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3 •The inner layer at the back of the eye is the retina, where light rays are interpreted into images. To complete the whole vision process, we need the optic nerves to transmit nerve impulses to the brain. The brain then adjusts and interprets the images of the world surrounding you. And that’s how we see.

4 •Iris This is the coloured, muscular section that adjusts the size of the pupil to control how much light enters the eye.

•Lens The lens is the main part of the eye that bends light rays on to the retina at the back of the eye.

Copyright © COG 2005 All rights reserved. No part of this book may be reproduced or copied in any form without written permission from the publisher.



Colour Vision Iris

Even though there is an incredible variation in body shapes and sizes, all humans share common features and the same working parts. The human body is built from trillions of cells. Similar cells group together to form tissues (a Latin word that means “woven”). Types of tissues include nervous tissue, muscle tissue, connective tissue and epithelial tissue. And when tissues group together, they can make organs that pump blood (heart), digest food (stomach) and breath air (lungs). The Bio Signs series is an assortment of 6 human anatomy models. The detailed models are the: Brain and Skull, Eye, Head and Torso, Heart, Skin, and Teeth and Gums.

•Vitreous Humour This is a clear fluid that fills up most of the eye to give it a spherical shape.


•Retina This is the back of the eye where light rays are interpreted into images. The retina consists of two light-sensitive cells called rod cells and cone cells. Rod cells are photoreceptors that detect shades of grey when it is dark. Cone cells are photoreceptors that detect colours.

Eyes are the body’s organs of sight. They are hollow fluid-filled spheres and their walls are made up of three different layers: •The Outer layer is the white sclera, which has a transparent window at the front of the eye called the cornea to let in light rays. •The middle layer includes the coloured iris.


Optic Nerve

Lens Vitreous Humour


This I Know Guide provides an introduction to: the Eye

•Optic Nerve The optic nerve runs out of the back of your eye and transmits nerve impulses directly to the brain.

A Light-Sensitive Retina The retina contains millions of

movement. In each eye, there is about 120 million rods. They are densest at the edge of the retina and do not need a lot of light to work. They are mainly used in dim light.


These are the main parts of the eye: •Cornea This is the front part of the eye and is filled with a clear fluid called aqueous humour. It can help bend light rays into the eye.


6 sensitive receptor cells. They respond to light entering your eye, which is why they are called photoreceptors. Photoreceptors contain visual pigments and when light strikes these pigments, they briefly lose their colour. This bleaching process produces nerve impulses, which are transmitted to the brain via the optic nerve. The optic nerve is connected to the retina where there are no photoreceptors. This part of the retina is known as the “blind spot”. There are two types of photoreceptor cells: rods and cones

Grey Vision and Movement Rod cells detect light in shades of grey and they can sense shape and

Cone cells see colour. There are three different types of cone cell, each of which contains a pigment that detects a different wavelength of light - green, red or blue. If the brain receives impulses from more than one type of cone at the same time, the result is mixed colours. In each eye, there is around 7 million cones. They are densest at the centre of your retina and need bright light to function. This is why, when it is dark, only shades of grey are detected.


Try This! Do you know that our eyes need to work together? If one eye looks at one thing while the other eye looks at something else, your brain can get very confused!


Secrets of the Lens Before light rays touch the retina, they travel through a disc-shaped lens. The lens bends light rays to focus them on to the retina. Muscles in the eye can change the shape of the lens, to help adjust the light-bending power. This allows the eye to focus on different objects.

3) Put your left hand against the left side of the tube near the end.

4) Keep both eyes open and slide your left hand towards your left eye.

Because people have two eyes, the brain also needs to merge the two slightly different images captured by each of the eyes into one. By doing so, the brain creates 3D images. Why Do Humans Need Two Eyes? Two eyes working together enable a person to see more accurately than one eye alone. Two eyes provide the brain with a remarkable amount of information.

9 Our eyes view an object from about the same height but from two separate positions. Therefore, two separate images are sent to the brain and then made into a single image. Thus, our brain is able to interpret the object in three dimensions and calculate how far away it is. This amazing system is called stereoscopic vision or 3D vision.


10 This experiment will show you why we need our eyes to work together.

1) Roll up a piece of paper into a tube, or use an empty cardboard tube.

2) Using your right hand, hold the tube in front of your right eye.

5) What happens? Do you have a



Why it works Obviously, you do not have a hole in your left hand! Your brain is interpreting the images from each of your eyes and combining them together. This is why your left hand appears “holey”. Do you know what causes vision defects?

•A slight misshaping of the eyeball can cause astigmatism (uneven focusing in different planes).

There are several reasons that may cause vision defects!

•A slight defect to the lens can cause either near-sightedness or far-sightedness.

1) Watching TV or looking at objects too closely or for too long.

•Near-sightedness is when faraway images are unfocused and appear blurred.

3) Having a family history of poor eyesight.

•Far-sightedness is when nearby images are unfocused and appear blurred.

2) Looking directly at the sun.


Obviously, you do not have a hole in your left hand! Your brain is interpreting the images from each of your eyes and combining them togethe...

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