152 



THE VITAL PROCESSES 



a digestive fluid known as the intestinal juice. The mem- 

 brane is thrown into many transverse, or circular, folds 

 which increase its surface and also prevent materials from 

 passing too rapidly through the intestine. One important 

 respect in which the small intestine differs from all other 

 portions of the food canal is that its surface is covered with 

 great numbers of minute elevations known as the villi. 

 The purpose of these is to aid in the absorption of the 

 nutrients as they become dissolved (Chapter XI). 



The muscular coat of the small intestine is made up of two 

 distinct layers the inner layer consisting of circular fibers 

 and the outer of longitudinal fibers. These muscles keep 

 the food materials mixed with the juices of the small intes- 

 tine, but their main purpose is to force the materials under- 

 going digestion through this long and much-coiled tube. 



The outer, or serous, coat of the small intestine, like that 

 of the stomach, is an extension from the general lining of 

 the abdominal cavity, or peritoneum. In fact, the intestine 

 lies in a fold of the peritoneum, somewhat as an arm in a 

 sling, while the peritoneum, by connecting with the back 

 wall of the abdominal cavity, holds this great coil of digest- 

 ive tubing in place (Fig. 64). The portion of the peri- 

 toneum which attaches the intestine to the wall of the 

 abdomen is called the mesentery. 



Most of the liquid acting on the food in the small intes- 

 tine is supplied by two large glands, the liver and the pan- 

 creas, that connect with it by ducts. 



The Liver is situated immediately below the diaphragm, 

 on the right side (Figs. 71 and 72), and is the largest gland 

 in the body. It weighs about four pounds and is separated 

 into two main divisions, or lobes. It is complex in struc- 

 ture and differs from the other glands in several particulars. 

 It receives blood from two distinct sources the portal vein 



