CHAPTER 8 The Special Senses occurs; that is, until the ciliary muscle contracts, causing the lens to become more spherical, the increased convexity causes the lens to refract the light waves more strongly, bringing the image into focus on the retina.
Proper imaging of light on the retina depends not only on the lens and cornea but also on the iris, which adjusts the amount of light that can enter the eye through the pupil. In this regard, the pupil is analogous to the aperture in a camera, which also controls the depth of field of the image and the amount of spherical aberration produced by the lens. When the pupil is constricted, the depth of field is increased, and the light is directed through the central part of the lens, where spherical aberration is minimal. Pupillary constriction occurs reflexively when the eye accommodates for near vision or adapts to bright light, or both. Thus when a person reads or does other fine visual work, the quality of the image is improved by adequate light.
Layers of the Retina
The 10 layers of the retina are shown in
Fig. 8.2 . The outermost portion is the pigmented epithelium (layer 1), which is just inside the choroid. The pigment cells have
Asanindividualages,theelasticityofthelensgraduallydeclines.Asaresult,accommodationofthelensfornearvisionbecomesprogressivelylesseffective,aconditioncalledpresbyopia. Ayoungpersoncanchangethepowerofthelensbyasmuchas14D.However,bythetimethatapersonreaches40yearsofage,theamountofaccommodationhalves,andafter50years,itcandecreaseto2Dorless.Presbyopiacanbecorrectedbyconvexlenses.
Defectsinfocuscanalsobecausedbyadiscrepancybetweenthesizeoftheeyeandtherefractivepowerofthedioptricmedia.Forexample,inmyopia (near-sightedness),theimagesofdistantobjectsarefocusedinfrontoftheretina.Concavelensescorrectthisproblem.Conversely,inhypermetropia (far-sightedness),theimagesofdistantobjectsarefocusedbehindtheretina;thisproblemcanbecorrectedwithconvexlenses.Inastigmatism, anasymmetryexistsintheradiiofcurvatureofdifferentmeridiansofthecorneaorlens(orsometimesoftheretina).Astigmatismcanoftenbecorrectedwithlensesthatpossesscomplementaryradiiofcurvature.
•Fig. 8.2 Layers of the Retina. Lightimpingingontheretinawouldbecomingfromthetopofthefigure,andwouldpassthroughallthesuperficiallayerstoreachthephotoreceptorrodsandcones.9 Nerve fiber layer Axons at surface of retina passing via optic nerve, chiasm and tract to lateral geniculate body Ganglion cell Bipolar cell Amacrine cell Horizontal cell Rod Cone Choroid Pigment cells Müller cell (supporting glial cell) 8 Ganglion cell layer 7 Inner plexiform layer 5 Outer plexiform layer 6 Inner nuclear layer 4 Outer nuclear layer 2 Photoreceptor layer 1 Pigment epithelium 3 Outer limiting membrane 10 Inner limiting membrane Inner segment Outer segment tentacle-like processes that extend into the photoreceptor layer (layer 2) and surround the outer segments of the rods and cones. These processes prevent transverse scatter of light between photoreceptors. In addition, they serve a mechanical function in maintaining contact between layers 1 and 2 so that the pigmented epithelium can (1) provide nutrients and remove waste from the photoreceptors; (2) phagocytose the ends of the outer segments of the rods, which are continuously shed; and (3) reconvert metabolized visual pigment into a form that can be reused after it is transported back to the photoreceptors.