3l6 COMPARATIVE ANATOMY 



to form the transverse colon. Reaching the lower end of the spleen on 

 the left side, it curves sharply downwards, to become the descending 

 colon. Passing down the left side to a point below the kidney, th^ 

 descending colon bends towards the median plane of the body and enters 

 the pelvic cavity, where it forms the sigmoid flexure. The rectum is 

 continuous with the sigmoid colon and extends to the anus. In the 

 rectum, a number of transverse folds of the wall tend to prevent fecal 



Fig. 265. — Transverse section of the human vermiform process. Note the absence 

 of villi and the abundance of nodules. Clear spaces in the submucosa are fat cells. 

 Only a part of the circular layer of the muscularis has been drawn. X20. (From 

 Bremer's "Text Book of Histology," after Sobotta.) 



matter from pressing into the anal canal. In the anal region, the layer of 

 circular muscles is thickened to form the sphincter ani, which, unlike that 

 of the lower rectum, is non-striated and not under control of the will. 

 The external sphincter of the anus, however, is striated and voluntary. 



Development of the Intestine. Except in the mouth and anal regions, 

 the mucous lining of the alimentary canal and the secretory epithelium of 

 the glands connected with it develop from the endoderm. Primarily, the 

 endoderm of the embryonic area is continuous with that which lines 

 the yolk-sac. In correlation with the development of head fold and tail 

 fold, a fore-gut and hind-gut are formed in connexion with the yolk-sac by 



