Ossicles

  • Themalleus(Latin:"hammer") articulates with the incus through the incudomalleolar jointand is attached to thetympanic membrane(eardrum), from which vibrational sound pressure motion is passed.
  • Theincus(Latin:"anvil") is connected to both the other bones.
  • Thestapes(Latin:"stirrup") articulates with the incus through the incudostapedial jointand is attached to the membrane of thefenestraovalis, the elliptical oroval windowor opening between the middle ear and thevestibuleof theinner ear. It is the smallest bone in the body.[1]

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 fenestraovalis, 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 a liquid presents resistance to the force of the sound waves traveling through the air. The ossicles give the eardrum amechanical advantagevia lever action and a reduction in the area of force distribution; the resulting vibrations would be much weaker 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 inpressureto 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 presence of the ossicles to concentrate the force of the vibrations improves the sensitivity to 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 (thetensor tympaniand thestapedius). 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.