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:
- situations where there is bright lighting
- for acute vision, i.e. receiving sharp detailed images
- colour vision
There are thought to be three distinct types of cones, each type being
sensitive to a specific band of wavelengths of light, which are 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.
Structure of the cones
The cones in the retina of the eye have a conical shape, hence their name. They 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
choroid, which is 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, for example 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.