bio

Nictitating membrane

A translucent or transparent third eyelid that sweeps sideways across the cornea, found in sharks, amphibians, reptiles, birds, and a handful of mammals — seals, sirenians, polar bears. What matters is what it actually does: it protects, rather than improving vision. It shields the eye from grit and from the shearing force of water at speed, and keeps the tear film intact — while still letting light through, so the animal never has to close its eyes. It does not correct the severe farsightedness water causes; that is the lens and the pupil's problem.