THE AUDITORY APPARATUS 217 



during the operation. Occasionally, when the lateral wall of 

 the aditus is being removed, the bone breaks away owing to the 

 weakness caused by the facial canal (aqueduct of Fallopius), 

 and a short piece of the facial nerve is exposed. This may be 

 bruised during the remaining steps of the operation, or it may 

 be pressed upon by the packing, etc., leading to a complete but 

 transitory facial paralysis. Any irritation of the nerve sets up 

 a spasm of the facial muscles, and the anaesthetist is able to warn 

 the surgeon. When the antrum and tympanic cavity are 

 syringed out, some of the fluid used may pass along the auditory 



Lateral semicircular canal Posterior semicircular canal 



Remains of posterior . Temporal line 



wall of external meatus / , r 



Wall of groove for 

 sigmoid part of 

 transverse sinus 



Tympanic plate ' 



Styloid process '^ Wa n ? f canalis 

 facialis 



FIG. 66. Dissection of the Tympanic (Mastoid) Antrum and the petro- 

 mastoid part of the Temporal Bone from the outer side. The arrow 

 is passing through the aditus from the tympanic antrum into the 

 tympanic cavity. 



tube into the nasal pharynx, and the anaesthetist must be pre- 

 pared to prevent it from passing down into the larynx. 



The mucous membrane of the tympanic cavity, together 

 with the incus and malleus, may require to be removed ; but 

 the stapes, which is firmly attached to the fenestra of the vesti- 

 bule (fenestra ovalis), is left in situ, as otherwise the internal 

 ear would be opened. 



After the antrum has been dealt with, the opening may be 

 enlarged upwards and forwards to explore the middle cranial 

 fossa (p. 231), or backwards to explore the transverse sinus 

 and cerebellum (p. 216), if the symptoms point to a spread of the 

 disease in either direction. 



