DIGESTION. 244 DIGESTION. (Fijr. 7). A single ixrU'iy I'liters its base, and passing up its centre, divides into a capillary plexus, which almost surrounds the villus imme- diately beneath the mucous membrane. From these arise small veins, which usually pass out of Fig. 7. Vertical section oT the coats of the small intes- tine, showing tite capiilnries and the be^nnin^ of the portal vein. The arteries are n<it seen, not hnvlnp: be«Mi penetratt-vi b.v the injwtion which has been thrown into the portal vein. H. veHwis of the villi : h, those of the tubes or follicles of Lieberkniui ; c, those of the muscular coat. the villus in two, three, or more trunks, and contribute to form the portal vein. See CiRCf- LATIOX. The villus also contains in its interior one or more laclvals, 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, sujigests the origin of their name. The tissue which occupies the cavity of the villus, in which the lacteals are imbedded, and which suppcuis 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 chiefly goes on. (2) They are most developed in that part of the intestine where chyle is first formed. (Ji) They are tur- gid, enlarged, and opaque during the process of eliylification, 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 trans- verse folds, which are termed the ralrula con- tiivcntes, from their valvular form and from their movements under water resembling the winking motion of the eyelids (Fig. 8). Each fold passes round three-foiirths or more of the put: 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 inch 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 Licberkuhirs follicles or tubes, which exist in the whole of the smaller intestine, there are otiier glandular or secreting structures, iiiibeildcd in the submucous tissue of certain por- tions of the intestinal tract, which require con- sideration. These are: (1) Brunner"s glands (q.v.), which occur only in '^'^ -^. the duodenum; (2) solitary glands, which seem to occur in all parts of the intestines, botli sinalLand large: and(.'i) Peycr's glands, which are usu- ally confined to the ileum. Hrunncr's r/lunds are most abundant at the pyloric end of the duodenum. In struc ture they resemble the pan creas. their ultimate elements l>eing btmches of vesicles, from which minute ducts arise, which coalesce and form larger ducts, through which the secretion is poured into the duodenum. It is believeil and Inl.l open tosho that they secrete a fluid simi- Rentes""'"' ""' lar to the pancreatic juice. The sulilary ylands occur in all jiarts of the intestine, but are perhaps more numerous in the jejunum than elsewhere. Each gland is a sinii>le mem- branous flask-shaped vesicle, the neck correspond- ing to the surface of the intestine, while the rounded base lies in the sidmiucous tissue. The neck presents no opening, and how the contents, which consist of nuclei and granular particles, are discharged into the intestine is not dearly known. As we never see them larger than a mustard-seed, we may presume that on attaining that size they burst. I'cycr's (ilands (Fig. P) Fio. 8. Small Inteii- tine distenthMl and hunleiied by alcohol. Flo. 9. Vertical section through a patch of Pe.rer'sgiauda In the do^. Jt. villi; /). tniies of I.ieberlvUhn : c. subinuconf* tissue, with theKlandsuf r,'.ver linlxHlded in it; </. ninsi'Ulur ami peritoneal coats ; e, apex of ono of the {glands pri>Jectiag among the tubes. are apparently mere aggregations of solitary glands, forming oval patches in the ileum. These patches vary in .size and number, being largest toward the c.Tcum, where their long diameter sometimes measures three or four inches, and smallest toward the jejunum, where their num- ber varies from fifteen to twenty, 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
