ALIMENTATION 113 



Gastric digestion is facilitated by movements of the pyloric half 

 of the stomach. During these movements, which consist of 

 rhythmic muscular contractions beginning at about the middle 

 of the stomach and extending to the pylorus, the fundus remains 

 quiescent. The stomach cavity is entirely shut off from the rest 

 of the alimentary canal by the contracted sphincter muscles at 

 the cardia and pylorus, except at certain intervals when the 

 pyloric sphincter relaxes and allows a portion of the stomach 

 contents to be pushed out into the duodenum. That portion 

 of the food subjected to the rhythmic contractions of the stomach 

 is gradually converted into a liquid mass known as chyme; and 

 it is thought that the liquefaction of the food may be one of the 

 factors involved in causing the relaxation of the pyloric sphincter, 

 since observations have been recorded indicating that when 

 liquid food alone is taken, it leaves the stomach in a few minutes. 

 In other words, the pyloric sphincter remains contracted as long 

 as the pyloric contents are solid, but relaxes when subjected to 

 the pressure of chyme of a certain degree of fluidity. It has also 

 been suggested that the chemical reaction of the chyme may be a 

 factor, but agreement is lacking on this point. After the expul- 

 sion of a quantity of the chyme, a corresponding quantity of food 

 is pushed out of the fundus, presumably by the tonic contractions 

 of the fundic muscles. 



Digestion in the Small Intestine. — In the intestine, the chyme 

 from the stomach is subjected to the action of enzymes contained 

 in the pancreatic juice and also enzymes produced by the small 

 intestine itself. The alkaline secretion from the pancreas con- 

 tributes four enzymes: two proteolytic enzymes, trypsin and 

 erepsin; an amylolytic enzyme, amylase; and a lipolytic enzyme, 

 lipase. From the intestinal mucosa comes a proteolytic enzyme, 

 erepsin, identical with the erepsin of the pancreatic fluid; various 

 inverting enzymes; and enterokinase, which activates trypsin- 

 ogen. All of these enzymes act in an alkaline or neutral or 

 faintly acid medium. 



When the acid chyme from the stomach is discharged into the 

 duodenum the pancreatic juice begins to flow into the duodenum. 

 A substance, pancreatic secretin, is formed by the action of acid 

 on the duodenal mucosa and is carried by the blood stream to the 

 pancreas, where it causes active secretion. It has a similar effect 

 on the liver, causing the secretion of bile. Pancreatic secretin, 



