MOVEMENTS OF THE ALIMENTARY CANAL 473 



sons suffering from indigestion, or certain nervous disorders, or 

 in users of tobacco. The upward rush of the acid stomach con- 

 tents into the esophagus gives rise to a burning sensation which is 

 generally known as "heart burn," although the heart has really 

 nothing whatever to do with it. 



Movements of the Stomach. When the stomach is empty of 

 food its normal condition is as a flabby pouch. Its walls are neither 

 much relaxed nor strongly distended. There are probably always 

 a small amount of liquid and some bubbles of swallowed air in the 

 stomach, even at the time when we speak of it as empty. Shortly 

 before the usual time for taking a meal the circular muscle coat 

 of the stomach goes into a state of tonus, probably as a result of 

 a flow of impulses over the vagus nerve, which is the motor nerve 

 of the organ. The effect of this tonus is to contract the stomach 

 until it is little more than a tube. Usually about this same time 

 the active contractions which give rise to hunger sensations (p. 209) 

 begin. As food enters the contracted stomach it makes room for 

 itself by stretching the 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 to some extent 

 in the pyloric regions of the stomach. Such food as is in the fundus 

 is not exposed directly, 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 

 to the pylorus. The fundic end is not involved at all in them. 



