The cytoarchitecture of the coulomotor nuclear complex (ONC) and of the trochlear nucleus (TN) was studied in the encephalon of 10 adult marmosets (Callithrix jacchus). All the encephala were fixed with Bouin, embedded in paraffin and submitted to 20-µm serial sections in the transversal, frontal and horizontal directions. The following staining methods were employed: Nissl, Kluver-Barrera, and Pal-Weigert, modified by Erhart [1951]. Morphologically we delimited four cell columns in the ONC of the marmoset, namely lateral, dorsal, anterior, and dorsal central. The lateral and dorsal central columns comparatively correspond, in man, to the somatic portion. The others to the visceral portion. The trochlear nucleus of the marmoset is compact, round in shape and can be divided in to central cellular, dorsoventral and ventrolateral groups. The neurons of both the nuclei, of the multipolar type, predominantly have a round shape but it is also possible to observe oval or triangular neurons. They have a central nucleus, the cramatine substance is not dense and is irregularly disposed. Some differences in the size of neurons can be observed among the columns of the ONC.

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