Tracts of the Posterior Column of the Cord (Figs. 142, 143). In the posterior column of the spinal cord, there are, first, two ascending and two descending tracts, derived from posterior nerve roots; second, the posterior fasciculus proprius, which is in the ventral part of the columna; and, third, the entry zone occupied by the incoming fibers of the posterior roots.
Entry Zone (Figs. 142 and 143),—Over the apex and along the medial surface of the posterior columna of gray substance the posterior roots of the spinal nerves enter the cord and divide T-like into ascending and descending branches. The name entry zone is well applied to this region. The presence of hori zontal fibers distinguishes the entry zone from the longitudinal tracts. The root-fibers of small caliber and many collaterals very soon enter the gray substance. The large fibers, in part, enter the dorsal nucleus, but the greater number form the longi tudinal tracts of the posterior column. The ascending fibers begin their upward course in the entry zone. As they ascend, the posterior gray column is crowded lateralward 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 trend in a dorso-medial direction, approaching the median septum, 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, to gether with the fibers of the posterior fasciculus proprius which rise in the posterior columna, constitute the longitudinal 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 dis tinct ascending tracts, a postero-medial, the fasciculus gracilis, and a postero-lateral, the fasciculus cuneatus (Fig. 142).
These two tracts are alike in constitution. They have the same function, carrying impulses that excite the following sensations: of posture and movement (muscle-sense), of two or more simul taneous contacts (tactile discrimination), of size, shape and form in three dimensions, of weight, of vibration, of roughness and texture (?), of light touch and pressure touch. The impulses produced by light touch and pressure also ascend another path in the opposite lateral column. The two ascending tracts of the posterior column of the spinal cord 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. 142 and 143). It ascends along the posterior median septum to the nucleus funiculi gracilis of the medulla. Up to the lower thoracic nerves it gradually enlarges, due to the acquisition of successive pos terior root-fibers; and in this region, excepting only the entry zone and the descending branches of the posterior roots, it comprises the whole extrinsic part of the posterior column. Its size is not increased in the upper three-fourths of the cord, where the posterior intermediate furrow separates it from the fasciculus cuneatus; it rather diminishes above the fifth thoracic nerve, as no root-fibers enter it above that level and collaterals leave it in every successive segment to end within the gray crescent. In depth it almost reaches the gray commissure, near which it intermingles with the posterior fasciculus proprius. Its fibers are the ascending rami of T-branched axones originating in the spinal ganglia.
Arriving at the clava, all the fibers of the fasciculus gracilis arborize and end in the nucleus funiculi gracilis.