The portion of the central nervous system within the spinal canal of the vertebral column, that is, the entire central nervous system except the brain. The spinal cord extends from the foramen magnum at the base of the skull to a variable level of the spinal canal; it terminates at the lumbar level in humans and extends well into the caudal region in fishes. Paired spinal nerves enter the spinal canal between each pair of vertebrae and connect with the spinal cord. The number of spinal nerves varies widely in vertebrates; in humans there are 31 pairs (8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal). In tetrapods there are spinal cord enlargements where the large spinal nerves extend to the limbs, a cervical enlargement cranially, and a lumbosacral enlargement caudally.