Hi pzh200707, I can see how you are concerned looking at that picture. It almost looks like retrothalamic part of that patient's quadrigeminal cistern is connected directly to his trigones. lol....and to make matters worse that animation incorrectly shows the lateral ventricle connected to the cisterns. The trigones of the lateral ventricles pass in close proximity to the quadrigeminal cisterns but they do not connect. If they did connect directly like that picture suggests then aqueductal stenosis would not be expected to cause significant hydrocephalus. The CSF in the lateral ventricles must first traverse the 3rd and 4th ventricles before reaching the cisterns.