If Kitty's eyes look a bit strange, you may be seeing her third eyelid, known more formally as her nictitating membrane. Yes, cats have three eyelids on each eye, as most animals do. In fact, humans are among the few mammals that don't have a third eyelid.
Kitty Has Three Eyelids
If you woke Kitty up from a nap, or perhaps sedated her for a car ride, you might see some translucent tissue covering up her beautiful eyes. That's probably her third eyelid, which is a membrane in the inside corner of her eye. About a hundred years ago, veterinary science thought the third eyelid was irrelevant; in fact there were descriptions detailing how to remove it so the eye could be examined more easily.
Three's Not a Crowd
Now we know better: Kitty's third eyelid has a purpose, which is to protect her cornea. Think about her wild ancestors who spent their time hunting prey, sometimes in tall grasses. The third eyelid protects a cat's eyes as she moves through her world, shielding her cornea from injury. The extra eyelid also acts a bit like a windshield wiper, removing debris from the eye and moistening it with tears.
Hidden From View—Mostly
When Kitty's alert, you won't see most of her third eyelid because it's hidden in her eye socket; you may see a bit of it in the corner of her eye. If she blinks, or is relaxed or asleep, the eyelid makes an appearance, thanks to a set of muscles that helps it move from the inner, lower corner of her eye to the upper, outer corner. The sympathetic nervous system and muscle cells within the eyelid also help it move.
Third Eyelid Conditions
If you see Kitty's third eyelid, it doesn't necessarily mean there's a problem. It may be quite the opposite: she may be very relaxed. But Kitty's third eyelid can also appear if her eyes are irritated. If she's dropped some weight, her third eyelids may become more prominent because the fat pads behind her eyes have shrunk, which can change her eyes' position in their sockets. Although this is rare, intestinal, neurologic or respiratory infections can affect the nerve control of the third eyelid. If one eyelid is making an appearance, it's often a sign that that specific eye is irritated or has suffered some trauma. The three most common third eyelid problems in cats are Horner's syndrome, haw syndrome ("haw" is an older term for the eyelid) and cherry eye, which vary in cause and treatment. If Kitty isn't tired or sedated, but her third eyelid is making itself known, it's best to have a vet take a look to make sure all is well with her eyes.