THIRD EYELID: Cats--and most other animals--have a third eyelid that comes in from the side to further protect their eyes from harm.
Image: COURTESY OF PAUL MILLER
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Veterinarian Paul Miller of the University of Wisconsin-Madison explains.
The inner eyelid of cats--more properly called the palpebra tertia but also known as the nictitating membrane, third eyelid or "haw"--has been regarded by some as a biological curiosity much like the human appendix or wisdom teeth. In fact, some veterinary articles in the early 1900s describe methods for removing this supposedly irrelevant structure so as to facilitate examination of the eye. Despite these perceptions, the third eyelid of cats plays an important role in maintaining the health of their eye surface. In fact, it is so important that among mammals and birds the norm is for a species to have a third eyelid and those lacking one--such as humans and some of our fellow primates--are the true oddities in nature.
The anatomy of the third eyelid is complex. It is a fold of tissue covered by a specialized mucous membrane (the conjunctiva) that faces the inner surface of the eyelids (palpebral surface) on one side and the cornea on the other side (bulbar surface). Embedded in the bulbar surface is a dense population of lymphoid follicles that are in contact with the surface of the eye and the tear film, a thin layer of liquid. These structures function as the lymph nodes of the eye, trapping unwanted dirt and detritus.
Between the two layers of conjunctiva is a dense T-shaped cartilage plate. The crossbar of this T cartilage stiffens the free edge of the third eyelid and is also curved so as to conform to the corneal surface. The stem of the T cartilage is surrounded by an accessory lacrimal gland, which produces a substantial portion of the tear film. The tiny ducts through which tears leave the gland of the third eyelid exit between the lymphoid follicles on the surface of the third eyelid, allowing these cells to dump their contents into the tear film and be widely distributed over the surface of the eye.
In cats, as in most species, the third eyelid is large enough to completely cover the cornea and acts much like a windshield wiper blade by removing debris from the surface and redistributing tears over the cornea. When the cat is alert, the bulk of the third eyelid is hidden within the eye socket and only a small portion is visible in the inner corner of the eye. When relaxed, during sleep or during blinking, however, retraction of the eyeball by a set of skeletal muscles causes the third eyelid to passively move across the ocular surface from the inner, lower corner of the eye to the upper, outer corner. Movement of the third eyelid in cats is also partially regulated by the sympathetic nervous system as well as by smooth muscle cells within the third eyelid. The former fact has been used extensively in the study of how certain drugs affect the sympathetic nervous system.
The exact function of the third eyelid in cats is not completely known but it is believed to help protect a very large cornea from injury as cats move through tall grass or capture prey. Additionally the presence of an accessory tear gland allows for even greater tear production and rinsing of the ocular surface than is found in primates. As this portion of the tear film flows over the lymphoid follicles covering the surface of the third eyelid, a variety of immunologic mediators, including secretory IgA and lactoferrin, are dumped into the tear film to bathe and immunologically protect the ocular surface from the stew of bacteria and fungi that inhabit the surface of even a normal eye.
The third eyelid is also believed to help keep the surface of the eye moister by holding the tear film against the cornea better than the eyelids do by themselves. Loss of the third eyelid through trauma or in the treatment of neoplasia frequently results in chronic irritation of the cornea and remaining conjunctiva. In view of this, the real question is not "Why do cats have a third eyelid?" but "Why don't people have a third eyelid?" In humans, the third eyelid has been reduced to a rudimentary fleshy bump in the inner corner of the eye. Although the exact reason why we lack a third eyelid is unknown, it may be related to the fact that humans do not typically capture prey by biting (as would a cat) or by rooting through vegetation (as would a horse). Thus, there may be no advantage for us in having this extra measure of protection for the surface of the eye.
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8 Comments
Add CommentI'm worried sick about my Maine Coon whose 3rd eyelid is covering more than 1/2 his eye. I've been to the ER when it wasn't AS bad, and she looked to see if anything was in his eye. Negative. She saw a corneal laceration,a faint one and said it was probably due to that. I was sent home with ointment, antibiotics and kept my next day appointment with my regular vet. He read what they did and in his busyness, felt that he's seen a lot of cats with their eye looking like that heal in a week. Yesterday & today, I see it as getting worse; redder, more swollen, but his temperature went down to normal and altho he's not his perky self, he does not seem to be in any pain. The ER was $350, vet visit $50, and I don't feel satisfied or good waiting for this to heal even though it may be protecting that scratch on his cornea. I'm on a limited income, but would starve first,except that I don't want to get different answers and just wait for "it" to heal. Maybe there is something behind the eye. Please help me and my precious Maine Coon. He doesn't mind ointments in his eye every 2 hours or antibiotics, but to me, it's looking worse and I've seen 2 Drs. Please help.
Reply | Report Abuse | Link to thisI too have a cat with similar eye. I am not sure what happened. She may have a virus. Did you get any help or did it clear up. Also the side of her face is no the same shape as the other side.
Reply | Report Abuse | Link to thisSometimes the best thing to do is nothing.
Reply | Report Abuse | Link to thisThank you, I was wondering what was the white thing in my cats eyes and i though extra eyelids but wasn't very confident in my own thoughts so this was a real eye opener for myself
Reply | Report Abuse | Link to thisrEGARDING THE COMMENTS LISTED ABOVE, IS THERE ANY ANSWERES OR REPLYS REGARDING THE EYE ISSUE IN CATS CALLED the palpebra tertia but also known as the nictitating membrane, third eyelid or "haw"--. mY CAT ALSO DEVELOPED THIS 3 DAYS AGO AND $95.00 AND ANTIBOTICE CREAM IS DOESN LOOK ANY BETTER. dOES THIS CONDITION GET BETTER??
Reply | Report Abuse | Link to thisCats are very resilient and have very good built in healing mechanisms, so the previous poster is correct, sometimes the best thing to do is nothing.
Reply | Report Abuse | Link to thisThat said, I often see the third eyelid partly extended in my cats when their eyes are irritated. Most often, it turns out to be fleas - cats can develop allergies to fleas even if they haven't previously had a reaction. They'll retract it soon after the fleas are treated.
If it's an allergic reaction, a vet can also administer a steroid shot for immediate relief.
HTH!
There are lots of things that can cause this problem. This happened to my cat - in one eye only. The 3rd eyelid was half way over his eye the first day, on the second it became inflamed and red. At that point I took him to the vet, who thought the cat had probably physically injured the eye. He was put on oral antibiotics and given antibiotic eye cream. After a week of treatment he was only slightly better - you could see very little difference - so the vet gave him a further course of both medicines. At the end of the second week the cat was 80% better, and by the end of three weeks he was back to normal. I worried about him a lot, particularly when he showed little sign of improvement, but he did get better in the end and he's fine now.
Reply | Report Abuse | Link to thisThanks for the article. But I wouldn't say that humans don't have a third eyelid because there is no longer an advantage to having one -- that's not how evolution works. For a feature or trait to disappear there needs to actually be an advantage to its removal.
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