cerebrospinal fluid (CSF), the fluid that flows through and protects the four ventricles of the brain, the subarachnoid spaces, and the spinal canal. It is composed mainly of secretions of the choroid plexuses in the lateral ventricles and in the third and the fourth ventricles of the brain and is clear and colorless. Changes in the carbon dioxide content of CSF affect the respiratory center in the medulla, helping to control breathing. A brain tumor may press against the cerebral aqueduct and shut off the flow of the fluid from the third to the fourth ventricle, causing fluid accumulation in the lateral and third ventricles, called internal hydrocephalus. Other blockages of the flow of CSF, such as those caused by blood clots, result in serious complications. Certain illnesses and various diagnoses may require microscopic examination and chemical analysis of CSF. Samples of the fluid may be removed by lumbar puncture between the third and the fourth lumbar vertebrae or from the cisterna magna.
