ABDOMINAL CAVITY 



327 



on the inner wall of the small intestine. They are folds of 

 the mucous membrane placed more or less transversely to 

 the long axis of the intestine. Note particularly that they are 

 permanent folds, and that no amount of stretching or distension 

 of the walls will cause their obliteration. On careful study 

 of the dried specimen three 

 main varieties of plicae cir- 

 culares may be recognised. 

 The great majority are in the 

 form of crescentic folds, which 

 extend for a variable distance 

 round the wall of the intestine; 

 others form complete rings 

 around the interior ; the third 

 variety, usually the least numer- 

 ous, are arranged in a spiral 

 manner, and take from one 

 to three spiral turns around 

 the wall of the intestine. 

 Each fold consists of two 

 layers of mucous membrane, 

 with a little intervening areolar 

 tissue derived from the sub- 

 mucous coat. The other coats 

 of the intestine take no part 

 in the formation of the plicae 

 circulares. In the upper part 

 of the jejunum the plicae cir- 

 culares are strongly developed, FlG - ^: -Typical part of 



showing few and small Plicae Circu- 



and placed so closely together j ares 

 that the intervals between 



them are hardly greater than the thickness of one of the folds. 

 Lower down, however, the plicae gradually diminish in numbers, 

 become more widely separated, more oblique in their direction, 

 and not nearly so large. Towards the middle of the ileum, 

 they become few and far between, and a little beyond that 

 point they usually disappear altogether (Figs. 150, 151). 



The chief function of the plicre circulares is to increase the absorbing 

 and secreting surface of the small intestine. 



Another characteristic of the mucous lining of the small 

 intestine is the presence of villi. They are minute projec- 



