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- - In most regions the pia and arachnoid are separated by only a few mm, but in some places the gap becomes much larger and accumulations of CSF, termed cisterns, are formed in the expanded subarachnoid space. Sometimes this happens because the arachnoid fails to follow the pia into cul de sacs formed by the brain's surface; the cisterna magna, superior cistern and interpeduncular cistern are good examples. Another cistern, the prepontine one, occurs because the large basilar artery, running in the subarachnoid space, pushes the arachnoid and pia apart, as suggested in Figure 1-16.