PAEALYSIS OF THE HYPOGLOSSAL NEEVE. 211 



THE HYPOGLOSSAL NERVE. 



As with other motor nerves, paralysis of the hypoglossal may 

 be of supranuclear, nuclear, or infranuclear origin. 



Supranuclear Paralysis. The cortical centre for the tongue 

 lies at the posterior extremity of the third left frontal convolution, 

 and is connected by the pyramidal fibres with the hypoglossal 

 nucleus of the opposite side. Lesions of the centre or of the 

 conducting tracts above the nucleus rarely affect the tongue 

 alone but usually give rise to a hemiplegia which is more or less 

 complete. The reasons for this have already been given when 

 dealing with the other motor cranial nerves. 



Nuclear Paralysis. The hypoglossal nucleus lies ventral to 

 the central canal of the cord, and when the canal opens up into 

 the fourth ventricle the nucleus lies subjacent to the trigonum 

 hypoglossi. (Fig. 17, p. 191.) The nucleus is implicated by the 

 same types of lesion as those already indicated when dealing with 

 the vago-glossopharyngeal nucleus, indeed the two nuclei being 

 closely adjacent usually suffer together. The results of intra- 

 cranial disease have led to the suggestion that the hypoglossal 

 nucleus gives rise to the motor fibres of the lips as well as to 

 those of the tongue. 



Infranuclear paralysis of the hypoglossal may be the result 

 of a lesion of the medulla where this is traversed by the issuing 

 nerve roots. In such a case there is every likelihood that the 

 adjacent pyramidal tract will be damaged so that a crossed 

 paralysis of tongue on side of lesion and limbs on the opposite 

 side of body will be produced. 



Lesions in the subdural space will implicate the nerve some- 

 where between its points of exit in the groove between the pyramid 

 and the olive and its passage through the anterior condyloid 

 foramen. This foramen is only separated from the jugular fora- 

 men by the bony eminence known as the eminentia innominata. 

 Given a lesion in this neighbourhood the adjacent vago-glosso- 

 pharyngeal fibres are almost certain to suffer at the same time 

 as the hypoglossal, so that a combined unilateral paralysis of the 



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