Anatomy of the three ossicles
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The ossicles (also called auditory ossicles) are three bones in either middle ear that are among the smallest bones in the human body. They serve to transmit sounds from the air to the fluid-filled labyrinth (cochlea). The absence of the auditory ossicles would constitute a moderate-to-severe hearing loss. The term "ossicle" literally means "tiny bone" and, though the term may refer to any small bone throughout the body, it typically refers the malleus, the incus and the stapes of the middle ear.
- The malleus (Latin: "hammer") articulates with the incus through the incudomalleolar joint and is attached to the tympanic membrane (eardrum), from which vibrational sound pressure motion is passed.
- The incus (Latin: "anvil") is connected to both the other bones.
- The stapes (Latin: "stirrup") articulates with the incus through the incudostapedial joint and is attached to the membrane of the fenestra ovalis, the elliptical or oval window or opening between the middle ear and the vestibule of the inner ear. It is the smallest bone in the body.
Studies have shown that ear bones in mammal embryos are attached to the dentary, which is part of the jaw. These are ossified portions of cartilage—called Meckel's cartilage—that are attached to the jaw. As the embryo develops, the cartilage hardens to form bone. Later in development, the bone structure breaks loose from the jaw and migrates to the inner ear area. The structure is known as the middle ear, and is made up of the stapes, incus, malleus, and tympanic membrane. These correspond to the columella, quadrate, articular, and angular structures in the amphibian, bird or reptile jaw. For this reason, researchers believe that mammals and reptiles share a common ancestry.
As sound waves vibrate the tympanic membrane (eardrum), it in turn moves the nearest ossicle, the malleus, to which it is attached. The malleus then transmits the vibrations, via the incus, to the stapes, and so ultimately to the membrane of the fenestra ovalis, the opening to the vestibule of the inner ear.
Sound traveling through the air is mostly reflected when it comes into contact with a liquid medium; only about 1/30 of the sound energy moving through the air would be transferred into the liquid. Think about the abrupt cessation of sound that occurs on a busy summer's day at the pool when you submerge your head underwater. This is because the relative incompressibility of liquids confers resistance to the force of the sound waves traveling through the air. The ossicles give the eardrum mechanical advantage via lever action and a reduction in the area of force distribution; the resulting vibrations would be much smaller if the sound waves were transmitted directly from the outer ear to the oval window. This compression of the area of force distribution allows a greater force per given area, which provides a large enough increase in pressure to transfer the energy into the liquid. This increase in pressure can compress the fluid found in the cochlea and thus transmit the stimulus. Thus, the use of the ossicles to concentrate the force of the vibrations improves the quality of sound and is a form of impedance matching.
However, the extent of the movements of the ossicles is controlled (and constricted) by two muscles attached to them (the tensor tympani and the stapedius). It is believed that these muscles can contract to dampen the vibration of the ossicles, in order to protect the inner ear from excessively loud noise (theory 1) and that they give better frequency resolution at higher frequencies by reducing the transmission of low frequencies (theory 2) (see acoustic reflex). These muscles are more highly developed in bats and serve to block outgoing cries of the bats during echolocation (SONAR).
Occasionally the joints between the ossicles become rigid. One condition, otosclerosis, results in the fusing of the stapes to the oval window. This reduces hearing and may be treated surgically.
The term ossicle dates to 1570 ossiculum, a diminutive of "bone" (Latin: os; genitive ossis). The malleus gets its name from Latin malleus, meaning "hammer", the incus gets its name from Latin incus meaning "anvil" from incudere meaning "to forge with a hammer", and the stapes gets its name from Modern Latin "stirrup," probably an alteration of Late Latin stapia related to stare "to stand" and pedem, an accusative of pes "foot", so called because the bone is shaped like a stirrup - this was an invented Modern Latin word for "stirrup," for which there was no classical Latin word, as the ancients did not use stirrups.
This article uses anatomical terminology; for an overview, see anatomical terminology.
- Meng, Jin. "The Journey From Jaw to Ear." Biologist. vol. 50. (2003) p. 154-158.
- Hill, R.W., Wyse, G.A. & Anderson, M. (2008). Animal Physiology, 2nd ed..
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