The subarachnoid space and the ventricles are filled with a colorless and transparent fluid – cerebrospinal fluid. The total volume of this fluid is 100-150 ml in normal individuals, its specific gravity is 1, it is weakly alkaline, and the number of cells in 1 cubic millimeter is below 8, mainly epithelial cells and lymphocytes. Cerebrospinal fluid production: In the central nervous system, cerebrospinal fluid is produced at a rate of 0.3 ml/min, with a daily secretion of 400-500 ml. The choroid plexus tissue within the brain is the main structure that produces cerebrospinal fluid. The choroid plexus is mainly located at the base of the lateral ventricles and the top of the third and fourth ventricles, and is structured as a network of capillaries covered by a layer of ventricular epithelium resembling microvilli. This microvilli is like a one-way open membrane that secretes cerebrospinal fluid only into the ventricular and subarachnoid spaces. It is also believed that the ventricular canal and brain parenchyma also have a role in producing cerebrospinal fluid. Circulation of cerebrospinal fluid: The flow of cerebrospinal fluid has a certain direction. The two lateral ventricular choroid plexuses are the most abundant and produce the most cerebrospinal fluid, which flows into the third ventricle via the interventricular foramen and then into the fourth ventricle via the midbrain aqueduct. The cerebrospinal fluid produced by each ventricular plexus converges to the fourth ventricle and flows into the subarachnoid space of the brain and spinal cord through the median and lateral foramina of the fourth ventricle. Finally, the cerebrospinal fluid leaks back into the superior sagittal sinus via the arachnoid granules adjacent to the sagittal sinus, allowing the cerebrospinal fluid to return to the venous system. The reflux (or absorption) of cerebrospinal fluid depends mainly on the pressure difference between intracranial venous pressure and cerebrospinal fluid and the effective colloid osmotic pressure across the blood-brain barrier. The blood vessels of the brain and spinal cord, the perineural space and the ventricular canal membrane are also involved in the absorption of cerebrospinal fluid. The cerebrospinal fluid is continuously produced and absorbed back into the veins, and plays the role of lymphatic fluid in the central nervous system, supplying brain cells with certain nutrients, transporting metabolites from brain tissue, and regulating the acid-base balance of the central nervous system. It also cushions the pressure of the brain and spinal cord, and has a protective and supportive effect on the brain and spinal cord. The properties and pressure of cerebrospinal fluid are affected by many factors. If the central nervous system is diseased and the metabolism of nerve cells is disturbed, the properties and composition of cerebrospinal fluid will be changed; if the circulatory path of cerebrospinal fluid is blocked, the intracranial pressure will be increased. Therefore, when the central nervous system is damaged, the detection of cerebrospinal fluid becomes one of the important auxiliary diagnostic tools.