The brain stem (truncus cerebri) comprises the rhombencephalon and the mesencephalon; caudally it grades into the spinal cord while rostrally it borders the diencephalon. In all vertebrates the brain stem harbours the centres of origin and termination of all cranial nerves, except for cranial nerve I. In cartilaginous fish the brain stem contains a fairly well developed reticular formation and a number of relay centres and their associated ascending and descending connections. The cerebellum, although ontogenetically and phylogenetically a derivative of the rostral part of the rhombencephalon, is not included in the brain stem and will be considered separately in Chap. 7. However, the cerebellum and the brain stem are strongly interconnected and many of the cerebellar afferent systems originate, and almost all cerebellar efferent fibres terminate, within the brain stem.