ANATOMY OF VERTEBRATES. 



allows the colour of the 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 plicated ciliary circle adheres to the 

 capsule of the lens. The retina is thick, and is continued to the 

 capsule 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 muscle 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 

 pass to be attached to the angles of the lower lid : this is transparent. 

 The eyes are small in Serpents : the sclerotic is fibre-carti- 

 laginous, but thin : the choroid resembles that in the Frog, but 



220 



with less brilliancy of the argentine 

 layer: the ciliary plica? are small and 

 feeble : there is a delicate falciform 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 Boidce, 

 and in the nocturnal species of Dipsa- 

 didce ; and is horizontal in most species 

 of Dryophis, especially those which have 



Dingrammatic section of the eye of a the muzzle pointed and prolonged. Bllt 



Viper, magu. ccxiv. 



the chief peculiarity in the ophidian 

 organ of vision is in its defensive part, fig. 220. The integu- 

 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 

 part of the conjunctive sac,/. At the exuviating period, the epi- 

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

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

 eyeball, a, before it is reflected, as at e, e, forward to line the 

 antocular tegument, c. 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, 

 but admitting a bristle in Python, leads to a slender membranous 

 duct, which dilates into a pouch communicating with the mouth 

 behind the premaxillary. In the Viper and other venomous 

 Serpents, the lacrymal canal opens into the nasal mcatus. The 



