Abstract

In cases of hydrocephalus, echoes from the region of the cerebral median segittal plane may show a fluttering variation both in amplitude and range. Evidence is presented that, in the case studied, these movements arose from the falx cerebri and that they were caused by ballistocardiac forces presumably setting the CSF in the enlarged lateral ventricles into resonance within the enlarged cranium. Similar movements would be expected in the lateral ventricular walls as well as the septum pellucidum when the latter is imperforate. It is suggested that the lowering of the resonant frequency of the ventricular CSF in cases of hydrocephalus with both large ventricles and large heads allows ballistic and acceleratory forces applied to the hydrocephalic head to cause large pressure changes between the two lateral ventricles with consequent lateral movement of the midline structures separating them and possible rupture of the septum pellucidum, as is commonly found in hydrocephalus.