374 FOOD AND DIGESTION 



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. 



At the taking of food or immediately thereafter the content of the stomach 

 begins to pass through the pyloric orifice into the intestine. But the pylorus 

 is quickly closed so completely that Little of the contents escape at this 

 time. The pylorus is automatically regulated as demonstrated by Cannon. 

 The acid gastric content in the duodenum sets up a local reflex that closes 

 the pylorus until the bile and pancreatic juice have neutralized the acid. 

 When an alkaline reaction is produced the pylorus relaxes and at the next 

 peristaltic wave of contraction is opened again. Indeed it is claimed that in 

 the human the pylorus takes more or less part in each peristaltic wave passing 

 from the stomach on over the duodenum. 



The Peristalsis of the Stomach. The char- 

 acter of stomach movements has been admirably 

 determined by recent observers using the X-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 sphinc- 

 ter 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 



clock-like regularity, in the cat one wave in ten 

 FIG. 261. Diagram to . . 



show the movement of food seconds, which requires in each case about twenty 



in the pylorus at times when seconds for its completion. In man they are 

 doubtless slower. These peristalses continue dur- 

 ing 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 

 mixed with the gastric juice, forming the chyme. Figure 261 gives an idea of 

 the movement of the food in the antrum. The peristaltic contractions carry 

 it forward, but if the valve does not open to permit passage to the duodenum, 

 then the pressure will force the chyme back through the center toward the 



