瞬膜
A [i]nictitating membrane[/i] or [i]third eyelid[/i] is a transparent or translucent additional eyelid present in some animals that can be drawn across the eye to protect and moisten it while maintaining vision.  Animals that possess a full nictitating membrane are tailless amphibians, some reptiles, birds, and sharks.  For most mammals the nictitating membrane only has a vestigial portion remaining, but mammals such as cats, camels, polar bears, seals and aardvarks possess full ones.

h4. External links

* "Wikipedia - Nictitating membrane":[https://en.wikipedia.org/wiki/Nictitating_membrane]
third eyelid