174 



PHYSIOLOGY. 



Mouth of Gland 



Epithelium 



I Principal 

 " Cells 



diminished, and the membrane is comparatively pale. 

 But as soon as food is introduced into the stomach the 

 blood flow is greatly increased, and the mucous membrane 

 becomes red. This blood supply gives the glands the ma- 

 terials with which they manufacture the gastric juice. At 



the same time the cells of 

 the glands are stimulated 

 to action, and the secre- 

 tion is poured out rapidly. 

 The alkaline saliva also 

 aids in stimulating the 

 secretion of the gastric 

 juice. 



The Work of the 

 Gastric Juice. The spe- 

 cial work of the gastric 

 juice is accomplished by 

 the pepsin, aided by the 

 acid ; these convert pro- 



teids into a soluble substance, called peptone, which can 

 be absorbed through the walls of the digestive tube into 

 the blood. 



Rennet and Rennin. Rennet, used in cheese making, is a familiar 

 substance obtained from the fourth stomach of the calf. When milk 

 enters the stomach it is curdled ; that is, the casein previously dissolved 

 in the liquid milk is coagulated. This curdling, or coagulation, is at- 

 tributed to a ferment in the gastric juice called rennin, and it seems to 

 be entirely distinct from pepsin. 



Churning Action of the Stomach. At the same time 

 all the food is soaked by the gastric juice, the process being 

 greatly assisted by the churning motion of the stomach 

 caused by the action of the muscular coat. The food is 

 thus gradually reduced to a pulpy mass called chyme. 



Connective Tissue 



Fig. 6 1 . Three Glands of the Stomach 

 Cardiac Pan. 



