THE ANATOMY OF THE WEDDELL SEAL. 129 



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 tsenia 

 thalami 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 tsenia 

 thalami. It formed a flattened area which did not project backwards with an over- 

 hanging border as in man. The habenula was situated partly to the lateral side and 

 partly to the mesial side of the taenia thalami. In other words, the taenia 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 stnatum was displayed by making horizontal 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 nucleus 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 internal 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 



(BOY. soc. Ems. TRANS., VOL. XLVIII., 841.) 



