2O2 COMPOSITION OF THE DIGESTIVE JUICES. [CH. VIII. 



obtained by extracting the washed and minced mucosa 

 with 3 parts by weight of 5 per cent, alcohol for 4 hours, 

 filtering and concentrating under reduced pressure. 



The products of action of pepsin differ with the nature 

 of the protein undergoing digestion. The distinguishing 

 feature of peptic digestion is that the final products consist 

 mainly of simple proteins called peptones and polypep tides. 

 It is true that in artificial digestions over long periods, 

 traces of amino-acids are produced. But this action is of 

 little physiological importance compared to the correspond- 

 ing action of trypsin and erepsin. An account of some of 

 the products formed during the peptic digestion of fibrin is 

 given on p. 53. 



It is possible that pepsin does not break the ordinary 

 pep tide linkage (see Note 5, Ex. 24). It is significant that 

 it has no action on any artificial polypeptide, many of 

 which are split into their constituent amino-acids by 

 trypsin or erepsin. The proteoses and peptones formed 

 by peptic digestion may be united in the intact protein 

 molecule by some special form of linkage which is readily 

 attacked by pepsin. This has not yet been satisfactorily 

 demonstrated, but from various considerations it would 

 seem to be highly probable. 



Pepsin can be estimated by a variety of methods. A 

 very well-known method is that of Mett. Egg-white is 

 drawn up into fine glass tubes and coagulated by heat. 

 Lengths are cut off, immersed in the solution for a stated 

 time, and the amount of egg-white digested determined 

 by measurement. Experience has shown that the length 

 of egg-white digested varies as the square root of the 

 amount of pepsin present, this relationship being known 

 as the " Schutz-Borissow law." The preparation of 

 satisfactory tubes is not a simple matter. J. Christiansen 

 (Biochem. Zeitschrift, XLVL, p. 257) has described a 

 method of standardising the egg tubes, but, perhaps in 

 spite of this, the method seems to be falling into disuse. 



The edestin method of Fuld (Ex. 250) is reliable, except 



