Anat. Also chiasm. [mod. L., a. Gr. χίασμα arrangement of two lines (sticks, etc.) crossed like the letter Χ (χῖ), decussation; χιάζ-ειν to mark with or like a chī (Χ, χ).] Intercrossing or decussation. Optic chiasma: the crossing or decussation of the fibers of the optic-nerves at the base of the brain; the optic commissure.
1839–47. Todd, Cycl. Anat., III. 769/1. The existence of a chiasma is not general throughout the animal series.
1873. Mivart, Elem. Anat., 387. This decussation of fibres is called the optic commissure or chiasma.
1880. Günther, Fishes, 104. In Palæichthyes the two nerves are fused together, immediately after their origin, into a chiasma.
