Everything you ever wanted to know about the 3rd eyelid
The 3rd eyelid is technically called the nictitating membrane. It’s a translucent third eyelid that some animals have. They use it to moisten their eyes while keeping watch for something or to protect their eyes during a moment of vulnerability, like while they are underwater. The 3rd eyelid is commonly found in birds, reptiles, amphibians, and fish, although there are some mammals that have a 3rd eyelid. The most commonly thought-of mammals that have a third eyelid are dogs and cats, although many marsupials also have a 3rd eyelid.
The 3rd eyelid is distinctive because it moves across the eye horizontally instead of vertically. Animals like beavers that spend quite a bit of time underwater use it to protect their eyes while they swim. Land animals tend to use their 3rd eyelid to keep sand, dust, and small rocks out of their eye while they run or hunt for prey. Birds actually use their 3rd eyelid to protect their eyes from the baby chicks while they’re feeding them. They also use it to cleanse their eyes when they’re diving from high up in the air.
The nictitating membrane has been disappearing from mammals for thousands of years, so that’s why people don’t have one. However, some researchers believe that certain eye muscles are actually remnants of this 3rd eyelid. This special eyelid may also be on the way out for dogs and cats. Our common house pets don’t have as much control over this membrane as their ancestors did, and animals that use it on a regular basis have complete control over it. However, you can easily see the membrane in dogs and cats by doing a careful examination of their eyes. Simply looking at your pet’s eyes should not reveal the membrane unless the animal has an eye disorder. However, when you take the animal to the vet, you will notice the doctor pulling open the eyes and pressing down on the eyeball very slightly. This causes the membrane to appear so they can examine it.
The nictitating membrane in some breeds of dogs can be prone to prolapse, which basically means that it starts to fall out of place. This causes a condition known as cherry eye, which is a very common congenital eye defect in some breeds of dogs and cats. The cats that are most often affected are the Bombay and Burmese breeds. As for dogs, the beagle, cocker spaniel, Chihuahua, bulldog, Pekingese, mastiff, and basset hound are especially prone to cherry eye, which shows up as a red mass in the inner corner of the eye. Some dog owners of this breed think the mass is a tumor, but actually it’s just caused by prolapsed of the dog’s 3rd eyelid. There is usually a discharge with this eye problem, and the eye becomes inflamed. The gland that becomes inflamed causes production of tears in dogs, so dryness of the eyes is also very common in dogs that develop this condition. The good news is that surgery to remove the gland eliminates the problem easily.