Donaldson, Gmvth of Central Nervous System. 369 



The observation as it stands, represents a special instance of the 

 phenomenon already observed by Bunge ('02) and Rubner 

 ('08 and '08A), that during the phase of rapid growth, immediately 

 following birth, the smaller mammals double their body weight 

 in a much shorter period of time than does man. The present 

 observation has moreover the interest of applying to an organ in 

 which it is probable that cell division has nearly ceased, so that 

 the increase in weight during this period, is due almost entirely 

 to the mere enlargement of the elements which are for the most 

 part neurones. 



It might be urged that to complete the demonstration, it should 

 be shown that during this interval, the same percentage of the limit- 

 ing brain weight had been attained by both forms. The facts are 

 these. The brain of the rat has a weight (calculated by formula 

 [i]) at 72 days of 1.6823 §"^-' ^^^ ^^ Z'^^ days (corresponding to 

 25 years in man) a weight of 1.9020 gm., so that at 72 days it has 

 attained approximately 88.4 per cent of its limiting weight. 



On the other hand, the human brain (mean of both sexes) has 

 at six years a weight of approximately 1215 gms., which accord- 

 ing to the value in table 10, is 92.2 per cent of its limiting weight 

 at 25 years, and 90.8 per cent of its calculated maximum weight 

 at 16 years. ^ Thus the human brain has attained a greater frac- 

 tion of both its limiting and its maximal weight. The discrep- 

 ancy seems to depend mainly on the fact that while the early 

 phases of body growth in the rat are similar to those in man, yet 

 the rat continues to grow for a relatively longer period after matur- 

 ity than man does, and at the same time, the weight of the brain 

 and spinal cord continues to increase with that of the body. This 

 difference in the later phase of body growth therefore is a 

 point which needs to be investigated. At the same time although 

 the early attainment of the maximum weight in man followed by 

 a slow decline in weight through later life, as brought out by sev- 

 eral investigators and specially studied by Pearl ('05), may be 

 a normal biological phenomenon, yet it must be frankly admitted 

 that the human records as they stand, are distinctly influenced 

 by the factors represented by the peculiarities of the "hospital 

 population" on the one hand, and the effect of disease, especially 



' It may be noted in passing that Handmanx ('o6, S. 14-17) finds the maximum brain weights in both 

 sexes between 15 to 17 years. 



