ANIMAL FLUIDS AND TISSUES. 435 



the stomach, but these are not secreted in the gastric juice itself,- but 

 are produced by some fermentative action from the food after it has 

 entered the stomach. The acidity of gastric juice itself is due to free 

 hydrochloric acid, present in quantities varying from 0.1 to 0.3, or 

 even 0.4 per cent. 



The nature of the decomposition resulting in the liberation of free hydro- 

 chloric acid is not known, but it may possibly be formed by the action of 

 sodium phosphate on calcium chloride : 



2(Na 2 HPO 4 ) -f 3CaCl 2 = = Ca 3 (PO 4 ) 2 + 4NaCl + 2HC1. 



Sodium Calcium Calcium Sodium Hydrochloric 



phosphate. chloride. phosphate. chloride. acid. 



According to others, the hydrochloric acid is liberated by the action of acid 

 sodium carbonate on sodium chloride : 



NaHC0 3 + Nad = Na^COg + HC1. 

 Sodium acid Sodium Sodium Hydrochloric 



carbonate. chloride. carbonate. acid. 



The above formulas show the reverse action of that which these substances 

 exert upon each other under common conditions, but it must be remembered 

 that the living cell is capable of decomposing matter generally in a manner 

 different from that which it suffers ordinarily. 



The average composition of pure gastric juice may be approxi- 

 mately stated thus : 



Water ......... 99.26 per cent. 



Pepsin and other organic matter .... 0.30 " 



Kennet ......... ? " 



Free hydrochloric acid ...... 0.22 " 



Alkali chlorides ....... 0.20 " 



Phosphates of calcium, magnesium, and iron . . 0.02 " 



Pepsin, besides hydrochloric acid, the most important constituent 

 of gastric juice, has been spoken of heretofore ; it has, in the pres- 

 ence of free hydrochloric acid, the power of converting proteids into 

 albumoses, and finally into peptones. Pepsin is not secreted by the 

 gastric tubules as such, but in a preliminary stage or pro-enzyme (pep- 

 sinogen), and is changed by the hydrochloric acid into pepsin. 



Another ferment, known as rennet, is found in the gastric secretion. 

 Like pepsin, it is secreted in a preliminary stage or pro-enzyme (rennet 

 zymogen). Rennet has the power of coagulating milk in neutral 

 solutions, that is, of precipitating the casein. 



Experiment 66. Open the stomach of a pig, sheep j or calf, recently killed 

 while fasting ; wash it rapidly in cold water, spread it out and scrape off the 

 mucous surface ; digest it under frequent stirring with about ten parts of water 

 for six hours, and filter. The solution contains pepsin which verity by its 

 dissolving action on coagulated albumin. 



