ACTION ON TROTEIDS. 331 



Von Wittich's Glycerine Method. (a) Glycerine extracts pepsin in a very 

 pure form. The mucous membrane is rubbed up with glass until it forms a pulp, 

 mixed with glycerine, and allowed to stand for eight days. The fluid is pressed 

 through cloth, and the filtrate mixed with alcohol, thus precipitating the pepsin, 

 which is washed with alcohol and afterwards dissolved in the dilute HC1, to form 

 an artificial digestive fluid. [The addition of a few drops of the glycerine extract 

 to dilute HC1, is sufficient for experiments on artificial digestion.] 



(6.) Or the mucous membrane may be placed for 24 hours in alcohol, and after- 

 wards dried and extracted for 8 days with glycerine. 



(f.) Wm. Roberts has used other agents for extracting enzyms (p. 295). 



Preparation of pure pepsin. Briicke pours on the pounded mucous membrane 

 of the pig's stomach a 5 per cent, solution of phosphoric acid, and afterwards 

 adds lime water until the acid reaction is scarcely distinguishable. A copious 

 precipitate, which carries the pepsin with it, is produced. This precipitate is 

 collected on cloth, repeatedly washed with water, and afterwards dissolved in very 

 dilute HCL A copious precipitation is caused in this fluid, by gradually adding to 

 it a mixture of cholesterin in four parts of alcohol and one of ether. The 

 cholesterin-pulp is collected on a filter, washed with water containing acetic acid, 

 and afterwards with pure water. The cholesterin-pulp is placed in ether to dis- 

 solve the cholesterin, and the ether is then removed. The small watery deposit 

 contains the pepsin in solution. 



Properties. Pepsin so prepared is a colloid substance ; it does not 

 react like albumin with the following tests, viz. : it does not give the 

 xanthoprotein reaction (p. 333), is not precipitated by acetic acid and 

 potassium ferrocyanide, nor by tannic acid, mercuric chloride, silver 

 nitrate, or iodine. In other respects it belongs to the group of 

 albumins. It is rendered inactive in an acid fluid by heating it to 

 55-60C. (Ad. Mayer). 



166. Process of Gastric Digestion. 



Chyme. The finely divided mixture of food and gastric juice is 

 railed chyme. The gastric juice acts upon certain constituents of this 

 chyme. 



L Action on Proteids. 



Pepsin and the dilute hydrochloric acid, at the temperature of the body, 

 transform proteids into a snlulle form, to which Lehmann (1850) gave 

 the name of " Peptone." During this change, they are first transformed 

 into a substance which has the characters of syntonin (Mulder). 

 Syntonin is an acid-albumin or albuminate; when neutralised by an 

 alkali [e.g., sodium carbonate], the albuminate is again precipitated. 

 An intermediate product is formed, a body which, as it were, stands 

 midway between albumin and peptone. This is called propeptone 

 (Schmidt-Miilheim), and is identical with Kiihne's hemialbuminose and 

 Meissner's parapeptone. It is not coagulated by heat, but is precipitated 



