THE MECHANISMS OF DIGESTION 483 



gastric pressure has risen to about %Lc,m. 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 

 of tone of the sphincter has been observed to be responsible for occasional 

 regurgitation of some of the gastric contents into the esophagus up to the 

 level of the heart or even to the base of the neck. The presence of the 

 gastric contents in the esophagus starts a peristaltic wave, which pushes 

 the material back again into the stomach. This peristaltic wave starts 

 in the absence of any other phases of the deglutition process, indicating 

 that it has been excited by the presence of the material in the esophagus 

 itself, and belongs, therefore, to the lower order of peristaltic wave, as 

 seen in the intestines but not in the upper half of the esophagus. Regur- 

 gitation of food into the esophagus occurs only when the intragastric 

 pressure is fairly high. It may last for a period of from twenty to thirty 

 minutes after the meal is taken, and disappears when the tonus of the 

 sphincter becomes increased as a result of the presence in the gastric 

 contents of free hydrochloric acid. 



Much information has been secured by listening with a stethoscope to 

 the sounds caused by swallowing and by observing with the x-ray the 

 shadows produced along the course of the esophagus when food impreg- 

 nated with bismuth subnitrate is taken. When a solid bolus is swal- 

 lowed only one sound is usually heard, but with liquid food there are 

 two, one at the upper end, due to the rush of the fluid and air, and 

 the other at the lower end (heard over the epigastrium), four or six 

 seconds later, due to the arrival here of the peristaltic wave with the 

 accompanying opening of the cardiac sphincter and the escape of the 

 fluid and air into the stomach. Sometimes, when the person is in the 

 horizontal position, this second sound may be broken up into several, 

 indicating that, unassisted by gravity, the fluid does not so readily pass 

 through the sphincter. The x-ray shadows yield results in conformity 

 with the above. After swallowing milk and bismuth, for example, the 

 shadow falls quickly to the lower end of the esophagus and then passes 

 slowly into the stomach. When the passage of a solid bolus is watched 

 by the x-ray method, its rate of descent will be found to depend on 

 whether or not it is well lubricated with saliva; a well lubricated bolus 

 takes from eight to eighteen seconds. 



Vomiting 



Vomiting is usually preceded by a feeling of sickness or nausea, and 

 is initiated by a very active secretion of saliva. The saliva, mixed with 



