Sheep Eye Dissection InstructionsThe anatomy of the human eye can be better shown and understood by the actual dissection of an eye. One eyeof choice for dissection, that closely resembles the human eye, is that of the sheep. Sheep eyes are removed atthe time the animal is slaughtered and then preserved for later use. Differences between the two eye types willbe mentioned as the dissection is completed.Begin the dissection by gathering the equipment and supplies listed here (sheep eye, dissecting pan, scissors,single edge razor blade, probe, forceps, paper towels and a notebook and pencil for recording information aboutthe eye as it is dissected).PART 1: External Anatomy of the eyeStep 1: Wash the sheep eye in running water to remove the preservative fluid. Dry the eye with paper toweling.Examine the front of the eye and locate the eye-lid, cornea, sclera (white of the eye) and fatty tissue. Examinethe back of the eye and find extrinsic muscle bundles (brown), fatty tissue (yellow) and the optic nerve. Thefour extrinsic muscles (humans have six) move the sheep eye while the fatty tissue cushions the eye. If the opticnerve is not visible use the probe to move the fatty tissue around until the nerve is exposed.Step 2: Use your scissors to cut away the eye-lid, muscle and fatty tissue from both the front and rear surfacesof the eye. Be careful not to remove the optic nerve. Cut along the surface of the sclera until all the tissue isremoved and your specimen looks similar to the photographs you see here. The sclera is very tough so you donot need to worry about cutting into this layer of the eye. When you have finished removing the tissuesurrounding the eye identify the sclera, cornea, optic nerve, and the remaining extrinsic muscle remnants. Thecloudy nature of the cornea is caused by the death of this tissue. It is transparent in the living state.IDENTIFY THESE STRUCTURES ON STUDENT LAB SHEET:Eye lidCorneaScleraOptic NerveExternal Eye MusclePart 2: Internal Posterior AnatomyStep 3: Place your eye specimen in the dissection pan. Turn the specimen sothe cornea is on the left and the optic nerve is on your right. Select a place tomake an incision of the sclera midway between the cornea and optic nerve.Use the point of a very sharp razor blade to make a small cut through thesclera. Fluid should ooze out of the eyeball when you have cut deeply enough.You will be reminded of how tough the sclera is when you make this cut.Fatty Tissue
Step 4: Insert the point of the scissors into the slit and cut the sclera with a shallow snipping motion. Turn theeye as you continue the cutting action. Cut the sclera all the way around the ball of the eye. You will need tosupport the eye in the palm of your hand while you complete this step of the dissection. Do not be surprised ifsome fluid from the eye oozes from the slit as you make this cut. Take the notes you need to record what youhave observed so far.Step 5: Arrange the two hemispheres of the eye as you see in the left photograph. Observe the semi-fluidvitreous humor that fills the central cavity of the eye. It is transparent in the living eye but might be cloudy inthe preserved specimen. The vitreous humor along with the aqueous humor helps to maintain the shape of theeye. More will be said about the aqueous humor later. The retina lines the posterior cavity of the eye andextends forward to the ciliary body. Use your probe to lift and pull the retina back from the underlying choroidlayer. See the photograph on the right side above. Notice that the retina is only firmly attached to the choroid atone place. This region is the optic disc or blind spot. Here the nerve fibers leave the retina and form the opticnerve which is directly behind the blind spot. Take the notes you need to record what you have observed so far.Step 6: Use your forceps to peel the retina away from the underlying choroid coat. The retina should remainattached at the blind spot. The choroid coat is dark and relatively thin. Use your forceps or probe to gentlyseparate the choroid from the outer sclera. Verify that the eye has three distinct layers, the retina, choroid andsclera. See left photograph above. The choroid contains an extensive network of blood vessels that bringnourishment and oxygen to itself and the other two layers. The dark color, caused by pigments, absorbs light sothat it is not reflected around inside of the eye. The tapetum lucidum, which is not found in the humaneye, functions to reflect light onto the retina. It especially helps animals with night vision since it can reflectlight even at very low intensities. It is shiny, glittering with a bluish color. In just a moment you will see that thechoroid extends forward to the ciliary body.IDENTIFY THESE STRUCTURES ON STUDENT LAB SHEET:Vitreous HumorRetinaBlind SpotChoroidLift up layers of retina & Choroid
