THE GASTRIC JUICE. 131 



acid. The ferment is here apparently digested by the pepsin, and it 

 is thus easily possible to obtain solutions of pepsin which are alto- 

 gether free from chymosin. In neutral solution it is more resistant, 

 and can be heated to a temperature of 50 C. ; at 70 C., however, 

 it becomes permanently inactive. In its dry state, on the other 

 hand, it can be heated to 110 C. without losing its activity. Alka- 

 lies when present beyond traces destroy the substance, as they do 

 pepsin. Like all other ferments, it is capable of effecting an exten- 

 sive reaction, even when present in small amount. The quantity of 

 ferment contained in 1 gramme of the dried and pulverized mucous 

 membrane of the fourth stomach of the calf, when dissolved in 

 water, is thus capable of coagulating 200 liters of milk in one 

 minute at a temperature of 50 C. 



Of the chemical nature of chymosin, nothing is known ; but, as 

 in the case of pepsin, the purer preparations do not give the usual 

 reactions of albumins. It is precipitated from its neutral solutions 

 by subacetate of lead, while the acetate and tannic acid are without 

 effect. Alcohol likewise precipitates the ferment and gradually 

 renders it inactive. Like pepsin, it is not dialyzable. 



Under normal conditions chymosin is always present in the gastric 

 juice of man. In certain diseases of the stomach, however, which 

 are associated with the death of its glandular elements, the ferment, as 

 also its zymogen, is lacking. 



Tests for Chymosin and Chymosinogen. To test for the presence 

 of chymosin, 5 or 10 c.c. of milk are treated with a few drops of 

 the filtered gastric juice and kept at a temperature of from 37 to 

 40 C. If coagulation occurs within ten or fifteen minutes, the 

 presence of chymosin may be assumed. Should the gastric juice, 

 however, be markedly acid, it is necessary first to neutralize it with 

 sodium hydrate. 



To test for chymosinogen, the milk is treated with 2-3 c.c. of a 

 1 per cent, solution of calcium chloride, and 10 c.c. of filtered 

 gastric juice which has been rendered feebly alkaline with sodium 

 hydrate. The mixture is kept at a temperature of from 37 to 40 

 C., when in the presence of the zymogen a thick cake of casein is 

 formed within a few minutes. 



Isolation of Chymosin. To isolate chymosin in comparatively 

 pure form, the following method, as suggested by Hammarsten, may 

 be employed : The mucous membrane of the fourth stomach of the 

 calf is carefully dissected off, washed with water, and extracted with 

 an 0.1 per cent, solution of hydrochloric acid, as already described. 

 The infusion is then neutralized and repeatedly shaken with 

 powdered magnesium carbonate until the pepsin has been removed. 

 The filtrate is treated with subacetate of lead, the precipitate 

 decomposed with very dilute sulphuric acid, and the acid filtrate 

 further precipitated with an aqueous solution of stearin soap. The 

 ferment is thus thrown down together with the fatty acids, from 

 which it is then separated by suspending the precipitate in water 



