782 REPORT — 1901. 



further e\idence, conclusions wbich have been drawn from the examination of 

 the few brains of eminent men that have been described. There cannot be a doubt 

 that the region in question is one which has extended greatly in the hnman brain, 

 but the association of high intellect with a special development of the region is a 

 matter on which I must confess I am at present somewhat sceptical. 



But it is not only in a backward direction that the parietal lobe in man has 

 extended its territory. It has likewise increased in a downward direction. There 

 are few points more striking than this in the evolution of the cerel^ral cortex of 

 man. In order that I may be able to make clear the manner in which this increase 

 has been brought about, it will be necessary for me to enter into some detail in 

 connection witli the development of a region of cerebral surface termed the insular 

 district. The back part of the frontal lobe is also involved in this downward 

 extension of surface area, and, sncK being the case, it may be as well to state that 

 the boundary which has been fixed upon as giving the line of separation between 

 the parietal "and frontal districts is purely artificial and arbitrary. It is a demar- 

 cation which has no morphological significance, whilst from a physiological point 

 of view it is distinctly misleading. 



The insular district in the foetal brain is a depressed area of an elongated 

 triangular form. The general surface of the cerebrum occupies, all round about it, 

 a more elevated plane, and thus the insula comes to be bounded by distinct walls, 

 like the sides of a shallow pit dug out in the ground. The upper wall is formed 

 by the lower margins of the frontal and parietal lobes, the lower wall by the 

 upper margin of the temporal lobe, and the front wail by the frontal lobe. 

 From each of these bounding walls a separate portion of cerebral cortex 

 grows, and these gradually creep over the surface of the insula so as to overlap 

 it, and eventually completely cover it over and exclude it from the surface, 

 in the same way that the lips overlap the teeth and gums. That which grows 

 from above is called the fronto-parietal operciihnn, while that which grows from 

 below is termed the temporal ojiercxlum. These appear very early, and are 

 responsible for closing over more than the hinder three-fourths of the insula. The 

 lower or temporal operculum is in the first instance more rapid in its growth than 

 the upper or fronto-parietal operculum, and thus it comes about that when their 

 margins meet more of the insula is covered by the former than by the latter. So 

 far the development is apparently precisely similar to what occurs in the ape. 

 The slit or fissure formed by the approximation of the margins of these two 

 opercula is called the Sylvian fissure, and it constitutes a natural lower boundary 

 for the parietal and frontal lobes which lie above it. At first, from the more 

 energetic growth of the lower temporal operculum, this fissure slants very 

 obliquely upwards and backwards, and is very similar in direction to the corre- 

 sponding fissure in the brain of the ape. But in the human brain this condition 

 is only temporary. Now begins that downward movement of the parietal lobe 

 and back part of the frontal lobe to which reference has been made. The upper or 

 fronto-parietal operculum, in the later stages of foetal life and the earlier months of 

 infancy, enters into a growth antagonism with the lower or temporal operculum, 

 and in this it proves the victor. The margins of the two opercula are tightly 

 pressed together, and, slowly but surely, the fronto-parietal operculum gains 

 ground, pressing down the temporal operculum, and thus extending the territory 

 of the frontal and parietal districts. This is a striking process in the brain 

 development of man, and it results in a depression of the Sylvian fissure or the 

 lower frontier line of the frontal and parietal lobes. Further, to judge from the 

 oblique direction of the Sylvian fissure in the brain of the ape, the process is 

 peculiar to man ; in the simian brain there is no corresponding increase in the area 

 of cerebral cortex uniier consideration. 



I do not think that it is difficult to account for this important expansion of the 

 cerebral surfixce. In the fore part of the region involved are placed the groups of 

 motor centres which control the muscular movements of the more important ])arts 

 of the body. These occupy a broad strip of the suiface which stretches across the 

 whole depth of the district concerned. Within this are the centres for the arm 

 and hand, for the face, the mouth and the throat, and likewise, to some extent, 



