The spinal cord is a cylindrical mass of nerve tissue encased within the bony vertebral column, extending from the medulla to the first or second lumbar vertebra. It contains important motor and sensory nerve pathways that exit and enter the cord via anterior and posterior nerve roots and spinal and peripheral nerves. The spinal cord also mediates reflex activity of the deep tendon (or spinal nerve) reflexes. Motor and sensory tracts and the deep tendon reflexes are further discussed on pp. 541-546).
The spinal cord is divided into five segments: cervical (C1-8), thoracic (T1-12), lumbar (L1-5), sacral (S1-5), and coccygeal.
Note that the spinal cord is not as long as the vertebral canal. The level of the nerve roots exiting the cord differs from the adjacent vertebral level. The lumbar and sacral roots travel the longest intraspinal distance. These roots fan out like a horse's tail at L1-2, giving rise to the term cauda equina. (To avoid injury to the cord, most lumbar punctures are performed at the L3-4 vertebral interspace.)
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