The corpus callosum is a broad, thick bundle of axons which runs sideways connecting the two cerebral hemispheres. It is a white matter structure beneath the cerebral cortex. The corpus callosum is composed of 250 million contralateral axonal projections. The posterior portion of the corpus callosum is called the splenium; the anterior is called the genu; between the two is the body. The rostrum is the portion of the corpus callosum that projects posteriorly following from the anteriormost genu.