BRAIN, DRAWING. BRAIN, DRAWING The brain is divided into two hemispheres bridged by a mass of white matter, the corpus callosum, made up of millions of nerve fibers. This, along with the brain stem and the cerebellum in the posterior area, constitute the encephalon. The surface of each hemisphere forms sinuous folds, separated by grooves, or sulci, which divide the brain into lobes which are named after the overlying skull bones: frontal, parietal, temporal and occipital.