THE HYPOGLOSSAL NERVE 271 



The external or spinal portion (fig. 183, 5), after issuing from the jugular 

 foramen, is directed backwards either across the front of or behind the internal 

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

 and joining amongst the fleshy fibres with the nerve furnished to the muscle from 

 the cervical plexus. Descending in the next place obliquely across the posterior 

 triangular space of 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 filaments to the trapezius, which extend 

 nearly to the lower border of the muscle. 



Varieties. The lower limit of the origin of the spinal part of the nerve was found by 

 Holl to range from the third to the seventh cervical nerve, but in the greater number of 

 cases it corresponded to the fifth or sixth nerve. It was seen by Sommering opposite the first 

 dorsal nerve. The spinal part of the nerve in one case pierced the dura mater below the first 

 cervical nerve, and re-entered the spinal theca higher up (Holl). The external portion of the 

 spinal accessory nerve sometimes passes beneath the sterno-mastoid without piercing the 

 muscle. In one instance this nerve has been seen terminating- in the sterno-mastoid muscle, 

 the trapezius being- supplied entirely by the third and fourth cervical nerves (Cumow). In 

 rare cases it has been observed sending a branch to join the descending cervical nerve. 



XII. HYPOGLOSSAL NERVE. 



The hypoglossal or twelfth cranial nerve arises from a nucleus placed ventro- 

 laterally to the central canal in the lower part of the medulla oblongata, and extend- 

 ing upwards beneath the trigonum hypoglossi of the fourth ventricle (fig. 178, n. 

 xn). Thence the fibres pass obliquely forwards and outwards, between the anterior 

 and lateral areas of the bulb, and form a series of from ten to fifteen fine roots which 

 emerge along the groove separating the pyramid from the olivary body. The fila- 

 ments are directed outwards above (or behind) the vertebral artery, and are usually 

 collected into two bundles which perforate the dura mater separately opposite the 

 anterior condylar foramen, and are united into a single trunk as they pass through 

 that opening. 



As it leaves the anterior condylar foramen the nerve is very deeply placed on the 

 inner side of the deep cervical vessels and the pneumo-gastric nerve. Winding 

 round the lower ganglion of the last, to which it is closely bound by connective 

 tissue, the hypoglossal nerve descends, inclining at the same time gradually forwards 

 between the internal carotid artery and jugular vein, to the lower border of the 

 digastric muscle. At this level it curves forwards round the commencement of the 

 occipital artery, the sterno-mastoid branch of which turns downwards over the nerve, 

 and is thence directed forwards above the hyoid bone to the under part of the tongue. 

 In the latter part of its course, it passes beneath the tendon of the digastric, the 

 lower end of the stylo-hyoid, and the mylo-hyoid muscles ; it crosses the external 

 carotid and the lingual arteries ; and it rests upon the hyo-glossus muscle, being- 

 accompanied by the ranine vein of the tongue. At the anterior border of the hyo- 

 glossus it is connected with the lingual branch of the fifth nerve, and then penetrates 

 the fibres of the genio-glossus muscle, dividing into branches which are distributed 

 to the muscular substance of the tongue. 



BRANCHES. While passing through the anterior condylar foramen, the hypoglossal 

 nerve gives off one or two minute recurrent twigs which ramify in the dura mater 

 around the foramen magnum, and in the diploe of the occipital bone. They were 

 thought by Luschka to be formed by recurrent fibres derived from the lingual nerve, 

 but it is possible that they consist of fibres which ascend from the communication 

 with the first cervical nerve, or from the sympathetic (Rudinger). The branches 

 arising from the nerve in the neck are partly filaments of communication with other 



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