518 



THE NERVOUS SYSTEM 



of the same side; but some fibers can be traced through the anterior com- 

 missure to cells of the anterior cornu of the opposite side. 



The Posterior Roots. The fibers of the posterior roots enter the spinal 

 cord to the inner or median side of the posterior cornu. The fibers, 

 as soon as they reach the cord, divide in a fork-like fashion, one branch 

 passing down a short distance, about three centimeters, the other branch 

 passing up for a shorter or longer distance. This upper branch some- 

 times reaches the whole extent of the cord, but generally it extends over 

 only one or two segments of the cord. The divisions of the posterior root 

 fibers give off in their course numerous collaterals, figure 368. The fibers 



Fl&. 363. Section of the Spinal Cord, Showing the Grouping of Nerve-Cells and the Course of 

 Nerve Fibers Entering in Posterior and Anterior Roots. (After Lenhossek.) 



of the posterior roots are divided into two sets, an internal or median, an ex- 

 ternal or lateral group. The lateral set consists mostly of small fibers which 

 enter the cord opposite the tip of the posterior horn. The fibers pass in part 

 to the marginal column of Lissauer, where they ascend and descend; in part 

 they penetrate the posterior horn, and come in relation with its cells. From 

 the median set some fibers pass to Clarke's column, others pass by way 

 of the posterior commissure to the median cells of the other side. Others 

 pass through the gray matter to the anterior horn cells of the same side. 

 Besides this, they are connected through collaterals with the intrinsic cells of 

 the gray matter at different levels of the cord. One can realize that each 

 nerve root has, in this way, an effective grip upon a large extent of the cord. 

 This is seen well by studying figures 361 and 363. 



