A cross-section of the brain showing the layers of the meninges from Gray's Anatomy

The meninges are a series of membranes that envelop and protect the central nervous system. They are made up of three layers:

  • The dura mater, a thick membrane that coats the inside of the skull;
  • The arachnoid mater, a spider web like, transparent, loose membrane that envelops the brain.
  • The pia mater, a thin, flexible membrane that surrounds the brain and spinal cord.

Some diseases that affect the meninges are meningitis and meningioma.

Bleeding in the brain is often characterized by the layer of the meninges that it penetrates. A subarachnoid hemmhorage is one where the bleeding remains within the arachnoid mater, which is impermeable to liquid. A bleed between the arachnoid mater and the dura mater is a subdural hematoma.

Meninges on Wikipedia

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