3G0 THE SMALL IXTESTINE. 



* 



thirds of the distance round the interior of tlie tube, and tliey follow 

 closely one upon another along the intestine. The largest are about 2i 

 inches long and ^ of an inch wide at the middle or broadest part. Large 

 and small ones are often found to alternate with each other. Some of 

 them are bifurcated at one end, and others terminate abruptly, appearing 

 as if suddenly cut off. Each consists of a fold of the mucous membrane, 

 that is, of two layers placed back to back, united together by the sub- 

 mucous areolar tissue. They contain no part of the circular or of the 

 longitudinal muscular coats. Being extensions of tlie mucous membrane, 

 they serve to increase the absorbent surface to which the food is exposed, 

 and are said to contribute to delay its passage along the intestine. 



Tliere are no valvulas conniventes quite at the commencement of the 

 duodenum ; a short distance 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 hatf 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 altogether disappear. 



The villi, peculiar to the small intestine, and giving to its internal 

 surface the velvety or villous appearance already spoken of, are 

 small processes of the mucous membrane, which are closely set on 

 every part of the inner surface of the small intestine, over the valvule 

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

 placing a piece of intestine, well cleansed from its mucus, under water, 

 and examining it with a simple lens. 



The villi are, as a rule, conical and flattened in form (figs. 255, 257): 

 some are more cylindrical (fig. 25G), sometimes with an enlarged or 

 clubbed extremity. Occasionally two or three are connected together at 

 their base. 



Their length varies from ;^th to ^rd of a line, or even more ; and the 

 broad flattened kinds are about ^th or ^rth of a line wide, and -j-u^h or 

 a\th of a line thick. They are largest and most numerous in the duo- 

 denum and jejunum, and become gradually shorter, smaller, and fewer 

 in number in the ileum. In the upper part of the small intestine 

 Ivrause estimated their number at from 50 to 90 in a square line, and 

 in the lower part at i'rom 40 to 70 in the same space : he calculates their 

 total number to be at least four millions. 



In structure a villus consists of a prolongation of the proper mucous 

 membrane, and is, like that, covered by epithelium and encloses a net- 

 work of blood-vessels, one or more lacteal vessels, and a few plain 

 muscular fibre-cells, these being all supported and held together by 

 retiform or lymphoid tissue, which at the surface under the epithelium 

 is condensed into a basement membrane upon which the epithelial cells 

 are set. Nerves have not yet been demonstrated in the villi, although 

 they are probably not wanting. Each villus receives, as a rule, one 

 small arterial twig, which runs up the centre to near the middle of the 

 villus, where it begins to break u]d into a number of capillaries. These 

 form near the surface, beneath the epithelium and limiting membrane, 

 a fine capillary network, from which the blood is returned for the most 



