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• The pia or pia mater is a very delicate membrane. It is closest to the brain and follows all the minor contours of the brain.
• The middle element of the meninges is the arachnoid, so named because of its spider web-like appearance.
• The dura mater is a thick, durable membrane, closest to the skull.

The subarachnoid space is the space that normally exists between the arachnoid and the pia.

Normally, the dura mater is attached to the skull, the arachnoid is attached to the dura mater, and the pia mater is attached to the brain. When the dura mater and the arachnoid separate through injury or illness, the space between them is the subdural space.

A subdural hematoma is, therefore, a separation of the arachnoid from the dura mater. The small veins that connect the dura mater and the arachnoid are torn, and blood can leak into this area. Other medical conditions that affect the menegies include meningitis, a viral or bacterial infection of the meninges.