THE SPINAL CORD 



829 



Filum (filum terminate; nervus impar). The delicate terminal thread called 

 the filum, continuous with the tapered end of the conns, is about 24 cm. (10 inches) 

 in length. As far as the level of the second sacral vertebral segment it is enclosed, 

 together with the cauda equina, in the tapering sheath of the dura. Within the 

 dural sac it is called the filum internum, in contradistinction to the filum externum, 

 which is an attenuated process of connective and glia tissue closely invested 

 by a prolongation of the dura, which finally attaches to the periosteum of the dor- 

 sum of the coccyx. The filum externum occupies one-third of the total length 

 of the filum. Morphologically, the filum is the caudal representative of the 

 cord, and its intradural portion is usually accompanied by slender fascicles 

 of nerve fibres, which are rudimentary second and third coccygeal pairs of spinal 

 nerves. 



Dorsal nerve root. 

 Central canal. 



Nuclei of spongio- 

 blasts. 



'euroblasts. 



Processes of neuroblasts 

 growing out to form 

 ventral nerve root. 



Ventral column. 



Fin. 601. Section of spinal cord of a four weeks' embryo. (His.) 



Fissures and Grooves. The spinal cord is a bilaterally symmetrical structure 

 and exhibits a deep ventral fissure and a slight dorsal groove partially subdividing 

 the cord into the right and left halves. The ventral (antero-median) fissure 

 extends throughout the entire length of the cord, being shallower in the cervical 

 and thoracic portions (less than one-third of the sagittal diameter) than in the 

 lumbar portion. It is surfaced by a fold of pia which conveys the more im- 

 portant nutritive vessels to the depths of the cord substance. In the depth of this 

 fissure lies the white (ventral) commissure of the cord. The dorsal groove has 

 been regarded, erroneously, as being analogous to the ventral fissure. Unlike the 

 ventral fissure, however, the pia does not descend into its depths, but passes 

 continuously over it. The shallow groove marks the site of a septum made up 

 of neuroglial and ependymal elements. An actual groove is best demonstrable 

 in part of the lumbar cord and in the medulla oblongata, where it constitutes 

 a veritable fissure between the clavse. 



Each lateral half is marked by the lines of root attachment of the spinal nerves. 

 The dorsal nerve root fascicles enter the cord in linear series and in a depression 

 or true sulcus termed the dorsolateral fissure. The ventral nerve root fascicles 

 emerge, irregularly scattered, out of a greater circumferential area, and no true 

 ventrolateral fissure can be said to exist. The line of emergence of the outermost 

 fascicles is usually taken as an arbitrary boundary between the ventral and lateral 

 districts of the cord, while the dorsolateral fissure more distinctly maps off the 



