Back and spinal cord: vertebral column, spinal meninges
Human Anatomy · MBBS
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A doctor needs cerebrospinal fluid to test for meningitis and inserts a needle in the lower back. The spinal cord ends around L1–L2, but the dural sac with CSF continues to S2. A lumbar puncture at L3–L4 (or L4–L5) enters the subarachnoid space below the cord — safe from cord injury — and CSF drips out for testing. This is why the needle goes in low, between the lumbar vertebrae, never higher.