A nictitating membrane, a third eyelid, is a translucent or opaque membrane found in some animals, including birds, reptiles, and certain mammals.
This membrane can move horizontally across the eye and serves various functions, such as protecting the eye, moistening it, and potentially aiding in camouflage. In many birds, the nictitating membrane can shield the eye while maintaining some degree of visibility. Its movement is involuntary for some animals; others consciously control it.
The featured birds, in order: Whooper Swan, Japanese Night Heron, Saddle-Billed Stork, Golden Pheasant, Green Aracari, Grey Heron, White-Tailed Eagle, Burmese Peacock-Pheasant, Toco Toucan, Common Peafowl, Secretary Bird, Victoria Crowned Pigeon, Cassowary, Rock Eagle Owl, Vulturine Guineafowl, Red-Tailed Hawk, Black-Necked Stilt, Shoebill.
Some historical and anatomical background from this 2017 Indian Journal of Ophthalmology case report:
“The first anatomical description of nictitating membrane in the eye was made by [Richard] Owen in 1866. He describes this as a membrane stiffened by cartilage and covered in conjunctiva which moves horizontally across the eye, from the inner to the outer corner, in birds. As the humans and most other primates have evolved over the years, the nictitating membrane in them has dwindled away to a small vestigial structure called as the plica semilunaris, membrana nictitans, or palpebra tertia. “
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