268 APPLIED BACTEEIOLOGY 



time to time, that the changes were due to heat only, to 

 grinding only, or to the action of strong acids, etc. 



Gastric juice exudes when the surface of the stomach 

 is touched. It is usually colourless, and always acid in 

 reaction. Human gastric juice has a specific gravity from 

 I'OOl to I-OIO, and contains less than 1 per cent, of solid 

 matter. On boiling, it is not coagulated, but its power is 

 totally destroyed. It may be kept for months unchanged. 



The power of gastric juice in dissolving proteids may be 

 traced to the pepsin it contains, which can only act in the 

 presence of an acid, preferably hydrochloric. 



These peptonising ferments convert coagulable albu- 

 minoids into soluble or diffusible albuminoids or peptones, 

 which are not precipitated on boiling. 



The Preparation of Pepsin. — The British Pharmacopoeia 

 directs that the mucous membranes of the stomachs be 

 washed, and then scraped off and dried. Pepsin may also 

 be obtained by digesting the membranes in glycerine, in 

 which the pepsin is soluble, and precipitating with alcohol. 



Peptonising ferments are often secreted by many bacteria, 

 a large number of which cause liquefaction of the gelatine 

 medium on which they grow. These ferments appear to 

 be more allied to trypsin than to papain. 



3. The Rennet Ferment. — The fourth stomach of the calf 

 has been long known for its milk-curdling properties, due 

 to a ferment termed chymosin. This ferment is invariably 

 present in the healthy human stomach, and is present in 

 many animals. The chymosin may be extracted from the 

 calf's stomach with water, but a better way is to digest with 

 weak acid for twenty-four hours and carefully neutralise. 

 Aqueous solutions of salicylic acid extract the ferment 

 well, and will keep. Alcohol precipitates the ferment in 

 an impure form. Prolonged contact with alcohol, especially 

 if strong, is said to destroy the ferment. Fixed caustic 



