Ossicles
Encyclopedia
The ossicles are the three smallest bone
s in the human body. They are contained within the middle ear
space and serve to transmit sound
s 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 "ossicles" literally means "tiny bones" and commonly refers to the auditory ossicles, though the term may refer to any small bone throughout the body.
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. However, the extent of the movements of the ossicles is controlled (and constricted) by certain 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 bat
s 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 quality of sound is improved by impedance matching
.
. 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 incus
, stapes, malleus
, and tympanic membrane. These correspond to the quadrate
, prearticular, articular
, and angular
structures in the reptile jaw. For this reason, researchers believe the similarity of the results shows that mammals and reptiles have a common ancestry.
Bone
Bones are rigid organs that constitute part of the endoskeleton of vertebrates. They support, and protect the various organs of the body, produce red and white blood cells and store minerals. Bone tissue is a type of dense connective tissue...
s in the human body. They are contained within the middle ear
Middle ear
The middle ear is the portion of the ear internal to the eardrum, and external to the oval window of the cochlea. The mammalian middle ear contains three ossicles, which couple vibration of the eardrum into waves in the fluid and membranes of the inner ear. The hollow space of the middle ear has...
space and serve to transmit sound
Sound
Sound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.-Propagation of...
s from the air to the fluid-filled labyrinth (cochlea
Cochlea
The cochlea is the auditory portion of the inner ear. It is a spiral-shaped cavity in the bony labyrinth, making 2.5 turns around its axis, the modiolus....
). The absence of the auditory ossicles would constitute a moderate-to-severe hearing loss. The term "ossicles" literally means "tiny bones" and commonly refers to the auditory ossicles, though the term may refer to any small bone throughout the body.
Etymology
Ossicle dates to c. 1570, from Latin Ossiculum, a diminutive of Latin Os "bone" (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, a 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.Anatomy
The ossicles are, in order from the eardrum to the inner ear (biologically/medically from superficial to deep), the malleus, incus, and stapes. Terms that, in Latin, are translated as the hammer, the anvil, and the stirrup.- The malleusMalleusThe malleus or hammer is a hammer-shaped small bone or ossicle of the middle ear which connects with the incus and is attached to the inner surface of the eardrum...
(hammer) articulates with the incus and is attached to the tympanic membrane (eardrumEardrumThe eardrum, or tympanic membrane, is a thin membrane that separates the external ear from the middle ear in humans and other tetrapods. Its function is to transmit sound from the air to the ossicles inside the middle ear. The malleus bone bridges the gap between the eardrum and the other ossicles...
), from which vibrational sound pressure motion is passed. - The incusIncusThe incus or anvil is the anvil-shaped small bone or ossicle in themiddle ear. It connects the malleus to the stapes. It was first described by Alessandro Achillini of Bologna.The incus transmits sound vibrations from the malleus to the stapes....
(anvil) is connected to both the other bones. - The stapes (stirrup) articulates with the incus and is attached to the membrane of the fenestra ovalis, the elliptical or oval windowOval windowThe oval window is a membrane-covered opening which leads from the middle ear to the vestibule of the inner ear.Vibrations that come into contact with the tympanic membrane travel through the three ossicles and into the inner ear...
or opening between the middle ear and the vestibuleVestibule of the ear-Definition:The vestibule is the central part of the osseous labyrinth, and is situated medial to the tympanic cavity, behind the cochlea, and in front of the semicircular canals.The etymology comes from the Latin vestibulum, literally an entrance hall....
of the inner earInner earThe inner ear is the innermost part of the vertebrate ear. In mammals, it consists of the bony labyrinth, a hollow cavity in the temporal bone of the skull with a system of passages comprising two main functional parts:...
.
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, the opening to the vestibule of the inner ear.The ossicles give the eardrum mechanical advantage
Mechanical advantage
Mechanical advantage is a measure of the force amplification achieved by using a tool, mechanical device or machine system. Ideally, the device preserves the input power and simply trades off forces against movement to obtain a desired amplification in the output force...
