40 
MR. W. BEVA.N LEWIS ON THE COMPARATIVE 
characteristic of motor areas. Again, we come upon regions where a nested arrange¬ 
ment of small ganglionic cells is associated with a six-laminated cortex. In fact, the 
nested arrangement and size of the individual cells appear to he invariably in inverse 
ratio to the depth of the small angular formation superimposed, so that, as we approach 
motor realms, the latter thins out and gradually disappears, whilst the nests become 
more and more perfect and embrace larger and still larger cells. On the other hand, 
as we recede from motor regions the nests thin out, become less perfect as groups, 
include cells of small dimensions, become more widely separated from each other, whilst 
the superimposed stratum gains in depth and increases in density by the close aggre¬ 
gation of its angular and pyramidal elements. In studying the arrangement of the 
layers of the cortex the above points appear to me to demand careful attention, and 
the eye should become perfectly familiarised with the varied appearances presented 
by transition zones of the cerebrum. 
First layer (Plate 6, A).—Over different regions of the cortex this stratum varies 
somewhat in depth: In the neighbourhood of the limbic lobe it averages '619 mm., 
and nearer the Sylvian fissure it becomes ‘418 mm. in depth. In ail points excejrt the 
relative proportion borne by the connective to the nervous elements, this layer closely 
resembles the corresponding layer in the higher Mammalia. The matrix is constituted 
of a fine areolation of connective fibrils, with which is associated an extremely delicate 
network of nerve fibrils. The latter are derived from the apex processes of nerve cells 
in subjacent layers, the long, delicate process of the large ganglionic cell being fre¬ 
quently traced up into this layer, where it is observed to divide into numerous sub¬ 
divisions. The connective elements include cell-forms of two kinds—the perivascular, 
and the neuroglia corpuscles, both forms nucleated, and measuring respectively 5/x and 
9/r in diameter. The larger elements are especially numerous just beneath the pia 
mater, and constitute a distinct belt of cells in many situations—irregular in form, 
and throwing off numerous delicate processes. These cells, known also as Deiter’s 
corpuscles, are found in such vast numbers in the Sheep that attention will be directed 
to their mode of distribution and significance when describing the first cortical layer 
of that animal. In all fresh preparations this layer presents us with three distinct 
regions differing somewhat in appearance and corresponding very nearly to its upper, 
middle, and lower third. In the upper division we find a band of medullated nerve 
fibres following the course of the convoluted surface and surrounded on all sides by 
processes from the cells of Deiter. The lower division shows us a coarse network 
formed by the division and decussation of the apex processes of the second layer of 
cells, which are often thrown oft’ at a very oblique angle from the cell. 
Second layer. —A shallow belt of densely congregated cells forms the second layer 
of the cortex (Plate 6, B). Its superficial and deep limits terminate somewhat abruptly, 
so that a fair estimate may be taken of its depth, which is found to be decidedly less 
than the corresponding layer in the human brain. The average depth may be given 
at ‘093 mm., increasing, however, in some regions to ‘116 mm. or even T38 mm. 
