136 DIGESTION. 



striped muscular fibres arranged in two principal layers. 

 Nearly all the principles contained in food are modified by the 

 gastric juice, and some are completely liquefied and absorbed 

 in the stomach. By the action of the gastric juice, the food, 

 comminuted and incorporated with the fluids of the mouth, 

 is further reduced to a pultaceous mass, which was formerly 

 called the chyme ; the muscular movements of the stomach 

 turning it over and over, so that it may become thoroughly 

 incorporated with the fluids. These movements have a ten- 

 dency to force the food, as it becomes sufficiently liquefied, 

 into the small intestine; and a large collection of circular 

 muscular fibres, called sometimes the pyloric muscle, stands 

 at the pylorus as a guard, allowing the liquid portions to 

 pass gradually through, but sending back the larger masses 

 to be further acted upon in the stomach. 



By these movements, a great portion of the food, prepared 

 by the action of the stomach, is slowly forced into the small 

 intestine. This tube, from fifteen to twenty feet in length, is 

 covered with peritoneum and loosely bound to the spinal 

 column by the mesentery, which is formed of the two folds 

 of the peritoneum, and is sufficiently long to allow of free 

 movements of the intestines over each other and in the ab- 

 dominal cavity, except the first few inches, where it is pretty 

 firmly attached to the posterior abdominal wall. The small 

 intestine commences by a dilated portion eight or ten 

 inches in length, called the duodenum. The remainder is 

 divided into the jejunum and the ileum. The former em- 

 braces the upper two-fifths of the intestine, but there is no 

 distinct line of separation between it and the ileum. The 

 mucous membrane lining the small intestine is thick, pro- 

 vided with an immense number of villi, and, particularly in 

 the upper portion, is thrown into transverse folds which are 

 called the valvulse conniventes. These disappear in the lower 

 part of the ileum. They are peculiar to the human subject. 

 Thickly set in the upper part of the duodenum, and scattered 

 through its lower portion and the upper part of the jejunum, 



