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Exploring the Anatomy of the Eye – 60 Informal PointsIntroduction

Our senses allow us to communicate with the outside world. What we smell, hear, taste, touch and see direct our actions. But it is vision that dominates our impressions of the outside world. We use this sense every moment when we are awake and we even dream in visual images while we sleep. We blink. We stare. We cry. Our eyes not only interpret what we see, but they allow us to communicate our feelings and emotions. Each of us sees the world differently. Through the miracle of the eye, we take in the world around us and make it our own.

Your eye has the amazing ability to convert light into a stream of nervous impulses. Light is directed through the eye and an image of what we see is projected on the retina, at the back of the eye. Neurons then take information about this image to the brain for processing and interpretation. Each structure in the eye plays a unique role in assuring that the image you see is the image before you.

One way to figure out how something works is to look inside it. In this activity, you will use your power of sight to learn about the power of sight. To investigate how the eye works, you will dissect a cow’s eye and observe its unique structure. As you complete the dissection, see if you can figure out the specific function of each part of your eye. Observe how all the parts of an eye work together to allow you to take in a brilliant sunset, an exciting sporting event, or a scary movie.

Procedure1. View the drawing of the eye found at the Exploratorium site

http://www.exploratorium.edu/learning_studio/cow_eye/eye_diagram.html and copy this labeled drawing into the space below, using color and labeling all parts or making a key.

2. Fill in the table below to describe the function of each eye part. Put the cursor over the labeled eye part of the pervious website to get the description. You may have to do some additional research for the functions. Some website that may help are:

Exploratorium: The Eye http://www.exploratorium.edu/learning_studio/cow_eye/eye_diagram.html

Neuroscience for Kids: The Eye http://faculty.washington.edu/chudler/bigeye.html

Eye Structure Description & Function

Retina

The layer of light sensitive cells at the back of the eye. The retina detects images focused by the cornea and the lens. The retina is connected to the brain by the optic nerve.

Cornea

A tough, clear covering over the iris and the pupil that helps protect the eye. Light bends as it passes through the cornea. This is the first step in making an image on the retina. The cornea begins bending light to make and image; the lens finishes the job.

Pupil

The pupil is the dark circle in the center of your iris. It’s a hole that lets light into the inner eye. Your pupil is round. A cow’s pupil is oval.

Aqueous Humor

A clear fluid that helps the cornea keep its rounded shape.

Iris

A muscle that controls how much light enters the eye. It is suspended between the cornea and the lens. A cow’s iris is brown. Human irises come in many color, including brown, blue, green and gray. It gives you your eye color!

Lens

A clear, flexible structure that makes an image on the eye’s retina. The lens is flexible so that it can change shape, focusing on objects that are close up and objects that are far away.

Vitreous Humor

The thick, clear jelly that helps give the eyeball its shape.

Sclera

The thick, tough, white outer covering of the eyeball.

Tapetum

The colorful, shiny material located behind the retina. Found in animals with good night vision, the tapetum reflects light back through the retina.

Optic Nerve

The bundle of nerve fibers that carry information from the retina to the brain.

The place where the optic nerve leaves the retina. Each eye has a blind spot where there are no light-sensitive cells.

Blind Spot

3. In preparation for the actual cow eye dissection visit the following website and list the 13 steps with their descriptions. These steps will be followed during the dissection. Website: http://www.exploratorium.edu/learning_studio/cow_eye/step01.html

StepNumber

Step Title Step Description

1

The Cow’s Eye Examine the outside of the eye. See how many partsof the eye you can identify. You should be able to find the whites (or sclera), the tough, outer covering of the eyeball. You should also be able to identify the fat and muscle surrounding the eye. You should be able to find the covering over the front of the eye (the cornea). When the cow was alive, the cornea was clear. In your cow’s eye, the cornea may be cloudy. You may be able to look through the cornea and see the iris, the colored part of the eye, and the pupil, the dark oval in the middle of the iris.

