Ossicular chain: malleus, incus and stapes
Sound passes from the eardrum along a chain of three tiny middle-ear bones, the ossicles, to the oval window, the opening into the fluid-filled inner ear.
Remember
The bone fixed to the eardrum is the malleus (hammer), so the order is malleus, incus (anvil), stapes (stirrup), spelling MIS, and the chain ends at the oval window.
Mnemonic
MIS
- M Malleus
- I Incus
- S Stapes
The first letters of the three bones spell MIS, and the Malleus is at the tympanic Membrane (two Ms together).
Sound is vibration of the air, and it makes the tympanic membrane (the eardrum) vibrate. Behind the membrane, the middle ear is an air-filled cavity. It contains three ossicles, the tiny middle-ear bones, joined in a chain.
The first bone is the malleus (the hammer-shaped bone). Its handle is fixed to the inside of the eardrum, so the malleus moves with the eardrum and receives the vibration first. The malleus forms a joint with the incus (the anvil-shaped bone), and the incus then joins the stapes (the stirrup-shaped bone, the last and smallest ossicle).
The stapes fits with its base into the oval window, a membrane-covered opening into the fluid-filled inner ear. Its movement sets the fluid of the cochlea (the snail-shaped hearing organ of the inner ear) moving, so the chain carries vibrations from air to fluid. In area, the eardrum is about 20 times larger than the base of the stapes, so the force gathered over the eardrum acts on a much smaller area and the pressure reaching the cochlear fluid is raised.
A separate structure, the Eustachian tube, joins the middle ear to the nasopharynx (the part of the throat behind the nose). It lets the pressure in the middle ear be equalised. Infection can spread along the same route from the nasopharynx to the middle ear.
A broken or fixed link anywhere in the chain means sound is passed on poorly. This is conductive hearing loss: sound is not carried efficiently to the cochlea, although the cochlea itself can be intact. It is a problem of the outer or middle ear.
Key facts
- Order of transmission
- malleus, then incus, then stapes, from the eardrum to the oval window.
- Input bone
- the handle of the malleus is attached to the eardrum, so the malleus receives the vibrations first.
- Middle link
- the incus lies between the malleus and the stapes, joining the two.
- Output bone
- the base of the stapes sits in the oval window and sets the cochlear fluid moving.
- Pressure route
- the Eustachian tube connects the middle ear to the nasopharynx and helps to equalise middle-ear pressure.
- If the chain breaks
- sound is no longer passed on efficiently, which is conductive hearing loss.
Exam traps
- The incus is not attached to the eardrum. It is the middle ossicle, joined to the malleus on one side and to the stapes on the other, so the vibration reaches it second.
- Malleus, then stapes, then incus has the wrong order. The head of the malleus forms a joint with the incus, and the incus is then joined to the stapes, so the incus comes between them.
- The tensor tympani muscle is tempting because it is linked to the malleus, yet it is not attached to the eardrum, and it dampens vibration in loud sound rather than carrying sound.
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