Biology - Coordination - Eyes
The cornea does about 70% of the work involved in converging
the light rays onto the retina. The lens will do the final adjusting. This is
called Accommodation. Accommodation is a reflex action. The stimulus is the
light, which is picked up by the rod and cone cells in the eye (the receptors).
The signal as to whether the image is too far-focussed or short-focussed is
sent via a sensory neurone to the CNS, which uses a relay neurone to send an
impulse down a motor neurone to the effector, the muscles, which will relax or
contract according to the original stimulus.
Accommodation is achieved through the relaxation and
contraction of the antagonistic pair of muscles, the ciliary muscles and the
suspensory ligaments.
Near Focusing: The ciliary muscles relax, and then the lens
is pulled thin by the strain on suspensory ligaments exerted by the sclera
under pressure from tissue fluid.
Far Focusing: The lens collapses more, due to its
elasticity, when the strain on suspensory ligaments is reduced and ciliary
muscles contract.
Long sighted people are unable to focus on near objects.
Either the cornea or lens doesn’t bend the light enough, or the eyeball is too
short. The image is brought into focus too far behind the retina.
Short sighted people cannot focus on distant objects. The
cornea or lens bends the light too much or the eyeball is too long. The image
is brought into focus in front of the retina.
With age, the lens loses flexibility and does not go back to
a round shape as easily. This is why some older people have to wear reading
glasses.
