Eye Dissection
Part 1 - Eye Basics
What is it?
•
Your eye is a complex organ that allows us to perceive light and
interact with our environment. Structurally it is considered a
highly specialized SIMPLE eye (because it only has one lens)
How does it work?
•
When light comes into your eye, it
passes through the cornea and the hole
of this iris or the pupil. Muscles in the
iris adjust the size of the pupil
according to the level of light. The
light is then focused through the lens
onto the retina, where light sensing
cells convert the image to an electrical
impulse that gets carried to the brain
via the optic nerve.
Why dissect a cow’s eye?
•
•
Cow eyes are very similar in structure
to the human eye.
Cow eyes are larger than human eyes
which makes it easier to identify and
study the internal structures of the eye.
Part 2 – The Outside – Muscles, Cornea & Optic Nerve
Identify Structures
Have students examine the eye as a whole. Ask them to make observations. Have them
identify the muscles of the eye, the sclera, the cornea and the Optic Nerve. Discuss each
of their functions.
Sideview
2
1. Fat – The fat that surrounds the eye provides a
cushion from the eye sockets of the skull.
2. Muscle – the muscles of the eye allow for
movement of the eye just like all the muscles in
our bodies. A cow’s eye has 4 muscles to move
the eye: up, down, left & right. Human eyes
5
have 6 muscles to allow for all around rotation.
Try making a circle with your eyes!
3. Sclera – Thick, tough, white outer covering of the
eyeball.
4. Cornea – A clear, tough covering over the iris
and the pupil that helps protect the eye and begins
focusing the light.
5. Optic Nerve – The bundle of nerve fibers that
4
3
1
carry information from the retina to the brain.
Part 3 – Inside the Eye – Dissection
Dissection
As you go through the steps of the dissection be sure to identify each structure and its
function.
Part 1 – Cornea, Aqueous Humor, Iris & Pupil
•
Students should use the point of their
scissors to poke a small hole at the edge
of the Cornea where it meets the Sclera.
STUDENTS SHOULD BE EXTRA CAREFUL
WITH THIS STEP IF THEY ARE HAVING
TROUBLE YOU CAN ASSIST THEM BY
MAKING THE FIRST INCISION FOR THEM
WITH YOUR SCALPEL.
•
Using the hole as a starting point, have
students cut around the edge of the
Cornea. Just behind the Cornea is the
Aqueous Humor, a clear fluid that helps
the cornea keep its rounded shape.
While the students are cutting this liquid
should spill or squirt out!
•
Carefully remove the Cornea section
from the rest of the eyeball and turn it
over. Behind the Cornea is the Iris.
Carefully peal the Iris off of the
Cornea and allow the students to
examine it. What do they notice?
Ridges, Hole, Color, etc. Remind
students that the Iris is a muscle that
opens and closes to allow the right
amount of light into the eye. The hole
in the Iris is the Pupil. The color of
the Iris is the color that we refer to
when we ask, “What color are your
eyes?”
Part 2 – Lens, Vitreous Humor, Retina & Tapetum
•
Have students look into the
remaining portion of the eye. The
Lens is the round, hard, ball-like
structure in the middle of the eye.
Have students remove it and
examine it up close. The Lens is a
clear, flexible structure that adjusts
the eye’s focus, allowing us to see
objects both near and far. It is
responsible for about 20% of our
focusing. NOTE: The Vitreous
Humor may come out with the lens.
•
If the Vitreous Humor did not come out with
the lens, have the students pour it out onto their
dissecting trays. NOTE: Tell the students to
pour SLOWLY in order to keep the Retina at the
back of the eye intact! It is very delicate! Ask
students what they think the Vitreous Humor is
for. The Vitreous Humor is the thick, clear jelly
that helps give the eyeball its shape. It is made
out of protein and water.
•
With the remaining hollow eye, cut a ½
inch slit on opposite sides of the eye and
CAREFULLY turn it inside out. This
should allow the Retina to hang from the
optic nerve (IF the Retina hasn’t already
been detached). Have the students
examine the Retina. The Retina is a net
of light sensitive cells that fans out over
the back of your eye. 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. Ask
students if they have ever heard of a blind spot in your
eye? Explain that the spot where the retina attaches to
the optic nerve has no light-sensitive cells. Therefore,
any light projected onto this part of the eye is not
detected – it’s a blind spot!
