FOOD AND DIGESTION 



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 pic- 

 ture is made clearer if one remembers that the food mass is retained almost 

 wholly in the fundus of the stomach. The pyloric portion of the stomach is 

 quite strongly muscular and quite definitely marked off by the strong trans- 

 verse band at its union with the fundus. 



The gastric juice is assisted in accomplishing digestion by the movements 

 of the stomach itself. When digestion is not going on, the stomach is uni- 

 formly contracted, its orifices not more firmly than the rest of its walls; but, 

 if examined shortly after the introduction of food, it is found closely encircling 

 its contents, and its orifices are firmly closed like sphincters. The cardiac 

 orifice, every time food is swallowed, opens to admit its passage to the stomach, 

 and immediately closes again. The pyloric orifice, during the taking of food 



FIG. 261. Diagram to Show the Movement of Food in the Pylorus at Times when the Pyloric 



Valve is Closed. 



and the first part of gastric digestion, is so completely closed that none of the 

 contents escape. 



The character of stomach movements has been admirably determined by 

 recent observers using the Roentgen-ray method. Thus Cannon working 

 with cats has shown that in from five to ten minutes after a meal slight rings 

 or constrictions occur in the pyloric antrum and travel slowly toward the 

 pyloric valve in the form of a peristaltic wave. Successive waves begin a little 

 further back toward the fundus each time and follow over the pyloric 

 antrum with clocklike regularity, in the cat one wave in ten seconds, which 

 requires in each case about twenty seconds for its completion. In man they 

 are doubtless slower. These peristalses continue during the whole period of 

 digestion, for as much as seven or even more hours. 



These peristaltic contractions aid the gastric juice in carrying away the 



