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. Movement of the third eyelid in cats is also partially regulated by the sympathetic nervous system as well as by smooth muscle cells within the third eyelid.
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.