Biology 12
Cow Eye
Lab
Cow Eye Dissection
____/40
1/10/2016
Name: ________________
Partners _______________
Purpose:
The mammalian eye consist of many cells and tissues that are a result of
millions of years of evolution. During this lab exercise we will take our
background from class and apply it to the anatomy of the vertebrate eye.
Materials:
- 1 cow eye
- scalpel
- dissection tray
- forceps
-dissection scissors
- probes
Procedure
External Features
1. The white part of the eye, the sclera, is a tough outer covering of the eyeball. The sclera gives the eye its
shape and helps to protect the delicate inner parts.
2. The blue covering over the front of the eye is the cornea. When the cow was alive, the cornea was clear.
Together with the lens, the cornea refracts light and helps the eye to focus. The cornea gives a larger
contribution to the total refraction than the lens. The curvature of the cornea is fixed while that of the
lens is changeable., In your cow's eye, the cornea may be cloudy,.
3. You may be able to look through the cornea and see the iris the coloured part of the eye, and the pupil,
the dark oval in the middle of the iris. At the back of the eye is the optic nerve. To see the separate fibers
of the eye pinch the nerve with your fingers. If you squeeze it hard enough then you may get myelin off
the nerve (nerve insulation remember). It is the nerve that transmits visual information from the eye to
the brain.
(I) List two functions of the sclera
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(II) The cornea covers and helps protect the eye. What are two other things it does?
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(III) What is the function of the optic nerve?
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Biology 12
Cow Eye
1/10/2016
4. Without moving your head, look up, down, look all around. Six muscles attached to your eyeball move
your eye so you can look in different directions. This allows us to roll our eyes. Cows only have 4 (which
actually aren’t seen in this lab) meaning that cows could only look right, left, up and down. They couldn’t
roll their eyes even if they didn’t agree with you!
5. The muscles of the eyes work as a team. One eye is generally dominant. Make a circle with your finger
and thumb. Hold it up and look through it so that you are looking at an object across the room (a poster,
computer, etc). Close each eye, each time looking through your circle with the eye that remains open. The
eye that is open that allows you to still see the object through it means that eye is dominant.
6. If you reach up and feel around your eye. You will feel the bone of your skull. There’s yellow fat
surrounding your eyeball to keep it from bumping up against the gone and getting it bruised.
(IV) What is the function of the muscles attached to your eye?
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(V) Which of your eyes is dominant? ________________________________________________________
(VI) What is the purpose of the layer of fat surrounding your eye?
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Internal Features
7. Remove the cornea
(a) Use a scalpel to make an incision in the cornea. Hold the scalpel in your palm so the blade does not
slip. Cut the cornea (puncture it) until the clear liquid that is under the cornea. This liquid is water like. It is
called the aqueous humor. It is made to keep the shape of the eye.
8. Use the scalpel (or scissors if the scalpel is dull) to cut the sclera (eye white) around the eye so the front of
the eye (including the cornea) is removed leaving the back of the eye separate.
9. The cornea is made of very tough material, helping it protect your eye. It also helps bend light coming in
to your eye. The cow’s cornea has many layers to make it strong to stop grass or vegetation damaging the
eye.
10. The next step is to pull out the iris. The iris is between the cornea and the lens. You will notice that
behind the cornea and iris is the lens and the vitreous humour, a jelly like substance that is made of water
and protein. It also helps the eye keep its shape.
11. The iris is the dark disk on the back of the cornea. The pupil, the opening in the iris, is oval in cows but in a
human this would be circular
Biology 12
Cow Eye
1/10/2016
12. Remove the vitreous humor and the lens so you can see the pupil and iris.
13. The hard object found within the vitreous humor is the lens. In a living cow this would be transparent, but
the denaturing of the protein in the lens starts to make it cloudy. As you look at the iris you should see a
ring of tiny Ciliary muscles that are clear and look very much like saran wrap that help move , contract,
and change the shape of the lens.
14. Remove the lens.
15. You should be able to take a probe and peel the iris material off so the iris (the coloured part of the eye).
(VII) List the two functions of the lens:
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(VIII) Describe the iris and explain its function.
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(IX) Describe the pupil
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(X) If you enter a dark room, what happens to your pupil? ______________________________________
16. Now we will look at the other half of the eye. You will see a white material lining the back of the eye. This
is the retina. The retina is covered with light sensitive cells (Rods and Cones). The retina is loosely
attached to the back of the eye. You should be able to detach and push it around with a probe. You will
notice it is attached at only one place.
(XI) Why is the retina attached at only once spot on the back of the eye? (What does this create?)
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17. Remove the retina with a probe. Behind the retina is a blue coloured material called the tapeatum.
Humans do not have this adaptation. When you look in your cat’s eyes at night you will notice their eyes
shine. Cows have this similar adaptation. It is designed to help the animal see at night. They glow because
light is reflected back from the eye so the light sensitive cells have a better chance of collecting light
Biology 12
Cow Eye
1/10/2016
17. (Continued)
(XII) Suggest a way that the tapetum could have evolved through natural selection.
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18. Find your Blind spot. Use the two eyes below. Hold one hand over your left eye and look directly at the
left hand dot (with your right eye). At first you should be able to see both dots clearly. Move the paper
closer to your face slowly until you can no longer see the right hand dot. If you move your eye you can
see it, but in this situation your brain fills in the gap and you don’t see the dot. As you get closer or further away the dot disappears.
(XIII) Does the blindspot in vertebrates have any evolutionary advantage? Answer yes or no but explain
your choice!
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Biology 12
Cow Eye
1/10/2016
Analysis
1. The development of the eye has frequently been used to argue against evolution. Its development
shows a very complex organ. Critics say that such complex organs could not be developed through
natural selection (and evolution). Write a response to this statement that supports evolution using the
eye as evidence.
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2. Complete the following matching exercise:
Match
Word
Numbers List
Definition
External
1. A clear structure that refracts light and can change in curvature
Retina
2. Tiny ring of muscles that change the shape of the lens
Cornea
3. The pigment ring of muscles that change the size of your pupil
Lens
4. Works with the lens to refract light and helps the eye focus
Optic Nerve
5. Move the eye around
Iris
6. Transmits signals from your eye to the brain
Sclera
7. Gives the eye its shape and protects the inner parts
Ciliary Muscles
8. A thin layer of cells that convert light into nerve signals
3. Dogs don’t have any cones in their eye. What effect would this have on the dog’s vision?
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4. Humans have three types of cone cells– one for red, one for yellow, and one for blue. Dogs only have
two types of cones (one for yellow, one for blue). What effect will this have on the dogs vision?
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Biology 12
Cow Eye
1/10/2016
5. Fish that live in the ocean deep often have nonfunctioning eyes, or eyes that work very differently than
our own. Explain, using natural selection, why this is the case.
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6. Label the following diagram using information learned in this lab.
(Gelly Like Substance)
I
J
A
B
C
H
D
(Controls Eye Movement)
G
E
F
A. ___________________________________
F. ___________________________________
B. ___________________________________
G. ___________________________________
C. ___________________________________
H. ___________________________________
D. ___________________________________
I. ___________________________________
E. ___________________________________
J. ___________________________________
Adapted From: http://www2.mbusd.org/staff/pware/labs/CowEyeDissection.pdf