The spinal accessory nerve [n. accessorius] is exclusively motor. It consists of two parts, the accessory or superior, and the spinal or inferior part.
The fibers of the accessory or superior portion [ramus internus] ("accessory vagus") spring chiefly from the inferior continuation of the nucleus ambiguus, in common with the motor fibers of the vagus above, and they pass through the reticular formation to the postero-lateral sulcus of the medulla, where they emerge as a series of filaments, below those of the vagus. The filaments pierce the pia mater and unite, as they pass outward in the posterior fossa of the cranium, to form a part of the nerve which enters the apertm-e in the dura mater common to the vagus and spinal accessory nerves. In the aperture this trunk is joined by the spinal portion of the nerve.
The spinal or inferior portion [ramus externus] arises from the ventro-lateral cells of the ventral horn of the cord as low as the fifth, and rarely the seventh, cervical nerve. The fibers pass dorsalward and lateralward from their origins through the lateral part of the ventral horn and through the lateral funiculus of white substance, and they emerge from the lateral aspect of the cord behind the ligamentum denticulatum, along an oblique line, the lower fibers passing out immediately dorsal to the ligament, and the upper close to and sometimes in association with the dorsal roots of the upper two spinal nerves. As the spinal fibers pass out of the surface of the cord they unite to form an ascending strand which enters the posterior fossa of the cranium, through the foramen magnum, and, turning lateralward, blends more or less intimately with the accessory portion. Thus combined, the nerve enters the jugular foramen in company with the vagus, but here it is again separated into its two branches, which contain chiefly the same fibers as the original superior and inferior parts.
The superior branch, or accessory portion of the nerve, gives one or more filaments to the jugular ganghon (ganglion of the root of the vagus), and then joins either the trunk of the vagus directly or its ganglion nodosum, the fibers of the branch being contributed to the pharyngeal, laryngeal, and cardiac branches of the vagus. Fibers corresponding to the white rami communicantes, absent in the cervical nerves, probably enter the cervical sympathetic ganglion through this ramus of the spinal accessory nerve. The fibers from the accessory to the vagus therefore probably include visceral motor and cardio-inhibitory fibers.
The inferior branch or the spinal portion runs backward and downward under cover of the posterior belly of the digastric and the sterno-mastoid. It usually crosses in front of and to the lateral side of the internal jugular vein and between it and the occipital artery; then it pierces the sterno-mastoid, supplies filaments to it, and interlaces in its substance with branches of the Second cervical nerve. It emerges from the posterior border of the sterno-mastoid slightly above the level of the upper border of the thyreoid cartilage, passes obliquely downward and backward across the occipital portion of the posterior triangle, and disappears beneath the trapezius about the junction of the middle and lower thirds of the anterior border of that muscle (fig. 743). In the posterior triangle it receives communications from the third and fourth cervical nerves, and beneath the trapezius its fibers form a plexus with other branches of the same nerves. Its terminal filaments are distributed to the trapezius and they can be traced almost to the lower extremity of that muscle.
The nuclei of origin, like other motor nuclei, are connected with the somsesthetic area of the cerebral cortex of the opposite side by the pyramidal fibers, and they are associated with the sensory nuclei of other cranial nerves by the medial longitudinal fasciculus, and with sensations brought in by the spinal nerves by the fibers of the fasciculi proprii.