Nictitating membrane
Overview
The nictitating membrane (from Latin nictare, to blink) is a transparent or translucent third eyelid
Eyelid
An 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 eye
Eye
Eyes are organs that detect light and convert it into electro-chemical impulses in neurons. The simplest photoreceptors in conscious vision connect light to movement...

 for protection and to moisten it while maintaining visibility. Some reptiles, birds, and shark
Shark
Sharks 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....

s have a full nictitating membrane; in many mammals, there is a small vestigial remnant of the membrane in the corner of the eye.
Encyclopedia
The nictitating membrane (from Latin nictare, to blink) is a transparent or translucent third eyelid
Eyelid
An 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 eye
Eye
Eyes are organs that detect light and convert it into electro-chemical impulses in neurons. The simplest photoreceptors in conscious vision connect light to movement...

 for protection and to moisten it while maintaining visibility. Some reptiles, birds, and shark
Shark
Sharks 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....

s have a full nictitating membrane; in many mammals, there is a small vestigial remnant of the membrane in the corner of the eye. Some mammals, such as camel
Camel
A camel is an even-toed ungulate within the genus Camelus, bearing distinctive fatty deposits known as humps on its back. There are two species of camels: the dromedary or Arabian camel has a single hump, and the bactrian has two humps. Dromedaries are native to the dry desert areas of West Asia,...

s, polar bear
Polar Bear
The 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 land carnivore and also the largest bear, together with the omnivorous Kodiak Bear, which is approximately the same size...

s, seal
Pinniped
Pinnipeds or fin-footed mammals are a widely distributed and diverse group of semiaquatic marine mammals comprising the families Odobenidae , Otariidae , and Phocidae .-Overview: Pinnipeds are typically sleek-bodied and barrel-shaped...

s, and aardvark
Aardvark
The aardvark is a medium-sized, burrowing, nocturnal mammal native to Africa...

s, 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, amphibian
Amphibian
Amphibians , are a class of vertebrate animals including animals such as toads, frogs, caecilians, and salamanders. They are characterized as non-amniote ectothermic tetrapods...

s, and 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
Homology (biology)
Homology forms the basis of organization for comparative biology. In 1843, Richard Owen defined homology as "the same organ in different animals under every variety of form and function". Organs as different as a bat's wing, a seal's flipper, a cat's paw and a human hand have a common underlying...

 to the nictitating membrane seen in some other mammals and other vertebrates. Only one species of primate
Primate
A primate is a mammal of the order Primates , which contains prosimians and simians. Primates arose from ancestors that lived in the trees of tropical forests; many primate characteristics represent adaptations to life in this challenging three-dimensional environment...

—the Calabar Angwantibo
Calabar Angwantibo
The 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 beaver
Beaver
The beaver is a primarily nocturnal, large, semi-aquatic rodent. Castor includes two extant species, North American Beaver and Eurasian Beaver . Beavers are known for building dams, canals, and lodges . They are the second-largest rodent in the world...

s and manatee
Manatee
Manatees are large, fully aquatic, mostly herbivorous marine mammals sometimes known as sea cows...

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 lion
Sea Lion
Sea lions are pinnipeds characterized by external ear-flaps, long fore-flippers, the ability to walk on all fours, and short thick hair. Together with the fur seal, they comprise the family Otariidae, or eared seals. There are six extant and one extinct species in five genera...

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 (89.4 m/s) 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 blindness
Snow blindness
Photokeratitis or ultraviolet keratitis is a painful eye condition caused by exposure of insufficiently protected eyes to the ultraviolet rays from either natural or artificial sources. Photokeratitis is akin to a sunburn of the cornea and conjunctiva, and is not usually noticed until several...

. 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 shaking-induced retinal injury.

The nictitating membrane in cats and dogs does not have many muscle fibers and 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
Prolapse
Prolapse literally means "to fall out of place", from the Latin prolabi meaning "to fall out". 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...

, resulting in a condition called cherry eye
Cherry eye
Cherry eye is the term used to refer to canine nictitans gland prolapse, a common congenital eye defect 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...

. 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
Classical conditioning
Classical conditioning is a form of conditioning that was first demonstrated by Ivan Pavlov...

in rabbits.

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
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