128 DIGESTION AND ABSORPTION 



(2) Chemical stimulation is brought about by the materials con- 

 tained in certain foods and by the stimulating materials contained 

 in the products of digestion. Certain foods such as meat juices 

 or extractives, by their actions upon the nerves of the stomach, 

 stimulate the gastric glands to pour forth their secretion. Such 

 substances are called secretagogues . Other foods such as milk and 

 bread do not seem to contain these substances. When these foods 

 are eaten, the secretion of juices is probably due to a mechanical 

 stimulus resulting from the presence of food in the stomach. When 

 certain digestive products are formed', they, in turn, stimulate a 

 further secretion of gastric fluid. The amount of secretion de- 

 pends upon the quantity and nature of the food to be digested. 



Gastric juice is a thin, nearly colorless liquid with an acid 

 reaction. It contains some inorganic salts, but the essential con- 

 stituents are hydrochloric acid and two enzymes, pejpsin and rennin. 

 Hydrochloric acid in normal gastric fluid is found in the propor- 

 tion of 0.2 to .5 per cent. Experiments have shown that a higher 

 concentration of hydrochloric acid is not favorable for the diges- 

 tive action of pepsin. It has the following functions : (1) it acti- 

 vates the yepsinogen (an inactive form of pepsin) of gastric 

 juice to form pepsin ; (2) it provides an acid medium which 

 is necessary for the pepsin to carry on its work; (3) it swells 

 the protein fibers, thus providing a larger surface for the action 

 of pepsin ; (4) it kills many bacteria that enter the stomach ; 

 and (5) it helps to regulate the opening and closing of the pylorus. 



Pepsin, which may be called gastric protease, is first formed by 

 the gastric glands as pepsinogen and is changed into pepsin when 

 it comes into contact with the hvdrochloric acid. It has the 

 power of changing proteins into the intermediate products called 

 proteoses and peptones. These are a simpler form of proteins, but 

 not yet simple enough to be absorbed. Therefore, they are called 

 intermediate products of digestion. 



Rennin, another enzyme found in gastric juice, acts upon casein, 



