Vi//i.— The interior of almost all the small intestine presents to the naked eye a texture very like that of velvet. For it is soft and shaggy : yields readily to pressure : and, on close examination, is evidently composed of innumerable short filaments, which are placed more or less vertically to the general inner sur face of the tube. These filaments, — the dense arrangement of which on a common surface causes this general velvety appearance,-- are thence usually named vi//i. Their form, and their situation, or office, might also be denoted by the name of intestinal or elzyleferous papillce.
We have seen that, in the stomach, the con fluent ridges intervening between the tubes are here and there raised into slight projec tions. These are rendered more prominent by artificial injection of the subjacent vessels, or even by that afflux of blood which ordi narily attends the digestive act. In the pylo ric extremity or the organ, these projections become more distinct. And just at its termi nation, some of them often assume the form of bluntly triangular and flattened folds.
In the upper part of the duodenum, the villi begin ; by processes which somewhat resemble the gastric elevations just alluded to, and occupy an analogous situation with respect to the intestinal tubes. At first, they may be described as flattened folds, the out line of which is a very obtuse triangle, that has a broad base about four or five times its height (Tiruth of an inch). In the lower part of the duodenum, this rudimentary form equal their own bulk. Here they also acquire their maximum length, which ranges from about it,th to viuth, or even Ath or of an inch. Their form, however, is still that of a flattened cone (comparefig. 259. andfig. 257. p.:317.) ; —the breadth of the base of which is about ith, and the depth about 46-th, its height. In the remainder of the intestine, the length of the villi gradually recedes to that which they possess in the lower part of the duodenum ; while their number also diminishes to a some what smaller extent. Throughout all this extent, the shapes and sizes of contiguous villi often present great varieties. But as a rule, the lower we descend in the examina tion of the intestine, the greater is the number of cylindrical forms we meet with. While towards the extremity of the ileum, the gradual diminution of their size renders many of them scarcely more than viuth of an inch in dia meter.
The villi cover the whole surface of the mucous membrane of the small intestine, in cluding its valvulw conniventes ; and they extend to the free margin of the valve which marks the commencement of the cmcurn and colon. The only exception to their pre
sence occurs in the agminate follicles, or " Peyer's patches." Here they are absent over the several follicles which together form each patch ; and become short, (a, fig. 272, p. 358.) blunt, irregular, or even confluent, where they occupy their interstices.
We have seen that each of the talvulx conniventes is a doubled fold of membrane, separated by a layer of areolar tissue. While the minute intestinal tube may almost be re garded as a mere membranous lamina, which is involuted so as to surround a cylindrical cavity, and is packed in a sparing fibrous invest ment. But the villus constitutes, as it were, a solid process of the mucous membrane. In accordance with this structure, it consists of an epithelium, a basement membrane, a stroma or basis of fibrous tissue, unstriped muscle, and numerous blood-vessels. And in ad dition to these constituents, which may be found under various modifications throughout the whole intestinal mucous membrane, the interior of each villus encloses one or more branches of the lacteal vessels which con tain the chyle.
The epithelium of the villi (a, fig. 259. and a, figs. 264,265,266.) consists of a single layer of cylindrical cells, vs hich,— as regards size, shape, and general appearance, — closely resemble those seen on the ridges between the tubes of the stomach. They are, how ever, even more delicate in their structure, as well as more conical in their shape.
And their contents are, even during fasting, soinev. hat darker and more granular. The nucleus, which occupies the same situation in both these varieties of cylindrical epi thelium, contains a single bright spot, or nucleolus : in rare instances, this appears to be double.
The basement-membrane (at b, fig. 260.) does not require any special mention. As in the gastric ridges, it is very closely at tached to the subjacent structures, espe cially to the vessels. But its continuity with the sirnilar structure forming the in testinal tubes sufficiently indicates that it is really a distinct membrane. And it is often demonstrated to be such by. the action of vvater ; which, after transuding it from the outer surface, raises the membrane, in the shape of a delicate transparent bulla, from the general mass of the villus beneath.