' .-f, .V, V ' 'i /}' ''’ '• '* ^??P'// V 4 iillliilpil^^ astssgsj : I Ifflliiil^^^ : I liif liltP^ I SiiiiiSiiil ,/ '.’y '' j '// / / '}!'.:’rl‘^';':-y{ '% •//,'■}, -v i/'/i f'h'-l'{t''i’l’0 j fpi}ii>m;!- ^ ■**>'■. *,' wA‘ '■' 1 \1 Jf- \ -y''J'Y. t > * }0}M ;■'■■ i::-.)';;0-'. '•-3 ,.vV^^'^■•'^■^>.^V;■v/^■''’'‘.'■'-^’■'- ■■'’V '-V;>';- 'V- V' ■' V " ' ' Dr. W.W. Lindahl 23 o 8 Bigelow Avenue North Seattle, Washington 98109 Digitized by the Internet Archive in 2016 https://archive.org/details/outlinesofphysio21mars PHYSIOLOGY. VOL. II. LONDON PRINTED BY SPOTTISWOODE AND CO. NEW-STREET SQUARE OUTLINES OF PHYSIOLOGY HUMAN AND COMPAEATIVE. BY JOHN MARSHALL, F.R.S. PROFESSOR OF SPROERT IX TNIVERSITY COLLEGE, LOXDOX : SURGEOX TO THE UXIVERSITY COLLEGE HOSPITAL. ILLUSTRATED BY NUMEROUS WOODCUTS. IN TWO VOLUMES. VOL. II. LONDON : LONGMANS, GEEEN, AND CO. 1867. The right of translation is resa'ved. CONTEXTS OF THE SECOND VOLUME. SPECIAL PHYSIOLOGY — continued. THE vegetative FUNCTIONS. PAGE Digestion 1 Sources, Varieties, and Nature of Human Food ... 2 Prehension and Preparation of Food 7 Mechanical Processes of Digestion 12 The Digestive Fluids 53 Chemical Processes of Digestion 82 Summary of the Chemistry of Digestion .... 107 Circumstances which modify Digestion .... 109 Helative Value of different Foods . . . . .113 The Organs and Function of Digestion in Animals . . 116 Absorption .151 The Absorbent Vessels and Glands ..... 153 Endosmosis, Exosmosis, Osmosis, and Dialysis . . .160 General Absorption . . . . . . . .165 Absorption of the Food 174 Intrinsic Absorption .182 The Absorbent System, and Absorption in Animals . . 186 Circulation 187 The Heart and Blood vessels . . . . . .188 Course and Causes of the Circulation ..... 200 Action of the Heart 203 Motion of the Blood through the Arteries . . . .222 VI CONTENTS OF THE SECOND VOLUME, The Pulse Motion of the Blood through the Capillaries „ ,, Veins Period of a complete Circulation Quantity of Blood in the Body The Uses of the Blood and its Circulation . Organs and Punction of the Circulation in Animals Nutrition Nutrition of the Chyle ...... „ „ Blood „ „ Organs and Tissues .... OflBces of the Blood and of its Constituents in Nutrition Haemorrhage or Loss of Blood Vitality of the Blood . The Coagulation of the Blood S.\NGUIFICA.TION The Blood Glands The Liver considered as a Blood Gland Glycogenic Function of the Liver .... Sanguification and the Blood Glands in Animals . Secretion Secretion in General General Function of the Liver ..... The Mammary Glands and Lactation .... Mucous Secretion and Mucus ..... Serous and Synovial Secretion Excretion The Eenal Excretion „ „ in Animals Special Secretions and Excretions in Animals The Skin and its Excretions ..... The Cutaneous Excretion in Animals .... Eespiration ......... The Organs of Eespiration ...... Mechanism of Eespiration ...... Changes of the Air in Eespiration .... Eifects of Eespiration on the Blood and Tissues . Conditions which modify tlie Chemical Processes of Eespira- tion Effects of Breathing other Gases than Air .... PAGE 234 242 247 2.57 260 263 264 27o 276 277 278 287 296 299 300 314 318 334 334 342 343 343 354 356 363 364 365 365 391 395 395 401 401 404 414 435 444 463 469 CONTENTS OF THE SECO]S’D VOLUME. Asphyxia .......... Suspended Eespiration and Animation .... Effects of Breathing impure Air ...... The Organs and Function of Eespiration in Animals . Anima.l Heat, Light, ant) Electricity ..... Animal Heat Effects of Cold on the Human Body ..... „ Heat „ ,, ..... Theories of Animal Heat Influence of the Nervous System on Animal Heat Uses of Animal Heat ....... Hybernation ......... Spontaneous Combustion ....... Evolution of Light „ Electricity ....... Statics and Dyna3iics of the Human Body .... Statics of the Human Body ....... Specific Gravity of the Body ...... Height of the Body Weight of the Body ....... Daily quantity of Food and its Composition Eelations between the Constituents of the Body and the daily Food ......... Destination of the Food in the living Economy Effects of Deprivation of Food ..... Dynamics of the Human Body ...... Measure of Heat, or Heat-Unit Mechanical CoefBcient, and Mechanical Equivalent of Heat Quantities of Heat developed by Combustion . Calorific Work of the Body ...... Daily Heat compared with the Quantity of Carbon and Hydrogen oxidised ....... Mechanical Work of the Body ..... Eelations of the Kinds of Food to the Modes of Work Value of Food as a source of Motor Power Transformation of Mechanical into Calorific Workin the Body Nutritive or Assimilative Work ..... Electric Work Nervous Force and Work ...... Eepeoduction Spontaneous Generation VOL. II. a VII PAGE 473 476 482 488 502 602 607 509 512 519 621 621 523 524 527 531 632 532 533 533 535 536 538 547 650 655 556 557 558 568 559 561 575 575 576 577 577 580 580 Vlll CONTENTS OF THE SECOND VOLUHE. PAGE The various Modes of Eeproduction 581 The Ovum generally ........ 590 The Ovaries and Ova of Birds ...... 693 The Mammalian Ovaries and Ova 597 The Ovaries and Ova of other Animals .... 600 Fertilization of the Ovum 601 Development 603 Changes in the Ovum, and first formation of the Embryo . 603 General Development of the Embryo and its Appendages . 608 Development of the Organs 615 Development of the Tissues 641 Animal and Vegetable Cells 641 Development of the several Tissues 646 Eegeneration and Eeparation 661 Growth 664 Decay and Death 665 Index 671 SPECIAL PHYSIOLOGY. SPECIAL PHYSIOLOGY. THE VEGETATIVE FUNCTIONS. The functions to be considered under this head, are the nutritive and reproductive functions. The former include digestion, absorption, chylification, circulation, nutrition, re- paration, sanguification, secretion, excretion, and respiration, together with the production of animal heat, muscular force, light, and electricity. DIGESTION. Amongst other phenomena produced by the waste of the solid constituents of the body, and the loss of the fluid, or watery, part of the tissues, are the special sensations of hunger and thirst, which have their seat, like other sensations, in the nervous system, and the phenomena of which have been already explained (vol. i. p. 443). These sensations of appetite, excite the desire to take food ; and by the process of digestion, the food, thus taken, is prepared for absorption, and conversion into blood. The term food includes all substances, received into the alimentary canal, and used for the support of life, either by supplying the waste constantly occurring in the living animal tissues, or by affording materials for the maintenance of the temperature of the body. Food, therefore, contains substances which have a certain chemical relation to the tissues which it supports. These tissues, besides containing water and saline substances, are composed of proximate or- ganic principles, having a highly complex chemical constitu- tion (vol. i. pp. 96 to 99). Food also consists, more or less, of substances having already the same, or a similar, chemical composition ; for the animal body, so far as is known, has no power of forming such proximate organic compounds out of VOL. II. B 2 SPECIAL PHYSIOLOGY. their component elements, or from the simpler combinations of these. Animals, indeed, are either carnivorous or herbivorous. The carnivorous, or flesh-devouring species, obviously live upon food possessing the same chemical composition as the fluids and tissues of their own bodies ; and as regards the herbivorous, or vegetable-feeding animals, their food also con- tains proximate principles, closely resembling those which exi.st in the animal body. Whatever the nature or source of the food of Animals, its proximate principles are, therefore, chemically similar ; and it is to the Vegetable Kingdom, that we must attribute the power of chemically combining, rmder the agency of solar light and heat, the elements derived from the simpler combinations of inorganic nature, into those complex organic proximate principles wdiich, thus elaborated in the living tissues of vegetables, constitute the nutriment of Animals. Hence, the Vegetable Kingdom derives its nourishment from, and depends upon, the Mineral Kingdom ; the Animal Kingdom derives its nourishment from, and depends upon, the Vegetable Kingdom ; whilst the decaying portions of the Vegetable King- dom which are unconsumed by animals, and the particles of the bodies of animals which undergo change during life, or decomposition after death, revert to the simpler chemical com- pounds of inorganic nature, wdiich, again, under the influence of the vito-chemical forces of the plant, are reintroduced into the stream of organic existence. Sources, Varieties, and Nature of Human Food. The food of Man may be either solid or fluid. If solid, it may be hard, so as to require to be broken by ma.stication. or soft, so as merely to need subdivision, before it is swallowed. Again, food may be derived from the inorganic or from the organic world ; or it may be classified according to its source, whether this be mineral, vegetable, or animal. Thus, the alkaline and earthy salts, the traces of iron, sulphur, and phosphorus, and the large quantity of water, are derived from the mineral kingdom. Vegetable food includes the roots, stems, leaves, fruits, and seeds of plants; also certain products of vinous decomposition, as the various alcohoh'c beverages, and lastly, condiments, vegetable acids, and vinegar or the product of the acetous fermentation. Animal food consists of all the digestible parts of animals, in which is comprised PROXIMATE CONSTITUENTS OF FOOD. nearly every tissue, with the exception of the horny textures and the hair, even the bones yielding nutriment on being boiled. Besides this, eggs and roe, milk, butter, butter-milk, curd, cheese, and whey, are comprehended in this category. The chemical constitution of food, however, is the point to which the greatest significance is to be attached ; and the most useful classifications are founded on a consideration of the different nutrient proximate chemical principles which it contains. Tims regarded, the multitudinous articles of diet consumed by man, under his extremely varied conditions of life, dependent on climate, social condition, national custom, or individual habit, consist of a comparatively small number of proximal e chemical constituents. The importance of these chemical distinctions of the food, was clearly indicated by Front, and has been since established by the researches of Liebig, and many other chemists. Front divided all nutrient substances into alhuminous bodies, such as the albumen, fibrin, and casein of animals, and the gluten and legumin of plants ; oleaginous substances, including the animal and vegetable fats and oils; and sacc/tanree matters, comprising the various kinds of sugar. According to him, the typical form of animal food, is that supplied, by nature, to the young of mammiferous animals and man, viz. milk, in which fluid, casein represents the albuminous kind of nutritive substances; butter, the oleaginous kind ; and sugar of milk, the saccharine kind. Be.sides these, milk also supplies water, and the mineral matters essential to the formation of the tissues. A more exhaustive classification of the nutritive substances contained in food, is that which follows : — 1. A/iimmohi substances. From the animal kingdom, men, whether derived from the white of eggs,from blood, or from the muscular or nervous tissues ; syntonin, or the fibrinous ele- ment of muscle, some of which is contained in the expressed juice of meat ; globulin, cruorin, and fibrin, from the blood ; casein, derived from milk ; and the vitellin of the yolk of eggs. The substance of the liver, pancreas, kidneys, and other glands, is also, in great part, albuminoid, mixed, however, especially in the first organ, with fat. The brain substance is also highly nutritive, containing both albuminoid and fatty matter. In this group, must be included, not only cruorin, or the colour- ing matter of the blood, but also niyochrome, or that of muscle, both of which have an extraordinary affinity for oxygen. From the vegetable kingdom, are obtained the albuminoid substance B 2 4 SPECIAL PHYSIOLOGY. gluten, sometimes called vegetable albumen, ■which is chiefly obtained from the seeds of the various kinds of com, and other grasses ; also leguniin, which has been compared to animal casein, and exists in large quantity in the seeds of peas, beans, lentils, and other leguminous plants. Vegetable albumen likewise exists, in small quantity, in the growing or soft tissues of the various succulent edible parts of vegetables and fruit, such as the cabbage, cauliflower, turnip, apple, pear, and orange. 2. Gelatinoid sub.stances. These, which are derived solelv from the animal kingdom, include jelbj of various kinds, ob- tained from the gelatin-yielding tissues of animals, such as isinglass, which is the dried sound, or air-bladder, of the sturgeon, the areolar and fibrous tissues, tendons, and bones ; also chondrin, or the jelly obtained from cartilages. These several tissues, however, are not supposed to contain gelatin or chondrin, when in their raw or uncooked state. Gelatinoid substances are present in broths, jellies, and ivory bone-dust. So far as their nutrient qualities are concerned, they must be distinguished from the albuminoid substances. 3. Oleaginous substances. These comprehend the animal fats and oils, stearin, margarin, pahnitin, and olein, the fattv matters of the bile and of the brain, and those of the yolk of eggs ; and also the fatty acids of butter, the butyric, capric, and caproic. To these must be added," the vegetable oils, whether solid or fluid, such as cocoa-nut oil, olive oil, and almond oil. 4. Amylaceous or starchy, gummy, and saccharine substances. These comprehend the different varieties of starch, such as potato starch, arrow-root, sago, tapioca, rice, and the starchy portion of wheat and other grain. The gummy substances include, besides all the natural gurus and mucilages of fruits and vegetables, the substance named dextrin, which results from the transformation of starch, cellulose or lignin, and also pectin, a constituent of succulent vegetables. The sugars are the com- mon, or cane sugar, and grape sugar, derived, as such, from vegetables, or produced by the transformation of starchy or gummy substances. There are also the sugar of honey, which is an animal preparation ; the glycogen, or animal starch, often present in flesh, but chiefly found in the substance of the liver ; inosite, or sugar of muscle; and lastly, the sugar of milk. lactose, or lactin, which, though usually formed in the animal economy, can also be artificially made, by acting upon starch with certain acids, at a high temperature. 5. Stimulating substances. These consist of three classes : PROXIMATE CONSTITUENTS OF FOOD. viz. first, the various kinds of spices or condiments, the active properties of wliich depend usually upon volatile or essential oils ; secondly, the parts of vegetables, whether the leaves or berries, which contain the alkaloids, thein, caffein, or theobro- min, which are found in tea, coffee, cocoa, and the Paraguay tea. With these should probably be associated, the substances named extractives, viz. cerehric acid, which exists in nervous substance, and also in corn, especially in Indian corn ; creatirc and creatinin, which are found in the juice of meat, in the brain, and in the blood, the former being converted in the system into the latter; both of these act either as stimu- lants, or by retarding chemical change and loss in the albu- minoid tissues. The thein and allied bodies certainly stimu- late the heart, muscles, and nervous system. Thirdly, there are the various alcoholic beverages made by the fermentation of saccharine substances, such as mead, beer, cider, wine, and the stronger alcoholic fluids or spirits distilled from various fermenting saccharine vegetable juices. These substances are probably not immediately nutritive, or able to supply the waste of material, but appear rather to act as stimuli to the nervous system, and also by preventing waste. To these may be added, the several ethers formed in ripe fruits, and in wines, from the action of the organic vegetable acids on alcohol. This class may also include certain organic vegetable acids, such as the acetic acid of vinegar, the tartaric, malic, racemic, oxalic, and citric, derived respectively, from grapes or raisins, apples, gooseberries, the esculent rhubarb, and the lemon, lime, and orange ; and lastly, the lactic acid existing in sauer-kraut, and in fermented cucumbers or beans, all of which are lavourite articles of diet with some nations. The prevalence of the desire for acids with the food, is remarkable. Lactic acid also e.xists in sour milk, which is much consumed, and in the juice of meat, together with paralactic and inosinic acids. 6. Saline, earthy, and mineral substances. These, which are, in certain proportions, essential articles of food, soda for the blood, potash for the muscles, and lime for the bones, consist of the chlorides of sodium and potassium, the phos- phates of soda, potash, and of magnesia, perhaps the alka- line sulphates, the pho.sphate and carbonate of lime, and oxide of iron. Minute traces of manganese and silica are also necessary, the latter being probably combined with fluorine. Such substances as alumina and copper, are probably adventi- tious ingredients, and of no essential importance as food. G SPECIAL PIITSIOLOCiY. 7. Water is tlie most abundant constituent of the animal body, and is a most essential article of food. From the many offices which it performs, dissolving the food, rendering it capable of absorption and entrance into the circulation, facilitating all nutritive, secretive, and excretive processes, and lastly, maintaining the due elasticity and flexibility of the tissues, and their susceptibility of vito-chemical changes, water may be regarded as a common vehicle, in which all other articles of diet are conveyed into, through, and from the animal economy. The albuminoid and gelatinoid nutrient substances, resemble each other very closely in composition; in addition to car- bon, hydrogen, oxj^gen, and sulphur, they contain nitrogen, and have therefore been named, nitrogenous or azotised food ; and. as these substances are especially concerned in the formation of the albuminoid and gelatin-jdelding tissues of the body, which indeed cannot be built ttp without them, they have been desig- nated nutritive or plastic food. Moreover, as they supply the waste which takes place in the muscular and other tissues, they have been likewise called flesh- fanning, tissue-fonning, or histo- genetic, food. On the other hand, the oleaginous and saccharine substances are composed of carbon, hydrogen, and oxygen only, and are therefore named non-nitrogenous or non-azotised food. The starchy, saccharine, and allied com- pounds, form the carbhydrates ; whilst the fatty substances, still richer in carbon, are named hydrocarbons. As neither of these is ever supposed to be convertible, by the addition of nitrogen, into nitrogenous, plastic, or flesh-forming food, but rather, owing to their richness in carbon and hj'drogen, and their poverty in oxj’gen, to be ultimatelv used for the purposes of maintaining the animal heat, either being first stored up in the body as fat, or being at once oxygenated through the respiratory process, they have been classed to- gether under the appellation of respiratory, calonflc, or heat- forming, food. These distinctions, which have been chiefly explained and advocated by Liebig, undoubtedly represent a general truth ; but they must be accepted with certain qualifications. In the first place, albuminoid substances may, it would seem, undergo metamorphosis, in the living body, into fatty or even starch- like substances, and so may nourish non-nitrogenotis, as well as fleshy or nitrogenous, tissues. Moreover, the nitrogenous tissues of the living body, especially those of the muscles and PREHENSION OF FOOD. 7 brain, themselves undergo a most active waste, i.e. a chemical decomposition, of which the essential feature is oxidation ; so that, to a certain extent, they too, in being decomposed, must contribute to the evolution of heat, subserve the respiratory process, and so far act as respiratory food. Again, chemical analysis shows, that in the brain especially, but also in muscular tissue, fatty matter is an important constituent, essential, indeed, to the composition of those tissues; moreover, starchy and saccharine matters exist in certain organs, and are convertible, in the living economy, into fat ; hence the non-nitrogenous, oleaginous, and saccharine substances must, also, be regarded as nutritive or plastic food. Even in young growing animal cells, liitty matter appears to be an essential element. Again, as regards gelatin, and the gelatin-yielding tissues, which, though they contain nitrogen, have a lower chemical constitution than the albuminoid sub- stances, it is not certain that they are convertible into, or capable of being made use of as, nutriment for the living tissues. It is now generally denied that they can be so converted into, or assimilated by, tissues which, like muscle and nerve, contain s}mtonin and albumen ; it is even doubted whether they can be directly assimilated as nutriment, even by the living gelatin-yielding tissues themselves, which, of course, have an identical chemical composition. Such substances may, therefore, possess very limited or no nutritive or plastic qualities ; and may merely be oxidised in the system, like the non-nitrogenous, respiratory food. The precise destination of the several elements of food is, however, not completely understood ; but neither of the two kinds of food, the nitro- genous, or the non-nitrogenous, is alone adequate to support animal or human life ; for perfect nutrition, the two must be taken together in certain proportions. The chemical composition of most of the nitrogenous and non-nitrogenous proximate constituents of animal substances used as food, is given in the tables at pages 96 and 98, vol. i. The closely similar composition of the nitrogenous and non- nitrogenous proximate constituents of vegetable substances used as food, is illustrated in the annexed table (p. 8). Prehension and Preparation of Food. In the lower animals, the important act of the prehension of food, is provided for, in every case, with the most admir- 8 SPECIAL PHYSIOLOGY. Analysis of Vegetable Troxirmte Constituents. C a s = -■ j p 'S 2 ■ti 2 f s S c a. ^ 1 Vegetable Albumen . . = 55-01 7-23 15-92 21-84 1 included | -j with the j Vegetable Fibrin or Gluten . = 54-6 7-2 15-8] 22-29 '' oxveen I Legumin, a similar compo- ■) 1 sition, but not well deter- - mined . . . . > 1 Thein, Caffein (CsH,„N^Oj) . = 49-4 5-2 28-9 16-5 Theobromin (C,HgN^02) . = Vegetable Oils, chiefly Oleic 46-7 4-4 31-1 17-8 1 acid, and Glycerin (p. 96). i 1 Starch. . .'j Dextrin or Gum . tx \ 44-4 6-2 49-4 ' Cellulose and Lig- j'' ® i nin . J CaneSuffar(C,oHooO,,). . = 42-1 6-4 5P5 Grape Sugar, Glu- i /p tt p -n _ cose, or Dextrose J ^ ® 40- 6-7 53-3 I Alcohol (C,H,0) . 52-2 13- 34-8 Ether (C^H,„0) . . . = 64-8.3 13-5 21-65 1 Vegetable Acids ; Citric (CgHgO,) . . = 3 / *0 4-2 58-3 1 Malic (C.HgOj) . 35-8 4-4 59-8 Tartaric (C^HgO^) . 32- 4- 64- ! Alkaline and Earthy Salts and \ Water, the same as in Ani- i mals ; but Land Plants con- ! i i i tain mostly Potash, and I Marine Plants mostly Soda. able perfection of contriyance. In Sian, however, the ami and hand are so wonderihlly organised for other, and higher, purposes (vol i. p. 239), that their prehensile action, in the gathering, or preparation, of food, and its conveyance to the mouth, are, though essential, only subordinate offices of the upper limb. The lips and tongue, which, in the Mammalia, are devoted, mainly at least, to the taking of food, are in Man also so employed ; but higher services are demanded of these PREPARATION OF FOOD. 9 parts, and we are accustomed to associate their mechanism more especially with the faculty of speech. Lastly, the jaws and teeth, although, in animals, they frequently constitute the most important, and, in the case of the lower Vertebrata, the sole organs of prehension, can hardly be said to fulfil, in Man, in addition to their proper office of mastication, a prehensile office in reference to the food. As regards the prehension of food, Man appears, indeed, almost at a mechanical disadvantage, in comparison with the animals beneath him, so far, at least, as concerns any special adaptation of the parts of the organism, employed for that purpose in animals. Nevertheless, he accomplishes this act with facility. In the choice and selection of food, Man, guided by his intelligence, possesses enormous advantages over the lower animals. He ranges through the whole domain of the organic kingdom, and by the arts of acclimatisation, breeding, cultiva- tion, and agriculture, has improved many species, both animal and vegetable, which, in their wild, and uncultivated condi- tion, are much inferior as sources of food. The improvement of the cereal, or corn plants, of vegetables and fruits, and of the ox, sheep, and pig, and also the acclimatisation of many gallinaceous birds, and the more recent resrdts of piscicrdture, and of attempts to breed the oyster, afford proofs of this statement. The use of fire for the preparation of food, is, like the employment of fire in general, peculiar to Man, who has, indeed, been designated a “ cooking animal.” The direct application of fire heat to food, develops peculiar empyreu- matic flavors and odours, in the cooked substance, whether this be animal or vegetable ; but the more important action of heat, whether applied directly, as in roasting or baking, or indirectly, through the agency of water, as in boiling, is to change the molar and molecular condition of the cooked sub- stances. Thus, the albuminoid bodies are more or less coagu- lated ; the gelatin-yielding tissues become swollen and partially gelatinised ; fat-cells are ruptured, and fats are rendered more fluid ; the various kinds of starch have their granules pulpi- fied, and the cellulose and lignin of vegetable tissue, are broken up, so as to liberate the contents of the cells. The general result of cooking, is to disintegrate, and separate the animal tissues into minuter portions, and to destroy the con- tinmty of vegetable textures. Cooking, therefore, produces 10 SPECIAL PHYSIOLOGY. hoth physical and chemical changes in the food, the tendency of which is to facilitate mastication, and the subsequent action of the digestive fluids, thus rendering them softer and more digestible. Man also has discovered and employed as drinks, numerous beverages, obtained from the natural products of nearly every climate, by the spontaneous, or the induced, alcoholic fer- mentation of saccharine matter, whether this saccharine matter exist ready formed, as in the juice of the grape, or other fruits, or whether it be artificially generated by the transformation of starch into sugar, as happens when barley is manufactured into malt. Besides consuming the immediate products of fennentation, in the shape of wine, beer, and other fermented liquors, distillation is had recourse to by Man, in order to procure, in a more concentrated state, the spirit, or alcohol, generated in that fermentation. Man, therefore, not only employs the art of cooking, but also the chemical processes of fermentation and distillation, in the preparation of food, using this term in its widest sense. The precise destination of alcohol in the system will be hereafter discussed. Other beverages are made by simple infusion or decoction, so as to dissolve out certain nutrient or stimulating substances, as from tea, roasted coffee, cocoa, and other vegetable products. Sugar is used in solution, in the sweetening or preservation of fruits, in cookery, and in preparing various articles of con- fectionery; it is a highly important and useful form of food. Common salt, being contained in the blood and tissue.s, is an essential article of food. Its use as a condiment, and also as a preservative, especially of animal substances employed as food, is very old and general. All animals are fond of sidt. Its injurious influence on the qualitv of the food preserved in it, has long been recognised, the continued use of such food, in the form of salted provisions, favouring the production of scorbutus or scurvy. Salt hardens the muscular and other tissues preserved in it, by abstracting water from them ; with this water, which appears in the brine, the soluble potash and magnesia salts, as well as the creatin and other ex- tractives, are likewise abstracted from the meat, and pass into the preservative liquor, thus leaving the meat de.stitute of many alimentary principles essential to health. Indirectly, this may be the cause of scurvy ; or that disease may partly de- pend on the direct action of the common salt taken in excess. The employment of vinegar as a condiment, and the use of THE DIGESTITE PROCESSES. 11 vegetable acids, those universally favourite articles of diet, aid in the solution of nitrogenous food, and possibly of the lime salts, but they can scarcely be regarded as possessing positive nutrient properties. Other condiments, and spices, serve to stimulate the secretion of the digestive fluids, and excite the movements of the alimentary canal. In the artificial preparation of food, so as to render it soluble, or more easy of solution, we assist the digestive function itself, which, in adapting nutrient substances, by a series of processes, for absorption into the tissues of the body, has, for its immediate aim, the minute subdivision and the solution of these substances. The process of digestion, accordingly, includes certain mechanical and chemical acts. The former have for their object, to triturate and comminute the food, to mix it with fluids and with the various secretions in the alimentary canal, to move it within and onwards through the several portions of that canal, and lastly, to expel from the body the un- absorbed residue. The latter are accomplished by the aid of the various digestive fluids poured into the alimentary canal. Considered in the order in which they take place within the body, the several processes necessary to digestion, are mastica- tion, or the chewing of the food, and insalivation, or the mixing it with saliva, which occur simultaneously in the mouth ; deglutition, or swallowing, in which the food is con- veyed through the pharynx and (esophagus, into the stomach ; gastric digestion, which takes place in the stomach, by aid of the gastric juice, also called chymificalion, and sometimes, though erroneously, digestion proper, for further true diges- tive processes occur in the intestine ; and, lastly, intestinal digestion itself, accomplished by aid of the bile, pancreatic juice, and intestinal juice, immediately preparatory to the proper act of absorption of the digested materials, by the lacteals, in which they appear as chyle. Absorption of certain constituents of the food, however, likewise occurs, more or less, through the capillaries of every part of the alimentarv canal. The residue of the food, or ingesta, together with the un- absorbed secretions, form the egesta, the expulsion of which, con.stitutes the function of defecation. The mechanical and chemical processes of digestion, require separate, and lengthened, consideration. 12 SPECIAL PHTSIOLOGT. MECHANICAL PUOCESSES OF DIGESTION. Mastication and Insalivation. The parts concerned in mastication, are the teeth and. jaws, the muscles which move the lower jaw upon the upper one, tlie muscles of the cheeks, the lips, the tongue, and palate. The teeth in Man, as in all Mammalia, are developed in two sets ; a first, less numerous, and smaller set, known as the milk, temporary, or deciduous teeth, and a second set, larger and more numerous, called the permanent teeth. The milk teeth are twenty in number, ten in each jaw. The five teeth, in either half of each jaw, commencing at the middle line, consist of two so-called incisor teeth, one canine, and two molar teeth. The formula of these teeth is thus written, — M2 Cl 14 Cl M2 M2 Cl 14 Cl M2‘ When these teeth are shed, they are succeeded, at intervals, by the permanent teeth, which are thirty-two in number, .sixteen in each jaw, eight in either half of each jaw ; viz. Fig. 84. Piff. 84 . Human teeth, i, lower lateral incisor, seen from behind, c, lower canine, seen from within, b. second upper bicuspid, seen sideways, m, second lower molar, seen from without, i', section of an incisor tooth, showing: the pulp cavity, extending from the point of the fang, the den- tine, or tooth substance, the enamel on the crown, and the layer of cement on the fang, m’, section of a molar tooth, showing the same ))arts, and the pulp cavity extending into each fang. (Blake.) commencing at the middle line, two incisors, one canine, two bicuspids, and three molars. The formula of these teeth is therefore, — M3 B2 Cl 14 Cl B2 M3 M3“'B2~1 Ti”I 4 Cl B2 M3' THE TEETH. 