66 THE EVOLUTION OF MAN 



rates with the result that they bear a different ratio to the total 

 body weight at different ages. Here is a field that needs much 

 further study inasmuch as this changing relationship in size 

 may have an important bearing on the incidence of disease at 

 different ages. The studies which have been made on the 



2 mo. (fetal) 



5 mo. 



Newborn 



2yrs. 



6yrs. 



12 yrs. 



25 yrs. 



Fig. 24. Changes in proportion during prenatal and postnatal growth. 

 After Stratz. From Morris's Human Anatomy. By permission of P. Blakis- 

 ton's Son & Co. 



relative growth of the different organs show that the same 

 proportionate weight of the skeleton, fat, and skin, which is 

 about 39 per cent of the total weight of the new-born, persists 

 in the adult; that the relative weight of the organs of circula- 

 tion, respiration, and alimentation is about twice as large in the 

 new-born as in the adult; that the weight of the muscles of the 

 new-born is relatively one half as large as in the adult, and, 

 that the central nervous system of the new-born is relatively 

 eight times as large as in the adult. It is possible that the very 

 rapid growth of the brain in early childhood accounts in part, 

 at least, for the nervous instability of that period. 



Considerable data have accumulated with regard to the 

 growth of bone. Membrane bones, such as the bones of the 



