;io THE PHYSIOLOGY OF REPRODUCTION 



representing the prepubertal and pubertal growth. Then there is a 

 further decline in the power of growth, which gradually diminishes. 

 The prepubertal growth of girls usually precedes that of boys, so 

 that between the ages of twelve and fifteen girls are often heavier 

 and taller than boys. Boys grow most rapidly at sixteen, girls at 

 thirteen or fourteen. Boys attain their full height at from twenty- 

 three to twenty-five years of age; girls at twenty or twenty-one. 

 In both sexes the weight of the body tends to increase until about 

 the fiftieth year or somewhat later, owing to an accumulation of fat, 

 but there are of course very many exceptions. 1 



That good nourishment and a healthy environment favour growth 

 is a fact recognised by all. So also systematic exercise has been 



56 66 7) 90 - K>6 



FIG. 180. (From Minot's Problem of Age, Growth, and Death, 

 G. S. Putnam & Sons, and John Murray.) 



found to increase both the weight and the height, 2 and it has been 

 shown further that well-developed children are more efficient 

 mentally and take better places at school than ill-developed and 

 badly-nourished ones. 3 



In horses and other domestic animals the effects of feeding on 

 growth and general development are remarkable. Thus it is said 

 that a highly-fed thoroughbred at two years old is " furnished " and 

 looks as old as an ordinary horse at four years old. 4 



1 See Minot, Popular Science Monthly, vol. Ixxi., 1907. Minot states that his 

 calculations are based on data supplied by Professor Donaldson. See also Lee, 

 Article "Reproduction," in Howell's American Text-book of Physiology, 2nd 

 Edition, London, 1900. 



2 Beyer, " The Influence of Exercise upon Growth," Jour, of Exp. Mad., 

 vol. i., 1896. 



3 Porter, " The Physical Basis of Precocity and Dulness," Trans. A cad. of 

 Science, St. Louis, vol. vi., 1893. 



4 Smith, loc. cit. For growth in pigs, see Hammond, Jour, of Agric. Sci., 1922. 



