146 Journal of Comparative Neurology, 



tire encephalon' (27.2 grms.) We assume however, as the rec- 

 ords in the Table are from persons having at least normal intel- 

 ligence, that moderate variations in the number of dendritic 

 branches belonging to the cortical cells and in the associated 

 terminals represent the greatest differences to be expected. We 

 therefore conclude that the largest part of the difference in 

 weight, as exhibited in the Table, is to be referred to the ax- 

 ones, with their medullary sheaths ; at the same time recogniz- 

 ing that variations in the mass of the axone are of no value in 

 increasing the complexity of the central system. 



Thus far in this argument, it has been tacitly assumed that 

 the encephala compared contained the same number of cortical 

 neurones distributed in the same way. Yet no one would care 

 to maintain the absolute correctness of such an assumption. 

 Probably the fact to which the assumption most nearly corres- 

 ponds is a comparatively slight variation in the number and dis- 

 tribution of the cortical neurones in the majority of the enceph- 

 ala from ordinary persons. In order to determine the value of 

 the explanation given above, the analysis of the gross results 

 may be carried one step further. 



Speaking in a general way, the cortical neurone grows in 

 such a manner that there is a relation between the mass of the 

 cell body (including its dendrons), the axone, and the terminals, 

 and, within certain limits, these mass relations can be inferred 

 one from the other. Nevertheless, the facts warrant us in as- 

 suming a rather wide normal variability in the mass of the ax- 

 one, and, as a consequence, there must follow variations in the 

 mass of the entire encephalon that are very considerable, be- 

 cause the axones form a very large part of it.^ 



' Taking the weight of the average male encephalon as 1360 grms., 2^ of 

 it would be 27.2 grms. 



2 If the mass of the nerve cell bodies be taken as 29^, of the encephalon, 

 one is at first tempted for the sake of brevity, to speak of the remaining 98% 

 as composed of the axones. This remainder, however, is composed of the blood- 

 vessels and sustentacular tissues as well as the axones, and we have not yet any 

 observations which enable us to give an exact value to these non-nervous con- 

 stituents. Hence it can only be said that the mass of the axones is very large, 

 forming as it does, the greater part of this 98^;^. 



