The cortex of the brain which contains nerve cell bodies. The gray matter is as opposed to the white matter, the part of the brain that contains myelinated nerve fibres. The gray matter is so named because it in fact appears gray. 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.'