White matter ( WM ; Meckel, 1817 ) : Since the 16th century the nervous system (Monro, 1783) has been divided more and more precisely into gray matter (Meckel, 1817) and white matter based on their appearance in freshly dissected material observed with the naked eye (macroarchitecture); since the 19th century this differentiation has been made at the histological (microarchitecture) level. White matter is the nervous system compartment that consists primarily of axons (Kölliker, 1896). Its name is derived from its appearance in fresh and fixed macroscopically observed material, its whitish color due to the presence of myelinated axons. However, white matter is often a mixture of myelinated axons and unmyelinated axons, and entire aggregates of axons can be unmyelinated. Therefore, as defined here white matter is a generic term for a nervous system volume where axons are the predominant neural component, although of course glia (Virchow, 1846) and parts of the circulatory system, vascular cells, are also present. White matter can contain scattered neurons (Waldeyer, 1891) that may be assigned either to an adjacent gray matter region, or to a new gray matter region embedded (nested) within the white matter; assignment depends on differentiable neuron types (Bota & Swanson, 2007) involved. There is often a fuzzy border of variable width and difficult to measure between gray matter and white matter. This use of the term was probably introduced by Meckel; see English translation (1832, vol. 2, pp. 152-154), also see Herrick (1915, p. 108). For early history see Clarke & O'Malley (1996, Ch. 10); for modern histological interpretation see Peters et al. (1991), Swanson (2003, pp. 60-66).