may also be shown, as already mentioned, by killing the animal and examining the contents of the alimentary canal. During the

digestion of muscular flesh and adipose tissue, the stomach contains masses of softened meat, smeared over with gastric juice, and also a moderate quantity of grayish grumous fluid, with an acid reaction. This fluid contains muscular fibres, isolated from each other, and more or less disintegrated by the action of the gastric juice. (Fig. 33.) The fat vesicles are but little or not at all altered in the stomach, and there are only a few free oil globules to be seen floating in the mixed fluids, contained in the cavity

of the organ. In the duodenum the muscular fibres are further disintegrated. (Fig. 34.) They become very much broken up, pale

and transparent, but can still be recognized by the granular markings and striations which are characteristic of them. The fat vesicles also begin to become altered in the duodenum. The solid granular fat of beef, and similar kinds of meat, becomes liquefied and emulsioned; and appears under the form of free oil drops and fatty molecules; while the fat vesicle itself is partially emptied, and becomes more or less collapsed and shrivelled. In the middle and lower parts of the intestine (Figs. 35 and 36)

these changes continue. The muscular fibres become constantly

more and more disintegrated, and a large quantity of granular debris is produced, which is at last also dissolved. The fat also

progressively disappears, and

the vesicles may be seen in the lower part of the intestine, entirely collapsed and empty.

In this way the digestion of the different ingredients of the food goes on in a continuous manner, from the stomach throughout the entire length of the small intestine. At the same time, it results in the production of three different substances, viz: 1st. Albuminose, produced by the

Fig. 35.

FROM MIDDLE OF SMALL INTESTINE.-a, a.

action of the gastric juice Fat vesicles, nearly emptied of their contents.

on the albuminoid matters;

2d. An oily emulsion, produced by the action of the pancreatic juice on fat; and, 3d. Sugar, produced from the transformation of starch by the mixed intestinal fluids. These substances are then ready to be taken up into the circulation; and the next change which they undergo, in the regular course of the vital processes, is that of absorption. This process will form the subject of the next chapter.

Fig. 36.

a

FROM LAST QUARTER OF SMALL INTESTINE. -a, a. Fat vesicles, quite empty and shrivelled.

CHAPTER VII.

ABSORPTION.

BESIDE the glands of Brunner and the follicles of Lieberkühn, already described, there are, in the inner part of the walls of the

Fig. 37.

intestine, certain glandularlooking bodies which are termed "glandulæ solitariæ," and "glandulæ agminatæ." The glandulæ solitariæ are globular or ovoid bodies, about one-thirtieth of an inch in diameter, situated partly in, and partly beneath, the intestinal mucous membrane. Each glandule (Fig. 37) is formed of an investing capsule, closed on all sides, and containing in its interior a

PATCHES, from Small Intestine of Pig. Magnified soft pulpy mass, which con

50 diameters.

Fig. 38.

GLANDULE AGMINATE, from Small Intestine of Pig. Magnified 20 diameters.

sists of minute cellular bodies, imbedded in a homogeneous substance. The inclosed mass is penetrated by capillary blood vessels, which penetrate through the investing capsule, inosculate freely with each other, and return upon themselves in loops, near the centre of the glandular body. There is no external opening or duct; in fact, the contents of the vesicle, being pulpy and vascular, as already described, are not to be regarded

as a secretion, but as constituting a kind of solid glandular tissue. The glandulæ agminatæ (Fig. 38), or "Peyer's patches," as they are sometimes called, consist of aggregations of similar globular or ovoid bodies, found most abundantly toward the lower extremity of the small intestine. Both the solitary and agminated glandules are evidently connected with the lacteals and the system of the mesenteric glands, which latter organs they resemble very much in their minute structure. They are probably to be regarded as the first row of mesenteric glands, situated in the walls of the intestinal canal.

Fig. 39.

Another set of organs, intimately connected with the process of absorption, are the villi of the small intestine. These are conical vascular eminences of the mucous membrane, thickly set over the whole internal surface of the small intestine. In the upper portion of the intestine, they are flattened and triangular in form, resembling somewhat the conical projections of the pyloric portion of the sto mach. In the lower part, they are long and filiform, and often slightly enlarged, or club-shaped, at their free extremity (Fig. 39), and frequently attaining the length of one-thirty-fifth of an inch. They are covered externally with a layer of columnar epithelium, similar to that which lines the rest of the intestinal mucous membrane, and contain in their interior two sets of vessels. The most superficial of these are the capillary blood vessels, which are supplied in each villus by a twig of the mesenteric artery, and which form, by their frequent inosculation, an exceedingly close and abundant network, almost immediately beneath the epithelial layer. They unite at the base of the villus, and form a minute vein, which is one of the commencing rootlets of the portal vein. In the central part of the villus, and lying nearly in its axis, there epithelium. 6. Bloodvessel. c. Lacteal is another vessel, with thinner and more

EXTREMITY OF INTESTINAL VILLUS; from the Dog.-. Layer of

transparent walls, which is the commencement of a lacteal. The precise manner in which the lacteal originates in the extremity of the villus is not know It commences near the apex, either by a

blind extremity or by an irregular plexus, passes, in a straight or somewhat wavy line, toward the base of the villus, and then becomes continuous with a small twig of the mesenteric lacteals.

The villi are the active agents in the process of absorption. By their projecting form, and their great abundance, they increase enormously the extent of surface over which the digested fluids come in contact with the intestinal mucous membrane, and increase, also, to a corresponding degree, the energy with which absorption takes place. They hang out into the nutritious, semi-fluid mass contained in the intestinal cavity, as the roots of a tree penetrate the soil; and they imbibe the liquefied portions of the food, with a rapidity which is in direct proportion to their extent of surface, and the activity of their circulation.

The process of absorption is also hastened by the peristaltic movements of the intestine. The muscular layer here, as in other parts of the alimentary canal, is double, consisting of both circular and longitudinal fibres. The action of these fibres may be readily seen by pinching the exposed intestine with the blades of a forceps. A contraction then takes place at the spot irritated, by which the intestine is reduced in diameter, its cavity obliterated, and its contents forced onward into the succeeding portion of the alimentary canal. The local contraction then propagates itself to the neighboring parts, while the portion originally contracted becomes relaxed; so that a slow, continuous, creeping motion of the intestine is produced, by successive waves of contraction and relaxation, which follow each other from above downward. At the same time, the longitudinal fibres have a similar alternating action, drawing the narrowed portions of intestine up and down, as they successively enter into contraction, or become relaxed in the intervals. The effect of the whole is to produce a peculiar, writhing, worm-like, or "vermicular" motion, among the different coils of intestine. During life, the vermicular or peristaltic motion of the intestine is excited by the presence of food undergoing digestion. By its action, the substances which pass from the stomach into the intestine are steadily carried from above downward, so as to traverse the entire length of the small intestine, and to come in contact successively with the whole extent of its mucous membrane. During this pas sage, the absorption of the digested food is constantly going on. Its liquefied portions are taken up by the villi of the mucous membrane, and successively disappear; so that, at the termination of the small intestine, there remains only the undigestible portion of the