Cones

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.