Sight, like the other four senses, is closely related to the other parts of our anatomy – particularly the brain. The eye works in much the same way as a camera – the iris and the pupil control how much light to let into the eye, and how we see depends upon this transfer of light. Once the light is passed through to the eye, the cornea and the lens help focus the light rays onto the back of the eye. The cells in the retina then absorb and convert the light to electrochemical impulses which are sent to the brain. In order to understand how the eye works, it’s important to understand basic eye anatomy.
The cornea is the transparent front segment of the eye that covers the iris, pupil, and anterior chamber. This part of the eye serves as the first and largest refracting medium; in fact, approximately 70 percent of the refractive power of the eye comes from the cornea. Corrective laser surgery actually reshapes the cornea and changes the focus, so those with poor eyesight are once again able to see clearly.
The pupil is the dark center opening in the iris that allows light to reach the retina. Under normal illumination, a human pupil ranges in size from two to five millimeters in diameter. The pupil changes in size to adjust for the amount of light available. This part of the eye works much like the aperture in most 35 mm cameras – the pupils shrink in bright lights, and they’ll grow in low light situations.
The retina is the part of the eye that converts images into electrical impulses. Once the images are converted, the retina sends them along the optic nerve to the brain, where they’re translated. This part of the eye consists of many layers that include rods and cones, and is highly sensitive to light. Within the retina are seven layers of cells and processes that translate light from the pupil into a signal.
This part of the eye is easily identifiable, as it’s the colored part of the eye. Not only does this pigmented tissue give color to the eye, but it regulates the amount of light entering the eye by changing the size of the pupil. In bright light, the iris constricts and makes the pupil opening smaller. In dim light, the iris dilates and makes the pupil opening larger to increase the amount of light entering your eye.
This transparent substance in the eye allows rays of light to converge or diverge onto the retina. Since our lenses deteriorate as we age, many individuals require reading glasses as a result. If necessary, the lens can be replaced –intraocular lenses are used to replace lenses clouded by cataracts. Because the lens is flexible and elastic, it can change its shape to focus on objects that are nearby or at a distance.
This bundle of over a million nerve fibers carries messages from the retina to the brain. The retina actually sees things upside down but, after the optic nerve carries images to the brain, the brain then turns these images right side up so we aren’t ‘seeing’ things upside down. Once these images are turned the proper way, the result is the sense of vision as we know it.