TYMPANIC CAVITY 523 



and the tympanic antrum. Anteriorly, on the medial wall of 

 the tympanic cavity, is the promontory, which marks the position 

 of the first turn of the cochlea. Above and posterior to the 

 promontory is the fenestra vestibuli. The fenestra cochleae lies 

 at the lower and posterior part of the promontory, in the anterior 

 part of a recess called the fossula fenestrae cochleae. Above 

 the fenestra vestibuli is a ridge caused by the posterior horizontal 

 part of the canalis facialis ; this becomes continuous, on the 

 medial wall of the aditus, with the vertical ridge which indicates 

 the position of the vertical part of the canal. Above the latter 

 is the horizontal ridge due to the lateral semicircular canal. 

 The dissector should open the canalis facialis to expose the 

 facial nerve ; then he should open the lateral semicircular canal, 

 and afterwards remove the bone above and posterior to it to 

 expose the walls of the superior and posterior semicircular 

 canals (Figs. 219, 220, 221). 



INTRAPETROUS PART OF THE FACIAL NERVE AND THE 

 ACOUSTIC NERVE. 



The facial and acoustic nerves have already been traced 

 into the internal acoustic meatus (p. 112). The dissector 

 should now open up the 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 acoustic meatus, and opens on the exterior of 

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

 mencement and termination it pursues a curved course, and 

 that, combined with the density of the bone, renders the dis- 

 section 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 acoustic 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 anterior surface of the petrous portion. 

 Make a second horizontal saw cut, immediately above the roof 

 of the internal acoustic 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 



