If you have ever glanced at your cat and seen a pale, filmy curtain drawn part way across the inner corner of an eye, you have met the third eyelid. It is standard feline equipment, and many owners go years without noticing it.
A brief flash of it in a cat who is drifting off to sleep means very little. A third eyelid that stays up for hours or days is a different signal, and it usually means something else in the body needs attention.
The anatomy behind that pale curtain
The formal name is the nictitating membrane. It is a thin, mobile fold of tissue that sits in the inner corner of each eye, closest to the nose, normally folded away out of sight.
Tucked at its base is a tear gland that supplies a real share of the moisture on the eye surface. The membrane also contains immune tissue, which is one reason it swells and becomes obvious when an eye is irritated or infected.
Three jobs it quietly handles
- Armor on demand. It sweeps across the eye faster than a blink, shielding the surface during a tussle or a dive into shrubbery. It also rides higher while a scratch heals.
- Tear supply. Its gland produces part of the tear film, so the membrane is genuinely involved in keeping the eye surface comfortable.
- Cleaning. Each pass works like a wiper blade, spreading tears and pushing dust toward the inner corner.
When a glimpse is nothing to worry about
The membrane is not hauled across by a muscle the way your own eyelids are. It moves largely in response to the position of the eyeball and the tone of the tissue behind it, so it drifts into view whenever a cat relaxes. You may see it in a cat who is half asleep, mid yawn, waking from a nap, or picked up suddenly, and in one coming out of sedation at the vet office.
The check is simple. Get your cat's attention with a toy or a sound. If the membrane retracts and stays retracted, and the eye looks clear and comfortable, that is typical.
What it means when it stays showing
A membrane that will not go back down is a symptom, not a diagnosis. The most useful thing to work out before you call is whether one eye is affected or both, because the two point in different directions.
One eye only
A single eye usually suggests a local problem: a corneal scratch, a grass seed or clump of litter trapped under a lid, an ulcer, an infection, inflammation inside the eye, raised pressure, or a knock to the face. Look for company symptoms such as squinting, watering, cloudiness, discharge, pawing at the face, or a sudden dislike of bright rooms.
One pattern is worth knowing. A droopy upper lid, a smaller pupil and a raised third eyelid all on the same side point to trouble with the nerve supply to that eye, which can trace back to an ear infection, an injury to the neck or chest, or other causes. It is not painful in itself, but it deserves a proper workup.
Both eyes
When both membranes are up in a cat whose eyes otherwise look white and comfortable, attention usually shifts to the rest of the body. Digestive upset, intestinal parasites, dehydration, weight loss, fever and viral illness can all show themselves this way. There is also a recognized syndrome in which both third eyelids protrude for a few weeks in a cat who is bright and eating well, often on the heels of a bout of diarrhea. It tends to settle on its own, but that is not something you can assume from your sofa.
Lumps, cherry eye, and things not to touch
Sometimes the gland at the base of the membrane slips out of position and shows up as a smooth pink swelling in the inner corner, often called cherry eye. It is far more common in dogs than cats. It is rarely a middle-of-the-night emergency, but it does need veterinary care, because a cat who loses that gland can end up with a chronically dry eye.
Do not push the tissue back, pry the lids apart for a better look, or reach for the medicine cabinet. Human eye drops, particularly redness relief products and leftover prescriptions, can cause serious harm in a cat's eye.
When to call, and what the visit looks like
Same-day care is the right call if you notice any of these:
- The eye is held shut, or your cat pulls away when you come near her face
- The surface looks cloudy, bluish, or has a visible dent or spot on it
- There is thick or colored discharge, or any bleeding
- The eye bulges, sinks, or looks a different size than the other one
- She is bumping into furniture, or is also lethargic, feverish, off her food or vomiting
At the appointment, expect a general physical exam alongside the eye exam. A vet may place a dye on the eye to reveal scratches, measure tear production, check the pressure inside the eye, and use numbing drops to inspect behind the membrane for trapped debris. Blood work is common when both eyes are involved, and treatment aims at whatever is driving the protrusion rather than the membrane itself.
Common questions
Can I gently push my cat's third eyelid back into place?
No. You risk scratching the cornea, pushing debris deeper, and hurting a cat who is already sore. Leave it alone and let a veterinarian examine the eye.
My cat's third eyelid shows only when she is sleeping. Is that a problem?
Usually not. If it disappears within a few seconds of her waking and focusing on something, and the eye stays clear and open, that is normal for the tissue.
How long should I wait before booking an appointment?
If the membrane is still visible after a day of normal activity, book. If there is pain, cloudiness, discharge, or your cat seems unwell in any other way, do not wait at all.
Your veterinarian is the right professional here, and the only one who can examine your cat's eyes. Animal eye problems can move quickly when they are left alone. Take a phone photo of both eyes each day so you can show how things have changed, note anything different about appetite or energy, and make the call. While eyes are on your mind, it may be worth checking whether your own exam is overdue and seeing which eye doctors near you work with your vision plan.