Dense fibrous dura, avascular arachnoid, delicate vascular pia
Expansion
Dura mater - dense fibrous connective tissue, two layers cranially: an outer periosteal layer and an inner meningeal layer, which separate to form the venous sinuses. Only the meningeal layer continues into the spinal canal. It is richly innervated by the trigeminal nerve, which is why it is pain sensitive while the brain is not.
Arachnoid mater - avascular, with a barrier layer of tight junctions. Trabeculae cross the subarachnoid space beneath it.
Pia mater - a single cell layer, adherent to the brain surface, following every sulcus, and highly vascular, accompanying vessels into the brain as the perivascular space.
The subarachnoid space lies between arachnoid and pia, containing cerebrospinal fluid and the major vessels, which is why a berry aneurysm bleeds there.
Arachnoid granulations project into the superior sagittal sinus and return CSF to the venous circulation. Their obstruction, by blood after subarachnoid haemorrhage or by protein after meningitis, causes communicating hydrocephalus.
The clinical layers map onto the haemorrhages: extradural (arterial, biconvex), subdural (bridging veins, crescentic), subarachnoid (aneurysmal, in the sulci).