11.7: The Auditory Ossicles
The auditory ossicles of the middle ear transmit sounds from the air as vibrations to the fluid-filled cochlea. The auditory ossicles consist of two malleus (hammer) bones, two incus (anvil) bones, and two stapes (stirrups), one on each side. These bones develop during the fetal stage and are the ones to ossify first. They are fully mature at birth and do not grow afterward.
The aptly named stapes look very much like a stirrup. The three ossicles are unique to mammals, and each plays a role in hearing. The malleus attaches at three points to the interior surface of the tympanic membrane. The incus attaches the malleus to the stapes. In humans, the stapes are not long enough to reach the tympanum. If we did not have the malleus and the incus, the tympanum vibrations would never reach the inner ear. These bones also function to collect force and amplify sounds.
Malleus
The malleus is placed most laterally and is the largest of the auditory ossicles. The parts of the malleus include the handle, the spatulate, the lateral and anterior processes, the neck and the head. The manubrium is the handle of the malleus that extends downward to embed into the tympanic membrane and then becomes narrow as it grows inferiorly. At the end of the manubrium lies the spatulate process, which is attached to the tympanic membrane's pars tensa, forming a concavity. Superior to the handle forming an oblique cone is the lateral process and the anterior process. The anterior process is also known as the Folian or Rau's process. On top of the pars flaccida of the tympanic membrane is the narrow neck of the malleus. Above the neck, the round head of the malleus fits into the epitympanic recess. At the incudomalleolar joint, the neck of the malleus posterolaterally articulates with the body of the incus. Three ligaments help in suspending the malleus: i) the anterior ligament (Casserio's ligament), which attaches the base of the neck of the malleus to the carotid wall of the tympanic cavity; ii) the superior ligament of malleus, which attaches the tegmental roof of the tympanic cavity with the head of the malleus; and iii) the lateral ligament of malleus, which attaches the Rivinus' notch or tympanic notch to the head or neck of the malleus.
Incus
The incus is suspended lateral to the stapes and medial to the malleus and articulate with both these ossicles via synovial joints. The incus has a body, short and long limbs, and a lenticular process. Anterolaterally, the malleus enters the body region of the incus. From the body, posteriorly, the short limb projects and attaches to the incus's posterior ligament. The long limb runs parallel to the manubrium. It bends to form the lenticular process by the incudostapedial joint with the head of the stapes.
Two ligaments suspending the incus are: i) the superior ligament of incus, which attaches its body to the tegmental roof of the tympanic cavity; and ii) the posterior ligament of incus, which attaches its short limb region to the posterior wall of the tympanic cavity.
Stapes
The smallest auditory ossicle is the stapes. The head of the stapes is called the capitulum. It articulates with the lenticular process of the incus through the incudostapedial joint. The capitulum is attached to the anterior and posterior crus of the stapes through a neck region. The anterior and posterior crus are attached to the oval-shaped base that is also called the footplate. The footplate connects to the oval window of the tympanic membrane via the stapedial annular ligament, a ring of fibrous tissue.
This text is partially adapted from Openstax, Anatomy and Physiology 2e, 5.4 Hearing and Openstax, Anatomy and Physiology 2e, 36.4 Hearing and Vestibular sensation
