268 S. WALTER RANSON 



collaterals are given off from the mediillated fibers and take a 

 more or less oblique course toward the posterior horn. But these, 

 for the most part, take a much lighter stain than the non-medul- 

 lated fibers. Occasionally medium sized bundles of non-medul- 

 lated fibers are seen in the entering root zone as at e in figure 10, 

 but such bundles usually have a direction nearly at right angles 

 to the medullated fibers, and are making their way toward the 

 tract of Lissauer. We shall see that the majority of non-medul- 

 lated fibers separate themselves from the medullated just before 

 the entrance into the cord. The scattered non-medullated fibers 

 as well 8ts the bundles of such fibers in the entering root zone, 

 were delayed in their separation from the medullated fibers; but 

 most of them finally find their way into the tract of Lissauer. 

 It is possible, however, that a few of the non-medullated fibers 

 pass medially to the tract of Lissauer and enter the fasciculus 

 cuneatus. 



A dorsal root as it enters the cord is broken up into a large 

 number of fila radicularia or rootlets. As each rootlet enters the 

 cord it is surrounded and constricted by an encircling band of 

 pia. Shortly before the rootlet reaches this constricting band, 

 the non-medullated fibers, which nearer the ganglion have been 

 distributed quite uniformly throughout the root, separate out from 

 the medullated ones and come to lie either at the periphery of 

 the radicles or along septa which divide the radicles into smaller 

 bundles. Li this way large flat bundles of non-medullated fibers 

 are formed; often a thin layer of such fibers is seen making a 

 complete tubular sheath at the periphery of the radicle. In serial 

 sections it is possible to trace these compact bundles into the 

 cord and see that they enter Lissauer's tract. It is this early 

 separation of these fibers from the main mass of the radicle that 

 causes the entering root zone to be composed almost entirely of 

 medullated fibers. 



We will now take several typical instances and show how these 

 fibers can be traced into Lissauer's tract. In the first sacral 

 segment (figs. 5, 6 and 7) the radicles are of good size and are 

 divided by connective tissue septa, running in a general antero- 

 posterior direction, into smaller fascicles which are displaced 



