IN THE STOMACH. 9 



gastric juice can be proved to contain free hydrochloric 

 acid, for the quantity of chlorine in it is considerably more 

 than sufficient to combine with all the metals present. 

 As regards human gastric juice the proof is less complete. 

 Lactic acid is present in chyme whenever carbohydrates 

 are being digested. It has been found that hydrochloric 

 acid can be replaced by phosphoric acid, as well as by 

 acetic and other acids of the same series. 



Pepsin, although resembling the albuminous bodies in 

 chemical composition, exhibits none of their distinctive 

 characters. It exists in gastric juice in a state of imperfect 

 solution, so that it can be removed from it by mechanical 

 means ; on this fact Briicke's method of preparing pure 

 pepsin is founded. It is capable cf taking part in the 

 digestion of albuminous substances, even in the smallest 

 quantity, provided that the liquid is not too dilute, and 

 that it does not contain too large a proportion of the 

 product of digestion peptone. In the process, the pep- 

 sin neither increases, diminishes, nor undergoes any loss of 

 activity. 



The substance commercially known as pepsin consists 

 of dried gastric mucous membrane, and hydrochloric acid. 

 A solution of pepsin which possesses its digestive proper- 

 ties is obtained by extracting fresh mucous membrane 

 with glycerine. 



In animals that die during digestion, the stomach 

 digests itself after death. During life such digestion is 

 prevented by the alkalinity of the tissues. Chyme, as 

 it leaves the stomach, contains remains of animal and 

 vegetable structure, unaltered starch grains, fat particles, 

 and (after milk diet) curd particles. It does not normally 

 contain bile. It yields a mixture of gases in which there 

 is much less oxygen than in common air, and a larger 

 proportion of CO 2 . 



Peptone differs from proteids in the readiness with which it diffuses 

 through animal membranes. It resembles them in chemical composition 



