THE MIDDLE EAR. 



699 



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

 the promontory; it is closed in the recent state by a membrane (membrana tym- 

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

 convex towards the cochlea. It consists of three layers: the external, or 

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

 from the lining membrane of the cochlea; and an intermediate, or 

 fibrous layer. 



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



Fig. 379. View of Inner Wall of Tympanum. (Enlarged.) 



mucous, 

 serous, 



Chorda 



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

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

 tympanic plexus. 



The rounded eminence of the aquseductus Fallopii is placed beneath the fenestra 

 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 tym- 

 panum above the fenestra ovalis, and behind that opening curves nearly verti- 

 cally 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 fenestra ovalis, 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, and 

 transmits the nerve which supplies the Stapedius. 



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 

 communicate with large irregular cavities contained in the interior of the mas- 

 toid process. These cavities vary considerably in number, size, and form; 

 they are lined by mucous membrane, continuous with that covering the cavity 

 of the tympanum. 



The anterior wall of the tympanum is wider above than below; it corresponds 

 with the carotid canal, from which it is separated by a thin plate of bone; it 

 presents for examination the 



Canal for the Tensor Tympani. Orifice of the Eustachian Tube. 



The Processus Cochleariformis. 



