ORGANS OF SIGHT IN REPTILES. 337 



free extremities of which the eyeballs are situated. In all 

 Plagiostomes the eyeball is supported on a cartilaginous peduncle : 

 this is short and broad in the Rays ; longer and cylindrical in the 

 Sharks ; in Selache it is articulated by a ball and socket synovia! 

 joint to a tubercle above, and external to the entry of the optic 

 nerve. 1 A fibrous ligament attaches the sclerotic to the wall of 



O 



the orbit in the Sturgeon and the Salmon. 



The space between the eyeball and the orbit contains a soft bed 

 of gelatinous and adipose substance : but there is no lacrymal 

 gland in Fishes. An apparatus to moisten the cornea was, of 

 course, unnecessary in animals perpetually moving in a liquid 

 medium. The cornea, which in most fishes is always exposed to 

 that medium, is flat ; it is, therefore, less liable to injury in the 

 rapid movements of the fish, and being level with the side of the 

 head, offers no impediment to those movements. This form of 

 cornea diminishes the capacity of the aqueous chamber ; but the 

 aqueous humour is needed only to float the free border of the iris ; 

 and to make up for the small quantity of that humour, the refrac- 

 tive power of the lens is maximised by its spherical form. To 

 compensate for the deviation from the spherical form of the eye- 

 ball, produced by the flattening of its fore-part, and the consequent 

 loss of power to resist external pressure, the sclerotic capsule is 

 cartilaginous or bony. 



65. Organs of Sic/lit in Reptiles.- The eyes are very small, 

 of simple structure, and concealed by the skin, which passes 

 smoothly over them with little other change than subtransparency 

 of texture, in both the ichthyo- and ophio-morphous Batrachia. 

 The sclerotic, in Proteus, is lined by some dark pigment, and 

 contains a minute spherical lens. It may serve to warn the 

 animal, wandering into light, to retreat to the safe darkness of its 

 native subterranean waters. The Axolotl has the eyeball better 

 developed, and provided with muscles ; but devoid of lids. In 

 the Newts there is a horizontal fold of integument over each eye- 

 ball : the retina is thick, although the optic nerve is small : the 

 choroid shows pigment : the pupil is transverse ; the lens is 

 spherical. The cornea is convex in the Land Salamander. In 

 Newts the eyeballs are retracted in water, and are less prominent 

 than in air ; for this purpose there is a kind of choauoid muscle, 

 besides the ordinary recti and obliqui. The eyeball is very small 

 in Pipa, and has no eyelid. In the Frog, the eyeball is propor- 

 tionally large, and is prominent ; the globe is spherical ; the 

 sclerotic of subcartilaginous hardness anteriorly ; elsewhere it 



1 xx. vol. iii. p. 175, prep. no. 1762. 

 VOL. I. Z 



