Filum terminale externum

The lower portion of the filum terminale (a pia mater extension) is known as the filum termianle externum, or the coccygeal ligament. It extends downward from the S2 vertebral level and attaches to the back of the first segment of the coccyx.

In adults, the spinal cord ends at the L1 vertebral level, but the protective coverings of the spinal cord continue to descend beyond this point. One of these coverings, called the pia mater, extends from the tip of the conus medullaris all the way to the bony coccyx. This continuation of the pia mater is called the filum terminale. It's worth noting that the arachnoid membrane, along with the space beneath it, i.e. the subarachnoid space, ends at the S2 level. However, the filum terminale passes through it to reach the coccyx. As a result, it can be divided into two parts: the upper part, known as the filum terminale internum, which stretches from the tip of the conus medullaris to the S2 level, and the lower part, from S2 to the coccyx (beyond the subarachnoid space), called the filum termianle externum.