Nictitating membrane

Rare card · No. 24305

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. Most Anura (tailless amphibians), some reptiles, birds, and sharks, and some mammals (such as cats, beavers, polar bears, seals, sheep, and aardvarks) have full nictitating membranes; in many other mammals, a small, vestigial portion of the nictitating membrane remains in the corner of the eye. It is often informally called a third eyelid or haw; the scientific terms for it are the plica semilunaris, membrana nictitans, or palpebra tertia. Unlike the upper and lower eyelids, the nictitating membrane moves horizontally across the eyeball. In many species, any stimulus to the eyeball (such as a puff of air) will result in reflex nictitating membrane response. This reflex is widely used as the basis for experiments on classical conditioning in rabbits.

Read on Wikipedia · Image Joxerra aihartza, CC BY-SA 3.0