Abstract Among vertebrates which are not too strongly nocturnal a local modification of the retina for maximal visual acuity is of widespread occurrence. The modified area (which is single except in some birds) may be small or extensive, a broad horizontal band across the fundus or a suprapapillar crescent, round or in the shape of a variously oriented oval, located centrally or temporally even as far as the periphery and on or slightly above or below the horizontal meridian. The generic term for such a region of high resolving power is area centralis—misnomer though this name often is. An area centralis may be distinguished from the surrounding retina only by the visual cells being more numerous in it per unit of retinal area. When this massing of visual cells is marked, however, it is usual to find several accompanying adaptations: The visual cells are slenderized, to facilitate their aggregation; they References 1. Kahmann, H.: Ueber das foveale Sehen der Wirbeltiere: I. Ueber die Fovea centralis und die Fovea lateralis bei einigen Wirbeltieren , Arch. f. Ophth. 135:265, 1936. 2. Walls, G. L., and Judd, H. D.: The Intra-Ocular Colour-Filters of Vertebrates , Brit. J. Ophth. 17:641 and 705, 1933.Crossref 3. The more detrimental phase of such extinction would of course be scattering, due to optical heterogeneity, rather than absorption ; but retinal tissue appears to be quite homogeneous optically. 4. Walls, G. L.: The Reptilian Retina: I. A New Concept of Visual-Cell Evolution , Am. J. Ophth. 17:892, 1934. 5. Walls, G. L.: Human Rods and Cones: The State of Knowledge , Arch. Ophth. 12:914 ( (Dec.) ) 1934.Crossref 6. There are several reasons for believing that rays entering the bases of the visual cells are confined within the latter by total internal reflection. Hence for the present purpose the external limiting membrane may be taken as the level of perception.
Archives of Ophthalmology – American Medical Association
Published: Dec 1, 1937
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