294 THE NERVOUS SYSTEM 



Functions. Both roots of this nerve are purely motor, but 

 communication with other nerves gives it a degree of sensibility. 

 The fibers from the medulla (accessory) go exclusively to the 

 muscles of the larynx and pharynx, which those from the cord 

 (spinal) go exclusively to the sterno-mastoid and trapezius; 

 and section of either root separately is followed by phenomena 

 corresponding to these facts. When both roots are divided 

 there is loss of voice, disturbance of deglutition, loss of cardiac 

 inhibition and partial paralysis of the sterno-mastoid and tra- 

 pezius. The loss of voice and disturbance in deglutition are 

 explained by the distribution of the fibers of the eleventh with 

 the pharyngeal and laryngeal branches of the tenth. The loss 

 of the power of the vagus to inhibit cardiac action is because the 

 fibers of the tenth which convey the inhibitory impulses are re- 

 ceived from the spinal accessory. The sterno-mastoid and 

 trapezius are only partially paralyzed because they receive motor 

 fibers also from the cervical plexus. 



Twelfth Nerve (Hypoglossal). 



Origin. This nerve supplies motion to the tongue. Its 

 apparent origin is by 10-15 filaments in the groove between the 

 anterior pyramid of the medulla and the olivary body. Its 

 deep origin is in the floor of the fourth ventricle under the lower 

 border of the fasciculus teres. 



Course and Distribution. The nerve passes through the 

 anterior condyloid foramen in two bundles which unite to form a 

 common trunk below. Running downward in company with 

 the internal carotid artery and internal jugular vein, it reaches a 

 point opposite the angle of the jaw, then runs forward, crosses 

 the external carotid, lies on the hyoglossus and is continued for- 

 ward in the genio-hyoglossus to the tip of the tongue. 



It communicates with the tenth, sympathetic, first and second 

 cervical and the lingual branch of the fifth. 



Its branches of distribution are (i) meningeal to the dura 



