The central part of the retina is the area where color vision and eyesight are most sensitive. It was so named because when one looks into the eyeball of a corpse, a yellow spot (approximately 2 millimeters in diameter) can be seen in the center of the cloudy retina. The macula is located on the ear side of the optic disc, about 15 degrees away in terms of visual angle, and the center of the macula is concave where the thickness of the retina is particularly reduced; this part is called the fovea centralis. The fovea centralis contains only cones. Objects in the outside world that one is trying to see are imaged onto this macula. When lesions occur in the macula, central vision is affected, causing problems with reading and other activities. Other symptoms include a central scotoma, a shadow in the center of the field of vision, and metamorphopsia, in which objects appear distorted. Diseases that can cause this include central retinitis, age-related macular degeneration, and hereditary macular dystrophies. Central retinal vein occlusion and diabetic retinopathy can also damage the macula, causing a sudden decline in vision. [Mizuo Matsui] [Reference items] | | | | |Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend |
網膜の中心部で、色覚や視力のもっとも鋭敏な部位である。死体の眼球をのぞくと、混濁した網膜の中心に黄色の斑点(径約2ミリメートル)が見えるところから名づけられた。黄斑は視神経乳頭の耳側で、視角にして約15度離れたところにあり、黄斑の中央は網膜の厚さがとくに減少してへこんでおり、この部を中心窩(か)という。中心窩には錐状体(すいじょうたい)だけがある。見ようとする外界のものは、この黄斑に像を結ぶようになっている。 黄斑に病変がおこると、中心視力が冒されるため、読書などに障害が出てくる。このほか、視野の中心部に陰が出る中心暗点、物がゆがんで見える変視症などの症状もみられる。病気としては中心性網膜炎や加齢黄斑変性症のほか、遺伝性の黄斑ジストロフィーなどがある。また、網膜中心静脈閉塞(へいそく)症や糖尿病網膜症などが、黄斑に障害を及ぼしてくることもあり、急激に視力が低下する。 [松井瑞夫] [参照項目] | | | | |出典 小学館 日本大百科全書(ニッポニカ)日本大百科全書(ニッポニカ)について 情報 | 凡例 |
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