2

Muscles Move the Eye and Fat cushions the eye

Cut away the fat and muscle. Without moving your head, look up. Look down. Look all around. Six muscles attached to your eyeball move your eye so you can look in different directions. Cows have only four muscles that control their eyes. They can look up, down, left, and right, but they can’t roll their eyes like you can.If you reach up and feel around your eye, you’ll feel the bone of your skull. There’s fat surrounding your eyeball to keep if from bumping up agains the bone and getting bruised.

3

The cornea protects the eye

A clear tough surface called the cornea covers the front of your eye and protects your eye. Use a scalpel to make an incision in the cornea. (Careful— Don’t cut yourself!) Cut until the clear liquid under the cornea is released. That clear liquid is the aqueous humor. It’s made of mostly of water and keeps the shape of the cornea; it gives your eye its shape!

4

Cutting the Sclera Now we are going to cut through the sclera and divide the eye in half, right around the middle. The cornea will be on the front half of the eye. The cornea is made of many layers of tissue. Use the scalpel to make an incision through the sclera in the middle of the eye.

5

Cutting the Eye in half Use your scissors to cut around the middle of the eye, cutting the eye in half. You’ll end up with two halves. On the front half will be the cornea.

The cornea is made of pretty tough stuff—it helps protect your eye. It also helps you see by bending the light that comes into your eye.

Once you have removed the cornea, place it on the board (or cutting surface) and cut it with your scalpel or razor. Listen. Hear the crunch? That’s the sound of the scalpel crunching through layers of clear tissue.The cow’s cornea has many layers to make it thick and strong. When the cow is grazing, blades of grass may poke the cow’s eye— but the cornea protects the inner eye.

6

The pupil lets in light IF you look at your eye in a mirror, you’ll see a colored circle with a black spot in the middle. The colored circle is the iris. The black spot in the middle of the iris is the pupil, a hole through the iris that lets light into the eye. In dim light, the pupil opens wide, letting lots of light in.

The next step is to pull out the iris. The iris is between the cornea and the lens. It may be stuck to the cornea or it may have stayed with the back of the eye. Find the iris and pull it out. It should come out in one piece. You can see that there’s a hole in the center of the iris. That’s the pupil, the hole that lets light into the eye. The iris contracts or expands to change the size of the pupil. In dim light, the pupil opens wide to let light in. In bright light, the pupil shuts down to block light out.

7

The lens Here is the back half of the eye. With the cornea and the iris out of the way, you can see the lens. It looks gray in the photo, but it’s really clear. The clear good around the lens is the vitreous humor. The eyeball stays round because its filled with this clear goo.

The back of the eye is filled with a clear jelly. That’s the vitreous humor, a mixture of protein and water. It’s clear so light can pass through it. It also helps the eyeball maintain its shape.

Now you want to remove the lens. It’s a clear lump about the size and shape of a squashed marble.

8

Looking through the lens

Here’s what you see if you look through the lens. Everything on the other side looks upside down and backwards. You can use the lens to make an image, a picture of the world. That’s what the lens does in your eye. It makes a picture of the world on your retina.

The lens of the cow’s eye feels soft on the outside and hard in the middle. Hold the lens up and look through it. What do you see?

9

The lens is a magnifier

Here, the lens works like a magnifying glass, making the words look bigger. The lens of the cow’s eye (like the lens of your eye) is shaped like the lens of a magnifying glass. It’s thicker in the middle than it is at the edges.

Put the lens down on a newspaper and look through it at the words on the page. What do you see?

10

The Retina Here’s the back of the eye with the lens and vitreous humor removed. It’s shaped like a bowl. On the inside of the bowl is a thin film with red blood vessels running through it. The retina contains light-sensitive cells that detect light.

Now take a look at the rest of the eye. If the vitreous humor is still in the eyeball, empty it out. On the inside of the back half of the eyeball, you can see some blood vessels that are part of a thin fleshy film. That film is the retina. Before you cut the eye open, the vitreous humor pushed against the retina so that it lay flat on the back of the eye. It may be all pushed together in a wad now.

The retina is made of cells that can detect light. The eye’s lens uses the light that comes into the eye to make an image, a picture made of light. That image lands on the retina. The cells of the retina react to the light that falls on them and send messages to the brain.

