Subarachnoid space

Immediately below the layer of arachnoid mater is a real space, termed the subarachnoid space. 

In life the subarachnoid space is filled with cerebrospinal fluid (CSF) and arachnoid granulations penetrate into the dural venous sinuses to drain any excess CSF.  

Cerebral arteries and veins also run through the subarachnoid space.