The organization of area 18 in the monkey. A Golgi study.

The organization of area 18 in monkey has been studied in Golgi preparations. In layer III large pyramidal cells with profusely ramified basal dendritic arbors are identified. They probably represent one of the major components originating association connections. Layer II and the upper part of layer III contain cells with spine-free dendrites having axons forming long bundles descending vertically to layers IV and V. Layer IV is characterized by the presence of granular cells with local axons and small pyramidal cells with ascending recurrent axons. Layer V contains various types of pyramidal cells including giant pyramidal neurons. The mean number and distribution of dendritic spines along apical shafts were computed in four varieties of pyramidal cells. Several neuronal parameters pertaining to the length of axons and dendrites were calculated. Three-dimensional aspects of cells were analyzed by means of a computer program which permits neurons to be displayed as perspective views from various positions. Attempts were made to correlate the present description with previous observations on this and other cortical visual areas.