The pia or pia mater is a very delicate membrane. It is closest to the brain and attached thereto.
The middle element of the meninges is the arachnoid.
The dura mater is 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 subdurmal 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.