270 S. WALTER HANSON 



projecting points and this band form part of the constricting ring. 

 Upon the surface of this band and separating it from the medul- 

 lated fibers of the bundle can be seen a layer of closely packed 

 non-medullated fibers, b. They can be followed for only a short 

 distance at b as they arch over the constricting band but between 

 b and c they can be traced over this band into Lissauer's tract. 

 Along the ventral surface {d) of the extra-medullary part of the 

 bundle and along the connective tissue septum (e) separating this 

 from the next root bundle the non-medullated fibers have accumu- 

 lated. Along the line a, b, c, d, e, there is indicated a peripheral 

 layer of non-medullated fibers which a little more than half sur- 

 rounds the bundle of entering medullated fibers. Whether a 

 similar layer is present on the dorso-medial surface of the radicle 

 it is impossible to say, since it is not likely that the fibers would 

 take the stain in this superficial position if they were present. 



The general principles which govern the entrance of the non- 

 medullated '■fibers into the cord are well illustrated by the radicle 

 from the seventh cervical dorsal root which is shown at three 

 levels in figures 9, 10 and 11. The lettering in these three figures 

 is the same as in figure 1 from the same segment and to which 

 reference should be made for orientation. Figure 9 is from a 

 section just above the level of entrance of the radicle in ques- 

 tion. The medullated fibers of this rootlet are seen cut obliquely 

 at a, and at b is indicated the tract of Lissauer. At 1 is seen a 

 bundle of non-medullated fibers which are arching ventro-laterally 

 over the obhquely coursing medullated fibers, a, of the entering 

 rootlet. Dorso-laterally bundle 1 is CQutinuous with bundle 2 

 of more vertically running fibers. A study of serial sections 

 shows that bundle 1 is derived from two sources. First there are 

 upon the upper surface of bundle a, as it enters the cord, a large 

 number of non-medullated fibers which turn upward and then 

 ventro-laterally to enter into the formation of bundle 1. A few 

 of the uppermost of these are seen at 3. The second group of 

 fibers entering into bundle 1 are derived from bundle 2. 



Tracing the same structures downward through the series we 

 find them arranged as in figure 10. Here bundle a, composed 



