How the Brain Works The Facts Visually Explained by DK (z-lib.org)

BRAIN FUNCTIONS AND THE SENSES

Seeing

Seeing

The eye provides us with probably the most

important of our five senses. It gathers the

light reflected by an object and delivers this

information to the brain via the optic nerve.

The structure of the eye

The eyeball is roughly 1 in (2.5 cm) in diameter. At the back of the

eye is the retina, which contains light-sensitive cells that connect

via neurons to the optic nerve. The space inside the eyeball is filled

with a jellylike substance. The front of the eye contains a hole (the

pupil), which has a clear lens behind it. Surrounding the pupil is a

circle of colored muscle, the iris, which controls how much light

enters the eye. The cornea, a clear membrane, covers them and

merges into the white outer membrane called the sclera.

Seeing things

The eye is capable of providing the

brain with an enormous amount

of detail about what it is looking

at. However, the image the brain

receives is inverted, so it has to be

flipped before we can understand it.

WHY DO MY EYES

CLOSE WHEN I SNEEZE?

When a nasal irritant triggers

the brain stem control center,

it causes widespread muscle

contractions, including those

in the eyelids. This makes

you blink momentarily.

Cornea is a

transparent layer

covering front of eye

Light rays start to refract

(bend) as they pass from

air and into cornea

L

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N

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IR

IS

P

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P

IL

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A

(^)
R
ET
IN
A
LIGHT
Lens and focusing
Behind the iris is the lens, where the
light rays are bent so the image forms on the
retina. The lens is connected to muscles that
allow it to change shape—it flattens for
distant objects and thickens for close objects.
2
Crossed-over rays
produce an inverted
image on retina
(^)
(^)
(^)
(^) S
CL
ER
A
(^) CH
OR
OI
D
Light enters the eye
Light passes through the cornea and
into the eye through the pupil. The pupil is
surrounded by a ring of colored muscle, the
iris, which can make the pupil contract or
dilate to vary the amount of light entering.
1
Choroid is a
blood-rich layer
that surrounds
retina
Lens is like a bag of
jelly that changes
shape to help
focusing
Iris is a ring
of muscle
Eyeball is encased
by sclera
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