K^i **<+ c ^ / v>~ <- sV. sf HAND-BOOK OF PHYSIOLOGY. Digitized by the Internet Archive in 2010 with funding from Columbia University Libraries http://www.archive.org/details/handbookofphysioOOkirk KIRKES HAND-BOOK OF FHY816WMY. HAND-BOOK PHYSIOLOGY. BY W. MORRANT BAKER; F.R.C.S. s to st. Bartholomew's hospital and consulting surgeon to the evelina hospital FOR SICK CHILDREN; LECTURER OX PHYSIOLOGY AT ST. BARTHOLOM - ITAL, AND LATE MEMBER OF THE BOARD OF EXAMINERS OF THE ROYAL COLLEGE OF SURGEONS OF ENGLAND. A>"T) VINCENT DORMER HARRIS, M.D. Loud., DEMONSTRATOR OF PHYSIOLOGY AT ST. BARTHOLOMEW'S HOSPITAL. (tMcbcntb Orbit ion. w :tz nearly f:tz htitzezd illtstzaz:: PHILADELPHIA : P. BLAKISTOX, SOX & CO., 1012, WALNUT STREET. l88+ VTli l[[ ^4 PREFACE TO THE ELEVENTH EDITION Ix the preparation of the present edition of Kirkes' Physiology, we have endeavoured to maintain its character as a guide for students, especially at an early period of their career ; and, while incorporating new facts and observa- tions which are fairly established, we have as far as possible omitted the controvertible matters which should only find a place in a complete treatise or in a work of reference. A large number of new illustrations have been added, for many of which we are indebted to the com I : Dr. Klein, Professor Michael Foster, Prole— Schafer, Dr. Mahomed, Mr. Gant, and Messrs. McMillan, who have been so good • allow various figures to be copied. Our thanks are also due to Mr. Win. Lapraik, F.C.S., who has kindly pre- pared a table of the absorption spectra oi the blood and bile, based upon his own observations ; as well as to Mr. S. K. Alcock for several careful drawings of microscopical preparations, and for reading several sheets in their passage through the pr< ss. Mr. Danielsson, of the firm of Lebon & Co., has executed all the new figures to our entire satisfaction : and for the skill and labour he has expended upon them we are much indebted to him. We are desirous also of acknowledging the help we have derived from the following works: — Klein's Histoid vi TREFACE TO THE ELEVENTH EDITION. M. Foster's Text Book of Physiology ; Pavy's Food and Dietetics ; Quain's Anatomy, Vol. II., Ed. ix. ; Wickham Legg's Bile, Jaundice, and Bilious Diseases ; Watney's Minute Anatomy of the Thymus; Rosenthal's Muscles and Nerves ; Cadiat's Traite D'Anatomie Generale ; Ranvier's Traite Technique D 'Histologic ; Landois' Lehrbuch der Physiologie des Menschen, and the Journal of Physiology. W. MOREANT BAKER, V. D. HARRIS. Wimpole Street, August, 1884. CONTENTS CHAPTER I. PACK The General and Distinctive Characters of Living Beings i CHAPTER II. Structural Basis of the Human Body 5 Cells ;h - Protoplasm 7 Nucleus I1 Intercellular Substance . 20 Fibres **• Tubules 21 CHAPTER III. Structure of the Elementary Tissues 21 Epithelium 22 Connective Tissues 33 Areolar Tissue 37 White Fibrous Tissue *&« Yellow Elastic Tissue 38 Gelatinous 39 Retiform or Adenoid 40 Neuroglia 4 1 Adipose 4 2 Cartilage 46 Bone 5 1 Teeth 67 CHAPTER IV. The Blood 78 Quantity of Blood 79 Coagulation of the Blood 80 viii CONTEXTS. PAGE The Blood— continued. Conditions affecting Coagulation 88 The Blood- Corpuscles 92 Physical and Chemical Characters of Bed Blood-Cells . . . lb. The White Corpuscles, or Blood-Leucocytes 98 Chemical Composition of the Blood 102 The [Serum 105 Variations in Healthy Blood under Different Circumstances . . 107 Variations in the Composition of the Blood in Different Parts of the Body 108 Gases contained in the Blood 109 Blood- Crystals 112 Development of the Blood 119 Uses of the Blood 123 Uses of the various Constituents of the Blood . ih. CHAPTER V. CIRCULATION OF THE BLOOD 124 The Systemic, Pulmonary, and Portal Circulations . . . . 125 The Forces concerned in the Circulation of the Blood . . .127 The Heaet ' \l>. Structure of the Valves of the Heart 135 The Action of the Heart 137 Function of the Valves of the Heart 139 Sounds of the Heart . 145 Impulse of the Heart 147 The Cardiograph . . . . . . . . . . 148 Frequency and Force of the Heart's Action 151 Influence of the Nervous System on the Action of the Heart . . 154 Effects of the Heart's Action 157 The Arteries. Capillaries, and Veins 160 Structure of the Arteries 161 Structure of Capillaries 164 Structure of Veins 168 Function of the Arteries 171 The Pulse 177 Sphygmograph 178 Pressure of the Blood in the Arteries, or Arterial Tension . .185 The Kymograph 186 Influence of the Nervous System on the Arteries . . . . 1 90 Circulation in the Capillaries 197 Diapedesis of Blood-Corpuscles 198 CONTENTS, IX Circulation in tin: Varus 201 Blood-pres8aie in the veins 202 :ity of the Circulation 203 ity of the Blood in the Arteries 204 ., Capillaries 206 r „ •• Veins #. Velocity of the Circulation as a whole ib. Peculiarities of the Circulation in Different Parts . 208 Circulation in the Brain *&« Circulation in the Erectile Structures 210 Agents concerned in the Circulation 211 Discovery of the Circulation 212 Proofs of the Circulation of the Blood ib. CHAPTER VI. Respiration Position and Structure of the Lungs . Structure of the Trachea and Bronchial Tubes Structure of Lobules of the Lungs Mechanism of Respiration .... Respiratory Movements .... Respiratory Rhythm Respiratory Sounds ..... Respiratory Movements of Glottis . Quantity of Air respired .... Vital or Respiratory Capacity Force exerted in Respiration Circulation of Blood in the Respiratory Organs Changes of the Air in Respiration Changes produced in the Blood by Respiration Mechanism of various Respiratory Actions . Influence of the Nervous System in Respiration Effects of Vitiated Air — Ventilation . Effect of Respiration on the Circulation . Apncea — Dyspnoea — Asphyxia . 214 215 218 221 227 ib. 233 ib. 234 ib. 235 236 ib. 238 244 247 249 252 253 25S CHAPTER VII. Foods 262 Classification of Foods 264 Foods containing chiefly Nitrogenous Bodies .... 265 „ „ „ Carbohydrate Bodies . . . . 267 CONTEXTS. Foods — continued. Foods containing chiefly Fatty Bodies Substances supplying the Salts liquid Foods Effects of Cooking .... Effects of an Insufficient Diet . Starvation Effects of Improper Food Effects of too much Food . Diet Scale 268 ib. ib. ib. 269 270 272 273 275 CHAPTER VIII. Digestion 276 Passage of Food through the Alimentary Canal . . 277 Mastication 278 Insalivation 279 The Salivary Glands and the Saliva ib. Structure of the Salivary Glands ib. The Saliva 283 Influence of the Xervous System on the Secretion of Saliva . . 285 The Pharynx 291 The Tonsils ib. The CEsophagus or Gullet 292 Swallowing or Deglutition .... ... 294 Digestion of Food in the Stomach 296 Structure of the Stomach 297 Gastric Glands 299 The Gastric Juice 303 Functions of the Gastric Juice 305 Movements of the Stomach 308 Vomiting 310 Influence of the Nervous System on Gastric Digestion . . .312 Digestion of the Stomach after Death 313 Digestion in the Intestines 315 Structure of the Small Intestine ib. Yalvulae Conniventes 317 Glands of the Small Intestine ib. The Villi 321 Structure of the Large Intestine 325 The Pancreas and its Secretion 328 CONTENTS. x i DlGBSTION IN THE lXTKSTINKS— nmtiniiol. Structure of the Liver 332 Functions of the Liver 338 The Bile ib. The Li ver as a Blood-elaborating Organ 347 Glycogenic Function of the Liver ....... 348 Summary of the Changes which take place in the Food during its Passage through the Small Intestine 352 Succus Entericus . . . 351 Summary of the Process of Digestion in the Large Intestine . . 355 Defecation 357 1 1 -os contained in the Stomach and Intestines . . . . 358 Movements of the Intestines 359 Influence of the Nervous System on Intestinal Digestion . . 360 CHAPTER IX. Absorption 361 The Lacteal and Lymphatic Vessels and Glands . ... ib. Lymphatic Glands 368 Properties of Lymph and Chyle 373 Absorption by the Lacteal Vessels 375 Absorption by the Lymphatic Vessels 376 Absorption by Blood-vessels 377 CHAPTER X. Animal Heat 382 Variations in Bodily Temperature ib. Sources of Heat 384 Loss of Heat 387 Production of Heat 390 Inhibitory Heat-centre 391 CHAPTER XI. Secretion 393 Secreting Membranes 394 Serous Membranes ib. Mucous Membranes 396 xii CONTENTS. PACE Secretion — eont in ued. Secreting Glands 39 s Process of Secretion 4 01 Circumstances influencing Secretion .... 403 Mammary Glands and their Secretion . . . . 405 Chemical Composition of Milk 409 CHAPTER XII. The Skin and its Functions 410 Structure of the Skin . . . ib. Sudoriparous Glands 416 Sebaceous Glands 417 Structure of Hair 418 Structure of Nails 421 Functions of the Skin 422 CHAPTER XIII. The Kidneys and Urine 422 Structure of the Kidneys . 428 Structure of the Ureter and Urinary Bladder .... 436 The Urine 437 The Secretion of Urine 450 Micturition 460 CHAPTER XIY. The Vascular Glands Structure and Functions of the Spleen . ib. i) Thymus . . . . . . . 466 JJ ;> » Thyroid .... . 468 >» •• n Supra-renal capsules . . 469 >' s? »» Pituitary Body • 472 5J •■> Pineal Gland . . ib. Functions of the Vascular Glands in general . ib. CHAPTER XV. Causes and Phenomena of Motion 474 Ciliary Motion ib. Amoeboid Motion 475 CONTENTS. xiii iw.r. Causes and Phenomena of Motion — continued. Muscular Motion 47° Plain or Onstriped Muscle ib. Btriated Muscle 47 s Development of Muscle 4^3 I'hvsiology of Muscle at rest 4 4 „ ,, in activity 4^8 Rigor Mortis 5°4 Actions of the Voluntary Muscles 5°7 „ „ Involuntary Muscles 511 Sources of Muscular Action 5 12 Electrical Currents in Nerves 5 r 3 CHAPTER XVI. The Voice and Speech 518 Mode of Production of the Human Voice ib. The Larynx 520 Application of the Voice in Singing and Speaking . . .526 Speech 530 CHAPTER XVII. Nutrition : The Income and Expenditure of the Human Body 533 Nitrogenous Equilibrium and Formation of Fat . . . . 538 CHAPTER XVIII. The Nervous System 540 Elementary Structures of the Nervous System . . . . ib. Structure of Nerve-Fibres 541 Terminations of Nerve-Fibres 547 Structure of Nerve-Centres 550 Functions of Nerve Fibres 552 Classification of Nerve-Fibres 554 Laws of Conduction in Nerve-Fibres 555 Functions of Nerve-Centres 558 Laws of Reflex Actions 560 Secondary or Acquired Reflex Actions 562 xiv CONTENTS. PAGE Cerebrospinal Nervous System 564 The Spiual Cord and its Nerves 565 The White Matter of the Spinal Cord 567 The Grey Matter of the Spinal Cord 568 Nerves of the Spinal Cord 569 Functions of the Spinal Cord 573 The Medulla Oblongata 583 Its Structure ib. Distribution of the Fibres of the Medulla Oblongata . . . 584 Functions of the Medulla Oblongata 587 Structure and Physiology of the Pons Varolii. Crura Cerebri. Corpora Quadrigemina. Corpora Geniculata. Optic Thai.ami. and Corpora Striata 590 Pons Varolii ib. Crura Cerebri ... ........ ib. Corpora Quadrigemina 592 Corpora Striata and Optic Thalami 593 The Cerebellum 595 Functions of the Cerebellum 596 The Cerebrum 600 Convolutions of the Cerebrum 601 Structure of the Cerebrum 604 Chemical Composition of the Grey and White Matter . . . 606 Functions of the Cerebrum . 608 Effects of the Piemoval of the Cerebrum 609 Localisation of Functions 610 Experimental Localisation of Functions 613 Sleep 617 Physiology of the Cranial Nerves n>. Physiology of the Third Cranial Nerve 620 .. ,, Fourth Cranial Nerve 621 ., „ Fifth or Trigeminal Nerve 622 ,, „ Sixth Nerve 627 „ ,, Facial Nerve 628 ., .. Glosso- Pharyngeal Nerve 630 ,, ., Pneumogastric Nerve 631 ., ., Spinal Accessory Nerve 635 ., Hypoglossal Nerve ib. Physiology of the Spinal Nerves 636 Physiology of the Sympathetic Nerve ib. Functions of the Sympathetic Nervous System .... 640 I ONTENTS. xv CHAPTER XIX. PAGE The Senses . . 646 Commit. 9 ions ib. satians 647 The Sense of Touch 651 The Sense of Taste 65S The Tongue and its Papillae 659 The Sense of Smell 666 The Sense of Hearing 671 Anatomy of the Organ of Hearing 672 Phy _ f Hearing 6-9 Functions of the External Ear . . . . . . ib. Functions of the Middle Ear : the Tympanum. Ossicula. and Fenestras 6S0 Functions of the Labyrinth 6S - Sensibility of the Auditory Nerve The Sense of sight The Eyelids and Lachrymal Apparatus The Structure of the Eye-ball .... Optical Appara: - Accommodation of the Eye .... Defects in the Apparatus Spherical Aberration Chromatic Aberration The Blind Spot Visual Purple Col Sensations Eeciprocal Action of different parts of the Ketina Movements of the Eye Simultaneous Action of the two'Ev - . 691 692 ib. 699 "03 7cS 710 7ii 713 716 7-3 n 729 CHAPTER XX. Generation and Development .... -,«=; / j u Generative Organs of the Female ^ Enimpregnated Ovum -, q the Ovum - V, Menstruation »., Corpus Luteum *.- xvi CONTEXTS. PAGE Generation and Development — continued. Impregnation of the Ovum 75° Male Sexual Functions *&« Structure of the Testicle *& Spermatozoa 75 2 The Semen 75^ Development 757 Changes of the Ovum up to the Formation of the Blastoderm . ib. Segmentation of the Ovum 759 Fundamental Layer- of the Blastoderm : Epiblast ; Mesbblast ; Hypoblast 760 First Rudiments of the Embryo and its Chief Organs . . . 761 Fcetal Membrane- 767 The Umbilical Vesicle 769 The Amnion and Allantois ib. The Chorion 771 Changes of the Mucous Membrane of the Uterus and Formation of the Placenta 773 Development of Organs 778 Development of the Vertebral Column and Cranium . . . ib. .. Face and Visceral Arches 782 ., Extremities 784 .. .. Vascular System 785 Circulation of Blood in the Foetus 796 Development of the Nervous System 798 .. Organs of Sense 802 Alimentary Canal . . . . . . 806 Respiratory Apparatus ..... 810 Wolffian Bodies. Urinary Apparatus, and Sexual Organs 810 CHAPTER XXI. On the Relation of Life to other Forces .... 819 APPENDIX. The Chemical Basis of the Human Body 844 CONTENTS. xv ii ArPENDIX B: PAGE Anatomical Weights and Measures 86 1 Measures of Weight ib. n .. Length ib. Sizes of various Histological Elements and Tissues . . . . 862 Specific Gravity of various Fluids and Tissues .... 863 Table showing the per-centage composition of various Articles of Food ib. Classification of the Animal Kingdom 864 ib. French Measurements of Length, Capacity, and Weight rendered into English Equivalents xviii Table for converting Degrees of the Fahrenheit Thermo- meter Scale into Degrees Centigrade .... ib. INDEX S67 TO BINDER. The Coloured Plate to face p. 115. XY111 *Table for converting Degrees of tie FAHRENHEIT Ther- mometer Scale into Degrees CENTIGRADE. Fahrenheit. Cbktigbadb. 5oo D 260° 401 205 392 200 383 195 374 190 356 180 347 175 338 170 329 165 320 160 311 155 302 150 284 140 275 135 266 130 248 120 239 115 230 110 212 100 203 95 194 90 176 80 167 75 140 60 122 50 113 45 105 40-54 104 40 100 37-8 98-5 36-9 95 35 86 30 77 25 68 20 50 10 41 5 32 Zero 23 - 5 14 -10 + 5 -15 - 4 -20 -13 —25 -22 -30 -40 -40 -76 -60 1 degree Fahr. = •54° C. 18 '.. „ = 1°C. 3-6 .. .. = 2° C. 4-5 .. .. = 2-5° C. 5-4 •• -, = 3° C. * Modified from Fownes' Chemistry. MEASUREMENTS. FRENCH INTO ENGLISH. 1 metre 10 decimetres 100 centimetres .000 millimetres LENGTH. \ = 39*37 English inches (or 1 yard and 3^ in.) i decimetre 10 centimetres 100 millimetres = 3'937 inches (or nearly 4 inches) 1 centimetre 10 millimetres 1 milli metre = '3937 or about (nearly § inch.) = nearly i inch. CAPACITY. 1. 000 cubic decimetres ~| , . .. ,, J- = 1 cubic metre 1. 000.000 cubic centimetres J 1 cubic decimetre \ or ( = 1 litre ! • ,. u ( Cmi fluid oz.. 1.000 cubic centimetres J ormther less than an English quart) WEIGHT. 1 gramme 10 decigrammes 100 centigrammes 1. 000 millierammes = i5"432349 grs. (or nearly 15*) i decigramme 10 centigrammes 100 milligrammes = rather more than ii grain 1 centigramme 10 decigrammes - = rather more than 5 3 grain millier i^nme = rather more tnan sb g rain Measure of 1 decimetre, or 10 centimetres, or 100 millimetres. Cranium. 7 Cervical Vertebrae. Clavicle. Scapula. 12 Dorsal Vertebrae. Humerus. 5 Lumbar Vertebrae. Hiuni. Ulnar. Eadius. 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 Holdex). Highest point of Crest of the Ilium . Anterior Su- perior Spine of the Hium. Symphysis Pubis. DIAGRAM OF THORACIC AND ABDOMINAL REGIONS. A . Aortic Valve. M. Mitral Valve. P. Pulmonary Valve. T. Tricuspid Valve. HANDBOOK OF PHYSIOLOGY. CHAPTER I. TEE GENERAL AXD DISTINCTIVE CHARACTERS OF LIVING BEINGS. Hitman Physiology is the science which treats of the life of man — of the way in which he lives, and moves, and has his being. It teaches how man is begotten and born ; how he attains ma- turity ; and how he dies. Having, then, man as the object of its study, it is unnecessary to speak here of the laws of life in general, and the means by which they are carried out, further than is requisite for the more clear understanding of those of the life of man in particular. Yet it would be impossible to understand rightly the working of a complex machine without some knowledge of its motive power in the simplest form ; and it may be well to see first what are the so- called essential* of life — those, namely, which are manifested by all living beings alike, bv the lowest vegetable and the highest © © " v © © animal — before proceeding to the consideration of the structure and endowments of the organs and tissue belonging to man. The essentials of life are these, — Birth, Growth and Development, Decline and Death. The term birth, when employed in this general sense of one of the conditions essential to life, without reference to any particular kind of living being, may be taken to mean, separation from a parent, with a greater or less power of independent life. Taken thus, the term, although not defining any particular ge in development, serves well enough for the expression of the fact, to which no exception has yet been proved to exist, that the capacity for life in all living beings is obtained by inheritance. B 2 GROWTH. [chap. I. 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 conditions for obtaining fresh material, will grow in a fashion as definitely characteristic and as easily to be foretold as that of a living creature. It is, therefore, necessary to explain the distinctions which exist in this respect between living and lifeless structures ; for the manner of growth in the two cases is widely different. Differences between Living and Lifeless Growth. — (i.) The growth of a crystal, to use the same example as before, takes place merely by additions to its outside : the new matter is laid on particle by particle, and layer by layer, and, when once laid on, it remains unchanged. The growth is here said to be superficial. In a living structure, on the other hand, as, for example, a brain or a muscle, where growth occurs, it is by addi- tion of new matter, not to the surface only, but throughout even- part of the mass ; the growth is not superficial but interstitial, (2.) All living structures are subject to constant decay ; and life consists not, as once supposed, in the power of preventing this never-ceasing decay, but rather in making up for the loss atten- dant 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 individual. Almost every part is changed by degrees ; but the change is so gradual, and the re- newal of that which is lost so exact, that no difference may be noticed, except at long intervals of time. A lifeless structure, as a crystal, is subject to no such laws ; neither decay nor repair is a necessary condition of its existence. That which is true of structures which never had to do with life is true also with re- spect to those which, though they are formed by living parts, are not themselves alive. Thus, an oyster-shell is formed by the living animal which it encloses, but it is as lifeless as any other mass of inorganic matter ; and in accordance with this circumstance its growth takes place, not interstitially, but 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 ohap. I.] DEVELOPMENT. 3 alteration in the chemical constitution of the material which is taken up and added to the previously existing mass. For example, when a crystal of common salt grows on being placed in a fluid which contains the same material, the properties of the salt are not changed by being taken out of the liquid by the crystal and added to its surface in a solid form. But the case is essentially different in living beings, both animal and vegetable. A plant, like a crystal, can only grow when fresh material is presented to it ; and this is absorbed by its leaves and roots ; and animals, for the same purpose of getting new matter for growth and nutrition, take food into their stomachs. But in both these cases the materials are much altered before they are finally assimilated by the structures they are destined to nourish. (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 without limit as to growth as without limit to life. Development is as constant an accompaniment of life as growth. The term is used to indicate that change to which, before maturity, all living parts are constantly subject, and by which they are made more and more capable of performing their several functions. For example, a full-grown man is not merely a magnified child ; his tissues and organs have not only grown, or increased in size, they have also developed, or become better in quality. No very accurate limit can be drawn between the end of de- velopment and the beginning of decline ; and the two processes may be often seen together in the same individual. But after a time all parts alike share in the tendency to degeneration, and this is at length succeeded by death. Differences between Plants and Animals. — It has been already said that the essential features of life are the same in all living things ; in other words, in the members of both the animal and vegetable kingdoms. It may be well to notice briefly the distinctions which exist between the members of these two king- doms. It may seem, indeed, a strange notion that it is possible to confound vegetables with animals, but it is true with respect to the lowest of them, in which but little is manifested beyond the essentials of life, which are the same in both. b 2 4 ANIMALS CONTRASTED [chap. i. (i.) Perhaps the most essential distinction is the presence or absence of power to live upon inorganic material. By means of their green colouring matter, chlorophyl — a substance almost exclusively confined to the vegetable kingdom, plants are capable of decomposing the carbonic acid, ammonia, and water, which they absorb by their leaves and roots, and thus utilizing them as food. The result of this chemical action, which occurs only under the influence of light, is, so far as the carbonic acid is concerned, the fixation of carbon in the plant structures and the exhalation of oxygen. Animals are incapable of thus using inorganic matter, and never exhale oxygen as a product of decomposition. The power of living upon organic as well as inorganic matter is less decisive of an animal nature ; inasmuch as fungi and some other plants derive their nourishment in part from the former source. (2.) There is, commonly, a marked difference in general chemical composition between vegetables and animals, even in their lowest forms ; for while the former consist mainly of cellulose, a substance closely allied to starch and containing carbon, hydrogen, and oxygen only, the latter are composed in great part of the three elements just named, together with a fourth, nitrogen ; the chief proximate principles formed from these being identical, or nearly so, with albumen. It must not be supposed, however, that either of these typical compounds alone, with its allies, is confined to one kingdom of nature. Nitrogenous compounds are freely produced in vegetable structures, although they form a very much smaller proportion of the whole organism than cellulose or starch. And while the presence of the latter in animals is much more rare than is that of the former in vegetables, there are many animals in which traces of it may be discovered, and some, the Ascidians, in which it is found in considerable quantity. (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 motion is now known, however, to exist in so many vegetable forms, that it can no longer be held as an essential distinction between them and animals, and ceases to be a mark by which the one can be distinguished from the other. Thus the zoospores of many of the Cryptogamia exhibit ciliary or amoeboid chap, ii.] WITH VEGETABLES. 5 movements (p. 9) of a like kind to those seen in animalcules* \ and even among the higher orders of plants, many, e.g., Dioncea Mttscipula (Venus'a fly-trap), and Mimosa Sensitwa (Sensitive plant), exhibit such motion, either at regular times, or on the application of external irritation, as might lead one were this tact taken by itself, to regard them as sentient beings. InhnvnT 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. It is necessary to compare, side by side, the lowest members of the two kingdoms, in order to understand rightly how faint are the boundaries between them. CHAPTEE II. STRUCTURAL BASIS OF THE HITMAN BODY. By dissection, the human body can be proved to consist of various dissimilar parts, bones, muscles, brain, heart, lungs, in- testines, &c., while, on more minute examination, these are found to be composed of different tissues, such as the connective, epithe- lial, nervous, muscular, and the like. Cells. — Embryology teaches us that all this complex organisa- tion has been developed from a microscopic body about y^ in. in diameter (ovum), which consists of a spherical mass of jelly-like matter enclosing a smaller spherical body (germinal vesicle). 6 STRUCTURAL BASIS OF THE HUMAN BODY. [chap. ii. Further, each individual tissue can be shown largely to consist of bodies essentially similar to an ovum, though often differing from it very widely in external form. They are termed cells : and it must be at once evident that a correct knowledge of the nature and activities of the cell forms the very foundation of physiology. Cells are, in fact, physiological no less than histological units. The prime importance of the cell as an element of structure was first established by the researches of Schleiden, and his con- clusions, drawn from the study of vegetable histology, were at once extended by Schwann to the animal kingdom. The earlier observers defined a cell as a more or less spherical body limited by a membrane, and containing a smaller body termed a nucleus, which in its turn encloses one or more nucleoli. Such a definition 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 re- garded as quite a secondary matter, while at the same time the cell-substance came gradually to be recognised as of primary im- portance. Many of the lower forms of animal life, e.g., the Rhizopoda, were found to consist almost entirely of matter very similar in appearance and chemical composition to the cell-sub- stance of higher forms : and this from its chemical resemblance to flesh was termed Sarcode by Dujardin. When recognised in vegetable cells it was called Protojilasm 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 Blastema, 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 present day is Protoplasm, and inasmuch as all life, both in the animal and vegetable kingdoms, is associated with protoplasm, we are justified in describing it, with Huxley, as the "physical basis of life." A cell may now T be defined as a nucleated mass of protoplasm,* of microscopic size, which possesses sufficient individuality to have a life-history of its own. Each cell goes through the same cycle * In the human body the cells range from the red blood-cell (g^in.) to the ganglion-cell (3^ in.). ohap. ii.] PROTOPLASM. 7 of changes as the whole organism, though doubtless in a much shorter time. Beginning with its origin from Borne pre-existing cell, it grows, produces other colls, 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 account of the vital activities of protoplasm. Protoplasm. — Physical characters. — Physically, protoplasm is viscid, varying in consistency from semi-fluid to strongly coherent. Chemical characters. — Chemically, living protoplasm is an ex- tremely unstable albuminoid substance, insoluble in water. It is neutral or weakly alkaline in reaction. It undergoes heat stiffening or coagulation at about 130°^. (54'5°C.), and hence no organism can live when its own temperature is raised beyond this point, though, of course, many can exist for a time in a much hotter atmosphere, since they possess the means of regu- lating their own temperature. Besides the coagulation produced by heat, protoplasm is coagulated by all the reagents which produce this change in albumen. If not-living protoplasm be subjected to chemical analysis it is found to be made up of nume- rous bodies* besides albumen, e.g., of glycogen, lecithin, salts and. water, so that if living protoplasm be, as some believe, an inde- pendent chemical body, when it no longer possesses life, it under- goes a disintegration which is accompanied by the appearance of these new chemical substances. When it is examined under the microscope two varieties of protoplasm are recognised — the hyaline, and the granular. Both are alike transparent, but the former is perfectly homogeneous, while the latter (the more common variety) contains small granules or molecules of various sizes and shapes. Globules of watery fluid are also sometimes found in protoplasm ; they look like clear spaces in it, and are hence called vacuoles. Vital or Physiological characters. — These may be conveniently treated under the three heads of — I. Motion ; II. Nutrition ; and III. Reproduction. I. Motion. — It is probable that the protoplasm of all cells is capable at some time of exhibiting movement; at any rate this phenomenon, which not long ago was regarded as quite a curiosity, * For an account of which, reference should be made to the Appendix. STRUCTURAL BASIS OF THE HUMAN BODY. [chap. ii. has been recently observed in cells of many different kinds. It may be readily studied in the Amoebic, in the colourless blood- cells of all vertebrata, in the branched cornea-cells of the frog, in the hairs of the stinging-nettle and Tradescantia, and the cells of Yallisneria and Chara. These motions may be divided into two classes — (a) Fluent and (b) Ciliary. Another variety — the molecular or vibratory — has also been classed by some observers as vital, but it seems exceedingly probable that it is nothing more than the well-known " Brownian " molecular movement, a purely mechanical phenomenon which may be observed in any minute particles e.g., of gamboge, suspended in a fluid of suitable density, such as water. Such particles are seen to oscillate rapidly to and fro, and not to progress in any definite direction. (a.) Fluent. — This movement of protoplasm is rendered percep- tible (i) by the motion of the granules, which are nearly always imbedded in it, and (2) by changes in the outline of its mass. If part of a hair of Tradescantia (fig. 1 ) 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 flowing steadily in definite directions, some coursing round the film which lines the interior of the cell-wall, and others flowing towards or away from the irregular mass 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 variations of form are produced. In the Amoeba, a minute animal consisting of a shapeless and structureless mass of sarcode, an irregular mass of protoplasm is gradually thrust out from the main body and retracted : a second mass is then protruded in another direction, and gradually the whole protoplasmic substance is, as it were, drawn into it. The Amoeba thus comes to occupy a new position, and when this is Fig. 1. — Cell of Tradescantia drawn at successive intervals of two minutes. The cell-contents consist of a central mass connected by many irregular processes to a peripheral film : the whole forms a vacuolated mass of protoplasm, which is continually changing its shape. (Schofield.) obap.ii.] PEOTOPLASMIC KOTION. 9 repeated several times we have locomotion in a definite direction, together with a continual change <»t' form. These movements when observed in other cells, such as the colourless blood-cor- puscles of higher animals (fig. 2) are hence termed amoeboid. Colourless blood-corpuscles were first observed to migrate, /.<".. past through the walls of the blood-vessels (p. 198). by Waller, whose observations were confirmed and extended to connective tissue corpuscles by the re- searches 1 if Recklinghausen, ( !ohnheim, and others, and thus the phenomenon of migration has been proved to play an important part in many normal, and pathological processes, especially in thai of inflammation. This amoeboid movement enables many of the lower animals to capture their prey, which they accomplish by simply flowing round and enclosing it. The remarkable motions of pigment-granules observed in the branched pigment-cells of the frog's skin by Lister are probably Fig. 2. — Human colourless blood-eorpiiscle, showing- its successive changes of outline within ten minutes when kept moist on a warm stage. (Schoneld.) 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.f/., light and electricity) they collect in the central mass, leaving the branches quite colourless. (b.) Ciliary action must be regarded as only a special variety of the general motion with which all protoplasm is endowed. The grounds for this view are the following : In the case of the Infusoria, which move by the vibration of cilia (microscopic 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 processes. The movements of protoplasm can be very largely modified or even suspended by external conditions, of which the following are the most important. 10 STRUCTURAL BASIS OF THE HUMAN BODY. [chap rr. i. Changes of temperature. — Moderate heat aets as a stimulant : this is readily observed in the activity of the movements of a human colourless blood-corpuscle when placed under conditions in which its normal temperature 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 colourless blood-cor- puscle is stimulated 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 move- ment, and by imbibition causes great swelling and finally bursting of the cells. In some cases, however, (myxomycetes) protoplasm can be almost entirely dried up, and is yet capable of renewing its motions when again moistened. Dilute salt-solution and many dilute acids and alkalies, 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 to become motionless. II. Nutrition. — The nutrition of cells will be more appro- priately described in the chapters on Secretion and Nutrition. Before describing the Reproduction of cells it will be necessary to consider their structure more at length. Minute Structure of Cells. — (a.) Cell-ivall. — We have seen (p. 6) that the presence of a limiting-membrane is no essential part of the definition of a cell. In nearly all cells the outer layer of the protoplasm attains a firmer consistency than the deeper portions : the individuality of the cell becoming more and more clearly marked as this cortical layer becomes more and more differentiated from the deeper portions of cell-substance. Side by side with this physical, there is a gradual chemical differentiation, till at length, as in the case of the fat-cells, we have a definite limiting membrane differing chemically as well as physically from the cell-contents, and re- CHAP. II.] MINUTE STRUCTURE OF CELLS. U maining as a Bhrivelled-up bladder when they have beeD removed. Such a membrane is transparent and structureless, flexible, and permeable to fluids. The cell-substance can, therefore, still be nourished by imbibi- tion through the cell-wall. In many cases (especially in fat) a membrane of some toughness is absolutely necessary to give to the tissue the requisite consistency. When these membranes attain a certain degree of thickness and independence they are termed capsules : as examples, we may cite the capsules of cartilage-cells, and the thick, tough envelope of the ovum termed the " primitive chorion." (b.) Cell content*. — In accordance with their respective ages, positions, and functions, the contents of cells are very varied. The original protoplasmic substance may undergo many trans- formations; thus, in fat cells we may have oil, or fatty crystals, occupying nearly the whole cell-cavity : in pigment cells we find granules of pigment ; in the various gland cells the elements of their secretions. Moreover, the original protoplasmic contents of the cell may undergo a gradual chemical change with advancing _■■ ; 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 replaced by the haemoglobin of the mature coloured blood-corpuscle. The minute structure of cells has lately been made the subject of careful investigation, and what was once regarded as homo- geneous protoplasm with a few scattered granules, has been stated to be an exceedingly complex structure. In colourless blood- corpuscles, epithelial cells, connective tissue corpuscles, nerve- cells, and many other varieties of cells, an intracellular network of very fine fibrils, the meshes of which are occupied by a hyaline interstitial substance, has been demonstrated (Heitzmann's net- work) (fig. 3). At the nodes, where the fibrils cross, are little Bwellings, and these are the objects described as granules by the older observers : but in some cells, e.f many ells ha shown to contain a \-nuclear network in every oilar to th I ribed ■bore as intra-eellnlar (fig. 3), the tut of which are occu- pied by Bemi-fluid protopla>m. III. Reproduction. — The life of individual cells is probably very short in comparison with that of the organism they com] and their constant decay and death necessitate constant repro- duction. The mode in which this takes place has 1' _ d the subject of great controversy. In the case of plants, all of whoe tisE a ire either cellular or composed of cells which are modified or have 1 in various . the theory that all new cells are derived from pre-existimr was early advanced and very generally accepted. But in the 3 of animal tise - Schwann and others maintained a theory of spontaneous or free cell formation. According- to this view a minute corpuscle (the future nucle- olus) springs np spontaneously in a structureless substance (blastema) very much as a crystal is formed in a solution. This nucleolus attracts the suiTOundin- molecules of matter to form the nucleus, and by a repetition of the process the substance and wall are produced. This theory, once almost universally current, was first disputed and finally overthrown by Remak and Virchow, whose researches established the truth expressed in the words -• Omnis cellula e cellula/' It will be seen that this view is in strict accordance with the truth established much earlier in Vegetable Histology that every cell is descended from some pre-existimr (mother-) cell. This derivation of cells from cells takes place by (1) gemmation^ or ■a or division. (1.) Gemmation. — This method has not been observed in the human body or the higher animals, and therefore requires but a og notice. It consists ssentially in the budding off and .rating of a portion of the parent cell. (2.) Fission or Din ision. — As examples of reproduction by fission, we may select the ovum, the blood cell, and cartilage cells. H STRUCTUllAL BASIS OF THE HUMAN BODY. [chap. 11. In the frog's ovum (in which the process can he most readily observed) after fertilization lias taken place, there is first some amoeboid movement, the oscillation gradually increasing until a permanent dimple a])] tears, 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, b). This process Fig. 4. — Diagram of an ovum (a) undergoing segmentation. In [b) it has divided into • two ; in (c) into f our ; and in (d) the process has ended in the production of the so- called " mulberry mass." (Frey.) being repeated by the formation of a second furrow at right angles to the first, we have four cells produced (c) : this 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 developed. Segmentation is the first step in the development of most animals, and doubtless takes place in man. Multiplication by fission has been observed in the colourless blood-cells of many animals. In some cases (fig. 5), the process ® ® 6 @ Fig. 5. — Blood-corpuscle from a young deer embryo, multiplying by fission. (Frey.) has been 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. Ml IP. II. J CELL DIVISION. 15 In cartilage (fig. 6), a prod mtially similar occurs, with the exception that (as in the ovum) the cells produced by fission remain in the original capsule, and in their turn undergo division, Fig. 6. — Diagram of a cartilagi cell undergoing fission within its capsule. The process of division is represented as commencing in the nucleolus, extending to the nucleus, and at length involving the body of the cell. (Frey.) s<» 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 to 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 divi- sion of cells does not take place by the simple constriction of their nuclei and surrounding protoplasm into two parts as above described (direct division), but is preceded by complicated changes in their nuclei (karyokinesis). These changes consist in a gradual re- arrangement of the intranuclear network of each nucleus, until tw< > 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 meshwork of fibrils, which stain deeply in carmine, imbedded 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 separated than they were and stain better. This is the stage of con- 1 6 STRUCTURAL BASIS OF THE HUMAN BODY. [chap. h. volution (fig. 7, b, c). The next step in the process is the arrange- ment of the fibrils into some definite figure by an alternate looping in and out around a central space, by which means the rosette or wreath stage (fig. 7, d) is reached. The loops of the rosette next Fig. 7. — Koryokinesis. a, ordinary nucleus of a columnar epithelial cell ; b, c, the same nucleus in the stage of convolution ; d, the wreath or rosette form ; e, the aster or single star ; f, a nuclear spindle from the Descemet's endothelium of the frog's cornea ; a, h, 1, diaster; k, two daughter nuclei. (Klein.) become divided at the periphery, 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 Y-shaped from the centre, forming a star (aster) or wheel (fig. 7, e), or perhaps from two centres, in which case a double star (diaster) results (fig. 7, G, h, and 1). After remaining almost unchanged for some time, the V-shaped fibres being first re-arranged 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 nucleus (mother nucleus) has gone, but in a reverse order, viz., the star, the rosette, and the convolution. During or shortly after the formation of the daughter nuclei the cell itself becomes constricted, and then divides in a line about midway between them. Functions of Cells. — The functions of cells are almost infinitely varied and make up nearly the whole of Physiology. They will chap, ii.] DECAY AND DEATH OP CEL1 17 l>e more appropriately considered in the chapters treating of the nis and systems of organs which the cells compot Decay and Death of Cells. There aretwo chief ways in which the comparatively brief existence of cells is brought to an end. Mechanical abrasion, (2) Chemical transformation. 1. The various epithelia furnish abundant examples of mecha- nical abrasion. As it approaches the free surface the cell be© more and more flattened and scaly in form and more horny in consistence, rill at length it is simply rubbed off. Hence we find epithelial 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 producing oil-globules in the secretion of milk, fatty degeneration of the muscular fibres of the uterus after the birth of the foetus, and of the cells of the < rraanan follicle giving rise to the ; ' corpus luteuni." (See chapter On ( feneration.) ( 1». ) Pigmentary degeneration from deposit of pigment, as in the epithelium of the air-vesicles of the lungs. (c.) Calcareous degeneration which is common in the cells of many cartilages. Having thus reviewed the life-history of cells in general, we may now discuss the leading varieties of form which they present. In passing, it may be well to point out the main distinctions It-ticeen (ini'iuii ninl 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 observed, is non-nitrogenous, and thus differs chemically as well as structurally from the contained ma-. Moreover, in vegetable cells (fig. 8, b). the protoplastic contents of the cell fall into two subdivisions : (1) a continuous film which lines the interior of the cellulose wall ; and (2) a reticulate mass containing the nucleus and occupying the cell-cavity ; \t< interstices are filled with nuid. In young vegetable cells such a distinction does not exist ; a finely granular proto- plasm occupies the whole cell-cavity (fig. 8, A). Another striking difference is the frequent presence of a large quantity of intercellular substance in animal tissues, while in vegetables it is com paratively rare, the requisite consistency being given to their tissues by the c !8 STRUCTURAL BASIS OF THE HUMAN BODY. [chap. ir. tough cellulose walls, often thickened by deposits of lignin. In animal cells this end is attained by the deposition of lime-salts in a matrix of inter- cellular substance, as in the process of ossification. Fig. 8. — (a). Young vegetable cells, showing cell-eavity entirely filled with granular proto- plasm enclosing a large oval nucleus, with one or more nucleoli. (b.) Older cells from same plant, showing distinct cellulose-wall and vacuolation of protoplasm. Forms of Cells. — Starting with the spherical or spheroidal (fig. 9, 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. 9, b). Of this, the primitive segmentation-cells may afford an example. Fig. 9. — Various forms of cells, a. Spheroidal, showing nucleus and nucleolus, b. Poly- hedral, c. Discoidal (blood-cells), d. Scaly or squamous (epithelial cells,. The discoid shape is seen in blood-cells (fig. 9, c), and the scale- like form in superficial epithelial cells (fig. 9, d). Some cells have a jagged outline (prickle-cells) (fig. 13). Cylindrical, conical, or prismatic cells occur in the deeper layers of laminated epithelium, and the simple cylindrical epithelium of the intestine and many gland ducts. Such cells may taper off at one or both ends into fine processes, in the former case being- caudate, in the latter fusiform (fig. 10). They may be greatly elongated so as to become fibres. Ciliated cells (fig. 10, d) must be noticed as a distinct variety : they possess, but only on their free surfaces, hair-like processes (cilia). These vary immensely in CHAP. II. | CLASSIFICATION OF CELLS. 19 si/.c, 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 Fig. 10.— Various forms of cetts. a. Cylindrical or columnar. l>. Caudate, c. Fusiform. d. Ciliated (from trachea), e. Branched, stellate. examples (fig. 10, e). In these cells the primitive branches by secondary branching may give rise to an intricate network of processes. Classification of Cells. — Cells may be classified in many ways. According to : — (a.) Form : They may be classified into spheroidal or polyhedral, discoidal, flat or scaly, cylindrical, caudate, fusiform, ciliated and stellate. (b.) Situation : — we may divide them into blood cells, gland cells, connective tissue cells, &c. (c.) Contents : — fat and pigment cells and the like. (d.) Function : — secreting, protective, contractile, &c. (e.) Origin: — hypoblastic, mesoblastic, and epiblastic cells. (See chapter on Generation.) It remains only to consider the various ways in which cells are connected together to form tissues, and the transforma- tions by which intercellular substance, fibres and tubules are produced. Modes of connection. — Cells are connected : — (1) By a cementing intercellular substance. This is probably always present as a transparent, colourless, viscid, albuminous c 2 20 STRUCTURAL BASIS OF THE HUMAN BODY. [chap. ii. substance, even between the closely apposed cells of cylindrical 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 substance. 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 the branched cells lie : through these branching spaces nutritive fluids can find their way into the very remotest parts of a non-vascular tissue. As a special variety of intercellular substance must be mentioned the basement membrane (membrana propria) 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 which may persist as a hyaline saccule after the gland-cells have all been discharged. (2) By anastomosis of their processes. This is the usual way in which stellate cells, e.g. of the cornea, are united : the individuality of each cell is thus to a great extent lost by its connection with its neighbours to form a reticulum : as an example of a network so produced, we may cite the stroma of lymphatic glands. Sometimes the branched processes breaking up into a maze of minute fibrils, adjoining cells are connected by an inter- mediate reticulum : this is the case in the nerve-cells of the spinal cord. Besides the Cell, which may be termed the primary tissue- element, there are materials which may be termed secondary or derived tissue-elements. Such are Intercellular substance, Fibres and Tubules. Intercellular substance is probably in all cases directly derived from the cells themselves. In some cases (e.g. cartilage), by the use of re-agents the cementing intercellular substance is, as it were, analysed into various masses, each arranged in concentric ayers around a cell or group of cells, from which it was probal try derived (fig. 6). Fibres. — In the case of the crystalline lens, and of muscle both . n.J TTJBUU 21 and noi imply a cell : in * by a multifile; - the nuclei. The various fibres and fibrflls anective gradual transfonuatioii of an originally homogeneous inter- cellular subsl Fil pea thus formed may undergo g chemical ;t^ well as physical transformation : this is notably the . It i i yellow el. - ie, in which the sharply defined el 3se8s _ _ vat power of resistance t" _ strik: _ rith the homog tter from which they are derive" 1. f which wen _ ally supposed i ructure- membrane, have now been proved to be composed of flat, thin Iges, 3 I lapillariea With these simple materials the various parts of the body are built up; the more elementary tissues bein_. - * speak, first compounded of them : while these again are variously mixed and interwoven to form ni'-re intricate combinations. Thus are constructed epithelium and its modifications, connec- tive tissue, fat. cartihiL'e, bone, the fibres of muscle and nerve. Arc. : and these, again, with the more simple structures before men- tioned, are usee - materials wherewith to fomi arteries, veins, and lympha: 3, e reting md vascular glands, lungs, heart, liver, and other pans of the body. CHAPTEB III. STRUCTURE OF THE ELEMENTARY TISSUES. In this chapter the leading characters and chief modifica: of two great groups of tissues — the Epithelial and Connective — •will be briefly described; while the Xerv.»us and Muscular, together with several other m _ - ialized . will be appropriately considered in the chapters treating of their physioL _ 22 STRUCTURE OF ELEMENTARY TISSUES. [chap. hi. Epithelium. Epithelium is composed of cells of various shapes held together by a small quantity of cementing intercellular substance. Epithelium clothes the whole exterior surface of the bod}', forming the epidermis with its appendages — nails and hairs; becoming continuous at the chief orifices of the body — nose, mouth, anus, and urethra — with the epithelium which lines the whole length of the alimentary and genito-urinary tracts, together with the ducts of their various glands. Epithelium also lines the cavities of the brain, and the central canal of the spinal cord, the serous and synovial membranes, and the interior of all blood- vessels and lymphatics. The cells composing it may be arranged in either one or more layers, and thus it may be sub-divided into (a) Simple and (b) Stra- tified or laminated Epithelium. A simple epithelium, for example, lines the whole intestinal mucous membrane from the stomach to the anus : the epidermis on the other hand is laminated throughout its entire extent. Epithelial cells possess an intracellular and an intranuclear net- work (p. n). They are held together by a clear, albuminous, cement substance. The viscid semi-fluid consistency both of cells and intercellular substance permits such changes of shape and arrangement in the individual cells as are necessary if the epithe- lium is to maintain its integrity in organs the area of whose free surface is so constantly changing, as the stomach, lungs, &c. 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 mem- brane 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. Classification of Epithelial Cells. Epithelial cells may be conveniently classified as : i. Squamo2cs, scaly, pavement, or tesselated. 2. Spheroidal, glandular, or polyhedral. 3. Columnar, cylindrical, conical, or goblet-shaped. 4. Ciliated. 5. Transitional. CHAP. III.] EPITHELIAL CELL- 23 Although, for convenience, epithelial cells are thus classified, yet tin- first three forms of cells arc sometimes met with at different depths in the same membrane. Ajs an example of such Fig. 11. — V m of Babbit? s cornea, a. Anterior epithelium, showing the different shapes of the cells at various depths from the free surface, b. Portion of the substance of cornea. (Klein.) a laminated epithelium showing these different cell-forms at various depths, we may select the anterior epithelium of the cornea (fig. 11). 1. Squamous .Epithelium (fig. 12). — Arranged (a) in several superposed layers (stratified or laminated), this form of epithelium covers (a) the skin, where it is called the Epidermis, and lines (b) tin 1 mouth, pharynx, and oesopha- gus, (c) the conjunctiva, (d) the vagina, and entrance of the urethra in both sexes ; while, as (b) a single layer, the same kind of epithelium forms (a) the pig- mentarv layer of the retina, and lines (b) the interior of the serous and synovial sacs, and (c) of the heart, blood- and lymph-vessels (Endothelium). It consists of cells, which are flattened and scaly, with an irregular outline : and, when laminated, may form a dense horny investment, as on parts of the palms of the hands and soles of the feet. The nucleus is often not apparent. The really cellular nature of even the dry and shrivelled scales cast off from the surface of the epidermis, can be proved by the application of caustic potash, which causes them rapidly to swell and assume their original form. Fig. 12. — Squamous epithelium scales from the inside of the mouth. X 260. (Henle.) 24 STRUCTURE OF ELEMENTARY TISSUES. [chap. hi. Squamous cells are generally united by an intercellular sub- stance ; but in many of the deeper layers of epithelium in the mouth and skin, the outline of the cells is very irregular. Such cells (fig. 13) are termed "ridge and furrow," " cogged" or " prickle" cells. These " prickles" are prolongations of the intra- cellular network which run across from cell to cell, thus joining them together, the interstices being filled by the transparent inter- cellular cement substance. "When this increases in quantity in in- flammation, the cells are unshed further apart and the connecting fibrils or "prickles" elongated, and therefore more clearly visi- Fig. 13. — Jagged cells of the middle layers of pavement epithelium, from a vertical section of the gum of a newborn infant. (Klein.) ble. H Squamous epithelium, e.g. the pigment cells of the retina, may have a deposit of pigment in the cell-substance. This pig- ment consists of minute mole- cules of melanin, 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 colourless pavement epithe- lial cells. Endothelium — The squamous epithelium lining the serous membranes, and the interior of blood-vessels, presents so many special features as to demand a special description ; it is called by a distinct name — Endothelium. The main points of distinction above alluded to are, 1. the very flattened form of these cells ; 2. their constant occurrence in only a single layer; 3. the fact that they are developed from the " mesoblast," while all other epithelial cells are derived from the . 14. — Pigment cells from Oc retina. A, cells still cohering, seen on their sur- face ; a, nucleus indistinctly seen. In the other cells the nucleus is concealed by the pigment granules. B, twp cells seen in pronle ; a, the outer or posterior part containing scarcely any pigment, x 370. (Henle.) b> are gradually appearing on the free surface of the epithelium, and are finally detached : these consist of the cell-contents which ar charged by the open mouth of the goblet, leaving the nucleus surrounded by the remains of the protoplasm in its narrow stem. _ L this transformation as a normal process which is continually going on during life, the discharged cell-contents con- tributing to form mucus, the cells being supposed in many - - - recover their original shape. The columnar epithelial cells of the alimentary canal pose structureless layer on their free surface : such a layer, appearing striated when viewed in section, is termed the ; * striated basilar border " (tig. 20, a. 4. ( led cells are generally cylindrical (fig. 21. b), but may r even almost squamous in shape (liu'. 21. This form of epithelium lines (a.) the whole of the respiratory tract from the larynx to the finest sub-divisions of the bronchi, -lie lower parts of the nasal pa— ges, and some portions of the generative apparatus — in the male (b.) lining tl efferentia " of the testicle, and their prol stations at - the lower end of the epididymis : in the feinak .nmencinu- about the middle of the neck of the uterus, and extending 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 spina] cord are clothed with ciliated epithelium in the child, but in the adult it is limited to the central canal of the cord. 30 STRUCTURE OF ELEMENTARY TISSUES. [chap. hi. The Cilia, or fine hair-like processes which give the name to this variety of epithelium, vary a good deal in size in different Fig. 21.— A. Spheroidal ciliated cells from the mouth of the frog. X 300 diameters. (Sharpey.) B. a. Ciliated columnar epithelium lining a bronchus, h. Branched connective-tissue corpuscles. (Klein and Noble Smith.) classes of animals, 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., the gill of a mussel, is ex- amined 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 swiftly 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. 5. Transitional Epithelium. — This temi 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 lami- nated ; in other words, the term 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. 21, b). The epithelium which is met with lining the urinary bladder and ureters is, however, the transitional par OB LP. in. | TRANSITIONAL EPITHELIUM. 31 excellence. In this variety there are two or three layers of cells, the upper being more or less flattened according to the full or Fig. 22.— E 'pith Hum of th> bladder; a, one of the cells of the first row; l>, a cell of the second row ; c, cells in situ, of first, second, and deepest layers. (Obersteiner.) collapsed condition of the organ, their under 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 narrower Fig. 23.— Transitional epithelial cells from a scraping of the mucous membrane of the bladder of the rabbit. (V. D. Harris.) 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. 22). It can be easily understood, therefore, that if a scraping of the mucous membrane of the bladder be teazed, and examined under the microscope, cells of a great variety of forms may be made out (fig. 23). Each cell contains a large nucleus, and the larger and superficial cells often possess two. Special Epithelium in Organs of Special Sense. — In addition to the above kinds of epithelium, certain highly specialized forms of epithelial cells are found in. the organs of smell, sight, and hearing, viz., olfactory cells, retinal rods and cones, auditory cells ; they will be described in the chapters which deal with their functions. 32 STRUCTURE OF ELEMENTARY TISSUES. [chap. hi. Functions of Epithelium According to function, epithelial cells may be classified as : — (i.) Protective, e.g., in the skin, mouth, blood-vessels, &c. (2.) Protective and moving — ciliated epithelium. (3.) Secreting — glandular epithelium ; or, Secreting formed ele- ments — epithelium 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 exposed to pressure, and developing various appendages, such as hairs and nails, whose structure and functions will be considered in a future chapter. Epithelium lines also the sensorial surfaces of the eye, ear, nose, and mouth, and thus serves as the medium through which all impressions from the external world — touch, smell, taste, sight, hearing — reach the delicate nerve-endings, whence they are conveyed to the brain. The ciliated epithelium which lines the air-passages serves not only as a protective investment, but also by the movements of its cilia is enabled to propel fluids and minute particles of solid matter so as to aid their expulsion from the body. In the case of the Fallopian tube, this agency assists the progress of the ovum towards the cavity of the uterus. Of the purposes served by cilia in the ventricles of the brain, nothing is known. (For an account of the nature and conditions of ciliary motion, see chapter on Motion.) The epithelium of the various glands, and of the whole intes- tinal tract, has the power of secretion, i.e., of chemically trans- forming certain materials of the blood ; in the case of mucus and saliva this has been proved to involve the transformation of the epithelial cells themselves ; the cell-substance of the epithelial cells of the intestine being discharged by the rupture of their envelopes, as mucus. Epithelium is likewise concerned in the processes of transuda- tion, diffusion, and absorption. It is constantly being shed at the free surface, and reproduced ohap. in.] THE CONNECTIVE TISSUES. 33 in the deeper Livers. The various stages of its growth and de- velopment can be well soon in a BOCtion of any laminated epithe- lium, such as the epidermis. The Connective Tissues. This group of tissues forms the Skeleton with its various con- nections — bones, cartilages, and ligaments — and also afford- a supporting framework and investment to various organs composed of nervous, muscular, and glandular tissue. Its chief function is the mechanical one of support, and for this purpose it is so inti- mately 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 connective tissues may be thus classified : — I. The Fibrous Connective Tissues. A. — Chief Forms. a. Areolar. b. White fibrous. c. Elastic. B. — Special Varieties. a. Gelatinous. I. Adenoid or Retiform. c. Neuroglia. d. Adipose. II. Cartilage. III. Bone. All of the varieties of connective tissue are made up of two parts, namely, cells and intercellular suhstance. Cells. — The cells are of two kinds. (a.) Fixed. — These are cells of a flattened shape, with branched processes, which are often united together to form a network : D 34 STRUCTURE OF ELEMENTARY TISSUES. [chap. in. 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 system of branching canals freely communicating (fig. 24). Fig. 24. — HorizontaZ'preparation of cornea of frog, stained in gold chloride; sho\ring the network of branched cornea corpuscles. The ground-substance is completely colour- less, x 400. (Klein.) To this class of cells belong the flattened tendon corpuscles which are arranged in long lines or rows parallel to the fibres (fig. 29). These branched cells, in certain situations, contain a number of pigment-granules, giving them a dark appearance : they form one variety of pigment-cells. Branched pigment-cells of this kind are found in the outer layers of the choroid (fig. 25). In many lower animals, such as the frog, they are found widely distributed, not only in the skin, but also in many internal parts, e.g., the mesentery and sheaths of blood-vessels. In the web of the frog's foot such pigment -cells may be seen, with pig- ment evenly distributed through 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 all over the OHAP. ill.] CONNECTIVE TISSUE. 35 processes. Thus the skin in the frog is sometimes uniformly dusky, and sometimes quite light-coloured, with isolated dark spots. In the choroid and retina the pigment-cells absorb light. (o.) Amwhoid cells, of an approximately spherical shape: they have a great general resemblance to colourless blood corpuscles (fig. 2), with which some of them arc probably identical. They consist of finely granular nucleated protoplasm, and have the property, not only of changing their form, but also of moving about, whence they are termed mi- gratory. They are readily dis- tinguished from the branched connective-tissue corpuscles by their free condition, and the absence of processes. Some are much larger than others, and are found especially in the Fig. 25. — Ramified pigment - ceUe, from "the tissue of the choroid coat of the eye. x 350. a, cell with pigment ; b, colourless fusiform cells. (Kolliker.) sublingual gland of the dog and guinea pig and in the mucous membrane of the in- testine. A second variety of these cells called plasma cells (Waldeyer) are larger than the amoeboid cells, apparently granular, less active in their movements. They are chiefly to be found in the inter- muscular septa, in the mucous and submucous coats of the intestine, in lymphatic glands. and in the omentum. Fig 26. — Flat, pigmented, branched, con- nective-tissue celh 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 pig- ment is withdrawn into the cell-body, so that thev appear smaller, blacker, and less branched, x 350. (Klein and Noble Smith.) Intercellular Substance. —This may be fibrillar, as in the fibrous tissues and certain varieties of cartilage ; or homogeneous* as in hyaline cartilage d 2 36 STRUCTURE OF ELEMENTARY TISSUES. [chap. nr. 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. 27), or may produce quite a felted texture by their interlacement. The indi- vidual fibres composing these fasci- culi are homogeneous, unbranched, and of the same diameter through- out. 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 salt, or potassium perman- ganate : these reagents possessing the power of dissolving the ce- menting interfibrillar substance (which is nearly allied to syn- tonin), and thus separating the fibres from each other. (b.) Yellow Elastic Fibres (fig. 28) are of all sizes, from ex- cessively fine fibrils up to fibres of considerable thickness : they are distinguished from white fibres by the following characters :— (1.) Their great power of re- sistance even to the prolonged action of chemical reagents, e.g., Caustic Soda, Acetic Acid, &c. (2.) Their well-defined outlines. (3.) Their great tendency to branch and form networks by anastomosis. (4.) They very often have a twisted corkscrew-like appearance, and their free ends usually curl up. (5.) They are of a yellowish tint and very Fig. 28. — Elastic fibres from the liga- elastic menta aubflava. x 200. (Sharpey.) Fig. 27. — Fibrous tissue of cornea, showing bundles of fibres with a few scat- tered fusiform cells lying in the inter-fascicular spaces, x 400. (Klein and Noble Smith.) p. in. J PIB] TIVE T. 37 Varieties of Connective Tissue. I. Viva, b I ■nnectivk T A. — Chief F r is.