Abstract
In recognition of the impact that a powerful new anatomical tool, such as the Golgi method, can have, this essay highlights the enormous influence that biocytin-filling has had on modern neuroscience. This method has allowed neurones that have been recorded intracellularly, 'whole-cell' or juxta-cellularly, to be identified anatomically, forming a vital link between functional and structural studies. It has been applied throughout the nervous system and has become a fundamental component of our technical armoury. A comprehensive survey of the applications to which the biocytin-filling approach has been put, would fill a large volume. This essay therefore focuses on one area, neocortical microcircuitry and the ways in which combining physiology and anatomy have revealed rules that help us explain its previously indecipherable variability and complexity.