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Arachnoid granulations (or arachnoid villi) are small protrusions of the arachnoid (the thin second layer covering the brain) through the dura mater (the thick outer layer). They protrude into the venous sinuses of the brain, and allow cerebrospinal fluid (CSF) to exit the sub-arachnoid space and enter the blood stream. Largest granulations lie along the superior sagittal sinus, a large venous space running from front to back along the center of the head (on the inside of the skull). They are, however, present along other dural sinuses as well. Smaller granulations are called ''villi'', large calcified ones are referred to as pacchionian bodies. ==Function== The arachnoid granulations act as one-way valves. Normally the pressure of the CSF is higher than that of the venous system, so CSF flows through the villi and granulations into the blood. If the pressure is reversed for some reason, fluid will pass back into the subarachnoid space (of the brain)(this have read 'will not'? ) . The reason for this is not known. It has been suggested that the endothelial cells of the venous sinus create vacuoles of CSF, which move through the cell and out into the blood. The importance of arachnoid granulations for the drainage of CSF is controversial. By some accounts, a large portion (perhaps the majority) of CSF is drained through lymphatics associated with extracranial segments of the cranial nerves. A large proportion of CSF is believed to leave the cranial vault through the axons of CN I (olfactory nerve) through their extension through the cribiform plate. On the inner surface of cranial bones, small pits called granular fovea are produced by arachnoid granulations. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「arachnoid granulation」の詳細全文を読む スポンサード リンク
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