474 THE HUMAN BODY 



In man they recur regularly, so long as food is in the stomach, at 

 intervals of about twenty seconds. For a considerable period 

 after food enters the stomach the pyloric sphincter, which guards 

 the exit into the small intestine, remains perfectly tight. During 

 this time the peristaltic waves crowd the food caught by them up 

 to the pylorus but cannot force any through. As the constriction 

 approaches the pylorus the food-mass in front of it escapes back 

 through the opening at its center, the waves not being deep enough 

 to close this entirely, and so the food in the central and pyloric 

 portions of the stomach is thoroughly churned. 



During this churning the food, already semi-liquid from the 

 mixture with saliva and with such liquid as was taken with the 

 meal, is mixed with the gastric juice and made still more liquid, 

 being called at this stage chyme. The effect of the gastric juice is 

 to give the food an acid reaction, stopping the action of ptyalin 

 and permitting that of the pepsin which it also pours out upon 

 the food. 



The Control of the Pyloric Sphincter. The way in which the 

 sphincter of the pylorus is regulated so that after the food has been 

 thoroughly mixed with gastric juice it opens and allows a small 

 amount to pass, and then promptly closes to give opportunity for 

 this to be influenced by the intestinal secretions before more is 

 admitted, is one of the most interesting adaptations that we know 

 of in the Body. The mechanism of this action is a special case 

 of a peculiar reflex which apparently obtains throughout the ali- 

 mentary canal, and is probably dependent on special properties 

 of the nerve plexus which is embedded therein. This so-called 

 myenteric reflex, is of such a sort that a stimulus applied to any 

 point along the alimentary canal causes a contraction of the 

 muscles immediately in front of (anterior to) the stimulated point, 

 and a relaxation of those immediately behind (posterior to) it. 

 The reflex was worked out first for the small intestine, and has 

 since been shown to apply to the other parts of the canal. It is a 

 so-called " local reflex," as the central nervous system has noth- 

 ing whatever to do with it. 



The adequate stimulus for arousing the reflex in the pyloric 

 sphincter is the presence of free hydrochloric acid. When there- 

 fore the originally alkaline food in the pyloric part of the stomach 

 has been completely neutralized by the acid of the gastric juice, 



