770 A MANUAL OF PHYSIOLOGY 



the postero-external column, sweeping obliquely through it to 

 gain the tract of Goll. In this tract some of them run right 

 on to the medulla oblongata, to end by arborizing among the 

 cells of the nucleus gracilis. Other fibres, both of Coil's and of 

 Burdach's tract, end at various levels in the cord, their collaterals, 

 and ultimately the main branches themselves, coming into rela- 

 tion 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 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). 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 rela- 

 tion 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 inter- 

 medio-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 degeneration 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 arboriza- 

 tions, some of which, as previously stated, represent the termina- 

 tions of posterior root-fibres and of their collaterals. The 



