XI N. Accessorius
—The accessory (or spinal accessory) nerve arises (Fig. 138, XI) by numerous rootlets from the lateral surface of the medulla and of the spinal cord as far caudad as the fifth to seventh cervical nerve. These spinal rootlets join to form a nerve which enters the cranium through the foramen magnum. After receiving the rootlets from the medulla, it leaves the cranial cavity along with the vagus and glossopharyngeal by the jugular foramen. Just outside the foramen (Fig. 156, c) it becomes involved in a plexus of fine branches which interconnect it with the vagus, sympathetic, and hypoglossal. It then turns laterad and caudad (Fig. 158, 1) and pierces the cleidomastoid muscle (d), to which it gives small branches. It then divides: one branch enters the sternomastoid muscle; the other passes caudad along the dorsal border of the levator scapulæ ventralis, sends branches to the clavotrapezius, and may be traced to the acromiotrapezius and spinotrapezius, which it supplies.