Nasal Cavity, Sinuses and Ear - LO 3
3. What are the middle ear ossicles? How do the middle ear ossicles function in hearing?
The middle ear ossicles are the three smallest bones of the human skeleton. The ossicles form a chain (each connected by its own synovial joint to the next) of bones that transmit sound via vibration from the tympanic membrane (attached to the malleus) to the oval window (which receives the footplate of the stapes). Each ossicle has its own distinctive shape:
Malleus:
hammer-shaped (head, neck, & manubrium)
attached to tympanic membrane (manubrium)
attachment for tensor tympani m. (manubrium)
articulate with incus (head)
close association with chorda tympani
largest of the ossicles
Incus:
anvil-shaped (body, short limb, long limb, facet for malleus)
articulates with malleus (facet for malleus)
articulates with stapes (lenticular process of long limb)
Stapes:
stirrup-shaped (head, neck, anterior & posterior crura, footplate)
articulates with incus (head)
attachment for stapedius m. (neck)
sits in oval window (footplate)
smallest of the ossicles
The middle ear ossicles work akin to a step-up electrical transformer: sound energy from the air is transmitted from the tympanic membrane through bone to the liquid contents of the cochlea, thus facilitating impedance coordination among these different media. As each medium (air, membrane, bone, and fluid) has a different innate resistance to conduct sound (acoustic impedance), the ossicles work to bridge the different acoustic impedances of air and cochlear fluid to facilitate hearing.
The middle ear ossicles have a fascinating evolutionary history. Well corroborated hypotheses of homology exist between the ossicles and lower jaw bones of ray-finned fishes and non-mammalian amniotes. The transition of these bones from elements of the jaw to elements of the middle ear is well documented via embryology, paleontology, and phylogenetics.