Eyeballs are weird. They’re these strange little blobs that sit inside your skull, and there’s nothing else in your body quite like them. They aren’t bone, but they aren’t soft like the tissue under your skin, either.
Most of your eyeball isn’t even that white stuff you see from the outside. Most of your eye is actually a clear, jelly-like type of collagen, the same kind of structural protein that gives your nose and ears their shape.
The whiteness you see when you look at your eye is the sclera the outer layer that holds the whole sac of goo together. But it’s just a tough, thin outer coat. The sclera covers everywhere except the very front of your eyeball, where it connects to the clear tissue that makes up your cornea. It’s clear so that it lets in light, just so you can actually see.
Then there’s all that stuff that helps the light get to the back of your eye. After it comes through the cornea, light reaches the colorful part of your eye next, the iris. Its main job is to control the size of your pupils so that the right amount of light gets further in.
Directly behind the iris is a lens, a hard, glass-like structure that does exactly what it sounds like: it focuses the light. Meanwhile, the retina, which is actually what does the seeing, is all the way in the back. It contains light-sensitive cells, as well as the system of nerves that are hooked up to your optic nerve to tell your brain what you’re looking at.
But there’s still a lot, a LOT of space left in your eyeball. And most of it, 80% of your eyeball by volume, is the clear, gel-like substance that sits between the retina and the lens. That gooey stuff is called the vitreous humor or the vitreous body, and it’s there as a shock absorber and for structural support.
Vitreous humor is latin for ‘glassy fluid,’ referring to the fact that the goo is clear. Like most gels, it’s mostly water, which is mixed with collagen fibers and a kind of sugar called hyaluronic acid. So yeah. Your eyeball is mostly sweet, gooey collagen.
There are also lots of other things sprinkled around in really low amounts: more than 120 distinct proteins have been found in the vitreous humor. It’s actually a littler gooier in the center, and firmer toward the edges. As you age, it all gets a bit more liquidy. That can cause it to separate from the retina in some places, casting those little moving shadows we call floaters.
There’s also another fluid, called the aqueous humor, in front of the lens, sandwiched between your iris and your cornea. That gel takes up most of the remaining 20% of your eyeball’s volume, and as the name suggests, it’s more watery than the vitreous humor. It’s there to nourish your cornea, in addition to making sure your eye remains the right shape. What it doesn’t contain is blood, or hemoglobin, or any other way of delivering oxygen to those see-through cells in the front. Which makes sense, because you wouldn’t want red stuff getting in the way of you seeing things.
The cornea doesn’t have blood, either, or it wouldn’t be clear. So it has to get the oxygen it needs another way: from the gas dissolved in your tears. Which means your cornea basically breathes on its own. Your eyeball. Breathes. There’s not a lot of oxygen in tears, but it’s just enough to give the corneal cells what they need to stay alive so the whole sack of goo doesn’t fall apart.