MOVEMENTS OF THE ALIMENTARY CANAL 427 



esophagus. It is part of the reflex act of swallowing and takes 

 place whenever the act occurs, whether there be any food-mass 

 to be conveyed to the stomach or not. The sphincter muscle at 

 the entrance of the stomach is ordinarily tightly contracted, hold- 

 ing the esophagus shut, and only opens when the peristaltic wave 

 coming down crowds it open, forcing the food-mass through into 

 the stomach. Liquids, which pass very quickly down the esoph- 

 agus (in 0.1 sec.), usually do not get into the stomach at once, but 

 are held by the sphincter until the arrival of the peristaltic wave 

 forces a passage for them. A curious fact is that two peristaltic 

 waves cannot be moving along the gullet simultaneously. If one 

 swallows a second time within six seconds of a former swallowing 

 the peristaltic wave started by the first is inhibited, promptly 

 fades out, and whatever food is at the sphincter must wait for the 

 arrival of the second wave to enter the stomach. 



Movements of the Stomach. When the stomach is empty its 

 muscular walls are so strongly contracted as to bring the mucus 

 layers into contact, leaving no cmp^y space. As food enters the 

 stomach it makes room for itself by stretching the stomach walls, 

 and the more food is taken, the more the stomach is distended. 

 One result of this manner of filling the stomach is that the food is 

 deposited in it in layers, the first food taken being next to the 

 walls, subsequent amounts being toward the center, and further 

 from the walls the more has entered before them. 



The gastric glands are located in the middle and pyloric regions 

 of the stomach. Such food as is in the fundus is not exposed di- 

 rectly, therefore, to the action of gastric juice, and so is not very 

 rapidly acidified. The action of salivary ptyalin, which is brought 

 to an end when the food becomes acid, may thus continue in the 

 fundic region for a considerable time after the food is swallowed, 

 especially in those portions of food which are swallowed late in 

 the meal. 



The movements of the stomach have been watched by means 

 of the X-rays. Food which has been mixed with bismuth subni- 

 trate is opaque to these rays and its movements in response to the 

 movements of the stomach walls can be readily followed. By 

 this means it has been learned that the walls of the stomach show 

 peristaltic waves; these begin at about the middle, in a strong 

 contraction of a ring of circular muscles at that point, and sweep 



