomes much more vascular during lactation; and all circumstances which give rise to an increase in the quantity of material secreted by an organ produce, coincidently, an increased supply of blood; but we have seen that a discharge of saliva may occur under extraordinary circumstances, without increase of blood-supply, and so it may be inferred that this condition of increased blood-supply is not abso- lutely essential. (2.) An increase %n the amount of the materials which the glands are designed to separate or elaborate, contained in the Uood supplied to them, increases the amount of any secretion. Thus, when an excess of nitro- genous waste is in the blood, whether from excessive exercise, or from destruction of one kidney, a healthy kidney will excrete more urea than it did before, (3.) Influence of the Nervous System on Secretion. — The process of secretion is largely influenced by the condition of the nervous system. The exact mode in which the influence is exhibited must still be 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, as was described at length in the case of the submaxillary gland, upon the secreting cells themselves; this may be called trophic influence. Its influence over se- cretion, 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 the saliva. Through the nerves, various conditions of the brain also influence the secretions. Thus, the thought of food may be sufficient to excite an abundant flow of saliva. And, probably, it is the mental state which SECRETION. 335 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 mental conditions, as in the cases in which, through grief or passion, the secretion of milk is al- tered, and is sometimes so changed as to produce irritation in the ali- mentary canal of the child, or even death (Carpenter). Relations between the Secretions. — 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. The Mammary Glands. Structure. — The mammary glands are composed of large divisions or lobes, and these are again divisible into lobules, the lobules being com- FIG. 228.— Dissection of the lower half of the female mamma during the period of lactation. ?s.— 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 mamilla 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 lac- tiferous ducts shown passing towards the mamilla where they open; 6. one of the sinus lactei or reservoirs; 7, some of the glandular lobules which have been unravelled; 7', others massed together '(Luschka). posed of the convoluted subdivision of the main ducts (alveoli). The 336 HANDBOOK OF PHYSIOLOGY. lobes and lobules are bound together by areolar tissue; penetrating be- tween 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. 228) 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 connective 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 (lobular) ducts, and open by small separate orifices through the nipple. At the points of junction of lobular ducts to form lactiferous ducts, and just before these enter the base of the nipple, the ducts are dilated (Fig. 228); and, during lactation, the period of active secretion by the gland, the dilatations form reservoirs for the milk, which collects in, and distends them. The walls of the gland-ducts are formed of areolar with some unstriped muscular tissue, and are lined internally by short columnar, and near the nipple by squamous epithe- lium. The alveoli consist of a membrana propria of flattened endothe- lial cells lined by low columnar epithelium, and are filled with fat globules. The nipple, which contains the terminations of the lactiferous ducts, is composed also of areolar tissue, and contains unstriped muscular fibres. Blood-vessels are also freely supplied 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. Blood-vessels, nerves, and lymphatics are plentifully supplied to the mammary glands; the calibre of the blood-vessels, as well as the size of the glands, varying very greatly under certain conditions, especially those of pregnancy and lactation. Changes in the Glands at certain Periods. — The minute changes which occur in the mammary gland during its periods of evolution (preg- nancy), and involution (when lactation has ceased), are the following: The most favorable period for observing the epithelium of the mam- mary gland fully developed is shortly before the end of pregnancy. At this period the acini which form the lobules of the gland, are found to be lined with a mosaic of polyhedral epithelial cells (Fig. 229), and sup- ported by a connective-tissue stroma. The rapid formation of milk during lactation results from a fatty metamorphosis of the epithelial cells. In the earlier days of lactation, epithelial cells partially transformed are discharged in the secretion: these are termed " colostrum cor- puscles," but later on the cells are completely transformed into fat be- fore the secretion is discharged. SECRETION. 337 After the end of lactation, the mamma gradually returns to its ori- ginal size (involution). The acini, in the early stages of involution, are lined with cells in all degrees of vacuolation. As involution proceeds the acini diminish considersbly in size, and at length, instead of a mosaic of lining epithelial cells (twenty to thirty in each acinus), we have five or six nuclei (some with no surrounding protoplasm) lying in an irregular heap within the acinus. During the later stages of involu- tion, large, yellow granular cells are to be seen. As the acini diminish in size, the connective tissue and fatty matter between them increase, and in some animals, when the gland is completely inactive, it is found to consist of a thin film of glandular tissue overlying a thick cushion of fat. Many of the products of waste are carried off by the lymphatics. During pregnancy the mammary glands and mammae undergo changes (evolution) which are readily observable. They enlarge, become harder and more distinctly lobulated: the veins on the surface become more prominent. The areola becomes enlarged and dusky, with pro- Fia. 229.— Section of mammary gland of bitch, showing acini, lined with epithelial cells of a polyhedral or short columnar form. X 200. (V. D. Harris.) jecting papillae; the nipple too becomes more prominent, and milk can be squeezed from the orifices of the ducts. This is a very gradual pro- cess, which commences about the time of conception, and progresses steadily during the whole period of gestation. The acini enlarge, and a series of changes occur, exactly the reverse of those just described under the head of Involution. The Mammary Secretion : Milk. .The secretion of the mammary glands, or milk, is a bluish- white opaque fluid with a pleasant sweet taste. It is a true emulsion. Under the microscope, it is found to contain a number of globules of various sizes (Fig. 230), the majority about ru-sinr °f an incn in diameter. They are composed of oily matter, probably coated by a fine layer of albumi- nous material, and are called milk-globules; while, accompanying these, are numerous minute particles, both oily and albuminous, which exhibit 22 338 HANDBOOK OF PHYSIOLOGY. 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- corpuscles. These, which appear to be small masses of albuminous and oily matter, are probably secreting cells of the gland, either in a state of fatty degeneration, or old cells which in their attempt 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. Colostrum-corpuscles have been seen to exhibit contractile movements and to squeeze out drops of oil from their interior. Chemical Composition. — In addition to the oil existing in numberless little globules, coated with a thin layer of albuminous matter, floating FIG. 230.— Globules and molecules of Cow's milk, x 400. in a large quantity of water, milk contains casein, serum-albumin , milk- sugar (lactose), and several salts. Its percentage composition has been already mentioned, but may be here repeated. Its reaction is alkaline: its specific gravity about 1030. Table of the Chemical Composition of Milk. Water, Solids, Proteids, including Casein and Serum-Albumin, . Fats or Butter, Sugar (with extractives), Human. 890 110 1000 Human, 35 25 48 Cow. 858 142 1000 Cow. 68 38 30 SECRETION. 339 Salts (chiefly potassium, sodium, and calcium, chlorides and phos- phates), 110 142 When milk is allowed to stand, the fat globules, being the lightest portion, rise to the top, forming cream. If a little acetic acid be added to a drop of milk under the microscope, the albuminous film coating the oil drops is dissolved, and they run together into larger drops. The same result is produced by the process of churning, the effect of which is to break up the albuminous coating of the oil drops: they then coalesce to form butter. Curdling of Milk — The curdling of milk is due to the coagulation of the casein which is kept in solution under normal conditions by the alkaline calcium phosphate. On the addition of an acid, such as acetic, the casein is precipitated. This occurs, too, if it be allowed to stand for some time, its reaction becomes acid: in popular language it "turns sour/' The change appears to be due to the conversion of the milk-sugar into lactic acid, by means] of a special micro-organism, Bacterium lactis; this causes the precipitation (curdling) of the casein: the curd contains the fat globules: the remaining fluid (whey) consists of water holding in solution albumen, milk-sugar, and certain salts. The same effect is produced in the manufacture of cheese, which is really casein coagulated by the agency of rennet (p. 254). When milk is boiled, the scum which forms consists chiefly of serum-albumin. Curdling Ferments. — The effect of the ferments of the gastric, pancreatic, and intestinal juices in curdling milk (curdling ferments) has already been mentioned in the Chapter on Digestion. The salts of milk are chlorides, sulphates, phosphates, and carbo- nates of potassium, sodium, and calcium. Traces of iron, fluorine, and silica are also found, and the gases, car- bonic acid, oxygen, and nitrogen. CHAPTER XL THE STRUCTURE AND FUNCTIONS OF THE SKIN. The skin serves — (1), as an external integument for the protection of the deeper tissues, and (2), as a sensitive organ in the exercise of touch; it is also (3), an important secretory and excretory, and (4), an absorbing organ; while it plays an important part in (5) the regulation of the temperature of the body. Structure. — The skin consists, principally, of a vascular tissue named the corium, derma, or cutis vera, and an external covering of epithelium termed the cuticle or epidermis. Within and beneath the corium are imbedded several organs with special function, namely, sudoriferous glands, sebaceous glands, and hair follicles ; and on its surface are sen- sitive papillae. The so-called appendages of the skin — the hair and nails — are modifications of the epidermis. A. Epidermis. — The epidermis is composed of several strata of cells of various shapes and sizes; it closely resembles in its structure the epi- thelium of the mucous membrane that lines the mouth. The following four layers may be distinguished in a more or less developed form. 1. Stratum corneum (Fig. 231, a), consisting of superposed layers of horny scales. The different thickness of the epidermis in different regions of the body is chiefly due to variations in the thickness of this layer; e. g.< on the horny parts of the palms of the hands and soles of the feet it is of great thickness. The stratum corneum of the buccal epithelium chiefly differs from that of the epidermis in the fact that nuclei are to be distinguished in some of the cells even of its most superficial layers. 2. Stratum lucidum, a bright homogeneous membrane consisting of squamous cells closely arranged, in some of which a nucleus can be seen. 3. Stratum granulosum, consisting of one layer of flattened cells which appear fusiform in vertical section: they are distinctly nucleated, and a number of granules extend from the nucleus to the margins of the cell. 4. Stratum Malpighii or Rete mucosum consists of many strata. The deepest cells, placed immediately above the cutis vera, are columnar with oval nuclei: this layer of columnar cells is succeeded by a number of layers of more or less polyhedral cells with spherical nuclei; the cells THE STRUCTURE AND FUNCTIONS OF THE SKIN. 341 of the more superficial layers are considerably flattened. The deeper surface of the rete mucosum is accurately adapted to the papillae of the true skin, being, as it were, moulded on them. It is very constant in thickness in all parts of the skin. The cells of the middle layers of the stratum Malpighii are almost all connected by processes, and thus form " prickle cells " (Fig. 27). The pigment of the skin, the varying quan- tity of which causes the various tints observed in different individuals and different races, is contained in the deeper cells of rete mucosum; the pigmented cells as they approach the free surface gradually losing their color. Epidermis maintains its thickness in spite of the constant wear and tear to which it is subjected. The columnar cells of the deep- FIG. 231. FIG. 232. FIG. 231.— Vertical section of the epidermis of the prepuce, a, stratum corneum, of very few layers, the stratum lucidum and stratum granulosum not being distinctly represented; ft, c, d, and e, the layers of the stratum Malpighii, a certain number of the cells in layers d and e showing signs of segmentation; layer c consists chiefly of prickle or ridge and furrow cells: /, basement mem- brane; g, cells in cutis vera. (Cadiat.) FIG. 232. -Vertical section of skin of the negro, a, a. Cutaneous papillae. 6. Undermost and dark-colored layer of oblong vertical epidermis-cells, c. Stratum Malpighii. d. Superficial layers, including stratum corneum, stratum lucidum, and stratum granulosum the last two not differen- tiated in fig. X250. (Sharpey.) est layer of the "rete mucosum " elongate, and their nuclei divide into two (Fig. 231, e). Lastly, the tipper part of the cell divides from the lower; thus from a long columnar cell are produced a polyhedral and a short columnar cell: the latter elongates and the process is repeated. The polyhedral cells thus formed are pushed up towards the free surface by the production of fresh ones beneath them, and become flattened from pressure: they also become gradually horny by evaporation and trans- 342 HANDBOOK OF PHYSIOLOY. formation of their protoplasm into keratin, till at last by rubbing they are detached as dry horny scales at the free surface. There is thus a constant production of fresh cells in the deeper layers, and a constant throwing off of old ones from the free surface. When these two pro- cesses are accurately balanced, the epidermis maintains its thickness. When, by intermittent pressure a more active cell-growth is stimulated, the production of cells exceeds their waste and the epidermis increases in thickness, as we see in the horny hands of the laborer. 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; for it serves as well to protect the sensi- tive and vascular cutis from injury from without, as to limit the evapo- ration of fluid from the blood-vessels. 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 latter will become brown, dry, and horn-like, while that of the former will almost retain its natural moisture. B. Cutis vera. — The corium or cutis vera, 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 are- olar tissue, interwoven in all directions, and forming by their interlace- ments, numerous spaces or areolae. These areolse are large in the deeper layers of the cutis, and are there usually filled with little masses of fat (Fig. 234): but, in the superficial parts, they are small or entirely ob- literated. Plain muscular fibres are also abundantly present. Papillae. — The cutis vera presents numerous conical elevations, or papillce, with a single or divided free extremity, which are more promi- FIG. 233.— Compound papillae from the palm of the hand, a, basis of a papilla: 6, 6, divisions or branches of the same; c, c, branches belonging to papillae, of which the bases are hidden from view. X 60. (KSlliker.) nent and more densely set at some parts than at others (Fig. 233). This is especially the case on the palmar surface of the hands and fingers, and on 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 THE STRUCTURE AND FUNCTIONS OF THE SKIN. 343 curved lines, separated from each other by depressions. Thus they may be easily seen on the palm, whereon each raised line is composed of a double row of papillae, and is intersected by short transverse lines or fur- rows 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 average length FIG. 234.— Vertical section of skin. A. Sebaceous gland opening into hair follicle. B. Muscular fibres. C. Sudoriferous or sweat-pland. D. Subcutaneous fat. E. Fundus of hair-follicle, with hair-papillae. (Klein and Noble Smith.) is about -j-J-jj- of an inch, and at their base they measure about ^-g- of an inch in diameter. Each papilla is abundantly supplied with blood, re- ceiving from the vascular plexus in the cutis one or more minute arterial twigs, which divide into capillary loops in its substance, and then re- unite into a minute vein, which passes out at its base. This abundant supply of blood explains the turgescence or kind of erection, which they undergo when the circulation through the skin is active. The majority, 344 HANDBOOK OF PHYSIOLOGY. 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 nerve-terminations in the skin are described under the Sen- sory Nerve Terminations. Glands of the Skin. — The skin possesses glands of two kinds; (a) Sudoriferous, or Sweat Glands; (b) Sebaceous Glands. (a) Sudoriferous, or Sweat Glands. — Each of these glands consists of a small lobular mass, formed of a coil of tubular gland-duct, surrounded by blood-vessels and imbedded in the subcutaneous adipose tissue (Fig. 234, C). From this mass the duct ascends, for a short distance, in a spiral manner through the deeper part of the cutis, then passing 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 FIG. 235.— Terminal tubules of sudoriferous glands, cut in various directions from the skin of the pig's ear. (V. D. Harris.) straight in their course (Fig. 234). The duct, which maintains nearly the same diameter throughout, is lined with a layer of columnar epithe- lium (Fig. 235) continuous with the epidermis; while the part which passes through the epidermis is composed of the latter structure only; the cells which immediately form the boundary of the canal in this part being somewhat differently arranged from those of the adjacent cuticle. The coils or terminal portions of the gland are lined with at least two- layers of short columnar cells with very distinct nuclei (Fig. 235), and possess a large lumen distinctly bounded by a special lining or cuticle. The sudoriferous 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, and of the sole of the foot. The glands by which the peculiar odorous matter of the axillae is secreted form a nearly complete layer under the cutis, and are like the ordinary THE STRUCTURE AND FUNCTIONS OF THE SKIN. 345 sudoriferous glands, except in being larger, and having very short ducts. The peculiar bitter yellow substance secreted by the skin of the ex- ternal auditory passage is named cerumen, and the glands themselves ceruminous glands; but they do not much differ in structure from the ordinary sudoriferous glands. (b) Sebaceous Glands. — The sebaceous glands (Fig. 236), like the sudoriferous glands, are abundantly 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 FIG. 236.— Sebaceous gland from human skin. (Klein and Noble Smith.) the various passages into the body, as the anus, nose, lips, and external ear. They are entirely absent from the palmar surface of the hand and the plantar surfaces of the feet. They are minutely lobulated glands composed of an aggregate of small tubes or sacculi filled with opaque white substances, like soft ointment. Minute capillary vessels overspread them; and their ducts 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 or more glands to each hair (Fig. 234). Hair.— A hair is produced by a peculiar growth and modification of the epidermis. Externally it is covered by a layer of fine scales closely imbricated, or overlapping like the tiles of a house, but with the free edges turned upwards (Fig. 237, A). It is called the cuticle of the hair. 34:6 HANDBOOK OF PHYSIOLOGY. Beneath this is a much thicker layer of elongated horny cells, closely packed together so as to resemble a fibrous structure. This, very com- monly, in the human subject, occupies the whole of the inside of the hair; but in some cases there is left a small central space filled by a sub- stance called the medulla or pith, composed of small collections of irreg- ularly shaped cells, containing sometimes pigment granules or fat, but mostly air. The follicle, in which the root of each hair is contained (Fig. 238), forms a tubular depression from the surface of the skin, descending into the subcutaneous fat, generally to a greater depth than the sudoriferous glands, and at its deepest part enlarging in a bulbous form, and often curving from its 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 folli- cles of the largest hairs, has the structure of vascular fibrous tissue. At the bottom of the follicle is a small papilla, or projection of true skin, FIG. 237.— Surf ace of a white hair, magnified 160 diameters. The wave lines mark the upper or free edges of the cortical scales. B, separated scales, magnified 350 diameters. (Kolliker.) and it is by the production and outgrowth of epidermal cells from the .surface of this papilla 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 integument (Fig. 238). This epidermal lining of the hair-follicle, or root-sheath of the hair, is com- posed of two layers, the inner one of which is so moulded on the im- bricated scaly cuticle of the hair, that its inner surface becomes imbri- cated 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 consistence. 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 THE STRUCTURE AND FUNCTIONS OF THE SKIN. 34T and projects beyond the extremity of the digit. The intermediate por- tion of the nail rests by its broad under surface on the front part of the matrix, which is here called the led of the nail. This part of the matrix is not uniformly smooth on the surface, but is raised in the form of longi- tudinal and nearly parallel ridges or laminae, on which are moulded the epidermal cells of which the nail is made up (Fig. 241). The growth of the nail, like that of the hair, or of the epidermis 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. Inas- -7 FIG. 238. FIG. 239. FIG. 238.— Medium-sized hair in its follicle, a, stem cut short; 6, root; c, knob; d, hair cuticle; e, internal, and /, external root-sheath; g, h, dermic coat of follicle; i, papilla; fc, fc, ducts of sebace- ous glands; 1. corium; m, mucous layer of epidermis; o, upper limit of internal root sheath. X 50. (Kolliker.) FIG. 239.— Longitudinal section of a hair follicle, a, Stratum of Malpighi, deep layer forming the external root-sheath, and continued to the surface of the papilla to form the medullary sheath of the hair; ft, second external sheath; c, internal root sheath; d, fibroid sheath of the hair: e, medullary sheath or medulla; /, hair papilla; pearance and in being able to perform amoeboid move- ments; others are red blood -corpuscles of normal appearance or variously changed; while there are also large cells containing either a pigment al- lied to the coloring matter of the blood, or rounded corpuscles like red blood-corpuscles. FIG. 264. FIG. 263.— Section of dog's spleen injected: c, capsule; tr, trabeculee; m, two Malpighian bodies with numerous small arteries and capillaries: a, artery, I, lymphoid tissue, consisting of closely- packed lymphoid cells supported by very delicate retiform tissue; a light space unoccupied by cells is seen all around the trabeculae, which corresponds to the '* lymph path " in lymphatic glands (Schofield.) FIG 264.— Reticulum of the spleen of a Cat, shown by injection with gelatin and silver nitrate (Cadiat.) The splenic artery, after entering the spleen by its concave surface, divides and subdivides, with but little anastomosis between its branches; at the same time its branches are sheathed by the prolongations of the fibrous coat, which they, so to speak, carry into the spleen with them. The arteries send off branches into the spleen-pulp which end in capil- laries, and these either communicate, as in other parts of the body, with THE VASCULAR GLANDS. 385 the radicles of the veins, or end in lacunar spaces in the spleen-pulp, from which veins arise. The walls of the smaller veins are more or less incomplete, and readily allow lymphoid corpuscles to be swept into the blood-current. The blood from the arterial capillaries is emptied into a system of inter- mediate passages, which are directly bounded by the cells and fibres of the network of the pulp, and from which the smallest venous radicles with their cribriform walls take origin (Frey). The veins are large and very distensible; the whole tissue of the spleen is highly vascular, and becomes readily engorged with blood: the amount of distention is, how- ever, limited by the fibrous and muscular tissue of its capsule and trabeculae, which forms an investment and support for the pulpy mass within. 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 -gL- to -g-1^ inch in diameter. These are the Malpighian corpuscles of the spleen, and are situated on the sheaths of the minute splenic ar- teries, of which, indeed, they may be said to be outgrowths (Fig. 263). For while the sheaths of the larger arteries are constructed of ordinary connective tissue, this has become modified where it forms an invest- ment for the smaller vessels, so as to be composed of adenoid tissue, with abundance of corpuscles, like lymph-corpuscles, contained in its meshes, and the Malpighian corpuscles are but small outgrowths of this cyto- genous or cell-bearing connective tissue. They are composed of cylin- drical masses of corpuscles, intersected in 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 Malpighian cor- puscle of the spleen is, therefore, very similar to that of lymphatic-gland substance. Functions. — With respect to the office of the spleen, we have the fol- lowing data: (1.) 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 ma- terial, seem to indicate that this organ is concerned in elaborating the al- buminous materials of the food, and for a time storing them up, to be gradually introduced into the blood, according to the demands of the general system. (2. ) It seems probable that the spleen, like the lymphatic glands, is engaged in the formation of blood-corpuscles. For it is quite certain that the blood of the splenic vein contains an unusually large amount of white corpuscles; and in the disease termed leucocythaemia, in which the pale corpuscles of the blood are remarkably increased in number, there 25 386 HANDBOOK OF PHYSIOLOGY. is almost always found an hypertrophied state of the spleen or of the lymphatic glands. 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 current of the circulation. (3.) There is reason to believe, that in the spleen many of the red corpuscles of the Uood, those probably which have discharged their office and are worn out, undergo disintegration; for in the colored portions of the spleen-pulp an abundance 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. This process appears to be as follows. The blood-corpuscles, becoming smaller and darker, collect together in roundish heaps, which may remain in this condition, or be- come each surrounded by a cell-wall. The cells thus produced may con- tain from one to twenty blood-corpuscles in their interior. These cor- puscles 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 en- veloped by a cell-wall or not. (4.) From the almost constant presence of uric acid, in larger quan- tities than in other organs, as well as of the nitrogenous bodies, xanthin, hypoxanthin, and leucin, in the spleen, some special nitrogenous meta- bolism may be fairly inferred to occur in it. (5.) Besides these, its supposed direct offices, the spleen, is believed to fulfil some purpose in regard to the portal circulation, 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 di- gestion is going on, and enlarges when that act is concluded, it is sup- posed 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 ob- struction to the passage of blood through the latter organ, and by its diminution when the congestion of the portal system is relieved by dis- charges 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 function. It is only necessary to mention that Schiff believes that the spleen manufactures a substance without which the pancreatic secretion cannot act upon proteids, so that when the spleen is removed the digestive ac- tion of the pancreatic juice is stopped. Influence of the Nervous System up^n the Spleen. — When the spleen THE VASCULAR GLANDS. 387 is enlarged after digestion, its enlargement is probably due to two causes, (1) a relaxation of the muscular tissue which forms so large a part of its framework; (2) a dilatation of the vessels. Both these phenomena are doubtless under control of the nervous system. It has been found by experiment that when the splenic nerves are cut the spleen enlarges, and that contraction can be brought about (1) by stimulation of the spinal cord (or of the divided nerves); (2) reflexly by stimulation of the central stumps of certain divided nerves, e.g., vagus and sciatic; (3) by local stimulation by an electric current; (4) the exhibition of quinine and some other drugs. It has been shown by the oncometer of Koy (Fig. 260), that the spleen undergoes rhythmical contractions and dilatations, due no doubt to the contraction and relaxation of the muscular tissue in its capsule and trabeculaB. It also shows the rhythmical alteration of the general blood-pressure, but to a less extent than the kidney. The Thymus. This gland must be looked upon as a temporary organ, as it attains its greatest size early after birth, and after the second year gradually FIG. 265. FIG. 266. FIG. 267. FIG. QG5.— Transverse section of a lobule of an injected infantile thymus gland, a, capsule of connective tissue surrounding the lobule; 6, membrane of the glandular vesicles; c, cavity of the lobule, from which the larger blood-vessels are seen to extend towards and ramify in the spheroidal masses of the lobule, x 30. (Kolliker.) FIG 266.— From a horizontal section through superficial part of the thymus of a calf, slightly magnified. Showing in the centre a follicle of polygonal shape with similarly shaped follicles round it. (Klein and Noble Smith.) FIG 267.— The reticulum of the Thymus. a, epithelial elements; 6, corpuscles of Hassall. (Cadiat.) diminishes, until in adult life hardly a vestige remains. At its greatest development it is a long narrow body, situated in the front of the chest 388 HANDBOOK OF THl'SIOLOGY. behind the sternum and partly in the lower part of the neck. It is of a reddish or grayish color, distinctly lobulated. Structure. — The gland is surrounded by a fibrous capsule, which sends in processes, forming trabeculae, which divide the glands into lobes, and carry the blood and lymph- vessels. The large trabeculaa branch into small ones, which divide the lobes into lobules. The gland is incased in a fold of the pleura. The lobules are further subdivided into follicles by fine connective tissue. A follicle (Fig. 2G6) is seen on section to be more or less polyhedral in shape, and consists of cortical and medullary portions, both of which are composed of adenoid tissue, but in the medullary portion the matrix is coarser, and is not so filled up with lymphoid corpuscles as in the cortex. The adenoid tissue of the cortex, and to a less marked extent that of the medulla, consists of two elements, one with small meshes formed of fine fibres with thickened nodal points, and the other inclosed within the first, composed of branched connective-tissue corpuscles (Watney). Scattered in the ade- noid tissue of the medulla are the concentric corpuscles of Hassall, which are protoplasmic masses of various sizes, consisting of a nucleated gran- ular centre, surrounded by flattened nucleated endothelial cells. In the reticulum, especially of the medulla, are large transparent giant cells. In the thymus of the dog and of other animals are to be found cysts, probably derived from the concentric corpuscles, some of which are lined with ciliated epithelium, and others with short columnar cells. Haemoglobin is found in the thymus of all animals, either in these cysts, or in cells near to or of the concentric corpuscles. In the lymph issuing from the thymus are cells containing colored blood-corpuscles and haemo- globin granules, and in the lymphatics of the thymus there are more colorless cells than in the lymphatics of the neck. In the blood of the thymic vein, there appears sometimes to be an increase in the colorless corpuscles, and also masses of granular matter (corpuscles of Zimmer- mann) (Watney). The arteries radiate from the centre of the gland. Lymph sinuses may be seen occasionally surrounding a greater or smaller portion of the periphery of the follicles (Klein). The nerves are very minute. Function.— The thymus appears to take part in producing colored corpuscles, both from the large corpuscles containing haemoglobin, and also indirectly from the colorless corpuscles (Watney). Respecting the thymus gland in the hybernating animals, in which it exists throughout life, as each successive period of hybernation ap- proaches, the thymus greatly enlarges and becomes 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 to serve for the storing up of materials which, being reabsorbed in inactivity of the hybernating period, may maintain the THE VASCULAR GLANDS. 339 respiration and the temperature of the body in the reduced state to which they fall during that time. It has been shown also to be a source of the red blood-corpuscles, at any rate in early life. The Thyroid. The thyroid gland is situated in the neck. It consists of two lobes one on each side of the trachea extending upwards to the thyroid carti- lage, covering its inferior cornu and part of its body; these lobes are connected across the middle line by a middle lobe or isthmus. The thy- roid is covered by the muscles of the neck. It is highly vascular, and varies in size in different individuals. Structure. — The gland is encased in a thin transparent layer of dense areolar tissue, free from fat, containing elastic fibres. This capsule sends in strong fibrous trabeculae, which inclose the thyroid vesicles — FIG. 268.— Part of a section of the human Thyroid, a, fibrous capsule; 6. thyroid vesicles filled with, e, colloid substance; c, supporting fibrous tissue; d, short columnar cells lining vesicles; /, arteries; gr, veins filled with blood; ft, lymphatic vessel filled with colloid substance, x (S. K Alcock.) which are rounded or oblong irregular sacs, consisting of a wall of thin hyaline membrane lined by a single layer of short cylindrical or cubical cells. These vesicles are filled with a coagulable fluid or transparent colloid material. The colloid substance increases with age, and the cav- ities appear to coalesce. In the interstitial connective tissue is a round- 390 HANDBOOK OF PHYSIOLOGY. meshed capillary plexus, and a large number of lymphatics. The nerves adhere closely to the vessels. In the vesicles there are in addition to the yellowish glassy colloid material, epithelium cells, colorless blood-corpuscles, and also colored corpuscles undergoing disintegration. Function. — There is little known definitely about the function of the thyroid body. It, however, produces colloid material of the vesicle, which is carried off by the lymphatics, and discharged into the blood, and so may contribute its share to the elaboration of that fluid. The destruction of red blood-corpuscles is also supposed to go on in the gland. In certain animals its removal appears to produce a peculiar condition in which mucin is deposited in its tissues. A similar condition, known as Myxoedema, and Cretinism are closely associated with disease or removal of the thyroid gland in the human subject. Supra-renal Capsules or Adrenals. These are two flattened, more or less triangular or cocked-hat shaped bodies, resting by their lower border upon the upper border of the kid- neys. FIG. 269.— Vertical section through part of the cortical portion of supra-renal of guinea-pig, a, capsule; 6, zona glomemlosa; c, xona fasciculata; d, connective tissue supporting the columns of the cells of the latter, and also indicating the position of the blood-vessels, x (S. K. Alcock.) Structure. — The gland is surrounded by an outer sheath of connec- tive tissue, which sometimes consists of two layers, sending in exceed- ingly fine prolongations forming the framework of the gland. The THE VASCULAK GLANDS. 391 gland tissue proper consists of an outside firmer cortical portion, and an inside soft dark medullary portion. (1.) The cortical portion is divided into (Fig. 269) an external nar- row layer of small rounded or oval spaces, the zona glomerulosa, made by the fibrous trabeculae, containing multinucleated masses of proto- plasm, the differentiation of which into distinct cells, cannot be made out. (b) A layer of cells arranged radially, the zona fasciculata (c). The substance of this layer is broken up into cylinders, each of which is surrounded by the connective-tissue framework. The cylinders thus produced are of three kinds — one containing an opaque, resistant, highly refracting mass (probably of a fatty nature); frequently a large number of nuclei are present; the individual cells can only be made out with difficulty. The second variety of cylinders is of a brownish color, and contains finely granular cells, in which are fat globules. The third variety consists of gray cylinders, containing a number of cells whose nuclei are filled with a large number of fat granules. The third layer of the cortical portion is the zona reticularis (not shown in Fig. 269). This layer is apparently formed by the breaking up of the cylinders, the elements being dispersed and isolated. The cells are finely granular, and have no deposit of fat in their interior: but in some specimens fat may be present, as well as certain large yellow granules, which may be called pigment granules. (2.) The medullary substance consists of a coarse rounded or ir- Fio. 270.— Section through a portion of the medullary part of the supra-renal of guinea-pig. The vessels are very numerous, and the fibrous stroma more distinct than in the cortex, and is moreover reticulated. The cells are irregular and larger, clean, and free from oil globules. X Co. K. Alcock.) regular mesh work of fibrous tissue, in the alveoli of which are masses of multinucleated protoplasm (Fig. 270); numerous blood-vessels; and an abundance of nervous elements. The cells are very irregular in shape 392 HANDBOOK: or PHYSIOLOGY. and size, poor in fat, and occasionally branched; the nerves run through the cortical substance, and anastomose over the medullary portion. Function. — Of the function of the supra-renal bodies, nothing can be definitely stated, but they are in all probability connected with the lymphatic system. Addison's Disease. — The collection of large numbers of cases in which the supra-renal capsules have been diseased, has demonstrated the very close relation subsisting between disease of those organs and brown discoloration of the skin (Addison's disease); 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. Pituitary Body. This body is a small reddish-gray mass, occupying the sella turcica of the sphenoid bone. Structure. — It consists of two lobes — a small posterior one, consist- ing of nervous tissue; an anterior larger one, resembling the thyroid in structure. A canal lined with flattened or with ciliated epithelium, passes through the anterior lobe; it is connected with the infundibulum. The gland spaces are oval, nearly round at the periphery, spherical to- wards the centre of the organ; they are filled with nucleated cells of various sizes and shapes not unlike ganglion cells, collected together into rounded masses, filling the vesicles, and contained in a semi-fluid gran- ular substance. The vesicles are inclosed by connective tissue, rich in capillaries. Function. — Nothing is known of the function of the pituitary body. Pineal Gland. This gland, which is a small reddish body, is placed beneath the back part of the corpus callosum, and rests upon the corpora quadrigemina. Structure. — It contains a central cavity lined with ciliated epithe- lium. The gland substance proper is divisible into— (1.) An outer cor- tical layer, analogous in structure to the anterior lobe of the pituitary body; and (2.) An inner central layer, wholly nervous. The cortical layer consists of a number of closed follicles, containing (a) cells of va- riable shape, rounded, elongated, or stellate; (#) fusiform cells. There is also present a gritty matter (acervulus cerebri), consisting of round particles aggregated into small masses. The central substance consists of white and gray matter. The blood-vessels are small, and form a very delicate capillary plexus. Function. — Of this there is nothing known. THE VASCULAR GLANDS. 393 The Coccygeal and Carotid Glands. These so-called glands are situated, the one in front of the tip of the coccyx, and the other at the point of bifurcation of the common carotid artery on each side. They are made up of a plexus of small arteries, are inclosed and supported by a capsule of fibrous tissue, which contains connective-tissue corpuscles. The blood-vessels are surrounded by one or more layers of cells like secreting-cells, which are said to be modified plasma-cells of the connective tissue. The function of these bodies is unknown. Functions of the Vascular Glands in General. The opinion that the vascular glands serve for the higher organiza- tion 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 organized blood is necessary. The bulk of the thymus gland; in proportion to that of the body, appears to bear almost a direct propor- tion 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 capsules, also, though they probably never cease to dis- charge some 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 proportionate size and apparent ac- tivity of function. The spleen more nearly retains its proportionate size, and enlarges nearly as the whole body does. Although the functions of all the vascular glands may be similar, in so far as they may all alike serve for the elaboration and maintenance of the blood, yet each of them probably discharges a peculiar office, in re- lation either to the whole economy, or to that of some other organ. Eespecting any special office of the thyroid gland, nothing reasonable has been hitherto suggested; nor is there any certain evidence concern- ing that of the supra-renal capsules, Bergman believed that they formed part of the sympathetic nervous system from the richness of their nervous supply. Kolliker looked upon the two parts as function- ally distinct, the cortical part belonging to the blood vascular system, and the medullary to the nervous system. CHAPTER XIV. THE MUSCULAR SYSTEM. I. STRUCTURE OF MUSCULAR TISSUE. THERE are two chief kinds of muscular tissue, differing both in minute structure as well as in mode of action, viz. (1.) the plain or non- striated, and (2 ) the striated. The striped form of muscular fibre is sometimes called voluntary muscle, because all muscles under the con- trol of the will are constructed of it. The plain or unstriped variety is often termed involuntary, because it alone is found in the greater num- ber of the muscles over which the will has no power. (i.) Unstriped or Plain Muscle. Distribution. — Unstriped muscle forms the proper muscular coats (1.) of the digestive canal from the middle of the O3sophagus to the in- FIG. 271.— Vertical section through the scalp with two hair sacs; a, epidermis; 6, cutis; c, mus- cles of the hair-follicles. (Kolliker. ) ternal sphincter ani; (2.) of the ureters and urinary bladder; (3.) of the trachea and bronchi; (4.) of the ducts of glands; (5.) of the gall-blad- der; (6.) of the vesiculse seminales; (7.) of the pregnant uterus; (8.) of the blood-vessels and lymphatics; (9.) of the iris, and some other parts. This form of tissue also enters largely into the composition (10.) of the tunica dartos, the contraction of which is the principal cause of the wrinkling and contraction of the scrotum on exposure to cold. Un- striped muscular tissue occurs largely also in the cutis generally, being especially abundant in the interspaces between the bases of the papillae. THE MUSCULAR SYSTEM. 395 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, whirh lie alongside the hair-follicles and sebaceous glands. They pass obliquely Fia. 272.— A, unstriped muscle cells from the mesentery of a newt. The sheath exhibits trans- verse markings. X 180. B, from a similar preparation, showing that each muscle cell consists of a central bundle of fibrils, F (contractile part), connected with the intra-nuclear network, N, and a sheath with annular thickenings, St. The cells show varicosities due to local contraction and on these the annular thickenings are most marked. X 450. (Klein and Noble Smith.) from without inwards, embrace the sebaceous glands, and are attached to the hair-follicles near their base (Fig. 271). Structure. — TJnstriated muscles are made up of elongated, spindle- FIG. 273.— Plexus of bundles of unstriped muscle cells from the pulmonary pleura of the guinea, pig. X 180. (Klein and Noble Smith.) A, branching fiores; B, their long central nuclei. shaped, nucleated cells (Fig. 272), which in their perfect form are flat, from about T^ir *° srVir °f an incn broad, and -g J¥ to -%%-$ of an inch in length — very clear, granular, and brittle, so that when they break they often have abruptly rounded or square extremities. Each cell of these consists of a fine sheath, probably elastic; of a central bundle of fibrils, representing the contractile substance; and of an oblong nucleus, which 396 HANDBOOK OF PHYSIOLOGY. includes within a membrane a fine network anastomosing at the poles of the nucleus with the contractile fibrils. The ends of fibres are usually single, sometimes divided. Between the fibres is an albuminous cementing material or endomysium in which are found connective-tissue corpuscles, and a few fibres. The perimysium is continuous with the endomysium in the fibrous connective tissue surrounding and separating the bundles of muscle cells. (2.) Striated or Striped Muscles, Distribution. — The striated muscles include the whole of the volun- tary muscles of the body, the heart, and those muscles neither completely Toluntary nor involuntary, which form part of the walls of the pharynx, and exist in certain other parts of the body, as the internal ear, urethra, etc. Structure. — All these muscles are composed of fleshy bundles called fasciculi, inclosed in coverings of fibro-cellular tissue or perimysium, by FIG. 274. FIG. 275. FIG. 274. — A small portion of muscle natural size, consisting of larger and smaller fasciculi, seen in a transverse section, and a. the same magnified 5 diameters. (Sharpey.) FIG. 275. — Muscular fibre torn across; the sarcolemma s;ill connecting the two parts of the fibre. (Todd and Bowman.) which each is at once connected with and isolated from those adjacent to it (Fig. 274). Each fasciculus is made up of several smaller bundles, similarly ensheathed. A bundle is made up of muscle fibres with small processes and connective-tissue cells between them or endomysium. Each muscular fibre is thus constructed: — Externally is a fine, trans- parent, structureless membrane, called the sarcolemma, which in the form of a tubular investing sheath forms the outer wall of the fibre, and is filled up 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. 275), and its presence is in this way best demonstrated. The "fibres, which are cylindriform or prismatic, with an average diameter of about ^1-0 of an inch, are of a pale yellow color, and apparently marked by fine stride, which pass transversely round them, in slightly curved or THE MUSCULAR SYSTEM. 397 wholly parallel lines. Each fibre is found to consist of broad dim bands of highly refractive substance representing the contractile portion of the muscle fibre— the contractile discs— alternating with narrow bright bands of a less refractive substance — the interstitial discs. After hardening, each contractile disc becomes longitudinally striated, the thin oblong rods thus formed being the sarcous elements of Bowman. The sarcous elements are not the optical units, since each consists of minute doubly- refracting elements— the disdiaclasts of Brucke. When seen in trans- verse section the contractile discs appear to be subdivided by clear lines into polygonal areas Cohnheim's fields (Fig. 278), each corresponding to one sarcous element prism. The clear lines are due to a transparent in- FlG. 276. FIG. 277. FIG. 276. Part of a striped muscle-fibre of a water beetle prepared with absolute alcohol. A, sarcolemma; B, Krause's membrance. The sarcolemma shows regular bulgings. Above and below Krause's membrane are seen the transparent " lateral discs." The chief mass of a muscular com- partment is occupied by the contractile disc composed of sarcous elements. The substance of the individual sarcous elements has collected more at the extremity than in the centre: hence this latter is more transparent. The optical effect of this is that the contractile disc appears to possess a " median disc " (Disc of Hensen). Several nuclei of muscle corpuscles, C and D, are shown, and in them a minute network, x 300. (Klein and Noble Smith.) FIG. 277. A. Portion of a medium-sized human muscular fibre, x 800. B. Separated bundles of fibrils equally magnified; a, a, larger, and 6, 6, smaller collections; c, still smaller; d, d, the smallest which could be detached, possibly representing a single series of sarcous elements. (Sharpey.) terstitial fluid substance pressed out of the sarcous elements when they coagulate. The sarcolemma is a transparent structureless elastic sheath of great resistance which surrounds each fibre (Fig. 275). 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 398 HANDBOOK OF PHYSIOLOGY. space formed of a pellucid substance continuous with them. According to Sharpey, even in a fibril so constituted, the ultimate anatomical ele- ments of the fibre are not isolated. His view was that each fibril with quadrangular sarcous elements is composed of a number of other fibrils still finer, so that the sarcous element of an ultimate fibril would be not quadrangular but as a streak. 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. 277). A fine black streak can usually be discerned passing across the inter- stitial disc between the sarcous elements: this streak is termed Krause's membrane: it is continuous at each end with the sarcolemma investing the muscular fibre (Fig. 276 B). Thus the space inclosed by the sarcolemma is divided into a series of compartments by the transverse partitions known as Krause's msm- FIG. 278. FIG. 279. Fio. 278.— Three muscular fibres running longitudinally, and two bundles of fibres in transverse section, M, from the tongue. The capillaries, C, are injected, x 150. (Klein and Noble Smith.) FIG. ^79.— Transverse section through muscular fibres of human tongue. The muscle-corpuscles are indicated by their deeply-stained nuclei situated at the inside of the sarcolemma. Each muscle- fibre shows the " Cohnheim's fields," that is, the sarcous elements in transverse section separated by clear (apparently linear) interstitial substance, x 450. (Klein and Noble Smith.) branes; these compartments being occupied by the true muscle substance. On each side (above and below) of this membrane is a bright border (lateral disc). In the centre of the dark zone of sarcous elements a lighter band can sometimes be dimly discerned: this is termed the middle disc of Hensen (see Fig. 276, A). In some fibres-, chiefly those from insects, each lateral disc contains a row of bright granules forming the granular layer of Flogel. The fibres contain nuclei, which are roundish ovoid, or spindle-shaped in different animals. These nuclei are situated close to the sarcolemma, their long axes being parallel to the fibres which contain them. Each nucleus is composed of a uniform network of fibrils, and is imbedded in a thin, more or less branched film of protoplasm. The nucleus and pro- toplasm together form the muscle cell or muscle-corpuscle of Max Schultze. THE MUSCULAR SYSTEM. 399 The sarcous elements and Krause's membranes are doubly refracting, the rest of the fibre singly refracting. (Briicke. ) According to Schafer, the granules, which have been mentioned on either side of Krause's membrane, are little knobs attached to the ends of " muscle-rods;" and these muscle-rods, knobbed at each end, and im- bedded in a homogeneous protoplasmic ground-substance, form the sub- stance of the muscles. This view of the structure of muscle requires further confirmation. Although each muscular fibre may be considered to be formed of a number of longitudinal fibrils, arranged side by side, it is also true that they are not naturally separate from each other, there being lateral cohe- sion, if not fusion, of each sarcous element with those around and in con- tact with it; so that it happens that there is a tendency for a fibre to split, not only into separate fibrils, but also occasionally into plates or FIG. 280.— From a preparation of the nerve-termination in the muscular fibres of a snake, a, End plate seen only broad surfaced. 6, End plate seen as narrow surface. (Lingard and Klein.) discs, each of which is composed of sarcous elements laterally adherent one to another. Muscular Fibres of the Heart (Figs. 92 and 93) form the chief, though not the only exception to the rule, that involuntary muscles are constructed of plain fibres; but although striated and so far resembling those of the voluntary muscles, they present these distinctions: — Each muscular fibre is made up of elongated, nucleated, and branched cells, the nuclei or muscle-corpuscles being centrally placed in the fibre. The fibres are finer and less distinctly striated than those of the voluntary muscles; and no sarcolemma can be usually discerned. Blood and Nerve Supply. — The voluntary muscles are freely supplied with blood-vessels; 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 them from 400 HANDBOOK OF PHYSIOLOGY. the cerebro-spinal system, and the unstriped muscles from the sympa- thetic or ganglionic system. The nerves terminate in the muscular fibre in the following ways: — (1.) In unstriped muscle, the nerves first of all form a plexus, called the ground plexus (Arnold), corresponding to each group of muscle bun- dles; the plexus is made by the anastomosis of the primitive fibrils of the axis-cylinders. From the ground plexus, branches pass of, and again anastomosing, form plexuses which correspond to each muscle FI&. 281— Two striped muscle-fibres of the hyoglossus of frog, o, Nerve end-plate; 6, nerve- fibres leaving the end-plate; c, nerve-fibres, terminating after dividing into branches d, a nucleus in which two nerve-fibres anastomose, x 600. (Arndt.) bundle — intermediary plexuses. From these plexuses branches consist- ing of primitive fibrils pass in between the individual fibres and anasto- mose. These fibrils either send off finer branches, or terminate them- selves in the nuclei of the muscle cells. (2.) In striped muscle the nerves end in motorial end-plates, having first formed, as in the case of unstriped fibres, ground and intermediary plexuses. The fibres are, however, medullated, and when a branch of THE MTJSCULAK SYSTEM. 