40 SENESCENCE AND REJUVENESCENCE 



The visible substratum of the organism, i.e., the protoplasm, 

 must consist fundamentally of such physiologically stable sub- 

 stances, for if this were not the case we should have merely a system 

 of chemical reactions, and no permanency of form or structure 

 could exist. Theoretically, at least, a distinction must be made 

 between the substratum of the cell or organism and the substances 

 which are decomposed and eliminated and which constitute the 

 source of energy. Practically, however, such a distinction cannot 

 be clearly made in most cases, for physiological stability is relative 

 rather than absolute and it is impossible to say in a given case to 

 what extent the substratum is itself involved in the chemical re- 

 actions. Still it is evident that the substances which accumulate 

 within the cell under given conditions as its visible or structural 

 substratum must be in general and under the existing conditions 

 less subject to decomposition into eliminable form than those sub- 

 stances which undergo breakdown and elimination. 



The organic colloids are in general physiologically stable sub- 

 stances. When once formed within the cells they do not diffuse 

 readily and cannot ordinarily escape except as they are decomposed 

 into eliminable substances. We know from studies of the metabo- 

 lism of the higher animals and from the amount of nitrogen- 

 containing food which is necessary for maintenance that in these 

 forms at least the breakdown of proteid molecules into completely 

 eliminable form constitutes only a fraction of the metabolic process 

 at any given time. Moreover, some of the nitrogenous substances 

 excreted may come from proteids of the food which have been 

 decomposed without forming a part of the substratum of the cells. 

 Undoubtedly also many chemical changes occur in the colloid 

 substratum which involve merely the transformation or exchange 

 of certain chemical groups and not the complete disruption of the 

 molecule. Chemical changes of this sort do not necessarily involve 

 the disintegration of the substratum as a whole, and it is probable 

 that cellular structures are often the seat of such changes without 

 undergoing any conspicuous morphological change. 



The fact that emulsoid colloids and particularly proteids are 

 the fundamental constituents of the substratum of living organisms 

 is a necessary consequence, first, of the formation of these substances 



