The cerebrum is divided into two hemispheres separated by the longitudinal fissure. The terms fissure and sulcus describe grooves in the surface of the cerebrum. Most of the time fissure refers to a larger groove than a sulcus, although they are somewhat interchangeable. In between grooves is found a raised area called a gyrus. Certain of these structures are consistent landmarks for the brain. The outer layer of the cerebrum is composed of gray matter and is called the cerebral cortex. The cerebral cortex is the area of conscious thought and perception. For this reason, and because it forms a cap over the rest of the brain the cerebral cortex has been called the "thinking cap". The cortex can be described as made up of regions called lobes. Each lobe bears the name of the bone lying above it. The central sulcus separates the parietal from the frontal lobe. The lateral fissure divides the parietal from temporal lobe. A short parieto-occipital fissure indicates the upper delineation of the parietal from occipital lobe. The largest fissure is the longitudinal fissure which divides the two hemispheres from one another.