CHAP. XXV.] THE INTESTINAL MUCOUS MEMBRANE. 227 



described certain permanent folds or valves, of semilunar shape, 

 which he demonstrated by moderately distending the rectum with 

 alcohol, which at the same time hardens its tunics, and thus displays 

 their condition in the state of repletion. Three is the average 

 number ; but sometimes a fourth is found, and at other times only 

 two are present. The largest and most constant valve is situated 

 opposite the base of the bladder, about three inches from the anus. 

 The fold next in frequency is placed at the upper end of the rectum ; 

 and the third occupies a position midway between these. When 

 a fourth is present, it is situated about one inch above the anus. 

 In the empty state of the intestine these folds overlap each other, 

 as Mr. Houston remarks, so effectually as to require considerable 

 manoeuvring to conduct a bougie or a finger along the cavity of 

 the intestine. Their use seems to be " to support the weight of 

 faecal matter, and prevent its urging towards the anus, where its 

 presence always excites a sensation demanding its discharge.* 



Of the Villi. Villi are minute processes of the mucous mem- 

 brane of the small intestine, to which they are exclusively confined 

 (fig. 157). They project from the free surface of the mucous mem- 

 brane into the cavity of the intestines, and seem admirably adapted 

 to become implanted, like so many little roots, in any semifluid or 

 fluid material which may fill the bowel. Villi are first found in the 

 duodenum, where they appear to develop themselves as elonga- 

 tions of the partitions between the cells into which Lieberkiihn's 

 tubes open. In the lower half of the duodenum, and the rest of 

 the small intestines, they are very numerous, and give to the sur- 

 face of the mucous membrane an appearance like that produced by 

 the pile of velvet. They are continued down to the ileo-ccecal 

 aperture, and cease abruptly at its margin, covering the surface of 

 the valve -segments next the ileum, but being wanting on the coecal 

 surface. 



A good view of the shape, arrangement, and number of the villi 

 may be obtained by examining a piece of villous mucous membrane 

 fixed under water. In man the villi are conical in shape, somewhat 

 flattened, and measure in length from apex to base from the -g^th to 

 the -5^-th. of an inch, They vary much in shape and size in the 

 lower animals : in the dog, cat and lion, they are long and almost 

 filiform ; in the sheep and rabbit they are small, flattened, and 

 conical ; in the turkey they are large and lamelliform. Most fishes 

 and reptiles are devoid of villi. 



In structure a villus resembles an everted Lieberkiihn's follicle ; 

 * Dub. Hosp. Rep. vol. v. p. 1 63. 



