Nictating 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 for protection and to moisten the eye while also keeping visibility. Various reptiles, birds, and sharks have a full nictitating membrane, whereas, in many mammals, there is a small vestigial remnant of the membrane present in the corner of the eye. Some mammals, such as polar bears, seals, and aardvarks, also have a full nictitating membrane. It is often called a third eyelid or haw and may be referred to as the plica semilunaris or palpebra tertia in scientific terminology.

Nictitating membranes are found in birds, reptiles, amphibians, fish, but are less common in mammals, mainly being present in monotremes and marsupials. In humans, the plica semilunaris and its associated muscles are thought to be the vestigial remnants of the nictitating membrane and its muscles. Charles Darwin reported that the plica semilunaris is slightly larger in Africans and Australian Aborigines than in other peoples. Only one species of primate&mdash;the Calabar angwantibo&mdash;is known to have a functioning nictitating membrane.

Unlike human 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. In polar bears it protects the eyes from snow blindness. In sharks it protects the eye while the shark strikes at its prey. Woodpeckers tighten their nictitating membrane a millisecond prior to their beak impacting the trunk of a tree in order to prevent their eyes from leaving their sockets.

In cats and dogs, the nictitating membrane 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.

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.