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.
XII. N. hypoglossus
([Fig. 156], b).—The twelfth or hypoglossal nerve arises from the ventral side of the medulla ([Fig. 138], XII), as previously [described]. It emerges from the skull through the hypoglossal canal. At first it passes ventrad, then gradually turns craniad, following thus a curved course and passing successively laterad of the vagus and sympathetic nerves ([Fig. 156], d and e), the common carotid artery, and the cranial cornu of the hyoid bone. A short distance from its origin the nerve gives off a ramus descendens ([Fig. 156], f), which receives a communicating branch from the first cervical nerve (j), and divides into two branches, one (g) to the thyrohyoid muscle (9), the other to the sternohyoid (8) and sternothyroid muscles (7). The main nerve passes beneath the mylohyoid muscle into the tongue, at first following the lingual artery, then laterad of it, then crossing it again. It sends branches to all the muscles of the tongue, and may be traced to its tip.
The hypoglossal nerve is involved in the plexus just outside of the jugular foramen, with which the vagus, sympathetic, and accessory nerves are also connected.
2. Spinal Nerves.
—The origin and general features of the spinal nerves have been described in connection with the account of the spinal cord ([page 337]). A description of their peripheral distribution will now be given.
A. Cervical Nerves.
—There are eight pairs of cervical nerves. The first leaves the vertebral canal through the atlantal foramen. The second passes out between the arches of the atlas and axis, not through a special intervertebral foramen, so that its ganglion lies among the muscles of the back of the neck. The others emerge through the intervertebral foramina, the eighth one from between the last cervical and first thoracic vertebræ.