11

The retina is attached in one spot

The retina is attached to the back of the eye at just one spot. It’s called your blind spot. Because there are no light-sensitive cells at that spot, you can’t see anything that lands in that place on the retina. At the blind spot, all the nerves from the retina join to form the optic nerve.

Use your finger to push the retina around. The retina is attached to the back of the eye at just one spot. Can you find that spot? That’s the place where nerves from all the cells in the retina come together. All these nerves go out the back of the eye, forming the optic nerve, the bundle of nerves that carries messages from the eye to the brain. The brain uses information from the retina to make a mental picture of the world.

The spot where the retina is attached to the back of the eye is called the blind spot. Because there are no light- sensitive cells at that spot, you can’t see anything that lands in that place on the retina.

12

The tapetum Here’s the inside of the back of the eye again. Behind the retina is a layer of shiny, blue-green stuff called the tapetum. This layer assists night vision by reflecting light back through the retina. You don’t have a tapetum, but cats and cows (and other animals) do. A cat’s eyes shine in the headlights of a car because of the tapetum.

Under the retina, the back of the eye is covered with shiny, blue-green stuff. This is the tapetum. It reflects light from the back of the eye.

Have you ever seen a cat’s eyes shining in the headlights of a car? Cats, like cows, have a tapetum. A cat’s eye seems to glow because the cat’s tapetum is reflecting light. If you shine a light at a cow at night, the cow’s eyes will shine with a blue-green light because the light reflects from the tapetum.

13

The optic nerve carries messages to the brain

At the back of the eye, you can see the optic nerve, which carries messages from the retina to the brain. You see the world because your lens makes a picture on your retina, the retina sends a message to your brain, and your brain turns that message into a mental picture of the world!

Look at the other side of the back of the eye. Can you find the optic nerve? To see the separate fibers that make up the optic nerve, pinch the nerve with a pair of scissorsor your fingers. If you squeeze the optic nerve, you may get some white goop. That is myelin, the fatty layer that surrounds each fiber of the nerve.

4. Use the table in #3 and the pictures from the website in #3 to complete an actual cow eye dissection. As you complete the dissection follow the directions below and answer the following questions.

5. Put on safety goggles and gloves. Obtain a cow’s eye, a dissecting tray and a set of dissecting instruments. Place the cow eye in the tray.

6. First, look at the external anatomy of the eye. See how many parts you can identify just by looking at your drawing.

7. Observe the fat attached to the outside of the eye. Remember the fat pads you built behind the eye of your Maniken®. What purpose do you think this fat serves?

8. Find muscle attached to the side of the eye. Why do you think the human eye is surrounded by six different muscles? What roles do these muscles play?

9. If the eyelid is still present, note its structure and the presence of eyelashes. What is the function of the eyelid and eyelashes?

10. Find the sclera, the tough outer covering of the eye also called the “whites” of the eye. Feel the sclera. Do you think this structure is designed for strength or clarity? Why?

11. Using your scissors, trim the fat and the muscle tissue from the eye.

12. Find the cornea, the covering over the front of the eye. When the cow was alive, the cornea was clear. Describe the membrane in the space below.

13. Note that the cornea does not have any blood vessels. Given the function of the cornea, why do you think it is important that the cornea is clear and free of blood vessels?

14. Look through the cornea and see the iris, the colored part of the eye. Note the pupil, the dark oval in the middle of the eye. What shape is the pupil in a human eye?

15. Orient the eye in the tray so the cornea is to the side. Fluid is going to flow out of the eye once you make a cut. You do not want to spray yourself or your partner. Use a scalpel to make an incision in the middle of the cornea. Cut until the liquid under the cornea is released. This watery fluid is called the aqueous humor.

16. Prepare to cut through the sclera, divide the eye in half, and remove the cornea. Use the scalpel to make an incision through the sclera on the side of the eye- about 2-3cm posterior to the cornea. Put the point of your scissors in the incision and carefully cut around the sclera to remove the front of the eye. Return to the video or the screen shots if you have trouble visualizing this step.

