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Hearing a sound involves a set of structures that extends from the ear all the way to the brain. The transfer of a sound involves many components, most of which are not visible. The outer ear, called the pinna, is like a funnel that traps and transmits sound waves to the other parts of the ear structure. This is the only structure in the mechanism of hearing that is visible to the unaided human eye.
The middle ear is where sound waves hit the first structure of the hearing process. This structure is the tympanic membrane, or eardrum. It acts like a drum in that it is a thin covering of skin that vibrates when sound waves hit it. Three tiny bones, the ossicles, are attached to the eardrum and form a chain that sends sound to the cochlea. Each of these bones is named for (and also resembles) a structure found in the human world. The bone that is attached to the tympanic membrane is the malleus, and looks like a hammer. The next in the chain is the incus, which resembles an anvil. The last one is the stapes, or stirrup, and is connected to the cochlea.
The cochlea is located in the inner ear and is shaped like a snail shell. It contains tiny hairs called cilia. When these hairs are hit by sound waves they send impulses along the auditory nerve. The auditory nerve is responsible for balance, and is one of twelve nerves connected to the brain.
The brain is the final step in the hearing process. In the hearing part of our brain sound is processed and given meaning according to our knowledge and past experiences. Our hearing mechanism is always at work taking in and processing the sounds around us.
Image: Universidad de Sevilla, Creative Commons 2.0