94 PRINCIPLES OF PHYSIOLOGY 



ments. The greater part of this nervo-muscu- 

 lar mechanism is beyond the control of the 

 will, after the food has passed sufficiently far 

 into the mouth, and so exquisite are its adap- 

 tations that only when food is swallowed 

 hurriedly, or with great gulps of liquid, is there 

 any danger of the matter entering the wrong 

 passage. 



49. In the stomach, which is simply a special 

 enlargement of the alimentary canal, the food 

 is subjected to three processes : (1) The 

 action of a temperature of about 98 F. ; 

 (2) a churning-like motion produced by slow 

 contractions of the muscular walls by which the 

 food is thoroughly mixed with the special 

 secretion of the stomach, the gastric juice ; 

 and (3), the chemical action of the gastric 

 juice itself. This juice is secreted by numerous 

 tubular glands in the mucous membrane, 

 It is a clear watery fluid containing a 

 minute quantity of various salts in solution, 

 a small amount (*2 per cent.) of hydrochloric 

 acid, and a special enzyme, pepsin. It is only 

 secreted in ordinary circumstances when food 

 enters the stomach, and by the contractions 



