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Cow's Eye Dissection

High School Explainer dissecting a cows eye in front of a group of fascinated kids
Cow's Eye Dissection




Watch: The steps of a cow's eye dissection

At the Exploratorium, we dissect cows’ eyes to show people how an eye works. This Web site shows photos and videos of a dissection. If you try this at home, wash your hands after the dissection. Wear latex gloves if you have cuts in your hands.

holding an intact cows eye
Step 1: The cow's eye
Here’s a cow’s eye from the meat company. The white part is the sclera, the outer covering of the eyeball. The blue is the cornea, which starts out clear but becomes cloudy after death.
holding a cows eye with muscles being demonstrated
Step 2: Muscles move the eye
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.
close up of scissors cutting away the fat from a cows eye
Step 3: Fat cushions the eye
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 it from bumping up against the bone and getting bruised. In the cow’s eye dissection, we cut away all the fat and muscle so that we can see the eyeball.
Using a razor to cut the cows eye
Step 4: The cornea protects the eye
A clear tough surface called the cornea covers the front of your eye and protects your eye. If you make a cut in the cornea, a clear fluid oozes out. That’s the aqueous humor, which is made of protein and water. The aqueous humor helps give the eye its shape.
Using scissors to cut the sclera in the middle of the cows eye
Step 5: 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. Watch the video to hear the crunch of a scalpel cutting through those layers.
Holding the ring of the iris from a cows eye
Step 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.
holding a dissected cows eye to expose the lens
Step 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 this photo, but it’s really clear. The clear goo around the lens is the vitreous humor. The eyeball stays round because it’s filled with this clear goo.
An explainer's face, seen upside down through the lens of a cow's eye
Step 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 backward. 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. (Learn more about that here.)
cow's eye lens magnifying text on a page
Step 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.
Holding a light blue cow's retina in hand
Step 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.
showing where the nerves connect with the cow's retina
Step 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.
showing the optic nerve at the back of the cow's eye
Step 12: 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!
holding the shiny blue-green tapetum of the cow's eye
Step 13: 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.



Learn: How does your eye work?

You see the world because light gets into your eyes. Your eye uses that light to make an image of the world inside your eye—just as a camera uses light to make a photograph.

Printable diagram

Diagram of the different parts of an eye

Experimenting with a Lens

To understand how your eye makes an image of the world, you need to know a little bit about lenses. Learn about lenses and experiment with a magnifying glass to discover how light makes an image of the world.

upside down candle flame projected on a wall through a magnifying glass
Upside-down candle
Using the lens of a magnifying glass, you can bend light to make an image of the world. Look at the photo on the left. We used a magnifying glass to produce an inverted image of a candle.
showing the candle in front of the magnifying glass
How it's done
This photo shows how the inverted image of the candle is made. You can try the same thing at home. For instructions on how to make an image with a magnifying glass, click here.

What's Going On?

Suppose you use your magnifying glass to make an image of a tree on a sunny day. You hold your lens between the tree and a piece of paper. You move the lens to just the right spot. Voilà! There’s an image of the tree. That image is made of light.

Sunlight bounces off the tree and spreads out in all directions. Your lens gathers the light shining out in all directions from each spot on that tree and bends that light so it all comes back together on a single spot on your piece of paper. So light shining from a leaf at the top of the tree ends up on one spot on your paper. Light shining from a spot on the tree’s trunk ends up in a different spot on your paper. All these spots of light blend together in your eye to make an image.

This works because the lens in your magnifying glass is carefully shaped to bend light in a particular, predictable way. The lens is shaped to bend light rays so that they come together and then spread apart to make an image.The lens of your magnifying glass is probably fat in the middle and thin at the edges. If you took the lens out of the magnifying glass, it would look like this image to the right. The surface of this lens is curved. It’s that curve that makes light bend when it shines through your lens.

drawing of a magnifying glass and it's lens

Lenses and light

We took this picture to show you what happens when several beams of light encounter a lens.

light against black background, encountering a curved lens and changing direction

When nothing gets in its way, light travels in a nice straight line. But when you put a chunk of clear glass or plastic in the way, light may not keep traveling straight. If light encounters a piece of glass or plastic at an angle, the light bends. The bigger the angle, the more the light bends. The light may bend again when it moves from inside the glass or plastic into the air. If the light meets the air at an angle, the light bends. The bigger the angle, the bigger the bend.