— Dist i lion. — This variety has a very wi- although the white fibres swell up and become homogeneous, certain elastic fibres may still be seen arranged in various directions, sometimes even appearing to pass in a more or circular or in a spiral manner round a small mass of gelatinous mass of changed white fibres. The cells of the tissue are arranged in no very regular manner, being tained in the spaces (areolae) between the fibres. They com- municate, however, with one another by their branched pro- -, and also apparently with the cells forming the walls of the capillary blood-vessels in their neighbourhood, connecting ther the fibrils in a certain amount of albuminou- substance. (6.) White Fibrous Ti^m*. Distribution. — Typically in tendon ; in ligaments, in the peri- 33 STRUCTURE OF ELEMENTARY TISSUES. [chap. hi. osteum 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 mi- croscope, the tissue appears to consist of long, often parallel, wavy bundles of fibres of different sizes. Sometimes the fibres inter- sect each other. The cells in tendons 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 generally marked by one or more lines or stripes when viewed longitudinally. This appearance is really produced by the laminar extension either projecting upwards or downwards. (c.) Yelloio Elastic Thsue. Distribution. — In the ligamentum nucha? of the ox, horse, and many other animals ; in the ligamenta subflava of man ; in the arteries, constituting 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. Fig. 29. — CauAdk tendon of young rat, shewing the arrangement, form, and structure of the tendon cells. X 300. (Klein.) cil.w. III.] FIBROUS CONXECTIVK TISSUES. 39 Structure. — Elastic tissue occurs in various forms, from ;i struc- tureless, elastic membrane to a tissue whose chief constituents ure bundles of elastic fibres crossing each other at different angles : these varieties may be classified as follows: — (((.) Fine elastic fibrils, which branch and anastomose to form a network : this variety of elastic tissue occurs chiefly in the skin and mucous membranes, in subcutane- ous and submucous tissue, in the lungs and true vocal cords. (b.) Thick fibres, sometimes cylindri- cal, sometimes flattened like tape, which branch and form a network : these are seen most typically m the ligamenta subflava and also in the ligamentuin nuchse of such animals as the ox and horse, in which it is largely developed. (c.) Elastic membranes with perfora- tions, e.g., Henle's fenestrated mem- brane : this variety is found chiefly in the arteries and veins. (d.) CoiltillUOUS, homogeneous elastic Fig. 30.— Transverse section of ten- . don from a cross section of the membranes, e.g., Bowman s anterior elas- tic 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 constituting this variety of connective tissue. tail of a rabbit, showing sheath, fibrous septa, and branched con- nective-tissue corpuscles. The spaces left white in the drawing- represent the tendinous fibres in transverse section, x 250. (Klein.) 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, but remains in the adult only in the vitreous humour of the eye. It may be best seen in the last-named situa- tion, 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 humour are rounded, and in the jelly of the enamel organ are stellate, imbedded in a soft jelly-like inter-cellular substance which forms 40 STRUCTURE OF ELEMENTARY TISSUES. [chap. hi. the bulk of the tissue, and which contains a considerable quantity of mucin. In the umbilical cord, that part of the jelly immediately surround- ing the stellate cells shows marks of obscure fibrillation. (6.) 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 glands of the tongue, in Peyer's patches and in the solitary glands of the intes- tines, and in the mucous membranes generally. Fig. 31. — Tissue of the jelly of Wharton from umbilical cord, a, connective-tissue cor- puscles; b, fasciculi of connective tissue ; c, spherical formative cells. (Frey.) Structure. — Adenoid or retiform tissue consists of a very delicate network of minute fibrils, formed originally by the union of processes of branched connective- Fig. 32. — Part of a section of o lymphatic gland, from ■which the corpuscles have been for the most part "removed, showing the adenoid reticulum. (Klein and Noble Smith.) tissue corpuscles the nuclei of which, however, are visible only during the early periods of development of the tissue (fig. 32). niAF. in.] DEVELOPMENT OF FIBROUS TISSUES. 41 The nuclei found on the fibrillar meshwork <1«» not form an essential part of it. The fibrils are neither white fibrous nor elastic tissue, as they are insoluble in boiling water, although readily soluble in hot alkaline solutions. (c.) Neuroglia. — This tissue forms the support of the Nervous elements 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 " mesoblast." These develop either into a network of branched cells, or into groups of fusiform cells (fig. 33). Fig. 35. — Portion of submucous tissue of gravid uterus of sow. «, branched cells, more or less spindle-shaped ; b, bundles of connective tissue. (Klein.) The cells are imbedded in a semi-fluid albuminous substance derived either from the cells themselves or from the neighbouring 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 connective-tissue corpuscles, or by the fibrils being developed from the outside layers of the protoplasm of the cells, which grow up again to their original size and remain im- bedded among the fibres. This process gives rise to fibres arranged in the one case in interlacing networks (areolar tissue), in the other in parallel bundles (white fibrous tissue). In the mature forms of purely fibrous tissue not only the remnants of the cell- substance, but even the nuclei may disappear. The embryonic tissue, from which elastic fibres are developed, is composed of fusi- form cells, and a structureless intercellular substance by the gradual fibrillation of which elastic fibres are formed. The fusi- 42 STRUCTURE OF ELEMENTARY TISSUES. [chap. hi. form 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 consider- able influence in the production 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 subsidiary but important use of supporting and connect- ing the various tissues and organs of the bod}-. In glands the trabecule of connective tissue form an interstitial framework 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 nucha3 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 without any muscular exertion ; and when the head has been lowered by the action of the flexor mus- cles of the neck, and the ligamentum nuchas thus stretched, the head is brought up again to its normal position by the relaxation of the flexor muscles which allows the elasticity of the ligamentum nucha? 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 exceptions being the subcutaneous tissue of the eyelids, penis, and scrotum, the nymphse, and the cavity of the cranium. Adipose tissue is also absent from the substance of many organs, as the lungs, liver, and others. Fatty matter, not in the form of a distinct tissue, is also widely present in the body, e.g., in the liver and brain, and in the blood and chyle. CD IF. III. J ADIPOSE TISSUE. 43 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 -^^ or 3-^ of an inch in diameter, each composed of a structureless and colourless membrane or bag, filled with fatty matter, which is liquid during life, but in part soli- dified after death (fig. 34). A nucleus is always present in some part or other of the cell-wall, but in the ordinary condition of the cell it is not easily or always visible. This membrane and the nucleus can generally be brought into view by staining the tissue : it can be still more satisfactorily demonstrated by extracting the contents of the fat-cells with ether, when the shrunken, shrivelled membranes remain behind. By mutual pressure, fat-cells come to assume a polyhedral figure (fig. 35). Fig ^4. — Ordinary fot-ceUs of a jot tract in the omentum of a rat. (Klein.) «s». ■£i<*. 35. — Group of fat-cells (fc) with capillary vessels (c). (Noble Smith.) The ultimate cells are held together by capillary blood-vessels 44 STRUCTURE OF ELEMENTARY TISSUES. [chap. hi. (% 35) > while the little clusters thus formed are grouped into small masses, and held so, in most cases, by areolar tissue. Fig. 36. — Blood-vessels of adipose tissue. A. Minute flattened fat-lobule, in ■which the vessels only are represented, a, the terminal artery; 0, the primitive vein; b, the fat-vesi- cles of one border of the lobule separately represented, x 100. b. Plan of the arrange- ment of the capillaries (c) on the exterior of the vesicles : more highly magnified. (Todd and Bowman.) The oily matter contained in the cells is composed chiefly of the compounds of fatty acids with glycerin, which are named oleirij 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 ordinary branched connective-tissue corpuscle and a mature fat-cell. The process of development is as follows : a few small drops of oil make their appearance in the protoplasm : by their confluence a larger drop is produced (fig. 37) : this gradually increases in size at the expense of the original protoplasm of the cell, which becomes correspondingly diminished in quantity till in the mature cell it only forms a thin crescent ic film, closely pressed against the cell-wall, and with a nucleus imbedded in its substance (figs. 34 and 37). Under certain circumstances this process may be reversed and fat-cells may be changed back into connective-tissue corpuscles. (Kolliker, Yirchow. ) OflAP. in.] ADIPOSE TISSUE. 45 Vessels and Serves, — A large number of blood-vessels ore found in adipose tissue, which subdivide until each lobule of fat contains a fine meshwork of capillaries ensheathing cadi individual fat- Fig. 37. — A lohitle of developing adipose tissue from an eight months' foetus, a. Sphe- rical or, from pressure, polyhedral cells with large central nucleus, surrounded by a finely reticulated substance staining uniformly with hematoxylin, b. Similar cells with spaces from which the fat has been removed by oil of cloves, c. Similar cells showing how the nucleus with enclosing protoplasm is being pressed towards peri- phery. 0. Nucleus of endothelium of investing capillaries. (McCarthy.) Drawn by Treves. Fig. 38. — Branched connective-tissue corpuscles, developing into fat-cells, (Klein.) globule. 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: — 46 STRUCTURE OF ELEMENTARY TISSUES. [chap. hi. 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. c. As a packing material, fat serves very admirably to fill up spaces, to form a soft and yielding yet elastic material wherewith to wrap tender and delicate structures, or form a bed with like qualities on which such structures may lie, 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 sub- stance of the bone. II. Cartilage. Cartilage or gristle exists in three different forms in the human body, viz., i, Hyaline cartilage, 2, Yellow elastic-cartilage, and 3, White jihro -cartilage. Structure of Cartilage.— All kinds of cartilage are composed of cells imbedded in a substance called the matrix : and the apparent differences of structure met with in the various kinds of cartilage are more due to differences in the character of the matrix than of the cells. Among the latter, however, there is also consider- able 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 perichon- drium. On the surface of the articular cartilage of the foetus, the perichondrium is represented by a film of epithelium ; but this is gradually worn away up to the margin of the articular surfaces, when by use the parts begin to suffer friction. Nerves are probably not supplied to any variety of cartilage. 1. Hyaline Cartilage. Distribution. — This variety of cartilage is met with largely < H IP. III.] CAKTILAGE. 47 articular ends of bones, the nasal cartilages, and in the human body — investing the and forming the costal cartilag those of the larynx with the excep- tion of the epiglottis and cornicula laryngis. The cartilages of the trachea and bronchi arc also hyaline. Structure, — Like other cartilages it is composed of cells imbedded in a matrix. The cells, which con- tain a nucleus with nucleoli, are irregular in shape, and generally grouped together in patches (fig. 39). 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 pearance like that of ground glass, and in man and the higher 2*# Fig - . 39. — Ordinary hyaline cartilage from trachea of a child. The car- tilage cells are enclosed singly or in paii-s in a capsule of hyaline sub- stance, x 150 diams. "(Klein and Noble Smith.) has a dimly granular ap- Fig. 40. — Freeh cartilage from the Triton. (A. Eollett.) animals has no apparent structure. In some cartilages of the frog, however, even when examined in the fresh state, it is 48 STRUCTURE OF ELEMENTARY TISSUES. [chap. hi. seen to be mapped out into polygonal 1 (locks or cell-territories, each containing a cell in the centre, and representing what is generally called the capsule of the cartilage cells (fig. 40). Hya- line cartilage in man has really the same structure, which can be demonstrated by the use of certain reagents. If a piece of human hyaline cartilage be macerated for a long time in dilute acid or in hot water 95 — H3°F. (35 to 45 C), the matrix, which pre- viously appeared quite homogeneous, is found to be resolved into a number of concentric lamella?, like the coats of an onion, arranged round each cell or group of cells. It is thus shown to consist of nothing but a number of large systems of capsules which have become fused with one another. The cavities in the matrix in which the cells lie are connected together by a series of branching canals, very much resembling those in the cornea : through these canals fluids may make their way into the depths of the tissue. In the hyaline cartilage of the ribs, the cells are mostly larger than in the articular variety, and there is a tendency to the development of fibres in the matrix. The costal cartilages also frequently become calcified in old age, as also do some of those of the larynx. Fat-globules may also be seen in many cartilages. 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 mate- rial 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 matrix is found in the human subject only in early foetal life, when it constitutes the chorda dorsalis. Nutrition of Cartilage. — 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 ill W. III.] CARTILAGE. 49 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 and nucleoli (fig. 41). The matrix in which they are seated is composed almost entirely of fine elastic fibres, which form an intricate interlacement about the cells, and in their general charac- ters are allied to the yellow variety of fibrous tissue : a small and variable quantity of hyaline intercellular substance is also usually present. A variety of elastic cartilage, sometimes called cellular, may be obtained from the external ear of rats, mice, or other small mam- mals. 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 directions and enclose them in a kind of network. Fig. 41. — Section of the epiglottis. (Baly«) Fig. 42.— Transverse section through the intervertebral cartilage of tail of mouse, showing lamelhe of fibrous tissue with cartilage cells arranged in rows between them. The cells are seen in profile, and being flattened, appear staff -shaped. Each cell lies in a capsule, x 350. (Klein and Noble Smith.) 3. White Fibro-Cartilage. Distribution. — The different situations in which white fibro-carti- lage is found have given rise to the following classification : — 1. Inter-articular fibro-cartilage, e.g., the semilunar cartilages of the knee-joint. 50 STRUCTURE OF ELEMENTARY TISSUES. [chap. 2. Circumferential or marginal, as on the edges of the aceta- bulum and glenoid cavity. 3. Connecting, e.g., the inter-vertebral fibro-cartilages. 4. 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 neighbouring tendon of the peroneus longus. Structure. — White fibro-cartilage (fig. 43), which is much more widely distributed throughout the body than the foregoing kind, is composed, like it, of cells '; f! """ "| and a matrix ; the latter, how- ever, being made up almost entirely of fibres closely re- sembling 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 an- Fig. tf.-White fibro-cartilage from an inter- Other portion, COlltillUOUS with vertebral ligament. (Klein and Noble ^ cartikgc ceUg may be prett y freely distributed. Functions of Cartilage. — Cartilage not only represents in the foetus 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 developed body, possessing at the same time strength and elasticity. It maintains the shape of tubes as in the larynx and trachea. It affords attachment to muscles and ligaments ; it binds bones together, yet allows a certain degree of movement, as between the vertebrae ; it forms a firm 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 embryonal tissue, consisting of cells with a very sma.ll quantity of chap, in.] BONE. 5 1 intercellular 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 bythe multiplication of its cellular elements and corresponding increase in its matrix. III. Bone. Chemical Composition. — Bone is composed of earthy and animal matter in the proportion of about 67 per cent, of the former to 33 per cent, of the latter. The earthy matter is com- posed chiefly of calcium phosphate, but besides there is a small quantity (about 11 of the 67 per cent.) 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 1 > y chemical action, as, for instance, by heat in one case and by the action of acids in another, that they can be separated. Their close union, too, is further shown by the fact that when by acids the earthy matter is dissolved out, or, on the other hand, when the animal part is burnt out, the shape of the bone is alike preserved. The proportion between these two constituents of bone varies in different bones in the same individual, and in the same bone at different ages. Structure. — To the naked eye there appear two kinds of struc- ture 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 1 >t her 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 carp** E 2 52 STRUCTURE OF ELEMENTARY TISSUES. [chap. hi. 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 yelloiv. Red marrow is that variety which occupies the spaces in the cancellous tissue ; it is highly vascular, and thus maintains the Fig. 44. — Cells of the red marrow of the guinea pig, highly magnified. 0, a large cell, the nucleus of which appears to be partly divided into three by constrictions ; b, 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 myelo- plaxe, with three nuclei ; e — i, proper cells of the marrow. (E. A. Schafer.) 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 undistinguishable 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 coloured 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. 44). Yelloiv 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 derived, as we shall presently show, large quantities of red blood- corpuscles. Periosteum and Nutrient Blood-vessels. — The surfaces of 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 mem- (HAP. III.] STRUCTURE OF BONE. 53 brane, that the 1 tones, especially their more compact tissue, are in great part supplied with nourishment, — minute branches from the periosteal vessels entering the little foramina on the surface of the hone, and finding their way to the Haversian canals, to be immediately described. The long bones are supplied also by a proper nutrient artery which, entering at some part of the shaft so as to reach the medullary canal, breaks up into branches for the supply of the marrow, from which again small vessels are distri- buted 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 dif- ferences of 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 little irregular spaces, approximately Fig. 45. — Transverse section of compact bony tissue (of humerus). Three of the Haversian canals are seen, with their concentric rings ; also the corpuscles or lacuna?, with the canaliculi extending from them across the direction of the lamella?. The Haversian apertures had got filled with debris in grinding down the section, and therefore appear Muck 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.) fusiform in shape, called lacunae f with very minute canals or canaliculi, as they are termed, leading from them, and anasto- mosing with similar little prolongations from other lacunae (fig. 45). 54 STRUCTURE OF ELEMENTARY TISSUES. [chap. hi. 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 arrangement shown in fig. 45, 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 lacuwe and canaliculi following the same concentric plan of distribution around the small hole in the centre, with which, indeed, they communicate. On making a longitudinal section, the central holes are found to be simply the cut extremities of small canals which run mm mm mm mm m mm m pi m ml* Am MM Fig. 46.— Longitudinal section of human ulna, showing Haversian canal, lacume, and canaliculi. (Eollett.) lengthwise through the bone, anastomosing with each other by lateral branches (fig. 46), and are called Haversian canals, after the name of the physician, Clopton Havers, who first accurately described them. The Haversian canals, the average diameter of which is T ^y 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 lacunae absorbing nutrient matter from the Haversian blood-vessels, and conveying it still more intimately to the very substance of the bone which they traverse. < MAI'. III.] STRUCTURE OF BONE, 55 The blood-vessels enter the Haversian canals both from without, by traversing the Bmall holes which exist on the surface of all bones beneath the periosteum, and from within by means of Bmall 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 dilatations. The lacunae are occupied by branched cells (hone-cells, or bone- oorpuscles) (fig. 47), which very closely resemble the ordinary branched connec- tive-tissue corpuscles ; each of these little masses of protoplasm ministering to the nutrition of the bone immediately surrounding it, and one lacunar corpuscle communicating with an- other, and with its sur- rounding 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 impreg- nated 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 spaees ; while the finely fibrillated structure of the bone-lamella), 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 lamella} can be clearly recognised. (1.) General or fundamental lamella); which are most easily traceable just beneath the periosteum, and around the medullary Fisr. 47. — Bone corpuscles with their process. - seen in a thin section of human bone. (Kollett.) STRUCTURE OF ELEMENTARY TISSUES. [chap, ill' 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 lamellae, filling the spaces between them, and conse- quently attaining their greatest develop- ment where the Haversian systems are few, and vice verso. The ultimate structure of the lamella: 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 f'///v'-'- .-■>"■' ' v'. {';,' ■'- : " ■'.'■'",V> r .v Fig. 48. — 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. (Sharper.) ST* Or - Fig. 49. — Lamella; torn off from a decalcified human parietal bone at some depth from the surface, a, a lamella, showing reticular fibres ; b, b, darker part, where several lamellae are superposed ; c, perforating fibres. Apertures through which perforating fibres had passed, are seen especially in the lower pail, a, a, of the figure. (Allen Thomson.) chap.ih.] DEVELOPMENT OF BONE. 57 liquely, but 1 Ing at the points of intersection, as if hi the 61 I rather than woven together (fig. 48). - irpey.) In many places these reticular lamellse are perforated by taper- ing fibres (Claviculi of Gagliardi), resembling in character the ordinary white or rarely the elastic fibrous tissue, which bolt the neighbouring lamellse together, and may be drawn out when the latter are torn asunder (fig. 49). These perforating fibres origin * 60m ingrowing pn - 3 of the periosteum, and in the adult still retain their connection with it. Development of Bone. — From the point of view of their development, all bones may be subdivided into two classes. (a.) Those which are ossified directly in membrane, e.g.. the - forming the vault of the skull, parietal, frontal. i 3 bos form, previous : ssification, is laid down in hyali tilage, e.g., humerus, femur. The pn — of development, pure and simple, may studied in bones which are not preceded by cartilage — ;; membrane- - " (e.g., parietal) : and without a knowledge of this — irlcation in membrane), it is impossible to understand the much more complex - ri a f changes through which such a structure a 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 or periosteum from which such a bone as the parietal is developed consists of two layers — an external fibrous, and an internal cellular or osL tic The external one consists f ordinary connective-tissue, being composed of layers of fibrous tissue with branched connective- sue corpuscles here and there between the bundles of fibi The internal layer consists : a network of tine fibrils with a lai _ number of nucleated cells, some of which are oval, others drawn out into a long branched pi as, and others branched : it is more richly supplied with capillaries than the outer layer. The rela- tively large number of its cellular elements, their variability in 3 and -hape, together with the abundance of its blood-? clearly mark it - - the portion of the periosteum which is im- mediately concerned in the formation of bone. In such a bone as the parietal, the deposition of bony matter. 53. STRUCTURE OF ELEMENTARY TISSUES. [chap. hi. which is preceded by increased vascularity, takes place in radiat- ing spiculse, starting from a " centre of ossification," and shooting- out in all directions towards the periphery ; while the bone in- creases in thickness by the deposition of successive layers beneath the periosteum. The finely fibrillar network of the deeper or osteogenetic layer of the periosteum becomes transformed into bone- matrix (the minute structure of which has been already (p. 55) described as reticular), and its cells into bone-corpuscles. On the young bony trabecule thus formed, fresh layers of cells (osteo- blasts) from the osteogenetic layer are developed side by side, Fig. so.— QsteoLhisf.*: from the parietal bone of a human embryo, thirteen weeks old. a, bony septa with the cells of the lacuna? ; h, layers of osteoblasts ; c, the latter in tran- sition to bone corpuscles. Highly magnified. (Gegenbaur.) lining the irregular spaces like an epithelium (fig. 50, L). Lime- salts are deposited in the circumferential part of each osteoblast, and thus a ring of osteoblasts gives rise to a ring of bone with the remaining uncalcified portions of the osteoblasts imbedded in it as bone-corpuscles (fig. 50). Thus, the primitive spongy bone is formed, whose irregular branching spaces are occupied by processes from the osteogenetic layer of the periosteum with numerous blood-vessels and osteo- blasts. Portions of this primitive spongy bone are re-absorbed ; the osteoblasts being arranged in concentric successive layers and thus giving rise to concentric Haversian lamella) 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 Haver- sian systems remaining as interstitial or ground-lamellaB (p. 56). I HAP. m. J DEVELOPMENT OF BONE, 59 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 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 typical example, we may consider the process by which the solid cartilaginous rod which represents it in the foetus is converted into the hollow cylinder of compact bone with expanded ends of cancellous tissue 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 purpose is indeed purely tem- porary ; and, after its calci- fication, it is gradually and entirely re-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 periosteum 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 CH Fig\ 51. — From a transversi section through part of foetal jaw near the extreme periosteum, in the state of spongy bone, p, fibrous layer of periosteum ; b, osteogenetic layer of perios- teum ; o, osteoblasts ; c, osseous substance, containing many bone corpuscles. X 300. (Schofiekl.) 6o STRUCTURE OF ELEMENTARY TISSUES. [chap. in. as usually exist between the embryonic and mature forms of connective-tissue. Between the hyaline cartilage of which the foetal femur consists r 4- pj . > ^ 2# — Ossifying cartilage showing loops of blood-vessels. and the bony tissue forming the adult femur, two intermediate stages exist — viz., calcined cartilage, and embryonic spongy bone. These tissues, which successively occupy the place of the foetal CHAP, III. J DEVELOPMENT OF BONE. 61 cartilage, are in bu< i entirely re-absorbed, and their place taken by true bone. The process by which the cartilaginous is transformed into the bony femur may be divided for the sake of clearness into the following six Btages : — Stage 1.— Vascularisation of the Cartilage.— Proc from the oeteogenetic <>r cellular layer of the perichondrium containing blood-vessels grow into the substance of the cartilage from the humerus of a fetal sheep. I ified trabecule are seen extending between the columns of cartilage cells, c, cai- tilage cells, x i p. v^harpey.) much as ivy insinuates itself into the cracks and crevices of a wall. Thus the substance of the cartilage, which previously contained no vessels, is traversed by a number of branched anastomosing channels formed bv the enlargement and coalescence of the spaces in which the cartilage-cells lie, and containing loops of blood-vessels (fig. 52) and spheroidal-cells which will become osteobh- 62 STRUCTURE OF ELEMENTARY TISSUES. [chap. hi. Stage 2.— Calcification of Cartilaginous Matrix. — Lime- salts are next deposited in the form of fine granules in the hyaline matrix of the cartilage, which thus becomes gradually transformed into a number of calcified trabecule (fig. 54, 5 ), forming alveolar spaces (primary areola-) containing cartilage cells. By the absorp- Yis. 54. — T, - • - ofaportii f, showing — i, fibrous layer of periosteum; 2, oncogenetic layer of ditto; 3, periosteal borne; 4, cartflage with matrix gradually becoming calcified, as at 5, with cells in primary areola? : beyond 5 the calcified matrix 'is being entirely replaced by spongy bone, x 200. ( V. D. Harris.) tion of some of the trabecule larger spaces arise, which contain cartilage-cells for a very Bhort 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 Cartilage. — The cells of the primary marrow arrange them- OHAP. in. | DEVELOPMENT OF BONE. 63 a — 0& C ?*;: ^^ff^ selves as a continuous layer like epithelium on the calcified trabecule and deposit a layer of bone, which ensheathes the calci- fied trabecules : these calci- fied trabecule?, encased in their sheaths of young bone, become gradually absorbed, so that finally Ave have tra- becule composed entirely of spongy bone, all trace of the original calcified car- tilage having disappeared. It is probable that the large multinucleated giant-cells termed "osteoclasts" b}^ Kolliker, which are derived from the osteoblasts by the multiplication of their nu- clei, are the agents by which the absorption of calcified cartilage, and sub- sequently of embryonic spongy bone, is carried on (fig. 55, 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 process of ossification into the intermediary carti- 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 extends into the hyaline matrix between them (figs. 52 and 53). 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 successive changes which occur at the centre of the shaft. Periosteal bone is now deposited in successive layers beneath the periosteum, i.e., at tlte Fig. 55. — A small isolated mass of bone next the periosteum of the lower ja"W of human fuetus. a, osteogenetic layer of periosteum. G, mul- tinuclear giant cells, the one on the left acting here probably like an osteoclast. Above c, the osteoblasts are seen to become surrounded by an osseous matrix. (Klein and Noble Smith.) 6 4 STRUCTURE OF ELEMENTARY TISSUES. [chap. hi. circumference of the shaft, exactly as described in the section on " ossification in membrane," and thus a casing of periosteal bone is formed around the embryonic endochondral spongy bone : this Fig. 56. — Transverse section through the tibia of a foetal kitten semi-diagrammatic. x 60. P, Periosteum. O, osteogenetic layer of the periosteum, showing the osteo- blasts arranged side by side, represented as pear-shaped black dots on the surface of the newly-formed bone. B, the periosteal bone deposited in successive layers beneath the periosteum and ensheathing E, the spongy endochondral bone ; represented as more deeply shaded. Within the trabecule? of endochondral spongy bone are seen the remains of the calcined cartilage trabecule represented as dark wavy lines. C, the medulla, with V, T, veins. In the lower half of the figure the endochondral spongy bone has been completely absorbed. (Klein and Noble Smith.) 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 trabecule becoming gradually thinned and its CHAP, HI.] DEVELOPMENT OK BONE. 65 meshes enlarging, and finally coalescing into one greal cavity — the medullary cavity of the shaft. Stage 5.— Absorption of the Inner Layers of the Perios- teal Bone. — The absorption of the endochondral spongy l>onc is now complete, 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 character. Stage 6.— Formation of Compact Bone. — The transforma- tion 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. 58). The Irregularities in the walls of the areolae in the spongy hone are ab- sorbed, while the osteo- blasts which line them are developed in concen- tric layers, each layer in turn becoming ossified till the comparatively large space in the centre is reduced to a well- formed Haversian canal (fig. 57). When once formed, bony tissue gr< > ws to some extent intersti- tially, as is evidenced by the fact that the la- cuna; 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 sification in membrane " are apt to mislead, since they seem to Fig. 57. — Transverse section of femur of a human embryo about eleven weeks old. a, rudimen- tary Haversian canal in cross section ; b, in lon- gitudinal section ; r, osteoblasts ; d, newly formed osseous substance of a lighter colour; e, that of greater age ; /, lacunto with their cells ; g, a cell still united to an osteoblast. (Frey.) 66 STRUCTURE OF ELEMENTARY TISSUES. [chap. hi. imply two processes radically distinct. The process of ossification, nowever, is in all cases one and the same, all true bony tissue being- formed from membrane (perichondrium or periosteum) ; but in the development of such a bone as the femur, which may be taken as the type of so-called " ossification in cartilage," lime-salts are deposited in the cartilage, and this calcified cartilage 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," calcifica- tion of the cartilaginous matrix precedes the real formation of bone. AVe must, therefore, clearly distinguish between calcifica- tion 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 ossification 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 atone or more points, termed "centres of ossification." The long- bones, eg., femur, humerus, &c, have at least three such points — one for the ossification of the shaft or dia/physis, and one for each articular extremity or epiphysis. Besides these three primary centres which are always present in long bones, various secondary centres may be superadded for the ossification of different processes. Growth of Bone. — Bones increase in length by the advance of the process of ossification into the cartilage intermediate between the diaphysis and epiphysis. The increase in length indeed is clue 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 un- altered though the bone has gradually increased in length, the growth having taken place beyond and not between them. If now one pin be placed in the shaft, and the other in the epiphysis, of a growing bone, their distance apart will increase as the bone grows in length. Thus it is that if the epiphyses with the intermediate cartilage be removed from a young bone, growth in length is no longer pos- sible ; while the natural termination of growth of a bone in length takes place when the epiphyses become united in bony continuity with the shaft. DEAF. Hi. J TEE! I!. 6? [ncrease in thickneu in the shaft of a long bone, occurs by the deposition of successive layers beneath the periosteum. [fa thin metal plate be inserted beneath the periosteum of a growing bone, it will soon l>e covered by osseous deposit, but if it he put between the fibrous and osteogenetic layers, it will never become enveloped in bone, for all the bone is formed beneath the latter. 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; notwithstanding their hardness they possess a considerable degree of elasticity, which often saves them from fractures. Teeth. The principal part of a tooth, viz., dentine, is called by some a connective tissue, and 011 this account the structure of the teeth is considered here. t>on of a human molar tooth ; c, cement ; d, dentine ; e, enamel p, pulp cavity. (Owen.) b. 1 lection. The letters indicate the i>ame &> in a. A tooth is generally described as possessing a crown, neck, and fang or fangs. F 2 68 STRUCTURE OF ELEMENTARY TISSUES. [chap. hi. The crown is the portion which projects beyond the level of the givm. The neck is that constricted portion just below the crown which is embraced by the free edges of the gum, and the fang in- cludes all below this. On making a longitudinal section through the centre of a tooth (figs. 58, 59), it is found to be principally composed of a hard matter, dentine or ivory ; while in the centre this dentine is hollowed out into a cavity resem- bling in general shape the outline of the tooth, and called the pulp cavity, from its containing a very vascular and sensitive little mass, composed of connective - tissue, blood-vessels, and nerves, which is called the tooth-pulp. The blood-vessels and nerves enter the pulp through a small opening at the extremity of the fang. Capping that part of the den- tine which projects beyond the level of the gum, is a layer of very hard calcareous matter, the enamel ; while sheathing the por- tion of dentine which is beneath the level of the gum, is a layer of true bone, called the cement or crnsta petrosa. 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 as we approach the crown, and the cement as we approach the lower end or apex of the fang. I. — Dentine. Chemical composition. — Dentine or ivory in chemical composition closely resembles bone. It contains, however, rather less animal Fig. 59. — Premolar tooth of cat in situ. Vertical section. 1. Enamel with decussating and parallel stripe. 2. Dentine 'with Sehreger's lines. 3. Cement. 4. Periosteum of the alve- olus. 5. Inferior maxillary hone showing canal for the inferior dental nerve and vessels which appears nearlv circular in transverse section. (Waldeyer.) Clf A !'. III.] TEETH: DENTINE. 69 matter ; the proportion in a hundred parts being about twenty- eight animal to Beventy-two of worthy. The former, like the animal matter of bone, may be resolved into gelatin \>y boiling. The earthy matter is made up chiefly of calcium phosphate, with a small portion of the carbonate, and traces of calcium fluoride ami gnesium phosphate. Structure. — Under the ml 1 denti] en to be finely channelled by a multitude of delicate tubes, which, by their inner ends, communicate with the pulp-cavity, and by their outer ex- tremities come into contact with the under part of the enamel and Fig. €•: .— - from the m root of an a, den tal tubuli ramifying and terminating, some of them in the inter- globular - - nd <-. which somewhat resemble bone laeun* ; d. inner layer of the cement with numerous set canaliculi ; e, outer layer of cement ; /, lacunie ; •j, canaliculi. cement and sometimes even penetrate them for a greater or less distance (fig. 60). In their course from the pulp-cavity to the surface of the dentine, the minute tubes form gentle and nearly parallel curves and divide and subdivide dichotomously, but without much S8ening of their calibre until they are approaching their peri- pheral termination. From their sides proceed other exceedingly minute secondary canals, which extend into the dentine between the tubules, and anastomose with each other. The tubules of the dentine, the average diameter of which at their inner and larger extremity is -j-foo of an inch, contain fine prolongations from the tooth-pulp, which Lrive the dentine a certain faint sensitiveness under ordi- nary circumstances and, without doubt, have to do also with its nutrition. These prolongations from the tooth-pulp are really processes of the dentine-cells or odontoblasts which are branched '0 8TEUCTUEE OF ELEMENTARY TISSUES. [chap. hi. cells lining the pulp-cavity ; the relation of these processes to the tubules in which they lie being precisely similar to that of the pro- cesses of the hone-corpuscles to the canaliculi of bone. The outer portion of the dentine, underlying both the cement and enamel. forms a more or less distinct layer termed the granular or inter- globular layer. It is characterised by / the presence of a number of minute ^rr-^rrn: ? ~~~:r&f cell-like cavities, much more closely packed than the lacunae in the cement, and coinmunicating with one another and with the ends of the dentine-tubes (fig. 60), and containing cells like bone- corpuscles. ' - - - .- - - /-■•-.- ■m E'fM II. — Enamel. Chemical composition. — The ena which is by far the hardest portion of a tooth, is composed, chemically, of the same elements that enter into the com- sition of dentine and bone. Its ani- mal matter, however, amounts only to about 2 or 3 per cent. It contai: _ r proportion of inorganic matter and is harder than any other tissue in the body. Structure. — Examined under the microscope, enamel is found com] of fine hexagonal fibres _ 61. 62) 50 x 00 of an inch in diameter, which are set on end on the surface of the dentine, and fit into corresponding de] in the same. They radiate in such a manner fin >m the dentine that at the top of the tooth they are more or vertical, while towards the sides they tend to the horizontal direc- tion. Like the dentine tubules, they are not Btraight, but dis] 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. Fig. ci. — of the enanf-l and a part tine, a, cuticular pellicle of the enamel : h. enamel fibres, or columns with fissures tween them and cross striae : c, larger cavities in the enamel, communicating with the mities of some of the tufoi i X 350. (Koll: OHAP. in.] DEVELOPMENT OF TEETH. n The enamel-prisms are connected together by a very quantity of hyaline cement-substance. In the deeper par enamel, between the prisms, are small lacunas, which com- municate with the " interglo- bular 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. III. — Orusta Petrosa. The crusta petrosa, or cement (fig. 60, c, d), is composed of true hone, and in it are la- cunse (/) and canaliculi (f the sub-epithelial tissue surrounding the enamel organ and interposed between the enamel .u-erm and the developing bony jaw, is composed of nucleated cells arranged in a meshwork, the outer or peripheral part being covered with a layer of columnar nucleated cells called odontoblasts. The odontoblasts form the dentine, while the remainder of the papilla forms the tooth-pulp. The method of the formation of the dentine from the odontoblae is as follows: — The cells elongate at their outer part, and th< - processes are directly converted into the tubules of dentine (fig. 64). 74 STRUCTURE OF ELEMENTARY TISSUES. [chap. hi. The continued formation of dentine proceeds by the elongation of the odontoblasts, and Their subsequent conversion by a process of calcification into dentine tubules. The most recently formed tubules are not immediately calcified. 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 between the cells of the superficial layer (fig. 64) 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 it is to form. As the dentine increases in thickness, the papillae dimi- nish, and at last when the tooth is cut, only a small amount of the papilla re- mains 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 constriction below the crown, or by division of the lower part of the papilla. The enamel cap is found later on to consist (fig. 65) of three parts : (a) an inner membrane, composed of a layer of columnar epithe- lium m contact with the dentine, called enamel cell*, and outside of these one or more layers of small polyhedral nucleated cells {stratum intermedium of Hannover) ; (I) an outer membrane of several layers of epithelium ; (c) a middle membrane formed of a Fig ! trans f tfu dental sac, pulp, fee, of a kitten, n. dental papilla or pulp ; b. 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 epithe- lial layer of the enamel organ : g, inner layer, and h. outer layer of dental sac. X 14. (Thiersch. chap. in. J DEVELOPMENT OF TEETH. 75 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 extremities, and the direct 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 arc used for the regeneration of the enamel cells, hut these and the middle membrane after a time disappear. The cells of the outer mem- brane give origin to the cuticle of the enamel. The cement or crusta petrosa is formed from the tissue of tie- tooth sac, the structure and function of which are identical with those of the osteogenetic layer of the periosteum. In this manner the first set of teeth, or the milk-teeth, arc formed ; and each tooth, by degrees developing, presses at length on the wall of the sac enclosing it and, causing its absorption, is cut, to use a familiar phrase. The temporary or milk-teeth have only a very limited term of existence. This is due to the growth of the permanent teeth, which push their way up from beneath, absorbing in their progress the whole of the fang of each milk-tooth and leaving at length only the crown as a mere shell, which is shed to make way for the eruption of the permanent teeth (tig. 66). The temporary teeth are ten in each jaw, namely, four incisors, two canines, and four molars, and are replaced by ten permanent teeth, each of which is developed in a way almost exactly similar to the manner of development already described, from a small process or sac set by, so to speak, from the enamel germ of the temporary tooth which precedes it, and called the cavity of reserve. The number of permanent teeth in each jaw is, however, in- creased to sixteen, by the development of three others on each side of the jaw after much- the same fashion as that by which the milk- teeth were themselves formed. The beginning of the development of the permanent teeth of course takes place long before the cutting of those which they are to succeed. One of the first steps in the development of a milk- tooth is the outgrowth of a lateral process of epithelial cells from its primitive enamel organ (fig. 63, c, f p). This epithelial out- growth ultimately becomes the enamel organ of the permanent 7 6 STRUCTURE OF ELEMENTARY TISSUES. [chap. tit. tooth, and is indented from below by a primitive dental papilla, precisely as described above. Fig. 66. — 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 till 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. (Quain.) The following formula shows, at a glance, the comparative ar- rangement and number of the temporary and permanent teeth : — Temporary Teeth MO. CA. IX. (A. MO. Upper 2 i 4 i 2 = io Lower 20 2 I 4 I 2 IO Permanent teeth MO. BI. CA. IN. CA. BI. MO. Upper 3 2 i 4 i 2 3= 16 Lower = 32 2 I 16 From this formula it will be seen that the two bicuspid teeth in the adult are the successors of the two molars in the child. They differ from them, however, in some respects, the temporary molar* having a stronger likeness to the permanent than to their imme- diate descendants, the so-called bicuspids. The temporary incisors and canines differ from their successors but little except in their smaller size. The following tables show the average times of eruption of the Temporary and Permanent teeth. In both cases, the eruption of , n\r. in.] ERUPTION OP THE TEETH. yy any given tooth of the lower jaw precedes, as a rule, that of the corresponding tooth of the upper. / mporary or Milk Teeth. The figures indicate in month* the age at which ca<-h tooth appears. CANINES. rCISOBS. CANINES. 24 12 iS 9 7 7 9 12 24 Per mi, 1 hi nt Teeth. The age at which each tooth is cut is indicated in this table in years. ■OLABS. BICUSPID. CANINE8. INi - - l LXINE8. BICUSPID. M< 17 12 12 17 to to 6 10 9 11 to 12 8778 n to 12 9 10 6 to to -5 13 13 25 The times of eruption put down in the above tables are only approximate : the limits of variation being tolerably wide. Some children may cut their first teeth before the age of six months and others not till nearly the twelfth month. In nearly all eases the two central incisors of the lower jaw are cut first ; these being suc- ceeded 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 indi- cated 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 number) make their appearance behind the milk- molars, and for a short time the child has four permanent and twenty temporary teeth in position at once. It is worthy of note that from the age of five years to the shedding of the first milk-tooth the child has no fewer than forty- eight teeth, twenty milk-teeth and twenty-eight calcified germ-, of permanent teeth (all in fact except the four wisdom teeth). jS THE BLOOD. [chap. iv. CHAPTER IV. THE BLOOD. The blood of man, us indeed of the great majority of verte- brate animals, is a more or less viscid fluid, of a red colour. The exact shade of red is variable, for whereas that taken from the arteries, from the left side of the heart or from the pulmonary veins, is of a bright scarlet hue, that obtained from the systemic veins, from the right side of the heart, or from the pulmonary artery, is of a mnch darker colour, and varies from bluish-red to reddish-black. To the naked eye, the red colour appears to belong- to the whole mass of blood, but on examination with the micro- scope it is found that this is not the case. By the aid of this instrument the blood is shown to consist in reality of an almost colourless fluid, called Liquor. Sanguinis or Plasma, in which are suspended numerous minute rounded masses of protoplasm, called Blood Corpuscles. The corpuscles are, for the most part, coloured, and it is to their presence that the red colour 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 trans- parent, and assumes a lake colour, in consequence of the colouring matter of the corpuscles having been, by these means, dis- charged into the plasma. The average specific gravity of blood at 6o°F. (15 C.) is 1055, tne extremes consistent with health being 1045- 106 2. The reaction of blood is faintly alkaline. Its. temperature varies within narrow limits, the average being ioo° F. (37 '8° C). The blood stream is slightly warmed by | ing through the muscles, nerve centres, and glands, but is some- what cooled on traversing the capillaries of the skin. Eecently drawn blood has a distinct odour, which in many cases is charac- teristic of the animal from which it has been taken; the odour mav be further developed by adding to blood a mixture of equal I arts of sulphuric acid and water. . ii.vi-. iv.] ANTITY OF BLOOD. Quantity of the Blood. — The quantity «»f blood in animal under normal conditi it relati .it. The methods employed it not so simple as migl I t I it sight be thought. For mple, it would to get an .mtc informa- ; <>n the point from the amount obtained by rapidly bl< an animal I th, for then an indefinite quanta! ild remain in the vess la, l well s in tJ Q ' other han«.l, would it be possible * I in a eon by less rapid bleeding a, am lii more prolonged, time would be all r the j - _ into the blood of lymph from the lymphatic toss m the tisa tea. In the form ae, therefore, we should nnder-eatimate, and in the latter over- >tal amount of the blood. - era! methods which have been employed, the u accurate appears to be the following. A small quantity of blood is taken from an animal by venesection : it is defibrinated an I measured, and used tni the minci:._- is sarefully filtered, and added * the diluted blood previou- f ined. and the whole is measured. The ik t st i in the process is the m] iris n of the colour of the diluted blood with that of standard solut: blood and wat known strength, imtil it is red to what stan- dard solution the diluted bloo: the amount of blood in the nding standard solution is known. .1 as tl. total quantity of diluted blood obtained from * animal, it Iculate the al amount of blood which the latl ntained, and to this is added the small amount which was withdrawn to make the standard soluti< This irives the total amount of blood which the animal contained. It is cent with the weight of the anim known. T rait of many experime:.* - 3 that the quan- tity of blood in various animal< ■ _ - _'_ to ^ of the total body weight. SO THE BLOOD. [OHAP. iv. An estimate of the quantity in man which corresponded nearly -with the above, was made some years ago from the following data. A criminal was weighed before and after decapitation \ the differ- ence 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 colour. This fluid was then also weighed ; and the amount of blood which it represented was calculated by com] taring the proportion of solid matter contained in it with that of the first blood which escaped on decapitation. Two experiments of this kind gave precisely similar results. (Weber and Lehmann.) It should be remembered, however, in connection with these estimations, that the quantity of the blood must vary, even in the same animal, very considerably with the amount of both the in gesta and egesta of the period immediately preceding the experi- ment ; and it has been found, indeed, that the quantity of blood obtainable from a fasting animal barely exceeds a half of that which is present soon after a full meal. Coagulation of trie Blood. — One of the most characteristic properties which the blood possesses is that of clotting or coagulating, when removed from the body. This phenomenon may be observed under the most favourable 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 ; this change next taking place, in a minute or two, at the sides of the vessel in which it is contained, and then extending 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 volume 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-coloured fluid, the serum, may be seen to make their appearance on the surface and, as they become more and more numerous, 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, ( II \l'. IV.] i o v.i i. \i \<>s. Hi which in the course of an hour or two floats in the liquid. The Bret dr rum appear on tin- surface about eleven or twelve minutes after the blood has been drawn ; and the fluid con- tinues to transude for from thirty-sis to forty-eight hours. The clotting of blood is due to the development in it of a sub- Btance called Jibrin, which appears as a meshwork (fig. 67) of fine fibrils. Tlii- m< sh- work entangles and encloses within it the blood corpuscles, clotting takes pli too quickly to allow them to >ink to the bottom of the plasma. The first clot formed, therefore, includes the whole of the consti- tuents of the blood in an apparently solid mass, but soon the fibrinous mesh work begins to contract, and the .serum which does not belong to the clot ueezed out. When the whole of the serum has transuded, 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-corpuscles, and clotted blood of serum and clot. Liquid Blood. Fig. 67.— /'•• ' "'■■ •• from a drop of human blood, after treatment with rosanilin. Ram i- Plasma Corp. Serum Fibrin Clot Clotted Blood Buflfy Coat.— 1 fader ordinary circumstances coagulation occurs, as we have mentioned above, before the red corpuscles have had G 82 THE BLOOD. [CHAP. iv. time to subside; and thus from their being entangled in the meshes of the fibrin, the clot is of a deep red colour 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. (o° 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 coloured corpuscles to sink to the bottom of the fluid. When clotting does occur, the upper layers of the blood, being free of coloured corpuscles and counting chiefly of fibrin, form a superficial stratum differing in appearance from the rest of the clot, in that it is of a grayish yellow colour. This i< known as the " huffy coat.''' Cupped appearance of the Clot. — When the bufly coat has been produced in the manner just described, it com m only contracts more than the rest of the clot, on account of the absence of coloured 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 pro- duces 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 astern, especially in inflammation, where the fibrin-forming con- stituents are in excess, and it is also well marked in chlorosis where the corpuscles are deficient in quantity. Formation of Fibrin. — In describing the coagulation of the blood in the preceding paragraphs, it was stated that this phe- nomenon was due to the development in the clotting blood of a meshwork of fibrin. This may be demonstrated by taking recently-drawn blood, and whipping it with a bundle of twigs ; the fibrin is found to adhere to the twigs as a reddish-white, stringy mass, having been thus obtained from the fluid nearly free from coloured corpuscles. The defibrinated blood no longer retains the power of spontaneous coagulability. The fibrin which makes its appearance in the blood when it is undergoing coagulation is derived chiefly, if not entirely, from the plasma or liquor sanguinis ; for although the colourless corpuscles are intimatelv connected with the process in a way which will be CBAP. iv.] PLASMA. $$ presently explained, the coloured corpuscles appear to take no active part in it whatever. This may be shown by experimenting with plasma free from coloured corpuscles. Such plasma may be procured by delaying coagulation in blood, by keeping it at a low temperature, 32 F, (o° (.'.), until the coloured corpuscles which aiv of 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 colourless plasma, in the lower Layers of which are many colourless corpuscles. The blood of the horse is specially suited for the purposes of this experiment ; and the upper stratum of colourless plasma derived from it, if decanted into another vessel and exposed to the ordinary temperature of the air, will coagulate just as though it were the entire blood, producing a clot similar in all respects to blood clot, except that it is almost colourless from the absence of red corpuscles. 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 seen to be due to fibrin fibrils developing in the fluid — first of all at the circumference of the containing vessel, and gradually extending throughout the mass. Again, if 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. Evidently, therefore, fibrin is derived from the plasma and not from the coloured corpuscles. In these ex- periments, 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.g., by allowing blood to flow direct from the vessels of an animal into a vessel containing a third or a fourth of the bulk of the blood of a saturated solution of a neutral salt (preferably of magnesium sulphate) and mixing care- fully, will answer the purpose and, just as in the other case the coloured corpuscles will subside leaving the clear superstratum of * Neutral saline solution commonly consists of a 75 solution of common salt (sodium chloride) in water. g 2 8/| Till-; BLOOD. [chap. iv. (salted) plasma. In order to cause this plasma to coagulate, it is necessary to get rid of the salts by dialysis, or to dilute it with several times its bulk of water. The antecedent of Fibrin. — If plasma he saturated with solid magnesium sulphate or sodium chloride, a white, sticky, precipitate called plasmine is thrown down, after the removal of which, by filtra- tion, the plasma will not spontaneously coagulate. This plasmine is soluble in dilute neutral saline solutions, and the solution of it speedily coagulates, producing a clot composed of fibrin. From this we see that blood plasma 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 con- verted into fibrin, when clotting of fluids containing it takes place. Nature of Plasmine. — There seems 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 col< air, which in diseased conditions may be greatly increased. It somewhat resembles serum in appearance, but in reality differs from it, and is probably identical with 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 tendency 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 mostly used) we may obtain, by saturating it with solid magnesium sulphate or sodium chloride, a white viscid substance as a precipitate which is called fibrinogen, which may be separated by filtration, and is then capable of being dissolved in water, as a certain amount of the neutral salt is entangled with the precipitate sufficient to produce a dilute saline solu- tion in which it is soluble. This body belongs to the globulin class of proteid substances. Its solution has no tendency to clot of itself. Fibrinogen may also be obtained as a viscid i hap. iv.] PARAGLOBULIN : FIBRIN FERMENT. S'- precipitate from hydrocele fluid by diluting it with water, and passing a brisk stream of carbon dioxide gas through the solu- tion. Now if serum be added t<» a solution of fibrinogen, the uii\t lire dots. From serum may be obtained another globulin very similar in properties to fibrinogen, if it be subjected to treatment similar to either of the two methods by which fibrinogen is obtained from hydrocele fluid ; this substance is called paraglobulin, and it may be separated by filtration and dissolved in a dilute saline solution in a manner similar to fibrinogen. If the solutions of fibrinogen and paraglobulin be mixed, the mixture cannot be distinguished from a solution of plasmine, and like that solution (in a great majority of eases) firmly clots whereas a mixture of the hydrocele fluid and serum, from which they have been respectively taken, no longer does so. In addition to this evidence of the compound nature of plasmine, it may lie further shown that, if sufficient care be taken, both fibrinogen and paraglobulin may be obtained from plasma : fibrinogen, as a flaky precipitate, by adding carefully 13 per cent, of crystalline sodium, chloride ; and after the removal of fibrinogen from the plasma by filtration, paraglobulin may be afterwards precipitated, on th further addition 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 saturated with sodium chloride or magnesium sulphate, and that the mixture of the two corresponds with plasmine. Presence of a Fibrin Ferment. — So far it has been shown that plasmine, the antecedent of fibrin in blood, 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 para- globulin ; 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). 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 paraglobulin which the former lacked ; this, however, is not the case, as hydrocele does not lack this body, and if paraglobulin, obtained from serum by the carbonic acid method, be added to it, it will not coagulate, neither will a mixture 86 THE BLOOD. [chap. iv. of solutions of fibrinogen and paraglobulin 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 paraglo- bulin, when obtained by the saturation method. From this it is evident that in plasmine there is something more than the two bodies above mentioned, and that this something is precipitated with the paraglobulin by the saturation method, and is not pre- cipitated 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 coagu- lated 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 extremely little, if any, proteid matter. Yet a little of this watery extract will determine 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, obtained by the carbonic acid method, to clot. This 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, belongs to that class of bodies which promote the union of other bodies, or cause changes in other bodies, without themselves entering into union or undergoing change, i.e. ferments. The third substance has, therefore, received the name Jibrin ferment. This ferment is developed in blood soon after it has been shed, and its amount appears to increase for a certain time afterwards (p. 92). The part played by Paraglobulin. — So far we have seen that plasmine is a body composed of three substances, viz., fibrinogen paraglobulin, and fibrin ferment. The question presents itself, are these three bodies actively concerned in the formation of fibrin ? OHAP. iv.] COAGULATION. 87 Bere we come to a point about which two distinct opinions pre- vail, and which it will be necessary to mention Schmidt holds that fibrin is produced by the interaction of the two proteid bodies, viz., fibrinogen and paraglobulin, brought about by the presence of a special fibrin ferment. Also, that when coagulation does not occur in serum, which contains paraglobulin and the fibrin ferment, the non-coagulation is accounted for by luck of fibrinogen, and 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 hbrino-plastic property. The other opinion, held by Hammersten, is 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 preventing the formation or precipitation of fibrin, this power of preventing coagulation being well known to belong to the free alkalies, to the alkaline carbonates, and to certain salts ; and he looks upon fibrin as formed from fibrinogen, which is either (1) decomposed into that substance with the pro- duction of some other substances ; or (2) bodily converted into it under the action of a ferment, which is frequently precipitated with paraglobulin. Influence of Salts on Coagulation. — It is believed that the presence of a certain but small amount of salts, especially of sodium chloride, is necessary for coagulation, and that without it, clotting cannot take place. Sources of the Fibrin Generators. — It has been previously remarked that the colourless corpuscles which are always present in smaller or greater numbers in the plasma, even when this has been freed from coloured corpuscles, have an important share in the production of the clot. The proofs of this may be briefly summarised as follows : — (1) That all strongly coagulable fluids contain colourless corpuscles almost in direct proportion to their coagulability; (2) That clots formed on foreign bodies, such as needles inserted into the interior of living blood-vessels, are preceded by an aggregation of colourless corpuscles ; (3) That plasma in which the colourless corpuscles happen to be scanty, $8 J in: blood. [.hap. iv. 