Why do cats have a third eyelid?

Unlike people, cats have three eyelids. If you watch a cat's eyes, you will notice that sometimes a little membrane (the third eyelid) covers up parts of the eye. It comes from the inside corner of the cat's eye (you shouldn't be messing with this by the way - you can see it without doing anything to the cat, just look every once in awhile). It is called a nictitating membrane. It cleans dirt and other junk off of the eye, and it also is used by the cat to narrow the amount of light that hits the cat's eyes (remember, cats can see better than people in the dark so their eyes are more sensitive to light).


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If you see this membrane and your cat is acting sick, it is a bad sign. Sometimes when cats are sick it will just come out and stay there. But that also happens when cats are happy or tired, so you should only worry if the cat is doing something else that seems sickly.

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