The Nictitating Membrane is a third eyelid that is present in many species, even domestic Dogs and Cats, and is used generally as an extra protection for the eye. The transparency of the membrane varies subject to the density of muscle fibers in the membrane, enabling vision to be maintained even when it is closed. The Peregrine Falcon, when diving for prey at impressive speeds in excess of 300 kilometers an hour, uses continuous blinking of the nictitating membrane to keep moisture over the eye and to clear debris. A Shark blinks the nictitating membrane just moments before striking for prey to protect the eye.
This photo is of an African Hawk-eagle (Aquila spilogaster) blinking the nictitating membrane as it was taking off.
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