Arachnoid Mater

The arachnoid mater (from Greek arachne — spider and eidos — form, likeness; Latin mater — mother, protector) is the middle layer of the three meninges protecting the central nervous system.

It is an extremely thin, transparent membrane without its own blood vessels, positioned strictly between the dura mater and the pia mater.

Etiology and pathophysiology

The membrane gets its name from numerous thin connective tissue strands (trabeculae). These arise from its inner surface and intertwine to form a dense spatial web, attaching to the underlying pia mater.

Unlike the pia mater, the arachnoid membrane does not enter the grooves and convolutions of the brain but rather spans across them like a stretched film. The space beneath this membrane is filled with cerebrospinal fluid. This creates an ideal hydrodynamic cushion, in which the brain resides and absorbs shocks. Externally, the arachnoid mater adheres closely to the dura mater but does not physically merge with it.

Clinical significance

In traumatology, this anatomical structure plays a critical role in the development of various types of hemorrhages. The venous vessels that penetrate it completely, when ruptured, spill blood over the membrane, forming a subdural hematoma. Damage to the arteries passing beneath it leads to a subarachnoid hemorrhage.

Additionally, in the event of skull fractures or severe impacts, the arachnoid mater itself may suffer tears. In such cases, cerebrospinal fluid under pressure escapes from underneath, accumulating as a subdural hygroma, a pathological water lens. This accumulation of fluid can compress the brain as intensely as a dense blood clot.