TRACT OF LISSAUER 113 



accumulated on the upper, lateral and lower surfaces of the 

 rootlet. At l.p.y.r. in the illustration one sees a large bundle of 

 non-medullated fibers from the layer on the lateral surface of 

 the rootlet running over the constricting ring and into the tract 

 of Lissauer. A study of the two sections just above shows that 

 this layer of non-medullated fibers is continuous with that on 

 the upper surface of the rootlet. These non-medullated fibers 

 on the upper surface of the roolet enter the cord between the 

 constricting ring and the medullated fibers, and turning lateral- 

 ward enter the tract of Lissauer. Followed downward in the 

 series, a continuous layer of non-medullated fibers is seen passing 

 over the lateral part of the constricting ring and entering the 

 tract of Lissauer, just as in the figure given. On the under 

 surface of the radicle one sees the fibers forming this part of the 

 peripheral layer turning sharply lateralward underneath the 

 entering medullated fibers to reach the tract of Lissauer. 



It is very difficult to follow the course of the non-medullated 

 fibers in the thoracic segments because of the close relation of the 

 entering radicles to the tract of Lissauer. In the fifth lumbar 

 segment one sees that as the rootlet passes over the apical ac- 

 cumulation of neuroglia to reach the medial side of the tract of 

 Lissauer numerous small bundles of non-medullated fibers are 

 given off which run ventrally through the apical neuroglia into 

 this tract. It is this arrangement of the non-medullated fibers 

 which was seen in Golgi preparations of newborn animals by 

 Kolliker and others. The lateral part of the root, which they 

 describe as consisting of very fine axons, is shown by pyridine- 

 silver preparations to be still present in the adult and to take the 

 same course into Lissauer 's tract. The few fine medullated 

 fibers which take the same course are by no means numerous 

 enough to account for the lateral bundle of the dorsal root as 

 seen in Golgi preparations. With the exception of a few small 

 bundles of non-medullated fibers which are running toward the 

 substantia gelatinosa, the entering root zone is free from non- 

 medullated fibers — few, if any, enter the cuneate fasciculus. 



The obhque and horizontal fibers of the tract pass forward 

 into the substantia gelatinosa, and between this and the tract nu- 



THE AMERICAN JOURNAL OF ANATOMY, VOL. 16, NO. 1 



