234 EVES 
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 upper 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-like 
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 Ratite, Birds-of-Prey, and Galline; but is absent in 
Parrots. The eyelids are moved by a circular 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 Nidifugz, or the blind condition prevails during 
a longer or shorter time in the young Nidicole. 
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 choanz 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 oz the eyeball, 
and is of an irregular, often considerable size. 
