This is certainly not an original idea of mine; Like the chicken wing dissection, it is a fairly standard middle school project, and you can find other presentations of it easily enough by searching the web. Nevertheless I wanted to include my own presentation and my own thoughts here.
We can expect that a cow, as a fellow mammal, has an eyeball that is pretty much like our own. Cow eyes are also readily available, as a byproduct of beef-production, and can be purchased inexpensively from science supply companies, such as Carolina Biological Supply, stored in preservative. Apparently it is also possible to obtain them fresh from processing plants, but I have never tried to do this. (The cornea and lens are normally cloudy in preserved samples, but may be clear if you can obtain the eyeball soon enough after removal.) Thus we can gain a pretty good idea what our own eyeballs are like by dissecting those of a cow.
Eyeballs do not float loose within their sockets, but are sewn in and turned around by various kinds of padding, connective tissue, and muscle. There is also a thick tough cord, which students will often identify as a tendon, extending from the back end of the eyeball. (Depending on how the eye was removed, this cord may be long and dangly or it may be a mere stub.) A cow’s eye, as normally provided, will not be a neatly trimmed eyeball but a messy agglomeration of flesh.
The first job of the dissector is to remove the connective tissue to produce a neatly trimmed eyeball, leaving the eyeball itself and the rear cord intact. The clear (or cloudy) dome on one side is the cornea, and marks the front of the eyeball. The cord, or “optic nerve”, is opposite to the cornea and marks the back of the eyeball. The connective tissue grows from the eyeball more or less in a circle around the “waist” or middle of the eyeball, and extends backwards forming a cylinder around the optic nerve. The most efficient way to remove the connective tissue is therefore to cut with a scissors in a circle around the middle of the eyeball.
When you are finished trimming, you should have something more or less like this:
In the side and rear views, you can see a ring of pink material around the waist of the eyeball. This is the remnant of the muscles that moved the eye. If you pinch the end of the optic nerve, you will be able to make it fray and ooze a little bit and you might be able to observe individual fibers.
To explore the interior of the eyeball, you need to cut it open. The neatest way of doing this is to cut it around the waist, separating it into a front half and a back half. However, it is also instructive to remove the cornea and look underneath it, and I think it works a little better to do this first, before bisecting the eyeball. The cornea is very tough, as a protective cover should be, and I think the best way to remove it is to make an incision down the middle with a scalpel, and then use a scissors to cut in a circle around the base of the cornea.
(The disadvantage of this method is that it does not remove the cornea intact–there is a slice down the middle. I once tried forgoing the scissors and just slicing around the base of the cornea with a scalpel, but it was much harder to control the position of the cut and not damage the material underneath. Also, you need a pair of scissors with a fairly strong shearing action. I find that the children’s scissors shown in the picture actually work fairly well, while the stainless steel dissection scissors I purchased from the supply company have a tendency to twist the fabric and greatly frustrate the user. I haven’t tried a pair of kitchen shears, but I imagine they work pretty well, too.)
When you make the initial incision into the cornea, you may notice a runny, watery juice leaking out. It may be clear, or it may be dirtied by something black. This is the “aqueous humor”. With the cornea removed, you can plainly see the object constituting the “pupil”, and a thin, wrinkly, ring-shaped curtain covering over the outer part of the pupil: the iris.
Once you have removed the cornea, you can proceed with the bisection of the eyeball. The wall of the eyeball, like the cornea, is pretty tough. You can either make an initial incision along the “waist” or “equator” with a scalpel, then continue around the waist with a scissors, or you can make the entire cut with a scalpel. Using care and a scalpel will help preserve the contents in the interior; Using scissors will be messier but will be much easier and more appropriate for young children.
The following picture shows the results of using care and a scalpel. In any case, students will easily notice that the interior is filled with jelly. The main advantage of carefully using a scalpel is that you may be able to preserve the jelly in a single, pretty blob. The jelly is the “vitreous humor”, and the blob is the “vitreous body”. It will adhere to the front half of the eyeball, but if you are careful, you can separate it from the back half so that it rests inside the front half like a scoop of ice cream in a cone.
Covering the back wall you will notice a pink skin, the retina. The retina is very delicate and fragile, and it is not fastened down. You can easily wipe it away, except for one particular spot. That spot is opposite to the optic nerve. The retina gathers together at this point and passes through the back wall and becomes the optic nerve.
Underneath the retina is a pretty, shiny, blue-green mirror, called the “tapetum”. This is what causes cow’s eyes to shine in headlights, and it is probably the most memorable part of an eye dissection for children. Unfortunately, this is one thing that cows and people do not have in common. Our eyes do not shine in headlights, because we do not have a tapetum. (We do have “red-eye” in flash photographs, but that is just a brightly illuminated retina, not reflection from a mirror.) Around the tapetum is a sooty, jet-black wall-coating. This is what a cow has where it does not have a tapetum, and this is what we have everywhere underneath our retina. This is the “choroid” and it serves the same function as dark walls in a theater. (If you observed dirty aqueous humor leaking out from underneath your cornea, can you now guess what might have dirtied it?)
The vitreous humor may or may not detach easily from the front wall of the eyeball. You may have to tease or tear it away, to reveal that the inside of the front wall of the eyeball looks something like this:
You can clearly see from the inside that the “pupil” is actually a lens (sometimes called a “crystalline lens” to be precise.) Like the cornea, it is supposed to be clear, but in preserved cow eyes it is usually a bit cloudy. Around the edge of the lens is a feathery ring, similar to the iris and actually part of the same fabric structure. If you want to remove the lens, you have to tear it free of this “lens-holding” fabric, known technically as the “ciliary body”. After you remove the lens, you might be tempted to try to wipe it clean, but if you rub too hard, you can tear the outer layer of the lens. If you slice completely through the lens with a scalpel, you can observe that it has layers all the way through, like an onion. If you are careful, you can tear the iris / lens-holder piece of fabric free from the wall of the eyeball and take a closer look at it. If you click the picture below to enlarge it and look closely at the black fabric, you should be able to distinguish the lens-holding ciliary body, the overlying feathery layer with a broader opening, from the iris, the thinner underlying layer with the narrower, somewhat oblong opening.