Did you know that cats have not one, not two, but three eyelids? The first two eyelids are similar to ours — one on the top and one on the bottom that meet in the middle of the eye when the lids are closed. The third eyelid, also known as the nictitating membrane, is a retractable membrane located in the inner corner of each eye (closest to the nose).

First, What Is Your Cat’s Third Eyelid?

“Most mammals, other than most primates, have third eyelids,” explains Nancy Bromberg VMD, MS, Dipl. ACVO, a board-certified veterinary ophthalmologist at VCA SouthPaws in Fairfax, Virginia. “When an animal blinks, the third eyelid sweeps across the corneas under the eyelids, acting like a windshield wiper to clear debris, pollens, dust, etc. There is also a lacrimal gland at the base of the third eyelid that produces up to 50 percent of the normal tears.” 

Why Don’t You Usually See a Cat’s Third Eyelid?

Usually, you don’t really see a cat’s third eyelid because it’s hidden from view when retracted inside the corner of the eye. Sometimes, you might be able to see the third eyelid if your cat is very relaxed. For instance, if your cat just woke up from a deep sleep or was sedated for a surgical procedure, you might catch a glimpse of the third eyelids of both eyes.

When Does Seeing Your Cat’s Third Eyelid Indicate a Problem?

Most often, however, if you can see your cat’s third eyelid, it indicates a problem — either something is wrong with the eye or third eyelid itself, or possibly another health issue might be at hand (often, a sick cat). Many cat eye conditions cause the third eyelid to stick out, including conjunctivitis or pink eye (inflammation of the eye membranes), corneal ulcers (damage to the corneas), glaucoma, uveitis (intraocular inflammation), masses growing on the third eyelid and Horner’s syndrome (a neurological disorder of the eye and facial muscles).

“The gland of the third eyelid sometimes gets inflamed and swollen and everts the third eyelid,” Dr. Bromberg explains. “This is commonly known as ‘cherry eye.’ It interferes with the normal function of the third eyelid and should be surgically repositioned. This condition is most common in the Burmese [cat breed].”

What Should You Do If You See Your Cat’s Third Eyelid?

If you see any part of one or more of your cat’s third eyelids, you should bring her into the veterinarian for an exam. When one third eyelid is showing and the other is not, that eye might be experiencing an issue. If both third eyelids are showing, and if your cat is acting sick, it could be an indication that your cat is ill with something not necessarily related to the eye.

It’s important not to delay seeking veterinary care because a cat who has a visible third eyelid is likely experiencing pain and/or discomfort. “A full ophthalmic examination should be done to check for corneal ulcers, elevated intraocular pressure, uveitis, masses, etc.,” Dr. Bromberg advises. “Once the cause is determined, appropriate treatment is prescribed.”

Top photograph: krblokhin/Thinkstock.

Read more about cat eyes on Catster.com:

• Cat Eye Discharge — What’s Normal and What’s Not
• Cat Pupils: Here’s What Your Cat’s Eyes Are Telling You
• 4 Things to Know About Cats With Blue Eyes