The term area PA of Bonin refers to an area of the cerebral cortex defined on the basis of cytoarchitecture in the macaque. It is a "narrow zone in the depth of the posterior wall" of the central sulcus that differs from the adjacent area FA of Bonin in that it is of "a thinner laminated type. The two granular layers appear and layer v clears. It may also be seen" on the medial surface of the hemisphere. The authors did not delineate area PA in their map of cortical areas. They regarded it as a transition zone between area FA and area PB of Bonin homologous to area PA as described by Economo in the human ( Bonin-1947 ).