Intro | Cornea | Optic Disk and Blind Spot | Fovea | Iris | Lens | Muscles (to move eye) | Muscles (to adjust lens) | Optic Nerve | Pupil | Retina
Part 1: Image-Mapped Tutorial
Part 2: Matching Self-Test
Part 3: Multiple-Choice Self-Test
Muscles (to adjust lens): The lens is made of a transparent material that is flexible enough to adjust in response to the contraction of muscles designed for this purpose. Through the process called accommodation, the curvature of the surface of the lens can be adjusted to provide a clearer image. When we focus on a distant object, the muscles contract to flatten the surface of the lens. The light rays are only slightly bent. In contrast, when we focus on a close object, the muscles relax so that the lens surface becomes rounder.
Nearsightedness occurs when close objects are seen clearly, but distant ones are not because the light of objects is focused short of the retina. This condition occurs when the eye is too long or when the cornea or lens bends the light at too wide an angle. Farsightedness occurs when close objects are blurry, but distant ones are clear. This condition occurs when the light of objects is focused behind the retina, which is usually caused by a short eye.
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The muscles that adjust the shape of the lens to accommodate objects of varying distance away are called the ciliary muscles. There are two divisions of this muscle group; the straight or longitudinal division and the circular or sphincter division. Contraction of the ciliary muscles changes the shape of the lens from concave in the direction of convex.