The nictitating membrane is attached at the eye's inner corner. It is a thin, translucent membrane that can be drawn horizontally across the eye to protect it, to remove sand and other debris or to provide moisture while permitting vision.
Nictitating membranes are also called third eyelids or haws. They are found in many species of mammals, birds, amphibians, reptiles and fish. In some species, nictitating membranes serve highly specialized functions. In polar bears, they protect against snow blindness, while in woodpeckers they stabilize the eye from shaking and damaging the retina as the woodpecker pecks. In cats and dogs, an exposed nictitating membrane is usually a sign of injury or poor health.