As pointed out above, seeing is done in the brain. It tries to help fill in the gaps. You have a blind spot in each eye (where the nerve bundle comes in) but it is at a different spot in each eye so it's an issue only when you close one eye and look a certain way. You can find the blind spot and make the tip of your finger disappear when you do it right but even in this case you still get a background image that seems to fit. This is just your brain making it up.