In the third month of development the spinal cord extends the entire length of the embryo, and spinal nerves pass through the intervertebral foramina at their level of origin (Fig. 19.13A). With increasing age, however, the vertebral column and dura lengthen more rapidly than the neural tube, and the terminal end of the spinal cord gradually shifts to a higher level. At birth, this end is at the level of the third lumbar vertebra (Fig. 19.13C). As a result of this disproportionate growth, spinal nerves run obliquely from their segment of origin in the spinal cord to the corresponding level of the vertebral column. The dura remains attached to the vertebral column at the coccygeal level.
In the adult, the spinal cord terminates at the level of L2 to L3, whereas the dural sac and subarachnoid space extend to S2. Below L2 to L3, a threadlike extension of the pia mater forms the filum terminale, which is attached to the periosteum of the first coccygeal vertebra and which marks the tract of regression of the spinal cord. Nerve fibers below the terminal end of the cord collectively constitute the cauda equina. When cerebrospinal fluid is tapped during a lumbar puncture, the needle is inserted at the lower lumbar level, avoiding the lower end of the cord.
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