THE HUMAN EYE AND HOW IT WORKS
HOW VISION OCCURS
Let's say you're looking at autumn leaves. Light reflects off the leaves and enters the eye through the cornea, then travels through several structures to the back of the eye, where the image reaches its sharpest focus. From there, the optic nerve carries the focused "leaves images" to the brain. Only then can you actually see the leaves. Let's take a closer look at this process. It might be helpful to think of seeing as four distinct processes, which are roughly similar to the way a camera processes an image.
1. Light Reflects Off an Object When you're admiring the beautiful orange and red foliage on a maple tree in autumn, you're actually seeing the light reflecting off them-just as a photograph is an image of light reflecting off an object. If this were not the case, then you'd be able to see objects at night, or photograph them, just as well as in daylight.
2. Light Enters the Eye As it reaches the eye, the form of energy that we call light first enters the clear, curved cornea. The curved surface of the cornea bends the incoming light so that the rays come together, like branches, instead of remaining parallel as they enter the eye. Thus the cornea does most of the focusing work of the eye.
After passing through the cornea, light is bathed in a thin layer of liquid, the aqueous humor, before it reaches the iris. The iris contracts or expands (dilates) around the pupil to regulate the amount of light allowed into the eye's interior. Sophisticated cameras have light-regulating mechanisms that do much the same thing.
When you walk out of a dark movie theater into bright daylight, you've probably experienced that "blinding" sensation before the iris has time to contract, adjusting to the difference in light.
3. Light Is Focused in the Lens
The lens, by changing its shape, also contributes to the eye's focusing work. The lens is round and usually somewhat flat. When you were younger, most likely your eye lenses were perfectly clear and quite flexible. Some eye specialists compare the lens to a small, clear gelcap with a thin but sturdy exterior. If you squeeze it in the middle, it gets thinner and flatter; if you squeeze the ends, it gets thicker and more rounded.
That's basically what happens to the eye lens when you focus on objects at varying distances, except that the work of "squeezing," or contracting, is done by muscles called ciliary muscles and the ligaments attached to them (zonules). These muscles and ligaments also hold the lens in place. When you focus on something close up, the ciliary muscles contract, making the lens thicker and rounder. As your focus moves outward to more-distant objects, the muscles relax and the lens becomes thinner and flatter. Thus, the lens projects a clear image onto the retina, at the back of the eye, whether the source of the image is near or far, or somewhere between.
This shape-changing adjustment to distance is called accommodation. Although the cornea does the initial focusing, it is the lens and its accommodation ability that allow you to focus well at different distances.
Once light passes through the lens, it enters the spacious cavity that occupies about two-thirds of the eye. This cavity is filled with a clear gel called vitreous, or vitreous humor, which helps the eye maintain its round shape.
4. Light Signals Are Interpreted by the Brain Finally, the light reaches its destination-the retina, which receives images in much the same way that camera film does. After the retina's specialized cells, rods, and cones have converted the image to signals that the brain can understand, the signals are finally carried to the brain through the nerve bundle at the back of eye-the optic nerve, which consists of millions of nerve fibers. The brain receives and interprets the signals, and it is at that point that we actually see.
The eye is sometimes described as an extension of the brain, and if there is severe damage to the optic nerve, the eye becomes useless. Vision occurs only when an image reaches the brain and is identified.
Eye-Structure Abnormalities
