41 
STRUCTURE OF THE CORTEX CEREBRI. 
Ill the human brain, on the other hand, the depth of this layer in the frontal lobes 
varies between ‘139 mm. and ’186 mm., whilst posterior to the fissure of Rolando 
it may increase to ’279 mm. The nerve-cells vary greatly in form, appearing as 
angular, pyriform, pyramidal, oval, or globose elements, which throw off several delicate 
processes, one or more thrown off from the apex towards the surface of the cortex being 
a frequent but not constant arrangement. I have elsewhere alluded to a striking 
character as common to these elements in the human brain,"' viz.: the relatively large 
size borne by the nucleus to the protoplasm of the cell as compared with larger nerve- 
cells of subjacent layers. We find the same character maintained by these cells in the 
cortex of the Pig. Another peculiarity is the frequent occurrence of the globose form 
of cell which throws oft' an apical process and several very delicate branches from its 
sides, into which branches, however, the cell protoplasm does not prolong itself as in 
other cells ; hence its contour is wholly unaffected by the branching. This form of cell 
is peculiar, and I shall allude to its possible relationships further on. In this layer 
the angular form of cell is most frequent; its peripheral branches diverge obliquely to 
the radial fasciculi, cross each other in all directions, and by their repeated divisions 
and ramifications form a dense network of fine fibres quite characteristic of the deeper 
portions of the first layer. In delicate sections this network can be traced upwards 
for a distance of '162 mm.— i.e., one-fourth the whole depth of the superficial layer 
of the cortex. These angular cells are peculiar in form, and are met with frequently, as 
we shall see, at deeper levels. The gradual blending of the second and third layers of 
the cortex as seen in the human brain is far less marked in the Pig, as over most 
regions of the cortex one layer abruptly terminates in the other. 
Third layer (Plate 6, C).—The depth of this layer, taken together with the second 
layer, averages '913 mm. This average was struck from measurements at thirty-six 
different points of the cortex of the vault, the variations lying between '744 mm. 
and 1'488 mm. The greatest depth is attained immediately below the summit of 
the convolution, whilst towards the sulci the layer gradually becomes shallower. The 
gradual increase in size of its constituent cells from above downwards, which is so 
prominent a feature in the human brain, is not observed here, the deeper cells of this 
layer being identical in point of size with the more superficial cells bordering upon the 
second layer. Not only are these cells very constant in size throughout the depth of 
this layer, but they vary very slightly in magnitude throughout the whole surface 
of the cortex. An approximate average from a very large number of measurements 
may be given as 23/x in length by 13/x in breadth, and the longest diameter of their 
nucleus as 9/x. On reference to the table of measurements we shall find that, whilst 
the upper series of cells in this layer in Man and the Carnivora do not reach this 
magnitude, the lower series in Man are far larger, whilst in the Cat the lower series 
corresponds exactly in size with those of the Pig. It is important to bear in mind the 
fact that the lowest stratum of the third layer in the Cat is formed of cells identical 
* “ The Cortical Lamination of the Motor Area of the Brain,” Proc. Royal Soc., No. 185. 
MDCCCLXX X. 
G 
