| While it seems somewhat counterintuitive that the spinal cord would end at the level of the first or second lumbar vertebra, (as is often the case in anatomy) the explanation lies in the embryological development of the spinal cord. In embryos, the spinal cord occupies the entire length of the vertebral canal, but during development the vertebral column grows faster than the spinal cord. Consequently, in an adult, not all spinal cord segments are found at the level of the vertebra of the same number. Because of the discrepancy between the spinal cord segment and the vertebral level, the spinal nerve roots of the more caudal spinal cord segments are rather long and descend for a significant distance before reaching their respective intervertebral foramina. It is these spinal nerve roots that compose the cauda equina beyond L1/L2. The fact that the spinal cord ends at L1/L2 is very useful in clinical practice in that it allows for spinal taps to be performed to sample CSF without the risk of puncturing the spinal cord. Below L1/L2 but above S2, there is still CSF contained in the dural sac (in what is called the lumbar cistern), but instead of a spinal cord there are only nerve roots, which float away from the positive pressure of a needle entering the space. Extreme caution must be taken in performing a spinal tap on an infant or a young child whose spinal cord may extend as far as vertebra L4 because their vertebral column has not reached its maximum length.
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