The spinal accessory is derived partly from the brain, and partly from the spinal cord.
The nucleus ambiguus in the medulla oblongata immediately below the glossopharyngeal and vagus.
The lateral aspect of the medulla in series with, and below, the vagus.
Anterior column of grey matter of the spinal medulla, near its lateral margin.
The filaments emerge from the lateral surface of the spinal cord as low as the sixth cervical nerve.
The spinal part ascends in the spinal canal between the ligamentum denticulatum and the posterior nerve roots, to enter the posterior cranial fossa through the foramen magnum. Having joined with its accessory portion, it leaves the skull through the middle compartment of the jugular foramen, lying posterior to the vagus and glossopharyngeal nerves. It passes over the internal jugular vein, then beneath the posterior belly of the digastric and stylo-hyoid muscles, to run below the transverse process of the atlas. At this level it is crossed superficially by the occipital artery. The nerve next pierces the sterno-mastoid obliquely, and emerging from the muscle, traverses the posterior triangle to ramify on the under surface of the trapezius.
The spinal accessory supplies the sterno-mastoid and the trapezius, communicating in the former muscle with the second cervical nerve, and in the latter with the third and fourth cervicals.