While many fibers in the lateral preoptic area generate terminal boutons and boutons of passage, roughly half of them continue laterally into the adjacent substantia innominata. It appears likely that this projection to and through the lateral preoptic area—or some fraction of it—represents a separate projection pathway arising from axons that extend ventrally from the lateral septal nucleus; other methods will be needed to determine the precise structural organization of this complex part of the descending projection system from ventral field CA1. In any event, whereas labeled field CA1 axons in caudal levels of the preoptic region typically run parallel to one another, fibers projecting specifically through the lateral preoptic area appear to descend caudoventrally from the septal region instead of rostroventrally through the medial preoptic area (from the column of the fornix; see above). Many fibers in the lateral preoptic region end abruptly in its caudoventromedial tip. Cajal (1909-1911) referred to axons descending from the region of the lateral septal nucleus to the preoptic region as lateral arcuate fibers.