REJUVENESCENCE AND DEATH 297 



believe that the effect of a change in mental occupation or of a 

 vacation after long-continued mental labor in a particular field is 

 in some slight degree a rejuvenescence of the nerve cells ? Many 

 facts indicate that a reasonable variety in mental occupation is a 

 factor in retarding mental senility. What we often call mental 

 fatigue may be something much less evanescent than fatigue in the 

 ordinary sense, but recovery may occur in time. Verworn ('09, 

 p. 557) has drawn a distinction between fatigue, resulting from 

 accumulation of substances which retard metabolism, and exhaus- 

 tion, resulting from lack of oxygen or other substances necessary 

 for metabolism. Recently Dolley ('14) has maintained that both 

 of these changes may bring about senility in the nerve cell. Ex- 

 haustion, I believe, resembles senility as death from asphyxiation 

 resembles death from old age. In both exhaustion and senility the 

 rate of oxidation may be decreased, but the factors involved and 

 the condition of the organism in the two cases are very different. 

 Recovery from exhaustion is then not the same sort of change as 

 rejuvenescence except as it involves increase in rate of oxidation. 

 Fatigue and recovery constitute a cycle resembling much more 

 closely the age cycle. As I have pointed out in chap, viii, it is im- 

 possible to draw the line sharply between age changes and various 

 other periodic or cyclical changes in the organism, and, although 

 the nervous system is without doubt a highly stable tissue, the very 

 definite physiologically regressive changes which occur in recovery 

 from mental fatigue or from long-continued mental activity of a 

 particular kind suggest that changes closely approaching rejuvenes- 

 cence occur. Even here development is not always and only pro- 

 gressive, as Minot and many others would have us believe, but is 

 made up of progressive and regressive changes with the balance 

 greatly in favor of the former. 



The occurrence of rejuvenescence in connection with starvation 

 in planarians raises the question whether any changes in this direc- 

 tion are associated with starvation in the higher animals. The 

 metabolism of starvation in man and the higher vertebrates has 

 been extensively studied by many investigators, 1 and there is 



1 See the bibliographies in the the article by Weber, "tiber Hungerstoffwechsel," 

 Ergebnisse d. Pkysiol., I, 1902, in the paper by Pembrey and Spriggs, '04, and in Bene- 

 dict's studies of starvation metabolism in man (Benedict, '07, '15). 