13 Bach tooth, fiq. 84, i to m, consists of an exposed part, called the crown or body, and of a part buried in the gum and jaw, named the root or fang ; at the junction of the crown and fang, is the slightly constricted cervix or neck. The several kinds of teeth differ in the form of their crowns, and in the number of their fangs ; hence their different designations. The incisor teeth, i, have wide, thin, crowns, slightly convex in front, and smooth or marked with longitudinal furrows, but somewhat concave, or bevelled off, on their hinder surface ; their edges, which, at first, present three small prominent points, are, when worn, long, narrow, and chisel-shaped, being well adapted for cutting purposes ; hence their name. The fang is long, single, and somewhat compressed from side to side. In the temporary teeth, but much more markedly in the permanent set, the upper incisors are larger, and occupy more space transversely, than the lower ones ; in the upper jaw, the middle incisors are larger than the lateral ones ; in the lower jaw, the reverse is the case. The canine teeth, c, larger and thicker than the incisors, are distinguished by the pointed character of their crowns, which are very convex in front, and a little hollowed behind, and also by the great size and length of their single fang, which presents, on its sides, a slight longitudinal furrow. The upper canines, popularly called the eye-teeth, are larger and longer than the lower ones, and on their posterior surface, close to the gum, is found a minute tubercle. The groove on the fang, and this posterior tubercle, foreshadow the subdivided fang and double crown of the bicuspid teeth. The canine teeth are so named from their large size in the dog, though they are still larger in the great feline animals ; in Man, they are more uniform in size with the neighbouring teeth, than in the larger Quadrumana and Carnivora. From their single point or cusp, which wears down with use, these teeth are sometimes called the cuspidate teeth. The bicuspid teeth, b, sometimes called premolai's, because they are placed before the molars, and also named the small or false molars, have a double crown furnished with two pointed cusps or tubercles ; viz. an outer higher, and an inner lower, one, between which is an irregular depression. The summit of the crown is quad- rangular, and compressed from side to side, contrasting with the pointed canines, and chisel-shaped incisors. The fang, in the lower bicuspids, is deeply grooved on each side, but in the upper ones, is cleft for a certain distance at the point. The molars or grinding teeth, m, are the largest of the entire set ; 14 SPECIAL PHYSIOLOGY. the first on each side of each jaw, are the largest, and the third, or last molars, which are also named the wisdom teeth ( dentes sapientiffi), from their late appearance, are the smallest. They have a large, nearly cuboid crown. In tiie upper molars, this presents four cusps or tubercles, placed at the angles of the upper surface, and separated by a crucial de- pression ; the first and second of these teeth have the internal anterior tubercle always the largest ; in the last irpper molars, the two internal tubercles are blended. The crowns of the lower molars are larger than those of the upper, and are dis- tinguished by having a fifth small cusp or tubercle placed between the outer and inner posterior cusps, rather nearer to the former than to the latter ; this fifth cusp is best marked in the last lower molar tooth. The grinding surface of the lower molars is nearly square ; that of the upper, rhom- boidal. In the lower jaw, the two anterior molars have two fangs, but these are broad, grooved on their surface, and sometimes subdivided at their points. In the upper jaw, the fangs of the two anterior molars are three in number, two outer and one inner fang, the latter being sometimes grooved or even subdivided. The fangs of the upper molars are more divergent than those of the lower ones. In the wisdom teeth, or last molars of each jaw, the fangs are generally connate or united into a mass, showing marks of subdivision into two fangs in the lower teeth, and three in the upper. The row of teeth, in each jaw, forms what is called the dental arch. In Man, it presents a broad, even curve, the upper dental arch being larger than the lower, so that usually it overlaps the latter when the teeth are closed, and thus saves the edges of the incisor teeth from unnecessary wear. The upper ifont teeth are inclined slightly forwards, and the back teeth, outwards; whilst the lower front teeth are vertical, and the lateral teeth directed somewhat inwards, an arrangement which corresponds with the greater size, and the overlapping of the upper dental arch. In Man, the entire series of teeth are characterised by being uninterrupted by any marked in- terval, hiatus, or diastema, and by their nearly even height, which however diminishes slightly from before backwards. In Mammiferous animals, the teeth are either of unequal height at different parts of the jaw, or are inteiTupted by larger or smaller intervals, or diastemata. The temporary teeth, though of course, in each case, of smaller size, have forms like those of the permanent teeth of THE STEUCTUEE OF THE TEETH. 5 the same name. The crowns of tlie incisors are chisel-shaped, those of the canines pointed, and those of the molars square, and provided with several cusps. The first upper molar, the largest of all, has three cusps, and the second foitr ; the first lower molar four, and the second five. The fiings of the temporary incisors and canines, are single ; those of the lower molars are two in number ; those of the upper, three. In both jaws, they are more divergent than those of the permanent teeth. The hard mass of a tooth is hollowed out, so as to form a cavity, called the pulp cavity, because, during life, it contains a soft substance named the pulp. This pulp cavity, fig. 84, i', m! , varies in shape with that of the tooth ; it occupies the base of the crown,, and is prolonged down each fang, in the form of a small canal, which opens at the point. The pulp consists of areolar tissue, supplied with vessels and nerves, which enter at the minute opening at the point of the fang ; it is the remains of the vascidar and nervous papilla, upon which the tooth is originally formed. The hard portion of the tooth surrounding the pulp, is composed of three substances; viz. the tooth substance, ivory, or dentine, the enamel, and the crusta petrosa, or cement (see fig. 84). The dentine forms the greater part of the tooth, imme- diately surrounds the pulp cavity, and corresponds, in form, with the tooth itself. Its hardness is owing to the large quantity of earthy matter which it contains, its chemical composition being 72 parts of earthy to 28 of animal matter; whilst ordinary bone shows a proportion of 66^ to 33^. The earthy salts contain 66'7 of phosphate of lime, 3'3 of carbonate of lime, 1’8 of phosphate of magnesia and other salts, and some traces of fluoride of calcium. The animal substance is converted into gelatin on being boiled. The dentine consists of microscopic tubes, called the dental tubuli, which have hard walls, and are embedded in an intermediate hard substance. These tubuli, originally de- scribed by Leeuwenhoek, commence by minute orifices on the walls of the pulp cavity, and proceed outwards in a slightly wavy course, close together ; they soon divide dichotomously, and reach the superficial portion of the dentine, near the sur- face of which they terminate in fine branches, in loops, or in minute dilatations from which still finer branches proceed, or else in minute dentinal cells. The diameter of the inner or larger ends of the tubes, is about the 4 5 ^ 0 ^^ of an inch ; their 16 SPECIAL PHTSIOLOGT. terminations are immeasurably fine. These tubuli might be compared to extremely minute Haversian canals, their finest terminal ramifications to the canaliculi, and the minute den- tinal cells to the corpuscles or lacunte of bone (vol. i. p. 47). The dentine is, indeed, regarded as modified bone. In Man, the dentinal cells are few in number, and very minute, so that their similarity to the lacunae of bone is not so striking as it is in the teeth of the horse and other animals, in Avhich they are larger and more numerous. In the recent state, the dental tubuli are occupied by minute processes of the tooth pulp, which serve the piirposes of nutrition, and perhaps also impart sensibility to the dentine. The substance of the walls of the tubuli, is comparatively thick ; its structure is not exactly known. The intermediate hard, or so-called inter- Fig. 85. Section of a portion of the crown of a tooth, magnified about sno diameters, d, the enamel, composed of wavy fibres, marked with faint cross lines ; the surface is bounded with a tine homogeneous layer. Be- neath the enamel, is a portion of the tooth substance, showing the ends of the tubercle of the dentine, and certain irregular spaces in it. (After Kblliker.) tubular substance, is shghtly granular, and contains the greater part of the earthy matter. When this is removed by an acid, the softened animal basis is said, by some, to consist of fibres running parallel with the tubes, by others, of minute corpuscles, arranged aroimd the tubes, and, according to ano- ther view, of fine lamellte disposed concentrically around the pulp cavity, across the direction of the tubules, Avhich are supposed to perforate the lamellae. The enamel, the hardest of the dental substances, and, indeed, of all known animal textures, is the dense white covering, which Fig. 8-5. THE STRUCTUIiE OF THE TfeETH. 17 protects the crowns of the teeth ; it is thickest on the edges of the incisor and canine, and on the ci’own of the molar teeth, and gradually becomes thinner towards the neck, where it terminates. It contains more earthy matter than any other animal tissue, viz. 96'5 percent., of which 89'8are phosphate of lime, with traces of fluoride of calcium, 4'4 carbonate of lime, and 1'3 phosphate of magnesia and other salts. The animal matter amounts to 3’5 per cent., the analysis showing a loss of 1 per cent. (Bibra). Berzelius estimated the animal matter at the remarkably low proportion of 2 per cent. The enamel, fig. 85, rf, is composed entirely of microscopic hexagonal prismatic fibres, or rods, arranged closely together upon the dentine ; they are fixed, by one extremity, to nrimrte depressions on the surface of the dentine, and, following a somewhat wavy course, present, at their outer ends, the appearance of a hexagonal mosaic pattern, where they form the free surface of the enamel. On the crowns of the teeth, the enamel fibres are vertical ; on the sides, they become first oblique, and then horizontal. Their diameter is g-jV-^th of an inch. ISlear the surface of the dentine, minute interstices are found between the enamel fibres, supposed to be for the purpose of nutritive permeation. In the growing tooth, by the action of an acid, the enamel may be separated into its microscopic elements, viz. into delicate prismatic nucleated cells, the walls of which coalesce, and which form moulds for the deposit of the earthy matter. In the perfectly developed tooth, the thin parietes of the cells become almost, or entirely, absorbed, and the prismatic earthy casts are blended together as the enamel fibres. On treating a growing tooth with an acid, an exceed- ingly delicate membrane or cuticle is found, covering the entire surface, which afterwards, becoming calcified and coherent with the ends of the subjacent fibres, forms an impenetrable protective covering to it. The crusta petrosa, or cement, fig. 84, i', m' , is a thin layer of true bone, wdiich covers the fang, being thinnest next to the enamel, and thickest along the grooves and near the point ; it becomes thicker in advanced age, and sometimes fills up the minute opening leading into the pulp cavity. The crusta petrosa contains lacunse and canaliculi ; the latter, in the deep layers, sometimes anastomose with the terminations of the dental tubuli ; in its thicker portions, it contains Haver- sian canals, surroiinded by concentric lamella;. Its outer surface is firmly attached to a fibro- vascular and sensitive VOL. II. o SPECIAL PHYSIOLOGY. membrane, called periodontal niemhrane, which is analogous to a periosteum, and serves to fasten the teeth in the alveoli or sockets of the jaw, being itself united to the periosteal mem- brane which lines the sockets. The dentine gives strength and solidity to the teeth, but being penetrated by processes of the sensitive pulp, and doubtless subject to nutritive changes, it is liable, when exposed, to sutler pain, and to undergo a process of decay resembling caries, which may even be repaired by an exuda- tion of dense iiregular dentinal substance. The dentine, though v'ery hard, would not bear constant attrition ; hence that singularly hard organised product, the enamel, is provided as a covering to the exposed parts of the teeth. This enamel, however, wears down, as is well seen in the incisor teeth, the primitively sharp, wavy, ornotched edge of which, soon becomes worn to an even chisel-like border. The enamel often exhibits minute fissures, and in the depressions between the cusps of the molar teeth, deep cracks, which are the usual seats of com- mencing caries in the subjacent dentine. As life advances, the crusta petrosa often forms little knobs of bone upon the fangs of the teeth ; and after a certain age, a deposit, partly resem- bling dentine and partly bone, named osteo-dentine, or secondanj dentine, is sometimes slowly formed in the tooth cavity, whilst the pulp itself necessarily wastes. This deposit is produced by a conversion of the pulp, and serves to strengthen and solidify the tooth, as its crown is being worn away ; in time, how- ever, this process ends by cutting off the vascular supply ot the pulp, and leads to that final stage, in which the remaining parts of the teeth drop out, and leave the edentulous jaw of old age. The teeth of Man, and of the Mammalia generally, are not parts of the endo-skeleton, but appendages developed in the mucous membrane of the mouth, which, like the armoiu-- plates of the armadillo, the bony scales of the crocodile, and the scales and spines of fishes, all appendages of the skin, belong to the exo-skeleton, or dermal skeleton. The mode of develoj^meut of the teeth, and the manner in which the milk teeth are shed, and the permanent teeth are cut. will be described in the section on Development. The period of the cutting or eruption of the temporary teeth is as folloAvs : — The milk teeth begin to appear at about the seventh month, and are completed at the expiration of the second year : but considerable difference exists in regard to the precise periods THE MILK TEETH. 19 of their eruption, frequently the first teeth appearing as early as the fifth or sixth month, and some infants being born with teeth. The annexed diagram shows the usual order and average time at which the milk teeth are cut, the numbers indicating months. I I 18 12 24 ji Tlie lower middle incisors appear first, and generally the lower teeth are cut before the corresponding teeth of the upper jaw. Before the cutting of the teeth, the edges of the jaw, previously sharp, hard, and pale, become rounded and swollen, and of a darker colour, and the apex of the future tooth appears, like a white line or spot, through the gum. The milk teeth, having, for a time, fulfilled the office of mastication, fall out, and are succeeded by the permanent set, destined to serve the same purpose through the remainder of life. Teeth, once formed, cannot increase in size. The miUc teeth, though sufficiently large for the infantile jaws, and strong enough to resist the action of the less powerful muscles working them against the softer food consumed in the earber periods of life, would not be strong enough for the fully developed jaws and muscles, and the harder food, of the adult. Hence, they are removed to make way for a larger set, which also, when once formed, undergo no change in size. Their formation, and calcification, commence, indeed, at very early periods of life, the ossification of the first perma- nent teeth beginning at the age of six months, and that of the last molars, or wisdom teeth, at about twelve years of age ; yet their size is proportionate to the dimensions of the future alveoli and jaws, and to the future wants of the still imde- veloped adult. The formation of the permanent teeth pre- sents one of the clearest examples of anticipative design in the animal economy ; for they are laid down, and their crowns even are fully formed, whilst the jaw itself is still too small 20 SPECIAL PHTSIOLOGT. for their proper accommodation, and their foture alveoli do not even exist. The erirption of the permanent teeth corresponds, generally, with that of the milk set. Thus, the permanent incisors .suc- ceed to the temporary incisors, the canines of the one set, to those of the other, and the two permanent bicuspids, to the two temporary molars. Tlie three permanent molars on each side are cut, like the milk teeth, directly through the gums. The cutting of the milk teeth, is doubtless, in many cases, though not necessarily, a painful process; it may even pro- duce refle.x nervous irritation, which may affect the digestive, circulatory, or muscular systems, causing diarrhoea, fever, con- / Fig. 86. Fig. 86. Left side of lower jaw, at the age of five years, having the bony substance partly removed, to show the second set of teeth, forming be- neath the temporary or milk teeth. t, temporary incisors, c, temporary canine, m, first and second milk molar, and first permanent molar. F, permanent incisors, forming in recesses or sacs witliin the jaw, below the milk incisors, o', permanent canine, if, permanent bicuspids, com- mencing below the two milk molars, which they replace. Hi', second permanent molar, rising beliind the first, which is already through the gum. Above and beliind this, is tiie sac of the wisdom tooth, or third permanent molar. vulsions, or paralysis. Lancing the gums of children, affords relief in two ways; it removes the tension of the inflamed gums, and also leads to the formation of a yielding and easily absorbed cicatrix, in place of the firmer tissue of the gums. The cutting of the ten anterior permanent teeth, is unattended by pain, for the crown of each, passes through an opening in the gum, left by the shedding of a milk tooth ; but the cutting of the permanent molar teeth, which have no precursory tern- THE PERMANENT TEETH. 21 porary teeth, is usually a painful process, more particularly the cutting of the wisdom teeth, the jaw and gums being frequently so cramped, that the tooth has not sufficient room to rise. At about the age of five years, immediately before the shed- ding of any of the milk teeth, the jaw bone contains more teeth than at any other period of life ; for, besides the milk teeth, all the permanent ones, except the wisdom teeth, are found in an advanced stage of growth embedded in the bone (see fig. 86, and description). The rudiments of the wisdom teeth first appear about the sixth year. The order and date of the eruption of the permanent teeth, in the lower jaw, are expressed in years, in the annexed diagram ; the corresponding teeth in the upper jaw appear usually, in each case, somewhat later. I I ^ 5-7 ^ c 7-9 B 9-12 ® ® 8-10 ® 10-12 . M 5_7 M ■ 12-14 “ 17-25 In accordance with the increased number and size of the permanent teeth, contemporaneous alterations take place in both jaws. In youth, the alveolar border is almost semi- circular, but in the adult, semi-elliptical ; it is, of course, shallow in the child, and deeper and broader in the adult ; its hinder part especially, enlarges for the accommodation of the permanent molars. At first, the wisdom teeth of the upper jaw, lie behind and above the second molars; in the lower jaw, these teeth are embedded in the base of the coronoid processes, but descend to their proper position, as the jaw elongates. In the infant, the angle formed behind by the lower jaw, is very obtuse ; in the adult, it is nearly a right angle ; but in old age, when the teeth have fallen out, it again becomes more SPECIAL PHYSIOLOGY. obtuse. The obtuseness of this angle, favours the approxima- tion of the edges of the jaws in the absence of teeth, both in infancy and old age. The use of the incisor teeth is to seize and divide, like scissors, the .softer portions of the food. The pointed canine teeth, stronger, and situated at the sides of the dental arches, also cut or pierce the Ibod ; whilst the bicuspids, and especially the molars, or grinders, are employed in bruising, crushing, triturating, and grinding it. The harder parts of our food are broken by the lateral, or posterior, teeth only. To ac- complish these purposes, the lower jaw is made moveable upon the upper one, which has no movement, except in conjunc- tion with the skull itself. By two projections placed at the summit of its back part, named condyles, the lower jaw ar- ticulates with the hinder part of two depressions in the temporal bones, named the glenoid fossce. The condyles of the lower jaw are flattened before and behind, and widened trans- versely; their long diameters are, hotv'ever, not quite transverse, but are inclined backwards and inwards, so that lines passing ihrough them, would meet at a point further back in the skull. Each condyle has a loose hinge and gliding movement, in the corresponding glenoid fossa ; but the two together form a firm hinge-joint, admitting also of movements, in which both condyles glide a little forwards and backwards, out of, and into, the fossa;. Moreover, when this motion is limited to one condyle, the lower jaw and teeth move sideways imder the upper ones, to the right hand or to the left, the point of the chin being carried in the same direction. For the better adaptation of the articular surfaces, and the greater security of the joint, a biconcave inter-articidar cartilage, thin or per- forated at its centre, and thicker at its margins, is inteqmsed between the condyle and the glenoid fossa, and is canned ■with the condyle, in all the movements of the jaw, especially in the backward and forward movements, in the lateral move- ments, and in extreme depression of the jaw, as in yawning, 'fhis latter motion is checked by the pterygo-maxillary liga- ment. Owing to the slight sliding movement of the cartilage, the axis of motion of the lower jaw is not at the joint, but a little below it, in a line Avith the grinding sm faces of the teeth. The force employed in moving the lower upon the upper jaw, is muscular, and the agents immediately concerned, are the muscles of mastication. In opening the mouth, the lower THE MUSCLES OF MASTICATION. 2.3 jaw partly descends by its own weight; but it is also drawn downwards by that portion of the digastric muscle, rvhich ascends from the sides of the hyoid bone, and is inserted into the hinder surface of the front part of the lower jaw. The platysma myoides, a muscle of the neck, may also assist in drawing the jaw down, and so likewise do the genio-hyoid and mylo-hyoid muscles, which ascend to it from the hyoid bone, this bone being fixed by the sterno-hyoid and omo- liyoid muscles, which ascend to it from the sternum and the scapulae, and also by the stylo-hyoids and the hinder portion of the digastrics, which descend to it from the styloid pro- cesses, and the inner part of the mastoid processes of the tem- poral bone. The external pterygoid muscles also draw the jaw forwards, and so aid in its opening. The closure of the ja^v is accomplished by muscular effort only, the muscles concerned being the most powerful of those of the head and face. The chief of these are the temporal muscles, which descend from the temporal fossae at the sides of the skull ; each arises from the fr-ontal, parietal, temporal, and sphenoid bones, passes beneath the zygomatic arch, and is attached to the so-called coronoid process, at the upper and anterior end of the ascending part of the lower jaw, about an inch and a half in fi'ont of the condyle or joint. The leverage with which these muscles act, is greater than if they had been attached nearer to the condyles ; their action is like that of a lever of the third order, in which strength is, to a certain extent, sacrificed to rapidity of motion. Another muscle of mastication, on each side, is the masseter, a very thick and powerful muscle, which descends from the lower border of the zygomatic arcli and neighbouring part of the malar bone, and is inserted into the outer surface of the lower jaw, near its angle, both on its ascending and horizontal joart. Each of these muscles consists of a superficial part, the fibres of which are directed downwards and backwards, and of a deep part, the fibres of Avhich descend obliquely forwards; whilst, there- fore, the Avhole muscle closes the jaw, the superficial part can draw this bone a little forwards, and the deeper part, slightly backwards. On the inner side of each ascending portion of the jaw, between it and the cavities of the mouth and pharynx, are two other strong mu.scles, named the external and internal pterygoids, which proceed from the so-called pterygoid pi-o- cesses of the sphenoid bones, and from the palate bones, and pass, the external one horizontally bticktvards and outwards, 24 SPECIAL PHYSIOLOGY. to the inner surface of the neck of the condyle of the lower jaw, and the internal one, obliquely backwards and downwards, to the inner surface of the ascending part and angle of the jaw. The latter muscles, on each side, co-operate with the temporals and masseters, in raising the jaw, and assist a little in drawing the bone forwards ; but the external pterygoids are the muscles chiefly concerned in executing this latter movement, as in protruding the chin. The backward movement is accomplished by aid of the posterior fibres of the temporal, and by the internal pterygoids. The external pterygoid of one side, causes the lateral motion of the bone upon its opposite condyle, and the lateral movement of the chin over to the other side. To accomplish the forward gliding movement of the interarticular cartilage, and, at the same time, to withdraw the two synovial membranes, situated above and below it. from the risk of pressure, certain fibres of the external ptery- goid muscle are fixed to the anterior edge of the interarticular cartilage, and also to both synovial membranes. The move- tnents of the masticatory muscles accelerate the flow of saliva and mucus into the mouth. The chief movement, employed in dividing or lacerating soft food, is a direct ascent of the lower jaw, accomplished by the temporal, masseter, and internal pterygoid muscles. In crushing harder food, or in the bad practice of cracking nuts with the teeth, the same movement occurs, the substance being placed far back between the molar teeth, not only because these teeth are broader and stronger than the rest, but because the muscular force is used with greater effect, the nearer to the fulcrum it is exerted. The advantage of having the molar teeth in the part of the jaw nearest to the fulcrum, is obviotis. A simple uj^ward movement of the lower jaw is insufficient for the purposes of mastication ; but the necessary' bruising and trituration of the food, are accomplished by its backward and forward movements, and especially by the lateral move- ment, combined with a slight backward and forward action, which cause a rotatory or grinding motion of the lower teeth upon the upper ones. IMastication is extremely important in the case of all solid, firm, or fibrous food, as well as of that which is hard and dry, preparing it, by comminution, for the action of the digestive ilirids; when it is hurriedly or imperfectly performed, dyspepsia often ensues. In the act of mastication, the saliva plays an important THE MUSCLES OF THE TOHGUE. 25 mechanical part, as, indeed, it also does in the movements of the tongue in speech. Poured into the mouth at various jioints, especially from the inner side of the cheeks near the molar teeth, it not only lubricates the mucous membrane, thus facilitating the requisite and constant motion of the food in the mouth, and moistens the teeth, so as to prevent the adhesion of the food by the clogging of their grinding surfaces, but, mixed with the food, it materially assists in softening it, and converting it into a pulpy mass, lit to pass down through the membranous gullet. In mastication, the food is also mixed with a small quantity of air. It has been observed that in the mastication of dry food, such as crusts or biscuits, a larger quantity of saliva is, for a time, secreted than in the case of softer food ; this is probably, in part at least, due to the more vigorous action of the muscles of mastication, excit- ing a general determination of vascular and nervous energy to the part's. It was found by Bernard, in experiments made by opening the oesophagus of a horse, that the mass of food swallowed, was usually mixed with about ten times its weight of saliva ; when the Whartonian ducts were tied, mastication was performed much more slowly, and the food mass, taken from the oesophagus, was drier, though covered with mucus, and weighed only three and a half times its original weight. Certain movements, which co-operate in mastication, are performed, within the dental arches, by the tongue, and on the outer side of these arches, by the buccinators, or cheek muscles, which compress the cheeks. These movements serve to place, and hold, the food between the teeth, to turn it, so that fresh portions may be subjected to the pressure of the teeth, and, finally, when it is fully masticated, to push or withdraw it from between the teeth, so that it may be swallowed. The tongue also aids in crushing soft masses of food, and forming them into suitable boluses to pass into the pharynx and gullet. The tongue is a muscular organ, composed of two symme- trical halves, separated from each other by a median fibrous septum, and covered by mucous membrane and a submucous fibrous stratum. The muscles of this organ are extrinsic and intrinsic. The former pass into the tongme, at its base and under surface, and connect it with neighbouring parts ; they are four in number in each half of the tongue, viz. the h jo-glossus, the genio-hyo-glossus, the stglo-glossus, and the gmlato-glossus, so named from their respective bony attachments. A few fibres of the superior constrictor muscle of the pharynx, are 2G SrECIAL PHYSIOLOGY. also connected with the side of the tongue. The intrinsic, or proper muscles of the tongue, are the superior lonpitndinal, the inferior longitudinal or lingualis, and the transverse. The Injo-glossus is a thin quadrilateral muscle, which, aris- ing from the hyoid bone, passes upwards to the side of the tongue, to be inserted between the stylo-glossus and the lingualis. Beneath the hyo-glos.sus, is a flat triangular muscle, the genio-hyo-glossus, the apex of which arises from the inner surface of the anterior portion of the lower jaw, its base being inserted into the hyoid bone, a small portion of the pharynx, and the entire length of the under surface of the tongue. The stglo-glossus arises fr-orn the styloid process of the temporal bone, and divides into two portions on the side of the tongue, one, longitudinal, blending with the lingualis, the other, oblique, decussating with the hyo-glossus. The palato-glossus, which, as previou.sly mentioned, forms, on each side, the anterior pillar of the soft palate, passes from the soft palate to the side and upper surface of the tongue, where it joins the fibres of the stylo-glossus. Of the intrinsic muscles, the superior longitudinal muscle occupies the upper surface of the tongue, close beneath the mucous membrane, extending from its apex to the hyoid bone ; some of the fibres are longitudinal, others oblique ; many of them are branched or itndergo subdivision, and are connected, at intervals, with the submucous and glan- dular structures. The inferior longitudinal, or lingualis. muscle reaches from the apex to the base of the tongue, lying between the hyo-glossus and the genio-hvo glossus, blending anteriorly with the fibres of the stylo-glossus. Between the superior longitudinal and the lingualis, are placed the trans- verse fibres ; internally, these are connected with the median fibrous septum, aud, passing outwards, they are inserted into the dorsum and margins of the tongue, where they intersect the other muscular fibres. These transverse fibres form the greater portion of the substance of the organ ; they are inter- mixed with a considerable quantity of fat. From the varied course of its component fibres, the tongue jtossesses the power of movement in all directions. For the act of sucking, the tongue is especially important. The lips of the infant being closely applied to the breast, the tongue is drawn back, and the threatened vacuum in the mouth is filled with milk, forced in by the atmospheric pressure on the breast, as well as by the elasticity of the THE FAUCES AND PALATE. 27 distended ducts of that organ. By means of the palate, uvula, and posterior pillars of the fauces, the respiratory passages through the nose and pharynx are shut off, so that air cannot enter the mouth by that path, and, moreover, respiration is not hindered, until the act of swallnwing takes place. Drink- ing, with the lips closed on the rim of any vessel, involves a similar mechanism ; but the fluid is often allowed to enter the mouth by its gravity only. In sipping, the fluid is drawn in by an inspiratory movement; and, most commonly, the act of drinking is performed partly by sipping, and partly by pour- ing the fluid into the mouth. In drinking from a stream, the lips are protruded and submerged, and a combination of suck- ing with oral inspiration, takes place. Deglutition. Deglutition, or the act of sivallowing, is that mechanical process, by which the food is passed from the mouth, through the opening called the fauces^ into the jiliary7ix, and thence along the gullet, into the stomach. This act is usually de- scribed as consisting of three stages : — first, that in which the food is forced backwards from the mouth, through the fauces, into the pharynx ; secondly, that in which it is made to traverse the middle and lower part of the pharynx to the gullet ; and thirdly, that in which it descends along the gullet, and enters the stomach. The first stage of deglutition is performed by aid of the tongue, the hinder part of the hard imlate and the soft palate, together with the so called pillars of the fauces. The hard palate is formed by parts of the superior maxillary and palate bones, covered by periosteum and a dense mucous membrane. The soft palate descends, like an apron, from the posterior border of the hard palate, and forms the upper margin and sides of the opening, seen on looking into the mouth, called the fauces. The arched border of this opening, forming the isthmus of the fauces, presents, in the middle line above, the pendulous body, named the uvula. Two prominent ridges on each side, are called the pillars of the fauces; the anterior pillars pass down on the sides of the tongue, the posterior pillars, on the sides of the pharynx ; between the two pillars, on each side, is a depression, in which are lodged the soft, pro- jecting, oval, or almond-shaped, somewhat rugose, glandular bodies, named the amygdalce (almonds), or tonsils. These 25 SPECIAL PHYSIOLOGY. bodies present a number of follicular depressions, the sides of which are surrounded by small closed spherical sacs, analogou.s to those of the so-called Peyer’s patches in the intestines ; they have thickish walls, lined by an epithelium, and contain a tenacious greyish white secretion ; sometimes they open on the stu’face. The mucous membrane of the under surface of the soft palate, is covered with a squamous epithelium, and possesses numerous compound racemose mucous glands. The mucous membrane of the upper surface, turned towards the superior part of the pharynx, is continuous with that of the nasal Ibssse, and, near the openings of the Eustachian tubes, has a ciliated columnar epithelium. Between the two layers of mucous membrane, which join at the free border of the soft palate, are found, besides areolar tissue, bloodvessels, lym- phatics, and nerves, a number of symmetrical muscles, by means of which, the soft, pendent, valve-like palate, is rapidly moved in various directions. Thus, the palate and uvula are raised by the levator palatl, a thin sheet of mus- cular substance, which descends from the petrous part of the temporal bone and from the Eustachian tube, to the back of the soft palate ; moreover, two small auxiliary muscles descend within the uvula, constituting together the so-called aci/gos uvulce inuscle, which elevates the uvula. Descending tfom the pterygoid processes of the sphenoid bone, and from the Eusta- chian tube, on each side, is a muscle, terminating below, in a little tendon, which turns beneath the Jiamular, or hooked-like end of the pterygoid process, and so, changing its direction, spreads out towards the middle line udthin the soft palate, and itnites with its fellow of the opposite side. This muscle, acting fr'om its point of reflexion over the hamular process, tightens and spreads out the soft palate, hence its name. circinnjlexus, ov tensor jjalati. The two pillars of the fauces, on each side, likewise contain small muscles ; those Avithiu the anterior pillars, are named, from descending to the tongue, the palato-glossi muscles ; and those Avithin the posterior pillars, from passing to the sides of the pharynx, the palato-phargngei muscles. These muscles draw the soft palate doAvnAvards, and either baclcAvards or forwards, in the direction of the tongue or palate ; by their joint action on the tAvo sides, they also con- tract the apertirre of the fauces to a triangular fissure, Avhich can then be completely closed by the uvula. By the A-ariously combined actions of the surroimding muscles, the fauces can THE PKARrXX. 