nerve of the face, ear, palate, and tongue, (8) the auditory or nerve of hearing, (9) the glosso-pharyngeal, nerve of sensation and taste, (10) the pneumogastric or vagus, which is both motor and sentory and governs respiration, the heart, and the stomach, (11) the spinal accessory, to the muscles of the soft palate, and (12) the hypoglossal, the motor nerve to the tongue.
The spinal nerves also are arranged in pairs: Eight cervical pairs, twelve dorsal or thoracic, five lumbar, five sacral, and one coccygeal, these titles denoting their point of origin near the vertebra of the same name. Each of these nerves arises by two roots, an anterior motor root from the anterior horn of gray matter and a posterior sensory root from the posterior horn, the latter having a ganglion upon it. After emerging from the cord the two roots unite to form the nerve, that the nerve may contain both motor and sensory fibers. The motor fibers are called efferent because they carry impulses from the cord, while the sensory are called afferent because they carry impulses back to the cord. After leaving the cord the nerves unite to form plexuses, which again divide into various nerve trunks and are distributed to the muscles.
The first cervical nerves pass out of the spinal column above the first cervical vertebra and the other cervical nerves below that and the succeeding vertebræ, while the other spinal nerves emerge each below the corresponding vertebra, as the first dorsal below the first dorsal vertebra, etc. After emerging they break up into a large anterior division and a small posterior division, the posterior branches supplying the spine and the dorsal muscles and skin, the anterior the rest of the trunk and the limbs. The cervical plexus is formed by the anterior divisions of the first four cervical nerves, the brachial plexus by the last four cervical and the first dorsal or thoracic nerves, the lumbar plexus by the four upper lumbar, and the sacral plexus by the last lumbar and the four upper sacral nerves.
