Allometric growth of the spinal cord
In the eighth week the spinal cord with its spinal cord skins (pia mater, arachnoïdea and dura mater) is the same length as the vertebral canal.
The sphere-shaped end of the spinal cord, the conus medullaris, is to be found at the level of the first coccyx vertebra. The spinal nerves thus emerge in a right angle from the spinal cord and leave the vertebral canal through the opposite foramina intervertebralia
From the 4th month the longitudinal growth of the spinal cord slows while the growth of the spinal column continues unabated. This allometric (uneven) growth leads to the development of the cauda equina ("horse tail"). It arises through the bundling of the dorsal and ventral fila radicularia that emerge from the lower spinal cord segments.
Since the spinal column and meninges extend faster than the spinal cord, the lumbosacral spinal nerves within the vertebral canal must for now stretch from its spinal cord segment longitudinally caudally until it reaches its corresponding intervertebral foramen. The conus medullaris is attached over a long filum terminale internum out of pia mater at the lower end of the dural sac.
At birth the conus medullaris lies at the level of the third lumbar vertebra (L3), in adults at the level of the first lumbar vertebra (L1/L2).
In adults the dural tube and the arachnoidea extend to the second sacral vertebra (S2). The dural tube is attached to the coccyx via the filum terminale externum.