The brain stem is located in the posterior cranial fossa and consists of medulla, pons, and midbrain (Fig. 5.1). We have included the diencephalon in this discussion on the brain stem as the majority of pathways from the brain stem and spinal cord terminate on the nuclei in the diencephalon. The gray and white matter in each level is intermixed and demonstrates a columnar organization. The discussion of each level will include the important functions, nuclei, and tracts found in that level. The constituent parts of the central nervous system are the spinal cord, brain stem, cerebellum, diencephalon, and cerebrum. The cerebellum is attached to the pons and medulla but will be discussed as part of the motor system in Chap. 11.