266* ORGANS OF THE SENSES. 



four recti and two ooliqui muscles as in higher vertebrata ; 

 they are very rarely directed to one side of the head, as in 

 the pleuronectes ; sometimes the eye-lids are so perfect as to 

 be provided with distinct orbicular or sphincter muscles 5 

 and in several fishes the upper edge of the iris extends down 

 in form of a vine leaf over the middle of the pupil. The 

 pearly lustre of the exterior choroid seems due to crystalline 

 spicula, and from the deficiency of internal pigment the eyes 

 of many cartilaginous fishes have an internal shining lustre, 

 like the tapetum of the eye of quadrupeds. 



The eyes of amphibious animals are generally interme- 

 diate in their form and structure between those of fishes 

 which receive the rays of light constantly through the dense 

 medium of water, and those of the higher air-breathing 

 classes which receive them through the rare medium of 

 the atmosphere. These organs are still therefore of great 

 size, and flat in front from the small quantity of aqueous 

 humour, especially in the perennibranchiate species which 

 more constantly reside in the water. From the imperfect 

 ossification of the orbit the eyes have great lateral extent 

 of motion, they are provided with an upper and lower eye-lid, 

 and a very complete membrana nictitans, and as in other 

 oviparous vertebrata the lower eye-lid is more developed and 

 more moveable than the upper. The eye is large and provided 

 with a thick and firm sclerotic, as in fishes, in the frog 

 and the toad, but is very small in the pipa where it is 

 destitute of eye-lids, and the crystalline lens is spherical, 

 as in most other amphibia and in fishes. The outer layer 

 of the choroid has the pearly lustre of that of fishes and 

 passes shining over the fore-part of the iris around a pupil 

 still almost motionless, the ciliary processes are not developed, 

 and the retina forms a thick and pulpy external layer like that 

 of fishes. The eyes appear to be still destitute of lachrymal 

 apparatus, as in the former class, and in the proteus the 

 organs of vision are covered over by the opaque integuments 

 of the head. 



The eyes of reptiles are more adapted to receive the 

 rays of light from the rare medium of the atmosphere than 

 those of fishes or amphibia, their cornea is therefore gene- 

 rally more convex, their aqueous and vitreous humours 

 more abundant, and their lens less spherical in form. They 



