[Neuron structure of the rat septum telencephali].

1. The neuronal structure of the septum of adult rats was studied after GOLGI impregnation. On the basis of cytoarchitectonic investigations the septum of the rat was divided into three big nuclei: nuclei septalis lateralis, septalis medialis, and tractus diagonalis (s. ANDY and STEPHAN). 2. The nc. septalis medialis contains cells with pyramidal, oval, round, spindle-shaped, fusiforme or triangular pericarya, while in the nc. septalis lateralis cells with pyramidal pericarya are absent. In the nc. tractus diagonalis four different forms of cells bodies occur: oval, triangular, round, and parymidal ones. 3. The existence of single, characteristic cell forms is not restricted to certain areas within a nucleus; the different kinds of cells are distributed irregularly and arranged ina different distance. 4. Mostly after a dichotome division the dendrites spread into all directions (bipolar to star-shaped dendritic arborisation). An orientation of the dendrites of single neurones into a special direction (i.e. to the ventricle) was not observed. 5. Neurons of the septum have dendritic fields with a mean diameter of 300 mum. The dendritic diameter regularly remains constant in the distance between two branchings and decreases only after the division of the dendrite. 6. The distribution of spines on the dendrites is typical for interneurons. Immediately after their origin from the pericaryon the dividing dendrites have a segment free of spines which extends up to the first outgrowth of a lateral dendrite. In neurons with poorly ramified dendrites this spines-free segment is absent; the first spines are found close by the pericaryon. Bferore and after a ramification of dendrites a different density of spines exists. 7. The neurons of the septum are morphologically and functionally classified as interneurons. Their dendritic fields correspond to those of star cells and are even similar to pyramidal cells. For the cell types described a great structural variability in the dentritic arrangement occurs.