Nervous System
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Eye Disorders
2.31 Image Forming – When the eyes view an image, each eye sees the image from a slightly different angle. About one-third of the visual fields overlap, but the other two-thirds differ. This overlap produces binocular vision, which enables people to perceive depth. Also, the optic nerves do not connect to only one hemisphere. About one-half of each optic nerve crosses over into the opposite side of the brain; because of this, each hemisphere receives visual information from both eyes. The brain combines all the information it receives to produce one image.
2.32 Where do tears come from and what is their purpose? – Tears are produced by a gland found above and to the outside of each eyeball. Each gland produces a watery secretion that mixes with a slightly oily secretion from accessory glands. The tear glands have between 10 and 12 ducts, and every time the eyes blink, tears are spread across the surface of the eye. Tears protect the eyes by lubricating them and washing away debris. Also, they contain an enzyme that helps prevent eye infection. Tears drain into two ducts in the inner corners of the eye. These ducts drain into the back of the nose.
2.33 Nearsight, Farsight and Astigmatism – Some people are nearsighted or farsighted; others have astigmatism. Why is this? For an image to appear clear, the lens must focus light rays directly onto the retina. The lens does this by using tiny muscles and ligaments to adjust its thickness. If the shape of the eyeball is too elongated or too short, the lens will be unable to change sufficiently to focus the image properly. In nearsighted people, the focal point is in front of the retina, resulting in poor distance vision. In farsighted people, the focal point is behind the retina, resulting in poor close vision. Astigmatism is caused by an irregularly shaped cornea. This causes the light rays to be focused erratically and to mostly miss the retina. Corrective lenses adjust the light ray’s focal point so that it is on the retina.
