CHAPTER VII 



FIBER TRACTS OF THE SPINAL CORD 



THE fibers composing the white substance of the spinal cord are not scat- 

 tered and intermingled at random, but, on the contrary, those of a given func- 

 tion are grouped together in more or less definite bundles. A bundle of fibers 

 all of which have the same origin, termination, and function is known as a fiber 

 tract. The funiculi of the spinal cord are composed of many such tracts of 

 longitudinal fibers, which, while occupying fairly definite areas, blend more or 

 less with each other, in the sense that there is considerable intermingling of the 

 fibers of adjacent tracts. It is convenient to have a name for certain obvious 

 subdivisions of the funiculi which contain fibers belonging to more than one tract. 

 Such a mixed bundle is properly called a. fasciculus. 



THE INTRAMEDULLARY COURSE OF THE DORSAL ROOT FIBERS 



The central end of a dorsal root breaks up into many rootlets or filaments 

 (fila radicularia) , which enter the spinal cord in linear order along the line of 

 the posterior lateral sulcus. As it enters the cord each filament can be seen to 

 separate into a larger medial and a much smaller lateral division. The fibers of 

 the medial division are of relatively large caliber and run over the tip of the 

 posterior column into the posterior funiculus (Fig. 72). Those of the lateral 

 division are fine and enter a small fascicle which lies along the apex of the pos- 

 terior column, the fasciculus dorsolateralis or tract of Lissauer. Very soon 

 after their entrance into the cord each dorsal root fiber divides in the manner of 

 a Y into a longer ascending and a shorter descending branch (Fig. 70). 



The ascending branches of the fibers of the medial division of the dorsal root 

 run for considerable but varying distances in the posterior funiculus; some from 

 each root reach the medulla oblongata, others terminate at different levels in the 

 gray matter of the spinal cord. At the level of their entry into the cord these 

 fibers occupy the lateral portion of the fasiculus cuneatus; but in their course 

 cephalad, as each successive root adds its quota, those from the more caudal 

 roots are displaced medianward. In this way the longer fibers come to occupy 

 the medial portion of the posterior funiculus (Fig. 71). In the cervical regior 



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