The body’s five senses feed the brain with millions of pieces of information every day. While all the senses are important, sight is perhaps the most obvious and most used of the senses. The eyes absorb the light and feed the images to the brain. It’s the brain that interprets the images and makes sense of them. The way the eye works by taking the light waves that come into it and transforming them into usable images is fascinating. The different sections of the eye work together to bring images of life to the brain.
The eye is actually made up of two primary parts. The sclera is the outer coating of the back part of the eye. This coating surrounds the iris, pupil, and other parts of the eye, protecting and enclosing them. The sclera is white, which is why most of the eye that is visible is white. The cornea and sclera are connected by a ring called the limbus.
The front part of the eye is the cornea. It is a transparent layer which acts much like a window. The cornea allows the light through to the back part of the eye. It is also much more curved than the back part of the eye. Interestingly, the cornea accounts for only about one-sixth of the mass of the eye.
The cornea also affects the clarity of the images which come through the eye. When the cornea is misshapen, the focus of the light coming in will be off. This is why some people need glasses or contacts. These re-focus the light correctly so the person can see clearly. In the same way, laser eye surgery reshapes the cornea so the person can see clearly without glasses.
Anterior and Posterior Chambers
Between the cornea and the back section of the eye which contains the iris and pupil, there is a small cavity of fluid. This area is called the anterior chamber of the eye. The fluid is aqueous humor. This fluid helps the cornea keep its shape and provides nutrition to this area of the eye. If the drain for this fluid becomes obstructed, too much of this fluid will accumulate, resulting in glaucoma.
The posterior chamber sits between the iris and the lens. It is also filled with aqueous humor. The fluid actually comes from the anterior chamber to the posterior chamber and then drains from there. This area should not be confused with the posterior segment, or cavity, which is behind the lens and is filled with a different substance.
Iris and Pupil
When people speak of someone’s eyes, they are usually referring to the iris and pupil. The iris is the colored part of the eye. Irises can be blue, brown, green, grey, or a mottling of these. Besides being aesthetically pleasing, the iris plays an important part of the eye’s function. It controls the amount of light which enters the eye.
The pupil is the dark center of the iris. It is the opening in the eye which allows the light to pass through to the back of the eye. Much like the shutter on a camera, the iris will open the pupil to let in more light when it is dark. The iris closes the pupil partway if the light is too bright. Continuing the camera analogy, the pupil is the aperture of the eye.
Just behind the pupil is the lens. Just like the lens on the camera, the lens in the eye focuses the light waves coming through the pupil onto the retina. The lens is also transparent, just like a camera lens. It is the deterioration of this lens which causes people to need reading glasses as they age.
Though it has a funny name, the vitreous humor is no laughing matter. The sclera, choroid, and retina are outer layers around the posterior, or vitreous cavity. This cavity is filled with a gel-like substance called vitreous humor. The vitreous humor helps give the eye its shape and helps protect the retina and optical nerve in the back of the eye.
The retina is the receptor of all the light images coming through the eye. It is a layer of nerves along the back of the eye. These special nerves receive the light and send electrical impulses to the brain. These are carried by the optical nerve to the part of the brain which interprets the images.
The images on the retina are actually upside down. The brain then reorients them, combines the images from both eyes, and sends the information along to other parts of the body. Without the brain interpreting the images, a person cannot see. This is why a head injury in the right place can result in blindness. The brain must also control and coordinate the movements of both eyes so that they work together.
The macula is actually part of the retina. The macula contains the special cells which can sense the light that is being focused on it. It also acts like a filter to the fovea. Not only does this play a crucial role in sending messages to the brain, but it also allows humans to see fine details. As humans age, the macula may also not work as well, a condition known as macular degeneration, which can lead to blindness.
The center of the macula which produces this sharp vision is the fovea. It is this sharpness that allows humans to read, drive, or do anything in which precise detail is important. This is also the area that detects subtle differences in color and provides the brain with sharp color images. The macula is thinner at the fovea, providing the light better access to it.
- The Human Eye: How it Works
- Interactive Diagram of the Eye
- How the Eye Works
- Webvision: Pictures of the Eye
- How the Eye Works
- Parts of the Eye
- Image Formation and Human Vision
- Physical Structures of the Eye
- The Sclera
- Facts About the Cornea
- Segments and Chambers of the Eye
- Model of the Iris and Pupil
- The Retina
- The Macula