GENERAL FEATURES The spinal cord is housed in the vertebral canal. It is continuous with the medulla below the pyramidal decussation and terminates as the conus medullaris at the second lumbar vertebra of the adult. The roots of 31 pairs of spinal nerves arise segmentaUy from the spina) cord. There are eight cervical pairs of spinal nerves (Cl through C8). The cervical enlargement (C5 through Tl) gives rise to the rootlets that form the brachial plexus, which innervates the upper limbs. There are 12 thoracic pairs of spinal nerves (Tl through T12). Spinal nerves emanating from thoracic levels innervate most of the trunk. There are five lumbar pairs of spinal nerves (LI through L5). The lumbar enlargement (LI through S2) gives rise to rootlets that form the lumbar and sacral plexuses, which innervate the lower limbs. There are five sacral pairs of spinal nerves (Si through S5). Spinal nerves at the sacral level innervate part of the lower limbs and the pelvis. There is one coccygeal pair of spinal nerves. The cauda equina consists of the dorsal and ventral roots of the lumbar, sacral, and coccygeal spinal nerves. Inside the spinal cord, gray matter is centrally located and shaped like a butterfly. It contains neuronal cell bodies, their dendrites, and the proximal parts of axons. White matter surrounds the gray matter on all sides. White matter contains bun- dies of functionally similar axons called tracts or fasciculi, which ascend or descend in the spinal cord