54 
ME. W. BEVAN LEWIS ON THE COMPARATIVE 
is maintained. Tlie third parietal convolution is six-laminated throughout, its fourth 
layer of small' pyramidal elements being very distinct and richly developed. The 
ganglionic cells are arranged in clusters towards the anterior half of this convolution, 
becoming larger and denser nests the nearer they approach the sigmoid gyrus. In this 
latter region the cells are of large size. Towards the posterior extremity of the third 
parietal convolution the clustered arrangement gives place to the laminar. Passing 
now to the fourth parietal or sagittal gyrus, we observe at the anterior extremity, 
where it impinges upon the crucial sulcus, the small pyramidal cells below the third 
layer are so scanty that nothing beyond a mere tendency to the formation of a separate 
stratum can here be discerned. The ganglionic cells, however, are very large and globose, 
and the nests rich and extensive. Except at the very extreme anterior portion of 
this convolution, the cortex is distinctly six-laminated throughout, and the cell-clusters 
become thinned posteriorly to a solitary or laminar formation, as far as the posterior 
pole of the hemisphere. 
Frontal lobe. —This lobe forms the extreme anterior pole of the hemisphere, and 
presents a well-marked fissure directed obliquely downwards and outwards—the fissure 
of Rolando, bounded externally by the ascending parietal, and internally by the 
ascending frontal convolution. The cortex of this lobe is five-laminated, except in the 
immediate neighbourhood of the fissure of PlOLANDo, where a transition lamination 
presents itself. 
Taking the structure of this lobe in the order of its layers, we notice that the first 
layer attains a depth of '325 mm., and differs in no essential points from that of the 
limbic lobe. The second layer is here scarcely appreciable, always poorly represented, 
and often absent over limited areas. It becomes distinctly defined towards the 
ascending parietal gyrus. The second and third layers have a depth of '930 mm. 
The pyramidal elements of the third layer increase in size with their depth, are most 
densely congregated in the upper half of the layer, whilst the lower half is left as a 
paler zone, in which towards the fissure of Rolando we can readily trace the gradual 
increase of angular and small pyramidal elements, which, as above stated, form in the 
ascending parietal a distinct belt dividing the cortex into six layers. 
Regional Distribution. 
It will be seen that we have in the foregoing description anticipated many of the 
facts regarding relative areas of different laminations and distribution of the ganglionic 
series. The whole superior arc of the great limbic lobe, from its origin near the 
olfactory lobe backwards as far as the retro-limbic annectant, presents us with the best 
example of the clustered arrangement of the ganglionic cells. These cell clusters are 
most thickly grouped, and most rich in cells in the anterior regions of this upper arc, 
attaining their maximum development in the vicinity of the crucial sulcus. The 
elements of this layer in the latter region differ much amongst themselves in contour 
