The human ear is a complex and wonderful 

instrument to say though that all it does is 

to enable us to hear is to really understate the things 

that is capable of.

It is first of all,

a collector of sound waves,

it catches the sound waves and directs them down into the 

external canal so that they can find their way on 

further into the inner ear where they can be received and 

interpreted.

It is also an energy transducer that is,

it changes the acoustic energy of the airborne 

signal to mechanical energy to 

hydraulic energy and finally to electrical energy.

It is also a sound amplifier that 

is,

it actually increases the loudness of the sound 

after it reaches the ear.

It's also a sound analyzer that is,

it allows us to be able to interpret the 

complex noise that we call speech.

And finally,

it contains the mechanism that 

enables us to be able to maintain our balance 

and to stay in an upright position.

Now,

I'd like to tell you some things about the ear.

But before we do that,

I'd like to review with you some of the basic notions 

of sound and sound.

Waves,

how they're generated where they come from 

and how they travel through the air 

sound sources can be of varying kinds.

Some examples are the reed of the clarinet,

the bell,

the human voice and a tuning fork.

All of them produce sound in the same basic 

way.

Let's review then the four essentials that we need for a sound to be 

created and to be heard.

First of all,

we need a vibrator such as a tuning fork.

Secondly,

we need some force to set the vibrator in motion.

Thirdly,

we need a medium such as the air to convey the 

wave motion created by the vibrator.

And finally,

we need the hearing mechanism itself which can receive 

and interpret the sound.

I think the best example to explain how a sound 

wave is generated is the tuning fork.

If you were to squeeze the tides of the tuning fork together 

and then let them snap apart,

you would of course generate a pure tone.

The reason why the tone is generated is 

because as the time springs back 

past its original resting position,

it pushes the molecules of the air together and 

causes them to bunch up,

it condenses them.

Then as it springs back again,

in the opposite direction,

it creates a vacuum or a rare faction 

where there are fewer molecules than there should be.

This constant repetition of condensation 

and rare faction increase in pressure,

decrease in pressure is the basis for the sound 

wave.

Now,

it's important to remember that it's not the molecules in 

the air themselves that travel.

But rather the pressure changes that travel through the 

air to the ear.

There are two basic characteristics of a sound wave that I 

think you should be familiar with.

The first is frequency,

frequency is analogous to pitch.

And it's defined as being the 

number of times the 

repetition of condensation and 

rare faction occurs in some unit time.

For example,

in 1 2nd,

the fewer number of times in a second 

that the conversation and rare faction cycle 

goes on.

Then the lower the pitch,

the greater the number of times that this condensation 

rare faction cycle goes on.

Then the higher the perceived pitch,

if you will look at the screen,

you will be able to see a representation of 

a pure tone that's been created by an 

oscilloscope.

The uppermost portion of the sound wave is that 

represents the condensation portion.

The lower most portion represents the rare 

faction portion.

You can see that these waves are some 

distance apart and probably 

represent a tone with a frequency of about 

250 cycles per second 

as we increase the tone so that the 

frequency is close to 1000 cycles per 

second,

the waves come closer together.

And we see more a greater number of 

repetitions of condensation and rare faction.

In a second.

The other aspect of a sound wave that I would want you to be 

familiar with is amplitude.

Amplitude simply relates to the fact that 

the strength of the particle or molecular energy 

is greater.

And so we have a louder sound.

This is represented on the screen by the 

sound wave increasing in amplitude,

you can see that the envelope becomes wider as the sound 

becomes louder.

Then as the sound becomes softer,

the envelope of the sound wave decreases and 

becomes smaller.

Now,

this example,

from the tuning fork is a fairly simple 

representation of a sound wave and it's 

normally not one that we run into every day.

I better a different example which I would like 

to show you is what the human voice would look 

like if you were to put it on a 

screen as I'm talking.

Now,

you can see the jumble of lines and angles on the 

screen representing my voice as it comes 

to you.

It's,

it's really a wonderful thing to consider the human ear that it 

can take such a jumble of noise and 

make sense out of it.

Now,

let's take a look at the year just how it's made and just 

how it works.

