What is "nictitating membrane"

Nictitate \Nic"ti*tate\, v. i. [See Nictate.] To wink; to nictate.

Nictitating membrane (Anat.), a thin membrane, found in many animals at the inner angle, or beneath the lower lid, of the eye, and capable of being drawn across the eyeball; the third eyelid; the haw.

n. a transparent protective fold of skin acting as an inner eyelid in birds, reptiles and some mammals

n. a protective fold of skin in the eyes of reptiles and birds and some mammals [syn: third eyelid]

Fully developed nictitating membranes are found in fish, amphibians, reptiles, birds and mammals, but are rare in primates. 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 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 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 crocodiles, it protects their eyes from water but also hinders their focus under water. 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.

The nictitating membrane in cats and dogs does not have many muscle fibers, so 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, which 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. The cherry eye is due to a gland called the gland of the third eyelid (nictitans gland), which prolapses. This gland helps to produce up to 50% of the tear film in certain animals. Another gland, called Harder's gland, is present in other animals. Birds can actively control their nictitating membrane.

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 in rabbits.