204 THE ORGANS OF SPECIAL SENSE 



postero-inferior corner of the promontory, there is a small 

 circular foramen, termed the fenestra cochlea (rotunda], which 

 is closed by a membrane. This membrane intervenes between 

 the middle ear and the perilymph of, the cochlea (p. 207), 

 and it lies practically opposite to the postero-inferior quadrant 

 of the membrana tympani. As the middle ear is normally 

 rather less than one-eighth of an inch wide at this point, in 

 performing paracentesis care must be taken lest the point of 



FIG. 79. The Facial Nerve traversing ihe Facial Canal in the Petrous 



Part of the Temporal Bone. 



IT. Facial nerve. 



the instrument pass across the middle ear and open into the 

 labyrinth. 



The facial canal, which transmits the facial nerve through 

 the petrous portion of the temporal bone, passes backwards 

 above the promontory. As the bony wall which separates it 

 from the cavity of the middle ear is extremely thin, it may 

 readily become necrosed in the course of otitis media, ex- 

 posing the facial nerve and leading to facial paralysis (p. 87). 



Two small muscles, termed the tensor tympani and the 



