844 THE CENTRAL NERVOUS SYSTEM 



in the nucleus cuneatus. In the posterior column, then, the 

 numerous fibres of the posterior roots which do not end in the spinal 

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

 nearest the median fissure (in the postero-median column), and those 

 from the higher roots farthest away from it (in the postero-external 

 column. Thus, in a section through the upper cervical region Coil's 

 column is almost entirely composed of fibres from the posterior limb, 

 while the column of Burdach consists of fibres from the anterior 

 limb. Other collaterals from the posterior root-fibres, and many 

 of the main root-fibres themselves, run into the anterior horn and 

 terminate in arborizations around its cells; some pass into the 

 posterior horn, and doubtless come into relation with its scattered 

 cells and, in the dorsal region, with the cells of Clarke's column. 

 Some of the posterior root-fibres and their collaterals also form 

 synapses with the cells of the intermedio-lateral tract. Other 

 collaterals and probably some axons cross the middle line in the 

 anterior and posterior commissures and end in the grey matter of the 

 opposite side. 



Connections of the Direct or Dorsal Cerebellar Tract. Since the 

 dorsal or direct cerebellar tract does not degenerate after section of 

 the posterior nerve-roots, but does degenerate above the level of the 

 lesion after section of the spinal cord, the nerve-cells from which its 

 axons arise must be situated somewhere or other in the cord. Now, 

 it has been observed that the vesicular column of Clarke first becomes 

 prominent in the lower dorsal region, and that in this same region 

 the direct cerebellar tract begins. Atrophy of the cells of Clarke's 

 column has sometimes in disease been shown to accompany de- 

 generation of the direct cerebellar fibres. After an experimental 

 lesion of these fibres in animals, some of the cells of the vesicular 

 column show the changes in the Nissl bodies and the other changes 

 which we have already described as occurring in nerve-cells whose 

 axons have been cut. After two or three months these cells may 

 be found almost completely atrophied (Schafer). Finally, axis- 

 cylinder processes have been seen sweeping out from Clarke's 

 column into the direct cerebellar tract (Mott). The evidence, then, 

 is complete that the cells of origin of this tract are in Clarke's column. 

 Clarke's cells are surrounded by arborizations, some of which, as 

 previously stated, represent the terminations of posterior root-fibres 

 and of their collaterals. The neurons whose axons run in the dorsal 

 cerebellar tract are therefore the second link in an afferent path. 

 The direct cerebellar tract runs right up to the cerebellum through 

 the restiform body, without crossing and without being further 

 interrupted by nerve-cells. The restiform body ends partly in the 

 dentate nucleus of the cerebellum, partly in the vermis, and among 

 the fibres which end in the vermis are those of the direct cerebellar 

 tract. In the dorsal cerebellar tract there is a definite stratification 



