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Sheep’s Eye Dissection
Background Information
The sheep’s eye is very similar to the human eye. The wall of the eyeball is made up of three layers.
The outer layer, the sclera, is a tough tissue that forms the white of the eye. At the front of the eye,
the sclera becomes thin and transparent to form the cornea. The middle layer, the choroid, is dark
and rich with blood vessels. At the front of the eye behind the cornea, the choroid is modified into the
iris. The pigment in the iris determines the color of our eyes. The opening in the center of the iris is
the pupil.
Directly behind the behind the iris is
the lens. The lens is attached to a
muscle that can change the shape of
the lens to focus on near and far
objects. The inner layer, the retina, is
sensitive to light. In the retina, the
rods and cones bundle together at
the optic nerve. The spot where the
optic nerve connects the brain to the
spinal cord has no rods or cones and
is also called the blind spot. A very
short distance away is a yellowish
spot, called the fovea. This is the site
of sharpest vision, because it has the
highest concentration of rods and
cones in the eye.
Lab Safety
 You MUST wear safety goggles. As long as Mrs. Oelfke is wearing hers, you must wear yours. If
you take your goggles off without permission, you will sit out of the lab and your grade will reflect
that.
 Tie back long hair and avoid wearing loose clothing. Roll up loose sleeves that might fall into the
dissection pan.
 You MUST wear gloves.
 You MUST wash your hands before leaving class.
 Avoid scratching your face…you might be spreading chemicals and/or pathogens.
 Aprons are available upon request.
Clean-up
 All animal matter should be disposed of in the special bucket. Do not throw any animal matter in
the trashcan or down the sink.
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 All of the tools and your tray need to be washed and rinsed off. There should be no “guts” left on
the tray or tools. Your lab bench must also be wiped down.
 Dry your tools and tray with a paper towel and neatly place them in the dissection tray.
 Remove your gloves properly and throw them away in the special bucket.
 Return the goggles neatly to the cabinet.
Pre-Lab
Respond to the following items in complete sentences on a separate sheet of paper.
1.
Using your class notes, describe the function of the pupil.
2.
What will we do during this lab to stay safe?
3.
Epilepsy affects one of every 200 Americans. Brain neurons normally produce small bursts of
impulses. During an epileptic seizure, large numbers of brain neurons send rapid, uncontrolled
bursts of nerve impulses. The body of an individual having a seizure may grow rigid and jerk or
convulse. From what you know about the brains control of muscles and posture, how would
you explain these symptoms?
An optional sheep eye dissection can be found online at:
http://science.jburroughs.org/resources/skeleton/eye/eyedissection.html
IF YOU DO NOT COMPLETE THESE PRE-LAB QUESTIONS, YOU WILL NOT DISSECT AN EYE.
Lab Procedure
Use the diagrams above to help you find the structures in the eye.
1.
Eye-dentify the following before cutting:
A. Sclera
B. Cornea
2.
Use the scissors to cut completely around the cornea, about 1 to 2cm away from its edge.
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Completely separate the eye into front and back.
3.
Identify the following from the front part of the eye.
A. Iris – Color will have been lost due to the chemicals used to preserve the eye.
B. Pupil – It really is a hole!
C. Carefully cut the cornea out. Separate it from the iris.
4.
In the front section of the eye, use a probe and tweezers to uncover the lens. It should not be
clear anymore because the chemicals used to preserve the eye. Try to locate any muscles that
connect to the lens. Remove the lens from the eye. It will be attached to the vitreous humor
(Jell-o™-like substance). This helps to support the eye and hold the lens in place.
5.
In the back section of the eye, observe the whitish retina. It is probably shriveled and may have
detached. If it is not detached, gently pull it off with forceps. The optic nerve will be easily seen
when you remove this (optic nerve “anchors” this down).
6.
Use scissors to find the optic nerve on the outside of the eye, in the back. It will be buried in fat
and connective tissue. Carefully remove the excess fat and connective tissue.
7.
Clean up. See the other side of this sheet for more details. Failure to clean up correctly will
result in a loss of points of this assignment.
Post-Lab
Answer the following questions on a separate sheet of paper in complete sentences.
4.
What if the cornea became cloudy during an animal’s lifetime? How might it affect the animal’s
ability to see?
5.
Write a paragraph of at least 5 sentences about your experience during the dissection. Use an
example from the dissection to explain if this experience is what you expected it to be like. How
will it help you to remember the parts of the eye? What could have made this experience
better?
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Sheep Eye Dissection Checklist
Find a group of two to three students for the dissection. Sit with them. No more than three. No
less than two.
Each GROUP needs (go get it):
A dissection tray – each tray should contain one pair of scissors, one pair of tweezers, one
probe, and a blue dissection mat
A Sheep Eye Dissection Guide
Each PERSON needs (go get it):
Safety goggles
Pair of gloves
Apron (OPTIONAL)
PERSONAL SAFETY
Put all books to the side. You want as much room to work as possible. Make sure you keep a
pencil and some paper out.
All hair needs to be tied back. The last thing you wanted is dead organ juice in your nice hair.
If you have long sleeves, consider rolling them up so they don’t get “juiced.”
If you have on loose clothing, consider taking off things like jackets or sweatshirts and tucking in
shirts.
If you have gum or candy, spit it out.
CLEAN-UP
Place all organ and organ parts in a special bucket. See Mrs. Oelfke.
Rinse out the dissection tray. Make sure that no organ parts are left in the tray or the sink.
Place a paper towel in the tray.
Place the blue dissection mat in the location Mrs. Oelfke indicates to dry.
All utensils must be cleaned and returned they way they were given to you – clean, dry, and
neatly placed in the dissection tray.
Used gloves also go into the bucket. Make sure you remove them properly.
Clean your table thoroughly. Make sure it is dry.
You MUST wash your hands before you leave.
If there is time left, work on your post-lab questions!
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