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Pia mater
Pia mater ( or 〔(Entry "pia mater" ) in ''(Merriam-Webster Online Dictionary )'', retrieved 2012-07-28.〕), often referred to as simply the pia, is the delicate innermost layer of the meninges, the membranes surrounding the brain and spinal cord. ''Pia mater'' is medieval Latin meaning "tender mother".〔 The other two meningeal membranes are the dura mater and the arachnoid mater. Pia mater is a thin fibrous tissue that is impermeable to fluid. This allows the pia mater to enclose cerebrospinal fluid. By containing this fluid the pia mater works with the other meningeal layers to protect and cushion the brain. The pia mater allows blood vessels to pass through and nourish the brain. The perivascular space created between blood vessels and pia mater functions as a lymphatic system for the brain. When the pia mater becomes irritated and inflamed the result is meningitis.
==Structure==
Pia mater is the thin, translucent, mesh-like meningeal envelope, spanning nearly the entire surface of the brain. It is absent only at the natural openings between the ventricles, the foramen of Majendie, and the foramina of Luschka. The pia firmly adheres to the surface of the brain and loosely connects to the arachnoid layer.〔(【引用サイトリンク】title=meninges )〕 Because of this continuum, the layers are often referred to as the pia arachnoid or leptomeninges. A subarachnoid space exists between the arachnoid layer and the pia, into which the choroid plexus releases and maintains the cerebrospinal fluid (CSF). The subarachnoid space contains trabeculae, or fibrous filaments, that connect and bring stability to the two layers, allowing for the appropriate protection from and movement of the proteins, electrolytes, ions, and glucose contained within the CSF.
The thin membrane is composed of fibrous connective tissue, which is covered by a sheet of flat cells impermeable to fluid on its outer surface. A network of blood vessels travels to the brain and spinal cord by interlacing through the pia membrane. These capillaries are responsible for nourishing the brain.〔Polzin, Scott. "Meninges". ''Gale Encyclopedia of Neurological Disorders''. 2005. Encyclopedia.com. 20 February 2011 . l〕 This vascular membrane is held together by areolar tissue covered by mesothelial cells from the delicate strands of connective tissue called the arachnoid trabeculae. In the perivascular spaces, the pia mater begins as mesothelial lining on the outer surface, but the cells then fade to be replaced by neuroglia elements.
Although the pia mater is primarily structurally similar throughout, it spans both the spinal cord’s neural tissue and runs down the fissures of the cerebral cortex in the brain. It is often broken down into two categories, the cranial pia mater (pia mater encephali) and the spinal pia mater (pia mater spinalis).
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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