INTRODUCTIOX. 
XXXIll 
shorter, the refracting power of the lens and cornea is decreased and 
diminished. Around the outer coat, a circle of scale-like plates of bone 
guards the eye from injury. But nature, in some birds, has used still 
greater precaution. An hard, osseous, scaly substance projects externally 
above and below the eye of the Puffin, and over the upper part of that of 
the Sparrowhawk and the-Merlin rises an internal, strong, bony projection. 
She has given to most birds a thin, transparent membrane, called the 
nictitating membrane ; which, by means of muscles affixed to the back of 
the sclerotica, can be contracted, or extended, at the will of the bird, so as 
to protect the whole of the exterior of the eye, and cleanse its surface, 
which otherwise, from its habits, would be obnoxious to perpetual injury. 
This membrane is sufficiently transparent for the admission of the rays of 
light ; and though covering, like a curtain, the whole of the exterior of the 
eye, it scarcely obstructs its sight. Though Nature has been so careful in 
endeavouring to protect this organ from injury, she has given to all 
birds, for their greater safety, two eyelids : the under of which, in most 
of the species, is the largest, and ascends to close the eye ; but the Swift, 
the Parrot, the Owl, and the Goatsucker, move both eyelids with equal 
ease. The latter, doomed to nocturnal activity and mid-day slumbers, 
possesses a peculiarly large, elastic, upper eye-lid, which it can fold over its 
eye in many plaits, to preclude the admission of the rays of light, and to 
permit it to doze in total darkness in the blaze of day. At the bottom, 
near the entrance of the optic nerve, lies a triangular receptacle, or purse, 
containing a dense, black fluid : which some persons have supposed to be 
requisite to suffocate the rays of light, as the pupil of the eye of most 
birds admits them in great abundance. Even in man, the inside of the 
choroides is tinged with black, that the rays which pass through it may 
not be reflected back again upon the retina to confound the object. The 
pupils of the eyes of most birds are round. In the nocturnal tribe, they 
sometimes appear elliptical, and in them they readily dilate and lessen, and 
the retina is more tender ; consequently, when they are exposed to a glare 
of Light, the pupil contracts to so small an aperture as to prevent the 
F 
