Thoroughly Vetted: Dogs and cats have an extra eyelid

If a pet’s third eyelid remains exposed, it means something is wrong with that eye. If a pet’s third eyelid remains exposed, it means something is wrong with that eye.

At a glance, you wouldn’t even know it’s there. Dogs and cats appear to have the same two eyelids that we do. But look closer, especially if you find your pet half asleep, and you might catch a glimpse of their third eyelid. Properly called the nictitans, it’s a pink sheet of tissue that stretches diagonally from the inner corner of each eye, but which stays drawn out of view under normal circumstances.

The nictitans isn’t unique to our household pets. This membrane is common across the animal kingdom, and is seen in a variety of mammals, birds, fish, reptiles and amphibians. It’s even translucent in some species, functioning as flip-up safety goggles. And while human beings don’t have a proper third eyelid, you can see the vestigial remnant of yours in the mirror—a little band of pink tissue called the plica semilunaris.

In dogs and cats, the third eyelid isn’t directly controlled by any muscles. Instead, it works as a spring-loaded protective device. When threatened or injured, the eye reflexively sinks backwards into its socket. This simply gives the nictitans enough room to pop up and do its job. Once the situation has passed, the eye returns to its normal position, forcing the nictitans back into its pocket.

This is important because if a pet’s third eyelid remains exposed, it means something is wrong with that eye. It could be a simple eye infection, or perhaps a scratched cornea. Sometimes I’ll find foreign material—perhaps a bit of grass or grit—stuck in there. And if both are exposed, it can indicate a wide variety of underlying causes ranging from dehydration to neuromuscular disease.

It’s not uncommon that I see an animal with an injured third eyelid. It’s relatively delicate, and the edge can easily be cut or torn. But these injuries are usually simple to manage, and serve as evidence that the membrane performed admirably. Better to have an injured third eyelid than an injured eyeball. These cases are the ocular equivalent of fender-benders.

At least a few times a year, I’m also likely to see a puppy with “cherry eye,” an ugly pink lump in the corner of one or both eyes. That lump is a wayward tear gland that is supposed to be tucked deep behind the third eyelid. In some dogs (and rarely in cats), the tissue meant to hold the gland in place is too loose, allowing it to pop up over the edge of the lid. Apart from being unsightly, this can damage the tear gland and cause chronic irritation, and usually requires surgical correction to put that little sucker back where it belongs.

Perhaps it’s a bit odd that our pets have a third eyelid that we don’t. But given its prevalence in other species, and how useful it can be in preventing injury, I think it’s more appropriate to wonder why we only have two.

Dr. Mike Fietz is a small animal veterinarian at Georgetown Veterinary Hospital. He received his veterinary degree from Cornell University in 2003 and has lived in Charlottesville since.

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