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,/. At the exuviating period, the c]u- 

 derm, c, becomes opake, and is shed in connection with that of the 

 head and body. The conjunctiva covers a great proportion of the 

 (>ycbull, a, before it is reflected, as at c, r, forward to line the 

 autocnlar tegument, r. The cavity, /", is large, and receives the 

 lacrymal secretion. In the Pythons and Colubers, a pore at the 

 lower and forepart of the cavity, very minute in many species, 

 Init admitting a bristle in Pi/tlioii, leads to a slender membranous 

 duct, which dilates into a pouch conununicating with the mouth 

 ))eliind the iiremaxillary. In the Viper and other ^■onomous 

 Scrpcnis, tlie lacrymal cnn:il opens into the nasal mealus. The 



