6 



THE SPINAL CORD. 



in the middle line amongst the nerves composing the cauda equina, and, reaching 

 the lower end of the sheath opposite to the second sacral vertebra, perforates the dura 

 mater, and receiving an investment from it, passes on to be attached with this to the 

 periosteum of the lower end of the sacral canal, or to the back of the coccyx. It is 

 a prolongation of the pia mater, enclosing for about half its length an enlarged 

 continuation of the central canal of the cord (see p. 9), with a little grey matter 

 near the upper end. Below the termination of the canal, the filum is mainly com- 

 posed of connective tissue, with blood-vessels prolonged 

 from the anterior spinal vessels, and on either side there 

 run in it three or four small bundles of medullated nerve- 

 fibres, some of which" have a few ganglion-cells. These 

 nerve-bundles are regarded by Rauber as representing 

 rudimentary coccygeal nerve-roots. They have no con- 

 nection with the coccygeal nerves proper. 



The filum terminale is distinguished by its silvery 

 hue from the nerves among which it lies. 



Fig. 6. VlEW FROM BEHIND OF THK LOWER END OF THE SPINAL 



COED WITH THE CAUDA EQUINA AND DUKAL SHEATH. (Allen 



Thomson.) \ 



The sheath has been opened from behind and stretched towards 

 the sides ; on the left side all the roots of the nerves are entire ; on 

 the right side both roots of the first and second lumbar nerves are entire, 

 while the rest have been divided close to the place of their passage 

 through the sheath. The bones of the coccyx are sketched in their 

 natural relative position to show the place of the filum terminale 

 and the lowest nerves. 



a, placed on the posterior median fissure at the middle of the 

 lumbar enlargement of the cord ; b, b. the terminal filament, drawn 

 slightly aside by a hook at its middle, and descending within the 

 dural sheath ; 6', b', its prolongation beyond the sheath and upon 

 the back of the coccygeal bones ; c, the dural sheath ; d, double 

 foramina in this for the separate passage of the ventral and dorsal 

 (anterior and posterior) roots of each of the nerves ; e, ligamentum 

 denticulatum ; Dx, and DXII, the tenth and twelfth thoracic (dorsal) 

 nerves ; Li, and Lv, the first and fifth lumbar nerves ; Si, and Sv, 

 the first and fifth sacral nerves ; Ci, the coccygeal nerve. 



Fissures. The spinal cord is incompletely divided 

 into a right and left half by two fissures which pass in 

 from the middle of the anterior and posterior surfaces, 

 and penetrate through the greater part of its thickness. 

 Of these two median fissures the anterior or ventral (fig. 

 7, 1) is wider and therefore more distinct than the pos- 

 terior or dorsal, although it does not, in most parts, 

 penetrate to more than one-third the thickness of the 

 cord, while the posterior fissure may reach more than 

 half-way from back to front. The anterior contains a 

 fold of the pia mater and also many blood-vessels, which 

 are thus conducted to the centre of the cord. At the 

 bottom of this fissure is a transverse connecting portion 

 of white substance named the anterior or white com- 

 missure. 



The posterior (fig. 7, 2) is not an actual fissure, for, 

 although the lateral halves of the cord are quite separate 

 dorsally, there is not so much a fold of the pia mater between them, as merely a 

 of connective tissue and blood-vessels prolonged from that membrane which 



