REPRODUCTION 527 



appear to hear very imperfectly. The reproductive organs only 

 attain full development at puberty, and degenerate and lose all or 

 much of their functional importance as years accumulate. Cer- 

 tain 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. 



As old age comes on a general decline sets in, the rib cartilages 

 become calcified, and lime salts arc laid down in the arterial walls, 

 which thus lose their elasticity; the refracting media of the eye be- 

 come more or less opaque; the physiological irritability of the 

 sense-organs in general diminishes; and fatty degeneration, di- 

 minishing their working power, occurs in many tissues. In the 

 brain we find signs of less plasticity; the youth in whom few lines 

 of least resistance have been firmly established is ready to accept 

 novelties and form new associations; but the longer he lives, the 

 more difficult does this become to him. A man past middle life 

 may do good, or even his best work, but almost invariably in 

 some line of thought which he has already accepted; it is ex- 

 tremely .rare for an old man to take up a new study or change his 

 views, philosophical, scientific, or other. Hence, as we live, we all 

 tend to lag behind the rising generation. 



Death. After the prime of life the tissues dwindle (or at least 

 the most important ones) as they increased in childhood. 



Before any great diminution takes place, however, a breakdown 

 occurs somewhere, the enfeebled community of organs and ti 

 forming the man is unable to inert the rontingrnrirs of life, and 

 death supervenes. " It is as natural to die as to be born," Bacon 

 wrote long since; but though we all know it, few reali/.e the fact 



