. 



(ana/. 



Aquadvtt. fctlopii 

 Atti< or 



ftgmen 



tympa.ni 



ms, 



. 



pyramid 



FIG. 178. A diagrammatic sketch showing the outer side of the petrous bone after removal of the squama 

 and the tympanic plate : the area over which the plate lies against the petrous is coloured pink, whereas 

 the area of petro-squamous junction is shown uncoloured. The meeting of the tympanic plate with the 

 jugular plate of the petrous bone makes the floor of the tympanic cavity which the removal of the super- 

 ficial bones has exposed, and the tympanic plate also forms the floor of the bony outer part of the 

 Eustachian tube. The inner wall of the tympanum forms the outer covering of the inner ear ; the 

 first turn of the cochlea makes an elevation, the promontory, on the front part of the wall, the vestibule- 

 is situated behind this and has the fenestrce associated with it, and the semicircular canals, which con- 

 stitute the hinder part of the inner ear, cause a slight prominence (external canal) in the aditus which 

 leads from the tympanum to the mastoid antrum. The seventh nerve runs out behind and above the 

 cochlea, and thus comes to the inner wall above and behind (X.) the promontory : here the " Fallopian 

 aqueduct," the bony canal which contains the nerve, turns back along the inner wall, above the fenestra 

 ovalis, to reach the posterior wall, down which it turns to end at the stylomastoid foramen. The 

 relative position of the facial nerve and the various parts of the inner ear. as seen in the tympanum, is 

 also illustrated in the arrangement of foramina in the floor of the internal auditory meatus (Fig. 177). 



