754 



THE NERVOUS SYSTEM. 



The Twelfth Pair (Figs. 405, 406). 



The Twelfth or Hypoglossal Nerve is the motor nerve of the tongue. 



Its superficial origin is by several filaments, from ten to fifteen in number, 

 from the groove between the pyramidal and olivary bodies of the medulla, in a 

 continuous line with the anterior roots of the spinal nerves. Its deep origin can 

 be traced to a nucleus of gray matter (trigonum hypoglossi] on the floor of the 

 fourth ventricle. 



The filaments of this nerve are collected into two bundles, which perforate the 

 dura mater separately, opposite the anterior condyloid foramen, and unite together 

 after their passage through it. In those cases in which the anterior condyloid 

 foramen in the occipital bone is double, these two portions of the nerve are sepa- 

 rated by the small piece of bone which divides the foramen. The nerve descends 

 almost vertically to a point corresponding with the angle of the jaw. It is at first 

 deeply seated beneath the internal carotid artery and internal jugular vein, and 

 intimately connected with the pneumogastric nerve ; it then passes forward 



To Dura-mater 



? To Ganglion on Trunk of Vagus 



To Anterior Belly of Oino-htioid 



To Sterno-hyoid 

 'TO Sterno-thyroid 

 Fo Posterior Belly of Omo-hyoid 



FIG. 405. Plan of the hypoglossal nerve. 



between the vein and artery, and lower down in the neck becomes superficial below 

 the Digastric muscle. The nerve then loops round the occipital artery, and crosses 

 the external carotid and its lingual branch below the tendon of the Digastric mus- 

 cle. It passes beneath the tendon of the Digastric, the Stylo-hyoid, and the Mylo- 

 hyoid muscles, lying between the last-named muscle and the Hyo-glossus, and com- 

 municates at the anterior border of the Hyo-glossus with the lingual (gustatory) 

 nerve ; it is then continued forward in the fibres of the Genio-hyo-glossus muscle 

 as far as the tip of the tongue, distributing branches to its muscular substance. 



