A common, general question about cats is How many eyelids do cats have? The answer to this question is that a cats eye has three eyelids. Cats have a third eyelid.
Even if someone looks close at a cat that is alert, they may not be able to detect how many eyelids cats have. The third eyelid may not be visible when the cat is awake and alert since almost all of it is covered by the cats outer eyelids.
The haw and nictitating membrane are other names for the third eyelid. Veterinarians may refer to the third eyelid as the palpebra tertia. A small part of the third eyelid may be visible in the inner corner of the cats eye. The nictitating membrane is often more visible in cats that have eyes that protrude more than usual.
Third eyelids may be more visible while the cat is resting if the cats eyes are partially open. If the cat sleeps with its eyes open slightly, the nictitating membranes might be visible.
The nictitating membrane closes over the eye from the inner corner of the cats eye. The purpose of the nictitating membrane is to cleanse the eye by wiping any dust or debris from the surface of the eye.
For cats in the wild, the nictitating membrane may be an important source of protection. Some cat owners have noticed the nictitating membrane closing while the cat is jumping. The instinctual closure of the third eyelid may protect the eye while the cat is running through grass or jumping.
The nictitating membrane adds moisture to the eye. The tears that are spread by the nictitating membrane flow over the underside of the membrane which is covered with lymph cells. The tears act as added protection by spreading substances from the immune system that help protect the eyes from infection.
Inside the membrane surfaces of the nictitating membranes is a layer of cartilage. Many mammals other than humans have nictitating membranes.
A nictitating membrane may partially cover the eye. This abnormal visibility of the third eyelid can be caused by a viral infection. A virus that causes the exposure of the third eyelid can also cause diarrhea, vomiting, and sneezing.
This viral infection is not usually serious. Cat owners should have their cats examined by a veterinarian whenever the cat has a change in health.