THE ACOUSTIC NERVE 91 



divided into two groups : (a) those referable to the cochlear 

 nerve, and (b) those referable to the vestibular nerve. 



(a) When the disease is slowly progressive in type, tinnitus 

 is the symptom which is first noticed. Later, gradually in- 

 creasing unilateral deafness becomes more apparent. In the 

 case of sudden haemorrhage into the labyrinth, deafness is 

 sudden in onset and is, in the first instance, complete. 



(I)) Involvement of the vestibular nerve causes attacks of 

 giddiness and vertigo. They may occur in the course of the 



FIG. 47. Section through Upper Part of Medulla Oblongata. 



1. Pyramidal tract. 



2. Olivary nucleus. 



3. Sensory nucleus of V. 



4. Nucleus of tractus solitarius. 



5. Nucleus ambiguus. 



6. Nucleus of vagus nerve. 



7. Nucleus of hypoglossal nerve. 



8. Sensory decussation. 



disease without any premonitory symptoms, and the patient 

 usually falls to the ground, although he does not necessarily 

 lose consciousness. 



The Glosso-pharyngeal or Ninth Cerebral Nerve con- 

 tains both motor and sensory fibres. The motor fibres arise 

 from the upper extremity of the nucleus ambiguus (p. 94) in the 

 upper part of the medulla oblongata. The sensory fibres arise 

 in the superior and the petrosal ganglia of the glosso-pharyngeal 

 nerve and establish connexions centrally with the tractus 



