208 HUMAN PHYSIOLOGY. 



ments 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 prolonged vocal 

 sound, as in singing. When the external branch alone is divided, in animals, 

 they experience shortness in breath during exercise, from a want of coordina- 

 tion 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 muscles regulating 

 the position and tension of the vocal bands; 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. 



Twelfth Nerve. Hypoglossal. 



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 condyloid foramen. 

 After emerging from the cranium, it sends filaments 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 fi- 

 nally distributed to the sternohyoid, sterno thyroid, omohyoid, thyrohyoid, 

 styloglossi, hyoglossi, geniohyoid, geniohyoglossi, and the intrinsic muscles of 

 the tongue. 



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

 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 on both sides abolishes all movements of the tongue 

 and interfere 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 protruded, owing to the 

 unopposed action of the geniohyoglossus on the sound side. 



Articulation is considerably impaired in paralysis of this nerve, great diffi- 

 culty being experienced in the pronunciation of the consonantal sounds. 



Mastication is performed with difficulty, from inability to retain the food 

 between the teeth until it is completely triturated. 



