SPINAL NERVES. 409 



oy ten or fifteen filaments, which being collected into two bundles, escape 

 from the cranium through the anterior condyloid foramen. The nerve 

 then passes forwards between the internal carotid artery and internal jugu- 

 lar vein, and descends along the anterior and inner side of the vein to a 

 point parallel with the angle of the lower jaw. It next curves inwards 

 around the occipital artery, with which it forms a loop, and crossing the 

 lower part of the hyo-glossus muscle to the genio-hyo-glossus, sends fila- 

 ments onwards with the anterior fibres of that muscle as far as the tip of 

 the tongue. It is distributed to the muscles of the tongue, and principally 

 to the genio-hyo-glossus. While resting on the hyo-glossus muscle it is 

 flattened, and beneath the mylo-hyoideus it communicates with the gusta- 

 tory nerve. 



At its origin the hypoglossal nerve sometimes communicates with the 

 posterior root of the first cervical nerve. 



The Branches of the hypoglossal nerve are : 



Communicating branches with the Pneumogastric, 



Spinal accessory, 

 First and second cervical nerves, 

 Sympathetic. 



Descendens noni, 



Thyro-hyoidean branch, 



Communicating filaments with the gustatory nerve. 



The Communications with the pneumogastric and spinal accessory take 

 place through the medium of a plexiform interlacement of branches at the 

 base of the skull, behind the internal jugular vein. The communications 

 with the sympathetic nerve are derived from the superior cervical ganglion. 



The Descendens noni is a long and slender twig, which quits the hypo- 

 glossal just as that nerve is about to form its arch around the occipital 

 artery, and descends upon the sheath of the carotid vessels. Just below 

 the middle of the neck it forms a loop with a long branch (communicans 

 noni) from the second and third cervical nerves. From the convexity of 

 this loop branches are sent to the sterno-hyoideus, sterno-thyroideus, and 

 both bellies of the omo-hyoideus ; sometimes also a twig is given off to 

 the cardiac plexus, and occasionally one to the phrenic nerve. If the 

 descendens noni be traced to its origin it will be found to be formed by a 

 branch from the hypoglossal, and one from the first and second cervical 

 nerves ; occasionally it receives also a filament from the pneumogastric. 



The Thyro-hyoidean nerve is a small branch, distributed to the thyro 

 hyoideus muscle. It is given off from the trunk of the hypoglossal near 

 the posterior border of the hyoglossus muscle, and descends obliquely 

 over the great cornu of the os hyoides. 



The Communicating filaments, with the gustatory nerve, are several 

 small twigs, which ascend upon the hyoglossus muscle near its anterior 

 border, and form a kind of plexus with filaments sent down by the gusta- 

 tory nerve. 



SPINAL NERVES. 



There are thirty-one pairs of spinal nerves, each arising by two roots, 

 dii anterior or motor root, and a posterior or sensitive root. 



The anterior roots proceed from a narrow white line, interior lateral 

 35 



