Childrens Illustrated Encyclopedia

201

EyEball

Three pairs of muscles turn the

eyeball to look up, down, and from

side to side and rotate it. Pads of

fat cushion the eye and the optic

nerve, which is stretched and

pulled by eye movements.

Rods and conEs

The retina contains millions

of light-sensitive cells called

rods and cones. The rods

work best in dim light, while

cones are sensitive to different

colors. Rods and cones

produce nerve signals when

clEaR and dEfEcTivE vision light falls on them.

clear vision depends on the lens bending light

rays to the correct angle so that the rays form

a sharp picture on the retina. in far-sighted

people, the eyeball is too short and nearby

objects appear blurry. in near-sighted people,

the eyeball is too long, making distant objects

out of focus. Glasses and artificial lenses, such

as contact lenses, help the eye’s own lens to

focus the rays correctly, thus correcting

defective vision.

innER EyE

inside the eye is the retina, which

contains about 120 million rod cells,

mainly around the sides, and seven

million cone cells, mainly in the

fovea. The image on the retina is

upside down, but the brain turns it

the right way up as it processes nerve

signals from the eye.

ouTER EyE

light rays enter the curved front of the eye called the

cornea, where they are partly focused. They pass through

the pupil, which enlarges in dim conditions to let in more

light and shrinks in bright conditions to protect the inside

of the eye from too much light. The rays are then focused

on to the retina by the lens.

nEaR-siGhTEdnEss

Rays are focused in

front of retina. a

concave lens corrects

the focus.

faR-siGhTEdnEss

Rays are focused

behind the retina.

a convex lens

corrects the focus.

as you REad This PaGE, you are using the two

organs of sight—the eyes. our eyes enable us to

learn a great deal about the world around us. Each

eyeball measures about 1 in (25 mm) across and sits

in the front of the skull in the eye socket, or orbit.

The eyes can swivel around in their sockets so that

you can see things above, below, and to the side.

Each eye has an adjustable lens and sees a slightly

different view of the same scene. The eyes work together, controlled by

the brain. This is called binocular vision. The lens of each eye allows rays

of light to enter from the outside and project a picture on to the retina—

the inner lining of the eye. The retina converts the light into nerve signals

that travel along optic nerves to the brain, where images are perceived.

Muscles

anchored at

back of eye

socket move

the eye.

Sclera

Pupil is a hole

within the iris.

Choroid, containing

nourishing blood vessels

Retina, bearing light-

sensitive cells

Sclera—tough

outer covering

Cornea is like a

transparent window in

the front of the eyeball.

The cornea partly

focuses light rays.

Conjunctiva—thin layer

covering white of the eye

Lens fine-focuses

light rays.

Blind spot, containing no

light-sensitive cells, where

optic nerve leaves eye

Optic nerve

to brain

Eyes

EyE sockETs

The eyelid and eyelashes protect

the front of the eye. When you blink, the

eyelids sweep moisture over the eyeball,

keeping it clean. The moisture is

produced in the tear glands above the

eyes. These glands also produce tears

when you cry. Tiny holes drain fluid

through tear ducts into the tear sac,

which empties

inside the nose.

Openings of

tear (lacrimal)

gland

Tear

duct

Tear

sac

Iris

makes

pupil

larger or

smaller.

Find out more

cameras

color

Ears

human body

light

EaGlE siGhT

a golden eagle has

extremely powerful

eyesight. it can see rabbits

and other prey from a

distance of more than

half a mile (1 km).

Eye muscle

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