Table : Various eye defects and their corrective measures
Eye defect Reasons Corrective measures- Myopia or near sightedness
In this eye defect, person has
difficulty in seeing distant
objects clearly.
(i) Either the eyeball is longer than normal
or antero–posteriorly elongated so that
the image of distant objects is formed in
front of retina.
(ii) Or there is high curvature of the lens
(more convex).
Parallel rays from distant objects get
focused in front of retina.concave or divergent lenses.2 Hypermetropia or long
sightedness
In this eye defect the person has
difficulty in seeing near objects.(i) Either the eyeball is shorter than normal
(ii) Or there is low convexity of the lens.
Light rays converge at a point behind the
retina.convex or convergent lenses.- Astigmatism
It is due to irregular cornea or
lens, causing the image to be
out of focus, producing faulty
vision
cornea has different curvatures in different
regions.
Blurred image as rays get focused at
different points.cylindrical lenses.- Presbyopia
(old age long sightedness). It
is a defect in accommodation
occurring in advancing age.
Failure of accommodation to
focus near objects.
Loss of elasticity of lens after the age of 40. convex or bifocal
lenses, trifocal or contact lenses.- Cataract
also known as safaid motia.
Lens loses its transparency,
more common in old age.
Opacity of lens occur.
Person loses the ability to see as light does
not pass through the lens.Surgical removal of lens, and then
either implantation of artificial lens
or use of spectacles with convex
lens and laser treatment of opacity.- Glaucoma
also known as kala motia,
common in old people.
Increased secretion of aqueous humour
and increased intraocular pressure in
the anterior chamber the eye. This exerts
pressure on the posterior chamber and
greatly reduces the blood supply to the
retina. Thus, lack of nutrients ultimately
damages the nerve cells of the retina
leading to blindness.cured by drugs if detected in time
by increasing the rate of drainage
of aqueous humour or the removal
of a portion of iris to improve the
flow of the fluid. If the pressure
is due to blockage of canal of
Schlemm, a new canal can be
created surgically.Intext Practice Questions
- Why is the blind sport in the eye devoid of ability of vision?
- Krause’s corpuscles are __ while Ruffini’s corpuscles are ____.
- Distinguish between Bowman’s glands and Meiboman glands.
- How is rhodopisn reformed in the dark?
- Explain the structure of Schneiderian membrane.