CHEMICAL MECHANISMS OF SECRETION 293 



starch and to some extent on fats, the action on fats being increased 

 by the addition of succus entericus. It is, in fact, normal pan- 

 creatic juice. 



6. Secretin rapidly disappears from the tissues, but cannot 

 be detected in any of the secretions. It is apparently not absorbed 

 from the lumen of the intestine. 



7. It is not possible to obtain a body resembling secretin from 

 any tissues of the body other than the mucous membrane of the 

 duodenum and jejunum. 



8. Secretin solutions, free from bile salts, cause some increase 

 in the secretion of bile. They have no action on any other glands. 



9. Acid extracts of the mucous membrane normally contain 

 a body which causes a fall of blood- pressure. This body is not 

 secretin, and the latter may be prepared free from the depressor 

 substance by acting on desquamated epithelial cells with acid. 



The Chemical Mechanism of Gastric Secretion Gastrin. It has 

 long been known that the introduction of certain substances into 

 the stomach provokes a secretion of gastric juice, and the effect 

 has been ascribed to a nervous mechanism stimulated by the effect 

 of absorbed substances upon peripheral nerve endings in the gastric 

 mucosa. Soon after the discovery of secretin it was shown by 

 Edkins that intravenous injection of extracts prepared in special 

 manner from certain parts of the gastric mucous membrane leads to 

 a flow of gastric juice. Edkins considers this action to be due to 

 a substance which he has named gastrin, and which acts as a 

 chemical excitant or " hormone " for the gastric secretion, in a 

 similar fashion to secretin in the case of the pancreatic juice. It 

 is hence possible that those substances shown by Pawlow to excite 

 the gastric secretion when introduced into the stomach so as not 

 to call forth a psychical flow, as by the use of a sound or, better, 

 through a gastric fistula without attracting the animal's attention, 

 produce their effect not by exciting peripheral nerve- endings in 

 the gastric mucosa, but by means of a chemical action upon the 

 secreting cells. This action may either be a direct one of the 

 substances themselves or, more probably, according to Edkins's 

 observations, an indirect action in which these substances, similarly 

 to hydrochloric acid in the case of the duodenal mucosa, set free 

 an active substance chiefly from the pyloric portion of the gastric 

 mucosa. This substance, after being absorbed by the blood-stream, 

 is carried to the secreting cells lying deeper in the mucosa, and also 



