The term brainstem (BST) refers to a composite substructure of the Brain (BRN). In structural Brain atlases, such as the Allen Brain Atlas - human Adult and Structural CNS Model - Macaque it is composed of the midbrain (MBR) and hindbrain (HBR), that is, the stem of the BRN that is detached from the forebrain (FBR). It is found in the human ( Carpenter-1983 ), macaque ( NeuroNames), rat ( NeuroNames ), and mouse ( NeuroNames ). A common alternate definition includes the diencephalon (IBR) of the FBR ( Swanson-2012 ). Other definitions have included different structures ( Anthoney-1994 ). Some have included parts of the diencephalon (IBR) and a few have included the cerebellum (CBL). Updated 4 Nov 2025.