30 ATLAS OF NERVE CELLS 



The neuraxons which enter the cord from the spinal ganglia pass in partly at the apex 

 of the posterior horn, and partly through the column of Burdach in the portion adjacent to 

 the posterior horn, hence known as the root-zone of the posterior column. On entering the 

 cord each neuraxon divides into two parts, which turn at an angle of 150 to the original 

 direction of the neuraxon, making a Y-shaped division ; and these pass up and down the 

 cord. Plate XII. shows this peculiar division of the fibres, as seen in a longitudinal section 

 of the cord through the root-zone. The fibres which pass downward appear to be short, 

 rarely extending downward more than three centimetres. They pass together in a bundle 

 which lies in the antero-lateral part of the column of Burdach, near the posterior horn, and 

 has been termed the comma-shaped bundle, from the shape of its cross-section. Those which 

 pass upward are of indefinite length, many in fact extending all the way to the medulla. 

 Both divisions give off collaterals which enter the gray matter. In a long organ like the 

 cord, which receives 31 pairs of nerves, it is evident that the higher the level, the greater 

 the number of these fibres in a cross-section ; for in the cervical region are to be found 

 fibres from every level below it. It has been found that a definite order of arrangement of 

 these long ascending fibres takes place. As each successive nerve root comes into the root- 

 zone, from below upward, it enters near to the posterior horn, and displaces the nerve fibres, 

 already ascending, inward and backward toward the posterior septum. Hence in the cervical 

 cord the fibres from the cervical nerves lie next the posterior horn, those from the dorsal 

 cord are further inward, and those from the lumbar and sacral cord are crowded against the 

 posterior septum. 



The arrangement of columns in the cord and their connections with the various cells are 

 shown in the diagram (Fig. 4). A, B, and C are three levels of the spinal cord. I., II., and 

 III. are three sensory nerve roots entering the cord at level B. They are seen to bifurcate, 

 to send their branches downwards and upwards to levels 4 and C, and to send collaterals 

 into the posterior horns at different levels, some of which terminate about the intrinsic cells 

 of the same side (/t) t and some of which cross in the gray commissure, to terminate about 

 the intrinsic cells of the opposite side (/). From these cells a neuraxon proceeds outward to 

 the antero-lateral ascending tract of Gowers (G), which passes upward to the brain. Cell d, 

 representing the Clarke column of cells, is also shown at the three levels, on the right side 

 only, and its neuraxon ascending the cord in the direct cerebellar column (D). Collaterals 

 from the posterior nerve roots terminate around these cells, as has been already shown in 

 Fig. i, page 16. These collaterals are shown on the left side only in this figure for the 

 sake of clearness. It is thus evident that impulses entering the spinal cord by the pos- 

 terior nerve roots are not only distributed to the gray matter of the cord itself, as shown 



