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Lesson 1 of 4
How we hear
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The human auditory system is truly amazing.
In this section you will learn the basics of how we hear.

The visible part of our hearing system, the external ear, is called the 'pinna'.
And like our fingerprints, no two are exactly alike.

You can keep referring back to the image above as you learn more about each of the main sections and parts of the hearing system.
More than meets the eye!

As this cutaway diagram shows, the auditory system is much more than just the visible pinna. Let's consider the various components, grouped into 3 main sections, the outer ear, middle ear and inner ear.
Outer ear
Middle ear
Inner ear
Outer ear
The outer ear consists of the visible external ear or pinna, and the ear canal which ends at the eardrum.
The pinna's shape is not just useful for holding up our glasses and decorating with earrings. It actually collects sound and funnels it to the ear canal and down to the eardrum.
The ear canal is about 2.5 centimeters (1 inch) in length and 0.7 centimeters (0.3 inches) in diameter. You can see from the diagram that the structure beneath the skin of the ear canal is different near the ear canal opening from near the eardrum. Near the canal opening there is cartilage; as the canal approaches the eardrum there is bone. The ear canal is curved and lined with fine hairs and has small glands which produce cerumen (earwax), all of which protect the deeper structures beyond.

Middle ear
The eardrum, more formally known as the tympanic membrane, is the transition point between the outer ear and the middle ear. Below is an image of a healthy eardrum. You can see the translucent membrane through which one of the middle ear bones on the other side is clearly visible.
There are actually 3 small bones, called ossicles, connected together and suspended inside the air-filled middle ear space. The ossicles are commonly known as 'hammer', 'anvil' and 'stirrup'. The actual scientific names are 'malleus', 'incus', and 'stapes'. These are the smallest bones in the human body. They are connected together with the malleus attached to the tympanic membrane (the eardrum, shown below) on one end, the incus in the middle and the stapes attached to the inner ear on the other end.
There is also a tube, called the Eustachian tube, which connects the middle ear space and the throat. This tube opens periodically to equalize pressure on both sides of the eardrum. Sometimes you feel this equalization, like when your ears "pop" after an elevation change. Inflammation of this tube is usually the cause of a buildup of fluid in the middle ear space which may result in ear infections, especially in younger children.
The main function of the middle ear is to change the sound waves collected by the outer ear into mechanical vibrations and transfer them on to the fluid-filled inner ear.

Inner ear
The stapes is connected to the inner ear structure known as the cochlea, the primary organ of hearing. The cochlea is a small, snail shaped structure, which is filled with fluid. Inside the cochlea this fluid surrounds a variety of other smaller structures, most important of which are the hair cells. The cochlea is only about 30mm long but contains over 30,000 of these tiny hair cells, a few of which you can see in the image below/right. In fact, they are not hairs at all - they are highly specialized sensory cells with microscopic fibers - much smaller than hair.
Vibrations from the middle ear cause the fluid in the inner ear, and hence these cells, to move, converting sounds from a mechanical vibration into an electro-chemical signal which is then sent on to the hearing nerve and up to the brain.
Attached to the cochlea, and also a crucial part of the inner ear, is the vestibular labyrinth, one of the key components of our balance system.

Putting it all together

Play Video
Play Video Click the 'play' button above to see a simple visualization of the outer, middle and inner ear sections working together to transmit sound to the brain.
Just for fun - Animals 'hear' in different ways. Mammals have a similar structure to us, but they sometimes have much larger and even movable external ears and can hear a different range of sounds than humans. Click below for some fun.

Fox hears the voices
Knowledge check
Take a short test to see how well you have learned the information.
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What is the primary organ of hearing?
Auditory receptor
this is just one of the three parts of the auditory analyzer
the main organ of hearing
Know the result
What structure often causes ear infections with inflammation?
separates the outer and middle ear
Inflammation of this tube is usually the cause of fluid buildup in the middle ear cavity, which can lead to infection of the ear, especially in young children.
these are three bones, interconnected and suspended in the middle ear cavity filled with air. Ossicles are known as the "malleus" (maleus), the "anvil" (incus), and the "stapes" (stapes).
Know the result
The smallest bones in the human body?
Ossicles are known as the "malleus" (maleus), the "anvil" (incus), and the "stapes" (stapes). These are the three smallest bones. The mass of each of these bones is from 2 to 4 mg, while they do not change with age.
They are really not big, but they are not the smallest bones in the human body.
The ethmoid bone - the bone of the medulla of the human skull, separates the nasal cavity from the cranial cavity.
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You should go through the whole lesson from the very beginning.
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You should carefully reread the oral part of the lesson.
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