As we look at the layers of the eye in a little bit more detail, we're going to talk about the fibrous layer first. And the fibrous layer is going to be the outermost layer of the eye. And we said that it is there really to protect the eye, to give it shape, and also to be an attachment site for muscles. Right? So as you look around, it's because you have muscles attached to this fibrous layer pulling that eyeball back and forth. Now it's called the fibrous layer because it's made of fibrous connective tissue and the fiber we're talking about here is collagen. So remember, collagen is strong like a rope. Really hard to break, but it's also very flexible. So that's what makes up this fibrous layer. The outer layer of your eye is really tough, but it's flexible; your eye can squish a little bit.
Alright. Next, we're going to look at our image here, and I just want to orient you to this image because we're going to be using this image over and over again for our different layers. We're just gonna highlight it differently for each one. Here we are looking at a transverse section of the eye. So this is a top down view of the eye, and specifically, we are looking at the right eye. So we can see here in blue all the way around the outside, we have this fibrous layer, but you'll see that we have color-coded a light blue and a dark blue. In light blue here, we have the cornea, and all around the back in darker blue, we have the sclera.
Alright. So let's start with the cornea. The cornea, you can see there, is the front section, and it is transparent because you have to look through it. Right? And so we said we're going to talk about these layers in the order that light hits them. So as your incoming light interacts with the eye, the first thing it needs to do is it needs to pass through this cornea to enter the eyeball. We're going to say that the cornea allows light to enter, and it also is going to protect the eye. The cornea is going to be highly innervated, and it's mostly innervated with pain receptors. So while it is really tough, it doesn't feel really tough. And we've all had the experience, right, getting a grain of sand or even just an eyelash in your eye. You know how darn uncomfortable that is. And that makes sense because you want a really nice, clear thing to look through. You don't want it to get all scratched up. So anything gets on that cornea, even something as simple as an eyelash that you wouldn't feel anywhere else on your body, it's deeply uncomfortable because you're going to spend a lot of time, very gently, trying to remove it.
The cornea, we are going to say, is continuous with the sclera. And by continuous, we just mean that those fibers run into from one to the other. Now, interestingly though, and you probably don't need to know this, but I find it very interesting that the cornea actually develops from a different embryological tissue than the sclera does. They develop from different tissues in the embryo, they grow together early on in the eye, and then become this continuous fibrous outer layer.
The last thing you need to know about the cornea here is that it is avascular. There are no blood vessels in your cornea because you have to look through it. You don't want to be looking through blood when you are seeing things. Okay. We'll now move to the rest of this fibrous layer, and that is going to be the sclera. The sclera is most of the outside of your eye, roughly 80% of the surface of your eye. We're going to say that it's really just the white of your eye is what you probably refer to it as and it is there to protect the eye, and also it gives the eye its shape. Right? So the eye is this collagen, this fibrous material, so you really want to think of it almost like a soccer ball. It's really tough, it has shape to it, but to keep that shape it needs to be inflated. And it's going to be inflated with that aqueous humor and the vitreous humor, and we'll talk about that in more detail coming up later.
Here, though, we're going to say that the sclera protects and shapes the eye, and it also anchors the eye muscles. So we've been saying that the eye muscles attach to the fibrous layer. Specifically, they attach to the sclera. Like the cornea, the sclera is innervated, but it is less sensitive. Right? And that makes sense. It's not quite as big a deal if you get a scratch on your sclera. So things on your sclera are uncomfortable, but not quite as uncomfortable as if you get it right on the cornea. The sclera, we said, is continuous with the cornea. The sclera is going to be continuous with the dura mater of the CNS. And if you remember from the central nervous system, the dura mater, that is the outer layer of the meninges, this really tough membrane that surrounds the brain and the spinal cord. In fact, dura mater literally means the tough mother. So this, the sclera actually surrounds the eye, and then it just comes back, and it surrounds the optic nerve and goes back and is continuous with that membrane that surrounds the brain.
Now interestingly, again, you probably don't need to know this, but the eyeball developmentally actually develops most of it develops as an outgrowth of that central nervous system. So it's almost like the eye is like a pimple of the brain, someone once described it to me as, which I kinda like. And you can think of that that that makes sense because it has this continuous layer with that dura mater that's surrounding the rest of the central nervous system.
The last thing that we need to know here is that the sclera contains blood vessels. And if you've ever gotten bloodshot eyes, you know that's true.
Okay. So that's the fibrous layer, the outer layer. Next, we'll talk about the middle layer or the vascular layer. But before we get there, we got example and practice problems. Give them a try.
