The external granular layer is very dense and contains small granular cells and small pyramidal cells. The external or medial pyramidal layer contains pyramidal cells arranged in row formation. The cell bodies of some association fibers are found here. The internal granular layer is thin, but its cell structure is the same as that of the external granular layer. The ganglionic layer contains small granular cells, large pyramidal cells as well as the cell bodies of some association fibers.
All layers are present in all parts of the cortex.
However, they do not have the same relative density in all areas. Depending upon the function of a particular area, some of these layers will be thicker than others in that location. The cortex wraps around the brain, covering its inferior surface and lining the gap between the right and left cerebral hemispheres, which is called the longitudinal or interhemispheric fissure . The part of the cortex covering the sides of the hemispheres is called lateral cortex while the part covering the sides of the hemispheres that lie within the longitudinal cerebral fissure are called medial cortex . Brodmann's Classification System Studies done by Brodmann in the early part of the twentieth century generated a map of the cortex covering the lobes of each hemisphere.