THE ANATOMY OF THE WEDDELL SEAL. 131 



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: length, 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 MURIE, 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 

 " relatively large." 



The optic tract followed the usual course from the optic chiasma backwards and 

 outwards to wind round the lateral aspect of the crus c~-ebri. 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 internum, 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 genicnlatum externum. So far as the eye could 

 judge, some of the fibres also reached the superior of the quadrigeminal bodies, but it 

 did not divide into the brachia which characterise its human arrangement. 



III. THE 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 cnira 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. 



(ROT. SOC. EDIX. TRAXS., VOL. XLVm., 843.) 



