Welcome to our Peculiar Pet Facts series, where we investigate the oddities of our pets and explore the science behind them.
It sounds (and looks) like it originates from science fiction, but the usually hidden third eyelid of cats and dogs has an important role in eye protection. It’s also associated with a condition known as “cherry eye.”
Nictitating Membranes
Third eyelids, also called nictitating membranes, are thin, translucent-to-opaque membranes found underneath the lower eyelid, in the corner of the eye closest to the nose. They are present in birds, reptiles and some mammals, including cats and dogs.
Why Do Cats and Dogs Have a Third Eyelid?
The purpose of the nictitating membrane is to protect the eye (particularly the cornea) from dust, dirt and other unwanted material. A gland underneath the membrane keeps the eye moist by producing tears. The nictitating membrane also has tissue associated with the immune system that can help heal eye wounds or infections by stretching across the injured eye.
A Visible Third Eyelid Is Not Normal
Nictitating membranes are not usually visible in cats and dogs unless there is a medical issue like an infection, cyst or other underlying condition. If you can see your cat or dog’s nictitating membrane, you should take them to a veterinarian to have their eye examined.
Cherry Eye Is No Pie
A common reason that pet parents notice their cat or dog’s third eyelid is when their pet develops “cherry eye” — a prolapse of the third eyelid gland. When the connective tissue that attaches the gland to the inner surface of the third eyelid is weak, the gland can protrude from behind the eyelid. It looks like a round or oval piece of pink tissue in the corner of the eye.
Any cat or dog can develop cherry eye, but it’s most common in Burmese and Persian cats and dogs with prominent eyes like cocker spaniels and English bulldogs. Cherry eye can’t be prevented, but it can be treated surgically by putting the gland back in place and suturing it into position.
As peculiar as they look, nictitating membranes are important for keeping cat’s and dog’s eyes healthy. If you’re concerned about your pet’s eyes or eyelids, have your pet checked by a veterinarian.
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