GROWTH AND SENESCENCE. 1003 



country especially by Bowditch, Porter, and Beyer.* An inter- 

 esting feature of the records collected by Bowditch is the proof 

 that the prepubertal acceleration of growth comes earlier in girls 

 than in boys, so that between the ages of twelve and fifteen 

 the average girl is heavier and taller than the boy. Later, the boy's 

 growth is accelerated and his stature and weight increase beyond 

 that of the girl. Robertson f has accumulated some facts which 

 indicate that the growth of the individual does not proceed with 

 a uniformly accelerated or a uniformly retarded velocity, but 

 rather in cycles, each of which, expressed graphically, takes the 

 form of an S-shaped curve. He believes that there are three, 

 possibly four, of these curves. The first begins with the implanta- 

 tion of the ovum and terminates about a year after birth. From 

 observations made on the weights of infants born before and after 

 the normal period he concludes that the increments of weight in 

 each case follow the same curve. An infant born after the normal 

 period has increased in weight while in the uterus as it would 

 have grown in the same period of extra-uterine life. The second 

 cycle starts toward the close of the first year, reaches its maximum 

 at five and one-half years, and then falls off. The third cycle 

 begins at the end of the second, reaches its maximum at puberty, 

 and terminates at adult life. The signs of old age may be de- 

 tected in other ways than by observations upon the rate of growth. 

 Changes take place in the composition of the tissues ; these changes, 

 at first scarcely noticeable, become gradually more obvious as old 

 age advances. The bones become more brittle from an increase in 

 their inorganic salts, the cartilages become more rigid and calca- 

 reous, the crystalline lens gradually loses its elasticity, the muscles 

 lose their vigor, the hairs their pigment, the nuclei of the nerve 

 cells become smaller, and so on. In every way there is increasing 

 evidence, as the years grow, that the metabolism of the living mat- 

 ter of the body becomes less and less perfect ; the power of the 

 protoplasm itself becomes more and more limited, and we may 

 suppose would eventually fail, bringing about what might be called 

 a natural death. As a matter of fact, death of the organism usually 

 results from the failure of some one of its-many complex mechanisms, 

 while the majority of the tissues are still able to maintain their exis- 

 tence if supplied with proper conditions of nourishment. The phys- 

 iological evidences of an increasing senescence warrant the view, 

 however, that death is a necessary result of the properties of living 

 matter in all the tissues except possibly the reproductive elements. 



1877, 



1893-94; 



1895. 



f Robertson, "American Journal of Physiology," 37, 1, 1915. 



