Download DISSECTING A COW`S EYE

IB Biology Year 2 / IHS
OCULAR ANATOMY LAB: THE FRESH BOVINE EYE!
Adapted from the San Francisco Exploratorium and other sources
Pre-Lab Assignment: In your journal, state the functions of the 14 mammalian eye
structures listed in the “Teacher’s Notes” section of Neurobio Objective E.2.2
Materials per team of two: fresh cow or calf eye*, dissecting tools, tray, gloves, ruler
& goggles, colored pencils & white paper OR camera and a color printer.
Procedure:
Make four labeled, informative sketches (see figures 1-4) on unlined white paper. You may make other sketches too!
Include a size reference for each sketch. Clear, labeled photographs printed in color may be submitted in lieu of
sketches.
1.
Examine the posterior and side surfaces of the whole eye.
__ Identify the optic nerve. Pinch it with a forceps – as you can see, it is fibrous (all those axons!).
__ Identify remnants of extrinsic eye muscles. (This may require trimming off some fat first.)
__ Sketch the front of the eyeball as figure 1. Label the sclera, iris, pupil & cornea. Draw the pupil shape
accurately!
2.
With a sharp scalpel, make your first incision in the sclera, just below the sclera-cornea boundary. Aqueous
and/or vitreous humor may leak out of the eye if you squeeze, so, don’t squeeze!
3.
Starting at the scalpel cut, use sharp scissors to cut all the way around the sclera. Keep scissor tips horizontal
and shallow. As the anterior half of the eye (the half with the cornea) starts to lift free, hold it in the center and
cut around it. Gently lift off the cornea-containing half and place it in the tray. (If the lens comes out at this
step, that’s okay, set it aside until step 5.)
__ Now cut off, examine and sketch the cornea as figure 2. Try to capture its shape in your sketch.
4.
Find and remove the muscle known as the iris. Note its shape.
5.
Let’s focus (pun intended) on the lens! Gently separate the lens from its attachment points (ciliary body,
suspensory ligaments) and/or vitreous humor. Try not to gouge or cut it
__ Rest the lens upon a pair of forceps. Try to read print with it. See if your answer depends on the distance
between the lens and what you are trying to read.
6.
Look into the gelatinous vitreous humor still occupying the lower half of the eye. At the “bottom” of the eye,
locate the blood vessels radiating over the surface of the retina.
__ Sketch the retinal vasculature as figure 3.
7.
Look for the retina as a pink layer below the blood vessels. Without the pressure of the intact vitreous, the
delicate retina detaches and looks wrinkled.
8.
Scoop out the vitreous body from the globe of the eye. Then, for better viewing, cut the sclera & turn the eye
inside-out.
__ Below the retina lies the choroid. In cows and many other animals, there is a tapetum - a highly reflective
layer of tissue within the choroid that is a blue-green, iridescent color. It allows cows to see well in dim
light (dawn/dusk).
__ Find the blind spot, the spot on the retina where the optic nerve exits the eye. Sketch the optic nerve
exiting the eye as figure 4.
9.
If time permits, look at eye structures (iris, tapetum, lens) with the stereoscope, or make a wet mount in saline
of a tiny piece of eye tissue (your choice) and view with the compound microscope. If pale, add stain.
Clean up: dispose of eye tissue in the tissue waste. Discard used gloves. Wash the tray, the tools & your hands!
Post-Lab Questions (Publish your responses on separate paper, rephrasing the questions in the answers.)
1. Why might the sclera of a grazing animal’s eye be tougher than our own?
2. What did the shape of the cornea remind you of, and why?
3. What color was the cow’s iris? How are human irises different?
4. Did the cow lens magnify print? What did it do to the image? Explain.
5. What is the function of the tapetum? Think ‘ecologically’ – the wild cousins of cows (deer & bison) have one,
too.
6. Which aspect of the cow eye dissection was most interesting to you, and why?
IB Biology Year 2 / IHS
Alternative Assignment, OCULAR ANATOMY LAB: THE FRESH BOVINE EYE
1.
2.
3.
4.
5.
Visit the website of the famed San Francisco Exploratorium at www.exploratorium.org
Search for: Cow’s eye dissection. Click on the first entry that comes up.
Choose Watch Online.
Select, Watch Step 1 Video – BUT, choose “Watch All Steps” instead.
The Cow Eye Dissection is about 8 minutes long.
You are still responsible for doing the pre-lab in your journal, sketching or finding photographs of the four figures
in the lab and turning in labeled, color copies (see lab handout), and publishing your answers to the 6 post-lab
questions.
IB Biology Year 2 / IHS
Alternative Assignment, OCULAR ANATOMY LAB: THE FRESH BOVINE EYE
1.
2.
3.
4.
5.
Visit the website of the famed San Francisco Exploratorium at www.exploratorium.org
Search for: Cow’s eye dissection. Click on the first entry that comes up.
Choose Watch Online.
Select, Watch Step 1 Video – BUT, choose “Watch All Steps” instead.
The Cow Eye Dissection is about 8 minutes long.
You are still responsible for doing the pre-lab in your journal, sketching or finding photographs of the four figures
in the lab and turning in labeled, color copies (see lab handout), and publishing your answers to the 6 post-lab
questions.
IB Biology Year 2 / IHS
Alternative Assignment, OCULAR ANATOMY LAB: THE FRESH BOVINE EYE
1.
Visit the website of the famed San Francisco Exploratorium at www.exploratorium.org
2.
3.
4.
5.
Search for: Cow’s eye dissection. Click on the first entry that comes up.
Choose Watch Online.
Select, Watch Step 1 Video – BUT, choose “Watch All Steps” instead.
The Cow Eye Dissection is about 8 minutes long.
You are still responsible for doing the pre-lab in your journal, sketching or finding photographs of the four figures
in the lab and turning in labeled, color copies (see lab handout), and publishing your answers to the 6 post-lab
questions.
IB Biology Year 2 / IHS
Alternative Assignment, OCULAR ANATOMY LAB: THE FRESH BOVINE EYE
1.
2.
3.
4.
5.
Visit the website of the famed San Francisco Exploratorium at www.exploratorium.org
Search for: Cow’s eye dissection. Click on the first entry that comes up.
Choose Watch Online.
Select, Watch Step 1 Video – BUT, choose “Watch All Steps” instead.
The Cow Eye Dissection is about 8 minutes long.
You are still responsible for doing the pre-lab in your journal, sketching or finding photographs of the four figures
in the lab and turning in labeled, color copies (see lab handout), and publishing your answers to the 6 post-lab
questions.