401 the intermediary plexus passes to enter a muscle-fibre, its primitive sheath becomes continuous with the sarcolemma, and the axis-cylinder forms a network of its fibrils on the surface of the fibre. This network lies imbedded in a flattened granular mass containing nuclei of several kinds; this is the material end-plate (Fig. 281). In batrachia, besides end-plates, there is another way in which the nerves end in the muscle- fibres, viz., by rounded extremities, to which oblong nuclei are attached. Development. — (1.) Unstriped. — The cells of unstriped muscle are derived directly from embryonic cells, by an elongation of the cell, and its nucleus; the latter changing from a vesicular to a rod shape. (2.) Striped. — Formerly it was supposed that striated fibres were formed by the coalescence oi several cells, but recently it has been proved, that each fibre is formed from a single cell, the process involving an enor- mous increase in size, a multiplication of the nucleus by fission, and a differentiation of the cell-contents. This view differs but little from the other, that the muscular fibres is produced, not by multiplication of cells, but by arrangement of nuclei in a growing mass of protoplasm (answering to the cell in the theory just referred to), which becomes gradually differentiated so as to assume the characters of a fully devel- oped muscular fibre. Growth of Muscle. — The growth of muscles both striated and non- striated, is the result of an increase both in the number and size of the individual elements. In the pregnant uterus the fibre cells may become enlarged to ten times their original length. In involution of the uterus after parturition the reverse changes occur, accompanied generally by some fatty infiltration of the tissue and degeneration of the fibres. II. THE CHEMICAL COMPOSITION OF MUSCLE. A. Proteids. — The principal substance which can be extracted from muscle, when examined after death, is a proteid body, called Myosin. This body appears to bear the same relation to the living muscle, as fibrin does to the living blood, since the coagulation of muscle after death is due to the formation of myosin. Thus if coagulation be delayed in muscles removed immediately from recently killed animals, by subjecting them to a temperature below 0° C., it is possible to obtain from them by expression a viscid fluid of slightly alkaline reaction, called muscle plasma (Kuhne,Halliburton). And muscle plasma, if exposed to the ordinary temperature of the air (and more quickly at 37-40° C.), undergoes coag- ulation much in the same way as does blood plasma, separated from the blood by the action of a low temperature, under similar circumstances. The appearances presented by the fluid during the process are also very similar to the phenomena of blood-clotting, viz., that first of all an in- creased viscidity on the surface of the fluid, and at the sides of the con- taining vessel, appears, which gradually extends throughout the entire mass, until a fine transparent clot is obtained. In the course of some hours the clot begins to contract, and to squeeze out of its meshes a fluid 402 HANDBOOK OF PHYSIOLOGY. corresponding to blood-serum. In the course of coagulation, therefore, muscle plasma separates into muscle clot and muscle serum. The mus- cle clot is the substance myosin. It differs from fibrin in being easily soluble in a 2 per cent solution of hydrochloric acid, and a 10 per cent solution of sodium chloride. It is insoluble in distilled water, and its solutions coagulate on application of heat. It is a body, therefore, belonging to the globulin class of proteids. During the process the re- action of the fluid becomes distinctly acid. The coagulation of muscle plasma can not only be prevented by cold, but also, as Halliburton has shown, by the presence of neutral salts in certain proportions; for example, of sodium chloride, of magnesium sul- phate, or of sodium sulphate. It will be remembered that this is also the case with blood plasma. Dilution of the salted muscle plasma will produce its slow coagulation, which is prevented by the presence of neu- tral salts if in strong solution. It is highly probable that the formation of muscle-clot is a ferment action (myosin ferment). The antecedent of myosin in living muscle has received the name of myosinogen, in the same way as the fibrin- forming element in the blood is called fibrinogen. Myosinogen is, how- ever, made up of two globulins, which coagulate at the temperatures 47° C. and 56° C. respectively. Myosin may also be obtained from dead muscle by subjecting it, after all the blood, fat, and fibrous tissue, and substances soluble in water have been removed, to a 10 per cent solution of sodium chloride, or 5 per cent solution of magnesium sulphate, or 10 to 15 per cent solution of ammonium chloride, filtering and allowing the filtrate to drop into a large quantity of water, when myosin separates out as a white flocculent precipitate. A very remarkable fact with regard to the properties of myosin has been demonstrated by Halliburton, namely, that a solution of muscle which has undergone rigor mortis, in strong neutral saline solution, pos- sesses very much the same properties as muscle plasma, and that if di- luted with twice or three times its bulk of water, myosin will separate out as a clot, which clot can be again dissolved in a strong neutral saline solution, and the solution can be again made to clot on dilution. This process can be often repeated; but in the fluid which exudes from the clot there is no proteid present. Myosin then when dissolved in neutral saline fluids is converted into myosinogen, but reappears on dilution of the fluid. Muscle clot is almost pure myosin; but it appears to be combined with a certain amount of salts, for if it be freed from salts, especially of those of calcium, by prolonged dialysis, it loses its solubility. If a small amount of calcium salts be added, however, it regains that property. Muscle serum is acid in reaction, and almost colorless. It contains three proteid bodies, viz. — (a.) A globulin (myo-globulin), which can be THE MUSCULAK SYSTEM. 403 precipitated by saturation with sodium chloride, or magnesium sulphate, and which can he coagulated at 63° C. (b.) Serum-albumin, which coagulates at 73° 0., but is not precipitated by saturation with either of those salts. And (c.) Myo-albumin, which is neither precipitated by heat, nor by saturation with sodium chloride or magnesium sulphate, but may be by saturation with ammonium sulphate. It is closely con- nected with, even if it is not itself, myosin ferment. Neither casein nor peptone has been found by Halliburton in muscle extracts. In extracts of muscles, especially of red muscles, there is a certain amount of Hce- moglobin, and also of a pigment special to muscle, called by McMunn Myo-hcematin, which has a spectrum quite distinct from hsemoglobin, viz., a narrow band just before D, two very narrow between D and E, and two other faint bands, near the violet, E b, and between E and F close to F (McMunn). B. Ferments. — In addition to muscle ferments, already mentioned, muscle extracts contain certain small amounts of pepsin and fibrin fer- ment, and also of an amyloly tic ferment. C. Acids, particularly sarco-lactic, also acetic and formic. D. Glycogen and Glucose, also Inosite. E. Nitrogenous crystalline bodies, such as Kreatin, Hypoxanthin, or carnin, Taurin and Urea, the last in very small amount. F. Salts, the chief of which is potassium phosphate. III. PHYSIOLOGY OF MUSCLE. Muscle may exist in three different conditions: A. during rest; B. during activity; and C. in rigor. A. Rest * ^X- Physical condition. — During rest or inactivity a muscle has a slight but very perfect Elasticity; it admits of being considerably stretched ; but returns readily and completely to its normal condition. In the liv- ing body the muscles are always stretched somewhat beyond their natural length; they are always in a condition of slight tension; an arrangement which enables the whole force of the contraction to be utilized in ap- proximating the points of attachment. It is obvious that if the muscles were lax, the first part of the contraction until the muscle became tight would be wasted. There is no doubt that even in a condition of rest Oxygen is abstract- ed from the blood, and carbonic acid is given out by a muscle; for the blood becomes venous in the transit, and since the muscles form by far the largest element in the composition of the body, chemical changes must be constantly going on in them as in other tissues and organs, al- though not necessarily accompanied by contraction. When cut out of 404: HANDBOOK OF PHYSIOLOGY. the body such muscles retain their contractility longer in an atmosphere of oxygen than in an atmosphere of hydrogen or carbonic acid, and dur- ing life, an amount of oxygen is no doubt necessary to the manifestation of energy as well as for the metabolism going on in the resting condi- tion. The reaction of living muscle in a resting or inactive condition is neutral or faintly alkaline. In muscles which have been removed from the body, it has been found that for some little time electrical currents can be demonstrated passing from point to point on their surface; but as soon as the muscles die or enter into rigor mortis, these currents disappear. The Method of Demonstration usually employed is as follows: — The frog's muscles are the most convenient for experiment; and a muscle of regular shape, in which the fibres are parallel, is selected. The ends FIG. 282.— Diagram of Du Bois Raymond's non-polarizable electrodes, a, glass tube filled with a saturated solution of zinc sulphate, in the end, c, of which is china clay drawn out to a point; in the solution a well amalgamated zinc rod is immersed and connected, by means of the wire which passes through A, with the galvanometer. The remainder of the apparatus is simply for conveni- ence of application. The muscle and the end of the second electrode are to the right of the figure. are cut off by clean vertical cuts, and the resulting piece of muscle is called a regular muscle prism. The muscle prism is insulated, and a pair of non-polarizable electrodes connected with a very delicate galva- nometer is applied to various points of the prism, and by a deflection of the needle to a greater or less extent in one direction or another, the strength and direction of the currents in the piece of muscle can be esti- mated. It is necessary to use non-polarizable and not metallic electrodes in this experiment, as otherwise there is no certainty that the whole of the current observed is communicated from the muscle itself, and is not derived from the metallic electrodes, in consequence of the action of the saline juices of the tissues upon them. The form of the non-polarizable electrodes is a modification of du Bois Reymond's apparatus (Fig. 282), which consists of a somewhat flattened glass cylinder a, drawn abruptly to a point, and fitted to a socket capable of movement, and attached to a stand A, so that it can be raised or lowered as required. The lower portion of the cylinder is filled with china clay moistened with saline so- THE MUSCULAR SYSTEM. 405 lution, part of which projects through its drawn-out point; the rest of the cylinder is fitted with a saturated solution of zinc sulphate into which dips a well amalgamated piece of zinc which is connected by means of a wire with the galvanometer. In this way the zinc sulphate forms a homogeneous and non-polarizable conductor between the zinc and the china clay. A second electrode of the same kind is, of course, neces- sary. In a regular muscle prism the currents are found to be as follows: — If from a point on the surface a line — the equator — be drawn across the muscle prism equally dividing it, currents pass from this point to points away from it, which are weak if the points are near, and increase in strength as the points are further and further away from the equator; the strongest passing from the equator to a point representing the mid- dle of the cut ends (Fig. 283, 2); currents also pass from points nearer the equator to those more remote (Fig. 283, 1, 3, 4), but not from points equally distant, or iso electric points (Fig. 283, 6, 7, 8). The cut ends FIG. 233. —Diagram of the currents in a muscle prism. (Du Bois Beymond.) are always negative to the equator. These currents are constant for some time after removal of the muscle from the body, and in fact re- main as long as the muscle retains its life. They are in all probability due to chemical changes going on in the muscles. The currents are diminished by fatigue and are increased by an in- crease of temperature within natural limits. If the uninjured tendon be used as the end of the muscle, and the muscle be examined without removal from the body, the currents are very feeble, but they are at once much increased by injuring the muscle, as by cutting off its tendon. The last observation appears to show that they are right who believe that the currents do not exist in muscles uninjured in situ, but that injury, either mechanical, chemical or thermal, will render the injured part electrically negative to other points on the muscle. In a frog's heart it has been shown, too, that no currents exist during its inactivity, but that as soon as it is injured in any way they are developed; the injured part being negative to the rest of the muscle. The currents which have 406 HANDBOOK OF PHYSIOLOGY. been above described are called either natural muscle currents or cur- rents of rest, according as they are looked upon as always existing in muscle or as developed when a part of the muscle is subjected to injury; in either case, up to a certain point, it is agreed that the strength of the currents is in direct proportion to the injury. B. Activity. The property of muscular tissue, by which its peculiar functions are exercised, is its Contractility, which 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 property, although commonly brought into action through the nervous system, is inherent in the muscular tissue. For — (1.) it may be manifested in a muscle which is isolated from the influence of the nervous system by division of the nerves supplying it, so long as the natural tissue of the muscle is duly nourished; and (2.) it is manifest in a portion of muscular fibre, in which, under the microscope, no nerve-fibre can be traced. (3.) Sub- stances such as urari, which paralyze the nerve-endings in muscles, do not at all diminish the irritability of the muscle. (4.) When a muscle is fatigued, a local stimulation is followed by a contraction of a small part of the fibre in the immediate vicinity without any regard to the dis- tribution of nerve-fibres. If the removal of nervous influence be long continued, as by division of the nerves supplying a muscle, or in cases of paralysis of long-stand- ing, 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 im- paired nutrition of the muscular tissue, which ensues through its inac- tion. The irritability of muscles is also of course soon lost, unless a supply of arterial blood to them is kept up. Thus, after ligature of the main arterial trunk of a limb, the power of moving the muscles is par- tially or wholly lost, until the collateral circulation is established; and when, in animals, the abdominal aorta is tied, the hind legs are ren- dered almost powerless. The same fact may be readily shown by compressing the abdominal aorta in a rabbit for about 10 minutes; if the pressure be released and the animal be placed on the ground, it will work itself along with its front legs, while the hind legs sprawl helplessly behind. Gradually the muscles recover their power and become quite as efficient as before. So, also, it is to the imperfect supply of arterial blood to the muscu- lar tissue of the heart, that the cessation of the action of this organ in asphyxia is in some measure due. Besides the property of contractility, the muscles, especially the striated, possess Sensibility by means of the sensory nerve-fibres distrib- THE MUSCULAR SYSTEM. 407 uted to them. The amount of common sensibility in muscles is not great; for they maybe 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 is shown in that their nerves can communicate to the mind an accurate knowl- edge of their states and positions when in action. By this sensibility we are not only made conscious of the morbid 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 en- abled to estimate and compare their weight and resistance by the effort of which we are conscious in measuring, moving, or raising them. The Phenomena of Muscular Contraction. The power which muscles possess of contraction may then be called forth by stimuli of various kinds, and these stimuli may also be applied directly to the muscle or indirectly to the nerve supplying it. There are distinct advantages, however, in applying the stimulus through the nerves, as it is more convenient, as well as more potent. The stimuli are of four kinds, viz. : — (1.) Mechanical stimuli, as by a blow, pinch, prick of the muscle or its nerves, will produce a contraction, repeated on the repetition of the stimulus; but if applied to the same point for a limited number of times only, as such stimuli will soon destroy the irritability of the preparation. (2.) Thermal stimuli. — If a needle be heated and applied to a muscle or its nerve, the muscle will contract. A temperature of over 100° F. (37.8° C.) will cause the muscles of a frog to pass into a condition known as heat rigor. (3.) Chemical stimuli. — A great variety of chemical substances will excite the contraction of muscles, some substances being more potent in irritating the muscle itself, and other substances having more effect upon the nerve. Of the former may be mentioned, dilute acids, salts of cer- tain metals, e. g., zinc, copper and iron; to the latter belong strong glycerin, strong acids, ammonia and bile salts in strong solution. (4.) Electrical Stimuli. — For the purpose of experiment electrical stimuli are most frequently used, as -the strength of the stimulus may be more conveniently regulated. Any form of electrical current may be employed for this purpose, but galvanism or the induced current is usually chosen. (1.) Galvanic currents are usually obtained by the employment of a continuous current battery such as^that of Daniell, by which an elec- trical current which varies but little in intensity is obtained. The bat- tery (Fig. 284) consists of a positive plate of well-amalgamated zinc im- mersed in a porous cell, containing dilute sulphuric acid; and this cell is again contained within a larger copper vessel (forming the negative 408 HANDBOOK OF PHYSIOLOGY. plate), containing besides a saturated solution of copper sulphate. The electrical current is made continuous by the use of the two fluids in the following manner. The action of the dilute sulphuric acid upon the zinc plate partly dissolves it, and liberates hydrogen, and this gas passes through the porous vessel, and decomposes the copper sulphate into cop- per and sulphuric acid. The former is deposited upon the copper plate, and the latter passes through the porous vessel to renew the sulphuric acid which is being used up. The copper sulphate solution is renewed by spare crystals of the salt, which are kept on a little shelf attached to the copper plate, and slightly below the level of the solution in the vessel. The current of electricity supplied by this battery will continue without variation for a considerable time. Other continuous current batteries, such as Grove's, may be used in place of Daniell's. The way in which the apparatus is arranged is to attach wires to the copper and zinc plates, and to bring them to a key, which is a little apparatus for connecting the wires of a battery. One often employed is Du Bois Rey- mond's (Fig. 287, D); it consists of two pieces of brass about an inch Fio. 284.— Diagram of a Daniell's battery. long, in each of which are two holes for wires and binding screws to hold them tightly; these pieces of brass are fixed upon a vulcanite plate, to the under surface of which is a screw clamp by which it can be secured to the table. The interval between the pieces of brass can be bridged over by means of a third thinner piece of similar metal fixed by a screw to one of the brass pieces, and capable of movement by a handle at right angles, so as to touch the other piece of brass. If the wires from the battery are brought to the inner binding screws, and the bridge connects them, the current passes across it and back to the battery. Wires are connected with the outer binding screws, and the other ends are approxi- mated for about two inches, but. being covered except at their points, are insulated, the uncovered points are about an eighth of an inch apart,, These wires are the electrodes, and the electrical stimulus is applied to the muscle, if they are placed behind its nerve, and the connection be- tween the two brass plates of the key be broken by depressing the handle of the bridge, and so raising the connecting piece of metal. The key is then said to be opened. (2.) An induced current is developed by means of an apparatus, called an induction coil, and the oneemployed for physiological purposes is mostly Du Bois Reymond's, the one seen in Fig. 285. THE MUSCULAR SYSTEM. 409 Wires from a battery are brought to the two binding screws d' and d, a key intervening. These binding screws are the ends of a coil of coarse covered wire c, called the primary coil. The ends of a coil of finer covered wire g, are attached to two binding screws to the left of the figure, one only of which is visible. This is the secondary coil, and is capable of being moved nearer to c along a grooved and graduated scale. To the binding screws to the left of ff, the wires of electrodes used to stimulate the muscle are attached. If the key in the circuit of wires from the battery to the primary coil (primary circuit) be closed, the cur- rent from the battery passes through the primary coil, and across the key to the battery, and continues to pass as long as thekey continues closed. At the moment of closure of the key, at the exact instant of the completion of the primary circuit, an instantaneous current of elec- tricity is induced in the secondary coil g, if it be sufficiently near; and the nearer it is to c, the stronger is the current induced. The current is only momentary in duration, and does not continue during the whole of the period whilst the primary circuit is complete. When, however, ff FIG. 285.— Du Bois Raymond's induction coil. the primary current is broken by opening the key, a second, also momen- tary, current is induced in g. The former induced current is called the making, and the latter the breaking shock; the former is in the opposite direction to, and the latter in the same as, the primary current. The induction coil may be used to produce a rapid series of shocks by means of another and accessory part of the apparatus at the right of the fig., called the magnetic interrupter. If the wires from a battery are connected with the two pillars by the binding screws, one below c, and the other, a, the course of the current is indicated in Fig. 286, the direc- tion being indicated by the arrows. The current passes up the pillar from c, and along the springs if the end of d' is close to the spring, the cur- rent passes to the primary coil c, and to wires covering two upright pillars of soft iron, from them to the pillar a, and out by the wires to the battery; in passing along the wire, #, the soft iron is converted into a magnet, and so attracts the hammer,/, of the spring, breaks the connec- tion of the spring with d' , and so cuts off the current from the primary coil, and also from the electro-magnet. As the pillars, #, are no longer 410 HANDBOOK OF PHYSIOLOGY. magnetized the cpring is released, and the current passes in the first di- rection, and is in like manner interrupted. At each make and break of the primary current, currents corresponding are induced in the secon- dary coil. These currents are opposite in direction, but are not equal in intensity, the break shock being greater. In order that the shocks should be nearly equal at the make and break, a wire (Fig. 286 e') con- nects e and d', and the screw d' is raised out of reach of the spring, and d is raised (as in Fig. 286), so that part of the current always passes through the primary coil and electro-magnet. When the spring touches d, the current in b is diminished, but never entirely withdrawn, and the primary current is altered in intensity at each contact of the spring with d, but never entirely broken. Record of Muscular Contraction under Stimuli.— The muscles of the frog are most convenient for the purpose of recording contractions. The frog is pithed, that is to say, its central nervous system is entirely destroyed by the insertion of a stout needle into the spinal cord, and the parts above it. One of its lower extremities is used in the following manner. The large trunk of the sciatic nerve is dissected out at the back of the thigh, and a pair of electrodes is inserted behind it. The FIG. 286.— Diagram of the course of the current in the magnetic interrupter of Du Bois Rey- xnoncTs induction coil. (He]mholz1s modification.) tendo Achillis is divided from its attachment to the os calcis, and a liga- ture is tightly tied round it. This tendon is part of the broad muscle of the thigh, (gastrocnemius), which arises from above the condyles of the femur. The femur is now fixed to a board covered with cork, and the ligature attached to the tendon is tied to the upright of a piece of metal bent at right angles (Fig. 287, B), which ir capable of movement about the pivot at its knee, the horizontal portion carrying a writing lever (myograph). When the muscle contracts, the lever is raised. It is necessary to attach a small weight to the lever. In this arrangement the muscle is in situ, and the nerve disturbed from its relations as little as possible. The muscle may, however, be detached from the body with the lower end of the femur from which it arises, and the nerve going to it may be taken away with it. The femur is divided at about the lower third. The bone is held in a firm clamp, the nerve is placed upon two electrodes con- nected with an induction apparatus, and the lower end of the muscle is connected by means of a ligature attached to its tendon with a lever which can write on a recording apparatus. To prevent evaporation this so-called nerve-muscle preparation is THE MUSCULAR SYSTEM. 411 placed under a glass shade, the air in which is kept moist by means of blotting paper saturated with saline solution. Effects of a Single Induction Shock. — With a nerve-muscle pre- paration arranged in either of the above ways, on closing or opening the key in the primary circuit, we obtain and can record a contraction, and if we use the clockwork apparatus revolving rapidly, a curve is traced such as is shown in Fig. 288. Another way of recording the contraction is by the pendulum myo- graph (Fig. 289). Here the movement of the pendulum along a certain arc is substituted for the clockwork movement of the other apparatus. The pendulum carries a smoked glass plate upon which the writing lever of a rnyograph is made to mark. The opening or breaking shock is sent Fia. 287.— Arrangement of the apparatus necessary for recording muscle contractions with a revolving cylinder carrying smoked paper. A, revolving cylinder; B, the frog arranged upon a cork- covered board which is capable of being raised or lowered on the upright, which also can be mov tendon of the gastrocnemi - , ed along a solid triangular bar of metal attached to the base of the recording apparatus— the don of the gastrocnemius is attached to the writing lever, properly weighted, by a ligature. The electrodes from the secondary coil pass to the apparatus— being, for the sake of conveni- ence, first of all brought to a key, D (Du Bois Reymond's); C. the induction coil; F, the battery (in this fig. a bichromate one) ; E, the key (Morse's) in the primary circuit. into the nerve-muscle preparation by the pendulum in its swing opening a key (Fig. 289, C.) in the primary circuit. Single Muscle Contraction.— The tracing obtained of a single muscle contraction (muscle curve) is seen in Fig. 288, and may be thus explained. The upper line (m) represents the curve traced by the end of the lever after stimulation of the muscle by a single induction-shock: the middle 412 HANDBOOK OF PHYSIOLOGY. line (I) is that described by the marking-lever, and indicates by a sudden drop the exact instant at which the induction-shock was given. The lower wavy line (t) is traced by a vibrating tuning-fork, and serves to measure precisely the intervals of time occupied in each part of the con- traction. It will be observed that after the stimulus has been applied, as indi- cated by the vertical line s, there is an interval before the contraction commences, as indicated by the line c. This interval, termed (a) the latent period, when measured by the number of vibrations of the tun- ing-fork between the lines s and c, is found to be about yfoj- sec. The latent period is longer in some muscles than in others, and differs also according to the condition of the muscle, being longer in fatigued mus- cles, and the kind of stimulus employed. During the latent period there is no apparent change in the muscle. FIG. 288. — Muscle curve obtained by means of the pendulum myograph. s. indicates the exact instant of the induction shock; c, commencement; and m x, the maximum elevation of lever; t> the line of a vibrating tuning-fork. (M. Foster.) The second part is the (b) stage of contraction proper. The lever is raised by the sudden contraction of the muscle. The contraction ist at first very rapid, but then progresses more slowly to its maximum, in- dicated by the line m x, drawn through its highest point. It occupies in the figure Tfo sec. (c) The next stage, stage of elongation. After reaching its highest point, the lever begins to descend, in consequence of the elongation of the muscle. At first the fall is rapid, but then be- comes more gradual until the lever reaches the abscissa or base line, and the muscle attains its precontraction length, indicated in the figure by the line c'. This stage occupies T|~o second. Very often after the main contraction the lever rises once or twice to a slight degree, producing curves, one of which is seen in Fig. 290. These contractions, due to the elasticity of the muscle, are called most properly (d) Stage of elastic after-vibration, or contraction remainder. THE MUSCULAR SYSTEM. 413 The muscle curve obtained from the heart resembles that of unstriped muscles in the long duration of the effect of stimulation; the descending curve also is very much prolonged. The greater part of the latent period is taken up by changes in the muscle itself, and the remainder occupied in the propagation of the shock along the nerve. Tetanus. — If we stimulate the nerve-muscle preparation with two induction shocks, one immediately after the other, when the point of stimulation of the second one corresponds to the maximum of the first, a second curve (Fig. 290) will occur, which will commence at the highest point of the first and will rise nearly as high, so that the sum of the Fia. 289.— Simple form of pendulum myograph and accessory parts. A, pivot upon which pen- dulum swings; 'B, catch on lower end of myograph opening the key, C, in its swing; Z>, a spring- catch which retains myograph, as indicated by dotted lines, and on pressing down the handle of which the pendulum swings along the arc to D on the left of figure, and is caught by its spring. height of the two curves almost exactly equals twice the height of the first. If a third and a fourth shock be passed, a similar effect will ensue, and curves one above the othe* will be traced, the third being slightly less than the second, and the fourth than the third. If a more numerous series of shocks occur, hov/ever, the lever after a time ceases to rise any further, and the contraction, which has reached its maximum, is main- tained. The condition which ensues is called Tetanus. A tetanus is really a summation of contractions, and unless the stimuli become very rapid indeed, the muscle will be then in a condition of vibratory con- traction and not of unvarying contraction. 414 HANDBOOK OF PHYSIOLOGY. If the shocks, however, be repeated at very short intervals, being 15 per second for the frog's muscle, but varying in each animal, the muscle contracts to its utmost suddenly and continues at its maximum contrac- FIG. 290.— Tracing of a double muscle-curve. To be read from left to right. While the muscle was engaged in the first contraction (whose complete course, had nothing intervened, is indicated by the dotted line), a second induction-shock was thrown in, at such a time that the second contrac- tion began just as the first was beginning to decline. The second curve is seen to start from the first, as does the first from the base line. (M. Foster.) tion for some time and the lever rises almost perpendicularly, and then describes a straight line (Fig. 292). If the stimuli are not quite so FIG. 291 . — Curve of tetanus, obtained from the gastrocnemius of a frog, where the shocks were sent in from an induction coil, about sixteen times a second, by the interruption of the primary current by means of a vibrating spring, which dipped into a cup of mercury, and broke the primary current at each vibration. rapid the line of maximum contraction becomes somewhat wavy, indi- FIG. 292.— Curve of tetanus, from a series of very rapid shocks from a magnetic interrupter. eating a slight tendency of the muscle to relax during the intervals be- tween the stimuli (Fig. 291). THE MCSCULAK SYSTEM. 415 Muscular Work. — We have seen that work is estimated by multi- plying the weight raised, by the height through which it has been lifted. It has been found that in order to obtain the maximum of work, a muscle must be moderately loaded: if the weight is increased beyond a certain point, the muscle becomes strained and raises the weight through so small a distance that less work is accomplished. If the load is still fur- ther increased the muscle is completely overtaxed, and cannot raise the FIG. 293.— Diagram of fatigue muscle-curves. (Ray Lankester.) weight. No work is then done at all. Practical illustrations of these facts must be familiar to every one. The power of a muscle is usually measured by the maximum weight which it will support without stretching. In man this is readily determined by weighting the body to such an extent, that it can no- longer be raised on tiptoe: thus the power of the calf-muscles is deter- mined. The power of muscle thus estimated depends of course upon its cross section. The power of a human muscle is from two to three times as great as a frog's muscle of the same sectional area. Fatigue of Muscle. — A muscle becomes rapidly exhausted from re- peated'stimulation, and the more rapidly, the more quickly the induc- tion-shocks succeed each other. This is indicated by the diminished height of the muscular contractions. It will be seen in Fig. 293 that the vertical lines, which indicate the extent of the muscular contraction, decrease in length from left to right. The line A B drawn along the tops of these lines is termed the " fatigue curve." It is usually a straight line. In the first diagram the effects of a short rest are shown: there is a pause of three minutes, and when the muscle is again stimulated, it contracts up to A', but the recovery is only temporary, and the fatigue 416 HANDBOOK OF PHYSIOLOGY. curve, after a few more contractions, becomes continuous with that be- fore the rest. In the second diagram is represented the effect of a stream of oxy- genated blood. Here we have a sudden restoration of energy: the muscle in this case makes an entirely fresh start from A, and the new fatigue curve is parallel to, and never coincides with the old one. A fatigued muscle has a much longer latent period than a fresh one. The slowness with which muscles respond to the will when fatigued must be familiar to every one. In a muscle which is exhausted, stimulation only causes a contraction producing a local bulging near the point irritated. A similar effect may be produced in a fresh muscle by a sharp blow, as in striking the biceps smartly with the end of the hand, when a hard muscular swelling is in- stantly formed. Accompaniments of Muscular Contraction. (1.) Heat is developed in the contraction of muscles. Becquerel and Breschet found, with the thermo-multiplier, about 1° Fahr. 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°. This estimate is probably high, as in the frog's muscle a consider- able contraction has been found to produce an elevation of temperature equal on an average to less than J° C. It is not known whether this development 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. (2.) Sound is said to be produced when muscles contract forcibly, as mentioned above. Wollaston showed that this sign 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 rumbling sound is heard, the height and loud- ness 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. (3.) Changes in Shape. — The 'mode of contraction in the trans- versely striated muscular tissue has been much disputed. The most probable account is, that the contraction is effected by an approxima- tion of the constituent parts of the fibrils, which, at the instant of con- traction, without any alteration in their general direction, become closer, flatter, and wider; a condition which is rendered evident by the approx- imation 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, THE MUSCULAR SYSTEM. 417 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 con- tents. 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 at- tended with any diminution 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 experiment is carefully performed, the level of the water in the tube remains the same, whether the muscle be con- tracted or not. In thus shortening, muscles appear to swell up, becoming rounder, more prominent, harder, and apparently tougher. But this hardness of FIG. 294.— The microscopic appearances during a muscular contraction in the individual fibrillae after Engelmann, 1. A passive muscle fibre; c to d = doubly refractive discs, with median disc a b in it; h and g are lateral discs; f and e are secondary discs, only slightly doubly refractive; fig. on right same fibre in polarized light; bright part is doubly refracted, black ends not so. 2. Transi- tion stage; and 3. Stage of entire contraction; hi each case the right-hand figure represents the effect of polarized light. (Landois after Engelmann.) 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 resistance ordinarily opposed to their contraction. When no resistance is offered, as when a muscle is cut off from its tendon, not only is no hardness perceived during contraction, but the muscular tis- sue is even softer, more extensile, and less elastic than in its ordinary uncontracted state. During contraction in each fibre it is said that the anisotropous or doubly refractive elements become less refractive and the singly refractive more so (Fig. 294). (4.) Chemical changes.— (a) The reaction of the muscle which is normally alkaline or neutral becomes decidedly acid, from the develop- ment of sarcolactic acid, (b) The muscle gives out carbonic acid gas and takes up oxygen, the amount of the CO, given out not appearing to be entirely dependent upon the 0 taken in, and so doubtless in part 418 HANDBOOK OF PHYSIOLOGY. arising from some other source, (c) Certain imperfectly understood chemical changes occur, in all probability connected with (a) and (#). Glycogen is diminished, and glucose, or muscle sugar (inosite) appears; the extractives are increased. (5.) Electrical changes.— When a muscle contracts the natural muscle current or currents of rest undergo a distinct diminution, which is due to the appearance in the actively contracting muscle of currents in an opposite direction to those existing in the muscle at rest. This causes a temporary deflection of the needle of a galvanometer in a direction op- posite to the original current, and is called by some the negative varia- tion of the muscle current, and by others a current of action. Conditions of Contraction. — (a) The irritability of muscle, as in^ dicated by length of latent -period, velocity and extent of contraction, is greatest at a certain mean temperature; (#) after a number of contrac- tions a muscle gradually becomes exhausted; (c) the activity of muscles after a time disappears altogether when they are removed from the body or the arteries are tied; (d) oxygen is used up in muscular contraction, FIG. 295.— Muscle-curves from the gastrocnemius of a frog, illustrating effects of alterations in temperature. but a muscle will act for a time in vacuo or in a gas which contains no oxygen: in this case it is of course using up the oxygen already in store; (e) the contraction is greater if the stimulus is applied to the nerve, than if it be applied to the muscle directly. Response to Stimuli. — The two kinds of fibres, the striped and the 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 the respective differences of structure, but to some degree, possibly, to their respective modes of connection with the nervous system. When irritation is applied 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 instanta- neous, and ceases on the instant of withdrawing the irritation. But when any part with unstriped muscular fibres, e. g.9 the intestines or bladder, is irritated, the subsequent contraction ensues more slowly, ex- tends beyond the part irritated, and, with alternating relaxation, con- tinues for some time after the withdrawal of the irritation. The differ- ence in the modes of contraction of the two kinds of muscular fibres may be particularly illustrated by the effects of the repeated stimuli THE MUSCULAK SYSTEM. 419 with the magnetic interrupter. The rapidly succeeding shocks given by this means to the nerves of muscles excite in all the transversely-striated muscles, except in the case of the heart, a fixed state of tetanic contrac- tion as previously described, which lasts as long as the stimulus is con- tinued, and on its withdrawal instantly ceases; but in the muscles with nnstriped fibres they excite a slow vermicular movement; which is com- paratively slight and alternates with rest. It continues for a time after the stimulus is withdrawn. In their mode of responding to these stimuli, all the skeletal muscles, or those with transverse striae, are alike; but among those with unstriped fibres there are many differences — 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. 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, and 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 blad- der 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 regu- lar, and more sustained; and they require no more stimulus than that of the air to excite them. The heart, on account, doubtless, of its striated muscle, 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 others respects, to be the connecting member of the two classes of muscles. All the muscles retain their property of contracting under the influ- ence 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 respiratory 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 irri- tability 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 fol- lowing 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 trunk, lower and upper extremities; lastly, in the right and left auricle of the heart. C. Rigor Mortis. After the muscles of the dead body have lost their irritability or capa- bility of being excited to contraction by the application of a stimulus, they spontaneously pass into a state of contraction, apparently identical with that which ensues during life. It affects all the muscles of the body; and, where external 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 420 HANDBOOK OF PHYSIOLOGY. of the trunk, is produced a general stiffening of the body, constituiug the rigor mortis or post-mortem rigidity. When this condition has set in, the muscle (a) becomes acid in reaction (due to development of sarco-lactic acid), (b) gives off carbonic acid in great excess, (c) Its volume is slightly diminished; (d) the muscul&Y fibres become shortened and opaque, and their substance sets firm. It comes on much more rapidly after muscular activity, and is hastened by warmth. It may be brought on, in muscles exposed for experiment, by the action of distilled water and many acids, also by freezing and thaw- ing again. Cause. — The immediate cause of rigor seems to be a chemical one,, namely, the coagulation of the muscle plasma. We may distinguish, three main stages — (1.) Gradual coagulation. (2.) Contraction of coagu- lated muscle-clot (myosin), and squeezing out of muscle-serum. (3.) Putrefaction. After the first stage, restoration is possible through the circulation of arterial blood through the muscles, and even when the second stage has set in, vitality may be restored by dissolving the coagti- lum of the muscle in salt solution, and passing arterial blood through its vessels. In the third stage recovery is impossible. Order of Occurrence. — The muscles are not affected simultaneously by rigor mortis. It affects the neck and lower jaw first; next, the upper extremities, extending from above downwards; and lastly, reaches the lower limbs; in some rare instances only, it affects the lower extremities 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 ex- tremities, and lastly, in the lower extremities. 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. Heat is developed during the passage of a muscular fibre into the condi- tion of rigor mortis. Since rigidity does not ensue until muscles have lost the capacity of being excited by external stimuli, it follows that all circumstances which cause a speedy exhaustion of muscular irritability, induce an early occur- rence 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 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 experi- ments of Brown-Sequard make it probable that the rigidity may THE MUSCULAR SYSTEM. 431 supervene immediately after death, and then pass away with such rapidity as to be scarcely observable. Experiments. — Brown-Sequard took five rabbits, and killed them by removing their hearts. In the first, rigidity came on in 10 hours, and lasted 192 hours; in the second, which was feebly electrified, it com- menced in 7 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-gal- vanic 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 ordinary electric discharge, the rigidity may ensue so early after death, and pass away so rapidly as to escape detection. The in- fluence exercised upon the onset and duration of post-mortem rigidity by causes which exhaust the irritability of the muscles, was well illus- trated 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 muscles which had been powerfully electrified just before death than those which had not been thus acted upon. The occurrence of rigor mortis is not prevented by the previous exist- ence 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 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 contraction of the muscles with unstriped fibres is shown by an experiment of Valentin, who found that if a gradu- ated tube connected with a portion of intestine taken from a recently- killed 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. 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 removed, 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. Actions of the Voluntary Muscles. The greater part of the voluntary musclss of the body act as sources of power for removing levers — the latter consisting of the various bones to which the muscles are attached. Examples of the three orders of levers in the Human Body.— .All levers have been divided into three kinds, according to the relative 422 HANDBOOK OF PHYSIOLOGY. position of the power, the loeight to be removed, and the axis of motion or fulcrum. In a lever of the first kind the poiuer 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. 296, 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 mentioned the act of raising the body FIG. 296. from the stooping posture by means of the hamstring muscles attached to the tuberosity of the ischium (Fig. 296). 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 wheel- barrow, or in stretching an elastic band, as in Fig. 297. In the human El. AST! FIG. 297. body the act of opening the mouth by depressing the lower jaw is an ex- ample of the same kind — the tension of the muscles which close the jaw representing the weight (Fig. 297). In a lever of the third kind the arrangement is — F.P.W., and the act of raising a pole, as in Fig. 298, is an example. In the human body there are numerous examples of the employment of this kind of leverage. The act of bending the fore-arm may be mentioned as an instance (Fig.. 298). The act of biting is another example. THE MUSCULAR SYSTEM. 423 At the ankle we have examples of all three kinds of lever. 1st kind — Extending the foot. 3d kind — Flexing the foot. In both these cases the foot represents the weight: the ankle joint the fulcrum, the power being the calf muscles in the first case and the tibialis anticus in the second case. 2d kind — When the body is raised on tip-toe. Here the ground is the fulcrum, the weight of the body acting at the ankle joint the weight, and the calf muscles the power. In the human bod}T, 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 to produce motion. It is also evident, however, from the same diagrams, that by the closeness of the power to the ful- crum a great range of movement can be obtained by means of a com- paratively slight shortening of the muscular fibres. The greater number of the more important muscular actions of the human body — those, namely, which are arranged harmoniously so as to FIG. 298. subserve some definite purpose or other in the animal economy — are de- scribed in various parts of this work, in the sections which treat of the physiology of the processes by which these muscular actions are resisted or carried out. There are, however, one or two very important and somewhat complicated muscular acts which may be 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 progres- sion, 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 astrag- alus, and with it, of course, the whole body, the weight of which is transmitted through the tibia to this bone (Fig. 298). 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 such a way, that it would fall prostrate were it not that the right foot is 424 HANDBOOK OF PHYSIOLOGY. 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 inclination forwards, it is plain that when the heel is raised by the calf-muscles, the whole body will be raised, and pushed obliquely for- wards and upwards. The successive acts in taking the first step in walk- ing are represented in Fig. 299, 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 prostrate. This advance of the other leg (in this case the right) is effected partly by its mechanically swinging forwards, pendulum-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; 2dty, the hamstring muscles, which slightly bend the leg on the thigh; and 3f%, 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. The second part of the act of walking, which has been just described is shown in the diagram (4, Fig. 299). When the right foot has reached the ground the action of the left leg has not ceased. The calf-muscles of the latter continue 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 mus- cles on the front 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 mam kinds of leverage are, therefore, employed in the act of walking, and if this idea be firmly grasped, the details will be understood with compara- tive ease. On kind of leverage employed in walking is essentially the same with that employed in pulling forward ^the pole, as in Fig. 298. And the other, less exactly, is that employed in raising the handles of a wheelbarrow. Now, supposing the lower end of the pole to be placed in the barrow, we should have a very rough and inelegant, but not alto- gether bad lepresentation of the two main levers employed in the act of walking. The body is putted forward by the muscles in front, much in THE MUSCULAR SYSTEM. 425 the same way that the pole might be by the force applied at p. (Fig. 298), 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 bar- row. The manner 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 timepushed and pulled forwards by the mus- cles of the other, may be gathered from the previous description. There is one more thing to be noticed especially in the act of walk- ing. Inasmuch as the body is being constantly supported 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 way, and the manner in which it is effected is one ele- Fio. 300. ment in the differences which exist in the walking of different people. Thus it may be done by an instinctive slight rotation of the pelvis on the head of each femur in turn, in such a manner that the centre of grav- ity 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. 299, 3, the pelvis is instinctively by various muscles, made to rotate on the head of the left femur 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 swinging forwards, and leav- ing all the work of support to fall on its fellow. Such a "rocking" movement of the trunk and pelvis, however, is accompanied by a move- ment of the whole trunk and leg over the foot which is being planted on the ground (Fig. 300); the action being accompanied with a compensa- tory outward movement at the hip, more easily appreciated by looking at the figure (in which this movement is shown exaggerated) than de- scribed. 426 HANDBOOK OF PHYSIOLOGY. Thus the body in walking is continually rising and swaying alter- nately 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-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 imperfect idea of the infinite number of combined and harmoniously arranged muscular con- tractions which are necessary for even the simplest acts of locomotion. Action of the Involuntary Muscles.— The involuntary muscles 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. Ex- amples of this action are to be found in the intestines, urinary bladder, heart and blood-vessels, gall-bladder, gland-ducts, etc. The difference in the manner of contraction of the striated and non- striated fibres has been already referred to (p. 418); and the peculiar vermicular or peristaltic action of the latter fibres has been described at p. 419. Source of Muscular Action. — It was formerly supposed that each act of contraction on the part of a muscle was accompanied by a correla- tive waste or destruction of its own substance; and that the quantity of the nitrogenous excreta, especially of urea, presumably the expression of this waste, was in exact proportion to the amount of muscular work per- formed. It has been found, however, both that the theory itself is erro- neous, 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 e wear and tear' which may be represented by a slight increase in the quantity of urea excreted; but it is not the correlative 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 to believe that the waste of muscle-substance can be expressed, with unimportant ex- ceptions, in any other way than by an increased excretion of urea, it is THE MUSCULAR SYSTEM. 