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.
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.