UTILISATION OF PROTEIDS IN THE ANIMAL 163 



does not permit that the question as to the nature of the 

 linkages or the relation of any one amino-acid to others 

 should be dealt with here. Current researches, and especially 

 the investigations being carried out in the laboratory of Emil 

 Fisher, are yielding us important information on points such 

 as these, but a discussion of them may be here dispensed with. 



It is easy to recognise that the heterogeneity displayed by a 

 large molecule, such as that we must attribute to proteids, offers 

 abundant opportunity for variation, and a recognition of such 

 complexity raises at once the question as to whether all pro- 

 teids are, after all, so nearly identical with each other as Liebig 

 believed. Recent research indicates clearly that they are not. 



While it is even possible that proteids of diverse origin 

 may display differences in the actual permutations and com- 

 binations — in the sorting, so to speak — of their constituent 

 amino-acids, we have, at present, no evidence to prove this ; 

 but that the constitution of one proteid may differ quantitatively 

 from that of others is now certain. We may sufficiently 

 illustrate this significant fact, by indicating the differences 

 which have been found between certain prominent dietetic 

 proteids and the proteids of the blood. Thus, comparing the 

 chief proteid of milk, caseinogen, with the last mentioned, we 

 find that it yields, when similarly treated, little more than 

 half the amount of leucine ; caseinogen gives about 10 per 

 cent., the blood proteids about 20 per cent. The propor- 

 tionate amount of cystine in the milk proteid is also decidedly 

 less, while i'ts content of glutaminic acid is notably greater. 

 The proteids of wheat are astonishingly rich in glutaminic 

 acid, one of them (gliadine) yielding nearly five times as much 

 as that got from blood proteids [Osborne and Harris]. The 

 mixture of proteids which we consume in white bread would 

 probably yield at least four times as much glutaminic acid as 

 would an equal weight of the proteids in our blood. Blood 

 is comparatively rich in tryptophane, whereas a proteid which 

 forms a large proportion of the widely used foodstuff, maize, 

 contains none at all. 



Such differences might be further illustrated, but the facts 

 given will suffice. They show fundamental variations in the 

 constitution of proteids which the elementary analyses relied 

 upon by Liebig did not bring to light. Having realised such 

 differences, it is no longer possible to conceive, with Liebig, 



