Download Sheep Eye Dissection

Name:
Date:
Hour:
Sheep Eye Dissection
Materials
Sheep eye
Dissecting Probe
Dissecting tray
Latex/Vinyl Gloves
Dissecting
Scissors
Paper towels
Dissecting Pin
Tweezers
Plastic Trash bag
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.
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, all four muscles and the optic nerve. Notice the
amount of external yellow, light and dark tissue around the eye.
This yellow tissue is fat and more brown tissue is muscle.
2. Look toward the back of the eye for a small circular nodule
which is the optic nerve. 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.Use the pin to pierce the sclera, about half way between the
pupil and the back of the eye. You will be cutting a half inch
square between the muscles of the eye and through the sclera.
Be careful when you cut so that you do not destroy the pupil and
lens. Caution: liquid may ooze out.
5. Careful how much pressure you press into the eye. Once the
square is cut place that section onto your tray.
A clear jelly like fluid will be inside the eye. This is the vitreous
humor. With the tweezers carefully remove the lens with the
humor attached and place onto your tray. This lens is convex
and will have a slightly gray color. In a live organism, the lens
would be transparent. Note: 3 points Extra Credit for successful
“Operation” of removal of lens with humors attached
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Extra Credit
____________________
Teacher Signature
6. After showing instructor/ teacher, remove the lens from the
vitreous humor. Dry it with a piece of paper towel. Feel and
examine how flexible it is. Why is the lens flexible?
____________________
7. Hold the lens up in front of your eye, look through it. Describe
how the lens changes your vision.
8. Observe the back half of the eye. You will see a dark
“reflective” lining. This is called the tapetum lucidum 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 tapetum lucidum. The point at which the retinal
nerve tissue connects to the optic nerve is the eye’s blind spot.
10.Separate a small portion of the retina with some tapetum
lucidum from the back portion of the eye and place it onto the
tray to examine.
11.Now through your opening observe the front half of the eye.
Place the eye with the cornea down.
12. In the front of the eye, or mixed with the vitreous humor will
be a black colored iris. This looks like the underside of a
mushroom. The purpose of the iris is a muscle to open and
close the pupil. Notice the pupil opening within the iris.
13.A ring of tiny ciliary muscles are located along the inner side
of the iris. These muscles connect to the lens.
14. 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.
15.The cornea would be transparent in a living organism. Why
does the cornea and lens have to be transparent?
16.ASSESSMENT: While you are working on your dissection I will
come by and try to ask each of you to identify one or more
parts of the eye. You will have 2 chances to place the probe
on the correct part. I will note who who answers correctly.
NO TALKING/HELPING during this time by any lab
partners.
17. Follow clean up procedures:
a) Wrap all remains in a paper towel and dispose of in the
plastic trash bag by the door
b) All dissecting instruments, tray and blue 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
Analysis Questions: To be answered at your seat.
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?
3. 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)
This lab is adapted from http://www.carver.org/middle_school/Teams/Ciarfella/Sci2.htm