X 



THE EYE AND ITS APPENDAGES 



The eye of birds is relatively large, and is 

 lodged in an orbit with indifferently developed 

 bony walls. The bony margin of the orbit is 

 incomplete, and the two orbital cavities, in 

 the dried skull, communicate with each other 

 through a defect in the interorbital septum 

 which forms a thin partition between them. 



Of the two eyelids, the lower is the better 

 developed and the more movable. A thin 

 nictitating membrane, or third eyelid, is mainly 

 concealed within the medial angle of the eye 

 when not in use. It is, however, so extensive 

 as to be capable of covering the whole anterior 

 surface of the eyeball, its movements being 

 effected with great rapidity through the action 

 of the pyramidal and quadrate muscles. As- 

 sociated with the membrane is a gland of some 

 size. A delicate and exquisitely sensitive 



97 7 



