584 THE HUMAN BODY 



system. For it is difficult to explain such effects as those of castra- 

 tion on any other basis than that the generative organs elaborate 

 some control-exercising substance of which the body is deprived 

 by castration. Perhaps the best known examples of hormones 

 concerned with reproduction are those that have to do with lacta- 

 tion. It has been proven that the development of the mammary 

 glands during pregnancy is caused by a hormone produced in the 

 body of the embryo. This hormone is attended apparently by 

 another one, which, while permitting the development of the 

 glands inhibits their active functioning. At the birth of the child 

 this second hormone is withdrawn, and the glands are thus left 

 free to pour forth their secretion. 



The Stages of Life. Starting from the ovum each human being, 

 apart from accident or disease, runs through a life-cycle which 

 terminates on the average after a course of from 75 to 80 years. 

 The earliest years are marked not only by rapid growth but by 

 differentiating growth or development; then comes a more station- 

 ary period, and finally one of degeneration. The life of various 

 tissues and of many organs is not, however, coextensive with that 

 of the individual. At birth numerous bones are represented mainly 

 by cartilage. The pancreas has not attained its full development; 

 and some of the sense-organs seem to be in the same case; at least 

 new-born infants appear to hear very imperfectly. The reproduc- 

 tive organs only attain full development at puberty, and de- 

 generate and lose all or much of their functional importance as 

 years accumulate. Certain organs have even a still shorter range 

 of physiological life; the thymus, for example, attains its fullest 

 development at the end of the second year and then gradually 

 dwindles away, so that in the adult scarcely a trace of it is to be 

 found. The milk-teeth are shed in childhood, and their so-called 

 permanent successors rarely last to ripe old age. 



During early life the Body increases in mass, at first very rapidly, 

 and then more slowly, till the full size is attained, except that girls 

 make a sudden advance in this respect at puberty. Henceforth 

 the woman's weight (excluding exceptional cases of accumulation 

 of non-working adipose tissue) remains about the same until the 

 climacteric. After that there is often an increase of weight for 

 several years due mainly to increased formation of fat; a man's 

 weight usually slowly increases until forty. 



