534 DR. J. MURIE ON THE ANATOMY OF THE SEA-LION. 
III. Sensory APPARATUS. 
1. Organ of Vision. The Eye and its Appendages. 
In treating of the external characters generally, the peculiarities and appearances of 
the outward portions of the organ of vision have been described in Part ii. It here 
remains for me to take into consideration the contents of the orbit. The osseous 
orbital cavity, as has been shown, is deficient in its posterior bony marginal ring. 
This deficiency as regards the orbital contents, however, is made good by a bridge 
of fibrous tissue stretching between the postorbital process of the frontal bone and the 
fronto-orbital spur of the malar bone, the fleshy fibres of the temporal muscle, more- 
over, materially strengthening this otherwise weak boundary. Circumferentially the 
eyeball and its muscles are well cushioned with fat ; but a small hemispherical separate 
mass surrounds the optic nerve and middle of the back of the eye. The latter portion 
doubtless relieves pressure on the optic nerve and vessels during contraction of the 
choanoid and other ocular muscles. 
a. Hyeball.—The globe of the eye is of good size in proportion to the body of the 
animal. ‘It is not perfectly spherical, but slightly wider across than from before back- 
wards, the average diameter being about 1 inch. The sclerotic is altogether remarkably 
dense and strong, and, as in the Earless Seals, of very unequal thickness. As in them, 
the middle portion or zone immediately behind the iris is thinnest, namely about a 
line deep, while in front, between this and the cornea, it increases to fully more than 
0-1 inch. Behind (Pl. LXXIX. fig. 49), the sclerotic bulges in a crescentic manner, 
both above and below the optic nerve. The portion above the nerve is slightly the 
longer and thicker of the two, being 1:08 inch thick at its middle, but only 0-1 at 
the level of the optic nerve’. To the naked eye or with an inch lens, the sclerotic 
tunic in the preserved specimens is seen to be composed of an interlacement of 
fine white glistening fibro-elastic tissue, resembling that of a thick tendon. At its 
junction with the cornea the network-like arrangement ceases, the cornea itself 
appearing as a continuation of the outermost of these, but in a compact linear series. 
The cornea is nearly circular in outline, 1 inch in diameter, and ranging from 35 to 35 
of an inch in thickness, the centre being the thinnest portion. It is only moderately 
convex. 
The choroidal portion of the middle tunic is a thin uniform layer, very vascular, and 
with an abundance of dark pigment, overlain internally by a large iridescent tapetum. 
As Leydig has observed in Carnivora, the tapetum is composed of irregularly shaped 
cells and granular matter. 
In the live animal the pupil is subject to great variations of size and shape (as illus- 
trated by the previous diagrams); but in the dissected eye it is found to be pyri- 
form, the narrow end below. The so-called sphincter of the pupil is very distinct 
‘ See Dr. Lightbody’s remarks on these structures in Mammalia, Journal of Anat. Cambridge, 1867, i. p. 15. 
