Throughout the nervous system, neurones are arranged so that their cell bodies are grouped together. Within the central nervous system these groupings are known as nuclei; in the peripheral nervous system the groupings are called ganglia (singular: ganglion ). They form the grey matter of the nervous system.

The axons of different nerve cells are also grouped together. These bundles of fibres form the white matter of the nervous system. They are called tracts in the central nervous system and simply nerves in the peripheral nervous system.

The image shows the spinal cord inside the vertebral column in some detail. You can see the grey and white matter.