MOVEMENTS OF THE STOMACH 327 



tion is important is proven, in laboratory experiments, by the decreased 

 length of time required for digestion when fibrin has first been soaked in 0.2 

 per cent hydrochloric acid and thus caused to swell with the absorption of 

 water before coming in contract with the pepsin. 



Circumstances Influencing Gastric Digestion. A temperature of 

 about 40 C. is most favorable to gastric digestion. The pepsin is de- 

 stroyed 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 nitric acid or the organic acids may be substi- 

 tuted 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, the casein being 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 from the fourth stomach of a calf, has long been 

 known, as it is used extensively to cause precipitation of casein in cheese manu- 

 facture. The ferment 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 w r ill 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, over-exertion 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 proteid 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 sus- 

 pended 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 

 proteid. Gastric juice dissolves soluble substances like salts, saccharides, 

 etc. Fats*and carbohydrates are not digested by gastric juice, in fact fats 

 tend to hinder the 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 



