In this issue:
The human eye is one of the most complex and sophisticated organs in the body. Despite being small and extremely delicate, the eye allows us to see our world without any conscious effort on our part. The eye automatically adjusts its light sensitivity, allowing us to see both in starlight and the brightest sunlight. Its unique automatic focusing system outstrips that of any camera. While a camera lens must be moved back and forth to adjust for distance, the lens of the human eye simply changes shape. (This, thankfully, means that we don't need long tubes sticking out of our eyes!) Each fragile part of the eye works together to provide information to the brain, and the brain interprets it instantaneously giving you a perfect image. It is an amazing process.
A cow eye is very similar to the eye of a human. By dissecting and examining the anatomy of a preserved cow eye, you can learn how your own eye forms images of the world and sends these images to your brain. If you are a bit squeamish about dissecting, just take a look at our photos to see what the eye looks like inside!
>> Use our eye dissection images to learn the parts of the eye. Includes labeled versions and fill-in-the-blank versions so you can test yourself!
This eye dissection kit comes with everything you need, or you can buy what you need individually:
Observation: External AnatomyLook carefully at the preserved cow eye. The most noticeable part of the eye is the large mass of gray tissue that surrounds the posterior (back) of the eye and is attached to the sclera. The second most noticeable part of the eye is the cornea, located in the anterior (front) part of the eye. Due to the fact that the eye has been preserved, the cornea is cloudy and bluish-gray in color. It may also be wrinkly and seem a bit 'deflated'. On the posterior side of the eye, nestled in the fat and muscle tissue, there is a noticeably round protuberance that feels stiffer than the surrounding tissue. This is the optic nerve, and it sends the images collected in the eye to the brain.
Dissection: Internal Anatomy
1. Place the cow eye on a dissecting tray. The eye most likely has a thick covering of fat and muscle tissue. Carefully cut away the fat and the muscle. As you get closer to the actual eyeball, you may notice muscles that are attached directly to the sclera and along the optic nerve. These are the extrinsic muscles that allow a cow to move its eye up and down and from side to side. Keep cutting close to the sclera, separating the membrane that attaches the muscle to it. After removing the excess tissue, the sclera and optic nerve should be exposed but still intact.
2. Using a sharp scalpel, cut through the sclera around the middle of the eye so that one half will have the anterior features of the eye (the cornea, lens, iris, and ciliary body) and the other half will contain the posterior features like the optic nerve. The inside of the eye cavity is filled with liquid. This is the vitreous humor. Depending on how the specimen was preserved, it will be either a dark liquid that will flow out easily, or a slightly gelatinous material that you can pour out to remove. (In a living eye, the vitreous humor is clear and gel-like.)
3. Flip the anterior half of the eye over so that the front of it is facing upward. Using a pair of sharp scissors, cut the cornea from the eye along the boundary where the cornea meets the sclera. When the scissors have cut in far enough, a clear fluid will start to seep out - this is the aqueous humor. After removing the cornea, pick it up and look through it. Although it is cloudy due to the degrading of the tissue, it is still fairly transparent. Notice the toughness and strength of the cornea. It is designed this way to protect the more delicate features found inside the eye.
4. With the front of the anterior half of the eye facing up, locate the iris. Notice how the iris is positioned so that it surrounds and overlaps the lens. This position allows the iris to open and close around the lens to allow different amounts of light into the eye. In bright light, the iris contracts to let in less light. In dim light, such as at night, the iris expands to let in more light.
5. Flip the anterior half over and examine it from the back. Locate the lens and ciliary body. The ciliary body surrounds the lens, changing the shape of the lens to help the eye focus.
6. Next, pull the lens out. While the cow was alive, the lens was clear and very flexible. In a preserved cow eye, the lens will most likely have yellowed and become very hard. However, it may still be possible to look through the lens and see its ability to magnify objects. Try this by placing the lens on a piece of paper with writing on it.
7. Inside the posterior half of the eye, there is a thin, tissue-like material that slides easily inside the sclera. This is the retina. The retina contains photoreceptor cells that collect the light entering the eye through the lens. These images are sent to the optic disc, the spot where the optic nerve attaches to the eye. At this place, there are no photoreceptor cells; there are only nerves sending images to the brain. This place in the eye is often referred to as the blind spot since no images can be formed here. See the next project to learn more about the blind spot.
