arachnoideus

Arachnoideus, in anatomical terminology, refers to the arachnoid, one of the three meninges that envelop the brain and spinal cord. It is the middle, delicate membrane that lies between the dura mater (outer) and the pia mater (inner). The term arachnoid derives from its spider-web-like appearance, as the membrane contains fine trabeculae that extend across the subarachnoid space to the pia.

The subarachnoid space, located between the arachnoid and the pia, contains cerebrospinal fluid (CSF) and major blood vessels that supply the brain. The arachnoid itself is avascular and serves as a protective barrier; its trabeculae help cushion neural tissue while allowing CSF and small vessels to circulate. Arachnoid granulations, or Pacchionian granulations, protrude into the dural venous sinuses and facilitate drainage of CSF into the bloodstream.

Clinically, the arachnoid membrane is involved in several conditions. Subarachnoid hemorrhage refers to bleeding into the subarachnoid space, often from ruptured blood vessels, while arachnoid cysts are fluid-filled sacs that can compress adjacent neural structures. Arachnoiditis denotes inflammation of the arachnoid, which can follow infections, trauma, or surgery and may lead to chronic pain or neurological symptoms.

In summary, the arachnoideus (arachnoid) describes a fragile, lace-like membranous layer of the meninges essential for CSF dynamics, protection, and the pathophysiology of various central nervous system conditions.