If you look at a slice of cortex under the microscope, there appear to be six physical layers (like a cake), owing to the different types, numbers, and arrangem

If you look at a slice of cortex under the microscope, there appear to be six physical layers (like a cake), owing to the different types, numbers, and arrangement of neurons in each. Canonically, the cortex is built out of columns, each of which repeat the same motif. Within a cortical column, signals enter a cortical region through layer IV, 'ascend' to other cortical areas via Layers II and III, and project elsewhere in the brain via Layer V/VI. Layer I mostly contains passing fibers going elsewhere. There are also "horizontal" or lateral connections between and within columns. This is sort of an abstraction though. It's often hard to clearly delineate the boundary between Layer II and III. Layer IV of primary visual cortex has many small sublayers (4C alpha), but it's very very small in others.