The =hypoglossal nerve= (N. hypoglossus) is purely motor and innervates
the muscles of the tongue (Fig. 437). Its root-fibers arise from the
ventral face of the medulla in linear series about 3 to 4 mm. lateral to
the posterior half of the pyramid (Fig. 499). The filaments converge to
three or four bundles which perforate the dura mater and unite to form
the trunk. The latter emerges through the hypoglossal foramen (Fig. 515)
and runs downward and backward between the guttural pouch and the
capsule of the atlanto-occipital articulation for a distance of a little
less than an inch (ca. 2 cm.). It then passes between the tenth and
eleventh nerves, turns downward and forward, crosses the external face
of the external carotid artery, and continues over the pharynx parallel
with the great cornu of the hyoid bone and behind the external maxillary
artery. It then crosses beneath the artery, runs forward on the external
face of the hyo-glossus muscle, and divides into its terminal branches
(Rami linguales). The smaller branch supplies the stylo-glossus,
hyo-glossus, and lingualis. The larger branch ramifies on the
genio-glossus and supplies the remaining muscles. Anastomoses occur with
branches of the lingual nerve.
In the first part of its course the nerve communicates with the superior
cervical ganglion and with the ventral branch of the first cervical
nerve, and gives filaments to the pharyngeal branch of the vagus and the
pharyngeal plexus.
The fibers of the nerve arise from the hypoglossal nucleus, an
elongated group of large multipolar cells situated chiefly under the
posterior part of the floor of the fourth ventricle close to the
median plane. The two nuclei are connected by commissural fibers. The
other central connections include: (_a_) communications by the medial
longitudinal fasciculus with the nuclei of termination of other
cranial nerves; (_b_) cortico-nuclear fibers which come from the
cortex by way of the internal capsule and the pyramids and go largely
to the nucleus of the opposite side; (_c_) fibers which join the
dorsal longitudinal bundle of Schütz, a tract which underlies the
floor of the fourth ventricle and is traceable forward below the
cerebral aqueduct.