28 OPHTHALMOLOGY FOR VETERINARIANS 



tinuous with a pad of fat which is insinuated between all 

 the muscles of the eye. Its internal surface is concave 

 and its external surface is convex. When the eye is 

 in its natural position only the margin of the membrane, 

 covered by conjunctiva, is perceptible, the rest being 

 buried in the ocular sheath; but when, by contraction 

 of the straight muscles, the globe presses upon the pad 

 of fat, the membrane is forced out and covers more or 

 less of the cornea. This movement, which is instan- 

 taneous, is for the purpose of removing any offending 

 agent from the surface of the eye. In some diseases, as 

 tetanus, the membrane is forced outward and remains 

 so. It is vulgarly called the ''haw of the eye" (Vaughn). 



The nictitans is very prominent in fowls and birds, as 

 well as in quadrupeds generally, while in the inhabitants 

 of the sea it is absent. Its function is associated with the 

 "gland of Harder," which is a sort of supplementary 

 lacrimal gland, and furnishes an abundance of unctuous 

 fluid in conjunction with the action of the membrana 

 nictitans. It is described as a ''reddish-yellow gland," 

 and is situated beneath the membrana nictitans, at 

 about the middle of its outer portion. 



As these organs are very essential for the protection of 

 the cornea, they should by no means be interfered with 

 surgically, as is too often done without sufficient reason. 



