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140 Human Body

Our eyes create the sense of vision.

They take in light rays, focus them

to create an image, and convert this

image into a stream of billions of

nerve impulses that travels to the

brain. The impulses are interpreted

in different parts of the brain, but

come together to create the detailed,

colour, 3-D image we “see”.

HOW DO WE SEE IN COLOUR?

The cells in the retina that are sensitive to light are

called rods and cones. Rods work best in dim light,

but cannot see colour. Cones can detect colours in

bright light. There are three types of cone, and each

type is sensitive to one of the primary light colours

• red, blue, and green. By combining information
  from all three, we are able to see all the colours
  of the rainbow.

WHAT IS THE PUPIL?

The pupil is a hole that lets light enter the eye.

It appears black, because light passes straight through

it without being reflected. Around the pupil is the iris

• a circle of coloured muscle that controls the pupil’s
  size. In dim light the pupil widens to take in extra
  light. In bright light it shrinks to protect the nerve
  cells at the back of the eye.

WHY DO WE HAVE TWO EYES?

Each eye sees the world from a different angle,

creating slightly different pictures. The brain combines

these two pictures into a single, three-dimensional

image. This is called binocular vision. Seeing in 3-D

helps you to judge the distance and size of objects

much more easily.

INSIDE THE EYE 3

There are two cavities in the eye. The smaller cavity

in front of the lens contains a watery substance,

called aqueous humour. The larger cavity behind the

lens contains a jelly-like substance, called vitreous humour.

This fluid fills the eye and creates the pressure that maintains

the eye’s shape. Blood vessels nourish the cells of the retina.

4 MRI SCAN

The two optic nerves cross over

just behind the eyes and run

deep into the brain. Each

contains about a million nerve

fibres leading from rods and

cones in the retina. The left

side of the brain processes

signals from the left side of each

retina; the right side of the brain

processes signals from the right side

of each retina.

FIND OUT MORE. Brain 139 • Colour 180 • Lenses 181 • Light 178–179 • Nervous System 138

The eye has a curved, transparent lens that bends light rays to create

an image. The eye focuses images on the retina, just as a projector

focuses images on a screen. To keep the image sharp, the lens

changes shape. Muscles around the lens make it fatter for nearby

objects and thinner for distant objects.

THE EYE’S EXTERIOR 3

Most of the eyeball lies

inside the eye socket, so

only the iris, pupil, and

sclera (the white) of

the eye can be seen. The

surface of the eye is kept

moist with tears, and

blinking keeps the eye

clean as tears are washed

across the surface.

Eye

The lens

is an elastic, transparent disc

that changes shape to adjust

the eye’s focus

HOW THE EYE FOCUSES

The cornea

is the curved front of

the eye, which helps

focus light

Optic nerves

cross and divide

to take signals

from both eyes

to each side of

the brain

The iris

is a ring of coloured

muscle that changes the

size of the pupil

Left visual

cortex

processes signals

from the left side

of both eyes

Ciliary muscles control

the shape of the lens

Elastic lens

changes shape to

focus light clearly

on the retina

Light rays

reflected from the

tree enter the eye

Pupil

Iris

Cornea

Retina