562 THE AUDITORY APPARATUS 



INTRAPETROUS PART OF THE FACIAL NERVE 

 AND THE ACUSTIC NERVE. 



The facial and acustic nerves have already been traced 

 into the internal acustic meatus (p. 215). The dissector 

 should now open up this meatus and follow the facial nerve 

 in its course through the petrous portion of the temporal bone. 

 The canal which it occupies is termed the canalis facialis 

 (O.T. aqueduct of Fallopius). It begins at the bottom of 

 the internal acustic meatus, and opens on the exterior of 

 the skull at the stylo-mastoid foramen. Between its com- 

 mencement and termination it pursues a complicated course, 

 and this, combined with the density of the bone, renders the 

 dissection very difficult. 



Dissection. On the side on which the middle ear has been opened from 

 the lateral aspect and the canalis facialis has already been partially opened 

 up, the dissector should complete the dissection of the intrapetrous part of 

 the facial nerve and should examine the acustic nerve. 



Separate the temporal bone from the other cranial bones which still 

 adhere to it, and fix it in the natural position (in a.vice if possible). Remove 

 the squamous portion by a horizontal saw cut at the level of the upper 

 surface of the petrous portion. Make a second horizontal saw cut, im- 

 mediately above the roof of the internal acustic meatus, and carry it 

 laterally into the tympanum, in which it should emerge immediately above 

 the already opened canalis facialis where the latter lies above the fenestra 

 vestibuli. Then, with the bone forceps or chisel, remove the remains of the 

 roof of the internal meatus and follow the facial nerve along the canalis 

 facialis to the hiatus canalis facialis, and so expose the ganglion geniculi. 

 Secure the branches which arise from the ganglion and then follow the nerve 

 posteriorly 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 in this part of the canal. 



Intrapetrous Portion of the Facial Nerve. As the facial 

 nerve traverses the petrous bone, it may be divided into four 

 stages, 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 acustic meatus. 



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



acustic meatus to the ganglion geniculi. 



