Neurons have two main parts--the axon and the cell body. The axons are collectively called "white matter" because their fatty coating (known as the myelin sheath) makes them look white when you're looking at a slice of brain.
"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.

White matter is nervous tissue composed mainly of myelinated nerve fibers. It makes up the conducting part of the brain and spinal cord. The medulla consists of both white and gray matter. The white matter consists mainly of nerve tracks that pass between the spinal cord and the various portions of the brain. It consists of fibers that connect the two hemispheres of the brain and the fibers that are part of both ascending and descending tracts. Basel gangala are located within the white matter and links the various areas of the brain. White matter of the cerebrum consists of myelinated neurons. There is a large band of white matter, called the corpus callosum, which connects the right and left hemispheres.

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