optic chiasm definition
Optic chiasma. The optic chiasm or optic chiasma is an X-shaped space, located in the forebrain, directly in front of the hypothalamus. Crucial to vision, the left and right optic nerves intersect at the chiasm, thus creating the hallmark X-shape. One-half of each nerve's axons (their long, threadlike portions) enters the opposite tract at this location, making it a partial decussation (crossing).