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 secre tion 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 expos ing 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.

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 4th, 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 dissolve, 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

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; 9, 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.

Two villi, denuded of epithelium, with the laoteal 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

Digestion.

we can satisfactorily do this, we must say a few words regarding the intestinal mucous membrane, with its various glands, etc., and on the changes which take place in it during digestion.

The mucous membrane of the small intestine resembles that of the stomach in so far as it is of considerable thickness, and consists in a great measure of laterally grouped tubes. The reader is referred to fig. 5, which exhibits a section of the mucous membrane of the small intestine in the dog. These tubes, which form the great mass of the middle portion of the section marked b, are commonly called the follicles of Lieberkuhn, although they were first described by Brunner. They are straight, nearly uniform in diameter throughout their entire length, and are parallel to one another, and perpendicular to the inner surface of the small intestine on which they open. Nothing is known of the exact nature of their secretion; but in association with the secretions of other glands, they combine to yield the intestinal juice whose characters and uses will shortly come under our notice.

The projecting bodies marked a in the figure are termed the villi; they are minute processes of the mucous membrane of the small intestine, and obviously serve to increase to a great extent the amount of absorbing mucous membrane. They first appear in the duodenum, where they seem to develop themselves as elongations of the partitions between the cells or pits into which the tubes open. Comparatively scanty in number at first, they become very numerous (covering the whole surface) in the further part of the duodenum and the rest of the small intestines, giving to the mucous membrane a velvet-like or pilous appearance; they finally cease at the ileo-cæcal valve, which forms the boundary between the small and large intestine. In man, they are conical in shape, and measure from th to th of an inch in length. They vary much in shape and size in the lower mammals and in birds. (In carnivorous animals, as the dog, they are longer and more filiform than in man.)

The structure of a villus (fig. 6) is somewhat complicated, but we must endeavor to explain it, because, without tolerably accurate knowledge on this point, no one can understand how most of the essential elements of food (the albuminates and fatty matters) make their way from the intestine to the blood. Each villus is provided with an abundant set of capillaries, which doubtless absorb fluid matters, which thus find their way directly from the bowels into the blood (fig. 7). A single artery enters its base, and passing up its center, divides into a capillary plexus, which almost surrounds the villus immediately beneath the mucous membrane. From these arise small veins, which usually pass 6 out of the villus in two, three, or more trunks, and contribute to form the portal vein. See CIR

FIG. 7.

C

CULATION.

The villus also contains in its interior one or more lacteals, which are vessels with club-shaped closed extremities, which absorb the chyle from the intestine. Their milk-white appearance, when they are filled with chyle, suggests the Vertical section of the coats of the small in- origin of their name. The tissue which occupies testine, showing the capillaries and the beginnings of the portal vein. The arter- the cavity of the villus, in which the lacteals are ies are not seen, not having been pene- imbedded, and which supports the capillary trated by the injection which has been plexus, is in a great measure made up of nuclei

thrown into the portal vein.

a, vessels of the villi; b, those of the tubes and granules, except at the free extremity, where or follicles of Lieberkuhn; c, those of the a vesicular structure, resembling very minute fat globules, is apparent.

muscular coat.

There is abundant evidence that the function of the villi is connected with absorption, and mainly with the absorption of chyle. 1. The villi exist only in the small intestine, where the absorption of food goes chiefly on. 2. They are most developed in that part of the intestine where chyle is first formed. 3. They are turgid, enlarged, and opaque during the process of chylification, and small and shrunken in animals that have been kept fasting for some time before death.

In addition to the villi, the mucous membrane of the small intestine presents numerous transverse folds, which are termed the valvula conniventes, from their valvular form and from their movements under water resembling the winking motion of the eyelids (fig. 8). Each fold passes round three fourths or more of the gut; and in the lower part of the duodenum, and in the jejunum (the parts in which they are most fully developed) they are often more than half an in. in depth; further on, they diminish in depth, length, and number, and in the lowest part of the ileum they can scarcely be traced. Their object clearly is to increase the extent of the absorbent mucous membrane.

