[Orientation of nerve fiber systems in the tangential plane of layer I of the rat neocortex].

The orientation of nerve fibers in the tangential plane of layer I has been studied in silver impregnated paraffin sections and in frozen sections of rat cerebral cortex. The neocortex was divided into 4-6 pieces which were embedded separately and serially cut tangentially to the surface. In these serial sections the orientation of the nerve fibers running parallel to the cortical surface was studied. A system of parallel fibers was revealed which form a distinct pattern in layer Ib that changes in a characteristic way from one region to the next. Diagrams of preferential fiber directions have been drawn into a reconstruction of the entire neocortex. It is assumed that the orientation of the tangential fibers is indicative of associative connections between primary thalamo-cortical projection areas.