The accessory nerve [XI] is a cranial nerve that carries BE fibers to innervate the sternocleidomastoid and trapezius muscles (see Diogo R et al. Nature 2015;520:466–473). It is a unique cranial nerve because its roots arise from motor neurons in the upper five segments of the cervical spinal cord. These fibers leave the lateral surface of the spinal cord and, joining together as they ascend, enter the cranial cavity through the foramen magnum (Fig. 8.54). The accessory nerve [XI] continues through the posterior cranial fossa and exits through the jugular foramen (Fig. 8.53). It then descends in the neck to innervate the sternocleidomastoid and trapezius muscles from their deep surfaces.