two inches in diameter and about five feet long (see illustration, p. 167), 

 Pig gut and calf gut are used as sausage casing. 



The wall of the intestine is thin and soft. The lining carries very small 

 glands, and the outer layer contains muscle cells. The muscles run around 

 the tube in rings, as in the esophagus, so that, as they contract, the diameter 

 of the intestine is reduced. Waves of contraction start at the forward end 

 (nearest the stomach) and pass backward along the whole length of the 

 small intestine. The contractions move some of the contained mixture along, 

 a short distance at a time. This movement is called peristalsis and is similar 

 to the swallowing movement of the gullet. In vomiting, the peristaltic 

 action of the food tube is reversed. 



On leaving the stomach the food mixture contains in solution all the 

 sugar that was there to begin with, all the sugar that was formed by the 

 digestive action of the saliva; it contains the peptones resulting from the 

 gastric digestion, and various mineral salts. This mixture contains what- 

 ever starch was not digested; any undigested proteins; and all the fats, 

 which are affected by neither the saliva ferments nor by the gastric enzymes. 

 In addition, there is a quantity of water, the acid remains of the juices, and 

 the fibers and cell walls of the food material. 



The fats and the remaining starches and proteins are digested in the 

 intestine. 



Intestinal Digestion^ Near the beginning of the intestine two small 

 ducts or tubes empty at a common opening. One of them leads from the 

 largest gland in the body, the liver; the other from the pancreas (see illus- 

 tration, p. 167). 



The juice secreted by the pancreas contains three important enzymes: 

 (1) an enzyme that converts starch into sugar; (2) an enzyme that digests 

 proteins into simpler compounds; (3) an enzyme that breaks up fats into 

 glycerin and fatty acids. 



The pancreatic juice thus contains ferments that digest all classes of or- 

 ganic nutrients. The fatty acids that result from the splitting combine with 

 other substances into "soaps". Soaps and glycerin dissolve in water and 

 are absorbed by cells lining the Intestine. Farther along, where the intestinal 

 fluid is acid, this kind of digestion is impossible. 



The liver produces bile, or gall, which contains no digestive enzymes. 

 But the bile neutralizes the acid of the gastric juice and so furthers the work 

 of the pancreatic enzymes, which are active only in an alkaline solution. 

 The bile also influences the diffusion of soaps and fatty acids into the cells 

 of the intestine. 



The bile consists largely of materials that are of no further use in the 

 body; the liver is thus also an excretory organ. 



^See No. 8, p. 184. 

 168 



