PHYSIOLOGY OF THE CKANIAL NERVES. 545 



The Twelfth or Hypoglossal Nerve. 



Origin and Connections. The hypoglossal nerve arises from two 

 large celled and one small celled, nuclei in the lowest part of the floor of 

 the 4th ventricle near the middle line. The fibres of origin are contin- 

 uous with the anterior roots of the spinal nerves. It is connected with 

 the vagus, the superior cervical ganglion of the sympathetic and with the 

 upper cervical nerves. 



Distribution. The hypoglossal or ninth nerve, or motor lingua, has 

 a peculiar relation to the muscles connected with the hyoid bone, includ- 

 ing those of the tongue. It supplies through its descending branch 

 (descendens noni), the sterno-hyoid, sterno-thyroid, and omo-hyoid; 

 through a special branch, the thyro-hyoid, and through its lingual 

 branches the genio-hyoid, stylo-glossus, hyo-glossus, and genio-hyo-glos- 

 sus and linguales. It contributes, also, to the supply of the submaxil- 

 lary gland. 



Functions. The function of the hypoglossal is exclusively motor, 

 except in so far as its descending branch may receive a few sensory fila- 

 ments from the first cervical nerve. As a motor nerve, its influence on 

 all the muscles enumerated above is shown by their convulsions when it 

 is irritated, and by their loss of power when it is paralyzed. The effects 

 of the paralysis of one hypoglossal nerve are, however, not very striking 

 in the tongue. Often, in cases of hemiplegia involving the functions of 

 the hypoglossal nerve, it is not possible to observe any deviation in the 

 direction of the protruded tongue; probably because the tongue is so 

 compact and firm that the muscles on either side, their insertion being 

 nearly parallel to the median line, can push it straight forwards or turn 

 it for some distance towards either side. 



Spinal Nerves. 



Functions. Little need be added to what has been already said of 

 these nerves (pp. 480, 481). The anterior roots of the spinal nerves are 

 formed exclusively of motor fibres; the posterior roots exclusively of 

 sensory fibres. Beyond the ganglia, all the spinal nerves are mixed 

 nerves, and contain as well sympathetic filaments. 

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