The auditory ossicles (ossicula auditus) are three small bones that transmit air vibrations from the tympanic membrane across the cavity of the middle ear to the inner ear. The most lateral and largest of the three bones is the malleus.
The malleus consists of a head; a wide, thin neck; and a manubrium, or handle. The manubrium is three-sided in cross-section. The side embedded in the substance of the tympanic membrane is wider and smoother than the other two; it is also slightly concave longitudinally. At the base of the manubrium, extending medially and slightly rostrally, is the muscular process of the malleus. This is provided with a tiny hook at its end, to which the m. tensor tympani attaches. The rostral process, or long process, is largely embedded in the tympanic membrane. It extends directly rostral from the neck of the malleus, arising at the same level as the muscular process. Opposite the muscular process at an angle of approximately 90 degrees with the rostral process is the short, lateral process. This is the most dorsal attachment of the manubrium to the tympanic membrane. The head of the malleus articulates with the body of the incus in the epitympanic recess, the most dorsal portion of the tympanic cavity.
The incus, measuring approximately 4 mm long and 3 mm high, is much smaller than the malleus. Its shape has often been likened to a human bicuspid tooth with divergent roots. The incus consists of a body, two crura and a process. It lies caudal to the malleus in the epitympanic recess where the head of the malleus articulates with the body of the incus. The crura are located on each side of a transverse ridge that forms the caudal limit of the recess. The short crus points caudally into the fossa incudis dorsal to this ridge. The long crus is also directed caudally but presents a small bone, the os lenticularis, which is in the articulation between the incus and the stapes. It extends rostrally and somewhat medially from the distal end of the incus. In some instances this connection ossifies to form the processus lenticularis.
The stapes consists of a head, two crura, a base, and a muscular process. It lies in a horizontal plane, the base facing medially. The head articulates with the incus via the os lenticulare or lenticular process. The base articulates with the fibrocartilaginous ring that covers the edge of the vestibular window. The stapes is the innermost ossicle and is the smallest bone in the body, being approximately 2 mm in length. The rostral and caudal crura are hollowed on their concave or opposed sides. A cross-section of a single crus appears as a narrow semicircle of bone. There is a thin stapedial membrane (membrana stapedis) that connects one crus to the other. In many mammals, there is an artery passing between the crura. The rostral crus is slightly longer than the caudal crus. Arising from the caudal crus near the head is a minute muscular process that provides attachment for the stapedius muscle.
Ligaments of the Ossicles
Several ligaments that attach the ossicles to the wall of the tympanic cavity were described by Getty et al. (1956) (ligg. ossiculorum auditus). A short but fairly well-defined lateral ligament of the malleus connects the lateral process of the malleus to the margins of the tympanic notch. The dorsal ligament of the malleus is a somewhat diffuse mass of ligamentous tissue that joins the head of the malleus to a small area on the roof of the epitympanic recess. The rostral ligament of the malleus is a short ligament attaching the rostral process of the malleus to the osseous tympanic ring just ventral to the canal by which the chorda tympani leaves the tympanic cavity. The body of the incus is attached to the roof of the epitympanic recess by the dorsal ligament of the incus. The caudal ligament of the incus attaches the short crus of the incus to the fossa incudis. An anular ligament (lig. anulare stapedis) attaches the base of the stapes to the cartilage that lines the vestibular window.
Muscles of the Ossicles
Two tiny muscles are associated with two of the ossicles. The m. tensor tympani is spherical, with its base in the fossa tensor tympani. The short tendon of insertion is attached to the hook on the apex of the muscular process of the malleus. Contraction of this muscle tends to draw the handle of the malleus medially, tensing the tympanic membrane. Innervation is by a branch from the mandibular nerve from the trigeminal nerve (cranial nerve V). The m. stapedius is the smallest skeletal muscle in the body, and its origin is in the fossa musculae stapedis. The body of the muscle lies largely medial to the facial nerve. Its tendon of insertion attaches to the muscular process of the stapes. Contraction of the stapedius muscle moves the rostral end of the base of the stapes caudolaterally and tenses the ossicle to reduce movement. This muscle is innervated by the stapedial branch of the facial nerve (cranial nerve VII).