55 



THE NERVOUS SYSTEM 



Course of the Fibers of the Spinal Nerve Roots. The Anterior 

 Roots. The anterior roots leave the cord in several bundles, which may be 

 called: i, Internal; 2, Middle; 3, External. All have their origin from the 

 groups of multipolar cells in the anterior columns. The internal fibers are 

 originated partly in the internal group of nerve cells of the anterior columns 

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

 missure to cells of the anterior column 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 column. The fibers, as 



FIG. 365. Section of the Spinal Cord, Showing the Grouping of Nerve Cells and the 

 Course of Nerve Fibers Entering in Posterior and Anterior Roots. Numerals 1-6 are 

 different types of the sensory fibers. Their collaterals, 7-12, connect with different regions 

 of the gray substance; 13 and 14, collaterals from descending tracts of the cord; r.a., 

 anterior root; a, anterior motor cells; b, cells contributing fibers to the various tracts; 

 c, commissural cells; d, Golgi cells. (After Lenhossek.) 



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

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

 up for a shorter or longer distance. This upper branch sometimes 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 of the posterior 

 roots are divided into two sets, an internal or median, an external 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 column, and come in relation with its cells. From 



