172 ORGANIC AGRICULTURAL CHEMISTRY 



brought about by enzymes, but in such actions several inter- 

 mediate products have been isolated, known as proteoses^ 

 peptones and polypeptides. In the classification of proteins 

 these are termed secondary derived proteins as they still possess 

 certain protein characters. The final products of both acid 

 and enzyme hydrolysis, the amino-acids, are definite nonprotein 

 compounds. 



The Stomach 



The digestion of protein food begins in the stomach, prac- 

 tically no digestion of these food constituents taking place in 

 the mouth, by the action of salivary enzymes. The protein 

 digestion in the stomach is brought about by the action of two 

 enzymes, gastric rennin and pepsin^ both contained in the 

 stomach juice known as gastric juice. 



Gastric Juice. — The gastric juice is secreted by two sets of 

 gastric glands present in two regions of the mucous lining of 

 the stomach, and flows into the stomach when certain stimu- 

 lation occurs. This stimulation is usually either chemical, 

 due to the character of the food mass, such as presence of water, 

 milk or food juices, or psychical, due to the psychic influence of 

 sight, thought or taste of food. It has been found that water 

 in the stomach stimulates the flow of gastric juice and that 

 water taken at mealtimes is beneficial rather than injurious 

 to the action of gastric digestion. This agrees also with the 

 fact previously referred to in connection with the influence of 

 water upon the action of salivary digestion. 



Gastric juice is a thin liquid varying considerably in specific 

 gravity from i.ooi to 1,010. It contains from 2 to 3 per cent 

 of solid matter. The chief substances present in gastric juice 

 are enzymes^ free hydrochloric acid and inorganic salts. The 

 distinctive property of gastric juice is its acidity due to the 

 free hydrochloric acid, and equaling 0.2 to 0.3 per cent. 



Enzymes. — The enzymes found in gastric juice are of three 

 classes, two of which act upon proteins and one, gastric lipase, 

 acts upon fats. The protein enzymes are : 



