158 PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



higher parts of the cord or brain are designated as ascending, since 

 normally the impulses conducted by them take this direction. They 

 constitute the afferent or sensory paths, and in case of injury to the 

 fiber or cell the secondary degeneration also extends upward. The 

 reverse, of course, holds true for the descending or motor paths. 

 The tracts may be divided also into long and short (or segmental) 

 tracts. The latter group comprises those tracts or fibers which 

 have a short course only in the white matter, extending over a dis- 

 tance of one or more spinal segments. Histologically the fibers of 

 these tracts take their origin from the tract cells in the gray matter 

 of the cord and after running in the white matter for a distance of 

 one or more segments they again enter the gray matter to terminate 

 around the dendritic processes of another neuron. These short 

 tracts may be ascending or descending, and the impulses that they 

 conduct are conveyed up or down the cord by a series of neurons, 

 each of whose axons runs only a short distance in the white matter, 

 and then conveys its impulse to another neuron whose axon in turn 

 extends for a segment or two in the white matter, and so on. 

 These tracts are sometimes described as association or short associa- 

 tion tracts, because they form the mechanism by which the activi- 

 ties of different segments of the cord are brought into association. 

 This method of conduction by segmental relays involving the par- 

 ticipation of a series of neurons may be regarded as the primitive 

 method. It indicates the original structure of the cord as a series 

 of segments, each more or less independent physiologically. The 

 short tracts in the mammalian cord lie close to the gray matter, 

 forming the bulk of what is known as the ground bundles. The 

 long tracts, on the contrary, are composed of those fibers, as- 

 cending or descending, which run a long distance, and, in fact, 

 extend from the cord to some part of the brain. It is known, 

 however, that, although the tracts as tracts extend from brain 

 to cord, many of the constituent fibers of these tracts may begin 

 and end in the cord or in the brain, as the case may be. Some 

 of the fibers of the long tracts are, therefore, so far as the. cord is 

 concerned, simply long association tracts which connect different 

 regions e. g., cervical and lumbar of the cord by a single 

 neuron, as the short association tracts connect different seg- 

 ments of the same region. It is said that in these long tracts 

 those fibers that have the shortest course lie to the inside that 

 is, nearest to the gray matter.* From the results of comparative 

 studies of the different vertebrates we may conclude that the long 

 tracts are a relatively late development in their phylogenetic his- 

 tory, and that in the most highly developed animals, man and the 



* Sherrington and Laslett, "Journal of Physiology/' 29, 188, 1903; and 

 Sherrington, ibid., 14, 255. 



