Cauda equina and filum terminale seen from behind. The dura mater has been opened and spread out, and the arachnoid has been removed. (Filum terminale labeled at center right.)
Sagittal section of vertebral canal to show the lower end of the medulla spinalis and the filum terminale.
Li, Lv. First and fifth lumbar vertebra.
SII Second sacral vertebra.
1. Dura mater.
2. Lower part of subarachnoid cavity.
3. Lower extremity of medulla spinalis.
4. Filum terminale internum.
5. Filum terminale externum.
6. Attachment of filum terminale to first segment of coccyx.
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 most inferior of the spinal nerves, the coccygeal nerve leaves the spinal cord at the level of the conus medullaris, superior to the filum terminale. However, adhering to the outer surface of the filum terminale are a few strands of nerve fibres which probably represent rudimentary second and third coccygeal nerves. Furthermore, the central canal of the spinal cord extends 5 to 6 cm beyond the conus medullaris, downward into the filum terminale.