A domestic cat has how many lids on each eye?

The inner eyelid of cats--more properly called the palpebra tertia but also known as the nictitating membrane, third eyelid or "haw"--has been regarded by some as a biological curiosity much like the human appendix or wisdom teeth. In fact, some veterinary articles in the early 1900s describe methods for removing this supposedly irrelevant structure so as to facilitate examination of the eye. Despite these perceptions, the third eyelid of cats plays an important role in maintaining the health of their eye surface. In fact, it is so important that among mammals and birds the norm is for a species to have a third eyelid and those lacking one--such as humans and some of our fellow primates--are the true oddities in nature.

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.