All parts of the brain › White matter

Where is the white matter?

White matter is the part of the brain and spinal cord made mainly of myelinated axons, the long fibres that link one area of grey matter to another; it looks pale because myelin is rich in lipid. In the cerebrum it is the inner bulk of each hemisphere, under the grey cortex, with deep grey nuclei such as the basal ganglia set inside it. [1][2]

The 3D brain needs JavaScript. The text below still says where the white matter is and what it does.

Lit: the white matter of both hemispheres, with the internal capsules and the corpus callosum.

Slice

Opens cut by a front (coronal) slice through the white matter. Drag to turn, pinch or scroll to zoom, tap any part to name it; .

Lit: the white matter of both hemispheres as the atlas draws it (its “white matter of left/right cerebral hemisphere”, which takes in the internal capsules) plus the corpus callosum. Smaller white-matter parts with their own pages, such as the anterior commissure and the fornix, are not lit here. [3]

Its parts, each lit on its own page: corpus callosum, internal capsule.

Where it is

Inside the cerebral cortex: grey matter makes up the outer layer of the hemispheres, and the white matter lies under it. [2][4]
White matter forms the bulk of the deep parts of the brain; clusters of grey matter such as the basal ganglia sit within the cerebral white matter. [1]
The central white core under the cortex of each hemisphere is the centrum semiovale; lower down it continues as the corona radiata, which narrows into the internal capsule. [5][6]
The corpus callosum, the brain’s largest white-matter structure, bridges the two hemispheres at the bottom of the longitudinal fissure. [1][7]

What it does

It carries signals between areas of grey matter: the white regions hold mostly axons, while the nerve cell bodies and dendrites sit in the grey matter (see grey and white matter side by side). [2][4]
Association tracts link cortical areas within the same hemisphere; the cingulum and the superior longitudinal fasciculus are examples. [8]
Commissural tracts link matching areas of the two hemispheres: the corpus callosum, the largest, and the anterior and posterior commissures. [8]
Projection tracts link the cortex with the striatum, the diencephalon, the brainstem and the spinal cord; the corticospinal tract, carrying motor signals down to the spinal cord, is one. [8]

Connections

The corona radiata, a sheet of rising and descending axons, carries most of the traffic to and from the cerebral cortex; the corticospinal, corticobulbar and corticopontine tracts run in it. [6]
Below it the fibres gather into the internal capsule, which passes between the caudate nucleus and thalamus on the inside and the putamen and globus pallidus on the outside. [9]

Inside it

Mostly myelinated axons; myelin’s lipid gives it its light colour. Fresh it looks pinkish-white; preserved brains, usually fixed in formaldehyde, look whiter. [1]
Grey matter is not really grey either: fresh it is pinkish from its blood, and it reads as grey mostly because it is darker than the white matter beside it. [2]

Other names

Substantia alba (Latin); the white core of each hemisphere is the centrum semiovale (semioval centre, centrum ovale). [1][5]

Size

One study estimated the total length of myelinated fibres at age 20 at about 176,000 km in men and 149,000 km in women, with the total falling by about 10% a decade afterwards. [1]
On this model the white matter of both hemispheres with the corpus callosum measures about 163 mm front to back, 100 mm top to bottom and 125 mm side to side (the box it fits in, not its volume). [3]

Sources

Facts describe anatomy and normal function only. The 3D model is the adult male BodyParts3D brain (credits and licence); real brains vary in size and shape, so its measurements describe one model, not anyone’s brain.