Download Eye Anatomy and Function Pre

STUDENT WORKSHEET
What’s in an Eye? Eye Anatomy and Function
Introduction: Do you wear glasses or contact lens? Do you know a family member that has
an eye disease, such as cataract or glaucoma? Have you ever wondered why you or a family
member may have an eye problem or disease? The eye is a fascinating organ that consists of
several components working together to allow for us to see. However, sometimes the eye can be
imperfect leading to eye problems and diseases. In this activity, you will dissect and identify
structures of a mammalian eye, Bos taurus. Bos taurus has a very similar eye to humans
although there are some differences that we will discuss. While identifying these structures, eye
problems and diseases that affect these structures will be discussed.
Pre-lab Questions:
Do animals need eyes? Why?
How does the eye function?
What are some vision problems and eye diseases that you are familiar with? Do you know
what causes these diseases?
1
Label the parts of the eye on this diagram:
2
What’s in an Eye? Eye Anatomy and Function
Laboratory Exercise
Materials:
Dissecting Microscope
Magnifying glasses
Mammalian Eye
Dissecting Kit
Dissecting Pan
Ruler
Gloves
Goggles
Eye Dissection Procedure:
Can you guess what organism this eye is from?
The major structures that you will identify are the cornea, sclera, optic nerve, aqueous humor,
pupil, iris, lens, conjunctiva, vitreous humor, choroid, and retina.
1. Observe the tissue surrounding the eye (Discussion I).
Discussion I: What type of tissue surrounds the eye?
Make an observation about the tissue surrounding the eye.
2. In this organism’s eye, there are four muscles that allow for the eye to move up and down
and side to side.
Do you think humans have less or more eye muscles? Why or why not?
Humans have ocular muscles, which allow for involuntary and voluntary eye movements that
contribute to our overall vision.
3
Why do you think these eye movements are important? Name one activity that would be
difficult if you could not move your eyes.
Observe the following optical illusion and describe what you see.
This optical illusion demonstrates how our eyes are constantly moving even when we
think our eyes are fixed on an object. These involuntary movements are an important
part of our vision.
3. Observe the outer structure of the eye. Identify the following: optic nerve, sclera, cornea.
In some cases, the conjunctiva, a transparent covering over the eye, is present.
4. Trim away excess tissue surrounding the eyeball on the sclera (see Activity I and
Discussion II).
Activity I: Cornea and Refractive Errors:
Make three observations about the surface of the eye.
1.
2.
4
3.
How many of you wear contacts or glasses? Are you nearsighted (myopia) or farsighted
(hyperopia)?
If you have myopia or hyperopia, use a white index card and punch out or cut a small hole
(pinhole) in the middle of the card. Place the card directly in the front and center of the eye.
Focus on any image that was not in focus before.
Do you notice any difference? What would be a major problem with making pinhole
glasses to correct for myopia and hyperopia?
Discussion II: Conjunctiva and Pink Eye
Conjunctivitis (pink eye) is the inflammation or infection of the conjunctiva transparent
membrane that covers the part of the eyelid and the anterior part of the sclera. This membrane is
important for providing blood supply to the outer surface of the eye. Exposure to viruses,
bacteria, allergens, or chemicals can cause inflammation of the small blood vessels in the
conjunctiva. The dilation of small blood vessels in the conjunctiva causes these vessels to
become more visible, causing the eye to appear pink or red.
Hemorrhaging of the blood vessels can also cause the eye to appear red.
5
5. Hold the eyeball gently with your thumb and forefinger at the cornea and near the optic
nerve.
6. Begin a cross-section of the eye by making an incision slightly behind the middle of the
eyeball through the sclera. Do not cut deep into the eyeball or squeeze it too tightly, so as
not to damage the interior structures. You may begin the cut with a scalpel and finish
with small scissors, or you may use a scalpel for the entire cut. If some of the vitreous
humor begins to come out of the eyeball as you cut through the sclera, let it come out
slowly. (Alternatively, you can slice around the cornea carefully and remove it to
see the iris and lens still suspended by the ciliary body.)
7. Once you have made a cut around the eyeball, separate the eye into halves. Let the
vitreous humor – gelatinous, transparent material found inside the eye behind the lensand any associated structures slowly slide out of the eye. You may need to tease the
vitreous humor gently away from the lining of the eye.
8. Look at the inside front portion of the eyeball. The lens may still be suspended in the
middle of the pupil (Activity II and Discussion III). Tip the front portion over and let
the lens and associated structures fall out. Again, depending on the viscosity of the
vitreous humor, you may need to ease the material loose from the inside of the eye.
Activity II: Lens and Accommodation
Once the lens is removed, make two observations of the lens.
1.
2.
Accommodation:
 Close one eye and stare at a point about 20 feet away.

Keep focusing on the point and raise one of your fingers into your line of sight just
below the point.
 How does your finger look?
 Now, change focus: look at the tip of your finger instead of the point 20 feet away.
 How does the distant image look?
6
As the lens ages, it becomes more rigid and the ability to accommodate reduces. This is
why as people get older; they wear bifocals or reading glasses. This condition is called
presbyopia and results in difficulty seeing objects that are near.
Based on what you know about correcting refractive errors, what kind of glasses (type of
lens) could be used to correct this problem?
Discussion III: Lens and Cataract
Imagine if the lens was opaque instead of transparent. How would that affect vision?
Cataract occurs when part of the lens becomes opaque inhibiting the amount of light that is
focused onto the retina. One cause of cataract is aggregated lens proteins causing a reduction in
transparency. The structure of the lens is like an onion with layers of cells packed tightly
together. The protein content in the cells is extremely high and the cells in the center part of the
lens do not have any organelles leading to an increase in the lens transparency and refractive
power. Because there are no organelles, the proteins in the lens are not replaced throughout life.
