Nictitating membrane

third eyelidnictating membranehawnictitatingnictitating eyelidsemilunar foldhawsnicitating membranetransparent third eyelid
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 from the medial canthus for protection and to moisten it while maintaining vision.wikipedia
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Eyelid

eyelidspalpebralpalpebra
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 from the medial canthus for protection and to moisten it while maintaining vision.
Eyelids can be found in other animals, some of which may have a third eyelid, or nictitating membrane.

Shark

sharksSelachimorphaselachians
Some reptiles, birds, and sharks have full nictitating membranes; in many mammals, a small, vestigial portion of the membrane remains in the corner of the eye.
To protect their eyes some species have nictitating membranes.

Camel

camelsCameluscamel meat
Some mammals, such as cats, camels, polar bears, seals and aardvarks, have full nictitating membranes.
If sand gets lodged in their eyes, they can dislodge it using their transparent third eyelid.

Vestigiality

vestigialvestigerudimentary
Some reptiles, birds, and sharks have full nictitating membranes; in many mammals, a small, vestigial portion of the membrane remains in the corner of the eye.
He listed a number of them in The Descent of Man, including the muscles of the ear, wisdom teeth, the appendix, the tail bone, body hair, and the semilunar fold in the corner of the eye.

Pinniped

sealsealsPinnipedia
Some mammals, such as cats, camels, polar bears, seals and aardvarks, have full nictitating membranes.
As in many mammals and birds, pinnipeds possess nictitating membranes.

Bird

birdsAvesavian
Fully developed nictitating membranes are found in fish, amphibians, reptiles, birds and mammals, but are rare in primates.
Instead the eye is lubricated by the nictitating membrane, a third eyelid that moves horizontally.

Eye

eyesoculareyeball
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 from the medial canthus for protection and to moisten it while maintaining vision.

Harderian gland

Harder's glandHarderianHarderian glands
A gland of the third eyelid (nictitans gland) or Harder's gland are attached to the nictating membranes of some animals and may produce up to 50% of the tear film.
The Harderian gland is a gland found within the eye's orbit that occurs in tetrapods (reptiles, amphibians, birds and mammals) that possess a nictitating membrane.

Plica semilunaris of conjunctiva

plica semilunarisPlica semilunaris of the conjunctivainside corner of the eye
Often called a third eyelid or haw, it may be referred to in scientific terminology as the plica semilunaris, membrana nictitans, or palpebra tertia.
It is the vestigial remnant of the nictitating membrane (the "third eyelid") which is drawn across the eye for protection, and is present in other animals such as birds, reptiles, and fish, but is rare in mammals, mainly found in monotremes and marsupials.

Crocodilia

crocodiliancrocodyliacrocodilians
In crocodiles, it protects their eyes from water but also hinders their focus under water.
When the animal completely submerges, the nictitating membranes cover its eyes.

Lemur

lemursLemuroideaLemurs of Madagascar
In most primate species, a plica semilunaris is present, although fully developed nictitating membranes can be found in lemurs and lorisoid primates.
Lemurs also have a third eyelid known as a nictitating membrane, whereas most other primates have a lesser developed plica semilunaris.

Cherry eye

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.
Cherry eye is a disorder of the nictitating membrane (NM), also called the third eyelid, present in the eyes of dogs and cats.

Woodpecker

Picidaewoodpeckersbird carpenter
Woodpeckers tighten their nictitating membrane a millisecond prior to their beak impacting the trunk of a tree to prevent shaking-induced retinal injury.
During the millisecond before contact with wood, a thickened nictitating membrane closes, protecting the eye from flying debris.

Human vestigiality

Human tailsvestigial remnantevolutionary remnants
* Human vestigiality
These included the muscles of the ear; wisdom teeth; the appendix; the tail bone; body hair; and the semilunar fold, in the corner of the eye.

Peregrine falcon

peregrine falconsFalco peregrinusperegrine
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 200 mph dives, they will blink repeatedly with their nictitating membranes to clear debris and spread moisture across the eyes.
To protect their eyes, the falcons use their nictitating membranes (third eyelids) to spread tears and clear debris from their eyes while maintaining vision.

Transparency and translucency

translucenttransparenttransparency
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 from the medial canthus for protection and to moisten it while maintaining vision.

Anatomical terms of location

ventraldorsalanterior
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 from the medial canthus for protection and to moisten it while maintaining vision.

Canthus

canthicanthalcanthoplasty
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 from the medial canthus for protection and to moisten it while maintaining vision.

Mammal

mammalsMammaliamammalian
Some reptiles, birds, and sharks have full nictitating membranes; in many mammals, a small, vestigial portion of the membrane remains in the corner of the eye.

Cat

domestic catcatsFelis catus
Some mammals, such as cats, camels, polar bears, seals and aardvarks, have full nictitating membranes.

Polar bear

polar bearsUrsus maritimuspolar
Some mammals, such as cats, camels, polar bears, seals and aardvarks, have full nictitating membranes.

Aardvark

aardvarksOrycteropus aferant bear
Some mammals, such as cats, camels, polar bears, seals and aardvarks, have full nictitating membranes.

Classical conditioning

conditioningPavlovian conditioningPavlovian
This reflex is widely used as the basis for experiments on classical conditioning in rabbits.

Fish

fishesfinfishichthyofauna
Fully developed nictitating membranes are found in fish, amphibians, reptiles, birds and mammals, but are rare in primates.

Amphibian

Amphibiaamphibiansamphibious
Fully developed nictitating membranes are found in fish, amphibians, reptiles, birds and mammals, but are rare in primates.