how many eyelids do dogs have

Dogs have three eyelids. In addition to the upper and lower eyelids like humans, dogs have a third eyelid called the nictitating membrane or haw. This thin, translucent membrane is located in the inner corner of the eye and serves to protect the eyes, keep them moist by distributing tears, remove debris, and help prevent infection. The third eyelid is not usually visible unless the dog is sleepy or if there is an eye issue. This third eyelid plays a vital role in maintaining the health and function of a dog's eyes.