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Anatomy of Hearing


It has been said that the universe itself is silent as without ears and brains the vibrations are never given meaning and elevated into sound. This begs the question 'when was the first sound ever heard?' and to some extent this can be answered by looking at the evolution of the ear.

Evolution of the Ear

The ear almost certainly began its evolution as a simple pressure sensing device in fish. This is still present in many fish and is known as the lateral line organ. It runs right the way down both sides of the fish and above it are a series of depressions. Even the most primitive fish seem to have had this organ in some form and it is believed that it enables the fish to sense currents and possibly moving objects.

The Labyrinth

Over time this organ began to develop into a bony structure that is still found in all vertebrates and is known as the labyrinth. The labyrinth consists of three (or very occasionally one or two) fluid-filled canals positioned at mutual right-angles to each other. It developed as a means of aiding balance - if an animal turns its head the fluid in the labyrinth will tend to lag behind this movement due to inertia. This difference is picked up by tiny hairs inside the canals which then send nerve impulses to the brain to signal the change in direction. Reflex reactions in the brain immediately respond to balance this movement. It is due in part to the labyrinth that extreme spinning or swaying can cause nausea and giddiness as the brain struggles to find the happy medium from the excessive sensory input it is receiving. This early function still survives in modern day man in the endolymph.

While the evolution so far was adequate for water dwelling creatures, a more complex set of changes began to occur probably around 3 million years ago. At this point in the earth's history the seas and and lakes were beginning to dry up and an extensive drought was to follow. This occurred slowly enough for evolution to respond with the result that creatures that were better adapted to an amphibious life were more likely to survive.

As the evolutionary trend towards amphibians progressed into the development of land living creatures, so the organs that were to become the ear changed to meet the new environment. Air is very different to water and so organs for sensing changes in water pressure were of little use on land. What was needed was a more sophisticated organ that could sense extremely small changes in air pressure. Any creatures whose evolution tended this way had an immediate advantage as these tiny differences in air pressure are caused by what we call sounds and so could be used to indicate food, danger etc.

As with any part of evolution a certain amount is always guesswork but the way the ear developed to meet this task was probably as follows:

Certain creatures developed a tendency for the bony section of the labyrinth to have a small area that was thinned down to almost skin.At some point this became thin enough to allow vibrations in the air to pass across into the fluid inside. At the same time the labyrinth grew in size and complexity. Amphibians such as frogs show what is believed to be a step along this path in that their labyrinth is swollen to the size of a visible bulge. Further evolution created from this part of the labyrinth the wonder of nature that is known as the cochlea.

Anatomy of the Ear
The Modern Ear

The way that the ear developed to achieve this sensitivity can be seen by looking at the hearing organs found in modern animals.

The Cochlea

The cochlea is often compared to stringed instruments such as the piano or harp. Both have 'strings' and a 'sounding board' but a human cochlea has roughly 24,000 'strings' and is about the size of a small pea. It is a snail or spiral shaped tube, covered in bone, with two canals and a duct subdivided by a thin partition called the cochlear partition, which runs the entire length of the spiral.

The basilar membrane lies on one side of the partition and the vestibular membrane lies on the other side. Sound vibrations reaching the inner ear are transmitted through the fluid of the cochlear canals (the tympanic canal and the vestibular canal) and around the cochlear duct which divides them. As the pressure of the waves flows over the basilar membrane, which is the vibrating wall of the cochlear duct, the fluid inside the duct is agitated. This movement of the fluid stimulates the organ of Corti, which sits on the membrane inside the cochlear partition.

The organ of Corti is a hearing sense organ and performs the actual transformation of mechanical vibration into nerve impulses. It has a gelatinous membrane and two sets of hair cells, the inner and outer receptor cells, which are located between the basilar and gelatinous membrane of the organ of Corti. When the basilar membrane vibrates it pushes the hair cells against the gelatinous membrane, causing them to produce a chemical that converts the movement into electrical impulses which are transmitted to the adjacent nerve fibres.

The massive advantage that the cochlea bestows on its owner is that it enables sound to be both heard and analysed into constituent tones. This means that (with the help of the brain) sounds can be distinguished and remembered. Not being able to hear a predator loudly approaching from behind you is a distinct disadvantage whereas recognising the sound of a large tiger creeping through the grass in your direction is a definite bonus.

From the Ear to the Brain

The signal passes from the ear to the brain by means of the auditory nerve.

From Sound to Music

Just being able to hear is an advantage but the more sensitive your hearing the better (unless you live by an airport). Sensitive hearing enables us to make much more use of the information contained in sound - Does it sound like a large animal in the undergrowth? Is it getting closer? What's that tune it's humming?

Together with the development of the brain and possibly language, the increased sensitivity of hearing gave rise to what we now call music. Quite why we find certain music pleasing is another subject but it is essentially the ability to analyse small vibrations in the air in terms of their constituent tones that makes music physically possible. Without the amazing apparatus of the ear we would be blissfully unaware that there were any vibrations out there at all, good or otherwise.

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