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SHEEP EYE DISSECTION PROCEDURES
The anatomy of the human eye can be better shown and understood by the actual dissection of an eye. One eye of choice for dissection, that closely resembles the human eye, is that of the sheep. Differences between the two eye types will be mentioned as the dissection is completed. Begin the dissection by gathering the equipment and supplies listed here.
Materials Needed: (sheep eye, dissecting pan, safety goggles, non-latex gloves, scissors, scalpel, probe, forceps, paper towels and a notebook and pencil for recording information about the eye as it is dissected.)
Step 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. Examine the back of the eye and find extrinsic muscle bundles, fatty tissue and the optic nerve. The four extrinsic muscles (humans have six) move the sheep eye while the fatty tissue cushions the eye. If the optic nerve is not visible use the probe to move the fatty tissue around until the nerve is exposed. Take the notes you need to record what you have observed so far.
Step 2: Use your scissors to cut away the eye-lid (if necessary), muscle and fatty tissue from both the front and rear surfaces of the eye. Be careful not to remove the optic nerve. Cut along the surface of the sclera until all the tissue is removed and your specimen looks similar to the photographs you see here. The sclera is very tough so you do not need to worry about cutting into this layer of the eye. When you have finished removing the tissue surrounding the eye identify the sclera, cornea, optic nerve, and the remaining external muscle parts. The cloudy nature of the cornea is caused by the non-living tissue. It is transparent in the living state.
Step 3: Place your eye specimen in the dissection pan. Turn the specimen so the cornea is on the left and the optic nerve is on your right. Select a place to make 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 the sclera. 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.
Step 4: Insert the point of the scissors into the slit made by the razor blade and cut the sclera with a shallow snipping motion. Turn the eye as you continue the cutting action. Cut the sclera all the way around the ball of the eye. You will need to support the eye in the palm of your hand while you complete this step of the dissection. Do not be surprised if some fluid from the eye oozes from the slit as you make this cut. Take the notes you need to record what you have observed so far.
Step 5: Arrange the two hemispheres of the eye as you see in the left photograph. Observe the semi-fluid vitreous humor that fills the central cavity of the eye. It is transparent in the living eye but might be cloudy in the preserved specimen. The vitreous humor along with the aqueous humor (found behind the cornea) helps to maintain the shape of the eye. The retina lines the posterior (back) side of the eye and extends forward to the ciliary body (beneath the iris, used to make aqueous humor). Use your probe to lift and pull the retina back from the underlying choroid layer (found in-between the sclera and the retina, used to nourish the back of the eye). See the photograph on the right side above. Notice that the retina is only firmly attached to the choroid at one place. This region is the optic disc or blind spot. Here the nerve fibers leave the retina and form the optic nerve which is directly behind the blind spot.
Step 12: Place all of your eye structures in a zip-lock bag. Wipe your dissection tray with an antibacterial cloth and put away all dissecting tools. Clean your lab area with another antibacterial cloth and work on laboratory questions. Wash your hands.
Observe the eye model. Examine the external muscles (FYI)
Sheep Eye Dissection Lab Sheet Name Per.
Follow instruction sheet for procedures. Date ________________
You and your partner will sign off (initial) on the chart as you find the structures. You should be able to explain their functions, if asked by teacher.
1. Write three observations you made when you examined the surface of the eye:
______________________________
______________________________
______________________________
Part 2: External Anatomy
Identify: Place your initials here cornea sclera optic nerve external eye muscle fatty tissue
Questions:
- Why don’t you have to worry about cutting into the sclera when you are removing the extrinsic muscles and fatty tissue?
- How does the fat tissue look different from the muscle tissue?
- What is the choroid’s function?
- Why is the cornea so cloudy?
- What is the tapetum lucidum? Do human have this structure? Why or why not- explain.
Part 5: Cornea & Lens Questions:
How does the thickness of the cornea compare to the thickness of the sclera?
What shape is the sheep’s pupil? How does it compare to the shape of the human pupil?
What color is your sheep’s iris (not the cornea)?
Describe the texture of the lens.
Part 6. Match the following parts of the eye to their function: (ciliary body, pupil,
sclera, iris, retina, lens, & tapetum lucidum)
____________________ Contains the photoreceptors for vision.
____________________ The colored portion of the eye.
____________________ This structure changes shape to focus light on the retina.
____________________ The opening in the iris through which light passes.
____________________ The iridescent portion of the choroid layer in nocturnal animals.
____________________ Consists of muscles, which control and shape the lens.
____________________ The white of the eye.
4. Use the pictures below to name the parts of the eye:
1. ________________________________________
2. ________________________________________
3. ________________________________________
4. ________________________________________
5. ________________________________________
6. ________________________________________
7. ________________________________________
8. ________________________________________
9. ________________________________________
10. ________________________________________
11. ________________________________________
12. ________________________________________
