CEREBRAL LOCALIZATION 855 



The middle cell lamina is much hypertrophied in the so-called projec- 

 tion areas of the cerebrum for example, in the visuosensory area (see 

 Fig. 220), where it is so thick that it is usually described as being divided 

 into two parts by a narrow fiber band (the line of Gennari). Diminution 

 in the layer occurs in the visuosensory area in long-standing cases of 

 atrophy. "It seems therefore primarily to subserve the function of re- 

 ceiving afferent impressions whether these arrive directly from the lower 

 sensory neurons or indirectly through other regions of the cerebrum. ' ' 



The fifth or inner cell lamina is the first to become differentiated, and 

 it is of extremely constant depth in the adult. It is not much affected in 

 amentia, unless when this is very severe, as in patients who are unable 

 to carry on the ordinary animal functions. In short, ' ' it subserves the lower 

 voluntary and instinctive activities of the animal economy" (Bolton). 



Taking the results as a whole, it appears that the region of the cortex 

 behind the Rolandic fissure consists of sensory areas and association 

 areas which may be immediately connected with them (visuopsychic and 

 intermediate postcentral) or more removed (in parietal lobe). The por- 

 tion in front of the Rolandic fissure, on the other hand, contains the 

 efferent areas, of which the precentral may be regarded as of lowest 

 grade. The motor discharges from it are conditioned upon impulses 

 coming partly from the adjacent intermediate precentral area, in which 

 again are elaborated those received from the sensory areas, and partly 

 from those coming from the prefrontal region, which is the most com- 

 plex zone of association. This last is indeed the supreme dominating 

 area. It coordinates or integrates the activities of the other association 

 areas and may be considered as the seat of the intellect. The evidence 

 for this high evolution of the prefrontal area is very strong. It is the 

 last portion of the cortex to be evolved and the first to undergo retro- 

 gression. In idiots and imbeciles the thickness of the pyramidal cell 

 layer in this region is directly proportional to the mental -power, and 

 in dementia degrees of retrogression occur that vary directly with the 

 existing grade of dementia. In normal brains this layer is the very 

 one which varies in depth in different individuals. Along with its high 

 development in the brain of man, as compared with that of other ani- 

 mals, goes hand in hand a great increase in the other association areas. 

 Thought is the product of integration between these various associa- 

 tion areas, and articulate and written language the outward manifesta- 

 tion of the process. It is owing to the relatively great extent and com- 

 plexity and constant development of the prefrontal lobe that man so far 

 excels even the highest apes in his intellectual activity, and it is owing 

 to the relative functional development of this lobe that individuals dif- 

 fer from one another in their mental powers. 



