Part of the metencephalon that lies in the posterior cranial fossa behind the brain stem. It is concerned with the coordination of movement[MESH]. A large dorsally projecting part of the brain concerned especially with the coordination of muscles and the maintenance of bodily equilibrium, situated between the brain stem and the back of the cerebrum , and formed in humans of two lateral lobes and a median lobe[BTO]. Brain structure derived from the anterior hindbrain, and perhaps including posterior midbrain. The cerebellum plays a role in somatic motor function, the control of muscle tone, and balance[ZFA].
from Scholarpedia - Cerebellum The cerebellum derives its name as a diminutive of the word "cerebrum". This is particularly explicit in German, where the cerebellum is called Kleinhirn ("small brain"). This structure, present in all vertebrates, occupies a position immediately behind the tectal plate and straddles the midline as a bridge over the fourth ventricle. In addition, it is the only region of the nervous system to span the midline without interruption. Technically, the output of the cerebellum is exclusively inhibitory through the Purkinje neurons onto the cerebellar nuclei, but the cerebellar nucleus exerts both excitatory and inhibitory influences, on the thalamus and on the Inferior Olive, respectively (Ruigrok and Voogd, 1995).
The cerebellum has undergone enormous elaboration throughout phylogeny, in fact, more so than any other region of the central nervous system (CNS), but has maintained its initial neuronal structure, almost invariant. Thus, its size but not its wiring has changed with CNS evolution. As an example, the cerebellar cortex in a frog has an area approximately 12 mm2 that is, 4 mm wide (in the mediolateral direction) and 3 mm long (in the rostrocaudal direction). In humans, the cerebellar cortex is a single continuous sheet with an area of 500 cm2 (1,000 mm long and 50 mm wide) . This is 4 x 10^3 times more extensive than that of a frog (Braitenberg & Atwood, 1958). The increase in cortical extent has resulted in folding into very deep folia (Figure 2), allowing this enormous surface to be packed into a volume of 6 cm x 5 cm x 10 cm. Because the cerebellar cortex extends mainly rostro-caudally, most of the foldings occur in that direction.