Where's the nictitating membrane located?

Eyes

Explanation:

Nictitating membrane is located in the eyes. It is knows as the "third eyelid". In some vertebrates, a thin membrane is found beneath the lower eyelid that has the ability to extend and cover the entire eyeball. It is transparent or translucent and thus allows vision to some extent even when it covers the eye.  It also removes dirt from the eye and maintain its moisture content.