736 
MR. W. BEVAN LEWIS ON THE COMPARATIVE 
posterior division of the brush may readily be traced into the inferior arc of the great 
limbic lobe, its fasciculi never rising above the level of the limbic sulcus. They are 
ultimately distributed to that peculiar region at the occiput characterised by the 
modified olfactory cortex. 
It now remains for us to follow up the extensions of medullated fasciculi from the 
olfactory lobe, and to trace the expansions into the distant cortical areas of the central 
olfactory and arcuate olfactory systems. Prior to doing so, it will be advisable to 
consider the structure and cortical connexions of the corpus striatum and callosal 
commissure. 
Corpus Striatum and its Cortical Connexions. 
Corpus striatum. —This ganglionic body exhibits in these animals, as in higher 
members of the animal kingdom, a marked division into lenticular and caudate 
segments, or, in other words, extra- and intra-ventricular nuclei, which assume also 
a higher level as compared with the thalamus, since the largely developed cornu 
ammonis and the bulky fibrous fornix commences at higher and anterior planes, and 
not, as in Man, behind the thalamus. These structures therefore intervene betwixt 
the cortex of the vertex and thalamus, and displace the latter to a lower level, so that 
sections carried horizontally through the upper part of the hemisphere will exhibit the 
striate body and cornu ammonis cut through in one and the same plane. Anteriorly 
the corpus striatum exhibits a large rounded head, which bends downwards to the 
base of the hemisphere within the confines of the superficial olfactory fasciculus. 
Posteriorly it becomes rapidly attenuated and is continued as a small cylindrical belt 
of grey matter—the tail of the caudate nucleus, which curves backwards and down¬ 
wards within the descending horn of the lateral ventricle to terminate in the inferior 
limbic arc, near the summit of the gyrus hippocampi (Plate 49, fig. 10, C). To study 
the form and relationship of the striate body, vertical sections should be examined 
passing through the septum pellucidum in front of the optic commissure. This 
ganglion is then seen as a large irregularly pyriform structure, its greater mass 
directed upwards, its upper and outer border nearly concentric with the surface of the 
hemisphere (Plate 49, fig. 12, 4),' y ' whilst the inner or intra-ventricular border lies 
partly adjacent to the thick foliole of the septum lucidum (fig. 8, b), and at the angle 
of union with the latter the transverse section of a compact band of medullated fibres 
is seen, the central olfactory fasciculus (Plate 49, fig. 8, h.). The true structure of the 
corpus striatum ends upon a level with this olfactory fasciculus, and is mapped out by 
a succession of fasciculi, which are cut across and lie in a continuous line from the 
central olfactory fasciculus to the outer margin of the ganglion. These fasciculi are 
of greater size than those beneath them, and are constituted by the strands of the 
“lyre” already alluded to. Beneath this line the striate nucleus still extends as far 
* The diagram (fig. 8) exhibits in outline a part of the caudate nucleus and its full basal relationships. 
