162 PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



matter chiefly in the posterior horn of the same side and end around 

 tract cells. 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 horn 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 com- 

 missure, or in the motor cells of the anterior horn, thus making a 

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

 through the posterior gray commissure, to end in the gray matter 

 of the opposite side. The larger fibers lying nearest to the median 

 line enter the column of Burdach 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 column of Clarke. The axons of the cells in the 

 column of Clarke in turn pass out of the gray matter to constitute 

 the ascending path in the lateral column known as the dorsal cere- 

 bellar (Flechsig's) tract. 



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. 62. 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, there- 

 fore, which runs upward in the column of Burdach (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 em- 

 bryo, but we have little exact information concerning their numeri- 

 cal value in the adult. The schema given in Fig. 71 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 

 Columns. The posterior columns 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 

 from fibers that arise from tract cells in the gray matter of the cord 

 itself. . It is convenient to speak of the former group as exog- 

 enous fibers, using this term to designate nerve fibers which 

 arise from cells placed outside the cord; and the latter group as 



