White matter is one of the two components of the central nervous system and consists mostly of myelinated axons. White matter tissue of the freshly cut brain appears pinkish white to the naked eye because myelin is composed largely of lipid tissue veined with capillaries. Its white color is due to its usual preservation in formaldehyde. A 20 year-old male has around 176,000 km of myelinated axons in his brain.
The other main component of the brain is grey matter (actually pinkish tan due to blood capillaries). A third colored component found in the brain that appears darker due to higher levels of melanin in dopaminergic neurons than its nearby areas is the substantia nigra.
Note that white matter can sometimes appear darker than grey matter on a microscope slide because of the type of stain used.
White matter is composed of bundles of myelinated nerve cell processes (or axons), which connect various grey matter areas (the locations of nerve cell bodies) of the brain to each other, and carry nerve impulses between neurons. Myelin acts as an insulator, increasing the speed of transmission of all nerve signals.
In the cerebral hemispheres two types of myelinated axons are identified: short-distance (10 –30 mm) fibers below the gray matter that follow its contours, and long distance (30–170 mm) fibers that are bundled into fasciculi in the deep white matter. There are also shorter intracortical (1–3 mm) unmyelinated fibers within the grey matter.
The total number of long range fibers within a cerebral hemisphere is 2% of the total number of cortico-cortical fibers and is roughly the same number as those that communicate between the two hemispheres in Corpus callosum. Schüz and Braitenberg note "As a rough rule, the number of fibres of a certain range of lengths is inversely proportional to their length"377
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