Cow Eye Dissection
Introduction:
All mammalian eyes are very similar in structure. Therefore, dissecting a sheep or cow eye is an excellent
activity for learning human eye structure. In this lab you will identify the similarities and differences of a
human and cow eye to better understand how the structures relate to their function.
Materials: cow eye, dissecting tray, probe, dissecting needle, knife, razor blade
Procedure:
Before you begin observing, write a title, purpose and hypothesis for the lab. Your hypothesis should predict
what differences you would expect to see in the cow eye as compared to the human eye and your reasoning.
(If…then…because….)
A. External Anatomy
1.
Obtain a mammalian eye, place it in the dissecting tray and dissect as follows:
a. Trim away the fat and other connective tissues but leave the stubs of the extrinsic muscles
and optic nerve. This nerve projects outward from the posterior region of the eyeball.
b. The conjunctiva lines the inside of the eyelid and is reflected over the anterior surface of the
eye, except for the cornea. Lift some of this thin membrane away from the eye with forceps
and examine it.
c. Locate and observe the cornea, sclera, and iris. Also note the pupil and its shape. Identify
any remaining eyelid and eye lashes. The cornea from a fresh eye will be transparent, when
preserved it becomes opaque. See how many additional parts you can identify. You should be
able to see the white of the eye, the clear covering of the eye, the fat and muscles surrounding
the eye.
2. Examine the posterior surface (the back). The light brown stumps are the extrinsic eye muscles and
are around the eyeball. Near the center, identify the nerve that runs to the brain.
3. Take down 10 descriptive observations.
4. Draw a diagram of the outer surface and label all the structures you can observe from the outside both
on the anterior and posterior sides. Your diagram should be a lateral view so you can indicate all the
structures observed. Identify your scale and title your diagram.
B. Internal Anatomy
1. Take down 10 observations of the internal anatomy after you cut it open. Use sharp scissors to
make a frontal section of the eye. To do this, cut through the wall about 1 cm from the margin of
the cornea and continue all the way around the eyeball. Try not to damage the internal structures
of the eye.
a. Gently separate the eyeball into anterior and posterior portions. Usually the jellylike
vitreous humor will remain in the posterior portion, and the lens may adhere to it. Place
the parts in the dissecting tray with their contents facing upward.
b. Examine the anterior portion of the eye and locate the ciliary body, which appears as a
dark, circular structure. Also note the iris and the lens if it remained in the anterior portion.
The lens is normally attached to the ciliary body by many suspensory ligaments which
appear as delicate, transparent threads.
c. Use a dissecting needle to gently remove the lens and examine it. If the lens is still
transparent, hold it up and look through it at something in the distance and note that the
lens inverts the image. The lens of a preserved eye is usually too opaque for this
experience. If the lens of the human eye becomes opaque, the defect is called a cataract.
d. Examine the posterior portion of the eye. Note the vitreous humor. This jelly like mass
helps to hold the lens in place anteriorly and helps to hold the retina against the choroid
coat.
e. Carefully remove the vitreous humor and examine the retina. This layer will appear as a
thin, nearly colorless to cream-colored membrane that detaches easily from the choroid
coat.
f.
Note the iridescent area of the choroid coat beneath the retina. This colored surface in
animals is called the tapetum fibrosum. It serves to reflect light back through the retina,
an action thought to aid in the night vision of some animals. The tapetum fibrosum is
lacking in the human eye.
g. Be ready to show your me the lens, iris, pupil, cornea, retina, optic disk, optic nerve, sclera
and vitreous humor. Once you have dissected and know the parts, call me over for an oral
quiz.
h. Clean up your dissecting tools, tray and desk. Discard the cow eye parts at the front of the
room in the bag provided. Make sure your tray is wiped and dry and your tools are clean.
With your partners, bring your cleaned tools and tray to me to get another stamp showing
you have cleaned up.
Discussion Questions
1. How is the cow eye similar to the human eye?
2. How is a cow eye different than the human eye?
3. Match the following:
a. Contains the photoreceptors for vision
b. The colored portion of the eye.
c. This structure changes shape to focus light on the retina.
d. The opening in the iris through which light passes.
e. The iridescent portion of the choroids layer found in nocturnal animals.
f. Consists of muscle, which control and shape the lens.
g. The white of the eye.
A. tapetum lucidum
B. lens
C. retina
D. sclera
E. ciliary body
F. pupil
G. iris
4. Explain what you think would happen to the eye if the pressure changed dramatically in the vitreous
humor.
5. Explain what rods and cones are and determine how a cow’s rods and cones may be different than a
human’s?
6. What is the function of the dark pigment in the choroid coat?
7. Describe the lens of the dissected eye. Explain how the lens works in both the human eye and the cow
eye.
Conclusion:
Write a conclusion for this lab. What did you learn? Re-visit your hypothesis.