CH. VIII.] PEPSIN. 203 



for solutions containing small amounts of pepsin and 

 relatively large amounts of salts and other substances. 

 These partially precipitate the edestin and render it 

 impossible to make satisfactory observations. 



For clinical purposes the author suggests that the 

 simplest method of estimating pepsin would be to determine 

 the time required for the clotting of " calcified " milk. It 

 is true that the method would not distinguish pepsin from 

 rennin, but it is improbable that rennin is secreted by the 



adult human stomach (see p. 207). 



/ 



For the following experiments use a 0-5 per cent, 

 solution of commercial pepsin (Armour's) in water. 



247. Place equal amounts of fresh washed fibrin in four test- 

 tubes labelled A, B, C, and D. 



To A add 5 cc. of pepsin and 5 cc. of 0-4 per cent. HC1. 



To B add 5 cc. of pepsin and 5 cc. of water. 



To C add 5 cc. of water and 5 cc. of 0-4 per cent. HC1. 



To D add 5 cc. of pepsin that has been boiled and then cooled, 

 and 5 cc. of 0-4 per cent. HC1. 



Place the four tubes in a water bath at 40 C. for at least thirty 

 minutes. 



Note that in 



A, the fibrin swells up, becomes transparent, and then dissolves ; 



B, the fibrin is unaltered; 



C, the fibrin swells up, becomes transparent, but does not dis- 

 solve ; 



D, the fibrin is like that in C. 



NOTE. These exercises show that neither 0-2 per cent. HC1 alone, nor 

 pepsin alone, can digest fibrin, but that pepsin in the presence of 0-2 per cent. 

 HC1 has this property. In D the ferment pepsin has been destroyed by boiling. 

 Fibrin is obtained by whipping blood at a slaughter house with a bundle of 

 twigs, feathers, etc. The blood must be whipped as soon as it is shed. The 

 mass of fibrin is placed on a sieve and thoroughly washed in running water to 

 remove the haemoglobin. It is then chopped up on a board into small 

 pieces. 



