Sense Organs: Otic (Hearing) Structures
In humans, hearing structures are also commonly known as the ear. The human ear is divided into three functional areas: the outer, middle and inner ear.
The outer ear includes the visible portion of the ear. This portion of the ear acts to collect and focus sound. Without the dish shaped portion of the ear, low intensity sound might escape detection. The visible portion of the ear also includes a structure known as the pinna (or auricle). This is visible as a hole. It leads to a passageway, typically about 0.98 in. (2.5 cm) in length that leads deeper into the ear. The passageway is commonly called the ear canal (or auditory canal). The ear canal is typically not a uniform diameter all down its length. Rather, it narrows about three-quarters of the way inward toward the eardrum to a diameter of typically a bit less than 0.098 in. (0.8 mm), and then widens near a taut membrane stretched across the end of the canal. The membrane is known as the eardrum. The ear drum is the boundary between the outer and middle portions of the ear.
The construction of the ear canal—open at one end and closed off at the other end by a membrane--is what enables sound vibrations to be amplified. An example of a similar effect is the resonance that can be achieved by the pipes of an organ.
The middle ear houses three small bones: the malleus (or hammer), incus (or anvil), and the stapes (or stirrup). The malleus is attached at one end to the inner surface of the eardrum and at the other end to the Incus. The latter is in turn attached at its other end to the stapes. The three bones form a "U" shape. They function by acting as levers, driven by the movement of the eardrum, with malleus pushing incus and incus pushing stapes. These three bones are located in a cavity. At the bottom of the cavity lies the Eustachian tube. This tube connects the middle ear with the nasopharynx and functions to equalize the pressure between the external air and the middle ear. An example of the Eustachian tube in action occurs when traveling in an airplane. The relief of the ear fullness upon swallowing or chewing gum is due to the equalization of air via the Eustachian tube.
The stapes bone of the middle ear contacts a structure termed the oval window. The oval window is the boundary between the middle and inner portions of the ear. It is another membrane, similar to the eardrum, but is approximately one-third the size of the eardrum. As with the eardrum, the oval window is designed to vibrate, and so to pass on and amplify sound waves.
Beyond the oval window lies the region of the inner ear. This region consists of a structure that is termed the cochlea. The cochlea looks something like a snail shell and is only the size of an average-size fingernail. The interior of the cochlea contains a looping channel, which is further divided by thin membranes into a triangular arrangement of two canals, the vestibular canal and the tympanic canal, and a duct called the cochlea duct. The membrane demarcating the vestibular canal and the cochlea duct is Reissner's membrane. The membrane demarcating the tympanic canal and the cochlear duct is the basilar membrane. The intersection of the triangular division houses a body called the organ of Corti.
The cochlea converts the sound energy to hydraulic energy by virtue of the fluid that circulates through it. The vestibular and tympanic channels contain perilymph, which is similar to spinal fluid. The cochlea duct contains endolymph, which is very similar to the fluid found within the cells of the body. These fluids and the Organ of Corti accomplish the sound-to-hydraulic energy conversion. In other words, sound energy is converted to waves in the cochlear fluids. The wave height provides the information for the nerve cells positioned around the cochlea, which telegraph the information to the brain.
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