THE DIGESTIVE SYSTEM 235 



appendix appears to be a rudiment of a more extended cecum functional 

 in the ancestors of man. 



The colon is divided into four regions, ascending, transverse, descend- 

 ing and sigmoid colon. The ascending colon passes up the right side of the 

 abdominal cavity as far as the liver, where it bends to the left to form the 

 transverse colon. Reaching the lower end of the spleen on the left side, it 

 curves sharply downward, to become the descending colon. Passing down 

 the left side to a point below the kidney, the descending colon bends 

 toward 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 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, (Fig. 221) 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 means of anterior and posterior intestinal portals (Fig. 72). 

 From the fore-gut develop pharynx, esophagus, stomach, and the anterior 

 part of the small intestine; from the hind-gut the remainder of the intestine. 

 Early in development, an allantois arises as a ventral outpocketing of the 

 hind-gut, with which it retains connexion by an allantoic stalk. The cloaca 

 is the posterior portion of the hind-gut into which allantois and intestine 

 open, and which is closed to the exterior by the cloacal membrane 

 (Fig. 72Z}). 



The later development of the intestine involves its elongation and 

 twisting. The opening into the yolk-sac becomes reduced to a slender 

 vitelline duct, which disappears during the second month. Becoming at 

 first too long for the body-cavity, a loop of the intestine pushes down into 

 the umbilical cord. In a six weeks' embryo, the beginning of a cecum is 

 indicated by a swelling posterior to the vitelline duct. Later a horizontal 

 septum grows backward to divide the cloaca into a dorsal rectum and a 

 ventral urogenital sinus. The septum forms the perineum of the adult. 

 During the second month, an anal canal is formed by the invagination of an 

 ectodermal proctodeum and the rupture of the cloacal membrane. (Fig. 

 217) The four layers of the intestinal wall develop as has been 

 described for the stomach. 



History of the Intestine. The intestine as a region for the digestion 

 and absorption of food is present in the great majority of animals from 