29 be closed, whether the palate be drawn upwards or downwards. By the approximation of the posterior pillars to the uvula, and by the simultaneous elevation of the palate, the middle part of the pharynx can be shut olf from its upper part, so that this latter, or the respiratory, portion, which communicates with the nasal fossa;, is separated from the middle part, through which the food has to descend. In the first stage of deglutition, the lower jaw is raised, the mouth is closed, and its cavity made smaller ; the mass of food, sufBciently masticated, and softened by the saliva, is placed between the tongue and the hard palate, and is then pressed backwards, by a movement of the tongue, beneath the slightly sloping soft palate, which is rendered tense by the circumfle.x muscles. The anterior pillars of the fauces are separated, to receive the mass, whilst the posterior pillars and the uvula, by being elevated and approximated in the manner just described, shut off the upper part of the pharynx and the posterior nasal openings. The tongue, becoming shorter and thicker, its pos- terior part is rendered convex, and, by means of the mylohyoid muscles, which form the muscular floor of the mouth, and also by the digastrics, stylohyoids, and thyrohyoids, is then forced rtpwards and backwards, and following the mass of food, propels it, through the fauces, into the middle portion of the pharynx ; thus is completed the first stage in the act of deglutition. The second stage of deglutition is performed through, and by, the ipharynx. This is a musculo-membranous sac, or bag, about 4^ inches in length, and wider above than below, which is suspended from the base of the skull, in front of the vertebral column, and behind the cavities of the nose, mouth, and larynx, with all of which it communicates. It is through the larynx, that the air passes to and from the lungs. On a level with the lower border of the larynx, the pharynx be- comes continuous with the oesophagus, or gullet. The pharynx, fig. 87, has seven openings leading into it. At its upper part in front, are the two posterior nares, w, or nasal openings ; at each side, are the apertures of the Eitstachian tubes, Avhich lead to the tympanic cavities of the ears ; these four openings are above the level of the soft palate. Below the soft palate, p, the pharynx opens, by the isthmus of the fauces, into the mouth ; lower down, beyond the root of the tongue, is the opening, e, into the larynx, I ; at its termination, is that leading into the oesophagus, o. The walls of the pharynx consist chiefly of three pairs of, so-called constrictor, muscles. 30 SPECIAL PHTSIOLOGT. supported by areolar tissue, and lined throughout by a mucous membrane, continuous with that of the nasal cavities, Eusta- chian tubes, mouth, larynx, and gullet. The constrictor muscles, named, from their relative positions, superior, middle, and inferior, overlap each other from below, that is, in the opposite direction to the slates of a roof, the inferior muscle being external to the middle one, and the middle one external to the upper one ; the superior muscle, which is open in front. Fig. 87. Pig. 87. Back view of the pharynx and part of the oesophagus, suspended from the base of the skull, and laid open behind, n, openings of the nasal cavities, called the posterior nares, separated by a median septum. p, soft palate, with the uvula depending from it, in the centre. Below this, the arches of the fauces, bounded by its posterior pillars : beneath this arch, is seen the back of the tongue, e, the epiglottis, or valve which protects the superior aperture of the larynx. I, the back of the larynx, seen in the opened part of the oesophagus, o, the oesophagus. t, the trachea, or windpipe. is, therefore, embraced, at its lower end, by the middle muscle, whilst this again is embraced by the inferior constrictor. Considered together, these constrictor muscles are attached, above, to the base of the skull ; in front and at the sides, to various parts of the bones of the skull and face, and also to a fibrous band passing from the styloid process of the temporal bone to the lower jaw ; still lower down, to the side of the SECOND STAGE OF DEGLUTITION. 31 tongue, to the stylohyoid ligament, and the hyoid hone ; and, lastly, to the thyroid and cricoid cartilages of the larynx. Posteriorly, the fibres of the constrictor muscles, sweeping backwards in a curved direction, meet at a raphe, or median line, along the back of the pharynx. Spreading out on each side of the pharynx, is the stylo-pharyngeus muscle, which descends from the styloid process, and also the palato- pJia7'!/iigeus, which passes down in the posterior pillar of the fauce.s. The upper portion of the pharynx, above the level of the soft palate, is exclusively respiratory, and its mucous membrane is covered with a columnar ciliated epithelium ; the middle portion, through which not only air, but food and drink pass, and the lower portion below the laryngeal aper- ture, which is devoted exclusively to the passage of food and drink, are covered with a squamous non-ciliated epithelium. Numerous simple and compound racemose mucous glands open upon the pharyngeal mucous membrane, and moisten it with their secretion. In the second stage of deglutition, the softened mass of food, forced, by the backward movement of the tongue, into the middle portion of the pharynx, is compressed, in rapid succes- sion, from above downwards, by the lower fibres of the supe- rior constrictors, and more especially by those of the middle and inferior constrictors, and thus is propelled rapkUg into the upper end of the gullet. At the same time, the upper fibres of the superior constrictors, and especially the fibres of the stylo-pharyngei muscles, draw upwards, and somewhat out- wards, the pharyngeal walls over the mass of food, as this is forced downwards, d'he super-position of the constrictors, one upon the other, from above downwards, facilitates the propulsion of the food in that direction ; moreover, the food itself meets with no obstruction from the edges of the two lower constrictors, as Avould have been the case, had the im- brication of the muscles been in the opposite direction. The second stage of deglutition is rapidly performed, because re- spiration is suspended during its occurrence. Provision must also be made, during this stage of deglutition, for the safe transit of drink and food through the pharynx into the gullet, without any drop or particle being forced uprvards into the nasal fossas, where it would excite irritation, or downwards into the larynx, whence it would descend into the windpipe, and cause coughing, difficulty of breathing, or suffocation. The posterior nares are accordingly protected by the elevation 32 SPECIAL PHYSIOLOGT. and tension of the soft palate above the middle portion of the pharynx, in the mode already described (p. 2U), so as to form an inclined plane, beneath which the food glides into the pharynx, as this ascends to receive it. At the same time, the opening into the larynx is protected by the epiglottis, a leaf- like valve, situated at the root of the tongue (vol. i. p. 250), fig. 87, e, fig. 9, e. This valve, in the ordinary condition of the parts, stands erect, with its free margin directed upwards : the larynx then communicates with the middle portion of the pharynx, and air can pass from the nose, and mouth, if that be open, to and from the windpipe and lungs. "When, how- ever, the tongue is raised, and pressed backwards at the end of the first stage of deglutition, the larynx is elevated, and the mass of food, or the portion of liquid, then swallowed, presses the previously erect epiglottis downwards and backwards, so as, together with certain folds of the mucous membrane connected with its borders, completely to close the opening into the larynx, whilst the food or drink is passing by it, into the lower portion of the pharynx. The moment the solid or fluid has thus passed down, the tongue resumes its previous position, the epiglottis is again erected by the elastic folds connecting it with the anterior part of the larynx and root of the tongue, and the air passage is once more free for the purposes of respiration. The third stage of deglutition is performed by aid of the muscular Avails of the gullet or oesophagus. This musculo- membranous tube is that portion of the alimentarv canal, which extends fi-om the pharymx down to the stomach. It measures about nine inches in length, and is the narrotvest ]-)art of the alimentary canal, being itself narrowed at its lower, but narrowest at its upper end. It descends through the loAver part of the neck and through the whole length of the thorax, and then, perforating the diaphragm, opposite the ninth dorsal vertebra, enters the abdominal cavity, and imme- diately opens into the stomach. It is supported tipon the vertebral column, being placed between the carotid jirteries, and behind the trachea, the heart, and the arch of the aorta ; beloAv the latter, it lies in the space between the two pleurte, to the right, and then in front, of the descending aorta ; it traverses the diaphragm through a special opening, named the oesophageal opening. The Avails of the oesophagus are composed of three coats, muscular, areolar, and mucous. The muscular coat consists of an external layer of longitucUnal THIRD STAGE OF DEGLUTITION. 33 fibres, and an internal layer of circular fibres ; at the tipper end of the oesophagus, these fibres are chiefly striated, and striated fibres are to be found in smaller numbers even down to its lower end ; but the great mass of the muscular coat consists of the plain, or unstriped, muscular fibres. The areolar coat is a soft distensible tunic, which supports the mucous coat. The mucous coat, reddish above, and pale below, is thick, and when the oesophagus is closed, it is thrown into numerous longitudinal phcEe ; in this state, a section across the tube presents no cavity, but, in its centre, a radiating or branching cleft, formed by the meeting of the plicated folds. The pharynx is permanently open, as far as the aperture lead- ing into the larynx, but its lower portion, and the whole length of the oesophagus, are habitually closed, their sides being always in contact, excepting when solids, fluids, or gases are passing through them ; they are examples of what are called potential cavities. When, however, any solid or fluid is passing down the oesophagus, the longitudinal plicse of its mucous coat are obliterated. This membrane is beset with papillae, and covered with a many-layered squamous epithelium, which, at the lower end of the cesophagits, at the line of junction with the stomach, abruptly changes its character, and presents a crenulated border. The mucous membrane of the oesophagus is provided, especially at its upper and lower ends, with small compound mucous glands. In the third and final stage of deglutition, the food, pressed down by the muscles of the pharynx, first distends the walls of the oesophagus, the muscular coat of which, however, speedily contracts above the morsel, and so urges it further downwards ; the part thus dilated, then contracts above the mass of food, which is thus driven on, and so, by a succession of similar acts, is propelled, in separate portions, into the stomach. This successive contraction of the muscular coat of the oesophagus, from above downwards, is called vermicular or peristaltic. The circular fibres contract, in a wave-like manner, from above downwards, and are the propulsive agents ; whilst the longitu- dinal fibres, drawing up and widening the walls of the oesopha- gus, over the sides of the morsel of food, facilitate its descent. Gravitation, though it may assist, has but little influence on, the downward movement of food or liquids. The resistance to be overcome, is slight, consisting only of the elastic pressure of the walls of the oesophagus and of the surrounding parts. Solid substances, and even fluids, are habitually swallowed by VOL. II. D .34 SPECIAL PHYSIOLOGY. the horse and other animals, against the force of gravity ; and certain clowns can perform the feat of eating and even drink- ing, whilst “ standing upon their heads.” The rate of motion of food through the msophagus, is not so rapid as that through the pharynx. Ordinarily, the movement causes only a slight sensation at the upper end of the oesophagus ; but if the morsel be too large, the act is painful, especially as the mass is passing through the diaphragmatic oesophageal opening. As the oeso- phagus receives fibres coming from the spinal accessor)' nerves, but reaching it through the pneumogastrics, division of the latter in the neck, paralyses the lower part of this tube, so that the food remains in it, and distends it. It also receives sympa- thetic nerve fibres. The three stages of deglutition are distinguished from each other in a remarkable manner, according to the mode in which they are regulated, or governed, through the nervous system. The, stage is voluntary \ we place the food between the tongue and the palate, and, by an effort of the will, pass it backwards through the fauces, into the pharynx. Even the accompanying movement of the soft palate, to shut off the nasal fosste, which is an associated movement, so deter- mined by habit as to be unconsciously perfonned, is neverthe- less a A'oluntary movement, or at least one Avhich, by trifling practice, may be voluntarily performed. The second stage is, however, Avholly involuntary and automatic, and is performed through the intervention of a reflex action, though it may be partly imitated by the Avill. No sooner has the food reached a certain part of the fauces, than it excites afferent nerves dis- tributed to that part, the impressions on the fibres of Avhich, being conveyed to a certain nervous centi-e, are reflected, through efferent fibres of other nerves, to the various and numerous muscles required to contract ; and, by the simul- taneous action of these, this stage of deglutition is rapidly penbrmed. Whilst, then, the first stage, which involves no obstacle to respiration through the nose and pharynx, is A'olun- tary and deliberate, the second stage, during Avhich respi- ration must be siispended, is involuntary and rapid, and, more- over, is not entrusted to moA-ements reqAiiring practice, habit, or attention, to ensure their perfect co-operation, but is per- formed as promptly, efficiently, and safely, the first time by the neAv-born infant, as at any after period of life. The accidental passage of food or drink into the air-passages, Avith its accom- panying inconA'eniences, incidentally proA'es the adA'antage of THE STOMACH. 35 the perfect performance of this movement. The afferent nerves concerned in this important reflex act, are those supplying the mucous membrane of the fauces and neighbouring parts of the pharynx, viz. the palatal branches of the fifth pair, and, chiefly, the pharyngeal branches of the glosso-pharyngeal and pneumo- gastric nerves ; the efferent or motor fibres are contained, sonre in the former, but mostly in the latter nerves, being, however, derived partly from the spinal accessory nerves (vol. i. p. 336). Some also belong to the hypoglossal, which governs the move- ments of the tongue, and certain muscles of the neck ; to the facial nerve, Avhich supplies the digastric and stylohyoid muscles ; and perhaps a few to the cervical spinal nerves. The reflex nervous centre is situated in the medulla oblongata, and upper part of the spinal cord. The third stage of deglutition is also entirely involuntary, and chiefly, if not wholly, reflex. Tlie afferent fibres concerned, are contained in the oesophageal branches of the pneumo-gastric nerves, and tlie efferent fibres are included in the same branches, derived partly, however, from the spinal accessory nerves. It is supposed by many, that the non-striated muscular fibres of the oesophagus, may be directly stimulated by the substances swallowed, without the intervention of any reflex nervous action. Movements of the Stomach. The stomach, figs. 13, 89, s, the dilated part of the alimentary canal, into which the oesophagus opens above, and Ifom which the small intestine leads below, is a musculo-membranous bag, of a peculiar shape, extending across the abdominal cavity, from left to right, in front of the vertebral column, just below the diaphragm and liver, immediately behind the anterior wall of the abdomen, and above the transverse colon. It is some- what pear-shaped, the wider end, fundus or cardiac end, fig. 89, 0 , being turned to the left side, and the smaller ov pyloric end, p, which ends in the small intestine, being turned to the right side. The oesophagus enters the stomach a little to the right of the cardiac end. The upper border of the stomach is concave, and is named the lesser curvature ; the lower border, convex, is called the greater curvature ; the left end of the stomach, beyond the entrance of the oesophagus, is named the great cul-de-sac, and a slightly dilated part of the convex border, towards the left end of the stomach, is called the lesser cul-de-sac. After death, the human stomach sometimes has D 2 .“56 SPECIAL PHTSIOLOGT. an honr-glass form, being constricted across its middle, or somewhat nearer its pyloric end. The stomach has two aper- tures, one named the cesopliageal or cardiac opening ; and the other the pyloric opening. It is attached, by its oe-sophageal end, to the diaphragm, and, by its pyloric end, to the back of the abdomen ; the lesser curvature is attached, by a double fold of the peritoneum, or lining membrane of the abdomen, to the under surface of the liver ; the left end, or great cul-de-sac, of the stomach, is connected, by a similar fold, with the spleen, and the greater curvature is loosely attached, by like folds, to the transverse colon. The greater curvature is the most movable part of the organ, which, when empty, is flattened on its anterior and posterior surfaces ; but, as its cavity is filled, it is tilted forwards and upwards, so that its anterior and posterior surfaces are then turned, respectively, obliquely upwards and forwards, and downwards and backwards, the oesophageal and pyloric ends remaining almost stationary. The .stomach de- scends with the diaphragm during inspiration, and ascends in expiration ; its state of distension affects the cavity of the chest, and, when over-distended, causes dyspnoea and palpitation of the heart. The capacity of the stomach is most variable, ranging from complete emptiness, with its walls in contact with each other, to a condition of full distension, in which it may hold three pints. ^Yhen moderately full, it measures 12 inches in length, by 4 in diameter. Its weight is about 44 ozs. The membranous walls of the stomach consist of four coats, viz. commencing from without, the serous, muscular, areolar, and mucous coats, all of which are held together by a more or less extensible areolar tis.sue. The serous coat, thin, trans- parent, and smooth, is a part of the peritoneal lining of the abdomen ; the anterior and posterior surfaces of the organ, are covered by distinct layers of the peritoneum, which, leaving it along its greater and lesser cummtures, become applied to each other, to form the double supporting folds named omenta, by which the stomach is held in connection with other parts. The serous coat is elastic, and thus accommodates itself to the variable state of distension of the organ, which is also facilitated by a loose interspace between the two peritoneal layers along its curvatures. The muscular coat, to which the serous coat adheres by fine areolar tissue, contains three layers of fibres, named, from their direction, longitudinal, circular, and oblique. The longitudinal fibres, which are next beneath the serous coat. THE COATS OF THE STOMACH. 37 are continuous with the longitudinal fibres of the oesophagus ; they , spread out over the stomach, being accumulated in great numbers along the lesser curvature, in smaller numbers along the greater curvature, and only thinly scattered upon the anterior and posterior surfaces of the organ. At the cnsopha- geal opening, they form the so-called stellate fibres, and, at the pylorus, they are again disposed in a uniform layer, and become continuous with the longitudinal fibres of the small intestine. The circular fibres, internal to the longitudinal ones, form thin circular fasciculi at the great cul-de-sac, and surround the whole extent of the stomach up to the pyloric end, where they are collected into a dense ring, which projects inwards, and forms an annular sphincter muscle. This projecting ring. Fig. 88. Fig. S8. Vertical section through the pyloric end of the stomach, and the curved part of the duodenum, to shosv the circular fold, or annular valve, at the pylorus, s, small part of the stomach, d, part of the duodenum. p, the pylorus, or pyloric opening of the stomach, with its annular valves, a, ends of the common bile duct, and the hepatic duct, entering the left side of the bend of the duodenum, to open internally by a common orifice. Much reduced in size. covered, on its interior, by the mucous membrane, constitutes the pylorus or pyloric valve (ttuAjj, a gate), fig. 88, p, the muscular fibres of which can partially, or completely, close the pyloric aperture of the stomach. The oblique muscular fibres do not, like the longitudinal and circular set, to which they are internal, extend over all parts of the stomach ; from around the CBSophageal opening, where they are continuous with the circular fibres of the oesophagus, and form a sort of sphincter, they may be followed for a short distance on the great cul-de-sac of the stomach, spreading obliquely down- wards on its anterior and posterior surfaces. The muscular .3 SPECIAL PHYSIOLOGY. fibres of the stomach are pale, and, for the most part, non- striated, though a few, in the longitudinal layer, present traces of indistinct stria. The areolar coat of the stomach, sometimes called, from its position, the submucous coat, consists of dense areolar tissue, containing some fatty tissue, and a delicate layer of imstriped muscular fibres. It supports the mucous coat, and, like it, is of greater extent, and less expansible, than the muscular and serous coats ; with the muscular coat, it is connected by very loose areolar tissue, so that in the empty condition of the stomach, it is thrown, together with the mucous membrane, into numerous irregular, but chiefly longitudinal, folds, called rugae. The bloodvessels, lymphatics, and nerves, belonging to the raucous coat, subdivide in the areolar coat, before they enter the mucous membrane. From the number of vessels in it, the areolar tunic was formerly named the vascular coat, and from its white colour, the nervous coat ; both terms, however, are objectionable. Its muscular fibres are sup- posed to assist, by their contraction, in the process of absorp- tion. The iunei'most,or mucous coat of the stomach, is a soft, pulpv, smooth, membrane, of a pale straw colom-, after death, but of a pink, or bright red, hue during life, being much darker during digestion. It is habitually moistened with mucus. It adheres firmly to the areolar or submucous coat, and follows the folds or rvgce seen in the empty stomach, but which are completely obliterated, wdien this organ is distended. The mucous membrane is provided with multitudes of glands, to be hereafter described, which secrete the gastidc juice. The bloodvessels and lymphatics are numerous. The nerves of the stomach are derived, partly from the large terminal branches of the pneumogastric or vagi nerves, which are joined by the splanchnic branches of the sympathetic, and partly also by the sympathetic branches, proceeding along the ai'teries from the coeliac or solar plexus. The stomach is a dilated portion, or diverticulum, of the alimentary canal, intended for the reception and retention of successive portions of fluid, and of masticated and insalivated solid food, in order that whilst the watery and dissolved parts are absorbed, the solid substances may be subjected to the action of the gastric juice. Besides these purposes, for which it is fitted by the extensibility of its serous and muscular coats, and by the loose rugse of its less expansible submucous and mucous THE MOVEMENTS OF THE STOMACH. 39 tunics, the stomach also, by aid of its muscular fibres, im- presses peculiar movements upon the food in its interior, and urges onwards through the pylorus, into the small intestine, those portions which are sufficiently softened and digested by the gastric juice. In these movements, the longitudinal fibres shorten the stomach ; the circular fibres lessen its diameter, acting peristaltically from its cardiac onwards to its pyloric end, Avhilst the oblicjue fibres draw the sides of the organ over the alimentary mass. When the stomach is empty, the several sets of fibres contract it in every direction, some narrowing it, and others shortening it, and so reduce it to its smallest possible dimensions. The pyloric part diminishes relatively less than the cardiac portion. When, however, the stomach contains food, its internal surface is kept in close contact with this, and the different fasciculi of each layer acting consecu- tively, give rise to complicated movements in certain directions. The combined result of these, is a remarkable rotatory^ or churning, motion, which urges the food from the great cul-de-sac along the lower border of the stomach, towards the pylorus, and thence back, along the upper border to the great cul-de-sac again, and so on : such rotation is said to occupy from one to three minutes (Beaumont). In order to prevent regurgitation of the food into the oesophagus, espe- cially during effort with the abdominal muscles, the cardiac orifice is kept closed by the circular fibres of the lower end of the oesophagus, aided by the edges of the opening in the diaphragm ; the pylorus is closed by its proper muscidar ring. As the outer layer of the alimentary mass becomes digested, and converted into a pulp, it is pressed, by the peristaltic action of the circular fibres, through the pylorus, and escapes at intervals, into the duodenum. As this pulpy portion is expelled, fresh layers of the food mass are brought into con- tact with the gastric walls ; towards the end of digestion, larger quantities pass the pylorus. Whilst the pylorus per- mits the passage out of the stomach, of the pulpy products of gastric digestion, such solid substances as do not yield lo the digestive process, are not allowed to pass, apparently because they excite the conti-action of the circular pyloric mus- cular fibres. Such substances, as well as fish bones, buttons, plum stones, or other bodies accidentally swallowed, remain in the stomach for some time after the evacuation of its digestible contents ; but after a certain delay, the pylorus relaxes, and allows them also to pass into the intestinal canal. The move- 40 SPECIAL PHYSIOX.OGY. ments of the stomach are partly reflex, being excited through the pneumogastric nerves, as is shoYm by experiments on animals ; but it would also seem probable that a direct stimu- lation of its muscular fibres may co-operate. The sphincter fibres at the cardiac end, appear to be under the government of the sympathetic nerves. It is not knoum whether the con- traction of the pylorus is a reflex act. The gastric movements aid in the function of digestion, by rotating the food in the stomach, thus exposing all parts of the digesting mass to the action of the gastric fluid, and by continually removing the softer parts from the surface, and expelling them gradually through the pylorus, so that fresh portions of that surface are then exposed. The pressure exercised upon the contents of the stomach, may further assist in the process of venous absorption. It is to be observed, however, that portions of food, placed in perforated metal tubes or balls, and introduced into the stomach, are neverthe- less digested. Movements of the Intestines. The intestinal canal, fig. 89, d to r, or portion of the alimentary canal extending from the stomach downwards, is divided into a longer and narrower part, called the small in- testine, d to i, and a wider and shorter part, named the large intestine, c to r. The small intestine extends from the pylorus p, to a valvular opening leading into the large intestine, c ; it measures about 2U feet in length, and becomes somewhat, though slightlv, narrower from above downwards. This long tube lies in coils. or convolutions, occupying the middle and lower part of the abdominal cavity, and the pelvis, fig. 13. It is supported by a broad double fold of the peritoneum, named the mesentery. which is attached, by a shorter posterior margin, to the back of the abdomen, but is connected by a longer anterior margin, with the back of the small intestine, so that both it and the intestine are thrown into folds, which are capable of constant change in form and position. The layers of the mesentery are prolonged over the intestine, and form its outer or serous coat ; and between these two layers, are contained the bloodvessels, lymphatics and hunphatic glands, and the nerves of the intes- tine, all of which help to support this part. The small intestine commences on the right side of the vertebral column, beneath the right lobe of the liver, and after THE SMALL INTESTINE. 41 undergoing its numerous convolutions, terminates in the lower part of the right side of the abdomen. For purposes of Fig. 89. 0 Eig. 89. Diagram, showing the abdominal portion of the alimentary canal, its subdivisions, and the general position of these in the abdomen, s, the stomach, o, the oesophageal, or cardiac end. p, the pylorus, d, d, the duodenum, or first portion of the small intestine, curving from right to left, j, coils of the jejunum, or second part of the small intestine. i, i, coils of the ileum, or third and last part of the small intestine, c, the CECcum, or first part of the large intestine, with its vermiform appendix, co, co, co, ascending, transverse, and descending portions of the colon. /, sigmoid flexure of the colon, r, straight intestine or rectum. The small intestine is seen to occupy the middle of the abdo- men, and to be surrounded on three sides by the large intestine. description, it is said to be composed of three portions : first, of a short portion named the duodenum, d, d {duodeni, 42 SPECIAL PHYSIOLOGY. twelve), because it corresponds in length to the width of twelve lingers placed side by side ; secondly, of a longer portion named the jejunum, j { jejunus, fasting), from its being usually found empty after death ; and, lastly, of a still longer portion named, from its numerous coils or convolutions, the ileum, i (e'i\£iv, to coil). The duodenum, d, d, is about 8 or 10 inches long ; it is the widest part of the small intestine, measuring from 1-^ to inches in diameter ; it is also the most fixed part, having no mesentery, the peritoneum merely covering it in front, except near the stomach. The duodenum describes a horse-shoe like curve, the convexity of which is turned to the right; first it Fig. 90. PiR. 90. Portion of the small intestine, dissected, to show the position of its several coats, s, the outer, smooth, serous or peritoneal coat. )«, the muscular coat, composed of an outer layer of longitudinal fibres, and an inner layer of circular fibres, c, the submucous and mucous coats united together. Much reduced in size. jiscends, for about 2 inches, towards the under surface of the liver and gall-bladder ; then, it descends in front of the right kidney ; next it passes from right to left, across the second lumbar vertebra, the attachment of the diaphragm, the ascending vena cava, and the aorta, and passing slightly up- wards, joins the jejunum, opposite a line corresponding vith the superior mesenteric artery and vein. In the concavity of the curve of the duodenum, is placed the right end or head of the pancreas, which is here attached to the intestine. The common bile duct and the j)micreatic duct, open into the duodenum. The jejunum, j, forms about two-fifths, and the ileum, i, i, THE VALVCL.E CONNITENTES. 43 the remaining three-fifths of the part of the small intestine below the duodenem. The jejunum occupies the middle and left regions of the abdomen ; whilst the ileum is placed in the middle, lower, and right regions, and, occasionally, partly descends into the pelvis. The termination of the ileum in the large intestine, c, is situated in the right iliac fossa. The jejunum has thicker and dark coloured coats, and is some- what wider than the ileum, the average diameter of the former being 1^ inch, that of the latter 1 inch. The membranous walls of the small intestine are composed, like those of the stomach, of four coats ; viz. the serous, muscular, areolar, and mucous coats. The serous coat, fig. 90, Fig. 91. ? Fig. 