Donaldson, Nerve Cell Bodies in the Cortex. 147 



Accepting- this conclusion, the differences in question can 

 be explained in one of three ways. In the larger brain of any 

 group, there is either a greater number of complete neurones, 

 or, second, a more generous development of the axones alone, 

 or, third, a possible combination of both of these conditions. 



In making a choice between these several explanations 

 thus suggested, a guide may be found in the observation that, 

 within a rather wide range of absolute weight, there is a re- 

 markable constancy in the proportional weights of the several 

 subdivisions (cerebrum, cerebellum and stem) of the enceph- 

 alon.' 



The conditions which cause differences in total weight must 

 therefore act harmoniously among the several subdivisions of 

 the encephalon. It has already been pointed out that the cor- 

 tex was calculated to weigh 658 grms. in the case of an aver- 

 age male brain. This is somewhat less than one half of the to- 

 tal weight of the encephalon (1360 grms.) If then, the varia- 

 tions in the total weight of the encephalon tend to be harmoni- 

 ously distributed, only about one half of the difference in the 

 total weight can in any case be attributed to variations in the 

 cortex. 



If then, it is granted that slight variations among the den- 

 drons and terminals of the cortex may determine wide physio- 

 logical differences as indicated by mental power, and further, 

 that the axones may vary widely in their mass, independent of 

 the rest of the neurone, we are left with a strong presumption 

 in favor of explaining the weight differences tabulateil above as 

 due mainly to differences in the axones, and therefore as practi- 

 cally insignificant for the physiological complexity of the cortex 

 in particular ; and the encephalon in general. 



Two special cases still remain to be mentioned. After the 

 age of six years, the encephalon often shows as great a weight 

 as in the prime of life.^ 



> P. 99, Table 14; p. loo, Table 15. 



»P. 104, Table 17. 

 P. 105, Figure 17. 



