WordNet

relating to or resembling a member of the class Arachnida (同)arachnidian, spidery, spiderlike, spiderly
the middle of the 3 meninges (同)arachnoid_membrane
a minute hairlike projection on mucous membrane

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絨毛(膜の表面の虫状の血管の多い小さな突起)

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出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2015/01/21 15:00:12」(JST)

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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 centre 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). 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.^[1]^[2]

Eponym

Occasionally, they are referred to by their old name: Pacchioni's granulations or pacchionian bodies, named after Italian anatomist Antonio Pacchioni.^[3]

References

^ Linden Forest Edwards (1934). Anatomy for physical education, descriptive and applied. P. Blakiston's son & co., inc. p. 80. Retrieved 23 June 2012.
^ Sir Henry Morris (1921). Morris's human anatomy. P. Blakiston's son & Company. p. 953. Retrieved 23 June 2012.
^ synd/392 at Who Named It?

Additional images

Left parietal bone. Inner surface.
Frontal bone. Inner surface.
CT scan of the head showing an arachnoid granulation

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English Journal

Overview of the CSF dual outflow system.

Pollay M1.
Acta neurochirurgica. Supplement.Acta Neurochir Suppl.2012;113:47-50. doi: 10.1007/978-3-7091-0923-6_10.
It was firmly established in the mid-twentieth century that the arachnoid villi represented an open pathway between the subarachnoid space and the dural venous sinuses. Intracellular and extracellular pathways within the villous structure provided the conduit for cerebrospinal fluid (CSF) and partic
PMID 22116422

Human spinal arachnoid villi revisited: immunohistological study and review of the literature.

Tubbs RS1, Hansasuta A, Stetler W, Kelly DR, Blevins D, Humphrey R, Chua GD, Shoja MM, Loukas M, Oakes WJ.
Journal of neurosurgery. Spine.J Neurosurg Spine.2007 Sep;7(3):328-31.
OBJECT: Few have described the relationship between arachnoid protrusions (villi) and adjacent spinal radicular veins, and the descriptions that do exist are conflicting. Some authors have even denied the presence of spinal arachnoid villi, suggesting that they play no role in cerebrospinal fluid (C
PMID 17877268

[Expressions of CD31 and vWF in arachnoid villus endothelium cell from experimental animal with Hydrocephalus].

Wang ER1, Tang J, Sun H, Mao BY.
Sichuan da xue xue bao. Yi xue ban = Journal of Sichuan University. Medical science edition.Sichuan Da Xue Xue Bao Yi Xue Ban.2007 Jun;38(3):408-12.
OBJECTIVE: The aim of this study was to investigate the CD-31 and vonWillebrand factor (vWF) expressions and the cell morphological alterations ofarachnoid villus endothelium in different stages of animal hydrocephalus.METHODS: Silicon oil was injected into the fourth ventricle of experimental canin
PMID 17593818

Japanese Journal

Block of arachnoid villus by subarachnoid hemorrhage

J Neurosurg 30, 651-657, 1969
NAID 10007741997

「クモ膜顆粒」

Arachnoid granulation
	Diagrammatic representation of a section across the top of the skull, showing the membranes of the brain, etc. ("Arachnoid granulation" label is at top right.)
Details
Latin	Granulationes arachnoideae
Identifiers
Gray's	p.878
TA	A14.1.01.205
FMA	FMA:77760
	Anatomical terminology