232 HUMAN PHYSIOLOGY. 



they experience shortness in 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 can not be prolonged. 



Function. Governs phonation by its influence upon the muscles 

 regulating the position and tension of the vocal bands ; influences the 

 movements of deglutition, inhibits the action of the heart, and con- 

 trols certain respiratory movements associated with sustained or pro- 

 longed muscular efforts and phonation. 



Twelfth Pair. Hypoglossal or Sublingual. 



Origin. From nerve-cells situated deep in the substance of the 

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

 the fourth ventricle. From this origin the fibers pass forward and 

 emerge from the medulla in the groove between the anterior pyramid 

 and the olivary body. 



Distribution. The trunk formed by the union of the different 

 filaments passes out of the cranial cavity through the anterior con- 

 dyloid foramen. After emerging from the cranium, it sends fila- 

 ments to the sympathetic and pneumogastric ; it anastomoses with 

 the lingual, branch of the fifth pair, and receives and sends filaments 

 to the upper cervical nerves. The nerve is finally distributed to the 

 sternohyoid, sternothyroid, omohyoid, thyrohyoid, styloglossi, hyo- 

 glossi, geniohyoid, geniohyoglossi, and the intrinsic muscles .of the 

 tongue. 



Properties. A purely motor nerve at its origin, but derives sensi- 

 bility outside the cranial cavity from anastomosis with the cervical 

 pneumogastric, and fifth nerves. 



Irritation of the nerve gives rise to convulsive movements of the 

 tongue and slight evidences of sensibility. 



Division of the nerve abolishes all movements of the tongue and 

 interferes considerably with the act of deglutition. 



When the hypoglossal nerve is involved in hemiplegia, the tip of 

 the tongue is directed to the paralyzed side when the tongue is pro- 

 truded, owing to the unopposed action of the geniohyoglossus on 

 the sound side. 



Articulation is considerably impaired in paralysis of this nerve, 

 great difficulty being experienced in the pronunciation of the con- 

 sonantal sounds. 