427 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 expression 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-nitro- genous 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 carbonic acid, at least, is. the result of chemical action in the system, and especially of the com- bustion of non-nitrogenous food, although, doubtless, of nitrogenous food also. We are, therefore, driven to the conclusion — that the sub- stance 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. ELECTRICAL CURRENTS is NERVES. The electrical condition of Nerves is so closely connected with the phenomena of muscular contraction, that it will be convenient to con- sider it in the present chapter. If a piece of nerve be removed from the body and su bjected to exam- ination in a way similar to that adopted in the case of muscle which has been described (p. 404), electrical currents are found to exist which cor- respond exactly to the natural muscle currents, and which are called natural nerve currents or currents of rest, according as one or other theory of their existence be adopted, as in the case with muscle. One point (equator) on the surface being positive to all other points nearer to the cut ends, and the greatest deflection of the needle of the galvano- meter taking place when one electrode is applied to the equator and the other to the centre of either cut end. As in the case of muscle, these nerve-currents undergo a negative variation when the nerve is stimulated, the variation being momentary and in the opposite direction to the natural currents; and are similarly known as the currents of action. The currents of action are propagated in both directions from the point of the application of the stimulus, and are of momentary duration. Rheoscopic Frog. — This negative variation may be demonstrated 428 HANDBOOK OF PHYSIOLOGY. by means of the following experiment. — The new current produced by stimulating the nerve of one nerve-muscle preparation may be used to stimulate the nerve of a second nerve-muscle preparation. The foreleg of a frog with the nerve going to the gastrocnemius cut long is placed upon a glass plate, and arranged in such way that its nerve touches in two places the sciatic nerve, exposed but preserved in situ, in the opposite thigh of the frog. The electrodes from an induction coil are placed behind the sciatic nerve of the second preparation, high up. On stimulating it with a single induction shock, the muscles not only of the same leg are found to undergo a twitch, but also those of the first prepa- ration, although this is not near the electrodes, and so the stimulation cannot be due to an escape of the current into the first nerve. This ex- periment is known under the name of the rheoscopic frog. Nerve-stimuli. — Nerve-fibres require to be stimulated before they can manifest any of their properties, since they have no power of them- selves of generating force or of originating impulses. The stimuli which are capable of exciting nerves to action, are, as in the case of muscle, very diverse. They are very similar in each case. The mechanical, chemical, thermal, and electric stimuli which may be used in the one case are also, with certain differences in the methods employed, effica- cious in the other. The chemical stimuli are chiefly these: withdrawal of water, as by drying, strong solutions of neutral salts of potassium, sodium, etc., free inorganic acids, except phosphoric; some organic acids; ether, chloroform, and bile salts. The electrical stimuli employed are the induction and continuous currents concerning which the observations in reference to muscular contraction should be consulted, p. 406. Weaker electrical stimuli will excite nerve than will excite muscle; the nerve stimulus appears to gain strength as it descends, and a weaker stimulus applied far from the muscle will have the same effect as a stronger one applied to the nerve near the muscle. It will be only necessary here to add some account of the effect of a constant electrical current, such as that obtained from DanielFs battery, upon a nerve. This effect may be studied witli the apparatus described before. A pair of electrodes are placed behind the nerve of the nerve- muscle preparation, with a Du Bois Reymond's key arranged for short circuiting the battery current, in such a way that when the key is opened the current is sent into the nerve, and when closed the current is cut off. It will be found that with a current of moderate strength there will be a contraction of the muscle both at the opening and at the closing of the key (called respectively making and breaking contrac- tions), but that during the interval between these two events the muscle remains flaccid, provided the battery current continues of constant in- intensity. If the current be a very weak or a very strong one the effect is not quite the same; one or other of the contractions may be absent. Which of these contractions is absent depends upon another circum- THE MUSCULAR SYSTEM. 429 stance, viz., the direction of the current. The direction of the current may be ascending or descending; if ascending, the anode or positive pole is nearer the muscle than the kathode or negative pole, and the current to return to the battery has to pass up the nerve; if descend- ing, the position of the electrodes is reversed. It will be necessary before considering this question further to return to the apparent want of effect of the constant current during the interval between the make and break contraction: to all appearances no effect is produced at all, but in reality a very important change is brought about in the nerve by the passage of this constant (polarizing) current. This may be shown in two ways, first of all by the galvanometer. If a piece of nerve be taken, and if at either end an arrangement be made to test the electrical condi- tion of the nerve by means of a pair of non-polarizable electrodes con- nected with a galvanometer, while to the central portion a pair of elec- trodes connected with a Daniell's battery be applied, it will be found that natural nerve-currents are profoundly altered on the passage of the con- Fio. 301.— Diagram illustrating the effects of curious intensities of the polarizing currents, n, n', nerve; a. anode; fc, kathode; the curves above indicate increase, and those below decrease of irritability, and when the current is small the increase and decrease are both small, with the neutral point near a, and so on as the current is increased in strength. stant current in the neighborhood. If the polarizing current be in the same direction as the latter the natural current is increased, but if in the direction opposite to it, the natural current is diminished. This change, produced by the continual passage of the battery-current through a por- tion of the nerve, is to be distinguished from the negative variation of the natural current to which allusion has been already made, and which is a momentary change occurring on the sudden application of the stim- ulus. The condition produced by the passage of a constant current is known by the name of Electrotonus. The other way of showing the effect of the same polarizing current is by taking a nerve-muscle preparation and applying to the nerve a pair of electrodes from an induction coil whilst at a point further removed from the muscle, electrodes from a Daniell's battery are arranged with a key for short circuiting and an apparatus (reverser) by which the bat- tery current may be reversed in direction. If the exact point be ascer- tained to which the secondary coil should be moved from the primary 430 HANDBOOK OF PHYSIOLOGY. coil in order that a minimum contraction be obtained by the induction ;shock, and the secondary coil be removed slightly from the primary, the induction current cannot now produce a contraction; but if the polariz- ing current be sent in a descending direction, that is to say, with the kathode nearest the other electrodes, the induction current, which was before insufficient, will prove sufficient to cause a contraction; whereby indicating that with a descending current the irritability of the nerve is increased. By means of a somewhat similar experiment it may be shown that an ascending current will diminish the irritability of a nerve. Sim- ilarly, if instead of applying the induction electrodes below the other electrodes they are applied between them, like effects are demonstrated, indicating that in the neighborhood of the kathode the irritability of the nerve is increased by a constant current, and in the neighborhood of the anode diminished. This increase in irritability is called katlielectrotonus, .and similarly the decrease is called anelectrotonus. As there is between the electrodes both an increase and a decrease of irritability on the pas- sage of a polarizing current, it must be evident that the increase must shade off into the decrease, and that there must be a neutral point where there is neither increase nor decrease of irritability. The position of the neutral point is found to vary with the intensity of the polarizing cur- rent— when the current is weak the point is nearer the anode, when strong nearer the kathode (Fig. 301); when a constant current passes into a nerve, therefore, if a contraction result, it may be assumed that it is due to the increased irritability produced in the neighborhood of the kathode, but the breaking contraction must be produced by a rise in irritability from a lowered state to the normal in the neighborhood of the anode. The contractions produced in the muscle of a nerve-muscle preparation by a constant current have been arranged in a table which is known as Pfliiger's Law of Contractions. It is really only a state- ment as to when a contraction may be expected: — Descending Current. Ascending Current. Make. Break. Make. Break. Weak Yes. Yes, Yes. No. Yes. No. Yes. Yes. No. No. Yes. Yes. Strong The difficulty in this table is chiefly in the effect of a weak current, but the following statement will explain it. The increase of irritability at the kathode is more potent to produce a contraction than the rise of irritability at the anode, and so with weak currents the only effect is a contraction at the make of both currents, and the descending current is THE MUSCULAR SYSTEM. 431 more potent than the ascending (and with still weaker currents is the only one which produces any effect), since the kathode is near the mus- cle; whereas in the case of the ascending current the stimulus has to pass through a district of diminished irritability, which with a very strong current acts as a block, but with a weak only slightly affects the contraction. As the current is stronger recovery from anelectrotonus is able to produce a contraction as well as kathelectrotonus, a contrac- tion occurs both at the make and the break of the current. The absence of contraction with a very strong current at the break of the ascending current may be explained by supposing that the region of fall in irrita- . bility at the kathode blocks the stimulus of the rise in irritability a t the anode. Thus we have seen that two circumstances influence the effect of the constant current upon the nerve, viz., the strength and direction of the current. It is also necessary that the stimulus should be applied sud- denly and not gradually, and that the irritability of the nerve be normal and not increased or diminished. Sometimes (when the nerve is spe- cially irritable ?) instead of a simple contraction a tetanus occurs at the make or break of the constant current. This is especially liable to oc- cur at the break of a strong ascending current which has been passing for some time into the preparation; this is called Ritter's tetanus, and may be increased by passing a current in an opposite direction or stopped by passing a current in the same direction. CHAPTER XV. NUTRITION ; THE INCOME AND EXPENDITURE OF THE HUMAN BODY. THE various physiological processes which occur in the human body have, with the exception of those in the nervous and generative systems, which will be considered in succeeding chapters, now been dealt with, and it will be as well to give in this chapter a summary of what has been considered more at length before. The subject may be considered under the following heads. (1.) The Evidence and Amount of Expenditure. (2.) The Sources and Amount of Income. (3.) The Sources and Objects of Expenditure. 1. Evidence and Amount of Expenditure. — There is complete evidence of Expenditure by the living body. From the table (p. 208) it will be seen how the various amounts of the excreta are calculated. a. From the Lungs there is exhaled every 24 hours, Of Carbonic Acid, about . . . 15,000 grains. Of Water, 5,000 " Traces of organic matter. b. From the Skin — Water, 11,500 grains. Solid and gaseous matter, . . . 250 " c. From the Kidneyi Water, 23,000 grains. Organic matter, 680 " Minerals and salines, .... 420 " d. From the Intestines — Water, 2,000 grains. Various organic and mineral substances, 800 ' ' In the account of Expenditure, must be remembered in addition the milk (during the period of suckling), and the products of secretion from the generative organs (ova, menstrual blood, semen); but, from their variable and uncertain amounts, these cannot be reckoned with the pre- ceding. Altogether, the expenditure of the body represented by the sum of these various excretory products amounts every 24 hours to — INCOME AND EXPENDITURE OF BODY. 433 Solid and gaseous matter, . .^ . 17,150 grains (1,113 grms.) Water (either fluid or combined with the solids and gaseous matter), . . 49,500 " (2,695 " ) The matter thus lost by the body is matter the chemical attractions of which have been in great part satisfied; and which remains quite use- less as food, until its elements have been again separated and re-arranged by members of the vegetable world. It is especially instructive to com- pare the chemical constitution of the products of expenditure, thus separated by the various excretory organs, with that of the sources of income to be immediately considered. It is evident from these facts that if the human body is to maintain its size and composition, there must be added to it matter corresponding in amount with that which is lost. The income must equal the expenditure. 2. Sources and Amount of Income. — The Income of the body consists partly of Food arid Drink, and partly of Oxygen. Into the stomach there is received daily: — Solid (chemically dry) food, . . . 8,000 grains (520 grms.) Water (as water, or variously combined with solid food), .... 35,000-40,000 " (2,444 " ) By the Lungs there is absorbed daily: — Oxygen, . 13,000 " (844 " ) The average total daily receipts, in the shape of food, drink and oxy- gen, correspond, therefore, with the average total daily expenditure, as shown by the following table. Income. Solid food, . . 8,000 grains. Water, . . 37,650 " Oxygen, . . 13,000 " 58,650 grains. (3,808 grms., or about 8J- Ib.) Expenditure. Lungs, . . 20,000 grains. Skin, . . . 11,750 " Kidneys, , . 24,100 " Intestines, . . 2,800 " . (Generative and mam- mary-gland products are supposed to be included) 58,650 grains, (about 3, 808 grms.) These quantities are approximate only. But they may be taken as fair averages for a healthy adult. The absolute identity of the two num- bers (in grains) in the two tables is of course diagrammatic. No such exactitude in the account occurs in any living body, in the course of any given twenty-four hours. But any difference which exists between the two amounts of income and expenditure at any given period, corresponds 28 434 HANDBOOK OF PHYSIOLOGY. merely with the slight variations in the amount of capital (weight of body) to which the healthiest subject is liable. The chemical composition of the food (p. 209) may be profitably com- pared with that of the excreta, as before mentioned. The greater part of our food is composed of matter which contains much potential energy; and in the chemical changes (combustion and other processes) to which it is subject in the body, active energy is manifested. 3. The Sources and Objects of Expenditure. — The sources of the necessary waste and expenditure in the living body are various and extensive. They may be comprehended under the following heads: — (1) Commoji wear and tear; such as that to which all structures, liv- ing and not living, are subjected by exposure and work; but which must be especially large in the soft and easily decaying structures of an animal body. (2) Manifestations of Force in the form either of Heat or Motion. In the former case (Heat), the combustion must be sufficient to maintain a temperature of about 100° F. (37.8° 0.) throughout the whole substance of the body, in all varieties of external temperature, notwithstanding the large amount continually lost in the ways previously enumerated. In the case of Motion, there is the expenditure involved in the (a) Ordinary muscular movements, as in Prehension, Mastication, Locomotion, and numberless other ways: as well as in (b) Various involuntary movements, as in Respiration, Circulation, Digestion, etc. (3) Manifestation of Nerve Force; as in the general regulation of all physiological processes, e. g., Respiration, Circulation, Digestion; and in Volition and all other manifestations of cerebral activity. (4) The energy expended in all physiological processes, e.g., Nutrition, Secretion, Growth, and the like. The total expenditure or total manifestation of energy by an animal body can be measured, with fair accuracy; the terms used being such as are employed in connection with other than vital operations. All state- ments, however, must be considered for the present approximate only, and especially is this the case with respect to the comparative share of expenditure to be assigned to the various objects just enumerated. The amount of energy daily manifested by the adult human body in (a) the maintenance of its temperature; (b) in internal mechanical work, as in the movements of the respiratory muscles, the heart, etc. ; and (c) in external mechanical work, as in locomotion and all other voluntary movements, has been reckoned at about 3,400 foot-tons. Of this amount only one-tenth is directly expended in internal and external mechanical work; the remainder being employed in the maintenance of the body's heat. The latter amount represents the heat which would be required to raise 48.4 Ib. of water from the freezing to the boiling point; or if INCOME AND EXPENDITURE OF BODY. 435 converted into mechanical power,, it would suffice to raise the body of a man weighing about 150 Ib. through a vertical height of 8£ miles. To the foregoing amounts of expenditure must be added the quite unknown quantity expended in the various manifestations of nerve-force, and in the work of nutrition and growth (using these terms in their widest sense). By comparing the amount of energy which should be produced in the body from so much food of a given kind, with that which is actually manifested (as shown by the various products of com- bustion, in the excretions) attempts have been made, indeed, to estimate, by a process of exclusion, these unknown quantities; but all such calculations must be at present considered only very doubtfully approxi- mate. Sources of Error. — Among the sources of error in any such calcula- tions must be reckoned, as a chief one, the, at present, entirely unknown extent to which forces external to the body (mainly heat) can be utilized by the tissues. We are too apt to think that the heat and light of the sun are directly correlated, as far as living beings are concerned, with the chemico-vital transformations involved in the nutrition and growth of the members of the vegetable world only. But animals, although comparatively independent of external heat and other forces, probably utilize them, to the degree occasion offers. And although the correlative manifestation of energy in the body, due to external heat and light, may still be measured in so far as it may take the form of mechanical work; yet, in so far as it takes the form of expenditure in nutrition or nerve- force, it is evidently impossible to include it by any method of estima- tion yet discovered; and all accounts of it must be matters of the purest theory. These considerations may help to explain the apparent discrep- ancy between the amount of energy which is capable of being produced by the usual daily amount of food, with that which is actually manifested daily by the body; the former leaving but a small margin for anything beyond the maintenance of heat, and mechanical work. In the foregoing sketch we have supposed that the excreta are exactly replaced by the ingesta. Nitrogenous Equilibrium and Formation of Fat. — If an animal, however, which has undergone a starving period, be fed upon a diet of lean meat it is found that instead of the greater part of the nitrogen being stored up, as one would expect, the chief part of it appears in the urine as urea, and continuing with the diet the excreted nitrogen ap- proximates more and more closely to the ingested nitrogen until at last the amounts are equal in both cases. This is called nitrogenous equilib- rium. There may, however, be an increase of weight which is due to the putting on of fat. If this is the case it must be apparent that the protoplasm of the tissues is able to form fat out of proteid material and to split it up into urea and fat. If fat be given in small quantities 436 HANDBOOK OF PHYSIOLOGY. with the meat, for a time the carbon of the egesta and ingesta are equal, but if the fat be increased beyond a certain point the body weight in- creases from a deposition of fat; not, however, by a mere mechanical deposition or filtration from the blood, but by an actual act of secretion by the protoplasm whereby the fat globules are stored up within itself: similarly as regards carbo-hydrates, if they are in small quantity, the carbon appears in the excreta, but beyond a certain amount a consider- able portion of it is retained in fat, having been by the protoplasm stored up within itself in that material. The amount of proteid material required to produce nitrogenous equilibrium is considerable, but it may be materially diminished by the addition of carbo-hydrate or fatty food, or of gelatin to the exclusively meat diet. It is of much interest to consider how the protoplasm acts in con- verting food into energy and decomposition products, since the sub- stance itself does not undergo much change in the process except a slight amount of wear and tear. We may assume that it is the property of protoplasm to separate from the blood the materials which it may re- quire to produce secretions, in the case of the protoplasm of secreting glands, or to enable it to evolve heat and energy, as in the case of the protoplasm of muscle. The substances are very possibly different for each process, and the decomposition products, too, may be different in quality or quantity. Proteid materials appear to be specially needed, as is shown by the invariable presence of urea in the urine even during starvation; and as in the latter case, there has been no food from which these materials could have been derived, the urea is considered to be de- rived from the disintegration of the nitrogenous tissues themselves. The removal of all fat from the body in a starvation period, as the first ap- parent change, would lead to the supposition that fat is also a specially necessary pabulum for the production of protoplasmic energy; and the fact that, as mentioned above, with a diet of lean meat an enormous amount appears to be required, suggests that in that case protoplasm ob- tains the fat it needs from the proteid food, which process must be evi- dently a source of much waste of nitrogen. The idea that proteid food has two destinations in the economy, viz., to form organ or tissue proteid which builds up organs and tissues, and circulating proteid, from which the organs and tissues derive the materials of their secretions, or for pro- ducing their energy, is a convenient one, and it is unlikely that proto- plasm would go to the expense of construction simply for the sake of immediate destruction. CHAPTER XVI. THE VOICE AND SPEECH. The Larynx.— In nearly all air-breathing vertebrate animals there are arrangements for the production of sound, or voice, in some parts of the respiratory apparatus. In many animals, the sound admits of being variously modified and altered during and after its production; and, in Cornumin: Cornu sup; •- ,«i, Sterno-hyoideua, m. Stcmo-hyoideua, »». Sterno-hyoidexia, . Crico-tliyroideus* g : crico-thyr. med; Cart: cricoidea«-. lag: crico-tracheae. • — Cart: tracheale. <£- Fio. 302. — The Larynx, as seen from the front, showing the cartilages and ligaments. The muscles, with the exceptions of one crico-thyroid, are cut off short. (Stoerk.) man, one such modification occurring in obedience to dictates of the cerebrum, is speech. It has been proved by observations on living subjects, by means of the laryngoscope (p. 441), 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 vibration of the inferior laryngeal ligaments, or true vocal cords (A, cv, Fig. 307) which bound the glottis, caused by currents of expired air impelled over their edges. If a free opening exists in the trachea, the sound of the voice ceases, but it returns if the opening is closed. An opening into the air-passages above the glottis, on the contrary, does not prevent the voice being produced. By forcing a current of air through the larynx in the dead subject, clear vocal sounds are elicited, though 438 HANDBOOK OF PHYSIOLOGY. 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 attach- ment, and be kept tense and so approximated that the fissure of the glot- tis may be narrow. The vocal ligaments or cords, therefore, are regarded as the proper organs for the production of vocal sounds: the modifications of these sounds being effected, as will be presently explained, by other parts — tongue, teeth, lips, etc., as well as by them. The structure of the vocal cords is adapted to enable them to vibrate like tense membranes, for they are essentially composed of elastic tissue; and they are so attached Carti~Wris6ergu Cart, SantorinL Cart, aryten. Trac. musciil. I jig"* nrlco-sryten, , LSg, Derafowa3cx>^-pQSt, Tsnp, Cormdnfer. Ug1. caratrcricD. jost. inf. S- 3?ats mcmbian. FIG. 303.— The larynx as seen from behind after removal of the muscles. The cartilages and ligaments only remain. (Stoerk.) 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. Thus it will be seen that the larynx is the organ of voice. It may be said to consist essentially of the two vocal cords and the various car- tilaginous, muscular, and other apparatus by means of which not only can the aperture of the larynx (rima glottidis), of which they are the lateral boundaries, be closed against the entrance and exit of the air to or from the lungs, but also by means of which the cords themselves 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. THE VOICE AND SPEECH. 439 Anatomy of the Larynx. — The principal parts entering into the formation of the larynx (Figs. 302 and 303) are — the thyroid cartilage; the cricoid cartilage; the two arytenoid cartilages; and the two true vocal oords (Fig. 307). The epiglottis (Fig. 303) has but little to do with the voice, and is chiefly useful in protecting the upper part of the larynx from the entrance o.f food and drink in deglutition. It also probably guides mucus or other fluids in small amount from the mouth around the sides of the upper opening of the glottis into the pharynx and ceso- phagus, thus preventing them from entering the larynx. The false vocal cords (cvs, Fig. 307), 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. Cartilages. — (a) The thyroid cartilage (Fig. 304, 1 to 4) does not form a complete ring around the larynx, but only covers the front por- tion, (b) The cricoid cartilage (Fig. 304, 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 (c) the aryte- noid cartilages (Fig. 304, 7), the connection between the cricoid below and arytenoid cartilages above being a joint with synovial membrane and ligaments, the latter permitting tolerably free motion between them. But although the arytenoid cartilages can move on the cricoid, they of course accompany the latter in all its movements, just as the head may nod or turn on the top of the spinal column, but must accompany it in all its movements as a whole. Joints and Ligaments. — The thyroid cartilage is also connected with the cricoid, not only by ligaments, but also by joints with synovial membranes; the lower cornua 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. Inas- much, too, as the arytenoid cartilages rest on the top of the back portion of the cricoid cartilage, and are connected with it by capsular and other ligaments, all movements of the cricoid cartilage must move the aryte- noid cartilages, and also produce an effect on the vocal cords. Intrinsic Muscles.— 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. 306) 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 arytenoid car- tilages on 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, on the other hand, have an opposite action — pulling the thyroid backwards, and the arytenoid and upper back part of the cricoid cartilages forwards, and thus relaxing the vocal cords. 440 HANDBOOK OF PHYSIOLOGY. The crico-arytenoidei postici muscles (Fig. 303) dilate the glottis, and separate the vocal cords, the one from the other, by an action on the arytenoid cartilage. 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 attached. These posterior crico-arytenoid muscles are opposed by the crico-ary- tenoidei laterales, which, pulling in the opposite direction from the other side of the axis of rotation, have of course exactly the opposite effect, and close the glottis. The aperture of the glottis can be also contracted by the arytenoid muscle (Fig. 305), which, in its contraction, pulls together the upper parts of the arytenoid cartilages between which it extends. Nerve Supply. — In the performance of the functions of the larynx the sensory filaments of the superior laryngeal branch of the vagi sup- * wy epigloto CartiTV Cart. San' . Arytefl pr. Crico-arytenoid. post. C'ornu inferior Lig. cerato-cric. Pars. post. inf. meml>rani Pars, caitflag FIG. 304. FIG. 305. FIG. 304.— Cartilages of the larynx seen from the front. Ito4, thyroid cartilage; 1, vertical ridge or pomum Adami; 2, right ala; 3, superior, and 4, inferior cornu of the right side; 5, 6, cricoid cartilage; 5, inside of the posterior part; 6, anterior narrow part of the ring; 7, arytenoid cartilages. X ^ FIG. 305.— The larynx as seen from behind. To show the intrinsic muscles posteriorly. (Stoerk ) ply that acute sensibility by which the glottis is guarded against the in- gress of foreign bodies, or of irrespirable gases. The contact of these stimulates the nerve filaments; and the impression conveyed to the medulla oblougata, whether it produce sensation or not, is reflected to the filaments of the recurrent or inferior laryngeal branch, and excites contraction of the muscles that close the glottis. Both these branches of pneumogastric co-operate also in the production and regulation of the voice; the inferior laryngeal determining the contraction of the muscles that vary the tension of the vocal cords, and the superior laryngeal con- veying to the mind the sensation 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 contrac- tion of the glottis in the acts of expiration and inspiration, and more THE VOICE AND SPEECH. 441 evidently iii those of coughing and other forcible respiratory move- ments. The laryngoscope is an instrument employed in investigating dur- ing life the condition of the pharynx, larynx, and trachea. It consists of a large concave mirror with perforated centre, and of a smaller mir- ror fixed in a long handle. It is thus used: the patient is placed in a chair, a good light (argand burner, or lamp) is arranged on one side of, and a little above his head. The operator fixes the large mirror round his head in such a manner, that he looks through the central aperture with one eye. He then seats himself opposite the patient, and so alters the position of the mirror, which is for this purpose provided with a ball and socket joint, that a beam of light is reflected on the lips of the pa- tient. The patient is now directed to throw his head slightly backwards, and to open his mouth; the reflection from the mirror lights up the cavity of the mouth, and by a little alteration of the distance between the operator and the patient the point at which the greatest amount of light is reflected by the mirror — in other words its focal length — is readily discovered. The small mirror fixed in the handle is then warmed, either by holding it over the lamp, or by putting it into a ves- sel of warm water; this is necessary to prevent the condensation of breath upon its surface. The degree of heat is regulated by applying' the back of the mirror to the hand or cheek, when it should feel warm without being painful. After these preliminaries the patient is directed to put out his tongue, which is held by the left hand gently but firmly against the lower teeth, by means of a handkerchief. The warm mirror is passed to the back of the mouth, until it rests upon and slightly raises the base of the uvula, and at the same time the light is directed upon it: an inverted image of the larynx and trachea will be seen in the mirror. If the dorsum of the tongue be alone seen, the handle of the mirror must be slightly lowered until the larynx comes into view; care should be taken, however, not to move the mirror upon the uvula, as it excites retching. The observa- tion should not be prolonged, but should rather be repeated at short in- tervals. The structures seen will vary somewhat according to the condition of the parts as to inspiration, expiration, phonation, etc.; they are (Figs. 307) first, and apparently at the posterior part, the base of the tongue, immediately below which is the arcuate outline of the epiglottis, with its cushion or tubercle. Thea are seen in the central line the true vocal cords, white and shining in their normal condition. The cords approxi- mate (in the inverted image) posteriorly; between them is left a chink, narrow whilst a high note is being sung, wide during a deep inspiration. On each side of the true vocal cords, and on a higher level, are the pink false vocal cords. Still more externally than the false vocal cords is the aryteno-epiglottidean fold, in which are situated upon each side three small elevations; of these the most, external is the cartilage of Wrisberg, the intermediate is the cartilage of Santorini. whilst the summit of the arytenoid cartilage is in front, and somewhat below the preceding, being only seen during deep inspiration. The rings of the trachea, and even the bifurcation of the trachea itself, if the patient be directed to draw a- deep breath, may be seen in the interval between the true vocal cords. 442 HANDBOOK OF PHYSIOLOGY. Movements of the Vocal Cords. — The placing of the vocal cordg in a position parallel one with the other, is effected by a combined action of the various intrinsic muscles which act on them — the thyro-arytenoi- dei having, without much reason, the credit of taking the largest share in the production of this effect. Fig. 307 is intended to show the various positions of the vocal cords under different circumstances. Thus, in ordinary tranquil breathing, the opening of the glottis is wide and triangular (B), becoming a little wider at each inspiration, and a little narrower at each expiration. On making a rapid and deep inspiration the opening of the glottis is widely dilated (as in c), and somewhat FIG. 306. FIG. 307. FIG. 306. -Lateral view of exterior of the larynx 8, thyroid cartilage; 9, cricoid cartilage ; 10, •crico-thyroid muscle; 11, crico-thyroid ligament; 12, first rings of trachea. (Willis.) FIG. 307.— Three laryngoscopic views of the superior aperture of the larynx and surrounding parts. 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', show in horizontal sections of the glottis the position of the vocal ligaments and arytenoid 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 ante- rior 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 San- torini; a, the tip or summit of the arytenoid cartilages; c v, the true vocal cords or lips of the rima glottidis; c v s, the superior or false vocal cords; between them tho ventricle of the larynx; in C, tr is placed on the anter.or wall of the receding trachea, and b indicates the commencement of the two bronchi beyond the bifurcation which may be brought into view in this state of extreme dilata- tion. (Quain after Czermak.) lozenge-shaped. At the moment of the emission of sound, it is narrowed, the margins of the arytenoid cartilages being brought into contact and THE VOICE AND SPEECH. 443 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. 307, 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 utterance of grave tones, on the other hand, the epiglottis is depressed and brought over them, and the arytenoid cartilages look as if they were trying to hide themselves under it (Fig. 308). 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 corre- sponds with the height of the note produced; but probably not always, FIG. 308.— View of the upper part of the larynx as seen by means of the laryngoscope 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. (Czermak.) for the width of the aperture has no essential influence 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 ary- tenoid cartilages. For if the arytenoid cartilages be approximated in such a manner that their anterior processes touch each other, 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; and the height or pitch of the note produced is the same whether the posterior part of the glottis be open or not, provided the vocal cords maintain the same degree of tension. THE VOICE IN SINGING ANP SPEAKING. Varieties of Vocal Sounds. — The laryngeal notes may observe three different kinds of sequence. The first is the monotonous, in which 4:4:4: HANDBOOK OF PHYSIOLOGY. the notes have nearly all the same pitch as in ordinary speaking; the variety of the sounds of speech being due to articulation in the mouth. In speaking, however, occasional syllables generally receive a higher in- tonation for the sake of accent. The second mode of sequence is the suc- cessive transition from high to low notes, and vice versa, without inter- vals; 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. In different individuals this 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 distinguished 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 singing the same note.' The qualities of the barytone and mezzo-soprano voices are less marked; the barytone being intermediate between the bass and tenor, the mezzo-soprano 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 voices depends on the different length of the vocal chords 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 THE VOICE AND SPEECH. 4:45 form of the resounding walls, which in the male larynx are much more extensive, and form a more acute angle anteriorly. The different quali- ties 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 varieties 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 of " timbre." The larynx of boys resembles the female larynx; their vocal cords before puberty are not two-thirds the length of the adult cords; 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 defect is owing to the ossification of the cartilages of the larynx and the altered condi- tion of the vocal cords; the want of steadiness arises from the loss of nervous power and command over the muscles; the result of which is here, as in other parts, a tremulous movement. These two causes combined render the voices of old people void of tone, unsteady, bleat- ing, and weak. In any class of persons arranged, as in an orchestra, according to the character 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 condi- tions that determine these distinctions are, however, quite unknown. They are 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 pro- 4:46 HANDBOOK OF PHYSIOLOGY. duced 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 iii such a manner as that one ends when the other begins, but they run in part side by side. Method of the Production of Notes.— 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 the falsetto notes were 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 glot- tis, which they believe assumes, at such times, the contour of the em- bouchure of a flute. Others (considering some degree of similaritv which exists between the falsetto notes and the peculiar tones called har- monic, 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 (a) on the degree to which the vocal cords can be made to vibrate; and partly (b) on the fitness for resonance of the membranes and cartilages of the larynx, of the pari- etes of the thorax, lungs, and cavities of the mouth, nostrils, and com- municating sinuses. It is diminished by anything which interferes with such capability of vibration. The intensity or loudness of a given note with 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 increasing the loundness of a note from the faintest "piano" to "fortissimo" without its pitch being altered, there must be some means of compen- sating 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 current of the breath through the glottis is increased. When a note is rendered fainter, the reverse of this must occur. THE VOICE AND SPEECH. 447" 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 ner- vous 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 strengthening 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-passages, the combination of which sounds by the agency of the cerebrum into different groups to designate objects, properties, actions, etc., constitutes language. 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 formation of the greater number of languages. Each language contains a certain number of such sounds, but in no one are all brought together. On the contrary, different languages are characterized by the prevalence in them of certain classes of these sounds, while others are less frequent or altogether absent. Articulate Sounds. — The sounds produced in speech, or the ar- ticulate sounds, are commonly divided into vowels and consonants, the distinction between which is, that the sounds for the former are gene- rated by the larynx, while those of the latter are produced by interrup- tion of the current of air in some part of the air-passages above the larynx. The term consonant has been given to these because several of them are not properly sounded, except consonantly with a vowel. T^hus, if it be attempted to pronounce aloud the consonants b, d, and g, or their modifications, p, t, k, the intonation only follows them in their combination with a vowel. To recognize the essential properties of the articulate sounds, it is necessary first to examine them as they are pro- duced in whispering, and then investigate which of them can also be uttered in a modified character conjoined with vocal tone. By this pro- cedure 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 independently of the voice, but are also capable of being ut- tered in conjunction with it. 44:8 HANDBOOK OF PHYSIOLOGY. All the vowels can tie expressed in a ivhisper without vocal tone, that 15, mutely. These mute vowel-sounds differ, however, 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 with the mouth closed, the nostrils being open, and the utterance of all vocal tone avoided. The 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, e, i, o, u, in all their modifi- cations. The cavity of the mouth assumes the same form for the articulation of each of the mute vowels as for the corresponding vowel when vocal- ized; the only difference in the two cases lies in the kind of sound emitted by the larynx. It has been pointed out that the conditions necessary for changing one and the same sound into the different vowels, are differences in the size of two parts — the oral canal and the oral open- ing; and the same is the case with regard to the mute vowels. By oral canal is meant here the space between the tongue and palate; for the pronunciation of certain vowels both the opening of the mouth and the space just mentioned are widened; for the pronunciation 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 opening. Size of oral canal, a as in " far " . 5 . . . 3 a " "name" 4 . . . 2 e " "theme" 3 ... 1 o " "go" 2 ... 4 00 " "cool" 1 ... 5 Another important distinction in articulate sounds is, that the utter- ance of some is only of momentary duration, taking place during a sud- den change in the conformation of the mouth, and being incapable of prolongation by a continued expiration. To this class belong b, p, d, and the hard g. In the utterance of other consonants the sounds may be continuous; they may be prolonged, ad libitum, as long as a particu- lar disposition of the mouth and a constant expiration are maintained. Among these consonants are h, m, n, f, s, r, 1. Corresponding differ- THE VOICE AND SPEECH. 449 ences in respect to the time that may be occupied in the irutterance ex- ist in the vowel sounds, and principally 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 letters), as in " can" and "fact," in "'dog" and " rotten/' cannot be prolonged. All sounds of the first or explosive kind are insusceptible of combina- tion with vocal tone ("intonation"), and are absolutely mute; nearly all the consonants of the second or continuous kind may be attended with "intonation." Ventriloquism. — The peculiarity of speaking, to which the term ventriloquism is applied, appears to consist merely in the varied modifi- cation of the sounds produced in the larynx, in imitation of the modifi- cations which voice ordinarily suffers from distance, etc. From the ob- servations of M tiller and Columbat, it seems that the essential mechanical parts of the process of ventriloquism consist in taking a full inspiration, then keeping the muscles of 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 Miiller, much of the ventriloquist's skill in imitating the voices coming from particular directions, consists in de- ceiving other senses than hearing. 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. Action of the Tongue in Speech. — The tongue, which is usually credited with the power of speech — language and speech being often employed as synonymous terms — plays only a subordinate, although very important 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 modi- fied; but the loss of speech is confined to those letters, in the pronuncia- tion of which the tongue is concerned. Stammering depends on a want of harmony between the action of the muscles (chiefly abdominal) which expel air through the larynx, and that of the muscles which guard the orifice (rima glottidis) by which it escapes, and of those (of tongue, palate, etc.) which modulate the sound to the form of speech. Over either of the groups of muscles, by itself, a stammerer may have as much power as other people. But he cannot harmoniously arrange their conjoint actions. 29 CHAPTER XVII. THE NERVOUS SYSTEM. I. THE STRUCTURE OF THE NERVOUS ELEMENTS. NERVOUS tissue is found under the microscope to consist essentially of two main elements, namely, of nerve fibres and nerve cells. When the nerve fibres are collected together into bundles they form nerve trunks or nerves. When nerve cells are collected together they form nerve ganglia, but in such ganglia nerve-fibres are also invariably found. A. Nerve Fibres. Varieties. — In most nerve-trunks two kinds of fibres are mingled, called (A) medullated or white fibres, and (B) non-medullated or gray fibres. (A.) Medullated Fibres. — Each medullated nerve-fibre is made up of the following parts: — (1.) An external sheath called the primitive nerve sheath, or nucleated sheath of Schwann; (2.) An intermediate or packing substance known as the medullary sheath, or white substance of Schwann; and (3.) internally the axis-cylinder, primitive band, axis band, or axial fibre. Although these parts can be made out in nerves examined some time after death, in a recent specimen the contents of the nerve-sheath appear to be homogeneous. But by degrees they undergo changes which show them to be composed of two different materials. The internal or cen- tral part, occupying the axis of the tube (axis-cylinder), becomes gray- ish, while the outer, or cortical portion (white substance of Schwann), becomes opaque and dimly granular or grumous, as if from a kind of coagulation. At the same time, the fine outline of the previously trans- parent cylindrical tube is exchanged for a dark double contour (Fig. 309, B), the outer line being formed by the sheath of the fibre, the inner by the margin of curdled or coagulated medullary substance. The granu- lar material shortly collects into little masses, which distend portions of the tubular membrane; while the intermediate spaces collapse, giving the fibres a varicose, or beaded appearance (Fig. 309, c and D), instead of the previous cylindrical form. The whole contents of the nerve- THE NEKVOUS SYSTEM. 451 tubules are 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. (1.) The external nucleated sheath of Schwann is a pellucid mem- brane, forming the outer investment of the nerve-fibre. Within this delicate structureless membrane nuclei are seen at intervals, surrounded by a variable amount of protoplasm. The sheath is structureless, like the sarcolemma, and the nuclei appear to be within it: together with the protoplasm which surrounds them, they are the relics of embryonic C TJ P FIG. 309. FIG. 310. FIG. 811. FIG. 309.— Primitive nerve-fibres. 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. (Wagner.) FIG. 310.— Two nerve-fibres of sciatic nerve. A. Node of Banvier. B. Axis-cylinder, c. Sheath of Schwann, with nuclei, x 300. (Klein and Noble Smith.) FIG. 311. — A node of Ranvier in a medullated nerve fibre, viewed from above. The medullary sheath is interrupted, and the primitive sheath thickened. Copied from Axel Key and Eetzius. X 750. (Klein and Noble Smitli.) cells, and from their resemblance to the muscle corpuscles of striated muscle, may be termed nerve-corpuscles. They are easily stained with logwood and other dyes. (2.) The medullary sheath or white substance of Schwann is the part to which the peculiar white aspect of some nerves is principally due. It is a thick, fatty, semi-fluid substance, as we have seen, possessing a double contour. It is said to be made up of a fine reticulum (Stilling, 452 HANDBOOK OF PHYSIOLOGY. Klein), in the meshes of which is imbedded the bright fatty material. It stains well with osmic acid. According to McCarthy, the medullary sheath is composed of small rods radiating from the axis-cylinder to the sheath of Schwann. Some- times the whole space is occupied by them, whilst at other times the rods appear shortened, and compressed laterally into bundles imbedded in some homogeneous substance. (3. ) The axis-cylinder consists of a large number of primitive fibrillce. This is well shown in the cornese, where the axis-cylinders of nerves break up into minute fibrils which go to form terminal networks, and also in the spinal cord, where these fibrillae form a large part of the gray matter. From various considerations, such as its invariable presence and unbroken continuity in all nerves, though the primitive sheath or the medullary sheath may be absent, there can be little doubt that the axis cylinder is the essential part of the fibre, the other parts having the subsidiary function of support and possibly of insulation. At regular intervals in most medullated nerves, the nucleated sheath of Schwann possesses annular constrictions (nodes of Ranvier). At these points (Figs. 310, 311), the continuity of the medullary white substance is interrupted, and the primitive sheath comes into immediate contact with the axis-cylinder. Size. — The size of the nerve-fibres varies; it is said that the same fibres may not preserve the same diameter through their whole length. The largest fibres are found within the trunks and branches of the spinal nerves, in which the majority measure from 14.4/j1 to 19/* in diameter. In the so-called visceral nerves of the brain and spinal cord medullated nerves are found, the diameter of which varies from 1.8/f to 3.6/*. In the hypoglossal nerve they are intermediate in size, and gen- erally measure 7.2yU to 10.8/*. (B.) Non-medullated Fibres. — The fibres of the second kind (Fig. 312), which constitute the whole of the branches of the olfactory and auditory nerves, the principal part of the trunk and branches of the sympathetic nerves, and are mingled in various proportions in the cere- bro-spinal nerves, differ from the preceding, chiefly in their fineness, being only about -J- to £ as large in their course within the trunks and branches of the nerves; in the absence of the double contour; in their contents being apparently uniform; and in their having, when in bun- dles, a yellowish-gray hue instead of the whiteness of the cerebro-spinal nerves. These peculiarities depend on their not possessing the outer layer of medullary substance; their contents being composed exclusively of the axis-cylinder. Yet, since many nerve- fibres may be found which appear intermediate in character between these two kinds, and since the 1 UL = .001 mm. THE NERVOUS SYSTEM. 453 large fibres, as they approach both their central and their peripheral . c. T., which corresponds to the peripheral portion of the posterior lateral column between the crossed pyramidal tract and the edge of the cord, can be traced upwards directly to the cerebellum and downwards as far as the mid-lumbar region; (4) Posterior median column or Fasciculus of Goll, is found on either side of the posterior commissure, and is traceable upwards and terminates as the fasciculus gracilis of the medulla. It is traceable downwards as far as the mid- dorsal region. The portion of the posterior column between the poste- rior median column and the posterior roots of the spinal nerves, known as (5) the Fasciculus cuneatus, Burdactts, or Poster o-external column, is composed of fibres of the posterior roots on their way to enter the gray substance and the posterior median column at different heights. The antero-lateral column contains fibres from the anterior cornua of the same as well as of the opposite side; (G) Lateral limiting layer (L. L. L.) 480 HANDBOOK OF PHYSIOLOGY. consists of fine fibres which pass into the gray matter at different levels; it probably consists of connecting fibres to connect the gray matter of different levels. These fibres have not a long course; (7) Anterior ground fibres (A. G. F.) are vertical fibres which probably connect the anterior cornua at different levels. Some fibres pass to the anterior commissure and connect with the anterior cornu of the opposite side; (8) Antero-lateral ascending tract is a tract which degenerates upwards. It is a sensory tract, and is connected with the posterior nerve-roots of the opposite side. Functions of the Spinal Nerve-Roots. — The anterior spinal nerve-roots are efferent or motor: the posterior are afferent or sensory. The fact is proved in various ways. Division of the anterior roots of one or more nerves is followed by complete loss of motion in the parts supplied by the fibres of such roots; but the sensation of the same parts remains perfect. Division of the posterior roots destroys the sensibility of the parts supplied by their fibres, while the power of motion continues unimpaired. Moreover, irritation of the ends of the distal portions of the divided anterior roots of a nerve excites muscular movements; irri- tation of the ends of the proximal portions, which are still in connection with the cord, is followed by no appreciable effect. Irritation of the distal portions of the divided posterior roots, on the other hand, pro- duces no muscular movements and no manifestations of pain; for, as already stated, sensory nerves convey impressions only towards the nerv- ous centres: but irritation of the proximal portions of these elicits signs of intense suffering. Occasionally, under this last irritation, muscular movements also ensue; but these are either voluntary, or the result of the irritation being reflected from the sensory to the motor fibres. Occasionally, too, irritation of the distal ends of divided anterior roots elicits signs of pain, as well as producing muscular movements: the pain thus excited is probably the result either of cramp or of so-called recur- rent sensibility. Recurrent Sensibility. — If the anterior root of a spinal nerve be divided, and the peripheral end be irritated, not only movements of the muscles supplied by the nerve take place, but also of other muscles, in- dicative of pain. If the main trunk of the nerve (after the coalescence of the roots beyond the ganglion) be divided, and the anterior root be irritated as before, the general signs of pain still remain, although the contraction of the muscles does not occur. The signs of pain disappear when the posterior root is divided. From these experiments it is be- }ieve$ that the stimulus passes down the anterior root to the mixed nerve, and returns to the central nervous system through the posterior root by means of certain sensory fibres from the posterior root, which loop back into the anterior root before continuing their course into the mixed nerve-trunk. THE CEREBRO-SPINAL NERVOUS SYSTEM. 