ABSTRACT
When we talk about the cerebral cortex, what we usually have in mind is the sheet of laminated grey matter surrounding the forebrain. This is where the phenomenon of areal diversification manifests itself most evidently. However, the white matter underneath it is an equally important constituent of the telencephalic cortex: hardly any other structural feature characterizes the cortex as much as the mass of fibres underlying it, mostly composed of axons of cortical neurones. The vast majority of these fibres connect the cortex to itself as is evident in the large amount of cortical white matter as compared to the thickness of the various subcortical fibre tracts (Braitenberg, 1974; Seitz, this volume). While the intrinsic connectivity within an area seems to be provided to a large extent by collaterals running within the grey matter (e.g. Yoshioka et al., 1992; Amir et al., 1993), the traffic between different areas is mainly the responsibility of the main axons, running through the white matter. This is certainly true for the majority of fibres in large brains, in spite of the fact that axon collaterals freely cross the borders between neighbouring
than a few millimeters across.
