Why Dissect? Dissection allows students to: Practice safety procedures Construct science process skills Organize the hierarchal relationships of tissues, organs, organ systems and organisms Compare similarities and differences among organisms Connect organism complexity to its diversity Dissection can be a controversial issue. Arguments both for and against dissection can be made related to personal safety, environmental safety, ethics and pedagogy. Opponents of dissection contend: Dissection depletes natural populations Dissection can be unsafe for students in very large or unruly classes Dissection preservatives are toxic and allergenic Dissection may be too much for young students who may be emotionally immature Dissection may focus too much on simple identification rather than on higher order thinking skills. Dissection proponents contend: Dissection can teach students a lot when structures around inquiry questions rather than identifying parts. Additionally dissection allows students to practice safety procedures, construct science process skills, organize the hierarchal relationships of tissues, organs, organ systems and organisms, compare similarities and difference among organisms and connect organism complexity to its diversity.

Interesting Facts Look all around without moving your head. Look up, look down, look side to side and all around. You have six muscles surrounding your eye that allows you to look in different directions. However, cows only have four muscles. Therefore, they can only look up, down, left and right. They are not able to roll their eyes like you can. This slide is designed to engage your students in learning about the cow eye, its structure and function. At this time, I would like to play a little game of Simon says. Simon says look up. Look down. No, Simon says look down. Look right, look left. No Simon says look right. Simon says look left. Now, roll your eyes all around. No, Simon says roll your eyes all around. It’s a bit like doing the hokey pokey isn’t it. What’s so cool about this is that humans have the ability to move their eyes in all directions because we have six muscles surrounding our eyes, allowing us to look in different directions. Cows on the other hand, only have four muscles surrounding their eyes. Therefore, cows can only look up, down, left and right. They cannot roll their eyes like you can. I bet the momma cows appreciate that as their babies become teenagers.

Interesting Facts Muscle Reach up and feel around your eye. Can you feel the bone of your skull? Fat surrounds your eyeball to keep it from bumping against the bone and getting bruised. The fat and muscle surrounding the cow eye is cut away when dissecting in order to see the eyeball. Feel around your eye. Can you feel the bone of your skull? Fat surrounds your eyeball to keep it from bumping against the bone and getting bruised. Fat is a yellowish-white tissue that cushions the eye from shock. As mentioned previously, cows have four muscles attached to the eye that allows the cow to look up, down, left and right. These muscles are reddish and flat. We will locate these structures momentarily. Fat Optic Nerve

External Anatomy Obtain a cow eye, place it in your dissecting pan and rinse with water. Observe the white part of the eye. This is the sclera. The sclera is the outer covering of the eyeball. Sclera Following verbal instructions, a short video will be shown that models the instructions provided on the next few slides. Now that you have a cow eye specimen, see if you can locate the yellowish-white fatty tissue. Also, see if you can identify the four reddish-brown muscles attached to the eye. These are often imbedded or surrounded by fat. Finally, identify the sclera. The sclera is a thick tough, white outer covering of the eyeball. Sclera Fat

External Anatomy Locate the eyelids. Observe the blue portion of the eye. This is the cornea. The cornea starts out clear but becomes cloudy after death. Look through the cornea and locate the iris. The iris is a round black tissue. Locate the pupil. The pupil is the round opening in the center of the eye. Most cow eye specimens will have eyelids attached. However, on rare occasions the eyelids may be removed. Take this time to identify the eyelids. Eyelids are located on top and bottom of the eye. They are moveable covers that protect the eye from dust, bright light and impact. Now observe the blue portion of the eye. This is the cornea. The cornea is a tough, clear covering over the iris and 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 an image; the lens finishes the job. The cornea starts out clear, but becomes cloudy after death. This may make it difficult to see the pupil. You will be able to identify these parts upon further internal investigation. Look through the cornea and locate the iris. The iris is a round, black muscle tissue that controls the amount of light that enters the inner part of the eye. It is suspended between the cornea and lens. In cow’s the iris is brown. Humans irises come in many colors including, brown, blue, green and grey. The iris can change shape, focusing on objects that are close up and objects that are far away. Now locate the pupil. The pupil is the round opening in the center of the eye. It is a dark circle in the center of the iris. The pupil allows light to enter the eye. The size of the pupil is controlled by the iris. Your pupil is round, but the cow’s pupil is oval. In dim light, the pupil enlarges to let in more light. In bright light, the pupil contracts to allow less light to enter the eye. Cornea Pupil Iris

