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 present in some animals that can be drawn across the
eye from the
medial canthus 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
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 ...
s, 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
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. ...
s,
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 spec ...
s,
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
The nictitating membrane 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 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
eyelids, the nictitating membrane moves horizontally across the eyeball.
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 lear ...
in rabbits.
Distribution
Fully developed nictitating membranes are found in
fish
Fish are Aquatic animal, aquatic, craniate, gill-bearing animals that lack Limb (anatomy), limbs with Digit (anatomy), digits. Included in this definition are the living hagfish, lampreys, and Chondrichthyes, cartilaginous and bony fish as we ...
,
amphibian
Amphibians are four-limbed and ectothermic vertebrates of the class Amphibia. All living amphibians belong to the group Lissamphibia. They inhabit a wide variety of habitats, with most species living within terrestrial, fossorial, arbo ...
s,
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 rhynchocephalia ...
s,
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 includin ...
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
lemurs and
lorisoid primates. Some mammals, such as
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. ...
s,
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 spec ...
s,
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, and many mammals retain 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 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
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 a ...
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 falcons 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
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.
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)
See also
*
Accessory abducens nucleus
Accessory abducens nucleus is a small cluster of neurons in the pontine reticular formation of the rat and rabbit.
In the rabbit, motoneurons that are involved in the nictitating membrane response are found in the accessory abducens nucleus.
Cro ...
*
Human vestigiality
References
External links
* {{youTube, dgh6ABfdlAU, Hawk, transparent eyelids (nicitating membrane slow motion video)
Eye
Vertebrate anatomy
fi:Vilkkuluomi