cassell's book of birds. 



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 fonvard and the 

 other backward, but the 

 membrane itself, being elas- 

 tic, is capable of being 

 drawn out like a thin sheet 

 of india-rubber, and of re- 

 turning 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 con- 

 nected by a tendon with a 

 muscle situated at the back 

 part of the eye. This ten- 

 don, though strong, is so 

 fine as not to obstruct the 

 sight, even when it passes 

 across the pupil, and the 



Both eyelids are divided through their middle, and everted, so as to display the Nictitating Mem- , , • 1 •. i 



brane, a, and the passage for the tears (fund* Ucrymalia), b. 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.) 



But this is not all. In the arrangement of the muscle which, though placed behind the eye, 

 draws the nictitating membrane in front of it, there is what justly deserves to be called a marvellous 

 piece of mechanical contrivance. The extent of contraction necessary to draw the membrane over 

 the whole front of the visual organ would require a much longer muscle than could have been placed 

 in such a situation ; in order to meet this difficulty, the tendon which draws forth the nictitating mem- 

 brane is made to pass through a loop in another muscle, as represented in the next wood-cut (Fig. 7), 

 where it is evident that, by the simultaneous contraction of both these muscles, the extent of their 

 action when drawing the nictitating membrane over the eye is considerably increased. Neither is this 

 the only advantage derived from so ingenious a contrivance ; were it not for the plan adopted, the 



Fig. 6. — EVE OF AN OWL, SHOWING THE ARRANGEMENT OF THE 

 NICTITATING MEMBRANE. 



