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 for protection and to moisten it while maintaining visibility. Some reptiles, birds, and sharks have full nictitating membranes; in many mammals, a small, vestigial portion of the membrane remains in the corner of the eye. Some mammals, such as camels, polar bears, seals, and aardvarks, have full nictitating membranes. Often called a third eyelid or haw, it may be referred to as the plica semilunaris or palpebra tertia in scientific terminology.
Fully developed nictitating membranes are found in birds, reptiles, amphibians, fish, and mammals, but are rare in primates. In humans, the plica semilunaris (also known as the semilunar fold) and its associated muscles are thought to be probably homologous to the nictitating membranes seen in some other mammals and other vertebrates. In most primate species, a plica semilunaris is present, although fully developed nictitating membranes can be found in lemurs and lorisiform primates.
Unlike the upper and lower eyelids, the nictitating membrane moves horizontally across the eyeball. It is normally translucent. In some diving animals, for example beavers and manatees, it moves across the eye to protect it while under water, and in these species it is transparent; in other diving animals, including sea lions, it is activated on land, to remove sand and other debris. This is its function in most animals. In birds of prey, it also serves to protect the parents' eyes from their chicks while they are feeding them, and when peregrine falcons go into their 200-mile-per-hour (320 km/h) dives, they will blink repeatedly with their nictitating membranes to clear debris and spread moisture across the eyes. In polar bears, it protects the eyes from snow blindness. In sharks, it protects their eyes while the shark strikes at its prey. Woodpeckers tighten their nictitating membrane a millisecond prior to their beak impacting the trunk of a tree to prevent shaking-induced retinal injury.
The nictitating membrane in cats and dogs does not have many muscle fibers, so consequently is not usually visible, and its being chronically visible should be taken as a sign of poor condition or ill health. It can, however, be seen clearly when gently opening the eye of the healthy animal when it is asleep, or pushing down/applying pressure on the eyeball will cause it to appear. In some breeds of dogs, the nictitating membrane can be prone to prolapse, resulting in a condition called cherry eye. Birds can actively control their nictitating membrane.
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.