294 THE RHOMBENCEPHALON. 



longitudinal bundles, the substantia reticularis alba and two 

 nuclei, the arcuate nucleus and the medial accessory olivary nucleus. 

 Excepting that part forming the lateral pyramidal tract and the 

 medial fillet, the anterior area is continued in the anterior column 

 of the spinal cord. It is naturally divided into a ventral and a 

 dorsal part, or the region of the pyramid and the region of the 

 substantia reticularis alba. 



The pyramid (pyramis) with the arcuate nucleus imbedded in 

 it and the arcuate fibers winding over it, occupies the ventral 

 portion of the anterior area (Figs. 92 and 94). It is the continua- 

 tion of the pyramidal tract and is composed of the axones of 

 cortical cells in the anterior central gyrus of the cerebrum. The 

 pyramidal tract diminishes in size as it descends through the pons 

 and medulla, because some of its fibers terminate in ramifications 

 about the cells of cerebral nerve nuclei. In the lower part of the 

 medulla, the pyramid breaks up into the anterior (direct) pyram- 

 idal tract (10 per cent, of the pyramid), which descends along 

 the anterior median fissure in the same side of the spinal cord; 

 and the lateral (crossed) pyramidal tract (80 per cent, of the pyra- 

 mid), which, after decussating with its fellow through the anterior 

 median fissure, runs down in the lateral column of the opposite 

 side of the cord (Fig. 95). The fibers of the anterior tract cross 

 in succession to the opposite side of the cord, through the white an- 

 terior commissure; and there, with the fibers of the lateral pyram- 

 idal tract, terminate in fibrillar end-tufts about the cell-bodies in 

 the gray matter. Thus the pyramid forms a crossed cerebral 

 tract for motor cerebral and spinal nerves. A small number of 

 pyramidal fibers (10 per cent.) diverge lateral ward from the 

 pyramid in the medulla and descend in the lateral funiculus of 

 the cord without decussation. They account for the weakness 

 on the well side, and for slight motion on the paralyzed side, 

 which are commonly observed in hemiplegia. If, as E. A. Schafer 

 claims, the fibers of the pyramidal tracts end in the posterior 

 columna of gray matter, then at least one neurone intervenes 

 between them and the motor neurones of the spinal nerves; but 

 without doubt, they enter into either direct or indirect relations 

 with those neurones. 



