146 ANATOMY AND PHYSIOLOGY 



and becomes the so-called sigmoid colon. It then enters the pelvis 

 to become the rectum. 



Surgical note. The ascending colon lies so close to the posterior abdominal 

 wall that there is no peritoneum behind it, and the descending colon also is bare 

 in a narrow strip at the back, consequently the surgeon may take advantage of 

 this condition to open the colon without wounding the peritoneum, in the 

 operation called lumbo-colotomy. 



The rectum is about five to seven inches long, very distensible, 

 and so called because it has no convolutions, but simply follows the 

 curve of the pelvic wall, lying in front of the sacrum and coccyx. 

 In the last inch or inch and a half it bends backward (perineal 

 flexure) to pass the tip of the coccyx. This is the anal canal, and 

 it ends at the opening called the anus (Fig. no). 



The portion above the anal canal is the widest part the rectal 

 pouch. 



The mucous membrane of the rectum is red, and usually pre- 

 sents two or three special folds about two or three inches above the 

 anus, called the rectal folds, or Houston's valves. 



The largest, a permanent fold, is on the right side about two and one-half 

 inches above the anus and called the third sphincter. Two smaller ones, not 

 permanent, are on the left side, above and below the former. 



The muscular coat has the two layers, circular and longitudinal. 

 The peritoneal coat covers the front and sides of the upper part only. 



The reaction of the fluids in the large intestine is alkaline. 



Sphincters of the anus. The circular fibers around the anal 

 canal form the internal sphincter. 



The external sphincter is a flat circular muscle just under the 

 skin around the anus. (Its contraction causes the radiating lines 

 in the skin.) The function of the sphincters is to guard and 

 control the anus. 



Clinical note. The point of a syringe should be passed in an upward and 

 forward direction through the anal canal, and then turned backward, 



RESUME. 



The alimentary tract begins with the mouth and ends with the large in- 

 testine, passing through the head, neck, thorax, and pelvis. It is practically 

 a long tube of mucous membrane surrounded by layers of muscle and held to 

 them by connective tissue. The mucous membrane contains glands which 

 secrete the digestive fluids. The muscle layers pass the food along, that it may 

 be acted upon in all portions of the tract; and wherever free motion accom- 



