The human ear is one of the most under-appreciated organs. Most of us take it for granted that we can listen to music whenever we want or hear our loved ones speaking to us. Many people do not realize just how complex the ear really is. It is made up of thee main parts: the outer ear, the middle ear, and the inner ear. Each of these plays a role in the process of processing sounds, and each is divided into several more essential pieces.
The outer ear is, as the name implies, the outermost part of the ear, which acts as a funnel to guide incoming air vibrations (sound) to the eardrum. It is composed of two parts: the pinna and the ear canal. The outer ear deals with sound localization, meaning it allows us to hear sounds differently based on their origin. The head itself blocks some sound waves, but the others reach the left or right ear at a different intensity. This enables our brains to decipher the location of a sound, because we can tell from which side of our body it is originating.
The pinna is the external part of the ear, made up of skin over cartilage, with the exception of the earlobe. When sound waves enter the ear, the shape of the pinna -- its whorls and recesses -- moves them through to the eardrum. The pinna also helps shield the ear from sounds coming from behind, which helps us locate the source of a sound.
The ear canal, like the pinna, is supported by cartilage at its opening. It is shaped like the letter S and is around three centimeters long in most adults. Bone supports the part of the ear canal that is further inside the head. This part of the ear is also covered in skin and features tiny hairs that guard the entrance to the ear canal. This, along with the secretions produced by the ear canal (earwax) helps to keep airborne contaminants and other particles out of the ear so they do not interfere with the hearing process.
The middle ear is located exactly where one would expect: between the external and inner ear. The tympanic membrane (more on that next) separates it from the ear canal. The middle ear's purpose is to transfer sound vibrations from the eardrum to the fluid in the inner ear. This process is carried out by a chain of small, movable bones called ossicles, as well as the small muscles that support and move them.
The tympanic membrane is what we refer to as the eardrum. It separates the ear canal from the middle ear. It is around one centimeter in diameter and curves inward slightly to form a concave shape. The eardrum vibrates in response to sound, which helps us hear. It is full of nerves, however, and is very sensitive to pain caused by both sound and physical touch. The tympanic membrane functions perfectly when there is equal pressure on both sides of it. When this pressure changes on one side, perhaps due to high altitudes, this can obstruct hearing and even cause pain.
The eardrum sits within the tympanic cavity, which also contains the three smallest bones in the body and two of the smallest muscles. The bones or ossicles are the malleus, incus, and stapes. The malleus is attached to the eardrum and extends into the tympanic cavity. The incus is connected to the malleus and the stapes, forming the center link in the chain. The stapes is an arched bone with a footplate held in place by a ring of tissue called the "oval window," which functions as the entrance to the inner ear. These bones work together to move sound into the inner ear from the eardrum.
The inner ear is the deepest part of the ear, nestled within the bony labyrinth, a maze-like passageway of bone. These bones are lined with a network of tubes called the membranous labyrinth. The inner ear contains a fluid cushion called perilymph located between the two labyrinths, as well as a fluid called endolymph within the membranous labyrinth. The inner ear also contains the vestibule, which is largely responsible for our sense of balance.
The cochlea is, for all intents and purposes, the most important part of the aural process. This is where the ear converts sound vibrations into what we perceive as hearing. It is shaped like a spiral snail shell and is around five millimeters tall and nine millimeters wide at the base. The cochlea winds around the modiolus, a piece of spongy bone, as the cochlea is not able to support itself on its own due to its fleshy, boneless form.
There are three chambers in the cochlea. Each of these is full of fluid and separated from the others by membranes. The uppermost layer is the scala vestibule and is filled with perilymph fluid. The bottom chamber, the scala tympani, is filled with this same liquid and covered by a secondary tympanic membrane (eardrum). The middle chamber, the scala media or cochlea duct, contains endolymph fluid.
The basilar membrane supports the organ of corti, which is the size of a green pea and works to translate vibrations into nerve impulses. It features hair cells with stiff microvilli on their surface; they are coated in the tectorial membrane, another jelly-like substance that assists in the process of hearing. These hair cells (along with other cells known as supporting cells) work to adjust the cochlea to different sound frequencies, helping the ear to process sounds more efficiently and accurately.
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