The arachnoidea mater, or simply arachnoidea, is part of the leptomeninges (soft meninges). It is the middle meningeal layer between the Dura mater and the Pia mater[^1].
Anatomy
Like the dura mater, the arachnoidea bridges the Sulci of the brain and derives its name from the prominent whitish pattern of fine fibers of collagenous connective tissue, which gives it a spiderweb-like appearance. Knob-shaped protrusions extend from the arachnoidea into the venous sinuses within the dura mater. These protrusions are referred to as arachnoid granulations or Pacchionian granulations and are a site of cerebrospinal fluid resorption. Intraoperative injuries to the Pacchionian granulations can result in severe venous bleeding. Beneath the arachnoidea lies the subarachnoid space, which is filled with cerebrospinal fluid and thus constitutes the outer cerebrospinal fluid space. Numerous superficial arteries and veins of the brain lie in this space between the arachnoidea and pia mater.
