482 DIGESTION 



tion. Thus, if swallowing movements are excited by stimulating the cen- 

 tral end of the superior laryngeal nerve, they can be instantly inhibited 

 by simultaneously stimulating the glossopharyngeal, and the respiratory 

 movements stop in whatever position they may have been at the time. 

 When the glossopharyngeal nerves are cut, the esophagus enters into a 

 condition of tonic contraction, which may last a day or so. This shows 

 that the inhibiting impulses are acting continuously. 



This inhibition of the esophagus is indeed a most important part of 

 the process when liquid or semiliquid food is swallowed. By contraction 

 of the mylohyoid muscle fluids are quickly shot down the dilated esopha- 

 gus, at the lower end of which, on account of the closure of the cardiac 

 sphincter, they accumulate until the arrival of the peristaltic wave which 

 has meanwhile been set up by stimulation of the pharynx. As the per- 

 istaltic wave approaches the cardia the sphincter becomes inhibited al- 

 lowing the fluids to be passed into the stomach. If the swallowing is 

 immediately repeated the esophagus remains dilated because peristalsis 

 is inhibited and the fluid collects above the closed cardiac sphincter 

 until the last mouthful is taken when the peristaltic wave passes over 

 the esophagus and sweeps the contents through the sphincter which is 

 at the same time relaxed. When a series of swallows in rapid succes- 

 sion, as in drinking, is made the cardiac sphincter may remain inhibited 

 throughout, so that the fluid passes directly into the stomach unassisted by 

 the peristaltic wave. 



The Cardiac Sphincter 



The passage between the esophagus and the stomach is guarded by 

 the cardiac sphincter or cardia. This exists in a permanently con- 

 tracted state, or tonus, superimposed on which from time to time are 

 rhythmic alternations of contraction and relaxation. This tonus is never 

 very pronounced. In man it is said that a water pressure of from 2 to 7 

 cm. applied to the esophageal side of the sphincter will drive air or 

 water into the stomach, this pressure being less than that of a column 

 of fluid filling the thoracic esophagus in the erect position. During 

 repeated deglutition the tonus becomes less and less marked, and after 

 a number of swallows the sphincter may become completely relaxed. 

 When this relaxation disappears, however, the sphincter becomes more 

 contracted than usual and remains so for a longer time. 



The tonic condition of the sphincter is controlled by the vagus nerve, 

 stimulation of which causes relaxation with an after-effect of strong 

 contraction. Mechanical or chemical stimulation of the lower end of the 

 esophagus increases the tonus of the sphincter. Forcing of the sphincter 

 from the stomach side requires a higher pressure than from the esopha- 

 geal. Eructation of gas, for example, does not take place until intra- 



