Dissection of the Cow Eye

DISSECTION OF THE COW EYE Please make sure to wear gloves and safety glasses when you are dissecting, and make sure to clean up thoroughly after the lab. Also, the cow eyes can be rather slippery, so use caution when handling and cutting them. You will need a scalpel and forceps. 1. First, identify the most external structures of the eye. Near the posterior pole, observe the optic nerve, which was obviously cut when the eye was removed. Additionally, you should be able to see a large mass of tissue attaching to the periphery of the eyeball. This tissue includes the extraocular fat and possibly several of the extraocular muscles. You do not need to identify individual muscles, but try to distinguish them from the fat. External view of the cow eye The most external layer of the wall of the eyeball is called the fibrous layer. It is a dense, avascular connective tissue that contains the contents of the eyeball. The posterior 90% of the fibrous layer is the opaque sclera. The anterior 10% covers the anterior pole of the eye and is called the cornea. Since light must pass through it, it is translucent. Once you have identified all of these structures, remove the extraocular fat and muscles from the eyeball. 2. Place the eyeball on the dissecting tray and gently hold it with two fingers at the cornea and the optic nerve. Cut a cross-section through the sclera slightly behind the middle of the eyeball so that the eyeball is separated into an anterior half and a posterior half. Only cut through the wall of the eyeball, not through the entire globe. The inside of the eyeball is filled with the gelatinous vitreous humor, if you cut too deep into the globe, this may start leaking out, which is not a problem. Once you have cut around the entire periphery of the eyeball, separate into two halves. Let the vitreous humor slide out of the eye. You may need to gently tease it loose from the inside of the eye. 3. The eyeball should now be separated into an anterior and a posterior half. Turn the anterior portion of the eyeball over on the dissecting tray so 1 that the front is facing up. Try to keep the lens attached to the anterior portion of the eye, although it may fall out. Using the scalpel or pointed scissors, cut around the periphery of the cornea, where it contacts the sclera. When the scissors have cut in far enough, a clear fluid will start to seep out - this is the aqueous humor. While cutting out the cornea, be careful to not accidentally cut the iris or the lens. 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 the back half. Locate the lens and ciliary body. The ciliary body surrounds the lens, allowing it to change the shape of the lens to help the eye focus on the object it is viewing. After you have finished examining the back of the anterior half of the eye, remove the lens and place it over text on a page to observe its magnifying properties. 6. Now focus on the posterior half of the eye. You should observe 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 from the outside world. Most of the retina is not attached to the eye. Instead, it is held in place by the vitreous humor that presses the retina flat against the inside of the eye. Use a pair of tweezers to gently lift the retina off the inside wall of the eye. 7. 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 layer 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. 2 8. This reflective material in the cow’s choroid coat is called 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. 3