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 coloured corpuscles, will, if removed, clot feebly, as it contains little of the fibrin factors; whereas the colourless plasma, especially the lower layers of it in which the colourless 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 colourless 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 colourless corpuscles. Although the intimate connection of the colourless 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 remains still 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 colourless corpuscles which the plasma contains. Many believe that the fibrinogen also is wholly or in part derived from them. Conditions affecting Coagulation. — The coagulation of the blood is hastened by the following means : — 1. Moderate warmth, — from about ioo° to 120° F. (37*8 — 49° C). 2. Rest is favourable to the coagulation of blood. Blood, of which the whole mass is kept in uniform motion, as when a closed vessel completely filled with it is constantly moved, coagulates very slowly and imperfectly. 3. Contact with foreign matter, and especially multipli- cation of the points of contact. Thus, coagulated fibrin may be quickly obtained from liquid blood by stirring it with a bundle of small twigs ; and even in the living body the blood will coagu- late upon rough bodies projecting into the vessels; as, for ex- ample, upon threads passed through them, or upon the heart's valves roughened by inflammatory deposits or calcareous accumu- lations. 4. The free access of air. — Coagulation is quicker in shallow than in tall and narrow vessels. chap, iv.] CONDITIONS AFFECTING COAGULATION. 89 5. The addition of loss than twice the bulk of water. The blood last drawn is said to coagulate more quickly than the • ret The coagulation of the blood is retarded, suspended, or prevented by the following means : — 1. Cold retards coagulation ; and s<» long as Mood is kept at a temperature, 32 l\ (o°(\), it will not coagulate at all. Freezing the blood, of course, prevents its coagulation ; vet it will coagu- late, though not firmly, if thawed after being frozen; and it will do so, even after it has been frozen for several months. A higher temperature than 120 F. (49 C.) retards coagulation or, by coagulating the albumen of the serum, prevents it altogether. 2. The addition of water in greater proportion than twice the bulk of the blood. 3. Contact with living tissues, and especially with the interior of a living blood-vessel. 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. Coagula- tion, however, ensues on dilution with water. The time during which blood can be thus preserved in a liquid state and coagulated by the addition of water, is quite indefinite. 5. Imperfect aeration, — as in the blood of those who die by asphyxia. G. In inflammatory states of the system the blood coagu- lates more slowly although more firmly. 7. Coagulation is retarded by exclusion of the blood from the air, as by pouring oil on the surface, etc. In vacuo, the blood coagulates quickly ; but 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 con- tact with the containing vessel. 8. The coagulation of the blood is prevented altogether by the addition 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 QO THE BLOOD. [chap. iv. anger. He says, " I have seen instances of them all." Doubtless lie 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 foundfclots in the hearts of hares and stags hunted to death, and of cocks killed in fighting. Cause of the fluidity of the blood within the living body. — Very closely connected with the problem of the coagula- tion of the blood arises 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 : — (i) Blood will coagulate within the living body under certain con- ditions, — 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 the roughness has been pro- duced by inflammation, as in endocarditis, and it is also equally true that aneurisms of arteries are sometimes spontaneously 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, Avithout the addition of any re-agent, 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 iimilar animal be ligatured in two places some inches apart, and after some time 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 chap, iv.] LGULATION. 91 another similarly prepared, as from one test-tube into anol thereby Buffering free exposure to the air, without coagulation occurring as long as the vessels retain their vitality, [fthe • theliol lining of the vein, however, be injured, the blood will oot 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 fluidity under the usual conditions of the blond within the Living body must reconcile the above apparently contradictory facts, and must at the same time be made to include all the other known facts concerning the coagulation of the blood. 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 move- ment j 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 supposes that the blood has no natural tendency to clot, but that its coagulation out of the body is due to the action of foreign matter with which it happens to be brought into contact, and in the body to conditions of the tissues which cause them to act towards it like foreign matter. 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. Support was once thought to be given to Briicke's and like theories by cases of injury, in which blood extravasated in the living body has seemed to remain uncoagulated for weeks, or even months, on account of its contact with living tissues. But the supposed facts have been shown to be without foundation. The blood-like fluid in such cases is not uncoagulated blood, but a mixture of serum and blood-corpuscles, with a certain proportion of clot in various stages of disintegration (Morrant Baker.) As the blood must contain the substances from which fibrin is formed, and as the re-arrangement of these substances occurs very quickly whenever the blood is shed, so that it is somewhat difficult to prevent coagulation, it seems more reasonable to hold with Briicke, that the blood has a strong tendency to clot, rather than with Lister, that it ha- ecial tendency thereto. t)2 THE BLOOD. [chap. iv. It has been recently suggested that the reason why blood does not coagulate in the living vessels, is that the factors which Ave have seen are necessary for the formation of fibrin are not in the exact state required for its production, and that the fibrin ferment is not formed or is not, at any rate, free in the living blood, but that it is produced (or set free) at the moment of coagulation by the disintegration of the colourless corpuscles. This supposition is certainly plausible, but 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 sufficient 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 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, produce clotting. There must be a distinct difference, therefore, if only in amount, between the normal disintegration of a few colourless corpuscles in the living uninjured 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 neighbourhood of the living uninjured blood vessels. The Blood Corpuscles or Blood-Cells. There are two principal forms of corpuscles, the red and the white, or, as they are now frequently named, the coloured and the colourless. In the moist state, the red corpuscles form about 45 per cent, by weight, of the whole mass of the blood. The proportion of colourless corpuscles is only as i to 500 or 600 of the coloured. Red or Coloured Corpuscles. — Human red blood-corpuscles are circular, biconcave disks with rounded edges, from ^Vo to __i__ inch in diameter, and xyJoo nicU m thickness, becoming flat or convex on addition of water. When viewed singly, they appear of a pale yellowish tinge; the deep red colour which they give to the blood being observable in them only when they are seen en masse. CHAP. IV. ] BLOOD-CORPUSI I.l>. They are composed of a colourless, structureless, and transparent filmy framework or stroma, infiltrated in all parts bya red « -< -1* »m i-ii j — matter termed haemoglobin. The stroma is tough and elasl that, as the cells circulate, they admit of elongation and other changes of form, in adaptation to the vessels, yet recover their natural Bhape as soon as they escape from compression The term cell, in the sense of a bag or sac, is inapplicable to the red blood corpuscle : and it must be considered, if not solid through- out, yet as having no such variety of consistence in different parts as to justify the notion of its being a membranous sac with fluid contents. The stroma exists in all parts of its substance, and the colouring-matter uniformly pervades this, and is not merely surrounded by and mechanically enclosed within the outer wall of the corpuscle. The red corpuscles have no nuclei, although, in their usual state, the unequal refraction of transmitted light giv< - 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 Fig. 6v At ", ". are two whi* sometimes flat or slightly convex, with a shining particle apparent like a nucleolus. In almost every specimen of Mood may Ik- 94 THE BLOOD. [chap, iv also observed a certain number of corpuscles smaller than the rest. They are termed microeytes, and are probably immature corpuscles. A peculiar property of the red corpuscles, exaggerated in inflam- matory blood, may be here again noticed, i.e., their great tendency 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. 68). 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. This tendency on the part of the red corpuscles, to form rouleaux, is probably only a physical phenomenon, comparable to the collection into somewhat similar rouleaux of discs of corks when they are partially immersed in water. (Norris.) Action of Reagents. — Considerable light has been thrown on the physical and chemical constitution of red blood-cells by study- ing the effects produced hy mechanical means and by various reagents : the following 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 indicates that there is a superficial pellicle partly differentiated from the 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 accompanied by any escape of cell contents ; the two halves, on the contrary, assume a rounded form, proving clearly that the corpuscles are not mere membranous sacs with fluid contents like fat-cells. Fluids. — Water. — When water is added gradually to frog's blood, the oval disc-shaped corpuscles become spherical, and gradually discharge their hsemoglobin, a pale, transparent stroma beinc left behind ; human red blood-cells change from a discoidal to a spheroidal form, and discharge their cell-contents, becoming quite transparent and all but invisible. Saline solution (dilute) produces no appreciable effect on the CHAP. IV.] Tin: COLOURED CORPUSCLES 95 Mammal*. Birds. Re] til.-s. Amphibia. Fish. Fig. 69. * The above illustration is some what altered from a drawing by Gulliver, in the Proceed. Z00L Societv, and exhibits the typical characters of the red blood-cells in the main divisions of the Yertebrata. The fractions are these of an inch, and represent the average diameter. In the case of the oval cells, only the long diameter is here given. It is remarkable, that although the size of the red blood-cells varies so much in the different classes of the vertebrate kingdom, that of the white corpuscles remains comparatively uniform, and thus they are, in some animals, much greater, in others much less than the red corpuscles existing side by side with them. 96 THE BLOOD. [chap. iv. red blood-cells of the fr< _ In the red blood-cells of man the discoid shape is exchanged for a spherical one, with s& £$ spinous projections, like a horse-chestnut (fig. 70). Their "^iv original forms can be at once restored by the use of Fig-. 70. carbonic acid. Acetic acid (dilute) causes the nucleus of the red blood cells in the frog to become more clearly defined ; if the action is prolonged, the nucleus becomes strongly granulated, and all the colouring matter seems to be concentrated in it, the Burrounding cell-substance and outline of the cell becom- ing almost invisible : after a time the cells lose their colour altogether. The cells in the figure (fig. 71 ) repre- sent the successive stages of the change. A similar loss of colour occurs in the red cells of human blood, which, however, from the absence of nuclei, seem to disappear entirely. Alkalies cause the red blood-cells to swell and finally disappear. 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. Tannin. — When a 2 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 : the colouring matter becomes at the same time concentrated in the nucleus, which grows more distinct (fig. 72). A somewhat similar effect i- produced on the human red blood-cell. (Robeit^. | Magenta, when applied to the red blood-cells of the frog, produces a similar little knob or knobs, at the same time staining the nucleus and causing the discharge of the haemoglobin. (Roberts.) 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 magenta, and a macula is formed at the point of escape. A similar macula is produced in the human red blood-cell. Boracic arid.- — A 2 per cent, solution applied to nucleated blood-cells (frog) will cause the concentration of all the colouring matter in the nucleus : the coloured body thus formed gradually chap. iv. 1 ACTION OF REAGENTS. gy quits its central position, and comes to be partly, sometimes entirely protruded from the surface of the now colourless cell (fig. 73). The result of this experi- ment led Briicke to distinguish the coloured con- tents of the cell (zooid) from its colourless stroma Fi (oocoid). When applied to the non-nucleated mammalian corpuscle its effect merely resembles that of other dilute acids. Gases — Carbonic acid. — If the red blood-cells of a frog be first exposed to the action of water-vapour (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 restored. The upper and lower cell in fig. 74 show the effect of carbonic acid ; the middle one the effect of the re-admission of air. These effects can be reproduced five or six times in succession. If, however, the action of the carbonic acid be much prolonged, the granulation of the nucleus becomes permanent ; it appears to depend on a coagulation of the para- globulin. (Strieker.) Ammonia. — Its effects seem to vary according to the degree of concentration. Sometimes the outline of the corpuscles becomes distinctly crenated ; at other times the effect resembles that of boracic acid, while in other cases the edges of the corpuscles begin to break up. (Lankester.) Heat— The effect of heat up to 120 — 140 F. ( 5 o c — 6o° C.) is to cause the formation of a number of bud-like processes (fig. 75). 1< '<•«! of frogs, newts, and other cold-blooded animals. Amoeboid movement.— A remarkable property of the colourless corpuscles consists in their capa- bility of spontaneously changing their shape. This - first demonstrated by Wharton Jones in the 1 •!< ■< .«1 of the skate. If a drop of blood be examined with a high power "f the microscope on a warm ji _•.', or, in other words, under conditions by which » of moisture is prevented, and at the same time the temperature is maintained at about that of the blood in its natural state within the walls of the living oo"' F. (37'8 C C), the colourless corpuscles will be rved slowly altering their shapes, and sending out pro., at various pans of their circumference. This alteration of shape, which can be m nveniently studied in the newt's blood, is called amoeboid, inasmuch as it strongly resembles the movement of the lowly organized amoeba. The | nes which are sent out are either lengthened or withdrawn. If lengthened, the proto- plasm of the whole corpuscle flows as it were into its pr and the corpuscle chang a to position; if withdrawn, protr of another process at a different point of the circumference speedily follows. The change of position of the corpuscle can also take place by a flowing movement of the whole mass, and in this 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 H 2 Thrre '. hlood-r.or- B. Thrt' hlood- rorpuscUs acted on by acetic acid ; the nuclei are very clearly ble. x 900. IOO THE BLOOD. [chap. iv. are especially likely to occur in a variety of the colourless corpuscle, which consists of masses of finely granular protoplasm with jagged outline, containing three or four nuclei, or of Large irregular masses Fig. 78. — "Human colourless blood-corpuscle, showing its successive changes of outline within ten minutes when kept moist on a warm stage. (Schotield.) of protoplasm containing from five to twenty nuclei. Another phenomenon may he 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 lengthening out and attenuation of the connection, and finally by its rupture, two cor- puscles result. The nuclei have previously undergone division. The cells so formed are said to be remarkably active in their move- ments. Thus we see that the rounded form which the colourless 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. Action of re-agents upon the colourless corpuscles.— Feeding the coiyuscles. — If some fine pigment granules, e.g., powdered vermilion, be added to a fluid containing colourless blood-cor- puscles, on a glass slide, these will be observed, under the micro- scope, to take up the pigment. In some cases colourless corpuscles have been seen with fragments of coloured ones thus embedded in their substance. This property of the colourless 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 colourless corpuscles possess of passing through the walls of the blood-vessels will be described later on. Enumeration of the Red and White Corpuscles. — Several methods are employed for pounting the blood-corpuscles, most of them depending, upon the same principle, i.e., the dilution of a minute volume of blood with a given volume of a colourless solution similar in specific gravity to blood serum, so that the size and shape ' II \ I ENUMERATION OF THE CORPUS* IOI of the corpuscles is altered as little as possible. A minute quantity of the well-mixed solution is theo taken, examined under the mien - ther in a flattened capillary tube I M.il or in a cell (Hayem & Nachet, G of known capacity, and the number of corpuscles in a measured length of the tul _ ven area of the cell is counted. The Length of the tube and the area of the cell are ascertained by means of a micron scale in the microsc Jar ; or in the i - vers 1 modifi- Fig. 79. — Ham' cation, by the division of the eell area into squares of known size. Having ascertained the number of corpuscles in the diluted 1. it is easy to rind out the number in a given volume of normal blood. Growers' modification of Havem & Nachetfs ■ instrument, called by him u Hccmacytomrter" appears to be the most convenient form of instrument for counting the cor- puscles, and as such will alone be described (fig. 79). It consists of a small pipette (a), which, when filled up to a mark on its stem, holds 995 cubic millimetres. It is furnished with an india- rubber tube and glass mouth-piece to facilitate filling and empty- a capillary tube (b) marked to hold 5 cubic millimetres, and 102 THE BLOOD. [chap. iv. also furnished with an india-rubber tube and mouthpiece ; a small glass jar (d) in which the dilution of the blood is performed ; a glass stirrer (e) 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 finger 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 ex- amined under a microscope with a power of about 400 diameters, and focussed until the lines dividing the cell into squares are visible. 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 milli- metre) squares 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 Yierordt and Welcker, is 5,000,000 in adult men, and rather fewer in women. Chemical Composition of the Blood in Bulk. Water 784 Solids- Corpuscles 130 Proteids (of serum) . . . . 70 Fibrin (of clot) 2-2 Fatty matters (of senim) . . . . 1-4 Inorganic salts (of serum) ... 6 Gases, kreatm, urea and other extractive ) matter, glucose and accidental sub- > 64. — stances ' 216 1,000 chap. iv. J CHEMICAL COMPOSITION. 203 Chemical Composition of the Red Corpuscles. — Anal of a thousand parts of moist blood corpuscles shows the following a^ the result : — Solid — I Organic 303SS ( Mineral Si 2 — ^12 1. 000 Of the solids the most important is Ha , th substance to which the blood owes its colour. It constitutes, as will be 5 from the appended Table, more than 90 per eeut. 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 eh rt n and lecithin. In 1000 parrs organic matter are found : — Haemoglobin ........ 905*4 Proteids 867 F: "~ 7'9 I.OOO' Of the inorganic salts of the corpuscles, with the iron omitted — In 1000 pans corpuscles (Schmidt) are found : — Potassium Chloride Phosphate sulphate Sodium Calcium _nesium 679 343 094 060 34i 7'2&2 The properties of haemoglobin will be considered in relation to the Gases of the blood. * An account of the proteid bodies, kc, will be found in the Appendix, an I should be referred to for explanation of the terms employed in the text. 104 THE BLOOD. [chap, iv, Chemical Composition of the Colourless Corpuscles.— In consequence of the difficulty of obtaining colourless corpuscles in sufficient number to make an analysis, little is accurately known of their chemical composition ; in all probability, however, the stroma of the corpuscles is made up of proteid matter, 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 (globulin) is soluble in a ten per cent, solution of sodium chloride, and the solution is 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 phosphate is in greatest amount. Chemical Composition of the Plasma or Liquor Sanguinis. — The liquid part of the blood, the plasma or liquor sanguinis in which the corpuscles float, may be obtained in the ways mentioned under the head of the Coagulation of the Blood. In it are the fibrin factors, inasmuch as when exposed to the ordinary tem- perature 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 senmi obtained from it. It may be freed from white corpuscles by filtration at a temperature below 41 F. (5°C). Fibrin. — The part played by fibrin in the formation of a clot has been already described (p. 81), and it is only necessary to consider here its general properties. It is a stringy elastic sub- stance belonging to the proteid class of bodies. It i.s insoluble in water and in weak saline solutions, it swells up into a trans- parent jelly when placed in dilute-hydrochloric acid, but does not dissolve, but in strong acid it dissolves, producing acid-albumin * ; it is also soluble on boiling in strong saline solutions. Blood contains only '2 per cent, of fibrin. It can be converted by the * 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. CHAP. !V.J COMPOSITION OF SERUM. IO: r pancreatic jui I peptone. It \ of liberating the oxygen from solutions of hydric 1 l< » . Tins may be Bhown by dipping a few Bhreda "f rihrin in tincture laiacum and then immersing them in a Bolntion of hydric ode. The fibrin h< bluish colour, from its haying liberated from the solution _ o, which oxidises the resin of guaiacum contained in tlie tincture and thus produces the © Salts of the Plasma. — In iooo parts plasma tl Sodium Chloride ...... Soda Sodium Phosphate ..... Potassium chloride ...... sulpha: ..... Icium phosphate ...... Magnesium phosphate ..... ere are 5-546 1-532 ■271 •359 •281 •298 ■218 5S05 Serum. — The serum is the liquid part of the blood or of the plasma remaining after t: 5 ration of the clot It is alkaline, yellowish, transparent fluid, with a specific gravity of from 1025 to 1032. In the usual mode of coagulation, -■nun remains in the clot, and the rest, squeezed from the Lot by ite 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 - there ia clot in coagulated blood. Chemical Composition of the Serum. about 900 Proteids : a. Serum-albumin ..... &. Paraglobulin ...... J Salts. — including fatty adds, eholesterin. lecithin ; and some soaps ....... Grape sugar in small amount ..... Kxtractives — kreatin. kreatinin. urea. a:c. Yellow pigment, which is independent of haemoglobin Gases — small amounts of oxygen, nitrogen, and car- bonic acid ........ So 20 I coo 106 THE BLOOD. [chap. iv. Water. — The water of the serum varies in amount according to the amount of food, drink, and exercise, and with many other circumstances. Proteids. — a. Serum-albumin is the chief proteid found in serum. It is precipitated on heating the seruni to 140 F. (6o° C), and entirely coagulates at (167 F. 75° C), and also by the addition of strong acids, such as nitric and hydrochloric ; by long contact with alcohol it is precipi- tated. It is not precipitated on addition of ether, and so differs from the other native albumin, viz., which the liquid is subjected. And conversely, if a liquid containing a gas in solution be exposed to an _ aoneofthe gas, the gas will he given up to the atmosphere until its amount in the liquid and in the atmosphere equal. This condition is called a condition of equal ten- The condition may be understood by a simple illustration. A large amount of carbonic acid gas is lissolved in a bottle of water by exposing the 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 there is a tendency of the _ - ' -.ape into the atmosphere, in order that the tension may be relieved : this causes the violent expulsion of the cork when the wire is removed, and if the water be placed in a 2 as th( gas will continue to be evolved until it U st all got rid of, 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. Now the oxygen of the blood does not obey this law of pressure. For if blood which contains little or no oxygen be exposed * succession of atmospheres containing more and more of that _ s, we find that the absorption is at first very great, but soon becomes relatively very small, not being therefore regularly in | >n to the increased amount (or tension) of the oxygen of the atmosph and that conversely, if arterial blood be submitted to regularly diminishing pressures 1 if oxygen, at first veiy little of the contained oxygen is uiven off to the atmosphere, then suddenly the _ - escapes with great rapidity, again disobeying the law of pressures. Very little oxygen can be obtained from serum freed from blood corpuscles, even by the strongest mercurial air-pump, neither can serum be made to absorb a large quantity of that gas ; but the small 112 THE BLOOD. [chai\ iv. 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 solu- tion. The chief solid constituent of the coloured corpuscles is haemoglobin, 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 favourable circum- stances, 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 haemoglobin, and not simply dissolved ; but inasmuch as it is comparatively 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 constitutes by far the largest portion of the coloured corpuscles. It is intimately distributed throughout their stroma, and must be dissolved out of it before it will undergo crystallization. Its percentage composition is C. 53*85 ; H. 7-32 ; N. 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 C^, H^, N I54 , Fe S 3 I79 . 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 possesses the power of crystallizing to very different extents (blood-crystals). In some animals the formation of crystals is almost spontaneous, whereas in others crystals are formed either with great difficulty or not at all. Among the animals whose blood colouring-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 favour the formation of the crystals. In many instances other means must be adopted, e.g., the addition of alcohol, ether, or chloroform, rapid freezing, and then thawing, an electric current, a temperature of 140 F. (6o° C), or the addition of sodium sulphate. CJIA1'. IV.] HEMOGLOBIN. 113 Human blood crystallizes with difficulty, as does also that of the ox. the pig, the sheep, and the rabbit Tig. 81.— Crystals of oxy-hamoglobin— prismatic from human blood. The forms of haemoglobin crystals, as will be seen from the appended figures, differ greatly. * 4> ' y • V •'• J J,£&- .. : l? \% ► > - i%. ^^^rt^ir^l:^^ ^^SSS^ST^SSSL 'dral, from blood of the guinea-pig. f ^oodofsauu^eV: On these hex* - IS plates, prismatic crystals, grouped inT stellate manner, not unfrequently occur [after Funke . Haemoglobin crystals are soluble in water. Both the crystals themselves and also their solutions have the characteristic colour of arterial blood. 114 THE BL00r) - [chap. iv. A dilute solution of haemoglobin gives a characteristic appear- ance with the spectroscope. Two absorption bands are seen between the solar lines d and e (see plate), one towards the red, with its middle line some little way to the blue side of d, is very intense, but narrower than the other, which lies near to the red side 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 unabsolved, and part of the red, and on further increase of strength the former disappears. If the crystals of oxy-heemoglobin be subjected to a mercurial air-pump they give off a definite amount of oxygen (i gramme giving off i*59 c. cm. of oxygen), and they become of a purple colour ; and a solution of oxy-haernoglobin may be made to give up oxygen and to become purple in a similar manner. This change may be also effected by passing through it hj-drogen or nitrogen gas, or by the action of reducing agents, of which Stokes's fluid* is the most convenient. With the spectroscope, a solution of deoxidized haemoglobin 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 absorbed. Even in strong solutions this latter ap- pearance is found, thereby differing from the strong solution of oxidised haemoglobin 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 deoxidised crystals or their solutions quickly absorb oxygen on exposure to the air, becoming scarlet. If solutions of blood be taken instead of solutions of haemoglobin, results similar to the whole of the foregoing can be obtained. * Stokes's Fluid consists of a solution of ferrous sulphate, to which ammonia has been added and sufficient tartaric acid to prevent precipita- tion. Another reducing agent is a solution of stannous chloride, treated in a way similar to the ferrous sulphate, and a third re-agent of like nature is an aqueous solution of ammonium sulphide. ABSORPTION SP 1 Spectrum of Aiv rraunhofer's 2. Blood; i. e a suronq solution of Oxyhemoglobin & reduced Haemogl:. 3. Blood more ii. 4Redu ed - - uogioLin G & Sulphuretted Hydrogen jam and cole it in Chlor 5 le compc 7 . [ fpectra draunfrom obscrixrtians 6i .V H . Z 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-corpuscles are developed. Here the nucleated embryonal 4 ■■©■■■ Fig. 86. — Jtorf o/tfc network of developing blood-vessels in the vascular area of a guinea-pig. hi, blood-corpuscles becoming free in an enlarged and hollowed out part of the net- work ; o, process of protoplasm. (E. A. Schiifer.) cells of the mesoblast, from which the blood-vessels and cor- puscles are to be formed, send out processes in various directions, and these joining together, form an irregular 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 becoming coloured, form the red blood corpuscles. The protoplasm of the cells and their branched network in which these corpuscles lie then becomes hollowed out into a system of canals enclosing fluid, in which the red nucleated corpuscles float. The corpuscles at first are from about aB x 6tf to xriny °^ an mcn m diameter, mostly spherical, and with granular contents, and a well-marked nucleus. Their nuclei, which are about -^Vo of an 120 THE BLOOD. [CHAP. IV. inch 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 colourless corpuscles of the fully developed blood, but are coloured. 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 colourless and 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 coloured nucleated cells begin very early in foetal life to be mingled with coloured 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 Red 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 I .-■ •- . — Bevelcj From the subcutaneous :>sue of a new-born rat. h, a cell containing hfenioglobin in a diffused form in the protoplasm : /<', one containing coloured globules of varying size and vacuoles ; h" . a cell filled ■with coloured eiobules of nearlv uniform size : f. r", developing fat cells. E. A. Schafer.) their origin are the following: — (i.) During fcetal life and possibly in some animals, 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 in the following way. Small globules, ' BAP. iv. | DEVELOPMENT OF THE BLOOD. 121 of varying Bize, ofoolouring matter arise in the protoplasm "f tho cells, and the cells themselves become branched, their branches joining the branches of similar cells. The culls next become vacuolated, and the red globules are free in a cavity filled with fluid (fig. S8) ; by the extension of the cavity of the cells into their processes anastomosing vessels are produced, which ultimately join with the previously existing vessels, and the globules, now having the size and appearance of the ordinary red corpuscles, are ^ J. — Further development of blood-corpuscles in <:oi, - "nd transformation of the latter into capillary bli - -. a, an elongated cell with a cavity in the proto- plasm occupied by fluid and by blood-corpuscles which are still globuiar; b, 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 discord ; c, shows the mode of union of a " htemapoietic " cell, which, in this instance, contains only one corpuscle, with the prolongation [hi] of a previously existing vessel ; a and c, from the new-born rat ; b, from the foetal sheep. E. A. Scnafer.) passed into the general circulation. This method of formation is called intracellular (Schafer). (2.) From the white corpuscles. — The belief that the red cor- puscles are derived from the white is still very general, although no new evidence has been recently advanced in favour of this view. It is, however, uncertain whether the nucleus of the white corpuscle becomes the red corpuscle, 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. 122 THE BLOOD. [chap. iv. (3.) From the medulla of bones. — Red corpuscles are to a very large extent derived during adult life from the large pale cells in the red marrow of bones, especially of the ribs (figs. 44, 89). These cells become coloured from the formation of haemoglobin chiefly in one part of their protoplasm. This coloured part becomes sepa- Fig. 89. — Coloured nucleated corpuscles, from the red marrow of the guinea-pig. (E. A. ScMfer.) rated 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 red as well as white corpuscles may be produced in the spleen. (5.) From Microcytes. — Hayem describes the small particles (microcytes), previously mentioned as contained in the blood (p. 94), and which he calls hsematoblasts, as the precursors of the red corpuscles. They acquire colour, 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 red corpuscles undergo disintegration in the spleen ; and indeed corpuscles in various degrees of degeneration have been observed in this organ. Origin of the Colourless Corpuscles. — The colourless corpuscles of the blood are derived from the lymph corpuscles, being, indeed, indistinguishable from them ; and these come chiefly from the lymphatic glands. Their number is increased by division. Colourless corpuscles are also in all probability derived from the spleen and thymus, and also from the germinating endothe- chap, iv.] r>i:> OF THE BLOOD, j 23 limn of serous membranes, and from connective tissue. The corpuscles are carried into the blood either with the lymph and ohyle, or pass directly from the Lymphatic tissue in which they have been formed into the neighbouring blood-vessels. Uses of the Blood. 1. 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. Uses of the various Constituents of the Blood. Albumen. — Albumen, which exists in so large a proportion among the chief constituents of the "blood, is without doubt mainly for the nourishment of those textures which contain it or other compounds nearly allied to it. Fibrin. — In considering the functions of fibrin, we may exclude the notion of its existence, as such, in the blood in a fluid state, and of its use in the nutrition of certain special textures, and look for the explanation of its functions to those circumstances, whether of health or disease, under which it is produced. In haemorrhage, for example, the formation of fibrin in the clotting of blood, is the means by which, at least for a time, the bleeding is restrained or stopped ; and the material or blastema which is produced for the permanent healing of the injured part, contains a coaguahle material identical, or very nearly so, with the fibrin of clotted blood. Fatty Matters.— The fatty matters of the blood subserve more than one purpose. For while they are the means, in part, by which the fat of the body, so widely distributed in the proper 124 CIRCULATION OF THE BLOOD. [chap. v. adipose and other textures, is replenished, they also, by their union with oxygen, assist in maintaining the temperature of the body. To certain secretions also, notably the milk and bile, fat is contributed. Saline Matter. — The uses of the saline constituents of the blood are, first, to enter into the composition of such textures and secretions as naturally contain them, and, secondly, to assist in preserving the due specific gravity and alkalinity of the blood, and in preventing its decomposition. The phosphate and carbonate of sodium, to which the blood owes its alkaline reaction, increase the absorptive power of the serum for gases. Corjyuscles. — The important use of the red corpuscles is in relation to the absorption of oxygen in the lungs, and its convey- ance to the tissues. How far the red corpuscles are actually concerned in the nutrition of the tissues is quite unknown. The relation of the colourless corpuscles to the coagulation of the blood has been already considered; of their functions, other than are concerned in this phenomenon, and in the regeneration of the red corpuscles, nothing is positively known. CHAPTEE Y. THE CIRCULATION OF THE BLOOD. The Heart is a hollow muscular organ containing four cham- bers, two auricles and two ventricles, arranged in pairs. On each side (right and left) of the heart is an auricle joined to and communicating with a ventricle, but the chambers on the right side do not directly communicate with those on the left side. The circulation of the blood is chiefly carried on by the contrac- tion of the muscular walls of these 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 by the arteries, and returned to the right side of the heart by the veins, the arteries and veins being continuous with each other at one end en. v.: THE HEART. 125 by means of the heart, and at the other by a fine network of v- called t!. The blood, th< s from the heart paaaea first into the arteries, then into the capill and lastly into the veins, by which it is conveyed back again to the heart, thus completing a n 1 lotion. F:_ . go. — Diagram of the Circulation. The right side of the heart d'.»es not directly coinruunicate with the left to complete the entire circulation, but the blood has to pass from the right side to the lungs, through the pulmonary art then through the pulmonary capUlary-vese Is aid through the pulmonary veins to the left side of the heart. Thus there are two circulations by which the bloc- ffl ; the one, a shorter circuit from the right aide ft heart t<> the lungs and back again to the left side of the heart : the other and larger circuit. from the le of the heart to all parts of the body and back 126 CIRCULATION OF THE BLOOD. [•HAP. V. again to the right side : but more strictly speaking, there is only one complete circulation, which may be diagrammaticallv represented by a double loop, as in the accompanying figure (fig. 90). 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 Pulmonary Artery. Diaphragm F - : — I lungs in situ. The front portion of the chest-wall, and the onter or yexs of the pleurce and pericardium have been removed. The lungs are partly collapsed. 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 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 completes a revolu- tion. This subordinate stream through the liver is called the Portal circulation. chap. v.J THE PERICABDIUM. l2 j The Forces concerned in the Circulation of the Blood.— (i) The principal force provided for constantly moving the blood through the course of the circulation is that of the muscular sub- stance of the heart ; Other assistant forces are (2) those of the elastic walls of the arteries, (3) the pressure of the muscles among which some of the veins run, (4) the movements of the walls of the chest in respiration, and probably, to some extent, (5) the interchange of relations between the blood and the tissues which occurs in the capillary system during the nutritive processes. The Heart. The Pericardium. — The heart is invested by a membranous sac — the pericardium, which is made up of two distinct parts, an rnal fibrous membrane, composed of closely interlacing fibres, which has its base attached to the diaphragm — 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 <»ut of the heart receive more or less investment from this sac. The heart is situated in the chest behind the sternum and costal cartilages, 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 backwards, inwards, ami towards the right. It rests upon the diaphragm, and its pointed apex, formed exclu- sively 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 from 128 CIRCULATION OF THE BLOOD. [chap. v. 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 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 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 itself with blood, as well as nerves and lymphatics imbedded in more or less fatty tissue. 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 respec- tively, as already incidentally mentioned, auricle and ventricle, which freely communicate one with the other ; the aperture of communication, however, being guarded by valves, so disposed as to allow blood to pass freely from the auricle into the ventricle, but not in the opposite direction. There are thus four cavities altogether in the heart — two auricles and two ventricles; the auricle and ventricle of one side being quite separate from those of the other (fig. 90). Right Auricle. — The right auricle is situated at the right part of the base of the heart us 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 CHA1 ( HAMBERS OF THE EEART. Inferior venae cavae, or greal veins, which convey the blood from all parta of the body to the heart Tin' former is directed down- wards and forwards, the latter upwards and inwards; between Fig. 92.— The right auricle ■', and a part of their right and anterior Us removed, -how their interior, |.— i, superior vena cava; 2, inferior vena it Bhort : 3, light auricle ; 3', placed in the fossa ovalis, below which is the Eustachian valve ; j ', is placed close to the aperture of the coronary vein ; — . — . placed in the auriculo- ventricular groove, where a narrow portion of the adja- cent 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 pulmonarv 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 entrances of these vessels is a Blight tubercle called tubercle of Lower. The opening of the inferior cava is protected and partly covered by a membrane called the Eustachian valve. In 130 CIRCULATION OF THE BLOOD. [chap, v, 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 fcetal 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 endo- cardium, and on the anterior wall of the auricle are similar elevations arranged parallel to one another, called musculi pectinati. Right Ventricle. — The right ventricle occupies the chief part of the anterior surface of the heart, as well as a small part of the posterior 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, is semilunar or crescentic (fig. 94) ; into it are two openings, the auriculo-ventricular at the base, and the opening of the pulmonary artery also at the base, but more to the left ; the part of the ventricle leading to it is called the conus arteriosus or infundibuium ; 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 columnee carnece (described at length p. 135). Left Auricle. — The left auricle is situated at the left and posterior part of the base of the heart, and is best seen from behind. It is quadrilateral, and receives on either side two pul- monary 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 interior 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-ventricular 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 auricles, which exists in foetal life, on the septum between them. 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 i H.\: CHAMBERS OF THE BEART 131 the valves corresponding to th the right Bide of ti viz. the bicuspid or mitral and the semilunar or Fig. 93. — The k/i 'ride opened, and a part of their anterior and removed. A.— The pulmonary artery has been divided at its commencement : opening into the left ventricle carried a short distance into the aorta between two of the segments of the semilunar valves : and the left part of the auricle with its appendix has been removed. The right auricle is out of view. 1, the two right pulmonary veins 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 ■ 2, a narrow portion of the wall of the auricle and ventricle preserved roun . auriculo-ventricular orifice ; 5. 5', the cut surface of the walls of the ventricle, seen to become very much thinner towards 3 ", at the ap- 1 the anterior wall of the left ventricle which has been preserved with the principal anterior columna carnea or musculus papillaris attached to it ; 5. 5, musculi papLL of the septum, between the two ventricles, within the cavity 01 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 axe hanging loosely toge- ther ; 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 rema ining attached to the aorta by 9, the cord of the duct sos; 10, the arteries rising from the summit of the aortic arch 'Allen Thomson, . 132 CIRCULATION OF THE BLOOD. [chap. v. Fig. 94. — Transverse section of bullock'' s heart in a state of cadaveric rigidity. «, cavity of left ventricle, b, cavity of right ventricle. (Dalton.) 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 co- lumnar carnese, 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 ventricle, which are nearly half an inch in thick- ness, are, w T ith the exception of the apex, twice or three times as thick as those of the right. Capacity of the Chambers. — The capacity of the two ven- tricles 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 auricles is rather less than that of the ventricles : the thickness of their walls is considerably less. The latter condition is adapted to the small amount of force which the auricles require in order to empty themselves into their adjoining ventricles ; the former to the circumstance of the ventricles being partly filled with blood before the auricles contract. Size and Weight of the Heart. — The heart is about 5 inches long, 3! inches greatest w T idth, and 2\ 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 diminishes in old age. Structure. — The Avails 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 in- serted between each auricle and ventricle, and forms the boundary of the auriculo-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, how- ever, quite separate from those of the ventricles, the bond of (II IP. \ . I STKrcTl'ltK OF T1IK EtEART. 133 connection between them being onlj the fibrous tissue of the auriculo-ventricular openings. Fig. 95.— Network of muscular fibres (striated) from the heart of ;t pig. The nuclei of the muscle-corpuscles are well shown, x 450. (Klein and Noble Smith.) The muscular fibres of the heart, unlike those of most of the involuntary muscles, are striated ; but although, in this respect they resemble the skeletal muscles, they have distinguishing characteris- tics of their own. The fibres which lie side by side are united at frequent intervals by short brandies (fig. 95). 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 certain con- ditions subdivided into oblong por- tions or "cells," the offsets from ^'£SS^ fSA.f^ 134 CIRCULATION OF THE BLOOD. [CHAP. V. winch are the means by which the fibres anastomose one with another (fig. 96). Endocardium.— As the heart is clothed on the outside by a thm transparent layer of pericardium, so its cavities are lined by a smooth and shining membrane, or endocardium, which is directly continuous 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 tessellated layer of flattened endothelial cells. Here and there unstriped muscular fibres are sometimes found in the tissue of the endocardium. Course of the Blood through the Heart.— The arrange- ment of the heart's valves is such that the blood can pass only in one direction, and this is as follows (fig. 97) :— From the right Fig- 9T-— Din yia in of the circulation through tht heart (Dalton). auricle the blood passes into the right ventricle, and thence into the pulmonary artery, by which it is conveyed to the capillaries of the lungs. From the lungs the blood, which is now purified and altered in colour, is gathered by the pulmonary veins and taken to the left auricle. From the left auricle it passes into the left ventricle, and thence into the aorta, by which it is distributed to the capillaries chap, v.] VALVES OF THE EEAET. 135 of every portion of the body. The branches of the aorta, from being distributed to the general system, are called spstt mic arterii and from these the blood passes into the systemic capillaries, where it again becomes dark and impure, and thence into the branch* of the systemic veins, which, forming by their union two lai trunks, called the superior and inferior vena cava, discharge their contents into the right auricle, whence we supposed the blood to start The Valves of the Heart. — The valve between the righl auricle and ventricle is named tricuspid (5. fig. 99), because it presents thra principal cusps or subdivisions, and that between the left auricle and ventricle bicuspid (or mitral), because it b two such portions (6, fig. 93). But in both valves there is between each two principal portions a smaller one ; so that more properly, the tricuspid may be described as consisting of six, and the mitral «>1' four, ]K »it ions. Eacli portion is of triangular form, its apex and sides lying free in the cavity of the ventricle, and its base, which oiitinuons with the bases of the neighbouring portion a to form an annular membrane around the anriculo-ventrieular open- ing, being fixed to a tendinous ring which encircles the orifice between the auricle and ventricle and receives the insertions of the muscular fibres of both. In each principal cusp may be distinguished a middle-piece, extending from its base to its apex, and including about half its width, which is thicker, and much tougher and tighter than the border-pieces or edges. AVhile the bases of the several portions of the valves are fixed to the tendinous rings, their ventricular surfaces and borders are fastened by slender tendinous fibres, the chorda: tendinece, to the walls of the ventricles, the muscular fibres of which project into the ventricular cavity in the form of bundles or columns — the columnce cameos. These columns are not all of them alike, for while some of them are attached along their whole length on one side, and by their extremities, others are attached only by their extremities ; and a third set, to which the name muscttli papillartt has been given, are attached to the wall of the ventricle by one extremity only, the other projecting, pa] 'ilia-like, into the cavity of the ventricle (5, fig. 93), and having attached to it chorda ten- dinece. of the tendinous cords, besides those which pass from the walls of the ventricle and the musculi papillares to the margins of ! 36 CIRCULATION OF THE BLOOD. [chap. v. 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 arc spread out in the substance of the valve, giving its middle piece its pecu- liar strength and toughness ; and from the sides numerous other more slender and branching cords arc 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 principal 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 con- siderably thicker and stronger than the tricuspid, in accordance with the greater force which it is called upon to resist. It has been already said that while the ventricles communicate, on the one hand, with the auricles, they communicate, on the other, with the large arteries which convey the blood away from the heart ; the right ventricle with the pulmonary artery (6, fig. 93), which conveys blood to the lungs, and the left ventricle with the aorta, which distributes it to the general system (7, fig. 93). And as the auriculo-ventricular orifice is guarded by valves, so are also the mouths of the pulmonary artery, and aorta (figs- 93> 99)- The semilunar valves, three in number, guard the orifice of each of these two arteries. They are nearly alike on both sides of the heart ; but those of the aorta are altogether thicker and more strongly constructed than those of the pulmonary artery, 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. 93). In the centre of the free edge of the valve, which contains a fine cord of fibrous tissue, is a small fibrous nodule, the corjms Arantii, and from this and from the attached border fine fibres extend into every part of the mid sub- CHAP, v.] ACTION OF THE II i:\KT. ^7 Stance Of the valve, except a small I u l i;t t < < I space jusl v, itliin 1 1 n ■ free edge, on each side of the corpus Arantii. Here the valve is thinnest, and composed of little more than the endocardium. Tims constructed and attached, the three semilunar valves are placed side by side around the arterial orifice of each ventricle, SO a- to form three little pouches, which can he separated by the hlood passing out of the ventricle, hut which immediately afterwards are pressed together so as to prevent any return (7, rig-. 93, and 7, fig. 99). This will he 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. Structure, of the Valves. — The valves of the heart are formed essentially of thick layers of closely woven connective and elastic tissue, over which, on every part, is reflected the endocardium. The Action of the Heart. The heart's action in propelling the blood consists in the suc- cessive alternate contraction (systole) and relaxation (diastole) of the muscular walls of its two auricles and two ventricles. Action of the Auricles. — The description of the action of the heart may best be commenced at that period in each action which immediately precedes the beat of the heart against the side of the chest. For at this time the whole heart is in a passive state, the walls of both auricles and ventricles are relaxed, and their cavities are being dilated. The auricles are gradually filling with blood flowing into them from the veins ; and a portion of this blood passes at once through them into the ventricles, the opening between the cavity of each auricle and that of its 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 commences at the entrance of the great veins into them, and is thence propagated towards the auriculo- ventricular opening ; but the last part which contracts is the auricular appendix. The 138 CIIICULATIOX OF THE BLOOD. [chap. v. effect of this contraction of the auricles is to quicken the flow of blood from them into the ventricles ; the force of their contraction not being sufficient under ordinary circumstances to cause any back-flow into 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 1 tack wards ; 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 vein-;. During the auricular contraction the force of the blood pro- pelled 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 auriculo-ventricular valves, which, when the ventricle is full, form a complete septum between it and the auricle. Action of the Ventricles. — The blood which is thus driven, by the contraction of the auricles, into the corresponding ven- tricles, 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 probabty always thoroughly empty themselves, differing in this respect from the auricles, in which, even after their complete contraction, a small quantity 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 diminish- ing the diameter of the base, especially in the plane of the auriculo- ventricular valves ; but the length of the heart as a whole is not CHAP. V.] IT.\(TI<>.\ OF THE IIKAltTS YAI.YKS. , yj altered. (Ludwig.) During the systole of the ventricles, boo, the aorta and pulmonary artery, being filled wit f i blood by the force of the ventricular action against considerable resistance, elongate as well us expand, and the whole heart moves slightly towards the right and forwards, twisting <>n its Long axis, and exposing inert' of the left ventricle anteriorly than is usually in front. When the systole ends the heart resumes its forme)- position, rotating to the left again as the aorta and pulmonary artery contract. Functions of the Auriculo-Ventricular Valves. — The distension 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 reflex current caused by the systole of the auricles. This elevation of the auriculo-ventricular valves is, no doubt, 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 especially marked in the neighbour- hood of the auriculo-ventricular rings, and thus 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 another, by the simultaneous contraction of the musculi papillares, whose chordae tendincae have a special mode of attachment for this object (p. 136). As in the case of the semilunar valves to be immediately described, the auriculo-ventricular valves meet not by their edges only, but by the opposed surfaces of their thin outer borders. The semilunar valves, on the other hand, which are closed in the intervals of the ventricle's contraction (fig. 92, 6), are forced apart by the same pressure that tightens the auriculo- ventricular valves; and, thus, the whole force of the contracting ventricles is directed to the expulsion of blood through the aorta and pulmonary artery. 140 CIRCULATION OF THE BLOOD. [chap. V. The form and position of the fleshy columns on the internal walls of the ventricle no douht help to produce this obliteration of the cavity during their contraction ; and the completeness of the closure may often be observed on making a transverse section of a heart shortly after death, in any case in which the contraction of the ri'ior mortis is very marked (fig. 94). 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 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 pulmonary circulation. It has been found to be equal to 23 mm. of mercury, and is quite independent of the aspiration or suction power of the thorax in aiding the blood-flow to the heart, to be described in the chapter on Respiration. Function of the Musculi Papillares. — The special function of the mv&nli papillares is to prevent the auriculo-ventricular valves from being everted into the auricle. For the chorda? tendinea? 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-ventricular 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 safety-valve action of this valve. The circumstances in which it usually happens are those in which chap. v.| SEMILUNAB VALVES. 141 the vessels of the lung arc al ready full enough when the right ventricle contracts, as <.;/., in certain pulmonary diseases, in very active exertion, ami in great efforts. In these eases, the tricuspid valve does qoI completely close, and the regurgitation of the blood may be indicated by a pulsation in the jugular veins synchronous with that in the carotid arteries. Function of the Semilunar Valves. — The arterial or semi- lunar valves are forced apart by the out-streaming blood, with which the contracting ventricle dilates the large arteries. The dilation of the arteries is, in a peculiar manner, adapted to bring the valves into action. The lower borders of the semi- lunar valves are attached to the inner surface of a 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 of tough, close-textured, fibrous tissue, with strong interwoven cords, and covered with endocardium. Hence, when the ventricle propels blood through the orifice and into the canal of the artery, the lateral pressure which it exercises is 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 from the ventricle is, that the wall of the first portion 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. 98, b). Their positions may be explained by the diagrams, in which the continuous lines represent a transverse section of the arterial walls, the dotted ones the edges of the valves, firstly, when the valves are nearest to the walls (a), and, secondly, when, the walls being dilated, the valves are drawn away from them (b). 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 circulation. Part of the blood thus forced back 142 CIRCULATION OF THE BLOOD. [chap. v. lies in the pouches (sinuses of Valsalva) (a, fig. 98, b) between the valves and the arterial walls ; and the valves are. by it pressed Fig. 98. — Sections of aorta, to show the action of the semilunar valves. A is intended to show the valves, represented by the dotted lines, pressed towards the arterial walls, represented by the continuous outer line, b (after Hunter) shows the arterial wall distended into three pouches («), and drawn away from the valves, which are straightened into the fomi of an equilateral triangle, as represented by the dotted lines. together till their thin lunated margins meet in three lines ra- diating from the centre to the circumference of the artery (7 and 8, fig. 99). Fig. 99. — Vieto of the base of the ventricular part oftfu heart, showing the relative position of the arterial and auriculo-ventricular orifices. — §. The muscular fibres of the ven- tricles 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 valves, and the auricles have been removed immediately above the auriculo-ventricular orifices. The semilunar and auriculo- ventricular "valves are in the nearly closed condition, i, i, 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, that of the left ; 5, 5', 5", the tricuspid valve : 6, 6', the mitral valve. In the angles between these segments are seen the smaller frii frequently observed ; 7, the anterior part of the pulmonary artery ; 8, placed upon the posterior part of the root of the aorta ; 9, the right, 9', the left coronary artery. (Allen Thomson.) The contact of the valves in this position, and the complete closure of the arterial orifice, are secured by the peculiar con- chap, v.l SEMILUNAR VALVES. 143 BtructioD of their borders before mentioned. Amoi sords which are interwoven in the substance of the valves, are two of greater strength and prominence than the real ; of which one extends along the free border of each valve, and the other forma 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 tree margin of its valve, and in the middle to the corpus Arantii ; they thus enclose a lunated space 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. When the valves are pressed down, all these parts or spaces of their surfaces come into contact, and the closure of the arterial orifice is thus secured by the apposition not of the mere ed'^s of the valves, but of all those thin lunated parts of each which lie between the free c _ and the cords next below them These parts are firmly pr- _ ther, and the greater the pressure that falls on them thee and more secure is their apposition. The corpora Arantii meet at Fig. 100. — I turn through the aorta at its junction with the left ventricle, a. Section of aorta, bb, Section of two valves, c, Section of wall of ventricle. Internal surface of ventricle. 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 pul- monary 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. 144 CIRCULATION OF THE BLOOD. [chap. v. It has been clearly shown that this pressure of the blood is not entirely sustained by the valves alone, but in part by the muscular substance of the ventricle (Savory). By making vertical sections (fig. ioo) through various parts of the tendinous rings it is possible to show clearly that the aorta and pulmonary artery, expanding towards their termination, are situated upon the outer edge of the thick upper border of the ventricles, and that consequently the portion of each .semilunar 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 ventricular 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 great venous trunks. They are thus filling during all the time in which the ventricles are contracting: and the contraction of the ventricles being ended, these also again dilate, and receive again the blood that flows into them from the auricles. By the time that the ventricles are thus from one-third to two-thirds full, the auricles are distended ; these, then suddenly contracting, fill up the ventricles, as already described (p. 137). Cardiac Revolution. — If we suppose a cardiac revolution divided into five parts, one of these will be occupied by the con- traction of the auricles, two by that of the ventricles, and two by repose of both auricles and ventricles. Contraction of Auricles . . . 1 + Kepose of Auricles . . .4 = 5 „ Ventricles . . 2 + ., Ventricles . .3 = 5 Kepose (no contraction of either auricles or ventricles) . . . 2 + Contraction (of either auri- — cles 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 * Savory's preparations, illustrating this and other points in relation to the structure and functions of the valves of the heart, are in the Museum of St. Bartholomew's Hospital. chap, v.] SOUNDS OF THE HEART. 