626 THE CRANIAL NERVES. 



in the foramen j ugulare. After issuing from the foramen, the nerve is directed 

 backwards across the internal jugular vein, in some cases over, in others under 

 it, and perforates the sterno-mastoid muscle, supplying this with branches, 

 and joining amongst the fleshy fibres with branches of the cervical plexus. 

 Descending in the next place across the neck behind the sterno-mastoid, the 

 nerve passes beneath the trapezius muscle. Here it forms a kind of plexus 

 with branches of the third and fourth cervical nerves, and distributes fila- 

 ments to the trapezius, which extend nearly to the lower edge of the muscle. 



NINTH PAIR OF NERVES. 



The hypoglossal or ninth cranial nerve is the motor nerve of the tongue, 

 and in part of some muscles of the neck. 



The filaments by which this nerve arises from the medulla oblongata are 

 collected into two bundles, which converge to the anterior condyloid 

 foramen of the occipital bone. Each bundle of filaments perforates the 

 dura mater separately within the foramen, and the two are joined after 

 they have passed through it. 



After leaving the cranium, this nerve descends almost vertically to the 

 lower border of the digastric muscle, whera, changing its course, it is 

 directed forwards above the hyoid bone to the under part of the tongue, 

 It lies at first very deeply with the vagus nerve, to which it is connected ; 

 but passing between the internal carotid artery and the jugular vein, it 

 curves forward round the occipital artery, and then crosses over the external 

 carotid below the digastric muscle. Above the hyoid bone it is crossed by 

 the lower part of the stylo-hyoid muscle and posterior belly of the digastric, 

 and rests on the hyo-glossus muscle. At the anterior border of the hyo- 

 glossus it is connected with the gustatory nerve, and is continued in the 

 fibres of the genio-hyo-glossus muscle beneath the tongue to the tip, 

 distributing branches upwards to the muscular substance. 



The principal branches of this nerve are distributed to the muscles 

 ascending to the larynx and hyoid bone, and to those of the tongue ; a few 

 serve to connect it with some of the neighbouring nerves. 



In animals the ninth nerve not unfrequently possesses a posterior root furnished with 

 a ganglion, in the same manner as that of a spinal nerve. 



CONNECTING BRANCHES. 



Connection with the pneumo-gastric. Close to the skull the hypoglossal nerve is 

 connected with the second ganglion of the pneumo-gastric by separate filaments, or 

 in some instances the two nerves are united so as to form one mass. 



Union with the sympathetic and first two spinal nerves. Opposite the first cervical 

 vertebra the nerve communicates by several twigs with the upper cervical ganglion 

 of the sympathetic, and with the loop uniting the first two spinal nerves in front of 

 the atlas. 



MUSCULAR AND LINGUAL BRANCHES. 



Descending branch of the ninth nerve. This branch (r. descendens noni), 

 leaves the ninth nerve where this turns round the occipital artery, or, some- 

 times, higher up. It passes downwards on the surface of the sheath of 

 the carotid vessels, gradually crossing from the outer to the inner side, 

 gives a branch to the anterior belly of the omo hyoid muscle, and joins 

 about the middle of the neck in a loop with one or two branches from 

 the second and third cervical nerves, forming the ansa hypoylossi. The 

 concavity of this loop is turned upwards ; and the connection between 

 the nerves is effected by means of two or more interlacing filaments, which 



