Structure and Functions (Magill’s Medical Guide, Sixth Edition)
The eye captures pictures from the environment and transforms them into neural impulses that are processed by the brain into visual images. The retina, with its light-sensitive cells, acts as a camera to “put the picture on film,” while neural processing in the brain “develops the film” and forms a visual image that is meaningful and informative for the individual.
The human eye originates during development, that is, while the individual is being formed as an embryo in the uterus. Eye formation begins during the end of the third week of development when outgrowths of brain neural tissue, called the optic vesicles, form at the sides of the forebrain region. The optic vesicle induces overlying embryonic tissue to thicken in one region, forming a primitive lens structure called the lens placode. The lens placode, in turn, induces the optic vesicles to form a cuplike structure, the optic cup, while the brain’s connection of the vesicles narrows into a slender stalk that forms the optic nerve. The inner part of the optic cup forms the neural or sensory retina, with its photoreceptors, while the outer part of the optic cup develops into the layers of tissues, or tunics, that make up the wall of the eyeball. The lens placode further condenses and solidifies by forming lens fibers that become transparent. The function of the lens will eventually be to focus light onto the retina. The major structures of the...
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Disorders and Diseases (Magill’s Medical Guide, Sixth Edition)
Commonly existing refractive errors are astigmatism, myopia, hyperopia, and presbyopia. Presbyopia is an anomaly that occurs with aging when the crystalline lens loses its ability to accommodate. Causes include thickening of the lens and changes in the attachment fibers that anchor the lens. Because of these alterations, the lens is not able to change its shape and the eye remains focused at a specific distance. To compensate for this problem, bifocal spectacles are normally prescribed, with the upper region of the lens focused for distant vision and the lower lens focused for near vision. Hyperopia, also called farsightedness, results when an eyeball is too short. Because light rays are not bent sufficiently by the lens system, the image is focused not on the retina but behind the retina. To compensate for this problem, spectacles with convex lenses are prescribed, which bring the focus point back on the retina. Conversely, myopia, or nearsightedness, results from an abnormally long eyeball. In this case, the lens system focuses in front of the retina. This abnormal vision can be corrected by spectacles with a concave lens. Astigmatism results from a refractive error of the lens system, usually caused by an irregular shape in the cornea or less frequently by an irregular shape in the lens. The consequence of this anomaly is that some light rays are focused in front of the retina and some behind the retina, creating a blurred...
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Perspective and Prospects (Magill’s Medical Guide, Sixth Edition)
Early physicians recognized the importance of good eyesight, but because of limited understanding they had minimal means to treat major eye disorders. During the Middle Ages, surgeons performed eye operations, including ones for cataracts in which the lens was pushed down and out of the way with a needle inserted into the eyeball. In the eighteenth century, this operation was improved when cataract lenses were extracted from the eye. In the early seventeenth century, Johannes Kepler described how light was focused by the lens of the eye on the retina, thus providing insight into why spectacles are valuable in cases of poor eyesight. In 1801, Thomas Young published a foundational text entitled On the Mechanics of the Eye. Hermann von Helmholtz in the nineteenth century invented the first ophthalmoscope, which allowed inspection of the interior structures of the eye. Young and Helmholtz also developed theories to explain the phenomenon of color vision. From the invention of the ophthalmoscope, the range of clinical observation was extended to the inside of the eyeball, allowing the diagnosis of eye disorders. The modern understanding of eyesight and vision is increasing with contributions from ongoing research.
Ophthalmology is the study of the structure, function, and diseases of the eye. An ophthalmologist is a physician who specializes in the diagnosis and treatment of eye disorders and diseases with...
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For Further Information: (Magill’s Medical Guide, Sixth Edition)
Buettner, Helmut, ed. Mayo Clinic on Vision and Eye Health: Practical Answers on Glaucoma, Cataracts, Macular Degeneration, and Other Conditions. Rochester, Minn.: Mayo Foundation for Medical Education and Research, 2002. A helpful handbook on all the medical, social, and emotional facets of vision impairment.
Guyton, Arthur C., and John E. Hall. Human Physiology and Mechanisms of Disease. 6th ed. Philadelphia: W. B. Saunders, 1997. Guyton is a nationally recognized authority on medical physiology, having written and edited numerous college-level and medical school textbooks on the subject. His writing style is understandable to the nonmedical specialist and student. This college-level text contains two chapters on the eye: The first deals with the optics of vision and the function of the retina; the second emphasizes the neurophysiology of vision.
Litin, Scott C., ed. Mayo Clinic Family Health Book. 4th ed. New York: HarperResource, 2009. Perhaps the best general medical text for the layperson, this book covers the entire medical field. While the information is derived from a wide variety of highly technical sources, the articles are written to be easily understood by a general audience.
National Foundation for Eye Research. http://www .nfer.org. Site provides consumers and professionals with access to developing technology for treating impaired vision.
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