ON THE THEORY OF METABOLISM 135 



The food proteid is distinguished in the first place by the ease with which 

 it is destroyed by the organs of the body. No other organic foodstuff can 

 compare with it in this respect. It is true that fat and carbohydrates, if 

 supplied in sufficient quantity, can reduce the destruction of proteid to a 

 certain extent, but in a general way this destruction bears about the same 

 relation to the proteid ingestion whether the N-free foodstuffs be eaten or not. 



The digested proteid passes into the blood as food proteid. If the quan- 

 tity absorbed is not too small, and if at the same time plenty of N-free food- 

 stuffs are present, a part of it may remain in the body undestroyed, but, as 

 a rule, the greater part of it is destroyed within twenty-four hours. 



In sharp contrast with this is the fact that the mass of living substance 

 has but a very slight influence on the amount of proteid destroyed. We have 

 seen (page 117) that there is no direct proportion between the weight of 

 the body and the amount of proteid metabolism, and (page 124) that beyond 

 a certain lower limit the body can maintain its proteid status on widely dif- 

 ferent quantities of proteid, provided only that the nonnitrogenous foodstuffs 

 be present in sufficient quantity. 



It follows, therefore, that the living substance is not destroyed either in 

 the metabolism of proteid or in that of the nonnitrogenous foodstuffs, but on 

 the whole is relatively stable. 



The doctrine of Voit, given preference here, that the food proteid is de- 

 stroyed first and the living substance only when the food proteid is not sufficient 

 for the needs of the body, is still disputed by many authors of high rank, who 

 advocate the view, first put forward by Liebig, but modified later by Pfliiger and 

 others. According to this view the living molecules of which the cells are com- 

 posed are continually being destroyed and built up again in the life process; 

 the cells as such do not break down, but their molecules are incessantly chang- 

 ing; the living molecule disintegrates much more easily than that of dead pro- 

 teid, and the latter is destroyed only after it has been transformed into living 

 molecules ; of itself it is much more stable than the living substance. 



It is indeed a matter of everyday experience that when an organ is taken 

 out of the living body, it dies within a relatively short time, and on the other 

 hand, that dead proteid in a dry condition can be preserved for any length of 

 time unchanged. But we are not to conclude from such observations that in 

 the living body, dead proteid is less destructible than living protoplasm. For 

 an extirpated organ, by the very act of its removal from the body, is placed in 

 altogether abnormal circumstances. In the living body the medium in which 

 the cells and the tissues carry on their activities is the lymph. Wherever this 

 fluid is wanting, or has not the proper temperature and the normal constitution, 

 is not renewed often enough or is not provided with sufficient oxygen, protoplasm 

 there represents a very destructible substance. But we are not justified by this 

 alone in maintaining that the living substance behaves in the same way, when 

 the lymph is perfectly normal. What we know is, that when the lymph is nor- 

 mal, the living substance carries on its functions ; and there is no ground for the 

 assumption that it is then less stable than the dead substances found in the fluid 

 by which it is bathed. 



If the living substance were always breaking down when it is active, what 

 a tremendous work of synthesis would be required in order to keep it restored 

 from the dead food proteid ! And if this were true, how should we explain the 

 extraordinary difficulty with which the adult body lays on proteid? If the food 



