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 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 iu 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 memi brave 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 itt diameter throughout their entire length, and are parallel to one another, and perpendie-1 ular 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 DM; 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-excal valve, which forms the boundary between the small and large intestine. In man, they are conical in shape, and measure from Ath to Atli 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 atillus (fig. 6) is somewhat complicated,, but we must endeavor to explain it, because, tolerably accurate knowledge on this point, no one can understand how most of the essential elements of food (the &humiliates 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 vil lus immediately beneath the mucous membrane. From these arise small veins, which usually pass out of the villus in two, three, or more trunks, and contribute to form the portal vein, See CIR CULATION.
The villus also contains in its interior one or more lacteal.% 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 origin of their name. The tissue which occupies the cavity of the villus, in which the lacteals are imbedded, and which supports the capillary plexus, is in a great measure made up of nuclei and granules, except at the free extremity, where a vesicular structure, resembling very minute fat globules, is apparent.
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 numer ous transverse folds, which are termed the valrvlv connive/des, 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 did lower part of the duodenum, and in the jejunum (the parts in which they are most fully devel oped) 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.