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Your eyes and sight

What would you do if you had a problem with your eyes?  Well, for a start you wouldn’t be able to read this, because you couldn’t see the words!  Your eyes work all of the time, from the very moment you wake up and every moment until you close your eyes to go to sleep.  They work to help you to make sense of the world, by taking in everything around you, from the shapes of people, places and things to colours, and movement.  Every image you see is sent to your brain so you can figure out what’s going on and take any action if it is necessary!

Your eye is quite complex and has lots of different parts.  You can only see a very small amount of your eye (which you can see in a mirror) but it’s not a good idea to go poking around to find the other bits!  Your eye is very delicate and it can be easily damaged so its best to leave examinations to the optician!

If you look at your eye in the mirror, one of the first things you’ll see is the ‘white’ or your eye.  This is called the sclera and it is like a thick, tough skin that covers the outside of most of your eyeball.  If you look closely, you will see tiny pink threads.  These are your blood vessels and they make sure your eye has enough oxygen to work properly. 

In the middle of your sclera, at the front, is a clear piece.  This is the cornea and it is clear so you can see!  Your cornea is like a dome, it curves and helps light to focus on the right part of the inside of your eye. 

Right in the middle of your cornea is your iris.  This is the coloured part of your eye and you’ll see that different people have different colours in their iris.  Some people even have lots of colours in their iris.  This doesn’t mean that some people have been eyesight than others, it just means that people have different coloured eyes!  Your iris has very strong muscles called the ciliary muscles which help you to change the shape of your lens (learn most about your lens below) so you can see things that are close by and far away.

Right in the middle of your iris is your pupil.  This is a hole in the middle of your iris that lets light into your eye.  There are very strong muscles, right around your pupil.  These make the pupil bigger when there is a little light and smaller when there is lots of light so your eye has the proper amount of light it needs to see.  You can see this if you stand in front of a mirror in a brightly lit room.  Close your eyes really tightly and count to 10.  When you open your eyes, you should see you pupils get much bigger and then return to a smaller size as they get used to the change in the level of light!

The parts of the eye that you cannot see are very important. Your lens focuses light onto your retina, which is like a cinema screen at the very back of your eye.  It works like the lenses on a pair of glasses and can changes the direction of light as it goes into your eye.  The very strong ciliary muscles in your iris change the shape of your lens.  When they squeeze tight, your lens gets all squashed and you can see things that are close to you.  When the ciliary muscles relax, your lens gets thinner and you can see things that are far away.  Isn’t that clever?

Your eye is like a camera – you point it at something you want to see and it takes a picture!  Instead of printing a picture like a camera, when light goes through your lens, an image of the thing you are looking at is printed on your retina, which sends signals to your brain to tell you what you are looking at.  Your retina has two different types of cells that help your brain figure out what you are looking at.  There are about 120 million rods in each of your retinas!  They see black, white and grey and help you to see in the dark.  Rods also help you to tell what shape something is.  There are much fewer cones in your eye, about 7 million in each eye.  Cones work in the light and tell you what colour something is.  There are three types of cones that see red, green or blue colours.  Together with the rods, they turn whatever you are looking at into signals for your brain!

The spot in your retina where the signal from your rods and cones is sent to your brain is called your blind spot.  There are no rods or cones here so you don’t have an image on this part of your retina.  Your optic nerve carries the messages from your rods and cones to your brain and your brain figures out what the image is.  This all happens too quickly for us to notice – it all happens immediately and we don’t notice all the different parts of the eye working!

Your eye is about the same size and shape as a table tennis ball – but what keeps it from getting squashed?  There are two different types of liquid in your eye.  The aqueous humour fills up the space around your lens and it looks and feels just like water.   The vitreous humour fills up the space inside your eyeball.  This is thicker, like a jelly and it makes sure that your eye stays the shape it should be!

Your eyes are very precious and you need to protect them.  Your body has defences that help to make sure your eyes are in the best shape they can be, all of the time.  Your eyelashes are very sensitive to air, dust, water or anything else that gets close to your eye.  If they feel something coming, they make sure your eyelids snap shut at an amazing speed so your eyes are protected.  Your eyebrows act like a ledge over your eyes and they keep dirt and sweat out of your eyes (if you get sweat into your eyes it can really sting!). 

The best protection your eyes have against dust and microbes are your tears!  The lacrimal gland in the corner of your eye produces tears and every time your blink, a very thin layer of your tears is spread over your eyeball.  Small ‘invaders’ like dust get washed into the corner of your eye, next to your nose, just like a windscreen wiper on the windscreen of your car.  If you look carefully, you will see the start of the tiny tube, the lacrimal duct, which carries your tears back inside where they travel down to the back of your nose.  If you are hurt or sad or sometimes even if you are happy, the tears flood this little tube and they can run down your cheeks.  Some of the tears still make it down to the back of your nose, which is why your nose runs when you cry!

So, what happens if your eyeball is a different shape to a table tennis ball?  If this happens, the shape of your eyeball can make it difficult for your lens to focus images onto your retina.  This isn’t a problem for people today because glasses have been invented!  If you wear glasses, the lenses have a special shape that is right just for you.  The shape depends on the shape of your eyeball and it means that you can see things close by or far away (depending on what your problem was) when you couldn’t before!  Contact lenses do the same job, the lenses are just much smaller and fit into your eye.

Another problem people sometimes have with their vision is colour blindness.  This doesn’t mean that they are blind, it just means that they have problems telling which colour is which.  This can happen if a person doesn’t have enough cone cells (remember, the ones that see colour in your retina), or they are a different type.  Usually, it is boys who have a problem telling colours apart but it is in their genes (no, not the denim jeans!), something that is passed on from their parents, so they can’t do anything about it!

Your eyes are your windows to the world. Make sure you look after them!



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