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 science fiction, but the usually hidden third eyelid of dogs has an essential role in eye protection. Just like it’s important to monitor your dog’s vision, it can be useful to understand the role of your dog’s third eyelid and what it looks like if it’s not working correctly. So here’s all you need to know about your dog’s third eyelid.
Dog Eye Anatomy 101
Along with upper and lower eyelids, dogs have a third eyelid that helps protect their eyes. Also called the nictitating membrane, the third eyelid is a thin, translucent-to-opaque membrane found underneath the lower eyelid, in the inner corner of the eye closest to the nose. Birds, reptiles and some mammals, including cats and dogs, have three eyelids.
What Is the Purpose of a Dog’s Nictitating Membrane and Third Eyelid Gland?
The role of third eyelids is to protect the eye (particularly the cornea) from foreign bodies like dust, dirt and other unwanted material. Along with the lacrimal gland which is located in the upper outside corner of their eye, a gland underneath the third eyelid keeps the eye moist by producing the liquid component of the tear film. The tear film is what spreads across the surface of the eye every time your dog blinks and helps to protect the eye. 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.
Help! I Can See My Dog’s Third Eyelid
A dog’s third eyelid is not usually visible unless there is a medical issue like an infection, cyst, prolapsed gland or other underlying condition. If you can see your dog’s third eyelid, you should have their eye examined by your veterinarian. In some cases they may refer your dog to a veterinary ophthalmologist.
Cherry Eye in Dogs
A common reason that pet parents notice their dog’s third eyelid is due to a condition called “cherry eye” — a prolapse of the third eyelid gland from its normal position. 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 third eyelid. It looks like a round or oval piece of pink tissue in the inside corner of the eye.
Any dog can develop cherry eye, but it’s most common in certain breeds 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.
The Third Eyelid: Peculiar but Necessary
As peculiar as they look, nictitating membranes are important for keeping your dog’s eyes healthy. If you’re concerned about your dog’s eyes or eyelids, have them checked by your vet.
