Cerebellum

The cerebellum sits low and toward the back, in a compartment of the skull called the posterior cranial fossa, behind the fourth ventricle, one of the fluid-filled cavities of the brain, and behind the pons and the medulla oblongata, the two parts of the brainstem that lie just in front of it. It is the largest part of the hindbrain, the name given to that posterior region of the brain.

It looks like two halves stuck together. Those are the two cerebellar hemispheres, joined in the middle by a strip called the vermis, a word that means worm and describes its appearance fairly well. And the whole organ is subdivided into three lobes, the anterior, the posterior, and the flocculonodular, separated by two fissures that run across it from side to side.

Cut it in half and the internal structure shows. On the outside there is a heavily folded layer of gray matter, the cerebellar cortex, and on the inside a body of white matter that branches like a tree; in fact it is called the arbor vitae, the tree of life. And the cerebellum is not loose: it is anchored to the brainstem by three bridges of fibers, the cerebellar peduncles.