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Digestion.

hills, etc., secure their prey; and in the chameleon among reptiles, and the woodpecker among birds, the tongue seems specially developed for prehensile purposes. In the elephant, this act is accomplished by the prolongation of the nostrils into the organ popularly known as the trunk. In other mammals (the ruminants and solipeds), the large pendulous lips are the organs employed. In birds, the bill (which is a modification of the lips) is always the prehensile organ of that class.

The prehension of fluids is effected in two ways: sometimes the liquid is poured into the mouth, and is allowed to fall into it by its own weight; in other cases, the tongue is used after the fashion of a piston, being drawn within the mouth so as to exhaust the anterior part of that cavity, and fluids are thus forced to enter by atmospheric pressure.

2. Mastication is effected in the cavity of the mouth by means of the teeth. This cavity is bounded superiorly by the palate or roof of the mouth, and in other directions by the cheeks, lips, and tongue. Projecting into its interior, above and below, is an arched series of teeth, which are firmly fixed by roots into corresponding sockets in the upper and lower jawbones. The upper jaw (and consequently the dental arch imbedded in it) is immovable, or only movable with the entire head; but the lower jaw, with its teeth, is capable of moving upwards, downwards, backwards, forwards, and laterally, by means of the powerful muscles of mastication. It is by the varied movements of the lower teeth against the upper, through the action of these muscles, that the food is broken down or masticated. For information regarding the structure, etc., of the teeth, see TEETH; see also DENTITION.

The operation of mastication is very important, since the more the food is broken down the more easily will it mix with the saliva and other fluids which participate in the digestive process.

3. Insalivation is effected by the admixture of the secretions of the three pairs of salivary glands (the parotids, the submaxillaries, and the sublinguals) and of the buccal mucus with the triturated food. A brief description of these structures is given in the article GLAND. The common saliva, formed by the combined secretion of these various secreting organs, is a colorless, turbid, viscid, inodorous, and tasteless fluid, which, after standing for some time, deposits a layer of pavement epithelium (see EPITHELIUM) and mucus corpuscles. In the normal state, its reaction is alkaline, but the degree of alkalinity varies, and is greatest during and after meals. Saliva does not contain more than five or six parts of solid constituents to 995 or 994 parts of water, the most important ingredients being an organic matter termed ptyaline, and sulphocyanide of potassium, neither of which substances occurs in any other solid or fluid of the body. The daily quantity of saliva secreted by an adult man is estimated at about 48 ounces, but determinations of this kind must be regarded only as approximations to the truth, since the activity of the salivary glands is dependent upon various influences and conditions. Thus, movement of the lower jaw, as in masticating, speaking, or singing, increases the secretion; as also do acrid and aromatic substances, and dry hard food; while the use of moist and soft food is accompanied by a scanty secretion.

The uses of the saliva in reference to digestion are partly mechanical and partly chemical. The mechanical uses are almost too apparent to require notice. The moistening of the dry food by the saliva serves the double purpose of adapting it for deglutition and of separating the particles, and thus allowing them to be more freely acted on by the other digestive fluids; moreover, from its viscidity, it lubricates the bolus of food, and thus facilitates deglutition; and it is probably also subservient to the sense of taste. The great chemical use of the saliva is to convert the amylaceous (or starchy) portion of the food into glycose or grape sugar, and thus to promote its absorption.

4. Deglutition is the act by which the food is transferred from the mouth to the stomach. The pharynx, or cavity into which the mouth leads, takes so slight a part in the digestive process, that we need scarcely allude to any anatomical details connected with it. It is sufficient to observe that between it and the mouth is the pendulous or soft palate, which is a movable muscular partition that separates the two cavities during mastication. As soon, however, as the latter act is accomplished, and the bolus is pressed backwards by the tongue, the soft palate is drawn upwards and backwards, so as to permit the passage of the food into the pharynx. The bolus or pellet of food having arrived near the esophagus or gullet (which is continuous inferiorly and posteriorly with the pharynx), is driven into it by the action of certain muscles, which almost surround the pharynx, and are termed its constrictor muscles. All voluntary action censes as soon as the food is pressed backwards by the tongue into the pharynx. It is impossible to recall the pellet, and it is necessarily carried on (without even our cognizance) into the stomach. On receiving the food forced into its upper extremity by the action of the constrictor muscles of the pharynx, the esophagus is dilated (for it usually lies in a collapsed state, with its walls in contact, or nearly so); this contact of the pellet with its mucous membrane causes its muscular walls to contract, and the food is thus driven, by a series of these contractions, into the stomach. The act of deglutition is now completed.

