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 Eyes are organs of the visual system. They provide living organisms with vision, the ability to receive and process visual detail, as well as enabling several photo response functions that are independent of vision. Eyes detect light and conv ...

from the medial canthus to protect and moisten it while maintaining vision. All Anura(tailless amphibians), and some reptiles, birds, and sharks have full nictitating membranes; in many

mammal Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or ...

s, a small, vestigial portion of the nictitating membrane remains in the corner of the eye. Some mammals, such as cats,

camel A camel (from: la, camelus and grc-gre, κάμηλος (''kamēlos'') from Hebrew or Phoenician: גָמָל ''gāmāl''.) is an even-toed ungulate in the genus ''Camelus'' that bears distinctive fatty deposits known as "humps" on its back. C ...

polar bear The polar bear (''Ursus maritimus'') is a hypercarnivorous bear whose native range lies largely within the Arctic Circle, encompassing the Arctic Ocean, its surrounding seas and surrounding land masses. It is the largest extant bear specie ...

seals Seals may refer to: * Pinniped, a diverse group of semi-aquatic marine mammals, many of which are commonly called seals, particularly: ** Earless seal, or "true seal" ** Fur seal * Seal (emblem), a device to impress an emblem, used as a means of a ...

and aardvarks, have full nictitating membranes. Often called a third eyelid or haw, it may be referred to in scientific terminology as the ''plica semilunaris'', ''membrana nictitans'', or ''palpebra tertia''.

Description

Nictitating membrane - Nickhaut - Membrana nittitante - (Balearica pavonina - Weltvogelpark 2013)

The nictitating membrane is a transparent or translucent third eyelid present in some animals that can be drawn across the

for protection and to moisten it while maintaining vision. The term comes from the Latin word '' nictare'', meaning "to blink". It is often called a ''third eyelid'' or ''haw'', and may be referred to in scientific terminology as the ''plica semilunaris'', ''membrana nictitans'', or ''palpebra tertia''. Unlike the upper and lower eyelids, the nictitating membrane moves horizontally across the eyeball. Third eyelid on a domestic dog

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.

Distribution

Fully developed nictitating membranes are found in fish,

amphibian Amphibians are tetrapod, four-limbed and ectothermic vertebrates of the Class (biology), class Amphibia. All living amphibians belong to the group Lissamphibia. They inhabit a wide variety of habitats, with most species living within terres ...

reptile Reptiles, as most commonly defined are the animals in the class Reptilia ( ), a paraphyletic grouping comprising all sauropsids except birds. Living reptiles comprise turtles, crocodilians, squamates (lizards and snakes) and rhynchocephalians ( ...

s, birds and mammals, but are rare in primates. In humans, the plica semilunaris (also known as the semilunar fold) and its associated muscles are

homologous Homology may refer to: Sciences Biology *Homology (biology), any characteristic of biological organisms that is derived from a common ancestor *Sequence homology, biological homology between DNA, RNA, or protein sequences * Homologous chrom ...

to the nictitating membranes seen in some other mammals and other vertebrates. In most primate species, a plica semilunaris is generally not present, although fully developed nictitating membranes can be found in lemurs and

lorisoid Lorisoidea is a Taxonomic rank, superfamily of nocturnal primates found throughout Africa and Asia. Members include the galagos and the Lorisidae, lorisids. As Strepsirrhini, strepsirrhines, lorisoids are related to the lemurs of Madagascar and ar ...

primates. Some mammals, such as

and aardvarks, have full nictitating membranes, and many mammals retain a small, vestigial portion of the membrane in the corner of the eye. A gland of the third eyelid (nictitans gland) or Harder's gland is attached to the nictating membranes of some animals and may produce up to 50% of the

tear film Tears are a clear liquid secreted by the lacrimal glands (tear gland) found in the eyes of all Mammal, land mammals. Tears are made up of water, electrolytes, proteins, lipids, and mucins that form layers on the surface of eyes. The different ty ...

Functions

Common Goldeneye showing nictitating membrane - crop

The nictitating membrane is normally translucent. In some diving animals, including sea lions, it is activated on land, to remove sand and other debris—its function in most animals. In crocodiles, it protects their eyes from water but also hinders their focus under water. In some diving animals, for example

beaver Beavers are large, semiaquatic rodents in the genus ''Castor'' native to the temperate Northern Hemisphere. There are two extant species: the North American beaver (''Castor canadensis'') and the Eurasian beaver (''C. fiber''). Beavers ar ...

s and manatees, it is transparent and moves across the eye to protect it while under water. Birds can actively control their nictitating membrane. In birds of prey, the membrane also serves to protect the parents' eyes from their chicks while they are feeding them, and when

peregrine falcon The peregrine falcon (''Falco peregrinus''), also known as the peregrine, and historically as the duck hawk in North America, is a Cosmopolitan distribution, cosmopolitan bird of prey (Bird of prey, raptor) in the family (biology), family Falco ...

s go into their dives, they will blink repeatedly with their nictitating membranes to clear debris and spread moisture across the eyes. Woodpeckers tighten their nictitating membrane a millisecond prior to their beak impacting the trunk of a tree to prevent shaking-induced retinal injury.Wygnanski-Jaffe T, Murphy CJ, Smith C, Kubai M, Christopherson P, Ethier CR, Levin AV. (2007) Protective ocular mechanisms in woodpeckers ''Eye'' 21, 83–89. The nictitating membrane can be used to protect the eye while attacking prey, as in sharks. It can also protect the eye from ultraviolet radiation, similar to its role in polar bears to prevent snow blindness.

Vestigiality

Nictitating membranes in cats and dogs do not have many muscle fibers, so they are not usually visible; chronic visibility should be taken as a sign of poor condition or ill health. The membrane can, however, be seen clearly when gently opening the eye of the healthy animal when it is asleep, or by pushing down/applying pressure on the eyeball, which will cause it to appear. In some breeds of dogs, the nictitating membrane can be prone to prolapse of the gland of the third eyelid, resulting in a condition called cherry eye.Artem Cheprasov
Why do cats have an extra eyelid?
Guru Magazine. 7 Feb 2013. (Accessed 26 Mar 2013)

References

External links