(arachnoid mater) n. the middle of the three membranes covering the brain and spinal cord (see meninges), which has a fine, almost cobweb-like, texture. Between it and the pia mater within lies the subarachnoid space, containing cerebrospinal fluid and large blood vessels; the membrane itself has no blood supply.
one of the thin-walled projections outwards of the arachnoid membrane into the blood-filled sinuses of the dura, acting as a one-way valve for the flow of cerebrospinal fluid from the subarachnoid space into the bloodstream. Large villi, known as arachnoid granulations (or Pacchionian bodies), are found in the region of the superior sagittal sinus. They may be so distended as to cause pitting of the adjacent bone.... arachnoid villus
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