758. How many pairs of nerves issue from the spinal cord? Explain fig. 128. Fig. 129.
759. The anterior roots arise from a narrow white line upon the anterior columns of the spinal cord. The posterior roots arise from a narrow gray band formed by the internal gray substance of the cord. They are larger, and the filaments of origin more numerous than those of the anterior roots. A ganglion is found upon each of the posterior roots in the openings between the bones of the spinal column through which the nerve passes.
Fig. 130.
Fig. 130. A section of the spinal cord, surrounded by its sheath. B, A spinal nerve, formed by the union of the motor root (C) and the sensitive root (D.) At D, the ganglion upon this root is seen.
760. After the formation of the ganglion, the two roots unite, and constitute a spinal nerve, which passes through the opening between the vertebræ on the sides of the spinal column. The nerves divide and subdivide, until their minute filaments ramify on the tissues of the different organs.
759. Give the origin of the anterior roots. Of the posterior roots. In what respect do the posterior roots differ from the anterior? 760. When do the two roots unite, and where do they pass?
761. The spinal nerves are divided into—