DIGESTION 255 



in reaction, and containing 15 per cent, of protein 

 (chiefly albumin) and 6 per cent, of ash. About 500 

 to 600 c.c. of it are produced in twenty-four hours. 



It is not yet determined whether there is a psychical 

 or * appetite ' production of pancreatic, as there is of 

 gastric, juice, but the chief flow is reached about three 

 hours after the taking of food, and is the result of a 

 chemical stimulus exerted on the pancreas by the 

 substance secretin, which is produced by the action of 

 acids upon the cells lining the upper part of the in- 

 testine. That the presence of acids is not essential for 

 the production of the stimulus, however, there can be no 

 doubt, for the secretion may still go on in cases in which 

 the production of hydrochloric acid by the stomach is 

 entirely arrested, and Pawlow has shown that fats are 

 also capable of calling out a flow of the juice. 



The pancreatic juice contains a proteolytic ferment 

 (trypsinogen), which is converted into trypsin by mixture 

 with the enterokinase of the intestinal juice. Unless 

 enterokinase be present, trypsinogen is inactive, which 

 explains the fact that the pancreas does not digest itself. 

 In addition to trypsinogen, the juice contains the fat- 

 splitting ferment (lipase) and a diastasic ferment, which 

 converts starch to maltose. Both of these are present in 

 the fresh juice, though the action of the fat-splitting 

 ferment is intensified by the presence of bile and 

 intestinal juice. 



Absence of pancreatic juice from the intestine inter- 

 feres to some extent with the absorption of protein, but 

 to a greater extent with that of fat, although the split- 

 ting up of fat seems still to go on to a considerable 



