STAGES OF LIFE. 



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 de- 

 velopment; then comes a more stationary 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 fibre 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 

 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. Tha 

 reproductive organs only attain full development at pu- 

 berty, and degenerate and lose all or much of their func- 

 tional importance as years accumulate. Certain organs 

 have even a still shorter range of physiological life; the 

 thymus, for example (p. 333), attains its fullest develop- 

 ment 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 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; 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 are laid down. 



