It's well-established that the CNS is completely submerged in CSF; but to what extent is this true regarding the finer spaces within the CNS itself? We used MRI to simultaneously map the presence of CSF within all peri-neural and peri-vascular spaces in vivo in humans. Our findings indicated that all peri-neural spaces surrounding cranial and spinal nerves, as well as all peri-vascular spaces, were filled with CSF. These findings suggest that anatomically, substance exchange between the brain parenchyma and outside tissues (i.e. lymphatics) can only occur through CSF, warranting further investigation into its cerebral waste clearance and immunomodulation implications.