THE SMALL INTESTINE 91 



to continue without compensatory secretion, a final stage 

 might be reached in which the cavity of the intestine would 

 be drained of all fluid, while the walls would be crusted 

 with a dry residue suggestive of boiler scale. 



The Pancreatic Juice. In considering the causes of 

 gastric secretion the importance of the central nervous 

 system called for emphasis. This is not true to the same 

 extent of the government of the pancreas, though here 

 also it is held that the nervous system plays a part. A 

 chemical means of control is better known. As has been 

 hinted before, there is an intimate relation between the 

 gastric activity and the later awakening of the pancreas 

 from its usual state of repose. The arrival of acid from 

 the stomach in the duodenum causes a timely outflow of 

 pancreatic juice. 



This might be supposed to be an instance of reflex action, 

 like the production of saliva when acid is taken into the 

 mouth. It has been shown, however, that it has another 

 explanation. The acid, striking into the lining membrane 

 of the duodenum, initiates a series of reactions which have 

 been studied in detail, and which lead at last to the for- 

 mation of a substance of quite definite chemical properties 

 called secretin. This finds its way into the circulation, 

 and, like a drug, is swept far and wide. It stimulates the 

 pancreas to produce its secretion, augments the formation 

 of bile by the liver, and probably excites the glands in the 

 wall of the intestine to greater activity. In the light of 

 these facts it becomes clear that a vigorous gas'tric digestion 

 with the strongly acid chyme which results goes far to 

 insure a normal intestinal process. 



The pancreatic juice in man is an abundant secretion 

 a pint or more daily and, in marked contrast with the 

 saliva and the gastric juice, it has relations with all three 

 principal classes of food-stuffs.. It contains an enzyme 

 which some have thought to be identical with the ptyalin 

 of the saliva, but generally called amylopsin or pancreatic 

 amylase. Thus the progress of starch digestion, inter- 

 rupted for a time in the stomach, is now renewed. The 



