64 PHYSIOLOGY FOR DENTAL STUDENTS. 



as a result of absorption from the stomach. This "hormone" 

 (see p. 124) is not merely absorbed food, for no gastric secretion 

 occurred when solutions of meat extract, or of peptone were in- 

 jected intravenously. It must therefore be some substance that 

 is absorbed into the blood from the mucous membrane of the 

 stomach, and which is produced in this as a result of the action 

 of the gastric contents on its cells. In confirmation of this view 

 it has been shown that boiled extracts of the mucous membrane 

 of the pyloric region of the stomach (made with water or weak 

 acid or solutions of peptone or dextrin) cause some gastric juice 

 to be secreted when they are injected in small quantities every 

 ten minutes into a vein, similar injections of the extracting fluids 

 themselves being without effect. 



We are now provided with the necessary facts upon which to 

 draw a completed picture of the mechanism of gastric secretion. 

 The satisfaction of taking food causes appetite juice to flow and 

 this soon digests some of the protein. The products of this diges- 

 tion, along with the extractive substances of the food, after some 

 time (which is probably quite short in the case of man), gain the 

 pylorus, where they act on the mucosa to produce some hormone, 

 which becomes absorbed into the blood and stimulates further 

 secretion of the juice. As digestion proceeds juice therefore con- 

 tinues to be secreted. The appetite juice sets the process agoing ; 

 it ignites gastric digestion. 



The Active Constituents of Gastric Juice. When there is no 

 food in the stomach, a certain amount of the mucous secretion 

 is present in it, and most of the gland cells are filled with zymo- 

 gen granules (see p. 40). An extract (made with glycerine) 

 of the mucosa in this resting condition exhibits no digestive 

 powers; but if the mucosa be first of all macerated with weak 

 hydrochloric acid, the extract becomes highly active, because it 

 contains large amounts of the proteolytic ferment pepsin. Other 

 cells in the stomach produce the necessary hydrochloric acid. 

 It may be concluded therefore that during the process of secre- 

 tion the zymogen granules in the cells are acted on by hydro- 

 chloric acid and converted to pepsin. In conformity with this, 

 it has been found that the secretion of a pouch of stomach pre- 



