CHAPTER III 



THE PROBLEM AND METHODS OF INVESTIGATION 

 THE NATURE OF THE PROBLEM 



Both morphological and physiological changes are involved in 

 the processes of senescence and rejuvenescence, and we may attack 

 the problems from either the morphological or the physiological 

 side. On the morphological side we may determine the changes 

 in physical properties, form, and structure of the substratum which 

 occur during senescence and rejuvenescence, and on the physio- 

 logical side we may investigate the changes in functional activity 

 and in metabolism. 



Concerning the morphological changes associated with senes- 

 cence, particularly in the higher animals and man, we already 

 possess a considerable body of facts. As regards the physiological 

 changes, we know that in the higher animals and man the rate of 

 metabolism per unit of substance undergoes in general a decrease 

 with advancing age from very early stages onward, and that 

 sooner or later a decrease in functional activity and a general 

 deterioration of the organism occurs. Our knowledge concerning 

 the lower animals is less complete. We are familiar with the general 

 course of development and differentiation in most forms, but the 

 morphological differences between young and old adults have 

 received comparatively little attention. Of the physiological 

 aspect of senescence in the lower forms we have little positive 

 knowledge. We know that in most forms growth is more rapid 

 in earlier stages and that in many plants and animals the length 

 of life under the usual conditions is more or less definite, and in 

 some forms we can observe a decrease in functional activity with 

 advancing age. On the other hand, some organisms live and 

 remain active for an indefinite period and apparently do not grow 

 old. Few attempts have been made, however, to determine by 

 analytic investigation the significance of these various facts and to 

 find a common basis for them. 



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