In addition to Lieberkuhn's follicles or tubes, which exist in the whole of the smaller intestine, there are other glandular or secreting structures, imbedded in the submucous tissue of certain portions of the intestinal tract, which require consideration. These

Digestion. are: 1. Brunner's glands, which occur only in the duodenum; 2. Solitary glands, which seem to occur in all parts of the intestine, both small and large; and 3. Peyer's glands, which are usually confined to the ileum.

Brunner's glands are most abundant at the pyloric end of the duodenum. In struct ure, they resemble the pancreas, their ultimate elements being bunches of vesicles, from which minute ducts arise, which coalesce and form larger ducts, through which the secretion is poured into the duodenum. It is believed that they secrete a fluid similar to the pancreatic juice. The solitary glands occur in all parts of the intestine, but are perhaps more numerous in the jejunum than elsewhere. Each gland is a simple membranous flask-shaped vesicle, the neck corresponding to the surface of the intestine, while the rounded base lies in the submucous tissue. The neck presents no opening, and how the contents, which consist of nuclei and granular particles, are discharged into the intestine, is not clearly known. As we never see them larger than a mustardseed, we may presume that, on attaining that size, they burst. Peyer's glands (fig. 9) are apparently mere aggregations of solitary glands, forming oval patches in the ileum. These patches vary in size and number, being largest towards the cæcum, where their long diameter sometimes measures 3 or 4 in., and smallest towards the jejunum; while their number varies from 15 to 20, or even more. Nothing certain is known regarding the uses of these solitary or aggregated glands; but as they are largest during the digestive process, we must infer that they are in some way connected with that function. Possibly the peculiar odor of the fæces may be due to their secretion. In typhoid or enteric fever, and in phthisis, these glands become ulcerated, which probably occasions the diarrhea so common in these diseases.

Brunner's glands are much more developed in the herbivora than in the carnivora; Peyer's, on the other hand, are most developed in the latter.

We have endeavored, in the preceding sentences, to give the reader some idea of the complicated structure of the mucous and submucous coat of the small intestines; we now proceed to notice the chief uses of the muscular coat of the intestine. This coat,

FIG. 8.

Small intestine distended and hardened by alcohol, and laid open to show the valvulæ conniventes.

FIG. 9.

Vertical section through a patch of Peyer's glands in the dog. a, villi; b, tubes of Lieberkuhn; c submucous tissue, with the glands of Peyer imbedded in it; d, muscular and peritoneal coats; e, apex of one of the glands projecting among the tubes.

as has been already mentioned, consists of two layers of muscular fibers-namely, circular and longitudinal fibers, of which the former lie next to the submucous coat. The peristaltic or vermicular action by which the substances which enter the duodenum from the stomach are moved onwards, is due to this muscular coat. A person who has once seen the abdomen of an animal laid open immediately after death, will have a better idea of the nature of this movement than can be afforded by any description. It commences about the pyloric third of the stomach, from whence successive wave-like movements are propagated through the entire length of the intestinal canal. It is the rapid succession of these alternate contractions and relaxations that impels the intestinal contents onwards, and occasion those movements which, from their resemblance to the writhings of a worm, have been termed vermicular. It is very probable that the rapidity of this movement varies in different individuals-those persons, for example, whose bowels act twice daily having a more rapid vermicular motion than those in whom the act of defecation occurs only once in the twenty-four hours.

We have now to consider the effects produced on the chyme by the different fluids with which it becomes mixed in the small intestine. These fluids are: 1. The bile; 2. The pancreatic juice; and, 3. The intestinal juice.

The bile (see BILE) is a faintly alkaline or neutral fluid, containing two essential con stituents, one of which is of a resinous nature, while the other is a pigment. The resinous constituent is not precisely identical in all kinds of bile, but it generally consists of a soda-salt whose acid is either glyco-cholic or tauro-cholic acid (q.v.), or of a mixture