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Sound conduction

The auricle, the external auditory canal, the tympanic membrane, the auditory ossicles, the annular ligament of the oval window, the membrane of the round window (secondary tympanic membrane), the labyrinth fluid (perilymph), and the main membrane take part in conducting sound vibrations.

In humans, the role of the auricle is relatively small. In animals with the ability to move their ears, the auricles help determine the direction of the sound source. In humans, the auricle, like a mouthpiece, only collects sound waves. However, in this respect, its role is insignificant. Therefore, when a person listens to quiet sounds, he puts his hand to his ear, due to which the surface of the auricle is significantly increased.

Sound waves, having penetrated the auditory canal, lead to a friendly oscillation of the eardrum, which transmits sound vibrations through a chain of auditory ossicles into the oval window and then the perilymph of the inner ear.

The eardrum responds not only to those sounds whose oscillation number coincides with its own tone (800-1000 Hz), but also to any sound. This resonance is called universal in contrast to acute resonance, when a second-sounding body (for example, a piano string) responds to only one specific tone.

The eardrum and auditory ossicles not only transmit sound vibrations entering the external auditory canal, but transform them, i.e., they transform air vibrations with large amplitude and low pressure into vibrations of the labyrinth fluid with small amplitude and high pressure.

This transformation is achieved due to the following conditions: 1) the surface of the eardrum is 15-20 times larger than the area of ​​the oval window; 2) the malleus and the anvil form an unequal lever, so the excursions made by the stirrup foot plate are about one and a half times less than the excursions of the handle of the malleus.

The general effect of the transforming action of the eardrum and the auditory ossicle lever system is expressed in an increase in sound strength by 25-30 dB. Violation of this mechanism with damage to the tympanic membrane and diseases of the middle ear leads to a corresponding decrease in hearing, i.e., 25-30 dB.

For the eardrum and the ossicular chain to function normally, it is necessary that the air pressure on both sides of the eardrum, i.e., in the external auditory canal and in the tympanic cavity, be the same.

This pressure equalization is due to the ventilation function of the auditory tube, which connects the tympanic cavity with the nasopharynx. With each swallowing movement, air from the nasopharynx enters the tympanic cavity, and thus, the air pressure in the tympanic cavity is constantly maintained at atmospheric level, i.e. at the same level as in the external auditory canal.

The sound-conducting apparatus also includes the muscles of the middle ear, which perform the following functions: 1) maintaining the normal tone of the eardrum and the chain of auditory ossicles; 2) protection of the inner ear from excessive sound irritations; 3) accommodation, t.
e. the adaptation of a sound-conducting apparatus to sounds of various strengths and heights.

With the contraction of the muscle that stretches the eardrum, the auditory sensitivity increases, which gives reason to consider this muscle to be "alarming." The stirrup muscle plays the opposite role - when it contracts, it restricts the movement of the stapes and thereby muffles too strong sounds.

The above-described mechanism for transmitting sound vibrations from the external environment to the inner ear through the external auditory canal, the eardrum, and the chain of auditory ossicles is an air conduction. But sound can be delivered to the inner ear and bypassing a significant part of this path, namely directly through the bones of the skull - bone conduction. Under the influence of environmental fluctuations, oscillatory movements of the bones of the skull, including the bone labyrinth, occur. These1 oscillatory movements are transmitted to the labyrinth fluid (perilymph). The same transmission occurs when the sounding body, for example, the tuning fork legs, comes into direct contact with the bones of the skull, as well as under the influence of high-frequency sounds with a small amplitude of vibrations.

The presence of osseous conduction of sound vibrations can be seen through simple experiments: 1) when the ears are tightly plugged with both fingers, that is, when the access of air vibrations through the external auditory canals is completely stopped, the perception of sounds is significantly impaired, but still occurs; 2) if the leg of the sounding tuning fork is attached to the crown of the head or to the mastoid process, the sound of the tuning fork will be clearly audible even with the ears closed.

Bone conduction is of particular importance in ear pathology. Thanks to this mechanism, the perception of sounds is provided, albeit in a sharply weakened form, in those cases when the transmission of sound vibrations through the outer and middle ear is completely stopped. Bone conduction is carried out, in particular, with complete blockage of the external auditory canal (for example, with sulfuric plug), as well as with diseases that lead to immobility of the auditory ossicle chain (for example, with otosclerosis).

As already mentioned, the vibrations of the tympanic membrane are transmitted through the chain of stones to the oval window and cause the perilymph to move, which propagates along the staircase of the vestibule to the tympanic staircase. These fluid movements are possible due to the presence of a round window membrane (secondary tympanic membrane), which, with each movement of the stirrup plate inward and the corresponding push of the perilymph, bulges toward the tympanic cavity. As a result of the movements of the perilymph, oscillations of the main membrane and the Corti organ located on it arise.
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