During myelography we observed the contrast material in the spinal subarachnoid space while we changed: (1) the intracranial blood volume by CO2 inhalation, hyperventilation, and jugular vein compression; (2) the intra-abdominal and intrathoracic pressure by forced expiration with glottis closed; and (3) the CSF volume by withdrawals and reinjections of fluid. The spinal dural sac enlarges with increases in volume of both intracranial blood and CSF. It partially collapses with reductions in volume of both intracranial blood and CSF. With increases in intra-abdominal and intrathoracic pressure, the thoracolumbar sac partially collapses, while the cervical sac enlarges. From these observations we conclude that the spinal dural sac is a dynamic structure, readily changing its capacity in response to prevailing pressure gradients across its walls. It acts as a reservoir for CSF, which moves to and fro through the foramen magnum in response to changes in cerebral blood flow. By its bladder-like ability to alter its capacity, the spinal dural sac provides the `elasticity' of the covering of the central nervous system.
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