STRUCTURE OP THE BRAIN IN RODENTS. 
737 
as the base, forming the modified structure referred to as the olfactory area, and which 
is here covered by a thin layer of cortex.* The outer margin of the striate body is 
sharply differentiated from the outer olfactory realm (lower limbic arc), and terminates 
abruptly at the inner margin of the superficial olfactory band, to which point run the 
fibres of the claustral formation (Plate 49, fig. 8, c). On the inner side, at its base, 
we find this olfactory area of the corpus striatum bounded not only by the central 
olfactory fasciculus, but by sheaves of fibres arising from the cortex of the olfactory 
lobe (Plate 49, fig. 8, G, g), and running as the arciform fibres up the septum lucidum 
to reach the under surface of the corpus callosum. In these anterior realms the 
internal capsule which separates the two main divisions of the striate body is not as 
yet clearly differentiated, and hence the large oval mass is constituted by the blending 
together of the caudate and lenticular nuclei, and the fasciculi here being chiefly pro¬ 
jections from the cortex of the frontal lobe are seen in transverse or oblique section. 
To obtain a good view of the fibres radiating from the frontal pole through the 
structure of the corpus striatum vertical sections must be taken across the hemi¬ 
spheres. At the summit or upper pole of the striate body (Plate 49, fig. 11, H) the 
fibres of the corpus callosum (D) meet those of the projection system from the cortex 
to the basal ganglia, and at their line of intersection mark off the intra-ventricular (K) 
from the extras ventricular portion (L). We have therefore in such sections a large 
ganglionic mass formed by the blending of the two striate nuclei, enclosed in a 
concentric manner by the hemisphere, and exhibiting the following systems of fibres : 
the callosal fasciculi (D) ; the striate radiations to cortex (C) ; the claustral formation 
(Plate 49, fig. 8, c) ; the external ( b ) ; median or deep (m) ; internal and central 
olfactory fasciculi (G and h). In vertical sections carried through the hemisphere of the 
Rabbit on a plane with the anterior commissure the distinction betwixt the two nuclei 
of the corpus striatum is very obvious. Here the caudate nucleus lies reclinate upon 
the sloping oblique side of the internal capsule, measuring from 9 mm. to 10 mm. in 
its longer, and 2*5 mm. in its shorter diameter, its upper pole directed upwards and 
outwards, receiving the radiating fibres from the cortex; its inferior pole directed 
downwards and inwards as a thin prolonged margin, exhibiting on its ventricular 
aspect the transverse section of the stria cornea (Plate 49, fig. 12, 4). Beneath the 
stria cornea with the structure of this inferior pole are nerve fasciculi, which in part 
pass downwards into the anterior perforated space (Plate 49, fig. 12, 2 ) (olfactory 
area), whilst a few arch inwards and are directly continuous with the anterior com¬ 
missure (Plate 49, fig. 12, 1 ). Beneath the sloping roof formed by the internal 
capsule lies the lenticular nucleus, which is distinctly wedge-shaped in form, its base 
directed outwards and curved concentrically to the surface of the hemisphere, its apex 
directed downwards and inwards terminates in the internal capsule (Plate 49, fig. 9, D). 
The lenticular nucleus appears to consist of outer and inner segment; the inner 
* Vide Plate 49, fig. 8. All structures below li. 
5 b 2 
