n :  any of three arteries that supply the spinal cord and its membranes and adjacent structures:  a  :  a single unpaired artery that is formed by the anastomosis of a branch of the vertebral artery on each side and that descends in the anterior median fissure of the spinal cord - called also anterior spinal artery   b  :  either of two arteries of which one arises from a vertebral artery on each side below the level at which the corresponding branch of the anterior spinal artery arises and which descend on the posterior lateral surface of the spinal cord along a line passing close to the entrance of the dorsal roots to the spinal cord - called also posterior spinal artery  .
 
 
 
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