SALIVARY AND GASTRIC DIGESTION 197 



The Sphincters. Where a circular zone of muscle is 

 habitually contracted to close a passage we say that a 

 sphincter exists. The idea is not so much of a definite 

 structure as of a special manifestation of irritability. 

 We say that sphincters are found at the cardia and the 

 pylorus, meaning that closure at these places is the rule 

 rather than the exception. The conditions which de- 

 termine whether there shall be contraction or relaxa- 

 tion at these localities have been much studied. Much 

 is found to depend upon the degree of acidity of the 

 stomach contents. 



The cardiac sphincter is not so tightly contracted just 

 after a meal as it is a little later. The rustic diner usually 

 concludes his repast by freeing his stomach of the air 

 he has swallowed and has no difficulty in doing so. The 

 X-ray has shown that for a short time after a meal 

 the food eaten by a cat slips out again and again into the 

 lower part of the esophagus to be returned methodically 

 by peristaltic waves. Very soon this escape becomes 

 impossible. The secretion of the gastric glands is 

 strongly acid and the effect of acid acting just below the 

 sphincter is to increase its tone. This is consistent 

 with the fact that a swallow of alkali, such as magnesia 

 or bicarbonate of soda, favors the relaxation of the 

 sphincter and the escape of gas. The central nervous 

 system appears to override the local influence of the 

 acid at the time of vomiting when the opening of the 

 sphincter occurs often in spite of high acidity. 



In general, it may be said of the pylorus that it reacts 

 in the same way as the cardiac opening of the stomach. 

 A period of strong closure follows each passage of acid 

 material to the duodenum. The mechanism thus pro- 

 vides that after each transfer of food from the stomach 

 to the intestine there shall be a period of constriction. 

 One cannot picture a simpler or better device to guard 

 against overdistention of the intestine; time is allowed 

 for each succeeding portion to make its way onward 

 before any more follows it. 