481 Functions of the Ganglia on Posterior Roots. — The ganglia act as centres for the nutrition of the nerves, since when the nerves are severed from connection with the ganglia, the parts of the nerves so severed degenerate, whilst the parts which remain in connection with them do not. Functions of the Spinal Cord. The power of the spinal cord, as a nerve-centre, may be arranged under the heads of (1) Conduction; (2) Transference; (3) Keflex action. (1) Conduction. — The functions of the spinal cord in relation to con- duction, may be best remembered by considering its anatomical connec- tions with other parts of the body. From these it is evident that, with the exception of some few filaments of the sympathetic, there is no way by which nerve-impulses can be conveyed from the trunk and extremi- ties to the brain, or vice versa, other than that formed by the spinal cord. Through it, the impressions made upon the peripheral extremi- ties or other parts of the spinal sensory nerves are conducted to the brain, where alone they can be perceived. Through it, also, the stimulus of the will, conducted from the brain, is capable of exciting the action of the muscles supplied from it with motor nerves. And for all these conductions of impressions to and fro between the brains and the spinal nerves, the perfect state of the cord is necessary; for when any part of it is destroyed, and its communication with the brain is interrupted, im- pressions on the sensory nerves given off from it below the seat of injury, cease to be propagated to the brain, and the brain loses the power of voluntarily exciting the motor-nerves proceeding from the portion of cord isolated from it. Illustrations of this are furnished by various examples of paralysis, but by none better than by the common paraplegia, or loss of sensation and voluntary motion in the lower part of the body, in consequence of destructive disease or injury of a portion, including the whole thickness, of the spinal cord. Such lesions destroy the com- munication between the brain and all parts of the spinal cord below the seat of injury, and consequently cut off from their connection with the brain the various organs supplied with nerves issuing from those parts of the cord. It is not probable that the conduction of impressions along the cord is effected (to any great extent), as was formerly supposed, through the gray substance, i. e., through the nerve-corpuscles and filaments connect- ing them. All parts of the cord are not alike able to conduct all im- pressions; and as there are separate nerve-fibres for motor and for sensory impressions, so in the cord, separate and determinate tracts serve to con. duct always the same kind of impression. Experimental and other observations point to the following conclu- 31 482 HANDBOOK OF PHYSIOLOGY. sions regarding the conduction of sensory and motor impressions through the spinal cord. It is important to bear in mind that the gray matter of the cord, even if it conduct some impressions giving rise to sensation, appears not to be sensitive when it is directly stimulated. The explanation probably is, that it possesses no apparatus such as exists at the peripheral terminations of sensory nerves, for the reception of sensory impressions. The Conducting Paths in the Spinal Cord. a. Sensory Impressions are conveyed to the spinal cord by the pos- terior nerve-roots, and generally speaking cross over to the opposite side, and are conveyed upwards in two or three paths, according to the nature of the sensory impulse. (1.) Sensibility to Pain is almost certainly conveyed upwards in that part of the lateral column which is called by Gowers the antero-lateral ascending tract (A L A T, Fig. 333). It is a tract of vertical fibres imme- diately in front of the crossed pyramidal and direct cerebellar tracts. The zone extends across the lateral column as a band which is largest in area near the periphery of the cord, where it fills up the angle between the crossed pyramidal and cerebellar tracts, and it reaches the surface of the cord in front of the latter tract; it then extends forwards in the periphery of the anterior column, almost to the anterior median fissure (Gowers). (2.) Sensibility to Touch (tactile sensibility) is probably conveyed up- wards, after decussating almost as soon as it enters the cord, in the pos- terior median column. (3.) Sensibility of the Muscles (muscular sensibility). — The path of muscular sensation does not decussate, but passes upwards probably in the posterior median column of the same side, passing up to it from the hinder part of the postero-external column, and according to Elechsigin the direct cerebellar tract. (4.) Sensibility to Temperature. — The path for sensations of tem- perature is probably near to that of sensibility to pain, in the lateral column. (5.) Sensory Impressions subserving Reflex Actions. — There is con- siderable probability that all the paths for cutaneous sensibility undergo interruption in the spinal cord, and do not pass straight up, as no ascend- ing tract of degeneration has been demonstrated so far when a lesion has been confined to the nerve-roots. If this be the <3ase, it is probable that the same fibres which convey sensation have also to do with the cutaneous reflexes. In the case of muscular reflexes, however, as the fibres pass upwards without interruption, the reverse is in all probability the case, THE CEREBRO-SPINAL NERVOUS SYSTEM. 483 and special afferent fibres, even if few in number, exist, which are em- ployed in the chain of such reflexes. T}. Motor Impressions. — Motor impressions are conveyed down- wards from the brain along the pyramidal tracts, viz., the direct or ante- rior, and the crossed or lateral, chiefly in the latter. Generally speaking, the impressions pass down on the side opposite to which they originate, having undergone decussation in the medulla; but some impressions do not cross in the medulla, but lower down, in the cord, being conveyed by the anterior or uncrossed pyramidal fibres, and decussate in the ante- rior commissure. The motor fibres for the legs partially pass downwards FIG. 334.— Diagram of the decussation of the conductors for voluntary movements, and those for sensation: a r, anterior roots and their continuations in the spinal cord, and decussation at the lower part of the medulla oblongata, mo; p r, the posterior roots and their continuation and decussation in the spinal cord ; gr, gr, the ganglions of the roots. The arrows indicate the direction of the nervous action; r, the right side; Z, the left side. 1, 2, 3, indicate places of alteration in a lateral half of the spino-cerebral axis, to show the influence on the two kinds of conductors, result- ing from section of the cord at any one of these three places. (After Brown-Sequard.) in the lateral columns of the same side. This is also probably the case with the bilateral muscles, i. e., muscles of the two sides acting together, such as the intercostal muscles and other muscles of the trunk, as well as the costo-humeral muscles. It is quite certain, as was just now pointed out, that the fibres of the anterior nerve roots are more numerous than the fibres proceeding down- wards from the brain in the pyramidal tracts, or the so-called pyramidal fibres. It is therefore probable that each pyramidal fibre, or set of fibres, 484 HANDBOOK OF PHYSIOLOGY. corresponds with an apparatus of ganglion cells in the anterior cornu either on the same level, or even above or below, that when this fibre, or set of fibres, is stimulated, very complex co-ordinated movements occur — such co-ordinated movements having been set up by impressions from a connected system of ganglion cells, sent out into the motor nerve fibres which arise from them. In other words, it appears to be probable that in the gray matter of the anterior cornua of various sections of the cord are contained the apparatus for various complicated co-ordinated move- ments. The apparatus of each co-ordinated movement may be set in motion either by sensory impressions passing to the cord, when the re- sult of movement would be a reflex action, or by an impression travelling downwards from the brain, and conveyed by one or more pyramidal fibres. Division of the anterior pyramids of the medulla at the point of de- cussation (2, Fig. 334), is followed by paralysis of motion, never quite absolute, in all parts below. Disease or division of any part of the cere- bro- spinal axis above the seat of decussation (1, Fig. 334) is followed by impaired or lost power of motion on the opposite side of the body; while a like injury inflicted below this part (3, Fig. 334) induces similar, never quite absolute no doubt, on the corresponding side. When one half of the spinal cord is cut through, complete anaesthesia of the other side of the body below the point of section results, but there is often greatly increased sensibility (hyperaesthesia) on the same side- so much so that the least touch appears to be agonizing. This condition may persist for several days. Similar effects may, in man, be the result of injury. In addition to the transmission of ordinary sensory and motor im- pulses, the spinal cord is the medium of conduction also of impulses to and from the Vaso-motor centre in the medulla oblongata, although it probably contains special vaso-motor centres of its own. It will be seen in Chapter XXI. that Gaskell considers that the white visceral branches from the spinal cord to the sympathetic system are con- nected with or arise from the posterior vesicular column of Clarke, and from the anterior lateral cells. Others think that the direct cerebellar tract arises from Clarke's column. ransference.— Examples of the transference of impressions in the cord have been given (p. 466); and that 'the transference takes place in the cord, and not in the brain, is nearly proved by the frequent cases of pain felt in the knee and not in the hip, in diseases of the hip; of pain felt in the urethra or glans penis, and not in the bladder, in calculus; or, if both the primary and the secondary or transferred impression were in the brain, both should be felt. THE CEREBRO-SPIXAL NERVOUS SYSTEM. 485 Reflex Action or Reflection. In man the spinal cord is so much under the control of the higher nerve-centres, that its own individual functions in relation to reflex ac- tion are apt to be overlooked; so that the result of injury, by which the cord is cut off completely from the influence of the encephalon, is apt to lessen rather than increase our estimate of its importance and individual endowments. Thus, when the human spinal cord is divided, the lower extremities fall into an}7 position that their weight and the resistance of surrounding objects combine to give them; if the body is irritated, they do not move towards the irritation; and if they are touched, the conse- quent reflex movements are disorderly and purposeless; all power of voluntary movement is absolutely abolished. In other mammals, how- ever, e. g.9 in the rabbit or dog, after recovery from the shock of the operation, which takes some time, reflex actions in the parts below will occur after the spinal cord has been divided, a very feeble irritation being followed by extensive and co-ordinate movements. In the case of the frog, and many other cold-blooded animals, in which experimental and other injuries of the nerve-tissues are better borne, and in which the lower nerve-centres are less subordinate in their action to the higher, the reflex functions of the cord are still more clearly shown. When, for example, a frog's head is cut off, its limbs remain in, or assume a natural position; they resume it when disturbed; and when the abdomen or back is irritated, the feet are moved with the manifest purpose of pushing away the irritation. The main difference in the cold-blooded animals being that the reflex movements are more definite, complicated, and effective, although less energetic than in the case of mammals. It might indeed be thought, on superficial examination, that the mind of the ani- mal was engaged in the acts; and yet all analogy would lead us to the belief that the spinal cord of the frog has no different endowment, in kind, from those which belong to the cord of the higher vertebrata: the difference is only in degree. And if this be granted, it may be assumed that, in man and the higher animals, many actions are performed as reflex movements occurring through and by means of the spinal cord, although the latter cannot by itself initiate or even direct them independently. Cutaneous and Muscle Reflexes. — In the human subject two kinds of reflex actions dependent upon the spinal cord are usually dis- tinguished, the alterations of which, either in the direction of increase or of diminution, are indications of some abnormality, and are used as a means of diagnosis in nervous and other disorders. They are termed respectively (a) Cutaneous reflexes, and (b) Muscle reflexes, (a) Cu- taneous reflexes are set up by a gentle stimulus applied to the skin. The subjacent muscle or muscles contract in response. Although these cutaneous reflex actions may be demonstrated almost anywhere, yet cer- tain of such actions as being most charactsristic are distinguished, e. g., 4:86 HANDBOOK OF PHYSIOLOGY. plantar reflex; gluteal reflex, i. e., a contraction of the glutens maximus when the skin over it is stimulated; ere master reflex, retraction of the testicle when the skin of the inside of the thigh is stimulated, and the like. The ocular reflexes, too, are important. They are contraction of the iris on exposure to light, aad its dilatation on stimulating the skin of the cervical region. All of these cutaneous reflexes are true reflex actions, but they differ in different individuals, and are more easily elicited in the young, (b) Muscle reflexes, or as they are often termed, tendon-reflexes, consist of a contraction of a muscle under conditions of more or less tension, when its tendon is sharply tapped. The so-called patella-tendon-reflex is the most well-known of this variety of reflexes. If one knee be slightly flexed, as by crossing it over the other, so that the quadriceps femoris is extended to a moderate degree, and the patella tendon be tapped with the fingers or the earpiece of a stethoscope, the muscle contracts and the knee is jerked forwards. Another variety of the same phenomenon is seen if the foot is flexed so as to stretch the calf muscles and the tendo Achilhs is tapped; the foot is extended by the contraction of the stretched muscles. It appears, however, that the tendon reflexes are not exactly what their name im- plies. The interval between the tap and the contraction is too short for the production of a true reflex action. It is suggested that the contrac- tion is caused by local stimulation of the muscle, but that this would not occur unless the muscle had been reflexly stimulated previously by the tension applied, and placed in a condition of excessive irritability. It is further probable that the condition on which it depends is a reflex spinal irritability of the muscle or (exaggerated) muscular tone, which is ad- mitted to be a reflex phenomenon. Inhibition of Reflex Actions. — The fact that such movements as are produced by irritating the skin of the lower extremities in the human subject, after division or disorganization of a part of the spinal cord, do not follow the same irritation when the mind is active and connected with the cord through the brain, is, probably, due to the mind ordinarily perceiving the irritation and instantly controlling the muscles of the irritated and other parts; for even when the cord is perfect, such invol- untary movements will often follow irritation, if it be applied when the mind is wholly occupied. When, for example, one is anxiously thinking, even slight stimuli will produce involuntary and reflex movements. So, also, during sleep, such reflex movements may be observed, when the skin is touched or tickled; for example, when one touches with the finger the palm of the hand of a sleeping child, the finger is grasped — the im- pression on the skin of the palm producing a reflex movement of the muscles which close the hand. But when the child is awake, no such effect is produced by a similar touch. Further, many reflex actions are capable of being more or less con- trolled or even altogether prevented by the will: thus an inhibitory ac- tion may be exercised by the brain over reflex functions of the cord and the other nerve centres. The following may be quoted as familiar ex- amples of this inhibitory action: — THE CEREBKO-SPINAL NERVOUS SYSTEM. 487 To prevsnt the reflex action of crying out when in pain, it is often sufficient firmly to clench the teeth or to grasp some object and hold it tight. When the feet are tickled we can, by an effort or will, prevent the reflex action of jerking them up. So, too, the involuntary closing of the eyes and starting, when a blow is aimed at the head, can be simi- larly restrained. Darwin has mentioned an interesting example of the way in which, on the other hand, such an instinctive reflex act may override the strongest effort of the will. He placed his face close against the glass of the cobra's cage in the Reptile House at the Zoological Gardens, and though, of course, thoroughly convinced of his perfect security, could not by any effort of the will prevent himself from starting back when the snake struck with fury at the glass. It has been found by experiment that in a frog the optic lobes and optic thalami have a distinct action in inhibiting or delaying reflex ac- tion, and also that more generally any afferent stimulus, if sufficiently strong, may inhibit or modify any reflex action even in the absence of these centres. On the whole, therefore, it may, from these and like facts, be con- cluded that reflex acts, performed under the influence of the reflecting power of the spinal cord, are essentially independent of the brain and may be performed perfectly when the brain is separated from the cord: that these include a much larger number of the natural and purposive movements of the lower animals than of the warm-blooded animals and man: and that over nearly all of them the mind may exercise, through the higher nerve centres, some control; determining, directing, hinder- ing, or modifying, them, either by direct action, or by its power over associated muscles. To these instances of spinal reflex action, some add yet many more, including nearly all the acts which seem to be performed unconsciously, such as those of walking, running, writing, and the like: for these are really involuntary acts. It is true that at their first performances they are voluntary, that they require education for their perfection, and are at all times so constantly performed in obedience to a mandate of the will, that it is difficult to believe in their essentially involuntary nature. But the will really has only a controlling power over their performance; it can hasten or stay them, but it has little or nothing to do with the actual carrying out of the effect. And this is proved by the circumstance that these acts can be performed with compkte mental abstraction: and, more than this, that the endeavor to carry them out entirely by the ex- ercise of the will is not only not beneficial, but positively interferes with their harmonious and perfect performance. Any one may convince him- self of this fact by trying to take each step as a voluntary act in walking 488 HANDBOOK OF PHYSIOLOGY. down stairs, or to form each letter or word in writing by a distinct exer- cise of the will. These actions, however, will be again referred to, when treating of their possible connection with the functions of the Sensory Ganglia. Morbid reflex actions. — The relation of the reflex action to the strength of the stimulus is the same as was shown generally in the action of ganglia, a slight stimulus producing a slight movement, and a greater, a greater movement, and so on; but in instances in which we must as- sume that the cord is morbidly more irritable, i. e., apt to issue more nervous force than is proportionate to the stimulus applied to it, a slight impression on a sensory nerve produces extensive reflex movements. This appears to be the condition in tetanus, in which a slight touch on the skin may throw the whole body into convulsion. A similar state is induced by the introduction of strychnia, and, in frogs, of opium, into the blood; and numerous experiments on frogs thus made tetanic, have shown that the tetanus is wholly unconnected with the brain, and de- pends on the state induced in the spinal cord. Special Centres in Spinal Cord. It may seem to have been implied that the spinal cord as a single nerve-centre, reflects alike from all parts all the impressions conducted to it. This, however, is not the case, and it should be regarded as we have indicated, as a collection of nervous centres united in a continuous column. This is well illustrated by the fact that segments of the cord may act as distinct nerve-centres, and excite muscular action in the parts supplied with nerves given off from them; as well as by the anal- ogy of certain cases in which the muscular movements of single organs are under the control of certain circumscribed portions of the cord. The special centres are the following: — (a.) Centre for Defcecation, or Ano-Spinal centre. — The mode of ac- tion of the ano-spinal centre appears to be this. The mucous membrane of the rectum is stimulated by the presence of faeces or gases in the bowel. The stimulus passes up by the afferent nerves of the haemor- rhoidal and inferior mesenteric plexus to the centre in the cord, situated in the lumbar enlargement, and is reflected through the pudendal plexus to the anal sphincter on the one hand, and on the other to the mus- cular tissue in the wall of the lower bowel. In this way is produced a relaxation of the first and a contraction of the second, and expulsion of the contents of the bowel follows. The centre in the spinal cord is par- tially under the control of the will, so that its action may be either in- hibited, or augmented or helped. The action may be helped by the abdominal muscles which are under the control of the will, although under a strong stimulus they may also be compelled to contract by reflex action. THE CEEEBRO-SPINAL NERVOUS SYSTEM. 489 (b. ) Centre for Micturition, or the Vesico-Spinal centre. — The vesico- spinal centre acts in a very similar way to that of the ano-spinal. The centre is also in the lumbar enlargement of the cord. It may be stimu- lated to action by impulses descending from the brain, or reflexly by the presence of urine in the bladder. The action of the brain may be voluntary, or it may be excited to action by the sensation of distention of the bladder by the urine. The sensory fibres concerned are the pos- terior roots of the lower sacral nerves. The action of the centre thus stimulated is double, or it may be supposed that the centre consists of two parts, one which is usually in action and maintains the tone of the sphincter, and the other which causes contraction of the bladder and other muscles. When evacuation of the bladder is to occur, impulses are sent on the one hand to its muscles and to certain other muscles, which cause their contraction, and on the other to the sphincter urethrae which procures its relaxation. The way having been opened by the re- laxation of the sphincter, the urine is expelled by the combined action of the bladder and accessory muscles. The cerebrum may act not only in the way of stimulating the centre to action, but also in the way of in- hibiting its action. The abdominal muscles may be called into action as in defecation. (c.) Centre for Emission of Semen, or Genito- Spinal centre. — The centre situated in the lumbar enlargement of the spinal cord is stimulated to action by sensory impressions from the glans penis. Efferent im- pulses from the centre excite the successive and co-ordinate contractions of the muscular fibres of the vasa deferentia and vesiculae seminales, and of the accelerator urinse and other muscles of the urethra; and a forcible expulsion of semen takes place, over which the mind has little or no con- trol, and which, in cases of paraplegia, may be unfelt. (d.) Centre for tlie Erection of the Penis. — This centre is also situ- ated in the lumbar region. It is excited to action by the sensory nerves of the penis. Efferent impulses produce dilatation of the vessels of the penis, which also appears to be in part the result of a reflex contraction of the muscles by which the veins returning the blood from the penis are compressed. (e.) Centre for Parturition. — The centre for the expulsion of the contents of the uterus in parturition is situated in the lumbar spinal cord rather higher up than the other centres already enumerated. The stimu- lation of the interior of the uterus by its contents may, under certain conditions, excite the centre to send out impulses which produce a con- traction of the uterine walls and expulsion of the contents of the cavity. The centre is independent of the will, since delivery can take place in paraplegic women, and also whilst a patient is under the influence of chloroform. Again, as in the cases of defecation and micturition, the 490 HANDBOOK OF PHYSIOLOGY. abdominal muscles assist; their action being for the most part reflex and involuntary. (/.) Centre for Movements of Lymphatic Hearts of Frog. — Volkmann has shown that the rhythmical movements of the anterior pair of lym- phatic hearts in the frog depend upon nervous influence derived from the portion of spinal cord corresponding to the third vertebra, and those of the posterior pair on influence supplied by the portion of cord oppo- site the eighth vertebra. The movements of the heart continue, though the whole of the cord, except the above portions, be destroyed; but on the instant of destroying either of these portions, though all the rest of the cord be untouched, the movementsof the corresponding hearts cease. What appears to be thus proved in regard to two portions of the cord, may be inferred to prevail in other portions also; and the inference is reconcilable with most of the facts known concerning the physiology and comparative anatomy of the cord. (g.) The Centre for the Tone of Muscles. — The influence of the spinal cord on the sphincter ani and sphincter urethra has been already men- tioned (see above). It maintains these muscles in permanent contrac- tion. The condition of these sphincters, however, is not altogether ex- ceptional. It is the same in kind though it exceeds in degree that condition of muscles which has been called tone, or passive contraction; a state in which they always when not active appear to be during health, and in which, though called inactive, they are in slight contraction, and certainly are not relaxed, as they are long after death, or when the spinal cord is destroyed. This tone of all the muscles of the trunk and limbs depends on the spinal cord, as the contraction of the sphincters does. If an animal be killed by injury or removal of the brain, the tone of the muscles may be felt and the limbs feel firm as during sleep; but if the spinal cord be destroyed, the sphincter ani relaxes, and all the muscles feel loose, and flabby, and atonic, and remain so till rigor mortis com- mences. This kind of tone must be distinguished from that mere firmness and tension which it is customary to ascribe, under the name of tone, to all tissues that feel robust and not flabby, as well as.Nto muscles. The tone peculiar to muscles has in it a degree of vital contraction: that of other tissues is only due to their being well nourished, and therefore compact and tense. All the foregoing examples illustrate the fact that the spinal cord is a collection of reflex centres, upon which the higher centres act by sending down impulses to set in motion, modify or control them. The move- ments or other phenomena of reflex action being as it were the function of the ganglion cells to which an afferent impression is conveyed by the posterior nerve-trunks in connection with them, and that the extent of the movement depends upon the strength of the stimulus, the position THE CEREBRO-SPINAL, NERVOUS SYSTEM, 491 in which it is applied as well as the condition of the nerve cells, the con- nection between the cells being so intimate that a series of co-ordinated movements may result from a single stimulation. Whether the cells possess as well the power of originating impulses (automatism) is doubt- ful, but this is possible in the case of (h.) Vaso-motor centres which are situated in the cord (p. 147), and of (i.) Sweating centres which must be closely, related to them, and possibly in the case of (/.) The centres for maintaining the tone of muscles. The Nutrition (a) of the muscles, appears to be under the control of the spinal cord. When the anterior motor nerve cells are diseased the muscles atrophy. In the same way (b) the bones and (c) joints are seriously affected when the cord is diseased. The former where the an- terior nerve cells are implicated do not grow, and the latter are disorgan- ized in some cases when the posterior columns are affected, (d) The skin too evidently is only maintained in a healthy condition as long as the cord and its nerves are intact. No doubt part of this influence which the cord exercises over nutrition is due to the relationship which it bears to the vaso-motor nerves. Within the cord are contained, for some distance, fibres (a) which regulate the dilatation of the pupil, (b) which have to do with the glycogenic function of the liver, (c) which control the nerve-supply of the vessels of the face and head, (d) which produce acceleration of the heart's action, and, in fact, all the other so- called sympathetic functions (see Chapter XXI.). B. The Medulla Oblongata. The medulla oblongata (Figs. 335, 336) is a column of gray and white nervous substance formed by the prolongation upwards of the spinal cord and connecting it with the brain. Structure. — The gray substance which it contains is situated in the interior and variously divided into masses and laminae by the white or fibrous substance which is arranged partly in external columns, and partly in fasciculi traversing the central gray matter. The medulla ob- longata is larger than any part of the spinal cord. Its columns are pyri- form, enlarging as they proceed towards the brain, and are continuous with those of the spinal cord. Each half of the medulla, therefore, may be divided into three columns or tracts of fibres, continuous with the three tracts of which each half of the spinal cord is made up.' The col- umns are more prominent than those of the spinal cord, and separated from each other by deeper grooves. The anterior, continuous with the anterior columns of the cord, are called the anterior pyramids; the pos- terior, continuous with the posterior columns of the cord, with the ad- dition of the funiculusof Eolando on each side (Fig. 337, fR), are called 492 HANDBOOK OF PHYSIOLOGY. the restiform bodies. On the outer side of the anterior pyramids 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, con- tinuous with the posterior median column of the cord, a small tract is marked off by a slight groove from the remainder of the restiform body, and called the posterior pyramid or fasciculus gracilis. The restiform columns, instead of remaining parallel with each other throughout the whole length 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, exposed by this divergence. FIG. 336. FIG. 335.— Anterior surface of the pons Varolii, and medulla oblongata. a, a, anterior pyra- mids; &, their decussation; c, c, olivary bodies; d, d, restiform bodies; e, arciform fibres; /, fibres passing from the anterior column of the cord to the cerebellum; g. anterior column of the spinal cord; h, lateral column; p, pons Varolii; i, its upper fibres; 5, 5, roots 9f the fifth pair of nerves. FIG. 336.— Posterior surface of the pons Varolii, corpora quadrigemina, and medulla oblongata. The peduncles of the cerebellum are cut short at the side, a, a, the upper pair of corpora quadri- gemina; 6, 6, the lower; /, /, superior peduncles of the cerebellum; c, eminence connected with the nucleus of the hypoglossal nerve: e, that of the glosso-pharyngeal nerve: i, that of the vagus nerve; d, d, restiform bodies; p, p, posterior pyramids; v, v, groove in the middle of the fourth ventricle, ending below in the calamus scriptorius; 7, 7, roots of the auditory nerves. On separating the anterior pyramids, and looking into the groove be- tween them, some decussating fibres of the lateral columns of the cord can be plainly seen. Distribution of the Fibres of the Medulla Oblongata. a. The anterior pyramid ef each side, although mainly composed 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 almost entirely the decussating strands THE CEREBROSP1NAL NERVOUS SYSTEM. 493 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 Pons to the Cerebrum. 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. b. The lateral column of the cord on each side of the medulla, in proceeding upwards, divides into three parts, outer, inner, and middle, which are thus disposed of: — 1. The outer fibres (direct cerebellar tract) go with the restiform tract to the cerebellum. 2. The middle (crossed FIG. 337.— Posterior view of the medulla, fourth ventricle, and mesencephalon (natural size). p.n., line of the posterior roots of the spinal nerves; p.m./., posterior median fissure; f.g., funiculus gracilis; cZ.,itsclava; /.c., funiculus cuneatus; f.R., funiculus of Rolando; r.6., restiform body; c.s., calamus scriptorius; I, section of ligula or taenia; part of choroid plexus is seen beneath it ; l.r., lateral recess of the ventricle; str., striae acusticae; i.f., inferior fossa; s./., posterior fossa; be- tween it and the median sulcus is the fasciculus teres; cbL, cut surface or the cerebellar hemi- , ___, ^ seen on the surface of the tegmentumf c., crusti; l.g ., lateral groove; c g.i", corpus geniculum in- ternus; th., posterior part of thalamus; p., pineal body. Theroman numbers indicate the corre- sponding cranial nerves. (E. A. Schafer.) pyramidal tract) 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, at first superficially but afterwards beneath the olivary body and the arcuate fibres, and then proceed along the floor of the fourth ventricle, o'n each side, under the name of the fasciculus teres. c. The posterior column of the cord is represented in the medulla by 494 HANDBOOK OF PHYSIOLOGY. i. the posterior pyramid, or fasciculus gracilis, which is a continuation of the posterior median column, and by ii. the restiform body, comprising the funiculus cuneatus and the funiculus of Rolando. The fasciculus gracilis (Fig. 337, /.#), diverges above as the broader clava to form one on either side the lower lateral boundary of the fourth ventricle, then tapers off, and becomes no longer traceable. The funiculus cuneatus, or the rest of the posterior column of the cord, is continued up in the medulla as such (Fig. 337, f.c); but soon, in addition, between this and the continuation of the posterior nerve-roots, appears another tract called the funiculus of Eolando (Fig. 337, f>R}. High up, the funiculus cune- atus is covered by a set of fibres (arcuate fibres), which issue from the anterior median fissure, turn upwards over the anterior pyramids to pass directly into the corresponding hemisphere of the cerebellum, being joined by the fibres of the direct cerebellar tract; the funiculus of n.c. FIG. 338. FIG. 339. FIG. SSS.-^ection of the medulla oblongata in the region of the superior pyramidal decussation; , internal convolutions of the parietal lobe; 13, upper surface of the cerebellum. (Sappey after Foville.) of the brain — including those of the arched white commissure or fornix which extends backwards from the septum lucidum, and consists of two lateral halves joined only in the middle, with two anterior pillars and two posterior crura — and of the third ventricle, refer to Fig. 342 and the description. The Internal Surface (Fig. 347) contains the following gyri and sulci: 506 HANDBOOK OF PHYSIOLOGY. Gyrus fornicatus, a long curved convolution, parallel to and curving round the corpus callosum, and swelling out at its hinder and upper end FIG. 346.— View of the brain from above (semi-diagrammatic). 81, end of horizontal ramus of fissure of Sylvius. The other letters refer to the same parts as in Fig. 344. (Ecker.) into the quadrate lobule (prsecuneus), which is continuous with the superior parietal lobule on the external surface. FIG. 347.— View of the right hemisphere in the median aspect (semi-diagrammatic). CC, cor- pus callosum longitudinally divided; Gf, gyrus fornicatus; H, gyrus hippocampi: h, sulcus hippo- campi; U, uncinate gyrus; cm, calloso-marginal fissure; Fl, median aspect of first frontal con- volution; c, terminal portion of sulcus centralis (fissure of Rolando); A, ascending frontal : B, ascending parietal convolution; PI', prsecuneus; Oz, cuneus; po, pa rieto-occipital fissure ; o, sulcus occipitalis transversus; oc, calcarine fissure; oc', superior; oc". inferior ramus of the same: D. gyrus descendens ; T4, gyrus occipito-temporalis lateralis (lobulus fusiformis); T5, gyrus occipito- temporalis medialis (lobulus lingualis). (Ecker.) THE CEREBRO-SPINAL NERVOUS SYSTEM. SOT Marginal convolution runs parallel to the preceding, and occupies the space between it and the edge of the longitudinal fissure. The two convolutions are separated by the calloso-marginal fissure. The internal perpendicular fissure is well marked, and runs down- wards to its junction with the calcarine fissure: the wedge-shaped mass intervening between these two is termed the cuneus. The calcarine fis- (wWlf&» FIG. 348. FIG. 349. FIG. 350. FIG. 34^.— The layers of the cortical gray matter of the cerebrum. (Meynert.^ FIG. 350.— (Drawn by G. Mimro Smith from ammonium bichromate preparations by E. C. Bonsfleld.) sure corresponds to the projection into the posterior cornu of the lateral ventricle, termed the Hippocampus minor. The temporo-sphenoidal lobe on its internal aspect is seen to end in a hook (uncinate gyrus). The notch round which it curves is continued up and back as the dentate or 508 HANDBOOK OF PHYSIOLOGY. hippocampal sulcus: this fissure underlies the projection of the hippo- campus major within the brain. There are three internal temporo-occi- pital convolutions, of which the superior and inferior ones are usually well marked, the middle one generally less so. The collateral fissure (corresponding to the eminentia collateralis) forms the lower boundary of the superior temporo-occipital convolution. All the above details will be found indicated in the diagrams (Figs. 346, 347). Structure. — The cerebrum is constructed, like the other chief divi- sions of the cerebro-spinal system, of gray and white matter; and, as in the case of the Cerebellum (and unlike the spinal cord and medulla ob- longata), the gray matter (cortex) is external, and forms a capsule or covering for the white substance. For the evident purpose of increasing its amount without undue occupation of space, the gray matter is vari- ously infolded so as to form the cerebral convolutions. The cortical gray matter of the brain consists of five layers (Mey- nert) (Fig. 348). 1. Superficial layer with abundance of neuroglia and a few small multipolar ganglion-cells. 2. A large number of closely packed small ganglion-cells of pyramidal shape. 3. The most important layer, and the thickest of all: it contains many large pyramidal ganglion-cells, each with a process running off from the apex vertically towards the free sur- face, and lateral processes at the base which are always branched. Also a median process from the base of each cell which is unbranched and becomes continuous with the axis-cylinder of a nerve-fibre. 4. Numerous ganglion-cells: termed the "granular formation" by Meynert. 5. Spindle-shaped and branched ganglion-cells of moderate size arranged chiefly parallel to the free surface (vide Fig. 348). According to recent observations by Bousfield, the fibres of the me- dullary centre become connected with the multipolar ganglion-cells of the fourth layer, and, from these latter, branches pass to the angles at the bases of the pyramidal cells of the third layer of the cortex (Fig. 350, a). From the apices of the pyramidal cells, the axis-cylinder processes pass upwards for a considerable distance, and finally terminate in ovoid corpuscles (Fig. 349), closely resembling, and homologous with, the cor- puscles in which the ultimate ramifications of the branched cells of Pur- kinje in the cerebellum terminate. Thus it would seem that the large pyramidal cells of the third layer are themselves homologous with the cells of Purkinje in the cerebellum. The white matter of the brain, as of the spinal cord, consists of bundles of medullated, and, in the neighborhood of the gray matter, of non-medullated nerve-fibres, which, however, as is the case in the cen- tral nervous system generally, have no external nucleated nerve-sheath, which are held together by delicate connective tissue. The size of the fibres of the brain is usually less than that of the fibres of the spinal cord: the average diameter of the former being about y^gir of an inch. THE CEKEBRO-SPINAL NEKYOUS SYSTEM. 509 Chemical Composition. — The chemistry of nerves and nerve cells has been chiefly studied in the brain and spinal cord. Nerve matter con- tains several albuminous and fatty bodies (cerebrin, lecithin, and some others), also fatty matter which can be extracted by ether (including cholesterin) and various salts, especially Potassium and Magnesium phosphates, which exist in larger quantity than those of Sodium and Calcium. The great relative and absolute size of the Cerebral hemispheres in the adult man, masks to a great extent the real arrangement of the sev- eral parts of the brain, which is illustrated in the two accompanying diagrams. From these it is apparent that the parts of the brain are disposed in a linear series, as follows (from before backwards): olfactory lobes, cerebral hemispheres, optic thalami, and third ventricle, corpora quadri- gemina, or optic lobes, cerebellum, medulla oblongata. This linear arrangement of parts actually occurs in the human foetus; and it is permanent in some of the lower Vertebrata, e. g., Fishes, in which the cerebral hemispheres are represented by a pair of ganglia intervening between the olfactory and the optic lobes, and considerably smaller than the latter. In Amphibia the cerebral lobes are further de- veloped, and are larger than any of the other ganglia. In Reptiles and Birds the cerebral ganglia attain a still further devel- opment, and in Mammalia the cerebral hemispheres exceed in weight all the rest of the brain. As we ascend the scale, the relative size of the cerebrum increases, till in the higher apes and man the hemispheres, which commenced as two little lateral buds from the anterior cerebral vesicle, have grown upwards and backwards, completely covering in and hiding from view all the rest of the brain. At the same time the smooth surface of the brain, in many lower Mammalia, such as the rabbit, is replaced by the labyrinth of convolutions of the brain. Weight of the Brain. — The brain of an adult man weighs from 48 to 50 oz. — or about 3 Ibs. It exceeds in absolute weight that of all the lower animals except the elephant and whale. Its weight, relatively to that of the body, is only exceeded by that of a few small birds, and some of the smaller monkeys. In the adult man it ranges from -g^--^ of the body weight. Variations. Age. — In a new-born child the brain (weighing 10-14 oz. ) is yV of the body weight. At the age of 7 years the brain already averages 40 oz., and about 14 years the brain not unfrequently reaches the weight of 48 oz. Beyond the age of forty years the weight slowly but steadily declines at the rate of about 1 oz. in 10 years. Sex. — The average weight of the female brain is less than the male: and this difference persists from birth throughout life. In the adult it amounts to about 5 oz. Thus the average weight of an adult woman's brain is about 44 oz. Intelligence. — The brains of idiots are generally much below the average, some weighing less than 16 oz. Still the facts at present col- lected do not warrant more than a very general statement, to which there are numerous exceptions, that the brain weight corresponds to some extent with the degree of intelligence. There can be little doubt that 510 HANDBOOK OF PHYSIOLOGY. the complexity and depth of the convolutions, which indicate the area of the gray matter of the cortex, correspond with the degree of intelligence. Weight; of the Spinal Cord.- The spinal cord of man weighs from 1-1£ oz. ; its weight relatively to the brain is about 1 : 36. As we de- scend the scale, this ratio constantly increases till in the mouse it is 1 : 4. In cold-blooded animals the relation is reversed, the spinal cord is the heavier and the more important organ. In the newt, 2:1; and in the lamprey, 75 : 1. Distinctive Characters of the Human Brain. — The following characters distinguish the brain of man and apes from those of all other animals, (a.) The rudimentary condition of the olfactory lobes, (b.) A perfectly defined fissure of Sylvius, (c. ) A posterior lobe completely FIG. 351. FIG. 352. Fra. 351.— Diagrammatic horizontal section of a Vertebrate brain. The figures serve both for this and the next diagram. Mb, mid brain: what lies in front of this is the fore-, and what lies behind, the hind-brain; Lt, lamina terminalis; Olf, olfactory lobes; Hmp, hemispheres; Th. E, thalamencephalon; Pn, pineal gland; Py, pituitary body; F. M, foramen of Munro; cs, corpus striatum; Th, optic thalamus; CO, crura cerebri: the mass lying above the canal represents the the corpora quadrigemina; Cb, cerebellum: I— IX., the nine pairs of cranial nerves; 1, olfactory ventricle; 2, lateral ventricle; 3, third ventricle; 4, fourth ventricle; +, iter a tertio ad quartum ventriculum. (Huxley.) FIG. 352.— Longitudinal and vertical Diagrammatic section of a vertebrate brain. Letters as before. Lamina terminalis is represented by the strong black line joining Pn and Py. (Huxley.) covering the cerebellum, (d.) The presence of posterior cornua in the lateral ventricles. The most distinctive points in the human Irain, as contrasted with that of apes, are: — (1.) The much greater size and weight of the whole brain. The brain of a full-grown gorilla weighs only about 15 oz., which is less than £ the weight of the human adult male brain, and barely ex- ceeds that of the human infant at birth. (2.) The much greater com- plexity of the convolutions, especially the existence in the human brain THE CEREBRO-SPDSTAL NERVOUS SYSTEM. 511 of tertiary convolutions in the sides of the fissures. (3.) The greater relative size and complexity, and the blunted quadrangular contour of the frontal lobes in man, which are relatively both broader, longer, and higher, than in apes. In apes the frontal lobes project keel-like (ros- trum) between the olfactory bulbs. (4.) The much greater prominence of the temporo-sphenoidal lobes in apes. (5.) The fissure of Sylvius is nearly horizontal in man, while in apes it slants considerably upwards. (6.) The distinctness of the external perpendicular fissure, which in apes is a well-defined almost vertical "slash," while in man it is almost obscured by the annectent gyri. Fio. 353.— Brain of the Orang, % natural size, showing the arrangement of the convolutions. Sy, fissure of Sylvius: R, fissure of Rolando; E P, external perpendicular fissure: Olf, olfactory lobe; C 6, cerebellum; P V, pons Varolii; M O, medulla oblongata As contrasted with the human brain, the frontal lobe is short and small relatively, the fissure of Sylvius is oblique, the temporo- sphenoidal lobe very prominent, and the external perpendicular fissure very well marked. (Gratio- let.) Most of the above points are shown in the accompanying figure of the brain of the Oraug. Functions of the Cerebrum. Speaking in the most general way, and for the present omitting the accumulating evidence in favor of the direct representation of the vari- ous co-ordinated movements of the muscles of the body in ganglia situ- ated in different parts of the cerebral cortex, it may be said that: — (1.) The Cerebral hemispheres are the organs by which are perceived those clear and more impressive sensations which can be retained, and regard- ing which we can judge. (2.) The Cerebrum is the organ of the will, in so far at least as each act of the will requires a deliberate, however quick determination. (3. ) It is the means of retaining impressions of sensible things, and reproducing them in subjective sensations and ideas. (4.) It is the medium of all the higher emotions and feelings, and of the 51 '2 HANDBOOK OF PHYSIOLOGY. faculties of judgment, understanding, memory, reflection, induction, imagination and the like. Evidence regarding the physiology of the cerebral hemispheres has been obtained, as in the case of other parts of the nervous system, from the study of Comparative Anatomy, from Pathology, and from Experi- ments on the lower animals. The chief evidences regarding the func- tions of the Cerebral hemispheres derived from these various sources, are briefly these: — 1. Any severe injury of them, such as a general concus- sion, or sudden pressure by apoplexy, may instantly deprive a man of all power of manifesting externally any mental faculty. 2. In the same general proportion as the higher mental faculties are developed in the Vertebrate animals, and in man at different ages and in different indi- viduals, the more is the size of the cerebral hemispheres developed in com- parison with the rest of the cerebro-spinal system. 3. No other part of the nervous system bears a corresponding proportion to the development of the mental faculties. 4. Congenital and other morbid defects of the cerebral hemisphere are, in general, accompanied by corresponding deficiency in the range or power of the intellectual faculties and the higher instincts. 5. Eemoval of the cerebral hemispheres in one of the lower animals produces effects corresponding with what might be antici- pated from the foregoing facts. Effects of the Removal of the Cerebrum. — The removal of the cerebrum in the lower animals appears to reduce them to the condition of a mech- anism without spontaneity. A pigeon from which the cerebrum has been removed will remain motionless and apparently unconscious unless dis- turbed. When disturbed in any way it soon recovers its former position; when thrown into the air it flies. In the case of the/ro^, when the cerebral lobes have been removed, the animal appears similarly deprived of all power of spontaneous move- ment. But it sits up in a natural attitude, breathing quietly; when pricked it jumps away; when thrown into the water it swims; when placed upon the palm of the hand it remains motionless, although, if the hand be gradually tilted over till the frog is on the point of losing his balance, he will crawl up till he regains his equilibrium, and comes to be perched quite on the edge of the hand. This condition contrasts with that resulting from the removal of the entire brain, leaving only the spinal cord; in this case only the simpler reflex actions can take place. The frog does not breathe, he lies flat on the table instead of sitting up; when thrown into a vessel of water he sinks to the bottom; when his legs are pinched he kicks out, but does not leap away. Unilateral Action. — Respecting the mode in which the brain dis- charges its functions, 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 numerou scases are recorded in which no men- THE CEREBRO-SPINAL NERVOUS SYSTEM. 513 tal defect was observed, although one cerebral hemisphere was so disor- ganized or atrophied that it could not be supposed capable of discharging its functions. The remaining hemisphere was, in these cases, adequate to the functions 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 hemisphere will suffice for these. In general, the brain 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 compression of either hemisphere, whatever effects are pro- duced in loss of sensation or voluntary motion, are observed on the side of the body opposite to that on which the brain is injured. Localization of Functions. — 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 ex- ercise of each of the mental faculties. But it is possible that each faculty has a special portion of the brain appropriated 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 compound organs or sys- tems in the bod}7, 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 men- tal functions are manifested 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 singular aptness in some study or pursuit; and it is a matter of daily observation that every one has his peculiar talent or propensity. 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 plurality 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 dis- ordered; it often happens that the strength of some is increased, while that of others is diminished; and in many cases one function only of the brain is deranged, while all the rest are performed in a natural manner. 4. The same opinion is supported by the fact that the several mental faculties are developed to their greatest strength at different periods of life, some being exercised with great energy in childhood, others only in 33 514 HANDBOOK OF PHYSIOLOGY. 