The incus, commonly known as the anvil, is one of three minute bones, or ossicles, situated within the air-filled space of the middle ear. Sound waves must transition from the air of the middle ear to the fluid-filled environment of the inner ear, a boundary where most acoustic energy would normally be reflected. The incus is a fundamental part of the mechanical system that overcomes this physical challenge. It acts as a bridge to ensure sound vibrations are successfully transmitted and amplified, initiating the process of hearing.
The Middle Ear’s Tiny Trio
The incus is the middle bone in a delicate, hinged chain of ossicles located in the middle ear cavity. This chain begins laterally at the eardrum and extends medially to the entrance of the inner ear. The incus is situated directly between the other two bones, which are the malleus (hammer) and the stapes (stirrup).
The malleus is the first bone in the sequence, connected firmly to the inside surface of the eardrum. The incus, which is shaped like an anvil, articulates with the head of the malleus at the incudomalleolar joint. This connection allows the incus to receive the mechanical vibrations that originate from the eardrum’s movement.
The other end of the incus, specifically its long limb, connects to the stapes at the incudostapedial joint. Together, the three bones form a complete, continuous mechanical linkage that spans the middle ear space. This assembly is necessary to efficiently conduct sound energy across the open gap between the eardrum and the fluid-filled cochlea.
The Anvil’s Pivotal Role in Sound Transmission
The primary function of the incus is to act as a crucial pivot point in the mechanical transmission of sound energy toward the inner ear. When the malleus vibrates in response to sound waves hitting the eardrum, the incus receives this motion and transfers it onward to the stapes. The entire ossicular chain works as a lever system, which is essential for a process known as impedance matching.
Impedance matching is the mechanism the middle ear uses to prevent the significant loss of sound energy that would occur if airborne sound directly struck the fluid of the inner ear. The fluid in the inner ear has a much higher impedance, or resistance to movement, than the air in the middle ear. Without this amplification, a vast majority of the sound energy would be lost due to reflection at this boundary.
The incus contributes to the necessary amplification through its role in the lever system. The malleus and incus bones are configured such that the arm of the malleus is slightly longer than the effective arm of the incus. This mechanical arrangement results in a gain of force, meaning the force of the vibration is multiplied by a factor estimated to be between 1.3 and 2.1 times.
This force multiplication is combined with a hydraulic effect. This effect is achieved by the difference in surface area between the large eardrum and the much smaller oval window where the stapes pushes into the inner ear. The combination of the lever action and the area difference concentrates the sound pressure by roughly 17 to 20 times. The incus is therefore integral to creating the high-pressure, low-displacement vibrations necessary to effectively drive the fluid within the cochlea and stimulate the sensory hair cells.
Protecting the Inner Ear
Beyond its primary role in sound transmission, the incus also plays a part in a protective mechanism called the acoustic reflex. This reflex is an involuntary action of tiny muscles in the middle ear that contract in response to loud sounds. The reflex helps to limit the amount of intense sound energy that reaches the delicate inner ear structures, preventing potential damage.
Two small muscles are involved: the stapedius and the tensor tympani. When a sound exceeds a certain threshold, these muscles contract rapidly. The tensor tympani muscle attaches to the malleus, and its contraction tenses the eardrum.
The stapedius muscle attaches to the stapes and pulls it away from the oval window. This combined action stiffens the entire ossicular chain, including the joints connected to the incus. The stiffening dampens the vibrations traveling through the incus and stapes, which reduces the transmission of low-frequency sound energy. This momentary reduction in mechanical efficiency safeguards the inner ear from the damaging effects of excessive noise exposure.