'll. Portion of the small intestine, laid open to show the smooth internal coat or mucous membrane, which is here tlu-own into numerous transverse double folds or ridges, which are permanent. These are the valvulae conniventes, v. A patch of the so-called Peyer’s glands, or glan- dulte agminatae, or aggregatm, with its little component round sacs, is shown at p. The oblong white piece of card, partly covering the patch of Peyer, and marked with an asterisk, ’, shows the relative size of the piece of mucous membrance represented in Pig. 9S. s, derived from the peritoneum, is thin and elastic, to permit of varioits degrees of distension ; Avhilst the smoothness and moisture of its free surface, facilitate the changes of form and position of the intestinal convolutions upon each other, and upon adjacent parts. The muscular coat, m, consists, as else- Avhere, of an external layer of longitudinal, and an internal layer of circular fibres. The longitudinal layer is thinner than the circular layer, and is most distinct along the free border of the intestine ; the circular fibres are arranged more closely together. The areolar or submucous coat, c, is loosely connected with the muscular coat, but more firmly with the mucous membrane, Avhich it supports. Thin crescentic 44 SPECIAL PHYSIOLOGY. extensions of tliis areolar coat project transversely, at intervals, into the interior of nearly every part of the small intestine, and. covered, on both sides and at their edges, by the closelv adherent mucous membrane, constitute the so-called vaJrulcp conniventes, fig. 91, v. These valves may be displayed by opening the intestine, and immersing it in ■water. In a portion of intestine inflated, dried, and laid open longitudinally, they are seen as transverse crescentic folds or ridges, -wader in the middle, and tapering at either end. Each extends about half or two-thirds around the interior of the tube : the longest are about two inches in length, and one-third of an inch wide at their broadest part, but most of them are smaller: the larger and smaller ones alternate; unlike the rugae of the stomach, thev are permanent, and not obbterated by distension : they do not contain any of the circular muscular fibres, as the pyloric valve does. Thev begin in the duodenum, about one inch below the pylorus : in the lower part of the duodenum, thev are veyy lairge, and succeed each other closely : about the middle of the jejunum, they begin to get smaller and wider apart ; in the lower half of the ileum, they become less dis- tinct, and in the lowest part of that intestine, they are altogether wanting. The mucous membrane of the small in- testine. which also covers the valvulse conniventes. is specially characterised by being everywhere closely beset with an immense number of minute thread-like processes, called villi •. when immersed in Avater. these stand up and produce a flocculent appearance, resembling the pile of velvet: hence this mucous membrane has been termed villous. It also contains the intestinal glands, to be presently described, and other glands to be noticed, with the lacteals. in the section on Absorption. The nerves of the small intestine are derived immediatelv from the sympathetic system : on their finest branches in the submucous areolar tissue, are foimd multitudes of the microscopic ganglia, elsewhere noticed ( vol. i. p. 325) : others exist between the circular and longitudinal muscular layers (Meissner, Auerbach). The movements of the small intestine, depending on the contraction of its longitudinal and circular fibres, afford the most perfect example of I'ermicuZar or pertsfa/b’c movements. Thev consist, in the healthy state, of slow, successive, wave- like contractions, chieflv of the circular fibres, from the upper to the lower part of the intestine. They are noticeable in very emaciated persons during life, but are powerfully ex- THE LARGE IXTESTIXE. 45 cited by exposure of the intestines to the air, especially when the abdominal aorta has been tied ; they continue for a short time after death, and even when the intestine is removed from the body. By narrowing the small intestine, they urge gently onwards from its upper to its lower end, the pulpy mixture of the ahmentary substances and digestive juices, gently compressing these soft materials against the mucous membrane, passing them on, over the numerous valvulce con- niventes, and so undoubtedly aiding in absorption. The progressive contractions of the longitudinal fibres, open and unfold the coils of the intestine, which otherwise might arrest the progress of its contents. The peristaltic movements of the intestines are influenced, both through the cerebro-spinal and sympathetic nervous systems ; this is shoum by experiments on animals, by irrita- tion of the solar plexus, spinal cord, and brain, and also by the peculiar effects of emotions on these movements ; they are accelerated by moderate stimulation, and retarded, or arrested or inhibited, by more powerful irritations. But, as they may continue after the intestine is removed from the body, it is possible that they are usually excited, either by the direct stimulation of the muscular fibres, or else, in a reflex manner, through the intervention of the minute nervous ganglia fotmd in the submucous tissue, and in the circular and longitudinal muscular layers. The stimuli which excite these motions are, in either case, the digested food, and the various digestive fluids ; of the latter, the bile is the most stimulating, and its importance as a regulator of the action of the alimentary canal, is well known. Besides these intrinsic movements, the small intestine is acted upon jointly by the diaphragm and the abdominal muscles, which subject it to various degrees of pressure, and more or less alter its general position in the abdomen ; such movements must aid in urging onwards the contents of the intestine. It has been estimated that the time occupied in the descent of the digested food along the small intestine, is about three horns. The large intestine, fig. 89, c to r, extends from the small in- testine to the termination of the alimentary canal. It mea- sures usually about five or six feet, i.e. aboirt one-fifth of the whole length of the intestinal canal. Though much shorter than the small intestine, it is considerably wider, measuring from 1^ to 2^ inches in width, being widest at its commence- 46 SPECIAL PHYSIOLOGY. ment, and gradually narrowing as it descends. It pursues a remarkable course ; commencing in the right iliac fossa, where the small intestine opens into it, it ascends along the rieht side to the under surface of the liver, then passes across, between the umbilicus and the pit of the stomach, to the left side of the abdomen, whence it de.scends to the left iliac fossa, and, having described a double or sigmoid curve, enters the pelvis, throitgh which it passes down, supported by the sacrum and coccyx. The large intestine is more or less ai-bitrarilv divided into three parts ; the first part named the ccecum. c, with its vermiform appendix ; the second part, the colon. CO to CO, again subdivided into the ascending, transverse, descending colon, and sigmoid flexure of the colon ; and the third part, or terminal portion, named the rectum, r. The ileiun, i, enters the inner or left side of the larcre in- testine, c, a short distance above the commencement of tin- latter, which forms, below the point of entrance, a pouch- like portion, about two inches in length, constituting the ccEcum, so named because it is a Mind pouch or cul-de-sac. fig. 92, c. Projecting from the lower and back part of the cjecum, is a narrow, coiled, and tapering, tube, about 4 inches in length, and about as thick as a worm, hence named the vermiform or worm- like appendix, fig. 92, a. It commrmicates with the ctecum by an opening, protected by a membranous ridge ; its outer end is closed. It may be regarded as a part of the cacum arrested in its growth, and is the homologue of the long cacum foimd in Mammalia generally, the orang-outanc, chim- panzee, and wombat being, however, exceptions. The cacum, and the ascending, transverse, and descending colon, with its sigmoid flexure, are distinguished from the small intestine, and also from the rectum, by their peculiar saccu- lated form. The sacculi of these parts, are aivanged in three longitudinal rows, separated from each other by three inter- mediate bands. Their presence depends upon a peculiar arrangement of the ccats of the intestine. These, as in the small intestine, are four in number, viz. proceeding from without inwards, the serous, the muscular, the areolar, and the mrrcous coats. The serous ox peritoneal coat, is complete in only certain portions of the great intestine, viz. in the transverse part of the colon, the sigmoid flexure, and the upper part of the rectum ; whilst the cajcum, the ascendinc: and descending colon, and the lower part of the recttim, are THE LARGE INTESTINE. 47 closely fixed behind, and therefore receive only a partial covering from the peritoneum. The muscular coat consists, as usual, of external longitudinal and internal circular fibres. On the vermiform appendix, both sets of fibres form uniform layers. On the sacculated pouch of the cascum, and through- out the whole length of the colon, however, the longitudinal fibres, thinly scattered over the sacculi, are chiefly collected into three long bundles, which form the three longitudinal bands between the sacculi. These bands, indeed, are shorter, from end to end, by nearly one-half, than the intermediate part of the intestine, which accordingly is puckered, and projects inwards in the form of sharp crescentic ridges between the dilated parts which form the sacculi. These sacculi become smaller and more scattered on the sigmoid flexure of the colon. On the rectum, the longitudinal fibres speedily form a thick stratum, evenly distributed over the whole circumference of the intestine, so that the sacculi disappear. The circular fibres cover the whole sitrface, but are accumulated in greater numbers on the ridges between the sacculi. Upon the rectum, however, they soon form a thick and uniform layer ; the lower portion of this is particularly well developed, con- stituting the internal sphincter muscle, which constricts the lower part of the bowel, and assists the external sphincter muscle, situated beneath the skin, around the aperture of the intestine, in keeping the bowel closed. The areolar or sub- mucous coat of the large intestine, is attached loosely to the muscular coat, but more intimately to the mucous membrane ; it is saccidated, and helps to maintain the ibrm of the intes- tine ; it supports the tender mucous coat, and furnishes a stratum, in which the bloodvessels, lymphatics, and nerves, ramify. The mucous coat, unlike that of the small intestine, follows strictly the form of the intestinal canal itself ; for it is not thrown into proper folds, like the valvulte conniventes, but only follows the concentric ridges between the sacculi. Moreover, it differs from the mucous membrane of the small intestine, in being somewhat thicker and paler, and in being perfectly smooth and entirely destitute of villi. In the CEecum and colon, it is of a greyish yellow colour, but in the rectum, it is darker, thicker, more vascular, and more loosely con- nected with the muscular coat. Its glands will be presently described. The nerves belong to the sympathetic system ; in the submucous coat, their fine branches present microscopic ganglia, which are also found outside the muscular coat. The 48 SPECIAL PHTSIOLOGT. movements of the large intestine are not retarded by irritation of the splanchnic nerves. At the junction of the lower end of the ileum, fig. 92, i, with the cfficum, c, and colon, co, there is found a very per- fect valve, the ileo-ccBcal valve, or valve of Tulp or Bauhin, composed of two semi-lunar segments, having their free edges directed towards the large intestine. The end of the ileum is somewhat flattened on its upper and under aspects, and is here inserted into the left side of the large intestine. The flattened part of the small intestine, carries in, with it, the side of the large intestine, and so forms the segments of the valve. Fia. 92. Fig. 92. The csecum, and the commencement of the ascending colon, laid open in front, to show the ileo-csecal or ileo-colic valve, at the junction of the small and large intestines, e, the cul-de-sac, named the caecum, or blind intestine, a, vermiform appendix of the caecum, co, part of the ascending colon, i, A piece of the ileum, or small intestine, entering the side of the large intestine, between the e*cum and colon, by a hori- zontal transverse fissure, bounded, above and below, by the crescentic segments of the ileo-caecal or ileo-colic valve. 3Iuch reduced in size. which consist therefore of the coats of both intestines, ex- cepting, however, the longitudinal muscular fibres and the j^eritoneal tunic. If the latter be carefully divided where it jiasses from one intestine to the other, the inserted part of the small intestine, may be drafvn out from the side of the large intestine, when the two segments of the ileo-c£ecal valve disappear, and the small intestine seems to open widely into the side of the large intestine. In the natural condition, the segments of this valve are placed one above the other, and leave, between their free edges, a narrow, nearly horizontal. COXTENTS OF THE LAEGE INTESTINE. slit, leading from the small into the large intestine. Each segment contains circular muscular fibres, areolar tissue, and two layers of mucous membrane, continuous with each other at the fi’ee edge of the segment. The mucous membrane of the surface turned towards the ileum, is covered ivitli villi ; whilst that turned toivards the large intestine, is destitute of those processes. Notwithstanding the active absorption Avhich takes place along the whole length of the small intestine, its contents retain a pulpy consistence. By the peristaltic action of the circular muscular fibres, they are pressed through the slit-like opening between the segments of the ileo-ctecal indve, having j^assed which, they are received into the pouch of the ca?cum, ■which now supports their weight, whilst the lateral position of the valve relieves it from pressure. Once having passed the valve, no force exerted upon the intestinal contents, can ever return them into the small intestine, the valve-segments, owing to the elasticity and muscularity of all the parts, meet- ing closely together under every change of dimensions. Even after death, when these parts are removed from the body, -water, poured into the colon, is, owing to the closure of the valve-seg- ments, comjfietely prevented from passing into the ileum. In the cfficum. the still pulpy residue of the processes of digestion and absorption, undergoes further inspissation, perhaps also further digestion. By the combined and comparatively slow peristaltic action of the longitudinal bands between the sac- culi, and of the circular fibres spread over the sacculi them- selves, it is pressed upwards into the ascending colon, and, in like manner, onwards from sacculus to sacculus of the ascend- ing, transverse, and descending colon, and, yet more slowly, through the sigmoid flexure of the colon into the rectum, ac- quiring, by gradual absorption, as it descends, its final state of inspissation, before it is expelled from the body. Undue pres- sure, or weight, is prevented by the sigmoid curve of the intestine. The external and internal sphincters, Avhich close the rectum below, are kept contracted, in a reflex manner, by the action of the spinal cord. In defsecation, these muscles are relaxed, whilst the intestine above contracts, the action being aided by expulsive efforts on the part of the abdominal and expiratory muscles generally, the diaphragm being fixed after closure of the glottis. The fibres surrounding the cardiac opening of the stomach, must also close that aperture simultaneously. VOL. II. E 50 SPECIAL PHYSIOLOGY. Vomiting. In the ordinar}' exercise of their functions in the digestive process, the oesophagus, the stomach, and the intestinal canal, manifest, as -we have seen, movements of the so-called peri- staltic kind, due to successive wave-like contractions of their muscular walls, excited, partly through the nervous system, but also, especially in the case of the intestines, by the direct stimulation of the food upon them. But, under certain con- ditions, an undue local stimtdation of the muscular fibres, or some wider irritation, operating through the nervous svstem, excites these organs to reversed, or so-called anti-peristaltic, action, often accompanied with powerful associated move- ments of the abdominal muscles, and Avith certain peculiar states of the diaphragm and muscles of respiration generally, so producing the acts of eructation, regurgitation, retching, and vomiting. The enictation of gaseous matters, depends chiefly on the contraction of the Avails of the stomach and oesophagus, aided slightly by that of the abdominal muscles and the diaphragm. The act of vomiting is a more general, and poAverful move- ment, and often invob^es a contraction of the small intestines; but it depends essentially on a similar mechanism. Though an exceptional phenomenon, and, in disease, often a serious or fatal symptom, it is, in many instances, beneficial, relieA'ing the stomach of indigestible, iiTitating, or poisonous substances, expelling Ironi it morbid secretions, or even inducing a state of exhaustion, in some Avay favourable to ultimate recovery. liefching is unsuccessftd vomiting. Eegurgitation is performed by the same mechanism as vomiting; but its effect is limited to the expulsion of smtdl portions only of the contents of the stomach. There are persons AA'ho possess a sort of poAver of rumination, sAvalloAving their food half cheAved, and, after a time, returning it to the month, AA'here it is fully masticated, and then re-swalloAved. The actual contraction of the stomach, in vomiting, is sometimes felt ; indeed, it has been witnessed. In a man, in Avhom the entire stomach protended through a Avound of the abdomen, forcible and repeated contractions of this organ, Avere observed to continue for half- an-hour, tiU it was entirely emptied of its contents (Lepine). As a preliminary con- dition to the inverted action of the fibres of the stomach ACTION OF THE MUSCLES IN VOMITING. 51 generally, the pyloric muscular ring contracts tightly, -whilst the oblique fibres surroimding the cardiac orifice are always, and necessarily, relaxed ; otherwise the contents of the stomach could not enter the oesophagus. The ineffectual attempts to vomit, sometimes noticed, before the actual ex- pulsion of the contents of the stomach, are due to the contrac- tion of these cardiac fibres, w'hich contraction ordinarily serves to retain the contents of the stomach, during any violent effort on the part of the abdominal muscles. The relaxation of these fibres, in vomiting, is immediately followed by an anti- peristaltic action of those of the oesophagus, movements which have been observed in the horse, after the injection of tartar emetic into its veins, and have been found to continue even when the oesophagus is separated from the stomach. It has been suggested, that the upward propulsion of the contents of the stomach or intestines, and of matters rising in the oesophagus, is due to a downward or peristaltic action meeting ■with resistance, and producing a central, or so-called axial cmrent upwards (Brinton) ; but this explanation is not gene- rally adopted, and antiperistaltic movements certainly occur in animals. The influence of the abdominal muscles in vomiting, is ob- vious, and, indeed, Magendie suggested, that these muscles and the diaphragm were alone concerned in this act, the stomach being, as it were, passive, and merely compressed by the descent of the diaphragm, and the backward movement of the abdominal nmscles. This view is supported by Bedard and Budge. The administration of tartar emetic, to an animal, or its injection into the veins, was said, by Magendie, never to produce contraction of the stomach. He found that, on drawing this organ out of the abdomen, no vomiting oc- curred ; but, as soon as it was replaced in its normal situation, the action of the abdominal muscles, or the pressure of the hand, immediately produced vomiting ; even after removal of the abdominal muscles, so as to leave only the linea alba, or the tendinous sti’ucture in the middle line of the abdominal ■walls, the descent of the diaphragm, according to that observer, still emptied the stomach. Moreover, on removing the stomach, and supplying its place by a bladder attached to tlie (Esophagus, the contents of the former were forced up- Avards by the contraction of the abdominal muscles. It is, hoAvever, generally believed, that these experiments merely prove, that the abdominal muscles are powerful agents in SPECIAL PHYSIOLOGT. 52 expelling the contents of the stomach into the oesophagus, just as they assist, most materially, in the expulsion of the contents of the other hollow viscera. They do not show a completely passive condition of the stomach itself, which organ, as just stated, has been seen to be able to empty its own contents. In experiments on animals, when the abdomen is opened, the movements of the stomach are frequently so feeble and rapid, that they might escape observation. It Avas supposed by Magendie, that the diaphragm is actively concerned in A'omiting, undergoing a movement of descent ; but, the associated acts necessary to vomiting, are expiratory, and the descent of the diaphragm is an inspiratorv movement (Alarshall HaU). At the moment of vomiting, the diaphragm, though moi'e or less contracted, is certainly fixed ; for, previous to each act of v'omiting, a poAverfiil inspiratorv effort occurs, and the diaphragm of course descends ; but the glottis is then closed, and any further movement, on the part of the diaphragm, is thus prevented, so that it probably remains passive in vomiting. During vomiting, as in the second stage of deglutition, cer- tain muscles draw the soft palate across the pharynx, and prevent the Ammited substances from passing into the posterior nares ; but Avhen the abdominal muscles act A'erA* poAverfulh', these are sometimes ejected through the nose. As elseAvhere mentioned, Ammiting is a refiex act, the pneumogastric nerAms being the afferent nerves, the medulla oblongata and cord, the excitable centres, and the nerA'es of the A^arious muscles concerned, the efferent nerA'es. Sometimes it is excito-motor, and mduced by a local stimulus, applied to the interior of the stomach itself, such as indigestible food, medicines, poisons, or diseased secretions ; it may also be due to morbid irritability of this organ, ft-om inflammation, ulcer- ation, or other disease ; or the cause of imtation may be distant, as in the intestines or some other part. In certain cases, as in sickness produced by a bloAv on the eye-ball or on the shin, by strangulation of the intestine, or by a calcrdus in the kidney, the reflex act is sensori-motor, or accompanied by sensations which are abvays of a painfid kind. The nausea and vomiting caused by tickling the fauces, by dis;igreeable tastes and odours, or by sickening sights, are likeAvise sensori- motor in their character. Sea-sickness is also an example of sensori-motor A'omiting. Emotional causes may likeAA'ise excite this act. Emetic medicines, Avhich operate just as readily THE DIGESTIVE FLUIDS. 53 when injected into the veins, as wdien introduced into the stomach, probably act directly on the reflex nervous centres concerned in vomiting; but they may operate on the extremities of the afferent nerves of the stomach. These are the pneumo- gastric nerves, irritation of which causes, amongst other results, contraction of the muscles of the abdomen, and vomiting. In the vomiting, named cerebral vomiting, which occurs after con- cussion of the brain, and in certain diseases of that organ, the cause of irritation is central. In some individuals, vomiting can be performed voluntarily, this power being either natural, or else acquired by practice. It is said that the act of vomiting but seldom occurs in the horse ; and it has been attempted to explain this, by reference to the structure of the cardiac end of the stomach ; but it would seem rather to be due to the very slight susceptibility of that animal to the action of emetic medicines. THE DIGESTIVE FLUIDS. The chemical processes concerned in the function of diges- tion, consist of peculiar reactions between the food and the various secretions of the alimentary canal. The digestive fluids, which are added to, and act chemically on, the food in its progress through the alimentary canal, are as follow: — first, the fluids of the mouth, consisting of the mucus secreted by the mucous membrane and glands of that cavity, and the saliva, the product of the three pairs of salivary glands, named parotid, submaxillar i/, and sublingual glands ; secondly, the secretion of the stomach, named the gastric juice , formed by minute gastric glands, or foUicles, embedded in the mucous membrane of that organ ; thirdly, the bile secreted by the liA^er, and poured into the duodenum; fourthly, the creatic juice secreted by the pancreas, and also added to the food in the duodenum ; and lastly, the mucus and the intes- tinal juices, secreted by the mucous glands, and by the so- called tubuli, which exist in vast numbers in the mucous membrane of every part of the small and large intestines. Each of these fluids exercises a special transmutation on one or more of the proximate constituents of the food, the tendency of such changes, being to convert those constituents, from an insoluble and unabsorbable condition, into a state of solution, or into a state in Avhich they can be absorbed, that being the ultimate object of the digestive process. 54 SPECIAL PIITSIOLOGT. Sources and Composition of the Buccal Mucus and Saliva. The mucous glands of the mouth are named, according to their position, labial, buccal, molar, palatal, and lingual. These are chiefly compound racemose glands, forming rounded masses beneath the mucous membrane, and opening into the mouth by their proper ducts. At the base of the tongue, are a few simple follicles, and some follicular depressions, having little closed sacs in their walls, like the follicles of the tonsils. The tonsils themselves probably also furnish some mucous secretion. Beyond the mouth, the pharynx possesses ntmie- rous simple follicles, and its upper part, compound racemose glands. Their secretion lubricates the parts, and also the surface of the food. It may likewise aid the saliva in its chemical action. Throughotit the whole length of the oeso- phagus, and especially in a circular group around its lower end, there are also numerous compound mucous glands, which perform similar offices. Of the three pairs of salivarg glands, the parotid glands are by far the largest, weighing from 5 to 8 drachms each. They are placed one on each side of the face, between the ear (-apd, near, ouc, ihrdc, the ear) and the lower jaw, which they overlap, being there supported by their ducts and blood-vessels, and bv a strong fascia. The facial nerves pass through the glands. The principal mass of each gland occupies the position above indi- cated, and likewise penetrates amongst the muscles and vessels of this region ; but a secondary or accessory portion, soda parotidis, extends forwards along the excretory duct. This canal, named the Stenonian duct, runs forward from the gland, ot'er the masseter muscle, passes obliquely through the buccinator muscle, and, opposite the second upper molar tooth, opens by amarrow orifice into the mouth. It is about 2|- inches long, and about the diameter of a crow-quill, but its orifice is very minute. The gland itself consists of numerous compressed lobes, held together by the ramified ducts and blood-vessels, and by areolar tissue. The lobes are again divided into lobules, each of which is a minute racemose gland, the branched ducts of which, terminate in vesicles, about of an inch iu diameter, fig. 42, c., each being sur- rounded by a network of capillaries. The saliva, secreted from the blood into these vesicles, flows along the smaller branches of the ducts, into the main canal or duct of Steno, and is thence poured into the mouth at a place suitable for THE SALIVARY GLANDS. 55 moistening the dry food, and for being mixed with the alimentary mass. The submaxiUcirij glands are placed, one on each side, beneath the horizontal part of the lower jaw, attached by their ducts and blood-vessels, and supported by the cervical fascia and certain muscles. Each gland is of a roundish shape, and weighs from 2 to 2^ drachms ; its struc- ture resembles that of the parotid. Its chief duct, thinner than that of the parotid, is named the Whartonian duct, and is about 2 inches long ; it runs forwards between the muscles, beneath the sublingual gland, to the side of the frienum ol' the tongue, where it opens upon a small eminence close to the duct of the opposite side. These glands, therefore, discharge their saliva, not outside the jaws, like the parotid glands, but in- side the lower dental arch, their secretion being pressed up into the mouth by the motions of the tongue. The suhlingual glands, the smallest of the salivary glands, are somewhat almond-shaped, and weigh each about one drachm ; they form two narrow oblong ridges, about 1-^ inch long, placed, one on each side, beneath the tongue. Their structure resembles that of the other salivary glands, but, instead of having a common duct, the several lobules open into from eight to twenty ducts, named the Rivinian ducts, some of which, including one large duct named the duct of Bartholin, joiir the Whartonian duct, as it runs for a certain distance immediately beneath the gland. The saliva from the sublingual glands, flows into the mouth, beneath the tip and sides of the tongue. The mechanical flow of the saliva into the mouth, is aided by the contraction of the muscles of the tongue and jaw engaged in mastication ; on opening the mouth before a look- ing-glass, and then turning up, and stiffening, the tongue, the saliva is sometimes seen to be ejected a considerable distance, from the orifices of the Whartonian ducts. The salivary glands all receive branches from the sympa- thetic nervous system ; the parotid glands are likewise supplied by the fifth pair (its auriculo-temporal branch) ; whilst the sublingual and submaxillary glands receive nervous filaments from the chorda-tympani branches of the facial nerves. The saliva flows intermittently ; and its secretion is excited through the nervous system, by the agency of which, the quantity of this and other secretions, is chiefly regulated (Vol. L, p. 333). Thus, the presence of food, especially of dry food, in the mouth, and even the introduction of food into the stomach through a gastric fistula, stimulates the flow of saliva; 56 SrECIAL PITYSIOLOGY. salt, vinegar, pepper, and other condiments, and partic>_darly tobacco, and the root of the pellitory of Spain, have a still more powerful effect ; these furnish examples of reflex stimulation of the salivary secretion. The afferent nerves concerned, are the gustatory branches of the fifth pair, and the glossopharyn- geal nerves ; the efferent nerve-fibres are contained in the chorda-tympani branches of the facial neiwes, or in the auri- culo-temporal branches of the fifth pair. The nervous centres are the submaxillary ganglia, and the cerebro-spinal axis. Besides this, the saliva is excited to flow by ideational or other mental stimuli, such as the sight of food, or even the thought of it. The act of speaking, and also that of vomiting, are preceded by a flow of saliva. Fear diminishes or arrests it. Irritation of the fourth ventricle, and the presence of certain substances in the blood, especially of mercury, likewBe increase the flow of this secretion. The effect of mercurialization in e.xciting a flow of saliva, is specific. The mode in -which the nervous system influences the secretion of saliva, has been elucidated by the interesting experiments of M. Bernard. When the sublingual and sub- maxillary glands, exposed in an animal, are at rest, little or no saliva being formed, the veins are seen to contain a moderate quantity of dark blood. On now stimulating the glands, in a reflex manner, by the application of vinegar to the tongue, the arteries sujjplying them dilate, the flow of blood through these vessels becomes quicker, even the veins pulsate, the venous blood is of a bright red colom-, and there occurs a copious flow of watery saliva. The afferent nerves concerned in this reflex act, are obviously branches of the gustatory and glossopharyngeal nerves ; the efferent fibres are contained in the chorda tympani ; for if either this or the facial, from which it is derived, be cut, the active phenomena, above described, all gradually cease, but they are again excited by imtation of the distal ends of the divided nerves. If the facial nerve be drawn out li'om the cranial cavity, irritation of the glosso- pharjmgeal no longer increases the flow of saliva. The efferent nerves of the parotid glands, are said, by Eckhard, to proceed, not from the chorda tympani or facial, but from the auriculo- temporal branch of the fifth pair. As already stated (Yol. I., p. 333), irritation of the symjDathetic branches supplying the sublingual and submaxillarj^ glands, has an opposite effect to that of stimulating the fibres of the chorda tympani ; the secretion from the glands, then becomes scanty and thick, the THE SALITA. 57 arteries small, the flow of blood through the gland diminished and retarded, and the venous blood dark. To explain these opposite phenomena, it is assumed that the sympathetic nervoiis centres cause a contraction of the muscular coats of the smaller arteries ; whilst the cerebro-spinal centres inhibit this power, and so induce a relaxed condition of the arterial coats. The efferent effect, conveyed through the chorda tympani nerve-fibres, is therefore not motor, but of a special kind, controlling, or inhibiting, the action of the sympathetic nerve-fibres and centres. This example will suffice to illustrate the mode in which secretion generally, is believed to be influenced through the nervous system. Lri- tation of the sympathetic nerves does not alter the quality, but only lessens the quantity, of the secretion of the parotid glands. According to Eckhard, great numbers of mucous corpuscles, exhibiting intrinsic movements, like those of the Amoeba, are found in the viscid secretion of the sublingual and submaxillary glands, after irritation of their sympathetic nerves ; such corpuscles, but in smaller number, exist, as we shall see, in ordinary saliva. The chemical composition of the saliva is, according to Dr. Wright, as follows : — Water . . 98-81 . = Ptyalin, or Salivin . -18^ Fatty matter -05 1 Albumen with Soda Mucus . Yi i- Solids . -26 * = Ashes . -41 1 Loss •12-1 100 98-81 1-19 100 - The saliva, thus constituted, is a transparent watery fluid, destitute of smell ; its specific gravity varies from 1002 to 1008. Besides fine granular pai'ticles, mucous corpuscles, derived, for the most part, from the lingual and tonsillar glands, and epithelial cells detached from the mouth, the saliva con- tains the so-called salivary corpuscles, spheroidal nucleated cells, somewhat resembling the white blood corpuscles, which undergo Amoeba-like changes in form, and exhibit a molecular movement in their interior. The quantity of saliva secreted in twenty- four hours by all the glands, has been estimated at from 1 to 3 lbs. : but it differs according to the nature of the food, and the intervals between the meals. Its flow is increased by mastication. 