BIOC33H3 Lecture Notes - Ophthalmoscopy, Near-Sightedness, Refractive Error
25 Mar 2013
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Document Summary
The visual system includes external tissues and structures surrounding the eye: external structures include the eyebrows, eyelids, eyelashes, lacrimal system, conjunctiva, cornea, sclera, and extraocular muscles. 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. 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.
