The middle auditory ossicle is called an anvil. He picks up the vibration of the hammer and gives it to the stirrup under mechanical reinforcement. Although the three ossicles are the smallest bones in humans, at the same time they are also very hard and firm in order to transmit vibrations as lossless as possible.
The incus (Incus), with its weight of about 27 milligrams, is the heavyweight among the three ossicles in the middle ear. As the middle limb of the three bones, which transmit the sound vibrations of the eardrum to the inner ear, it is connected with the hammer over the Articulatio incudomallearis joint and with the stirrup over the tiny joint Articulatio incudostapedia. The transmission of the vibrations on the stirrup takes place by utilizing the leverage.
Because the lever arm from the pivot point to the stirrup is shorter than the lever arm from the hammer joint to the pivot point, the deflection of the anvil at the junction with the stirrup becomes shorter, but stronger by a factor of 1.3. Another mechanical amplification by a factor of 17 then takes place by transferring the vibrations to the oval window, which, with an area of 3.2 mm², only reaches one seventeenth of the area of the eardrum (55 mm²).
The mechanical amplification with the total factor 22 (1.3 x 17) is necessary because the sound pulses from the compressible, gaseous, air with large amplitudes and low sound pressures on the incompressible, fluid, medium perilymph in the inner ear with low amplitudes but high sound pressure must be transferred. Like the two remaining ossicles, the anvil is composed of the hardest and most elastic bone material, so that only little loss due to deformation occurs during vibration transmission.
The anvil can be anatomically divided into the body (corpus) and two legs, the long (crus longum) and the short leg (crus breve). The main mass - and thus the center of gravity - is concentrated in the area of the body. There is also the fulcrum, so that in the vibration transmission and amplification very little mass must be accelerated. The long leg ends in the lenticular process (lenticular process), which is articulated to the stirrup.
The anvil is - like the other two ossicles - covered by a mucous membrane. The two tiny muscles in the middle ear, the tympanic tensor (Tensor tympani muscle) and the stapes muscle (stapedius muscle) have only indirect effects on the anvil. The two muscles take a protective function of the inner ear against very loud noises, such. B. a bang, true.
While the stapedius muscle can attenuate the efficiency of sound transmission through tension, good vibration transmission of air vibration to the eardrum requires tensing of the tympanic clamp - similar to tensing the fur of large drums and timpani in a symphony orchestra. The anvil itself plays a more or less passive role as an intermediary.
The main task and function of the anvil is, in conjunction with the remaining ossicles, to transmit the vibrations of the eardrum caused by airborne sound under mechanical reinforcement to the cochlea in the inner ear. This applies to the audible frequency range, which - depending on the sound pressure - is located at about 40 Hz to less than 20, 000 Hz. The frequency must not be changed and the different sound pressure (volume) must also be taken into account analogously.
By leveraging the anvil amplifies the vibrations transmitted by the hammer by a factor of 1.3. Because the anvil as a middle limb within the auditory ossicles has no direct connection to the two small muscles of the middle ear, the eardrum tensioner and the stirrup muscle, the vibration transmission is largely passive. Through the best possible transmission of the sound vibrations, the ossicles also have a certain protective function for the sensory cells in the cochlea.
For very loud noises that are above the pain threshold or a sudden bang, the two muscles in the inner ear for a reflex-like deterioration of the sound transmission (Stapediusreflex), so that in the short term a kind of conductive hearing loss is produced to protect the sensory cells in the inner ear. Here, too, the anvil acts as a passive link in a mechanical "forced chain".
Middle ear infections are the most common problems associated with the conduction of sound in the three auditory ossicles. The occurring inflammatory processes can reduce the function of the mechanical vibration transmission in the efficiency, so that adjusts a transient acoustic conduction hearing loss. The hearing problems usually recur as soon as the otitis media has healed and there has not yet been irreversible damage to the middle ear or eardrum.
In many cases, in the course of the middle ear inflammation, a tympanic effusion occurs, an accumulation of serous, mucoid, bloody or purulent fluid in the tympanic cavity immediately below the ossicles, which can additionally restrict the transmission of vibrations. Left untreated, otitis media can lead to chronic deafness if the inflammatory processes lead to permanent ossification of the ossicles, leading to sclerotization. Such sclerotization, also known as calcification of the ossicles, is often the cause of hearing problems in the elderly.
Interestingly, if neuronal problems occur with the trigeminal nerve, the lateral nerves of which not only innervate most facial muscles, but also the two tiny muscles in the middle ear, the stapedius reflex is omitted under very loud noises. Very loud noises are thus perceived as painful at significantly lower sound pressure levels and there is no protective mechanism for the sensory cells in the cochlea.