The cerebellum, an integral structure in transmitting sensorimotor signals in the brain, was the first system that Cajal studied. Through careful observation of its stained tissues, he first became convinced that neurons were distinct cells rather than part of a contiguous network. The cerebellum also served as one of the systems in which he would develop his law of dynamic polarization. The section shown here was taken from a sample perpendicular to the vividly branching Purkinje cell dendritic arbor that many think of when envisioning the cerebellum. The mossy fibers in the granular layer (C) transmit information from motor centers in the cerebral cortex to the granule cells (d), which send axons into the molecular layer (A). These axons split (e) to give rise to the parallel fibers (c), which contact Purkinje cells (b). Purkinje cells integrate these inputs in their large cell bodies, which occupy a distinct layer (B) and project axons to the deep cerebellar nuclei, where they provide inhibitory input. In more recent years, Cajal’s studies of the cerebellum have been corroborated by more modern techniques that have also refined our understanding of the fine organization of the cerebellum.