360 THE SPINAL CORD. 



udinal tracts of the posterior column. The ascending -fibers 

 begin their upward course in the entry zone. As they ascend they 

 are crowded toward the median septum by the entrance of new 

 root-fibers in the succeeding upper segments. So the fibers 

 near the septum are those which enter low down in the cord and 

 those close to the posterior columna are of recent entrance into 

 the cord. The descending T-branches and collaterals of the 

 posterior roots begin their descent also in the entry zone. Some 

 of them run a long course, even from the sixth cervical to the 

 sacral segments (Collier). The greater number are much shorter. 

 From above downward they are crowded in a dorso-medial direc- 

 tion, like the ascending fibers; and, after a considerable downward 

 course, they plunge forward through the white column to end in 

 the posterior columna. These ascending and descending fibers, 

 whose origin is in the spinal ganglia on the posterior roots of the 

 spinal nerves, together with the fibers of the posterior fasciculus 

 proprius which rise in the posterior columna, constitute the longit- 

 udinal tracts of the posterior column of the spinal cord. That 

 posterior column is undivided by any sulcus below the eighth 

 thoracic segment, where the posterior intermediate furrow and 

 septum fade away; but, above that level, it is subdivided into 

 two distinct ascending tracts, a postero-medial, the fasciculus 

 gracilis, and a postero-lateral, the fasciculus cuneatus (Fig. 102). 

 These two tracts are alike in constitution. They have the same 

 function, carrying impressions of the muscular and tactile senses. 

 They differ only in length; the fibers of the fasciculus gracilis 

 come from the spinal nerves below the eighth thoracic, while those 

 of the fasciculus cuneatus come entirely from thoracic and cervical 

 nerves. 



Fasciculus Gracilis (Ascending Postero-medial Tract, Goll's 

 Column). This tract may be said to begin at the entrance of the 

 posterior root of the coccygeal nerve (Figs. 102 and 103). It 

 ascends along the posterior median septum to the nucleus funiculi 

 gracilis of the medulla. Up to the lower thoracic nerves it gradu- 

 ally enlarges, due to the acquisition of successive posterior root- 

 fibers; and in this region, excepting only the entry zone and the 

 descending branches of the posterior roots, it comprises the whole 



