direct aspiration (fluid withdrawal) of cerebrospinal fluid (CSF) through a hollow needle. The needle is inserted in the lower back, usually between the third and fourth lumbar vertebrae, into the subarachnoid space of the spinal cord where the CSF is located. Lumbar puncture is generally performed to obtain pressure measurements and to withdraw CSF in order to secure a sample of CSF for cellular, chemical, and bacteriologic examination; to administer spinal anesthetics or antibiotics; to inject air or a radiopaque or water-soluble contrast medium substance for myelography; or to inject a radioactive substance to assist in the diagnosis of CSF leak or hydrocephalus.