145 occupies very much the same time, aboul /,, Bee, 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- ventrieular valves are closed, and the arterial valves are open during the whole time of the ventricular contraction, while, during the dilation and distension of the ventricles the latter valves are shut, the former open. Thus whenever the auriculo- ventrienlar valves are open, the arterial valves are closed and id. Sounds of the Heart. When the ear is placed over the region of the heart, two sounds may he heard at every beat of the heart, which follow in quick cession, and are succeeded by a pause or period of silence. The first sound is dull and prolonged ; its commencement coincides with the impulse of the heart, and just precedes the pulse at the wrist. The second is a shorter and sharper sound, with a some- what flapping 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 rela- tive length of time occupied by each sound, as compared with the other, is a little uncertain. The difference may be best appre- ciated by considering the different forces concerned in the pro- duction of the two sounds. In one case there is a strong, compa- ratively slow, contraction of a large mass of muscular fibres, urging forward a certain quantity of fluid against considerable 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 lubb — dup (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 dilatation of the auricles, (3) the closure of the auriculo- ventricular valves, (4) the opening of the semilunar valves, and (5) the propulsion of blood into the arteries. The sound is suc- ceeded, in about one-thirtieth of a second, by the pulsation of the 146 CIRCULATION OF THE BLOOD. [chap. v. facial arteries, and in about one-sixth of a second, by the pulsa- tion of the arteries at the wrist. The second sound, in point of time, immediately follows the cessation of the ventricular con- traction, and corresponds with (a) the closure of the semilunar valves, (b) the continued dilatation of the auricles, (c) the commenc- ing dilatation of the ventricles, and (d) the opening of the auriculo- ventricular valves. The %>ause immediately follows the second sound, and corresponds in its first x>art with the completed disten- sion of the auricles, and in its second with their contraction, and the completed distension of the ventricles ; the auriculo-ventricular valves being, all the time of the pause, open, and the arterial valves closed. Causes. — The chief cause of the first sound of the heart appears to be the vibration of the auriculo-ventricular valves, 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. The effect may be intensified by the muscular sound produced by contraction of the mass of muscular fibres which form the ventricle. The cause of the second sound is more simple than that of the first. It is probably due entirely to the sudden closure and conse- quent vibration of the semilunar valves when they are pressed down across the 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 appreciated. In these experi- ments 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 audible. 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 chap, v.] SOUNDS AND [MPTJLSE OF THE BEABT. 14- one than the firet may be, that the semilunar valves arc n<,t covered in by the thick layer of fibres composing the walls of the heart to such an extent as arc the auricukhventricular. It might be expected therefore that their vibration would be more sily 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 ami tic stethoscope placed on it, as a slight sound preceding and continued into the louder sound of the ven- tricular contraction. The Impulse of the Heart. — At the commencement of each ventricular contraction, the heart maybe 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 over- laps 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 ventricular systole, the apex of the heart is'situated upon the diaphragm and against the chest-wall in the fifth intercostal space. When the ventricles contract, their walls become hard and tense, since to expel their contents into the arteries is a distinctly labo- rious action, as it is resisted by the tension within 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- L 2 148 CIRCULATION OF THE BLOOD. [chap. v. tinctly because of the partial rotation of the heart, already spoken of, along its long axis towards the right. The move- ment produced by the ventricular contraction may be registered by means of an instrument called the cardiograph, and it will be found to correspond almost exactly with a tracing obtained by the same instrument applied over the contracting ventricle itself. The Cardiograph (fig. 101) consists of a cup-shaped metal box over the open front of which is stretched an elastic membrane upon Fig. 101. — Cardiograph. (Sanderson's. 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 box 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 impulse of the heart. The box or tympanum com- municates by means of an air-tight elastic tube (/) with the interior of a second tympanum (fig. 102, b), in connection with which is a long and light lever (a). The shock of the heart's impulse being communicated to the ivory knob, and through it to the first tympanum, the effect is, of course, at once transmitted by the column of air in the elastic tube to the interior of the second tympanum, also closed, and through the elastic and mov- able lid of the latter to the lever, which is placed in connection chap, v.] CARDIOGRAPH. 149 with a registering apparatus, which cod aerally of a cylinder or dram covered with smoked pi solving aooording b . tn which the movement of the column of air in the first tympanum is conducted by the tube, '. and from which it is communicated by the lever, '7, to a revolving cylinder, so that the tracing of the movement of the impulse beat is obtained. definite velocity by clock-work. The point of the lever writes upon the paper, and a tracing of the hearts impulse is thus obtained. B}' placing three small india-rubber air-bags in the interior respec- tively of the right auricle, the right ventricle, and in an intercostal Fig. 103. — Tracing 0/ the impulse <>j tUt heart of m i Maiey.] space in front of the heart of living animals (horse), and placing these bags, by means of long narrow tubes, in communication with three levers, arranged one over the other in connection with a registering apparatus (fig. 104), MM. Chauveau and Marey have been able to measure with much accuracy the variations of the endocardial pressure and the comparative duration of the contractions of the auricles and ventricles. By means of the same apparatus, the synchronism of the impulse with the contraction of the ventricles, is also well shown ; and the causes of the several vibrations of which it is really composed, have been discovered i;o CIRCULATION OF THE BLOOD. [(HAP. V In the tracing (fig 105), the intervals between the vertical lines represent periods of a tenth of a second. The parts on which any Fig. 104. — Apparatus of MM. Chan vau and Marey for estimating the variations of endo- cardial pressure, and production of impulse of the heart. given vertical line foils represent, of course, simultaneous events. Thus. — it will be seen that the contraction of the auricle, indicated bj the upheaval of the tracing at A in first tracing, causes a slight increase of pressure in the ventricle (a' in second tracing), and Fig. 10;. — Tracings of (i), Intra-aurieular, and [2), Tntra-ventricular pressures, and (3), of the impulse of the heart, to he read from left to right, obtained by Chauveauand Marey's apparatus. produces a tiny impulse (a" in third tracing). So also, the closure of the semilunar valves, while it causes a momentarily increased pressure in the ventricle at d', does not fail to affect chap, v.] FREQUENCY OF THE HEART'S ACTION. 151 the pressure in the auricle i>, and to leave its mark in the tracing of the impulse also, i> . The large upheaval <»f the ventricular and the impulse traci between a' and \<\ and a." and d", are caused by the ventricular i- infraction, while the smaller undulations, between b and 1 . B'and c', b" and c", are caused by the vibrations consequent on the tightening and closure of the auriculo-ventricular valves. Although, ii<> doubt, the method thus described may show a perfectly correct view of the endo-cardiac pressure variations, it should he recollected that the muscular walls may grip the air- . 'Ven after the complete expulsion of the contents of the chamber, ami bo the lever might remain for a too long time in the position of extreme tension, and would represent on the tracing not only, as it ought to do, the auricular or ventricular pressure on the blood, but, also afterwards, the muscular pressure exerted upon the bags themselves. (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 arteria pulse, to vary even in health. The chief are age, temperament, sex, food and drink, exercise, time of day, posture, atmospheric pressure, temperature. 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 iu extreme old age, thus : — Before birth the average number of pulses in a minute is 150 Just after birth from 140 to 130 During the first year During the second year . .... During the third year About the seventh year . . . . . . About the fourteenth year, the average number of pulses in a minute is frorn .... In adult age In old age In decrepitude Temperament and See. — In persons of sanguine temperament, the heart 130 to "5 115 to 100 100 to 90 90 to S5 85 to So So to 70 70 to 60 75 to 65 I5 2 CIRCULATION OF THE BLOOD. [chap. V. acts somewhat more frequently than in those of the phlegmatic ; and in the female sex more frequently than in the male. Food and Brink. Exercise. — After a meal its action is accelerated, and still more so during bodily exertion or mental excitement ; it is slower during sleep. Diurnal Variation. — It appears that, in the state of health, the pulse is most frequent in the morning, and becomes gradually slower as the day advances : and that this diminution of frequency is both more regular and more rapid in the evening than in the morning. Posture. — It is found that, as a general rule, the pulse, especially in the adult male, is more frequent in the standing than in the sitting posture, and in the latter than in the recumbent position ; the difference being greatest between the standing and the sitting posture. The effect of change of posture is greater as the frequency of the pulse is greater, and, accordingly, is more marked in the morning than in the evening. By supporting the body in different postures, without the aid of muscular effort of the indi- vidual, it has been 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 exer- tion was rendered unnecessary. (Guy.) Atmospheric Pressure. — The frequency of the pulse increases in a cor- responding ratio with the elevation above the sea. 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. (0° C). When heat is gradually applied to it, both the speed and force of the heart's con- tractions increase till they reach a maximum. If the temperature is still further raised, the beats become irregular and feeble, and the heart atlength 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 0, 5 C). At 113 — 114 F. (45 C), the rabbit's heart ceases to beat. Eelative Frequency of the Pulse to that of Respiration. — In health there is observed a nearly uniform relation between the frequency of the pulse and of the respirations ; the proportion being, on an average, one respiration to three or four beats of the heart. The same relation is generally maintained in the cases in which the pulse is naturally accelerated, as after food or exercise ; but in disease this relation usually ceases. In many affections accompanied with increased frequency of the pulse, the respiration is, indeed, also accelerated, yet the degree of its accele- ration may bear no definite proportion to the increased number of the heart's actions : and in many other cases, the pulse becomes more frequent without any accompanying increase in the number ohap. v.] FORCE OF THE HEARTS ACTION 153 of respirations ; or, the respiration alone may l>e accelerated, the Qumber of pulsations remaining stationary, or even falling below the ordinary standard. The Force of the Ventricular Systole and Diastole.— The fora of the left ventricular systole is more than double thai exerted by the contraction of the right: this difference in the amount of force exerted by the contraction of the two ventricL 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 pres- sure of about f of an inch (- 17 to — 16 mm.), and in the left ventricle from 2 inches to ± of an inch ( — 52 to — 20 mm.). Part of this 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 dilata- tion is about 4 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 circulation does not fall upon the right side of the heart, but is assisted by the left side. The Force of the Auricular Systole and Diastole.— The maximum pressure within the right auricle is about |- of an inch (20 mm.) of mercury, and is probably somewhat less in the left. It has been found that during diastolo the pressure within both auricles sinks considerably below that of the atmo- sphere ; 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 of 154 CIRCULATION OF THE BLOOD. [chap. v. work." The unit of work is defined to be the energy expended in raising a unit of weight (i lb.) through a unit of height (i ft.). In England, the unit of work is the " foot-pound" in France, the ii hUograffnmetre." The work done by the heart at each contraction can be readily found by multiplying -the weight of blood expelled by the ventricles by the height to which the blood rises in a tube tied into an artery. This height was found to be about 9 ft. in the horse, and the estimate is nearly correct for a largo artery in man. Taking the weight of blood expelled from the left ventricle at each systole as 6 oz., i.e., f lb., 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 \ kilogrammetre, 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 almost if not quite immediately after removal from the body, and are, therefore, unfavourable for the study of the nervous mechanism which regulates their action. Observations have, hitherto, there- fore, been principally directed to the heart of cold-blooded animals, e.g., the frog, tortoise, and snake, which will continue to beat under favourable conditions for many hours after removal from the body. Of these animals, the frog is the one mostly employed, and, indeed, until recently, it was from the study of the frog's heart that the chief part of our information was obtained. If removed from the body entire, the frog's heart will continue to beat for many hours and even days, and the beat has no apparent 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 beat may be induced by stimulating the heart by means of a blunt needle ; but the time before the stimulus applied produces its result (the latent period) is very prolonged, and as in this way the cardiac beat is like the contraction of unstriped muscle, the method has been likened to a peristaltic c< ntraction. chap, v.] I.\'Fl.ri:.M i: OF NERVOUS SYSTEM. 155 There is much uncertainty about the nervous mechanism of the beat of the frog's heart, but what has just been said shows, al any rate, two things; firstly, that as the heart will beat when removed from the body in a way differing not at all from the normal, it must contain within itself the mechanism of rhythmical con traction ; and secondly, that as it can beat without the presence of fluid within its chambers, the movement cannot depend merely on reflex excitation by the entrance of blood. The nervous appa- ratus existing in the heart itself consists of collections of microscopic ganglia, and of nerve-fibres proceeding from them. These ganglia Yet Fig. io6.—TT(nrt of froff. (Burdon-Sanderson after Fritsche.] Front view to the left, back new to the right. A A. Aorta 1 . V. cs. Vente cav«? superiores. At s, left auricle. At d, right auricle. Ven., ventricle. B. nr. Bulb us arteriosus. S. v., Sinus venosus. V. c. t'., Vena cava inferior. J\ h., Venie hepatica.'. V. p., Vente pul- inonales. are demonstrable as being collected chiefly into three groups ; one is in the wall of the sinus venosus (Remak's) ; a second, near the junction between the auricle and ventricle (Bidder's) ; and the third in the septum between the auricles. Some very important experiments seem to identify the rhyth- mical contractions of the frog's heart with these ganglia. If the heart be removed entire from the body, the sequence of the con- traction of its several beats will take place with rhythmical regularity, viz., of the sinus venosus, the auricles, the ventricle, and bulbus arteriosus, in order. If the heart be removed at the junction of the sinus and auricle, the former will continue to beat, but the removed portion will for a short variable time stop beating, and then resume its beats, but with a rhythm different to that of the sinus : and, further, if the ventricle be removed, it will take a still longer time before recommencing its pulsation after its removal than the larger portion consisting of the auri- cles and ventricle, and its rhythm is different from that of the unremoved portion, and not so regular, nor will it continue to 156 CIRCULATION OF THE BLOOD. [chap. v. pulsate so long : during the period of stoppage a contraction will occur if the ventricle be mechanically or otherwise stimulated If the lower two-thirds or apex of the ventricle be removed, the remainder of the heart will go on beating regularly in the body, but this part will remain motionless, and will not beat spontaneously, although it will respond to Btimuli. 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 con- tinue their movements of 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 possess the power of automatic movement. Although the theory that the pulsations of the rest of the heart are dependent upon that of the sinus, and to stimuli proceeding from it, when connection is. maintained, and only to reflex stimuli when removal has taken place, cannot be absolutely upheld, yet it is evident that the power of spontaneous contraction is strongest in the sinus, less strong in the auricles, and less so still in the ventricle, and that, therefore, the sinus ganglia are probably important in ex- citing the rhythmical contraction of the whole heart. This is expressed in the following way : — " The power of independent rhythmical contraction decreases regularly as we pass from the sinus to the ventricles," and " The rhythmical power of each seg- ment of the heart varies inversely as its distance from the sinus/" (Gaskell.) It has been recently shown that, under appropriate stimuli, even the extreme apex of the ventricle in the tortoise may take on rhythmical con- tractions, or in other words may be " taught to beat '' rhvthmicallv. (Gaskell.) Inhibition of the Heart's Action. — Although, under ordinary conditions, the apparatus of ganglia and nerve-fibres in the sub- stance of the heart forms the medium through which its action is excited and rhythmically maintained, yet they, and, through them, the heart's contractions, are regulated by nerves which pass to them from the higher nerve-centres. 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 chap, v.] [NHIBITION OF THE HEART. I ;; nerves when a record i> \»/\wj: taken of the heart If a single induction shock be Bent into th< . the heart, after a short interval, <■ - ~ beating, but after the bud- od 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 application of the electric-current. The stoppage of the heart may occur apparently in one of two ways, either by diminution of the strength of the systole or by increasing the length of the diastole. The stoppage of the heart may be brought about by the application of the electrodes to any part of the vagus, but most effectually if they are applied near the position of Remak's ganglia. It is supposed that the fibres of the vagi, therefore, terminate there in inhibitory ganglia in the heart-walls, and that the inhibition of the heart's beats, by means of the vagus, is not a simple action, but that it is produced by stimulating centres in the heart itself. These inhibitory centres are paralyze! by atropin, and then no amount of stimulation of the vagus, or of the heart itself, will produce any effect upon the cardiac beats. Urari in large doses paralyzes the vagus fibres, but in this case, as the inhibitory action can be pro- duced by direct stimulation of the heart, it is inferred that this drug does not paralyze the ganglia themselves. Muscarin and pilocarpin appear to produce effects similar to those obtained by stimulating the vagus fibres. If a ligature be tightly tied round the heart over the situation of the ganglia between the sinus and the auricles, the heart stops beating. This experiment (Stannius") would seem to stimulate the inhibitory ganglia, but for the remarkable fact that atropin does not interfere with its success. If the part (the ventricle) below the ligature be cut off, it will begin and continue to beat rhyth- mically : this may be explained by supposing that the stimulus •tion induces pulsation in the part which is removed from the influence of the inhibitory ganglia. So far, the effect of the terminal apparatus of the vagi lias been considered; there is. however, reason for believing that the vagi nerves are simply the media of an inhibito - lining influence over the action of the heart, which is conveyed thr _ them from a centre in the medulla oblongata which is always in operation, and, because of its restraining the heart's act: 158 CIRCULATION OF THE BLOOD. [chap. v. 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 increased reflexly, producing slowing or stoppage of the heart, through influence passing from it down the vagi. As an example of the latter, the well-known effect on the heart of a violent blow on the epigastrium may be referred to. The stoppage of the heart's action is due to the conveyance of the stimulus by fibres of the sympathetic to the medulla oblongata, and its subsequent reflection through the vagi to the inhibitory ganglia of the heart. It is also believed that the power of the medullary inhibitory centre may be reflexly lessened, producing accelerated action of the heart. Acceleration of Heart's Action.— Through certain fibres of the sympathetic, the heart receives an accelerating influence from the medulla oblongata. These accelerating nerve-fibres, issuing from the spinal cord in the neck, reach the inferior ceiwical ganglion, and pass thence to the cardiac plexus, and so to the heart. Their function is shown in the quickened pulsation which follows stimulation of the spinal cord, when the latter has been cut off from all connection with the heart, excepting that which is formed by the accelerating filaments from the inferior cervical ganglion. Unlike the inhibitory fibres of the pneumogastric, the accelerating fibres are not continuously in action. The accelerator nerves must not, however, be considered as direct antagonists of the vagus ; for if at the moment of their maximum stimulation, the vagus be stimulated with minimum currents, inhibition is produced with the same readiness as if these were not acting. The connection of the heart with other organs by means of the nervous system, and the influences 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 com- pression of the brain, the irregularities and palpitations caused by dyspepsia or hysteria, are good evidence of the connection of the heart with other organs through the nervous system. The action of the heart is no doubt also very materially affected chap, v.] THE AETERIES. 159 by the nutrition oi its walls l>y a sufficient supply of healthy blood sent to them, and it is not unlikely that the apparently contradictory effect of poisons may be explained by supposing that the influence of some of them is either partially or entirely directed t<> the muscular tissue itself, and not to the nervoue apparatus alone. As will be explained presently, the heart exerci a considerable influence upon the condition of the pressure of blood within the arteries, but in its turn the blood-pressure within the arteries reacts upon the heart, and lias a distinct effect upon its contractions, increasing by its increase, and rice versd, the force oi the cardiac beat, although the frequency is diminished as the blood- pressure rises. The quantity (and quality?) of the blood contained in each chamber, too, has an influence upon its systole, and within normal limits the larger the quantity the stronger the contraction. Rapidity of systole does not of necessity indicate strength, as two weak contractions often do no more work than one strong and prolonged. In order that the heart may do its maximum 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 overloaded stomach, or what not, the pulsations become irregular and feeble. 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 its course 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 various organs and tissues it passes on its way. In the abdomen it divides 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 vessels 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 com- municate with other arteries (anastomose or inosculate). The branches are usually given off at an acute angle, and the area of i6o CIRCULATION OF THE BLOOD. [CHAI*. V. 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 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 circulation. Structure.— The walls of the arteries are composed of three principal coats, termed the external or tunica adventitia, the middle or tunica media, and the internal coat or tunica intima. The external coat or tunica adventitia (figs. 107 and in, t. a.), the strongest and toughest part of the wall of the artery, is formed of areolar tissue, with which is mingled throughout a network of elastic fibres. At the inner part of this outer coat the elastic network forms in most arteries so distinct a layer as to be sometimes called the external elastic coat (fig. 123, e. e.). The middle coat (fig. 107, m) is composed of both muscular and elastic fibres, with a certain proportion of areolar tissue. In the larger arteries (fig. no) its thickness is comparatively as well as absolutely much greater than in the small, consti- tuting, as it does, the' greater part of the arterial wall. The muscular fibres, which are of the unstriped variety (fig. 109) are arranged for the most part transversely to the long- axis of the artery (fig. 107, m) ; while the elastic element, taking also a transverse direction, is disposed in the form of closely interwoven and branching fibres, which inter- sect in all parts the layers of muscular fibre. In arteries of various size there is a difference in the proportion of the muscular and elastic element, elastic tissue preponderating in the largest Fig. 107. — Minute arte review- ed in longitudinal section. e. Nucleated endothelial membrane, with faint nuclei in lumen, looked at from above, i. Thin elas- tic tunica intima. m. Mus- cular coat or tunica media. a. Tunica adventitia. (Klein and Noble Smith.) X 250. < II A I STRUCTURE OF ARTERIES. 161 arteries, while this condition is reversed in those of medium and small size. The internal coat is formed bj layers of elastic tissue, consisting in part of coarse longitudinal branching fibres, and in part of a Hi 108. — Portion of j from the femoral artery, x :m. a, b, r. PerforatioiLS. (Henle.) very thin and brittle membrane which possesses little elasticity, and is thrown into folds or wrinkles when the artery contracts. This latter membrane, the striated or fenestrated coat of Htnle (fig. 1 08), is peculiar in its tendency to curl up, when peeled off /I, J Fig. 109. — Muscular fbre-edls from human aruries, magnified 350 diameters. (Kulliker.) n. Nucleus, b. A fibre-cell treated with acetic acid. from the artery, and in the perforated and streaked appearance which it presents under the microscope. Its inner surface is lined with a delicate layer of endothelium, composed of elongated cells M 1 62 CIRCULATION OF TIIE BLOOD. [chap. v. (fig. ii2, a), 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. Immediately external to the endothelial lining of the artery is fine connective tissue, sub-endothelial layer, with branched cor- &s 536 h^r-^'S^riS 1 &&-. Fig. no. Transverse, section of aorta through internal and about Turff th? middle coat. a. Linm * endothelium with the nuclei of the cells only shown, h. Subepithelial layer °of connective tissue, c, d. Elastic tunica intima proper, with fibrils running circularly or longitudinally. «, /. Middle coat, consisting of elastic fibres arranged longitudinally, with muscle-fibres cut obliquely, or longitudinally. (Klein.) puscles. Thus the internal coat consists of three parts, (a) an endothelial lining, (6) the sub-endothelial layer, and (c) elastic layers. Vasa Vasorum. — The walls of the arteries, with the possible exception of the endothelial lining and the layers of the internal coat immediately 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, extend for some distance into the I BAP. v. ] STRUCTURE OF ARTERIES. 163 middle, l>ut do not reach the internal ooat These nutrient vet died Ml 4 CIRCULATION OF THE BLOOD. [chap. v. are continuous with vessels which distinctly ensheath them — perivascular lymphatic sheatlis (fig. 121). Lymph channels are said to be present also in the tunica media. Fig. 113. — Blood-vessels from mesocolon of rabbit, a. Artery, with two branches, showing' tr.n. nuclei of transverse muscular fibres; 1. n. nuclei of endothelial lining; t. a. tunica adventitia. v. Venn. 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.) Nervi Vasorum. — Most of the arteries are surrounded by a plexus of sympathetic nerves, which twine around the vessel very much like ivy round a tree : and ganglia are found at frequent intervals. The smallest arteries and capillaries are 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. 122). It is through these plexuses that the calibre of the vessels is regulated by the nervous system (p. 190). The Capillaries. Distribution. — In all vascular textures, except some parts of the corpora cavernosa of the penis, and of the uterine placenta, OHAP. v.] CAI'l l.l.A III KS. t6s ;ui(l of the Bpleen, tin- transmission of the blood from the minute branohes of the arteries to the minute veins is effected through a network of microscopic vessels, called capillaries. These may be seen in all minutely injected preparations ; and during life, in any transparent vascular parts, such as the web <>f the frog's foot, the tail or external branchiae of the tad pole, or the wing of the hat. The branches of the minute arteries form repeated anastomoses with each other, and give oft' the capillaries which, by their anastomoses, compose a conti- nuous and uniform network, from which the venous radicles take their rise (fig. 114). The point at which the arteries terminate and the minute veins com- mence, cannot be exactly defined, for the transition is gradual ; but the capillary network has, nevertheless, this peculiarity, K&- iw—Biood-vt 1 " ' intestinal 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 arteries 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 radiate, flattened and nucleated cells, so joined and dovetailed together as to form a continuous transparent membrane (fig. 115). Outside these cells, in the larger capillaries, there is a structureless, or very finely fibrillatcd membrane, on the inner surface of which they arc 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 endothelial cells which compose the capillary wall, pxeudo-stomata may be seen resembling those in serous membranes (p. 367) villus, representing the arrangement of capil- laries between the ultimate venous and arterial branches ; a, a, the arteries; b, the vein. 1 66 CIRCULATION OF THE BLOOD. [chap. v. The endothelial cells are often continuous at various points with processes of adjacent connective-tissue corpuscles. Fig. 115. — Capillary blood-vessels from the omentum of rabbit, showing the nucleated endo- thelial membrane of which they are composed. (Klein and Xoble Smith.) 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 dif- ferent textures of the body, the most common size being about 3-^ooth of an inch. Among the smallest may be mentioned those of the brain, and of the follicles of the mucous mem- brane of the intestines ; among the largest, those of the skin, and espe- cially those of the medulla of bones. The size of capillaries varies neces- sarily in different animals in relation to the size of their blood corpuscles : thus, in the Proteus, the capillary circulation can just be discerned with the naked eye. The form of the capillary network presents considerable variety in the different textures of the body : the varieties consisting principally of modifications of two chief kinds of mesh, the rounded and the elongated. That kind of Fig. 116. — Network of capillary vessels of the air-cells of the horse's limy magnified, a, a, capillaries pro- ceeding from b, b, terminal branches of the pulmonary artery. (Frey.) t II IF. v. I CAPILLARIES. 167 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. 116), most glands, and mucous membranes, and the cutis. The meshes of this kind of network are not quite circular but more or less angular, some- times presenting a nearly regular quadrangular or polygonal form, but being more frequently irregular. The capillary network with elongated meshes (fig. 117) is observed in parts in which the vessels are arranged among bundles of tine tubes or fibres, as in muscles and nerves. In such parts, the meshes usually have the form of a parallelo- gram, 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 accord- ing 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 into some prominent organ, and returning after forming one or more loops, as in the papilla? 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, yet very small. In the human liver the interspaces are of the same size, or even smaller than the capillary vessels themselves. In the human lung they are smaller than the vessels ; in the human kidney, and in the kidney of the dog, the diameter of the injected capillaries, compared with that of the interspaces, is in the pro- portion 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 Fig. 117. — Injected capil- lary vessels of musrl, seen with a low mag- nifying power. (Sharpey.) 1 63 CIRCULATION OF THE BLOOD. [chap. v. Less numerous than in some other parts. Their diameter, accord- ing 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 growing 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, probably, very little vital change occurs after they are once formed. The Veins. Distribution. — The venous system begins in small vessels which are slightly larger than the capillaries from which they spring. These vessels are gathered up into larger and larger trunks until they terminate (as regards the systemic circulation) in the two vena? cayse and the coronary veins, which enter the right auricle, and (as regards the pulmonary circulation) in four pulmonary A-eins, which enter the left auricle. The capacity of the veins diminishes as they approach the heart : but, as a rule, the capacity of the veins exceeds by several times (twice or three times) that of their corresponding arteries. The pulmonary veins, however, are an exception to this rule, as they do not exceed in capacity the pulmonary arteries. The veins are found alter death as a rule to be more or less collapsed, and often to contain blood. The veins are usually distributed in a superficial and a deep set which communicate frequently in their course. Structure. — In structure the coats of veins bear a general re- semblance to those of arteries (fig. nS). Tims, they possess an auter t middle, and internal coat. The outer coat is constructed of THE VEINS I 69 arclar tissue like that of the . but ifl thicker. In some veins it contains muscular fibre-cells, which are arranged longitu- dinally. The i" soat is considerably thinner than that of the arteries ; and, although it contains circular unstriped musculai ■^mmm§ €.1 Fiu 118— Tt • ' arten a,, J vein of the mucoid membrane 01 .< cnuo^s epiglottis : the contrast between the thick-willed artery and the thin-walled vein is well -huwn. A. Arterr, the letter is placed in the lumen of the vessel, e. hu- doth-lial cells with nuclei clearly visible: these cells appear very thick from th* contracted state of the vessel. Outside it a double wavy line marks the elastic tunic 1 intim 1 m Tunica media forming the chief part of arterial wall and consisting 01 un«triped musculai: fibres circularly arranged : their nuclei are well seen. a. Fart ; ot UV tunica ndventitia .-howine bundles of connective-ti-- - m >e- -tion, with tn- orcular nuclei of the connective-tissue corpuscles. This coat gradually merge- the surrounding connective-ti-ue. V. In the lumen of the vein. The other letter- indicate the same as in the artery. The muscular coat of the vein m « seen to De mu.-h thinner than that of the artery. Klein and Noble Smith. 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 vena cava and pulmonary veins, the middle coat is replaced, lor Borne distance from the heart, by circularly arranged striped muscular fibres, continuous with those of the auricles. The rfcoat of veins is less brittle than the corresponding c«»at of an artery, but in other resp rts a mblee it closely. 170 CIRCULATION OF THE BLOOD. [chat*, v. Valves. — The chief influence which the veins have in the circulation, is effected with the help of the valves, which arc placed in all veins subject to local pressure from the muscles between or near which they run. The general construction of these valves is similar to that of the semilunar valves of the aorta and pul- monary 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. 119). In the smaller veins, Fig. 119. — 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. 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 inaction, when the venous blood is flowing in its proper direction, they lie by the sides of the veins ; but when in action, they close together like the valves of the arteries, and offer a complete barrier to any backward move- ment of the blood (figs. 119 and 120). Their situation in the superficial veins of the forearm is readily discovered by pressing along its surface, in a direction opposite to the venous current, i. e., from the elbow towards the wrist ; when little swellings < HA)'. V.] VEINS. 171 (fig. 119, c) appear in the position of each pair of valves. These swellings at once disappear when the pressure is relaxed. Valves are not equally uumeroue in all veins, and in many they are absent altogether. They are most numerous in the veins of the extremities, and more so in those of the leg than the arm. They are commonly absent in veins of less than a line in diameter, and. as a general rule. There are few or none in those whieh are not subject to muscular pressure. Among those veins which have Fig. 120.— a, mm wiik wfcei open. b. km unfk vi: stream of blood passing olf by lateral channel. (Dalton.) no valves may be mentioned the superior and inferior vena cava, the trnnk and branches of the portal vein, the hepatic and renal veins, and the pulmonary veins : those in the interior of the cranium and vertebral column, those of the bones, and the trunk and branches of the umbilical vein are also destitute of valves. Circulation in the Arteries. Functions of the External Coat of Arteries. — The ex- ternal coat forms a strong and tough investment, which, though capable of extension, appears principally designed to strengthen the arteries and to guard against their excessive distension by the force of the heart's action. It is this coat which alone prevents i;2 CIRCULATION OF THE BLOOD. [chap. v. the complete severance of an artery when a ligature is tightly applied ; the internal and middle coats being divided. In it, too, i"isr. 121. — Surfat ■<- view of an artery from the mesentery or a j''<>:/, ensheathed in. a periva — cular lymphatic vessel, o. The artery, with its circular muscular coat (media' indicated by broad transverse markings, with an indication of the adventitia outside /. Lymphatic vessel ; its wall is a simple endothelial membrane. (.Klein and Noble Smith. the little vasa vasorum (p. 162) find a suitable tissue in which to subdivide for the supply of the arterial coats. Functions of the Elastic Tissue in Arteries. — The pur- pose 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 con- tents of the ventricles are forced into the arteries more quickly than they can be discharged into and through the capillaries. The blood therefore, being, for an instant, resisted in its onward course, a part of the force with which it was impelled is directed against the sides of the arteries ; under this force their elastic- walls dilate, stretching enough to receive the blood, and as they stretch, becoming more tense and more resisting. Thus, by chap. v.| FUNCTIONS OF A.KTERIES. 173 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 elasticity, to resume their former calibre ; (f>.) It equalizes the current of the blood by maintaining pressure on it in the arteries during the periods Fig. 122. — li'in/ijication of nerves and termination in the muscular coat of a small artery of the frog 1 (Arnold). 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 intervals of almost complete rest during the inaction of the ventricles. But in the actual condition of the arteries, the force of the successive contractions of the ventricles is expended partly in the direct propulsion of the blood, and partly in the dilatation of the elastic arteries ; and in the intervals between the con- tractions 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 semi-lunar arterial valves (p. 141), which takes place at the very commencement of the recoil of the arterial walls. 174 CIRCULATION OF TEE BLOOD. [chap. v. By this exercise of the elasticity of the arteries, all the force of the ventricles is made advantageous to the circulation ; for that part of their force which is ex- pended 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 ventri- cles. 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 ven- tricles. It is by this equalizing influence of the successive branches of every artery that, at length, the intermittent accele- rations 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 resist- ance which is offered to the blood-stream in these vessels (p. 197), is a necessary agent. Were there no greater obstacle to the escape of blood from the larger arteries than exists to its entrance into them from the heart, the stream would be intermittent, notwith- standing the elasticity of the walls of the arteries. (c.) By means of the elastic tissue in their walls (and of the muscular tissue also), 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; (77.) The elastic tissue Fig. 123. — Transversa section through a large branch of the inferior mesenteric artery of a pig. e, endothelial memhrane ; i, tu- nica elastica interna, no subendothelial layer is seen ; m, muscular tunica media, containing only a few wavy elastic fibres ; ee, tunica elastica externa, dividing the media from the connec- tive tissue adventitia, «. (Klein and Noble Smith.) x 350. chap, v.] FUNCTIONS OP AKTK1UKS. ^5 assists in restoring the norma] state after diminution of its calibre, whether this lias been caused l>y 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 OOntracted by means of their muscular element, after death, regain their average patency on the cessation of post-mortem rigidity (p. 177). (('■) By means of their elastic coat the arteries are enabled to adapt themselves to the different movements of the several parts of the body. Tension of Arteries. — 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 removed. The extent to which the divided extremities of arteries retract is a measure of this tension, not of their elasticity. (Savory.) Functions of the Muscular Coat. — The most important office of the muscular coat is, (l) that of regulating the quantity of" blood to be received by each part or organ, and of adjusting it to the requirements of each, according to various circumstances, but, chiefly, according to the activity with which the functions of each are at different times performed. The amount of work done by each organ of the body varies at 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 exercise of function, such a part requires a larger supply of blood than is sufficient for it during the times when it is comparatively inactive. It is evident that the heart cannot regulate the supply to each part at different periods ; neither could this be regulated by any general and uniform contraction of the arteries ; but it may be regulated by the power which the arteries of each part have, in their muscular tissue, of contracting so as to diminish, and of passively dilating or yielding so as to permit an increase of the supply of blood, according to the require- ments 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 pro- pulsion, and while the large and merely clastic arteries distribute it and equalise its stream, the smaller arteries, in addition, regu- I7 6 CIRCULATION OF THE BLOOD. [chip. v. late and determine, by means of their muscular tissue, the propor- tion of the whole quantity of blood which shall be distributed to each part. It must be remembered, however, that this regulating func- tion of the arteries is itself governed and directed by the nervous system (vaso-motor centres and fibres). Another function of the muscular element of the middle coat of arteries 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 consider- ablv even from hour to hour, and can never be quite constant; and were the elastic tissue only present, the pressure exercised by the walls of the containing vessels on the contained blood would be sometimes very small, and sometimes inordinately great. The presence of a muscular element, however, provides for a certain unifomiitv 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 escape of blood. It is only in consequence of such contraction and coagulation that we are free from danger through even verv 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 assists, 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. Simple elasticity could not effect this. (2.) When an artery is cut acro>s. 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 v.| THE PUIi i;; ipparently, in man than in the lower animal*, ontraction is dne in pan . l>ut in _vnerally increased by the application of cold, or of any simple stimulating B, or by mechanically irritating the cut ends of the artery, as by picking or twisting them. 1 contractile property of art- .tinues many hours after death, and thus affords an opportunity "f distinguishing it from thei an artery of a recently killed animal is exposed, mal may lx? thus completely clo-ed : in this - for a time, varying from a few hours to two days : then it di in, and permanently retains the same size. -f the contractile property after death wa> well si in an observation of Hunter, whieh may be mentioned as proving, also, the greater degree of • •untractility j assessed by the smaller than by the larger arteries. Having injected the uterus of a cow. which had been removed from the animal upwar nty-four hours, he found, after the lapse of another day. that the larger vessels had become much more turgid than when he u them, and that the smaller arteries had contracted so as to Hon lia«-k into the larger • The Pulse. If one extremity of an elastic tnbe In? fastened t«. a syringe, and the other be bo constricted as i<> present an obstacle t<> the e of fluid, we shall have a rough model of what is pn in the livin_ : — -The syringe representing the heart, the elastic tube the arteries, and the contracted orifice the arterioles .lest art< b nd capillaries. If the apparatus be filled with water, and if a finger-tip be placed on any part of the elastic tube, there will he felt with every action of the syringe, an impulse or beat, which corresponds exactly with what we feel in the arteries «»f the living body with every contraction of the heart, and call the The pulse is essentially caused by an expans which is due to the Injection of blood into an already full aorta : which blood expanding the vessel produces the pulse in it, almost coin- cideutly with the systole of the left ventricle. As the force of the left ventricle, however, is n<-t expended in dilating the aorta only, the wave of blood | nei n, 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 proceeds. The distension of each artery increases both its length and its diameter. In their elongation, the arteries change their form, the 178 CIRCULATION OF THE BLOOD. [chap. v. straight ones becoming slightly curved, and those already curved becoming more so, but they recover their previous form as well as their diameter when the ventricular contraction ceases, and their elastic walls recoil. The increase of their .curves which accom- panies the distension of arteries, and the succeeding recoil, may be well seen in the prominent temporal artery of an old person. In feeling the pulse, the finger cannot distinguish the sensation pro- duced by the dilatation from that produced by the elongation and curving ; that which it perceives most plainly, however, is the dilatation, or return, more or less, to the cylindrical form, of the artery which has been partially flattened by the finger. The pulse — due to any given beat of the heart — is not per- ceptible at the same moment in all the arteries of the body. Thus, — it can be felt in the carotid a very short time before it is perceptible in the radial artery, and in this vessel again before the dorsal artery of the foot. The delay in the beat is in 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 -i 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. 206) 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 what may be called the form of the pulse by the sphygmograph (figs. 124 and 125). The principle on which the sphygmograph acts is very simple (see fig. 124). 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 time that it is just strong enough to resist its making any sudden jerk, and in the interval of the beats also to assist in bringing it back to its original position. For ordinary purposes the instrument is bound on the wrist (fig. 125). It is evident that the beating of the pulse with the reaction of CHAP. \ . | THE SPHYGMOGRAPH. *79 the Bpring will cans.' an up-and-down movement of the lever, the pen of which will write the effect on a Bmoked card, which is T BUTTOH. Fig. 124. — Diagram oftht mode 0/ action of (he Bphy autograph. made to move by clockwork in the direction of the arrow. Thus a tracing of the pulse is obtained, and in this way much more Fig. 125. — The Sphyrjmograph applied to the arm. delicate effects can be seen, than can be felt on the application of the finger. The pulse-tracing differs somewhat according to the artery upon which the sphygmograph is applied, but its general characters are much the same in all cases. It consists of : — A sudden upstroke (fig. 126, 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 distinct 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 x 3 l8o CIRCULATION OF THE BLOOD. [. hap. v. 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 "-'ire (c), and in addi- tion there may be one or more Blight ascents after the dicro- tic, called post dicrotic (e). The explanation of these tracings presents some difficul- ties, not, however, as regards the two primary factors, viz., Fisr. 126. — Diagram of pidse-tradnff. a, .-, ■, -, -, -, npetroke; b, down-stroke; ■-, predi- the upstroke and downstroke, erotic- -wave; d, dicrotic; e, post i ^i n dicrotic wave. 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 signifying the recovery of the arteries by their recoil. It may be demonstrated on a system of elastic tubes, such as was described above, 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 or more sphygmographs upon such a system of tubes at increasing distances from the pump, we may demonstrate that the rise of the lever commences first in that nearest the pump, and is higher and more sudden, 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 percussion waves. The tidal wave occurs whenever fluid is injected into an elastic tube (fig. 127, b), and is due to the expan- sion of the tube and its more gradual collapse. The percussion wave occurs (fig. 127, a) when the impulse imparted to the fluid is more sudden ; this causes an abrupt 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 PULSE-TRAI . l8l double, th< tion of the I - ti'lal **▼«■ '' Fig. 127.— Diagram of the formation of the puU— A. I EI » B > tidal wave ; C, dicrotic wave. Mahonied. is most marked in tracings from I rg rteries, especially when their tone is deficient. In tracings, on the other hand, from fig. i:. — 1 toJ artery, somewhat deficient in tone. (Sander -11. Arteries of medium b . -, .. 'he radial, the upstroke a snally single. In this " asion-impnlse is not sufficiently -' radial aritry, with double apex, ^sanaeraocj ■trong to jerk up the lever and produce an effect distinct from that of the systolic I hich immediately follows it. and 1 82 CIRCULATION OF THE BLOOD. [chap. v. which continues and completes the distension. In cases of feeble arterial tension, however, the percussion-impulse may be traced by the sphygmograph, not only in the carotid pulse, but to a less extent in the radial also (fig. 129). The interruptions in the downstroke are called the hatacrotic waves, to distinguish them from an interruption in the upstroke, called the anacrotic wave, which is occasionally met with in cases in which the predicrotic or tidal wave is higher than the percus- sion wave. Fig. 130. — Anna-otic pulse from a case of aortic aneurism. A, anacrotic wave (or percussion wave). B, tidal or predicrotic wave, continued rise in tension (or higher tidal wave). There is considerable difference of opinion as to whether the dicrotic wave is present in health generally, and also as to its cause. The balance of opinion appears to be in favour of the belief of its presence in health, although it may be very faint ; while, at any rate, in certain conditions not necessarily diseased, it becomes so marked as to be quite plain to the unaided finger. Such a pulse is called dicrotic. Sometimes the dicrotic rise exceeds the initial upstroke, and the pulse is then called hyper- dicrotic. As to the cause of dicrotism, one opinion is that it is due to a recovery of pressure during the elastic recoil, in consequence of a rebound from the periphery, and it may indeed be produced on a schema hj obstructing the tube at a little distance beyond the spot where the sphygmograph is placed. Against this view, how- ever, 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, 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. The generally accepted notion among clinical observers, is that the dicrotic wave is due to the rebound from the aortic valves causing a second wave ; but the question cannot be CHAP, v. I ITISK-'I aACINGS. 183 considered settled, and the presenoe of marked dicrotism in 1 of haemorrhage, of anaemia, and of other weakening conditions, as well as its presence in eases of diminished pressure within the arteries, would imply that it might, at any rate sometimes, be due &K3S? 3, dicr^fcpuEe ": 4 aAd P 5 , the tidal wave very exaggerated, from high tension. (Mahomed.) 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 arterial walls. Waves may be produced in any elastic tube when a fluid is being driven through it with an 1 84 CIRCULATION OF THE BLOOD. [ohap, v. intermittent force, such waves being called waves of oscillation (M. Foster). They have received various explanations. In an arterial schema they vary with the specific 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 normal wave to be due to oscillation ; but, as just mentioned, even if this be the case, as is most likely, with post-dicrotic waves, the dicrotic 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 indicates closure of the aortic valves. Of the three main parts then of a pulse-tracing, viz., the per- cussion 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 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 jiercussion wave is increased by : — i. 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 con- traction of the Heart; 2. Large volume of blood; 3. Comparative emptiness of vessels ; 4. Diminished outflow or slow capillary circulation ; and diminished by the reversed conditions. The dicrotic wave is increased by : — 1. Sudden contraction of the Heart ; 2. Comparative emptiness of vessels ; 3. Increased outflow or rapid capillary circulation ; 4. Elasticity of the aorta ; 5. Relaxation of 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 I KAP. v. | ARTERIAL TENSION. 18. too great, khe oharacters of bhe pulse may be almosl entirely obscured, or the arterj may be entirely obstructed, and qo tracing is obtained ; and mi the other hand, if the pressure be too Blight, a verv small part of the characters maj be represented on the tracing. The Pressure of the Blood within the Arteries (producing arterial tension). It will be understood from the foregoing that the arteries in a norma] condition, are continually on the stretch during life, and in consequence of the injection of more Mood at each systole of the ventricle into the elastic aorta, this stretched condition is exaggerated each time the ventricle empties itself. This condition of the arteries is due to the pressure of blood within them, because of the resistance pre- sented by the smaller arteries and capillaries (peripheral resistance) to the emptying of the arterial system in the intervals between the contractions of the ventricle, and is called the condition of arterial tension. On the other hand, it must be equally clear 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, the elastic recoil sending on the blood after the immediate effect of the systole has passed ; so that, when an artery is cut across, the blood is projected forwards by this force for a considerable distance ; at each ventricular systole, a jet of blood escaping, although the stream does not cease flowing during the diastole. The relations which exist between the arte- ' IL ries and their contained blood are obviously of the utmost importance to the carrying on of the circulation, and it therefore becomes necessary 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 132. — Diagram of mer- curial »"- """ U r. i86 CIRCULATION OF THE BLOOD. [chap. v. short horizontal limb of this (fig. 132, 1) is connected, by means of an elastic tube and cannula, with the interior of an artery ; a solution of sodium or potassium carbonate being previously intro- duced 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 (2); and the depth to which the latter sinks, added to the height to which it rises in the other (3), will give the height of the mercurial column which the blood- pressure balances; the weight of the soda solution being sub- tracted. Fig. 1.53. — Diagram of mercurial kymograph, a. revolving cylinder, worked by a clockwork arrangement contained in the box (b), the speed being regulated by a fan above the box ; cylinder supported by an upright [b), and capable of being raised or lowered by a screw [a), by a handle attached to it; d, c, e, represent mercurial manometer, a somewhat different form of which is shown in next figure. For the estimation of the arterial tension at any given moment, no further apparatus than this, which is called Poiseuille's hcemo.d5 I by inhibiting or augmenting the action of the local nervous mechanism already referred to ; and as th< y are in connection with the central nn-v tern, it i> through this arrai I that thai o i> capable "f innuenci maintaining the normal local tone. it may also 'I that the local nerve i lv«B may be directly affected by the condition of blood nourishing them. The following table may serve as a summary of the effect of the nen stem upon the arteries and so upon the blood- rare : — A. An increase of the blood-pressure may be produced: — (i.) By stimulation of the - - 'tor centre in medulla, either a. J> , as by carbonated or deoxygenated blood. 0. Indirectly, by inipres-ions descending from the cerebrum, . in sudden pallor. -,. Reflexly, by stimulation of sensory nerve- anywhere. (2.) By stimulation of the centres in spinal cord. Possibly directly or indirectly, certainly reflexly. (3.) By stimulation of the local centres for each vascular area, by the vaso-constrictor nerves, or directly by means of altered blood. B. A decrease of the blood-pressure may be produced : — (1.) By stimulation of the vaao-motor centre in medulla, either (a.) Directly, as by oxygenated or aerated blood. (£.) Indirectly, by impressions descending from the cere- brum — e.g.. in blushing. (7.) Reflexly, by stimulation of the depressor nerve, and consequent dilatation of vessels of splanchnic area, and possibly by stimulation of other sensory nen the sensory impulse being interpreted as an indication for diminished blood-pressure. (2.) By stimulation of the centres in spinal cordL Possibly directly. indirectly, or reflexly. (3.) By stimulation of local centres for each vascular area by the vaso-dilator nerve, or directly by means of altered blood. 4. Chan the blood — a. As regards quantity. At rirst it would appear that one of the ea-siest ways to diminish the blood-pressure would be to remove blood from the v. - by bleeding ; it has been found by experiment, however, that although the blood-pres bilst L stractiona of blood are taking place, as soon as the bleeding ceases it rapidly, and Bpeedily becomes normal: that is to say, mile- - an amount of blood has been taken as to be positively o 2 196 CIRCULATION OF THE BLOOD. [chap. v. dangerous to life, abstraction of blood has little effect upon the blood-pressure. The rapid return to the normal pressure is due not so much to the withdrawal of lymph and other fluids from the body into the blood, as was formerly supposed, as to the regu- lation of the peripheral resistance by the vaso-motor nerves ; in other words, the small arteries contract, and in so doing maintain pressure on the blood and favour its accumulation in the arterial system. This is due to the stimulation of the vaso-motor centre from diminution of the supply of blood, and therefore of oxygen. The failure of the blood-pressure to return to normal in the too great abstraction must be taken to indicate a condition of exhaustion of the centre, and consequently of want of regulation of the peripheral resistance. In the same way it might be thought that injection of blood into the already pretty full vessels would be at once followed by rise in ' the blood-pressure, and this is indeed the case up to a certain point — the pressure does rise, but there is a limit to the rise. Until the amount of blood injected equals about 2 to 3 per cent, of the body weight the pressure continues to rise gradually ; but if the amount exceed this proportion, the rise does not continue. In this case therefore, as in the opposite when blood is abstracted, the vaso-motor appa- ratus must counteract the great increase of pressure by dilating the small vessels, and so diminishing the peripheral resistance, for after each rise there is a partial fall of pressure ; and after the limit is reached the whole of the injected blood displaces, as it were, an equal quantity which passes into the small veins, and remains within them. It should be remembered that the veins are capable of holding the whole of the blood of the body. The amount of blood supplied to the heart both to its substance and to its chambers, has a marked effect upon the blood-pressure. b. As regards quality. The quality of the blood supplied to the heart has a distinct effect upon its contraction, as too watery or too little oxygenated blood must interfere with its action. Thus it appears that blood containing certain substances affects the peripheral resistance by acting upon the muscular fibres of the arterioles themselves or upon the local centres, and so altering directly, as it were, the calibre of the vessels. 5. Respiratory changes affecting the blood-pressure will be considered in the next Chapter. chap, v.] CIRCULATION IN THE CAPILLARE U jj Circulation in the Capillaries. When seefl in any transparent part of a living adult animal by means of the microscope (fig. 140), the blood flows with a constant equable motion ; the red blood-corpuscles moving along, mostly in single file, and bending in various ways to accom- modate themselves to the tortuous course of the capillary, but instantly recovering their normal outline on reaching a wider seL It is in the capillaries that the chief resistance is offered to the progress of the blood ; for in them the friction of the blood is greatly increased by the enormous multiplication of the surface with which it is brought in contact. At the circumference of the stream in the larger capillaries, but chiefly in the small arteries and veins, in contact with the walls of the vessel, and adhering to them, there is a layer of liquor sanguinis which ap- pears to be motionless. The exist- ence of this still layer, as it is 1 • • e i i ^i f j.i rig. 140.— i (C.) in the web termed, is inferred both from the the nog's foot connecting a , p, ., , . , small artery (A) vrith a small general fact that SUCh an one exists veinV after Allen Thomson). in all fine tubes traversed by fluid, and from what can be seen in watching the movements of the blood- corpuscles. The red corpuscles occupy the middle of the stream and move with comparative rapidity ; the colourless lymph-cor- puscles run much more slowly by the walls of the vessel ; while next to the wall there is often a transparent space in which the fluid appears to be at rest ; for if any of the corpuscles happen to be forced within it, they move more slowly than before, rolling lazily along the side of the vessel, and often adhering to its wall. Part of this slow movement of the pale corpuscles and their occasional stoppage may be due to their having a natural tendency to adhere to the walls of the vessels. Sometimes, indeed, when the motion of the blood is not strong, many of the white corpi 'llect in a capillary vessel, and for a time entirely prevent the passage of the red corpuscles. CIRCULATION OF THE BLOOD. [CHAP. v. 198 Intermittent flow in the Capillaries.— When the peripheral resistance is greatly diminished by the dilatation of the small arteries and capillaries, so much blood passes on from the arteries into the capillaries at each stroke of the heart, that there is not sufficient remaining in the arteries to distend them. Thus, the intermittent current of the ventricular systole is not converted into a continuous stream by the elasticity of the arteries before the capillaries are reached ; and so intermittency of the flow occurs in capillaries and veins and a pulse is produced. The same phe- nomenon may occur when the arteries become rigid from disease, and when the beat of the heart is so slow or so feeble that the blood at each cardiac systole has time to pass on to the capillaries before the next stroke occurs, the amount of blood sent at each stroke bein^ insufficient to properly distend the elastic arteries. Diapedesis of Blood-Corpuscles. — Until within the last few years it has been generally supposed that the occurrence of any transudation from the interior of the capillaries into the midst of the surround- ing tissues was confined, in the absence of injury, strictly to the fluid part of the blood ; in other words, that the corpuscles could not escape from the circulating stream, unless the wall of the containing blood-vessel were ruptured. It is true that an English physiologist, Augustus Waller, affirmed, in 1846, that he had seen blood-corpuscles, both red and white, pass bodily through the wall of the capillary vessel in which they were contained (thus confirming what had been stated a short time previously by Addison); and that, as no opening could be seen before their escape, so none could be observed after- wards — so rapidly was the part healed. But these observations did not attract much notice until the phenomena of es- cape of the blood-corpuscles from the capillaries and minute veins, apart from mechanical injury, were re-discovered by Professor Cohnheim in 1867. mf&n Fig. 141. — A large capillary from the fro;/' eight hours after irritation had been set up, showing emigration of leucocytes. a, Cells in the act of travers- ing the capillary wall ; b, some already escaped. (Frey.) coup. v.