How the Eye Works
Vision is a complex sense composed of many elements. The human eye, elegant in its detail and design, represents a gateway to the process we call vision. The eyeball, or globe, is spherical in shape and about 1 inch across. It houses many structures that work together to facilitate sight.
The human eye is comprised of layers and internal structures, each of which performs distinct functions. The outside layer of the eye is comprised largely of a tough, white, protective tissue called the sclera. The sclera helps maintain the shape of the eyeball. At the front of the eye is an equally tough but clear structure called the cornea, which is responsible for letting light into the eye and bending light.
Going from outside to inside, the next layer of the eye is the choroid, which carries the blood supply necessary to nourish the eye's internal structures. Finally, there is the layer called the retina, lining the inside of the eye, which is sensitive to light and receives stimulation to its specialized cells.
The eye has a number of protective features. The eyelids, eyelashes and eyebrows are all designed to protect the eye from dirt and dust that might enter it and cause damage. The globe sits inside the orbital cavity, a bony pocket lined with fatty tissue as a cushion. Together these provide additional protection against injury. Six muscles attach at various points to the sclera and enable the globe to move in many directions inside the orbit.
In order for vision to take place, a succession of processes must occur involving the structures within the eye and the brain:
The first part of this chain is that light rays must travel through the eye to ultimately focus on the retina. There are a number of structures involved in the bending or refracting of light so that it focuses properly. Light first passes through the clear cornea at the front of the eye, and then through a watery substance called the aqueous humor which fills the small chambers located behind the cornea. As light continues on its pathway it passes through the pupil, a round opening in the center of the iris. The iris is the part of the eye that gives the eye its color. It also is made up of specialized muscles that are able to change the size of the pupil from very small (about 2 mm) to large (about 8 mm), regulating the light that is entering.
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