nictitate : Idioms & Phrases
Nictitating membrance
(Anat.) , a thin membrance, found in many animals at the inner angle, or beneath the lower lid, of the eye, and capable of being drawn across the eyeball; the third eyelid; the haw.
Webster 1913
nictitating membrane
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noun a protective fold of skin in the eyes of reptiles and birds and some mammals
third eyelid.