PHYSICAL AND CHEMICAL CHARACTERS. 327 



ment, in which we cannot see the intervention of 

 clearly accidental and abnormal disturbing agents. 

 Death appears to be the termination of a breaking-up 

 proceeding by insensible degrees in consequence of the 

 progressive accumulation of very small inappreciable 

 perturbations. This slow breaking up is adequately 

 expressed by the term growing old, or senescence. 

 The alterations by which it is betrayed in the cell are 

 especially atropJiic, but they are also accompanied, 

 however, by different forms of degeneration. An 

 extremely important question arises on this subject, 

 and that is whether the phenomena of senility have 

 their cause in the cell itself, if they are inevitably 

 found in its organization, and therefore if old age and 

 death are natural and necessary phenomena. Or, on 

 the other hand, should we consider them as due to a 

 progressive alteration of the medium, the character of 

 which would be accidental although frequent or 

 habitual ? This, in a word, is the problem which has 

 so often engaged the attention of philosophical 

 biologists. Are old age and death natural and 

 inevitable phenomena ? 



The recent experiments of Loeb and Calkins, and 

 all similar observations, tend to attribute to the 

 phenomenon of growing old the character of a 

 remediable accident. But the remedy has not been 

 found, and the animal finally succumbs to these slow 

 transformations of its anatomical elements. We then 

 say that it dies of old age. 



Metchnikojfs Theory of Senescence. Objections. 

 Metchnikoff has proposed a theory of the mechanism 

 of this general senescence. The elements of the 

 conjunctive tissue, phagocytes, macrophages, which 

 exist every \vhere around the specialized and higher 



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