What Is a Cat’s Third Eyelid, and What Does It Do?

A cat’s third eyelid is a thin membrane at the inner corner of each eye, nearest the nose. Also called the nictitating membrane, it sits beneath the familiar upper and lower lids. It helps protect the eye’s surface and spread tears, while a gland within it contributes tear fluid. Merck Veterinary Manual describes this extra layer of protection in cats.

The third eyelid is normally largely hidden. If it becomes newly prominent, swollen, or otherwise different, contact your veterinarian promptly rather than assuming that a protective structure must be harmless when it shows. Pain needs immediate veterinary contact.

Where the Third Eyelid Sits

The upper and lower eyelids are folds of skin that close over the eye. The third eyelid lies underneath them, between the eyeball and the lower lid at the nose-side corner. You may also see it called the haw. These names refer to the same structure, not to separate layers of the eye. Cornell Feline Health Center describes its position and role.

Although it looks like a thin sheet, the membrane has internal support: a T-shaped piece of cartilage gives it structure. Its covering is conjunctiva, the thin tissue that also lines the inside of the eyelids. The American College of Veterinary Ophthalmologists (ACVO) explains this construction.

Descriptions such as pale pink or whitish pink are useful starting points, but color varies. ACVO notes that the third eyelid can also have brown or black pigmentation. Color alone cannot tell you whether a newly changed eye is healthy, and you should not pull the lids apart to inspect the membrane.

How It Helps Protect the Eye

The membrane’s position is closely linked to the eyeball’s position in its socket. In the protective response described by ACVO, a muscle called the retractor bulbi draws a threatened or painful eye backward into the socket. As the eye moves back, the third eyelid slides across its surface like a curtain.

That movement helps shield the front of the eye. For a concrete example, Merck’s feline eye guide describes protection from scratches as a cat moves through brush. The upper and lower lids also protect the eye through blinking, so the third eyelid is one part of the protective system. It does not make the eye injury-proof.

This explains a protective mechanism, not a way to test your cat’s eye at home. There is no need to provoke the movement or try to uncover the membrane.

How the Membrane and Tear Gland Work Together

Keeping the eye moist involves both making tears and distributing them. The third eyelid participates in these jobs in different ways:

• The membrane helps spread tears. Its movement distributes moisture over the eye’s surface, alongside the work of the outer eyelids.
• The gland contributes tear fluid. The gland within the third eyelid produces part of the watery component of tears. It is associated with the membrane but is a distinct structure.

ACVO describes tear distribution and the associated gland; Merck explains how that gland fits into the wider tear system.

Tears contain water, oil, and mucus. The main lacrimal glands also supply watery fluid, cells in the conjunctiva supply mucus, and glands in the outer eyelids supply oil. Together, these components help maintain a protective layer of moisture. The third eyelid therefore contributes to a shared system rather than supplying all the tears by itself.

The distinction also helps explain the term cherry eye: it refers to prolapse of the third eyelid’s gland, not simply to any visible third eyelid membrane. Different problems can affect this area, and a veterinarian must identify what has changed. ACVO’s overview distinguishes these structures and problems.

What About Seeing It During Sleep?

A specific example helps explain why context matters. In a 2017 Cats Protection veterinary Q&A, an owner asked about a kitten sleeping with her outer eyelids slightly open. The veterinarian explained that the third eyelids could be seen through that opening and regarded that particular sleep appearance as normal. The kitten’s accompanying head shaking was separately referred for a veterinary check.

That case-specific answer cannot establish that every visible third eyelid is harmless. A newly conspicuous membrane while your cat is awake, swelling, discomfort, or another eye change needs veterinary advice. Do not use a sleep example or a photograph of another cat to decide that your own cat’s changed eye needs no assessment.

When to Contact Your Veterinarian

Contact your veterinarian promptly for a newly prominent, swollen, or changed third eyelid, even without obvious pain. ACVO recommends assessment for sudden changes, swelling, or protrusion. PDSA’s guidance on eye problems in cats advises contacting a veterinarian as soon as an eye problem is noticed and immediately if the cat is in pain. Do not wait for the change to persist before calling.

Other signs affect urgency:

• Seek urgent veterinary care for squinting or holding an eye closed, yellow, green, or bloody discharge, cloudiness, or a newly bulging eye.
• Seek emergency veterinary care for severe injury to the eyeball or an eyeball displaced out of its socket.

These distinctions follow VCA’s eye-issue guidance; they are examples, not a complete list of eye emergencies. Suspected changes in vision should also be reported without delay, as Cornell advises.

An eye’s appearance or a photograph alone cannot establish the cause. Veterinary assessment may require examination equipment and tests that assess the eye beyond what you can see at home. Merck’s examination overview describes that professional work.

As a practical precaution, do not try to expose, touch, or push the membrane back. Do not start human or leftover eye medication unless your veterinarian specifically directs its use for this cat and this problem. Follow existing veterinary instructions rather than changing prescribed treatment yourself.

For the separate question of caring for fur around an otherwise healthy eye, see How to Clean Your Cat’s Eyes Safely. That guide is not a treatment for a newly prominent third eyelid; a changed eye belongs with your veterinarian.

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