Begins in the cervical segments of the spinal cord (C1 through C6) where a large number of thin nerve fibre bundles in a row from the bottom up protrude out of the lateral cord. The bundles merge into a unit that is close to the surface of the spinal cord and eventually disappears into the cranial cavity through the large neck hole (foramen magnum). The extra nerve reaches up to the place where the vagus nerve emerges from the medulla oblongata and then joins this nerve out of the cranial cavity where it motorically supplies a neck and a shoulder-neck muscle (m.sternocleidomastoid =nick muscle; m.trapezius = coat muscle).
N.accessorius actually consists of two components:
(a) a part of the spinal cord; and
(b) a part coming out of the medulla containing nerve fibres intended for N. vagus.
The two parts (α+b) merge into one unit inside the skull cavity. The fusion runs alongside and together with n. vagus out of the skull cavity. Once outside, the part that started from the medulla passes into the n. vagus and the actual n. accessorius, which now consists only of nerve fibers coming from the spinal cord, find their way to the nodal and cape muscles.