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Visual and Auditory Systems

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Biological Sciences
Stephen Reid

Chapter 21:Visual and Auditory Systems STRUCTURES AND FUNCTIONS  The visual system includes external tissues and structures surrounding the eye. o External structures include the eyebrows, eyelids, eyelashes, lacrimal system, conjunctiva, cornea, sclera, and extraocular muscles. o Internal structures include the iris, lens, ciliary body, choroid, and retina.  The cornea, aqueous humor, lens, and vitreous must all remain clear for light to reach the retina and stimulate the photoreceptor cells.  Refraction is the ability of the eye to bend light rays so that they fall on the retina. When light does not focus properly, it is called refractive error.  Types of refractive errors are myopia (nearsightedness) and hyperopia (farsightedness).  Astigmatism is caused by corneal unevenness resulting in visual distortion. Presbyopia is a type of hyperopia due to aging.  The auditory system consists of peripheral and central systems. o Peripheral system includes the external, middle, and inner ear and is involved with sound reception and perception. o The central system (brain and its pathways) integrates and assigns meaning to what is heard.  Presbycusis can result from aging or insults from a variety of sources. Tinnitus, or ringing in the ears, may accompany the hearing loss that results from the aging process.  External and middle ear portions conduct and amplify sound waves from the environment. Problem
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