The structure of the eye

In this video, we're going to talk about the structure of the eye. And we're going to do that by drawing a cross-sectional diagram of the eyeball. The first thing we're going to draw is the white part of the eye, which is known as the sclera. So I'm just drawing that in. And I'm going to label is sclera. And the sclera is a thick, fibrous tissue that basically forms the substance of the eyeball. So the white part of the eye is the sclera, and it serves to protect the eye, and it also serves as an attachment point for muscles so that you can move your eyeball around when you're looking at different things. So this white part is the sclera. The next thing we're going to look at-- so let's imagine that this is the front of the eyeball, and this is the back of the eyeball. So when you're looking at somebody, you're looking at them this way. So the first thing you look at is something known as the cornea. The cornea is actually transparent, so you can't see it. But it basically serves to protect the front of the eye, and it also serves to bend light just a little bit. So let me label this "cornea." The cornea is actually protected. It's really sensitive, so has to be protected by a thin layer of cells. It's a thin layer of epithelial cells. And it is known as the conjunctiva. So this thin layer of epithelial cells protects the cornea from friction. So when you rub your eyeball, you don't scratch your cornea, and that's because it's actually protected by the conjunctiva. It also helps moisturize the cornea and protects it from dust and debris so that the cornea is protected. So when you're looking at someone's eye, you look at the cornea. And the next thing you actually look at-- you can't even see-- is the aqueous humour. So inside the cornea-- imagine over here-- basically, it's a chamber, and it's filled with a fluid, and that fluid is known as the aqueous humour. The aqueous humour is basically just water and salt, it fills in this anterior chamber of the eye. So "humour" means chamber, "aqueous" is water, so it's the water chamber, the anterior chamber of the eye. So the next thing that you would see-- so let's imagine a light ray comes in. And as I mentioned, the cornea kind of bends the light ray just a little bit. The next thing this light ray would hit would be the lens. The lens is back here. It's a bi-convex lens, which means that it's curved on both ends. And so when the light ray hits the lens, it bends a little bit more. So it'll bend a little bit more. And the lens actually can change shape, so it can either get thinner or thicker, depending on whether an object is nearby or far away. And so the thing that actually makes the lens thinner or thicker is known as the ciliary body. And the ciliary body is composed of a few things. The first are these ligaments that are connected to the lens, and they're known as suspensory ligaments. And the suspensory ligaments are connected are connected to the lens, and they're also connected to the ciliary muscles. It's on both sides of the lens. And the ciliary muscle and the suspensory ligaments change the shape of the lens, and they form this bigger structure known as the ciliary body, which also secretes this aqueous humour over here. So the cornea bends the light, and the lens bends it a little bit further. And what's interesting is when you dive under water, everything becomes really blurry. And the reason is when light is emitted-- I'll just draw the sun over here, happy sun. So when the sun emits a light ray, it actually comes in, and it's passing through air normally, and when it hits the cornea, it gets bent a certain amount. But when you're underwater, the air is replaced by water, and so the light ray actually gets bent a slightly different amount. So let's say this is how much it would get bent if there was air outside, and this how much it gets bent when there's water. And so that's why everything is a little bit blurry, because the light rays aren't converging where they should be. So when you wear goggles when you're underwater-- so let's imagine that you're wearing goggles underwater-- what the goggles serve to do is they add a little layer of air in front of the cornea so that when the light hits the cornea, it bends the correct amount instead of bending this abnormal amount, which it normally would if there's water right outside here. The next thing that we want to look at is something known as the iris. The iris is actually the part of the eye that is colored. So if somebody has blue eyes or if they have green eyes, that's because the iris is pigmented differently. The iris is actually two different muscles that contract and expand. And when they contract and expand, the size of the hole right here actually gets bigger and smaller. Let me just label the iris. And this hole is known as the pupil. Label that here. I know there are a lot of words, but just bear with me. They should all make sense after we draw the entire diagram. So the pupil is just a term for the hole that is controlled by-- the size of the hole is controlled by the iris. If it's really dark outside, you want the hole to be really big so that you can get the maximum amount of light rays entering the back of the eye. And when it's really bright outside, you want the iris to contract so that the pupil constricts, and there's less light entering the eye so that you can actually focus in on what you're looking at. So the light ray hits the lens and gets bent, and now it's passing through something known as the vitreous humour. So the vitreous humour makes up this posterior chamber of the eye. From here to here is the anterior chamber. And from here to the back of the eye is the posterior chamber of the eye. And the posterior chamber is composed of the vitreous humour. So "humour" means chamber, and "vitreous" is a jelly-like substance within the posterior chamber. So it's composed of water and some salt and some protein. And the main protein is albumin in this part of the eye. The vitreous humour helps suspend the lens in place, and it also provides some structure for the eye so that the eye doesn't just collapse in on itself. It's also transparent, so the light is able to just flow right through. So once the light ray comes towards the back of the eye, it will hit a structure known as the retina. The retina coats the entire back of the eyeball. And so the retina is composed of a bunch of different cells known as photo receptors that actually take this light ray and convert it into a neural impulse that the brain can understand. The retina is tinted red, it's reddish in color, and that's why when you take a photo of someone at night when the flash goes off, what happens is the flash goes off, and the light way enters the back of the eye and actually bounces off of the retina and gets picked up by the camera. So it causes something known as the red-eye effect. And some cameras actually have something that reduces the red-eye, a red-eye reduction feature. And so what happens there is you have two flashes that go off. The first flash simply serves to constrict the iris so that the pupil actually gets smaller. And then the second flash goes off, and when the second flash goes off to actually take the photo, the pupil is lot smaller, so there's less light actually getting reflected off the retina. So it reduces the red-eye effect. So the retina sends fibers through the back of the eye so that the fibers can actually go to the brain. And these fibers form something known as the optic nerve. So this is the optic nerve. And let me just label this "retina." So the retina sends fibers through the optic nerve, which goes to the brain, and then you're able to make sense of what you're looking at. The next thing that we want to talk about, the next structure, is just inside of the retina, and it's just like this. It's a membrane known as the choroid. And the choroid is basically a network of blood vessels that nourishes the retinal cells and nourishes other cells within the eye. So there's little kind of blood vessels that come off of the choroid that nourish all the cells in the retina and other parts of the eye. The choroid is also pigmented black. So that's why, when we look at somebody's eye, if you look through the pupil, it's actually really dark. It's black. And that's because you're looking at the choroid, which is actually pigmented black. And in other species, like cats, they don't have pigment, so the choroid is actually shiny. And what that serves to do is it gives them better night vision. So you can imagine a light ray coming in, kind of off to the side over here. And in humans, if this light ray wasn't absorbed by the retina, it would be absorbed by the choroid, because the choroid is black, and black absorbs light. But in cats, it's shiny, so the light ray would actually reflect off the choroid and hit the retina again and be absorbed by the retina. So it gets two chances to absorb the light ray and actually enhances night vision in other animals, like cats, for example. So the choroid is this network of blood vessels, and it's pigmented black. And I just wanted to point out one more feature of the retina. There's a little dimple over here. And this dimple is filled with cones. And these cones actually allow you to see in really rich detail, a rich amount of detail when you're looking at something. And it's known as the fovea. And the fovea is actually in the very center of a broader region known as the macula. So the macula is just an anatomical name for a particular region of the eye. In the very center of the macula is the fovea, which is rich in cones and lets you see in really high levels of detail. So these are all the structures of the eye.