The nerves emerging from the spinal cord; there are 31 pairs, each arising from the cord by rootlets that converge to form two roots, anterior (ventral or motor) and posterior (dorsal or sensory); the latter type is provided with a circumscribed enlargement, the spinal (dorsal root) ganglion; the two roots unite in the intervertebral foramen, and the mixed spinal nerve almost immediately divides again into anterior and posterior (primary) rami, the former supplying the anterolateral trunk and the limbs, the latter the true muscles and overlying skin of the back.