OLD AGE MODIFICATIONS 263 



the other hand, the Virginia oyster lays forty millions of 

 eggs, but these are not cared for. Accordingly the number 

 of young stands in close relation to their chances of 

 coming to maturity, and (according to this theory) the 

 length of time required to produce a sufficient number of 

 offspring to replace the parents is closely correlated with 

 the average length of life of the mature individual. It is 

 difficult to see, however, how the correlation actually fixes 

 the limit of the life of any individual. 



Old Age Modifications. — Infancy, youth, middle 

 life, and old age are merely stages in the process of grow- 

 ing old. During infancy the chemical changes in the cells 

 go on at a rapid rate, cell divisions occur at frequent in- 

 tervals, the protoplasm increases to a great extent, and 

 the body grows to a marked degree. Beyond this point 

 the increase in weight in the human species is much 

 slower and, after gradually slowing down, stops at about 

 twenty years in the case of the female and twenty-four 

 in the male. During this time the proportions of the 

 body undergo changes which continue throughout life, 

 and are usually so definite that they accurately indicate 

 the age of the individual. 



Inner changes also are characteristic of the body at 

 various stages of its growth. In old age the bones con- 

 tain a relatively large amount of mineral salts and become 

 brittle. The cartilage also is impregnated with mineral 

 salts and in consequence becomes less elastic. The 

 muscles lose their vigor, and the hair its pigment. The 

 lens of the eye loses its elasticity and the ability to dis- 

 tinguish near objects clearly. The metabolism of the 

 body becomes more imperfect, and what appear to be 

 degenerative changes affect the nervous system. 



Immediate Causes or Death. — Popularly it is be- 

 lieved that the vital machine " wears out " or " runs 

 down," but beyond this point no details of the process are 

 forthcoming. The physician is inclined to state that a 

 man is as old as his arteries, meaning that if the charac- 



