The Ossicles and their Function

Here the ossicles and contents of the middle ear will be discussed in more detail. Figure 3.5 depicts the position of middle ear and related structures. The TM, the bone of the tympanic ring, and outer (cortical) mastoid bone have been removed in this depiction.

The ossicles—the malleus, incus, and stapes—transmit sound vibrations from the large tympanic membrane into the small oval window, at a huge mechanical advantage. This system is really an "impedance-matching" one that enables sound waves in the air, at relatively low energy, to transfer sound waves into the fluid-filled medium of the inner ear, which offers higher resistance to sound flow. A fish, for example, has no need for a mid-

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Fig. 3.5 Middle ear and related structures:

I. Manubrium of malleus, near the short process above; 2. Head of malleus; 3. Body of incus; 4. Long process of incus; 5. Stapes footplate; 6. Stapedius tendon; 7. Round window; 8. Promontory; 9. Fibrous annulus of the TM; 10. Chorda tympani nerve;

II. Pyramidal eminence leading to stapedius muscle and tendon; 12. Location of lateral semicircular canal bulging within the mastoid antrum (this cavity is covered over here); 13. Vertical portion of facial nerve; 14. Eustachian tube.

(Source: Becker W, Naumann HH, Pfaltz CR. Ear, Nose, and Throat Diseases. Stuttgart: Thieme; 1994)

The Middle Ear 19

dle ear—sound waves already come in from a fluid medium. Thus, there is only a membrane between the ocean and its fluid-filled inner ear. Take this fish out of water, and it will have about a 30-dB hearing loss for air-transmitted sounds. If a human had no middle ear mechanism, he too would have about a 30-dB conductive loss. This situation is actually simulated with a fluid-filled middle ear cavity, or, alternatively, a tympanic perforation in combination with a disruption of the ossicles. In either situation, we are trying to transmit unenhanced sound waves from air directly into a fluid medium. The large ratio of TM/oval window surface area, further strengthened by the mechanical lever action of the ossicles, overcomes this potential loss.

Conductive hearing losses of varying degrees result from different impairments of the middle ear system. Examples include perforations, scarring, negative pressure, fluid effusions, ossicular fixation, ossicular disruption, and, of course, external canal blockages. Contrary to popular belief, a moderate-sized perforation of the TM does not cause a very large hearing loss—there is still ample drum left, along with the mechanical lever action of the ossicles. The tiny perforation of a ventilating tube that is placed to correct middle ear fluid causes no measurable loss, and of course, may correct a sizable loss related to the middle ear trouble that it remedies. The worst conductive losses are on the order of 50-60 dB HL. These might be seen with a congenital atresia of the canal and middle ear, with dense outer obstruction as well as associated deformity of the ossicles.

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