| nictitating membrane | A transparent fold of skin lying deep to the other eyelids at the mesial side, which may be drawn over the front of the eyeball. It is the so-called third eyelid, found in reptiles and birds generally and in many mammals. (12 Dec 1998) |
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| nictitating spasm | Involuntary spasmodic winking. Synonym: spasmus nictitans, winking spasm. (05 Mar 2000) |