The Third Eye Font Size: [small] [medium] [large] Cats' upper and lower eyelids, like those of humans, sheathe the eyeballs. For further protection, all cats have an opaque, white third eyelid, called the nictitating membrane, between the lower lid and inside corner of each eye. This layer helps moisten the eye and clear dust from the surface of the cornea. When dozing, this third eyelid closes, perhaps to act as a shade. As soon as the sleeping cat is alerted by any sound, the nictitating membrane flicks back to the inside corner of the eye. Hidden from view behind the outer eyelids of a sleeping cat and barely noticeable in the corner of the eyes while the cat is awake, the nictitating membranes are sometimes visible in felines that doze with open eyelids. While most cats do sleep with their eyes completely closed, some rest with their eyes partially open. Cats with very short noses, such as Persians, may not be physically able to close their eyes completely because the eyes bulge more, causing the nictitating membranes to be visible during sleep. The membranes also are seen when cats blink, which, quite mysteriously, they do infrequently — sometimes as little as once every few minutes. < back to original page