ON THE FUNCTIONS OF THE CEREBRUM 295 



in the visuo-psychic and prefrontal regions of the cerebral cortex. 

 In the visuo-psychic region this layer develops earlier than in the 

 prefrontal region, but later than in the visuo-sensory area, and 

 eventually attains to practically the same adult depth as in the 

 former. In this region it is of practically constant adult depth, 

 and does not vary in measurable depth according to the mental 

 capacity of the individual. In the prefrontal region the outer 

 cell lamina develops later than in the other regions referred to 

 It is the only cell layer of the cortex cerebri which varies definitely 

 in measurable depth in " normal " brains. It is under-developed to 

 different degrees according to the mental capacity of the individual 

 in persons exhibiting various grades of mental subevolution ; and 

 it undergoes degrees of retrogression which correspond to the 

 amount of dementia existing in cases which permanently suffer 

 from diminution or loss of their mental powers. 



The second, pyramidal, or outer cell lamina of the human cerebral 

 cortex, therefore, subserves the " psychic " or associational junctions 

 of the cerebrum. These functions are pre-eminent in the prefrontal 

 region (centre of higher association) ; they are less important in 

 the visuo-psychic region (region of lower association) ; and they are 

 of least importance in the visuo-sensory region (projection sphere). 

 These three regions are therefore of different grades in the hierarchy 

 of cerebral function. 



The results obtained by Watson during his histological investiga- 

 tion of the cerebral cortex of the mammalia supply the complement 

 from the phylogenetic aspect to the above (ontogenetic) conclusions. 

 Watson finds that in the insectivora the pyramidal or outer cell layer 

 is in a rudimentary condition, though the lower layers of the cortex 

 approximate in depth to that of these layers in the normal human 

 adult. Further, the pyramidal or outer cell layer is better developed 

 in the rodents than in the insectivores ; it is again better developed 

 in the ungulates and in the carnivores than in the rodents ; and it 

 is strikingly more developed in the primates than in the carnivores. 



He therefore functionally correlates this layer with the educa- 

 bility and general intelligence which appear in an increasing degree 

 during the ascent of the mammalian scale. 



Watson remarks : 



" The supra-granular (pyramidal) layer which is, relatively to 

 the infra-granular cortex, so poorly developed at birth is slow in 

 reaching maturity, and is, even at its best, in certain lower mammals, 



