252 ANATOMY OF VERTEBRATES. 



The tapetal layer is fibrous, extensive, of almost metallic bright- 

 ness ; in most of a fine green colour ; in a few of a bluish tint, 

 with certain portions, generally toward the bottom of the eye, 

 white : in the Ox the tapetum occupies a broad transverse 

 tract of the choroid. The pupil is transversely oblong, with 

 the upper border somewhat festooned in the Camel, Ox, and 

 Sheep. 



In the Garni vora the relative size of the eyes increases from 

 the Bears to the Cats. The tapetal layer exists in most, and 

 consists of obscurely nucleated cells. In the nocturnal Badger 



o 



it is silvery white ; in the Dog arid Wolf whitish, edged with 

 blue ; in most felines of an amber, or golden, or greenish hue, 

 with a lighter tract of crescentic form, curving round the lower 

 part of the entry of the optic nerve. In the Lion, the greater 

 extent of tapetum is below the nerve ; only a small portion above : 

 the general form of the whole tapetum is broadly crescentic in 

 Felines. In the small crepuscular Cats the pupil contracts to 

 a vertical slit ; in the larger diurnal felines it is circular. The 

 optic nerve penetrates more nearly the axis of the eyeball in 

 Carnivores than in Ruminants : the ciliary folds are long, espe- 

 cially in the Lynx, in which the retina does not reach the 

 meridian of the eyeball : it is also very thin. 



In the nocturnal Quadrumana the main modifications of the eye- 

 ball have been noted ; the large and prominent cornea, the unusu- 

 ally convex lens, the broad iris and circular pupil, and the patch 

 of tapetum, are well exemplified in the dissection of the eyes of 

 Stenops gracilis, in xx, vol. iii. p. 158, no. 1706. I found also a 

 delicate tapetum at the back of the eye in Chiromys ; but the 

 light is less brightly reflected from the living eyes of the Aye- 

 aye than from those of the slow Lemurs. The lens is almost 

 spherical in Perodicticus. In no Lemurine has the retinal spot 

 been found ; but there seems to be a minute fold or crease in its 

 place. This spot, fig. 201, A, due to a thinning there of the 

 retina, defined by a yellowish border, accompanied, usually, in 

 the dead eye, with a slight crease, and situated in or very near 

 the axis of vision, exists in the catarrhine Quadrumana as in 

 Man. The sclerotic seems, in most, to be somewhat thinner than 

 in Man and to take more readily the stain of the choroidal pig- 

 ment after death. In no Quadrumana above the Lemurs is there 

 a tapetum. 



The human eyeball is in some individuals a sphere ; in most 

 the antero-posterior is rather less than the transverse dia- 