External Anatomy On the opposite side of the cornea, locate the optic nerve. The optic nerve is a white cord on the back of the eye about 3 mm thick. Our final external anatomy investigation involves identifying the optic nerve. The optic nerve is located on the opposite side of the cornea. It is a white cord on the back of the eye that is approximately 3 millimeters thick. The optic nerve is a bundle of nerve fibers that carry information from the retina to the brain. The point at which the optic nerve leaves the retina is called the blind spot. Each eye has a blind spot where no light sensitive cells exist. We will investigate this again when we look at the internal anatomy of the eye. Now lets watch a short video clip to review what we’ve done so far. Please feel free to follow along with your specimen should you have one. Optic Nerve

Incision 1 Sclera Now remove the fat and muscle surrounding the eye using caution not to remove the optic nerve. Place the eye in the dissecting pan so it is again facing you. Using your scalpel, pierce the white part of the eye (sclera) just behind the edge of the cornea. Make a hole large enough for your scissors. Following verbal instructions, a short video will be shown that models the instructions provided on the next 3-4 slides. Place the cow eye in the dissecting pan so that the cornea is facing you. Using your scalpel, pierce the sclera, which is the white tissue just behind the edge of the cornea. Some fluid may ooze out when the eye is poked. This fluid is the aqueous humor.

Incision 2 Using your scissors, carefully cut around the eye using the edge of the cornea as a guide. Lift the eye and turn it as needed to make the cut and be careful not to squeeze the liquid out of the eye. Now, insert your scissors into the punctured tissue and carefully cut around the eye using the edge of the cornea as a guide. You will probably need to lift the eye and turn it as needed to complete the incision. Try to be cautious not to squeeze the aqueous humor or clear fluid out of the eye.

Internal Anatomy If a fluid should ooze out, it is called the aqueous humor. This fluid is clear and made of protein and water. The aqueous humor helps give the eye its shape. Should fluid escape from the eye, just remember this is the aqueous humor. This fluid is clear and composed of protein and water. The function of the aqueous humor is to give the eye and cornea its rounded shape. Aqueous Humor

Internal Anatomy After completing the cut, carefully remove the front of the eye and lay it in your dissecting tray. Place the back part of the eye in the pan with the inner part facing upward. After you have successfully dissected the eye around the cornea, carefully lay each portion on your dissecting try. Make sure you place the eye so that the inner portion of the eye is facing upward. Now let’s watch a video to review the procedures thus far. Please feel free to follow along with your specimen should you have one.

Internal Anatomy Observe the tough tissue of the removed cornea. Cut across the cornea with your scalpel to note its thickness. Observe the iris. The iris is the black tissue of the eye that contains curved muscle fibers Locate the ciliary body. It is located on the back of the iris that has muscle fibers to change the shape of the lens. Again, these sets of slides will be accompanied by a short video that models the instructions provided. Let’s see if we can identify some of the parts we discussed externally. Observe the tough tissue of the cornea. This is the clear tissue that can be seen through the opening called the pupil. The pupil is the hole that lets light into your inner eye. It is the dark circle in the center of your iris. Remember this the cornea may be cloudy instead of clear because the tissue changes upon death. Cut across the cornea with your scalpel and note its thickness. Why do you think this tissue is so thick? (USE WAIT TIME). What is the main function of the cornea? (USE WAIT TIME). Remember, the cornea protects the eye from injury. Therefore, its structure or thickness relates to its function, which is to protect. Now, observe the iris. The iris is the black tissue of the eye to contains curved muscle tissue. Remember the iris controls the amount of light that enters the eye. Therefore, the iris must be able to contract. This is why it is made of muscle tissue. This is yet another example of how the structure relates to its function. As mentioned previously, in humans the iris can come in a variety of colors. Finally, let’s locate the ciliary body. It is located on the back of the iris. It is composed of muscle fibers that help change the shape of the lens. Can you explain how the structure of the ciliary body relates to it’s function? (USE WAIT TIME). Ciliary Body Pupil Iris

