Anatomy of the cerebellum is located at the base of the brain, with the large mass of the cerebral cortex above it and the portion of the brainstem called the pons in front of it. It is separated from the overlying cerebrum by a layer of tough dura mater; all of its connections with other parts of the brain travel through the pons. Anatomists classify the cerebellum as part of the metencephalon, which also includes the pons; the metencephalon in turn is the upper part of the rhombencephalon or "hindbrain". Like the cerebral cortex, the cerebellum is divided into two hemispheres; it also contains
Anatomy of the cerebellum uniform, with connections organized into a rough, three-dimensional array of perpendicular circuit elements. This organizational uniformity makes the nerve circuitry relatively easy to study. To envision this "perpendicular array", one might imagine a tree-lined street with wires running straight through the branches of one tree to the next. There are three layers to the cerebellar cortex; from outer to inner layer, these are the molecular, Purkinje, and granular layers. The function of the cerebellar cortex is essentially to modulate information flowing through the deep nuclei. The microcircuitry of the cerebellum is schematized in Figure 5. Mossy and climbing fibers carry