512 PROCESSES INFERRED FROM INDIRECT OBSERVATION 



OLD AGE AND SENESCENCE. 



The leading characteristic of old age is the low average Metabolic 

 Rate of the tissues. From maturity to old age the calorific output 

 steadily diminishes, the total reduction, according to Du Bois, being 

 about thirteen per cent, by eighty years of age in men. This dimin- 

 ished metabolism, if it is not accompanied by a corresponding diminu- 

 tion of intake, may lead to the formation of extensive deposits of fat 

 and the Obesity which occurs in a certain percentage of elderly indi- 

 viduals. In general, however, the decreased metabolic rate is accom- 

 panied by a progressive loss of body-weight. In man the senescent 

 loss of body-weight begins relatively early, but proceeds very slowly, 

 so that it only becomes notable at an age in excess of the mean duration 



9 



1C 

 in 



5 10 15 20 25 30 40 50 60 70 80 00 100 110 120 130 140 150 



WEEKS 



FIG. 45. Growth curve of normal female white mice from four weeks until death 

 of the last surviving animal. The vertical cross-mark indicates average duration of 

 life. 



of life. In the mouse, on the contrary, the Senescent Loss of Weight 

 is relatively sudden and rapid and is quite marked before the mean 

 duration of life is attained. This is illustrated by the accompanying 

 curve (Fig. 45), which displays the growth and senescence of female 

 white mice from four weeks until the termination of the observations by 

 the death of the last surviving animals. Deaths from epidemic infec- 

 tion were excluded by the technique of the experiments. The terminal 

 fluctuations of the curve are due to the irregularly occurring deaths of 

 animals in which the process of senescence has been most rapid and 

 which have lost most weight. The survivors therefore represent an 

 earlier or less complete stage of senescence than those which have died, 

 and each group of late deaths is consequently accompanied by a rise 

 in the weight-curve of the survivors. Each rise, however, is succeeded 

 by a fall, which is even more rapid than the preceding one, indicating 



