GASTRIC JUICE 129 



the HCl and of the products of protein breakdown upon 

 the pyloric mucosa. The substance thus formed is absorbed 

 into the blood-stream, and, being conveyed back to the 

 stomach by the arterial blood, stimulates the glands to 

 continued activity. 



In the process of gastric secretion there are therefore 

 two factors. The first is a nervous reflex which starts the 

 secretion ; the second is the stimulus of a chemical substance 

 or hormone which continues it. Thus the hormone which 

 is the cause of the continuation of secretion is produced 

 as the result of the initial secretion. This hormone has 

 been called gastrin or gastric secretin. 



The gastric juice produced by the chemical method 

 differs from the juice of vagal origin in that it is adapted 

 to the kind of food present in the stomach. It is most 

 abundant with meat, while the presence of fats may alto- 

 gether inhibit its formation. 



Gastric Juice 



Gastric juice has the following active constituents — ■ 



1. Pepsin. — This is formed in the gastric glands in an 

 inactive form — pepsinogen, which is converted into pepsin 

 on contact with hydrochloric acid. Pepsin requires free 

 HCl, not only for its formation, but also for its digestive 

 action ; it is rapidly killed by alkahes. It causes an in- 

 complete hydrolysis of protein, the end-products formed 

 normally being proteoses and peptones. 



2. Rennin. — This ferment, by converting the soluble 

 caseinogen to insoluble casein, causes the coagulation of 

 milk. Rennin is believed by some to be identical with 

 pepsin. 



3. Gastric Lijjase. — Present in small quantities, it effects 

 hydrolysis of finely divided fat. 



4. Hydrochloric Acid. — -This is secreted by the oxyntic 

 cells of the fundus. Its functions will be described later. 