•
Carefully peal the Tapetum from what’s left of
the eye. The Tapetum is the colorful, shiny
material located behind the retina. It is found in
animals that have good night vision because it
reflects light back through the retina. Ask
students if they have ever seen cow’s eyes
glow? Have they seen any other animals’ eye’s
glow in the dark? CATS!
Background
Vision:
Light, and vision, are the driving force behind most of our life here on earth, so it is very
important that we have an organ with which to sense light and use it. “Some say that it is
the development of stereoscopic vision, along with the development of the large human
brain and the freeing of hands from locomotion, that have allowed humans to evolve to
such a high level” (howstuffworks.com).
When light comes into your eye, it first passes through the cornea and through the hole of
the iris, called the pupil. Muscles in the iris adjust the size of the pupil according to the
level of light. The pupil dilates (grows larger) when there is less light, to allow as much
light into the eye as possible.
After passing through the pupil, light is focused through the lens onto the retina. The lens
is responsible for bending rays of light even closer together before they reach the retina.
It is also responsible for adjusting itself to view distant or nearby objects.
The image projected onto the retina by the lens is actually inverted (upside-down), but
your brain will revert it for you (to right-side up). The process of bending light and
focusing it through the cornea, pupil, and lens to reach the retina (without losing any of
it) is called refraction.
The retina is the light sensing structure of the eye and is made up of two different types of
cells, called the rods and cones. The retina contains 100 million rods and 7 million
cones. Rods are in-charge of low light vision, while cones sense color and detail. When
light hits these cells, it begins a chemical reaction that creates an electrical impulse in the
optic nerve, which sends signals to your brain.
The retina is lined with a black pigment called melanin, in order to lessen the amount of
reflection. Its central area is called the macula, which has a concentration of only cones.
The macula is responsible for sharp, detailed vision.
Measuring Vision:
Vision is measured by comparing what you can see at a distance of 20 feet with what
most people can see at 20 feet. If you have 20/20 vision, then you can stand 20 feet away
from an eye chart and see what “normal” people can see. If you have 20/10 vision, you
can see what most people see at 10 feet when you are 20 feet away. Or if you have
20/100, you can see at 20 feet what normal people see at 100 feet. If you see at 20/200,
you are legally blind in the United States.
FLOATERS
Seeing floaters (spots) before one or both eyes is a frequent adult complaint. Floaters are
usually most noticeable against a white homogeneous background and seem to move
slowly when the eye is still. Floaters maintain their relative position in the visual field
with eye movement. They result from contraction of the vitreous gel and its separation
from the surface of the retina (posterior vitreous detachment). This leads to
macroscopic opaque aggregates of vitreous fibers, which can be seen floating in the
vitreous. Since the vitreous gel is denser where it attaches to the optic nerve, floaters are
usually more apparent in this area. Although floaters usually are without significance, in
a few patients they may indicate a tear in the retina. They are more prevalent in highly
myopic and older persons and tend to become less noticeable with time.
A minute vitreous hemorrhage or vitritis (inflammation in the vitreous) may also
produce floaters. Retinal detachments may be preceded by a shower of "sparks" or
lightning flashes (photopsia) and may be accompanied by a shower of floaters. Only after
the retina actually separates from its underlying structure (the retinal pigment epithelium)
does a curtain of visual loss move across the visual field.
Although floaters usually are not associated with serious disease, they warrant meticulous
examination of the entire retina and media after dilation with a short-acting mydriatic or
cycloplegic drug (eg, cyclopentolate 1%, 1 drop, repeated in 5 to 10 min, or if wider
dilation is required and the patient does not have hypertension and is not receiving an
oral -blocker, phenylephrine 2.5%, 1 drop, repeated in 5 to 10 min). Examination is best
performed by indirect ophthalmoscopy, a technique used by ophthalmologists. Vitreous
floaters can be seen with a high plus lens by looking into the red reflex at a distance of 15
to 30 cm (6 to 12 in). Repeated examinations are warranted if the complaint continues, if
vision is affected, or if apprehension persists. Floaters of recent onset or those
accompanied by flashes of light should be evaluated by an ophthalmologist. Disturbance
of vision always demands an explanation.
ERRORS OF REFRACTION
When light rays reach an angulated surface, like the lens, they are bent. This is called
refraction. In emmetropia, no optical defect exists, and parallel light rays (eg, originating
from a distant object) entering the eye focus clearly on the retina. In ametropia, an optical
defect exists in one or a combination of the following forms:
¾ Myopia, or nearsightedness, means you cannot see objects that are far away. Light
rays are focused in front of or before the retina, either because an eye is too long, or
because a lens system has too much power to focus. A concave lens will fix the
problem, and spread out the light before it hits the eye.
