While humans have only 2 eyelids, cats actually have 3.In cats, as in most species, the third eyelid is large enough to completely cover the cornea and acts much like a windshield wiper blade by removing debris from the surface and redistributing tears over the cornea. When the cat is alert, the bulk of the third eyelid is hidden within the eye socket and only a small portion is visible in the inner corner of the eye. When relaxed, during sleep or during blinking, however, retraction of the eyeball by a set of skeletal muscles causes the third eyelid to passively move across the ocular surface from the inner, lower corner of the eye to the upper, outer corner…
The exact function of the third eyelid in cats is not completely known but it is believed to help protect a very large cornea from injury as cats move through tall grass or capture prey. Additionally the presence of an accessory tear gland allows for even greater tear production and rinsing of the ocular surface than is found in primates. As this portion of the tear film flows over the lymphoid follicles covering the surface of the third eyelid, a variety of immunologic mediators, including secretory IgA and lactoferrin, are dumped into the tear film to bathe and immunologically protect the ocular surface from the stew of bacteria and fungi that inhabit the surface of even a normal eye.
The third eyelid is also believed to help keep the surface of the eye moister by holding the tear film against the cornea better than the eyelids do by themselves. Loss of the third eyelid through trauma or in the treatment of neoplasia frequently results in chronic irritation of the cornea and remaining conjunctiva.
Link & Image: Scientific American
Tags: Eyelid | Cat | Nature
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