STRUCTURE OF THE BRAIN IN RODENTS. 
713 
sulcus, and separated by medullated fasciculi from tire corpus striatum, they appear to 
terminate superficially near the inner border of tlie superficial olfactory band. Here, 
as will be mentioned further on, they seem to communicate with a superficial arciform 
system. The axis of these cells is arranged parallel to the surface of the cortex. 
Gyrus hippocampi (Plate 49, fig. 1, B). The structure of the cortex of this region is in 
most respects identical with that of the anterior segment of the limbic arc just described. 
The layers are three in number and alike in constitution, but the development of the 
third or large pyramidal layer is a more marked feature here. Whereas the second or 
small pyramidal layer was the only fully developed formation in the outer olfactory 
cortex, in the gyrus hippocampi the third or large pyramidal cells form a well 
differentiated layer, increasing in size and number towards the posterior extremity of 
this gyrus. Here they are identical in appearance with the pyramidal cells of the 
third cortical layer of human brain, showing in vertical sections six or seven branches 
and a long apical process. The depth of this layer also increases from before backwards. 
Inner Olfactory Type 
Near the inner margin of the superficial olfactory fasciculus we find the commence¬ 
ment of a thin and peculiar cortex, which spreads over the anterior perforated space 
to the inner margin of the hemisphere, where, with a slight bend upon itself, it becomes 
continuous with the septum lucidum (Plate 49, figs. 1, 2, P). This cortex covers 
an area the relationship of which will be more particularly described further on, and 
which we shall denominate, after Gratiolet, the olfactory area, or in contradistinction 
to the remaining cortex extending from it to the limbic sulcus—the inner olfactory 
area. The coarse or naked-eye appearances of this cortex are peculiar, as is also its 
histological constitution. Vertical sections carried through the hemisphere in this 
region will exhibit a wavy disposition of the second layer, which assumes a folded or 
duplicate arrangement wholly peculiar to this realm of the brain. In these V or W- 
shaped duplicatures the superficial layer of the cortex does not participate, so that at 
one time the second layer is close to the surface and at other times quite remote. The 
layers constituting the cortex here are three :— 
1. Superficial or peripheral zone. 
2. Layer of granule cells. 
3. Layer of spindle cells. 
1 . Peripheral zone .—The first layer has an average depth of ’232 mm., but from 
the duplicatures of the subjacent layer its depth will, of course, be subject to great 
variation. In structure it is similar to the same layer in other regions of the cortex, 
differing, however, in being perforated by numerous large vessels, which pass through 
* This region is seen in diagram (Plate 49, fig. 8), also by black area at base, Plate 49, fig. 1. 
4 Y 2 
