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The cisterna magna or cerebellomedular cistern is one of the three main openings in the subarachnoid space between the arachnoid layers and the pia mater of the meninges surrounding the brain. The openings are collectively referred to as cisterns. The cisterna magna is located between the cerebellum and the dorsal surface of the medulla oblongata. Cerebrospinal fluid flows through the fourth ventricle into the cisterna magna through the lateral openings and median opening.

The other two main cisterns are the pontine cistern located between the protrusion and the medulla, and the interpeduncular cistern located between the cerebral peduncles.

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