An attempt is made at showing that the cerebral cortex has to be envisaged as a mosaic of columnar units of remarkably similar internal structure and surprisingly little variation of diameter (200-300 $\mu $m). A systematic investigation is made of various types of interneurons and also of the local connections of the pyramidal cells by the combined application of four techniques: (i) the classical Golgi procedures, (ii) electron microscope studies of various types of synapses, (iii) light and electron microscope studies in chronically isolated cortical slabs of the local synapses that persist under such circumstances, and (iv) serial reconstruction-under the electron microscope-of clearly identified Golgi-stained interneurons (and pyramidal cells) which has revealed a hitherto unexpected degree of specificity in local connectivity. Most interneurons are not only highly specific with respect to the arborization pattern of their axons and to the size and shape within which they establish synapses with specific sites of certain other neurons, but also with respect to the character, preferential localization, and origin of the synapses that they receive. On the basis of this kind of information the local neuron network of the cerebral cortex can be defined as an intricate system of repetitive but mutually interpenetrating spatial modules of specific interneuron arborizations. As a consequence the synapses of each individual interneuron are distributed in such geometrically defined modules of cortical space. Some of the interneuron types and the corresponding spatial modules can be identified with considerable confidence as being of an excitatory, and others of an inhibitory, nature. By using recent information on the mode of termination of various afferent pathways of the cortex and on the cellular origin of the main efferent pathways, tentative models for the cortical neuron chains can be proposed.