| DIAPEDESIS. 199 Cohnheim's experiment demonstrating the passage of the cor- puscles through the wall of the blood-vessel, is performed in the following manner. A frog is urarized, that is to say, paralysis is produced by injecting under the skin a minute quantity of the poison called urari ; and the abdomen having been opened, a portion of small intestine is drawn out, and its transparent mesen- tery spread out under a microscope. After a variable time, occu- pied by dilatation, following contraction of the minute vessels and accompanying quickening of the blood-stream, there ensues a re- tardation of the current, and blood-corpuscles, both red and white, begin to make their way through the capillaries and small veins. "Simultaneously with the retardation of the blood-stream, the leucocytes, instead of loitering here and there at the edge ot the axial current, begin to crowd in numbers against the vascular wall. In this way the vein becomes lined with a continuous pave- ment of these bodies, which remain almost motionless, notwith- standing that the axial current sweeps by them as continuously as before, though with abated velocity. Now is the moment at which the eye must be fixed on the outer contour of the vessel, from which, here and there, minute, colourless, button- shaped elevations spring, just as if they were produced by budding out of the wall of the vessel itself. The buds increase gradually and slowly in size, until each assumes the form of a hemispherical projection, of width corresponding to that of the leucocyte. Eventually the hemisphere is converted into a pear-shaped body, the small end of which is still attached to the surface of the vein, while the round part projects freely. Gradually the little mass of protoplasm removes itself further and further away, and, as it does so, begins to shoot out delicate prongs of transparent pro- toplasm from its surface, in nowise differing in their aspect from the slender thread by which it is still moored to the vessel. Finally the thread is severed and the process is complete." (Burdon Sanderson.) The process of diapedesis of the red corpuscles, which occurs under circumstances of impeded venous circulation, and conse- quently increased blood-pressure, resembles closely the migration of the leucocytes, with the exception that they are squeezed through the wall of the vessel, and do not, like them, work their way through by amscboid movement. 200 CIRCULATION OF THE BLOOD. [chai\ v. Various explanations of these remarkable phenomena have been suggested. Some believe that minute openings (stigmata or pseudo-stomata) between contiguous endothelial cells (p. 165) pro- vide the means of escape for the blood-corpuscles. But the chief share in the process is to be found in the vital endowments with respect to mobility and contraction of the parts concerned — both of the corpuscles (Bastian) and the capillary wall (Strieker). Burdon-Sanderson remarks, "the capillary is not a dead conduit, but a tube of living protoplasm. There is no difficulty in under- standing how the membrane may open to allow the escape of leucocytes, and close again after they have passed out ; for it is one of the most striking peculiarities of contractile substance that when two parts of the same mass are separated, and again brought into contact, they melt together as if they had not been severed." Hitherto, the escape of the corpuscles from the interior of the blood-vessels into the surrounding tissues has been studied chiefly in connection with pathology. But it is impossible to say, at pre- sent, to what degree the discovery may not influence all present notions regarding the nutrition of the tissues, even in health. Vital Capillary Force. — The circulation through the capillaries must, of necessity, be largely influenced by that which occurs in the vessels on either side of them — in the arteries or the veins ; their intermediate position causing them to feel at once, so to speak, any alteration in the size or rate of the arterial or venous blood-stream. Thus, the apparent contraction of the capillaries, on the application of certain irritating substances, and during fear, and their dilatation in blushing, may be referred to the action of the small arteries, rather than to that of the capillaries them- selves. But largely as the capillaries are influenced by these, and by the conditions of the parts which surround and support them, their own endowments must not be disregarded. They must be looked upon, not as mere passive channels for the pas- sage of blood, but as possessing endowments of their own (vital capillary force), in relation to the circulation. The capillary wall is actively living and contractile ; and there is no reason to doubt that, as such, it must have an important influence in connection with the blood-current. Blood- Pressure in the Capillaries. — From observations upon the web of the frog's foot, the tongue and mesentery of the ohap. v.] CIBCULATION l.\ THE VEINS. 201 frog, the tails of newts, and small fishes (Roy and Brown), as well as upon the skin of t he finger behind the nail (Kries), by careful estimation of the amount of pressure required to empty the vessels of blood under various conditions, it appears that the blood- pressure is subject to variations in the capillaries, apparently following the variations of that of the arteries; and that up to -.< certain point, as the extra vascular pressure is increased, so docs the pulse in the arterioles, capillaries, and venules become more and more evident. The pressure in the first case (web of the frog's foot) lias been found to be equal to about 14 to 20 mm. of mercury ; in other experiments to be equal to about ± to |- of the ordinary arterial pressure. The Circulation in the Veins. The blood-current in the veins is maintained by the slight vis a tergo remaining of the contraction of the left ventricle. Very effectual assistance, however, to the flow of blood is afforded by the action of the muscles capable of pressing on such veins as have valves. The effect of such muscular pressure may be thus explained. When pressure is applied to any part of a vein, and the current of blood in it is obstructed, the portion behind the seat of pressure becomes swollen and distended as far back as to the next pair of valves. These, acting like the semilunar valves of the heart, and being, like them, inextensible both in themselves and at their margins of attachment, do not follow the vein in its dis- tension, but are drawn out towards the axis of the canal. Then, if the pressure continues on the vein, the compressed blood, tend- ing to move equally in all directions, presses the valves down into contact at their free edges, and they close the vein and prevent regurgitation of the blood. Thus, whatever force is exercised by the pressure of the muscles on the veins, is distributed partly in pressing the blood onwards in the proper course of the circula- tion, and partly in pressing it backwards and closing the valves behind (fig. 128, A and B). The circulation might lose as much as it gains by such compression of the veins, if it were not for the numerous anasto- moses by which they communicate, one with another ; for through 202 CIRCULATION OF THE BLOOD. [chap. v. these, the closing up of the venous channel by the backward pressure is prevented from being any serious hindrance to the circulation, since the blood, of which the onward course is arrested by the closed valves, can at once pass through some anastomosing channel, and proceed on its way by another vein. Thus, therefore, the effect of muscular pressure upon veins which have valves, is turned almost entirely to the advantage of the circulation ; the pressure of the blood onwards is all advantageous, and the pressure of the blood backwards is prevented from being a hindrance by the closure of the valves and the anastomoses of the veins. The effects of such muscular pressure are well shown by the acceleration of the stream of blood when, in venesection, the muscles of the fore-arm are put in action, and by the general acceleration of the circulation during active exercise : and the numerous movements which are continually taking place in the body while awake, though their single effects may be less striking, must be an important auxiliary to the venous circulation. Yet they are not essential ; for the venous circulation continues un- impaired in parts at rest, in paralysed limbs, and in parts in which the veins are not subject to any muscular pressure. Rhythmical Contraction of Veins. — In the web of the bat's wing, the veins are furnished with valves, and possess the remark- able property of rhythmical contraction and dilatation, whereby the current of blood within them is distinctly accelerated. (Wharton Jones.) The contraction occurs, on an average, about ten times in a minute ; the existence of valves preventing regurgi- tation, the entire effect of the contractions was auxiliary to the onward current of blood. Analogous phenomena have been fre- quently observed in other animals. Blood-Pressure in the Veins. — The blood-pressure gradu- ally falls as we proceed from the heart to the arteries, from these to the capillaries, and thence along the veins to the right auricle. The blood-pressure in the veins is nowhere very great, but is greatest in the small veins, while in the large veins towards the heart the pressure becomes negative, or, in other words, when a vein is put in connection with a mercurial manometer the mercury will fall in the area furthest away from the vein and will rise in the area nearest the vein, having a tendency to suck in rather chap, v.j TELOCITY OP THE I Il:< I'l.ATl 20^ than t<> push forward. In the reins in the Deck this tend -uck in aii cially marked, and is the cane operations in that region. The amount of pressure in the brachial vein is said to Bupport 9 nun. of mercury, whereas the pressure in the vein- of the neck is about equal to a negative pressure of - 3 t<> - S mm. The variations of venous pressure during le and diastole of the heart are very Blight, and a distinct pulse is seldom .seen in vein- I under very extraordinary circumstances. The formidable obstacle to the upward current of the blood in the veins of the trunk and extremities in the erect posture supposed to be {•resented by the gravitation of the blood, has no real existence, since the pressure exercised by the column of blood in the arteries, will be always sufficient to support a column of venous blood of the same height as itself: the two columns mutually balancing each other. Indeed, so l<>nu as both arteries and veins contain continuous columns of blood, the force of gravitation, whatever be the position of the body, can have no power to move or resist the motion of any part of the blood in any direction. The lowest blood- - have, of course, to bear the greatest amount of pressure; the pressure on each part being directly proportionate to the height of the column of blood above it : hence their liability to tension. But this pressure bears equally on both arteries and veins, and cannot either move, or resist the motion of, the fluid they contain, so long as the columns of fluid are of equal height in both, and continuous. Velocity of the Circulation. The velocity of the blood-current at any given point in the various divisions of the circulatory system is inversely propor- tional to their sectional area at that point. If the sectional area of all the branches of a vessel united were always the same - that of the vessel from which they arise, and if the aggregate sectional area of the capillary vessels were equal to that of the aorta, the mean rapidity of the bkxxTs motion in the capillaries would be the same as in the aorta and largest arteries : and if a similar correspondence of capacity existed in the veins and arteries, there would be an equal correspondence in the rapidity of the circulation in them. But the arterial and venous systems may be 204 CIRCULATION OF THE BLOOD. [chap, v. represented by two truncated cones with their apices directed towards the heart ; the area of their united base (the sectional area of the capillaries) being 400 — 800 times as great as that of the truncated apex representing the aorta. Thus the velocity of blood in the capillaries is at least —^ of that in the aorta. Velocity in the Arteries.— The velocity of the stream of blood is greater in the arteries than in any other part of the circulatory system, and in them it is greatest in the neighbourhood of the heart, and during the ventricular systole ; the rate of movement diminishing during the diastole of the ventricles, and in the parts of the arterial system most distant from the heart. Chauveau has estimated the rapidity of the blood-stream in the carotid of the horse at over 20 inches per second during the heart's systole, and nearly 6 inches during the diastole (520 — 150 mm.). Estimation of the Velocity. — Various instruments have been de- vised for measuring the velocity of the blood-stream in the arteries. Ludwig's " Stromvhr" (fig. 142) consists of an U-shaped glass tube dilated at a and a', and whose extremities, h and i, are of known calibre. The bulbs can be filled by a common opening at k. The instrument is so contrived that at b and b' the glass part is firmly fixed into metal cylinders, which are fixed into a circular horizontal table, c c, capable of hori- zontal movement on a similar table d d' about the vertical axis marked in figure by a dotted line. The opening in c c, when the instru- ment is in position, as in fig., corresponds exactly with those in d d' ; but if c c' be turned at right angles to its present position, there is no communication between h and a, and i and a', but h communicates directly with i ; and if turned through two right angles c' communicates with d, and c with d', and there is no direct connection between h and i. The experiment is performed in the following way : — The artery to be experi- mented upon is divided and connected with two cannulse and tubes which fit it accurately with h and i — h the central end, Fig. 142. — Ltidvrig's Stromuhr. ( HAT. \ VELOCITY IN THE ARTERIES. 205 and i the peripheral ; the bulb a is filled with olive oil up to a point rather Lower than /■, and or' and the remainder of a ua filled with defibrinated blood ; the tube on k is then carefully olamped \ the tubes d and ct are also filled with defibrinated blood. When everything is ready, the blood is allowed to How into a through //, and it pushes before it the oil, and thai the defibrinated blood into the artery through /, and replaces it in a ; when the blood reaches the former level of the oil in a, the disc c c is turned rapidly through two right angles, and the blood flowing through (I into a again displaces the oil which is driven into a. This is repeated Bevera] times, and the duration of the experiment noted. The capacity of a and a! is known; the diameter of the artery is also known by its corresponding with the cannulas of known dia- meter, and as the number of times a has been filled in a given time is known, the velocity of the current can be calculated. Chauveau's instrument (fig. 143) consists of a thin brass tube, 0, in one side of which is a small perforation closed by thin vul- Fig. 143. — Diagram of Chauveau's Instrument, o. Brass tube for introduction into the lumen of the artery, and containing' an index-needle, which passes through the elastic membrane in its side, and moves by the impulse of the blood-current, c. Graduated scale, for measuring the extent of the oscillations of the needle. canised indiarubber. Passing through the rubber is a tine lever, one end of which, slightly flattened, extends into the lumen of the tube, while the other moves over the face of a dial. The tube is inserted into the interior of an artery, and ligatures applied to fix it, so that the movement of the blood may, in flowing through the tube, be indicated by the movement of the outer extremity of the lever on the face of the dial. 206 CIRCULATION OF THE BLOOD. [chap. v. The Hcematochometer of Vierordt, and the instrument of Lortet, resemble in principle that of Chauveau. Velocity in the Capillaries. — The observations of Hales, E. H. Weber, and Valentin agree very closely as to the rate of the blood-current in the capillaries of the frog ; and the mean of their estimates gives the velocity of the systemic capillary circulation at about one inch (25 mm.) per minute. The velocity in the capil- laries of warm-blooded animals is greater. In the dog ■£§ to yj^ inch (-5 to 75 nun.) a second. This may seem inconsistent with the facts which show that the whole circulation is accomplished in about half a minute. But the whole length of capillary vessels, through which any given portion of blood has to pass, probably does not exceed from -^th to J^th of an inch (.5 mm.); and therefore the time required for each quantity of blood to traverse its own appointed portion of the general capillary system will scarcely amount to a second. Velocity in the Veins. — The velocity of the blood is greater in the veins than in the capillaries, but less than in the arteries : this fact depending upon the relative capacities of the arterial and venous systems. If an accurate estimate of the proportionate areas of arteries and the veins corresponding to them could be made, we might, from the velocity of the arterial current, calcu- late that of the venous. An usual estimate is, that the capacity of the veins is about twice or three times us great as that of the arteries, and that the velocity of the blood's motion is, there- fore, about twice or three times as great in the arteries as in the veins, 8 inches (about 200 mm.) a second. The rate at which the blood moves in the veins gradually increases the nearer it approaches the heart, for the sectional area of the venous trunks, compared with that of the branches opening into them, becomes gradually less as the trunks advance towards the heart. Velocity of the Circulation as a whole. — It would appear that a portion of blood can traverse the entire course of the circu- lation, in the horse, in half a minute. Of course it would require longer to traverse the vessels of the most distant part of the extremities than to go through those of the neck : but taking an average length of vessels to be traversed, and assuming, as we may, that the movement of blood in the human subject is not slower chap. v.| VELOI ITY OF THE CIBCULATION. 207 than in the horse, it may 1"' concluded thai half ;t minute repi Benta the.a^ ite. Satisfactory data for these estimates are afforded by the results of experiments I scertain the rapidity with which ; in- troduced into the blood are transmitted from one part >>i the vascular Bystem to another. The time required for the j of a solution of potassium ferrocyanide, mixed with the blood, from one jugular vein (through the right side of the heart, the pulmonary vessels, the left cavities of the heart, and the general circulation) to the jugular vein of the opposite Bide, varies from twenty to thirty seconds. The same Bubstance was transmitted from the jugular vein to the great saphena in twenty seconds; from the jugular vein to the masseteric artery, in between fifteen and thirty seconds ; to the facial artery, in one experiment, in between ten and fifteen seconds ; in another ex- periment in between twenty and twenty-five seconds: in its transit from the jugular vein to the metatarsal artery, it occupied between twenty and thirty seconds, and in one instance more than forty seconds. The result was nearly the same whatever was the rate of the heart's action. In all these experiments, it is assumed that the substance injected moves with the blood, and at the same rate, and does not move from one part of the organs of circulation to another by diffusing itself through the blood or tissues more quickly than the blood moves. The assumption is sufficiently probable, to be considered nearly certain, that the times above mentioned, as occupied in the passage of the injected substances, are those in which the portion of blood, into which each was injected, was carried from one part to another of the vascular system. Another mode of estimating the general velocity of the circu- lating blood, is by calculating it from the quantity of blood supposed to be contained in the body, and from the quantity which can pass through the heart in each of its actions. But the conclusions arrived at by this method are less satisfactory. For the estimates both of the total quantity of blood, and of the capacity of the cavities of the heart, hav< t only approxi- mated to the truth. Still the most careful of the estimates thus made accord very nearly with those already mentioned ; and it 208 CIRCULATION OF THE BLOOD. [chap. v. may be assumed that the blood may all pass through the heart in from twenty-five to fifty seconds. Peculiarities of the Circulation in Different Parts.— The most remarkable peculiarities attending the circulation of blood through different organs are observed in the cases of the brain, the erectile organs, the lungs, the liver, and the kidney. i. In the Brain. — For the due performance of its functions, the brain requires a large supply of blood. This object is effected through the number and size of its arteries, the two internal carotids, and the two vertebrals. It is further necessary that the force with which this blood is sent to the brain should be less, or at least should be subject to less variation from external circum- stances than it is in other parts, and so the large arteries are very tortuous and anastomose freely in the circle of Willis, which thus insures that the supply of blood to the brain is uniform, though it may by an accident be diminished, or in some way changed, through one or more of the principal arteries. The transit of the large arteries through bone, especially the carotid canal of the temporal bone, may prevent any undue distension ; and uniformity of supply is further insured by the arrangement of the vessels in the pia mater, in which, previous to their distribution to the sub- stance of the brain, the large arteries break up and divide into innumerable minute branches ending in capillaries, which, after frequent communications with one another, enter the brain, and cany into nearly every part of it uniform and equable streams of blood. The arteries are also enveloped in a special lymphatic sheath. The arrangement of the veins within the cranium is also peculiar. The large venous trunks or sinuses are formed so as to be scarcely capable of change of size ; and composed, as they are, of the tough tissue of the dura mater, and, in some instances, bounded on one side by the bony cranium, they are not compressible by any force which the fulness of the arteries might exercise through the substance of the brain ; nor do they admit of distension when the flow of venous blood from the brain is obstructed. The general uniformity in the supply of blood to the brain, which is thus secured, is well adapted, not only to its functions, but also to its condition as a mass of nearly incompressible sub- stance placed in a cavity with unyielding walls. These conditions of the brain and skull have appeared, indeed, to some, enough to chap, v.] PECULIARITIES OF THE CIRCl'I, ATIUX. 209 justify the opinion tliat the quantity of blood in the bruin must be tit all times the same. It was found that in animals bled fco death, without any aperture being made in the cranium, the brain became pale and anaemic like other parts. And in death from strangling or drowning, congestion of the cerebral vessels ; while in death by pmssic acid, the quantity of blood in the cavity of the cranium was determined by the position in which the animal was placed after death, the cerebral vessels being congested when the animal was suspended with its head downwards, and com- paratively empty when the animal was kept suspended by the ears. That, it was concluded, although the total volume of the contents of the cranium is probably nearly always the same, yet the quantity of blood in it is liable to variation, its increase or diminution being accompanied by a simultaneous diminution or increase in the quantity of the cerebro-spinal fluid, which, by readily admitting of being removed from one part of the brain and spinal cord to another, and of being rapidly absorbed, and as readily effused, would serve as a kind of supplemental fluid to the other contents of the cranium, to keep it uniformly filled in case of variations in their quantity (Burrows). And there can be no doubt that, although the arrangements of the blood-vessels, to which reference has been made, ensure to the brain an amount of blood which is tolerably uniform, yet, inasmuch as with every beat of the heart and every act of respiration, and under many other circumstances, the quantity of blood in the cavity of the cranium is constantly varying, it is plain that, were there not pro- vision made for the possible displacement of some of the contents of the unyielding bony case in which the brain is contained, there would be often alternations of excessive pressure with insufficient supply of blood. Hence we may consider that the cerebro-spinal fluid in the interior of the skull not only subserves the mechanical functions of fat in other parts as a packing material, but by the readiness with which it can be displaced into the spinal canal, provides the means whereby undue pressure and insufficient supply of blood are equally prevented. Chemical Composition of Cerebro-spinal Fluid. — The cerebro-spinal fluid is transparent, colourless, not viscid, with a saline taste and alkaline reaction, and is not affected by heat or acids. It contains 981 — 984 parts water, sodium chloride, traces of potassium chloride, of sulphates, carbonates, p 2I0 CIRCULATION OF THE BLOOD. [chap. v. alkaline and earthy phosphates, minute traces of urea, sugar, sodium lactate, fatty matter, cholesterin, and albumen (Flint). 2. In Erectile Structures. — The instances of greatest variation in the quantity of blood contained, at different times, in the same organs, are found in certain structures which, under ordinary cir- cumstances, are soft and flaccid, but, at certain times, receive an unusually large quantity of blood, become distended and swollen by it, and pass into the state which has been termed erection. Such structures are the corpora cavernosa and corpus spongiosum of the penis in the male, and the clitoris in the female ; and, to a less degree, the nipple of the mammary gland in both sexes. The corpus cavemosum penis, which is the best example of an erectile structure, has an external fibrous membrane or sheath ; and from the inner surface of the latter are prolonged numerous tine lamellae which divide its cavity into small compartments looking like cells when they are inflated. Within these is situated the plexus of veins upon which the peculiar erectile property of the organ mainly depends. It consists of short veins which very closely interlace and anastomose with each other in all directions, and admit of great variation of size, collapsing in the passive state of the organ, but, for erection, capable of an amount of dilatation which exceeds beyond comparison that of the arteries and veins which convey the blood to and from them. The strong fibrous tissue lying in the intervals of the venous plexuses, and the external fibrous membrane or sheath with which it is connected, limit the distension of the vessels, and, during the state of erection, give to the penis its condition of tension and firmness. The same general condition of vessels exists in the corpus spongiosum urethras, but around the urethra the fibrous tissue is much weaker than around the body of the penis, and around the glans there is none. The venous blood is returned from the plexuses by comparatively small veins ; those from the glans and the fore part of the urethra empty themselves into the dorsal veins of the penis ; those from the cavemosum pass into deeper veins which issue from the corpora cavernosa at the crura penis ; and those from the rest of the urethra and bulb pass more directly into the plexus of the veins about the prostate. For all these veins one condition is the same; namely, that they are chap, v.] PECULLLRITIEfl OF THB CIRCULATION. 2 II liable to the pres f muscles when the 3 "' The ilea chiefly concerned in this action ai lerator uriine. Erection results from the distension of the as plexuses with blood. The principal exciting cause in the erection of the penis is nervous irritation, originating in the part :. or derived from the brain and spinal cord. The nervous influence is communicated to the penis by the pudic nerves, which ramify in its vascular tissue : and after their division in the fa the penis is no longer capable of erection. This influx of the blood is the first condition necessary for erection, and through it alone much enlargement and turgescence <»f the penis may ensue. But the erection is probably not com- plete, nor maintained for any time except when, together with this influx, the muscles already mentioned contract, and by 1 sing the veins, stop the efflux of blood, or prevent it from being as great as the influx. It appears to be only the most perfect kind of erection that Is the help of muscles to compress the veins; and none such can materially assist the erection of the nipples, or that amount of turgescence, just Killing short of erection, of which the spleen and many other parts are capable. For such turgescence nothing more seems necessary than a large plexiform arrangement of the veins, and such arteries as may admit, upon occasion, augmented quantities of blood. (3, 4. 5). The circulation vr>. the Lungs, Liver, and Kidney* will be described under those heads. Agents concerned in the circulation. — Before quitting subject it will be as well to bring together in a tabular form the various agencies concerned in maintaining the circulation. 1. The Systole and Diastole of the Heart, the former pumping • the aorta and so into the arterial system a certain amount of :. and the latter to some extent sucking in the blood from the veins, 2. Tli- elastic and muscular coats of the . which serve to keep up an equable and continuous stream. 3. The so-called vital capillary force. 4. The pressure of the : with and the :it rhythmic contraction of the veins. 5. Aspiration of the thorax during inspiration, by means of which p 2 212 CIRCULATION OF THE BLOOD. [chap. v. the blood is drawn from the large veins into the thorax (to he treated of in next Chapter). Discovery of the Circulation. Up to nearly the close of the sixteenth century it "was generally believed that the blood passed from one ventricle to the other through foramina in the " septum ventriculorum." These foramina are of course purely imaginary, but no one ventured to dispute their existence till Servetus boldly stated that he could not succeed in finding them. He further asserted that the blood passed from the Right to the Left side of the heart by way of the lungs, and also advanced the hypothesis that it is thus " revivified," re- marking that the Pulmonary Artery is too large to serve merely for the nutrition of the lungs (a theory then generally accepted). Realdus, Columbo, and Caesalpinus added several important observations. The latter showed that the blood is slightly cooled by passing through the lungs, also that the veins swell up on the distal side of a ligature. The existence of valves in the veins had previously been discovered by Fabricius of Aquapendente, the teacher of Harvey. The honour of first demonstrating the general course of the circulation belongs by right to Harvey, who made his grand discovery about 1618. He was the first to establish the muscular structure of the heart, which had been denied by many of his predecessors ; and by careful study of its action both in the body and when excised, ascertained the order of contraction of its cavities. He did not content himself with inferences from the anatomy of the parts, but employed the experimental method of injection, and made an extensive and accurate series of observations on the circulation in cold- blooded animals. He forced water through the Pulmonary Artery till it trickled out through the Left Ventricle, the tip of which had been cut off. Another of his experiments was to fill the Right side of the heart with water, tie the Pulmonary Artery and the Venas Cavae and then squeeze the Right ventricle : not a drop could be forced through into the Left ventricle, and thus he conclusively disproved the existence of foramina in the septum ventriculorum. " I have sufficiently proved,*' says he, " that by the beating of the heart the blood passes from the veins into the arteries through the ventricles, and is distributed over the whole body.*' " In the warmer animals, such as man, the blood passes from the Right Ventricle of the Heart through the Pulmonary Artery into the Lungs, and thence through the Pulmonary Veins into the Left Auricle, thence into the Left Ventricle.'" Proofs of the Circulation of the Blood. — The following are the main arguments by which Harvey established the fact of the circulation : — 1. The heart in half an hour propels more blood than the whole mass of blood in the body. 2. The great force and jetting manner with which the blood spurts from an opened artery, such as the carotid, with every beat of the heart. / 1 + posterior membranous part of the trachea ; h. ',' , SICieS 01 Hie tracnea ^aOOUt right and left bronchi. ( Allen Thomson.) *. t WO-thirds of its cirCUmfer- ence), and are deficient behind ; the interval between their poste- rior extremities being bridged over by a continuation of the fibrous membrane in which they are enclosed (fig. 145). The C1I.U'. VI.] THE TRAHIKA. 219 cartilages of the trachea and bronchial tubes arc of th< hy variety. Immediately within this tube, at the back, is a layer of unstriped muscular fibres, which extends, transverse///. 1 ■• - . the end the cartilaginous rings to which they are attached, and op] the intervals between them, also ; their evident function being I - . :-- ; . 7 yf.—i . o. columnar ciliated epithelium ; b and <-, proper structure of the mucous membrane, containing elastic fibres cut across trans'*. - .bmucou.-^ tissue containing mucous glands, e, separated from the hyaline cartilage, .7, by a fine tibruus tissue, f\ /(.external investment of fine fibrous tissue. (S. K. Alcock.) diminish, when required, the calibre of the trachea by appi mating the ends of the cartilages. Outside these are a few V tudiiwl bundles of muscular tissue, which, like the preceding, arc attached both to the fibrous and cartilaginous framework. The mucous membrane aists of adenoid tissue, separate] 220 EESPIEATIOX. [cha*. vi. from the stratified columnar epithelium which lines it by a homo- geneous basement membrane. This is penetrated here and there by channels which connect the adenoid tissue of the imccosa with the intercellular substance of the epithelium. The stratified columnar epithelium is formed of several layers of cells (fig. 147), of which the most superficial layer is ciliated, and is often branched downwards to join connective-tissue corpuscles ; while between these branched cells are smaller elongated cells prolonged up towards the surface and down to the basement membrane. Be- neath 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 substance of the fibrous framework of the trachea ; their ducts perforating 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. 145), resemble the trachea exactly in structure, and in the arrangement of their cartilaginous rings. On entering the sub- stance 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 sub-divide, 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 sub-divisions of the lungs, called lobules. All the larger branches still have walls formed of tough mem- brane, containing portions of cartilaginous rings, by which they are held open, and unstriped muscular fibres, as well as longi- tudinal bundles of elastic tissue. They are lined by mucous mem- brane, the surface of which, like that of the larynx and trachea, is covered with ciliated epithelium (fig. 148). The mucous mem- brane 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, CHAP. VI. | THE LUNGS. 221 until, in the smaller bronchia] tubes, they arc represented only by minute and scattered cartilaginous flakes. And when the bronchi, 1>\ successive branches are reduced to about -J ti of an inch in diameter. they lose their cartilaginous element altogether, and their walls are formed only of a tough fibrous clastic membrane, with circular muscular fibres ; they are still lined, however, by a thin mucous T7K Fig. 148. — Transverse section of a bronchus, about \ inch in diameter, e. Epithelium (ciliated) ; immediately beneath it is the mucous membrane or internal fibrous layer, of varying thickness ; m, muscular layer ; s, m, submucous tissue ; /', fibrous tissue • r, cartilage enclosed within the layer's of fibrous tissue ; g, mucous gland. (F. e! Schulze.) membrane, with ciliated epithelium, the length of the cells bearing the cilia having become so far diminished, that the cells are now almost cubical. In the smaller bronchi the circular mus- cular fibres are more abundant than in the trachea and larger bronchi, and form a distinct circular coat. The Lungs and Pleura. — The Lungs occupy the greater por- tion of the thorax. They are of a spongy elastic texture, and on section appear 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 pleura, one layer of which adheres closely to the surface of the lung, and provides it with its smooth and slippery covering, while the other adheres to the inner surface of the chest-wall. The continuity of the two layers, which form a closed sac, as in the case of other 222 RESPIRATION. [chap. VI. serous membranes, will be best understood by reference to fig. 149. The appearance of a space, however, between the pleura which covers the lung {visceral layer), and that which lines the inner sur- face of the chest {parietal layer), is inserted in the drawing only for the sake of distinctness. These layers are, in health, every- where in contact, one with the other ; and between them is only just so much fluid as will ensure the lungs gliding easily, in their , «,, . J>ericairZium~ -, PuJhrtVttn t ••-"- Pulm 1 Pulm^Veirv Left Lung CEsojiliagul BrencJw.s Pig". 149. — Transverse section of the chest (after Gray). 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 considerable space is left between the lung and the chest- wall. In other words, the natural elasticity of the lungs would cause them at all times to contract away from the ribs, were it not that the 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, atmospheric 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 con- sists of an outer or denser layer and an inner looser tissue. The former or 'pleura proper consists of dense fibrous tissue with elastic < HAW VI 'J HE LUNGS. 223 vered l>\ endothelium, the cells <>f which are largi . flat, hya- line, and transparent when the lung is expanded, bul become smaller, thicker, and granular when the lung collapses. In the pleura is a lymph-canalicular system; and connective tissue corpus are found in the fibres and tissue which forms its groundwork. The inner, looser, or subpleura] tissue contains lamellae of fibrous connective tissue and connective tiss b between them. Numerous lymphatics are to l»e met with, which form a dense plexus 1 • -. many of which contain valve-. They are simple endothelial tubes, and take origin ill the lymph-canalicular BJ of the pleura proper. Scattered bundles of unstriped muscular nr in the pulmonary pleura. They are especially strongly developed on those parts (anterior and internal surfaces of lungs) which move most freely in respiration: their function is doubt- n to aid in expiration. 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 a; the right lung into three lobes, and the left into two. Each of these lobes, again, is composed of a large num- ber of minute parts, called lobules. Each pulmonary lobule may be considered a lung in miniature, consist- - it does, of a branch of the bronchial tub', air-cells, blood vess nerves, and lymphatics, with a sparing amount of areolar le. On entering a lobule, the small bronchial tube, the structure of which has been just described (a, fig. 150), divides and sub-divides; its walls at the same time becoming thinner and thinner, until at length thev are formed only of a thin membrane of areolar and elastic tissue, lined by a layer of squamous epithelium, not pro- vided with cilia. At the same time, they are altered in shape : Fig. 150. — Ciliary epithelium of the human I <7, Layer of longitudinally arranged elastic fibres "; b, basement membrane ; <-, deepest cells, circular in form ; d, intermediate elon- gated cells ; e, outermost layer of cells fully developed and bearing cilia. X 350. ::ker.; 224 RESPIRATION. [CHAP. VI. Fig. 151. — Terminal branch of a bronchial tube, with its infundibula and air-cells, from the margin of the lung of a monkey, injected with quicksilver, a, terminal bronchial twig; b b, infundibula and air-cells, x 10. (F. E. Schuke.) each of the minute terminal branches -widening out funnel-wise, and its walls being pouched out irregularly into small saccular dilatations, called air-cells (fig. 151, b). Such a funnel-shaped terminal branch of the bron- chial tube, with its group of pouches or air-cells, has been called an infundihulum (figs. 151, 152), and the irregular oblong space in its centre, with which the air-cells com- municate, an intercellular pas- sage. The air-cells, or air-vesicles, may be placed singly, like re- cesses 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 vesicles, all communicating with one another, open by a common orifice into the tube. The vesi- cles are of various forms, accord- ing to the mutual pressure to which they are subject; their walls are nearly in contact, and they vary from ■£§ to ^ 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 contigu- ous air-vesicles, or between the vesicles and the bronchial pas- sages. Numerous fibres of elastic tissue are spread out between contiguous air-cells, and many of these are attached to the outer surface of the fine membrane of Fig. 152. — Two small infundibula or groups of air-cells, a a, with air-cells, b b, and the ultimate bronchial tubes, c c, with which the air-cells communicate. From a new-bom child. (Kolliker.) I MAP. \ I. | THE LUNGS, 22 which each cell is composed, imparting to it additional strength, and the power of recoil after distension. The cells are lined by a layer of epithelium (fig. 153), not provided with cilia. Outside the cells, ;i network of pulmonary capillaries is spread out so denseli (fig. 1 54), that the interspaces or meshes arc even narrower than the vessels, which arc, on an average, .;,,',,,, <>f an inch iii diameter. a ■ Fig. 153. — From a section of lung of « 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. Circular muscular 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.) 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 exposure 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 Q 226 RESPIRATION. [CHAP. VI. nearer to the circumference. The vesicles of adjacent lobules do not communicate ; and those of the same lobule or proceeding from the same intercellular passage, 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, (6) the bronchial arteries. The former conveys venous blood to the lungs for its arterial nation, and this Fis 1=4. — Capillary net-ivork of Vie pulmm •-- '& in the human lung. x 60. (Kolliker.) blood takes no 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 interlobular connective tissue, &c. ; the blood of the bronchial vessels being returned chiefly through the bronchial and partly through the pulmonary veins. Lymphatics. — The lymphatics are arranged in three sets : — 1. Irregular lacunaB in the walls of the alveoli or air-cells. The lymphatic vessels 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 chap, v!.] INSPIRATION. 227 sinuses pass in towards the root of the lung to reaol 'in bronchial glands. .v . -The nerves of tlic lung arc to be traced from the anterior and posterior pulmonary plexuses, which are formed by branches of the vagus and sympathetic. The nerve* follow the course of the vessels and bronchi, and in the walls of* the latter many small ganglia arc 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 arc called Inspiration and Expiration respectively. For the inspiration of air into the lungs it is • vident 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 increase of capacity there will be of course a diminution of the pressure of the air in the lungs, and a fresh quantity will enter through the larynx and trachea to equalise the pressure on the inside and outside of the chest. For the expiration of air, on the other hand, it 1- 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 pressures within and without the chest arc again equal It both cases the air passes through the trachea and larynx, whether in entering 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. We have now to consider the means by which the respiratory movements 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 Q 2 228 RESPIRATION. [CHAP. VI. 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 posticus superior. (a.) The vertical diameter of the chest is increased by the con- traction and consequent descent of the diaphragm, — the sides of the muscle descending most, and the central tendon remaining, comparatively unmoved ; while the intercostal and other muscles, by acting at the same 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 Fig. 155. — Diagram of axes of movement of ribs. 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 up- ward movement by their attachment to the spine. The movement of the front extremities of the ribs is of necessity accompanied by an upward and forward movement of the sternum to which they OH \r. VI.] INSINUATION. 229 are attached, the movement being greater at the lower end than at the upper end of the latter bone. Th> axes of rotation in these movements are twoj one cor- responding with a line drawn through the two articulations which the rib forms with the spine (a b, fig. 155) ; and the other, with a line drawn from one of these (head of rib) to the sternum (A B, fig. 155, and fig. 156); 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. 156. — Diagram of movement of a rib in inspiration. (fig. 156, A); and thus an additional means is provided for increasing the antero-posterior diameter of the chest. The muscles by which the ribs arc raised, in ordinary quiet inspiration, are the external intercostals, and that portion of the internal intercostals 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, 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 230 RESPIRATION. [chap. VI. C'/, n' t^t %J D no fixed point towards which they can pull the bones to which they are attached. A very simple apparatus will explain this apparent anomaly and make their action plain. Such an apparatus is shown in rig. 157. A B is an upright bar. representing the spine, , 5 with which are jointed two parallel ,;>;'[ bars, C and D, which represent two s's' ij of the ribs, and are connected in A y? front by moveable joints with an- il- other upright, representing the ster- num. 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 inter- costal fibres (fig. 157, C D). i.e , passing downwards and forwards, they raise them (fig. 157 C D') ; while on the other hand, if placed in imitation of the position of the internal intercostals- (tig.158 , E F), -i.e., passing downwards and backwards, they depress them (fig. 158, E' F'). The explanation of the foregoing facts is very simple. The intercostal 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. 157) 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 attachment C 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 oppo- site action to that of the external. In contracting they must pull down the ribs, because the points E and F (fig. 158) can only be brought nearer one to another (fig. 158, E' E F') by such an alteration in their position. On account of the oblique position of the cartilages of the ribs with refer- ence to the sternum, the action of the inter-cartilaginous fibres of the internal U J5 Fig. 157. — Diagram of apparatus showing the action of the external intercostal muscles. Fig. 158. — Diagram of apparatus showing the action of the internal intercostal muscles. chap, vi.] EXPIRATION. 231 intercostala must, of course, on tlic foregoing principles, resemble that of the external intercostala. In tranquil breathing, the expansive movements of the lower part of fche 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 impossible 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 sternomastoid, the serratus magnus, the pectorales, and the trapezius. Types of Respiration. — The expansion of the chest in inspi- tation presents some peculiarities in different persons. In young children, it is effected chiefly by the diaphragm, which being highly arched in expiration, 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 walls being here more manifest than that of any other part, it is usual to call this the abdominal type of respiration. 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 more so in the upper, part of the chest (superior costal type). (See figs. 159, 160.) B. Expiration. — From the enlargement produced in inspira- tion, the chest and lungs return in ordinary tranquil expiration, by their elasticity ; 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 lungs is suffi- cient, in ordinary quiet breathing, to expel air from the chest in 232 JRESPIRATIOX. [CHAP. VI. the intervals of inspiration, and no muscular power is required. In all voluntary expiratory efforts, however, as in speaking, sing- ing, 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 muscles are brought into action. By far the chief of these are Fig. 159. — The changes of the thoracic and abdominal walls of the mule during respv- ration. The back is supposed to be fixed, in order to throw forward the respira- tory movement as much as possible. The outer black continuous line in front represents the ordinary breathing move- ment : the anterior margin of it being the boundary of inspiration, the poste- rior margin the limit of expiration. The line is thicker over the abdomen, since the ordinary respiratory move- ment is chiefly abdominal : thin over the chest, for there is less movement over that region. The dotted line indi- cates the movement on deep inspiration, during which the sternum advances while the abdomen recedes. Fig. 160. — 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 Hutchinson.) The posterior continuous line represents in both figures the limit of forced expi- ration. the abdominal 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 there- fore we must conclude that the abdominal muscles are assisted in .:•. vi.] SPIRATO] S 90TJX] 233 their action bj I g ter part of the the triangularit -' . the a itai lumborum. When by thi 3, the chest a eon squ< to _ ter, it ag tin, relaxation of the muscles, returns t< » the normal din. vinue of its elasticity. The construction of the -- tils, tin fore, admirably adapts them : ling gainst an well undue contraction as undue dilatation. In the natural condition of the ] ts, 1 _- never con- tract to the utmost, but are always more 'less u a th< stretch," og kept closely in contact with the inner surface of the walls the chest by atmospheric press . aid can contract away from these only when, by some means or other, - making an open- ing into the pleural cavi: y the eftusion of fluid there, the pre— d the exterior and interior of the lungs becomes equal. Thus, under ordinary circumstances, the dilatation of the lungs - dependent on that of the boundary walls the chest, th iter surfat of the one being inclose contact with the inner - of the other, and obliged to follow it in all lovements. Respiratory Rhythm. — The its : .-ion and contraction of the ch st> i . under ordinary circumstances, a nearly equal time. The act of inspiring air, however., - illy 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 i t the next inspiration. The respiratory rhythm may be thus expressed : — Inspiration ...... 6 Expiration . . . . . . . 7 or S A very sligli: pause. Respiratory Sounds. — If the ear be placed in c ^ itli the wall of the chest, or be separated from it only by a good conductor of sound, a faint torv murmur is heard during inspiration. This sound a somewhat in different parts — being loudest or o tsesf in the neighbourhood i and large bronchi (tracheal and bronchial breathing;, and fading off into a faint - og as the ear is pi from tL (vesicular breathing). It is best in children, and in them 234 INSPIRATION. [chap. vt. 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 distension (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. — 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 dilatation of the nostrils is well seen, although under ordinary conditions it may not be noticeable. The opening at the upper part of the larynx, however, or rima glottidis (fig. 297), is dilated at each inspiration, for the more ready passage of air, and becomes smaller at each expiration ; its condition, therefore, corresponding during respiration with that of the walls of the chest. There is a further likeness between the two acts in that, under ordinary circumstances, 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 contraction of the glottis, considerable muscular power exercised. Terms used to express Quantity of Air breathed.— Breathing or tidal air, is the quantity of air which is habitually and almost uniformly changed in each act of breathing. In a healthy adult man it is about 30 cubic inches. 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. 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. 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 esti- mated at about 100 cubic inches. obat, vi.] RESPIRATORY. CAPACITY. 235 The total quantity of air which passes into and oul of the Lungs of an adult, at rest, in 24 hours, is about 686,000 cubic inch) 'This quantity, however, is largely increased by ei srtion ; tin- average amount tV>r a hard-working labourer in the same tin being 1,568,390 cubic inches. Respiratory <'hown 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 lbs. The elastic force overcome in ordinary inspiration i-. according to the same authority, equal to about 170 lbs. Douglas Powell has shown that within the limits of ordinary tranquil respiration^ the elastic resilience of the walls of the favours inspiration ; and that it is only in deep inspiration that the ribs and rib-cartilages offer an opposing force t<> their dilata- tion. 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 expansion. Under all circumstances, of course, the elastic tissue of the lungs opposes inspiration, and favours expiration. Functions of Muscular Tissue of Lungs. — It is possible that the contractile power which the bronchial tubes and air-vesicles 238 RESPIRATION. [chap. vi. possess, by means of their muscular fibres may (1) assist in expira- tion ; bnt 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 contracts upon and gradually expels collec- tions of mucus, which may have accumulated within the tubes, and 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 muscular 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 clastic 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 Atmospliere. — 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, carbonic acid, and watery vapour, with, commonly, traces of other gases, as ammonia, sulphuretted hydrogen, &c. Of every 100 volumes of pure atmospheric air, 79 volumes (on an average) consist of nitrogen, the remaining 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 vapour varies greatly according to the temperature and other circumstances, but the atmosphere is never without some. In this country, the average quantity of watery vapour in the atmosphere is 1*40 per cent. Composition of Air which 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 vapour ; 6, the addition of a minute amount of organic matter and of free ammonia. chap, yi.] RESPIRATORY CHANGES OF AIE. 239 1. The expired air, heated by its contact with tin: interior of the lungs, is (at least in most climates) hotter thai] the inspired air. It>. 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 inhaled, it nearly acquires that of the blood before it i> expelled from the chest. 2. The Carbonic Acid in respired air is always increased; but the quantity exhaled in agiveo time is subject to change from various circumstances. From every volume of air inspired, about 4*8 per cent, of oxygeo 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 carl ion excreted from the lungs is about 173 grains per hour, or rather more than 8 ounces in twenty-four hours. These quantities must be considered approximate only, inasmuch as various circumstances, 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. Respiratory: movements. External temperature. Season of year. Condition of respired air. Atmospheric conditions. Period of the day. Food and drink. Exercise and sleep. a. Ar/e 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 awhile, gradually diminishes, and from fifty to extreme age it goes on diminishing, 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. \\ hen 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 respira- tion are perf ormed, the smaller is the proportionate quantity of carbonic acid contained in each volume of the expired air. Although, however, the pro- portionate quantity of carbonic acid is thus diminished during frequent respiration, yet the absolute amount exhaled into the air within a given 240 RESPIRATION. [uHAje. vi. time is increased thereby, owing to the larger quantity 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 various 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 io° F. causes a diminution of about two cubic inches in the quantity of carbonic acid exhaled per minitte. d. Season of tJie Year. — The season of the year, independently of tempe- rature, 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 tlie 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. /. Ilyyrometrie State of Atmosphere.-— The amount of carbonic acid exhaled is considerably influenced by the degree of moisture of the atmo- sphere, much more being given off when the air is moist than when it is dry. (Lehmann.) g. 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 absorded by night to meet the requirements of the day. If the total quantity of carbonic acid exhaled in 24 hours be represented by 100, 52 parts are exhaled during the day, and 48 at night. While, similarly, 33 parts of the oxygen are absorbed during the day, and the remaining 67 by night. (Pettenkofer and Voit.) h. Food and Brink. — By the use of food the quantity is increased. Avhilst 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 alcohol tends rather to increase than to lessen respiratory changes, and the amount therefore of carbonic acid expired ; ram, ale, and porter, also sherry, have very similar effects. On the other hand, brandy, whisky, and gin, particularly the latter, almost always lessened the respiratory changes, and consequently the amount of carbonic acid exhaled. (Edward 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 labour on the treadwheel, still further increases the amount of the acid exhaled. (Edward Smith.) A larger quantity is exhaled when the barometer is low than when it is high. chap.yl] CHANGES OF THE AIR. 241 3. The oxygen is diminished, and its diminution is generally proportionate to the increase of the carbonic acid. For every volume of carbonic acid exhaled into the air, 1-17421 volumes of oxygen arc absorbed from it, and 1346 cubic inches, or 636 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 estimate, 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 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 of in forming some of the carbonic acid and water given off from the skin, and in combining with sulphur and phosphorus to form part of the acids of the sulphates and phosphates excreted in the urine, and probably also, with the nitrogen of the decomposing nitrogenous tissues. (Bence Jones.) 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 experi mented with. It has often been discussed whether Nitrogen is absorbed by or exhaled from the lungs during respiration. At present, all that can be said on the subject is that, under most circumstances, animals appear to expire a very small quantity above that which exists in the inspired air. During prolonged fasting, on the con- trary, a small quantity appears to be absorbed. 5. The watery vapour 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 circumstances, (1), by the quantity of air respired ; for the greater this is, the greater also will be the quantity of moisture R 242 RESPIRATION. [chap. vi. exhaled. (2), by the quantity of watery vapour contained in the air previous to its being inspired ; because 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 vapour required to saturate the air ; (4), by the length of time which each volume of inspired air is allowed to remain in the lungs ; for although, during ordinary respiration, the expired air is always saturated with watery vapour, yet when respiration is performed very rapidly the air has scarcely time to be raised to the highest tem- perature, or be fully charged with moisture ere it is expelled. The quantity of water exhaled from the lungs in twenty-four hours ranges (according to the various modifying circumstances already mentioned) from about 6 to 27 ounces, the ordinary quantity being about 9 or 10 ounces. Some of this is probably formed by the chemical combination of oxygen with hydrogen in the system ; but the far larger proportion of it is water which has been absorbed, as such, into the blood from the alimentary canal, and which is exhaled from the 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 animals. 6. A small quantity of ammonia is added to the ordinary constituents of expired air. It seems probable, however, both from the fact that this substance cannot be always detected, and from its minute amount when present, that the whole of it may be derived from decomposing particles of food left in the mouth, or from carious teeth or the like ; and that it is, therefore, only an accidental constituent of expired air. 7. The quantity of organic matter in the breath is about 3 grains in twenty-four hours. (Ransome.) The following represents the kind of experiment by which the foregoing facts regarding the excretion of carbonic acid, water, and organic matter, have been established. A bird or mouse is placed in a large bottle, through the stopper of which two tubes pass, one to supply fresh air, and the other to carry 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 becomes turbid and soon quite milky from the precipitation of calcium carbonate ; and it finally passes CHAJ.YL] METHOD OF THE RESPIRATORY CHANGE 243 through strong sulphuric acid, which, by turning brown, indicates the pre- Bence of organic matter, The watery rapoui 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, experi- ments 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 considered. It remains to consider how it is that the blood absorbs oxygen from, and gives up carbonic acid to, the air of the alveoli. In the first place, it must be remem bered 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 stationary air by diffusion, and so supplies to it new oxygen. The amount of oxygen in ex- pired air, which may be taken as the average composition of the mixed air in the lungs, is about 1 6 to 17 per cent. ; in the pulmo- nary 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 difficult 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 quan- tity 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 grm. haemoglobin). On the other hand, if the substance 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 r 2 244 RESPIBATION. [chap. vi. it neither takes up nor gives out oxygen ; 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, (or 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 temperature, 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 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. Respiratory Changes 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 permit 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 intervening. 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 junction, and incapable of closing the orifice of either of the veins between which they are placed. The pul- chap, vi.] VABI0U8 RESPIRATORY ACTION8, 245 monary circulation also is unaffected by changes of atmospheric pressure, and is not exposed to the influence of tin- pressur muscles : the force by which it is accomplished, and the coure the blood arc alike simple. Changes produced in the Blood by Respiration. — The most obvious change which the blood of the pulmonary artery undergoes in its passage through the lungs is 1st, that of colour. the dark crimson of venous blood being exchanged for the bright scarlet of arterial blood; 2nd, and in connection with the pre- ceding change, it gains oxygen; $rd, it loses carbonic acid; 4^, it becomes slightly cooler (p. 239); 5^, it coagulates sooner and more firmly, and, apparently, contains more fibrin (see p. 108). The oxygen absorbed into the blood from the atmospheric air in the lungs is combined chemically with the haemoglobin of the red blood-corpuscles. In this condition it is carried in the arterial blood to the various parts of the body, and brought into near relation or contact with the tissues. In these tissues, and in the blood which circulates 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 accompanying diagram (fig. 161) 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 inter- fered with. From the same diagram it will be seen that the lungs communicate with the exterior of the body through the glottis, and further on through the mouth and nostrils — through either of them separately, or through both at the same time, according to the position of the soft palate. The stomach communicates 246 RESPIRATION. [chap. vr. with the exterior of the body through the oesophagus, pharynx, and mouth ; while below the rectum opens at the anus, and the bladder through the urethra. All these openings, through which the hollow viscera communicate with the exterior of the body, are Fig. 161. guarded by muscles, called sphincters, which can act independently of each other. The position of the latter is indicated in the diagram. 