THE TYMPANIC MEMBRANE 



1087 



wall of the meatus, forming, as a rule, with the superior wall, an angle of 140 

 degrees. It varies, however, greatly in form, size, and obliquity. Viewed from 

 the meatus, it appears as a semitransparent membrane, which sometimes has a 

 reddish tinge. It is drawn medially and made funnel-shape by the manubrium of 

 the malleus, but the walls of the funnel bulge toward the meatus (fig. 834). The 

 most depressed point at its centre, the umbo, is slightly inferior and posterior to 

 the centre of the membrane, and corresponds to the tip of the manubrium (fig. 

 832). From it a whitish streak, the malleolar stria, caused by the manubrium 

 shining through, passes superiorly toward the circumference. At the superior 

 end of the stria is a slight projection, the malleolar prominence, formed by the 

 lateral process of the malleus. From it two folds, the anterior and posterior 

 Fig. 831. — Frozen Coronal Section of the Right Ear. (Somewhat Enlarged.) 



Prominence of facial canal 



Medial tympanic wall 



Tegmen tympani 

 Recessus epitympanicus 



Dura mater ^ 

 Auricular cartilage \ 



Stapes 



Facial nerve 



Fenestra vestibuli 



Cochlea 

 , Facial, and cochlear 

 I vestibular nerves 



Cavum conchffi 



Cartilage of meatus j 



Parotid gland 



Interaal carotid artery 



plicae, stretch to the extremities of the tympanic sulcus (fig. 832). The small 

 triangular area of the membrane bounded by the plicae, is termed the pars flaccida 

 (Shrapnell's membrane). It is thin and flaccid, and is attached directly to the 

 petrous bone in the tympanic notch (notch of Rivinus). The larger part of the 

 tympanic membrane, thepars tensa, is inferior to the plicae and is tightl}" stretched. 

 Its thickened margin, the limbus, is attached by a fibro -cartilaginous annulus 

 to the tympanic sulcus, and at the spines of the tympanic ring is continuous with 

 the plicae. 



Structure of the tympanic membrane. — The tympanic membrane is about .1 mm. thick, 

 and consists of four layers. The lateral cutaneous layer, relatively thick, is a continuation 

 of the skin lining the external auditory meatus. Next to it is a radiate fibrous layer, composed 

 of connective tissue, the fibres of which are attached to the manubrium of the malleus and radiate 

 from it. Medial to it is the circular fibrous layer, which has its fibres arranged concentrically 

 and is especially thick at the circumference. It is closely bound to the radiate layer. The 

 mucous layer, which is a continuation of the mucosa of the tympanic cavity, covers the medial 

 surface of the membrane smoothly, except where the manubrium of the malleus causes a pro- 

 jection. The fibrous layers are attached to the fibro-cartilaginous ring and are not present 

 in the pars flaccida. 



