226 DIGESTION. [CHAP. xxv. 



about three-fourths or five-sixths of its circumference, gradually 

 diminishing in depth towards each extremity, but sometimes bifur- 

 cating and coalescing by one or both subdivisions with the fold 

 above and below. In the lowest part of the duodenum, and in 

 the jejunum, the valvulce conniventes acquire their highest develop- 

 ment. Here they lie very close together, and many of them pass 

 nearly round the intestine ; they are deeper, also, here than else- 

 where, being sometimes half or three-fourths of an inch in depth. 

 In the ileum they gradually diminish in length and in depth, 

 frequently not passing round more than one-half the circumference 

 of the intestine, and measuring not more than one-fourth or one- 

 eighth of an inch in depth ; and in the lowest part of the ileum 

 they almost entirely disappear. 



It is remarkable, that these folds are peculiar to the human 

 subject. No other animal, so far as we know, exhibits any ar- 

 rangement of transverse folds of the intestinal mucous membrane 

 resembling them. 



The folds of mucous membrane in the large intestine are the par- 

 titions between the cells of the colon ; they exhibit much uniformity 

 of shape, although they vary very much in size ; they are least 

 developed in the sigmoid flexure. 



At the junction of the ileum with the coecum there are two 

 folds which bound the slit-like aperture of communication between 

 the small and the large intestine. These are the segments of the 

 ileo-ccecal valve. The aperture is a simple slit which passes nearly 

 horizontally from before backwards, and is bounded on all sides by 

 mucous membrane. Its lower border is formed by the free edge of 

 a deep semilunar fold of mucous membrane, enclosing submucous 

 tissue, and a few circular muscular fibres ; and another fold, of 

 much less depth, but of similar shape and constitution, forms its 

 upper lip. This latter fold has a more horizontal direction than the 

 former, which is nearly vertical. The folds coalesce in front of 

 and behind the aperture, and form small bands, called frcena, and 

 which follow for a short distance the course and direction of the 

 segments of the valve. The free margins of these two segments 

 come in apposition in the distended state of the ccecum and close 

 the aperture, or at least diminish and constrict it so much as, in 

 general, to prevent the reflux of any but liquid or much-subdivided 

 matters into the small intestine. 



In the rectum there are folds of various sizes and directions, 

 which are most numerous in the empty state of the gut, and are 

 effaced by its distension. The late Mr. Houston, of Dublin, has 



