Externally, the outer surface of the brain, or cerebral cortex, is composed of six layers of cell bodies referred to as gray matter. Internally, the myelinated axonal processes of these cells extend into the cerebral hemispheres. Because of the whitish appearance of these large bundles of myelinated axons, they are referred to as white matter. In the brain, gray matter is predominantly located on the cortical surface and the white matter runs deep inside the cerebral hemispheres; the opposite is true for the spinal cord, where the white matter is superficial to the gray matter. Topographically, the surface of the cerebral hemispheres has a series of elevations called gyri and infoldings referred to as sulci, both of which significantly increase the surface area of the brain.