The eye is a really complex organ. If we simplify a lot its functions, it can be compared to a camera: like a camera it is provided with
objective lens, diaphragm, film and focus lens.
Cornea and crystalline constitute the objective lens having the function of conveying the rays of light on the "film" that is the retina.
Between them, you can see the iris, which has different colors according to the individuals. In the middle of iris, there is a hole capable of contracting and widening according to the environment lighting: the pupil. This structure is the eye "diaphragm" that regulates the amount of light which shall enter inside it.
Thanks to cornea and crystalline lens and to the pupil (diaphragm), the image is clearly focused on the retina: this last coats the eye internal rear part and acts as a film in a camera, in order to fix the image.
The above described structures are separated by two other important components of the eye: the aqueous humor and the vitreous body. The aqueous humor is a liquid substance produced inside the eye, sited between cornea and crystalline lens, while the vitreous body is a gelatinous body sited between crystalline lens and retina.
The focusing is not the less important component. For obtaining a good picture, the camera shall have a device for focusing the image on a film, through fore and back displacement of the objective lens: in the eye this function is regulated by the crystalline lens and by the suspensor apparatus consisting of the cilial muscle and the zonular fibers. In fact, the cilial muscle supporting the crystalline lens tightens or slackens, thus producing a change in the crystalline lens shape, according to the distance from which an image is to be focused. This process is called accommodation. When the eye looses this capability, we have a condition of presbyopia, that is the difficulty of near reading. This is a normal physiological condition after the age of 40 years, related to the failure of the cilial muscle which is no longer capable of contracting thus producing the crystalline lens shape change.
In other words, the light crosses cornea, aqueous humor, pupil, crystalline lens and vitreous body and hits retina, thus producing the visual stimulus. The visual stimulus is transformed into electrical impulses by the retina cells and conveyed through the optic nerve up to the brain who decodes them, thus shaping the images.
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Whatever alteration affecting one of these structures implies a defective images shaping.
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In the following chapters, we shall analyze the main pathologies affecting the eye, that are:
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