TRACTS OF THE POSTERIOR COLUMN.
The posterior column is composed, in largest part, of the continuations of the sensory posterior root-fibres, that proceed from the spinal ganglia (Fig. 150). The cells of the latter give off a nerve-process, which soon divides into two branches, one passing peripheral ward and the other centralward. The centrally coursing branches enter the spinal cord, as the pos terior root, as two more or less distinct bundles. One of these is made up of fine fibres, lies lateral and passes toward the substantia gelatinosa Ro landi; the other consists of coarse fibres, lies medial and passes toward the posterior column. The en trance zone of the lateral bundle, between the apex of the posterior horn and the periphery of the cord, is known as Lissauer's marginal zone; that of the medial bundle, to the inner side of the posterior horn, is called the radicular zone. Immediately upon entering the spinal cord, the fibres of both bundles undergo a branching. Both result ing branches assume a longitudinal direction and during their course, respectively up or down, give off numerous collaterals to the gray substance of the cord. The descending branch is the thinner and ends, after a short course, within the gray substance. According to their length, the ascending fibres are short, medium or long. The short fibres pass into the gray substance after a very limited course; those of medium length proceed farther upward, but like wise end within the cord by bending over into the gray substance; while the long fibres ascend to the medulla oblongata, where they end within the poste rior column nuclei, the nucleus gracilis and cuneatus.
The fibres entering the cord below are dis placed more and more towards the mid-line by the fibres entering at higher levels ; those fibres, therefore, that on entering the cord occupy the lateral part of the posterior column,as they ascend soon collectively constitute the middle and, finally, the innermost part of the column. Hence, as already noted, the posterior column exhibits in the cervical region of the cord, a subdivision into the medial fasciculus gracilis or Golfs column and the lateral fasciculus cunealus or Burclach' s demarcation not emphasized in the lower part of the cord. Goll's column consists essentially of fibres that come from the lower segments of the cord and is nothing more than the continuation of the laterally situated fibres of the lower segments, which during their ascent have been pushed toward the mid-line by the new increments of fibres entering at higher levels. Or, we may
say, in the cervical region of the cord, Goll's column is composed of fibres which ascend from the lower parts of the cord and conveys sensory fibres from the lower extremities and the lower half of the trunk, while Burdach's column carries sensory fibres that enter the spinal cord from the upper half of the trunk and the upper extremities (Fig. 151).
The terminal arborization of the ascending fibres and of the collaterals occurs in almost all parts of the gray substance of the same side of the cord ; a small part of the fibres passes, by way of the posterior commissure, to the opposite side to end within the posterior horn. The short fibres and the collaterals of the lateral bundle, in particular, end within the homolateral posterior horn and also within the central field ; the main fibres and collaterals of the median bundle, which end within the cord, arborize around the cells of Clarke's column, the cells of the intermediate zone and the anterior horn cells. The collaterals from the posterior column, which break up around the anterior horn cells, constitute the reflex collateral:.
The descending branches of the fibres of the posterior column, entering the root zone medial to the posterior horn, caudalward form a bundle that in cross-section appears comma-shaped. The fibres of this field, the comma bundle of Schultze, after a short course enter the gray substance.
In addition to the chief fibres, within the posterior column are others which arise in the posterior horns of the cord, as the axones of association-cells. They course within the ventral part of the posterior column and in cross-section appear as the ventral field (Fig. 149).
Finally to be noted are fibres that extend from the cervical region as far as the conus terminalis. In the upper regions, they lie dorsally at the periphery of the poste rior column, more within the area of Goll's column ; farther below, they migrate toward the septum posterius and, within the sacral region, in cross-section appear as a small medial oval field. This part has been termed the oval bundle of the posterior column. It corresponds to the bandelette mediale of Gombault and Philippe, the dorso-medial sacral field of Obersteiner and the tractus cervico-lumbalis dorsalis of Edinger.
The chief tracts descending to and ascending from the spinal cord are represented in Figs. 152 and 153.