SPINAL CORD AS A PATH OF CONDUCTION. 167 



as the tract of Lissauer, lying between the periphery of the cord 

 and the substantia gelatinosa of the posterior column. These fibers, 

 partly medullated and partly non-medullated, terminate chiefly 

 round the small cells found in the substantia gelatinosa.* The 

 larger fibers of the root lying to the median side fall into two groups : 

 Those lying laterally (2, 3, 4) enter the gray matter of the posterior 

 column and end in tract cells (2) whose axons are distributed to 

 the same side of the cord, or in tract cells whose axons (3) pass 

 to the other side through the anterior white commissure, or (4) 

 in the motor cells of the anterior column, thus making a typical 

 reflex arc. Some of the fibers of this group may also pass through 

 the posterior commissure, to end in the gray matter of the oppo- 

 site side. The larger fibers lying nearest to the median line 

 enter the fasciculus cuneatus and run forward in the cord, some of 

 them (6) continuing upward to the medulla, and some of them 

 (5), after a shorter course, turning into the gray matter to end in 

 the cells of the dorsal nucleus. The axons of the cells in the 

 dorsal nucleus in turn pass out of the gray matter to constitute 

 the ascending path in the lateral funiculus, known as the cerebello- 

 spinal fasciculus. 



This general outline of the mode of ending in the cord of the 

 fibers of the posterior root is complicated further by the fact that 

 these fibers are supposed to give off collaterals after entering the 

 cord. The course of the typical fiber in the posterior root is 

 represented in Fig. 66. According to this diagram, the root 

 fiber, after entering the cord, makes a Y or T division, one branch 

 passing downward or posteriorly for a short distance, the other, 

 longer division, passing upward or anteriorly. Each of these 

 main stems may give off one or more lateral branches, sensory 

 collaterals. A main stem, therefore, which runs upward in the 

 fasciculus cuneatus (6) to terminate in the medulla oblongata 

 may give off collaterals at various levels which terminate in the 

 gray matter of the cord, either around tract cells or around the 

 anterior root cells, forming in the latter case a simple reflex arc. 

 The existence of collaterals upon the root fibers within the cord 

 has been demonstrated in the human embryo, but we have little 

 exact information concerning their numerical value in the adult. 

 The schema given in Fig. 76 must, therefore, be accepted as an 

 entirely diagrammatic representation of the chief possibilities 

 of the mode of ending of the fibers of the posterior root by way 

 of their collaterals as well as by way of the main stems. 



Ascending (Afferent or Sensory) Paths in the Posterior 



Funiculi. — The posterior funiculi are composed partly of fibers 



derived directly from the posterior roots (6 in schema) and arising, 



therefore, from the cells in the posterior root ganglia, and partly 



*Ranson, "The American Journal of Anatomy," 16, 97, 1914. 



