The malleus, incus, and stapes are three minute bones collectively known as the auditory ossicles. These structures are the smallest bones found in the human body. Functioning as a mechanical bridge, the ossicles receive sound vibrations and transmit them to the inner ear. They convert airborne sound waves into the fluid-based signals the brain can interpret.
The Three Major Divisions of the Ear
To understand the location of the ossicles, it helps to first recognize the three distinct anatomical divisions of the human ear. The outer ear consists of the visible pinna and the ear canal, which serves to capture sound waves and direct them inward. This canal ends at the tympanic membrane, more commonly known as the eardrum, which acts as the boundary between the outer and middle ear.
The middle ear is a small, air-filled cavity situated just behind the eardrum. It is within this space that the ossicles reside. This chamber is separated from the inner ear by a thin partition of bone.
The inner ear houses the fluid-filled structures responsible for both hearing and balance, including the spiral-shaped cochlea and the semicircular canals.
The primary role of the outer and middle ear is to conduct and process sound for successful transfer to the inner ear. The separation into these three regions allows sound energy to be efficiently channeled and enhanced. The air-filled middle ear provides the space necessary for the ossicles to vibrate freely.
Identifying the Malleus, Incus, and Stapes
The malleus, incus, and stapes are found exclusively within the air-filled space of the middle ear cavity. They are arranged in a specific chain-like sequence that extends from the eardrum to the inner ear. Each bone is named for its rough resemblance to a common object, which aids in identification: the malleus is the hammer, the incus is the anvil, and the stapes is the stirrup.
The first bone in this chain is the malleus, or hammer, which has a slender handle that is directly attached to the inner surface of the tympanic membrane. When the eardrum vibrates from an incoming sound wave, the malleus receives this movement first. The head of the malleus then articulates with the second bone, the incus.
The incus, or anvil, is centrally positioned and acts as the mechanical link between its two neighbors. It is suspended within the middle ear cavity and receives vibrations from the malleus. The incus transfers that motion onto the stapes through a small joint.
The stapes, or stirrup, is the last bone in the chain. Its flat, oval-shaped footplate is seated within the oval window. This oval window is a membrane-covered opening that marks the entrance to the fluid-filled cochlea of the inner ear.
How the Ossicles Transmit Sound
The three ossicles work in concert to transfer and amplify sound vibrations. When an airborne sound wave strikes the eardrum, the membrane vibrates, initiating motion in the attached malleus. This mechanical energy is then sequentially passed along the interconnected chain from the malleus to the incus and finally to the stapes.
This chain of movement involves a sophisticated mechanical advantage. The ossicles operate using a lever-like action, which, combined with the difference in surface area between the large eardrum and the much smaller oval window, focuses the force. This design results in a substantial increase in pressure applied to the oval window.
This amplification is necessary because sound waves move from the air of the middle ear into the dense fluid of the inner ear. Without this pressure increase, most sound energy would reflect off the fluid, causing significant loss of intensity. The mechanical boost provided by the ossicles ensures enough energy is transmitted into the cochlear fluid to stimulate the sensory receptors.