Sheep Eye Dissection
Precautions
1. Do not leave your lab station.
2. Treat the lab equipment with respect.
3. Do not stab or cut the dissection pan gel
4. In the event of cut, wash the skin immediately with water and inform
your teacher.
Materials
Sheep eye
Lab Apron
Tweezers
bag
Latex/Vinyl Gloves
Dissecting probe
Dissecting tray
Safety goggles
Dissecting scissors
Paper towels
Plastic Trash
The parts of the human eye
Procedure
1. Prior to making any incisions(cuts), examine the outside layer of the
sheep eye. Identify the following parts: The sclera, cornea, and the optic
nerve. Notice the amount of external yellow, light and dark tissue
around the eye. This tissue is fat and muscle.
2. Use the scissors to cut away the excess fat and muscle from around
the eye so the sclera and the optic nerve are clearly exposed. Be careful
when cutting around the optic nerve so that you did not cut it away along
with the fat. The optic nerve will be a “tough” tube like structure coming
out of the back of the eye.
3. The sheep eye has 4 external attached muscles which control the eye
movement. The human eye has six. The additional muscles allow
humans the ability to “roll” and move their eyes in all directions.
4. Although the muscles of each eye work collectively as a team, the eyes
themselves do not focus or work together until months after birth. Most
people have one dominant eye.
To find out which of your eyes is dominant, form a circle with your
thumb and index finger. Hold that position and place your hand in
front of you. With both eyes, look at an object through the circle.
Continue to hold that position and close one eye then open it. Close
the other eye, open it. The eye that you could still see the object with is
your dominant eye.
Which eye is your dominant eye? Left
Right (circle one)
5. Use the probe to pierce the sclera, about half way between the pupil
and the back of the eye. You will be cutting the eye in half, through
the sclera. Be careful when you cut so that you do not destroy the
pupil and lens.
6. Follow a circular pattern around the sclera, rotate the eye while
continuing to carefully cut the eye in half.
7. A clear jelly like fluid will be inside the eye. This is the vitreous
humor. Carefully lay the two sides of the eye in the dissection pan.
8. Observe the back half of the eye. You will see a dark reflective lining.
This is called the tapetum and is not found in the human eye. This is
why an animal’s eyes glow in the dark. This reflective covering
reflects light back toward the lens and allows the animals to see better
in limited light.
9. Attached to the optic nerve is a wrinkled saclike structure connected
to the back of the eye. This is the retina and is considered the
innermost layer of the eye. Just as the tapetum surrounds the inside
of the sclera, the living tissue of the retina would have been smooth
and would have rested against the tampetum. The point at which the
retinal nerve tissue connects to the optic nerve is the eye’s blind spot.
10.Separate the retina from the back portion of the eye.
11.Remove the tapetum from the tough shell like outer layer of the
sclera and see if you can find the choroid.
12.Now observe the front half of the eye. Place the eye with the cornea
down.
13. In the front of the eye, or mixed with the vitreous humor will be a
small lens. This lens is convex and will have a slightly gray color. In a
live organism, the lens would be transparent.
14.Remove the lens from the vitreous humor. Dry it with a piece of
paper towel. Feel and feel how flexible it is.
Why is the lens flexible?
15.Hold the lens up in front of your eye, look through it.
Describe how the lens changes your vision.
16. A ring of tiny ciliary muscles are located along the inner side of the
iris. These muscles connect to the lens.
17. The pupil surrounded by the iris is on the front of the eye is the
actual opening located in the center of the pupil. The pupil is covered
and protected by the transparent living tissue of the cornea.+
18.The cornea would be transparent in a living organism. Use the
tweezers and hold up the front part of the eye. Look through the
sheep pupil and cornea.
How does your vision change?
Why does the cornea and lens have to be transparent?
19.ASSESSMENT: When you are finished with the dissection, raise your
hand. I will come by and ask you to identify one part of the eye.
You will have 2 chances to place the probe on the correct part. I will
record who answers correctly. NO TALKING during this time by any
lab partners.
20. Follow clean up procedures:
a) Wrap all remains in a paper towel and dispose of in the plastic
trash bag
b) All dissecting instruments, tray and gel must be thoroughly
washed in water.
c) The gel should be in the dissection pan, cover with a clean dry
paper towel.
d) All used gloves are to be disposed of the trash bag.
e) All lab aprons and goggles returned to the designated place.
f) Nothing is in the sinks at your lab station. If any material is in the
sink, use a paper towel to remove and dispose of the paper towel
in the trash bag.
g) Lab stations are clean and free of water/waste
h) Each person washes their hands with soap and water
This lab is adapted from
http://www.carver.org/middle_school/Teams/Ciarfella/Sci2.htm
Analysis Questions:
1. Why does the retina have to be smooth? Why not wrinkled? (Think
about reflecting light off of a wrinkled piece of aluminum foil and a
smooth piece.)
2. Think about learning the parts of the eye. Teachers can use a diagram,
a model, or dissection to show students the parts. In your opinion,
use examples from your own experience, which method (diagram,
model, dissection) do you think helped you to understand the parts of
the eye the most? Why?