The studies were carried out on the mesencephalons of adult guinea pigs. The preparations were made by means of the Golgi technique, as well as the Nissl and Klüver-Barrera methods. Four types of neurons were distinguished in the substantia nigra (SN) of the guinea pig: 1. Bipolar neurons of two kinds: the neurons of the first kind have elongated, fusiform perikarya (25–40 µm), whereas the cells of the second kind have rounded and oval perikarya (15–22 µm). These neurons possess two dendritic trunks which arise from the opposite poles of the cell body and run for a relatively long distance. The bipolar neurons are the most numerous in the pars compacta of SN. 2. Triangular neurons with three primary dendrites arising conically from a perikaryon (20–35 µm). They are the most often observed type of neurons in the pars reticulata of SN. 3. Multipolar neurons with quadrangular or oval perikarya (22–35 µm) and 4–5 dendritic trunks which spread out in all directions. 4. Pear-shaped neurons (perikarya 15–25 µm), which have one or two primary dendritic trunks arising from one pole of the cell body. In all the types of neurons an axon originates either from the dendritic trunk or from the soma and is observed only in its initial segment.
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