GASTRIC JUICE 247 



10. The Nature and Action of Gastric Juice.— Pure 



gastric juice is a clear, acid fluid and consists of little 

 more than water, containing a few saline matters in solu- 

 tion, and its acidity is due to the presence of free 

 hydrochloric acid to the extent of "4 per cent. It 

 possesses, however, in addition a small quantity of a 

 peculiar substance called pepsin, a soluble ferment 

 'or enzyme in many respects similar to, though very 

 different in its efl'ects from ptyalin. 



It is easy to ascertain the properties of gastric juice 

 experimentally, by putting a small portion of the mucous 

 memljrane of a stomach into water made acid by the 

 addition of "2 — "5 per cent, of hydrochloric acid and con- 

 ta.ining small pieces of meat, hard-boiled egg, or other 

 proteids, and keeping the mixture at a temperature of 

 about 40° C. (104' F.). After a few hours it will be 

 found that the white of egg, if not in too great quantity, 

 has become dissolved : while all that remains of the meat 

 is a pulp, consisting chiefly of the connective tissue and 

 fatty matters which it contained. This is artificial 

 digestion, and it has been proved by experiment that 

 precisely the same operation takes place when food 

 undergoes natural digestion within the stomach of a 

 living animal. 



It takes a very long time (some days) for the dilute acid 

 alone to dissolve proteid matters, and hence the solvent 

 power of gastric juice must be chiefly attributed to the 

 pepsin ; moreover gastric juice which has been boiled, in 

 which case all the ferment it contains is "killed" (see 

 p. 243), is quite mactive although it contains the usual 

 amount of acid. 



Thus gastric juice dissolves pi-oteins, and the character- 

 istic protein which is formed during the solvent action of 

 the juice is called peptone, and has pretty much the 

 same characters whatever the nature of the protein which 

 has been digested. 



