by Professor Anthony A. Goodman M.D., Montana State University
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.
The eyes are very complex structures, but as far as humans go, we don’t have a top-of-the-line eye in evolution. What we do have is something very special. As predators, we have binocular vision, which is extremely important to our survival.
The predator needs to be able to judge distances very accurately. With our eyes, we can judge a little bit of that distance by the size of that object and how far away it is from us. Looking at something and focusing on a very near object, the brain knows and interprets unconsciously that that object is closer. If the object is moved farther away and we look at it in the distance where our eyes are almost parallel, our brain will know that distance, because it’s good at judging distance.
If vision is lost in one eye, we lose the ability to have depth perception. 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. Sometimes surgeons run into this when doing laparoscopic surgery, which is done through a fisheye lens on a television screen. Viewed through a camera, 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, forcing them to learn to judge distance. Binocular vision is very important to our survival and our ability to function in fine-detailed ways.
The Conjunctiva, Cornea, and Sclera
Let’s look first at the different parts of the structure of the eye, beginning with the cornea. The cornea has a shape that is part of a hemispheric dome and has the function of protecting the eye. It’s curved so that it can focus light. Even before the 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. Sight is a very important sense and the body does a great deal to preserve it.
This is a transcript from the video series Understanding The Human Body: An Introduction to Anatomy and Physiology. Watch it now, on Wondrium.
The next layer is called the sclera. It 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, but 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 and structure to the eyes.
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The Iris, Pupil, and More
Going more internally, we come to the iris. Just like a camera shutter, 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 its usual length, but it can’t forcibly lengthen. To be able to open and close the iris, you need two sets of muscles. 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 metabolically inactive and receives 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, runs through the choroids, and comes out onto the retina. You can actually see through the retina and see those blood vessels. When we look at the eye, we get a good view by looking in through the cornea and the lens to the back of the eye, which allows us to see this whole area.
One of the other parts of the eye that we can see is the retina. The retina is the film that light hits, producing the vision for the brain, and is the beginning of our perception of light.
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How to Find Your Blind Spot
The eye is a complicated system of muscles and structures. Following the retina, the optic disc is where the optic nerve comes through the brain and enters the eye. The nerve then spreads out on both sides, in 360 degrees all around the retina. All the impulses are channeled from the retina back to the brain; it also carries the blood supply. The drawback is the nerve is a single artery and a single vein that supplies all the blood for the eye. We only have one supply vessel from which the capillaries of the choroid radiate. It this vessel is damaged, 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, leaving us with only one blood supply vessel.
Where the nerves and the vessels penetrate the retina is known as the blind spot, for 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 so much located in that area; it’s the exit of the nerves and the vessels, and the retina just doesn’t have room to function. There is 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. 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. Now you can put the dot right in front of your blind spot. When both eyes function, you’re totally unaware of it.
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The Retina’s Receptors
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, and 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. Prey animals may do the same as well as the predatory animals, because they’ve unconsciously adapted to seeing best in dark situations.
Humans have about 120 million rods and 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. Humans don’t see color in dim night vision because the cones just don’t function. They don’t have the energy or chemistry they need.
There’s a very thin area of the retina called the macula lutea. It’s the very center of the retina where the most important part of our light is going to come in. It’s called the central fovea, which is a depression. You can 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
The ophthalmologists and other medical doctors look at the eye as a window on the whole neurovascular system. It’s the only place in the body we can look in and see things functioning. Almost all of us have been faced with an ophthalmoscope: the doctor puts it right to his eye, gets 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. Many diseases are diagnosed by their retinal damage; for example, diabetes can be diagnosed with 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. This is what the doctor is looking for. Doctors also look for the lags of reflex. The pupil dilates at death because of that last rush of adrenaline. Then, when you shine light in, there’s no reflex. It’s a good indicator that there’s something 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.
The eye also has several chambers; it’s not just one big balloon filled with water. In front of the iris is the anterior chamber and a posterior chamber that sits 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 place 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 or the choroids, and have a flowing out into the eye, bulging out. This may disrupt its blood supply and that area of the retina may die, leaving a bigger, different blind spot in that portion.
Ophthalmologists 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. With that information, the doctor can measure the shape of the visual field and detect blind spots. Machines can do that measurement now as well.
If the retina comes off, even if it does not become devascularized and it still can function, it would be as if you hung a sheet to see a movie and somebody made a bulge in that sheet. It would distort the vision in that part of the eye, leaving you with 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.
Common Questions About the Eye
The eye works in a step-by-step process. First, light enters the cornea and then the pupil. It then travels through a lens, which directs the light onto your retina—think of how light is focused by a magnifying glass. The retina then reads and sends the signal into your brain.
Our eyes see things thanks to several magnificent parts of the eye: the cornea, the pupil, the lens, and the retina. These pieces receive and focus light into a signal that the brain can understand.
The process of seeing involves both perceiving light (an external phenomenon) and translating the light into a signal the brain can interpret (and internal process). The cornea, pupil, lens, and retina all work together to make this process work.
The eyes work with the brain to capture and translate visual images in the form of light. Think of the eye as a camera capturing a photograph. The data from that image transfer along the optic nerve to the brain (like transferring files via a memory stick), and the brain then downloads and makes sense of the information (like opening a photo on your computer).