Reference > Anatomy of the Human Body > Page 777
of the medulla spinalis, and may be regarded as dorsal projections of this, each being covered superficially by the fibers of the corresponding fasciculus. On transverse section (Fig. 694) the nucleus gracilis appears as a single, more or less quadrangular mass, while the nucleus cuneatus consists of two parts: a larger, somewhat triangular, medial nucleus, composed of small or medium-sized cells, and a smaller lateral nucleus containing large cells.

FIG. 690– Deep dissection of brain-stem. Ventral view. (See enlarged image)

  The fibers of the fasciculus gracilis and fasciculus cuneatus end by arborizing around the cells of these nuclei (Fig. 692). From the cells of the nuclei new fibers arise; some of these are continued as the posterior external arcuate fibers into the inferior peduncle, and through it to the cerebellum, but most of them pass forward through the neck of the posterior column, thus cutting off its head from its base (Fig. 693). Curving forward, they decussate in the middle line with the corresponding fibers of the opposite side, and run upward immediately behind the cerebrospinal fibers, as a flattened band, named the lemniscus or fillet. The decussation of these sensory fibers is situated above that of the motor fibers, and is named the decussation of the lemniscus or sensory decussation. The lemniscus is joined by the spinothalamic fasciculus (page 792), the fibers of which are derived from the cells of the gray substance of the opposite side of the medulla spinalis.
  The base of the posterior column at first lies on the dorsal aspect of the central canal, but when the latter opens into the fourth ventricle, it appears in the lateral part of the rhomboid fossa. It forms the terminal nuclei of the sensory fibers of


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