CRANIAL NERVES. 103 



which enters the cranial cavity through the foramen magnum, where it is 

 at times joined by fibres from the posterior roots of the two upper cervical 

 nerves, and sends filaments to the ganglion of the root of the pneumo- 

 gastric. After emerging from the cranial cavity through the jugular fora- 

 men, it sends a branch to the pneumogastric, and receives others in return, 

 and also from the 2d, 3d and 4th cervical nerves. It divides into two 

 branches: (i) An internal or anastomotic branch, made up of filaments 

 coming principally from the medulla oblongata, and is distributed to the 

 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 pneumogastric 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 and sterno-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. 



