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SCHEMATIC OF THE MENINGES AND THEIR RELATIONSHIPS TO THE BRAIN AND SKULL


SCHEMATIC OF THE MENINGES AND THEIR RELATIONSHIPS TO THE BRAIN AND SKULL
The meninges provide protection and support for neural tissue in the central nervous system. The innermost membrane, the pia mater, adheres to every contour of neural tissue, including sulci, folia, and other infoldings. It adheres tightly to glial end foot processes of astrocytes; this association is called the pialglial membrane. 

The arachnoid mater, a fine, lacy membrane external to the pia, extends across the neural sulci and foldings. The space between these two membranes is the subarachnoid space, a space into which the cerebrospinal fluid flows, providing buoyancy and protection for the brain. Arteries and veins run through the subarachnoid space to and from the central nervous system. The rupture of an arterial aneurysm in a cerebral artery results in a subarachnoid hemorrhage. The dura mater, usually adherent to the inner arachnoid, is a tough protective outer membrane. It splits into two layers in some locations to provide channels, the venous sinuses, for return flow of the venous blood. The arachnoid granulations, one-way valves, extend from the subarachnoid space into the venous sinuses, especially the superior sagittal sinus, allowing cerebrospinal fluid to drain into the venous blood and return to the heart. Blockage of these arachnoid granulations (e.g., in acute purulent meningitis) can result in increased intracranial pressure. Cerebral arteries and veins traverse the subarachnoid space. The veins, called bridging veins, drain into the dural sinuses. As they enter the sinus, these bridging veins are subject to tearing in cases of head trauma. If there is atrophy in the brain, as occurs with age, these veins may tear with relatively minor head trauma; in younger adults, more severe head trauma is needed to tear these bridging veins. Such tearing permits venous blood to accumulate in the subdural space as it dissects the inner dura from the arachnoid. This process may be gradual (chronic subdural hematoma) in older individuals or may be abrupt (acute subdural hematoma) with severe head trauma. A subdural hematoma, especially when it occurs acutely, may be life-threatening as the result of increased intracranial pressure caused by accompanying edema and by the accumulation of the blood in the hematoma itself. The dura is closely adherent to the inner table of the skull. A skull fracture may tear a branch of the middle meningeal artery, permitting arterial blood to dissect the dura from the skull, resulting in an epidural hematoma.
SCHEMATIC OF THE MENINGES AND THEIR RELATIONSHIPS TO THE BRAIN AND SKULL

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