The brain stem has many basic vital functions, including regulation of heart rate, breathing, sleeping, and eating. It is divided into three parts, the midbrain, the pons, and the medulla oblongata. The medulla oblongata (sometimes just called the medulla) connects to the spinal cord and is only slightly wider than the spinal cord.
The cerebellum (“little brain”) is the small structure that appears to be a separate structure under the brain, but is actually integrated with the rest. The cerebellum receives information from the sensory systems, the spinal cord, and other parts of the brain and then regulates motor movements.
When dissected sagittally, there is branched white matter in the middle of the cerebellum. This white matter is called the arbor vitae, because it looks tree-like.