How Do Your Eyes Work?

The human eye is a spectacularly complex organ that is fascinating to behold. Take a closer look at the structure and function of your eyes.

close up photograph of a woman's eye

The eyes are very complex structures, and we actually don’t have a top-of-the-line eye in evolution. We have a very good eye and we do have something very special. Predators, such as we, have binocular vision; it’s extremely important. The predator needs to be able to judge distances very accurately. We can judge a little bit of that distance by the size of that object and how far away it is from us. We can look with our two eyes and focus on a very near object and the brain knows and interprets unconsciously that the angle is very sharp here and that that object is closer. We can move that object farther away and look at it in the distance where our eyes are almost parallel, and the brain will know that distance, because it’s very good at judging distance.

If you lose vision in one eye, you lose the ability to have depth perception. And if you hold a finger out, close one eye and try to touch it in one shot, you’ll probably miss. You’ll then be able to make corrections and hit it. And we run into this sometimes as surgeons when we started doing laparoscopic surgery, which was done through a fisheye lens on a television screen, and all of a sudden our depth perception was gone and we had to learn how to judge distance purely by size. Neurosurgeons who work through a very small hole can only get one eye looking down that hole at a time. And they have to learn to judge distance. So binocular vision is very important to our survival and to our ability to function in fine-detailed ways.

Diagram of the Eye
Fig 1. Diagram of the Eye (click to enlarge)

The Conjunctiva, Cornea, and Sclera

Let’s look first at the different parts of the structure of the eye. First is the cornea. Now the cornea has a shape. It is part of a hemispheric dome. It has the function of protecting the eye and it is curved so that it can focus light. Even before light gets to the lens, there’s a little focusing going on in the cornea. The cornea only contains pain fibers, so that anything that touches the cornea is perceived as pain, and reflexes close the eyes and create tearing to get rid of it, a sudden wash of tears to flood anything away. Obviously sight is a very important sense and the body does a great deal to preserve it.

The next layer is called the sclera. The sclera encircles the eye, also known as the globe. It’s a very, very hard layer that’s almost continuous with the cornea in front. But in the back it’s hard and it’s not transparent, it’s white. It is what we mean when we hear, “Wait until you see the whites of their eyes.” The sclera is a supportive and protective coat and it maintains the shape, it provides some structure to the eyes.

The Iris, Pupil, and More

Image of an eye, showing the sclera (white), the Iris (blue), and the pupil (black)
Image of an eye, showing the sclera (white), the Iris (blue), and the pupil (black)

Going more internally, we come to the iris. It’s the shutter of the lens of our eyes. It’s just like a camera shutter. And it is composed of two kinds of muscle fibers. When a muscle contracts, it contracts in only one direction. It can only shorten and then relax to usual length, but it can’t forcibly lengthen. In order to be able to open and close the iris, you need two sets of muscles. So in the iris, we have radial fibers and circular fibers.

The pupil is really nothing; it’s just the hole in the iris. If you need more light, the sympathetic fibers of the nervous system will fire. Or if you put a drop of adrenaline in the eye, it will diffuse right across the cornea. It will cause contraction of the radial fibers and open the pupil, allowing in more light. If, on the other hand, you fire the parasympathetic fibers, you get constriction of these muscles and relaxation of the radial fibers and these circular muscles will narrow the pupil.

In this area we also have the ciliary body. We have a ciliary process and ciliary muscles that attach to the lens.

The lens is the second avascular structure in the eye. It’s very metabolically inactive and it gets all of its nutrition and excretion of waste from diffusion. It has shape and it focuses light that comes through it.

The next layer in from the sclera is the choroids. The choroid layer is almost entirely blood vessels. This layer supplies the vascular supply to the retina, which is the inner layer. Blood comes in and it runs through the choroids and comes out onto the retina. We can actually see through the retina and see those blood vessels. When we look at the eye, we get a very good view by looking in through the cornea and the lens to the back of the eye—we can see this whole area.

One of the things we can see is the retina. The retina is basically the film that light hits. It produces the vision for the brain. It’s the beginning of our perception of light.

How to Find Your Blind Spot

rods-and-cones-diagram
Fig 2. This graph depicts the density of the eye’s rods and cones, as well as the eye’s blind spot, located on the nasal side of the eye. Because of the presence of the optic disc (where nerves and blood vessels exit), there is no room for the rods and cones that act as the receptors for vision. (click to enlarge)

There are more structures here you really want to know about. The optic disc is where the big optic nerve comes through the brain and enters the eye. The nerve then is spread out on both sides, actually in 360 degrees all around the retina. All the impulses are going to be channeled from the retina back here to the brain. It also carries the blood supply. Here’s a single artery and a single vein supplying all the blood for the eye—bad situation, isn’t it? We only have one supply vessel from which the capillaries of the choroid radiate. If you knock off this vessel, that’s it for the eye. Nature has chosen not to put too many vessels in this area because every vessel takes up room where we might be receiving vision. So we ended up with only one blood supply vessel.

Where the nerves and the vessels penetrate the retina is known as the blind spot. There’s a good reason. The blind spot is not right in the middle of our axis of vision; it’s off to the nasal side. That blind spot is there because there’s too much stuff going on. It’s the exit of the nerves and the vessels, and the retina just doesn’t have any room to function. So we have a small blind spot on each eye, but they’re not symmetrical. Therefore, each eye has a little overlap so our blind spots are canceled. So when we look around, we’re not aware of any blind spot.

