274 



SENESCENCE AND REJUVENESCENCE 



during each period the weight or other growth component of the 

 body increases. The rate of growth is measured by the propor- 

 tional or percentage increments in given periods, consequently the 

 rate of growth may remain constant or may even decrease, while 

 the absolute increments of growth become successively larger. In 

 fact, the latter possibility is realized during a large 

 part of the growth period in the higher vertebrates. 

 Many students of growth have failed entirely to 

 recognize the fact that the absolute increment is 

 not a correct measure of the rate of growth, and 

 have therefore reached incorrect conclusions. 



The curves presented in Figs. 109 and no 

 show the percentage increments of weight in boys 

 and girls from the first to the twenty-third year. 1 

 The very great decrease in the annual percentage 

 increment is at once apparent. During the first 

 year after birth the percentage increment of weight 

 is 200 per cent in boys and 187 per cent in girls. 

 During the second year it is only 22 per cent in 

 boys and 28 per cent in girls. From this time on 

 it decreases slowly with slight irregularities and 



Percent 

 200 



180 

 160 

 140 



120 



100 



80 



60 



40 



20 



Years 



2 3456 7 8 9 10 n 12 13 14 15 16 17 18 19 20 21 22 23 



FIG. 109. Curve showing the decrease in the rate of growth in boys from the 

 first to the twenty- third year: each vertical interval indicated on the axis of ordinates 

 represents 20 per cent increment in weight, each horizontal interval on the axis of 

 abscissae one year. Fom Miihlmann's tables (Muhlmann, 'oo) calculated from 

 Quetelet's data. 



with a distinct but slight increase at the age of puberty, after which 

 it falls again. Various other data from different sources, including 

 statistics on the increment of body-length, monthly increments of 

 weight during the first year, decrease in weight during later life, 

 etc., all show that in man the rate of growth decreases, and that 



1 The curves are based on the percentage increments determined by Miihlmann 

 ('oo) from the statistics in Quetelet's U Anthropometrie (1835 and 1840). 



