526 THE HUMAN BODY 



various processes in the functioning of the reproductive mechan- 

 ism are many of them very striking and they have long been the 

 subject of investigation. The development of the so-called second- 

 ary sexual characters at puberty, where in a few weeks the vocal 

 cords change and hair develops over various parts of the body, is a 

 good example of the sort of interrelations that occur in this sys- 

 tem. The fact, known for centuries, that castration in early life 

 prevents the appearance of the secondary sexual characters, shows 

 that they are directly dependent on the reproductive organs. Be- 

 fore the idea of hormone action had crystallized to its present form 

 some such mechanism had been postulated for the reproductive 

 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. Attempts have been made to demonstrate the pro- 

 duction of hormones by the testes and ovaries by experimental in- 

 troduction of extracts of them into the blood ; but as yet such ex- 

 periments have failed to yield very positive results. There can be 

 little doubt, however, that the control of the secondary sexual 

 characters, the stimulation of the mammary glands to activity at 

 the end of pregnancy, and other reproductive functions are medi- 

 ated by hormones, produced by the generative organs, . or in cer- 

 tain instances by the developing fetus. 



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. During life all the formed elements of the Body 

 are constantly being broken down and removed; either molecularly 

 (i. e., bit by bit while the general size and form of the cell or fiber 

 remains unaltered), or in mass, as when hairs and the cuticle are 

 shed. The life of many organs, also, does not extend from birth to 

 death, at least in a functionally active state. At the former period 

 numerous bones are represented mainly by cartilage. The pan- 

 creas has not attained its full development; and some of the sense- 

 organs seem to be in the same case; at least new-born infants 



