BIOLOGY CT-2 TEXT

6.03 The eye

How you see things

1 Light goes from an object to

your eye.

2 Light is bent as it passes

through your eye.

3 An upside-down picture of

the object is focused on a

layer called the retina at the

back of your eye. The retina

is made of cells sensitive to

light.

4 The retina sends messages

along the optic nerve to your

brain. Your brain allows you

to see a picture of the object

which is the right way up.

How eyes are protected

Your eyes are set in holes called orbits in your skull. So all except the

front of each eye is protected by bone.

There are tear glands behind

the top eyelid. They make tears

that wash your eye clean when

you blink. Tears are produced

faster if dust or smoke get into

your eyes.

The iris is the coloured part of

the eye. It is a ring of muscle

with a hole called the pupil in

the middle. It protects the eyes

from bright light.

The pupil lets light into the eye.

If the light is too bright the iris

muscle makes the pupil smaller.

In dim light the iris muscle

make the pupil bigger.

The eyelashes form a net in

front of the eye which protects it

from dust.

The white of the eye is a tough

protective layer called the

sclerotic.

The parts of the eye

The conjunctiva is a thin clear

skin which covers the front of

the eye.

Ciliary muscles change the

shape of the lens during

focusing.

The yellow spot is the most

sensitive part of the retina. It

lets you see colour.

The iris controls the amount of

light entering the eye.

The front part of the eye is filled

with a watery liquid called

aqueous humour.

The cornea is a clear window in

the sclerotic in front of the iris.

It lets light into the eye.

The lens helps focus a picture

on the retina. The lens is clear

and can change shape.

The suspensory ligaments hold

the lens in place.

The sclerotic layer is the tough,

white protective layer of the

eye.

The back of the eye is filled with

a jelly called vitreous humour.

The choroid is a black layer that

stops light being reflected round

the inside of the eye.

The retina is a layer of cells

which are sensitive to light.

They send messages to the

brain.

The blind spot is where blood

vessels and nerves join the

eyeball. It has no light-sensitive

cells, so it sends no messages to

the brain.

6.06 The nervous system

Your nervous system is like a

manager inside your body. Its job

is to control and coordinate the

parts of your body so that they

work together, doing their jobs at

the right time.

Your nervous system coordinates

muscles so that you can do things

which need thought, like cycling,

dancing or reading.

It also coordinates things which

you don't need to think about, like

heartbeat and breathing.

The nervous system consists of the

brain, the spinal cord, and

millions of nerves. Together the

brain and spinal cord are called

the central nervous system.

The brain is at the top of the

nervous system. It is protected

by the skull.

The spinal cord is a very thick

nerve. It hangs from the brain

down through the hollow

middle of the backbone.

Millions of nerves branch from

the central nervous system.

They carry messages called

nerve impulses around the

body.

A nerve is a bundle of nerve

fibres.

Nerve

Nerve cells (neurones)

The nervous system is made up of nerve cells called neurones. Most body

cells are rounded but neurones are not. They are stretched out into long

thin nerve fibres that can be over a metre long. Nerve impulses travel

along nerve fibres in only one direction.

Sensory neurones carry impulses from sense organs

into the central nervous system.

Motor neurones carry impulses from the central

nervous system to effector organs, the muscles and

glands which respond to a stimulus.

How the nervous system works

If you sat on a drawing pin you would jump up yelling with pain. This is an

example of a stimulus and a response. The stimulus is pain. The response is

jumping and yelling. Your nervous system controls the response.

1 A stimulus is received by a sense

organ — the receptor. This sends

nerve impulses along sensory

neurones to the central nervous

system — the coordinator of this

sequence of events. It works out

(coordinates) an appropriate

response to the stimulus and

sends impulses along a relay

neurone and motor neurone to

effector organs. These are the

muscles or glands which carry

out the response.

Relay neurone

2 There is a tiny gap called a

synapse where the nerve fibre of

one neurone meets the cell body

of another. A chemical is released

by the first neurone which

causes the impulse to jump this

gap to the next neurone.

3 Only strong stimuli generate

enough impulses to cause these

chemicals to be released and let

impulses cross a synapse. Nerve

impulses must cross many

synapses as they pass from

sensory neurone fibres through

the central nervous system. This

is why very weak stimuli do not

cause a response in an effector

organ.

Summary

The sequence of events between a stimulus and appropriate response can

be summarized as follows:

Stimulus Receptor —4 Coordinator Effector organ Response

6.04 Vision

The light that goes into your eye has to be bent or focused on to the retina

to let you see clearly.

Most of the bending of light takes place as it passes through the cornea

and aqueous humour. The lens bends it a little more to make a perfectly

clear picture on the retina. The ciliary muscles change the shape of the

lens to bend light. A fat lens bends light more than a thin lens.

To see a near object

Light from a near object needs to be bent more than light from a distant

object. So the lens is made much fatter.

If the lenses in your eyes don't

focus properly, wearing glasses

will help.

To see a near object:

1 The ciliary muscles are in a

circle round the lens. When they

contract, the suspensory

ligaments become slack, which

allows the lens to become fatter

in shape.

2 The lens can now bend light

enough to make a clear picture

on the retina of a near object.

To see a far object:

Light from a distant object needs to be bent very little. So the lens is

stretched to make it thin.

I When the ciliary muscles relax,

pressure inside the eyeball

stretches the lens into a thin

shape. the ligaments taut

2 The thin, flat lens bends light

just a little, to make a clear

picture on the retina of a distant

object.

Three-dimensional vision

Each of your eyes gets a slightly

different view of an object.

Your brain puts these two views

together, so that you see the

object as three-dimensional rather

than flat.

Three-dimensional vision helps

you judge how far away an

object is.

Two eyes

Rabbits, chickens, fish, and many

other animals have eyes which

look sideways, and not forwards

like yours. Each eye sees a

different view. They can even see

what is happening behind them.

This is useful if other animals hunt

you for food!

Rabbits can see what's going on at

each side ..

. but owls look straight ahead.

More about the retina

There are tsvo kinds of light-sensitive cells in the retina. They are called

rods and cones.

Cones only work in bright light,

but give a very clear picture and

are sensitive to colour. The

yellow spot in the middle of the

retina is made entirely of cones.

If you want to see something very

clearly you look straight at it, so

that its picture falls on the yellow

spot.

The rest of the retina is mostly

rods, with a few cones. Rods do

not give as clear a picture as

cones and are not sensitive to

colour. But rods work in dim

light.

This explains why you don't see

colours clearly in dim light.