A neurosurgeon orders a spinal tap (also known as a lumbar puncture) to obtain cerebrospinal fluid. The needle is inserted into the spinal cavity , specifically into the subarachnoid space within the spinal canal in the lumbar region of the back, usually between the lumbar vertebrae L3-L4 or L4-L5. This space contains cerebrospinal fluid that surrounds the spinal cord
. The spinal cavity is a subdivision of the dorsal body cavity, which houses the spinal cord and cerebrospinal fluid. The needle passes through the skin, ligaments, and dura mater to reach this fluid-filled space without damaging the spinal cord, which ends higher up around the L1 vertebra
. Thus, the needle is inserted into the spinal cavity (spinal canal) in the lumbar region during a spinal tap.
