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 the 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 bone 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 

 one eye is moved independently 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, the air-bladder closely approaches in structure the 

 lowest form of the lung in air-breathing Vertebrata, as 



