(6) We must HOW follow the progress of tho semi-fluid mass, known as the chyme, from the stimmell into the small intestine, and notice the changes which are collectively impressed upon it, and are known as chylifica lion or intestinal di gestion. Ilia before we can satisfactorily do this, we must say a few words re garding the intestinal mucous membrane, with its various glands, etc., and on the changes which take place in it during digestion.
The moor; membrane of the small intestine resembles that of the stomach in so far as it is of eonsiderable thick ness, and consists in a great measure of laterally grouped tubes. The reader is referred to Fig. 5, which exhibits a sect mu of the mucous mem brane of the small intestine in the dog. The 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 de scribed by Brunner. They are straight, nearly uniform in diameter throughout their en tire length, and are parallel to one another, and perpen dicular 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 bine to yield the intestinal juice whose ters and uses will shortly come under our notice.
The projecting bodies marked a in the figure are termed the rilli ; 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 num ber at first, they become very (cover ing 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 lams appearance: they finally (Paso at the ileo-ewcal valve, which forms the boundary be tween the small and large intestine. In man, they are conical in shape, and measure front one-forty-fifth to one-sixtieth 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 nail more filiform than in man.) The structure of a villa: /Fig. 61 is somewhat emnplicated, but We n1114 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 albuini nates and fatty matters) make their way from the intestine to the blood. Each villus is pro
vided with an abundant set of capillaries., which doubtless absorb fluid matters, which thus find their way ilireetly from the bowels into the blood (Fig. . A single artery inters it, 11-e, and parsing up it- centre, divides into a capillary plexus. which :111110.-t surrounds the villa: imm• diately bem.it It the mucous membrane. From these arise small veins, NVII1V11 pass out of the villus in two, three. or more trunks, and contribute to the portal Ieiu. See CIRCU LATION.
The villas also contains in its interior one or more biricals. which are vessels with club-shaped closed extremitie,, 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 oeculdes the cavity of the villas, in which the lacteal., are imbedded. and which supports the capillary plexus, is in a great measure maple up of nuclei and grannies, except at the free extremity, whore vesieular 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: I I I The villi exist only in the small intestine, where the absorption of food chiefly goes on. l'2i They are most developed in that part of the intestine where chyle is first formed. (3) They are tur gid. enlarged, and opaque during the process of chyliticat ion. and small and shrunken in animals that liav• been kept fast big for some time before death.
In addition to the villi. the mucous membrane of the small intestine presents numerous trans verse folds. bird' are termed the rat ruhr ron virt nit s from their valvular form and from their movements under water resembling, the winking motion of the eyelids ( Fig. ti). Each fold passes round thrce-fonrths or more of the gut and in the lower part of the duodenum, and in the jejunum (the parts in they tire most hilly developed), they are often more than half an inch in depth: further on they diminish in depth. length, and number, and in the lov.e•t part of the ileum they can seareely be traced. Their objeet clearly is to increase the extent of the absorbent mucous membrane.