is otherwise known commonly as the third eyelid is a thin fold of semi-transparent membrane which can be pulled across the eye independently of other eyelids. Nictitating membranes are typically observed in vertebrates.

NICTITATING MEMBRANE: "Some creatures, such as birds and cats have a nictitating membrane, which is a semi-transparent membrane which covers the eyes and can be moved independently from other eyelids."
Cite this page: N., Sam M.S., "NICTITATING MEMBRANE," in, April 7, 2013, (accessed October 21, 2021).