The cerebral cortex is the highly convoluted external surface of the brain packed in tightly in the cranium. This inordinately creased and folded brain surface arose during evolution as the volume of the cortex increased considerably more rapidly than the cranial volume. The cerebral cortex consists of more than 100 billion neurons. More than two-thirds of the cortical surface is buried in the grooves, called "sulci" and "fissures;" with the latter being deeper than the former. It has a grey color, hence the name "grey matter". Grey matter is formed by neurons and their unmyelinated fibers.
In evolutionary terms, the most recent part of cerebral cortex is called the neocortex, which is made up of six layers of neurons; this is the seat of human intelligence and thoughts. The more ancient part of the cerebral cortex is called the hippocampus, which has three cellular layers, and is divided into subfields.
The cerebral cortex is divided into right and left hemispheres. It encompasses about two-thirds of the brain mass and lies over and around most of the other brain structures. The fissures divide the cerebral cortex into lobes; frontal, temporal, parietal, and occipital. Each one of these lobes have a specific function. For example, there are specific areas involved in vision, hearing, touch, movement, and smell. Other areas are critical for thinking and reasoning.