Tlie Evolution of Optics. 277 



ample, sharks a nictitating membrane or third eyelid is 

 developed. This is a thin membrane situated at the inner 

 angle of the eye, and which can be drawn across the globe 

 in a direction at right angles to the external lids. We find 

 in some species of reptiles a muscle by which the eye is 

 retracted at will, being prominent when in the air, and with- 

 drawn from danger when in the water. 



The eye of the lizard is peculiar in that it possesses a 

 macula, and the two eyes have an independent movement. 

 Many reptiles present nic- 

 titating membranes, and 

 those living upon land a 

 lachrymal apparatus. 



The eyes of birds (Fig. 

 6) are conspicuous for 

 their large size and prom- 

 inence, as we would nat- 

 urally expect from the 

 habits of the animal. In 

 some aquatic species we 

 find a flat cornea, but, as 

 a rule, the anterior seg- 

 ment of the eye is promi- 

 nent and the cornea con- FIG. 6. 

 vex. 



The coats of the eye are supplemented by a layer of osse- 

 ous plates, which are situated in the sclera ; there also ex- 

 ists a peculiar muscular arrangement which makes traction 

 upon these scleral plates, compressing the globe and render- 

 ing prominent the cornea, thus quickly adapting the eye for 

 near vision. It is alleged that in birds the act of accommo- 

 dation is largely dependent upon this bulging forward of 

 the anterior segment of the eye to produce near vision, and 

 it is possible that the pecten, which is a vascular loop ex- 

 tending into the interior of the eye, is of service in keep- 

 ing up the intra-ocular tension during such extreme changes 

 in form. 



In mammals we never find any development of bone in 

 the sclerotic, and the eyes are usually well protected by a 

 bony orbit. In animals of nocturnal habits the cornea is 

 proportionately larger and more convex ; the lens is likewise 

 more globular, thus adapting the eye for vision at nearer 

 range. In aquatic mammals the eyes more nearly resemble 

 those of fishes in form, the cornea being flat and the lens 



