nictitating membrane
anatomy
Also known as: third eyelid
Quick Summary
The nictitating membrane, often called the "third eyelid," is a thin membrane found in many vertebrates. It is located at the inner angle or beneath the lower lid of the eye and can extend across the eyeball. This transparent membrane helps to keep the eyeball moist and clean. The term "nictitate" relates to winking and the development of this specialized eyelid structure, which has been observed in animals since the early 18th century.12
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