SALIVARY AND GASTRIC DIGESTION 199 



that juice effects. We have seen that it has much to 

 do with the regulation of the two sphincters. We 

 shall find later that the awakening of the pancreas to 

 activity at a proper time depends to a great degree 

 upon the amount of this acid. Still another function is 

 expressed when we say that the acid makes gastric 

 juice antiseptic. The microorganisms which may have 

 been swallowed in the food are not all killed by the 

 secretion but many of them arc while others are prob- 

 ably weakened. It is likely that many germs which 

 might cause disease are destroyed in the stomach. 

 The suggestion has been made that infected drinking 

 water is more dangerous than similarly infected food 

 because water alone induces only a slight flow of gastric 

 juice. 



Rennin. Gastric juice has one curious property which 

 has been familiar from ancient times. This is the power 

 to curdle milk. Extracts made from the lining of the 

 calf's stomach have been used for ages in making cheese. 

 Under the influence of such preparations the bulk of the 

 protein in milk, the casein, is changed from a soluble to 

 an insoluble form. This is opposed to our general 

 conception of digestion as a process in which the solubility 

 of the products is steadily increased. The curd has 

 later to be resolved like any solid food. No satisfactory 

 reason for the existence of the coagulating agent can 

 be offered. An enzyme is assumed to be responsible 

 and it is called rennin. 



Peptic Digestion. The chief action of the gastric 

 juice is upon proteins. These compounds have been 

 briefly characterized in Chapter II. They are present 

 in most foods but most abundantly in meats, eggs, 

 milk, legumes, and cereals. In the last two classes of 

 staples they are associated with starch. The greater 

 part of the proteins we eat are in a solidified or coagulated 

 form as a result of heating. The effect is familiarly 

 observed in the boiling of eggs. Whether coagulated or 

 not they require extensive digestion and this is begun 