The year is first of all located in the petrus 

portion of the temporal bone and this is 

the hardest bone in the body and nature designed 

it that way to be sure and protect the inner ear from 

damage.

As a matter of fact,

the word petrus derives from the Latin word 

petrus,

which means rock,

the outer ear is connected to the skull by a 

series of muscles ligaments and other 

tissue.

Now,

most of the muscles that attach the outer 

ear to the skull 

are vestigial.

That is at one time,

we used to use them the same way an animal uses 

them now to turn his ears to localize 

sound.

But uh they are no longer of any 

real use to us at this time.

The outer ear is the first thing that 

receives the sound.

It's ovoid in shape and contains a 

number of depressions and humps.

Its function is to collect the 

sound waves as they hit it and direct them 

down and into the external canal.

It provides us with about 10 decib of 

amplification.

Now,

this amount of amplification is really not 

significant.

As a matter of fact,

if we were to lose the outer ear,

it would not cause any real loss of 

hearing for us.

So even the outer ear itself is not that useful 

to us today and serves primarily a 

cosmetic function.

Now,

as we move from the outer ear 

into the external canal,

remember now that the sound waves coming through 

the air are collected by the outer ear 

and directed down into the canal where they 

finally will hit the ear drum.

The external canal is about an inch long and 

it's about a quarter to a half inch in diameter.

The tissue that uh is made that makes 

up the wall of the external canal is the same 

tissue that is connected with the tissue 

that makes that up that tissue of the ear,

the hand,

the back and so forth.

There are two things about the external canal that I think you should be 

familiar with.

One is that it contains a number of hairs 

along this portion here.

And the function of those hairs is to protect it 

from insects intruding into the external 

canal.

In addition to that,

there are also in here,

a number of glands which are called saruman.

It's glands and they will 

produce saruman or what's more 

popularly known as earwax.

Now,

this earwax or saruman is very bitter 

tasting and repugnant material.

And it also serves the function of protecting the 

ear from the intrusion of insects.

Now,

the sound wave comes down 

from the article into the external 

canal and finally,

it comes up to the middle ear,

the middle ear is made up of the ear 

drum,

The articular chain which is made up of three 

tiny bones called the,

asse.

the Incas and the stay 

peas.

The function of the middle ear 

is to transform the 

acoustic energy into mechanical 

energy and also to increase or 

amplify the amount of sound that that 

is reaching the ear.

And it does this by means of what is 

called the a real ratio.

That is if we take the ratio of 

the area of the Eardrum to the 

ratio of the area of this foot plate.

Here,

we find that the increase in sound is 

about 18 times.

Now,

it's necessary for this to occur because 

the sound after it leaves,

the middle ear will go into the inner ear,

which is filled with fluid.

And it's necessary to overcome the impedance which 

is offered by the fluid,

the fluid in the inner ear.

There are two muscles in the middle ear,

which we should be familiar with.

And those are called the tensor tympani.

And the step PDS muscle,

the tensor tympani originates here and comes 

across the middle ear space to the handle of 

the malas.

The step pds muscle originates on the 

posterior wall behind these bones 

and it inserts right here in the head of the stay peas.

These two muscles serve a protective function 

that is whenever a sound which is loud enough to 

cause damage to the ear occurs,

they contract very quickly and they 

pull the articular 

chain off its normal course of action.

And so it protects the inner ear from uh 

from damage.

Now,

when the sound wave it comes 

down into the external canal and it hits the 

eardrum,

it causes the eardrum to vibrate at the same 

frequency and amplitude as the sound as the 

original sound.

The articular chain is connected to the eardrum.

And so that also vibrates at the 

same frequency and amplitude.

And so the sound wave is now conducted across this 

middle ear space up to the inner 

ear as as of yet 

though no,

no hearing has taken place.

The only thing that's really happened is that the sound 

wave has been conducted up to the inner 

ear.

The other part of the inner ear that you should be familiar 

with is something called the U station 

tube.

And this is located in the 

anterior inferior portion 

of the middle ear space.

The station tube runs from the middle 

airspace down to the nasal firing.

So to the back of your throat And it 

serves two functions.

