356 



THE HUMAN BODY AND ITS FOOD 



giludinal 



Muscle 



FIG. 281. 

 Two Kinds of Muscles. 



provided with lengthwise muscles (Fig. 281). As these 

 can be shortened by contraction, the esophagus can carry 

 out movements similar to those described for the earth- 

 worm (cf. 336). The lengthwise muscles shorten the 

 tube, and pull it over the food, while the circular muscles, 



by contracting, form a 

 ring behind the food, and 

 push it along. 



The stomach has both 

 of these kinds of muscles, 

 and in addition it has 

 oblique muscles. As a 

 result of the action of 

 these three kinds of 

 muscles, the food re- 

 ceived from the esopha- 

 gus is forced around and around in the stomach. It is 

 thus mixed thoroughly with the gastric juice, the 

 secretion of the glands of the stomach. 



364. The Gastric Juice. The gastric juice is chiefly 

 water, but it contains the important ferments rennin and 

 pepsin, and also hydrochloric acid, in solution. From 5 

 to 10 pints are secreted daily. Rennin coagulates the 

 casein (a proteid; 357) of milk, forming "clots" like 

 those of sour milk. The proteids are insoluble in water, 

 and cannot pass from the alimentary canal into the blood; 

 but the pepsin changes them into simpler substances, 

 called peptones and proteoses, which are soluble. 



The hydrochloric acid of the stomach (cf. 214) carries out several 

 important reactions. Thus, pepsin acts best when hydrochloric 



