THE ANATOMY OF THE WEDDELL SEAL. 843 
of an adult brain, the complete object was in an undisturbed position. The peduncle 
was very thin, flattened from above downwards, and measured 6 mm. in the transverse 
direction. It was closely enveloped in the pia mater, and extended backwards on the 
vermis of the cerebellum to terminate in a disc-like expansion 12 mm. in width. The 
discoid part was flattened upon its cerebellar surface, while it was slightly conical on 
the opposite side. From the commencement of the peduncle to the extreme edge of 
the disc it measured 25 mm., of which the peduncle represented 15 mm. and the disc 
10 mm. Numerous vessels travelled between the pineal body and the pia mater. 
These two adult brains had been preserved in precisely the same way, and therefore it 
would appear as if the pineal body of the Weddell seal underwent a gradual reduction 
in size subsequent to birth, but that the shrinkage is not accompanied by any marked 
shortening in the total length of the object. Similar facts have been recorded by 
TURNER in connection with the pineal body of the elephant seal, in which the 
measurements were :—leneth, 16 mm.; greatest breadth, 8 mm.; greatest vertical 
diameter,6 mm. In two specimens taken from the walrus the dimensions were, in one 
case, 30 mm. long and 18 mm. wide; in the other case, 29 mm. long and 13 mm. 
wide. There is thus satisfactory evidence that, so far as the seals are concerned, the 
pineal body attains an unusual size as compared with other mammals; although in the 
case of Otaria jubata, described by Murti, the size of this structure may not have been 
so noteworthy as in the specimens above detailed, otherwise such a competent observer 
could scarcely have confined his account of its size to the statement that it was 
“yelatively large.” 
The optic tract followed the usual course from the optic chiasma backwards and 
outwards to wind round the lateral aspect of the crus cerebri. Thereafter—owing to its 
relations to the hippocampus major, as already described—it became compressed into a 
somewhat triangular band upon the under side of the thalamus, and sweeping past the 
corpus geniculatum imternum, with which it became closely associated, it continued its 
course, spreading out certain of its fibres towards the pulvinar, but reserving a bundle 
of considerable bulk for the corpus geniculatum externum. So far as the eye could 
judge, some of the fibres also reached the superior of the qguadrigeminal bodies, but it 
did not divide into the brachia which characterise its human arrangement. 
III. Tae MESENCEPHALON. 
The mesencephalon presented the corpora quadrigemina on its dorsal aspect, and 
each one of these was quite distinctly defined from the other by longitudinal and 
transverse furrows. On its ventral surface the crura cerebri were also well marked. 
Latterly, the corpus geniculatum internum constituted a large oval elevation, larger 
than either of the corpora quadrigemina and separated from them by a deep furrow 
through which many vessels entered the brain substance. The aqueduct of Sylvius 
(fig. 2) was a fairly wide canal, and was not reduced to a T-shaped chink as in man. 
TRANS. ROY. SOC. EDIN., VOL. XLVIII., PART IV. (NO. 30). 123 
