Large-cavity CSF compartments and bulk flow. Extracellular fluid enables volume transmission (convection) of fluid from ventricles to SAS [7, 27]. CSF formed by lateral, 3rd and 4th ventricle CPs flows from the lateral to 3rd ventricle via the cerebral aqueduct and 4th ventricle to SAS in cisterna magna. Then CSF is transmitted by bulk flow through cisternal foramina (Magendie and Luschka) into basal cisterns. CSF is also convected from ventricles through velae channels to the quadrigeminal and ambient cisterns . Thereafter fluid is convected to the SAS of the spinal cord and brain convexities. As CSF flows through the ventriculo-subarachnoid system, there are diffusional and bulk flow exchanges between CSF and brain [16, 24, 168, 169, 289], depending upon region-specific gradients for concentration and hydrostatic pressure that promote widespread distribution of CSF-borne materials . Normally CSF is readily distributed from the ventricles to arachnoidal drainage sites. In hydrocephalus, flow pathways can be disrupted at multiple points.