"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.
From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.