TYMPANIC CAVITY OR MIDDLE EAR 



835 



anterior and posterior malleolar plicae. The small triangular portion of the membrane 

 (Fig. 710) situated above these folds is thin and lax, and constitutes the pars flaccida 

 (O.T. membrane of Shrapriell) ; the main portion of the membrane is, on the other 

 hand, tightly stretched and termed the pars tensa. A small orifice, sometimes 

 seen in the pars flaccida, is probably either a pathological condition or has been 

 produced artificially during manipulation. The manubrium mallei is firmly fixed to 

 the medial surface of the membrana tympani, the central portion of which is drawn 

 towards the tympanic cavity so that its lateral surface is concave. The deepest 

 part of this concavity corresponds with the lower end of the manubrium of the 

 malleus, and is named the umbo membranae tympani. 



The membrana tympani consists of three layers : (1) a lateral, the stratum 

 cutaneum ; (2) an intermediate, the membrana propria ; (3) a medial, the stratum 

 mucosum. 



The stratum cutaneum is continuous with the skin of the meatus, and consists 

 of a thin layer of cutis covered with epidermis. The cutis is thickest near the 

 circumference ; the epidermis, on the other hand, is thickest near the centre of the 

 membrane. 



The membrana propria consists of two sets of fibres : (a) a lateral, the stratum 

 radiatum, situated immediately under the stratum cutaneum, and radiating from 

 the manubrium of the malleus to the annulus fibrocartilagineus ; (6) a medial, the 

 stratum circulare, the fibres of which are numerous near the circumference, but 

 scattered and few in number near the centre of the membrane (Fig. 710). Both 

 radial and circular fibres are absent from the pars flaccida, which consists only of 

 the cutaneous and mucous strata. Gruber pointed out that, in addition to the 

 radial and circular fibres, there exists, next the stratum mucosum, a series of 

 dendritic or branched fibres, which are best developed in the posterior part of the 

 membrane. 



The stratum mucosum is continuous with the mucous lining of the tympanic 

 cavity. It is thicker over the upper part of the membrane than near its centre, 

 and is covered with pavement epithelium. 



Otoscppic Examination of the Membrana Tympani (Fig. 711). The membrana tympani, 

 in the living, is of a " pearl-gray " colour, but may present a reddish or yellowish tinge, depending 

 upon the condition of its mucous lining and on the condition of the cutaneous lining of the meatus ; 

 the posterior segment is usually 

 clearer than the anterior. At the 

 an tero- superior part, close to its 

 periphery, a whitish point appears 

 as if projecting towards the meatUS ; Membrana flaccida |M -^IM^I^BE malleus 



this is the processus lateralis of the Anterior malleolar 



malleus. Passing downwards and [_ Postero-superior 



backwards from this point to the 9k i quadrant 



umbo is a ridge caused by the Manubrium mallei 

 manubrium mallei, the lower ex- 

 tremity of which appears rounded. 

 Two ridges, corresponding with the 

 malleolar plicae, extend from the 

 processus lateralis of the malleus, 

 one forwards and upwards, the other 

 backwards and upwards. Behind, 

 and near the lower extremity of the 

 manubrium mallei, is a reddish or 

 yellowish spot, due to the promontory of the medial wall of the tympanic cavity shining through. 

 [f the membrane is very transparent, the long crus of the incus may be visible behind the upper 

 part of the manubrium mallei, and reaching downwards as far as its middle. From the lower 

 end of the manubrium mallei, the " cone of light " or " luminous triangle " extends downwards 

 and forwards, its apex being directed towards the handle ; this triangle varies in size in different 

 people. A line prolonging the manubrium downwards divides the membrane into two parts, 

 while another, drawn at right angles to this through the umbo, will subdivide it into quadrants, 

 viz., postero-superior, postero-inferior, antero-superior, and antero-inferior ; this subdivision is 

 useful in enabling the otologist to localise and describe accurately the seat of lesions in the 

 membrane. 



Vascular and Nervous Supply of the Membrana Tympani. The arteries are arranged in 



two sets, one on the cutaneous and another on the mucous surface ; they anastomose by means 



: small branches which pierce the membrane, especially near its periphery. The first set is 



derived ehiefly from the deep auricular branch of the internal maxillary, whilst those on the 



54 a 



Antero-superior 

 quadrant 



Antero-inferior 

 quadrant 



Posterior 

 malleolar plica 

 Lateral process of 



Postero-inferior 

 quadrant 



Cone of light 



FIG. 711. LEFT TYMPANIC MEMBRANE (as viewed from the external 

 acoustic meatus). x 3. 



