conjunctiva

noun

1. A clear mucous membrane that lines the inner surface of the eyelid and the exposed surface of the eyeball or sclera.

   “There is a third inner eyelid, highly developed and of beautiful mechanism: this is the nictitating membrane, or "winker" (nictito, I wink), a delicate, elastic, translucent, pearly-white fold of the conjunctiva. While the other lids move vertically and have a horizontal commissure, the winker sweeps horizontally or obliquely across the ball, from the side next the beak to the opposite.”