[TA] the region of the forebrain that lies inferior to the anterior half or so of the lentiform nucleus, extending in the frontal plane from the lateral preopticohypothalamic zone laterally over the optic tract to the amygdala (amygdaloid body); rostrally it tapers off over the superior border of the olfactory tubercle, and caudally it ends where the internal capsule reaches the surface to form the cerebral peduncle or pes pedunculi. Notable among its polymorphic cell population is the large-celled basal nucleus of Meynert. These magnocellular elements within the innominate substantance are present in the medial septum and the diagonal band of Broca, but they occur in largest numbers ventral to the globus pallidus. Histochemical evidence indicates that magnocellular elements distribute cholinergic fibers widely in the cerebral cortex and that these cells undergo selective degeneration in Alzheimer disease. SYN: substantia innominata [TA].