Nictitating Membrane

The nictitating membrane, also known as the third eyelid, is a translucent or transparent membrane present in the eyes of many animals but vestigial in humans.

Function

Protects and moistens the eye while maintaining visibility
Moves horizontally across the eyeball
Common in birds, reptiles, amphibians, and some mammals (e.g., cats, dogs)

In Humans

Humans retain only a vestigial remnant called the plica semilunaris in the inner corner of the eye
The plica semilunaris has no significant function