THE CENTRAL NERVOUS SYSTEM 



681 



these appearances seems to be as follows. It may be seen in pre- 

 parations of the cord impregnated by Golgi's method that the fibres 

 of the posterior roots soon after their entrance into the cord divide 

 into two processes, one of which runs up and the other down in the 

 posterior column, or in the adjoining portion of the posterior horn. 

 From both of these collaterals are given off at intervals. The 

 descending branches run downwards only for a short distance, 

 and the degeneration in the comma tract seen after section of 

 the cord is due to the division of these branches. Many of the 

 ascending branches pass up for a short distance in the postero- 

 external column, sweeping obliquely through it to gain the tract of 



FIG. 244. POSTERIOR ROOTS 



ENTERING SPINAL CORD (at 



the left of the figure). (From a 

 preparation stained with aniline 

 blue-black.) 



FIG. 245. BRANCHING OF POS- 

 TERIOR ROOT FIBRES IN CORD 

 (DONALDSON, AFTER CAJAL). 

 Collaterals, Col, are seen coming 

 off from the two main branches of 

 the root-fibres, D R, and ending 

 in arborisations. 



C, C, cells in the grey matter of the 

 cord, whose axons also give off col- 

 laterals. 



Goll. In this tract some of them run right on to the medulla 

 oblongata, to end in a collection of grey matter, the nucleus gracilis. 

 Others must end at various levels in the cord, their collaterals, and 

 ultimately the main branches themselves, coming into relation with 

 nerve-cells in the grey matter. When the cervical posterior roots 

 are cut, many of the degenerated fibres remain in Burdach's column 

 up to the medulla, where they terminate in the nucleus cuneatus. 

 In the posterior column, then, the fibres of the posterior roots 

 which do not form synapses with nerve-cells in the spinal cord 

 are arranged in layers, the fibres from the lower roots being 



