THE ANATOMY OF THE WEDDELL SEAL. 841 
and for its extreme thinness under the body of the fornix, where it covered the optic 
thalami and formed one of the roof structures in relation to the mesial or 3rd ventricle, 
for which it likewise provided the usual choroid plexuses. It transmitted numerous 
vessels into the upper or dorsal surface of the optic thalamus, to which it was closely 
adherent, but especially so at the hinder part. 
The third ventricle was situated as usual between the optic thalami, and its most 
noteworthy character was the large size of the middle commissure (fig. 2). The position 
of its anterior and posterior commissures did not call for special comment, and the 
structural arrangements and composition of its boundaries were not in any way peculiar. 
The optic thalami formed large well-developed masses, and, as already described, no 
part of their upper surfaces was visible within the lateral ventricles until the fornix 
and velum interpositum were removed. When the upper surface of the optic thalamus 
was fully displayed, it presented certain very interesting features. At its postero- 
lateral end—that is, close to the entrance to the descending horn of the lateral ventricle, 
but upon the upper surface of the optic thalamus—the corpus geniculatum externum 
constituted a well-marked elevation which was related to the fornix as previously 
explained. Along the mesial margin of its upper surface, a flattened ridge—the tana 
thalam or stalk of the pineal body—ran backwards towards the anterior end of the 
mesencephalon above the posterior commissure of the 3rd ventricle and the entrance 
to the aqueduct of Sylvius, where it was joined by its fellow from the opposite thalamus, 
and thus formed the peduncle of the pineal body. 
The pulvinar was situated between the corpus geniculatum externum and the teenia 
thalami. It formed a fiattened area which did not project backwards with an over- 
hanging border asin man. The habenula was situated partly to the lateral side and 
partly to the mesial side of the tenia thalami. In other words, the teenia thalami ran 
across the surface of the habenula. Considered as a whole, the habenula formed a 
narrow pyriform projection whose wider end was directed backwards and presented 
itself on the lateral wall of the 3rd ventricle high up in the interval between the middle 
and posterior commissures. 
The corpus striatum was displayed by making Nereranial transverse sections from 
the surface of the insula towards the mesial plane so as to include the caudate nucleus, 
but it was not until the lower levels of the island of Reil were reached that definite 
evidence of striation was observed. The grey substance of the surface convolutions and 
that of the caudate nucieus were always distinctly seen. but it was only after the sections 
had been subjected to the staining influence of a saturated solution of bichromate of 
potash for forty-eight hours that the other grey masses were clearly visible. 
The lenticular nucleus occupied its usual position on the postero-lateral aspect of 
the head of the caudate nucleus. Its mesial border was convex and separated from the 
caudate nucleus and the optic thalamus by the imternal capsule. ‘this band was quite 
definite, but very narrow; and it presented the characteristic anterior and posterior 
limbs with an intermediate genu. The lateral margin of the lenticular nucleus in its 