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 medium of the parts of the brain appropriated to them. Unconscious Cerebration. — In connection with the above, some remarkable phenomena should be mentioned which have been described as depending on an unconscious action of the brain. It must be within the experience of every one to have tried to recol- lect some particular name or occurrence; and after trying in vain for some time the attempt is given up and quite forgotten amid other occu- pations, when suddenly, hours or even a day or two afterwards, the desired name or occurrence unexpectedly flashes across the mind. Such occurrences are supposed by many to be due to the requisite cerebral processes going on unconsciously, and, when the result is reached, to our all at once becoming conscious of it. That unconscious cerebration may sometimes occur, is likely enough; and it is paralleled by the unconscious walking of a somnambulist. But many cases of so-called unconscious cerebration are better explained by the supposition that some missing link in the chain of reasoning cannot at the moment be found; but is afterwards, by some chance combination of events, suggested, and thus the mental process is at once, with the memory of what has gone before, completed. Again, in the vain endeavor to solve a difficult, or it may be an easy problem, the reasoner is frequently in the condition of a man whose wearied muscles could never, before they have rested, overcome some obstacles. In both cases, — of brain and muscle, after renewal of their textures by rest, the task is performed so rapidly as to seem instan- taneous. Sleep. — All parts of the body which are the seat of active change require periods of rest. The alternation of work and rest is a necessary condition of their maintenance, and of the healthy performance of their functions. These alternating periods, however, differ much in duration in different cases; but, for any individual instance, they preserve a gen- eral and rather close uniformity. Thus, as before mentioned, the periods of rest and work, in the case of the heart, occupy, each of them, about half a second; in the case of the ordinary respiratory muscles the periods are about four or five times as long. In many cases, again (as of the voluntary muscles during violent exercise) while the periods during active exertion alternate very frecuently, yet the expenditure goes far ahead of the repair, and, to compensate for this, an after repose of some hours become necessary; the rhythm being less perfect as to time, than in the case of the muscles concerned in circulation and respiration. Obviously, it would be impossible that, in the case of the Brain, there should be short periods of activity and repose, or in other words, of con- sciousness and unconsciousness. The repose must occur at long inter- vals; and it must therefore be proportionately long. Hence the necessity THE CEREBRO- SPINAL NERVOUS SYSTEM. 515 for that condition which we call Sleep ; a condition which, seeming at first sight exceptional, is only an unusually perfect example of what occurs, at varying intervals, in every actively working portion of our bodies. A temporary abrogation of the functions of the cerebrum imitating sleep, may occur, in the case of injury or disease, as the consequence of two apparently widely different conditions. Insensibility is equally pro- duced by a deficient and an excessive quantity of blood in the cranium (coma); but it was once supposed that the latter offered the truest analogy to the normal condition of the brain in sleep, and in the absence of any proof to the contrary, the brain was said to be during sleep con- gested. Direct experimental inquiry has led, however, to the opposite conclusion. By exposing, at a circumscribed spot, the surface of the brain of living animals, and protecting the exposed part by a watch-glass. Durham was able to prove that the brain becomes visibly paler (anaemic) during sleep; and the anaemia of the optic disc during sleep, observed by Hugh- lings Jackson, may be taken as a strong confirmation, by analogy, of the same fact. A very little consideration will show that these experimental results correspond exactly with what might have been foretold from the analogy of other physiological conditions. Blood is supplied to the brain for two partly distinct purposes. (1.) It is supplied for mere nutrition's sake. (2.) It is necessary for bringing supplies of potential or active energy u. e., combustible matter or heat) which may be transformed by the cerebral corpuscles into the various manifestations of nerve-force. During sleep, blood is requisite for only the first of these purposes; and its supply in greater quantity would be not only useless, but, by supply- ing an excitement to work, when rest is needed, would be positively harmful. In this respect the varying circulation of blood in the brain exactly resembles that which occurs in all other energy transforming parts of the body; e. g., glands or muscles. At the same time, it is necessary to remember that the normal anaemia of the brain which accompanies sleep is probably a result, and not a cause of the quiescence of the cerebral functions. What the immediate cause of this periodical partial abrogation of function is, however, we do not know, Somnambulism and Dreams.— What we term sleep occurs often in very different degrees in different parts of the nervous system; and in some parts the expression cannot be used in the ordinary sense. The phenomena of dreams and somnambulism are examples of differ- ing degrees of sleep in different parts of the cerebro-spiual nervous system. In the former case the cerebrum is still partially active; but the mind-products of its action are no longer corrected by the reception, on the part of the sleeping sensorinm, of impressions of objects belong- ing to the outer world; neither can the cerebrum, in this half-awake condition, act on the centres of reflex action of the voluntary muscles, so as to cause the latter to contract — a fact within the painful experience of all who have suffered from nightmare. In somnambulism the cerebrum is capable of exciting that train of reflex nervous action which is necessary for progression, while the nerve- centre of muscular sense (in the cerebellum ?) is, presumably, fulJy awake; but the sensorium is still asleep, and impressions made on it are 516 HANDBOOK OF PHYSIOLOGY. not sufficiently felt to rouse the cerebrum to a comparison of the differ- ence between mere ideas or memories and sensations derived from exter- nal objects. The Motor Centres of the Cerebral Cortex. The experiments upon the brains of various animals by means of elec- trical stimulation have demonstrated that there are definite regions of the cerebral cortex the stimulation of which produces definite movements of co-ordinated groups of muscle of the opposite side of the body. It had long been well-known that the cerebral hemispheres could not be excited by mechanical, chemical, or thermal stimuli, but Fritsch and Hitzig were the first to show that they are amenable to electric irritation. They employed a weak constant current in their experiments, applying a pair of fine electrodes not more than -^ in. apart to different parts of the cerebral cortex. The results thus obtained have been confirmed and extended by Ferrier and many others. The fundamental phenomena observed in all these cases may be thus epitomized: — (1). Excitation of the same spot is always followed by the same move- ment in the same animal. (2). The area of excitability for any given movement is extremely small, and admits of very accurate definition. (3). In different animals excitations of anatomically corresponding spots pro- duce similar or corresponding results. The various definite movements resulting from the electric stimula- tion of circumscribed areas of the cerebral cortex, are enumerated in the description of the accompanying figures of the dog and monkey's brain. In the case of the dog, the results obtained are summed up as follows, by Hitzig: — (a). One portion (anterior) of the convexity of the cerebrum is motor; another portion (posterior) is non-motor. (#). Electric stimulation of the motor portion produces co-ordinated muscular contraction on the opposite side of the body. (c). With very weak currents, the contrac- tions produced are distinctly limited to particular groups of muscles; with stronger currents the stimulus is communicated toother muscles of the same or neighboring parts, (d). The portions of the brain inter- vening between these motor centres are inexcitable by similar means. With regard to the facts above mentioned/all experimenters are agreed, but there is still considerable diversity of opinion as to their explanation. In applying the facts ascertained by these experiments to elucidate the physiology of the human brain, we must remember that the method of electric stimulation is an artificial one, differing widely from the ordi- nary stimuli to which the brain is subject during life. Effects of Stimulation of Various Regions of a Monkey's Brain.— THE CEREBRO-SPINAL NERVOUS SYSTEM. 517 According to the observations of Ferrier, confirmed and extended by later experimenters, stimulation of various parts of the monkey's brain, as in- dicated by the numbers in Figs. 356, 357, produces movements of definite muscles, thus: — Stimulation of the districts marked 1, causes movements of hind foot; of 2, chiefly adduction of the foot; of 3, movements of hind foot and tail; s FIG. 355. FIGS. 354 and 355.— Brain of dog, viewed from above and in profile. F, frontal fissure, some- times termed crucial sulcus, corresponding to tiie fissure of Rolando in man; S, fissure of Sylvius, around which the four longitudinal convolutions are concentrically arranged; 1, flexion of head on the neck, in the median line ; flexion of head on the neck, with rotation towards the side of the stimulus; 3, 4, flexion and extension of anterior limb; 5, 6, flexion and extension of posterior limb; 7, 8, 9, contraction of orbicularis oculi, and the facial muscles in general. The unshaded part is that exposed by opening the skull. (Dalton.) of 4, of latissimus dorsi; of 5, extension forward of arm; a, I, c, d, movements of hand and wrist; of 6, supination and flexion of forearm; of 7, elevation of the upper lip; of 8, conjoint action of elevation of 518 HANDBOOK OF PHYSIOLOGY. upper lip and depression of lower; of 9, opening of mouth and protru. sion of tongue; of 10, retraction of tongue; of 11, action of platysma; of 12, elevation of eyebrows and eyelids, dilatation of pupils, and turn- ing head to opposite side; of 13, eyes directed to opposite side and up wards, with usually contraction of the pupils; of 13', similar action, but eyes usually directed downwards; of 14, retraction of opposite ear, head turns to the opposite side, the eyes widely opened, and pupils dilated; of 15, stimulation of this region, which corresponds to the tip of the un- cinate convolution, causes torsion of the lip and nostril of the same side. It is thus seen that the motor areas chiefly correspond with the as- cending frontal and ascending parietal convolutions, and that the move- ments of the leg are represented at the upper part of these convolutions, FIGS. 356 and 357.— Diagram of monkey's brain to show the effects of electric stimulation of cer- tain spots. (According to Ferrier.) then follow from above downwards the centres for the arms, the face, the lips, and the tongue. According to the further researches of Schafer and Horsley, electrical stimulation of the marginal convolution internally at the parts corre- sponding with the ascending frontal and parietal convolutions, from before backwards, produces movements of the arm, of the trunk, and of the leg. A good deal of doubt was thrown upon the experiments of Terrier by Goltz and other observers, from the results of excising the so-called motor areas of the dog's brain. It was found that the part might be sliced away or washed away with a stream of water, but that no perma- THE CEREBRO-SPINAL NERVOUS SYSTEM. 519 nent paralysis ensued. Burden-Sanderson, too, showed that stimulation of different points in a horizontal section, through the deeper parts of the hemispheres, produces the same effects as stimulation of the so-called '• centres." More extensive observations, however, have confirmed Ferrier's origi- nal statement, at any rate with regard to the monkey's brain. Destruc- tion of the motor areas for the arm produces permanent paralysis of the arm of the opposite side, and similarly of that for the leg, paralysis of the opposite leg. If both areas are destroyed permanent hemiplegia ensues. Paralysis of so extensive and permanent character does not, however, appear the rule when the brain of a dog is used instead of that of the monkey. It is suggested that in the animal lower in the scale, the functions which in the monkey are discharged by the cortical centres may be subserved by the basal ganglia. Motor ial Areas of the Human Brain. — It is naturally of great im- FIG. 358.— Motorial areas of the brain. A.F., ascending frontal convolution; A P., ascending parietal; F.R., fissure of Rolando; F. Sy., sylvian fissure. (After Gowers.) portance to discover how far the result of experiments upon the dog and monkey hold good with regard to the human brain. Evidence furnished by diseased conditions is not wanting to support the general idea of the existence of cortical motorial centres in the human brain (Fig. 358). So far, however, it has been possible to localize motor functions in the frontal and ascending parietal convolutions, only to the convolu- tions which bound the fissure of Rolando, and to those on the inner side of the hemispheres which correspond thereto. The position of the centres is probably much the same as in the monkey's brain — those for the leg above, those for the arm, face, lips, and tongue from above downwards. Destruction of these parts causes paralysis, corresponding to the district affected, and irritation causes convulsions of the muscles of the same part. Again, a number of cases are on record in which aphasia, or the loss of power of expressing ideas 520 HANDBOOK OF PHYSIOLOGY. in words, has been associated with disease of the posterior part of the lower or third frontal convolution on the left side. This condition is usually associated with paralysis of the right side (right hemiplegia). This district of the brain is now generally known as the motor area; and there seems no doubt whatever that from this area pass the nerve- fibres which proceed to the spinal cord, and are there represented as the pyramidal tracts. This is the reason, no doubt, thau movements are produced on stimu- lation of the white matter after the superficial gray matter of the ani- mal's brain has been sliced off. Motor tracts in the brain. — These motor fibres are connected with the pyramidal cells of the cortex, and are indeed their continuations. It will be necessary, therefore, to trace them from the cortex down- FIG. 359. FIG. 360. Fro. 359.— Diagram to show the connecting of the Frontal Occipital Lobes with the Cerebellum, etc. The dotted lines passing in the crusta (TOG), outside the motor fibres, indicate the connection between the temporo-occipital lobe and the cerebellum. F.C., the fronto-cerebellar fibres, which pass internally to the motor tract in the crusta; I.F , fibres from the caudate nucleus to the pons. FR., frontal lobe; Oc., occipital lobe; AF , ascending frontal; AP., ascending parietal convolutions; PCF., pre-central fissure in front of the ascending frontal convolution ; FR., fissure of Rolando; IFF., inter-parietal fissure; a section of cms is lettered on the left side. SN., substantia nigra; PY, pyramidal motor fibre, which on the right is shown as continuous lines converging to pass through the posterior limb of ic., internal capsule (the knee or elbow of which is shown thus *) upwards into the hemisphere and downwards through the pons to cross at the medulla in the anterior pyramids. (Gowers.) FIG. 360.— Diagram to show the relative positions of the several motor tracts in then' course from the cortex to the crus. The section through the convolutions is vertical; that through the internal capsule, I, C, horizontal; that through the crus again vertical. C,N, caudate nucleus; O, TH, optic thalamus; L2 and L3, middle and outer part of lenticular nucleus; /, a, I, face, arm, and leg fibres. The words in italic indicate corresponding cortical centres. (Gowers.) wards. From the motor area of the cortex they converge to the internal capsule, a comparatively narrow band of fibres passing first of all be- tween the two parts of the corpus striatum, namely, the intra-ventric.u- lar portion, or caudate nucleus, and the extra-ventricular portion, or lenticular nucleus, and then between the optic thalamus internally and THE CEREBROS FINAL NERVOUS SYSTEM. 52t The relations of the inter the lenticular nucleus externally (Fig. 3GO). nal capsule are most important. Corpora Striata.— (1. ) The corpora striata are situated in front of the optic thalami, partly within and partly without the lateral ventricle. Each corpus striatum consists of two parts. (a.) An mtra-ventricular portion (caudate nucleus) which is conical in shape, with the base of the cone forwards; it consists of gray matter, with white substance in its centre, (b. ) An extra-ventricular portion (lenticular nucleus), which is separated from the other portion by a layer of white material, which forms a portion of the internal capsule, _ the anterior limb. The lenticular nucleus is seen, on a horizontal sec- tion of the hemisphere, to consist of three parts, separated from one another by white matter, of which the smallest is inside, each part some- what resembling in shape a wedge. The upper and internal surface is- in relation with the caudate nucleus, being separated from it by the ante- FIG. 361.— Vertical section through the cerebrum and basic ganglia to show the relations of the latter, co, cerebral convolutions; c.e., corpus cailosum; v.L, lateral ventricle; /, fornix; vIIL, third ventricle; n.c., caudate nucleus; th, optic thalamus; n.l.} lenticular nucleus; c.t., internal capsule; cl., claustrum; c.e., external capsule; m, corpus mammillare; t.o., optic tract; s.t.t, stria terrai- nalis; n.a., nucleus amygdalse; cm, soft commissure. (Schwalbe.) rior limb of the internal capsule. The remainder of the internal surface is in relation to the optic thalamus, being separated from it by theposterior limb of the internal capsule. The anterior and posterior limbs of the internal capsule meet at an acute angle, which is known as the knee of the internal capsule. The horizontal section is wider in the centre than at the end. On the outside is the gray lamina (claustrum) sepa- rated by a thin white layer — external capsule — from the lenticular nucleus. Optic Thalami. — (2.) The Optic Thalami are oval in shape, and rest upon the crura cerebri. The upper surface of each thalamus is free, and of white substance, it projects into the lateral ventricle. The posterior surface is also white. The inner sides of the two optic thalami are in partial contact, and are composed of gray material uncovered by white, and are, as a rule, connected together by a transverse portion. 522 HANDBOOK OF PHYSIOLOGY. In the internal capsule the fibres which pass onwards and downwards to the pyramidal tracts of the spinal cord do not occupy more than a small section, namely, that part known as the knee, and the anterior two-thirds of the posterior segment (Fig. 360). In this district the fibres for the face, arm, and leg, are in this relation: those for the face and tongue are just at the knee, and below or behind them come first the fibres for the arm and then those for the leg. The posterior third of the posterior segment is occupied by the sensory fibres. Following the fibres downwards from the internal capsule it is found that those which are motor in function descend in the crusta of the cms on either side, where they are collected into the upper part of the mid- dle third, and that they then pass through the pons to form the anterior pyramids of the medulla. The fibres then either decussate in the middle line, passing over to the opposite side to become the lateral or crossed pyramidal tract of the lateral column of the cord, or remain as the direct pyramidal tract of the anterior column on either side of the anterior fissure. The direct pyramidal tracts, it will be remembered, decussate by degrees in the cord. This pathway of the pyramidal fibres is demonstrated by their degen- eration when any lesion separates the fibres from their corresponding cortical cells, as, for example, a hemorrhage into the corpus striatum of sufficient extent — but the interruption may take place anywhere in the whole course of the tract. If the whole of these fibres on one side is destroyed transversely, above the decussation, hemiplegia of the opposite side, more or less complete, results. The idea which was formerly held, that some of these fibres pass through the corpus striatum does not appear to be supported by sufficient evidence. They have an interrupted course. The reason why a hemorrhage into the corpus striatum pro- duces hemiplegia appears to be because of the almost certain pressure which such a lesion exerts upon the fibres of the internal capsule. Sensory paths in the brain. — The knowledge which we possess of the distribution of the sensory fibres in the brain is not nearly so definite as that which has been obtained of the motor tracts. As we have seen, the course of the sensory fibres even in the cord is not by any means com- pletely understood. Supposing such fibres to be contained chiefly in the anterior part of the lateral columns and in the posterior columns of the cord, having previously crossed over to the opposite side of the cord to that from whence they came, they probably proceed in the posterior half of the medulla, chiefly in the formatio reticularis, and in the correspond- ing part of the pons, beneath the corpora quadrigemina to the tegmen- tum of the crus. In this they pass above the locus niger, and enter the posterior third of the posterior limb of the internal capsule (sensory crossway}. From this district the fibres pass on into the white matter of the brain and probably extend into the so-called motorial areas already THE CEKEBRO-SPINAL NERVOUS SYSTEM. 523 spoken of situated in the posterior frontal and anterior parietal regions. Some of the fibres pass into the optic thalamus. The fibres of the fifth nerve join the tegmentum, and so in the internal capsule are included with the other sensory fibres. This is also probably the case with the other nerves of special sense, — smell, vision, and hearing. Cerebro-cerebellar fibres. — The tracts of fibres connecting the cere- bellum with the cerebrum are in all probability at least three in number. (a. ) Fibres situated in the crusta to the inside of the pyramidal fibres (Fig. 360). These pass upwards in the anterior limb of the internal capsule and proceed into the anterior frontal lobes. In the other direc- tion they descend to the pons, and appear to end in the gray matter within it. But it is very likely that from this gray matter fibres pro- ceed, to the lateral and posterior parts of the opposite side of the cerebellum. As the fibres degenerate downwards they conduct in the same direction, but are arrested at the pons, where they are interrupted by gray matter. (#.) Fibres which in the crusta are situated outside the pyramidal tract do not enter the internal capsule, but at once pro- ceed to the occipital and temporo-sphenoidal lobes. These fibres pro- ceed downwards to the cerebellum, being interrupted in the pons, and from thence proceed to the upper surface of the opposite side of the cerebellum near the middle lobe, (c.) The third tract is situated (Fig. 359, i, F) beneath the pyramidal fibres and above the locus niger. The fibres pass from the corpus striatum chiefly from the caudate nucleus to the pons and thence to the cerebellum. Functions of the Corpora Striata. — The idea that the corpora striata are concerned in the transmission of motor impulses, or that they are the great motor ganglia at the base of the brain, rests upon insufficient evidence. It has been already incidentally mentioned that lesions of the corpora striata produce hemiplegia only because of the pressure effects they exercise upon the internal capsule close by. The caudate nucleus is connected with the opposite side of the cere- bellum by fibres which conduct downwards, and the lenticular nucleus is connected with the cerebellum by fibres from the tegmentum and supe- rior cerebellar peduncles which conduct upwards. It is suggested that the corpora striata are central organs analogous to the cerebral cortex itself. " The analogy to those parts of the cortex that are connected with the cerebellum is rendered still greater by the fact that a lesion, even an ex- tensive lesion, may exist in either the caudate or lenticular nucleus, and so long as it does not interfere with the functions of the motor or sen- sory parts of the internal capsules it causes no persistent symptoms/' (Growers.) Functions of the optic thalami. — That the optic thalami are the great sensory centres at the base of the brain — which was a view held by many until recently — does not seem to be based upon sufficiently accurate ob- 524 HANDBOOK OF PHYSIOLOGY. servations. Some fibres from the tegmentum enter it no doubt, but the main body skirts the ganglion on either side, and does not enter it. Fibres connect the optic thalamus with the superior peduncle of the cerebellum of the opposite side. Fibres connect it with the optic nerves. From the optic thalamus of either side fibres pass to the lenticular nucleus as well as to all parts of the cerebral cortex. Lesions of the optic thalamus do not of themselves produce loss of sensation. If such a symptom follows, it is due to pressure upon, or injury to the posterior limb of the internal capsule. The optic thalamus is connected with visual sensations, and may be a reflex-centre for some of the higher reflex actions. Of the functions of the external capsule and of the claustrum nothing definite is known. The Cerebellum. The Cerebellum (7, 8, 9, 10, Fig. 341), is composed of an elongated central or lobe portion, called the vermiform processes, and two hemi- spheres. Each hemisphere is connected with its fellow, not only by FiG, 362.— Cerebellum in section and fourth ventricle, with the neighboring parts. 1, median groove of fourth ventricle, ending below in the calamus scriptorius, with the longitudinal emi- nences formed by the fasciculi teretes, one on each side; ^, 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 cerebellum, or processus e cerebello ad cerebrum (or ad testes); 6, 6, fillet to the side of the crura cerebri; 7, 7, lateral grooves of the crura cerebri; 8, corpora quadrigemina. (From Sappey after Hirschfeld and Leveille.) 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 Yarolii), while the superior crura with the valve of Vieussens con- nect it with the cerebrum (5, Fig. 361), and the inferior crura (formed by the prolonged restiform bodies) connect it with the medulla oblon- gata (3, Fig. 361). THE CEKEBKO-SPINAL NERVOUS SYSTEM. 525 Structure. — The cerebellum is composed of white and gray matter, the latter being external, like that of the cerebrum, and like it. infolded, so that a larger area may be contained in a given space. The convolu- tions of the gray matter, however, are arranged after a different pattern as shown in Fig. 362. Besides the gray substance on the surface, there is, near the centre of the white substance of each hemisphere, a small cap- sule of gray matter called the corpus dentatum (Fig. 363, cd), resembling very closely the corpus dentatum of the olivary body of the medulla ob- longata (Fig. 363, o). If a section be taken through the cortical portion of the cerebellum, the following distinct layers can be seen (Fig. 364) by microscopic exami- nation. (1.) Immediately beneath the pia mater (p m) is a. layer of considera- ble thickness, which consists of a delicate connective tissue, in which are scattered several spherical corpuscles like those of the granular layer of the retina, and also an immense number of delicate fibres passing up FIG. 363.— Outline sketch of a section of the cerebellum, showing the corpus dentatum. The section has been carried through the left lateral part of the pons, so as to divide the superior pe- duncle 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; g, corpora quadrigemina; sp, superior peduncle of the cerebellum divided; m jo, middle pe- duncle or lateral part of the pons Varolii, with fibres passing from it into the white stem; a v, con- tinuation 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. (Allen Thomson.) %. towards the free surface and branching as they go. These fibres are the processes of the cells of Purkinje. (2.) The Cells of Purkinje (p). These are a single layer of branched nerve-cells, which give off a single unbranched process downwards, and numerous processes up into the ex- ternal layer, some of which become continuous with the scattered cor- puscles. (3.) The granular layer (g), consisting of immense numbers of corpuscles closely resembling those of the nuclear layers of the retina. (4.) Nerve-filre layer (/). Bundles of nerve-fibres forming the white matter of the cerebellum, which, from its branched appearance has been named the " arbor vitae." Functions.— The physiology of the Cerebellum may be considered in its relation to sensation, voluntary motion, and the instincts or higher faculties of the mind. Its supposed functions, like those of every other 526 HANDBOOK OF PHYSIOLOGY. part of the nervous system, have been determined by physiological ex- periment, by pathological observation, and by its comparative anatomy. (1.) With the exception of its middle lobe, it is itself insensible to irritation, and may be all cut away without eliciting signs of pain (Longet). Its removal or disorganization by disease is also generally un- FIG. 364.— Vertical section of dog's cerebellum; p m, pia mater; p, corpuscles of Purkinje, which are branched nerve-cells lying in a single layer and sending single processes downwards and more numerous ones upwards, which branch continuously and extend through the deep "molecular layer " towards the free surface; g, dense layer of ganglionic corpuscles, closely resembling nuclear layers of retina; /, layer of nerve-fibres, with a few scattered ganglionic corpuscles. This last layer ( // ) constitutes part of the white matter of the cerebellum, while the layers between it and the free surface are gray matter. (Klein and Noble Smith.) accompanied by loss or disorder of sensibility; animals from which it is removed can smell, see, hear, and feel pain, to all appearance, as per- fectly as before (Flourens ; Magendie). Yet, if any of its crura be THE CEREBKO-SP1NAL NERVOUS SYSTEM. 527 touched, pain is indicated; and, if the restiform tracts of the medulla oblongata be irritated, the most acute suffering appears to be produced. It cannot, therefore, be regarded as a principal organ of sensation. (2.) Co-ordination of Movements. — In reference to motion, the ex- periments of Longet and many others agree that no irritation of the cerebellum produces movement of any kind. Remarkable results, how- ever, are produced by removing parts of its substance. Flourens (whose experiments have been confirmed by those of Bouillaud, Longet, and others) extirpated the cerebellum in birds by successive layers. Feeble- ness and want of harmony of muscular movements were the consequence of removing the superficial layers. When he reached the middle layers, the animals became restless without being convulsed; their movements 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 pre- serving their equilibrium. 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 and sensation, therefore, were not lost, but merely the faculty of combining the actions of the muscles; and the endeavors of the animal to maintain 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 before referred to, Flourens inferred that the cerebellum belongs neither to the sensory nor the intellectual apparatus; and that it is not the source of voluntary movements, although it be longs to the mot or 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 sup- port from comparative anatomy; animals whose natural movements re- quire most frequent and exact combinations of muscular actions being those whose cerebellaare most developed in proportion to the spinal cord. We must remember, too, that the cerebellum is connected with the posterior columns of the cord as well as with the direct cerebellar tract, both of which probably convey to the middle lobe muscular sensations. It is also connected with the auditory nerves. Movements of the eyes also occur on direct stimulation of the middle lobe. It seems, therefore, to be connected in some way with all of the chief sensory impulses which have to do with the maintenance of the equilibrium. Foville supposed that the cerebellum is the organ of muscular 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 528 HANDBOOK OF PHYSIOLOGY. will upon them; and it must be admitted that all the facts just referred to are as well explained on this hypothesis as on that of the cerebellum being the organ for combining movements. A harmonious combination of muscular actions must depend as much on the capability of appreciat- ing the condition of the muscles with regard to their tension, and to the force with which they are contracting, as on the power which any special nerve-centre may possess of exciting them to contraction. And it is be- cause the power of such harmonious movement would be equally lost, whether the injury to the cerebellum involved injury to the seat of mus- cular sense, or to the centre for combining muscular actions, that ex- periments on the subject afford no proof in one direction more than the other. Forced Movements. — The influence of each half of the cerebellum is directed to muscles on the opposite side of the body; and it would ap- pear 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 move- ments ensue (forced movements). The animals 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 generally from the side on which the injury has been inflicted. The rotations sometimes take place with much rapidity; as often, according to Magendie, as sixty times in a minute, and may last for several days. Similar movements have been observed in men; as by Serres in a man in whom there was apoplectic effusion in the right crus cerrebelli; and by Bellhomme in a woman, in whom an exostosis pressed on the left crus. They may, perhaps, be explained by assuming that the division or injury of the crus cerebelli produces para- lysis 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 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 paralysis of the body is thus made almost complete. Other varieties of forced movements have been observed, especially those named " cir- cus movements," when the animal operated upon moves round and round in a circle; and again those in which the animal turns over and over in a series of somersaults. Nearly all these movements may result on section of one or other of the following parts; viz. crura cerebri, me- dulla, pons, cerebellum, corpora quadrigemina, corpora striata, optic thalami, and even, it is said, of the cerebral hemispheres. THE CEREBKOSPINAL NERVOUS SYSTEM. 529 Sensory Centres in the Cerebral Cortex. Experimental lesions of various portions of the cerebral cortex and stimulation of such parts appears to show that the special senses are in some way represented at definite spots in the convolutions. Thus (a) the visual or optic centre is localized in the occipital lobe on either side on the outer convex part (Fig. 358). This has been demon- strated in the dog's brain by Munk. In the human brain there seems to be a very complex mechanism about this centre. The optic nerve- fibres having partially decussated in the chiasma pass in the optic tract to the optic thalami, and thence to the cortical substance of the occipi- tal lobe. Hemianopia, restriction of the field of vision of opposite sides of the two eyes, may be produced, either by a lesion of one optic tract, in which are (chiefly) the crossed fibres from the nasal portion of the retina of the opposite eye and the uncrossed fibres of the external por- tion of the retina of the corresponding eye; or of the occipital centre. Part of the fibres of the optic tract pass to the corpora geniculata and to the corpora quadrigemina. Each of these so-called half-vision cen- tres of opposite sides, situated in the occipital lobes, appears to be in connection with a higher centre in which the retinae of both eyes are represented, but especially that of the opposite eye. If both occipital lobes be extensively diseased total blindness results. (b) The Olfactory centre, is said to be localized in the anterior ex- tremity of the uncinate gyrus. The fibres, however, appear to be con- nected with a centre on the same side; others cross over to a centre on the opposite side. (c) The Auditory centre, is situated (according to Ferrier and Munk) in the monkey's brain in the first temporo-sphenoidal convolution. The auditory fibres pass up the pons in which they cross, and then in the superior portion of the tegmentum through the hinder portion of the internal capsule to this centre. Destruction of the entire region causes deafness of the opposite ear. (d) The centre for Taste has not yet been localized. According to Gowers, it is quite probable that the whole of the taste-fibres belong to the fifth nerve. Those which are distributed to the anterior parts of the tongue in the chorda tympani, coming from that nerve through the Vid- ian, which passes from the spheno -palatine ganglion to the facial, and those which are distributed to the back of the tongue through the glosso- pharyngeal, being derived from the otic ganglion of the fifth nerve through the small petrosal nerve and the tympanic plexus. 34 CHAPTER XIX. PHYSIOLOGY OF THE CRANIAL NERVES. THE Cranial nerves are commonly enumerated as nine pairs; but the number is in reality twelve pairs, the seventh nerve consisting as it does, of two nerves, and the eighth of three. All arise (superficial ori- gin) from the base of the encephalon, in a double series which extends from the under surface of the anterior cerebral lobes to the lower end of the medulla oblongata. Traced into the substance of the brain and medulla, the roots of the nerves are found to take origin from various &-VVV Fio. 365.— Fourth ventricle, with the medulla oblongata and the corpora quadrigemina. The roman numbers indicate superficial origins of the cranial nerves, while the other numbers indicate their deep origins, or the position of their central nuclei. 8, 8', 8", 8'", auditory nuclei nerves; t, funiculus teres; A, B, corpora quadrigemina; c g, corpus geniculatum; p, c, pedunculus cerebri; m, c, p, middle cerebellar peduncle; s, c, p, superior cerebellar peduncle; t, c, p, inferior cerebellar pe- duncle; I, c, locus caeruleus; e, £, eminentia teres; a, c, ala cinerea; a, n, accessory nucleus; o, obex; c, clava; /, c, funiculus cuneatus; /, gr, funiculus gracilis. masses of gray matter, which are all connected one with another, and with the cerebral hemispheres. The roots of the olfactory and of the optic nerves have been already mentioned. The third and fourth nerves arise from gray matter be- neath the corpora quadrigemina; and the roots of origin of the remainder of the cranial nerves can be traced to gray matter in the medulla oblon- PHYSIOLOGY OF THB CKANIAL NEKVES. 531 gata in the floor of the fourth ventricle, and in the more central part of the medulla, around its central canal, as low down as the decussation of the pyramids. According to their several functions, the cranial nerves may be thus arranged: — A. Nerves of special sense, . . Olfactory, Optic, Auditory, part of the Glosso-pharyn- geal, and part of the Fifth. B. Nerves of common sensation, The greater portion of the Fifth. 0. Nerves of motion, Third, Fourth, lesser division of the Fifth, Sixth, Facial, and Hypoglossal. D. Mixed nerves, ...... Glosso-pharyngeal, Vagus, and Spinal accessory. The physiology of the First, Second, and Eighth will be considered with the organs of Special sense. The Third Nerve, or Motor Oculi. Functions. — The Third nerve, or motor oculi, which arises in three distinct bands of fibres from the gray matter beneath the aqueduct of 3 FIQ. 366.— Diagram of a longitudinal section through the pons, showing the relation of the nuclei for the ocular muscles. CQ, corpora quadrigemina; 3, third nerve; in., its nucleus; 4, fourth nerve; iv., its nucleus, the posterior part of the third; 6, sixth nerve. The probable position of the centre and nerve fibres for accommodation is shown at a and a' ; for the reflex action of iris, at 6, and &'; for the external rectus muscles, at c, c'. The lines beneath the floor of the fourth ven- tricle indicate fibres, which connect the nuclei. (Gowers.) Sylvius near the middle line in conjunction with the fourth nerve. It supplies the levator palpebrae superioris muscle, and all of trie muscles of the eye-ball, but the superior oblique, 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 motor filaments to the iris and ciliary muscle. The fibres which subserve the three functions, accommodation, contraction of the pupil, and nerve-supply to the external ocular muscles, arise from three distinct groups of cells. When the third nerve is irritated within the skull, all those muscles to which it is distributed are convulsed. When it is paralyzed or di- vided the following effects ensue: — (1) the upper eyelid can be no longer 532 HANDBOOK OF PHYSIOLOGY. raised by the levator palpebrse, but droops (ptosis) and remains gently closed over the eye. under the unbalanced influence of the orbicularis palpebrarum, which is supplied by the facial nerve: — (2) the eye is turned outwards (external strabismus) 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, diplopia, is often experienced when a single object is within view of both the eyes: (3) the eye cannot be moved either upwards, dowmvards, or inwards: (4) the pupil becomes dilated (mydriasis), and insensible to light: (5) the eye cannot accommodate for short distances. Contraction and Dilatation of the Pupil. — The relation of the third nerve to the muscles of the iris is of peculiar interest. Under ordinary circumstances the contraction of the iris is a reflex action, which is pro- duced by the stimulus of light on the retina which is conveyed by the optic nerve to the brain (probably to the corpora quadrigemina or me- dulla), 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 centre is destroyed or much compressed. But when the optic nerve is divided, the contraction of the iris may be ex- cited 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 the contraction of the iris. The contraction of the iris thus shows all the characters of a reflex act, and in ordinary cases requires the concurrent action of the optic nerve, its centre, and the third nerve; and, probably also, considering the peculiarities of its perfect mode of action, of the ophthalmic gan- glion. But, besides, both irides will contract under the reflected stimu- lus of light falling upon one retina or under irritation of one optic nerve only. Thus in amaurosis of one eye, its pupil may contract when the other eye is exposed to a stronger light; and generally the contraction of each of the pupils appears to be in direct proportion to the total quan- tity 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 volun- tary 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 ex- cluding those outermost rays of light which would be too far divergent PHYSIOLOGY OF THE CRANIAL NEBVES. 533 to be refracted to a clear image on the retina; and the dilatation in look- ing straight forwards, as in looking at a distant object, permits the ad- mission of the largest number of rays, of which none are too divergent to be so refracted. The Fourth Nerve, or Trochlearis. Functions. — The Fourth nerve, Nervus trochlearis, or Patheticus, is exclusively motor, and supplies only the trochlearis or obliquus supe- rior muscle of the eyeball. It arises from above the fourth ventricle from the valve of Vieussens, but its fibres can be traced to the lower part of the nucleus of the third (Fig. 366) nerve. It decussates with its fellow between its deep and superficial origins. The Fifth Nerve, or Trigeminus. Functions. — The Fifth or Trigeminal nerve resembles, as already stated, the spinal nerves, in that its branches are derived through two roots; namely, the larger or sensory, in connection with which is the Gasserian ganglion, and the smaller or motor root, which has no gan- glion, and which passes under the ganglion of the sensory root to join the third branch or division which ensues from it. The fibres of origin of the fifth nerve appear to come from under the floor of the fourth ven- tricle. The motor root to the inside of the sensory, about the middle of each lateral half. The sensory fibres, however, can be traced down in the medulla as far as the upper part of the cord, these latter fibres bring- ing sensory impressions from the tongue. In addition to these sensory fibres, coming from the nucleus and the spinal cord, there are it is said others coming from the cerebellum. The motor centre is connected with the cerebral cortex of the opposite side. Fibres for the motor root also come from the corpora quadrigemina along the aqueduct of Sylvius. The first and second divisions of the nerve, which arise wholly from the lar- ger root, are purely sensory. The third division being joined, as before said, by the motor root of the nerve, is of course both motor and sen- sory. (a.) Motor Functions. — 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. Filaments are also supplied to the tensor tympani and tensor palati. The motor function of these branches is proved by the violent contraction of all the muscles of masti- cation in experimental irritation of the third, or inferior maxillary divi- sion 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 re- tention of the power of these muscles, when all those supplied by the 534: HANDBOOK OF PHYSIOLOGY. facial nerve lose their power through paralysis of that nerve. The last instance proves best, that though the buccinator muscle gives passage to, and receives some filaments from, a buccal branch of the inferior divi- sion of the fifth nerve, yet it derives its motor power from the facial, for it is paralyzed together with the other muscles that are supplied by the facial, but retains 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 sensory, or in part motor also, must remain for the present doubtful. From the fact that this muscle, besides its other functions, acts in concert or harmony with the muscles of mastication, in keeping the food between the teeth, it might be supposed 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, sensory; although it is not quite certain that it does not give a few motor filaments to the buccinator muscle. (b.) Sensory Functions. — 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 included the organs of the special sense, from which common sensations are conveyed through the fifth nerve, and their special sensations through their sev- eral nerves of special sense. The muscles, also, of the face and lower jaw acquire muscular sensibility, through the filaments of the ganglionic portion of the fifth nerve distributed to them with their proper motor nerves. The sensory function of the branches of the greater division of the fifth nerve is proved, by all the usual evidences, such as their dis- tribution in parts that are sensitive and not capable of muscular con- traction, the exceeding sensibility of some of these parts, their loss of sensation when the nerve is paralyzed or divided, the pain without con- vulsions 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. Other Functions. — In relation to muscular movements, the branches of the greater or ganglionic portion of the fifth nerve exercise a manifold influence on the movements of the muscles of the head and face, and other parts in which they are distributed. They do so, in the first place (a), 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 sensi- bility 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 PHYSIOLOGY OF THE CRANIAL NERVES. 535 that when the fifth nerve is divided, animals are always slow and awk- ward 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 muscular movements, by (b) conveying sensations of the state and position of the skin and other parts: which the mind perceiving, is enabled to deter- mine appropriate acts. Thus, when the fifth nerve or its infra-orbital branch is divided, the movements of the lips in feeding may cease, or be imperfect. Bell supposed that the motion of the upper lip in grasping FIG. 367.— General plan of the branches of the fifth pair. %.— 1, lesser root of the fifth pair; 2, greater root passing forwards into the Gasserian ganglion ; 3, placed on the bone above the ophthal- mic 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 inferior maxillary nerve, giving off the anterior auricular and muscular branches, and continued by the inferior dental to the lower jaw, and by the gustatory to the tongue; a, the sub- maxillary gland, the submaxillary ganglion placed above it in connection with the gustatory nerve; 6, the chorda tympani; 7, the facial nerve issuing from the stylomastoid foramen. (Charles Bell.) 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, arid used the tongue for the prehension of the food. Mayo corrected this error. He found, indeed, that after the infra-orbital 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, 536 HANDBOOK OF PHYSIOLOGY. therefore, justly attributed the phenomena in Bell's experiments to the loss of sensation in the lips; the animal not being able to feel the food, and, therefore, although it had the power to seize it, not knowing how and where to use that power. The fifth nerve has also (c), 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 irrita- tion of the nose excites no sneezing. Through its ciliary branches and the branch which forms the long root of the ciliary or ophthalmic ganglion, it exercises also (d), some influence on the movements of the iris. When the trunk of the ophthalmic portion is divided, the pupil be- comes, 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 stimulus of light. How the fifth nerve thus affects the iris is unexplained; 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 ophthalmic division to the iris; or, as has been ingeniously suggested, the influence of the fifth nerve on the movements of the iris may be ascribed to the affection of vision in con- sequence of the disturbed circulation or nutrition in the retina, when the normal influence of the fifth nerve and ciliary ganglion is disturbed. In such disturbance, increased circulation making the retina more irritable might induce extreme contraction of the iris; or under moderate stimu- lus of light, producing 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. Trophic influence. — 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 special or trophic influence on the nutrition of all these organs; although, in part, the effect of the section of the nerve is only indirectly destructive by abolishing sensation, and therefore the natural safeguard which leads to the protection of parts from external injury. 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 ensues 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 ir- PHYSIOLOGY OF THE CRANIAL NERVES. 537 ritation of the nose), may be at the same time lost or gravely impaired; so may the hearing, and commonly, whenever the fifth nerve is para- lyzed, the tongue loses the sense of taste in its anterior and lateral parts, and according to Growers in the posterior part as well. In relation to Taste, — The loss of tactile sensibility as well as the sense of taste, is no doubt due (a) to the lingual branch of the fifth nerve being a nerve of tactile sense, and also because with it runs the chorda tympani, which is one of the nerves of taste; partly, also, it is due (#), to the fact that this branch supplies, in the anterior and lateral parts of the tongue, a necessary condition for the proper nutrition of that part; while (c), it forms also one chief link in the nervous circle for reflex action, in the secretion of saliva. But, deferring this question un- til 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 sensibility to their several peculiar impressions for the reception and conduction of which they are purposely constructed and supplied with special nerves besides the fifth. The facts observed in these cases 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 unrea- sonable to believe, that, in paralysis of the fifth nerve, their tissues may be the seats of such changes as are seen in the laxit}% the vascular con- gestion, oedema, and other affections of the skin of the face and other tegumentary parts which also accompany the paralysis; and that these changes, which may appear unimportant when they affect external parts, are sufficient to destroy that refinement of structure by which the or- gans of the special senses are adapted to their functions. The Sixth Nerve, or Abducens. Functions. — The Sixth nerve, Nervus abducens or ocularis externus, is also, like the fourth, exclusively motor, and supplies only the rectus externus muscle. It arises from the floor of the fourth ventricle from the anterior region in the deeper part. It is connected (Fig. 367) with the nuclei of the third, fourth, and seventh nerves. It is nearer the middle line than the nuclei of the fifth. The rectus externus is convulsed, and the eye is turned outwards, when the sixth nerve is irritated; and the muscle is paralyzed when the nerve is divided. In all such cases of paralysis, the eye squints inwards, and cannot be moved outwards. 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 communi- 538 HANDBOOK OF PHYSIOLOGY. cations with the sympathetic, and the subsequent distribution of its filaments after joining the sixth nerve,, are quite unknown. The Seventh or Facial Nerve. Functions. — The facial, or portio dura of the seventh pair of nerves, arises from the floor of the central part of the fourth ventricle to the outside of and deeper down than the sixth nucleus. It may be con- nected with the hypoglossal nucleus. There are two roots, the lower and smaller is called the portio intermedia, 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; 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, probably the levator palati and azygos uvulae; by its tympanic branches it supplies the stapedius and laxator tympani, and, through the otic gan- glion, the tensor 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 to the mucous membrane of its an- terior two-thirds; and by branches given off before it comes upon the face, it supplies the muscles of the external ear, the posterior part of the digastricus, and the stylo-hyoideus. Besides its motor influence, the facial is also, by means of the fibres which are supplied to the submaxillary and parotid glands, a secretory nerve. For, through the last-named branches, impressions may be con- veyed which excite increased secretion of saliva. Symptoms of Paralysis of Facial Nerve. — When the facial nerve is divided, or in any other way paralyzed, 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 palpebrae; and the conjunc- tiva, thus continually exposed to the air and the contact of dust, is liable to repeated inflammation, 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. 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 dis- ease 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 com- monly at the same time impaired through the inability to draw air briskly towards the upper part of the nasal cavities in which part alone the olfactory nerve is distributed; because, to draw the air perfectly in PHYSIOLOGY OF THE CRANIAL NERVES. 