THE HYPOGLOSSAL NERVE 107 



The terminal branches of the hypoglossal nerve are dis- 

 tributed to the muscles of the tongue. This group includes 

 the stylo-glossus, the hyo-glo?sus, the genio-glossus and the 

 intrinsic muscles. 



The fibres which are distributed by the facial nerve to the 

 orbicularis oris muscle are said to arise in the hypoglossal 

 nucleus. They then ascend to join the facial nerve of the 

 same side (p. 87). 



SUPRA-NUCLEAR LESIONS produce little effect on the tongue 

 muscles, owing to their bilateral representation in the cerebral 

 cortex. If they occur in the internal capsule, a true deviation 

 of the protruded tongue may be present, owing to weakening 

 of the genio-glossus (vide infra]. 



NUCLEAR LESIONS are usually bilateral (bulbar paralysis, 

 p. 108). The tongue lies motionless in the floor of the mouth 

 and deglutition is, therefore, practically impossible. The 

 orbicularis oris is also affected, and the combined paralysis 

 of tongue and lips has a serious effect on the speech. 



INFRA-NUCLEAR LESIONS. In rare cases, the fibres of the 

 hypoglossal may be interfered with as they traverse the 

 medulla oblongata. The site of the lesion is usually indicated 

 by a crossed paralysis, the limbs being affected on the opposite 

 side of the body, owing to injury of the pyramidal tract above 

 the decussation (Fig. 52). 



In unilateral hypoglossal paralysis, the characteristic sign 

 is deviation of the protruded tongue to the paralysed side. 

 This is principally due to the action of the unopposed genio- 

 glossus of the sound side, which arises from the posterior 

 aspect of the symphysis menti and spreads backwards and 

 laterally into the tongue. Owing to the lateral inclination 

 of some of its fibres, the unopposed genio-glossus drags the 

 lateral border of the dorsum of the tongue towards the median 

 plane and thus the tongue, a.s a whole, is pulled over to the 

 side of the lesion. Difficulty in the pronunciation of the 

 lingual consonants also accompanies unilateral paral>sis of 

 the tongue muscles, and speech is therefore rendered indistinct. 



