nictitating membrane

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nictitating membrane

(in reptiles, birds, and some mammals) a thin fold of skin beneath the eyelid that can be drawn across the eye
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

Nictitating Membrane

 

a third eyelid, located in the inner corner of the eye in some Euselachiae (fishes) and the majority of terrestrial vertebrates. The nictitating membrane is formed from a fold of the conjunctiva and is located behind the upper and lower eyelids. In terrestrial vertebrates, a transparent nictitating membrane periodically covers the cornea to clean and moisten it. Nictitating membranes are most highly developed in reptiles and birds. Among mammals, a third eyelid is absent in Echidna and Cetacea; apes and humans have rudimentary nictitating membranes in the form of semilunar folds.

The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.

nictitating membrane

[′nik·ə‚tād·iŋ ′mem‚brān]
(vertebrate zoology)
A membrane of the inner angle of the eye or below the eyelid in many vertebrates, and capable of extending over the eyeball.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.