THE DIGESTIVE JUICES 73 



acid originates by the interaction of sodium chloride and sodium 

 dihydrogen phosphate, as is shown in the following equation : 



NaH 2 P0 4 + NaCl=Na 2 HP0 4 + HC1 



[sodium di- [sodium [ilisodium [hydro- 



hydrogeu chloride] hydrogen chloric 



phosphate] phosphate] acid] 



The sodium dihydrogen phosphate in the above equation is prob- 

 ably derived from the interaction of the disodium hydrogen phos- 

 phate and the carbonic acid of the blood, thus : 



Na 2 HP0 4 + C0 2 + H 2 0=NaHC0 3 + NaH 2 PO 4 . 



Other theories have tried to explain the formation of such a strong 

 acid as hydrochloric by the law of ' mass action.' We know that by 

 the action of large quantities of carbonic acid on salts of the mineral 

 acids the latter may be liberated in small quantities. We know, 

 further, that small quantities of acid ions may be continually formed 

 in the organism by ionisation. But in every case we can only make 

 use of these explanations if we assume that the small quantities of 

 acid are carried away as soon as they are formed, and thus give room 

 for the formation of fresh acid. Even then it is impossible to 

 explain the whole process. A specific action of the cells is no doubt 

 exerted, for these reactions can 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 constituents of the 

 blood, and the hydrochloric 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 

 gastric juice of man and dog : 



