Thirty-one spinal nerves pass out on each side of the spinal cord through the intervertebral foramina. They correspond in name with the bones with which they are associated, thus :
Cervical (8), Dorsal (12), Lumbar (5), Sacral (5), Coccygeal (1).
Each nerve has two roots, an anterior and a posterior, which arise from two cornua, or horns, formed by projections backward and forward of the gray matter of the cord. The anterior root is composed of motor fibres, the posterior root of sensory fibres. On the posterior nerve roots there are swellings, called ganglia ; immediately beyond these ganglia the two roots unite and form a nerve trunk, which is very short, as it immediately divides into the anterior and posterior primary divisions. Each division contains fibres from both roots, so all the spinal nerves in their distribution are mixed nerves— that is, they contain both sensory and motor fibres.
The motor or efferent fibres of the anterior root are distributed to the muscles, and cause their contractions. If the anterior root of the nerves supplying a certain part be injured, loss of power results in that part, though feeling remains.
The sensory or afferent fibres end in the skin, and convey impressions of feeling to the brain ; on entering the spinal cord they cross over to the opposite side, and by it are conducted upwards. If the posterior root of the nerves supplying 3. certain part be injured, loss of feeling will result in that part, though power of movement remains.
If a nerve composed of efferent and afferent fibres be injured, sensation and movement are both lost in the part supplied by that nerve. If the spinal cord be injured, there will be loss of sensation and of power of movement in the body below the injury ; but the parts above the injury will continue to have their ordinary power.