(1) in general, the predominantly small-celled gray matter adjoining or surrounding the central canal of the spinal cord and the third and fourth ventricles of the brainstem; (2) in particular, the thick sleeve of gray matter surrounding the cerebral sylvian aqueduct in the midbrain, rostrally continuous with the posterior nucleus of the hypothalamus; in sections stained for myelin it stands out from the adjoining tectum and tegmentum by the paucity of its myelinated fibers. SYN: substantia grisea centralis [TA], periaqueductal gray substance.