The white matter comprises all types of conduction pathways and has the task of conveying nervous information between different processing sites (nuclei and bark) in the CNS. The white matter can be compared to the computer's various so-called buses.

White matter mainly consists of:

1/ tightly packed myelinated axons of varying diameter from 0.6 to 15-20 μm (micrometers; 1 μm = 0.001 mm;

2/ Groups of unmyelinated axons

3/ oligodendrocytes (oligodendroglia)

4/ astrocytes (astroglia)

5/ microglia

6/ sparingly with blood vessels

The white matter of the large brain has the volume approx. 350 cm

^{3}.

One might then ask:

"How long is the total nerve fibre length in the white matter of the cerebrum?"

Answer: the total nerve fibre length in the white matter of the cerebrum should be at least 25,000 kilometres.

(A cm 3 corresponds to a cube with side 1 cm. 350 cm 3 can be said to correspond to a

^{3.5}meter long string with a cross-sectional area of 1 cm

^{2}.

^{}Since it goes 10 mm on 1 cm and 1000 μm on 1 mm, 1 cm corresponds to 2 100

^{}000 000 (hundreds of million) μm

^{2}. You can then calculate how many cross-cut nerve fibers fit within the surface 1 cm

^{2}. Suppose the nerve fibers have circular cross-section and are on average 4 μm thick (radius = 2, which is probably on the upper edge). The cross-sectional area of such a nerve fibre can then be calculated using the formula: surface area = radius squared (r

^{2}) x pi(π = 3.14);

2 x 2 x π(3.14) = 12.56 μm

^{2}

The number of nerve fibers/1 cm

^{2}= 100,000,000 / 12.56 = approx. 8 000 000.

Their total length in meters = 8,000,000 x 3.5 = 28,000,000 meters.)