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. However, the extent of the movements of the ossicles is controlled (and constricted) by certain muscles attached to them (the tensor tympani
Tensor tympani
The tensor tympani, the larger of the two muscles of the tympanic cavity, is contained in the bony canal above the osseous portion of the auditory tube...
and the stapedius
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....
). 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
Acoustic reflex
The acoustic reflex is an involuntary muscle contraction that occurs in the middle ear of mammals in response to high-intensity sound stimuli....
). These muscles are more highly developed in bat
Bat
Bats are mammals of the order Chiroptera "hand" and pteron "wing") whose forelimbs form webbed wings, making them the only mammals naturally capable of true and sustained flight. By contrast, other mammals said to fly, such as flying squirrels, gliding possums, and colugos, glide rather than fly,...
s and serve to block outgoing cries of the bats during echolocation
Animal echolocation
Echolocation, also called biosonar, is the biological sonar used by several kinds of animals.Echolocating animals emit calls out to the environment and listen to the echoes of those calls that return from various objects near them. They use these echoes to locate and identify the objects...
(SONAR).
Occasionally the joints between the ossicles become rigid. One condition, otosclerosis
Otosclerosis
Otosclerosis is an abnormal growth of bone near the middle ear. It can result in hearing loss.-Clinical description:Otosclerosis can result in conductive and/or sensorineural hearing loss...
, results in the fusing of the stapes to the oval window. This reduces hearing and may be treated surgically.
The quality of sound is improved by impedance matching
Impedance matching
In electronics, impedance matching is the practice of designing the input impedance of an electrical load to maximize the power transfer and/or minimize reflections from the load....
.
Development
Studies have shown that ear bones in mammal embryos are attached to the dentary, which is part of the jawJaw
The jaw is any opposable articulated structure at the entrance of the mouth, typically used for grasping and manipulating food. The term jaws is also broadly applied to the whole of the structures constituting the vault of the mouth and serving to open and close it and is part of the body plan of...
. These are ossified portions of cartilage
Cartilage
Cartilage is a flexible connective tissue found in many areas in the bodies of humans and other animals, including the joints between bones, the rib cage, the ear, the nose, the elbow, the knee, the ankle, the bronchial tubes and the intervertebral discs...
-- called Meckel's cartilage
Meckel's cartilage
The cartilaginous bar of the mandibular arch is formed by what are known as Meckel’s cartilages also known as Meckelian cartilages; above this the incus and malleus are developed....
-- 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
Middle ear
The middle ear is the portion of the ear internal to the eardrum, and external to the oval window of the cochlea. The mammalian middle ear contains three ossicles, which couple vibration of the eardrum into waves in the fluid and membranes of the inner ear. The hollow space of the middle ear has...
, and is made up of the incus
Incus
The incus or anvil is the anvil-shaped small bone or ossicle in themiddle ear. It connects the malleus to the stapes. It was first described by Alessandro Achillini of Bologna.The incus transmits sound vibrations from the malleus to the stapes....
, stapes, malleus
Malleus
The malleus or hammer is a hammer-shaped small bone or ossicle of the middle ear which connects with the incus and is attached to the inner surface of the eardrum...
, and tympanic membrane. These correspond to the quadrate
Quadrate bone
The quadrate bone is part of a skull in most tetrapods, including amphibians, sauropsids , and early synapsids. In these animals it connects to the quadratojugal and squamosal in the skull, and forms part of the jaw joint .- Evolutionary variation :In snakes, the quadrate bone has become elongated...
, prearticular, articular
Articular
The articular bone is part of the lower jaw of most tetrapods, including amphibians, sauropsids and early synapsids. In these animals it is connected to two other lower jaw bones, the suprangular and the angular...
, and angular
Angular
The angular is a large bone in the lower jaw of amphibians and reptiles , which is connected to all other lower jaw bones: the dentary , the splenial, the suprangular, and the articular...
structures in the reptile jaw. For this reason, researchers believe the similarity of the results shows that mammals and reptiles have a common ancestry.