MOVEMENTS OF THE STOMACH 353 



by a temperature of 55 (neutral) to 65 C. (acid solution) and its action is 

 retarded and suspended by low temperatures. It is inactive in neutral 

 or alkaline solution, for an acid medium is necessary. Hydrochloric is the 

 best acid for the purpose, but other mineral acids or the organic acids may 

 be substituted for the hydrochloric. Excess of peptone delays the action, and 

 the removal of the products of digestion facilitates the process. 



Action of Rennin. Milk is curdled by the action of gastric juice, the 

 casein being first precipitated, and then dissolved. The curdling is due to a 

 special ferment of the gastric juice, rennin, and is not due to the action of the 

 free acid alone. The effect of rennin, which is obtained commercially 

 from the fourth stomach of the calf, has long been known, as it is used ex- 

 tensively to cause precipitation of casein in cheese manufacture. The fer- 

 ment rennin is active in a neutral solution as well as in acid. 



Time Occupied in Gastric Digestion. Under ordinary conditions, 

 from three to four hours may be taken as the average time occupied by the 

 digestion of a meal in the stomach. But many circumstances will modify 

 the rate of gastric digestion. The chief are: The nature of the food taken 

 and its quantity (the stomach should be fairly filled, not distended) ; the time 

 that has elapsed since the last meal, which should be at least enough for the 

 stomach to be quite clear of food; the amount of exercise previous and 

 subsequent to a meal (gentle exercise being favorable, overexertion injurious, 

 to digestion); the state of mind; and the bodily health. 



Summary of Changes in the Food in Gastric Digestion. Briefly 

 summarizing the action of gastric juice, the facts appear as follows: Gastric 

 juice has a specific digestive action on protein foods of all kinds, converting 

 them into the more soluble proteases and peptones. The action is due to an 

 enzyme, pepsin, acting in and with an acid, hydrochloric acid. Digestion 

 takes place best at the temperature of the body, is destroyed by high heat 

 and suspended by cold, o C. Putrefaction is prevented by gastric juice. 

 Milk is first coagulated by a special enzyme, rennin, and then digested as any 

 other protein. Gastric juice dissolves soluble substances like salts, saccha- 

 rides, etc. Fats and carbohydrates are not digested by gastric juice; in fact, 

 fats tend to hinder the action. 



Pancreatic Digestion in the Stomach. Boldyroff has recently shown 

 that after the ingestion of fats or fatty foods in sufficient amounts, that the 

 secretion of gastric juice is inhibited and that the presence of the pancreatic 

 and intestinal secretions can be demonstrated in the stomach contents. Under 

 these conditions, the pancreatic juice is present in amounts sufficient to have 

 a considerable proteolytic and fat-splitting action. 



MOVEMENTS OF THE STOMACH. 



Attention has been called to the fact that the stomach is a muscular sac 

 capable of holding quite a large mass of food. During a full meal as much 



