812 THE INTESTINE. 



The internal or mucous coat is characterised by presenting all over its 

 inner surface a finely flocculeiit or shaggy appearance, like the pile upon 

 velvet, owing to its being covered with multitudes of minute processes, 

 named -villi ; hence it is also named the villous coat. It is one of the 

 most vascular membranes in the whole body, and it is naturally of a red- 

 dish colour in the upper part of the small intestine, but becomes paler, and 

 at the same time thinner towards the lower end. The mucous tissue con- 

 tains beneath its basement membrane, a thin muscular layer, demonstrated 

 easily in animals, but scarcely recognisable in man. It presents for con- 

 sideration, 1, the large folds called valvulce conniventes ; 2, the villi and 

 epithelium ; 3, the glands. 



1. Valvulce Conniventes. The folds and wrinkles found upon the inner 

 surface of the oesophagus and stomach may be completely obliterated by full 

 distension of those parts of the alimentary canal. In the lining membrane 

 of the small intestine, however, there exist, besides such effaceable folds, 

 other permanent ones, which cannot be obliterated, even when the tube is 

 forcibly distended. These permanent folds are the valvulce conniventes t or 

 valves of Kerkring. They are crescentic projections of the mucous mem- 

 brane, placed transversely to the course of the bowel, each of them reaching 

 about one-half or two-thirds of the distance round the interior of the tube, 

 and they follow closely one upon another along the intestine. 



The largest of these valves are about two and a half inches long and one- 

 third of an inch wide at the middle or broadest part; but the greater number 

 are under these dimensions. Large and small valves are often found to alter- 

 nate with each other. Some of them are bifurcated at one end, and others 

 terminate abruptly, appearing as if suddenly cut off. Each valve consists of 

 a fold of the mucous membrane, that is, of two layers placed back to back, 

 united together by the submucous or areolar tissue. They contain no part 

 of the circular and longitudinal muscular coats. Being extensions of the 

 mucous membrane, they serve to increase the absorbent surface to which the 

 food is exposed, and at the same time they contribute to delay its passage 

 along the intestine. 



There are no valvulse conniventes quite at the commencement of the 

 duodenum ; about an inch or somewhat more from the pylorus they begin 

 to appear ; beyond the point at which the bile and pancreatic juice are 

 poured into the duodenum they are very large, regularly crescentic in form, 

 and placed so near to each other that the intervals between them are not 

 greater than the breadth of one of the valves : they continue thus through 

 the rest of the duodenum and along the upper half of the jejunum ; below 

 that point they begin to get smaller and farther apart ; and finally, towards 

 the middle of the ileum, having gradually become more and more irregular 

 and indistinct, sometimes even acquiring a very oblique direction, they alto- 

 gether disappear. 



2. Villi. The villi, peculiar to the small intestine, and giving to its 

 internal surface the velvety or villous appearance already spoken of, are 

 small, elongated, and highly vascular processes, which are found situated 

 closely together on every part of the mucous membrane, over the valvulao 

 conniventes, as well as between them. They are best displayed by placing 

 a piece of intestine, well cleansed from its mucus, under water, and examin- 

 ing it with a simple lens. The prevalent form of the villi is that of minute, 

 flattened, bell-shaped membranous processes ; others are conical or cylindrical, 

 or even clubbed, or filiform at the free extremity. A few are compound as 

 if two or three villi were connected together at their base. 



