Medially, the posterior and anterior roots divide into rootlets, which attach to the spinal cord.
A spinal segment is the area of the spinal cord that gives rise to the posterior and anterior rootlets, which will form a single pair of spinal nerves. Laterally, the posterior and anterior roots on each side join to form a spinal nerve.
Each spinal nerve divides, as it emerges from an intervertebral foramen, into two major branches: a small posterior ramus and a much larger anterior ramus (Fig. 2.61):