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Jan. 25, 2010

Cow’s Eye Dissection

by Science Friday Education

Click to enlarge images

Science Friday Video: Laser Eye Surgery in Six Minutes
 
Each year, millions of Americans pay to have their eyeballs poked, prodded, suctioned, sliced and zapped with a laser. In exchange, their vision is corrected. In this video, we go inside an operating room at Acuity Laser Eye and Vision Center in Bethlehem, Pennsylvania, to watch Dr. Steven Vale, who has done over 20,000 surgeries, perform lasik eye surgery. See for yourself!
 
Grade Level: 6th – 8th grade
Subject Matter: Life Science
National Standards: NS.5-8.1, NS.5-8.3
 
Overview
The human eye may be only about the size of a ping-pong ball, but it is an amazingly complex sensory organ that requires all of its components to function properly in order for a person to have optimal vision. Each part of the eye works together with the others to process light rays into electrical impulses, or messages that are transmitted to the brain. The brain interprets this information and allows us to be aware of our surroundings. Our eyes and brain are able to capture and interpret millions of images a day, shaping our view of the world.
 
In this experiment, students will dissect a cow’s eye to learn the anatomy of the eye, and compare and contrast a cow’s eye and a human eye.
 
Activity Materials
N.B.: If your budget does not allow for each student to conduct a cow’s eye dissection, then divide students into groups or conduct the dissection as a demonstration. Review safety rules and proper use of scalpel and scissors with students before beginning activity. Students should perform dissection only under adult supervision.
 
Cow’s eye, preferably one for each student. Available at your local butcher or online.
Scissors, one pair for each student or group of students
Scalpel, one for each student or group of students
Cutting board, one for each student or group of students
Latex gloves, one pair for each student
 
Vocabulary
Pupil: circular opening in the center of the pigmented, or colored, iris of the eye, through which light passes to the retina.
Iris: an involuntary muscle that controls how much light enters the eye by changing the size of the pupil.
Retina: light-sensitive membrane that lines the back wall of the eyeball.
Rods and cones: light-sensitive cells on the sensory layer of the retina. Cones allow a person or animal to see color, and rods enable them to see shades of gray and motion.
Aqueous humor: a clear fluid that helps the cornea maintain its rounded shape.
Cornea: a tough, clear covering over the iris and the pupil that helps protect the eye.
Lens: a clear, flexible structure that bends light to project an image onto the retina.
Optic nerve: the bundle of nerve fibers that transmits information from the retina to the brain.
Vitreous humor: the thick, clear jelly that helps give the eyeball its shape.
Depth perception: the ability to judge how near or far an object is.
 
