The third eyelid is called the nictitating membrane. It’s purpose is to protect the eye. It also helps in the production and distribution of tears.
Sometimes, however, the cat will have their third eyelid up and partially over their eye when totally relaxed and resting. This is normal and can be excluded when using the third eyelid for assessing whether your cat is ill. When the third eyelid is over the eye, under these circumstances, it will quickly retract when the cat is alert.
Haw Syndrome is quite a common condition which causes the third eyelid to protrude. It affects cats that are otherwise healthy and which are under 2 years of age. It often occurs after a gastrointestinal illness. Apparently, the protrusion clears up without treatment, within a few months. A veterinarian can prescribe eye drops, which will reduce the amount the third eyelid covers the eye.
A part of the structure of the third eyelid is a gland which is important in the production of tears. Cherry eye refers to the condition when this gland protrudes from the eye or it becomes prolapsed (falls out of place). Another description is, “eversion of the gland of the nictitans” (eversion: turns inside-out).
Apparently, this condition is more likely to affect Burmese cats amongst the cat breeds. The cartilage of the third eyelid folds over, turning the third eyelid inside out. It can cause the cornea (the surface of the eye) to become ulcerated. Veterinarian attention must be sought.
A rare autonomic nervous system disorder called “Key-Gaskell Syndrome” causes the third eyelid to prolapse. This is a serious condition.
The nictitating membrane can protrude across the eye due to infection in the tissue behind the eyeball, bleeding behind the eye or a tumor. These conditions also cause the eye to bulge.
The eye can retract (become sunken) and the muscles surrounding the eye can spasm. If this occurs in both eyes the advice is to suspect a “systemic” illness i.e. a viral respiratory infection. If one eye is affected suspect: chronic weight loss and dehydration.
Conclusion: We should expect to see an eye without seeing the nictitating membranee except sometimes when the cat is resting or sleeping. If it is visible routinely or for long periods, I would take him/her to the veterinarian for a checkup.