The Inner Eyelids of Cats

You may have noticed something unusual if you’ve ever watched your cat blink or when they’re fast asleep. Upon inspection, you might have caught a glimpse of a thin membrane as it slides over your cat’s eye. This membrane, called the palpebra tertia or, more commonly, the nictitating membrane, is your cat’s inner or third eyelid. Wild cats like mountain lions and bobcats also have this additional eyelid.

But what is the purpose of this extra eyelid? The third eyelid is large enough to cover the cornea and essentially acts an internal wiper blade. The membrane works to clear away dust and other debris that may accumulate on the cat’s eyes as it moves through tall grass or dust. The third eyelid keeps the cat’s eye moist, while also protecting it from harmful bacteria or fungi that might otherwise occupy the surface of the eye.

When cats are awake and alert, the third eyelid is hidden away within the eye socket. Only a small portion of it is visible in the corner of its eye. However, when the cat is relaxed or asleep, its eye retracts into the skull slightly, resulting in passive movement of the membrane across the surface!

Denise Peterson

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