539 this direction, the action of the dilators and compressors of the nostrils should be perfect. Lastly, the sense of taste is impaired, or may be wholly lost in para- lysis 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 tym- pani. This result, which has been observed in many instances of disease of the facial nerve in man, appears explicable on the supposition that the chorda tympani is the nerve of taste to the anterior two-thirds of the tongue, its fibres being distributed with the so-called gustatory or lin- gual branch of the fifth. Some look upon the chorda as partly or en- tirely made up of fibres from the fifth nerve, and not strictly speaking as a branch of the facial; others consider that it receives its taste fibres from communications with the glosso-pharyngeal. Together with these effects of paralysis of the facial nerve, the mus- cles of the face being all powerless, the countenance acquires on the paralyzed side a characteristic, vacant look, from the absence of all ex- pression: the angle of the mouth is lower, and the paralyzed half of the mouth looks longer than that on the other side; the eye has an unmean- ing stare. All these peculiarities increase, the longer the paralysis lasts; and their appearance is exaggerated when at anytime the muscles of the opposite side of the face are made active in any expression, or in any of their ordinary functions. In an attempt to blow or whistle, one side- of the mouth and cheek acts properly, 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 pow- erless, and food lodges between the cheek and gum. The Ninth, or Glosso-Pharyngeal Nerve. The glosso-pharyngeal nerves (ix., Fig. 341), in the enumeration of the cerebral nerves by numbers according to the position in which they leave the cranium, are considered as divisions 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 their anatomy or physiology. Distribution. — The glosso-pharyngeal nerve gives filaments through its tympanic branch (Jacobson's nerve), to the fenestra ovalis, and fe- nestra 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 pneu- mogastric, and soon after it leaves the cranium, with the sympathetic, digastric branch of the facial, and the accessory nerve, the glosso-pharyn- geal nerve parts into the two principal divisions indicated by its name,. HANDBOOK OF PHYSIOLOGY. 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. Functions. — The glosso-pharyngeal nerve contains some motor fibres, together with those of common sensation and the sense of taste. 1. Its motor influences are distributed to the glosso-pharyngeal, the stylo-pharyngei, palato-glossi, and constrictors of the pharynx. Besides being (2) a nerve of common sensation in the parts which it supplies, and a centripetal nerve through which impressions are con- veyed to be reflected to the adjacent muscles, the glosso-pharyngeal is also a nerve of special sensation; being the nerve of taste (from its fibres derived from the fifth, Gowers), in all the parts of the tongue and palate 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 not themselves, strictly speaking, nerves of this special function, but through their connection with the fifth nerve. For very numerous ex- periments and cases have shown that when the trunk of the fifth nerve is paralyzed or divided, the sense of taste is completely lost in the supe- rior surface of the anterior and lateral parts of the tongue, at the back of the tongue, on the soft palate and palatine arches. The loss is in- stantaneous after division of the nerve; and, therefore, cannot be ascribed wholly to the defective nutrition 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. The Tenth or Pneumogastric Nerve. The Vagus or Par Vagum. The origin of the Vagus nerve is in the lower half of the calamus scriptorius in the ala cinerea (Fig. 365). Its nucleus very probably represents the cells of Clarke's posterior vesicular column of the spinal cord. In origin it is closely connected with the glosso-pharyngeal, spinal accessory, and the hypoglossal. It supplies sensory branches, which accompany the sympathetic on the middle meningeal artery, and others which supply the back part of the meatus and the adjoining part of the external ear. It is connected with the petrous ganglion of the glosso-pharyngeal, by means of fibres to its jugular ganglion; with the spinal accessory which supplies it with its motor fibres for the larger and upper portion of the oesophagus, and with its inhibitory fibres for the heart; also with the hypoglossal, with the superior cervical ganglion of the sympathetic and with the cervical plexus. PHYSIOLOGY OF THE CRANIAL NERVES. l±\ Distribution. — The Pneumogastric nerve, Vervus Vagus, or Par Varfum (1, Fig. 368), has, of all the cranial and spinal nerves, the most various distribution, and influences the most various functions, either through its own filaments, or through those which, derived from other FIG. 368.— View of the nerves of the eighth pair, their distribution and connections on the left side. 2/5.-1, pneumogastric nerve in the neck; 2, ganglion of its trunk; 3, its uni<5n with the spinal accessory; 4, its union with the hypoglossal; 5, pharyngeal branch; 6, superior laryngeal nerve; 7, external laryngeal; 8, laryngeal plexus; 9, inferior or recurrent laryngeal; 10, superior cardiac branch; 11, middle cardiac; IsJ, plexiform part of the nerve in the thorax; 13, posterior pulmonary plexus; 14, lingual or gustatory nerve of the inferior maxillary; 15, hypoglossal, passing into th muscles of the tongue, giving its thyro-hoid branch, and uniting with twigs of the lingual; 16, glosso-pharyngeal nerve; 17, spinal accessory nerve, uniting by its inner branch with the pneumo- gastric and by its outer, passing into the sterno-mast rid muscle; ,18, second cervical nerve; 19, ; 20, fourth; 21, origin of the phrenic nerve; 22, 23, fifth, sixth, seventh, and eighth cervical us 24 suerior cervical anerhon of the third; , , , , , nerves, forming with the first dorsal the brachial plexus; 24, superior cervical ganerhon sympathetic; 25, middle cervical ganglion; 26, inferior cervical ganglion united with the first dorsal glnglion; 27, 28, 29, 30, second, third, fourth, and fifth dorsal ganglia. (From Sappey after Hirsch- feld and Leveille.) 542 HANDBOOK OF PHYSIOLOGY. nerves, are mingled in its branches. The parts supplied by the branches of the vagus nerve are as follows: — (1.) By its pharyngeal branches, which enter the pharyngeal plexus, a large portion of the mucous membrane, and, probably, all the muscles of the pharynx. (2.) By the superior laryngeal 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. (3.) By the inferior laryngeal nerve, the mucous membrane and mus- cular fibres of the trachea, the lower part of the pharynx and larynx, and all the muscles of the larynx except the crico-thyroid. (4.) By its cesopJiageal branches, the mucous membrane and muscular coats of the (Esophagus. (5.) Through the cardiac nerves, moreover, the branches of the vagus form a large portion of the supply of nerves to the heart and the great Arteries derived from both the trunk and the recurrent nerve. (6.) Through both the anterior and the posterior pulmonary plexuses to the Lungs. (7.) Through its gastric branches and to the Stomach, by its termi- nal branches passing over the walls of that organ. (8.) Through hepatic and splenic branches the Liver and the Spleen are partly supplied with nerves. Communications. — Throughout its whole course, the vagus contains both sensory 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 combining with it. This is particularly the case with the filaments of the sym- pathetic nerve, which are abundantly added to nearly all its branches. The likeness to the sympathetic which it thus acquires is further in- creased 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 communicates with the vagus is the accessory nerve, whose internal branch joins its trunk, and is lost in it. Functions. — The particular functions which the branches of the pneumogastric nerve discharge in the several parts to which they are dis- tributed, may be thus summarized. They show that — 1. The pharyn- geal branch is the principal motor nerve of the pharynx and soft palate, and is most probably wholly motor; the chief part of its motor fibres being derived from the internal branch of the accessory nerve. 2. The inferior or recurrent laryngeal nerve is the motor nerve of the larynx. PHYSIOLOGY OF THE CRANIAL NEKVES. 54:3 3. The superior laryngeal nerve is chiefly sensory: the muscles supplied by it being the crico-thyroid, the arytenoid in part (?), and the inferior constrictor of the pharynx. 4. The motions of the oesophagus, the stomach and part of the small intestines are dependent on motor fibres of the vagus, and are probably excited by impressions made upon sensi- tive fibres of the same. 5. The cardiac branches communicate, from the centre in the medullary channel, impulses (inhibitory) regulating the action of the heart. 6. The pulmonary branches form the principal channel by which the sensory impressions on the mucous surface of the trachea, bronchi and lungs that influence respiration, are transmitted to the medulla oblongata; and some fibres also supply motor influence to the muscular portions of the fibres of the trachea and bronchi. 7. Branches to the stomach and intestine not only convey motor but also vaso-motor impulses to those organs. 8. The action of the so-called de- pressor branch (p. 149) in inhibiting the action of the vaso-motor centre has already been treated of, and also the influence of the vagus in stimu- lating the secretion of the salivary glands, as in the nausea which pre- cedes vomiting. To summarize, therefore, the many functions of this nerve, it may be said that it supplies (1) motor influence to the pharynx and oesophagus, stomach and small intestine, the larynx, trachea, bronchi and lung; (2) sensory and in part (3) vaso-motor influence in the same regions; (4) inhibitory influence to the heart ; (5) inhibitory afferent impulses to the vaso-motor centre ; (6) excito-secretory to the salivary glands; (7) excito-motor in coughing, vomiting, etc. Effects of Section. — Division of both vagi, or of both their recurrent branches, is often very quickly fatal in young animals; but in old ani- mals the division of the recurrent nerve is not generally fatal, and that of both vagi is not always fatal, and, when it is so, death ensues slowly. This difference is, probably, because, the yielding of the cartilages of the larynx in young animals permits the glottis to be closed by the atmo- spheric pressure in inspiration, and they are thus quickly suffocated unless tracheotomy be performed. 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 mus- cles are paralyzed, a portion at its posterior part remains open, and through this the animal continues to breathe. In the case of slower death, after division of both the vagi, the lungs are commonly found gorged with blood, oedematous, or nearly solid, with a kind of low pneumonia, 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 nerves exercise on the movements of the air-cells and bronchi; yet, since they are not always produced in one lung when its 544 HANDBOOK OF PHYSIOLOGY. nerve is divided, they cannot be ascribed wholly to the suspension of organic nervous influence. Kather, they may be ascribed to the hindrance to the passage of blood through the lungs, in consequence of the dimin- ished supply of air and the excess of carbonic acid in the air-cells and in the pulmonary capillaries; in part, perhaps, to paralysis of the blood- vessels, leading to congestion; and in part, also, 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 sensibility, is no longer stimulated or closed in consequence of their contact. References to other functions of Vagi.— Regarding the influence of the vagus, see also Heart (p. 131), Arteries (p. 149 1, Salivary Gland (p. 237), Glottis and Larynx (p. 440), Respiration (p. 197), Pharynx and (Esophagus (p. 245), Stomach (p. 256). The Eleventh or Spinal Accessory Nerve. Origin and Connections. — The nerve arises by two distinct origins — one from a centre in the floor of the 4th ventricle, partly but chiefly in the medulla, and connected with the vagus nucleus; the other, from the outer side of the anterior corner of the spinal cord as low down as the 5th or 6th cervical vertebra. The fibres from the two origins come to- gether at the jugular foramen, but separate again into two branches, the inner of which, arising from the medulla, joins the vagus, to which it supplies its motor fibres, consisting of small medullated or visceral nerve- fibres, whilst the outer consisting of large medullated fibres, supplies the trapezius and sterno-mastoid muscles. The small-fibred branch probably arises from a nucleus which corresponds to the posterior vesicular column of Clarke. The principal branch of the accessory nerve, its external branch, then supplies the sterno-mastoid and trapezius muscles; and, though pain is produced by irritating it, is composed almost exclusively of motor fibres. The internal branch accessory nerve supplies chiefly viscero-motor fila- ments to the vagus. The muscles of the larynx, all of which, as already stated, are supplied, apparently, by branches of the vagus, are said to derive their motor nerves from the accessory; and (which is a very sig- nificant fact) Vrolik states that in the chimpanzee the internal branch of the accessory does not join the vagus at all, but goes direct to the larynx. Among the roots of the accessory nerve, the lower or external, arising from the spinal cord, appears 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 sensory as well as motor fibres. PHYSIOLOGY OF THE CKANIAL NERVES. 545 The Twelfth or Hypoglossal Nerve. Origin and Connections. — The hypoglossal nerve arises from two large celled and one small celled, nuclei in the lowest part of the floor of the 4th ventricle near the middle line. The fibres of origin are contin- uous with the anterior roots of the spinal nerves. It is connected with the vagus, the superior cervical ganglion of the sympathetic and with the upper cervical nerves. Distribution. — The hypoglossal or ninth nerve, or motor lingua, has a peculiar relation to the muscles connected with the hyoid bone, includ- ing those of the tongue. It supplies through its descending 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-hyo-glos- sus and linguales. It contributes, also, to the supply of the submaxil- lary gland. Functions. — The function of the hypoglossal is exclusively motor, except in so far as its descending branch may receive a few sensory fila- ments from the first cervical nerve. 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, however, not very striking in the tongue. Often, in cases of hemiplegia 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 forwards or turn it for some distance towards either side. Spinal Nerves. Functions. — Little need be added to what has been already said of these nerves (pp. 480, 481). The anterior roots of the spinal nerves are formed exclusively of motor fibres; the posterior roots exclusively of sensory fibres. Beyond the ganglia, all the spinal nerves are mixed nerves, and contain as well sympathetic filaments. 35 CHAPTER XX. THE SENSES. General Considerations. — Through the medium of the Nervous sys- tem the mind obtains a knowledge of the existence both of the various parts of the body, and of the external world. This knowledge is based upon sensations resulting from the stimulation of certain centres in the brain, by irritations conveyed to them by afferent (sensory) nerves. Under normal circumstances, the following structures are necessary for sensation: (a) A peripheral organ for the reception of the impression; (b) a nerve for conducting it; (c) a nerve-centre for feeling or perceiving it. Classification of Sensations. — Sensations may be conveniently classed as (1) common and (2) special. (1.) Common Sensations. — Under this head fall all those general sensations which cannot be distinctly localized in any particular part of the body, such as Fatigue, Discomfort, Faintness, Satiety, together with Hunger and Thirst, in which, in addition to a general discomfort, there is in many persons a distinct sensation referred to the stomach or fauces. In this class must also be placed the various irritations of the mucous membrane of the bronchi, which give rise to coughing, and also the sen- sations derived from various viscera indicating the necessity of expelling their contents; e. g., the desire to defsecate, to urinate, and, in the female, the sensations which precede the expulsion of the foatus. We must also include such sensations as itching, creeping, tickling, tingling, burning, aching, etc., some of which come under the head of pain : they will be again referred to in describing the sense of Touch. It is impos- sible to draw a very clear line of demarcation between many of the com- mon sensations above mentioned, and the sense of touch, which forms the connecting link between the general and special sensations. 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 common to many nerves; e. g., all the sensory spinal nerves, the vagus, glosso- pharyngeal, and fifth cerebral nerves, and in its impressions being com- municable through many organs. Among common sensations must also be ranked the muscular sense, which has been already alluded to. It is by means of this sense that we become aware of the condition of con- THE SENSES. 547 traction or relaxation of the various muscles and groups of muscles, and thus obtain the information necessary for their adjustment to various purposes — standing, walking, grasping, etc. This muscular sensibility is shown in our power to estimate the differences between weights by the different muscular efforts necessary to raise them. Considerable delicacy may be attained by practice, and the difference between 19£ oz. in one hand and 20 oz. in the other is readily appreciated. This sensibility with which the muscles are endowed must be care^ fully distinguished from the sense of contact and of pressure, of which the skin is the organ. When standing erect, we can feel the ground (contact), and further there is a sense of pressure, due to our feet being pressed against the ground by the weight of the body. Both these are derived from the skin of the sole of the foot. If now we raise the body on the toes, we are conscious (muscular sense) of a muscular effort made by the muscles of the calf, which overcomes a certain resistance. (2.) Special Sensations. — Including the sense of touch, the special senses are five in number — Touch, Taste, Smell, Hearing, Sight. Difference between Common and Special Sensations. — The most im- portant distinction between common and special sensations is that by the former we are made aware of certain conditions of various parts of our bodies, while from the latter we gain our knowledge of the external world also. This difference will be clear if we compare the sensations of pain and touch, the former of which is a common, the latter a special sensation. "If we place the edge of a sharp knife on the skin, we feel the edge by means of our sense of touch; we perceive a sensation, and refer it to the object which has caused it. But as soon as we cut the skin with the knife, we feel pain, a feeling which we no longer refer to the cutting knife, but which we feel within ourselves, and which com- municates to us the fact of a change of condition in our own body. By the sensation of pain we are neither able to recognize the object which caused it, nor its nature." General Characteristics : Seat. — In studying the phenomena of sen- sation, it is important clearly to understand that the Sensorium, or seat of sensation, is in the Brain, and not in the particular organ (eye, ear, etc.) through which the sensory impression is received. In common parlance we are said to see with the eye, hear with the ear, etc. , but in reality these organs are only adapted to receive impressions which are conducted to the sensorium, through the optic and auditory nerves re- spectively, and there give rise to sensation. Hence, if the optic nerve is severed (although the eye itself is per- fectly uninjured), vision is no longer possible; since, although the image falls on the retina as before, the sensory impression can no longer be conveyed to the sensorium. When any given sensation is felt, all that we can with certainty affirm is that the sensorium in the brain is excited. 548 HANDBOOK .OF PHYSIOLOGY. The exciting cause may be (in the vast majority of cases is), some ob- ject of the external world (objective sensation); or the condition of the sensorium may be due to some excitement within the brain, in which case the sensation is termed subjective. The mind habitually refers sensations to external causes; and hence, whenever they are subjective (due to 'causes within the brain), we can hardly divest ourselves of the idea of an external cause, and an illusion is the result. Illusions. — Numberless examples of such illusions might be quoted. As familiar cases may be mentioned, humming and buzzing in the ears caused by some irritation of the auditory nerve or centre, and even musi- cal sounds and voices (sometimes termed auditory spectra); also so-called optical illusions: persons and other objects are described as being seen, although not present. Such illusions are most strikingly exemplified in cases of delirium tremens or other forms of delirium, in which cats, rats, creeping loathsome forms, etc., are described by the patient as seen with great vividness. Cases of Illusions. — 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 sen- sations of light and luminous flashes; in the auditory nerve, the sensa- tion of humming and ringing sounds; in the olfactory nerve, the 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 aurium; " and in the common sensory nerves, the sensation of creeping over the surface. So, also, among external causes, the stimu- lus 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 in the other parts of the body, a perception of peculiar jar- ring or of the mechanical impression, or a shock like it. Experiments seem to have proved, however, that none of the nerves of special sense possess the faculty of common sensibility. Thus, Ma- gendie observed that when the olfactory nerves, laid bare in a dog, were pricked, no signs of pain were manifested; and other experiments of his seem to show that both the retina and optic nerve are insusceptible of pain. Further, the optic nerve is insusceptible to the stimulus of light when severed from its connection with the retina which alone is adapted to receive luminous impressions. Sensations and Perceptions. — The habit of constantly referring our sensations to external causes, leads us to interpret the various modi- fications which external objects produce in our sensations, as properties THE SENSES. 549 of the external bodies themselves. Thus we speak of certain substances as possessing a disagreeable taste and smell; whereas, the fact is, their taste and smell are only disagreeable to us. It is evident, however, that on this habit of referring our sensations to causes outside ourselves (per- ception), depends the reality of the external world to us; and more especially is this the case with the senses of touch and sight. By the co- operation of these two senses, aided by the others, we are enabled gradu- all}r to attain a knowledge of external objects which daily experience confirms, until we come to place unbounded confidence in what is termed the " evidence of the senses." Judgments. — We must draw a distinction between mere sensations, and the judgments based, often unconsciously, upon them. Thus, in looking at the near object, we unconsciously estimate its distance, and say it seems to be ten or twelve feet off: but the estimate of its distance is in reality a judgment based on many things besides the appearance of the object itself; among which may be mentioned the number of the in- tervening objects, the number of steps which from past experience we know we must take before we could touch it, and many others. Sensation of Motion is, like motion itself, of two kinds — progressive and vibratory. The faculty of the perception of progressive motion is possessed chiefly by the senses of vision, touch, and taste. Thus an im- pression 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 vibrations, is perceived by several senses, but especially by those of hearing and touch. Sensations of Chemical Actions. — 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 influ- ence 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 per- ceived not only by the sense of smell, but also by the senses of touch and taste. Thus, the vapors of horse-radish and mustard, and acrid suffo- cating gases, act upon the conjunctiva and the mucous membrane of the lungs, exciting through the common sensory nerves, merely modifica- tions of common feeling; and at the same time they excite the sensa- tions of smell and of taste. 550 HANDBOOK OF PHYSIOLOGY. THE SPECIAL SENSES. I. Touch. Seat. — 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 modi- fication 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 sensory cerebral nerves. But, although all parts of the body supplied with sensory nerves are thus, in some degree, organs of touch, yet the sense is exercised in per- fection only in those parts the sensibility of which is extremely delicate, e. g., the skin, the tongue, and the lips, which are provided with abun- dant papillae. A peculiar and, of its own kind in each case, a very acute sense of touch is exercised through the medium of the nails and teeth. To a less extent the hair may be reckoned an organ of touch; as in the case of the eyelashes. The sense of touch renders us conscious of the presence of a stimulus, from the slightest to the most intense degree of its action, by that indescribable something which we call feeling, or common sensation. 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 the 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. Varieties. — (a) The sense of Touch, strictly so-called (tactile sensi- bility), (b) the sense of Pressure, (c) the sense of Temperature. These when carried beyond a certain degree are merged in (d) the sen- sation of Pain. Various peculiar sensations, such as tickling, must be classed with pain under the head of common sensations, since they give us no infor- mation as to external objects. Such sensations, whether pleasurable or painful, are in all cases referred by the mind to the part affected, and not to the cause which stimulates the sensory nerves of the part. The sensation of tickling may be produced in many parts of the body, but with especial intensity in the soles of the feet. Among other sensations belonging to this class, and confined to particular parts of the body, may be mentioned those of the genital organs and nipples. (a) Touch proper. — In almost all parts of the body which have delicate tactile sensibility the epidermis, immediately over the papillae, THE SENSES. 551 is moderately thin. When its thickness is much increased, as over the heel, the sense of touch is very much dulled. On the other hand, when it is altogether removed, and the cutis laid bare, the sensation of contact is replaced by one of pain. Further, in all highly sensitive parts, the papillae are numerous and highly vascular, and usually the sensory nerves are connected with special End-organs. The acuteness of the sense of touch depends very largely on the cutaneous circulation, which is of course largely influenced by external temperature. Hence the numbness, familiar to every one, produced by the application of cold to the skin. Special organs of touch are present in most animals, among which may be mentioned the antennas of insects, the "whiskers" (vibrissae) of cats and other carnivora, the wings of bats, the trunk of the elephant, and the hand of man. Judgment of the Form and Size of Bodies. — By the sense of touch the mind is made acquainted with the size, form, and other external charac- ters of bodies. And in order that these characters may be easily ascer- tained, 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 sensi- bility, generally, as the act of seeking, following, or examining odors, does to the sense of smell. The hand 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 the body; the power it possesses of opposing the thumb to the rest of the hand, and the relative mobility of the fin- gers; and lastly from the abundance of the sensory terminal organs which it possesses. In forming a conception of the figure and extent of a surface, the mind multiplies the size of the hand or fingers used in the inquiry by the number of times which it is contained in the surface tra- versed; and by repeating this process with regard to the different dimen- sions of a solid body, acquires a notion of its cubical extent, but, of course, only an imperfect notion, as other senses, e. g., the sight, are required to make it complete. Acuteness of Touch. — The perfection of the sense of touch on differ- ent parts of the surface is proportioned to the power which such parts possess of distinguishing and isolating the sensations produced by two points placed close 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 different contiguous points will act on only 552 HANDBOOK OF PHYSIOLOGY. one nervous fibre, and hence be confounded, and perhaps produce but one sensation. Experiments have been made to determine the tactile properties of different parts of the skin, as measured by this power of distinguishing distances. These consist of touching the skin, while the eyes are closed, with the points of a pair of compasses sheathed with cork, and in ascertaining how close the points of compasses might be brought to each other, and still be felt as two bodies. Table of variations in the tactile sensibility of different parts. — The measurement indicates the least distance at which the two Hunted points of a pair of compasses could be separately distin- guished. (E. H. Weber.) Tip of tongue, . . . , • -j Palmar surface of third phalanx of forefinger, . iV '* Palmar surface of second phalanges of fingers, Eed surface of under-lip, . . . . i " Tip of the nose, . . . . . ^ " Middle of dorsum of tongue, . . . £ (C Palm of hand, . . . . . . X « Centre of hard palate, . . . . \ " Dorsal surface of first phalanges of fingers, . . T\ " Back of hand, . . . . . 1-i- " Dorsum of foot near toes, . . . lj " Gluteal region, . . . . . 1^ '• Sacral region, . . . . . 1£ " Upper and lower parts of forearm, . . . 1£ " Back of neck near occiput, . . . 2 " Upper dorsal and mid-lumbar regions, . . 2 " Middle part of forearm, . . . 2-J- " Middle of thigh, ..... 24 " Mid-cervical region, . . . . 2J " Mid-dorsal region, . . . . . 2$ " Moreover, in the case of the limbs, it was found that before they were recognized as two, the points of the compasses had to be further separated when the line joining them was in the long axis of the limb, than when in the transverse direction. According to Weber the mind estimates the distance between two points by the number of unexcited nerve-endings which intervene be- tween the two points touched. It would appear that a certain number of intervening unexcited nerve-endings are necessary before two points touched can be recognized as separate, and the greater this number the more clearly are the points of contact distinguished as separate. By practice the delicacy of a sense of touch may be very much increased. A familiar illustration occurs in the case of the blind, who, by constant practice, can acquire the power of reading raised letters the forms of which are almost if not quite undistinguishable, by the sense of touch to an ordinary person. THE SENSES. 553: The power of correctly localizing sensations of touch is gradually derived from experience. Thus infants when in pain simply cry, but make no effort to remove the cause of irritation, as an older child or adult would, doubtless on account of their imperfect knowledge of its exact situation. By long experience this power of localization becomes perfected, till at length the brain possesses a complete "picture" as it were of the surface of the body, and is able with marvellous exactness to localize each sensation of touch. Illusions of Touch. — The different degrees of sensitiveness possessed by different parts may give rise to errors of judgment in estimating the distance between two points where the skin is touched. Thus, if blunted points of a pair of compasses (maintained at a constant distance apart) be slowly drawn over the skin of the cheek towards the lips, it is almost impossible to resist the conclusion that the distance between the points is gradually increasing. When they reach the lips they seem to be con- siderably further apart than on the cheek. Thus, too, our estimate of the size of a cavity in a tooth is usually exaggerated when based upon sensation derived from the tongue alone. Another curious illusion may here be mentioned. If we close the eyes, and place a small marble or pea between the crossed fore and middle fingers, we seem to be touching two marbles. This illusion is due to an error of judgment. The marble is touched by two surfaces which, under ordinary circumstances, could only be touched by two separate marbles, hence, the mind taking no cognizance of the fact that the fingers are crossed, forms the conclusion, that two sensations are due to two marbles. (b) Pressure. — It is extremely difficult to separate touch proper from sensations of pressure, and, indeed, the former may be said to de- pend upon the latter. If the hand be rested on the table and a very light body such as a small card placed on it, the only sensation produced is one of contact; if, however, an ounce weight be laid on the card an additional sensation (that of pressure) is experienced, and this becomes more intense as the weight is increased. If now the weight be raised by the hand, we are conscious of overcoming a certain resistance; this consciousness is due to what is termed the "muscular sense." The esti- mate of a weight is, therefore, usually based on two sensations, (1) of pressure on the skin, and (2) the muscular sense. The estimate of weight derived from a combination of these two sen- sations (as in lifting a weight) is more accurate than that derived from the former alone (as when a weight is laid on the hand); thus Weber found that by the former method he could generally distinguish 19£ oz. from 20 oz., but not 19f oz. from 20, while by the latter he could at most only distinguish 14^ oz. from 15 oz. It is not the absolute, but the relative, amount of the difference of weight which we have thus the fa.culty of perceiving. 554 HANDBOOK OF PHYSIOLOGY. 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 influence necessary for the production of a certain degree of movement. 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 or muscular action, or of nervous energy, which we had ex- erted 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 distinguished from the actual sensation of fatigue in the muscles. So, with regard to the ideas derived from sensations of touch combined with movements, it is doubtful how far the consciousness of the extent of muscular movement is obtained from sensations in the muscles them- selves. The sensation of movement attending the motions of the hand is very slight; and persons who do not know that the action of particu- lar muscles is necessary 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. (c) Temperature. — The whole surface of the body is more or less sensitive to differences of temperature. The sensation of heat is distinct from that of touch; and it would seem reasonable to suppose that there are special nerves and nerve-endings for temperature. At any rate, the power of discriminating temperature may remain unimpaired when the sense of touch is temporarily in abeyance. Thus if the ulnar nerve be compressed at the elbow till the sense of touch is very much dulled in the fingers which it supplies, the sense of temperature remains quite un- affected. The sensations of heat and cold are often exceedingly fallacious, and in many cases are no guide at all to the absolute temperature as indicated by a thermometer. All that we can with safety infer from our sensations of temperature, is that a given object is warmer or cooler than the skin. Thus the temperature of our skin is the standard; and as this varies from hour to hour according to the activity of the cutaneous circulation, our estimate of the absolute temperature of any body must necessarily vary too. If we put the left hand into water at 40° F. and the right into water at 110° F., and then immerse both in water at 80° F., it will feel warm to the left hand but cool to the right. Again, a piece of metal THE SENSES. 555 which has really the same temperature as a given piece of wood will feel much colder, since it conducts away the heat much more rapidly. For the same reason air in motion feels very much cooler than air of the same temperature at rest. Perhaps the most striking example of the fallaciousness of our sensa- tions as a measure of temperature is afforded by fever. In a shivering fit of ague the patient feels excessively cold, whereas his actual tempera- ture is several degrees above the normal, while in the sweating stage which succeeds it he feels very warm, whereas really his temperature has fallen several degrees. In the former case the cutaneous circulation is much diminished, in the latter much increased; hence the sensations of cold and heat respectively. In some cases we are able to form a fairly accurate estimate of abso- lute temperature. Thus, by plunging the elbow into a bath, a practised bath-attendant can tell the temperature sometimes within 1° F. The temperatures which can be readily discriminated are between 50°-115° F. (10°-15° C.); very low and very high temperatures alike produce a burning sensation. A temperature appears higher according to the extent of cutaneous surface exposed to it. Thus, water of a tem- perature which can be readily borne by the hand, is quite intolerable if the whole body be immersed. So, too, water appears much hotter to the hand than to a single finger. The delicacy of the sense of temperature coincides in the main with that of touch, and appears to depend largely on the thickness of the skin; hence, in the elbow, where the skin is thin, the sense of tempera- ture is delicate, though that of touch is not remarkably so. Weber has further ascertained the following facts: two compass points so near to- gether on the skin that they produce but a single impression, at once give rise to two sensations, when one is hotter than the other. More- over, of two bodies of equal weight, that which is the colder feels heavier than the other. 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 com- pare 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 temperatures, experi- enced one after the other, better than between temperatures to which the two hands were simultaneously subjected. This power of comparing present with past sensations diminishes, however, in proportion to the time which has elapsed between them. After-sensations left by impres- sions on nerves of common sensibility or touch are very vivid and dura- ble. As long as the condition into which the stimulus has thrown the organ endures, the sensation also remains, though the exciting cause 556 HANDBOOK OF PHYSIOLOGY. should have long ceased to act. Both painful and pleasurable sensations afford many examples of this fact. 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, etc., may be produced by internal causes. Neuralgic pain, the sensation of rigor, formication or the creeping of ants, and the states of the sexual organs occurring during sleep, afford striking examples of subjective sensations. The mind has a remarkable power of exciting sensations 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; numerous examples of this influence might be quoted. II._Taste. Conditions necessary. — The conditions for the perceptions of taste are: — 1, the presence of a nerve and nerve-centre with special endow- ments; 2, the excitation of the nerve by the sapid matters, which for this purpose must be in a state of solution. The nerves concerned in the production of the sense of taste have been already considered (pp. 537 and 540). The mode of action of the substances which excite taste consists in the production of a change in the condition of the gustatory nerves, and the conduction of the stimulus thus produced to the nerve- centre; and, according to the difference of the substances, an infinite va- riety of changes of condition of the nerves, and consequently of stimula- tions of the gustatory centre, may be induced. 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 tongue and fauces are dry, sapid substances, even in solution, are with difficulty tasted. 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 mechanical impressions. Thus, a small current of air directed upon the tongue gives rise to a cool saline taste, like that of saltpetre; and 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. Seat. — The principal seat (apparent seat, that is, to our senses) of the sense of taste is the tongue. But the results of experiments as well as ordinary experience show that the soft palate and its arches, the THE SENSES. 557 uvula, tonsils, and probably the upper part of the pharynx, are also 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 adduced that the sense of taste is conferred upon them by this nerve. In most, though not in all persons, the anterior parts of the tongue, especially the edges and tip, are en- dowed with the sense of taste. The middle of the dorsum is only feebly endowed with this 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 sensi- tive parts. The Tongue. Structure. — The tongue is a muscular organ covered by mucous mem- brane. The muscles, which form the greater part of the substance of the tongue (intrinsic muscles) are termed linguales; and by these, which are attached to the mucous membrane chiefly, its smaller and more delicate movements are chiefly performed. By other muscles (extrinsic muscles), as the genio-hyoglossas, the styloglossus, etc., the tongue is fixed to surrounding parts; and by this group of muscles its larger movements are performed. The mucous membrane of the tongue resembles other mucous mem- branes in essential points of structure, but contains papillce, more or less peculiar to itself; peculiar, however, in details of structure and arrange- ment, not in their nature. The tongue is beset with numerous mucous follicles and glands. The use of the tongue in relation to mastication and deglutition has already been considered. The larger papilla of the tongue are thickly set over the anterior two-thirds of its upper surface, or dorsum (Fig. 369), and give to it its characteristic roughness. In carnivorous animals, especially those of the cat tribe, the papillae attain a large size, and are developed into sharp recurved horny spines. Such papillae cannot be regarded as sensitive, but they enable the tongue to play the part of a most efficient rasp, as in scraping bones, or of a comb in cleaning fur. 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 diversities of form; but three principal varieties, differing both in seat and general characters, may usually be distinguished, namely, the (1) circumvallate, the (2) fungi- form, and the (3) 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 blood-vessels and nerves. In details of structure, however, they differ considerably one from another. 558 HANDBOOK OF PHYSIOLOGY. The surface of each kind is studded by minute conical processes of mucous membrane, which thus form secondary papillae. Simple papillae also occur over most other parts of the tongue not occupied by the compound papillae, and extend for some distance behind the papillae circumvallatae. They are commonly buried beneath the epithelium; hence they are often overlooked. The mucous mem- brane immediately in front of the epiglottis is, however, free from them. Fia. 369.— Papillar surface of the tongue, with the fauces and tonsils. 1, 1, circumvallate papillae, in front of 2, the foramen caecum; 3, fungiform papillae; 4, filiform and conical papillse; 5, transverse and oblique rugas; 6, mucous glands at the base of the tongue and in the fauces; 7, tonsils; 8, part of the epiglottis; 9, median glosso-epiglottidean fold (fraenum epiglottidis). (From Sappey.) (1.) Circumvallate. — These papillae (Fig. 370), eight or ten in num- ber, are situate in two V-shaped lines at the base of the tongue (1, 1, Fig. 369). They are circular elevations from -g^th to TVth 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 THE SENSES. 559 elevation and the ring being formed of close-set simple papillae (Fig. 370). (2.) Fungiform. — The fungiform papillae (3, Fig. 369) are scattered chiefly over the sides and tip, and sparingly over the middle of the dor- sum, of the tongue; their name is derived from their being usually nar- rower at their base than at their summit. They also consist of groups FIG. 370.— Vertical section of a circumvallate papilla of the calf. 1 and 3, epithelial layers cov- ering it; 2. taste goblets; 4 and 4', duct of serous gland opening out into the pit in which papilla is situated ; 5 and 6, nerves ramifying within the papilla,. (Engelmann.) of simple papillae (A. Fig. 371), each of which contains in its interior a loop of capillary blood-vessels (B.), and a nerve fibre. (3.) Conical or Filiform. — These, which are the most abundant papillae, are scattered over the whole surface of the tongue, but espe- cially over the middle of the dorsum. They vary in shape somewhat, FIG. 371.— Surface and section of the fungiform papillae. A, the surface of a fungiform papilla, partially denuded of its epithelium; p, secondary papillse ; e, epithelium. B, section of a fungiform papilla with the blood-vessels injected; a, artery; v, vein; c, capillary loops of similar papillee in neighboring structure of the tongue ; d, capillary loops of the secondary papillae; e, epithelium. (From Kolliker, after Todd and Bowman.) 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 pro- jections, hair-like in appearance, and in some instances in structure also (Fig. 371). From their peculiar structure, it seems likely that these papillae have a mechanical function, or one allied to that of touch rather 560 HANDBOOK OF PHYSIOLOGY. than of taste; the latter sense being probably seated especially in the other two varieties of papillae, the circumvallate and the fungiform. The epithelium of the tongue is stratified with the upper layers of the squamous kind. It covers every part of the surface; but over the i'ungiform papillae forms a thinner layer than elsewhere. The epithe- lium covering the filiform papillae is extremely dense and thick, and as before mentioned, projects from their sides and summits in the form of long, stiff, hair-like processes (Fig. 372). Many of these processes bear a close resemblance to hairs. Blood-vessels and nerves are supplied freely to the papillae. The nerves in the fungiform and circumvallate FIG. 372.— Two filiform papillse, one with epithelium, the other without 35/1.— d, the substance of the papillae dividing at their upper extremities into secondary papillae; a, artery, and v, vein, di- viding into capillary loops; e, epithelial covering, laminated between the papillse, but extended into hair-like processes, /, from the extremities of the secondary papillae. (From Kolliker, after Todd and Bowman.) papillae form a kind of plexus, spreading out brush-wise (Fig. 370), but the exact mode of termination of the nerve-filaments is not certainly known. Taste Goblets. — In the circumvallate papillae of the tongue of man peculiar structures known as gustatory luds or taste goblets, have been discovered. They are of an oval shape, and consist of a number of closely packed, very narrow and fusiform, cells (gustatory cells'). This central core of gustatory cells is inclosed in a single layer of broader fu- THE SENSES. 561 siform cells (encasing cells]. The gustatory cells terminate in fine spikes not unlike cilia, which project on the free surface (Fig. a, 373). These bodies also occur side by side in considerable numbers in the epithelium of the papilla foliata, which is situated near the root of the tongue in the rabbit, and also in man. Similar taste*goblets have been observed on the posterior (laryngeal) surface of the epiglottis. It seems probable, from their distribution, that these taste-goblets are gustatory in function, though no nerves have been distinctly traced into them. Other Functions. — Besides the sense of taste, the tongue, by means also of its papillae, is endued (2) especially at its side 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 pressure, and conse- quently of the form of surfaces. The tongue may lose its common sen- sibility, and still retain the sense of taste, and vice versa. This fact renders it probable that, although the senses of taste and of touch may FIG. 373.— Taste-goblet from dog's epiglottis Oaryngeal surface near the base), precisely similar in structure to those found in the tongue, a, depression in epithelium over goblet; below the letter are seen the fine hair-like processes in which the cells terminate ; c, two nuclei of the axial (gusta- tory) cells. The more superficial nuclei belong to the superficial (encasing) cells; the converging lines indicate the fusiform shape of the encasing cells. X 400. (Schofield.) 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 seem to prove that the lingual branch of the fifth nerve is the conductor 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 functions of the tongue in connection with (3) speech, (4) mas- tication, (5) deglutition, (6) suction, have been referred to in other chapters. 36 562 HANDBOOK OF PHYSIOLOGY. Taste and Smell: Perceptions. — The concurrence of common and special sensibility in the same part makes it sometimes difficult to deter- mine 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. Much of the perfection of the sense of taste is often due to the sapid substances being also odorous, and exciting the simultaneous 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. Varieties of Tastes. — Among the most clearly defined tastes are the sweet and bitter (which are more or less opposed to each other), the acid, alkaline, and saline tastes. Acid and alkaline taste may be excited bv electricity. If a piece of zinc be placed beneath and a piece of copper above the tongue, and their ends brought into contact, an acid taste (due to the feeble galvanic current) is produced. The delicacy of the sense of taste is sufficient to discern 1 part of sulphuric acid in 1000 of water; but it is far surpassed in acuteness by the sense of smell, After-taste. — 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, 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 conso- nance 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 per- ception 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 impossible to discriminate between them. The simple contact of a sapid substance with the surface of the gus- tatory 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, friction, and motion between the tongue and palate. THE SENSES. 563 Subjective Sensations of Taste. — The sense of taste seems capable of being excited only by external 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 external causes, such as changes in the nature of the secretions of the mouth. III.— Smell. Conditions necessary. — (1.) The first conditions essential to the sense of smell are a special nerve and nerve-centre, the changes in whose con- dition are perceived in sensations of odor, for no other nervous structure 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 centre may cause another peculiar sensation through the nerves of taste, and may produce an irritating and burning sensation on the nerves of touch; but the sensation of odor is yet separate and distinct from these, though it may be simultaneously perceived. (2.) The second condition of smell is a peculiar change produced in the olfactory nerve and its centre by the stimulus or odorous substance. (3.) The material causes of odors are, usually, in the case of animals living in the air, either solids suspended in a state of extremely 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 mucous membrane before they can be immediately applied to, or affect the olfactory nerves; therefore a further condition necessary for the perception of odors is, that the mucous membrane of the nasal cavity be moist. When the Schneiderian membrane is dry, the sense of smell is impaired or lost; in the first stage of catarrh, when the secretion of mucus within the nostrils is lessened, the faculty of perceiving odor is either lost, or rendered very imperfect. (4.) 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 inter- rupting respiration we prevent the perception of odors, and by repeated quick inspiration, assisted, as in the act of sniffing, by the action of the nostrils, we render the impression more intense. An odorous substance in a liquid form injected into the nostrils appears incapable of giving rise to the sensation of smell; thus Weber could not smell the slightest odor when his nostrils were completely filled with water containing a large quantity of eau-de-Cologne. Seat. — The human organ of smell is formed by the filaments of the olfactory nerves, distributed in the mucous membrane covering the 564 HANDBOOK OF PHYSIOLOGY. 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 FIG. 374.— Nerves of the septum nasi, seen from the right side. ?£.—!, the olfactory bulb; 1, the olfactory nerves passing through the foramina of the cribriform plate, and descending to be distrib- uted on the septum; 2, the internal or septal twig of the nasal branch of the ophthalmic nerve; 3, naso-palatine nerves. (From Sappy, after Hirschfeld and Leveille.) of the nasal cavities beneath the cribriform plates of the ethmoid bones (Figs. 374 and 376). The olfactory region is covered by cells of cylin- FIG. 375. FIG. 376. FIG. 375.— Section through the olfactory mucous membrane of the new-born child, a, non-nu- clear; and 6, nucleated portions of the epithelium; c, nerves; dd, glands, marked out by Schultze as Bowman's. (M. Schultze.) FIG. 376.— Nerves of the outer walls of the nasal fossae. 3/5.— 1, network of the branches of the olfactory nerve, descending upon the region of the superior and middle turbinated bones ; 2, ex- ternal twig of the ethmoidal branch of the nasal nerves; 3, spheno-palatine ganglion; 4, ramifica- tion of the anterior palatine nerves; 5, posterior, and 6, middle divisions of the palatine nerves; 7, branch to the region of the inferior turbinated bone; 8, branch to the region of the superior and middle turbinated bones; 9, naso-palatine branch to the septum cut short. (From Sappey, after Hirschfeld and Leveille.) drical epithelium, prolonged at their deep extremities into fine branched processes, but not ciliated; and interspersed with these are fusiform THE SENSES. 565 {olfactory) cells, with both superficial and deep processes (Fig. 377), the latter being probably connected with the terminal filaments of the olfac- tory nerve. The lower, or respiratory part, as it is called, of the nasal fossae is lined by cylindrical ciliated epithelium, except in the region of the nostrils, where it is squamous. The branches of the olfactory nerves retain much of the same soft and grayish texture which distinguishes those of the olfactory tracts within the cranium. Their filaments, also, are peculiar, more resembling those of the sympathetic nerve than the filaments of the other cerebral nerves do, containing no outer white sub- stance, and being finely granular and nucleated. The sense of smell is derived exclusively through those parts of the nasal cav- ities in which the olfactory nerves are distributed; the accessory cavities or sinuses communicating with the nostrils seem to have no relation to it. Air impreg- nated with the vapor of camphor was injected into the frontal sinus through a fistulous opening and odorous substances have been injected into the antrum of High- more; 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 obtained by filling their cavities with fat, is here attained, as it is in many bones of birds, by their being filled with air. Other Functions of the Olfactory Region. — All parts of the nasal cavities, whether or not they can be the seats of the sense of smell, are endowed with com- mon sensibility by the nasal branches of the first and sec- ond divisions 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 of smell is lost, while the mucous membrane of the nose remains susceptible of the vari- ous modifications of common sensation 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, horse-radish, mustard, etc., 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 eyelids. It was because the common sensibility of the nose to these irritating substances remained after the destruction of the FIQ. 877.