170 LECTURES ON THE CENTRAL NERVOUS SYSTEM. 



Knowledge obtained by experiment and at the bedside, confirms these late 

 discoveries. If one-half of a spinal cord is divided the sensation of the skin is 

 lost below the point of division, and that not on the same but on the opposite 

 side. We had previously been at a loss to harmonize this fact with our knowl- 

 edge that posterior root-fibres were continued directly (without decussation) in 

 the posterior columns. The phenomenon, however, was at once explained when 

 we discovered that a considerable portion of each root passed, in some way not 

 clearly understood, to the opposite side shortly after entering the cord. 



You will at once be struck by the remarkable analogy ex- 

 isting between the central fibres of the anterior and posterior 

 roots. In the former we have (in relation to the root) a direct 

 tract (the lateral pyramidal tract) and a crossed tract (the 

 anterior pyramidal tract). 



The principal distinction between the motor and sensory 

 tracts consists in the different mechanisms which are interposed 

 between the cells from which they originate and the point of 

 their decussation. 



While there is good cause to believe that this mechanism is 

 very simple in the anterior horn, apparently residing in the cell 

 itself, we see, in the case of the posterior roots, a whole net-work, 

 itself filled with small cells, thrust in between the cells of the 

 spinal ganglia and the decussation. 



We shall have occasion again to refer to this important 

 matter when we study the oblongata. 



We must now consider that portion of the posterior root 

 which does not arise from the cells of the spinal ganglion but is 

 connected with the columns of Clarke. Pathological observa- 

 tions go to show that this portion has probably nothing to do 

 with conveying sensory impressions from the skin. The central 

 connecting fibres from the columns of Clarke do not pass 

 upwart^ in the general sensory tract. They send off fibres which 

 pass out laterally from the gray matter, and reach the direct 

 lateral cerebellar tracts in the periphery of the white substance, 

 and in these tracts they pass to the cerebellum. These fibres 

 are apparently of importance in the co-ordination of movements. 

 We not only sec disturbances in co-ordination of gait and 



