70 BIOLOGY OF DEATH 



was found to be relatively unstable.. They further found 

 that the long-lived animals represent a relatively stable 

 group, highly resistant to external disturbing factors, 

 and showing a more or less marked but not invariable 

 tendency to early overgrowth and relative paucity of 

 tissue accretion in late life. The short-lived animals are 

 on the contrary relatively unstable, sensitive to external 

 disturbing factors, and, as a rule, but not invariably, dis- 

 play relatively deficient early growth and a tendency to 

 rapid accretion of tissue in later life. 



In interpreting these results, Robertson and Ray be- 

 lieve that the differences are based upon the fact that 

 in early or embryonic life the outstanding characteristic 

 of the tissues is a high proportion of cellular elements, 

 whereas in old age there is a marked increase in connective 

 tissues. They further point out that connective tissue 

 elements are ultimately dependent upon cellular tissues 

 for their support, and that the connective tissues are 

 expensive to maintain. They believe that the reason that 

 the substance tethelin (cf. Chap. VII infra) prolongs life 

 is because it accelerates the metabolism of the cellular 

 elements to the detriment of the connective tissue ele- 

 ments. Longevity on this view is determined not by the 

 absolute mass of living substance, but by the relative 

 proportions of parenchymatous to sclerous tissues. 



SENESCENCE 



The facts presented in this and the preceding chapter 

 clearly make it necessary to review with some care the 

 current conception of senescence. Senescence, or grow- 

 ing old, is commonly considered to be the necessary prel- 

 ude to "natural," as distinguished from accidental death. 



