GROWTH AND SENESCENCE 



43 



Age 



Figure 23. The rise and decline of basal 

 metabolism with age. The area referred 

 to in the chart was obtained by Meeh's 

 formula. The numerical values are tentative 

 as neither the areas nor the values for 

 metabolism were definitely established. 



representation of the changing vitality with age. Indeed, it 

 seems to have certain features of a probability curve. But it is, 

 nevertheless, a useful curve giving us a bird's-eye view of the 

 age changes. 



The curves in Figure 22 

 similarly represent the chang- 

 ing resistance with age against 

 death due to breaking down of 

 the nervous system and of the 

 excretory system. Here again 

 the maximum resistance is at 

 thirteen years which is repre- 

 sented by 100 per cent. The 

 resistance is then seen to go 

 down, in the case of the upper 

 curve, to 73 per cent at twenty 

 years, 30 per cent at thirty 

 years, 20 per cent at forty 

 years, 10 per cent at fifty years, 

 5 per cent at sixty years, and 

 about 1 per cent at eighty 

 years. From the formula on 

 the chart, after the age of fif- 

 teen years, the resistance de- 

 clines at the rate of 7 per cent 

 per year. In the lower curve, 

 the vitality declines at the rate 

 of about 9 per cent per year. 



Figure 23 shows the rise 

 and decline of basal metabolism per unit area with age. The 

 basal metabolism per unit area appears to be at a maximum at 

 the end of the infantile period, namely, at two and one-half 

 years, then it declines at the rate of about 2 per cent per year. 



Figure 24 exhibits the rising and declining curves of vitality 

 with age in another animal form, namely, the dairy cow, and as 



Figure 24. The rise and decline of milk pro- 

 duction in Holstein-Friesian cattle. 



