134 PHYSIOLOGICAL CHEMISTRY 



longer time. Digestion in the stomach thus is started by a 

 copious flow of juice incited by nervous impulses, either psychic 

 or reflex. The process is continued by a flow which is incited 

 by the presence of the food itself in the stomach by means of a 

 chemical substance, gastrin, produced in the mucous mem- 

 brane, given off into the blood, which upon reaching the gastric 

 glands, incites them to secrete. 



The amount of gastric juice secreted varies greatly. It is 

 roughly proportional to the amount of food eaten. From 2-3 

 liters a day has been estimated as an average amount. 



General Character of the Secretion. The character and com- 

 position of the gastric juice vary with the nature and amount 

 of food taken. Perhaps this is due to the fact that all of the 

 glands producing gastric juice may not be secreting all the 

 time. The juice first secreted has a greater digestive activity 

 than that secreted later, and any sudden increase in the amount 

 of flow also is accompanied by increased digestive power. The 

 digestive agents apparently accumulate in the gland cells dur- 

 ing rest ; the first secretion would thus be more active. 



Gastric juice from dogs or man is a clear, watery fluid, and 

 is colorless unless mixed with bile which sometimes gets into 

 the stomach through the pylorus. The specific gravity ranges 

 from 1.002-1.0059. It contains the salts which are found in the 

 blood serum, and some organic material. Its most important 

 constituents are hydrochloric acid and the enzymes pepsin, 

 rennin, and a lipase. The chief digestive activity of the stom- 

 ach is on proteins, since pepsin and rennin both act on that 

 group. 



Hydrochloric Acid. The first stage of the digestion of pro- 

 teins in the stomach consists in their conversion into acid meta- 

 protein by the action of the hydrochloric acid present in the 

 gastric juice. Variations in the amount of acid occur, and if its 

 concentration departs sufficiently from the normal, grave diges- 

 tive disturbances result. Variations in the amount of hydro- 

 chloric acid accompany certain pathological conditions. For 

 these reasons, the acid of the gastric juice long has been the sub- 



