STRUCTURE OF THE BRAIN IN RODENTS. 
705 
pyramidal layer of the Rabbit—the cells remain small to their lowest level. In form, 
these cells are usually pyramidal or pyriform, whilst the small irregular, angular cell 
is met with chiefly on the outskirts of this layer where it joins the outer cortical zone. 
The apex process of these cells divides and subdivides into a delicate network of fibrils, 
those which are deeper seated undergoing the primary division at some distance from 
the cell, those nearer the cortical zone undergoing instant division, so that instead of a 
single elongated apex process the apex of the cell itself is bifid or horned. A similar 
feature pertains to the subjacent layer, the apex process of which may be traced often 
into the outer cortical zone ere primary division occurs. It therefore appears probable 
that whilst the greater bulk of nerve-fibre element of the lower four or five layers is 
formed by the numerous secondary processes of the nerve-cell/" the outer cortical zone 
derives its nervous element chiefly from the apex process of cells of underlying strata. 
The fact that the bifurcate or horned cell is met with at the commencement of this 
layer in sparse detached clumps has been alluded to above as significant probably of a 
rudimentary form of the second layer of higher animals. Now it has been shown that 
this latter stratum is also extremely defective and in great part entirely absent in 
the cortex of the Sheep ; in the Pig it is also but a shallow belt averaging *093 mm. 
deep, increasing in. certain regions to '139 mm., or even *186 mm., whilst in Man its 
greatest development is fully *279 mm. in depth. It appears clear, therefore, that the 
development of this belt bears an important relation to the position of the animal in. 
the scale of organisation. The average size of these small pyramids in the Rabbit is 
13p,longby 9/x broad, with a nucleus of S/x in length—the largest do not exceed 
17ix X 12[x. This closely approximates to the upper series of the third layer in Man, 
which in the ascending frontal gyrus measure 12p,X8/x, and in the ascending parietal 
gyrus 15/x X 11 fx. 
This second layer attains its greatest depth in that portion of the upper limbic arc 
lying in front of the corpus callosum, gradually diminishing in depth backwards, so 
that behind the corpus callosum it is but ‘372 mm. as contrasted with *883 mm., the 
depth of the anterior region. In the direction of the extra-limbic mass, i.e., over the 
exposed aspect of the hemisphere, the depth of this layer is often increased to 1 ‘023 mm., 
being double the depth in the limbic lobe on a corresponding plane. 
3. Ganglionic layer .—Immediately beneath the small pyramids of the second layer 
at the frontal end of this upper arc we find the far larger cells of the ganglionic layer. 
These cells, whilst differing widely from those of the ganglionic layer in Man, 
approximate to them sufficiently in many features to justify us in regarding them as 
representatives of the same series. Thus they are the largest and most characteristic 
cells in the cortex ; they lie immediately upon the spindle layer, with a slight pale 
poorly-celled zone extending above and below it as in Man; their processes are 
peculiarly elongated, extending right through the pyramidal series into the outer 
* By secondary processes are meant all extensions from the cell exclusive of the apex and basal or axis 
cylinder process, 
4x2 
