TYMPANUM 



antrum. (O On the medial wall of the aclitus and the anterior part of the 

 medial waff of the tympanic antrum is a horizontal ridge which 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 outer surface of the 

 tympanic membrane (p. 549). 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 



Superior semicircular canal 

 Fenestra vestibuli 



Canalis facialis (posterior ', 



horizontal part) , 



Lateral semicircular canal 



' Temporal line 



.-' Posterior semicircular canal 



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. 246. Dissection of the Tympanic Cavity and the semicircular 

 canals from the outer side. 



be cut away without fear of injury to any important structure. The 

 dissection should be completed by the removal of the tympanic membrane 

 and ossicles, and when this has been done a very complete view will be 

 obtained of the medial walls of the tympanic cavity, the aditus, 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 fenestne 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 vertical semicircular canals 

 (Figs. 245, 246). 



VOL. II 36 



