Hockenbury Discovering Psychology 5th txtbk

94 CHAPTER 3 Sensation and Perceptioncornea(CORE-nee-uh) A clear membrane coveringthe visible part of the eye that helps gatherand direct incoming light.pupilThe opening in the middle of the iris thatchanges size to let in different amounts oflight.iris(EYE-riss) The colored part of the eye, whichis the muscle that controls the size of thepupil.lensA transparent structure located behind thepupil that actively focuses, or bends, light asit enters the eye.accommodationThe process by which the lens changesshape to focus incoming light so that it fallson the retina.How We SeeThe Human Visual SystemSuppose you’re watching your neighbor’s yellow and white tabby cat sunning himselfon the front steps. How do you see the cat? Simply seeing a yellow tabby cat involvesa complex chain of events. We’ll describe the process of vision from the object to thebrain. You can trace the path of light waves through the eye in Figure 3.3.First, light waves reflected from the cat enter your eye, passing through thecornea, pupil, and lens. The cornea, a clear membrane that covers the front of theeye, helps gather and direct incoming light. The sclera, or white portion of the eye,is a tough, fibrous tissue that covers the eyeball except for the cornea. The pupil isthe black opening in the eye’s center. The pupil is surrounded by the iris, the coloredstructure that we refer to when we say that someone has brown eyes. The irisis actually a ring of muscular tissue that contracts or expands to precisely control thesize of the pupil and thus the amount of light entering the eye. In dim light, the iriswidens the pupil to let light in; in bright light, the iris narrows the pupil.Behind the pupil is the lens, another transparent structure. In a process calledaccommodation, the lens thins or thickens to bend or focus the incoming light sothat the light falls on the retina. If the eyeball is abnormally shaped, the lens maynot properly focus the incoming light on the retina, resulting in a visual disorder. InIrisPupilPath of lightCorneaLensRetinaThin membrane lining back ofeyeball; contains rods and cones.FoveaPoint of highest visual acuity;cones concentrated here.Optic discPoint where optic nerveleaves eye; no rods orcones in this part of retina,creating a blind spot.Optic nerveGanglion cellsBipolar cellsOptic nerve fibersAxons of ganglion cellsform fibers of optic nerve.SensoryreceptorcellsRodConeOptic nerveDelivers signal to optic chiasm,thalamus, and visual cortex of brain.Close-up of retinaFigure 3.3 Path of Light in a Human Eye Light waves pass through the cornea, pupil, andlens. The iris controls the amount of light entering the eye by controlling the size of thepupil. The lens changes shape to focus the incoming light onto the retina. As the light strikesthe retina, the light energy activates the rods and cones. Signals from the rods and cones arecollected by the bipolar cells, which transmit the information to the ganglion cells. The ganglioncell axons are bundled together to form the optic nerve, which transmits the informationto the brain. The optic nerve leaves the eye at the optic disk, creating a blind spot in ourvisual field. (For a demonstration of the blind spot, see Figure 3.4 on page 96.)