Dogs have three eyelids. While you can see the upper and lower lids, the third eyelid only appears when there's issues with the eye. That's when you'll see that reddish or pink membrane emerge and start covering the eye.
The Nictitating Membrane
Formally known as the nictitating membrane, a dog's third eyelid serves as eye protection. This membrane helps keep your dog's eye clean, as it contains a tear-producing gland that provides lubrication. If the third eyelid starts appearing, examine your dog's eye carefully. It can show up because of eye trauma or infection, or if the dog is fighting an illness elsewhere in the body. It's a sign that your dog needs veterinary attention.
Some dogs always have slightly visible third eyelids, known as haws. It's just a small pink area in the corner of the eye. Haws don't affect the dog, but are considered a conformational flaw if you intend to compete in the show ring.
Cherry eye occurs when the third eyelid pops out, resembling a cherry at the eye's inside corner. It's actually a prolapsed gland of nictitating membrane. Besides unsightliness, the dog's eye no longer tears properly, so dry eye can result. Not only is this condition painful, but your dog can eventually lose vision. Take your dog to the vet at the first sign of cherry eye.
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