— Epithe- lial and olfactory cells of man. The letters are placed on the free surface, EE, epithelial cells; Olf., olfactory cells. (Max Schultze.) 566 HANDBOOK OF PHYSIOLOGY. olfactory nerves, that Magendie was led to the erroneous belief that the fifth nerve might exercise this special sense. Varieties of Odorous Sensations. — Animals do not all equally perceive the same odors; the odors most plainly perceived by an herbivorous ani- mal 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. Her- bivorous animals are peculiarly sensitive to the latter, and have a nar- rower sensibility to animal odors, especially to such as proceed from other individuals 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 sus- ceptibility to various odors is more uniform and extended. The cause of this difference lies probably in the endowments of the cerebral parts of the olfactory apparatus. The delicacy of the seuse of smell is most remarkable; it can discern the presence of bodies in quantities so minute as to be undiscoverable even by spectrum analysis; TumrthnrTnr °^ a grain of musk can be distinctly smelt (Valentin). Opposed to the sensation of an agreeable odor is that of a disagreeable or disgusting odor, which corresponds to the sensations of pain, dazzling and disharmony of colors, and dissonance 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 sever- ally 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 dif- ference in the acuteness of the sense of smell as among others in the acuteness of sight. We have no exact proof that a relation of harmony and disharmony 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 perceptive 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 plea- sure in them. Subjective Sensations. — The sensations of the olfactory nerves, inde- pendent of the external application of odorous substances, have hitherto- been little studied. The friction of the electric machine produces a smell like that of phosphorus. Hitter, too, has observed, that when gal- vanism is applied to the organ of smell, besides the impulse to sneeze, THE SENSES. 567 and the tickling sensation excited in the filaments of the fifth nerve, a smell like of ammonia was excited by the negative pole, and an acid odor by the positive pole; whichever of these sensations were produced, it remained constant as long as the circle was closed, and changed to the other at the moment of the circle being opened. Subjective sensations occur frequently in connection with the sense of smell. Frequently a person smells something which is not present, and which other persons cannot smell; this is very frequent with nervous people, but it occasion- ally happens to every one. In a man who was constantly conscious of a bad odor, the arachnoid was found after death 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. IV.— Hearing. Anatomy of the Ear. — For descriptive purposes, the Ear, or Organ of Hearing, is divided into three parts, (1) the external, (2) the middle, FIG. 378 —Diagrammatic view from before of the parts composing the organ of hearing of the leftside. The temporal bone of the left side, with the accompanying soft parts, has been detach- ed from the head, and a section has been carried through it transversely, so as to remove the front of the meatus externus, half the tympanic membrane, the upper and anterior wall of the tympanum and Eustachian tube. Ths meatus internus has also been opened, and the bony labyrinth exposed by the removal of the surrounding parts of the petrous bone. 1, the pinna and lobe; 2, 2', meatus externus; 2', membrana tympani; 3, cavity of the tympanum; 3', its opening backwards into the mastqid cells; between 3 and 3', the chain of small bones; 4, Eustachian tube; 5, meatus internus, containing the facial (uppermost) and the auditory nerves; 6, placed on the vestibule of the la by- rinth above the fenestra ovalis; a, apex of the petrous bone; b, internal carotid artery; c, styloid process; d, facial nerve issuing from the stylo-mastoid foramen; e, mastoid process ;/, squamous part of the bone covered by integument, etc. (Arnold.) 568 HANDBOOK OF PHYSIOLOGY. and (3) the internal ear. The two first are only accessory 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. 378). (1) External Ear. — The external ear consists of the pinna or auri- cle, and the external auditory canal or meatus. The principal parts of the pinna (Fig. 379) are two prominent rims inclosed one within the other (helix and antihelix), and inclosing a cen- tral hollow named the concha; in front of the concha, a prominence di- rected backwards, 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 membrana 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 extending 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 substance called ceru- men, or ear-wax. (2.) Middle Ear or Tympanum. — The middle ear, or tympanum (3, Fig. 378), 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, and lined with mucous membrane, which forms a commu- nication between the tympanum and the pharynx. It opens into the cavity of the pharynx just behind the posterior aperture of the nostrils. The cavity of the tympanum 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 (4, Fig. 378). The walls of the tympanum are osseous, except where apertures in them are closed with membrane, as at the fenestra rotunda, and fe- nestra 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 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 SEXSES. 569 The ossicles or small bones 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 membrana tym- pani; the line of attachment being vertical, includ- ing the whole length of the handle, and extending from the upper border to the centre of the mem- brane. The head of the malleus is irregularly rounded; its neck, or the line of boundary between it and the handle, supports two processes; a short conical one, which receives the insertion of the tensor tympani, and a slender one, processus gra- cilis, which extends forwards, and to which the laxator tympani muscle is attached. 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, di- rected backwards, has a free end lodged in a de- pression in the mastoid bone; the other, curved downwards and more pointed, articulates by means of a roundish tubercle, formerly called os orbicu- lar-e, 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 stir- rup, is attached the stapedius muscle. " The ossicula of aquatic mammalia are very bulky and relatively large, especially in the true seals and the sirenia (Manatee and Dugongj. In the cetacea the stapes is generally ankylosed to the fenestra ovalis, the malleus is always ankylosed to the tympanic bone, yet the membrana FIG. 379.— Outer sur- face of the pinna of the right auricle. 1, helix; 2, fossa of the helix; 3, anti- helix; 4, fossa of the an- tihelix; 5, antitragus; 6, tragus; 7, concha; 8, lobule. %. FIG. 380. FIG. 381. FIG. 382. FIG. 380 —The hammer-bone or malleus, seen from the front. 1, The head; 2, neck; 3, short pro- cess; 4, long process. (Schwalbe) FIG. 381. — The incus, or anvil-bone. 1, body; 2, ridged articulation forthe malleus; 4, processus brevis, with 5, rough articular surface for ligament of incus; 6, processus magnus, with articulat- ing surface for stapes; 7, nutrient foramen. (Schwalbe.) FIG. 382. — The stapes, or stirrup-bone. 1, base; 2 and 3, arch; 4. head of bone, which articulates •with orbicular process of the incus; 5, constricted part of neck; 6, one of the crura. (Schwalbe.) tympani is well formed, and there is a manubrium, often ill-developed, but always attached to the membrane by a long process. In the Otariae or Sea-lions, where the ossicula are far smaller relatively, and less solid than in whales, manatees, and the earless true seals, there are well-formed, 570 HANDBOOK OF PHYSIOLOGY. movable external ears. The ossicula seem to be vestigial relics utilized for the auditory function. In land animals they vary in shape accord- ing to the type of the animal rather than in relation to its acuteness of hearing. I have never found a muscular laxator tympani in any ani- mal, but the tensor exists as a ligament in whales where the malleus is fixed." (Alban Doran.) 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 membrane closing the fenestra ovalis. The malleus, also, is movable 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 tym- pani muscles. The stapes is movable on the process of the incus, when the stapedius muscle acting, draws it backwards. The axis round which the malleus and incus rotate is the line joining the processus gracilis of the malleus and the posterior (short) process of the incus. Fio. 383.— Interior view of the tympanum, with membrana tympani and bones in natural posi- tion. 1, Membrana tympani; 2, Eustachian tube; 3, tensor tympani muscle; 4, lig. mallei superior; 5, lig. mallei super.; 6, chorda-tympanic nerve; a, 6, and c, sinuses about ossicula. (Schwalbe.) (3.) The Internal Ear. — The proper organ of hearing is formed by the distribution of the auditory nerve within the internal ear, or labyrinth, 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 membranous labyrinth. The membranous laby- rinth contains a fluid called endolymph ; while outside it, between it and the osseous labyrinth, is a fluid called perilymph. The osseous labyrinth consists of three principal parts, namely, the vestibule, the cochlea, and the semicircular canals. The Anatomy of the Internal Ear. — The vestibule is the middle cavity of the labyrinth, and the central organ of the whole auditory ap- THE SENSES. 5T1 paratus. 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 (2, Fig. 384), an opening filled by the base of the stapes, one of the small bones of the ear; in its posterior and superior walls, five open- ings by which the semicircular canals communicate with it: in its ante- rior 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 ciqu&ductus vestibuli, a canal leading to the posterior margin of the petrous bone, with uncertain contents and unknown purpose. The semicircular canals (Figs. 384, 385), are three arched cylin- driform 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 before 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. FIG. 384. FIG. 385. FIG. 384.— Right bony labyrinth, viewed from the outer side. The specimen here represented is prepared by separating piecemeal the looser substance of the petrous bone from the dense walls which immediately inclose the labyrinth. 1, the vestibule; 2, fenestra ovalis; 3, superior semicir- cular canal; 4, horizontal or external canal; 5, posterior canal; *, ampullae of the semicircular canals; 6, first turn of the cochlea; 7, second turn; 8, apex; 9, fenestra rotunda. The smaller fig- ure in outline below shows the natural size. 2^/1. (Sommering. ) FIG. 385.— View of the interior of the left labyrinth. The bony wall of the labyrinth is removed superiorly and externally. 1, fovea hemielliptica; 2, fovea hemispherica; 3. common opening of the superior and posterior semicircular canals; 4, opening of the aqueduct of the vestibule; 5, the superior, 6. the posterior, and 7, the external semicircular canals; 8, spiral tube of the cochlea, (scala tympani); 9, opening of the aqueduct of the cochlea; 10, placed on the lamina spiralis in the scala vestibuli. 2>^/l. (Summering/) The cochlea (6. 7, 8, Figs. 384 and 385), a small organ, shaped like a common snail-shell, is seated in front of the vestibule, 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 mentioned, communicates with the vestibule; another called fenestra rotunda, is separated by a mem- brane from the cavity of the tympanum; the third is the orifice of the 572 HANDBOOK OF PHYSIOLOGY. aquceduclus cochlece, a canal leading to the jugular fossa of the petrous bone, and corresponding, at least in obscurity of purpose and origin, to the aquaeductus vestibuli. The spiral canal is divided into two passages, or scalas, by a partition of bone and membrane, the lamina spiralis. The osseous part or zone of this lamina is connected with the modiolus; the membranous part, with a muscular zone, 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 scalae are, therefore, in cor- respondence with this arrangement, named scala vestibuli and scala tympani (Fig. 386). At the apex of the cochlea, the lamina spiralis «nds in a small hamulus, the inner and concave part of which, being de- tached from .the summit of the modiolus, leaves a small aperture named helicotrema, by which the two scalse, separated in all the rest of their length, communicate. Besides the scala vestibuli and scala tympani, there is a third space between them, called scala media or canalis membranaceus (CO. Fig. FIG. 386. FIG. 387. FIG. 386.— View of the osseous cochlea divided through the middle. 1, central canal of the modiolus; 2, lamina spiralis ossea; 3, scala tympani; 4, scala vestibuli; 5, porous substance of the modiolus near one of the sections of the canalis spiralis modioli. 5/1. (Arnold.) FIG. 387.— Section through one of the coils of the cochlea (diagrammatic). S T, scala tym- pani; 8 V, scala vestibuli; C C. canalis cochleae or canalis membranaceus; R, membrane of Reiss- ner; I s o, lamina spiralis ossea; I I s, limbus laminae spiralis; s s, sulcus spiralis; n c, cochlear nerve; g s, ganglion spirale; t, membrana tectoria (below the membrana tectoria is the lamina reticularis) ; 6, membrana basilaris; Co, rods of Corti; I sp, ligamentum spirale. (Quain.) 387). In section it is triangular, its external wall being formed by the wall of the cochlea, its upper wall (separating it from the scala vesti- buli) by the membrane of Eeissner, and its lower wall (separating it from the scala tympani) by the basilar membrane, these two^meeting at the outer edge of the bony lamina spiralis. Following the turns of the cochlea to its apex, the scala media there terminates blindly; while towards the base of the cochlea it is also cjosed with the exception of a very narrow passage (canalis reunions) uniting it with the sacculus. The scala media (like the rest of the membranous labyrinth) contains endo- lymph. Organ of Corti. — Upon the basilar membrane are arranged cells of various shapes. About midway between the outer edge of the lamina spiralis and the outer wall of the cochlea are situated the rods of Corti. Viewed side- THE SENSES. 573 ways, the rods of Corti are seen to consist of tin external and internal pillar, each rising from an expanded foot or base on the basilar mem- brane (o. n. Fig. 388). They slant inwards towards each other, and each ends in a swelling termed the head; the head of the inner pillar overlying that of the outer (Fig. 388). Each pair of pillars forms, as it were, a pointed roof arching over a space, and by a succession of them, a little tunnel is formed. It has been estimated that there are about 3,000 of these pairs of pil- lars, in proceeding from the base of the cochlea towards its apex. They are found progressively to increase in length, and become more oblique; in other words the tunnel becomes wider, but diminishes in height as we approach the apex of the cochlea. Leaning, as it were, against these external and internal pillars are certain other cells, of which the exter- nal ones, hair cells, terminate in small hair-like processes. Most of the above details are shown in the accompanying figure (Fig. 388). This FIG. 388.— Vertical section of the organ of Corti from the dog. 1 to 2, homogeneous layer of the so-called membrana basilaris; u, vestibular layer; v, tympanal layer, with nuclei and protoplasm; a, prolongation of tympanal periosteum of lamina spiralis ossea; c, thickened commencement of the membrana basilaris near the point of perforation of the nerves h • d, blood-vessel (vas spirale); e, blood-vessel; /, nerves; w instead of 14.4yw to 19yU. They are a peculiarity of the spinal nerve roots chiefly in the thoracic region, but are also to be found in the 3d and 3d sacral nerves, and constitute there the nervi erigentes which pass directly to the hypogastric plexus, and not first of all into the lateral chain. From this plexus branches pass upwards into the inferior mesenteric ganglia and downwards to the bladder, rectum and generative organs. These nerves, called by Gaskell pelvic splanchnic nerves, differ from the rami viscerales of the thoracic region only in not communicating with the lateral ganglia; the branches which pass upwards from the thoracic re- gion to the neck, he calls cervical splanchnics, and the splanchnics proper abdominal splanchnics. The white rami viscerales of the upper cervical and cervico-cranial regions do not rim with their corresponding gray rami, but form, Gaskell thinks, the internal branch of the spinal accessory nerve, which contains small medullated fibres similar to those of the visceral branches in the thoracic region. This branch passes into the ganglion of the trunk of the vagus. Small visceral fibres exist too in the roots of the vagus, and in those of the glosso-pharyngeal in connection with the ganglion of the trunk and ganglion petrosum, as well as in the -chorda tympani, in the small petrosal and in other cranial visceral nerves. Functions. — The functions of the sympathetic system are not by any means completely understood. Indeed, until within the last few years, what could be said about them was of a very vague kind. The remarka- ble researches of Gaskell 'have, however, done much to clear up the former confusion; and in the following account the description of the functions of the sympathetic as given by that observer, will be to a great extent followed. A. Functions of the nerve fibres. — The efferent nerve fibres of the sympathetic system supply (a) the muscles of the vascular sys- tem, to which they send vaso-motor fibres, i. e., vaso-constrictor and cardiac augmentor or accelerator, and vaso-inhibitory fibres, i. e., vaso- dilator and cardiac inhibitory; (b) the muscles of the viscera, to which they send both viscero-motor and viscero-inhilitory fibres, (c) The se- cretory gland-cells. 628 HANDBOOK OF PHYSIOLOGY. (a) i. Vaso-motor or Vaso-constrictor and Cardio-augmentor F, bres. — The vaso-motor nerves for all parts of the body come from the central nervous system, and pass out from the spinal cord in the white rami vis- cerales of the thoracic region from the 2d thoracic to the 2d lumbar nerve roots inclusive, as fine medullated fibres; they then pass to the lateral or main sympathetic chain, become non-medullated, and are dis- tributed to their muscles either directly or through terminal ganglia. Thus the augmentor nerves of the heart arise in the thoracic rami, pass upwards, and are distributed to the heart through the ganglion stellatum or inferior cervical ganglion; the vaso-motor nerves for the arm pass out of the cord below the origin of the roots of the brachial plexus, in the anterior roots of the 2d and lower thoracic nerves, and reach that plexus by the same ganglion; the vaso-motor nerves of the foot leave the spinal cord high up, and reach the sympathetic lateral ganglia above the origin of the sciatic nerve, into which they pass through the abdominal sympa- thetic. In all cases the nerves lose their medulla in the ganglia. Simi- larly the vaso-motor nerve supply for the blood-vessels of the head and neck and of the abdomen is derived from the cervical and abdominal splanchnics respectively, or from the corresponding rami efferentes of the upper lumbar ganglia. The lateral sympathetic chain G-askell proposes to call the chain of vaso-motor ganglia. ii. Vaso-inhibitory or Vaso-dilator, and Cardio-inhibitory Fibres.— Of these, which are doubtless as widely distributed as the vaso-motor fibres, we have distinct proof in the existence of fibres separate from vaso- motor, e. g., in the inhibitory nerve of the heart, the cardio-vagus; in the chorda tympani; in the small petrosal, and in the nervi erigentes. These nerve-fibres, as far as we know at present, leave the central nervous system among the fine medullated nerves of the cervico-cra- nial and sacral rami communicantes, do not enter the lateral ganglia, but pass without losing their medulla into the collateral or terminal gan- glia. (b.) i. Viscera-motor Fibres. — These fibres, upon which depend the peristaltic movements of the thoracic portion of the oesophagus, and of the stomach, and intestines, arise from the central nervous system, as the fine medullated fibres of the upper portion of the cervical region, not in the spinal nerve roots of that region, but as the bundles of fibres which may be called the rami viscerales of the vagus and accessory nerves. They pass to the ganglion of the trunk of the vagus, where they lose their medulla. ii. Viscero-Inhibitory Fibres. — It appears that the nerve- supply to the circular muscles of the alimentary canal and its appendages is con- tained in the abdominal splanchnics, and consists of those fibres which THE SYMPATHETIC NERVOUS SYSTEM. 629 have not passed through the lateral chain, and which therefore retain their medulla until they reach the proximal or collateral chain. c. Glandular Nerve Fibres. — A double nerve supply,, in all proba- bility coinciding with the supply to the visceral muscles, has been demonstrated in the cases of the submaxillary, parotid, and lachrymal glands, and in these cases the course of the fibres is very similar to that of the corresponding fibres for the vaso-muscular supply. Thus the sympathetic supply for these glands passes along with the vaso-motor fibres from the cervical splanchnic (or sympathetic trunk), and superior cervical ganglion; whilst the cerebro-spinal supply comes from the rarni viscerales of the cranial nerves in conjunction with the vaso-dilator fibres. Central Origin of the Rami Viscerales. — There appears to be the strongest presumption that the white rami of the thoracic region arise in the spinal cord in, or are connected with, the cells of the posterior vesic- ular column of Clarke. This conclusion is based upon the fact that these special cells are found in the three regions already mentioned, and in those only where the white rami of fine medullated fibres exist, viz., in the cervico-cranial regions, in the spinal accessory, in the thoracic region, and in the sacral region. But it is probable that the fibres are also connected with the cells of the lateral horn of the gray matter of the spinal cord, and its representative in the medulla, the antero-lateral nucleus of Clarke. B. Structure and Functions of the Ganglia. — The sympathetic ganglia all contain — (1.) nerve-fibres traversing them; (2.) nerve-fibres originating in them; (3.) nerve- or ganglion-corpuscles, giving origin to these fibres; and (4.) other corpuscles that appear free. In the sympa- thetic ganglia of the frog, ganglion-cells of a very complicated structure have been described by Beale, and subsequently by Arnold. The cells are inclosed each in a nucleated capsule: they are pyriformin shape, and from the pointed end two fibres are given off, which gradually acquire the characters of nerve-fibres: one of them is straight, and the other (which sometimes arises from the cell by two roots) is spirally coiled around it. According to Gaskell the functions of the main sympathetic ganglia are the following:— (1.) They effect the conversion of medullated into non-medullated fibres; (2.) They possess a nutritive influence over the nerves which pass from them to the periphery; (3.) They increase the number of fibres at the same time as they cause the removal of the medulla. As regards their possession of the usual properties of nerve- centres little or nothing is certainly known. It appears unlikely that they possess the reflex functions of the spinal centres. Respecting the general action of the peripheral ganglia of the sympa- thetic, in reflex or other actions, little need be said, since they may be 630 HANDBOOK OF PHYSIOLOGY. taken as examples by which to illustrate the common modes of actk>k of all nerve-centres. Indeed, complex as the sympathetic 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 illustra- tion of all the nervous actions with which the cerebrum is unconnected. The parts principally supplied with sympathetic nerves are usually capable of none but involuntary movements, and when the cerebrum acts on them at all, it is only through the strong excitement or depress- ing 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 statements; for the heart and stomach, though supplied in large 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 sympathetic nerve continue to move, though more feebly than before, 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 motions of the parts supplied with nerves from the sympathetic are shown to be, in a measure, independent of the brain and spinal cord; this independent maintenance of their action being, without doubt, due to the fact that they contain, in their own substance, the apparatus of ganglia and nerve-fibres by which their motions are immediately gov- erned. It seems to be a general rule, at least in animals that have both cere- bro-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 respiration are maintained in uniform rhythmic action chiefly by the reflecting and combining power of the medulla oblongata, so are those of the heart, stomach, ^and intes- tines, 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 the respiratory muscles — if these nerves of the medulla oblongata itself be directly stimulated, the movements that follow are convulsive and disorderly; but if the medulla be stimulated through a THE SYMPATHETIC NERVOUS SYSTEM. 631 centripetal nerve, as when cold is applied to the skin, then the impres- sions 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-ordi- nation of movements is to be ascribed, must be reckoned those which lie in the very substance of the organs; such as those of the heart. Those also may be included which have been found in the mesentery close by the intestines, as well as in the muscular and submucous tissue of the stomach and intestinal canal, and in other parts. Respecting the influence of the sympathetic system on the various physiological processes, the sections on the Heart, Arteries, Animal Heat, Salivary Glands, Stomach and Intestines should be referred to. Influence of the Nervous System in general on Nutrition.— It has been held that the nervous system cannot be essential to a healthy course of nutrition, because in plants, in 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 ner- vous system, nutrition 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 sys- tem) will always be present and blended in a more and more intimate relation with all the rest: according to the general law, that the inter- dependence of parts augments with their development. The reasonableness of this assumption is proved by many facts show- ing the influence of the nervous system on nutrition, and by the most striking of these facts being observed in the higher animals, and espe- cially in man. The influence of the mind in the production, aggravation, and cure of organic diseases 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 portions of the nervous centres, or to individual nerves, are frequently followed by defective nutrition of the parts supplied by the injured nerves, or deriv- ing their nervous influence from the damaged portions of the nervous centres. Thus, lesions of the spinal cord are sometimes quickly followed by gangrene of portions of the paralyzed parts. After such lesions also, the repair of injuries in the paralyzed parts may take place less com- pletely than in others; as, in 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, in which having, in salamanders, 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, it was found that the end of the tail was reproduced more slowly 632 HANDBOOK OF PHYSIOLOGY. than in other salamanders in whom the spinal cord was left uninjured above the point at which the tail was amputated. Illustrations of the same kind are furnished by the several cases in which division or de- struction 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 degeneration 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. So many and varied facts leave little doubt that the nervous system exercises an influence over nutrition as over other organic processes; and they cannot be easily explained by supposing that the changes in the nutritive processes are only due to the variations in the size of the blood- vessels supplying the affected parts, although this is, doubtless, one im- portant element in producing the result. As a contribution towards the explanation of the nervous mechanism of nutrition comes in Gaskell's theory of katabolic and anabolic nerves. He supposes that every tissue is supplied with two sets of nerves, the- former of which corresponds with the motor nerve, the viscero-motor, and the cardio-augmentor, by the stimulation of which an increase of the- metabolism takes place, and which is followed by exhaustion. It may be accompanied either by contraction of a muscle or by an increase of contraction. Sujch a nerve is excellently illustrated by the sympathetic augmentor or accelerator nerve of the heart, on stimulation of which an increase in the force and frequency of the heart takes place, followed after a time by exhaustion. A katabolic nerve stimulates the destructive- metabolism which is always going on in a tissue. The anabolic nerve is the exact opposite of the katabolic nerve in function. It subserves con- structive metabolism. Stimulation of the nerve produces diminished activity, repair of tissue, and building up. An example of this kind of nerve is seen in the cardiac vagus, stimulation of which produces inhibition. Inhibition must generally be looked upon as an anabolic process. It will be seen that the results of stimulation of the nerves to the salivary glands, discussed in a former chapter, appear to support the- theory that the processes of constructive and destructive metabolism are under the control of separate nerve fibres. In the case of the submaxil- lary gland, for example, if the sympathetic fibres be stimulated, a kata- THE SYMPATHETIC NERVOUS SYSTEM. 633 bolic effect is produced, and the materials of secretion are formed at the expense of the protoplasm (this action in the case of the gland Heiden- hain calls trophic}] if on the other hand the chorda tympani or the secretory nerve be stimulated, two things happen, one being the discharge of water and the materials of secretion from the gland cells, and the other the building up or reconstruction of the protoplasm of the cells. A part of this action at any rate is anabolic, and similar to the action of inhibitory nerves. CHAPTEE XXII. THE REPRODUCTIVE ORGANS. BEFORE describing the method of Reproduction or the way in which the species is propagated, it will be advisable to describe the structure of those organs which in either sex are concerned in reproduction, and FIG. 424. — Diagrammatic view of the uterus and its appendages, as seen from behind. The uterus and the upper part of the vaerina have been laid open by removing the posterior wall: the Fallopian tube, round ligament, and ovarian ligament 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 rug® termed arbor vitee; v, upper part of the vagina; od, Fallopian tube or oviduct; the nar- row communication of its cavity with that of the cornu of the uterus on each size is seen: I, round ligament; lo, ligament of the ovary; o, ovary; i, wide outer part of the right Fallopian tube: fi, its fimbriated extremity; po. parovarium; ft, one of the hydatids frequently found connected with the broad ligament. %. (Allen Thomson.') which are called the genital or generative organs or the sexual appa- ratus. A. The Genital Organs of the Female. The female organs of generation (Fig. 424) 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 conducting 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, or vagina, with its appendages, for the reception of the male or- THE REPRODUCTIVE ORGANS. 635 gan in the act of copulation, and for the subsequent discharge of the foetus. a. The Ovaries. — The ovaries are two oval compressed bodies, FIG. 425.— View of a section of the ovary of the cat. 1, outer covering and free border of the ovary; 1', attached border; Si, the ovarian stroma, presenting a fibrous and vascular structure; 3, granular. substance lying external to the fibrous stroma; 4, blood vessels; 5, ovigerms in their earli- est 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 the cells constituting the proligerous disc; 9. the most advanced follicle containing the ovum, etc. ; 9', a follicle from which the ovum has accidentally escaped; 10, corpus luteum. 6/1. (Schron.) situated in the cavity of the pelvis, one on each side, enclosed in the folds of the broad ligament. Each ovary measures about an inch and a *•••»»• , hypoblast, consisting of a single layer of flattened cells; M, "for- mative cells." They are seen on the right of the figure, passing in between the epiblast and hypo- blast to form the mesoblast; A, white yelk granules. Many of the large ' • formative cells " are seen containing these granules (Strieker). and meet in the middle line. Anterior to the headfold is a second fold parallel to it, which is the commencing amnion. * The medullary canal is bounded by its two folds or longitudinal eleva- tions, laminae dorsales, which are folds consisting entirely of cells of the epiblast: these grow up and arch over the medullary groove (Fig. 446) till after some time they coalesce in the middle line, converting it from an open furrow into a closed tube — the neural canal or the primi- tive cerebro-spinal axis. Over this closed tube, the walls of which con- sist of more or less cylindrical cells, the superficial layer of the epiblast is now continued as a distinct membrane. The union of the medullary folds or laminae dorsales takes place first about the neck of the future embryo; they soon after unite over the re- gion of the head, while the closing in of the groove progresses much more slowly towards the hinder extremity of the embryo. The medullary groove is by no means of uniform diameter throughout, but even before the dorsal laminae have united over it, is seen to be dilated at the anterior DEVELOPMENT. 665 extremity and obscurely divided by constrictions into the three primary vesicles of the brain. The part from which the spinal cord is formed is of nearly uniform calibre, while towards the posterior extremity is a lozenge-shaped dila- tation, sinus rhomboidalis, which is the last part to close in (Fig. 447). Whilst the changes which have been described are taking place in the area pellucida, which has enlarged to a certain extent, the area opaca has considerably extended. The hypoblast and mesoblast have also been prolonged laterally, not by mere extension, but also from the ger- true amnion; a', reflected layer of amnion, sometimes termed " false amnion;1' sp, backward limit of splanchnopleure folds, along which run the omphalomesaraic veins uniting to form 7i, the heart, which is continued forwards into 6a, the bulbus arteriosus; d, the fore-gut lying behind the heart, and having a wide crescentic opening between the splanchnopleure folds; HB, hind-brain; MB\ mid-brain; pv, protovertebrae lying behind the fore-gut; me line'of junction of medullary folds ani of notochord; ch, front end of notochord; vpl, vertebral plates; pr, the primitive-groove at its cau- dal end (Foster and Balfour). minal wall, which is the thickened edge of the blastoderm, together with formative cells of the yelk; on each side of the notochord and medullary canal, the mesoblast remains as a longitudinal thickening. It now however splits horizontally into two layers or laminae (parietal and visceral) : of these the former, when traced out from the central axis, is seen to be in close apposition with the epiblast and gives origin 666 HANDBOOK OF PHYSIOLOGY. to the parietes of the trunk, while the latter adheres more or less closely to the hypoblast, and gives rise to the serous and muscular walls of the alimentary canal and several other parts (Fig. 450). The united parietal layer of the mesoblast with the epiblast is termed Somatopleure, the united visceral layer and hypoblast, Splanchno- pleure. The space between them is the pleuro-peritoneal cavity, which becomes subdivided by subsequent partitions into pericardium, pleura, and peritoneum. FIG. 450.— Transverse section through dorsal region of embryo chick (45 hrs.). One-half of the section is represented: if completed it would extend as far to the left as to the right of the line of the medullary canal (A/c). A, epiblast; C, hypoblast, consisting of a single layer of flattened cells; Me, medullary canal; Pv, proto vertebra; Wd, Wolfflan duct: So, somatapleure; Sp, splanch- nopleure; pp, pleuro-peritoneal cavity; ch, notochord; ao, dorsal aorta, containing blood-cells; v, blood-vessels of the yolk-sac (Foster and Balfour). The splitting of the mesoblast extends almost to the medullary canal, but a portion on either side ( p. v. Fig. 450) remains undivided, the vertebral plate. The divided portion is known as the lateral plate. The longitudinal thickening of the vertebral plate is seen after awhile to be divided, at right angles to the medullary canal by bright trans- FIG. 451.— Diagrammatic longitudinal section through the axis of an embryo. The head-fold has commenced, but the tail-fold has not yet appeared; FSo, fold of the somatopleure; FSp, fold of the splanchnopleure ; the line of reference, FSo, lies outside the embryo in the "moat," which marks off the overhanging head from the amnion; D, inside the embryo, is that part which is to become the fore-gut; FSo and Fsp, are both parts of the head-fold, and travel to the left of the figure as development proceeds; op, space between somatopleure and splanchnopleure, pleuro-peri- toneal cavity; Am, commencing head-fold of amnion; NC, neural canal; Ch, notochord; Ht, heart; A, B, C, epiblast, mesoblast, hypoblast (Foster and Balfour). verse lines into a number of square segments. These segments, which are the surface appearance of cubes of mesoblast, are the mesoblastic somites or protovertebrae. The first three or four of these proto- vertebrae make their appearance in the cervical region, while one or two DEVELOPMENT. 66T more are formed in front of this point; and the series is continued back- ward till the whole medullary canal is flanked by them (Fig. 449). That which is first formed corresponds to the second cervical vertebra. From these somites the vertebrae and the trunk muscles are derived. Head and Tail Folds. Body Cavity. — Every vertebrate animal consists essentially of a longitudinal axis (vertebral column) with a neural canal above it, and a body-cavity (containing the alimentary canal) beneath. We have seen how the earliest rudiments of the central axis and the neural canal are formed; we must now consider how the general body- cavity is developed. In the earliest stages the embryo lies flat on the FIG. 452.— Diagrammatic section showing the relation in a mammal between the primitive alimen- tary canal and the membranes of the ovum. The stage represented in this diagram corresponds to that of the fifteenth or seventeenth day in the human embryo, previous to the expansion of the allantois; c, the villous chorion ; a, theamnion; a', the place of convergence of the amnionand reflexion of the false amnion, a" a", or outer or corneous layer; e, the head and trunk of the embryo, comprising the primitive vertebrae and cerebro-spinal axis; t, i, the simple alimentary canal in its upper and lower portions. Immediately beneath the right hand i is seen the foetal heart, lying in the anterior part of the pleuro-peritoneal cavity; v, the yolk-sac or umbilical vesicle; vi, the vitello-intestinal opening; u, the allantois connected by a pedicle with the anal portion of the alimentary canal (Quaim. surface of the yelk, and is not clearly marked off from the rest of t\i& blastoderm: but gradually the head-fold or cresceiitic depression (with its concavity backwards) is formed in the blastoderm, limiting the head of the embryo; the blastoderm is, as it were, tucked in under the head, which thus comes to project above the general surface of the membrane:, a similar tucking in of blastoderm takes place at the caudal extremity, and thus the head and tail folds are formed (Fig. 452). Similar depressions mark off the embryo laterally, until it is com- 668 HANDBOOK OF PHYSIOLOGY. pletely surrounded by a sort of moat which it overhangs on all sides, and which clearly defines it from the yelk. This moat runs in further and further all round beneath the over- hanging embryo, till the latter comes to resemble a canoe turned upside- down, the ends and middle being, as it were, decked in by the folding or tucking in of the blastoderm, while on the ventral surface there is still a large communication with the yelk, corresponding to the well or undecked portion of the canoe. This communication between the embryo and the yelk is gradually contracted by the further tucking in of the blastoderm from all sides, till it becomes narrowed down, as by an invisible constricting band, to a mere pedicle which passes out of the body of the embryo at the point of the future umbilicus. The downwardly folded portions of blastoderm are termed the vis- ceral plates. Thus we see that the body-cavity is formed by the downward folding of the visceral plates, just as the neural cavity is produced by the up- ward growth of the dorsal laminas, the difference being that, in the visceral or ventral laminae, all three layers of the blastoderm are con- cerned* The folding in of the splanchnopleure, lined by hypoblast, pinches off, as it were, a portion of the yelk-sac, inclosing it in the body-cavity. This forms the rudiment of the alimentary canal, which at this period ends blindly towards the head and tail, while in the centre it communi- cates freely with the cavity of the yelk-sac through the canal termed vitelline or omphalo-mesenteric duct. The yelk-sac thus becomes divided into two portions which communi- cate through the vitelline duct, that portion within the body giving rise, as above stated, to the digestive canal, and that outside the body remain- ing for some time as the umbilical vesicle (Fig. 453, ys). The hypoblast forming the epithelium of the intestine is of course continuous with the lining membrane of the umbilical vesicle, while the visceral plate of the mesoblast is continuous with the outer layer of the umbilical vesicle. All the above details will be clear on reference to the accompanying •diagrams. At the posterior end of the embryo chick, when the amniotic fold is commencing to be formed, and the hind fold of the splanchnopleure has commenced, there remains for a time a communication between the neural canal and the hind gut, which is called the neurenteric canal. It passes in at the point where the notochord falls into the primitive streak. The anterior part of the primitive streak becomes the tail swell- ing, the posterior part atrophies, and the corresponding lateral part of the blastoderm forms part of the body-wall of the embryo. The ante- rior part of the medullary canal having been completely roofed in; the DEVELOPMENT. 669 foremost portion undergoes dilatation, and a bulb, or first cerebral vesicle results. From either side of this dilatation a process, the cavity of which is in communication with it, is separated off; these processes are the optic vesicles. Behind the first cerebral vesicle two other vesicles now arise, and at the posterior part of the head two small pits, the au- ditory pits, are to be seen. The folding of the head, it should be rec- ollected, is the cause of the inclosure below the neural canal (Fig. 451) of a canal ending blindly, which has in front the splanchnopleure, and which is just as long as the involution of that membrane. This canal is the fore-gut. In the interior of the splanchnopleure fold below it (as seen in Fig. 451) in the pleuro-peritoneal cavity the heart is formed, at the point where the splanchnopleure makes its turn forwards. It arises as a thickening of the mesoblast on either side as the two splanch- nopleure folds diverge, and of a thickening of the mesoblast at the point of divergence. So that at first the rudiment of the heart is like an in- verted V, which by the gradual coming together of the diverging cords is converted into an inverted Y. The cylinders become hollowed but, and are thus converted into tubes, which then coalesce. Layers are separated off towards the interior, which become the epithelial lining, and the mass of the mesoblast sur- rounding this afterwards forms the muscle and serous covering, whilst at first the rudimentary organ is attached to the gut by a mesoblastic mesen- tery, the mesocardium. FCETAL MEMBRANES. Umbilical Vesicle or Yelk-sac. — The splanchnopleure, lined by hypoblast, forms the yelk-sac in Reptiles, Birds, and Mammals; but in Amphibia and Fishes, since there is neither amnion nor allantois, the wall of the yelk-sac consists of all three layers of the blastoderm, inclosed, of course, by the original vitelline membrane. The body of the embryo becomes in great measure detached from the yelk sac or umbilical vesicle, which contains, however, the greater part of the substance of the yelk, and furnishes a source whence nutriment is derived for the embryo. This nutriment is absorbed by the numerous vessels (omphalo-mesenteric) which ramify in the walls of the yelk-sac, forming what in birds is termed the area vasculosa. In Birds, the contents of the yelk-sac afford nourishment until the end of incubation, and the omphalo-mesenteric vessels are developed to a corresponding degree; but in Mammalia the office of the umbilical vesicle ceases at a very early period, the quantity of the yelk is small, and the embryo soon becomes independent of it by the connections it forms with the parent. Moreover, in Birds, as the sac is emptied, it is gradually drawn into the abdomen through the umbilical opening, which then closes over it: but in Mammalia it always remains on the outside; and as it is emptied it 670 HANDBOOK OF PHYSIOLOGY. contracts (Fig. 455), shrivels up, and together with the part of its duct external to the abdomen, is detached and disappears either before or at the termination of intra-uterine life, the period of its disappearance varying in different orders of Mammalia. When blood-vessels begin to be developed, they ramify largely over the walls of the umbilical vesicle, and are actively concerned in absorb- ing its contents and conveying them away for the nutrition of the em- bryo. FIG. 453.— Diagrams showing three successive stages of development. Transverse vertical sec- tions. The yelk-sac, ys, is seen progressively diminishing in size. In the embryo itself the medul- lary canal and notochord are seen in section, a', in middle figure, the alimentary canal, becoming pinched off, as it were, from the yelk-sac; a', in right hand figure, alimentary canal completely closed; a, in last two figures, amnion; oc, cavity of amnium filled with amniotic fluid; pp, space between amn ion and chorion continuous with the pleuro-peritoneal cavity inside, the body; vt, vitellme membrane; ys, yelk-sac, or umbilical vesicle (Foster and Balfour). At an early stage of development of the foetus, and some time before the completion of the changes which have been just described, two im- FIG. 454. FIG. 455. a, area pellucida; 6, area vasculosa; FIG. 454. — Diagram showing vascular area in the chick c, area vitellina. FIG. 455.— Human embryo of fifth week with umbilical vesicle; about natural size (Dalton). The human umbilical vesicle never exceeds the size of a small pea. portant structures, called respectively the amnion and the allantois, begin to be formed. Amnion. — The amnion is produced as follows: — Beyond the head- and tail-folds before described (p. 667), the somatopleure coated by epi- blast, is raised into folds, which grow up, arching over the embryo, not only anteriorly and posteriorly but also laterally, and all converging DEVELOPMENT. 671 towards one point over its dorsal surface (Fig. 453). The growing up of these folds from all sides and their convergence towards one point very closely resembles the folding inwards of the visceral plates already described, and hence, by some, the point at which the amniotic folds meet over the back has been termed the amniotic umbilicus. The folds not only come into contact but coalesce. The inner of the two layers forms the true amnion, while the outer or reflected layer, sometimes termed the false amnion, coalesces with the inner surface of the original vitelline membrane to form the subzonal membrane or false chorion. This growth of the amniotic folds must of course be clearly distinguished from the very similar process, already described by which the walls of the neural canal are formed at a much earlier stage. The cavity between the true amnion and the external surface of the embryo becomes a closed space, termed the amniotic cavity (ac, Fig. 453). At first, the amnion closely invests the embryo, but it becomes gradually distended with fluid (liquor amnii), which, as pregnancy ad- vances, reaches a considerable quantity. This fluid consists of water containing small quantities of albumen and urea. Its chief function during gestation appears to be the mechan- ical one of affording equal support to the embryo on all sides, and of protecting it as far as possible from the effects of blows and other inju- ries to the abdomen of the mother. The embryo up to the end of pregnancy is thus immersed in fluid, which during parturition serves the important purpose of gradually and evenly dilating the neck of the uterus to allow of the passage of the foe- tus: when this is accomplished the amniotic sac bursts, and the ' ( waters " escape. On referring to the diagrams (Fig. 453), it will be obvious that the cavity outside the amnion (between it and the false amnion) is continu- ous with the pleuro-peritoneal cavity at the umbilicus. This cavity is not entirely obliterated even at birth, and contains a small quantity of fluid ("false waters "), which is discharged during parturition either before, or at the same time as the amniotic fluid. Allantois. — Into the pleuro-peritoneal space the allantois sprouts out, its formation commencing during the development of the amnion. Growing out from or near the hinder portion of the intestinal canal (c, Fig. 456), with which it communicates, the allantois is at first a solid pear-shaped mass of splanchnopleure; but becoming vesicular by the projection into it of a hollow out-growth of hypoblast, and very soon simply membranous and vascular, it insinuates itself between the amni- otic folds, just described, and comes into close contact and union with the outer of the two folds, which has itself, as before said, become one 672 HANDBOOK OF PHYSIOLOGY. with the external investing membrane of the egg. As it grows, the allantois develops muscular tissue in its external wall and becomes ex- ceedingly vascular; in birds (Fig. 457) it envelops the whole embryo- taking up vessels, so to speak, to the outer investing membrane of the egg, and lining the inner surface of the shell with a vascular membrane, by these means affording an extensive surface in which the blood may be aerated. In the human subject and other Mammalia, the vessels carried out by the allantois are distributed only to a special part of the outer membrane or false chorion, where, by interlacement with the vascular system of the mother, a structure called the placenta is developed. In Mammalia, as the visceral laminae close in the abdominal cavity, the allantois is thereby divided at the umbilicus into two portions; the outer part, extending from the umbilicus to the chorion, soon shrivelling; while the inner part, remaining in the abdomen, is in part converted into the urinary bladder; the portion of the inner part not so converted, extending from the bladder to the umbilicus, under the name of the urachus. After birth the umbilical cord, and with it the external and FIG. 456. Fia. 457. FIG. 456.— Diagram of fecundated egg. a, umbilical vesicle; 6, amniotic cavity; c, allantois. (Dalton.) FIG. 457.— Fecundated egg with allantois nearly complete, a, inner layer of amniotic fold; &, outer layer of ditto ; c, point where the amniotic folds come in contact. The allantois is seen pene- trating between the outer and inner layers of the amniotic folds. This figure, which represents only the amniotic folds and the parts within them, should be compared with Figs. 453, 459, in which will be found the structures external to these folds. (Dalton.) shrivelled portion of the allantois, are cast off at the umbilicus, while the urachus remains as an impervious cord stretched from the top of the urinary bladder to the umbilicus, in the middle line of the body, imme- diately beneath the parietal layer of the peritoneum. It is sometimes enumerated among the ligaments of the bladder. It must not be supposed that the phenomena which have been suc- cessively described, occur in any regular order one after another. On the contrary, the development of one part is going on side by side with that of another. The C.horion. — It has been already remarked that the allantois is a structure which extends from the body of the foetus to the outer investing membrane of the ovum, that it insinuates itself between the two layers of the amniotic fold, and becomes fused with the outer layer, which has DEVELOPMENT. 673 itself become previously fused with the vitelline membrane. By these means the external investing membrane of the ovum, or the true chorion, as it is now called, represents three layers, namely, the original vitelline membrane, the outer layer of the amniotic fold, and the allantois. Very soon after the entrance of the ovum into the uterus, in the human subject, the outer surface of the chorion is found beset with fine processes, the so-called villi of the chorion (a, Figs. 458, 459), which give it a rough and shaggy appearance. At first only cellular in structure, these little outgrowths subsequently become vascular by the development in them of loops of capillaries (Fig. 460); and the latter at length form the minute extremities of the blood-vessels which are, so to speak, conducted from the foetus to the chorion by the allantois. The function of the villi of the chorion is evidently the absorption of nutrient matter for the foetus; and this is probably supplied to them at first from the fluid matter, secreted by the follicular glands of the uterus, FIGS. 458 and 459.