The beams of light shine through the clear plastic of the lens and encounter the curved surface where the plastic of the lens meets the air. Beams of light that meet this surface straight on keep on going straight. But some beams of light meet the curved surface at an angle. Those beams bend. This lens is curved on one side and flat on the other. The lens in your eye is curved on both sides, but it works in the same way.

Light and your eye

Once you know how lenses work, you can follow the path of light through your eye and find out how your eye uses light to make an image. How does light let you see these words? Here’s how:

diagram of how the eye sees an object in the world
  1. Suppose you’re looking out the window on a sunny day and you see a tree. You see that tree because light from the sun hit that tree. Some of that light reflected from the tree—it bounced off the tree like a ball bouncing off a wall.
  2. Some of that reflected light hit you right in the eye. That reflected light goes through the clear cornea of your eye. As it goes through the cornea, it bends a little.
  3. The light shines through your pupil, the dark hole in the middle of your eye.
  1. The light shines through the lens of your eye. The lens in your eye bends the light that has reflected from that tree to make a perfect little upside-down picture of the tree on the back of your eyeball. (Find out more about lenses and how bending light makes pictures.)
  2. At the back of your eyeball, there’s a layer of cells that are sensitive to light called the retina. When the picture of the tree shines on the retina, the light-sensitive cells send messages to your brain.
  1. Your brain takes the information from your retina and puts it together to make an image of the tree in your mind.
  2. Weird, isn’t it? You think you see the tree—but what you see is the light that bounced off the tree and got into your eye. Or if you really want to get picky, what you really see is the fixed-up picture that your brain makes up from the mixed signals it gets from your eye. Amazing!

Eye Links

light refracting through a glass bottle
Perception Snacks: These perception and eye-related activities are miniature versions of our museum exhibits.
Paul Doherty’s Eye Activities: Paul Doherty’s eye activities are part of the Exploratorium Teacher Institute’s month-long workshop for high school science teachers.
Pictures from Light: With a lens, you can bend light to make pictures of the world.
Human Body Explorations: This Exploratorium publication contains several eye-related activities, including Tunnel of Light, Something in Your Eye, and A Hole New Experience.
Close up of someone's eye looking to their upper left
Museum Exhibits
Seeing Collection: Learn more about the Exploratorium’s revitalized and expanded exhibit collection on vision and human perception.
Eye Exhibits: The Exploratorium has developed over 50 exhibits relating to the eye and perception. Many of these can be found in our partner museums.
Pathway: Looking Without Seeing: Can you always trust your eyes? This Field Trip Pathway will test the limit of your visual perception.
Pathway: Eyeballs: Investigate how we see things using this Field Trip Pathway.
Light spreading out against a dark surface
Other Eye Links
Eye-D Aim High Webcast: In this Exploratorium Webcast, middle school students from the Aim High program explore the topic of retinal scans.
Not Fade Away: Joel Deutsch shares his journey into blindness as his retina slowly deteriorates due to retinitis pigmentosa .
Sighting the First Sense: This student-created Thinkquest site provides an interactive introduction to the eye, vision, and perception.
The Joy of Visual Perception: This online book presents copious information and illustrations on diverse topics relating to human vision and perception.
Cut Out Dissection!: This page from PETAkids presents ethical arguments against animal dissection.



Do your own dissection


Download step-by-step instructions below for doing your own cow’s eye dissection. Instructions include an eye diagram, a glossary, and color photos for each step.

coweye.pdf (326.92 KB)

Producer: Noah Wittman

Content & Writing: Pat Murphy

Design & Implementation: Yael Braha

Photography: Amy Snyder

Videographer: Elisabeth M. Spencer

Video Producer: Dia Felix

Explainer: Kathryn Fleming

Editorial: Martha Steele

Content Review: Rilla Chaney, Paul Doherty, Karen Kalumuck, Darlene Librero, Linda Shore


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