Figure 1.

CSF is produced in the choroid plexus of the lateral, third, and fourth ventricles. The CSF flows unidirectionally from the lateral ventricle to the third ventricle, to the fourth ventricle, and then to the subarachnoid space in the region of the basal meninges. Ventricular sampling of the CSF is from the lateral ventricles, and lumbar sampling is from the lumbar cisterns in communication with the subarachnoid space. Only a minority of CSF flows into the lumbar cisterns, with the rest reabsorbed by the arachnoid villi of the dural venous sinuses. Abbreviation: CSF, cerebrospinal fluid.