The Nictitating membrane 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 while 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 ardvarks 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 more advanced applications.
Nictitating membranes are found in birds, reptiles, amphibians, fishes, but is less common in mammals, mainly being present in monotremes and marsupials. In humans, the nictitating membrane is the pink lump in the inner corner of the eyes; it is permanently folded into that corner and no longer functions, apparently rendered redundant by evolution some time in the past. Its associated muscles are also vestigial (see human vestigiality). Those of Africans and Australian Aborigines have been said to have slightly larger than other peoples. Only one species of primate -- the Calabar angwantibo -- 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. 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.
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