5. Stomachal digestion or chymification is the next process to be considered. The whole of the alimentary canal (q. v.), (fig. 1) below the diaphragm (q.v.), or great muscular partition which separates the cavity of the chest from that of the abdomen or belly,

Digestion.

possesses the following points in common, in relation to structure: The stomach, the small intestine, and the large intestine, are all lined by mucous membrane, have a muscular coat, consisting of two sets of distinct fibers-namely, circular fibers which surround the tube or viscus after the manner of a series of rings, and longitudinal fibers running in the same direction as the intestine itself-and are invested with a serous membrane, the peritoneum (see SEROUS MEMBRANES), which at the same time retains the viscera in their proper position, and permits their necessary movements.

The human stomach is an elongated curved pouch, lying almost immediately below the diaphragm, and having the form of a bagpipe. It is very dilatable and contractile, and its function is to retain the food until it is duly acted upon and dissolved by the gastric juice, which is secreted by glands lying in its inner or mucous coat, and then to transmit it, in a semi-fluid or pulpy state, into the duodenum. Its average capacity is about five pints. The parts of it which have received special names are the greater curvature (fig. 1) b, the lesser curvature, upon its upper border, and the cardiac, e, and pyloric, d, extremities.

The mucous membrane, or lining coat of the stomach, is thick and soft, and lies in irregular folds, in consequence of the contraction of the muscular coat, unless when the organ is distended with food. On opening the stomach, and stretching it so as to remove the appearance of folds, we perceive even with the naked eye, but better with a lens, numerous irregular pits or depressions, irregular in shape, and averaging about both of an inch in diameter. To see them properly, the mucus with which they are filled must be washed out (fig. 2, A). These pits are so shallow as not to dip into the mucous membrane to a greater extent than th or th of the thickness. The rest of the thickness is chiefly made up of minute tubes, running parallel to one another, and vertically to the surface of the stomach (fig. 2, B). These are the gastric tubes or glands which secrete the gastric juice from the blood in the capillaries which abound in the

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mucous membrane. They pass in twos, threes, or fours from the bottom of each pit, and usually subdivide into several tubes, which, after running a more or less tortuous course, terminate in blind or closed extremities. These tubes are filled with epithelial cells, whose contents are composed of granules, with which oil-globules are often mixed, and each tube is invested with capillaries, which usually run in the direction of its long axis. In the pyloric or duodenal end of the stomach, these tubes (at least in the dog and several other animals whose stomachs have been carefully examined in a perfectly fresh state) are considerably wider than those which we have described, and differ from them also in other respects; and hence some physiologists believe that while they collectively secrete the gastric juice, one set may secrete the acid fluid and the organic matter

Digestion, termed pepsine, and the other mucus; the free acid and the pepsine are, as we shall shortly see, the two essential constituents of the gastric juice.

When food is introduced into the stomach, three special phenomena are induced in that viscus: 1. There are certain movements induced which are dependent on its muscular coat; 2. The mucous membrane is altered in appearance; and 3. There is the secretion of the gastric juice. Each of these phenomena requires a brief notice.

On killing an animal while the act of digestion is going on, and at once laying open its abdomen, we find that the stomach is in a contracted state, firmly embracing its contents, and with both its orifices so closed as to prevent the escape of the food, this contraction being due to the stimulation of the muscular coat by the food. If we examine the movements of the stomach during digestion, which we can do either by exposing the stomach of a living animal, or by sending a magneto-electric current through this organ in an animal just killed, we perceive that, in the cardiac half or two thirds, the movements are extremely slow, the muscular coat apparently contracting on the food, and progressively sending it towards the pylorus; whilst in the pyloric end of the stomach the movements are more energetic and rapid, resembling the peristaltic or vermicular movement, which we shall presently describe as occurring in the intestinal canal. When the transverse constriction has reached the firmly shut pylorus, a relaxation lasting about a minute ensues, followed by a repetition of the circular contractions. The movements which these contractions impress upon the food are described by Dr. Beaumont in the following terms: "The food entering the cardiac end of the stomach, c, turns to the left, descends into the splenic extremity, 8, and follows the great curvature towards the pyloric end, d. It then returns in the course of the smaller curvature, and makes its appearance again at the cardiac aperture in its descent into the great curvature to perform similar revolutions. These revolutions are effected in from one to three minutes." This account, given by Dr. Beaumont, is based on the observations which he made in the stomach of Alexis St. Martin, a Canadian, with a fistulous opening into the stomach, whose case is referred to in the article DIET. Dr. Brinton, however, adopts a modified view, which is probably the correct one. He supposes that the semi-fluid food entering at c (fig. 3), the cardiac orifice, goes in the directions marked a, partly along the greater and partly along the lesser curvature; and that these two currents of food meet at the closed pylorus, when they are both reflected into the direction b, forming a central or axial current, occupying the real axis of the stomach which unites the two apertures. The mutual interference of these currents at their borders causes a uniform admixture of the various substances composing them, while the reflection of the upper and lower currents into one another insures an equal contact of all the mass with the secreting surface of the mucous membrane.

