266 PHYSIOLOGY FOR DENTAL STUDENTS. 



THE TWELFTH OR HYPOGLOSSAL NERVE. The twelfth in-rvc 

 or hypoglossal is entirely efferent, being the motor nerve of the 

 tongue muscles and of most of the muscles attached to the hyoid 

 bone. When it is paralyzed, as in bulbar paralysis, swallowing 

 of food becomes impossible, the tongue cannot be protruded 

 and soon atrophies because of the removal of the trophic in- 

 fluence of the nerve cells. Rarely the paralysis is unilateral, 

 but this is because of lesions higher up in the nervous system 

 than the medulla and so situated that they destroy the con- 

 nection of the fibers which run from the higher motor centers 

 in the cerebrum to the hypoglossal nucleus. Such lesions neces- 

 sarily involve fibers of the same type running to the nerve cells 

 of the spinal cord, so that hemiplegia (p. 248) accompanies and 

 is on the same side as the tongue paralysis. When a patient 

 with such a lesion attempts to put out the tongue, it is directed 

 towards the affected side but it shows no atrophy. 