¾ Hyperopia, or farsightedness, means you cannot see objects that are close; for
instance, you might have trouble reading the paper. Light rays are focused behind
the retina, either because an eyeball is too short, or a lens system is too weak or has
too little focusing power. A convex lens can help focus the light rays before they
reach the eye. This is the most common refractive error in eyes.
¾ Astigmatism is an uneven curvature of the cornea and causes distortion of vision.
A cylindric corrective lens (a segment cut from a cylinder) is used that has no
refractive power along one axis and is concave or convex along the other axis.
¾ Anisometropia, a significant difference between the refractive errors of the two
eyes (usually > 2 diopters), is seen occasionally. When the refractive errors are
corrected with lenses, differences in image size (aniseikonia) are produced and can
lead to difficulties in fusion and even to suppression of one of the images.
¾ Presbyopia, a hyperopia for near vision that develops with advancing age, results
from a physiologic change in the accommodative mechanism by which the focus of
the eye is adjusted for objects at different distances. Beginning in adolescence, the
lens substance gradually grows less pliable and eventually cannot change shape
(accommodate) in response to the action of the ciliary muscles. As a result, the
person becomes unable to focus well for near vision (objects closer than 30 to 60 cm
[1 to 2 ft]) but usually does not need corrective glasses until he reaches his early to
mid-40s.
Detecting Color:
The eye is sensitive to vast ranges of light intensity. As you know, after light passes
through the pupil, it enters the eye and hits the retina. The rods and cones on the retina
are what detect an image’s colors. Three different types of cones are sensitive to colors,
red, green, or blue. Rods only register in black-and-white. At night, only the rods are
sensitive enough to respond to images, so the night world is actually seen in black-andwhite. However, most people do not see in that way because their brains remember
having seen the same scenes in color during the day (Cole, 68). Additionally, rods can
detect some green, so in a dark room green objects may appear brighter than red ones.
Color Blindness:
Color blindness is an inherited disorder, in which a person has trouble differentiating
between certain colors. Most often, a person simply has trouble distinguishing between
red and green. The inability to see any color, or seeing only in gray, is very rare. The is
disorder affects more men than women (8% of men, and 0.4% of women), as the capacity
for color vision is located on the X chromosome.
VOCABULARY WORDS
Aqueous humor
A clear fluid that helps the cornea keeps 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.
Ciliary body
Muscles that control the shape of the lens for near and far vision.
Cones
One type of photoreceptor cells in the retina. They are responsible for daylight and color
vision.
Cornea
A clear, tough covering over the iris and the pupil that helps protect the eye and begins
focusing the light.
Fovea
A dimple in the retina where cones are concentrated and vision is most acute.
Iris
A muscle that controls the amount of light that enters the eye. It is suspended between the
cornea and the lens.
Lens
A clear, flexible structure that 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 carry information from the retina to the brain.
Retina
The layer of light-sensitive cells lining the inner eyeball. 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.
Suspensory ligaments
Fibers that connect the ciliary body to the lens.
Tapetum
The colorful, shiny material located behind the retina. Found in animals that have good
night vision, it reflects light back through the retina.
Vitreous humor
The thick, clear jelly that helps give the eyeball its shape.
ACTIVITIES:
Blind Spot Activities:
One of the coolest experiments to try is the demonstration of the blind spot. The
blind spot is the area of the retina that lacks receptors, which respond to light.
Therefore, an image that falls on this region will NOT be seen. It is in this region
that the optic nerve exits the eye on its way to the brain. To find your blind spot,
stare at the circle and cross images.
Close your right eye, or hold your hand in front of it to block what you see with
that eye. Then, with your left eye, stare at the black circle from a few feet away.
Slowly, really slowly, move your head closer to the image. At a certain distance
the cross will disappear from sight. This is when the cross falls on top of your
blind spot or the image is on your retina. Try it with both eyes. The cross
disappears because it falls on the optic nerve head, the hole in the photoreceptor
sheet.
So, as you can see, you have a pretty big blind spot, at least as big as the spot in
the diagram. What's particularly interesting though is that you don't SEE it. When
the spot disappears you still don't SEE a hole. What you see instead is a continuous
white field (remember not to LOOK at it; if you do you'll see the spot instead).
What you see is something the brain is making up, since the eye isn't actually
telling the brain anything at all about that particular part of the picture.