lissauer's tract in the cat 271 



chiefly of medullated fibers, is seen entering the cord. At 4 a 

 bundle of non-medullated fibers can be seen running over the 

 constricting ring to pass directly into the tract of Lissauer on 

 entering the cord. At 5 are seen a few non-medullated fibers 

 gathering along a line which represents the beginning of a line of 

 separation between two fascicles of the root. Bundle 2 is larger 

 here than in the preceding section and is placed between bundle 

 a and another bundle more dorsally situated. It is composed of 

 fibers which have separated out from these two bundles and 

 especially from bundle a, and, instead of running directlj^ across 

 the bundle of medullated fibers to reach Lissauer's tract, have 

 turned upward or downward on the dorsal surface of bundle a. 

 In this way a vertical bundle of large size is formed, which traced 

 upward is seen to arch ^-entro-laterally over the upper surface of 

 bundle a to form part of bundle 1, figure 9, and so to reach the 

 tract of Lissauer. Traced downward, the descending fibers which 

 enter into its composition are seen to turn ventro-lateralh" below 

 bundle a to run into Lissauer's tract (fig. 11). Here they form 

 part of a rather wide band of fibers on the medial and under 

 surface of bundle a. A part of this band is seen at 6, figure 11. 

 But the band is much wider than is indicated in the figure. In 

 the succeeding sections the band is seen to underlie bundle a 

 to the very edge of the cord. ,This band is composed in part of 

 fibers derived from bundle 2, figure 10, and in part from fibers 

 entering the cord as a laj^er upon the under surface of bundle a. 

 It thus appears that the non-medullated fibers separate out 

 from the bundles of entering root fibers. They occupy the pe- 

 ripher}' of the bundles as these enter the cord and then take the 

 route of least resistance to reach Lissauer's tract. In th'e case 

 of the fibers on the dorso-medial surface of such entering root 

 bundles this path of least resistance is usually around the bundle 

 of entering medullated fibers rather than through it. For this 

 reason these fibers form bundles arching over or under the bundle 

 of medullated fibers. For those non-medullated fibers occupying 

 the ventro-lateral portion of the surface of the entering dorsal 

 root bundle the path to Lissauer's tract is direct (fig. 10, 4). In 



