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.