in vertebrates, regions of the brain conforming to the organization of the spinal cord, as distinct from such suprasegmental structures such as the cerebral cortex and the cerebellum, kitsamas tähenduses hõlmab piklikaju, silda ja keskaju, laiemas tähenduses kõiki arengulooliselt vanemaid osi (ka vaheaju, põhimikutuumi, haistmisaju), mida vastandatakse otsaju mantlile