34 GROWTH 



velopment of pelvis and mammary glands, and growth of 

 hair on the pubes. In boys it is marked by the appearance of 

 pigmented hair in the pubic regions and on the face, and the 

 "breaking" and then deepening of the voice. Simultaneous 

 with these changes there occurs, under certain conditions of 

 nutrition, an acceleration in growth in weight and in height, 

 changing from a gain of 9 per cent per year to a gain of, per- 

 haps, 12 per cent per year, depending on nutritive conditions. 



While there is no direct evidence to this fact, it seems rea- 

 sonably certain that the acceleration in growth at this age is as- 

 sociated with the development of the sex function. This idea is 

 inferred from the fact that emasculated children do not show 

 some of the changes associated with puberty, and from the re- 

 sults obtained by Edgar Allen, the anatomist at the University 

 of Missouri, and by other investigators, who found that sub- 

 stances from several genital tissues injected into spayed animals 

 produced an accelerated growth of certain tissues. However, 

 this accelerated growth initiated at puberty does not last very 

 long. About two years after the onset of puberty, the rate of 

 growth rapidly decreases, and by the age of about twenty-two 

 years growth in weight and height is practically completed. 



The age of twenty-three, at any rate, marks the end of our 

 physical growth. There is no doubt that from the standpoint of 

 physical strength we are at our best between twenty and thirty 

 years of age. The proof of this statement is furnished by Fig- 

 ures 1 8 and 19 and by the fact that it is between these ages that 

 championships are generally held in such of the competitive 

 sports which require great sudden physical exertion as, for ex- 

 ample, prize fighting. 



From thirty years on, the decline in the physical powers is 

 unmistakable. The age of fifty usually marks the termination of 

 the phenomenon of menstruation and consequently the end of 

 the reproductive period in woman. In man the reproductive 

 period is not terminated as abruptly as in woman, but the de- 

 cline is unmistakable from about thirty-five years on. The de- 



