Materials Needed:

• Two cow eyes --- PUT ONE IN THE FREEZER A FEW HOURS BEFORE DISSECTION
  
• One new single-edged razor blade, scalpel or X-acto blade
  
• Small, sharp, pointed scissors
  
• Tweezers
  
• A magnifying glass
  
• A cutting board
  
• Wax paper and paper towels
  
• Small plastic containers to store dissected parts
  
• Surgical gloves
  
  

Where to find cow eyes

Cow eyes may be ordered at a butcher shop or purchased directly from a slaughterhouse It possible purchase eyes with muscles and fat intact Cow eyes picked up the day of the dissection will be fresher and easier to cut. You may also find frozen cow eyes even easier to cut Ask the meat-processing establishment to keep as much of the orbital contents as possible attached to the eyeball. This will include orbital fat and the muscle cone Usually the meat processor extracts only the eyeball itself and a small amount of the orbital contents.

Observe:       

The eyeball has much fat attached to it. This serves as a cushion for the eye as it sits inside the bony orbit.
Cows have four muscles that move the eyes up, down, left and right. From the insertion point, they run backwards behind the eye. They can be used in combination to move the eye up and to the right or down and left. A human has six muscles.

The cornea bulges (convex) forward. There appears to be a chamber or space between the back of the convex cornea and the front of the iris or colored part of the eye that forms the pupil. This space is known as the anterior chamber and contains a clear liquid called aqueous humor. The posterior chamber lies behind the iris and also contains aqueous humor. The aqueous is secreted by the ciliary processes that lie at the outer edge of the posterior chamber. Before you begin your dissection, can you find the following structures externally visible:

• Cornea
  
• anterior chamber
  
• iris
  
• pupil
  
• sclera
  
• extraocular muscles
  
• the optic nerve.
  

Dissection Procedure

CAUTION: WHILE THE EYE IS A VERY DELICATE ORGAN, THE OUTER
STRUCTURES ARE VERY FIBROUS AND TOUGH. YOU MAY FIND IT DIFFICULT TO CUT EVEN WITH A SHARP INSTRUMENT. BE CAREFUL AND ALWAYS CUT AWAY FROM YOUR FINGERS.

We will use the frozen eyeball first. Our purpose is to make one cut that will allow us to remove the cornea and iris. We are not particularly concerned about the zonules that hold the lens in place with this eyeball so make sure you get deep enough to free the cornea and iris. Do not remove the cornea and iris now. We will allow it to thaw a little more. Now is a good time to cut through the equator of the eyeball. Again, using a sharp cutting tool cut around the middle of the globe. Cut deep enough to get into the vitreous cavity. Once we remove the vitreous and everything forward of it, we will have a good view of the posterior structures including the retina and the beginning of the optic nerve.

The perpendicular cut

When everything thaws, we should be able to lift the cornea then the iris with a pair of tweezers. The cornea may be preserved in contact lens saline solution. I would just keep the iris moist and in the refrigerator until needed. Note how thin they both are especially the iris.

The Cornea
The Iris

The lens of the eye should be sitting right in the center. Cut around the lens to free the vitreous attachments. Once you have it free, remove it and clean off the excess materials so that it looks like a lens. It may be stored in saline also.

The vitreous will probably be pretty soupy by now so remove the sclera anterior to the cut you made earlier and allow the vitreous to drain out of the posterior shell. You should have a view of the very thin retina and the very small optic nerve. Keep this section moist and in a refrigerator until you are ready to use it.

The Retina

The retina is the most important structure of the eye. In the human eye, this thin, Saran-wrap-like layer contains one million rods and six million cones. Rods detect only black and white and function more under lower light conditions such as our present at night. There are three types of cones. They are sensitive to red, green and blue light. They obviously provide color vision and in general provide higher visual resolution (more acute vision) than is provided by rods. The fovea, located at the center, contains the highest concentration of cones in the retina. Ultimately, little fibers from the receptors collect and form the optic nerve. This is the white nerve visible on the retina. Through a chemical process the retina takes light energy and creates an electrical impulse that is transmitted to the visual center of the brain via the optic nerve and optic tract.

Now turn to the second, unfrozen eyeball. Assuming we obtained good specimens of the cornea and iris from the other eye, we will be primarily interested in demonstrating an intact lens that is still supported by the zonules. The zonules are the suspensory attachments that support the lens and cause the lens to change focus when the cow reads. Using a very sharp single-edged razor blade or scalpel, cut along the border between the sclera and the cornea. Your cutting instrument should be parallel to your cutting surface. When you first penetrate the cornea, the clear aqueous humor will begin to leak out. Do not cut deep. Keep the tip of the blade visible through the cornea. Continue to cut around the cornea. Scissors may be handy here if they are pointed and sharp. Remove the cornea once it is free. Preserve it if you didn't get a good specimen from the first eye.

Parallel Cut into Limbus

Look at the pupil. The pupil is formed by the very thin and fragile iris. It is sitting right on the lens which we want to disturb as little as possible. Unless you did not get a good specimen from the other eye, we are not interested in preserving this iris. Place the end of one side of your tweezers under the edge of the pupil and raise the iris. Close down the tweezers and lift the iris away from the lens and note where the edge of the iris inserts at the peripheral edge. We want to cut the iris here, but we do not want to cut too deep and disturb the zonules or ciliary body where the zonules attach. Keep the iris lifted and cut 360 degrees to free the iris.

Ideally, you will now see the lens of the eye. The lens is suspended by zonules. The zonules are controlled by the ciliary muscle. By contracting and relaxing the ciliary muscle the zonules pull on the elastic lens which allows the eye to increase its power for viewing closer objects.

Lens Zonules Ciliary Body

Did you note the flexibility of the lens you removed earlier? As we age, our lens grows harder and becomes less flexible. Humans need reading glasses to help with the loss of focusing ability as they age. Cows probably just give up reading.

Optic Nerve

View the outside of the eye. Note the optic nerve. Pinch the nerve and note the small fibers present. Remember there are one million fibers in the human optic nerve. Does there seem to be an organizational pattern to fibers? There doesn't appear to be, but actually there is. Fibers from the temporal or ear side of the retina stay on the same side of the brain as the eye is. Whereas, fibers that serve receptors on the nasal side cross over to the opposite side and join the temporal fibers from the other eye. Together they form the optic tract which winds its way to the brain's visual center at the back of the head. The actual crossing over takes place above the roof of the mouth. The significance is that objects on the left side of our vision are seen by the right side of our retinas and are ultimately imaged by the right side of our brain. The opposite is true for objects seen to the right.