CH. XXXIII.] NERVES OF THE GASTRIC GLANDS 511 



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. 



In recent years, the composition and action of the gastric juice 

 has been studied by Pawlow. By an ingenious surgical operation, he 

 succeeded in separating from the stomach of dogs a diverticulum 

 which pours its secretion through an opening in the abdominal wall ; 

 the nerves of this small stomach are intact, and the amount of juice 

 that can be collected from it when it is active amounts to several 

 hundred cubic centimetres in a few hours. Pawlow thus obtained a 

 pure gastric juice, which enabled him to study its action and com- 

 position. It is clear, colourless, has a specific gravity of 1003 1006, 

 and is feebly dextro-rotatory. It contains 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 precipitate of pepsin, and this 

 carries down with it the acid in loose combination, especially in the 

 layers first deposited. Its percentage composition is very similar 

 to that of a protein, only it contains chlorine in addition to the usual 

 elements. The numbers agree closely with those obtained by Kiihne, 

 who used ammonium sulphate as the precipitant. 



Pepsin stands apart from nearly all other enzymes 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 is derived by 

 fermentative processes from the food. 



The digestive powers of the acids are proportional to their dissociation and the 

 number of H ions liberated. The anions, however, modify this by having different 

 powers of retarding the action. The greater suitability of hydrochloric over lactic 

 acid, for instance, in gastric digestion is due to the fact that the former acid more 

 readily undergoes dissociation. 



The Innervation of the Gastric Glands. 



As long ago as 1852 Bidder and Schmidt showed in a dog with 

 a gastric fistula that the sight of food caused a secretion of gastric 

 juice; and in 1878 Kichet observed that in a man with complete 

 occlusion of the gullet the act of mastication caused a copious flow 

 of gastric juice. There could therefore have been no doubt that the 



