Many species of land animals have a nictitating membrane, which can move across the eyeball to give the sensitive eye structures additional protection in particular circumstances. It is often called a third eyelid or haw. This article or section does not cite its references or sources. ...

The nictitating membrane of a chicken.

Nictitating membranes are found in crocodiles, lizards, birds, frogs, most species of sharks, and some species of mammals. In humans, the nictitating membrane is the apparently useless pink lump in the inner corner of the eyes; it is permanently folded into that corner and no longer functions, apparently rendered redundant by evolution some time in the past. Unlike human 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. In polar bears it protects the eyes from snow blindness. In sharks it protects the eye while the shark strikes at its prey. Image File history File links This is a chicken blinking. ... Binomial name Gallus gallus A chicken (Gallus gallus) is a type of domesticated bird which is often raised as a type of poultry. ... Genera Crocodylus Osteolaemus Tomistoma See full taxonomy. ... Families Many, see text. ... Orders Many - see section below. ... Distribution of frogs (in black) Suborders Archaeobatrachia Mesobatrachia Neobatrachia - List of Anuran families A frog is an amphibian in the order Anura (meaning tail-less from Greek an-, without + oura, tail). ... Orders Carcharhiniformes Heterodontiformes Hexanchiformes Lamniformes Orectolobiformes Pristiophoriformes Squaliformes Squatiniformes Sharks are a group (superorder Selachimorpha) of fish, with a full cartilaginous skeleton, a streamlined body plan, with normally 5, but up to 7 (depending on species) gill slits along the side of, or beginning slightly behind, the head (in some... Orders Subclass Multituberculata (extinct) Plagiaulacida Cimolodonta Subclass Palaeoryctoides (extinct) Subclass Triconodonta (extinct) Subclass Eutheria (includes extinct ancestors)/Placentalia (excludes extinct ancestors) Afrosoricida Artiodactyla Carnivora Cetacea Chiroptera Cimolesta (extinct) Creodonta (extinct) Condylarthra (extinct) Dermoptera Desmostylia (extinct) Embrithopoda (extinct) Hyracoidea Insectivora Lagomorpha Litopterna (extinct) Macroscelidea Mesonychia (extinct) Notoungulata (extinct) Perissodactyla Pholidota Plesiadapiformes... This article or section does not cite its references or sources. ... Trinomial name Homo sapiens sapiens Linnaeus, 1758 Humans, or human beings, are biologically classified as bipedal primates belonging to the mammalian species Homo sapiens (Latin for wise man or thinking man) under the family Hominidae (the great apes). ... An eyelid is a thin fold of skin and muscle that covers and protects an eye. ... Species C. canadensis C. fiber Beavers are semi-aquatic rodents native to North America and Europe. ... Species Trichechus inunguis Trichechus manatus Trichechus senegalensis Manatees (family Trichechidae, genus Trichechus) are large aquatic mammals sometimes known as sea cows. ... Genera Eumetopias Zalophus Otaria Neophoca Phocarctos A sea lion is any of several marine mammals of the family Otariidae. ... Binomial name Ursus maritimus Phipps, 1774 The Polar Bear (Ursus maritimus), also known as the white bear, northern bear, or sea bear, is a large bear native to the Arctic. ... Snowblind redirects here. ... Orders Carcharhiniformes Heterodontiformes Hexanchiformes Lamniformes Orectolobiformes Pristiophoriformes Squaliformes Squatiniformes Sharks are a group (superorder Selachimorpha) of fish, with a full cartilaginous skeleton, a streamlined body plan, with normally 5, but up to 7 (depending on species) gill slits along the side of, or beginning slightly behind, the head (in some...

In cats and dogs, the nictitating membrane is not usually visible, and its appearance is a sign of poor condition or ill health. Trinomial name Felis silvestris catus (Linnaeus, 1758) The cat, also called the domestic cat or house cat, is a small feline carnivorous mammal of the subspecies Felis silvestris catus. ... Trinomial name Canis lupus familiaris (Linnaeus, 1758) This article is about the domestic dog. ...

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. Classical conditioning (also Pavlovian conditioning or respondent conditioning) is a type of associative learning. ... Genera Pentalagus Bunolagus Nesolagus Romerolagus Brachylagus Sylvilagus Oryctolagus Poelagus Rabbits are small mammals in the family Leporidae, found in many parts of the world. ...

External links

• BioMEDIA site including an animation of a seal's nictitating membrane
• "Nictating Membrane" from PetCat.US