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It is challenging to specify the role of the default mode network (DMN) in human behaviour. Contemporary theories, based on functional magnetic resonance imaging (MRI), suggest that the DMN is insulated from the external world, which allows it to support perceptually-decoupled states and to integrate external and internal information in the construction of abstract meanings. To date, the neuronal architecture of the DMN has received relatively little attention. Understanding the cytoarchitectural composition and connectional layout of the DMN will provide novel insights into its role in brain function. We mapped cytoarchitectural variation within the DMN using a cortical type atlas and a histological model of the entire human brain. Next, we used MRI acquired in healthy young adults to explicate structural wiring and effective connectivity. We discovered profound diversity of DMN cytoarchitecture. Connectivity is organised along the most dominant cytoarchitectural axis. One side of the axis is the prominent receiver, whereas the other side remains more insulated, especially from sensory areas. The structural heterogeneity of the DMN engenders a network-level balance in communication with external and internal sources, which is distinctive, relative to other functional communities. The neuronal architecture of the DMN suggests it is a protuberance from the core cortical processing hierarchy and holds a unique role in information integration.