THE TYMPANUM. 633 



The fenestra ovalis is a reniform opening, leading from the tympanum into 

 the vestibule ; its long diameter is directed horizontally, and its convex border 

 upwards. The opening in the recent state is closed by the lining membrane 

 common to both cavities, and is occupied by the base of the stapes. This mem- 

 brane is placed opposite the membrana tympani, and is connected with it by the 

 ossicula auditus. 



The fenestra rotunda is an oval aperture, placed at the bottom of a funnel- 

 shaped depression, leading into the cochlea. It is placed below and rather behind 



Fig. 314. View of Inner Wall of Tympanum. (Enlarged.) 



Cl.erda 



the fenestra ovalis, from which it is separated by a rounded elevation, the pro- 

 montory ; it is closed in the recent state by a membrane (membrana tympani 

 secundaria, Scarpa). This membrane is concave towards the tympanum, convex 

 towards the cochlea. It consists of three layers; the external or mucous, 

 derived from the mucous lining of the tympanum ; the internal or serous, from 

 the lining membrane of the cochlea ; and an intermediate or fibrous layer. 



The promontory is a rounded hollow prominence, formed by the projection 

 outwards of the first turn of the cochlea ; it is placed between the fenestrae, and 

 furrowed on its surface by three small grooves, which lodge branches of the 

 tympanic plexus. 



The rounded eminence of the aquseductiLS Fallopii is placed between the fenestni 

 ovalis and roof of the tympanum ; it is the prominence of the bony canal in which 

 the portio dura is contained. It traverses the inner wall of the tympanum above 

 the fenestra ovalis, and, behind that opening, curves nearly vertically downwards 

 along the posterior wall. 



The pyramid is a conical eminence, situated immediately behind the fenestra 

 ovalis, and in front of the vertical portion of the eminence above-described ; it is 

 hollow in the interior, and contains the Stapedius muscle ; its summit projects 

 forwards towards the vestibular fenestra, and presents a small aperture, which 

 transmits the tendon of the muscle. The cavity in the pyramid is prolonged into 

 a minute canal, which communicates with the aquaeductus Fallopii. 



The posterior wall of the tympanum is wider above than below, and presents 

 for examination the 



Openings of the Mastoid Cells. 



These consist of one large irregular aperture, and several smaller openings, 

 situated at the upper part of the posterior wall ; they lead into canals, which com- 

 municate with large irregular cavities contained in 'the interior of the mastoid 



