The
optic chiasm or
optic chiasma (
Greek , "crossing", from the Greek 'to mark with an X', after the Greek letter '
Χ',
chi) is the part of the
brain where the
optic nerves (CN II) partially cross. The optic chiasm is located at the bottom of the brain immediately below the
hypothalamus. The optic chiasm is found in all vertebrates, although in cyclostomes (lampreys and hagfishes) it is located within the brain.