In the middle ear of the human ear, there are three ossicles that are hinged together and transmit the mechanical vibrations of the eardrum to the cochlea in the inner ear. The middle ossicle is called the incus. It receives the vibrations of the hammer and transmits them to the stapes with mechanical amplification. Although the three ossicles are the smallest bones in humans, at the same time they are also very hard and strong in order to transmit vibrations with as little loss as possible.
What is the anvil?
Weighing about 27 milligrams, the incus is the heavyweight among the total of three ossicles in the middle ear. As the middle limb of the three ossicles that transmit sound vibrations from the eardrum to the inner ear, it is connected to the malleus by the joint articulatio incudomallearis and to the stapes by the tiny joint articulatio incudostapedia. The vibrations are transmitted to the stirrup by using the leverage effect. Because the lever arm from the fulcrum to the stirrup is shorter than the lever arm from the hammer joint to the fulcrum, the deflection of the anvil at the connection point to the stirrup is shorter but stronger by a factor of 1.3. A further mechanical amplification by a factor of 17 then occurs through the transmission of the vibrations to the oval window, which, with an area of 3.2 qmm, reaches only one seventeenth of the area of the eardrum (55 qmm). The mechanical amplification with a total factor of 22 (1.3 x 17) is necessary because the sound impulses must be transferred from the compressible, gaseous, air with large amplitudes and low sound pressures to the incompressible, liquid, medium perilymph in the inner ear with low amplitudes but high sound pressures. The incus, like the other two ossicles, is composed of the hardest and most elastic bone material, so there is little loss due to deformation during vibration transmission.
Anatomy and structure
The anvil can be divided anatomically into the body (corpus) and two legs, the long leg (crus longum) and the short leg (crus breve). The main mass – and therefore the center of gravity – is concentrated in the body area. The center of rotation is also located there, so that very little mass has to be accelerated during vibration transmission and amplification. The long limb ends in the lenticular process, which is hinged to the stapes. The incus is covered by a mucous membrane, as are the other two ossicles. The two tiny muscles in the middle ear, the tympanic tensor (Musculus tensor tympani) and the stapes (Musculus stapedius) have only indirect effects on the incus. The two muscles perform a protective function of the inner ear against very loud sounds, such as a bang. While the stapedius muscle can weaken the efficiency of sound transmission by tensioning it, good vibration transmission of airborne vibration to the eardrum requires tensioning of the eardrum tensioner – comparable to tensioning the skin of large drums and timpani in a symphony orchestra. The anvil itself plays more or less a passive role as an intermediate link.
Function and tasks
The main task and function of the anvil is to transmit, in conjunction with the other ossicles, the vibrations of the eardrum caused by airborne sound to the cochlea in the inner ear under mechanical amplification. This applies to the audible frequency range, which – depending on the sound pressure – is located at about 40 Hz to below 20,000 Hz. The frequency must not be changed and the different sound pressure (loudness) must also be taken into account analogously. By means of a leverage effect, the incus amplifies the vibrations transmitted by the hammer by a factor of 1.3. Because the incus, as the middle limb within the ossicles, has no direct connection to the two small muscles of the middle ear, the tympanic tensor and the stapes muscle, the transmission of vibrations is largely passive. By transmitting the sound vibrations in the best possible way, the ossicles also have a certain protective function for the sensory cells in the cochlea.In the case of very loud noises above the pain threshold or a sudden bang, the two muscles in the inner ear cause a reflex-like deterioration of sound transmission (stapedius reflex), so that a kind of conductive hearing loss is established for a short time to protect the sensory cells in the inner ear. The anvil also acts here as a passive link in a mechanical “chain of constraint.”
Diseases
Middle ear inflammation forms the most common problems associated with sound conduction of the three ossicles. The inflammatory processes that occur can reduce the efficiency of the function of mechanical vibration transmission, resulting in temporary conductive hearing loss. The hearing problems usually disappear as soon as the middle ear inflammation has healed and no irreversible damage has occurred in the middle ear or to the eardrum. In many cases, tympanic effusion, an accumulation of serous, mucous, bloody, or purulent fluid in the tympanic cavity just below the ossicles that can further limit vibration transmission, develops during the course of middle ear infection. If left untreated, otitis media can lead to chronic hearing loss if the inflammatory processes result in permanent stiffening of the ossicles, or sclerotization. Such sclerotization, also known as calcification of the ossicles, is often the cause of hearing problems in older individuals. Interestingly, if neuronal problems occur with the trigeminal nerve, the 5th cranial nerve, whose lateral branches innervate not only most facial muscles but also the two tiny muscles in the middle ear, the stapedius reflex fails to respond to very loud sounds. Very loud sounds are thus already perceived as painful at much lower sound pressures, and there is no protective mechanism for the sensory cells in the cochlea.
Typical and common ear disorders
- Ear drum injuries
- Ear flow (otorrhea)
- Otitis media
- Ear canal inflammation
- Mastoiditis
- Ear furuncle
