534 DE. J. MTJUIE OX THE ANATOMY OF THE SEA-LIOX. 



III. Sensory Appakatus. 

 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 canity, 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. Cii-cumferentially 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 nerv^e and vessels during contraction of the 

 choanoid and other ocular muscles. 



a. Eyeball. — 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 

 n-1 inch. Behind (PI. 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 01 at 

 the level of the optic nerved 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 -^ to -^ 

 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. 



