INCOME OF MATTER 107 



in the blood is carried to the tissues, and there 

 it is metabolized for the upbuilding of proto- 

 plasm. As we now know there are many 

 tissue-enzymes, it may be that in the living cell 

 these enzymes, in a sense, re-digest this portion 

 of proteid, changing it again into amino-acid 

 bodies, and that these are used for upbuilding 

 protoplasm. But there may be an excess of 

 protein, and it may then be thrown aside as 

 creatin and other bodies found in muscle-juice. 

 This portion we may call tissue-protein. Prob- 

 ably it has characters of its own different from 

 those of the other portion of the protein which 

 we must now consider. This second portion of 

 protein, which we may term excess-protein, is 

 decomposed by cells in the liver, passes prob- 

 ably through many stages and is ultimately 

 voided by the kidneys in the form of urea. A 

 rich protein diet always causes an increase in 

 the amount of urea eliminated. There are, how- 

 ever, critical objections to this view. Is this 

 complicated process merely an arrangement 

 for getting rid of excess of protein ? One 

 can hardly imagine this to be the case. At 

 present we are not yet in a position to state 



