Dissecting an Eyeball

Look inside the mammalian organ of vision with this popular and easy dissection activity.

Cows have four limbs, two ears and two eyes, and a chest and a belly, much like we do. Compared to an octopus or a beetle, a cow seems like merely a variation on the same design as people. As a fellow mammal, we can expect that a cow 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. The cornea and lens of long-deceased animals are usually cloudy, but the interior of the eyeball is still intact, and quite fascinating, and even pretty in its own way. Apparently, if you can obtain eyeballs soon enough after removal, perhaps by obtaining them fresh from processing plants, the corneas and lenses will still be clear, but I have never been able to do this. Also, I usually purchased my eyeballs submerged in liquid preservative, in bulk pails from Carolina. (One is reminded of a witch's cauldron when one opens the pail.) I once ordered them individually wrapped in plastic, but I didn't like those as well. I thought they were stiff and rubbery and hard to dissect well. I recommend buying the eyeballs in bulk pails, especially if you are buying enough for an entire classroom. You can also buy sheep eyeballs, which are cheaper, but also smaller, and I think the larger cow eyeballs are better for most purposes.

In any case, it is possible to obtain cow eyeballs without too much trouble, and we can gain a pretty good idea what our own eyeballs are like by dissecting them.

Trimming the Eyeballs

Living eyeballs do not float loose within their sockets. They are sewn in and turned around by various kinds of connective tissue, padding, and muscle. There is also a thick, tough cord 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.

Thus 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. Students will often identify the cord on the other end as a tendon. It certainly resembles one, because it is tough and cord-like. But it does not grow out of a muscle and it does not pull on a bone, as a tendon is supposed to do. The official modern name is the optic nerve. (Although in a way, the kids were right — the word “nerve” actually derives from a Greek root word meaning “sinew” or “tendon”.) The optic nerve sits 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.

Trimming the connective tissue from around a preserved cow eyeball
Trimming the Eyeball

When you are finished trimming, you should have something more or less like this:

Front, side, and back views of a preserved cow eyeball, with the muscles and connective tissue neatly trimmed away
A Trimmed Eyeball

In the side and rear views, you can see a ring of pink material around the waist of the eyeball. These are the remnants of the muscles that moved the eye (the extraocular muscles). If you pinch the end of the optic nerve, you might be able to make it fray and ooze a little bit and you might be able to observe individual fibers.

Removing the Cornea

To explore the interior of the eyeball, you need to cut it open. However, it is also instructive to remove the cornea and look underneath it at the front structure of the eye (the ones you normally see when you look into someone's eyes). And I think it works a little better to do this first, before cutting the whole eyeball open. 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 to not damage the material underneath. Also, you need a pair of scissors with a fairly strong shearing action. I found that the children's scissors shown in the picture actually worked quite well, while the stainless steel dissection scissors I purchased from the supply company had a poor shearing action, and would tend to twist the fabric without cutting it, and greatly frustrate the user. I haven't tried a pair of kitchen shears, but I imagine they work pretty well, too.)

Removing the cornea from a preserved cow eyeball with scissors
Removing The Cornea

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 fluid is the aqueous humor. With the cornea removed, you can plainly see the object forming the “pupil”, as well as a thin, wrinkly, ring-shaped curtain covering over the outer part of the pupil, namely the iris.

The front of a cow's eyeball with the cornea removed
The Eyeball with the Cornea Removed

Opening the Camera

Once you have removed the cornea, you can proceed to cut open the eyeball. The neatest way of doing this is to cut through it in a circle around the waist, separating it into a front half and a back half. However, the wall of the eyeball, like the cornea, is very tough. (The wall is technically known as the sclera, and the cornea is simply the transparent front portion of the sclera.) 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 avoid damaging this jelly and observe its natural shape, like something from a jelly mold. You may notice that it lives inside the eyeball as a single, pretty, smooth-surfaced orb. The jelly substance is the vitreous humor, and the round blob of jelly is called the vitreous body. It tends to adhere to the front half of the eyeball, but if you are careful, you can gently separate it from the back half so that it rests inside the front half like a scoop of ice cream in an ice cream cone.

The front and back halves of a dissected eyeball, showing the retina and vitreous body
The Front and Back Halves of the Eyeball

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, and that spot is right where the optic nerve comes out of the eyeball. The retina folds and gathers together at this point and passes through the back wall, and when it comes out the other side, it becomes the optic nerve. Where is the other end of the optic nerve? Where did it go before it was cut? The optic nerve goes out the back of the eye socket, and into the bottom of the brain. You might be able to find the other end if you are ever able to observe a preserved brain.

