Here, Cajal illustrates a possible flow of information within the cerebral cortex, focusing on 4 pyramidal neurons.  Inputs (E,F) come in several forms, including projections to apical dendrites (D) and to intermediate layers (A). After relay through other cortical neurons (a), deeper layer neurons (C) project out of the cortex. While there is strong anatomical evidence for these canonical paths of information flow, in recent years we have learned that the high degree of connectivity and diversity of cells in the cortex makes the fundamental computation of the cortex quite complex: a single cortical column has hundreds of thousands of such pyramidal cells, connected by millions of synapses. Cajal’s depiction of this circuit presages the 'canonical cortical microcircuit' of Douglas and Martin by nearly 100 years.  The field is currently engaged in an effort to extend the canonical circuit ideas to the entire cortical network: understanding how the massive local connectivity builds on this canonical circuit backbone to create cortical computation.