The eleventh cranial or spinal accessory nerve consists of an upper and lower portion. The accessory or upper portion is smaller. It arises deeply from the floor of the fourth ventricle in common with the ninth and tenth cranial nerves. Superficially, it arises by fine filaments from the side of the medulla oblongata below the origin of the vagus nerve. It then emerges with it through the same groove. The spinal or lower portion is larger. It arises by several filaments form the side of the spinal cord, between the ligamentum denticulatum and the posterior roots of the spinal nerves as low down as the sixth cervical nerve. It enters the cranial cavity through the foramen magnum of the occipital bone. It then exits through the middle compartment of the jugular foramen. The spinal accessory nerve is a motor nerve. It supplies the sternocleidomastoid and the trapezius muscles.

Motor

Turns heads to opposite side (sternocleidomastoid) Elevates and rotates scapula (trapezius)

Jugular foramen

Neck

Radix spinalis nervus accessorius

Nervus accessorius [XI]

Pars spinalis nervus accessorius