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What is a dog's nictitating membrane?

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Quick Answer

The nictitating membrane is an opaque or transparent third eyelid that is present in some animals, including dogs and cats. It functions as a protective barrier from environmental hazards such as water, sand or debris. The nictitating membrane, which moves horizontally across the eyeball, also moistens the eye.

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Full Answer

In dogs, exposure of the nictitating membrane can be a sign of a health problem, indicating that the eyeball has recessed back into the socket. When the connective tissues supporting the nictitating membrane are weakened, a healthy tear gland can prolapse and cause cherry eye. This abnormal tear gland can irritate the surface of the eye and lead to conjunctivitis.

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