Definitions for nictitating membrane
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nictitating membrane, third eyelid(noun)
a protective fold of skin in the eyes of reptiles and birds and some mammals
a transparent protective fold of skin acting as an inner eyelid in birds, reptiles and some mammals
The nictitating membrane is a transparent or translucent third eyelid present in some animals that can be drawn across the eye for protection and to moisten it while maintaining visibility. 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. Some mammals, such as camels, polar bears, seals 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. Fully developed nictitating membranes are found in fish, amphibians, reptiles, birds and mammals, but are rare in primates. In humans, the plica semilunaris 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 present, although fully developed nictitating membranes can be found in lemurs and lorisoid primates. Unlike the upper and lower eyelids, the nictitating membrane moves horizontally across the eyeball. It is normally translucent. In some diving animals, for example beavers and manatees, it moves across the eye to protect it while under water, and in these species it is transparent; in other diving animals, including sea lions, 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-mile-per-hour dives, they will blink repeatedly with their nictitating membranes to clear debris and spread moisture across the eyes. In polar bears, it protects the eyes from snow blindness. In sharks, it protects their eyes while the shark strikes at its prey. Woodpeckers tighten their nictitating membrane a millisecond prior to their beak impacting the trunk of a tree to prevent shaking-induced retinal injury.
U.S. National Library of Medicine
A fold of the mucous membrane of the CONJUNCTIVA in many animals. At rest, it is hidden in the medial canthus. It can extend to cover part or all of the cornea to help clean the CORNEA.
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From our Multilingual Translation Dictionary
- blikhimna, augnhimna, innra augnalokIcelandic
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"nictitating membrane." Definitions.net. STANDS4 LLC, 2015. Web. 27 Nov. 2015. <http://www.definitions.net/definition/nictitating membrane>.