— a, chorion with villi The villi are shown to be best developed in the part of the chorion to which the allantois is extending; this portion ultimately becomes the pla- centa; 6, space between the two layers of the amnion; c, amniotic cavity; d, situation of the intes- tine, showing its connection with the umbilical vesicle; e, umbilical vesicle;/, situation of heart and vessels; gr, allantois. in which they are soaked. Soon, however, the foetal vessels of the villi come into more intimate relation with the vessels of the uterus. The part at which this relation between the vessels of the foetus and those of the parent ensues, is not, however, over the whole surface of the chorion: for, although all the villi become vascular, yet they become indistinct or disappear except at one part where they are greatly developed, and by their branching give rise, with the vessels of the uterus., to the formation of the placenta. To understand the manner in which the fatal and maternal blood- vessels come into relation with each other in the placenta, it is necessary briefly to notice the changes which the uterus undergoes after impregna- tion. These changes consist especially of alterations in structure of the superficial part of the mucous membrane which lines the interior of the 43 674 HANDBOOK OF PHYSIOLOGY. uterus, and which forms, after a kind of development to be immediately described, the membrana decidua, so called on account of its being dis- charged from the uterus at birth. Formation of the Placenta. The mucous membrane of the human uterus, which consists of a matrix of connective tissue containing numerous corpuscles (adenoid tissue), and is lined internally by columnar ciliated epithelium, is abun- dantly beset with tubular glands, arranged perpendicularly to the sur- face (Fig. 461). These follicles are very small in the unimpregnated uterus, but when examined shortly after impregnation, they are found elongated, enlarged, and much waved and contorted towards their deep and closed extremity, which is implanted at some depth in the tissue of the uterus, and may dilate into two or three closed sacculi (Fig. 461). FIG. 460. FIG. 461. FIG. 461.— Section of the lining membrane of a human uterus at the period of commencing preg- nancy showing the arrangement and other peculiarities of the glands, d, d, d, with their orifices, a, a, a, on the internal surface of the organ. Twice the natural size. The glands are lined by columnar ciliated epithelium, and they open on the inner surface of the mucous membrane by small round orifices set closely together (a, a, Fig. 461). On the internal surface of the mucous membrane may be seen the circular orifices of the glands, many of which are, in the early period of pregnancy, surrounded by a whitish ring, formed of the epithelium which lines the follicles (Fig. 462). Coincidently with the occurrence of pregnancy, important changes occur in the structure of the mucous membrane of the uterus. The epithelium and sub-epithelial connective tissue, together with the tubular glands, increase rapidly, and there is a greatly increased vascularity of the whole mucous membrane, the vessels of the mucous membrane be- coming larger and more numerous; while a substance composed chiefly of nucleated cells fills up the interfollicular spaces in which the blood- DEVELOPMENT. 675 vessels are contained. The effect of these changes is an increased thickness, softness, and vascularitj of the mucous membrane, the super- ficial part of which itself forms the membrana decidua. The object of this increased development seems to be the production of nutritive materials for the ovum; for the cavity of the uterus shortly becomes filled with secreted fluid, consisting almost entirely of nucleated cells in which the villi of the chorion are imbedded. When the ovum first enters the uterus it becomes imbedded in the structure of the decidua, which is yet quite soft, and in which soon afterwards three portions are distinguishable. These have been named the decidua vera, the decidua reflexa, and the decidua serotina. The first of these, the decidua vera, lines the cavity of the uterus; the second, or decidua reflexa, is a part of the decidua vera which grows up around the ovum, and, wrapping it closely, forms its immediate investment. FIG. 462. FIG. 463. FIG. 462.— Two thin segments of human decidua after recent impregnation, viewed on a dark ground : they show the openings on the surface of the membrane. A, is magnified six diameters, and .a, twelve diameters. At 1, the lining of epithelium is seen within the orifices, at 2 it has escaped. (Sharpey.) FIG. 463.— Diagram of an early stage of the formation of the human placenta, a, embryo; 6, amnion; c, placental vessels; d, decidua reflexa; e, allantois; /, placenta! villi; g, mucous mem- brane. CCadiat.) The third, or decidua serotina, is the part of the decidua vera which becomes especially developed in connection with those villi of the chorion, which, instead of disappearing, remain to form the foetal part of the placenta. In connection with these villous processes of the chorion, there are developed depressions or crypts in the decidual mucous membrane, which correspond in shape with the villi they are to lodge; and thus the chori- onic villi become more or less imbedded in the maternal structures. These uterine crypts, it is important to note, are not, as was once sup- posed, merely the open mouths of the uterine follicles. As the ovum increases in size, the decidua vera and the decidua reflexa 676 HANDBOOK OF PHYSIOLOGY. gradually come into contact, and in the third month of pregnancy the cavity between them has quite disappeared. Henceforth it is very diffi- cult, or even impossible, to distinguish the two layers. The Placenta. — During these changes the deeper part of the mu- cous membrane of the uterus, at and near the region where the placenta is placed, becomes hollowed out by sinuses, or cavernous spaces, which communicate on the one hand with arteries and on the other with veins of the uterus. Into these sinuses the villi of the chorion protrude, push- ing the thin wall of the sinus before them, and so come into intimate relation with the blood contained in them. There is no direct com- FIG. 464. FIG. 466. FIG. 464.— Diagrammatic view of a vertical transverse section of the uterus at the seventh or eighth week of pregnancy, c, c, c', cavity of the uterus, which becomes the cavity of the decidua, opening at c, c, the cornua, into the Fallopian tubes, and at c' into the cavity of the cervix, which is closed by a plug of mucus; d v, decidua vera; dr, decidua reflexa, with the sparser villi im- bedded in its substance; d s, decidua serotina, involving the more developed chorionic villi of the commencing placenta. The foetus is seen lying in the amniotic sac; passing up from the umbilicus is seen the umbilical cord and its vessels, passing to their distribution in the villi of the chorion ; also- the pedicle of the yelk sac, which lies in the cavity between the amnion and chorion. (Allen Thom- son.) FIG. 465. Extremity of a placenta! villus. a, lining membrane of the vascular system of the mother ; b, cells immediately lining a ; d, space between the maternal and foetal portions of the vi- lus; e, internal membrane of the villus, or external membrane of the chorion;/, internal cells of the villus, or cells of the chorion; g, loop of umbilical vessels. (Qoodsir.) munication between the blood-vessels of the mother and those of the foetus; but the layer or layers of membrane intervening between the blood of the one and of the other offer no obstacle to a free interchange of matters between them. Thus the villi of the chorion containing foatal DEVELOPMENT. 677 blood, are bathed or soaked in maternal blood contained in the uterine sinuses. The arrangement may be roughly compared to filling a glove with foetal blood, and dipping its fingers into a vessel containing mater- nal blood. But in the foetal villi there is a constant stream of blood into and out of the loop of capillary blood-vessels contained in it, as there is also into and out of the maternal sinuses. It would seem that, at the villi of the placental tufts, where the foetal and maternal portions of the placenta are brought into close relation with each other, the blood in the vessels of the mother is separated from that in the vessels of the foetus by the intervention of two distinct sets of nucleated cells (Fig. 465). One of these (b) belongs to the maternal portion of the placenta, is placed between the membrane of the villus and that of the vascular system of the mother, and is probably designed to separate from the blood of the parent the materials destined for the blood of the foetus; the other (/) belongs to the foetal portion of the placenta, is situated between the membrane of the villus and the loop of vessels contained within, and probably serves for the absorption of the material secreted Vy the other sets of cells, and for its conveyance into the blood-vessels of the foetus. Between the two sets of cells with their investing membrane there exists a space (d), into which it is probable that the materials secreted by the one set of cells of the villus are poured in order that they may be absorbed by the other set, and thus conveyed into a foetal vessel. Not only, however, is there a passage of materials from the blood of the mother into that of the foetus, but there is a mutual interchange of materials between the blood both of foetus and of parent; the latter sup- plying the former with nutriment, and in turn abstracting from it ma- terials which require to be removed. Alexander Harvey's experiments were very decisive on this point. The view has also received abundant support from 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 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, 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 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 ex- periments, repeated on other animals with like results, leave no doubt of the rapid and direct transmission of matter from the foetus to the mother through the blood of the placenta. The placenta, therefore, of the human subject is composed of &fcetal part and a maternal part, — the term placenta properly including all that 678 HANDBOOK OF PHYSIOLOGY. entanglement of foetal villi and maternal sinuses, by means of which the blood of the foetus is enriched and purified after the fashion necessary for the proper growth and development of those parts which it is de- signed to nourish. The importance of the placenta is at once apparent if we remember that during the greater portion of intra-nterine life the maternal blood circulating in its vessels supplies the foetus with both food and oxygen. It thus performs the functions which in later life are discharged by the alimentary canal and lungs. 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 after-birth; 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 membrane 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 dis- charges or the lochia, which occur at this period. A new mucous membrane is of course gradually developed, as the old one, by its transformation into 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 placenta, contains the remnants of other structures which in the early stages of the development of the em- bryo 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 umbilicus. (2.) The umbilical vesicle with its duct and appertaining omphalo-mesenteric blood-vessels. (3.) The remains of the allantois, and continuous with it the urachus. (4.) The umbilical vessels, which, as just remarked, ulti- mately form the greater part of the cord. THE DEVELOPMENT OF THE ORGANS. It remains now to consider in succession the development of the several organs and systems of organs in the further progress of the em- bryo. The accompanying figure (Fig. 466) shows the chief organs of the body in a moderately early stage of development. The Vertebral Column and Cranium —The primitive part of the vertebral column in all the Vertebrata is the chorda dor sails (noto- chord), which consists entirely of soft cellular cartilage. This cord tapers to a point at the cranial and caudal extremities of the animal. In DEVELOPMENT. 679 the progress of its development, it is found to become inclosed in a membranous sheath, which at length acquires a fibrous structure, com- posed of transverse annular fibres. The chorda dorsahs 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 state of hyaline cartilage or bone, 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 only traces of it persist in the adult animal. In many Fish no true vertebrae are developed, and there is every gra- dation from the amphioxus, in which the notochord persists through life and there are no vertebrae, through the lampreys in which there are a few scattered cartilaginous vertebrae, and the sharks, in which many of ss N*» FIG. 466.— Embryo chick (4th day), viewed as a transparent object, lying on its left side (mag- nified). C H, cerebral hemispheres; FB, fore-brain or vesicle of third ventricle, with Pn, pineal gland projecting from its summit; MB, mid-brain; C b, cerebellum; IV. V, fourth ventricle; L, lens; chs, choroidal slit; Cen V, auditory vesicle; s m, superior maxillary process; IF, 2F, etc., first, second, third, and fourth visceral folds; V, fifth nerve, sending one branch (ophthalmic) to the eye, and another to the first visceral arch; VII, seventh nerve, passing to the second visceral arch; G.Ph, glosso-pharyngeal nerve, passing to the third visceral arch; Pa, pneumogastric nerve, pass- ing towards tne fourth visceral arch ; i v, investing mass; ch, notochord; its front end cannot be seen in the living embryo, and it does not end as shown in the figure, but takes a sudden bend down- wards, and then terminates in a point; Ht, heart seen through the walls of the chest; MP, muscle plates; W, wing, showing commencing differentiation of segments, corresponding to arm, forearm, and hand; HL, hind-limb, as yet a shapeless bud, showing no differentiation. Beneath it is seen the curved tail. (Foster and Balfour.) the vertebrae are partly ossified, to the bony fishes, such as the cod and herring, in which the vertebral column consists of a number of distinct ossified vertebrae, with remnants of the notochord between them. In Amphibia, Eeptiles, Birds, and Mammals, there are distinct vertebrae, which are formed as follows: — Thje rnesoblastic somites, which have been already mentioned (p. 666); send processes downwards and inwards to surround the notochord, 680 ' HANDBOOK OF PHYSIOLOGY. and also upwards between the medullary canal and the epiblast covering it. In the former situation, the cartilaginous bodies of the vertebrae make their appearance, in the latter their arches, which inclose the neu- ral canal. The vertebrae do not exactly correspond in their position with the protovertebrae: but each permanent vertebra is developed from the con- tiguous halves of two protovertebrae. The original segmentation of the protovertebrae disappears, and a fresh subdivision occurs in such a way that a permanent invertebral disc is developed opposite the centre of each protovertebra. Meanwhile the protovertebrae split into a dorsal and ventral portion. The former is termed the musculo-cutaneous plate, and from it are developed all the muscles of the back together with the cutis of the dorsal region (the epidermis being derived from the epiblast). The ventral portions of the protovertebrae, as we have already seen, give rise to the vertebrae and heads of the ribs. The chorda is now inclosed in a case, formed by the bodies of the vertebrae, but it gradually wastes and disappears. Before the disappear- ance of the chorda, the ossification of the bodies and arches of the verte- brae 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 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 pro- cess, 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. At an early period the head is bent downwards and forwards round the end of the chorda dorsalis in such a way that the middle cerebral vesicle, and not the anterior, comes to occupy the highest position in the head. Pituitary Body. — In connection with this must be mentioned the development of the pituitary body. It is formed by the meeting of two out-growths, one from the foetal brain, which grows downwards, and the other from the epiblast of the buccal cavity, which grows up towards it. The surrounding mesoblast also takes part in its formation. The con- nection of the first process with the brain becomes narrowed, and per- DEVELOPMENT. 681 sists as the infundibulum, while that of the other process with the buc- cal cavity disappears completely at a spot corresponding with the future position of the body of the sphenoid. Cranium. The first appearance of a solid support at the base of the cranium ob- served by Muller in fish, consists of two elongated bands of cartilage (trabeculae cranii), one on the right and the other on the left side, which are connected with the cartilaginous capsule of the auditory apparatus, and which diverge to inclose the pituitary body, uniting in front to form the septum nasi beneath the anterior end of the cerebral capsule. Hence, in the cranium, as in the spinal column, there are at first de- veloped at the sides of the chorda dorsalis two symmetrical elements, which subsequently coalesce, and may wholly inclose the chorda. The brain-case consists of three segments: occipital, parietal, and frontal, corresponding in their relative position to the three primitive cerebral vesicles; it may also be noted that in front of each segment is developed a sense-organ (auditory, ocular, and olfactory, from behind forwards). The basis crauii consists at an early period of an unseg- mented cartilaginous rod, developed round the notochord, and continued forward beyond its termination into the trabeculce cranii, which bound the pituitary fossa on either side. In this cartilaginous rod three centres of ossification appear: basi- occipital, basi-sphenoid, and p re-sphenoid, one corresponding to each segment. The bones forming the vault of the skull, viz., the frontal, parietal, squamous portion of temporal and the squamo-occipital, are ossified in membrane. The Visceral Clefts and Arches. As the embryo enlarges, the heart, which at first occupied a position close to the cranial flexure, is carried further and further backwards until a considerable intervening part exists between it and the head, in which the mesoblast is undivided. This becomes the neck. On a sec- tion it is seen that in it the whole three layers are represented in order, and that there is no interval between them. In the neck thus formed soon appear the visceral or branchial clefts on either side, in series, across the axis of the gut not quite at right angles. They are four in number, the most anterior being first found. At their edges the hypo- blast and the epiblast are continuous. The anterior border of each cleft forms a fold or lip, the branchial or visceral fold. The posterior bor- der of the last cleft is also formed into a fold, so that there are four clefts and five folds, but the three most anterior are far more prominent 682 HANDBOOK OF PHYSIOLOGY. than the others, and of these the second is the most conspicuous. The first fold nearly meets its fellow in the middle line, the second less nearly, and the others in order still less so. Thus in the neck there is a triangular interval, into which by the splitting of the mesoblast at that part the pleuro-peritoneal cavity extends. The branchial clefts and arches are not all permanent. The first arch gives off a branch from its front edge, which passes forwards to meet its fellow, but these offshoots do not quite meet, being separated by a process which grows downwards from the head. Between the branches and the main first fold is the cavity of the mouth. The branches represent the superior maxilla, and the main folds the mandible or lower jaw. The central process, which grows down, is the fronto-nasal process. In this way, the so-called visceral arches and clefts are formed, four on each side (Fig. 467, A). From or in connection with these arches the following parts are de- veloped:— FIG. 467.— 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 is the first cleft; 7. 8, 9, second, third, and fourth arches and clefts. B. Anterior view of the head of a human foetus 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 process; 7, the lower jaw; x, the tongue; 8, first branchial cleft becoming the meatus auditorius externus. The first arch (mandibular) contains a cartilaginous rod (Meckel's cartilage), around the distal edge of which the lower jaw is developed , while the malleus is ossified from the proximal end. When the maxillary processes on the two sides fail partially or com- pletely to unite in the middle line, the well-known condition termed cleft palate results. When the integument of the face presents a similar deficiency, we have the deformity known as hare-lip. Though these two deformities frequently co-exist, they are by no means always neces- sarily associated. The upper part of the face in the middle line is developed from the so-called frontal-nasal process (A, 3, Fig. 467). From the second arch are developed the incus, stapes, and stapedius muscle, the styloid pro- cess of the temporal bone, the stylo-hyoid ligament, and the smaller cornu of the hyoid bone. From the third visceral arch, the greater DEVELOPMENT. 683: cornu and body of the hyoid bone. In man and other mammalia the fourth visceral arch is indistinct. It occupies the position where the neck is afterwards developed. A distinct connection is traceable between these visceral arches and certain cranial nerves: the trigeminal, the facial, the glosso-pharyngeal,. and the pneumogastric. The ophthalmic division of the trigeminal sup- plies the trabecular arch; the superior and inferior maxillary divisions supply the maxillary and mandibular arches respectively. The facial nerve distributes one branch (chorda tympani) to the first visceral arch, and others to the second visceral arch. Thus it divides, inclosing the first visceral cleft. Similarly, the glosso-pharyngeal divides to inclose the second visceral 7F flL FIG. 468.— Embryo chick (4th day), viewed as a transparent object, lying on its left side (mag- nified). OH, cerebral hemispheres; FB, fore-brain or vesicle of third ventricle, with Pn, pineal gland projecting from its summit: MB, mid-brain; C 6, cerebellum; IV. V, fourth ventricle; Z,, lens; chs, choroidal slit: Cen.V, auditory vesicle; .s m, superior maxillary process; IF. 2F, etc., first, second, third, and fourth visceral folds; V, fifth nerve, sending one branch (ophthalmic) to the eye, and another to the first visceral arch; VII, seventh nerve, passing to the second visceral arch;. G.Ph, glosso-pharyngeal nerve, passing to the third visceral arch; Pg, pneumogastric nerve, pass- ing towards the fourth visceral arch ; iu, investing mass; ch. notochord; its front end cannot be seen in the living embryo, and it does not end as shown in the figure, but takes a sudden bend down- wards, and then terminates in a point; Ht, heart seen through the walls of the chest; MP, muscle plates; TF, wing, showing commencing differentiation of segments, corresponding to arm, forearm, and hand; S S, somatic stalk; Al, allaritois; fiTL, hind-limb, as yet a shapeless bud, showing no differentiation. Beneath it is seen the curved tail. (Foster and Balfour.) cleft, its lingual branch being distributed to the second, and its pharyn- geal branch to the third arch. The vagus, too, sends a branch (pharyngeal) along the third arch, and in fishes it gives off paired branches, which divide to inclose succes- sive branchial clefts. The Extremities. The extremities are developed in a uniform manner in all vertebrate animals. They appear in the form of leaf -like elevations from the pari- etes of the trunk (see Fig. 468), at points where more or less of an arch HANDBOOK OF PHYSIOLOGY. will be produced for them within. The primitive form of the extremity is nearly the same in all Vertebrata, whether it be destined for swim- ming, crawling, walking, or flying. In the human fo3tus the fingers are .at first united, as if webbed for swimming; but this is to be regarded not so much as an approximation to the form of aquatic animals, as the primitive form of the hand, the individual parts of which subsequently become more completely isolated. The fore-limb always appears before the hind-lirnb, and for some time continues in a more advanced state of development. In both limbs alike, the distal segment (hand or foot) is separated by a slight notch from the proximal part of the limb, and this part is subsequently divided again by a second notch (knee or elbow-joint). The Vascular System. — At an early stage in the development of the embryo chick, the so-called " area vasculosa " begins to make its appearance. A number of branched cells in the mesoblast send out pro- FIG. 469.— 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, beneath which are two other visceral arches separated by the branchial clefts; 10, rudiment of the upper extremity; 11, that of the lower extremity; 12, the umbilical cord; 15, the eye; 16, the ear; 17, cerebral hemispheres; 18, optic lobes, corpora quadrigemina. (Muller.) cesses which unite so as to form a network of protoplasm with nuclei at the nodal points. A large number of the nuclei acquire a red color; these form the red blood-cells. The protoplasmic processes become hollowed out in the centre so as to form a closed system of branching canals, in the walls of which the rest of the nuclei remain imbedded. In the blood-vessels thus formed, the circulation of the embryonic blood commences. According to Klein's researches, the first blood-vessels in the chick are developed from embryonic cells of the mesoblast, which swell up and become vacuolated, while their nuclei undergo segmentation. These cells send out protoplasmic processes, which unite with corresponding ones from other cells, and become hollowed, give rise to the capillary DEVELOPMENT. 685 wall composed of endothelial cells; the blood-corpuscles being budded off from the endothelial wall by a process of gemmation. Heart. — About the same early' period the heart makes its appearance as a solid mass of cells of the splanchno-pleure in the manner before in- dicated. At this period the anterior part of the alimentary tube ends blindly beneath the notochcord. It is beneath the posterior end of this fore-gut that the heart begins to be developed. The heart when first formed is FIG. 470.— Capillary blood-vessels of the tail of a young larval frog, a, capillaries permeable to blood; 6, fat granules attached to the walls of the vessels, and concealing the nuclei; c, hollow pro- longation of a capillary, endmg in a point; d, a branching cell with nucleus and fat- granules; it communicates by three branches with prolongation of capillaries already formed ; e, e, blood cor- puscles still containing granules of fat. x 350 times. (Kolliker.) made up of two not quite complete tubes which coalesce to form one, and so when the cavity is hollowed out in the mass of cells, the central cells float freely in the fluid, which soon begins to circulate by means of the rhythmic pulsations of the embryonic heart. These pulsations 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, and long before muscular fibres or ganglia have 686 HANDBOOK OF PHYSIOLOGY. been formed in the cardiac walls. At first they seldom exceed from fif- teen to eighteen in the minute. The fluid within the cavity 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 walls are composed are transformed into fibrous and muscular tissues, and into epithelium. In the developing chick it can be observed with the naked eye as a minute red pulsating point before the end of the second day of incubation. Blood-vessels. — Blood-vessels appear to be developed in two ways, ac- cording to the size of the vessels. In the formation of large blood-vessels, masses of embryonic cells similar to those from which the heart and other structures of the embryo are developed, arrange themselves in the position, form, and thickness of the developing vessel. Shortly after- wards the cells in the interior of a column of this kind seem to be de- FIG. 461. FIG. 462. FIG. 471.— Development of capillaries in the regenerating tail of a tadpole, a, 6, c, d, sprouts and cords of protoplasm. (Arnold.) FIG. 472.— The same region after the lapse of 24 hours. The " sprouts and cords of protoplasm " have become channelled out into capillaries. (Arnold.) veloped into blood-corpuscles, while the external layer of cells is con- verted 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, pass- ing between the main artery and vein, and are produced by the junction of prolongations, sent from both the artery and vein, with certain elon- gated or star-shaped cells, in the substance of the tail. When these arches are formed and are permeable to blood, new prolongations pass from them, join other radiated cells, and thus form secondary arches. In this manner, the capillary network extends 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 DEVELOPMENT. 687 within its meshes. The prolongations by which the vessels communi- cate 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 permeable to blood, it enlarges, and a cavity then forms in its interior (see Figs. 470, 472). This tissue is well calculated to illus- trate the various steps in the development of blood-vessels from elon- gating and branching cells. In many cases a whole network of capillaries is developed from a net- work of branched, embryonic connective-tissue corpuscles by the joining of their processes, the multiplication of their nuclei, and the vacuolation of the cell-substance. The vacuoles gradually coalesce till all the parti- FIG. 473.— Capillaries from the vitreous humor of a foetal calf. Two vessels are seen connected by a '• cord " of protoplasm, and clothed with an adventitia, containing numerous nuclei; a, inser- tion of this " cord " into the primary walls of the vessels. (Frey.) tions are broken down, and the originally solid protoplasmic cell-substance is, so to speak, tunnelled out into a number of tubes. Capillaries may also be developed from cells which are originally spheroidal, vacuoles form in the interior of the cells gradually becoming united by fine protoplasmic processes: by the extension of the vacuoles into them, capillary tubes are gradually formed. Morphology Heart. — When it first appears, the heart is approximately tubular in form, being at first a double tube, then a single one. It receives at its two posterior angles the two omphalo-mesenteric or vitel- line veins, and gives off anteriorly the primitive aorta (Fig. 474). The junction of the two veins which pass into the auricle becomes removed farther and farther away from the heart, and the vessel thus formed is •called sinus venosusnear to the auricle, and ductus venosus farther away, or if it be called by one name, that of meatus venosus may be used. 688 HANDBOOK OF PHYSIOLOGY. It soon, however, becomes curved somewhat in the shape of a horse- shoe, with the convexity towards the right, the venous end being at the same time drawn up towards the head, so that it finally lies behind and somewhat to the right of the arterial. It also becomes partly divided by constrictions into three cavities. Of these three cavities which are developed in all Vertebrata, that at the venous end is the simple auricle, with the sinus venosus, that at the arterial end the bulbus arteriosus, and the middle one is the simple ven- tricle. FIG. 474.— Foetal heart in successive stages of development. 1, venous extremity; 2, arterial extremity; 3, 3, pulmonary branches; 4,ductus arteriosus. (Dalton.) These three parts of the heart contract in succession. The auricle and the bulbus arteriosus at this period lie at the extremities of the horse- shoe. The bulging out of the middle portion inferiorly gives the first indication of the future form of the ventricle (Fig. 475). The great FIG. 475.— Heart of the chick at the 45th, 65th, and 85th hours of incubation. 1, the venous trunks; *, the auricle; 3, the ventricle; 4, the bulbus arteriosus. (Allen Thomson.) curvature of the horse-shoe by the same means becomes much more de- veloped than the smaller curvature between the auricle and bulbus; and the two extremities, the auricle and bulb, approach each other superiorly, so as to produce a greater resemblance to the later form of the heart, whilst the ventricle becomes more and more developed inferiorly. The heart of Fishes retains these four cavities, no further division by inter- nal septa into right and left chambers taking place. In Amphibia, also, the heart throughout life consists of the three muscular divisions which are so early Jormed in the embryo and the sinus venosus; but the DEVELOPMENT. 689 auricle is divided internally by a septum into a pulmonary and systemic auricle. In reptiles, not merely the auricle is thus divided into two cavities, but a similar septum but incomplete is more or less developed in the ventricle. In Birds and Mammals, both auricle and ventricle undergo complete division by septa; whilst in these animals as well as in reptiles, the bulbus aortae is not permanent, but becomes lost in the ven- tricles. The septum dividing the ventricle commences at the apex and extends upwards. The subdivision of the auricles is very early fore, shadowed by the outgrowth of the two auricular appendages, which occurs before any septum is formed externally. The septum of the auri- cles is developed from a semilunar fold, which extends from above down- wards. 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 partition of the auricles is first commencing, the two venae cavae 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 entrance of the inferior cava. Subsequently, however, the septum, growing from the anterior wall close to the upper end of the ventricular septum, becomes directed more and more to the left of the vena cava inferior. During the entire period of fcetal life, there remains an opening in the septum, which the valve of the foramen ovale, devel- oped in the third month, imperfectly closes. The bulbus arteriosus, which is originally a single tube, becomes gradually divided into two by the growth of an internal septum, which springs from the posterior wall, and extends forwards towards the front wall and downwards towards the ventricles. This partition takes a some- what spiral direction, so that the two tubes (aorta and pulmonary artery) which result from its completion, do not run side by side, but are twisted round each other. As the septum grows down towards the ventricles, it meets and coa- lesces with the upwardly growing ventricular septum, and thus from the right and left ventricles, which are now completely separate, arise respec- tively the pulmonary artery and aorta, which are also quite distinct. The auriculo-ventricular and semilunar valves are formed by the growth of folds of the endocardium. At its first appearance, as we have seen, the heart is placed just be- neath the head of the foetus, and is very large relatively to the whole body; but with the growth of the neck it becomes further and further removed from the head, and is lodged in the cavity of the thorax. Up to a certain period the auricular is larger than the ventricular 44 690 HANDBOOK OF PHY3IOLOGY. division of the heart; but this relation is gradually reversed as develop- ment proceeds. Moreover, all through foetal life, the walls of the right ventricle are of very much the same thickness as those of the left, which may probably be explained by the fact that in the foetus the right ventri- cle has to propel the blood from the pulmonary artery into the aorta, and thence into the placenta, while in the adult it only drives the blood through the lungs. Arteries. — The primitive aorta arises from the bulbus arteriosus and divides into two branches which arch backwards, one on each side of the foregut and unite again behind it, and in front of the notochord into a single vessel. This gives off the two omphalo-mesenteric arteries, which distribute branches all over the yolk-sac; this area vasctilosa in the chick attaining •pn FIG. 476.— Diagram of the aortic arches in the mammal, showing transformations which give rise to the permanent arterial vessels. A, primitive arterial stem or aortic bulb, now divided into A, the ascending part of the aortic arch, and p, the pulmonary; a, a', right and left aortic roots; A', de- scending aorta; 1, 2, 3, 4, 5, the five primitive aortic or branchial arches; /, II, III, IV, the four branchial clefts which, for the sake of clearness, have been omitted on the right side. The per- manent systemic vessels are deeply, the pulmonary arteries lightly, shaded; the parts of the primitive arches which are transitory are simply outlined; c, placed between the permanent com- mon carotid arteries; ce, external carotic arteries; ci, internal carotid arteries; s, right subclavian, rising from the right aortic root beyond the fifth arch; v, right vertebral from the same, opposite the fourth arch; v', s\ left vertebral and subclavian arteries rising together from the left, or per- manent aortic root, opposite the fourth arch, p, pulmonary arteries rising together from the left fifth arch; d, outer or back part of left fifth arch forming ductus arteriosus; pn,pn', right and left pneumogastric nerves descending in front of aortic arch, with their recurrent; branches represented diagrammatically as passing behind to illustrate the relations of these nerves respectively to the right subclavian artery (4), and the arch of the aorta and ductus arteriosus (d). (Allen Thomson, after Rathke.) a large development, and being limited all round by a vessel known as the sinus terminalis. The blood is collected by the venous channels, and returned through the omphalo-mesenteric veins to the heart. Behind this pair of primitive aortic arches, four more pairs make DEVELOPMENT. 691 their appearance successively, so that there are five pairs in all, each one running along one of the visceral arches. These five are never all to be seen at once in the embryo of higher animals, for the two anterior pairs gradually disappear while the poste- rior ones are making their appearance, so that at length only three remain. In Fishes, however, they all persist throughout life as the branchial arteries supplying the gills, while in Amphibia three pairs persist throughout life. In Keptiles, Birds, and Mammals, further transformations occur. In Reptiles the fourth pair remains throughout life as the perma- nent right and left aorta; in Birds the right one remains as the perma- nent aorta, curving over the right bronchus instead of the left as in Mammals. i This substance occurs in the body in combination as in the biliary acids, but is never free. Glycocholic acid, when treated with weak acids, with alkalies, or with baryta water, splits up into cholic acid and glycin, or amido-acetic acid. Thus: 026 H43N06 4- H20 = Ca6 H40 05 + C2 H5 N02. Glycocholic acid + water = cholic acid + glycin, and under similar circumstances Taurocholic acid splits up into cholic acid and taurin:— C26 H45 03 NSOa + H20 = C26 H40 05 + C2 H7 NS03, or amido-isethionic. Taurocholic acid 4- water = cholic acid and taurin. Glycin occurs also in hippuric acid. It can be prepared from gelatin by the action of acids or alkalies; it can also be obtained from hippuric acid. Sarcosin or Methyl I n TT TSTO f- PH /NH CH* ^ It is Glycin, f°i*r»v.Vf= H»\CO OH J' stituent of kreatin, and also of caffeine, but has never been found free in the human body. It may be obtained from these bodies by boiling with baryta water. 0" [ °.H»N0.( = CH..OH.CH1CH,.CH(NH1) CO OH occurs normally in many of the organs of the body and is a pro- duct of the pancreatic digestion of proteids. It is present in the urine in certain diseases of the liver in which there is loss of substance, espe- cially in acute yellow atrophy. It occurs in circular oily discs or crystallizes in plates, and can be prepared either by boiling horn shavings, or any of the gelatins with sulphuric acid, or out of the products of pancreatic digestion. Amido-sulplionic Acids. Taurin, or Amido- ) n Tr ATQ^ I n TT /SO,H\ • isethionic Acid, [ °>H'NS04 = C°H Senses, special, 546 organs of, development of, 700 Sensory centres in cerebral cortex, 529 Sensory impressions, conduction of, 458 by spinal cord, 482 in brain, 522 nerves, 458 paths, 482 Septum between auricles, formation of, 689 between ventricles, formation of, 689 luciduin, 689 Serine, 79, 735 Serous fluid, 327 Serous membranes, 326 arrangement of, 326 communication of lymphatics with, 302 epithelium, 20 fluid secreted by, 327 functions, 326 lining joints, etc., 326 et seq. visceral cavities, 326 structure of, 326 Serum, of blood, 78 albumin, 735 separation of, 78 Seventh cerebral nerve, 538 Sex, influence on blood, 89 influence on production of carbonic acid, 188 relation of, to respiratory movements, 181 Sexual organs and functions in the female, 634 in the male, 640 Sighing, mechanism of, 195 Sight, 584. See Vision. Silica, parts in which found, 753 Silicon, 753 Singing, mechanism of, 196 et seq. Single vision, condition of, 620 Sinus pocularis, 712 rhomboidalis. 712 urogenitalis, 712 Sinuses of dura mater, 162 etseq. of Valsalva, 104 Sixth cerebral nerve, 537 Size of field of vision, 611 Skatol, 272 Skeleton. See Frontispiece. Skin, 340 absorption by, 352 of metallic substances, 352 of water, 352 cutis vera of, 342 epidermis of, 340 evaporation from, 350 excretion by, 350 exhalation of carbonic acid from, 351 of watery vapor from. 350 functions of, 348 respiratory, 351 glands, 344 Skin — continued. papillae of, 342 perspiration of, 350 rete mucosum of, 340 sebaceous glands of, 345 structure of, 340 sudoriferous glands of, 344 Sleep, 514 Smell, sense of, 563 conditions of, 563 delicacy, 566 different kinds of odors, 566 impaired by lesion of facial nerve, 539 impaired by lesion of fifth nerve, 537 internal excitants of, 567 limited to olfactory region, 563 structure of organ of, 564 subjective sensations, 566 varies in different animals, 566 Sneezing, centre, 497 mechanism of, 195 Sniffing, mechanism of, 196 smell aided by, 563 Sobbing, 196 Sodium, 752, 753 in human body, 752, 753 sulphindigotate, 378 Solitary glands. See Peyer's. Soluble ferments, 746 Somatopleure, 666 Somnambulism, 515 Sonorous vibrations, how communicated in ear, 575 et seq. in air and in water, ib. See Sound. Soprano voice, 444 Sound, binaural sensations, 583 conduction of by ear, 575 heart, 121 movements and sensations produced by. 584 perception, of direction of, 582 of distance of, 583 permanence of sensation of, 583 production of, 582 subjective, 584 Source of water, 752 Spasms, reflex acts, 498 Speaking, 447 mechanism of, 196, 447 Special senses, 547 Spectrum-analysis of blood, 84 Speech. 447 function of tongue in, 549 Spermatozoa, development of, 642 form and structure of, 652 function of, 657 motion of, 652 Spherical aberration, 602 correction of. 602 Spheroidal epithelium, 23 Sphincter ani. See Defaecation. Sphygmograph, 138 tracings, 139 et seq. 780 INDEX. Spinal accessory nerve, 544 Spinal cord, 472 automatism, 471 canal of, 473 centres in, 488 a collection of nervous centres, 488 columns of, 474 commissures of, 474 conduction of impressions by, 481 etseq. course of fibres in, 479 decussation of sensory impressions in, 483 development of, 696 effect of injuries of, on conduction of impressions, 484 et seq. fissures and furrows of, 474 functions of, 481 of columns, 482 influence on lymph-hearts, 490 on sphincter ani, 488 on tone, 490 morbid irritability of, 488 nerves of, 477 reflex action of, 485 in disease, 488 inhibition of, 486 special centres in, 488 structure of, 472 et seq. transference, 484 weight, 510 relative, 510 white matter, 475 gray matter, 476 Spinal nerves, 477 origin of, 479 et seq. physiology of, 480 Spirometer, 184d Splanchnic nerves, 149, 627 Splanchnopleure, 666 Spleen, 383 functions, 385 hilus of, 383 influence of nervous system, 386 Malpighian corpuscles of, 385 pulp, 383 et seq. stroma of, 383 structure of, 383 trabeculse of, 383 et seq. Splenic vein, blood of, 90 Spot, germinal, 636 Squamous epithelium, 20 Stammering, 449 Stannius' experiments, 131 Stapedius muscle, 569 function of, 579 Stapes, 569 Starch, 236, 748 digestion of in mouth, 235 .Starvation, 215 appearances after death, 216 effect on temperature, 215 loss of weight in, 215 period of death in, 216 symptoms, 216 Steapsin, 273 Stearic acid, 750 Stearin, 750 Stercorin, 284 allied to cholesterin, 284 Stereoscope, 624 Stimuli, protoplasmic, 5 St. Martin, Alexis, case of, 251 Stomach, 245 blood-vessels, 250 development, 704 et seq, digestion in, 252 circumstances favoring, 253 products of, 253 digestion after death, 257 glands, 248 lymphatics, 250 movements, 255 influence of nervous system, 256 mucous membrane, 247 muscular coat, 246 nerves, 256 ruminant, 245 secretion of, 250. See Gastric fluid. structure, 246 temperature, 251 Stomata, 22, 303 Stratum intermedium (Hannover), 228 Striated muscle, 396 Stroma fibrin, 61 Stromuhr, 159 Structural basis of human body, 15 Submaxillary gland, 238 Succus entericus, 289 functions of, 290 Sucking, mechanism of, 196 centre, 497 Sudoriferous glands, 344 their distribution, 344 number of, 344 their secretion, 350 Suffocation, 204 et seq. Sugar. See Glucose. tests, 236 Sulphates, 753 in tissues, 753 in urine, 371 Sulphuretted hydrogen, 751 Suprarenal capsules, 390 development of, 709 disease of, relation to discoloration of skin, 392 structure, 390 Sun, a source of energy, Chap. XXIV. Swallowing. 244 centre, 497 nerves engaged, 245 Sweat, 350 Sympathetic nervous system, 465, 625 conduction of impressions by, 629 distribution, 465 divisions of, 465 fibres, differences of from cerebro-spinal fibres, 452 functions, 627 et seq. T81 Sympathetic nervous system — continued. ganglia of, 629 action of, 629 et seq. co-ordination of movements by, 630 structure, 625 in substance of organs, 625 influence on blood-vessels, 147 et seq. heart, 133 involuntary motion, 627 et seq. salivary glands, 238 et seq. secretion, 629 structure of, 625 Synovial fluid, secretion of, 328 membranes, 328 Syntonin, 253 Systemic circulation. See Circulation. vessels, ib. Systole of heart, 115 T. Table of diet, 219 Taste, 556 after-tastes, 562 centre, 529 conditions for perception of, 556 connection with smell, 562 impaired by injury " of facial nerve, 539 of fifth nerve, 537 nerves of, 540 seat of, 556 subjective sensations, 563 varieties, 562 Taste-goblets, 560 Taurin, 742 Taurocholic acid, 280 Teeth, 221 development, 226 eruption, times of, 222 structure or, 223 et seq. temporary and permanent, 221 et seq. Tegmentum, 499 Temperament, influence on blood, 89 Temperature, 316 average of body, 316 changes of, effects of, 324 et seq. circumstances modifying, 316 of cold-blooded and warm-blooded ani- mals, 317 in disease, 317 loss of, 320 maintenance of, 329 of Mammalia, Birds, etc., 317 of paralyzed parts, 323 regulation of, 319 of respired air, 188 sensation of variation of, 323. See Heat. Temporo-maxillary fibro-cartilage, 230 Tendon-reflex, 486 Tendons, structure of, 32 cells of, 32 Tenor voice, 444 Tension, arterial, 143 Tension of gases in lungs, 192 Tensor tympani muscle, 579 office of, 579 Tesselated epithelium, 19 Testicle, 640 development, 709 descent of, 710 structure of, 640 et seq. Tetanus, 413 Thalamencephalon, 698 Thalami optici, function of, 523 Thermogenic nerves and nerve-centres, 323 Thirst, 214 Thoracic duct, 298 contents, 309 Thymus gland, 387 function of, 388 structure, 387 Thyro-arytenoid muscles, 439 Thyroid cartilage, structure and connec- tions of, 439 Thyroid-gland, 389 function of, 390 structure, 389 Timbre of voice, 444 Tissue, adipose, 37 areolar, cellular, or connective, 34 elastic, 33 fatty, 37 fibrous, 32 gelatinous, 35 retiform, 35 Tissues, connective, 29 elementary structure of, 30 et seq. erectile, 163 Tone of blood-vessels, 148 of muscles, 490 of voice, 444 Tongue, 557 aetion of, in deglutition, 244 in sucking, 196 action of, in speech, 449 epithelium of, 560 influence of facial nerve on, 539 motor nerve of, 545 an organ of touch, 561 papillae of, 558 parts most sensitive to taste, 560 structure of, 557 Tonic centres, 498 Tonsils, 241 Tooth-ache, radiation of, sensation in, 467 Touch, 550 after sensation, 555 conditions for perception of, 550 connection of, with muscular 553 co-operation of mind with, 555 hand an organ of, 551 illusions, 553 782 INDEX. Touch— continued. modifications of, 550 a modification of common sensation, 550 special organs, 550 subjective sensations, 556 the tongue an organ of, 550 various degrees of, in different parts, 552 Touch-corpuscles, 463 Trachea, 170 Tracts in the spinal cord, 479 Tradescentia Virginica, movements in cells of, 4 Tragus, 568 Transference of impressions, 466 Traube-Hering's curves, 203 Tricuspid valve, 103 safety-valve action of, 118 Trigeminal or fifth nerve, 533 effects of injury of, 535 Trophic nerves, 536 Trypsin, 272 Trypsinogen, 271 Tubercle of Lower, 99 Tubes, Fallopian, 638. See Fallopian tubes. Tubular glands, 330 Tubules, 18 Tubuli seminiferi, 641 uniferi, 354 et seq. Tunica albuginea of testicle, 640 Tympanum or middle ear, 568 development of, 702 functions of, 576 membrane of, 577 structure of, 577 use of air in, 576 Types of respiration, 181 Tyrosin, 272, 743 U. Ulceration of parts attending injuries of nerves, 631 Ulnar nerve, effects of compression of, 459 Umbilical arteries, 694 cord, 678 vesicle, 669 Unconscious cerebration, 514 Unorganized ferments, 746 TJnstriped muscular fibre, 394 development, 401 distribution, 394 structure, 395 Urachus, 672 Urate of ammonium, 367 of sodium, 367 Urea, 364, 743 apparatus for estimating quantity, 366 chemical composition of, 364 identical with cyanate of ammonium, ib.t 365 Urea — continued. properties, 364 quantity, 366 in relation to muscular exertion, 380 sources, 379 Ureter, 359 Uric acid, 366, 744 condition in which it exists in urine, 367 forms in which it is deposited, 368 proportionate quantity of, 367 source of, 381 tests, 368 variations in quantity, 368 Urina sanguinis, potus, et cibi, 363 Urinary bladder, 360 development, 709 nerves, 360 structure, 360 Urinary ferments, 362 Urine, 361 abnormal, 364 analysis of, 361 chemical composition, 361 coloring matter of, 368 cystin in, 372 decomposition by mucus, 362 effect of blood-pressure on, 374 expulsion, 382 extractives, 370, 381 flow of, into bladder, 381 gases, 373 hippuric acid in, 368 mucus in, 370 oxalic acid in, 372 physical characters, 361 pigments, 368 quantity of chief constituents, 362 reaction of, 362 in different animals, 362 made alkaline by diet, 362, 371 saline matter, 371 secretion, 373 effects of posture, etc., on, 382 rate of, 382 solids, 364 variations of, 362 specific gravity of, 363 variations of, 363 urates, 367 urea, 364 uric acid in, 366 variations of specific gravity, 363 of water, 366 Urobilin, 368 Urochrome, 368, 745 Uroerythrin, 369 Uromelanin, 369 Uses of blood, 94 Uterus, 639 change of mucous membrane of, 673 et seq. development of, in pregnancy, 673 f ollicular glands of, 674 masculinus, 712 structure, 639 INDEX. 783 TJtriculus of labyrinth, 574 Uvula in relation to voice, 447 Yagina, structure of, 639 Vagus nerve. See Pneumogastric. Yalve, ileo-caecal, structure of, 266 Valves of heart, 103 action of, 117 bicuspid or mitral, 103 semilunar, 104 tricuspid, 103 of lymphatic vessels, 303 of veins, 114 Valvulse conniventes, 267 Vas deferens, 640 Yasa efferentia of testicle, 641 recta of testicle, 641 vasorum, 109 Yascolar area, 669 Vascular glands, 383 in relation to blood, 393 several offices of, 393 Vascular system, development of, 684 Yaso-constrictor nerves, 151 Yaso-dilator nerves, 151 Vaso-motor influence on blood pressure, 147 etseq. Vaso-motor nerves, 147 effect of section, 148 et seq. influence upon blood-pressure, 148 Yaso-motor nerve-centres, 149, 498 reflection by, 149 Vegetables and animals, distinctions be- tween, 10 Veins, 112 blood-pressure in, 157 circulation in, 156 et seq. rate of, 159 cardinal, 692 collateral circulation in, 114 cranium, 162 development, 692 distribution, 112 effects of respiration on, 200 influence of expiration, 202 inspiration, 200 influence of gravitation in, 158 parietal system of, 693 et seq. pressure in, 157 rhythmical action in, 157 structure of, 113 systemic, 112 umbilical, 694 valves of, 114 velocity of blood in, 160 visceral system of, 692 et seq. Velocity of blood in arteries, 159 in capillaries, 160 in veins, 160 of circulation, 158 of nervous force, 458 conditions modifying, 458 Vena ports, 275 Venae hepaticse advehentes, 692 revehentes, 692 Ventilation, 199 Ventricles of heart, 99 capacity of, 116, 127 contraction of, 116 dilatation of, ib., 127 force of, 128 of larynx, office of, 447 lateral, 505 Ventriloquism, 449 Vermicular movement of intestines, 293 Vermiform process, 266 Vertebrae, development of, 678 Vertebral plate, 666 Vesicle, germinal, 636 Graafian, 636 Vesicula germinativa, 636 Yesiculae seminales, 643 functions of, 654 structure, 643 Vestibule of the ear, 570 Vibrations, conveyance of, to auditory nerve, 575 et seq. Vidian nerve, 538 Yilli in chorion, 673 in placenia, 677 Villi of intestines, 264 action in digestion, 265 Visceral arches, development of, 681 connection with cranial nerves, 683 laminae or plates, 668 Visceral plates, 668 Yiscero-inhibitory nerves, 628 motor, 628 Vision, 584 angle of, 611 at different distances, adaptation of eye to, 596 et seq. centre, 529 corpora quadrigemina, the principal nerve-centres of, 500 correction of aberration, 602 et seq. of inversion of image, 609 defects of, 600 et seq. distinctness of, how secured, 598 et seq. duration of sensation in, 605 estimation of the form of objects, 613 of their direction, 613 of their motion, 613 of their size, 612 field of, size of, 611 focal distance of, 599 impaired by lesion of fifth nerve, 537 influence of attention on, 614 modified by different parts of the retina, 617 purple, 608 single, with two eyes, 619 Visual direction, 613 Vital or respiratory capacity of chest, 184 Vital capillary force, 156 Vitellin, 738 Vitelline duct, 704 membrane, 658 784 INDEX. Vitelline spheres, 658 Vitiated air, effects of, 199 Vitreous humor, 594 Vocal cords, 438 et seq. action of, in respiratory actions, 184 et seq. approximation of, effect on height of note, 446 longer in males than in females, 444 position of, how modified, 442 vibrations of, cause voice, 438 Voice, 443 of boys, 445 compass of, 445 conditions on which strength depends, 446 human, produced by vibration of vocal cords, 443 in eunuchs, 445 influence of age on, 445 of arches of palate and uvula, 447 of epiglottis, 443 of sex, 444 of ventricles of larynx, 447 of vocal cords, 443 in male and female, 444 cause of different pitch, 444 modulations of, 444 natural and falsetto, 445 peculiar characters of, 444 varieties of, 443 et seq. Vomiting, 258 action of stomach in, 258 centre, 497 nerve-actions in, 259 voluntary and acquired, 259 Vowels and consonants, 447 Vulvo-vaginal or Duverney's glands, 640 W. Walking, 423 Water, 751 absorbed by skin, 352 by stomach, 254 amount, in blood, variations in, 89, 90 Water — continued. exhaled from lungs, 190 from skin, 350 forms large part of human body, 751 influence of on decomposition, 733 in urine, excretion of, 363 variations in, 363 loss of, from body, 752 uses, 752 quantity in various tissues, 752 source, 752 vapor of, in atmosphere, 190 Wave of blood, causing the pulse, 137 velocity of, 137 White corpuscles, 94. See Blood-corpus- cles, white ; and Lymph-corpuscles. White fibre-cartilage, 42 fibrous tissue, 32 Willis, circle of, 162 Wolffian bodies, 706 et seq. Wooldridge, 69 Work of heart, 128 X. Xanthin, 370, 744 Xantho-proteic reaction, 734 Y. Yawning, 196 Yelk, or vitellus, 637 changes of, in Fallopian tube, 658 cleaving of, 658 constriction of, by ventral laminae, 633 Yelk-sac, 669 et seq Yellow elastic fibre, 33 fibro-cartilage, 42 spot of Sommering. 588 Young-Helmholtz theory, 615 Z. Zimmermann, corpuscles of, 38b Zona pellucida 658 • "m 56564 -.Handbook of physiology, by W.Morrant Baker & Vincent Dormer Harris. 12tft-ed. re- arranged, rev. ftlld re-writ "ten. 56;