THE RECOVERY FROM DEPRESSION 489 



Having thus attempted to make clear that the nerve cell's 

 differentiation is not an active structural factor in senescence, 

 I can suggest how that fact does not antagonize the generaliza- 

 tion of Conklin and Child that accumulation of the products of 

 metabolism and of differentiation is the basis of senescence. The 

 reason from the point of view of differentiation why the nerve 

 cell remains unimpaired through long years of life, rejuvenescent 

 rather than senescent, is that the specific products of its differen- 

 tiation are entirely consumed in the functional reaction and the 

 by-products eliminated. The high metabolic rate which Child 

 ('15, p. 281) offers to explain its stability is the objective cyto- 

 logical fact. It is continuously forced to the renewal of the 

 superficial substances, such as the chromatin and nucleolar sub- 

 stance, which are the final products of its differentiation, "^here 

 is no absolute rest for the nerve cell, even without actual func- 

 tion, for it is necessarily in some degree of metabolic 'tone' as 

 long as it exists. Hence the condition of rapid interchange 

 between plasma and nucleus (Conklin), of rapid metabolism 

 (Child) , the condition of youth, is sufficiently fulfilled, unless the 

 inner exhaustion goes too deeply or unless external conditions 

 depress it. 



There is no rejuvenescence in actual fact, for there is no dedif- 

 ferentiation, and a recovery from a profound depression or an 

 immediate exhaustion involves in either case the same substance 

 as preexisted. Since the factor of metabolism which conditions 

 rejuvenescence is so nearly a constant, provided the function 

 which conditions it in the nerve cell is properly balanced, the 

 stability of its protoplasm holds, the status quo remains essentially 

 youthful, and it is not placed in need of the actuality of rejuve- 

 nescence, which is for it impossible. The return to the normal 

 after fatiguing or exhausting nervous activity or after depression 

 is simply a recovery and not a rejuvenescence. Of course, the 

 elimination of the waste products of metabolism is possibly to 

 be conceded as a daily rejuvenescence in the most superficial 

 sense, but it is not only probably an infinitesimal factor under 

 normal conditions, in respect to that, but also the more important 

 phase here is the recovery, regrowth of materials which these 



