Gathering the Stimulus: The Eye
Eyes come in many forms. Some invertebrates have simple eyespots that merely sense light or dark; others have complex, multicellular organs with crystalline lenses. In vertebrates, the actual detection of light is done by cells called photoreceptors. These cells are located on the retina, a layer of tissue lining the back of the eyeball. Before the light reaches the retina, however, several mechanisms are needed to control the amount of light reaching the photoreceptors and to ensure a clear and sharply focused retinal image.
The iris is a smooth, circular muscle surrounding the pupillary opening—the open-ing through which light enters the eye. Adjustments in the iris are under reflex control and cause the pupil to dilate (grow larger) or contract, thus allowing considerable con-trol over how much light reaches the retina.
In the mammalian eye, the cornea and the lens focus the incoming light just like a camera lens does (Figure 4.24). The cornea has a fixed shape, but it begins the process of bending the light rays so they’ll end up properly focused. The fine-tuning is then done by adjustments of the lens, just behind the cornea. The lens is surrounded by a ring of ligaments that exert an outward “pull,” causing the lens to flatten somewhat; this allows the proper focus for objects farther away. To focus on a nearby object, con-traction of a muscle in the eye reduces the tension on the ligaments and allows the lens to take on a more spherical shape.
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