The Eyes and Vision
While most people take vision for granted, sight should obviously be a prized possession. Of all the sensory organs, the eyes are the most highly developed. A greater proportion of the brain is used to process sight in a human being than touch, smell, and hearing combined.
Rays of light enter the eye through the pupil and pass through the lens. This light stimuli is converted by photoreceptors (rods and cones) in the retina in an electrical impulse. The impulse is in turn relayed to the visual cortex via the optic nerve.
How the Eye is Constructed
Look in the mirror at your eyes and the first thing you see is the circular iris which determines eye color. In the middle of this is the pupil. The "white" of the eye is the largest part of the eye, also known as the vitreous body, which is a transparent, gel-like mass kept in shape by connective tissue casing called the sclera.
This tissue blends with the transparent, curved cornea at the front of the eye. A thin mucous membrane called the conjunctiva covers the sclera, the cornea and the inside of the eyelids and is kept moist by lacrimal fluid. The lacrimal glands above the outer corner of the eye produce this fluid and the fluid is distributed over the front part of the eye by blinking. Dust particles are washed out of the eye by this fluid.
The iris is made up of ring-shaped muscle fibers and is located in front of the lens. The pupil is located in the middle of the iris and is very much like the aperture on a camera, adjusting itself to the available lighting conditions by constricting and expanding.
The lens, which is transparent, is located between the iris and the vitreous body. The lens is tasked to concentrate the light rays entering the eye in such a way that a clear image can be formed on the retina. Lens fibers hold the lens in place. A ring-shaped group of muscles (ciliary muscles) are capable of changing the curvature of the lens, thereby regulating the refraction of light.
Aqueous humor is formed in the ciliary body, a connective tissue appendage of the ciliary muscles. Small ducts allow aqueous humor to drain in the front section of the eye. The production and draining of this aqueous humor generally balance each other so a constant intraocular pressure is allowed to prevail.
The innermost layer of the eye is composed of the retina. Light stimuli is absorbed by the retina with its sensory cells and relays them via the nervous system to the brain. These nerve cells form the optic nerve, which extends from the back of the eyeball to the cerebral cortex.
There are several problems that can afflict the eyes (other than diseases) resulting in blurring or distortion of vision. Common vision problems involve difficulty with focusing, including myopia (nearsightedness), hyperopia (farsightedness), presbyopia (middle-age vision) and astigmatism.
Other problems can include strabismus (crossed eyes), amblyopia (lazy eye) and a deficiency of color vision (color blindness).
Problems such as those listed above can be present alone or with other conditions, and can be restricted to one eye or can exist in both eyes. Some of these conditions are preventable: most of these conditions can be improved with surgery, eyeglasses or contact lenses.
Many negative conditions of the eye (such as conjunctivitis, blepharitis and dry eye) can cause a great deal of discomfort. Diseases of the eye include glaucoma, macular degeneration and (on rare occasions) cancer. All of these diseases can lead to cancer.
Causes of Vision Problems
Many vision problems which involve focusing derive from an abnormal shape of one or more parts of the eye, or the changes that commonly take place with aging. Other conditions are caused by heredity.
Preventing Problems with the Eyes
Some conditions affecting the eyes may not be preventable. However, a healthy diet high in nuts, fish, and vegetables may help to lower the risk for certain eye diseases such as macular degeneration.
Avoiding overexposure to the sun's rays and protecting the eyes from physical injury (due to sports, exercise, etc.) may be the best ways to ensure eye disorders do not occur.
- McCracken, T. & Walker, R. New atlas of human anatomy, London : Constable (2001)
- Parker, S. The Concise Human Body Book, Dorling Kindersley Limited (2009)
- Winston, R. et al. Human: The Definitive Visual Guide, DK Publishing, Inc (2004)
- Ullmann, H.F. Atlas of Anatomy, Elsevier GmbH, Munich (2009)
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The Eyes and Vision Disorders
A leading cause of blindness, glaucoma typically does not develop until after the age of 40 and then becomes increasingly more common as individuals age. This condition affects 2 out of 100 people and is thought to be the cause of blindness in some 13% of the population
A stye, whose medical name is hordeolum, is simply a bacterial infection of an eyelash follicle. Typically caused by staphylococcal bacteria, the infection causes a bump filled with pus to form either on the inside or the outside of the eyelid. This bump continues to grow for approximately one week and then usually subsides, rupturing and then healing.
The macula is the middle area of the retina which allows human beings to see very detailed images in the center field of their vision. The macula is the most sensitive part of the retina and is the nerve-rich portion of the back of the eye that is necessary for sight. When these cells start to deteriorate (usually after the age of 60), the resulting blurred or distorted vision is called macular degeneration.
Cataract condition can be hard to detect because cataracts are painless. However, over a period of years the vision becomes progressively more blurred. Seeing in dim light or at night becomes especially difficult. Images may be distorted and colors may fade.