|
The Sense of Sight / All About the Eyes
Page: 17
The image of an object brought close to the eye would be formed behind the retina if there were no change in the focal length of the eye. This change to bring the image of an object upon the retina is called accommodation. The point nearer than which accommodation is no longer effective is called the near point of accommodation. In very young people, the near point of accommodation is quite close to the eye, namely about seven centimetres (about three inches) in front at 10 years old; at 40 years the distance has increased to about 16 centimetres (about 6 inches), and at 60 years it is 100 centimetres or one metre (39 inches). Thus, a 60-year-old would not be able to read a book held at the convenient distance of about 40 centimetres (16 inches), and the extra power required would have to be provided by convex lenses in front of the eye, an arrangement called the presbyopic correction.
It is essentially an increase in curvature of the anterior surface of the lens that is responsible for the increase in power involved in the process of accommodation. A clue to the way in which this change in shape takes place is given by the observation that a lens that has been taken out of the eye is much rounder and fatter than one within the eye; thus, its attachments by the zonular fibres to the ciliary muscle within the eye preserve the unaccommodated or flattened state of the lens; and modern investigations leave little doubt that it is the pull of the zonular fibres on the elastic capsule of the lens that holds the anterior surface relatively flat. When these zonular fibres are loosened, the elastic tension in the capsule comes into play and remolds the lens, making it smaller and thicker. Thus, the physiological problem is to find what loosens the zonular fibres during accommodation. The ciliary muscle has been described earlier, and it has been shown that the effect of contracting its fibres is, in general, to pull the whole ciliary body forward and to move the anterior region toward the axis of the eye by virtue of the sphincter action of the circular fibres. Both of these actions will slacken the zonular fibres and therefore allow the change in shape. As to why it is the anterior surface that changes most is not absolutely clear, but it is probably a characteristic of the capsule rather than of the underlying lens tissue. Defective accommodation in presbyopia is not due to a failure of the ciliary muscle but rather to a hardening of the substance of the lens with age to the point that readjustments of its shape become ever more difficult. Take this preliminary to see if your condition could respond to treatment.
Not sure on your treatment options? For a limited time you can schedule a
to talk with a licensed doctor or clinician regarding your condition.
Locate a health care practitioner to treat your condition.
Additional information regarding conditions of the eyes can be found here.
|
