VISION TESTS
V
THE SENSE OF VISION
Vision starts in the retina, the membrane at the back of the
eye that contains the light-sensitive rod and cone cells. Much
of the rest of the eye is concerned with focusing light, in the
right quantities, on to the retina. Huge amounts of data
are sent from the retina, through the optic nerves, to the
brain for analysis.
Visual cortices
Optic tract
Optic chiasma
Optic nerve
Pathways to the brain
Signals pass from the eyes
along the optic nerves to the
optic chiasma and then to the
visual cortices in the brain. There
is some crossover of nerve fibres
atthe optic chiasma, so both
sides ofthe brain receive signals
from both eyes.
Visual region of
Image
received by
the left eye
Stereoscopic vision
The two eyes receive slightly different views of all but the most
distant objects; information from the two images is compared and
processed in the brain to give a single 3-D interpretation ofthe object.
Image
received by
the right eye
IMAGE RECEPTION
The light-rays from an object
stimulate a group of receptors in
the retina within an area that has
the same shape as the object but is
upside down. The brain automatically
interprets the image the right way up.
Lens
\ Object
Image on
retina
EYEBALL MOVEMENTS
In order to maintain the image of any moving object on the
The muscles act to swivel the eyeball (here, the right) in
centre ofthe retina, precise eyeball movements, which are
the directions indicated. The muscles always act in groups
achieved by the sixmuscles shown below, are necessary.
to achieve movement.
I n f e r i o r o b l i q u e
Upwards, outwards,
and anticlockwise
rotation
L a t e r a l r e c t u s
Outwards
S u p e r i o r o b l i q u e
Downwards, outwards,
and clockwise rotation
S u p e r i o r r e c t u s
Upwards, inwards,
and clockwise rotation
M e d i a l r e c t u s
Inwards
I n f e r i o r r e c t u s
Downwards, inwards,
and anticlockwise
rotation
792
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