The short answer to this question is that a cat has three eyelids. The reason for this dates back to the cat's historic development and its need then and now for additional protection for one of its most valuable assets: eyesight.
The Third Eyelid
Technically a cat's third eyelid is known as a nictitating membrane. Cats aren't the only critters whose eyes are equipped with this protective feature: birds, reptiles, camels, sharks, llamas and kitty's nemesis -- the dog -- also have this transparent inner eyelid that closes to moisten and protect the eye. VetInfo gives a bit more medical insight, describing the third eyelid as a tiny triangle of pink tissue called the "haw."
As reported in "USA Today," feline geneticists have confirmed what folklore long suspected: the domestic history and development of cats as a human companion began in the Middle East. That's also where there is a tremendous amount of sand that regularly gets blown around by powerful wind storms. It is theorized that the third eye was nature's useful adaptation, allowing kitty to protect his sensitive eyes from the damage of blowing sand while still being able to see to navigate his surroundings.
Its Use Today
The third eyelid continues to be a functional asset for today's modern cat. "Scientific American" describes this physical attribute that is large enough to completely cover the cat's cornea as acting much like a windshield wiper blade, removing debris from the surface and redistributing tears over the cornea. It serves to protect the eye from damage as a cat moves through tall grass or other potentially hazardous environments. It also gives added protection yet seeing ability when cats are capturing prey, or as in the case of a house cat, chasing toys.
Diseases of The Third Eyelid
For the most part, the third eyelid of a feline is not visible. According to VetInfo, it can become visible when a cat is relaxed or sleeping. However, in most cases, the third eyelid becomes a literal eye sore when it is clearly visible. Various ailments cause it to become noticeable. According to Wed MD's Healthy Cats, these include: Horner's syndrome, which is a sunken eye; cherry eye, which occurs when the cartilage folds over itself, causing an unsightly to you and uncomfortable to your cat condition that if left untreated can lead to the formation of ulcers on the cornea; and Haw syndrome, which is a protrusion of the third eyelid that oddly enough is the result of another health problem, such as a gastrointestinal disorder. Veterinarians often look at a cat's third eyelid for initial visual clues to his overall health, as problems in other parts of the cat's body are often expressed by an abnormal color or swelling of the third eyelid.
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