Regions and organization of the brain
Within this section, we will discuss the anatomy of the brain in terms of the four major regions of the adult brain. The four regions are as follows: cerebral hemispheres, diencephalon, brain stem (midbrain, pons, and medulla oblongata), and the cerebellum.
The most basic way to describe the layout of the brain is a central cavity, surrounded by grey matter (mostly neuron cell bodies), surrounding white matter (myelinated fiber tracts). Both the cerebral hemispheres and the cerebellum have an outer layer of grey matter that is called the cortex. This layout changes, though, within the brain stem. The brain stem lacks the cortex, but grey matter nuclei are scattered throughout the white matter. At the lowest portion of the brain stem, the cortex surrounding the white matter then returns.