444 THE NERVOUS SYSTEM. 



supply all the motor filaments which the branches of the pneu- 

 mogastric contain. 



Among the roots of the accessory nerve, the lower, arising 

 from the spinal cord, appear to be composed exclusively of 

 motor fibres, and to be destined entirely to the trapezius and 

 sterno-mastoid muscles ; the upper fibres, arising from the 

 medulla oblongata, contain many sensitive as well as motor 

 fibres. 



Physiology of the Hypoglossal Nerve. 



The hypoglossal or ninth nerve, or motor linguce, has a pe- 

 culiar relation to the muscles connected with the hyoid bone, 

 including those of the tongue. It supplies through its de- 

 scending 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-hyoglossus and linguales. 

 It contributes, also, to the supply of the submaxillary gland. 



The function of the hypoglossal is, probably, exclusively 

 motor. 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, how- 

 ever, not very striking in the tongue. Often, in cases of hemi- 

 plegia 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 for- 

 wards or turn it for some distance towards either side. 



Physiology of the Spinal Nerves. 



Little need be added to what has been already said of these 

 nerves (pp. 390 to 392). The anterior roots of the spinal 

 nerves are formed exclusively of motor fibres ; the posterior 

 roots exclusively of sensitive fibres. 



Beyond the ganglia all the spinal nerves appear to be 

 mixed nerves, and to contain as well sympathetic filaments. 



Of the functions of the ganglia of the spinal nerves nothing 

 very definite is known. That they are not the reflectors of 

 any of the ascertained reflex actions through the spinal 

 nerves, is shown by the reflex movements ceasing when the 

 posterior roots are divided between the ganglia and the spinal 

 cord. 



