Notice in this magnified view of the spinal cord that the gray matter contains many nerve cell bodies (pointed to by the red arrows), which gives the region its darker color.  The gray matter functions as the integrative component of the spinal cord and is responsible for mediating spinal reflexes.  The white matter is comprised of primarily myelinated axons along with fewer numbers of unmyelinated fibers.  The axons of the white matter are organized into tracts.  A tract is a bundle of axons within the central nervous system (CNS) that have the same origin, termination and function.  Tracts carry impulses to and from the brain and to other levels of the spinal cord.  The lighter color of the tracts is due to the white color of the fatty myelin that surrounds most of the axons.