The anterior and posterior surfaces of the spinal cord have several longitudinally running fissures and sulci. Along the midline on the anterior surface of the spinal cord is a deep separation, the anterior median fissure (eFig. 9.47). Posteriorly the spinal cord has a shallower separation, the posterior median sulcus, which is flanked on either side by a posterolateral sulcus (eFig. 9.47). Emerging from the spinal cord are a series of rootlets, which coalesce to form anterior and posterior roots at the corresponding cord segment (eFig. 9.48). These anterior and posterior roots converge to form 31 pairs of spinal nerves, which extend the length of the spinal cord (eFig. 9.44). Along the length of the spinal cord two regions are enlarged to accommodate the numerous neurons innervating the upper and lower extremities. The cervical enlargement extends from C5 to T1 and innervates the upper extremities, whereas the lumbar enlargement extends from L2 to S3 and innervates the lower extremities (eFig. 9.44).