42 STUDIES IN DIGESTION 



that they may fail to make progress even when the 

 operation has apparently not been postponed too long. 

 Sometimes they will request that they should still 

 be allowed to take food into the mouth "just to 

 taste it." Evidently they lack the first secretion of 

 gastric juice due to the relish with which the food is 

 tasted and swallowed, and digestion may in conse- 

 quence be very imperfect. This may be overcome 

 by the simple device of adding some form of extrac- 

 tives to the feed, such as beef-tea, gravy, soup, or a 

 meat essence. Thus the chemical mechanism is 

 brought into play though the nervous reflex fails. 

 Excellent practical results have been obtained by 

 this expedient. 



THE SECRETION OF PANCREATIC JUICE. 



This was first thought by Pawlow to be due to a 

 reflex through the vagus, but it has been shown by 

 Bayliss and Starling that the stimulus is in reality 

 chemical. When the hydrochloric acid of the gastric 

 juice touches the mucous membrane of the duodenum, 

 a soluble chemical substance is formed called 

 " secretin," which passes into the rootlets of the 

 portal vein, is carried to the liver and heart, and 

 thence all over the body. Some of it in due course 

 reaches the pancreas, and a flow of pancreatic juice 

 is at once instituted and continues as long as the 

 acid contents of the stomach continue to enter the 

 duodenum. The secretion acts chemically on the 

 pancreatic cells, liberating steapsin from pro-steapsin, 

 amylopsin from pro-amylopsin, and trypsinogen from 

 pro-trypsinogen. There is some evidence that secretin 