First of all,

it provides a constant source of oxygen 

to the middle ear space.

The second thing it does is to is to provide an 

equalization of pressure whenever there are 

changes in altitude.

If you were to experience a 

severe change in altitude,

for example,

the pressure on the outside of your ear 

would be less than the pressure on your,

on the inside of your ear.

And it would force the ear drum outward.

If you yawn,

then you open the station tube and it allows the air 

to come up into the middle ear.

And so it equalizes this pressure.

I'm sure if you've flown in an airplane,

you have probably experienced this and have had 

to equalize that pressure in order to get rid of the 

uncomfortable or stuffy feeling that you find in your 

ear.

Now,

once the sound wave again,

has crossed the middle ear,

it enters the inner ear 

it is in the inner ear that the essential organ of 

hearing is contained.

The inner ear contains also the balance 

mechanism.

And this mechanism is made up of the three 

semicircular canals,

the sack you'll and the you trickle.

And this mechanism,

all of these parts working together provide 

us with a sense of balance to keep ourselves upright.

This yellow line that you see here is the 

vestibular portion of the auditory nerve.

The other portion of the inner ear or the labyrinth is 

called the cochlea.

And this is a spiral or snail shaped 

bony labyrinth within which the 

essential organ of hearing is located.

This portion of the inner ear is called the 

vestibule.

And you can see that there are three channels here.

This is the scale of vestibular or the 

vestibular channel.

The scallop media or the middle channel and the 

skeleton pony or the tim panic channel.

This area in here is filled with a fluid 

which is called parallel.

And this middle channel floats 

in that parallel and it protects it from 

bumping up against the walls of the bony 

uh capsule.

The fluid which is inside the scallop 

media is called indolent.

And this is much different than the parallel 

and arises from somewhere near the cortex.

Now,

when,

when we hear 

the acoustic signal,

which has been changed to uh mechanical 

energy now has to come into the inner 

ear and be somehow changed into hydraulic 

energy and then into electrical energy.

Now,

what I would like to do is to show you a 

more detailed version of what the cochlea 

looks like and just what it's made up of.

Now,

this picture over here is a much 

more detailed version of the cochlea.

Right here.

We have the auditory branch 

of the 8th cranial nerve coming on 

up into the cochlea and branching 

off at every level.

You can see the snail shaped appearance 

of the cochlea.

And this picture shows it as though it had been 

cut straight down the middle.

The arrows that you see indicate the 

direction uh the force,

the direction of the force of the 

energy which is created by the sound wave,

it moves on up and then at the 

top turns around comes back down.

Again.

Now,

here's an even more 

detailed 

picture of of the cochlea.

This is just one section of it.

And again,

you can see a fiber from the eighth cranial 

nerve coming on through the spiral lamanna to 

connect with the essential organ of hearing.

This is the scalar vestibular,

the scalar tympani and the 

scalar media within 

the scalar media contains,

as I said,

the essential organ of hearing,

the scalar media is bounded by 

prisoners membrane here,

the base layer membrane here 

and the spiral ligament 

here 

within the scalar media,

we find the sectorial 

membrane 

and the organ of corti,

the pink area 

in the organ of corti.

You'll see hair cells 

located along its whole length.

Each of these hair cells is connected 

to the nerve of hearing 

when they say no 

comes from the middle ear into the inner ear.

A number of things happen.

First of all,

the stay peas moves inward 

as it does,

it compresses the fluid in the inner ear.

Now that pressure has to be released somewhere.

And right down here in the skeleton pony is something called a 

round window.

The staples moves in through the over window,

compresses.

The fluid causes it to move.

That pressure is released at the 

round window.

So the signal again comes 

by way of the article to the external canal.

It has changed to mechanical energy and 

amplified across the time panic membrane of 

secular change structure.

And now it is causing the fluid of the inner ear 

to move.

And so now it has been changed into hydraulic 

energy.

You can see the dotted lines here 

and they indicate that as that fluid 

is moving through the inner ear,

it causes this membrane to bulge 

inward.

That membrane causes the fluid in here to 

move.

That in turn causes the tech tutorial membrane to 

bend downward.

