746 



THE EAR. 



which is from before backwards, with a slight inclination downwards in 

 front, is occupied in the recent state by the base of the stapes, and the 

 annular ligament connected with that process of bone. Above the fenestra 

 ovalis, and between it and the roof of the tympanum, a ridge indicates 

 the position of the aqueduct of Fallopius, as it passes backwards, contain- 

 ing the portio dura of the seventh nerve. Below it is a larger and more 

 rounded elevation, caused by the projection outwards of the first turn of 

 the cochlea, and named the promontory, or tuber cochleae ; it is marked by 

 grooves, in which lie the nerves of the tympanic plexus. 



Fig. 501. 



Fig. 501. INNER WALL OP THE OSSEOUS TYMPANUM AS EXPOSED BY A LONGITUDINAL 

 SECTION OP THE PETROUS AND MASTOID BONE (from Gordon). 



1, opening of the tympanum into the inastoid cells ; 2, fenestra ovalis ; 3, fenestra 

 rotunda ; 4, promontory ; 5, aqueduct of Fallopius, or canal of the facial nerve ; 6, 

 junction of the canal for the chorda tympani with the aqueduct ; 7, processus cochleari- 

 formis ; 8, groove above it for the tensor tympani muscle ; 9, Eustachian tube j 10, 

 anterior orifice of the carotid canal. 



Below and behind the promontory, and somewhat hidden by it, is a 

 slightly oval aperture named fenestra rotunda) which lies within a funnel- 

 shaped depression. In the macerated and dried bone the feuestra rotunda 

 opens into the scala tympani of the cochlea ; but, in the recent state it is 

 closed by a thin membrane. 



The membrane closing the fenestra rotunda the secondary membrane of 

 the tympanum (Scarpa) is rather concave towards the tympanic cavity, 

 and is composed of three strata like the membrana tympani ; the middle 

 layer being fibrous, and the outer and inner derived from the membranes 

 lining the cavities between which it is interposed, viz., the tympanum and 

 the cochlea. 



The posterior wall of the tympanum presents at its upper part one larger, 

 and several smaller openings, which lead into irregular cavities, the mastoid 

 cells, in the substance of the mastoid process of the temporal bone. These 

 cells communicate freely with one another, and are lined by mucous mem- 

 brane continuous with that which clothes the tympanum. Behind the 

 fenestra ovalis, and directed forwards, is a small conical eminence, called 

 the pyramid, or eminentia papillaris. Its apex is pierced by a foramen, 

 through which the tendon of the stapedius muscle emerges from a canal 

 which turns downwards in the posterior wall of the tympanum, and joins 

 obliquely the descending part of the aqueduct of Faliopius. 