58 SPECIAL PHYSIOLOGY. but is aiTested by the cessation of that movement. The saliva from the parotid gland, is very thin and watery, and becomes more abundant during mastication; that from the submaxillary, and especially from the sublingual gland, is more viscid, and flows more constantly, for purposes of speech. The parotid glands, when active, are said to secrete fi-om eight to ten times their own weight in one hour. When first secreted, and especially during active secretion, the sahva is alkaline ; that of the submaxillary gland is less so than that of the parotid. In fasting, the moisture of the mouth is nearly neutral, or even acid, at that time consisting probably almost entirely of mucus, ptyalin or salivin, the most important consti- tuent of the saliva, is an albuminoid substance. Of the salts, the tribasic phosphate of soda is probably the cause of the alka- linity of the secretion ; besides this, there are found chlorides of sodium and potassium, sulphate of soda, phosphates of lime and magnesia, and oxide of iron. The tartar of the teeth is formed by a deposit of these earthy salts, mixed with mucus, and the remains of bacteria or vibrios ; it contains 20 per cent, of animal matter. Urea has also been found in the fluids of the mouth, and fraces of ammonia, the results of decomposi- tion. Thus far, the salts of the healthy saliva resemble those of the blood ; but it contains a pecuhar and remarkable Siilt, named the sulphocyanide of potassium, which strikes a deep red colour, with a solution of a persalt of iron. Source and composition of the Gastric Juice. When the soft pulpy nurcous membrane of the stomach is examined under a moderate magnifying power, it presents a delicate honeycomb appearance (fig. 93), caused by numerous, shallow, hexagonal, or polygonal, depressions, named the cells or alveoli of the stomach ; near the pylorus, these measure ■j-^th of an inch in width, but elsewhere are smaller and less distinct, measuring only -j-J ^yth to ^-g-g-th of an inch. Between the alveoli, are slightly elevated ridges, upon which, especially near the pyloric end of the stomach, are minute processes, which somewhat resemble vdlli, and are more distinct in the influit. No lacteals, however, have been detected in them. At the bottom of the alveoli are clusters of minute spots (tig. 93), which are the oritices of tubular follicles. These follicles, called the gastric glands or tuhuli, secrete the gastric juice ; they are arranged, side by side, in little groups (tig. 94), perpeu- THE GASTEIC GLAKDS. .09 dicularly to the surface of the membrane, and form almost its entire substance. At the pyloric end of the stomach, where the mucous membrane is thickest, the tubuli are the longest, measuring nearly ^th of an inch in length ; towards the car- diac end, where the mucous membi'ane is thinnest, they are less thickly set, and become gradually shorter, measuring only th of an inch in length ; their average diameter is about .j^.iyth to -g-^th of an inch, the orifices, c, being somewhat naiTower. Each follicle is somewhat dilated, or flask-shaped, at its deeper or blind end ; the larger follicles are sometimes convoluted or varicose, and sacculated at the blind end, or jn Fig. S3. Minute portion of the surface of the mucous membrane of the human stomach, showing the polygonal depressions or alveoli, with the elevated ridges between them. At the bottom of the alveoli, are seen the open mouths of clusters of the tubuli of the stomach, or gastric tubuli. Jlagnified 60 diameters. (After Boyd.) Fig. 94. Perpendicular section through a small piece of the mucous mem- brane of the stomach, to show the clusters of the gastric tubuli. a, neck of a single tubule, d, dilated end or fundus, filled with glandular epi- thelial cells, c, orifices of the tubuli, at the bottom of the alveoli, m, muscular bundles of the muscular coat. (After Kolliker.) Magnified 40 times. even subdivided into two, or, sometimes, as in tlie pyloric por- tion of the stomach, into as many as six or eight short saccu- lated tubuli. These tubuli consist of extensions of the gastric mucous membrane. The upper third of each tubule, next to its orifice, is lined by columnar epithelial cells (fig. 95, a), arranged perpendicularly on the basement membrane. This epithelium is continuous with that at the bottom of the alveoli, and on the interalveolar ridges, and indeed is similar 60 SPECIAL PHYSIOLOGY. to that lining the stomach generally. In the lower two-thirds of each tubule, the epithelium' changes its character, being composed of soft, roundish, oval, or compressed nucleated cells, b, which, very much larger than the cylindrical epithelial cells, and distended with granular matter, almost or completely block up the cavity of the tubule. These soft epithelial cells are named the peptic cells, because in, or by, them, the gastric jmce, or, at least, its characteristic animal substance, called pepsin^ appears to be formed. Some of these cells are present as microscopic elements of the gastric jmce. The tubuli, Avhich are said to number about five millions, are sometimes Fig. 95. Pig. 95. Single gastric tubiilus, or peptic glanJ, more highly magnified, a, neck of the tubule, lined with columnar epithelium. 6, dilated lower end, or fundus, of the tubule, filled with oval nucleated glandular epithelial ceUs, or peptic cells. 3Iagnified 70 diameters. named the peptic glands. Tliey are surrounded by a fine capillary network; minute arteries and veins pass up and down between them, and end in a capillary plexus on tlie bottom of the alveoli, and on the interalveolar ridges. The unstriped muscular fibres found in the submucous coat, are placed immediately beneath these glands, and probably assist in expelling their secretion. Besides these proper gastric or peptic glands, there are found, especially near the pylorus, clusters of larger simple and com- pound mucous glands, which are lined throughout with cylin- drical epithelium, and are supposed to secrete gastric mucus. THE SECRETION OF THE GASTRIC JUICE. G1 In certain conditions of the stomach, especially during and after digestion, and also in irritation and inilanimation of this organ, and nearly always in the stomachs of infants, nume- rous small, milky-white, elevated spots are seen scattered over the mucous membrane. These consist of lenticular closed sacs, not opening on thesurlace; they are filled with a white, senn- fluid and finely granular substance. They resemble the closed sacs of the tonsils, and of the so-called solitary and agminated glands of the small intestine, to be hereafter noted ; like them, they are now considered to be appendages of the absorbent system. The lymphatics of the stomach form a fine net-Avork near the surface of the mucous membrane, and coarser plexuses in the submucous coat, all intimately connected together. The gastric juice, during the digestive process, or under the excitement of condiments, small stones, and other irritant bodies, exudes from every part of the mucous membrane of the stomach, Avhich then assumes a bright red hue. The secretion pouring from the tubules, oozes from the alveoli in minute drops, Avhich speedily run together, and cover the Avhole mucous membrane. This has been seen by Dr. Beaumont and others, in the case of Alexis St. Martin, a Canadian voy- ageur, the interior of Avhose stomach Avas exposed by a gim- shot injury. The condition of the stomach, and the formation of the gastric juice, as of other secretions, are influenced by the nervous system. It Avas shoAAUi, by Dr. John Eeid, that the diAUsion of both pneumogastric nerves, in the neck of a dog, in the first instance, arrested digestion ; but that, if the animal lived sufficiently long, the process might be restored ; for then, generally, the state of emaciation, Avhich folloAved the ex- periment, Avas removed, acid and partly digested food Avas A'omited, and absorption and chylification took place. This restoration of function Avas not due to reunion of the divided neiwes, for portions of the nerves Avere removed, or care AA^as taken to keep the cut ends apart. Bernard also found, that, on division of these nerves, the stomach became pale, its Avails relaxed, and the formation of gastric juice Avas instantly arrested, digestion being thus stopped. On the other hand, galvanising these nerAms increased the gastric secretion. Ac- cording to Longet, hoAAmver, the pneumogastric nerves are rather the motor nen'es of the stomach, their division, as he believes, chiefly affecting the movements of that organ ; for he found, that milk, introduced into the stomach one or two 62 SPECIAL PHTSIOLOGY. days after the operation, always became coagulated; whilst, although large portions of food were only acted upon on the surface, owing to the paralysis of the muscular libres, and the necessary absence of the churning movements of the stomach, yet small portions were actually digested. By Budge, it is believed, that the very decided effect of division of these pneuniogastric nerves on digestion, noticed by Eeid and Bernard, was owing to those nerves having been cut in the neck, so as to interfere ^vith respiration, and thus disturb the whole economy ; for, he’ observed that, on dividing them in the rabbit, close to the cardiac orifice of the stomach, no interfer- ence with the appetite, the gastric secretion, or digestion, oc- curred. Although, therefore, the secretion of the gastric juice appears to be influenced by the cerebro-spinal nervous system, through the pneumogastric nerves, it cannot be said to be dependent upon it. The effects of mental emotion, in aiTesting digestion, sufficiently prove this influence. It has been stated by Bernard, that galvanism applied to the sympathetic nerves of the stomach, causes an immediate cessation of its secretion, this effect being the reverse of what happens, when the pneumogastric nerves are so stimulated. If these two results are confirmed, they would cori-espond with those already detailed (p. 56), as to the effects of stimulation of the sympathetic nerves and the chorda tympani, on the secretion of the sublingual and submaxillary glands. Neither division of the splanchnic nerves, nor section of the pneumo- gastrics upon the stomach, that is to say, after the latter have received the fibres from the former nerves, has appeared to interfere natch, or at all, with the gastric secretion (Schiff and others) ; even the coeliac plexus, and the neighbouring ganglia, have been removed without permanent effect (Budge). It would seem impossible, however, in any such experiments, to remove, or divide, all the sjmipathetic nerves of the stomach. Finally, the influence of this part of the nervous system, on the gastric secretion, is imcertain ; and it is not yet shown that the secre- tion is either arrested by, or depends on, the sympathetic system. The quantity of the gastric juice secreted, appears to be enormous. In dogs, the daily quantity has been calculated as TTyth (Corvisart), or -j^th (Lehmann), part of the weight of the body ; the latter ratio would give 14 lbs., in a man of 140 lbs. weight, a quantity equal to rather more than 11 pints daily. That this estimate, however large, is not extreme, is THE GASTRIC JUICE. 63 shown by the fact that, in a case of gastric fistula, in a Avoman, the estimated daily quantity Avas 30^ lbs. av., the weight of her body being 116 lbs. From observations on dogs, having artificial gastric fistulse, the secretion appears to be less abun- dantly excited by mechanical, than by chemical or special irritants, such as salt or pepper ; acid food excites a less abundant fiow than food made slightly alkaline ; but alkali in the solid state, induces an abundant secretion of mucus. Too poAverful mechanical irritation has a similar effect, lessening, or arresting, the secretion of proper gastric juice, and, in both cases, vomiting, and the passage of bile into the stomach, may take place. PoAverful chemical irritants arrest digestion, and cause signs of inflammation. The effect of cold Avater, or ice, is, after first causing the gastric membrane to be pale, ultimately to increase the Aoav of blood to it, and to excite a very active secretion ; ice, in larger quantity, causes shivering, and delays digestion. A high temperature, even a small quantity of boiling water, produces collapse and death Avithin four hours, causing redness, turgescence, and ecchymosis of the mucous membrane (Bernard). Dr. Beaumont found that, on injecting into the human stomach only 2 ozs. of water at 50°, the tem- perature of this organ Avas depressed to less than 70°, and required more than half-an-hour to regain its normal standard, viz. about 100°. The specific gravity of the gastric juice, in Man, is 1002'5 ; in the dog, 1005. The quantity of solids is about '5 per cent. It is a colourless, or pale yelloAV, transparent, slightly viscid, and strongly acid fluid, having a faint smell. It resists putrefaction, and is rendered turbid on boiling. Its com- position, mixed Avith a little saliva, is as folloAvs (Schmidt) : — Water . 994-4 Pepsin, Awth other organic matter . 3-2 Salts ....... 2-2 Free hydrochloric acid •2 1000- The gastric juice of the dog, contains ten times as much free acid, and five times as much organic matter ; that of the sheep, six times as much acid, and a little more organic matter; that of the horse, is somewhat more concentrated. The small quantity of solid matter in the gastric juice, is remarkable, considering its extremely active poAvers. The pepsin, its characteristic constituent, is a neutral, albuminoid, G4 SPECIAL PHTSIOLOGT. substance, slightly soluble in water, forming, on evaporation, a greyish viscid mass, and having a strong affinity for acids. It is precipitated by tannin, acetate of lead, caustic alkalies, alum, and alcohol. The saline matters consist chiefly of alkaline and earthy chlorides and phosphates. A small amount of lactic acid exists in the gastric juice, but Avhether as a pro- duct of secretion, or of decomposition, is not certain ; by Bernard and others, it is even believed to be the special acid of the gastric juice. Acetic, butyric, and other volatile acids are certainly the result of changes in the food. The presence of free hydrochloric acid is undoubted, inasmuch as chlorine is found in the gastric juice in larger quantity than the bases wlricli could combine with it ; and moreover, this acid has been obtained by the method of dialysis, and therefore inde- pendently of chemical decomposition (Graham). Its existence affords a singular example of the liberation of a mineral acid from its strongly combined base, by an organic process in the living animal economy. The source of this acid is probably chloride of sodium, or common salt ; and the seat of its de- composition, like that of the formation of the pepsin, is pro- bably the soft glandular epithelial cells, or peptic cells ; but it has been suggested, that it may be secreted by the columnar epithelial cells of the upper part of the tubuli and gastric mucous membi’ane generally (Brinton). It is supposed bv Brlicke, that the pepsin is neutral when contained in the peptic cells, and becomes acidified only after its escape from these cells ; for the pepsin obtained from the gastric mucous membrane of the animal, after its acidity has been removed by washing, is neutral. It has also been shown by Bernard, that, Avhereas the introduction of lactate of iron, and ferrocyanide of potassium into the blood of a living animal, produces no blue colour in the blood, tissues, or secretions generally, nor even in the gasfric glands, yet the surface of the mucous membrane of the stomach is stained blue. Other parts of the body, moreover, become blue on the application of an acid. This experiment, therefore, also favours the supposition that the acid of the gastric juice is formed near, or at, the surface. It is uncertain whether the separation of the hydrochloric acid, is a direct result of an act of secretion by secreting cells, or whether it is a secondary product of a decomposition, in- duced by the action of some other intermediately formed fl-ee organic acid. The quantity of solid matter in the gastric juice, and the relative amount of organic and saline consti- THE LIVEE. 65 tuents, differ in different animals. It is universally acid, but the nature of the acid, as well as that of the organic peptic agent, may vary in certain cases, according to the species, age, and diet of the animal. When the stomach is at rest, its mucous secretion is neutral or alkaline, semi-opaque, and more viscid than the gastric juice. Source and Composition of the Bile. The liver is a solid organ of a dark reddish brown colour, measuring 10 or 12 inches from side to side, about 7 inches from front to back, and about 3 inches in thickness at its posterior margin, its anterior edge being, however, thin. Its average bulk has been differently estimated at 88 or 100 cubic inches ; its weight varies from 50 to 60 ounces. It is the largest secreting gland in the body, and, with the exception of the lungs, occupies more space tlian any other organ. It secretes the bile, the importance of which office is shown by the fact, that the liver is found in all the Vertebrate, and in most of the non -Vertebrate animals. The substance of the liver has a sp. gr. of 1050 to 1060. It has an acid re-action ; its composition, in Man, in 100 parts, is said to be as follows (Beale). The extractive matters men- tioned include the amyloid substance named glycogen, a certain quantity of sugar, with traces ofinosite, hypoxanthin, xanthoglobulin, urea, and uric acid. Water Fatty matters . 3-82'! Albumen 4-67 Extractive matters . 5-40 Alkaline salts . 1-17 Earthy salts . •33 Vessels, &c., insoluble in water 16-03-J Total solids 68-58 31-42 100 - The liver is placed in the uj)per part of the abdomen, beneath the diaphragm, reaching from back to front, and from the right side partly over into the left. Its upper surface is smooth and convex, and is adapted closely to the diaphragm. Its thick posterior border rests on the pillars of the diaphragm and on the vertebral column, being hollowed out opposite the latter, and presenting also a deep notch for the ascending vena cava. The thin anterior border is con- cealed, in the recumbent posture, by the lower ribs and their 66 SPECIAL PHYSIOLOGY. cartilages, bnt descends a little below these parts, in standing, especially during inspiration, when the diaphragm descends (see fig. 13). This border is slightly notched, a little to the left of the middle line. The right border of the liver, nearly as thick as its posterior border, descends lower than the left, and is in contact with the diaphragm ; the left border, thinner even than the anterior margin, extends upwards to the cardiac end of the stomach. The under surface, fig. 9G, is concave and very uneven, presenting various slight depressions, where it touches the stomach, the duodenum, the bend of the ascend- ing and transverse colon, the right kidney, and its supra- renal capsule ; this surface is also marked by special fossaj or fissures for the lodgment of the gall-bladder, and for the entrance and exit of bloodvessels, lymphatics, nerves, and ducts. The greater part of the surface of the liver, is covered by the peritoneum, by which its slight changes of position in the abdomen, are facilitated. At certain points, this serous mem- brane passes, in the form of folds, to the abdominal walls, and thus aids in supporting or suspending the liver. These folds constitute four of the five ligaments of the liver. The broad, suspensory, or falciform, ligament is a triangular double fold.' attached by one border to the diaphragm, and to the anterior wall of the abdomen as far as the umbilicus, and by the other, to the upper surface of the liver, as far as the notch in its anterior margin ; the remaining border is free, and extends from the notch in the liver, to the umbilicus. This latter border contains a dense fibrous cord, named the round ligament, ligamentum teres, fig. 96, a, which is formed by the remains of the umbilical vein, a structure Avhich becomes obliterated after birth. A considerable portion of the thick posterior border of the liver, is attached, by areolar tissue, to the diaphragm, and is therefore not covered by peritoneum, which, instead, passes from one part to the other, forms the so-called coronary ligament, and thus helps to suspend the liver to the diaphragm. The right and left lateral ligaments are triangular peritoneal folds, strengthened by intermediate fibrous tissue, which pass from each side of the liver to the diaphragm. The liver is described as consisting of five lobes. Thus, it is divided by the notch in its anterior margin, and by the line of attachment of the suspensory ligament to its upper surface, into a right, I, and left lobe, V, the former being quadrangular in shape, and the latter somewhat triangular, and constituting THE LITER. 67 only about one-fifth of the entire organ. A deep fissure on the under surface, also marks the limit between these lobes. On its under surface, the light lobe is further divided into the following smaller lobes : viz. the Spigelian lobe, a pyramidal Fig. 96. Tip:. 96. View of the under surface of the liver and stomach, lifted up, to show the duodenum, pancreas, and spleen, and their mutual relations, s. the under or posterior surface of the stomach, which is lifted up. 0 , the oesophagus, p, the p,yloru.s. d, the horse-shoe curve of the duo- denum. or first part of the small intestine. I, under side of tlie right lobe of the liver. V, I', under side of the left lobe, the liver being turned up. a, small piece of the round and suspensory ligament of the liver. ff, under side of the gall-bladder, ending below in the cystic duct : this is joined by the hepatic duct, formed by the union of a right and left duct, from the two lobes of the liver. The common duct, resulting from the union of the cystic and hepatic ducts, the ductus communis chole- dochus, or common bile duct, passes down, as shown by the dotted lines, behind the duodenum, to end with the pancreatic duct, also shown by dotted lines, by a common orifice, on a papilla, in the duodenum, b, the )>aiicreas, attached to the curve of the duodenum ; it is partly di'sected to show its Central duct, with its branches, the eiul of it being indicated by dotted lines, as above described, m, the spleen, attached to the left end of the stomach and pancreas: its anterior notched border is seen. The drawing indicates the dark colour of the spleen and liver, and the white Colour of the pancreas. mass situated near the. hinder border ; the caudate or tailed lobe, passing forwards from the Spigelian lobe; and, lastly, the square or quadrate lobe, placed between the gall-bladder and the line of demarcation between the right and left lobes.' F 2 68 SPECIAL PHYSIOLOGY. The liver also presents, on its under surface, five fossa or fssures, in which are contained important vessels and ducts. The longitudinal fissure passes, from before backwards, be- tween the right and left lobes, and is divided into two parts; the anterior part is named the nmhilical fissure, which con- tains, before birth the umbilical vein, but afterwards, the round ligament, the fibrous cord, left after the obliteration of that vein ; the posterior part, called the fissure of the ductus venosus, contains, before birth, the large vein so named, and subse- quently the fibrous cord remaining after its closure. Thirdly, extending nearly at right angles from the junction of the umbilical fissure with the fissure of the ductus venosus, to about the centi-e of the right lobe, is the transverse ov portal fissure, or porta, the gate, sometimes called the hilus of the liver ; through this the chief bloodvessels, lymphatics, neiwes, and ducts of the liver pass in and out (see fig. 96). The principal A'essel which enters here, is a large vein, the vena portce or portal vein. The fourth fissure is the notch on the posterior border of the liver, which joins the fissure for the ductus venosus, and lodges the ascending vena cava and the orifices of the so-called hepatic veins. The fifth fissure receives the upper side of the gall-bladder. The liver possesses three sets of bloodvessels, two convey- ing blood to it ; viz. the portal vein and the hepatic arterg, and a third set, the hepatic veins, which carry the blood fi-om it. The liver in Man, and in the Vertebrata generally, is re- markable for being supplied, partly by venous, and partly by arterial, blood, for the portal vein, contrary to the u.«ual office of a vein, conveys blood into the liver. This portal vein, fig. 97, p, is formed by the union of the veins of the abdominal organs of digestion and sanguification, excepting the Ih'er itself, viz. by those of the stomach, s, small intestine, i, large intestine, co, except the lower two-thirds of the rectum, r, of the gall-bladder, pancreas, d, and spleen, in. The veins, from these parts, unite to form the superior mesenteric and splenic veins, which join to constitute the vena portae. The venous trunk thus formed, p, is of great size, being more than half an inch in diameter. It ascends to the under surface of the liver, and entering the portal fissure, there divides into a right and left branch, for the corresponding lobes of the liver, in the substance of which it ramifies Like an artery. The hepatic ai’tery, which also conveys blood to the liver, is a branch of the cceliac axis, a short trunk given olF from the abdominal Fig. 97. Fif?. 97. Diagram to show the large vessels concerned in the so-called nortal circulation. The trunk, or body, is supposed to be divided down the middle lino, so as to show the cavity of the thorax or chest, above the arched diaphrag u, and that of the abdomen below it. In the abdomen, l, is the liver ; s, the stomach ; d, a section of the duodenum, and pan- creas ; i, the small intestine ; co, a part of the colon ; r, the rectum ; m, the lower end of the spleen; and k, the right kidney. The blood to all these parts, is supplied through arteries which are branches of the abdominal aorta, marked a. From the rectum, r, and the kidney, k, the blood is returned by veins, which end in the great ascending vein, named the ascending vena eava, marked e, which conveys the venous blood directly through the diaphragm, and into the right side of the heart, o. But the blood from the stomach, s ; spleen, m ; duodenum and pancreas, d \ small intestine, i\ and large intestine, co (excepting the rectum, r), is collected by venous branches, which end in a large venous trunk, named the vena portae, or portal vein, p, by which this venous blood is conveyed to, and distributed by branches through, the liver. From this organ, it is collected by other veins, which unite to form the hepatic veins, h, which then join the ascending vena cava, c, and so reach the right side of the heart. 70 SPECI^VL PHYSIOLOGY. aorta, a ; it enters the liver, by the side of the vena port®, at the portal fissrrre, and, like that vein, divides into a right and left branch for the corresponding lobes. The hepatic veins, which convey the blood from the liver, converge from all parts of the organ, to the notch in its posterior border, where they enter the ascending vena cava, by two or three main trunks, and thus the blood from the liver, mixed Avith that from the loAver half of the body, ascends to the heart. The liver, like all secreting glands, is proA'ided Avith ducts. These, named the hepatic ducts, form, as they issue from the gland, tAvo principal trunks, one from the right, the other from the left lobe. They emerge at the bottom of the portal fissure, Avhere the tAvo chief divisions of the portal A-ein and hepatic artery enter, and then unite to form a single duct named the hepatic duct, ductus choledochus, or hile duct. HaA’ing de- scended for about tAvo inches, this joins another duct, proceed- ing from the gall-bladder, fig. 9G, g, named the cystic duct, and so forms the ductus communis choledochus, or common hile duct. This latter duct is about three inches long, and tAvo or three lines AA'ide ; passing doA\Ti behind the duodenum, d, it reaches the left or concaA^e border of the intestine, where it comes in contact Avith the pancreas, and soon after, Avith the duct of that gland, or pancreatic duct, fig. 88, a, fig. 96. The two ducts then pass together, and obliquely, through the Avails of the duodenum, for about three-quarters of an inch, and finalljq opposite the junction of the middle and loAver parts of the duodenum, about three inches beloAv the pylorus, open upon a slight eminence of the mucous membrane, by a common and .slightly constricted orifice, provided Avith a kind of sphincter. Sometimes, hoAvever, the biliary and pancreatic ducts open separately into the duodenum. The lymphatics of the liver are either superficial or deep ; the former ramify upon its simface, the latter emerge at the ])ortal fissure. The nen-es are comparatively feAv in number; they are derh'ed chiefly from the sympathetic system, and. as usual Avith those nerves, are supported on the arteries. The pneumo-gastric nerves, especially the left, also supply a feAv branches to the liver. The right phrenic nerA-es send filaments to the peritoneal coat. Beneath the partial peritoneal invest- ment, the liA^er possesses a proper areolar coat, which covers its Avhole surface, and, at the portal fissrue, passes into the interior of the organ, and becomes continuous with a loose areolar tissue, named the capsule of Glisson, to be presently described. THE HEPATIC LOBULES. 71 The proper substance of the liver, is firm, and presents, on a section, a reddish-broAvn mottled aspect. It is composed of a multitude of compressed 2:>olyhedral masses, about the size of a pin’s head, measuring from -g^th to ^brth of an inch in diameter, named the hepatic lobules ; they cause the granular appearance of the torn surface of the liver. These little portions of gland substance, are held together by the ultimate ramifications of the bloodvessels and ducts, and also by a fine areolar tissue, occupying the inter- lobular spaces, and named the interlobular tissue, which is itself connected, on the surface of the gland, with the areolar coat. The hepatic lobules are closely arranged around cer- tain canals, which commence at the portal fissure, branch out in aU directions through the gland, becoming smaller and smaller as they proceed, and ultimately lose them- selves in tlie interlobular spaces. These are portal canals, which contain not only the branches of the portal vein, but also those of the hepatic artery, and hepatic ducts, the deep lymphatics, and the nerves. Surrounding and supporting those vessels, ducts, and nerves, is found the loose areolar tissue, named Glisson’s capsule, which, outside and beyond the portal canal, is continuous with the interlobirlar tissue. A transverse section through a portal canal, shows a roundish space in the gland-substance, occupied chiefly by a section of a portal vein, with which, however, are associated one or two branches of the hepatic artery, and hepatic duct, the whole being embedded in the capsule of Glisson; the arteries are smaller than the duct ; the canal also contains lymphatics, invisible, rmless injected, and nerves supjDorted upon the arteries ; in the smallest portal canals, the parts are not so distinct. The hepatic veins do not lie in the portal canals, but pursue a separate course through the liver, the branches of these being seen, on a section, passing along through the gland, immediately surrounded by the lobules. As the portal veins diverge from the portal fissure, Avhilst the hepatic veins converge to the posterior border of the gland, their branches cross each other ; moreover, they have very different relations to the hepatic lobules. Each minute lobule has one aspect, which is named its base, whilst its other surfaces are called its sides. The bases of all the lobules rest upon the so-called sublobular veins, which are branches of the hepatic vein, the inner surface of rvhich, as shown Avhen they are opened, is marked by the polygonal outlines of the bases of the lobules. When divided trans- 72 SPECIAL PHYSIOLOGY. versely, the lobules are polyhedral ; when cut longitudinally, they present a foliated appearance, and are seen to be sup- ported on the sublobular hepatic veins, like sessile leaves upon a leaf-stalk. The sides of the lobules are turned toAvards each other in the interlobular spaces, towards the portal canals, or to the surface of the liver. The portal veins, ramifying in the portal canals, give off branches Avhich enter the interlobular spaces, and are hence named interlobular veins-, from these, still finer branches penetrate the sides of the lobules, and end, Avithin them, in the so-called lobular venous plexus, or lobular capillary netivork. From this netAvork, proceeds a small A^ein, occupying the centre of each lobule, named the intra-lobular vein, and belonging to the hepatic venous system ; it opens by a minute orifice, situated in the middle of the base of the lobule, into the cor- responding sublobular vein. It Avill thus be seen, that the blood of the portal A'ein, is conveyed, by the portal interlobular veins, to the sides of the lobules, and tlnis reaches their internal vascidar plexus, from Avhich it is collected by the hepatic intralobular A^eins, and so passes out, at the bases of the lobides, into the sublobular hepatic veins, by Avhich it is ultimately conveyed aAvay. From the pecAiliar distribution of the branches of the portal and hepatic venous systems, in each lobule, it folloAvs that a con- gested state of either, influences the mottled coloirr of the hver in a characteristic manner. Thus Avhen the hepatic system is congested, a rather freqrtent occurrence, the centre of each lobule is dark, and the circumference paler ; Avhilst in portal congestion, Avhich is rare, and occurs chiefly in children, the centre of each lobule is pale, and the marginal part dark. From the great size of the portal vein, as compared Avith the hepatic artery, it is eAudent that the Ih'er is chiefly supphed by venous blood. But eA-en the arterial blood furnished to this organ, by the hepatic artery, appears to become venous and portal, before it reaches the plexus Avithin the lobule. The hepatic artery is a nutrient vessel, supplying the frameAvork. and not the secreting tissixe, of the liA-er ; its branches termi- nate in a capillary netAvork, in the coats of the bloodvessels and ducts, in the areolar tissue of the capsule of Glisson, the interlobrdar tissue, and the areolar coat of the hA'er; from these parts, the blood, noAv become A'enous, is beheA-ed to be returned into the smaller portal veins, and in this indirect manner only, to reach the hepatic lobules. According to this THE GALL BLADDEE. 