THE MIDDLE EAR OR TYMPANIC CAVITY 



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iter chordae anterius (canal of Huguier) is placed at the medial end of the 

 petrotympanic fissure; through it the chorda tympani nerve leaves the tympanic 

 cavity. 



The Tympanic Membrane (membrana tympani) (Figs. 909, 910) separates the 

 tympanic cavity from the bottom of the external acoustic meatus. It is a thin, 

 semitransparent membrane, nearly oval 

 in form, somewhat broader above than 

 below, and directed very obliquely down- 

 ward and inward so as to form an angle 

 of about fifty-five degrees with the floor 

 of the meatus. Its longest diameter is 

 downward and forward, and measures 

 from 9 to 10 mm.; its shortest diameter 

 measures from 8 to 9 mm. The greater 

 part of its circumference is thickened, 

 and forms a fibrocartilaginous ring which :s 

 fixed in the tympanic sulcus at the inner 

 end of the meatus. This sulcus is defi- 

 cient superiorly at the notch of Rivinus, 

 and from the ends of this notch two bands, 

 the anterior and posterior malleolar folds, 

 are prolonged to the lateral process of the 

 malleus. The small, somewhat triangular 

 part of the membrane situated above these 

 folds is lax and thin, and is named the 

 pars flaccida ; in it a small orifice is some- 

 times seen. The manubrium of the malleus 

 is firmly attached to the medial surface of 

 the membrane as far as its center, which 

 it draws toward the tympanic cavity; the 

 lateral surface of the membrane is thus 

 concave, and the most depressed part of 

 this concavity is named the umbo. 



Structure. The tympanic membrane is com- 

 posed of three strata: a lateral (cutaneous), an 

 intermediate (fibrous), and a medial (mucous). 



FIG. 910. The tympanic membrane viewed from 

 within. (Testut.) The malleus has been resected 

 immediately beyond it^ lateral process, in order to 

 show the tympanomalleolar folds and the membrana 

 flaccida. 1. Tympanic membrane. 2. Umbo. 3. 

 Handle of the malleus. 4. Lateral process. 5. Anterior 

 tympanomalleolar fold. 6. Posterior tympanomalleolar 

 fold. 7. Pars flaccida. 8. Anterior pouch of Troltsch. 

 9. Posterior pouch of Troltsch. 10. Fibrocartilaginous 

 ring. 11. Petrotympanic fissure. 12. Auditory tube. 

 13. Iter chordae posterius. 14. Iter chordae anterius. 

 15 Fossa incudis for short crua of the incus. 16. Pro- 

 minentia styloidea. 



The cutaneous stratum is derived from the 



integument lining the meatus. The fibrous stratum consists of two layers: a radiate stratum, 



Chorda tympani. 



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FIG. 911. View of the inner wall of the tympanum (enlarged.) 



the fibers of which diverge from the manubrium of the malleus, and a circular stratum, the 

 fibers of which are plentiful around the circumference but sparse and scattered near the center 



