144 Journal of Comparative Neurology. 



extent may be fairly correlated with the variations in mental 

 power. 



If this value be given to the cell bodies of the cortex, in- 

 cluding their dendrons, it is necessary also to find an interpre- 

 tation for the other portions of the neurones which these bodies 

 represent. For this purpose, the neurone may be resolved into 

 three portions. First, the receiving portion ; second, the con- 

 ducting; third, the transmitting. These correspond first to the 

 cell body and its dendrons; second, to the axone; and third, to 

 the ends of the branches or the terminals. 



By means of the first portion, the neurone is rendered re- 

 sponsive to nerve impulses brought .into its neighborhood. By 

 means of the second, inpulses arising in the cell body are con- 

 ducted to the ends of the axone, and by means of the third, the 

 terminals, these impulses stimulate other neurones. 



It is therefore on the dendrons and the terminals, i.e., the 

 receiving and transmitting portions of the neurone, that the 

 complexity of the encephalon is dependent. The axone, acting 

 merely to conduct, may vary widely both in length and circum- 

 ference (and hence in mass), without modifying in any way the 

 complexity ef the parts which it connects. Hammarberg's ob- 

 servations demonstrate that there is a decided mental deficiency 

 in those cases where the receiving portions^ of the cortical neu- 

 rones fail to develop fully. Even when the fully developed cor- 

 tex is compared with the poorly developed, the difference be- 

 tween the two masses of the cell bodies is comparatively small, 

 for we have seen that the total weight of these cell bodies when 

 normal is only 1 3 grms. Thus the change in the mass of these 

 elements corresponding to a moderate degree of mental defici- 

 ency (say 20% =2.6 grm.), would be easily within the limits 

 of error for the brain weight as ordinarily taken. 



Furthermore, if differences of the same sort underlie the 

 grades of intelligence exhibited by normal individuals, these dif- 



' Probaly there was in these cases corresponding deficiency in the develop- 

 ments of the terminals but on this point his preparations do not give any in- 

 formation. 



