The normal anatomy of nucleus sphericus in the common boa (Constrictor constrictor) is described here on the basis of routine light microscopic and Golgi material. Nucleus sphericus is a cup-shaped aggregation of cells lying in the forebrain posterior to the anterior commissure and consisting of three concentric layers. The marginal layer is subependymal, is continuous rostrally with the anterior commissure, and contains fusiform cells with discoidal dendritic fields tangent to the mural layer. Marginal cell axons entwine mural cell dendrites. The mural layer forms the walls of the cup and appears as a closed ring of cells in transverse sections. Mural cell dendrites extend radially into the marginal and hilar layers and horizontally within the mural layer. All of these dendrites are profusely covered by dendritic spines with conical heads and slender stalks. Mural cell axons ascend to the marginal layer. The hilar layer forms the cup's cavity containing scattered cells whose dendrites extend in all directions and whose axons approach the cup's mouth. Afferent fibers in the nucleus run predominately at right angles to mural cell dendrites bearing varicosities at irregular intervals.