What is the function of the auditory bones

Structure of the ear and function of hearing

Outer ear

From the outside only the outer ear and the entrance to the auditory canal are visible, the middle and inner ear are inside the head. To the outer ear count the cartilaginous auricle(Auricula) and the funnel-shaped outer ear canalwhich consists partly of cartilage and partly of bone. The auricle is shaped individually in every person. Their shape ensures that the sound - depending on which direction it comes from - is passed on with a slight delay. In this way we can tell whether a sound is e.g. B. comes from above or below. At the lower end of the auricle is the fleshy one Earlobe(Lobulus). Since the earlobe is well supplied with blood but is hardly sensitive to pain, individual drops of blood are often taken for laboratory tests.

The outer ear canal is 2–2.5 cm long and about 7 mm in diameter. It is lined with skin whose sweat and sebum glands are that Ear waxForm (cerumen). This moisturizes the skin of the ear canal and protects the ear canal from water, pathogens and insects.

Middle ear and eardrum

The Eardrum, a wafer-thin flexible membrane (about 0.8 cm2 large), forms the boundary between the outer ear and the middle ear. It closes the middle ear to the outside and protects it from dirt, pathogens and mechanical damage. In addition, the incoming sound causes the eardrum to vibrate and transmits it.

in the Middle ear is the Tympanic cavity, an air-filled cavity between the eardrum and the inner ear. In the tympanic cavity sits a chain made up of three tiny movable ones Auditory ossicles:hammer, anvil and stirrup. The bones are fused with the eardrum and transmit its vibrations through the oval windows to the inner ear, where the actual hearing organ is located. The tympanic cavity is about that Eustachian tube(Eustachian tube, Tuba auditiva) connected directly to the throat. The middle ear and ear trumpet are lined with a mucous membrane like the pharynx. The ear trumpet opens when swallowing and yawning and ensures ventilation of the middle ear and the equalization of pressure between the middle ear and the nasopharynx (e.g. on an airplane). In addition, the tympanic cavity is connected to the Mastoid process(Mastoid) des Temporal bonewhich lies behind the auricle in the skull bone and contains air-filled cavities lined with mucous membrane. They are hollow to save bone mass and therefore weight. The mastoid process can be felt from the outside behind the auricle.

The tympanic cavity in the middle ear is microscopic (its volume is only 1 cm3) and contains the chain of ossicles.
Georg Thieme Verlag, Stuttgart

Inner ear

The Inner ear lies in a highly branched cavity system within the Petrous bone, which also belongs to the temporal bone. It is therefore also known as the bony labyrinth. It consists of three sections: the Forecourt(Vestibule), den Semicircular canals (Canales semicirculares) and the Cochlea(Cochlea).

That sits in the forecourt and the three arcades Balance organ(Vestibular organ). It is used together with z. B. the eyes of orientation in space, the maintenance of head and body posture and the gaze stabilization when moving the head.

The spiral-shaped turns of the bony snail are filled with a liquid, the perilymph. A delicate tube system, which is filled with endolymph, runs between the windings, separated by a wafer-thin membrane. The real thing is in it Hearing organ(Organ of Corti).

The conversion of sound and movement into nerve impulses takes place according to the same pattern in the auditory and equilibrium organs: Both have sensory cells from which fine hairs protrude into the fluid. They bend when the endolymph moves, thus converting mechanical impulses into nerve signals. These signals are transmitted by fibers of the Hearing andEquilibrium nerve(VIII. Cranial nerve, vestibulocochlear nerve) forwarded to the brain.

Structure of the ear
Georg Thieme Verlag, Stuttgart

Sound perception and volume

Sound is transmitted in waves through vibrations in the air. The number of oscillations per second is called the frequency of a tone, she will be in hertz(Hz) measured. The frequency determines the perception of the Pitch: We perceive sound waves with a low frequency as low tones, high frequencies as high. The range of the frequency spectrum that can be heard by humans is between 16 and 20,000 Hz, depending on age, and human speech between 750 and 3,000 Hz.

In addition to the frequency, the sound pressure, the extent of the vibration, is also decisive, it gives the volume and is in decibel(dB) measured. At 0 dB, a sound is barely perceived by healthy people. The sound pressure level grows logarithmically - we perceive an increase of 10 dB as roughly doubling. Excessive sound pressure causes pain and discomfort, the pain threshold is ~ 125 dB. A high level of noise exposure leads to permanent hearing impairment (inner ear hearing loss) after a relatively short time.

In the human ear, the sound is transmitted to the inner ear via the auditory canal and the middle ear through air vibrations (air conduction) as well as through vibrations of the skull bone (bone conduction).

Modern hearing aids are getting smaller and smaller, and manufacturers often offer the same model in different designs and sizes. While the "in-the-ear" device fills the entire auricle, the "in-the-canal" device (top right) closes off with the ear canal and can hardly be seen. The “Complete-In-Canal” device is even smaller (bottom left). Scheme drawing of an implantable hearing aid (middle ear implant).
Georg Thieme Verlag, Stuttgart


Prof. Dr. med. Gerhard Grevers, Dr. Ute Koch, Thilo Machotta, Dr. med. Arne Schäffler in: Gesundheit heute, edited by Dr. med. Arne Schäffler. Trias, Stuttgart, 3rd edition (2014). Revision and update: Dr. med. Sonja Kempinski | last changed on at 15:53