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Nictitating Membranes

12-17-13 crow 054If you look closely, you’ll see that part of this American Crow’s eye appears white. This white (actually transparent) part of the eye is referred to as a “third eyelid,” or nictitating membrane. Many animals, including some fish, mammals, reptiles, amphibians and birds, have nictitating membranes. Instead of moving up or down like the upper and lower eyelids, they move horizontally across the eye, much like windshield wipers. These membranes lie under a bird’s eyelids. When not in use, they are curled up in the inner corners of a bird’s eyes, but can be quickly drawn across the eyes to protect, clean and moisten them without shutting out the light.

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3 responses

  1. Jean Harrison

    Neat photo, Mary

    December 17, 2013 at 6:54 pm

  2. dellwvt

    For goodness sake – how interesting! Do all birds have this adaptation?

    December 18, 2013 at 2:47 am

    • Yes, I believe all birds do have a nictitating membrane. Most of them “blink” with it, not with their upper or lower eyelid.

      December 18, 2013 at 2:13 pm

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