The ossicles (also called auditory ossicles) are three
bone
A bone is a rigid organ that constitutes part of the skeleton in most vertebrate animals. Bones protect the various other organs of the body, produce red and white blood cells, store minerals, provide structure and support for the body, ...
s in either
middle ear that are among the smallest bones in the human body. They serve to transmit
sound
In physics, sound is a vibration that propagates as an acoustic wave, through a transmission medium such as a gas, liquid or solid.
In human physiology and psychology, sound is the ''reception'' of such waves and their ''perception'' by ...
s from the air to the fluid-filled
labyrinth
In Greek mythology, the Labyrinth (, ) was an elaborate, confusing structure designed and built by the legendary artificer Daedalus for King Minos of Crete at Knossos. Its function was to hold the Minotaur, the monster eventually killed by ...
(
cochlea
The cochlea is the part of the inner ear involved in hearing. It is a spiral-shaped cavity in the bony labyrinth, in humans making 2.75 turns around its axis, the modiolus. A core component of the cochlea is the Organ of Corti, the sensory o ...
). The absence of the auditory ossicles would constitute a moderate-to-severe
hearing loss
Hearing loss is a partial or total inability to hear. Hearing loss may be present at birth or acquired at any time afterwards. Hearing loss may occur in one or both ears. In children, hearing problems can affect the ability to acquire spoken l ...
. The term "ossicle" literally means "tiny bone". Though the term may refer to any small bone throughout the body, it typically refers to the
malleus,
incus, and
stapes (hammer, anvil, and stirrup) of the middle
ear.
Structure
The ossicles are, in order from the eardrum to the inner ear (from superficial to deep): the
malleus,
incus, and
stapes, terms that in
Latin
Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through ...
are translated as "the
hammer,
anvil, and
stirrup
A stirrup is a light frame or ring that holds the foot of a rider, attached to the saddle by a strap, often called a ''stirrup leather''. Stirrups are usually paired and are used to aid in mounting and as a support while using a riding animal ...
".
* The
malleus ( la, "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 ( la, "anvil") is connected to both the other bones.
* The
stapes ( la, "stirrup") articulates with the incus through the
incudostapedial joint and is attached to the membrane of the ''
fenestra ovalis'', the elliptical or
oval window
The oval window (or ''fenestra vestibuli'' or ''fenestra ovalis'') is a membrane-covered opening from the middle ear to the cochlea of the inner ear.
Vibrations that contact the tympanic membrane travel through the three ossicles and into the in ...
or opening between the middle ear and the
vestibule
Vestibule or Vestibulum can have the following meanings, each primarily based upon a common origin, from early 17th century French, derived from Latin ''vestibulum, -i n.'' "entrance court".
Anatomy
In general, vestibule is a small space or cavity ...
of the
inner ear
The inner ear (internal ear, auris interna) is the innermost part of the vertebrate ear. In vertebrates, the inner ear is mainly responsible for sound detection and balance. In mammals, it consists of the bony labyrinth, a hollow cavity in th ...
. It is the smallest bone in the body.
Development
Studies have shown that ear bones in mammal embryos are attached to the
dentary, which is part of the lower
jaw. These are ossified portions of
cartilage
Cartilage is a resilient and smooth type of connective tissue. In tetrapods, it covers and protects the ends of long bones at the joints as articular cartilage, and is a structural component of many body parts including the rib cage, the neck ...
—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.
Evolution
Function
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 (oval window), 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. This is observed from the abrupt cessation of sound that occurs when the head is submerged underwater. This is because the relative incompressibility of a liquid presents resistance to the force of the sound waves traveling through the air. The ossicles give the eardrum a
mechanical advantage via lever action and a reduction in the area of force distribution; the resulting vibrations are stronger but don't move as far. This allows more efficient coupling than if the sound waves were transmitted directly from the outer ear to the oval window. This reduction in the area of force application allows a large enough increase in
pressure
Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country a ...
to transfer most of the sound energy into the liquid. The increased pressure will compress the fluid found in the cochlea and transmit the stimulus. Thus, the lever action of the ossicles changes the vibrations so as to improve the transfer and reception 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
The stapedius is the smallest skeletal muscle in the human body. At just over one millimeter in length, its purpose is to stabilize the smallest bone in the body, the stapes or strirrup bone of the middle ear.
Structure
The stapedius emerges fr ...
''). 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).
Clinical relevance
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 using a passive
middle ear implant.
History
There is some doubt as to the discoverers of the auditory ossicles and several anatomists from the early 16th century have the discovery attributed to them with the two earliest being
Alessandro Achillini and
Jacopo Berengario da Carpi. Several sources, including
Eustachi and
Casseri, attribute the discovery of the malleus and incus to the anatomist and philosopher
Achillini. The first written description of the malleus and incus was by
Berengario da Carpi
Jacopo Berengario da Carpi (also known as Jacobus Berengarius Carpensis, Jacopo Barigazzi, Giacomo Berengario da Carpi or simply Carpus; c. 1460 – c. 1530) was an Italian physician. His book "''Isagoge breves''" published in 1522 made him the mo ...
in his ''Commentaria super anatomia Mundini'' (1521), although he only briefly described two bones and noted their theoretical association with the transmission of sound.
[O'Malley, C.D. ''Andreas Vesalius of Brussels, 1514–1564.'' Berkeley: University of California Press, 1964. p. 120] Niccolo Massa's ''Liber introductorius anatomiae'' described the same bones in slightly more detail and likened them both to little hammers.
A much more detailed description of the first two ossicles followed in
Andreas Vesalius' ''
De humani corporis fabrica
''De Humani Corporis Fabrica Libri Septem'' (Latin, lit. "On the fabric of the human body in seven books") is a set of books on human anatomy written by Andreas Vesalius (1514–1564) and published in 1543. It was a major advance in the history ...
'' in which he devoted a chapter to them. Vesalius was the first to compare the second element of the ossicles to an anvil although he offered the molar as an alternative comparison for its shape. The first published description of the stapes came in Pedro Jimeno's ''Dialogus de re medica'' (1549) although it had been previously described in public lectures by
Giovanni Filippo Ingrassia
Giovanni Filippo Ingrassia or Ioannis Philippi Ingrassiae (1510–1580) was an Italian physician, student of Vesalius, professor at the University of Naples, ''Protomedicus'' of Sicily and a major figure in the history of medicine and human anatomy ...
at the
University of Naples as early as 1546.
The term ''ossicle'' derives from , a
diminutive
A diminutive is a root word that has been modified to convey a slighter degree of its root meaning, either to convey the smallness of the object or quality named, or to convey a sense of intimacy or endearment. A ( abbreviated ) is a word-form ...
of "bone" ( la, os;
genitive
In grammar, the genitive case ( abbreviated ) is the grammatical case that marks a word, usually a noun, as modifying another word, also usually a noun—thus indicating an attributive relationship of one noun to the other noun. A genitive can a ...
). 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.
See also
*
*
*
References
External links
The middle ear and the ossicles
{{Authority control
Auditory system
Bones of the head and neck