<P> The brain and spinal cord are enveloped by a layer of fluid, 125 - 150 ml in total (in adults) which acts as a shock absorber and provides a medium for the transfer of nutrients and waste products . The majority is produced by the choroid plexus in the brain and circulates from there to other areas, before being reabsorbed into the circulation (predominantly by the arachnoid granulations). </P> <P> The cerebrospinal fluid can be accessed most safely in the lumbar cistern . Below the first or second lumbar vertebrae (L1 or L2) the spinal cord terminates (conus medularis). Nerves continue down the spine below this, but in a loose bundle of nerve fibers called the cauda equina . There is lower risk with inserting a needle into the spine at the level of the cauda equina because these loose fibers move out of the way of the needle without being damaged . The lumbar cistern extends into the sacrum . </P> <P> The person is usually placed on their side (left more commonly than right). The patient bends the neck so the chin is close to the chest, hunches the back, and brings knees toward the chest . This approximates a fetal position as much as possible . Patients may also sit on a stool and bend their head and shoulders forward . The area around the lower back is prepared using aseptic technique . Once the appropriate location is palpated, local anaesthetic is infiltrated under the skin and then injected along the intended path of the spinal needle . A spinal needle is inserted between the lumbar vertebrae L3 / L4, L4 / L5 or L5 / S1 and pushed in until there is a "give" as it enters the lumbar cistern wherein the ligamentum flavum is housed . The needle is again pushed until there is a second' give' that indicates the needle is now past the dura mater . The arachnoid membrane and the dura mater exist in flush contact with one another in the living person's spine due to fluid pressure from CSF in the subarachnoid space pushing the arachnoid membrane out towards the dura . Therefore, once the needle has pierced the dura mater it has also traversed the thinner arachnoid membrane . The needle is then in the subarachnoid space . The stylet from the spinal needle is then withdrawn and drops of cerebrospinal fluid are collected . The opening pressure of the cerebrospinal fluid may be taken during this collection by using a simple column manometer . The procedure is ended by withdrawing the needle while placing pressure on the puncture site . The spinal level is so selected to avoid spinal injuries . In the past, the patient would lie on their back for at least six hours and be monitored for signs of neurological problems . There is no scientific evidence that this provides any benefit . The technique described is almost identical to that used in spinal anesthesia, except that spinal anesthesia is more often done with the patient in a seated position . </P> <P> The upright seated position is advantageous in that there is less distortion of spinal anatomy which allows for easier withdrawal of fluid . Some practitioners prefer it for lumbar puncture in obese patients, where lying on their side would cause a scoliosis and unreliable anatomical landmarks . However, opening pressures are notoriously unreliable when measured in the seated position . Therefore, patients will ideally lie on their side if practitioners need to measure opening pressure . </P>

Where is the needle inserted for a spinal tap