256 THE DIGESTIVE SYSTEM 



below in the right iliac fossa. This mesentery contains the 

 blood-vessels, nerves and lymph-vessels of the intestine and a 

 large number of lymph glands. The latter commonly become 

 enlarged in tabes mesenterica and throw recognisable shadows 

 in radiograms. It is characteristic of them that they are 

 irregularly placed and that, owing to the mobility of the 

 mesentery, they occupy different positions in radiograms taken 

 at different times. 



Taken together, the jejunum and ileum form a tube about 

 20 feet in length, but the two parts are not clearly marked off 

 from one another. In the jejunum, the mucous membrane 

 is thrown into numerous transverse folds which are termed the 

 plicae circulares (valvulae conniventes). They serve to in- 

 crease the size of the absorptive area without unduly increasing 

 the length of the intestine. The plicae circulares decrease in 

 number in the lower part of the jejunum, and they are almost 

 absent in the lower part of the ileum. 



In the ileum, collections of lymphoid tissue, termed the 

 intestinal tonsils (Peyer's patches), form elongated oval areas 

 in the mucous membrane. They are especially well marked 

 in the terminal part of the ileum and in the caecum. In typhoid 

 fever these areas are the site of small circular ulcers, the 

 confluence of which may form a typical ovoid patch, corre- 

 sponding in outline to the shape of the area. In tuberculous 

 disease the intestinal tonsils may be the site of chronic 

 ulceration. In this condition the ulcer tends to spread in 

 the direction of the intestinal blood- and lymph-vessels, i.e. 

 at right angles to the long axis of the gut, and when such 

 an ulcer heals the accompanying cicatricial changes result in 

 the formation of annular strictures. 



Under certain conditions (p. 277), the terminal portion of 

 the ileum may become kinked in such a way as to cause 

 serious obstruction to the passage of the intestinal contents. 

 As a result of this obstruction, the small intestine becomes 

 abnormally distended and the weight of the gut drags the 

 duodeno-jejunal flexure, which is fixed in position, in a down- 



