In meditating on the ventricular system we meditate on the spaces in between.
Some meditations are focused on the bones, feeling into the shape of your frontal bone and sensing the sensations that exist in that area. Frontal bones, temporal bones, sphenoid bones are akin to trees and sand banks. For the ventricular system we are sensing into the rivers and streams that run between those trees and those sand banks; the waters that touch one tree and then flow to the next and the next and the next.
Message-carrying waters, and cleaning waters.