nictitating membrane

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pli·ca sem·i·lu·na·'ris of con·junc·ti·'va

[TA]
1. the semilunar fold formed by the palpebral conjunctiva at the medial angle of the eye;
2. a fold of the conjunctival mucous membrane found in many animals; normally partially hidden in the medial canthus of the eye when at rest, it may be extended to cover part or all of the cornea in a winklike action to clean the cornea, as in birds. Synonym(s): membrana nictitans, nictitating membrane, palpebra III, palpebra tertia, third eyelid
Farlex Partner Medical Dictionary © Farlex 2012

nictitating membrane

(nĭk′tĭ-tā′tĭng)
n.
A transparent inner eyelid in many vertebrates that closes to protect and moisten the eye. Also called third eyelid.
The American Heritage® Medical Dictionary Copyright © 2007, 2004 by Houghton Mifflin Company. Published by Houghton Mifflin Company. All rights reserved.

nictitating membrane

the third eyelid that occurs in birds, reptiles, and some mammals, sharks and amphibians. It is transparent and flicks over the eyeball from the inner (anterior) corner of the eye to clean and moisten the eyeball without shutting out the light. In some diving birds such as auks, there is a lens-like window in the membrane which adjusts the focus of the eye under water.
Collins Dictionary of Biology, 3rd ed. © W. G. Hale, V. A. Saunders, J. P. Margham 2005