subarachnoid cisterns

Cerebellomedullary cistern (Cisterna magna) - the largest of the subarachnoid cisterns.

Some major subarachnoid cisterns:

Although they are often described as distinct compartments, the subarachnoid cisterns are in fact not truly anatomically distinct.

It follows from this that in certain areas around the brain the pia and arachnoid are separated widely; in such regions are formed cavities called the subarachnoid cisterns.