*The nictitating membrane is a transparent or translucent third eyelid present in some animals that can be drawn across the eye for protection and to moisten the eye while also keeping visibility...In sharks it protects the eye while the shark strikes at its prey. Woodpeckers tighten their nictitating membrane a millisecond prior to their beak impacting the trunk of a tree in order to prevent their eyes from leaving their sockets.

*From http://en.wikipedia.org/wiki/Nictitating_membrane




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