73 view, amongst the sources of the portal blood, must be included, not only the stomach, intestinal canal, pancreas, gall-bladder, and spleen, but also the non-secreting part of the liver itself. The secreting portion of the liver is composed, in each loljule, first of the lobular capillary network or venous plexus, already mentioned, as interposed between the termination of the portal and the commencement of the hepatic venous sj'stems ; secondly, of an intermediate gland- sub stance or paren- chyma, occupying the interstices of this capillary network ; and, thirdly, of the commencements of the hepatic or biliary ducts. The gland-substance consists of roundish, or flattened, poly- hedral, nucleated cells, having a delicate cell-wall, one or two bright vesicular nuclei with nucleoli, and certain faintly yellowish, semi-fluid, amorphous, granular contents, in which are commonly found larger or smaller globules of oily matter. These very peculiar cells, are named the hepatic cells ; they vary from to xwoth of an inch in diameter. They are the true secreting gland-cells of the liver, their contents closely resembling the bile, which is secreted by them. The relations of these cells to the finest commencements of the biliary ducts, and the mode of commencement of those ducts, are difficult points for investigation. The clusters of the hepatic cells occupy the interstices of the lobular venous plexris, and, whatever may be their relation to the finest com- mencements of the ducts, or in whatever mode the bile, formed within these cells, passes into the ducts, the hepatic cells themselves lie outside the venous plexus, and this has no direct communication with the ducts. The hepatic cells, moreover, are an-anged in lines or roAvs, Avhich radiate, amongst the blood\'essels, from the centre toAvards the circum- ference of the lobule. By most anatomists, these rows of cells are said to be supported on a thin basement membrane, Avhich is continuous Avith the Avails of the commencing efferentbihary tubes or ducts, so that the liver might be regarded as a com- plex gland, having ramified anastomosing ducts (Beale and Eetzius). According to another vicAV, hoAvever, the hepatic cells are merely arranged around the network of the lobular plexus, and are unsupported by a proper basement membrane (Kolliker). The gall-bladder . — The hepatic, cystic, and common bile ducts, already described, are composed of a strong areolar coat, containing a fcAv muscular fibres, and lined by a mucous membrane covered Avith a columnar epithelium ; in the finest 74 SPECI*U. PHYSIOLOGY. ducts, the epithelium is squamous. The walls of these ducts present generally minute racemose mucous glands, the open- ings of which, are arranged in rows within the ducts. The cystic duct which leads to the gall-bladder, has, in its interior, a series of obhque crescentic projecting ridges or folds, follow- ing each other closely, so as to present the appearance of a spiral valve. The gall-hladcler, fig. 96, is a pear-shaped sac, from 3 to 4 inches long, about 1 inch across at its -widest part, and holding rather more than one fluid ounce. It is lodged in a fossa on the under surface of the liver ; its larger end or fundus, projects beneath the anterior border of the gland; whilst its narrow end or neck, directed, beneath that organ, upwards, backwards, and to the left, is continuous Avith the cystic duct. Its upper surface is attached to the liver by areolar tissue and bloodvessels; the rest is covered by the peritoneum, Avhich therefore furnishes it Avith a partial serous coat. Its proper AAmlls are composed of interlacing bands of Avhite, fibrous, and areolar tissue, intennixed with elastic fibres, and longitudinal and circular unstriped muscular fibres. Within this areolar coat, is the mucous coat, Avhich has a peculiar pitted or alveolar aspect, OAving to the presence of innumerable fine ridges, Avhich bound polygonal depressions of various size and form ; at the bottom of the largest depres- sions, there are seen, by aid of a lens, the orifices of fine recesses resembhng mucous follicles. The mucous membrane of the gall-bladder is usually of a deep yeUoAv colour, and is lined by a columnar epithelium. The gall-bladder forms a sort of receptacle, or reservoir for such bile as is not immediately required for the pirrposes of digestion. It has been shoAvn, in animals, in AA'hich artificial openings, or fistula, haAm been made into the hepatic duct, that bile is being constantly secreted by the liver. In the interA-als betAveen the process of digestion, the secretion is slow ; but, during digestion, the bile is secreted Amry rapidly, and at once passes along the hepatic duct, and common bUe duct, into the duodenum ; such bile is named hepatic bile. The period of most rapid secretion, in animals, has been variously stated to be from one or tAvo hom-s, to ten or tAvehm hours sifter eating. According to observations made by Dalton on a dog, the quantity increases suddenly after eating, reaches its maximum in an hour, and then gradually declines ; a far larger quantity enters the intestine during the first hour, than in any other THE BILE. 75 equal period. Abstinence lessens the quantity veiy much. In the intervals between digestion, hoAvever, the bile being secreted more scantily, has not sufficient force to pass through the narrow orifice of the common duct, and thus more or less of the secretion enters the gall-bladder ; there, it undergoes inspissation, losing water, and receiving much mucus from the gall-bladder, some having been already added to it, in the ducts. It thus becomes darker, and more viscid, and, in this condition, it is called cystic bile. The mechanical effect of the spiral folds in the cystic duct, on the passage of the bile into, or out of, the gall-bladder, is probably to favour its entrance, and somewhat check its escape. During digestion, both cystic and hepatic bile are believed to be employed, and it is supposed that, at that period, the former is pressed out of the gall-bladder, partly by the distended stomach, and partly by the contraction of its own muscular fibres, stimulated in a reflex manner, by the acid chyme passing over the orifice of the common bile duct, the sphincter-like margin of which may be at the same time relaxed. The analyses of bile present some discrepancies which may depend on the difference between the hepatic and the cystic bile. Speaking generally, the bile is a yelloAvish, or yellowish- green, viscid fluid, having a peculiar smell, and a bitter taste. In carnivorous animals, its colour is brownish-yellow ; in herbivorous animals, it is generally greenish. The quantity of bile secreted by a man in twenty-four hours, is uncertain. In dogs, with artificial biliary fistulas, the quantity secreted daily is about oz. to every pound weight of the animal, or -j-^nd part of its weight (Killliker, H. Muller). Supposing the weight of a man to be 140 lbs., this would give 70 ozs. or 4 lbs. 6 ozs. avoirdupois in a day, of which about ^th, or nearly 3 ozs., would be solid matter. This estimate, however, appears very high. Bidder and Schmidt calculate the daily quantity secreted by man to be 56 ozs. ; Nasse and Platner’s observa- tion on the dog, would give a total daily quantity for man of 33^ ozs. ; whilst others again have estimated it at only from 17 to 24 ozs. The specific gravity of the cystic bile in man, varies from 1026 to 1032 ; that of hepatic bile is of course less. The cystic bile of man, contains about 10 per cent, of solid matter ; while the bile, from an artificial fistula in the bile duct of an animal, i.e. hepatic bile, contains from 3 to 5 per cent. only. 76 SPECIAL PHYSIOLOGY. The analysis of cystic ox gall by Berzelius, gives the follow- percentage composition : — Water ...... . 90-44 Bilin, with fat and colouring matters . 8- I Mucus, chiefly cystic .... ■3 \ 9-56 1-26) Salts ....... 100 00 The analyses by other chemists, show a similar composition, but, according to Strecker, the bilin of Berzelius is a compound substance. Its two characteristic constituents are the colour- less conjugated fatty acids, named gli/cockolic or cholic, and tau- rocholic ; the one formed by the combination of a nitrogenous body, named ^/_ycoci«, or glycocoU, and acid ; the other, formed by the union of the same acid with another nitrogenous body, which contains sulphur, named taurin. The chemical relations of these substances may be seen, by comparing their atomic compositions (Vol. L, p. 98). Cholalic acid crystallises in white tetrahedra ; dissolved in sulphuric acid, with the addition of sugar, it yields a purple violet colour, the reaction of the so-called Pettenkofer’s test for bile. Glycocoll, obtainable also by the action of acids or alkalies upon glue and some other animal substances, forms hard, transparent, colourless, crystals, soluble in water, but nearly insoluble in alcohol and ether. Taurin crystallises in white hexagonal prisms, inodorous and almost tasteless ; it contains the large proportion of one-fourth its weight of sulphur ; it leaves much sulphurous acid on being burnt, and gives off sulphuretted hydrogen when decomposed. Both glycocoll and taurin are neutral substances, having a tendency to unite Avith acids, to form, as in the bile, conjugated acids. Glycocholic, or cholic acid, consists of fine crystalline needles, soluble in water and alcohol, but very slightly so in ether. haA’ing a bitter SAveet taste, and a strong acid reaction. Taurocholic acid has not yet been obtained in a crystalline form. In the bile, the glycocholic and taurocholic acids, Avhich form from 4 to 7 per cent, of that secretion, are ahvays united with soda, as glycocholate and taurocholate of soda. The bile, hoAveA'er, occasionally contains an e.xcess of some base ; for, though often neutral, it may be feebly alkaline. The substance of the liver has, or rapidly acquii'es after death, an acid reaction. The proportions of glycocholic and taurocholic acids, A’ary in the bile of different animals, but are tolerably constant in each species. In the dog, the glycocholic acid is scanty, and sometimes THE CONSTITUENTS OF THE BILE, 77 absent. In the pig, another allied acid is found, named lujocholic, and a small quantity of an acid analogous to the taurocholic. In the goose, a different allied acid exists, named tauro-clienolic. Although varied in different animals, and present in variable proportions, the characteristic constituent of the bile is, in all cases, a soda salt of some fatty acid, resem- bling the acids of fatty and resinous bodies. The sulphuretted and nitrogenous body, taurin, is always present. The next most characteristic constituent of the bile, is its colouring matter, named by different chemists, cholepyrrhm, hilipyrrliiv, and biliphcein. This forms about 5 per cent, of the secretion. According to Berzelius, two modifications of colouring matter exist in bile. The one, a yellowish colouring substance, was named by him hilifulvin ; it seems to coincide with the cholepyrrhin and biliphajin of other writers. It is uncrystallisable, insoluble in water, only slightly soluble or insoluble (Briicke) in alcohol, but especially so in caustic alkalies, and in chloroform. It affords a peculiar reaction with nitric acid or nitrates, Avhich, when added in small quantities to the yellow alkaline solution, first produce a green colour, then blue, violet, and red, and finally yelloAv again, oAving, it is supposed, to the occurrence of different degrees of oxidation. The other coloirring matter of the bile, smaller in quantity, is yreetii and hence Avas named by Berzelius, biUverdin ; it Avas supposed by him, though not proved so, to be identical with chlorophyll. It is insoluble in chloroform, slightly so in alcohol, and insoluble in AA'ater ; it appears to be a more highly oxi- dised form of bilifulvin. These colouring matters are closely allied to the hsematin, or cruorin of the blood ; but neither these, nor the fatty acids of the bile, pre-exist in the blood ; they are formed in the liver by the hepatic cells. In addition to these, its essential constituents, bile contains about 1 per cent, of ordinary fats, margarin and olein, or alkaline margarates and oleates. It also presents traces of cholesterin, the fatty or resinoid body, Avhich likeAvise exists in nervotrs substance, in the blood, and in certain diseased exitdations. Cholesterin crystallises in brilliant colourless plates, insoluble in Avater, soluble in boiling alcohol and in ether, and absolutely resisting saponification. In the living body, it is probably held in solution by fluid fats. The bile contains about 1 per cent, of salts, its ashes yielding, besides soda in large proportion, traces of potash, magnesia, and lime, in combination Avith phosphoric acid and chlorine. The SPECIAL PHTSIOLCGT. mucus foimd in bile, indicated by the presence of mucous and epithelial cells, is an adventitious substance, derived from the vails and follicles of the bile ducts or gall-bladder. Besides being engaged in the formation of biliary substances, partly intended for use in the digestive process, and partly destined, as we shall hereafter explain, to be thrown out of the body as excrementitious matters, the liver has recently been discovered to perform another most remarkable office in the economy, viz. that of separating from the blood by its cells, a substance named glycogen, or animal starch, which has the property of being rapidly transformed into glucose, or grape sugar. This sugar is supposed to enter the hepatic blood, to proceed with it to the heart, and thence to the lungs, to be oxidised in the resjiiratory process, and aid in the develop - ment of heat. This glycogenic or sugar-forming function of the liver, will be more fully noticed in the section on Secretion. Sources and Composition of the Pancreatic Juice. 1\\Q p>ancreas (vdi' rpeac, all flesh), or abdominal sweetbread, is a long, narrow, pinkish, gland, flattened before and behind, having its right, larger end lodged in the concavity of the duodenum; whilst its left, pointed extremity, touches the spleen. Its shape lias been compared to that of a dog’s tongue, or of a hammer. It crosses over the front of the first lumbar vertebra, behind the lower border of the stomach, and is held in place by its attachment to the duodenum, by its bloodvessels, nerves, lymphatics, and ducts, by areolar tissue, connecting it with adjacent parts, and by a peritoneal layer. It is about 6 or 8 inches long, 1-^ inch broad, and from an inch to 1 inch thick, being thicker at its larger end. It usually weighs between 2 j and 3^ ozs , but sometimes as much as G ozs. In structure, the pancreas resembles the salivary glands, and has been termed the abdominal salivaiy gland. Its numerous lobes and lobules are compressed, and are held together by the vessels, ducts, and interlobular areolar tissue. Each lobule, like those of the parotid gland, fig. 42, c, consists of a branched duct, ending in rounded vesicles, surrounded l«y networks of capillaries. The ducts from the numerous lobes, join a principal duct, which runs through the gland from left to right. This duct, the pancreatfc duct, or canal of Wirsung, who discovered it in the human body, in 1G42, is THE PANCREATIC JUICE. 79 about the size of a small quill ; it emerges from the larger end of the gland, and, accompanied by the common bile duct, passes, with it, obliquely through the walls of the duodenum, and, about 3 inches below the pylorus, opens into the intestine by a common orifice Avith the bile duct, or sometimes by a separate aperture. Occasionally there exists a supplementary pancreatic duct, Avhich enters the duodenum about an inch from the chief duct. The secretion from the pancreas, or the ’pancreatic juice, is a somewhat A'iscid, transparent, colourless, and inodorous fluid. The quantity secreted daily, in animals, varies, according to different observers, from 15 to 35 grains per hour for each pound Aveight of the body ; so that in a man Aveighing 140 pounds, the quantity secreted Avould be from 4^ ozs. to 11 ozs. per hour. The secretion is probably not continuous, and its quantity increases as digestion goes on, the activity of the process being, by some, referred to the absorption of albu- minoid substances already digested. From these fluctuations, it is impossible to estimate correctly the quantity formed daily ; AA’hich has been differently estimated at from 7 ozs. to 16^ lbs. Statements, almost as discrepant, have been made concerning the gastric jirice and bile, correct results, as regards these internal secretions, not being so attainable as in the case of the saliA'a. The collection of these fluids, by aid of artificial fistulaj, in animals, is open to the objection, that the conditions, especially of the nerves, Avhich goA-ern the quantity of the secretion, are not healthy. The total quantity of the digestiA^e fluids poured into the alimentaiy canal, after taking food, is, liOAveAmr, much greater than AA'as formerly supposed, and, in comparison AA’ith the blood circulating in the body, is A’erv great. The solid constituents of the pancreatic juice, as estimated from cases of artificial fistulaj in animals, vary from 1 '5 to 6, or even 10 per cent.; the more i-apid the secretion, the less solid matter it contains. Its most peculiar constituent is an albuminoid substance named ptancreatm, the special composi- tion of AAdiich is not yet determined. Like salivin, this sub- stance is soluble in Avater, coagulable by heat, and jirecipitable by alcohol, but may again be dissolved in Avater ; unlike albumen, it is precipitated by sulphate of magnesia. To the pancreatin, are attributed the peculiar digestive properties of the pancreatic juice, Avhich differ, in one respect most remark- ably, from those of the saliva. The pancreas, indeed, resembles so SPECIAL PHYSIOLOGY. the salivary glands anatomically, but not physiologically ; for its secretion is nuich more viscid, is coagulated by strong mineral acids, and does not contain sulpho-cyanide of potas- sium. Its salts, about "5 to 1' per cent., are chiefly chloride of sodium and phosphate of lime and magnesia. Like the salh'a, it is alkaline, but more strongly so ; as digestion proceeds, it becomes more alkaline, but less viscid and coagulable. On standing, it speedily becomes neutral and then acid ; it soon puti-efies, but may be preserr'ed for a few days, at a tempera- ture of 45° ; its properties are destroyed by a heat slightl}' above that of the body. It contains the debris of a few nucleated cells. Sources and Composition of the Intestinal Juices. The mucous membrane of the small intestine, is provided with two kinds of secreting glands, named respectively, after their discoverers, glands of Brunner and the glands, follicles, or crgpts of Lieherkiilin. The secreted products of all these glands, constitute the succus entericus. Brunner'' s glands are found in the duodenum, being most abundant near the pylorus, and disappearing lower down, very few being present at the commencement of the jejunum. They are compound racemose glands, like the buccal and labial glands, and appear to bear the same relation to the pancreas as those glands do to the salivary glands. They secrete a viscid alkaline mucus. The follicles or crypts of LieherJcuhn are found throughout the small and large intestines. They consist of multitudes of minute tubuli, closed at their deep extremities, but opening on to the sm'face of the mucous membrane, perpendicularly to which they are arranged, more or less closely together. In the small intestine, they measure from -g^j^th to ii^ch in length, and about :^^th of an inch in diameter. Their orifices are seen, fig. 98, by aid of a lens, in all parts of the small in- testine, even on the valvulas conniventes, between the villi, and also in little circlets, around the closed sacs of the so-called agmlnated glands. Their total number has been estimated at several millions. They are sometimes flask-shaped, but never subdivided, like the gastric glands; they are lined with a columnar epithehmu, fig. 99, and are suiTounded bv capillaries. They contain a transparent granular fluid, the intestinal THE INTESTINAL TDBULI AND JUICE. juice, proper-, sometimes they are distended -with opaque mucus, and desquamated epithelial cells destitute of fat. The composition of the intestinal juice, is not well known ; it pro- bably differs from ordinary muctis, and has special properties; it is colourless and viscid, and is usually described as being strongly alkaline, but, according to others, it is acid in a great part of the small intestine ; it contains from 2 to 3'5 per cent, of solid matter, in which is included an organic substance, precipitable by alcohol and resoluble in water, but forming- insoluble precipitates with metallic .salts. Attempts have been made to collect it, from animals, by liga- turing previously emptied portions of intestine, or by forming Fig. 98. Fig. 99. Eig. 98. Portion of the border of a Peyer’s patch, magnified aliout twelve diameters. It shows the minute pointed processes named the villi of the small intestine, found both on the general surface, and also on the lighter part or Peyer’s patch. On this latter, are seen the rounded or oval sacs, constituting the agminated .glands, with the villi between, not upon, them. Around the borders of these, are circlets of the orifices of the intestinal tubnli, or crypts of Lieberkuhn, others of which are seen, scattered over the general surface between the villi. (After Boehm.) Pig. 99. Diagrammatic vertical section of one sac, and a part of another, from a patch of Peyer, with the surrounding i)arts. g, the sac with its granular contents. /, one of the intestinal tubnli, crypts, or follicles of Lieberkuhn, of which three others are seen, on the other side of the sac. V, the intestinal villi, on the surface of the mucous membrane, covering the patch. in, cut ends of the circular muscular fibres ; be- neath these, the longitudinal fibres, and the serous or peritoneal cover- ing of the intestine. (After Kdlliker.) Magnified forty diameters. artificial intestinal fistulfe ; but the fluid so obtained, must differ from the normal secretion. The quantity daily secreted in Man is uncertain, but is doubtless considerable, especially after meals. VOL. II. G 82 SPECIAL PHTSIOLOGT. The tubuli or crypts of Lieberkuhn of the large intestine, are longer, wider, more numerous, and more closely arranged than those of the small intestine. The entire surface presents, when examined with a lens, a cribriform aspect, due to the numerous orifices, which, in the lower part of the intestine, are almost visible to the naked eye ; they are lined with columnar epithelium. Besides these crypts, there are found, scattered over the mucous membrane of the large intestine, small depressions, resembling saccular glands; they were formerly described as solitary glands, but they are lined with a columnar epithelium only, and are placed over certain closed sacs, exactly similar to those of the so-called solitary glands of the .stomach and small intestine, and of the agminated glands of the latter. The intestinal juice of the large intestine, resembles, so far as is known, that of the small intestine, being composed partly of mucus, but chiefly of a special secretion, which is said to be alkaline, though, in the caecum, the intestinal contents are acid. CHEMICAL PROCESSES OF DIGESTION. ACTION OF THE DIGESTIVE FLUIDS, WITH HE.\T. As already stated, the purpose of the digestive process in the animal economy, is the reduction of alimentary substances into a soluble and absorbable condition, a state of solution, or of exceedingly minute subdivision and suspension in a fluid, being an essential condition, antecedent to the absorption of any nutrient substance into the living ti.ssues. Food, as we have seen, considered chemically, consists of water, alkaline and earthy salts, and certain importairt organic proximate constituents, which are classified into non-nitrogenous and nitrogenous substances. Of these, the water, the natural medium of sohrtion or suspensioir of the solid alimentary substances, arrd likerdse the saline substances, both alkaline and earthy, which are mostly dissolved in it, correspond with the water which forms three- fourths of the soft tissues of the body, and with the rvater and salts of the blood : they are directly absorbed rvithoirt any digestive change. The organic constituents, whether non- nitr'ogenous or nitrogenous, are some of them soluble, and some insoluble in water at the temperatiu-e of the interior of the body, viz. about 102°. The soluble non-nitrogenous bodies PROBLEMS OF DIGESTION. 83 are pectin, gum, dextrin, sugars, alcohol, organic acids, and ethers. Tlie soluble nitrogenous substances are certain forms of albumen, fibrin, casein, gelatin, and chondrin ; the albuminoid principles of the digestive fluids, viz. saliviu, pepsin and pan- creatin, which are probably in a state of solution in the living body; creatin and creatinin ; cerebricacid; and thein, caffein and theobromin. Many of these also are possibly directly absorbed. The insoIuhJe organic constituents are the non-nitrogenous cellulose, starch, and fatty matters ; and the nitrogenous solid forms of albumen, syntouin, casein, fibrin, gluten and legumin, and the gelatin and chondrin-yielding tis.sues. All these, how- ever soft or minutely divided, must be dissolved, before they can be absoi’bed. They are the most abundant constituents of our food : in all kinds of bread, and biscuit, in cooked potatoes, rice, sago or tapioca, the quantity of insoluble starch is greater than that of soluble starch, gum, dextrin, or sugar ; in cooked meat, poultry, fish, and eggs, and also in cheese, the albuminoid constituents are all solidified ; the vegetable gluten and legumin are either solid, or are coagulated by cooking; and even the fiuid or finely granular casein of milk, is first precipitated or citrdled in the stomach, by the action of the acid gastric juice. Indeed, undissolved, though minute, granules of amyloid, insoluble oleoid, and solidified albuminoid substances, con- stitute the most nutritive forms of food. In a chemical sense, these substances are instable com- pounds ; they have a high atomic constitution, and are easily broken up by powerful chemical agents, by elevated tempera- tures, fermentation, or putrefaction. Nevertheless, under ordi- nary circumstances, they are insolirble in water at the heat of the body, and are decomposable, or rendered soluble, only by the action of agents and temperatures, which would be destructive to living animal tissues. Thus, starch is rendered mucilaginous only at the temperature of 160° ; it is changed into dextrin at a still more elevated temperature ; and it is convertible into a sugar, by the highly corrosive sulphuric acid. Of the fats, mar- garin and stearin become fluid only at temperatures higher than that of the body, viz. 114°, and 118° ; none of them are easily miscible with, or can be kept suspended, in minute particles, in rvatery flirids ; and to render any of these soluble in water, they must be saponified by the action of caustic alkalies, which are destructive to living tissues. The solid albuminoid prin- ciples, so far from lieing soluble even in boiling water, have their component particles knit still more firmly together, bv- G 2 84 SPECIAL PHYSIOLOGY. being boiled ; and putrefaction alone Avill dissolve them, — a condition inconsistent Avith their retention of nutritive pro- perties, and, indeed, conv'erting them into noxious products. The first problem of digestion, hoAvcA'er, is to render such substances, Avhich, in this point of vieAv, are refractory, soluble at a temperature, and by means of agents, compatible AA’ith the life and integrity of the digestiA’e organs themselves. But. secondly, starch, even Avhen dissolved, so as to form a soluble mucilage, and also albumen, AA'hen perfectly soluble as in the Avhite of egg, are too tenacious to pass readily through moist membranes, and belong to the so-called colloid bodies, Avhich have a feeble permeating poAver, in compari.son Avith the so- called crystalloid substances (Graham) ; AA-hilst oil, likeAvise, passes through moist membranes, only under considerable pressure. According!}', in the process of digestion, starch is not only dissolved, but is converted into the ciystalloid, and highly permeating substance, sugar ; albuminoid bodies are converted into a substance named albuminose, AA'hich, though not shoAvn to be crystallisable, nevertheless, permeates moist membt'anes Avith great facility ; Avhilst fatty matters are either emulsified, decomposed, or dissolved. The.se transmu- tations are daily accomplished, within the body, at its proper temperature, in modes at present only hypothetically explained, by the re.spective actions of the salivin, pepsin, pancreatin, and conjugated fatty acids, of the saliva, gastric juice, pancreas, and bile. Action of the Saliva and other Fluids of the iifouth. The salrfa, the chief fiiiid poured into the mouth, acts first, by its AA'atery basis, as a soh'ent, contributing thus also, to the perfection of the sense of taste. It dissolves saline substances, the organic acids, alcohols, and ethers, gum, sugar, and the, soluble albuminoid and gelatinoid bodies. Secondly, and most importantly, the saliva changes the starch granules, first intodex- triu, and then into soluble and crystalloid dextrose, glucose or grape-sugar, ready for absorption. Dextrin has the same atomic constilirtion as starch, CellioOg, AA'hilst grape sugar, C6H12O6, appears to be produced from it, bv the taking up of 1 atom of AA'ater ILO. No eA'olution of gas takes place, as occau's in alco- holic fermentation. The change is more rapid than fermenta- tion. On adding some salrfa to a Aveak solution of boiled starch, and immediately testing it Avith iodine, the blue colom- of ACTION OF THE GASTRIC SECRETIONS. 85 iodide of starch fails to appear ; or, on mixing saliva with a small quantity of cooked starch, already rendered blue by iodine, the colour is discharged. (Vintschgau.) These facts prove that the starch is changed ; its conversion into sugar, is shown by e.xaniination with a polariscope, or by boiling the fluid, after adding a slightly alkaline solution of tartrate of copper, when a yellowish red precipitate of o.xide of copper is thrown down, indicating the presence of grape-sugar (Tromm.er’s test). The parotid saliva is, by itself, able to convert starch into sugar ; that of the submaxillary and sublingual glands accom- plishesthe change, when combined with the mucus of the mouth, which, indeed, has, by some, been regarded as the sole agent in this transformation. A mixture of all the fluids of the mouth appears, however, to form the most active combination for this purpose. Besides the saliva and buccal fluids, the pancreatic juice possesses this jjroperty in great perfection ; but the gastric juice and the bile do not. Most animal membranes also, such as the mucous menibrane of the mouth, intestines, and even the bladder, particularly if they are in a state of commencing decomposition, exhibit this power. The constituent of the saliva, to which this peculiar power of transmutation is due, is the salivin orpti/alin, which is said to actcatalytically, or hy presence, or contact; forif this albuminoid substance be precipitated by alcohol, collected on a filter, and re-dissolved in water, it will still effect the transformation very rapidly, and will convert 2,000 times its own weight of starch into sugar. Neither dilute alcohol or acids, nor, it is said, even a boiling lieat, ai-rest altogether the action of salivin. Finally, although the action of saliva is more rapid and com- plete on cooked starch, yet grains of ra^v starch, masticated and mixed with saliva in the mouth, and then maintained at a temperature of 100°, at length break down, and are converted into sugar. The saliva has no specific action on gum, pectin, cellulose or fattymatters, unless it may, to a slight degree, emul- sify the latter, nor yet on albuminoid or gelatinoid substances. Action of the Gastric Juice, and Mucus of the Stomach. It is the gastric juice, secreted by the peptic glands, which accomplishes the act of gastric digestion ; the secretion of the racemose glands, lined with columnar epithelium, found near the pyloric end of the stomach, is supposed not to participate SPECIAL PnXSIOLOGT. in this office, but it may act in the further conversion of starch into sugar. In this stage of digestion, albuminoid and gelatinoid substances are specially acted upon, and are reduced to a pulpy mixture, containing the so-called alhmni- nose or peytone. The solid or insoluble forms, such as coagu- lated albumen, syntonin, and fibrin, are slowlv dissolved ; certain of the soluble forms, as the casein in milk, and the albumen in vegetable juices, are first precipitated, and then dissolved ; whereas fluid albumen, as the raw white of egg, remains in solution whilst it is beiiiGr converted into albumi- O nose. xUbuminose resembles the albuminoid bodies in chemi- cal composition, though differences will probably hereafter be detected in it. 'Whatever the peculiarity of the albimiinoid body, whether it be albumen, syntonin, fibrin, or casein, gluten or legumin, it is transformed into an almost identical albumi- nose. Moreover, this albuminose, or peptone, possesses pro- perties which distinguish it from the albuminoids. Thus, it is no longer coagulable by heat, nor by the action of nitric acid, though still precipitable by tannic acid, metallic salts, and strong alcohol ; it is soluble in all propoi-tions in water, so miTch so, that the act of digestion of the albuminoids, or their conversion into albuminose, has been referred hvpotheti- cally to a kind of hj'dration of the albuminoids, or a taking uj) by them, of certain atoms of water, just as the hydration of starch or dextrin, appears to be a step in their conversion into sugar. Gelatin, and the gelatin-yielding tissues, furnish a special kind of peptone, a viscid fluid, Avhich does not, accord- ing to some, gelatinize or stiffen in the cold. The tran.sforma- tion of albuminoid and gelatinoid substances- into the ultimate albumen and gelatin-peptones, is not sudden, but is charac- tei'ised by intermediate stages, in Avhich less soluble forms of these substances appear, named pavapeptones. Parapeptone is precipitated, in the form of flocculi, from the peptone, Avhen their mixed acid solution is neuti-alised by an alkali : it is insoluble in tvater, though gradually dissolved In" Aveak acid and alkaline solutions. The peptone, as already said, is highly sohrble in Avater, and precipitable by tannic acid, alcohol, and metallic salts. When a sohition of peptone is injected into the blood of an animal, it does not appear in tlie renal excretion ; but Avhen albumen, dissoh'ed in A'ery AA'eak hydrochloric acid, is employed in a similar manner, albumen is found in the urine. These facts indicate that a ti'ue metamorphosis is effected in the albimiinoid constituents AETIFICIAL DIGESTION. of food. The peptone ultimately produced, is not only freely soluble in water, but most i-eadily permeates moist animal membranes, and hence is a substance admirably fitted for ab- sorption. The gastric juice has no peptic action upon either the amylaceous or oleaginous constituents of food. The agent by which the gastric juice dissolves, or excites the solution of, albuminoid and gelatinoid substances, is the jteculiar animal substance, itself albuminoid, the yjepsi’n ; but the free acid contained in it, is also essential to the digestive process. Dilute hydrochloric, or other acid, of the strength of that present in the gastric juice, possesses by itself no digestive property, though it renders the tissues semi-trans- parent, and dissolves out earthy matter from bones. Again, pepsin alone, obtained pure by jrrecipitation from the gastric juice by means of alcohol, filtration, and re-solution in tvater, also possesses no digestive power ; nor does pure gastric juice, provided that its acid be carefully neutralised ; for small pieces of meat, or albumen, placed in such solutions, do not digest, but after a time putrefy. These, and many other, facts concerning the rapidity and results of digestion, have been established by experiments, amongst the most interesting in Physiology, on artificial diges- tion, i.e., by subjecting different substances to the action of different digestive fluids, under exactly like conditions. The temperature employed may A'ary from 96° to 102°. During natural digestion, the temperature of the stomach of Alexis 8t. Martin, Avas found to be from 100° to 101° F.; Avhilst during fasting, it Avas 98° or 99°. (Dr. F. Smith.) An artificial digestive fluid may be obtained directly from the human or animal stomach, by first exciting the How of gastric juice, and then causing vomiting ; or it may be collected Ifom artificial gastric fistulse in animals. A digestive fluid may, hoAvever, be more conA'eniently', and less cruelly, obtained from the gastric mucous membrane of the recently killed sheep, calf, ox, or pig, especially if the animal be slaughtered Avdiilst digestion is going on in the stomach. Finely cut portions, or scrapings, of the mucous membrane, are to be macerated in 20 times their Aveight of cold Avater for 24 hours, with frequent agitation of the mixture. A temperature as loAv as 50° is desirable, to preA’ent the pepsin, extracted from the membrane, from exhausting itself, more or less, in the digestion of that membrane itself. The fragments of the mu- 88 SPECIAL PHYSIULOGT. cous membrane being allowed to subside, the supernatant fluid is poured off, forming a solution of pepsin extracted from the peptic cells, but containing only a slight and insuffi- cient quantity of free acid ; for the pepsin is stored up in the pepiic cells, so that it may be extracted by water after death, whilst the acid of the gastric juice is probably secreted only when reqttired, perhaps by the columnar epithelial cells; it therefore ceases on the death of the animal. Hence the solution of pepsin as above prepared, requires an addition of hydro- chloric acid, to make it dige.st actively. Too little, or too much, acid diminishes its peptic properties. Ten minims, i.e. about 13 drops of the pure hydrochloric acid of commerce, to every ounce of the digestive fluid, is said to be the best proportion. The inefficiency of the acid, and of the solution of pepsin, septirately employed, and the powerful effect of the two together, may be thus strikingly illustrated. Three fluids are to be prepared, one, of hydrochloric acid and water, in the proportion of 13 drops to the ounce ; a second, of the above described solution of pepsin, exactly neutralised by carbonate of soda; and a third, of the same sohttion, acidified with hydro- chloric acid, in the proper proportions. In equal quantities of these fluids, contained in glass jars of the same size, are suspended the legs of fowls, or the fore-limbs of rabbits, either cooked or uncooked, one in each jar ; the jars are then placed in a Avater-bath, and maintained at a temperature ranging between 96° and 102°, for 24 hours. At the end of that period, the limb suspended in the hydrochloric acid and Avater, is fotmd to be slightly SAVollen, fiale and semi-transparent, Avhil.st the solution, itself of a yelloAvish tint, is quite clear, and free from deposit. The limb submitted to the action of the neutralized solution of pepsin, Avhich is itself slightly turbid, appears sodden, but its surface is noAvhere dissolved ; the fluid itself is darker, but not more turbid. In the acid solution of pepsin, hoAA'ever, all the soft parts of the digested limb are, as it u-ere, eaten axcay and pulpijied, or dissolved ; the tendons disappear first, then the muscles, next the ligaments, and lastly, even the bones and cartilages are more or less attacked, the slight residual mass contrasting stiongl}' Avith the. uiidissolved and SAA'ollen limbs in the other tAvo solutions; moreover, the fluid itself has a broAvnish colour, andpresents a soft flocculent or pulpy grumous sediment, several inches deep, Avhich, on the slightest agitation, mixes easily Avith the fluid above, and resembles the digested contents of the stomach, after taldng animal food. GASTRIC DIGESTION. 