Nictitating membrane
Nictitating Membrane
The Nictitating Membrane (pronounced: nik-ti-tey-ting mem-brane), also known as the third eyelid or palpebra tertia, is a translucent or opaque third eyelid that can be drawn across the eye for protection and to moisten it while maintaining visibility.
Etymology
The term "nictitating" is derived from the Latin word "nictare", meaning "to blink". The term "membrane" comes from the Latin "membrana", meaning "a thin layer or sheet".
Function
The Nictitating Membrane is present in certain animals such as birds, reptiles, and mammals. It moves horizontally across the eye, unlike the upper and lower eyelids, which move vertically. It serves to protect and moisten the eye while still allowing some vision. Some species have a clear nictitating membrane, while others have a membrane that is opaque or semi-opaque.
In Humans
In humans, the Plica Semilunaris, a small fold of tissue in the corner of the eye, is a vestigial remnant of the nictitating membrane.
Related Terms
- Plica Semilunaris: The vestigial remnant of the nictitating membrane in humans.
- Eyelid: A thin fold of skin that covers and protects the eye.
- Blinking: The rapid closing and opening of the eyelid.
- Vestigiality: The process of retaining structures that have lost some or all of their ancestral function.
External links
- Medical encyclopedia article on Nictitating membrane
- Wikipedia's article - Nictitating membrane
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