SECT. III.] NERVOUS SYSTEM. 223 



Besides the fibres of the motor roots, which, in the cord, arc 

 directly continued into the elements of the anterior and lateral 

 columns, others, according to recent investigations, appear to exist, 

 which stand in connection with the nerve-cells, and, accordingly, 

 seem thus to take their origin in the spinal marrow itself. 



The posterior nerve-roots (fig. 93, k) pass horizontally, or some- 

 what obliquely upwards, from the sulcus lateralis posterior' 

 through the longitudinal fibres of the region of conjunction of the 

 lateral and posterior columns, and through the latter, in which 

 they run generally in a beautifully arched manner, as far as the 

 posterior horns. Here they separate into larger and smaller bun- 

 dles (of 001'" to o - 02'"), and each separately is continued, di- 

 rcetly and without entering into any direct connection with nerve- 

 cells through the substantia gelatinosa into the substantia grisea. 

 In the latter, they pursue different directions. One set of them 

 bend in a curved manner, or nearly at a right angle, upwards and 

 downwards (Clarke), run farther longitudinally in the most poste- 

 rior part of the substantia grisea, close in front of the substantia 

 gelatinosa, and seem partly to join the posterior and lateral 

 columns in order to extend further as longitudinal fibres, but 

 partly to enter again the grey substance in a horizontal direc- 

 tion. A second part of the sensitive roots penetrate always in 

 bundles further forwards between the just mentioned longitudinal 

 bundles in the grey substance itself, and are finally lost in the 

 anterior horns, in the grey or posterior, and the anterior commis- 

 sure. The grey commissure stands in connection with the sensitive 

 roots, but its fibres also spread out in the direction of the lateral 

 white columns and the anterior grey horns. 



In the substantia gelatinosa, the fibres of the sensitive roots never 

 measure more than 0-004'"; m tne g re y substance, cooi'" to 

 0"003'"; in the grey commissures, only 00008'" to o - ooi2"'; and 

 in the posterior and lateral columns again, 0*0012 to 0.004'". 

 Several authors are of opinion, that certain of the fibres of the 

 sensitive roots are also connected with the nerve-cells in the spinal 

 marrow ; still this point is not nearly so well ascertained as in the 

 case of the anterior roots. Besides the fibres connected with the 

 motory and sensitive nerves, there are seen in the grey substance 

 a good many nerve-tubes, which cannot be directly traced to the 

 roots, although they may possibly belong to them. 



The central canal, which some authors, as Stilling and Wagner, 

 regard as constant, seems to be sometimes obliterated in the adult. 

 When it exists, it is well marked in the entire length of the cord 



