448 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 inhibitory impulses are tonic in nature. 



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

 the process when liquid or semiliquid food is swallowed. 'By the contrac- 

 tion of the mylohyoid muscle, fluids are quickly shot down the distended 

 esophagus, 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. 

 If the swallowing is immediately repeated, as is usually the case in 

 drinking, the esophagus remains dilated because peristalsis is inhibited, 

 and the fluid lies outside the cardiac orifice until the last mouthful has 

 been taken. 



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- 

 gastric pressure has risen to about 25 cm. of water. In deep anesthesia, 

 however, intragastric pressure may rise considerably higher without 

 forcing the sphincter. 



In animals fed with starch paste impregnated with subnitrate of bis- 

 muth and then examined by means of the x-rays, the variation in degree 



