AUDITOEY NEEVE. 205 



indication of nerve deafness. The presence of the internal 

 carotid artery in a canal in the petrous bone probably accounts 

 for the pulsatile tinnitus which occurs in some cases. 



Inflammatory disease of the labyrinth may originate in it 

 or extend to it from the middle ear through the foramina in 

 the inner wall of the tympanum, or pass from within the skull 

 along the sheath of the auditory nerve, for the subarachnoid 

 space around the auditory nerve communicates with the perilymph 

 spaces of the internal ear. Such an extension is not uncommon 

 in leptomeningitis. 



A combination of nerve deafness with facial paralysis and 

 suppuration of the middle ear indicates that the inflammation 

 involves the bony labyrinth. Paralysis of the same two 

 nerves with a healthy tympanum would lead to the inference 

 that the lesion was in the internal meatus or at the base of the 

 brain. New growths in the petrous bone or posterior fossa of 

 the skull, gummatous and other varieties of meningitis in this 

 locality, and occasionally an aneurysm may involve the facial and 

 auditory nerves at the base. Sometimes the sixth and other 

 adjacent nerves are also paralysed. It has been observed that 

 the facial nerve (portio dura) is more resistant to the effects of 

 surrounding inflammation than the eighth nerve (portio mollis). 



The fibres of the vestibular nerve are derived from the semi- 

 circular canals, the utricle and the saccule. After traversing the 

 local ganglia in the petrous bone they are continued through the 

 internal meatus towards the restiform body. Up to this point, 

 as already mentioned, both parts of the auditory nerve are likely 

 to suffer together. On reaching the surface of the brain the 

 vestibular fibres diverge from the cochlear fibres to enter the pons 

 on the ventral aspect of the restiform body, which thus becomes 

 interposed between the two portions of the auditory nerve. The 

 vestibular fibres terminate in certain nuclei in the outer angle of 

 the fourth ventricle, including Deiter's nucleus. (Fig. 17, p. 191.) 

 The fibres are thus brought into connection with other parts of the 

 mechanism of equilibration, including the cerebellum, the ocular 

 nuclei and the antero-lateral columns of the spinal cord. These 



