|
The fovea centralis (the term ''fovea'' comes from the Latin, meaning ''pit'' or ''pitfall'') is a small, central pit composed of closely packed cones in the eye. It is located in the center of the macula lutea of the retina.〔"Relation Between Superficial Capillaries and Foveal Structures in the Human Retina" - (with nomenclature of fovea terms), Masayuki Iwasaki and Hajime Inomara, - ''InvestigativeOphthalmology & Visual Science'' (journal), - volume 27, pages 1698-1705, 1986, IOVS.org, webpage: - (IOVS-fovea-capillaries ). -〕 The fovea is responsible for sharp central vision (also called foveal vision), which is necessary in humans for activities where visual detail is of primary importance, such as reading and driving. The fovea is surrounded by the ''parafovea'' belt, and the ''perifovea'' outer region.〔 The parafovea is the intermediate belt, where the ganglion cell layer is composed of more than five rows of cells, as well as the highest density of cones; the perifovea is the outermost region where the ganglion cell layer contains two to four rows of cells, and is where visual acuity is below the optimum. The perifovea contains an even more diminished density of cones, having 12 per 100 micrometres versus 50 per 100 micrometres in the most central fovea. This, in turn, is surrounded by a larger peripheral area that delivers highly compressed information of low resolution following the pattern of compression in foveated imaging. Approximately half of the nerve fibers in the optic nerve carry information from the fovea, while the remaining half carry information from the rest of the retina. The parafovea extends to a radius of 1.25 mm from the central fovea, and the perifovea is found at a 2.75 mm radius from the fovea centralis.〔"eye, human."Encyclopædia Britannica. 2008. Encyclopædia Britannica 2006 Ultimate Reference Suite DVD〕 ==Structure== Fovea (or ''Fovea centralis'') is the depression in the inner retinal surface, about 1.5 mm wide, the photoreceptor layer of which is entirely cones and which is specialized for maximum visual acuity. Fovea also grossly corresponds to the retinal avascular zone (which means without any blood vessels). This allows the light to be sensed without any dispersion or loss. This anatomy is responsible for the depression in the center of the fovea. The foveal pit is surrounded by the foveal rim that contains the neurons displaced from the pit. This is the thickest part of the retina. The fovea is located in a small avascular zone and receives most of its oxygen from the vessels in the choroid, which is across the retinal pigment epithelium and Bruch's membrane. The high spatial density of cones along with the absence of blood vessels at the fovea accounts for the high visual acuity capability at the fovea.〔Provis et al., Prog Retin Eye Res Vol35 pp63-81〕 The center of the fovea is the foveola – about 0.2 mm in diameter – or central pit where only cone photoreceptors are present and there are virtually no rods.〔 The central fovea consists of very compact cones, thinner and more rod-like in appearance than cones elsewhere. These cones are very densely packed (in a hexagonal pattern). Starting at the outskirts of the fovea, however, rods gradually appear, and the absolute density of cone receptors progressively decreases. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Fovea centralis」の詳細全文を読む スポンサード リンク
|