Here's what you should know about your cat's third eyelid—and what to do if it's showing.
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close up of a cat's eye and his third eyelid; how many eyelids to cats have?
Credit: schankz / Shutterstock

When a cat sleepily squints her eyes at you, it's like a little furry hug and a sweet "I love you." But what if the third eyelid that slides across her eyes like a curtain makes an appearance, too? And, hold up, how many eyelids does a cat have?

It depends who you ask. That's why we took a closer look with the help of a feline ophthalmologist (aka a cat eye doctor). "The third eyelid is not a true eyelid per se," says Kristin Fischer, DVM, DACVO, and a small animal ophthalmologist at Animal Eye Care Associates in Oak Ridge, Tenn. Technically, it's a membrane and it turns out, cats aren't the only ones who have (or had) a third eyelid.

Cat Eye Anatomy

Your cat doesn't see the world the same way we do, and she can thank more than her catty sense of curiosity for that.

"There are many differences between a cat eye and a human eye," Fischer says, and many of them help your fierce hunter prowl at dawn and dusk. "The first, and most noticeable, is the shape of their pupil." The elongated pupil is a common trait among ambushing predators and improves their depth perception.

Cats' eyes also glow, unlike ours. That's because they have a tapetum lucidum, or a reflective membrane located in the back of the eye. This gives the rods, or light-detecting cells, a second chance at absorbing light and provides better (but not perfect) night vision. Cats' eyes have more rods than ours, but they have fewer color-detecting cells, or cones.

infographic showing top eyelid, bottom eyelid, and third eyelid
Credit: Kailey Whitman

Like us, cats have two sets of true eyelids—an eyelid at the top and bottom of each eye. "In addition to the upper and lower eyelids that you see, cats also have a third eyelid or nictitating membrane," Fischer says.

Why Do Cats Have a Third Eyelid?

Scientists say there's evidence humans and other primates once had a third eyelid. But, because we're not stalking prey on all fours through the grasslands, it dwindled down to the small vestigial structure called the plica semilunaris.

"Different from humans, cats are hunters and typically live in areas where their eyes may be at increased risk for trauma from plants and grasses or other animals," Fischer explains. "The nictitating membrane or third eyelid serves a protective purpose and helps keep the eye clear of debris and protected from trauma." Plus, she adds, there's a gland in the third eyelid that produces and distributes tears.

Should You Be Able To See Your Cat's Third Eyelid?

Sometimes, but only a little bit of the third eyelid, Fischer says. You can see a cat's third eyelid peek out when they're resting or sedated. It could be pink, non-pigmented, black, or brown.

"Cats have a small amount of smooth muscle within their third eyelid, meaning that they do have a bit of control of the position," Fischer says. But the position and movement of the third eyelid is mostly dictated by the position of the eye.

Typically, the third eyelid "sits hidden in a space near the inside corner of the eye." If the third eyelid is protruding, Fischer says there's a medical reason, like an infection or irritation.

When Your Cat's Third Eyelid Is Cause for Concern

If your cat's third eyelid is showing more than usual, it's red or swollen, or causing your cat to scratch at her eyes, it's worth a trip to the vet to assess the situation. According to Fischer, common cat eye conditions that might cause your cat's third eyelid to show include:

  • Corneal ulceration: a scratch or tear
  • Conjunctivitis: also known as pink eye
  • Uveitis: inflammation within the eye
  • Glaucoma: elevated pressure within the eye
  • Prolapse of the third eyelid gland, also known as cherry eye
  • Irritation caused by tumors or masses
  • Eyelid defects
  • Certain neurological conditions

No need to rush to a feline ophthalmologist, Fischer says. First, visit your regular veterinarian. Chances are, they'll be able to diagnose, treat, or manage the underlying condition. If your cat needs more advanced treatment (like surgery), isn't responding to treatment, or if the underlying cause can't be readily identified, then your vet can refer you to a specialist like Fischer.