89 Phosphoric, sulphuric, and even nitric acid maybe employed in the artificial digestive fluid, but they are not so suitable as hydrochloric. Very strong acids, metallic salts, caustic alkalies, alum, tannin, and strong alcohol, destroy its digestive proper- ties, and so does a temperature of 120°. A strongly acid artificial gastric juice is better suited for the digestion of some sub- stances, such as coagulated albumen, the solid syntonin of cooked muscle, and legumin ; whilst fibrin is more quickly dissolved in a feebly acid juice, even 1 drop to 1 oz. of fluid. (Brlicke.) The strongly acid natuial gastric juice of the Carnivora, acts most quickly on the firmer animal albumen, but the less acid secretion of the Herbivora, most quickly on the softer vegetable gluten. The hitman gastric juice has a feebler power even than that of the herbivora ; its acidity declares itself immediately on the introduction of food into the stomach, and increases, for a time, as digestion goes on, when the less digestible food requires to be attacked ; ivhen the stomach is empty, the acidity quite disappears. Tlie ]tower of the gastric juice to dissolve animal substances, is well illustrated by the softening or digestion of the coats ot the .stomach by its own secretion, after death, often noticed both in men and animals dying whilst digestion is going on : all the coats of the stomach may be thus perlbrated ; in the human body, the effects may simulate the action of a corrosive poison. The immunity of the living gastric mucous membrane, or its power of resisting the solvent action of its own secretion, has been variously explained. According to one view, the epithelium and mucus constitute a sufficient protection ; for when the former is detached, the subjacent tissue is said to be attacked, in the living stomach, as well as after death. The ‘ vitality’ of the mucous membrane (the sum ofits vital actions), has been supposed to enable it to re.sist solution ; and this resist- ance necessarily ceases on the death of the part. A more recent view, founded on many experiments, attributes the non-solution of the living mucous membrane, to the protecting influence of the blood in the capillaries, which is supposed to maintain, so long as the circulation continues, the alkalinity ofthe tissues, a chemical condition incompatible, as we have seen, with peptic digestion. (Pavy.) The digestive action of the fluids of the living stomach was shown long ago by Spallanzani, Stevens, Tiedemann, Gmelin, and others, who induced dogs to swallow pieces of sponge 90 SPECIAL PHYSIOLOGY. fastened to strings, and afterwards withdrawing them, obtained a quantity of fresh gastric juice, which slowly dissolved food, kept in it at a temperature of 100°. But the most direct evidence of the solvent joower of gastric juice, is that obtained by Dr. Beaumont, who emjDloyed the fluid collected from the stomach of the Canadian voyageur, Alexis St. Martin. ^Vith that fluid, the process of solution was very rapid. Three drachms of boiled beef ])laced in an ounce of fluid, main- tained at a temperature of 100°, began to digest in 40 minutes ; in 60 minutes, a pulpy deposit began to form in 2 hours, the areolar tissue was digested, leaving the muscular fibres dis- connected or loosened ; in G hour.s, these M’ere nearly all digested; and in 10 hours, the meat was completely dissolved : the gastric juice, from being transparent, tvas now the colour of whey, and contained a meat-coloured sediment. Digestion was still more rapidly accomplished, when a similar piece of beef, attached to a thread, was placed in Alexis St. Martin's stomach ; for, althoitgh at the end of one hour, its condition appeared much the same as that of the piece of beef digested in the gastric fluid out of the body, at the expiration of two liours, it was completely dissolved.- From these and other experiments, it is evident, that, with the excejttion of the rapidity of the two processes, artificial and natural digestion are identical in character. The rapidity of natural, as compared with artificial digestion, may probably be explained, partly by the more powerful action of a con- tinuously fresh supply of gastric juice, and partly by the, constant removal of the outer pulpified layer of the nutrient mass, by the incessant pressure and motion of this mass by means of the muscular coats of the stomach. Artificial digestion is much accelerated by occasional agitation. The mere quantity of fluid employed in natural digestion, must also be very important. It has been shown, from experiments on the gastric juice of the dog, that 20 ozs. of fluid are needed for the digestion of 1 oz. of albumen. The daily quantity of gastric juice secreted by a man, 140 lbs. in weight, has been estimated at 14 lbs. or 11 pints imperial. A pint of saliva, which is a moderate estimate, and 2 pints of Avater consumed as beverage, would make a total of 14 pints of fluid, employed in the gastric digestion of the daily solid food; beyond the stomach, 21 pints of bile, 1-j pint of pancreatic juice, and one pint of intestinal juice are added. The total quantity of fluid employed in the digestive THE PEOPERTIES OF PEPSIN. 91 process in 24 hours, certainly exceeds the quantity of blood in the bodjq which, taken at -j^th part of the weight of the latter, would be, for a Man weighing 140 lbs., less than 11 lbs., or 9 pints. It is evident, therefore, that the large quantities of fluid, daily secreted for the purposes of diges- tion, can only be supplied by a circular movement of the same aqueous particles, in succe.ssive acts of secretion, absorption, re-secretion, and re-absoiqition. The water which leaves the blood, to form part of the digestive juices, re-enters the blood with the absorbed food, once more leaves it in the newly formed digestive juices, and is again re-absorbed, until diges- tion is complete. This continued irrigation of the food, com- bined with the activity of the freshly formed gastric juice, must greatly contribute to the rapidity of natiual digestion. That the pepsin of the gastric jiiice, is the special agent in the gastric digestion of albuminoid and gelatinoid substances, is easily shown. By evaporating the natural gastric juice, or the artificial solution of pepsin, to a viscid consistence, and adding strong alcohol to it, the pepsin is precipitated in whitish flocculi, which may then be sejDarated by filtration, from the other constituents of the gastric juice, dried at a low temperature, and preserved for months. The dried pepsin thxrs obtained, forms a firm, greyish, mass, or powder ; it is easily soluble in, or miscible with, water, and 1 grain dissolved in so large a quantity as 60,000 grains, i. e. 6^ pints of acidified water, still possesses digestive properties. Pepsin, whether dry or dissolved, as in natural or artificial gastric juice, loses its digestive power, if it be subjected to a tempe- rature a little above that of the body, for example, a heat even of 120°. It is likewise rendered inactive, by strong chemical reagents. It is remarkable that alcohol, which pre- cipitates it, and temjxorarily suspends its digestive properties, does not destroy them ; for on sufficient dilution with watei’, it is redissolved, and again becomes active. The energy of pepsin, like that of salivin, in converting starch into sugar, is catalytic. The action of contact or pressure, exhibited by both these substances, differs from that of the yeast ferment in the alcoholic fermentations, in not causing the evolution of any gas, and in not being continually reproduced. It is said, how- ever, by some, that the pepsin does not itself imdergo waste in the process of digestion ; but the power of a given quantity is certainly limited. Salivin and pepsin have been regarded as albuminoid bodies in a state of change, and cajrable of 92 SPECIAL PHASIOLOGT. inducing changes in other albuminoids, with which they are brought into contact. Putrescent albuminoid substances, as is well known, can propagate putrescent changes to fresh albu- minoid substances, and can also convert starch into sugar, or one form of sugar into another. But pepsin is not a putrescent body, nor is the peptone, produced by its action on albumen, putrefied. On the contrary, it has been shown by experiment, that fresh gastric juice, applied to putrid meat, first arrests putrefaction, removing its signs, and then digests the meat. Like fermentation and putrefaction, however, digestion is retarded by low temperatures, altogether arrested at a tem- perature of 34°, and is stopped by high temperatures, by the action of absolute alcohol, strong acids, alkalies, and metallic salts. The action of the gastric juice varies, according to the character of the food, its state of comminution or subdivision, and its condition of dryness or moisture. In order to deter- mine the time required for the solution of different nutritive substances, these have been introduced, inclosed in perforated tubes of metal or glass, into the stomachs of animals, and then have been withdrawn ; or, animals have been fed with such substances, and afterwards killed at certain intervals. The most important observations, however, are those made by Dr. Beaumont in the human subject. In the stomach of Ale.xis St. Martin, a mixed meal of animal and vegetable food, was nearly all dissolved into a pulp, within an hour ; and the stomach was completely emptied in 2-1- hours. A breakfa.st, consisting of three hard boiled eggs, some pancakes with coffee, being taken at 8 o’clock, the stomach was empty at 10‘15. Two roasted eggs and three apples, eaten at 1 1 o’clock on the same day, had disappeared at 12T5. Roast pig and vegetables, afterwards eaten at 2 P.Jt., were half dissolved at 3, and had disappeared at 4'30. It w'as further observed, that a meal, consisting of boiled dried cod-fish, potatoes, parsnips, bread and butter, eaten at 3 o’clock, was about half digested at 3‘30, the bread and parsnips having disappeared, the fish being separated into threads, and the potatoes being least altered; ;it 4 o'clock, very few pieces of the fibre of the fish were found, but some of the potato was still perceptible ; at 4'30 all was completely pulpified ; and at 5 o’clock, the stomach was empty. Again it was found, that rice and tripe were digested in 1 hour; that eggs, salmon, trout, apples, and venison, took 14 hour; tapioca, barley, milk, liver, and fish. CHTMIFICATION. 93 2 lunrs; turkey, lamb, potatoes, pork, 2-J- hours; beef, mutton, and poultry, from 3 to 3-^ hours ; and veal a little longer. The order in which each separate article of food is mentioned above, indicates its relative digestibility, at least in the stomach of Alexis St. Martin. As a rule, animal substances are more rapidly digested than vegetable substances. I'he rate of digestion of different sub- stances corresponds with the relative necessity for their being acted on by the gastric juice. Tims, those which require the most digestion by that fluid, necessarily remain the longest, Avhilst those which are merely liberated, but are not dissolved in it, pass out sooner ; and fluids, with their soluble ingredients, disappear the most quickly. In cases of fistulous openings in the dog, and in Man, it has been found that fibrin is digested in half an hour, casein in 1.J- hour, gelatin in 2 hours, coagu- lated albumen in 6 hours, and tendons in 10 hours. During gastric digestion, the muscular tissue breaks up first into its fasciculi, and then into fibres, the striai of which gradually disappear, the sarcolemma, as well as its .«arcous contents, being dissolved ; fragments of the fibres, however, pass into the intestine, and there undergo further, though, it may be, incomplete, digestion. Yellow elastic tissue appears to resist the action of the gastric juice ; tendinous fibres di.ssolve slowly; white areolar fibres are totally dissolved. The cor- puscles of cartilage are not digested, but the inter-cellular substance undergoes solution. The areolar fibres of adipose tissue disappear, and frequently also the walls of the fat-cells ; but their fatty contents are commonly said to resist the action of the gastric juice ; fat, however, may begin to be broken up into the fatty acids. (Marcet.) Of v^egetable tissues, the cellulose or lignin of the cell-walls, including the dotted, annular, and spiral ducts, for the most part resist the action of the ga.‘'tric fluid, which is also inoperative upon starch grains, though it does not interfei’e with, or totally arrest, the action of the swallowed saliva, and of the mucus of the stomach, upon starch. Chlorophyll, the gTeen colouring matter of plants, appears to resist digestion ; but the pectinous and albuminoid contents of vegetable cells, are completely dissolved. Chymification and Chyme. The general product of digestion in the stomach, resulting from the combined admixture with the food, and the action 94 SPECIAL PHYSIOLOGY. upon it, of the saliva, the muras of the mouth and stomach, and the gastric juice itself, is called the chyme , ; the process of its formation is named ch/pnification. The chyme is a thick, pulpy, grumous, fluid, containing the food thus far digested, together with partially digested, and indigestible, matters; it hasa stronc: sour smell and taste, find an acid reaction. The degree of acidity of the chyme varies, however, according to the quantity of acid, such as lactic or acetic acid, in the food, and also ac- cording to the relative quantities of saliva and gastric juice con- tained in it, much gastric juice rendering it more acid, and an excess of saliva less so. The colour of the chyme depends on the food, being whitish in an infant fed on milk and farinaceous food, but of a brownish hue when meat is eaten, or gi-eenisli after vegetable diet ; sometimes also, it is tinged with bile, which has ascended into the stomach. The presence of saliva, mucus, and gastric juice, is indicated by characteristic microscopic nucleated cells. The composition of chyme, like its colour, also varies with the nature of the food. With ordinary diet, it consists of a mixture of the saline, amylaceous, -saccharine, albuminoid, gelatinoid, and fa tty matters of the food, in different conditions of conversion or solution. The starch, partly changed into dextrin and sugar in the mouth, continues to undergo transformation in the stomach, even more rapidly, because the vegetable cells are loosened or dissolved, so as to set tree the starch grains. The conversion of starch into sugar in the stomach, is due to the saliva swallowed with it, for, in an animal, ligatttre of the oesophagus, which prevents the continued entrance of s;iliva into the stomach, arrests this transforma- tion. A good deal of starch always passes from the stomach, undissolved. The albuminoid and gelatinoid substances are represented in the chjnne, by albuminose or the albumen and gelatin-peptones ; whilst the fatty matters of animal tissues, perhaps to a small extent decomposed, are loosened from the fat cells, and, as well as the fatty matter of butter or cheese, are reduced to minute particles, interntixed with the rest of the chyme. The characters of the chyme depend, however, not only on solvent actions, but also on the process of absorption, which begins in the stomach, as soon as that organ contains fluid or dissolved matters. Owing to the escape of chyme into the intes- tine, the quantity actually in the stomach, at anv one time, is small; and, owing to absorption, the quantitj' which passes into the duodenum, is much less than the quantity of fluid swallowed USES OF THE BILE. 95 and secreted for the purposes of gastric; digestion. Even the soluble constituents of the chyme, are constantly being removed by absorption. The soluble constituents of our solid and fluid food, such as saline matters, sugar, alcohol, and thein, and also the soluble products of digestion, such as sugar, and the albumen and gelatin-peptones, mixed with some salivin and pepsin, are greedily absorbed, with the water of the chyme, by the blood-vessels of the mucous membrane of the stomach, and are then conveyed thi'ough the portal vein, into the liver. The chyme itself, therefore, at any one moment, does not represent the simple product of the digestion of food, but the joint product of the double process of digestion and ab.sorption. In comparison with the food taken, it necessarily contains a larger proportion of fatty matter, than of saline, saccharine, amylaceous, albuminoid, or gelatinoid substances ; for the latty substances have undergone little, or no, chemical change, and no absorption from the stomach, rvhereas the others have been more or less dissolved, altered, and absorbed. The semi-fluid product is, moreover, constantly being forced forwards, drop by droji, through the pylorus into the duodenum, where it undergoes further changes, now to be considered. Action of the Bile. The bile performs a most important part in the intestinal digestive process; but its action does not depend on the presence of an albuminoid substance, like salivin, pe]isin, or paucreatin. Its importance is shown by its highly complex composition, and by its containing substances which, unlike the urea and uric acid of the renal excretion, do not pre-exist in the blood, but are formed in the hepatic cells. Secondly, the bile, as already stated, (pp. 74-5), is much more abundantly secreted during the process of digestion than at any other period ; and although this may be due to the accomj^anying activity of the portal circulation, jmt the general adaptation of means to ends in the animal economy, suggests the conclusion that the secre- tion is most required at that particular time. Lastly, the situation at which the bile is discharged into the alimentary canal, immediately below the stomach, and therefore very high up in tlie intestine, seems to indicate its special adaptation to the further digestion of some important constituent of the chyme. Nevertheless, as we shall hereafter see, a large portion of the solid constituents of the bile, is remov’ed from the body. 96 SPECIAL PHYSIOLOGY. and this fluid must, to a gi-eat extent, be regarded as an ex- crementitious fluid, serving to eliminate carbon, hydrogen, and sulphur. The bile also serves certain sujiplementary non- chemical uses. Thus, it excites the mucous membrane of the intestine, and so probably causes an increased secretion of mucus and intestinal juice. It moreover, stimulates, either directly, or through the nerves, the contractile fibre-cells of the mucous membrane and its villi, as well as those of the muscular coat of the intestine ; the former action, probably, promotes ab- sorption by the villi : whilst the latter excites the intestinal peristaltic action, and so aids in the onward movement of the intestinal contents. It is well-known that a scanty suppl}- of bile may lead to constipation, whilst an excess of that fluid induces diarrhoea : hence, it may be inferred, that a proper quantity helps to maintain the healthy action of the intestines. The inspissated bile of the ox is used as an aperient medicine. As regards the chemical action of the bile, e.xperiments, made outside the body, by digesting v^ariou.s constituents of food in that fluid, at a temperature of 100°, show that it has an exceedingly feeble action in changing starch into su?ar ; cane sugar is slowly converted by it into lactic acid ; it neither dissolves albuminoid substances, nor saponifies or dissolves fat. Albuminoid and gelatinoid bodies, although stained, are other- wise unaltered ; fatty matters, agitated with bile, form an im- perfect opaque emulsion, but after a time, if left undisturbed, separate themselves entirely from that fluid, unchanged. Bile is said to arrest the actions of sjiliva and gastric juice, even when these have already commenced, upon starch and albumi- noid substances. Indeed, the bile and the gastric juice de- compose each other, when mixed out of the body ; but this does not seem to be the case, Avhen the gastric juice is already combined Avith peptone. In living animals, in Avhich biliary fistulffi have been established, so that the bile, prevented from entering the intestinal canal, escapes at the surface of the bod\'. amylaceous, albuminoid, and gelatinoid substances are still completely digested. With regard to fatty matters, hoA\*ever, the bile, appears, in some Avay, to assist in, or to determine, their absor[ition. It has been as.sumed that the bile is a sapo- naceous compound, and that it dissolves tatty matters directlv, like an ordinaiy soap; but soaps contain more or less free alkali, Avhich assists in dissolving additional fat, whilst the alkaline reaction of bile, even Avhen present, depends pro- bably on phosphate of soda. Experiment shoAvs, hoAvever, USES OF THE BILE. 97 that the bile is highly important for the proper digestion of latty matters. In animals -with biliary fistuliE, the chyle col- lected from the lacteals, or absorbents of the intestines, contains but a small quantity of fat ; half, or even more, of the fat taken with their food, passes itnchanged from the alimentary canal ; and, as a consequence of this, the bodies of such animals are very lean. According to the observations of Blondlot and others, animals thus treated, may live even as long as five years. Again, after ligature of the biliary duct in an animal, which prevents the descent of bile into the intestine, the fluid in the lacteals is clear and deficient in fat, instead of presenting its characteristic milky-white colour, and fatty molecular contents. In what mode the bile contributes to the absorption of fat, is not yet known. It certainly does not appear to act chemically, by decomposing or dissolving neutral fats ; nor does it make, with oily matters, a permanent emulsion. It probably co-operates with the pancreatic juice. It has also been shown, that fatty matters permeate moist animal membranes more readily than usual, if they be first wetted with bile, or with an alkaline solution. Since provision is made in the saliva and gastric juice, for the complete digestion of amyloid, gelatinoid, and albuminoid substances, and, as we .shall presently show, in describing the action of the pancreatic juice, of fatty matters also, the bile may be supposed to possess no exclusive digestive power, but rather to be superadded, in order to complete some particular part of the digestive process. As the contents of the upper part of the duodenum, like those of the stomach, are strongly acid, whilst those of the small intestine generally, become gradually alkaline in their descent, it was formerly thought that the bile, then regarded as a very alkaline fluid, was concerned in neutralising the acid of the chyme ; but it is now known that the bile is but feebly alkaline, or sometimes even neutral, and the alkalinity gradu- ally acquired by the contents of the small intestine, is attri- buted partly to the pancreatic and intestinal j uices, and partly to the evolution of ammonia, from .slow decomposition. The bile not only imparts a bright yellow colour to the chyme in the duodenum, but it further appears to exercise an anti-putrescent action, thus preventing, or retarding, a fetid decomposition of the contents of the intestine ; for, in the absence of bile from the alimentary canal, these frequently become decomposed, causing flatulence and diarrhosa. In VOL. II. u 98 SPECIAL PHYSIOLOGY. experiments made with bile, out of the body, it is found that various fermenting processes are arrested by that fluid. The colouring matter, with the cholesterin, and a certain portion of the other constituents of the bile, are foimd, more or less altered, in the residue of digestion ; but by far the larger portion of its characteristic, conjugated glycocholic and taurocholic acids, is absorbed by the mucous membrane of the intestine. These acids, when boiled with hydrochloric or other acids, are decomposed into cholalic acid, on the one hand, and glycocoll and taurin, on the other. The cholahc acid is then changed into choloidinic acid, and this again into a resinoid substance, soluble in ether, but insoluble in Avater, called dyslysin. These decompositions, due, atomically, to loss of certain atoms of water, are shoAvn below : — Cholalic acid .... Choloidinic acid, and 1 water . . + H,0 Dyslysin, and 2 water .... C.^jHjgOg + Similar decompositions appear to occur in the Ih'ing body ; for, in the small intestine, the acids of the bile are set free from their soda salts, by the hydrochloric and other acids in the chyme. At the commencement of the ilemn, they are already half decomposed, and partially absorbed : at the end of the ileum, they are AvhoUy decomposed ; whilst, in the contents of the large intestine, only dyslysin is present. Action of the Pancreatic Juice. The pancreatic juice, or so-called abdominal saliva, possesses, like that fluid, in a remarkable degree, the poAver of conA-erting starch into dextrin and grape sugar. The fresh juice is capa- ble of so converting more than four times its weight of starch ; the substance of the gland, macerated in Avater, also exhibits this poAver. Hence, probabl}', it aids in the metamorphosis of the starch, Avhich has escaped tlie action of the saliA a. This, hoAvever, is a secondary use of the pancreatic fluid ; for the mucus of the stomach and intestine, and the intestinal juice, also subserve this purpose ; moreover, the pancreas is as large in carniA^orous mammalia, the natural food of Avhich contains no starchy matter, as it is in the herbiA'orous species. The action of the pancreatic juice on gelatinoid substances, has not been specially studied ; but opinions differ as to its poAver oAmr albuminoid bodies. It is by most authorities maintained, that it does not digest these substances, because it ACTION OF THE PANCREATIC JUICE. 99 does not dissolve them in experiments out of the body. When the pancreatic juice, or the infusion of the gland substance employed, has undergone a kind of putrefaction, such solu- tion may occur ; but this is a condition not present in the living body. Moreover, any albuminoid substances mace- rated in water, will putrefy and slowly dissolve, and such jmtrefied soluble matter rapidly sets up similar changes in fresh albuminoid substances. Corvisart and Meissner believe, however, that the pancreatic juice is able to peptonise albuminoid substances, but that it only possesses this pro- perty when they have been previously mixed with gastric juice and bile, or when they are slightly acidified ; or, as Bernard supposes, only after a certain quantity of the digested food has already passed into the circulation, so as to supjtly the blood with materials suitable for the secretion of some special product, needed for a very powerful pancreatic juice. Extirpation of the pancreas affords no certain information concerning the use of its secretion. According to some, the removal of the gland is followed by the absence of white chyle in the lacteals, and the presence of undigested fat in the contents of the large intestine ; at the same time, emaciation occurs. According to others, when this gland is extirpated, neither a total arrest of nutrition, nor death by starvation, necessarily i'ollow, every constituent of the food still undergoing more or less perfect digestion, the office of the j^ancreatic juice being fulfilled by other secretions. Complete degeneration of the pancreas in Man, the liver and other organs remaining healthy, does not necessarily interfere with the digestive process ; but, in certain diseases of this gland, fatty matter has been observed to pass undigested through the alimentary canal. The use of the pancreatic juice, seems, indeed, to be subsidiary, or comple- mentary, to the other digestive fluids ; for it aids the saliva in the conversion of starch into sugar. It is said by some to be able, Avith the gastric juice, to dissolve albuminoid matters, and if, as is generally believed, its chief office is to digest fatty matters, it must co-operate, in some manner, rvith the bile. The effects of the pancreatic juice, on fatty matters, have been shown by experiments out of the body, and by observa- tions on living animals. If either the fluid, obtained fresh from pancreatic fistulee in animals, or a Avatery infusion of the substance of the gland just taken from an animal killed during the digestive process, be agitated Avith a neutral fat, and the mixture be maintained at a temperature of 100°, the fatty H 2 100 SPECIAL PHYSIOLOGY. substance is most perfectly emulsified, the action being much more complete and durable than if saliva, bile, intestinal juice, or any other animal fluid, had been employed. The emulsion lasts as long as eighteen hours, after -which the fat sepa- rates. It is found, however, that a portion of the olein, mar- gariu, and stearin, is now decomposed, having been rapidly separated into the corresponding fatty acids and glycerin. These effects are most marked when the pancreatic juice is collected a short time after digestion has begun, all the characters of the secretion being then most evident. This remarkable decomposition is usually attributed to the pancreatin, combined with the operation of the free alkali of the fluid, just as pepsin with an acid, effects the transformation of albuminoid substances. The pancreatic juice no longer pos- sesses this power of decomposing neutral fats into their fatty acids and glycerin, in the presence of ordinary acids, which desti’oy its alkalinity ; hence it has been urged, that the acidity of the chyme must prevent this peculiar decomposition. But the pancreatic fluid and the intestinal juice ai-e strongly alka- line, and moreover, the bile may here interpose, and, by means of the soda present in combination with its conjugated acids, may neutralise the acids of the chyme, and so permit the decomposition of the neutral fats of the food by pancreatic juice; for this process may not be interfered with by the acids of the bile, which are themselves fatty. That the action of the pancreatic juice is, in some important way, aided bv the bile, and conversely, that the action of the bile is seconded l)y that of the pancreatic juice, is highly probable, from the fact that they are discharged into the intestinal canal so near together, generally, indeed, at a common orifice. It happens, however, that in the rabbit, the chief pan- creatic duct enters the small intestine rather more than twelve inches below the bile duct, which opens as usual a little below the pylorus ; another smaller duct also exists, but it is almost impermeable. This arrangement has been taken advantage of, in physiological experiments on the use of the pancreatic fluid. The most interesting and important of these, is one originally performed by Bernard, and since repeated by others. A rabbit is crammed with fiit, or its stomach is injected with oily matter, and it is afterwards killed, whilst digestion is going on ; it is then found that the fatty matters in the intes- tine, above the entrance of the pancreatic duct, though mixed with the bUe, are yet unchanged ; whilst below that point they ACTION OF THE PANCREATIC JUICE. 