42 



GROWTH 



Pep cent of 



► maximum 



100 



w 60 



£ 40 

 %c 



"S 20 



■H o 



S--20 



ance against pneumonia is greatest at this age, that is, there are 

 fewer deaths due to pneumonia at the age of thirteen than dur- 

 ing any other age. After the age of thirteen, the resistance to 

 pneumonia declines. A man of forty-five is seen to have ap- 

 proximately the same resistance as a child of three years, and 

 has only about 1 1 per cent of the resistance possessed by a child 



of thirteen years. At seventy, we 

 can see the resistance against pneu- 

 monia to be only 2 per cent of the 

 resistance of a child of thirteen. 



The formula on the chart gives a 

 quantitative statement of the course 

 with which the resistance to pneu- 

 monia changes with age. The expo- 

 nent in the first term of the formula 

 multiplied by ioo, that is 5.0, is the 

 percentage decline per year in re- 

 sistance to pneumonia beginning 

 with the age of fifteen years. 



The curve gives us a fair idea of 

 growth and senescence as measured 

 by the rise and decline of resist- 

 ance to pneumonia with age. The 

 formula representing the rise and 

 decline of this curve would be con- 

 sidered as a law comparable, for ex- 

 ample, to the law of gravitation if the curve were obtained on a 

 homogeneous population and if the formula were shown to 

 have some universal significance. But, of course, this curve was 

 not obtained on a homogeneous population. The individuals in- 

 cluded in the census were of many different types, some possess- 

 ing a vigorous constitution and high resistance to pneumonia, 

 others weak and of slight resistance to pneumonia. As a result 

 there was a selective action, a weeding out of the less fit indi- 

 viduals, and the curve, for this reason, is not a strictly accurate 



gwo 



5 20 

 

 -20I 



YPS. 10 20 30 40 50 60 ?0 80 



Figure 22. The rise and decline of 

 vitality with age as measured by the 

 reciprocals of mortality due to the 

 breakdown of the nervous and the 

 excretory systems. 



