352 'Journal of Comparative Neurology and Psychologv. 



the following coefficients of correlation between body weight and 

 head measurements: 



Length Width 

 of head. of head. 



Boys 0-43 0.32 



Girls 0.41 0.33 



Thus in both these series from man, the correlation is less per- 

 fect than in the albino rat. However, it must be remembered 

 that the determination of the true body weight, especially when 

 it must be taken postmortem, is much more difficult to make in 

 man than in the rat. 



In 535 records (357 male, 178 female) the age of the rat is 

 known, and a similar calculation of the coefficient of correlation 

 between age and brain weight in the male, gives a much smaller 

 value, 0.5177 ± 0.0261, a result which might have been expected 

 from the fact that the body weight of the rat is so easily modified 

 by food and other external conditions. In this case also the coeffi- 

 cient of correlation for man is much less than for the rat. 



An examination of either of the charts (i and 3) shows that 

 between the body weights of 50 to 100 gms. the observations 

 tend to sag below the theoretical curve. For this "sag" no expla- 

 nation has yet been found. There is of course no cogent reason 

 for expecting that the increase in the weight of the brain must con- 

 form to a simple formula, yet it does conform to such a formula, 

 except at the body weights of 50 to 100 gms, and we are therefore 

 justified in expecting that this deviation may sometime be ex- 

 plained. 



In order to distinguish between the period of early rapid growth, 

 and the later period of slow growth of the brain, a determination 

 has been made of the limits within which the mature brain changes 

 in weight in a simple relation to the body weight. 



Taking as a standard the theoretical brain weight of the heaviest 

 group (315 gms.), as given in column D, table i, and calculating the 

 values for each successive group below this, it is found that as far 

 as the group with a body weight of 105 gms. the brain weight 

 diminishes nearly in proportion to the 7th root of the body weight. 

 The calculated values based on the 7th root of the body weight, 

 are given in column E of table i. 



For this distance the straight line formed by the 7th roots of the 

 body weight runs as a chord, of which the logarithmic curve forms 



