The indusium griseum is a thin gray layer of neuronal cells on the outer surface of the corpus callosum. The layer of glial tissue is between the pia and corpus callosum. It is part of the supracallosal gyrus and contains four longitudinal bundles. MRIs demonstrate great variability in the morphology among individuals. There are no connections to the hippocampus. In fact, the function of the indusium griseum is completely unknown.