Canthus in the context of "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. 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.