Do Cats Really Have a Third Eyelid?

Posted on 10 Dec 2015 16:36 by EricT

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You've heard about this third eyelid that cats have, but when you look, you don't see it. Do they have three eyelids? And, if so, where is the third one?

Cats don't exactly have a third eyelid, technically speaking. What they do have is something called the nictitating membrane or haw. This membrane is an enlarged fold of conjuntiva that is located at the inner corner of the eye. Other scientific names for it are membrana nictitans and plica semilunaris. Many other animals, including dogs, rabbits, cows, horses, rats, and mice, have a nictitating membrane. Birds and reptiles have them too, although theirs works a bit different than other in other animals, since they can voluntarily pull them over the eye.

Where Is the Cat's Third Eyelid?

If you look very closely, you may see a sliver of silver to pale pink at the inside of each of your cat's eyes. This is a protective membrane that helps shield his eyes from glare. It also helps clear away dust from the cornea and keep it moist. Your cat has no conscious control of this "third eyelid." When a cat is hunting, fighting, or roaming through tall grass or brush, or in the harsh sunlight, and protection of the eye is needed, this third eyelid will unfold to cover the eye and help protect it from injury.

How Does it Work?

Normally, the third eyelid is kept retracted by the eyeball itself. The eyeball protrudes in the socket so that the membrane is kept down in the corner. when protection is needed, a special muscle in the eye pulls the eye back into the socket, ever so slightly, but just enough to allow the third eyelid to slide across the eye.

Cat's vary in how much the third eyelid is exposed when at rest. Some cats, however, will have a bit of it protruding when they are just relaxing, and you may notice it then, covering, perhaps a third of the inner eye. When your cat becomes alert, the membrane will quickly retract back.


Cat with third eyelid protrusion in both eyes

Protrusion of third eyelid of both eyes in cat Image by NikaJiadze via wikimediaImage Credit


Third Eyelid Protusion May Indicate Eye Problems or Other Illness

However, sometimes, when the nictitating membrane becomes visible it can indicate an eye condition or other illness. When the eye is injured or irritated, cats use the same muscle to pull it back, so that the membrane can slide across the eye to protect it. Damage to the third eyelid itself can also cause it to become prominent.

When this happens you may notice swelling and other abnormal appearance. If you notice the third eyelid in your cat simply because you start looking for it, this may be normal for your cat. If you are wondering, visit your veterinarian to have your cat's eyes checked. It is important, in this case, for you to know what is normal for your pet.

If, however, you do not normally notice the third eyelid, and it becomes conspicuous, schedule a vet appointment as soon as possible, as this is likely the result of an eye problem, or even a systemic illness such as respiratory infection.

Sometimes, foreign objects can get caught under the third eyelid. Outdoor cats may get pieces of grass or other debris caught. When something gets caught under this membrane, it will scrape against the cornea and cause great pain and damage to the eye. In some cases, it may be possible for you to remove the object yourself, by pulling the third eyelid forward and up, and grasping the foreign object to remove it. This could be harmful to the eye if you are are not careful, and since your cat's eye will be painful and your cat will be in distress, it is very unlikely she will let you do this. A veterinarian, then, is a better choice.

Tumor, eye infection, and other eye conditions could also cause the nictitating membrane to protrude. If the protruding third eyelid is only visible in one eye, then there is probably a problem with that eye. If it is visible in both eyes, and this is not normal for your cat, this may indicate a systemic illness. Your cat's eye(s) may be water and his eyelids may seem to tightly close involuntarily, similar to your own eyes when they are bothering you or something is stuck in one of them.

Cats can easily have their third eyelid injured if they fight or play roughly with other cats. The membrane can be punctured or lacerated by the other cat's claw. Superficial punctures or shallow lacerations will probably resolve on their own, although a topical antibiotic solution may be warranted to ward off infection. More serious lacerations may need to be sutured.

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