15. The Special Senses
The Lens and Focusing Light on the Retina
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As we talk about the lens and how we focus light on the retina, we now wanna think physiologically what happens in the eye to do this. And this process of changing our focus, we're gonna call accommodation. Alright. So accommodation is changing our focal distance. And to understand how this works, we first just wanna note that there's actually 2 structures in the eye that are gonna refract light. Well, first is the cornea. The cornea, that front of your eye, that actually has the most refraction, but it's not flexible. Right? So it bends light the most entering your eye, but because it's not flexible, it can't focus. You can't change the way it bends the light. So if we have our r here, it's gonna enter the eye. It's gonna refract at the cornea, but the thing that's gonna do the most work in focusing, that's gonna be the lens. Because the lens can change shape, and that is what's gonna get things to focus on a really clear point, giving you a really clear image when you look at things. Now to understand how the lens works, you need to understand that the lens well, we said that it's this flexible disk. That flexible disk is normally stretched. To be flatter. Alright? So there's tension on all the way around on the outside of that lens, pulling it flatter, pulling it thinner. Now, remember a flatter lens, that allows us to see things that are farther away. So at rest, your eye is just primed to see things that are farther away. To see things closer to you, to focus on things close-up, your eye has to do some work. So that's what we wanna talk about. What is the work that your eye has to do to change that focus to allow you to see things that are close-up? But we're gonna say to focus on near object on near objects, the eye actually uses multiple systems. So we're gonna talk about more than the lens right here, but we'll start talking about the lens. Alright. So we're going to call this lens accommodation. And we're going to say in lens accommodation, the ciliary muscles contract. When the ciliary muscles contract, that releases tension on the lens. When the tension is released, the lap that flexible lens lens, it bulges. It becomes more round, or I'm gonna say here more convex. Right? And we said a more convex or rounder lens is able to focus on things that are closer up. Right. So remember those ciliary muscles, a circular muscle surrounding the lens. The ciliary zonials are suspending the lens in between there. And just to look at this, we're gonna zoom in now on this structure. So now we have this front view of these ciliary muscles, the ciliary zonials, and suspending the lens in the middle. Now the idea that a muscle contracts to release tension is sometimes a little counterintuitive. Normally, I think when a muscle contracts, it puts tension on something. But you see, this ciliary muscle, it's a circular muscle. So when it contracts, it squeezes in like a sphincter and the space on the inside gets smaller. It's connected to those ciliary zonules, so as it squeezes in, it pulls on the ciliary zonules less and less. So we can see that here. This muscle is gonna squeeze in, and those ciliary zonules are gonna get a little bit of slack in them. That allows that lens to bulge out because there's no longer tension on it. As it bulges out, it becomes rounder, and you're focusing on things that are closer up. Now to see something farther away again, the opposite happens. Those lenses or, I'm sorry, those muscles just relax. That puts the tension back on the lens. It flattens out. Alright. I'm gonna move this out of the way just so I have more room here, but we'll leave that up there for a reference. Okay. So that's lens accommodation. That's what happens with your lens. The next thing that your eye needs to do is called the accommodation pupillary reflex. So your pupil, just like that ciliary muscle, is going to constrict. K? So the muscle and the iris is gonna contract, make those pupils smaller. That's gonna block light from hitting the edges of the lens. Now remember, when you're looking at something closer, that thing that is closer, the light is coming at your eye at more of an angle, so it's more likely to hit the edges of the lens. The edge of the lens doesn't focus as well as the center of the lens. So to block that, the pupil just closes down a little bit, and it keeps the light hitting the center of the lens so it's able to focus nicely. Now if you see something farther away, the opposite happens. It just opens up, and that light is hitting your eye more straight on, so it's not gonna be hitting the edge of the lens nearly as much as if you're looking at something close-up. You don't need to worry about as much. Okay. The final thing that we're gonna do here, we're gonna call eyeball convergence. And this you're probably familiar with. Right? It just means that as you look at something, as it's closer and closer to your face, your eyes cross, And that is to focus the image on the fovea. Right? That fovea, that center of your vision on the retina, to keep that image focused on the fovea of both eyes, they need to cross to keep things focused, everything in the center of your vision. Alright. The last thing I wanna note here, as people get older, their lenses become less flexible. Now, that's true of a lot of things in the body. Right? As folks get older, things become less flexible. They don't bend as well. They don't spring back as well. It's true for the lens. So the lens is normally stretched flat, so if it becomes less flexible, it no longer bulges out. We're gonna say that it loses its ability to accommodate. So that means as you get older, you lose your ability to see things that are close-up, and this happens to everyone roughly around age 50. You see older people, they try and look at their phone, they start holding it farther and farther away from themselves, or they put on their readers. Right? No shame in asking help from your friends. It's gonna happen to all of us. I'll see you in the next video.
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