go THE NERVOUS SYSTEM 



to determine whether the deafness is due to the condition of 

 the nerve or to the condition of the conducting apparatus. If 

 otoscopic examination is not sufficient, Weber's test may be 

 employed. The base of a vibrating tuning-fork is applied to 

 the vertex in the median plane. If the unilateral deafness is 

 due to an affection of the nervous mechanism, the tuning-fork 

 will be heard only, or much more distinctly, on the sound side ; 

 on the other hand, if the conducting apparatus is at fault, the 

 tuning-fork will be heard better on the affected side. 



(b) The VESTIBULAR NERVE carries afferent fibres from the 

 vestibule and the semicircular ducts (canals). It runs with 

 the cochlear nerve from the bottom of the internal acoustic 

 meatus to the surface of the brain-stem, where they become 

 separated by the restiform body. The fibres of the vestibular 

 nerve terminate in several nuclei within the medulla oblongata 

 and from these nuclei new fibres arise which connect the 

 nerve with the cerebral cortex, the cerebellar cortex and the 

 grey matter of the spinal medulla (spinal cord). 



NOTE. In the above descriptions of the connexions of the cochlear and 

 the vestibular nerves, it has been deemed unnecessary to incorporate any 

 more than a rough outline. For a more detailed account the reader must 

 consult the standard text- books of Anatomy or Neurology. 



Both the cochlear and the vestibular nerves are affected 

 (i) in lesions of the acoustic nerve and (2) in inflammation or 

 haemorrhage into the membranous labyrinth (p. 208). 



1. The acoustic nerve may be involved in cerebral tumours 

 of the cerebello-pontine angle (p. 22), in purulent exudates 

 in the cisterna pontis, or in syphilitic meningitis in the 

 neighbourhood of the internal acoustic meatus. The symptoms 

 directly referable to the acoustic nerve are the same as those 

 described in the following paragraph. 



2. Pathological conditions of the labyrinth give rise to a 

 train of symptoms, which are grouped together under the name 

 of Meniere's disease. The lesion may be of the nature of a 

 progressive inflammation or it may take the form of a 

 haemorrhage into the labyrinth. The symptoms are naturally 



