Cats have a total of three eyelids: the top and bottom lids that help Felix blink and an extra third eyelid, tucked inside the corner of the eye. This third eyelid is called a nictitans and is basically an extra layer of protection for the cornea. Knowing how much cats like to fight with the neighboring cats, this extra eyelid acts as a shield to protect the cornea. Sometimes the third eyelid can become elevated with trauma (such as a corneal ulcer, which is a scratch on the clear surface of the eye) or can pop up with general malaise (from severe dehydration or weight loss) or infections (such as upper respiratory viruses). You normally shouldn’t see the third eyelid at all, but if it pops up, something is wrong, and you should take your cat to a vet for closer inspection.
Material from It's a Cat's World... You Just Live In It, available at Amazon.com. More information available at www.drjustinelee.com. Copyright © 2008 Justine Lee Veterinary Consulting, LLC.