Can Cats See Color?

10. Why do cats have an extra eyelid?

Like most animals, cats have an additional set of eyelids located in each eye. These are, not surprisingly, known as "third eyelids," or "nictitating membranes," to give them their technical name. "Nictitate," which is rarely used these days, means wink, and that is precisely what these eyelids do: wink across the eye like a windshield wiper each time the animal blinks. Because animals are a lot closer to the ground than we are, their eyes are exposed to a lot more dirt and debris and therefore need an extra level of protection. We, humans, are actually the odd ones out here, with our two sets of eyelids: one on top, and one on the bottom.