The nictitating membrane (from Latin '' nictare'', to blink) is a transparent or

translucent In the field of optics, transparency (also called pellucidity or diaphaneity) is the physical property of allowing light to pass through the material without appreciable scattering of light. On a macroscopic scale (one in which the dimensions ...

third

eyelid An eyelid is a thin fold of skin that covers and protects an eye. The levator palpebrae superioris muscle retracts the eyelid, exposing the cornea to the outside, giving vision. This can be either voluntarily or involuntarily. The human eye ...

present in some animals that can be drawn across the eye from the medial

canthus The canthus (pl. canthi, palpebral commissures) is either corner of the eye where the upper and lower eyelids meet. More specifically, the inner and outer canthi are, respectively, the medial and lateral ends/angles of the palpebral fissure. T ...

to protect and moisten it while maintaining vision. All Anura(tailless amphibians), and some reptiles, birds, and

shark Sharks are a group of elasmobranch fish characterized by a cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not fused to the head. Modern sharks are classified within the clade Selachi ...

s have full nictitating membranes; in many mammals, a small,

vestigial Vestigiality is the retention, during the process of evolution, of genetically determined structures or attributes that have lost some or all of the ancestral function in a given species. Assessment of the vestigiality must generally rely on co ...

portion of the nictitating membrane remains in the corner of the eye. Some mammals, such as

cat The cat (''Felis catus'') is a domestic species of small carnivorous mammal. It is the only domesticated species in the family Felidae and is commonly referred to as the domestic cat or house cat to distinguish it from the wild members of ...

s, camels, polar bears, seals and

aardvark The aardvark ( ; ''Orycteropus afer'') is a medium-sized, burrowing, nocturnal mammal native to Africa. It is the only living species of the order Tubulidentata, although other prehistoric species and genera of Tubulidentata are known. Unlik ...

s, 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

third

present in some animals that can be drawn across the eye 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

s, 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 Classical conditioning (also known as Pavlovian or respondent conditioning) is a behavioral procedure in which a biologically potent stimulus (e.g. food) is paired with a previously neutral stimulus (e.g. a triangle). It also refers to the learni ...

in rabbits.

Distribution

Fully developed nictitating membranes are found in

fish Fish are aquatic, craniate, gill-bearing animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish as well as various extinct related groups. Approximately 95% of ...

, amphibians, reptiles,

bird Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweig ...

s and mammals, but are rare in

primate Primates are a diverse order of mammals. They are divided into the strepsirrhines, which include the lemurs, galagos, and lorisids, and the haplorhines, which include the tarsiers and the simians ( monkeys and apes, the latter including ...

s. In humans, the plica semilunaris (also known as the semilunar fold) and its associated muscles are homologous 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

lemur Lemurs ( ) (from Latin ''lemures'' – ghosts or spirits) are wet-nosed primates of the superfamily Lemuroidea (), divided into 8 families and consisting of 15 genera and around 100 existing species. They are endemic to the island of Madagas ...

s and lorisoid primates. Some mammals, such as camels, polar bears, seals and

s, have full nictitating membranes, and many mammals retain a small,

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 land mammals. Tears are made up of water, electrolytes, proteins, lipids, and mucins that form layers on the surface of eyes. The different types of ...

Functions

Common Goldeneye showing nictitating membrane - crop

The nictitating membrane is normally translucent. In some diving animals, including

sea lion Sea lions are pinnipeds characterized by external ear flaps, long foreflippers, the ability to walk on all fours, short and thick hair, and a big chest and belly. Together with the fur seals, they make up the family Otariidae, eared seals. ...

s, it is activated on land, to remove sand and other debris—its function in most animals. In

crocodiles Crocodiles (family Crocodylidae) or true crocodiles are large semiaquatic reptiles that live throughout the tropics in Africa, Asia, the Americas and Australia. The term crocodile is sometimes used even more loosely to include all extant memb ...

, it protects their eyes from water but also hinders their focus under water. In some diving animals, for example beavers and

manatee Manatees (family Trichechidae, genus ''Trichechus'') are large, fully aquatic, mostly herbivorous marine mammals sometimes known as sea cows. There are three accepted living species of Trichechidae, representing three of the four living speci ...

s, 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 falcons go into their dives, they will blink repeatedly with their nictitating membranes to clear debris and spread moisture across the eyes.

Woodpecker Woodpeckers are part of the bird family Picidae, which also includes the piculets, wrynecks, and sapsuckers. Members of this family are found worldwide, except for Australia, New Guinea, New Zealand, Madagascar, and the extreme polar regions. ...

s 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

s. It can also protect the eye from

ultraviolet radiation Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30 PHz) to 400 nm (750 THz), shorter than that of visible light, but longer than X-rays. UV radiation i ...

, similar to its role in polar bears to prevent

snow blindness Photokeratitis or ultraviolet keratitis is a painful eye condition caused by exposure of insufficiently protected eyes to the ultraviolet (UV) rays from either natural (e.g. intense sunlight) or artificial (e.g. the electric arc during welding) ...

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 In medicine, prolapse is a condition in which organs fall down or slip out of place. It is used for organs protruding through the vagina, rectum, or for the misalignment of the valves of the heart. A spinal disc herniation is also sometimes ca ...

of the gland of the third eyelid, resulting in a condition called

cherry eye Cherry eye is a disorder of the nictitating membrane (NM), also called the third eyelid, present in the eyes of dogs and cats.Gelatt, K. N. (2000). ''Essentials of Veterinary Ophthalmology''. Baltimore: Lippincott Williams & Wilkins. Cherry eye ...

.Artem Cheprasov
Why do cats have an extra eyelid?
Guru Magazine. 7 Feb 2013. (Accessed 26 Mar 2013)

References

External links