TRANSFERENCE OF FOOD MATERIALS 129 



the fatty acids set free. By measuring the amount of standard 

 alkali necessary to neutralise the fatty acids set free and to 

 once more render the solution faintly alkaline to phenol- 

 phthalein the rate of hydrolysis of fats can be estimated. 



$he secretion of pancreatic juice can be brought about by 

 a nervous reflex just like the secretion of saliva or gastric 

 juice, but the predominant influence is a chemical stimulus. 

 Acid in the duodenum causes a secretion of pancreatic juice 

 even after all the nervous connections of the pancreas have 

 been severed. Bayliss and Starling showed that this was due 

 to a substance which they called secretin.* In the mucous 

 membrane of the duodenum is a substance called prosecretin, 

 which on treatment with acid is turned into secretin. If the 

 mucous membrane of the duodenum is boiled with dilute acid 

 and the extract carefully neutralised, injection of the solution 

 into the circulation causes a secretion of pancreatic juice. 



The regulation of pancreatic secretion by secretin is adapted 

 to secure complete neutralisation of the acid gastric contents. 

 The passage of acid into the duodenum causes the formation of 

 secretin. This causes a secretion of pancreatic juice which 

 neutralises the acid. So long as there is any unneutralised 

 acid, secretin is formed, and when all the acid is neutralised 

 the formation of secretin ends. The secretin formed by the 

 last amount of acid must be absorbed and pancreatic secretion 

 continues until this secretin is used up. Hence there is 

 always a slight excess of pancreatic juice over that necessary 

 to neutralise the acid. 



In the absence of acid it has been found that other sub- 

 stances can cause the formation of secretin. Thus as is usual 

 in biological processes there is provision against the failure 

 of the usual mechanism. Soaps seem able to produce secretin 

 from the mucous membrane of the duodenum (Babkin). 



Secretin is said to have a slight effect in causing secretion 

 by the cells of the intestinal glands and of the liver. 



There is an interesting transition from ihe secretion of 

 saliva which is due to purely nervous influences, through the 

 gastric secretion which is mainly nervous and followed by some 

 chemical stimulus, to the pancreatic secretion which is mainly 

 chemical. These differences correspond to the many sen- 

 sations associated with food in the mouth and just before it 

 passes into the stomach and the almost complete absence of 

 sensation associated with food in the duodenum. 



* W. M. Bayliss and E. H. Starling, Journ. Physiol., 1902,'vol. 28, 

 P- 325- 



