354 FOOD AND DIGESTION 



as one to two liters or more of semisolid 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- 

 ach is quite strongly muscular and quite definitely marked off by the strong 

 transverse band at its union with the fundus. 



Acid Closure of the Cardiac and Pyloric Orifices. The gastric 

 juice is assisted in accomplishing digestion by the movements of the stomach 

 itself. When digestion is not going on, the stomach is uniformly 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 immedi- 

 ately closes again. This closure of the cardiac orifice is accomplished by a 

 local reflex. The stimulus is the acid secretion covering the mucous mem- 

 brane in the immediate neighborhood. 



The pyloric orifice, during the taking of food and the first part of gastric 

 digestion, is also so completely closed that little of the contents escape. 

 This valve, too, is automatically regulated, as demonstrated by Cannon. 

 The acid gastric content that escapes into the duodenum starts to flow down 



the duodenum. This sets up a local reflex stimulus 

 that closes the pylorus until the bile and pancreatic 

 juice have neutralized or made the chyle alkaline. 

 The Peristalsis of the Stomach. The char- 

 acter of stomach movements has been admirably 

 determined by recent observers using the Rontgen- 

 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 py- 

 loric antrum and travel slowly toward the pyloric 



valve in the form of a peristaltic wave. Successive 



FIG. 261. Diagram to waves begin a little further back toward the fundus 

 Show the Movement of Food eac h time and follow over the pyloric antrum with 

 in the Pylorus at Times when . , , .. , . . , 



the Pyloric Valve is Closed. clock-like regularity, in the cat one wave m 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 

 softened layers of food by propelling it into the pylorus. There it is thoroughly 



