Usually called the ‘third eyelid’, the nictitating membrane is present in some animals that can be drawn across the eye.

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In birds of prey, the membrane also serves to protect the parents’ eyes from their chicks while they are feeding them, and when peregrine falcons go into their 200-mile-per-hour (320 km/h) dives, they will blink repeatedly with their nictitating membranes to clear debris. Woodpeckers tighten their nictitating membrane a millisecond prior to their beak impacting the trunk of a tree to prevent retinal injury.

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The nictitating membrane protects the polar bears’ eyes from UV radiation as the white landscape reflects sunlight can damage the eye which can lead to temporary loss of vision — a condition known as snow blindness.

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The plica semilunaris is a fold of conjunctiva at the inner corner of the human eye. Its likeness to the nictitating membrane, or third eyelid, of other animals led to the idea that it might be the vestige of such a structure, which is still part of the eye in some primates.

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The nictitating membrane is a transparent or translucent third eyelid present in some animals.

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