THE NERVOUS SYSTEM 



1313 



Filum Terminale. — This delicate glistening thread is also known 

 as the central ligament of the cord. It Ues in the median hne 

 between the lumbar, sacral, and coccygeal nerves of either side, 

 which constitute the cauda equina, and it extends from the apex 

 of the conus meduUaris to the back of the first coccygeal segment. 

 It is about 6 inches in length. As low as the back of the body of 

 the second sacral segment it is situated within the theca, but at 

 that level it pierces the theca, from which it receives an investment, 

 and then passes to be attached to the back of the first coccygeal 



^■xrtJ — Fiist Lumbar Vertebra 



Conns MeduUaris 



- Intratbecal Portion of 

 Filum Terminale 



First Sacral Vertebra 



-' Eztiathecal Portion of 

 Filiun Terminale 



Fig. 548. — Lumbar and Sacral Portions of Spinal Canal, showing 

 Lateral View of Conus Medullaris, Filum Terminals, and Theca 

 (Testut). 



segment, where it blends with the periosteum. The intrathecal 

 portion is known as the fdutn terminale internum, and the extrathecal 

 portion as the filum terminale externum. 



Structure. — The filum terminale internum in its upper half consists 

 of pia mater, prolonged from the conus medullaris of the spinal cord. 

 This encloses grey matter, within which, over about the upper third, 

 there is a continuation of the central canal of the cord. The lower 

 half consists chiefly of connective tissue. The filum terminale 

 extemimi is a mere fibrous filament, invested by a prolongation of 



83 



