The haw or nictitating membrane is a thin, third eyelid present in many animals. It moves across the eye horizontally, reaching from the inner corner to the outside of the eye and back. When not in use, the haw folds into the corner of the eye and is barely visible. Humans once had them, but they are now permanently folded away into the pinkish bulb at the inner corner of the eye.
The haw serves various purposes depending on the species under consideration, but in all cases, it protects the eye from injury of one form or another. Specialized uses have evolved over time as required by each species. As sharks attack to feed, the eyes roll back into the head and the haw moves across the eye to protect it from thrashing prey. It serves as a basic shield from injury. This is also true of birds of prey when feeding their chicks. The eyelid protects the eye from over-eager youngsters pecking for food.
Diving animals like beavers and manatees utilize the haw to protect the eyes when underwater. This type of eyelid is transparent and serves the same purpose as built-in goggles. The polar bear’s is specially suited to filter ultra-violet rays in order to prevent snow blindness. You could say the polar bear has naturalized sunglasses.
Mammals that live between sand and sea, such as sea lions and harbor seals, use the haw to wipe away sand and grit from the eyes. These eyelids function like built-in windshield wipers. Lizards, crocodiles, rabbits, birds and many other animals also have a haw. In most cases, it cleans the eye, keeps it from drying out, and protects it from wind and dust.
While cats and dogs also have a haw, if it is continually visible, it is usually a sign of injury or illness. A cat’s is white and is sometimes drawn halfway across the eye when relaxing. However, a listless cat with a visible haw or a cat that does not withdraw it should be taken to a veterinarian without delay. A dog’s haw rises almost diagonally across the eye and is dark in color, though it may become reddish if infected.
The haw is a popular feature of aliens in science fiction, giving creatures a reptilian feel. The human-like Vulcans of Star Trek are also attributed a third eyelid in sci-fi culture.