The nictitating membrane (from Latin nictare, to blink) is a transparent or Translucentthird Eyelidpresent in some animals that can be drawn across the Eyefor protection and to moisten the eye while also keeping visibility. Various reptiles, birds, and Sharkhave a full nictitating membrane, whereas, in many Mammals, there is a small Vestigialremnant of the membrane present in the corner of the eye. Some mammals, such as Camel, Polar_bear, Pinniped, and Aardvark, also have a full nictitating membrane. It is often called a third eyelid or haw and may be referred to as the Plica_semilunaris_of_the_conjunctivaor palpebra tertia in scientific terminology.

Nictitating membranes are found in Bird, Reptile, Amphibian, Fish, but are less common in Mammal, mainly being present in Monotremeand Marsupial.Answers.comIn humans, the plica semilunaris and its associated muscles are thought to be the vestigial remnants of the nictitating membrane and its muscles.Answers.comCharles Darwin reported that the plica semilunaris is slightly larger in Africans and Australian Aborigines than in other peoples.Answers.comOnly one species of Primate---the Calabar_angwantibo---isknown to have a functioning nictitating membrane.Answers.com File:Haliaeetus_leucocephalus_LC0198.jpg File:Haliaeetus_leucocephalus_LC0198.jpg

The nictitating membrane (mid-blink) of a Bald_Eagle

The plica semilunaris is thought to be the vestigial remnant of a nictitating membrane in humans.

Unlike the upper and lower eyelids, the nictitating membrane moves horizontally across the eyeball. It is normally translucent. In some diving animals, for example Beaverand Manatee, it moves across the eye to protect it while under water, and in these species it is transparent; in other diving animals including Sea_lion, 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_falconsgo into their 200 mph dives, they will blink repeatedly with the nictitating membrane to clear debris and spread moisture across the eye. In polar bears it protects the eyes from Snow_blindness. In sharks it protects the eye while the shark strikes at its prey. Woodpeckerstighten their nictitating membrane a millisecond prior to their beak impacting the trunk of a tree in order to prevent their eyes from leaving their sockets.Answers.com

The nictitating membrane in Catand Dogdoes not have many muscle fibers and consequently is not usually visible, and its being chronically visible should be taken as a sign of poor condition or ill health. It can, however, be seen clearly when gently opening the eye of the healthy animal when it is asleep, or pushing down/applying pressure on the eyeball will cause it to appear. In some breeds of dogs, the nictitating membrane can be prone to Prolapse, resulting in a condition called Cherry_eye. Birds can actively control their nictitating membraneAnswers.com

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_conditioningin Rabbit.Answers.com

i hope that answers your question =D

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