One of the members of the areas of the central nervous system that are primarily made up of myelinated axons, also called tracts. Long thought to be a passive tissue, white matter affects learning and brain function by modulating the distribution of action potentials, acting as relays, and coordinating communication between different brain regions. The white matter got its name for its relatively light appearance, due to the lipid content in myelin. Its white color in finished samples is due to its usual preservation in formaldehyde. Otherwise, it looks pinkish-white to the naked eye, since myelin consists mainly of lipid tissue riddled with capillaries.