THE E4R 



1539 



Mastoid 

 Ancrum 



Pyram-d, with Open- 

 ing for Stapedius 



Fenestra CK-alis 

 Aqueduct of Fallopius 

 at Hiatus Fallopii 



The promontory {tuber cochlea) is situated below ' the fenestra 

 ovalis, between it and the fenestra rotunda, and slightly anterior 

 to both. It is a rounded prominence which is produced hy the first 

 turn of the cochlea, and it presents grooves for the nerves of the 

 tympanic plexus. 



The fenestra rotunda is situated in a funnel-shaped depression, 

 called the fossula rotunda, below and behind the promontory. It 

 leads into the scala tympani of the cochlea, and in the recent state 

 is closed by the secondary membrane of the tympanum. 



The sinus tympani is a depression situated behind the promontcwy,. 

 and between the fossula ovalis and fossula rotunda. 



The roof of the tympanum is formed by a thin plate of bone,, 

 called the legmen tympani, which forms 

 part of the superior surface of the petrous- 

 part of the temporal bone. 



The floor is formed by a 

 thin plate of bone which 

 separates the tympanum 

 from the jugular fossa. 



The anterior wall is 

 narrow, owing to the 

 descent of the roof, and 

 the inclination towards 

 each other of the outer 

 and inner waUs. It pre- 

 sents the orifices of two 

 canak, the upper of 

 which lodges the tensor 

 tympani muscle, whilst 

 the lower is the osseous 

 part of the Eustachian 

 tube. The two orifices 

 are separated by the 

 margin of Xht processus 

 cochlearifor mi s. 



The posterior wall is 

 formed by the anterior 

 or tympanic surface 

 of the petrous part of the temporal bone. It presents, from above 

 downwards, the following parts : (i) the opening of the mastoid 

 antrum, which communicates with the attic of the tympanum, or 

 epitympanic recess ; (2) a depression called the fossa inoudis, 

 which receives the short process of the incus ; (3) a small conical 

 projection, called the pyramid, the summit of which presents an 

 opening for the passage of the tendon of the stapedius muscle ; 

 (posteriorly the canal within the pyramid, which contains the 

 stapedius muscle, passes downwards in the posterior wall of the 

 tympanum, and communicates with the descending part of the 

 aqueduct of Fallopius, which contains the facial nerve, thus ex- 

 plaining how the branch of that nerve to the stapedius reaches the 



Outlet of 

 Caiotid Canal 



Promontory 

 Fenestra Rotunda 



Sinns Tympani 



bullet of Aqueduct of FallopitlS- 

 at Stylo- mastoid Foramen 



Fig. 644.- — SECTtON THROUGH THE PeTROUS 



AND Mastoid Portions of the Tempor.\l 

 Bone, showing the Tympanum and Mas- 

 toid Cells. 



