THE AUDITORY APPARATUS 



213 



by the upward spread of infection from the tympanum along 

 the veins which traverse the suture and terminate in the superior 

 petrosal sinus. The narrow Floor of the tympanum is formed 

 by the bone of the jugular fossa,, and lies below the level of the 

 lower border of the tympanic membrane. It is pierced by a 

 few small veins, which pass from the mucous membrane lining 

 the cavity to join the internal jugular vein. Septic thrombosis 

 of the latter may follow this course in otitis media. On the 

 Medial Wall, which separates the tympanum from the internal 

 ear, a slight elevation is present. It is produced by the first 

 coil of the cochlea, and is termed the promontory. The fenestra 

 of the cochlea (fenestra rotunda) is situated below and behind 



Tympanic antrum 



Fenestra vestibuli 



Incus 



Head of malleus 

 Tegmen tympani 

 Chorda tympani 



Auditory tube 



Promontory 



Fenestra cochleae 



Facial canal 



I T-" Tympanic membrane 

 Facial canal 



FIG. 65. Section through a Left Temporal Bone along the line of the 

 Tegmen Tympani. 



the promontory, and in life it is closed by a membrane which 

 separates the tympanic cavity from the scala tympani of the 

 cochlea. The fenestra of the vestibule (fenestra ovalis) is placed 

 at a slightly higher level. It is closed by the base (foot-plate) 

 of the stapes. The canal for the facial nerve produces a ridge, 

 which passes backwards above the promontory and downwards 

 behind it (Fig. 65). The Lateral Wall is formed mainly by 

 the tympanic membrane (p. 211). In the upper and medial 

 part of the Posterior Wall the epitympanic recess communicates 

 with the tympanic (mastoid) antrum through a short passage, 

 which is known as the aditus. The medial wall of the aditus 

 is crossed by the descending part of the ridge produced by the 

 facial nerve, and just above the ridge there is a slight promi- 

 nence due to the lateral semicircular canal. It is through the 



14 b 



