Introduction

In humans, the cerebellum overlies the posterior parts of the pons and medulla, occupying a large part of the posterior fossa. Structurally, the cerebellum consists of four pairs of deep nuclei, embedded in white matter, and is surrounded by a cortical mantle of gray matter. Unlike in the cerebral cortex, the cytoarchitecture of the cerebellum is remarkably uniform. This chapter reviews the fundamental aspects of the macroscopic and microscopic anatomy of the cerebellum, with emphasis on aspects of functional importance.

Evolution

Cartilaginous fish have a transversal eminence at the level of the octavo-lateral line system (Schnitzlein and Faucette, 1969). Even in the lower species, the afferent pathway is not limited to the VIIIth nerve. Trigeminal somesthetic projections and a genuine spinocerebellar tract are present. The cytoarchitecture of the cerebellum has evolved with few refinements.

In most teleosts and amphibians the cerebellum is a single leaf with lateral auricles (Fig. 2.1). In reptiles and birds, this original leaf duplicates in the rostral direction as a foliated fan-like structure, named the vermis because of its worm-like appearance. The posterior portions (the flocculus and paraflocculus) and their lateral expansions (the auricles) are the oldest phylogenetic part, making up the vestibulocerebellum. From birds to primates the number of folia increases significantly (22 in pigeons and 260 in humans). In parallel with this rostro-caudal expansion and the development of the cerebral cortex, there has been a continuous medio-lateral expansion of the hemispheres. The area of transition between the vermis and the hemispheres is called the intermediate region.