472 THE HUMAN BODY 



to contract in proper sequence. The pharyngeal muscles, although 

 of the striped variety, are but little under the control of the will; 

 it is extremely difficult to go through the movements of swallow- 

 ing without something (if only a little saliva) to swallow and thus 

 excite the movements reflexly. Many persons, after having got 

 the mouth completely empty cannot perform the movements of 

 the second stage of deglutition at all. On account of the reflex 

 nature of the contractions of the pharynx, any food which has 

 once entered it must be swallowed : the isthmus of the fauces is a 

 sort of Rubicon; food that has passed it must continue its course 

 to the stomach, although the swallower learnt immediately that 

 he was taking poison. The third stage of deglutition is that by 

 which solid food is passed along the gullet, and is comparatively 

 slow. The movements of the eosphagus are of the kind known 

 as peristaltic. Its circular muscular fibers contract behind the 

 morsel and narrow the passage there; and the constriction then 

 travels along to the stomach, pushing the food in front of it. 

 Simultaneously the longitudinal fibers, at the point where the 

 food-mass is at any moment and immediately in front of that, 

 relax, tending to widen the passage. This peristaltic wave re- 

 quires about six seconds in man for its passage along the 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 ring of smooth muscle of the circular 

 coat at the entrance of the stomach acts as a sphincter (cardiac 

 sphincter). This is ordinarily tightly contracted when there is 

 food in the stomach, holding the esophagus shut, and only opens 

 at the approach of the peristaltic wave to allow the food-mass 

 to pass through into the stomach. Liquids, which pass very 

 quickly down the esophagus (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 opens a passage for them. The relaxation 

 of the cardiac sphincter under these circumstances does not open 

 a free communication between the stomach and the throat, for 

 there is always a descending peristaltic wave holding the esophagus 

 closed. This is important because, as we shall see, the stomach 

 contents are under pressure, and would be forced up into the esoph- 

 agus were the sphincter to relax with no peristaltic wave present. 

 As a matter of fact this sometimes happens, particularly in per- 