If you draw an X on the wall and a short way over put a dot, cover your left eye and look at the X and then move that dot in and out, at some point the dot will disappear.

You can prove you have a blind spot. If you draw an X on the wall and a short way over put a dot, cover your left eye and look at the X and then move that dot in and out, at some point the dot will disappear. And now you can put the dot right in front of your blind spot. When both eyes function, you’re totally unaware of it.

The Retina’s Receptors

Photograph of a retina
Photograph of a retina

The retina actually contains all the receptors for vision. It retains something called rods and cones. The rods, of which there are about 120 million in each eye, are photoreceptors shaped like a cylinder, and they receive black and light information only. They can see in shades of gray and they can see in very dim light. If you go out at night, one thing you’ll notice is that when it gets very dark, you lose your color vision. The cones are the color receptors. Now the rods increase in number in your peripheral vision.

The reception of the rods is best for light in our peripheral field, in darkness. There are lots of rods in our peripheral field and not a lot of cones. Hunters know in dark settings not to look directly at their prey. That’s because they’re going to get better night vision in their peripheral vision. They’ll look a little bit away from their prey, and see their prey better in their peripheral vision. I think the prey animals do the same, as well as the predatory animals because they’ve unconsciously adapted to seeing best in dark situations.

We have about 120 million rods; we have about 6 million cone-shaped receptors in each eye. These receptors are for color vision and they need much more bright light conditions to be able to see. We don’t see color, as I said, in dim night vision, because the cones just don’t function. They don’t have the energy; they don’t have the chemistry they need.

There’s a very thin area of the retina called the macula lutea. It’s the very center of the retina and where the most important part of our light is going to come in. It’s called the central fovea, which is a depression. And you can actually see a little depression in the retina. It provides the clearest vision. It’s basically where we can see almost to the back of the retina. It’s important for vision, and it’s the place we see best.

A Window into Your Neurovascular System

photograph of a doctor using an ophthalmoscope
Doctor using an ophthalmoscope

The ophthalmologists and other medical doctors look at the eye as really a window on the whole neurovascular system. It’s the only place in the body we can actually look in and see things functioning. We’ve all been, I’m sure, faced with an ophthalmoscope. The doctor puts it right to his eye, gets really close to your face and shines a light right in. Sometimes the ophthalmologist, with the help of a vasodilator—a sympathomimetic drug like adrenaline to open the eye up—looks back and can see the entire surface of the retina and can see if there are any problems in the retina. And there are a lot of diseases that are diagnosed by their retinal damage; for example, diabetes can be diagnosed by retinal damage. The doctor can also see the vessels and blood circulating in the veins and arteries back there.

When somebody dies, the pressure drops and these vessels collapse. In places between the collapsed blood vessels it looks like little boxes all lined up in a train; we call those boxcars. When you look at the dilated pupil of somebody who you think is dead (you can’t feel a pulse), you look at that eye and you see boxcars and no movement, you know that patient is dead. That’s what the doctor is looking for. We also look for the lags of reflex. The pupil dilates at death because of that last rush of adrenaline and then when you shine light in, there’s no reflex. That’s really a good indication that there’s something really wrong with the nervous system.

Inside the Chambers of the Eye

About every 90 minutes there’s new fluid going in and old fluid coming out.

We also have several chambers in the eye.  The eye is not just one big balloon filled with water. We have something called an anterior chamber in front of the iris and a posterior chamber that’s right behind the iris, all of which are surrounding the lens and are in front. These chambers are filled with something called the aqueous humor—a very rubbery solution that exchanges rapidly. About every 90 minutes there’s new fluid going in and old fluid coming out. It bathes the lens and uveal tract and the cornea with a fresh supply of nutrients and oxygen all the time, in lieu of a blood supply.

The rest of the globe is filled with something called the vitreous, and the vitreous is not so watery, it’s much more jellylike. The vitreous does not exchange. You have most of the vitreous that you’re going to have for your whole life. It’s not like the aqueous humor. What it does is it presses against the very delicate, filmy retina, keeping the retina under pressure all the time, flat against the surface where it needs to be.

What happens when there’s an error there? For example, if you have an injury—sudden acceleration, blunt injury to the eye—you can get bleeding or just deceleration that can tear a piece of the retina off the back wall, off the choroids, and have a flowing out into the eye, bulging out. Now, this may disrupt its blood supply and that area of the retina may die. You’ll have a bigger, different blind spot in that portion.

photograph of a doctor administering a visual field test
Doctor administering a visual field test

We measure people’s visual fields. A person looks at a blank screen with a grid on it and looks straight ahead. The doctor brings in different colored objects and the person tells him when he or she sees it, and the doctor can measure the shape of the visual field and detect blind spots. Also we have machines that can do that now.

If the retina comes off, even if it does not become devascularized and it still has the ability to function, it would be as if you hung a sheet to see a movie and somebody made a bulge in that sheet. It would really distort the vision in that part of the eye. So you would have blurred vision. The lens, after all, is using its ability as a focusing agent to focus a point of light precisely on each point of the retina. Once you move the retina, the lens is still focusing on where it thinks the retina is, and you have blurred vision in that area.

From the lecture series Understanding The Human Body: An Introduction to Anatomy and Physiology
Taught by 
Professor Anthony A. Goodman M.D., Montana State University

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