II. INTESTINAL ABSORPTION.

FIG. 89.

The intestinal absorbents form a special department of the lymphatic system aiding nutrition. On account of the white chyle seen as a milky fluid through their transparent walls, they have been called lacteals. Their functions are to take up the nutrient fluid from the intestinal cavity, and to drain the tissue in which they lie. In order to fulfill these functions, they are arranged in a particular way, especially adapted to the peculiar construction of the mucous membrane lining this part of the alimentary tract, which must be

briefly described before the mechanism Diagram of relation of the of absorption can be understood.

epithelium to the lacteal radical in villus. The protoplasmic epithelial cells supposed to be connected to the absorbent vessel by adenoid tissue. (After Funke.)

They consist of nipintestinal cavity, so

The most striking characteristic of the lining membrane of the small intestine is the existence of villi, which are only found in this part of the alimentary canal. ple-shaped processes, projecting into the closely set that they have the appearance of the pile of velvet ; and being just visible to the naked eye, they give the mucous membrane, when washed and held under water, a peculiar velvety look. By means of these villi, and also of the ring-like folds of mucous membrane in the upper part of the small intestine, the extent of surface over which the chyme has to travel is greatly increased.

The surface of the villi is covered over with a simple layer of columnar epithelial cells in continuity with the epithelium lining the rest of the intestinal tract. The free surface of these cells is marked by a clear striated margin composed of a row of minute rods closely packed together, while the deep-seated end of the cells is branched, and appears to be prolonged into the substance of the villus and in some way to be connected with the supporting retiform tissue. Some of the cells are seen to swell upon the reagents, owing to their containing mucus,

addition of certain

which gives them a peculiar goblet shape; hence they are called goblet cells. These occur at intervals, and some observers consider that they form a distinct variety, differing from the neighboring cells just as the border cells of the stomach glands differ

The body of the villus is composed of a very delicate kind of

Section of Intestine of a Dog in which the blood vessels (c) and the lacteals (d) have been injected. The blind ending or simple loop of the black lacteal is seen to be surrounded by the capillary network of the blood vessels. (Cadiat.)

connective tissue, forming a slender frame in which a little cagelike network of blood vessels surrounds a central lacteal radicle. The interstices of this connective tissue are filled with pale protoplasmic cells, like those formed in the lymph. Under the basement membrane forming the foundation of the epithelium are some unstriated muscle cells which embrace the villus and are

Section of Single Lymph follicle of the Small Intestine, showing (a) follicle covered with epithelium (6), which has fallen from the villi (c); (d) Lieberkühn's follicles; (e) Muscularis mucosa. (Cadiat.)

able to squeeze it and empty the vessels that lie within it. The lacteal radicles which lie in the villi are sometimes double, and have a communication with the lymph spaces of the connective tissue. They frequently branch as they pass down from the villi to reach the dense network of lacteal vessels which lies beneath the mucous membrane. (Fig. 93.)

Section through the Intestinal Wall in the neighborhood of the grouped lymph follicles (f) (Peyer's patch), showing the upper] narrow (6) and the deep, wide (c) lymphatic plexuses.

At irregular intervals throughout the submucous tissue are found masses of lymphoid tissue similar to those seen in packets within a lymph gland or in other lymph follicles. These are either isolated (solitary glands) or collected into groups (agminated glands or Peyer's patches). Though called glands by anatomists, it should be borne in mind that they are in no way

connected with the secretion of any of the intestinal juices, but belong to the absorbing arrangements of the intestine. Around these solitary and grouped lymph follicles are spaces and networks from which the lacteal vessels arise (Fig. 93).

MECHANISM OF ABSORPTION.

Formerly absorption was supposed to take place by means of the blood vessels alone. After the discovery of lymph and chyle vessels by Caspar Aselli the belief in the direct absorption by the blood vessels was abandoned, and all the work of absorption was attributed to the lymphatics. Now, however, ample evidence exists to show that substances capable of absorption can make their way into the blood vessels of any part not protected by an impermeable covering like the horny layer of the skin, and thus be carried directly to the general circulation. The share taken by the blood vessels in interstitial absorption in the tissues is not defined, and when no impediment to the lymph flow exists is probably insignificant.

In the absorption from the alimentary tract the blood vessels appear to take a considerable part.

How far the tissue interspaces and the local lymph channels, many of which surround the blood vessels, aid in the passage of substances into the blood currents is not known; but they probably have some such effect, for the experiments showing direct absorption by the blood vessels leave the local lymph channels in operation, and at the same time the normal flow of lymph toward the thoracic duct is more or less hindered.

We can easily imagine that a surface covered by a single layer of epithelial cells, with numerous blood vessels and a good supply of absorbents beneath them, is capable of absorbing materials in solution; and we know that large quantities of fluids and solutions of various materials are absorbed from the stomach and large intestine, partly, no doubt, by means of the lacteals or lymphatics, and partly by the minute blood vessels themselves.

The small intestine, however, seems to be the part of the alimentary tract which is especially adapted for taking up the materials elaborated from the food.