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Henry Gray (1825–1861).  Anatomy of the Human Body.  1918.
 
  The iter chordæ 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 downward 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 is fixed in the tympanic sulcus at the inner end of the meatus. This sulcus is deficient 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 sometimes 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.


FIG. 910– The tympanic membrane viewed from within. (Testut.) The malleus has been resected immediately beyond its 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 Tröltsch. 9. Posterior pouch of Tröltsch. 10. Fibrocartilaginous ring. 11. Petrotympanic fissure. 12. Auditory tube. 13. Iter chordæ posterius. 14. Iter chordæ anterius. 15. Fossa incudis for short crus of the incus. 16. Prominentia styloidea. (See enlarged image)



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


Structure.—The tympanic membrane is composed of three strata: a lateral (cutaneous), an intermediate (fibrous), and a medial (mucous). The cutaneous stratum is derived from the integument lining the meatus. The fibrous stratum consists of two layers: a radiate stratum, 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

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