spinal puncture, surgical penetration of the spinal canal by a hollow needle introduced between two of the lumbar vertebrae. The arrangement permits injection of antibiotics or anesthetics (see anesthesia) as well as dyes to facilitate X-ray studies. It also allows withdrawal of cerebrospinal fluid (CSF), the plasmalike liquid cushioning the brain and spinal cord, in which case the procedure is known as a spinal tap. Examination of the CSF is useful in diagnosing disease of the central nervous system. The fluid is first tested for pressure; a high reading may signal inflammation or tumor. If pressure is normal, a small sample can be taken. It is then analyzed for antibodies, white blood cells, cellular debris, bacteria, and other organisms. Unusual concentrations may indicate disorders such as spinal meningitis, polio, or cancer. The concentrations of protein, sugar, and other chemical components are also determined. Excessive protein may be a sign of spinal tuberculosis.