68 PHYSIOLOGY FOR DENTAL STUDENTS. 



gradually contracts on the food which has been forced into it, 

 thus tending to eject it through the pyloric sphincter, or, if this 

 is closed, to cause it to pass back again as an axial stream into 

 the proximal part of the pylorus, which has been called the 

 pyloric vestibule (see Fig. 5). These waves occur every fifteen 

 to twenty seconds, three or four being present in the pyloric 

 vestibule at the same time. They become more marked as diges- 

 tion proceeds, and are accompanied by a gradual diminution in 

 size of the body of the stomach. Their function, besides carrying 

 the food towards the outlet of the stomach, is to keep it properly 

 mixed with the gastric juice. 



The Opening of the Pyloric Sphincter. The mere pressure 

 with which the contents of the vestibule are thus driven, with 

 each peristaltic wave, against the pyloric sphincter does not, 

 however, in itself serve to open it; for half an hour after feed- 

 ing with protein, for example, no food may pass the sphincter, 

 although during this time there may have been' well over a hun- 

 dred peristaltic waves. Nor is it the consistency of the food 

 which controls the opening. It must therefore be some chemical 

 property which the food acquires during its stay in the stomach. 

 This has definitely been shown by Cannon to be the presence of 

 free acid. By measuring the length of the skiagram shadow in 

 the intestines after feeding cats with bismuth-impregnated foods 

 rendered acid or alkaline, it could be clearly shown that acid 

 hastened the initial discharge, whereas alkalies retarded it, and 

 observations through a fistula in the vestibule showed that any 

 delay in the appearance of acid in the contents was associated 

 with a delay in the opening of the sphincter. But the sphincter 

 does not remain open; it quickly closes after a little chyme, as 

 the half digested food is called, has got through it. This clos- 

 ure is due to the free acid acting on the duodenum, where it 

 stimulates afferent nerve endings that cause the sphincter to 

 close and to keep closed so long as any acid remains in the duo- 

 denum. Whenever -this acidity has become neutralized by the 

 alkali present in the bile and pancreatic juice, the acid on the 

 stomach side again becomes operative and the sphincter opens. 

 We must conclude that the pyloric sphincter is under the con- 



