 The optic chiasm is formed by the union of the two optic nerves.The nasal fibers of each optic nerve decussate (cross) across the chiasm to the contralateral side while the temporal fibers course posteriorly to form the optic tract on the ipsilateral side.This arrangement allows the left half of the visual field to end up on the right side of the brain and the right half of the visual field to locate to the left side.