It’s true, while we have a single upper eyelid and a single lower eyelid, dogs have a third. Our eyelids help to protect us from dust, light, and debris. For dogs, it’s the same, but dogs have three eyelids because the third has a special job. Most guardians are not aware of this fun fact and those that do know, are often still surprised the first time they actually see it.
Dogs Have Three Eyelids, But What Is The Third Eyelid?
The third eyelid is called a nictitating membrane. It’s also called a haw. Regardless of breed, all dogs have three eyelids. The membrane is in the inner corner of the eye, underneath the regular two eyelids we are familiar with. When it’s closed, it is drawn horizontally across the eye to the upper, outer part of the eye. Color can vary, but it mostly depends on the dog’s breed and health. For some dogs, it will be cloudy and maybe yellowed. Others will have one that is red or pink. And for other dogs, it will be clear.
Fun Fact: Dogs have two tear glands in each eye, but humans only have one.
What Does The Third Eyelid Do?
Dogs have three eyelids and the third is not just decorative. Similar to the first two eyelids, this one keeps moisture in and debris out. It is a sort of windshield-wiper that keeps the eye clear and healthy. It is also responsible for producing about a third of dogs tears because it has tear glands at its base. Those glands and the third eyelid are crucial for fighting infections. The glands will produce antibodies and fluid to carry those antibodies to the eye. Those antibodies and the third eyelid help to wash away foreign objects and irritants. The third eyelid also helps to protect the cornea, which is the outermost layer of the eye. The cornea is critical for eyesight, but does not have any blood vessels to nourish or protect the eye from infections. Besides keeping our pups in good health, the third eyelid is also helpful when hunting prey, particularly in bad weather or dense, leafy environments.
Fun Fact: The tear gland in dogs produces 60% of their tears. The tear gland that supports the third eyelid produces 40% of our dog’s tears.
What do I need to Know About My Dog’s Third Eyelid?
Well, the first thing to know is that the third eyelid is completely normal in dogs. Dogs have three eyelids because they need the extra moisture and eye covering. In fact, they aren’t the only ones with triple protection. Birds, cats, camels, fish, and reptiles also have three protective eyelids. Some cats also have a third eyelid, but not all of them. It depends on their breed. A healthy membrane helps to make sure our dogs have a healthy eyes. If we notice discoloration, discharge, swelling, itchiness, or unusual blinking, a quick visit to the vet should help clear any infections before they become a problem that can damage eyesight or spread to other areas or pets. The third eyelid is really only visible if our dog suddenly open his eyes, like when they are startled awake or when they are first opening their eyes after a long, deep sleep. If our puppy's third eyelid is visible or isn’t receding as usual, even when she’s awake and alert, it could mean there is a medical problem of some kind.
Dogs are amazing animals in many ways, but the fact that dogs have three eyelids can be especially fascinating. What are some of your favorite fun facts about dogs? Share it with us on Facebook .
Further Reading:
- "Everyday Medicine: The Third Eyelid," AMC Animal Medical Center
- "Structure of the Canine Eye," Whole Dog Journal
- "Dogs and Tear Stains," WebMD