The thirty-one pairs of nerves which leave the vertebral canal by the openings between the vertebrae are called spinal nerves, in contradistinction to the cranial nerves, which pass through the base of the skull. They are attached to the spinal marrow by two bands, the anterior and posterior roots, which unite together in the intervertebral canal to form the trunk of the nerve. Just before the junction of the two roots the posterior one is enlarged by a ganglionic swelling.
The spinal nerves are all "mixed nerves," that is to say, they contain both efferent and afferent fibres; but these two sets of fibres are separate in the roots of each nerve, the posterior root containing only afferent, and the anterior only efferent fibres. The spinal nerves are thus joined to the spinal marrow by two nervous cords, each one of which is functionally distinct. About seventy years ago Charles Bell discovered that the anterior roots were motor, and the posterior sensory channels. Hence, the anterior are commonly spoken of as the motor roots, and the posterior as the sensory roots of the spinal nerves. The experiments to show this difference are simple, but require delicate manipulation.
If the anterior roots of the nerves supplying the hind leg of a recently-killed frog be divided, the muscles of the limb are cut off from the centres in the spinal cord, and the leg hangs limply, and does not move if pinched when the frog is suspended; whereas the limb on the sound side, upon which the anterior roots are intact, will move energetically when the motionless one is irritated. If the distal ends of the divided anterior roots be stimulated, the muscles of the paralyzed limb are thrown into action; but stimulation of the proximal end gives no result. If the two webs of this frog be compared, the blood vessels running across the transparent part of the web on the injured side will be found to be fuller than those in the web of the other limb, but if the distal ends of the motor roots be stimulated, the dilated blood vessels return to their normal calibre. By these experiments we are shown that efferent fibres carrying impulses to the muscular walls of the vessels are contained in the anterior roots of the spinal nerve, together with fibres to the skeletal muscles.