4 
ment in birds. It seems to act not only as a scraper of the cornea 
for cleansing purposes—like the true lid margins—but probably 
enables some birds, like the eagle, for example, to see through it, 
with a degree of distinctness, even in the blinding sunshine. When 
in the pursuit of food or when engaged in fighting, the bird may 
at short notice draw the curtain of his nictitating membrane and 
thus prevent serious damage to his cornea. When the eye is at rest 
and the true lids are separated, the avian nictitating membrane 
shows only at the inner canthus. Law 1 well describes this third 
lid as an “elastic fibrocartilage prolonged at its inner end as a 
thick prismatic stem and expanded anteriorly into a broad, thin 
expansion with a perfectly smooth, even border, fitting accurately 
on the rounded surface of the eyeball and covered by the mucosa. 
Its thick, deep extremity is continuous, with an abundant cushion 
of adipose tissue which fills the depth of the orbit and extends 
between the muscles.” 
In those mammals—the hoofed animals, for example—which are 
constantly grazing among thistles, nettles and other spiky plants, 
the cornea needs some protection other than lids, and this is prob¬ 
ably found in the membrana nictitans. On the other hand, in the 
marine mammals—whale, seal, etc.—in which a third eyelid can 
serve no useful purpose, it is wanting. This shutter-like mem¬ 
brane is drawn over the cornea chiefly by the musculus pvramidalis, 
attached to it in much the same way as the true lids are moved by 
their muscles. 
The Gland of Harder. —That the cornea of birds may be con¬ 
stantly disinfected and its surface cleaned of foreign bodies of all 
kinds, the gland of Harder, a supplementary lacrimal gland, 
furnishes a copious supply of tears, which are forced out and 
sprayed over the eyeball as the third eyelid sweeps over the anterior 
surface of the globe. This gland is placed beneath the membrana 
nictitans and functionates simultaneouslv with it. 
Both the membrana nictitans and Harder’s gland occasionally 
suffer at the hands of would-be veterinary surgeons. In most acute 
diseases of the eye in birds and in many general conditions, the 
third lid falls down over the cornea. The ignorant operator, in his 
effort to relieve the sick animal, removes the “cataract” without, it 
1. Ophthalmic Record, 1005, p. 440. 
