204 CLINICAL APPLIED ANATOMY. 



tensor tympani increases the sensitiveness of the ear to musical 

 tones. Neuralgic pain at the back of the ear and in the teeth 

 and gums of the paralysed side may accompany facial paralysis. 

 The anatomical explanation is obscure but it has been supposed 

 to be due to irritation of sensory fibres of the fifth nerve which 

 have joined the facial nerve in the Fallopian canal. 



The pathology of facial spasm is obscure, but it is sometimes 

 due to reflex irritation from the territory of the fifth nerve and 

 particularly from its orbital branches. Exceptionally irritation 

 of the facial nerve near its origin from the pons or of the cortical 

 facial centres originates the spasm. 



THE AUDITORY NERVE. 



The auditory nerve consists of two distinct portions ; of these 

 one is called vestibular and is concerned in equilibration or 

 orientation of the body, and the other is called cochlear and con- 

 cerned in hearing. The ampullary and cochlear portions of the 

 labyrinth in which these nerves respectively commence are 

 continuous, and the combined nerves form a common trunk as 

 far as the surface of the pons ; the nuclei of termination in the 

 fourth ventricle are also closely adjacent, but the ultimate central 

 terminations of the two nerves are widely separated. Hence in 

 labyrinthine lesions and lesions of the common auditory trunk 

 both portions of the nerve are equally exposed to injury, and it 

 is usual to find that impairment of hearing is associated with 

 vertiginous sensations. The latter, especially if of sudden onset, 

 may be so obtrusive that for some time the accompanying deaf- 

 ness may be overlooked, the more so since in labyrinthine lesions 

 the defect may only be marked in the case of the higher pitched 

 sounds. 



The organ of hearing being enclosed in the petrous bone can 

 receive and appreciate vibrations directly from the bone. This 

 fact is taken advantage of when a tuning fork or watch is applied 

 directly to the mastoid or vertex in testing nerve conduction, the 

 failure to appreciate vibrations applied in this way being an 



