. On the anatomy of vertebrates. Vertebrates; Anatomy, Comparative; 1866. 338 ANATOMY OF VERTEBRATES. 220 allows the colour of tlie choroid to be seen through it: the cornea is very convex. The choroid has an argentine or nacreous layer externally, and a dark pigment internally ; the former gives the bright colour to the iris in both Frogs and Toads. The pupil is subrhomboidal. A slightly j^licated ciliary circle adheres to the capsule of the lens. Tiae retina is thick, and is continued to the capsvile of the crystalline, which forms a small spheroid lens. Besides the usual muscles of the eyeb
. On the anatomy of vertebrates. Vertebrates; Anatomy, Comparative; 1866. 338 ANATOMY OF VERTEBRATES. 220 allows the colour of tlie choroid to be seen through it: the cornea is very convex. The choroid has an argentine or nacreous layer externally, and a dark pigment internally ; the former gives the bright colour to the iris in both Frogs and Toads. The pupil is subrhomboidal. A slightly j^licated ciliary circle adheres to the capsule of the lens. Tiae retina is thick, and is continued to the capsvile of the crystalline, which forms a small spheroid lens. Besides the usual muscles of the eyeball, there is a choanoid muscle ; the eyes are strongly retracted when the Frog dives. The chief nictitating lid is the lower one ; the upper eyelid merely follows the movements of the eyeball when it is turned down. A small mviscle arising from the lower and back part of the eyeball sends two tendons through the choanoid, which wind over the sides of the ball to a pulley at each angle of the orbit, through which they ])ass to beattached to the anglesof the lower lid: this is transparent. The eyes are small in Serpents: the sclerotic is fibro-carti- lngino\is, but tliin : the choroid resemldes that in the Frog, but with less brilliancy of the argentine layer: the ciliary plica; are small and feeble : there is a delicate falcifonii pro- cess, without pigment: the lens is more spheroid than in Lizards: the pupil is round in most Serpents; but is a ver- tical slit in venomous Snakes, in Boidcp, and in the nocturnal species of Dipsa- (lidce; and is horizontal in most species oi Drijoplds, especially those which have tlie muzzle pointed and prolonged. But the chief pecidiarity in the ophidian oro-an of vision is in its defensive part, fi"'. 220. The intesru- ment, c, is continued from the surrounding circles of scales, d, directly over the eye: it consists of a layer of transparent epiderm, and a thin layer of chorium, which adheres to the outer ])art of the conjunctive sac,/.
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