FOOD AND DIGESTION 365 



the investigations of Starling to be due to the formation of a 

 material which has been called seeretin, in the epithelium 

 lining the intestine, under the influence of an acid. 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, made with dilute hydrochloric acid, of the lining 

 membrane of the upper part of the small intestine, leads to a flow 

 of pancreatic juice. This seeretin is not destroyed by boiling, 

 and is soluble in strong alcohol. It is therefore not of the 

 nature of an enzyme. 



But while seeretin 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. 



The influence of the pancreas in the general metabolism will 

 be considered later (p. 410). 



B. Secretion of the Intestinal Wall (Suceus Enterieus) 



This is formed in the Lieberkiihn's follicles of the intestine, 

 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 together the ends from which this piece 

 has been taken away so as to make a continuous tube. On 

 mechanically irritating the mucous membrane, a pale yellow 

 clear fluid is secreted, which contains native proteins and mucin, 

 and is alkaline in reaction from the presence of sodium carbonate. 



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, laetase, 

 which splits milk sugar. (3) Erepsin, an enzyme which seems 

 to act more powerfully than trypsiu in splitting peptones into 

 their component non-protein crystalline constituents such as 

 the di-amino acids and non-amino acids, e.g. leucin and ty rosin. 

 The object of this is not at present fully understood. It may be 

 that the nitrogen of the protein is largely treated as a waste 

 product and thrown off. But feeding experiments on dogs and 

 rats seem to show that these non- protein derivatives of proteins 



