MOVEMENTS OF THE ALIMENTARY CANAL 429 



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

 and excess acid begins to accumulate the pyloric sphincter is stim- 

 ulated, but from the stomach side, that is, in front, and accord- 

 ing to the working of the myenteric reflex a stimulus from that 

 side produces relaxation. 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 in- 

 testinal side, namely, from behind, and therefore 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 intestinal 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 

 emptied. The tune required for emptying the stomach com- 

 pletely varies with different foods and under different bodily con- 

 ditions. An average meal is probably all out of the stomach about 

 six hours after eating. 



Movements of the Small Intestine. The food entering the 

 small intestine is subjected to two sorts of movements whose 

 combined effect is to churn it very thoroughly and to move it 

 slowly along the gut so as to make room for more to come in from 

 the stomach. The churning is effected mainly by movements of 

 the intestine known as rhythmic segmentation. In these move- 

 ments rings of the circular muscle coat about an inch apart con- 

 strict simultaneously, splitting the contained food into a scries of 

 segments; an instant later these constrictions disappear, and now 

 ones, midway between the first, are formed, by which the food is 

 again segmented, but in a shifted position. These rhythmic move- 

 ments may recur as often as thirty times a minute. Their effort i.* 