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 inspiratory part of this act, the descent of the diaphragm presses the abdominal viscera downwards, and of chap, vi.] VARIOUS RE8PIEATORY ACTION& 247 coin^e tliis pressure tends to evacuate the contents of >uch as communicate with the exterior of the body. Enasmuch, how- r. as their various openings are guarded by sphincter muscl -. in a state of constant tonic contraction, there of their contents, and air simply enters the lungs. In the second, or expiratory part of the act of sighing, there is also pressure mode on the abdominal viscera in the opposite direction, by the elastic or muscular recoil of the abdominal walls : but the pres- sure is relieved by the escape of air through the open glottis, and the relaxed diaphragm is pushed up again into its original position. The sphincters of the stomach, rectum, and bladder, act as before. Hiccough resembles sighing in that it is an inspiratory act ; but the inspiration is sudden instead of gradual, from the diaphragm acting suddenly and spasmodically ; and the air. there- fore suddenly rushing 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 an inspiration, and this is followed by an expiration : but when the lungs have been tilled by the preliminary inspiration, instead of the air being easily let out again through the glottis, the latter is momentarily closed by the approximation of the vocal cords, and then the abdominal muscles, strongly acting, push up the viscera against the diaphragm, and thus make pressure on the air in the lungs until its tension is sufficient to burst open noisily the vocal cords which oppose its outward passage. In this way a considerable force is exercised, and mucus or any other matter that may need expulsion from the lungs or trachea is quickly and sharply expelled by the outstreaming current of air. Now it is evident on reference to the diagram (fig. 161), that pressure exercised by the abdominal muscles in the act of cough- ing, acts as forcibly on the abdominal viscera as on the lungs, inasmuch as the viscera form the medium by which the upward pressure on the diaphragm is made, and of necessity there is quite as great a tendency to the expulsion of their contents as of the air in the lungs. The instinctive, and if necessary, volun- : .ly increased contraction of the sphincters, however, prevents any escape at the openings guarded by them, and the pressure is effective at one part only, namely, the rima glottidis. 248 RESPIRATION. [chap. vi. 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 contraction of the pillars of the fauces and descent of the soft palate, chiefly through the nose, and an}- offending matter is thence expelled. Speaking. — In speaking, there is a voluntary expulsion of air through the glottis by means of the expiratory muscles ; and the vocal cords are put, by the muscles of the larynx, in a proper position and state of tension for vibrating as the air passes over them, and thus producing sound. The sound is moulded into words by the tongue, teeth, lips, &c. — the vocal cords producing the sound only, and having nothing to do with articulation. Singing, — Singing resembles speaking in the manner of its production; the laryngeal muscles, by variously altering the posi- tion and degree of tension of the vocal cords, producing the different notes. Words used in the act of singing are of course framed, as in speaking, by the tongue, teeth, lips, etc. Sniffing- — Sniffing is produced by a somewhat quick action of the diaphragm and other inspiratory muscles. The mouth is, how- ever, closed, and by these means the whole stream of air is made to enter by the nostrils. The alee nasi are, commonly, at the same time, instinctively dilated. Sobbing. — Sobbing consists in a series of convulsive inspira- tions, at the moment of which the glottis is usually more or less closed. Laughing. — Laughing is a series of short and rapid expirations. Yawning. — Yawning is an act of inspiration, but is unlike most of the preceding actions in being always more or less in- voluntary. 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 character of yawning depends probably on the fact that the muscles concerned are themselves at all times more or less involuntary, and require, therefore, something beyond the exercise of the will to set them in action. For the same reason, yawning, like sneezing, cannot be well performed voluntarily. chap, vi.] RESPIRATORY CENTRE, 249 Sucking. — Slicking is not properly a respiratory act, but it may be most conveniently considered in tins 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 equili- brium on the inside and outside of the mouth as best it may. The communication between the mouth and pharynx is com- pletely shut oft' by the contraction of the pillars of the soft palate and descent of the latter so as to touch the back of the tongue ; and the equilibrium, therefore, can be restored only by the entrance of something through the mouth. The action, indeed, of the tongue and floor of the mouth in sucking may be compared to that of the piston in a syringe, and the muscles which pull down the os hyoides and tongue, to the power which draws the handle. Influence of the Nervous System in Respiration. — Like all other functions of the body, the discharge of which is neces- sary to life, respiration must be essentially an involuntary act. Else, life would be in constant danger, and would cease on the loss of consciousness for a few moments, as in sleep. But it is also necessary that respiration should be to some extent under the control of the will. For were it not so, it would be im- possible to perform those voluntary respiratory acts which have been just enumerated and explained, as speaking, singing, and the like. The respiratory movements and their rhythm, so far as they are involuntary and independent of consciousness (as on all ordinary occasions) are under the governance of a nerve-centre in the medulla oblongata corresponding with the origin of the pneumogastric nerves ; that is to say, the motor nerves and through them, the muscles concerned in the respiratory move- ments, are excited by a stimulus which issues from this part of the nervous system. How far the medulla acts automatically, i.e., how far the stimulus originates in it, or how far it is merely a nerve-centre for re/lex action, is not certainly known. Probably, as will be seen, both events happen; and, in both cases, the stimulus is the result of the condition of the blood. The respiratory centre is bilateral or double, since the respira- 250 RESPIRATION. [chap. vi. tory movements continue after the medulla at this point is divided in the middle line. As regards its supposed automatic action, it has been shown that if the spinal cord be divided below the medulla, and both vagi be divided so that no afferent impulses can reach it from below, the nasal and laryngeal respiration continues, and the only possible eourse of the afferent impulses would be through the cranial nerves ; and when the cord and medulla are intact the division of these pro- duces no effect upon respiration, so that it appears evident that the afferent stimuli are not absolutely necessary for maintaining the re- spiratory movements. But although automatic in its action the respiratory centre may be reflexly excited, and 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 be cut the respiratory action is still slower. The influence of the vagus trunk upon it is twofold, for if the nerve be divided below the origin of the superior laryngeal branch and the central end be 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 follows from a tetanus of the respira- tory muscles (diaphragm). Whereas if the superior laryngeal branch be divided, 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 be increased, stops, but not in inspiration as in the other case, but in expiration. Thus the vagus trunk contains fibres which slow and fibres which accelerate respiration. If we adopt the theory of a doubly acting respiratory centre in the floor of the medulla, one tending to produce inspiration and the other expiration, and acting in antagonism as it were, so that there is a gradual increase in the tendency to produce respiratory action, until it culminates in an inspiratory effort, which is followed by a similar action of the expiratory part of the centre, producing an expiration, we must look upon the main trunk of the vagus as aiding the inspiratory, and of the superior laryngeal as aiding the expira- tory 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 (hap. vi.] STIMULATION OF RESPIBATORY CENTER 25 1 as the respiration is slowed on division of the vagi, and not quickened or affected manifestly on simple division of the superior larygneal, it must be supposed that the vagi fibres are always in action, whereas the superior larygneal fibres are not. It appears, however, that there are. in some animals at all events, subordinate centres in the spinal cord which arc able, under certain conditions, to discharge the function of the chief medullary centre. The centre in the medulla may be influenced not only by afferent impulses proceeding along the vagus and laryngeal ner but also by those proceeding from the cerebrum, as well as 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, as is exemplified by the deep inspira- tion which follows the application of cold to the surface of the skin, and by the sneezing which follows the slightest irritation of the nasal mucous membrane. At the time of birth, the separation of the placenta, and the consequent non-oxygenati'on of the foetal blood, are the circumstances which immediately lead to the issue of automatic impulses to action from the respiratory centre in the medulla oblongata. But the quickened action which ensues on the application of cold air or water, or other sudden stimulus, to the skin, shows well the intimate connection which exists between this centre and other parts which are not ordinarily connected with the function of respiration. Methods of Stimulation of Respiratory Centre. —It is now necessary to consider the method by which the centre or centres are stimulated themselves, as well as the manner, in which the afferent vagi impulses are produced. The more venous the blood, the more marked are the inspira- tory impulses, and if the air is prevented from entering the che in a short time the respiration becomes very laboured. Its cessation is followed by an abnormal rapidity of the inspiratory acts, which make up even in depth for the previous stoppage. The condition caused by obstruction to the entrance of air, or by any circum- stance by which the oxygen of the blood is used up in an abnor- mally quick manner, is known as dyspnoea^ and as the aeration of the blood becomes more and more interfered with, not only are the ordinary respiratory muscles employed, but also those extraor- 252 RESPIBATIONi [cbap. vl dinary muscles which have been previously enumerated (p. 231), so that as the blood becomes more and more venous the action of the medullary centre becomes mure and more active. The ques- tion arises as to what condition of the venous blood causes this increased activity, whether it is due to deficiency of oxygen or excess of carbonic acid in the blood. This 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 therefore cannot be due to the accumulation of carbonic acid, and secondly, that if plenty of oxygen be supplied, dyspnoea proper does not occur, although the carbonic acid of the blood is in excess. The respiratory centre is evidently stimulated to action by the absence of sufficient oxygen in the blood circulating in it. The method by which the vagus is stimulated to conduct afferent impulses, influencing the action of the respiratory centre, appears to be by the venous blood circulating in the lungs, or as some say by the condition of the air* in the pulmonary alveoli. And if either of these be the stimuli it will be evident that as the condition of venous blood stimulates the peripheral endings of the vagus in the lungs, the vagus action which tends to help on the discharge of inspirator}' impulses from the centre, must tend also to increase the activity of the centre, when the blood in the lungs becomes more and more venous. Xo doubt the venous condition of the blood will affect all the sensory nerves in a similar manner, but it has been shown that the circulation of too little blood through the centre is quite sufhcient by itself for the purpose ; as when its blood supply is cut off increased inspiratory actions ensue. Effects of Vitiated Air.— Ventilation. — We have seen that the air expired from the lungs contains a large proportion of carbonic acid and a minute amount of organic putrescible matter. Hence it is obvious that if the same air be breathed again and again, the proportion of carbonic acid and organic matter will constantly increase till fatal results are produced ; but long before this point is reached, uneasy sensations occur, such as headache, languor, and a sense of oppression. It is a remarkable fact that the organism after a time adapts itself to such a vitiated atmosphere, and that a person soon comes to breathe, without CHAP, vi.] EFFECT ON THE CIBCULATION. 253 sensible inconvenience, an atmosphere which, when he Brsi entered it, felt intolerable. Such an adaptation, however, can only take place at the expense of & depression of all the vital functions, which must be injurious if long continued or often repeated. This power nf adaptation is well illustrated bj 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 Hie 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. The adaptation above spoken of is a gradual and eontinuous one : thus a bird which will live one hour in a pint of air will live three hours in two pints; and if two birds of the same species, age, and size, be placed in a quantity of air in which either, separately, Avould survive three hours, they will not live ih hour, but only i\ hour. From what has been said 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, itc), and with this allowance the air can only be main- tained 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. Inasmuch as the hgart and great vessels are situated in the air-tight thorax, they are exposed to a certain alteration of pres- sure when the capacity of the latter is increased ; for although the expansion of the lungs during inspiration tends to counter-balance this increase of area, it never quite does so, since part of the pres- sure 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 con- siderably less than the atmospheric pressure. It has been ascer- tained that the amount of the pressure used up in the way above described, varies from 5 or 7 mm. of mercury during the pause, and 254 RESPIRATION. [chap. vr. to 30 mm. of mercury 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 accom- panying diagram how, if there were no lungs in the chest, but Fig. 162.— Diagram of an apparatus illustrating the effect of inspiration upon the heart and great vessete within the thorax.— I, the thorax at rest ; II, during inspiration ; d, repre- sents the diaphragm when relaxed ; d' when contracted (it must be remembered that this position is a mere diagram), i.e., when the capacity of the thorax is enlarged: h, the heart ; v, the veins entering it, and a, the aorta ; a?, U, the right and left lung; t, the trachea; m, mercurial manometer in connection 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. (Landois.) 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-vascular pressure. The diminution of pressure within the chap, vi.] 1 . 1 1 1 :« T ON tin: < [BCULATION. 255 veins passing to the right auricle and within the right auricle itself, will draw the blood into the thorax, and bo assist the circu- lation : this suction action aiding, though independently, the suction power of the diastole of the auricle about which we have previously spoken (p. 153)- The effect of sucking more blood into the right auricle will, caterif 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, and this tends to increase the arterial tension. The effect of the diminished pressure upon the pul- monary vessels will also help towards the same end, i.e., an increased flow through the lungs, so that as far as the heart and its veins are concerned inspiration increases the blood pressure in the arteries. The effect of inspiration upon the aorta and its branches within the thorax would be, however, contrary ; for as the pressure 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 insufficient to counteract the increase of arterial tension produced by the effect of inspiration upon the veins of the chest, and the balance of the whole action would be in favour of an increase of arterial tension during the inspiratoiy period. But if a tracing of the variation be taken at the same time that the respiratory movements are 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 inspira- tion (fig. 163). As regards the effect of expiration, the capacity of the chest is diminished, and the intra-thoracic pressure returns to the normal, which is not exactly equal to the atmospheric, pressure. The effect of this 011 the veins is to increase their intra-vascular pres- sure, 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 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 256 RESPIRATION. [chap. vi. 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. / Pig. 163. — 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 undula- tions, the slower beats on the descent being very marked ; b 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 c. 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 . ) 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 con- dition being produced by pressure on the small pulmonary vessels. We may summarise this mechanical effect therefore, and say that inspiration aids the circulation and so increases the arterial tension, and that although expiration does not materially aid the circulation, yet under ordinary conditions neither does it obstruct. Under extraordinary conditions, 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 nervous 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 two-fold. In the first place the cardio-inhibitory centre is believed to be stimulated during the fall of blood pressure, producing a slower rate of heart-beats during expiration, which will be noticed in the tracing (fig. 163), CHAP. VI.] TEATJBE-HERING'S CUEVES. 257 the undulations during the decline of blood-pressure being l< but less frequent This effect disappears when, by sectioD of the vagi, the effect of the centre is cut off from the heart. In the Fig 1 . 164. — 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 of 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 respiration was being- kept up, but the vagi having been divided, the pidsations on the ascent and descent of the undulations do not differ ; when artificial respiration ceased these undulations for a 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. Foster.) second place, the vaso-motor centre is also believed to send out rhythmical impulses, by which undulations of blood pressure are produced independently of the mechanical effects of respiration. The action of the vaso-motor centre in taking part in pro- s 258 RESPIRATION. [chap. vi. during 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 respiration 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 Travbe-Herinefs curves. They 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 eliminated by the poison and by the cessation of artifi- cial respiration, and the effect of the cardio-inhibitory centre be the division of the vagi. It may be presumed therefore that the vaso-motor centre, as well as the cardio-inhibitory, must be con- sidered to take part with the mechanical changes of inspiration and expiration in producing the so-called respiratory undulations < «f blood-pressure. Cheyne-Stohes' breathing. — This is a rhythmical irregularity in respira- tions -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 succes- sive breath is deeper than the preceding until a climax is reached, then comes in a prolonged sighing expiration, succeeded by a pause, after which the next group begins. Apncea.— Dyspnoea.— Asphyxia. As blood which contains a normal proportion of oxygen excites the respiratory centre (p. 252), and, as the excitement and conse- quent 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 tem- porary cessation. This condition of absence of breathing is termed apnoea* and it can be demonstrated, in one of the lower animals, * This term has been, unfortunately, often applied to conditions of dyspnoea or asphyxia ; but the modern application of the term, as in the text, is the more convenient. CHAP, vi.] ASPHYXIA. 259 by performing artificial respiration to the extent of saturating the Mood with oxygen. When, on the other hand, the respiration is stopped, by, e.g., interference with the passage of air to the lungs, or by supplying aii- devoid of oxygen, a condition ensues, which passes rapidly from the state of dyspnoea (difficult breathing) to what is termed 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 intro- ducing instead of ordinary air a gas devoid of oxygen, or, again, by interference with the due interchange of gases between the air and the blood. Symptoms of Asphyxia. — The most evident symptoms of asphyxia or suffocation are well known. Violent action of the respiratory muscles and, more or less, of all the muscles of the body ; lividity of the skin and all other vascular parts, while the veins are also distended, and the tissues seem generally gorged with blood ; convulsions, quickly followed by insensibility, and death. The conditions which accompany these symptoms are — (1) More or less interference with the passage of the blood through the pulmonary blood-vessels. (2) Accumulation of blood in the right side of the heart and in the systemic veins. (3) Circulation of impure (non-aerated) blood in all parts of the body. Cause of Death from Asphyxia. — The causes of these conditions and the manner in which they act, so as to be incom- patible with life, may be here briefly considered. (1) The obstruction to the passage of blood through the lungs is not so great as it was once supposed to be ; and such as there is occurs chiefly in the later stages of asphyxia, when, by the violent and convulsive action of the expiratory muscles, pressure is indirectly made on the lungs, and the circulation through them is proportionately interfered with. (2) Accumulation of blood, with consequent distension of the right side of the heart and systemic veins, is the direct result, at s 2 2 6o RESPIRATION. [chap. vi. 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. (6) The increased arterial tension is followed by inhibition of the action of the heart, and, thus, the latter, con- tracting less frequently, and gradually enfeebled also by deficient supply of oxygen, becomes over-distended by blood which it cannot expel. At this stage the left as well as the right cavities are distended with blood. The ill effects of these conditions are to be looked for partly in the heart, the muscular fibres of which, like those of the urinary bladder or any other hollow muscular organ, may be paralysed by over-stretching ; and partly in the venous congestion, and consequent interference with the function of the higher nerve- centres, especially the medulla oblongata. (3) The passage of non-aerated blood through the lungs and its distribution over the body are events incompatible with life, in one of the higher animals, for more than a few minutes ; the rapidity with which death ensues in asphyxia being due, more particularly, to the effect of non-oxygenized blood on the medulla oblongata, and, through the coronary arteries, on the muscular substance of the heart. The excitability of both nervous and muscular tissue is dependent on a constant and large supply of oxygen, and, when this is interfered with, is rapidly lost. The diminution of oxygen, it may be here remarked, has a more direct influence in the production of the usual symp- toms of asphyxia than the increased amount of carbonic acid. Indeed, the fatal effect of a gradual accumulation of the latter in the blood, if a due supply of oxygen be maintained, resembles rather that of a narcotic poison. In some experiments performed by a committee appointed by the Medico- Chirurgical Society to investigate the subject of Suspended Animation, it was found that, in the dog, during simple asphyxia. i.e. t by simple privation of air, as by plugging the trachea, the average duration of the respiratory movements after the animal had been deprived of air, was 4 minutes 5 seconds ; the extremes being 3 minutes 30 seconds, and 4 minutes 40 seconds. The average duration of the heart's action, on the other hand, was 7 minutes 1 1 seconds : the extremes being 6 minutes 40 seconds, and chap. vi. J ASPHYXIA. 2(3! 7 minutes 45 seconds. It would Beem, therefore, that on an average, the heart's action continues for 3 minutes 15 seconds after the animal has cea make respiratory efforts. A very similar relation was observed in the rabbit Recovery never took place after the heart's action had ceased. The results obtained by the committee on the subject of drowning were very remarkable, especially in this respect, that whereas an animal mav recover, after simple deprivation of air for nearly four minutes, yet, after submersion in water for 1$ minute, recov \a to be impossible. This remarkable difference was found to be due. not to the mere submersion, nor directly to the si E the animal, nor to depression of temperature, but t 1 the two facts, that in drowning, a free passage is allowed to air out of the lungs, and a free entrance of water into them. It is probably to the entrance <>f water into the lungs that the speedy death in drowning is mainly due. The results of post-mortem examination strongly support this view. On examining the lungs of animals deprived of air by plugging the trachea, they were found simply congested ; but in the animals drowned, not only was the congestion much more intense, accompanied with 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 de- barred 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 insufficient supply of oxygen from the directly poisonous action of such a gas as carbonic oxide, which is present to a considerable amount in common coal-gas. The fatal effects often produced by this gas (as in accidents from burning charcoal stoves in small close rooms), are due to its entering into combination with the haemoglobin of the blood-corpuscles (p. 117), and thus expelling the oxygen. 262 FOOD. [CHAP. VII. CHAPTER VII. FOOD. In order that life may be maintained it is necessary that the 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 replacing the waste of the tissues. And to arrive at a reasonable estimation of the proper diet in twenty-four hours it is necessary to consider the amount of the excreta daily eliminated from the body. The excreta contain chiefly carbon, hydrogen, oxygen, and nitro- gen, but also 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 the principal part in forming the excreta, in large amount. We have seen in the last Chapter that carbonic acid and ammonia, i.e., the elements carbon, oxygen, nitrogen, hydrogen, are given off from the lungs. By the excretion of the kidneys — the urine — many elements are discharged from the blood, especially nitrogen, hydrogen, and oxygen. In the sweat, the elements chiefly represented are carbon, hydrogen, and oxygen, and also in the feces. 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 con- tained in these various excreta in twenty-four hours may be represented in the following way (Landois) : Water. C. H. N. 0. By the lungs . . By the skin . . By the urine . . By the faeces . . 330 660 1700 128 248-8 2'6 9-8 20* 3 3 3' 1 is-8 3' 651-15 7-2 III 12" Grammes . . 2818 28r2 &s 188 68r4I CHAP. VII.] CLASSIFICATION. 2 6$ 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 oxydation (i.e., 32*89 hydrogen and 263-41 oxygen). There are twenty-six grammes of salts got rid of by the urine and six by the faxes. As the water can be supplied as such, the losses of carbon, nitrogen, and oxygen are those to which we should direct our attention in supplying food. For the sake of example, we may now take only two elements, carbon and nitrogen, and, if we discover what amount of these is respectively discharged in a given time from the body, we shall be in a position to judge what kind of food will most readily and economically replace their loss. The quantity of carbon daily lost from the body amounts to about 281*2 grammes or nearly 4,500 grains, and of nitrogen 1 8 -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 elements 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 com- pounds 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 substances, not of one alone, and we must therefore turn to the available food-stuffs. For the sake of convenience they may be classified as follows : 264 FOOD. [chap. vii. A. ORGANIC. I. Nitrogenous, consisting of Proteids, e.g. albumen, casein, syntonin, gluten, legumin and their allies ; and Gelatins, which include gela- tin, elastin, and chondrin. All of these contain carbon, hydrogen, oxygen, and nitrogen, and some in addition, phosphorus and sulphur. II. Non-Nitrogenous, comprising : (1.) Amyloid or saccharine bodies, chemically known as carbo-hydrates, since they contain carbon, hydrogen, and oxygen, with the last two elements in the proportion to form water, i.e.. H 2 0. To this class belong 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. L Mineral and saline matter. II. Water. To supply the loss of nitrogen and carbon, it is found by expe- rience that it is necessaiy to combine substances which contain :i 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 quantity 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 ordi- nary 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 substances can be properly digested without previous exposure to heat and to other manipulations which constitute the process of cooking. It will be well, therefore, to consider the composition of the various substances employed as food, and then to consider how they are affected by cooking. ii \p. vii.] NITROGENOUS FOODS. 265 A.— Poods containing principally nitrogenous bodies. I. — Flesh of Animals, especially of the ox (beef, veal), slice]) (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, 1 6*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-albu- min, gelatin (from the interstitial fibrous connective tissue) ; elastin (from the elastic tissue), as well as haemoglobin. (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., kreatin. Besides, there are sarcolactic and inositic acids, taurin, xanthin, and others. (4) Salts, chiefly of potassium, calcium, 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. Table of Per-centage Composition of Beef, Mutton, Pork, and veal. — (letheby.) "Water. Albumen. Fat. Salts. Beef. — Lean . Fat . . Mutton. — Lean „ Fat Veal .... Pork.— Fat 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 nitro- 72 I9-3 3* 5i 51 14-8 29-8 4"4 72 183 4'9 4-8 53 12*4 31-1 3-5 . 63 16-5 158 47 39 9-8 48-9 23 78 iS i 2-9 I" 77 161 5'5 i*4 75 9'9 13-8 1 '3 7574 1172 2*42 273 266 FOOD. [chai\ vii. genous substances, and also fish — salmon, eels, &c., and shell-fish, e.g., lobster, crab, mussels, oysters, shrimps, scollop, cockles, &c. Table of Per-cextage Composition of Poultry and Fish. — (Lethebt.) "Water. Albumen. Fats. Salts. Poultry 74 21 3S 11 (Singularly devoid of fat, and so generally eaten with bacon or pork.) White Fish .... Salmon Eels (very rich in fat) . . Oysters Even now the list of fleshy foods is not complete, as nearly all animals have been occasionally eaten, and we may presume that the average composition of all is nearly the same. 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) Albuminous substances in the form of casein and, in small amount, of serum-albumin. (2) Fats in the cream. (3) Carbo- hydrates 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 fat according as the cheese is made of skim milk, (skim cheese), 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 albnmen is split up with formation of fat. (/3) Cream, which consists of the fatty globules incased in casein, and which being of low specific oTavity float to the surface. (y) Butter, or the fatty matter deprived of its casein envelope by the process of churning. (S) Butter-mill:, or the fluid obtained from cream after butter has been formed ; very rich therefore in nitrogen, (e) Whey, or the fluid which remains after the precipitation of casein ; this contains sugar, salt, and a small quantity of albumen. chap, vii.] CARBOHYDRATE AND FATTY FOODS. 267 Table of Composition' of Milk, Butteb-milk, Cream, and Cheese. — (I.ltiii.uv and 1'avi Nitrogenous matters. I" Lactose. Salts. Wa* Milk {Cow) . 41 39 5'2 •s 86 Buttermilk 41 7 6-4 •8 88 Cream . 27 267 2*8 1-8 66 - . — Skim . . 44-8 63 — 4"9 44 Cheddar . . 28-4 311 Non-nitrogenous matter and loss. 4*5 36 „ Neufchatel (Fresh) S* 4071 36-58 •5i 36 Salts. "Water 1-6 7S i*3 52 III. Eggs. — The yelk and albumen of eggs are in the same relation as food for the embryoes of oviparous animals that milk is to the young of mammalia, and afford another example of the natural admixture of the various alimentary principles. Table of the Per-centage Composition of Fowls* Egg>. Nitrogenous substances. Fats. White . . . . 20*4 — Yelk 16- 307 IV. Leguminous fruits are used by vegetarians, as the chief source of the nitrogen of the food. Those chiefly used are peas, leans, lentil*, atc, they contain a nitrogenous substance called legumin, allied to albumen. They contain about 2 5 '30 per cent, of this nitrogenous body, and twice as much nitrogen as wheat. B. Substances supplying principally carbohydrate bodies. a. Bread, made from the ground grain obtained from various so-called cereals, viz., wheat, rye, maize, barley, rice, oats, Arc, 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 Per-centage Composition of Bread and Flour. Nitrogenous Carbo- matters. hydrates. Fats. Salts. "Wat< 8l 5i' r6 2"3 37 108 7o-S5 n* 17 15 Bread Hour Various articles of course are made from flour, e.g., macaroni, biscuits, 7i 55 64 55 52 55 44 >l 42 55 42 55 39 55 34 55 33 55 3i 55 22 55 16 55 10 55 2 ,. (ne (2.) The effect of starvation on the temperature of the various animals experimented on by Chossat was very marked. For some time the variation in the daily temperature was more marked than its absolute and continuous diminution, the daily fluctua- tion amounting to 5 or 6° F. (3 C), instead of i° or 2 F. (■5 to i° 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. (16-5° 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 differences of time with regard to the period of the fatal result are attended by the same ultimate loss of heat, but the effect of the application of external warmth to animals cold and dying from starvation, is more effectual in reviving them than the administration of food. In other words, an animal exhausted by deprivation of nourishment is unable so to digest food as to use it as fuel, and therefore is dependent for heat on 272 FOOD. [chap. vii. its supply from without. Similar facts are often observed in the treatment of exhaustive diseases in man. (3.) The symptoms produced by starvation in the human sub- ject are hunger, accompanied, or it may be replaced by pain, referred to the region of the stomach ; insatiable thirst ; sleep- lessness ; general weakness and emaciation. The exhalations both from the lungs and skin are fetid, indicating the tendency to decomposition which belongs to badly-nourished tissues ; and death occurs, sometimes after the additional exhaustion caused by diarrhoea, often with symptoms of nervous disorder, delirium or convulsions. (4.) In the human subject death commonly occurs within six to ten days after total deprivation of food. But this period may be considerably prolonged by taking a very small quantity of food, or even water only. The cases so frequently related of survival after many days, or even some weeks, of abstinence, have been due either to the last-mentioned circumstances, or to others 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 beino* least noticeable in the brain. The stomach and intestines are empty and contracted, and the walls of the latter appear remarkably thinned and almost transparent. The various secre- tions 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 and drink as usual ; but in the course of the second week, they began to get thin, although their appetite continued CHAP. VII.] STARVATION. 273 good, and they took daily between six and eight ounces of sugar. The emaciation increased during the third week, and they became feeble, and lost their activity and appetite. At the same time an ulcer formed on each cornea, followed" by an escape of the humours of the e} r e : this took place in repeated experiments. The animals still continued to eat three or four ounces of sugar daily ; but became at length so feeble as to be incapable of motion, and died on a day varying from the thirty-first to the thirty-fourth. On dissection, their bodies 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 phenomena produced were the same, except that no ulcera- tion of the cornea took place ; the effects were also the same with butter. The experiments of Chossat and Letellier prove the same \ and in men, the same is shown by the various diseases to which 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 insufficient for the maintenance of health. The experiments of the Academies of France and Amsterdam were equally conclusive that gelatin alone soon ceases to be nutritive. Savory's observations on food confirm and extend the results obtained by Magendie, Chossat, and others. They show that animals fed exclusively on non-nitrogenous diet speedily emaciate and die, as if from starvation • that life is much more prolonged in those fed with nitrogenous than by those with non-nitrogenous food ; and that animal heat is maintained as well by the former as by the latter — a fact which proves, if proof were wanting — that nitrogenous elements of food, as well as non-nitrogenous, may be regarded as calorifacient. 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 x 274 FOOD. [chap. vii. action of the intestines. In other cases, the nnabsorbed portions undergo putrefactive changes in the intestines, which are ac- companied by the production of gases, such as carbonic acid, carbnretted and sulphuretted hydrogen ; a distended condition of the bowels, accompanied by s}miptorns of indigestion, is the result. An excess of the substances required as food may however undergo absorption. It is a well-known fact that numbers of people habitually eat too much ; especially of nitrogenous food. Dogs can digest an immense amount of meat if fed often, and the amount of meat taken by some men would supply not only the nitrogen, bnt 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 processes 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 by the liver into urea, and so easily excreted by the kidneys, affording one variety of what is called luxus consumption ; but after a time the organs, especially the liver, will yield to the strain of the over-work, and will not reduce the excess of nitrogenous material into urea, but into other less oxidised products, such as uric acid ; and general plethora and gout may be the result. This state of things, however, is delayed for a long time, if not altogether obviated, when large meat-eaters take a considerable 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 in addition to dyspepsia, with acidity and flatulence. There is 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 should contain about the same amount of the carbon and nitrogen that are got rid of by the excreta. Without doubt these desiderata may be satisfied in CHAP, vii.] NOEMAL DIET. 2 7t numerous ways, and it would be simply absurd to believe that the diet of every adult should be exactly similar. The age, sex, strength, and circumstances of cadi individual should ultimately determine his diet. A dinner of bread and hard cheese with an onion contain all the requisites for a meal; but such diet would he 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., replace- ment 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 anyone 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 : — Table of Water and Food required for a Healthy Adult. (Parkes.) In laborious occupation. At rest. Nitrogenous substances, e.g., flesh . 6 to 7 oz. av. 2-5 oz. Fats 3'5 ^ 4-5 oz. 1 oz. Carbo-hydrates 16 to 18 oz. 12 oz. S alts i-2 to i*5 oz. '5 oz. 267 to 31 oz. 16 oz. The above is the dry food; but as this is nearly always combined with 50 to 60 per cent, of water, these numbers should be doubled, and they would then be 52 to 60 oz., and 32 oz. of so called solid food, and to this should be added 50 to 80 oz. of fluid. 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.— £lb. of cooked meat, £lb. potatoes, bread and beer. Tea. — 1 pint of tea, bread and butter. Supjper. — Bread and butter, beer. t 2 2j6 DIGESTION. [chap. yiii. Daily allowance to each patient. — 2 pints cf tea, with milk and sugar ; 14 oz. bread ; i lb. of cooked meat : § lb. potatoes : 2 pints of beer, 1 oz. butter. 31 oz. solid, and 4 pints (80 oz.), liquid. CHAPTER VIII. 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 thern to be absorbed into the system by the blood vessels and lymphatics, without which absorp- tion they would be useless for the purposes they have to fulfil ; almost the whole of the food undergoes various 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. Thence it passes into the intestines, where it meets with the bile, the pancreatic juice and the intestinal juices, all of which exercise an influence upon that portion of the food not absorbed from the stomach. By this time most of the food is capable of absorption, and the residue of undigested matter leaves the body in the fomi of faeces by the anus. The course of the food through the alimentary canal of man will be readily seen from the accompanying diagram (fig. 165). The Mouth is the cavity contained between the jaws and inclosed by the cheeks laterally, and by the lips in front ; behind it opens into the pharynx by the fauces, and is separated from the nasal cavity by the hard palate in front, and the soft palate behind, which form 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 and lateral boundaries. The whole of the mouth is lined with mucous membrane, covered by stratified squamous epithelium, which is continuous in front along the lips with the OHAP. VIII. 1 COURSE TAKEN BY THE FOOD. 277 epithelium of 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 Got 1 1 Blotddei ■■', Ltver tufiteduJi . — Pharynx •J? • *\ "Ion Y>5V <^ "~ Petri erects -KjC 'Jj2=^ij\vA St ? ,notd Intestine II j> J! Fig. 165.— Diagram of the Alimentary Canal. The small intestine of man is from about 3 to 4 times as long as the large intestine. 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 now consider, in detail, the process of digestion, as it takes place in each stage of this journey of the food through the alimentary canal. 2 yS DIGESTION. [chap. viii. 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 crushing the softer portions of the food against the hard palate, gums, &c, and thus supplementing the action of the teeth, and partly by returning the morsels of food to the action of the teeth, again and again, as they are squeezed out from between them, until they have been sufficiently chewed. The simple up and down, or biting movements of the lower jaw, are performed by the temporal, masseter, and internal 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 external pterygoids act together, the lower jaw is pulled directly forwards, so that the lower incisor teeth are brought in front of the level of the upper. Temporo-maxillary Fibro-cartilage. — The function of the inter-articular fibro-cartilage of the temporo-maxillary joint in mastication may be here mentioned, (i) As an elastic pad, it serves well to distribute the pressure caused by the exceedingly powerful action of the masticatory muscles. (2) It also serves as a joint-surface or socket for the condyle of the lower jaw, when the latter has been partially drawn forward out of the glenoid cavity of the temporal bone by the external pterygoid muscle, some of the fibres of the latter being attached to its front surface, and consequently drawing it forward with the condyle which moves on it. Nerve-mechanism of mastication. — As in the case of so many other actions, that 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 nerves chiefly con- cerned are the sensory branches of the fifth and the glossopharyn- geal, and the motor branches of the fifth and the ninth (hypoglos- sal) cerebral nerves. The nerve-centre through which the reflex action occurs, and by which the movements of the various muscles P. viii.] SALIVARY CLAN 279 arc harmonised, is situate in the medulla oblongata, In bo far as mastication is voluntary or mentally perceived, it becomes so under the influence, in addition to the medulla oblongata, of the cerebral hemisphen Insalivation. — The act of mastication is much assisted by the saliva which is secrete! by the salivary -lands in largely incn amount during the pn 38, and the intimate incorporation of which with the food, as it is being chewed, is termed insalivation. The Salivary Glands. The salivary glands are the parotid, the sub j mattllary i and the tub-lingual, 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 usually described as com- pound tubular glands. They are made up of lobules. Each '/>L 1M* Mm Fig. 166.— Section of submaxillary gland of don. Showing gland-cells, b, and a duct, a, in section. (KGlliker.) lobule consists of the branchings of a subdivision of the main duct of the gland, which are generally more or less convoluted towards their extremities, and sometimes, according to son. - rvera, 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 280 DIGESTION. [chap. VIII. Fig. 167. — From a section through a true salivary gland, a, the gland alveoli, lined with albuminous " salivary cells ;" b, intralobular duct cut transversely. (Klein and Noble Smith.) joined together by processes to produce a fenestrated membrane, the spaces of which are occupied by a homogeneous ground-sub- stance. Within, upon this mem- brane, which forms the tube, the nucleated salivary secreting cells, of cubical or columnar form, are arranged parallel to one an- other surrounding a middle central canal. The granular appearance which is 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. Con- necting 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 con- nected to form larger lobules (lobes), in a similar manner. The alveoli pass into the intralobular ducts by a narrowed portion (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 presents a fine longitudinal 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 consists of one or more layers of columnar epithelium, containing an intracellular network of fibres arranged longitudinally. Varieties. — Certain differences in the structure of salivary glands may be observed according as the glands secrete pure saliva, or saliva mixed with mucus, or pure mucus, and therefore OHAP. VIII.] SALIVARY GLANDS. 28l 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 submaxillary of the rabbit and guinea- pig (fig. 167). In this kind the alveolar lumen is small, and the cells lining the tubule are short, granular columnar cells, with nuclei presenting the intranuclear network. During 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 mucus-secret- ing glands, as the sublingual of man and 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 columnar cells with nuclei near the base- ment membrane. The cell substance is made up of a fine network, which in the resting state Fig. 168. — From a section through a mucous rjland in a quiescent state. The alveoli are lined with transparent mucous cells, and outside these are the demilunes of Heidenhain. The cells should have been represented as more or less granular. (Heidenhain.) contains a transparent substance called mucigen, during which the cell does not stain well w T ith logw r ood (fig. 168). When the gland is secret- ing, mucigen is converted into mucin, and the cells sw r ell up, appear more transparent, and stain deeply in logw r ood (fig. 169). During rest, the cells become smaller and more granular from having discharged their contents, and the nuclei appear more distinct. (b) Semilunes of Heidenhain (fig. 168), which are crescentic masses of granular parietal cells found here and there between the basement membrane and the central cells. These cells are small, and have a very dense reticulum, the nuclei are spherical, and increase in size during secretion. In the mucous gland there 282 DIGESTION. [chap. viii. . - me large tubes, lined with large transparent central cells, and have besides a few granular parietal cells; other small tubes are lined with small granular parietal cells alone; and a third variety are lined equally with each kind -■gggt-: --■ '- °f cell. (3) In the muco-sali- vary or mixed glands, as the ^^■^^^■BSH^^R human submaxillary gland, part of the gland presents the 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 salivary glands, they Fig. 169. — A part 0/ a section through a mucotis . " latum, are contained in the connective The alveoli are lined with small granular . _ . , ,. . . .. cells. Lavdovski.) tissue of the alveoli principally, and in certain glands, especi- ally 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, penetrate 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 trabecular between the alveoli, 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 [/lands, and often with air bubbles, which, being retained by its viscidity, make it frothy. When obtained from the parotid ducts, and free from mucus, saliva is a transparent watery fluid, the specific gravity of which varies from 1004 to 1008, and in which, when examined with the microscope, are found floating a number of minute particles, derived from the secreting ducts and vesicles of the glands. In the impure or mixed saliva are found, besides these particles, numerous epithelial scales separated from the surface of the mucous membrane of the mouth and tongue, and the so-called salivary corpuscles, discharged probably from the mucous glands of the mouth and the tonsils, which, when the saliva is collected in a deep vessel, and left at rest, subside in the form of a white CHAP, viii.] SALIVA. 283 opaque matter. Leaving the supernatant salivary fluid transparent and colourless, or with a pale bluish-grey tint. In reaction, the saliva, when first Becreted appears to be always alkaline. During Easting, the saliva, although secreted alkaline, shortly becomes neutral ; and it docs BO especially when secreted slowly and 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 5-90 Ptyalin 1*41 Fat 007 epithelium and Proteids (including Serum-Albumin, Globulin. Mucin, &c.) 2*13 Salts :— Potassium Sulpho-Cyanate Sodium Phosphate Calcium P'hosphatc . Magnesium Phosphate . Sodium Chloride Potassium Chloride 2-29 5'90 The presence of potassium sulphocyanate (or tltioajanate) (CN K S) in saliva, may be shown by the blood-red colouration which the fluid gives with a solution of ferric chloride (Fe. 2 CL 6 ), and which is bleached on the addition of a solution of mercuric chloride (HgCL). Rate of Secretion and Quantity. — The rate at which saliva is secreted is subject to considerable variation. When the tongue and muscles concerned in mastication are at rest, and the nerves of the mouth are subject 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). Uses of Saliva. — The purposes served by saliva are (1) me- chanical and (2) chemical. I. Mechanical. — (1) It keeps the mouth in a due condition of moisture, facilitating the movements of the tongue in speaking, and the mastication of food. (2) It 284 DIGESTION. [chap. viii. serves also in dissolving sapid substances, and rendering them capable of exciting the nerves of taste. But the principal me- chanical 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 kinds of food 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 hr aided 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. — Saliva has the power of converting starch into glucose or grape-sugar. When saliva, or a portion of a salivary gland, is added to starch paste in a test-tube, and the mixture kept at a temperature of ioo° F. (3 7 "8° C), the starch is very rapidly transformed into grape-sugar. There is an intermediate stage in which a part or the whole of the starch becomes dextrin. Test for Glucose. — 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 ; and when common raw starch is masticated and mingled with saliva, and kept with it at a temperature of 90 or ico° F. (30° — 37'8° C), the starch-grains are cracked or eroded, and their contents are transformed in the same manner as the starch-paste. Saliva from the parotid is less viscid, less alkaline, clearer, and more watery than that from the submaxillary. It has, moreover, a less powerful action on starch. Sublingual saliva is the most viscid, and contains 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 before this age on starchy food, corn-flour, &c, which they are unable to render soluble and capable of absorption.' ni.u'. viii.] ACTION OF SALIVA. 285 Action of Saliva on Starch. — This action is due to the pre- sence in the saliva of the body called ptyalin. It is a nitrogenous body, and belongs to the order of ferments, which are bodies who exact chemical composition is unknown, and which .arc capable of producing by their presence changes in other bodies, without themselves undergoing change. Ptyalin is called a hydrolytic ferment, that is to say, it acts by adding a molecule of water to the body changed. The reaction is supposed to be as follows : 3 C H lo O 5 + 3 H 2 = C H 12 + 2 (C H lo O 5 ) + 2 II 2 = 3 C 6 H 12 O . Starch + Water Glucose Dextrin Glucose. But it is not unlikely that the action is by no means so simple. In the first place, recent observers believe that a molecule of starch must be represented by a much more complex formula ; next, that the stages in the reaction are more numerous and extensive ; and thirdly, that the product of the reaction is not true glucose, but maltose. Maltose is a sugar more akin to cane- than grape- sugar, of very little sweetening power, and with less reducing power over copper salts. Its formula is C ia H 22 lx . The action of saliva on starch is facilitated by : (a) Moderate heat, about ioo° F. (37*8° C). (b) A slightly alkaline medium. (c) Removal of the changed material from time to time. Its action is retarded by : (a) Cold ; a temperature of 32 F. (o° C.) stops it for a time, but does not destroy it, whereas a high tem- perature above 140 F. (6o° C.) destroys it. (6) 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 : it has no apparent influence on any of the other ternary principles, such as sugar, gum, cellulose, or on fit, and seems to be equally destitute of power over albuminous and gelatinous substances. Influence of the Nervous System. — The secretion of saliva is under the control of the nervous system. It is a reflex action, and in ordinary conditions 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 glossopharyngeal part of the eighth pair of nerves, which are dis- tributed to the mucous membrane of the tongue and pharynx. 286 DIGESTION. [chap. viii. 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 these sensory nerves 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 sensory nerves besides those mentioned, may produce reflexly the same effects. 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 experience to accompany the preparation of a meal, may also, in the hungry, stimulate the nerve centre to action. In addi- tion 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 impressions from the cerebrum also are capable of stimulating the centre to action or of inhibiting its action. Secretion may be excited by direct stimulation of the centre in the medulla. A. On the Submaxillary Gland. — The submaxillary gland has been the gland chiefly employed foi* the purpose of experimentally demonstrating the influence of the nervous system upon the secre- tion 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 portio dura of the seventh pair of nerves), in the canal through coup, viii.] SECRETION OF SALIYA. 2.S7 which it passes in the temporal bone, in it- passage from the interior of the skull to the face; and (2) branches of the sympa- thetic nerve from the plexus around the facial artery and branches to the gland. The chorda ( fiu. 170, eh. t), after quitting the temporal bone, passes downwards and forwards, under c the externa] pterygoid muscle, and joins at an acute angle the lingual or gustatory uerve, proceeds with it for a short distance, and then passes along the submaxillary gland duct (fig. 170, sm.d.), t<> which it is distributed, giving branches to the submaxillary ganglion (fig. 170, sm. ///.), and sending others to terminate in the superficial muscle of the tongue. If this nerve be expos 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 glossopharyngeal will produce a flow of saliva. But if the peripheral end of the divided 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 sympa- thetic filaments (mere division producing no apparent effect), the arteries contract, 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 cir- culation 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 peripheral end of the divided chorda produces all the vascular effects as before, without any secretion of saliva accompanying them. Again, if an animals head be cut off, and the chorda be rapidly exposed and stimulated 2S8 DIGESTION. [CHAP. VIII. with an interrupted current, a secretion of saliva ensues for a short time, although the blood supply is necessarily absent. These experiments serve to prove that the chorda contains two sets of nerve fibres, one set (yaso-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 pro- ducing an increased supply of blood to the gland ■ while another set, which are paralyzed by injection of atropin, directly stimulate Fi°\ 170. — Diagrammatic representation of the submaxillary gland 0/ the dog tvith 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 (s7n. d.), of which a cannula has been tied. The sublingual gland and duct are not shown. n.L, n. I'., the lingual or gustatory nerve ; eh. t., eh. t'., the chorda tympani proceeding from the facial nerve, becoming conjoined with the lingual at n. 7'., and afterwards diverging and passing to the gland along the duct; sm. gl., submaxillary ganglion with its roots; n.L, the lingual nerve proceeding to the tongue; a. car., the carotid artery, two branches of which, a. am. n. and /•. 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 jugular vein ; r. sym., the conjoined vagus and sympathetic trunks ; gl. cer. s., the superior-cervical ganglion, two branches of which fomnng 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. (M. Foster. ) the cells themselves to activity, whereby they secrete and dis- charge the constituents of the saliva which they produce. These latter fibres very possibly terminate in the salivary cells them- selves. If, on the other hand, the sympathetic fibres be divided, stimulation of the tongue by sapid substances, or of the trunk of the lingual, or of the glosso-pharyngeal continues to produce a flow .hat. mil] IM-I.l'K.WK OF THE SERYOU8 8Y8TEM. of saliva. From these experiment that the chorda tympani nerve is the principal nerve through which efferent im- pulses proceed from the cenl bo excite the secretion of this gland. The sympathetic fibres appear to act principally as a vaso-con- strictor nerve, and t<> exult the action of the local v.. centres. The sympathetic is more powerful in this direction than the chorda. There is not sufficient evidence in favour of the belief that the submaxillary ganglion is ever the nerve centre which controls the secretion of the submaxillarv gland. - />. On the Parotid Gland. — The nerves which influence secre- tion in the parotid gland are branches of the facial (lesser super- ficial 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 oblongata, are. as in the of the submaxillary gland, the fifth, and the glossopharyn- geal The pneumogastric nerves convey a further stimulus to the tion 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 appeal's to be divided into two stages, which differ somewhat according to the class to which the gland belongs, viz., (r) the true salivary, or (2) the mucous type. In the former case, it has been noticed, as has been already described (p. 281), that during the rest which follows an active secretion the lumen of the alveoli becomes smaller, the gland cells larger, and very granular. During secre- tion 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. 171). 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 manu- factures the elements of its own secretion, and which are stored up in the form of granules in the cell during rest, the second ting of the actual discharge of these granules, with or 290 DIGESTION. [chap. VIII. 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 Fig. 171. — Alveoli of true salivary gland. A, at rest ; B, in the first stage of secretion ; C, after prolonged secretion. (Langley.) first stage of the process, and when stimulated the protoplasm is extremely active in manufacturing the granules, whereas the chorda tympani is 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 occur when atropin has been subcutaneously injected. In the mucous-secreting gland, the changes in the cells during secretion have been already spoken of (p. 281). 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 con- stituents 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 active exercise of their functions. The source whence the cells obtain the materials chap, viii.] THE pharynx. 291 of their secretion, is the blood, <>r, to Bpeak more accurately, the plasma, which is filtered off from the circulating blood into the interstices 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. 165). 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 externally, and are lined internally by a strong faseia (pharyngeal aponeurosis), on the inner aspect of which is areolar (submucous) tissue and mucous membrane, continuous with that of the mouth, and, as regards the part concerned in swallowing, is identical with it in general structure. The epithelium of this part of the pha- Fig . T72 .—Lhi ( piai foUich or rynx, like that of the mouth, is stratified mucous "memSanl^th onrl en n. -.m mi « its papillee ; h, lymphoid ana Squamous. tissue, with several lym- The pharynx is well supplied with phoidsacs. (Frey.) mucous glands (fig. 174). The Tonsils. — Between the anterior and posterior arches of the soft palate are situated the Tonsils, one on each side. A tonsil consists of an elevation of the mucous membrane presenting 12 to 15 orifices, which lead into crypts or recesses, in the walls of Avhich are placed nodules of adenoid or lymphoid tissue (fig. 173). 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. 173, e). Into the crypts open a number of ducts of mucous glands. The viscid secretion which exudes from the tonsils serves to lubricate the bolus of food as it passes them in the second part 1 >f the act of deglutition. D 2 292 DIGESTION. [CHAP. VIII. The (Esophagus or Gullet. The (Esophagus or Gullet (fig. 165), the narrowest portion of the alimentary canal, is a muscular and mucous tube, nine or ten Fig. 173. — Vertical section through a erupt of the human tonsil, o, entrance to the crypt, •which is divided below by the elevation -which does not quite reach the surface ; l>, stratified epithelium ; c, masses of adenoid tissue ; d, mucous glands cut across ; e, fibrous capsule. (V. D. Harris.) 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 muscular coat (fig. 175, force it back to the entrance of the pharynx. (2.) The second act is the most complicated, because the food must pass bythe posterior orifice of the oose 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 contracting on it and then behind it. The root of the tongue being retracted, and the larynx being raised with the pharynx and carried for- wards 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 simul- taneous contraction of its own muscles : so that, even when the epiglottis is destroyed, there is little danger of food or drink pass- ing into the larynx so long as its muscles can act freely. At the same time, the raising of the soft palate, so that its posterior edge touches the back part of the pharynx, and the approximation of the sides of the posterior palatine arch, which move quickly in- wards like side curtains, close the passage into the upper part of the pharynx and the posterior nares, and form an inclined plane, along the under surface of which the morsel descends ; then the pharynx, raised up to receive it, in its turn contracts, and forces it onwards into the oesophagus. (3.) In the third act, in which the food passes through the oesophagus, every part of that tube, as it receives the morsel and is dilated by it, is stimulated to con- tract : 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 contrac- tion of the oesophagus is slow, and attended with pain. Division of both pneumogastric nerves paralyses the contractile power of the oesophagus, and food accordingly accumulates in the tube. The second and third parts of the act of deglutition are in- voluntary. Nerve Mechanism. — The nerves engaged in the reflex act of deglutition are : — sensory, branches of the fifth cerebral supplying the soft palate; glosso-pharyngeal, supplying the tongue and 296 DIGESTION. [chap. vni. 