Why don't you notice the hole in each eye's vision?

You don't notice it because your brain fills the gap in with a guess. Each eye has a small patch of the retina with no light detectors, where the optic nerve leaves the eye. Light landing there is simply not detected. But the brain interpolates the gap using the surrounding detail and the view from your other eye, so what you perceive is not a hole but a seamless scene.

That patch is called the optic disc. It is the exit door for about a million nerve fibres heading to the brain, and because the door has no rods or cones on it, it is a blind spot. In people it sits roughly 12 to 15 degrees to the outer side of where you are looking, and it is about 7.5 degrees high and 5.5 degrees wide. That is a bigger patch than most people would guess, and the other eye's view helps to cover it.

Cover one eye and the brain still doesn't leave a hole. Show a pattern of texture that runs across the blind spot, and you see the texture continue straight through it. This is called filling-in: what you see there is the brain's best guess, based on what surrounds the gap.

The discovery has a royal story. In the 1660s the French scientist Edme Mariotte found it. Until then, people expected the point where the optic nerve enters the eye to be the most sensitive part of the retina, and Mariotte showed the opposite. He is said to have presented the finding at the court of King Louis XIV.

All vertebrates share the flaw. The eyes of cephalopods such as the octopus evolved separately and don't have it, because their optic nerve approaches the receptors from behind.