Why does each hemisphere's visual cortex see the opposite half of the world with both eyes? · Grey Matter

Each retina is split down the middle: its nasal half sees the outer part of the field on its own side, its temporal half the inner part toward the other side.

Why does each hemisphere's visual cortex see the opposite half of the world with both eyes?

Each retina is split down the middle: its nasal half sees the outer part of the field on its own side, its temporal half the inner part toward the other side. At the chiasm the nasal fibres cross and the temporal ones stay, so the left optic tract collects everything both eyes see of the right half of the world, and the right tract the left half. That arrangement brings the two eyes' views of the same point to the same hemisphere, which is what the visual cortex needs to compare them and see depth.