The Nature and Criteria of Senescence 



genates, Pearce, 1936; mouse lymphoid tissue, Victor and 

 Potter, 1935) 2 uptake has been reported to decline with age. 

 Calorimetric experiments on the whole mammal indicate a 

 general decline in heat production with increasing age (Sonden 

 and Tigerstedt, 1895; Benedict and Root, 1934; Magnus-Levy 

 and Falk, 1899; Shock, 1942, 1948; Benedict, 1935; Boothby 

 et aL, 1936; Kise and Ochi, 1934). This decline, however, like 

 that of growth-energy, is greatest in early life, and relatively 

 slight in man after the. age of 50 (Shock, 1953). It does not 

 parallel the senescent increase in mortality. There is also gross 

 individual variation. Kunde and Norlund found (1927) no 

 significant decrease in the basal metabolism of dogs up to 12 

 years of age. In rats, Benedict and Sherman (1937) found a 

 slight decrease in heat production with increasing age, measured 

 in the same individuals, but with the onset of senescence the body 

 weight itself declined, so that the metabolism per unit body 

 weight appeared to increase. In man O a uptake per litre intra- 

 cellular fluid shows no decrease with age (Shock, Watkin and 

 Yiengst, 1954). A fuller bibliography is given by Shock (1951, 

 1953). It is not so far possible, in most organisms, to base 

 intelligible estimates of individual senescence upon changes in 

 metabolic rate. 



1-2 Forms of senescence 



Increase in death-rate and decrease in resistance after a 

 certain age might be expected in a number of model systems. 

 The curve of failure rate for mechanical devices such as lamp 

 bulbs, telephone switchboards (Kurtz and Winfrey, 1931), or 

 radar units bears a superficial resemblance to the mortality 

 curve of a senescent population, both in cases where all-or-none 

 failure results from wear or from the passage of time (lamp 

 filament failure, crystallization of metals, changes in condenser 

 dielectrics) or where wear is cumulative and inefficiency in- 

 creases to the point of failure (frictional wear, decline of cathode 

 emission). The resemblance to biological senescence is closest 

 in cases where several coincident processes ultimately become 

 self-reinforcing. The 'death-rate' of motor-cars, plotted by 

 Griffin (1928) and Pearl and Miner (1935) is closely similar to 

 that of wild-type Drosophila (Fig. 6). 



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