17. Note the thickness of the cornea. Place the cornea on your tray and cut into it with a scalpel. Listen to the crunch. The many layers of the cornea help protect your eye.

18. Gently pull the iris out from the eye. Since you have removed the cornea, this colored part should be visible and should come out in one piece. Note the hole in the middle. Remember, this is the pupil.

19. What happens to the pupil when you shine a light into a person’s eye? Why?

20. What is the function of the pupil?

21. Find the lens of the eye suspended in a jelly-like fluid called the vitreous humor. With your fingers or the scalpel, carefully remove the lens. It is a clear lump inside the vitreous humor.

22. Now see what the lens can do. Hold the lens up in the direction of your partner and look through it. What do you notice about the image you see? NOTE: If the eye is not fresh, you may have trouble seeing through the lens.

23. Put the lens down on your paper and look through it to words on the page. If you are using a preserved eye, you may need to shine a light on the lens to see the words. What do you notice?

24. Examine the posterior half of the eyeball. If the vitreous humor is still in the eyeball, carefully remove this fluid and observe the thin lining that detaches easily from the inner surface of the eyeball. This is the retina.

25. If the retina is not already hanging from the back of the eye, use forceps to gently pull the retina away from the back wall.

26. Note that the retina is the sensory layer of the eyeball which contains receptors for sight, called rods and cones. The eye uses light coming in to make an image, a picture which lands on the retina. Seeing the state of the retina after removing the fluid, what do you think would happen to the retina if this fluid was not present? How would this affect vision?

27. Notice how the retina is attached at one point called the optic disk. The optic disk is the point at which all the nerves from the retina converge and exit the eye via the optic nerve. Light can not stimulate this point on the retina; this point is referred to as the blind spot.

28. Locate the optic nerve. You may find this spot by locating a patch of white goop. This goop is the myelin around the neurons of the optic nerve. What is the function of this myelin?

29. Note that the optic nerve takes messages back to the brain and interprets the signal projected on the retina. Which area(s) of the brain is (are) responsible for processing these signals? NOTE: You can refer back to your brain diagram.

30. Observe the iridescent layer of the inner eyeball behind the retina. This is the tapetum. Humans do not have this layer in their eyes. Based on the function of this structure, why don’t we see a tapetum in the human eye?

31. Discard the tissue as directed by your teacher and clean your tray and instruments.

Conclusion Questions1. Trace the path of light from the time it enters the eye to the time it leaves the eye to travel to the brain.

You should refer to your labeled eye diagram.

light through the eye to the cornea aqueous humor through pupil lens vitreous humor retina optic nerve brain the occipital lobe

2. Is the response of your pupil a reflex or a voluntary action? Describe how this response is controlled by the nervous system.

A reflex because the smooth cells of the radial muscle, controlled by the sympathetic nervous system contract, dilating the pupil. The pupil constricts when the circular muscle, controlled by the parasympathetic nervous system, contracts. It responds to the amount of light in the environment so that as much can be seen as possible.

3. Describe two ways in which the cow eye and the human eye differ? How are these differences linked to function?

Cow eyes have a tapetum, which reflects light so that the cow can see in the dark. Humans do not have a tapetum because they are not nocturnal. Cow eyes also have oval shaped pupils so that they can see more. Cows have eyes on the side of their head, so the pupils need to be larger to accustom to not being able to see a full 360 degrees like humans, who have round pupils.

4. Describe how what you see can impact other human body systems. Provide at least five specific examples of how your communication with the outside world through your eyes initiates a response in another body system. For example, when you see a ball whizzing towards you, your muscular and skeletal systems are activated to move your limbs and catch the ball.

1. The eye sees good food and signals the nervous system to be happy and hungry,2. The eye sees that you are underwater and signals to the respiratory system to hold your breath.3. The eyes see a bee and signal to the muscular system to swat it.4. The eye sees something gross and signals to the digestive system to vomit.5. The eye sees a dog and signals to the skeletal system to pet it.

5. Describe at least three specific ways that we communicate with the outside world and at least three ways (other than sight) in which the world communicates to us.

We communicate with the world through speech, audio, and touch and the world communicates with us by providing stimuli for these senses to pick up.