Side by side
| Compared | Grey matter | White matter |
|---|---|---|
| Where it is | On the outside: the cortex covering the surface of the cerebral hemispheres, plus deep clusters in the cerebrum such as the thalamus, hypothalamus and basal ganglia (putamen, globus pallidus). [2][1] | On the inside: it forms the bulk of the deep parts of the brain, wrapped by the cortex and wrapped around the deep grey nuclei; the corpus callosum carries it across the midline. [3][1] |
| What it does | Holds the neuron cell bodies, the dendrites and the synapses between them: the places where signals are received and processed. [2][1] | Carries signals between grey-matter areas: association tracts within one hemisphere, commissural tracts between the two, projection tracts down to the brainstem and spinal cord. [6][3] |
| Connects with | Its areas are joined to each other by white-matter tracts; a CNS cluster of cell bodies such as the thalamus is called a nucleus. [1] | Under the cortex it gathers in the centrum semiovale, then the corona radiata, then the internal capsule on its way to the brainstem. [7][8] |
| Size measured from the model | 180 mm front to back, 117 mm top to bottom, 141 mm side to side [4] | 163 mm front to back, 100 mm top to bottom, 125 mm side to side [4] |
Size is the box each part fits in on this one model (grey matter (both hemispheres); white matter (both hemispheres)), not its volume. Real brains vary.
How far apart are they?
Measured from the model [4], from the grey matter (both hemispheres) to the white matter (both hemispheres):
- Centre to centre: 4 mm in a straight line.
- The centre of the white matter (both hemispheres) is 4 mm further forward, 1 mm higher and the same distance from the midline than the centre of the grey matter (both hemispheres).
- Nearest points of the two shapes: under 1 mm apart.
A “centre” here is the average position of the points on the part’s 3D surface, so for a curved part such as a C shape it can sit outside the part itself. The nearest-points figure is measured between the corners of the simplified meshes, so a gap of 2 or 3 mm or less is within the model’s coarseness: read it as “touching or nearly so”.
How to tell them apart
- The colours are what fresh, unstained tissue shows: grey matter can look pinkish from its blood, and it reads as grey mostly because it is darker than the white matter beside it. [1]
- In the spinal cord the arrangement is the other way round: the white matter is on the outside and the grey matter forms columns inside. [3][2]