Anatomy of the cerebellum major part of the cerebellum by mass, but in other vertebrates, it is typically the spinocerebellum. In amphibians, lampreys, and hagfish the cerebellum is little developed; in the latter two groups it is barely distinguishable from the brain-stem. Although the spinocerebellum is present in these groups, the primary structures are small paired nuclei corresponding to the vestibulocerebellum. The cerebellum follows the general groups-of-three pattern found in anatomy, with three major input and output cerebellar peduncles (fiber bundles). These are the superior (brachium conjunctivum), middle (brachium pontis), and inferior (restiform and juxtarestiform bodies) cerebellar peduncles. There are three sources of input
Cerebellum are, however, a number of respects in which the structure of the cerebellum is compartmentalized. There are large compartments that are generally known as "zones"; these can be divided into smaller compartments known as "microzones". The first indications of compartmental structure came from studies of the receptive fields of cells in various parts of the cerebellar cortex. Each body part maps to specific points in the cerebellum, but there are numerous repetitions of the basic map, forming an arrangement that has been called "fractured somatotopy". A clearer indication of compartmentalization is obtained by immunostaining the cerebellum for certain types of