Eyesight 101: How Our Eyes Work
"There are so many things going on in this amazing contraption that gives us a wealth of sensory information from our environment."
Today's show with Dr. Keller Wortham, MD, is the first in a four-part series on eyesight and vision health. Through the course of this series, we'll discuss a few common causes of vision loss, and how we can prevent and treat them. In part one, we're talking about what sight is: the components of the eye and how it works. You don't want to miss this fascinating series!
- 02:55: The Mechanics of the Eye
- 04:26: Cornea
- 05:23: Pupil and Iris
- 06:43: The Lens
- 07:18: Ciliary Body
- 08:22: Anterior and Posterior Segments
- 09:18: Retina
- 11:16: Macula
- 13:31: Other Accessories
- 14:28: Wrap-Up
Sight is so important to us. We get so much information from the world by what we see. And this may come as a surprise, but many people who suffer from vision loss weren’t born blind. In fact, many cases of blindness are preventable or even treatable (at least in the beginning).
The Mechanics of the Eye
The eyes are pretty incredible incredible parts of the human body — they’re so specific and incredible that there's a medical specialty dedicated to them. So today we’re going to take a closer look (no pun intended) at how this amazing body part works.
The mechanics of vision work a lot like a camera, which uses light in the environment, channeling it in through a lens, projecting onto the back of the camera (either film or a digital chip), and then using that information to create an image. So how does that happen in the eye?
The first part of our eye is something called the cornea. The cornea is a thin, clear rounded area on the top and front of the eye, covering the iris. The cornea is similar to the fisheye part of a camera lens. The purpose of the cornea is to get the light from a wider area of our environment and refract it, or bend it, towards our eyes. So, we're getting more light input bent towards a smaller space, and that cornea is very important for improving the amount of light coming into the eye. The cornea isn't something that you can really see but it's a very small bulge on the eye. If you have ever put a contact in your eye, that's where the contact rests; right on top of that cornea. So light passes through the cornea, and then makes its way towards the pupil.
Pupil and Iris
The pupil (as most of you already know) is the little black dot in the middle of our eye that changes size. At least that’s what it appears to be; in reality, it’s not a black dot, it's actually a hole in the middle of our iris. The iris is what gives our eyes their color — it’s the green, blue, or brown part of your eye. The iris is a smooth muscle that opens and closes to control the size of the pupil, and it responds to light in our environment.
So, as more light comes in, the iris closes, the pupil becomes smaller, and less light gets through to the back of the eye. It’s similar to how the aperture of a camera works. The aperture tightens or opens up to make the shutter hole larger or smaller, to control the amount of light getting into the back of the camera.
After we get through the cornea, through the hole in our iris which is our pupil, we enter the lens. This is really the main part of the eye that's responsible for focusing, much like the lens in a camera. A camera lens is really two panes of glass, and the amount of space between them changes as you focus. The lens in our actual eye does this a little bit differently; it's kind of like a little disk, which can become narrower or thicker, using a tiny muscle called the ciliary body.
The ciliary body is the tiny muscle that helps stretch the lens to make it thinner so we can see farther away, or contracts the lens to make it thicker so we can focus on things closer to us. Now that little muscle has to work harder and harder to contract the lens, to make it thicker and thicker and allow us to see things up close. So if you've ever tried to read something up close and felt like you were getting a headache, or felt your eyes getting tired, that’s because the tiny muscle that has to bend that lens is working overdrive to be able to see something up close for a long period of time. The ciliary body is made up of tiny fibers attaching to the lens that can pull it or relax it to help the lens thicken or thin out.
Anterior and Posterior Segments
The cornea, pupil, iris, and lens are the components of what we call the anterior segment of the eye. That whole area is bathed in a liquid we call the aqueous humor, because it's very watery. After light has come in through the anterior segment of the eye, and passed through the lens, it then passes to the posterior segment of the eye. The posterior segment has a huge amount of fluid, called the vitreous humor, and the vitreous is much thicker — almost gelatinous. That thick liquid in the main globe part of our eye is what gives the eye its shape and volume so that it can stay in the eye socket and continue its functions.
After the light passes through the lens, it passes through the vitreous humor to the very back of the eye, and in the very back of the eye there's this thin layer of very specialized tissue called the retina. This layer of tissue has the ability to receive all the light coming in and then eventually change that into something that our brain recognizes as an image.
Let’s go back to our camera analogy: at the back of the camera, after light travels through the lens and through the space at the other side of the lens, it gets to the film or digital chip, which takes the light coming in and uses electric signals to translate it into an image. That's basically what our retina does. It contains a bunch of very specialized cells, called rods and cones. The rods are cells that can detect very low levels of light, but tend to give us something like a black and white or grayscale image. The cones, which need higher levels of light, can actually perceive and transmit colors. So your retina has the ability to take the light coming in and use these specialized cells to change light energy into chemical energy by moving chemicals around in these cells, and from there translate it into electrical energy. The electrical energy goes through the millions of nerve fibers at the back of the retina, and then travels to the brain through something called the optic nerve. This is what allows your brain to perceive an image from that light coming in.
Now, the retina has one specialized area, called the macula. The macula is the center portion of the retina, that's very dense in these nerve fibers and cone cells. This is similar to the megapixels of your camera, in that there’s higher function in this particular area. Your macula is what allows you to focus very clearly in the center of your field of vision.
So if you're looking at someone’s face, that person’s face is projecting onto the macula of your retina, so you can see it very clearly. But you might notice that you're not looking at directly, even if they are on the same plane of focus, are not quite as clear until you move your eyes to the side to look at them. That's because your macula is really giving that visual specificity to the thing you’re looking at. In order to see a specific area, you have to move the macula slightly aside to follow it. If you're reading in a book, as another example, you can look at one word and see it very clearly, but the words in that same sentence right before and right after are going to be a little blurry until you move your eyes to scan across them.
As we mentioned previously, the lens helps us focus light onto the retina almost like a projection screen. And again, your lens is going to allow you to see things in focus in one particular visual plane but things behind that plane or in front of that plane are going to be blurry to you. So, if you're looking at your telephone screen right now, or your computer, chances are that the things in the room around you are pretty blurry. That's because the lens is contracting and expanding to allow you to see a specific visual field, and then the macula within that visual field is allowing you to see a specific area until you move around to check out other things.
So that is basically the major mechanics of vision. Of course our eyes also have a lot of other accessories that help us see:
- We have the conjunctiva — if you have ever heard of conjunctivitis, that's inflammation of the conjunctiva. The conjunctiva is the very thin layer of tissue that protects our eye from dust, debris, and dehydration. It covers the surface of our eye, and the underside of our eyelids.
- We have no less than six specialized muscles around the eye that allow it to look in all different directions and those are regulated by various nerves within our brain.
- We have a lacrimal duct and gland that help lubricate our eye, to keep it moist.
- We have eyelids that close to protect our eye, and they do that not only involuntarily if we have to blink but also voluntarily when we choose to close our eyes.
There are so many things going on in this amazing contraption that gives us a wealth of sensory information from our environment.
So, as you can see (seriously, no pun intended), the eye is a very wonderful machine. But there are a lot of areas where things can go wrong, and that's what we will get into in the next three parts of this series, where we'll talk about some places where vision can start to go bad and what you can do to prevent it.