Ear (External, Middle, and Internal)
The ear is the means by which the sounds of the world reach us. The external ear that is evident on humans and other creatures is only a part of the ear. The middle ear and the inner ear have different structures and contributions to the overall ability of the ear to detect sound.
The external ear consists of a hole in the side of the skull, known as the auricle or pinna, and of the ear canal. The auricle helps focus the incoming sound waves. The hole leads into the auditory canal, a roughly cylinder-shaped, small diameter canal that is about 0.98 in. (2.5 cm) long. Towards the inner end, the canal widens slightly and ends at the eardrum. The ear canal can be thought of as a shaped tube with a resonating column of air inside it, having open and closed ends, similar to the construction of an organ pipe.
This analogy is apt, for the ear canal enhances the sound vibrations that have traveled in from the outside. The canal can resonate, or vibrate, typically at frequencies that the ear hears most sharply. The vibration increases the wavelength of the sound waves traveling down the canal. The amplified waves eventually contact the ear drum, which is positioned at the inner end of the canal, and marks the boundary between the outer ear and the middle ear.
The ear drum is a membrane. It is capable of vibration, which occurs when the sound waves contact it. The vibrational energy of the ear drum is converted to mechanical vibrations in the solid materials of the middle ear. These solid materials are three bones: the malleus, incus and stapes. The bones form a system of levers that are linked together and are driven by the eardrum. The outer malleus pushes on the incus, which in turn pushes on the stapes. This further amplifies the sound vibrations, typically 2-3 fold. Muscles are positioned around the bones, the smallest muscles in the body, and 'dampen down' the mechanical vibrations if they become too pronounced. They are a form of safety device, restricting movement of one or more of the bones. This protects against the creation of too great a vibration from a very loud sound. Muscle movement is triggered when sound exceeds a certain level.
The stapes bone is in close proximity to a structure called the oval window, which defines the boundary between the middle and inner ears. The oval window is another membrane--15 to 30 times smaller than the eardrum--that covers an opening at the base of the inner ear structure known as the cochlea. Mechanical vibration from the stapes is transferred to the oval window, which further amplifies the sound by 15 to 30 times.
By the time a sound wave has reached the cochlea, it has undergone amplification in the ear canal, the middle ear triplet bone arrangement, and at the oval window. The amplification of the original signal can be upwards of 800 times by this point.
The cochlea contains three fluid-filled regions. The vestibular and tympanic canals contain perilymph; a liquid almost identical to spinal fluid. The third region, the cochlea duct, contains endolymph; a fluid similar to that found within cells. These regions are separated by thin membranes (Reissener's membrane between the vestibular canal and cochlea duct, and the basilar membrane between the tympanic canal and the cochlear duct). Reissener's membrane is only two cell walls thick.
Any rupture of these delicate structures impairs the function of the cochlea, which is to convert the incoming mechanical sound pressure to hydraulic pressure. The cochlear membranes vibrate with the incoming mechanical sound energy. Their vibration is converted to hydraulic pressure (akin to waves in water) that is then transmitted along the entire structure of the cochlea. The waves can be of different heights. The different wave shapes determine which nerve fibres positioned around the basilar membrane will send a signal to the brain. This enables sounds of different frequencies to be distinguished and such information routed to the brain for interpretation.
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