Longitudinal tract : A white matter tract coursing essentially along the longitudinal axis (Barclay, 1803) of the nervous system (Monro, 1783); if a longitudinal tract crosses the median plane (Henle, 1855) in an animal with bilateral symmetry, the segment related to that plane is a decussation. The basically transverse and longitudinal organization of nervous system tracts was emphasized early on by Spencer (1881) and Cajal (1899-1904, vol. 1, p. 12), and more recently in the orthogon theory of Reisinger (1925, 1972).