REJUVENESCENCE AND DEATH 301 



way or another the continued existence of the whole mechanism. 

 But the facts indicate that even in such organisms some degree of 

 regression and rejuvenescence may occur. 



LENGTH OF LIFE AND DEATH FROM OLD AGE 



When the rate of metabolism becomes so low in consequence of 

 advancing senescence that the cell or organism can no longer 

 synthesize its metabolic substratum in sufficient amount to com- 

 pensate the losses, atrophy begins and must sooner or later end in 

 the destruction of the physiological mechanism, which is death. 



In a complex organism like man, different cells and tissues grow 

 old at different rates, and death from old age of the organism as a 

 whole does not by any means imply the death of all its cells. Death 

 of cells apparently from old age occurs from early stages of develop- 

 ment throughout the whole life history, and we also know that 

 most of the cells of the body do not die when death of the individual 

 occurs. The individual dies when some tissue or organ which is 

 essential for its continued existence reaches the point of death, 

 and since the parts are incapable of dedifferentiation and a new 

 individuation, the other organs or cells die sooner or later because 

 of lack of nutrition or oxygen, or because of the accumulation of 

 toxic products of metabohsm. 



So-called physiological death in the higher animals is then due 

 to the breakdown of the physiological mechanism of the indi- 

 vidual at some essential point, and not to the simultaneous death 

 of all parts. As regards this fact different authorities are agreed, 

 but wide differences of opinion exist as regards the organ or organs 

 responsible for breakdown of the mechanism. Miihlmann ('00, 

 '10, '14) and Ribbert ('08) maintain that physiological death is 

 essentially a death of the brain; Lorand ('11), that the glands of 

 internal secretion are primarily responsible; and Demange ('86) 

 and Metchnikoff ('03, '10) regard arteriosclerosis as the most 

 important factor in death. 



Without attempting any extended discussion of these and other 

 views, it may be pointed out that the growth of the central nervous 

 system begins and is completed earlier, and that its development is 

 apparently more continuously progressive, with less rejuvenescence, 



