Another admirable contrivance peculiar to the feathered race, is the existence of a thin, semi-transparent veil, which, when requisite, can be instantaneously drawn over the front of the eye. This apparatus, generally known as the "nictitating membrane," is useful for a variety of purposes; it sweeps over the eye to cleanse it from dust, it diffuses the tears which keep it bright and polished, it will act as a screen to shut out the too great intensity of light, so that with its assistance the eagle can confront the sun even at noon-day; it will likewise defend the eye from sudden injuries, and yet, even when drawn like a curtain over the pupil, not shut out the light. The commodious manner in which this membrane lies folded up in the inner corner of the eye, and the quickness with which it executes its purpose, are known to every observer; but what is equally admirable, though not quite so obvious, is the employment of two kinds of material, and the combination of two kinds of force, by which the movements of this membrane are effected. It is not, as in ordinary cases, by the action of two antagonist muscles, the one pulling it forward and the other backward, but the membrane itself, being elastic, is capable of being drawn out like a thin sheet of india-rubber, and of returning to its former position when the force acting upon it is removed. Such being its nature, in order to adapt it for its office it is connected by a tendon with a muscle situated at the back part of the eye. This tendon, though strong, is so fine as not to obstruct the sight, even when it passes across the pupil, and the muscle which moves it being situated deeply within the orbit, derives from its situation the advantage of not only being secure from injury, but of being out of the way, which it hardly would have been in any position that could be assigned to it in the front of the eye, where its function really lies. When this muscle contracts, the membrane, by means of the communicating tendon, is instantly drawn, as it were, by a thread, over the transparent cornea, and when the muscle ceases to act, the elasticity of the membrane is sufficient to bring it back into its former position. (See Fig. 6.) [Illustration: Fig. 6.--EYE OF AN OWL, SHOWING THE ARRANGEMENT OF THE NICTITATING MEMBRANE. Both eyelids are divided through their middle, and everted, so as to display the Nictitating Membrane, _a_, and the passage for the tears (_puncta lacrymalia_), _b_.]