What is the function of the three small bones that connect the tympanic membrane to the oval window group of answer choices?

Updated by: David C. Dugdale, III, MD, Professor of Medicine, Division of General Medicine, Department of Medicine, University of Washington School of Medicine. Also reviewed by David Zieve, MD, MHA, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.

Índice Show

The anatomy of the middle ear
The bones of the middle ear
The oval window
The round window
The Eustachian tube

Douglas E. Vetter, Assistant Professor of Neuroscience at the Tufts University Sackler School of Biomedical Sciences, sounds out an answer to this query.

The hammer, anvil and stirrup—also known as the malleus, incus, and stapes, respectively, and collectively, as "middle ear ossicles"—are the smallest bones in the human body. Found in the middle ear, they are a part of the auditory system between the eardrum and the cochlea (the spiral-shaped conduit housing hair cells that are involved in transmitting sound to the brain). To understand the role of these bones in hearing requires an understanding of levers. This is because the middle ear ossicles are arranged and interact with each other as a lever system.

All levers generate a mechanical advantage. They are used to exert a large force over a small distance at one end of the lever by applying a smaller force over a longer distance at the opposite end. The leveraging capabilities of the middle ear ossicles are needed to generate the large forces that allow us to hear.

As terrestrial animals, we live in a gaseous environment. But, our inner ear is filled with fluid, and this represents a problem. As an example, most people have first hand knowledge of hearing underwater. If someone screams at you from above the water's surface, the sounds are tremendously muted, making it difficult to understand or even hear at all. That is simply because most of the sound is reflected off the water's surface.

So how do we take in airborne sounds, which are simply vibrations of the air molecules, and get them past the air-fluid interface between our ear canal and the inner ear? We need a system to use those air vibrations to push against the surface of the inner ear fluid.

When the eardrum vibrates as sound hits its surface, it sets the ossicles into motion. The ossicles are arranged in a special order to perform their job. Directly behind and connected to the eardrum—which is essentially, a large collector of sound—is the hammer. The hammer is arranged so that one end is attached to the eardrum, while the other end forms a lever-like hinge with the anvil. The opposite end of the anvil is fused with the stirrup (so anvil and stirrup act as one bone). The stirrup then connects with a special opening in the cochlea called the "oval window." The footplate of the stirrup—the oval, flat part of the bone that resembles the part where one would rest ones foot in an actual stirrup—is loosely attached to the oval window of the cochlea, allowing it to move in and out like a piston. The piston-like action generates vibrations in the fluid-filled inner ear that are used to signal the brain of a sound event. Without the middle ear ossicles, only about 0.1 percent of sound energy would make it into the inner ear.

Overcoming the problem of getting airborne sound into the fluid-filled inner ear is solved by two main mechanisms: the concentration of energy from the large eardrum onto the small stirrup footplate situated in the oval window; and the lever-like action between the hammer and the anvil-stirrup complex. In cats, for example, the simple concentration of forces from the eardrum to the stirrup increases pressure at the oval window to about 35 times what is measured at the eardrum. The lever action of the middle ear bones imparts a further mechanical advantage to the system—occurring because the anvil is shorter than the hammer—and further increases pressure by roughly 35 percent. In this way we overcome the problem of getting airborne vibrations into the pressurized, fluid-filled inner ear.

Not all animals have this same middle ear bone configuration. In fact, reptiles, amphibians and birds, have a middle ear that contains just one bone, called the columella, which connects the eardrum directly to the oval window of the cochlea. When we examine the most sensitive frequency for hearing in these animals, they do very well for sounds around 1,000 hertz (1 kHz) but quickly lose their ability to hear well at higher frequencies. On the other hand, animals with three middle ear bones tend to hear at much higher frequencies. For humans, our hearing can extend to 20 kHz, although most of our lives are spent attending to sounds between 4 and 8 kHz.

The human ear consists of three regions called the outer ear, middle ear, and inner ear. The oval window, also known as the fenestra ovalis, is a connective tissue membrane located at the end of the middle ear and the beginning of the inner ear.

The fenestra ovalis connects the tiny bones of the middle ear to the scala vestibuli, which is the upper part of the cochlea. (The cochlea is the central organ of the inner ear.) The bone of the middle ear that actually connects to the fenestra ovalis is called the stirrup, or stapes.

The middle ear functions to transmit the motion of the eardrum (or tympanic membrane) to the inner ear. This increases the pressure on the connective tissue of the oval window. This pressure is ultimately transmitted through the stapes, which presses against the fenestra ovalis, to the cochlea. From there, it travels through the auditory nerve to the brain, which processes the sound.

The middle ear is the part of the ear between the eardrum and the oval window. The middle ear’s function is to transmit sound from the outer ear to the inner ear.

The anatomy of the middle ear

The middle ear consists of three bones: the hammer (malleus), the anvil (incus) and the stirrup (stapes), the oval window, the round window and the Eustachian tube.

The bones of the middle ear

The eardrum, which is located in the outer ear, is very thin. It measures approximately 8-10 mm in diameter and is stretched by means of small muscles. The pressure from sound waves makes the eardrum vibrate.

The vibrations are transmitted further into the ear via three bones in the middle ear: the hammer (malleus), the anvil (incus) and the stirrup (stapes). These three middle ear bones form a kind of bridge, and the stirrup, which is the last bone that sounds reach, is connected to the oval window.

The oval window

What is the oval window? In the middle ear, the oval window is a membrane covering the entrance to the cochlea in the inner ear. When the eardrum vibrates, the sound waves are transferred to the middle ear bones and travel via the hammer and anvil to the stirrup and then on to the oval window.

When the sound waves are transmitted from the eardrum to the oval window, the middle ear is functioning as an acoustic transformer amplifying the sound waves before they move on into the inner ear. The pressure of the sound waves on the oval window is some 20 times higher than on the eardrum.

The pressure is increased due to the difference in size between the relatively large surface of the eardrum and the smaller surface of the oval window. The same principle applies when a person wearing a shoe with a sharp stiletto heel steps on your foot: The small surface of the heel causes much more pain than a flat shoe with a larger surface would.

The round window

In the middle ear, the round window vibrates in opposite phase to vibrations entering the inner ear through the oval window. In doing so, it allows fluid in the cochlea to move.

The Eustachian tube

What is the Eustachian tube? Another important middle ear function is carried out by the Eustachian tube, which is also found in the middle ear. It connects the ear with the rearmost part of the palate. The Eustachian tube’s function is to equalise the air pressure on both sides of the eardrum, ensuring that pressure does not build up in the ear. The tube opens when you swallow, thus equalising the air pressure inside and outside the ear.

In most cases the pressure is equalised automatically, but if this does not occur, it can be brought about by making an energetic swallowing action. The swallowing action will force the tube connecting the palate with the ear to open, thus equalising the pressure.

Built-up pressure in the ear may occur in situations where the pressure on the inside of the eardrum is different from that on the outside of the eardrum. If the pressure is not equalised, a pressure will build up on the eardrum, preventing it from vibrating properly. The limited vibration results in a slight reduction in hearing ability. A large difference in pressure will cause discomfort and even slight pain. Built-up pressure in the ear will often occur in situations where the pressure keeps changing, for example when flying or driving in mountainous areas.

The middle ear is only one part of the fascinating apparatus that enables us not only to hear, but also to maintain our balance. On our website, you can learn about all parts of the human ear, understand the functions and parts of the outer ear and the inner ear.