522 THE AUDITORY APPARATUS 



and more compact bone is exposed. When that stage of the 

 dissection is completed, the dissector should note the following 

 points : (i) In the anterior boundary of the exposed area is 

 the compact posterior wall of the bony part of the external 

 meatus. (2) Posteriorly is a broad projecting ridge indicating 

 the position of the groove which lodges the sigmoid part of the 

 transverse sinus. (3) At the upper and deeper part of the area 

 are the medial wall of the tympanic antrum and the aditus 

 leading into ] the tympanic cavity. (4) The intervening area is 

 occupied by the remains of the mastoid air-cells, which may 

 extend downwards to the tip of the mastoid process. They are 

 continuous above with the cavity of the , tympanic antrum. 

 (5) On the medial wall of the aditus and the anterior part of the 

 medial wall of the tympanic antrum is a horizontal ridge which 



Lateral semicircular canal 

 Superior semicircular canal I 



Fenestra vestibuli | Temporal line 



Canalis facialis (posterior ! ^, ^ f > Posterior semicircular canal 



horizontal part) 



End of canal for 

 tensor tympani 



End of auditory tube 

 Styloid process 



Vertical part of canalis facialis 

 Remains of posterior wall of external meatus 



FIG. 221. Dissection of the Tympanic Cavity and the semicircular 

 canals from the outer side. 



indicates the position of the lateral semicircular canal of the 

 labyrinth, and, below it, on the medial wall of the mouth of the 

 aditus, is a vertical ridge indicating the position of the 'canalis 

 facialis, which lodges the important facial nerve. 



The next stage of the dissection consists in the removal of 

 the posterior wall of the external meatus, and the exposure of 

 the lateral surface of the tympanic membrane (p. 514). After 

 the tympanic membrane has been examined, a seeker should 

 be passed through the aditus into the tympanic cavity, and its 

 handle should be allowed to rest on the lower part of the exposed 

 area ; then, whilst the seeker remains in position, the remainder 

 of the posterior wall and the upper boundary of the external 

 meatus, from the level of the seeker to the level of the roof of 

 the tympanic antrum, can be cut away without fear of injury 

 to any important structure. The dissection should be com- 

 pleted by the removal of the tympanic membrane and ossicles, 

 and when this has been done a very complete view wiJJ be 

 obtained of the medial walls of the tympanic cavity, the aditus 



