As we continue talking about the vascular layer of the eye, we now want to talk about control of light entering the eye in a lot more detail. And remember, light enters through the pupil, but the pupil is just a hole that's in the center of the iris. So to understand how we control the light entering the eye, we need to understand how the iris works in more detail. Well, the iris is the color of your eye, so we're going to start off by saying that that color of the iris just comes from the pigment melanin. And you'll remember from the integumentary system that melanin is a brown pigment, and it's the same thing that gives your skin its color. People with very dark skin colors have a lot of melanin. People with very fair or pale skin have very little melanin in their skin. The same thing happens in the eyes. We're going to say that more pigment equals brown eyes. And as we look up here, we have this sort of scale of eye color here. We have 5 eyes laid out from blue to green to darker and darker browns. And as we go to the right on this scale, we see that it says, "High melanin," we're putting more and more melanin in the eyes. We can see that the eyes go from sort of a light brown to darker and darker browns. And some people have very dark brown eyes because they just have a lot of melanin in their eyes. But you'll notice as we go to the left on the scale, it's not like skin color. It doesn't just get paler or a lighter brown. Eventually, it sort of looks kinda greenish or blue. So we're going to say here that less pigment is going to equal a green or blue color of the eyes. But, importantly, we're still just talking about melanin. There's nothing blue or green in the irises of people who have blue or green eyes. So I have fairly blue eyes. If you were to gouge out my eye, rip out my iris, and lay it on the table, you would see it would look probably just kind of like this unpigmented, pale, beige tissue. The reason it looks blue in my eye is the same reason that the sky is blue. Blue is a shorter wavelength light. So like all light, when the light passes through materials of different densities, in this case the elastic tissue of the eye, it's going to refract or bend. But shorter wavelengths refract or bend more than longer wavelengths. So that blue light hits the iris, starts bending, and more of the blue light bends and comes back out of the eye for people to see, the eye looks blue. If I had more melanin in my eye, that blue light would just get absorbed and my eyes would look brown.

What we really want to talk about here, though, is how light gets past the iris, though, and that's going through the pupil. So we're going to say here that changing the size of the pupil changes the amount of light coming into the eye, and of course, we change the size of the pupil by changing the size of the iris. Now before we get into the details here, I just want to watch a video of this, a super close-up vision, video of the eye, so that we can sort of think about how all this is working. So we're going to see here someone whose eye or whose pupil is dilated. So the pupil is going to be really big starting out, and then that iris is going to squeeze in and make the pupil smaller. Here we go. We have this big pupil and you can see that iris squeezing in the elastic tissue with the melanin in it, squeezing in with it, getting pulled along with it. Now, I'm going to play this again for you one more time. And as we do, I want you to think about how the muscles in this iris must be working. How must those iris be those muscles in the iris be arranged to squeeze in in this way? And then if we wanted to open up the pupil, how must those muscles be arranged and how would they work as well?

So now that we're thinking about that, let's look back at our page. So there are 2 basic actions that the iris can do. We can have constriction of the pupil—the iris can squeeze inwards. Or we can have dilation of the pupil. The iris can pull outwards and make that pupil bigger. We are going to talk about 2 muscles here, and they're pretty easy to remember the names because they're just named after the action. So to constrict the pupil, we have the pupillary constrictor muscle, and to dilate the pupil, we have the pupillary dilator. So let's start with this pupillary constrictor muscle. We can see here we have well, we have two images of the iris here where we removed that elastic tissue off of the iris. And so now we're just looking at the muscles, and we can see this pupillary constrictor in the middle here. This is this sort of circular-shaped muscle. And this circular muscle well, like all circular muscles, when it squeezes in, it constricts in and makes the hole in the middle smaller. Alright? So that is going to make that iris bigger, the pupil smaller. Now in contrast, on the outside of the iris here, we have these muscles, this pupillary dilator muscle that's almost arranged like spokes on a wheel, and we're going to call this a radial shape, sort of pulling outwards. And so to open up this iris, to dilate the pupil, these muscles are going to pull outwards.

Okay? So now why would they do that? In what cases would each different muscle contract? Well, the pupillary constrictor is going to contract for bright lights. That makes sense. If it's really bright out, you want to shut down that pupil so you don't get too much light in the eye and damage the retina. It's also going to do it though for close vision. And why that is, we'll talk about in more detail when we talk about focusing light on the retina using a lens. For now, though, just remember that for close vision, that's another thing that's going to cause this iris to close down, this pupillary constrictor muscle to contract. Alright. Well, in contrast, the dilation of the pupil, the pupillary dilator, that's going to happen for dim lights. And that makes sense if it's not very bright. Need to open up the pupil, let more light into the eye, and also for distant vision. Again, why for distant vision? We'll talk about when we talk about focusing light on the retina using the lens. For now, just know that it happens when you're looking at something far away. That pupil is going to be bigger.

Control of the pupil is by the autonomic nervous system. This is something that you have no conscious control of. So for innervation, we want to think about which division of the autonomic nervous system is controlling it. For the constriction, that's going to be the parasympathetic division of the autonomic nervous system. And remember, the parasympathetic division of the autonomic nervous system, we sometimes think of as our rest and digest nervous system. So when I think about it that way, I sort of think, well, if I want to rest and digest, I kind of want to just shut things down. I want to reduce the total amount of sensory information coming into my body. So I'm just gonna kinda turn those headlights down and just, you know, block stuff out. Now in contrast, the pupillary dilators are going to be controlled by the sympathetic division of the automatic nervous system. Remember, the sympathetic division, that is like your fight or flight response. So again, I think of it this way. If I, you know, am in my fight or flight, I'm scared, I need to fight something, I want as much sensory information coming into my body as possible. So I open up that pupil, I let a lot of the light in so I can see everything super clearly. All right. Associated with that, we can think of what emotional states are going to lead to constriction or dilation. So, that pupillary constrictor muscle is going to contract in states of boredom. Right? Shut things down, rest and digest, but also things like an unpleasant sight. And I think of that again as if I don't want to see something I can kind of shut things down a little bit and reduce the total amount of information coming in. In contrast, these dilators, you want to open up the pupil, well, for things like fear, desire, problem-solving. All these states where you really want a lot of sensory information coming in so that you can really process everything that's going on. Alright. So with that, we're done talking about the vascular. But like always, we have an example and practice problems to follow. Give them a try.