354 NEUROLOGY, OR ANATOMY OF NERVOUS SYSTEM 



bodies. The deep origin is from a nucleus in the floor of the 

 fourth ventricle. The filaments form two bundles which pierce 

 the dura separately and unite in the anterior condylar foramen. 

 The nerve descends behind the internal carotid artery and 

 internal jugular vein, closely bound to the vagus, then passes 

 forward between the artery and vein, and becomes superficial 

 below the digastric, curving around the occipital artery. It 

 now crosses the external carotid and lingual arteries, runs 

 between the mylohyoid and hyoglossus, communicates with 

 the gustatory nerve, and, after piercing the genioglossus, 

 breaks up into filaments to the substance of the tongue. 



Branches of communication pass to the vagus, superior 

 cervical ganglion of the sympathetic, to the loop between the 

 first and second cervical, and to the gustatory nerves. 



Branches of Distribution. Dcscendens hypoglossi leaves the 

 nerve as it crosses the occipital artery, descends within or in 

 front of the carotid sheath, and, joining the ansa hypoglossi, 

 forms a loop from which the sternohyoid and thyroid and 

 both bellies of the omoh3 T oid are supplied. Its origin may be 

 traced to the first and second cervical nerves. 



The thyrohyoid branch crosses the great cornu of the hyoid 

 bone, to supply the muscle. 



Muscular branches pass to the styloglossu^s, hyoglossus, 

 geniohyoglossus, and geniohyoid muscles. 



Meningeal branches run to the posterior fossa, leaving the 

 nerve at the foramen. 



THE SPINAL NERVES 



The spinal nerves consist, on each side, of eight cervical, 

 twelve dorsal, five lumbar, five sacral, and one coccygeal, 

 in all thirty-one pairs, which arise from the cord by two roots, 

 anterior and posterior. The latter are the larger and are supplied 

 with ganglia. The suboccipital or first cervical nerve has no 

 ganglion. The two roots unite just beyond the ganglion, 

 and the resulting trunk divides into two divisions, anterior 

 and posterior, each containing fibers from both roots. The 

 posterior division divides into an external and an internal branch. 

 The anterior divisions in the dorsal region remain separate, but 

 elsewhere they unite into plexuses. They are larger than the 

 posterior. Each division is connected with the sympathetic. 



