arachnoid

Arachnoid refers to the arachnoid mater, the middle layer of the three meninges that surround the brain and spinal cord. The name comes from the Greek arachne, meaning spider, reflecting its delicate, web-like appearance. The arachnoid lies between the dura mater (the outer layer) and the pia mater (the inner layer) and together with the other meninges helps protect and cover the central nervous system.

The arachnoid is a thin, avascular membrane that forms a network of collagen and fibrous tissue. It creates a spiderweb-like arrangement called the arachnoid trabeculae, which span the subarachnoid space and connect to the pia mater. The subarachnoid space contains cerebrospinal fluid (CSF) and the major cerebral vessels. This space plays a key role in CSF circulation and provides a cushion for the brain and spinal cord.

Arachnoid granulations, or arachnoid villi, protrude into the dural venous sinuses and facilitate the absorption of CSF into the venous system. The arachnoid layer also contributes to barrier functions that separate CSF from surrounding dural tissues.

Clinical relevance includes arachnoid cysts, which are fluid-filled sacs that can develop within the arachnoid space and may cause neurological symptoms if they compress adjacent structures. Arachnoiditis is inflammation of the arachnoid, often resulting from infection, hemorrhage, trauma, or surgery and can lead to scarring and chronic pain. In general, integrity of the arachnoid and the subarachnoid space is important for normal CSF dynamics and central nervous system protection.

The term arachnoid is also used to describe structures with a spider-web-like appearance in the body.