564 THE AUDITORY APPARATUS 



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 anteriorly, and corresponds to the 

 ganglion geniculi. 



The fourth stage is vertical, and arches downwards, posterior 

 to the pyramid, to gain the stylo-mastoid foramen. 



The branches which spring from or join the facial nerve 

 during its passage through the temporal bone are : 



1. The greater superficial petrosal nerve, "| 



2. Communicating twig to the smaller superficial I from ganglion 



petrosal, j geniculi. 



3. External superficial petrosal nerve, 



4. Nerve to stapedius. 



5. Chorda tympani. 



6. Communicating twigs to the auricular branch of vagus. 



The great superficial petrosal nerve has been examined 

 already (p. 212). Its origin from the ganglion geniculi of 

 the facial can now be seen. 



The communicating branch to the small superficial petrosal 

 arises from the ganglion geniculi, and unites with the fibres 

 of the tympanic nerve which issue from the tympanic plexus. 



The external petrosal nerve is not always present. It 

 joins the sympathetic plexus which accompanies the middle 

 meningeal artery. 



The nerve to the stapedius muscle arises from the facial as 

 it arches downwards posterior to the pyramid. It enters the 

 base of the pyramid and thus reaches the stapedius muscle. 



The communicating twigs to the auricular branch of the 

 vagus arise a short distance above the stylo-mastoid foramen. 



Chorda Tympani. The chorda tympani represents to a 

 large extent the sensory fibres set free from the trunk of 

 the facial nerve. It is the largest branch given off by the 

 facial during its passage through the canalis facia/is. It takes 

 origin a short distance above the stylo-mastoid foramen, and 

 arching upwards and anteriorly, in a narrow canal in the 

 petrous portion of the temporal bone (the canaliculus chordae 

 tympani), it appears in the tympanum by passing through 

 the tympanic aperture of the canaliculus chordae below the 

 base of the pyramid, and close to the posterior margin of 



