**1. Introduction**

The cerebrospinal fluid (CSF) is an ultrafiltrate of plasma, which resides in two compartments within the central nervous system (**Figure 1**). The ventricular system comprises four interconnected cavities in the brain and contains a network of ependymal cells forming the choroid plexus which has been believed to be the site of production of the CSF. The ventricular system is continuous with the central canal of the spinal cord (from the fourth ventricle) and allows the CSF to continuously bathe the cranium and the spine. The subarachnoid spaces form openings termed as subarachnoid cisterns which separate the arachnoid and the pia mater, thereby creating an anatomic space between the two meninges. These cisterns are filled with cerebrospinal fluid and form the second compartment where the CSF flows within the cranial cavity.

**Figure 1.** *The anatomy of the fluid compartments of the brain: ventricular and cisternal systems.*
