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. |
