160 



PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



Of these tracts, those of Burdach and Goll, Flechsig and Gowers, 

 represent ascending or sensory paths, while the direct and the 

 crossed pyramidal tracts form a related descending or motor path. 

 It will be convenient to describe first the connections and physio- 

 logical significance of these tracts and then refer to the newer work 

 concerning less definitely established ascending and descending 

 paths. 



The Termination in the Cord of the Fibers of the Posterior 



Root. All sensory fibers 

 G c- from the limbs and trunk 



enter the cord through the 

 posterior roots. Inasmuch 

 as these roots are superfi- 

 cially connected with the 

 posterior columns, the older 

 observers naturally sup- 

 posed that these columns 

 form the pathway for 

 sensory impulses in the 

 cord. That this supposi- 

 tion is not entirely cor- 

 rect was proved by experi- 

 mental physiology. Section 

 of the posterior columns 

 causes little or no obvious 

 loss of sensations in the 

 parts below the lesion. 

 Histological investigation 

 has since shown that only 

 a portion of the fibers en- 

 tering through the posterior 

 root continues up the cord in 

 the posterior column; some 

 and indeed a large propor- 

 tion of the whole number 

 enter into the gray matter 

 and end around tract cells, 

 whence the path is con- 

 tinued upward by the axons of these latter cells in the lateral or 

 anterolateral columns. The several ways in which the posterior 

 root fibers may end in the cord are indicated in Fig. 71. 



The posterior roots contain fibers of different diameters and 

 those of smallest size (1) are found collected into an area known 

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

 and the tip of the posterior horn. These fibers enter the gray 



Fig. 71. Schema to show the terminations 

 of the entering fibers of the posterior root: 1, 

 Fibers entering zone of Lissauer and terminating 

 in posterior horn ; 2, fiber terminating around a 

 tract cell whose axon passes into white column of 

 same side ; 3, fiber terminating around a tract cell 

 whose axon passes to opposite side (commissural 

 cell) ; 4, fiber terminating around motor cell of 

 anterior horn (reflex arc) ; 5, fiber terminating 

 in tract cell of column of Clarke; 6, fiber (exog- 

 enous) passing upward in posterior column to 

 terminate in the medulla oblongata. 



