The corpus callosum (/ˈkɔːrpəs kəˈloʊsəm/; Latin for "tough body"), also known as the callosal commissure, is a wide, flat bundle of neural fibers about 10 cm long beneath the cortex in the eutherian brain at the longitudinal fissure. It connects the left and right cerebral hemispheres and facilitates interhemispheric communication. It is the largest white matter structure in the brain, consisting of 200–250 million contralateral axonal projections.