CSF tends to accumulate in certain large subarachnoid areas of the brain and spinal cord. These are called cisterns in the brain (Fig-13). The cisterna magna, just below the cerebellum, is the largest of these. Others include the pontine cistern, anterolateral to the pontomedullary border, the interpeduncular cistern, anterior to the midbrain and inferior to the diencephalon, the chiasmatic cistern surrounding the optic chiasm, the superior cistern, between the cerebellum and the inferior colliculi of the posterior midbrain, and the cistern of the vein of Galen, posterior to the diencephalon. The subarachnoid space is relatively large below the level of the second lumbar vertebra and contains a relatively large amount of CSF. Because this is below the termination of the spinal cord, a needle can be safely passed into the subarachnoid space in this region to withdraw a CSF sample or to make an injection. Less commonly the cisterna magna is used for this purpose.