As light enters the eye and travels back to the retina, we said that the vascular layer or the iris is controlling how much light gets into the eye. But, of course, it needs to pass through the middle of the eye to make it back to the retina. And what it's passing through, we are going to call the optic components. So remember, we're saying the optic components of the eye are there to transmit, and importantly in the case of the lens, to focus the light as it travels through the eye. Now, before we go through the different parts we're going to talk about here, let's orient ourselves to our image. We have this transverse section of an eyeball here, this top down view of the eye, and it's the right eye. And you can see here we have 3 different structures color coded in these different shades of pink or purple. In the front here, filling up the front of the eye, we have the aqueous humor in that sort of purplish color. We have this light pink disc shape here, that's going to be the lens. And then in the back, filling up the back of the eye, we have this brighter pink, that's going to be the vitreous humor or sometimes called the vitreous body.
Let's start with the aqueous humor. The aqueous humor fills up the anterior, just our fancy word for front. So the anterior or front portion of the eye, and you can see that here. This aqueous humor is here in between, this purple color between the iris and the cornea and also sort of surrounding the lens there as well. This aqueous humor is going to be a liquid, and it's going to be made by the ciliary body. So remember when we talked about the ciliary bodies, we have these ciliary processes on the back here, and that produced this aqueous humor, and this aqueous humor very slowly sort of flows out, comes out through the pupil there, and then it leaves the eye down here sort of at the edges of the cornea there. And so when I say that it flows, it moves very slowly. It's not, like, actually flowing, but you can think it's always sort of being replenished. It's always sort of making a little bit more and it's draining out in the sides. Now the job of this, other than just letting light to pass through and to fill up the eye, is to nourish the lens and the cornea. So the lens and the cornea both have living cells in them, but they're both avascular. There's no blood vessels there because you need to see through them. So those cells that are living there need to get their oxygen and nutrients from somewhere. They get it from this aqueous humor. Now aqueous humor doesn't have a lot of oxygen or nutrients in it. It's not, you know, nowhere near what blood does, but those cells aren't that metabolically active, so it has enough to keep them alive and enough to help let them do the jobs they need to do.
As the light moves through the aqueous humor, it's then going to hit the lens. The lens is this transparent, flexible disc, so this round disc shape here, and its real job is to focus the light as it comes in. So light needs to be focused, so it makes a really clear picture as it moves back and hits the retina. Now we're going to talk a lot about how the lens focuses light in a future video coming up. Right now, we just want to figure out what its structure is. We are going to say that it is avascular, and so it doesn't have any blood vessels, again, because you need to see through it, you want it to be really clear. It's going to be made up of 2 layers. First, we have the lens epithelium, made of a layer of cuboidal cells. So you have a layer of cuboidal cells on the outside, and the inside is going to be these lens fibers made of crystalline. And you can just remember that term, crystalline protein, because crystalline well, it makes sort of this crystal like structure that light can pass through and be focused.
As light passes through that lens, it then has to travel back to the retina, and to do that it's going to travel through this vitreous humor or sometimes called the vitreous body. This is going to fill the posterior, just our sort of fancy word for back there. So the whole back portion of the eye is going to be filled with this vitreous humor. And unlike the aqueous humor, this vitreous humor is a gel. And so remember, the vitreous humor is so it's always being replaced. Some is always draining out of the eye and new liquid is being produced. Well, this aqueous humor I'm sorry, this vitreous humor is a gel. So this is actually made once during fetal development, and then you get the same vitreous humor for your entire life. Now there's a lot of liquid in it, so there's liquid that diffuses in and out, but the gel substance, the stuff that makes it a gel, is just made in fetal development and you get it once. Its job, other than just, you know, letting light pass through, is to really fill up that eye and maintain eye shape. Remember I sort of describe the eye kind of like an inflated soccer ball? It's this vitreous humor and also that aqueous humor that's inflating the ball.
So that's all we're going to talk about, the aqueous and the vitreous humor. We are going to talk about the lens in more detail in terms of how it focuses light. We'll do that coming up, but, of course, first, we have example and practice problems. We'll see you there.
