Cats and dogs have a third eyelid. Normally you don’t see it, as it stays tucked down in the corner of the eye. There are tear-producing glands on the back side, and periodically it sweeps across the eye like a windshield-wiper, spreading the tears during the blinking process. When the outer lids droop, they close together. When the third eyelid (nictitating membrane) droops, it slides out over the eye. Most of the time, it just indicates that the pet is feeling crummy. If an animal is really feeling low, it may cover most of the eye, which leads to frantic phone calls saying "My dog’s eyes have rolled back into his head!!" Of course, that couldn’t really happen, as it would rip off your optic nerves when the eyeball rolled around 180 degrees. The pale third eyelid can resemble the white of the eye (sclera) and that’s what fakes people out.
This cat has Haws syndrome (and I can’t find out who Dr. Haws was, so let me know if you know. Of course, it could be called "haws syndrome" because "haw" is another name for the third eyelid. But that’s so mundane… and it doesn’t really explain anything. Why not just call it "third eyelid syndrome"? But I digress). The cat seems to feel fine except for the third eyelids showing. This is believed to be caused by intestinal irritation, as in an animal with a heavy worm burden, or colitis. The mechanism is poorly understood (I don’t think it’s the same as "You’re so full of ____ your eyes are brown", but since we don’t really know…).
There are lots of write-ups on this if you Google it, but I just happened to get this cool picture, so I’m adding yet another piece of informational flotsam to the web. The syndrome usually resolves over a few weeks as mysteriously as it appeared. De-worming may help,if that’s what the problem is. Some may benefit from treatment for intestinal inflammation. I wouldn’t give them an enema to lower the "brown" content, though.