UTILISATION OF PROTEIDS IN THE ANIMAL 161 



and go through a vast amount of muscular exercise. He is 

 compelled to consume force merely in order to supply matter 

 for respiration." 



This remarkable conception of Liebig's, that proteid is only 

 metabolised during the mechanical activity of organs was in 

 reality disproved during his lifetime, for quantitative studies of 

 the course and rate of excretion showed that the elimination of 

 nitrogen from the body during any given period is closely and 

 immediately related to the contemporary ingestion of proteid, 

 and much less closely, if at all, to the muscular work being done 

 at the time. But to quote Liebig is not often to quote effete 

 opinion, and his doctrines regarding the utilisation of proteid 

 influenced physiological thought long after experiment had 

 proved much of them to be faulty. The direct utilisation of 

 proteid, indeed, was in its essence hardly questioned by 

 physiologists till lately. But Liebig's teaching has probably 

 influenced medical and semi-popular opinion still more ; and has 

 been at the bottom of the idea that the value of a dietary to 

 an active animal necessarily increases with the amount of 

 proteid contained in it. It is true that this last opinion in a 

 modified form is still held by no less an authority than Pfluger, 

 and it must be referred to again later. 



In any case, Liebig's teaching as to the simplicity of the 

 phenomena involved in proteid assimilation is no longer to be 

 accepted ; his conception of the proteid we eat entering the 

 tissues as intact proteid is being replaced by a belief that the 

 process of assimilation is much more subtle and elaborate, and 

 his view that the whole of the proteid absorbed from the bowel 

 is, stricto sensu, assimilated, appears to be giving way to an under- 

 standing that the organism deals with the material in a highly 

 discriminative and selective manner, true assimilation of proteid 

 being confined to a small portion of what is actually consumed. 



The first task to be undertaken here is to show how our 

 views have become thus modified. 



While Liebig, Mulder, and the earlier workers did not per- 

 haps fully realise it, a recognition of the complexity of the proteid 

 molecule has long existed ; but it remained for quite recent 

 analytical studies to demonstrate fully how great is this com- 

 plexity. Recent work has not perhaps led to any exaggeration 

 of our ideas as to the magnitude of the molecule, but has shown 

 how heterogeneous are the details of structure within it. When 



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