Accommodation

When a normal eye views an object, parallel rays of light are refracted to a point, or focus, on the retina (see fig. 10.34). If the degree of refraction remained constant, movement of the object closer to or farther from the eye would cause corresponding movement of the focal point, so that the focus would either be behind or in front of the retina.

The ability of the eyes to keep the image focused on the retina as the distance between the eyes and object varies is called accommodation. Accommodation results from contraction of the ciliary muscle, which is like a sphincter muscle that can vary its aperture (fig. 10.32). When the ciliary muscle is relaxed, its aperture is wide. Relaxation of the ciliary muscle thus places tension on the zonular fibers of the suspensory ligament and pulls the lens taut. These are the conditions that prevail when viewing an object that is 20 feet or more from a normal eye; the image is focused on the retina and the lens is in its most flat, least convex form. As the object moves closer to the eyes,

Fields of vision

Fields of vision

Eye Physiology

■ Figure 10.31 The image is switched right-to-left on the retina.

The left side of the visual field is projected to the right half of each retina, while the right side of each visual field is projected to the left half of each retina.

the muscles of the ciliary body contract. This muscular contraction narrows the aperture of the ciliary body and thus reduces the tension on the zonular fibers that suspend the lens. When the tension is reduced, the lens becomes more rounded and convex as a result of its inherent elasticity (fig. 10.33).

Ciliary Muscle And Lens

Figure 10.32 The relationship between the ciliary muscle and the lens. (a) A diagram, and (b) a scanning electron micrograph (from the eye of a 17-year-old boy) showing the relationship between the lens, zonular fibers, and ciliary muscle of the eye.

Part (b) from "How the Eye Focuses" by James F. Koretz and George H. Handleman. Copyright © 1988 by Scientific American, Inc. All rights reserved.

Figure 10.32 The relationship between the ciliary muscle and the lens. (a) A diagram, and (b) a scanning electron micrograph (from the eye of a 17-year-old boy) showing the relationship between the lens, zonular fibers, and ciliary muscle of the eye.

Part (b) from "How the Eye Focuses" by James F. Koretz and George H. Handleman. Copyright © 1988 by Scientific American, Inc. All rights reserved.

Length Suspensory Ligament Eye

■ Figure 10.33 Changes in the shape of the lens permit accommodation. (a) The lens is flattened for distant vision when the ciliary muscle fibers are relaxed and the suspensory ligament is taut. (b) The lens is more spherical for close-up vision when the ciliary muscle fibers are contracted and the suspensory ligament is relaxed.

The ability of a person's eyes to accommodate can be measured by the near-point-of-vision test. The near point of vision is the minimum distance from the eyes at which an object can be brought into focus. This distance increases with age; indeed, accommodation in almost everyone over the age of 45 is significantly impaired. Loss of accommodating ability with age is known as presbyopia (presby = old). This loss appears to have a number of causes, including reduced flexibility of the lens and a forward movement of the attachments of the zonular fibers to the lens. As a result of these changes, the zonular fibers and lens are pulled taut even when the ciliary muscle contracts. The lens is thus not able to thicken and increase its refraction when, for example, a printed page is brought close to the eyes.

Essentials of Human Physiology

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Responses

  • priamus
    When ciliary muscle relaxes zonular fibers?
    5 years ago
  • alice
    How an eye focuses on an image?
    5 years ago
  • john
    Is accommodation of a lens relaxed?
    5 years ago
  • holfast chubb-baggins
    What shape is the lens in the eye?
    5 years ago
  • luigia
    What is the near point for a 17 year old for eyes?
    5 years ago
  • luwam
    What is the physiology of the visual fields?
    5 years ago
  • romola
    When the ciliary muscle is relaxed, the suspensory ligament is?
    5 years ago
  • Irvin Bronson
    How the eye focuses accommodation?
    5 years ago
  • jennifer
    What is accommodation in anatomy and physiology?
    12 months ago

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