524 THE AUDITORY APPARATUS 



branches which arise from the ganglion and then follow the 

 nerve backwards above the fenestra vestibuli. The greater part 

 of the vertical portion of the canal has already been opened from 

 the lateral aspect ; the remainder can now be displayed by 

 means of two saw cuts (i) a frontal section (vertical transverse) 

 carried medially from the lateral surface of the bone to the 

 posterior border of the stylo-mastoid foramen ; (2) a sagittal 

 cut (vertical antero-posterior) carried from the posterior surface 

 of the bone to meet cut (i). The portion of bone between the 

 two cuts must then be removed, and the dissection must be 

 completed with bone forceps. Three branches are given off 

 from the facial nerve in the terminal part of the canal. 



Intrapetrous Portion of the Facial Nerve. As the facial 

 nerve traverses the petrous bone, it may be looked upon as 

 consisting of four parts, which differ from one another in the 

 relations they present and in the direction which they take. 

 They are : 



1. A part within the internal acoustic meatus. 



2. A very short part which extends from the bottom of the internal 



acoustic meatus to the ganglion geniculi. 



3. A part which occupies that portion of the canalis facialis which runs 



along the medial wall of the tympanic cavity. 



4. A part which extends vertically downwards to the stylo-mastoid 



foramen. 



First Part. In the internal acoustic meatus, the facial 

 nerve runs almost directly laterally, in company with the 

 acoustic nerve. In that stage of its course it lies in relation 

 to the upper and anterior part of the acoustic nerve, and 

 its motor and sensory roots join. At the bottom of the 

 internal acoustic meatus it enters the canalis facialis. 



Second Part. The second part of the facial nerve is very 

 short. It runs laterally, with a slight inclination forwards 

 between the vestibule and cochlea, and very soon ends in 

 the swelling termed the ganglion geniculi. 



Third Part. At the ganglion geniculi, the facial nerve 

 bends suddenly and proceeds backwards and slightly 

 downwards, in that portion of the canal which runs along 

 the upper part of the medial wall of the tympanic cavity, 

 immediately above the fenestra vestibuli (O.T. ovalis). 



The first three portions of the facial nerve are nearly 

 horizontal, and pursue a somewhat V-shaped course. The 

 apex of the V is directed forwards, and corresponds to the 

 ganglion geniculi. 



Fourth Part, The fourth part is vertical, and passes down- 



