74 ESSENTIALS OF CHEMICAL PHYSIOLOGY 



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. A compound of the two 

 substances called pepsin-hydrochloric 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, however, is 

 derived by fermentative processes from the food. 



Pawlow has shown that in dogs the secretory fibres for the gastric 

 glands are contained in the vagus nerves. 



By an ingenious surgical operation he succeeded in separating off 

 from the stomach a diverticulum which pours its secretion through an 

 opening in the abdominal wall. This small stomach was found to 

 act in every way like the main stomach of the animal. The pure juice 

 so obtained is clear and colourless : it has a specific gravity of 1003 

 to 1006. It is feebly dextro-rotary, and gives some of the protein 

 reactions. It contains from 0*4 to 0'6 per cent, of hydrochloric acid. 

 It is strongly proteolytic, and inverts cane sugar. When cooled to 

 C. it deposits a fine precipitate of pepsin : this settles in layers, 

 and the layers first deposited contain most of the acid, which is 

 loosely combined with and carried down by the pepsin. Pepsin is 

 also precipitable by saturation with ammonium sulphate (Kuhne). 

 Elementary analysis gave the following results : 



Pepsin precipitated by cold Precipitated by (NH 4 ) 2 S0 4 



Carbon . . . 50-73 per cent. 50'37 



Hydrogen . . 7 '23 6-88 



Chlorine . . I'Ol to 1-17 0'89 



Sulphur . . . 0-98 1-34 



Nitrogen . . . not estimated 14-55 to 15'0 



Oxygen . . . the remainder the remainder. 



The juice is most abundant in the early periods of digestion, but 

 it continues to be secreted in declining quantity as long as any food 

 remains to be dealt with. When there is no food given there is no 

 juice. But sham feeding with meat will cause it to flow. 



The larger the proportion of protein in the diet, the more abundant 



