234 EYE 



midal muscle pulls the nictitating membrane obliquely over the 

 anterior surface of the eye, from the outer lower towards the upper 

 inner angle. Contraction of the quadrate muscle adjusts this 

 motion, and at the same time prevents pressure of the optic nerve. 

 During relaxation of the muscles the nictitating membrane with- 

 draws through its own elasticity. 



The wpipefr and lower eyelids are simply folds of the skin, being 

 attached to the orbital margins and hanging over the eye. The 

 outer surface of these lids is sometimes covered with fine down-liko 

 feathers, as in Sula, more frequently, however, it is naked. The 

 margins of the lids carry sometimes rudimentary feathers without 

 barbs ; such eyelashes being especially well developed in the Ostrich, 

 the Amazon Parrots, in the Hornbills, and Crotophaga. The inner 

 surface of the lids is transformed into a sort of mucous membrane, 

 and is continuous with the conjunctiva. In most birds only the 

 lower eyelid is movable, and contains frequently a rather large 

 saucer -shaped cartilage, the so-called " tarsus palpebralis " ; the latter 

 is large in liatitse. Birds -of -Prey, and Gallinas ; but is absent in 

 Parrots. The eyelids are moved by a circulai- sphincter, a levator, 

 and a, depressor muscle, which partly arise from the walls of the orbit, 

 and are supplied by the oculomotor and trigeminal nerves. In all 

 birds the margins of the upper and lower eyelids are fused to- 

 gether during the greater part of their embryonic stage. The lids 

 become separated either shortly before the bird is hatched, as is 

 the case in most Nidifugae, or the blind condition prevails during 

 a longer or shorter time in the young Nidicolse. 



Glands of the eye. The surface of the cornea is kept moist and 

 bathed by the secretions of two glands situated within the orbit. 



The lacrymal gland rests as a mostly small roundish and reddish 

 body upon each eyeball near the outer or hinder corner of the eye, 

 and opens upon the inner wall of the eyelids through a small 

 slit ; the secretion, the tears spread over the cornea, and are col- 

 lected near the inner corner of the eye through two slits into the 

 wide naso-lacrymal canal, which, situated below the skin and 

 between the lacrymal and nasal bones, opens into the nasal cavity 

 immediately above the choange or inner nares. 



Besides these lacrymal and the Nasal glands (q.v.) birds 

 possess a pair of so-called Harderian glands, which produce a slimy 

 fluid, which escapes below the nictitating membrane at the inner 

 corner of the eye. This gland is yellowish white, always placed 

 within the orbit upon the median and upper surface of the eyeball, 

 and is of an irregular, often considerable size. 



