The
nictitating membrane (from Latin
nictare, to blink) is a transparent or translucent third
eyelidAn eyelid is a thin fold of skin that covers and protects an eye. With the exception of the prepuce and the labia minora, it has the thinnest skin of the whole body. The levator palpebrae superioris muscle retracts the eyelid to "open" the eye. This can be either voluntarily or involuntarily...
present in some animals that can be drawn across the
eyeEyes are organs that detect light, and send electrical impulses along the optic nerve to the visual and other areas of the brain. Complex optical systems with resolving power have come in ten fundamentally different forms, and 96% of animal species possess a complex optical system...
for protection and to moisten the eye while also keeping visibility. Various reptiles, birds, and
sharkSharks are a type of fish with a full cartilaginous skeleton and a highly streamlined body. The earliest known sharks date from more than 420 million years ago, before the time of the dinosaurs....
s 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.
The
nictitating membrane (from Latin
nictare, to blink) is a transparent or translucent third
eyelidAn eyelid is a thin fold of skin that covers and protects an eye. With the exception of the prepuce and the labia minora, it has the thinnest skin of the whole body. The levator palpebrae superioris muscle retracts the eyelid to "open" the eye. This can be either voluntarily or involuntarily...
present in some animals that can be drawn across the
eyeEyes are organs that detect light, and send electrical impulses along the optic nerve to the visual and other areas of the brain. Complex optical systems with resolving power have come in ten fundamentally different forms, and 96% of animal species possess a complex optical system...
for protection and to moisten the eye while also keeping visibility. Various reptiles, birds, and
sharkSharks are a type of fish with a full cartilaginous skeleton and a highly streamlined body. The earliest known sharks date from more than 420 million years ago, before the time of the dinosaurs....
s 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
camelCamels are even-toed ungulates within the genus Camelus. The dromedary or Arabian camel has a single hump, and the Bactrian camel has two humps. They are native to the dry desert areas of western Asia, and central and east Asia, respectively...
s,
polar bearThe polar bear is a bear native largely within the Arctic circle encompassing the Arctic Ocean, its surrounding seas and surrounding land masses. It is the world's largest carnivore species found on land. It is also the largest bear, together with the omnivore Kodiak bear which is approximately...
s,
sealPinnipeds or fin-footed mammals are a widely distributed and diverse group of semi-aquatic marine mammals comprising the families Odobenidae , Otariidae , and Phocidae...
s, and
aardvarkThe Aardvark is a medium-sized, burrowing, nocturnal mammal native to Africa. It is sometimes called "antbear", "anteater", "Cape anteater" , "earth hog" or "earth pig"...
s, 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
birdBirds are winged, bipedal, endothermic , vertebrate animals that lay eggs. There are around 10,000 living species, making them the most numerous tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Birds range in size from the Bee Hummingbird to the ...
s,
reptileReptiles, or members of the class Reptilia, are air-breathing, generally "cold-blooded" amniotes that generally have skin covered in scales or scutes. They are tetrapods and lay amniote eggs, whose embryos are surrounded by the amnion membrane...
s,
amphibianAmphibians , such as frogs, toads, salamanders, newts, and caecilians, are ectothermic animals that metamorphose from a juvenile water-breathing form, to an adult air-breathing form. Though amphibians typically have four limbs, the Caecilians are notable for being limbless. Unlike other land...
s,
fishA fish is any aquatic vertebrate animal that is typically ectothermic , covered with scales, and equipped with two sets of paired fins and several unpaired fins...
, but are less common in
mammalMammals are a class of vertebrate animals whose females are characterized by the possession of mammary glands while both males and females are characterized by sweat glands, hair, three middle ear bones used in hearing, and a neocortex region in the brain.Mammals are divided into three main...
s, mainly being present in
monotremeMonotremes are mammals that lay eggs instead of giving birth to live young like marsupials and placental mammals ....
s and
marsupialMarsupials are an infraclass of mammals, characterized by a distinctive pouch , in which females carry their young through early infancy.- History :...
s. 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
primateA primate is a member of the biological order Primates , the group that contains lemurs, lorisids, galagos, tarsiers, monkeys, and apes, with the last category including great apes. With the exception of humans, who inhabit every continent on Earth, most primates live in tropical or subtropical...
—the
Calabar angwantiboThe Calabar Angwantibo , also known as the Calabar Potto, is a strepsirrhine primate from the Lorisidae family. It shares the genus Arctocebus with the Golden Angwantibo...
—is known to have a functioning nictitating membrane.
Unlike the upper and lower eyelids, the nictitating membrane moves horizontally across the eyeball. It is normally translucent. In some diving animals, for example
beaverThe beaver is a primarily nocturnal, large, semi-aquatic rodent. Castor includes two extant species, Castor canadensis and Castor fiber . Beavers are known for building dams, canals, and lodges . They are the second-largest rodent in the world...
s and
manateeManatees are large, fully aquatic marine mammals sometimes known as sea cows. They are noted for their rather friendly nature, large size and paddle-like flippers. The name manatí comes from the Taíno, a pre-Columbian people of the Caribbean, meaning "breast"...
s, it moves across the eye to protect it while under water, and in these species it is transparent; in other diving animals including
sea lionSea lions are any of seven species in seven genera of modern pinnipeds including one extinct species . Sea lions are characterized by the presence of external ear pinnae , long front flippers, and the ability to walk on all four flippers on land...
s, 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 mph dives, they will blink repeatedly with the nictitating membrane to clear debris and spread moisture across the eye. In polar bears it protects the eyes from
snow blindnessSnow blindness is a painful eye condition, caused by exposure of unprotected eyes to the ultraviolet rays in bright sunlight reflected from snow or ice or less commonly from sea or sand. Fresh snow reflects about 80% of UV radiation compared to a dry, sandy beach or sea foam...
. 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
catThe cat , also known as the domestic cat or housecat to distinguish it from other felines and felids, is a small carnivorous mammal that is valued by humans for its companionship and its ability to hunt vermin and household pests...
s and
dogThe dog is a domesticated form of the Gray Wolf, a member of the Canidae family of the order Carnivora. The term is used for both feral and pet varieties. The domestic dog has been one of the most widely kept working and companion animals in human history...
s, the nictitating membrane no longer has as many muscle fibers as that of their ancestors, like birds, which actively control their nictitating membrane. Consequently, it 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
prolapseProlapse literally means "To fall out of place". In medicine, prolapse is a condition where organs, such as the uterus, fall down or slip out of place. It is used for organs protruding through the vagina or the rectum, or for the misalignment of the valves of the heart...
, resulting in a condition called
cherry eyeCherry eye is the term used to refer to canine nictitans gland prolapse, a common eye condition in various dog breeds where the gland of the third eyelid known as the nictitating membrane prolapses and becomes visible. Commonly affected breeds include the Bulldog, chihuahua, Cocker Spaniel,...
.
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 conditioningClassical conditioning is a form of associative learning that was first demonstrated by Ivan Pavlov. The typical procedure for inducing classical conditioning involves presentations of a neutral stimulus along with a stimulus of some significance...
in
rabbitRabbits are small mammals in the family Leporidae of the order Lagomorpha, found in several parts of the world. There are seven different genera in the family classified as rabbits, including the European rabbit , Cottontail rabbit , and the Amami rabbit...
s.