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How do humans see?

The eye is a ball covered with a tough outer membrane. The covering in front is clear and curved. This curved area is the cornea, which focuses light while protecting the eye.

After light passes through the cornea, it travels through a space called the anterior chamber (which is filled with a protective fluid called the aqueous humor), through the pupil (which is a hole in the iris, the colored part of the eye), and then through a lens that performs more focusing. Finally, light passes through another fluid-filled chamber in the center of the eye (the vitreous) and strikes the back of the eye, the retina.

Like the film in a camera, the retina records the images focused on it. But unlike film, the retina also converts those images into electrical signals, which the brain receives and decodes.

One part of the retina is specialized for seeing fine detail. This tiny area of extra-sharp vision is called the macula.
The individual components of the eye work in a manner similar to a camera. Each part plays a vital role in providing clear vision. So think of the eye as a camera with the cornea, behaving much like a lens cover. As the eye's main focusing element, the cornea takes widely diverging rays of light and bends them through the pupil, the dark, round opening in the center of the colored iris. The iris and pupil act like the aperture of a camera.

Next in line is the lens which acts like the lens in a camera, helping to focus light to the back of the eye. Note that the lens is the part which becomes cloudy and is removed during cataract surgery to be replaced by an artificial implant nowadays.

The very back of the eye is lined with a layer called the retina which acts very much like the film of the camera. The retina is a membrane containing photoreceptor nerve cells that lines the inside back wall of the eye. The photoreceptor nerve cells of the retina change the light rays into electrical impulses and send them through the optic nerve to the brain where an image is perceived. The center 10% of the retina is called the macula. This is responsible for your sharp vision, your reading vision. The peripheral retina is responsible for the peripheral vision. As with the camera, if the "film" is bad in the eye (i.e. the retina), no matter how good the rest of the eye is, you will not get a good picture.

The human eye is remarkable. It accommodates to changing lighting conditions and focuses light rays originating from various distances from the eye. When all of the components of the eye function properly, light is converted to impulses and conveyed to the brain where an image is perceived.
Laura C. Fine, MD
Ophthalmology
Sight is not fully developed at birth; the brain and eyes have to learn to work together in the first months of life. Once sight is well developed, the eyes and the brain team up to provide virtually instantaneous visual information.

Consider what happens when you walk through a parking lot and spot your car. First, you are actually seeing the light reflected off the car that enters your eye; some light must be present in order to see.

If the image is clear, it means that light thrown off the surfaces of the automobile hits your cornea (the clear part of the eye's protective covering), where it is refracted, or bent, inward and is then sent through the aqueous fluid until it reaches the lens. The light rays are then bent further, passed through the vitreous fluid (clear, gel-like substance that fills the space behind the lens and supports the shape of the rear portion of the eye), and projected onto the retina (the innermost layer of the eye) as a flat, upside-down image.

The light is absorbed by the retina and turns into electrical energy, which the optic nerve then conveys to the visual area of the brain. Data about your car -- its size, shape, color, and position -- are sent along the optic nerve as impulses, a sort of neurologic code that the brain deciphers. Although the image is upside down on the retina, the brain automatically turns it right side up.

Although it is possible to see with only one eye, you generally rely on binocular vision -- vision with both eyes -- for depth perception. You get a three-dimensional view of your vehicle because the brain interprets what is seen from your two eyes (each with a slightly different perspective) as a single image.

If a flashy car nearby catches your attention, you instantly shift your gaze without a thought. The external muscles of the eyes are synchronized to keep the eyes aligned and to coordinate their movement.
Dr. Mehmet Oz, MD
Cardiology (Cardiovascular Disease)
To see, your eye essentially takes information from outside sources and passes it along to your brain. How does it do this? Very carefully.

First, light travels through the cornea (the clear covering of the eye) to the iris, which is the colored part of the eye. Behind the iris sits the lens, which is shaped like a camera lens. This remarkable organ not only changes shape to focus light, but also filters out some parts of the light spectrum that may be harmful to the eye.

The cornea and lens focus light to form the image (an upside-down image) on the back of the surface of the eye-your retina. Once it reaches the retina, it's then sent through the optic nerve and rotated another 180 degrees so your brain can determine that what you're seeing is right-side up.
YOU: The Owner's Manual, Updated and Expanded Edition: An Insider's Guide to the Body that Will Make You Healthier and Younger

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The macula is the central area of one of the most important parts of your eye -- the retina. The retina is a thin layer of light-sensitive tissue that lines the back of the eye. Light rays are focused onto the retina, where they are transmitted to the brain and interpreted as the images you see. The macula is the portion of the retina responsible for clear, detailed vision.

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Important: This content reflects information from various individuals and organizations and may offer alternative or opposing points of view. It should not be used for medical advice, diagnosis or treatment. As always, you should consult with your healthcare provider about your specific health needs.