CORRELATION OF BRAIN AND SPINAL CORD. 515 



Both brain and spinal cord are more closely associated with 

 body weight than with body length. Although the differences 

 are slight and not statistically valid, the consistency of their 

 direction and their presence in both sexes puts into the relation 

 a significance that cannot be denied. It is a relation that might 

 be expected by virtue of the fact that the developmental processes 

 productive of weight in the parts of the organism are more 

 closely allied than the developmental processes productive of 

 weight are allied to those of length. This result would seem on 

 the face of it to detract somewhat from Donaldson's (5) dictum 

 that "body length is a better datum than body weight from 

 which to infer the weight of the brain or spinal cord." The 

 objection is negatived, however, by the fact that in a normal 

 population the variability in body weight is greater than that 

 in body length. This will be discussed presently. 



The high degree of positive correlation of brain weight and 

 spinal cord weight with body weight and body length allows the 

 extension of Donaldson's (6) conclusion that the weight of the 

 spinal cord can be inferred from body length or body weight 

 with a high degree of accuracy, to include the brain. 



With one exception (r\ 3 ) the degree of association between the 

 several pairs of variables is greater in the male than in the female 

 rat. The degree of difference is, however, statistically valid 

 only in the case of the body weight-body length correlation 

 (ri 2 ). These figures in Table I., it must be remembered, are 

 indices of the degree of association between pairs of variables, 

 when interfering influences assumed to be exerted by the other 

 variables are still present. Such being the case, and if the 

 general trend of sex difference is accepted as significant because 

 of its consistency, it is suggestive of a greater independence of 

 the central nervous system of the female from the general factors 

 contributive to interstructural and inter-organ association as 

 carried by the body as a whole. This inference is supported by 

 the results of the analysis by the method of partial correlations. 

 The use of this method is allowable here because the regressions 

 are in all cases essentially linear. 



In Table II. are given the correlation coefficients of the several 

 pairs of variables after the removal of the assumed influences 



