Dogs have a unique “third eyelid” called the nictitating membrane. This transparent membrane is found in the inner corner of a dog’s eye and can slide across the eye for added protection or moisture. It helps to keep the eye clean and lubricated, while also shielding it from debris and injury. In some cases, the membrane can be seen when a dog is sleeping or relaxed, as it may partially cover the eye.
First Friend Fact #83
Tony Bierman, "First Friend Fact #83," OBTESA, Accessed December 21, 2024, http://esbt.us/e3.