The Anatomy and Functions of the Middle-ear Structures
The middle ear consists of three ossicles and the tympanic membrane,which can be considered as either part of the outer or middle ears. It will be discussed here as part of the middle ear, as its function has been shown to be integrally related to the middle-ear impedance matching.
The Tympanic Membrane: – Is a thin, cone-shaped membrane that separates the external ear from the. Its function is to transmit sound from the air to the ossicles inside the middle ear, shape-pearly grey, shiny, translucent, with no bulging or retraction.
The acoustic energy from the sound travelling down the external
Figure 3 : normal tympanic membrane
auditory meatus (EAM) is translated into motion of the tympanic membrane. This motion is directly related to the compression and rarefaction motion of the air molecules travelling as the sound wave. Thus, when compression occurs, the tympanic membrane moves medially; and when rarefaction occurs, the tympanic membrane moves laterally. Thus, the tympanic membrane moves in-and-out or back-and-forth. ( Clark, J.G et al, 2002)
Middle-ear Ossicles There are three small bones, or ossicles, in the middle ear, called the malleus (hammer), incus (anvil) and stapes (stirrup);
Figure 4: middle ear ossicles
The malleus, or hammer, is attached to the tympanic membrane. Like the hammer from which it derives its name, the malleus has two parts, the head and the arm, called the manubrium. The lateral end of the manubrium is attached to the centre, or umbo, of the tympanic membrane. Thus, movement or oscillation of the tympanic membrane is transmitted directly to the malleus. The malleus is also suspended from the bony walls of the middle ear at several points, so that it can move as the vibrations of the tympanic membrane are transmitted to it. Thus, as the tympanic membrane moves medially, the malleus does the same because of this direct vibration. The head of the malleus is rounded like a mallet and is attached to the head of the incus, the next middle-ear ossicle, via ligaments.
The head of the incus is indented to form a ball–in–socket attachment. attachment allows free motion of the malleus and incus.The movements of the incus are in opposition to those of the malleus, i.e.as the malleus moves medially the incus moves laterally. This causes a lever-type action between these two ossicles. This lever is one of the important mechanical devices involved in the functioning of the middle ear. The head of the incus leads to the body and the long process of the incus .
The stapes or stirrup, is attached to the end of this process. The attachment between the incus and the stapes is such that any movement of the incus leads to movement of the stapes in the same direction. Therefore, as the incus moves in, the stapes moves in as well.The stapes is the third and last ossicle,making up what is called the ossicular chain. The stapes bone has fourmain parts: the head,neck, arms or crura, and the footplate.The footplate is the end part of the stapes and resembles the part of a stirrup into which the foot is put when mounting a horse.This footplate is oval in shape and fits in an oval-shaped opening in the medial bony wall, separating the middle ear from the inner ear,called the oval window. In anatomical terminology, a window is an open space in a bony wall. The ligaments that hold the footplate in the oval window allow this part of the stapes to rock in and out of the inner ear. This rocking motion sets up waves in the fluid of the inner ear. ( Katz, J., et al ,2001)