THE FOOD AND DIGESTION 349 



any true rennin, although it produces a modified clotting of 

 milk, under certain conditions. 



The influence of the pancreas in the general metabolism 

 will be considered later (p. 390). 



3. Physiology of Pancreatic Secretion. The secretion 

 of pancreatic juice is not constant, but is induced when 

 the acid chyme passes into the duodenum. This occurs 

 even when all the nerves to the intestine have been 

 cut, and it appears to be due to the formation in the 

 epithelium lining the intestine, under the influence of an 

 acid, of a material which has been called secretin. This 

 is absorbed and, on being carried to the pancreas, stimulates 

 it to secrete. It has been shown that the injection into the 

 blood of an extract of the lining membrane of the upper part 

 of the small intestine, made with dilute hydrochloric acid, 

 leads to a flow of pancreatic juice. This secretin is not 

 destroyed by boiling, and is soluble in strong alcohol. It is 

 therefore not of the nature of an enzyme. 



But while secretin seems to play so important a role, 

 it has been found that stimulation of the vagus nerve, 

 after a latent period of two minutes, increases pancreatic 

 secretion, so that it must be concluded that the secretion 

 of the fluid is, to a certain extent, under the control of the 

 nervous system. 



C. Secretion of the Intestinal Wall (Succus Entericus). 



This is formed in the Lieberkiihn's follicles of the intes- 

 tine, and it may be procured by cutting the intestine across 

 at two points, bringing each end of the intermediate piece to 

 the surface, and connecting the ends from which this piece 

 has been taken away. On mechanically irritating the 

 mucous membrane, a pale yellow clear fluid is secreted, 

 which contains native proteids and mucin, and is alkaline 

 in reaction from the presence of carbonate of soda. 



Action. The succus entericus contains: (1) An enzyme 

 which splits some disaccharids, as maltose and cane sugar, 

 into monosaccharids, but does not seem to act on lactose. 

 (2) In the intestine of animals taking milk a special zymin, 



