112 GENERAL ZOOLOGY 



Chewing thoroughly mixes saliva with the food, but swallowing 

 takes place before all the action of ptyalin can be completed in 

 the mouth. The reaction of saliva is neutral or slightly acid, 

 but its activity is stopped by concentrations of hydrochloric acid 

 as low as 0.003 per cent and also by strong alkali. The gastric 

 juice contains enough acid (0.50 per cent of HCL) to stop the 

 action of ptyalin on food in the stomach ; nevertheless the action 

 of ptyalin may continue in the stomach because the food may 

 remain undisturbed in the fundus for an hour or more and 

 remain unmixed with acid. If a rat is fed with food of different 

 colors, the stomach removed, frozen, and sectioned, it is found 

 that the food is deposited in concentric layers, the first layer 

 against the wall and each later layer inside the preceding one. 

 Since time is required for the complete mixing of the food with 

 gastric juice, ptyalin might well continue its action particularly 

 on food swallowed toward the end of the meal. 



Gastric Digestion. — The acidity of pure gastric juice, is about 

 0.5 per cent, while the acidity of the contents of -the stomach 

 during digestion is about 0.2 per cent. The drop in acidity during 

 digestion is due to neutralization and dilution by the stomach 

 contents and also apparently by regulated regurgitation of the 

 alkaline contents of the duodenum during gastric digestion. 



Gastric juice is secreted by the stomach in response to the 

 stimulations of nerve endings of taste and odor, produced during 

 the chewing and swallowing of food. This constitutes the 

 appetite secretion. In addition there is evidence of a chemical 

 stimulation of the gastric glands brought about by the action of 

 food on the stomach mucosa, producing a substance, gastric 

 secretin, which is absorbed by the blood and carried to the 

 gastric glands, where it causes secretion of the gastric juice. 



The principal enzyme of the stomach, pepsin, acts only in an 

 acid medium, upon proteins, reducing them to simple and more 

 soluble forms known as peptones. The curdling of milk brought 

 about by rennin, the coagulating enzyme of the gastric juice, 

 takes place in two steps: (1) the conversion of casein into para- 

 casein, and (2) the reaction of the paracasein with calcium salts 

 of the milk to form the curd, which is an insoluble protein. The 

 latter then undergoes proteolytic digestion by pepsin. Digestion 

 of protein is begun in the stomach and carried to completion in 

 the intestine. 



