The function of anvil and hammer
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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.
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