MOVEMENTS OF THE ALIMENTARY CANAL 475 



and excess acid begins to accumulate, the pyloric sphincter is 

 stimulated, but from the stomach side, and according to the work- 

 ing of the myenteric reflex a stimulus from that side produces 

 relaxation of a region just posterior to it. As soon as the sphincter 

 relaxes under this stimulation that part of the food lying in the 

 pylorus is forced through into the intestine, but it carries with it 

 the free acid with which the food is mixed and stimulates the 

 sphincter from the intestinal side, namely, from behind, and there- 

 fore tends to cause it to close. A feature of the myenteric reflex is 

 that where, as just described, a point is simultaneously stimulated 

 from in front and from behind, the stimulus causing contraction, 

 that from behind, is dominant. Therefore as soon as food enters 

 the intestine the sphincter of the pylorus contracts and prevents 

 more from passing. Before it will relax again the acid on its in- 

 testinal side must be neutralized; but this is rapidly done by the 

 strongly alkaline bile and pancreatic juice, and so as fast as the 

 food in the intestine is mixed with these juices more is admitted 

 from the stomach. 



The fundus of the stomach, which stores the bulk of the food 

 while that in the pylorus is being thus treated and passed on to 

 the intestine, is on the stretch all the time, so that as fast as food 

 is passed out through the pyloric sphincter more is pushed to the 

 pylorus from the fundus until at last the stomach is wholly emp- 

 tied. The time required for emptying the stomach completely 

 varies with different foods and under different bodily conditions. 

 An average meal is probably all out of the stomach about four 

 to six hours after eating. 



An interesting incidental feature of this mechanism is that it 

 operates automatically to pass quickly on into the small intestine 

 carbohydrate food stuffs, which undergo no digestive action in 

 the stomach, while proteins, upon which the pepsin of gastric juice 

 acts, remain long enough to ensure their thorough mixture with 

 the juice. This differentiation depends on the fact that the acid 

 does not enter any chemical combination with carbohydrates, and 

 therefore begins to appear in excess as soon as the alkali present 

 has been neutralized. Proteins, on the other hand, do combine 

 chemically with the acid, and there can be no excess, therefore, 

 until this combination has occurred. Meanwhile thorough mix- 

 ture with pepsin is taking place. This difference does not show, 



