Cats and eyelids

The third eyelid is really only visible in a cat if the cat is unwell in some way. In some ways it is a protective mechanism to give extra protection to the eye if the cat has got a sore eye, or been in a fight. But it also comes up if the cat has a high temperature or fever, or has some other illness. In a small minority of cats there can be a birth defect that means the "third eyelid" is always half covering their eye even though the cat is perfectly healthy, but on the whole, if you see your cat with this eyelid visible, he/she ought to be taken to the vets for a check up.

Humans, dogs and cats have a lot of features in common. We share many of the same reactions to stress and pain, but there is one thing that cats, canines and most other animals have that we don’t, and that is the third eyelid. What is the third eyelid? Well, medically it is known as the “nictitating membrane.” In English it is an eyelid that provides extra protection for the eye. Rather like the wiper on our windshield. Our windshield wipers are usually connected to a container of water to clean our windshield, the third eyelid contains a gland that provides tears to keep the eye moist, among other things. It also provides antibodies and an enzyme that can inactivate bacteria. http://ezinearticles.com/?Understanding-Cherry-Eye/The-Third-Eyelid&id=433536