A new interpretation of the origin of extradural space negative pressure.

The negative pressure in the extradural space is the result of the interaction of hydrostatic forces created between the vertebral canal (a stiff-walled cylinder) and the dura mater, with its content of cerebro-spinal fluid (CSF), which behaves as a cylinder that can collapse according to the position of the subject. In order to corroborate this hypothesis, we have designed a model formed by a sealed, water-filled, glass cylinder ("vertebral canal"). In the interior there is a collapsible, non-distendible, water-filled cylinder ("dural sac"), in free communication with a reservoir which represents the CSF contained in the cranial cavity. Changes in the position of the model produce subatmospheric pressure in the space between both cylinders ("extradural space"), as happens in man.