260 
LIFE, IN ITS HIGHER FORMS. 
density of water being so nearly the same as that of the 
aqueous and vitreous humours, these have scarcely any 
power to refract the rays of light; and hence a high mag- 
nifying power is given to the crystalline lens. Its form is 
that to which the very highest possible power is assigned 
— a perfect sphere, and the density of its texture is very 
great. But as tho power of a lens and the nearness of 
its focal point are in the same ratio, it was needful to 
bring the retina, or curtain on which the image is painted, 
very close to the lens ; and this is done by diminishing 
the vitreous humour behind it, and thus flattening the 
sphere ; while a provision is made for maintaining this 
shape in certain plates of bono or cartilage, imbedded in 
the tough coat of the eye, called the sclerotica. 
The eye is never protected by an eyelid in fishes ; the 
pupil is very large and incapable of contraction; and 
another peculiarity is, that (at least in many species) the 
ono eye is moved indepeudently of the other. 
The last organ we have space to notice at present is the 
air-bladder, which is found in most of the bony fishes. It 
is usually of a lengthened form, attached beneath the 
spine; but its shape is subject to some variety. Thus, in 
the Hedgehog fishes it is two-lobed, like a Dutchman’s 
breeches; sometimes it is a double sac; in the great Card 
family, and in the Electric Eels, it is divided into two 
compartments by a transverse partition, which, in the 
former case, is perforated to allow an intercommunication. 
In one of the Cat-fishes ( Pangasius ), it is divided into four 
compartments, and in others into many irregular cells. 
Thus, tho air-bladder closely approaches in structure the 
lowest form of tho lung in air-breathing Vertebrata, as 
