Sheep Eye Dissection

Materials: Forceps, scissors, paper towel, dissecting probe, preserved sheep eye, hand lens, dissection tray.

Note: Pay special attention to structures when they are bolded. Before you continue dissecting, make sure you pause and find them on your checklist. Write down any observations about the structures before you continue to the next.

Procedure - External Structure Observation: 1. Obtain a sheep eye, place it in your dissecting pan. 2. Rotate the eye until the large bulge is on the top of the eye. The eye is now in the position it would be in a body as you face the body. 3. On the outside of the eye, locate the following parts: fat: surrounds the eye & cushions it from shock optic nerve: a white cord on the back of the eye about 3mm thick just toward the nasal side; carries messages between the eye & brain muscles: reddish, flat muscles found around the eye to raise, lower, & turn (right & left) the eye. 4. Turn the eye so that it is facing you & examine these structures on the front surface of the eye: eyelids: two moveable covers that protect the eye from dust, bright light, and impact sclera: this is the tough, white outer coat of the eye that extends completely around the back & sides of the eye cornea: a once clear covering (preservative often makes this appear cloudy) over the front of the eye that allows light to come into the eye iris: round black tissue through the cornea that controls the amount of light that enters the inner part of the eye (may be colored in humans) pupil: the oval opening in the center of the eye that allows light to enter and whose size is controlled by the iris

Procedure - Internal Structure Observation: 5. Place the eye in the dissecting pan so it is again facing you. Using the tip of your probe, GENTLY pierce the white part of the eye or sclera about 1 cm away from the edge of the cornea. Make a hole large enough for your scissors. Sheep Eye Dissection 6. Using your scissors, carefully cut around the eye using the edge of the cornea as a guide. Pick up the eye in your hand & turn it as needed to make the cut. Be careful not to squeeze the liquid out of the eye. 7. After completing the cut, carefully remove the front of the eye and place the front portion of the eye face down in your dissecting pan. Place the back part of the eye in the pan with the inner part facing upward. Hold the front portion of the eye up to the light and observe the cornea. The cornea is not completely transparent, but transparent in a living eye. Carefully remove the lens and aqueous and vitreous humor from the eyeball. Use your forceps & probe to carefully lift & work around the edges of the lens to remove it. Also remove the jellied contents and notice the difference between aqueous humor and vitreous humor. 8. Now examine the back portion of the eye, emptied of its jellied contents. You should have the cut portion facing up. You should notice a thin, badly wrinkled, whitish tissue on the inside along the back. This is the retina. The retina in a living eye is smooth. NOTE: The retina can be removed for further examination. Observe where it attaches to the back of the eye. This is the blind spot leading to the optic nerve. 9. At the back of the eyeball is a bluish layer called the tapetum lucidium. This layer acts as a reflective surface and is only found in certain animals. Push the tapetum aside at its cut edge to find the choroid layer directly below. 10. Examine the solid, round, yellowish structure that fell out when you opened the eyeball. This is the lens. It is covered with a layer of fine muscle fibers that control the shape of the lens. Hold it up to the light. The lens does not appear completely transparent now, but it is transparent in a living eye. 11. Locate the following internal structures of the eye and check them off on your checklist: cornea: observe the tough tissue of the removed cornea; cut the cornea in half to note its thickness aqueous humor: fluid in front the eye that runs out when the eye is cut iris: brown tissue of the eye that contains curved muscle fibers ciliary body: located on the back of the iris that has black muscle fibers to change the shape of the lens lens: can be seen through the pupil, hard, solid structure vitreous humor: jelly-like fluid inside the back cavity of the eye behind the lens retina: tissue in the back of the eye where light is focused

 tapetum: reflective layer under retina 12. Dispose of your materials according to the directions from your teacher and clean up your work area. Wash your hands before leaving the lab.