The filum terminale ("terminal thread"), is a delicate strand of fibrous tissue, about 20 cm in length, proceeding downward from the apex of the conus medullaris. It is one of the modifications of pia mater. It gives longitudinal support to the spinal cord and consists of two parts:

• The upper part, or filum terminale internum, is about 15 cm long and reaches as far as the lower border of the second sacral vertebra. It is continuous above with the pia mater and contained within a tubular sheath of the dura mater. In addition, it is surrounded by the nerves forming the cauda equina, from which it can be easily recognized by its bluish-white color.
• The lower part, or filum terminale externum, closely adheres to the dura mater. It extends downward from the apex of the tubular sheath and is attached to the back of the first segment of the coccyx in a structure sometimes referred to as the coccygeal ligament.