Arachnoid Mater

The arachnoid mater is the middle of three layers that make up the meninges, or membranes, that surround the spinal cord and brain. The dura mater is the outermost layer of the meninges. The innermost layer that contours closely to the many folds and crevices of the brain is called the pia mater.


The arachnoid mater has delicate, cobweb-like filaments that are attached to the top of the pia mater, connecting them structurally. Combined, the two layers are known as the leptomeninges. The void in between the arachnoid and pia mater is called the subarachnoid space. This space holds the central nervous system’s cerebrospinal fluid. The clear fluid has several purposes, including removing metabolic waste from the central nervous system, making the heavy brain more buoyant, and protecting brain tissues from hitting the skull.


Related conditions


  • Arachnoid cysts – Cysts of cerebrospinal fluid-filled sacs can develop in the arachnoid mater in the brain or spinal cord, with causes generally unknown. Arachnoid cysts are more common around the brain, but spinal cysts can also occur. While many cysts may be asymptomatic, a spinal cyst in the right location might impinge upon the spinal cord or nearby nerve root. This could cause pain in the area of impingement, as well as weakness, numbness, spasms, and tingling radiating down the arms or legs.
  • Subarachnoid hemorrhage – This type of hemorrhage results from bleeding in the subarachnoid space. It can be caused by trauma or by an aneurysm, leading to severe disability or death.
  • Subdural hematoma – If the arachnoid and dura maters separate – an occurrence that is usually caused by severe trauma – a hematoma (collection of blood) can form as the resulting torn capillaries release blood into the space created.


What’s in a name?


The term “arachnoid” is derived from the Greek word for spider. The arachnoid mater earned its name due to its spider web-like appearance. Regardless of where the name came from, the arachnoid mater’s thin, transparent fibers are vital to the protection and function of the central nervous system.