Nictitating membrane



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 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 as the  plica semilunaris  or palpebra tertia in scientific terminology.

Fully developed nictitating membranes are found in birds, reptiles, amphibians, fish, and mammals, but are rare in primates.[1 ] [2 ] In humans, the plica semilunaris (also known as the semilunar fold) and its associated muscles are thought to be probably homologous to the nictitating membranes seen in some other mammals and other vertebrates.[3 ] In most primate species, a plica semilunaris is present, although fully developed nictitating membranes can be found in lemurs and lorisiform primates.[4 ] [5 ]