 Area 15, like all Brodmann areas, is defined on the basis of cytoarchitecture of the region of cortex.The cortex in area 15 is thinner than in the rest of the insula and temporal lobe.The molecular layer (I) is unusually wide; the external granular layer (II) and the external pyramidal layer (III) are less dense, and the internal granular layer (IV) is totally absent, so that the medium-sized pyramidal cells of layer III and the internal pyramidal layer (V) merge with a few isolated granular cells scattered at their boundary.