From the grey matter of the floor of the fourth ventricle, and the antero-lateral part of the grey matter around the central canal. The hypoglossal nucleus extends from immediately above the decussation of the pyramids to the level of the striae medullares.
The nerve issues from the front of the medulla between the pyramid and olivary body. There are numerous filaments which form two groups. Each group occupies a separate compartment of dura mater.
The nerve bundles unite in the anterior condyloid foramen (hypoglossal canal), as a complete trunk, which reaches the neck behind the internal jugular vein and the internal carotid artery. Proceeding downwards betAveen these two vessels, the nerve passes beneath the posterior belly of the digastric and the stylo-hyoid. It then hooks round the occipital artery, crosses the external carotid and the first part of the lingual superficially, to enter the submaxillary triangle. In this triangle the nerve disappears beneath the mylo-hyoid, resting upon the hyoglossus just below Wharton's •duct, and reaches the tongue medial to the lingual artery.
(a) Recurrent, to the dura mater of the posterior fossa.
(b) Descending, arises as the hypoglossal crosses the internal carotid. It lies on the anterior part of the carotid sheath, and is joined by the descending cervical (from the second and third cervicals). The nerve thus formed is termed the ansa hypoglossi; it supplies the omo-hyoid, sternohyoid, and sterno-thyreoid.
(c) Thyreo-hyoid, innervates the thyreo-hyoid muscle.
(d) Lingual, supplies the hyoglossus, genio-hyoid,- genio-hyoglossus, and the intrinsic muscles of the tongue. The infra-hyoid muscles and the genio-hyoid are supplied by fibres derived from the first three cervical nerves.
With (a) the lingual; (b) nodal ganglion of the vagus; (c) superior cervical ganglion of the sympathetic; (d) loop between first and second cervicals, and (e) the descending cervical.
Plate V. Eighth Pair Of Nerves