Definition of Gray matter

Gray matter: The cortex of the brain which contains nerve cell bodies. The gray matter is in contrast to the white matter, the part of the brain that contains myelinated nerve fibers. The gray matter is so named because it in fact appears gray. The white matter is white because that is the color of myelin, the insulation covering the nerve fibers. In "The Mysterious Affair at Styles" (1920), Agatha Christie first quoted the fictional Belgian detective Hercule Poirot in regard to his gray matter: "'This affair must be unravelled from within.' He tapped his forehead. 'These little grey cells. It is "up to them"-as you say over here.'"