164 LECTURES ON THE CENTRAL NERVOUS SYSTEM. 



columns convey upward all the fibres of any posterior root. 

 Many of the fibres pass into the gray matter directly after the 

 root has entered the spinal cord, and others pass first into the 

 posterior horn and then into the gray matter. Therefore, rela- 

 tively few of those fibres which entered the posterior columns 

 below are to be found in the upper part of the cord. Experi- 

 mentally we have ascertained that the area of degeneration due 

 to section of a posterior root becomes continually smaller as it 

 is traced upward. At the same time it moves inward. 



At the upper part of the spinal cord the fimiculus cuneatus 

 receives fibres which do not come directly from the posterior 

 roots. Their origin is uncertain. 



But, as you have been told before, all the sensory fibres do 

 not terminate in the cells of the spinal ganglia ; a part pass 

 through the ganglia apparently to end in the cells of the column 

 of Clarke, their first terminal station. These fibres remain 

 normal when the root is cut across, for the reason that they are 

 not divided from their own cells in that experiment. They do 

 degenerate, however, if the continuity of the spinal cord itself 

 is broken. In fact, in the latter case (Fig. 99), we see still 

 another tract of fibres degenerate. It is that shown in the 

 peripheral portion of the lateral columns at 6, in Fig. 100. 

 This tract can be traced as far as the worm of the cerebellum. 

 It is owing to embryological researches (Flechsig) that we are 

 able to recognize and distinguish this direct lateral cerebellar 

 tract from other tracts in the lateral column. In the first weeks 

 of life, before the pyramidal tract has become medullated, the 

 cerebellar tract lies like a delicate white border along one-half 

 of the periphery of the lateral column. 



Just in front of the cerebellar tract lies a tract which projects like a wedge 

 from the periphery of the lateral column toward the centre. It is not indicated 

 in the diagrams, but is apparently independent, for it may degenerate (upward) 

 by itself. It is called the fasciculus antero-lateralis. 



By the study of secondary degeneration and embryology, 

 therefore, we have discovered the following systems of white 



