The cerebrum makes up approximately 85% of the weight of the human brain. It is divided into right and left hemispheres by the longitudinal fissure
, a large groove that runs front-to-back, down the middle of the brain. The two hemispheres are connected by the corpus callosum
, a large bundle of white matter
tracts in the middle of the brain.
The cerebral cortex is a 3-4mm-thick layer of gray matter that covers each cerebral hemisphere. The fact that the cortex is gray matter means that it is composed of unmyelinated cells. The cortex contains billions of interconnected neurons, and is probably the physiological site of higher cognitive functions, such as consciousness and personality.
The inner mass of the cerebrum is composed of several kinds of myelinated fibers (white matter) that form a wide variety of connections. Projection fibers connect the cerebrum to other parts of the brain (such as the brainstem). Association fibers connect different parts of the same cerebral hemisphere. Commissural fibers connect one side of the brain to the other side.
Each cerebral hemisphere can be divided into five different lobes, separated by prominent fissures. Four of these lobes are visible externally, and are named after the bones of the skull which cover them. They are the frontal lobe, parietal lobe, temporal lobe, and occipital lobe. The fifth lobe, the insula, is located just behind the temporal lobe, and is not visible from the outside. The Fissure of Rolando (central sulcus) separates the frontal and parietal lobes. The parieto-occipital fissure separates the parietal lobe from the occipital lobe. The Fissure of Sylvius (lateral fissure) is a deep groove near the side of the brain. It separates the temporal lobe from the frontal and parietal lobes, as well as from the insula.