The hindbrain is derived from the embryonic hindbrain vesicle. It is continuous rostrally with the midbrain and caudally with the spinal cord. The cerebellum is an outgrowth from the rostral hindbrain. Modern studies of gene expression reveal that the hindbrain is logically divided into 12 segmental components—the isthmus plus rhombomeres 1–11. The isthmus is a forgotten part of the mammalian brain. The isthmus is in fact the most rostral part of the hindbrain, not a caudal part of the mensencephalon. The isthmic region plays a crucial role in early development as a secondary organizing center, specifiying the fate of the area rostral to it (midbrain), and the area caudal (rhombencephalon and cerebellum). The isthmus of the mammalian brain is a distinct but relatively thin segment of the brain, separating the mesencephalon from the first rhombomere. The rhombomeres are transverse developmental units of the embryonic hindbrain. The rostral rhombomeres (r1–r6) are initially marked by transverse constrictions in the neural tube wall. The cerebellum is an outgrowth of the dorsal parts (the rhombic lip region) of the isthmus and rhombomere 1. The sensory nuclei of the hindbrain include somatosensory nuclei, auditory nuclei, vestibular nuclei, and the nuclei associated with visceral sensation. The primary nucleus receiving visceral sensation is the nucleus of the solitary tract. The hindbrain contains a number of distinctive monoamine nuclei that have important roles in controlling behavioral states and cardiovascular function. The major groups contain adrenergic, noradrenergic, dopaminergic, serotoninergic neurons. The hindbrain is a corridor between the forebrain and midbrain on one side and the spinal cord on the other.