8. Most of the retina is not attached to the eye. Instead, it is held in place by fluids in the eye. The tissue of the retina gathers at the back of the eye where it forms into the optic nerve. This is the only place where the retina is attached to the eye. Use a pair of tweezers to gently lift the retina off the inside wall of the eye. It is very delicate, so it may tear. Underneath the retina you will find a very shiny and colorful tissue. This is the choroid coat. The choroid coat is also known as the vascular tunic because it supplies the eye with blood and nutrients. In a human eye, the choroid coat is very darkly colored to minimize the reflection of light which would cause distorted images.
9. Notice that the choroid coat in the cow's eye is very colorful and shiny. This reflective material is the tapetum lucidum, and its reflective properties allow a cow to see at night by reflecting the light that is absorbed through the retina back into the retina. (While this does allow the cow to see better at night than humans can, it distorts the clarity of what the cow sees because the light is reflected so much.) The tapetum lucidum is also responsible for the 'glowing' eyes of animals, such as cats, when a small amount of light reflects off the tapetum lucidum in an otherwise dark room.
| Try It Out: |
Cow Eye Dissection Kit
Get an inside view of how the eye works! A cow eye dissection is a memorable way to learn about anatomy. This complete dissection kit comes with a preserved cow eye, an illustrated dissection guide, #22 broad-blade scalpel, scissors, and a sturdy disposable dissecting tray. You'll get see how a cow's eye is like a human eye as you see parts from the iris and lens to the colorful choroid layer.
The spot where your optic nerve connects to your retina is called the optic disc. There are no photoreceptor cells on this disc, so when an image hits that part of your retina, you can't see it. This is your blind spot. You don't notice this blind spot in every-day life, because your two eyes work together to cover it up. To find it, draw a filled-in, 1/4'-sized square and a circle three or four inches apart on a piece of white paper.
Hold the paper at arm's length and close your left eye. Focus on the square with your right eye, and slowly move the paper toward you. When the circle reaches your blind spot, it will disappear! Try again to find the blind spot for your other eye. Close your right eye and focus on the circle with your left eye. Move the paper until the square disappears.
What happened when the circle disappeared? Did you see nothing where the circle had been? No, when the circle disappeared, you saw a plain white background that matched the rest of the sheet of paper. This is because your brain 'filled in' for the blind spot - your eye didn't send any information about that part of the paper, so the brain just made the 'hole' match the rest. Try the experiment again on a piece of colored paper. When the circle disappears, the brain will fill in whatever color matches the rest of the paper.
The brain doesn't just match colored backgrounds. It can also make other changes to what you see. Try drawing two filled-in rectangles side by side with a circle in between them. A few inches to the right of this, draw a square.
Close your right eye and focus your left eye on the square. Move the paper until the circle disappears and the two separated bars become one bar. How did that happen? The circle in between the bars fell on your blind spot. When it disappeared, the brain filled in for the missing information by connecting the two bars!
Here is one final experiment with your blind spot. In this instance the brain doesn't match the blind spot with its immediate white background, but instead with the pattern surrounding it. Draw a line down the center of your page. On one side draw a small square and on the other draw rows of circles. Color the center circle red and all the others blue.
Close your left eye and look at the square with your right eye. As you move the paper, the red circle should disappear and be replaced by a blue one!
The brain is very busy when your eyes are working, and not just in hiding your blind spot. The image that forms on your retina is upside down and backwards; your brain has to flip it right-side up. The photoreceptor cells on your retina are sensitive to red, blue, and green - so your brain calculates the percentages of those three colors in order to determine what colors you see: purple, or orange, or something else!
Tears. There are two situations when we cry: when there is an irritant in our eye that needs to be washed away, and when we are under the influence of strong emotion. Interestingly, the tears are chemically different for each situation. When we cry from emotion our tears contain chemicals that are not present in our other tears. These include manganese (a chemical depressant), leucine-enkephalin (an endorphin which helps control pain), and adrenocorticotrophic hormone (a hormone produced by bodies under stress). When we release these toxins, we feel chemically and physically less depressed. A good cry makes us feel better!
Check out all our online dissection guides.
Watch this animated explanation of how the eye works.
Try this amazing animated color vision experiment.