DIGESTION. 199 



tion of the vagus depends upon the extent to which the contraction at the 

 moment is inhibited by the splanchnic. Thus after the splanchnics are 

 divided, stimulation of the vagus causes a much more pronounced contrac- 

 tion than would otherwise be the case; a fact that indicates that the 

 splanchnic nerve-center is in a state of tonus or tonic activity and therefore 

 exerting a constant inhibitor effect on the muscle-fibers. Stimulation of the 

 peripheral end of the divided splanchnic causes an arrest or inhibition of 

 a preexisting contraction. 



The nerve-centers regulating the contraction and relaxation of the 

 muscle walls of the intestine are doubtless excited to activity by nerve impulses 

 transmitted through afferent nerves, probably the vagus, from the mucous 

 surface of the small intestine. These centers are also influenced by nerve 

 impulses descending from the cerebrum, though the route they take is not 

 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. 



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

 right angle, crosses the abdominal cavity to the spleen, bends again, and 

 descends to the left iliac fossa. 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 18 centimeters in length. Within three 

 centimeters of its termination 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. 



