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.
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.