MOVEMENTS OF THE STOMACH 373 



pyloric valve is least vigorously active. The presence of bile under these 

 conditions is usually taken as indicative of this regurgitation. Under 

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

 have a considerable proteolytic and fat-splitting action. 



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: i 

 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. 

 2. The lipase in gastric juice produces a small amount of fat cleavage, tend- 

 ing to convert the fats into fatty acids and glycerin in which condition they 

 are absorbed. The presence of fat tends to inhibit the gastric digestion of 

 proteins. 3. Milk is first coagulated by a special enzyme, rennin, and then 

 digested as any other protein. 4. Gastric juice dissolves soluble substances 

 like salts, saccharides, etc. 



5. The enzyme, ptyalin, continues the digestion of the carbohydrates in 

 the stomach so long as the food remains neutral or alkaline, but they are 

 not digested under the influence of any gastric enzymes. However, maltase 

 is present in the gastric juice and aids in the last step in carbohydrate hydroly- 

 sis. It is significant that outside the body 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 the acid of natural gastric juice. 



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 

 as one to two liters or more of semi-solid food is packed away in the organ in 

 a comparatively short space of time. The gastric juice is secreted by the 

 mucous membrane which surrounds the surface of the food mass. The result 

 is that the secretion begins to soften and digest the food over its surface, thus 

 tending to liquefy and erode away layer after layer of the food mass. The 

 picture is made clearer if one remembers that the food mass is retained al- 

 most wholly in the fundus of the stomach. The pyloric portion of the stom- 



