As we learn about our sense of sight, we're going to spend a lot of time talking about the structure of the eyeball. Now, to me and to a lot of people, the structure of the eye is one of the most amazing products of evolution. We have this structure that's able to focus electromagnetic radiation in a way that we're able to perceive vibrant colors and see three-dimensional images. That all works kind of blows my mind, but it does work, and we largely understand how it works. And like so many things, it comes back to structure. So we're going to start by saying that the eyeball is this irregularly shaped, hollow sphere. It's basically this small ball with some bumps on it, and the outside of that ball is made up of 3 layers that surround what we're going to call on the inside, the optic components. Now, as we go through this, we are going to go through all of this information in a lot more detail. Right now, we just want to step back and see the entire picture so that when we get to that detail, you have a better frame of reference. So for structure, we're going to start with the outside and work our way in. We're going to start with the fibrous layer. That is the outer layer and it's there for structural protection and also for muscle attachments. So if you move your eyeballs back and forth, that's because there are muscles attached to this fibrous layer pulling on the eye back and forth. Now, you can also touch your eyeball or poke yourself in the eye. I don't recommend it. But if you do, they'll probably be okay because this fibrous layer is on the outside and it's really tough providing protection. Now, as we look at our image here, we see a cross section of the eye. And in blue, we see this fibrous layer going all the way around. We can see, though, that we have 2 shades of blue here. In the front, we have that light blue. That's the cornea. The cornea is the front of your eyeball that is clear so that you can see through it. The rest of it, we have here in this darker blue. That darker blue represents the sclera, sometimes you just call that the white of your eye.
So, as we move in from the fibrous layer, we reach the vascular layer. The vascular layer is the middle layer, and sometimes it's called the uvea. Now, I'm not going to call it the uvea going forward, but you should be familiar with that name because sometimes it comes up. The vascular layer is there to control the light and also provide blood supply. As we look at the eye here, you can see going all the way around inside that fibrous layer, we have that vascular layer. And that's going to be made up of 3 smaller parts, The iris. The iris is here in front. That's the color of your eye, and it's responsible for controlling how much light actually enters the eyeball. We have the ciliary body, which you see here on the sides there. The ciliary body is largely responsible for changing the shape of the lens. It has muscles in it that can pull or relax, and when it does that, it changes the shape of the lens and allows your eye to focus. And then going around the back, we have what's called the choroid. And again, this is the vascular layer, so that's where a lot of that blood supply for the eye is. The choroid also has some pigment in it because when light comes into the eye, you don't want it bouncing all around. Any light that isn't absorbed by those photoreceptor cells, you want absorbed in the back of the eye so it doesn't cause artifacts in your vision. That's the job of the choroid.
As we move in from the vascular layer, we reach the inner layer, and the inner layer is often just referred to as the retina. The retina is where you have your photosensitive cells and your nerve cells. So for vision, this is really where the rubber hits the road. These are the cells that are picking up the light and then sending those messages to the brain. So we can see in our image here all around the back, you have those photoreceptor cells, but then to get the image to the brain, you need these nerve cells that are connecting all these photoreceptor cells running through the retina, and then they're going to run out and down the optic nerve so that the image can get to your brain. All right. We just really have one structure here. That's the retina. But the retina is a really complex structure that we're going to go into a lot of detail on going forward.
Finally, inside the eye, we have what we're going to call the optic components. We're saying this is the inside of the eye, and we're going to say that this transmits and focuses light. So these are the clear components that fill up your eye, and there are really three things here. We have the aqueous humor, the vitreous humor, and the lens. All right. So the aqueous and the vitreous humor. The aqueous humor is here in purple. It's a liquid that fills up the front of the eye. The vitreous humor is here in pink. It's a gel-like substance that fills up the majority of the eyeball. Again, both of them are clear, and really their job is just to fill up that hollow space of the eye and allow light to pass through. Then we have the lens. The lens is this relatively hard but flexible structure there, and it is there to focus the light. The light that enters passes through the lens. It's focused on the retina so we can see a clear image. All right. We just went through this sort of outline from outside to inside. As we go through this going forward, we're going to approach it as light passes through the eye. The one change that we have then is we're going to talk about the optic components before we talk about the retina. I'm looking forward to it. I'll see you there.