If you are familiar with using lenses to cast images of bright things, then you will notice that the cow's eyeball, and presumably yours, has a similar arrangement. When you cast an image with a lens, you hold up the lens and face it towards something bright, and behind it at just the right distance you place a screen of some kind, and then the lens will cast an image of the bright thing onto the screen. In your eyeball, the cornea and/or the “crystalline lens” acts like the lens, and when you point it at a bright scene, it casts an image on the retina, which acts like the “screen” or image-catching object. Then what? You use the skin on your fingers and around your body to feel things in the world around you...maybe this special sensitive skin inside your eyeball “feels” the picture somehow, and then sends it to the brain through the optic nerve?

(The active-minded students will often express consternation when they realize how their eye works, because they will have noticed that when lenses project images, the image is always upside-down. It boggles their minds that the pictures inside their retinas are not the right way up. When you look at a person, that person's head touches the bottom of your retina, and that person's feet land on the top of your retina. Unfortunately, I never did succeed in finding a good way of helping students through this. I did remember reading once about a study in which people wore special glasses that inverted the picture. They were helpless at first, but eventually they got used to it, and they could go about their daily business just as well as anyone else. But then the glasses were removed and they could see “right-side up” again, they became confused and helpless again, until they could re-adjust back to normal vision. I never did find out if this was a real study, but discussing it with my students at least helped them to grasp that there are issues of perceptual processing and “getting used to it”. As a side note, many of the scientists originally involved in figuring out how the eye works had similar difficulties in accepting that images on retinas could be upside-down.)

The Jelly, the Tapestry, and the Picture-Adjustors

Returning to our dissection, underneath the retina is a beautiful, shiny, blue-green mirror, officially 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. This is also 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 pink 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 movie theater. (If you observed dirty aqueous humor leaking out from underneath the cornea, can you now guess what might have dirtied it?)

The back wall of a dissected eyeball, showing choroid, tapetum, and retina
The Back Wall of the Eyeball

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:

The interior of a dissected eyeball, showing the crystalline lens, and the ciliary body
The Interior Front Wall of the Eyeball

If the students haven't poked the “pupil” loose yet, they should be able to see now from the inside that the “pupil” is actually another 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. So which lens actually casts the picture: the cornea, or the crystalline lens? If you submerge a magnifying glass in water, you may observe that it doesn't magnify nearly so well. There is less difference between glass and water than between glass and air, and a glass lens in water is thus not nearly as strong or powerful as a lens in air. The “crystalline lens” is surrounded by aqueous and vitreous humor, while the cornea has air on the outside. Knowing this, can you guess whether the cornea or the lens does most of the image-casting work in an eyeball? The cornea actually does most of the work, and the crystalline lens submerged inside the eyeball is only there to make fine adjustments to the picture.

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 from this “lens-holding” fabric, known technically as the ciliary body. There are tiny muscles in this fabric that pull on the lens, changing the shape, re-focusing the picture, and allowing us to see near things as well as far things. You can feel the strain in these muscles when you struggle to read something very close to your eye.

If you tear the lens free from this fabric, 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 look closely at the black fabric in the photo below, you may be able to distinguish the lens-holding ciliary body (the overlying feathery layer with a broader opening) from the separate layer of the iris (the thinner layer underneath with the narrower, somewhat oblong opening).

The ciliary body of a dissected eyeball
The Fabric Around the Lens

Why do we have an iris, a separate layer of fabric lying over the top of the crystalline lens? As you probably realize from observing it from the outside, the iris is like a curtain over a window, drawing back to let more light into the “movie theater”, or pulling closed to let in less light. If you've never noticed this before, try staring at your own eyes in a mirror. Allow your eyes to adjust to the dark, by holding them closed and/or turning off all of the lights. Then suddenly turn all the lights on. It may be even more dramatic if you suddenly shine a flashlight into your eyes at the same time. If you can watch your eyes as they go from darkness to bright light, you will notice that the pupils suddenly shrink. (This transition happens very fast, as a protective mechanism, but the reverse process of opening up and adapting to darkness is much slower. Also, if you are a teacher, you will probably realize that you can easily adapt this demonstration to a classroom by having the children partner up and stare into each other's eyes. Also, as long as I'm on the subject, I'll mention a similar fascinating project you can try: Construct some kind of divider to keep the bright light out of one eye, but allow it to surprise the other, and observe both irises when only one of them is suddenly exposed to bright light. The one that stays in darkness will nevertheless suddenly shrink in sympathy with its partner, presumably as a protective mechanism. But keep watching. What do you think will happen next?)