n :  the thick longitudinal cord of nervous tissue that in vertebrates extends along the back dorsal to the bodies of the vertebrae and is enclosed in the vertebral canal formed by their neural arches, is continuous anteriorly with the medulla oblongata, gives off at intervals pairs of spinal nerves to the various parts of the trunk and limbs, serves not only as a pathway for nervous impulses to and from the brain but as a center for carrying out and coordinating many reflex actions independently of the brain, and is composed largely of white matter arranged in columns and tracts of longitudinal fibers about a large central core of gray matter somewhat H-shaped in cross section and pierced centrally by a small longitudinal canal continuous with the ventricles of the brain - called also medulla spinalis  .
 
 
 
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