The eye is a sensory organ that reacts to light. It allows for the conscious perception of light, differentiation of color and depth perception through stereoscopic vision (the use of both eyes to produce a coherent, three-dimensional image).
The sclera is the white, opaque part of the eye that encloses most of the eyeball and is surrounded by the conjunctiva, a thin mucous membrane that covers the outer surface of the eye everywhere except on the cornea.
The cornea is the clear, outermost part of the eye through which light enters. If we compare the eye to a camera, the sclera would be the camera’s outer shell, and the cornea would be the outer, main lens. The cornea does most of the focusing for the eye at distance.
Beneath the cornea is the iris, which gives color to our eyes, and the pupil, the aperture through which light enters the inside of the eye. Light is brought into sharp focus by the eye’s natural, crystalline lens. A muscle surrounding the lens squeezes the lens in order to make it more convex; it behaves like a zoom lens in a camera, increasing the eye’s refractive power so that we may clearly see objects directly in front of us or read. Beneath the lens, the clear gelatinous mass that fills the inside of the eye to provide structural support to the eye is the vitreous.
The retina is a membrane that lines the rear of the globe and converts visual signals into electrical impulses. In our camera analogy, the retina is the film of the camera. The cornea and the lens bring light into sharp focus on the macula, a specific region of the retina.
Rod and cone cells, photoreceptor cells on the retina, sense actual photons (particles of light) and differentiate their frequencies, which we interpret as the various colors. This information travels along the optic nerve to the brain’s visual cortex. We have a limited knowledge of how visual signals are processed at present and, in particular, of how the brain copes with the limited “channel capacity” of the optic nerve. Research to provide a more thorough understanding of how our minds produce a detailed, continuous image is ongoing.
Six orbital muscles outside of the eye control movement. Four are responsible for movement up, down and sideways, and the other two keep the eyes aligned as the head moves. The eyelids, the thin folds of skin which cover and protect the eyes from debris and perspiration, are also responsible for spreading secretions on the eye and for the tear film that lubricates the cornea, which must always be kept moist.