Human sight is a complex sense composed of many complementary elements that work together. The miraculous human eye, elegant in its detail and design, represents a gateway to vision.
The eyeball, or globe, is spherical in shape and about 1 inch across. It holds many structures that perform different functions and work together to facilitate sight.
The outside layer of the eye is made up largely of a tough, white, protective tissue called the sclera. The sclera helps maintain the spherical integrity of the eyeball. At the front of the eye is an equally tough but clear structure called the cornea, which is responsible for allowing light into the eye and then bending, or refracting, it. There is a layer called the retina, lining the inside back, which is sensitive and responsive to the light entering the eye.
The eye also has a number of ingeniously protective features. The eyelids, eyelashes and eyebrows are designed to protect from dirt and dust that could get in and create damage. The globe sits inside an orbital cavity, a bony pocket lined with fatty tissue, which serves as a cushion to hold the eye. These features protect the eye against injury. There are also six muscles, attached to different points on the sclera, which permit the globe to move in many directions inside the orbit.
The Journey of Vision
- In order for vision to occur, a series of processes must take place involving all of these structures within the eye and others within the brain. Let’s go on a journey and follow light rays as they travel through the eye to ultimately reach the retina. All of these structures are needed to bend, or refract, the light so that it focuses properly. It’s a real “team” effort!
- Light first passes through the cornea at the front of the eye, and then through a watery substance called the aqueous humor which fills small chambers behind the cornea.
- As the light continues, it passes through the pupil, a round opening in the center of the iris. The iris is the colored part of the eye and it has specialized muscles that function like the f-stop on a camera, changing the size of the pupil from very small to large, and regulating the amount of entering light.
- The lens is the next structure that the light penetrates; it’s attached to muscles which contract or relax in order to change the lens shape. Changing its shape is what produces the clarity of images that we see at different distances. (The ability to focus decreases as we age and that’s why many adults over 40 years old need reading glasses or bifocals.)
- Next, the light passes through the large back portion of the eye that is filled with a clear, jelly-like substance called the vitreous. From there, the light finally reaches the retina, where rod and cone cells are stimulated to release split-second chemical reactions converting the light to electrical impulses. Cone cells (typically about 7 million) are in greatest concentration in the central part of the retina called the macula. This area is responsible for producing sharp, detailed vision and color vision. An even greater number of rod cells (about 100 million) are found in the retina, away from the macula, and these allow us to see in dimly lit settings. But with less detail or resolution than the macula.
- When all parts of the visual system are working, the eyes are able to move together, adapt to light and dark environments, perceive colors and accurately evaluate an object’s location in space. Our eyes are sensitive to contrast differences, and can provide detailed vision, measured as visual acuity. “Normal” visual acuity is reported as 20/20. As the second number of this expression gets higher, it tells us that vision is weaker than “normal.” For instance, “legal blindness” is described as 20/200. An easy way to understand the meaning of these numbers is that the eye being tested sees at 20 feet what a “normal” eye would see at 200 feet. People who are “legally blind” may still see well enough to do some of the things they need to do in daily life.
Too Far or Too Close
No two sets of eyes are the same. Some eyeballs are too long, or have too much focusing power, causing myopia (nearsightedness). Others are too short, or have too little focusing power, causing hyperopia (farsightedness). Some eyeballs have uneven curvature, called astigmatism. There are different ways to correct such “mechanical” problems including eyeglasses, contact lenses or refractive surgery. Other problems are caused by disease or injury, and are not correctable by these means. People whose vision is impaired due to diseases such as macular degeneration, glaucoma, cataract, diabetic retinopathy and others can sometimes be helped by vision rehabilitation.
The Role of Your Brain
Even when all structures of the eye work well, vision cannot happen without the brain’s proper interpretation of the electrical impulses sent to it by the retina. The optic nerve is a bundle of retinal fibers that exits the rear of the eye and transmits electrical impulses to the brain where they are interpreted in an area called the visual cortex.
Corneal Research Strengthens Sight
At the Cornea Research Foundation of America, we’re investigating what causes the most prevalent corneal problems, such as Fuchs dystrophy and keratoconus, and we’re developing treatments that address the underlying problem, instead of just treating the symptoms. We’re also improving cornea transplant techniques by developing methods that are more targeted and less invasive, so that patients recover vision faster. We have trained over 600 surgeons from around the globe in advanced transplant techniques so that people around the world can benefit.
Helping people regain vision and return to a normal life is an immensely rewarding endeavor made possible through the dedication of our staff and the generous support of our donors. You can help by making a tax-deductible gift to the Foundation. Your help will support the ongoing research that is making a difference in the lives of people around the world.