166 PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



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

 tracts. The latter group comprises those tracts or fibers which 

 have only a short course 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 anterior and lateral 

 proper fasciculi. The long tracts, on the contrary, are com- 

 posed of those fibers, ascending 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 their constituent 

 fibers 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, there- 

 fore, so far as the cord is concerned, simply long association 

 tracts which connect different regions e. g., cervical and lum- 

 bar of the cord by a single neuron, as the short asso- 

 ciation tracts connect different segments 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 history, and that in the 

 most highly developed animals, man and the anthropoid apes, 

 these long tracts are more conspicuous and form a larger per- 

 centage of the total area of the cord. A physiological corollary 

 of this conclusion should be that in man the independent activity 

 of the cord is less marked than in the lower vertebrates, and 

 this deduction is borne out by facts (see p. 145). 



* Sherrington and Lasiett, "Journal of Physiology," 29, 188, 1903; and 

 Sherrington, ibid., 14, 255. 



