The ossicles include the malleus, the incus, and the stapes. The stapes has a footplate that inserts into the oval window. Behind the oval window is a fluid-filled component of the inner ear, the vestibule. It is continuous with a tubular structure known as the scala vestibuli. Inward movement of the tympanic membrane by a sound pressure wave causes the chain of ossicles to push the footplate of the stapes into the oval window (see
Fig. 8.17B ). This movement of the stapes footplate in turn displaces the fluid within the scala vestibuli. The pressure wave that ensues within the fluid is transmitted through the basilar membrane of the cochlea to the scala tympani (described later), and it causes the round window to bulge into the middle ear.
The tympanic membrane and the chain of ossicles serve as an impedance-matching device. The ear must detect sound waves traveling in air, but the neural transduction mechanism depends on movement in the fluid-filled cochlea, where acoustic impedance is much higher than that of air. Therefore, without a special device for impedance matching, most sound reaching the ear would simply be reflected, as are voices from shore when a person is swimming under water. Impedance matching in the ear depends on (1) the ratio of the surface area of the large tympanic membrane advantage of the lever system formed by the ossicles. This impedance matching is sufficient to increase the efficiency of energy transfer by nearly 30 dB in the range of hearing from 300 to 3500 Hz.
The middle ear also serves other functions. Two muscles are found in the middle ear: the tensor tympani attached to the malleus and the stapedius attached to the stapes. When these muscles contract, they damp movements of the ossicles and decrease the sensitivity of the acoustic apparatus. This action can protect the acoustic apparatus against damaging sounds that can be anticipated. However, a sudden explosion can still damage the acoustic apparatus because reflex contraction of the middle ear muscles does not occur quickly enough. The chamber of the middle ear connects to the pharynx through the eustachian tube. Pressure differences between the external ear and middle ear can be equalized through this passage. If fluid collects in the middle ear, as during an infection, the eustachian tube may become blocked. The resulting pressure difference between the external ear and middle ear can produce painful displacement of the tympanic membrane and, in extreme cases, cause rupture of the tympanic membrane. Unequalized pressure changes as a result of flying or diving can also cause discomfort.
to that of the smaller oval window and (2) the mechanical
The inner ear includes the bony and membranous labyrinths. The bony labyrinth is a complex but continuous series of spaces in the temporal bone of the skull, whereas the membranous labyrinth consists of a series of soft tissue spaces and channels lying inside the bony labyrinth. The cochlea and the vestibular apparatus are formed from these structures. The cochlea is a spiral-shaped organ (see
Fig. 8.17A ). In humans, the spiral consists of 23 4 turns from a broad base to a narrow apex, although its internal lumen is small at the base and wide at the top. The apex of the cochlea faces laterally (see