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You need to know about the nictitating membrane, also called the "third eyelid." A lot of reptiles and birds have this third eyelid, but most mammals just have a little bit of one that doesn't really do much. Except for a few mammals, like camels, for example, who use this membrane to cover their eyes and protect them from sand. Polar bears have it, too, which keeps them from getting snow blindness. One interesting thing about the nictitating membrane is that it goes across the eye from side to side, instead of up and down like regular eyelids.
Dogs and cats have a third eyelid that we don't often see, if you do see it frequently, though, it probably means the dog or cat is sick.
Anyway, there is a gland in the third eyelid that makes tears to help keep the eye from getting all dry and scratchy. Sometimes the gland prolapses, it sticks up and covers part of the eye. If it's not fixed, it can get infected and maybe even damage a dog's eye.
Nobody knows exactly why the gland prolapses, maybe because the tissue that's supposed to hold it where it belongs is weak which could be inherited from a sire or dam.
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