914 



NEUROLOGY 



it again. As its exit from the jugular foramen, it runs backward in front of the 

 internal jugular vein in 66.6 per cent, of cases, and behind in it 33.3 per cen". 

 (Tandler). The nerve then descends obliquely behind the Digastricus and Stylo- 

 hyoideus to the upper part of the Sternocleidomastoideus; it pierces this muscle, 

 and courses obliquely across the posterior triangle of the neck, to end in the deep 

 surface of the Trapezius. As it traverses the Sternocleidomastoideus it gives severe,! 

 filaments to the muscle, and joins with branches from the second cervical nerve. 

 In the posterior triangle it unites with the second and third cervical nerves, while 

 beneath the Trapezius it forms a plexus with the third and fourth cervical nerves, 

 and from this plexus fibers are distributed to the muscle. 



IJypoglossal X. 

 Vagus X. 



Glossopharyngeal JV. 





FIG. 794. Hypoglossal nerve, cervical plexus, and their branches. 



THE HYPOGLOSSAL NERVE (N. HYPOGLOSSUS; TWELFTH NERVE) 



(Figs. 794, 795). 



The hypoglossal nerve is the motor nerve of the tongue. 



Its fibers arise from the cells of the hypoglossal nucleus, which is an upward 

 prolongation of the base of the anterior column of gray substance of the medulla 

 spinalis. This nucleus is about 2 cm. in length, and its tipper part corresponds 

 with the trigonum hypoglossi, or lower portion of the medial eminence of the rhom- 

 boid fossa (page 779). The lower part of the nucleus extends downward into the 

 closed part of the medulla oblongata, and there lies in relation to the ventro-lateral 



