98 HUMAN PHYSIOLOGY 



it does not reach the stomach. This secretion stops after 

 the cutting of the vagi. 



The secretion is dependent upon psychical conditions, as 

 it can be brought about by the mere sight of food. After 

 section of the vagi, secretion still occurs when food is placed 

 in the stomach itself. Whether this secretion is a reflex 

 process in which the vagus does not participate, or whether 

 it is due to direct stimulation of the glands, is not known. 



The secretion of the gastric juice is, therefore, called forth 

 by the sight and deglutition of food and is then continued 

 by the presence of the food in' the stomach. 



4. PANCREATIC SECRETION 



\. Composition of pancreatic juice. The pancreatic juice 

 from a newly placed fistula in the pancreatic duct is a clear, 

 thick fluid having a specific gravity of 1.03. Because of its 

 sodium carbonate (0.2$) it has a strong alkaline reaction. 

 Sometimes it coagulates spontaneously. From a permanent 

 fistula the secretion is not so thick (sp. gr. i.oi). 



The pancreatic juice obtained by a temporary fistula con- 

 tains about 90$ water, that by a permanent fistula 98$. 

 The solids contain from 0.6-0.9$ ash, also organic sub- 

 stances, especially proteids (from a temporary fistula about 

 io#). The secretion from a temporary fistula is frequently 

 so rich in proteids that, on heating, it coagulates to a solid 

 mass. Pancreatic juice also contains leucin, fat, soaps in 

 small quantities, and the following three characteristic 

 ferments : 



(a) A diastatic ferment, which acts upon starch like the 

 ptyalin of saliva. 



(ti) Trypsin, a ferment splitting proteids up into proteoses. 

 Trypsinogen, the antecedent of trypsin, is changed to trypsin 

 during secretion and also by the action of oxygen or organic 

 acids. 



(c] Steapsin, a fat-splitting ferment, which splits the 

 neutral fats into glycerin and fatty acids. 