FIG. 3.

Diagram to show the general direction of movement impressed on the semifluid food in the stomach.

aa, the hemispherical or surface current, carrying the semi-fluid food towards the closed pylorus, where it is reflected into b, the central current, which unites the cardiac (c) and pyloric (d) openings.

The changes in the mucous membrane are mainly the following: The inner surface of the healthy fasting stomach is of a paler pink tint than after the introduction of food, and while in the latter case the reaction of the moisture on the surface is very acid, in the former it is neutral, or even alkaline. Dr. Beaumont found (in the case of Alexis St. Martin) that, on the introduction of food into the stomach, the vessels of the mucous membrane became more injected, and that its color became changed from a pale pink to a deep red. A pure colorless and slightly viscid fluid, with a well-marked acid reaction, was then observed to distill from the surface of the membrane, and to collect in drops, which trickled down the walls, and mixed with the food.

That the gastric juice, which is the term applied to the acid fluid which Dr. Beaumont saw exuding from the mucous membrane, and which is secreted or formed in the gastric tubes which we have already described, is capable of exerting a solvent action on food, is proved by numerous experiments. It was first ascertained by Reaumur (1752), who obtained some of this fluid by making animals swallow sponges with a string attached, by which he could withdraw them. He thus showed that alimentary substances out of the body were altered by this fluid in the same manner as they are changed in the stomach, and disproved the favorite theory of that period, which ascribed all the changes which the food underwent in the stomach to a species of trituration. The subject of artificial digestion, or digestion out of the body, has, since that period, been carefully investigated by many observers, and there is now no doubt that the changes which the food undergoes in the stomach are essentially chemical, and not mechanical.

Two years before Beaumont's experiments, Dr. Prout had ascertained not only that an acid fluid is secreted by the gastric mucous membrane of rabbits, hares, horses, dogs, etc., during digestion, but that the acid is the muriatic or hydrochloric acid, and it was supposed that the solvent action of the gastric juice was due to this source. But experi

Digestion.

ments showed that the solvent action is not due simply to the acid of the gastric juice, and that the latter must contain some other ingredient which, either alone or in com bination with the acid, can exercise this power. It was then discovered that the addition of a portion of the gastric mucous membrane to water acidified with hydrochloric acid produced a perfect digestive fluid, due attention being paid to the temperature, which should be kept at about 100°, or about the normal temperature of the interior of the animal body. Later observations showed that we can obtain from the gastric mucous membrane the special organic matter on which its digestive power depends, and to this substance the name of pepsine has been given. The two essential elements of the gastric juice are then: 1. A free acid, which in some cases seems to be hydrochloric alone, and in others a mixture of hydrochloric and lactic acids; and 2. An organic matter, which is found on analysis to be highly nitrogenous, and to be allied to the albuminates, and which we term pepsine. The best analysis of human gastric juice is that made by Schmidt of Dorpat, who, in 1853, had an excellent and rare opportunity of examining it in the case of an Esthonian peasant, Catharine Kütt, aged 35 years, and weighing about 118 lbs., in whom there had existed for three years a gastric fistula or opening, three or four lines in diameter, under the left breast, between the cartilages of the ninth and tenth ribs. The introduction of dry pease and a little water into the stomach, through the opening, occasioned (even in the morning, on an empty stomach) the secretion of from 5 to 7 ozs. of a clear limpid fluid with an acid reaction, which, however, was much less strong than Schmidt had observed in previous experiments on the gastric juice of dogs and sheep, in which he had artificially established similar fistulous openings. The following table gives the mean of two analyses of the gastric juice of Catharine Kütt, with corresponding mean results of the same fluid in the sheep, a purely herbivorous animal, and in the dog, a purely carnivorous animal.

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The only impurity that could affect these analyses, is the saliva that possibly might have been swallowed.

The quantity of the gastric juice secreted in 24 hours was determined by Bidder and Schmidt (Die Verdauungs-säfte, etc.) in the sheep to be th, and in the dogth of the weight of the body. If the latter ratio were true for men, a man of ten stone weight would secrete about 14 lbs. of this fluid daily. In the case of Catharine Kütt, the mean daily quantity amounted to no less than 31 lbs., or to more than a fourth part of the weight of her body. On this calculation, a man of ten stone would daily secrete 37 lbs. of gastric juice.

