Functional Modules in the flocculus of the rabbit cerebellum

Many studies, using different anatomical and physiological techniques, have aimed at subdiViding the cerebellum into distinctive parts that exert control over specific motor systems for reflex and voluntary movements, posture, and muscle tone. In view of the homogeneity of the cerebellar cortex, delineation can not be based on cytoarchitectonic differences as in the cerebral cortex. In the first half of this century the macroscopic subdivision of the cerebellum into lobes, lobules, and folia was used to distinguish functional units. The comparative anatomical studies of Balk (1906) stressed the importance of the subdivision of the mammalian cerebellum into vermis and hemispheres. Bolk considered the vermis and hemispheres as continuous, relatively independent "folial chains". separated by the paramedian sulcus. Each of the folia! chains can be subdivided into lobules, separated by deep, transverse fissures, and/or changes in the direction of the folial chain. In the anterior lobe and the adjoining part of the posterior lobe (the simple lobule) the transverse fissures continue uninterrupted from the vermis into the hemisphere. The separation into vermis and hemisphere is more complete in the caudal posterior lobe. In some regions the cortex between vermis and hemisphere is completely interrupted and white matter comes to the surface in the paramedian sulcus. Bolk {1906) used the variation among species in the size of different segments of the folial chains of vermis and hemispheres, in combination with differences in motor behaviour, to relate parts of the cerebellum to the control of specific motor behaviours. The comparative studies by Larseil (1934, 1937) showed the constancy in the development of the transverse lobular pattern throughout many mammalian species. This approach eventually culminated in the subdivision of the cerebellum into ten, transverse lobules, each of which was given a Roman numeral (Larsen, 1952).