White substance, named for its relatively light appearance resulting from the lipid content of myelin, refers to axon tracts and commissures.
White substance 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 in prepared specimens is due to its usual preservation in formaldehyde.
White substance, long thought to be passive tissue, actively affects how the brain learns and functions. While grey matter is primarily associated with processing and cognition, white substance modulates the distribution of action potentials, acting as a relay and coordinating communication between different brain regions.