The uses of this fluid in reference to digestion are clear. It serves not only to dis solve, but also to modify the nitrogenous elements of the food (such as albumen, fibrin, casein, and, in short, all animal food except fat, and the blood-forming portion of vege table food), converting them into new substances, termed peptones, which, although they coincide in their chemical composition, and in many of their physical properties, with the substances from which they are derived, differ essentially from them in their more ready solubility in water, and in various chemical relations. Thus, albumen is converted by the gastric juice into albumen-peptone, fibrin into fibrin peptone, etc. According to the investigations of Meissner, the albuminates are simultaneously decomposed or broken up into peptones and substances which he terms parapeptones, which latter are not further changed by the action of the gastric juice, but are converted into peptones by the action of the pancreatic juice, with which they come in contact in the duodenum. All the best observers agree that the gastric juice exerts no apparent action on the non-nitrogenous articles of food-namely, the fats and the carbo-hydrates (sugar, starch, etc.); as, however, the fats exert a favorable influence on the digestion of nitrogenous matters, it is probable that they undergo some slight, although not appreciable, modification. Gelatine and the gelatinous tissues are, as far as is known, the only nitroge nous articles of food which are not converted into peptones and parapeptones by the action of the gastric juice.

Although the main object of the gastric juice is to dissolve the albuminates, etc. (e.g., the contents of the egg, flesh, cheese, etc.), it appears from the experiments of Lehmann, Schmidt, and others, that it cannot dissolve the quantity necessary for the due nutrition of the organism. According to Lehmann, gastric juice can only dissolve th of its weight of coagulated albumen, while Schmidt makes the quantity as low as th. Now, since a dog secretes about th of its weight of gastric juice daily, it would only be able-even taking Lehmann's estimate, which is more than twice as high as Schmidt's-to digest 5 parts of dry or coagulated albumen for every 1000 parts of its

Digestion.

weight; but a dog, in order to keep in condition on an exclusive flesh diet-and this is its natural food-should take 50 parts of flesh, containing 10 parts of dry albuminates, for every 1000 parts of its weight. Hence its gastric juice only suffices for the digestion of Half the albuminates necessary for nutritiona result which is in accordance with the observed fact, that a considerable portion of the albuminates enters the duodenum in an undissolved state, and which will be explained when we consider the part which the intestinal juice-the fluid secreted by the various glands lying in the mucous membrane of the small intestine-takes in the digestive process. On comparing the experiments made on dogs with those made on Catharine Kütt, it appears

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FIG. 4.

that in the human subject the gastric digestion The under surface of the stomach and liver, of the albuminates is much more imperfect than even in the dog.

The process of gastric digestion is slow. According to Beaumont's researches on Alexis St. Martin, the mean time required for the digestion of ordinary animal food, such as butcher's meat, fowl, and game, was from two hours and three quarters to four hours.

which are raised to show the duodenum and pancreas.

st, stomach; p, its pyloric end; 1, liver; g, gall-bladder; d, duodenum, extending from the pyloric end of the stomach to the front, where the superior mesenteric artery (sm) crosses the intestines; pa, pancreas; sp, spleen; a, abdominal aorta.

The next point to be considered is: What becomes of the matters that are thoroughly dissolved in the stomach? Are they absorbed, without passing further down the canal? or do they pass through the pyloric valve into the duodenum, and are they finally taken up by the lacteals? Two of our highest authorities in physiological chemistry, Frerichs and Donders, maintain that the absorption of the peptones commences in the stomach: but the view generally adopted is, that the albuminates, etc., which are converted into peptones, are for the most part taken up by the lacteals. The rapidity with which

FIG. 5.

Vertical and longitudinal section of the small intestine in the lower part of the jejunum, showing the general arrangement of its coats.

a, villi; b, intestinal tubes or follicles of Lieberkuhn; c. submucous areolar tissue; d, circular muscular fibers; e, longitudinal muscular fibers.

Ба db FIG. 6.

Two villi, denuded of epithelium, with the lacteal vessels in their interior.

a, limitary membrane of the villus: b, basis of the same; c, dilated blind extremity of the central lacteal; d, trunk of the same.

aqueous solutions of iodide of potassium, the alkaline carbonates, lactates, citrates, etc., pass into the blood, and thence into the urine, saliva, etc., shows that the absorption of fluids must take place very shortly after they are swallowed, and there is little doubt that the blood-vessels (capillaries) of the stomach constitute the principal channel through which they pass out of the intestinal tract into the blood. As the veins of the stomach, which are formed by the union of these capillaries, contribute to form the portal vein (see CIRCULATION, ORGANS OF), the absorbed matters pass directly to the liver, and probably stimulate it to increased secretion (fig. 4).

6. We must now follow the progress of the semi-fluid mass known as the chyme, from the stomach into the small intestine, and notice the changes which are collectively impressed upon it, and are known as chylification or intestinal digestion. But before

U. K. IV.-53

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