Cow eye dissection
Overview
You will work in small groups to dissect a preserved cow eye, identify the structures, and describe to the function of
each part of the eye. Your group will present your dissection to your teacher by pointing out each part of the eye with
your probe and describing its functions. You should also describe the path light takes as it moves through the eye.
Everyone in the group should participate in the presentation, and you should use your notes as little as possible during
the presentation.
Pre-lab
1)
To prepare for the dissection, label the following diagram with the names and functions of each part.
2)
Describe the path light takes as it enters and moves through the light. Underline any structure that refracts the
light.
3)
Describe which part(s) of the eye are affected in each of the following conditions
a) Cataracts
b) Macular degeneration
c) Glaucoma
d) Refractive errors (near-sightedness, far-sightedness, or astigmatism)
Dissection Instructions
1. Obtain a preserved cow eye, dissecting scalpel, probe, scissors, a tray, and gloves
2. Examine the external surface of the eye, noticing the thick cushion of adipose tissue. Identify
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the optic nerve as it leaves the eyeball
the cut remnants of the extrinsic eye muscles
the conjunctiva
the sclera
the cornea (preservation chemicals turn the cornea opaque – normally, it is clear)
3. Trim away most of the fat and connective tissue, but leave the optic nerve intact. Holding the eye with the cornea
facing downward, carefully make an incision with a sharp scalpel into the sclera about ¼ of an inch above the cornea.
The sclera is very tough, so this will require substantial force. Be prepared for the eyeball to squirt. Then, use scissors to
cut around the circumference of the eyeball paralleling the corneal edge.
4. Carefully lift the anterior part of the eyeball away from the posterior portion. The vitreous body should remain with
the posterior part of the eyeball. Move some of the vitreous humor aside to view the choroid tunic (In cows, this
appears iridescent due to a special layer called the tapetum lucidum – the tapetum lucidum is what creates ‘eye-shine’
in most mammals, but it is not present in humans).
5. Examine the anterior part of the eye and identify the following structures:
 ciliary body: black pigmented body that appears in a halo encircling the lens
 lens: biconvex structure that is opaque in preserved specimens
 suspensory ligament: a halo of delicate fibers attaching the lens to the ciliary body
6. Carefully remove the lens and identify the iris, the anterior continuation of the ciliary body.
7. Examine the posterior portion of the eyeball. Remove the vitreous humor, and examine the retina. It appears as a
white, probably crumpled membrane that separates easily from the pigmented choroid. Find the optic disc, the point
where the optic nerve attaches to the retina, as well.
Presentation
You will present your dissection to your teacher by pointing out the structures listed below and describing their
functions. You should also be sure to describe the path light takes as it goes through the eye. All members should be
involved in the presentation, and you should try to do it using your notes as little as possible.
Be sure to practice as a group before presenting to your teacher!
Structures to identify:
 cornea
 sclera
 retina
 optic nerve
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optic disc
lens
pupil
ciliary body
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iris
vitreous humor
Rubric
Pre-Lab Questions
Achievement Descriptor
Level
0
The pre-lab questions were not completed before the lab began or little attempt has been
made to accurately answer the questions.
1
The answers to the pre-lab questions partially accurate.
2
The answers to the pre-lab questions were mostly accurate.
3
The answers to the pre-lab questions were completely accurate.
Group Work & Lab Skills
Achievement Descriptor
Level
0
Group members did not follow safe lab procedures.
1
Group members had difficulty working together. Scholars used equipment carefully and
followed proper clean up and disposal procedures.
2
Group members mostly worked together effectively during the dissection and presentation.
Scholars used equipment carefully and followed proper clean up and disposal procedures.
3
All group members worked together safely and effectively during the dissection and
presentation. Scholars used equipment carefully and followed proper clean up and disposal
procedures.
Presentation
Achievement Descriptor
Level
0
Scholars showed little ability to identify structures and functions.
1
Scholars correctly identified some structures and functions while using notes and/or some
prompting from the teacher or other group members.
2-3
Scholars correctly identified most structures and functions with some use of notes and/or
some prompting from the teacher or other group members.
4-5
Scholars correctly identified each structure and describe its function with minimal use of
notes.
Achievement Grade
Level
11
100
Achievement
Level
7
Grade
10
95
6
70
9
85
5
65
8
80
4
60
75