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 mastication ; 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 hypo- glossal, the muscles of the tongue. The nerve-centre by which the muscles are harmonised in their action, is situate in the medulla oblongata. In the movements of the oesophagus, the ganglia contained in its walls, with the pneuino-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 stomach, it consists of a dilatation of the alimentary canal placed between and continuous with the oesophagus, which enters its larger or cardiac end on the one hand, and the small intes- tine, which commences at its narrowed end or pylorus, on the other. It varies in shape and size according to its state of distension. The Ruminants (ox. sheep, deer, &c.) posse>» very complex stomachs ; in most of them four distinct cavities are to be distinguished (fig. 176). 1. The Pavndi or Rumen, a very large cavity which occupies the cardiac end. and into which large quantities of food are in the first instance swal- lowed with little or no mastication. 2. The Reticulum, or Honeycomb stomach, so called from the fact that its mucous membrane is disposed in a number of folds enclosing hexagonal cells. 3. The Psalteriwm, orManyplies, 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 successively 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 a?sophagus) into the manyplies, and thence into the abomasum. It will thus be seen that the first two stomachs • li \r. VIII.] Til I! STOMACH. 297 (paunch and reticulum) have chiefly the mechanical functions of storing ami moistening (lie Eodder: tin' third (manyplies) probably act-; as u strainer, only allowing the finely divided portions "I' Eood to pass on into the fourth stomach, where the gastric juice is Becreted ami the proci digestion carried on. The mucous membrane of the firs! three stomachs Fig. 176. — Stomach 0/ sheep. a>, oesophagus ; Ru, ramen; Set, reticulum ; Pa, psalterium, or manyplies; A, abomasum ; Jjk, duodenum ; v shallow polygonal depressions, the diameter of which varies generally from ...'...th to -,,tli of an inch ; but near the pylorus Is as much as 1( 1 ll) th of an Inch. They are separated by slightly elevated ridges, which sometimes, (.'specially in certain morbid states .. — The chief or cubical cells of the peptic glands, and the corre- sponding cells of the pyloric glands during the early stag a stion, if hardened in alcohol, appear swollen and granular, and stain readily. A- a later stag the cells become smaller, but more granular and stain even more readily. The parietal cells swell up, but are otherwu not altered during digestion. The _ onles, however, in the alcohol- hardened specimen, are believed not exist in the living cells, but to have been precipitated by the hard- ening re-agent : for if examined dur- ing life they appear to be confined to the inner zone of the cells, and the outer zone is free from grannli s, whereas during rest the Cull _ nnlar throughout. These granules the substance from which pepsin is glands become larger, more They are directlv continu- K'fe % = Section showmg the pyloric glands, s, free surface ; a, ducts of pyloric gland* : ■ . neck of game ; m, the gland alveoli ; van, muscularis mucosae. (Klein and Noble Smith.) are thought to be peps:. forme . which is 302 DIGESTION. [CHAP, vm. during rest stored chiefly in the inner zone of the cells and dis- charged into the lumen of the tube during secretion. (Langley.) Lymphatics. — Lymphatic vessels surround the gland tubes to a greater or less extent. Towards the fundus of the peptic glands are found masses of lymphoid 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 •p- jgQ p] an n f ftp blood-vessels of the stomach, as they would be seen in a vertical section. a arteries passing' up from the vessels of submucous coat; b, capillaries branching between and around the tubes ; c, superficial plexus of capillaries occupying the ridges of the mucous membrane ; d, vein formed by the union of veins which, having collected the blood of the superficial capillary plexus, are seen passing down between the tubes. (Brinton.) the closelv packed glandular tubes, anastomosing around them by means of a fine capillary network, with oblong meshes. Con- tinuous 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 inter-tubular capillary plexus, they open finally into the venous network in the submucous tissue. (map. vin.] DIGESTION IX THE STOMACH. 303 Ntrvet.- The nerves of the stomach are derived from the pneumogastrio and sympathetic, and form a plexus in the submucous and muscular coats, containing many ganglia (Remak, Meissner). Digestion in the Stomach. Gastric Juice.— The functions of the stomach are to secrete a digestive fluid (gastric juice), to the action of which the food is next subjected after it has entered the cavity of the stomach from the oesophagus ; to thoroughly incorporate the fluid with the fond by means of its muscular movements: and to absorb such sub- stances as are capable of 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 substance 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 of Gastric Juice. — The first accu- rate analysis of gastric juice was made by Front : but it does not appear to have been collected in any large quantity, or pure and separate from food, until the time when 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, snch as the bulb of a thermometer, into the stomach, excited at once the secretion of gastric fluid. This was drawn off, and was often obtained to the extent of nearly an ounce. The introduction of alimentary substances caused a much more rapid and abundant secretion than did other mechanical irritants. Xo increase of temperature could be detected during the most active secretion : the thermometer introduced into the stomach always stood at 100' F. (37 'S° C.) except during muscular exertion, when 3C>4 DIGESTION. [chap. vm. 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 investigated 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 introduction of some hard matter, such as dry peas, and the secretion was removed by means of an elastic tube. The fluid thus obtained was found to be acid, limpid, odourless, with a mawkish taste — with a specific gravity of 1 002, or a little more. It contained a few cells, seen with the microscope, and some fine granular matter. The analysis of the fluid obtained in this is given below. The gastric juice of dogs and other animals obtained by the introduction into the stomach of a clean sponge through an artificially made gastric fistula, shows a decided difference in composition, but possibly this is due, at least in part, to admixture with food. Chemical Composition of Gastric Juice. Dog's. Human, Water 971-17 994*4 Solids 28-82 539 Solids — Ferment — Pepsin ..... 175 319 Hydrochloric acid (free) 27 •2 Salts- Calcium, sodium, and potassium, chlorides ; and calcium, magnesium, and iron, phos- phates ....... S-57 2-19 The quantity of gastric juice secreted daily has been variously estimated ; but the average for a healthy adult may be assumed to range from ten to twenty pints in the twenty-four hours. The acidity of the 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. The amount of hydrochloric acid varies from 2 to '2 per 1000 parts. In health}' gastric juice the amount of free acid may be as much as '2 per cent. ohap, nil.] GA8TBIC JTJI< 305 Aj regards the formatioD of pepsin and acid, the formi 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, ;i- no acid is formed by the pyloric glands in which this variety of cell is absent. The ferment Pepsin (p. 305) can be procured by digesting' por- tions of the mucous membrane of the stomach in cold water, after they have been macerated for some time in water at a temperature 8o° — ioo : I". 2y° — 37*8° C). The warm water dissolves various substances as well ;ts some of the pepsin, but the cold water takes up little else than pepsin, which is contained in a greyish-brown viscid fluid, on evaporating the cold solution. The addition of alcohol throws down the pepsin in greyish-white floccnli. Glycerine 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 glycerine. Functions. — The digestive power of the gastric juice depends on the pepsin and acid contained in it, both of which are, under ordinary circumstances, 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 accord- ing to the nature of the food, yet always presenting a character- istic thick, pultaceous, grumous consistence, with the undigested portions of the food mixed in a more fluid substance, and a strong, disagreeable acid odour and taste. The chief function of the gastric juice is to convert proi into peptones. This action may be 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 albumin cannot be preci- pitated on boiling, but that if the solution be neutralised with an alkali, a precipitate of acid-albumin is thrown down. After a while the proportion of acid-albumin gradually dimin that at last scarcely any precipitate results on neutralization, and finally it is found that all the albumin has been changed into another proteid 306 DIGESTION. [chap. vm. substance which is not precipitated on boiling or on neutraliza- tion. This is called peptone. Characteristics of Peptones. — Peptones have certain characteristics which distinguish them from other proteids. i. They are diffu- sible, i.e., they possess the property of passing through animal membranes. 2. They cannot be precipitated by heat, nitric, or acetic acid, or potassium 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 diftusibility peptones differ remarkably from egg- albumin, and on this diftusibility depends one of their chief uses. Egg-albumin as such, even in a state of solution, would be of little service as food, inasmuch as its indiffusibility would effec- tually prevent its passing by absorption into the blood-vessels of the stomach and intestinal canal. Changed, however, by the action of the gastric juice into peptones, albuminous matters diffuse readily, and are thus quickly absorbed. After entering the blood the peptones are very soon again modified, so as to re-assume the chemical characters of albumin, a change as necessary for preventing their diffusing out of the blood-vessels, as the previous change was for enabling them to pass in. This is effected, probably, in great part by the agency of the liver. Products of Gastric Digestion. — The chief product of gastric digestion 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 undoubtedly 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 Avill not act without the presence of acid. The pro- duction of two forms of peptone is usually recognised, called chap, vim. 1 G ISTRIC DIGESTION. 307 respectively anii-peptone and &m*-peptone. Their different chemical properties have not yet been made out, but they are distinguished by this remarkable fact, thai the pancreatic juice, while possessing no action over the former, is able to convert the latter into leucin and tyrosin. Pepsin acts the pari of a hydro- lytic ferment proteolytic), and a])pcars to cause hydration of albumin, peptone being a highly hydrated form of albumin. Circumstances favouring Gastric Digestion. 1. — A tem- perature of about ioo° F. (37-8° (\); at 32 F. (o° 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 proteids are converted by the gastric juice into peptone-, and, therefore, whether they be taken into the body in meat, £8, 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 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 precipitation 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 niacin, and the horny tissues, keratin generally are unaffected. (>n the amylaceous articles of food, and upon pure oleaginous principles 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, bj which means the fat is able to mingle more uniformly with the other constituents of the chynu . The gastric fluid acts as a general solvent for some of the saline constituents 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 x 2 308 DIGESTION. [cHAi-. viir. 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 putrefaction 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 antici- pated, not only from the proneness to decomposition of organic matters, such as those used as food, especially under the in- fluence 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 favour- able, over-exertion injurious to digestion) ; the state of mind (tranquillity of temper being essential, in most cases, to a quick and due digestion) ; the bodily health ; and some others. Movements of the Stomach. — The gastric fluid is assisted in accomplishing its share in digestion by the movements of the stomach. In granivorous birds, for example, the contraction of the strong muscular gizzard affords a necessary aid to digestion, by grinding and triturating the hard seeds which constitute part of the food. But in the stomachs of man and other Mammalia the motions of the muscular coat are too feeble to exercise any such mechanical force on the food ; neither are they needed, for mastication has already done the mechanical work of a gizzard ; and experiments have demonstrated that substances enclosed in perforated tubes, and consequently protected from mechanical influence, are yet digested. The normal actions of the muscular fibres of the human chap, tiii.] GA8TRIC DIGESTION. 309 mach appear to have a three-fold pui (1) to adapt the mach to the quantity of food in it, bo that its walls may be in contact with the food on all Bides, and, at the same time, may • a certain amount of compression upon it: (2) to k- • the orifices of the stomach closed until the food is digested ; and to perform certain peristaltic movements, whereby the food, it becomes chymified, is gradually propelled towards, and ultimately through, the pylorus. In accomplishing this latter end, the movements without doubt materially contribute towai 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 Bhortly 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 illowed, opens to admit its passage to the stomach, and imme- diately again closes. The pyloric orifice, during the first pan of _ -trie digestion, is usually so completely closed, that even when the stomach is separated from the intestines, none of its contents ape. But towards the termination of the digestive process, the pylorus seems to offer less r sistance 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 advanced, and were continued until it was completed. The contraction of the fibres situated towards the pyloric end of the stomach seems to be more energetic and more decidedly ristaltic than those of the cardiac portion. Thus, it was found in the case of St. Martin, that when the bulb of the thermo- meter was placed about three inches from the pylorus, through the gastric fistula, it was tightly embraced from time to time, and drawn towards the pyloric orifice for a distance of three or four inches. The object of this movement appeal's to be. just said, to carry the food towards the pylorus as fast as it formed into chyme, and to propel the chyme into the 310 DIGESTION. [chap. vnr. duodenum ; the undigested portions of food being kept back until they are also reduced into chyme, or until all that is digestible has passed out. The 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 pre- dominant action 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 mus- cular coats of the stomach, not merely is chymified food gradually propelled through the pylorus, but a kind of double current is continually kept up among the contents of the stomach, the circumierential 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, namely, towards the cardiac orifice : in this way is kept up a constant circulation of the contents of the viscus, highly con- ducive to their free mixture with the gastric fluid and to their ready digestion. Vomiting. — The expulsion of the contents of the stomach in vomitingr, like that of mucous 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, it 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 inert the effect. It has been frequently stated that the stomach itself is quite passive during vomiting, and that the expulsion of its contents is i bap. vim.] VOMITING. 3! i effected solely by the pi upon it when the • of the abdomen is diminished bythe contraction of the diaphragm, and subsequently of the abdominal muscles. The experirj an ssess the power of vomiting at will, without applying any undue irritation to the stomach, but simply voluntary effort. It seems also, that tins power may be acquired by those who do not naturally possess it, and by continual prac- tice may become a habit. There are i - i of rare occurrence in which persons habitually swallow their food hastily, and nearly nnmasticated, and then at their leisure i _ _.:ate it, piece by piece, into their mouth, remasticate, and again swallow it, like members of the ruminant order of Mammalia. 312 DIGESTION. [chap. viii. The various nerve-actions concerned in vomiting are governed by a nerve-centre situate in the medulla oblongata. The sensory nerves are the fifth, glossopharyngeal and vagus principally ; but, as well, vomiting may occur from stimulation of sensory nerves from many organs, e.g., kidney, testicle, kc. 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 the spinal nerves. Influence of the Nervous System on Gastric Digestion. — The normal movements of the stomach during gastric digestion CO o are directly connected with the plexus of nerves and ganglia con- tained in its walls, the presence of food acting as a stimulus which is conveyed to the ganglia 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 de- rived, probably, from the splanchnic nerves. The exact function of the vagi in connection with the move- ments of the stomach is not certainly known. Irritation of the vagi produces contraction of the stomach, if digestion is proceed- ing ; 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 dividing their vagic nerves, the process of diges- tion 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 impulse 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 the process of digestion, being occasionally followed by death from inanition ; yet the digestive powers of the stomach may be completely restored after the operation, and the formation of chyme and the nutrition of / f these nerves excited an active secretion of the fluid, while a like stimulus applied to the sympathetic nerves issuing from the semilunar ganglia, caused a diminution and even complete arrest of the secretion. The influence of the 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 maybe produced, and the contents of the stomach may in part escape into the cavity of the abdomen. This pheno- menon 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 prevent its tempe- rature from falling, not only the stomach, but many of the sur- rounding 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 1 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 arc 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 expe- riments of Pavy's, in which the mucous membrane of the stomachs of dogs was dissected off for a small space, and, on killing the animals some days afterwards, no sign of digestion of the stomach was visible. " Upon one occasion, after removing the mucous membrane, and exposing the muscular fibres over a space of about an inch and a half in diameter, the animal was allowed to live for ten days. It ate food every day, and seemed scarcely affected by the operation. Life was destroyed whilst digestion was being 314 DIGESTION. [chap. viii. carried on, and the lesion in the stomach was found very nearly repaired : new matter had been deposited in the place of what had been removed, and the denuded spot had contracted to much less than its original dimensions." Pavy believes that the natural alkalinity of the blood, which circulates so freely during life in the walls of the stomach, is sufficient to neutralize the acidity of the gastric juice; and as may Fig. 181. — Auerbach's nerve-plexus in small intestine. The plexus consists of fibrillated substance, and is made up of trabecular of various thicknesses. Nucleus-like elements and ganglion-cells are imbedded in the plexus, the whole of which is enclosed in a nucleated sheath. (Klein.) 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 the experiments adduced in favour of this theory are open to many objections, 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 intestine. It must be confessed that no entirely satisfactory theory has been yet stated. The Intestines. The Intestinal Canal is divided into two chief portions, named from their differences in diameter, the (I.) small and (II.) large CHAP. VIII.] THE INTESTINES. 315 intestine (fig. 165). These are continuous with each other, and communicate by means of an opening guarded by a valve, the Ueo-ccecal valve, which allows the passage of the products of digestion tV.au the -mall into the large bowel, but not, under ordinary circumstances, in the opposite direction. /. Th Small I at- sin'. — The Small Intestine, the average Length of which in an adult is about twenty feet, has been divided, for convenience 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 thum, which forms three- tifths of the rest of the canal. Structure. — The small intestine, like the stomach, is con- structed of f«.ur principal coats, viz., the serous, muscular, sub- mucous, and mucous. (1.) The serous coat, formed by the visceral layer of the peri- toneum, and has the structure of serous membranes in general. (2.) The muscular coats consist of an internal circular and an external longitudinal layer : the former is usually considerably Fig. 182. — Horizontal section of a si tall fragment of the mucous nt«m&raii0, including one entire crypt of Lieberkiihn and parts of several others : a, cavity of the tubular glands or crypts : b, one of the lining epithelial cells ; r, the lymphoid or retiform spaces, of ■which some are empty, and others occupied by lymph cells, as at (/. 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 (Auerbach's 3i6 DIGESTION. [(HAT. VIII. plexus, plexos myentericus) (fig. 181) similar in structure to Meissner's (in the submucous tissue), but with more numerous ganglia. This plexus regulates 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-medullated 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 pro- bable 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 valvulo? connive ntes ; the villi; and the glands. general structure of mucous membrane of intestines resembles that of the stomach (p. 298), and, like it, is lined on its inner surface by columnar epithelium. Adenoid tissue (fig. 182, c and d) enters largely into its construction ; and on its deep surface is the mus- cularis mucosas (m m, fig. 183), the fibres of which are arranged in two layers : the outer longitudinal and the inner circular. Fig. 183. — 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 "striated basilar border," while inside the villus the adenoid tissue and un- striped muscle - cells are seen ; If, Lieberkuhn's follicles ; m m, muscularis mucosae, sending up fibres between the follicles into the villi ; rnn, submucous tissue; containing [gm), ganglion cells of Meissner's plexus. (Schofield.) The the the (II LP. VIII. | (.LANDS OF SMALL LYI'LSTI \ L. 317 Valvulse Connivontes. — The valvulce conniventes (fig, 184) commence in the duodenum, about one or two inches beyond the pvlonis, and becoming larger and more numerous immediately beyond the entrance of the bile duct, continue thickly arranged and well developed throughout the jejunum ; then, gradually diminishing in size and number, they cease near the middle of the ileum. They are formed by a doubling inwards of the mucous membrane j the crescentic, nearly circular, folds thus formed being arranged transversely to the axis of the intestine, and each individual fold seldom extending around more than | or § of the bowel's circumference. Unlike the rugse in the oesophagus and stomach, they do not disappear on distension 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. 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 (tig. 184). Their functions are probably less — Besides (1) offering a largely increased surface for secretion and absorption, they probably (2) prevent the too rapid passage of the very liquid products of gastric digestion, immediately after their escape from the stomach, and (3), by their projection, and consequent interference with an uniform and untroubled current of the intestinal contents, probably assist in the more perfect mingling of the latter with the secretions poured out to act on them. Glands of the Small Intestine.— The glands are of three principal kinds :— viz., those of (1) Lieberkuhn, (2) Brunner, and (3) Peyer. (1.) The (/lands or crypts of Lieberlciihi are simple tubular de- pressions of the intestinal mucous membrane, thickly distributed Fig. 184. — Piece ofsmn/l in- testine [previously di$- tended "/»/ hardened by alcohol) laid open to show the normal posi- tion of the valvule con- niventes. 3i8 DIGESTION. [chap. VIII. Fig. 185. — Tranverse section through four crypts of Lieberkuhn from the large intestine of the pig. They are lined by columnar epithelial cells, the nuclei being placed in the outer part of the cells. The divisions between the cells are seen as lines radiating from L, the lumen of the crypt; G, epithelial cells, which have become transformed into goblet cells. X 350. (Klein and Noble Smith.) over the whole surface both of the large and small intestines. In the small intestine they are visible only with the aid of a lens ; and their orifices appear as mi- nute dots scattered between the villi. They are larger in the large intestine, and increase in size the nearer they approach the anal end of the intestinal tube ; and in the rectum their orifices may be visible to the naked eye. In length they vary from ^ to ^ of a line. Each tubule (fig. 186) is constructed of the same essential parts as the intestinal mucous membrane, viz., a fine membrana propria^ or basement membrane, a layer of cylindrical epithelium lining it, and capillary blood- vessels covering its exterior, the free surface of the columnar 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 frequently occur (fig. 185). (2.) Brumier's glands (fig. 188) 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 submucous 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 under- goes a similar change during secretion; but they are more Fig. 186. — A gfand of Lieberkilhn in lon- gitudinal section. (Brinton.) « BAP. VIII.] SMALL INTESTIXK. 319 branched and convoluted and their ducts are longer. (Watney.) The duct of each gland passes through the muscularis mucosa, ami opens on the surface of the mucous membrane. Fig. 187. — T of injected Peya's glands 'from Kolliker). The dra-n-ing"was taken from a preparation made by Frey : it represents the fine capillary-looped net- work spreading from the suiToundin? blood-vessels into the interior of three of Fever's si iles from the intestine of the rabbit . (3.) The [/lands of Peyer occur chiefly but not exclusively in the imall intestine. They are found in greatest abundance in the lower part of the ileum near to the ileo-c?ecal valve. They are met with in two conditions, viz., either scattered singly, in which case they are termed ' £/> Intestine. — The Large Intestine, which in an adult is from about 4 to 6 feet Long, is subdivided for descriptive purposes into three portions (fig. 165) viz. : — the caecum, a short wide pouch, communicating with the lower end of the small intestine through an opening, guarded by the ileo-c&calvalxe ; the . continuous with the caecum, which forms the principal part «.f the large intestine, and is divided into an ascending, transverse, and descending portion \ and the rectum, which, after dilating at its lower part, again contracts, and immediately afterwards opens externally through the o.nv.<. Attached to the caecum is the small appendix verm iform is. Structure. — Like the small intestine, the large is constructed of four principal coats, viz., the serous, muscular, submucous, and mucous. The serous coat need not be here particularly described. Connected with it are the small processes of peritoneum, contain- ing fat, called appendices epiploicae. The fibres of the muscular coat, like those of the small intestine, are arranged in two layers — the outer longitudinal, the inner circular. In the csecum and eolon, the longitudinal fibres, besides being, as in the small intestine, thinly disposed in all parts of the wall of the bowel, are collected, for the most part, into three strong bands, which being shorter, from end to end, than the other coats of the 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 longitu- dinal bands spread out and mingle with the other longitudinal fibres, forming with them a thicker layer of fibres than exists on any other part of the intestinal canal. The circular muscular til-res 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 326 DIGESTION. [CHAP. VIII. the form of valvules conniventes. Its general microscopic structure resembles that of the small intestine : and it is bounded below by the muscularis mucosce. The general arrangement of ganglia and nerve-fibres in the large intestine resembles that in the small (p. 315). Glands of the Large Intestine— The glands with which the large intestine is provided are of two kinds, (1) the tubular and (2) the lymphoid. jHo IQ , Horizontal section through a portion of the mucous membrane of the large intestine, °'i/e, and the central zone dimini - 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 supplied to it by the blood. The granules are thought to be not the ferment itself, but material from which, under certain condi- tions, the ferments of the gland are made, and therefore called Zymogen, Pancreatic Secretion. — The secretion of the pancreas has been obtained 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 removed from an animal killed during digestion, posa 3fi - the active properties of pancreatic secretion. It is made by first de- hydrating 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, placing it in strong glycerin. A ' X o o glycerin extract is thus obtained. It is a remarkable fact, how- ever, that the amount of the ferment trypsin greatly in- creases if the gland be exposed to the air for twenty-four hours before placing in alcohol ; indeed, a glycerin extract made from gland immediately upon removal from the body often appears to contain none of that 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 conversion of the «>ne into the other only takes place by degrees. Dilute acid appears to t and accelerate the conversion, and if a recent pancreas be rubbed up with dilute acid before dehydration, a glycerin extract made afterwards, even though the gland may have been only recently removed from the body, is very active. 330 DIGESTION. [chap. vm. Properties. — Pancreatic juice is colourless, transparent, and slightly viscid, alkaline in reaction. It varies in specific gravity from ioioto 1015, according to whether it is obtained from a permanent fistula — then more watery — or from a newly-opened duct. The solids vary in a temporary fistula from 80 to 100 parts per thousand, and in a permanent one from 16 to 50 per thousand. Chemical Composition of the Pancreatic Secretion. From a permanent fistula. (Bernstein.) Water ■ . . . 975 Solids — Ferments : Proteids, including Serum — Albumin, ) Casein, Leucin and Tyrosin. Fats / 17 and Soaps . . . . . ) Inorganic residue, especially Sodium ~| 8 Carbonate J 2 5 1000 Functions. — (1.) It converts proteids into peptones, the interme- diate product being not akin to syntonin or acid-albumin, as in gastric digestion, but to alkali-albumin. Kiihne believes that the intermediate products, both in the peptic and pancreatic digestion of proteids, are two, viz., antialbumose and hemialbu- mose, and that the peptones formed correspond to these, viz., antipeptone and hemipeptone. The hemipeptone is capable of being converted by the action of the pancreatic ferment — trypsin — into leucin and tyrosin, 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 fsecal substances, of which indol and naph- thilamine are the most important. (Kiihne.) When the digestion goes on for a long time the indol is formed in considerable quantities, and emits a most disagreeable faecal odour, which was attributed to putrefaction till Kiihne showed its true nature. All the albuminous or proteid substances which have not been converted 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. chap, vin.] PANCREATIC SECRETION. 3 5I (2.) Nitrogenous bodies other than /■ ■ 1 >sr in mi exactly similar manner to that which happens with the saliva. As mentioned befoiv. it seems not unlikely that glucose is not formed at once from starch, but that certain dextrinea are intermediate products. If the BUgar which is at first formed, as is stated by some chemists* be not glucose but maltose, at any rate the pancreatic juice after a time completes the whole change of starch into glucose. There is a distinct amylolytic ferment (Amylopsin) in the pan- creatic juice which cannot be distinguished from ptyalin. (4.) Oils and fats are both emulsified and split up into their fatty acids and glycerin by pancreatic secretion. Even if part of this action is due to the alkinity of the medium, it is probable that there is a third distinct ferment (Steapsin) which facilitates the change. 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 intes- tines. In nearly all these cases, indeed, the liver was coincidently diseased, and the change or absence of the bile might appear to contribute to the result ; }*et the frequency of extensive disease of the liver, unaccompanied by fatty discharges from the intes- tines, favours 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. (5.) It ])Ossesses the property of curdling mUJc, containing a special (rennet) ferment for that purpose. The ferment is dis- tinct from trypsin, and will act in the presence of an acid ( W. Roberts). Conditions favourable to the Action of the Pancreatic Juice. — These are similar to those which are favourable to the action of the saliva, and the reverse (p. 285). 33^ DIGESTION. [chap. VIII. The Liver. The Liver, the largest gland in the body, situated in the abdomen, chiefly on the right side, is an extremely vascular organ, and receives its supply of blood from two distinct vessels, the portal vein and hepatic artery, while the blood is returned from it into the vena cava inferior by the hepatic veins. Its secretion, the bile, is conveyed from it b} T the hepatic duct, either directly into the intestine, or, when digestion is not Fi°-. 196. — The under surface of tic liver, a. b., gall-bladder ; h. d., common bile-duct : *H. a., hepatic arteiy. v. p., portal vein; l. q.. lobulus quadratus ; l. s., lobulus spigelii ; l. c, lobulus caudatus ; d. v., ductus venosus ; u. v.,imibilical vein. (Noble Smith.) 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 peri- toneum, and beneath this is a very fine coat of areolar tissue, con- tinuous over the whole surface of the organ. It is thickest where the peritoneum is absent, and is continuous on the general surface of the liver with the fine, and, in the human subject, almost imperceptible, areolar tissue investing the lobules. At the transverse fissure it is merged in the areolar investment called Glisson's capsule, which, surrounding the portal vein, I 1I.W. VIII. J THE LIVER. 3-> *> hepatic artery, and hepatic duct, aa they enter at thia part, ac- companies them in their b ran< jhinga through the substance of the liver. Structure. — The liver is made up of small roundish or oval portions called lobules, each of which is aboul ',, of an inch in diameter, and com] osed of the minute branches of the portal vein, hepatic artery, hepatic duct, and hepatic vein ; while the intcr- stices of these vessels are filled by the liver cells. The hepatic cells (fig. 197), which form the glandular or secreting part of the liver, are of a spheroidal form, somewhat polygonal from mutual pressure about -^ to T ^o inch iu diameter, possessing Fi„ 107.— A. I - B. Ditto, con- taining various .-bced particles of fat. Fig 198 —I "?' canal, containing a portal vein, hepatic artery and hepatic duct, from the pig. p,' branch of vena port«>, situate in a portal .anal lormed amongst the lobules of the liver, I 1, and giving off vaginal branches ; there are also seen within the large portal vein numerous orihees of the smallest interlobular veins arising directly from it ; a, hepatic artery ; d, hepatic duct, x 5. (Kiernan.) one, sometimes two nuclei. The cell-substance contains numerous fatty molecules, and some yellowish-brown granules of bile-pigment. 334 DIGESTION. [CHAP. VIII. 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 enter 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. 198. Running 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). To take the distribution of the portal vein first : — In its course through the liver this vessel gives off small branches Fig. 109. — Cross section of a lobule of the human liver, in which the capillary network between the portal and hepatic veins has been fully injected, i, section of the ;'/^/77-lobular vein; 2, its smaller branches collecting blood from the capillary network; 3. inter- lobular branches of the vena porta? with their smaller ramifications passing inwards towards the capillary network in the substance of the lobule, x 60. (Sappey.) which divide and subdivide between the lobules surrounding them and limiting them, and from this circumstance called inter-lobukur 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 CHAl". VIII.] THE LIVER. 335 Is, converges finally to a single small vein, occupying the centre of the lobule, and hence called mtfra-lobular. Tliis arrange nient is well Been in fig. 199, which represents a transverse section of a lobule. The small inlro-lobular veins discharge their contents into veins called ^-lobular (hhh, fig. 200) ; while these again, by their union, form the main branches of the hepatic veins, which leave Fig. 200. — 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 (/) 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 ; ?', mouth of the intralobular veins, opening into the sublobular veins ; %'. intralobular veins shown passing up the centre of some divided lobules ; /, /, cut sm-face of the liver ; c, c, wails of the hepatic venous canal, formed by the polygonal bases of the lobules, x 5. (Kiernan.) the posterior border of the liver to end by two or three principal trunks in the inferior vena cava, just before its passage through the diaphragm. The swMobular 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 nib4obular veins by means of the small intralobular veins is well- 33$ DIGESTION. [CHAP. VIII. seen in the diagram (fig. 200 and in fig. 201), which represent the parts as seen in a longitudinal section. The appearance has been likened to a twig having leaves with- out footstalks — the lobules representing the leaves, and the sub-lobidar vein the small branch from which it springs. On a transverse section, the appearance of the intra-lobular veins is that of 1, ibui*. ^S- j 99j Av hil° both a transverse and Longitudinal section are exhibited in fig. 176. The hepatic artery, the function of which is to distribute blood for nutri- tion to Glisson'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 b} T small branches either into the ramifications of the portal vein, or into the capillary plexus of the lobules which connects the inter- and wrtra-lobular veins. Lobule Fig. 201. — Diagram showing the manner in which the lobules of the ■ •" on the sublobular (After Kiernan.) p h Fig. 202.— 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 Halting : it shows nearly the whole of two lobules, and parts of three others ; p, portal branches running in the interlobular spaces ; h, hepatic veins penetrating and radiating from the centre of the lobules, x 45. (Kolliker.) The hepatic duct divides and subdivides in a manner very like that of the portal vein and hepatic artery, the larger branches en \i\ \ in. | STRUCTURE <>!•' LIVER. 337 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 appeal' to be always bounded by hepatic cells on all sides, and are thus separated from the nearest blood-capillary by at least the breadth of one cell (figs. 203 and 204). The Gall-bladder .—The ( ! all-bla. 1 der (g, b, fig. 196) is a pyriform bag, attached to the under surface of the liver, and supported also by the peri- toneum, 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 princi- pal coats. (1) Externally (excepting that part which is in contact with the liver), is the serous coat, which has the same structure as the peritoneum with which it is continuous. Within this is (2) the fibrous or areolar coat, constructed of tough fibrous Fig". 203. — Portion of a lobnlr of liver, a, bile capillaries between liver-cells, the network in which is well seen ; b, blood capillaries, x 350. (Klein and Noble Smith.) Fig. 204. — 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 corpuscles of the blood are recognized by their circular contour: vp, corre- sponds to an interlobular vein in immediate proximity with which are the epithelial cells of the biliary ducts, to which, at the lower part of the figures, the much larger hepatic cells suddenly succeed. (E. Hering.) 338 DIGESTION. [chap. viii. and clastic tissue, with which is mingled a considerable number of plain muscular fibres, both longitudinal and circular. (3) In- ternally the gall-bladder is lined by mucous membrane, and a layer of columnar epithelium. The surface of the mucous mem- brane presents to the naked eye a minutely honeycombed ap- pearance 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. 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 ma} 7 be included the Glycogenic Function. I. The Secretion of Bile. Properties of the Bile. — The bile is a somewhat viscid fluid, of a yellow or reddish-yellow colour, a strongly bitter taste, and, when fresh, with a scarcely perceptible odour : it has a neutral or slightly alkaline reaction, and its specific gravity is about 1020. Its colour and degree of consistence vary much, apparently inde- pendent of disease ; but, as a rule, it becomes gradually more deeply coloured and thicker as it advances along its ducts, or when it remains long in the gall-bladder, wherein, at the same time, it becomes more viscid and ropy, of a darker colour, 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 140-8 iooo-o oh ip. viii.] Bl LE. ^-?o Bile salts or Bilin 91-5 Fat 9-2 Cholesterin ......... 26 Mucus and colouring matters . . . . . . 29^8 Baits 7.7 1408 Bile salts, or Bilin, can be obtained as colourless, exceedingly deliquescent crystals, soluble in water, alcohol, and alkaline solu- tions, giving to the watery solution the taste and general characters of bile. Thc} r consist of sodium salts of glycocholic and tauro- cholic acids. The former salt is composed of cholic acid conjugated with glycin (see Appendix), the latter of the same acid conjugated with taurin. The proportion 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 car- nivora contains taurocholate alone ; inhuman bile both are present in about the same amount (glycocholate in excess }). Preparation of Bile Salt. — Bile salts may be prepared in the following 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. Separate the alcoholic extract by filtration, and to it add perfectly anhydrous 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 ciystals 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 z 2 340 DIGESTION. [chap. vnr. of the acid are re-dissolved. Tf to the solution a drop of solu- tion of cane sugar be added, a fine purple colour is developed. The re-action 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 colour similar to the above is produced by the action of sulphuric acid and sugar on albu- men, the crystalline lens, nerve tissue, oleic ncid. pure ether, cholestcrin. morphia, codeia and amylic alcohol. The spectrum of Pettenkofer's reaction, when the fluid is moderately diluted, shows four hands — 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 the fourth near D. The yellow colouring matter of the bile of man and the Carnivora is termed Bilirubin or Bilifulvin (c l6 h i8 x 2 o,) crystallizable and insoluble in water, soluble in chloroform or carbon disulphate ; a green colouring matter, Biliverdin (c l6 h 20 x 2 o.), which always exists in large amount in the bile of Herbivora, being formed from bilirubin on exposure to the air, or by subjecting the bile to any other oxidizing 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 colouring matter of the bile is re-absorbed, and circulates with the blood, giving to the tissues- the yellow tint characteristic of jaundice. The colouring matters of human bile do not appear to give characteristic 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). There seems to be a close relationship between the colour- matter of the blood and of the bile, and it may be added, between these and that of the urine (urobilin), and of the faeces (ster- cobilin) 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 Urobilin is identical with JIt/drobitiri/hin, a sub- stance which is obtained from bilirubin by the action of sodium CHAP. VIII. 1 BILE. 341 amalgam, or bj the action of sodium amalgam on alkaline hsematin; both urobilin and hydrobilirubin giving a characteristic absorption band between band F. They are also identical with stercobilin, which is formed in the alimentary canal from l»ile pigments. A common test (Gmelin's) for the presence of Ul pigment con BiBts <>f the addition of a small quantity of nitric acid, yellow with nitrous acid ; if bile be present, a play of colours is produced, beginning with green and passing through blue and violet to red, and lastly to yellow. The spectrum of Gmelin's test gives a black band extending from near b to beyond F. Fatty substances are found in variable proportions in the bile. Besides the ordinary saponifiable fats, there is a small quantity of Cholesterin, a so-called non-saponi- tiable fat, which, with the other live fats, is probably held in solu- tion by the bile salts. It is a body belonging to the class of monatomic alcohols (c 26 h 44 o), and ciystallizes in rhombic plates {tig. 205). It is insoluble in water and cold alcohol, but dissolves easily in boiling alcohol or ether. It gives a red colour with strong Bulphuric acid, and with nitric acid and ammonia ; also a play of colours beginning with blood red and ending with green on the addition of sulphuric acid and chloroform. Lecithin (c 44 h 90 xro 9 ), a phosphorus-containing body and Neurin (c s h i5 no 2 ), are also found in bile, the latter probably as a decomposition product of the former. The Mucus in bile is derived from the mucous membrane and glands of the gall-bladder, and of the hepatic ducts. It consti- tutes the residue after bile is treated with alcohol. The epithe- lium with which it is mixed may be detected in the bile with the microscope in the form of cylindrical cells, either scattered or still held together in layers. To the presence of the mucus is probably to be ascribed the rapid decomposition undergone by the Fi#\ 205. — ( 'rystalline scales of ciholesti rin. 342 DIGESTION. [chap. yiii. bilin ; for, according to Berzelius, if the mucus be separated, bile will remain unchanged for many days. The Saline or inorganic constituents of the bile are similar to those found in most other secreted fluids. It is possible that the carbonate and neutral phosphate of sodium and potassium, found in the ashes of bile,- are formed in the incineration, and do not exist as such in the fluid. Oxide of iron is said to be a common constituent of the ashes of bile, and copper is generally found in healthy bile, and constantly in biliary calculi. Gas — A certain small amount of carbonic acid, oxygen, and nitrogen, may be extracted from bile. Mode of Secretion and Discharge. — The process of secreting bile is continually going on, but appears to be retarded during fisting, and accelerated 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, Avhereby 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 dis- charged 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. (Blondlot, Bidder and Schmidt.) The bile is formed in the hepatic cells ; then, being discharged into the minute hepatic ducts, it passes into the larger trunks, and from the main hepatic duct may be carried at once into the duodenum. But, probably, this 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 accu- mulates till, in the next period of digestion, it is discharged into the intestine. The gall-bladder thus fulfils what- appears to be its chief or only office, that of a reservoir ; for its presence enables bile to be constantly secreted, }-et insures its employment in the service of digestion, although digestion is periodic, and the secre- tion of bile constant. The mechanism by which the bile passes into the gall-bladder is simple. The orifice through which the common bile-duct com- municates with the duodenum is narrower than the duct, and appears to be closed, except when there is sufficient pressure chap, viii.] SECRETION OF BILE. 343 behind to force the bile through it. The pressure exercised upon the bile secreted during the intervals of digestion appears insuffi- cient to overcome the force with which the orifice of the duct is closed ; and the bile in the common duct, finding no exit in the intestine, traverses the cystic duct, and so passes into the gall- bladder, being probably aided in this retrograde course by the peristaltic action of the ducts. The bile is discharged from the gall-bladder and enters the duodenum on the introduction of food into the small intestine : being pressed on by the contrac- tion of the coats of the gall-bladder, and of the common bile- duct also; for both these organs contain 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 common bile-duct. Bile, as such, is not pre-formed in the blood. As just observed, it is formed 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 f/all-stone along the hepatic duct) it is absorbed in great excess into the blood, and, circulating 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 -?- inch of mercury (16 mm.) the secretion 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 doubtless varying, like that of the gastric fluid, in pro- portion 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. Uses. — (1) As an excre?nentitious substance, the bile may serve especially as a medium for the separation of excess of carbon and hydrogen from the blood ; and its adaptation to this purpose is well-illustrated by the peculiarities attending its secretion and disposal iu the foetus. During intra-utcrine life, the lungs and 344 DIGESTION. [chap. viij. 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 secre- tion ; the meconium, or fa?ces of the foetus, containing all the essential principles of bile. ('•imposition of Meconium (Frerichs) : Biliary resin 15 '6 Common fat and cholesterin . . . . . 15 "4 Epithelium, mucus, pigment, and salts . . 69/0 lOO'O In the foetus, therefore, the main purpose of the secretion of bile must be the purification of blood by direct excretion, i.e., by separation from the blood, and ejection from the body without further change. Probablv all the bile secreted in fcetal life is incorporated in the meconium, and with it discharged, and thus the liver may be said to discharge a function in some sense vicarious of that of the lungs. For, in the foetus, nearly all the blood coming from the placenta passes through the liver, previous to its distribution to the several organs of the body ; and the abstraction of carbem, hydrogen, and other elements of bile will purify it, as in extra-uterine life it is purified by the separation of carbonic acid and water at the lungs. The evident disposal of the foetal bile by excretion, makes it highly probable that the bile in extra-uterine life is also, at least in part, destined to lie discharged as excrementitious. The analysis of the freces of both children and adults shows 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 colouring, and some of its fatty matters, and to only a very slight degree, its salts, almost all of which have been re-absorbed from the intestines into the blood. i HAP. \ 111. USES OF BILE. 345 The elementary composition of bile salts shows, however, Buch 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 excretion 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 carbon 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. A substance, which has been discovered in the fieces, and named stt rcorin is closely allied to cholesterin ; and it lias 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). 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 excretory, not only for such hydro-carbonaceous matters as may need expulsion from any portion of the blood, but that it serves for the direct purification of the stream which, arriving by the portal vein, has just gathered up various substances in its course through the digestive organs — substances which may need to be 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 nutrition, but which would be positively injurious if allowed to mingle with the general mass of the blood. The liver, therefore, may be supposed placed in the only road by which such matters can pas.-, unchanged into the general current, jealously to guard against their further progress, and turn them back again into an excretory channel. The frequency with which metallic poisons are either excreted by the liver, or intercepted and retained, often for a considerable 346 DIGESTION. [chap. vm. time, in its own substance, may be adduced as evidence for the probable truth of this supposition. (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 excretion, 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 intestinal 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 ruixed- A similar indication is furnished also by the fact that the secre- tion of bile is most active, and the quantity discharged into the intestines much greater, during digestion than at any other time; although, without doubt, this activity of secretion during dieres- tion 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 temporarily, (to be afterwards re- absorbed,) or permanently. Respecting 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 unaffected, chyle was no longer well formed ; the contents of the lacteals consisting of clear, colourless fluid, instead of being opaque and white, as they ordinarily are, after feeding. (6.) It is probable, also, that the moistening of the mucous mem- brane of the intestines by bile facilitates absorption of fatty matters through it. (c.) The bile, like the gastric fluid, has a considerable anti- septic power, and may serve to prevent the decomposition of food chap. viu. | USES OF BILE. • 347 during the time of its Bojourn in the intestines. Experiments show that the contents of the intestines are much more foetid after the common bile-duct lias been tied than at other tim< moreover, it is found that the mixture of bile with a fermentine fluid stops or spoils the process of fermentation. ((/.) 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 th< gastric parapeptones and j>ej>tones, tor/ether with the pepsin which is mixed up with them, as soon as the contents of the stomach meet it in the duodenum. 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. Nothing is known with certainty respecting the changes which the re-absorbed portions of the bile undergo. That they are much changed appears from the impossibility of detecting them in the blood ; and that part of this change is effected in the liver is probable from an experiment of Magendie, who found that when he injected bile into the portal vein, a dog was unharmed, but was killed when he injected the bile into one of the systemic vessels. II. The Liver as a Blood-elaborating Gland. 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. 348 DIGESTION, [chap. viii. that the peptones, conveyed from the alimentary canal by the blood of the portal vein, require to be submitted to the influence of the liver before they can he 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 albu- minous part of the blood. Albuminous matters are also subject to decomposition by the liver in another way to be immediately noticed (p. 349). The formation of urea by the liver will he again referred to (p. 457). Glycogenic Function. — One of the chief uses of the liver in connection with elaboration of the blood is comprised in what is known as its glycogenic function. The important fact that the liver normally forms glucose or grape sugar, or a substance readily con- vertible into it, was discovered by Claude Bernard in the course of some experiments which he undertook for the purpose of finding out in what part of the circulatory system the saccharine matter dis- appeared, which was absorbed from the alimentary canal. With this purpose 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. He then fed a dog with meat only, and, to his surprise, still found sugar 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 venous 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 experi- ments, that a liver, removed from the body, and from which all chap, viu.] GLYCOGENIC FUNCTION OF LIVER. 349 sugar had been completely washed away by injecting stream of water through its 1.1 L-vessels, will be found, after the lapse of r few hours, to contain Bugar in abundance. This \ production of sugar w;is a fact which could only !>■' explained in the supposition that the liver contained a Bubstance, readily con- vertible into sugar in the course merely of post-mortem decom- position : and this theory was proved correct by the 3 ry of a substance in the liver allied to starch, and now generally termed glycogen. We may believe, therefore, that the liver does ii"t form Bugar 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, glycogen is produce I by the liverwheu neither starch nor sugar is present in the E it> amount is much less under such a diet. . 1 rerage amount of Glycogen in the Um- of Dogs under various Diets (Pavy). Diet. Amount of Glycogen in Liver. Animal food 7-19 per cent. Animal food with sugar (al»out \ lb. of sugar daily) 145 ,. v getable diet (potatoes, with bread or barley-meal] 1723 The dependence of the formation of glycogen on the food taken is also well shown by the following results, obtained by the same experimenter : — J verage quantity of Glycogen found hi the Liter of Rabbits after Fasting anil after a <■ t Starch ami Sugar respectively. Average amount of Glycogen in I. After fasting for three days .... Practically absent. ., diet of starch and grape-sugar . .154 j>er cent. .. cane-sugar 16-9 Regarding these facts there is no dispute. All are agreed that glycogen 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 up into glycogen, which is temporarily stored in the liver, and urea, which is ex- creted by the kidney-. Destination of Glycogen. — There are two chief theories on 350 DIGESTION. [chap. viii. the subject of the destination of glycogen, (i.) That the conver- sion of glycogen into sugar takes place rapidly during life by the agency of a ferment also formed in the liver : and the sugar is conveyed away by the blood of the hepatic veins, and soon under- goes 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 transformation. 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 examination be delayed for a very short time after death, sugar in abundance ma}' be found in such blood ; (b), that the liver, like the venous blood in the heart, is, at the moment of death, completely free from sugar, although afterwards its tissue speedily becomes saccharine, unless the formation of sugar be pre- vented by freezing, boiling, or other means calculated to interfere with the action of a ferment on the amyloid substance of the organ. Instead of adopting Bernard's view, that 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 and Glycogen. — 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 colouring matters. The decoction is then filtered and may be tested for glucose (p. 284). Glycogen (c 6 h io o 5 ) is an amorphous, starch-like substance, odourless and tasteless, soluble in water, insoluble in alcohol. It is converted into glucose by boiling with dilute acids, or by con- tact with any animal ferment. 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 pulpy mass, and subsequently boiled in distilled water for about a quarter of an hour. The glycogen is precipitated from the filtered decoction by i hap. vin.] <.i.y i:.\. the addition of alcohol. Glycogen has been found in many other :h;ui the liver. Si 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 ( i such an extent that sugar would be present not only in the hepal veins, but in the blood generally, Such is frequently the the sugar when in excess in the blood being secreted by the kidneys, and thus appearing in variable quantities in the urine (( rlycosuria). Influence of the Nervous System in producing Glyco- suria. — Glycosuria may be 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 possibly 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 medulla down the spinal cord as far as — in rabbits — the fourth dorsal vertebra, and thence to the 1 thoracic ganglion. Many other circumstances will cause glycosuria. 1: has been observed after the administration of various drugs, after the in- jection of urari, poisoning with carbonic oxide gas, the inhalation pf ether, chloroform, etc., the injection of oxygenated bl< into the portal venous system. It has been observed in man after injuries to the head, and in the course of various disease 3. The well-known disease, diabetus m in which a large quantity of sugar is persistently secreted daily with the urine, has. doubtless, - Ioe relation to the normal glycogenic function of the liver; but the nature of the relationship is at present quite unknown. The Intestinal Secretion, or Suecus Entericus. — On account of the difficulty in isolating the secretion of the glands in the wall of the intestine (Brunner's and Lieberkuhn's) from other secretions poured into the canal (gastric juice, bile, and 352 DIGESTION. [cHAr. yiil pancreatic secretion), but little is known regarding the composi- tion of the former fluid (intestinal jnice, succus entericus). It is said to be a yellowish alkaline fluid with a specific gravity of ion, and to contain about 2*5 per cent, of solid matters (Thiry). Functions. — The secretion of Brnnner's glands is said to be able to convert proteids into peptones, and that of Lieberkiihn's is be- lieved to convert starch into sugar. To these functions of the succus entericus the powers of converting cane into grape sugar, and of turning cane 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 : — 2 G 12 H 22 1X + 2 H 2 = C 12 H 2t 12 + C 13 H 24 12 Saccharose Water Dextrose Lfevulose The conversion is probably effected by means of a hydrolytic ferment. (Inversive ferment, Bernard.) The length and complexity of the digestive tract seem to be closely con- nected with the character of the food on which an animal lives. Thus, in all carnivorous animals, such as the cat and dog, and pre-eminently in car- nivorous 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 requiring that they should absorb as much as possible from their intestines. Herbivorous animals, on the other hand, and the ruminants especially. have very long intestines (in the sheep 30 times the length of the body)- which is no doxibt to be connected with their lowly nutritious diet. In, others, such as the rabbit, though the intestines are not excessively long, this is compensated by the great length and capacity of the caecum. In man. the length of the intestines is intermediate between 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 < hap. vin.] stmmakv OF DIGESTION. 353 of food that reach it through the oesophagus, but especially t<» 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 con- version of starch into sugar, which began in the mouth, lias been interfered 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 dis- appear by absorption into the blood-vessels, and the same thing has befallen such fluids as may have been swallowed, — wine, water, etc. 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 albu- minous 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 faeces. On the entrance of the chyme into the duodenum, it is sub- jected 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 hy their .id mixture with the gastric chyme, its acidity becomes less and less until at length, at about the middle of the small intestine, the reaction becomes alkaline and continues so as far as the ileo- cecal valve. The special digestive functions of the small intestine may be taken in the following order : — (i.) 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 pro- cess, however, allotted respectively to the bile, to the pancreatic secretion, and to the intestinal juice, is still uncertain, — probably the pancreatic juice is the most important. The fat is changed A A 354 DIGESTION. [chap. vin. 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. (6). It is emulsionised, i.e., its particles are minutely subdivided and diffused, so that the mixture 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 intes- tine, but a small part, which is saponified, is also absorbed by the blood-vessels. (2.) The albuminous substances which have been partly dis- solved 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 w r ith the gastric peptones and parapeptones, as soon as the chyme meets with bile. By these means the pancreatic ferment trypsin is enabled to pro- ceed with the further conversion of the parapeptones into peptones,, and of part of the peptones (hemipeptone, Kiihne) into leucin and ty rosin. Albuminous substances, which are chemically altered in the process of digestion (peptones), and gelatinous matters similarly changed, are absorbed by both the blood-vessels and lymphatics of the intestinal mucous membrane. Albuminous matters, in a state of solution, which have not undergone the peptonic change, are 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 conver- sion 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 intestine ; 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. chap, vin.] 8UMMABY OF DIGE8TION. 355 rater, wine, ale, 0., which mayhavi rption in the Btomach, are absorbed probably r< their entrance into the intestine ; the fluidity of the contents of the latter being preserved more by the constant secretion of fluid I the intestinal glands, pancreas, and liver, than by any given portion of fluid, whether swallowed or secreted, remaining l<-ng unabsorbed. From this fact, th. it may be gathered that there is a kind of circulation constantly proceeding from th< intestines 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 .secre- tion 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 colour, and has a distinctly fecal odour. This odour depends upon the formation of indol. In this state it passes through the ileo-caecal opening into the large intestine. Summary of the Digestive Changes in the Large Intestine. The changes which take place in the chyme in the large in- line are probably only the continuation of the same changes that occur in the course of the food'- a£ ige through the upper part of the intestinal canal. From the absence of villi, however, we may conclude that absorption, especially of fatty matter, in great part completed in the small intestine ; while, from the still half-liquid, pultaceous eonsistence of the chyme when it first enters the caecum, there can be no doubt that the absorp- tion of liquid is not by any means concluded. The peculiar odour, moreover, which is acquired after a short time by the contents >>t' the Luge bowel, would seem to indicate a further chemical change in the alimentary matters or in the digestive fluids, or both. The acid reaction, which had disappeared in the .