The Eye and It's Conditions
To many of us the eye and how it functions is more of a mystery than the computer you're using to view this page. While sight is arguably one of our most valued senses, how sight is achieved and how to maintain our sight rarely comes to bear on our thoughts or in our daily conversations. Browse this section to learn how your eyes work, and how to preserve your gift of sight.
How it Works
Technically, your eyes are part of your brain, and 70% of the brain’s volume is involved in processing data related to vision. The anatomy of the eye is what makes that collection and processing possible. If one part is damaged by disease or infection, the abilty of the eye to capture and process data is dramatically affected, hurting your vision. An annual comprehensive exam, can catch problems before that become untreatable.
The cornea is the clear window through which light enters the eye. The cornea adds significant focusing power to the optical system of the eye.
The cornea is unique in that it has no blood vessels. It gets its oxygen from the tear film. The water part of the tears dissolve oxygen into the water, which transports the oxygen to the cornea. It is important if you wear contact lenses to use lenses that transmit enough oxygen, and not to wear lenses overnight if they are not silicone hydrogel lenses.
Oxygen deprivation due to improper contact lens wear will cause structural changes in the cornea that can permanently damage vision by causing scarring and angioneogenesis, the ingrowth of blood vessels into the cornea in response to hypoxia. Because the cornea needs to be transparent, blood vessels growing into it are damaging to vision, and do not resolve or go away.
The iris diaphragm is the colored part of the eye, and is the light intensity regulation system of the eye. In dim light the pupil dilates to allow more light to enter the eye, an constricts in bright light to decrease light intensity. The iris is in the anterior chamber, formed by the cornea, iris, and lens. The anterior chamber is filled with a fluid called aqueous humor. It is a liquid constantly produced and constantly drained.
The iris is susceptible to inflammatory conditions that can be difficult to control and can be recurrent. Iritis is controlled with steroids, and is more common in people with autoimmune disease.
The color of the iris is the result of the presence of a variety of pigments, and the mix of pigments determines the color of the iris. Pigment that is shed by the iris can be a problem when it accumulates in the drainage channel that carries aqueous humor out of the eye. Aqueous humor is constantly produced and constantly drained, and if anything blocks the drainage channel, pressure in the eye can rise to dangerous levels, causing glaucoma.
The opening in the iris is called the pupil. The size of the pupil is constantly changing, based on the amount of light present, and the visual task at hand. When the eye accommodates for close vision, the pupil constricts also.
The diameter of the pupil affects the clarity of the retinal image. When the pupil dilates at night to admit more light, the light moving through the more peripheral parts of the optical system are subject to the aberrations present in the periphery of the cornea-lens system, and the image is less clear.
The lens of the eye is very flexible when we are young, and can change shape to focus at virtually any distance. With age, the lens looses flexibility, and near vision is affected, requiring reading glasses or bifocals. This condition is know as Presbyopia. There are currently no medications or treatments that can restore flexibility to the lens.
Age related changes of the lens also cause changes in the structure of the lens that reduces its clarity and transparency. This is the process that causes cataracts. Cataracts are not growths, they are normal structural changes. The lens of the eye is differentiated skin, and like all of our skin, it undergoes age-related change. To correct the problem, the lens of the eye is removed, and an intra-ocular implant lens is put in its place. It is a highly successful surgery, and can restore vision to astonishing levels of clarity. It is not painful, and recovery is very rapid.
The vitreous is a gel-like substance that fills the posterior chamber, the larger of the two chambers of the eye. The vitreous is in direct contact with the retina. With age, the vitreous tends to shrink, which can cause the vitreous to separate from the retina. A vitreous detachment is not serious and does not require treatment, but may produce the perception of flashing lights, especially with movement of the eye.
The retina is the business end of the eye. The retina is a very complex structure, and contains the photoreceptor array and image enhancing nerve structures, along with a very significant blood supply. The miles of capillaries (minute blood vessels) that supply the working elements of the retina are very susceptible to breakdown and leakage in people who have diabetes or macular degeneration. Repair of these problems almost never restores vision to its former level, so prevention is very important.
The macula is a tiny area of the retina that contains all the cone receptors, about 7 million of them. The cones are the color vision receptors, and provide the sharpest images and color perception. When you look directly at an object, you are pointing the macula at it. This is why deterioration in the macula, such as macular degeneration, causes loss of central vision and degrades color perception.
The Optic Disc and Optic Nerve
The optic disc is the opening in the back of the eye through which the nerve bundle, called the optic nerve, exits the eye on its way to the brain. The optic nerve bundle collects all the data generated by the rods and cones, and conducts them to the occipital cortex of the brain, where visual perception takes place.
As the nerve bundles from across the retina turn to move through the optic disc, they make an angle of nearly 90 degrees. It is at this turning point where they exit the eye that these structures are subject to damage caused by elevated pressure inside the eye, as in glaucoma. The pressure creates physical damage to the nerve structures, and closes down the all-important blood supply, causing the nerve tissue to die. It never regenerates, so preventing the damage is critical.