166 



PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



bundles are composed of mixed ascending and descending fibers 

 which have not yet been separated satisfactorily into specific tracts, 

 although some attempts in this direction have been made, as is 

 described briefly below. At the apex of the posterior horn, or 

 column of gray matter, there is found a small group of fibers known 

 as the tract of Lissauer, which is composed apparently chiefly of 

 ascending fibers. 



The Termination in the Cord of the Fibers of the Posterior 

 Root. — All fibers conveying afferent impulses from the skin of the 

 limbs and trunk, from the muscles and joints, and from the vis- 

 cera and internal mem- 

 .(> T branes enter the cord 



through the posterior roots. 

 Inasmuch as these roots 

 are superficially con- 

 nected with the pos- 

 terior funiculi, the older 

 observers naturally sup- 

 posed that this portion 

 of the white matter of the 

 cord forms the pathway for 

 sensory impulses passing to 

 the brain. That this sup- 

 position is not entirely cor- 

 rect was proved by experi- 

 mental physiology. Sec- 

 tion of the posterior fu- 

 niculi causes little or no 

 obvious loss of sensations 

 in the parts below the 

 lesion. JHistological inves- 

 tigation has since shown 

 that only a portion of the 

 fibers entering through the 

 posterior root continue up 

 the cord in the posterior 

 funiculi; some and indeed 

 a large proportion of the 

 whole number enter into 

 the gray matter and end 

 around tract cells, whence the path is continued upward by the 

 axons of these latter cells, mainly in the lateral or anterolateral 

 funicuh. The several ways in which the posterior root fibers 

 may end in the cord are indicated in Fig. 76. 



The posterior roots contain fibers of different diameters, and 

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



Fig. 76. — Schema to show the terminations 

 of the entering fibers of the posterior root : 1, 

 Fibers entering zone of Lissauer and terminating 

 in posterior column; 2, fiber terminating around a 

 tract cell whose axon passes into white matter 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 column (reflex arc); 5, fiber terminating 

 in tract cell of dorsal nucleus; 6, fiber (exog- 

 enous) passing upward in posterior funiculus to 

 terminate in the medulla oblongata. 



