The two layers are the inner and outer dura mater. The venous sinuses into which venous blood drains are channels between the two layers of dura mater. Arachnoid A thin fibrous membrane forming the middle of the three coverings of the central nervous system. Its smooth outer surface lies in close proximity, but not attached, to the inner surface of the dura mater. A CNS hemorrhage in which blood collects in this space is referred to as the subdural hemorrhage.
Subarachnoid space is the space between the arachnoid and the pia mater, the innermost mennix.
A thin fibrous membrane firmly attached to the brain surface. It faithfully follows the gyri and sulci of the cerebral surface. It also envelopes the cerebellum but does not follow the cerebellar contour by dipping into every cerebellar sulcus and gyrus. Unlike the arachnoid, the pia mater contains blood vessels. The pia mater and the arachnoid are collectively called leptomeninges as distinguished from dura mater or pachymeninx.