DIGESTION 127 



the mixture thicker and sticky. It colours it yellowish-brown. 

 Under the influence of pancreatic juice the remaining proteins 

 and proteoses are rapidly converted into peptones, some of 

 which are shaken down by the violent action of erepsin into 

 simpler bodies, such as leucin and tyrosin, etc. The chyme 

 becomes alkaline, grey, and thin. All undigested starch is 

 changed into maltose, and this into dextrose. Cane-sugar 

 is converted into dextrose and levulose. These sugars 

 are absorbed into the blood. Milk-sugar, if not converted 

 into lactic acid, remains as lactose (C 12 H 22 O n ), in which 

 condition it is absorbed without " inversion." Fats are split 

 by a ferment of the pancreatic juice into fatty acid and 

 glycerin ; some of the fatty acid combines with alkali to form 

 soap, but of this we shall have more to say later on. 



The duct common to the liver and the pancreas opens into 

 the second part of the duodenum. The organs which produce 

 bile and pancreatic juice are comparatively remote from the 

 place where their secretions come into contact with the food. 

 By what mechanism are they thrown into activity when the 

 assistance of their secretions is required ? As in the case of the 

 stomach, the agent is a hormone, a chemical messenger. The 

 hormone, termed " secretin," is formed by the cells of the 

 mucous membrane of the duodenum when acid comes in 

 contact with them. It is absorbed by the blood, which carries 

 it to the pancreas and the liver. When it reaches the pancreas, 

 it acts as a most powerful stimulant to the discharge of accumu- 

 lated ferments, and to the production of an additional supply. 

 It stimulates the liver to pour forth bile. At present we are 

 in ignorance as to the chemical nature of this hormone. It is 

 not a projeid substance, nor is it a ferment. If scrapings from 

 the mucous membrane of the duodenum be crushed with sand 

 and hydrochloric acid, the mixture boiled, neutralized with 

 carbonate of soda, and filtered, the clear, colourless liquid 

 which results has a powerful effect upon the pancreas, when 

 injected, in even small quantities, into the blood. Apparently, 

 the cells of the duodenal mucousTmembrane are constantly 

 producing and accumulating a substance which is converted 

 into secretin when acted on by acid. It is not necessary for 

 the acid to stimulate the living cells.:") If the mucous membrane 

 is ground up with sand and salt-solution, the filtrate is inactive ; 