And now you see the hair cells here which are connected 

between the organ of corti and the tech 

tutorial membrane.

When the tech tutorial membrane bends downward,

it distorts or bends these hair 

cells out of shape as soon as it does 

that,

that triggers off an electrical 

signal,

Which is then picked up by the fibers 

of the 8th cranial nerve and 

carried on up through the central nervous system to 

the cortex.

The movement of the fluid,

of course,

being pressure continues on.

And you can see the basilar membrane has 

bent down also 

that causes this fluid to continue 

moving.

And if we can go back over here again for a minute,

it would be released here at the oval 

window.

Let's take a look at 

schema ties version of 

the inner ear.

Remember the inner ear is coiled 

snail shaped of 2.5 turns.

And so we've schema ties it a little bit here in order to 

show you in some detail,

just how it works.

The sound comes down through the the external 

canal hits the 

eardrum that is connected to the articular 

chain.

Now,

the foot plate of the stay peas moves into 

the the vestibule and 

it compresses this fluid in here.

Now,

you can see that at this point,

this continuation or movement of fluid 

through the inner ears taking place,

risers membrane is bending downward.

The fluid continues to compress 

it causes the basal membrane to bend 

and comes right on down through and finds its release 

here at the round window.

These arrows here indicate 

the pressure which is not 

necessary here or not,

uh which is not necessary at this 

point going on up around through the 

whole system and eventually coming out through the round 

window.

The basil er end of the 

inner ear contains those 

uh hair cells and fibers 

which are responsible for low pitched 

sounds.

I beg your pardon for high pitched sounds.

The a pickle end up here is responsible 

for those uh hair cells and 

fibers which are responsible for low 

pitched sounds.

Once the the 

hair cells have been excited and trigger off this 

electrical response,

they will excite the fibers of the eighth 

cranial nerve And they all come 

together in a bunch 

of,

of nerve fibers,

which is called the spiral ganglion.

And then they find their way on up through the 

central nervous system to the cortex where the 

sound is interpreted.

Now,

what we've been discussing so far is the way we hear 

by what is called air conduction.

That is the signal travels from the air 

directly into the ear canal into the inner 

ear.

This is the way that we normally hear in most everyday 

situations.

There is another way though that we can hear and it's called hearing by 

bone conduction.

Now,

if we can take a look at this chart,

again,

recall that the inner ear is 

encased in a capsule of bone and that bone is 

part of the skull.

When a sound wave is strong enough to cause the 

bones of the skull to vibrate,

those vibrations can cause the 

fluid in the inner ear to 

move and to carry out the 

same essential process as 

happens by air conduction.

Now,

this is normally not important for us as long as 

the outer ear and the middle ear are intact.

But for those persons who have some 

pathology of the outer and middle ear,

who may perhaps need a hearing aid,

we can provide a hearing aid that would put a 

vibrator right here on the skull and 

that would cause the bones of the skull to vibrate 

at an in an amplified way,

excite the inner ear.

And so allow him to be able to hear 

normally.

Now,

let's just take a look again at the 

whole process of hearing by air conduction.

Remember that we have a signal which is generated,

for example,

by a tuning fork,

it generates a number of compensations and 

rare factions changes in 

pressure.

Those pressure changes find their way through the 

air are collected by the external 

ear and forced down into the ear canal.

They reached the eardrum secular 

change structure where they are changed to mechanical 

energy and amplified.

They then find their way into the inner ear when the 

foot plate of the staples moves in and out in 

time to the signal that came into the year 

originally.

And that causes the fluid to move,

causes the membrane structure to move.

And so excites the nerves of hearing from 

there.

They then find their way on up through the central nervous system 

to the cortex where they're received 

analyzed and interpreted.

Now,

that's essentially how the ear works.

Uh it's come a long way since the time when it 

was a simple system for that in a 

fish millions of years ago.

So that the fish could detect the pressure changes in the 

water and maintain an upright and balanced 

position.

We've learned a lot about it since the early days of,

since 1950,

with,

with the invention of the electron microscope,

but we still have a lot more to learn and I'm 

sure that in the next few years we'll,

we'll see a lot more being added to our store of knowledge 

about the year and how we hear.