56 



ESSENTIALS OF CHEMICAL PHYSIOLOGY 



hardly be considered to occur in the blood generally but rather in 

 the oxyntic cells, which possess the necessary selective powers in 

 reference to the saline constituents of the blood, and the hydro- 

 chloric acid, as soon as it is formed, passes into the secretion of the 

 gland in consequence of its high power of diffusion. 



COMPOSITION OF GASTRIC JUICE 



The following table gives the percentage composition of the 

 gasti'ic juice of man and the dog : — 



One sees from this how much richer in all constituents the gastric 

 juice of the dog is than that of man. Carnivorous animals have 

 always a more powerful gastric juice than other animals : they have 

 more work for it to do ; but the great contrast seen in the table is, 

 no doubt, partly due to the fact that the persons from whom it has 

 been possible to collect gastric juice have been invalids. In the 

 foregoing table one also sees the great preponderance of chlorides 

 over other salts : apportioning the total chlorine to the various 

 metals present, that which remains over must be combined with 

 hydrogen to form the free hydrochloric acid of the juice. 



Pepsin stands apart from nearly all other ferments by requiring 

 an acid medium in order that it may act. Probably a compound of 

 the two substances called pepsin-hydrochloi'ic acid is the really 

 active agent. Other acids may take the place of hydrochloric acid, 

 but none act so well. Lactic acid is often found in gastric juice : 

 this appears to be derived by fermentative processes from the 

 food. 



Some recent very valuable work performed by Pawlow on dogs 

 has enabled him not only to show that the secretory fibres for the 

 gastric glands are contained in the vagus or pneumogastric nerves, 

 but also to obtain a gastric juice free from any admixture with saliva 



