Homologous with a bone of the lower jaw joint in reptiles, and derived in the embryo from the cartilage of the first pharyngeal arch
The malleus — the reptilian articular bone, formed from Meckel's cartilage
Anatomy · Eye, ear & senses
Malleus
The 'hammer' — the outermost auditory ossicle
Middle ear · ossicular chain
The longest and outermost of the three middle-ear bones. It is club-shaped, with a rounded head that articulates with the incus, a slender neck, and a long handle, the manubrium, that is fixed into the inside of the tympanic membrane. Two small processes and the insertion of the tensor tympani muscle mark its surface. It develops from the first pharyngeal arch and corresponds to a jaw bone in reptiles.
How to spot them
- ▸Shaped like a mallet: rounded head, narrow neck, long handle (manubrium)
- ▸The head has a saddle-shaped facet for the incus, forming the incudomallear joint
- ▸The manubrium is embedded in the tympanic membrane down to the umbo, with the lateral process a small bump near the top
- ▸Tensor tympani inserts on it; the chorda tympani nerve passes across its neck
What to know
A muscle running from a canal above the pharyngotympanic tube, turning through a bony pulley, pulls this bone inwards and tightens the drum
The tensor tympani inserts on the manubrium of the malleus, supplied by the mandibular nerve
The handle is stuck to the inside of the drum, ending at the point of greatest concavity
The manubrium of the malleus reaches the umbo
Its rounded head sits in the roof of the middle cavity, articulating with a bone shaped like a tooth by a saddle joint
The head of the malleus and the incudomallear joint
The taste nerve from the front of the tongue runs between its neck and the next bone in the chain
The chorda tympani crosses the neck of the malleus, between it and the long process of the incus
Has a short lateral process, seen through the drum as a small white bump, and a slender anterior process attached to the wall by a ligament
Lateral and anterior processes of the malleus
About 8 mm long, the longest of the three ossicles, which together span less than a centimetre
The malleus is the longest ossicle; the stapes is the shortest
Key ideas
- The three bones of the ossicular chain act as a lever: the malleus handle is slightly longer than the long process of the incus, giving a force gain of about 1.3 on top of the area ratio of the drum to the oval window.
- The evolution of the mammalian middle ear is one of the best-documented examples in the fossil record: the reptilian jaw-joint bones, articular and quadrate, became the malleus and incus.
Quick-fire facts
- •Latin malleus means 'hammer'
- •It develops from the first pharyngeal arch; the stapes arises largely from the second arch
- •The reptilian articular bone became the malleus and the quadrate became the incus, fossils such as the cynodonts show the transition
- •Tensor tympani is innervated by the nerve to tensor tympani, a branch of the mandibular division of the trigeminal nerve; stapedius is supplied by the facial nerve
- •Ossification begins in the fetus and the bone is adult size at birth
- •Attached to the wall of the middle ear by superior, anterior and lateral ligaments
- •Erosion of the malleus and incus by cholesteatoma or chronic infection is a cause of conductive hearing loss
- •The tensor tympani contracts during chewing and yawning and in response to loud sound, stiffening the drum
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Sources
- TeachMeAnatomy — The Middle Ear — Parts, Bones, Muscles
- Kenhub — Tympanic membrane: anatomy, structure and function