An X-shaped structure formed by a crossing-over of the two optic nerves (see EYE) which run from the back of the eyeballs to meet in the mid line beneath the brain. Nerve fibres from the nasal part of the retina cross to link up with fibres from the outer part of the retina of the opposite eye. The linked nerves form two separate optic tracts which travel back to the occipital lobes of the brain.