114 HUMAN PHYSIOLOGY. 



muscles of the pharynx through the pharyngeal nerves coming from the 

 pneumogastric ; to all the muscles of the larynx, except the crico-thyroid 

 through the inferior laryngeal nerve; to the heart, by filaments which 

 reach it through the pneumogostric nerve. (2) An external branch, which 

 is distributed to the sterno-cleido-mastoid and trapezius muscles ; these 

 muscles also receiving filaments from the cervical nerves. 



Properties. At its origin it is a purely motor nerve, but in its course 

 exhibits some sensibility from anastomosing fibres. 



Destruction of the medullary root, by tearing it from its attachment by 

 means of forceps, impairs the action of the muscles of deglutition, and 

 destroys the power of producing vocal sounds by paralysis of the laryngeal 

 muscles, without, however, interfering with the respiratory movements of 

 the larynx ; these being controlled by other motor nerves. The normal 

 rate of movement of the heart is also impaired by destruction of the 

 medullary root. 



Irritation of the external branch throws the trapezius andsterno-mastoid 

 muscles into convulsive movements, though section of the nerve does not 

 produce complete paralysis, as they are also supplied with motor influence 

 from the cervical nerves. The sterno-mastoid and trapezius muscles per- 

 form movements antagonistic to those of respiration, fixing the head, neck 

 and upper part of the thorax, and delaying the expiratory movement during 

 the acts of pushing, pulling, straining, etc., and in the production of a pro- 

 longed vocal sound, as in singing. When the external branch alone is 

 divided, in animals, they experience shortness of breath during exercise, 

 from a want of coordination of the muscles of the limbs and respiration ; 

 and while they can make a vocal sound, it cannot be prolonged. 



Function. Governs phonation by its influence upon the vocal move- 

 ments of the glottis ; influences the movements of deglutition, inhibits the 

 action of the heart and controls certain respiratory movements associated 

 with sustained or prolonged muscular efforts and phonation. 



I2th Pair. Hypoglossal or Sublingual. 



Apparent Origin. By two groups of filaments from the medulla ob- 

 longata, in the grooves between the olivary body and the anterior pyramid. 



Deep Origin. From the hypoglossal nucleus situated deeply in the 

 substance of the medulla, on a level with the lowest portion of the floor, of 

 the 4th ventricle; some decussating filaments have been traced to a higher 

 encephalic centre. 



Distribution. The trunk formed by a union of the root filament 



