Donaldson, Groivth of Central Nervous System. 367 



ORDT ('90), we give in the following table his observed values, and 

 also the values obtained from a smoothed curve based on these 

 data. The curve represented by Vierordt's observations is 

 given in "The growth of the brain" (Donaldson '95) and also 

 in the American Text-Book of Physiology (Donaldson '01). 

 The smoothed curve which passed with the least deviation through 

 the rough curve has been drawn, and then the values given by the 

 smoothed curve were determined for each year. These values are 

 entered in table 10 under "calculated" in columns D and F, and 

 from these, of course, the smoothed curves can be reconstructed. 



To so reduce the values of the human records as to make them 

 comparable when plotted with those from the rat, it was necessary 

 to divide them by 700. In chart 5 the weight of the human brain 

 thus reduced is compared with that of the rat, the span of human 

 life being taken as thirty times that of the rat, and the time inter- 

 vals entered accordingly. When thus plotted, it is seen that the 

 two curves are similar in form. Moreover, if we determine the 

 age at which the rapid growth of the brain ceases in the rat, which 

 is at a body weight of 105 gms. (see p. 352), it is found to fall at 

 about 70 days in the male, and 73 days in the female and it is 

 evident that the average date, 72 days, corresponds very nearly 

 with six years in man. 



Between birth and 72 days the rat brain has increased in weight 

 (mean of both sexes combined) from .02517 gm. to 1.6823 gm. 

 or 6.3 times, while in the corresponding interval, the human brain 

 has increased in weight (mean of both sexes combined) from 383 

 to 1215 gms., or 3.2 times. 



We know, however, that the rat is born relatively much less 

 mature than the child. The comparison as it stands, is therefore 

 hardly fair. If we determine for the rat the initial brain weight, 

 which at 72 days would give an increase similar to that observed 

 in man (3.2 times) we find the required w^eight to be .525 gm., or 

 approximately the weight of the rat's brain between five and six 

 days. Therefore, between the age of five and six days — at which 

 time the rat's brain is certainly more comparable with the human 

 brain than at birth — and 72 days, the brain of the rat increases 

 in weight in the same proportion as does the human brain between 

 birth and six years. 



This relation, although derived from a treatment of the data 

 which is admittedly rough, is very suggestive, but it will be hardly 