Many of your lens proteins were present when your lens developed in utero. Thus, it is really
important to protect our eyes especially from UV even at a young age to prolong the lifetime of
these proteins and to prevent damage to the proteins.
Although there is a surgery to remove the cataractous lens and replace it with a plastic lens,
cataract still remains the leading cause of blindness worldwide. It affects more than 50% of
individuals in the US over the age of 80 regardless of race and gender.
9. Observe the vitreous body, lens, and associated structures. The hyaloid fossa is an
indention in the center of the vitreous body that supports the lens. Surrounding the
hyaloid fossa is the zonula ciliaris, made up of suspensory ligaments that suspend the lens
and stretch it to focus. You will also notice dark lines around the hyaloid fossa. These
lines are pigment from the iris.
10. Pick up the lens with a pair of forceps. Pat it dry with a paper towel. Note: You may
want to place the lens on some printed text on paper and observe its ability to magnify.
This works best if you allow it to dry overnight.
11. Turn the front half of the eyeball over so that you are looking at the cornea. Cut the front
of the eye around the outside of the cornea (where the cornea meets the sclera) to remove
the cornea. This cut works best if you make the initial incision with the scalpel but then
use scissors to finish.
7
12. Place the cornea on your dissecting tray and cut it in half to observe its thickness.
13. Insert the forceps through the opening created and carefully separate the edge of the iris
from the inner surface of the eye. You may be able to remove the iris intact.
14. Pick up the back half of the eyeball and observe the structures on the inside. Identify the
retina, which contains the cone and rod photoreceptor cells. Follow this mass of nerve
cells to their convergence at the back of the eye where the optic nerve begins. This is
called the blind spot (Activity III).
Activity III: Where’s my blind spot?
What does your blind spot represent? Why is it called a blind spot?
Cover your right eye and with your left eye, look at the + and slowly bring the image (or move
your head) closer while looking at the +. At a certain distance, the dot will disappear from sight.
This is when the dot falls on the blind spot of your retina. Now, reverse this process by closing
your left eye and looking at the dot with your right eye. Move the image slowly closer to you
and the + should disappear.
8
Why is it hard to find your blind spot?
Cover your right eye and with your left eye, look at the red circle and slowly bring the image (or
move your head) closer while looking at the red circle. Remembering the distance where you
found your blind spot from the previous figure, describe what happens to the blue line.
15. Turn the back of the eyeball over and observe the optic nerve on the outside of the eye.
Pinch the nerve with your forceps the see the separate fibers of the nerve.
16. Look at the interior of the back portion of the eyeball again. Move the retina so that you
can see the dark, metallic-looking tissue at the back of the eye. This is the choroid, a thin
layer that lies between the retina and the sclera. The portion of the choroid that appears
iridescent blue and green with shades of yellow is called the tapetum.
17. Once you have observed all the structures of the eye – dispose according to your
institution’s instructions (Discussion IV).
Discussion IV: How to protect your eyes?
1. What parts of the eye that you observed in the lab are designed to protect the inside of the eye?
What properties do these parts have that enable them to provide protection?
2. What do you think are the parts of the eye that are the most easily damaged or injured?
Explain your answer.
9
Review Questions:
1. As you observed, the retina connected with the optic nerve at the back of the eye.
Why is this connection essential? What would happen if the retina became disconnected from the
optic nerve or if the optic nerve became damaged?
2. What do you think are the most important parts of the eye for seeing things effectively?
Explain why you think this.
3. List in the correct order the parts of the eye that light travels through as it passes through the
eye to the retina.
GLOSSARY
Aqueous humor - clear fluid filling the area between the lens and the cornea, composed mostly
of water and helps the cornea keep its rounded shape.
Blind spot - The area where the optic nerve leaves the retina. Each eye has a blind spot where
there are no photoreceptor cells.
Blood vessels - Tiny arteries and veins that carry blood to the retina.
Choroid – thin, dark sheet of tissue between the retina and the sclera.
Cones - One type of photoreceptor cells in the retina. They are responsible for daylight and color
vision.
Cornea – A transparent covering over the iris and the pupil that helps protect the eye and begins
focusing the light. In the preserved specimen, the cornea is cloudy.
Hyaloid fossa – indention in the center of the vitreous body that supports the lens
Fovea - A region of the retina where cones are highly concentrated and vision acuity is the
highest.
10
Iris - A muscle that controls the amount of light that enters the eye. It is suspended between the
cornea and the lens.
Lens - A biconvex transparent flexible structure that focuses the light coming into the cornea and
pupil, adjusts the eye's focus, allowing us to see objects both near and far. It is responsible for
about 20 percent of our focusing.
Optic nerve - The bundle of nerve fibers that sends information from the retina to the brain.
Retina – light-sensitive portion of the eye composed of receptor cells called rods and cones. It
detects images focused on the back of the eye by the lens and the cornea. The retina is connected
to the brain by the optic nerve.
Rods- One type of photoreceptor cells in the retina. They respond to dim light.
Sclera - The thick, tough, white outer covering of the eyeball. It is an opaque sheet of
connective tissue that protects inner structures of the eyeball and helps maintain rigidity.
Tapetum - The iridescent portion of the choroid tissue that is located behind the retina. Found in
animals that have good night vision, it reflects light back through the retina.
Vitreous body – the cavity between the retina and the back of the lens.
Vitreous humor – viscous fluid that fills the vitreous body and helps give the eyeball its shape.
Zonula ciliaris – ligaments that suspend the lens and stretch it to focus for near and far vision
Eye Dissection
11