Have you ever noticed that Siamese cats are cross-eyed? In fact, that's the only way a Siamese cat can see straight.
Unlike the eyes of some animals such as rabbits, a cat's eyes both point forwardjust like oursso most of what it sees it sees with both eyes. In order to see clearly, however, the brain has to coordinate the signals it gets from a group of nerve-endings called the "retina" on the back of each eye.
For every spot on the retina of one eye, there's a spot on the retina of the other eye that has to see the same thing. Let's say, for example, that those spots are focused on a mouse.
For the brain to interpret what it sees as one mouse instead of two, the nerves that detect the mouse in one eye have to go to the same part of the brain as the nerves in the other eye that detect the same mouse. If the eyes send the mouse-images to two different parts of the brain, the cat sees two mice instead of one.
And that's what's wrong with the eyes of a Siamese cat. Instead of being lined up in the back of the eye, the center of the left retina is shifted to the right and the center of the right retina is shifted to the left. So if a Siamese cat's eyes were pointed straight ahead, it's retinas would be looking in different directions, sending a very confused message to the brain.
By turning its eyes in, a Siamese cat looks cross-eyed, but its retinas are now lined up like a normal cat's, sending the brain a clearer picture.