Brodmann area 13 is a subdivision of the cerebral cortex as defined on the guenon monkey and on the basis of cytoarchitecture. Brodmann area 13 is found in humans, however it seems to act as a bridge between the lateral and medial layers of the brain. Thus it is sometimes misidentified as not being a Brodmann area.
The external granular layer (II) is relatively dense. The external lamina pyramidalis externa (III) has a central stripe of less cellular density that separates two sublayers, IIIa and IIIb. The internal granular layer (IV) is sufficiently wide and dense to separate clearly sublayer IIIb from layer V. The boundary between layers V and VI is defined by larger ganglion cells, more pyramidal in shape, in layer V giving way to smaller, more spindle-shaped cells that become denser and more homogeneous deeper in layer VI. Often the spindle cells are arrayed horizontally as in the claustrum (VICl), which Brodmann considered a likely extension of layer VI beyond the extreme capsule (VICe) (Brodmann-1905).