Cones are one of two
types of light-sensitive photo-detector cells found in the
"Jacob's Membrane"
(Layer 9.) of the retina of
the human eye.
The other type of photo-detector cells are called rods.
Cones are especially important for vision in bright lighting situations,
for acute vision (i.e. receiving sharp detailed images), and for colour
vision.
There are thought to be three distinct types of cones, each type being
sensitive to a specific band of wavelengths of light (usually described
in terms of the primary colours red, green, and blue). Perception of
other colours is explained in terms of combinations of the three bands
of wavelengths detected by the three types of visual cones. Each
human eye contains approx. 6-7 million cones - compared
with approx. 125 million rods.
The cones have a conical shape (hence their name),
and are positioned with the broad end of the cone in contact with the
Membrana Limitans Externa (Layer 8. of the retina),
hence the broad end of the cones is pointed towards the pupil while the
narrow end is directed
towards the choroid -
behind the retina, at the back of the eye.
In exactly the same way as for the rods, each cone is composed of an
outer
and
an
inner
segment. These segments have different properties of refraction
(the
extent
to
which they bend the light passing through them), and interaction with colouring
reagents (concerning staining by various chemicals). They also have slightly
different physical structures - e.g. the outer segments are marked by transverse
straiae and has faint longitudinal markings.
In both cases, the optical and
chemical properties of the segments of the cones are the same as those of the
corresponding
segments
of the rods. |
