Chap. XIV] THE DIGESTIVE SYSTEM 271 



colon until, reaching the left side, it curves beneath the lower 

 end of the spleen (left colic or splenic flexure) and passes downward 

 as the descending colon. Reaching the left iliac region on a level 

 with the margin of the crest of the ileum, it makes a curve like the 

 letter S, — hence its name of sigmoid flexure, — and finally ends 

 in the rectum. 



The rectum. — The rectum is from six to eight inches (15 to 

 20 cm.) long; it passes obliquely from the left until it reaches 

 the middle of the sacrum, then it follows the curve of the sacrum 

 and the coccyx, and finally arches slightly backward to its ter- 

 mination at the anus. 



The anus is the aperture leading from the rectum to the exterior 

 of the body. It is guarded, and except during defecation is kept 

 closed by the contraction of two involuntary circvilar muscles 

 called, respectively, the internal and external sphincters. 



Coats of the large intestine. — The large intestine has the usual 

 four coats except in some parts where the serous coat only par- 

 tially covers it, and the rectum, where the serous coat is lacking. 

 The muscular coat consists of two layers of fibres, one arranged 

 longitudinally and the other circularly. Beginning at the appen- 

 dix, the longitudinal fibres are arranged in three ribbon-like bands, 

 which extend the whole length of the colon to the rectum, and these 

 bands being shorter than the rest of the tube, the walls are puck- 

 ered between them. The third coat consists of submucous areolar 

 tissue, and the fourth or inner coat consists of mucous membrane. 

 The mucous coat possesses no villi and no circular folds. It 

 contains numerous tubular glands and solitary lymph nodules 

 which closely resemble those of the small intestine. 



Functions. — The functions of the large intestine are three. 



(1) The process of digestion is continued. This is due to the pres- 

 ence of bacteria, and to the digestive juices with which the food 

 became mixed in the small intestine. (2) The process of absorp- 

 tion is continued, and (3) the waste products are removed from the 

 body. 



ACCESSORY ORGANS OF DIGESTION 



The accessory organs of digestion are : (1) the salivary glands, 



(2) the tongue, (3) the teeth, (4) the pancreas, and (5) the liver. 

 The first three have been described. 



