Endocrine System
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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.
2.34 Colour Blindness – A condition that is almost always genetic in origin, is the inability to distinguish colours. It is caused by an abnormality of one or more of the three types of light-sensitive pigments in the cone cells. The inability to distinguish red light from green light is the most common type of colour blindness. About 10 percent of all males have some degree of colour blindness while less than 1 percent of females are affected. Total colour blindness is extremely rare.
2.35 Cataracts – When a person has cataracts, the lens of the eye is no longer transparent. Light cannot pass through the lens easily, so vision is diminished. Although ageing is the most common cause of cataracts, they can also result from a reaction to a drug, from radiation including solar exposure, and from disease. In the past, people with cataracts became blind; however, today the condition can be corrected surgically. The faulty lens is removed and replaced with an artificial one. Glasses or contact lenses are then prescribed to adjust the vision.
2.36 Cornea Scar – Some diseases cause scarring of the cornea. When this occurs, blindness may result because light cannot pass through the damaged cornea. This condition can be corrected by a corneal transplant. The central portion of the scarred cornea is removed and replaced with a cornea from an organ donor.
Hearing and Equilibrium
2.37 When you think of your ears, you may only consider your external ear, the part that sticks out on each side of your head. But you also have a middle ear and an inner ear, the portions responsible for sensing sound.
