"White matter" is the portions of the brain and spinal cord which are white and composed of the long, thin extensions (called axons and dendrites) of neurons. This is in contrast to gray matter, which is the portions that are gray and composed of the cytons (main bodies) of neurons. In the spinal cord, the white matter borders the gray matter, making up the outer layers of nerve tissue. In the brain, the white matter carries the nerve impulses originating in the gray matter.