Part 3: Anterior Interior AnatomyStep 7: Use your forceps and probe to remove the vitreous humor from the anterior hemisphere of the eye. Seeright photograph above. This will take some time and effort as the semi-fluid material separates easily. It helpsto turn the hemisphere on edge and to use a scrapping motion to remove the fluid. Try not to disturb the lensthat is just below the vitreous humor.Step 8: Removal of the vitreous humor reveals the lens, ciliary body, and suspensory ligaments. In the normalcondition the lens is transparent except, when as a condition of aging, the lens turns cloudy. The cloudycondition, called cataract, prevents or reduces the amount of light reaching the retina. Cataract can be treated byremoving the lens and replacing it with a stiff artificial one. The normal lens is convex shaped and somewhatelastic. It is held in place by the suspensory ligaments that in turn join with the smooth muscle containing ciliarybody. When the smooth muscle fibers contract the resulting force flattens the lens and the degree of bending ofthe light rays is reduced. Relaxation of the smooth muscle results in a thickening of the lens and a greaterbending of the rays of light.Step 9: Remove the lens by pulling it free from its attachments. Note the shape ofthe lens, its stiffness and opaqueness. Suspensory ligaments may also be visiblealong the edge of the lens. Take the notes you need to record what you haveobserved so far.Step 10: When the lens is removed, an opening, allowing light to enter the eye is seen. This opening, the pupilis located in the center of the iris. Two muscle layers of the iris regulate the size of the pupil. One layerincreases the pupil size with decreasing light intensity and the other layer reduces pupil size with increasinglight intensity. Note the oblong shape of the sheep pupil, in humans the pupil is circular. The back side of theiris can be seen just above the pointer in the photograph. Part of the iris is being lifted by the pointer but the iriscontinues all the way around the pupil opening.A second cavity or space is present between the iris and the cornea. This space is filled with a second semiliquid fluid, the aqueous humor. This fluid, like the vitreous humor helps to maintain the shape of the eye.Glaucoma is a condition where the fluid pressure becomes too high causing eye damage.IDENTIFY THESE STRUCTURES ON STUDENT LAB SHEET:Suspensory LigamentIrisCiliary Body/MuscleLensPupilAqueous Humor
Part 4: Removal of Cornea and LensStep 11: Remove the cornea from the front eye hemisphere. Use a razorblade to puncture a small slit at the boundary between the cornea andsclera. Then insert the scissors into the slip and cut all the way around thecornea to remove it. Notice the thickness of the cornea. How does itcompare to the thickness of the sclera? Carefully observe the front side ofthe iris and pupil. What shape is the sheep’s pupil? How does it compare tothe shape of the human pupil? Notice that the back or posterior color of theiris is black while the anterior or front of the iris is colored. What color is your sheep’s eye?Step 12: Find the lens. Wash all the ligaments and vitreous gel from the lens. Use your fingernails to tear thelens apart and observe the texture of the lens.IDENTIFY THESE STRUCTURES ON STUDENT LAB SHEET: Color of Anterior view of Iris
Lab: Sheep Eye Dissection Student Sheet The teacher will sign off on the following parts as you find them. You should be able to explain their function when you show them to your teacher.Part 1: External Anatomy (Follow Steps 1 & 2.)Identify: External Eye Muscle Cornea Sclera Optic Nerve Fatty TissueQuestions:1. Why don’t you have to worry about cutting into the sclera when you are removing the extrinsic muscles andfatty tissue?2. How does the fat tissue look different from the muscle tissue?Part 2: Posterior Anatomy (Follow Steps 3-6)Identify: Vitreous Humor Retina Blind Spot Choroid Lift up layers of retina & ChoroidQuestions:3. What is the “Posterior”?4. How did you find out that the sclera is so tough?Part 3: Anterior Anatomy (Steps 7, 8, 9. 10)Identify: Ciliary Body (Muscle) Lens Iris (Back Side) Pupil Suspensory LigamentQuestions:5. What is the “Anterior”?6. How do ciliary bodies (muscles) help you see?7. What is a cataract?8. Which structure of the eye would be just behind the pupil opening?9. What is “Glaucoma”?Part 4: Cornea and Lens (Step 11&12)Identify: Iris (color on front side) Lens textureQuestions:10. How does the thickness of the cornea compare to the thickness of the sclera?11. What color is your sheep’s eye?12. Describe the texture of the lens.13. What shape is the sheep’s pupil? How does it compare to the shape of the human pupil?