MIDDLE EAR 



875 



The membrane of Shrapnell is extremely thin, and consists only of the outer 

 and inner layers, the fibrous layer being absent. 



The handle of the malleus descends between the two inner layers, accompanied 

 by the vessels, which are often clearly visible in the living memV^rane. 



The Tympanic Cavity or middle ear is a narrow space which intervenes be- 

 tween the external auditory meatus and the labyrinth of the internal ear (figs. 496 

 and 498a). It is a narrow irregular cavity, varying in width from a twelfth to a 

 sixth of an inch (2 to 4 mm.), and having a total height of a little more than half 

 an inch (15 mm). It consists of two parts, a lower and narrower, bounded exter- 

 nally by the tympanic membrane, and an upper and wider part, called the attic, or 

 epitym|)anum, which is continued l^ackwards as the mastoid antrum into the mas- 

 toid air cells. It is prolonged downwards and forwards into the Eustachian tube, 

 through which it communicates with the pharynx. It contains air, and it is crossed 

 from without inwards liy a movable chain of bones — the tympanic ossicles — which 

 conduct viln-ations from the membrana tympani to the internal ear. 



Fig. 498. — Internal View of Right Membkana Tympani. (Enlarged.) 



Suspensory ligament of malleus INCUS 



HEAD OF MALLEUS 



CHORDA TYMPANI 

 NER VE 



Tendon of tensor 

 tympani 



HANDLE OF MALLEUS 

 Tensor tympani muscle 



EUSTACHIAN TUBE 



POSTERIOR PORTION OF 

 RECESS FOR OSSICLES 

 ABOVE MEMBRANE 



FOOTPIEGE OF STAPES 



LENTICULAR PROCESS (OS 

 ORBICULARE) 



Posterior portion of 

 membrana tympani 



The roof is a thin plate of the petrous part of the temporal bone, which 

 separates the cavity from the middle fossa of the skull. 



The floor is narrower than the roof. It is formed by the tympanic plate, 

 and it separates the cavity from the jugular fossa. 



The outer wall is formed below by the tympanic membrane, and above by 

 a part of the squamous portion of the temporal bone. 



The chorda tympani nerve., which traverses the outer wall, enters by a minute 

 orifice, the aperture of the iter chordae posterius, and leaves the cavity by the 

 iter chordae anterius, which leads into the canal of Huguier. The Glaserian 

 fissure lies immediately below the latter orifice, but these minute openings are 

 hidden by the mucous lining. 



The inner wall is vertical, and looks directly outwards. It presents, antero- 

 inferiorly, a rounded prominence, the promontory, marked by grooves for the tym- 

 panic plexus, and caused by the first turn of the cochlea. In front of tlie promon- 

 tory, is the commencement of the Eustachian tube (fig. 498a), above is the outer 

 extremity of the canal for the tensor tympani muscle bounded by a prominent 

 ridge of bone called the cochleariform process, and behind it is a recess, the sinus 

 tympani, which intervenes between the fenestra ovalis above and the fenestra ro- 