What To Do
  1. Start the lesson by having the students watch the Science Friday Video, “Laser Eye Surgery in Six Minutes.” Review with students the parts of the human eye that were operated on, that they saw in the video. Tell students that they are going to conduct a cow’s eye dissection to learn more about the human eye and how it works.
  2. Place the cutting board on a flat surface. Hand out a pair of latex gloves and a cow’s eye to each student who will be performing the dissection. Have all students put latex gloves on and begin examining the outside of the eye. Ask students to describe the texture of the eye. Can they identify any parts of the eye? Why do they think there is so much fatty tissue around the eye? Tell students that the fat protects the eye from being damaged by the surrounding bone.
  3. Have students slowly and carefully use the scissors to cut away the fatty tissue. Underneath, students will find red muscle tissue. Ask students what do they think the muscles do. Tell the students that cows have four muscles to move their eyes up, down, left and right, whereas humans also have an additional two muscles that allow clockwise and counter-clockwise eye movement.
  4. Have the students locate the cornea, a cloudy covering over the iris and pupil. Tell students that the cornea focuses most of the light that enters the eye. The cornea may be cloudy after death, but would have been translucent when the cow was alive. Have the students make an incision in the cornea. A clear liquid will leak out from the incision. This clear liquid is called the aqueous humor and helps the cornea retain its shape and nourishes the front part of the eye. Ask the students to notice how the human cornea bulges out by looking at another person’s eyes.
  5. Have students continue to cut the entire cornea out. Place the cornea on the cutting board and cut it with the scalpel. What did they hear as they tried cutting through the cornea? Tell students that the cornea is made of layers of interlocking fibers that strengthen the cornea so that it can protect the eye from harmful particles.
  6. Located underneath the cornea is the iris. Tell students to carefully pull the entire iris out and describe what they see. Can they explain the function of an iris? The iris is an involuntary muscle that will change the size of the hole in the center of it, called the pupil. Ask the students why is it necessary for the pupil to get bigger or smaller? How is the shape of a cow’s pupil different than that of a human?
  7. Ask students to feel and look for a small clear ball that looks like a marble. That is the lens. What does the lens on a camera or microscope do? How is this lens similar to the lens in a human eye? Have the students remove the lens and use the scalpel to peel off the different layers of the lens. Tell students that in the human eye, the lens grows in layers until the age of puberty.
  8. Ask students to squeeze out the jelly-like substance from the back of the eye. This is the vitreous humor. Ask students to explain what they think it is for. How is it similar to the aqueous humor? Tell students that the vitreous humor has a lot of protein in it, which gives it a jelly-like texture. Once the vitreous humor has been squeezed out, have the students compare how the eye felt before the dissection and how it feels now. What do they think is the purpose of the vitreous humor?
  9. Ask students to look for a cream colored layer behind the vitreous humor, towards the back of the eye. This is the retina, consisting of millions of light-sensitive cells (called cones and rods) that make up the surface of the retina. Ask the students why they think these light-sensitive cells are important.
  10. Ask students to move the retina around until they find a spot where the retina is attached to the back of the eye. Have the students describe what they see in that area. This bundle of nerve fibers is the called the optic nerve. Based on how the optic nerve seems to stretch towards the back of the cow’s head, what do they think the optic nerve does?
  11. After completing the dissection, have students place their cow’s eye and latex gloves in a plastic bag and dispose in the garbage. Equipment, work area and hands should be washed thoroughly.
What's Happening?
Our eyes need light in order to see. Light first enters the eye through the cornea, a tough covering of tissue that protects the outer layer of the eye. The light will then pass through the pupil, the small opening in the center of the iris.
 
In order to achieve the best vision possible, the iris will cause the pupil to change size depending on the brightness of the light. When the light is dim, the pupil will increase size to allow more light to enter the eye. In bright light, the pupil will decrease in size to avoid overexposure to light.
 
After passing through the pupil, the light rays will pass through the lens. The lens bends the light rays and works like a camera by focusing on an object and projecting that image onto the retina. However, since light will bend after passing through a lens, the image of the object on the retina will be upside-down. On the surface of the retina are two types of receptors called cones and rods. Cones allow us to see color and rods allow us to see shades of gray and motion. The image is then converted into an electrical impulse that travels through the optic nerve to the brain. The brain then interprets the information that it receives; even though the image is upside-down the brain will process the image right side up. However, the brain can sometimes visually perceive an image that differs from reality. This is known as an optical or visual illusion.
 
Topics for Science Class Discussion
  • What happens to the lens when someone is nearsighted or farsighted?
  • Describe the conditions in the eye that cause cataracts and glaucoma.
  • What are the three different types of optical illusions? Demonstrate examples of each type.
  • What is a blind spot and where is it located in the eye?
Extended Activities and Links
  • Draw an illustration that demonstrates how the individual components of the eye are similar to how a camera works.
  • Assign students to investigate various eye irregularities or conditions. Have them create a class presentation, explaining how these conditions occur and how to prevent them.
  • Ask students to define depth perception. Have students experiment with depth perception by holding a pencil sideways in each hand, so that the erasers of each pencil face one another. Have students close one eye and try to touch the erasers together. What happened? Why is it difficult to have the erasers connect? Have students try it with both eyes open. Did they achieve better depth perception with one or two eyes?
  • Explore how images are produced on the retina of the human eye as an object is moved closer or further away:
  • Students can research and explore the diversity of eyes in the animal kingdom:
  • Try these online activities from the Exploratorium that experiment with sensory perception:
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