-mall bowel, again becomes very manifest in the caecum — probably from acid fermentation-processes in ><-meof the materials the food. A A 2 35^ DIGESTION. [CHAP. VIII. There seems no reason to conclude that any special * secon- dary digestive' process occurs in the caecum or in any other part of the large intestine. Probably any constituent of the food which has escaped digestion and absorption in the small bowel may be digested in the large intestine; and the power of this part of the intestinal canal to digest fatty, 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, and the completion of the changes which were proceeding in the small intestine, — the process being assisted by the secretion of the numerous tubular glands therein present. Fgeces. — By these means the contents of the large intestine, as they proceed towards the rectum, become more and more solid, and losing their more liquid and nutrient parts, gradually acquire the odour and consistence characteristic of faxes. 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 defaecation. 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"°o .Solids 267-00 Special excrementitious constituents : — Excretin. excre- x toleic acid (Marcet), and stercorin (Austin Flint). Salts : — Chiefly phosphate of magnesium and phosphate 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, un- 267-00 digested muscular fibres or fat, and the like, with insoluble substances accidentally introduced with the food. Mucus, epithelium, altered colouring matter of bile, fatty acids, etc. Varying quantities of other constituents of bile, and de- rivatives from them. chap, viii.] DEFECATION. 357 Length of Intestinal Digestive Period. — The time occu- pied by the journey of a given portion of food from the Btomach bo the anus, varies considerably even in health, and on this account, probably, it is that such different opinions have been expressed in regard to the subject. About twelve hours ari occupied by the journey of an ordinary meal through the small intestine, and twenty-four to thirty-six hours by the pass;. through the large bowel. (Brinton.) Defsecation. — Immediately before the act of voluntary expul- sion of faeces (defalcation) 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 con- tracted as in expiration ; 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 con- tents 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 instinc- tively or involuntarily yields (see fig. 144). Nervous Mechanism of Defsecation. — The anal sphincter muscle is normally in a state of tonic contraction. The nervous centre which governs this contraction is probably situated in the lumbar region of the spinal cord, inasmuch 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 immediately after the opera- tion. By an effort of the will, acting through the centre, the con- traction may be relaxed or increased. In ordinary cases the apparatus is set in action by the gradual accumulation of faaces in the sigmoid flexure and rectum pressing against the sphincter and causing its relaxation; this sensory impulse acting through the brain and reflexly through the spinal centre. Peristaltic action, especially of the sigmoid flexure in pressing onwards the fax-es against the sphincter, is a very important part of the act. 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 353 DIGESTION. [CHAP. VIII. occasion, in a post-mortem examination, either to lay open the intestines, or to let out the gas which they contain, must have been struck by the small space afterwards occupied by the bowels, and by the large degree, therefore, in which the gas, which naturally 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, probably, a mechanical one. Sources. — The sources of the gas contained in the stomach and bowels may be thus enumerated : — i. Air introduced in the act of swallowing either food or saliva ; 2. Gases developed by the decomposition of alimentary matter or of the secretions and excretions mingled with it in the stomach and intestines ; 3. It is probable that a certain mutual interchange occurs between the gases con- tained 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 intestines may be the proper excretory organs for many odorous and other substances, either absorbed from the air taken into the lungs in inspiration, or absorbed in the upper part of the alimentary canal, again to be excreted at a portion of the same tract lower down — in either case assuming rapidly a gaseous form after their excretion, and in this way. perhaps, obtaining a more ready egress from the body. It is probable that, under ordinary circumstances, the gases of the stomach and intestines are derived chiefly from the second of the sources which have been enumerated (Brinton). Composition of Gases contained in trie Alimentary Canal. (Tabulated from various authorities by Brixton.) Whence obtained. Stomach . . . . Small Intestines Csecum . . . . Colon Rectum . . . . Expelled per anvm Composition by Volume. Oxygen. Xitrog. Carbon. Acid. Hydrog. Carburet. Hydrogen. Sulphuret. Hydrogen, j II 71 14 4 — — — 32 30 3« — I - trace. 66 35 12 57 8 6 13 8 — 46 43 — 11 22 4i .9 19 i hap. viii.] MOVEMENTS OF THE l.YJ Kstixks. 3^ Movements of the Intestines. — It remains only to consider the manner in which the food and the several secretions mingled with it arc moved through the intestinal canal, so as to be slowly subjected to the influence of fresh portions of intestinal 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 muscular fibres contract first, or more than the circular; they draw a portion of the intestine upwards, or, as it were, backwards, over the sub- stance to be propelled, and then the circular fibres of the same portion contracting in succession from above downwards, or, as it were, from behind forwards, press on the substance into the portion next below, in which at once the same succession of action next ensues. These movements take place slowly and, in health, are commonly unperceived by the mind ; but they are perceptible when they are accelerated under the 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 of the contents of the large into the small in- testine by the ileo-crecal valve. Besides, — the orifice of communi- cation between the ileum and caecum (at the borders of which orifice are the folds of mucous membrane which form the valve) is encircled with muscular fibres, the contraction 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 propor- tion to the greater strength required for the onward moving of the faces, which are gradually becoming firmer. The greatest strength is in the rectum, at the termination of which the circular unstriped muscular fibres form a strong band called the internal sphincter ; while an external sphincter muscle with striped fibres is placed rather lower down, and more externally, and as we 360 DIGESTION. [chap, vnu 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 conditions of the mind 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 pneumo-gastric 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 ganglionic plexus before referred to. Influence of the Nervous System on Intestinal Digestion. — As in the case of the oesophagus and stomach, the peristaltic movements of the intestines are directly due to reflex action through the ganglia and nerve fibres distributed so abundantly in their walls (p. 315); the presence of chyme acting as the stimulus, and few or no movements occurring when the intes- tines 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 coeliac and other abdominal plexuses. The splanchnic nerves are in relation to the intestinal move- ments, inhibitory — these movements being retarded or stopped when the splanclmics are irritated. As the vasomotor nerves of the intestines, the splanclmics are also much concerned in intes- tinal digestion. chap.ix.] LYMPHATICS AND LACTEALS -f,i CHAPTEE IX. ABSORPTION. The process of Absorption has, for one of its objects, the intro- duction 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 both these offices, i.e., in both absorption from without and absorption from within, the process manifests some variety, and a very wide range of action ; and in both two sets of vessels are, or may be, concerned, namely, the Blood-vessels, and the Lymph-vessels or Lymphatics to which the term Absorbents has been also applied. The Lymphatic Vessels and Glands. Distribution. — The principal vessels of the lymphatic system are, in structure and general appearance, like very small and thin- walled veins, and like them are provided with valves. By one extremity they commence by fine microscopic branches, the lymphatic capillaries or lymph-capillaries, in the organs and tissues of the body, and by their other extremities they end directly or indirectly in two trunks which open into the large veins near the heart (fig. 206). Their contents, the lymph and chyle, 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 arc mingled with the stream of blood, and form part of its consti- tuents. Remembering the course of the fluid in the lymphatic vessels, viz., its passage in the direction only towards the large veins in the neighbourhood of the heart, it will readily be seen from fig. 206 that the greater part of the contents of the lymphatic system of vessels passes through a comparatively large trunk called the thoracic duct, which finally empties its contents into the blood-stream, at the junction of the internal jugular and sub- clavian veins of the left side. There is a smaller duct on the right side. The lymphatic vessels of the intestinal canal arc 362 ABSORPTION. [chap. IX. called lacteal*, because, during digestion, the fluid contained in them resembles milk in appearance; and the lymph in the lacteals Lymphatics of head and neck, right. Eight internal jugular vein. Right subclavian vein. Lymphatics of right arm. Eeceptaculum chyli. Lymphatics of lower extremities. sniH ■c : ^'^m v - ' ,; 'fil^ ^ iffifsHu v .>\M%0^— fe mJ9miWL^iwM^M R|\l 2K M/^ffl Bra P -V ^'SJYi'% tat sF/fJ; f — : |£ §U>' ! " C1 — : «r^ii{^BMm Lymphatics of head and neck, left. Thoracic duct. Left subclavian vein. Thoracic duct. Lacteals. Lymphatics of lower extremities. Fig. 206.— Diagram of the principal groups of lymphatic vessels (from Quain). during the period of digestion is called chyle. There is no essen- tial distinction, however, between lacteals and lymphatics. Iu some parts of their course all lymphatic vessels pass through certain bodies called lymphatic glands. Lymphatic vessels are distributed in nearly all parts of the body. Their existence, however, has not yet been determined in the placenta, the umbilical cord, the membranes of the ovum, or in any of the non-vascular parts, as the nails, cuticle, hair and the like. • HAP. IX. | ORIGIN OF LYMPH CAPILLAEIES. 03 Origin of Lymph Capillaries. -The lymphatic capillariet commence most commonly either in closely-meshed networks, or m irregular lacunar spaces between the various structur< which the different 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 Fig. 207.— Lymphatics of central tendon of rabbit's diaphragm, stained with silver nitrate The ground substance has been shaded diagrammatic-ally to bring out the lympha- tics clearly. I. Lymphatics lined by long narrow endothelial cells, and showing v valves at frequent intervals (Schofield) . ° ' 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 arc found to communicate freely with regular lymphatic vessels. In many eases, 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 endo- thelium. The lacteals offer an illustration of another mode of origin namely, in blind dilated extremities (figs. 192 and 193); but there is no essential difference in structure between these and the lymphatic capillaries of other parts. ;64 ABSORPTION, [CHAP. IX. Structure of Lymph Capillaries. — The structure of lym- phatic capillaries is very similar to that of blood-capillaries : their walls consist of a single layer of endothelial cells of an elongated Fig. 208. — Lymphatic v of the head and neck and the upper port of the trunk (Mascagm). &■ — 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 principal lymphatic vessels and glands are shown on the side of the head and face, 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 mediastinal glands, as well as the cardiac and the deep mammary lymphatics, is also shown. form and sinuous outline, which cohere along their edges to form a delicate membrane. They differ from blood capillaries mainly in their larger and very variable 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 or is strained through the walls of hap. ix.] STEUCTUEE OP LTMPfl CAPILLAEIES. 3^5 the blood-capillaries, so as to u rarrounding and occupies the interspaces which exist among their different : »/ the ha '. z - Two small glands at the bend, of the arm. 6. Radial lymphatic vessels. 7. Ulnar lymphatic Palmar arch of lymphatics. 9, 9'. Outer and inner - 1 t^phalic vein. ■'.. Eadial vein. e. Median vein. /'. Ulnar vein. The lymp':. are represented as lying on the deep : b Mascagni.) Fig. 210. — Sup€ ncial lymphatics of right groin and upper part of thigh, ±. I. Upper inguinal glands. 2. 2'. Lower inguin-.il or femoral glands. 5, 3'. Plexus of lymphatics in the :rse of the long saphenous vein. Mu ..mi.) elements. These same interspaces have been shown, as just stated, i-m the beginnings of the lymph-capillaries ; and the latter, therefore, are the means of collecting the exuded blood plasma, -66 ABSORPTION. [chap. ix. and returning that part which is not directly absorbed by the tissues into the blood-stream. For many years, the notion of the existence of any such channels between the blood - vessels and lymph-vessels as would admit blood-corpuscles, has been given up ; observations having proved that, for the passage of such corpuscles, it is not necessary to assume the presence of any special channels at all, inasmuch as blood-corpuscles can pass bodily, without much difficulty, through the walls of the blood-capillaries and small veins (p. 199), and could pass with still less trouble, probably, through the comparatively ill-defined walls of the capillaries which contain lymph. It is worthy of note that, in many animals, both arteries and veins, espe- cially the latter, are often found to be more or less completely ensheathed in large lymphatic channels. In turtles, crocodiles, and many other animals, the abdominal aorta is enclosed in a large lymphatic vessel. Stomata. — In certain parts of the body openings exist by which lymphatic capillaries directly communicate with parts hitherto sup- posed to be closed cavities. If the peritoneal cavity be injected with milk, an injection is obtained of the plexus of lymphatic vessels of the central tendon of the diaphragm (fig. 207) ; and on remov- ing a small portion of the central tendon, with its peritoneal surface uninjured, and examining the process of absorption under the microscope, the milk-globules run towards small natural openings or stomata between the epithelial cells, and disappear by passing vortex-like through them. The stomata, which have a roundish outline, are only wide enough to admit two or three milk- globules abreast, and never exceed the size of an epithelial cell. Pseudostomata. — When absorption into the lymphatic system takes place in membranes covered by epithelium or endothelium through the interstitial or intercellular cement-substauce, it is said to take place through pseudostomata. Demonstration of Lyvqrfiatic* if Diaphragm. — The stomata on the peri- toneal surface of the diaphragm are the openings of -hort vertical canals which lead up into the lymphatics, and are lined by cells like those of germinating endothelium (p. 27). By introducing a solution of Berlin blue into the peritoneal cavity of an animal shortly after death, and sus- pending it. head downwards, an injection of the lymphatic vessels of the diaphragm, through the stomata on its peritoneal surface, may readily be obtained, if artificial respiration be carried on for about half an hour. In this wav it has been found that in the rabbit the Lymphatics are arranged LP. ix. | STOMATA AND PSEUDOSTOMATA. 367 between the tendon bundles of the centrum tendineum ; and they are hence termed interfascicular. The centrum tendineum Is coated by endo- thelium on its pleural and peritoneal surfaces, and its substance consif Fig. 211. — Peritoneal surface of septum eisterna lymphatica magna of frog. The stomata, some of which are open, some collapsed, are surrounded by germinating endothelium. X 160. (Klein. 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 can be little doubt that during life the flow of lymph along these channels is chiefly caused by the action of the diaphragm during respiration. As it descends in inspiration, the spaces between the radiating tendon bundles dilate, and lymph is sucked from the peritoneal cavit}-. 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 lymphatic- towards the pleural surface. (Klein.) It thus appears probable that during health there is a continued sucking in of lymph from the perito- neum 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 impedi- ment to absorption, an accumulation of fluid necessarily takes place (ascites). Stomata have been found in the pleura; and as they may be uned to exist in other serous membranes, it would seem as if the serous cavities, hitherto supposed closed, form but a large lymph-sinus or widening out, so to speak, of the lymph-capillary Bystem with which they directly communicate. ^65 ABSORPTION. ['hap. ix. Structure of Lymphatic Vessels. — The larger vessels are very like veins, having an external coat of fibre-cellular tissue, with elastic filaments ; within this, a thin layer of fibre-cellular tissue, with plain muscular fibres, which have, principally, a cir- cular direction, and are much more abundant in the small than in the larger vessels ; and again, within this, an inner elastic layer of longitudinal fibres, and a lining of epithelium ; and numerous valves. The valves, constructed like those of veins, and with the free edges turned towards the heart, are usually arranged in pairs, and, in the small vessels, are so closely placed, that when the ressels are full, the valves constricting them where their edges are attached, give them a peculiar beaded or knotted ap- pearance. Current of the Lymph. — With the help of the valvular mechanism (i) all occasional pressure on the exterior of the lym- phatic and lacteal vessels propels the lymph towards the heart : thus muscular and other external pressure accelerates the flow of the lymph as it does that of the blood in the veins. The actions of (2) the muscular fibres of the small intestine, and probably the laver of organic muscle present in each intestinal villus, seem to ssist in propelling the chyle : for, in the small intestine of a mouse, the chyle has been seen moving with intermittent propul- sions 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 vis a forgo 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 vessel's own walls. The respiratory movements, also, (5) favour the current of lymph through the thoracic duct as they do the current of blood in the thoracic veins (P- 253)- Lymphatic Glands are small round or oval compact bodies varying in size from a hempseed to a bean, interposed in the course of the lymphatic 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 mmibers in the mesen- tery, and along the great vessels of the abdomen, thorax, anc\ neck ; in the axilla and groin ; a few in the popliteal space, but not further down the lesr, and in the arm as far as the elbow. «ii.\p. IX.] LYMPHATIC GLANDS. 369 Some lymphatics do not, however, pass through glands before entering the thoracic duet. 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 con- cave (hilus) ( fig. 2 1 2, a), the capsule scuds processes inwards in which Fig. 212. — Section of a mesenteric gland from the ox, slightly magnified, a, Hilus; b (in the central part of the figure), medullary substance ; <•, cortical substance with indis- tinct alveoli ; d, capsule (Kulliker). the blood vessels are contained, and these join with other processes called trabecules (fig, 215, t.r.) 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 section, even when examined with the naked eye, to be made up of two parts, an outer or cortical (fig. 212, c } c), which is light coloured, and an inner of redder appearance, the medullary portion (fig. 212). In the outer or cortical part of the gland (fig. 215, c) the intervals between the trabecular are comparatively large and more or less triangular, the intercommunicating spaces being termed alveoli ; whilst in the more central or medullary part a finer meshwork is formed by the more free anastomosis of the trabecular processes. In the alveoli of the cortex and in the meshwork formed by the trabecular in the medulla, is contained the proper gland structure. In the former it is arranged as follows (fig. 215) : occupying the central and chief part of each alveolus, is a more or less wedge-shaped mass (l.h.) of adenoid tissue, densely packed with lymph corpuscles ; but at the periphery surrounding the central portion and immediately next the capsule and trabe cular, is a more open meshwork of adenoid tissue' constituting the b 1; 37o ABSORPTION. [CHAP. IX. lymph sinus or channel (l.s.), and containing fewer lymph corpuscles; The central mass is enclosed in endothelium, the cells of which J ? v. < . Fig. 213. — Front a vertical section through the capsule, cortical sinus and peripheral portion of follicle of a human compound lymphatic gland. The section had been shaken, so as to get rid of most of the lymph corpuscles. A. Outer stratum of capsule, consisting of bundles of fibrous tissue cut at various angles. B. Inner stratum, showing fibres of connective tissue with nuclei of flattened connective-tissue corpuscles. Beneath this (between B and C) is the lymph-sinus or lymph-path, containing a reticulum coated by flat nucleated endothelial cells. C. Fine nucleated endothelial membrane, marking boundary of the lymph-follicle. The rest of the section from C to E is the adenoid tissue of the lymph-follicle, which consists of a fine reticulum, E, with numerous lymph corpusles, D. They are so closely packed that the adenoid reticulum is invisible till the section has been shaken so as to dislodge a number of the lymph-corpuscles x 350 (Klein and Noble Smith) . join by tHeir processes, the processes of the adenoid framework of the lymph sinus. The trabecule are also covered with endothe- lium. The lining of the central mass does not prevent the passage of fluids and even of corpuscles into the lymph sinus. The framework 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 bifurcates (fig. 215) or divides into two or more smaller rounded or cord-like masses and here joining with those from the other alveoli, form a much closer arrangement of the gland tissue (fig. 214, a) than in the cortex; spaces (fig. 214, b), are left within those anastomosing cords, in which are found portions of the trabecular meshwork and the continuation of the lymph sinus (6, c). CHAP. ix. J LYMPHATIC - ) Fatty matter a trace 3'6oi '92 Salts -585 711 -44 chap. EC] ABSOBPTION l:V LACTEAIA 37c From the above analyses of lymph and chyle, it app that they contain essentially the Banie constituents thai are found in the blood. Their composition, indeed, differs from thai of the blood in degree rather than in kind. They do not, how- by accident, contahi coloured corpuscles, entity. — The quantity which would pass into a cat's blood in twenty-four hours has been estimated to be equal to about one-sixth ofthe weight of the whole body. And, since the estimated weight of the blood in rats is to the weight of their bodies as 17, 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 scries of experiments, the quantity of lymph traversing the thoracic duct of a dog in twenty- four hours was found t<> be about equal t<> two-thirds of the blood in the body. (Bidder and Schmidt) Absorption by the Lacteals. — During the passage of the chyme along the whole tract of the intestinal canal, its com- pletely digested parts are absorbed by the blood-vessels and lac- teals distributed in the mucous membrane. The blood-vessels appear to absorb chiefly the dissolved portions of the food, and these, including especially the albuminous and saccharine, they imbibe without choice : whatever can mix with the blood passes into the vessels, as will be presently described. But the lacteals appear to absorb only certain constituents of the food, including particularly the fatty portions. The absorption by both sets of vessels is carried on most actively but not exclusively, in the villi of the small intestine ; for in these minute processes, both the capillary blood-vessels and the lacteals are brought almost into contact with the intestinal 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 (Watney), and especially those which clothe the surface of the villi. Thence, the fatty particles are passed on into the interior of the lacteal vessels (fig. 216, a), but how they pass, and what laws govern their so doing, are not at present 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 376 ABSORPTION, [chap. ix. of the bile, and the pancreatic and intestinal secretions, which moisten the absorbing surface. For it has been found by experi- ment, that the passage- of oil through an animal membrane is made much easier when the latter is impregnated with an alkaline fluid. Absorption by the 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 lymphatic capillaries. The lymph is, without doubt, identical in great part, with the liquor sanguinis, which, as before remarked, 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 the 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 correspond 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 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 afterwards found in the lymphatics. Some other matters than those originally contained in the exuded 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 re-organised and adapted again for nutrition ; and these may be absorbed by the lymphatics, and elaborated with the other contents of the lymph in passing through the glands. Lymph-Hearts. — In reptiles and some birds, an important auxiliary to the movement of the lymph and chyle is supplied in certain muscular sacs, named lymph-hearts (fig. 217), and it has been shown that the 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 anterior and two posterior ; in the frog, the posterior lymph-heart on each side is situated in the ischiatic re- gion, just beneath the skin ; the anterior lies deeper, just over the transverse CHAP. IX. J ABSORPTION BY BLOODS : : the third vesxi , the orifices of tl tg guard t the mph. Pi conveys t he lymph directly into the :i. In the f: ferior lymphatic heart, on each n lymph into a branch of the ischiatic by the - ■ the lymph is forced into a branch of the jugular vein, which isfi rior surface, and which becomes turgid time that the sac contra its, Bl d - ted from passing from th he lymphatic heart by a valve atite muscular coat of these hearts is of variable thick some cases it can only I - ered by means of the microscope ; but in every case it is cunij jtripedfil _ contractions of the hearts are rhythmical, Fig. 217. — Lgmj - Python bi> The external cellular coat. 5. The thick muscular coat. Four muscular columns run across its cavity, which communicates with three lymphatics 1 — only one ^nd with two veins 2. 2 . 6. The smooth lining membrane of the 1 - A small appendage, or auricle, the cavity of which is continuous with that of the rest of the organ (after B. Weber . occurring about sixty times in a minute, slowly, and. in comparison with 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 Yolkniann 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 desl while destruction of this portion, though all other pans of the nervous centres were uninjured, instantly •-•d the heart's movements. The posterior, or ischiatic, pair of lymph- hearts were found to be governed, in like manner, by the portion of spinal cord 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. Absorption by Blood-vessels. — In the absorption by the 3/8 ABSORPTION. [CHAP. IX. lymphatic or lacteal vessels just described, there appears some- thing 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 he 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 absorp- tion are determined solely by their physical or chemical properties and conditions, and by those of the blood and the walls of the blood-vessels. Osmosis. — The phenomena are, indeed, to a great extent, com- parable to that passage of fluids through membrane, which occurs quite independently of vital conditio >ns, 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. 218). If now the bell be filled with a solution of a salt — say sodium chloride, 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 important influence, as might be anticipated, on the rapidity and duration of the osmotic current. Thus, if a piece of ordinary bladder be used as the septum between water and alcohol, the current is almost solely from the water to the alcohol, on account of the much greater afnnitv 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. Fig. 218.— End osmometer. < hat. ix.] 0SM08IS. 3-9 Osmosis by Blood-vessels. — Absorption by blood - vessels is the consequence of Iheir walls being, like tin- membranous I'tiini <>t* the endosmometer, porous and capable of imbibing iiuMs, ami of the blood being bo composed that most fluids will mingle with it. The process of absorption, in an instructive, though very imperfect degree, may l>c observed in any portion of vascular tissue removed from the body, [f 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 lias 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 colouring matter of the blood (together with which chemistry would detect some of the albumen and other parts of the liquor sanguinis) will spread more widely every day. The same will happen if the piece of tissue be placed in a saline solution instead of water, or in a solution of colouring or odorous matter, either of which will give their tinge or smell to the blood, and receive, in exehange, the colour of the blood. Colloids and Crystalloids. — Various substances have been classified according to the degree in which they possess the pro- perty 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 those which pass with difficulty, on account of their, physically, glue- like characters, colloids. (Graham.) 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 characterised 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 characterised by qualities the reverse of those just mentioned as belonging to colloids. Alcohol, sugar, and ordinary saline substances are exanrples of crystalloids. Rapidity of Absorption. — The rapidity with which matters 3 So ABSORFTIOX. [chap. ix. may be absorbed from the stomach, probably by the blood-vessels chiefly, and diffused through the textures of the body, may be gathered from the history of some experiments. From these it appears that even in a quarter of an hour after being given on an empty stomach, 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 humour of the eye. 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 1 o minutes ; or, if the stomach be full at the time of taking the dose, in 20 minutes. It may sometimes be detected in the urine, moreover, for six, seven, or eight days. (Bence Jones.) Some experiments on the absorption of various mineral and vegetable poisons, have brought to light the singular fact, that, in some cases, absorption takes place more rapidly from the rectum than from the stomach. Strychnia, for example, when in solution, produces its poisonous effects much more speedily when introduced into the rectum than into the stomach. When introduced in the solid form, however, it is absorbed more rapidly from the stomach than from the rectum, doubtless because of the greater solvent property of the secretion of the former than of that of the latter. (Savory.) 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 of 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 ab- sorbed must, as a general rule, be in the liquid or gaseous state, or, if a solid, must be soluble in the fluids with which it is brought chap, rx.] CONDITIONS FOB AB80RPTTON. 3S1 utact. II- :. the marks of * _. and the discoloration produced by diver nitrate taken internally, remain. Mercury may rbed even in the metal] ; and in tli may into and remain in the blood from them; and such subetai edingly finely-divided char when taken into the alimentary canal, have been found in the nteric veins; the insoluble materials of ointments may also ibbed into the blood-vest U ; but there are no facte to d mine how these various substances effect their passage. Oil, minutely divided, as in an emulsion, will p ly into blood- ve— Is, - it will through a filter moistened with water: and. without doubt, fatty matters find their way into the blood-w— Lb - well as the lymph -v..— Lb f the intestinal canal, although the latter seem to be specially intended for their ■ 2. The less dense the fluid to be - sd, the more speedy, - eral rule, is its absorption by the living blood-vessels. Hence the rapid absorption <:>f water from the stomach : also of weak saline solutions: but with strong solutions, there appears less rption into, than effusion from, the blood- • 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 injc I into a dog's veins to repletion, poison is absorbed very slowly; but when the tension of the - Is is diminished by bleeding, the poison acts quickly. So, when cupping-g] - 3 laced over a poisoned wound, they retard the absorption of the poison not only by diminishing the velocity of the circulation in the part, but by filling all its vessels too full to admit more. On the same ground, absorption is the quicker the more rapid the circulation of the blood : n< »t because the fluid to be absorbed is more quickly imbibed into the tissue-, or mingled with the blood, but because as East as it enters the blood, it is earned away from the part, and the blood being constantly renewed, is con- stantly as fit as at the first for the reception of the substance to be absorbed. o 52 AJTEMAL HEAT. [chap. x. CHAPTEE X. 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 arm-pit — the most convenient situation, under ordinary circumstances, for examination by the thermometer — the average temperature is 98-6° F. (36*9° C). 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 maintaining uniformity of temperature in different parts. Circumstances causing Variations in Temperature.— The chief circumstances by which the temperature of the healthy body is influenced are the following : — Age ; Sex : Period of the day ; Exercise ; Climate and Season ; Food and Drink. Age. — The average temperature of the new-born child is only about i c F. (*54 c C.) above that proper to 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 b} T about the same amount from puberty to fifty or sixty years of age. In old age the tem- perature again rises, and approaches that of infancy ; but although this is the case, yet the power of resisting cold is less in them — ciim'. x.| VABIATI0N8 IN TJSMPEBATTTRE. 383 exposure to a low temperature causing a greater reduction of heat than in young persons. The same rapid diminution of temperature has been observed to occur in the new-born young of most carnivorous and rodent animals when th< removed from the parent, the temperature of the atmosphere beii 5o J and 53-5' P. (io°-i2° ('.) ; whereas while Lying close to the b the. mother, their temperature is only 2 or 3 degrees I', lower than b same law applies to the young of birds. . — The average temperature of the female would appear to be very slightly higher than that of the male. Period of the Day, — The temperature undergoes a gradual alteration, to the extent of about i° to 1-5° F. (-54 — -8° C.) in the course of the day and night ; the minimum being at night or in the early morning, the maaimum late in the afternoon. Exercise. — Active exercise raises the temperature of the body fn»m i° to 2° F. (-54° — ro8°C). This may be partly ascribed to generally increased combustion-processes, and partly to the fact, that every muscular contraction is attended by the development of one or two degrees of heat in the acting muscle ; and that the heat is increased according to the number and rapidity of these contractions, and is quickly diffused by the blood circulating from the heated muscles. Possibly, also, some heat may lie generated in the various movements, stretchings, and recoilings <»f the other tissues, as the arteries, whose elastic walls, alternately dilated and contracted, may give out some heat, just as caoutchouc alternately stretched and recoiling becomes hot. But the heat thus developed cannot be great. The great apparent increase of heat during exercise depends, in a great measure, on the increased circulation and quantity of blood, and, therefore, greater heat, in parts of the body (as the skin, and especially the skin of the extremities), which, at the same time that they feel more acutely than others any changes of temperature, are, under ordinary conditions, by some degrees colder than organs more centrally situated. Climate cend Season. — The temperature of the human body is the same in temperate and tropical climates. (Johnson, Boileau, Purnell.) 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. (Wunderlich.) Food and Drink. — The effect of a meal upon the temperature 384 ANIMAL HEAT. [chap. x. of a body is but small. A very Blight rise usually occurs. Cold alcoholic drinks depress the temperature somewhat ("5° to i° F.). AVarm alcoholic 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 standard to a greater extent than would be anticipated from the Blight effect of external conditions during health. Thus, in some diseases, as pneumonia and typhus, it occasionally rises as high us 106" or ic; : F. (41 — 4i'6° < '.". i : and considerably higher tempe- ratures 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, average- ioi° (38.3° C.) The extremes recorded by them were 96' and 106°. the former in the nar- whal, the latter in a bat (Vespertilio pipistrella). In Birds, the average is as high as I07 D (41 "2" C. : the highest temperature, 111*25° (4-6' 2 ' C.) ; being in the small species, the linnets. &c. Among Eeptiles. while the medium they were in was 75 : (23-9'' C.) their average temperature, was 82*5° (3 1 '2° C). As a general rule, their temperature, though it falls with that of the surrounding medium, is. in temperate media, two or more degrees higher ; and though it rises also with that of the medium, yet at very high degrees it ceases to do so. and remains even lower than that of the medium. Fish and invertebrata present, as a general rule, the same temperature as the medium in which they live, whether that be high or low ; only among fish, the tunny tribe, with strong hearts and red meat-like muscles, and more blood than the average of fish have, are generally 7" (3*8' C.) warmer than the water around them. The difference, therefore, between what are commonly called the warm and the cold-blooded animals, is not one of absolutely higher or lower tem- perature ; for the animals which to us in a temperate climate, feel cold (being like the air or water, colder than the surface of our bodies), would in an external temperature of ioo° (37*8° C.) have nearly the same tempera- ture 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 dis- turbance or die. Sources and Mode of Production of Heat in the Body. — The heat which is produced in the body arises from com- bustion, and is due to the fact that the oxygen of the atmosphere thai'. x.| PBODUCTIOH OP HEAT. taken into th< is comb ith the carbon and h j Any changes which occur in the protoplasm of tl suiting in an exhibition of their function, is attended by tin- evolution of heat and also by the production "f carbonic tu and water ; and the more active the changes, the greater the b at produced and the greater the amount of the carbonic acid and water formed. But in order that the protoplasm may perform its func- tion, the waste of its own- » structive metabolism), must be repaired by the supply of food material, and therefore for the produc- i of heat it is necessary to supply food. In the tissues, therefore, tw«> pr lessee 1 continually going on: the building up of the protoplasm from the t nstructive metabolism), which is not accompanied by the evolution of heat but possibly by the ?• - , and the oxidation of the protoplastic materials, resulting in the productioi rgy, by which heat is produced and carbonic acid and water are evolved. Some heat will also he 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 pro- se 3, which ultimately issue in the production of carbonic acid and water, are as simple as the bare statement «»f the fact might seem to indicate. But complicated as the vari< 3 stages of combustion may be, the ultimate result is as simple as in ordinary combustion outside the body, and the products .. the same. .' - ime amount of heat will be evolved in the union of any friven 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 chair.'- which occur in the living body. And *ince the heat thus arising will be distributed wherever the blood is earned, every j of the body will be heated equally, or nearly a This the ry. that the maintenance of the temperature of the living body depends on continual chemical change, chiefly by oxi.lnri-.il, of combustible materials existing in the tissu long stablished by the demonstration that the quantity of and hydrogen which, in a given time, unites in the bt wit _ sufficient : ant for the amount of heat lerated in the animal within the same time : an amount capable maintaining the temperature of the body at from 98° — ico c F. c c 386 ANIMAL HEAT. [< hap. x. (36-S° — 37*S : C), notwithstanding a large loss by radiation 1 and evaporation. It should be remembered that 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 accom- panied 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 : — (i), 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 demonstrated 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 that even neglecting the heat pr< duced by the quiet metabolism of muscular tissue, the amount of heat generated by muscular activity supplies the principal part of the t"tal heat produced within the body. (2), In the Secreting glands, and principally in the liver as being the larg si and most active. It lias been found by experiment that the blood leaving the glands is & nsiderably warmer than that entering them. The metabolism in the glands is very active and, as we have seen, the more active the metabolism the greater the heat produced. 131. In tlve Brain; the venous blood having a higher temperature than the arterial. It must lie remembered, 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 t.> 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 inconsideral ile. Regulation of trie 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) variation in the amount of heat got rid of. and (2) variations in the amount of heat produced or introduced chap, x.] LOSS OF BEAT, 387 into the body. In healthy warm-blooded animals the loss and g in of heat are so nearly balanced one by the other that, under all ordinary circui " 5, an uniform temperature, within * or three _ s, is preserve 1. I. Methods of Variation in the amount of Heat got rid of. — The loss of heat from the human body is principally regu- lated by the amount lost by radiation and conduction fi surface, and by means of the constant evaporation of water from the same part, and (2) to a much less -_ from the air- 38 _ - : iii each act I spiration, heat is lost to a greater or stent according to the temperature of the atmosphei 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 mine and feces which leave the body at about its own temperature are also means by which a small amount is I st // U from +'■■ > vftfo Body: (lie Skin. — By far the nmst important loss of heat from the body, — probably 70 or So per cent, of the whole amount, is that which takes place by liation, conduction, and evaporation from the skin. The means by which the skin is able to act as one of the m< st important organs for regulating the temperature of the blood, are — 1 1 . that it offers a large surface for radiation, conduction, and evaporation : (2), that it contains a larg m int i blood; . that the quantity of blood contained in it is the greater under those circumstances which demand a loss of heat from the body, and For the circumstance which directly termines the quantity of blood in the skin, is that which _ rerns the supply of blood to all the tissues and organs of the body, namely, the power of the vaso-motor nerves to cause a ss tension of the muscular element in the walls of the arteries, and. in correspondence with this, a lessening increase 1 >f the calibre of the vessel, accompanied by a less or greater current of blood. A warm or hot atmosphere- its on the nerve fibres of the skin, as to lead them to cause in turn a relaxation of the muscular fibre of the blo< - ssels; 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. c c 2 7 g8 ANIMAL HEAT. [chap. x. the blood-vessels shrink, and in accordance 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 vasomotor centre in the medulla and spinal cord. Thus, by means of a Belf-regulating apparatus, the skin becomes the most important of the means by which the temperature of the body is regulated. In connection with loss of heat by the skin, reference has been made to that which occurs both by radiation and conduction, and by evaporation: and the subject of animal heat lias been considered almost solely with regard to the ordinary case of man living in a medium older than his body, and therefore losing- heat in all the ways mentioned. The importance of the means however, adopted, so to speak, by the skin for regulating the temperature of the body, will depend on the conditions by which it is surrounded; an inverse proportion existing in most cases between the loss by radiation and conduction on the one hand, and by evaporation on the other. Indeed, the small loss of heat bv evaporation in cold climates may go far to compensate for the irreater loss bv radiation: as, on the other hand, the great amount of fluid evaporated in hot air may remove nearly as much heat as is commonly lost by both radiation and evaporation in ordinary temperatures; and thus, it is possible that the quantities of heat required for the maintenance of an uniform proper tempera- ture in various climates and seasons are not so different as they, at first thought, seem. Many example may be given of the power which the body possesses of resisting tie effects af a l>> far as is known, iu 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 lest, are altogether subordinate, in the power of regulating the temperature, to the skin. By Clothing. — The influence of external coverings for the body must not be unnoticed. In warm-blooded animals, they are 3 ro ANIMAL HEAT. [chap. x. always adapted, among other purposes, to the maintenance of uniform temperature ; 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 Pro- duced. — It may seem to have been assumed, in the foreg< dug 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 regu- lating 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 com- bustion : 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 climates, such as the several fatty and oily substances, abounds in carbon and hydrogen, and is fitted to combine with the large quantities of oxygen which, breathing cold dense air, they absorb from their lungs. (b.) By Exercise. — In exercise, we have an important means of raising the temperature of our bodies (p. 383). (c.) By Influence of the Kercous System. — The influence of the nervous svstem in modifying the production of heat must be very important, as upon nervous influence depends the amount of the metabolism of the tissues. The experiments and observations which best illustrate it are those showing, first, that when the supply of nervous influence to a part is cut off, the temperature of that part falls below its ordinary degree ; and, secondly, that < hap. x.l REGULATION OF TEMPERATURE. 391 when death is caused bj severe injury to, or removal of, the nervous centres, the temperature <>f the body rapidh falls, even though artificial respiration be performed, the circulation main- tained, and to all appearance the ordinary chemical changes of the body be completely effected. It has been repeatedly noticed, that after division of the nerves of a limb its tempe- rature lulls ; and this diminution of beat 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 perspiration, which is excited by passions of the mind ; in the sudden rush of heat to the face, which is not a mere sensation; and in the equally rapid diminution of temperature in the depressing passions. But none of these instances suffice to prove that heat is generated by mere nervous action, independent of any chemical change; all are explicable, on the supposition that the nervous system alters, by its power of controlling the calibre of the blood-vessels, 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. Inhibitory heat-centre. — Whether a centre exists which regulates the production of heat in warm-blooded animals, is still unde- cided. Experiments 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 temperature, and warm- blooded animals then re-act to variations of the external tempe- rature just in the same way as cold-blooded. These experiments seem to suggest that there is a centre, to which, under normal circumstances, the impression of cold is conveyed, and from which 392 ANIMAL HEAT. [chap. X. 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 urarised animals, as the nerves to the muscles, the metabolism of which is so important 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 altera- tion 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 circum- stances too powerful, either in raising or lowering the heat of the body, to be controlled by the proper regulating apparatus. Walther found that rabbits and dogs, when tied to a board and exposed to a hot sun. reached a temperature of 114*8° F., and then died. Cases of sunstroke furnish us with 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 inconsistent with the continuance of life. The effect <»f mere loss of bodih- temperature in man is less well known than the effect of heat. From experiments by Walther, it appears that rabbits can lie cooled down to 48° F. (8"o/ C), before they die, if artificial respiration be kept up. Cooled down to 64 F. ( 1 7'S" C), they cannot recover unless external warmth be applied together with the employment of artificial respiration. Babbits net cooled below 77° F. (25 C.) recover by external warmth alone. < hap. xi. 1 SECRETION. 303 rilAlTEK XI. SECRETION. Secretion is the process by which materials arc separated from the blood, and from the organs in which they are formed, for the purpose either <>f serving Borne ulterior office in the economy, or of being discharged from the body as useless injurious. In the former case, the separated materials are termed •etions ; in the latter, they arc termed excretions. Most of the secretions consist of Bubstances which, probably, do DOt pre-exist in the same form in the blood, but require special organs and a process of elaboration for their formation, e.g., the liver for the formation of bile, the mammary gland for the forma- tion of milk. The excretions, on the other hand, commonly or chiefly consist of substances 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 orpins by which any of them is elaborated, it is no longer formed. - sometimes occur in which the secretion continues to be formed by the natural organ, but not being able to escape to- wards 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 arc not instances of true vicarious secre- tion, 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 excretions is as complex as that of the parts con- cerned in the formation of secretions. And since the differences of the two proa 8S< - of separation, corresponding with those in the several purposes and destinations of the fluids, are not yet ascer- 394 SECRETION. [CHAP. XI. tained, 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 basement-membrane ; certain cells ; and blood-vessels. These three structural elements are arranged together in various wavs: but all the varieties may be classed under one or other of two principal divisions, namely, membranes and glands. Organs and Tissues of Secretion. The principal secreting membranes are (i) the Serous and Synovial membranes; (2) the Mucous membranes ; (3) the Mam- mary gland ; (4) the Lachrymal gland ; and (5) the Skin. (1) Serous Membranes. — The serous membranes are espe- cially- distinguished by the characters of the endothelium covering their free surface : it always consists of a single layer of polygonal cells. The ground sub- stance of most serous membranes consists of connective - tissue cor- puscles of various forms lying in the branching spaces which constitute the " lymph canalicular system" (p. 363), and interwoven with bundles of white fibrous tissue, and numerous delicate elastic fibrillse, together with blood- vessels, nerves, and lymphatics. In relation to the pro- cess of secretion, the layer of connective 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 instances, as in the arachnoid covering the dura mater, Fig. 210. — Section of synovial membrane, a, endothelial covering of elevations of the membrane : b, sub- serous tissue containing fat and blood-vessels ; <■, Ligament covered bv the synovial membrane. (Cadiat.; chap. xi. | SEBOUS FLUID. 395 and in the Interior of the ventricles of the brain. The primary membrane and epithelium are always present, an 1 are concerned in the formation of the fluid by which tin- free surface of the membrane is moistened. Scnms membranes are of two principal kinds ; [*/. 'rim-,' wind, line visceral cavities, —-the arachnoid, pericardium, pleural, perito- neum 9 and tunica vaginaies. 2nd. The synovial membranes lining tin- joints, and the sheaths of tendons and ligaments, with which, also, arc usually included the. synovial bursi e, or bursasmuco9ce y whetherthese be subcutaneous, or situated beneath tendons that glide over bones. The serous membranes form closed sacs, and exist wherever the tree 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 membranes is to furnish a smooth, moist surface, to facilitate the movements of the invested organ, and to prevent the injurious effects of friction. This purpose is especially manifested in joints, in which free and extensive movements take place ; and in the stomach and intestines, which, from the varying quantity and movements of their contents, are in almost constant motion upon one another and the walls of the abdomen. Serous Fluid. — The fluid secreted from the free surface of the serous membranes is, in health, rarely more than sufficient to ensure 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 manifest disease, is probably such as has transuded after death, or in the last hours of life. An excess of such fluid in any of the serous sacs constitutes dropsy of the sac. The fluid naturally secreted by the serous membranes appears to be identical, in general and chemical characters, with the serum of the blood, or with very dilute liquor sanguinis. It is 396 ' SECRETION. [chap. xi. of a pale yellow or straw-colour, slightly viscid, alkaline, mid, on account of the presence of albumen, eoagulable 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 transu- dation, 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, coloured with the bile. But there is reason for supposing that the fluid of the cerebral ventricles and of the arachnoid sac are exceptions to this rule : for they differ from the fluids of the other serous sacs not only in being pellucid, colourless, and of much less specific gravity, but in that they seldom receive the tinge of bile when present in the blood, and are not coloured by madder, or other similar substances introduced abundantly into the blood. Synovial Fluid : Synovia. — It is also probable that the formation of synovial fluid is a process of more genuine and elabo- rate secretion, by means of the epithelial cells on the surface of the membrane, and especially of those which are accumulated on the edges and processes of the synovial fringes; for, in its peculiar density, viscidity, and abundance of albumin, 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 as in the tympanum, to periosteum ; in the intestinal canal, it is connected with a firm submucous mem- brane, which on its exterior gives attachment to the fibres of the muscular coat. The mucous membranes line certain prin- cipal tracts — Gastro-Pulmonaiy and Genito-Urinary ; the former being subdivided into the Digestive and Respiratory 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, i ii u\ si.] MUCOUS MEMBB V.NES 307 and oesophagus, to the stomach, and is thenco 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 Eusta- chian 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 tin 1 glottis, whence it is continued, through the larynx and trachea, t<> the bronchi and their divisions, which it lines as far as the branches of about gL of an inch in diameter, and continuous with it is a layer of delicate epithelial membrane which extends into the pulmonary cells. 3. The Genito-urinary tract, which lines the whole of the urinary passages, from their external orifice to the termination of the fcubuli 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 abdomen at the fimbriae of the Fallopian tubes. Structure. — Along each of the above tracts, and in different portions of each of them, the mucous membrane presents certain structural peculiarities adapted to the functions which each part has to discharge; yet in some essential characters mucous mem- brane 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 basement membrane, and beneath this, a stratum of vascular tissue of vari- able thickness, containing lymphatic vessels and nerves which in different cases presents either out-growths in the form of papillae and villi, or depressions or involutions in the form of glands. But in the prolongations of the tracts, where they pass into gland- ducts, these constituents are reduced in the finest branches of the ducts to the epithelium, the primary or basement-membrane, and the capillary blood-vessels spread over the outer surface of the latter in a single layer. 398 SECRETION. [chap. xr. The primary or basement-membrane is a thin transparent layer, simple, 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 mem- branes, 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 between the epithelium- cells of mucous membranes and gland-cells. It thus appears, that the tissues essential to the production of a secretion are, in their simplest form, a membrane, having on one surface blood-vessels, and on the other a layer of cells, which may be called either epithelium-cells or gland-cells. Mucous Fluid : Mucus. — From all mucous membranes there is secreted either from the surface or from certain special glands, or from both, a more or less viscid, greyish, or semi-transparent fluid, of alkaline reaction and high specific gravity, named mucus. It mixes imperfectly with water, but, rapidly absorbing liquid, it swells considerably when water is added. Under the microscope it is found to contain epithelium 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 Glands.— The structure of the elementary portions of a secreting apparatus, namely epithelium, simple membrane, and blood-vessels having been alread}' described in this and previous chapters, we may proceed to consider the manner in which they are arranged to form the varieties of secretin;/ 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 general plan of structure, by which they arc distinguished from all other textures of the body ; espe- cially, 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 elaborate, as secreting cells, the substances to be discharged from them. Glands arc provided also with lymphatic vessels and nerves. The distri- chap. >.;. I SECRETING GLAXDS. 399 bution of the former is not peculiar, and need not be here con- sidered. Nerve-fibres are distributed both to the blood-vessels of the gland and to its ducts ; and, in some glands, to the secreting cells also (p. 282). Varieties. — 1. The simple tubule, or tubular gland (a, fig. 220), examples of which are furnished by some mucous glands, the follicles of Lieberkiihn (fig. 186), 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 mem- brane, and is lined with secreting cells arranged as an epithelium. To the same class may be referred the elongated and tortuous sudoriferous glands. The compound tubular glands (i>, fig. 220) form another division. These consist of main gland-tubes, which divide and sub-divide. Each gland may consist of the subdivisions of one or more main tubes. The ultimate sub-divisions 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. -. The aggregate or racemose glands, in which a number of vesicles or acini are arranged in groups or lobules (0, fig. 220). The Meibomian follicles arc 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 con- nection 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 t<> form one cavity lined or tilled with secreting cells which also occupy recesses from the main vity. 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, whatever ondary arrangement there may be, nil have the same essential character of rounded groups of vesicles containing gland-cells, and opening by a common central cavity into minute ducts, which 400 SECRETION, [CHAP. XI. 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. 1 a^j SSSfP^ fig. 220. — Plans of ' exU ecre&ng membrane by inversion or formoft A. -imple gland*, viz. .<-/, straight tub*- : h, sac ; ; , coiled tube. B, multilocular er. i-, of tubular form : Jar. I . i i saccular compound gland; rn, entire eland, showing branched duct and lobular structure ; «. a lobule, detached with 0, branch of duct proceeding from it. L>, compound tubular gland (Sharpey . Among these varieties of structure, all the secreting glands arc- alike in some essential points, besides those which they have in common -with all truly secreting structures. They agree in presenting a large extent of secreting surface within a compara- tively small space ; in the circumstance that while one end of the -mm'. xi.] PE0CE8S OF SECRETION. 401 gland-dud opens <»n a free surface, the opposite cud is always closed, having do direct communication with blood vessels, or any other canal ; and in an uniform arrangement of capillary 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 proc< are concerned which may be spoken of as I. Physical) and II. Chemical, 1. Physical />/-o, » 6 f ~ ° <&&& o oSfe q8$8g % k: a <&& °y Q 00 °" c V Pig. 224. — Globules and m 400. seen to exhibit contractile movements and to squeeze out drops of oil from their interior (Strickei Chemical Composition — Milk is in reality an emulsion con- sisting of numberless little globules of fat, coated with a thin layer of albuminous matter, floating in a large quantity of water which contains in solution 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. Cows. • • • 858 . . 142 Human Water . 890 Solids I IO IOOO Proteids. including Casein and Serum-Albumin 35 Fats or Butter . . . 25 Sugar (with extractiv 4 s Salts . 2 IOOO 68 3* no 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 albu- 410 THE SKIN. [..-hap. xr. minous 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. — If milk be allowed to stand for sonic time, its reaction becomes acid : in popular language it " turns sour." This change appears to be due to the conversion of the milk-sugar into lactic acid, which causes the precipitation of the casein (curdling) : the curd contains the fat globules : the remain- ing fluid (whey) consists of water holding in solution albumin, 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. 307). "When milk is boiled, a scum of serum- albumin forms on the surface. 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 carbonates of potassium, sodium, calcium. CHAPTEE XII. THE SKIX AND ITS FUNCTIONS. The skin serves — (1), as an external integument for the pro- tection of the deeper tissues, and (2), as a sensitive organ in the exercise of touch ; it is also (3), an important excretoiy, and (4), an absorbing organ ; while it plays an important part in (5) the regulation of the temperature of the body. Structure of the Skin. — 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 » BA2. XII.] STRUCTUKK OF 'III K EPLDEfiMIS. 4 II Bpeda] function, namely red _ glandfl, and hair follicles ; and on its Surface BJ The so- called aji pendagee of the akin — the hair and nails — are modifica- US of the enideri: Epidermis. — The epidermis is composed of several strata of cells of various sliaj.es, and closely resembles in its structure that which lines the mouth. The following four layers may be dis- tinguished. 1. Stratum eomeum (fig. 225. a), consisting of many superposed layers of horny Bcales. The different thickness of the epidermis in dif- ferent 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 thick- ness. The stratum eomeum of the buc- cal 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 homogene- ous 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. :. N> d ..1 ^ Fig. 225. — Vertical section of Hu . -tratum eomeum, of very few layers, the stratum lueidum and stratum granuiosum not being distinctly represented : b, c. d, and ■% the layers of the stratum Malpighi, a certain number of the cells in layers