The substantia innominata (plural: substantiae innominatae), or substantia innominata of Meynert, is an area of mixed grey and white matter located within the anterior perforated substance in the basal forebrain. It contains the acetylcholine-rich basal nucleus of Meynert.
The substantia innominata is a thin band located in the posterior part of the anterior perforated substance in the basal forebrain. It lies directly below the subcommissural part of the globus pallidus. Medially, the grey matter is known as the diagonal gyrus, or the diagonal band of Broca, which makes up the medial margin of the anterior perforated substance.
Best visualized as a thin strip immediately below the globus pallidus on thin slice coronal T2-weighted images.