DIGESTION 203 



clearly defined. It is well known that mental states markedly influence the 

 contraction in one direction or another. 



It has also been experimentally determined that the introduction of 

 various acids and gases into the intestinal canal is followed by an increase in 

 the contraction. It is probable therefore that the gases, acids, and perhaps 

 other compounds as well, developed by bacterial action also act as excitants 

 to muscle activity. 



The Large Intestine. The large intestine is that portion of the ali- 

 mentary canal situated between the termination of the ileum and the anus. 

 It varies in length from one and a quarter to one and a half meters, in 

 diameter from three and a half to seven centimeters. It is divided into 

 the cecum, the colon (subdivided into an ascending, transverse, and descend- 

 ing portion, including the sigmoid flexure), and the rectum. 



The cecum is situated in the right iliac fossa. It is that dilated portion 

 of the large intestine below the orifice of the small intestine. The posterior 

 and inner wall presents a small opening which leads into a narrow round 

 process about ten centimeters in length the vermiform appendix. The 

 opening of the small intestine into the cecum is narrow and elongated and 

 bordered by two folds of mucous membrane strengthened by fibrous and 

 muscle-tissue. These folds constitute the so-called ileo-cecal valve. When 

 the cecum is distended the margins of these folds are approximated and 

 effectually prevent the return of material into the small intestine. 



The closure of this opening is now attributed to the activity of a sphincter 

 muscle, the ileo-colic, the contraction of which is regulated by the nerve 

 system, by way of the splanchnic nerves. 



The colon ascends to the under surface of the liver, where it bends at a 

 right angle, forming the hepatic flexure, crosses the abdominal cavity to 

 the spleen, forming the splenic flexure, descends to the left iliac fossa, and 

 bends again. At this point it turns upon itself to form the sigmoid flexure. 

 The rectum is a dilated pouch, situated within the true pelvis. It measures 

 from 15 to 1 8 centimeters in length. Within three centimeters of its termina- 

 tion at the anus it presents a constriction formed by a circular band of smooth 

 muscle-fibers known as the internal sphincter. The margin of the anus is 

 also surrounded by bands of striated muscle-fibers known collectively as the 

 external sphincter. 



The walls of the large intestine consist of three coats: viz.: serous, 

 muscular, and mucous. 



The serous is a reflection of the general peritoneal membrane. 



The muscle is composed of both longitudinal and circular fibers. The 

 longitudinal fibers are collected into three narrow bands which are situated 

 at points equidistant from one another. At the rectum they spread out so 

 as to surround it completely. As the longitudinal bands are shorter than 

 the intestine itself, its surface becomes sacculated, each sac being partially 

 separated from adjoining sacs by narrow constrictions. The circular fibers 

 are arranged in the form of a thin layer over the entire intestine. Betwet 

 the sacculi, however, they are more closely arranged. The sacculi have beer 

 termed haustra from their supposed function, that of absorbing or drawm 

 water from the intestinal contents thus imparting to them a certain degree 

 of consistency. In the rectum the circular fibers are well developed, and at 



