STRUCTURE OF THE BRAIN IN RODENTS. 
715 
cells of which are no longer pyramidal but swollen, inflated, globose or flask-sliaped 
(Plate 50, fig. 6, B). They are also greatly increased in size, measuring 37/xX32/r, 
or 37/r X 23/r ; whilst others are more elongated, measuring 46p.X27/x. Their nucleus 
has a diameter of 13 /jl. Their dimensions are thus more than double those attained by 
the elements of the second layer in other regions of the brain, whilst their irregular 
mode of branching as a bifurcate cell gives this formation the aspect of an unusually 
rich development of the ordinary second layer. Scattered amongst these larger cells, 
and descending as a distinct layer, are small pyramidal cells measuring but 23/xX 13/x, 
so that in this region the series of larger cells is placed superficial to the smaller 
elements. As regards either pyramidal series in other regions of the cortex, this latter 
feature is never observed—the upper series being invariably composed of smaller cells. 
Immediately beneath this third layer of pyramidal cells is a clear white belt devoid of 
nerve cells, and to this succeeds a. belt of spindle cells (Plate 50, fig. 6, D, E). The 
cortex of this modified olfactory type is therefore composed of five layers :— 
1. Peripheral cortical zone (depth ‘418 mm.). 
2. Layer, of large inflated cells (depth *279 mm.). 
3. Small pyramidal cells (depth - 372 mm.). 
4. Pale belt devoid of nerve cells (depth ’184 mm.). 
5. Spindle series (depth '372 mm.). 
The region embraced by this formation is that portion of cortex at the extreme 
posterior extremity of the lower limbic arc where the upper limbic arc unites with the 
gyrus hippocampi (light-shaded area from T-W, Plate 49, fig. 1). It extends as far 
as the upper extremity of the limbic fissure seen on the median posterior aspect of the 
hemisphere, being bounded externally by the limbic sulcus (C) and internally by the 
gradually vanishing granule formation of the modified upper limbic cortex (dotted area, 
Plate 49, fig. 1). Sections carried horizontally through the occiput at this site will 
therefore exhibit from without inwards four types of lamination :— 
1. Five-laminated type of extra-limbic cortex. 
2. Modified olfactory type. 
3. Modified upper limbic type. 
4. Type of cornu ammonis. 
Independent of the peculiar arrangement of cells and disposition of the layers in 
this region, a characteristic feature is acquired in the presence of two arciform stripes 
of medullated fibres, disposed as in the modified upper limbic cortex, one above the 
second layer and the other immediately beneath the same. They will be more 
minutely described in the section on the central projections of the olfactory lobe. For 
the present it will suffice to bear in mind the important fact that both these modified 
occipital regions of the great limbic lobe are characterised by the double stripe ofintra- 
cortical arciform medulla , and by the dense granule belt of the one, as contrasted with 
the great inflated cell formation of the second layer in the other. 
