TJIE HYPOGLOSSAL .VA'AT/',' 987 



two spinal nerves. As the .spinal fi I ires- pass out of (lie surface of the con! they unite 

 to form an ascending strand which enters t he posterior fossa of the cranium, through 

 the foramen magnum, and, turning outwards, blends more or les> intimately wit h I he 

 accessory portion. Thus combined, the nerve enters the juiriilar foramen in com- 

 pany with the vagus, hut here it is attain separated into its internal and external 

 branches, which contain chielly the same fibres as the original su|K>rior and inferior 

 parts. 



The internal branch, or accessory portion of the nerve, gives one or more 

 filaments of communication to the jugular ganglion (ganglion of the root of (lie 

 vagus), and then joins either the trunk of t he vagus directly or its ganglion nodo.sum, 

 the fibres of the branch being contributed to the pharyngeal, larynireal. ami cardiac 

 branches of the vagus. Fibres corresponding to the white rami communicantes. 

 absent in the cervical nerves, probably enter the cervical sympathetic ganglion 

 through this ramus. 



The external branch or the spinal portion runs backwards and downwards under 

 cover of the posterior belly of the digastric and the sterno-mastoid. It usually 

 crosses in front of and to the outer side of the internal jugular vein and between it and 

 the occipital artery; then it pierces the sterno-mastoid, supplies filaments to 

 it, and anastomoses in its substance with branches of the second cervical nerve. 

 It emerges from the posterior border of the sterno-mastoid slightly above the 

 level of the upper border of the thyreoid cartilage, passes obliquely downwards and 

 backwards across the occipital portion of the posterior triangle, and disappears 

 beneath the trapezius about the junction of the middle and lower thirds of the an- 

 terior border of that muscle (fig. 707) . In the posterior t riangle it receives communi- 

 cations from the third and fourth cervical nerves, and beneath the trapezius its fibres 

 form a plexus with other branches of the same nerves. Its terminal filaments are 

 distributed to the trapezius and they can be traced almost to the lower extremity of 

 that muscle. 



Central Connections. The nuclei of origin, like other motor nuclei, are connected with the 

 mmiiPsthetic area of the cortex eerebri by the pyramidal fibres, and they are associated with the 

 sensory nuclei of other cerebral nerves lay the medial longitudinal fasciculus, and the fibres of 

 the fasciculi proprii. 



THE TWELFTH NERVE-THE HYPOGLOSSUS 



The hypoglossal nerves are exclusively motor; they supply the genio-hyoidei 

 and the extrinsic and intrinsic muscles of the tongue except the glosso-palatini. The 

 fibres of each nerve issue from the cells of an elongated nucleus which lies in the floor 

 of the central canal in the lower half of the medulla and in the floor of the fourth 

 ventricle in' the upper half beneath the trigonum hypoglossi. This nucleus is the 

 upward continuation of the ventral group of cells of the ventral horn of the spinal 

 cord. From their origin the fibres run ventralwards and somewhat lateralwards, 

 probably joined in the medulla by a few fibres from the nucleus ambiguus which is a 

 segment of the upward prolongation of the lateral group of cells of the ventral horn. 

 The conjoined fibres issue from the medulla in the sulcus between the pyramid and 

 the olivary body, in a series of from ten to sixteen root filaments, which pierce the 

 pia mater and unite with each other to form two bundles (fig. 698). These bundles 

 pass forwards and outwards to the hypoglossal (anterior condyloid) foramen, where 

 they pierce the arachnoid and dura mater. In the outer part of the foramen the 

 t wo bundles unite to form the trunk of the nerve. At its commencement, at the 1 

 of the skull, the trunk of the hypoglossus lies on the inner side of the \ami<. but 

 as it descends in the neck it turns gradually around the back and the outer side of 

 the latter nerve, lying between it and the internal jugular vein, and a little above 

 the level of the hyoid bone it bends forwards, and crosses external to the internal 

 carotid artery, the root of origin of the occipital artery, the external carotid, and the 

 loop formed by the first part of the lingual artery (fig. 707). After crossing the 

 lingual artery it proceed- forwards on the outer surface of the hyo-glossus, crossing 

 to the inner side of the posterior belly of the diga.-tric. and the stylo-hyoid muscles. 

 It disappears in the anterior part of the submaxillary region between the mylo- 

 hyoid and the hyo-glossus, and divides into its terminal branches between the 

 latter muscle and the genio-glossus. 



As it descends in the neck the trunk lies deeply between the internal jugular vein 



