Diagram showing the positions of the three principal subarachnoid cisternæ. (On diagram, is visible at lower right.)
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The cisterna magna (or cerebellomedullary cistern) is a large cistern between the cerebellum and medulla.
As with all cisterns of the brain, this cistern is an opening in the subarachnoid space created by the separation of the dura mater and arachnoid layer.