101 begin to be emulsified ; moreover, the lactesils, or absorbent vessels of the intestine, above the point of entrance of the pancreatic duct, although bile is there mixed with the food, are filled only with a clear transparent fluid, and, indeed, are mostly invisible ; whilst, at a point immediately below the entrance of the pancreatic duct, those vessels are charged with their characteristic milk-white fluid, containing fatty particles. To this apparently precise and unexceptionable experiment, it has been objected, by those who dispute the influence of the pancreatic ji;ice in the digestion of fiitty matters, that the difference in the contents of the lacteals, above, and be- low, the entrance of the pancreatic duct, depends upon the time that has elapsed before the animal is killed, after being fed with fat. AVhen this is done within two hours, it is said, that the lacteals given off above the pancreatic duct, are found filled with white chyle ; but if the animal be killed from lour to six hours after, the lacteals below the pan- creatic duct are alone filled. These results are not in accordance with the experience of most observers, who fully confii'm those obtained by Bernard. It is further stated by that experimenter, that ligature of the pancreatic duct, arrests the emulsification and absorption of fat, but this, again, is disputed by others. The opposite conditions observed by them, are explained by Bernard, on the supposition, either that the smaller pancreatic duct escaped ligature, or that certain minute glands of the duodenum, in jmrt supplied the place of the pancreas. It has been asserted, that if the small intestine be tied, in cats and puppies, below the entrance of the pancreatic duct, and oil, mixed with milk, be injected into the bowel below the ligature, the lacteals, after a time, become filled with white chyle (Frerichs). It is also said, that after the formation of a pancreatic fistula in a cow, chyle collected from a fistula, subsequently made in the thoracic duct, con- tains almost as much fat as that of other cows in which no pancreatic fistula has been established (Colin and Lasaigne). Furthermore it is objected, that no large amount of saponified fat, is found either in the contents of the intestine, or in the chyle itself, as might be expected, if the pancreatic juice decomposed the neutral fats, and so rendered them absorbable (Bidder and Schmidt). To conclude : first, the pancreatic juice exercises a positive power of converting starch into sugar, and so may aid in digestion. Secondly, its digestive power over albuminoid 102 SPECIAL PIITSIOLOGT. and gelatinoid bodies, when it is fresh, is very slight, but more marked when it is acidified, or Avhen it co-operates with the gastric juice. Thirdly, it possesses a remarkable poAver of emulsifying fat, and rendering it absorbable, more marked eA^en than that of the bile. Lastly, Avhilst, out of the body, it not only emulsifies, but also decomposes the neutral fats into their fatty acids and glycerin, it is uncertain Avhether this decompo- sition actually takes place within the body. A case has been recorded, in which a calf’s pancreas, taken internally, aided the assimilation of fat ; and, quite recently, preparations of pancreatin made from animals, like those of pepsin long employed, have been administered medicinally. Action of the Intestinal Juices. Oiving to the mixture of secretions in the intestinal canal, it is difficult to determine the dige.stiA'e properties of the intestinal juices. Portions of food, enclo.sed in perforated tubes, have been introduced through artificial openings, into the small intestine, the duodenum being first tied, in order to prevent the saliva, gastric juice, bile, and pancreatic juice, fi'om passing doA\m. Other experiments haA-e been made, by isolating portions of the small intestine and its contents, bA' including them betAveen tAvo ligatures. Various kinds of food have also been subjected to artificial digestion, outside the body, at a proper temperature, in the juices of the small or large in- testine, or Avith portions of the mucous membrane macerated in Avater. From such experiments, seA^eral conclusions are obA’ious. The strongly alkaline intestinal juice certainly converts starch into sugar, many believing that this change is chiefly accom- plished in the small intestine ; sugar itself here also passes into lactic and butyric acids ; it acts still more poAverfully in the solution of albuminoid substances. Lastly, it is also more or less capable of forming an emulsion Avith fat, and so of aiding the pancreatic juice, or even of supplying its place. It aa-ouM seem, therefore, that the intestinal juice operates as an auxiliaiw digestive agent upon the three principal constituents of our food. Its effects do not appear to be interfered with by those of the other digestive fluids. The action of the secretion of Brunner’s glands, and that of the intestinal juice, separately, are quite unknoAA’u. CHANGES OF FOOD IN THE INTESTINES. 103 Changes of the Food in the Small and Large Intestine. Contents of those Intestines. In considering the changes in the food, which occur in any given part of the intestine, it must be remembered, that the fluids, poured into the alimentary canal higher up, are still present, in greater or Jess quantity, in the intestinal contents lower down, and doubtless exercise some digestive influence. Thus, gastric juice, and even sahva, must be present in the upper part of the duodenum, and more or less pancreatic juice and bile, in the lower part of the small intestine. The venous pulpy chyme, poured trom the stomach into the small intestine, is acid, and brownish or variously coloured; but, on its admix- ture with the bile and pancreatic juice, it assumes a bi’ight yellowish colour and becomes much more opaque, owing to the addition of the biliary colouring substances, the decompo- sition of the acids by the bile, and the gradual emulsification of the fatty substances by the pancreatic juice. The contents of the upper part of the small intestine are still acid, partly from the acid of the gastric juice, and partly from the acids of the bile, which are set fi-ee by the former ; but their acidity is gradually diminished, not only by the alkaline pancreatic juice, but also, and chiefly, by the even more powerfully alkahne intes- tinal juice. The hydrochloric acid of the gastric juice, is probably soon neutralised, and is then absorbed into the blood, as chloride of sodium or common salt. At the lower end of the ileum, the reaction of the residual intestinal contents, is generally stated to be alkaline ; but near that point, in a case of accidental fistula in the human subject, it has been found acid, notwithstanding the alkaline condition of the mucous membrane. The contents of the csecum are said to be acid ; but those of the large intestine generally, to be alkaline. Much, however, depends on the nature of the food ; for, from the formation of acetic or lactic acid, during the use of an excess of vegetable diet, the contents of the whole intestinal canal may be acid. In carnivora, the contents of the cacum, from the presence of ammonia, exhibit an alkaline reaction, whilst in the herbivora, they are always acid, from the pre- sence of lactic acid. The chemical composition of the contents of the small intestine, is dependent on the nature of the food taken. It must also vary at different parts of the canal, according to the 104 SPECIAL PHTSIOLOGY. composition and quantity of the secretions mixed with it, and according to the relative quantity and nattire of the substances which have been absorbed fi'om it. Thi;s, the contents of the first part of the duodenum, consist of the acid chyme, with bile and pancreatic juice, i. e., of a mixture of the food taken, whether this be bread, milk, meat, or eggs, together A\-ith saliva, gastric juice, bile, pancreatic juice, and mucus, minus a certain amount of the water and dissolved substances, which have already been absorbed. These substances, which are almost exclusively absorbed by the blood vessels, consist of suline matters, unaltered starch, sugar, whether pre-existing in the food, or produced by conversion of starch, dissolved albuminoid and gelatinoid substances, in the shape of albu- minose and gelatin-peptone, sabvin, pepsin, creatin and other extractive matter.s, and lastly, traces of alcoholic, etherial, acid, and various sapid, substances. The sugar found here, is said, by some, to be grape sugar, the conversion of cane sugar into grape sugar being chiefly accomplished, in this part of the alimentary canal, by the agency of the intestinal juice. No fatty matter is yet absorbed, but it all remains in the contents of the upper part of the duodeniim. Even after the admixture of the bile and pancreatic juice, all the substances, just enume- rated, still continue to undergo solution and absoiqrtion, and the fatty matters, also now emulsified and rendered absorbable, are gradually taken up, together with some of the fatty acids of the bile. The contents of the small intestine are thus pro- gressively robbed of all their dissolved or emulsified nutrient substances, in which they become by degrees poorer. Finally, passing into the large intestine, they acquire a greater con- sistence and a darker hue. The contents of the large intestine have been supposed to undergo an imperfect secondary digestion in the ca“cum ; and there are reasons for believing, that such a process, due to the action of lactic or other acid.s and of the intestinal juices, may, especially after heavy meals, be continued along the rest of the intestine. This may explain the digestion and absorption of the nutrient substances in enemas, by means of which the system, as is well known, may be for a long time supported. Whether starch is changed, or fat emulsified, is uncertain. The final residue consists chiefly of the insoluble or undigested portions of the food, broken down into small fragments. In it are found, particles of vegetable matter, such as unaltered starch-grains, woody fibre, remains of vegetable epidermic, COXTE>^TS OF THE LAEGE IMTESTINE. 105 and other cells, with portions of spiral and annular ducts. Of animal substances, there are present, portions of yellow elastic tissue, cartilage cells, unchanged fat, epidermoid, and epithelial cells, unchanged fragments of fibrous tissue, such as portions of tendon or fascia, and muscular fibres more or less altered, though having escaped complete digestion ; besides this, there are certain earthy salts, especially the ammonio-magnesian- phosphate, with the phosphates of magnesia and of lime. The neutral salts of the vegetable acids, such as the citrates, tartrates, malates, and benzoates of potash or soda, appear partially in the contents of the lower part of the large intestine, as carbonates, the rest having been absorbed, also, it is said, chiefly in the form of carbonates. Furthermore, the fecal mass contains colouring matters and other substances left from the almost completely changed or decomposed bile, such as cholalic and choloidinic acids, traces of cholesterin, and especially the substance named dyslysin, also a crystallisable substance containing sulphur, named excrefme (Marcet), traces of stearic, margaric, and a peculiar fatty acid called excreteric (Marcet'), with some animal matter, probably the residue of the pancreatic and the mucous secretions, especially of those of the larger intestine. It appears certain, indeed, that the glandular apparatus of the intestines, serves to excrete, and thirs eliminate from the blood, products of the decomposition of the tissues, which would be injurious if retained in it; these must be present in the fecal substance, and may in great part explain its odour. The small intestine, with its villous mucous membrane, is adapted to the function of absorp- tion ; but the non-villous mucous coat of the large intestine appears better adapted for excretory purposes. The percentage composition of the ashes of the daily quantity of feculent matter removed from the body, varie.s, according to the food, from 2 to 10 oz. ; the average quantity is about 6 oz., of which three-fourths are water. The percentage composition of the ashes, after burning, is as follows : — Chloride of sodium, alkaline sulphates, and phosphate of soda (or potash) ...... 4 Phosphate of lime and phosphate of magnesia . 81'o Sulphate of lime ....... 4'5 Phosphate of iron ....... 2 Silica ......... 8 100 - The contents of the stomach invariably include a certain lOG SPECIAL PHYSIOLOGY. quantity of atmospheric air (4 nitrogen to 1 oxygen), •which has been mixed with the food and saliva in the mouth, and swallowed with them. The decomposition of the amylaceous and saccharine food, into lactic and butyric acid, may cause the evolution of carbonic acid and hydrogen. The oxygen, and especially the carbonic acid, being more soluble in water, would be more easily absorbed than the nitrogen and hydro- gen ; but the nitrogen may also pass into the blood. An interchange of the other gases with carbonic acid from the blood, may take place, by what might be termed intestinal respiration. In the small intestine, the carbonic acid and hydrogen relatively increase in quantity, the nitrogen remain- ing about the same ; tvhilst the oxygen di.sappears. On including a loop of the small intestine of a living animal, between ligatures at two different points, the gases of the blood, oxygen, carbonic acid, and nitrogen, have been found to pass iiito the interior of the intestine ; so that these gases may be both absorbed from, and excreted into, the intestinal canal. In the large intestine, besides carbonic acid, as the principal gas, carburetted hydrogen may appear, owing to the slow decomposition of its contents ; nitrogen abounds after a tlesh diet, and hydrogen after a milk diet ; lastly, though, it would seem, but seldom, as a consequence of the decomposition of the albuminoid substances containing sulphur, or of the taurin of the bile so rich in that element, or possibly from the de-oxidation of sulphates, small quantities of sulphuretted hydrogen gas are evolved. These two last-mentioned gases may also be absorbed into the blood; indeed, it has been shown experimentally, that animals may be quickly poisoned by injecting sulphuretted hydrogen into the large intestine. The time taken by different articles of diet to descend through the alimentary canal, varies. Laxative medicines may pass in four hours, carbonate of iron in twelve hours, the coloiu'ing matter of spinach and other vegetables in eighteen hours, and grape-jjips and cherrt'-stones in from three to fom days. It has been shown that it is useless, and perhaps imprudent, to administer purgatives immediately after the accidental swallowing of buttons, coins, or stones ; it is better to administer thick tenacious food for a day or two. and then give a dose of castor oil. SUMMARY OF THE DIGESTITE CHANGES. 107 Summary of the Chemistry of Digestion. We have seen that the digestible and absorbable parts of food, consist chiefly of the carbhydrates, or the amylaceous, gummy, and saccharine substances; of hydrocarbons, or fats and oils ; of nitrogenous gelatinoid and albuminoid substances, and extractive matters ; of hydrocarbonaceous alcohol and organic acids ; of saline substances, and of 'water. Part of the starch is converted, by the saliva in the mouth, into glucose or grape sugar; this change still goes on in the stomach, even in the presence of the gastric juice ; it is com- pleted, or, according, to some, chiefly accomplished, in the interior of the small intestine, by the continued action of the saliva and by the superadded agency of the pancreatic and intestinal juices. Cooked starch is changed more rapidly than ra’w starch, the cells of which sometimes escape digestion ; the emptied envelopes of the starch grains commonly remain undigested. Cane sugar (Bouchardat), and milk-sugar (Leh- mann), are for the most part converted into grape sugar, in the stomach, more particxtlarly in the intestine ; small quantities of cane sugar are said to be absorbed without change (Bernard). The grape sugar, thus formed from starch and other sugars, or that which may be contained in the food, is principally absorbed as such by the blood vessels; but it appears partially to be changed, especially when abundantly taken, within the alimentary canal, into lactic acid, and this again into butyric acid, with an accompanying separation of carbonic acid and hydrogen. Thus : — 1 1 Butyric acid 1 Grape sugar = 2 Lactic acid = 1 2 Carbonic acid (4 Hydrogen I fCA02 = 2C3H,03 = 2 CO, (4 H The albuminoid bodies begin to be digested in the stomach, by the gastric juice ; whilst their solution is continued, and completed, in the small intestine, by the additional action of the intestinal juice. Pluid albumen, and especially vegetable albumen, and coagulated fibrin, are easily digested ; coagtdated albumen, casein, gluten, and legumin, more slowly. Casein is first precipitated in a flocculent form, and then dissolved. 108 SPECIAI. PHTSIOLOGY. All albuminoid substances are converted at once into albumi- nose or albumen-peptone. Gelatin and gelatin- jdelding tissues are converted into gelatin-peptone. These peptones, and also the saliva, pepsin, and pancreatin, are absorbed from the stomach, as well as from the small inte.stine, and chiefly by the blood vessels. Fats, whether pure, and merely melted by the heat of the stomach, or whether forming part of an organised tissue, and set free by the digestion of the enveloping areolar tissue and walls of the adipose cells, coalesce, into small drops, in the stomach and upper part of the duodenum. In the small intestine, so long as its contents remain acid, the fats are merely emulsified by the pancreatic juice, aided possibly by the bile ; in the lower portion of the small intestine, however, ■where the intestinal contents become more or less alka- line, certain quantities of the fat are probably decomposed into their fatty acids and glycerin, by the further action of the pancreatic juice, and may even be saponified by the strongly alkaline intestinal juice. Thus emulsified, decom- posed, or saponified, all but a small residue of the fatty matters are absorbed by the lacteals of the intestines. Alcohol, in all its forms, ethers, and other soluble acid and sapid bodies are absorbed imchanged, along the whole surface of the alimentary canal, chiefly, if not entirely, by the blood vessels. This absorption begins even in the mouth, otherwise these st;bstances would produce no flavour. The organic acids probably decomposed into carbonates. The extractive matters, creatin and creatinin, the cerebric acid, those which are uncrystallisable, and perhaps some of the cruorin and myochrome, are also probably absorbed without change, by the blood vessels. The saline constituents of the food are chiefly absorbed without alteration ; the soluble ones, from the mouth, stomach, and intestinal canal generally ; whilst the less soluble phos- phates of magnesia and lime, appear rather to be dissolved in the large intestine. Any carbonates contained in the food or drink, must be decomposed by the acids of the gastric juice, by the lactic acid of the food, and by the acids resulting from the decomposition of saccharine matters. The salts formed by such organic acids with soda or potash, are either absorbed into the blood, and there converted into carbonates, or they are thus changed in the intestinal canal. Water remains undecomposed, and is absorbed freely during HOW DIGESTION IS INFLUENCED. 109 the digestive process, constituting the natural menstruum, in -wliich the different soluble substances are dissolved, and in which the fatty matters are suspended. Of substances, the digestion of which is doubtful, may be mentioned vegetable mucus, gums, pectin, and cellulose. The three former, though soft and tender substances, miscible but probably not actirally soluble iu water, are said, by some, indeed, neither to be capable of being absorbed, nor jmt to be so chemically changed, as to become so. The softer kinds of cellulose, such as that contained in the growing tissue.s of green vegetables, in the tuber of the potato, and in the pulp of fruits, are supposed to be dissolved in small quantity, if not for nutrient purposes, yet in order to set free their starchy, gummy, saccharine, and albuminoid contents. Herbivorous animals, however, certainly digest large quantities of cellulose and vegetable pectin, by changing them into sugar. Chloro- phyll, speaking generally, is indigestible. Though putrescent meat, such as high game, may be first sweetened, and then digested by the gastric juice, yet certain decomposing sub- stances, like poisonous or fermenting sausages, cannot be corrected by the juices of the stomach, but excite vomiting and diarrhcna, and, when absorbed, often prove fatal. Circmnstances tvhich modify Digestion. The rate of gastric digestion of certain articles of diet, has already been mentioned (p. 92). It pai-tly depends on the relative solubility of the various proximate constituents of the food ; but it may also be greatly modified by other circum- stances, such as the quantity, consistence, and peculiar mix- tures of the food, its condition of subdivision, its absolute quantity, the relative quantity of its different constituents, the absence or presence of stimulating substances, the conditions of the nervous system, the state of sleeping or waking, the condition of the body as regards health, habit, individual peculiarities, bodily fatigue, and even the condition of the mind. Rest and exercise also affect the digestive process. The more rapidly and perfectly the constituents of any given kind of food, are capable of being dissolved, the more easily such food is digested, and vice versa. As a rule, bread, not too new, nearly all kinds of meat, poultry and white fish, eggs, milk, jelly,and the gelatin-forming ti.ssues, and well-boiled potatoes, are easy of digestion ; whilst new bread and potatoes, no SPECIAL PHYSIOLOGY. fatty meats, fat, tendons, cartilage, cheese, and green vegetables are more difficult of digestion. Hard-boiled eggs are, of course, more difficult to digest than the fine coagulum of albumen formed in a custard, or in the gravy of meat, owing obviously to the difference of consistence and degree of subdivision in the two cases. Mashed potatoes and finely grated cheese, and soft cream- and milk- cheeses, are more easily and rapidly digested than plain boiled potatoes or hard cheese. Again, all vegetable substances too much matured, and therefore com- posed of cells having harder cell walls, are more difficult to digest, and hence require much cooking, and artificial sub- division, to burst and break down the cells, and permit the digestive juices to enter their interior, and act on their con- tents. Carrots, turnips, cabbages, celery, artichokes, aspara- gus, and onions, may be classed in this category.- Even the cooking of flour, and of all other amylaceous articles of diet, helps digestion in an extraordinary degree, by bursting or swelling the fecida or starch grains. Large quantities of adi- pose tissue, intermixed with muscitlar tissue, probably impede the penetration of the gastric juice, and so render too fat meats, such as pork, and also oily fish, as, for example, salmon, com- paratively indigestible. It has been found that the flesh of animals living in a wild state, is more digestible than that of the allied tame species, probably owing to the more fatty muscular tissues of the latter. A large quantity of fat, in the shape of fatty tissue, taken with other food, may have the same effect of interfering with digestion ; but such fatty tissue is far preferable to fat itself, and more easy of digestion, because it is contained in areolar tissue, and is divided into minute spherules -within the fine adipose cells, so that the gastric juice percolates it with comparative facility. Hence suet and cooked fat are more digestible than the melted fat derived from them, and swimming on the suriace of gra-iy. Pure solid fats having a granulated texture, especially cold butter, the particles of ^vhich adhere together, as it were, only by certain points of contact, are more easily digested than the same fats taken in a melted condition, such as oiled butter, in which, the oleaginous particles havm completely coalesced. It is pos- sible, also, that the heating of fatty matters determines slight chemical changes, inconsistent with easy digestion. But per- haps the most objectionable effect of fat, is that which occurs in certain processes of cooking, in which it saturates heated or dried albuminoid, gelatinoid, or amj-laceous substances, and HOW DIGESTION IS INFLUENCED. Ill SO preoccupies their interstices, as to render them extremely difficult of penetration by the gastric juice, which is aqueous, as in the case of buttered toast, or greasy hot dishes of any kind. Moreover, OAving to the liigh temperature in roasting or baking, the substances above mentioned, as Avell as the fats themselves, sometimes undergo peculiar chemical changes, by which acrolein, or other pyrogenic compounds are per- haps developed. These latter conditions are met tvith in the burnt parts of roasted joints, in over-roasted, baked, or fried parts of the skin of poultry or of fish, and especially in greasy and burnt pie -crust. It Avould seem that animal albuminoid substances, held in solution, as in soups and broths, are not more easily digested than the same substances in a solid form ; lor the water requires to be almost entirely absorbed, before the nutrient principles can be converted into peptones. Hence, solid food, even in the case of many invalids, is more suitable than bulky fluid food. It is said that dextrin, introduced into the system, favours the digestion of albumen (SchilF) ; this affords an illustration of the adA^antage of mixed diets. Too large a quantity of food, at any one meal, also renders digestion proportionally difficult. When the digestive poAvers are Aveak, the bad effect of quantity is much more obA'ious. It is believed that the secretion of the gastric juice especially, is regulated, as to quantity, more by the demands of the body, than by the amount of food taken ; hence, an excess of food, not only remains undigested, but acts as an irritant to the stomach itself, lessening its further secreting poAver, and, if passed on into the duodenum, causing more or less disturbance to the system. At the same time, some solid substance is essential or favourable to digestion ; hence, perhaps, the habit of certain nations, mixing, Avith their scanty food, some indi- gestible material, such as saAA"-dust or earth, which can only increase its buffi. After a very heavy meal has been digested, the stomach secretes but a A^ery Aveak gastric juice (Schiff). The effects of cold AA^ater, or ice, in repressing the secretion of the gastric juice, and so retarding the digestive process, have been already mentioned ; the reduction of the tempera- ture of the stomach, and the retardation of the capillary cir- culation, afibrd an explanation of these facts ; taken in large quantities, Avith or after food, ices and iced beverages must suspend digestion. On the other hand, digestion is un- doubtedly favoured by moderate quantities of alcohol, also by 112 SPECIAL PIITSIOLOGT. salt, vinegar, lemon juice, pickles, sauces, and spices, these substances acting as stimulants to the secreting processes necessary for digestion, especially to that of the gastric juice ; vinegar, moreover, contributes, by its acidity, to swell and pulpify albuminoid substances. Lemon juice yields, in addi- tion, potash salts to the blood. AYines and beers also contain potash, magnesia, and lime ; the red wines e.specially, yield small quantities of tannin, and traces of iron. Severe exercise of the body, or active employment of the mind, too soon after a meal, hinders digestion ; even moderate exertion of the body is not desirable immediately after a full meal, rest being found decidedly to favour digestion ; but persons of sedentary habits digest slowly. Sleep is said to i-etard this process, but otherwise does not interfere with it. Mental emotion may arrest digestion, perhaps, by putting a stop to the secretion of gastric juice. Digestion, as already men- tioned, requires, for its due performance, the secretion of large quantities of the digestive fluids, and this can only be accom- plished by an incresed supply of blood to the organs concerned in this function ; hence, any acts which determine the blood strongly to the brain or muscles, interfere with it. Habit has an extraordinary effect in modifying the digestive power in particular instances ; thus, infants or invalids, who have been habitually fed on fluid and easily digested food, are inconvenienced, or injured, by the use of hard food difficult of digestion, and can only by degrees acquire, or regain, a stronger digestive power. Those persons, even, who are accustomed to take food of a dry and hard nature, and requiring strong digestive powers, have their digestive organs deranged by the use of soft and succulent food, which they can only properly digest after a kind of education. A certain effort in the di- gestive act, is probably beneficial, as it is natural to the system. Custom, and differences of climate, explain the well known national peculiarities of diet, and also the fact that, as a rule, a foreign dietary, unless modified, or gradually adopted, is less adapted to the digestive powers of individuals of different nations and climates. Finally, the effects of individual differences, or, as they are called, idinsyncracies, are truly remarkable in the case of the digestive functions. In certain instances, particuku-, and per- haps not otherwise difficultly digestible, substances invariably produce the most serious pain and disorder ; whilst substances ordinarily indigestible, may perhaps be readily digested. VALUE OF DIFFERENT FOODS. 113 Thus, for example, oysters, lobsters, crabs, and salmon, will each produce, in different persons, severe attacks of indigestion, and even give rise to eruptions on the skin. In some persons, strawberries are known to produce a similar effect ; and to others, cucumber is almost a certain poison. Relative Value of different Foods. The following Table, chiefly from Vierordt, exliibits the com- position of a few of the great variety of articles of food consumed by Man. It shows the total amount of solids, and the propor- tions of organic proximate constituents, salts, and water, in each article of diet ; also the relative amount of its nitrogenous and non-nitrogenous constituents, and, as regards the latter, the respective quantities of oleaginous, amylaceous, and saccharine matters. The relative value of different articles of diet, for plastic or tissue-forming purposes, for calorific or respiratory purposes, or for maintaining the proper saline constitution of the blood, is thus shown, so far as their chemical composition is concerned; but this alone affoixls no sufficient indication for the practical choice of diet in individual cases, so much depending on the physical characters and mode of prepar- ation of food, as well as on the age and idiosyncracies of the individual. The total quantity of solids, shown in the first column of the following Table, reveals the highly nutritive quality of legu- minous and cereal food, butter, cheese, and eggs, in comparison with meat ; but such general comparisons are inexact, for the proportions of non-nitrogenous and nitrogenous substances, in each kind of food, are not taken into account. As regards the latter, cheese is the most nutritious diet, then the leguminous seeds, next meat, and then, in order, the yolk of egg, flour, the white of egg, and bread. As regards fat, the order of nutritive value is, butter, yolk of egg, and cheese. Starch and sugar are most abundant in wheat, next in the leguminosa and the in- ferior cereals, less so in potatoes, and least in the succulent vegetables and fruits. Cheese is an extraordinarily concentrated diet; the legu- minosa are highly nutritious, especially those grown in hot countries, but they require a thorough preparation and good cooking ; the great merit of bread, is its soft, porous, permeable, and well-cooked substance ; the advantage of meat consists in its concentrated, yet succulent, tender, and easily digested VQL. n. I SPECIAL PHTSIOLOGT. 1 14 substance, and in its containing the very elements of the tissues and the blood, even fat, creatin, and the potash salts. Potatoes are a weak food, one pound being only equal to about six ounces of bread, and four ounces and a half of lentils ; they are not much more nutritious than the succulent vegetables, but, like these and fruits, they contain, which bread does not, potash, so essential to the muscles ; hence, perhaps, their utility in preventing and curing scurvy. A well selected vegetarian diet is quite equal to the main- tenance of life and health ; the Japanese, the Hindoos, and the lazarroni of Naples, subsist chiefly on a vegetable diet. The macaronis and vermicellis are composed of gluten, Avith but a small proportion of starch. Indian com, and also Avheat, though not in such quantity, contain cerebric acid, a remarkable nitrogenous compound, found in the nervous substance, of very high atomic constitution. Broth is a very Aveak nutriment, even AAdien some strong farinaceous element is added to it ; so is beef tea, if improperly prepared. Meat contains prin- ciples Avdiich may be extracted, some better by cold water, others by Avarm Avater, and others, again, by boiling ; it should, therefore, be cut into small pieces, be submitted for three hours each time, in succession, to half its A\-eight of co/rf, of and of boiling Avater ; the fluids, strained off from the first and second macerations, are to be mixed Avith that strained off hot from the third or boiling process, and the mixture should be just brought to a boiling heat to cook it ; the fat should be skimmed off; a feAv drops of some acid, with salt, will increase the flavour. Thus prepared, beef-tea contains albumen, ti'aces of syntonin, fibi'in, cruorin, and myochrome, in a flocculent state ; and gelatin, creatin, cerebric acid, perhaps glycogen, i nosite, paralactic, lactic, and inosinic acids, and salts of potash, soda, and magnesia, in a state of solution ; nearly all the .syntonin remains in the shrunken meat; the fat is never absolutely removed. Beef-tea, if good, is a light, nutritious, easily assimilated, conserA'ative, and stimulating food. The now much used extractum carnis or extract of meat, is the in- spissated juice of meat, and resembles a viscid beef-tea ; but it contains no gelatin, and no glycogen or sugar ; to be truly nourishing, it requires the addition of some albuminoid and amylaceous materials. Malt liquors are more nutritious than Aveak beef-tea. Alcohol stimulates, and develops heat; it seems to be partly digested and oxidised, though a great jjortion escapes unchanged by the lungs, skin, and kidneys. TABLE OF ANALYSES OF FOOD, 115 Remarks. Also M 3 -ochroine and Cruorin, and often Glyco- Strong broth, 3 per cent, of solids. [gen. The Starch is Glycogen. Cerebric Acid. The white is twice the weight [of the yolk. Some volatile fatty acids. Indian corn especially contains Cerebric Acid. Some of the Sugar is Inosite. Organic Acid 1* Organic Acid *5-l* Organic Acid *7 Organic Acid *5, Phosphates. Alcohol 5-12, .^Ether, traces. In Red Wine, Tannic Acid. Alcohol 2-5. Oil of Hops. ^ .*■ h- 00 .* t'-COb-b-iO'*tOOCO>-' C2 00 CO ^CO .•■'Tf'i— (T^WOOh(^■^WO^-l 86-92 90-93 pH CO ^ ^ IC liO lOb-uO ...... r1 I-1 1 -H pH *o?j ‘S0Al!^O'Br!^Xa; c<» « rH »o ■ •uiuSit; ‘9soinn90 CO 10> CO WS-^C^COCOCOCOp-lr- ( •Xminn-o ‘auuuqoong St. or Su. 1-2 Su. traces. La. 4 (M . O CO 40 p tc lO _ CO . . . ^ 00 pH 00 p « p p J . . . pHphQ , “i -«2 P P • p p* p ^ « A -p “ «2 CO CC CQ 02 02