Nictitating membrane

The nictitating membrane (from Latin nictare, to blink) is a transparent or translucent third eyelid present in some animals that can be drawn across the eye from the medial canthus to protect and moisten it while maintaining vision.

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 it while maintaining vision.

[3] Fully developed nictitating membranes are found in fish, amphibians, reptiles, birds and mammals, but are rare in primates.

Woodpeckers tighten their nictitating membrane a millisecond prior to their beak impacting the trunk of a tree to prevent shaking-induced retinal injury.

In some breeds of dogs, the nictitating membrane can be prone to prolapse of the gland of the third eyelid, resulting in a condition called cherry eye.