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Human Body^141

HOW DO EARS HELP US BALANCE?

The semi-circular canals contain fluid that moves

whenever the head moves, making tiny blobs of jelly

sway about and triggering nerve cells. There are also

two fluid-filled chambers, called the utricle and

saccule, which contain blobs of jelly that sway with

gravity, telling the brain which way is up and down.

The ear is the organ of hearing

and balance. The outer ear collects

sound and directs it to the sensory

structures deep inside the skull.

HOW DOES SOUND TRAVEL THROUGH THE EAR?

Sound travels into the ear as vibrations in air. The

eardrum picks up the vibrations and transmits them

to tiny bones in the middle ear. These bones pass the

vibrations to the fluid-filled cochlea in the inner ear.

4 RODS AND CONES
Magnified thousands of times,
this picture shows rods (in blue)
and a shorter cone (in green) in
the retina. Each retina has more than
100 million rods and up to 7 million
cones. The cones are concentrated mainly
in an area called the fovea, in the middle of
the retina. The fovea creates a detailed image
of whatever the eyes are looking at directly.

WHAT HAPPENS IN THE COCHLEA?

Sound vibrations travel along the cochlea’s spiral,

fluid-filled tube, creating waves of pressure. These

stimulate tiny hair cells on a structure called the

organ of Corti, which sends signals to the brain.

FIND OUT MORE. Brain 139 • Sound 176–177

INNER EAR 1

The inner ear consists of the semi-circular canals, the

saccule and utricle, which detect movement, and the

cochlea, which contains the sound receptors.

Ear

INSIDE THE EAR 1

Sound vibrations push the eardrum

against the bones of the middle ear,

which pass the signals to the inner ear.

The inner ear

is made up of the tiny

organs that detect sound,

balance, and movement

The retina

is a layer of light-

sensitive cells lining

the back of the eye

The eardrum

passes vibrations to

tiny bones inside

the middle ear

The middle ear

has three tiny bones,

which carry vibrations

to the inner ear

The optic nerve

contains nerve

cells carrying

signals from the

retina to the brain

Nerve fibres carry

Semi-circular signals to the brain

canals contain

balance

receptors

Saccule

and utricle

detect gravity

Cochlea contains

receptor cells that

detect vibrations

Ear canal

Outer ear

eye

ear