Internal Anatomy Retina Locate the lens. It can be found by looking through the pupil. Using your scalpel and dissecting needle, carefully lift and work around the edges of the lens to remove it. The gel the lens is suspended it is called the vitreous humor. Let’s identify and discuss some parts we haven’t discussed yet. Locate the lens. It can be found by looking through the pupil. Using your scalpel and dissecting needle, carefully remove it from the eye. The lens is a clear, flexible structure that makes an image on the eye’s retina. It is so flexible that it can change shape in order to focus on that are close up or on objects that are far away. The lens flattens in order to focus on objects that are far away. The muscle controlling the shape of the lens (also called the cilliary body) relaxes and fibers attach to the lens stretch it out a little bit. When observing something close, the muscle controlling the shape of the lens or the cilliary body contracts, reducing the tension on the fibers that stretch out the lens, letting the lens form its naturally bulging shape so that it can focus light from a nearby object on the retina. The lens only focuses 20% of the light and it acts as a magnifier. If the lens is not cloudy, students could place the lens over printed material to observe how it magnifies the print. The lens was surrounded by a thick, clear, jelly-like substance. This is the vitreous humor. The vitreous humor helps give the eyeball its shape. Like the aqueous humor, the vitreous humor is clear so that light can pass through it and it is made of protein and water. This jelly helps maintain pressure that helps hold the retina on the back of the eye. It is important that the vitreous humor keep the retina laying flat on the back of the eye. If the retina becomes folded, the signal from the retina to the brain will be confused or distorted. This is called a detached retina. Lens Vitreous Humor

Internal Anatomy Observe the back of the eye after the lens has been removed. It should be shaped like a bowl. On the inside of the bowl is a thin film with red blood vessels running through it. This is the retina. The retina is attached to the back of the eye at just one spot. It is called your blind spot. Use forceps to separate the retina from the back of the eye to see the layer beneath it. Retina Now that the lens and aqueous humor have been removed, observe the back of the eye. It should be shaped like a bowl. On the inside of the bowl is a thin film with red blood vessels running through it. This is the retina. The retina is composed of photoreceptor cells called rods and cones that are sensitive to light. Rods function mainly in dim light and provide black and white vision, while cones support daytime vision and the perception of color. The retina receives signals and sends those signals to the brain through the fibers of the optic nerve. The retina is attached to the back of the eye at just one spot. This is called your blind spot. Use forceps to separate the retina from the back of the eye to see the layer beneath it and identify the blind spot by inserting a probe. Again, the blind spot is the place where the optic nerve leaves the retina. At this point there are no light sensitive cells. It is important to note that each eye has a blind spot. This is an excellent point to trace the hierarchal relationships that exist in the structure of the cow eye. The eye is an organ which his made of various tissues which are composed of cells. For example: the eye (an organ) has a retina (tissue) that is composed of rods and cones (cells). Can you give another example of this hierarchical relationship? (USE WAIT TIME). Organs are part of organ systems, which system do you believe the eye is a part of? (USE WAIT TIME). If you said the nervous system you would be correct. Blind Spot

Internal Anatomy The layer beneath the retina is called the tapetum lucidum. It is a shiny, blue-green layer designed to assist night vision by reflecting light back through the retina. Humans do not have a tapetum lucidum. This structure is responsible for the shining of animals eyes at night. Tapetum Have you ever seen an animals eyes shine at night? There’s a reason for that. Just underneath the retina, there is a shiny, blue-green layer in the called the tapetum. The tapetum is designed to assist night vision by reflecting light back through the retina. Therefore its function is to improve an animals ability to see at night. Humans do not have a tapetum because their survival is not as dependent upon seeing at night. Can you think of some examples of animals that would need a tapetum? (USE WAIT TIME). Many animals have this layer, dogs, cats, opossums. You could definitely name any nocturnal animal and be correct. It’s interesting that cows have this layer since they really do not hunt at night. Do you think this adaptation was necessary for cows for some reason long ago? (USE WAIT TIME). Now lets watch a short video clip to review what we’ve done so far. Please feel free to follow along with your specimen should you have one.