Taken at Arizona Sonoran Desert Museum 2009 ~ Going through the shots I noticed this particular capture of the 3rd eyelid on this raptor ~ a favorite of mine as well as many!
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 the eye while also keeping visibility. Various reptiles, birds, and sharks have a full nictitating membrane, whereas, in many mammals, there is a small vestigial remnant of the membrane present in the corner of the eye. Some mammals, such as camels, polar bears, seals, and aardvarks, also have a full nictitating membrane. It is often called a third eyelid or haw and may be referred to as the plica semilunaris or palpebra tertia in scientific terminology.
Nictitating membranes are found in birds, reptiles, amphibians, fish, but are less common in mammals, mainly being present in monotremes and marsupials. Only one species of primate—the Calabar angwantibo—is known to have a functioning nictitating membrane.