Nictitating membrane
The nictitating membrane is a transparent or translucent third eyelid present in some animals that can be drawn across the eye from the medial canthus
Functions in birds
In birds of prey, the membrane also serves to protect the parents’ eyes from their chicks while they are feeding them
When peregrine falcons go into their 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.
Its function in polar bear
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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