THE COURSE OF THE FIBERS IN" THE SPINAL COED. 355 



The portions of the dorsal roots Just described lie to the median side 

 of the point of the posterior horn. Lateral to it (4) lie small bundles of 

 coarse fibers, which may be traced through the substantia gelatinosa, and the 

 rest of the horn anterior to it, up to the large cells of the anterior horn, 

 around which they arborize. This tract is usually regarded as the shortest 

 reflex path (Fig. 33). 



More laterally still (5) are root-fibers which enter the gray matter after 

 a longer or shorter course. These fibers split, just on entering the periphery 

 of the gray matter, or soon afterward into an ascending and a descending 

 branch. Many of them, especially large-fibered, cross through the sub- 

 stantia gelatinosa Kolandi before splitting. Prom these two divisions there 

 pass out numerous collaterals in the gray matter, especially of the posterior 

 horn, where they subdivide into small, thin tufts, around the cells there 

 found. The more delicate fibers divide generally at the periphery of the 

 posterior horn. So there arises, between the point, or extremity, of the pos- 

 terior horn and the periphery of the cord, a field which is filled with these 

 ascending and descending thin fibrils (zona marginalis; z. terminalis). 

 From this zone pass continually fine fibrils into a net-work lying between it 

 and the gelatinous substance, — zona spongiosa, — and from this net-work 

 arise again other fine fibers, which pass through the substantia gelatinosa, 

 and reach the fibrous tangle of the posterior horn. Probably they then enter 

 into similar relationship to the cells there, as the coarse fibers, just described. 



It must, however, not be overlooked, that much of what has just been stated 

 regarding the fibrous tracts of the posterior spinal roots has not, with certainty, been 

 ascertained to be true in man. As far as they have been sought, though, correspond- 

 ing arrangements have been found, as in those mammals that have been thoroughly 

 examined. 



So far, therefore, can the sensory tracts be followed into the cord: As 

 the most important of them are established, first, those entering the posterior 

 columns and ascending in them toward the brain; second, those ending in 

 Clarke's vesicular column; third, those — comprising the greater mass of the 

 lateral root-fibers — which, after a greater or less course, arborize aroimd 

 cells of the anterior and posterior horns. Besides these there are fibers 

 known which pass into the lateral mixed zone, some coming from the an- 

 terior horn, some going to it. 



As to those fibers which arrive at the posterior horns and the gray 

 matter just in front of these, it has been, in all probability, determined how 

 they enter into relationship with higher centers. From the ganglion-cells 

 around which the root-fibers arborize arises a second set of fibers. The axis- 

 cylinders of these cells turn forward and inward to the anterior commissure, 

 cross over in it to the anterior or lateral column of the opposite side, in which 

 they ascend farther. What portion of the antero-lateral column is really to 



